Simultaneous determination of ascorbic acid,dopamine and uric acid based on tryptophan functionalized graphene

A new type of tryptophan-functionalized graphene nanocomposite (Trp-GR) was synthesized by utilizing a facile ultrasonic method via π–π conjugate action between graphene (GR) and tryptophan (Trp) molecule. The material as prepared had well dispersivity in water and better conductivity than pure GR. The surface morphology of Trp-GR was characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and Raman spectroscopy. The electrochemical behaviors of ascorbic acid (AA), dopamine (DA), and uric acid (UA) were investigated by cyclic voltammetry (CV) on the surface of Trp-GR. The separation of the oxidation peak potentials for AA–DA, DA–UA and UA–AA was about 182 mV, 125 mV and 307 mV, which allowed simultaneously determining AA, DA, and UA. Differential pulse voltammetery (DPV) was used for the determination of AA, DA, and UA in their mixture. Under optimum conditions, the linear response ranges for the determination of AA, DA, and UA were 0.2–12.9 mM, 0.5–110 μM, and 10–1000 μM, with the detection limits (S/N = 3) of 10.09 μM, 0.29 μM and 1.24 μM, respectively. Furthermore, the modified electrode was investigated for real sample analysis.     

Simultaneous determination of ascorbic acid,dopamine, uric acid and tryptophan on gold nanoparticles/overoxidized-polyimidazole composite modified glassy carbon electrode

A novel electrode was developed through electrodepositing gold nanoparticles (GNPs) on overoxidized-polyimidazole (PImox) film modified glassy carbon electrode (GCE). The combination of GNPs and the PImox film endowed the GNPs/PImox/GCE with good biological compatibility, high selectivity and sensitivity and excellent electrochemical catalytic activities towards ascorbic acid (AA), dopamine (DA), uric acid (UA) and tryptophan (Trp). In the fourfold co-existence system, the peak separations between AA–DA, DA–UA and UA–Trp were large up to 186, 165 and 285 mV, respectively. The calibration curves for AA, DA and UA were obtained in the range of 210.0–1010.0 μM, 5.0–268.0 μM and 6.0–486.0 μM with detection limits (S/N = 3) of 2.0 μM, 0.08 μM and 0.5 μM, respectively. Two linear calibrations for Trp were obtained over ranges of 3.0–34.0 μM and 84.0–464.0 μM with detection limit (S/N = 3) of 0.7 μM. In addition, the modified electrode was applied to detect AA, DA, UA and Trp in samples using standard addition method with satisfactory results.     

Synthesis of Pt/ionic liquid/graphene nanocomposite and its simultaneous determination of ascorbic acid and dopamine

A water-soluble and electroactive composite - Pt nanoparticles/polyelectrolyte-functionalized ionic liquid (PFIL)/graphene sheets (GS) nanocomposite was synthesized in one pot. The structure and composition of the Pt/PFIL/GS nanocomposite were studied by means of ultraviolet-visible (UV-vis) and X-ray photoelectron spectra (XPS). Scanning electron microscopy (SEM) and transmission electron microscope (TEM) images reveal Pt nanoparticles are densely dispersed on the transparent thin PFIL-functionalized graphene sheets. The obtained Pt/PFIL/GS nanocomposite-modified electrode was fabricated to simultaneously determine ascorbic acid (AA) and dopamine (DA) by cyclic voltammetry. It is worthwhile noting that the difference between the two peak potentials of AA and DA oxidation is over 200 mV, which leads to distinguishing AA from DA. The detection of increasing concentrations of AA in the presence of DA and the oxidation of continuous addition of DA in the presence of AA were also studied using differential pulse voltammetry. The proposed sensor in real sample analysis was also examined in human urine samples. Three independent oxidation peaks appear in urine sample containing AA and DA. Therefore, the Pt/PFIL/GS nanocomposite might offer a good possibility for applying it to routine analysis of AA and DA in clinical use.     

