•A poly(6-thioguanine) film modified glassy carbon electrode (P6-TG/GCE) had been fabricated.•P6-TG/GCE showed catalytic activities toward the electro-oxidations of DA, UA, XA and HXA.•P6-TG/GCE could separate the overlapped oxidation waves of DA, UA, XA and HXA into four anodic peaks.•P6-TG/GCE could be used for the simultaneous determinations of DA, UA, XA and HXA in real samples.A novel and simple electrochemical sensor based on the electro-polymerized film of 6-thioguanine (6-TG) modified on glassy carbon electrode (GCE) has been fabricated and used for the selective and simultaneous determination of dopamine (DA), uric acid (UA), xanthine (XA), and hypoxanthine (HXA) in 0.1 mol L− 1 phosphate buffer solution (PBS, pH 7.0) by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques. The results show that the polymerized 6-TG modified GCE (P6-TG/GCE) not only exhibits electro-catalytic effects toward electrochemical oxidations of DA, UA, XA and HXA with negatively shifted oxidation potentials and enhanced peak current responses, but also resolves the sluggish and overlapped voltammetric responses of DA, UA, XA and HXA into four strong and well-defined oxidation peaks using both CV and DPV, which can be applied for the selective and simultaneous determination of DA, UA, XA and HXA in their mixture. The surface morphology of the P6-TG film has been investigated by using a scanning electron microscope (SEM). The P6-TG film exhibits a nearly homogeneous surface structure with irregular and continuous holes and sticks standing on an electrode surface, which is helpful for the electrooxidations of analytes. Under the optimum conditions, the linear dependences of DPV current responses are observed for DA, UA, XA and HXA in the concentration ranges of 1–200 μmol L− 1, 2–1600 μmol L− 1, 1–500 μmol L− 1 and 2–800 μmol L− 1 with the correlation coefficients of 0.9986, 0.9997, 0.9997 and 0.9998, respectively. The detection limits are 0.05 μmol L− 1, 0.06 μmol L− 1, 0.30 μmol L− 1 and 0.10 μmol L− 1 for DA, UA, XA and HXA, respectively (S/N = 3). The different electrochemical behaviors of DA, UA, XA and HXA at various scan rates indicate that the electrode reaction of DA is an adsorption-controlled process, and those of UA, XA and HXA are diffusion-controlled processes at P6-TG/GCE. The application of P6-TG/GCE are demonstrated by simultaneously determining the concentrations of DA, UA, XA and HXA in human urine and serum samples by using standard adding method with satisfactory results.

Poly(2-mercaptobenzothiazole) (PMBT) modified glassy carbon electrode (PMBT/GCE) has been fabricated and employed for the selective and simultaneous determinations of dopamine (DA), uric acid (UA) and nitrite (NO2−) in 0.1 mol L−1 phosphate buffer solution (PBS, pH 6.0). The PMBT modified GCE not only exhibits electro-catalytic effects toward the electrochemical oxidations of DA, UA and NO2− with negatively shifted oxidation potentials and enhanced peak current responses, but also resolves the weak and overlapped voltammetric responses of DA, UA and NO2− into three strong and well-defined oxidation peaks using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques, which can be applied for the selective and simultaneous determinations of DA, UA and NO2− in their mixture. The surface morphology of PMBT film has been investigated by using scanning electron microscope (SEM). The SEM image shows the formation of continuous PMBT film composed of nano-scaled particulates with the diameter varying in the range of 15-25 nm. Under the optimum conditions, the linear concentration dependences of DPV peak current responses are observed for DA, UA and NO2− in the concentration ranges of 0.8-45 μmol L−1, 1.0-165 μmol L−1 and 60-1000 μmol L−1, with the correlation coefficients of 0.9996, 0.9994 and 0.9979, respectively. The detection limits (S/N = 3) are 0.05 μmol L−1, 0.10 μmol L−1 and 0.30 μmol L−1 for measuring DA, UA and NO2−, respectively. The modified electrode has been used for simultaneous determinations of DA, UA and NO2− in human urine and serum samples using standard adding method with satisfactory results.

