Designing of a functional paper-tip substrate for sensitive surface-enhanced Raman spectroscopy (SERS) detection.

A simple and flexible fabrication method of paper SERS substrate was developed by nanoparticles (NPs) droplet self-assembly at the paper tip with a temperature gradient (PTTG). We turned the drawback of the coffee ring effect into an effective way of preparing paper SERS substrate. When the NPs droplets were continuously dripped onto the PTTG, NPs were densely and uniformly distributed at the paper-tip front based on the combination of gravity and the coffee ring effect, which could achieve 91.2-fold improvement of SERS performance compared to a flat filter paper. Meanwhile, the analytes could also be enriched at the paper-tip front, which could achieve 9.3-fold signal enhancement compared to the paper-tip tail. Thus, the PTTG realized an excellent signal amplification for SERS detection. The paper-tip SERS substrate combined with a portable Raman spectrometer yielded an excellent analytical enhancement factor of 1.15 × 10 with the detection limit of 10 nM Rhodamine 6G (R6G). The whole fabrication procedure was completed within 2 h, and the paper-tip substrate showed a satisfactory substrate-to-substrate reproducibility with a relative standard deviation (RSD) of 5.13% (n = 10). It was successfully applied for quantitatively detecting real samples of oxytetracycline and malachite green with recoveries of 83.84-105.25% (n = 3). Meanwhile, we further evaluated the SERS performance of the PTTG using a laboratory-based Raman spectrometer, and it could realize the detection as low as 10 pM R6G. The proposed paper-tip substrate would offer a promising potential application for the on-site SERS analysis of food safety and environmental health.