The present invention relates to precursor molecules for the synthesis of D-luciferin, or blocked D-luciferin and derivatives thereof, which are functionalized in different positions. Further, the use of functionalized precursor molecules for the synthesis of D-luciferin or blocked D-luciferin and methods of synthesising blocked D-luciferin and D-luciferin or derivatives thereof are described. Also described is a kit of parts for screening assay comprising said functionalized precursor molecules to D-luciferin or blocked D-luciferin and derivatives thereof and the uses of such a screening assay for the detection of molecules for the study of molecular uptake and for the detection of a reducing environment. Methods of detection of bio- molecules such as metabolites, activity of enzymes and proteases and methods for determining a sequence within a peptide of protein cleaved by specific enzymes are described. Also a method of determining the cysteine and derivatives thereof concentration in a cell or tissue sample is described. Precursor molecules of D-luciferin according to the invention are advantageous as they are small and travel easily around live systems and allow tissue cells or organs to be targeted specifically. Further, the methods and screening assays described allow the simultaneous study of multiple biological processes described above in one experiment only.