ROLE OF VARIOUS AIR-STABLE OXIDATION STATES OF GOLD IN NANOPARTICLE GROWTH, SUPRA-ARCHITECTURE, AND RUTHENIUM PHOTOCHEMISTRY

Abstract

The thesis entitled “Role of various air-stable oxidation states of gold in nanoparticle growth, supra-architecture, and ruthenium photochemistry” has been divided into four chapters. Chapter 1 presents a basic introduction to the nucleation and growth of gold nanoparticles (NPs), followed by their properties, surface functionalization, assembly, and applications. Among all the noble metal NPs, special emphasis has been given to gold nanoparticles (AuNPs) due to their distinct physical and chemical properties like fascinating color, tunable size, surface plasmon resonance (SPR), high surface-to-volume ratio, and less cytotoxicity. Therefore, AuNPs have been enormously utilized in sensing, catalysis, optoelectronics, and therapeutics. Recently, special attention has been devoted to the development of anisotropic AuNPs with varying surface functionalities and novel properties by manipulating the nucleation and growth of these NPs. The nucleation, growth, role of different small molecules and biomolecules induced seed-mediated growth, surface functionalization, self-assembly, and biomedical application of AuNPs have been briefly discussed (Scheme A). Additionally, the catalytic aspects of Au+ and Au3+ were correlated to the photo-substitution (PS) in ruthenium(II) polypyridyl complexes (Ru(II) PPCs). The last section of this chapter discussed the existing research gap in the above field followed by the scope of the thesis.Scheme A. Targeted objectives using different oxidation states of gold.Chapter 2 describes the role of reducing agents and biomolecules on the growth reaction of gold nanorods This chapter is divided into two sections. Section A describes the seed-mediated growth reaction of gold nanorods: Role of reducing agents and amino acids. The cetyl-trimethylammonium bromide (CTAB) stabilized Gold nanorods (GNR) were employed as seeds in the hydroxylamine (HA)-mediated growth reactions. The variation of reducing agents suggested HA being the weakest reducing agent compared to ascorbic acid (AA) and hydroquinone (HQ). Unlike the seed-independent growth of GNRs in the case of AA and HQ, the weak reducing power of HA resulted in the formation of unique gold snowflake-like structures. The seed-independent formation of NPs was mostly observed during the growth reaction of GNR with basic amino acids (Arginine- Arg and Lysine- Lys), considering their greater binding affinity to the gold surface, which was reflected in the zeta potential (ζ) analysis. A similar reaction with basic amino acids and HA as reducing agents suggested selective seed-dependent growth. The presence of neutral amino acids could only induce faster gold reduction without providing any stabilization to the newly formed NPs against aggregation in the case of AA and HQ. But a similar reaction with HA clearly demonstrated the formation of a bigger snowflake-like gold supra-architecture. This gold snowflake structure was characterized by Ultraviolet-visible (UV-vis) spectroscopy, powdered X-ray diffraction (PXRD), transmission electron microscope (TEM), high-resolution TEM (HRTEM), and X-ray photoelectron spectroscopy (XPS). The above analysis suggested that the assembly and merging of GNR led to the formation of snowflake-like architectures. Section B describes the synthesis and application of supramolecular architectures from GNR via growth reaction. Lowering the basic amino acid concentration during the growth reaction of GNRs resulted in a similar snowflake-like gold nanoarchitecture. However, prior incubation of 5′-amine modified nucleic acid (PMR, NH2−C6H12-5′-ACATCAGT-3′) to GNRs produced sea urchin-like supra-structures selectively in the case of Arg after the growth reaction. Strong hydrogen bonding and cation-pi interaction between PMR and Arg cause the above structural change. The above nucleic acid-induced growth reaction strategy was extended to discriminate two structurally close α-helical RRR (Ac-(AAAAR)3A−NH2) peptides and the lysine mutated KKR (Ac−AAAAKAAAAKAAAARA−NH2) peptide with a partial helix at the amino terminus. The non-covalent interactions between PMR and the arginine residues of α-helical RRR peptide led to the formation of unique sea urchin-like supra-structures, which distinguish themself from the snowflake structures formed in the case of 310-helial KKRpeptide. Simulation studies also confirmed a greater number of hydrogen bonding and cation-π interactions between the Arg residues and PMR that support the possible structural change in the case of RRR peptide. Chapter 3 describes the role of methionine in the growth reaction of gold nanostructures This chapter is divided into two sections. Section A describes methionine (Met) mediated inhibition of secondary nucleation during nonclassical growth of gold nanoparticles via assembly and merging. Conventional steps for seed-mediated growth of noble metal nanostructures involve classical and nonclassical nucleation. The seed surface catalyzes Au+ to Au0 reduction during secondary nucleation thus providing in-plane growth on the seed. Here, a unique Metcontrolled seed-mediated growth reaction of AuNPs was demonstrated, which proceeded via impeding secondary nucleation. The