A new bioanalytical method for urea determination by using the immobilized urease on the highly luminescent ZnS quantum dots (QDs), which functioned as the pH fluorescent label has been developed. Bioconjugation of ZnS QDs to urease via amide bond was performed to obtain the ZnS QDs-urease bioconjugate. Upon enzymatic hydrolysis of urea by the immobilized urease, a pH change to a more alkaline condition has led to the deprotonation of ZnS QDs, and an increase in the fluorescence intensity can be observed. The fluorescence intensity of the urease conjugated ZnS QDs changed linearly with the urea concentrations between 4 � 10?9 M and 4 � 10?3 M (R2 = 0.992) at pH 6 with a calibration sensitivity of 179.46 intensity/decade. No noticeable influence by the Ca2+, Mg2+, K+ and Na+ ions on the response of the fluorescent pH bioprobe. Urea determination in soil sample by using the urea bioprobe was in good agreement with the standard DMAB (p-dimethylaminobenzaldehyde) UV�vis spectrophotometric method. By employing the strategy of ZnS QDs-urease bioconjugation, the stable covalent link between the two substances has appeared to widen the dynamic range and lower the detection limit for urea compared to free enzyme and QDs in solution for bioassay of urea concentration. � 2016

A regenerable electrochemical DNA biosensor based on a new type of acrylic microspheres and gold nanoparticles (AuNPs) composite coated onto a screen printed electrode (SPE) has been successfully developed for specific determination of the 35 S promoter from cauliflower mosaic virus (CaMV 35S) gene in soybean. DNA probe was immobilised onto acrylic microspheres via covalent bonding. The presence of modified gene in soybean can be detected via hybridisation of CaMV 35S gene-modified DNA with immobilised DNA probe, which was monitored by differential pulse voltammetry of anthraquinone-2-sulfonic acid monohydrate sodium salt (AQMS) as redox indicator during hybridisation event. The peak current signal of AQMS was linearly related to the target CaMV 35S gene concentration over the range of 2 × 10-15 to 2 × 10 -9 M (R2 = 0.982) with a very low concentration detect limit (7.79 × 10-16 M). The recovery test showed satisfactory results of 94.6 ± 5.1-105.4 ± 4.9% (n = 5) when the biosensor was used for the determination of genetically modified (GM) DNA sequences extracted from GM soybean samples. The DNA biosensor showed good reproducibility (relative standard deviation (RSD) below 5.0%, n = 5) and regenerability (RSD below 5.0%, n = 7). The biosensor response was stable up to 45 days of storage period at 4 C. The main advantages of this biosensor design are very low detection limit and capability of reusing the biosensor for at least seven times after regeneration with mild sodium hydroxide.