Dual-functional poly(2-methyl-2-oxazoline)/poly(2-(dimethylamine)ethyl methacrylate) mixed brushes with switchable protein adsorption and antibacterial properties

Abstract

Herein, binary mixed brushes consisting of poly(2-methyl-2-oxazoline) (PMOXA) and poly(2-(dimethylamine)ethyl methacrylate) (PDMAEMA) with different chain lengths were fabricated by successive grafting of NH2-terminated PMOXA and SH-terminated PDMAEMA onto polydopamine-anchored substrates. The mixed-brush coating was characterized by variable-angle spectroscopic ellipsometry, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, zeta potential measurements, water contact angle, and atomic force microscopy. The mixed brushes showed tunable surface charge, wettability, and surface roughness, depending on the degree of PDMAEMA swelling under varying pH and ionic strength (I). Then the adsorption behaviors of pepsin, bovine serum albumin (BSA), γ-globulin, and lysozyme, four very different proteins with regard to isoelectric point, on the mixed brushes coating were studied by using fluorescence microscopy and surface plasmon resonance. When the chain length of PDMAEMA was about twice as long as PMOXA, the mixed brushes not only had high adsorption capacity for pepsin, BSA, and γ-globulin but also had a desorption efficiency of 86.9%, 87.1%, and 93.5%, respectively. It is explained that electrostatic attraction between the protonated PDMAEMA and positively charged acidic proteins (pepsin and BSA, whose isoelectric points were below the pKa of PDMAEMA) would drive the intensive adsorption (at pH 3, I = 10−3 mol·L−1 for pepsin, and pH 5, I = 10−5 mol·L−1 for BSA), while desorption was dominated by the hydrophilic PMOXA when PDMAEMA was shrinking (at pH 7, I = 10−1 mol·L−1 for pepsin, and pH 9, I = 10−1 mol·L−1 for BSA). Furthermore, the isoelectric precipitation led to the adsorption of neutral protein (γ-globulin, whose isoelectric point was near the pKa of PDMAEMA) at pH 7, I = 10−5 mol·L−1, while electrostatic repulsion and antifouling PMOXA triggered the desorption of γ-globulin at pH 3, I = 10−1 mol·L−1. However, alkaline protein (lysozyme, whose isoelectric point was higher than the pKa of PDMAEMA) exhibited slight adsorption on PMOXA/PDMAEMA mixed brushes under test conditions, regardless of whether PMOXA or PDMAEMA occupied the outermost layer. The antibacterial property of the mixed brushes against Escherichia coli was investigated. PMOXA/PDMAEMA mixed brushes showed significant bactericidal activity at pH 3, I = 10−3 mol·L−1, while the rinse of pH 9, I = 10−1 mol·L−1 solution could remove most of the residual bacteria. This work not only enables controlled adsorption of proteins with different isoelectric points but also ensures that the surface of the coating is minimized from bacterial contamination.