Polymer brushes provide an ideal platform for studying biofouling and screening anti-biofouling materials. In the present work, high-density poly(ionic liquid) brushes based on imidazolium salts were successfully grafted to surfaces via surface-initiated ring-opening metathesis polymerizations from catecholic initiator and their anti-microbial activity was evaluated. Very uniform poly(ionic liquid) coatings with thickness up to 80 nm were obtained on TiO₂. Various characterization techniques including infrared spectroscopy, X-ray photoelectron spectroscopy, thermogravimetric analysis and transmission electron microscopy were used to characterize poly(ionic liquid) brushes modified TiO₂ nanomaterials. Subsequently, the anti-bacterial and anti-biofouling properties of poly(ionic liquid) brushes were evaluated. It was found that poly(ionic liquid) brushes can obviously resist adhesion of Chlorella spores and the counter-anions have a key impact on the anti-microbial property. The hexafluorophosphate anion poly(ionic liquid) coated TiO₂ nanomaterials have excellent anti-bacterial properties compared to pristine TiO₂ nanoparticles against both E. coli and S. aureus.