Chenxin Cai

Nonenzymatic Electrochemical Detection of Glucose Based on Palladium−Single-Walled Carbon Nanotube Hybrid Nanostructures

A new electrocatalyst, palladium nanoparticle-single-walled carbon nanotube (Pd-SWNTs) hybrid nanostructure, for the nonenzymatic oxidation of glucose was developed and characterized by X-ray diffraction (XRD) and the transmission electron microscope (TEM). The hybrid nanostructures were prepared by depositing palladium nanoparticles with average diameters of 4-5 nm on the surface of single-walled carbon nanotubes (SWNTs) via chemical reduction of the precursor (Pd(2+)). The electrocatalyst showed good electrocatalytic activity toward the oxidation of glucose in the neutral phosphate buffer solution (PBS, pH 7.4) even in the presence of a high concentration of chloride ions. A nonenzymatic amperometric glucose sensor was developed with the use of the Pd-SWNT nanostructure as an electrocatalyst. The sensor had good electrocatalytic activity toward oxidation of glucose and exhibited a rapid response (ca.3 s), a low detection limit (0.2 +/- 0.05 microM), a wide and useful linear range (0.5-17 mM), and high sensitivity (approximately 160 microA mM(-1) cm(-2)) as well as good stability and repeatability. In addition, the common interfering species, such as ascorbic acid, uric acid, 4-acetamidophenol, 3,4-dihydroxyphenylacetic acid, and so forth did not cause any interference due to the use of a low detection potential (-0.35 V vs SCE). The sensor can also be used for quantification of the concentration of glucose in real clinical samples. Therefore, this work has demonstrated a simple and effective sensing platform for nonenzymatic detection of glucose.

Detection of Glucose Based on Direct Electron Transfer Reaction of Glucose Oxidase Immobilized on Highly Ordered Polyaniline Nanotubes

An amperometric glucose biosensor based on the direct electron transfer of glucose oxidase (GOx) was developed by electrochemically entrapping GOx onto the inner wall of highly ordered polyaniline nanotubes (nanoPANi), which was synthesized using anodic aluminum oxide (AAO) membrane as a template. The cyclic voltammetric results indicated that GOx immobilized on the nanoPANi underwent direct electron transfer reaction, and the cyclic voltammogram displayed a pair of well-defined and nearly symmetric redox peaks with a formal potential of -405 +/- 5 mV and an apparent electron transfer rate constant of 5.8 +/- 1.6 s(-1). The biosensor had good electrocatalytic activity toward oxidation of glucose and exhibited a rapid response (approximately 3 s), a low detection limit (0.3 +/- 0.1 microM), a useful linear range (0.01-5.5 mM), high sensitivity (97.18 +/- 4.62 microA mM(-1) cm(-2)), higher biological affinity (the apparent Michaelis-Mentan constant was estimated to be 2.37 +/- 0.5 mM) as well as good stability and repeatability. In addition, the common interfering species, such as ascorbic acid, uric acid, and 4-acetamidophenol, did not cause any interference due to the use of a low detection potential (-0.3 V vs SCE). The biosensor can also be used for quantification of the concentration of glucose in real clinical samples.

Facile synthesis of nitrogen-doped graphene for measuring the releasing process of hydrogen peroxide from living cells

Modulating the electronic characteristics of graphene is of great technological importance for improving and expanding its applications. Chemical doping with other elements is a promising way to achieve this goal. This work reports a facile synthesis of nitrogen-doped graphene (N-graphene) at low temperature. This method, which involves the steps of graphite oxidation, exfoliation, and chemical reduction with the use of hydrazine as a reducing agent, can simultaneously realize the reduction of graphene oxide and doping graphene with nitrogen atoms. The spectroscopic results demonstrate that N-graphene with N/C atomic ratio up to ∼4.5% can be prepared, and the doping N atoms consist of pyridinic, pyrrolic, graphitic, and oxidized nitrogen structures with the surface atomic compositions of ∼28%, 49%, 19%, and 4%, respectively. The prepared N-graphene exhibits superior electrocatalytic activity toward H2O2 reduction, and the contribution of the doped N atoms to the enhanced electrocatalytic activity is explained in detail based on density functional theory (DFT) calculations. Moreover, N-graphene is further used to study the dynamic process of H2O2 (a common representative of reactive oxygen species, ROS, in living cells) release from living cells such as neutrophil, RAW 264.7 macrophage, and MCF-7 cells. The results presented here open a new way to synthesize N-graphene, and also developed a new platform for a reliable collection of kinetic information on cellular ROS release. The approach established in this work could be potentially useful in study of downstream biological effects of various stimuli in physiology and pathology.

