Surface-enhanced optical absorption applications

Instrumentation. In order to employ local enhancement of infrared absorption by surface plasmon polaritons that cause locally enhanced surface electromagnetic fields, a suitable optical arrangement is needed [295]. Surface enhanced infrared absorption spectroscopy can also be observed in the transmission mode [285, 296]. However, since no application of this approach in spectroelectrochemistry has been reported so far, it is not discussed further. 

More complicated dependences are observed when two layers are located on the surface of the ATR element. The optical properties of a hemicylin-drical IRE-thin (d < 50 nm) metal hhn-hlm system, called the Kretschmann configuration [84] (Fig. 2.36a), were actively investigated in the seventies and eighties (see, e.g.. Ref. [85]) regarding the possibility of SEW excitation at the metal-outer layer interface. However, even without exploiting this and surface-enhanced infrared absorption (SEIRA) (Section 3.9.4) effects, optical enhancement may be achieved in the ATR spectrum of a layer deposited on metal. Because of this, the Kretschmann configuration has found wide application in the investigation of nanolayers located on the metal surfaces, especially at the metal-solution interface (Section 4.6.3). 

This article provides some general remarks on detection requirements for FIA and related techniques and outlines the basic features of the most commonly used detection principles, including optical methods (namely, ultraviolet (UV)-visible spectrophotometry, spectrofluorimetry, chemiluminescence (CL), infrared (IR) spectroscopy, and atomic absorption/emission spectrometry) and electrochemical techniques such as potentiometry, amperometry, voltammetry, and stripping analysis methods. Very few flowing stream applications involve other detection techniques. In this respect, measurement of physical properties such as the refractive index, surface tension, and optical rotation, as well as the a-, //-, or y-emission of radionuclides, should be underlined. Piezoelectric quartz crystal detectors, thermal lens spectroscopy, photoacoustic spectroscopy, surface-enhanced Raman spectroscopy, and conductometric detection have also been coupled to flow systems, with notable advantages in terms of automation, precision, and sampling rate in comparison with the manual counterparts. 

The revolution in Raman spectroscopy has been slow to come to the college chemistry classroom and laboratory. Standard undergraduate textbooks attempt to cover modern Raman spectroscopy, but achieve mixed results. Textbooks typically devote far less space to Raman scattering than to infrared absorption. The student is often left with the impression that Raman spectroscopy is an esoteric branch of vibrational spectroscopy, useful only for its selection rules or for measurements in aqueous solution. Almost entirely missing is a sense of excitement over such contemporary topics as Raman microscopy and Raman imaging, ultrasensitive surface-enhanced Raman spectroscopy, or industrial process control, and the many other applications enabled by fiber-optic probes. 

Freely suspended liquid droplets are characterized by their shape determined by surface tension leading to ideally spherical shape and smooth surface at the subnanometer scale. These properties suggest liquid droplets as optical resonators with extremely high quality factors, limited by material absorption. Liquid microdroplets have found a wide range of applications for cavity-enhanced spectroscopy and in analytical chemistry, where small volumes and a container-free environment is required for example for protein crystallization investigations. This chapter reviews the basic physics and technical implementations of light-matter interactions in liquid-droplet optical cavities. 

While the linear absorption and nonlinear optical properties of certain dendrimer nanocomposites have evolved substantially and show strong potential for future applications, the physical processes governing the emission properties in these systems is a subject of recent high interest. It is still not completely understood how emission in metal nanocomposites originates and how this relates to their (CW) optical spectra. As stated above, the emission properties in bulk metals are very weak. However, there are some processes associated with a small particle size (such as local field enhancement [108], surface effects [29], quantum confinement [109]) which could lead in general to the enhancement of the fluorescence efficiency as compared to bulk metal and make the fluorescence signal well detectable [110, 111]. 

It is not surprising therefore that the optical properties of small metal particles have received a considerable interest worldwide. Their large range of applications goes from surface sensitive spectroscopic analysis to catalysis and even photonics with microwave polarizers [9-15]. These developments have sparked a renewed interest in the optical characterization of metallic particle suspensions, often routinely carried out by transmission electron microscopy (TEM) and UV-visible photo-absorption spectroscopy. The recent observation of large SP enhancements of the non linear optical response from these particles, initially for third order processes and more recently for second order processes has also initiated a particular attention for non linear optical phenomena [16-18]. Furthermore, the paradox that second order processes should vanish at first order for perfectly spherical particles whereas experimentally large intensities were collected for supposedly near-spherical particle suspensions had to be resolved. It is the purpose of tire present review to describe the current picture on the problem. 

---