Direct Evidence for the Polymeric Nature of Polydopamine

Inspired by the adhesive proteins of mussels, polydopamine (pDA) has emerged as one of the most widely employed methods for functionalizing material surfaces, fueled in part by the versatility, simplicity, and spontaneity of pDA film deposition on most materials upon immersion in an alkaline aqueous solution of dopamine. However, the rapid adoption of pDA for surface modification over the last decade stands in stark contrast to the slow pace in understanding the composition of pDA. Numerous attempts to elucidate the formation mechanism and structure of this fascinating material have resulted in little consensus mainly due to the insoluble nature of pDA; which renders most conventional methods of polymer molecular weight characterization ineffective.([1]) Here, we employed the non-traditional approach of single molecule force spectroscopy (SMFS) to characterize pDA films. Retraction of a pDA coated cantilever from an oxide surface shows the characteristic features of a polymer with contour lengths up to 200nm. pDA polymers are generally weakly bound to the surface through much of their contour length, with occasional “sticky” points. Our findings represent the first direct evidence for the polymeric nature of pDA and provide a foundation upon which to understand and tailor its physicochemical properties.