Photoacoustic Circular Dichroism as a Window Into Wavelength-Dependent Chiroptical Effects

Optical activity of chiral materials is a crucial parameter for advancing our knowledge of light-smatter interactions and for driving innovations in optical device technology. Conventional chiroptical spectroscopy methods, such as circular dichroism (CD), are widely used to probe biomolecular structures in the UV-Visible range but often suffer from scattering and diffraction limitations. In contrast, photoacoustics convert absorbed optical energy into acoustic signals, thereby offering enhanced sensitivity and deeper penetration when employed in the near-infrared (NIR) region. We investigate the photoacoustic circular dichroism (PACD) response of optically active D-glucose in the NIR-II (1400-1600 nm) range and Naproxen, a NSAID drug (in the 1300-1600 nm), using a microfluidic device with a 700 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\mu$</tex-math></inline-formula>m channel and sample volume of 0.3mL. PACD from our system demonstrated measurable phase changes at glucose concentrations as low as 120 mg/dL and enabled sensing across practical depths of 1-2 mm. We further introduced a metric termed Circular Differential Absorption Intensity (CDAI), which provides a more robust photoacoustic-based circular dichroism spectrum. Furthermore, optical rotary dispersion (ORD) was derived from CDAI, offering wavelength-resolved optical rotation data. This is the first demonstration of optical rotary dispersion (ORD) derived from photoacoustic measurements, enabling deeper spectral insights into chiral systems. The integration of optical and acoustic methodologies not only advances our understanding of chiral systems but also opens new avenues for biomolecular characterization, and lab-on-a-chip diagnostics and sensing applications.