Mercaptothiadiazole capped gold nanoparticles as fluorophore for the determination of nanomolar mercury(ii) in aqueous solution in the presence of 50 000-fold major interferents

Abstract

The present work describes the determination of picogram Hg(II) using 2,5-dimercapto-1,3,4-thiadiazole stabilized gold nanoparticles (DMT–AuNPs) by a spectrofluorimetry method. DMT–AuNPs show emission maximum at 773 nm with excitation at 514 nm. They show a large stock shift (259 nm), narrow emission profile and good photostability. While adding 10 μM Hg(II) the red color solution of DMT–AuNPs changes to purple and the UV-visible spectrum of DMT–AuNPs band at 514 nm was decreased. This is due to aggregation of DMT–AuNPs and it was confirmed by high resolution transmission electron microscopy (HR-TEM). UV-visible spectra of DMT–AuNPs in the presence of nanomolar concentrations of Hg(II) do not show any significant changes at 514 nm. However, the emission intensity of DMT–AuNPs was enhanced during adding even at picomolar concentration of Hg(II) 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. The binding constant (K_A = 2.6514 × 10⁴ mol⁻¹ L) value suggested that there is a strong binding force between Hg(II) and DMT–AuNPs. The present fluorophore showed an extreme selectivity towards Hg(II). The emission intensity was increased linearly against a wide range of Hg(II) concentration from 1 × 10⁻¹² to 1 × 10⁻⁷ M and a detection limit of 0.64 pg L⁻¹ Hg(II) (S/N = 3) was achieved for the first time using DMT–AuNPs by spectrofluorimetry method. The proposed method was successfully applied for the determination of Hg(II) in environmental samples. The obtained results were validated by ICP-AES.