Surface-Enhanced Raman Spectroscopy as a Low-Cost Alternative for the Indirect Detection of Glyphosate-Based Products

Abstract

Commercial glyphosate-based herbicides are widely used in agriculture. They can be environmental contaminants and may pose a risk to human health through water and the consumption of agricultural products. However, methods for detecting glyphosate in different matrices are expensive and tedious procedures because it is a challenging molecule to detect. Therefore, in this study, we employed surface-enhanced Raman spectroscopy (SERS) with silver nanoparticles (AgNPs) as a sensitive, simple, and rapid method to indirectly detect glyphosate in commercial glyphosate-based products with the ninhydrin reaction. The glyphosate-ninhydrin reaction product exhibits a band at approximately 567 nm in the visible light spectrum, and Raman analysis reveals two distinct peaks at 660 and 790 cm⁻¹ in the standard. This confirms that the peaks are consistent with commercial glyphosate-based products and directly related to the concentration of glyphosate. The proposed SERS method may be practical for analyzing environmental samples with relatively high concentrations when compared to regulated levels in some matrices or to those reported in other scientific studies. Although its application is mainly geared toward detection in high ranges, it is a functional method that can be adjusted to improve its sensitivity and adapt to different analytical conditions. Our results provide an effective strategy for detecting this pollutant, which is crucial for monitoring, controlling, and preventing population exposure. A reliable and straightforward glyphosate detection method can thus support environmental safety and public health.

Graphical abstract

This is a visual representation of the abstract.

Keywords

Surface-enhanced Raman spectroscopy SERS pesticide environmental detection nanoparticles silver nanoparticles AgNPs ninhydrin reaction

Introduction

Glyphosate (N-(phosphonomethyl)glycine) is a broad-spectrum, non-selective herbicide belonging to the organophosphate class of agrochemicals. Its mode of action is post-emergent, meaning it is applied after weed germination and prior to crop establishment.¹ This herbicide is the most widely used globally. It was synthesized in the 1970s by Monsanto and is currently owned by Bayer, which acquired it in 2018.² In 2015, the IARC (International Agency Against Cancer) classified this herbicide as a probable type 2A carcinogen. However, there is limited epidemiological evidence in humans and significant evidence of animal cancer.³ Residues of glyphosate and its metabolite aminomethylphosphonic acid (AMPA) have been detected frequently in sediments, soils, drains, ditches, rainfall, streams, and rivers; and less often in ponds, lakes, groundwater, and wetlands, as well as agri-food products.^4,5 As a result, glyphosate residues present in agri-food products may enter and accumulate in the human body through dietary intake.

Glyphosate is one of the most challenging pesticides to determine, especially in polar matrices such as water. It lacks chemical groups (such as chromophoric groups) that can be directly analyzed by traditional colorimetric and fluorescence techniques.^6,7 The above complicates the development of selective and sensitive sensors for glyphosate residue detection.^8–10 Therefore, developing a rapid, inexpensive, and efficient analytical method for glyphosate in agri-food products is of great importance. In this regard,¹¹ demonstrated the high sensitivity and simplicity of a method for glyphosate quantification that couples the ninhydrin reaction with surface-enhanced Raman spectroscopy (SERS). The approach achieved a detection limit as low as 1.43 × 10⁻⁸ mol L⁻¹, highlighting its strong potential for rapid and ultra-sensitive glyphosate analysis.

The SERS effect occurs when an analyte is positioned near a nanoscale-structured metal surface under optimal conditions. In this way, the intensity of Raman signals can be enhanced by several orders of magnitude.¹² To date, gold and silver are two of the most widely used materials for fabricating active nanosubstrates in SERS.⁷ Metallic nanoparticles (NPs) exhibit high sensitivity and analytical capability in detecting the presence of analytes.¹³ The analytical properties of metallic NPs are significantly dependent on the size and shape of the NPs.¹⁴ Constant increases in Raman signal intensity have been observed in the range of 10⁴ to 10⁶ and can even reach high values such as 10⁸ to 10¹⁴ due to the spatially localized surface plasmon resonance of hot spots, which favor notable local increases in electromagnetic field intensity.⁷

Therefore, given the importance of glyphosate use in Mexico and its prohibition, it is necessary to evaluate glyphosate residues that could be accumulating and entering the food chain. The objectives of this study were (i) to evaluate the indirect detection method for glyphosate through the reaction with ninhydrin in SERS, and (ii) to detect glyphosate in commercial formulations commonly sold in Mexico using SERS.

