Gelatin-Based Highly Stretchable, Self-Healing, Conducting

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Gelatin based Highly Stretchable, Self-healing, Conducting, Multiadhesive and Antimicrobial Ionogels Embedded with Ag2O Nanoparticles Gagandeep Singh, Gurbir Singh, Krishnaiah Damarla, Pushpender K Sharma, Arvind Kumar, and Tejwant Singh Kang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.7b00719 • Publication Date (Web): 11 Jul 2017 Downloaded from http://pubs.acs.org on July 12, 2017

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ACS Sustainable Chemistry & Engineering

Gelatin based Highly Stretchable, Self-healing, Conducting, Multi-adhesive and Antimicrobial Ionogels Embedded with Ag2O Nanoparticles Gagandeep Singh,†,# Gurbir Singh,†,# Krishnaiah Damarla,‡ Pushpender K. Sharma,$ Arvind Kumar, ‡ Tejwant S. Kang†,* †

Department of Chemistry, UGC-centre for Advance Studies – II, Guru Nanak Dev University, Amritsar, 143005,

India. $

Department of Biotechnology, Sri Guru Granth Sahib World University, Fatehgarh Sahib,140406, India.

‡

AcSIR, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar, 364002, India.

#These authors contributed equally. *To whom correspondence should be addressed: email: [email protected]; [email protected] Tel: +91-183-2258802-Ext-3207

Abstract The poly-ionic nature of a protein, Gelatin (G), derived from partial hydrolysis of collagen, is utilized to prepare ionogels (IGs) in conjunction with aqueous mixtures of a polar ionic liquid (IL), 1-ethyl-3-methylimidazolium ethylsulfate, [C2mim][C2OSO3]. Highly polar nature of IL-H2O mixture (50/50 v/v %) supported the high solubility of G, where the IGs are prepared by dissolving equal amount of G to IL-H2O mixture (50/50 v/v %) in a step-wise manner at 45 oC while stirring. The combination of IGs with Ag2O nanoparticles (NPs) prepared in-situ, via photo-reduction of AgNO3 led to induction of antimicrobial activity in IGs, while enhancing the mechanical properties. The prepared IGs show fast self-healing (< 1 minute) and multi-adhesive nature along with reversible stretching efficiency and high conductivity. The conductivity (2 mS.cm-1) of prepared IG is highest among all biopolymer based IGs reported, till date. The multi-adhesive and highly conducting nature, transparency, inherent shape-memory effect and mechanical stability of the prepared the IGs are expected to be utilized in various electrical and bio-electronic applications. Moreover, these properties can be controlled by tuning the morphology of Ag2O NPs and water content in IGs. The method used for preparation of IGs provides a new way for easy, green and economical preparation of antimicrobial IGs at a reduced

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temperature, where no harmful reducing agent or UV light is used for in-situ preparation of Ag2O NPs. Keywords: Biopolymer, Ionogel, Self-sustainable, Mechanical properties, Rheology Introduction Ionic liquids (ILs) have attracted a great interest from various groups of researchers, owing to their remarkable physico-chemical properties such as high conductivity, high thermal and chemical stability, wide electrochemical window, negligible vapor pressure, non-flammability,1 and has been exploited for diverse applications.2-7 The affinity of ILs for various polymers8 and biopolymers9-11 facilitated the formulation of ionogels (IGs). IGs are the materials, where IL is trapped in the polymer network, and reflect the properties of ILs (conductivity, thermal stability and electrochemical window etc.). IGs have been established to be useful in different applications due to their electrolytic nature, which includes batteries, fuel cells, photovoltaic cells, electrochromic windows, actuators and capacitors etc.12-17 Based on the nature of gelling material, both inorganic (silica-based)18-20 and organic (polymer21-35/biopolymer36-47 based) IGs have been reported. The properties of IGs depend both on nature of gelling agent as well constituent ions of IL. Recently, organic IG exhibiting high toughness and large conductivity, formed by sequential triblock copolymer self-assembly followed by chemical cross-linking have been reported.29 The high tensile strength of IGs have enabled the researchers to cut these IGs by hand, which could also stick to different surfaces.30 These IGs, owing to easy handling and stick ability to various substrates have been employed successfully to devise high capacitance gate dielectrics for transistor-gating experiments. Similarly, magnetic IGs,31 IGs with high IL loading capacity,32 IGs based on IL compatible cyclic carbonate network,33 photo-responsive34 and proton-responsive35 IGs have also been reported. In general, these IGs have been found to be


