Label-free detection of early-stage protein aggregation using a fiber-optic mechanical resonant probe.

Early detection of protein aggregates offers valuable insights into the pathophysiology of a variety of neurodegenerative diseases. However, fluorescence methods, which serve as the "golden standard" for detecting protein aggregation, rely on fluorophores that bind to aggregated proteins, potentially interfering with the aggregation process. Here, we propose a fiber-optic mechanical resonant probe (FOMRP) for the label-free detection of early-stage protein aggregation by monitoring the mechanical properties of the protein solution. The FOMRP was fabricated by a tapered optical fiber and was driven by a piezoelectric actuator. The mechanical vibration of FOMRP was monitored with a fiber Bragg grating. We found that the FOMRP supports a series of nonlinear mechanical vibrations. We investigated the energy transfer mechanism between the fundamental and harmonic vibration modes. The 1/2 order subharmonic vibration was found to be highly sensitive to the external environment and was subsequently employed for viscosity measurement. The FOMRP was further applied to monitor early-stage protein aggregation, showing higher sensitivity than the conventional fluorescence method. This approach provides a promising tool for label-free protein aggregation monitoring and has potential applications in molecular and cellular mechano-assays.