Functionalized nanocomposites based on various type of graphene nanomaterials including graphene, graphene oxides (GOs), and doped graphene (oxides) are widely used as materials for various sensors that can display high sensitivity, selectivity and stability. This review with 347 references summarizes advances in the preparation and functionalization of graphene nanocomposites for the application of electrochemical sensors and biosensors. Following a general introduction into the field, the article is divided into subsections on (a) the synthesis and functionalization of nanocomposites (made from graphene, various kinds of GOs, heteroatom-doped GOs), (b) on methods for functionalization of composites (with other carbon nanomaterials, metal nanoparticles, metal oxide and metal sulfide nanoparticles), (c) on functionalization with inorganic materials including polyoxometalates, hexacyanoferrates, minerals), (d) on functionalization with organic materials such as amino acids, surfactants, organic dyes, ionic liquids, macrocycles (including cyclodextrins, crown ethers and calixarenes), and (e) on functionalization with organometallics and with various other organic compounds, (f) on functionalizations with polymers such as conventional polymers, polyelectrolytes, conducting polymers, molecularly imprinted polymers, (g) on functionalization with biomolecules including proteins and nucleic acids. Other subsections cover flexible graphene and GO based nanocomposites and 3D composites. Application of graphene and GO nanocomposites are then covered in a in large section that comprises electrochemical sensors and biosensors (based on voltammetry, amperometry, potentiometry, impedimetry, electrochemiluminescence, photoelectrochemistry, field effect transistors, electrochemical immunosensors) with specific subsections on gas sensors, enzymatic biosensors and gene sensors. A concluding section covers current challenges and perspectives of graphene and GO based (bio)sensing.

•The photoelectrochemical properties of NO was first investigated.•A novel photoelectrochemical sensor of NO was developed.•C60 was functionalized by polyhydroquinone (PH2Q), forming an electron donor–acceptor linked supramolecular system.•An enhanced cathodic photocurrent response of NO was obtained on a PH2Q-C60 modified ITO electrode under visible light radiation.•The main interferent NO2− has no apparent photoelectrochemical response on the NO photoelectrochemical sensor.An electron donor–acceptor linked supramolecular system composed of polyhydroquinone (PH2Q) and fullerene (C60) was prepared and characterized by different techniques like TEM, UV–vis, XRD and photoelectrochemical photocurrent detection techniques. The photoelectrochemical properties of nitric oxide (NO) were first investigated. An enhanced cathodic photocurrent response was obtained on the PH2Q-C60 modified ITO electrode under visible light radiation with NO as electron acceptor. The photoelectrochemical process of NO was discussed and a novel photoelectrochemical sensor of NO was developed. The influence of the main interferents, especially the significant interfering species NO2− was proved to be negligible. The photoelectrochemistry of NO and the new NO sensor may have potential applications in photoelectrochemistry and biological analysis field.

Electrochemical sensing has been demonstrated to represent an efficient way to quantify nitric oxide (NO) in challenging physiological environments. A sensing interface based on nanomaterials opens up new opportunities and broader prospects for electrochemical NO sensors. This review (with 141 refs.) gives a general view of recent advances in the development of electrochemical sensors based on nanomaterials. It is subdivided into sections on (i) carbon derived nanomaterials (such as carbon nanotubes, graphenes, fullerenes), (ii) metal nanoparticles (including gold, platinum and other metallic nanoparticles); (iii) semiconductor metal oxide nanomaterials (including the oxides of titanium, aluminum, iron, and ruthenium); and finally (iv) nanocomposites (such as those formed from carbon nanomaterials with nanoparticles of gold, platinum, NiO or TiO2). The various strategies are discussed, and the advances of using nanomaterials and the trends in NO sensor technology are outlooked in the final section.

We describe an electrochemical sensor for nitric oxide that was obtained by modifying the surface of a nanofiber carbon paste microelectrode with a film composed of hexadecyl trimethylammonium bromide and nafion. The modified microelectrode displays excellent catalytic activity in the electrochemical oxidation of nitric oxide. The mechanism was studied by scanning electron microscopy and cyclic voltammetry. Under optimal conditions, the oxidation peak current at a working voltage of 0.75 V (vs. SCE) is related to the concentration of nitric oxide in the 2 nM to 0.2 mM range, and the detection limit is as low as 2 nM (at an S/N ratio of 3). The sensor was successfully applied to the determination of nitric oxide released from mouse hepatocytes.

This work reported the rapid in situ detection of ultratrace 2,4-dinitrotoluene (DNT) solids on various substrates by a sandwiched paper-like electrochemical sensor. The sensor, prepared by a simple electroless deposition method without using special instruments, possessed a unique thin-film structure of an insulated polyvinylidene fluoride (PVDF) membrane in between two gold (Au) conducting layers. The resulting gold–PVDF sandwich (GPVDFS) array exhibited excellent flexibility, porosity and electrochemical performance as a highly integrated dual-electrode sensor platform. The infiltration of nonvolatile ionic liquid (IL) electrolytes containing ferrocene (Fc) into the GPVDFS array produced a paper-like electrochemical sensor, which can directly detect ultratrace DNT solids on various substrate surfaces (e.g., plant leaves, gloves and metal knives) with detection limit as low as 0.33 ng/mm2. The critical role of Fc in the detection of DNT at this dual-electrode sensor was explored. The compensating electrochemical oxidation of Fc at the counter/reference electrode was found to be essential to the reduction of DNT at the working electrode when IL electrolytes were employed. The present work thus demonstrated the promising applications of paper-based porous electrode arrays in developing IL-based electrochemical sensors for the in situ detection of analyte solids in complicated environments.

Recent progress in nanotechnology has promoted research in the surface functionalization of graphene oxide (GO) nanosheets owing to their large specific surface area and abundant functional groups. In the present work, cage-structured fullerene (C60) was attached non-covalently to laminar GO through simple grinding, which not only achieved the efficient solubilization of C60 in water but also retained the original physicochemical characteristics of GO and C60. The C60–GO nanocomposite could be dispersed in water with solubility up to 5 mg mL−1 and stability for more than one month. By further combining the redox reversibility of phosphotungstic acid (PTA), a novel electrochemical sensing film (PTA–C60–GO) was fabricated by one-step electropolymerization on a pretreated home-made graphite electrode (GE). This PTA–C60–GO/GE exhibited a sensitive electrochemical response for the direct oxidation of cis-jasmone (CJ), a well-known component of plant volatiles. Under optimal conditions, the oxidation current of CJ increased linearly with its concentration in the range of 0.3–50.0 μmol L−1 with a detection limit of 0.1 μmol L−1. This sensing platform was applied to the determination of the CJ content in rice spikelet samples.

•C60 or C70 is supported on GO via noncovalent conjunction through facile grinding.•GO–C60/C70 composite has solubility as high as 5 mg mL−1 for months.•GO–C60/C70 can be one-step electrodeposited with phosphotungstic acid (PTA).•PTA–GO–C60/C70 film electrode show catalytic activity for the redox of biomolecules.A versatile approach to the dispersion of C60/C70 in water with high concentration and stability by graphene oxide (GO) was reported here. By simply grinding with GO, C60/C70 can be readily dissolved in water with solubility up to 5 mg mL−1 and stability for more than three months. Transmission electron microscopy indicated that C60/C70 formed a uniform and high-density layer on the surface of GO, which may be achieved through the strong π–π interaction between fullerenes and GO, as supported by Fourier transform infrared spectroscopy, Raman and ultraviolet–visible spectroscopy spectra. Through a simple electrodeposition method, two novel phosphotungstic acid–graphene oxide–fullerene hybrids (i.e., PTA–GO–C60 and PTA–GO–C70) were formed on glassy carbon electrodes. The resulting hybrid modified electrodes exhibited enhanced electrocatalytic activity for the oxidation of a variety of small biomolecules, including dopamine, ascorbic acid, uric acid, l-tryptophan, tyrosine, indole-3-acetic acid, salicylic acid and 6-benzylaminopurine, reflected by the remarkably enlarged peak currents and apparently reduced oxidation overpotentials as compared with those on either PTA or PTA–GO modified electrodes.

