Oxidation Of Uric Acid Research Articles

The doping level of boron-doped diamond electrodes affects the voltammetric sensing of uric acid

In this work, the electrochemical oxidation and subsequent determination of uric acid was investigated using boron-doped diamond electrodes with various B/C ratios (0–2000 ppm).

Interfacing Electrochemically Reduced Graphene Oxide with Poly(erichrome black T) for Simultaneous Determination of Epinephrine, Uric Acid and Folic Acid

Graphene is well known for a variety of sensing applications due to its unique charge transport properties and ultra-thinness. Here we report, interfacing of electrochemically reduced graphene oxide (ERGO) with the polymer of erichrome black-T (PEBT) as a sensing platform for the simultaneous voltammetric determination of epinephrine (EP), uric acid (UA) and folic acid (FA). The reduction of graphene oxide (GO) and the polymerization of EBT was enabled using cyclic voltammetry (CV). Electrochemical impedance spectroscopy (EIS) and CV have been probed to study the electron transfer kinetics of the modified electrodes. An enhanced sensitivity towards the EP, UA and FA electrochemical oxidation has been observed after the modification of GCE with PEBT/ERGO. The sensor is able to detect simultaneously EP, UA and FA in their mixture at well-defined peak potential resolution. The proposed method allowed for the determination of EP, UA and FA in the presence of other interfering species with the limit of detection of 0.4 μM, 1 μM and 4.0 μM respectively. The fabricated PEBT/ERGO/GCE successfully used for the simultaneous determination of the analytes in blood sera samples with acceptable recovery rates.

Allantoin as an independent marker associated with carotid intima-media thickness in subclinical atherosclerosis.

Allantoin is the main product of uric acid oxidation and was found to be augmented in atherosclerotic plaque in human autopsy and in animal models of atherosclerosis. Uric acid is abundant in human plasma and is prone to oxidation in inflammatory conditions such as atherosclerosis. In this study, we found a significant increase in plasma uric acid (P=0.002) and allantoin (P=0.025) in participants of the Brazilian Longitudinal Study of Adult Health (ELSA-Brasil) that presented common carotid intima-media thickness (c-IMT) within the 75th percentile (c-IMT≥P75). Multiple linear regression showed an association of c-IMT with uric acid (β=0.0004, P=0.014) and allantoin (β=0.018, P=0.008). This association was independent of age, the traditional risk factor LDL/HDL ratio, and non-traditional risk factors: pulse pressure, neck circumference, and the inflammatory marker myeloperoxidase. The independent and strong association of allantoin with c-IMT shows that it might be a useful marker, along with other traditional risk factors, to evaluate an early stage of atherosclerosis.

Uricase based fluorometric determination of uric acid based on the use of graphene quantum dot@silver core-shell nanocomposites.

The authors describe a fluorescent "turn-on" assay for detection of uric acid (UA) based on the use of graphene quantum dots coated with a shell of silver (GQD@Ag). The fluorescence of the GQDs is quenched by the silver shell. However, if the silver shell was removed via etching with H2O2 (which is produced by uricase catalyzed oxidation of UA), the fluorescence of the GQDs is restored. The resulting increase in fluorescence at 466nm depends directly on the concentration of H2O2, which, in turn, depends on the concentration of UA. The method allows UA to be quantitated with a 2μM detection limit. It was applied to the analysis of human urine samples and exhibited satisfactory results. The method is cost-effective, sensitive and selective for UA. In our perception, it provides a useful tool in clinical analysis and may be extended to other assays based on the use of oxidases. Graphical abstract Schematic of the reduction of Ag(I) and the growth of a silver shell on the surface of graphene quantum dots (GQDs) to form a GQD@Ag nanocomposite whose fluorescence is quenched. Uricase catalyzes the oxidation of uric acid (UA) to produce allantoin and H2O2 which etchesthe silver shell. This results in the release of GQDs and increased fluorescence, allowing quantitative analysis of UA.

