Electrochemistry provides better understanding of polymer brushes as smart coatings

Functional polymer brushes have a tremendous interest for surface engineering, thanks to their stimuli-responsive behavior, allowing applications in the field of switchable wettability/adhesion processes, controlled release and sensor development. However, surface-attached polymer brushes are very difficult to characterize with standard techniques like GPC and NMR due to the very small amount of polymer grafted (~0.01 mg cm-2). Thus, more sophisticated (and expensive) techniques, i.e. XPS, are needed in order to achieve a satisfactory knowledge of the sample under study. In this work [1] we demonstrate how grafted-from hydrophilic polymer brushes can solve the problem of the development of on-chip electrochemical microsensors, impaired by the electrochemical inertness of native oxide-coated silicon wafer. Silicon substrates were functionalized with the alkoxysilane-type BIB-APTES initiator. Polymer brushes with different composition were then grown by SI-ARGET ATRP with different feed ratios of the monomers 2-hydroxyethyl methacrylate (HEMA) and 2-aminoethyl methacrylate hydrochloride (AMA). Both homopolymer (PHEMA, PAMA) and copolymer (PHEMA-co-PAMA 80:20, PHEMA-co-PAMA 50:50) and micropatterned (by using remote photocatalytic lithography[2]) brushes were obtained and characterized. Every step of the grafting procedure, from surface pretreatment to functionalization with initiator and eventually the grafted brushes, was studied using electrochemical techniques, in particular electrochemical impedance spectroscopy (EIS). In this way, the homogeneity and density of the initiator layer and of the resulting brushes, their composition and thickness and also the post-functionalization reactions could be easily investigated. Brushes with different loading of cationic groups could be differentiated through their marked reactions to an anionic redox probe. Noteworthy, the brushes were grown on silicon substrate which is an atypical electrode material due to its very poor electrochemical response. Grafted-from brushes allowed the reaction of ferrocyanide at the silicon surface, behaving as «tentacles» to capture the redox probe and keep it in proximity of the silicon surface. Micropatterning was found to improve the electrochemical behavior of the system. Our results thus pave the way for the development of on-chip electrochemical devices and sensors. Applications for drug-delivery and microfluidics can be envisaged as well. REFERENCES [1] G. Panzarasa, G. Soliveri, V. Pifferi, J. Mater. Chem. C, Accepted. [2] G. Panzarasa, G. Soliveri, K. Sparnacci, S. Ardizzone, Chem. Commun., 51, 2015, 7313–7316.