Responsive Polymers as Sensors, Muscles, and Self-Healing Materials.

Top Curr Chem (2015) DOI: 10.1007/128_2015_626 # Springer International Publishing Switzerland 2015

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials Qiang Matthew Zhang and Michael J. Serpe

Abstract Responsive polymer-based materials can adapt to their surrounding environment by expanding and shrinking. This swelling and shrinking (mechanotransduction) can result in a number of functions. For example, the response can be used to lift masses, move objects, and can be used for sensing certain species in a system. Furthermore, responsive polymers can also yield materials capable of self-healing any damage affecting their mechanical properties. In this chapter we detail many examples of how mechanical responses can be triggered by external electric and/or magnetic fields, hygroscopicity, pH, temperature, and many other stimuli. We highlight how the specific responses can be used for artificial muscles, self-healing materials, and sensors, with particular focus on detailing the polymer response yielding desired effects. Keywords Artificial muscles Mechanochemistry Responsive polymers Selfhealing materials Sensors

Contents 1 Introduction 2 Artificial Muscles 2.1 Dielectric Elastomers 2.2 Ionic Polymers 2.3 Conducting Polymers 2.4 Liquid Crystal Elastomers 3 Self-Healing Polymers 3.1 Thermoresponsive Cycloaddition-Based Polymeric Smart Materials 3.2 Dynamic Covalent Bonds 3.3 Hydrogen Bonding

Q.M. Zhang and M.J. Serpe (*) Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada, T6G 2G2 e-mail: [emailprotected]

Q.M. Zhang and M.J. Serpe 3.4 Metal–Ligand Coordination 3.5 π–π Stacking Interactions 4 Mechanotransducing Sensors 4.1 Mechanotransducing pH Sensors 4.2 Mechanotransducing Chemical Sensors 4.3 Mechanotransducing Biosensors 5 Summary and Conclusions References

1 Introduction Sea cucumbers rapidly change their stiffness by several orders of magnitude in the face of danger, Venus flytraps close their leaves and trap insects in response to stimulation of their trigger hairs, and octopus, squid, and cuttlefish quickly change their color for camouflage or to warn potential predators [1]. These are just a few of the many examples of nature’s ability to respond to external stimuli. Polymer scientists and engineers have been working very hard to develop novel materials that can mimic this incredible behavior. To accomplish this, stimuli-responsive polymers are often the materials of choice. Stimuli-responsive polymers (also called smart/intelligent polymers) are capable of significantly changing their properties such as shape, mechanical properties, solubility, permeability, optical properties, and electrical properties upon the application of a stimulus; common stimuli include temperature, pH, light, magnetic field, electrical field, sonication, solvents, ions, enzymes, and specific organic compounds [2–8]. Several classes of stimuli-responsive polymers have been reported. Polysaccharides, proteins, and nucleic acids represent one group of stimuli-responsive biopolymers which can be found widely in living systems [9–11]. Indeed, these materials belong to the oldest class of substances in Nature with stimuli-responsive properties. However, for many years, synthetic polymers with stimuli-responsive properties have attracted a similar level of attention within the scientific community [12]. Table 1 lists some examples of stimuli-responsive polymers, and the stimuli that can be used elicit a response [13]. Of all stimuli-responsive polymers, temperature-responsive polymers are the best known and most studied. Among those, polymers that exhibit a lower critical solution temperature (LCST) have found the widest applicability [14]. The LCST is a fascinating phenomenon found for various polymer solutions. Polymer solutions often exhibit both an LCST and an upper critical solution temperature (UCST). For the LCST, at temperatures below the LCST the polymer is completely miscible in the solvent, whereas at temperatures above the LCST a phase separation occurs. In fact, the most investigated temperature-responsive polymer featuring a LCST in water is poly(N-isopropylacrylamide) (pNIPAm). The LCST of pNIPAm is ~32 C,

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials Table 1 Abbreviated names of polymers with their associated responsivities Polymer BIS PAA PAAEM PAm PBA PDEA PDMS PDPA PEO PGMA PHEMA PHFBMA PLG PLLA PMMA PMPC PNaA PNaVBA PNCL PNIPAM PPO PSMA PVIm

Bisacrylamide Poly(acrylic acid) Poly(acetoacetoxyethyl methacrylate) Poly(acrylamides) Poly(butyl acrylate) Poly[2-(diethylamino) ethyl methacrylate] Poly(dimethylsiloxane) Poly[2-(diisopropylamino) ethyl methacrylate] Poly(ethylene oxide) Poly(glycerol monomethacrylate) Poly(hexyl ethyl methacrylate) Poly(hexafluoro butylmethacrylamide) Poly (glutamic acid) Poly(L-lactides) Poly(methyl) methacrylate Poly[2-(methacryloyloxy) ethyl phosphorylcholine] Poly(sodium acrylate) Poly(sodium-4-vinylbenzoate) Poly(N-vinylcaprolactone) Poly(N-isopropylacrylamide) Poly(propylene oxide) Poly(stearyl methacrylate) Poly(N-vinylimidazole)

Types of stimulus pH

pH E-field, bT-field pH T-field

T-field pH

E-field, T-field pH pH T-field, pH T-field T-field pH

Reproduced with permission from [13] a Electrical field b Thermal field

which is close to human body temperature [15]. Consequently, by altering the temperature of pNIPAm in water, its solubility can be tuned, e.g., pNIPAm transitions from hydrophilic (soluble) to hydrophobic (insoluble) as the LCST is exceeded. Other N-substituted polyacrylamides [16, 17] and other classes of polymers such as poly(oligoethyleneoxide-(meth)acrylate)s [18] or poly(2-oxazoline)s [19] have also been shown to exhibit thermoresponsivity and exhibit an LCST. Another class of responsive polymers that have been found to be useful is light responsive polymers. The response of the polymer, and its reversibility, depends greatly on the functional groups making up the polymer. For example, some photochromic molecules undergo a reversible isomerization upon irradiation [20]. This process is usually accompanied with a polarity change as well as a color change in the chromophoric units. Such phenomena can be observed in chemical compounds such as azobenzene [21], spiropyran [22], and triphenylmeth-

Q.M. Zhang and M.J. Serpe

ane leuco [23]. Irreversible light responsivity is achieved when polymers contain photocleavable units, which subsequently yield the polymer response [24]. For example, the ester bond of o-nitrobenzyl ester (ONB) [25, 26] irreversibly breaks upon exposure to UV light. Photocleavage reactions are generally initiated by UV-light; however, under certain conditions, near infrared (NIR)-light can also be used for cleavage of ONB-groups [25]. Mechanochemistry, even though a relatively new topic, has matured considerably over recent years. It has implications for material wear, abrasion, friction, and lubrication; this ultimately affects the performance of materials and their properties. Most important for this submission are volume/conformational changes of responsive polymers, which can yield a specific function [27]. While this kind of mechanochemistry is not a result of molecular scale bond breaking and reformation, it is nevertheless mechanochemistry. This is because the system’s volume/conformational changes are in response to the environment, which changes the chemistry of the system and yields a response. For example, as pointed out in examples below, polymers composed of a weak acid can be ionized in a pH-dependent fashion; the “chemical reaction” of base with the polymer causes its ionization and a "mechanochemical" swelling response. Up to now, most reviews on responsive polymers focus on its applications such as drug delivery and biomaterials [28]. In this review, we showcase specific responsive polymers and their use as muscles, self-healing materials, and sensors.

2 Artificial Muscles The ability to mimic the muscles in humans, both for the improvement of the quality of human life and in some cases simply for our amusem*nt, has been a topic of great interest for some time. The performance of the emerging polymer actuators exceeds that of natural muscle in many respects, making them particularly attractive for real world applications. For example, muscle-like behavior is desirable in medical devices, prostheses, robotics, toys, biomimetic devices, and micro/ nanoelectromechanical systems [29, 30]. Several “smart materials” have been proposed as artificial muscles, such as shape memory alloys, magnetostrictive alloys, piezoelectrics, and responsive polymers [31]. Polymer-based artificial muscle technologies are being developed, which produce high strains and stresses in response to electrostatic forces, electrostriction, ion insertion, and molecular conformational changes [32]. Materials used include elastomers, conducting polymers, ionically conducting polymers, and gels.

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 1 Process for preparing S-IPN films: (a) solution is drop cast onto treated glass; (b) solvent is allowed to evaporate and the RTV silicone cures at room temperature; (c) film is peeled off of the glass substrate; (d) film is stretched biaxially by 100 100%; (e) HTV silicone is cured at 180 C for 30 min; (f) film is relaxed and some prestrain is preserved. Reproduced with permission from [34]

2.1

Dielectric Elastomers

Dielectric elastomers (DEs) behave as compliant capacitors, expanding in area and shrinking in thickness when a voltage is applied [33]. They consist of a thin elastomeric film coated on two sides with compliant electrodes. When an electric field is applied across the electrodes, a stress is generated on the film because of electrostatic attraction between opposite charges on these electrodes as well as repulsion from similar charges on each electrode. This stress causes the film to contract in thickness and expand in area. Most elastomers used are essentially incompressible, so any decrease in thickness results in a concomitant increase in the planar area. An all-silicone pre strain-locked interpenetrating polymer network (all-S-IPN) elastomer has been developed as a muscle-like actuator [34]. The elastomer was fabricated using a combination of two silicones: a soft room temperature vulcanizing (RTV) silicone as the host elastomer matrix, and a more rigid high temperature vulcanizing (HTV) silicone to preserve the prestrain in the host network (Fig. 1). In the S-IPN fabrication procedure, the RTV and HTV silicones