Experimental

Materials and Methods

The silver nanoparticles (AgNPs) were purchased from Sigma-Aldrich (Sigma Chemical Co. Ltd., USA). According to manufacturers’ specifications, AgNPs contain sodium citrate as a stabilizer. Glyphosate (N–(Phosphonomethyl)glycine, C₃H₈NO₅P, CAS: 1071–83–6, > 95%) was obtained from Cayman Chemical Company. Ninhydrin (2,2–dihydroxyindane–1,3–dione, C₉H₆O₄, CAS: 485–47–2, 99%), sodium molybdate (Na₂MoO₄, CAS: 7631–95–0, > 98%), and glacial acetic acid (C₂H₄O₂, CAS: 64-19-7, > 99.7%) were purchased from Sigma Aldrich Chemical Co. Sodium acetate was obtained from Química Suastes, S.A. de C.V. Ultrapure water was used throughout the experiments.

Commercial Glyphosates

The commercial formulations of glyphosate were purchased at two distribution points, (i) in Miguel Aleman, belonging to the agricultural zone of the coast of Hermosillo, Sonora (Centella Fuerte, Credit and Glyfos), and (ii) in the city of Hermosillo (Faena Clásico). The specifications of each product are: (i) Faena Clásico (formulated, registered, and distributed by Monsanto commercial S. de R.L. de C.V.), 363 g of glyphosate acid L⁻¹. (ii) Centella Fuerte (formulated by Shandong Weifang Rainbow Chemical Co, Ltd. Registry importer: Rainbow Agro Science, S.A. de C.V., distributed in Mexico by Nay-Chem S.A. de C.V.), 360 of glyphosate acid L⁻¹. (iii) Credit (registered and distributed by Nufarm Grupo México, S. R.L. de C.V), 356 g of glyphosate acid L⁻¹. (iv) Glyfos (formulated and registered by Sintesis y Formulaciones de Alta Tecnología, S.A. de C.V., distributed by FMC Agroquímica de Mexico, S. de R.L. de C.V), 360 of glyphosate acid L⁻¹.

Characterization of AgNPs and Glyphosate

The ultraviolet–visible (UV–Vis) absorption spectrum of the AgNPs dispersion, glyphosate, and glyphosate reaction were measured on a Thermo Scientific Multiskan Go spectrophotometer (Thermo Fisher Scientific Inc, Massachusetts USA). The hydrodynamic diameter, polydispersity index, and Zeta potential were measured using dynamic light scattering (DLS). We used a Litesizer 500 equipped with a semiconductor laser diode of 40 mW, 658 nm (Anton Paar GmbH, Austria). Zeta potential was measured by laser Doppler electrophoresis. Three replicate measurements were carried out at 25 °C.

Detection of Glyphosate Based on Ninhydrin Reaction

To detect glyphosate, we used the technique described by Xu et al.,¹¹ which involves a reaction with ninhydrin. A 5% (ω/V) ninhydrin solution was prepared by dissolving 2.50 g ninhydrin in a 50 mL volumetric flask and diluting to 50 mL with H₂O. Ninhydrin working reagent (NWR): A 5% ninhydrin solution + water + acetate buffer (0.4 mol·L⁻¹, pH 5.5) (2:1:1, V/V/V). A 5% (ω/V) Na₂MoO₄ solution was prepared by dissolving 2.50 g Na₂MoO₄ in H₂O in a 50 mL volumetric flask and diluting to 50 mL with H₂O. A stock solution of 1.0 × 10⁻³ mol L⁻¹ glyphosate was prepared by dissolving 8.45 mg glyphosate in a 50 mL volumetric flask and then diluting to 50 mL with H₂O.