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transparent, flexible, and conducting with desired mechanical strength, which is a prerequisite for their use in industrial applications. IGs discussed till here were prepared by using synthetic polymers. No doubt, these IGs exhibit desired mechanical and electrical properties however the synthesis of such polymers for preparing IGs is not an easy task for everyone. Moreover, the replacement of synthetic polymers by readily available biopolymers for preparation of IGs would make the IGs relatively greener in nature. In this regard, there is lesser utilization of naturally occurring polysaccharides such as cellulose,36 agarose,37-39 starch,40 chitin,41,42 chitosan,43 and guar-gum etc.,44-47 for preparation of IGs. These materials are greener and abundant in nature, contrary to the synthetic polymers utilized for preparation of IGs. However, the biopolymer based IGs have shown low gel-strength as revealed by rheological measurements.37-40,42 Cellulose based IGs produced by gelation of microcellulose with an IL, 1-ethyl-3-methylimidaozlium methylphosphate, have been reported.36 The unique properties of these IGs such as ease of their transfer from one substrate to another, transparency, flexibility, and high specific capacitance made them suitable for flexible electronics. Similarly, agarose, a neutral polysaccharide, generally used for preparation of hydrogels, have been found to form IGs37-39 with self-healing property39 and have a potential to be utilized in super-capacitors.39 However, the relatively slower self-healing process, which takes about 5-6 hours, and the poor stretch-ability of agarose based IGs could limit their application. The hybrid IGs comprising an IL, 1-butyl-3-methyl imidazolium chloride, [C4mim][Cl], and mixture of agarose and chitosan as active components and exhibiting mechanical properties depending upon relative content of agarose and chitosan, have been reported.38 Temperatureinduced shapeable films using elastic IGs comprising [C4mim][Cl] and guar gum via solvent extraction using ethanol, exists in literature.46 The relatively lower solubility of agarose, chitosan



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and guar-Gum in ILs along with the absence of electrostatic forces of interaction between these biopolymers and ILs seems to be a reason for formation of relatively weaker IGs as compared to those comprising synthetic polymers.29 This disadvantage can be overcome by using polyampholyte such as gelatin (G), which also is inexpensive, readily available, and has high solubility in many of the ILs, although water plays an important role in its solubilization in ILs and IG formation.48 G is obtained from collagen by its denaturation, and is composed mainly of single polypeptide chain. A small amount of double and triple helices remains present but G lacks any tertiary or quaternary structure.49 The presence of positively charged (lysine and arginine, 7.5%), negatively charged (glutamic and aspartic acid, 12%), neutral (glycine, proline, and hydroxyproline, 58%) and hydrophobic (leucine, isoleucine, methionine, and valine, 6%) amino acid residues makes it prone to undergo different set of interactions with solvent or electrolytes. The expected set of electrostatic, hydrophobic and H-boding interactions between G and ILs leads to formation of IGs. Depending on the nature of IL (hydrophilic/hydrophobic), G forms compact rigid solid, viscous liquid, and IGs of different shapes, which depends on the shape of the mould.48 These IGs have shown to exhibit mixed ionic and electronic conduction, which have shown to be suitable for preparation of electrochromic windows. On the other hand, the stability of bio-based IGs towards microbial degradation is a matter of concern for their applications in device formulation, which has not been addressed in open literature to best of our knowledge. The IGs can be made resistant towards microbial degradation by incorporation of antimicrobial metal or metal oxide nanoparticles (NPs) such as Ag or Ag2O NPs. Further, the elastic nature of IGs and fast selfhealing is highly desirable from the view point of their use in different electronic applications. Most importantly, the adhering power of IGs on a given substrate for device fabrication has not