Preparation of silver hexacyanoferrate nanoparticles and its application for the simultaneous determination of ascorbic acid, dopamine and uric acid

A silver hexacyanoferrate nanoparticles/carbon nanotubes modified glassy carbon electrode was fabricated and then successfully used for the simultaneous determination of ascorbic acid, dopamine and uric acid by cyclic voltammetry. A detailed investigation by transmission electron microscopy (TEM) and electrochemistry was performed in order to elucidate the preparation process and properties of the nanocomposites. The size of silver hexacyanoferrate nanoparticles was examined by TEM around 27 nm. Linear calibration plots were obtained over the range of 4.0 × 10⁻⁶-7.8 × 10⁻⁵, 2.4 × 10⁻⁶-1.3 × 10⁻⁴ and 2.0 × 10⁻⁶-1.5 × 10⁻⁴ mol L⁻¹ with detection limits of 4.2 × 10⁻⁷,1.4 × 10⁻⁷ and 6.0 × 10⁻⁸ mol L⁻¹ for ascorbic acid, dopamine and uric acid, respectively. The practical analytical utilities of the modified electrode were demonstrated by the determination of ascorbic acid, dopamine and uric acid in urine and human blood serum samples.     

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.     

Sodium dodecyl sulfate modified carbon nanotubes paste electrode as a novel sensor for the simultaneous determination of dopamine,ascorbic acid,and uric acid

A novel modified multiwall carbon nanotubes paste electrode with sodium dodecyl sulfate as a surfactant (SDS) has been fabricated through an electrochemical oxidation procedure and was used to electrochemically detect dopamine (DA), ascorbic acid (AA), uric acid (UA), and their mixture by cyclic voltammetry (CV) and differential voltammetry (DPV) methods. Several factors affecting the electrocatalytic activity of the hybrid material, such as the effect of pH, of the scan rate and of the concentration were studied. The bare carbon nanotubes paste electrode (BCNTPE) and SDS-modified carbon nanotubes paste electrode (SDSMCNTPE) were characterized using Field Emission Scanning Electron Microscopy (FESEM) and Energy-Dispersive X-ray spectroscopy (EDX). Using the CV procedure, a linear analytical curve was observed in the 1 × 10⁻⁶–2.8 × 10⁻⁵ M range with a detection limit at 3.3 × 10⁻⁷ M in pH 6.5, 0.2 M phosphate buffer solutions (PBS).     

Simultaneous electrochemical sensing of ascorbic acid,dopamine and uric acid at anodized nanocrystalline graphite-like pyrolytic carbon film electrode

Nanocrystalline graphite-like pyrolytic carbon film (PCF) electrode fabricated by a non-catalytic chemical vapor deposition (CVD) process was used for the simultaneous electrochemical sensing of ascorbic acid (AA), dopamine (DA), and uric acid (UA). The electrode was studied with respect to changes in electrocatalytic activity caused by a simple and fast electrochemical pretreatment. The anodized electrode exhibited excellent performance compared to many chemically modified electrodes in terms of detection limit, linear dynamic range, and sensitivity. Differential pulse voltammetry (DPV) was used for the simultaneous determination of ternary mixtures of DA, AA, and UA. Under optimum conditions, the detection limits were 2.9 μM for AA, 0.04 μM for DA, and 0.03 μM for UA with sensitivities of 0.078, 5.345, and 6.192 A M⁻¹, respectively. The peak separation was 219 mV between AA and DA and 150 mV between DA and UA. No electrode fouling was observed and good reproducibility was obtained in all the experiments. The sensor was successfully applied for the assay of DA in an injectable drug and UA in human urine by using standard addition method.     

Simultaneous determination of ascorbic acid, dopamine, uric acid and xanthine using a nanostructured polymer film modified electrode

This paper describes the simultaneous determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and xanthine (XN) using an ultrathin electropolymerized film of 2-amino-1,3,4-thiadiazole (p-ATD) modified glassy carbon (GC) electrode in 0.20 M phosphate buffer solution (pH 5.0). Bare GC electrode failed to resolve the voltammetric signals of AA, DA, UA and XN in a mixture. On the other hand, the p-ATD modified electrode separated the voltammetric signals of AA, DA, UA and XN with potential differences of 110, 152 and 392 mV between AA-DA, DA-UA and UA-XN, respectively and also enhanced their oxidation peak currents. The modified electrode could sense 5 μM DA and 10 μM each UA and XN even in the presence of 200 μM AA. The oxidation currents were increased from 30 to 300 μM for AA, 5 to 50 μM for DA and 10 to 100 μM for each UA and XN, and the lowest detection limit was found to be 2.01, 0.33, 0.19 and 0.59 μM for AA, DA, UA and XN, respectively (S/N = 3). The practical application of the present modified electrode was demonstrated by the determination of AA, UA and XN in human urine samples.     

