Turn-On Selectivity in Inherently Nonselective Gold Nanoparticles for Pb²⁺ 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.