Simultaneous electrochemical determination of dopamine, uric acid and ascorbic acid using palladium nanoparticle-loaded carbon nanofibers modified electrode

Palladium nanoparticle-loaded carbon nanofibers (Pd/CNFs) were prepared by electrospinning and subsequent thermal treatment processes. Pd/CNFs modified carbon paste electrode (Pd/CNF-CPE) displayed excellent electrochemical catalytic activities towards dopamine (DA), uric acid (UA) and ascorbic acid (AA). The oxidation overpotentials of DA, UA and AA were decreased significantly compared with those obtained at the bare CPE. Differential pulse voltammetry was used for the simultaneous determination of DA, UA and AA in their ternary mixture. The peak separation between UA and DA, DA and AA was 148 mV and 244 mV, respectively. The calibration curves for DA, UA and AA were obtained in the range of 0.5-160 microM, 2-200 microM, and 0.05-4mM, respectively. The lowest detection limits (S/N=3) were 0.2 microM, 0.7 microM and 15 microM for DA, UA and AA, respectively. With good selectively and sensitivity, the present method was applied to the determination of DA in injectable medicine and UA in urine sample.     

Hollow nitrogen-doped carbon microspheres pyrolyzed from self-polymerized dopamine and its application in simultaneous electrochemical determination of uric acid, ascorbic acid and dopamine

Hollow nitrogen-doped carbon microspheres (HNCMS) as a novel carbon material have been prepared and the catalytic activities of HNCMS-modified glassy carbon (GC) electrode towards the electro-oxidation of uric acid (UA), ascorbic acid (AA) and dopamine (DA) have also been investigated. Comparing with the bare GC and carbon nanotubes (CNTs) modified GC (CNTs/GC) electrodes, the HNCMS modified GC (HNCMS/GC) electrode has higher catalytic activities towards the oxidation of UA, AA and DA. Moreover, the peak separations between AA and DA, and DA and UA at the HNCMS/GC electrode are up to 212 and 136 mV, respectively, which are superior to those at the CNTs/GC electrode (168 and 114 mV). Thus the simultaneous determination of UA, AA and DA was carried out successfully. In the co-existence system of UA, AA and DA, the linear response range for UA, AA and DA are 5-30 μM, 100-1000 μM and 3-75 μM, respectively and the detection limits (S/N = 3) are 0.04 μM, 0.91 μM and 0.02 μM, respectively. Meanwhile, the HNCMS/GC electrode can be applied to measure uric acid in human urine, and may be useful for measuring abnormally high concentration of AA or DA. The attractive features of HNCMS provide potential applications in the simultaneous determination of UA, AA and DA.     

Carbon nanotubes-polymer-redox mediator hybrid thin film for electrocatalytic sensing

Electrocatalytic sensing of NADH using a hybrid thin film derived from multi-wall carbon nanotubes (CNTs), Nafion (Nf) polymer and electrogenerated redox mediator is described. The redox mediator was electrochemically generated by the oxidation of serotonin on the hybrid thin film modified glassy carbon electrode (GC/Nf-CNT). Controlled potential electrolysis of serotonin at 0.1 V in neutral solution results in the generation of the redox mediator 5,5'-dihydroxy-4,4'-bitryptamine (DHB) on the hybrid thin film. The electrogenerated DHB has redox active quinone-imine structure and was electrochemically characterized by studying the pH dependent redox response. DHB on the hybrid thin film exhibits reversible redox peak at -0.05 V and the formal potential shifts by -55 mV while increasing the solution pH by 1 unit. The quinone-imine structure of DHB efficiently catalyzes the oxidation of NADH with a decrease in the overpotential of about 500 mV compared to the unmodified electrode. The CNTs of the hybrid thin film facilitates the mediated electrocatalytic oxidation of NADH. The hybrid thin film modified electrode exhibits stable amperometric response and it linearly responds to NADH (0.5-400 microM). This hybrid thin film modified electrode could detect NADH as low as 0.1 microM at -0.05 V with a sensitivity of 11.1 nA/microM in physiological pH.     

