Optimizing the Heavy Metal Ion Sensing Properties of Functionalized Silver Nanoparticles: The Role of Surface Coating Density

We present a colorimetric sensor based on functionalized silver nanoparticles for the detection of metal ions in aqueous solutions. The interaction between the target metal ion and the functionalizing agent triggers the aggregation of these nanoparticles, and the consequent change in optical properties allows the detection/quantification of the analyte. In detail, this work describes the synthesis of AgNPs by a chemical reduction method, and the production of mercaptoundecanoic acid functionalized NPs with different surface densities (multi-, full-, and two partial layers). UV-Vis spectroscopy was used to monitor the functionalization processes, and to investigate the aggregation behavior of each AgNPs@11MUA sensor upon titration with the metal ions of interest, namely Ni2+, Zn2+, Co2+, Cd2+, Mn2+, and Cu2+. The resulting UV-Vis raw data obtained for each layer density were submitted to principal component analysis to dissect the role of the metal ions in NP aggregation and in establishing the sensitivity and selectivity of the AgNPs@11MUA sensor. Interestingly, we observed an increase in sensor sensitivity and selectivity at a lower density of the functionalizing agent on the AgNPs’ surface, which results in characteristic colors of the NP suspension upon titration with each metal ion.