An α-D-mannose ligand equipped with a terminal carboxylic acid through a hydrophilic PEG3 spacer. The carboxylic acid can be activated to NHS, sulfo-NHS, or pentafluorophenyl esters and reacted with amine-functionalized platforms or lysine residues on proteins and peptides to form amide bonds. This compound provides an α-D-mannose building block for glycoconjugate synthesis and the construction of probes for mannose-recognizing receptors and lectins, including the mannose receptor (CD206) and DC-SIGN, with relevance to immune-targeting and pathogen-recognition research.