Inner filter effect in fluorescence spectroscopy: As a problem and as a solution

This article discusses three aspects of inner filter effect (IFE) in fluorescence spectroscopy. (i) First, IFE as undesirable in fluorescence measurements: IFE results in non-linear fluorescence response of the analyte under study and it has been verified that IFE cannot be eliminated; it can either be minimized or corrected for intensity loss. Over the years, researchers have proposed many intensity correction methods to avoid IFE related issues. Often analysts using fluorescence spectroscopy, knowingly or unknowingly, ignore IFE or use an inappropriate intensity correction method. Herein, we have highlighted the basis and significances of various correction models that are proposed since 1970s to till date. (ii) Second, IFE mediated concentration dependent red shift (CDRS) as an analytical tool: the conventional fluorescence measurements and IFE correction strategies cannot be applied in analysis of optically dense multi-fluorophoric samples like oils, petrochemicals, biological samples and food samples etc. The strategy for exclusive inclusion of IFE and IFE induced CDRS as characteristics of the system for development of fluorescence based assay, towards maximizing fluorescence sensitivity of optically dense multi-fluorophoric systems, have been discussed. (iii) Third, IFE based sensing: when the sample contains chromophores, which absorb either at the excitation or at the emission wavelength range of the fluorophore, then the chromophores act as a filter. Thus tuning either the absorber or fluorophore concentration will lead to development of fluorescence based assay for a selective analyte. Principles and protocols are described to identify whether a sensing event is due to IFE or any other fluorescence mechanism. Additionally, a brief description is given on advanced findings and progresses made in sensing of for various classes of analytes in the recent past, using IFE concept. The second and third aspect combined together serve as a tool towards enhancing sensitivity of fluorescence measurement.