Analytical Chemiluminescence/Chemiluminescence sensors

D5. Chemiluminescence sensors
Chemiluminescence has the advantage of lower background emission than fluorescence, avoiding noise caused by light scattering. However, because chemiluminescence reagents are irreversibly consumed, chemiluminescence sensors have shorter lifetimes than fluorescence sensors and their signals have a tendency to drift downwards due to consumption, migration and breakdown of reagents. Reagent immobilization onto suitable substrates plays an important role in the development of chemiluminescence sensors. Selectivity and sensitivity as well as lifetime of chemiluminescence sensors depends on the choice of reagent and substrate and on the method of immobilization.

Chemiluminescence reagents are typically aqueous solutions of ions and so can be immobilized by convenient procedures onto ion exchange resins, giving high surface coverage, and released quantitatively by appropriate eluents. Analytes can also react directly with immobilized reagents. These properties have been widely used to prepare chemiluminescence sensors containing immobilized luminol or other reagents, which typically would be packed into a flow cell positioned in front of the window of a photomultiplier. “Bleeding” columns of anion/cation exchange resins with co-immobilized luminol and metal ions such as Co2+, Cu2+ or [Fe(CN)6]3– can detect and measure analytes such as hydrogen peroxide, though this arrangement causes unnecessary dilution of samples and reagents, which impairs detectivity. Immobilized tris-(2, 2/-bipyridyl)ruthenium(II) can be regenerated from tris-(2, 2/-bipyridyl)ruthenium(III) and can be used for at least six months.

Immobilization of enzymes can be used to produce highly active and selective chemiluminescence sensors from which enzyme is not consumed, though their operational stability is limited. Encapsulation of reagents in sol-gel silica involves little or no structural alteration and is very suitable for chemiluminescence sensors because of its optical transparency and chemical stability. For example, encapsulated horseradish peroxidase displays high activity and long life, as does sol-gel immobilized haemoglobin. Chemiluminescence sensors constructed from plant and animal tissues have advantages of cost, activity, stability and lifetime; examples are soyabean tissue in sensors for urea and spinach tissue in sensors for glycolic acid.

Molecular imprinted polymers have been found to be very useful materials for fabrication of chemiluminescence sensors, both as molecular recognition agents and as chemiluminescence reaction media. Analytes that can be successfully detected in this way include 1,10-phenanthroline and dansylated amino-acids. Metal oxide particles can sometimes be entrapped onto membranes or in columns, including chemiluminescence flow cells. This affords a simple fabrication method producing long-lived sensors. Manganese dioxide has been immobilized in this way on sponge rubber for the assay of the drug, alangin, using manganese(IV) chemiluminescence.

Chemiluminescence has been detected from surface reactions on nanoparticles, opening up the possibility of chemiluminescence nanosensors of good stability and durability. Coumarin C343, a fluorescent dye, has been conjugated to silica nanoparticles entrapped in sol-gel silica to produce nanosensors capable of enhancing the weak chemiluminescence associated with lipid peroxidation.