Ultraviolet-Visible UV-vis Absorption Spectroscopy

The interaction of photons of ultraviolet and visible (UV-vis) radiation with dean and adsorbate-covered surfaces is central to many surface science techniques. Those techniques that measure photon interactions through processes such as photon reflection or scattering or through subsequent processes such as photoelectron emission are described elsewhere. In this section, an overview of UV-vis absorption and its applications in surface and interface science are given. 

UV-vis spectrophotometers are composed of UV and visible radiation sources, monochromators, detectors, and associated optics. Handling UV radiation becomes 

The polychromatic white light of a UV-vis source is monochromated by reflection from a diffraction grating or transmission through a prism and then irradiates the sample. The light intensity transmitted or reflected from the sample (depending on whether the sample is, for instance, nanoparticles in solution or molecular species clustered on or dispersed across a surface) is measured and compared to that from a reference sample to produce the absorption spectrum. 

The detectors used in UV-vis spectroscopy are typically photomultiplier tubes, photodiodes, or charge-coupled devices (CCDs). Photomultipliers are very sensitive to UV and visible radiation and have fast response times, but with amplification factors of 10 electrons per incident photon, they are limited to measuring low light intensities. Photodiode or CCD arrays allow parallel detection of many wavelengths of radiation simultaneously if the array is positioned at the output focal plane of the monochromator. With many hundreds or thousands of chaimels, UV-vis spectra can be can be acquired very rapidly. 

To illustrate the applications of UV-vis absorption spectroscopy in surface and interface science, a selection of recent case studies of nanoparticle systems are described with references to relevant review articles. 

---

The symmetry of the LB films was determined by polarized ultraviolet-visible (UV-Vis) absorption spectroscopy, optical rotation, and second-harmonic generation. All studies showed that the constructed LB films are anisotropic in the plane of the film and that the symmetry of the film is C2 with the twofold rotation axis perpendicular to the film plane. For example, when the SH intensity is plotted as a function of the azimuthal rotation angle (rotation around an axis perpendicular to the plane of the film), the twofold symmetry becomes evident (Figure 9.23). Isotropic films generate an SH signal independent of the azimuthal rotation angle. On the other hand, the LB... 

The photochemical and thermal stabilities of Ru complexes have been investigated in detail [8,153-156]. For example, it has been reported that the NCS ligand of the N3 dye, cri-Ru(II)(dcbpy)2(NCS)2 (dcbpy = 2,2 -bipyridyl-4,4 -dicarboxylic acid), is oxidized to produce a cyano group (—CN) under irradiation in methanol solution. It was measured by both ultraviolet-visible (UV-vis) absorption spectroscopy and nuclear magnetic resonance (NMR) [8,153]. In addition, the intensity of the infrared (IR) absorption peak attributed to the NCS ligand starts to decrease at 135°C, and decarboxylation of N3 dyes occurs at temperatures above 180°C [155]. Desorption of the dye from the 2 surface has been observed at temperatures above 200°C. 

Using ultraviolet/visible (UV/Vis) absorption spectroscopy, it is possible to measure the protein concentration using Beer s Law A = e c, where A is the measured absorbance of a solution, e is the absorptivity of the protein, is the pathlength of the cell used to determine the absorbance, and c is the protein concentration. Proteins typically exhibit two strong, broad absorption bands in the UV/Vis part of the spectrum. The first and most intense band is centered at 214 nm and arises from absorption of light by the peptide backbone. The second absorption band is typically found at 280nm. This band arises from absorbance from the aromatic side chains of Trp, Tyr, and Phe. Disulfide bonds may exhibit weak absorption in this range as well. 

Time-resolved spectroscopic techniques are important and effective tools for mechanistic photochemical studies. The most widely used of these tools, time-resolved ultraviolet-visible (UV-Vis) absorption spectroscopy, has been applied to a variety of problems since its introduction by Norrish and Porter [1] over 50 years ago. Although a great deal of information about the reactivity of organic photochemical intermediates (e.g., excited states, radicals, carbenes, and nitrenes) in solution at ambient temperatures has been amassed with this technique, only limited structural information can be extracted from such investigations because absorption bands are usually quite broad and featureless. Questions of bonding, charge distribution, and solvation (in addition to those of dynamics) are more readily addressed with time-resolved vibrational spectroscopy. 

