Qian Cao

Electrochemical determination of melamine using oligonucleotides modified gold electrodes

A novel and simple electrochemical method for determination of melamine is developed based on oligonucleotides film modified gold electrodes. The electrochemical probe of ferricyanide was used to investigate the interactions between oligonucleotides and melamine. Results of cyclic voltammetries, differential pulse stripping voltammetries, electrochemical impedance spectrometry and atomic force microscope, proved that melamine might interact with oligonucleotides mainly through electrostatic and hydrogen-bonding interactions. The interactions between oligonucleotides and melamine lead to the increase in the peak currents of ferricyanide, which could be used for electrochemical sensing of melamine. The redox peak currents of ferricyanide were linear with the concentration of melamine in the range from 3.9x10(-8) to 3.3x10(-6)M with a linear coefficiency of 0.990. The detection limit was 9.6x10(-9)M. The proposed electrochemical biosensor is rapid, convenient and low-cost for effective sensing of melamine. Particularly, the proposed method was applied successfully to the determination of melamine in milk products, and the recovery was 95%.

Hydrogen-bonding-induced colorimetric detection of melamine by nonaggregation-based Au-NPs as a probe

A colorimetric, label-free, and nonaggregation-based gold nanoparticles probe has been developed for the detection of melamine. Gold nanoparticles were generated using 3,5-dihydroxybenzoic acid as a reducer without adding gold nanoparticle seeds at room temperature. Upon the addition of melamine, the reducer 3,5-dihydroxybenzoic acid can interact with melamine through strong hydrogen-bonding interaction. Consequently, the formation of gold nanoparticles was interrupted by melamine since there was not enough reducer for the reduction of Au(3+) ion. And the color change from purple to yellowgreen with increasing melamine concentration was observed. The plasmon absorbance of the formed Au-NPs allows the quantitative detection of melamine. A sensitive linear correlation existed between the absorbance and the logarithm of melamine concentration ranging from 1x10(-9) M to 1x10(-5) M with a linear coefficiency of 0.993. The system has a high selectivity to melamine with a low detection limit of 8x10(-10) M.

Electrochemical immunosensor for casein based on gold nanoparticles and poly(L-Arginine)/multi-walled carbon nanotubes composite film functionalized interface.

In this paper, a novel electrochemical immunosensor for the determination of casein based on gold nanoparticles and poly(L-Arginine)/multi-walled carbon nanotubes (P-L-Arg/MWCNTs) composite film was proposed. The P-L-Arg/MWCNTs composite film was used to modify glassy carbon electrode (GCE) to fabricate P-L-Arg/MWCNTs/GCE through electropolymerization of L-Arginine on MWCNTs/GCE. Gold nanoparticles were adsorbed on the modified electrode to immobilize the casein antibody and to construct the immunosensor. The stepwise assembly process of the immunosensor was characterized by cyclic voltammetry and differential pulse voltammetry. Results demonstrated that the peak currents of [Fe(CN)(6)](3-/4-) redox pair decreased due to the formation of antibody-antigen complex on the modified electrode. The optimization of the adsorption time of gold nanoparticles, the pH of supporting electrolyte and the incubation time were investigated in details. Under optimal conditions, the peak currents obtained by DPV decreased linearly with the increasing casein concentrations in the range from 1 × 10(-7) to 1 × 10(-5) g mL(-1) with a linear coefficiency of 0.993. This electrochemical immunoassay has a low detection limit of 5 × 10(-8) g mL(-1) and was successfully applied to the determination of casein in cheese samples.

Colorimetric detection of melamine during the formation of gold nanoparticles

A sensitive and simple colorimetric method for determination of melamine was reported based on the seedless production of gold nanoparticles (Au-NPs). Au-NPs were synthesized by using pyrocatechol-3,5-disodiumsulfonate (PD) as reducer without adding nanoparticles seeds and stabilizing agent. PD can form intramolecular hydrogen-bonding in solution by adjacent sulfurnate and α-phenolic hydroxyl group, resulting in its weak reducing capacity and thus the synthesis of Au-NPs was slow. While in the presence of melamine, PD reacted with melamine through hydrogen-bonding. Therefore, the intramolecular hydrogen-bonding of PD was interrupted by melamine, and the α-phenolic hydroxyl group was free to reduce Au(3+), hence, the synthesis of Au-NPs was accelerated. Especially, the presence of melamine led to a shift in the surface plasmon bond and a color change of Au-NPs from green to yellow. Results showed that the absorbance ratio (A(436)/A(600)) was linear with the logarithm of melamine concentration in the range of 4.8×10(-9) to 1.6×10(-6) M with a correlation coefficient of 0.9949. The detection limit (3σ) obtained by UV-vis spectrum was 6.4×10(-10) M (i.e., 0.08 ppb). The proposed method was applied successfully to the determination of melamine in pretreated liquid milk products, and the recoveries were from 93% to 107%.

