Available online at www.sciencedirect.com Colloids and Surfaces B: Biointerfaces 62 (2008) 232–237 Spectral force analysis using atomic force microscopy reveals the importance of surface heterogeneity in bacterial and colloid adhesion to engineered surfaces Huilian Ma, Charles J. Winslow, Bruce E. Logan ∗ Department of Civil and Environmental Engineering, 212 Sackett Building, the Pennsylvania State University, University Park, PA 16802, USA Received 2 August 2007; received in revised form 9 October 2007; accepted 10 October 2007 Available online 22 October 2007 Abstract Coatings developed to reduce biofouling of engineered surfaces do not always perform as expected based on their native properties. One reason is that a relatively small number of highly adhesive sites, or the heterogeneity of the coated surface, may control the overall response of the system to initial bacterial deposition. It is shown here using an approach we call spectral force analysis (SFA), based on force volume imaging of the surface with atomic force microscopy, that the behavior of surfaces and coatings can be better understood relative to bacterial adhesion. The application of vapor deposited TiO 2 metal oxide increased bacterial and colloid adhesion, but coating the surface with silica oxide reduced adhesion in a manner consistent with SFA based on analysis of the “stickiest” sites. Application of a TiO 2 -based paint to a surface produced a relatively non-fouling surface. Addition of a hydrophilic layer coating to this surface should have decreased fouling. However, it was observed that this coating actually increased fouling. Using SFA it was shown that the reason for the increased adhesion of bacteria and particles to the hydrophilic layer was that the surface produced by this coating was highly heterogeneous, resulting in a small number of sites that created a stickier surface. These results show that while it is important to manufacture surfaces with coatings that are relatively non-adhesive to bacteria, it is also essential that these coatings have a highly uniform surface chemistry. © 2007 Elsevier B.V. All rights reserved. Keywords: Bacterial adhesion; Surface heterogeneity; AFM force volume imaging 1. Introduction Bacterial adhesion and subsequent cell growth that leads to biofouling is a common problem faced by various engineered systems, including window glasses [1], water distribution pipes [2–4], water treatment facilities [5,6], cooling towers [7], marine surfaces [8,9], medical device surfaces [10], to name a few. Hence, understanding the basis of bacterial adhesion, especially the initial adhesion process, is essential for taking appropri- ate measures to reduce biofouling. Once a bacterium reaches the vicinity of a target surface via transport, the likelihood of adhesion is potentially determined by the net interaction force between the bacterium and the surface. Conventionally, these forces are characterized based on the physicochemical and ther- ∗ Corresponding author. Tel.: +1 814 863 7908; fax: +1 814 863 7304. E-mail address: blogan@psu.edu (B.E. Logan). modynamic properties of the interacting surfaces, in terms of the average surface charge (or surface potential) and hydrophobic- ity (or contact angles), which are commonly used for (extended) DLVO calculations to assess adhesion [11–13]. The relative importance of these surface properties in governing adhesion has been confirmed from numerous studies [14–20]. For instance, it has been shown that hydrophilic surfaces are more resistant to bacterial adhesion than hydrophobic surfaces [19,20]. These findings have provided useful guidance in developing strate- gies to manufacture substrata with suitable surface properties to reduce biofouling in practical situations. Our primary interests here are to understand bacterial adhe- sion to glasses and coatings, as various coatings can be applied to glass surfaces to reduce fouling by modifying surface properties. Available approaches to reduce fouling include making the glass surfaces less prone to accumulation of bacteria (or other dirt) or making it easier to clean. In this respect, titanium dioxide (TiO 2 ) has gained considerable attention to date and been broadly used 0927-7765/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2007.10.007