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.     

Gold Electrode Modified with Self‐Assembled Monolayer of Cysteamine‐Functionalized MWCNT and Its Application in Simultaneous Determination of Dopamine and Uric Acid

The voltammetric behavior of dopamine (DA) and uric acid (UA) on a gold electrode modified with self‐assembled monolayer (SAM) of cysteamine (CA) conjugated with functionalized multiwalled carbon nanotubes (MWCNTs) was investigated. The film modifier of functionalized SAM was characterized by means of scanning electron microscopy (SEM) and also, electrochemical impedance spectroscopy (EIS) using para‐hydroquinone (PHQ) as a redox probe. For the binary mixture of DA and UA, the voltammetric signals of these two compounds can be well separated from each other, allowing simultaneous determination of DA and UA. The effect of various experimental parameters on the voltammetric responses of DA and UA was investigated. The detection limit in differential pulse voltammetric determinations was obtained as 0.02 µM and 0.1 µM for DA and UA, respectively. The prepared modified electrode indicated a stable behavior and the presence of surface COOH groups of the functionalized MWCNT avoided the passivation of the electrode surface during the electrode processes. The proposed method was successfully applied for the determination of DA and UA in urine samples with satisfactory results. The response of the gold electrode modified with MWCNT‐functionalized SAM method toward DA, UA, and ascorbic acid (AA) oxidation was compared with the response of the modified electrode prepared by the direct casting of MWCNT.     

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.     

Fabrication of layer-by-layer modified multilayer films containing choline and gold nanoparticles and its sensing application for electrochemical determination of dopamine and uric acid

A novel electrochemical sensor has been constructed by use of a glassy carbon electrode (GCE) coated with a gold nanoparticle/choline (GNP/Ch). Electrochemical impedance spectroscopy (EIS), field emission scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the properties of this modified electrode. It was demonstrated that choline was covalently bounded on the surface of glassy carbon electrode, and deposited gold nanoparticles with average size of about 100 nm uniformly distributed on the surface of Ch. Moreover, the modified electrode exhibits strong electrochemical catalytic activity toward the oxidation of dopamine (DA), ascorbic acid (AA) and uric acid (UA) with obviously reduction of overpotentials. For the ternary mixture containing DA, AA and UA, these three compounds can be well separated from each other, allowing simultaneously determination of DA and UA under coexistence of AA. The proposed method can be applied to detect DA and UA in real samples with satisfactory results.     

A Novel Functionalized Single‐Wall Carbon Nanotube Modified Electrode and Its Application in Determination of Dopamine and Uric Acid in the Presence of High Concentrations of Ascorbic Acid

Multilayer films of negatively charged single‐wall carbon nanotubes (SWCNTs) and positively charged cetylpyridinium bromide (CPB) have been deposited on a glassy carbon electrode (GCE) using layer‐by‐layer (LBL) technique. The assembled multilayer films have been investigated by scanning electron microscopy (SEM), electrochemical impedance spectroscopy (EIS), and quartz crystal microbalance (QCM) measurements. The voltammetric signal of dopamine (DA), uric acid (UA), and ascorbic acid (AA) could be observed well‐separated with the assembled SWCNTs/CPB multilayer films in pH 7.0 PBS. The oxidation peak potentials of DA, UA, and AA are centered at about 169 mV, 292 mV and ?10 mV on differential pulse voltammograms (DPVs), respectively. The peak‐to‐peak potential separation was 123 mV, 179 mV, and 302 mV for DA‐UA, DA‐AA, and UA‐AA in DPVs, respectively. This permits the simultaneous detection of DA and UA in the presence of AA.     