A simple approach to the mass production of nanoporous gold electrode arrays on cellulose membranes for electrochemical sensing of oxygen using ionic liquid (IL) electrolytes was established. The approach, combining the inkjet printing of gold nanoparticle (GNP) patterns with the self-catalytic growth of these patterns into conducting layers, can fabricate hundreds of self-designed gold arrays on cellulose membranes within several hours using an inexpensive inkjet printer. The resulting paper-based gold electrode arrays (PGEAs) had several unique properties as thin-film sensor platforms, including good conductivity, excellent flexibility, high integration, and low cost. The porous nature of PGEAs also allowed the addition of electrolytes from the back cellulose membrane side and controllably produced large three-phase electrolyte/electrode/gas interfaces at the front electrode side. A novel paper-based solid-state electrochemical oxygen (O2) sensor was therefore developed using an IL electrolyte, 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF6). The sensor looked like a piece of paper but possessed high sensitivity for O2 in a linear range from 0.054 to 0.177 v/v %, along with a low detection limit of 0.0075% and a short response time of less than 10 s, foreseeing its promising applications in developing cost-effective and environment-friendly paper-based electrochemical gas sensors.

cis-Jasmone is a plant activator in terms of production of defence-related volatile semiochemicals. The redox and quantitative analysis of cis-jasmone are practically significant for the investigation of physiological processes in plants. In this work, the electrochemical characteristics of cis-jasmone in different acid media were investigated at a multi-wall carbon nanotube/Nafion composite film modified glassy carbon electrode (MWNT/Nafion/GCE) by cyclic voltammetry, chronocoulometry and infrared spectroscopy. The redox mechanism of cis-jasmone in strong acid media, i.e., a remarkable catalytic oxidation process for the reduction product of cis-jasmone at MWNT/Nafion/GCE, was demonstrated to be completely different from the cases in weakly acidic or neutral solution. The standard rate constant k0, the diffusion coefficient D and the surface coverage Γ were calculated and discussed. The sensitive anodic stripping voltammetric response of cis-jasmone was employed to determine its content in the extract of rice spikelet. The electrochemical redox and quantitative analysis of cis-jasmone are therefore of great significance for studying not only the physiological processes of cis-jasmone in plants but also the electrochemical redox mechanism of other α,β-unsaturated ketones.Highlights► The electrochemical characteristics of cis-jasmone in strong acid media were different from those in weakly acidic or neutral solution. ► A remarkable catalytic redox process for cis-jasmone was demonstrated at MWNT/Nafion/GCE. ► The redox mechanism of cis-jasmone in strong acid media has been proposed.

Platinum nanoparticles were electrodeposited onto a film of dihexadecyl hydrogen phosphate deposited on a glassy carbon electrode (GCE) and modified with dispersed acetylene black. Scanning electron microscopy and electrochemical impedance spectroscopy revealed that this nanocomposite has a uniform nanostructure and a large surface area that enables fast electron-transfer kinetics. The modified GCE showed high electrocatalytic activity for the oxidation of nitric oxide (NO). Under optimal conditions, the oxidation peak current of nitric oxide is linearly related to the concentration of NO in the concentration range between 0.18 and 120 μM, and the detection limit is as low as 50 nM (at an S/N of 3). The modified electrode was successfully applied to sensing of NO as released from rat liver.

In this work, a novel phosphotungstic acid/neutral red intercalated montmorillonite composite (PTA/NR–MMT) film was prepared by layer-by-layer assembling on a graphite electrode (GE). The PTA/NR–MMT/GE exhibited excellent electrocatalytic effect for the oxidation of alkylene group in methyl jasmonate (MeJA). The synergistic effect of the good conductivity and high adsorption capacity of MMT and the solid super-acidity of PTA was believed responsible for the high electrocatalytic activity of this nanohybrid. Under optimal working conditions, MeJA showed a wide calibration range of 5.0 × 10−7 to 8.0 × 10−5 M with a low detection limit of 2.0 × 10−7 M by using the derivative differential pulse voltammetry. This novel electrocatalytic system can be applied to the determination of MeJA in spikelet samples of rice.Highlights► We prepare a phosphotungstic acid/neutral red intercalated montmorillonite (PTA/NR–MMT) nanocomposite. ► PTA/NR–MMT is used as highly active catalyst for the oxidation of alkylene group in methyl jasmonate (MeJA). ► PTA/NR–MMT nanohybrids can be used in the electrocatalytic oxidation of alkylene group-containing organics. ► MeJA contents in spikelet samples of rice are successfully determined.

A novel electrochemical system for the sensitive determination of cis-jasmone on an electrodeposited Nafion film modified glassy carbon electrode (e-Nafion/GCE) in the presence of cetyltrimethylammonium bromide (CTAB) was described. The redox mechanism of cis-jasmone on e-Nafion/GCE was studied by voltammetry, chronocoulometry and infrared spectroscopy. The property and interface behavior of the e-Nafion film were characterized by electrochemical impedance spectrometry. The standard rate constant k0, the diffusion coefficient D and the surface coverage Γ were calculated and discussed. Under optimal working conditions, the anodic stripping peak current of cis-jasmone linearly increased with its concentration in the range of 6.0 × 10−7 to 1.0 × 10−4 mol/L, with a detection limit of 4.0 × 10−7 mol/L. The proposed method was applied to the determination of cis-jasmone in spikelet samples of rice and the good recoveries showed that it may have potentially valuable applications.Highlights► Electrochemical redox mechanism of cis-jasmone. ► Electrodeposited Nafion film modified glassy carbon electrode (e-Nafion/GCE). ► Enhancement effect of cetyltrimethylammonium bromide. ► Sensitive determination of cis-jasmone by anodic stripping voltammetry.

A disposable electrochemical sensor for the determination of indole-3-acetic acid (IAA) based on nanocomposites of reduced graphene oxide (rGO) and poly(safranine T) (PST) was reported. The sensor was prepared by coating a rGO film on a pre-anodized graphite electrode (AGE) through dipping–drying and electrodepositing a uniform PST layer on the rGO film. Scanning electron microscopic (SEM) and infrared spectroscopic (IR) characterizations indicated that PST–rGO formed a rough and crumpled composite film on AGE, which exhibited high sensitive response for the oxidation of IAA with 147–fold enhancement of the current signal compared with bare AGE. The voltammetric current has a good linear relationship with IAA concentration in the range 1.0 × 10−7–7.0 × 10−6 M, with a low detection limit of 5.0 × 10−8 M. This sensor has been applied to the determination of IAA in the extract samples of several plant leaves and the recoveries varied in the range of 97.71–103.43%.

An amperometric sensor for the determination of indole-3-acetic acid (IAA) based on the CeCl3-DHP film modified gold electrode was developed. CeCl3 was dissolved into water in the presence of dihexadecyl hydrogen phosphate (DHP). The IAA sensor was prepared via evaporating solvent of the CeCl3-DHP dispersion on the gold electrode surface. The amperometric response of IAA on the CeCl3-DHP film modified gold electrode was investigated. The experimental results indicate that the passivation of the electrode due to the adsorption of the oxidation product of IAA decreases significantly at the CeCl3-DHP film modified gold electrode, in contrast to that at the bare and the DHP modified gold electrode. The experimental parameters were optimized and an electrochemical method for the determination of IAA was established. The oxidation peak current is linearly with the concentration of IAA from 1 × 10−7 to 2 × 10−5 mol l−1 and the detection limit is 3 × 10−8 mol l−1. The relative standard deviation of eight measurements is 3.2% for 5 × 10−7 mol l−1 IAA. The IAA in plant leaves were extracted and determined by the IAA sensor.

A simple method has been developed to prepare porous Au film-modified glassy carbon electrode (PAu/GCE). By using a simple electrodeposition process, a dense porous Au (PAu) film possessing good adhesion, large surface area, and mechanical integrity, was obtained. The surface characterization studies confirm that the formation of porous film constituted of Au nanoparticles. It was found, from the CV studies, that the prepared PAu modified electrode shows excellent catalytic activity for the electro-oxidation of adrenaline (AD) in a neutral medium. As to the electrochemical response of redox of adrenaline/adrenalinequinone couple in 0.1 M pH 7.0 phosphoate buffer solution (PBS), at the PAu/GCE, the anodic peak potential Epa shifted by 50 mV negatively in the negative direction, compared to that on the Au film modified glassy carbon electrode (Au/GCE), indicating the extraordinary activity of PAu in electrocatalysis for the oxidation process of AD. The application of the modified electrode for the determination of AD in pharmaceutical preparations indicates that the PAu/GCE has good sensitivity and reproducibility.

We report a simple strategy for fabricating a thin-film filtering membrane electrode (FME) of single-walled carbon nanotubes (SWCNTs) by vacuum filtration. A SWCNT–FME, comprised of SWCNTs and mixed cellulose ester (MCE), is applicable in two detection modes. The face-mode SWCNT–FME directly employs the exposed SWCNT sensing layer and possesses remarkable properties as thin-film electrochemical sensor platforms, including good flexibility, wide potential window, large surface area and excellent analytical performance resembling traditional carbon nanotube-based electrochemical sensors. The back-mode SWCNT–FME has a tunable selectivity produced by combining the separation function of the outer MCE membrane with the high surface area of the inner SWCNT sensing layer. As an example of surface functionalization, an electrodeposition method was proposed for the controlled decoration of a SWCNT–FME with either low-density discrete flower-like gold nanoparticles (GNPs) or continuous GNP films. The resulting SWCNT–GNP composites were proved to possess high surface area and excellent electrocatalytic activity for the oxidation of glucose.

