Towards the Photonic Nose: A Novel Platform for Molecule and Bacteria Identification

Adv. Mater. 2010, 22, 1351–1354 2010 WILEY-VCH Verlag G O N The olfactory system has been recognized for its ability to identify airborne molecules using a combinatorial response, in which a library of activated olfactory receptor neurons gives a unique fingerprint for each type of odorant. Mimics of the olfactory system, dubbed artificial noses that enable identification of vapor phase compounds are currently of great technological interest for odor analysis in areas that include flavor and fragrance, food and beverage, packaging, pharmaceutical, cosmetic and perfume, narcotic and disease diagnostics. While artificial noses for the detection of gas-phase molecules based on modulation of electrical and gravimetric properties are well documented, by contrast, optical noses based on modulation of optical properties are scarcer, even with a well-developed field of optical sensing materials available. Herein, we present the concept of the photonic nose, a novel combinatorial sensing platform whose operating principle is based on molecule-induced modulation of the optical Bragg diffraction properties of a pixelated nanoparticle 1D photonic crystal, in which each pixel has different surface-energy properties enabled through selective chemical functionalization. The photonic nose is a straightforward and low-cost, environmentally friendly and defect-tolerant combinatorial colorimetric sensor that is able to detect and discriminate vapor species like small molecules and bacteria volatiles with a simple digital-camera color-imaging system. These are considered the first steps towards the use of a photonic nose in chemical sensing and disease diagnostics. The development of an artificial nose based on the modulation of optical signals was first reported by Dickinson et al., using a fluorescence-based approach with a fiber-optics array. The use of self-encoded bead-assisted detection was a major breakthrough, opening the way to a number of applications such as attomolar DNA detection. Rakow and Suslick were the first to report an artificial nose based on a colorimetric approach by using an array of metal porphyrins, in which each type of porphyrin shows a different coordination constant with the vapor analytes, leading to unique color-change patterns upon binding of vapor-phase ligands or solvatochromic-induced effects. Further development of the concept with the incorporation of a larger variety of sensing species allowed discrimination of 100 volatile organic compounds. The approach described herein is distinct to the above and is instead based on the use of functional 1D photonic crystals comprised of multilayers of alternating refractive index, also known as Bragg stacks (BS). Functionality can be introduced into BS by the incorporation of (meso)porosity into the layered structure. This approach provides photoniccrystal architectures with a high surface area and tunable color upon infiltration and capillary condensation of solvent vapors. The color changes are typically monitored by reflectivity or transmissivity measurements, as the change in the effective refractive indexes upon infiltration results in a shift in the position of the Bragg diffraction peak. For the present study, we have employed a porous BS based on alternating SiO2 and TiO2 nanoparticulate layers, deposited by a simple spin-coating process at 2000 rpm, followed by 15min calcination steps at 450 8C after every bilayer deposition (a representative scanning electron microscopy (SEM) image and reflectance spectrum can be found in Fig. S1 of the Supporting Information). The obtained multilayered films are then laterally patterned by selective etching with a patterned mask put in conformational contact with the film surface, generating an array of nine 3mm 3mm squares. The squares are separately functionalized with different alkoxysilanes for the attainment of a combinatorial array with distinct surface energy characteristics, generating a proof-of-concept 3 3 array of surface functionalized BS. For this purpose, we incorporated the surface functionalities ethyl (Et), butyl, (Bu), hexyl (Hex), octyl (Oct), CF3(CF2)3(CH2)2–(CF4), CF3(CF2)5(CH2)2–(CF6), and CF3(CF2)7 (CH2)2–(CF8) as well as leaving one nonfunctionalized (NF) square. The effect of surface functionalization on the modulation of the optical properties upon vapor infiltration was effectively probed by environmental spectroscopic ellipsometry, using water-saturated nitrogen gas as probe. While we have employed pixels with different hydrophobicities as a proof of concept, the platform is highly versatile as any chemical or biochemical functionality could be incorporated, in principle, by use of the surface chemistry of silicon and metal oxides. Towards a cost-effective and simple platform for combinatorial measurements of color changes in BS arrays exposed to different saturated atmospheres, we have implemented, for the first time, color imagery analysis as an alternative to the conventional optical spectroscopic probe methods. A very similar approach has been reported for the analysis of colorimetric artificial noses. The vapor exposure experiments were performed in a simple configuration consisting of a sealed chamber containing the sample, which is connected to a solvent inlet. A digital camera and