Maoguo Li

Multifunctional carbon nanotubes for direct electrochemistry of glucose oxidase and glucose bioassay

Polydopamine (Pdop) has recently been shown to adsorb to a wide variety of surfaces and serves as an adhesion layer to immobilize biological molecules. In this work, the multifunctional carbon nanotube (CNT) composites were prepared though the oxidation of dopamine at room temperature and subsequent electroless silver deposition by mildly stirring. The stable immobilization and direct electron transfer of glucose oxidase were achieved on the composite film modified glassy carbon electrode. The resulting electrode gave a well-defined redox peaks with a formal potential of about -482 mV (vs. SCE) in pH 7.0 buffer. The electron transfer rate constant was estimated to be 3.6 s(-1), due to the combined contribution of Pdop, CNTs and Ag nanoparticles with the help of Nafion. Furthermore, the method for detecting of glucose was proposed based on the decrease of oxygen caused by the enzyme-catalyzed reaction between glucose oxidase (GOD) and glucose. The linear response to glucose ranging from 50.0 μM to 1.1 mM (R(2)=0.9958), with a calculated detection limit of 17.0 μM at a signal-to-noise ratio of 3. The low calculated apparent Michaelis-Menten constant (K(M)(app)) was 5.46 mM, implying the high enzymatic activity and affinity of immobilized GOD for glucose. It can reasonably be expected that this observation might hold true for other noble metal nanostructure-electroactive protein systems, providing a promising platform for the development of biosensors and biofuel cells.

A flow injection chemiluminescence method for the determination of fluoroquinolone derivative using the reaction of luminol and hydrogen peroxide catalyzed by gold nanoparticles

Based on the enhancement of chemiluminescence (CL) of luminol-hydrogen peroxide-gold nanoparticles system by fluoroquinolones (FQs), a novel and rapid CL method is reported for the determination of FQs derivatives. Under the optimum conditions, the CL intensity is proportional to the concentration of FQs derivative in solution. The corresponding linear regression equations are established over the range of 0.08-1.28mug/mL for norfloxacin, 0.013-1.32mug/mL for ciprofloxacin, 0.014-1.4mug/mL for lomefloxacin, 0.029-1.46mug/mL for fleroxacin, 0.02-1.0mug/mL for ofloxacin and 0.01-1.44mug/mL for levofloxacin, respectively. The limits of detection (S/N=3) are 3.2, 9.5, 7.0, 9.0, 8.0, and 8.0ng/mL with the relative standard deviation (n=11) 4.3, 1.5, 1.9, 1.3, 1.6 and 2.1% for norfloxacin, ciprofloxacin, lomefloxacin, fleroxacin, ofloxacin and levofloxacin, respectively. This proposed method has been applied to detect FQs derivatives in human urine successfully.

Horseradish peroxidase immobilization on carbon nanodots/CoFe layered double hydroxides: Direct electrochemistry and hydrogen peroxide sensing

Carbon nanodots and CoFe layered double hydroxide composites (C-Dots/LDHs) were prepared via simply mixing C-Dots and CoFe-LDHs. The as-prepared composites were used for the immobilization of horseradish peroxidase (HRP) on the glass carbon (GC) electrode. The electrochemical behavior of the HRP/C-Dots/LDHs/GC electrode and its application as a H2O2 biosensor were investigated. The results indicated that HRP immobilized by C-Dots/LDHs retained the activity of enzyme and displayed quasi-reversible redox behavior and fast electron transfer with an electron transfer rate constant ks of 8.46 s(-1). Under optimum experimental conditions, the HRP/C-Dots/LDHs/GC electrode displayed good electrocatalytic reduction activity and excellent analytic performance toward H2O2. The H2O2 biosensor showed a linear range of 0.1-23.1 μM (R(2) = 0.9942) with a calculated detection limit of 0.04 μM (S/N = 3). In addition, the biosensor exhibited high sensitivity, good selectivity, acceptable reproducibility and stability. The superior properties of this biosensor are attributed to the synergistic effect of HRP, C-Dots and CoFe-LDHs, which has been proved by investigating their electrochemical response to H2O2. Thus the C-Dots and LDHs composites provide a promising platform for the immobilization of redox enzymes and construction of sensitive biosensors.