Electrocatalytic activity of a cobalt hexacyanoferrate modified glassy carbon electrode toward ascorbic acid oxidation

A cobalt hexacyanoferrate (CoHCF) modified glassy carbon (CoHCF/GC) electrode was prepared electrochemically. The voltammetric responses of CoHCF are stable and the electrochemical behaviour is related to the concentrations of supporting electrolyte and counterions. The CoHCF/GC electrode shows electrocatalytic activity toward the oxidation of ascorbic acid in phosphate buffer solution. The electrocatalytic rate constant of the CoHCF/GC electrode for the oxidation of ascorbic acid is determined using rotating disk electrode measurements.

Aptamer-Guided Silver–Gold Bimetallic Nanostructures with Highly Active Surface-Enhanced Raman Scattering for Specific Detection and Near-Infrared Photothermal Therapy of Human Breast Cancer Cells

The aptamer (S2.2)-guided Ag-Au nanostructures (aptamer-Ag-Au) have been synthesized by photoreduction and validated by ultraviolet-visible light (UV-vis) spectra and transmission electron microscopy (TEM) images. Differential interference contrast (DIC), fluorescence, and TEM images, and surface-enhanced Raman scattering (SERS) spectra indicated that the aptamer-Ag-Au nanostructures can target the surface of human breast cancer cells (MCF-7) with high affinity and specificity. This targeting is completed via the specific interaction between S2.2 aptamer (a 25-base oligonucleotide) and MUC1 mucin (a large transmembrane glycoprotein, whose expression increased at least 10-fold at MCF-7 cells in primary and metastatic breast cancers). However, the nanostructures cannot target HepG2 (human liver cancer cells) or MCF-10A cells (human normal breast epithelial cells), because these cells are MUC1-negative expressed. Moreover, the synthesized nanostructures exhibited a high SERS activity. Based on these results, a new assay for specifically detecting MCF-7 cells has been proposed. This assay can also discriminate MCF-7 cells from MCF-10A cells and different cancer cell lines, such as HepG2 cells. In addition, the aptamer-Ag-Au nanostructures have a high capability of adsorpting near-infrared (NIR) irradiation and are able to perform photothermal therapy of MCF-7 cells at a very low irradiation power density (0.25 W/cm(2)) without destroying the healthy cells and the surrounding normal tissue. Therefore, the proposed assay is significant for the diagnosis of tumors in their nascent stage. The synthesized nanostructures could offer a protocol to specifically recognize and sensitively detect the cancer cells, and would have great potential for application in the photothermal therapy of the cancers.

An electrochemical approach for detection of DNA methylation and assay of the methyltransferase activity

This work develops an electrochemical approach for rapid detection of the genomic DNA methylation level, assay of methyltransferase activity, and evaluation and screening of the inhibitors of methyltransferase. This method may be a help for the discovery of anticancer drugs.

