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The characterization, identification and analysis of nanoparticles—and nanomaterials in general—requires a multi-targeted approach when it comes to the techniques used. There are many different aspects and properties of nanoparticles that need to be deduced, including the size (and size distribution), shape, degree of functionality and the different properties that they exhibit. To obtain all these different pieces of information, many different techniques are needed. In this article, we look at how nuclear magnetic resonance (NMR) spectroscopy and infrared (IR) spectroscopy can be used to together to identify and characterize nanoparticles.
NMR and IR are concerned with the identification of the different aspects and properties of a nanoparticle through the analysis of a spectrum. There are other quantitative methods out there which can be used (including other spectroscopy methods) as well as various qualitative methods, such as high-powered electron microscopy techniques (although many of these can now be used for quantitative data as well). Overall, NMR and IR are two techniques that can be utilized together to build up a big picture of the structure, features and properties of the nanoparticles being analyzed.
NMR is a well-established technique for the analysis of many organic molecules, as it recognizes the number of protons (bonded hydrogen atoms) on a carbon-based molecule, which enables the scientist to deduce the structure by the splitting and intensity of the spectra. There are also many different variations of the technique.
NMR can also be used to deduce many of the features of a nanoparticle, including the size and shape of the nanoparticle, as well as the surface area (and specific surface area) of the nanoparticle, the elemental composition, the chirality of nanoclusters, the density, arrangement, composition, binding, mass and the surface composition of the ligands which are attached to the surface of nanoparticle, the electron core structure and how the ligands on the surface influence the shape and size of the nanoparticle (as it is a well-known fact that the capping ligands used can direct the synthesis of the nanoparticle). So, NMR not only offers scientists a way of determining the physical features of the nanoparticle itself, but how some of the different molecular components can be changed and tuned to provide the nanoparticle with different features. This also includes measuring the growth kinetics of the nanoparticle.
NMR can also be used to see how the surface ligands of diamagnetic and antiferromagnetic nanoparticles interact with particles of differing electronic properties. However, NMR cannot be used to characterize the features and properties of ferromagnetic nanoparticles, because the nanoparticles cause a large magnetic saturation which affects the local magnetic field used in NMR. NMR can also be used to measure how the nanoparticles interact with other substances to give insights into how the nanoparticle will behave in different environments—especially with how gases absorb on to the surface of the nanoparticle. These are just some of the more common ways in which NMR is utilized, as there many specific and smaller ways in which NMR is utilized by scientists to build up a complete picture of the nanoparticles being analyzed.
IR spectroscopy—and in particular Fourier-transform IR (FTIR) spectroscopy—is another technique that can deduce some information about nanoparticles—although the amount of information as a stand-alone technique only generally yields information about the surface composition of the nanoparticle and the ligand binding interactions. However, when it is used in conjunction with other techniques (NMR and beyond), the information gained from the IR spectra can help to provide more detailed information on the nanoparticle’s structure, elemental composition and the key properties that the nanoparticle has. A lot of the information gained from IR is about the surface, and in particular the ligands, and IR can be used to determine composition, density, arrangement, mass, and surface composition of the ligands used—which enables the ligand binding interactions to be deduced.
There are in fact many different types of IR, and whilst conventional IR and FTIR are used for the above purposes, ultraviolet-visible-near infrared (UV-Vis-NIR) spectroscopy can be used to determine the optical properties of nanoparticles. Diffuse reflectance FTIR spectroscopy (DRIFTS) is another variation which enables the physicochemical properties of catalytic nanoparticles to be determined, whereas FTIR combined with an attenuated total reflectance (ATR) probe can be used to study how certain nanoparticles oxidize. But again, these variations are a few of the more common examples, and the combination of NMR alongside the various IR spectroscopy methods enables researchers to not only build a picture of the nanoparticles themselves, but also how they behave in various environments.
Sources and Further Reading
“Characterization techniques for nanoparticles: comparison and complementarity upon studying nanoparticle properties”- Thanh N. T. K. et al, Nanoscale, 2018, DOI: 10.1039/C8NR02278J