The major drawback of currently used MnO2 film sensor is the loss of electrical conductivity due to the formation of a poorly conductive MnO2 layer. To overcome this problem, a coating in which the Au is alloyed with MnO2 has been developed. The fabrication of the codeposited film electrode of Au and MnO2 by using a cyclic voltammetric (CV) method was described, and systematic physical and electrochemical characterization was performed. This MnO2/Au film electrode enhanced MnO2 electrocatalytic activity. The oxidation process of glucose at the codeposited MnO2/Au shows a well-defined peak at 0.27 V in alkaline aqueous solution. In contrast, the glucose oxidation at Au modified glassy carbon electrode (GCE) just shows a shoulder wave at 0.42 V. The experimental results indicate that the modification of MnO2 on the surface of GCE significantly improved the electrocatalytic activity towards the oxidation of glucose. Further study shows that the MnO2/Au could also effectively catalyze the oxidation of hydrogen peroxide in pH 7.0 phosphate buffer solution (PBS).

A novel biocompatible composite film based on a water-insoluble surfactant, didodecyldimethylammonium bromide (DDAB), and a hydrophobic room-temperature ionic liquid (RTIL), 1-hexyl-3-methyl-imidazolium hexafluorophosphate (HIMIMPF6), for the immobilization of biocatalytical proteins was reported. Differential scanning calorimetry (DSC) showed that the DDAB–HIMIMPF6 composite film has higher thermal stability than the DDAB film alone. SEM images indicated that different microstructures existed between the DDAB film and the composite film, indicating the interaction between DDAB and RTILs. This composite can be used as the immobilization matrix of proteins and other biomacromolecules. Heme-proteins, including hemoglobin (Hb), myoglobin (Mb) and horseradish peroxidase (HRP), were used as model proteins for studying the electrochemical behaviors of the resulting biocatalytical composite films. In the case of Hb, a pair of well-defined quasi-reversible redox peaks was obtained when the composite film containing Hb was modified on a glassy carbon electrode. The formal potential (E°′), the surface coverage (Γ*) and the electron transfer rate constant (ks) were calculated as −0.308 V, 1.32 × 10−11 mol cm−2 and 11.642 s−1, respectively. While, these parameters for Hb on DDAB films alone were −0.309 V, 7.20 × 10−12 mol cm−2 and 2.748 s−1, respectively. Therefore, the composite are more suitable for the direct electron transfer between Hb than DDAB alone. The native conformation and bioactivity of Hb adsorbed on the composite film was proved to be maintained, reflected by the unchanged ultraviolet–visible (UV–vis) as well as the catalytic activity toward hydrogen peroxide (H2O2) and nitric oxide (NO) compared with the free Hb molecules. Furthermore, Hb on the composite film are more sensitive for the detection of hydrogen peroxide (H2O2) and nitric oxide (NO) than that on DDAB film alone. The linear range of H2O2 on Hb/DDAB–RTILs/GC electrode is from 0.5 to 57.5 μM with linear regression equations I(μA) = 0.149 + 0.00904C(μM), while, the linear range of H2O2 on Hb/DDAB/GC electrode is from 0.5 to 57.5 μM with linear regression equations I(μA) = 0.0938 + 0.00553C(μM). For NO, its linear range on Hb/DDAB–RTILs/GC electrode is from 1.8 to 21.6 μM with linear regression equations I(μA) = 0.0937 + 0.0232C(μM). But its linear range on Hb/DDAB/GC electrode is from 1.8 to 21.6 μM with linear regression equations I(μA) = 0.0285 + 0.0167C(μM). Similar results were observed for Mb and HRP in the DDAB–HIMIMPF6 composite film.

The direct electrochemical determination of methyl jasmonate (MeJA) at a nano-montmorillonite modified glassy carbon electrode (nano-MMT/GCE) is reported. The modified electrode, prepared by a simple casting−drying method and characterized by scanning electron microscope (SEM) and electrochemical impedance spectra (EIS), was proved to process a uniform nanostructured surface with a large surface area and a fast electron transfer rate. This electrode exhibited a sensitive electrochemical response for the direct oxidation of MeJA in 0.1 mol L−1 HClO4, which could be further improved by using a derivative square wave voltammetry technique. Thus, a simple and fast electrochemical method for the determination of MeJA is proposed. Under optimal working conditions, the oxidation current of MeJA linearly increased with its concentration in the range of 7.0 × 10−7−1.0 × 10−3 mol L−1 with a detection limit of 5.0 × 10−7 mol L−1. This method had been applied to the determination of MeJA content in wheat spikelet samples.

In this work, Au–Ag alloy nanoparticles were biosynthesized by yeast cells and applied to fabricate a sensitive electrochemical vanillin sensor. Fluorescence microscopic and transmission electron microscopic characterizations indicated that the Au–Ag alloy nanoparticles were mainly synthesized via an extracellular approach and generally existed in the form of irregular polygonal nanoparticles. Electrochemical investigations revealed that the vanillin sensor based on Au–Ag alloy nanoparticles modified glassy carbon electrode was able to enhance the electrochemical response of vanillin for at least five times. Under optimal working conditions, the oxidation peak current of vanillin at the sensor linearly increased with its concentration in the range of 0.2–50 μM with a low detection limit of 40 nM. This vanillin sensor was successfully applied to the determination of vanillin from vanilla bean and vanilla tea sample, suggesting that it may have practical applications in vanillin monitoring system.

A novel methyl jasmonate (MeJA) sensor based on phosphotungstic acid/graphene oxide (PTA/GO) nanohybrid was developed by layer-by-layer assembling on a pre-anodized graphite electrode. Owing to the synergistic effect of the good conductivity, high surface area of GO and the solid super-acidity of PTA, the MeJA electrochemical sensor exhibited excellent electrocatalytic activity for the oxidation of alkylene group in MeJA, displaying as a wide linear response from 5.0 × 10−7 to 8.0 × 10−5 M and a low detection limit of 2.0 × 10−7 M in 0.1 M HClO4 solution.

A novel hemoglobin (Hb) electrode has been presented through adsorbed Hb onto a gold colloids modified carbon paste electrode. Ultraviolet–visible (UV–vis) spectra suggested that the conformation of Hb in the resulting films retained its native structure. Cyclic voltammetry displayed that the adsorbed Hb showed a pair of well-defined redox peaks at about −0.351 V versus SCE in pH 7.0 buffer solutions. Gold colloids can enhance the electron transfer of Hb efficiently on the modified electrode, which was testified by electrochemical impedance spectroscopy (EIS). Hb on that modified electrode was proved to exhibit good electrocatalytic activity for the reduction of nitric oxide (NO). Based on this, a sensitive amperometric nitric oxide biosensor which showed a good response to the reduction of NO without any mediator was developed. Under optimum conditions, the biosensor responded linearly to NO in the concentration range from 9.0 × 10−7 to 3.0 × 10−4 M with a detection limit of 1.0 × 10−7 M at 3σ. The response showed Michaelis–Menten behavior at higher NO concentrations and the apparent Michaelis–Menten constant is estimated to be 105.8 μM, showing an good affinity for NO. Moreover, this studied biosensor showed high sensitivity and selectivity, good reproducibility, and long-term stability.

We report a simple method for the stable dispersion of multi-walled carbon nanotubes (MWNTs) in water by vanillin and controllable surface addition onto carbon fiber microelectrodes (CFE) via electropolymerization. We have characterized these polyvanillin-carbon nanotube (PVN-MWNT) composite films with techniques including scanning electron microscopy (SEM), infrared spectroscopy (IR) and voltammetry. These investigations showed that the films have a uniform porous nanostructure with a large surface area. This PVN-MWNT composite-modified CFE (PVN-MWNT/CFE) exhibited a sensitive response to the electrochemical oxidation of nitrite. Under optimal working conditions, the oxidation peak current of nitrite linearly increased with its concentration in the range of 0.2 μM–3.1 mM, with the system exhibiting a lower detection limit of 50 nM (S/N = 3). We successfully applied the PVN-MWNT/CFE system to the determination of nitrite from lake water. The efficient recovery of nitrite indicated that this electrode was able to detect nitrite in real samples.

An ultrathin film of gold was grafted on human hair by a chemical liquid deposition method under ambient conditions. The method consisted of the assembling of gold nanoparticles (GNPs) on the adsorptive sites of human hair as seeds and the growth of isolated GNP seeds into a continuous gold layer. The resulting gold film coated hair possessed good conductivity and flexibility, and can be used as a novel gold hair microelectrode (GHME). This electrode inherited some merits of both hair and gold nanoparticles, for instance, good mechanical property, excellent biocompatibility and high surface area. GHME was also proven to exhibit sensitive electrochemical responses toward dopamine and nitric oxide, foreseeing its promising applications in the fields of biomedical analysis.

