Polyaromatic Luminescent Nanocrystals for Chemical and Biological Sensors

Recently, we have engineered new hybrid organic-inorganic materials through a simple and generic preparation of stable organic nanocrystals grown in sol-gel thin films prepared by spin-coating. 1-3 This process is based on the confined nucleation and growth of dyes in the pores of sol-gel networks. These nanocomposite coatings not only combine the optical properties of organic phases (luminescence, NLO properties, etc.) with those of amorphous inorganic materials (high stability, convenient processing, and shaping) but also the advantages of nanocrystals: preparation of low diffusing samples for visible or IR laser beams and higher stability than dispersed molecules. The aim of this work is to design a new type of fluorescent nanosensors through the preparation of organic luminescent nanocrystals grown in silicate films. Indeed, these nanocomposite coatings exhibit an open porosity which allows a great reactivity between the nanocrystals and their chemical surroundings. Besides, sol-gel thin films are already involved in the fabrication of gas sensors. 4 The adjustment of sol-gel porosity at the nanometer scale would allow the protection of these nanosensors against unwanted interactions , the silicate matrix playing thus the role of a filter in size, charge, and polarity. Moreover, organic nano-crystals exhibit higher photostability and luminosity than isolated molecules dispersed in solutions or in solid matrixes such as sol-gel or polymers. We have selected two polyaromatic dyes, rubrene (5,6,11,12-tetraphenyl-naphthacene) and tetracene (2,3-benzantracene), which exhibit high fluorescence efficiency in the crystal state. We have separated the detection and signalization functions, as previously made for latex: 5 the polyaro-matic nanocrystals act as the signaling part while the probe molecules adsorbed at the surface of the nano-crystals constitute the detection function. The first molecular probe selected was Methylene Blue (MB, (7-(dimethylamino)-phenothiazin-3-ylidene)-dimethylam-monium) used in aqueous solutions as a redox sensor. The sol-gel thin films are prepared by spin-coating. We start from solutions containing a solvent, the dye, silicon alkoxides, and water, which are inserted in airtight flasks. To specify the role of the host sol-gel matrix, several alkoxide precursors have been used in this work. Thus, thin films are obtained from TMOS (tetramethoxysilane), and equimolar alkoxide mixtures of TMOS/MTMOS (methyltrimethoxysilane) or TMOS/ MDMS (methyldimethoxysilane). The TMOS and TMOS/ MTMOS sols are synthesized under acid (HCl, pH ≈ 1.5) catalyzed conditions by one-step hydrolysis and condensation of the alkoxide precursors for 3 days at 80 °C with one water molecule per-OR function (h) [H 2 O]/[-OR]) 1). In the case of TMOS/MDMS sols, a two-step hydrolysis-condensation is used under neutral conditions: 12 h at 60 °C with h) 0.2 followed by 3 days at 60 °C with h made up to 0.6. Tetrahydrofuran (THF) has been used to dissolve the dyes, rubrene or tetracene, and to mix water of hydrolysis with alkoxide precursors; the molar ratio solvent/alkoxide s is set to 5 for all the thin films studied here. The 60-80 °C heating allows rapid dissolution of the organic powder and enhancement of the hydrolysis and condensation kinetics, leading to the formation of silicate chains dispersed in the solution. This sol aging is necessary to control the sol viscosity, around 10-20 cps, to obtain high-quality films, to ensure the control of particle growth, and to avoid the crystal coalescence. The resulting sols, which are stable for several weeks, are deposited at room temperature by spin-coating onto thin microscope slides with a rotation speed of 4000 rpm. Thus, films of around 0.5-µm thick have been prepared. These nanocomposite coatings are then stabilized by annealing at 200 °C. The dye concentration, expressed as the molar ratio d) organic/alkoxide ≈ 10-3-10-2 , depends on the dye solubility in THF, the nature of the host matrix, and the desired particle sizes. The latter was targeted here between 250 and 500 nm in diameter to facilitate the nanocrystal observations and characterizations by optical confocal microscopy. Rubrene and tetracene nanocrystals were first visualized through fluorescence confocal microscopy (FCM) excited by the 488-nm beam of a 15-mW Argon laser. The confocal laser scanning microscope is a LSM410 from Zeiss. Figure 1 shows a typical FCM image of our nanocomposite thin films. One can see well-dispersed and embedded organic nanocrystals in the silicate coating. However, we are limited in FCM by the weak resolution of this technique: around 200 nm in the x-y plane and 500-600 nm in the thickness, z. For this reason, we have also characterized the films by transmission electron microscopy (TEM). In this case, the coatings are deposited onto sodium chloride crystal plates. These substrates are dissolved in water and the films are then placed on a copper grid for TEM characterizations. Thus, one can observe well-defined molecular crystals with a spherical shape and narrow size distribution, ranging for example between 300 and 500 nm for the sample displayed in Figure 2.. † CNRS. ‡ ENS-Cachan. (1) Ibanez, A.; Maximov, S.; Guiu, A.; Chaillout, C.; Baldeck, P. L.