Micromolar Hg(II) induced the morphology of gold nanoparticles: a novel luminescent sensor for femtomolar Hg(II) using triazole capped gold nanoparticles as a fluorophore

In this paper, we report the morphological changes of gold nanoparticles induced by micromolar Hg(II) and the determination of femtomolar Hg(II) by a luminescent method. 3,5-Diamino-1,2,4-triazole capped gold nanoparticles (DAT-AuNPs) were synthesized by a wet chemical method. The HR-TEM images show that the spherical structure of DAT-AuNPs was changed into a chain-like structure after the addition of micromolar Hg(II) due to the strong coordination of DAT-AuNPs with Hg(II). The binding of Hg(II) with DAT-AuNPs was confirmed by XPS. The XPS of Hg5p shows two peaks at 69.3 eV for 5p1 and 74.35 eV for 5p3, suggesting that mercury was present as Hg(II), coordinated with DAT-AuNPs. The DAT-AuNPs show the emission maximum at 776 nm while exciting at 520 nm and the emission intensity was enhanced after the addition of even nanomolar Hg(II). The quantum yield estimated for DAT-AuNPs in the presence of Hg(II) was 1.5-fold higher than that of free DAT-AuNPs. This suggests that Hg(II) induced the fluorescence properties of DAT-AuNPs due to photoinduced electron transfer and metal binding-induced conformational restriction upon complexation. Based on the enhancement of emission intensity, the concentration of Hg(II) was determined. Further, the DAT-AuNPs showed an extreme selectivity towards the determination of 10 nM Hg(II) in the presence of a 50 000-fold higher concentration of common interferents. The emission intensity increases linearly in the concentration range of 1 x 10(-7) to 5 x 10(-13) M Hg(II) and the detection limit was found to be 0.75 fM L-1 Hg(II) (S/N = 3). This method was successfully utilized for the determination of Hg(II) in environmental samples.