Daizhi Kuang

A graphene oxide-based electrochemical sensor for sensitive determination of 4-nitrophenol.

A graphene oxide (GO) film coated glassy carbon electrode (GCE) was fabricated for sensitive determination of 4-nitrophenol (4-NP). The GO-based sensor was characterized by scanning electron microscope, atomic force microscopy and electrochemical impedance spectroscopy. The electrochemical behaviors of 4-NP at the GO-film coated GCE were investigated in detail. In 0.1M acetate buffer with a pH of 4.8, 4-NP yields a very sensitive and well-defined reduction peak at the GO-modified GCE. It is found that the GO film exhibits obvious electrocatalytic activity toward the reduction of 4-NP since it not only increases the reduction peak current but also lowers the reduction overpotential. Based on this, an electrochemical method was proposed for the direct determination of 4-NP. Various kinetic parameters such as transfer electron number, transfer proton number and standard heterogeneous rate constant were calculated, and various experimental parameters were also optimized. Under the optimal conditions, the reduction peak current varies linearly with the concentration of 4-NP ranging from 0.1 to 120 μM, and the detection limit is 0.02 μM at the signal noise ratio of 3. Moreover, the fabricated sensor presented high selectivity and long-term stability. This electrochemical sensor was further applied to determine 4-NP in real water samples, and it showed great promise for simple, sensitive, and quantitative detection of 4-NP.

Green synthesis of silver nanoparticles–graphene oxide nanocomposite and its application in electrochemical sensing oftryptophan

A new kind of nanocomposite based on silver nanoparticles (AgNPs)/graphene oxide (GO) was conveniently achieved through a green and low-cost synthesis approach using glucose as a reducing and stabilizing agent, and the synthetic procedure can be easily used for the construction of a disposable electrochemical sensor on glassy carbon electrode (GCE). The nanocomposite was detailedly characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR) and electrochemical impedance spectroscopy (EIS). The experimental results demonstrated that the nanocomposite possessed the specific features of both silver nanoparticles and graphene, and the intrinsic high specific area and the fast electron transfer rate ascribed to the nanohybrid structure could improve its electrocatalytic performance greatly. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were employed to evaluate the electrochemical properties of AgNPs/GO/GCE towards tryptophan, and the AgNPs/GO film exhibited a distinctly higher activity for the electro-oxidation of tryptophan than GO film with tenfold enhancement of peak current. The oxidation mechanism and the kinetic parameters were investigated, and analysis operation conditions were optimized. Under the selected experimental conditions, the oxidation peak currents were proportional to tryptophan concentrations over the range of 0.01 μM to 50.0 μM and 50.0 μM to 800.0 μM, respectively. The detection limit was 2.0 nM (S/N=3). Moreover, the proposed method is free of interference from tyrosine and other coexisting species. The resulting sensor displays excellent repeatability and long-term stability; finally it was successfully applied to detect tryptophan in real samples with good recoveries, ranging from 99.0% to 103.0%.

A highly efficient Cu-catalyst system for N-arylation of azoles in water

6,7-Dihydroquinolin-8(5H)-one oxime (L3) was found to serve as a superior ligand for the CuI-catalyzed N-arylation of imidazoles with aryl iodides, bromides, and electron-deficient chlorides in water. Moreover, the CuI/L3 catalyst system enabled the coupling reactions to take place smoothly with high yields under a low catalyst loading (0.1–1 mol% CuI and 0.2–2 mol% L3).

