圓偏振雷射光化學還原製作銀掌性奈米結構及其於表面增強拉曼光譜測量的應用

Abstract

本研究以圓偏振雷射光製作具掌性(chiral)之銀奈米結構，在顯微鏡系統中將雷射光透過圓偏振片，並聚焦照射於硝酸銀及抗壞血酸水溶液，利用溶液中已還原之銀原子團誘發表面電漿子效應產生熱電子促進光化學還原反應，同時電磁波使銀原子團間產生光力吸引驅動銀原子簇自組裝，並且圓形極化促使銀原子簇受到光力矩使其成螺旋分布，進而生成具有掌性的奈米立體結構，並將此結構應用於表面增強拉曼光譜(SERS)測量。實驗過程皆在室溫(25°C)及一大氣壓下進行，透過調整溶液前置反應時間、雷射光的功率、照射時間及波長等參數控制銀奈米結構的型貌。FE-SEM影像顯示，在照射前溶液前置反應5分鐘後進行照射能產生較細緻的奈米立體結構。若增加前置反應時間則會因為純化學還原反應已經趨於完備，光化學還原反應介入的影響降低，進而影響微結構型貌的複雜性，降低其對SERS的效能。在SERS測量上，本研究使用不同濃度的羅丹明6G(Rhodamine 6G, R6G)做為檢測樣本，實驗結果顯示此三維銀結構可測量不同濃度(10-6 M至10-10 M)的R6G。 實驗證明圓偏振雷射光驅動光化學還原反應可生成具掌性的三維銀奈米結構，除可提升SERS靈敏度外，其圓二色性(circular dichroism, CD)更可以應用於檢測具掌性的L或D型分子。

This study demonstrates the fabrication of chiral silver nanostructures using circularly polarized laser light. The laser, passed through a circular polarizer and focused in a microscope system, irradiates a silver nitrate and ascorbic acid solution. The plasmonic effect of the reduced atomic clusters in the solution generates hot electrons, which further drive the photochemical reduction process. Simultaneously, the optical forces of the circularly polarized light drive the self-assembly of silver atom clusters, and the optical torque causes them to distribute helically, forming chiral three-dimensional nanostructures. These structures are applied for surface-enhanced Raman spectroscopy (SERS) measurements. The experiments are conducted at room temperature (25°C) and one atmosphere pressure. The morphology of the silver nanostructures is controlled by adjusting the pre-reaction time of the solution, laser power, irradiation time, and wavelength. FE-SEM images show that irradiating the solution after a 5-minute pre-reaction produces finer nanostructures. Increasing the pre-reaction time reduces the influence of photochemical reduction due to the completion of the pure chemical reduction reaction, affecting the complexity of the microstructure and decreasing its SERS efficiency. For SERS measurements, different concentrations of R6G (Rhodamine 6G) are used as test samples. The results indicate that these three-dimensional silver structures can detect R6G at concentrations ranging from 10-6 M to 10-10 M. The experiment demonstrates that circularly polarized laser-driven photochemical reduction can produce chiral three-dimensional silver nanostructures, enhancing SERS sensitivity. Additionally, their circular dichroism (CD) can be applied to detect chiral L- or D-type molecules.