tris-NTA is a His-tagged protein ligand.1,2 It has been used alone, or conjugated to fluorophores, for the purification and detection of His-tagged proteins.
References:
[1]. Huang, Z., Hwang, P., Watson, D.S., et al.Tris-nitrilotriacetic acids of subnanomolar affinity toward hexahistidine tagged moleculesBioconjug. Chem.20(8)1667-1672(2009).
[2]. Glembockyte, V., Wieneke, R., Gatterdam, K., et al.Tris-N-nitrilotriacetic acid fluorophore as a self-healing dye for single-molecule fluorescence imagingJ. Am. Chem. Soc.140(35)11006-11012(2018).