Electroanalytical applications of cationic self-assembled monolayers: square-wave voltammetric determination of dopamine and ascorbate

Gold electrodes modified with cationic self-assembled monolayers (SAMs) of 2,2'-dithiobisethaneamine (CYST) and 6,6'-dithiobishexaneamine (DTH) were used for the simultaneous determination of dopamine (DA) and ascorbate (AA). The cationic SAM modified electrodes have several advantages over the bare electrode for the oxidation of AA. A very large (approximately 450 mV) decrease in the overpotential for the oxidation of AA when compared with the bare electrode has been observed at the cationic monolayer-modified electrode. The electrostatic interaction of negatively charged AA with the monolayer shift the oxidation peak potential of AA to less positive potential and enhances the peak current. On the other hand, the positively charged DA is repelled from the monolayer and the oxidation potential shifts to more positive potential when compared to the bare electrode. The electrochemical oxidation of AA at the mixed monolayer of CYST and diethyl disulfide (DEDS) supports the influence of cationic terminal group of the monolayer on the oxidation of AA. Since the oxidation of AA occurs well before the oxidation potential of DA is reached, the homogeneous catalytic oxidation of AA by the oxidized DA has been advantageously eliminated at the monolayer-modified electrode. The cationic self-assembled monolayers successfully detect DA in the presence of high concentration of AA. The sensitivity of the electrode modified with CYST monolayer was found to be 0.036 and 0.021 microA/microM towards AA and DA, respectively.     

Variable coordination mode of chloranilic acid. Synthesis, structure, and electrochemical properties of some osmium complexes

Reaction of chloranilic acid (H2ca) with [Os(bpy)2 Br2] (bpy = 2,2'-bipyridine) affords a dinuclear complex of type [{Os(bpy)2}2 (ca)]2+, isolated as the perchlorate salt. A similar reaction of H2ca with [Os(PPh3)2 (pap)Br2] (pap = 2-(phenylazo)pyridine) affords a dinuclear complex of type [{Os(PPh3)2 (pap)}2 (ca)]2+ (isolated as the perchlorate salt) and a mononuclear complex of type [Os(PPh3)2 (pap)(ca)]. Reaction of H2ca with [Os(PPh3)2(CO)2(HCOO)2] gives a dinuclear complex of type [{Os(PPh3)2(CO)2}2 (r-ca)], where r-ca is the two electron reduced form of the chloranilate ligand. The structures of the [{Os(PPh3)2 (pap)}2 (ca)](ClO4)2, [Os(PPh3)2 (pap)(ca)], and [{Os(PPh3)2(CO)2}2 (r-ca)] complexes have been determined by X-ray crystallography. In the [{Os(bpy)2}2 (ca)]2+ and [{Os(PPh3)2 (pap)}2 (ca)]2+ complexes, the chloranilate dianion is serving as a tetradentate bridging ligand. In the [Os(PPh3)2 (pap)(ca)] complex, the chloranilate dianion is serving as a bidentate chelating ligand. In the [{Os(PPh3)2(CO)2}2 (r-ca)] complex, the reduced form of the chloranilate ligand (r-ca(4-)) is serving as a tetradentate bridging ligand. All the four complexes are diamagnetic and show intense metal-to-ligand charge-transfer transitions in the visible region. The [Os(PPh3)2 (pap)(ca)] complex shows an Os(II)-Os(III) oxidation, followed by an Os(III)-Os(IV) oxidation on the positive side of a standard calomel electrode. The three dinuclear complexes show two successive oxidations on the positive side of SCE. The mixed-valent Os(II)-Os(III) species have been generated in the case of the two chloranilate-bridged complexes by coulometric oxidation of the homovalent Os(II)-Os(II) species. The mixed-valent Os(II)-Os(III) species show intense intervalence charge-transfer transitions in the near-IR region.     

