Self-Healing Plasmonic Metal Liquid as a Quantitative Surface-Enhanced Raman Scattering Analyzer in Two-Liquid-Phase Systems.

Liquid-state interfacial plasmonic systems are emerging as an alternative for the quantitation and practicability of the surface-enhanced Raman scattering (SERS) technique in analytical science, especially for complex liquid-phase systems. Here we show a general strategy for the three-dimensional (3D) self-assembly of gold nanoparticle (GNP) arrays on a spherical oil-water (O-W) interface, denoted as a plasmonic metal liquid (PML). The PML has excellent self-healing and shape-adaptive features; it can be transferred into containers of any shape; and it presents fast, quantitative, and multiplex SERS capability. Accurate control of nanoparticle density (PD) on the 3D interface enables tunable SERS strength. In situ synchrotron radiation small angle X-ray scattering (SR-SAXS) provides evidence that the interfacial PD is quantifiable and can be precisely regulated in the range of 24-216 particles/μm, which produces optimizable Raman enhancement. The strongest SERS signal is achieved at 167 particles/μm with GNP diameters of approximately 64 nm. In particular, the O phase acts not only as the assembly media for spherical PML arrays but also as the extracting agent for targets with different natures in complex media. Moreover, the O phase with continuous-phase features generates inherent and sharp SERS fingerprints and provides an effective internal standard (IS) for calibrating the fluctuation of samples and measuring conditions. By virtue of the triple roles of the O phase, the PML platform exhibits excellent mechanical stability, detection sensitivity, and signal reproducibility. This study demonstrates the concept of a fast and quantitative liquid-state SERS platform in common cuvettes on a portable Raman device that is as simple as a spectrophotometer.