Glucose biosensor based on glucose oxidase immobilized on a nanofilm composed of mesoporous hydroxyapatite, titanium dioxide, and modified with multi-walled carbon nanotubes

AbstractWe report on a highly sensitive glucose biosensor that was fabricated from a composite made from mesoporous hydroxyapatite and mesoporous titanium dioxide which then were ultrasonically mixed with multi-walled carbon nanotubes to form a rough nanocomposite film. This film served as a platform to immobilize glucose oxidase onto a glassy carbon electrode. The morphological and electrochemical properties of the film were examined by scanning electron microscopy and electrochemical impedance spectroscopy. Cyclic voltammetry and chronoamperometry were used to characterize the electrochemical performances of the biosensor which exhibited excellent electrocatalytic activity to the oxidation of glucose. At an operating potential of 0.3xa0V and pH 6.8, the sensor displays a sensitivity of 57.0xa0μAxa0mM−1xa0cm−2, a response time of <5xa0s, a linear dynamic range from 0.01 to 15.2xa0mM, a correlation coefficient of 0.9985, and a detection limit of 2xa0μM at an SNR of 3. No interferences are found for uric acid, ascorbic acid, dopamine and most carbohydrates. The sensor is stable and was successfully applied to the determination of glucose in real samples.n FigureMesoporous hydroxyapatite, titanium dioxide and multi-walled carbon nanotubes were ultrasonically mixed to form a rough nanofilm, and a new glucose biosensor was fabricated based on this nanofilm. The biosensor had great bioelectrocatalytic activity to glucose oxidation, and it exhibited a high sensitivity, wide linear dynamic range and high selectivity for glucose determination.

Electrochemical tyrosine sensor based on a glassy carbon electrode modified with a nanohybrid made from graphene oxide and multiwalled carbon nanotubes

AbstractWe report on a glassy carbon electrode that was modified with a composite made from graphene oxide (GO) and multiwalled carbon nanotubes (MWCNT) that enables highly sensitive determination of L-tyrosine. The sensor was characterized by transmission electron microscopy and electrochemical impedance spectroscopy, and its electrochemical properties by cyclic voltammetry, chronocoulometry and differential pulse voltammetry. The GO/MWCNT hybrid exhibits strong catalytic activity toward the oxidation of L-tyrosine, with a well defined oxidation peak at 761xa0mV. The respective current serves as the analytical information and is proportional to the L-tyrosine concentration in two ranges of different slope (0.05 to 1.0xa0μM and 1.0 to 650.0xa0μM), with limits of detection and quantification as low as 4.4xa0nM and 14.7xa0nM, respectively. The method was successfully applied to the analysis of L-tyrosine in human body fluids. The excellent reproducibility, stability, sensitivity and selectivity are believed to be due to the combination of the electrocatalytic properties of both GO and MWCNT. They are making this hybrid electrode a potentially useful electrochemical sensing platform for bioanalysis.n FigureA new L-tyrosine electrochemical sensor was fabricated based on graphene oxide and multiwalled carbon nanotube. The prepared sensor exhibits excellent electro-catalysis to the oxidation of L-tyrosine, and can improve determination sensitivity and decrease detection limit. This sensor was successfully applied to detect L-tyrosine in human fluids with satisfactory results.

Preparation of 2D molecularly imprinted materials based on mesoporous silicas via click reaction

The two-dimensional (2D) molecular imprinting approach has attracted extensive research interest in recent years due to its potential advantages such as simple construction, fast template removal and rapid mass transfer. In this study, a new 2D imprinting approach based on the combination of mesoporous silica materials and molecular imprinting technology is reported. 2D molecularly imprinted materials (MIMs) for cholesterol were prepared by using cholesterol as the template, azide modified β-cyclodextrin (azide-β-CD) as the functional monomer and alkynyl-modified SBA-15 (alkyne-SBA-15) as the skeleton. In this method, azide-β-CD molecules were first assembled around the templates by formation of template-monomer complexes, and thus the mutual positions of azide-β-CD molecules were fixed. Then, azide-β-CD molecules were anchored to the walls of the nano-pores of SBA-15 via click chemistry. After removal of the template molecules, the resulting cavities, i.e., recognition sites were formed in the nano-pores of mesoporous silicas. The synthesized MIM was characterized by FT-IR, X-ray diffraction (XRD), elemental analysis (EA), thermal gravimetric analysis (TGA), scanning electron microscopy (SEM) and so on. Binding kinetic experiments demonstrated that the 2D imprinting approach can improve site accessibility for the template effectively. The 2D MIM exhibited binding affinity and specificity for the template, as revealed by equilibrium binding experiments. When using MIM as a stationary phase for HPLC, baseline separation of cholesterol from other compounds can be achieved. In addition, the use of 2D imprinting significantly reduced the peak broadening and tailing.

