Strategies for SERS Detection of Organochlorine Pesticides

Abstract

Organochlorine pesticides (OCPs) embody highly lipophilic hazardous chemicals that are being phased out globally. Due to their persistent nature, they are still contaminating the environment, being classified as persistent organic pollutants (POPs). They bioaccumulate through bioconcentration and biomagnification, leading to elevated concentrations at higher trophic levels. Studies show that human long-term exposure to OCPs is correlated with a large panel of common chronic diseases. Due to toxicity concerns, most OCPs are listed as persistent organic pollutants (POPs). Conventionally, separation techniques such as gas chromatography are used to analyze OCPs (e.g., gas chromatography coupled with mass spectrometry (GC/MS)) or electron capture detection (GC/ECD). These are accurate, but expensive and time-consuming methods, which can only be performed in centralized lab environments after extensive pretreatment of the collected samples. Thus, researchers are continuously fueling the need to pursue new faster and less expensive alternatives for their detection and quantification that can be used in the field, possibly in miniaturized lab-on-a-chip systems. In this context, surface enhanced Raman spectroscopy (SERS) represents an exceptional analytical tool for the trace detection of pollutants, offering molecular fingerprint-type data and high sensitivity. For maximum signal amplification, two conditions are imposed: an efficient substrate and a high affinity toward the analyte. Unfortunately, due to the highly hydrophobic nature of these pollutants (OCPs,) they usually have a low affinity toward SERS substrates, increasing the challenge in their SERS detection. In order to overcome this limitation and take advantage of on-site Raman analysis of pollutants, researchers are devising ingenious strategies that are synthetically discussed in this review paper. Aiming to maximize the weak Raman signal of organochlorine pesticides, current practices of increasing the substrate's performance, along with efforts in improving the selectivity by SERS substrate functionalization meant to adsorb the OCPs in close proximity (via covalent, electrostatic or hydrophobic bonds), are both discussed. Moreover, the prospects of multiplex analysis are also approached. Finally, other perspectives for capturing such hydrophobic molecules (MIPs-molecularly imprinted polymers, immunoassays) and SERS coupled techniques (microfluidics-SERS, electrochemistry-SERS) to overcome some of the restraints are presented.

Keywords: POPs; SERS; multiplex analysis; organochlorine pesticides; preconcentration; selectivity; sensitivity.

Figure 1

Structures of the listed organochlorine pesticides. DDE—dichloro-diphenyl-dichloro-ethylene; 2,4-D—2,4-dichloro-phenoxyacetic acid.

Figure 2

Strategies for increasing the electromagnetic surface enhanced Raman spectroscopy (SERS) enhancement. (A) (a) Schematic illustration of the capsule; (b,c) Representative TEM micrographs of the capsules; (d) STEM/XED analyses of the capsule composition. Reproduced with permission of [42]. Copyright John Wiley and Sons, 2019. (B) (a) FESEM image of the chestnut-like Au nanocrystal film, (b) local magnified image. Reproduced with permission of [19]. (C) (a–c) SEM images of F-NPs substrate with 100 nm of Ag coating film. Reproduced with permission of [47]. Copyright IOP publishing, 2020. (D) Representative TEM images of (a) the Fe₃O₄@pNIPAM nanohybrid materials containing Ag seeds; (b) the finalFe₃O₄xAgNPs@pNIPAMcomposite microgels. Reproduced with permission of [50]. Copyright American Chemical Society, 2011.

Figure 3

Strategies for increasing the affinity toward the analyte. (A) (a) The Suzuki cross coupling reaction occurred between HCH and 4-MPBA via covalent linkage (C–C) on the modified substrate. (b) The Raman spectra of solid HCH (curve I), the SERS spectra of γ-HCH (10⁻⁶ M) obtained on the naked ANHC substrate (curve II), 4-MPBA modified ANHC substrate (curve III), 4-MPBA modified ANHC substrate immersed into 10⁻⁶ M HCH solution (curve IV). Reproduced with permission of [53]. Copyright Elsevier, 2019. (B) Schematic representation of the mechanism of sensing PCP using cysteamine modified substrates. (C) (a) Scheme displaying the pesticide hosting (in this case, for aldrin) in the dithiol layer organized in interparticle gaps; (b) SERS spectra of the analyzed pesticides (10⁻⁵ M) on DT8-functionalized AgNPs, showing the C–Cl stretching bands of the fingerprint region (300–400 cm⁻¹) and the C–S stretching bands of DT8 in the deduced multilayer highly ordered conformation employed as the reference band for quantitative analysis. Reproduced with permission of [23]. Copyright American Chemical Society, 2015.