Takeyoshi Okajima

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.

Simultaneous electroanalysis of dopamine and ascorbic acid using poly (N,N-dimethylaniline)-modified electrodes.

Glassy carbon (GC) electrode is modified with an electropolymerized film of N,N-dimethylaniline (DMA). This polymer (PDMA) film-coated GC electrode is used to electrochemically detect dopamine (DA) in the presence of ascorbic acid (AA). Polymer film has the positive charge in its backbone, and in neutral solution DA exists as the positively charged species whereas AA exists as the negatively charged one. In cyclic voltammetric measurements, favorable ionic interaction (i.e., electrostatic attraction) between AA and PDMA film causes a large negative shift of the oxidation potential for AA compared to that at the bare electrode. Oxidation potential for DA is positively shifted due to the electrostatic repulsion. The PDMA film shows hydrophobicity by incorporating uncharged hydroquinone molecule within the film. DA is also incorporated into the film due to hydrophobic attraction even though DA has a positive charge. The responses of DA and AA at polymer-modified electrodes largely change with the concentration of the monomer (i.e., 0.2, 0.1 and 0.05 M DMA) used in electropolymerization and thus with the film thickness. Hydrophobicity of the polymer film shows great influence on the voltammetric responses of both DA and AA. In square wave voltammetric measurements, the PDMA film-coated electrode can separate the DA and AA oxidation potentials by about 300 mV and can detect DA at its low concentration (e.g., 0.2 microM) in the presence of 1000 times higher concentration of AA, which is close to the physiological level. AA oxidizes at more negative potential than DA. The electrode response is not affected by the oxidized product of AA. So unlike the bare electrode, the fouling effect as well as the catalytic oxidation of AA by the oxidized form of DA are eliminated at the PDMA film-coated GC electrode. The electrode exhibits the stable and sensitive response to DA.

Electrochemical Reduction of Oxygen on Gold Nanoparticle-Electrodeposited Glassy Carbon Electrodes

The electrochemical reduction of oxygen (O 2 ) on Au nanoparticle-electrodeposited glassy carbon electrodes (GCEs) has been performed in 0.1 M phosphate buffer solution (pH 7.2). Two well-separated electrochemical reduction peaks for O 2 on GCE were observed at about -750 and -2000 mV vs. Ag/AgCl/KCl (sat.) i.e., a peak separation of ca. 1250 mV. Those two peaks were attributed to the two-step four-electron reduction of O 2 to H 2 O through H 2 O 2 . A remarkable decrease of the separation of the two peaks (down to 550 mV) along with a significant positive shift of the two reduction peaks of O 2 to -350 and -9880 mV, respectively, were observed upon loading of a very minute amount of Au nanoparticles onto the GCE (typically 2.78 X 10 - 7 g cm - 2 ). Further positive potential shift of the two peaks along with a concurrent decrease of the peaks separation could be achieved by controlling the extent of Au loading on the GCE. Au-electrodeposited GCE with an equivalent Au film thickness of 10 nm showed almost the same behavior toward the O 2 reduction as the bulk Au electrode. These observations were interpreted in view of the increase of the effective (real) surface area of the Au film by the increase of its thickness, as indicated by scanning electron micrographs in addition to the characteristic cyclic voltammogram for Au nanoparticle-electrodeposited GCEs in N 2 -saturated 0.05 M H 2 SO 4 .

Multiple voltammetric waves for reductive desorption of cysteine and 4-mercaptobenzoic acid monolayers self-assembled on gold substrates

The reductive desorption of cysteine and 4-mercaptobenzoic acid monolayers formed on polycrystalline and single-crystalline gold substrates has been investigated. Three cathodic waves observed at polycrystalline gold electrodes are correlated with reductive desorption from each small domain of the low index faces, i.e. (111), (100) and (110), over the surface. The stability of the adsorbates differs substantially depending on the surface crystallographic orientation, in the order (111) < (100) < (110). Thus, the reductive desorption occurs at certain potentials, leading to an apparent peak separation on their cyclic voltammograms. Partial desorption of the adsorbate via control of the potential would be a simple and reliable method of fabricating functional electrodes designed for use as electrocatalysts or sensors, on which only the small domains of the (111) face could be selectively exposed to an electrolyte.