Multi-walled carbon nanotube multilayers were modified onto a newly proposed gold hair microelectrode via a simple layer-by-layer assembling method. The resulting electrode showed a sensitive oxidation response to estradiol with detection limit as low as 1.0 × 10−8 mol/L, foreseeing a promising approach to the fabrication of high-sensitive microsensors.

A novel amperometric nitric oxide sensor based on poly(eosin b)-ionic liquid composite (PEB-IL) films was described. Conducting PEB was firstly electrodeposited on the surface of a glassy carbon electrode, and then a composite of 1-hexyl-3-methylimidazolium hexafluorophosphate (HIMIMPF6) and didodecyl dimethyl ammonium bromide (DDAB) was coated on the PEB film. The resulting PEB-IL composite has the advantages of fast response and high selectivity for the determination of nitric oxide. Cyclic voltammetry (CV) and amperometric methods were employed to study the electrochemical behavior of the PEB film electrode and its electrochemical response toward NO. The results showed that the PEB-IL composite modified electrode exhibited an excellent anti-interference ability and a wide linear relationship over a NO concentration range of 3.6 × 10−8 to 1.3 × 10−4 mol/L with a low detection limit of 1.8 × 10−8 mol/L (S/N = 3). This sensor was successfully applied to the monitoring of NO release in rat kidney.

Metal and semiconductor nanoparticles exhibit unique optical, electrical, thermal and catalytic properties. Therefore, they have attracted considerable interest and have been employed for construction of various electrochemical sensors. This minireview gives a general view of recent advances in electrochemical sensor development based on metal and semiconductor nanoparticles covering genosensors, protein and enzyme-based sensors, gas sensors and sensor for other organic and inorganic substances. Different assay strategies based on metal and semiconductor nanoparticles for biosensor and bioelectronic applications are presented, including electrochemical, electrical, and magnetic signal transduction techniques. Electrochemical transduction principles provide signal changes in conductance, charge, potential and current. We have paid much attention to the potential-based and current-based sensors herein. Lastly, a brief introduction is given into advances concerning the role of nanoparticles, quantum dots and nanowires for nanomedicine, such as drug delivery and discovery.

Spherical copper selenide nanoparticles (NPs) were prepared by a simple reaction of sodium selenosulfate with metal copper at room temperature in alkaline Na2SeSO3 aqueous solution. It is a galvanic process that operates on a coupled anodic copper oxidation and selenosulfate reduction. 1-Thioglycerol is found to catalyze this reaction. With gold and graphite as the positive electrodes, nanocrystallites of nonstoichiometric copper selenide (Cu2 − xSe) and stoichiometric copper selenides (CuSe) were produced, respectively. The XRD study shows that the produced CuSe and Cu2 − xSe are in the pure hexagonal phase and clausthalite phase, respectively. Transmission electron microscopy images show that the diameters of the produced CuSe and Cu2 − xSe NPs are in the range of 10∼20 and 5∼15 nm, respectively.

A novel nitric oxide (NO) electrochemical sensor was fabricated by electropolymerization with two-pulse potential method in aqueous solution containing poly (p-phenylenevinylene) derivative. The experimental results showed this sensor exhibits apparent current response to NO by cyclic voltammetry. Two anodic peaks potential are significantly shifted negatively compared with bare glass carbon electrode (GCE). Chronocoulometry were used to further validate the electron transfer process of NO oxidization on this sensor. The results indicated that oxidation of NO proceeds in two steps. The first step leading to NO2− involves one-electron transfer process and the second step corresponding to the oxidation of NO2− to NO3− involves two-electron transfer process. The amperometric response is linear with NO concentration ranging from 1.8 × 10− 7 to 1.0 × 10− 4 mol L− 1 with a linear coefficient of 0.9993. The detection limit of NO is 2.3 × 10− 8 mol L− 1 (S/N = 3). After possible interferences were tested, NO release from the rat heart cells stimulated by L-arginine was measured using this sensor. The result is satisfactory.

A new noncovalent approach for the dissolution of MWNTs in water by azocarmine B (ACB) is reported. Through a simple electro-polymerization procedure, a novel electrochemical NO sensor based on water-soluble MWNTs and polyazocarmine B (PACB) nanofilm electrode was prepared, which showed excellent electrocatalytic activity towards the oxidation of nitric oxide (NO). The oxidation current linearly increased with the nitric oxide concentration in the range of 2.2 × 10−7–1.2 × 10−4 mol L−1 with a low detection limit of 2.8 × 10−8 mol L−1. The sensor has the merit of good stability, reproducibility, high sensitivity and selectivity, and it can be used to monitor NO released from rat liver cells effectively.

A novel electrochemical method for the determination of sulphide at a multi-walled carbon nanotube-dihexadecyl hydrogen phosphate composite film coated glassy carbon electrode (MWNTs-DHP/GCE) based on in situ synthesis of methylene blue (MB) was established.

The direct electron-transfer of myoglobin in a new zwitterionic gemini surfactant film with glassy carbon electrode surface has been investigated. A pair of well-defined and quasi-reversible voltammetric peaks was observed at −0.34 and −0.30 V due to the direct electron-transfer of the redox couple of Mb (FeIII/FeII). The voltammetric responses of myoglobin–surfactant film under different pH and scan rate conditions were obtained. The presence of hydrogen peroxide changed the typical electrochemical behaviors in terms of bioelectrocatalysis of myoglobin to hydrogen peroxide, and a higher sensitive electroanalytical method for the determination of hydrogen peroxide has been developed.

A novel approach that uses nature biological tissues, fish blood, for the study of the direct electron-transfer of hemoglobin and its catalytic activity for H2O2 and NO2− is observed. The direct electron-transfer of hemoglobin in red blood cells in fish blood on glassy carbon electrode was observed for the first time. By simply casting fish blood on GC electrode surface and being air-dried, a pair of well-defined redox peaks for HbFe (III)/HbFe (II) appeared at about − 0.36 V (vs SCE) at the fish blood film modified GCE in a pH 7.0 phosphate buffer solution. Ultraviolet visible (UV/VIS) characterization and the enhancement of the redox response of Hb by adding pure Hb in fish blood suggested that Hb preserved the native second structures in the fish blood film. Optical micrographs showed that the RBCs retained its integrity in blood. Hb in blood/GCE maintained its activity and could be used to electrocatalyze the reduction H2O2 and NO2−.

In this work, a sensitive electrochemical microsensor of nitric oxide (NO) was reported. The microsensor was constructed by coating PBPB composite on carbon fiber microelectrodes (CFME). The NO microsensor displayed excellent electrochemical activity toward the oxidation of NO and have the virtue of good stability, reproducibility and high sensitivity. Under optimal working conditions, the oxidation current of NO at this microsensor exhibited a good linear relationship with NO concentration in the range of 3.6 × 10−8 to 8.9 × 10−5 mol/L with a low detection limit of 3.6 × 10−9 mol/L (S/N = 3). The microsensor was successfully applied to the direct and real-time detection of NO release from biological samples, foreseeing the promising applications of this microsensor in fields like biology and medicine.

Mirror-like PbS films have been deposited by chemical deposition on glass substrates from alkaline chemical bath containing lead nitrate, sodium thiosulfate and 1-thioglycerol, which was used to catalyze the hydrolysis of thiosulfate. Nanostructure characterization was carried out by x-ray diffraction and scanning electron microscopy in order to determine the average crystallite size (61 nm) and study the surface morphologies of the as-deposited films.

A novel method for the electrochemical synthesis of covellite (CuS) nanoparticles (NPs) in aqueous phase was developed. In this experiment, thioglycerol (TG) is used as the catalyst for the hydrolysis of sodium thiosulfate, the sulfur source for the synthesis of CuS. Cu foil, which acts as the sacrificing anode, is oxidized to Cu2+ by applying a potential of 0.5 V while OH- was produced on the cathode surface at the same time. The production of OH- facilitates the reaction between Cu2+ and thiosulfate under the catalysis of TG. The evolution of hydrogen bubbles effectively prevents the deposition of copper sulfide on the cathode. Copper sulfide sols of “golden-brown”, and “dark-green” forms can be obtained by varying the concentration of TG. The “golden-brown” copper sulfide sols are also observed to convert to the green form with time, and the rate of this conversion process is faster at higher temperatures. X-ray diffraction (XRD) and chemical analysis indicate that the “dark-green” form of product is pure hexagonal phase CuS. The obtained CuS NPs were covered by a layer of TG as suggested by Fourier transform infrared (FTIR) data. The size and morphology of the particles are studied by transmission electron microscope (TEM).