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been reported with exception of “Cut and Stick” IG applicable for fabrication of high capacitance of gate dielectrics29 and a bio-adhesive ion gel.47 In brief, the objective of the present work is to formulate Ag2O NPs doped G based multi-functional IGs employing G and an IL, 1-ethyl-3methylimidazolium ethylsulfate, [C2mim][C2OSO3], in the presence of water. The IGs are designed to overcome the challenges of reported biopolymer based IGs, which include lower mechanical strength, slow self-healing, poor-elasticity, and anticipated biodegradation. The prepared IGs with diverse properties are expected to be utilized for different electrical applications. Besides this, the work reports an economical and green method for preparation of IGs doped with in-situ prepared Ag2O NPs. A low temperature preparation of Ag2O NPs in IGs by not using any harmful organic solvent or reducing agent, make the process an economical and greener one. Experimental: A detailed account of materials used and methods employed for the preparation and characterization of G based hydrogel and various IGs is provided as Annexure S1 (Supporting Information). Result and discussion Preparation and characterization of Ag2O NPs doped IGs As discussed above, to counter the current obstacles for the industrial use of biopolymer based IGs, herein, gelatin (G), in combination with relatively polar ionic liquid (IL), 1-ethyl-3methylimidazolium ethylsulfate [C2mim][C2OSO3], has been exploited to prepare IGs. The conjunction of IGs with in-situ prepared Ag2O nanoparticles (NPs) via photo-reduction method, led to induction of antimicrobial activity in IGs while providing mechanical strength. In brief, the hydrogel (Figure 1a) as well as IG (Figure 1b) were prepared by step wise addition of G in small proportions to water and IL-H2O mixture (50/50 v/v %), respectively, while stirring at a



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relatively lower temperature i.e. 45 oC (Annexure S1, Supporting Information). The choice of temperature is driven by two factors: (i) to keep a check on evaporation of water during stirring; and (ii) not to allow a large increase in viscosity of the forming solution of G in IL-H2O mixtures, required for proper stirring.

Figure 1. The outlook of (a) hydrogel; (b) ionogel; (c,d) ionogel having Ag2O nanoparticles (IG@Ag2O 6h and 12h, respectively); as well as (e) SEM and (f) TEM image of Ag2O NPs (IG@Ag2O, 12h); (g) X-ray diffractogram of Ag2O NPs (IG@Ag2O NPs); and (h) UV-Vis spectra of IG@Ag2O NPs.

Different mass ratios of gelatin to IL-H2O mixtures were tested for formation of IGs as the relative amount of constituents effect the properties of IGs as can be seen from Table S1 (Supporting Information). After complete dissolution of G, hydrogel and transparent IG were formed at 25 oC (Figure 1 and Figure S1, Supporting Information). The IGs imbedded with Ag2O nanoparticles (NPs) (Figure 1c,d and S1, Supporting Information) was prepared following the same method using aqueous solution of AgNO3 (10 mmol L-1) in place of water, keeping the solution in dark till the dissolution of G takes place. Subsequently the viscous solution was kept under sunlight (1h to 12h, while stirring at 45 oC) for in-situ preparation of Ag2O NPs via photoreduction.50 For clarity of understanding, henceforth abbreviation "IG" and “IG@Ag2O” will be


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used for “ionogel” and “ionogel having Ag2O NPs”, respectively, whereas “IGs” will be used for all of the investigated ionogels. The transparent nature of IG and IGs@Ag2O as compared to hydrogel (Figure 1 and Figure S1, Supporting Information) indicates the phase homogeneity and the compatibility of G with both IL as well as Ag2O NPs. It is mentioned that the IGs prepared by dissolving equal amount of G and IL-water (50 / 50 v/v%) by weight have been investigated. The presence of Ag2O NPs in IGs@Ag2O were confirmed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction, and UV-Vis measurements (Figure 1, and Figure S2, Supporting Information). The increased duration of exposure of sun light from 1h to 12h, led to increase in size and change in shape of Ag2O NPs embedded in IGs@Ag2O. Spherical NPs (15-25 nm, 1h) transforms to bead like (100-200 nm, 12h) via growth of spherical sheet-like (80-150 nm, 6h) NPs. The formed Ag2O NPs in IGs@Ag2O shows cubic (primitive) crystal structure as revealed by XRD (JSPDS No. 76-1393) (Figure 1g) and selected area electron diffraction (SAED) pattern (S2d-g, Supporting Information). The crystallinity of NPs is found to be enhanced with increased irradiation time as evidenced from SAED pattern. The templating nature of both IL51and G52 as well as irradiation time is supposed to affect the size, morphology and crystallinity of NPs along with providing stability towards aggregation. The UV-Vis measurements show the enhancement in absorption at 390 nm, which corresponds to Surface Plasmon Resonance of Ag2O53 NPs, with increased broadness. This suggests the growth of Ag2O NPs in IGs@Ag2O with time. It is important to mention that Ag2O NPs were prepared at 45 oC under sun light contrary to that prepared at 100 oC,38 which supports the greener nature of the process. The formation pathway of Ag2O NPs is expected to be similar to that of Ag NPs prepared under sunlight.50 Mechanical Properties of IGs



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Mechanical properties of IG and IGs@Ag2O has been determined at 25 oC using rheometry. The G′ (storage modulus) is found to be greater than G′′ (loss modulus) (Figure 2a) throughout the investigated frequency range without any crossover between G′ and G′′, indicating solid-like behavior of IGs possessing elasticity.54 With some exceptions in low frequency range (up to 10 rad s-1), the IG and IGs@Ag2O have shown behavior typical of a chemically cross-linked polymer gels despite being physical gels.54

Figure 2. Variation of G′ (storage modulus) and G′′ (loss modulus) as a function of (a) frequency at a strain value of 5%; and (b) strain employing a frequency of 1Hz 25 oC.