Improved photodynamic efficiency for methylene blue from silica-methylene blue@tannic acid-Fe(III) ions complexes in aqueous solutions

To avoid multidrug resistance and tumour recurrence, photodynamic therapy (PDT) was emerging as an alternative therapy and its efficiency was related to photosensitizer (PS) efficiency, oxygen concentration and light characteristic. Methylene blue (MB) molecules as PSs were loaded in silica (silica-MB) and followed by encapsulation by coordination complexes of tannic acid (TA) and Fe(III) ions. In comparison with those of silica-MB, decreased condensation of Si-O-Si, shifted infrared absorbance frequencies of chemical bands, delayed thermal degradation and modulated release behavior of MB were observed for silica-MB@TA with a core–shell structure. Although MB dimers were dominantly released from silica-MB, release of MB monomers from silica-MB@TA was significantly promoted, which was described by the Higuchi model. The promotion of release of MB monomers from silica-MB@TA indicated the well control of aggregate states of MB by the encapsulation of TA and Fe(III) ions complexes. Through monitoring the oxidation of uric acid, generation efficiency of singlet oxygen (1O2) by MB released from silica-MB@TA was fairly higher than that from silica-MB. A facile method to encapsulate silica-MB with complexes of TA and Fe(III) ions was herein demonstrated to raise the generation efficiency of singlet oxygen.

Development of Electrochemical Sensor for Uric Acid Detection in Intestinal Closed Loop

Uric acid is a ulitimate metabolite of purine nucleotides in humans, because the uricase had been lost completely. So, the serum uric acid level is higher than other mammalians. The advantage of the higher serum uric acid levels has been suggested to be antioxidant activity of uric acid. On the other hand, hyperuricemia is an abnormally high level of uric acid in the serum that leads to uric acid precipitation in tissues and the main risk factors for the onset of gout, hypertension, chronic kidney disease, atherosclerosis, diabetes and so on. There are two important physiological mechanisms caused hyperuricemia: (a) increase production of the uric acid from dietary and endogenous substrates that raise purine levels and (b) reduced renal and extra-renal excretion of uric acid. It is commonly accepted that two thirds of the urate is excreted from the kidney into urine. The remaining one third is excreted via extra-renal excretion, mainly intestine. Recently, importance of the uric acid excretion from intestine has been gathering attention because the considerable inpact of urate transporter function in the intestine on the serum uric acid level has been revealed gradually. A novel concept in hyperuricemia was proposed as “extra renal underexcretion type”.[1] Intestinal closed loop method is one of the basic techniques to investigate the excretion of uric acid via trasporters in intestin. However, it needs troublesome procedure and time consuming process. If it is possible to detect the concentration of excreted uric acid with electrochemical sensor, dynamic analysis of urate excretion via the urate transporters will easily be available. In this study, we developed an electrode to detect uric acid sensitively. Gold beads electrode was prepared according to the literature.[2] 6-Ferrocenyl-1-hexanethiol which works as a mediator for the uric acid oxidation was used to make self -assenbled monolayers (SAMs) on gold surface. 6-Hydroxyl-1-hexananethiol, 5-Carboxyl-1-pentanethiol and 6-Amino-1-hexanethiol were also used to construct mixed SAMs. Gold beads electrodes were soaked in the ethanol solution containing these alkanethiols. The SAM-coated electrode was used as working electrode (C.E: Pt wire, R.E.: Ag/AgCl). Uric acid dissolved in phosphate buffer (pH 7.4) was added to the measurement system. Cyclic voltammograms were measeured in buffer by addition of uric acid (-0.2 to +0.5 V). Oxdation current based on the uric acid oxidation was observed with 6-Ferrocenyl-1-hexanethiol modified electrode at around +0.45 V. The oxidation current was increased with increasing of uric acid concentration. When the mixed SAMs-coated electrodes were examined, similar responses were observed regardress the component of SAMs. However, no response was observed from the SAM-coated electrode without 6-Ferrocenyl-1-hexanethiol. Responsible change was observed at oxidation current by 0.1 mg/mL change of uric acid. Furthermore, ammperometric measurement showed the step-like current change according to the added uric acid concentration.

Electrocatalytic Detection of Uric Acid on Nitrogen‐Doped Graphene Modified Electrode and Its Application

Nitrogen‐doped graphene (NG) was used to modify a glassy carbon electrode (GCE), which was applied to the voltammetric detection of uric acid (UA). Because of the presence of NG on the GCE surface, the modified electrode showed excellent electrocatalytic ability for UA oxidation with increase of the peak current and decrease of the peak potential. Compared to that of the traditional GCE, the overpotential of the oxidation peak decreased by 50 mV. Electrochemical performance of UA was studied on NG/GCE in detail, which showed a quasi‐reversible, diffusional‐controlled process. Electrochemical parameters including the charge transfer coefficient (α), the number of electron transfer (n), and the diffusional coefficient (D) were calculated. At the optimal conditions, the oxidation peak current was linear, with UA concentration from 0.1 to 10.0 μmol/L as revealed by differential pulse voltammetry, and the detection limit was estimated as 0.042 μmol/L (3σ). The UA content in the urine sample of a healthy volunteer was determined by the proposed method with a recovery of 98.7–102.3%.

