DOI: 10.1021/la1007473 11657 Langmuir 2010, 26(14), 11657–11662 Published on Web 06/21/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Synthesis of TiO 2 Nanocrystals with a High Affinity for Amine Organic Compounds Ricardo H. Gonc -alves,* ,† Wido Herwig Schreiner, ‡ and Edson R. Leite* ,† † Department of Chemistry, Federal University of S~ ao Carlos, 13565-905 S~ ao Carlos, SP, Brazil, and ‡ Departament of Physics, Federal University of Paran  a, 81531-990, Curitiba PR, Brazil Received February 20, 2010. Revised Manuscript Received April 14, 2010 This article describes a different approach to the colloidal synthesis of TiO 2 nanocrystals using a polymer melt as a solvent. This approach allowed us to obtain a colloidal dispersion with a high degree of stability in a polymeric solvent, resulting in a transparent colloid. Using this method, it was possible to obtain the TiO 2 nanocrystal with Brønsted acid sites and polymer chains chemically anchored on the nanocrystal surface. The acid surface of those nanocrystals has the chemical property to react in the presence of amine organic compounds and to maintain the colloidal stability. In this way, TiO 2 nanocrystals were combined with a molecular probe containing amine functional groups such as polyaniline. Through the combination of the molecular probe and inorganic nanocrystals, we obtained a hybrid material with interesting chemical, optical, and electronic behavior, making it a promising material for photovoltaic, photochromic, and sensor devices. Introduction Titanium oxide nanocrystals have attracted considerable atten- tion because of their importance in a wide variety of applications including photocatalysts, biosensors, and mainly in solar cells. 1-4 The synthesis of colloidal TiO 2 nanocrystals with controlled size and shape is the key step in these technological applications. The nonaqueous synthesis route of TiO 2 nanocrystals is a sophisti- cated method for producing nanoparticle with a narrow size distribution and high crystallinity. 5-11 Nevertheless, this method does not ensure the formation of colloids. The most important factor in colloid formation is the stability of the system, which depends on particle size and mainly the surface chemical compo- sition. In particular, the control of a colloidal surface is not a simple task, and many studies have revealed the achievement of the colloidal stability of TiO 2 nanocrystals via chemical surface modification using a specific ligand, such as carboxylic acid, organosilanes, or phosphorus organometallics. 12-16 Organic mo- lecules with carboxylic acid groups have been the principal compound used to modify to the surface of inorganic nanocryst- als because this compound acts as a chelating agent. 17 In many situations, the carboxylic acid is only a functional group anchor- ing the specific part of the molecule. 18 However, there is a huge variety of amine compounds with interesting biological and chemical properties that could be directly anchored to the TiO 2 surface, resulting in a material with different functionality. For instance, some ruthenium organometallics or bioligands 19,20 could be chemically incorporated onto the TiO 2 surface without the addition of a carboxylic acid functional group to the molec- ular structure. However, the amine group shows a weak affinity for the surface of TiO 2 obtained in aqueous media or via the synthesis method that uses surfactants as ligands. 21,22 The inherent acid-base property of the solid surface is a critical parameter in evaluating the interactions with chemical com- pounds. 23 Miyamoto et al. have shown that the terminal hydroxyl groups on the surface of vanadium pentoxide act as Brønsted acid sites for ammonia adsorption and that these acid sites are more energetically stable. 24 Typically, the chemical strategy for increas- ing the affinity between the inorganic surface and amine com- pounds is to increase the degree of dissociation of the Brønsted acid sites on the solid surface. In addition, the number of the acid sites depends on the surface area, crystallographic planes, and residual ligands. 25-27 In general, the synthesis approach used in *Corresponding authors. E-mail:ricardohg.ufscar@gmail.com and derl@ power.ufscar.br. (1) Fujishima, A.; Honda, K. Nature 1972, 238, 37–38. (2) Paunesku, T.; Rajh, T.; Wiederrecht, G.; Maser, J.; Vogt, S.; Stoji cevi c, N.; Proti c, M.; Lai, B.; Oryhon, J.; Thurnauer, M.; Woloschak, G. Nat. Mater. 2003, 2, 343–346. (3) Tachikawa, T.; Tojo, S.; Fujitsuka, M.; Majima, T. Langmuir 2004, 20, 2753–2759. (4) O’Regan, B.; Gr€ atzel, M. Nature 1991, 353, 737–740. (5) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613–1614. (6) Tang, J.; Redl, F.; Zhu, Y.; Siegrist, T.; Brus, L. E.; Steigerwald, M. L. Nano Lett. 2005, 5, 543. (7) Garnweitner, G.; Niederberger, M. J. Mater. Chem. 2008, 18, 1171–1182. (8) Feldmann, C.; Jungk, H. O. Angew. Chem., Int. Ed. 2001, 40, 359–362. (9) Wu, B.; Guo, C.; Zheng, N.; Xie, Z.; Stucky, G. D. J. Am. Chem. Soc. 2008, 130, 17563–17567. (10) Tae, E. L.; Lee, K. E.; Jeong, J. S.; Yoon, K. B. J. Am. Chem. Soc. 2008, 130, 6534–6543. (11) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235–245. (12) Zhang, H.; Penn, R. L.; Hamers, R. J.; Banfield, J. F. J. Phys. Chem. B 1999, 103, 4656. (13) Ye, L.; Pelton, R.; Brook, M. A. Langmuir 2007, 23, 5630–5637. (14) Iijima, M.; Kobayakawa, M.; Yamazaki, M.; Ohta, Y.; Kamiya, H. J. Am. Chem. Soc. 2009, 131, 16342–16343. (15) Li, J.; Zhu, L. H.; Wu, Y. H.; Harima, Y.; Zhang, A. Q.; Tang, H. Q. Polymer 2006, 47, 7361–7367. (16) Jun, Y. W.; Casula, M. F.; Sim, J. H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981–15985. (17) Moser, J.; Punchihewa, S.; Infelta, P. P.; Gr€ atzel, M. Langmuir 1991, 7, 3012–3018. (18) Gao, F.; Wang, Y.; Zhang, J.; Shi, D.; Wang, M.; Humphry-Baker, R.; Wang, P.; Zakeeruddin, S. M.; Gr€ atzel, M. Chem. Commun. 2008, 2635–2637. (19) Rajh, T.; Saponjic, Z.; Liu, J.; Dimitrijevic, N. M.; Scherer, N. F.; Vega- Arroyo, M.; Zapol, P.; Curtiss, L. A.; Thurnauer, M. C. Nano Lett. 2004, 4, 6. (20) Tracewell, C. A.; Cua, A.; Stewarr, D. H.; Bocian, D. F.; Brudvig, G. W. Biochemistry 2001, 40, 193. (21) Fleming, G. J.; Idriss, H. Langmuir 2004, 20, 7540–7546. (22) Boddenberg, B.; Eltzner, K. Langmuir 1991, 7, 1498–1505. (23) Kantoh, T.; Okazaki, S. Bull. Chem. Soc. Jpn. 1981, 54, 3259–3264. (24) Yin, X.; Han, H.; Gunji, I.; Endou, A.; Ammal, S. S. C.; Kubo, M.; Miyamoto, A. J. Phys. Chem. B 1999, 103, 4701–4706. (25) Soria, E.; Rom an, E.; Williams, E. M.; de Segovia, J. L. Surf. Sci. 1999, 433-435, 543. (26) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1216.