Selective detection of As(III) at the Au(111)-like polycrystalline gold electrode.

Selective electrochemical detection of As(III) using a highly sensitive platform based on a Au(111)-like surface is described. The Au(111)-like surface was achieved for the first time by the partial reductive desorption of n-butanethiol (n-BT) from polycrystalline gold (poly-Au), on which a self-assembled monolayer (SAM) of n-BT was formed previously, which allows the selective blockage of the Au(100) and Au(110) surface domains by n-BT while the Au(111) domain remains bare. Square wave anodic stripping voltammetry (SWASV) using the Au(111)-like poly-Au electrode confirms the successful detection of As(III) without any interference from Cu(II). The fabricated electrode is stable and highly sensitive even in the presence of Cu(II), and it shows a linear response for As(III) up to 15 μM. The detection limit (S/N = 3) toward As(III) is 0.28 ppb, which is far below the guideline value given by World Health Organization (WHO). The electrode was applicable for the analysis of spiked arsenic in tap water containing a significant amount of various other ion elements. The results indicate that the Au(111)-like poly-Au electrode could be promising for the electrochemical detection of trace level of As(III) in real samples without any interference from Cu(II).

A facilitated electron transfer of copper--zinc superoxide dismutase (SOD) based on a cysteine-bridged SOD electrode.

The direct electrochemical redox reaction of bovine erythrocyte copper--zinc superoxide dismutase (Cu(2)Zn(2)SOD) was clearly observed at a gold electrode modified with a self-assembled monolayer (SAM) of cysteine in phosphate buffer solution containing SOD, although its reaction could not be observed at the bare electrode. In this case, SOD was found to be stably confined on the SAM of cysteine and the redox response could be observed even when the cysteine-SAM electrode used in the SOD solution was transferred to the pure electrolyte solution containing no SOD, suggesting the permanent binding of SOD via the SAM of cysteine on the electrode surface. The electrode reaction of the SOD confined on the cysteine-SAM electrode was found to be quasi-reversible with the formal potential of 65 +/- 3 mV vs. Ag/AgCl and its kinetic parameters were estimated: the electron transfer rate constant k(s) is 1.2 +/- 0.2 s(-1) and the anodic (alpha(a)) and cathodic (alpha(c)) transfer coefficients are 0.39 +/- 0.02 and 0.61 +/- 0.02, respectively. The assignment of the redox peak of SOD at the cysteine-SAM modified electrode could be sufficiently carried out using the native SOD (Cu(2)Zn(2)SOD), its Cu- or Zn-free derivatives (E(2)Zn(2)SOD and Cu(2)E(2)SOD, E designates an empty site) and the SOD reconstituted from E(2)Zn(2)SOD and Cu(2+). The Cu complex moiety, the active site for the enzymatic dismutation of the superoxide ion, was characterized to be also the electroactive site of SOD. In addition, we found that the SOD confined on the electrode can be expected to possess its inherent enzymatic activity for dismutation of the superoxide ion.

Stability of Superoxide Ion in Imidazolium Cation-Based Room-Temperature Ionic Liquids

The stability of superoxide ion (O(2)(*-)) generated chemically by dissolving KO(2) in dried dimethyl sulfoxide solutions containing imidazolium cation [e.g., 1-ethyl-3-methylimidazolium (EMI(+)) and 1-n-butyl-2,3-dimethylimidazolium (BMMI(+))] based ionic liquids (ILs) was investigated with UV-visible spectroscopic, NMR, and voltammetric techniques and an ab initio molecular orbital calculation. UV-visible spectroscopic and cyclic voltammetric measurements reveal that the O(2)(*-) species reacts with BMMI(+) and EMI(+) cations of ILs to form hydrogen peroxide. The pseudo first order rate constant for the reaction of BMMI(+) and O(2)(*-) species was found to be about 2.5 x 10(-3) s(-1). With a molecular orbital calculation, the O(2)(*-) species is understood to attack the 2-position (C-2) of the imidazolium ring (i.e., BMMI(+)) to form an ion pair complex in which one oxygen atom is bounded to C-2 and the other to the hydrogen atom of -CH(3) group attached to C-2. Eventually, the ion pair complex of BMMI(+) cation and O(2)(*-) species undergoes a ring opening reaction as evidenced with (1)H NMR measurement.

