The construction and destruction of gold nanoparticle assembly at liquid-liquid interface for Cd²⁺ sensing

The analyte-induced aggregation of plasmonic nanoparticles (NPs) in the bulk solution have been widely employed in optical sensors. However, in trace detection, the slow diffusion kinetics of NPs and the ultralow concentration of analytes significantly limit the binding opportunity of the analytes, thus impose penalties on the sensitivity, reproducibility and response time of the sensors. Herein, we propose a novel sensing method with two working modes that based on the construction/destruction of NP arrays at the liquid-liquid interface (LLI). For the turning-on mode, the emulsion state of the two immiscible liquid phases and specific binding between Cd²⁺ and cysteine (Cys) assemble NPs into a liquid plasmonic mirror at the LLI whose reflectance has a positive relationship with the concentration of Cd²⁺. For the turning-off mode, the assembled NP arrays are intentionally destructed with the introduction of Cd²⁺ which aggregates neighboring NPs, reduces the interfacial reflectance. Compared with the conventional bulk aggregation method in a single phase, the NPs here that scavenge Cd²⁺ in the aqueous phase are condensed onto the LLI, boosting the local NP concentration and the diffusion kinetics for several orders. The prototypes achieve the limit of detection as 29.3 ± 0.03 μg L^-1, and are applicable in real-life samples.