Magnetically controlled sensor swarms

Suspensions of fluorescent chemical sensors containing magnetic components were assembled into groups or "swarms" of sensors using magnetic tweezers. The tweezers control the size and position of the swarm magnetically, without direct mechanical contact. In addition, the tweezers can reversibly assemble the particles within a swarm into chains and control the orientations of these chains. These sensing swarms represent a novel sensor architecture that lies between that of a mechanically rigid fiber sensor and that of an unguided suspension of sensor nanoparticles. The larger number of particles within a swarm makes the swarm brighter, impart more force and enable a higher terminal velocity than possible for individual particles or dilute suspensions. Magnetic pH sensitive particles were produced by precipitating iron oxide inside hollow organically modified silica particles, and subsequently swelling the shell with a hydrophobic pH sensitive dye. Swarms of these pH sensors were magnetically guided through a pH gradient in a capillary while measuring their fluorescence spectral emission. This new class of tools shows promise for chemical sensing while providing a means to simultaneously modulate fluorescence intensity, apply mechanical forces, produce singlet oxygen and measure chemical and physical properties of fluids and surfaces. (c) 2006 Elsevier B.V. All rights reserved.