The electrochemical response of phenol at acetylene black (AB)-dihexadecyl hydrogen phosphate (DHP) composite modified glassy carbon electrode in the presence of cetyltrimethylammonium bromide (CTAB) was investigated. In this system, a sensitive oxidation peak at 0.62 V (SCE) was obtained. The electrode process and the influence of CTAB on the oxidation of phenol were explored by chronocoulometry and linear sweep voltammetry (LSV). Experimental conditions for the determination of phenol were optimized. In the range of 5.0 × 10−7 to 1.2 × 10−5 M, the phenol concentration was linear with the oxidation peak current and the detection limit was found to be 1.0 × 10−7 M for 3 min accumulation. The method was applied for the determination of phenol in lake water and the results were satisfactory.

The direct electrochemistry of hemoglobin (Hb) incorporated in methacrylic acid (MAA) film on a paraffin-impregnated graphite electrode (PIGE) was described. A pair of well-defined and quasi-reversible cyclic voltametric peaks are obtained. The formal potentials (E0′) linearly depend on the pH of solution, indicating that the electron transfer was proton-coupled. Ultraviolet-visible (UV-Vis) spectra showed that the secondary structure of Hb in the MAA film was similar to individual Hb. The immobilized Hb retained its biological activity well and exhibited a nice response to the reduction of both NO2−, and H2O2, on the basis of which a new biosensor has been developed.

Carbon nanofibers (CNFs) were electrodeposited on indium tin oxide (ITO) electrodes by using a DC electric field from N,N′-dimethylformamide (DMF). An improved dispersion of CNFs has been found in DMF solution compared to ethanol and acetonenitrile. After treated by concentrated H2SO4/HNO3, CNFs were dispersed uniformly and stably in DMF. During the electrodeposition process, CNFs moved towards anode indicating the negative charge of the nanofibers. Effects of electric field strength, CNF concentration in the suspension, and the solvents used for CNF dispersion were examined on the deposition nature of CNFs.

Water-soluble multi-walled carbon nanotubes (MWNTs) were prepared by the strong adsorption of Congo red (CR) on MWNTs. The CR-functionalized MWNTs (MWNTs–CR) had a high solubility, a high purity and a special property of strong rebundling when dried, capable of forming uniform and compact MWNTs films with a 3D network structure of nanosizes on a glassy carbon electrode (GCE). Compared with GCE, the electrochemical response of estradiol at a MWNTs–CR modified glassy carbon electrode (MWNTs–CR/GCE) was greatly enhanced, which was further amplified by the addition of trace cetyltrimethylammonium bromide (CTAB) in solution, along with the accomplishment of antifouling capacity of the modified electrode. The weak hydrophobic adsorption of surfactants on the hydrophobic and smooth surface of MWNTs was found to be the key for simultaneously improving the sensitivity and antifouling capacity of carbon nanotube-based electrochemical sensors by surfactants.

This work reports on the direct electrochemistry of the xanthine oxidase (XO) from buttermilk, a mononuclear molybdenum enzyme that comprises four redox active cofactors: a five-coordinate mononuclear Mo ion, two [2Fe-2S] clusters, and a flavin adenine dinucleotide (FAD) group. The Mo, [2Fe-2S] and FAD redox responses are obtained from the enzyme immobilized on an activated single-wall carbon nanotubes (SWNTs) modified glassy carbon electrode using protein film voltammetry. The formal potentials of which are −0.61 V, −0.47 V and −0.37 V (vs. SCE) at pH 5.0, respectively. Upon addition of nitrate to the electrochemical cell, a steady-state voltammogram and i–t amprometric response were observed, indicating XO can catalyze the reduction of nitrate.

A sensitive electroanalytical method for detection of sodium nitroprusside (SNP) was investigated, on the basis of the enhanced electrochemical response of SNP at an acetylene black electrode (ABE) in the presence of cetyltrimethylammonium bromide (CTAB). Voltammetric studies showed that SNP exhibited a pair of quasi-reversible redox peaks at ABE in the presence of CTAB while no any redox peak was observed in the absence of CTAB. This was attributed to the enhanced adsorption of SNP at AB through electrostatic interactions between SNP and CTAB as well as hydrophobic adsorption of CTAB at the hydrophobic surface of ABE. Under optimal working conditions, the reduction current was proportional to the concentration of SNP in the range of 1.0 × 10−7 to 1.0 × 10−5 mol/L and 1.0 × 10−5 to 1.0 × 10−2 mol/L. A low detection limit of 5.0 × 10−9 mol/L was obtained for 2 min accumulation at open circuit (S/N = 3). The proposed method was successfully applied to the determination of SNP in pharmaceutical dosage forms.

The direct electrical communication between hemoglobin (Hb) and GCE surface was achieved based on the immobilization of Hb in a cationic gemini surfactant film and characterized by electrochemical techniques. The cyclic voltammograms showed that direct electron transfer between Hb and electrode surface was obviously promoted and then a novel unmediated nitric oxide (NO) biosensor was constructed in view of this protein-based electrode. This modified electrode showed an enzyme-like activity towards the reduction of NO and its amperometric response to NO was well-behaved with a rapid response time and displaying Michaelis–Menten kinetics with a calculated Kmapp value of 84.37 μmol L−1. The detection limit was estimated to be 2.00 × 10−8 mol L−1. This biosensor was behaving as expected that it had a good stability and reproducibility, a higher sensitivity and selectivity and should has a potential application in monitoring NO released from biologic samples.

Hemoglobin (Hb) was immobilized on glassy carbon (GC) electrode by a kind of synthetic water-soluble polymer, poly-α,β-[N-(2-hydroxyethyl)-l-aspartamide] (PHEA). A pair of well-defined and quasi-reversible cyclic voltammetric peaks was achieved, which reflected the direct electron-transfer of the Fe(III)/Fe(II) couple of Hb. The formal potential (E°′), the apparent coverage (Γ*) and the electron-transfer rate constant (ks) were calculated by integrating cyclic voltammograms experimental data. Scanning electron microscopy (SEM) demonstrated the morphology of Hb-PHEA film very different from the Hb and PHEA films. Ultraviolet visible (UV–vis) spectroscopy showed Hb in PHEA film remained its secondary structure similar to the native state. In respect that the immobilized protein remained its biocatalytic activity to the reduction of hydrogen peroxide (H2O2), a kind of mediator-free biosensor for H2O2 could be developed. The apparent Michaelis-Menten constant (Kmapp) was estimated to be 18.05 μM. The biosensor exhibited rapid electrochemical response and good stability. Furthermore, uric acid (UA), ascorbic acid (AA) and dopamine (DA) had little interferences with the amperometric signal of H2O2, which provide the perspective of this H2O2 sensor to be used in biological environments.

In biosensors based on direct electron transfer in redox proteins, efficient electron-transfer pathways between the immobilized redox protein and the electrode surface have to be established so to allow a fast electron transfer and concomitantly avoiding free-diffusing redox species. In this review, prerequisites for the direct electron transfer of redox proteins and immobilization of redox proteins on the electrode surfaces are addressed. Based on the specific nature of different proteins and non-manual immobilization procedures, possible biosensor designs are discussed, namely biosensors based on (1) ferritin; (2) cytochrome c; (3) myoglobin; (4) hemoglobin; (5) horseradish peroxidase; (6) catalase; (7) glucose oxidase; and (8) xanthine oxidase.

A novel sensor for the determination of nitrite anion (\( {\text{NO}}_{{\text{2}}} ^{ - } \)) was fabricated by electrodeposition of toluidine blue. The sensor exhibited good catalytic activity toward the electrochemical oxidation of nitrite. Amperometry was carried out to determine the concentration of \( {\text{NO}}_{{\text{2}}} ^{ - } \). A linear response in the range from 1.0×10−7 to 1.52×10−5 M with a substantially low detection limit of 5×10−8 M (S/N=3) was obtained. The proposed sensor had a feature of high sensitivity of 4.7×105 μA M−1 cm−2. The possible interference from several common ions was tested. This sensor was applied to the amperometric determination of nitrite in food samples, and the results were consistent with those obtained with the standard spectrophotometric procedure.

A simple and sensitive sensor is described for the determination of acetylspiramycin (ASPM) based on a single-wall carbon nanotubes (SWNTs)-dihexadecyl hydrogen phosphate (DHP) film coated glassy carbon electrode (GCE). Compared with a bare GCE, the SWNTs-DHP film modified GCE exhibits excellent enhancement effects on the electrochemical oxidation of ASPM. A well-defined oxidation peak of ASPM occurs at 0.89 V in 0.1 mol·L−1 phosphate buffer (pH 5.5), which was used to determine ASPM. The electrochemical behavior of ASPM at the SWNTs-DHP modified GCE was examined by cyclic voltammetry and differential pulse voltammetry. The experimental parameters were optimized and a direct electrochemical method for the determination of ASPM is proposed. Under optimum conditions, the oxidation peak current is linear to the concentration of ASPM in the range of 5.0–100 µg·mL−1, with a detection limit of 1 µg·mL−1. The SWNTs-DHP film modified electrode also provides an efficient way of eliminating interferences from some inorganic species in the solution. This sensor was successfully utilized to determine ASPM in drugs.

