Hydrogel Nanocomposite Towards Optical Sensing of Spermine in Biomedical and Real-Life Food Samples and Remediation of Toxic Dyes from Wastewater.

Nanocomposites such as graphene oxide (GO) have been incorporated into hydrogels to enhance conventional hydrogels' properties and develop new functions. Unique and strong molecular interactions between GO and low molecular weight gelators allow the fabrication of various functional hydrogels suitable for different applications. In the present study, we report a stable and soft nanocomposite hydrogel comprising a pyrene-based chiral amphipath having an amino acid (l-phenylalanine) core with pendant oligo-oxyethylene hydrophilic chains and GO. The mechanical and viscoelastic properties of the nanocomposite hydrogel were thoroughly studied using various spectroscopic, microscopic, and mechanical techniques. Even without GO, native hydrogels could form a self-supported thermoreversible and thixotropic hydrogel composed of the fibrillar network. Unlike native hydrogels, the morphological investigation of nanocomposite gels shows the presence of cross-linked nanosheet-like structures. The combined effect of π-π stacking and H-bonding interactions is the driving force for the formation of such composite hydrogels. Moreover, the nanocomposite hydrogels possess significantly superior mechanical stiffness than the native hydrogels. Interestingly, the thixotropic properties observed with the parent gel were retained even in the presence of carbon nanomaterials (GO). The nanocomposite hydrogel could be employed in the optical sensing of a biogenic polyamine, spermine, resulting in a visible gel-to-sol transition. The superior electrostatic interaction between the GOs and spermine molecules might have led to the release of entrapped fluorogenic dyes from the hydrogel network and a turn-on emission response. The sensory system was employed to analyze spermine content in human urine samples and decomposed food items. A gel-coated paper strip was also developed for onsite detection of the spermine. The nanocomposite hydrogel was further utilized to remove toxic organic dyes such as methylene blue (MB) and rhodamine B (RhB) from the aqueous media. The nanocomposite hydrogel thus showed excellent dye removal capabilities and was also found to be recyclable. Calculations of different mechanical parameters suggest that the dye removal efficiency of the nanocomposite hydrogel was better for MB than for RhB.

A biocompatible hydrogel as a template for oxidative decomposition reactions: a chemodosimetric analysis and in vitro imaging of hypochlorite.

The self-assembly properties of new biocompatible, thermoreversible fluorescent hydrogels, composed of amino acid residues, e.g., l-phenylalanine (PyL-PheOx) and l-tyrosine (PyL-TyrOx), have been reported. Spectroscopic investigations indicate that PyL-PheOx forms π-stacked 'compact' aggregates, while 'loose' aggregates with stronger CT characteristics are observed for PyL-TyrOx. Both the compounds showed the presence of fibrous networks in the self-assembled state. Circular dichroism spectral studies indicate the formation of M-helical and P-helical structures for PyL-PheOx and PyL-TyrOx, respectively. A striking gel-to-sol transition, caused by oxidative decomposition, is explicitly noticed in the presence of hypochlorite. A mechanistic investigation reveals the oxidation of the acyl aroyl hydrazine core of the gelators in the presence of ClO-. In addition to this, change in the fluorescence emission intensity of the hydrogel in the presence of ClO- is utilized for the analysis of commercial bleach samples. Gel-coated paper strips are also developed for the on-site detection of ClO-. Furthermore, the system is utilized for imaging hypochlorite in live mammalian cells.

Molecular design of amphiphiles for Microenvironment-Sensitive kinetically controlled gelation and their utility in probing alcohol contents.