For different IGs, both G′ and G′′ follows the order: IGs@Ag2O (12h) > IG > IGs@Ag2O (1h) > IGs@Ag2O (6h). This signifies the dominant effect of presence as well as shape and size of Ag2O NPs in IGs. This could minimize the effect produced by aging of gel, where gels generally stiffen with aging.55 Further, in the same frequency range, a marginal and large decrease in G′ and G′′, respectively, in case of IGs@Ag2O (1h and 6h) indicates the relaxation of polymer network due to change in physical cross-links, while maintaining the macroscopic gel structure. The presence of Ag2O NPs in agarose-chitosan composite IGs have been found to weaken the IG as revealed by frequency sweep measurements contrary to our observations.38 This contrasting behavior is assigned to difference in nature of constituent ions of ILs and the gelator used. The IG and IGs@Ag2O have shown large viscoelastic regime, where gels remains stable towards structural deformation under strain as revealed by dynamic strain sweep measurements (Figure


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2b). The critical value of strain (γc) is found to be in the range of 57 to 138 %, which increases while going from IG to IGs@Ag2O (12h). The γc for IG and IGs@Ag2O is larger as compared to agarose-chitosan composite IGs (γc = 7-10 %)38 but is comparable to IGs formed by functionalized agarose (γc = 85 %).39 Interestingly, despite being ionic in nature, both G′ and G′′ of IGs decrease beyond γc, showing an elastic response which is typical of hydrogels.56 This is contrary to earlier reports on IGs, where G′ decreases and G′′ increases beyond γc, due to break up of gel micro-structure.34 Beyond γc, a sudden decrease in G′ indicates the collapse of gelnetwrok to a quasi-liquid state.56 The IG and IGs@Ag2O also exhibit a fair amount of thixotropy i.e. rapid recovery of its mechanical properties after a large amplitude oscillatory break-down (Figure 3a-d).

Figure 3. Variation of G′ (storage modulus) and G′′ (loss modulus) during large oscillatory break-down measurements for (a) IG; (b) IGs@Ag2O (1h); (c) IGs@Ag2O (6h); and (d) IGs@Ag2O (12h) at 25 oC under oscillatory force (γ = 5% or 0.1% and frequency = 1 Hz).

The high stretch-ability of IG and IGs@Ag2O (12h, as a representative) is also tested via stretching the gels by mechanical force using hand (Video S1, Supporting Information), where the gels (disc shaped, thickness 4 mm and diameter 25 mm) were found to regain their original



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shape after repeated stretching for at least 100 cycles. The dimensions of un-stretched and fully stretched IGs as calculated from the images by using Image J software are provided in Table S2 (Supporting Information). The IG and IGs@Ag2O NPs has been found to exhibit ≈280-470 % increase in length (stretched) as compared to their diameter (un-stretched), whereas an increase of ≈135-270 % in area of stretched IGs as compared to un-stretched ones, have been observed. Figure S3 (Supporting Information) shows the variation of G′ and G′′ as a function of strain, approximately up to 3000%, at a frequency of 1Hz . The cross-over between G′ and G′′ gives the value of strain that corresponds to break down of IGs. As can be seen from Table S3 (Supporting Information), high value of strain (1800-2800%), which for different IGs varies as: IGs < IGs@Ag2O (1h) < IGs@Ag2O (6h) < IGs@Ag2O (12h), indicates that the investigated IGs have very high gel strength. This could be assigned to presence of stronger interactions between IL, G and water. Oscillatory break-down measurements for the investigated IGs were performed under unstrained (Strain 1%, Frequency 1Hz) and strained (Strain 2500-3000%, Frequency 1Hz) conditions, considering the break down point of IGs. Under the oscillatory force (γ = 25003000%, frequency = 1 Hz), G′ decreases from a high magnitude to a lower one, where tan δ (G′′/G′) increases, while going from IG to IGs@Ag2O (12h), suggesting the increased extent of quasi-liquid state. Interestingly, when the amplitude is decreased (γ = 1%, frequency = 1 Hz), G′ immediately approaches at its initial value, indicating the regain of quasi-solid state. An increase in the values of tan δ under strain (Table S3, Supporting Information) going from IG to IGs@Ag2O (12h) suggest an increase in quasi-liquid like state, which attains a quasi-solid state under unstrained conditions. On comparing different IGs, the values of tan δ suggests that IGs@Ag2O (12h) exhibit more pronounced quasi-liquid state under strain as compared to other IGs. This behavior is quite interesting as the investigated gels have same composition with the