Direct electron transfer of hemoglobin modified with quantum dots (QDs) (CdS) has been performed at a normal graphite electrode. The response current is linearly dependent on the scan rate, indicating the direct electrochemistry of hemoglobin in that case is a surface-controlled electrode process. UV–vis spectra suggest that the conformation of hemoglobin modified with CdS is little different from that of hemoglobin alone, and the conformation changes reversibly in the pH range 3.0–10.0. The hemoglobin in a QD film can retain its bioactivity and the modified electrode can work as a hydrogen peroxide biosensor because of its peroxidase-like activity. This biosensor shows an excellent response to the reduction of H2O2 without the aid of an electron mediator. The catalytic current shows a linear dependence on the concentration of H2O2 in the range 5 × 10−7–3 × 10−4 M with a detection limit of 6 × 10−8 M. The response shows Michaelis–Menten behavior at higher H2O2 concentrations and the apparent Michaelis–Menten constant is estimated to be 112 μM.

The direct electrochemistry of hemoglobin can be achieved by immobilizing hemoglobin onto the surface of yeast cells through electrostatic attractions on a glassy carbon electrode.

A new noncovalent approach for the dissolution and exfoliation of SWNTs in water by a rigid, planar and conjugated diazo dye, Congo red (CR), is reported. Using a simple physical grinding treatment, the mixture of SWNTs and CR can be dissolved in water with a solubility as high as 3.5 mg/ml for SWNTs. The complete elimination of free CR from the mixture hardly changed this excellent solubility. High-resolution transmission electron microscope images showed that the SWNTs bundles were efficiently exfoliated into individual SWNTs or small ropes, which was further demonstrated by the increased π-plasmon UV–vis absorption and the upshift of the radical breathing mode in Raman scatterings. The π-stacking interaction between adsorbed CR and SWNTs was considered responsible for the high solubility. The uninterrupted structure of SWNTs with CR functionalization was ascertained by the unchanged disorder-induced Raman scattering.

A versatile electrochemical platform for characterizing the adsorption of neutral and positively charged surfactants on hydrophobic surfaces was established using methylene blue (MB) as the probe. As a rigid, planar and electroactive species, MB can intercalate inside the regular self-assembled monolayers (SAMs) of n-hexanethiol and exhibit well-defined electrochemical responses. The adsorption of surfactants on the hydrophobic SAMs through the intercalation interaction between the hydrophobic tails of surfactants and the SAMs might change the density of the SAMs and influence the electrochemical behaviors of MB, providing a simple but effective approach for characterizing surfactant adsorption on hydrophobic surfaces. As an example, the adsorptive behaviors of cetyltrimethylammonium bromide (CTAB), a positively charged surfactant, and Triton X-100, a neutral surfactant, on hydrophobic surfaces were investigated by cyclic voltammetry and electrochemical impedance spectroscopy. The results showed that these surfactants generally experienced three different adsorptive behaviors: the monomer adsorption at low concentrations, the loose monolayer adsorption at intermediate concentrations and the dense monolayer adsorption at high concentrations. In the case of CTAB, a new additional submonolayer adsorptive behavior between the monomer and the loose monolayer adsorption was observed for the first time, due to its rather long hydrophobic tail.

A sensitive sensor for the determination of hydrogen peroxide (H2O2) was fabricated with MnO2 nanoparticles (nano-MnO2) and dihexadecyl hydrogen phosphate (DHP) composite film. The sensor exhibits high sensitivity, due to the enhanced oxidation of hydrogen peroxide (H2O2) at the MnO2 nanoparticles and its uniform dispersion in DHP film. Amperometry was carried to determine the concentration of H2O2. The dependence of the response current on H2O2 concentration was explored under optimal conditions and an excellent linear concentration range of 1.2 × 10−7–2.0 × 10−3 M with a substantially low detection limit of 8.0 × 10−8 M was obtained. The proposed nano-MnO2/DHP sensor has the feature of high sensitivity of 2.66 × 105 μA M−1 cm−2. This sensor has been applied to measure the concentration of H2O2 in tooth paste and hair dye. According to the principle of Sensors with injectable recognition elements (SIRE), trace concentration of glucose in urine samples can also be calculated.

A biocompatible silk fibroin (SF) film provided a feasible microenvironment for heme-proteins to direct electron transfer on graphite electrodes (GE). Myoglobin (Mb), hemoglobin (Hb), horseradish peroxidase (HRP), and catalase (Cat) in corporated in SF films exhibited a pair of well-defined, nearly reversible cyclic voltammetric peaks, corresponding to the reaction of hemeFe (III) + e → hemeFe (II). The formal potential (E0), the apparent coverage (Γ) and the electron transfer rate constant (ks) of four proteins in SF films were evaluated by analyzing the cyclic voltammograms (CVs) of heme-proteins. The formal potential was pH dependent, suggesting that proton ion was involved in the reaction. Ultraviolet visible (UV–vis) spectra and reflectance absorbance infrared (RAIR) spectra indicated that heme-proteins in SF films were not grossly denatured. The structure of heme-proteins–SF films was investigated using scanning electron microscopy (SEM) and RAIR. It indicated that there existed intermolecular interaction between heme-proteins and SF and this governed their different morphology in SF films. Hydrogen peroxide and nitric oxide were catalytically reduced by the heme-proteins in SF films, showing the potential applicability of the heme-proteins–SF films as the new type of biosensors based on the protein film voltammetry.

The electrochemical behaviors of dioxygen (O2) were studied by using rotate ring–disk electrode (RRDE) and other electrochemical methods at bare glassy carbon electrode (GCE) and single-walled carbon nanotubes (SWNTs)–dihexadecyl hydrogen phosphate (DHP) film modified GCE. The results showed that the electrochemical reduction of dioxygen was considered to proceed by a two-step two-electron reduction pathway at both bare GCE and SWNTs–DHP film modified GCE in 0.1 mol/L air-saturated sodium hydroxide (NaOH). Maybe because each reaction rate for two cases was different the cyclic voltammograms measurements exhibited different behaviors. The detection of ring current confirmed the presence of middle product hydrogen peroxide (H2O2). Furthermore, larger current and more positive reduction potential indicated that SWNTs showed a catalytic effect towards the electrochemical reduction of dioxygen.

Water-soluble single-walled carbon nanotubes (SWNTs) were prepared via noncovalent functionalization by Congo red through a physical grinding treatment. Based on the unique property of strong rebundling when dried, water-soluble SWNTs were firmly immobilized on the surface of a glassy carbon electrode by a simple casting method. The prepared films were characterized by scanning electron microscopy (SEM), cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results showed that water-soluble SWNTs formed uniform films with porous network structures of nanosizes on the electrode surface, which were stable in neutral and acidic solutions but were unstable in basic media. In addition, the structural properties and the negative charge density of these films can be conveniently controlled by choosing proper solvents during the washing procedure, providing a simple approach for adjusting their properties to specific applications. For instance, the films prepared from water-soluble SWNTs by N,N′-dimethyl formamide (DMF) washing (SWNTs–CRDMF) had looser structures and lower negative charge density than those by water washing (SWNTs–CRwater), which allowed the entry and the succedent electrochemical reactions of both positively and negatively charged species inside the films. The potential applications of these films in electroanalytical chemistry were examined. The enhanced response of dopamine (DA) and the separation of DA oxidation potential from those of uric acid (UA) and ascorbic acid (AA) at these films demonstrated that the water-soluble SWNTs were the ideal materials for constructing SWNTs-based electrochemical sensing films.

The heme-protein including myoglobin (Mb), hemoglobin (Hb) and horseradish peroxidase (HRP) were immobilized on normal graphite electrode by using N,N-dimethylformamide (DMF). The proteins undergo direct electron-transfer reactions. The current is linearly dependent on the scan rate, indicating that the direct electrochemistry of heme-protein in that case is a surface-controlled electrode process. The E°s are linearly dependent on solution pH (redox-Bohr effect), indicating that the electron transfer was proton-coupled. Ultraviolet–visible (UV–vis) and reflection–absorption infrared (RAIR) spectra suggest that the conformation of proteins in the presence of DMF are little different from that proteins alone the conformation changes reversibly in the range of pH 3.0–10.0. The catalytic activity of proteins were examined by hydrogen peroxide and nitrite.

A novel copper incorporated self-assembled monolayers (SAMs) modified gold electrode (Cu/SAMs) for determination of glucose was developed by electrodepositing Cu particles on the SAMs of hexanethiol. The scanning electron microscopic (SEM) images showed that copper formed orbicular particles of nanosizes on the SAMs, which was much different from the fractal-like particles of copper formed at gold electrode. The Cu/SAMs film electrode exhibited high sensitivity to glucose oxidation and depressed responses towards some interferents of glucose in blood like uric acid and ascorbic acid. Under optimal working conditions, the oxidation current of glucose was proportional to the concentration of glucose in the range from 3.0 μM to 10 mM by amperometry with a low detection limit of 0.7 μM glucose (S/N = 3). This electrode was successfully applied to the determination of glucose in rat blood and the results were satisfactory.

