Optosensor Based on Nanocomposite Sensing Probes and Composite Adsorbents for the Determination of Trace Organic Compounds in Foods and Beverages

Abstract

This thesis aimed to develop optosensor using composite sensing probes and composite adsorbents for the determination of trace organic compounds in food and beverage samples. This thesis is composed of two parts including the optosensor and sample preparation method, which are divided into four sub-projects. The first part focused on the development of optosensor using nanocomposite probes based on the fluorescence quenching of molecularly imprinted polymer (MIP) composited with quantum dots (QDs) for the detection of organic compounds including lomefloxacin, mafenide and sulfisoxazole. Molecularly imprinted polymer was synthesized via a sol-gel polymerization method consisting of template molecules (lomefloxacin, mafenide and sulfisoxazole), a functional monomer (3-aminopropyltriethoxysilane) and a cross-linker (tetraethyl orthosilicate). After template removal from MIP layer, the specific imprinted cavities are generated that are complementary to the templates in terms of functional groups, shape and size. For the first sub-project, the fabricated fluorescent probe combined the high selectivity of molecularly imprinted polymer, excellent sensitivity of cadmium telluride quantum dots (CdTe QDs) and high adsorption ability of polyaniline (PANI) and graphene oxide (GOx). The nanocomposite PANI-GOx-MIP-CdTe QDs probe was successfully fabricated and employed for the detection of lomefloxacin. The fluorescence emission of the developed fluorescent probe was linearly reduced with increasing concentrations of lomefloxacin from 0.10 to 50.0 micrograms per liter, and the developed probe provided a low detection limit of 0.07 micrograms per liter. The developed optosensor can be utilized to detect lomefloxacin in milk, chicken meat and egg samples, and the obtained recoveries ranged from 81.5 to 99.6% with relative standard deviations (RSDs) below 7%. For the second sub-project, dual magnetic composite fluorescent probes were developed and fabricated by incorporating metal organic framework (MIL-101) with graphene quantum dots (GQDs) or cadmium telluride quantum dots into a magnetic molecularly imprinted polymer (MMIP) for the enrichment and simultaneous detection of mafenide and sulfisoxazole. The dual MIL101-MMIP-GQDs and MIL101-MMIP-CdTe QDs probes can detect mafenide and sulfisoxazole at the same time since their fluorescence emission intensities were different at 435 and 572 nm, respectively. Under optimal condition, the emission intensities of dual fluorescent probes were linearly decreased with increasing mafenide and sulfisoxazole concentrations ranging from 0.10 to 25.0 micrograms per liter with low detection limits of 0.10 micrograms per liter. The developed dual probes successfully detected ultra-trace levels of mafenide and sulfisoxazole in milk with satisfactory recoveries of 80.4 to 97.9% and RSDs lower than 5%. In addition, the accuracy of these developed optosensors was confirmed by comparing with that of the high performance liquid chromatography (HPLC) technique. The analytical results produced by the developed optosensor not only agreed well with HPLC method but also exhibited superior sensitivity. The developed optosensor of both sub-projects is a reliable and effective method for the determination of trace lomefloxacin, mafenide and sulfisoxazole in food and beverage samples. The outstanding properties of the developed optosensor are high selectivity, good sensitivity, simple fabrication, short analysis time, uncomplicated measurement and cost-effectiveness. The second part focused on the development of composite adsorbents for the simultaneous extraction and enrichment of multiple target compounds including phthalate esters and nonsteroidal anti-inflammatory drugs (NSAIDs) coupled with chromatographic analysis. For the third sub-project, a composite adsorbent of graphene oxide, metal organic framework and silica-modified magnetite (Fe3O4-SiO2) incorporated into alginate hydrogel fiber was fabricated and utilized as dispersive magnetic solid-phase extraction (D-MSPE) adsorbent for the extraction, enrichment and determination of four phthalate esters including dibutyl phthalate (DBP), bis(2-ethylhexyl) phthalate (DEHP), benzyl butyl phthalate (BBP) and di-n-octyl phthalate (DNOP). Alginate hydrogel was employed as a non-toxic and biodegradable supporting material, which can be created in various shapes and readily entrap other composite materials. GOx and MIL-101 were utilized as the primary adsorption materials to adsorb phthalate esters through hydrogen bonding, hydrophobic and pi-pi interactions. The Fe3O4-SiO2 nanoparticles incorporated in hydrogel fiber facilitated rapid isolation of adsorbent from sample solution using an external magnet. The applied composite GOx/MIL-101/Fe3O4-SiO2 alginate hydrogel fiber for the extraction of phthalate esters and quantitative analysis by HPLC method exhibited wide linear response in the range of 5.0 to 250.0 micrograms per liter for DEHP and DNOP and 3.0 to 250.0 micrograms per liter for BBP and DBP. The limit of detections were achieved in the range of 3.0 to 5.0 micrograms per liter. The developed method was successfully utilized to extract and determine phthalate esters in tea, water and juice samples, and the acquired recoveries were satisfactory in the range of 80.7 to 89.9% with RSDs below 8%. In addition, the developed composite hydrogel adsorbent had excellent stability which can be reused up to 16 times. The fourth sub-project was a composite magnetic adsorbent of graphene quantum dots, silica-modified magnetite and mesoporous carbon (MPC) embedded into a molecularly imprinted polymer for the extraction of NSAIDs. The composite GQDs/Fe3O4-SiO2/MPC/MIP adsorbent was successfully fabricated and applied as D-MSPE adsorbent for the extraction, enrichment and determination of three NSAIDs. The selectivity of the adsorbent can be enhanced with MIP, which produced specific imprinted cavities for NSAIDs. In addition, GQDs and MPC enabled increasing adsorption ability between the composite adsorbent and NSAIDs via pi-pi stacking, hydrophobic interaction and hydrogen bonding. The extracted NSAIDs were identified and determined by HPLC. Under optimal condition, the developed method exhibited good linearity with ranges of 0.5 to 100.0 micrograms per liter for diflunisal and mefenamic acid and 1.0 to 100 micrograms per liter for flurbiprofen. The limit of detections were acquired from 0.5 to 1.0 micrograms per liter. The satisfactory recoveries of composite magnetic MIP adsorbent for NSAIDs extraction in milk samples were achieved from 81.4 to 93.7% with RSDs lower than 7%. Furthermore, the fabricated GQDs/Fe3O4-SiO2/MPC/MIP adsorbent can be effectively used to extract NSAIDs up to 6 cycles. The distinctive advantages of the developed composite adsorbents are high extraction efficiency, simple preparation, low solvent consumption, good reproducibility and reusability. In conclusion, the developed optosensor with composite fluorescent probes and the developed D-MSPE with composite adsorbents in this thesis were successfully applied for the determination of trace organic compounds in foods and beverages with good analytical performances. The developed methods have numerous advantages including simple fabrication, good sensitivity, high selectivity, low toxicity, ease of use and cost-effectiveness. In addition, they can be adapted as alternative strategies for the determination of other organic compounds in a variety of complex samples.