The mixed working reagent was NWR+5% Na₂MoO₄+ standard glyphosate solution (1:1:1, V/V). The mixed working solutions were heated in boiling water for 30 min; 1000 μL of the producing solutions were mixed with 10 μL Ag colloid. From this mixture, 20 μL were taken for SERS measurements (Figure 1). The following concentrations were prepared from the glyphosate stock solution: 2, 4, 6, 8, and 10 μg mL⁻¹, to perform a standard curve at low concentrations. At each concentration, the reaction was carried out and measured in SERS.

Figure 1.

Schematic representation of the procedure for the indirect detection of glyphosate by the ninhydrin–glyphosate reaction.

Preparation of Commercial Glyphosate Solutions and Detection of Glyphosate

In Mexico, agricultural workers generally disregard the label recommendations of commercial glyphosates. As far as we know, and depending on the region, they apply between 100 and 200 mL of glyphosate per 20 L of water in the spray backpacks. In this regard, for the four commercial glyphosates, we took an average of what is commonly applied, 150 mL of glyphosate per 20 L of water, and with this ratio, we made the solutions.

For the detection of glyphosate in commercial formulations, we replaced the glyphosate standard in the mixed working reagent with the solutions of each of the commercial formulations, NWR +5% Na₂MoO₄+ commercial glyphosate solution (1:1:1, V/V).

In addition, to obtain a SERS spectrum of the commercial glyphosates without reaction with ninhydrin, we took 20 μL of each glyphosate and performed a SERS measurement.

SERS Measurement

Raman spectra were obtained using a confocal Raman microspectrometer (Witec Alpha300 RA, Germany), equipped with a 600 groove/mm grating to disperse the scattered light. A 532 nm Nd:YAG laser with 20 mW output power was used as the excitation source. Sample preparation involved placing them on a calcium fluoride (CaF₂) substrate (13 mm Ø × 1.0 mm, Crystan Ltd., UK), prior to analysis. The laser beam was focused on the samples using a 50X objective lens.

Spectral data were collected over 5 accumulations, each with an acquisition time of 10 seconds, ensuring optimal signal quality. For each sample (2, 4, 6, 8, and 10 µg mL⁻¹), the average of five spectra acquired from independent reactions was used for statistical analysis (n = 5) and the relative standard deviation (%RSD) was calculated.

Results and Discussion

Characterization of Glyphosate and AgNPs

Figure 2a shows the UV–Vis absorption spectrum of the standard glyphosate solution contains no absorption bands in the visible region. This result coincides with what was reported by Mikac et al.¹⁵ On the other hand, when the glyphosate–ninhydrin reaction is performed, the product of this reaction shows an absorption maximum of around 567 nm. This result is in agreement with the data previously presented,¹¹ who reported an absorption maximum of 570 of the reaction product glyphosate–ninhydrin.

Figure 2.

Characterization of glyphosate and AgNPs. (a) UV–Vis absorption spectra of the glyphosate standard, the glyphosate–ninhydrin reaction product (initial glyphosate concentration: 1.0 × 10⁻³ mol L⁻¹), and the Ag nanoparticles (AgNPs). The glyphosate standard shows no significant absorbance in the visible region, while the derivatized glyphosate exhibits two characteristic absorption, confirming the formation of the reaction product. The AgNPs display a pronounced surface plasmon resonance, consistent with the expected optical behavior of colloidal silver nanoparticles of this size. (b) Hydrodynamic size distribution of the AgNPs obtained through dynamic light scattering (DLS). The measurement reveals a narrow and symmetrical particle indicating good colloidal dispersion and agreement with the nominal diameter reported by the supplier and (c) zeta potential distribution of the AgNPs. The colloidal dispersion exhibits a mean zeta potential, reflecting a highly negative surface charge that contributes to electrostatic stabilization of the nanoparticles in suspension.

