Surface Directed Growth of a Stable Free Radical Polymer Layer

Abstract

Molecular radicals such as nitric oxide (NO) play a role in numerous important biological processes. NO, however, is inherently unstable, and there is considerable interest in stable radicals with analogous behaviour, such as (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO), and their potential as biologically active surface coatings. Here we show that it is possible to grow stable TEMPO monolayers with an ordered arrangement and specific orientation of the nitroxide group. A combination of high-resolution atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations reveal that strong dipole-dipole interactions between neighbouring TEMPO molecules determine their orientation, resulting in an anti-parallel arrangement with upwards and downwards facing nitroxide groups. Surface coating is made possible using plasma polymerisation, a surface molecular engineering technique suitable for growing functional coatings of highly stable organic molecules. Typically, plasma polymerisation is considered a surface-independent process, with the plasma power and pressure dominating the deposited film's properties. We therefore test this assumption and its impact on TEMPO layers by studying initial stage growth on multiple material surfaces. We show that whilst plasma polymer growth creates ordered layers on gold and graphite surfaces, there is substantial substrate-dependence with less ordered growth on other materials up to a film thickness of 30 nm, suggesting variation in molecular packing and retention. Beyond that thickness, films convergence to a flat, uniform, surface-agnostic structure. These findings establish the utility of plasma polymerisation as a method for ordered growth and demonstrate the ability to direct the orientation of TEMPO molecules and NO free radicals within a thin film.