Selective Colorimetric Sensing of Fluoride Ion in Water by 4-Quinonimine Functionalized Gold Nanoparticles

The stable aqueous colloid of 4-quinonimine (4-QI) functionalized gold nanoparticles (AuNPs) was successfully employed for the selective colorimetric sensing of fluoride ion in water. The significant decrease in intensity of the surface Plasmon absorption band at 535 nm was observed in ultra violet visible spectra of the AuNPs towards increasing concentration of fluoride ion. This decrease of intensity was distinguished with in the pH range from 3 to 10 as compare to the decrease of intensity by addition of water or chloride ion. From the straight line fitting calibration plot, the sensitivity of the 4-QI functionalized AuNPs was obtained as similar to 340 mu M-1 with a detection limit of 0.17 mu M for fluoride ion and only similar to 64 mu M-1 with a detection limit of 0.93 mu M for chloride ion. An immediate selective decolorization of 0.5 mL 4-QI functionalized AuNPs colloid of 880 mM was observed from purple-red by addition of 1 mL aqueous fluoride ion solution of 175 mM. No such decolorization was noted for the other anions, cations or biomolecules of same concentration at same volume. The sensing mechanism of fluoride ion was explained by considering the aggregation of AuNPs caused by the breaking of dipolar interaction between 4-QI and AuNPs.