Martin M. F. Choi

An improved sensitivity non-enzymatic glucose sensor based on a CuO nanowire modified Cu electrode

CuO nanowires have been prepared and applied for the fabrication of glucose sensors with highly enhanced sensitivity. Cu(OH)(2) nanowires were initially synthesised by a simple and fast procedure, CuO nanowires were then formed simply by removing the water through heat treatment. The structures and morphologies of Cu(OH)(2) and CuO nanowires were characterised by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The direct electrocatalytic oxidation of glucose in alkaline medium at CuO nanowire modified electrodes has been investigated in detail. Compared to a bare Cu electrode, a substantial decrease in the overvoltage of the glucose oxidation was observed at the CuO nanowire electrodes with oxidation starting at ca. 0.10 V vs. Ag/AgCl (saturated KCl). At an applied potential of 0.33 V, CuO nanowire electrodes produce high and reproducible sensitivity to glucose with 0.49 microA/micromol dm(-3). Linear responses were obtained over a concentration range from 0.40 micromol dm(-3) to 2.0 mmol dm(-3) with a detection limit of 49 nmol dm(-3) (S/N = 3). The CuO nanowire modified electrode allows highly sensitive, low working potential, stable, and fast amperometric sensing of glucose, thus is promising for the future development of non-enzymatic glucose sensors.

Electrogenerated Chemiluminescence Behavior of Graphite-like Carbon Nitride and Its Application in Selective Sensing Cu2+

This paper reports for the first time the electrogenerated chemiluminescence (ECL) behavior of graphite-like carbon nitride (g-C(3)N(4)) with K(2)S(2)O(8) as the coreactant. The possible ECL reaction mechanisms are proposed. The spectral features of the ECL emission and photoluminescence (PL) of g-C(3)N(4) are compared, and their resemblance demonstrates that the excited states of g-C(3)N(4) from both ECL and photoexcitation are the same. The effects of K(2)S(2)O(8) concentration, pH, g-C(3)N(4)/carbon powder ratio, and scan rate on the ECL intensity have been studied in detail. Furthermore, it is observed that the ECL intensity is efficiently quenched by trace amounts of Cu(2+). g-C(3)N(4) is thus employed to fabricate an ECL sensor which shows high selectivity to Cu(2+) determination. The limit of detection is determined as 0.9 nM. It is anticipated that g-C(3)N(4) could be a new class of promising material for fabricating ECL sensors.

Simultaneous determination of L-ascorbic acid, dopamine and uric acid with gold nanoparticles-β-cyclodextrin-graphene-modified electrode by square wave voltammetry.

Graphene decorated with gold nanoparticles (AuNPs-β-CD-Gra) has been synthesized by in situ thermal reduction of graphene oxide and HAuCl(4) with β-cyclodextrin (β-CD) under alkaline condition. The AuNPs-β-CD-Gra product was well characterized by infrared spectroscopy, X-ray powder diffraction, scanning electron microscopy, high-resolution transmission electron microscopy, and selected area electron diffraction. This material was used to fabricate an AuNPs-β-CD-Gra-modified glassy carbon electrode (GCE) which showed excellent electro-oxidation of l-ascorbic acid (AA), dopamine (DA) and uric acid (UA) in 0.10 M NaH(2)PO(4)-HCl buffer solution (pH 2.0) by square wave voltammetry (SWV). Three well-resolved oxidation peaks of AA and DA and UA were obtained. The AuNPs-β-CD-Gra/GCE exhibits linear responses to AA, DA and UA in the ranges 30-2000, 0.5-150 and 0.5-60 μM, respectively. The detection limits (based on S/N=3 and preconcentration time=3.0 min) for AA, DA and UA are 10, 0.15 and 0.21 μM, respectively. The AuNPs-β-CD-Gra/GCE has been successfully applied to determine UA in human urine with satisfactory results. Our work provides a simple, convenient and green route to synthesize AuNPs on Gra which is potentially useful in electroanalysis.