Electrodeposited nanoporous ruthenium oxide for simultaneous quantification of ascorbic acid and uric acid using chronoamperometry at two different potentials

Nanoporous ruthenium dioxide (RuO2) was simply prepared by electrodeposition with potential cycling in RuCl3 solution in the presence of reverse micelles of Triton X-100. The electrodeposited RuO2 on Au substrate electrode showed high electroactivity for the oxidations of L-ascorbic acid (AA) and uric acid (UA), while AA oxidation started from a less positive potential compared to UA oxidation. Simultaneous quantitative analysis of AA and UA was successfully carried out via chronoamperometric measurements at two different applied potentials using RuO2 deposited electrode. Due to the oxidation potential difference between AA and UA at RuO2 electrode, only AA was oxidized at an applied potential of 0.20V vs. SCE and both AA and UA were oxidized at 0.32V vs. SCE. AA concentration was determined from the current measured at 0.20V; and then UA concentration was estimated from the current measured at 0.32V using the predetermined AA concentration. Electrodeposited nanoporous RuO2 electrode exhibited current responses linearly proportional to AA and UA concentrations in a wide concentration range (0–1mM) and fast response time (≤1s). The current sensitivities were 342.8±26.3μAmM−1cm−2 for AA at 0.20V; 377.8±18.4μAmM−1cm−2 for AA at 0.32V; and 344.2±11.6μAmM−1cm−2 for UA at 0.32V. In addition, RuO2 electrode showed a high selectivity over interferents such as 4-acetamidophenol (AP), dopamine (DA), and glucose. Reasonable stability and repeatability of RuO2 electrode were also confirmed. Selective quantification of AA and UA in an arbitrary mixture and a human urine sample demonstrated that RuO2 electrode has practical utility in real sample analysis with satisfactory results.

3DGH-Fc based electrochemical sensor for the simultaneous determination of ascorbic acid, dopamine and uric acid

In this work, we have been successfully developed a new electrochemical sensor for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) using ferrocene hybrid (Fc)/three dimensional graphene hydrogel (3DGH) nanocomposite. The composite materials were characterized using different techniques, such as scanning electron microscopy (SEM), atomic force microscopy (AFM), ultraviolet-visible spectrometer (UV–vis). The electrochemical detection methods using for the individual and simultaneous detection of AA, DA and UA were cyclic voltammetry (CV), differential pulse voltammetry (DPV) and amperometric (i–t). Due to the excellent properties and unique structure of the nanocomposite, this sensor exhibited sharp peaks for the oxidation of AA, DA and UA as compared with the bare glassy carbon electrode (GCE). The linear responses of AA, DA and UA were observed from 20 to 450μM, 10–180μM and 8–400μM with detection limits (S/N=3) of 0.183μM (AA), 0.042μM (DA) and 0.067μM (UA), respectively. In addition, the developed 3DGH-Fc/GCE electrode showed well anti-interference ability, great stability and excellent reproducibility. Moreover, the analytical application of this electrochemical sensor was successfully conducted for the determination of these species in human serum samples. These features implicate the potential applicability of the developed 3DGH-Fc/GCE biosensor in the development of is promising for the simultaneous determination of DA, AA and UA.

Development of a PrGO-Modified Electrode for Uric Acid Determination in the Presence of Ascorbic Acid by an Electrochemical Technique.

An attractive electrochemical sensor of poly(3,4-ethylenedioxythiophene)/reduced graphene oxide electrode (PrGO) was developed for an electrochemical technique for uric acid (UA) detection in the presence of ascorbic acid (AA). PrGO composite film showed an improved electrocatalytic activity towards UA oxidation in pH 6.0 (0.1 M PBS). The PrGO composite exhibited a high current signal and low charge transfer resistance (Rct) compared to a reduced graphene oxide (rGO) electrode or a bare glassy carbon electrode (GCE). The limit of detection and sensitivity of PrGO for the detection of UA are 0.19 μM (S/N = 3) and 0.01 μA/μM, respectively, in the range of 1–300 μM of UA.