NMR involves the absorption of radiowaves by the nuclei of some combined atoms in a molecule that is located in a magnetic field. NMR can be considered a type of absorption spectroscopy, not unlike ultraviolet/visible (UV/VIS) absorption spectroscopy. Radiowaves are low-energy electromagnetic radiation with frequencies on the order of 10 Hz. The SI unit of frequency, 1 Hz, is equal to the older frequency unit 1 cycle per second (cps) and has the dimension of inverse seconds, s . The energy of radiofrequency (RF) radiation can therefore be calculated from... 

In a large portion of routine and discovery-oriented analyses, mass spectrometry (MS) is used as a qualitative technique. The obtained qualitative data enable detection and structural elucidation of molecules present in the analyzed samples. However, modern chemistry and biochemistry heavily rely on quantitative information. In biochemistry it is often sufficient to conduct quantification of analytes in biofluids every few hours, days, or even weeks. In the real-time monitoring of highly dynamic samples, it is necessary to collect data points at higher frequencies. When it comes to selection of techniques for quantitative analyses, especially in the monitoring of dynamic samples, MS has not generally been favored. In fact, the performance of MS in quantitative analysis is worse than that of optical spectroscopies - especially, ultraviolet-visible (UV-Vis) absorption and fluorescence spectroscopy. 

Spectroscopy produces spectra which arise as a result of interaction of electromagnetic radiation with matter. The type of interaction (electronic or nuclear transition, molecular vibration or electron loss) depends upon the wavelength of the radiation (Tab. 7.1). The most widely applied techniques are infrared (IR), Mossbauer, ultraviolet-visible (UV-Vis), and in recent years, various forms ofX-ray absorption fine structure (XAFS) spectroscopy which probe the local structure of the elements. Less widely used techniques are Raman spectroscopy. X-ray photoelectron spectroscopy (XPS), secondary ion imaging mass spectroscopy (SIMS), Auger electron spectroscopy (AES), electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spectroscopy. 

In principle, absorption spectroscopy techniques can be used to characterize radicals. The key issues are the sensitivity of the method, the concentrations of radicals that are produced, and the molar absorptivities of the radicals. High-energy electron beams in pulse radiolysis and ultraviolet-visible (UV-vis) light from lasers can produce relatively high radical concentrations in the 1-10 x 10 M range, and UV-vis spectroscopy is possible with sensitive photomultipliers. A compilation of absorption spectra for radicals contains many examples. Infrared (IR) spectroscopy can be used for select cases, such as carbonyl-containing radicals, but it is less useful than UV-vis spectroscopy. Time-resolved absorption spectroscopy is used for direct kinetic smdies. Dynamic ESR spectroscopy also can be employed for kinetic studies, and this was the most important kinetic method available for reactions... 

From these results and others from UV-vis absorption spectroscopy and differential pulse polarography (data not shown), Garbin et al. (2007) concluded that HS can act as photocatalyst to pesticide photolysis in aqueous solution only for specific ranges of concentration (as seen in Figure 16.12), which in turn depended on the HS and pesticide chemical characteristics. Under ultraviolet and visible radiation, this photocatalysis is based on photogeneration of -OH radicals, and the susceptibility of pesticide molecules to -OH attacks defines the efficiency of the photocatalysis. 

Ultraviolet-visible (UV-Vis) spectrophotometric detectors are used to monitor chromatographic separations. However, this type of detection offers very little specificity. Element specific detectors are much more useful and important. Atomic absorption spectrometry (AAS), inductively coupled plasma-atomic emission spectroscopy (ICPAES) and inductively coupled plasma-mass spectrometry (ICP-MS) are often used in current studies. The highest sensitivity is achieved by graphite furnace-AAS and ICP-MS. The former is used off-line while the latter is coupled to the chromatographic column and is used on-line . 

In addition to the IR, Raman and LIBS methods previously discussed, a number of other laser-based methods for explosives detection have been developed over the years. The following section briefly describes the ultraviolet and visible (UV/vis) absorption spectra of EM and discusses the techniques of laser desorption (LD), PF with detection through resonance-enhanced multiphoton ionization (REMPI) or laser-induced fluorescence (LIF), photoacoustic spectroscopy (PAS), variations on the light ranging and detecting (LIDAR) method, and photoluminescence. Table 2 summarizes the LODs of several explosive-related compounds (ERC) and EM obtained by the techniques described in this section. 

