Ag Nanoparticles@Au Nanograting Array as a 3D Flexible and Effective Surface-Enhanced Raman Scattering Substrate

Surface-enhanced Raman scattering (SERS) is a powerful analytical technique for chemical identification, but it remains a great challenge to realize the large-scale and well-controlled fabrication of sensitive and repeatable SERS substrates. Here, we report a facile strategy to fabricate centimeter-sized periodic Au nanograting (Au-NG) decorated with well-arranged Ag nanoparticles (Ag-NPs) (denoted as Ag-NPs@Au-NG) as a three-dimensional (3D) flexible hybrid SERS substrate with high sensitivity and good reproducibility. The Au-NG patterns with periodic ridges and grooves are fabricated through nanoimprint lithography by employing a low-cost digital versatile disc (DVD) as a master mold, and the Ag-NPs are assembled by a well-controlled interface self-assembly method without any coupling agents. Multiple coupling electromagnetic field effects are created at the nanogaps between the Ag-NPs and Au-NG patterns, leading to high-density and uniform hot spots throughout the substrate. As a result, the Ag-NPs@Au-NG arrays demonstrate an ultrahigh SERS sensitivity as low as 10-13 M for rhodamine 6G with a high average enhancement factor (EF) of 1.85 × 108 and good signal reproducibility. For practical applications, toxic organic pollutants including crystal violet, thiram, and melamine have been successfully detected with high sensitivity at a low detection limit, showing a good perspective in the rapid detection of toxic organic pollutants.

Quantifying the effect of non-ionic surfactant alkylphenol ethoxylates on the persistence of thiabendazole on fresh produce surface

BACKGROUND

Understanding the role of adjuvants in pesticide persistence is crucial to develop effective pesticide formulations and manage pesticide residues in fresh produce. This study investigated the impact of a commercial non-ionic surfactant product containing alkylphenol ethoxylates (APEOs) on the persistence of thiabendazole on apple and spinach surfaces against the 30 kg m-3 baking soda (sodium bicarbonate, NaHCO3 ) soaking, which was used to remove the active ingredient (AI) in the cuticular wax layer of fresh produce through alkaline hydrolysis. Surface-enhanced Raman scattering (SERS) mapping method was used to quantify the residue levels on fresh produce surfaces at different experimental scenarios. Four standard curves were established to quantify surface thiabendazole in the absence and presence of APEOs, on apple and spinach leaf surfaces, respectively.

RESULTS

Overall, the result showed that APEOs enhanced the persistence of thiabendazole over time. After 3 days of exposure, APEOs increased thiabendazole surface residue against NaHCO3 hydrolysis on apple and spinach surfaces by 5.39% and 10.47%, respectively.

CONCLUSION

The study suggests that APEOs led to more pesticide residues on fresh produce and greater difficulty in washing them off from the surfaces using baking soda, posing food safety concerns. © 2023 Society of Chemical Industry.

Surface ligand modified silver nanoparticles-based SERS sensing platform for ultrasensitive detection of the pesticide thiram in green tea leaves: roles of coating agents in sensing performance

Silver nanoparticles (AgNPs) have been regarded as a highly promising substrate for surface-enhanced Raman scattering (SERS) sensors. In this study, we focused on the electrochemical synthesis method by developing three kinds of AgNPs using three different electrolytes: citrate (e-Ag-C), oleic acid (e-Ag-O) and fish mint (Houttuynia cordata Thunb.) extract (e-Ag-bio). The as-prepared AgNPs were characterized and then employed as SERS substrates to detect the pesticide thiram. The obtained results show that e-Ag-O exhibits the best SERS performance. The effect of the coating agent was explained by chemical and electromagnetic enhancements (CM and EM). Although thiram could absorb onto e-Ag-C at the highest level, allowing its Raman signal to be best enhanced via the CM, the smallest interparticle distance of e-Ag-O could have resulted in the largest improvement in the EM. Using e-Ag-O to develop SERS-based sensors for thiram, we obtain the impressive detection limit of 1.04 × 10-10 M in standard samples and 10-9 M in tea leaves. The linear ranges are from 10-4 M to 10-7 M and from 10-7 M to 10-9 M, covering the maximum residue levels for plant commodities established by the United States Environment Protection Agency and European Food Safety Authority (2-13 ppm ∼7.7 × 10-6 M to 5 × 10-5 M).

