catalisi

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CATALYSIS

The Luminol's reaction can be triggered through a wide range of catalysts, more or less specific for a particular oxidizing species and with varying efficiencies.

Bio-catalysis and chemi-catalysis [6,7]

Peroxidase enzymes, and particularly the one extracted from horseradish, are usually considered as the most effective catalyst, working at pH relatively close to neutral (810) and having a quite high specificity for a particular co-oxidant, the hydrogen peroxide. This chemiluminescence production system is the basis of biochemistry-based analytical systems such as biosensors, immunoassays, immunosensors and microarrays. The reaction is then used to reveal any interaction with a peroxidase-labelled entity, or to detect hydrogen peroxide first produced by a specific reaction.
The complex mechanism of the peroxidase-catalysed reaction and the stoichiometry remain hypothetical. Roughly speaking, the reaction sequence leading to light generation can be divided into two main processes. First, in the course of a series of enzymatic steps and in the presence of luminol (LH-) and hydrogen peroxide, horseradish peroxidase (HRP) is successively converted into intermediary complexes (complex I and complex II) before being regenerated to free peroxidase. These enzymatic steps produce luminol radicals (L-, LH.) which then enter a complex chemical pathway to finally generate luminol hydroperoxide (LO2H), the precursor of the light emitter (excited 3-aminophthalate ion).

Finally, transition metal cations (Co2+, Cu2+, Cr2+, Fe2++, Fe3+, Hg2+, Mn4+, Ni2+) and their complexed forms (e.g. ferrocene, ferricyanide) can be used as catalyst with mitigated performances which are linked to the relatively low signal to noise ratio obtained and the required elevated pH of the reaction.
A metal ion catalys decomposes the hydrogen peroxide (or other oxidizing molecule). When transition metal cations are used to catalyse the reaction, these are involved in the production of luminol radicals, in the same way as HRP is, but under harsher pH conditions.

Electro-catalysis[8,9]

The electrochemical oxidation of luminol is usually considered as the second most efficient way of triggering the reaction. The scheme is similar to the peroxidasecatalysed one with hydrogen peroxide as co-oxidant and a large range of working pH.
In a mechanistic study of this ECL reaction, Sakura (a chemist) had proposed that luminol was first oxidised at the Platinum electrode surface and then reacted, mole to mole, with hydrogen peroxide. The theoretical ratio (photon produced)/(H2O2 consumed) is then 1, whereas it is only 0.5 for the peroxidase-catalysed reaction.


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