Flexible fabrication of a paper-fluidic SERS sensor coated with a monolayer of core-shell nanospheres for reliable quantitative SERS measurements.

In recent years, paper-based Surface-enhanced Raman spectroscopy (SERS) substrates have received extensive attention in the field of rapid analysis. However, obtaining quantitative SERS results is still challenging because of the inferior uniformity originating from the irregular morphology of the filter paper. In this work, a novel paper-fluidic SERS sensor was developed and its in-depth applications in the real-word quantitative analysis of contaminants in complex matrices were demonstrated. In particular, the Au@Ag core-shell nanospheres were labeled with an internal standard molecule to successfully normalize the fluctuation of the SERS signal caused by the microstructure of the filter paper, which could significantly improve the detection accuracy and accomplish the SERS quantitative analysis. In addition, a facile and robust strategy for the fabrication of a paper-based SERS sensor, which uses a dropper and mask to transfer the nanoparticle monolayers, was developed. This convenient and flexible approach effectively achieved a precise patterned assembly of nanoparticles on the filter paper. Furthermore, the paper-fluidic SERS sensor was fabricated by cutting and packaging for two detection modes, i.e., lateral-flow and vertical-flow, which generates the functionalization of the paper-based SERS substrate. Both detection modes integrated sample pretreatment and sample enrichment with SERS detection were applied to accurately detect the pesticide thiram in a complex sample of orange juice with pulp. In summary, this paper-fluidic SERS sensor with a simple preparation process and integrated functions is an ideal candidate for real sample analysis without pretreatment.