Due to its many advantages, including low detection limits, high resolution, and minimal interference from water, enabling the real-time detection of trace substances, surface-enhanced Raman scattering (SERS) has garnered considerable attention and has been applied to various fields, including physical chemistry, biomedical science, and environmental monitoring. However, challenges remain for achieving quantitative analysis using SERS, primarily due to the uncertainties in the number of target molecules adsorbed on the enhancement substrate, uneven electromagnetic enhancement caused by the inherent nonuniformity of metal nanostructures, and molecular orientation fluctuations induced by chemical interactions. Currently, internal standard methods are primarily used for quantitative analysis. Internal standard methods involve the addition of internal standards or markers to achieve quantitative detection of analytes, partly addressing signal fluctuation issues. However, limitations such as the choice of internal standards, substrate materials, and preparation processes constrain their widespread practical applications. Carbon nanotubes (CNTs) exhibit distinctive 2D Raman characteristic peaks around 2696 cm^-1, far from the characteristic peaks of common probe molecules, making them natural internal standards for SERS calibration detection. In addition, when the strong electromagnetic enhancement effect of silver nanoparticles (AgNPs) is leveraged, the combination of CNTs and AgNPs can synergistically enhance performance. Therefore, in this study, a chemical self-assembly method was employed to prepare AgNPs/CNTs composite structures. Rhodamine 6G and malachite green were used as probe molecules in Raman self-calibration experiments to investigate the self-calibrated Raman enhancement characteristics of AgNPs/CNTs composite structures.

First, an electromagnetic simulation using finite-difference time-domain (FDTD) solutions was employed to analyze the structures of AgNPs/CNTs composite structures and AgNPs structures. SERS substrates based on AgNPs/CNTs composite structures were fabricated using a chemical self-assembly method. The prepared SERS substrates were characterized using SEM to assess their structural properties. Following characterization, a series of Raman spectroscopy measurements were conducted on the SERS substrates with AgNPs/CNTs composite structures using different concentrations of the target molecules R6G and MG. The obtained Raman spectral data were used to calculate the enhancement factors (k) for R6G and MG at various concentrations using the formula k_i=I_i(analyte)/I_i(2D). A relationship curve (k-C curve) was established by plotting these values, and linear fitting was performed using Origin software to evaluate the quantitative detection capabilities of the AgNPs/CNTs composite structures.

Electromagnetic simulation results show that the electric field intensity around the AgNPs/CNTs composite structure is stronger than that around the AgNPs structure, with a maximum electric field intensity of E_max=162.975 V/m (Fig. 2). The calculated electromagnetic enhancement factor (F_EM) for this structure is approximately 7.05×10⁸. By contrast, for the AgNP structure, the maximum electric field intensity is E_max=138.481 V/m, resulting in an F_EM of approximately 3.68×10⁸. In addition, the AgNPs/CNTs composite structure demonstrates detection limits for R6G and MG of as low as 10^-12 and 10^-9 mol/L, respectively. Linear fitting of the obtained k-C relationship curves shows high goodness of fit at the R6G characteristic peaks around 613 cm^-1 [R²=95.48%, Fig. 7(a)] and 773 cm^-1 [R²=96.15%, Fig. 7(b)]. Similarly, for MG, the linear fitting results show R² values of 95.11% at the characteristic peak 806 cm^-1 [Fig. 9(a)] and 95.77% at 915 cm^-1 [Fig. 9(b)].

The study utilized CNTs as natural internal standards for achieving quantitative SERS detection. The synergistic effect of these two components was investigated based on the strong electromagnetic enhancement of AgNPs. Simulation analysis of the electric field distribution around the AgNPs/CNTs composite structures and AgNPs structures revealed that the electromagnetic enhancement factor of the composite structure is approximately twice that of the AgNPs structure. The AgNPs/CNTs composite structures were prepared using chemical reduction and self-assembly methods. Self-calibration experiments were conducted using R6G and MG as probe molecules. Based on the normalized k-value method, experimental results demonstrate high linear fitting coefficients (R²) of 95.48% and 96.15% at the characteristic peaks of R6G at 613 cm^-1 and 773 cm^-1, respectively. Similarly, for MG, linear fitting coefficients are 95.11% at 806 cm^-1 and 95.77% at 915 cm^-1. This initial study achieves self-calibrated SERS detection through the use of AgNPs/CNTs composite structures.