Detecção de glifosato em água por reconhecimento de padrões em espectroscopia assistida por nanopartículas de prata fabricadas por ablação a laser

Abstract

Water is a natural resource that, although abundant, has been under great limitation in its availability due to human activity, mainly agriculture. Ensuring compliance of the water that is distributed to the population on levels considered safe for the presence of contaminants is therefore a public health issue and has attracted great attention. In order to detect potentially harmful substances, complex analytical chemistry procedures are used to verify and issue conformity reports used for decision making regarding its potability. In this scenario there is a demand for auxiliary methods to guide sampling and to perform sample screening. Glyphosate is the most widely used non-selective systemic herbicide in the world. Such a substance has received increasing attention, mainly due to its wide use and controversy regarding its potentially carcinogenic effect. This work presents a method for the detection of glyphosate directly in aqueous medium from the spectroscopic interrogation assisted by silver nanoparticles. With the use of Raman scattering, the region of spectral signature of molecules in aqueous solution is accessible in the visible region of the electromagnetic spectrum. However, the detection limit for this type of interrogation is impaired by the low intensity of the optical signal generated. The enhancing of Raman scattering by a nanostructured metallic body is a technique that allows the detection of traces of substances by means of their vibrational spectrum. Silver spherical nanoparticles, ranging from 5 to 20 nm in diameter, were produced by Pulsed Laser Ablation in Liquid (PLAL) technique, resulting in a colloidal solution stabilized by citrate ions, used as a surfactant. An experimental apparatus composed of two optical fiber spectrophotometers, and pumping by laser and wideband radiation sources, was implemented to interrogate water samples with the potential presence of glyphosate. The vibrational spectrum was obtained by measuring the surface enhanced Raman scattering (SERS) of the silver nanoparticle clusters, formed by the analyte-mediated aggregation. By means of the UV-Vis extinction spectrum, the state of aggregation of the nanoparticles in colloidal solution was measured. From this aggregation, it was possible to measure one of the analyte concentration dependent enhanced Raman scattering band. A mechanism based on the interaction between the analyte and the substrate has been proposed. From the results, the tuning of the nanoparticles production, as well as their interaction with the samples was carried out. The limit of detection (LOD) of 6.0 and 7.5 μM (1.0 and 1.3 ppm) for the two interrogation techniques, UV-Vis and Raman, respectively, was achieved. A pattern recognition system based on the Partial Least Squares Discriminant Analysis (PLS-DA) method has been implemented for the classification of samples. The system employs the low level fusion of the spectroscopic data, UV-Vis and Raman, in a unique feature space. The system was trained and validated with deionized water samples and tested with fresh water samples with addition of glyphosate. Considering a decision limit of 10 μM, a classification accuracy of 0.85 was obtained.