Nanopore Sensors

Although the fabrication of nanoporous alumina membranes using anodic oxidation of the aluminum has been known for years [1], it has only recently been used for nanotechnology, as a mask to etch the material below [2] or as a template to deposit another material [3]. The oxidized layer consists in a lattice of columnar hexagonal cells, each one with a cylindrical pore at the center. Size, density and length of the pore can be tuned by controlling the electrolyte parameters. A high density of pores (10-100 nm of diameter and 20-200 nm of distance, i.e., 109-1012 pores/cm2) over a macroscopic area (1-10 cm2) with small variations within the pore sizes can be typically achieved. Schullerís group has a wide experience in the use of such nanoporous membranes as nanotemplates [3]. On the other hand, porous silicon (Si) thin films are used as gas sensors and biosensors, based in optical interferences (photonic crystals) [4, 5]. The large surface area associated with the porous structure allows a significant change in the refractive index when gas or biological molecules are adsorbed on the pore walls. The advantage of porous alumina as a sensor is that the parameters can be precisely controlled and the alumina is extremely stable. The goal of this interdisciplinary project, in collaboration with Prof. Prof. Michael J. Sailor of the Department of Chemistry and Biochemistry at the UCSD, is the use of porous alumina as sensor [4] (gases in Chemistry and antibodies in Biology).

Fig. 1. Plan-view SEM images of some of the porous alumina samples, with pore diameters: A) 10 ±2, B) 18 ±4, C) 33 ±7 and D) 61 ±12 nm. The scale bar is 100nm.

So far, several achievements were obtained in gas sensing: 1) We have been able to control of the sensor response (gas adsorption) as a function of the characteristics of the nanopores; 2) The sensor response is reproducible and stable of for a variety of organic vapors; 3) We have observed an interesting phenomenon, hysteretic capillary condensation, that occurs at pressures below the saturation pressure of the gas inside the nanopores, since the surface tension makes the liquid phase energetically more favorable [5]. This phenomenon has both a basic (nanoscale effect) and an applied interest (the gas sensor response is magnified). It can be explained by the Kelvin equation; 4) We have observed a dependence of the hysteresis with the Van der Waals solid-fluid interactions. The diameter of the nanopores (10-100 nm) is suitable for biosensing, since it is of the order of protein and antibody size. The proper functionalization of the pore walls allows the detection of specific biomolecules by using optical interferometry, a very sensitive technique. The biomolecules are in liquid, which is placed into the nanopores.

Fig. 2. Change in effective optical thickness (2nL) as a function of the relative pressure of analyte (solid squares for isopropanol and open circles for toluene). Curves were obtained by first increasing (adsorption) and then decreasing (desorption) the relative pressure in discrete steps. Pore diameters of the samples are 10±2 nm (a) and 27±3 nm (b). Horizontal lines correspond to the change in 2nL measured by using the analytes in liquid phase (solid line for isopropanol, dashed line for toluene).

Finally, we have patented the idea of using capillary condensation in order to make this porous alumina a drug delivery nanocontainer. We plan to use this phenomenon in order that drugs can be transported and delivered in nanoporous alumina microchips, which would be introduced into the body.

[1] J. P. OíSullivan and G.C. Wood, Proc. Roy. Soc. Lond. A 317, 511 (1970).

[2] H. Masuda and K. Fukuda, Science 268, 1466 (1995).

[3] C.-P. Li et al., J. Appl. Phys. 100, 074318 (2006).

[4] J. Gao et al., Langmuir 18, 2229 (2002).

[5] C. Pacholski et al., J. Am. Chem. Soc. 127, 11636 (2005).

[6] S. D. Alvarez, C.-P. Li, C. E. Chiang, I. K. Schuller, M. J. Sailor, Am. Chem. Soc. Nano 3, 3301 (2009).

[7] F. Casanova, C. E. Chiang, C.-P. Li, and Ivan K. Schuller, Appl. Phys. Lett. 91, 243103 (2007).

(c) 2007 Ivan K. Schuller       -       designed by Thomas Gredig