Sunil K. Srivastava

Study of mechanism of enhanced antibacterial activity by green synthesis of silver nanoparticles

The extensive use of silver nanoparticles needs a synthesis process that is greener without compromising their properties. The present study describes a novel green synthesis of silver nanoparticles using Guava (Psidium guajava) leaf extract. In order to compare with the conventionally synthesized ones, we also prepared Ag-NPs by chemical reduction. Their optical and morphological characteristics were thoroughly investigated and tested for their antibacterial properties on Escherichia coli. The green synthesized silver nanoparticles showed better antibacterial properties than their chemical counterparts even though there was not much difference between their morphologies. Fourier transform infrared (FTIR) spectroscopic analysis of the used extract and as-synthesized silver nanoparticles suggests the possible reduction of Ag(+) by the water-soluble ingredients of the guava leaf like tannins, eugenol and flavonoids. The possible reaction mechanism for the reduction of Ag(+) has been proposed and discussed. The time-dependent electron micrographs and the simulation studies indicated that a physical interaction between the silver nanoparticles and the bacterial cell membrane may be responsible for this effect. Based on the findings, it seems very reasonable to believe that this greener way of synthesizing silver nanoparticles is not just an environmentally viable technique but it also opens up scope to improve their antibacterial properties.

A chitosan-based polyaniline–Au nanocomposite biosensor for determination of cholesterol

A polyaniline–gold (PAni–Au) nanocomposite is chemically synthesized and impregnated in a chitosan matrix for immobilization of cholesterol oxidase on an indium tin oxide-coated glass plate for development of cholesterol biosensors. The PAni–Au nanocomposite is characterized for structural and thermal properties. Further the PAni–Au nanocomposite is used for cholesterol oxidase immobilization and linkage is examined by FTIR. The nanocomposite is used to form a modified bioelectrode by immobilizing cholesterol oxidase in the chitosan matrix and examined under an optical microscope for uniformity and morphological studies. A modified bioelectrode (ChOx/PAni–Au–CH/ITO) is used to detect cholesterol by using a voltammetric technique with a redox mediator. The sensor exhibits linearity in a wide range of 50–500 mg dL−1 with a detection limit of 37.89 mg dL−1, a sensitivity of 0.86 μA mg dL−1 and a shelf life of more than 3 weeks when stored at 4 °C. Voltammetric studies have also been carried out with common possible interferents. Responses are recorded to get enzyme–substrate kinetics for the biosensor and found as Km = 10.84 mg dL−1. The low value of Km indicates that the prepared nanocomposite facilitates the enzymatic reaction and enzymatic activity. Excellent immobilization and enzyme–substrate reactions show a distinct advantage of this matrix over other matrixes used for cholesterol biosensors. The novelty of the prepared electrode lies in its reusability, higher sensitivity, better shelf life, and accuracy.

Protein-conjugated quantum dots interface: binding kinetics and label-free lipid detection.

We propose a label-free biosensor platform to investigate the binding kinetics using antigen-antibody interaction via electrochemical and surface plasmon resonance (SPR) techniques. The L-cysteine in situ capped cadmium sulfide (CdS; size < 7 nm) quantum dots (QDs) self-assembled on gold (Au) coated glass electrode have been covalently functionalized with apolipoprotein B-100 antibodies (AAB). This protein conjugated QDs-based electrode (AAB/CysCdS/Au) has been used to detect lipid (low density lipoprotein, LDL) biomolecules. The electrochemical impedimetric response of the AAB/CysCdS/Au biosensor shows higher sensitivity (32.8 kΩ μM(-1)/cm(2)) in the detection range, 5-120 mg/dL. Besides this, efforts have been made to investigate the kinetics of antigen-antibody interactions at the CysCdS surface. The label-free SPR response of AAB/CysCdS/Au biosensor exhibits highly specific interaction to protein (LDL) with association constant of 33.4 kM(-1) s(-1) indicating higher affinity toward LDL biomolecules and a dissociation constant of 0.896 ms(-1). The results of these studies prove the efficacy of the CysCdS-Au platform as a high throughput compact biosensing device for investigating biomolecular interactions.

pH-dependent Raman study of pyrrole and its vibrational analysis using DFT calculations.