A novel low-powered uric acid biosensor based on arrayed p-n junction heterostructures of ZnO thin film and CuO microclusters

An array of p–n junction heterostructures is fabricated by loading the surface of n-type semiconducting ZnO thin film with p-type CuO microclusters (μCs) for realization of uric acid biosensor without any external mediator. Uricase is immobilized onto the prepared CuO/ZnO arrayed heterostructure in order to prepare the sensing electrode. The electrochemical performance of the prepared p-n junction based biosensor is investigated by means of cyclic voltammetry. A stable and well-defined peak corresponding to oxidation of uric acid is observed at a very low potential of 0.03V in a mediator free solution. The largely decreased sensing potential may be attributed to the substantial enhancement of surface reaction kinetics in the formed p-n junction heterostructure due to built-in electric field which facilitate charge transport. A linear sensing response is observed over a range of 0.05–1.0mM uric acid concentration with a high sensitivity of 1.74mA/mM. Moreover, the prepared biosensor has good affinity towards uric acid (Km∼ 0.05mM) and possesses long shelf life ( >20 weeks). The low operating potential (0.03V) of CuO/ZnO arrayed p-n heterojunction reduces the energy consumption of developed biosensor and efficiently minimizes interference from common oxidizable interferents, thereby, making the biosensor highly selective. Thus, CuO μCs/ZnO thin film based p-n junction heterostructure with internal electric field is a novel matrix for developing low powered biosensors to meet future energy demands.

In-situ Generation of Au Nanostructures During Enzyme Free Oxidation of Uric Acid: A New Recognition at an Old Problem

Distinct evolution of Au nanostructures was noted spectrophotometrically at localized surface plasmon resonance band centre at around 580nm in a reaction mixture of uric acid and gold chloride solution at room temperature. In-situ formation of Au nanostructures during oxidation of uric acid was confirmed via transmission electron microscopy, energy dispersive X-ray, ultraviolet-visible spectroscopy and X-ray diffraction analyses. The ensuing Au particles exhibit brick red colour appearance and were not monodispersed. They dominantly constitute nearly spherical as well as rod like elongated structures to about 1000nm in length. Nanozyme behaviour of the produced Au particles was not observed. This formation of Au nanostructures during the oxidation reaction might intuitively illustrate traditionally practice chrysotherapy mechanism of hyperuricemia.

Catalytic Mechanisms for Cofactor-Free Oxidase-Catalyzed Reactions: Reaction Pathways of Uricase-Catalyzed Oxidation and Hydration of Uric Acid.

First-principles quantum mechanical/molecular mechanical (QM/MM)-free energy calculations have been performed to uncover how uricase catalyzes metabolic reactions of uric acid (UA), demonstrating that the entire reaction process of UA in uricase consists of two stages-oxidation followed by hydration. The oxidation consists of four steps: (1) chemical transformation from 8-hydroxyxythine to an anionic radical via a proton transfer along with an electron transfer, which is different from the previously proposed electron-transfer mechanism that involves a dianion intermediate (UA2-) during the catalytic reaction process; (2) proton transfer to the O2- anion (radical); (3) diradical recombination to form a peroxo intermediate; (4) dissociation of H2O2 to generate the dehydrourate. Hydration, for the most favorable pathway, is initiated by the nucleophilic attack of a water molecule on dehydrourate, along with a concerted proton transfer through residue Thr69 in the catalytic site. According to the calculated free energy profile, the hydration is the rate-determining step, and the corresponding free energy barrier of 16.2 kcal/mol is consistent with that derived from experimental kinetic data, suggesting that the computational insights into the catalytic mechanisms are reasonable. The mechanistic insights not only provide a mechanistic base for future rational design of uricase mutants with improved catalytic activity against uric acid as an improved enzyme therapy, but also are valuable for understanding a variety of other cofactor-free oxidase-catalyzed reactions involving an oxygen molecule.

