Priyadarshi Chakraborty

Improved mechanical and photophysical properties of chitosan incorporated folic acid gel possessing the characteristics of dye and metal ion absorption

The folic acid (F) gel, prepared in a 1:1 dimethyl sulfoxide (DMSO)–water mixture (v/v), at 0.2% (w/v) concentration has a nano-fibrous network morphology. FTIR spectra indicate the presence of intermolecular H-bonding interactions in the gel. The folic acid–chitosan (C) hybrid (FC) gel (prepared by mixing 0.1 ml of 1% (w/v) C solution with 2 ml 0.2% (w/v) F solution) appears as a yellow transparent semi-solid mass consisting of a nano-fibrous network with a higher density of branches. The fibrillar diameter of the FC hybrid gel (10.5 nm) is much lower than that of F gel (17.3 nm). The FTIR band at 3390 cm−1 of F gel shows a shift to 3435 cm−1 in the FC hybrid gel indicting a hydrogen bonding interaction between C and F. The fluorescence intensity of the FC hybrid gel is enhanced by 2.75 times than that of F gel. The shear viscosity of FC hybrid gel is 3 orders higher than that of the F gel and both gels exhibit shear thickening at a low shear rate (<10−3 s−1) but above that, shear-thinning occurs. In the creep phase, the FC hybrid gel exhibits strain recovery but the F gel does not. The complex modulus (G*) of both systems initially exhibit a sharp increase, followed by a slow increase with time (t) and dG*/dt varies with C concentration in the FC hybrid gel showing a maximum. The WAXS pattern of the FC hybrid xerogel does not exhibit any crystalline peak, suggesting that the additive (C) inhibits the crystallization of F. The dye absorption by the hybrid gel material is maximum for Eosin Yellow, (83.5%), however, Methyl Orange and Methylene Blue are absorbed to a lesser extent (∼40%). The FC hybrid gel absorbs 55% of Cu2+, 67.2% of Cr3+ and 49% of Co2+ from their respective solutions.

Robust hybrid hydrogels with good rectification properties and their application as active materials for dye-sensitized solar cells: insights from AC impedance spectroscopy

Dihybrid (GP) and trihybrid (GPPS) hydrogels constructed by 5,5′-(1,3,5,7-tetraoxopyrrolo[3,4-f]isoindole-2,6-diyl)diisophthalic acid (P), graphene oxide (GO) and P, GO, and PEDOT:PSS are studied. The gelation occurs through H-bonding and π–π stacking interactions. A magnificent improvement occurs in the mechanical properties of the hybrid gels compared to the P gel. The dc-conductivity of GPPS xerogels is 4–5 orders of magnitude higher compared to the GP and P xerogels. The current–voltage (I–V) characteristic curves of the GP and GPPS gels exhibit rectification properties with the highest rectification ratio of 61; however, the xerogels exhibit semiconducting nature. The GP and GPPS xerogels exhibit photoresponse behavior, and the on–off cycles display a stable photocurrent for the latter system. Dye-sensitized solar cells (DSSCs) are fabricated taking the GPPS gels as active materials and the power conversion efficiency (PCE) increases with the increase of PEDOT:PSS concentration showing a maximum PCE of 4.5%. The IPCE curve shows an absorption range of 360–700 nm with the maximum absorbance of ∼57%. Impedance spectroscopy indicates a considerable difference in the Nyquist plots between the xerogels and gels; the latter exhibit a semicircle with an additional spike corresponding to the Warburg impedance. The Cole–Cole plot of the DSSC consists of three semicircles and the equivalent resistance–capacitance circuits elucidate the PCE values.

