The bifunctional APC-Biotin conjugate demonstrates excellent avidin binding and strong fluorescence upon binding to a protein or nucleic acid target. It has avidin-binding properties similar to native biotin in terms of high affinity, fast association, and non-cooperative binding to avidin and streptavidin tetramers. APC-Biotin conjugate can be used for detecting and quantifying biotin binding sites of avidin, streptavidin or neutravidin. APC (allophycocyanin), isolated from Spirulina sp. blue-green algae, might be one of the brightest NIR fluorophores. The APC-Biotin conjugate is prepared from crosslinked APC, thus has higher stability.