Q.M. Zhang and M.J. Serpe

were codissolved in a common solvent, cast into thin films, and the RTV silicone allowed to cure before applying prestrain and finally curing the HTV silicone to lock in the prestrain. A performance improvement of the prestrain-locked silicones over standard silicone films was achieved, with a linear strain of 25% and an area strain of 45% when tested in a diaphragm configuration. The process can also be used to improve electrode adhesion and stability as well as improve the interlayer adhesion in multilayer actuators. The improved interlayer adhesion showed a longlife (>30,000 cycles at >20% strain) and repeated high-performance actuation (>500 cycles at ~40% strain) of prestrained free-standing multilayer actuators. A number of approaches have been explored for increasing the dielectric constant of DEs. This is most commonly achieved by adding a high dielectric constant filler material to an elastomer host, such as aluminum oxide, titanium oxide, and barium titanium oxide [35–37]. For example, Standard Thai Rubber 5 L (STR 5 L)/ aluminum oxide (Al2O3) composite (60 wt% Al2O3) was modified by cross-linking with 1 wt% dicumyl peroxide (DCP)/Irganox 1076 and cured at 170 C [35]. The deflection responses of the dielectric elastomer actuators were investigated under electrical field strengths of 0 and 650 V/mm at room temperature (27 C). The devices deflect toward the anode side at the electrical field strength of 200 V/mm; the degree of bending increases monotonically with increasing electrical field strength up to 650 V/mm [38]. Upon removal of the applied electric field, the device returns to its original position and shape (Fig. 2). The X-ray diffraction pattern of the film suggests that the Al2O3 generates dipole moments in the elastomer matrix, while scanning electron microscope images of the composites show that Al2O3 particles were uniformly distributed in the natural rubber matrix. A polymer-based device capable of lifting many times its own mass was fabricated by drying a solution of the polycation poly(diallyldimethyl ammonium chloride) (pDADMAC) on a flexible surface coated with charged poly(N-isopropylacrylamide)-based microgels (Fig. 3) [39–41]. Upon drying of the pDADMAC solution on the microgel-modified surface, it bends. The authors proposed that the microgels serve as “glue” that allows the contraction of the pDADMAC layer to be translated to the underlying substrate. That is, there are multiple, electrostatic interactions between pDADMAC and the charged microgels. Therefore, the contraction of the pDADMAC layer upon drying can be translated to the substrate through its interaction with the microgels, and the microgels transfer the contraction to the Au-coated surface, thereby pulling the sides of the substrate up. Flexible surfaces then curl up into scroll-like structures, which can be opened up at high humidity. These arms are able to lift relatively large masses, and resist forces many times their own mass.

2.2

Ionic Polymers

An alternative means of producing actuation in a polymer is to employ ions that are mobile within the polymer phase. An applied field drives the motion of these ions and entrained solvent, leading to swelling or contraction when the ions enter or

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 2 Photographs of deflection responses under electrical field strength of 100–650 V/mm of the dielectric elastomer compounds of formula 8: (a) measured at 0 V/mm; (b) measured at 100 V/ mm; (c) measured at 200 V/mm; (d) measured at 300 V/mm; (e) measured at 400 V/mm; (f) measured at 500 V/mm; (g) measured at 600 V/mm; (h) measured at 650 V/mm; (i) after switching off the electrical field for 60 s. The tip of the red arrow is pointing at the device. Reproduced with permission from [38]

leave regions of the polymer. If the polymer phase is electrically conductive, then the ions serve to balance charge generated on these conductors as the potential is changed, creating very strong local fields (but overall low voltage). The voltages employed in these materials are low (1–7 V), but the energies are nonetheless high

Q.M. Zhang and M.J. Serpe

Fig. 3 The preparation process (a) and strength test (b) of the device. Reproduced with permission from [39]

because of the close spacing between ions and electronic charges and the large amount of charge that can be transferred. A high-performance electro-active artificial muscle was prepared using pendent sulfonated chitosan (PSC) and functionalized graphene oxide (GO), which exhibited strong electro-chemomechanical interactions with ionic liquid (IL) in an open air environment (Fig. 4) [42]. The GO-PSC-IL nano-biopolymer membrane shows an increased tensile strength with ionic exchange capacity up to 83.1% which increased ionic conductivity over 18 times. The high ionic conductivity results in twice the bending actuation compared to the pure chitosan actuator under the same electrical input signals. The GO-PSC-IL actuators could show robust and high-performance actuation at low applied voltages (5 V) which are required in realistic applications. Ionic polymer–metal composites (IPMCs) have received enormous research interest as unique electroactive polymers (EAPs) over the past decade because of their soft and flexible structure, relatively large electromechanical bending, and low driving voltage [43, 44]. With the ability to operate in an aqueous environment and closely mimic the motion of biological muscles, IPMC materials are particularly attractive for artificial muscle applications [43]. In one example, a nanostructured electrode surface was fabricated using platinum nanothorn assemblies and a Nafion membrane [37]. The nanostructured actuator shows a new way to achieve highly enhanced electromechanical performance over existing flat/featureless electrodes. The authors demonstrated that the formation and growth of the nanothorn assemblies at the electrode interface lead to a dramatic improvement (three- to fivefold increase) in actuation range, as shown in Fig. 5. These advances are attributed to the high capacitance of the nanothorn assemblies, which increases significantly the charge transport speed during the actuation process.

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 4 Schematic illustration for pendent sulfonated chitosan and graphene oxide-pendent sulfonated chitosan-ionic liquid (GO-PSC-IL) nano-biopolymer actuator. Reproduced with permission from [42]

2.3

Conducting Polymers

Conducting polymers are typically semiconducting when undoped and conducting when doped with donor or acceptor ions [45]. The conformational changes of conducting polymer-based artificial muscles are a result of electrochemical ion insertion and removal, possibly along with other associated solvating species. One example utilizing conducting polymers showed that interpenetrating polymer networks (IPN) could be prepared using a hot pressing method to combine a polytetrahydrofuran network for mechanical resistance and a poly(ethylene oxide) (PEO) as a solid polymer electrolyte [46]. After interpenetration of two poly

Q.M. Zhang and M.J. Serpe

Fig. 5 Transported charge/displacement correlation. Peak-to-peak displacement vs transported charge (a) at 0.1 Hz, 61 V and (b) at 0.1 Hz, 3 V AC square-wave input. Reproduced with permission from [43]

Fig. 6 (a) Beam shaped IPN macroactuator (△E ¼ 2 V). (b) Oxidation-reduction process of conducting polymer. Reproduced with permission from [46]

(3,4-ethylenedioxythiophene) (PEDOT) electrodes in both faces of the IPN film and swelling in an ionic liquid, a 20 μm thick conducting IPN actuator was obtained, which moved with large displacement at a 125 Hz fundamental frequency (Fig. 6). A humidity-sensitive conducting polymer actuator made up of PEDOT doped with poly(4-styrenesulfonate) (PEDOT/PSS) was fabricated [47]. Water vapor sorption and electro-active actuating behavior of free-standing PEDOT/PSS films were investigated by sorption isotherm and electromechanical analyses. The non-porous PEDOT/PSS film, with a specific surface area of 0.13 m2 g1, sorbed water vapor of 1,080 cm3(STP) g1, corresponding to 87 wt%, at relative water

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials Fig. 7 Time profiles of electric current, surface temperature, and length change of PEDOT/PSS films (50 mm long, 2 mm wide, and 10 μm thick) under 10 V measured at 50% RH and various temperatures. Reproduced with permission from [47]

vapor pressure of 0.95. As temperature increased from 25 C to 40 C, the actuator exhibited a lower sorption degree, which is an indicator of an exothermic process. Isosteric heat of sorption decreased with increasing water vapor sorption and the value reached 43.9 kJ mol1, consistent with the heat of water condensation (44 kJ mol1). Upon application of 10 V, the film underwent 2.46% contraction at 5 C, which was caused by desorption of water vapor, and the contraction slightly decreased to 2.10% at 45 C (Fig. 7). The speed of contraction was an order of magnitude faster than that of expansion and less dependent on the temperature. This behavior is attributed to water vapor in the film desorbing at high temperature. In contrast, the higher the temperature the faster the film expansion because the diffusion coefficient at higher temperature is increased. A series of strong and flexible polymer films were developed by combining both a rigid matrix (polypyrrole (PPy)) and a dynamic network (polyol-borate (PEE)) [16]. Upon water sorption and desorption, the borate ester of PPy can be hydrolyzed and reformed, which changes the mechanical properties of the composite at different humidity (Fig. 8). Intermolecular hydrogen bonding between the PEE network and PPy also modulates intermolecular packing of the polymer composite, altering its mechanical properties in response to water. The polymer composite exhibits fast, reversible, and dramatic mechanical deformation and recovery in response to environmental moisture, visually reminiscent of “fast twitch” muscle activity.

Q.M. Zhang and M.J. Serpe

Fig. 8 Characterization of PEEPPy composite films. (a) A PEEPPy composite film (black) is composed of PPy polymer chains (gray lines) and a PEE-borate network (red lines). The structure changes (involving H bonds and borate ester bonds) in response to water (blue dots) sorption and desorption. (b) PEE-PPy weight change (red) synchronizes with air humidity change (black). (c) ATR-IR spectra showing H/D exchange between the PEE-PPy film and water vapor. Top to bottom: before D2O exposure and 0, 1, 2, 3, and 4 min after D2O exposure. Dashed lines indicate the three pairs of shifting peaks. (d) A PEE-PPy film maintains its flexibility and mirror-like surface after 6 months of open storage. Reproduced with permission from [48]

2.4

Liquid Crystal Elastomers

Liquid-crystalline polymers (LCPs) contain mesogens which are attached to the polymer backbones with uniform alignment. Once the molecular alignment of the mesogens is disordered by external stimuli such as heat [49] and/or electricity [50], LCPs show contraction along the mesogen’s alignment direction. For example, UV light can cause photoisomerization of azobenzene moieties; the conformation change of azobenzene moieties causes the mesogens to become disordered. By incorporating azobenzene chromophores into LCPs using monofunctional (monomer) and difunctional (crosslinker) azobenzene monomers (Fig. 9), deformation can be induced upon irradiation with UV light [51]. Photoinduced motion of the 10 μm thick LCP films was examined as shown in Fig. 10. To irradiate the LCP films under the same experimental condition, part of the LCP film was covered with a glass substrate and irradiated from above. By irradiation with UV light at 240 mW cm2, LCP films with a high azobenzene

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 9 Chemical structures of monofunctional (monomer) and difunctional (crosslinker) azobenzene monomers. Reproduced with permission from [51]

content (a46) bent beyond 90 toward the UV light source (Fig. 10B-a), while LCP films with a low azobenzene content (a42 and a20) are not capable of bending to such an extent (Fig. 10B-b). The high azobenzene content films reverted completely back to the initial flat conformation (Fig. 10B-c). The bending behavior is affected by the thickness of the film and UV light intensity. When 20 μm thick LCP films or UV light with low intensity (25 mW cm2) were used, the films did not exhibit such a “reverting motion.” A plausible mechanism for the unbending behavior is the relaxation of the gradient of the deformation along the depth direction as described in Fig. 10c. Because the incident light penetrates deep areas from the surface of the film, photoinduced contraction occurs at the opposite side of the film and at the irradiated surface. Consequently, the bent films revert to the initial shape. A plastic belt was prepared by connecting both ends of the LCP laminated film and then the belt was placed on a pulley system as illustrated in Fig. 11a [52]. By irradiating the belt with UV light from top right and visible light from top left simultaneously, a rotation of the belt was induced that drove the two pulleys in a counterclockwise direction at room temperature, as shown in Fig. 11b. A plausible mechanism of the rotation is as follows: Upon exposure to UV light, a local contraction force is generated at the irradiated part of the belt near the right pulley along the alignment direction of the azobenzene mesogens. This contraction force is parallel to the long axis of the belt and acts on the right pulley, leading it to rotate in the counterclockwise direction. At the same time, as the other side is irradiated by visible light, there is a local expansion force at the irradiated part of the belt near the left pulley. That causes a counterclockwise rotation of the left pulley. These contraction and expansion forces generated simultaneously at the different parts along the long axis of the belt give rise to rotation of the pulleys and belt in the same direction. The rotation then brings new parts of the belt to be exposed to UV and to visible light, which enables the motor system to rotate continuously.

