Non-invasive high-resolution acoustic microscopy technique using embedded nanostructures
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Non-invasive high-resolution acoustic microscopy technique using embedded nanostructures Daniel Wulin, and Shriram Ramanathan SEAS, Harvard University, 29 Oxford St, Cambridge, MA, 02138 ABSTRACT An opto-acoustic system capable of operating at frequencies greater than 1 GHz with novel biological applications is proposed. Metallic spheres with radii on the order of hundreds of nanometers dispersed inside a bio-matrix can be used to generate in-situ ultra-high frequency acoustic waves whose normal mode frequencies can be calculated using Lambís theory for acoustic oscillations of elastic spheres. The frequency and amplitude of the resulting acoustic waves can be related to the physical properties of the metallic spheres and the surrounding biomatrix: the acoustic waves produced by the metallic spheres are well-suited to high resolution acoustic imaging. We anticipate that our approach will open up new nanoscale techniques to study cells non-invasively. INTRODUCTION Acoustic microscopy has several demonstrated applications in materials science and biology [1,2]. Unlike conventional scanning acoustic microscopy that utilizes external transducers to generate the ultrasound probes, optoacoustic systems are non-invasive and biological samples can be probed nondestructively at a variety of depths. Certain opto-acoustic systems offer the possibility of real-time imaging [1-3]. Typical ultrasound biomicroscopy applications operate in the 40-60 MHz regime: maximum resolution correspondingly of the order of microns [3,5]. This is due to drastic attenuation of high frequency components of ultrasound waves in liquid media [4]. The potential of gigahertz (GHz)-range ultrasound biomicroscopy is enormous. Noninvasive nanometer scale imaging would offer a new and powerful tool to investigate biological samples at the cellular level. Understanding the relationship between a cellís structural and functional properties yields critical information: the efficiency of drug treatments, the evolution of disease states, and even the existence of disease in a cell [6]. For example, the mechanical properties of cells are of critical importance because they affect the functioning of the cell and can vary between healthy and diseased cells [6,7]. GHz range acoustic biomicroscopy would extend nondestructive imaging of cellular structure to the nanometer scale for the first time and open up new directions for biological imaging research. In this study, metallic particles with radii on the order of tens to hundreds of nanometers are considered as potential sources of high frequency acoustic waves to probe and image biological samples. Typical cells and cell structures range between on tens of nano meters and hundreds of micrometers: these length scales are specifically considered in our approach with applications on the cellular scale in mind. This new approach offers the possibility of generating very high frequency acoustic pulses very close (or dispersed within) to the sample being investigated, which minimizes the distance that the
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