Anal. Chem. 2004, 76, 939-946
Poly(amidoamine) Dendrimers as Nanoscale Diffusion Probes in Sol-Gel Films Investigated by Total Internal Reflection Fluorescence Spectroscopy Karla S. McCain, Peter Schluesche, and Joel M. Harris*
Department of Chemistry, University of Utah, 315 South 1400 East, Salt Lake City, Utah 84112-0850
Three generations of poly(amidoamine) dendrimers were dye-labeled and chemically modified to have terminal carboxyl groups and used as variably sized probes to study diffusion in thin sol-gel films. Total internal reflection fluorescence spectroscopy experiments, both correlation and concentration-jump measurements, were employed to measure the relative populations and effective diffusion coefficients of dendrimers in the films. For films prepared from small (27-nm) silica particles, larger dendrimers could be completely excluded from penetrating the solgel structure. In films made of larger (150-nm) particles with correspondingly larger pores, concentration-jump experiments showed that larger dendrimers are excluded from more of the intraparticle pore space than small dendrimers. Similarly, fluorescence-correlation measurements showed that the diffusion of smaller dendrimers exhibited greater tortuosity than larger dendrimers in the interparticle pores of the film. The smaller dendrimers explore a greater volume of smaller, more convoluted pores, whereas larger dendrimers penetrate a smaller volume of larger, more open pores. Sol-gels are complex, high surface area materials which are produced by the hydrolysis of metal alkoxides and which contain a range of porosities.1,2 They are important in many applications, including chromatography,3,4 catalysis,5 and as supports for sensors.6-10 The complicated porosity of sol-gel materials influences the way in which molecules diffuse within them, and understanding this process is important in all of these areas of application. For example, in order for a sol-gel sensor to register * To whom correspondence should be addressed. E-mail: harrisj@ chemistry.chem.utah.edu. (1) Brinker, C. J.; Scherer, G. W. Sol-Gel Science: The Physics and Chemistry of Sol-Gel Processing; Academic Press: Boston, 1990. (2) Iler, R. K. The Chemistry of Silica; John Wiley and Sons: New York, 1979. (3) Hayes, J. D.; Malik, A. Anal. Chem. 2000, 72, 4090-4099. (4) Nakanishi, K.; Shikata, H.; Ishizuka, N.; Koheiya, N.; Soga, N. J. High Resolut. Chromatogr. 2000, 23, 106-110. (5) Rolison, D. R. Science 2003, 299, 1698-1701. (6) Han, L.; Niemczyk, T. M.; Lu, Y.; Lopez, G. P. Appl. Spectrosc. 1998, 52, 119-122. (7) Wang, C.; Li, C.; Lin, Y.; Chau, L. Appl. Spectrosc. 2000, 54, 15-19. (8) Makote, R.; Collinson, M. M. Anal. Chim. Acta 1999, 394, 195-200. (9) Kao, H. P.; Yang, N.; Schoeniger, J. S. J. Opt. Soc. Am. A 1998, 5, 21632171. (10) Han, L.; Niemczyk, T. M.; Lu, Y.; Lopez, G. P. Appl. Spectrosc. 1998, 52, 119-122. 10.1021/ac0351015 CCC: $27.50 Published on Web 01/13/2004
© 2004 American Chemical Society
a response, the analyte molecule must diffuse through the material until it encounters a reagent molecule that produces a signal. So how quickly mass transport can occur determines the response time for the sensor. Diffusion in sol-gel films is complicated because of their intricate and heterogeneous pore structure. In the past, methods for examining diffusion in sol-gel materials have relied on slow time scale experiments, such as leaching,11 electrochemistry,12 and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR).13 The ATR-FTIR experiment showed a fast (
