Metal-Capped Brownian and Magnetically Modulated Optical Nanoprobes (MOONs): Micromechanics in Chemical and Biological Microenvironments

Abstract

Modulated optical nanoprobes (MOONs) are microscopic (spherical and aspherical) fluorescent particles designed to emit varying intensities of light in a manner that depends on particle orientation. MOONs can be prepared over a broad size range, allowing them to be tailored to applications including intracellular sensors, using submicrometer MOONs, and immunoassays, using 1−10 μm MOONs. When particle orientation is controlled remotely, using magnetic fields (MagMOONs), it allows modulation of fluorescence intensity in a selected temporal pattern. In the absence of external fields, or material that responds to external fields, the particles tumble erratically due to Brownian thermal forces (Brownian MOONs). These erratic changes in orientation cause the MOONs to blink. The temporal pattern of blinking reveals information about the local rheological environment and any forces and torques acting on the MOONs, including biomechanical forces as observed in macrophages. The rotational diffusion rate of Brownian MOONs is inversely proportional to the particle volume and hydrodynamic shape factor, for constant temperature and viscosity. Changes in the particle volume and shape due to binding, deformation, or aggregation can be studied using the temporal time pattern from the probes. The small size and the large number of MOONs that can be viewed simultaneously provide local measurements of physical properties, in both homogeneous and inhomogeneous media, as well as global statistical ensemble properties.