Xishan Guo

Direct electron transfer glucose biosensor based on glucose oxidase self-assembled on electrochemically reduced carboxyl graphene

A glucose biosensor based on direct electron transfer of glucose oxidase (GOD) self-assembled on the surface of the electrochemically reduced carboxyl graphene (ERCGr) modified glassy carbon electrode has been reported. X-ray photoelectron spectroscopy (XPS) analyses of ERCGr indicate most of the oxygen-containing groups such as epoxy/ether groups and hydroxyl groups in the carboxyl graphene were eliminated, while carboxylic acid groups remained. GOD was immobilized on the ERCGr modified glassy carbon electrode via self-assembly. The cyclic voltammetric result of the electrode shows a pair of well-defined and quasi-reversible redox peaks with a formal potential of -0.467 V and a peak to peak separation of 49 mV, revealing that the direct electron transfer between GOD and the electrode has been achieved. The proposed biosensor exhibits a linear response to glucose concentrations ranging from 2 to 18 mM with a detection limit of 0.02 mM. Moreover, this facile, fast, environment-friendly and economical preparation strategy of ERCGr may be extended for the preparation of other graphene based enzyme electrode biosensors.

Fast and sensitive detection of Pb2+ in foods using disposable screen-printed electrode modified by reduced graphene oxide.

In this study, reduced graphene oxide (rGO) was electrochemically deposited on the surface of screen-printed carbon electrodes (SPCE) to prepare a disposable sensor for fast detection of Pb2+ in foods. The SEM images showed that the rGO was homogeneously deposited onto the electrode surface with a wrinkled nanostructure, which provided 2D bridges for electron transport and a larger active area for Pb2+ adsorption. Results showed that rGO modification enhanced the activity of the electrode surface, and significantly improved the electrochemical properties of SPCE. The rGO modified SPCE (rGO-SPCE) was applied to detect Pb2+ in standard aqueous solution, showing a sharp stripping peak and a relatively constant peak potential in square wave anodic stripping voltammetry (SWASV). The linear range for Pb2+ detection was 5∼200 ppb (R2 = 0.9923) with a low detection limit of 1 ppb (S/N = 3). The interference of Cd2+ and Cu2+ at low concentrations was effectively avoided. Finally, the rGO-SPCE was used for determination of lead in real tap water, juice, preserved eggs and tea samples. Compared with results from graphite furnace atomic absorption spectroscopy (GFAAS), the results based on rGO-SPCE were both accurate and reliable, suggesting that the disposable sensor has great potential in application for fast, sensitive and low-cost detection of Pb2+ in foods.

O2 plasma-functionalized SWCNTs and PEDOT/PSS composite film assembled by dielectrophoresis for ultrasensitive trimethylamine gas sensor

A novel gas sensor based on composite films of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT/PSS) and single-walled carbon nanotubes (SWCNTs) was fabricated for the detection of fishy trimethylamine (TMA) vapor. The SWCNTs were functionalized by O2 plasma treatment to improve their solubility in the polymeric matrix, and alternative current dielectrophoresis was utilized for the first time to assemble the PEDOT/PSS-SWCNTs composite film to enhance the response to TMA molecules. The high resolution transmission electron microscopy (HR-TEM) images showed that the SWCNTs maintained their bulk structure after O2 plasma functionalization. The scanning electron microscopy (SEM) images of the composite film showed that the oxidized SWCNTs were orderly arranged and uniformly dispersed into the polymer by dielectrophoresis. Compositional analyses of SWCNTs by X-ray photoelectron spectroscopy (XPS) suggested that O2 plasma functionalization could remove amorphous carbon from the nanotube surface and introduce more hydrophilic oxygen-containing groups, leading to the improvement of SWCNTs solubility in the polymeric matrix. Gas sensitivities of the composite films largely relied on the treatment conditions. Compared to the raw or acid-treated SWCNTs-doped composite films, the film doped with SWCNTs modified by O2 plasma at 30 W for 3 min exhibited the most sensitive and stable response characteristics to ppb-level TMA gas.

