A silica-based SERS chip for rapid and ultrasensitive detection of fluoride ions triggered by a cyclic boronate ester cleavage reaction.

Chemical sensing for the convenient detection of trace aqueous fluoride ions (F) has been widely explored with the use of various sensing materials and techniques. It still remains a challenge to achieve ultrasensitive but simple, rapid, and inexpensive detection of F for environmental monitoring and protection. Here we reported a novel surface-enhanced Raman scattering (SERS) nanosensor, fluorescein phenylboronic acid covalently linked to 1,4-dimercapto-2,3-butanediol modified Au@Ag NPs by a cyclic boronate ester (Flu-PBA-Diol-Au@Ag NPs), for the rapid and ultrasensitive detection of F. Once the Flu-PBA approached the surface of Au@Ag NPs, the Raman signals of Flu-PBA were remarkably enhanced due to the strong SERS effect. However, the presence of F will induce the cleavage reaction of the cyclic boronate ester into the trifluoroborate anion (3F-Flu-PBA) and diol. The 3F-Flu-PBA molecules exfoliated from the surface of Au@Ag NPs, and the SERS signals of the nanosensor were quenched. Following the sensing mechanism, a silica-based SERS chip has been fabricated by the assembly of Flu-PBA-Diol-Au@Ag NPs on a piece of silicon wafer. The silica-based SERS chips showed high sensitivity for aqueous F, and the limit of detection (LOD) could reach as low as 0.1 nM. Each test using the SERS chip only needs a droplet of 20 μL sample and is accomplished within ∼10 min. The silica-based SERS chip has also been applied to the quantification of F in tap water and lake water.