Electrochemical determination of dopamine in the presence of ascorbic acid and uric acid using the synergistic effect of gold nanoflowers and L-cysteamine monolayer at the surface of a gold electrode

This paper describes a facile and effective method to synthesize gold nanoflowers (AuNFs) by a controllable electrodeposition method induced by a L-cysteamine (L-Cys) monolayer self-assembled on the surface of a gold electrode. The AuNFs/L-Cys/Au electrodes were characterized by field emission scanning electron microscopy (FE-SEM), cyclic voltammetry, and AC impedance spectroscopy methods. This obtained AuNFs/L-Cys/Au electrode exhibits excellent electrocatalytic activity towards the oxidation of dopamine (DA) due to the synergistic effect of AuNFs and a L-Cys monolayer. Differential pulse voltammetry (DPV) experiment results show that the oxidation peak of DA is separated from the oxidation peaks of ascorbic acid (AA) and uric acid (UA), which can be used to detect DA in the presence of AA and UA, and the results are satisfactory.     

Docosyltrimethylammonium chloride modified glassy carbon electrode for simultaneous determination of dopamine and ascorbic acid

A glassy carbon electrode (GCE) modified with docosyltrimethylammonium chloride (DCTMACl) is used for simultaneous determination of dopamine (DA) and ascorbic acid (AA) using differential pulse voltammetry (DPV) technique in 0.10 mol·L^?1 phosphate buffer solution of pH 5.0. The cationic surfactant DCTMACl modified film has a positive charge. DA exists as the positively charged species, whereas AA is the negatively charged one in the solution. Thus, at DCTMACl film-modified GCE, the oxidation peak potential of AA shifts toward less negative potential and the peak current of AA increases a little, while the oxidation peak potential of DA shifts toward more positive potential and peak current decreases greatly in comparison with that on bare electrode. The two anodic peaks are separated around 200 mV. Under optimal conditions, the catalytic peak currents obtained from DPV increase linearly with concentrations of DA and AA in the ranges of 1.0?×?10^?5 to 1.0?×?10^?3?mol·L^?1. This electrode has good reproducibility, high stability in its voltammetric response, and low detection limit (micromolar) for both AA and DA. The modified electrode has been applied to the determination of DA and AA in injection.     

Exploiting micellar environment for simultaneous electrochemical determination of ascorbic acid and dopamine

A simple and reliable method for simultaneous electrochemical determination of ascorbic acid (AA) and dopamine (DA) is presented in this work. It was based on the use of the cationic surfactant cetylpyridinium chloride (CPC) that enables the separation of the oxidation peaks potential of AA and DA. Cyclic voltammetry (CV) as well as pulse differential voltammetry (PDV) were used in order to verify the voltammetric behaviour in micellar media. In the cationic surfactant CPC, a remarkable electrostatic interaction is established with negatively charged AA, as a consequence, the oxidation peak potential shifted toward less positive potential and the peak current increased. On the other hand, the positively charged DA is repelled from the electrode surface and the oxidation peak potential shifts toward more positive potential in comparison to the bare electrode. Therefore, the common overlapped oxidation peaks of AA and DA can be circumventing by using CPC. Parameter that affects the E_pa and I_pa such as CPC concentration and pH were studied. Under optimised conditions, the method presented a linear response to AA and DA in the concentration range from 5 to 75 μmol L⁻¹ and 10 to 100 μmol L⁻¹, respectively. The proposed method was successfully applied to the simultaneous determination of AA and DA in dopamine hydrochloride injection (DHI) samples spiked with AA.     

Multilayer assembly of calf thymus DNA and poly(4-vinylpyridine) derivative bearing [Os(bpy)(2)Cl](2+): redox behavior within DNA film

Polyelectrolyte multilayer (PEM) films containing polycationic osmium (Os) bipyridyl (bpy) complex-attached poly(4-vinylpyridine) (PVP) derivative [Os(bpy)(2)Cl](2+)-PVP (Os-PVP) and polyanionic calf thymus DNA (CT-DNA) on the surface of gold (Au) electrodes were prepared using a layer-by-layer self-assembly method, and their redox properties were studied. Os complex shows different redox behavior with CT-DNA film in comparison with other PEM film which is composed of ordinary polymers. A cyclic voltammetric study suggested that the outermost polyanionic DNA layer does not hinder the redox reaction of Os complex within the Os-PVP/CT-DNA multilayer film, which may be helpful to understand the electron transfer mechanism with the DNA film. For all the Os-complex-containing PEM layers studied, a diffusion-free electron transfer from the Os complex moieties in these films to the electrode surface was observed. An electrocatalytic oxidation of ascorbic acid (AA) by this DNA-containing PEM film-covered electrode was also proposed.     

