C. Retna Raj

Gold nanoparticle arrays for the voltammetric sensing of dopamine

Abstract Gold (Au) nanoparticles immobilized on an amine-terminated self-assembled monolayer (SAM) on a polycrystalline Au electrode were successfully used for the selective determination of dopamine (DA) in the presence of ascorbate (AA). Well-separated voltammetric peaks were observed for DA and AA at the nano-Au electrode (Au nanoparticle-immobilized electrode). The oxidation potential of AA is shifted to less positive potential due to the high catalytic activity of Au nanoparticle. The reversibility of the electrode reaction of DA is significantly improved at the nano-Au electrode, which results in a large increase in the square-wave voltammetric peak current with a detection limit of 0.13 μM. The coexistence of a large excess of AA does not interfere with the voltammetric sensing of DA. The nano-Au electrode shows excellent sensitivity, good selectivity and antifouling properties.

Gold nanoelectrode ensembles for the simultaneous electrochemical detection of ultratrace arsenic, mercury, and copper.

Simultaneous electrochemical detection of As(III), Hg(II), and Cu(II) using a highly sensitive platform based on gold nanoelectrode ensembles (GNEEs) is described. GNEEs were grown by colloidal chemical approach on thiol-functionalized solgel derived three-dimensional silicate network preassembled on a polycrystalline gold (Au) electrode. GNEEs on the silicate network have been characterized by field emission scanning electron microscopy, X-ray diffraction, diffuse reflectance spectroscopy, and electrochemical measurements. Square wave anodic stripping voltammetry (SWASV) has been used for the detection of As(III) and Hg(II) without any interference from Cu(II) at the potentials of 0.06 and 0.53 V, respectively. The GNEE electrode is highly sensitive, and it shows linear response for As(III) and Hg(II) up to 15 ppb. The detection limit (signal-to-noise ratio = 4) of the GNEE electrode toward As(III) and Hg(II) is 0.02 ppb, which is well below the guideline value given by the World Health Organization (WHO). The potential application of the GNEE electrode for the detection of As(III) in a real sample collected from the arsenic-contaminated water in 24 North Parganas, West Bengal is demonstrated. The GNEE electrode has been successfully used for the simultaneous detection of As(III), Cu(II), and Hg(II) at sub-part-per-billion level without any interference for the first time. The nanostructured electrode shows individual voltammetric peaks for As(III), Cu(II), and Hg(II) at 0.06, 0.35, and 0.53 V, respectively. The analytical performance of the GNEE electrode is superior to the existing electrodes.

Voltammetric detection of uric acid in the presence of ascorbic acid at a gold electrode modified with a self-assembled monolayer of heteroaromatic thiol

Fabrication of a voltammetric sensor based on a self-assembled monolayer (SAM) of a heteroaromatic thiol, mercaptobenzimidazole (MBI) for the determination of uric acid (UA) in the presence of ascorbic acid (AA) is described. The MBI monolayer facilitates the oxidation processes of AA and UA. The orientation of the monolayer on the electrode surface controls the voltammetric behavior of AA and UA. The voltammetric signals of UA and AA are well separated with a potential difference of 270 mV and AA does not interfere with the measurement of UA at the monolayer-modified electrode. Linear calibration curves were obtained for UA over the concentration range 1–300 μM in neutral pH with a detection limit (3σ) of 1 μM in the presence of 100-fold excess of AA. The practical utility of the present monolayer-modified electrode is demonstrated by measuring UA in human serum. The results obtained at the MBI monolayer are compared with those at the monolayers of structurally related aromatic thiols.

Pt nanoparticle-based highly sensitive platform for the enzyme-free amperometric sensing of H2O2