Raman spectra of pyrrole in aqueous medium at different pH values, 2.5, 5.5, 7.5 and 10.5 were recorded in the two spectral regions, 1,040-1,160 cm(-1) and 3,300-3,360 cm(-1) and pH dependence of the linewidth, peak position and intensity of the Raman bands corresponding to the ring breathing and symmetric nu(N-H) stretching modes were examined. A linear pH dependence of the peak positions for the ring breathing mode and a maximum at nearly neutral pH (7.5) for the symmetric nu(N-H) normal mode is observed, whereas the linewidth (FWHM) shows almost no variation with the change of pH. A slight decrease in the wavenumber position of the nu(N-H) mode at pH value >7.5 indicates that the influence of deprotonation is small, which results from a weak interaction between the reference molecule and the surrounding environment. The density functional theory (DFT) calculations were made primarily to obtain the optimized geometry and vibrational spectra of pyrrole in the ground electronic state using B3LYP functional and the highest level basis set 6-311++G(d,p). The assignments of the normal modes of pyrrole were made on the basis of potential energy distribution (PED). The calculations were also performed on protonated and deprotonated structures of pyrrole.

Hydrogen-bonding and self association investigated in the binary mixture (C6H5CN + CH3OH)via concentration dependent Raman study of the CN stretching mode of benzonitrile (C6H5CN) and ab-initio calculations

The Raman study of (C6H5CN + CH3OH) binary mixture has been presented. The isotropic part of the Raman spectra, Iiso are analyzed in the CN stretching region. For neat C6H5CN, the Iiso shows a double peak structure, which has been explained in terms of self association. A quantum chemical calculation on the optimized structures and wavenumbers of different modes of neat C6H5CN, self associated C6H5CN and the hydrogen-bonded C6H5CN⋯HOCH3 complex reveals that the wavenumber position of the CN stretching mode is blue shifted due to both the self association and the hydrogen-bonding with CH3OH. The Raman spectra of binary mixtures with different mole fractions of the reference system (C6H5CN), C = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, as well as neat liquid have been explained in terms of self association and hydrogen-bonding. A variation of intensity ratio of the peak assigned to the hydrogen-bonded complex to the main band with concentration exhibits a regular trend. The dephasing of the CN stretching mode in the free C6H5CN molecules seems to be governed predominantly by the concentration fluctuation model, but other effects like diffusion and motional narrowing may also have some small influence.

Raman scattering and DFT calculations used for analyzing the structural features of DMSO in water and methanol

We report the concentration dependent Raman spectra of neat dimethyl sulfoxide [(CH3)2SO, DMSO] and its binary mixtures with water (W)/methanol (M) in both ν(SO) and ν(C–H) regions. The ν(SO) line profile of neat DMSO was resolved into four component peaks at 1036, 1044, 1054 and 1064 cm−1 and assigned to different dimeric species of neat DMSO. A careful analysis of the Raman spectra of DMSO with water in the ν(SO) region at different concentrations reveals that upon dilution, an additional peak is observed at ~1017 cm−1 (lower side of the peak ~1036 cm−1) which is attributed to the hydrogen bonding of DMSO with water. For the highest dilution case (χ = 0.1 of DMSO), hydrogen bonded species and symmetric stretching of the dimeric ν(SO) mode were obtained which suggest that DMSO exists in dimer form even at low concentration of DMSO. The significant blue shifting of the C–H frequency due to C–H⋯O hydrogen bonding was also obtained in the case of both solvents. Our experimental results imply the existence of the dimer form of DMSO in neat as well as at χ = 0.1 of DMSO. In order to simulate and validate our experimental findings, detailed ab initio and density functional theory (DFT) calculations were also performed to obtain the ground state geometries of neat DMSO, self-associated dimer, trimer and their hydrogen bonded complexes with water and methanol. Our calculated structure of DMSO in dimer and trimer form reports a more accurate and stable geometry, in comparison to earlier calculation on these structures. Overall in this study nice spectra–structure correlations were obtained.