Direct electrochemiluminescence detection of oxidized DNA in ultrathin films containing [Os(bpy)2(PVP)10]2+

Direct electrochemiluminescence (ECL) involving oxidized DNA was demonstrated in ultrathin films of cationic polymer [Os(bpy)2(PVP)10]2+ [PVP = poly(vinyl pyridine)] assembled layer-by-layer with DNA or oligonucleotides. Electrochemically oxidized Os(II) sites generated ECL from films containing oxo-guanines on DNA formed by chemical oxidation using Fenton reagent. Films combining DNA, [Ru(bpy)2(PVP)10]2+, and [Os(bpy)2(PVP)10]2+ had Os(II) sites that produced ECL specific for oxidized DNA, and Ru(II) sites gave ECL from reaction with oxo-adenines, chemically damaged DNA, and possibly from cleaved DNA strands.     

Application of a gold electrode,modified by a self-assembled monolayer of 2-mercaptodecylhydroquinone,to the electroanalysis of hemoglobin

A gold electrode modified by a self-assembled monolayer of 2-mercaptodecylhydroquinone (H(2)Q(CH(2))(10)SH) was applied to investigate the electrochemical response of hemoglobin in aerated buffer solutions. Compared with a bare gold electrode, the monolayer of H(2)Q(CH(2))(10)SH could suppress the reduction wave of dissolved oxygen in the buffer while effectively promoting the rate of electron transfer between hemoglobin and the electrode. Thus, a convenient way for electroanalysis of hemoglobin in air was achieved at the H(2)Q(CH(2))(10)SH/Au electrode. A linear relationship existed between peak current and concentration of hemoglobin in the range 1 x 10(-7)-1 x 10(-6) mol L(-1).     

Electrocatalytic activity of three-dimensional monolayer of 3-mercaptopropionic acid assembled on gold nanoparticle arrays

3-Mercaptopropionic acid (MPA) was assembled on gold nanoparticle arrays to form three-dimensional monolayer. The electrochemical behavior of small biomolecules such as NADH, ascorbic acid (AA), uric acid (UA) and dopamine (DA) on the as-prepared three-dimensional monolayer was studied. The cyclic voltammetric results indicated that three-dimensional MPA monolayer promoted the electron transfer between NADH and electrode, which was similar to two-dimensional MPA monolayer assembled on planar gold electrode. However, to the electrooxidation of AA, although two-dimensional MPA monolayer exhibited a blocking effect, three-dimensional MPA monolayer showed an obvious promotion. The catalytic activity of three-dimensional MPA monolayer towards UA and DA was also observed, which was attributed to its three-dimensional structure that might effectively prevent the poison of the electrode surface by the oxidation products.     

Electrocatalytic response of dopamine at a dl-homocysteine self-assembled gold electrode

The gold electrode self-assembled with the homocysteine monolayer (Hcy/Au) is demonstrated to catalyze the electrochemical response of dopamine (DA) by cyclic voltammetry. A pair of well-defined redox waves was obtained and the calculated standard rate constant (k_s) is 2.1×10⁻² cm/s at the self-assembled electrode. The reduction peak of DA can be used to determine the concentration of DA in presence of ascorbic acid (AA) owing to the Hcy/Au also catalyzing the electrochemical oxidation of AA.     