Electrochemical reduced graphene oxide-poly(eriochrome black T)/gold nanoparticles modified glassy carbon electrode for simultaneous determination of ascorbic acid,dopamine and uric acid

This work reports on the preparation of electrochemically reduced graphene oxide (ERGO)-poly(eriochrome black T) (pEBT) assembled gold nanoparticles for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) in PBS pH 6.0. Characterisations of the composite were carried out by scanning electron microscopy (SEM), Fourier transform infrared (FTIR), electrochemical impedance spectroscopy (EIS) and cyclic voltammetry. As a result of the synergistic effect, the modified glassy carbon electrode (GCE) possessed an efficient electrochemical catalytic activity with a high selectivity and sensitivity in oxidising AA-DA and DA-UA as compared to the bare GCE. The peak separations of AA and DA, DA and UA were 183 mV and 150 mV, respectively. The linear response ranges for AA, DA and UA were 10–900 μM, 0.5–20 μM and 2–70 μM with detection limits of 0.53 μM, 0.009 μM and 0.046 μM (S/N = 3), respectively. The sensitivity of ERGO-pEBT/AuNPs was measured as 0.003 µA/μM, 0.164 µA/μM and 0.034 µA/μM for AA, DA, and UA, respectively. The modified electrochemical sensor was used in the determination of AA, DA, and UA in vitamin C tablets and urine sample with good recovery.     

Preparation and characterization of PtAu hybrid film modified electrodes and their use in simultaneous determination of dopamine, ascorbic acid and uric acid

A novel biosensor was fabricated by electrochemical deposition of platinum and gold nanoparticles (nanoAu) with l-Cysteine on glassy carbon electrode. It was found that the nanoAu particle size distribution range was (50-80 nm), and the platinum particle size range was (200-300 nm). The hybrid film could be produced on gold and transparent indium tin oxide electrodes for different kind of studies such as electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) and electrochemical studies. The PtAu hybrid film was applied to the electro catalytic oxidation of dopamine (DA), ascorbic acid (AA) and uric acid (UA) at pH 4.0 using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The modified electrode was quite effective not only to detect DA, AA and UA individually but also in simultaneous determination of these species in a mixture. The overlapping anodic peaks of DA, AA and UA were resolved into three well-defined voltammetric peaks in CV and DPV. The catalytic peak currents obtained from CV and DPV increased linearly with concentration. The relative standard deviation (% R.S.D., n = 10) for AA, DA and UA were less than 2.0% and DA, AA and UA can be determined in the ranges of 0.103-1.65, 0.024-0.384 and 0.021-0.336 mM, respectively. In addition, the modified electrode also shows good sensitivity, and stability. Satisfactory results were achieved for the determination of DA, AA and UA in dopamine injection solution, vitamin C tablets and human urine samples.     

Electrochemical behavior of dopamine and ascorbic acid at osmium(II) complex cationic monolayer modified gold electrodes.

The electrooxidation of dopamine (DA) and ascorbic acid (AA) was studied using gold electrodes modified by a cationic self-assembled monolayer of [Os(bpy)2(bpy-(CH2),3SH)]2+ by cyclic voltammetry. At an [Os(bpy)2(bpy-(CH2)13SH)]2+/Au electrode, the oxidation peak of DA shifted to a much more positive potential as compared with that of a bare gold electrode, while the oxidation peak potential of AA showed a slightly negative shift due to their different electrostatic interactions with the cationic monolayer. Thus, a sufficient potential difference was achieved for distinguishing the electrochemical responses of DA and AA. However, when CH3(CH2)11SH was mixed into the cationic monolayer, the enhanced packing of the mixed monolayer blocked the access of DA or AA to the electrode, resulting in further positive shifts for both oxidation-peak potentials.     

Square wave voltammetric sensing of uric acid using the self-assembly of mercaptobenzimidazole

The self-assembled monolayer of a heterocyclic thiol, mercaptobenzimidazole (MBI) on gold (Au) electrode is successfully utilized for the voltammetric sensing of uric acid (UA). The self-assembly of MBI separates the voltammetric signal of UA from the interfering ascorbate (AA). Selective detection of UA in the presence of a large excess of AA or the simultaneous detection of UA and AA is achieved at the MBI monolayer-modified electrode. This electrode can detect as low as 1 microM of UA in the presence of 100-fold excess of AA with excellent reproducibility. The practical utility of the electrode is demonstrated by measuring the concentration of UA in human serum.     

