Joan J. Carvajal

Light-Emitting Nanofibers Operate as Subwavelength and Multiple Point Sources

Light-Emitting Nanofibers Operate as Subwavelength and Multiple Point Sources Light-emitting sources with constrained dimensions may play an important role in sensing and lab-on-a-chip applications operating without external optics, enhancing the sensitivity and improving the signal-to-noise ratio of detected fluorescence. J.M. MoranMirabal and co-workers from Cornell University produced point illumination sources based on fibers of [Ru(bpy)3] (PF6-)/PEO, with electroluminescent ionic transition metal complexes (iTMCs) embedded in a polymer electrolyte on interdigitated electrodes, as reported in the February 14 issue of Nano Letters (p. 458, DOI: 10.1021/nl062778+). To a solution of 50 mM ruthenium(II) tris(bipyridine) in dry acetonitrile filtered through a 450 nm polycarbonate membrane, the researchers added poly(ethylene oxide) (PEO) as the carrier polymer, whose concentration tuned the viscosity of the electrospinning solution and affected the size of the fibers with diameters ranging from 150 nm to several microns. Fibers were electrospun at 8–10 kV on silicon substrates with a 300–600 nm thermal oxide insulating layer and with micropatterned gold interdigitated electrodes (IDEs) on top, using a microfabricated tip coated with gold and keeping the tip-to-substrate distance between 25mmand 40mm. Fibers deposited on a device having a 5 μm interelectrode spacing emitted light confined to a planar region 540 × 540 nm2 with the out-of-plane dimension limited by the thickness of the fiber when applying a dc bias across the IDEs in a dry nitrogen atmosphere. These devices showed additional emission zones as the voltage increased, resulting in multiple light sources within a fiber. This represents an advantage to develop multiple light sources in parallel, with emission sites defined by interelectrode gaps. Fibers deposited on a device with 500 nm interelectrode spacing showed a single homogeneous emission zone confined to planar regions of 240 × 325 nm2 or smaller with the emission spectrum centered at 600 nm, which implies that these devices operate as subwavelength point sources, as well as a reduced turn-on voltage. For the electroluminescent fibers deposited on 5 μm IDEs, emission could be detected by a CCD camera when applied voltages were ~10 V and by eye in a dark room at 100 V. For the electroluminescent fibers deposited on 500 nm IDEs, emission could be detected at voltages as low as 2.6 V. Relatively long lifetimes were achieved during continuous operation at high voltages, which could be improved, in air, by encapsulation of the light-emitting fibers or substitution of the carrier polymer. By using other ionic transition metal complexes with emission at different wavelengths in the visible spectrum, nanoscopic light-emitting sources can be produced that excite multiple fluorescent tags, enabling the full integration of excitation and detection mechanisms on lab-ona-chip devices. JOAN J. CARVAJAL