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exception of presence and/or variation in shape as well as size of Ag2O NPs. It is natural to assume that the ions constituting IL, in swollen network of G, are weakly bound with G. Their mobility along with absence (IG) and negligible presence of Ag2O NPs (IGs@ Ag2O, 1h) leads to such behavior. The smaller size and spherical shape of Ag2O NPs in IGs@Ag2O (1h) is assumed not to fit well in aggregating G matrix, however under strain, there could be entanglement of G network encompassing the Ag2O NPs giving rise to physical cross-links, which results in strengthening of gel. The presence of large sized sheet-like (IGs@Ag2O, 6h) and bead-like (IGs@Ag2O, 12h) Ag2O NPs (Figure S2, supporting information) seem to undergo expulsion from gel network under strain leading to collapse of gels. Importantly, our IGs exhibit highly elastic nature and show unique quasi-liquid and quasi-solid like behavior as evidenced by oscillatory strain sweep measurements, which have never been observed before in any type of IGs based either on biopolymer or polymer. The variation in rheological behavior of IGs can be linked to their internal structure. Internal Structure and Surface Morphology of IGs The scanning electron microscopic (SEM) images of two representative gels i.e. IG (Figure 4a) and IGs@Ag2O (12h) (Figure 4b) in dried form provided information about varying internal structure of gels, which resulted in different mechanical properties. As can be seen from Figure 4a, IG comprises of long helical fibers of G with dimensions of ≈10-20 and ≈1-5 µm in length and breadth, respectively, presumably formed by a large number of polypeptide chains of G as happens in case of G hydrogels.54 At the same time, the presence of amorphous fraction in dried gels cannot be ruled out as evidenced by the presence of a broad peak in 15-25o 2θ range in the XRD patterns of hydrogel, ionogel and ionogel@Ag2O (Figure S4, Supporting Information).57-59 Intensity of the observed peak decreases as we move from pure gelatin toward IGs and follows



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the order pure gelatin > hydrogel > IG ≈ IG@Ag2O (1h, 6h and 12h). This indicates the decrease in amorphous content in dried gels, which decreases while going from hydrogel to IGs. This could be due to lesser amount of water in the IG and IGs@Ag2O than in hydrogels of G. It is quite reasonable that the constituent ions of IL screens the electrostatic repulsions between similarly charged amino acid residues on G backbone and mediates their self-assembly leading to larger strands of G as shown in Scheme S1 (Supporting Information).

Figure 4. Scanning electron microscopy (SEM) image of dried gels of (a) IG; and (b) IGs@Ag2O (12h), as representative. The Ag2O NPs could not be observed due to leaching into solvent from the gels. Enlarged view in Figure 4a shows the presence of helical structures of G in IG.

The presence of Ag2O NPs of varying size and shape, although small enough to alter the selfassembly of G chains at level of fibrils, could play an important role in case of IGs@Ag2O (12h), where much larger sheet like structures having dimension of ≈100-200 and ≈10-20 µm in length and breadth, respectively, are observed. Conformational change of G in IG and IGs@Ag2O (12h) in comparison to hydrogels of G has been monitored by Fourier Transformed Infrared Spectroscopy (FTIR) exploiting characteristic bands of G and IL as shown in Figure S5a (Supporting Information). The ratio of intensities of Amide III (1235 cm-1 in hydrogel) to Amide I (1634 cm-1 in hydrogel) band are used to find the loss/gain of secondary structure by random coil/α-helix or triple helix formation, respectively, in IGs.60 A decrease in the ratio of intensity of