Fabrication of heterogeneous bimetallic nanochains through photochemical welding for promoting the electrocatalytic hydrogen evolution reaction

Heterogeneous bimetallic nanochains (NCs) have gained significant attention in the field of catalysis due to their abundant active sites, multi-component synergistic catalytic, and exotic electronic structures. Here, we present a novel approach to synthesize one-dimensional heterogeneous bimetallic nanochains using a local surface plasmon resonance (LSPR) based strategy of liquid-phase photochemical welding method containing self-assembly and subsequent welding processes. Initially, we introduce additives that facilitate the self-assembly and alignment of Au nanoparticles (NPs) into orderly lines. Subsequently, the LSPR effect of the Au NPs is stimulated by light, enabling the second metal precursor to overcome the energy barrier and undergo photodeposition in the gap between the arranged Au NPs, thereby connecting the nano-metal particles. This strategy can be extended to the photochemical welding of Au NPs-Ag and Au NRs. Using electrocatalytic hydrogen evolution reaction (HER) as a proof-of-concept application, the obtained one-dimensional structure of Au5Pt1 NCs exhibit promoted HER performances, where the mass activity of the Au5Pt1 nanochains is found to be 4.8 times higher than that of Au5Pt1 NPs and 10.4 times higher than that of commercial 20 wt% Pt/C catalysts. The promoted HER performance is benefited from the electron conduction ability and abundant active sites.

Advancing Mycotoxin Detection: Multivariate Rapid Analysis on Corn Using Surface Enhanced Raman Spectroscopy (SERS)

Mycotoxin contamination on food and feed can have deleterious effect on human and animal health. Agricultural crops may contain one or more mycotoxin compounds; therefore, a good multiplex detection method is desirable to ensure food safety. In this study, we developed a rapid method using label-free surface-enhanced Raman spectroscopy (SERS) to simultaneously detect three common types of mycotoxins found on corn, namely aflatoxin B1 (AFB1), zearalenone (ZEN), and ochratoxin A (OTA). The intrinsic chemical fingerprint from each mycotoxin was characterized by their unique Raman spectra, enabling clear discrimination between them. The limit of detection (LOD) of AFB1, ZEN, and OTA on corn were 10 ppb (32 nM), 20 ppb (64 nM), and 100 ppb (248 nM), respectively. Multivariate statistical analysis was used to predict concentrations of AFB1, ZEN, and OTA up to 1.5 ppm (4.8 µM) based on the SERS spectra of known concentrations, resulting in a correlation coefficient of 0.74, 0.89, and 0.72, respectively. The sampling time was less than 30 min per sample. The application of label-free SERS and multivariate analysis is a promising method for rapid and simultaneous detection of mycotoxins in corn and may be extended to other types of mycotoxins and crops.

Understanding the impact of a non-ionic surfactant alkylphenol ethoxylate on surface-enhanced Raman spectroscopic analysis of pesticides on apple surfaces

Pesticide active ingredients (AIs) are often applied with adjuvants to facilitate the stability and functionality of AIs in agricultural practice. The objective of this study is to investigate the role of a common non-ionic surfactant, alkylphenol ethoxylate (APEO), on the surface-enhanced Raman spectroscopic (SERS) analysis of pesticides as well as its impact on pesticide persistence on apple surfaces, as a model fresh produce surface. The wetted areas of two AIs (thiabendazole and phosmet) mixed with APEO were determined respectively to correct the unit concentration applied on apple surfaces for a fair comparison. SERS with gold nanoparticle (AuNP) mirror substrates was applied to measure the signal intensity of AIs with and without APEO on apple surfaces after a short-term (45 min) and a long-term (5 days) exposure. The limit of detection (LOD) of thiabendazole and phosmet using this SERS-based method were 0.861 ppm and 2.883 ppm, respectively. The result showed that APEO decreased the SERS signal for non-systemic phosmet, while increased SERS intensity of systemic thiabendazole on apple surfaces after 45 min pesticide exposure. After 5 days, the SERS intensity of thiabendazole with APEO was higher than thiabendazole alone, and there was no significant difference between phosmet with and without APEO. Possible mechanisms were discussed. Furthermore, a 1% sodium bicarbonate (NaHCO₃) washing method was applied to test the impact of APEO on the persistence of the residues on apple surfaces after short-term and long-term exposures. The results indicated that APEO significantly enhanced the persistence of thiabendazole on plant surfaces after a 5-day exposure, while there was no significant impact on phosmet. The information obtained facilitates a better understanding of the impact of the non-ionic surfactant on SERS analysis of pesticide behavior on and in plants and helps further develop the SERS method for studying complex pesticide formulations in plant systems.

