CXCR4 and neoplasm: In this study, a CXC ligand 12 (CXCL12)-derived cyclic peptide (sequence motif cyclo[K(1,5-pentanedioic acid)R-(2-Nal)-GY]) functionalized NaGdF4 NDs (termed as anti-CXCR4-NaGdF4 NDs) were synthesized for MRI-guided biotherapy of tumor through immobilization of the CXCL12-derived cyclic peptide on tryptone coated 5 nm NaGdF4 NDs (termed as Try-NaGdF4 NDs) via covalent bond.