Ridhima Chadha

DFT and surface-enhanced Raman scattering study of tryptophan–silver complex

Surface-enhanced Raman scattering (SERS) study of tryptophan was carried out in silver hydrosol. The surface adsorption properties of tryptophan were investigated due to its biological importance. Tryptophan is an essential amino acid needed for the normal growth in infants and for nitrogen balance in adults. DFT calculations using B3LYP functional with LANL2DZ basis set was carried out to support the experimental Raman and SERS data. The strong enhancement of 1343 cm(-1) band, assigned to the CO(2) sym. stretching vibration in the SERS spectrum along with a red shift of 63 cm(-1), manifests that chemical mechanism contributes to the SERS activity. Moreover, the observed features in the SERS spectrum as well as theoretical calculations infer that tryptophan is chemisorbed to the silver surface directly through the oxygen and nitrogen atoms of the carboxylate and amino groups with an edge-on orientation with the indole ring lying nearly perpendicular to the silver surface. The SERS enhancement factors for various Raman vibrations of tryptophan were found to be of the order of 10(5)-10(6).

Role of Surfactant in the Formation of Gold Nanoparticles in Aqueous Medium

The stability of gold nanoparticles is a major issue which decides their impending usage in nanobiotechnological applications. Often biomimetically synthesized nanoparticles are deemed useless owing to their instability in aqueous medium. So, surfactants are used to stabilize the nanoparticles. But does the surfactant only stabilize by being adsorbed to the surface of the nanoparticles and not play significantly in moulding the size and shape of the nanoparticles? Keeping this idea in mind, gold nanoparticles (GNPs) synthesized by l-tryptophan (Trp) mediated reduction of chloroauric acid (HAuCl4) were stabilized by anionic surfactant, sodium dodecyl sulphate (SDS), and its effect on the moulding of size and properties of the GNPs was studied. Interestingly, unlike most of the gold nanoparticles synthesis mechanism showing saturation growth mechanism, inclusion of SDS in the reaction mixture for GNPs synthesis resulted in a bimodal mechanism which was studied by UV-Vis spectroscopy. The mechanism was further substantiated with transmission electron microscopy. Zeta potential of GNPs solutions was measured to corroborate stability observations recorded visually.

Reduction and aggregation of silver ions in aqueous citrate solutions.

Radiolytic reduction of Ag(+) ions and the subsequent formation of Ag clusters were studied in aqueous citrate solutions. Pulse-radiolysis studies show that the presence of citrate in the solution affects the early processes, via complexation of Ag(+) ions with the carboxyl moieties of the citrate. The ratio of citrate to Ag(+) determines the kinetic consequences of the reduction and agglomeration processes. The complexation reduces somewhat the rate of reduction by hydrated electrons. However, when all the ions are complexed to the citrate, the surface plasmon absorption band becomes broader, albeit small, but nevertheless it provides extreme stability to the formed nanoparticles.

Phase-transfer and film formation of silver nanoparticles.

In this article, a simple method for either transfer of silver nanoparticles from formamide to chloroform or to form a film at their interface is demonstrated. The transfer of the particles is a two-step size-dependent process. The size distribution of the colloidal hydrophobic silver particles in chloroform was almost the same as that before its transfer. Particles can be isolated by evaporation of chloroform. During evaporation, the hydrophobic particles become hydrophilic (charged) due to the formation of bilayer of CTAB over their surface. The isolated particles can be re-dispersed easily in polar solvents such as water and methanol. Nanocrystalline film of Ag is also prepared at the formamide-chloroform interface using suitable stabilizers in two immiscible layers. The nanocrystals have been characterized by various microscopic and spectroscopic techniques. The free standing film could be easily transferred on solid support.

Adsorption and sub-nanomolar sensing of thioflavin T on colloidal gold nanoparticles, silver nanoparticles and silver-coated films studied using surface-enhanced Raman scattering

Raman and surface-enhanced Raman scattering (SERS) studies of thioflavin T (ThT) in solid, solution, gold nanoparticles (GNPs), silver nanoparticles (SNPs) and silver-coated films (SCFs) were investigated. Concentration-dependent SERS spectrum of ThT in GNPs and SNPs indicated the existence of two possible structures, one with the torsional angle (φ) between benzothiazole and dimethylaminobenzene rings being 37° and the other with φ=90°. The SERS spectrum of ThT in SCFs were similar to the Raman spectrum of solid and solution that suggests φ=37°. In this paper, the high sensitivity of the SERS technique was employed for sub-nanomolar (picomolar) sensing of ThT.

