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A sensitive aptamer/protein binding-triggered sandwich assay for thrombin is described. It is based on electrochemical-chemical-chemical redox cycling using a glassy carbon electrode (GCE) that was modified with WSe2 and gold nanoparticles (AuNPs). The AuNPs are linked to thrombin aptamer 1 via Au-S bonds. Thrombin is first captured by aptamer 1 and then sandwiched through the simultaneous interaction with AuNPs modified with thrombin-specific aptamer 2 and signalling probe. Subsequently, the DNA-linked AuNP hybrids result in the capture of streptavidin-conjugated alkaline phosphatase onto the modified GCE through the specific affinity reaction for further signal enhancement. As a result, a linear range of 0–1 ng mL−1 and a detection limit as low as 190 fg mL−1 are accomplished. The specificity for thrombin is excellent. Conceivably, this strategy can be easily expanded to other proteins by using the appropriate aptamer.

We have developed an electrochemical method for the determination of 17β-estradiol. A glassy carbon electrode was modified with a composite made from copper sulfide nanosheets, gold nanoparticles, and glucose oxidase. The copper sulfide nanosheet was prepared by a single-step hydrothermal process, and its properties were characterized by X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy and transmission electron microscopy. Finally, an estradiol-specific aptamer was assembled on the electrode. The copper sulfide nanosheet on the electrode surface acts as a relatively good electrical conductor. Glucose oxidase acts as an indicator, and the dual modification of glucose oxidase and gold nanoparticles for signal amplification. The determination of 17β-estradiol was performed by differential pulse voltammetry of glucose oxidase because the signal measured at typically −0.43 V depends on the concentration of 17β-estradiol because addition of 17β-estradiol at electrode hinders electron transfer. A linear relationship exists between the peak current and the logarithm of concentration of 17β-estradiol in the 0.5 pM to 5 nM range, with a 60 f. detection limit (at 3σ/S). The method displays good selectivity over bisphenol A, 1-aminoanthraquinone and naphthalene even if present in 100-fold concentrations.

We report on the modification of a glassy carbon electrode with a composite consisting of silver nanoparticles (AgNPs), polydopamine, and graphene to give an electrochemical sensor for catechol. The composite was characterized by transmission electron microscopy, and the electrochemical behavior of catechol at the modified electrode was studied by cyclic voltammetry. The electrochemical response is greatly enhanced and thought to result from a combination of beneficial effects including the good conductivity and large surface area of the AgNPs, the high conductivity of graphene, the synergistic effects of the composite, and the increased quantity of catechol that is adsorbed on the surface of the electrode. Differential pulse voltammetric responses are proportional to the concentration of catechol between 0.5 and 240 μM levels of catechol, and the detection limit is 0.1 μM (S/N = 3). The performance of the sensor was evaluated with catechol-spiked water samples, and recoveries range from 96.5 % to 103.1 %. The results indicated that the composite presented here is a promising substrate for use in electrochemical sensing.

We report on the modification of a graphene paste electrode with gold nanoparticles (AuNPs) and a Nafion-L-cysteine composite film, and how this electrode can serve as a platform for the construction of a novel electrochemical immunosensor for the detection of hepatitis B surface antigen (HBsAg). To obtain the immunosensor, an antibody against HBsAg was immobilized on the surface of the electrode, and this process was followed by cyclic voltammetry and electrochemical impedance spectroscopy. The peak currents of a hexacyanoferrate redox system decreased on formation of the antibody-antigen complex on the surface of the electrode. Then increased electrochemical response is thought to result from a combination of beneficial effects including the biocompatibility and large surface area of the AuNPs, the high conductivity of the graphene paste electrode, the synergistic effects of composite film, and the increased quantity of HBsAb adsorbed on the electrode surface. The differential pulse voltammetric responses of the hexacyanoferrate redox pair are proportional to the concentration of HBsAg in the range from 0.5–800 ng mL−1, and the detection limit is 0.1 ng mL−1 (at an S/N of 3). The immunosensor is sensitive and stable.

A simple, cost-effective and efficient method was developed for the determination of glycine, gamma-aminobutyrate and taurine in rat brain using graphene as a sorbent for solid-phase extraction. The analytes were eluted from a graphene-packed solid-phase extraction cartridge with methanol, derivatized at their amino groups with the fluorescent label 4-carboxy-2,6-dimethylquinoline N-hydroxysuccinimide ester, and then separated and fluorescently detected by HPLC. The type and volume of eluent, sample pH, extraction time and sample volume were optimized with respect to sensitivity and precision. Under optimal conditions, linear response is obtained in the concentration range from 0.1 to 50 μg g−1, with correlation coefficients of >0.990. The limits of detection are 23.4 ng g−1 (gamma-aminobutyrate), 45.3 ng g−1 (glycine) and 67.5 ng g−1 (taurine) (S/N = 3). The results reveal the potential of graphene as a sorbent in the analysis of biological samples.

