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A kind of delayed predator-prey system with harvesting is considered in this paper. The influence of harvesting and delay is investigated. Our results show that Hopf bifurcations occur as the delay τ passes through critical values. By using of normal form theory and center manifold theorem, the direction of Hopf bifurcation and the stability of the bifurcating periodic solutions are obtained. Finally, numerical simulations are given to support our theoretical predictions. [ABSTRACT FROM AUTHOR]

The discovery of carbon nanotubes (CNTs) about a decade ago has brought fascinating evolutions in electronics, material industry, as well as bio-techniques for DNA analysis, gene therapy, drug delivery etc. It has also dramatically promoted the development of DNA biosensing techniques, especially electrochemical DNA biosensor. The application of CNTs in electrochemical DNA biosensors includes two main aspects: on one hand, using CNTs as a novel substrate not only enables immobilization of DNA molecules but also serves as a powerful amplifier to amplify signal transduction event of DNA hybridization. On the other hand, CNTs can also be employed as a powerful carrier to pre-concentrate enzymes or electroactive molecules for electrochemical sensing of DNA hybridization as a novel indicator. In this review, we place emphasis on recent studies of CNTs-based electrochemical DNA biosensors based on these two aspects, with advantages and disadvantages of each aspect introduced herein. [ABSTRACT FROM AUTHOR]

Imaging of localized hybridization of nucleic acids immobilized on a glass DNA microarray was performed by means of generation collection (GC) mode scanning electrochemical microscopy (SECM). Amine-tethered oligodeoxynucleotide probes, spotted on the glass surface, were hybridized with an unmodified target sequence and a biotinylated indicator probe via sandwich hybridization. Spots where sequence-specific hybridization had occurred were modified by streptavidin-horseradish-peroxidase-(HRP)-wrapped SiO2 nanoparticles through the biotin-streptavidin interaction. In the presence of H2O2, hydroquinone (H2Q) was oxidized to benzoquinone (BQ) at the modified spot surface through the HRP catalytic reaction, and the generated BQ corresponding to the amount of target DNA was reduced in solution by an SECM tip. With this DNA microarray, a number of genes could be detected simultaneously and selectively enough to discriminate between complementary sequences and those containing base mismatches. The DNA targets at prepared spots could be imaged in SECM GC mode over a wide concentration range (10-7-10-12 M). This technique may find applications in genomic sequencing. [ABSTRACT FROM AUTHOR]

Kaempferol, apigenin, rutin, quercetin, luteolin and ferulic acid are separated and detected in Chinese herbal tea using capillary zone electrophoresis coupled to amperometric detection. The phosphate running buffer also contains ß-cyclodextrin (ß-CD), which assists in separation and gives excellent separations within 20 min and detection limits as low as 10 ng mL−1 (S/N = 3). The effects of working electrode potential, pH and concentration of running buffer, concentration of ß-CD, separation voltage and injection time were investigated. The method was applied to analyze tea samples with recoveries in the range of 90.0 to 107.0%. The method offers high separation efficiency, short analysis time, small sample consumption, and good repeatability. [ABSTRACT FROM AUTHOR]

A novel, simple and sensitive DNA biosensor based on DNA–poly(amidoamine) (PAMAM) dendrimer nanoconjugates was developed by using the electrochemical impedance spectroscopy (EIS) technique. In this context, the assay relies on the hybridization of the single-stranded DNA (ssDNA) probe covalently conjugated on a mercaptoacetic acid self-assembled monolayer on gold electrodes, with the generation 4.5 (G-4.5) PAMAM–target DNA complex in solution. Once the double-stranded DNA (dsDNA) formed on the gold electrodes, G-4.5 PAMAM bearing carboxyls on the periphery was anchored on the hybrids; the changes of interfacial electron-transfer resistance (Ret) of the electrodes were measured using an Fe(CN)63−/4−redox probe by electrochemical impedance spectroscopy. The results showed that only a complementary sequence could form a dsDNA–PAMAM with the DNA–PAMAM probe and give an obviously enlarged Retvalue. The non-complementary and three-base mismatched sequence exhibited negligible impedance change compared with the blank measurement (the blank measurement means: ssDNA probe-modified gold electrode was directly measured by EIS). The unique spherical structure combining with more negative charges on the G-4.5 PAMAM periphery anchored on the hybrids could significantly amplify the hybridization signal (Retvalue), and the detection limit for measuring the full complementary sequence is down to pM level. [ABSTRACT FROM AUTHOR]

The simultaneous determination of three isomers of phenylenediamines ( o, m, and p-phenylenediamine) and two isomers of dihydroxybenzenes (catechol and resorcinol) in hair dyes was performed by capillary zone electrophoresis coupled with amperometric detection (CZE–AD). The effects of working electrode potential, pH and concentration of running buffer, separation voltage, and injection time on CZE–AD were investigated. Under the optimum conditions the five analytes could be perfectly separated in 0.30 mol L−1 borate–0.40 mol L−1 phosphate buffer (pH 5.8) within 15 min. A 300 μm diameter platinum electrode had good responses at +0.85 V (versus SCE) for the five analytes. Their linear ranges were from 1.0 × 10−6 to 1.0 × 10−4 mol L−1 and the detection limits were as low as 10−7 mol L−1 ( S/ N = 3). This working electrode was successfully used to analyze eight kinds of hair dye sample with recoveries in the range 91.0–108.0% and RSDs less than 5.0%. These results demonstrated that capillary zone electrophoresis coupled with electrochemical detection using a platinum working electrode as detector was convenient, highly sensitive, highly repeatable and could be used in the rapid determination of practical samples. [Figure not available: see fulltext.] [ABSTRACT FROM AUTHOR]

A controllable solid-state electrochemiluminescence (ECL) film based on efficient and stable quenching of ECL of ruthenium(II) tris-(bipyridine) (Ru(bpy)32) by oxidizing ferrocene (Fc) at the electrode is developed. The ECL intensity is correlated to the distance which is controlled by the conformation of the ferrocene-labeled DNA molecular beacon (Fc-MB) between the Fc and Ru(bpy)32immobilized on the electrode. The conformation adjustment is conducted viacomplementary DNA hybridizing with the bases in the loop of the Fc-MB and changing the temperature of the Fc-MB and the resultant double-stranded DNA (dsDNA). Those events all result in change of the ECL intensity. With such characteristics, the solid-state Ru(bpy)32-ECL film has the potential to be applied to reagentless DNA ECL biosensors and to calculate thermodynamic parameters of equilibrium constants of MB binding and the stem-loop formation. [ABSTRACT FROM AUTHOR]

A simple and rapid capillary electrophoresis (CE) with electrcochemical detection (ED) method has been established for the simultaneous determination of seven active ingredients in the stems and roots of Gaultheria leucocarpa var. yunnanensis and its medicinal preparation, including (+)-catechin, rutin, gentisic acid, vallinic acid, salicylic acid, quercetin, and protocatechuic acid. The effects of working potential, pH, and concentration of running buffer, separation voltage, and injection time on CE–ED are systematically investigated. Under the optimum conditions, the seven analytes could be completely separated within 23 min in a borax running buffer (pH 8.7). A good linear relationship is obtained over three orders of magnitude with detection limits (signal-to-noise ratio = 3) ranging from 5 × 10–8 g/mL to 3 × 10–7 g/mL for the analytes. The proposed method is successfully used in the analysis of real samples after a relatively simple extraction procedure, and the assay results are satisfactory. [ABSTRACT FROM AUTHOR]