A turn-on highly selective and ultrasensitive determination of copper (II) in an aqueous medium using folic acid capped gold nanoparticles as the probe.

This paper describes a 'turn-on' fluorescent determination of Cu(II) in an aqueous medium using folic acid capped gold nanoparticles (FA-AuNPs) as the probe. The FA-AuNPs were synthesized by the wet chemical method and were characterized by UV-visible, fluorescence, HR-TEM, XRD, zeta potential, and DLS techniques. The FA-AuNPs show an absorption maximum at 510 nm and an emission maximum at 780 nm (λ(ex): 510 nm). On adding 10 μM Cu(II), the wine-red color of FA-AuNPs changed to purple and the absorbance at 510 nm decreased. The observed changes were ascribed to the aggregation of AuNPs. This was confirmed by DLS and HR-TEM studies. Interestingly, the emission intensity of FA-AuNPs was enhanced even in the presence of a picomolar concentration of Cu(II). Based on the enhancement of the emission intensity, the concentration of Cu(II) was determined. The FA-AuNPs showed an extreme selectivity towards the determination of 10 nM Cu(II) in the presence of 10,000-fold higher concentration of interferences except EDTA and the carboxylate anion. A good linearity was observed from 10 × 10(-9) to 1 × 10(-12) M Cu(II), and the detection limit was found to be 50 fM l(-1) (S/N = 3). The proposed method was successfully applied to determine Cu(II) in real samples. The results obtained were validated with ICP-AES.