Rapid Detection of Chloramphenicol Residues in Aquatic Products Using Colloidal Gold Immunochromatographic Assay

A colloidal gold immunochromatographic assay (GICA) was developed for rapid detection of chloramphenicol (CAP) residues in aquatic products. A nitrocellulose (NC) membrane was used as the carrier, and the polyclonal CAP antibody was used as the marker protein. The average diameter of as-prepared colloidal gold nanoparticles (AuNPs) was about 20 nm. The optimal pH value of colloidal gold solutions and the amount of the antibody of CAP were 8.0 and 7.2 μg/mL, respectively. The CAP antibody was immobilized onto the conjugate pad after purification. The CAP conjugate and goat anti-rabbit IgG (secondary antibody) were coated onto the NC membrane. Next, the non-specific sites were blocked with 1% bovine serum albumin. The minimum detectable concentration of CAP in standard solution is 0.5 ng/mL, with good reproducibility. For the real samples from crucian carps injected with a single-dose of CAP in the dorsal muscles, the minimum detectable concentration of CAP residues was 0.5 μg/kg. The chromatographic analysis time was less than 10 min, and the strip had a long storage lifetime of more than 90 days at different temperatures. The strips provide a means for rapid detection of CAP residues in aquatic products.

Polyaniline nanofiber gas sensors by direct-write electrospinning

Electrospum polyaniline nanofibers doped with palladium nanoparticles have been demonstrated for hydrogen sensing in the formation of a chemresistor. These nanofibers have been deposited as single nanofibers across two gold electrodes by means of near-field electrospinning without using the conventional lithography process. These palladium nanoparticle based chemresistors have recorded 1.8% resistance change in the environment with 0.3% hydrogen concentration. These nanofibers sensors are flexible with good reversibility and potentially versatile by doping with different particles. As such, nanofiber-based sensors could be promising for environmental and industrial applications.

Capacitive monitoring of the antigen-antibody reactions enhanced by nanogold

An immunosensor for measuring antigen-antibody binding by nanogold particles enhancement was presented here. A gold plate electrode was modified by a SAM (self-assembled monolayer) of 11-mercaptoundecanioc acid. Then sheep anti-rabbit IgG was immobilized onto the end of the 11-mercaptoundecanioc acid. With that the gold electrode was exposed to the mixed solution of rabbit IgG and nanogold labeled sheep anti-rabbit IgG. The differential capacitance of the electrical double layer was monitored to detect the antigen-antibody reaction. The result showed the capacity of the electrical double layer by nanogold enhancement was largely increased compared to that in traditional capacitive immunosensor. The detection sensitivity is up to 10nF/mug/ml

Annealing nano-to-micro contacts for improved contact resistance

Nano-to-micro electrical contact resistance between carbon nanotubes (CNTs) and larger-scale silicon systems are investigated using both low-and high-power annealing techniques. Carbon nanotubes locally synthesized and suspended between two silicon microbridges are used as the test platform. The annealing technique involves Joule heating of either the CNT/silicon system or the secondary silicon bridge only at low or high input power for various times. Of the 15 samples tested, results show that the contact resistance decreases for 60% of the samples and two of the 15 samples show a decrease in contact resistance greater than 50%. Higher power and longer time anneals show the greatest improvement in reducing the contact resistance. This technique can potentially reduce the contact resistance for integration of CNTs with MEMS or microelectronics systems.

Development of a novel electronic tongue system using sensor array based on polymer films for liquid phase testing

A novel electronic tongue system had been developed recently in our labs, which was based on impedance measurements of a hybrid sensor array of different polymer films either conjugated conducting polymers (CPs) or rendered conductive polymers by the addition of carbon black (CB) or single wall carbon nanotubes (CNT). In this work, preliminary experiments were carried out on a series of measurements of liquids containing different concentrations of acid (HCl), salt (NaCl), glucose, ethanol and a bitter substance (sodium dehydrocholate).The impedance data have been processed using principal component analysis(PCA) algorithm. The preliminary results demonstrate the feasibility of artificial taste sensing using the polymer films sensory array

Oxygen plasma functionalized PEDOT/PSS-SWCNTs thin film deposited by electrophoresis for gas sensing

In this paper, the oxygen plasma treated single-walled carbon nanotubes (SWCNTs) were combined with the poly (3,4-ethylenedioxythiophene) / poly (styrenesulfonate) (PEDOT/PSS) solution to prepare the PEDOT/PSS-SWCNTs composite. Effects of the SWCNTs concentration and the alternative current dielectrophoresis (AC-DEP) assembly of the hybrid film on conductive characteristics and gas sensing properties of the modified electrode were investigated. It was found that SWCNTs treated by oxygen plasma could stably and homogeneously disperse into the PEDOT/PSS matrix which acted as a conductive dispersant. With the concentration of SWCNTs increasing gradually in the range of 1–20 mg/mL, the resistance of the modified electrode normally decreased while its gas sensitivity varied. Compared with the common drop-coating, AC-DEP assembly could improve the conductivity and gas sensing properties of the composite film. The PEDOT/PSS-SWCNTs composite film with SWCNTs of 5 mg/mL and deposited by AC-DEP, allowed the as-modified electrode for the rapid and sensitive response to NH3 gas with the concentration range of 2–20 ppm.