Analytical Chemiluminescence/Micellar enhancement

C1. Micellar enhancement

Well-defined mechanistic principles have emerged to rationalize micellar enhancement of chemiluminescence. The review of Lin and Yamada focuses on how micelles may be used to improve chemiluminescence signals by changes that affect the reaction rate. These occur in the microenvironment (i.e. polarity, viscosity and/or acidity, etc.), in the chemical and photophysical pathway and in the solubilization, concentration and organization of the solute/reactant. We shall now use these principles as a framework for discussing this work and it will become clear that they are highly inter-related rather than mutually exclusive.

There follow examples of micellar enhancement which have been explained by changes in the microenvironment. In the interaction of sulfite groups in drugs with dissolved oxygen in presence of acidic rhodamine 6G, the surfactant Tween 60 can enhance chemiluminescence by 200%, attributable to a microenvironment that leads to an increase in the fluorescence quantum yield of rhodamine 6G and prevents quenching by oxygen. Sensitization of IO₃/H₂O₂ chemiluminescence in the presence of various surfactants at various concentrations has been explained by changes in the microenvironment rather than by solubilization, electrostatic effects or changes in pH. In the chemiluminescence reaction of luminol with hypochlorite in cetyltrimethylammonium chloride (CTAC) micelles, the light reaction in micellar media results in chemiexcitation yields which are higher than those in the corresponding homogeneous aqueous media due to the less polar microenvironment of the micellar stern region but the actual chemiluminescence quantum yields are lower due to quenching, both chemical and photophysical.

In some cases there is evidence of changes in chemical or photophysical pathways or rates of particular reactions. In the system of lucigenin reduced by fructose, glucose, ascorbic acid or uric acid, the cationic surfactant cetyltrimethylammonium hydroxide (CTAOH) increases the chemiluminescence intensity better than cetyltrimethylammonium bromide (CTAB) due to the superiority of CTAOH in micellar catalysis of the rate-limiting step of the lucigenin-reductant reaction. In permanganate chemiluminescence for the analysis of uric acid in the presence of octylphenyl polyglycol ether, there is an alteration in the local microenvironment allowing the solute to associate with the micellar system and this affects various photophysical rate processes. A small amount of surfactant added to the luminol-gold(III)-hydroxyquinoline system, can stabilize gold(III) in aqueous solution, accelerate the reaction rate and hence increase chemiluminescence intensity. The surfactant Triton X-100 can accelerate the chemiluminescence reaction between colloidal manganese dioxide (MnO₂) and formic acid in perchloric acid but CTAB or sodium dodecyl sulfate (SDS) cannot.

Sometimes the micelles have their enhancing effect by changing the local concentrations and organization of the reactants. The determination of iron(II) and total iron by the effect on the luminol/hydrogen peroxide system is enhanced by tetradecyltrimethylammonium bromide (TTAB) in the presence of citric acid. An iron(II)-citric acid anion complex is formed and concentrated at the surface of the cationic micelle. This then reacts with hydrogen peroxide at that surface, increasing the rate of the chemiluminescence reaction. The effect of cationic surfactant on the copper-catalysed chemiluminescence of 1,10-phenanthroline with hydrogen peroxide is that 1,10-phenanthroline concentrates in the centre of the micelles, but superoxide anion radicals are attracted to the surface where the reaction occurs more easily.

Some cases of micellar enhancement are explained by facilitation of energy transfer. Greenway et al. found that a non-ionic surfactant helps to overcome the pH imbalance between codeine (in acetate buffer) and Ru(bipy₃³⁺ (in sulfuric acid and Triton X-100). The reacting species are enclosed within a micelle which enabled easier energy transfer. CTAB micellar complexes enhance the signal in the presence of fluorescein in the luminol-hydrogen peroxide system. The effect on energy transfer arises because the aminophthalate anion energy donors and the fluorescein anion acceptors will be located at distances approximately corresponding to diameter of micelle (1-3 nm). Since, the transfer of electron excitation energy in solutions can be realized up to a distance of 7-10 nm (Förster mechanism, see chapter C2) (ADD LINK), the concentration of both species in the micelle is very effective for energy transfer. The same explanation applies to the chemiluminescence reactions of luminol and its related compounds in the presence of CTAC, which is also enhanced by intramicellar transfer of electronic excitation energy. Intramicellar processes of energy transfer can easily be modified by altering surfactant concentration and optimized in order to reach maximum conversion of chemical energy to emitted light. The procedure is generally applicable, the effectiveness varying a little with different chemiluminescence reactions, acceptors of electron excitation energy, catalysts and surfactant enhancers.