Electrochemical copolymerization of o-phenylenediamine (OPD) with o-methoxy aniline (OMA) in 1.0 M HCl solution has been studied using cyclic voltammetric (CV) and in situ ultraviolet–visible (UV–vis) spectroelectrochemical techniques employing the indium tin oxide (ITO)-coated transparent glass as working electrode. The electrochemical features for the individual polymerizations of OPD and OMA are differently displayed in the copolymerization of OPD with OMA. The in situ UV–vis spectroelectrochemical results indicate that OPD and OMA are firstly oxidized to form the o-phenylenediamine cation radicals (OPDCR) and o-methoxy aniline cation radicals (OMACR) during the electrochemical copolymerization of OPD with OMA, then, mixed dimers/oligomers intermediates are formed by the cross-reaction between OPDCR and OMACR. The absorption peaks located at 403–422 nm and 482–492 nm in UV–vis spectra are assigned to these intermediates, which can also be confirmed by the results of CV and derivative cyclic voltabsorptogram (DCVA). The different voltammetric characteristics between the homopolymerization and copolymerization processes exhibit the occurrence of the copolymerization, and the differences between the copolymerization of OPD and OMA with different concentration ratios show the dependence of copolymerization on the concentration ratios of OPD and OMA in the feed. Furthermore, the copolymerization of OPD with OMA shows clear dependence on the applied potential. The copolymer has also been characterized by using Fourier transform infrared spectroscopy (FT-IR) and fluorescence spectroscopy. A plausible mechanism for the copolymerization of OPD with OMA has been suggested.

A poly(2-amino-4-thiazoleacetic acid-co-3-amino-5-mercapto-1,2,4-triazole) (PATA-AMT) film modified glassy carbon electrode (GCE) has been fabricated by using electrochemical copolymerization of 2-amino-4-thiazoleacetic acid (ATA) and 3-amino-5-mercapto-1,2,4-triazole (AMT) and used for electro-catalytic oxidations of dopamine (DA), uric acid (UA) and nitrite ion (NO2−) in 0.1 mol L−1 phosphate buffer solution (PBS, pH 6.0). The surface morphology of the copolythiazole film has been investigated by using scanning electron microscope (SEM). The SEM images show the formation of a non-periodic nano-network structure with a diameter of the fibril varying over the range of 55–85 nm. The copolythiazole film (PATA-AMT) modified GCE (PATA-AMT/GCE) not only exhibits strong electro-catalytic activities toward oxidations of DA, UA and NO2− with negatively shifted oxidation overpotentials and enhanced peak current responses, but also can resolve the sluggish and overlapped voltammetric waves of DA, UA and NO2− into three sensitive and well-defined oxidation peaks by both cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which can be applied for the selective and simultaneous determinations of DA, UA and NO2− in a mixture. Under the optimum conditions, the linear concentration dependences of the DPV current responses are observed for DA, UA and NO2− in the concentration ranges of 0.85–390 μmol L−1, 1.95–1200 μmol L−1 and 2.0–1240 μmol L−1 with the correlation coefficients of 0.9993, 0.9993 and 0.9977, respectively. The detection limits are 0.2 μmol L−1, 0.25 μmol L−1 and 0.5 μmol L−1 for DA, UA and NO2−, respectively (S/N = 3). The different electrochemical behaviors of DA, UA and NO2− at various scan rates indicate that the electrode reaction of DA is an adsorption-controlled process, and those of UA and NO2− are diffusion-controlled processes at PATA-AMT/GCE. The practical application of the modified electrode has been investigated by the simultaneous determinations of DA, UA and NO2− in human urine and serum samples by using standard adding method with satisfactory results.

Cu nanoparticles (nano-Cu)–poly(sulfonazo III) (PSA III) modified glassy carbon electrode (nano-Cu–PSA III/GCE) had been fabricated and successfully used for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The performance of the nano-Cu–PSA III/GCE towards the electrooxidation of AA, DA and UA had been investigated by cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The results showed that the modified electrode not only exhibited excellent electrocatalytic activity toward the electrooxidation of AA, DA and UA in a phosphate buffer solution (PBS, pH 4.0), but also could separate the overlapped oxidation waves of these species at the bare GCE into three sharp and strong oxidation peaks at 0.23 V, 0.37 V and 0.54 V using CV and 0.16 V, 0.33 V and 0.49 V using DPV, corresponding to the electrooxidation of AA, DA and UA, respectively. The separations of anodic peak potentials for AA–DA, DA–UA and AA–UA at nano-Cu–PSA III/GCE are large enough for the simultaneous detection of AA, DA and UA. Under the optimum conditions, the linear calibration curves were obtained over the range of 0.30–730, 0.02–65 and 0.25–107 μmol L−1 with detection limits of 0.15, 0.01 and 0.10 μmol L−1 for AA, DA and UA (S/N = 3), respectively. Due to the good selectivity and high sensitivity, nano-Cu–PSA III/GCE had been used for the simultaneous determination of AA and UA in urine samples and DA in human serum samples with satisfactory results.Highlights► Nano-Cu-PSA III composite film modified GC electrode (nano-Cu-PSA III/GCE) was fabricated. ► Nano-Cu-PSA III/GCE showed electrocatalytic activities towards the oxidation of AA, DA and UA. ► Nano-Cu-PSA III/GCE could separate the overlapped oxidation waves of AA, DA and UA into three anodic peaks. ► Nano-Cu-PSA III/GCE had been used for the simultaneous determination of AA, DA and UA in real samples.