在抗坏血酸存在下用L-赖氨酸修饰玻碳电极测定多巴胺

采用电化学氧化法制备了L-广赖氮酸单分子层修饰玻碳电极,研究了多巴胺（DA）和抗坏血酸（AA）在该电极上的电化学行为。结果表明,L-广赖氨酸单分子层修饰玻碳电极不仅能改善多巴胺和抗坏血酸的电化学行为,而且能将多巴胺和抗坏血酸二者在裸电极上的完全重叠的单氧化峰分开成为两个完全独立的氧化峰,循环伏安（CV）图上峰间距为507mV,差分脉冲伏安（DPV）图上峰间距为460mV,由此可实现在AA的共存下对样品中的DA进行选择性测定。     

The cyanoimido ligand as an oxo analogue. Novel approaches to the preparations of cyano(imino)-aza-phosphorus(V) and N-cyanoaziridine

The known Os(IV)-cyanoimido complexes, mer-Et4N[OsIV(bpy)(Cl)3(NalphaCNbeta)] (mer-[OsIV=N-CN]-) (bpy = 2,2'-bipyridine) and trans-[OsIV(tpy)(Cl)2(NalphaCNbeta)] (trans-[OsIV=N-CN]) (2,2':6',2' '-terpyridine), have formal electronic relationships with high oxidation state Ru and Os-oxo and -dioxo complexes. These include multiple bonding to the metal, the ability to undergo multiple electron transfer, and the availability of nonbonding electron pairs for donation. Thermodynamic, oxo-like behavior is observed for mer-[OsIV=N-CN]- in the pH-dependence of its Os(VI/V) to Os(III/II) redox couples in 1:1 (v/v) CH3CN:H2O. Oxo-like behavior is also observed in the reaction between mer-[OsVI(bpy)(Cl)3(NalphaCNbeta)]PF6 and benzyl alcohol to give mer-[OsIV(bpy)(Cl)3(NalphaCNbetaH2)]PF6 and benzaldehyde. The reaction is first order in each reactant with kbenzyl(CH3CN, 25.0 +/- 0.1 degrees C) = (8.6 +/- 0.2) x 102 M-1 s-1. Formal NCN degrees transfer, analogous to O-atom transfer, occurs in reactions with tertiary phosphine and hexenes. In CH3CN under N2, a rapid reaction occurs between trans-[OsIV=N-CN] and PPh3 (kPPh3(DMF, 25.0 +/- 0.1 degrees C) = 4.06 +/- 0.02 M-1 s-1) to form the nitrilic N-bound Os(II)-(N-cyano)iminophosphorano product, trans-[OsII(tpy)(Cl)2(NalphaCNbetaPPh3)] (trans-[OsII-NalphaC-Nbeta=PPh3]). It undergoes solvolysis at 45 degrees C after 24 h to give trans-[OsII(tpy)(Cl)2(NCCH3)] and (N-cyano)iminophosphorane (NalphaC-Nbeta=PPh3). The analogue to epoxidation, N-cyanoaziridination of cyclohexene and 1-hexene by mer-[OsIV=N-CN]- and trans-[OsIV=N-CN], occurs at Nbeta to give the Os(IV)-N-cyanoaziridino complexes, mer-Et4N[OsII(bpy)(Cl)3(NalphaCNbetaC6H10)] and trans-[OsII(tpy)(Cl)2(NalphaCNbetaC6H11)], respectively. Oxidation to mer-[OsV(bpy)(Cl)3(NalphaCNbeta)]- greatly accelerates N-cyanoaziridination of cyclohexene, which is followed by slow solvolysis to give mer-[OsIII(bpy)(Cl)3(NCCH3)] and N-cyanoaziridine (NC-NC6H10). The Os-(N-cyano)aziridino complexes are the first well-characterized examples of coordinated cyanoaziridines.     