Electrochemical Approach for Detection of Extracellular Oxygen Released from Erythrocytes Based on Graphene Film Integrated with Laccase and 2,2-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid)

This work develops a novel electrochemical approach for detection of the extracellular oxygen released from human erythrocytes. The sensing is based on the bioelectrocatalytic system of graphene integrated with laccase (Lac) and 2,2-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) toward the reduction of oxygen. ABTS and laccase are assembled on the surface of graphene, which is synthesized by a chemistry route, utilizing the pi-pi and electrostatic interactions of these components. Transmission electron microscopy (TEM), atomic force microscopy (AFM), and FT-IR spectroscopy demonstrate that graphene has been successfully synthesized, and ABTS and laccase have been effectively assembled on a graphene surface with the formation of Lac-ABTS-graphene hybrid. The voltammetric results indicate that ABTS can be used as a redox mediator when it is in immobilized form. The hybrid deposited on the glassy carbon (GC) electrode is demonstrated to be a good bioelectrocatalyst for the reduction of oxygen with inherent enzyme activity, accepted stability, high half-wave potential (ca.670 mV vs NHE), and unimpeded electrical communication to the copper redox sites of laccase. Therefore, this study has not only established a novel approach of detection of extracellular oxygen but also provided a general route for fabricating a graphene-based biosensing platform via assembling enzymes/proteins on a graphene surface.

Electrocatalytic oxidation of dopamine at a cobalt hexacyanoferrate modified glassy carbon electrode prepared by a new method

A stable electroactive thin film of cobalt hexacyanoferrate (CoHCF) was electrochemically deposited on the surface of a glassy carbon (GC) electrode with a new and simple method. The cyclic voltammograms of the CoHCF Film modified GC (CoHCF/GC) electrode prepared by this method exhibit two pairs of well-defined redox peaks, at scan rates up to 200 mV s(-1). The advantage of this method is that it is easy to manipulate and to control the surface coverage of CoHCF on the electrode surface. The modified electrode shows good electrocatalytic activity towards the electrochemical reaction of dopamine (DA) in a 0.1 mol dm (3) KNO3 + phosphate buffer solution (pH 7.0). The rate constant of the electrocatalytic oxidation of DA at the CoHCF/GC electrode is determined by employing rotating disk electrode measurements.

Electrocatalysis of NADH oxidation with electropolymerized films of azure I

Abstract A stable electroactive thin film of poly(azure I) (PAI) was deposited on the surface of a glassy carbon electrode by cyclic voltammetry from azure I (AI) in an aqueous solution. Cyclic voltammograms of PAI indicate the presence of two redox couples and the formal potentials of both shift linearly towards the negative direction with increasing solution pH. The PAI-modified glassy carbon electrode shows excellent electrocatalytic activity toward NADH oxidation in phosphate buffer solution (pH 7.0), with an overpotential of more than 460 mV lower than of the bare electrode. The catalytic rate constant of the modified glassy carbon electrode for the oxidation of NADH is determined by cyclic voltammetry and rotating disk electrode measurements.

Low-Potential Detection of Endogenous and Physiological Uric Acid at Uricase−Thionine−Single-Walled Carbon Nanotube Modified Electrodes

This work develops and validates an electrochemical approach for uric acid (UA) determinations in both endogenous (cell lysate) and physiological (serum) samples. This approach is based on the electrocatalytic reduction of enzymatically generated H(2)O(2) at the biosensor of uricase-thionine-single-walled carbon nanotube/glassy carbon (UOx-Th-SWNTs/GC) with the use of Th-SWNTs nanostructure as a mediator and an enzyme immobilization matrix. The biosensor, which was fabricated by immobilizing UOx on the surface of Th-SWNTs, exhibited a rapid response (ca. 2 s), a low detection limit (0.5 +/- 0.05 microM), a wide linear range (2 microM to 2 mM), high sensitivity (approximately 90 microA mM(-1) cm(-2)), as well as good stability and repeatability. In addition, the common interfering species, such as ascorbic acid, 3,4-dihydroxyphenylacetic acid, 4-acetamidophenol, etc., did not cause any interference due to the use of a low operating potential (-400 mV vs saturated calomel electrode). Therefore, this work has demonstrated a simple and effective sensing platform for selective detection of UA in the physiological levels. In particular, the developed approach could be very important and useful to determine the relative role of endogenous and physiological UA in various conditions such as hypertension and cardiovascular disease.