The UV–Vis spectrum of AgNPs shows a maximum absorption peak centered around 437 nm (Figure 2a). The AgNPs used in this study have a polydispersity index of 20.8% and a mean hydrodynamic diameter of 67.79 nm, as determined using DLS (Figure 2b). Zeta potential measurements revealed that the AgNPs were negatively charged, with a Zeta potential value of –47.73 mV (Figure 2c). It has been reported that the SERS intensity increases as the size of AgNPs increases, e.g. 60 nm are approximately twice as intense as those produced at 10 and 30 nm.^16,17 Therefore, we used AgNPs of around 60 nm, which have a maximum absorption peak at around 430 nm.^18,19 In addition, the value obtained for the Z potential indicates a high level of colloidal stability in solution.²⁰

SERS Spectra of the Purple Product Generated in the Ninhydrin–Glyphosate Reaction, Raman Characterization of Commercial Glyphosates

The reaction between glyphosate and ninhydrin forms a purple product that is produced by nucleophilic substitution, the N atom of glyphosate combining with ninhydrin, forming a new C=N bond and linking glyphosate and ninhydrin.²⁰ The product of the reaction is active SERS and correlates directly with the concentration of glyphosate.

The Raman spectrum of the reaction product between glyphosate and ninhydrin was measured with an excitation wavelength of 532 nm, as shown in the Figure 3a. Two strong peaks were distinguished around 660 and 790 cm⁻¹. Our results are similar to those obtained by Xu et al.,¹¹ who reported these same peaks of the glyphosate–ninhydrin reaction product, at 633, 532 and 785, although the spectra obtained in their study of 532 and 784 were low, and in our study the 532 nm laser is of good intensity and high quality. Therefore, the 532 nm laser line is an excitation wavelength that works for SERS measurements. Figure 3a also shows that the technique is very sensitive and can indirectly detect very low concentrations of glyphosate. The SERS intensity at 660 cm⁻¹ showed a good linear relationship with the glyphosate–ninhydrin reaction product concentrations in the range of 2, 4, 6, 8, and 10 μg mL⁻¹ (y = –4.109 + 0.0790 × C(μg mL⁻¹), R² = 0.987) (Figure 3b).The reproducibility of the measurements was evaluated by calculating the percent relative standard deviation (RSD) from five independent spectra for each concentration (n = 5). The %RSD values ranged from 4.30% to 10.30%, indicating good signal stability and acceptable reproducibility for SERS measurements, because they are within the acceptable limits of RSD <20%.²¹

Figure 3.

(a) Raman spectra obtained for standard glyphosate solutions at concentrations of 2 to 10 μg mL⁻¹ (1.18 × 10⁻⁵ m L⁻¹, 2.37 × 10⁻⁵ m L⁻¹, 3.55 × 10⁻⁵ m L⁻¹, 4.73 × 10⁻⁵ m L⁻¹, 5.91 × 10⁻⁵ m L⁻¹), showing characteristic bands in the 660–790 cm⁻¹ region. (b) Calibration curve constructed from the Raman spectra of the standard, showing the relationship between signal intensity and glyphosate concentration. The linear equation and the coefficient of determination indicate the method's capacity for spectroscopic quantification. (c) SERS spectra of the same commercial products without reaction, used as a reference to identify signals attributable to excipients, additives, or interferences inherent to the original formulation. (d) SERS spectra corresponding to commercial products formulated with glyphosate after chemical reaction, used to explore the spectral response induced by the interaction with the substrate and possible active components.

