Cavity-Amplified Scattering Spectroscopy Reveals the Dynamics of Proteins and Nanoparticles in Quasi-transparent and Miniature Samples

The ability to measure the size and distribution profile of molecules and particles is of great importance in physics, biology, and chemistry. In order to achieve this goal, it is important to use highly efficient light scattering methods, especially those that can allow particle sizing up to nanoscale level. Among them, dynamic light scattering (DLS) technique is widely used to probe the material dynamics in numerous research and industrial applications. Such applications include particle sizing, characterization of viscoelastic properties of materials and measurement of thermal motion spectrum and distribution of colloidal particles.

The Brownian motion of molecules or particles in suspension causes the scattering of the laser light at different intensities, which can be analyzed to obtain Brownian motion velocity, allowing determination of particle sizing and distribution using Stokes-Einsten relationships. Another method, diffusing wave spectroscopy (DWS), is often used to overcome the limitations of DLS, especially for applications involving high scattering in nonergodic media. Unfortunately, both DWS and DLS fail to accurately perform such important measurements and assess motion spectrum when samples fail to scatter adequate light due to very small scattering cross sections or low concentrations of scatterers.

To overcome these limitations, Professor Francois Amblard from Ulsan National Institute of Science and Technology in collaboration with Dr. Guillaume Graciani and Dr. John T. King from Seoul National University developed a different approach for amplifying the light scattering efficiency. Hence, cavity-amplified scattering spectroscopy (CASS) was developed. Its feasibility was validated by being used to reveal the dynamics of nanoparticles and proteins in quasi-transparent and miniature samples. Their work is currently published in the research journal, ACS Nano.

In their approach, weakly scattering samples were embedded inside a closed high-albedo Lambertian cavity to obtain improved amplification. The cavity was injected with highly coherent laser light to produce a three-dimensional (3D) isotropic and homogeneous light field. The cavity wall functioned to reflect light forcing back to the sample thousands of times to expand the average photon path while maintaining it shorter than the coherence light. Leveraging the design and setup of the CASS method, it was used to study the fast dynamics of dilute samples, and perform measurements at concentration conditions lower than measurement conditions of typical DLS. Thus, it was possible to study colloidal objects with low scattering efficiencies.

The research team compared the performance of the presented CAS method to classical techniques like DLS and DWS. The authors showed that the proposed CASS method elongated the photon scattering path length through the sample by 2 to 3 orders of magnitude. Due to this elongation and efficient amplification, the sensitivity limits of DWS and DLS were significantly increased by 10⁴-fold compared to classical techniques. Additionally, CASS could be used to measure the short-time dynamics of low-turbidity systems.

The sensitivity gain was demonstrated by measuring the diffusion coefficient as well as the particle sizes ranging from 20 μm to 5 nm with very low volume fractions and solvent refractive index mismatches of approximately 0.01. In addition to current applications of light scattering techniques, it was worth noting that CASS method represented a major milestone towards the study of short-time dynamics problems like nanoparticles, proteins, and ballistic limit of Brownian motion.

In summary, a miniaturized CASS setup was described. Its advantages include enhanced efficiency, enhanced sensitivity and reduce probe volume. Due to these benefits, light scattering applications could be potentially extended to miniaturized microfluidic samples as well as weekly scattering samples. In a statement to Advances in Engineering, the lead and corresponding author, Professor Francois Amblard explained the presented CASS setup is a promising method for performing light scattering in conjunction with microfluidics for extended environmental and industrial applications.