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Label-Free Nanoscale Photoacoustic Tomography (nPAT) for Single Cell Imaging

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P Samant

P Samant1*, A Hernandez1 , S Conklin1 , K Frazer2 , L Xiang1 , (1) University of Oklahoma, Norman, OK, (2) University of Oklahoma Health Sciences Center, Oklahoma City, OK


MO-AB-FS4-1 (Monday, July 31, 2017) 7:30 AM - 9:30 AM Room: Four Seasons 4

Purpose: Super-resolution microscopy has made invaluable strides recently, providing novel avenues with which optical imaging entered the nanoscale. However, there does not yet exist a 3D label-free imaging modality that can image single cells at nanoscale resolutions beyond the diffraction limit. The purpose of this study is to introduce nanoscale photoacoustic tomography, a novel imaging modality that obtains optical absorption with high frequency ultrasound resolution in order to enable label-free 3D axial super-resolution imaging.

Methods: Theoretical models were first built to ensure that a strong photoacoustic signal could be generated without inducing a high temperature rise in a biological sample (red blood cell). Next, a photoacoustic imaging system was built from an ultrashort (7ps) pulsed laser (532nm), two photodiode detectors, an optical delay line, and beam geometry/polarization components. The laser set-up featured a pump-probe imaging motif with a chopped pump beam and confocal pump and probe beams. The system was configured to shine a focused pump beam on a thin steel sample (10┬Ám) in order to obtain the first signal before proceeding to imaging of biological samples. Simultaneously, theoretical analysis was performed to estimate the resolution, sensitivity, and penetration depth of the system.

Results: The theoretical analysis demonstrated the thermal safety of the system, the system was also calculated to be capable of detecting an acoustic pressure wave at a minimum sensitivity of 25Pa. The resolution of the system, calculated via use of a point spread function of an absorber with linear thermal expansion, was ~10nm in the axial direction. In experiment, the system was able to obtain a detectable and repeatable signal on the order of GHz.

Conclusion: nPAT allows for label-free high-resolution imaging in biological samples. We demonstrate via theoretical calculation and experimental data that this method has the potential for nanometer level resolution obtains optical absorption contrast.

Funding Support, Disclosures, and Conflict of Interest: The authors gratefully acknowledge the University of Oklahoma Research Council and College of Engineering, and the IBEST-OUHSC seed grant at The University of Oklahoma for funding.

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