White Fluorescence from Core–Shell Silica Nanoparticles

Abstract

Silica nanoparticles (NPs) are versatile materials with unique properties. Since the first reports of their colloidal fabrication [1,2] and functionalization [3][4][5] with light-emitting molecules, their photophysical properties have been extensively studied. [6,7] One of the most fascinating advances in this area of research deals with the incorporation of multiple dyes in one nanoparticle, which allows the formation of ultrabright nanoobjects. [8] From the accumulation of different photoresponsive molecules inside the silica core, complex photophysical events may arise, such as excimer formation, [9,10] photoswitchable fluorescence emission, [11] or resonance energy transfer. [12,13] As a result, new luminescent nanoobjects with multiple color emission under a single excitation wavelength are accessible and find applications in sensors, [14,15] biolabeling, [16] and emitting displays.In the meantime, much attention has been directed to the fabrication of white-light-emitting devices. In most devices, the white fluorescence arises from the mixing of various dyes having blue, green, and orange fluorescence simultaneously. [17][18][19][20][21][22] Such displays require a careful control of each color contribution and of the energy transfer between the different dyes to obtain the pure white color according to the CIE standards. This issue has been overcome by Park et al. [23] by blocking any energy transfer between two linked fluorophores with blue and yellow emission. Multiple dye-doped silica NPs may be an interesting alternative to prepare such white displays, as many different light-emitting fluorophores can be hosted in the same unit.s-Tetrazines are aromatic heterocycles that have a structure similar to a benzene ring, where four carbon atoms are replaced by nitrogen. We [24] and others [25] have explored the photophysical and electrochemical properties of these molecules. Indeed, some tetrazines substituted with heteroatoms [26] showed a high fluorescence quantum yield, a good photostability, and a long fluorescence lifetime. Furthermore, stetrazines are extremely electron-deficient and can be reversibly reduced, leading to nonfluorescent species. [27] These photophysical and electrochemical properties make these fluorophores attractive candidate for sensing purposes. [28] In a recent study, we attached tetrazine derivatives onto the surface of silica nanoparticles and evaluated the sensing properties of these nanoobjects. [29] Another recent study from our group [30] showed that a naphthalimide chromophore covalently linked to a tetrazine was able to act as an antenna, increasing the brightness of the tetrazine. Herein we report a new white-light-emitting system based on bichromophoric silica NPs doped by a naphthalimide dye in the inner core and by a tetrazine derivative on the outer shell.Reaction between 1,8-naphthalic anhydride and ethanolamine provided N-(hydroxyethyl)-1,8-naphthalimide 1 rapidly and in good yield. Intermediate 1 was reacted with triethoxy(3-isocyanatopropyl)silane, providing the des...

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