A general strategy for label-free homogeneous bioassays based on selective recognition and silver ion-mediated conformational switch

Simple, sensitive, and cost-effective detection of biomolecules is important to clinical analysis and treatment. Herein, a novel fluorescence turn-off method for the homogeneous detection of DNA and proteins was developed using selective recognition reaction and silver ion-mediated conformational switch. In this work, we discovered the new phenomenon that cadmium telluride (CdTe) quantum dots (QDs) could selectively differentiate silver ions (Ag+) from the cytosine (C)-Ag+-cytosine (C-Ag+-C) hairpin structure, owing to the ability of Ag+ to quench CdTe QDs which the C-Ag+-C complex didn't possess. With this method, the C-Ag+-C hairpin probe formed, followed by the reaction with the target DNA, thus the C-Ag+-C was opened and free Ag+ were released. This initiated the selective recognition reaction with CdTe QDs, leading to the decrease in the CdTe QDs fluorescence intensity. Therefore, the DNA concentration could be quantitatively measured by indirectly monitoring the fluorescence signal of CdTe QDs. Furthermore, the application of this sensor was extended to the detection of prostate-specific antigen (PSA) by employing its aptamer as the recognize elements, with the similar strategy used for the target DNA assay. Under the optimized conditions, this strategy achieved good analytical performance for both DNA assay with a limit of detection of 0.12 nM and linear dynamic range of 5-100 nM, and PSA assay with a limit of detection of 0.01 ng/mL and the linear dynamic range of 0.1-2.5 ng/mL. This new fluorescence method is simple in design and operation, with no need for oligonucleotide labeling and separation, and it has great potential to be applied in clinical diagnosis.