Chemistry Applications of UV/VIS Spectroscopy

Chemistry, a branch of science so universally applicable, includes fields of study spread across the map. These divergent studies in the field of chemistry indicated that chemists and researchers could be working in similarly diverse industries. However, some technologies and techniques are shared by industrialists and researchers that span the field of chemistry.

UV/VIS spectroscopy is one such common tool that can be seen in the toolbox of many chemists. This resourceful measurement technology has secured its own place in the chemistry lab.

UV/VIS in the Spot Light

UV/VIS spectroscopy implies absorption/reflection measurements carried out in the ultraviolet and visible light spectrum. According to the Beer-Lambert law, which links the attenuation of light to the characteristics of the material through which the light passes, a sample’s absorbance is directly proportional to the concentration of the absorbing analyte. This axiom can be frequently observed at work in analytical chemistry for the quantification of analytes, monitoring reactions and processes, and for detecting specific organic compounds.

Atmospheric Chemistry

Atmospheric chemistry has a vital role in gaining an in-depth knowledge of the mechanisms of local climate conditions and factors contributing to the global radiative balance. Researchers investigate a category of compounds known as secondary organic aerosols (SOAs) which are the reactive products of gas-phase photooxidation of naturally occurring as well as human-made volatile organic compounds (VOCs). Although various studies have been performed on the reaction mechanisms in the propagation of SOAs, Dr. Kun Li and a team of fellow researchers from the Institute of Chemistry at the Chinese Academy of Sciences and the Beijing National Laboratory for Molecular Sciences closely observed the optical characteristics of these aerosols under differing reactive conditions ^[1].

The absorptive and scattering properties, the direct components of the refractive index, are dependent more on the aerosol composition than on the particle size or concentration. In-depth knowledge of the connection between the chemical composition of aerosol pollutants and their optical characteristics enables a considerably accurate prediction of the global radiative effects of localized reactive conditions.

The study by Dr. Li’s team involved testing the optical characteristics of SOA particles produced in the lab in a Teflon smog chamber using various different precursor compounds and under differing NO_x levels. The AvaSpec-2048L spectrometer was used to show that the particles were nonabsorbent at a wavelength of 532 nm. Retrieval of refractive indices (RI) for each sample at that wavelength resulted in values ranging from 1.38 to 1.59 depending on the precursor compound that produced the SOA as well as the concentration of NO_x. However, it was independent of the particle size or the concentration of SOA. Eventually, their study suggests that various environmental models might overestimate the refractive index, and hence the global radiative effects.

Colloids and Nanoparticle Reactions

The study of nanoparticles is one research area in chemistry that has attained considerable attention in the recent years. The size of the nanoscale particles ranges between 1 and 100 nm. These particles are enveloped by an interfacial layer of ions and inorganic and organic compounds, which can interact with other substances. Although nanoparticles can occur in powder form or as a solid matrix, usually, they are found in colloidal form, scattered in an aqueous solution or gel.

Nanoparticles can be produced by either disintegration of larger particles or by a controlled chemical reaction assembly process. These microscopic particles can be used for gene therapy, DNA probes, molecular tagging, and also cancer treatment. They can be found in consumer products such as anti-microbial or heat-resistant coatings, anti-glare/non-scratch eyeglasses, sunscreens, and also in our food supply. The processes involved in the chemical assembly of nanoparticles can be volatile, and the reproducibility and stability of a reaction rely on several factors such as solution temperature and humidity, as well as the concentration and purity of the reagents. Any investigation of nanoparticles essentially starts with their synthesis, and one of the most broadly approved methods for reaction monitoring is UV/VIS spectroscopy.

A team of scientists from the Department of Physics, Kaunas University of Technology, Kaunas, Lithuania, investigated colloidal silver nanoparticles developed through silver salt reduction. The chemical reduction of the silver salt solution was observed all through the reaction process with the help of the AvaSpec-2048L UV/VIS spectrometer in the wavelength range of 300–700 nm. A wide absorption band of 350–550 nm is exhibited by colloidal silver, with an absorption peak at 445 nm. Once the formation of nanoparticles starts, absorption increases; subsequently, the size of the particles starts to increase, and the absorption peak shifts toward the red wavelengths. The absorption peak’s stabilization indicates that the formation of new nanoparticles has ceased. The data gathered at the time of the reaction process also enabled the scientists to calculate the average particle size ^[2].

Figure 1. DLS distributions of NHC with different sizes while the concentration of SDS used is from 0.1 mg mL^-1 (purple) to 0.02 mg mL^-1 (red)

Another research team from the School of Chemistry and Chemical Engineering at the Beijing Institute of Technology, investigating nano-hydrogel colloidal (NHC) array photonic crystals for detecting humidity, relied on UV/VIS spectroscopy for carrying out the precipitation polymerization synthesis process for developing their NHCs. The innovative design of this colloidal gel humidity sensor taps the water absorbing characteristics of this hydrogel to enlarge and change volume in reaction to environmental stimulus. As the particles enlarge, the particle size changes, resulting in a shift in the absorption bands toward the red. This is displayed as a change in color which is visible to the naked eye and can be tuned, thus covering the complete visible wavelength range from 400 to 760 nm corresponding to a 20%–99.9% range in humidity. These experiments were supported and corroborated with the AvaSpec-ULS2048-TEC spectrometer and could pave the way for innovative sensor technologies that can be used for a range of inorganic and organic molecular detection sensors ^[3].

