Surface-enhanced Raman scattering (SERS) is the significant enhancement of Raman spectral intensity of the target molecules adsorbed on metal nanostructures with rough surfaces under the excitation of incident light waves. SERS enables rapid and non-destructive analysis based on the unique fingerprint features of analytes, achieving high specificity and spatial resolution at the single-molecule level. It has been widely applied to various fields such as biology, chemistry, and life sciences. In a three-dimensional SERS platform, the laser confocal volume is three-dimensional space, meaning that within the same three-dimensional laser confocal area, the three-dimensional substrate has higher effective utilization. Many researchers have demonstrated that the porous anodic aluminum oxide (AAO) template is an excellent SERS substrate. However, AAO-based SERS substrates still face challenges such as complex preparation processes and reliance on large-scale equipment. We primarily utilize the liquid-liquid interface self-assembly technique to prepare morphology and size-controllable monolayer Ag nanoparticles (AgNPs) and assemble them onto AAO, creating a novel flexible and open nanocavity-assisted SERS substrate. On this substrate, we conduct experiments for detecting R6G molecules at ultralow concentrations and multiple molecules simultaneously.

First, in 40 mL deionized water, 170 mg of polyvinyl pyrrolidone (PVP) and 170 mg of AgNO₃ solid are added sequentially, and the mixture is continuously stirred using a magnetic stirrer. After completely dissolving the solids, 400 μL 5 mol/L NaCl solution is added to the mixed solution, and the stirring is continued at room temperature in the dark for 15 min to produce an AgCl colloid solution. Next, 2.8 mL 0.5 mol/L NaOH solution and 2.5 mL AgCl colloid solution are added sequentially to 20 mL 50 mmol/L L-ascorbic acid (AA) solution. The mixture is stirred at room temperature in the dark for two hours. The prepared solution is centrifuged at 4000 r/min for 45 min and sonicated for 30 min to remove residual organic substances, especially PVP, and this process is repeated at least four times. The resulting AgNPs colloid is stored at 4 °C. Subsequently, 5 mL AgNPs colloid is added to a petri dish, and then 5 mL n-hexane is added to form an oil-water interface. 500 μL 0.1 mmol/L (3-Mercaptopropyl) trimethoxysilane (MPTMS) is added to the n-hexane layer, and the presence of MPTMS plays a crucial role in forming dense packing and a monolayer. Ethanol is slowly (0.5 mL/min) added to the AgNPs colloid, making AgNPs in the colloid gradually adsorb onto the oil-water interface. After the n-hexane evaporates, a layer composed of AgNPs can be observed on the upper surface of the solution. Finally, the AAO is fully immersed in the AgNPs colloid and then pulled out vertically, which leads to a large-area coverage of AgNPs on the AAO and creates an AAO-AgNPs composite structure.

SEM analysis of the substrate [Figs. 1(c) and (d)] shows that Ag particles are concentrated inside the AAO template pores. Random statistical analysis of 100 Ag particles reveals an average particle size of 35.65 nm [Fig. 1(e)]. The average gap between 50 randomly selected Ag particles is measured to be 1.14 nm [Fig. 1(f)], and the SERS performance of the prepared samples using rhodamine 6G (R6G) is evaluated as the analyte molecule. The main Raman characteristic peaks of R6G are located at 611, 772, 1363, and 1650 cm^-1 [Fig. 2(a)]. With the increasing R6G concentration, the Raman spectral intensity also rises accordingly. The maximum enhancement factor (AEF) is calculated to be 2.38×10¹⁰. Importantly, even at an R6G concentration of 10^-16 mol/L, typical Raman characteristic peaks can still be detected [Fig. 2(b)]. Thus, the detection limit of AAO-AgNPs as an SERS substrate reaches 10^-16 mol/L. Additionally, the relative standard deviation (RSD) of each dataset is calculated to quantify the substrate's uniformity, yielding RSD values of 6.46% at 611 cm^-1 and indicating good sample uniformity. Furthermore, Raman tests are conducted on samples stored at room temperature after 24, 72, 120, and 168 h by employing 10^-8 mol/L R6G as the analyte molecule [Figs. 3(c) and (d)] to assess the time stability of the samples. The Raman spectral intensity shows no significant changes compared to the original sample, indicating good time stability. Additionally, mixed solutions containing 10^-8 mol/L R6G, 10^-6 mol/L CV (crystal violet), 10^-4 mol/L MG (malachite green), and 10^-8 mol/L thiram solution are also tested, which shows that the substrate possesses good capability for practical molecular detection [Figs. 3(e) and (f)].

We conduct preparation, numerical analysis, characterization, and testing of the AAO-AgNPs composite structure, yielding significant findings. The structure demonstrates an extremely low detection limit (10^-16 mol/L) and an RSD of 6.46% in R6G molecule detection, with a maximum analytical enhancement factor of approximately 2.38×10¹⁰. Furthermore, the structure exhibits excellent multi-molecule detection ability. The AAO template features low cost, high sensitivity, high reproducibility, and multi-molecule detection, becoming a promising candidate for applications in SERS sensors. Future research can combine various types of AAO templates, and investigate different forms of metal nanostructures integrated with AAO templates and optical fibers to meet the demands of long-distance and flexible SERS technology applications.