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.

A filtration-assisted approach to enhance optical detection of analytes and its application in food matrices.

Analysis of target analytes in food and environmental samples often required sophisticated instrumentation, which restrains the accessibility and portability of the analysis. Herein, we developed an instrument-free approach for rapid quantification of target analytes. The reported filtration-assisted approach enables image analysis of aggregates formed via interaction between analytes and silver nanoparticles (AgNPs). Two model analytes were chosen for aggregating AgNPs, potassium phosphate for neutralizing the charges and a di-thiol molecule (2,2'-(ethylenedioxy) diethanethiol (EDT)) for cross-linking. The mixtures of AgNPs and analytes were filtered onto filter membranes and analyzed using grey color intensity analysis. Based on the AgNPs-EDT platform, we demonstrated the detection of 1 µg/mL acrylamide in instant coffee and biscuit matrices was achievable. The filtration-assisted method provides a simple, fast and inexpensive approach for optical detection and quantification of analytes in food matrices.

Headspace Characterization and Quantification of Aromatic Organosulfur Compounds in Garlic Extracts Using Surface-Enhanced Raman Scattering with a Mirror-in-a-Cap Substrate.

BACKGROUND: Surface-enhanced Raman scattering (SERS) has been deployed in the analysis of food at solid and aqueous states. However, its capability has not been fully explored in headspace profiling. OBJECTIVE: To develop an innovative SERS method for analyzing headspace volatile compounds in foods. METHODS: A volatile-capture device was developed by depositing a film of silver nanoparticles in a vial cap to capture the volatiles released from a model flavor compound (garlic). RESULTS: SERS peaks at 1632, 1400, 1291, 1191, 731, and 577 cm-1 were identified in the headspace of the garlic sample, which was representative of an organosulfur compound (diallyl disulfide), and its concentration was determined at 135 ppm, which was comparable to the value determined using GC. Preparation and analysis could be carried out in <10 min for the SERS method. The sensitivity of the SERS method (10 ppm), however, was slightly less than that of the GC method (5 pm). CONCLUSIONS: The SERS method was able to quantify the concentration of diallyl disulfide in the headspace of a raw garlic ethanolic extract. Compared to GC, the SERS method had a much shorter analysis time and simpler sample preparation procedure than GC when analyzing large numbers of samples. HIGHLIGHTS: The innovative "mirror-in-a-cap" substrate was simpler and faster than other reported SERS substrates used for this purpose. Additionally, SERS has much better portability and the potential for real-time monitoring of changes in the garlic headspace concentration during manufacturing and processing.

Rapid capture and SERS detection of triclosan using a silver nanoparticle core - protein satellite substrate.

Triclosan (TCS) is a synthetic antimicrobial compound that has been widely used in consumer products. However, increasing evidence suggests adverse effects of TCS to human health and environment, raising great public concerns. The existing methods for detecting TCS are limited to time-consuming and complicated procedure. Here, we developed a rapid method for capture and detection of TCS using surface-enhanced Raman spectroscopy (SERS) based on a silver nanoparticle (Ag NP) core - protein satellite nanostructure. Bovine serum albumin (BSA) assembled on Ag NPs as satellites configuration could anchor a large number of TCS molecules close to the surface of Ag NPs, producing amplified SERS signals. As low as 50 nM TCS standard was successfully detected within 30 min. We also demonstrated its capability for TCS detection in pond water. The developed SERS method holds a great promise for rapid screening of TCS in environmental and food samples.

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.

In situ colorimetric detection of glyphosate on plant tissues using cysteamine-modified gold nanoparticles.

Monitoring the levels of pesticides on plant tissues is important for achieving effective protection of crops after application, as well as ensuring low levels of residues during harvest. In this study, a simple, rapid, and fieldable colorimetric method for detecting the pesticide glyphosate (Gly) on the plant tissues in situ using cysteamine-modified gold nanoparticles (AuNPs-Cys) has been developed. The aggregation of AuNPs-Cys in the presence of Gly results in a consequent color change from red to blue (or purple), which could be observed visually on the surface of plant tissues. With the naked eye, we successfully detected Gly spiked on the surface of spinach, apple, and corn leaves in situ. Further verification and quantification were achieved using surface-enhanced Raman spectroscopy (SERS) which uses AuNPs-Cys as the substrate. Moreover, application of this method was demonstrated through the evaluation of the Gly distribution on plant tissues which could greatly facilitate the development of precision agriculture technology.

A facile solvent mediated self-assembly silver nanoparticle mirror substrate for quantitatively improved surface enhanced Raman scattering.

Herein, a facile solvent mediated silver nanoparticle (AgNP) mirror was introduced as a reproducible and sensitive surface-enhanced Raman scattering (SERS) substrate. The AgNP mirror was fabricated based on commercially available citrate coated AgNPs via a facile and unique water-solvent interface assembly method without using any electrical equipment. By adding 0.2 mg ml-1 citrate coated AgNPs dropwise into a mediating solvent, self-assembled AgNP mirrors were formed instantly and gradually settled down at the bottom. The mediating solvent is the key for this fabrication, which should contain a mixture of a non-polar (e.g. hexane, cyclohexane, and iso-octane) and a polar (e.g., acetonitrile, acetone, methanol, and ethanol) organic solvents that have a lower density than water. The fabricated AgNP mirror showed a uniform monolayer arrangement which produced highly reproducible and sensitive signals. The coefficient of variance (i.e., percentage relative standard deviation) for AgNP mirrors was 7.3% compared to 33.0% for AgNP aggregates. A lower limit of detection (0.008 ppm) and a higher coefficient of determination (0.9981) for the detection of the fonofos pesticide were also obtained from the AgNP mirror compared to that from the AgNP aggregates. The practical application of this nanoscale AgNP mirror sensing substrate was also confirmed by its accurate quantification of 0.5 ppm fonofos in apple juice and green tea (i.e., 105.06% and 106.16% recovery value, respectively).

Ag2O/TiO2 Nanocomposite Heterostructure as a Dual Functional Semiconducting Substrate for SERS/SEIRAS Application.

Surface-enhanced Raman spectroscopy (SERS) and surface-enhanced infrared absorption spectroscopy (SEIRAS) are complementary and powerful techniques for molecular characterization and detection. However, studies on substrates that can enhance both Raman and IR singles are extremely scanty. Here, we reported a hybrid semiconductor material (Ag2O/TiO2) coupled with a portable solid support served as a dual functional platform for both SERS and SEIRAS applications. A facile two-step deposition method was used to synthesize Ag2O/TiO2 nanocomposite on a flexible polymeric membrane without bringing any external chemical capping agent and background signal. The presence of Ag2O was proposed to enrich the photogenerated electrons onto TiO2 surface and facilitate the photon-induced charge transfer (PICT) between TiO2 and adsorbate. The heterostructure of Ag2O/TiO2 could bring additional enhancement. The enhancement factor from such hybrid semiconducting substrate was at least one or two orders of magnitude over traditional semiconducting materials and comparable to noble metals. Additionally, this substrate enabled the ultratrace detection regardless of the more Raman- or IR-active molecules and displayed distinct quantitative capacities for SERS and SEIRAS. High reproducibility of the SERS/SEIRAS spectra further confirmed the reliability and reproducibility of our substrates.