Rheological and fluorescent properties of riboflavin–poly(N-isopropylacrylamide) hybrid hydrogel with a potentiality of forming Ag nanoparticle

The riboflavin (R) and poly(N-isopropylacrylamide) (PNIPAAM) hybrid hydrogels have been prepared using the free radical polymerization of N-isopropylacrylamide in the presence of N,N′-methylene bisacrylamide as a cross linker for 1, 2 and 3 mM concentrations of R. The invariance of storage (G′) and loss (G′′) moduli over a wide range of angular frequency, where G′ > G′′ for R–PNIPAAM system characterize it to be behaving as a gel in the hybrid state. Both G′ and G′′ decrease with the increase of R concentration, but the decrease is four times higher in the former than in the latter. R–PNIPAAM gel has a higher critical strain value than PNIPAAM gels and it gradually increases with the increase in R concentration indicating that R is acting as a supramolecular cross-linker. The fluorescent intensity of R–PNIPAAM gels increases with the increase in R concentration and its variation with temperature at different pH shows an increase in the intensity value with temperature, showing the maximum at ∼30 °C because of the coil-to-globule transition of PNIPAAM chains, suggesting that the R–PNIPAAM gel can be used to be a probe for temperature detection. The sensitivity index of the fluorescent temperature sensing of R–PNIPAAM gel is moderate and it is highest at pH 7. R becomes gradually released from the R–PNIPAAM gel when dipped into water (pH 7) at 20 °C, initially at a slower rate, then at a higher rate, and finally it shows a leveling for the release of 19% of embedded R at 24 h of aging. Silver nanoparticles that are grown in the R–PNIPAAM gels by immersing the slashed gel pieces in AgNO3 solution stay at both the surface and the interior of the R–PNIPAAM fibres, maintaining the gel structure intact with a decrease in critical strain and causing a quenching of the fluorescence intensity of R–PNIPAAM gels. The average size and size distribution of AgNPs linearly increase with R concentration at a constant AgNO3 concentration.

Enhancement of Energy Storage and Photoresponse Properties of Folic Acid–Polyaniline Hybrid Hydrogel by in Situ Growth of Ag Nanoparticles

Electrically conductive hydrogels are a fascinating class of materials that exhibit multifarious applications such as photoresponse, energy storage, etc., and the three-dimensional micro- and nanofibrillar structures of the gels are the key to those applications. Herein, we have synthesized a hybrid hydrogel based on folic acid (F) and polyaniline (PANI) in which F acts as a supramolecular cross-linker of PANI chains. The gels are mechanically robust and are characterized by field-emission scanning electron microscopy, transmission electron microscopy, and spectroscopic, rheological, and universal testing measurements. The hybrid xerogel exhibit a BET surface area 238 m2 g-1, conductivity of 0.04 S/cm, specific capacitance of 295 F/g at a current density of 1A/g, and photocurrent of ∼2 mA under white-light illumination. Silver nanoparticles (AgNPs) are in situ grown to elegantly improve the conductivity, energy storage, and photoresponse capability of the gels. The formation of AgNPs drastically improves the specific capacitances up to 646 F/g (at current density 1A/g), excellent rate capability (403 F/g at 20 A/g), and stable cycling performance with a retention ratio of 74% after 5000 cycles. The AgNPs embedded gel exhibits dramatic enhancement of photocurrent to 56 mA, and its time-dependent photoillumination corroborates faster rise and decay of current compared to those of folic acid-polyaniline hydrogel.

Conducting hydrogel of a naphthalenetetracarboxylic dianhydride derivative and polyaniline: different electronic properties in gel and xerogel states

