Fermi level equilibration of Ag and Au plasmonic metal nanoparticles supported on graphene oxide.

For the first time, the process of Fermi level equilibration has been studied and compared for plasmonic metal nanoparticles (PMNPs) supported on conducting substrates i.e. graphene oxide (GO) sheets. The extent of Fermi level equilibration has been monitored by recording the changes in the position and intensity of the surface plasmon resonance (SPR) band of Ag and Au PMNPs supported on reduced graphene oxide (rGO). Ag PMNPs supported on rGO show larger variation in the SPR band position and intensity as compared to rGO supported Au PMNPs. The average shift in the chemical potential has been determined through the changes in the SPR band position for Ag, Ag@rGO, Au, and Au@rGO, which are approximately -1812 ± 70 mV, -171 ± 20 mV, -96 ± 8 mV and -29 ± 4 mV, respectively. The calculated values of the shift in chemical potential suggest that Ag and its rGO composite are more prone to Fermi level equilibration as compared to the Au and Au@rGO composite. The electrochemical (galvanostatic) charging/discharging (GCD) measurements also brace the observations from the chemical charging/discharging method with minor variations due to the measurements under two different conditions; particulate films in the case of the former versus the dispersed phase in the case of the latter. Moreover, the average capacitance associated with single nanoparticles (Ag and Au) is estimated using the capacitance values determined from GCD curves and the approximate number of nanoparticles determined from the quantity of PMNPs used in the deposited films for GCD measurements. These values are in close agreement with the quantized double layer capacitance values of monolayer protected clusters reported in the literature. A similar inference is also drawn from the enzyme-less glucose sensing activity of these nanostructures, where Ag and Ag@rGO show better activity in terms of lower values of the limit of detection (LOD) and the limit of quantification (LOQ).

Voltammetric trace determination of mercury using plant refuse modified carbon paste electrodes.

Citrus limon peel (kitchen waste) and Leucaena leucocephala seeds (agricultural waste) were used as a modifier for fabrication of modified carbon paste electrode for determination of mercury in aqueous sample using differential pulse anodic stripping voltammetry. Mercury was adsorbed on electrode surface at open circuit and anodic stripping voltammetric scan was run from -0.5 to 0.5 V. Various electrochemical parameters including amount of modifier, supporting electrolyte, accumulating solvent, pH of the accumulating solvent, and accumulation time were investigated. The effect of presence of other metal ions and surfactants was also studied. In comparison C. limon peel proved to be a better modifier than L. leucocephala seed biomass. This was justified by electrode characterization using cyclic voltammetry that indicated decrease in resistance of electrode when C. limon peel was used as modifier and increase when modifier was L. leucocephala seeds. Maximum current response was obtained using 5% C. limon peel biomass, hydrochloric acid as supporting electrolyte, acetate buffer of pH 6 as an accumulating solvent, 10-min accumulation time, and scan rate of 50 mV/s. Linear calibration curves were obtained in the concentration range 100 to 1,000 µg L(-1) of mercury for accumulation time of 10 min with limit of detection of 57.75 µg L(-1) and limit of quantification of 192.48 µg L(-1). This technique does not use mercury as electrode material and, therefore, has a positive environmental benefit.

Supersensitive Detection of Anions in Pure Organic and Aqueous Media by Amino Acid Conjugated Ellman's Reagent.

The last few decades witnessed a remarkable advancement in the field of molecular anion receptors. A variety of anion binding motifs have been discovered, and large number of designer molecular anion receptors with high selectivity are being reported. However, anion detection in an aqueous medium is still a formidable challenge as evident from only a miniscule of synthetic systems available in the literature. We, herein, report 5,5'-dithio-bis(2-nitrobenzoic acid) (Ellman's reagent) appended with amino acids as supersensitive anion sensors that can detect F- and H2PO4- ions in both aqueous as well as organic media. Interestingly, the sensors showed a dual response to anions, viz., chromogenic response in organic medium and electrochemical response in aqueous solutions. Various spectroscopic techniques such as UV-vis and 1H NMR are used to investigate the binding studies in acetonitrile, whereas electrochemical methods such as cyclic voltammetry (CV) and differential pulse voltammetry (DPV) are employed to explore the anion binding in water. The host-guest complex stoichiometry and binding constants are calculated using the BindFit software. The geometry of host-guest complex has been optimized by the density functional theory (DFT) method. These molecules are versatile sensors since these function in both water and acetonitrile with extremely low limit of detection (LOD) up to 0.07 fM and limit of quantification (LOQ) up to 0.23 fM. To our knowledge, the present system is the first example of a sensor that can detect the lowest concentration of anions in water quantitatively. The minimalistic design strategy presented here opens up the innumerable possibilities for designing dual anion sensors in a one fell swoop.