Fabrication of microstructured binary polymer brush "corrals" with integral pH sensing for studies of proton transport in model membrane systemsElectronic supplementary information (ESI) available: Experimental section, together with additional experimental data including the evolution of dry brush ellipsometric thickness versus time of OEGMA grown from silicon wafers functionalized with CMPTS followed by irradiation and reaction with glycerol and 2-bromoisobutyryl bromide; XPS measurements for

• Main Authors: Madsen, J, Ducker, R. E, Al Jaf, O, Cartron, M. L, Alswieleh, A. M, Smith, C. H, Hunter, C. N, Armes, S. P, Leggett, G. J
• Format: Journal Article
• Language: English
• Published: 21.02.2018
• ISSN: 2041-6520; 2041-6539
• DOI: 10.1039/c7sc04424k

Binary brush structures consisting of poly(cysteine methacrylate) (PCysMA) "corrals" enclosed within poly(oligoethylene glycol methyl ether methacrylate) (POEGMA) "walls" are fabricated simply and efficiently using a two-step photochemical process. First, the C-Cl bonds of 4-(chloromethyl)phenylsilane monolayers are selectively converted into carboxylic acid groups by patterned exposure to UV light through a mask and POEGMA is grown from unmodified chlorinated regions by surface-initiated atom-transfer radical polymerisation (ATRP). Incorporation of a ratiometric fluorescent pH indicator, Nile Blue 2-(methacryloyloxy)ethyl carbamate (NBC), into the polymer brushes facilitates assessment of local changes in pH using a confocal laser scanning microscope with spectral resolution capability. Moreover, the dye label acts as a radical spin trap, enabling removal of halogen end-groups from the brushes via in situ dye addition during the polymerisation process. Second, an initiator is attached to the carboxylic acid-functionalised regions formed by UV photolysis in the patterning step, enabling growth of PCysMA brushes by ATRP. Transfer of the system to THF, a poor solvent for PCysMA, causes collapse of the PCysMA brushes. At the interface between the collapsed brush and solvent, selective derivatisation of amine groups is achieved by reaction with excess glutaraldehyde, facilitating attachment of aminobutyl(nitrile triacetic acid) (NTA). The PCysMA brush collapse is reversed on transfer to water, leaving it fully expanded but only functionalized at the brush-water interface. Following complexation of NTA with Ni 2+ , attachment of histidine-tagged proteorhodopsin and lipid deposition, light-activated transport of protons into the brush structure is demonstrated by measuring the ratiometric response of NBC in the POEGMA walls. Binary polymer brush microstructures incorporating ratiometric fluorescent pH indicators enable in situ studies of light-activated transmembrane proton transport by proteorhodopsin.