Rusciano G, De Luca A C, Sasso A, Pesce G
Dipartimento di Scienze Fisiche, Università di Napoli "Federico II", Complesso Universitario Monte S. Angelo, Via Cinthia, I-80126 Napoli, Italy.
Anal Chem. 2007 May 15;79(10):3708-15. doi: 10.1021/ac070050n. Epub 2007 Apr 20.
Raman spectroscopy has become a powerful tool for microscopic analysis of organic and biological materials. When combined with optical tweezers (Raman tweezers), it allows investigating single, selected micrometric particles in their natural environment, therefore, reducing unwanted interferences from the cover plate. A general problem affecting both Raman spectrometers and Raman tweezers systems is the background caused by the environment surrounding the sample under investigation. In this paper, we report on a novel method that allows acquiring Raman spectra of a single trapped particle (polystyrene microspheres) free from any background contribution. The method is based on the use of two collinear and copropagating laser beams: the first is devoted to trapping (trap laser), while the second one is used to excite the Raman transitions (pump laser). The trap laser moves the trapped particle periodically, by means of a galvomirror, back and forth across the pump laser. The back-scattered photons are analyzed by a spectrometer and detected by a photomultiplier; finally, the resulting signal is sent to a lock-in amplifier for phase-sensitive detection. The purpose of the present work is to give a detailed description of our method and to supply a systematic study concerning the formation of the Raman signal. We trap polystyrene beads and study the dependence of the Raman signal on several parameters, such as height from the coverslip surface, the bead size, the modulation amplitude, and the pump laser intensity. Our results establish a direct and practical approach for background suppression in the spectroscopic analysis of optical trapped microsized samples.
拉曼光谱已成为用于有机和生物材料微观分析的强大工具。当与光镊（拉曼光镊）结合使用时，它能够在其自然环境中研究单个选定的微米级颗粒，从而减少来自盖玻片的不必要干扰。影响拉曼光谱仪和拉曼光镊系统的一个普遍问题是由被研究样品周围环境引起的背景。在本文中，我们报告了一种新颖的方法，该方法能够获取单个捕获颗粒（聚苯乙烯微球）的拉曼光谱，且无任何背景贡献。该方法基于使用两束共线且同向传播的激光束：第一束用于捕获（捕获激光），而第二束用于激发拉曼跃迁（泵浦激光）。捕获激光借助一个振镜使捕获的颗粒周期性地在泵浦激光上来回移动。背向散射光子由光谱仪分析并由光电倍增管检测；最后，所得信号被发送到锁相放大器进行相敏检测。本工作的目的是详细描述我们的方法，并提供关于拉曼信号形成的系统研究。我们捕获聚苯乙烯珠并研究拉曼信号对几个参数的依赖性，例如距盖玻片表面的高度、珠的尺寸、调制幅度和泵浦激光强度。我们的结果为光学捕获的微米级样品的光谱分析中的背景抑制建立了一种直接且实用的方法。
