Detection of emerging contaminants in solutions via surface amplified Raman scattering (SERS)

Author(s)

Marcelo José Dos Santos Oliveira

• Document type: Master's Dissertation
• Press: Presidente Prudente. 2023-09-06.
• Institution: Universidade Estadual Paulista (Unesp). Faculdade de Ciências e Tecnologia. Presidente Prudente
• Defense date: 2023-08-31
• Advisor: Carlos José Leopoldo Constantino

Abstract

The level of aggregation of metallic nanoparticles is one of the factors influencing the detection of substances of interest through the technique of Surface-Enhanced Raman Scattering (SERS), which can impact the reproducibility and sensitivity of analyses. In this context, this study aimed to evaluate the effects of two sample preparation methods (external standard method (ESM) and standard addition method (SAM)) for detecting the fungicide thiabendazole (TBZ) as the target molecule using the SERS technique with silver colloids (AgNPs). By employing extinction spectroscopy in UV-Vis, zeta potential measurements, dynamic light scattering (DLS), and transmission electron microscopy (TEM), it was observed that both methods induced significant colloidal aggregation of TBZ, albeit with distinct characteristics. The critical concentration (cc) at which intense aggregation occurred was determined to be 9.0x10-6 mol/L for the ESM method and 2.3x10-6 mol/L for the SAM method. Adsorption isotherms obtained via SERS (plotting SERS intensity against TBZ concentration) exhibited a direct correlation with colloidal aggregation. In both the APE and AMP methods, at low concentrations (< cc), a TBZ layer was formed, with adsorption favored by increased TBZ concentration (referred to as "cooperative adsorption"). However, at higher concentrations (> cc), the adsorption isotherm plateaued for the ESM method (indicating an excess of TBZ dispersed in the colloid, possibly leading to multilayer formation) and exhibited a sharp decline for the SAM method (indicating degradation of colloidal dispersion or precipitation). Regarding aggregate morphology, the ESM method at low concentrations exhibited more compact aggregates measuring up to 500 nm, while at high concentrations, the aggregates were more branched and exceeded 2 μm in size. Conversely, the SAM method displayed more branched aggregates at low concentrations, with dimensions of up to 2 μm, and more compact aggregates exceeding 2 μm at high concentrations. From these morphological observations, in relation to TBZ concentration and considering the diffusion-limited colloid aggregation (DLCA) and reaction-limited colloid aggregation (RLCA) mechanisms, it can be inferred that the ESM method follows the RLCA mechanism at low concentrations and DLCA at high concentrations, whereas the reverse is true for the SAM method. Lastly, in quantitative terms, despite the aforementioned differences and under identical experimental conditions, the limits of detection for TBZ were very similar for both methods (ESM method: 5.9x10-8 mol/L; SAM method: 4.9x10-8 mol/L). (AU)

• FAPESP's process: 21/05460-6 - Detection of emerging contaminants in solutions via surface amplified Raman scattering (SERS)
• Grantee: Marcelo José dos Santos Oliveira
• Support Opportunities: Scholarships in Brazil - Master