Microwave-assisted synthesis of BSA-stabilized and HSA-protected gold nanoclusters with red emission

A microwave (MW)-assisted synthesis method for preparation of fluorescent BSA-stabilized and HSA-protected gold nanoclusters (BSA–AuNCs and HSA–AuNCs) has been developed. The reaction time can be shortened from tens of hours to several minutes, thanks to the superheating and non-thermal effects of the MW energy. The optimal experimental conditions including concentrations of BSA, HAuCl4 and NaOH and the MW programme are investigated in detail. The as-synthesised BSA–AuNC product is well characterized by UV-vis spectroscopy, fluorescence spectroscopy, HRTEM, XPS, and IR spectroscopy. The HSA–AuNCs display strong red emission which is efficiently quenched by nitrogen oxides (NOx), demonstrating its great potential in determining the intracellular concentration of NOx. The proposed MW-assisted synthesis method is simple, fast and should be applicable to prepare various protein- or enzyme-protected metal nanoclusters.

Facile synthesis of nitrogen-doped carbon dots for Fe3+ sensing and cellular imaging

A fast and facile approach to synthesize highly nitrogen (N)-doped carbon dots (N-CDs) by microwave-assisted pyrolysis of chitosan, acetic acid and 1,2-ethylenediamine as the carbon source, condensation agent and N-dopant, respectively, is reported. The obtained N-CDs are fully characterized by elemental analysis, transmission electron microscopy, high-resolution transmission electron microscopy, Fourier transform infrared spectroscopy, Raman spectroscopy, X-ray diffraction pattern, X-ray photoelectron spectroscopy, UV-vis absorption, and photoluminescence spectroscopy. Doping N heteroatoms benefits the generation of N-CDs with stronger fluorescence emission. As the emission of N-CDs is efficiently quenched by Fe(3+), the as-prepared N-CDs are employed as a highly sensitive and selective probe for Fe(3+) detection. The detection limit can reach as low as 10 ppb, and the linear range is 0.010-1.8 ppm Fe(3+). The as-synthesized N-CDs have been successfully applied for cell imaging and detecting Fe(3+) in biosystem.

Biosensors for determination of glucose with glucose oxidase immobilized on an eggshell membrane

A glucose biosensor using an enzyme-immobilized eggshell membrane and oxygen electrode for glucose determination has been fabricated. Glucose oxidase was covalently immobilized on an eggshell membrane with glutaraldehyde as a cross-linking agent. The glucose biosensor was fabricated by positioning the enzyme-immobilized eggshell membrane on the surface of a dissolved oxygen sensor. The detection scheme was based on the depletion of dissolved oxygen content upon exposure to glucose solution and the decrease in the oxygen level was monitored and related to the glucose concentration. The effect of glutaraldehyde concentration, pH, phosphate buffer concentration and temperature on the response of the glucose biosensor has been studied in detail. Common matrix interferents such as ethanol, d-fructose, citric acid, sodium benzoate, sucrose and l-ascorbic acid did not give significant interference. The resulting sensor exhibited a fast response (100s), high sensitivity (8.3409mgL(-1) oxygen depletion/mmolL(-1) glucose) and good storage stability (85.2% of its initial sensitivity after 4 months). The linear response is 1.0x10(-5) to 1.3x10(-3)molL(-1) glucose. The glucose content in real samples such as commercial glucose injection preparations and wines was determined, and the results were comparable to the values obtained from a commercial glucose assay kit based on a spectrophotometric method.

Inhibition of beta 1–40 amyloid fibrillation with N-acetyl-l-cysteine capped quantum dots

One of the primary factors that induce Alzheimers disease (AD) is the deposition of beta-amyloid (Abeta). The Abeta molecules can self-assemble to form neurotoxic aggregates with various morphologies, such as dimers, oligomers, protofibrils and fibrils. For this aspect, we demonstrated that the amyloid fibrillation can be inhibited by quenching the nucleation and elongation processes with a low concentration of water dispersed N-acetyl-L-cysteine capped quantum dots (NAC-QDs). Based on the concentration dependence of NAC-QDs on the seeded fibril growth, there is a remarkable inhibition effect when the NAC-QDs concentration is increased by 100-fold from 10(-9) to 10(-7) M. The NAC-QDs concentration required to show inhibition effect is much lower than that of the amyloid peptide concentration (50 microM). The step-like change suggests that the inhibition effect of NAC-QDs displays a threshold response. The inhibition is likely due to the intermolecular attractive interactions such as the hydrogen bonding between NAC-QDs and amyloid fibrils resulting in the blockage of the active elongation sites on the fibrils.

