Chapter 1. Supramolecular Gels and their Applications Supramolecular gels are viscoelastic materials composed of a solid like three dimensional fibrillary network that is embedded in a liquid. Supramolecular gels are derived from low molecular weight compounds (typically MW < 3000). In the 1990s, the investigations on gels were mainly focused on designing new gelator molecules. However, during the last decade, research focus shifted towards designing functional gels and their applications. As a result of extensive work in this area, gels have been found to have varied applications in the templated synthesis of inorganic nanomaterials, hybrid materials, light harvesting systems, as responsive system and sensors, and also in drug delivery, tissue engineering etc. This chapter gives an introduction to supramolecular hydrogels/organogels and relevant bile acid chemistry touching upon the gelation properties of the bile acid derivatives. Diverse applications of the supramolecular gels are also illustrated with several examples. Scheme 1. Various applications of functional supramolecular gels Chapter 2. Bile Acid derived novel Hydrogelators Part 1. Hydrogelation of Bile acid protected Amino acids and Hybrid Materials Hydrogels from low molecular weight molecules have significant importance in biomedical applications. In this chapter, we report injectable hydrogel formation from bile acid conjugates of various amino acids. Hydrogel formation was found to be dependent on multiple factors such as bile acid backbone structure, linkage between the bile acid and the amino acid, pH etc. Single crystal structures of lithocholyl phenylalanine, lithocholyl-glycine, lithocholyl-L valine and lithocholyl-L alanine were also determined. Finally, the hydrogel frameworks were utilized to produce hybrid materials with Gold and ZnO nanoparticles. Scheme 2. (a) Crystal structure of LC-LF-OH gelator molecule, (b) photograph of gel, (c) SEM and (d) AFM image of LC-LF-OH xerogel Part 2. Hydrogelation of bile acid-dipeptide conjugates and in situ synthesis of silver and gold nanoparticles in the hydrogel matrix Fabricating supramolecular hydrogels with embedded metal nanostructures are important for the design of novel hybrid nanocomposite materials for diverse applications such as bio sensing and chemo sensing platforms, catalytic and antibacterial functional materials etc. Supramolecular self-assembly of bile acid-dipeptide conjugates have led to the formation of new supramolecular hydrogels. Gelation of these molecules depends strongly on the hydrophobic character of the bile acids. Ag+ and Au3+ salts were incorporated in the hydrogels, and photo reduction and chemical reduction led to the in situ generation of Ag and Au NPs in these supramolecular hydrogels without the addition of any external stabilizing agent. The color, size and shape of silver nanoparticles formed by photo reduction depended on the amino acid residue on the side chain. Furthermore, the hydrogel-Ag nanocomposite was tested for its antimicrobial activity. Scheme 3. Bile acid based dipeptide hydrogelators and soft hybrid materials Chapter 3. Sonogels of bile salts of In(III): use in the formation of self-templated indium sulfide nanostructures In this chapter, facile hydrogel formation by Indium(III) cholate and deoxy cholate are reported. When In(III) solution was added to aqueous solutions of sodium cholate and sodium deoxy cholate and sonicated, the mixtures formed gels. The gels thus obtained were translucent/turbid and thermos irreversible. Rheological measurements showed that all of them could be classified as viscoelastic soft solids. Scanning electron microscopy and atomic force microscopy showed typical entangled three dimensional fibrous networks. The In-Ch hydrogel were further used to prepare nanostructured In2S3 in which the cholate units possibly acted as a surfactant to confine the growth of the Nano flakes. Scheme 4. In-Ch hydrogel (Photograph and SEM image of In-Ch gel) Chapter 4. Palladium-Hydrogel Nanocomposite for C-C Coupling Reactions Supported metallic nanoparticles are important composite materials owing to their enormous potential for applications in various fields. This chapter describes the in situ formation of palladium nanoparticles in a calcium-cholate (Ca-Ch) hydrogel by reduction with sodium cyan borohydride. The hydrogel matrix appeared to assist the controlled growth as well as stabilization of palladium nanoparticles. The palladium nanoparticle/Ca-Ch hydrogel hybrid was characterized by scanning and transmission electron microscopy, atomic force microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Furthermore, PdNP/Ca-Ch hybrid xerogel was shown to act as an active catalyst for Suzuki reaction under aqueous aerobic conditions, up to 4 cycles. This PdNP/Ca-Ch xerogel retained its catalytic activities on storage for several months. Scheme 5. Palladium-hydrogel nanocomposite for C-C coupling reactions in water Chapter 5. Sensitization of Terbium/Europium in self-assembled cholate hydrogel: An approach towards the detection of amine vapours "Luminescent" lanthanides have intrinsic low molar absorptivity, although this problem can be addressed by complexing the lanthanide ion with suitable chelating ligands which improve the luminescence properties drastically. However the design of such systems often involves careful planning and laborious synthetic steps. It is therefore desirable to have a simpler way to sensitize lanthanides with high efficiency. It was observed in our group that trivalent lanthanides formed hydrogels on the addition of sodium cholate. This chapter describes the discovery of the several biphenyl derivatives (such as 4-biphenylcarbaxaldehyde, 4-acetylbiphenyl) for sensitization of Tb(III) and Eu(III) in lanthanide hydrogels. Sensitization of Tb(III) and Eu(III) were observed by doping was characterized by scanning and transmission electron microscopy, atomic force microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Furthermore, PdNP/Ca-Ch hybrid xerogel was shown to act as an active catalyst for Suzuki reaction under aqueous aerobic conditions, up to 4 cycles. This PdNP/Ca-Ch xerogel retained its catalytic activities on storage for several months. Scheme 6. Schematic representation of the sensitization process (the arrangement of themolecules in the gel fiber is arbitrary)(For figures pl refer the abstract pdf file)

