Microbubble nano-interrogation using the atomic force microscope

Abstract

The Atomic Force Microscope (AFM) is a versatile tool that provides spatial and force resolution of the order of Angstroms and sub- nanonewtons, respectively; therefore, it is ideally suited to study morphology and mechanical properties of materials at the nanometer scale. This paper introduces the use of the AFM to study the topography and the mechanical properties of individual biSphere ® microbubbles (Point Biomedical Corp, San Carlos, CA, USA) microbubbles. The Bioscope AFM (Veeco, Santa Barbara, CA, USA) was used for the measurements: a) Tapping mode AFM was used for imaging, with cantilevers resonating at 5-8 kHz (NP-20, Veeco). b) Contact mode AFM was used for mechanical measurements using tipless cantilevers (NSC-20, MikroMash, Spain) with spring constants ranging from 0.03-15 N/m. The resolution of the topography of the microbubble was as small as 20 nm. Such figures are comparable with data from electron microscopy, but AFM provides superior flexibility as it can interrogate microbubbles in various chemical and physical environments (in liquid at a range of different temperatures, pressures and viscosities). Optimal measurements of the stiffness of microbubbles ranging in size from 1-10 µm was provided by cantilevers between 0.3-0.6 N/m as cantilevers with lower spring constant were too soft for the measurements, and higher spring constant would destroy the microbubbles. The stiffness (effective spring constant) of the microbubbles ranged between 1 and 5 N/m. These measurements can be translated to shell properties by using an appropriate model for the shape and structure of the microbubbles. I. INTRODUCTION The progress of microbubble research has been slower than originally envisaged, partly due to its evolution into a multidiscipline scientific area. Perhaps the most significant obstacle in the development of microbubble technology remains the lack of understanding of microbubble behaviour. The introduction of fast acquisition optical imaging (1-4) and single microbubble scatter studies are steps towards improving that knowledge (5-6). However, neither optical nor acoustical techniques can assess microbubble physical parameters. The introduction of atomic force microscopy (AFM) (7) has provided an unprecedented spatial and force resolution of the order of Angstroms and sub-nanonewtons respectively. It is therefore ideally suited to study the morphology and mechanical properties of materials at the nanometer scale. We propose here the use of the AFM to nano-interrogate microbubbles.