Plasmonic Array at the Liquid-Liquid Interface: A Dual-Mode Optical Sensing Platform for Multianalytes.

Analyte-triggered nanoparticle (NP) assemblies in bulk colloidal suspension have been extensively utilized in various optical sensors. Nevertheless, the assembling process is still limited by the slow diffusion dynamics of NPs and the low concentration of analytes in trace detections, which hinders further improvement of the sensitivity and repeatability of the sensors. In this work, by functionalizing the gold NPs with specific ligands, we constructed a dual-mode optical sensing platform for multianalytes based on the plasmonic NP array at the liquid-liquid interface. Through emulsification, the NP diffusion kinetics are boosted for several orders, and the NPs are condensed from the bulk aqueous phase to the liquid-liquid interface as a plasmonic array. The as-formed metasurface generates major reflectance and surface-enhanced Raman scattering changes in response to analytes, providing two optical sensing modes. As prototypes, cysteine and glucose are selected as the target molecules, achieving the limit of detection as 193 ± 2 and 297 ± 12 pM, respectively.

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.