Microwave-assisted non-aqueous homogenous precipitation of nanoball-like mesoporous α-Ni(OH)2 as a precursor for NiOx and its application as a pseudocapacitor

A novel facile and template-free method to prepare a nanoball-like NiOx material has been developed. The preparation involves the synthesis of an α-Ni(OH)2 precursor by microwave heating and calcination of the precursor. The NiOx material possesses a nanoball-like structure with a large surface area. The pseudo-capacitive properties of the NiOx material were evaluated by cyclic voltammetry and electro-chemical impedance spectroscopy in 6 M KOH solution. The specific capacitances were 951, 796, 713 and 650 F g−1, corresponding to the scan rates of 2.0, 5.0, 10 and 20 mV s−1, respectively. The NiOx material also showed excellent cycling stability and maintained 92% of its maximum specific capacitance after 1000 cycles. The hybrid supercapacitor based on this NiOx material exhibited moderate energy density and power density.

Separation and preconcentration of persistent organic pollutants by cloud point extraction.

Persistent organic pollutants (POPs) are recognized as a class of poisonous compounds which pose risks of causing adverse effects to human health and the environment. Thus, it is very important to detect POPs in environmental and biological samples. The identification and determination of very low levels of POPs in complex matrices is extremely difficult. Recently a promising environmentally benign extraction and preconcentration methodology based on cloud point extraction (CPE) has emerged as an efficient sample pretreatment technique for the determination of trace/ultra-trace POPs in complex matrices. The purpose of this paper is to review the past and latest use of CPE for preconcentrating POPs and its coupling to different contemporary instrumental methods of analysis. First, the comparison of various extraction techniques for POPs is described. Next, the general concept, influence factors and other methods associated with CPE technique are outlined and described. Last, the hyphenations of CPE to various instrumental methods for their determination are summarized and discussed.

Homocysteine-protected gold-coated magnetic nanoparticles: synthesis and characterisation

An easy and simple two-step reaction is employed to synthesise a new type of ligand-protected (homocysteine-protected) gold-coated iron oxide nanoparticle (homocys-Au-Fe3O4). Fe3O4 nanoparticles are used as the central core to prepare homocys-Au-Fe3O4 in aqueous state without precipitation and aggregation of nanoparticles. Fe3O4 nanoparticles are initially prepared and subsequently coated with Au layers under hot citrate reduction of HAuCl4. The citrate monolayer of the nanoparticles is then ready for place-exchange with homocysteine molecules to produce the well dispersed homocys-Au-Fe3O4 nanoparticles. These homocys-Au-Fe3O4 nanoparticles have been fully characterised by X-ray photoelectron spectroscopy, visible absorption spectroscopy, magnetic susceptibility measurements, Fourier transform infrared spectroscopy, thermogravimetric analysis, atomic absorption spectroscopy, energy dispersive X-ray spectroscopy, X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy and high-resolution transmission electron microscopy. The homocys-Au-Fe3O4 nanoparticles show good paramagnetic properties and are coated with ultra-thin layers of gold atoms (∼0.5 nm) having an average diameter of ca. 12 nm. These magnetic nanoparticles are well dispersed in water and stable at physiological pH without precipitation. The visible absorption spectrum of homocys-Au-Fe3O4 can be altered by pH. These nanoparticles are aggregated in an acidic environment but dissociated at high pH conditions in a reversible manner. This article has provided important insights into the design of new water-soluble magnetic nanoparticles for biomedical, analytical and catalytic applications.