pubs.acs.org/Langmuir © 2009 American Chemical Society
Solid-State Dendrimer Sensors: Probing the Diffusion of an Explosive Analogue Using Neutron Reflectometry Hamish Cavaye,† Arthur R. G. Smith,† Michael James,§,^ Andrew Nelson,§ Paul L. Burn,*,† Ian R. Gentle,*,‡ S.-C. Lo,† and Paul Meredith† †
Centre for Organic Photonics & Electronics, The University of Queensland, Brisbane, Queensland 4072, Australia, ‡ School of Chemistry and Molecular Biosciences, The University of Queensland, Brisbane, Queensland 4072, Australia, §Bragg Institute, Australian Nuclear Science and Technology Organisation, PMB1, Menai, NSW 2234, Australia, and ^School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia Received May 18, 2009. Revised Manuscript Received June 26, 2009
Determining how analytes are sequestered into thin films is important for solid-state sensors that detect the presence of the analyte by oxidative luminescence quenching. We show that thin (230 ( 30 A˚) and thick (750 ( 50 A˚) films of a first-generation dendrimer comprised of 2-ethylhexyloxy surface groups, biphenyl-based dendrons, and a 9,9,90 ,90 -tetran-propyl-2,20 -bifluorene core, can rapidly and reversibly detect p-nitrotoluene by oxidative luminescence quenching. For both the thin and thick films the photoluminescence (PL) is quenched by p-nitrotoluene by ∼90% in 4 s, which is much faster than that reported for luminescent polymer films. Combined PL and neutron reflectometry measurements on pristine and analyte-saturated films gave important insight into the analyte adsorption process. It was found that during the adsorption process the films swelled, being on average 4% thicker for both the thin and thick dendrimer films. At the same time the PL was completely quenched. On removal of the analyte the films returned to their original thickness and scattering length density, and the PL was restored, showing that the sensing process was fully reversible.
Introduction Oxidative photoluminescence (PL) quenching utilizing conjugated polymers as the sensing material has proved to be one of the best of the many methods for sensing explosive analytes.1-5 For oxidative luminescence quenching to work efficiently the sensing material needs to be highly luminescent in the solid-state, have an affinity for the analyte, and have a lowest unoccupied molecular orbital (LUMO) energy sufficiently higher than the LUMO of the analyte.6 The sensing of analytes using the quenching of the PL is, in principle, a straightforward process. In the absence of an analyte, photoexcitation of the luminescent conjugated polymer leads to an exciton that can decay radiatively. However, in the presence of an analyte the excited electron of the exciton can be transferred to the LUMO of the analyte, and hence the PL is quenched. While luminescent conjugated polymers have been successfully used in sensors for explosive analytes,4,6-10 there is a number of issues that make them difficult to work with, including complex morphologies, reproducibility of syntheses, polydispersity, and the need to make relatively elaborate structures to reduce the close packing of the polymer chains.11 In addition, apart from *To whom correspondence should be addressed. (1) Thomas, S. W.; Joly, G. D.; Swager, T. M. Chem. Rev. 2007, 107(4), 1339– 1386. (2) Trogler, W. C. NATO ASI Workshop, Electronic Noses & Sensors for the Detection of Explosives; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2004. (3) Yinon, J. Forensic and Environmental Detection of Explosives; John Wiley & Sons: Chichester, 1999. (4) Toal, S. J.; Trogler, W. C. J. Mater. Chem. 2006, 16, 2871–2883. (5) Moore, D. S. Rev. Sci. Instrum. 2004, 75(8), 2499–2512. (6) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 11864–11873. (7) Sanchez, J. C.; Trogler, W. C. J. Mater. Chem. 2008, 18(26), 3143–3156. (8) Chen, L.; McBranch, D.; Wang, R.; Whitten, D. Chem. Phys. Lett. 2000, 330 (1-2), 27–33. (9) Sohn, H.; Sailor, M. J.; Magde, D.; Trogler, W. C. J. Am. Chem. Soc. 2003, 125(13), 3821–3830. (10) Chang, C.-P.; Chao, C.-Y.; Huang, J. H.; Li, A.-K.; Hsu, C.-S.; Lin, M.-S.; Hsieh, B. R.; Su, A.-C. Synth. Met. 2004, 144(3), 297–301. (11) Yang, J.-S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 5321–5322.
12800 DOI: 10.1021/la9017689
a recent report where the polymer was coated onto the outside of an optical fiber,12 most sensing has used very thin polymer films, of order