An aminoisophthalic acid derivative of naphthalenetetracarboxylic dianhydride (NDI) is synthesized to produce a hydrogel via acid–base treatment. NDI is used to make co-assembled hydrogels with polyaniline (PANI) by in situ polymerization of aniline, and these are named NP0.5, NP1.0 and NP1.5 according to the amount (ml) of anilinium chloride solution. The NDI gel exhibits a fibrillar network morphology, and in the NDI-PANI gels a change in fiber texture occurs. The FTIR and NMR spectra indicate the presence of supramolecular interactions in the NDI-PANI gels and the PANI produced in the gel is in the emeraldine salt (ES) form which is also supported by UV-vis spectroscopy results. X-ray diffraction indicates the NDI gel has a lamellar structure and the lamellar distance increases significantly in the NDI-PANI gels. A schematic model of the supramolecularly organized structure with alternating NDI and PANI chains is proposed for fiber formation. Rheological study indicates storage (G′) and loss (G′′) moduli to be invariant with frequency and a 213- and 112-time increase in G′ and G′′ is observed for the NP1.5 gel. Also, its elasticity increases by 22 463%, yield stress by 905% and stiffness by 88%, as compared to those of the NDI gel. The NP1 xerogel exhibits the highest dc conductivity (5.2 × 10−3 S cm−1) and the current (I)–voltage (V) curves of the xerogels are different from those of the gels; the former exhibits the characteristics of a semiconductor–metal junction and the latter exhibits negative differential resistance with rectification properties, simultaneously. The Nyquist plots of the impedance values also indicate a difference between the xerogel and gel state; in the former only semicircles are observed, but in the latter a semicircle and a sloping line are noticed. The difference in the I–V properties in the xerogel and gel state is attributed to the diffusion controlled movement of the charge carriers through the water molecules in the gels and it is supported by the sloping line in the Nyquist plot of the gels.

Integration of poly(ethylene glycol) in N-fluorenylmethoxycarbonyl-l-tryptophan hydrogel influencing mechanical, thixotropic, and release properties.

Polyethylene glycol (PEG) is incorporated to improve the mechanical properties of N-fluorenylmethoxycarbonyl-l-tryptophan (FT) hydrogel producing the hybrid (FTP) gels designated as FTP1, FTP2.5, etc. having PEG concentrations of 0.05 and 0.125% (w/v), respectively. Both the FT and FTP1 gels exhibit fibrillar network morphology; the fibers of the FTP1 gel are thinner than those of the FT gel. FTP gels exhibit a magnificent improvement in mechanical properties, and the storage and complex moduli increase with a maximum of ∼2800% for the FTP2.5 gel. Creep recovery experiment exhibits a maximum strain recovery of 90% for the FTP1 gel. The thixotropic property is observed for both FT and FTP gels and the rate of recovery increases with increase of PEG concentration; the latter acts as a molecular adhesive to the gel fibers bringing back the network structure easily. Gelation of FT causes a 5-fold increase of fluorescence intensity due to molecular aggregation, and with increase of FT concentration the ratio of fluorescence intensities at 470 and 395 nm increases. Exploiting the thixotropic property of FT and FTP hybrid gels, doxorubicin (DOX) is successfully encapsulated, and tunable release of DOX using appropriate amount of PEG in the gel matrix under physiological conditions is observed.

Stimuli-responsive, thixotropic bicomponent hydrogel of melamine-Zn(II)-orotate complex

Zinc(II)-orotate complex (ZnOA) has been synthesised, and it is used to produce supramolecular complex with melamine (M) at 1:1 molar ratio. During sonication for 5–6 min, the supramolecular complex is transformed into a bi-component hydrogel (S-gel). The spectroscopic tools such as FTIR and UV–vis spectra suggest that the H-bonding interaction between M and ZnOA produces a supramolecular complex which further forms J-aggregates during gelation. The circular dichroism spectra indicate the change in ellipticity with temperature that arises from the molecular dissymmetry during stacking of the ZnOAM complexes. The microscopic images indicate that the hydrogel network is formed with short helical nano-fibres. The energy-minimised structure of ZnOAM complex exhibits a well-defined d value of 11.3 Å obtained from X-ray scattering of the xerogel. The frequency-dependent rheological experiment (storage modulus (G′) is greater than loss modulus (G″)) characterises it as a gel. The gel shows thixotropic and stimuli-responsive behaviour.