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Amide III / Amide I band, while going from hydrogel to IG and IG to different IGs@Ag2O (Figure S5b, Supporting Information) indicates the loss of secondary structure suggesting the depletion of helical content of G in IGs. This is supported by a decrease in intensity of the broad XRD peak corresponding to helical content of G, while going from hyrogel to IGs@Ag2O (12h) (Figure S4, Supporting Information). The position of FTIR bands corresponding to [C2OSO3] anion of IL (S=Osym st. 1018 cm-1)61 and [C2mim] cationic head group (C(2)Hstr = 3165 cm-1; and C(4,5)Hstr = 3114 cm-1)61 remains constant in IGs indicating evenness of electrostatic interactions between ILs and G, while going from IG to IG@Ag2O. The CH2(str) = 2980 and 2940 cm-1 of alkyl chain of IL or of alkyl groups of amino acid residues of G, don’t shift while going from IG to IGs@Ag2O indicating negligible effect of shape or size of Ag2O NPs owing to their very small content. The presence of hydrophobic interactions between alkyl chains of IL templating Ag2O NPs and hydrophobic parts of G could draw more IL between G polypeptide chains disturbing the helix formation while strengthening the gel via bridging the helices with random coils leading to more physical cross-links between helices as shown in Scheme S1 (Supporting Information). It is stated that, in general, our IGs are mechanically stronger than other reported IGs based on biopolymers, till date.37-47 Self-healing, shape-memory and multi-adhesive nature of IGs On the other hand, the self-healing materials are being developed at a rapid pace due to many potential applications,62,63 and till date, there exist only two examples of biopolymer based selfhealing IGs, one based on functionalized Agarose39 and other based on Guar Gum.47 The hydrogel of G does not self-heal (Figure 5a and b), whereas IG (Figure 5c and d) and all of the IGs@Ag2O NPs (Figure 5e and f, IGs@Ag2O, 12h, as a representative) self-heals without the need of any external stimuli or plasticizer. Due to lower number density of Ag NPs, their



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presence in IGs@Ag2O is not hindering the self-healing process. The IGs have shown much improved self-healing behavior in terms of time taken for self-healing, which is crucial from application point of view. These IGs undergo self-healing at room temperature (25 oC) in < 1 minute, when two cut pieces of IGs are kept in contact with each other. This time is much less as compared to IG based on guar-gum47 and functionalized Agarose39, which took 5-6 h and 60 minutes, respectively, at room temperature. The process of self-healing can be repeated at least for 100 cycles of stretching of IGs with marginal increase in time taken by the IGs for selfhealing, which remains with in 1 minute (Figure S6, Supporting Information).

Figure 5. The pictures of (a) cut hydrogel; (b) hydrogel showing no self-healing; (c) cut IG; (d) self-healed IG; (e) cut IG@Ag2O (12 h); (f) Self-healed IG@Ag2O (12 h); (g-i) twisted IG@Ag2O (12h) regaining its shape after twisting for 5 minutes; (j-n) adhesion of IG and IG@Ag2O (12h) on different surfaces; (o) disc shaped IG@Ag2O (12 h) and (p) fully stretched IG@Ag2O (12 h). Different color of same ionogel is due to variation in light intensity during photography.


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The self-healed IGs regain their mechanical properties as evidenced by large amplitude oscillatory break down and regain measurements (Figure S7, Supporting Information). When joined together, the cut-pieces of IGs form a long slice, which can withhold its weight when suspended vertically (Figure S8, Supporting Information). The nature of the surface (freshly cut or uncut) or absence (IG) /presence (IG@Ag2O) of Ag2O NPs does not affect the self-healing process. It is observed that the extent of self-healing as observed visually decreases with decrease in water content and temperature (Table S2, Supporting Information). The IGs undergo self-healing at water content between 15-50% (w/w) at temperatures between 25-40 oC. At temperatures lower than 25 oC, the IGs become hard and at temperatures above 40 oC, the IGs either become very soft (40-55 oC) or become a viscous paste (55-65 oC). This suggests that the IL, G, temperature and the presence of water plays an important role in self-healing. The improved self-healing in agarose based IG in open atmosphere due to moisture uptake by gel supports our assumption of possible role of water in the self-healing39 which has shown to increase the mobility of ionic species across two fused surfaces leading to self-healing.47 In IGs, the multiple set of interactions mainly hydrophobic, H-bonding and electrostatic forces of interaction between ions of IL and G could play their role in self-healing.39 The role of alkyl chain appended to ions of IL in self-healing via hydrophobic interactions with G could be negligible owing to very small size of the alkyl chain. H-bonding interactions between two cut surfaces of IGs solely cannot govern the self-healing. This is evidenced by no self-healing of G hydrogel (Figure 5a and b), however the role of water in self-healing of IGs cannot be lined out completely. Therefore, electrostatic interactions between G and IL or between G chains mediated by ions of IL seem to play a major role in self-healing. Earlier, it has been shown that stronger electrostatic interactions between positively charged guanidinium based binders and negatively