The electrochemical behaviors of adrenaline at the acetylene black electrode in the presence of sodium dodecyl sulfate (SDS) were investigated by cyclic voltammetry and electrochemical impedance spectroscopy (EIS). The results indicated that the electrochemical responses of adrenaline were apparently improved by SDS, due to the enhanced accumulation of protonated adrenaline via electrostatic interaction with negatively charged SDS at the hydrophobic electrode surface. This was verified by the influences of different kinds of surfactants on the electrochemical signals of adrenaline. The electrochemical parameters of the adrenaline oxidation were explored by chronocoulometry. Under optimal working conditions, the oxidation peak current at 0.57 V was proportional to adrenaline concentration in the range of 5.0 × 10−8 to 7.0 × 10−6 mol/L, with a low detection limit of 1.0 × 10−8 mol/L for 70 s accumulation by differential pulse voltammetry (DPV). This method was applied to determine adrenaline in the hydrochloride injection sample. The results are satisfying compared with that by the standardized method of high performance liquid chromatography (HPLC).

The voltammetric responses of troxerutin were investigated at polyvinylpyrrolidone (cross-linked) (PVP) modified carbon paste electrode (CPE) in 0.1 mol/L KCl by several electrochemical techniques. A well-defined oxidation peak was observed at about 0.97 V. Compared with poor responses of troxerutin at bare electrode that at this modified electrode has been greatly improved. It is PVP that enhances the adsorption of troxerutin to electrode surface based on their hydrophobic property. Under some optimized experimental conditions, a simple and sensitive electroanalytical method was developed for the quantitative analysis of troxerutin. A very low detection limit of 5.0 × 10−9 mol/L was obtained for 5 min accumulation at open circuit (S/N = 3). This proposed method was successfully applied to the detection of troxerutin in pharmaceutical dosage forms and satisfying results had been obtained.

The colloidal Au nanoparticles–deoxyribonucleic acid (Aunano–DNA) film modified glassy carbon electrode (GCE) has been fabricated and the electrochemical reduction of dioxygen (O2) at this modified GCE has been studied in 0.2 mol/L air-saturated acetate buffer (pH = 5.2) using cyclic voltammetry (CV), chronocoulometry (CC), linear scan voltammetry (LSV) and rotating disk electrode (RDE) as diagnostic techniques. The modified electrode shows excellent enhancement effect towards the reduction of dioxygen to hydrogen peroxide (H2O2), and the overpotential is lower than that at bare GCE. A well-defined dioxygen reduction peak appeared at about − 0.24 V. Based on experimental results, a reaction mechanism is proposed and discussed.

A novel poly(toluidine blue)-modified electrode has been constructed for the determination of nitric oxide in biological sample. The electrochemical behavior of poly(toluidine blue) film electrode and its electrocatalytic activity toward NO were studied in detail by cyclic voltammetry. Possible interferences were tested and evaluated after further coated with Nafion. The poly(toluidine blue) and Nafion composite film-modified electrode exhibits a good linear relationship over a NO concentration of 1.8 × 10−7 to 8.6 × 10−5 mol/L, and the detection limit is 1.8 × 10−8 mol/L (S/N = 3). NO release from the rat liver homogenate stimulated by l-arginine was studied, and the responses were decreased by the nitric oxide synthase inhibitor Nω-nitro-l-arginine.

The direct electrochemistry of hemoglobin can be performed by immobilizing hemoglobin in a water-soluble quantum dots (CdSe–ZnS) film on glassy carbon electrode.

A novel voltammetric sensor for O,O-dimethyl-(2,4-dichlorophenoxyacetoxyl)(3′-nitrophenyl)methinephosphonate (Phi-NO2) based on molecularly imprinted polymer (MIP) film electrode is constructed by using sol–gel technology. The sensor responds linearly to Phi-NO2 over the concentration range of 2.0 × 10−5 to 1.0 × 10−8 mol L−1 and the detection limit is 1.0 × 10−9 mol L−1 (S/N = 3). This sensor provides an efficient way for eliminating interferences from coexisting substances in the solution. The high sensitivity, selectivity and stability of the sensor demonstrates its practical application for a simple and rapid determination of Phi-NO2 in cabbage samples.

The electrochemical reduction of dioxygen on the single-walled carbon nanotubes–dihexadecyl phosphate (SWNTs–DHP) film electrode has been investigated. Two well-defined dioxygen reduction peaks were observed at about −0.39 and −0.93 V in 0.1 M air-saturated NaOH at the SWNTs–DHP film electrode suggesting two two-electron reduction steps of dioxygen. The numbers of electron transferred and transfer coefficients in the dioxygen reduction process were calculated. The influences of some parameters on the reduction of dioxygen were examined including the concentration and amount of nanotubes, and ratio of SWNTs vs. DHP. Based on the experiment results under the conditions employed here, a possible mechanism is proposed and discussed.

Myoglobin (Mb), hemoglobin (Hb) and horseradish peroxidase (HRP) were incorporated in lecithin (PC) film on glassy carbon (GC) electrode by the method of vesicle-fusion. A pair of well-defined and quasi-reversible cyclic voltammetric peaks was obtained, which reflected the direct electron transfer of heme proteins. UV–Vis and reflectance absorption infrared (RAIR) spectroscopy showed that proteins in PC films remained at their secondary structure similar to their native states. Scanning electron microscopy (SEM) demonstrated the interaction between the proteins and PC would make the morphology of protein-PC films very different from the PC films alone. The immobilized proteins retained their biocatalytic activity to the reduction of NO and hydrogen peroxide, which provide the perspective to be the third generation sensors.

This paper describes glucose nanosensors based on the co-electrodeposition of a poly(vinylimidazole) complex of [Os(bpy)2Cl]+/2+ and glucose oxidase (GOD) on a low-noise carbon fiber nanoelectrodes (CFNE). The SEM image shows that the osmium redox polymer/enzyme composite film is uniform. The film modified CFNE exhibits the classical features of a kinetically fast redox couple bound to the electrode surface. A strong and stable electrocatalytic current is observed in the presence of glucose. Under the optimal experimental conditions, the nanosensor offers a highly sensitive and rapid response to glucose at an operating potential of 0.22 V. A wide linear dynamic rang of 0.01–15 mM range was achieved with a detection limit of 0.004 mM. Compared with the conventional gold electrode, the nanosensor possessed higher sensitivity and longer stability. Successful attempts were made in real time monitoring rabbit blood glucose levels.

A novel sensor for the determination of parathion based on p-tert-butylcalix[6]-1,4-crown-4 as functional monomer was fabricated by sol–gel method using molecularly imprinted technology. The electrochemical behavior of parathion at the imprinted p-tert-butylcalix[6]arene-1,4-crown-4 sol–gel film sensor was characterized by cyclic voltammetry, linear sweep voltammetry, chronoamperometry and alternating current impedance spectroscopy. A fast response of parathion can be obtained after being incubated in 0.1 M phosphonate buffer solution contain appropriate amount of parathion for 20 min. A linear response over parathion concentration in the range of 5.0 × 10−9 to 1.0 × 10−4 M was exhibited with a detection limit of 1.0 × 10−9 M (S/N = 3). This imprinted film sensor has been applied in determination of parathion in real sample and the results were consistent well with that obtained by high performance liquid chromatography.

A novel and easy route for the deposition of a thin film of carbon nanotubes onto an electrode surface by electropolymerization is described. Multi-wall carbon nanotubes (MWNTs) were “dissolved” in aqueous alizarin red S (ARS, 3,4-dihydroxy-9,10-dioxo-2-anthracenesulfonic acid, sodium salt) solution, and a very stable and well-distributed aqueous MWNTs–ARS solution was obtained. A thin film of MWNTs–ARS was successfully deposited onto the electrode surface by an in situ electropolymerization in aqueous MWNTs–ARS solution. The MWNTs–ARS thin film was characterized by scanning electron microscopy, Raman spectroscopy, UV–Vis spectroscopy and electrochemical techniques.

A novel carbon nanotube-modified glassy carbon electrode was described for the direct determination of pyridoxine. The electrochemical behavior of pyridoxine was investigated, and a well-defined oxidation peak with high sensitivity was observed at the modified electrode. Owing to the unique structure and extraordinary properties of multi-wall carbon nanotube (MWNT), the MWNT-modified glassy carbon electrode shows obvious electrocatalytic activity to the oxidation of pyridoxine, since it greatly enhances the oxidation peak current of pyridoxine as well as lowers its oxidation overpotential. Based on this, a very sensitive and simple voltammetric method was developed for the measurement of pyridoxine. A sensitive linear voltammetric response for pyridoxine was obtained in the concentration range of 5 × 10−7–1 × 10−4 mol/L, and the detection limit is 2 × 10−7 mol/L using differential pulse voltammetry. Compared with other voltammetric methods, this proposed method possesses many advantages such as very low detection limit, fast response, low cost and simplicity. The practical application of this new analytical method was demonstrated with pyridoxine drugs.

