A convenient and versatile protocol to encapsulate lanthanide doped upconverting nanoparticles by an amphiphilic polymer shell containing photoresponsive diarylethene chromophores was developed. The assemblies are all water-soluble and fluoresce in the visible region of the spectrum when excited with 980 nm near-infrared light. The fluorescent emission can be selectively and reversibly modulated by alternatively irradiating the photoresponsive nanoparticles with UV light and visible light, which triggers ring-closing and ring-opening reactions of the chromophores, respectively. Fluorescence lifetime experiments suggest that the quenching mechanism is a combination of energy transfer and emission-reabsorption processes. These photoresponsive upconverting nanoparticles have the potential to advance bioimaging and other applications in nanophotonics.

Sun block for nanoparticles: Unintentional photorelease triggered by UV light is a problem in photodynamic therapy. Encapsulating upconverting nanoparticles containing photoswitches in a UV-blocking amphiphilic polymer shuts down the one-photon process and only allows two-photon-driven photochemistry. Thus, UV light is blocked while NIR light can reach the nanoparticle core and trigger photorelease.

Sonnenschutz für Nanopartikel: Unbeabsichtigt durch UV‐Licht ausgelöste Freisetzung stört bei der photodynamischen Therapie. Das Einschließen von hochkonvertierenden Nanopartikeln, die Photoschalter enthalten, in ein UV‐Strahlung abblockendes amphiphiles Polymer unterbindet den Einphotonenprozess und lässt nur die zweiphotonengesteuerte Photochemie zu. Nur NIR‐Licht kann daher den Nanopartikelkern noch erreichen und die Photofreisetzung auslösen.

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