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Sensing & Measurement

Tailoring a versatile chemo-optical sensing technique

Novel mesoporous Bragg stacks consisting of spin-coated multilayers of mesoporous TiO2 and SiO2 have a sensitive tunable chemical detection ability.
27 February 2007, SPIE Newsroom. DOI: 10.1117/2.1200702.0627

The simple, rapid, and continuous in situ monitoring of various chemical and biological species generates significant interest for medical, pharmaceutical, environmental, and food technologies. Optical sensing is often the most attractive detection technique due to the advantages of small size, light weight, and high sensitivity.1 Recent developments in integrated chemo-optical sensors have focused attention on porous thin films as a platform for enhancing sensitivity, due to their large surface-to-volume ratio and the capillary condensation of vapor within their pores.1,2 However, their sensing enhancement is still limited as a result of the thickness of the sensing layer, which can then only modulate a fraction of the light propagating through the sensors.3 Most promising are multilayer silicon-based photonic crystals, known as Bragg stacks, which exhibit periodic variation in their effective refractive index.1 A rather attractive way to expand their intrinsic capability for sensing applications would be to make all their layers out of surfactant and block copolymer mesoporous materials, since these have broadly tunable composition, pore size, versatile mesostructures, and functionalizable surfaces.4,5

As a proof-of-concept, we prepared mesoporous Bragg stacks (MBSs) by alternately spin-casting mesoporous TiO2 (meso-TiO2) and mesoporous SiO2 (meso-SiO2) into a multilayer stack. We selected this combination because of the high refractive-index contrast of these materials and well-established synthesis methods. The novelty of our fabrication approach lies in producing an ABAB multilayered film consisting of mesoporous layers by use of a simple yet effective method to alternate layer deposition by spin casting and subsequent thermal treatment.5

Our MBSs exhibited intense colors as seen in Figure 1. The color can be reversibly altered by introducing or removing a given pore analyte. This induces a change in optical properties by modifying the effective MBS refractive index. As shown in Figure 1(c), the reflectance maxima of MBSs respectively consisting of two, four, and six layers correlate linearly with analytes’ refractive indices. These measurements were compared to the reflectance of a Bragg stack consisting of a single layer of meso-TiO2 and dense SiO2/TiO2 stacks, intended to represent a conventional chemo-optical sensor assembly. The calculated sensitivities (??/?n) demonstrated the superior performance of MBSs, by more than a factor of two, relative to the sensitivity of the conventional Bragg reflector having only a single porous layer as the chemical sensing element.

Figure 1. Digital images of a four-layer mesoporous Bragg stack (MBS), (a) in air and (b) in ethanol. (c) Reflectance maxima obtained from two-layer (MBS2), four-layer (MBS4), and six-layer (MBS6) mesoporous Bragg stacks in air (1), water (2), ethanol (3), 2-propanol (4), and 1-butanol (5) as a function of analyte's refractive index. MBS1 is the response of a single meso-TiO2 layer deposited on a Bragg reflector consisting of dense SiO2/TiO2 stacks synthesized without a structure-directing template.

In addition to this enhanced sensitivity, our MBSs exhibit tunable selectivity in the detection of analytes. A comparison of their respective response to various alcohols and alkanes in Figure 1(c) clearly shows that linear correlation between the reflectance maximum and the analyte's refractive index only occurs within the same analyte family.5 We also demonstrated that selectivity and sensitivity could be altered by changing the MBS composition.5 Our results suggest that MBS devices can be tailored for particular applications by selecting the appropriate number and composition of the mesostructured layers.

In summary, mesoporous Bragg stacks were assembled by alternating multiple coatings of meso-TiO2 and meso-SiO2. We showed that such mesoporous metal-oxide diffractive optical elements exhibit enhanced sensitivity to a change of the analyte's refractive index when compared to a conventional single-layered Bragg stack. We also demonstrated that MBS sensitivity and selectivity were highly dependent on the properties of their constituent mesoporous metal-oxide layers. This all bodes well for the development of a new generation of MBS-based chemo-optical and bio-optical sensors.

Geoffrey A. Ozin
Materials Chemistry Research Group, University of Toronto
Toronto, ON, Canada 
Sung Yeun Choi
Materials Science Division, Argonne National Laboratory
Argonne, IL 

1. M. D. Marazuela, M. C. Moreno-Bondi, Fiber-optic biosensors - an overview,
Anal. Bioanal. Chem.
 372, no. 5-6, pp. 664-682, 2002. doi:10.1007/s00216-002-1235-9.
2. M. J. Sailor, J. R. Link, “Smart dust”: nanostructured devices in a grain of sand,
Chem. Commun.
 11, pp. 1375-1383, 2005. doi:10.1039/b417554a.
4. M. Antonietti, G. A. Ozin, Promises and problems of mesoscale materials chemistry or why meso?,
Chem. Eur. J.
 10, no. 1, pp. 28-41, 2004.
5. S. Y. Choi, M. Mamak, G. von Freymann, N. Chopra, G. A. Ozin, Mesoporous Bragg stack color tunable sensor,
Nano Lett.
 6, no. 11, pp. 2456-2461, 2006.