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.     

Zeolite A functionalized with copper nanoparticles and graphene oxide for simultaneous electrochemical determination of dopamine and ascorbic acid

A novel Cu-zeolite A/graphene modified glassy carbon electrode for the simultaneous electrochemical determination of dopamine (DA) and ascorbic acid (AA) has been described. The Cu-zeolite A/graphene composites were prepared using Cu²⁺ functionalized zeolite A and graphene oxide as the precursor, and subsequently reduced by chemical agents. The composites were characterized by X-ray diffraction, Fourier transform infrared spectra and scanning electron microscopy. Based on the Cu-zeolite A/graphene-modified electrode, the potential difference between the oxidation peaks of DA and AA was over 200 mV, which was adequate for the simultaneous electrochemical determination of DA and AA. Also the proposed Cu-zeolite/graphene-modified electrode showed higher electrocatalytic performance than zeolite/graphene electrode or graphene-modified electrode. The electrocatalytic oxidation currents of DA and AA were linearly related to the corresponding concentration in the range of 1.0 × 10⁻⁷–1.9 × 10⁻⁵ M for DA and 2.0 × 10⁻⁵–2.0 × 10⁻⁴ M for AA. Detection limits (<!-- no-mfc -->S/N<!-- /no-mfc --> = 3) were estimated to be 4.1 × 10⁻⁸ M for DA and 1.1 × 10⁻⁵ M for AA, respectively.     

Simultaneous determination of dopamine, ascorbic acid and uric acid at poly (Evans Blue) modified glassy carbon electrode

A sensitive and selective electrochemical method for the determination of dopamine using an Evans Blue polymer film modified on glassy carbon electrode was developed. The Evans blue polymer film modified electrode shows excellent electrocatalytic activity toward the oxidation of dopamine in phosphate buffer solution (pH 4.5). The linear range of 1.0 x 10(-6)-3.0 x 10(-5) M and detection limit of 2.5 x 10(-7) M were observed in pH 4.5 phosphate buffer solutions. The interference studies showed that the modified electrode exhibits excellent selectivity in the presence of large excess of ascorbic acid and uric acid. The separation of the oxidation peak potentials for dopamine-ascorbic acid and dopamine-uric acid were about 182 mV and 180 mV, respectively. The differences are large enough to determine AA, DA and UA individually and simultaneously. This work provides a simple and easy approach to selectively detect dopamine in the presence of ascorbic acid and uric acid in physiological samples.     

聚乙烯亚胺功能化石墨烯修饰电极同时测定抗坏血酸、 多巴胺、尿酸和色氨酸

制备了聚乙烯亚胺(PEI)功能化的石墨烯(G)修饰电极以实现抗坏血酸(AA)、 多巴胺(DA)、 尿酸(UA)和色氨酸(Trp)的分离及同时测定. 采用红外光谱(FTIR)、 紫外-可见吸收光谱(UV-Vis)、 X射线粉末衍射仪(XRD)和透射电子显微镜(TEM)对电极修饰材料进行了表征, 并优化了该修饰电极同时测定AA, DA, UA和Trp的实验条件. 在聚乙烯亚胺功能化石墨烯修饰的玻碳电极(PEI-G/GCE)上实现了AA, DA, UA 和Trp氧化峰的分离, AA-DA, DA-UA和UA-Trp的氧化峰电位差分别为298, 130和350 mV. 该修饰电极对AA, DA, UA和Trp的检测线性范围分别为50~5800, 30~2570, 0.05~400和6~1000 μmol/L; 检出限分别为16.67, 10, 0.017和2 μmol/L.     

