Raman Spectroscopy of a Single Microdroplet
Single organic particles studied in analytical chemistry such as aerosol particles, microdroplets, and microcapsules have micrometer to nanometer order diameters. The molecules in such small particles frequently exhibit unusual properties. For instance, t
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22.1
Introduction
Single organic particles studied in analytical chemistry such as aerosol particles, microdroplets, and microcapsules have micrometer to nanometer order diameters. The molecules in such small particles frequently exhibit unusual properties. For instance, the chemical reactions in microdroplets are different from those in bulk, because the ratio of the surface area to the volume of a microdroplet is much larger than that of a container commonly used for bulk materials [1]. The chemical analysis of single particles has progressed with the development of techniques for trapping. There are two major trapping techniques for single particles. One uses an electric field and the other uses optical radiation pressure. The electrodynamic balance technique is based on a.c. and d.c. current electric fields and can trap one or more charged aerosol particles [2]. Raman spectroscopy has been used in studies of aerosol particles trapped by this technique in order to determine concentrations of environmentally important inorganic ions contained in aerosol particles, such as nitrate and sulfate ions in single droplets in gases [3]. Several approaches have been utilized to enhance Raman scattered light from aerosol particles to improve signal-to-noise ratios in Raman spectra. Morphorogy-dependent resonances have been used to elastically enhance Raman scattered light because the size and refractive index of a laser-trapped particle can lead to light wave interference in the particle [4]. A resonance Raman scattering technique has also been shown to greatly enhance Raman scattered light intensity from dyes in aerosol particles [5]. The laser trapping technique has been widely used to manipulate single particles in the micrometer range. This was first reported in 1970 by Ashkin, who used the force of radiation pressure generated from two laser beams to grab a small particle [6]. Later a more practical method that used a single laser beam and a conventional microscope, which is called optical tweezers, was reported by Chu and coworkers [7]. The optical tweezers initially used visible laser light. Nowadays, low-energy near-infrared laser light ranging from 700 to 1100 nm wavelength is widely used to capture and manipulate single organic and biological particles, because it causes lower photochemical damage to samples than visible laser light [8]. Another advantage of this technique, one that is particularly attractive to biological scientists, is that it allows one to H. Masuhara et al. (eds.), Single Organic Nanoparticles © Springer-Verlag Berlin Heidelberg 2003
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laser beam
vi
Raman scattering (Stokes)
vi-v
Rayleigh scattering
vi
Raman scattering (anti-Stokes)
vi+ v
[b] Fig. 22.1. Diagram for laser-trapping and Raman scattering of a single particle
manipulate not only biological cells but also cellular organelles and vesicles within cells [9]. Optical tweezers have been used with spectroscopic techniques to characterize molecules contained in single particles. Chemical reactions within single m
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