Gold Nanorod-Based Surface-Enhanced Raman Spectroscopy for the Rapid and Selective Detection of Aconitine in Various Complex Matrices

Aconitine (AC) is an alkaloid toxin and the primary toxic ingredient of plants such as Chuanwu, Caowu, and Epiphyllum (1). It mainly affects the vagus nerve and damages the peripheral nerves, resulting in symptoms that occur first in the nervous and circulatory systems (2), followed by those in the digestive system. Clinical symptoms include mouth, tongue, and limb numbness, as well as a generalized feeling of tightness, which may lead to severe acute poisoning requiring medical intervention (3). The effects of toxins on the vagus nerve can lead to various arrhythmias owing to increased ectopic pacing point autonomy (4,5). Oral administration of only 0.2 mg of pure AC can be toxic, and doses of 3–5 mg can be lethal. Given its harmful nature, it is extremely important to study the residues of AC in food or drugs to prevent human poisoning. (6). Current detection methods for AC, such as thin-layer chromatography, high-performance liquid chromatography (7), gas chromatography, and mass spectrometry (8,9), are often limited by long detection times and their requirements for complex preprocessing and specialized technicians, making them inconvenient for rapid and specific detection of AC in physiological samples. Therefore, rapid, simple, specific, and sensitive AC detection methods for physiological samples are urgently required (7,10–12).

The increasing frequency of poisoning incidents involving complex matrices emphasizes the need for rapid and effective toxin detection therein (13–15). However, the realization of this goal is hindered by the complex composition of such matrices and presence of impurities, such as proteins and lipids (16,17).Existing methods for detecting targets in complex matrix substrates often rely on large-scale instruments and specialized extractants for pretreatment. For example, Jiang and associates (18) optimized a refrigerant centrifugation method and combined it with gas chromatography–electron ionization mass spectrometry (GC-EI-MS) to pretreat phthalic acid esters in suet oil. Song and coauthors (19) developed a green and efficient supercritical fluid chromatography–mass spectrometry method for the separation and determination of nine biogenic amines from soy sauce extracts.

Surface-enhanced Raman spectroscopy (SERS) has garnered significant attention owing to its high sensitivity, selectivity, and unique peak recognition. It has been applied in various fields, including drug (20), pesticide (21), biomolecule (22), and virus detection (23,24). The enhancement effect in SERS signals is mainly a result of the electromagnetic interaction between light and metal, which is significantly amplified by the laser field via plasma resonance excitation (25,26). To observe this phenomenon, the target molecules must be adsorbed on, or neara metal surface. However, the complex compositions of complex matrices mean that there are numerous interfering substances during target detection, making direct and sensitive SERS detection challenging (27–29). Consequently, the aim of this study was to detect targets during the pretreatment process in complex matrices.

Considering the oil-soluble nature of AC, it can be isolated and enriched from complex matrices via liquid–liquid microextraction. In this study, a nonpolar solvent was utilized to isolate and enrich AC from complex matrices while precipitating proteins because solubility is greater for solvents and solutes with similar polarities. A handheld Raman spectrometer was employed to analyze the isolated target on the cetyltrimethylammonium bromide–gold nanorod (CTAB–AuNR) SERS substrate. The strong affinity between the acetonitrile solvent and SERS substrate ensured that AC was detected with high sensitivity and stability at concentrations as low as 1 × 10⁻⁶ M. This method is advantageous because of its low cost, simplicity, and ease of use, making it suitable for the rapid screening and monitoring of toxicants in poisoning emergencies, public safety, and first-response medicine.

Materials and Methods
Preparation of AuNRs
Preparation of Gold Seeds
To a conical flask containing 10 mL of 0.1 M CTAB, 52.5 μL of a 2% HAuCl₄ masterbatch was added and the mixture was stirred with a magnetic stirrer (500 rpm, 25–27 °C) until the HAuCl₄ had completely dissolved. This was followed by rapid addition of a freshly prepared 0.01 M NaBH₄ (0.6 mL) solution using ice water, and stirring was continued at 25–27 °C for approximately 3 min. Subsequently, the flask was placed in a constant-temperature water bath (maintained at approximately 27 °C) for over 2 h to allow gold seed formation.

Preparation of the Growth Solution
The conical flask containing 10 mL of 0.1 M CTAB was kept in a water bath at 27 °C. Subsequently, 125 μL of 0.008 M AgNO₃ was added, and the mixture was gently shaken. This was followed by the addition of 103 μL of 2% HAuCl₄ solution until the solution changed color. Finally, 60 μL of 0.1 M ascorbic acid (AA) was added to the solution with gentle shaking, and after a few seconds, the solution became colorless.

Later, 12 μL of the gold seed solution was added to the conical flask containing the growth solution, which was gently agitated for approximately 10 s. The mixture was incubated at room temperature for more than 6 h.

