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A colorimetric sensor has been developed in this work to sensitively detect α-glucosidase activity and screen α-glucosidase inhibitors (AGIs) utilizing unmodified gold nanoparticles (AuNPs). The sensing strategy is based on triple-catalytic reaction triggered by α-glucosidase. In the presence of α-glucosidase, aggregation of AuNPs is prohibited due to the oxidation of cysteine to cystine in the system. However, with addition of AGIs, cysteine induced aggregation of AuNPs occurs. Thus, a new method for α-glucosidase activity detection and AGIs screening is developed by measuring the UV–vis absorption or visually distinguishing. A well linear relation is presented in a range of 0.0025–0.05 U mL−1. The detection limit is found to be 0.001 U mL−1 for α-glucosidase assay, which is one order of magnitude lower than other reports. The IC50 values of four kinds of inhibitors observed with this method are in accordance with other reports. The using of unmodified AuNPs in this work avoids the complicated and time-consuming modification procedure. This simple and efficient colorimetric method can also be extended to other enzymes assays.

The combined flow injection–capillary electrophoresis (FI–CE) system, described previously in this series, was used in connection with a drug dissolution testing system to automatically monitor the dissolution process of multicomponents. Samples from the dissolution medium were withdrawn at fixed intervals through an on-line membrane filter to load the sample loop of an injection valve, from which 25 μl samples were injected into a carrier buffer and transported into a split–flow interface coupling the FI and CE systems. Trimethoprim (TMP) and sulphamethoxazole (SMZ), the two active components in a sulphatrim tablet formulation dissolved in 0.1 mol l−1 HCl, were introduced into a short silica separation capillary of 14.5 cm effective length by electrokinetic means, separated at a constant voltage of 1.0 kV by capillary zone electrophoresis, using a phosphate running buffer (pH 6.5), and the separated constituents recorded continuously, using an UV detector at 224 nm. The signals were recorded within a testing period of 65 min. By partially overlapping the separation zones of neighboring samples, a high sample throughput of 60 h−1 was achieved with single-vessel dissolution and 48 (24 duplicates) h−1 with dual-vessel dissolution. The reproducibility of the FI–CE system, obtained using TMP and SMZ standards within the testing period was 1.6 and 0.8% relative standard deviation (RSD) with peak height evaluation, and 2.0 and 1.1% RSD (n=72), respectively, for peak area. Good agreement of results was obtained between those using the reported method and a standard liquid chromatography method.

A combined flow injection (FI)–capillary electrophoresis (CE) system with a split-flow interface reported previously was used for on-line preconcentration of pseudoephedrine by solid phase extraction on a micro-column packed with C18. Aqueous samples were loaded on the column at 1.0 ml min−1 for 90 s and eluted by a mixed eluent of 40% 62.5 mM acetate buffer (pH 4.0) and 60% acetonitrile at 0.2 ml min−1. A zone-sampling approach was employed to sample 45 μl of the most concentrated zone of the eluate, which was injected into the FI–CE split-flow interface using a phosphate buffer carrier, with a 25 μl water zone preceding the sampled eluate zone to achieve enhanced field amplification effects. Sensitivity enhancements of 60 fold (180 fold with 4 min sample loading) were thus achieved with a sample throughput of 9 h−1 and 4.7% relative standard deviations (n = 10, 2 μg ml−1 pseudoephedrine) for peak area evaluation, without significant degradation of column efficiency.