Layer-by-layer assembled carbon nanotubes for selective determination of dopamine in the presence of ascorbic acid

Multilayer films of shortened multi-walled carbon nanotubes (MWNTs) are homogeneously and stably assembled on glassy carbon (GC) electrodes using layer-by-layer (LBL) method based on electrostatic interaction of positively charged poly(diallyldimethylammonium chloride) (PDDA) and negatively charged shortened MWNTs. The assembled MWNT multilayer films were studied with respect to the electrocatalytic activity toward ascorbic acid (AA) and dopamine (DA) and were further applied for selective determination of DA in the presence of AA. Scanning electron microscopy (SEM) used for characterization of MWNT films indicates that the assembled MWNTs are almost in a form of small bundles or single nanotubes on the electrodes. Cyclic voltammetric results with assembled MWNT electrode indicate that the strategy based on the LBL method for assembling the MWNT multilayer films on substrate well retains the electrochemical catalytic activity of the MWNTs toward AA and DA, offering some advantages particularly attractive for analytical applications, such as the form of MWNTs assembled on the substrate, i.e., small bundles or single tubes, homogeneity and stability of the as-assembled MWNT films. These features make the assembled MWNTs relatively potential for selective and sensitive determination of DA in the presence of AA.     

Simultaneous voltammetric detection of dopamine and uric acid at their physiological level in the presence of ascorbic acid using poly(acrylic acid)-multiwalled carbon-nanotube composite-covered glassy-carbon electrode

The use of poly(acrylic acid) (PAA)-multiwalled carbon-nanotubes (MWNTs) composite-coated glassy-carbon disk electrode (GCE) (PAA-MWNTs/GCE) for the simultaneous determination of physiological level dopamine (DA) and uric acid (UA) in the presence of an excess of ascorbic acid (AA) in a pH 7.4 phosphate-buffered solution was proposed. PAA-MWNTs composite was prepared by mixing of MWNTs powder into 1 mg/ml PAA aqueous solution under sonication. GCE surface was modified with PAA-MWNTs film by casting. AA demonstrates no voltammetric peak at PAA-MWNTs/GCE. The PAA-MWNTs composite is of a high surface area and of affinity for DA and UA adsorption. DA exhibits greatly improved electron-transfer rate and is electro-catalyzed at PAA-MWNTs/GCE. Moreover, the electro-catalytic oxidation of UA at PAA-MWNTs/GCE is observed, which makes it possible to detect lower level UA. Therefore, the enhanced electrocatalytic currents for DA and UA were observed. The anodic peak currents at approximately 0.18 V and 0.35 V increase with the increasing concentrations of DA and UA, respectively, which correspond to the voltammetric peaks of DA and UA, respectively. The linear ranges are 40 nM to 3 microM DA and 0.3 microM to 10 microM UA in the presence of 0.3 mM AA. The lowest detection limits (S/N=3) were 20 nM DA and 110 nM UA.     

Overoxidized polypyrrole film directed single-walled carbon nanotubes immobilization on glassy carbon electrode and its sensing applications

In this paper, the films of overoxidized polypyrrole (PPyox) directed single-walled carbon nanotubes (SWNTs) have been electrochemically coated onto glassy carbon electrode (GCE). Electroactive monomer pyrrole was added into the solution containing sodium dodecyl sulfate (SDS) and SWNTs. Then, electropolymerization was proceeded at the surface of GCE, and a novel kind of conducting polymer/carbon nanotubes (CNTs) composite film with the orientation of CNTs were obtained correspondingly. Finally, this obtained polypyrrole (PPy)/SWNTs film modified GCE was oxidized at a potential of +1.8 V. It can be found that this proposed PPyox/SWNTs composite film modified GCE exhibited excellent electrocatalytic properties for some species such as nitrite, ascorbic acid (AA), dopamine (DA) and uric acid (UA), and could be used as a new sensor for practical applications. Compared with previous CNTs modified electrodes, SWNTs were oriented towards the outside of modified layer by PPyox and SDS, which made the film easily conductive. Moreover, this proposed film modified electrode was more stable, selective and applicable.     

Simultaneous determination of dopamine and serotonin on a glassy carbon electrode coated with a film of carbon nanotubes

A chemically modified electrode based on the carbon nanotube film-coated glassy carbon electrode (GCE) is described for the simultaneous determination of dopamine (DA) and serotonin (5-HT). The multiwall carbon nanotube (MWNT) film-coated GCE exhibits a marked enhancement effect on the current response of DA and 5-HT and lowers oxidation overpotentials. The responses of DA and 5-HT merge into a large peak at a bare GCE, but they yield two well-defined oxidation peaks at the MWNT film-coated GCE. The experimental parameters were optimized, and a direct electrochemical method for the simultaneous determination of DA and 5-HT was proposed. The interference of ascorbic acid (AA) was investigated, and the results showed that a large excess of AA did not interfere with the voltammetric responses of DA and 5-HT. The modified electrode has been successfully applied for the assay of 5-HT and DA in human blood serum.     

