Kinetically-Controlled Growth of Chestnut-Like Au Nanocrystals with High-Density Tips and Their High SERS Performances on Organochlorine Pesticides

Strategies for SERS Detection of Organochlorine Pesticides

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.

Highly reusable nanoporous silver sheet for sensitive SERS detection of pesticides

Surface-enhanced Raman spectroscopy (SERS) enables pesticide detection at the point-of-need, but its practical application is limited by expensive and disposable SERS substrates. Here, we report a reusable nanoporous silver (NPAg) sheet for the SERS detection of organochlorine pesticides, aiming to maximize the cost-efficiency of substrate regeneration. The NPAg sheet is prepared by a reduction-induced decomposition method without chemical induced random aggregations. This SERS substrate is sensitive to various analytes regardless of their affinity to a metal surface such as rhodamine B, dichlorodiphenyl-trichloroethane (DDT), and lindane due to its large surface area and the coral rock-like morphology. The SERS signal of lindane, a typical organochlorine pesticide, is identified and quantified with a minimum detectable concentration of 3 × 10^-7 M (87 ppb), which is below the maximum residue limits in various foods set by the regulators across the world. More importantly, after a few minutes of ultrasonic cleaning in water, the NPAg sheet can be reused at least 20 times with a reproducible SERS activity. Furthermore, the NPAg sheet remains stable in terms of its sensitivity and reusability after several months of bare strorage. Therefore, the NPAg sheet as a SERS substrate holds great promise for mass production and convenient applications in low-cost pesticide analysis.

Porous zeolite imidazole framework-wrapped urchin-like Au-Ag nanocrystals for SERS detection of trace hexachlorocyclohexane pesticides via efficient enrichment

A core-shell configuration of the zeolite imidazole framework (ZIF-8) wrapped urchin-like Au-Ag alloyed nanocrystals (UAANs) were designed and fabricated via adding the pre-formed plasmonic nanoparticles into the ZIF-8 precursor solution with hexadecyltrimethyl ammonium bromide (CTAB). The UAANs are about 100 nm in size with high-density tips. The ZIF-8 shell layer is nanoporous and can be controlled in thickness from 10 nm to 40 nm by the CTAB concentration. Importantly, such ZIF-8 wrapped UAANs can be used as the highly efficient surface enhanced Raman scattering (SERS) substrates for detection of the trace hexachlorocyclohexane (HCH) molecules. The ZIF-8 shell layer with an appropriate thickness (-∼20 nm) can evidently increase the SERS performance of the UAANs to the trace γ-HCH and α-HCH. Such wrapping-enhanced SERS effect significantly increases, by a power function, with the decreasing HCH concentration, especially in the concentration below 10^-6 M, which is attributed to the ever-increasing enrichment effect to the HCH molecules. The detection limit is down below 1.5 ppb. This work presents a highly efficient substrate for the SERS-based detection of the trace HCH, and also displays the potential application in the SERS detection of volatile small molecules.