3 Self-Healing Polymers Self-healing polymers are defined as polymeric materials capable of reforming bonds, and healing, in response to damage [53]. This research is driven by the vision that, in the future, damaged materials may not have to be replaced so often,

Q.M. Zhang and M.J. Serpe

Film Glass Substrate Alignment Direction

B a a46

600 s

15 s

b a42

c a20

C UV Light

Bending toward Light Source

Reversion by Reduction of the Deformation Gradient

Reversion to the Initial Flat Shape

Fig. 10 Schematic illustration of the experimental setup (A), photographs of LCP films exhibiting photoinduced deformation (B), and schematic illustrations of the photoinduced deformation

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 11 (a) Schematic illustration of a light-driven plastic motor system used in this study, showing the relationship between light irradiation positions and a rotation direction. (b) Series of photographs showing time profiles of the rotation of the light-driven plastic motor with the LCP laminated film induced by simultaneous irradiation with UV (366 nm, 240 mW cm2) and visible light (>500 nm, 120 mW cm2) at room temperature. Diameter of pulleys: 10 mm (left), 3 mm (right). Size of the belt: 36 5.5 mm. Thickness of the layers of the belt: PE, 50 μm; LCE, 18 μm. Reproduced with permission from [52]

⁄ Fig. 10 (continued) mechanism (flat shape: film before UV irradiation; yellow: film; red: mesogens) (C). The series of photographs in (B) show the motion of LCP films by irradiation with UV light (366 nm, 240 mW cm2) at room temperature: the first frame, before irradiation; the second frame, after irradiation for 5 s; the third frame, after continuous irradiation with UV light. The films showed different photoinduced deformation: (a) bending alone; (b) partly unbending after bending by 90 ; (c) completely unbending. Size of the film: 2 3 mm 10 μm. The white dashed line in the photographs describes the edges of the film. Reproduced with permission from [51]

Q.M. Zhang and M.J. Serpe

See Also

Xi Jinping’s Russian Lessons

Q.M. Zhang and M.J. Serpe

similar to DNE because it contains ester groups capable of hydrogen bonding with an epoxy matrix. An optimized blend of the monomers was encapsulated using a urea-formaldehyde microencapsulation procedure, and the resulting capsules were used for in situ self-healing experiments. Improved healing efficiency was observed for samples containing the DCPD/DNE capsules.

3.4

Metal–Ligand Coordination

Because of their optical and photophysical properties, metal complexes offer many advantages compared to other systems [74]. Reversibility and tunability by incorporating different metal ion and ligand substituents make coordination chemistry particularly attractive. A series of metallosupramolecular polymers have been reported, comprising an amorphous poly(ethylene-co-butylene) core with 2,6-bis(19-methylbenzimidazolyl) pyridine ligands at the termini that coordinated metal ions through ligand binding (Fig. 19). These polymers can be mended by exposure to light [75]. Upon exposure to ultraviolet light, the metal–ligand motifs are electronically

Fig. 19 Mechanism and synthesis of photohealable metallosupramolecular polymers. (a) Proposed optical healing of a metallosupramolecular, phase separated network. (b) Synthesis of macromonomer 3 and polymerization by addition of Zn(NTf2)2. DEAD, diethyl azodicarboxylate. Reproduced with permission from [75]

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 20 (a) DOPA was randomly grafted onto polyallylamine. There are several pH responsive parts of the polymer. The amine side chain itself is involved in an acid/base equilibrium with a pKa around 9.3–9.7, which has been determined by potentiometric titrations. (b) Furthermore, the catechol can be oxidized to the quinone form or it can be cross-linked by FeIII in the pH-dependent manner. Reproduced with permission from [77]

excited and the absorbed energy is converted into heat. It caused temporary disengagement of the metal–ligand motifs and a concomitant reversible decrease in the polymers’ molecular mass as well as viscosity, thereby allowing healing of mechanical damage. There is increasing evidence that metal–ligand coordination plays an important role in the dynamics of biological rearrangements. Molecular engines that generate mechanical energy can be powered by chemical processes, resulting in swelling and shrinking of macromolecular segments caused by metal–ligand interactions. Recent studies took advantage of coordination between Fe3+ and catechol ligands, which resulted in pH-induced crosslinked self-healing polymer with near-covalent elastic moduli [76]. By attaching dopamine (DOPA) to an amine-functionalized polymer, a multiresponsive system is formed upon reaction with iron (Fig. 20) [77]. The degree of polymer crosslinking is pH controlled through the pH-dependent DOPA/ iron coordination chemistry. That is, when the solution is made more basic, the hydrogels can self-heal. Close to the pKa value, or more precisely the pI (isoelectric point) value, of the polymer, the gel collapses because of reduced repulsion between polymer chains. Thereby a bistable gel-system is obtained. The polymer system closely resembles mussel adhesive proteins and thus also serves as a model system for mussel adhesive chemistry.

3.5

π–π Stacking Interactions

π–π stacking interactions were utilized in the development of thermally triggered reversible self-healing supramolecular polymer networks. This was achieved using end-capped π-electron-deficient groups which interacted with other π-electron-rich aromatic molecules [78]. Upon heating, the π–π stacking interactions were interrupted, enabling π-electron-rich aromatic molecules to disengage from

Q.M. Zhang and M.J. Serpe

Fig. 21 Molecular structure of healing polymer blend 1 · 2 (1:3 w/w ratio). Reproduced with permission from [80]

π-electron-deficient groups and flow because of the presence of a flexible “soft” spacer [79]. Thus, repair of damage and restoration of mechanical strength was achieved by reformation of π–π stacking. A supramolecular healable polymer blend was formed via π–π interactions, which is comprised of a π-electron-rich pyrenyl end-capped oligomer and a chain-folding oligomer containing pairs of π-electron poor naphthalene-diimide (NDI) units as well as cellulose nanocrystals (CNCs) for reinforcement (Fig. 21) [80]. All the nanocomposites could be rehealed upon exposure to elevated temperatures. It was found that the healing rate was reduced with increasing CNC content. The best combination of healing efficiency and mechanical properties was obtained with the 7.5 wt% CNC nanocomposite. It exhibited a tensile modulus enhanced by as much as a factor of 20 over the matrix material alone and could be fully rehealed at 85 C within 30 min. Thus it is demonstrated that supramolecular nanocomposites can afford greatly enhanced mechanical properties over the unreinforced polymer, while still keeping efficient thermal healing.

4 Mechanotransducing Sensors “Sensor” is derived from the Latin word “sensus”, which directly translated means “sense,” or to “sense something.” When one thinks of “sensing,” we can turn to the basic five human senses: sight, hearing, taste, smell, and touch. These senses allow the human body to receive signals or sense stimuli from the environment and react or respond to them [81]. If a sense gives the ability to receive and respond to a signal, we can transfer this to the technical level and define a sensor as: “A device that receives and responds to signals and stimuli from the environment”. There has been a consistent increase in research output related to the development and application of optical chemical sensors in the late twentieth century. The development of novel molecular sensors capable of detecting environmental changes (temperature, pH, concentration of enzyme or ionic species, etc.) has been pursued

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

in medical, biological, and environmental applications [9, 82]. In this section, we highlight pH sensors, chemical sensors, and biosensors.

4.1

Mechanotransducing pH Sensors

pH is a key parameter to measure for a broad range of applications, e.g., in life sciences, food and beverage processing, soil examination, and marine and pharmaceutical research to name a few. The development of an optical pH sensor, which can be used in real world applications, is not trivial. pH-sensitive hydrogels can be weakly acidic or weakly basic, depending on the nature of the ionizable moieties on the polymer backbone. Therefore, charges can be generated on the polymer backbone in a pH-dependent fashion. The charges formed on the polymer backbone at certain pHs yields the polymer response caused by electrostatic interactions and osmotic pressure effects [83]. In one example, a poly(acrylic acid-co-isooctyl acrylate) hydrogel was coupled with a magnetoelastic sensor, the resonance frequency of which corresponds with applied mass load [84]. The gels were found to respond rapidly to changes in solution pH, reversible swelling and shrinking, leading to mass changes; the mass changes can be detected as a shift in the resonance frequency of the sensor (Fig. 22). The response of the pH sensor is accurately fitted using the equation fr ¼ 49.891 1.04 pH + 0.048 pH2. The average change in sensor resonance

Fig. 22 Resonance frequency of the pH sensor, as it is repeatedly cycled between pH 4.03 and 7.02 solutions. The pH-sensitive polymer layer is approximately 1.36 μm thick. Reproduced with permission from [84]

Q.M. Zhang and M.J. Serpe

Fig. 23 Schematic of the cantilever/polymer structure with associated dimensions. Reproduced with permission from [85]

