Nucleic acid-functionalized upconversion luminescence biosensor based on strand displacement-mediated signal amplification for the detection of trivalent chromium ions.

Background: Rapid industrial development has generated serious pollution, including the presence of toxic and harmful heavy metal ions. Among them, trivalent chromium ion (Cr) is a very important element that poses a threat to life and health in our industrial wastewater pollution. Thus, it is important to develop efficient fluorescence methods for Cr detection. In this study, an upconversion luminescence biosensor for detecting Cr was constructed based on a DNAzyme, strand displacement reaction (SDR), and DNA-functionalized upconversion nanoparticles (UCNPs).

Results: The sulfonate-rich poly (sodium 4-styrene sulfonate) (PSS) was modified onto the surface of UCNPs, forming UCNPs@PSS. Then, NH-Capture probe DNA (NH-Cp) was further modified onto the UCNPs@PSS surface through sulfonylation, resulting in UCNPs@PSS@NH-Cp. The DNAzyme activated by Cr triggered the release of the primer probe (Pp), which initiated the SDR system cycle, thereby releasing a tetramethylrhodamine (TAMRA)-modified signal probe (TAMRA-Sp). Finally, UCNPs@PSS@NH-Cp bound to TAMRA-Sp through complementary base pairing, causing UCNPs and TAMRA to approach each other. Because of the luminescence resonance energy transfer (LRET) mechanism, the upconversion luminescence (UCL) signal of the UCNPs was quenched by TAMRA, enabling the detection of Cr by the change of I/I ratio. This biosensor has good stability, selectivity, and sensitivity, with a linear range of 0.5-75 nM and a detection limit of 0.135 nM for Cr.

Significance And Novelty: Firstly, based on LRET between UCNPs and TAMRA, the quantitative analysis of Cr is achieved through the changes of ratio fluorescence. Secondly, the specificity of the biosensor is improved by utilizing the specific recognition of DNA enzymes. Thirdly, the signal amplification technology of the SDR cycle greatly improves the sensitivity of biosensor. This biosensor will be useful for future environmental safety monitoring and biopsy of biological fluids.