Solvent dependent dimercaptothiadiazole monolayers on gold electrode for the simultaneous determination of uric acid and ascorbic acid

Dimercaptothiadiazole compound, 2,5-dimercapto-1,3,4-thiadiazole (DMcT) forms ‘thin’ monolayers on Au electrode when it was adsorbed from methanol, ethanol or DMSO solutions while it forms ‘thick’ layers on Au electrode from an aqueous solution under identical experimental conditions. Thick DMcT layers formed from aqueous solution effectively blocks the redox reaction of couple in contrast to thin DMcT monolayers. The monolayer thickness did not vary when structurally related DMcT compounds, 5-methyl-1,3,4-thiadiazole-2-thiol or 5-amino-1,3,4-thiadiazole-2-thiol was adsorbed from aqueous and non-aqueous solutions. This indicates that the presence of two thiol groups in DMcT plays a crucial role in the formation of thick and thin DMcT layers on Au electrode when it was adsorbed from aqueous and non-aqueous solutions. Methanol, ethanol, or DMSO solution of DMcT is considered as strong acid because these solvents are able to deprotonate DMcT into DMcT⁻ and thus thin monolayers formed on Au electrode. The deprotonating ability of these solvents was further verified from the observed absorption spectrum characteristic of DMcT⁻ species. On the other hand, an aqueous solution of DMcT is less acidic due to weak deprotonation of DMcT by water and thus DMcT forms thick layer on Au electrode. Interestingly, thin DMcT monolayers formed from non-aqueous solvents separates the voltammetric signals of uric acid and ascorbic acid while thick DMcT layers formed from aqueous solution fails to separate them.     

Poly(3,4-ethylenedioxythiophene-co-(5-amino-2-naphthalenesulfonic acid)) (PEDOT-PANS) film modified glassy carbon electrode for selective detection of dopamine in the presence of ascorbic acid and uric acid

Poly(3,4-ethylenedioxythiophene-co-(5-amino-2-naphthalenesulfonic acid)) (PEDOT-PANS) film modified glassy carbon electrode was prepared by electrochemical polymerization technique. The properties of modified electrode was studied. It was found that the electrochemical properties of modified electrode was very much dependent on the experimental conditions, such as monomer oxidation potential and pH. The modified electrode surface was characterized by scanning electron microscopy (SEM). The PEDOT-PANS film modified electrode shows electrocatalytic activity toward oxidation of dopamine (DA) in acetate buffer solution (pH 5.0) and results in a marked enhancement of the current response. The linear sweep voltammetric (LSV) peak heights are linear with DA concentration from 2 × 10⁻⁶ to 1 × 10⁻⁵ M. The detection limit is 5 × 10⁻⁷ M. More over, the interferences of ascorbic acid (AA) and uric acid (UA) were effectively diminished. This work provides a simple and easy approach for selective determination of dopamine in the presence of ascorbic acid and uric acid.     

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.     

Selective detection of dopamine in the presence of ascorbic acid and uric acid by a carbon nanotubes-ionic liquid gel modified electrode

The electrochemistry of dopamine (DA) was studied by cyclic voltammetry at a glassy carbon electrode modified by a gel containing multi-walled carbon nanotubes (MWNTs) and room-temperature ionic liquid of 1-octyl-3-methylimidazolium hexafluorophosphate (OMIMPF₆). The thickness of gel on the surface of the electrode has to be controlled carefully because the charging currents increase with the modified layer being thicker. The anodic peaks of DA, ascorbic acid (AA) and uric acid (UA) in their mixture can be well separated since the peak potential of AA is shifted to more negative values, while that of UA is shifted to more positive values due to the modified electrode. At pH 7.08 the three peaks are separated ca. 0.20 and 0.15 V, respectively; hence DA can be determined in the presence of UA and more than 100 times excess of AA. Under optimum conditions linear calibration graphs were obtained over the DA concentration range 1.0 × 10⁻⁶ to 1.0 × 10⁻⁴ M. The detection limit of the current technique was found to be 1.0 × 10⁻⁷ M based on the signal-to-noise ratio of 3. The modified electrode has been successfully applied for the assay of DA in human blood serum. This work provides a simple and easy approach to selectively detect dopamine in the presence of ascorbic acid and uric acid.     

Simultaneous voltammetric detection of ascorbic acid, dopamine and uric acid using a pyrolytic graphite electrode modified into dopamine solution

