Page 59 - Analytical Electrochemistry 2d Ed - Jospeh Wang

44 STUDY OF ELECTRODE REACTIONS

Spectroelectrochemical experiments can be used to probe various adsorp-
tion=desorption processes. In particular, changes in the absorbance accruing from
such processes can be probed utilizing the large ratio of surface area to solution
volume of OTEs with long optical path length (29). Additional information on such
processes can be obtained from the Raman spectroelectrochemical experiments
described later.
In addition to transmission experiments, it is possible to use more sensitive
re¯ectance protocols. In particular, in internal re¯ectance spectroscopy (IRS) the
light beam is introduced to the electrode at an angle, and the spectrum is recorded
from the re¯ected beam at the solid±solution interface. Prisms are used to allow the
radiation enter and leave. In addition to its higher sensitivity, IRS is less prone to
solution resistance effects.
Infrared spectroelectrochemical methods, particularly those based on Fourier
transform infrared (FTIR) spectroscopy can provide structural information that UV-
visible absorbance techniques do not. FTIR spectroelectrochemistry has thus been
fruitful in the characterization of reactions occurring on electrode surfaces. The
technique requires very thin cells to overcome solvent absorption problems.
Besides its widespread use for investigating the mechanism of redox processes,
spectroelectrochemistry can be useful for analytical purposes. In particular, the
simultaneous pro®ling of optical and electrochemical properties can enhance the
overall selectivity of different sensing (30) and detection (31) applications. Such
coupling of two modes of selectivity is facilitated by the judicious choice of the
operating potential and wavelength.

2-2.3 Other Spectroelectrochemical and Spectroscopic Techniques

In addition to UV-visible absorption measurements, other spectroscopic techniques
can be used for monitoring the dynamics of electrochemical events or the fate of
electrogenerated species. Particularly informative are the couplings of electrochem-
istry with electron spin resonance, nuclear magnetic resonance, and mass spectro-
scopy. A variety of specially designed cells have been constructed to facilitate such
studies, and several reviews have been published (32±36). Electrochemilumines-
cence (ECL) is another useful technique for studying the fate of electrogenerated
radicals that emit light. It involves the formation of light-emitting excited-state
species as a result of fast and highly energetic electron-transfer reactions of reactants
formed electrochemically (37,38). Various organic and inorganic substances (e.g.,
2‡
polycyclic hydrocarbons, nitro compounds, luminol, Ru bpy† ) can produce ECL
3
upon electron transfer from electrodes, in connection with the formation of radical
ions. The electrogenerated radicals behave as very strong redox agents, and their
reactions with each other or with other substances are suf®ciently energetic to be
able to populate excited states. ECL experiments are usually carried out by recording
the spectra of the emitted light using a deoxygenated nonaqueous medium (e.g.,
highly puri®ed acetonitrile or dimethylformamide). Analytical applications of ECL
relying on the linear dependence of the ECL intensity and the reactant concentration
have also been realized (39).