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Necessary stability conditions depending on the Lyapunov matrix for neutral type systems with commensurate delays are presented. In order to obtain them further results are introduced: the computation of the Lyapunov-Krasovskii functional of complete type by a new Cauchy formula and the introduction of identities that relate the fundamental and the Lyapunov matrix of the system. The main result is illustrated by some examples.

The paper deals with the computation of delay Lyapunov matrices for linear exponentially stable time-invariant systems with several pointwise delays. It is shown that the matrices depend continuously on the system delays. Based on this theoretical result, an algorithm of approximation of the delay Lyapunov matrices with a prescribed accuracy is presented. The result is illustrated by an example.

A simple method for the rapid determination of prostate-specific antigen (PSA) in serum is reported using a lateral flow immunoassay with gold nanoparticles as the label. The method uses the intensity of colored test lines to determine PSA from 0.3 to 30 ng/mL. The limit of detection was 0.3 ng/mL and the coefficient of variation was less than 10%. The analysis time was approximately 20 min. The novel method showed good correlation with enzyme linked immunosorbent assay (ELISA) measurements of prostate-specific antigen concentration in human serum with a linear regression coefficient of 0.985. The developed system was stable for at least 12 months when stored from +4 to 30°C and has potential application for clinical practice.

Polyclonal antibodies against conjugates of chloramphenicol succinate and chloramphenicol base with proteins were obtained and characterized in direct ELISA. Antiserum against a conjugate of chloramphenicol (CAP) base with BSA (direct coupling) was very specific and showed cross-reactivity only with CAP succinate (11.3%) and CAP base (4.6%); whereas, antisera against a conjugate of CAP succinate with a protein recognized CAP succinate strongly as an initial compound. In direct ELISA, antisera against a conjugate of CAP succinate with KLH (homologous assay) and CAP base with BSA (heterologous assay) showed similar sensitivity: IC50 were 1.3 and 1.5 ng mL−1, respectively. Applicability of the immunoreagents obtained was shown in the analysis of CAP residues in milk (3.5% fat content). Detection limit of 0.3 ng mL−1 was obtained for milk diluted 5 times.

A range of ELISAs in indirect and direct formats for the determination of bisphenol A (BPA) was developed. Bisphenol A carboxymethyl ether (BPA-CME), (BPA-CPE) and 4,4-bis(4-hydroxyphenyl)valeric acid were coupled with bovine serum albumin (BSA) (immunogens for the production of polyclonal antibodies), ovalbumin (OVA) and horseradish peroxidase. In general, the indirect assay was more sensitive and specific than the direct one. Using heterologous combinations of immunoreagents in the indirect assay allowed to increase assay sensitivity and specificity. The highest sensitivity was obtained for the antibodies produced against a conjugate of BPA-CPE with BSA and a conjugate of 4,4-bis(4-hydroxyphenyl)valeric acid with OVA. The detection limit of BPA in phosphate buffer was 0.03 ng mL−1. The assay developed was also the most specific towards BPA. Maximum cross-reactivity values did not exceed 11% for 4-cumylphenol, 5% for bisphenol E and 2% for bisphenol S. Finally, the developed assay was used to analyze sur...

The influence of immunoreagents' structure on assay performance was investigated and a range of ELISAs for streptomycin in direct and indirect format was developed. Streptomycin was conjugated with proteins (bovine serum albumin (BSA) for immunization and ovalbumin (OVA) for immobilization on a plate) by two different methods. Streptomycin was derivatized with carboxymethoxylamine (CMO) and then coupled to a protein or the protein was activated with adipic acid dihydrazide (ADH) and then coupled with streptomycin. A conjugate with horse-radish peroxidase was synthesized using streptomycin-ADH derivative. With the indirect ELISA the most sensitive assay for polyclonal antisera against streptomycin-oxime-BSA in combination with homologous and heterologous conjugates (limit of detection 2.5 ng ml-1) was developed, whereas for a combination 'antisera against streptomycin-ADH-BSA/heterologous conjugate' higher background level of a calibration curve was observed. Besides the level was very high (about 60%) for...

An indirect competitive ELISA for the detection of chloramphenicol (CAP) in food of animal origin (milk, meat, eggs) is described. Influence of immunoreagent structure and composition on the assay sensitivity and specificity was investigated. Two CAP derivatives were used for conjugation with proteins: CAP succinate and a diazo derivative of CAP. Molar incorporation of CAP into the coating conjugates was also varied. To eliminate matrix effect on the assay results, a special casein-containing buffer was used for milk samples, whereas for meat and egg samples a 50-fold dilution of the buffer extracts was needed. The method developed permits CAP concentrations to be determined in the range 0.08–100 μg 1−1. The detection limit is 0.08 μg kg−1. Recovery in different food samples averages between 70 and 130%. The method can be applied for inspection of food of animal origin for CAP residues.

Chemiluminescent immunoassay with two types of a multispot membrane strip has been developed for the simultaneous determination of several pesticides in water samples. Three types of antibodies with different cross‐reactivity towards s‐triazines were immobilized in three separate spots of the first strip (called a ‘triazine membrane strip') by means of a photoimmobilization technique. The second strip (called a ‘pesticide membrane strip') contained two pesticide‐specific spots with immobilized antibodies, one specific to 2,4‐dichlorophenoxyacetic acid and another with a high extent of cross‐reactivity toward different s‐triazines. Each membrane strip contained two additional spots. One was employed as a positive control or cut‐off spot, and the other one was used to check the background luminescence. The immunoassay with a multispot membrane strip allowed us to estimate semi‐quantitatively the concentration for a particular analyte by a comparison of the recorded light intensities of the analyte spot and ...

A sensitive competitive immunoassay with chemiluminescent detection for the determination of 2,4‐dichlorophenoxyacetic acid (2,4‐D) was carried out in a microtiter and membrane strip format. The application of a portable scanning luminometer for the detection of light emission in polystyrene strips and from the surface of a porous membrane resulted in an increase in the sensitivity of the assay and a wider detection range when compared to conventional colorimetric detection. With the assay in microtiter strip format, 2,4‐D can be determined within the range 0.005–1 μg l‐1 with an IC50 at 0.04 p.g l‐1. Different immobilization techniques involving adsorption and covalent immobilization on different kinds of modified nylon were examined to optimize the attachment of specific antibodies to the surface of porous membranes. Photoimmobilization on positively charged nylon modified with p‐azidobenzoic acid was found to result in higher sensitivity. The application of a membrane strip format allows the quick quan...

New method for visual enzyme immunoassay of some model antigens in solution by using covalently photoimmobilized antibodies has been developed. This approach is based on the quantitative photoimmobilization of antibodies on the surface of porous matrixes. It is easy to control the dimensions and shape of the activated zones and the quantity of the active groups on it by this technique. A strip of the membrane impregnated with p-azidobenzaldehyde was illuminated by the light. A s a result, .the quantity of aldehyde groups developed on the surface of membrane is proportional to the time of illumination. After the covalent immobilization of antibodies, the membrane has separate zones with an exact surface concentration of antibodies. The antigens of different types were assayed: human IgG, human chorionic gonadotropin, Shigella Sonnei. The lowest detection limit was 1 μg/ml, 20 U/L 1×104 cells/ml. The method allows measure of thyroxire concentrations in the range 50 to 200 nM, the precision of repli...