Atmospheric chamber at University College Cork. Credit: Tomas Tyner, UCC.

Free radicals are atoms, molecules or ions that possess an unpaired electron. They play a key role in many chemical, biological, environmental processes. Hence their detection and characterisation are important for many areas of science.

Although some radicals are very stable, most are reactive intermediates with a very wide range of lifetimes, from nanoseconds to hours or longer. This makes their detection particularly challenging. Common approaches include direct (spectroscopic) detection, and indirect analysis, based on a selective reaction of a radical with a specific sensor compound.

Direct radical detection

Electron paramagnetic resonance (EPR) spectroscopy is a technique similar to NMR, it relies on the interaction of the spin of the unpaired electron with the magnetic field. This method is widely used for free radical detection but there are some disadvantages, for instance:

• Although EPR is more sensitive than NMR, radicals are often present at concentrations too low for EPR detection,
• EPR instruments are large, expensive and not very widely available,
• EPR spectra provide limited information about the structure of the radical, particularly for the functional groups further away from the unpaired electron.

Apart from EPR, some mass spectrometry and laser spectroscopy methods can detect radicals directly. These techniques are also expensive and not all are widely available. Some techniques are limited to specific radical types. They are usually best suited for radical detection in the gas phase, and there are many examples of their use in atmospheric chambers, e.g., to measure hydroxyl, peroxyl and other radicals.

Our recent blog (The Challenge of Measuring Radicals in the Atmosphere by Dr Adrien Gandolfo) discusses spectroscopic detection of atmospheric radicals in more detail.

Indirect radical detection

The challenges with direct spectroscopic detection of radicals has led to the development of a range of indirect methods. In these methods, a radical undergoes a fast and selective reaction with a specifically-designed sensor compound. The product of this reaction can then be analysed by a variety of analytical techniques.  Here are some examples:

• Salicylic acid is often used as a sensor for hydroxyl radicals as the reaction product can be readily detected by fluorescence [1].
• Hydroxylamines are used to detect oxidising radicals as the product of their oxidation (a nitroxide) is a stable radical which can be detected by EPR [2].
• Carbon-centred radicals are often detected by their reaction with nitroxides which gives a stable product usually either isolated from the reaction mixture or analysed by mass spectrometry (MS) [3].
• A very common and general method of radical detection is spin trapping. In this approach, radicals react with nitrone or nitroso-based spin traps to yield nitroxide products which are stable free radicals that can be analysed by EPR or MS [4,5]. This is a very powerful method which has been used to study free radicals in many different systems but it has many shortcomings including false positives, limited structural information, poor stability of spin traps and their reaction products etc. Hence scientists are still working on developing new methods for capturing free radicals. For instance, Criegee intermediates which have a partial diradical character, have been reported to undergo a cycloaddition with spin traps thus allowing to detect and quantify these elusive species [6].
• Here at the University of York, we have recently reported a new type of spin trap specifically designed for MS analysis, and we used these traps to study radical intermediates in a range of different gas- and liquid-based systems [7].

Radical detection in the RADICAL project

A simple radical sensor cannot rely on expensive instruments and therefore has to be based on indirect detection. The RADICAL project targets highly reactive ∙OH and ∙NO₃ radicals, and therefore the sensor compounds in this project are designed to be highly inert so that they will not react with non-radical components of the atmosphere. The only possible reaction will then be with the highly reactive radicals which should enable us to detect these radicals selectively. Radical reactions are not reversible, therefore the sensors will need to be replaced once a significant proportion of the sensor compound has reacted.

In order to optimise sensitivity and specificity of the radical sensors, the Chechik group at the University of York are currently investigating how the chemical and physical properties of the sensor compounds affect their reactivity with ∙OH radicals. We use atmospheric plasma to generate hydroxyl radicals, and a range of model sensor coatings, e.g., simple alkanes, unreactive perfluoroalkanes, somewhat more reactive ethers, ketones and aromatic compounds.

Preliminary results are encouraging!

Find out more:

Find our latest RADICAL research on our open access RADICAL project repository on Zenodo.

About the author: Victor Chechik is Professor in the Department of Chemistry, University of York. His research group specialises in the mechanistic chemistry of free radicals, EPR spectroscopy and functional nanoparticles. Victor leads the functionalisation work package within the RADICAL Project.