Simultaneous determination of catecholamines, uric acid and ascorbic acid at physiological levels using poly(N-methylpyrrole)/Pd-nanoclusters sensor

An interesting electrochemical sensor has been constructed by the electrodeposition of palladium nanoclusters (Pd_nano) on poly(N-methylpyrrole) (PMPy) film-coated platinum (Pt) electrode. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), and scanning electron microscopy were used to characterize the properties of the modified electrode. It was demonstrated that the electroactivity of the modified electrode depends strongly on the electrosynthesis conditions of the PMPy film and Pd_nano. Moreover, the modified electrode exhibits strong electrocatalytic activity toward the oxidation of a mixture of dopamine (DA), ascorbic acid (AA), and uric acid (UA) with obvious reduction of overpotentials. The simultaneous analysis of this mixture at conventional (Pt, gold [Au], and glassy carbon) electrodes usually struggles. However, three well-resolved oxidation peaks for AA, DA, and UA with large peak separations allow this modified electrode to individually or simultaneously analyze AA, DA, and UA by using differential pulse voltammetry (DPV) with good stability, sensitivity, and selectivity. This sensor is also ideal for the simultaneous analysis of AA, UA and either of epinephrine (E), norepinephrine (NE) or l-DOPA. Additionally, the sensor shows strong electrocatalytic activity towards acetaminophen (ACOP) and other organic compounds. The calibration curves for AA, DA, and UA were obtained in the ranges of 0.05 to 1 mM, 0.1 to 10 μM, and 0.5 to 20 μM, respectively. The detection limits (signal/noise [S/N] = 3) were 7 μM, 12 nM, and 27 nM for AA, DA, and UA, respectively. The practical application of the modified electrode was demonstrated by measuring the concentrations of AA, DA, and UA in injection sample, human serum, and human urine samples, respectively, with satisfactory results. The reliability and stability of the modified electrode gave a good possibility for applying the technique to routine analysis of AA, DA, and UA in clinical tests.     

Simultaneous determination of the monoamine neurotransmitters and glucose in rat brain by microdialysis sampling coupled with liquid chromatography-dual electrochemical detector

A new type of chemically modified electrode based ring-disk electrode as the dual electrochemical detector (DECD) for high-performance liquid chromatography (HPLC) to simultaneously determine the monoamine neurotransmitters and glucose is described. The ring electrode was modified with an ion-exchange polymer-overoxidized polypyrrole (OPPy) and the disk electrode was modified with nano Au colloid and glucose oxidase (GOD). The electrochemical behaviors of dopamine (DA) and ascorbic acid (AA) at the OPPy chemically modified electrode (CME) were investigated by differential pulse voltammetry (DPV). It was found that the CME could permeate dopamine cations and repelled the ascorbate anions, which could be used to determine the monoamine neurotransmitters and avoid the interference of AA. The electrochemical behavior of glucose at the Nafion/GOD-Au colloid/GC CME was investigated by amperometry and flow injection analysis (FIA). It was found that the sensitivity of the CME increased apparently in determination of glucose. In order to obtain better separation and current responses of the analytes in HPLC-DECD, several operational parameters have been investigated. Under the optimum conditions, the method showed good stability and reproducibility. The application of this method coupled with microdialysis sampling for in vivo simultaneous determination of monoamine neurotransmitters and glucose in rat brain was satisfactory.     

Enhancing dopamine detection using a glassy carbon electrode modified with MWCNTs, quercetin, and Nafion

Glassy carbon (GC) electrode was modified using multi-wall carbon nanotubes (MWCNTs), quercetin (Q) and Nafion^® in this sequence. The thus modified electrode was used for the detection of dopamine (DA) in the presence of equimolar ascorbic acid (AA). It is demonstrated in this study that MWCNTs can increase the current response of DA by five-fold and Q can reduce the oxidation overpotential of DA by about 60 mV, compared to these parameters obtained with a bare GC electrode. It is also shown that a layer of Nafion^® can virtually eliminate the interference of AA for the detection of DA. The GC/MWCNTs/Q/Nafion^® electrode (hereafter also called composite electrode) shows a current density of about 900 μA cm⁻² for DA, compared to the value of 80 μA cm⁻² of the GC electrode and to the value of 390 μA cm⁻² of the GC/MWCNTs electrode. The 11-fold enhancement in the sensitivity of the GC electrode for DA determination is attributed to the composite modification of the electrode, and is substantiated through various cyclic voltammetric experiments. Cyclic voltammetry (CV) and linear sweep voltammetry were used to characterize the electrodes. Calibration curves of batch and flow systems were obtained by amperometry for the detection of DA. Additionally, the composite modified electrode was tested with a human serum sample for the determination of DA and was found to be promising at our preliminary experiments.     