Fig. 24 Equilibrium cantilever deflection as a function of solution pH. The solid line is experimental results and a sensitivity of 5 105 pH for a 10 nm bending deflection resolution can be obtained. The dotted line is obtained with the cantilever and polymer modeled as a composite beam with no slip at the boundary. Small deflections with respect to the length are assumed. Polymer elastic modulus of 85 MPa is used to fit the model to experiments. The inset shows a three-dimensional plot of the deflection of the cantilever/polymer at pH 7.0, obtained from the model. Reproduced with permission from [85]

frequency is 506 Hz/pH, approximately 1%/pH of the resonance frequency value, from pH 4.4 to 8.5. A pH sensor with ultrahigh sensitivity could be fabricated using microcantileverbased technology (Fig. 23) [85]. Silicon-on-insulator wafers were used to fabricate cantilevers on which a polymer consisting of poly(methacrylic acid-co-ethylene glycol) was patterned using UV-initiated free-radical polymerization. When the pH around the cantilever was increased above the pKa of poly(methacrylic acid), negative charges were formed in the polymer network, which resulted in its expansion and caused the microcantilever to bend. Excellent mechanical amplification of polymer swelling as a function of pH change within the dynamic range was obtained, with a maximum deflection sensitivity of 1 nm/5 105 △pH [85]. At low pH, the cantilever is bent downwards as the hydrogel is swollen because of the liquid around it, when compared to the dry state. As the pH is increased above

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

5.7, the polymer swells rapidly and eventually the cantilever touches the bottom of the well. Between pHs of about 5.9 and 6.5 (dynamic range of the sensor), the slope of the deflection at the tip vs pH curve, and hence the sensitivity, is maximum at about 18.3 μm/pH (Fig. 24). If a laser-based deflection measurement system is used, such as the one used in conventional atomic force microscope, where deflections of 1 nm can easily be detected, the above number translates to a sensitivity of 5 105 pH for a 1-nm bending detection resolution. These sensitivities are among the highest reported for any micro-scale pH detectors. The color changes of some sensors can be detected by the naked eye, making them especially useful and inexpensive. One unique class of materials is obtained from the self-assembly of colloidal particles to form crystalline arrays (CCA) [86]. These arrays exhibit bright, visible colors similar to the opal gemstone, which can be “locked” into place by polymerization of a hydrogel into the interstices of the array to form a soft photonic material which possess the volume-phase transition phenomenon of polymer gels [87]. Specifically, when light impinges on the periodic structure, it is reflected/refracted/diffracted from each interface formed by the particles. This light, under suitable conditions, interferes constructively/ destructively and reflects/transmits certain wavelengths of light according to the well-known Bragg condition, modified for the photonic crystals, given by [88–91] mλ ¼ 2 n d sin θ;

ð1Þ

where m is the diffraction order, λ the wavelength of the reflected light, n the mean refractive index of the periodic structure, d the lattice period of the crystalline direction of propagation of light, and θ the angle between the incident light and diffraction crystal planes. The pH-dependent volume-phase transitions of a hydrogel network surrounding a CCA could be monitored using Bragg diffraction/color [92]. The pH-dependent charges on the polymer network induce the polymer to change volume, and hence size. The hydrogel size changes yield a change in the lattice spacing of the CCA, and a concomitant color change. The Young’s modulus of the hydrogel could be determined from the CCA hydrogel Bragg diffraction to determine the elastic restoring forces [93]. They describe a detailed hydrogel volume-phase model, which accurately models swelling with no adjustable parameters. Finally, the results demonstrate that carboxylated CCA photonic crystals are excellent pH and ionic strength sensors. Poly(hydroxyethyl methacrylate)-based photonic materials were also prepared and used as a pH sensor [94]. To accomplish this, the surface of monodisperse silica particles was coated with a thin layer of polystyrene. Surface charge groups were attached by a grafting polymerization of styrene sulfonate. The resulting highly charged monodisperse silica particles were self-assembled into a CCA in deionized water. Polymerization of hydroxyethyl methacrylate (HEMA) occurred around the CCA to form a HEMA-CCA. Hydrofluoric acid was utilized to etch out the silica particles to produce a three-dimensional periodic array of voids in the HEMACCA. The authors also fabricated a CCA by utilizing a second polymerization to incorporate carboxyl groups into the HEMA CCA. They were also able to model

Q.M. Zhang and M.J. Serpe

Fig. 25 Schematic illustration of experimental procedures. Reproduced with permission from [96]

the pH dependence of diffraction of the HEMA-CCA by using Flory theory. An unusual feature of the pH response is a hysteresis in response to titration to higher or lower pH. The kinetics of equilibration is very slow because of the ultralow diffusion constant of protons in the carboxylated CCA as predicted earlier by the Tanaka group [95]. In spite of its simplicity, the long response time of the hydrogel photonic crystal materials has limited their utility as sensors. This is because of the slow diffusion of analytes in the hydrogel to influence the optical properties. New fabrication procedures of these hydrogel photonic crystal sensors were used to improve the response time. The Lee Group [96] demonstrated a mechanically robust and fast responsive photonic crystal pH sensor, which was fabricated by templated photopolymerization of hydrogel monomers within the interstitial space of a selfassembled colloidal photonic crystal, as shown in Fig. 25. By optimization of photopolymerization conditions, these pH sensors show a response time of less than 10 s upon a pH change. The fast response behavior is a result of the void spaces in the structure. Most of the ionic species are expected to diffuse rapidly through the voids, and then into the hydrogel, resulting in an aqueous diffusion limited response time. Experiments revealed that the device’s response was reproducible over many cycles, and the response was consistent for >6 months. Mangeney’s group made a novel photonic crystal pH sensor for fast pH response by incorporating a planar defect inside the photonic crystals, as shown in Fig. 26a [97]. Figure 26b shows the typical SEM images of the used colloidal-

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 26 (a) Schematic illustration of the defect-containing direct opal and inverse opal hydrogel (IOH) films. (b) SEM images of (a) the colloidal-crystal template with embedded planar defect layer of larger particles and (b) the resulting inverse opal hydrogel film with a defect layer of larger macropores. The resulting materials consists of a three-dimensional, highly-ordered, and interconnected macroporous array of poly(methacrylic acid) which is sensitive to pH. Reproduced with permission from [97]

Fig. 27 (a) Schematic of a traditional Fabry-Perot etalon (d, distance between two mirrors; n, refractive index of the dielectric). (b) Schematic structure and proposed mechanism for our poly (N-isopropylacrylamide) microgel based etalons fabricated by sandwiching (b) a microgel layer between (a, c) two reflective Cr/Au surfaces, all on (d) a cover glass. Reproduced with permission from [102]

crystal template with an embedded planar defect layer and the resultant photonic polymer hydrogel films. The Serpe group recently reported on structures that are colored, and their color is capable of changing in response to stimuli [98–101]. These materials were constructed by “painting” temperature and pH responsive poly(N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgels onto an Au coated glass substrate [99]. Following further treatment, another Au layer is deposited onto the microgel layer. As can be seen in Fig. 27 [102], this yields a mirror–dielectric– mirror structure akin to a classic Fabry–Perot etalon. When the device was immersed in water, the pNIPAm-co-AAc microgels swell and separate the Au layers from one another. Light impinging on the structure scan enter the microgel-based cavity and resonate between the two Au layers, which results in

Q.M. Zhang and M.J. Serpe

Fig. 28 Photographs of an etalon with solutions of various pH spotted on a single surface (a, c, e, i) 25 C and (b, d, f, g, h) 37 C. (f) 3 min after heating; (g) 5 min after heating; (h) 6 min after heating. In each panel, the scale bar is 5 mm. Reproduced with permission from [102]

specific wavelengths of light being reflected/transmitted according to the following equation: mλ ¼ 2 n d cos θ;

ð2Þ

where λ is the wavelength maximum of the peak(s), m is the peak order, n is the refractive index of the dielectric, d is the spacing between the mirrors, and θ is the angle of incidence [98, 103]. Once AAc groups are deprotonated in high pH solution, the microgel layer swells because of Coulombic repulsion in the microgels [102]. Figure 28 shows the visual color change which can be observed as a function of pH, as well as the fact that different regions on a single device can be modulated independently, which could have implications for display device technology [102]. F€ orster resonance energy transfer (FRET) is a promising method for sensing at a molecular level where FRET is generally referred to as an energy transfer between fluorescent donor and acceptor [104]. Because the efficiency of FRET is very sensitive to the distance between FRET donor and acceptor, FRET has been traditionally used for monitoring single molecular events such as the conformational transition of macromolecules by labeling fluorophores at specific sites [105]. A polymeric pH sensor with a FRET donor and a FRET acceptor attached to both ends of a pH-sensitive polymeric linker has been reported [106]. The pH sensor exhibits a blue color corresponding to the emission of the FRET donor at pHs higher than 7.6, but the pH sensor emits green light when the pH is lower than 6.8. This is a direct result of FRET, induced by the conformational change of polymeric linker bringing the FRET pair together (Fig. 29).

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 29 (a) Diagrammatic representation of the induced FRET triggered by pH. (b) Fluorescence images of solutions at (A) pH 7.6 and (B) pH 6.8 when the solutions are irradiated at 330 nm. Both solutions have a concentration of 2.0 102 g/L. Reproduced with permission from [106] O HO

SC4H9

S 2

S HS

O HO

O (CH2)7CH=CH(CH2)7CH3 + HS

O S

SC4H9

S S

O S

n N 7

O mS

6 SC4H9

O O S

S n S N

SC4H9

Fig. 30 Synthesis of (PyMMP-b-P2VP)-CdSe/ZnS core-shell quantum dots. Reproduced with permission from [107]

Relatedly, cadmium selenide/zinc sulfide (CdSe/ZnS) core-shell quantum dots (QDs) were coated with pH responsive and fluorescent poly((1-pyrene)methyl-2methyl-2-propenoate))-b-poly(2-vinylpyridine) (PyMMP-b-P2VP) block copolymers [107]. The synthesis and chemical structure of (PyMMP-b-P2VP)-CdSe/ ZnS QDs is shown in Fig. 30. FRET between these two distinguishable chromophores of red emitting CdSe/ZnS cores and blue emitting polypyrene-based shells is governed by the interspacing between them, which can be controlled through the

Q.M. Zhang and M.J. Serpe

Fig. 31 (a) Photographic image of (PyMMP-b-P2VP)-QDs solutions under irradiation at 365 nm using a UV lamp. (b) PL spectra of (PyMMP-b-P2VP)-QDs. Reproduced with permission from [107]

Fig. 32 Structure of (a) P2VP-OQD and (b) PAA-BQD. (c) Schematic illustration of the conformation and behavior of MQD-GO at a given pH value. Reproduced with permission from [108]

pH-dependent conformation of the P2VP chains [107]. Furthermore, the emission intensity ratio between the QDs and pyrene blocks can easily be balanced and controlled by simply tuning the number of repeating pyrene units in the (PyMMP-bP2VP) chain. Therefore, polymer-coated QDs can exhibit not only ratiometric pH-dependent fluorescence spectra but also a clear color change from blue to purple to red as the solution pH is increased. These colors match well with the corresponding photoluminescence (PL) spectra shown in Fig. 31. A versatile platform for an efficient graphene oxide (GO)-based optical sensor which exhibits distinctive ratiometric color responses has been developed [108]. A key strategy for generating a colorimetric, wide pH range sensor is to use two different blue- and orange-colored QDs anchored to a single GO sheet, as shown in Fig. 32. The pH-dependent emissions of the blue and orange QDs were controlled

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

by using linkers of two different pH-responsive polymers which changed their conformation in response to a different, but complementary, range of pH values [108]. The conformational changes caused the QDs to approach the GO surface, which influenced their emission properties. In addition, the GO-based sensor exhibits excellent dispersion stability in aqueous media and reversibility, all of which satisfy the critical requirements for a pH sensor.