Self-assembly of colloidal nanoparticles into 2D arrays at water-oil interfaces: rational construction of stable SERS substrates with accessible enhancing surfaces and tailored plasmonic response

Self-assembly at water-oil interfaces has been shown to be a cheap, convenient and efficient route to obtain densely packed layers of plasmonic nanoparticles which have small interparticle distances. This creates highly plasmonically active materials that can be used to give strong SERS enhancement and whose structure means that they are well suited to creating the highly stable, reproducible and uniform substrates that are needed to allow routine and accurate quantitative SERS measurements. A variety of methods have been developed to induce nanoparticle self-assembly at water-oil interfaces, fine tune the surface chemistry and adjust the position of the nanoparticles at the interface but only some of these are compatible with eventual use in SERS, where it is important that target molecules can access the active surface unimpeded. Similarly, it is useful to transform liquid plasmonic arrays into easy-to-handle free-standing solid films but these can only be used as solid SERS substrates if the process leaves the surface nanoparticles exposed. Here, we review the progress made in these research areas and discuss how these developments may lead towards achieving rational construction of tailored SERS substrates for sensitive and quantitative SERS analysis.

A dual-functional PDMS-assisted paper-based SERS platform for the reliable detection of thiram residue both on fruit surfaces and in juice

In this work, a dual-functional SERS platform was developed via a paper-based SERS substrate with the aid of hydrophobic polydimethylsiloxane (PDMS) for effective and reliable measurements of thiram on fruit surfaces and in juice. Specifically, the paper-based SERS substrate was fabricated by coating with a core-shell Au@Ag nanorod monolayer, and the SERS performance was optimized compared with multilayer adsorption. Moreover, a versatile SERS platform was constructed by simply pasting the paper-based substrate in reverse onto PDMS using polymethyl methacrylate (PMMA) tape. On the one hand, this detection platform was able to realize sample enrichment due to the excellent hydrophobicity of PDMS, thereby increasing the sensitivity of measurements, and its function was successfully displayed through the identification of thiram in orange juice. On the other hand, PDMS could also play a supporting role and the paper-based substrate reversely stuck on PDMS was able to extract samples on the side without nanoparticles, which greatly avoids damage to nanoparticles on the substrate. Therefore, the accuracy of analysis was significantly improved and the inspection of thiram on the surface of an orange was demonstrated based on this function. As a result, this proposed SERS platform provides a new strategy for preparing multifunctional SERS sensors for the on-site monitoring of chemical contaminants in the food-safety field.

Mapping of Pesticide Transmission on Biological Tissues by Surface Enhanced Raman Microscopy with a Gold Nanoparticle Mirror

We presented an improved surface-enhanced Raman scattering (SERS) mapping technique for the imaging of pesticides on biological samples including tomato leaves, fruits, and mouse skin using a gold nanoparticle mirror as the SERS substrate. The gold nanoparticle mirror was fabricated using 50 nm commercial citrate-capped gold nanoparticles upon the interface of water and a mediating solvent that was prepared using acetonitrile and hexane. The properties of the gold nanoparticle mirror were compared with gold nanoparticles, and the mirror displayed higher sensitivity with a limit of detection of 0.07 μg/cm² and better reproducibility with a relative standard deviation of 5.48% for the SERS mapping of pesticide (ferbam) on biological samples. The gold mirror-based SERS mapping technique was also used to investigate pesticide transmission from tomato fruit surfaces to mouse skin after 1 mg/cm² of pesticides was administered upon the fruit, and the results showed that about 23% of the pesticide was transmitted from the fruit to the mouse skin. We also found that pesticides on the contaminated hand could not be completely removed by routine rinsing with tap water for 2 min. This study provides an effective approach for the imaging of pesticides on biological tissues that would facilitate research on pesticide behaviors both on and in biological systems.

Combining Headspace Solid-Phase Microextraction and Surface-Enhanced Raman Spectroscopy To Detect the Pesticide Fonofos in Apple Juice

We developed an innovative approach that couples headspace solid-phase microextraction (SPME) with surface-enhanced Raman spectroscopy (SERS) to detect a volatile pesticide (i.e., fonofos) in a liquid complex matrix (i.e., apple juice). A gold nanoparticles-coated fiber was fabricated by reducing gold(III) on a chemically etched stainless steel wire to extract pesticide, using SPME. The fabricated fibers were then tested by a headspace-SPME method and a dip-SPME method, followed by SERS detection of fonofos in water and apple juice samples. Using the headspace-SPME method, we can detect as low as 5 ppb of fonofos in water and apple juice, compared with the dip-SPME method, which cannot detect lower than 10 ppb in water and 50 ppb in apple juice. This study demonstrated the potential capability of the headspace-SPME-SERS method for rapid (within 30 min) and sensitive detection of volatile and vaporizable compounds in complex matrices. The developed method could be a potential alternative approach to the gas chromatography method. Future work is needed to optimize the fiber by minimizing signal variation, and it should be tested in a variety of targeted compounds and matrices.