Simultaneous determination of dopamine, ascorbic acid, and uric acid using carbon ionic liquid electrode

A recently constructed carbon composite electrode using room temperature ionic liquid as pasting binder was employed as a novel electrode for sensitive, simultaneous determination of dopamine (DA), ascorbic acid (AA), and uric acid (UA). The apparent reversibility and kinetics of the electrochemical reaction for DA, AA, and UA found were improved significantly compared to those obtained using a conventional carbon paste electrode. The results show that carbon ionic liquid electrode (CILE) reduces the overpotential of DA, AA, and UA oxidation, without showing any fouling effect due to the deposition of their oxidized products. In the case of DA, the oxidation and reduction peak potentials appear at 210 and 135mV (vs Ag/AgCl, KCl, 3.0M), respectively, and the CILE shows a significantly better reversibility for dopamine. The oxidation peak due to the oxidation of AA occurs at about 60mV. For UA, a sharp oxidation peak at 340mV and a small reduction peak at 250mV are obtained at CILE. Differential pulse voltammetry was used for the simultaneous determination of ternary mixtures of DA, AA, and UA. Relative standard deviation for DA, AA, and UA determinations were less than 3.0% and DA, AA, and UA can be determined in the ranges of 2.0x10(-6)-1.5x10(-3), 5.0x10(-5)-7.4x10(-3), and 2.0x10(-6)-2.2x10(-4)M, respectively. The method was applied to the determination of DA, AA, and UA in human blood serum and urine samples.     

Simultaneous and sensitive determination of a quaternary mixture of AA, DA, UA and Trp using a modified GCE by iron ion-doped natrolite zeolite-multiwall carbon nanotube

The evaluation of a novel modified glassy carbon electrode modified with iron ion-doped natrolite zeolite-multiwalled carbon nanotube for the simultaneous and sensitive determination of ascorbic acid (AA), dopamine (DA), uric acid (UA) and tryptophan (Trp) has been described. The measurements were carried out using cyclic voltammetry in buffer solution with pH 1. This modified electrode exhibits potent and persistent electroxidation behavior followed by well-separated oxidation peaks towards AA, DA, UA and Trp with increasing of the oxidation current. For the quaternary mixture containing AA, DA, UA and Trp, the 4 compounds can well separate from each other at the scan rate of 100 mVs(-1) with a potential difference of 270 mV, 150 mV and 260 mV for the oxidation peak potentials of AA-DA, DA-UA and UA-Trp, respectively, which was large enough to simultaneous determine AA, DA, UA and Trp. The catalytic peak current obtained, was linearly dependent on the AA, DA, UA and Trp concentrations in the range of 7.77-833 μM, 7.35-833 μM, 0.23-83.3 μM and 0.074-34.5 μM and the detection limits for AA, DA, UA and Trp were 1.11, 1.05, 0.033 and 0.011 μM, respectively. The analytical performance of this sensor has been evaluated for simultaneous detection of AA, DA, UA and Trp in human serum and urine samples.     

Electrochemical imprinted sensor for determination of oleanic acid based on poly (sodium 4-styrenesulfonate-co-acrylic acid)-grafted multi-walled carbon nanotubes-chitosan and cobalt hexacyanoferrate nanoparticles

A novel sensitive and selective imprinted electrochemical sensor for the determination of oleanic acid was constructed on a carbon electrode by stepwise modification of functional multi-walled carbon nanotubes, cobalt hexacyanoferrate nanoparticles and a thin imprinted sol-gel film. The fabrication of a homogeneous porous poly (sodium 4-styrenesulfonate-co-acrylic acid)-grafted multi-walled carbon nanotubes/SiO(2)-chitosan nanocomposite film was conducted by controllable electrodeposition technology. The surface morphologies of the modified electrodes were characterized by scanning electron microscope. The performance of the imprinted sensor was investigated by cyclic voltammetry, square wave voltammetry and electrochemical impedance spectroscopy in detail. The imprinted sensor displayed high sensitivity and selectivity towards oleanic acid. A linear relationship between the sensor response signal and the logarithm of oleanic acid concentrations ranging from 1.0×10(-8) to 1.0×10(-3) mol L(-1) was obtained with a detection limit of 2.0×10(-9) mol L(-1). It was applied to the determination of oleanic acid in real capsule samples successfully.     