Effect of SDS concentration on colloidal suspensions of Ag and Au nanoparticles

We present a kinetic study of the effects of sodium dodecyl sulfate (SDS) concentration on reduction and aggregation of Ag(+) and Au(3+) ions in aqueous solutions. There are distinct differences between the surface plasmon absorption bands of Ag nanoparticles at different concentrations of SDS. The results reveal the existence of two competing SDS-induced processes: stabilization of the Ag nanoparticles due to adsorption and aggregation of the Ag nanoparticles due to increase in ionic strength. However, SDS induced aggregation of Au nanoparticles is negligible because of less surface passivity as evident from eaq(-) reaction with AuCl4(-). Nevertheless, the average size of the Ag and Au nanoparticles remains almost similar at all SDS concentrations. UV-Vis spectrophotometry and transmission electron microscopy are used to characterize the nanoparticles. Moreover, it is shown that these SDS-capped Ag, Au and Au/Ag bimetallic nanoparticles could function as catalysts for the reduction of o-nitro aniline in the presence of NaBH4.

Synthesis of pH sensitive gold nanoparticles for potential application in radiosensitization.

Synthesis of gold nanoparticles (GNPs) for in-vivo applications involves reduction of chloroauric acid by a biologically important molecule which itself undergoes redox reaction during physiological processes in human body. But the GNPs often prepared by this method are not stable enough. In order to stabilize these particles, surfactants are used which may or may not be compatible for in-vivo applications. Is there any other way to stabilize these particles in solution? In this work, the answer to this question is explored and a detailed study of the mechanism of the formation of GNPs is done to understand the basis of stabilization of nanoparticles without using a stabilizer. Chloroauric acid is reduced by L-tryptophan (Trp) in buffered medium and the formation mechanism is studied both visually and by UV-vis spectroscopy. The pH dependent structure of Trp was found to play an important role not only in the formation of stable GNPs but also in the stabilization of redispersed nanoparticles. pH sensitive property of the synthesized GNPs was utilized to make the GNPs accumulate at polar-non-polar liquid-liquid interface similar to hypoxic tumor tissue environment. Mechanistic study of the formation of GNPs by gas chromatography throws light on intermediate decarboxylation process. On the other hand, fluorescence study gives information about the interaction of Trp with GNPs.

Surface selective binding of 2,5-dimercapto-1,3,4-thiadiazole (DMTD) on silver and gold nanoparticles: a Raman and DFT study

The binding affinity of 2,5-dimercapto-1,3,4-thiadiazole (DMTD) on noble metal silver (Ag) and gold (Au) nanoparticles (NPs) was investigated using the surface-enhanced Raman scattering (SERS) technique in combination with density functional theoretical (DFT) calculations. DMTD contains different anchoring groups; firstly the thiadiazole ring nitrogen (N) atoms, secondly the ring sulphur (S) atom, thirdly the thiocarbonyl S atoms and fourthly the ring π electrons. SERS combined with DFT calculations provided useful insight into the interaction between the molecule and metal, thus, illustrating the active sites of DMTD and the metal substrates being directly involved in binding. Raman, SERS and DFT studies were further exploited to study the tautomeric conformations of DMTD present in solid, aqueous solution and on the Ag and Au NP surfaces. The Raman spectrum indicated the existence of mainly the dithiol tautomer in the solid state with little contribution from the dithione and the thione–thiol forms. The pH-dependent Raman study of DMTD in aqueous solution clearly demonstrated the existence of different tautomers of DMTD with the changes in acidity or basicity. The SERS study of DMTD on the Ag NP surface with support from DFT calculations clearly suggested the abundance of mainly the thiol–thiolate tautomer at neutral pH while the dithiol form was predominant at acidic pH. The concentration-dependent SERS study on the Ag NP surface at acidic pH, indicated the preferential existence of the dithione form at low concentrations while the dithiol form is predominant at higher concentrations. On the Au NP surface at acidic pH, the abundance of the dithiol tautomer with slight contributions from the thione–thiol and the dithione forms are indicated. The binding is mainly through the thiadiazole ring N atom in case of Ag NPs whereas it is from the thiocarbonyl or the thiadiazole ring S atom in case of Au NPs. This selective binding observed in case of DMTD may be further extended in designing novel plasmonic nanostructures.