J. Phys. Chem. C 2007, 111, 7623-7628
7623
Synthesis and Characterization of Butoxylated Silica Nanoparticles. Reaction with Benzophenone Triplet States Anabel E. Ruiz,†,‡ Paula Caregnato,† Valeria B. Arce,†,‡ Marı´a de las M. Schiavoni,‡ Vero´ nica C. Mora,† Mo´ nica C. Gonzalez,† Patricia E. Allegretti,*,‡ and Daniel O. Ma´ rtire*,† Instituto de InVestigaciones Fisicoquı´micas Teo´ ricas y Aplicadas (INIFTA), UniVersidad Nacional de La Plata, Casilla de Correo 16, Sucursal 4, (1900) La Plata, Argentina and LADECOR, Departamento de Quı´mica, Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, Calle 47 y 115, (1900) La Plata, Argentina ReceiVed: February 7, 2007; In Final Form: March 22, 2007
Butoxylated silica nanoparticles (BSN) were prepared by esterification of the silanol groups of fumed silica nanoparticles with butanol. These particles were characterized by FTIR, BET, TEM, and TOC. BSN suspensions in water:acetonitrile mixtures were used as quenchers of benzophenone (BP) phosphorescence in time-resolved experiments at the excitation wavelengths of 266 and 337 nm. The phosphorescence signals obtained in the presence of the nanoparticles were fitted to biexponential decays. Both decays were accelerated in the presence of increasing amounts of BSN. A model including the reversible adsorption of BP on BSN and supported by computer simulations accounts for the observed results.
Introduction groups,1
The surface of silica carries silanol mSi-OH, which can be chemically modified to produce a covalently bonded organic layer which affects the chemical and physical properties of the silica.2 The success of such materials for a given application depends on close control of the surface properties and the chemistry of the bonded layers. This type of surface modification presents direct interest in applications such as catalysis, chromatography, controlled drug delivery, and membranes. The well-known esterification reaction of silanol groups on the silica surface with various alcohols to produce surfacealkoxylated silica, mSi-OR, is an attractive and possibly less expensive route to materials with a functionality similar to that of their silane-based counterparts.3-6 Surface silanol esterification by alcohols is a reversible process, involving H-bonding of the alcohol to the silica surface followed by a condensation; see reaction 1.
mSi-OH + ROH(l) f mSi-OH a H-OR f mSi-OR + H2O (1) The reaction is pushed toward completion by removing the water produced by refluxing the xylene-water azeotrope and further percolation through CaH2.4 Ossenkamp et al.3 reported that the hydrolytic behavior of alkoxylated silica in boiling water is strongly dependent on the initial RO surface density: the hydrolysis of samples with a higher initial RO surface density is slower. The photophysical and photochemical behavior of adsorbed molecules is observed to be different from that of molecules in the gas phase and in solution. Recent investigations focused on * Authors to whom correspondence should be addressed. D.O.M.: Telephone: -54 221 4257430/7291. Fax: -54 221 4254642. E-mail:
[email protected]. P.E.A.: Telephone: -54 221 4243104. Fax: -54 221 4254533. E-mail:
[email protected]. † INIFTA. ‡ LADECOR.
discovering the effect of the surface nature on the photophysics and photochemistry of adsorbed molecules.7-11 Upon excitation of benzophenone (BP) in the (n, π*) band in various matrixes, phosphorescence with a high quantum yield was observed. Since the BP phosphorescence spectrum is affected by its molecular environment, this molecule may be considered one of the most useful candidates to act as a “molecular probe” in studying the properties of solid surfaces.12 We employed here a modification of the procedures employed in refs 3 and 4 to prepare butoxylated fumed silica nanoparticles, BSN, with a very high initial surface density of butoxy groups. These particles were characterized by Fourier transform infrared spectroscopy (FTIR), Brunauer-Emmett-Teller analysis (BET), transmission electron microscopy (TEM), and total organic carbon (TOC) and used in acetonitrile/water suspensions as quenchers of the phosphorescence of benzophenone (BP). Experimental Methods Materials. Fumed silica (Sigma; specific surface area (SSA) ) 390 ( 40 m2/g, particle size estimated from the SSA ) 0.007 µm) was dried in a crucible for 15 h at 120 °C and then in a muffle furnace for 3 h at 250 °C and stored in a desiccator. The solvents o-xylene and butanol (Baker, analytical grade) were distilled onto molecular sieves, which had been dried at 250 °C for 4 h. Ethyl acetate (Ciccarelli, p.a.), CaH2 (Fluka), CaCl2 (SigmaAldrich), benzophenone (BP; 99%, Aldrich), acetonitrile (Baker, analytical grade), and KOH and KH2PO4 (Merck) were used without further purification. Distilled water (>18 MΩ cm-1,
