Histidine-mediated tunable peroxidase-like activity of nanosized Pd for photometric sensing of Ag+

In this work, we proposed a new and facile strategy for the high-performance colorimetric detection of Ag+ based on the tunable peroxidase-like activity of Pd nanoparticles (NPs) mediated by histidine (His). Bare Pd NPs possess intrinsic catalytic ability to trigger the oxidation of colorless 3,3',5,5'-tetramethylbenzidine (TMB) to blue TMBox in the presence of H2O2. After being decorated by His, the formed His-Pd nanozyme exhibits significantly improved mimetic activity due to its favorable physicochemical characteristics including smaller size, interactions between Pd and His, and better hydrophilicity, and such that an enhanced TMB color reaction is observed. When Ag+ exists, it is able to despoil the His modifier from the His-Pd surface via the specific interaction between Ag+ and His, resulting in the bald Pd again with weak activity. With this principle, highly sensitive determination of Ag+ in a linear scope of 30-300 nM was achieved with a limit of detection down to 4.7 nM. In addition, the colorimetric sensor based on the tunable enzymatic activity of nanosized Pd adjusted by His could provide excellent selectivity for Ag+ sensing against common interferents. Reliability of the strategy for practical detection of Ag+ was also demonstrated.