The ability to perform on-site chemical analysis has become increasingly crucial across various domains in recent times. Surface-enhanced Raman scattering (SERS) technology offers a simple and portable detection approach, making it highly promising for chemical analysis compared with conventional methods such as mass spectrometry and liquid chromatography. SERS technology can be categorized into two types based on the substrate used: flexible and rigid SERSs. Conventional rigid substrates have limitations in terms of universality, hardness, and fragility, thereby restricting their application in certain specialized environments. Conversely, paper-based SERS substrates are flexible and are easily fabricated. They exhibit properties such as flexibility, stretchability, and foldability, thereby expanding the potential application scenarios of SERS technology. The existing methods for fabricating paper-based SERS substrates use various types of substrates such as spray coating, direct immersion, and inkjet printing. However, these techniques require improvements in terms of performance, procedure simplicity, and limitations of the fabrication conditions. Furthermore, there is a need for further experimental analysis and exploration of SERS substrates, including multimolecule detection and investigation of flexible substrate properties, which can provide valuable insights into the application potential of SERS substrates. In this study, a liquid-liquid interface self-assembly technique is utilized to optimize SERS substrates by controlling the size of silver nanoparticles (AgNPs). In addition, the multimolecular detection ability and flexibility characteristics of SERS substrates are explored, and the detection performance of the substrate to be tested is discussed.

We used a liquid-liquid interface self-assembly technique to fabricate flexible SERS substrates by transferring a monolayer of AgNPs onto the surface of Whatman No.1 filter paper. The experimental procedure comprised the following steps. 1. First, we prepared a silver sol solution by reacting silver nitrate with sodium chloride under dark conditions to obtain a silver chloride colloid solution. Subsequently, in a dark and alkaline environment, we reacted the silver chloride colloid with ascorbic acid to reduce it into AgNPs. Finally, we subjected the prepared silver sol solution to centrifugation and ultrasound steps, repeating the washing process four times. 2. We placed the silver sol and hexane solution in a beaker, creating an immiscible water/hexane interface. Then, we sequentially added MPTMS and anhydrous ethanol, causing the AgNPs to float at the liquid interface. Notably, during this process, absolute ethanol should be added slowly. Finally, we transferred the AgNPs onto the surface of the filter paper. 3. We used the prepared flexible SERS substrate in Raman experiments to detect probe molecules.

The fabricated paper-based SERS substrate exhibits several advantages, including excellent detection performance, low cost, short preparation time, and controllable particle size, making it a highly attractive candidate for SERS applications. By investigating the impact of AgNP size on Raman experiments, we observed that a particle size of 20 nm exhibited the best detection performance. At this specific particle size, the substrate achieved the lowest detection concentration of 10^-10 mol/L for R6G molecules (Fig.4), accompanied by a maximum enhancement factor of 5.66 × 10⁸ and a relative standard deviation of 10.9% [Fig.5(a)].To further explore the potential application scenarios of the prepared SERS substrate, experimental analysis was performed to evaluate its multimolecule detection capabilities [Fig.5(b)] and flexible properties (Fig.6). This analysis confirmed the ability of the substrate to recognize and distinguish various molecules while also demonstrating its capability to detect target substances even in a bent detection environment. Hence, this type of SERS substrate is expected to become commercially viable chemical detection test paper, similar to litmus paper, pH paper, starch potassium iodide, and other such substrates, finding applications in analytical chemistry, biological detection, and various other fields.

This study successfully fabricated a flexible SERS substrate on the surface of Whatman No.1 filter paper using self-assembly techniques. The experimental results show that the Raman enhancement performance reaches its optimum when the AgNP size is 20 nm. The substrate with 20 nm NP size exhibits a detection limit of 10^-10 mol/L for R6G molecules, with a maximum enhancement factor of 5.66 × 10⁸ and a relative standard deviation of 10.9%. Furthermore, the fabricated flexible SERS substrate can detect mixed solutions of various molecules and exhibits excellent flexibility and recoverability. In addition, in this study, the substrate was characterized using scanning electron microscopy, and the electromagnetic field enhancement characteristics were numerically analyzed using finite-difference time-domain simulation software. The obtained simulation results were then compared with the experimental data to validate the findings.