Selective Electrochemical Determination of Dopamine with Molecularly Imprinted Poly(Carbazole‐co‐Aniline) Electrode Decorated with Gold Nanoparticles

Dopamine (DA) is a significant neurotransmitter in the central nervous system, coexisting with uric acid (UA) and ascorbic acid (AA). UA and AA are easily oxidizable compounds having potentials close to that of DA for electrochemical analysis, resulting in overlapping voltammetric response. In this work, a novel molecularly imprinted (MI) electrochemical sensor was proposed for selective determination of DA (in the presence of up to 80‐fold excess of UA and AA), relying on gold nanoparticles (Au_nano)‐decorated glassy carbon (GC) electrode coated with poly(carbazole (Cz)‐co‐aniline (ANI)) copolymer film incorporating DA as template (DA imprinted‐GC/P(Cz‐co‐ANI)‐Au_nano electrode, DA‐MIP‐Au_nano electrode). The DA recognizing sensor electrode showed great electroactivity for analyte oxidation in 0.2 mol L^?1 pH 7 phosphate buffer. Square wave voltammetry (SWV) was performed within 10^?4–10^?5 mol L^?1 of DA, of which the oxidation peak potential was observed at 0.16 V. The limit of detection (LOD) and limit of quantification (LOQ) were 2.0×10^?6 and 6.7×10^?6 mol L^?1, respectively. Binary and ternary synthetic mixtures of DA‐UA, DA‐AA and DA‐UA‐AA yielded excellent recoveries for DA. Additionally, DA was quantitatively recovered from a real sample of bovine serum spiked with DA, and determined in concentrated dopamine injection solution. The developed SWV method was statistically validated against a literature potentiodynamic method using a caffeic acid modified‐GC electrode.     

Microstructures by Selective Desorption of Self‐Assembled Monolayer from Polycrystalline Gold Electrodes

Potential‐controlled partial reductive desorption of a self‐assembled monolayer of mercaptopropionic acid (MPA) formed on polycrystalline gold electrodes is used to expose subdomains of bare gold to the electrolyte solution. Two sets of cathodic waves are observed in the reduction scan with MPA self‐assembled on a polycrystalline gold electrode. The origin of the two waves is ambiguous but there are indications that the waves are correlated with reductive desorption from the (111) and then simultaneously (100) and (110) index faces of a polycrystalline gold electrode. Consecutive reduction scans with reversing the potential direction after the first peak (but before the onset of the second wave) results in disappearance of the first wave. The exposed domains are then blocked by an assembling process of longer chain alkanethiols to create a mixed self‐assembled monolayer on polycrystalline gold electrodes. Desorption of the remaining MPA creates a partially blocked electrode and the blocking behavior towards hexacyanoferrate(III) is analyzed using the theory of partially blocked electrodes and indicates an array of interacting centers. The approach of partial reductive desorption may be exploited for use in biosensing applications where the exposed gold domains could be used for anchoring of DNA probes.     

Poly(4-amino-1-1'-azobenzene-3, 4'-disulfonic acid) coated electrode for selective detection of dopamine from its interferences

Here, we described a new method for electrochemically selective detection of dopamine (DA). In this report, for the first time, electrochemical polymerization of 4-amino-1-1'-azobenzene-3,4'-disulfonic acid (acid yellow 9 dye (AY)) was carried out onto the surface of glassy carbon (GC) electrode and indium tin oxide coated electrode (ITO) from acidic solution containing AY monomers. A polymerized film of acid yellow on the surface of a glassy carbon electrode was characterized by cyclic voltammetry (CV). The redox response of the poly(AY) film on the GC electrode showed a couple of redox peak in 0.1M sulfuric acid solution and the pH dependent peak potential was -58mV/pH which was close to the Nernst behavior. The poly(AY) film-coated GC electrode (GC/PAY) exhibited excellent electrocatalytic activity towards the oxidations of dopamine (DA) in 0.1M phosphate buffer solution (PBS, pH 7.0) and increased the anodic peak current three time higher than bare GC electrode. GC/PAY did not reduce the considerable overpotential for oxidation of DA when compare to bare GC electrode. However, in contrast to other polymer modified electrode, due to the strong negatively charged back bone of poly(AY) highly repelled the important interference of DA, such as ascorbic acid (AA), uric acid (UA) and reduced form of nicotinamide adenine dinucleotide (NADH) in 0.1M PBS (pH 7.0) and did not showed any response for oxidation of these interferences. This behavior makes the GC/PAY for selective detection of DA in the presence of higher concentrations AA, UA and NADH. Using differential pulse voltammetry the calibration curves for DA were obtained over the range of 1-100muM with good selectivity and sensitivity. The proposed method provides a simple method for selective detection of DA from its interferences.     

