No abstract

Imaging at the single-molecule level reveals heterogeneities that are lost in ensemble imaging experiments, but an ongoing challenge is the development of luminescent probes with the photostability, brightness and continuous emission necessary for single-molecule microscopy. Lanthanide-doped upconverting nanoparticles overcome problems of photostability and continuous emission and their upconverted emission can be excited with near-infrared light at powers orders of magnitude lower than those required for conventional multiphoton probes. However, the brightness of upconverting nanoparticles has been limited by open questions about energy transfer and relaxation within individual nanocrystals and unavoidable tradeoffs between brightness and size. Here, we develop upconverting nanoparticles under 10 nm in diameter that are over an order of magnitude brighter under single-particle imaging conditions than existing compositions, allowing us to visualize single upconverting nanoparticles as small (d = 4.8 nm) as fluorescent proteins. We use advanced single-particle characterization and theoretical modelling to find that surface effects become critical at diameters under 20 nm and that the fluences used in single-molecule imaging change the dominant determinants of nanocrystal brightness. These results demonstrate that factors known to increase brightness in bulk experiments lose importance at higher excitation powers and that, paradoxically, the brightest probes under single-molecule excitation are barely luminescent at the ensemble level.

Porous semiconductors have garnered significant attention for their novel chemistry and potential applications as high surface area and optically active substrates [ 1-5 ]. Porous silicon in particular has long been studied for its potential applications in optoelectronics and sensing as a result of its light-emitting properties [ 6-10 ]. In addition, they can also serve as drug or gene delivery matrix because of their good biocompatibility [11][12] . Porous silicon is typically synthesized by applying a voltage bias to a silicon substrate immersed in an aqueous or ethanoic hydrofluoric acid (HF) solution. Surface and charge instabilities at the solid-solution interface are thought to nucleate pore formation, and accelerated etching of silicon at the pore tips propagates the voids into the substrate. The resulting pore networks and remaining silicon scaffold form the structure of porous silicon [ 13,14 ]. The synthetic method described in this study, on the other hand, relies on an electroless metal deposition process to provide the current flux necessary for porous silicon formation. Electroless metal deposition and subsequent sacrificial etching of the surrounding silicon lattice has been previously observed [ 15 ] and exploited to controllably etch arrays of silicon nanowires [ 16 ]. We have now found that it is possible to etch arrays of single-crystalline mesoporous silicon nanowires without the application of an external voltage.