Plant polyphenol-based coordination polymers (CPs) with ultra-small particle size and tailorable compositions are highly desired in biomedical applications,but their synthesis is still challenging due to the sophisticated coordination assembly process and unavoidable self-oxidation polymerization of polyphenol.Herein,a general ligand covalent-modification mediated coordination assembly strategy is proposed for the synthesis of water-dispersible CPs with tunable metal species (e.g.,Gd,Cu,Ni,Zn,Fe) and ultra-small diameter (8.6-37.8 nm) using nontoxic plant polyphenol (e.g.,tannic acid,gallic acid) as a polymerizable ligand.Polyphenol molecules react with formaldehyde firstly,which can effectively retard the oxidation induced self-polymerization of polyphenol and lead to the formation of metal ions containing CPs colloidal nanoparticles.These ultrafine nanoparticles with stably chelated metal ions are highly water dispersible and thus advantageous for bioimaging.As an example,ultra-small Gd contained CPs exhibit higher longitudinal relaxivity (r1 =25.5L mmol-1 s-1) value with low r2/r1 (1.19) than clinically used Magnevist (Gd-DTPA,r1 =3.7L mmol-1 s-1).Due to the enhanced permeability and retention effect,they can be further used as a positive contrast agent for T1-weighted MR imaging of tumour.