4.2

Mechanotransducing Chemical Sensors

The emission of gaseous pollutants such as sulfur oxide, nitrogen oxide, and toxic gases as a result of a variety of industrial processes has become a serious environmental concern, especially in specific parts of the world. As a result, sensors are needed to monitor the concentration of various contaminants, which can prevent or limit uncontrolled releases of toxic compounds. In one example, polyaniline (PAN) films were prepared by Langmuir–Blodgett (LB) and self-assembly (SA) techniques [109]. NO2 is an oxidizing gas, which on contact with the π-electron network of polyaniline results in the transfer of an electron from the polymer to the gas. When this occurs, the polymer becomes positively charged. The charge carriers thus created give rise to the increased conductivity of the films, and a concomitant decrease in resistance. The response time to NO2 and the relative change of resistance of films increase with increase of the number of film layers, as show in Fig. 33. Thin films allow more contact with NO2 than thick films, which increases the response time. Quartz crystal microbalance (QCM)-based SO2 gas sensors were fabricated using amino-functionalized poly(styrene-co-chloromethyl styrene) derivatives [110]. SO2 can be absorbed by the polymer’s amine groups, which causes the mass to change such that it can be detected by the QCM. The sensor reaches equilibrium within 50 min in 50 ppm SO2. The sensors exhibit complete

Fig. 33 Plots of the relative change of resistance of polyaniline-based films prepared by LB technique with various number of layers vs response time at 20 ppm NO2. (a) PAN; (b) PAA-AA. Reproduced with permission from [109]

Q.M. Zhang and M.J. Serpe Fig. 34 Typical transient responses of the copolymerized propylene– butyl-film-coated sensor for various gases with concentrations of 5,000 ppm. Δf: quartz resonator frequency. Reproduced with permission from [111]

Fig. 35 Fabrication of a two-dimensional (2D) photonic crystal for sensing applications. (1, 2) PS particles self-assembled into a 2D close-packed array, (3) A hydrogel film is polymerized around the 2D array, (4) The swollen hydrogel with the embedded 2D array is peeled from the glass substrate. (5) Diffraction from the 2D array/hydrogel sandwich is monitored visually. Reproduced with permission from [112]

reversibility at 70 C. A similar system for the detection of aromatic gases (e.g., toluene and p-xylene) was also reported [111]. The copolymerized propylene–butyl film was chosen as the sensing membrane coated on the QCM [111]. The system exhibits high sensitivity and excellent selectivity for aromatic solvent gases such as toluene and xylene (Fig. 34). In order to detect heavy metals, a single layer of polystyrene particles was immobilized in hydrogel thin films, which contained molecular recognition agents. In this case, 4-acryloylamidobenzo-18-crown-6 (4AB18C6) was the active component in the hydrogel network, which responds specifically to Pb2+ (Fig. 35) [112]. When exposed to Pb2+ solution, 4AB18C6 selectively complexed Pb2+, which increased the charge density in the polymer network. This resulted in

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials Fig. 36 Visible extinction spectra showing how diffraction depends on glucose concentration for a 125-mm-thick CCA glucose sensor. Reproduced with permission from [114]

hydrogel swelling and correspondingly increased the distance between PS particles and resulted in a color change of the thin film. In another example, the Asher group showed that a periodic array of polystyrene spheres could be locked in a polyacrylamide hydrogel [113]. The enzyme glucose oxidase was incorporated into this system, which worked as a glucose sensor (Fig. 36) [114]. These materials were shown to exhibit Bragg diffraction in the visible wavelengths. Glucose solutions can then cause the hydrogel to swell, resulting in a red shift of the diffracted light. The hydrogel swelling is a result of the formation of a reduced glucose oxidase upon glucose turnover. The oxidized glucose oxidase is uncharged at neutral pH; however, the reduced glucose oxidase is anionic at pH 7. The reduced glucose oxidase is reoxidized by O2, as detailed in the below reactions. GOx ðoxÞ þ glucose ! GOx ðredÞ þ gluconic acid þ H þ H þ þGOx ðredÞ þ O2 ! H2 O2 þ GOx ðoxÞ: No response occurs for similar concentrations of sucrose or mannose because of the enzyme selectivity. Bisphenol A (BPA) is a common compound used in the synthesis of many plastics and epoxy resins. However, recent studies have determined BPA is an emerging contaminant, which can disrupt the endocrine system and potentially cause cancer [115]. By molecular imprinting, Gao’s group created numerous nanocavities in polymethyl methacrylate (PMMA) spheres, which can specifically target BPA (Fig. 37). The monodisperse PMMA spheres can be made into a CCA-based optical sensor. When the sensor is exposed to BPA solution, binding

Q.M. Zhang and M.J. Serpe

Fig. 37 Experimental procedures for the reflectometric detection of BPA using an imprinted nanocavity opal photonic crystal sensor. Reproduced with permission from [117]

occurs because of the hydrogen bonding and spatial effects, and the recognition process can swell the microspheres, resulting in smaller average refractive index and decreasing the diffraction peak intensity [117]. Li’s group also developed imprinted photonic polymers for detecting the pesticide atrazine (Fig. 38) [116]. This pesticide has recently shown up as a contaminant in drinking water and consumption of this pesticide above the maximum contaminant level (MCLs) has been associated with adverse human health effects. To accomplish this, the authors deposited silica colloids onto a glass substrate to form a 3D ordered array as a template. Pre-gel solution containing template molecule (atrazine) was filled into the void space of the 3D ordered array. After polymerization, silica and the atrazine molecular templates were removed from the hydrogel film. The system now had highly ordered porous arrays with specific nanocavities capable of recognizing atrazine through noncovalent interactions. When exposed to different concentrations of atrazine, hydrogen bonding occurred between atrazine and gels, and hence the gel swelled, resulting in a visual color change.

4.3

Mechanotransducing Biosensors

One of the most common and well-known sensors for the detection of biomolecules is the blood glucose biosensor. Initially described by Clark and Lyons in 1962, the sensor relies on monitoring the oxygen concentration in solution (using an oxygen selective electrode); the amount of oxygen can be related to glucose concentration from the reaction of glucose oxidase and oxygen [118]. With glucose sensing

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

3. Removal of silica particies and atrazine molecules

PMMA

2. Polymerization

Glass

1. Infiltration of pre-polymerization complex

PMMA

Hierarchical porous structure

Cl N

Pre-polymerization complex using MAA or AA as functional monomer

HO N

Cl N

HO N

H N

H O

H N

Pure buffer

10–12M

10–11M

10–10M

10–9M

10–8M

10–7M

10–6M

Fig. 38 (a) Schematic illustration of the procedure used for the preparation of the molecularly imprinted photonic polymer (MIPP). (b) Color change induced by exposure to atrazine at different concentrations. Reproduced with permission from [116]

solved, attention has shifted to the development of sensors that can detect specific DNA sequences, metal ions, small molecules, proteins, and cells. Biosensors were recently prepared by attaching DNA to synthetic polymer backbones [119]. Two strands of acrydite-modified DNA, S1 (50-acrydite-AAAACTCATCTGTGAAAG AACCTGGGGGAGTATTGCGGAGGAAGGT-30) and S2 (50-acryditeAAACCCA GGTTCTTCTAGAGGGAGAC-30), were copolymerized with linear polyacrylamide polymers to form polymer strands P-S1 and P-S2 in a transparent liquid form, respectively (Fig. 39) [120]. S1 was designed to contain an ATP aptamer fragment. A crosslinker L1 (50-GGGAGACAAGGATAAATCCTT CAATGAAGTGGGTCTCCCTCTACTCACAGATGAGT-30), containing a cocaine aptamer fragment, was designed to hybridize with S1 and S2. The polymers transform into a gel as the hybridization proceeds during the mixture of P-S1, P-S2 with L1. In the presence of cocaine and ATP, the specific aptamer–target recognition causes the DNA hydrogels to undergo a macroscopic gel–sol transition. In a related example, polyacrylamide hydrogel-based sensors functionalized with a thymine rich DNA which can simultaneously detect and remove mercury from water were fabricated [121]. Specifically, in the absence of Hg2+, the DNA is in a random coil conformation, and the addition of SYBR Green I gives a weak fluorescence (Fig. 40a, yellow line). In the presence of Hg2+, the DNA forms a hairpin structure yielding a ninefold emission increase. Using the naked eye, the

Q.M. Zhang and M.J. Serpe

Fig. 39 (a) Photograph of aptamer-crosslinked hydrogels visualized with trapped BSA-GNPs before and after addition of cocaine and ATP. (b) UV–vis spectrum of the results of the AND logic gate. (c) Photograph of selectivity of the system to cocaine. (d) Photograph of selectivity of the system to ATP. The input stimulus concentration was 1 mM. Reproduced with permission from [120]