Pyrolytic graphite electrodes (PGE) were modified into dopamine solutions using phosphate buffer solutions, pH 10 and 6.5, as supporting electrolyte. The modification process involved a previous anodization of the working electrode at +1.5 V into 0.1 mol L⁻¹ NaOH followed by other anodization step, in the same experimental conditions, into dopamine (DA) solutions. pH of the supporting electrolyte performed an important role in the production of a superficial melanin polymeric film, which permitted the simultaneous detection of ascorbic acid (AA), (DA) and uric acid (UA), ΔE_AA-DA = 222 mV; ΔE_AA-UA = 360 mV and ΔE_DA-UA = 138 mV, avoiding the superficial poisoning effects. The calculated detection limits were: 1.4 × 10⁻⁶ mol L⁻¹ for uric acid, 1.3 × 10⁻⁵ mol L⁻¹ for ascorbic acid and 1.1 × 10⁻⁷ mol L⁻¹ for dopamine, with sensitivities of (7.7 ± 0.5), (0.061 ± 0.001) and (9.5 ± 0.05) A mol⁻¹ cm⁻², respectively, with no mutual interference. Uric acid was determined in urine, blood and serum human samples after dilution in phosphate buffer and no additional sample pre-treatment was necessary. The concentration of uric acid in urine was higher than the values found in blood and serum and the recovery tests (92-102%) indicated that no matrix effects were observed.     

Enhanced sensing of dopamine in the present of ascorbic acid based on graphene/poly(p-aminobenzoic acid) composite film

Graphene/p-aminobenzoic acid composite film modified glassy carbon electrode (Gr/p-ABA/GCE) was first employed for the sensitive determination of dopamine (DA). The electrochemical behavior of DA at the modified electrode was investigated by cyclic voltametry (CV), differential pulse voltametry (DPV) and amperometric curve. The oxidation peak currents of DA increased dramatically at Gr/p-ABA/GCE. The modified electrode was used to electrochemically detect dopamine (DA) in the presence of ascorbic acid (AA). The Gr/p-ABA composite film showed excellent electrocatalytic activity for the oxidation of DA in phosphate buffer solution (pH 6.5). The peak separation between DA and AA was large up to 220 mV. Using DPV technique, the calibration curve for DA determination was obtained in the range of 0.05-10 μM. The detection limit for DA was 20 nM. AA did not interfere with the determination of DA because of the very distinct attractive interaction between DA cations and the negatively Gr/p-ABA composite film. The proposed method exhibited good stability and reproducibility.     

Simultaneous determination of ascorbic acid, uric acid and neurotransmitters with a carbon ceramic electrode prepared by sol-gel technique

A sol-gel carbon composite electrode (CCE) has been prepared by mixing a sol-gel precursor (e.g. methyltrimethoxysilane) and carbon powder without adding any electron transfer mediator or specific reagents. It was demonstrated that this sensor can be used for simultaneous determination ascorbic acid, neurotransmitters (dopamine and adrenaline) and uric acid. Direct electrochemical oxidation of ascorbic acid, uric acid and catecholamines at a carbon composite electrode was investigated. The experimental results were compared with other common carbon based electrodes, specifically, boron doped diamond, glassy carbon, graphite and carbon paste electrodes. It was found that the CCE shows a significantly higher of reversibility for dopamine. In addition, in comparison to the other electrodes used, for CCE the oxidation peaks of uric acid, ascorbic acid and catecholamines in cyclic and square wave voltammetry were well resolved at the low positive potential with good sensitivity. The advantages of this sensor were high sensitivity, inherent stability and simplicity and ability for simultaneous determination of uric acid, catecholamines and ascorbic acid without using any chromatography or separation systems. The analytical performance of this sensor has been evaluated for detection of biological molecules in urine and serum as real samples.     

A quercetin-modified biosensor for amperometric determination of uric acid in the presence of ascorbic acid

The present work reports a quercetin-modified wax-impregnated graphite electrode (Qu/WGE) prepared through an electrochemical oxidation procedure in quercetin-containing phosphate buffer solution (PBS), for the purpose of detecting uric acid (UA) in the presence of ascorbic acid (AA). During modification quercetin was oxidized to the corresponding quinonic structure, and in the blank buffer solution the electrodeposited film exhibits a voltammetric response anticipated for the surface-immobilized quercetin. Retarding effect of the film towards the reaction of anionic species was found; therefore the pH of sample solutions was selected to ensure the analyte in molecular form. At suitable pHs the Qu/WGE shows excellent electrocatalytic effect towards the oxidation of both AA and UA, and separates the voltammetric signal of UA from AA by about 280 mV, allowing simultaneous detection of these two species. A linear relation between the peak current and concentration was obtained for UA in the range of 1-50 μM in the presence of 0.5 mM AA, with a detection limit 1.0 μM (S/N = 3). This sensor was stable, reproducible and outstanding for long-term use.