interaction between the freshly generated Au+ and the thioether group of Met restricted the secondary nucleation process of further seed-catalyzed Au+ reduction. This incomplete conversion of Au+ resulted in a significant enhancement of the surface Zeta potential (ζ) even at low Met concentrations. Growth of in situ-generated smallnucleated gold nanoparticles (nAuNPs) took place on the parent seed surface, followed by their segregation from the seed. The self-assembly of these nAuNPs could be attributed to the aurophilic interaction of Au+. The time-dependent growth of smaller constituent particles through oriented attachment and merging within the same self-assembly validated the nonclassical growth. This growth strategy was successfully extended towards three bio-inspired decameric peptides with varying Met residues, which confirmed the similar nucleation, growth strategy, and assembly process even in the presence of a single Met residue in the peptide. Section B describes the Met-mediated growth reaction of positively charged GNR towards the formation of a unique marigold-like gold supra structure (MGS). Varying three important parameters, such as growth time and concentration of Met and Au3+, revealed the combination of freshly generated small nucleated particles (fNPs) and GNR towards fabricating the unique MGS containing disk and ray floret parts. Strong interaction between the Met and (111) plane of Au0 controlled the orientation of the (111) plane parallel to the direction of growth. This preferential interaction directed the assembly of gold nanostructures (AuNSs) and the merging of fNPs with concave GNR (cGNR) through the Au (200) plane to fabricate the external arrangement of the ray floret structure. The structural selectivity was attributed to the electron donating capacity of the thioether functional group of Met(S) to Au+, generated prior to secondary nucleation. As confirmed by XPS and ζ-potential analysis, the above interaction controls the Met concentration-dependent inhibition of further Au+→Au0 reduction. The growth strategy of GNR was further validated with a Met-enriched peptide to produce disk and ray florets. Chapter 4 describes 1H-pyrazole-3,5-dicarboxylic acid and lead(II) or zinc(II) as the external stimuli for the development of AuNPs-based supra-structures and the role of Au+/Au3+ redox potential towards the restriction of PS in selective ruthenium(II) polypyridyl complexes. This chapter has been divided into two sections. Section A describes 1H-pyrazole-3,5-dicarboxylic acid (PDC) and Pb(II)/Zn(II) combination for the self-assembly of gold nanoparticles. The affinity of sulfur and nitrogen functional groups towards the surface of AuNPs is known. The pyrazole nitrogen weakly interacted with the AuNPs surface, and bivalent metal ions like Zn(II) or Pb(II) simultaneously interacted with the carboxylate group, thereby triggering the assembly of the AuNPs, which resulted in Au/Zn@PDC and Au/Pb@PDC hybrid supra-plates and supra-spheres, respectively at room temperature. The time-dependent absorption and Field emission scanning electron microscopy (FESEM) studies demonstrated the assembly process. The variation of anions suggested that the perchlorate anion was the best choice for the well-distinguished morphology of the hybrid supra-plates and supra-spheres. This result experimentally demonstrated the role of small molecules and metal ions towards the stimuli-induced self-assembly of AuNPs. The hybrid supra-plates and supra-spheres were further characterized by XRD, XPS, and energy dispersive X-ray analysis (EDAX). Section B describes the role of Au(I) and chloride ions in the photoirradiation of Ru(II) polypyridyl complexes (PPCs). Chloride (Clˉ) ion-induced PS reaction in [Ru(PPC)3]2+ is wellknown. This PS reaction was revisited in the presence of Au(I) complex and Clˉ ions in acetonitrile (MeCN) and methanol (MeOH) for four [Ru(PPC)3](PF6)2 complexes 1–4, where PPCs are 2,2’-bipyridine (Bpy), 1,10-phenanthroline (Phen), 2,2’-bipyrazine (Bpz) and 1,4,5,8- tetraazaphenanthrene (Tap), respectively. The complexes were classified as –N (1 and 2) and +N (3 and 4) group ligands based on the availability of unchelated aromatic nitrogen atoms. Unlike the traditional way, the Clˉ ions were introduced by treating HAuCl4 and glutathione (GSH), which produced excess Clˉ ions by reducing Au3+ to Au+. Complexes 1–4 were introduced to the above reaction mixture separately and photoirradiated for 48 h under a 455 nm light source. In both the solvents, 3 and 4 with +N group ligands only showed PS reactions. but –N group complexes did not show any PS reaction against their previously reported PS in the presence of Clˉ ions. Mechanistic investigation with complex 1 demonstrated an equilibrium between Au3+ and Au+ and that of glutathione oxidized and reduced form, which restricts the PS reaction in –N group complexes. Similar reactions in the absence of GSH generated single crystals of [Ru(Bpy)3](AuCl4)2 (1’) or [Ru(Bpz)3]2(AuCl6)(AuCl4)3 (3’). The selective PS reaction in 3’ may be attributed to the exclusive presence of [AuCl6]ˉ in the octahedral voids, which may act as a chloride source in the solution, whereas [AuCl4]ˉ is common in both 1’ and 3’. The thesis ends with an overall conclusion and future scope for further research in this area.