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.

Electrochemical Preparation of a Au Crystal with Peculiar Morphology and Unique Growth Orientation and Its Catalysis for Oxygen Reduction

Gold nanocrystals with a peculiar pin-like morphology have been obtained on glassy carbon (GC) electrodes by electrochemical deposition from 0.5 M H 2 SO 4 solution containing 1.0 mM Na[AuCl 4 ] in the presence of cysteine as an additive. The amount of such electrodeposited Au particles as well as the particle size increased with the increase of deposition time (t d ). Crystalline facets of Au(100) and Au(110) were significantly enriched in the presence of cysteine during the electrodeposition, and the Au crystals grew along a unique crystalline orientation ofAu(lll). Finally, Au crystals (with the average diameter of ca. 100 nm and length of ca. 1660 nm at t d = 2000 s) with a peculiar pin-like morphology were obtained, in contrast to the plump Au particles prepared in the absence of cysteine. The cathodic peak current density for the O 2 reduction in acidic electrolyte (0.5 M H 2 SO 4 ) at the Au crystals-electrodeposited GC electrode prepared in the presence of cysteine was larger than that at the electrode prepared in its absence. This behavior could be reasonably explained in view of the higher specific surface area of the Au crystals prepared in the presence of cysteine than those prepared in the absence of cysteine.

Electrodeposition of Gold at Glassy Carbon Electrodes in Room-Temperature Ionic Liquids

The cyclic voltammetric behavior of [AuCl 4 ]¯ on glassy carbon (GC) and gold electrodes in room-temperature ionic liquids, i.e., ⇆-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF 4 ) and 1-n-butyl-3-methylimidazolium tetrafluoroborate (BMIBF 4 ) has been examined. A series of two-electron (2e-) and one-electron (1e-) reductions of the [AuCl 4 ] - ¯[AuCl 2 ] - ¯Au redox system could be observed at GC electrode. For example, the cathodic and anodic peaks corresponding to the [AuCl 4 ]¯/[AuCl 2 ]¯ redox couple were observed at ca. 0.2 and 1.2 V vs a Ag wire quasi-reference electrode, respectively, in EMIBF 4 , while those observed at -0.5 and 0.5 V were found to correspond to the [AuCl 2 ]¯/Au redox couple. The disproportionation reaction of the 2e-reduction product of [AuCl 4 ]¯, i.e., [AuCl 2 ]¯ to [AuCl 4 ]¯ and Au metal, was also found to occur significantly. A single reduction peak corresponding to the three-electron (3e-) reduction of [AuCl 4 ]¯ to Au metal was observed at Au electrode. The electrodeposition of Au nanoparticles was carried out on GC electrode in these ionic liquids containing [AuCl 4 ]¯ by applying potential-step electrolysis in a different potential range, i.e., the potential was stepped from 0.4 V to 0 and -1.0 V, at which the reduction of [AuCl 4 ]¯ to [AuCl 2 ]¯ and Au, respectively, takes place. The results obtained demonstrate that the electrodeposition of gold may occur via a disproportionation reaction of [AuCl 4 ]¯ to [AuCl 2 ]¯ and Au as well as via a series of the reductions of [AuCl 4 ]¯ to [AuCl 2 ]¯ and further, [AuCl 2 ]¯ to Au. The size and morphology of the prepared Au nanoparticles as well as the relative ratio of the Au(111), Au(llO), and Au(100) crystalline orientation domains constituting the polycrystalline Au nanoparticles electrodeposited were found to largely depend on the stepped potential (i.e., 0 and -1.0 V). Interestingly, the Au nanoparticles prepared by a potential-step electrolysis from 0.4 to 0 V are enriched in the Au(110) single-crystalline domain.