Chapter 1. General Introduction This chapter gives an introduction to supramolecular organo/hydrogels and the related bile acid chemistry touching upon the gelation properties of the bile acid derivatives. Diverse applications of the supramolecular gels are illustrated with several examples. In the concluding section of this chapter, a brief introduction on the semiconductor nanocrystals is provided. Finally, the content of the thesis is outlined. Chapter 2. Bile Acid Derived Novel Organo/hydrogelators Part 1. Bile Acid Derived Organo/hydrogelators With a Basic Side Chain Cationic analogues of bile acids which showed remarkable gelation properties in water were reported from our laboratory. This led us to investigate the aggregation behaviour of some of the lithocholic and deoxycholic acid derivatives having a basic side-chain. Figure 1. Bile acid based organo/hydrogelators containing a basic side-chain. In this part, an organogelator 1 and a hydrogelator 2 derived from parent bile acids have been described with respect to their gelation properties, morphology, thermal and mechanical stability of the gels. The organo/hydrogels were shown to be responsive to acid-base stimuli as the organogel formed only in the protonated state and the hydrogel formed in the neutral form of the tertiary amines. The xerogel fibres obtained from the organogel were found to be solid-like and stable up to 200 oC as confirmed by variable temperature polarizing optical microscopy. The non-fluorescent organogel was doped with a fluorescent dye (coumarin 153) to design a novel dye-organogel composite material which was investigated with laser scanning confocal fluorescence microscopy showing the dye molecules were uniformly deposited on the organogel fibres. Part 2. Serendipitous Organogelation by Dimeric Bile Acid Esters This section highlights our work on the organogelators based on a number of dimeric esters consisting of different bile acid units. Figure 2. The three different dimeric bile acid esters as organogelators. In this part, three bile acid derived dimeric esters (1, 2 and 3) were shown to possess organogelation properties in aromatic and halogenated aromatic solvents. We studied the morphological features and rheological properties of these organogels. Next, the organogel matrix was exploited to generate and stabilize gold nanoparticles and prepare AuNP/gel hybrid material. Chapter 3. Cholate Hydrogels and Soft Gel-nanoparticle Hybrid Materials Sodium cholate does not form gel in water under any condition as compared to other sodium salts of other bile acids such as sodium deoxycholate and lithocholate which show pH-dependent gelation behaviour. Figure 3. Metal cholate hydrogels derived from sodium cholate and a variety of metal ions. In this chapter, super hydrogelation of sodium cholate induced by a variety of metal ions (Ca2+, Cu2+, Co2+, Zn2+, Cd2+, Hg2+ and Ag+) is highlighted with respect to their morphology and mechanical strength/stability. The calcium cholate supramolecular system showed the presence of helically twisted nanofibres which were utilised in the synthesis of soft hybrid materials containing metal (Au and Ag) and metal sulphide (CdS, ZnS, HgS, etc.) nanoparticles. Chapter 4. Cadmium Deoxycholate and Highly Luminescent CdSe Nanocrystals Bile acid derivatives have very high chemical and thermal stability owing to the presence of a rigid steroidal nucleus. We explored the possibility of utilizing the bile salt derived from Cd as a metal complexes as precursor to high quality nanocrystals (NCs) which can only be accessed at high temperatures (>200 oC). Figure 4. Synthesis of high quality CdSe NCs from cadmium deoxycholate. In this chapter, the synthesis of high quality CdSe nanocrystals is discussed using a novel bile acid based precursor: cadmium salt of 7-deoxycholic acid, which has high thermal stability and can be conveniently used at very high temperatures (>300 oC) required for the synthesis of high quality nanocrystals. Syntheses were done both by ‘injection’ and ‘non-injection’ modes. The as-prepared nanocrystals have high photoluminescence quantum yield, multiple excitons, narrow size-distributions and zinc blende/wurtzite crystalline cores. Appendix. Steroidal Thiols in Design of Novel Quantum dot (QD)/Gel Hybrid Materials Bile acid derived steroidal thiols were reported to be efficient capping agents for silver and gold nanoparticles from our laboratory. So, we wanted to check whether they could stabilize the semiconductor nanocrystals as well. Figure 5. Steroidal thiols as stabilizers of semiconductor quantum dots. In this short report, we describe the efficient capping by bile acid derived thiols of group II-VI semiconductor nanocrystals/quantum dots (QDs) (CdS, CdSe). After synthesizing the thiol capped QDs, we tried to disperse the capped nanoparticles into the gel fibres. The hybrid gels showed the presence of nanoparticles inside the fibres as observed by transmission electron microscopy, although the photoluminescence of the QDs was very low in the gel matrix, which might be due to the inefficient surface passivation of the nanoparticles in the gel.