Source: http://aoot.osa.org/ome/abstract.cfm?uri=ome-9-3-1416
Timestamp: 2019-04-19 12:25:21+00:00

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4These authors contributed to this work equally.
The current effort utilizes methacrylate based hydrogels derived from polystyrene-co-poly(propargyl)acrylate (PS-pPA) colloidal particles with encapsulated stilbene, an organic scintillator, as an x-ray activated imaging agent. These nanoparticles self-assemble into electrostatically stabilized crystalline colloidal arrays (CCAs). Upon photo-polymerization into the hydrogel, the material remains ordered where the crystal is stabilized and more mechanically robust. Upon x-ray stimulation, stilbene/PS-pPA hydrogels emit blue light. While stilbene is an x-ray active material, it remains a poor emitter under x-ray irradiation. However, the color and amount of light emitted from stilbene/PS-pPA could be manipulated through judicious choice in fluorophores that form FRET pairs to span the visible spectrum. To this end, a copper(I) catalyzed azide/alkyne cycloaddition (CuAAC) reaction was employed to covalently attach an azide modified naphthalimide (AzNap) derivative to the particles all while being in hydrogel form. Stilbene and AzNap are FRET pairs with one another, which resulted in an increase of the total luminescence of the system; these hydrogels now emit green light. In addition, the original stilbene/PS-pPA hydrogels could be functionalized with both AzNap and an azide modified rhodamine B derivative (AzRhod), which are FRET pairs with each other, through CuAAC reactions in the hydrogel. These hydrogels emit orange light and the overall luminescence is similar to that of the AzNap functionalized hydrogels even through two energy transfers. These fully organic hydrogels may be suitable alternatives to toxic inorganic materials in x-ray based imaging techniques.
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Fig. 1 Crystalline colloidal array (CCA) composed of electrostatically assembled polystyrene-co-poly(propargyl acrylate) with stilbene encapsulated inside the particle in (a) liquid form and (b) encapsulated in a methacrylate based hydrogel that emits blue light upon x-ray stimulation (emitter series n°1). (c) Methacrylate based hydrogel composed of polystyrene-co-poly(propargyl acrylate) with stilbene encapsulated inside the particle with 2-(3-azidopropyl)-6-(piperidin-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (AzNap) covalently attached to the particles through a copper(I) catalyzed azide/alkyne cycloaddition (CuAAC) performed in the hydrogel (emitter series n°2). Emitter series n°2 emits green light upon x-ray irradiation. (d) Methacrylate based hydrogel composed of polystyrene-co-poly(propargyl acrylate) with stilbene encapsulated inside the particle with AzNap and N-(9-(2-((2-(2-(2-azidoethoxy)ethoxy)ethoxy)carbonyl)phenyl)-6-(diethylamino)9,9a-dihydro-3H-xanthen-3-ylidene-N-ethylethanaminium (AzRhod) covalently attached to the particles through copper(I) catalyzed azide/alkyne cycloaddition (CuAAC) reactions performed in the hydrogel; emitter series n°3 emits orange light upon x-ray stimulation.
Fig. 2 (a) X-ray radioluminescence spectrum of stilbene in its crystalline powder form. (b) Optical photographs of stilbene/PS-pPA liquid CCA illuminated under white light (left) and under x-ray irradiation (right). Under x-ray excitation, the droplet appears a bright blue color while the droplet appears white under white light illumination. (c) Optical photographs of stilbene/PS-pPA liquid CCA illuminated under white light (left) and under x-ray stimulation (right). The droplet has been diluted on the right side, and an opalescent line can clearly be seen in the white light image (left), while the diluted portion of the CCA results in a dark blue color, and the undiluted portion results in a bright blue color under x-ray irradiation (right). X-ray radioluminescence (blue) and reflection (red) spectra of stilbene/PS-pPA liquid CCA with rejection wavelength at (d.1) 423 nm, (d.2) 470 nm, (d.3) 480 nm, and (d.4) 530 nm. X-ray irradiation performed with an AmpTek Mini-X x-ray unit equipped with a tungsten target operating at 50 kV and 70 μA.
Fig. 3 Optical photographs of stilbene/PS-pPA methacrylate-based hydrogels (emitter series n°1) under (a) white light and (b) x-ray irradiation. X-ray radioluminescence (blue) and reflection (red) spectra of emitter series n°1 corresponding to reflection wavelength at (c) 483 nm, (d) 510 nm, and (e) 530 nm. X-ray irradiation performed with an AmpTek Mini-X x-ray unit equipped with a tungsten target operating at 50 kV and 70 μA.
Fig. 4 Optical photographs of stilbene/PS-pPA methacrylate-based hydrogels functionalized with AzNap (emitter series n°2) through a CuAAC reaction in the hydrogel under (a) white light and (b) x-ray irradiation. X-ray radioluminescence (blue) and reflection (red) spectra of emitter series n°2 at rejection wavelength corresponding to (c) 478 nm and (d) 541 nm. X-ray irradiation performed with an AmpTek Mini-X x-ray unit equipped with a tungsten target operating at 50 kV and 70 μA.
Fig. 5 Optical photographs of stilbene/PS-pPA methacrylate-based hydrogels functionalized with AzNap and AzRhod (emitter series n°3) through CuAAC reactions in the hydrogel under (a) white light and (b) x-ray irradiation. X-ray radioluminescence (blue) and reflection (red) spectra of emitter series n°3 corresponding to rejection wavelength at (c) 490 nm and (d) 532 nm. X-ray irradiation performed with an AmpTek Mini-X x-ray unit equipped with a tungsten target operating at 50 kV and 70 μA.
Fig. 6 Synthetic scheme to yield 2-(3-azidopropyl)-6-(piperidin-1-yl)-1H-benzo[de]isoquinoline-1,3(2H)-dione (AzNap).
Fig. 7 Synthetic scheme to yield N-(9-(2-((2-(2-(2-azidoethoxy)ethoxy)ethoxy)carbonyl)phenyl)-6-(diethylamino)9,9a-dihydro-3H-xanthen-3-ylidene-N-ethylethanaminium (AzRhod).

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