Simultaneous determination of ascorbic acid and dopamine in the presence of uric acid on ruthenium oxide modified electrode

RuOx x nH2O film was electrochemically synthesized conveniently using cyclic voltammetric technique. The film formation was ascertained by the Electrochemical quartz crystal microbalance (EQCM) method and 45 ng of deposit per cycle was obtained. Stoichiometric ratio of the ruthenium and ruthenium oxide have been studied with different pH of phosphate buffer. The stability of the modified electrode in the presence of different cations and anions with different concentrations and pH were examined. Electrochemical studies have shown that the ascorbic acid (AA) and dopamine (DA) catalytic oxidation on ruthenium oxide modified electrode (RME) with a span of 300 mV separation even in the presence of uric acid (UA) with a large decrease in their respective over potential compared with bare glassy carbon electrode (GC). Accidentally, the reversible redox properties of the AA have been expediently studied on the RME using cyclic voltammetry and this peculiarity was interrogated through rotating ring disc electrode (RRDE) experiments. RRDE experiment results are conformed to the CV studies result and thus reversible redox property of AA have been reiterated. Amperometric detection under stirred condition up to approximately 0.8mM of AA and DA was carried out at free of electrode fouling. Interestingly, the regeneration of used RME electrode even after many consequent analysis, 100% was obtained.     

Determination of dopamine in the presence of ascorbic acid using poly(3,5-dihydroxy benzoic acid) film modified electrode

A polymerized film of 3,5-dihydroxy benzoic acid (DBA) was prepared on the surface of a glassy carbon electrode (GCE) in neutral solution by cyclic voltammetry (CV). The poly(DBA) film-coated GCE exhibited excellent electrocatalytic activity toward the oxidation of dopamine (DA). A linear range of 1.0 × 10⁻⁷ to 1.0 × 10⁻⁴ M and a detection limit of 6.0 × 10⁻⁸ M were observed in pH 7.4 phosphate buffer solutions. Moreover, the interference of ascorbic acid (AA) was effectively eliminated. This work provides a simple and easy approach to selective detection of DA in the presence of AA.     

Highly sensitive and selective method to detect dopamine in the presence of ascorbic acid by a new polymeric composite film

A new approach was applied to modify gold electrode with a unique polymer composite for selectively detecting dopamine (DA), a neurotransmitter, in the presence of an electroactive species of ascorbic acid (AA). After self-assembly of 11-mercaptoundecanoic acid (MUA) monolayer on gold surface, polyethylene glycol (PEG) was used to perform electrochemical esterification with MUA. In general, AA is the main interference of DA detection in a biological system. The resulting composite layer showed high sensitivity to detect DA but selectively blocked the interference from AA. Furthermore, for the first time, an interesting mechanism was demonstrated from our experimental results, namely, that the catalytic effect of AA on DA is limited by DA concentration when AA/DA>1. The modified electrode showed good reproducibility (+/-2% relative standard deviation), a low detection limit (10 nM), a fast response time (<2s), high sensitivity (86 nA/microM), a wide dynamic range of detection (20 microM), and great selectivity (without AA interference). The discovery is very promising for applications of detection of DA in a physiological environment where a high concentration of AA always exists.     

Overoxidized polypyrrole/multi-walled carbon nanotubes composite modified electrode for in vivo liquid chromatography–electrochemical detection of dopamine

Overoxidized polypyrrole/multi-walled carbon nanotubes (OPPy/MWNTs) modified electrode has been developed for sensitively detecting dopamine (DA). OPPy films developed outside MWNTs might have a porous morphology. Thus, OPPy/MWNTs films developed by this method do not reject ascorbic acid (AA). However, OPPy/MWNTs modified electrode shows largely enhancing oxidative current responses of DA. When combined with liquid chromatography, it not only obtains a low detection limit of 7.5 × 10^?10 mol L^?1 for DA, but also improves the selectivity of DA detection. Mechanisms for the enhancement are also well discussed in this paper. With this approach, microdialysis has been employed for successful assessment of DA in rat striatum.     