Simultaneous Detection of Dopamine and Uric Acid on Indium Tin Oxides Modified with Cost‐effective Gas‐phase Synthesized Single Walled Carbon Nanotubes

AbstractCost‐effective gas‐phase synthesized single walled carbon nanotubes (SWCNTs) were first employed for the surface modification of indium tin oxides (ITO) via electrostatic coating of poly‐l‐lysine (PLL). Compared with control substrates of bare ITO and SWCNT‐PLL‐slide glasses, SWCNT‐PLL‐ITO, with high catalytic properties associated with large surface areas, showed significant improvement of electro catalytic activity toward the oxidation of dopamine (DA) and uric acid (UA). The cyclic voltammetric (CV) peak separation of oxidation for both DA and UA was estimated to be ∼ 0.13 V, which renders them capable of simultaneously detecting DA and UA in a mixed solution. The differential pulse voltammetry (DPV) technique was further performed to obtain a linear detection range of DA from 1.0 μM to 100 μM, and a detection limit of 1.0 μM was confirmed in the mixed solution. Furthermore, the practical analytical ability of the method was finally confirmed by selective detection of both DA and UA in human urine samples without any preliminary treatment, implying the feasibility of applying bio‐sensors in a low‐cost manner.

Directed evolution to improve the catalytic efficiency of urate oxidase from Bacillus subtilis.

Urate oxidase is a key enzyme in purine metabolism and catalyzes the oxidation of uric acid to allantoin. It is used to treat hyperuricemia and gout, and also in a diagnostic kit. In this study, error-prone polymerase chain reaction and staggered extension process was used to generate a mutant urate oxidase with improved enzyme activity from Bacillus subtilis. After several rounds of mutagenesis and screening, two mutants 6E9 and 8E279 were obtained which exhibited 2.99 and 3.43 times higher catalytic efficiency, respectively. They also exhibited lower optimal reaction temperature and higher thermo-stability. D44V, Q268R and K285Q were identified as the three most beneficial amino acid substitutions introduced by site-directed mutagenesis. D44V/Q268R, which was obtained through random combination of the three mutants, displayed the highest catalytic activity. The Km, kcat/Km and enzyme activity of D44V/Q268R increased by 68%, 83% and 129% respectively, compared with that of wild-type urate oxidase. Structural modeling indicated that mutations far from the active site can have significant effects on activity. For many of them, the underlying mechanisms are still difficult to explain from the static structural model. We also compared the effects of the same set of single point mutations on the wild type and on the final mutant. The results indicate strong effects of epistasis, which may imply that the mutations affect catalysis through influences on protein dynamics besides equilibrium structures.

Simultaneous electrochemical determination of ascorbic acid, dopamine and uric acid based on reduced graphene oxide-Ag/PANI modified glassy carbon electrode

An electrochemical method for simultaneous determination of ascorbic acid(AA), dopamine(DA) and uric acid(UA) was developed based on RGO-Ag/PANI modified glassy carbon electrode(RGO-Ag/PANI/GCE, RGO= reduced graphene oxide, PANI=polyaniline). Owing to the synergetic effect among RGO, Ag and PANI, the RGO-Ag/PANI based sensor not only improved the electrochemically catalytic oxidation of AA, DA and UA, but also resolved the overlapping anodic peaks. Under the optimized conditions, the calibration curves were obtained in the concentration ranges of 50—1000, 5—200 and 20—350 μmol/L, and the detection limits(S/N=3) of 0.5, 0.2 and 0.2 μmol/L were obtained for AA, DA, and UA, respectively, in neutral phosphate buffer solutions(PBS). The RGO-Ag/PANI/GCE sensor was successfully applied to the determination of AA, DA, and UA in bovine serum samples and showed high selectivity, sensitivity, and reproducibility.

Urate hydroperoxide oxidizes human peroxiredoxin 1 and peroxiredoxin 2

Urate hydroperoxide is a product of the oxidation of uric acid by inflammatory heme peroxidases. The formation of urate hydroperoxide might be a key event in vascular inflammation, where there is large amount of uric acid and inflammatory peroxidases. Urate hydroperoxide oxidizes glutathione and sulfur-containing amino acids and is expected to react fast toward reactive thiols from peroxiredoxins (Prxs). The kinetics for the oxidation of the cytosolic 2-Cys Prx1 and Prx2 revealed that urate hydroperoxide oxidizes these enzymes at rates comparable with hydrogen peroxide. The second-order rate constants of these reactions were 4.9 × 105 and 2.3 × 106 m-1 s-1 for Prx1 and Prx2, respectively. Kinetic and simulation data suggest that the oxidation of Prx2 by urate hydroperoxide occurs by a three-step mechanism, where the peroxide reversibly associates with the enzyme; then it oxidizes the peroxidatic cysteine, and finally, the rate-limiting disulfide bond is formed. Of relevance, the disulfide bond formation was much slower in Prx2 (k3 = 0.31 s-1) than Prx1 (k3 = 14.9 s-1). In addition, Prx2 was more sensitive than Prx1 to hyperoxidation caused by both urate hydroperoxide and hydrogen peroxide. Urate hydroperoxide oxidized Prx2 from intact erythrocytes to the same extent as hydrogen peroxide. Therefore, Prx1 and Prx2 are likely targets of urate hydroperoxide in cells. Oxidation of Prxs by urate hydroperoxide might affect cell function and be partially responsible for the pro-oxidant and pro-inflammatory effects of uric acid.

