Characterization and Applications of Modulated Optical Nanoprobes (MOONs)
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Characterization and Applications of Modulated Optical Nanoprobes (MOONs) Jeffrey N. Anker1, Caleb J. Behrend1, Brandon H. McNaughton1, Teresa Gail Roberts1, Murphy Brasuel2, Martin A. Philbert3, and Raoul Kopelman1 1 University of Michigan Chemistry Department, Ann Arbor, Michigan 48109-1055. 2 Colarado College Chemistry Department, Colorado Springs, Colorado 80903. 3 University of Michigan School of Public Health, Ann Arbor, Michigan 48109. ABSTRACT Modulated optical nanoprobes (MOONs) are microscopic (spherical and aspherical) particles designed to emit different fluxes of light in a manner that depends on particle orientation. When particle orientation is controlled remotely using magnetic fields (MagMOONs) it allows modulation of fluorescence intensity in any selected pattern including square and sinusoidal waves. The broad range of sizes over which MOONs can be prepared allows them to be tailored to applications from intracellular sensors using submicron MOONs to immunoassays using larger MOONs (1-10µm). In the absence of external fields, or material that responds to external fields, the particles tumble erratically due to Brownian thermal forces. These erratic changes in orientation cause the MOONs to blink. The temporal pattern of blinking contains information about the local rheological environment and any forces and torques acting on the MOONs.
INTRODUCTION In order to visualize physical and chemical dynamics within microscopic environments and even within living cells, a variety of nanoprobes have recently been developed1. One type of sensor utilizes fluorescence indicator and reference dyes encapsulated in a polymer matrix to measure local chemical concentrations2,3. These nano-sensors have several advantages over conventional molecular probes: First, the polymer matrix protects the dye from interferences such as quenching due to protein binding. For instance, Xu et al. showed that addition of albumin protein far below physiological protein concentrations resulted in near saturation of response from an oxygen sensing fluorophore; however, encapsulation of the fluorophore reduced interference to a negligible level4. In the case of cellular applications, the encapsulation of the dye also protects the cell from potentially toxic effects. A second strength of nanoparticle sensors is that the matrix provides a separate sensing phase, distinct from the surrounding environment, allowing multi-component sensor and effector systems to be prepared. These systems include enzyme coupled5 and ionophore coupled6 fluorescent sensors, photodynamic effectors, MRI contrast enhancing agents, and molecularly targeted systems1. In addition to MRI enhancement, incorporation of magnetic material allows particles and swarms of particles to be guided and oriented with external magnetic field gradients and field directions7-9. Most fluorescent nanospheres and microspheres emit light uniformly in all directions. By carefully constructing nano-systems that emit anistropically, it becomes possible to monitor particle orient
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