Gold nanoparticles on a thiol-functionalized silica network for ascorbic acid electrochemical detection in presence of dopamine and uric acid

A simple preparation methodology able to stabilize gold nanoparticles and to obtain an electrode which detects ascorbic acid, uric acid, and dopamine by different techniques is presented. A 3-mercaptopropyl-functionalized silica network was synthesized using the sol–gel method. Gold nanoparticles (nAu) were immobilized on the material at synthesis by adding a sol of these previously prepared particles to the reaction mixture. The electrochemical behavior of the SiO₂/MPTS/Au carbon paste electrode was studied using cyclic voltammetry in the presence of a hexacyanoferrate probe molecule. The presence of nAu in the functionalized silica network changes the electrochemical characteristics of the material, favoring the electron transfer process of this complex ion. The SiO₂/MPTS/Au electrode was proven to be an efficient tool in the simultaneous determination of ascorbic acid (H₂AA), dopamine (DA), and uric acid (UA) using square wave voltammetry techniques. With the nAu on the electrode, an increase in the peak current related to the redox process of the H₂AA, DA, and UA was observed. The separations of the anodic peak potentials between DA/H₂AA and UA/H₂AA were 310 and 442?mV, respectively. The results obtained show that the SiO₂/MPTS/Au electrode can be used in the simultaneous determination of H₂AA, DA, and UA.     

Detection of Uric Acid in the Presence of Dopamine and High Concentration of Ascorbic Acid Using PDDA Modified Graphite Electrode

The properties of graphite electrode (Gr) modified with poly(diallyl dimethyl ammonium chloride) (PDDA) for the detection of uric acid (UA) in the presence of dopamine (DA) and high concentration of ascorbic acid (AA) have been investigated by cyclic voltammetry, differential pulse voltammetry and chronoamperometry. The polymer modified graphite electrode was prepared by a very simple method just by immersing the graphite electrode in PDDA solution for 20 minutes. The PDDA/Gr modified electrode displayed excellent electrocatalytic activity towards the oxidation of UA, DA and AA compared to that at the bare graphite electrode. The electrochemical oxidation signals of UA, DA and AA are well resolved into three distinct peaks with peak potential separations of 220 mV, 168 mV and 387 mV between AA‐DA, DA‐UA and AA‐UA respectively in cyclic voltammetry studies and the corresponding peak potential separations are 230 mV, 130 mV and 354 mV respectively in differential pulse voltammetry. The lowest detection limits obtained for UA, DA and AA were 1×10^?7 M, 2×10^?7 M and 800×10^?9 M respectively. The PDDA/Gr electrode efficiently eliminated the interference of DA and a high concentration of AA in the determination of UA with good selectivity, sensitivity and reproducibility. The modified electrode was also successfully applied for simultaneous determination of UA, DA and AA in their ternary mixture.     

柠檬酸修饰电极对抗坏血酸、多巴胺和尿酸的同时检测

研究柠檬酸(CA)修饰玻碳电极(CA/GC)在抗坏血酸(AA)、多巴胺(DA)和尿酸(UA)混合体系中的循环伏安(CV)行为.结果表明,AA、DA和UA在CA/GC电极上氧化峰电流增大,且三者氧化峰电位明显分离(ΔEp(DA,AA)=170 mV,ΔEp(DA,UA)=130 mV,ΔEp(AA,UA)=300 mV).据此,可同时检测AA、DA和UA.在优化的实验条件下,AA、DA和UA的氧化峰电流与其浓度分别在2.0×10-6～1.5×10-3mol.L-1,6.0×10-7～1.0×10-3mol.L-1和6.0×10-7～1.0×10-3mol.L-1范围内呈线性关系.该电极重现性好,可用于盐酸多巴胺针剂DA、VC片剂AA及人体尿液UA的测定.     

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.     

