Surface treatment of silicate based glass: base Piranha treatment versus 193nm laser processing J. Canning*, I. Petermann, K. Cook Interdisciplinary Photonics Laboratories, 222 Madsen Building F09, School of Chemistry, The University of Sydney, Camperdown, Sydney, NSW 2006 * john.canning@sydney.edu.au ABSTRACT Contact angle measurements of water on pathology grade borosilicate glass microscope slides before and after base piranha treatment are compared to treatment with 193nm laser irradiation. 193nm irradiation in the presence of hydrogen was also explored. Within experimental resolution, the observed changes in contact angle as a result of treatment either with base Piranha solution or with laser processing are identical. The contact angle, a, in both cases is reduced from a = (27 ± 6)º to a = (8 ± 3)º with treatment. However, for the piranha base method, there is an observed reversal over time either fully recovering or partially recovering within hours. By contrast, with laser processed, the increased surface wettability is retained with no change for more than 15 hours. In all cases, surface functionalisation, as measured by contact angle, with (3-mercaptopropyl)trimethoxysilane (MPTS) is found to be largely independent of any processing. We conclude that the method of contact angle as a means for qualitatively asserting improvements in attachment is unjustified. Keywords: Silica, surface functionalisation, contact angle, surface wetting, hydrophobicity, hydrophilicity, hydroxylation, sensing, biosensors 1. INTRODUCTION Surface functionalisation is proving to be one of the key challenges in enabling practical deployment of chemical and biological sensors, particularly for those based on optics where the impact of optical energy on the duration and survivability of the films can add other challenges. Despite considerable work on surface treatment and modification, only recently are methods becoming much more systematic in attempting to both qualitatively identify successful and robust surface functionalisation and to quantify the degree of this functionalisation. To date, it has often been automatically assumed in many instances that seemingly obvious and logical chemical interactions must lead to enhanced surface functionalisation. In addition to simply not always being verified, much of these have also not been tested for long term performance. Quantification of surface functionalisation often involves characterisation of molecular and nano-scaled films using time consuming and/or expensive tools or facilities, including fluorescence and confocal microscopy, impedance spectroscopy, atomic force microscopy (AFM), scanning electron and tunneling microscopy (SEM, STM) and so on - these methods often preclude any routine testing and therefore a great deal of recipes are inferred to occur by expectation and a measurable improvement in a desired (but indirectly relevant) result, such as enhanced detection of a particular species. In many cases it is simply assumed to work because the underlying chemistry suggests it should; however, the practical need to understand why reproducibility and sensitivity has not been so straightforward to demonstrate in practical sensors is calling for a greater emphasis on quantitative confirmation of many of these processes. A relative straightforward qualitative measure increasingly used is contact angle measurement, where a contact angle < 90º is characteristic of a hydrophilic surface whilst one > 90º is characteristic of a hydrophobic surface. An implicit assumption in this last approach is that there is a chemical change in the surface wettability (or hydrophobicity) when the contact angle changes after some treatment. For example, changes in contact angle was one tool used to confirm that there was surface modification of a polysaccharide based hydrogel, used in tissue engineering, when poly(L-lysine) was deposited onto the gel [1]. In general, the contact angle increases when an organic species is successfully attached to the surface regardless of substrate used. The reasons for this are either or both the increased chemical hydrophobicity of end groups (particularly relevant with silica and other materials capable of supporting electrical double layers generated by Third Asia Pacific Optical Sensors Conference, edited by John Canning, Gangding Peng, Proc. of SPIE Vol. 8351, 83512N · © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.915823 Proc. of SPIE Vol. 8351 83512N-1 Downloaded From: http://spiedigitallibrary.org/ on 03/18/2014 Terms of Use: http://spiedl.org/terms