Study on electrochemical behavior of tryptophan at a glassy carbon electrode modified with multi-walled carbon nanotubes embedded cerium hexacyanoferrate.

Electrochemical behavior of cerium hexacyanoferrate (CeHCF) incorporated on multi-walled carbon nanotubes (MWNTs) modified GC electrode is investigated by scanning electron microscopy (SEM) and electrochemical techniques. The CeHCF/MWNT/GC electrode showed potent electrocatalytic activity toward the electrochemical oxidation of tryptophan in phosphate buffer solution (pH 7.0) with a diminution of the overpotential of 240mV. The anodic peak currents increased linearly with the concentration of tryptophan in the range of 2.0x10(-7) to 1.0x10(-4)M with a detection limit of 2.0x10(-8)M (at a S/N=3). And the determination of tryptophan in pharmaceutical samples was satisfactory.

Amperometric Sensor Used for Determination of Thiocyanate with a Silver Nanoparticles Modified Electrode

A novel electrode modified with silver nanoparticles was fabricated. It is found that the reducibility of silver nanoparticles is higher than for bulk silver by comparing a silver nanoparticles modified electrode with a silver micro-disk electrode. When SCN- was added, a new oxidation peak occurred and the anodic peak current of silver nanoparticles decreased. The new anodic peak current is proportional to the thiocyanate concentration in the range of 5.0×10-7∼4.0×10-4 mol/L in pH 6.0 NaH2PO4-Na2HPO4 buffer solutions (PBS). The detection limit (S/N=3) is 4×10-8 mol/L. This method has been applied to the determination of saliva (smoker and non-smoker).

Bifunctional polydopamine@Fe3O4 core–shell nanoparticles for electrochemical determination of lead(II) and cadmium(II)

The present paper has focused on the potential application of the bifunctional polydopamine@Fe3O4 core-shell nanoparticles for development of a simple, stable and highly selective electrochemical method for metal ions monitoring in real samples. The electrochemical method is based on electrochemical preconcentration/reduction of metal ions onto a polydopamine@Fe3O4 modified magnetic glassy carbon electrode at -1.1 V (versus SCE) in 0.1 M pH 5.0 acetate solution containing Pb(2+) and Cd(2+) during 160 s, followed by subsequent anodic stripping. The proposed method has been demonstrated highly selective and sensitive detection of Pb(2+) and Cd(2+), with the calculated detection limits of 1.4×10(-11)M and 9.2×10(-11) M. Under the optimized conditions, the square wave anodic stripping voltammetry response of the modified electrode to Pb(2+) (or Cd(2+)) shows a linear concentration range of 5.0-600 nM (or 20-590 nM) with a correlation coefficient of 0.997 (or 0.994). Further, the proposed method has been performed to successfully detect Pb(2+) and Cd(2+) in aqueous effluent.

Selective “turn-on” fluorescent sensing for biothiols based on fluorescence resonance energy transfer between acridine orange and gold nanoparticles

This study reports a novel fluorescence resonance energy transfer (FRET) system between acridine orange (AO) and gold nanoparticles (AuNPs), in which AO acts as the donor and AuNPs as the acceptor. In this system, AO is noncovalently self-adsorbed on AuNPs, which induces fluorescence quenching of AO as a result of FRET between AO and AuNPs. The fluorescence of AO switches to “turn-on”(restore) upon the addition of thiols due to the strong interactions between the thiols and gold nanoparticles, which leads to the dissociation of AO from the surfaces of AuNPs and thus its fluorescence “turn-on”. Based on the enhanced fluorescence, a homogenous assay method for sensing thiols is proposed. Under optimal conditions, the enhanced fluorescence intensity displays a linear relationship with the concentration of cysteine ranging from 2.5 × 10−9 M to 1 × 10−7 M with a detection limit of 0.72 nM. This method also demonstrates a high selectivity to other thiol-containing amino acids due to the strong affinity of thiols to gold, which allows the analysis of the total amount of thiol-containing amino acids in samples. The proposed approach demonstrates the feasibility of the AuNPs-based “turn-on” fluorescence sensing for total thiols in human plasma samples with satisfactory results.

