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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 2 — Jan. 28, 2013
  • pp: 2018–2023
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Ultra-highly sensitive optical gas sensors based on chemomechanical polymer-incorporated fiber interferometer

Mi-Kyung Bae, Jung Ah Lim, Sangsig Kim, and Yong-Won Song  »View Author Affiliations


Optics Express, Vol. 21, Issue 2, pp. 2018-2023 (2013)
http://dx.doi.org/10.1364/OE.21.002018


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Abstract

We demonstrate a novel optical sensor for use in explosive gas detection, having a simple structure, ultrahigh sensitivity, room-temperature sensing/refreshing operation, and no local power requirements. The sensor relies on a fiber Fabry-Pérot interferometer prepared using poly(4-vinylpyridine), which induces cavity expansion upon absorption of nitrobenzene, thereby shifting the phase matching conditions of the resonating modes. An estimated sensitivity limit as low as 5 ppb was achieved.

© 2013 OSA

1. Introduction

In the present work, we developed an optical gas sensor for the detection of very low concentrations of nitrobenzene (NB), an explosive gas. Because of its low vapor pressure at room temperature, reliable NB gas sensors have not been available despite the need [2

2. P. C. Chen, S. Sukcharoenchoke, K. Ryu, L. Gomez de Arco, A. Badmaev, C. Wang, and C. Zhou, “2,4,6-Trinitrotoluene (TNT) chemical sensing based on aligned single-walled carbon nanotubes and ZnO nanowires,” Adv. Mater. (Deerfield Beach Fla.) 22(17), 1900–1904 (2010). [CrossRef] [PubMed]

,12

12. J. Villatoro, D. Luna-Moreno, and D. Monzon-Hernandez, “Optical fiber hydrogen sensor for concentrations below the lower explosive limit,” Sens. Actuators B Chem. 110(1), 23–27 (2005). [CrossRef]

]. The sensor design described here relies on a fiber Fabry–Pérot interferometer (FFPI) fabricated from poly(4-vinylpyridine) (P4VP), which can convert molecular interactions between the polymer and the explosive gas into a defined volume expansion, which translates into changes in the phase-matching conditions of the FFPI resonance [10

10. J. Liu, Y. Sun, and X. Fan, “Highly versatile fiber-based optical Fabry-Pérot gas sensor,” Opt. Express 17(4), 2731–2738 (2009). [CrossRef] [PubMed]

,13

13. W. E. Tenhaeff, L. D. McIntosh, and K. K. Gleason, “Synthesis of poly(4-vinylpyridine) thin films by initiated chemical vapor deposition (iCVD) for selective nanotrench-based sensing of nitroaromatics,” Adv. Funct. Mater. 20(7), 1144–1151 (2010). [CrossRef]

]. The sensor design ensures an ultrahigh sensitivity, relies on a simple fabrication process, and permits room-temperature operation, while retaining all of the advantages of conventional optical sensors. NB gas was detected at a concentration of 10 ppm by achieving a >40 nm shift in the FFPI spectrum. The maximum sensitivity was estimated at approximately 5 ppb, in view of the optical spectrum analyzer (OSA) resolution, 0.02 nm. The sensor could be refreshed at room temperature without the need for supplemental thermal energy.

2. Formation and characterization of an FFPI prepared using P4VP

A schematic illustration of the operation of a P4VP-based FFPI is presented in Fig. 1
Fig. 1 Schematic illustration of the FFPI sensing mechanism based on a chemomechanical P4VP layer. (a) The initial FFPI conditions provided a reference reflected spectrum with a fixed periodicity determined by the interference between the light beams reflected from multiple mirrors. (b) The interaction of NB molecules with the P4VP chains expanded the volume of the P4VP layer to alter the phase-matching conditions in the FFPI. The periodic spectrum then shifts to a longer periodicity, resulting in a spectral red-shift in the individual peaks. The shifts indicate the presence of gas molecules.
. The silica/P4VP and P4VP/air interfaces formed a FFPI by acting as partial mirrors, m1 and m2, from which the incident broadband light was reflected. As illustrated in Fig. 1(a), in the absence of the NB gas, the interference between the reflected beams corresponded to survival of specific selected wavelengths, R1 and R2. The spectral singularity depended on the geometric and density factors of the FFPI. As the polymer layer underwent a chemomechanical transition upon absorption of the NB gas, the physical length of the light path increased from L0 to L0 + ΔL via conformational changes in the P4VP polymer chains. The surviving wavelengths then shifted to R1’ and R2’ with the new set of mirrors m1’ and m2’, as illustrated in Fig. 1(b). This spectral shift was sensitive to the presence of NB gas.

