..:: Metrohm DropSens ::.. Spectroelectrochemistry

SPECTROELECTROCHEMISTRY

Spectroelectrochemistry is a powerful technique that combines the advantages of spectroscopy and electrochemistry. Despite being a well developed technique, how much do I know about spectroelectrochemistry? Who were the researchers who developed this technique? Why can it be so powerful? If you are interested in knowing much more about this hybrid technique, its background, basics theoretics and historical facts, check our periodical articles on spectroelectrochemistry.

:: INFORMATION MEANS KNOWLEDGE

:: SPECTROELECTROCHEMISTRY BACKGROUND

:: SPECTROELECTROCHEMICAL SET-UP

:: CLASSIFICATION OF THE SPECTROELECTROCHEMICAL TECHNIQUES

INFORMATION MEANS KNOWLEDGE

Spectroelectrochemistry Metrohm DropSens

Currently, there is a dense net of communication channels allowing us to get information from different media as well as new perspectives. It is said that the more information we have, the better. When we are well informed we are able to make wiser decisions. As researchers, we want to understand why things happen in a specific way and have a perfect knowledge about the materials, reactions and methodologies we study.

At the same time, we yearn to make the most of every trial.

How can we get much more information from our experiments?

Different points of view

When we get data from just one perspective, we have biased information. If you see the image below, you clearly see a bike.

However, if you see the image from another perspective... the result is completely different.

Are we wrong when we think it is a bike? Certainly not, we jump to a conclusion by taking into account half of the information.

Coupling techniques: spectroscopy and electrochemistry

Electrochemistry allows us to correlate the electrical current generated during an electrochemical reaction with the concentration of a substance. The processes taking part on the electrode are well described by different equations that provide us with a deep understanding of the electrochemical processes.

Spectroscopy studies the interactions between matter and electromagnetic radiation. Spectroscopic methods are based on measuring the radiation absorbed, emitted or scattered by molecular or atomic species of interest. Depending on the excitation wavelength the acquired spectra will provide us with information about the chemical structure, bonds, but also about molecule concentration.

When we combine both techniques, while oxidation states are changed electrochemically at an electrode, spectral measurements on the solution adjacent to the electrode are made simultaneously. Undergoing reaction is characterized spectroscopically and allows to establish relationships between mechanisms and structures. The limited structural information available from the electrochemical response is complemented by optical monitoring.

Combining of both techniques is a good way of getting formal reduction potentials or electron stoichiometries of redox couples and biological redox complexes. Following a polymerization reaction, identifying polymer structure and stability, identification of reaction intermediates or defining product structures after electrochemical reaction are just a few examples of the potential of this hybrid technique.

If you are interested in Spectroelectrochemistry, do not hesitate to follow our articles on LinkedIn and on our website: www.metrohm-dropsens.com

SPECTROELECTROCHEMISTRY BACKGROUND

In-situ vs ex-situ experiments

When combining optical and electrochemical measurements, one simple approach could be run the electrochemical reaction and afterwards carry out the optical characterization of the solution or the electrode surface. Moreover, we could also remove the electrode from the electrochemical cell and characterize it optically. This kind of measurement is considered an ex-situ experiment. However, we do not get information while the electrochemical reaction is taking place. We are therefore unable to detect optically any change related to the molecules produced or consumed during the electrochemical process just close to the electrode’s surface.

Ex-situ experiments: after performing an electrochemistry measurement, electrode and sample could be optically studied

On the other hand, in-situ experiments lead us to get optical information while the electrode is immersed in the solution under electrochemical control. In this way, two independent signals but both related to the same process are obtained. This way we register dynamic information: reliable data under specific electrochemical conditions. In-situ spectroelectrochemistry is a dual technique, that is, an authentic multi-response technique.

In-situ experiments: electrochemical and spectroscopical measurements are performed simultaneously

Which could be the way to record an optical signal at the same time we are performing an electrochemical analysis?

Back to the ‘60s

The development of spectroelectrochemistry is related to the possibility of performing analysis through optically transparent electrodes. During 1960’s T. Kuwana et al. developed transparent electrodes and marked the beginning of UV-VIS absorption and electrochemical measurements. The first spectroelectrochemistry published paper (Anal. Chem. 1964, 36 (10), pp 2023-2025) describes the use of tin oxide-coated glass surfaces, electrodes optically transparent that make possible monitoring the changes in absorbance of different electroactive species during the electrolysis.