Fig. 40 (a) DNA sequence of acrydite-Hg-DNA and fluorescence signal generation for Hg2+ detection. The 50 -end is modified with an acrydite group for hydrogel attachment. (b) Covalent DNA immobilization within a polyacrylamide hydrogel and interaction with Hg2+ and SYBR Green I produces a visual fluorescence signal. (c, d) Chemical reaction schemes of Hg2+ binding with thymine base pairs (c) and polyacrylamide in hydrogel (d), where “Gel” in the molecular formula denotes the hydrogel matrix. Reproduced with permission from [121]

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 41 Schematic representation of the fabrication of GCE/PTH/ChOx/HRP biosensor (a) and the reaction processes at the GCE/PTH/ChOx/HRP biosensor (b). The crystal structures of cholesterol oxidase (ChOx) and horseradish peroxidase (HRP) enzyme was obtained from Protein Data Bank (ID: 2IOK.pdb and IHCH.pdb, respectively). Reproduced with permission from [122]

detection limit in water solution is 10 nM Hg2+. This sensor can be regenerated using a simple acid treatment, which removes Hg2+ from water within 1 h. A simple and inexpensive cholesterol biosensor was fabricated by immobilizing cholesterol oxidase (ChOx) and horseradish peroxidase (HRP) onto a poly (thionine) modified glassy carbon electrode (GCE/PTH) (Fig. 41) [122]. Hydroquinone (HQ) was used as a mediator to promote the electron transfer between the enzyme and the electrode. It results in excellent electrocatalytic activity of immobilized HRP for H2O2 reduction, which was produced from cholesterol by the enzymatic reaction with ChOx. The linear range for cholesterol spanned from 25 to 125 μM, with a detection limit and a sensitivity of 6.3 μM and 0.18 μA/cm2/μ M, respectively. The highly reproducible and sensitive GCE/PTH/ChOx/HRP sensor exhibited an interference-free signal for cholesterol detection with excellent recoveries for real sample analysis. There has recently been a growing interest in the use of block copolymer photonic gels for biosensing applications[123]. Block copolymers offer the flexibility of fabricating 1D, 2D, and 3D photonic materials through self-assembly. Some photonic gels are extremely sensitive to changes in charge, the dielectric environment, and biomolecules such as proteins and DNA. Exposure of the photonic gels to species that can alter the electrostatic and dielectric environment of the

Q.M. Zhang and M.J. Serpe

Fig. 42 Preparation of biotinylated photonic gels. Reproduced with permission from [123]

Fig. 43 Color changes of the inside-biotinylated photonic gels in response to streptavidin; (a) [S] ¼ 0 M; (b) [S] ¼ 0.5 mM; (c) [S] ¼ 10 mM. Reproduced with permission from [123]

photonic gels can change the photonic gel’s optical properties dramatically [123]. Kang and coworkers have reported a photonic gel sensor using polystyrene-b-quaternized poly(2-vinyl pyridine) (PS-b-QP2VP) modified with biotin molecules for detecting streptavidin (Fig. 42) [123]. This was achieved by on-gel (Fig. 42a) and in-gel (Fig. 42B) modification with biotin via the conventional carbodiimide coupling reaction. An apparent visual color change with streptavidin binding was observed for the in-gel photonic materials (Fig. 43). The color change was induced by binding of a streptavidin to multiple biotin molecules, which acted as a crosslinker, and resulted in the deswelling of the gels.

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

Fig. 44 Streptavidin (the analyte) is added to an excess amount of biotin-modified poly (allylamine hydrochloride) (PAH). The streptavidin–biotin–PAH complex is then removed from solution using biotin modified magnetic particles, leaving behind free, unbound PAH. The unbound PAH is subsequently added to a pNIPAm-co-AAc microgel-based etalon immersed in aqueous solution at pH 7.2 which renders both the microgel layer and the PAH charged. As a result, the etalon’s spectral peaks shift in proportion to the amount of PAH–biotin that was added. This, in turn can be related back to the original amount of streptavidin added to the PAH–biotin. Reproduced with permission from [124]

The Serpe group recently developed etalon-based biosensors [124]. They first showed that biotinylated polycationic polymer can penetrate through the Au overlayer of an etalon and cause the layer of negatively charged microgels to collapse. The extent of peak shift depends on the amount of biotinylated polycation added to the etalon; high polycation concentration yields a large shift, and vice versa. This phenomenon can be exploited to sense the concentration of streptavidin in solution at μM concentrations, as detailed in Fig. 44. Etalons are very interesting because, unlike most biosensors, a large signal is obtained for low analyte concentration, as can be seen in Fig. 45 [124]. The Serpe group went on to show that a similar concept can be used to detect μM concentrations of target DNA in solutions [125, 126]. This detection originates

Q.M. Zhang and M.J. Serpe

Fig. 45 Cumulative shift of the etalon’s reflectance peak upon addition of the indicated amounts of streptavidin to PAH–biotin100:1. The pNIPAm-co-AAc microgel-based etalon was soaked in pH 7.2 throughout the experiment, while the temperature was maintained at 25 C. Each data point represents the average of at least three independent measurements, and the error bars are the standard deviation for those values. Reproduced with permission from [124]

from the penetration of polyanionic DNA into the etalon’s positively charged microgel layer. DNA interacts electrostatically with the positively charged microgels and crosslinks them. The crosslinking results in shrinking of the confined microgel between Au layers resulting in the peaks shifts, as predicted in (2). The extent of shift can be related to the concentration of target DNA present in the sample solution.

5 Summary and Conclusions Various responsive polymer-based systems were reviewed here, and their utility as artificial muscles, self-healing materials, and sensors detailed. These polymers, expanding in area and shrinking in thickness during exposure to stimuli, have been investigated as artificial muscles. Stimuli include electrostatic forces, pH, light, magnetic fields, and others. Self-healing polymeric materials were also detailed, where damage to materials can “automatically” result in a healing response. We have shown examples using reversible covalent interactions or non-covalent interactions such as hydrogen/ion bonding or π–π stacking. We have also detailed many examples of sensors that use mechanotransduction mechanisms to yield responses from materials which can be quantified and related to the concentration of various species in solutions. This review has detailed how mechanochemistry can be used to yield desired functions, and has illustrated the tremendous opportunities for both fundamental and technological advances. Furthermore, many of the examples detailed in this

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials

chapter are not only useful for the specific applications detailed here, but are responsible for pushing the field of polymer science forward. While much progress has been made, many challenges still exist which prevent the use of these materials in everyday applications. For example, regarding sensing, polymers with enhanced sensitivities to specific species need to be developed and investigated. Furthermore, response times need to be enhanced, and the strength of the muscles improved. Additionally, many of the systems described here are prohibitively expensive for real world applications, so less expensive functional components therefore need to be identified. Finally, biocompatibility is a major issue; many of these materials are difficult to use in biological settings. Regardless of the challenges, continuous development of new responsive polymers and related technologies makes us optimistic about the future positive impacts these materials can have on human life. Acknowledgements MJS acknowledges funding from the University of Alberta (the Department of Chemistry and the Faculty of Science), the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canada Foundation for Innovation (CFI), the Alberta Advanced Education & Technology Small Equipment Grants Program (AET/SEGP), Grand Challenges Canada, and IC-IMPACTS.

References 1. Derby CD (2007) Escape by inking and secreting: marine molluscs avoid predators through a rich array of chemicals and mechanisms. Biol Bull 213:274 2. Rapoport N (2007) Physical stimuli-responsive polymeric micelles for anti-cancer drug delivery. Prog Polym Sci 32:962 3. Schmaljohann D (2006) Thermo-and pH-responsive polymers in drug delivery. Adv Drug Deliv Rev 58:1655 4. Ionov L (2010) Actively-moving materials based on stimuli-responsive polymers. J Mater Chem 20:3382 5. Liu F, Urban MW (2010) Recent advances and challenges in designing stimuli-responsive polymers. Prog Polym Sci 35:3 6. Stuart MAC, Huck WT, Genzer J, Mu¨ller M, Ober C, Stamm M, Sukhorukov GB, Szleifer I, Tsukruk VV, Urban M (2010) Emerging applications of stimuli-responsive polymer materials. Nat Mater 9:101 7. Roy D, Cambre JN, Sumerlin BS (2010) Future perspectives and recent advances in stimuliresponsive materials. Prog Polym Sci 35:278 8. Meng H, Li G (2013) A review of stimuli-responsive shape memory polymer composites. Polymer 54:2199 9. Zhang J, Zhang M, Tang K, Verpoort F, Sun T (2014) Polymer-based stimuli-responsive recyclable catalytic systems for organic synthesis. Small 10:32 10. Huo M, Yuan J, Tao L, Wei Y (2014) Redox-responsive polymers for drug delivery: from molecular design to applications. Polym Chem 5:1519 11. Gil ES, Hudson SM (2004) Stimuli-reponsive polymers and their bioconjugates. Prog Polym Sci 29:1173 12. Zhai L (2013) Stimuli-responsive polymer films. Chem Soc Rev 42:7148

