Abstract
Surface-enhanced Raman spectroscopy (SERS) has been widely investigated in many applications. However, only little work has been done on using SERS for the detection of volatile organic compounds (VOCs), primarily due to the challenges associated with fabricating SERS substrates with sufficient hotspots for signal enhancement and with the surface interfacially compatible for the VOCs. This study investigated the phase transfer of Ag-nanospheres (AgNSs) from the aqueous phase to the non-aqueous phase by electrostatic interaction induced by cationic surfactants, and the feasibility of the transferred AgNSs as SERS substrates for the determination of methyl salicylate VOC. Results indicated that one of three cationic surfactants, tetraoctylammonium bromide (TOAB) dissolved in organic solvent showed successful phase transfer of the AgNSs confirmed by several characterization analyses. The complex formed by hydrophobic interaction between the transferred AgNSs and Tenax-TA adsorbent polymer was able to be utilized as a SERS substrate, and the volatile of methyl salicylate could be easily determined from SERS measurements at 4 h static volatile collection. Therefore, the proposed new techniques can be effectively employed to areas where many VOCs relevant to food and agriculture need to be analyzed.
Highlights
Plant-emitted volatile organic compounds (VOCs) are important constituents that may indicate a plant’s physiological damage, potentially induced by abiotic or biotic stresses
Some of the added cationic surfactants may have started to move to the water/dichloromethane interface, and some of them may have interacted with the sodium dodecyl sulfate (SDS) right after the second vortexing with added dichloromethane
The added cationic surfactants may have been attracted to the anionic surfactant by electrostatic interaction, and the hydrophobic tails of the cationic surfactants may have been exposed to the water phase, which is not a favorable situation
Summary
Plant-emitted volatile organic compounds (VOCs) are important constituents that may indicate a plant’s physiological damage, potentially induced by abiotic or biotic stresses. Surface-enhanced Raman spectroscopy (SERS) using a metallic nanostructure for signal enhancement is considered a potential alternative in field applications due to its reasonable sensitivity, selectivity, and fast response [1]. To determine VOCs with SERS, key configuration requirements involve (1) using as many “hotspots” on nanoparticles as possible to intensify the weak Raman signal from the VOCs [2], and (2) making the surface properties of the nanoparticle favorable to the VOCs so they can be adsorbed on the surface due to enhanced interfacial compatibility [3]. Most 3D substrates have been designed to make each nanoparticle aggregate unique, because signal enhancement at the gap between particles can be much stronger than at the surface of a single nanoparticle [5]. Research on fabricating nanoparticle aggregates as a SERS substrate has been done by several research groups
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