Oxidation of ammonia in osmium polypyridyl complexes

The oxidations of cis- and trans-[OsIII(tpy)(Cl)2(NH3)](PF6), cis-[OsII(bpy)2(Cl)(NH3)](PF6), and [OsII(typ)(bpy)(NH3)](PF6)2 have been studied by cyclic voltammetry and by controlled-potential electrolysis. In acetonitrile or in acidic, aqueous solution, oxidation is metal-based and reversible, but as the pH is increased, oxidation and proton loss from coordinated ammonia occurs. cis- and trans-[OsIII(tpy)(Cl)2(NH3)](PF6) are oxidized by four electrons to give the corresponding OsVI nitrido complexes, [OSVI(typ)(Cl)2(N)]+. Oxidation of [Os(typ)(bpy)(NH3)](PF6)2 occurs by six electrons to give [Os(tpy)(bpy)(NO)](PF6)3. Oxidation of cis-[OsII(bpy)2(Cl)(NH3)](PF6) at pH 9.0 gives cis-[OsII(bpy)2(Cl)(NO)](PF6)2 and the mixed-valence form of the mu-N2 dimer [cis-[Os(bpy)2(Cl)2[mu-N2)](PF6)3. With NH4+ added to the electrolyte, cis-[OsII(bpy)2(Cl)(N2)](PF6) is a coproduct. The results of pH-dependent cyclic voltammetry measurements suggest OsIV as a common intermediate in the oxidation of coordinated ammonia. For cis- and trans-[OsIII(tpy)(Cl)2(NH3)]+, OsIV is a discernible intermediate. It undergoes further pH-dependent oxidation to [OsVI(tpy)(Cl)2(N)]+. For [OsII(tpy)(bpy)(NH3)]2+, oxidation to OsIV is followed by hydration at the nitrogen atom and further oxidation to nitrosyl. For cis-[OsII(bpy)2(Cl)-(NH3)]+, oxidation to OsIV is followed by N-N coupling and further oxidation to [cis-[Os(bpy)2(Cl)2(mu-N2)]3+. At pH 9, N-N coupling is competitive with capture of OsIV by OH- and further oxidation, yielding cis-[OsII(bpy)2(Cl)(NO)]2+.     

The remarkable reactivity of high oxidation state ruthenium and osmium polypyridyl complexes

There is a remarkable redox chemistry of higher oxidation state M(IV)-M(VI) polypyridyl complexes of Ru and Os. They are accessible by proton loss and formation of oxo or nitrido ligands, examples being cis-[RuIV(bpy)2(py)(O)]2+ (RuIV=O2+, bpy=2,2'-bipyridine, and py=pyridine) and trans-[OsVI(tpy)(Cl)2(N)]+ (tpy=2,2':6',2' '-terpyridine). Metal-oxo or metal-nitrido multiple bonding stabilizes the higher oxidation states and greatly influences reactivity. O-atom transfer, hydride transfer, epoxidation, C-H insertion, and proton-coupled electron-transfer mechanisms have been identified in the oxidation of organics by RuIV=O2+. The Ru-O multiple bond inhibits electron transfer and promotes complex mechanisms. Both O atoms can be used for O-atom transfer by trans-[RuVI(tpy)(O)2(S)]2+ (S=CH3CN or H2O). Four-electron, four-proton oxidation of cis,cis-[(bpy)2(H2O)RuIII-O-RuIII(H2O)(bpy)2]4+ occurs to give cis,cis-[(bpy)2(O)RuV-O-RuV(O)(bpy)2]4+ which rapidly evolves O2. Oxidation of NH3 in trans-[OsII(tpy)(Cl)2(NH3)] gives trans-[OsVI(tpy)(Cl)2(N)]+ through a series of one-electron intermediates. It and related nitrido complexes undergo formal N- transfer analogous to O-atom transfer by RuIV=O2+. With secondary amines, the products are the hydrazido complexes, cis- and trans-[OsV(L3)(Cl)2(NNR2)]+ (L3=tpy or tpm and NR2-=morpholide, piperidide, or diethylamide). Reactions with aryl thiols and secondary phosphines give the analogous adducts cis- and trans-[OsIV(tpy)(Cl)2(NS(H)(C6H4Me))]+ and fac-[OsIV(Tp)(Cl)2(NP(H)(Et2))]. In dry CH3CN, all have an extensive multiple oxidation state chemistry based on couples from Os(VI/V) to Os(III/II). In acidic solution, the OsIV adducts are protonated, e.g., trans-[OsIV(tpy)(Cl)2(N(H)N(CH2)4O)]+, and undergo proton-coupled electron transfer to quinone to give OsV, e.g., trans-[OsV(tpy)(Cl)2(NN(CH2)4O)]+ and hydroquinone. These reactions occur with giant H/D kinetic isotope effects of up to 421 based on O-H, N-H, S-H, or P-H bonds. Reaction with azide ion has provided the first example of the terminal N4(2-) ligand in mer-[OsIV(bpy)(Cl)3(NalphaNbetaNgammaNdelta)]-. With CN-, the adduct mer-[OsIV(bpy)(Cl)3(NCN)]- has an extensive, reversible redox chemistry and undergoes NCN(2-) transfer to PPh3 and olefins. Coordination to Os also promotes ligand-based reactivity. The sulfoximido complex trans-[OsIV(tpy)(Cl)2(NS(O)-p-C6H4Me)] undergoes loss of O2 with added acid and O-atom transfer to trans-stilbene and PPh3. There is a reversible two-electron/two-proton, ligand-based acetonitrilo/imino couple in cis-[OsIV(tpy)(NCCH3)(Cl)(p-NSC6H4Me)]+. It undergoes reversible reactions with aldehydes and ketones to give the corresponding alcohols.     