Poly(2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole) (PAPT) modified glassy carbon electrode (GCE) was fabricated and used for the simultaneous determinations of dopamine (DA), uric acid (UA) and nitrite (NO2−) in 0.1 mol L−1 phosphate buffer solution (PBS, pH 5.0) by using cyclic voltammetry and differential pulse voltammetry (DPV) techniques. The results showed that the PAPT modified GCE (PAPT/GCE) not only exhibited electrocatalytic activities towards the oxidation of DA, UA and NO2− but also could resolve the overlapped voltammetric signals of DA, UA and NO2− at bare GCE into three strong and well-defined oxidation peaks with enhanced current responses. The peak potential separations are 130 mV for DA–UA and 380 mV for UA–NO2− using DPV, which are large enough for the simultaneous determinations of DA, UA and NO2−. Under the optimal conditions, the anodic peak currents were correspondent linearly to the concentrations of DA, UA and NO2− in the ranges of 0.95–380 μmol L−1, 2.0–1,000 μmol L−1 and 2.0–1,200 μmol L−1 for DA, UA and NO2−, respectively. The correlation coefficients were 0.9989, 0.9970 and 0.9968, and the detection limits were 0.2, 0.35 and 0.6 μmol L−1 for DA, UA and NO2−, respectively. In 0.1 mol L−1 PBS pH 5.0, the PAPT film exhibited good electrochemical activity, showing a surface-controlled electrode process with the apparent heterogeneous electron transfer rate constant (ks) of 25.9 s−1 and the charge–transfer coefficient (α) of 0.49, and thus displayed the features of an electrocatalyst. Due to its high sensitivity, good selectivity and stability, the modified electrode had been successfully applied to the determination of analytes in serum and urine samples.

Three-dimension (3D) belt-like polyaniline (PAN) network has been prepared via electrochemical polymerization of aniline on p-phenylenediamine (PDA) functionalized glassy carbon electrode (GCE) using a three-step electrochemical deposition procedure. PDA was covalently binded on GCE via the formation of carbon–nitrogen bond between amine cation radical and the aromatic moiety of GCE surface using electrochemical oxidation procedure. X-ray photo-electron spectroscopy (XPS) and cyclic voltammetry have been performed to characterize the attachment of PDA on GCE. The images of scanning electron microscope (SEM) show that the 3D belt-like PAN network is uniform. The width and thickness of the PAN belt varies in the range of 1.5–5.5 μm and 0.1–0.8 μm, respectively. The distance between the belt-contacts ranges from 2.5 to 15 μm. The 3D belt-like PAN network modified GCE (PAN–PDA/GCE) exhibits an improved electro-activity of PAN at an extended pH up to 7.0. The PAN–PDA/GCE not only immobilizes but also leads to a direct electrochemical behavior of cytochrome c (Cyt c). The immobilized Cyt c maintains its activity, showing a surface-controlled electrode process with the electron-transfer rate constant (ks) of 14.8 s−1 and electron-transfer coefficient (α) of 0.48, and could be used for the electrocatalytic reduction of hydrogen peroxide (H2O2).

Polyaniline (PAN) nano-particles, nano-fibrils, and nano-network have been synthesized via electrochemical polymerization of aniline using a three-step electrochemical deposition procedure on α-alanine (ALA)-monolayer functionalized glassy carbon electrode (GCE). The structure and properties of PAN nano-structures have been characterized using field emission scanning electron microscope (SEM), Fourier transform infrared spectra (FT-IR), and electrochemical techniques. The 3-dimensional (3D) PAN nano-network/ALA composite film coated GCE (PAN–ALA/GCE) leads to the direct electrochemistry of horse heart cytochrome c (Cyt c) immobilized on this electrode surface. The immobilized Cyt c maintains its activity, showing a surface-controlled electrode process with the electron transfer rate constant (ks) of 21.9 s−1 and the charge-transfer coefficient (α) of 0.37, and could be used for the electrocatalytic reduction of hydrogen peroxide (H2O2). The steady-state current response increases linearly with H2O2 concentration from 2.5 × 10−5 to 3.0 × 10−4 mol l−1. The detection limit (3δ) is 8.2 × 10−6 mol l−1.

