Magnetic-Actuation-Enhanced Indirect Photoacoustic Sensing for Real-Time Homogeneous Immunoassay

Photoacoustic (PA) sensing has garnered growing interest as a volumetric optical-acoustic transduction modality that mitigates optical scattering and enables sensitive detection in complex matrices. However, conventional PA assays rely on diffusion-dominated biochemical reaction kinetics and endpoint-based signal acquisition, which limit their capacity for rapid and real-time monitoring. Here, we report a magnetic-actuation-enhanced indirect PA sensing platform that integrates rotating magnetic fields (RMFs) with real-time PA detection to accelerate and continuously monitor operationally homogeneous immunoassays (without separation or washing steps). Functionalized magnetic nanoparticles (MNPs) were actuated by the RMF into dynamic chained or clustered structures, thereby enhancing collision frequency with target analytes and promoting target-induced aggregation. The resultant MNP aggregates caused interparticle light shielding, enhanced light scattering of the particle system, and underwent rapid sedimentation, thereby modulating the PA signal amplitude, which enabled continuous volumetric PA monitoring. Using cardiac troponin I as a clinical target, the platform achieved detection limits of 0.9 pM (22 pg/mL) in buffer and 2 pM (48 pg/mL) in 50% serum, outperforming conventional passive or endpoint assays. Clinical serum validation showed strong concordance with a clinical reference method, underscoring the platform's utility in point-of-care diagnostics.