Leveraging Partial Coherence to Enhance Nanoparticle Detection Sensitivity and Throughput in Interferometric Scattering Microscopy

Interferometric-based microscopies stand as powerful label-free approaches for monitoring and characterizing chemical reactions and heterogeneous nanoparticle systems in real time with single-particle sensitivity. Nevertheless, coherent artifacts, such as speckle and parasitic interferences, together with limited photon fluxes from spatially incoherent sources, pose an ongoing challenge in achieving both high sensitivity and throughput. In this study, we systematically characterize how partial coherence affects the signal contrast and background noise level in inline holography microscopes operated in a reflection geometry, a category that encompasses interferometric scattering microscopy (iSCAT). This approach offers a route to improve the signal-to-noise ratio in the detection of single nanoparticles (NPs), irrespective of their size and composition or the light source used. We first validate that lasers can be modified into partially coherent sources with performance matching that of spatially incoherent ones while providing higher photon fluxes. Second, we demonstrate that tuning the degree of partial coherence not only enhances the detection sensitivity of both synthetic and biological NPs but also affects how signal contrasts vary as a function of the focus position. Finally, we apply our findings to single-protein detection, confirming that these principles extend to differential imaging modalities, which deliver the highest sensitivity. Our results address a critical milestone in the detection of weakly scattering NPs in complex matrices, with wide-ranging applications in biotechnology, nanotechnology, chemical synthesis, and biosensing, ushering in a new generation of microscopes that push both the sensitivity and throughput boundaries without requiring beam scanning.