Pyrene-laced 'fluorescent' amphiphiles have been designed with a C4-alkyl chain as 'hydrophobic spacer' and sugar moiety as hydrophilic residue. The amphiphile containing acetylacetohydrazide as a bridging unit (1) showed gel formation both in water and ethanol medium, while no gelation was observed with compound (2) with acetohydrazide as bridging unit. Accordingly, the self-assembly behavior of 1 was thoroughly investigated both in water and EtOH medium (as well as in their mixtures) in addition to their physicochemical properties. Though the gel formation kinetics was slower in water medium, the hydrogel showed superior mechanical strength (and thixotropic behavior) and improved thermal stability compared to the ethanolic gel. In water, the percolating H-bonding network with high cohesive force restricts the dynamics of the probe molecules, leading to some extent limited intermolecular interactions. However, hydrophobic interactions in water medium can result in the formation of rather 'compact' nano-assembly (H-type aggregation) with prominent π-π stacking interactions. Such unique solvent-dependency of 1 was further utilized to screen commercially available beverages in terms of their alcohol contents and probe adulteration in beverages. Besides, the hydrogel also showed excellent stimuli responsiveness, a gel-to-sol transition, accompanied by fluorescence color change, was observed in the presence of insulin.

A two-component charge transfer hydrogel with excellent sensitivity towards the microenvironment: a responsive platform for biogenic thiols.

A two-component charge transfer (CT) hydrogel has been derived from a supramolecular heteroassembly of a pyrene amino acid conjugate (PyHisOH, donor) with a 4-chloro-7-nitrobenzofurazan (NBD-Ox, acceptor) derivative in aqueous medium. The mechanical stiffness, as well as the thermal stability of the CT hydrogels largely depend on the relative ratios of donor and acceptor units as well as on their overall concentration. Moreover, the gel-to-sol transition is found to be susceptible to various external stimuli such as heat, pH, metal ions, etc. Circular dichroism and morphological investigation reveal the formation of left-handed helical fibers in the CT gel network. XRD studies show the lamellar packing of the interactive units in the 3D network of the CT hydrogel. The determination of different rheological parameters confirms the viscoelastic as well as the thixotropic nature of the CT gel. Furthermore, the CT gel is employed for turn-on sensing of biogenic thiols, cyan fluorescence was observed with cysteine/homocysteine, while blue fluorescence with glutathione. Nucleophilic attack at the NBD moiety leads to the formation of thermodynamically stable amino-linked derivatives for cysteine or homocysteine and kinetically controlled thiol-linked adduct for glutathione. Thus, the current system presents a unique opportunity, where a CT hydrogel sample is involved for discriminating biogenic thiols via specific chemodosimetric interactions.

A thermo-responsive supramolecular hydrogel that senses cholera toxin via color-changing response.

A Pyrene-based amphiphile with C4-alkanoyl spacer and lactose (PyLac) self-assembles in the aqueous media to form an injectable hydrogel. It shows preferential binding with Cholera Toxin (CT) via its terminal galactose residue, and hence can be employed for the selective detection of CT via color-changing response.

Modulation of Excited-State Proton-Transfer Dynamics inside the Nanocavity of Microheterogeneous Systems: Microenvironment-Sensitive Förster Energy Transfer to Riboflavin.

The excited-state proton-transfer efficiency of a tetraarylpyrene derivative, 1,3,6,8-tetrakis(4-hydroxy-2,6-dimethylphenyl)pyrene (TDMPP), was investigated thoroughly in the presence of various surfactant assemblies, such as micelles and vesicles. The confined microheterogeneous environments can significantly retard the extent of the excited-state proton-transfer process, resulting in a distinguishable optical signal compared to that in the bulk medium. Physical characteristics of the surfactant assemblies, such as order, interfacial hydration, and surface charge, influence the proton transfer process and allow multiparametric sensing. A higher degree of interfacial hydration facilitates the proton-transfer process, while the positively charged head groups of the surfactants specifically stabilize the anionic form of the probe (TDMPP-O*). Furthermore, Forster energy transfer from the probe to riboflavin was studied in a phospholipid membrane, wherein the relative ratio of the neutral versus anionic forms (TDMPP-OH/TDMPP-O*) was found to influence the extent of energy transfer. Overall, we demonstrate how an ultrafast photophysical process, that is, the excited-state proton transfer, can be influenced by the microenvironment.