Thus, different transparent electrodes were developed for going ahead with this combination of optical and electrochemical experiments. From the first antimony doped tin oxide over glass to different thin films of gold or platinum on quartz, germanium electrodes for IR wavelength or directly gold and platinum micromeshes where the wholes provided transparency to light are the most commonly used OTEs.

Optically Transparent Screen-Printed Electrodes where working electrode is made of ITO, PEDOT, Au, or C.

SPECTROELECTROCHEMICAL SET-UP

The main drawback when combining different instruments is the complexity of connecting different instruments and softwares. Moreover, data handling becomes difficult. In spectroelectrochemistry synchronization between both types of measurements is critical: If the techniques are not applied exactly at the same time, misinterpretations of the results are easily achieved. Finally, the cell is rather complex as it should be suitable for optical and electrochemical measurements. A compromise between optimal measurement conditions with both techniques should be achieved. A high expertise is required and, despite its potential, spectroelectrochemistry could be discouraging.

Classical set-up

Classical set-up for performing spectroelectrochemistry measurements combines a potentiostat with a light source and spectrometer.

Classical set-up: independent instruments and softwares.

How to get the best from this hybrid technique?

The best solution is having an integrated system that easily combines potentiostat/galvanostat, spectrometer and light source in a unique instrument. If everything is controlled by one software an encouraging approach to spectroelectrochemistry is possible.

Metrohm Electrochemistry offers different solutions according to your requirements. You can add spectroelectrochemistry to your existing Autolab electrochemical setup: An integrated solution for UV-VIS spectroelectrochemistry. SPELEC range of instruments combines light source, spectrometer and potentiostat/galvansotat in just one unit. One software has a control over all systems for a perfect synchronization, allowing also an easy data analysis. Advanced solutions for advanced research.

This set-up is compatible with your spectroelectrochemical cell but also you can now perform spectroelectrochemistry with screen-printed electrodes.

CLASSIFICATION OF THE SPECTROELECTROCHEMICAL TECHNIQUES

We have already seen one of the most important differences in spectroelectrochemistry: in-situ and ex- situ techniques. However, the spectroelectrochemical techniques can be organized according to other criteria, among others: the regime, the electromagnetic radiation configuration, the spectroscopic technique or the spectral range.

· Regime

In bulk electrolysis, the optical information of the bulk solution is obtained. In semi-infinite diffusion configuration, only the diffusion layer, where the electrochemical reactions take place, is studied.

Bulk and semi-infinite diffusion regime.

· Electromagnetic radiation configuration

In normal configuration, the light arrives in normal arrangement to the electrode surface and is transmitted or reflected depending the working electrode. In parallel configuration, the light beam goes to the electrode surface. Furthermore, combination of normal and parallel arrangements is known as bidimensional spectroelectrochemistry.

Normal reflection, parallel and bidimensional configuration.

· Spectroscopic technique

According to the Electromagnetic radiation-materia interaction, spectroscopic techniques are based on measuring the radiation absorbed, emitted or scattered by molecular or atomic species. The sample is excited by electromagnetic radiation and the amount of light absorbed (or transmitted), emitted or scattered is measured. These changes on the light energy are related to materia’s electron transitions and the measured light can be related to different properties and characteristics of the molecules under study.

Different possibilities of interaction between radiation and material.

· Spectral range

One of the most commonly used classification is based on the spectral range. The electromagnetic spectrum is the range of frequencies of electromagnetic radiation and their respective wavelengths and energies. It can be divided in seven different areas as shown in the graphic below:

Electromagnetic spectrum

As a general approach, the main regions are radio waves, microwaves, infrared, visible, ultraviolet, X- rays and gamma rays. According to the energy associated with each region, changes of spin, orientation, configuration, electron distribution or nuclear configuration can be studied. Furthermore, the light source and spectrometer must be in the corresponding wavelength range.

In the next publication, fundamental aspects of UVVIS, Raman and NIR spectroscopy will be explained with more details.