Electrochemiluminescence : a powerful method for bioanalysis

Electrochemiluminescence (ECL) or electrogenerated chemiluminescence is a light-emitting phenomenon triggered by an initial electrochemical reaction. This electron transfer reaction, occurring directly at the surface of an electrode, induces a cascade of chemical reactions. A "co-reactant" species such as tri-n-propylamine (TPrA), for example, will bring the phosphor into the excited state, which is most often the ruthenium(II) tris-bipyridine complex $\left(Ru{\left(bpy\right)}_3^{2\text{+}}\right)$ . On returning to the ground state, the phosphor emits a photon at a characteristic wavelength. The resulting light intensity constitutes the analytical signal, which is directly proportional to the number of luminophores present.

This form of light emission has found numerous bioanalytical applications, thanks to its remarkable intrinsic characteristics, which make it a powerful, high-performance method. In fact, no excitatory light source is needed to induce ECL, so there's no need to separate the excitatory and emitting wavelengths, as is the case with fluorescence. As a result, the background noise almost matches that of the photodetector, and is therefore extremely low. ECL is therefore a particularly sensitive measurement method. Other advantages of ECL include high signal linearity over several orders of magnitude, good selectivity, high luminophore stability and in situ generation of reagents in a physiological environment. In addition, ECL-inducing inorganic complexes can be easily conjugated to molecules of biological interest such as antibodies, DNA or RNA. The luminophore is thus classically used as a marker for medical diagnostics. More than 150 ECL-based immunotests are currently on the market, mainly from two companies: Roche Diagnostics and MesoscaleDiscovery. These immunotests are used to diagnose a wide range of pathologies, from heart disease and infectious diseases to thyroid dysfunction and tumor marker detection. According to the Business Overview Report of Roche Diagnostics, the leader in ECL diagnostics, more than 1.3 billion immunoassays based on ECL technology were marketed by the company in 2013. It is therefore essential to fully understand the reaction mechanisms involved in order to be able to improve analytical performance and also to propose new ECL-based measurement methods. This article describes how the detailed study of the ECL mechanisms involved in these immunoassays is carried out using original imaging approaches. On the one hand, this enables us to understand some of the limitations...