Electrochemical release of amine molecules from carbamate-based, electroactive self-assembled monolayers

In this paper, carbamate-based self-assembled monolayers (SAMs) of alkanethiolates on gold were suggested as a versatile platform for release of amine-bearing molecules in response to the electrical signal. The designed SAMs underwent the electrochemical oxidation on the gold surface with simultaneous release of the amine molecules. The synthesis of the thiol compounds was achieved by coupling isocyanate-containing compounds with hydroquinone. The electroactive thiol was mixed with 11-mercaptoundecanol [HS(CH(2))(11)OH] to form a mixed monolayer, and cyclic votammetry was used for the characterization of the release behaviors. The mixed SAMs showed a first oxidation peak at +540 mV (versus Ag/AgCl reference electrode), indicating the irreversible conversion from carbamate to hydroquinone groups with simultaneous release of the amine molecules. The analysis of ToF-SIMS further indicated that the electrochemical reaction on the gold surface successfully released amine molecules.     

Formation and reactivity of the Os(IV)-azidoimido complex, PPN[Os(IV)(bpy)(Cl)(3)(N(4))

Reactions between the Os(VI)-nitrido complexes, [OsVI(L2)(Cl)3(N)] (L2 = 2,2'-bipyridine (bpy) ([1]), 4,4'-dimethyl-2,2'-bipyridine (Me2bpy), 1,10-phenanthroline (phen), and 4,7-diphenyl-1,10-phenanthroline (Ph2phen)), and bis-(triphenylphosphoranylidene)ammonium azide (PPNN3) in dry CH3CN at 60 degrees C under N2 give the corresponding Os(IV)-azidoimido complexes, [OsIV(L2)(Cl)3(NN3)]- (L2 = bpy = [2]-, L2 = Me2bpy = [3]-, L2 = phen = [4]-, and L2 = Ph2phen = [5]-) as their PPN+ salts. The formulation of the N42- ligand has been substantiated by 15N-labeling, IR, and 15N NMR measurements. Hydroxylation of [2]- at Nalpha with O<--NMe3.3H2O occurs to give the Os(IV)-azidohydroxoamido complex, [OsIV(bpy)(Cl)3(N(OH)N3)] ([6]), which, when deprotonated, undergoes dinitrogen elimination to give the Os(II)-dinitrogen oxide complex, [OsII(bpy)(Cl)3(N2O)]- ([7]-). They are the first well-characterized examples of each kind of complex for Os.     

A bis-bipyridine osmium(II) complex with an N,S-chelating 2-aminoethanesulfinate: photoinduced conversion of an amine to an imine donor group by air oxidation

Treatment of [Os(bpy)(2)Cl(2)] (bpy = 2,2'-bipyridine) with 2-aminoethanethiolate was accompanied by air oxidation to give [Os(2-aminoethanesulfinato-N,S)(bpy)(2)](+) ([1](+)), which was further oxidized by air to be converted into [Os(2-iminoethanesulfinato-N,S)(bpy)(2)](+) ([2](+)) under photoirradiation. Complex [2](+) was reverted back to [1](+) by treatment with BH(4)(-).     