Q.M. Zhang and M.J. Serpe 13. Ahn S-k, Kasi RM, Kim S-C, Sharma N, Zhou Y (2008) Stimuli-responsive polymer gels. Soft Matter 4:1151 14. Dimitrov I, Trzebicka B, Mu¨ller AHE, Dworak A, Tsvetanov CB (2007) Thermosensitive water-soluble copolymers with doubly responsive reversibly interacting entities. Prog Polym Sci 32:1275 15. Schild H (1992) Poly (N-isopropylacrylamide): experiment, theory and application. Prog Polym Sci 17:163 16. Maeda Y, Nakamura T, Ikeda I (2001) Changes in the hydration states of poly (N-alkylacrylamide) s during their phase transitions in water observed by FTIR spectroscopy. Macromolecules 34:1391 17. Idziak I, Avoce D, Lessard D, Gravel D, Zhu X (1999) Thermosensitivity of aqueous solutions of poly (N,N-diethylacrylamide). Macromolecules 32:1260 18. Lutz JF (2008) Polymerization of oligo (ethylene glycol)(meth) acrylates: toward new generations of smart biocompatible materials. J Polym Sci A Polym Chem 46:3459 19. Hoogenboom R (2009) Poly (2‐oxazoline)s: a polymer class with numerous potential applications. Angew Chem Int Ed 48:7978 20. Dai S, Ravi P, Tam KC (2009) Thermo-and photo-responsive polymeric systems. Soft Matter 5:2513 21. Zhang QM, Li X, Islam MR, Wei M, Serpe MJ (2014) Light switchable optical materials from azobenzene crosslinked poly (N-isopropylacrylamide)-based microgels. J Mater Chem C 2:6961 22. Liu D, Chen W, Sun K, Deng K, Zhang W, Wang Z, Jiang X (2011) Resettable, multi‐readout logic gates based on controllably reversible aggregation of gold nanoparticles. Angew Chem Int Ed 50:4103 23. Zhang QM, Xu W, Serpe MJ (2014) Optical devices constructed from multiresponsive microgels. Angew Chem Int Ed 53:4827 24. Schumers JM, Fustin CA, Gohy JF (2010) Light‐responsive block copolymers macromol. Rapid Commun 31:1588 25. Kumar S, Dory YL, Lepage M, Zhao Y (2011) Surface-grafted stimuli-responsive block copolymer brushes for the thermo-, photo-and pH-sensitive release of dye. Macromolecules 44:7385 26. Yan B, Boyer J-C, Branda NR, Zhao Y (2011) Near-infrared light-triggered dissociation of block copolymer micelles using upconverting nanoparticles. J Am Chem Soc 133:19714 27. May PA, Moore JS (2013) Polymer mechanochemistry: techniques to generate molecular force via elongational flows. Chem Soc Rev 42:7497 28. Mura S, Nicolas J, Couvreur P (2013) Stimuli-responsive nanocarriers for drug delivery. Nat Mater 12:991 29. Madden JD, Vandesteeg NA, Anquetil PA, Madden PG, Takshi A, Pytel RZ, Lafontaine SR, Wieringa PA, Hunter IW (2004) Artificial muscle technology: physical principles and naval prospects. IEEE J Oceanic Eng 29:706 30. Madden JD, Schmid B, Hechinger M, Lafontaine SR, Madden PG, Hover FS, Kimball R, Hunter IW (2004) Application of polypyrrole actuators: feasibility of variable camber foils. IEEE J Oceanic Eng 29:738 31. Colgate JE, Lynch KM (2004) Mechanics and control of swimming: a review. IEEE J Oceanic Eng 29:660 32. Mirfakhrai T, Madden JD, Baughman RH (2007) Polymer artificial muscles. Mater Today 10:30 33. Brochu P, Pei Q (2010) Advances in dielectric elastomers for actuators and artificial muscles. Macromol Rapid Commun 31:10 34. Brochu P, Stoyanov H, Niu X, Pei Q (2013) All-silicone prestrain-locked interpenetrating polymer network elastomers: free-standing silicone artificial muscles with improved performance and robustness. Smart Mater Struct 22:055022

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials 35. Zhang Z, Liu L, Fan J, Yu K, Liu Y, Shi L, Leng J (2008) In: The 15th international symposium on: smart structures and materials & nondestructive evaluation and health monitoring. International Society for Optics and Photonics p 692610 36. Lotz P, Matysek M, Lechner P, Hamann M, Schlaak HF (2008) In: The 15th international symposium on: smart structures and materials & nondestructive evaluation and health monitoring. International Society for Optics and Photonics p 692723 37. Pei Q, Rosenthal MA, Pelrine R, Stanford S, Kornbluth RD (2003) Smart structures and materials. International Society for Optics and Photonics p 281 38. Tangboriboon N, Datsanae S, Onthong A, Kunanuruksapong R, Sirivat A (2013) Electromechanical responses of dielectric elastomer composite actuators based on natural rubber and alumina. J Elastom Plast 45:143 39. Islam MR, Li X, Smyth K, Serpe MJ (2013) Polymer-based muscle expansion and contraction. Angew Chem Int Ed 52:10330 40. Islam MR, Serpe MJ (2014) Poly (N-isopropylacrylamide) microgel-based thin film actuators for humidity sensing. RSC Adv 4:31937 41. Li X, Serpe MJ (2014) Understanding and controlling the self-folding behavior of polymerbased muscles. Adv Funct Mater 24:4119 42. Jeon JH, Cheedarala RK, Kee CD, Oh IK (2013) Dry‐type artificial muscles based on pendent sulfonated chitosan and functionalized graphene oxide for greatly enhanced ionic interactions and mechanical stiffness. Adv Funct Mater 23:6007 43. Jo C, Pugal D, Oh I-K, Kim KJ, Asaka K (2013) Recent advances in ionic polymer–metal composite actuators and their modeling and applications. Prog Polym Sci 38:1037 44. Palmre V, Pugal D, Kim KJ, Leang KK, Asaka K, Aabloo A (2014) Nanothorn electrodes for ionic polymer-metal composite artificial muscles. Sci Rep 4:6176 45. Kumar D, Sharma R (1998) Advances in conductive polymers. Eur Polym J 34:1053 46. Khaldi A, Plesse C, Soyer C, Cattan E, Vidal F, Legrand C, Teyssie´ D (2011) Conducting interpenetrating polymer network sized to fabricate microactuators. Appl Phys Lett 98:164101 47. Okuzaki H, Hosaka K, Suzuki H, Ito T (2010) Effect of temperature on humido-sensitive conducting polymer actuators. Sensor Actuat A Phys 157:96 48. Ma M, Guo L, Anderson DG, Langer R (2013) Bio-inspired polymer composite actuator and generator driven by water gradients. Science 339:186 49. Thomsen DL, Keller P, Naciri J, Pink R, Jeon H, Shenoy D, Ratna BR (2001) Liquid crystal elastomers with mechanical properties of a muscle. Macromolecules 34:5868 50. Lehmann W, Skupin H, Tolksdorf C, Gebhard E, Zentel R, Kru¨ger P, L€ osche M, Kremer F (2001) Giant lateral electrostriction in ferroelectric liquid-crystalline elastomers. Nature 410:447 51. Kondo M, Sugimoto M, Yamada M, Naka Y, J-i M, Kinosh*ta M, Shishido A, Yu Y, Ikeda T (2010) Effect of concentration of photoactive chromophores on photomechanical properties of crosslinked azobenzene liquid-crystalline polymers. J Mater Chem 20:117 52. Yamada M, Kondo M, Mamiya Ji YY, Kinosh*ta M, Barrett CJ, Ikeda T (2008) Photomobile polymer materials: towards light‐driven plastic motors. Angew Chem Int Ed 47:4986 53. Syrett JA, Becer CR, Haddleton DM (2010) Self-healing and self-mendable polymers. Polym Chem 1:978 54. Russell T (2002) Surface-responsive materials. Science 297:964 55. Descalzo AB, Martı´nez‐Ma´~ nez R, Sancenon F, Hoffmann K, Rurack K (2006) The supramolecular chemistry of organic–inorganic hybrid materials. Angew Chem Int Ed 45:5924 56. Becer CR, Hahn S, Fijten MW, Thijs HM, Hoogenboom R, Schubert US (2008) Libraries of methacrylic acid and oligo (ethylene glycol) methacrylate copolymers with LCST behavior. J Polym Sci A Polym Chem 46:7138 57. Ladmiral V, Legge TM, Zhao Y, Perrier S (2008) “Click” chemistry and radical polymerization: potential loss of orthogonality. Macromolecules 41:6728

Q.M. Zhang and M.J. Serpe 58. Nurmi L, Lindqvist J, Randev R, Syrett J, Haddleton DM (2009) Glycopolymers via catalytic chain transfer polymerisation (CCTP), Huisgens cycloaddition and thiol–ene double click reactions. Chem Commun 2727 59. Liu YL, Chen YW (2007) Thermally reversible cross‐linked polyamides with high toughness and self‐repairing ability from maleimide‐and furan‐functionalized aromatic polyamides. Macromol Chem Phys 208:224 60. Oehlenschlaeger KK, Mueller JO, Brandt J, Hilf S, Lederer A, Wilhelm M, Graf R, Coote ML, Schmidt FG, Barner‐Kowollik C (2014) Adaptable hetero Diels–Alder networks for fast self‐healing under mild conditions. Adv Mater 26:3561 61. Klukovich HM, Kean ZS, Iacono ST, Craig SL (2011) Mechanically induced scission and subsequent thermal remending of perfluorocyclobutane polymers. J Am Chem Soc 133:17882 62. Yang Y, Urban MW (2013) Self-healing polymeric materials. Chem Soc Rev 42:7446 63. Deng G, Tang C, Li F, Jiang H, Chen Y (2010) Covalent cross-linked polymer gels with reversible sol–gel transition and self-healing properties. Macromolecules 43:1191 64. Nicolay¨ R, Kamada J, Van Wassen A, Matyjaszewski K (2010) Responsive gels based on a dynamic covalent trithiocarbonate cross-linker. Macromolecules 43:4355 65. Arisawa M, Yamaguchi M (2003) Rhodium-catalyzed disulfide exchange reaction. J Am Chem Soc 125:6624 66. Yoon JA, Kamada J, Koynov K, Mohin J, Nicolay¨ R, Zhang Y, Balazs AC, Kowalewski T, Matyjaszewski K (2011) Self-healing polymer films based on thiol–disulfide exchange reactions and self-healing kinetics measured using atomic force microscopy. Macromolecules 45:142 67. Amamoto Y, Otsuka H, Takahara A, Matyjaszewski K (2012) Self‐healing of covalently cross‐linked polymers by reshuffling thiuram disulfide moieties in air under visible light. Adv Mater 24:3975 68. Kantor SW, Grubb WT, Osthoff RC (1954) The mechanism of the acid-catalyzed and basecatalyzed equilibration of siloxanes. J Am Chem Soc 76:5190 69. Zheng P, McCarthy TJ (2012) A Surprise from 1954: siloxane equilibration is a simple, robust, and obvious polymer self-healing mechanism. J Am Chem Soc 134:2024 70. Maes F, Montarnal D, Cantournet S, Tournilhac F, Corte´ L, Leibler L (2012) Activation and deactivation of self-healing in supramolecular rubbers. Soft Matter 8:1681 71. Montarnal D, Tournilhac F, Hidalgo M, Couturier J-L, Leibler L (2009) Versatile one-pot synthesis of supramolecular plastics and self-healing rubbers. J Am Chem Soc 131:7966 72. fa*ghihnejad A, Feldman KE, Yu J, Tirrell MV, Israelachvili JN, Hawker CJ, Kramer EJ, Zeng H (2014) Adhesion and surface interactions of a self-healing polymer with multiple hydrogen-bonding groups. Adv Funct Mater 24:2322 73. Wilson GO, Caruso MM, Schelkopf SR, Sottos NR, White SR, Moore JS (2011) Adhesion promotion via noncovalent interactions in self-healing polymers. ACS Appl Mater Interfaces 3:3072 74. Schubert US, Eschbaumer C, Hien O, Andres PR (2001) 40 -Functionalized 2,20 , 60 ,200 -terpyridines as building blocks for supramolecular chemistry and nanoscience. Tetrahedron Lett 42:4705 75. Burnworth M, Tang L, Kumpfer JR, Duncan AJ, Beyer FL, Fiore GL, Rowan SJ, Weder C (2011) Optically healable supramolecular polymers. Nature 472:334 76. Holten-Andersen N, Harrington MJ, Birkedal H, Lee BP, Messersmith PB, Lee KYC, Waite JH (2011) pH-Induced metal-ligand cross-links inspired by mussel yield self-healing polymer networks with near-covalent elastic moduli. PNAS 108:2651 77. Krogsgaard M, Behrens MA, Pedersen JS, Birkedal H (2013) Self-healing mussel-inspired multi-pH-responsive hydrogels. Biomacromolecules 14:297 78. Burattini S, Greenland BW, Merino DH, Weng W, Seppala J, Colquhoun HM, Hayes W, Mackay ME, Hamley IW, Rowan SJ (2010) A healable supramolecular polymer blend based on aromatic π–π stacking and hydrogen-bonding interactions. J Am Chem Soc 132:12051

