Features contributing to fluorescence of molecules have been discussed earlier. In contrast to organic species fewer inorganic materials show natural fluorescence. The present article discusses some processes that can help in inducing fluorescence to otherwise non- fluorescent inorganic species.
Inorganic species can be grouped under three main groups with regards to fluorescence behavior:
Lanthanides such as Ce,Pr,Nd and Uranium compounds are endowed with natural fluorescence and can be estimated in solution directly using fluorimetric techniques. Such species are fluorescent due to transitions of electrons from d to f shells or f to f shells. The f to f shell transitions result in sharp fluorescence transitions.
Reaction with Inorganic Reagents
Metals like Sn, Pb,Sb, Bi on reaction with mineral acids at low temperature like HCl or HBr exhibit fluorescence behavior \(Na_2WO_4\) can infuse fluorescence behavior in lanthanides in acidic range whereas phosphoric acid can enhance fluorescence of Uranyl compounds. Interference from other species in sample matrix can lead to fluorescence quenching and should be removed prior to analysis..
Reaction with Organic Reagents
Complexation reaction with organic species is the most popular approach for introducing fluorescence. Most reagents are capable of chelate formation or a ring structure with the metal ion.
Strongly interfering species can be removed from the sample matrix by extraction of active ingredient prior to analysis using solvent extraction techniques.
A number of normally non-fluorescent organic molecules combine with inorganic metal ions resulting in fluorescent products Some examples of such organic compounds commonly used for determination of metal ion concentrations are Oxine, Rhodamine, 8-Hydroxyquinoline, etc.
Combination of one ion with one ligand group results in a primary complex, for example,complex formation of Al with alizarin. When a metal ion combines with two ligands a ternary complex is formed, eg, Sn with rhodamine.
Certain anions such as \(CN^-\), \(F^-\) or \(S^2^-\) can be analysed on substitution reactions with a complexing agent.The anion reacts with the central metal atom of the metal-ligand complex and sets free the ligand thereby quenching the fluorescence of the metal-ligand complex. The degree of quenching is an indirect measure of the anion concentration.
The above mentioned approaches can be helpful in imparting fluorescent properties to normally non-fluorescent inorganic species and render their high sensitivity determinations in complex inorganic matrices such as minerals.