Multisite effects on intervalence charge transfer in a clusterlike trinuclear assembly containing ruthenium and osmium

The intervalence charge transfer (IVCT) properties of the mixed-valence forms of the diastereoisomers of the dinuclear [[Ru(bpy)2](mu-HAT)[M(bpy)2]]5+ (M = Ru or Os) complexes and the trinuclear heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8; HAT = 1,4,5,8,9,12-hexaazatriphenylene; bpy = 2,2'-bipyridine) species display a marked dependence on the nuclearity and extent of oxidation of the assemblies, while small differences are also observed for the diastereoisomers of the same complex in the dinuclear cases. The mixed-valence heterochiral [[Ru(bpy)2]2[Os(bpy)2](mu-HAT)]n+ (n = 7, 8) forms exhibit IVCT properties that are intermediate between those of the diastereoisomeric forms of the localized hetero-dinuclear complex [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and the borderline localized-to-delocalized homo-trinuclear complex [[Ru(bpy)2]3(mu-HAT)]n+ (n = 7, 8). The near-infrared (NIR) spectrum of the +7 mixed-valence species exhibits both interconfigurational (IC) and IVCT transitions which are quantitatively similar to those in [[Ru(bpy)2](mu-HAT)[Os(bpy)2]]5+ and are indicative of the localized mixed-valence formulation [[Ru(II)(bpy)2]2[Os(III)(bpy)2](mu-HAT)]7+. The +8 state exhibits a new band attributable to an IVCT transition in the near-infrared region.     

The Influence of Gold Nanoparticles on Simultaneous Determination of Uric Acid and Ascorbic Acid

A three-dimensional L-cysteine (L-cys) monolayer assembled on gold nanoparticles (GNP) providing simultaneous detection of uric acid (UA) and ascorbic acid (AA) was studied in this work. The cyclic voltammetry demonstrated that, at a bare glassy carbon electrode (GCE) or planar gold electrode, the mixture of UA and AA showed one overlapped oxidation peak; whereas when the electrode was modified with GNP, the oxidation peaks for UA and AA were separated. While a GNP modified electrode was further modified with L-cys monolayer (L-cys/GNP/GCE), namely, three-dimensional L-cys monolayer, a better separation for UA and AA response was obtained. Interestingly, the L-cys monolayer-modified planar gold electrode presented a block effect on the oxidation of AA, which was facilitated by the three-dimensional L-cys monolayer attributed to its distinct structure. The pH of solution presented a noticeable effect on the separation of UA and AA at GNP modified electrodes with or without L-cys monolayer. Wide concentration ranges from 2 × 10^?6?1 × 10^?3 M to UA and 2 × 10^?6?8 × 10^?4 M to AA could be obtained at L-cys/GNP/GCE.     

Application of 2-(3,4-dihydroxyphenyl)-1,3-dithialone self-assembled monolayer on gold electrode as a nanosensor for electrocatalytic determination of dopamine and uric acid

The electrooxidation of dopamine (DA), uric acid (UA) and their mixture on a gold electrode modified by a self-assembled monolayer of 2-(3,4-dihydroxyphenyl)-1,3-dithialone has been studied by cyclic voltammetry (CV), chronoamperometry and differential pulse voltammetry (DPV). CV was used to investigate the redox properties of the modified electrode at various scan rates and the apparent charge transfer rate constant (k(s)), and transfer coefficient (α) were calculated. The mediated oxidation of DA at the modified electrode under the optimum condition (pH = 7.0) in CV occurs at a potential about 220 mV less positive than that of the unmodified gold electrode. The values of electron transfer coefficients (α), catalytic rate constant (k) and diffusion coefficient (D) were calculated for DA, using electrochemical methods. DPV exhibited a linear dynamic range over the concentration range of 0.2-250.0 μM and a detection limit (3σ) of 0.07 μM for DA. The modified electrode was used for simultaneous determination of DA and UA by DPV. The results showed that the electrode is highly efficient for the catalytic electrooxidation of DA and UA, leading to a remarkable peak resolution (~350 mV) for two compounds. The electrode was used for the determination of DA in an injection sample.