Phosphonate-Substituted Ruthenium(II) Bipyridyl Derivative as a Photoelectrochemical Probe for Sensitive and Selective Detection of Mercury(II) in Biofluids

A ruthenium(II) bipyridyl derivative photoelectrochemical probe, Ru-1, is synthesized and coupled with TiO2 nanoparticles (Ru-1/TiO2) for the specific recognition and highly sensitive photoelectrochemical (PEC) detection of Hg2+ in a series of biofluids. The probe is designed with a chromophore, a thiocyanate recognition unit, a pi-conjugated photoelectron-transfer pathway, and a phosphonate anchor. TiO2 nanoparticles with strong affinity to phosphonate and suitable conduction band energy are used as intermediate layers to increase the Ru-1 adsorption amount and amplify the photocurrent response. Under irradiation, the Ru-1/TiO2/fluorine-doped tin oxide (FTO), with strong visible light harvesting capacity, aqueous stability, and efficient photoelectron transfer, shows a high and stable photocurrent response. In the presence of Hg2+, however, the specific Hg2+ and NCS coordination changes the photophysical properties of Ru-1, imposing the probe with a wider band gap, a weaker absorbance, and a poorer photoelectron and hole separation efficiency, thus resulting in a significant photocurrent decrease. On the basis of the Hg2+-induced photocurrent change, the Ru-1/TiO2/FTO shows good selectivity and high sensitivity toward the PEC detection of Hg2+, with wide linear ranges from 10(-12) to 10(-7) and 10(-7) to 10(-3) g/mL, and a low limit of detection of 0.63 pg/mL. The PEC probe is recyclable and accurate for selective detection of Hg2+ in urine, serum, and cell extracts. The whole analysis can be completed within 15 min. These good analytical performances indicate that the PEC method might have great potential for the onsite detection of small molecules in biosystems.