The present disclosure relates to compounds that are useful as near-infrared fluorescence probes, wherein the compounds include i) a pteroyl ligand that binds to a target receptor protein, ii) a dye molecule, and iii) a linker molecule that comprises an amino acid or derivative thereof. The disclosure further describes methods and compositions for incorporating the compounds as used for the targeted imaging of tumors. Conjugation of the amino acid linking groups increase specificity and detection of the compound. Methods and compositions for use thereof in diagnostic imaging are contemplated.