Gelatin-Based Highly Stretchable, Self-Healing, Conducting

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Research Article pubs.acs.org/journal/ascecg

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 S. Kang*,† †

Department of Chemistry, UGC-Centre for Advance Studies − II, Guru Nanak Dev University, Amritsar 143005, India AcSIR, CSIR-Central Salt and Marine Chemicals Research Institute, G. B. Marg, Bhavnagar 364002, India § Department of Biotechnology, Sri Guru Granth Sahib World University, Fatehgarh Sahib 140406, India ‡

S Supporting Information *

ABSTRACT: The polyionic nature of 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]. The 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 stepwise manner at 45 °C while stirring. The combination of IGs with Ag2O nanoparticles (NPs) prepared in situ, via photoreduction of AgNO3 led to induction of antimicrobial activity in IGs, while enhancing the mechanical properties. The prepared IGs show fast self-healing ( IG > IGs@Ag2O (1 h) > IGs@Ag2O (6 h). This signifies the dominant effect of presence as well as the 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 Furthermore, in the same frequency range, a marginal and large decrease in G′ and G″, respectively, in the case of

different electrical applications. The work also 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 not using any harmful organic solvent or reducing agent makes the process an economical and greener one.

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EXPERIMENTAL SECTION

A detailed account of materials used and methods employed for the preparation and characterization of G-based hydrogel and various IGs is provided in the Supporting Information.

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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, G, in combination with relatively polar IL, 1-ethyl-3-methylimidazolium ethylsulfate, [C2mim][C2OSO3], has been exploited to prepare IGs. The conjunction of IGs with in situ prepared Ag2O nanoparticles (NPs) via photoreduction 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 stepwise addition of G in small proportions to water and IL−H2O mixture (50/50 v/v %), respectively, while stirring at a relatively lower temperature, i.e., 45 °C (Supporting Information). The choice of temperature is driven by two factors: (i) to keep a check on evaporation of water during stirring and (ii) to prevent a large increase in viscosity of the forming solution of G in IL−H2O mixtures, required for proper stirring. 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. After complete dissolution of G, hydrogel and transparent IGs were formed at 25 °C (Figures 1 and S1). The IGs imbedded with Ag2O nanoparticles (NPs) (Figures 1c,d and S1) were prepared following the same method using an aqueous solution of AgNO3 (10 mmol L−1) in place of water, keeping the solution in dark until the dissolution of G takes place. Subsequently, the viscous solution was kept under sunlight (1−12 h, while stirring at 45 °C) for in situ preparation of Ag2O NPs via photoreduction.50 For clarity of understanding, henceforth the abbreviations “IG” and “IG@Ag2O” will be 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 the nature of hydrogel (Figures 1 6570

DOI: 10.1021/acssuschemeng.7b00719 ACS Sustainable Chem. Eng. 2017, 5, 6568−6577

Research Article

ACS Sustainable Chemistry & Engineering

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

variation of G′ and G″ as a function of strain, approximately up to 3000%, at a frequency of 1 Hz. The crossover between G′ and G″ gives the value of strain that corresponds to break down of IGs. As can be seen from Table S3, a high value of strain (1800−2800%), which for different IGs varies as IGs < IGs@ Ag2O (1 h) < IGs@Ag2O (6 h) < IGs@Ag2O (12 h), indicates that the investigated IGs have very high gel strength. This could be assigned to presence of stronger interactions among IL, G, and water. Oscillatory breakdown measurements for the investigated IGs were performed under unstrained (strain 1%, frequency 1 Hz) and strained (strain 2500−3000%, frequency 1 Hz) conditions, considering the break down point of IGs. Under the oscillatory force (γ = 2500−3000%, 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 (12 h), 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) going from IG to IGs@Ag2O (12 h) 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 suggest that IGs@Ag2O (12 h) exhibits more pronounced quasi-liquid state under strain as compared to that of other IGs. This behavior is quite interesting as the investigated gels have same composition with the 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 a swollen network of G, are weakly bound with G. Their mobility along with absence (IG) and negligible presence of Ag2O NPs (IGs@ Ag2O, 1 h) leads to such behavior. The smaller size and spherical shape of Ag2O NPs in IGs@Ag2O (1 h) 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 sheetlike (IGs@Ag2O, 6 h) and beadlike (IGs@Ag2O,

