A label-free fluorescent probe for Hg²⁺ and biothiols based on graphene oxide and Ru-complex

A novel, selective and sensitive switch-on fluorescent sensor for Hg(2+) and switch-off fluorescent probe for biothiols was developed by using [Ru(bpy)₂(pip)](2+) as the signal reporter and graphene oxide (GO) as the quencher. Due to the affinity of GO towards single-stranded DNA (ss-DNA) and [Ru(bpy)₂(pip)](2+), the three components assembled, resulting in fluorescence quenching. Upon addition of Hg(2+), a double-stranded DNA (ds-DNA) via T-Hg(2+)-T base pairs was formed, and [Ru(bpy)₂(pip)](2+) intercalated into the newly formed ds-DNA. Then, [Ru(bpy)₂(pip)](2+) and ds-DNA were removed from the surface of GO, resulting in the restoration of fluorescence. Subsequently, upon addition of biothiols, Hg(2+) was released from ds-DNA, due to the higher affinity of Hg(2+) to the sulfur atoms of biothiols, which could induce ds-DNA unwinding to form ss-DNA. Then ss-DNA and [Ru(bpy)₂(pip)](2+) were adsorbed on the surface of GO, the fluorescence of [Ru(bpy)₂(pip)](2+) was quenched again. Therefore, the changes in emission intensity of [Ru(bpy)₂(pip)](2+) directly correlated to the amount of detection target (Hg(2+) or biothiols) in solution. The assay exhibited high sensitivity and selectivity, with the limits of detection for Hg(2+), cysteine (Cys) and glutathione (GSH) to be 2.34 nM, 6.20 nM and 4.60 nM, respectively.