Simultaneous Determination of Ascorbic Acid, Dopamine and Uric Acid By a Novel Electrochemical Sensor Based on N2/ Ar RF Plasma Assisted Graphene Nanosheet/Graphene Nanoribbon

A highly selective electrochemical method was developed for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA) using a glassy carbon electrode modified by radio frequency nitrogen/ argon plasma (N2/Ar) mediated graphene sheets / graphene ribbons (GS/GR).The performance of the electrode towards was investigated by cyclic voltammetry, different pulse voltammetry (DPV) and chronoamperometry. The excellent electrocatalytic activity toward the oxidation of AA, DA and UA was demonstrated with a well-separated voltammetric peaks between AA/DA, DA/UA and AA/UA are 220,150 and 372 mV (vs. Ag/AgCl) in DPV, respectively. The detection limits (S/N = 3) of 0.00053 µM, 0.00025 µM, 0.00057µM were obtained for AA, DA and UA in PBS, respectively. Due to the synergistic effect between the high conductivity of graphene hybrid and the catalytic activity of N2, the as-prepared composite showed excellent performance. Additionally, the N2/Ar/GS/GR/GCE electrode presented well anti-interference ability, stability and reproducibility.

Fabrication of Nano Poly Cresol Red over Glassy Carbon Electrode and its Application in Selective Determination of Uric acid in the Presence of Ascorbic Acid

A selective electrochemical method for the determination of uric acid was developed by using nano poly cresol red modified glassy carbon electrode. This new material has been characterized by Scanning Electron Microscopy, cyclic voltammetry and Differential pulse voltammetry. This modified electrode shows excellent electrocatalytic activity towards the oxidation of uric acid in the presence of ascorbic acid in (pH 7.0). Compare to bare electrode the modified electrode able to resolve the both ascorbic acid and uric acid in the solution mixture. The oxidation current of uric acid with the modified electrode is two folds higher that the bare glassy carbon electrode. Using differential pulse voltammetry method, the oxidation current is linear with the uric acid concentration in the range 30 µM - 575 µM with the detection limit of 5 µM (S/N = 3). Moreover uric acid in real samples can be determined using nano poly cresol red without any pretreatment of samples giving satisfactory results.

Individual and simultaneous voltammetric determination of ascorbic acid, uric acid and folic acid by using a glassy carbon electrode modified with gold nanoparticles linked to bentonite via cysteine groups

A voltammetric sensor for both the individual and the simultaneous determination of ascorbic acid (AA), uric acid (UA) and folic acid (FA) is described. It is based on a glassy carbon electrode (GCE) that was modified with bentonite (Bnt) that was first functionalized with cysteine (Cys) to which gold nanoparticles were linked. The resulting material (referred to as Au-Cys-Bnt) and the other materials were characterized by UV-vis spectroscopy, powder X-ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray analysis and electrochemical methods. The XRD peak positions of bentonite and Cys-functionalized bentonite prove the incorporation of Cys into bentonite. The XPS spectrum of Au-Cys-Bnt confirms the interaction of gold nanoparticles with the thiol group of Cys. The modified GCE displays high electrocatalytic activity for the oxidation of AA, UA and FA, typically at 0.19, 0.41, and 0.73 V (vs. SCE), respectively. Differential pulse voltammetric data show a linear response that covers the 1 μM to 25 mM concentration range for AA, the 1 to 200 μM concentration range for UA, and two linear ranges for FA, one from 5 to 100 μM and one from 100 μM to 1.5 mM. The sensor was applied to the determination of AA, UA and FA in (spiked) multi-vitamin syrup, bird serum and milk samples.