IGs@Ag2O (1 and 6 h) 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 remain stable toward structural deformation under strain as revealed by dynamic strain sweep measurements (Figure 2b). The critical value of strain (γc) is found to be in the range of 57−138%, which increases while going from IG to IGs@Ag2O (12 h). The γ c for IG and IGs@Ag2O is larger as compared to agarose−chitosan composite IGs (γ c = 7−10%)38 but is comparable to that of IGs formed by functionalized agarose (γ c = 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 microstructure.34 Beyond γ c, a sudden decrease in G ′ indicates the collapse of gel-netwrok 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 breakdown (Figure 3a−d). The high stretching ability of IG and IGs@Ag2O (12 h, as a representative) is also tested via stretching the gels by mechanical force using hand (Video S1), where the gels (disc shaped, thickness 4 mm and diameter 25 mm) were found to regain their original shape after repeated stretching for at least 100 cycles. The dimensions of unstretched and fully stretched IGs as calculated from the images by using ImageJ software are provided in Table S2. The IG and IGs@Ag2O NPs has been found to exhibit ∼280−470% increase in length (stretched) as compared to their diameter (unstretched), whereas an increase of ∼135−270% in area of stretched IGs as compared to unstretched ones, have been observed. Figure S3 shows the 6571

DOI: 10.1021/acssuschemeng.7b00719 ACS Sustainable Chem. Eng. 2017, 5, 6568−6577

Research Article

ACS Sustainable Chemistry & Engineering

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

Figure 5. 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 (12 h) regaining its shape after twisting for 5 min; (j−n) adhesion of IG and IG@Ag2O (12 h) 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.

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 the 15−25° 2θ range in the XRD patterns of hydrogel, ionogel and ionogel@Ag2O (Figure S4).57−59 Intensity of the observed peak decreases as we move from pure gelatin toward IGs and follows the order of pure gelatin > hydrogel > IG ≈ IG@Ag2O (1, 6, and 12 h). This indicates the decrease in amorphous content in dried gels, which decreases while going from hydrogel to IGs. This could be due to a smaller amount of water in the IG and IGs@Ag2O than that 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

12 h) Ag2O NPs (Figure S2) 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 (12 h) (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 6572

DOI: 10.1021/acssuschemeng.7b00719 ACS Sustainable Chem. Eng. 2017, 5, 6568−6577

Research Article

ACS Sustainable Chemistry & Engineering Scheme 1. Proposed Mechanism for Self-Healing of IG or IG@Ag2Oa

a

(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.

without the need of any external stimuli or plasticizer. Due to lower number density of Ag NPs, their presence in IGs@Ag2O does not hinder 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 °C) in IGs@Ag2O (6 h) > IG > IGs@Ag2O (12 h) in low frequency range (0.1−20 Hz), whereas there is a crossover between IG and IGs@Ag2O (6 h) 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 (12 h). The enhanced mechanical properties of IGs@Ag2O (6 and 12 h) 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 (1 h) do not hinder the movement of ions, whereas larger NPs in IGs@Ag2O (6 and 12 h) could interfere with the movement of constituent ions of IL. In the temperature range of 25−70 °C, the ionic conductance of gels lies in the range of 1.45−9.1 mS cm−1 (Figure S9b). If we compare ionic conductance at 25 °C, different gels exhibit conductance in the range 1.4−2 mS cm−1, which is higher than that reported for gelatin-based IG with same IL (0.12−0.25 mS cm−1),48 and agarose-based IGs (0.1− 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 ca. 2-fold lower than that of the IL and IL−H2O mixture of same composition at a given temperature (Figure S9) and is due to impeded motion of ions because of wrapping G network where free volume is expected to play an important role. Importantly, the IGs remain highly conducting (Figure S9c) with a marginal loss in conductivity, where conductivity decreases by 1−1.5% even after 100 cycles of stretching. Antimicrobial Properties of IGs. Furthermore, the immobilization of Ag2O NPs in IG imparted stability to gels toward 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, respectively (T ≈ 25−35 °C, humidity ≈ 50−60%), whereas IGs@Ag2O have shown reluctance toward 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

binders and negatively 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 but rather mediate 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. 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 shape-memory effect, which regains their original shape in