SURFACE-ENHANCED RAMAN SPECTROSCOPY FOR DETECTING AGRICULTURAL VOLATILE ORGANIC COMPOUNDS COLLECTED WITH ADSORBENTS

Abstract

Volatile organic compounds (VOCs) emitted from any crop contain useful information that can tell its physiological status, and especially, herbivore-induced plant VOCs can be considered as an important phenome that can alert the infestation at the early stage. Therefore, main objective for this dissertation was to develop a reliable sensor to detect agricultural VOCs, and it consisted of total four research items: (1) fundamental analysis of VOCs induced from the interaction between major crop and major insect pest by adsorbent-gas chromatography/mass spectrometry (GC/MS) (2) development of adsorbent coupled Raman spectroscopy for detecting agricultural VOCs (3) phase transfer of Ag-nanospheres (AgNSs) from aqueous to non-aqueous for fabricating surface-enhanced Raman spectroscopy (SERS) substrate (4) Development of adsorbent-SERS substrate for collecting and detecting agricultural VOCs and its application. The VOCs from two major crops of sorghum and cotton were analyzed by adsorbent-GC/MS in response to major insect pests of sugarcane aphid (Melanaphis sacchari) and beet armyworm (Spodoptera exigua) respectively, and they showed that the pattern of VOCs induction was totally different depending on the feeding habit by the insect. However, GC/MS technique always required an hour to perform one analysis, so new detection system of adsorbent-Raman spectroscopy was finally developed to overcome that shortcoming. Based on the fundamental analyses, four different VOCs were selected for proof of concept study with the developed system, and they were successfully tested by the system, showing high sensitivity down to ppm level and high selectivity from simultaneous detection of four different VOCs. To improve the functionality of the detection system, especially sensitivity, SERS technique needed to be employed rather than standard Raman, so phase transfer of AgNSs was investigated, and electrostatic interaction by surfactants made successful transferred AgNSs with good interfacial compatibility with any organic chemicals. Based on the transferred AgNSs, adsorbent-coated SERS substrate was finally developed for detecting agricultural VOCs, and much enhanced interfacial compatibility between the AgNSs and the adsorbent polymer made it possible to detect the VOCs produced from different VOCs sources: aromas from three teas and caterpillar-induced cotton VOCs.