The polymerization of o-phenylenediamine (OPD) on l-tyrosine (Tyr) functionalized glassy carbon electrode (GCE) and its electro-catalytic oxidation towards ascorbic acid (AA) had been studied in this report. l-Tyrosine was first covalently grafted on GCE surface via electrochemical oxidation, which was followed by the electrochemical polymerization of OPD on the l-tyrosine functionalized GCE. Then, the poly(o-phenylenediamine)/l-tyrosine composite film modified GCE (POPD-Tyr/GCE) was obtained. X-ray photo-electron spectroscopy (XPS), field emission scanning electron microscope (SEM), and electrochemical techniques have been used to characterize the grafting of l-tyrosine and the polymerization and morphology of OPD film on GCE surface. Due to the doping of the carboxylic functionalities in l-tyrosine molecules, the POPD film showed good redox activity in neutral medium, and thus, the POPD-Tyr/GCE exhibited excellent electrocatalytic response to AA in 0.1 mol l−1 phosphate buffer solution (PBS, pH 6.8). The anode peak potential of AA shifted from 0.58 V at GCE to 0.35 V at POPD-Tyr/GCE with a greatly enhanced current response. A linear calibration graph was obtained over the AA concentration range of 2.5 × 10−4–1.5 × 10–3 mol l−1 with a correlation coefficient of 0.9998. The detection limit (3δ) for AA was 9.2 × 10−5 mol l−1. The modified electrode showed good stability and reproducibility and had been used for the determination of AA content in vitamin C tablet with satisfactory results.

Poly(aniline-co-o-aminophenol) (PAN–OAP) has been synthesized by electrochemical copolymerization of aniline (AN) and o-aminophenol (OAP) on glassy carbon electrode (GCE). FT-IR, UV–vis and electrochemical characterizations indicate the formation of the PAN–OAP and it has the structure of a head-to-tail coupling of AN and OAP units. When the concentration ratio of OAP–AN is more than 1:20, the voltammetric behaviors for the copolymerization of AN and OAP are similar, while the copolymerization rate decreases with increasing of OAP concentration in the solution. The PAN–OAP can maintain its electroactivity in neutral and even in alkaline media, although the electroactivity of the copolymers decrease with increasing of the concentration ratio of OAP–AN in solution. The PAN–OAP film coated GCE exhibits excellent electrocatalytic responses towards the electro-oxidation of ascorbic acid (AA) in phosphate buffer solution of pH 6.8. The anode peak potential of AA shifts from 0.53 V at bare GCE to 0.21 V at PAN–OAP/GCE with greatly enhanced current response. A linear calibration graph is obtained over the AA concentration range of 5 × 10−4–1.65 × 10−2 mol L−1 using cyclic voltammetry. Chronoamperometry has also been employed to investigate the electro-oxidation of AA. The PAN–OAP/GCE shows good stability and reproducibility.

Self-doped polyaniline (PAN) film on platinum electrode surface has been synthesized via electrochemical copolymerization of aniline with orthanilic acid (OAA). Fourier transform infrared, UV–Vis, and elemental analysis indicate the formation of the copolymer and that the copolymer has the structure of a head-to-tail coupling of aniline and OAA units. It was found that the internal doping of PAN with OAA can extend the electroactivity of PAN in neutral and even in alkaline media. The obtained self-doped PAN (PAN-OAA)-coated platinum electrode is shown to be a good surface for the electrooxidation of ascorbic acid (AA) in phosphate buffer solution of pH 7. The anode peak potential of AA shifts from 0.63 V at bare platinum electrode to 0.34 V at the PAN-OAA-modified platinum electrode with greatly enhanced current response. A linear calibration graph is obtained over the AA concentration range of 5–60 mM using cyclic voltammetry. Rotating disk electrode voltammetry and chronoamperometry have been employed to investigate the electrooxidation of AA. The PAN-OAA-modified platinum electrode shows good stability and reproducibility.

Gold nanoparticles have been attached on glassy carbon electrode surface through sulfhydryl-terminated monolayer and the gold nanoparticles-immobilized glassy carbon electrodes have been applied to the electrocatalytic oxidation of ascorbic acid, reducing the overpotential by about 200 mV with obviously increased current response. Due to its strong electrocatalytic activity towards ascorbic acid, the gold nanoparticles modified electrode can resolve the overlapped voltammetric waves of ascorbic acid and dopamine into two well-defined voltammetric peaks with peak-to-peak separation in potentials of about 300 mV. This can be used to allow the selective determination of ascorbic acid in the presence of dopamine. The catalytic current obtained from differential pulse voltammetry is linearly dependent on ascorbic acid concentration over the range of 6.5 × 10−6 to 1.45 × 10−4 M with correlation coefficient of 0.998 in the presence of dopamine. The detection limit (3σ) for AA was found to be 2.8 × 10−6 M. The simultaneous determination of ascorbic acid and dopamine in their binary mixture has also been investigated. The modified electrode shows good selectivity, stability and anti-fouling properties. The proposed methods have been used for the selective determination of ascorbic acid in the presence of dopamine and for the simultaneous determination of both them in their mixtures with satisfactory results.