Magnetically Controlled Reversible Photomagnetic Nanoactuators for Dynamic Contrast Enhancement in Optical Coherence Tomography.

Optical coherence tomography (OCT) is a high-resolution imaging modality that detects scattered optical signals from light-tissue interaction. However, the ability of OCT to visualize specific molecular and cellular events is hindered by the lack of effective exogenous contrast agents capable of producing noticeable imaging contrast. Here, photomagnetic nanoactuators with ≈90% scattering efficiency in the second near-infrared window, capable of reversible magnetic field-mediated modulation of optical scattering and, therefore, OCT signal, are proposed. These nanoactuators consist of a magnetite core and gold shell, functioning as a reversible magnetic actuator and an optical scatterer, respectively. When exposed to the external magnetic field, photomagnetic nanoactuators are assembled into chain structures via magnetostatic interactions between nearby nanoactuators, reducing their optical scattering and, consequently, the OCT signal. Upon removal of the magnetic field, the nanoactuators are disassembled, restoring their scattering property and OCT signal. It is demonstrated that this magnetically controlled OCT response allows enhanced particle-associated signal detection with suppressed background signal via image subtraction before and after magnetic field treatment, both in vitro and in vivo. The developed nanoactuator platform offers a strategy for dynamic enhancement of OCT contrast, potentially broadening the utility of OCT for noninvasive cell tracking and molecular diagnostic imaging.