Abstract:
Fluoride ions are critical in water quality management. While trace amounts support dental
and skeletal health, excessive intake can cause dental and skeletal fluorosis, and low intake increases
the risk of dental decay. Given fluoride’s health and industrial significance, detecting
trace levels of fluoride is essential for ensuring both health safety and product quality. Accurate
detection of trace levels remains challenging due to limitations, such as low sensitivity and interference
from common ions. Therefore, there is an increasing demand for a highly sensitive
and selective method for quantifying very low levels of fluoride. This study presents a cell-free
aptamer-based biosensor using Surface-Enhanced Raman Spectroscopy (SERS) for quantifying
fluoride concentrations in the picomolar to nanomolar range. The sensor utilises a derivative of
the fluoride-responsive riboswitch (FRS) aptamer domain from Bacillus cereus as the fluoride
recognition element. The FRS aptamer is immobilised on a silver nanoparticle (AgNP), which
serves as the SERS substrate, with glutathione acting as a capping agent and methylene blue as
the reporter molecule. Mg2+ ions were employed to facilitate structural folding of the aptamer,
enabling its specific binding to fluoride. Using carbodiimide chemistry (EDC/NHS), the aptamer
was covalently attached to silver nanoparticles (AgNPs), with glutathione (GSH) serving
as a capping agent to facilitate stable conjugation. The working model for fluoride detection
relies on the conformational changes in the aptamer triggered by anion binding, which brings
methylene blue closer to the silver nanoparticle surface. This proximity enhances the SERS
signal at the characteristic 1623 cm-1 Raman band. The parameters for the fluoride detection
assay were optimised, with methylene blue (3 ppm), glutathione (10-7 M), and aptamer (30nM),
ensuring efficient surface coverage without compromising signal amplification. Calibration experiments
conducted with fluoride concentrations ranging from 5 × 10-12M to 1×10-9M showed
a strong linear relationship (R2 = 0.96) for fluoride. This sensor offers a promising platform for
accurate, trace-level fluoride detection, making it suitable for environmental monitoring, industrial,
and healthcare applications.
