title: Investigating the molecular mechanisms of in-plane mechanochemistry on cantilever arrays.
abstract: Free-standing cantilevers, which directly translate specific biochemical reactions into micromechanical motion, have recently attracted much attention as label-free biosensors and micro/nano robotic devices. To exploit this mechanochemical sensing technology, it is essential to develop a fundamental understanding of the origins of surface stress. Here we report a detailed study into the molecular basis of stress generation in aqueous environments focusing on the pH titration of model mercaptohexadecanoic acid self-assembled monolayers (SAMs), using in situ reference cantilevers coated with nonionizable hexadecanethiol SAMs. Semiautomated data analysis and a statistical model were developed to quantify cyclic deprotonation/protonation reactions on multiple arrays. In-plane force titrations were found to have the sensitivity to detect ionic hydrogen bond formation between protonated and nonprotonated carboxylic acid groups in the proximity of the surface pK1/2, which generated a mean tensile differential surface stress of +1.2 +/- 0.3 mN/m at pH 6.0, corresponding to 1 pN attractive force between two adjacent MHA molecules. Conversely, the magnitude of compressive differential surface stress was found to increase progressively with pH >/= 7.0, reaching a maximum of -14.5 +/- 0.5 mN/m at pH 9.0, attributed to enhanced electrostatic repulsion between deprotonated carboxylic acid groups. However, striking differences were observed in the micromechanical responses to different ionic strength and ion species present in the aqueous environment, highlighting the critical role of counter- and co-ions on surface stress. Our findings provide fundamental insights into the molecular mechanisms of in-plane mechanochemistry, which may be exploited for biosensing and nanoactuation applications.
authors.affiliation: London Centre for Nanotechnology and Department of Medicine, University College London, 5 University Street, London WC1E 6JJ, UK.
authors.forename: Moyu
authors.initials: M
authors.lastname: Watari
authors.forename: Jane
authors.initials: J
authors.lastname: Galbraith
authors.forename: Hans-Peter
authors.initials: HP
authors.lastname: Lang
authors.forename: Marilyne
authors.initials: M
authors.lastname: Sousa
authors.forename: Martin
authors.initials: M
authors.lastname: Hegner
authors.forename: Christoph
authors.initials: C
authors.lastname: Gerber
authors.forename: Mike A
authors.initials: MA
authors.lastname: Horton
authors.forename: Rachel A
authors.initials: RA
authors.lastname: McKendry
mesh_terms: D015374:Biosensing Techniques / Q000379:methods*; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D009994:Osmolar Concentration; D010169:Palmitic Acids / Q000737:chemistry*; D011522:Protons; D012680:Sensitivity and Specificity; D013997:Time Factors
chemical_list: C450319:16-mercaptohexadecanoic acid; D010169:Palmitic Acids; D011522:Protons