Nation-离子液体-碳纳米管复合膜修饰电极的制备及用于抗坏血酸、多巴胺及尿酸的同时测定

制备了一种新颖的Nation-离子液体一多壁碳纳米管复合膜修饰电极,并研究了抗坏血酸（AA）、多巴胺（DA）和尿酸（uA）在该修饰电极上的电化学行为．该修饰电极结合了多壁碳纳米管良好的导电性、离子液体优良的催化性能及Nation的高选择性等优点,对AA、DA和UA的氧化具有很好的催化和分离效果,实现了AA、DA和UA的同时测定．在三者共存体系中,AA和DA、DA和UA的氧化峰电位差分别为148和167mV．对AA、DA和UA的同时检测,线性范围分别为5-3200、1～1100和1-300gmol／L,检出限分别为1．66、0．33和0．33gmol／L．该修饰电极选择性好、稳定性高、重现性好,有望用于实际样品中AA、DA和UA的同时检测．     

Polystyrene sulphonate wrapped multiwalled carbon nanotubes modified graphite electrode for simultaneous determination of ascorbic acid, dopamine and uric acid

Graphite electrode is modified by casting multi-walled carbon nanotubes (MWCNTs) wrapped with polystyrene sulphonate (PSS) onto the surface of the bare graphite electrode. The modified electrode was characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The behavior of the modified electrode towards the oxidation of ascorbic acid (AA), dopamine (DA) and uric acid (UA) has been determined by cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometry (CA). The modified electrode showed better electrocatalytic activity towards AA, DA and UA compared to bare graphite electrode. The electrochemical oxidation signals of AA, DA and UA are well separated into three distinct peaks with peak potential difference of 222, 128 and 350 mV between AA-DA, DA-UA and AA-UA respectively in CV studies and corresponding peak potential separation in DPV are 228, 120 and 348 mV. This modified electrode was successfully used for simultaneous determination of AA, DA and UA in ternary mixture.     

Electropolymerized film of functionalized thiadiazole on glassy carbon electrode for the simultaneous determination of ascorbic acid, dopamine and uric acid

The present study reports the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) in 0.20 M phosphate buffer solution (pH 5.0) using electropolymerized ultrathin film of 5-amino-2-mercapto-1,3,4-thiadiazole (AMT) on glassy carbon (GC) electrode. The bare GC electrode does not separate the voltammetric signals of AA, DA and UA. However, electropolymerized AMT (p-AMT) modified GC electrode not only resolved the voltammetric signals of AA, DA and UA but also dramatically enhanced their oxidation peak currents when compared to bare GC electrode. The enhanced oxidation currents for AA, DA and UA at p-AMT modified electrode are due to the electrostatic interactions between them and the polymer film. Using amperometric method, we achieved the lowest detection of 75 nM AA, 40 nM DA and 60 nM UA at p-AMT modified electrode. The amperometric current was linearly increased from 200 nM to 0.80 mM for each AA, DA and UA and the lowest detection limit was found to be 0.92, 0.07 and 0.57 nM, respectively (S/N = 3). The practical application of the modified electrode was demonstrated by the determination of DA in dopamine hydrochloride injection.     

Simultaneous Determination of Dopamine and Ascorbic Acid at an In‐Site Functionalized Self‐Assembled Monolayer on Gold Electrode

The in‐site functionalization of 4‐aminothiophenol (4‐ATP) self‐assembled monolayer on gold electrode at physiological pH yields a redox active monolayer of 4′‐mercapto‐N‐phenylquinone diimine (MNPD). The functionalized electrode exhibits excellent electrocatalytic responses towards dopamine (DA) and ascorbic acid (AA), reducing the overpotentials by about 0.22 V and 0.34 V, respectively, with greatly enhanced current responses. Due to its different catalytic activities toward DA and AA, the modified electrode resolves the overlapping voltammetric responses of DA and AA into two well‐defined voltammetric peaks by differential pulse voltammetry (DPV), which can be used for the simultaneous determination of these species in a mixture. The catalytic peak current obtained from DPV was linearly related to DA and AA concentration in the ranges of 5.0×10^?6?1.25×10^?4 M and 8.0×10^?6?1.3×10^?4 M with correlation coefficient of 0.999 and 0.998, respectively. The detective limits (3σ) for DA and AA were found to be 1.2×10^?6 M and 2.4×10^?6 M, respectively. The modified electrode shows good sensitivity, selectivity and stability, and has been applied to the determination of DA and AA simultaneously in samples with satisfactory results.