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials 79. Greenland BW, Burattini S, Hayes W, Colquhoun HM (2008) Design, synthesis and computational modelling of aromatic tweezer-molecules as models for chain-folding polymer blends. Tetrahedron 64:8346 80. Fox J, Wie JJ, Greenland BW, Burattini S, Hayes W, Colquhoun HM, Mackay ME, Rowan SJ (2012) High-strength, healable, supramolecular polymer nanocomposites. J Am Chem Soc 134:5362 81. Buenger D, Topuz F, Groll J (2012) Hydrogels in sensing applications. Prog Polym Sci 37:1678 82. Roy I, Gupta MN (2003) Smart polymeric materials: emerging biochemical applications. Chem Biol 10:1161 83. Robinson DN, Peppas NA (2002) Preparation and characterization of pH-responsive poly (methacrylic acid-g-ethylene glycol) nanospheres. Macromolecules 35:3668 84. Ruan C, Zeng K, Grimes CA (2003) Anal Chim Acta 497:123 85. Bashir R, Hilt J, Elibol O, Gupta A, Peppas N (2002) Micromechanical cantilever as an ultrasensitive pH microsensor. Appl Phys Lett 81:3091 86. Xia Y, Gates B, Yin Y, Lu Y (2000) Monodispersed colloidal spheres: old materials with new applications. Adv Mater 12:693 87. Weissman JM, Sunkara HB, Tse AS, Asher SA (1996) Thermally switchable periodicities and diffraction from mesoscopically ordered materials. Science 274:959 88. Yeh P (1988) Optical waves in layered media. Wiley, New York 89. Hu L, Serpe MJ (2013) Controlling the response of color tunable poly (N-isopropylacrylamide) microgel-based etalons with hysteresis. Chem Commun 49:2649 90. Schacher FH, Rupar PA, Manners I (2012) Functional block copolymers: nanostructured materials with emerging applications. Angew Chem Int Ed 51:7898 91. Ye X, Qi L (2011) Two-dimensionally patterned nanostructures based on monolayer colloidal crystals: controllable fabrication, assembly, and applications. Nano Today 6:608 92. Lee K, Asher SA (2000) Photonic crystal chemical sensors: pH and ionic strength. J Am Chem Soc 122:9534 93. Marchetti M, Prager S, Cussler EL (1990) Thermodynamic predictions of volume changes in temperature-sensitive gels. 1. Theory. Macromolecules 23:1760 94. Xu X, Goponenko AV, Asher SA (2008) Polymerized polyHEMA photonic crystals: pH and ethanol sensor materials. J Am Chem Soc 130:3113 95. Mafe´ S, Manzanares JA, English AE, Tanaka T (1997) Multiple phases in ionic copolymer gels. Phys Rev Lett 79:3086 96. Shin J, Braun PV, Lee W (2010) Fast response photonic crystal pH sensor based on templated photo-polymerized hydrogel inverse opal. Sensor Actuat B Chem 150:183 97. Griffete N, Frederich H, Maıˆtre A, Chehimi MM, Ravaine S, Mangeney C (2011) Photonic crystal pH sensor containing a planar defect for fast and enhanced response. J Mater Chem 21:13052 98. Sorrell CD, Carter MCD, Serpe MJ (2011) Color tunable poly (N-isopropylacrylamide)-coacrylic acid microgel–Au hybrid assemblies. Adv Funct Mater 21:425 99. Sorrell CD, Carter MCD, Serpe MJ (2011) A “paint-on” protocol for the facile assembly of uniform microgel coatings for color tunable etalon fabrication. ACS Appl Mater Interfaces 3:1140 100. Islam MR, Serpe MJ (2013) Polyelectrolyte mediated intra and intermolecular crosslinking in microgel-based etalons for sensing protein concentration in solution. Chem Commun 49:2646 101. Islam MR, Serpe MJ (2013) Penetration of polyelectrolytes into charged poly (N-isopropylacrylamide) microgel layers confined between two surfaces. Macromolecules 46:1599 102. Hu L, Serpe MJ (2012) Color modulation of spatially isolated regions on a single poly (N-isopropylacrylamide) microgel based etalon. J Mater Chem 22:8199

Q.M. Zhang and M.J. Serpe 103. Sorrell CD, Serpe MJ (2011) Reflection order selectivity of color-tunable poly(N-isopropylacrylamide) microgel based etalons. Adv Mater 23:4088 104. Gopich IV, Szabo A (2007) Single-molecule FRET with diffusion and conformational dynamics. J Phys Chem B 111:12925 105. Hong SW, Kim KH, Huh J, Ahn C-H, Jo WH (2005) Design and synthesis of a new pH sensitive polymeric sensor using fluorescence resonance energy transfer. Chem Mater 17:6213 106. Hong SW, Ahn C-H, Huh J, Jo WH (2006) Synthesis of a PEGylated polymeric pH sensor and its pH sensitivity by fluorescence resonance energy transfer. Macromolecules 39:7694 107. Paek K, Chung S, Cho C-H, Kim BJ (2011) Fluorescent and pH-responsive diblock copolymer-coated core–shell CdSe/ZnS particles for a color-displaying, ratiometric pH sensor. Chem Commun 47:10272 108. Paek K, Yang H, Lee J, Park J, Kim BJ (2014) Efficient colorimetric pH sensor based on responsive polymer–quantum dot integrated graphene oxide. ACS Nano 8:2848 109. Xie D, Jiang Y, Pan W, Li D, Wu Z, Li Y (2002) Fabrication and characterization of polyaniline-based gas sensor by ultra-thin film technology. Sensor Actuat B Chem 81:158 110. Matsuguchi M, Tamai K, Sakai Y (2001) SO2 gas sensors using polymers with different amino groups. Sensor Actuat B Chem 77:363 111. Nanto H, Dougami N, Mukai T, Habara M, Kusano E, Kinbara A, Ogawa T, Oyabu T (2000) A smart gas sensor using polymer-film-coated quartz resonator microbalance. Sensor Actuat B Chem 66:16 112. Zhang JT, Wang L, Luo J, Tikhonov A, Kornienko N, Asher SA (2011) 2-D array photonic crystal sensing motif. J Am Chem Soc 133:9152 113. Asher SA, Holtz J, Liu L, Wu Z (1994) Self-assembly motif for creating submicron periodic materials. Polymerized crystalline colloidal arrays. J Am Chem Soc 116:4997 114. Holtz JH, Asher SA (1997) Polymerized colloidal crystal hydrogel films as intelligent chemical sensing materials. Nature 389:829 115. Staples CA, Dome PB, Klecka GM, Oblock ST, Harris LR (1998) A review of the environmental fate, effects, and exposures of bisphenol A. Chemosphere 36:2149 116. Wu Z, Ca T, Lin C, Shen D, Li G (2008) Label-free colorimetric detection of trace atrazine in aqueous solution by using molecularly imprinted photonic polymers. Chem A Eur J 14:11358 117. Guo C, Zhou C, Sai N, Ning B, Liu M, Chen H, Gao Z (2012) Detection of bisphenol A using an opal photonic crystal sensor. Sensor Actuat B Chem 166:17 118. Clark LC, Lyons C (1962) Electrode systems for continuous monitoring in cardiovascular surgery. Ann N Y Acad Sci 102:29 119. Khimji I, Kelly EY, Helwa Y, Hoang M, Liu J (2013) Visual optical biosensors based on DNA-functionalized polyacrylamide hydrogels. Methods 64:292 120. Yin B-C, Ye B-C, Wang H, Zhu Z, Tan W (2012) Colorimetric logic gates based on aptamercrosslinked hydrogels. Chem Commun 48:1248 121. Dave N, Chan MY, Huang P-JJ, Smith BD, Liu J (2010) Regenerable DNA-functionalized hydrogels for ultrasensitive, instrument-free mercury(II) detection and removal in water. J Am Chem Soc 132:12668 122. Rahman MM, X-b L, Kim J, Lim BO, Ahammad A, Lee J-J (2014) A cholesterol biosensor based on a bi-enzyme immobilized on conducting poly (thionine) film. Sensor Actuat B Chem 202:536 123. Lee E, Kim J, Myung J, Kang Y (2013) Modification of block copolymer photonic gels for colorimetric biosensors. Macromol Res 20:1219 124. Islam MR, Serpe MJ (2013) Label-free detection of low protein concentration in solution using a novel colorimetric assay biosensor. Bioelectron 49:133 125. Islam MR, Serpe MJ (2014) Polymer-based devices for the label-free detection of DNA in solution: low DNA concentrations yield large signals. Anal Bioanal Chem 406:4777 126. Islam MR, Serpe MJ (2014) A novel label-free colorimetric assay for DNA concentration in solution. Anal Chim Acta 843:83

Responsive Polymers as Sensors, Muscles, and Self-Healing Materials. - PDF Download Free (2024)

References