SERS Experiment
Specific pre-treatments are necessary to effectively detect AC in oily matrices. First, the toxicant standard sample was added to an oily matrix using the standard addition method: 100 μL of an AC standard sample and 200 μL of an oil mixture were used. Next, ammonia was added to adjust the pH to 9.5 and ether was mixed with the target solution in a 1:1 volume ratio for 1 min with shaking. After stratification, 10 μL of the supernatant was transferred into 1.5 mL centrifuge tubes. When the ether was volatilized, acetonitrile was added to re-dissolve it and SERS detection was performed.

Experimental Characterization
The morphology of the AuNRs was confirmed via scanning electron microscopy (SEM, FEI, Quanta 200 FEG). UV–visible (UV–vis) absorption spectra of the AuNRs were collected using a UV-2600 spectrophotometer (Shimadzu, Japan). Raman spectra were collected using a portable Raman spectrometer (OCEAN HOOD, SEED3000) with an excitation wavelength of 785 nm, laser power of approximately 100 mW, and integration time of 1 s for each spectrum. SERS detection of the samples was performed using a handheld Raman spectrometer (CASAFER-18T1) with an excitation wavelength of 785 nm, laser power of approximately 100 mW, and integration time of 1 s for each spectrum.

Results and Discussion
Morphological Characterization of AuNRs

Figure 1a (open images in a new tab to enlarge) shows the UV–vis spectrum of the AuNR solution. In addition to the characteristic absorption peak at 520 nm for gold gels, a stronger absorption peak was observed at 708 nm. The wavelength of this peak is dependent on the ratio of the rod axis, shape, and size. The average aspect ratio (AR) of the AuNRs can be calculated from the horizontal coordinate of this absorption peak (29): λ_max = 96AR + 418, where λ_max represents the peak position of the long-wave longitudinal surface plasmon resonance absorption peak with a uniform scale of nm. The value of λ_max observed in this study was 755 nm, and the average AR was calculated to be 3.5. The scanning electron microscopy image of gold nanoparticles in Figure 1b shows a AuNR length-to-diameter ratio (AR) of 3.2–3.8, indicating a homogeneous distribution consistent with the UV–vis analysis.

Sensitivity and Reproducibility of AuNRs as SERS Substrates

To evaluate the sensitivity and reproducibility of the synthesized AuNRs as SERS substrates, crystal violet (CV) was used as the probe molecule. Figure 2a shows the SERS spectra demonstrating the use of AuNRs as substrates for detecting different concentrations of CV. Distinct characteristic peaks at 439, 797, 938, 1171, and 1615 cm⁻¹ are observed, and the characteristic peaks of CV can still be observed at concentrations as low as 10⁻¹⁰ M. The SERS signals of CV are shown in Figure2b. The SERS signal decreased with the CV concentration, c, and in the c range of 10⁻⁶ M to 10⁻⁹ M, a clear linear relationship, satisfying I = −7016.8522(−lg[c]) + 64094.3303 and R² = 0.9770, is exhibited. As shown in Figure S1, the peak intensities of the peaks at 1619 cm⁻¹ in the 50 CV spectral plots at 10⁻⁶ M had a relative standard deviation (RSD) of 13.86%, indicating good SERS detection signal homogeneity for the AuNR substrate (30).

Detection of AC in Standard Solutions
The chemical structural formula of AC is shown in Figure S2. Raman analysis of the solid AC powder was performed, as shown in Figure S3 and Table SI. Characteristic peaks at 620 cm⁻¹ (out-of-plane C–H bending vibration), 1000 cm⁻¹ (out-of-plane aromatic ring C–H deformation vibration), 1605 cm⁻¹ (aromatic ring C–C vibration), and 1724 cm⁻¹ (C=O deformation) are observed in the resultant spectrum (Figure S3). As shown in Figure 3, SERS was then performed on the AC standard at concentrations of 10⁻⁴ to 10⁻⁶ M. The characteristic peaks of the AC standard solutions can be observed at 619, 1001, 1025, and 1724 cm⁻¹, indicating Raman shifts due to the displacement of the target molecules with noble metal nanoparticles. The characteristic peaks of AC are clearly visible even at a concentration of 10⁻⁶ M. The detection limit of AC in ethanol standard solution using the AuNR substrate is also attained at 10⁻⁶ M. This demonstrates the ability of the proposed method to detect AC in an ethanol medium based on stable SERS with signals, suggesting the possibility of applying the SERS technique to the detection of AC in complex media.