We describe a method for solid-phase extraction of biogenic thiols using multi-walled carbon nanotubes as adsorbent, and their subsequent determination via HPLC and fluorescence detection. The fluorogenic reagent N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-yl)methyl) iodoacetamide was applied to derivatizate the thiols. The type of eluent and its volume, the sample pH, extraction time and sample volume were optimized. The calibration curves of the thiols are linear in the range from 0.5 to 200 nM (for glutathione), 0.02 to 5 nM (for cysteine), and 2 to 500 nM (for acetylcysteine), and the correlation coefficients range between 0.9955 and 0.9997. The respective limits of detection are 20 pM, 4 pM and 80 pM (at an SNR of 3), and the limits of quatification are 67 pM, 13 pM, and 267 pM (at an SNR of 10). Recoveries range from 85.0% to 113.1% for human urine and plasma samples spiked with the three thiols, and relative standard deviations are in the range from 2.1 to 7.4%.

We present a modified glassy carbon electrode as a sensing platform for glucose. It is based on a composite film prepared from Ni(II) ion, quercetin and graphene. The sensor was characterized by cyclic voltammetry. The electron transfer coefficient, reaction rate constant and catalytic rate constant were determined and found to be 0.53, 5.4 s−1 and 2.93 × 103 M−1 s−1, respectively. The catalytic current depends linearly on the concentration of glucose in the range from 3 to 900 μM, with a detection limit of 0.5 μM (at an S/R of 3). The sensor exhibits good reproducibility, stability, fast response, and high sensitivity.

A new strategy is described to construct disposable electrochemical immunosensors for the assay of human immunoglobulin. It is based on a carbon paste electrode constructed from chitosan nanoparticles modified with colloidal gold. The stepwise assembly process of the immunosensor was characterized by means of cyclic voltammetry and electrochemical impedance spectroscopy. Assay conditions that were optimized included the amount of chitosan nanoparticles in the preparation of carbon paste electrode, antibody concentration, and the incubation time of the antibody immobilization. Using hexacyanoferrate as a mediator, the current change increased with the concentration of human immunoglobulin G. A linear relationship in the concentration range 0.3 to 120 ng mL−1 was achieved, with a detection limit of 0.1 ng mL−1 (S/N = 3). The method combines the specificity of the immunological reaction with the sensitivity of the gold colloid amplified electrochemical detection, and it has potential application in clinical immunoassay.

A rapid, sensitive and simple electrochemical method is developed for the determination of trace-level ofloxacin, based on the excellent properties of multi walled carbon nanotubes (MWNT). The MWNT/Nafion film-coated glassy carbon electrode (GCE) is constructed and the electrochemical behavior of ofloxacin at the electrode is investigated in detail. Compared with the poor electrochemical signal at the unmodified GCE, the electrochemical response of ofloxacin at the MWNT/Nafion film-coated GCE is greatly improved. The determination conditions, such as the amount of MWNT–Nafion suspension, the pH values of the supporting electrolyte, accumulation potential and time, as well as scan rate are optimized. Under the chosen conditions, the concentration of ofloxacin shows excellent linear relationships with the oxidation peak current in the range of 0.5–100 µmol L−1, with a low detection limit of 1 × 10−7 mol L−1 (S/N = 3) for 20 sec accumulation at −1.2 V. The proposed method has been successfully applied to determine ofloxacin in pharmaceuticals with excellent repeatability and higher sensitivity.

A rapid and highly sensitive fluorogenic method has been developed for the determination of total primary aliphatic amines with a novel fluorogenic derivatizing reagent, 3-(4-fluorobenzoyl)-2-quinolinecarboxaldehyde (FBQCA). FBQCA selectively reacts with amines to form a highly fluorescent isoindole in buffer of pH 9.7 and in the presence of potassium cyanide solution. The fluorescence intensity of derivatized products are detected at λex/λem = 480/546 nm. The results of the experiments showed a linear dynamic range of 1 × 10−8 to 6 × 10−6 mol L−1 with a detection limit (S/N = 3) of 6 nmol L−1. The method was successfully used for the direct spectrofluorimetric determination of aliphatic amines in water samples with recoveries of 95–104%.

A rapid, highly sensitive and selective fluorogenic method for the determination of traces of nitrite is described. It is based on the reaction of weakly fluorescent 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacence (DAMBO) and nitrite in acidic aqueous solution to give 1,3,5,7-tetramethyl-8-(5-benzotriazolyl)-difluoroboradiaza-s-indacene (DAMBO-T), which is highly fluorescent. The optimum reaction conditions and other analytical parameters are investigated to enhance the sensitivity of the method. The fluorescence enhancement at 507 nm is linearly related to the concentration of nitrite in the range of 6.0 × 10−9–5.0 × 10−7 mol L−1 with a correlation coefficient of R = 0.9995 (n = 10) and a detection limit of 1.0 × 10−10 mol L−1. The R.S.D. is 1.12% (n = 10). The method is applied to the determination of nitrite in human saliva samples with the recoveries of 96. 24–105.30%.