We demonstrate an integrated planar Bragg grating sensor for the detection of oxygen by modification of the surface with a silica sol-gel containing immobilized methylene blue

Gates, J.C.

Grossel, M.C.

Parker, R.M.

Smith, P.G.R.

Wales, D.J.

English

Southampton (e-Prints Soton)

Integrated planar Bragg grating oxygen sensor D.J. Wales, R.M. Parker, J.C. Gates, M.C. Grossel, P.G.R. Smith Optoelectronics Research Centre & School of Chemistry University of Southampton Southampton, U.K. dw405@soton.ac.uk  Abstract— We demonstrate an integrated planar Bragg grating sensor for the detection of oxygen by modification of the surface with a silica sol–gel containing immobilized methylene blue.  Keywords:  oxygen  sensing;  Bragg  gratings;  evanescent  field; methylene blue; silica sol–gel. I.   INTRODUCTION The  detection  of  oxygen  is  required  in  a  diverse  set  of scientific  fields.  We  shall  present  a  planar  integrated  Bragg grating  sensor  with  a  chemically  modified  overclad  for  the detection of oxygen in gaseous or liquid environments.  A.  Oxygen sensing Quantitative and qualitative oxygen sensing is required for the understanding of many chemical processes across a diverse set  of  fields  including  medicine,  environmental  science  and chemical  synthesis.  For  example,  in  zymology,  quantitative oxygen  sensing  is  required  to  determine  and  understand  the enzyme kinetics of particular fermentation processes [1]. Typically  oxygen  sensing,  in  the  liquid  phase,  can  be achieved  through  the  use  of  modified  Clark  electrodes  [2]. However, there are many limitations associated with the Clark electrode  including  long  –  term  sensitivity,  as  the  active electrolyte  within  the  probe  is  consumed  over  time,  and potential  electrical  interference  issues  [2].  Furthermore, interrogation is unsuitable in flammable environments. Optical  fiber  and  integrated  optics  chemical  sensors overcome these limitations of electrical signal based sensing methods  because  optical  fibers  are  relatively  inert  to environmental noise and no spark risk. B.  Bragg grating chemical sensors It  has  been  previously  shown  that  an  optical  waveguide containing a Bragg grating is an effective method of detecting minute changes in refractive index [3]. Removal of the part of the waveguide’s clad allows the evanescent field of the optical mode to interact with an analyte. The  device  works  by  detecting  the  associated  effective index change when the analyte medium is altered. The change in effective index across the Bragg grating causes a shift in the Bragg  wavelength  of  the  grating.  The  relationship  between effective  index,  neff,  and  Bragg  wavelength,    is  shown  in Equation  (1),  neff  where    is  the  period  of  the  Bragg  grating.  For  a  silica waveguide  with  a  Bragg  grating  in  the  telecoms  region, isnm. II.  FABRICATION OF PLANAR BRAGG GRATING SENSORS The  waveguides  are  fabricated  in  flame  hydrolysis deposited layers of silica. Doping of the core layer allows a channel  waveguide  to  be  defined  using  a  process  known  as Direct  Grating  Writing  is  a  UV  laser  technique  that  allows simultaneous  definition  of  channel  waveguide  and  Bragg grating. This system differs from conventional direct writing by using an interferometer to generate a ~6 micron spot with a periodic interference pattern. Precise temporal modulation of the UV beam results in the generation of a periodic refractive index with the waveguide.  Reference [4] provides a thorough review  on  the  fabrication  and  development  of  planar  Bragg grating sensors. An aqueous solution of hydrofluoric acid is used to etch a “window” into the overclad (see Figure 1) and thus exposing the  Bragg  grating  to  the  surrounding  environment  [3].  In addition,  a  ~  50  nm  thick  layer  of  tantalum  pentoxide  is sputtered on to the exposed Bragg grating. The thin layer of higher refractive index tantalum pentoxide (relative to silica) “pulls”  the  evanescent  wave  of  the  mode  into  the  analyte medium and thus increases the sensitivity of the device [3]. The  Planar  Bragg  grating  devices  are  “pigtailed”  to polarizing maintaining fiber and typically interrogated using a broad band optical source and an optical spectrum analyzer.           Figure 1.   (a) Photograph of a pigtailed etched Bragg grating device (b) Schematic of a chemical sensing integrated planar Bragg grating waveguide III.  