In this article, AZoNano offers a comprehensive guide to diffuse reflectance spectroscopy, including its working principle and applications.
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Why is Diffuse Reflectance Spectroscopy Used?
One of the main techniques for evaluating the optical properties of materials is optical absorption spectroscopy, or UV-vis spectroscopy.
Optical absorption spectroscopy is a very versatile technique that can provide a variety of qualitative and quantitative information on materials and nanoparticles.1 Some examples include nanoparticle sizing and concentration determination2, or determination of band gaps.3
Most absorption spectroscopy measurements are performed in transmission geometries. What this means is the light from the source is passed through the sample and the absorption is measured by detecting the change in the incident versus transmitted light intensities.
However, many materials are simply not transparent to visible light. Strongly absorbing materials, which is a desirable property for many photovoltaic applications, will completely attenuate the light from a standard lamp source at just a few hundred nanometers of thickness.4
Measuring opaque samples then requires a different approach. Rather than measuring the optical properties through absorption measurements in transmission geometries, diffuse reflectance spectroscopy can be used.
What is Diffuse Reflectance Spectroscopy?
Diffuse reflectance spectroscopy is a type of absorption where the light reflected from the sample is recorded, rather than the beam transmitted through the sample. Much like more standard transmission measurements, diffuse reflectance spectroscopy can be used for the quantitative and qualitative analysis of samples and, depending on the wavelengths of light used, be used to look at the electronic or vibrational structure of the sample.5
One advantage of diffuse reflectance spectroscopy over other reflectance methods is that it is designed to work for surfaces that are not perfectly smooth and reflective. For other reflectance methods, the surface cannot be too rough or uneven as this leads to too much unwanted scattering background that will obscure the sample signal.
Diffuse reflectance spectroscopy measurements are reasonably straightforward to perform and can even be used on materials such as textile and paper.
How Does Diffuse Reflectance Spectroscopy Work?
A diffuse reflectance spectroscopy measurement requires a light source, the sample, and a detector to collect the reflected light. Additional optical components to ensure good beam collimation or high collection efficiencies may also be included with the diffuse reflectance spectroscopy instrument as well as dispersive elements.
Dispersive elements spatially separate the wavelength components of a beam of light and allow them to be detected simultaneously using an array detector like a CMOS or CCD camera.
The advantage of using dispersive components in diffuse reflectance spectroscopy is it means that there does not need to be any moving components, such as a monochromator for selecting input or detection wavelengths which helps with miniaturization of the diffuse reflectance spectroscopy instrument and can reduce acquisition times.
Where diffuse reflectance spectroscopy can be significantly more complex than traditional transmission absorption measurements is in the data analysis.5
Reflection geometries typically allow for better signal-to-noise in measurements as there is overall better transmission through the spectrometer, but how to convert the signal into something that can be interpreted in terms of the electronic or vibrational structure of the sample can be challenging.
Kubelka-Munk theory is one solution that works in the case of semi-infinite samples to calculate a scattering coefficient from diffuse reflectance spectroscopy measurements which, in turn, can be interpreted in terms of band gaps and other optical properties of a variety of material types.6
An additional consideration with reflectance measurements is there is usually an angular dependence of the scattering parameters, so measurements may need to be made over a variety of angles to fully reconstruct all the desired information about the sample properties.
Applications
Diffuse reflectance spectroscopy is used in a number of industries. Historically, diffuse reflectance spectroscopy has been used to largely study surface properties and, in particular, to evaluate dye and pigment behavior on textiles.7
The ability to make rapid, non-destructive measurements makes diffuse reflectance spectroscopy an ideal method for evaluation and quality control of manufacturing processes. The pharmaceutical industry uses diffuse reflectance spectroscopy to evaluate the active pharmaceutical ingredient in therapeutics, such as tablets, once they are in their final administrable form.8
The non-destructive nature of diffuse reflectance spectroscopy and its sensitivity to different colors has made diffuse reflectance spectroscopy also a useful tool in the heritage sciences.9 Improvements in the quality of portable spectrometers have meant diffuse reflectance spectroscopy can be used to evaluate the material composition of fragile artworks in situ, with minimal risk of damage to the artwork.10
Commercial Landscape
There are a number of commercial suppliers of portable and laboratory-based diffuse reflectance spectroscopy instruments. The non-invasive nature of diffuse reflectance spectroscopy measurements and lack of need for specific sample preparation means that this method is also highly appealing for use in real-time measurement and feedback systems for process optimization.
Already the method is in routine use in many manufacturing industries and research laboratories, but the growing demands of smart industry and ability to make quick measurements may see even wider uptake of this approach.
In the future, with the growing interest and drive to develop new point-of-care medical diagnostics, medical diagnosis for cancers is likely to become a more significant application of diffuse reflectance spectroscopy.11
References and Further Reading
Tissue, B. M. (2002). Ultraviolet and visible absorption spectroscopy. Characterization of Materials. doi.org/10.1002/0471266965.com059
Khlebtsov, N. G. (2008). Determination of size and concentration of gold nanoparticles from extinction spectra. Analytical Chemistry, 80(17), pp.6620–6625. doi.org/10.1021/ac800834n
Makuła, P., et al. (2018). How To Correctly Determine the Band Gap Energy of Modified Semiconductor Photocatalysts Based on UV-Vis Spectra. Journal of Physical Chemistry Letters, 9(23), pp.6814–6817. doi.org/10.1021/acs.jpclett.8b02892
Hrostea, L., et al. (2018). Optical and Morphological Properties of P3HT and P3HT: PCBM Thin Films Used in Photovoltaic Applications. IOP Conference Series: Materials Science and Engineering, 374(1), pp.1–7. doi.org/10.1088/1757-899X/374/1/012015
Blitz, J. P. (1998). Diffuse Reflectance Spectroscopy. Wiley. https://www.wiley.com/en-us/Modern+Techniques+in+Applied+Molecular+Spectroscopy-p-9780471123590
Alcaraz de la Osa, R., et al. (2020). The extended Kubelka–Munk theory and its application to spectroscopy. ChemTexts, 6(1), pp.1–14. doi.org/10.1007/s40828-019-0097-0
Frei, R. W. (1976). A Diffuse Reflectance Spectroscopy; Applications, Standards, and Calibration (With Special Reference to Chromatography). Journal of Research of the National Bureau of Standards - A. Physics and Chemistry, 80A, pp.551–565.
Li, P., et al. (2012). Rapid and nondestructive analysis of pharmaceutical products using near-infrared diffuse reflectance spectroscopy. Journal of Pharmaceutical and Biomedical Analysis, 70, pp.288–294. doi.org/10.1016/j.jpba.2012.07.013
Dal, J. S. A., & Fontana, F. R. (2020). Reflectance imaging spectroscopy in heritage science. La Rivista Del Nuovo Cimento, 43(10), pp.515–566. doi.org/10.1007/s40766-020-00011-6
Arrizabalaga, I., et al. (2014). Applicability of a Diffuse Reflectance Infrared Fourier Transform handheld spectrometer to perform in situ analyses on Cultural Heritage materials. Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, 129, pp.259–267. doi.org/10.1016/j.saa.2014.03.096
Akter, S., et al. (2018). Medical applications of reflectance spectroscopy in the diffusive and sub-diffusive regimes. Journal of Near Infrared Spectroscopy, 26(6), pp.337-350. doi.org/10.1177/096703351880663
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