Electrocatalytic activity of carbon spheres towards NADH oxidation at low overpotential and its applications in biosensors and biofuel cells

The excellent electrocatalytic activity of a micro-structured carbon material, carbon hollow spheres (CS), to the oxidation of dihydronicotinamide adenine dinucleotide (NADH) is demonstrated here. Compared to conventional bare glassy carbon electrodes, a substantial decrease by 450 mV in the overpotential of NADH electrooxidation was observed using CS coatings, with oxidation starting at ca. −0.10 V (vs.Ag/AgCl, pH 7.0). The CS-coated glassy carbon electrode (CS/GC) thus allows highly sensitive and direct amperometric detection of NADH at lower potential, ranging from 0.20 to 100 μM with a high sensitivity of 7.3 ± 0.2 nA μM−1 (i.e., 103.3 ± 2.8 nA μM−1 cm−2), low detection limit of 0.08 ± 0.03 μM, and minimization of surface fouling. With lactate dehydrogenase (LDH) as a model, a lactate biosensor with the LDH-CS/GC electrode was constructed and the biosensor shows rapid and highly sensitive amperometric response to lactate ranging from 0.5 to 12 μM with a detection limit of 3.7 ± 0.2 μM, a sensitivity of 4.1 ± 0.2 nA μM−1 (i.e., 57.9 ± 2.8 nA μM−1 cm−2), good reproducibility and excellent stability. Furthermore, the promoted direct electron transfer (DET) of bilirubin oxidase (BOD) on CS/GC electrode was investigated, and thus a membrane-less lactate/oxygen biofuel cell was assembled by using LDH-CS/GC as bioanode for lactate oxidation and BOD-CS/GC electrode for oxygen reduction, with a high open-circuit potential of 0.60 V. Such ability of CS to decrease the NADH oxidation overpotential and promote DET of blue-copper oxidases suggests great promise for dehydrogenase-based amperometric biosensors and biofuel cells.

MgFe-layered double hydroxide modified electrodes for direct electron transfer of heme proteins

In this study, the Fe-based layered double hydroxides (Mg(3)Fe LDH) were used to immobilize heme proteins including hemoglobin (Hb), myoglobin (Mb) and horseradish peroxidase (HRP) for fabrication of heme/Mg(3)Fe LDH film on glassy carbon electrode (Mg(3)Fe-heme/GCE). The possible role of iron in framework of LDH to promote direct electron transfer (DET) of heme proteins was investigated using an LDH containing non-iron as a reference. Hb was selected as a model protein for studying the electrocatalytic activity of immobilized heme in LDH film. The Mg(3)Fe-Hb/GCE displayed an enhanced electrocatalytic reduction towards H(2)O(2). The biosensor showed a very low detection limit (0.036 μM) and apparent Michaelis-Menten constant (7.98 μM). This work outlines that Fe-based LDH modified electrode provides a promising platform for immobilization of heme proteins and development of sensitive biosensors.

A Nanocomposite Modified Electrode: Electrocatalytic Properties and Its Sensing Applications to Hydrogen Peroxide and Glucose

A phenosafranine (PS) functionalized multiwalled carbon nanotubes (MWNTs) nanocomposite film (PS-MWNTs) modified glassy carbon (GC) electrode was prepared by noncovalent adsorption. The resulting electrode, PS-MWNTs/GC, shows good electrocatalytic activity to the reduction of hydrogen peroxide, and a non-enzyme amperiometric hydrogen peroxide sensor was built. The as-prepared electrode also exhibits excellent electrocatalytic activity to oxygen reduction. By combining the electrocatalytic activity of PS-MWNTs nanocomposite to oxygen reduction with the bioactivity of glucose oxidase (GOx) to glucose oxidation, a glucose biosensor was constructed by further cross-linking GOx on the PS-MWNTs/GC electrode.