Simple and sensitive electrochemical method for the determination of metronidazole, based on a nanostructured film coated glassy carbon electrode (GCE), is described. Multi-walled carbon nanotubes (MWNT) was dispersed into water in the presence of a hydrophobic surfactant to give very stable and homogeneous MWNT suspension, and a MWNT-film coated GCE was achieved via evaporating solvent. Metronidazole yields a well-defined reduction peak whose potential is −0.71 V at the MWNT-film coated GCE in pH 9.0 Britton–Robinson buffer. Compared with bare GCE, the MWNT-film modified GCE significantly enhances the reduction peak current of metronidazole. All the experimental parameters were optimized for the determination of metronidazole. The detection limit is 6×10−9 mol/l at 2 min accumulation. This method has been successfully used to determine metronidazole in the drugs. Furthermore, results obtained by the proposed method have been compared with spectrophotometric method.

An anthraquinone (AQ) improved Na-montmorillonite nanoparticles (nano-SWy-2) chemically modified electrode (CME) has been developed for the simultaneous determination of trace levels of cadmium (II) and lead (II) by differential pulse anodic stripping voltammetry (DPASV). This method is based on a non-electrolytic preconcentration via ion exchange model, followed by an accumulation period via the complex formation in the reduction stage at −1.2 V, and then by an anodic stripping process. The mechanism of this design was proposed and the analytical performance was evaluated with several variables. Under the optimized working conditions, the detection limit was 3 and 1 nM for Cd2+ and Pb2+, respectively. The calibration graphs were linear in the concentration ranges of 8×10−9 to 1×10−6 mol L−1 (Cd2+) and of 2×10−9 to 1×10−6 mol L−1 (Pb2+). Many inorganic species did not interfere with the assay significantly; the high sensitivity, selectivity, and stability of this nano-SWy-2-AQ CME were demonstrated. The applications for the detection of trace levels of Cd2+ and Pb2+ in milk powder and lake water samples indicate that it is an economical and potent method.

Electrochemical behavior of thyroxine at a polyvinylpyrrolidone modified carbon paste electrode in the presence of cetyltrimethyl ammonium bromide was described. Thyroxine underwent totally irreversible oxidation at this system and a well-defined peak at 0.42 V was obtained. The influence of various surfactants on the oxidation of thyroxine was examined by cyclic voltammetry. Chronocoulometry was also used to investigate the electrode process. In the range 2×10−7 to 9×10−6 mol/l, the thyroxine concentration was linear with the oxidation peak current and a low detection limit of 8×10−8 mol/l was obtained for 5 min accumulation.

A simple and effective chemically modified carbon paste electrode (CMCPE) for the simultaneous determination of lead(II) and cadmium(II) was developed in this work. The electrode was prepared by the addition of diacetyldioxime into a carbon paste mixture. Pb2+ and Cd2+ were preconcentrated on the surface of the modified electrode by complexing with diacetyldioxime and reduced at a negative potential (−1.10 V). Then the reduced products were oxidized by differential pulse stripping. The fact that two stripping peaks appeared on the voltammograms at the potentials of −0.65 V (Cd2+) and −0.91 V (Pb2+) demonstrates the possibility of simultaneous determination of Pb2+ and Cd2+. Under the optimized working conditions, calibration graphs were linear in the concentration ranges of 1.0×10−7–1.5×10−5 mol l−1 (Pb2+) and 2.5×10−7–2.5×10−5 mol l−1 (Cd2+), respectively. For 5 min preconcentration, detection limits of 1×10−8 mol l−1 (Pb2+) and 4×10−8 mol l−1 (Cd2+) were obtained at the signal noise ratio (SNR) of 3. To evaluate the reproducibility of the newly developed electrode, the measurements of 5×10−7 mol l−1 Pb2+ and Cd2+ were parallel carried out for six times at different electrodes and the relative standard deviations were 2.9% (Pb2+) and 3.2% (Cd2+), respectively. Interferences by some metals were investigated. Only Ni2+ and Hg2+ apparently affected the peak currents of Pb2+ and Cd2+. The diacetyldioxime modified carbon paste electrode was applied to the determination of Pb2+ and Cd2+ in water samples. The results indicate that this electrode is sensitive and effective for the simultaneous determination of Pb2+ and Cd2+.

An amperometric sensor for the determination of indole-3-acetic acid based on the multi-wall carbon nanotubes film coated glassy carbon electrode was developed. Multi-wall carbon nanotubes (MWNT) has been dispersed into water in the presence of hydrophobic surfactant such as dihexadecyl hydrogen phosphate (DHP). The indole-3-acetic acid sensor was achieved via evaporating solvent of the MWNT-DHP dispersion on the GCE surface. The oxidation peak current of indole-3-acetic acid (IAA) increases significantly at the MWNT-DHP film coated GCE, in contrast to that at the bare and the DHP-coated GCE, and the oxidation peak potential is at 0.68 V (versus SCE), which can be used for the detection of IAA. The electrochemical behaviors of IAA at the MWNT-DHP modified GCE have been examined by cyclic voltammetry. The experimental parameters were optimized and an electrochemical method for the determination of IAA was established. The oxidation peak current is linearly with the concentration of IAA from 1×10−7 to 5×10−5 mol/l and the detection limit is 2×10−8 mol/l. The relative standard deviation of eight measurements is 3.6% for 5×10−6 mol/l IAA. The IAA in plant leaves were extracted and determined by the IAA sensor.

A method is described for the determination of estrogens (estradiol, estrone and estriol) by stripping voltammetry. These estrogens yield one well-defined oxidation peak at the Nafion-modified glassy carbon electrode in the presence of cetyltrimethylammonium bromide (CTAB). The peak current is proportional to the concentration of estradiol in the range from 2.5×10−8 to 1.5×10−6 mol/l, and the detection limit is 1×10−9 mol/l after accumulation of 6 min. The total amounts of estrogens in the blood serums were determined using the voltammetric method, and the average recovery was 106.04%. The mechanism of the oxidation of estradiol was investigated by electrochemical techniques and UV spectra.

In this paper, the voltammetric properties of diethylstilbestrol (DES) at the carbon paste electrode were described. The oxidation peak currents of DES increase significantly in the presence of surfactant cetylpyridine bromide (CPB), compared with that in the absence of CPB. Based on this fact, a voltammetric technique for determining DES is proposed. The accumulation potential has no effects on the peak current of the DES. An open-circuit accumulation is carried out. The experimental parameters, such as pH value of buffer, scan rate, and accumulation time were optimized. The interferences of some metal ions and organic compounds have also been studied, and some metal ions almost do not interfere with the determination of DES. Using this voltammetric method, DES in the injection sample was measured. The results show that this voltammetric method is reliable for the practice determination of DES.

In this paper, a new voltammetric method for the determination of phenol is described. In pH 8.00 phosphate buffer and in the presence of long-chain cationic surfactant—cetyltrimethylammonium bromide—phenol has a very sensitive oxidation peak at 0.47 V (vs. SCE) on the Nafion-modified glassy carbon electrode (GCE). The experimental parameters, such as supporting electrolyte and pH values, amounts of Nafion, varieties and concentration of surfactants, accumulation potential and time, as well as scan rate were optimized. The peak current is linear with the concentration of phenol in the range from 8×10−9 to 1×10−5 M, and the detection limit is 1×10−9 M after being accumulated at −0.50 V (vs. SCE) for 3 min. Trace levels of phenol in water samples were determined by using this voltammetric method, the average recovery was calculated to be 99.56%.

Recent progress in nanotechnology has promoted research in the surface functionalization of graphene oxide (GO) nanosheets owing to their large specific surface area and abundant functional groups. In the present work, cage-structured fullerene (C60) was attached non-covalently to laminar GO through simple grinding, which not only achieved the efficient solubilization of C60 in water but also retained the original physicochemical characteristics of GO and C60. The C60–GO nanocomposite could be dispersed in water with solubility up to 5 mg mL−1 and stability for more than one month. By further combining the redox reversibility of phosphotungstic acid (PTA), a novel electrochemical sensing film (PTA–C60–GO) was fabricated by one-step electropolymerization on a pretreated home-made graphite electrode (GE). This PTA–C60–GO/GE exhibited a sensitive electrochemical response for the direct oxidation of cis-jasmone (CJ), a well-known component of plant volatiles. Under optimal conditions, the oxidation current of CJ increased linearly with its concentration in the range of 0.3–50.0 μmol L−1 with a detection limit of 0.1 μmol L−1. This sensing platform was applied to the determination of the CJ content in rice spikelet samples.