Cyanobacterium metallothionein decorated graphene oxide nanosheets for highly selective adsorption of ultra-trace cadmium

Abstract

Graphene oxide (GO) nanosheets were decorated with a cysteine-rich metal-binding protein, cyanobacterium metallothionein (SmtA). The SmtA–GO composites were characterized by means of FT-IR, AFM and TGA, giving rise to a SmtA binding amount of 867 mg g−1. The SmtA–GO composites exhibit ultra-high selectivity toward the adsorption of cadmium, i.e., the tolerant concentrations for the coexisting metal and anionic species were 1–800 000 fold improved after SmtA decoration with respect to bare GO. The SmtA–GO composites were then assembled onto the surface of cytopore microbeads and used for highly selective adsorption and preconcentration of ultra-trace cadmium. In comparison with bare GO (carboxyl-rich GO) loaded cytopore (GO@cytopore), SmtA–GO loaded cytopore (SmtA–GO@cytopore) shows a 3.3-fold improvement over the binding capacity of cadmium, i.e. 7.70 mg g−1 for SmtA–GO@cytopore compared to 2.34 mg g−1 for that by GO@cytopore. A novel procedure for selective cadmium preconcentration was developed using SmtA–GO@cytopore beads as a renewable sorption medium incorporated into a sequential injection lab-on-valve system, with detection by graphite furnace atomic absorption spectrometry (GFAAS). The cadmium retained on the SmtA–GO surface was eluted with a small amount of nitric acid. An enrichment factor of 14.6 and a detection limit of 1.2 ng L−1 were achieved within a linear range of 5–100 ng L−1 by using a sample volume of 1 mL. The procedure was validated by analyzing cadmium in certified reference materials and a series of environmental water samples.