13 Dec 2016

Using infrared spectroscopy to examine functionalized nanoparticles

Nanoparticles are of significant interest in various fields such as catalysis, energy storage and cosmetics, due to their large surface area to volume ratio.

For example, nanoparticles are used in catalysis to improve the production rate in commercial processes, and in electrode structures to develop better batteries. They’re also useful in cosmetics and coatings, and in nanocomposites where the surface properties of the individual nanoparticle influence the behaviour of the entire composite [1].

If you want more information on using Specac's spectrometer accessories, get in touch for a price and demonstration.

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TEM (a, b, and c) images of prepared mesoporous silica nanoparticles with mean outer diameter: (a) 20nm, (b) 45nm, and (c) 80nm. SEM (d) image corresponding to (b). The insets are a high magnification of mesoporous silica particle.

But this large surface area means the surface properties of nanoparticles dominate, and need to be thoroughly understood. This is critical if we are to maximize the potential of nanoparticles in technology. Hence, probing their surface using a variety of analytical techniques is necessary. Not only this, but controlling and functionalizing the nanoparticle surface is also important for tailoring its properties for specific applications.

Self-Assembled Monolayers (SAMs)

Self-assembled monolayers (SAMs) involve the use of organic functional groups that coordinate to the nanoparticle surface. Functionalizing in this manner allows scientists to tune the nanoparticle surface with well-defined properties, allowing for a high-level of control and flexibility. In fact, it has been suggested that self-assembly is the most effective and versatile technique to achieve surface functionalization [2].

For instance, in organic electronics, surface functionalization using SAMs stabilizes the device and enhances its performance [2]. And in battery electrodes, self-assembled organic layers with gold nanoparticles were found to have better anti-fouling properties compared to those formed by electro-deposition [3].

Examining SAMs using FTIR Spectroscopy

For improvements to be made to SAM functionalization, nanoparticle surfaces need to be studied and understood. And a variety of techniques are available for this purpose [4].

However, one technique in particular that offers several benefits is Fourier Transform Infrared (FTIR) spectroscopy. The main advantage of this technique is you can gain valuable insight into the functional groups of a particular system. This is because molecular vibrational excitation energy is in the range of 1013 – 1014 Hz, which corresponds to infrared radiation [5].

This means that IR spectroscopy is ideal for observing the vibrational transitions of self-assembled functional groups coordinated to nanoparticle surfaces, and both qualitative and quantitative analysis are possible.

In particular, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) allows the in-situ study of interfaces to probe the surface adsorption of functional groups on nanoparticles [4]. A benefit of FTIR is that it allows you to study a layer of nanoparticles coated on the ATR element, while also changing the overlying phase.

The molecular data obtained using this technique allows you to determine conformational and structural changes of the coordinating self-assembled functional groups on the nanoparticle surface [6]. ATR-FTIR spectroscopy can also be used as a quantitative surface analytical tool.

Of course, like most analytical techniques, FTIR spectroscopy works best when combined with other methods, and can serve as a wonderful complement to UV-Vis and NMR spectroscopy. This is because every technique has specific advantages and drawbacks [4]. By combining such techniques, it’s possible to collect detailed information on the interface between the functionalized nanoparticle surface and the surrounding environment.

Case Studies using FT-IR

FTIR spectroscopy was used to analyze the adsorption of carbonate in moderately hard reconstituted water (MHRW) on TiO2 nanoparticles [4]. In this particular case,

FTIR showed that HCO3- preferentially binds to the nanoparticle surface, and then changes to form adsorbed CO32-. FTIR also showed that these adsorbed species are bound in both monodentate and bidentate modes.

The most important finding was that the surface functionality of the TiO2 nanoparticle is different with the adsorbed layer of carbonate. This can alter the nanoparticle surface charge and influence toxicity.

This further confirms the importance of understanding the nanoparticle surface using techniques like FTIR. The surface charge of the nanoparticle can change due to the adsorption of ligands and functional groups. This can affect toxicity and cellular uptake, as well as aggregation behaviour. This study is one of many showing that FT-IR spectroscopy is an analytical tool that can be used effectively for the probing of a functionalized nanoparticle surface, and its interface with the surrounding medium.

Infrared accessories for SAM and nanoparticle analysis

The Golden Gate ATR accessory is a high-performance, single reflection, monolithic diamond product from Specac. It’s ideal for the FTIR analysis of SAM functionalized nanoparticle surfaces because of its range of sampling options, which include standard ambient temperature experiments as well as a reaction cell for heated and cooled experiments

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The Golden Gate ATR spectrometer accessory is also the world’s most versatile infrared sampling system, and is suitable for high-throughput quantitative and qualitative analysis of surface functionalized nanoparticles.

Check out #SpectroscopySolutions for more.

References

[1]. Lesley E. Smart and Elaine A. Moore, Solid State Chemistry: An Introduction, 2005, 3rd Edition, CRC Press Taylor and Francis

[2]. Stefano Casalini, Carlo Augusto Bortolotti, Francesca Leonardi and Fabio Biscarini, Self-Assembled Monolayers in Organic Electronics, Chem. Soc. Rev. 2016 DOI: 10.1039/C6CS00509H

[3]. Safura Taufik, Abbas Barfidokht, Muhammad Tanzirul Alam, Cheng Jiang, Stephen G. Parker and J. Justin Gooding, An Antifouling Electrode Based on Electrode–Organic Layer–Nanoparticle Constructs: Electrodeposited Organic Layers Versus Self-Assembled Monolayers, J. Electroanal. Chem. 2016, 779, 229–235

[4]. Imali A. Mudunkotuwa, Alaa Al Minshid and Vicki H. Grassian, ATR-FTIR Spectroscopy as a Tool to Probe Surface Adsorption on Nanoparticles at the Liquid–Solid Interface in Environmentally and Biologically Relevant Media, Analyst, 2014, 139, 870

[5]. Peter Atkins and Julio de Paula, Elements of Physical Chemistry, 2009, 5th edition, Oxford University Press

[6]. Vicki H. Grassian, ATR-FTIR Spectroscopy as a Tool to Probe Adsorption on Nanoparticle Surfaces at the GasSolid and Liquid-Solid Interface http://www.susnano.org/images/sessions2013/3B_4_UpdatedSNOGrassian.pdf