Electrochemical sensing of melamine with 3,4-dihydroxyphenylacetic acid as recognition element

A new electrochemical sensor for melamine with 3,4-dihydroxyphenylacetic acid as the recognition element is established. The results of Fourier Transform Infrared (FT-IR) spectra demonstrate that melamine may interact with 3,4-dihydroxyphenylacetic acid to form a complex mainly through the hydrogen-bonding interaction. The electrochemical behavior of 3,4-dihydroxyphenylacetic acid in the presence of melamine was studied. The anodic peak currents of 3,4-dihydroxyphenylacetic acid obtained by differential pulse voltammetry are linear with the logarithm of melamine concentrations in the range from 1.0 x 10(-8) to 5.0 x 10(-6) M with a linear coefficiency of 0.997. The detection limit is 3.0 x 10(-9) M. The proposed method displayed an excellent sensitivity and was successfully applied to the determination of melamine in milk products.

Colorimetric Assay for Determination of Lead (II) Based on Its Incorporation into Gold Nanoparticles during Their Synthesis

In this report, we present a new method for visual detection of Pb2+. Gold nanoparticles (Au-NPs) were synthesized in one step at room temperature, using gallic acid (GA) as reducer and stabilizer. Pb2+ is added during the gold nanoparticle formation. Analysis of Pb2+ is conducted by a dual strategy, namely, colorimetry and spectrometry. During Au-NPs synthesis, addition of Pb2+ would lead to formation of Pb-GA complex, which can induce the aggregation of newly-formed small unstable gold nanoclusters. Consequently, colorimetric detection of trace Pb2+ can be realized. As the Pb2+ concentration increases, the color turns from red-wine to purple, and finally blue. This method offers a sensitive linear correlation between the shift of the absorption band (Δλ) and logarithm of Pb2+ concentration ranging from 5.0 × 10−8 to 1.0 × 10−6 M with a linear fit coefficient of 0.998, and a high selectivity for Pb2+ detection with a low detection limit down to 2.5 × 10−8 M.

Chiral Induction, Memory, and Amplification in Porphyrin Homoaggregates Based on Electrostatic Interactions

Supramolecular chirality in two configurational homoaggregates of anionic meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) can be induced by D- and L-alanine in acidic water (see picture). The chirality can be further memorized and enforced through strong electrostatic interactions between TPPS aggregates and achiral poly(allylamine) [PAA].Supramolecular chirality in two configurational homoaggregates of anionic meso-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) can be induced by D- and L-alanine (Ala) in acidic water, respectively. The induced supramolecular chirality can be further memorized and enforced, even after complete removal of Ala or in the presence of excess Ala with the opposite configuration, through strong electrostatic interactions with achiral poly(allylamine) [PAA]. The ionic chiral interactions between TPPS and Ala or PAA are characterized by means of UV/Vis absorption and circular dichroism spectrometry. Fluorescence spectroscopy and atomic force microscopy are used as complementary techniques. On the basis of the comprehensive experimental results, a possible mechanism for chiral induction, memory, and amplification of TPPS homoaggregates by chiral amino acids and achiral PAA is proposed. Thus, we demonstrate a novel strategy to realize chiral memory in supramolecular systems by polyelectrolytes through hierarchical electrostatic self-assembly.

Transformation, memorization and amplification of chirality in cationic Co(III) complex–porphyrin aggregates

Λ- and Δ-enantiomers of cis-[CoBr(NH3)(en)2]Br2 were obtained by absolute asymmetric synthesis. The interactions between the achiral 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (TPPS) and two chiral cationic Co(III) complexes were studied by UV-vis spectroscopy, circular dichroism (CD), fluorescence spectroscopy and atomic force microscopy (AFM). It was found that the cationic Co(III) complexes can induce the formation of chiral porphyrin J-aggregates, that the metal-centered chirality can be transferred to the J-aggregates, and that the chirality was able to be memorized and amplified in the porphyrin J-aggregates. Therefore, this should be a good chiral model for understanding similar processes in physical and biological systems.

Aggregation of an anionic porphyrin with chiral metal complexes and the competitive binding influences of a surfactant and a polyelectrolyte

The non-covalent interactions of chiral metal complexes with the achiral 5,10,15,20-tetrakis (4-sulonatophenyl) porphyrin (H4TTPS2−) have been investigated by UV-vis and circular dichroism (CD) spectra. The results show that under acidic environments, only the chiral complex cations ([CoBr(NH3)(en)2]+, [Co(en)3]3+, [Ru(phen)3]2+) could interact with H4TTPS2− to form chiral aggregates, accompanied with the metal-centered chirality information transferred to the formed J-aggregates. However, the chiral complex anion ([Co(edta)]−) does not cause the self-assembly process. The competitive binding interactions between an achiral water-soluble cationic surfactant (N-hexadecyltrimethyl ammonium chloride, CTAC) and a cationic polyelectrolyte (polyallylamine, PAA) with the chiral metal complex H4TTPS2− J-aggregates, respectively, were also investigated. It was found that chiral-symmetry-breaking phenomena occur in the cationic surfactant induced event. In the case of a cationic polyelectrolyte, it could change the conformational flexibility of the H4TTPS2− aggregates. These results may lead us to understand the possible mechanism of the supramolecular self-assembly process by the non-covalent interactions.