SYNTHESIS OF MULTISTIMULUS-RESPONSIVE CYTIDINE ̵ 5' ̵ MONOPHOSPHATE MOLECULE ̵ MEDIATED SMART SUPRAMOLECULAR NANOHYBRID HYDROGELS ̵ THEIR MULTIFUNCTIONAL FEATURES AND APPLICATIONS

Abstract

Hydrogels represent a 3D crosslinked network of natural/synthetic polymers with hydrophilic structure providing them the porous morphology. It renders them to attain capabilities for adsorbing the toxic metal ions, and molecule/drug which have been extensively explored in the past in diverse fields such as environmental, food, cosmetics, drug delivery and biomedical. Polymer based hydrogels are well known since ancient times to produce the crosslinked 3D network involving polymeric units through covalent linkage(s), but generally exhibit poor reversibility and degradability. In recent years designing of small molecule based hydrogels has been drawing increasing attention in which 3D network is generally attained in the process of self-assembly involving non-covalent interactions. Such hydrogels, unlike to polymer based hydrogels containing covalent bond(s), are completely reversible when subjected to different physical or chemical stimuli. Synthesis of hybrid materials consisting of organic and inorganic phase(s) is considered to be innovative approach to design new material with unique features, in which the interface of the two phases is observed to play a significant role. These features could be utilized for designing of hybrid hydrogels, in which the functional groups of organic molecule as low molecular weight gelators (LMWG) upon interacting with inorganic components would furnish them not only the multiple crosslinking sites but would also provide them with the characteristic features of organic(s) and inorganic(s). Such materials with enhanced properties are expected to find tremendous commercial potential for biomedical systems. The interaction of organic gelator with inorganic nanostructured materials might further improve the structural network in the hydrogels by indroducing flexibility and new functionalites, which would contribute to their above characteristic features such as porosity, providing new binding sites on their surface and adsorption capabilities. In the literature on biomolecules-mediated hydrogels, a number of investigations have involved biopolymer(s) as hydrogelator(s) viz. polysaccharides (chitosan and methyl cellulose), and polypeptides. In recent reports several studies have fouced on purines based nucleotides (adenosine monophosphate (AMP), guanosine monophosphate (GMP) and inosine monophosphate (IMP)) molecules for gelation employing their fairly high concentrations. Somehow, we did not come across any reports on hydrogels synthesis using pyrimidine based nucleotides, which could serve as a missing building block in predicting the behavior of nucleic acid based hydrogels. An analysis of such behavior could be utilized for i their usage for environmental and biomedical applications such as removel of pollutants, drug delivery, tissue engineering, enzyme immobilization, and wound healing. In the above context the synthesis of hydrogels has been performed involving a natural biocompatible pyrimidine based nucleotide molecule, cytidine-5'-monophosphate (5'-CMP) as an organic gelator, which in its pure form neither produce any hydrogels, even under saturation conditions nor by changing any external stimuli like pH (4, 7 and 9) and temperature (5 to 80 °C). In the present work 5'-CMP molecule-mediated soft supramolecular hydrogels have been synthesized by forming its nanohybrids using iron oxide polymorph, akaganeite (β-FeOOH). The multifunctional features of as-synthesized hydrogels have further been improved by performing their modification using other inorganic components like silver nanoparticles (Ag NPs) and Zn2+ ions/ Ag NPs. The entire work presented in this thesis has been divided into the six chapters. A brief account of the contents of these chapters has been furnished below: The first chapter provides a brief introduction of hydrogels, their classification and multidiscipilinary applications. Specifically, it presents an overview of supramolecular hydrogel systems comprising polymeric and LMWG consisting of organics/biocompatible molecules. It included synthesis of hydrogels, their properties and applications. This chapter also outlines the aim(s) and objective(s) of the present work. The second chapter describes the experimental details covering the used materials and reagents, equipment and techniques. It also provides a brief description of the instruments used for the characterization of the as-synthesized hydrogels comprising nanostructures in terms of their phase/structure, size and shape. The characteristics of hydrogels have been monitored by recording their morphology, surface area, swelling behavior and viscoelasatic properties. Interactions between/among organic and inorganic phases have been analysed by using different spectroscopic techniques. The formulae used for the evaluation of different parameters have been mentioned. A breif account of methodology used for the synthesis of colloidal samples has also been discussed. The third chapter has been divided into two sections: Section A and Section B. The Section A reports the novel synthesis of 5'-CMP molecule-mediated pH responsive porous supramolecular hydrogels consisting of β-FeOOH@5'-CMP core-shell type nanohybrids as a building block (SPC3). As-synthesized hydrogels (SPC3H) has been fully characterized in terms of its stability, size of the core, shape/morphology along with its ii optical, magnetic and rheological features. Interestingly, the gelation in the self-assembly is induced by the puckering of