Surface-enhanced Raman scattering (SERS) technology analyzes the “fingerprint” of molecular vibrations and is a highly sensitive, selective, and non-destructive testing method. Currently, SERS technology has wide applications in fields such as food safety, environmental monitoring, and biomedicine. Silver is commonly used in SERS technology due to its wider range of electric field enhancement, higher SERS enhancement factor, and lower cost compared to gold nanoparticles. The SERS enhancement properties of metal nanostructures can be modified by controlling their size, shape, and crystallinity, and different microstructures of silver nanoparticles can lead to varied SERS detection outcomes. Silver nanotrees (AgNTs) are a unique type of silver nanoparticles with a three-dimensional structure that enhances SERS sensitivity and selectivity by increasing surface area and local electric fields. Traditional methods of SERS substrate preparation often face issues such as complex procedures, high detection costs, high detection limits, and low sensitivity. The electroless deposition method for preparing silver nanotrees offers a cost-effective and simple method, resulting in substrates with high stability, uniformity, and the ability to detect various molecules. This study establishes a foundation for developing high-sensitivity SERS substrates and expanding the application of SERS technology in environmental and food safety monitoring.

The silver nanotrees SERS substrate is prepared using an electroless deposition method. First, silicon wafers are cleaned ultrasonically in acetone, anhydrous ethanol, and deionized water for 15 min each. The wafers are then treated with a mixture of ammonia, hydrogen peroxide, and water at a 1∶1∶5 volume ratio, heated until boiling and immersed for 15 min to clean them. Then cleaned wafers are rinsed with deionized water and subjected to hydroxylation in a sulfuric acid and hydrogen peroxide mixture at 80 ℃ for 30 min. Silver nanotrees are then grown on the wafers by mixing HF solution, ethylene glycol, and silver nitrate, stirring the mixture, and immersing the silicon wafers. Silver nanotrees develop on the wafers due to electroless deposition, where the electrochemical reduction of silicon and silver ions form nanoscale electrolytic cells without additional power sources. The silver icons, with a higher redox potential than silicon, gain electrons and form nanoparticles, with the silicon reacting with hydrofluoric acid to release electrons. As diffusion and polymerization processes occur, silver nanotrees gradually form on the surface of silicon wafers [Fig. 1(a), Fig. 1(b)]. Over time, the nanotrees began to branch, with secondary and tertiary dendrites emerging. This branching enhances the three-dimensional structure of the nanotrees and increases the substrate’s hot spot density.

The SERS substrates are characterized using scanning electron microscope (SEM), X-ray energy dispersive spectrum (EDS), and UV-visible absorption spectroscopy. The microstructure of the substrates at different reaction times (15, 20, 25, 30, 35 min) is studied [Figs. 1(c)?(e)]. Silver is identified as the primary element contributing to SERS enhancement [Figs. 2(a)?(d)]. By adjusting the reaction time, the shape, size, and plasmon resonance frequency of the silver nanotrees are controlled, leading to significant enhancement of Raman scattering signals. The spatial distribution of electromagnetic field intensity on the substrate surface is simulated and analyzed using finite-difference time-domain (FDTD), revealing a theoretical SERS enhancement factor of about 1.35×10¹¹ [Figs. 4(a)?(d)]. Raman characterization indicates that the substrate prepared with a reaction time of 25 min exhibits optimal performance, with SERS enhancement factor for rhodamine 6G (R6G) of 2.32×10¹¹ and a detection limit for R6G of 10^-13 mol/L [Fig. 6(a), Fig. 6(b)]. Raman analysis also confirms the substrate’s excellent uniformity and high stability. This substrate is used to detect a mixture of probe molecules with concentrations of 10^-10 mol/L R6G, 10^-8 mol/L crystal violet (CV), and 10^-6 mol/L malachite green (MG), demonstrating the silver nanotrees’ good sensitivity in SERS applications [Figs. 7(a)?(c)].

In this paper, we propose a silver nanostructured SERS substrate with a tree-like structure for detecting trace amounts of R6G and identifying mixed solutions of various molecules. The substrate achieves a detection limit as low as 10^-13 mol/L for R6G and is characterized by a simple, uniform, stable, and repeatable preparation method. This silver natotrees SERS substrate holds promise for applications in environmental monitoring and biomedical fields. Future research will focus on integrating SERS substrates with microfluidic technology to further enhance sensitivity and stability while enabling the detection of solutions and gases.