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charged clay nanoparticles governs the self-healing of hydrogels.56 Even hydrogels of G in the presence of electrolytes such as calcium phosphate has been shown to exhibit self-healing behavior.64 Therefore, the dynamic diffusion of ionic species42,62 along with the G chains mediated by the presence of water, where the ions of IL not only screen the electrostatic repulsions between G chains, rather mediates the interaction between G chains as shown in Scheme 1. The speedy recovery of IGs to their original structure similar to other gels based on electrostatic interactions55,65 supports the role of electrostatic interactions in self-healing process.

Scheme 1. Proposed mechanism for self-healing of IG or IG@Ag2O (a) cut IG@Ag2O; (b) self-healing IG@Ag2O via IL mediated screening of columbic repulsions and other attractive interactions (H-bonding and hydrophobic interactions); and (c) self-healed IG@Ag2O.

Besides this, the role of H-bonding interactions along with π-π interactions between imidazolium cation of IL and amino acid residues of G cannot be ruled out. The investigated IGs show shapememory effect, which regains their original shape in < 2 minute after twisting (Figure 5) and retains the property to show shape-memory effect even after 100 cycles of stretching, with marginal increase in time required to regain the shape (Figure S6, Supporting Information). On the other hand, the utility of IGs as solid electrolyte in different electronic devices is expected to be enhanced by solvent free processing. Such processing to devise electronic devices can be performed by adopting “cut and stick” strategy,22 where IGs would be cut mechanically and


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made to stick to different substrates. Our IGs are found to be able to stick to a variety of investigated surfaces such as paper, quartz, polymers (polypropylene), steel, and even at human skin tissue showing bio-adhesive nature (Figure 5j-n). The fixing ability of gels on quartz, and steel at high temperature was investigated and IGs have been found to adhere to respective surface up to 45 oC for 15 minutes, after which gels became soft and remain adhered to the surface. On keeping the IGs for a long time at this temperature in open atmosphere, there can be loss of water leading to decreased adhering capacity, which is not expected to be hindrance for use of IGs under controlled conditions. Conductivity measurements Frequency dependent impedance characteristics of the IGs are important for their use in electrochemical applications. The phase angle (tan φ = Z′′ / Z′, here Z′ and Z′′ are real [Re(Z)] and imaginary component of impedence Z as [-Img(Z)]) vs. frequency plots (Figure 6a), where no constancy in phase angle in any frequency region suggest strong dependence of electrochemical behavior on frequency. b

a

Figure 6. Dependence of (a) phase angle (tan φ = Z′′ / Z′); and (b) ionic conductance upon applied frequency at 25 o C for different gels.

Relatively small values of phase angle (-5 to -15 in lower frequency range of 0.1 to 1 Hz and -3 to -5 in higher frequency range of 10000-100000 Hz) have been observed, which attains



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maximum values of -10 to -27 in the frequency range of 10-100 Hz, depending on the gel under investigation. A lower value of phase angle is indicative of dominant resistor-like behavior.36,66 An increase in phase angle with frequency in the frequency range of 0.1 to 100 Hz, depending on gel (Figure 6a), indicates increasing capacitor like behavior at the cost of decrease in resistor like behavior. The conductance of the IGs has been deduced from Nyquist plots (Figure S9a, Supporting Information) and is shown in Figure 6b. Different IGs exhibit conductance in the order: IGs@Ag2O (1h) > IGs@Ag2O (6h) > IG > IGs@Ag2O (12h) in low frequency range (0.1 to 20 Hz), whereas there is a crossover between IG and IGs@Ag2O (6h) near a frequency of 10 Hz. The variation in conductance is ascribed to increased interactions between G and IL as evidenced by FTIR spectroscopy and thickening of G bundles entrapping IL resulting in lower mobility of ions, while going from IG to IGs@Ag2O (12h). The enhanced mechanical properties of IGs@Ag2O (6h and 12h) indicate the stronger interactions prevailing between G-IL-Ag2O in IGs@Ag2O. It is natural to assume that much smaller spherical Ag2O NPs in IGs@Ag2O (1h) don’t hinder the movement of ions, whereas larger NPs in IGs@Ag2O (6h and 12h) could interfere with the movement of constituent ions of IL. In the temperature range of 25 to 70 oC, the ionic conductance of gels lies in the range of 1.45 to 9.1 mS cm-1 (Figure S9b, Supporting Information). If we compare ionic conductance at 25 0C, different gels exhibit conductance in the range 1.4 to 2 mS cm-1, which is higher than that reported for gelatin based IG with same IL (0.12 to 0.25 mS cm-1)48, and agarose based IGs (0.1 to 0.7 mS cm-1).37 Such behavior can be accounted for variation in base IL, as well as nature and amount of the gelator. It is important to mention that the conductivity of IGs is ≈ 2 fold lower than that of the IL and IL-water mixture of same composition at a given temperature (Figure S9, Supporting Information) and is due to impeded motion of ions because of wrapping G network where free volume is expected to play


