Dispersive Liquid-Liquid Microextraction (DLLME) Coupled with Droplet Evaporation on an Omniphobic Nano/Micro Structured Porous Microfiber Membrane for Surface-Enhanced Raman Spectroscopy

This study demonstrates an ultrasensitive method for surface-enhanced Raman spectroscopy (SERS) based on dispersive liquid-liquid microextraction (DLLME) coupled with droplet evaporation on a functionalized omniphobic microfiber membrane. The omniphobicity of the membrane was achieved by the fabrication of fractal structures using silica nanoparticles and lowering surface energy. The approach involves the preconcentration of the analyte in a ternary mixture of water-oil-ethanol, followed by collecting microemulsion droplets in an omniphobic porous microfiber membrane. A supraparticle of plasmonic nanoparticles formed from droplet evaporation on the membrane acts as hotspots for SERS detection. This method achieves exceptionally low detection limits of 10^-6-10^-14 M for four nonvolatile and three volatile analytes of significance to food safety and environment after oil spill. The intensity of SERS signals depends on the partition coefficient of the analyte in water and oil microdroplets, the response of functional groups in the chemical structure of the analytes to Raman scattering, and the wetting properties of the microfiber membrane. The preparation method demonstrated in this work may have broad applications in the detection of pesticides in environmental monitoring, pharmaceutical waste, and hydrocarbon pollutants.