Detection of AC in Complex Matrices
The sensitive detection of toxins and toxicants in oily matrices is challenging because of the slow evaporation rate of oily substances (13–16). Additionally, oily matrices can lead produce grease stains on SERS substrate surfaces, which can affect the SERS measurements. Furthermore, the detection of AC in kitchen spices and medicinal wines poses unique challenges owing to the detection environment (18–19). To achieve efficient and cost-effective extraction, common solvents with different polarities, such as acetonitrile and ether, have been employed to extract AC from complex matrices. The feasibility of the solvent extraction strategy for the in situ SERS detection of AC in complex matrices was evaluated by adding standard samples with varying AC concentrations to different complex matrices. As can be seen in Figures S4 and S5, AC has good solubility in ethanol and acetonitrile, and clear and well-designated characteristic peaks can be obtained. Therefore, the detection of AC in complex systems was carried out using acetonitrile for complex solubilization.

In the case of grain wine, AC was added and thoroughly mixed to create an experimental wine sample, and the same volume of grain wine was added to another centrifuge tube to serve as the control sample. The experimental and control sample solutions were then evaporated in a water bath at approximately 40 °C for approximately 10 min. Subsequently, 40 μL of 2% HCl was added, and the tubes were agitated for 30 s. Then, ammonia was added to render the solution alkaline (pH 9.5). Next, ether was added at a volume ratio of approximately 1:1, and after layering, 10 μL of the ether layer was collected and placed in a new centrifuge tube. After evaporation, acetonitrile was added to re-dissolve the solution for the SERS measurements, which are shown in Figure 4a. An obvious characteristic AC peak is seen at 1001 cm⁻¹ in the spectra of edible wine. At the concentration of 8 × 10⁻⁴ M, this peak is significantly more intense than it is at the other three concentrations, and at the concentration of 4 × 10⁻⁵ M, the peak is notably less intense. This indicates that the detection limit of this substrate for AC is 4 × 10⁻⁵ M. Furthermore, the detection limit of this substrate for AC in ethanol is also 4 × 10⁻⁵ M; thus, the proposed method was demonstrated to be capable of detecting AC in ethanol. This provides evidence for the implementation of our next detection method for AC in mixtures.

Water-soluble seasonings, salt and chicken essence, were first dissolved in ultrapure water, and then the AC standard was added to simulate real poisoning scenarios. The pretreatment method used for the grain wine samples was also applied to these samples, with the additions of 2% HCl to adjust the pH and ether for extraction. The resulting supernatant was collected and dissolved in acetonitrile for the SERS analysis.To prepare a soy sauce sample, AC was added directly to the soy sauce, and the mixture was evenly mixed via shaking. Sufficient 2% HCl was added to obtain an acidic pH (to slow the rate of AC hydrolysis) to create the experimental soy sauce sample. The same amounts of soy sauce and 2% HCl were placed in another centrifuge tube to serve as a soy sauce control sample. The pretreatment method used to extract AC from food wine was used to establish the generality of this method for extracting AC from complex substrates.

SERS detection of AC in complex matrices, including edible wine, salt, chicken essence, and soy sauce was demonstrated, as shown in Figure 5a–d. The SERS signals for AC in the soy sauce, chicken essence, and salt systems were notably more intense than those in the edible wine system; however, this did not affect the detection of AC on the gold nanorod substrate. The use of AuNRs as SERS substrates enabled the detection of AC in various media. To assess the practicality of the method, a handheld Raman spectrometer was used to measure the SERS signals from soy sauce spiked with AC solution (laser wavelength 785 nm, integration time 5 s, laser power 250 mW), as shown in Figure S6. Figure S6c shows the SERS spectra of 1 × 10⁻⁴ M AC and the AC standard data in the database, and the matching coefficient is 98.17; a magnified view of the spectrum is shown in Figure S6d.These results provide a promising starting point for the practical detection of AC using Raman spectroscopy in real-life scenarios.

Conclusion
In this study, a rapid and efficient method for in situ detection of the toxic substance AC in complex matrices was successfully developed using surface-enhanced Raman spectroscopy (SERS) combined with solvent extraction. The proposed approach utilizing AuNR substrates exhibited high sensitivity for detecting AC at concentrations as low as 1 ppm. The simplicity, speed, and cost-effectiveness of this method indicate that it is promising as a tool for emergency response, public safety, and first-response medicine applications. This research provides valuable insights into analytical chemistry, advances the field of toxicant and toxin detection, and opens new avenues for practical toxin detection in real-life scenarios.

Data Availability
Data will be made available on request.

Funding
This work was supported by the Suzhou University doctoral research start-up fund (2024BSK011); Bio-based Functional Materials and Composite Technology Research Center (No. 2021XJPT06); Anhui Province "Four New" Research and Reform Practice Project (2022SX150); Research and Development Fund of Suzhou University (2021fzjj08).

Declaration of Interest
There are no conflicts to declare.

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Miao Qin, Mingwen Ma, Likun Deng, Guangkuo Tong, and Cong Wangare with the Key Laboratory of Spin Electron and Nanomaterialsof Anhui Higher Education Institutes at the School of Chemistry and Chemical Engineering of Suzhou University, in Suzhou, Anhui, China. Direct correspondence to: mqin@mail.ustc.edu.cn or congwang@ahszu.edu.cn