ribose sugar in 5'-CMP molecule enhancing the intra-/inter- molecular non-covalent interactions between 5'-CMP and β-FeOOH within the building blocks as well as among different building blocks in relatively a narrow range of pH (5.5-7; 7- 8.5) and temperature (35-40 °C). These interactions have been analyzed by IR, Raman and CD spectroscopy. The viscoelastic nature of the optimized sample is revealed by fairly high values of viscosity (5.5 × 103 cP); storage modulus (G') (4 × 104 Pa), loss modulus (G'') (2.3 × 102 Pa) and yield strain (8.1%) with a linear viscoelastic region upto about 4%. A decrease in viscosity with increasing shear rate demonstrates its hydrogel-like behavior matching to the soft supramolecular hydrogels. The mechanism of gel formation has been thoroughly analyzed. The pH, temperature as well as sonication induced spontaneous transition(s) of sol into hydrogels and its complete reversal into sol suggesting it to be a smart material. In Section B, as-synthesized SPC3H in Section 3A under physiological conditions of pH (7) and temperature (37 °C) exhibited self-healing and injectable features. Thixotropic measurements showed that the hydrogel takes about 740 s for self-healing. The efficient loading of methylene blue (MB) (217 mg/g) on SPC3H and its release (> 98 %) in aqueous medium of different pHs is observed to occur over a period of more than 3 to 32 days, suggesting the as-synthesized hydrogels to be sustainable with a significant biological and environmental potential. The fourth chapter has been divided into two sections: Section A and Section B. Section A reports the synthesis of multistimulus-responsive supramolecular hydrogels derived by in situ coating of silver nanoparticles (Ag NPs) on colloidal cytidine-5' monophosphate-capped β-FeOOH nanohybrids (β-FeOOH@5'-CMP) under physiological conditions forming polycrystalline building block (Ag NPs-coated β-FeOOH@5'-CMP). The presence of Ag in the binary nanohybrids induces the puckering of ribose sugar bringing a change in its conformation from C2'-endo to C3'-endo, which enhanced the supramolecular interactions among different moieties of other building blocks to construct porous network of hydrogels in the self-assembly via the formation of micellar structure. Such supramolecular network in hydrogel is also evidenced by the reversible sol ⇌ gel transformation under multistimulus-responsiveness in a narrow range of pH, temperature and sonication. Thus, the as-synthesized hydrogels represent a unique superparamagnetic nanosystem consisting of all greener components (5'-CMP/β-FeOOH/Ag) with superior viscoelastic properties, % iii swelling (580) and displaying multistimulus-responsiveness (pH/temperature/sonication), suggesting it to be a smart supramolecular hydrogel system. Section B reports the shear thinning behavior of as-synthesized SCA3H under higher strain and converting back into the gel under lower strain, suggests its potential for self healing and injectability. The presence of Ag NPs in the hydrogel enhanced its loading capabilities for MB (590 mg g-1) and its controlled release over ̴ 2 to 30 days at varied pHs. It displayed efficient surface-enhanced Raman scattering (SERS) allowing to detect MB like drug molecule at ≤ 10-12 M and demonstrated antibacterial activity for both Gram positive and Gram negative bacterial strains. These features suggest its vast potential for biomedical and environmental applications. The fifth chapter has also been divided into two sections: Section A and Section B. Section A reports the synthesis of ternary nanohybrids (Zn2+-Ag NPs@β FeOOH@5'-CMP) (CISZ2) consisting of greener components as a building block with hydrophobic tail (containing Zn2+ ions, Ag NPs and β-FeOOH) and hydrophilic head (5'-CMP). The presence of Zn2+ ions and Ag NPs in the nanohybrids introduces new coordination sites and induce the puckering of ribose sugar in 5'-CMP to generate the solid-like network in the self-assembly via micellar formation involving building blocks to produce hydrogel (CISZ2H). Extensive cross-linking among organic and inorganic moieties render the production of these hydrogels rapidly in less than a period of 2 min with unique physicochemical features of improved mechanical strength ( ̴ 71000 Pa) with a large water retention capability (600%). It exhibited multistimulus- responsiveness in relatively a narrower range of pH, temeperature and sonication. Section B reports the shear thinning behavior of CISZ2H under higher strain and its transformation back into the gel under lower strain by performing thixotropic measurements, suggesting its potential for self-healing and injectability. The sol ⇌ gel transformation is observed to occur repetitively in a period of about 450 s. The co-doped Zn2+ ions and Ag NPs in CISZ2H imparts it with enhanced MB loading capabilities (1067 ± 20 mg/g), sustained drug release at different pHs over a period of about 2 to14 days, magnificent SERS activity and antibacterial features. It, thereby, makes the as-synthesized hydrogels consisting of all greener/ less toxic components to be safer for drug delivery, wound healing, sensing and tissue engineering. These features qualify it to be a smart soft material for advanced applications with enormous future potential. iv The sixth chapter presents the summary of results obtained in the third, fourth and fifth chapters. It also provided a comparative summary and conclusions drawn form these chapters. Among different hydrogel systems, CISZ2H demonstrated the best characteristic features of soft supramolecular hydrogels. The future plans for extension and practical implementation of this work have also been suggested.