Polarized Light Based Photoacoustics for Deep-Tissue Chiral Biomolecular Sensing

Abstract

Sensing methodologies are indispensable for detecting and quantifying physical, chemical, and biological parameters with precision, especially for biomedical applications. These techniques enable early disease detection, real-time physiological monitoring, and personalized healthcare, significantly improving patient outcomes. Among these, photoacoustic sensing, which converts optical energy into ultrasound signals could enable deep tissue biomolecular sensing. This thesis presents an innovative approach that combines polarized light, photoacoustic sensing, and microfluidic technologies for biomolecular detection, focusing on chiral biomolecules that are optically active. Near Infra-red-II (NIR-II) wavelengths (1000-1700nm) are known for less optical scattering compared to visible wavelength regimes and offers deep tissue penetration. Interference from scattering effects can be avoided while using NIR-II wavelengths. Integration of polarization and photoacoustics offered the advantages of detecting from deeper tissues with less scattering interference that will normally arises from pure optical methods. Chiral molecule sensing is typically performed using techniques like chromatography, electrophoresis, enzymatic assays, mass spectrometry, and chiroptical methods. While polarimetry allows for in-vivo sensing up to 1 mm depth using UV-visible light, it is limited by dominant light scattering beyond this depth. We propose that photoacoustic sensing in the Near-Infrared (NIR)-II window can enable deep tissue sensing as acoustic waves scatter less than light. To achieve this, we developed a Photoacoustic Polarization Enhanced Optical Rotation Sensing (PAPEORS) system, capable of estimating optical rotation from photoacoustic signals and correlating it with chiral molecular concentration for depths up to 3.5 mm. We also analyzed the optical rotation estimation using different polarization illumination’s with photoacoustic sensing. Non-invasive potential of the system was also demonstrated with preliminary in-vivo experiments. Our study concluded that PAPEORS holds promise for in-vivo sensing and easy miniaturization utilizing single wavelength. Non-invasive glucose sensing presents significant challenges due to the high scattering and complex optical properties of biological tissues. To address this, we explored evaluation of depth-dependent photoacoustic signals at the the Near-Infrared (NIR)-II window, leveraging different polarization states—vertical, 45° linear, and circular—alongside Monte Carlo simulations. This approach revealed an optimal sensing depth of 3–3.2 mm, where maximum optical rotation and strong linear correlations with glucose concentration was observed. These findings establish a foundation for precise, non-invasive glucose detection while also providing a calibration framework applicable to other chiral biomolecules. To further understand the wavelength-dependent behaviour of optical rotation, we extended our investigation to Photoacoustic Circular Dichroism (PACD) spectroscopy in the NIR-II range (1400–1600 nm) for D-glucose. By analysing optical rotation, circular dichroism, and optical rotary dispersion, we demonstrated the system’s ability to probe the chiral nature and optical activity of glucose molecules. The differential absorption of left- and right-circularly polarized light in serum samples validated PACD as a viable tool for biomolecular characterization. Building on these insights, we developed an innovative opto-acoustic microfluidic glucose sensing system that integrated polarized light incidence with photoacoustic detection. Designed to mimic blood vessel dimensions, this microfluidic platform enabled precise optical rotation measurements in serum-like and human blood samples. Proofof- concept studies involving diabetic and healthy volunteers achieved an 88% prediction accuracy, highlighting the potential of this system for real-time, non-invasive glucose monitoring. In summary, the thesis introduced advanced photoacoustic sensing methodologies that combine polarized light, optical activity, and microfluidic integration for deeptissue biomolecular detection. These innovations would pave the way for real-time, costeffective, and highly accurate diagnostic tools, offering new possibilities for personalized healthcare and improved disease management.