UV resonance Raman spectroscopic monitoring of supramolecular complex formation: peptide recognition in aqueous solution

The formation of a supramolecular complex between a tetrapeptide and an artificial receptor , is monitored at submillimolar concentrations in water by UV resonance Raman spectroscopy. Using 275 nm excitation, we selectively probe the carboxylate binding site (CBS) within the receptor, a moiety which is very efficient in binding the carboxy terminus of peptides in aqueous media. Complexation of the receptor with the tetrapeptide involves the formation of a H-bond enforced ion pair, resulting in significant changes in the corresponding UV resonance Raman spectra. Our qualitative interpretation is based on experimental reference and calculated Raman spectra on model systems. First preliminary calculations show that for a quantitative analysis, also the distinct contributions of multiple CBS conformers must be considered in addition to the H-bond induced changes upon complexation.

Functional graphene–gold nanoparticle hybrid system for enhanced electrochemical biosensing of free cholesterol

Realizing the unavailability of fast and reliable diagnostic techniques, especially for cholesterol measurement, the present work reports the development of cost effective bioelectrodes based on a reduced graphene oxide–functionalized gold nanoparticle (∼25 nm) hybrid system (RGO–Fn Au NPs). The electrodes fabricated by the electrophoretic deposition technique attest a synergistically enhanced electrochemical sensing ability of 193.4 μA mM−1 cm−2 for free cholesterol detection, which is much higher than that of the traditional RGO system. The electrochemical impedance studies (EIS) show low charge transfer resistance, RCT, for the hybrid system which is 57% and 60% lower than those of RGO and Au NPs respectively. Also higher loading capacity and enhanced kinetics have been realized for the hybrid system, owing to lower Km value (0.005 mM) and enhanced rate constant (3.8 × 10−4 cm s−1) in comparison with RGO and Au NPs. Moreover, the RGO–Fn Au NP platform promises a wider range of cholesterol detection (0.65–12.93 mM), while simultaneously being capable of detecting as low as 0.34 mM of free cholesterol. Apart from better sensitivity, loading capacity, kinetics and detection range, the system also has appreciable selectivity and stability. This supports its potential to be brought on field in the near future for cost effective and reliable detection from the complex system of human serum.

Excited state dipole moments and geometries of the bases of nucleic acids

Abstract CNDO/s-CI and VE-PPP methods have been employed to calculate the dipole moments of the bases of nucleic acids in the ground and excited states. A component analysis in terms of μhyb(σ), μch and μπ has been done using the CNDO/s-CI method and these results have been compared with those obtained by the CNDO/2 and IEHT methods. It is observed that while the CNDO/2 and CNDO/s-CI methods give almost the same total dipole moments, component-wise their predictions are very different. Dipole moments of the molecules have also been studied for the lowest excited singlet and triplet π* ← π states. It is observed that the conventional method of calculating dipole moments using changes of only the net charges in the excited state does not give correct results for uracil and thymine, for which experimental results are available. Considering deformed non-planar excited state geometries for these molecules, the observed excited state dipole moments have been explained. A method has been suggested to include the effects of non-planarity while calculating the properties of a complex molecule in a π* ← π excited state. For adenine, guanine and cytosine, the excited state dipole moments are found to be smaller than the ground state values.

Partially reduced graphene oxide-gold nanorods composite based bioelectrode of improved sensing performance

The present work proposes partially reduced graphene oxide-gold nanorods supported by chitosan (CH-prGO-AuNRs) as a potential bioelectrode material for enhanced glucose sensing. Developed on ITO substrate by immobilizing glucose oxidase on CH-prGO-AuNRs composite, these CH-prGO-AuNRs/ITO bioelectrodes demonstrate high sensitivity of 3.2 µA/(mg/dL)/cm(2) and linear range of 25-200 mg/dL with an ability to detect as low as 14.5 mg/dL. Further, these CH-prGO-AuNRs/ITO based electrodes attest synergistiacally enhanced sensing properties when compared to simple graphene oxide based CH-GO/ITO electrode. This is evident from one order higher electron transfer rate constant (Ks) value in case of CH-prGO-AuNRs modified electrode (12.4×10(-2) cm/s), in contrast to CH-GO/ITO electrode (6×10(-3) cm/s). Additionally, very low Km value [15.4 mg/dL(0.85 mM)] ensures better binding affinity of enzyme to substrate which is desirable for good biosensor stability and resistance to environmental interferences. Hence, with better loading capacity, kinetics and stability, the proposed CH-prGO-AuNRs composite shows tremendous potential to detect several bio-analytes in the coming future.