Colorimetric detection of Hg(ii) based on the gold amalgam-triggered reductase mimetic activity in aqueous solution by employing AuNP@MOF nanoparticles

Abstract

Although the potential of gold amalgam as a nanoenzyme has been demonstrated, its practical utility has been limited by its low catalytic activity caused by the aggregation of Au nanoparticles (Au NPs). Thus, there is a need to further engineer Au NPs to prevent aggregation and then to achieve higher enzyme activities for the detection of Hg²⁺ ions. Metal organic frameworks (MOFs), as one kind of promising material, have attracted particular attention due to their unique characteristics of uniform cavities and very high porosity. Herein, a hybrid material of Au nanoparticles and a MOF (AuNP@MOF), constructed by immobilization of Au NPs uniformly on the cavity surface of an iron-5,10,15,20-tetrakis (4-carboxyl)-21H,23H-porphyrin-based MOF (Fe-TCPP-MOF), has been successfully synthesized. Based on Hg²⁺ ion triggered Au catalysis of methylene blue (MB) reduction, a colorimetric method for highly sensitive and selective detection of Hg²⁺ ions has been established. The Hg²⁺ ions were first bound to the Au NP surface to form gold amalgam, and then the catalytic activity of Au NPs was initiated. This detection method showed the advantages of a fast response time, and high sensitivity and selectivity. The response time and the limit of detection were as low as 2 s and 103 pM, respectively, benefiting from the uniform cavities and the large specific surface area of Fe-TCPP-MOF, which ensure: (1) uniform dispersion of the Au NPs on the surface of the cavity; and (2) a higher chance of interaction of mercury and MB owing to the gathering effect of Fe-TCPP-MOF.