Turn-On Selectivity in Inherently Nonselective Gold Nanoparticles for Pb ²⁺ Detection by Preferential Breaking of Interparticle Interactions

Establishing a “precise” control over different interparticle interactions holds the promise of introducing inherently absent properties to nanosystems. In this direction, our aim is to introduce the notion of selectivity in inherently nonselective carboxylate-functionalized gold-nanoparticles ([−] AuNPs) toward strongly binding divalent metal ions (M2+). The common practice is to use the ability of M2+ ions to trigger the aggregation process in a kinetically trapped dispersed solution of [−] AuNPs. Aggregation of NPs being a thermodynamically favorable process will result in a uniform and nonselective turn-off response from most of the strongly binding divalent ions with [−] AuNPs. Our approach for identification is to use the preferential abilities of various M2+ ions to break a thermodynamically stable inter-nanoparticle precipitate containing [+] and [−] AuNPs (nanoionic precipitates). Importantly both [+] and [−] AuNPs, independently, are “blind” in terms of selectivity toward divalent ions. Remarkably, a hybrid system composed of such nonselective nanoparticles can discriminate between the hard-to-distinguish pair of Pb2+ and Cd2+ ions. Among different ions tested, Pb2+ can break the electrostatic interactions in [+]–[−] Au nanoionic precipitates and displace [+] AuNP to solution, thereby turning on the plasmonic wine-red color. The dominance of interaction energy for [−] AuNP–Pb2+ complexation over the inter-nanoparticle interactions is accountable for the selective discrimination of Pb2+ from other M2+ ions. A precise variation in strengths of different interparticle interactions helped in tuning both the selectivity and sensitivity of our identification protocol.