||Au Nanoparticles on Ordered ZrO2 Nanopores to Improve the Effect of SERS and Apply for Trace Detection of Pesticides
||Department of Materials Science and Engineering
surface-enhanced Raman scattering
殘留於農產品中的微量農藥，其神經毒性對人體健康的風險逐漸受到重視，而表面增顯拉曼散射技術（SERS）對樣品製備的低需求使其成為農產品品質控制的快速檢測方法之一。在本研究中，SERS活性基板藉由熱蒸鍍金奈米粒子（Au NPs）於有序多孔二氧化鋯（pZrO2）層內完成，而此有序多孔二氧化鋯層則由聚苯乙烯奈米球體的犧牲輔助基板以及溶膠-凝膠法製備而成。藉由最佳化的雷射選擇與分子探針羅丹明紅（R6G）的選用，SERS的增顯因子（EF）可測得為7*107，而SERS效應主要原因來自於金奈米粒子間產生的熱區效應以及金與二氧化鋯的交界面因電荷轉移而產生的電磁效應。除此之外，多孔結構的凹面性質使得入射雷射光散射能與表面電磁場互相疊加，其有序的多孔結構也使SERS量測呈現一致性。本研究之Au/pZrO2基板在檢測益滅松以及加保利兩種農藥可測得最低極限為0.3 ppm以及0.2 ppm。在混合農藥檢測中，益滅松因其磷酸基對二氧化鋯的高親和力以及硫基對金的親和性，使得SERS基板對益滅松具有高選擇性。因此，Au/pZrO2基板在快速檢測微量農藥方面具有很高的潛力。
The presence of trace amounts of pesticides in agricultural products for human consumption has gained increasing concerns regarding the health risks they pose due to their neurotoxic nature. One of the sensitive and rapid detection methods that has been developed for quality control of these products is surface-enhanced Raman scattering (SERS) since it requires minimal to no sample preparation. In this study, a SERS-active substrate was fabricated wherein thermally evaporated gold nanoparticles (Au NPs) were deposited onto an ordered porous ZrO2 (pZrO2) layer that was produced through sol-gel method with an assisting template of polystyrene nanoparticles. With an optimized substrate-laser wavelength combination and Rhodamine 6G (R6G) as the probe molecule, an enhancement factor (EF) of 7.0 x 107 was obtained. The presence of hot spots in Au-Au interparticle gaps and the formation of electromagnetic fields on the Au-ZrO2 interfaces due to charge transfer between Au and ZrO2 are major factors that contribute to the SERS effect; in addition, the concave nature of the pores allowed the incident light to scatter in a way that it lead to further overlap of the electromagnetic fields. The substrate also exhibited homogeneity in terms of SERS measurements owing to its ordered morphological features. Furthermore, Au/pZrO2 substrates were also able to detect pesticides i. e. phosmet and carbaryl, down to low concentrations (0.3 ppm and 0.2 ppm, respectively). Multiplex detection of the pesticides was also demonstrated but with a selectivity to phosmet as its phosphoric groups has a strong affinity to ZrO2 aside from the affinity of its sulfur constituent to the Au component of the substrate. The Au/pZrO2 substrate has thus demonstrated a high potential in the rapid detection of trace amounts of pesticide.
Table of Contents
Table of Contents V
List of Tables VII
List of Figures VIII
Chapter 1 Introduction 1
1.1 Introduction 1
1.2 Motivation 4
1.3 Objective 5
Chapter 2 Literature survey 7
2.1 Principles of Raman scattering and spectroscopy 7
2.1.1 Classical theory aspect 7
2.1.2 Quantum theory aspect 9
2.1.3 Vibrational modes 11
2.2 Surface-Enhanced Raman scattering (SERS) 12
2.2.1 Enhancement mechanisms 13
(a) Electromagnetic enhancement mechanism 13
(b) Chemical enhancement mechanism 18
2.3 SERS substrates 19
2.3.1 Forms of SERS substrates 20
2.3.2 SERS substrates involving porous oxides 22
2.3.3 Characteristics and preparation of zirconia 25
2.4 Application of SERS substrates in pesticide detection 27
Chapter 3 Materials and methods 30
3.1 Substrate Preparation 30
3.1.1 Formation of porous ZrO2 thin film 30
3.1.2 Deposition of Au nanoparticles (Au NPs) onto the ZrO2 substrates 33
3.2 Substrate characterization 33
3.3 Raman spectroscopy 34
3.3.1 SERS substrate evaluation 34
3.3.2 Application of the SERS substrates on the detection of pesticides 35
Chapter 4 Substrate structure and morphology 36
4.1 Formation of the porous ZrO2 support substrate 36
4.1.1 Polystyrene nanoparticle template 36
4.1.2 Morphology of porous ZrO2 38
4.2 Physical properties of the Au nanoparticle-porous ZrO2 hybrid substrate 40
4.2.1 Surface morphologies 40
4.2.2 Compositional analysis 43
(a) Energy-dispersive X-ray spectroscopy (EDS) 43
(b) X-ray diffraction (XRD) analysis 44
(c) Raman spectroscopy 45
(d) Work function measurements 47
4.3 Summary 48
Chapter 5 SERS effects of the Au/pZrO2 substrates 50
5.1 Evaluation of SERS effects provided by the Au/pZrO2 substrates 50
5.1.1 Substrate and laser wavelength optimization 50
(a) Effect of variation in Au NP size 51
(b) Laser excitation wavelengths on the SERS effect 52
(c) SERS effect imparted by components: Au NPs and ZrO2 54
5.1.2 Uniformity of measurements and signal enhancement 56
(a) Reproducibility and homogeneity 56
(b) Enhancement factors 58
5.1.3 Sensitivity 59
5.2 SERS mechanism with respect to substrate design 60
5.3 Applicability of Au/pZrO2 substrates to pesticide detection 62
5.3.1 Analysis of pesticide standard solutions 62
(a) Phosmet 62
(b) Carbaryl 66
5.3.2 Multiplex detection of pesticides 68
5.4 Summary 71
Chapter 6 Conclusion and future works 73
6.1 Conclusion 73
6.2 Recommendations for prospective work 74
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