Highly sensitive electrochemical platform based on polymer supported Pt nanoparticles (nPts) for the amperometric sensing of H(2)O(2) at sub-nanomolar level without any redox mediator or enzyme is developed. The nPts are generated by the chemical reduction of precursor pre-organized on the electrode surface and characterized by field emission scanning electron microscopy, X-ray diffraction, spectral and electrochemical measurements. The cationic polymer poly(diallyldimethylammonium) chloride was used to assist the pre-organization of metal precursor. nPts on the electrode surface have an average size of 17 nm. The nanoparticles show excellent electrocatalytic activity towards oxidation of H(2)O(2) at less positive potential than the polycrystalline Pt electrode. Unlike the polycrystalline Pt electrode, the nanoparticle-based electrode does not undergo deactivation by surface oxides and other species in solution. Particle loading on the electrode surface controls the electrocatalytic activity. The nanoparticle-based electrode is highly sensitive (9.15 microA/mM) and display linear response up to 3 mM. It could detect 0.5 nM (S/N=5) of H(2)O(2) under hydrodynamic condition in neutral solution and the electrode is highly stable. The detection limit achieved is significantly lower than the other nanoparticle-based electrodes. The excellent performance of the electrode is ascribed to the good catalytic activity of the particle and ensemble behavior of the nanoparticle-modified electrode. The analytical performance of the electrode in the development of glucose biosensor is demonstrated. The biosensor is used for the sensing of glucose in the micromolar level in neutral pH.

Facile Single-Step Synthesis of Nitrogen-Doped Reduced Graphene Oxide-Mn3O4 Hybrid Functional Material for the Electrocatalytic Reduction of Oxygen

Development of efficient electrocatalyst based on non-precious metal that favors the four-electron pathway for the reduction of oxygen in alkaline fuel cell is a challenging task. Herein, we demonstrate a new facile route for the synthesis of hybrid functional electrocatalyst based on nitrogen-doped reduced graphene oxide (N-rGO) and Mn3O4 with pronounced electrocatalytic activity towards oxygen reduction reaction (ORR) in alkaline solution. The synthesis involves one-step in situ reduction of both graphene oxide (GO) and Mn(VII), growth of Mn3O4 nanocrystals and nitrogen doping onto the carbon framework using a single reducing agent, hydrazine. The X-ray photoelectron (XPS), Raman and FTIR spectral, and X-ray diffraction measurements confirm the reduction of GO and growth of nanosized Mn3O4. The XPS profile reveals that N-rGO has pyridinic (40%), pyrrolic (53%), and pyridine N oxide (7%) types of nitrogen. The Mn3O4 nanoparticles are single crystalline and randomly distributed over the wrinkled N-rGO sheets. The hybrid material has excellent ORR activity and it favors the 4-electron pathway for the reduction of oxygen. The electrocatalytic performance of the hybrid catalyst is superior to the N-rGO, free Mn3O4 and their physical mixture. The hybrid material shows an onset potential of -0.075 V, which is 60-225 mV less negative than that of the other catalyst tested. It has excellent methanol tolerance and high durability. The catalytic current density achieved with the hybrid material at 0.1 mg cm(-2) is almost equivalent to that of the commercial Pt/C (10%). The synergistic effect of N-rGO and Mn3O4 enhances the overall performance of the hybrid catalyst. The nitrogen in N-rGO is considered to be at the interface to bridge the rGO framework and Mn3O4 nanoparticles and facilitates the electron transfer.

A rapid room temperature chemical route for the synthesis of graphene: metal-mediated reduction of graphene oxide

A rapid and facile route for the synthesis of reduced graphene oxide sheets (rGOs) at room temperature by the chemical reduction of graphene oxide using Zn/acid in aqueous solution is demonstrated.

Highly sensitive and selective electrochemical detection of sub-ppb level chromium(VI) using nano-sized gold particle.

Gold nanoparticle based nanostructured electrode has been developed for the amperometric detection of ultratrace amount of toxic Cr(VI). The nano-sized Au particles have been grown on a conducting substrate modified with sol-gel-derived thiol functionalized silicate network and used for the electroanalysis of Cr(VI). The nanostructured interface show well-defined voltammetric peak for the reduction of Cr(VI) at approximately 0.4 V. The voltammetric behavior of Cr(VI) strongly depends on the coverage of nanoparticle on the electrode surface. Constant potential amperometry has been used for the detection of Cr(VI) at well below the guideline value set by World Health Organization (WHO). This electrode is highly sensitive (30+/-0.2 nA/ppb) and the detection limit (S/N=9) was 0.1 ppb. Cr(III) and coexisting other metal ions and surface active agent present in water do not interfere with the amperometric measurement of Cr(VI). This nanostructured electrode is highly stable and it can be used for continuous measurement of Cr(VI) without using any pretreatment or activation procedures. The accuracy of the measurement has been validated by measuring the concentration of Cr(VI) in the certified reference material (CRM).