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron-sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 

Ultraviolet-visible (UV-Vis) spectroscopy was used to monitor synthesis and decomposition of dithiiranes 25a with an absorption maximum at 442 nm, and 25b with an absorption maximum at 438 nm <1995TL1867>. The UV-Vis spectra of dithiiranes 21, 8, 23a, and 22 reveal the absorption maximum in a range of450-455 nm due to the S-S bond <2003JOC1555>. UV-Vis spectroscopic data for dithiiranes are collected in Table 1. 

Analysis is an integral part of research, clinical, and industrial laboratory methodology. The determination of the components of a substance or the sample in question can be qualitative, quantitative, or both. Techniques that are available to the analyst for such determinations are abundant. In absorption spectroscopy, the molecular absorption properties of the analyte are measured with laboratory instruments that function as detectors. Those that provide absorbance readings over the ultraviolet-visible (UV-vis) light spectrum are commonly used in high-performance liquid chromatography (HPLC). The above method is sufficiently sensitive for quantitative analysis and it has a broader application than other modes of detection. 

As already pointed out, the most direct consequence of a reduction in the nanocrystallite size on the electronic structure of semiconducting materials is a pronounced increase in the band gap due to the quantum confinement effect. While there are several ways to quantitatively understand this phenomenon from a theoretical standpoint, the experimental determination of the band gap variation as a function of size is most directly performed by ultraviolet-visible absorption spectroscopy, with the experimental absorption threshold corresponding to the direct band gap in the material. As the band gap shifts to higher energy, the blue-shift in the absorption edge signals the formation of progressively smaller sized nanocrystals. Therefore, UV-Vis absorption spectroscopy has played an immensely important role in the study of these systems and we discuss the essential aspects in Section 11.3. 

Ultraviolet-Visible Spectroscopy Ultraviolet-visible (UV-VIS) molecular absorption spectrophotometry (often called light absorption spectrophotometry or just UV-visible spectrophotometry) is a technique based on measuring the absorption of near-UV or visible radiation (180-770 nm) by molecules in solution.35,36 Reference standard characterization by UV-VIS spectophotometry includes determining the absorption spectra and the molar extinction coefficient. These two spectral characterizations are used as identifiers of reference standards. 

Spectrophotometric analyses are the most common method to characterize proteins. TTie use of ultraviolet-visible (UV-VIS) spectroscopy is t rpically used for the determination of protein concentration by using either a dye-binding assay (e.g., the Bradford or Lowry method) or by determining the absorption of a solution of protein at one or more wavelengths in the near UVregion (260-280 nm). Another spectroscopic method used in the early-phase characterization of biopharmaceuticals is CD. 

Finally, it has been shown that sulfuranyl radicals, for example, 52 (cf. Scheme 6), exhibit absorption spectra, which can be detected with ultraviolet-visible (UV-Vis) spectroscopy absorption maxima were found to be sensitive to the a- versus 7t-electronic configurations of these radicals and thereby to be a sensitive guide in the assignment of the electronic stmcture of sulfuranyl radicals <1985JOC2516>. 

Spectra by the Thousands 575 Infrared Spectra 576 Characteristic Absorption Frequencies Ultraviolet-Visible (UV-VIS) Spectroscopy Mass Spectrometry 584 Molecular Formula as a Clue to Structure 589 Summary 590 Problems 593... 

Products were characterized by Fourier transform infrared spectrophotometry-attenuated total reflectance (FTIR-ATR), ultraviolet visible (UV-Vis) spectrophotometry, scanning electron microscopy (SEM), and broadband dielectric/impedance spectroscopy (BDS). New absorption bands were observed corresponding to the conjugated pol5mieric units by FTIR-ATR and UV-Vis spectrophotometric analysis. The influence of concentration of PEDOT-PSS and PEDOT on the composite electrospun nanofibers was studied by EIS. Morphologies of electrospun nanofibers were also investigated by SEM. 

Ultraviolet—visible (UV—Vis) spectroscopy (Section 13.8) A type of optical spectroscopy that measures the absorption of light in the visible and ultraviolet regions of the sp>ectrum. Visible-UV spectra primarily provide structural information about the kind and extent of conjugation of multiple bonds in the compound being analyzed. 

---