Nandita Maiti

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.

Investigation on the adsorption characteristics of anserine on the surface of colloidal silver nanoparticles

The surface-enhanced Raman scattering (SERS) studies of anserine (beta-alanyl-N-methylhistidine) was carried out on colloidal silver nanoparticles to understand its adsorption characteristics. The experimentally observed Raman bands were assigned based on the results of DFT calculations. The studies suggest that the interaction of anserine is primarily through the carboxylate group with the imidazole ring in an upright position with respect to the silver surface. Concentration dependent SERS studies suggest a change in orientation at sub-monolayer concentration.

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.

Raman and XPS study on the interaction of taurine with silver nanoparticles

This is the first report on the surface-enhanced Raman scattering (SERS) and X-ray photoelectron spectroscopic (XPS) studies of taurine adsorbed on the surface of silver nanoparticles. Taurine, the sulphonic acid analogue of β-alanine participates in numerous biological and physiological functions in the human body and is a potent antioxidant. The interpretation of the SERS spectrum based on the “surface selection rules”, XPS studies and DFT calculations has clearly indicated that the gauche tautomer of taurine is predominant on the silver nanoparticle surface. Taurine is directly bound to the silver surface through the sulphonate group along with interactions from the NH2 group that lies in close proximity to the metal surface. The binding of taurine through the oxygen atoms of the sulphonate group is confirmed by the XPS studies which show the presence of 71% Ag–O in taurine functionalized silver nanoparticles.

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.

Metal nanoparticle catalyzed charge rearrangement in selenourea probed by surface-enhanced Raman scattering

The adsorption behavior of selenourea (SeU) on noble metal silver (Ag) and gold (Au) nanoparticles (NPs) was investigated using the surface-enhanced Raman scattering (SERS) technique in combination with X-ray photoelectron spectroscopy (XPS) and density functional theoretical (DFT) calculations. SeU contains two different anchoring groups, firstly, a selenium atom and secondly, two amino groups. SERS, XPS and DFT studies provided useful insight into the metal–molecule interaction, thus, illustrating the active sites of SeU and the metal substrates being directly involved in binding. The SERS and DFT results provided further insight into the tautomerism of SeU, suggesting that as a solid, the selenone form is exclusively found. On the Ag NP surface, signatures of the selenol form were predominant which indicates charge rearrangement within the molecule while on the Au NP surface, contributions from both selenone and selenol tautomers were observed. In addition, the preferential binding affinity of the anchoring groups in SeU towards Ag and Au was exploited for the synthesis of metal–molecule–metal sandwiched nanoassemblies that further suggested possible charge rearrangement within the molecule.

Investigating structural aspects to understand the putative/claimed non-toxicity of the Hg-based Ayurvedic drug Rasasindura using XAFS.

XANES- and EXAFS-based analysis of the Ayurvedic Hg-based nano-drug Rasasindura has been performed to seek evidence of its non-toxicity. Rasasindura is determined to be composed of single-phase α-HgS nanoparticles (size ∼24 nm), free of Hg(0) or organic molecules; its structure is determined to be robust (<3% defects). The non-existence of Hg(0) implies the absence of Hg-based toxicity and establishes that chemical form, rather than content of heavy metals, is the correct parameter for evaluating the toxicity in these drugs. The stable α-HgS form (strong Hg-S covalent bond and robust particle character) ensures the integrity of the drug during delivery and prevention of its reduction to Hg(0) within the human body. Further, these comparative studies establish that structural parameters (size dispersion, coordination configuration) are better controlled in Rasasindura. This places the Ayurvedic synthesis method on par with contemporary techniques of nanoparticle synthesis.

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.