Glucose biosensor prepared by glucose oxidase encapsulated sol-gel and carbon-nanotube-modified basal plane pyrolytic graphite electrode

A new glucose biosensor has been fabricated by immobilizing glucose oxidase into a sol-gel composite at the surface of a basal plane pyrolytic graphite (bppg) electrode modified with multiwall carbon nanotube. First, the bppg electrode is subjected to abrasive immobilization of carbon nanotubes by gently rubbing the electrode surface on a filter paper supporting the carbon nanotubes. Second, the electrode surface is covered with a thin film of a sol-gel composite containing encapsulated glucose oxidase. The carbon nanotubes offer excellent electrocatalytic activity toward reduction and oxidation of hydrogen peroxide liberated in the enzymatic reaction between glucose oxidase and glucose, enabling sensitive determination of glucose. The amperometric detection of glucose is carried out at 0.3 V (vs saturated calomel electrode) in 0.05 M phosphate buffer solution (pH 7.4) with linear response range of 0.2-20 mM glucose, sensitivity of 196 nA/mM, and detection limit of 50 microM (S/N=3). The response time of the electrode is < 5s when it is stored dried at 4 degrees C, the sensor showed almost no change in the analytical performance after operation for 3 weeks. The present carbon nanotube sol-gel biocomposite glucose oxidase sensor showed excellent properties for the sensitive determination of glucose with good reproducibility, remarkable stability, and rapid response and in comparison to bulk modified composite biosensors the amounts of enzyme and carbon nanotube needed for electrode fabrication are dramatically decreased.     

Electrochemistry and detection of some organic and biological molecules at conducting poly(3-methylthiophene) electrodes

Electrodes modified by the electrodeposition of poly(3-methylthiophene) were used as chemical sensors for some organic and biological molecules of industrial and medicinal interest. The electrochemical behaviors of ferri/ferrocyanide, catechol, ascorbic acid, hydroquinone, dopamine epinephrine, acetaminophen, p-aminophenol and NADH were examined by cyclic voltammetry. The results showed that the proposed modified surface catalyzes the oxidation of these compounds. Differential pulse and square wave techniques were used for the analysis of binary mixture of ascorbic acid with catechol, NADH, dopamine and p-aminophenol. Voltammetric peak resolution was also demonstrated for a ternary mixture of ascorbic acid, catechol and p-aminophenol. Polymer coated electrode was also used in an amperometric detector for flow injection analysis of most of the aforementioned compounds. The responses of the polymer electrode were 4-10 times larger as compared to those of platinum. The modified electrode displayed excellent response stability for successive injections and detection limits were 10 ppb for catechol, dopamine, epinephrine, NADH and p-aminophenol, 1 ppb for acetaminophen and 100 ppb for ascorbic acid. Voltammetric peak positions were affected by the nature of the electrolyte and its pH. Also, film thicknesses were shown to be a factor affecting both the current magnitudes and oxidation peak potential of NADH.     

Au-nanoclusters incorporated 3-amino-5-mercapto-1,2,4-triazole film modified electrode for the simultaneous determination of ascorbic acid, dopamine, uric acid and nitrite

A novel biosensor has been constructed by the electrodeposition of Au-nanoclusters (nano-Au) on poly(3-amino-5-mercapto-1,2,4-triazole) (p-TA) film modified glassy carbon electrode (GCE) and employed for the simultaneous determination of dopamine (DA), ascorbic acid (AA), uric acid (UA) and nitrite (NO₂⁻). NH₂ and SH groups exposed to the p-TA layer are helpful for the electrodeposition of nano-Au. The combination of nano-Au and p-TA endow the biosensor with large surface area, good biological compatibility, electricity and stability, high selectivity and sensitivity and flexible and controllable electrodeposition process. In the fourfold co-existence system, the linear calibration plots for AA, DA, UA and NO₂⁻ were obtained over the range of 2.1–50.1 μM, 0.6–340.0 μM, 1.6–110.0 μM and 15.9–277.0 μM with detection limits of 1.1 × 10⁻⁶ M, 5.0 × 10⁻⁸ M, 8.0 × 10⁻⁸ M and 8.9 × 10⁻⁷ M, respectively. In addition, the modified biosensor was applied to the determination of AA, DA, UA and NO₂⁻ in urine and serum samples by using standard adding method with satisfactory results.