Optical sensing of biomedically important polyionic drugs using nano-sized gold particles.

A simple optical method for the sensing of biomedically important polyionic drugs, protamine and heparin based on the reversible aggregation and de-aggregation of gold nanoparticles (AuNPs) is described. The polycationic protamine induces the aggregation of negatively charged citrate-stabilized AuNPs, resulting in a shift in the surface plasmon (SP) band and a consequent color change of the AuNPs from red to blue. Addition of polyanionic heparin dissipates the aggregated AuNPs due to its strong affinity to protamine and the blue color changes to the native color. The color change was monitored using UV-vis spectrophotometry. The aggregation and de-aggregation was confirmed by transmission electron microscopic (TEM) measurements. The degree of aggregation and de-aggregation is proportional to the concentration of added protamine and heparin, allowing their quantitative detection. The change in the absorbance and SP band position has been used to monitor the concentration of protamine and heparin. This optical method can quantify protamine and heparin as low as 0.1 microg/ml and 0.6 microg/ml, respectively and the calibration is linear for a wide range of concentration.

Emerging new generation electrocatalysts for the oxygen reduction reaction

The design and development of a new economically viable electrocatalyst for the cathodic reduction of oxygen in fuel cells and metal–air batteries is of significant interest. The high cost, scarcity and lack of durability of traditional Pt-based electrocatalysts limit the widespread implementation of fuel cells for practical applications. The emergence of non-Pt and metal-free electrocatalysts for the oxygen reduction reaction (ORR) is promising in the development of energy conversion devices. In this review, we discuss the emerging new electrocatalysts, non-precious transition metals, metal nitrides and carbides and the nanoscale carbon-based metal-free electrocatalysts, for the ORR. Although the actual ORR mechanism and the active site of these catalysts are not well understood, their catalytic activity is undoubtful. The porosity and chemical and electronic environments of the catalysts control their activity. The activity of these catalysts is discussed in terms of onset potential, durability and their tolerance towards anode fuels. The metal-free heteroatom-doped carbon-based electrocatalysts are highly active in alkaline medium, paving the way for the development of alkaline fuel cells, though their long time durability in an actual fuel cell stack is not well explored. The challenges in the use of these catalysts and the lack of fundamental understanding of the catalytic activity are addressed.

Pt–Pd alloy nanoparticle-decorated carbon nanotubes: a durable and methanol tolerant oxygen reduction electrocatalyst

We describe the decoration of multiwalled carbon nanotubes (MCNTs) with Pt-Pd alloy nanoelectrocatalysts of three different compositions and their electrocatalytic performance toward the oxygen reduction reaction (ORR). The decoration of the MCNTs involves polymer-assisted impregnation of metal precursors PtCl(6)(2-) and PdCl(6)(2-) and the subsequent reduction of the impregnated precursors by a modified polyol route. The composition of the catalyst was controlled by tuning the molar ratio of the precursors during their impregnation. Electron probe microscopic analysis shows that the catalysts have compositions of Pt(46)Pd(54,) Pt(64)Pd(36) and Pt(28)Pd(72). The Pt(46)Pd(54) and Pt(64)Pd(36) catalysts have truncated octahedral and icosahedral shapes with a size ranging from 8 to 10 nm. On the other hand, the catalyst of Pt(28)Pd(72) composition has a spherical/quasispherical shape with a size distribution of 1-2 nm. The XPS measurement confirms the signature of metallic Pt and Pd. The Pt(46)Pd(54) catalyst has a pronounced electrocatalytic activity toward the ORR with a specific and mass activity of 378 μA cm(Pt-Pd)(-2) and 64 μA μg(Pt-Pd)(-1), respectively at 0.8 V. Moreover, the Pt(46)Pd(54) nanoelectrocatalyst is highly durable and it retains its initial catalytic activity even after 1000 extensive cycles. Interestingly, this catalyst has a very high tolerance toward methanol and it does not favor the oxidation of methanol in the potential window of 0.1-1.4 V. The electrocatalytic activity of the alloy electrocatalyst is compared with commercially available Pt black and MCNT-supported spherical Pt nanoparticles. The catalytic activity of the Pt(46)Pd(54) nanoelectrocatalyst is higher than the other catalysts. The Pt(46)Pd(54) catalyst outperforms the electrocatalytic activity of all other catalysts.