Development of a Lensless Compact Optical System for Ultrasensitive Nanomaterial Detection in Liquid Media via Exploiting Multiple Light Scattering.

Accurate detection of nanoparticles in liquids is essential across drug delivery, diagnostics, and environmental monitoring. Dynamic light scattering (DLS), the current gold-standard technique, relies on single-scattering events and requires prior knowledge of sample temperature and optical properties, limiting its sensitivity at low particle concentrations and increasing system cost and complexity. Here, we present a lensless, cost-effective speckle-based method that exploits multiple light scattering to overcome these limitations. A custom-machined aluminum sample holder uniformly illuminates the entire sample volume with a randomized coherent light field via repeated diffuse reflections. Subtle nanoparticle motions modulate the phase of the scattered light, producing time-varying speckle patterns captured by a CMOS camera without any imaging optics. Temporal correlation analysis of the speckle time series quantifies nanoparticle dynamics without requiring prior sample information. The method detects nanoparticles at concentrations as low as 10