Synthesis and Characterization of Magneto-fluorescent Nanoparticles with Excited- state Intramolecular Proton Transfer E. M. N. de Oliveira * , F. L. Coelho † , M. L. Zanini ** , L. F. Campo † , and R. M. Papaléo *** * Programa de Pós-Graduação em Engenharia e Tecnologia de Materiais, ** Faculdade de Química, *** Faculdade de Física, Pontifícia Universidade Católica do Rio Grande do Sul, Av. Ipiranga 6681, 90619-900 Porto Alegre, Brazil, papaleo@pucrs.br † Instituto de Química, Universidade Federal do Rio Grande do Sul, Av. Bento Gonçalves 9500, 91501- 970 Porto Alegre, RS, Brazil, leandra.campo@ufrgs.br ABSTRACT Excited-state intramolecular proton transfer (ESIPT) is a particular well known reaction that has been very little studied in magnetic environments, which limits its application in fluorescent and magnetic resonance multimodal imaging. We report the photophysical behavior of an ESIPT dye of the benzothiazole class in solutions containing bare iron oxide or silica-coated iron oxide nanoparticles. The nanoparticles presented superparamagnetism and stability in neutral pH medium. The bare iron oxide nanoparticles provoked the fluorescence quenching of the dye, whilst the core-shell silica-iron oxide system preserved the ESIPT emission. The silica network gives a protection against fluorescence quenching of the dye. The ESIPT dye in this case showed only an emission band assigned to the keto tautomer. Our silica-coated magneto-nanoparticles have a great potential for use as a multifunctional nanoprobe for bioimaging. Keywords: magnetic nanoparticles, iron oxide, ESIPT dye, contrast agents. 1 INTRODUCTION Magnetite (Fe3O4), maghemite (-Fe2O3) and hematite (-Fe2O3) are the main forms of superparamagnetic iron oxide nanoparticles (SPIONs). Because of their good biocompatibility, stability and size-dependent properties, SPIONS play an important role in research and development of techniques such as magnetic field controlled targeted delivery or contrast agents in magnetic resonance (MRI). [1-3] To improve the efficiency and versatility of SPIONs used in biomedical applications, hybrid magnetic nanoparticles incorporating multiple functionalities are being developed, and there is a focus on materials that have both fluorescent and magnetic properties. Fluorescent magnetic nanoparticles include a variety of materials based on a magnetic core coated with an inorganic layer or an organic polymer. [4,5,6,7] A single nanoparticle may contain many dye molecules, providing, in principle, better performance in imaging contrast. [8] A convenient method for coating SPIONs is silanization to give the core-shell type structure. [9,10] Different dyes that emit in the near infrared region (NIR) have been used in the construction of core-shell SPIONs@SiO2, [11] such as fluorescein isothiocyanate, [12] rhodamines, [4] fluorescent polymers, [,13] and rare earth elements. However, the use of fluorescent compounds by ESIPT (excited-state intramolecular proton transfer) has not been fully explored so far. The fluorescence of these compounds is remarkably distant from the excitation wavelength, due to the small difference in the energy between the fundamental and excited states of the tautomer with the proton transferred. [14,15] ESIPT molecules have been used as membrane probes, [16] optical sensors [17,18] and photoactive materials. [19,20] Our strategy of design of the core-shell silica SPIONs nanoparticles to graft the ESIPT fluorescent dye resulted in nanoprobes with preserved ESIPT process, and thus with suitable properties to be used as a multimodal contrast agent for MRI and optical imaging. 2 EXPERIMENTAL The iron oxide nanoparticles (Fe3O4) were synthesized using an alkaline co-precipitation method with iron salts and then stabilized with citrate groups. [21,22] The silica- coated iron oxide nanoparticles (Fe3O4@SiO2) were prepared by hydrolysis and condensation of TEOS using the sol-gel process. [23,24] The 2-[5’-N-(3-triethoxysilyl) propylurea-2’-hydroxyphenyl]benzothiazole silanized (HBT) molecule was synthesized according to the procedure developed by Campo et. al. [25] . To prepare the Fe3O4@SiO2-HBT nanoparticles (NPs), the Fe3O4@SiO2 NPs and the HBT dye were dissolved in ethyl acetate and stirred at reflux temperature for 48 hours. The resulting solid was purified (washing with ethanol until all the non reactive HBT was removed), and dried in vacuum. The morphology and size of the NPs were analyzed by transmission electron microscopy (TEM - JEM-2010 200 kV, JEOL). The size distribution and zeta potential in aqueous dispersions were analyzed by a ZEN3600 zetasizer (Malvern). The crystallinity was measured in a X-ray diffractometer (XRD – Shimadzu 7000), using Cu Kα radiation (λ=0.1540 Å). The chemical structure of the NPs Advanced Materials: TechConnect Briefs 2016 131