INTEGRATED BRAGG GRATING OXYGEN SENSOR Cao  et  al.  has  previously  demonstrated  an  optical  fiber based oxygen sensor incorporating methylene blue (MB) in a silica sol – gel [5]. It was shown that the absorption spectra of an  evanescent  field  changed  relative  to  the  concentration  of oxygen passed over the fiber. Therefore it was proposed that a We would like to thank the EPSRC for funding this research (a)  (b) planar  integrated  Bragg  grating  could be  used to  interrogate MB in a similar way. Replacement of the overclad layer above the Bragg grating with a silica sol–gel, containing immobilized MB,  could  be  used  as  oxygen  sensor  in  gaseous  and  liquid phases. This device differs from Cao’s system as it detects the change in refractive index of MB.  A.  Methylene blue MB  is  an  organic  dye  that  forms  intense  blue  colored solutions. Amongst other uses, MB is the main reagent of the notable “blue bottle” experiment in chemistry; the oscillating reaction between methylene blue and its reduced form, leuco-methylene blue (LMB) [6]. The oxidation of LMB to MB, due to ambient oxygen present, exhibits an explicit color change from colorless to blue, as shown in Figure 2. B.  Silica sol- gels The photochemistry of MB in a silica sol-gel matrix has been  investigated  [7].  Silica  sol-gels  are  inorganic  materials that consist of a porous silica matrix [8]. Thin glass films of silica sol-gel can be formed through conversion of the initial colloidal suspension of silica, known as a sol, into thin layers of a viscous silica gel by spin or dip-coating. Additional heat treatment  promotes  further  densification  into  solid  materials [8]. Examples of these silica sol-gel thin glass films have been used in optical applications. IV.  PRELIMINARY RESULTS It  was  believed  that  the  significant  color  change  upon oxidation of an aqueous solution of LMB to MB would have an associated change in refractive index, detectable by a Bragg grating  sensor. The  sensor consisted of  four  Bragg  gratings, three etched sensor gratings and one buried reference grating.   A blue colored aqueous solution of MB (of concentration 3x10-4  mol/L)  was  reduced  to  colorless  LMB  in  a  nitrogen environment.  An  exposed  planar  Bragg  grating  device  was immersed  in  this  stirred  solution  and  before  exposing  to  a restricted flow of air. Gradual color change from colorless to blue was observed, suggesting that LMB was oxidized to MB, confirmed  by  UV-visible  spectroscopy.  The  average  Bragg wavelength shift monitored on oxidation is shown in Figure 3.  Figure 2.   UV-visble spectra comparing methylene blue and leuco-methylene blue showing the loss of the absorption peak at ~660 nm.  Figure 3.   A shift of ~80 pm  in Bragg wavelength during the oxidation of aqueous leuco-methylene blue to methylene blue was observed when exposed to a restricted flow of air. This demonstrates the high sensitivity to changes in oxygen concentration of the simple Bragg device. The data is corrected for temperature variation. A Bragg wavelength shift of ~80 pm, corresponding to an effective index change of ~ 7 x 10-5, was observed during the oxidation of LMB to MB. The restricted air flow allowed for gradual oxidation over ~12 hours, highlighting the sensitivity to  subtle  changes  in  oxygen  concentration.  Temperature compensation  was  achieved  using  a  localized  buried  Bragg grating  within  the  device  and  a  scaling  factor  of  0.7  was applied to account for the different thermo-optic coefficients for silica and aqueous solvent.   V.  CONCLUSIONS: Here  we  show  preliminary  results  that  a  planar  Bragg grating can detect changes in the refractive index of methylene blue  in  solution  on  exposure  to  oxygen.  We  will  further develop Bragg grating sensors by immobilizing methylene blue into a sol-gel overlayer that will enhance sensitivity and allow for gas phase sensing. We shall present our latest work towards the production of a robust and reversible oxygen sensor. REFERENCES [1]  V. Bellon – Maurel, O. Orliac, Christen, P., “Sensors and measurements in solid state fermentation: a review,” Process Biochemistry, vol. 38, pp. 881 – 896, 2003 [2]  G.  Rao,  S.B.  Bambot,  S.C.W.  Kwong,  H.  Szmacinski,  J.  Sipior,  R. Holavanahali,  G.  Carter,  “Application  of  Fluorescence  Sensing  to Bioreactors,” in Topics in Fluorescence Spectroscopy: Probe design and chemical sensing, vol. 4, J.R. Lakowicz, Ed. New York: Plenum Press, 1994, pp. 417 – 444 [3]  R.M.  Parker,  J.C.  Gates,  M.C.  Grossel,  P.G.R.  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Application of Fluorescence Sensing to Bioreactors,”

Dual fluorescence of methylene blue encapsulated in silica matrix,”

In vacuo measurement of the sensitivity limit of planar Bragg grating sensors for monolayer detection,”

Integrated planar Bragg grating oxygen sensor

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