Complementary Measurement Methods

Although UV/VIS spectroscopy by itself is a robust tool, this method is often used in combination with other measurement technologies. The application of complementary techniques can yield richer sample data for researchers when compared to using either of the techniques alone. Instead, these techniques can serve as a check of accuracy, where sampling discrepancies are specified by data deviations between the techniques.

UV/VIS Spectroscopy with Attenuated Total Reflection (ATR)

Another spectroscopic technology that is often used in combination with UV/VIS spectroscopy is attenuated total reflectance (ATR). In analytical chemistry, when used with optically dense, or strongly light absorbing, material, ATR technologies can produce data related to a sample by using the electromagnetic evanescent field. During ATR, a beam of infrared (IR) light is allowed to pass through a crystal that has a high refractive index. When the sample comes into contact with the crystal, the incidence beam is reflected from the interior surface of the crystal in contact with the sample. An electromagnetic field produced by the vibrations of this reflection permeates into the sample, uncovering an absorption band at which the evanescent wave gets attenuated.

Dr. Thomas Bürgi from the Department of Physical Chemistry, University of Geneva, Switzerland, combined ATR with UV/VIS absorbance measurements to investigate oxidation reactions at the point of catalytic interfaces at an in-depth molecular level. While performing the experiments, Dr. Bürgi gathered simultaneous and synchronized ATR and UV/VIS spectra. Varying reaction conditions produced complementary information from the combination of measurement techniques. Attenuated total reflection (ATR) was performed by using the AvaSpec-ULS2048 spectrometer to recognize dissolved reaction products, whereas UV/VIS spectroscopy was highly sensitive to changes in the catalyst ^[4].

UV/VIS Spectroscopy with Attenuated Total Reflection (ATR), and Raman

Dr. Ursula Bentrup and a group of researchers from the Leibniz Institute of Catalysis at the University of Rostock investigated the production of mixed oxide catalyst precursors by using multiple methodologies during their experiments for gaining an in-depth knowledge of the way in which the modification of synthesis parameters has an impact on the structure and crystallinity of the precursors and the performance of the resultant catalysts.

The chemical reactions were monitored by using the in-situ UV/VIS measurements to track the formation and extinction of reactant species. In combination with Raman, the scientists were able to track changes at an atomic level, finding out a gradual change in crystallinity of Co²⁺ from octahedral to tetrahedral. The researchers used the AvaSpec-ULS2048 spectrometer together with a specialized ATR probe and noticed that absorption bands for molybdenum at 838, 879, 930, 1442, and 1632 nm, and also a nitrate band around 1338 nm, became broader and more intense with increase in precipitate formation. Such a combined technique of investigation has a high potential to perform real-time monitoring of complex reactions in liquid phase, such as the precipitate of solids ^[5].

UV/VIS Spectroscopy with Nuclear Magnetic Resonance

A technique that is frequently combined with UV/VIS Spectroscopy is nuclear magnetic resonance (NMR). German scientists reporting their research in the Angewandte Chemie journal investigated the efficiency of integrated UVNMR methodology in reaction monitoring to gain an understanding of the acid-based chemistry of strongly hydrogen-bonded complexes in aprotic solutions ^[N*]. Such slow reactions are highly sensitive to temperature, concentration, and the type of solvent used. The use of the AvaSpec-ULS2048 spectrometer for UV/VIS spectroscopy enabled the scientists to monitor reaction states by observing changes in the spectra surrounding the phenol group absorption bands observed at 315 nm in low-pH solutions and shifting to 400 nm in high-pH solutions. In the meantime, the concurrent use of NMR techniques provides an understanding of the hydrogen geometry of the reaction product ^[6].

Avantes, a Customer’s Partner in Chemistry

Since chemistry plays a vital role in several aspects of everyone’s lives, it is guaranteed that it can be found at the forefront of various developments and advances. Moreover, it can be trusted that Avantes will be supportive as well. Avantes’ experience in developing customized systems for chemistry applications for several decades implies that scientists can rely on the company for ease of use and reliability. Customers can contact a sales engineer to know about the Avantes Advantage.

References and Further Reading

1. Li, Kun, et al. "Optical properties of secondary organic aerosols generated by photooxidation of aromatic hydrocarbons." Scientific reports 4 (2014): 4922.
2. Šileikaitė, Asta, et al. "Analysis of silver nanoparticles produced by chemical reduction of silver salt solution." Mater. Sci 12.4 (2006): 1392-1320.
3. Wang, Zhe, et al. "Self-assembly of a nano hydrogel colloidal array for the sensing of humidity." RSC Advances 8.18 (2018): 9963-9969.
4. Bürgi, Thomas. "Combined in situ attenuated total reflection infrared and UV–vis spectroscopic study of alcohol oxidation over Pd/Al2O3." Journal of Catalysis 229.1 (2005): 55-63.
5. Bentrup, Ursula, et al. "Linking simultaneous in situ WAXS/SAXS/Raman with Raman/ATR/UV–vis spectroscopy: comprehensive insight into the synthesis of molybdate catalyst precursors." Topics in Catalysis 52.10 (2009): 1350-1359.
6. Tolstoy, Peter M., et al. "Combined NMR and UV/Vis spectroscopy in the solution state: study of the geometries of strong OHO hydrogen bonds of phenols with carboxylic acids." Angewandte Chemie International Edition 48.31 (2009): 5745-5747.

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