Fluorogenic molecularly imprinted polymers with double recognition abilities synthesized via click chemistry

This paper reports a new strategy for the preparation of molecularly imprinted polymer (MIP) based composite materials with double recognition abilities through the exploitation of click chemistry. Combining the inherent molecular recognition ability of MIPs and the affinity binding ability of boronic acid ligands for saccharides, a boronic acid-attached MIP with double recognition abilities was prepared. An alkyne modified 2,4-dichlorophenoxyacetic acid (2,4-D) imprinted polymer was first synthesized using a two-stage precipitation polymerization. An azide-contained boronic acid was then linked to the clickable 2,4-D imprinted polymers through copper-catalyzed azide-alkyne cycloaddition (CuAAC). The boronic acid-attached MIPs displayed recognition ability for 2,4-D and affinity binding ability for saccharides at physiological pH. The intensity of fluorescence emission of the boronic acid-attached MIPs was found to increase when increasing amounts of a cis-diol compound (i.e., fructose) were added.

An enhanced electrochemical sensing platform integrated with graphene oxide and iron hydroxide colloid for sensitive determination of phloroglucinol

A novel electrochemical sensing platform was fabricated with nanohybrid consisting of graphene oxide and iron hydroxide colloid. With high surface area and excellent electrical conductivity of the nanohybrid, the prepared electrochemical sensor exhibited preeminent electrocatalytic activity towards the oxidation of phloroglucinol, resulting in the increase of the oxidation peak current and decrease of the oxidation overpotential. The nanohybrid was used as an enhanced electrochemical sensing platform for sensitive determination of phloroglucinol. The oxidation mechanism of phloroglucinol at this sensing platform was investigated in detail, the determination conditions were optimized, and the kinetic parameters were also calculated. Under the optimized conditions, the oxidation peak current was proportional to phloroglucinol concentration in the range from 5.00 to 100.00 nmol L-1 with limits of detection and of quantification of 3.45 and 11.51 nmol L-1, respectively. This sensing platform displayed long-term stability, high reproducibility and super anti-interference capability. It was employed to detect phloroglucinol in environmental water samples with good recoveries. The excellent performance, operational simplicity and low expense make the graphene-based hybrid attractive in the sensor construction.

Preparation of L-Tryptophan Electrochemical Sensor Based on Graphene Oxide/Carbon Nanotubes Nanocomposite Modified Electrode

A single-layer graphene oxide(GO) was prepared by Hummers method,and then mixed with multiwalled carbon nanotubes(MWCNT) in ultrasound bath to form stable GO/MWCNT nanocomposites.A novel L-tryptophan(L-Trp) electrochemical sensor was fabricated based on GO/MWCNT modified glassy carbon electrode.The modified electrode was characterized by transmission electron microscope(TEM),cyclic voltammetry(CV) and alternating current impedance(EIS),and the electrochemical behaviors and kinetic properties of L-Trp on the modified electrode were also investigated.The experimental results showed that a sensitive oxidation peak of L-Trp appeared at GO/MWCNT modified electrode(Epa=0.956 V),and the oxidation reaction was an irreversible process containing two electrons and two protons transfer.This electrode process controlled by the adsorption step,and the standard rate constant was 9.613×10-6 cm/s.Trace levels of L-Trp can be determined by this oxidation peak.In phosphate buffer of pH 6.0,the oxidation peak currents were linearly dependent on the L-Trp concentrations in the range of 1.00×10-6-1.00×10-4 mol/L with accumulation time of 25 s at 0.600 V and scan rate of 100 mV/s.The correlation coefficient was 0.995 and the detection limit was 3.50×10-7 mol/L.The prepared electrochemical sensor had favorable stability and could be applied to the quick determination of L-Trp in human serum,and the recovery of standard addition was from 97.8% to 104.2%.