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an important role. Importantly, the IGs remain highly conducting (Figure S9c, Supporting Information) with a marginal loss in conductivity, where conductivity decreases by 1-1.5% even after 100 cycles of stretching. Antimicrobial properties of IGs Further, the immobilization of Ag2O NPs in IG imparted stability to gels towards microbial degradation owing to antimicrobial properties of Ag2O NPs.67-70 It was observed that both hydrogel and IG have been eaten up by microbes, when kept in open atmosphere for 10 and 30 days (T ≈ 25-35 oC, Humidity ≈ 50-60%), respectively, whereas IGs@Ag2O have shown reluctance towards microbial degradation under similar conditions for at least 6 months. In line with the visual observations, antimicrobial investigations have shown that the hydrogel followed by IG (Figure S10), exhibited a negligible zone of clearance indicating their ineffectiveness towards vulnerability to both Gram Positive and Gram Negative bacteria. It is natural that microbes eat up the G, preferentially. Contrarily, IGs@Ag2O have shown almost equal effectiveness in case of both Gram positive and Gram negative bacterium and diffused completely in the well created in the solidified LB-agar plate. The evaluation of variation in zone of clearance, a marker of effectiveness of antimicrobial activity,69 indicates that IGs@Ag2O (1h) are relatively more antimicrobial followed by IGs@Ag2O (6h) and IGs@Ag2O (12h). This can be correlated with decreasing surface area of Ag2O NPs, while transforming from spherical IGs@Ag2O (1h) to elongated bead like NPs IGs@Ag2O (12h). The shape dependent antimicrobial activity of Ag2O NPs have been observed, which is in line with the fact that more Ag+ ions are exposed to bacterial cells with increase in surface area.71 The surface charge present on the atomic planes of Ag2O NPs of different shapes could also affect the antimicrobial activity.71 The investigated IGs retain their antimicrobial activity for at least 100 cycles of



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stretching as the stretching of IGs don’t alter the content as well as shape and size of Ag2O NPs, responsible for antimicrobial activity. It is anticipated that IGs if prepared by using antimicrobial ILs72 would be much more effective and further studies in this regard are going on in our group. Conclusion The doping of IG comprising IL, 1-ethyl-3-methylimidzolium ethylsulfate, [C2min][C2OSO3], and gelatin, G, with in-situ prepared Ag2O nanoparticles (NPs) of varying size and shape via photo-reduction method was achieved. The prepared IG and IGs doped with Ag2O NPs (IGs@Ag2O) showed good mechanical strength along with remarkable stretch-ability as suggested by rheological measurements and observed by naked eye. The IGs show efficient selfhealing behavior, where thixotropy and conducting properties are retained by IGs even after selfhealing. The high content of water in IG as compared to that earlier reported IGs48 led to improvement in conductivity of IG and IGs@Ag2O. Further, the Ag2O NPs in IGs@Ag2O imparted antimicrobial property to IGs@Ag2O expected to provide stability towards microbial attack. The present results fill the large gap in terms of good mechanical properties, fast selfhealing behavior, remarkable stretch-ability, shape-memory effect, antimicrobial nature, and multi-adhesive (including bio-adhesive) nature of IGs. This work along with other reports on biopolymer based IGs36-47 is expected to open up a new research avenue of utilizing easily available biomaterials in combination with antimicrobial ILs or functional ILs to prepare and use antimicrobial/functional IGs in different applications. New research on economical and greener methods for incorporation of a variety of NPs in IGs is expected to take place following this work, where one pot preparation of Ag2O NPs inside IGs is achieved. Supporting Information


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For Table of Contents Use Only

Multi-functional gelatin based ionogels are prepared and characterized, which shows long shelflife while retaining the physico-chemical properties.


Gelatin-Based Highly Stretchable, Self-Healing, Conducting

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