Simple, rapid, and sensitive on-site detection of Hg in water samples through combining portable evanescent wave optofluidic biosensor and fluorescence resonance energy transfer principle.

Rapid and sensitive detection of Hg in the environment and drinking water is vital because of its non-degradability, bioaccumulation, and high toxicity. Herein, we report a portable evanescent wave optofluidic biosensor (EWOB) for simple sensitive detection of Hg using fluorescence labeled poly-A DNA strand (CY-A14) and quencher labeled poly-T DNA strand (BQ-T14) as signal reporter and biorecognition element, respectively. Both CY-A14 and Hg can competitively bind with BQ-T14 based on DNA hybridization and the specifical binding of Hg and T bases of DNA to form T-Hg-T mismatch structure, respectively. Higher concentration of Hg lead to less CY-A14 bound to BQ-T14 and thus a higher fluorescence intensity. The influence of several key environmental factors on Hg biosensor, such as pH, temperature, and ionic strength, was investigated in details because they were essential for practical applications of Hg biosensor. Under optimal conditions, a detection cycle for a single sample, including the measurement and regeneration, was less than 10 min with a Hg detection limit of 8.5 nM. The high selectivity of the biosensor was showed by evaluating its response to various potentially interfering metal ions. Our results clearly demonstrated that the portable EWOB could serve as a powerful tool for rapid and sensitive on-site detection of Hg in real water samples. The EWOB is also potentially applicable to detect other heavy metal ions or small molecule targets for which DNA/aptamers could be applied as specific biosensing probes.