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Fluorescence quenching mechanism of tryptophan (Trp) by norfloxacin (NORF) was investigated by spectrofluorometric method at physiological pH. Stern−Volmer quenching constant is positively related with temperature shows the dynamic type of quenching. Life time of Trp decreases by the addition of NORF supports dynamic type of quenching. Fluorescence quenching method was successfully applied for direct determination of NORF from pharmaceutical sample.

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Fluorescence resonance energy transfer (FRET) between tryptophan (Trp) and quinine sulphate (QS) has been investigated in aqueous medium at room temperature by absorption, emission, and time resolved fluorescence spectroscopy. Absorbance spectral characteristics rule out the possibility of ground state complex formation between them. Time resolved decay indicates dynamic type of quenching. Saturation of efficiency of energy transfer reveals the existence of FRET. Efficiency of energy transfer and distance between donor and acceptor pair were determined according to Förster's non radiative energy transfer theory.

The high intake of antitubercular isonicotinyl hydrazide (INH) is hazardous to human body. Hence, there is a need for its detection. For this reason, we study interaction between human hemoglobin (HHb) and INH by applying fluorescence and UV–vis spectroscopic techniques. Fluorimetric as well as UV–vis results showed the formation of complex between HHb and INH. The proposed HHb‐based fluorescence method was applied directly for the determination of INH in pharmaceutical sample.