Using Infrared Spectroscopy to Study Self Assembled Monolayers

Image Credit: Kateryna Kon/Shutterstock.com

Due to their large surface area to volume ratio, nanoparticles are expected to radically impact a number of fields such as catalysis, cosmetics, and energy storage.

For instance, nanoparticles are used in electrode structures to develop better batteries, and in catalysis to enhance the production rate in commercial processes. They are also useful in coatings and cosmetics, and in nanocomposites where the surface properties of the individual nanoparticle control the behavior of the entire composite.¹

However, this large surface area means the surface properties of nanoparticles play a huge role and have to be understood in detail. This is important if we are to increase the potential of nanoparticles in technology. In addition to this, functionalizing and controlling the nanoparticle surface is also important as this allows researchers to customize their properties for specific applications.

What are Self-Assembled Monolayers? (SAMs)

Self-assembled monolayers (SAMs) consist of organic functional groups that spontaneously coordinate to the nanoparticle surface and form a coating layer. By varying the properties of the coordinating organic molecules researchers are able to tweak the surface properties of nanoparticles. This provides a high level of flexibility and control over nanoparticles behavior.

It has been suggested that self-assembly is the most effective and versatile method of achieving surface functionalization.²

For example, in organic electronics, surface functionalization using SAMs not only stabilizes the device but also improves its performance.² In battery electrodes, self-assembled organic layers with gold nanoparticles were observed to have better anti-fouling properties than those formed by electro-deposition.³

Diagram of a layer of sulfur based organic ligands assembling on a gold surface. Molecules 2014.

Examining SAMs using FTIR Spectroscopy

In order to enhance SAM functionalization, it is important to study and understand nanoparticle surfaces. A number of methods are available for this purpose.⁴

Fourier transform infrared (FT-IR) spectroscopy is one method that provides a number of advantages. FT-IR spectroscopy allows valuable insight into the different functional groups that are present in a system by measuring the vibrational frequencies of the chemical bonds involved.

The vibrational excitation energy of molecules is in the range of 10¹³–10¹⁴ Hz, which corresponds to infrared radiation.⁵ This means that IR spectroscopy can be used to observe the vibrational transitions of self-assembled functional groups coordinated to nanoparticle surfaces, and both quantitative and qualitative analysis can be performed.

FTIR enables the in-situ analysis of interfaces to investigate the surface adsorption of functional groups on nanoparticles.⁴ An advantage of FTIR is that it enables users to analyze a layer of nanoparticles coated on the ATR element, while also altering the overlying phase.

The molecular data obtained through this technique enables users to establish structural and conformational changes of the coordinating self-assembled functional groups on the surface of nanoparticles ^[6]. Also, ATR-FTIR spectroscopy can be used as a quantitative surface analytical tool.

FTIR spectroscopy can serve as a wonderful complement to UV-Vis and NMR spectroscopy, and like most analytical methods works best when coupled with other techniques. This is because every method comes with specific benefits and drawbacks ^[4] . By integrating such methods, comprehensive information can be collected on the interface between the functionalized nanoparticle surface and the surrounding environment.

Case Studies using FT-IR

FT-IR spectroscopy was used to test the adsorption of carbonate in moderately hard reconstituted water (MHRW) on TiO₂ nanoparticles.⁴ In this specific case, FTIR revealed that HCO₃- preferentially binds to the surface of nanoparticles, and then changes to form adsorbed CO₃^2-.

In addition, FTIR spectroscopy revealed that these adsorbed species are bound in both bidentate and monodentate modes.

The most significant finding was that the surface functionality of the TiO₂ nanoparticle is different from the adsorbed carbonate layer. This can modify the surface charge of the nanoparticle and influence toxicity to humans.

This further confirms the significance of understanding the nanoparticle surface using the FTIR method. The nanoparticle surface charge can vary due to the adsorption of functional groups and ligands. This can influence cellular uptake, toxicity, and also aggregation behavior.

This research is one of several revealing that FT-IR spectroscopy is an analytical tool that can be effectively used to probe a functionalized nanoparticle surface and its interface with the surrounding medium.

Functionalization of nanoparticle surfaces will allow specific disease-causing cells to be targeted. Image Credit: Kateryna Kon/Shutterstock.com

Infrared Accessories for SAM and Nanoparticle Analysis

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

The reaction cell accessory for Specac's Golden Gate™ allows for the in situ analysis of SAMs

As the world’s most versatile infrared sampling system, the Golden Gate ATR accessory is ideal for high-throughput qualitative and quantitative analysis of functionalized nanoparticle surfaces.

References and Further Reading

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

This information has been sourced, reviewed and adapted from materials provided by Specac.

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