The improvement generated by SERS is due to two mechanisms: (i) electromagnetic fields and (ii) chemical mechanism (CM). The first mechanism is based primarily on the improvement of the local electromagnetic field near the metal surface, induced by localized surface plasmon resonance (LSPR).²² The CM is generated by charge transfer between the analyte and the substrate surface, altering the polarizability of the molecule and enhancing Raman scattering.²³ In this study, Ag was used, a plasmonic metal that typically produces significant SERS enhancement due to its associated field intensification in the proximity of the electromagnetic “hot spot” structure.²⁴ In this sense, the enhancement of the SERS signal is mainly caused by electromagnetic effects. However, in addition to this effect, additional intensification mechanisms may contribute. Non-resonant enhancement arises from chemical bond interactions between molecules and the metal substrate, inducing alterations in electronic structures. Resonant enhancement occurs when adsorbed molecules form new adsorbate–substrate complexes with surface atoms, creating additional electronic transition pathways.²⁵

We measured the Raman spectra of the unreacted commercial glyphosates with ninhydrin to discard the characteristic peaks of the reaction as shown in Figure 3c. It can be observed that the Credit and Centella commercial products share a strong peak around 830 cm⁻¹, it is likely that this peak can be attributed to a surfactant used in both commercial products, these surfactants are used to facilitate field application. Even more peaks are observed in Credit and Centella, which may be due to impurities in these two products. Interestingly, the commercial product Faena does not show peaks, indicating it is a purer product. To our knowledge, in Mexico Faena is the commercial glyphosate-based product most appreciated by farmers due to its higher effectiveness.

The Raman spectra of the reaction product of each of the commercial glyphosates with ninhydrin are shown in Figure 3d. The characteristic peaks of the glyphosate standard (660 and 790 cm⁻¹) are observed. From the above, we confirm that by reacting with ninhydrin, we can indirectly measure the concentration of glyphosate in commercial formulations and in environmental matrices such as water and food.

There are different analytical techniques for detecting glyphosate in different matrices, but the SERS technique has proven to be a quick and easy method for facilitating the detection of herbicides.^26,27

Finally, according to López-Castaños et al.,²⁸ extrapolation of the methodology to food may be complicated by the known reaction of ninhydrin with amino acids. However, this reaction is different from the classical amino acid coloration reaction with ninhydrin, in which no decarboxylation or dealdehyde reaction takes place, because the food–ninhydrin reaction product is formed following a different pathway in the presence of molybdate.²⁹

Challenges and Future Directions

In this study, the main objective was to demonstrate the feasibility of the SERS approach for detecting the glyphosate–ninhydrin complex and to establish the linear relationship within the evaluated working range. At this stage, the purpose was not to develop or comprehensively validate a quantitative analytical method, but rather to present a proof of concept that would allow the behavior of the system to be characterized under controlled experimental conditions. This preliminary approach lays the groundwork for subsequent stages aimed at determining key analytical parameters and fully validating the method for its eventual application in real matrices.

Assessment in matrices such as soil, water, and food is particularly complex, given that concentrations reported worldwide tend to be relatively low. However, the technique can be optimized to achieve more sensitive detection limits, which would allow for more accurate and reliable identification of these compounds in environmental and agri-food conditions.

Conclusion

The detection of glyphosate indirectly by reaction with ninhydrin and using SERS is a simple, sensitive, and rapid technique. The results showed that glyphosate has no bands in the visible light region, but the glyphosate–ninhydrin reaction product does.

The reaction product of commercial glyphosates with ninhydrin confirmed the characteristic peaks in SERS that are directly related to glyphosate concentration.

Further studies are needed, especially in different matrices, soil, water, and food. In the case of food, it would be interesting to determine commercial glyphosate residues, since many agricultural products may have a higher antioxidant content that can generate fluorescence and obscure the bands in SERS measurements.

Footnotes

Data Availability

All data generated or analyzed during this study are included in this published article [and its supplementary information files].

Declaration of Competing Interests

The authors declare that they have no competing interests.

Funding

This research was funded by the National Council for Science and Technology in Mexico (CONACYT) Grant 309959 PRONACES Toxic Agents, and Grant A1-S-29697 to DM-F, and a postdoctoral scholarship to Magín González-Moscoso I1200/224/2021.

Acknowledgments

We would like to thank PhD. Martín Pedroza-Montero (RIP) for his invaluable help, which was fundamental to the development of this research.

ORCID iDs

Monica Acosta-Elías:

Sofía Navarro-Espinoza:

Diana Meza-Figueroa:

Magín González-Moscoso:

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