frameworks with gases: Towards Novel Exhaust Gas Sensors, Characterisation of Porous Materials 7 (CPM7), 3rd â 6th May 2015, Delray Beach, Florida, United ...
Optical Chemical Sensors & Materials D. J. Wales1,2, R. M. Parker1,2, J. C. Gates2, P. G. R. Smith2 & M. C. Grossel1 1Department
Fig.1: Simultaneous writing of a channel waveguide and Bragg gratings within the Ge doped silica core layer of a “topless” planar waveguide
of Chemistry & the 2Optoelectronics Research Centre, University of Southampton, UK
• Integrated optical sensors are planar lightwave circuits that combine optical components, such as optical waveguides and Bragg gratings, to fulfil the sensing/detection function. • Optical waveguides and Bragg gratings can be written simultaneously into a planar silica wafer with a UV laser set-up (Figure 1), to create a planar Bragg grating refractometer chip (Figure 2) • A Bragg grating reflects at one particular wavelength, the Bragg wavelength, and transmits all others. • The reflected Bragg wavelength, λB, is dependent on the refractive index, neff, to which the Bragg grating is exposed. • The observed shift in Bragg wavelength can be used to detect changes in the refractive index of this environment, as per λB = 2 neffΛ (Figure 3). • The Bragg grating refractometer sensors were produced in two ‘flavours’: “topless” and “etched”
“Topless” sensor chips • Some of the power of a propagating optical mode extends normally to the topmost waveguiding laver in a “topless” sensor – the evanescent wave. • Maximum interaction with the evanescent wave and target analyte/s is desired for largest sensor response. • This has been achieved through 3-D sensing materials – porous sol-gels and polymer layers. Covered Bragg gratings sense and uncovered Bragg gratings act as reference.
“Etched” sensor chips
Fig.7: Schematic of an etched Bragg grating sensor.
Fig.4: Illustration of a “topless” Bragg grating refractometer sensor chip and evanescent wave of the optical mode normal to the waveguiding layer of the “topless” device.
Fig.8: A schematic of the Bragg grating sensor showing the effect of a tantalum pentoxide (Ta2O5) high index overlayer. The optical mode is ‘pulled out’ further from the waveguiding core.
Hygrometer (humidity sensor)
Fig.5: A complex relationship between relative humidity and the average Bragg wavelength shift
• A simple mesoporous aluminosilicate thin film enabled operation across the range 0– 100 %RH & maintained long-term sensitivity. • Sensitivity of the device = (0.69 ± 0.05) %RH/pm, in the range 0–60 %RH within a gas flow system (Figure 5). • Long-term measurements, in static ambient lab conditions, were in excellent agreement with a commercial humidity sensor • Thermal compensation afforded by the onchip reference gratings was sufficient to prevent any drift in calibration.
Volatile Organic Compound detection • Bragg grating chip partially coated with a thin-film of a hydrophobic polysiloxane. • Upon exposure to VOCs, both negative & positive Bragg wavelength shifts were measured. • Attributed to a combination of swelling and/or hydrocarbon solvent filling the free volume within the polymer film. • QSPR approach utilised to create a multiple variable linear regression model • Model indicated that the degree of swelling was due to the VOC physico-chemical properties. • Allows for prediction of the Bragg wavelength shift that would be measured upon exposure to other VOCs.
Fig.2: Schematic of a planar integrated Bragg grating refractometer, illustrating the location of the eight Bragg gratings along the waveguide and their approximate Bragg wavelength. The readout system is connected to the chip using standard optical fibre and is robustly pigtailed using Fig.3: Reflectance spectrum of a Gaussian apodised standard telecomm assembly techniques. The reflectance Bragg grating demonstrating spectrum of a Gaussian apodised Bragg grating high side-lobe suppression demonstrating high side-lobe suppression
Fig.6: Correlation between predicted Bragg wavelength shift for each solvent and the corresponding measured Bragg wavelength shift. Training set = black and test set = red. The dashed line is a guide for the eye only.
• Predicted ∆∆λB= (-34.21×Log10KOW)(65.11×nHbA)+(103.15×RDBE)+25.44
• Etching away the covering silica layer exposes the Bragg gratings and the evanescent wave encounters the analyte. • A high index overlayer of Ta2O5 on the surface dramatically enhances the sensitivity for monolayer detection. • Used the TM mode to monitor temperature fluctuations and, in conjunction with the thermo-optic constants for the liquids of the system, a temperature-insensitive Bragg grating sensor can be fabricated
Microfludics system • High precision refractive index sensor integrated with a chemically resistant microfluidic flow system. • Cycling between different solvents showed that the previous solvent determined the nature of the refractive index profile across the transition in composition (Figure 9). • Demonstrated a device that exploited this effect for the unambiguous composition measurement of a binary solvent system. • The silica surface of the sensor can be chemically modified. Functionalising with (3aminopropyl)triethoxysilane results in a monolayer of primary amine moieties on the surface (Figure 10). • That allows for any active sensing head group to be attached via facile peptide coupling, which facilitates the discrimination/detection of different analytes and chemical properties such as cations, anions, organic/inorganic contaminants. • The sensor device was modified with a crown ether-functionalised supramolecular surface and the ability to detect binding of Group I cations was assessed (Figure 11). • Determined that for small chemical species, simple additive changes in film-thickness no longer explain change in n.