Nanoengineering of a Supramolecular Gel by Copolymer Incorporation: Enhancement of Gelation Rate, Mechanical Property, Fluorescence, and Conductivity

In the quest to engineer the nanofibrillar morphology of folic acid (F) gel, poly(4-vinylpyridine-co-styrene) (PVPS) is judiciously integrated as a polymeric additive because of its potential to form H-bonding and π-stacking with F. The hybrid gels are designated as F-PVPSx gels, where x denotes the amount of PVPS (mg) added in 2 mL of F gel (0.3%, w/v). The assistance of PVPS in the gelation of F is manifested from the drop in critical gelation concentration and increased fiber diameter and branching of F-PVPSx gels compared to that of F gel. PVPS induces a magnificent improvement of mechanical properties: a 500 times increase of storage modulus and ∼62 times increase of yield stress in the F-PVPS5 gel compared to the F gel. The complex modulus also increases with increasing PVPS concentration with a maximum in F-PVPS5 gel. Creep recovery experiments suggest PVPS induced elasticity in the otherwise viscous F gel. The fluorescence intensity of F-PVPSx gels at first increases with increasing PVPS concentration showing maxima at F-PVPS5 gel and then slowly decreases. Gelation is monitored by time-dependent fluorescence spectroscopy, and it is observed that F and F-PVPSx gels exhibit perfectly opposite trend; the former shows a sigmoidal decrease in fluorescence intensity during gelation, but the latter shows a sigmoidal increase. The gelation rate constants calculated from Avrami treatment on the time-dependent fluorescence data manifest that PVPS effectively enhances the gelation rate showing a maximum for F-PVPS5 gel. The hybrid gel exhibit 5 orders increase of dc conductivity than that of F-gel showing semiconducting nature in the current-voltage plot. The Nyquist plot in impedance spectra of F-PVPS5 xerogel exhibit a depressed semicircle with a spike at lower frequency region, and the equivalent circuit represents a complex combination of resistance-capacitance circuits attributed to the hybrid morphology of the gel fibers.

Injectable Hydrogel of Vitamin B9 for Controlled Release of Both Hydrophilic and Hydrophobic Anticancer Drugs

Folic acid (FA), vitamin B9, is a good receptor of drugs triggering cellular uptake via endocytosis. FA is sparingly soluble in water. Herein, a new approach for the formation of FA hydrogel by the hydrolysis of glucono‐δ‐lactone in PBS buffer under physiological conditions has been reported. The gel has a fibrillar network morphology attributable to intermolecular H‐bonding and π‐stacking interactions. The thixotropic property of the gel is used for the encapsulation of both hydrophilic [doxorubicin (DOX)] and hydrophobic [camptothecin (CPT)] drugs. The loading of DOX and CPT into the gel is attributed to the H‐bonding interaction between FA and drugs. The release of DOX is sustainable at pH 4 and 7, and the Peppas model indicates that at pH 7 the diffusion of the drug is Fickian but it is non‐Fickian at pH 4. The release of CPT is monitored by fluorescence spectroscopy, which also corroborates the combined release of both drugs. The metylthiazolyldiphenyltetrazolium bromide assay of FA hydrogel demonstrates nontoxic behavior and that the cytotoxicity of the DOX‐loaded FA hydrogel is higher than that of pure DOX, with a minimal effect on normal cells.

Influence of Chain Length on the Self-Assembly of Poly(ε-caprolactone)-Grafted Graphene Quantum Dots

The multifarious applications of graphene quantum dots (GQDs) necessitate surface modifications to enhance their solution processability. Herein, we report the synthesis and self-assembly of GQDs grafted with poly(ε-caprolactone) (PCL) of different degrees of polymerization (3, 7, 15, and 21) produced from ring-opening polymerization. Optical and morphological studies unveil the transformation of the assemblies from J-aggregates to H-aggregates, accompanied by an alteration in morphology from toroid to spheroid to rodlike structures with increasing chain length of PCL. Functionalized GQDs with lower chain lengths of PCL at higher concentration also assemble into liquid-crystalline phases as observed from birefringent textures, which are later correlated to the formation of columnar hexagonal (Colh) mesophases. However, no such behavior is observed at higher chain lengths of PCL under identical conditions. Therefore, it is evident that the variation in the PCL chain length plays a crucial role in the self-assembly, which is primarily triggered by the van der Waals force between the polymer chains dictating the π stacking of GQDs, resulting in different self- aggregated behavior.