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Label-free colorimetric apta-assay for detection of <i>Escherichia coli</i> based on gold nanoparticles with peroxidase-like amplification

In this work, aptamers against E. coli with better performance were obtained via cell systematic evolution of ligands by exponential enrichment (cell-SELEX) and dissociation constants (Kd) of aptamers were estimated to range from 133.87 to 199.44 nM. Furthermore, the selected aptamer was employed for label-free colorimetric detection of E. coli using gold nanoparticles (AuNPs) with peroxidase-like activity to catalyze the oxidation of tetramethylbenzidine (TMB) by hydrogen peroxide (H2O2) to produce color development. This colorimetric apta-assay started with an aptamer-bacteria binding step, and the concentration of residual aptamers after binding depended on the amount of target bacteria. Then, the amount of separated residual aptamers determined the degree of cetyltrimethylammonium bromide (CTAB)-inhibited catalytic activity of AuNPs, which resulted in a color change from dark blue to light blue. Owing to the excellent peroxidase activity of AuNPs, they could emit strong visible color intensity in less than 1 minute to improve visual detection sensitivity. Under optimized conditions, the sensitivity of detection was 5 × 103 CFU mL-1 visually and 75 CFU mL-1 using the UV-vis spectrum with a linear range from 5 × 102 to 1 × 106 CFU mL-1. And it had shown a good recovery rate in real samples of water, juice and milk compared with classical counting methods.

High-throughput photo-chemiluminescence imaging for HIV DNA determination based on a sulfur-doped graphitic carbonitride photocatalyst

In this work, a novel photo-chemiluminescence (PCL) array imaging technique was developed to detect HIV DNA sequences using water-dispersed ultrathin sulfur-doped g-C3N4 porous nanosheets (SCNNSs) as photocatalysts, with complementary chains of HIV DNA as the biorecognition elements. The PCL response was enhanced when a suitable amount of SCNNSs was used. The large specific surface area and π-conjugated structure of the SCNNSs provided a good platform for immobilizing the complementary chains of HIV DNA. When DNA complementary chains were present, some of the catalytically active sites of SCNNSs were blocked, and the PCL of the platform was weakened. When the HIV DNA was added, the DNA double chain was far away from the surfaces of the SCNNSs because the stacking interactions between the formed dsDNA and SCNNSs were weak. Therefore, the addition of the target HIV DNA sequence noticeably restored the signal. In the range of 5.00 × 10-8 M to 200 × 10-8 M, the enhanced PCL response was linearly related to the concentration of the HIV DNA sequence, and the detection limit (3S/N) was 1.50 × 10-8 mol L-1. In addition, the combination of SCNNSs with complementary chains of HIV DNA successfully produced a high-performance PCL imaging sensor. In these proof-of-concept experiments, we demonstrated that our method was fast, portable, and ultra-sensitive, with high throughput.