Fig.9: In the methanol– water transition system, transients in refractive index were observed that were an order of magnitude larger in amplitude than the difference between the bulk fluids.
Fig.10: General synthetic pathway for surface functionalisation. A monolayer of amines are covalently grafted on to the surface, which allows any chemical moiety (blue circle) to be attached via peptide coupling.
Fig.11: Bragg wavelength shifts of the crown ether monolayer sensor as a function concentration for LiBr, NaCl, KCl and RbCl salts in methanol, plotted on a logarithmic x-axis. Error bars show the peak deviation between measurements of ±2 pm.
1.C. Holmes, J. C. Gates, L. G. Carpenter, H. L. Rogers, R. M. Parker, P. A. Cooper, F. R. Mahamd Adikan, C. B. E. Gawith, P. G. R. Smith and S. Chaotan, Meas. Sci. Technol., 2015, 26, 112001. 2.D. J. Wales, Ph.D Thesis, University of Southampton, 2013. 3.D. J. Wales, R. M. Parker, J. C. Gates, P. G. R. Smith and M. C. Grossel, Sensors Actuators B Chem., 2013, 188, 857–866. 4.D. J. Wales, R. M. Parker, P. Quainoo, P. A. Cooper, J. C. Gates, M. C. Grossel and P. G. R. Smith, Manuscript in Preparation 5.R. M. Parker, J. C. Gates, M. C. Grossel and P. G. R. Smith, Appl. Phys. Lett., 2009, 95, 173306. 6.R. M. Parker, J. C. Gates, M. C. Grossel and P. G. R. Smith, Sensors Actuators B Chem., 2010, 145, 428–432. 7.R. M. Parker, J. C. Gates, D. J. Wales, P. G. R. Smith and M. C. Grossel, Lab Chip, 2013, 13, 377–85. 8.R. M. Parker, D. J. Wales, J. C. Gates, J. G. Frey, P. G. R. Smith and M. C. Grossel, Analyst, 2014, 139, 2774–2782.
D. J. Wales3, J. Grand6, Valeska P. Ting3, Richard Burke4, Karen Edler5, Andrew D. Burrows5, S. Mintova6 & Chris R. Bowen4 3Department
of Chemical Engineering, 4Department of Mechanical Engineering & 5Department of Chemistry, University of Bath, UK 6Laboratoire Catalyse et Spectrochimie, ENSICAEN, France
• MOFs and zeolites can be tailored for specific & selective interactions with gases; attractive as gas sensing materials/gas selective filters. • For exhaust gas sensing a materials needs to exhibit decent thermal stability (>150 oC), hydrolytic stability and chemical stability. • Current examples of MOFs and zeolites used for the detection of gases typically found in exhaust gas streams were reviewed by our group in Chem. Soc. Rev. • However, many of the specialist MOFs in the literature would not be suitable for exhaust gas sensing. • Therefore common and novel MOFs and zeolites were screened with a variety of techniques. Common MOF results shown.
Fig.13: In-situ FT-IR spectroscopy experimental set-up .
• Many common MOFs were screened with insitu FT-IR (Figure 13). • For in-situ FT-IR, MOFs were deposited as thin films on Si wafers. • IR spectra collected at 25 oC after an in-situ heat treatment at 200 oC under Ar flow • Analyte gases in Ar carrier gas • Key exhaust gases were investigated: oxygen (O2), carbon dioxide (CO2), carbon monoxide (CO), nitrogen dioxide (NO2) and nitric oxide (NO) (Figure 14). • Concentrations of exhaust gases in Ar carrier gas ranging from 1—10000 ppm • At each concentration a spectrum was collected every 60 s for 15 min —> this allowed for kinetics to be investigated Fig.14: Spectral features that could be attributed to adsorption of the
1.Dominic J. Wales, Julien Grand, Valeska P. Ting, Richard D. Burke, Karen Edler, Chris Bowen, Svetlana Mintova, Andrew D. Burrows, “Gas Sensing using Porous Materials for Automotive Applications”, Chem. Soc. Rev., 2015,44, 4290-4321 2.Dominic J. Wales, Julien Grand, Valeska P. Ting, Richard D. Burke, Karen Edler, Andrew D. Burrows, Svetlana Mintova, Chris R. Bowen, In-situ characterisation of the interaction of metal-organic frameworks with gases: Towards Novel Exhaust Gas Sensors, Characterisation of Porous Materials 7 (CPM7), 3rd – 6th May 2015, Delray Beach, Florida, United States of America.
gases on the metal ions or physisorption using literature IR data. .