Figure 2
Fig. 2 Experimental procedure used to prepare the FFPI sensor head, highlighting the simplicity and reliability of the design.
shows that the advantages of this sensor design include the simplicity of the device structure and fabrication process, and the high reliability of the sensing operation. Dimethylformamide (DMF) was used as a solvent to dissolve the P4VP powder to a concentration of 14.5 wt%. The high solubility of P4VP precluded the need for additional chemical or physical treatments, thereby simplifying the process. As shown in the Fig., the FFPI resonance cavity was fabricated by drop-casting 0.2 μl of the P4VP layer onto the end facet of an optical fiber ferrule. Prior to application of the P4VP drop, the surface of the fiber ferrule was cleaned with ethanol and acetone to prepare a physically homogeneous P4VP/SiO2 interface. After coating, the device was heated at 80°C for 2 h to remove any remaining solvent. The device was subsequently aged at room temperature for 4 h to complete the preparation of an optical mirror surface morphology on the P4VP layer.

The swelling of P4VP in the presence of NB reflects the P4VP chain rearrangements induced upon intermolecular diffusion, which disrupted the weak π-π interactions among the P4VP chains. These effects were consistent with the Fourier Transform Infrared Spectroscopy (FT-IR) analysis. As shown in Fig. 3(a)
Fig. 3 Characteristics of the P4VP layers. (a) FT-IR curves of a pure P4VP layer, an NB gas, and a NB + P4VP layer, demonstrating that the volume expansion arose from the ‘physical mixing’ among molecules. SEM images of (b) a P4VP-coated fiber ferrule, (d) a P4VP layer prior to NB absorption, and (d) a P4VP layer after swelling. The images provide direct support that the P4VP volume expanded in the presence of the NB gas. Ellipsometric analysis characterized the changes in the (e) refractive index and (f) physical thickness (beam path length). The insets in (e) show an image of the NB gas chamber and a schematic illustration of the experimental set-up.
, the P4VP peaks observed in the absence or presence of NB were preserved, and no new peaks were generated by, for example, the formation of chemical bonds between the P4VP and NB. The FT-IR curve of the NB-absorbed P4VP layer presented major peaks at 2931 cm–1, 1598 cm–1, 1416 cm–1, and 996 cm–1, corresponding to the P4VP-only peaks. The NB peaks at 1522 cm–1, 1346 cm–1, and 680 cm–1 were attributed to N = O nitro, C-N amine, and C-H bond stretches, respectively. These results indicated that the NB-P4VP interactions were not associated with any critical energy barriers to absorption or evaporation. As a result, the detection and refreshing of the NB gases were fast and reversible processes at room temperature. The thickness change in the P4VP layer resulted from the volume expansion, as verified using field-emission scanning electron microscope (FE-SEM) under low vacuum conditions (0.8 ~1.2 Torr) as shown in Fig. 3(b)3(d). Figure 3(b) shows the as-coated P4VP layer on a fiber ferrule, the end facet of which was treated with optical polish. A magnified view (see Fig. 3(c)) of the layer revealed an initial thickness of 44.6 μm prior to gas absorption. Exposure of the P4VP layer to the NB gas resulted in an increase in the thickness to 47.3 μm corresponding to the layer swelling process (see Fig. 3(d)). Unfortunately, the precise evaluation of the NB concentration was hindered by the sample loading process under the vacuum conditions of the SEM chamber. Spectroscopic ellipsometry was combined with a home-built NB gas chamber to facilitate the collection of information about the P4VP layer upon NB absorption. The sample was prepared by drop-casting the P4VP solution (14.5 wt%) onto a SiO2 wafer with the final sample size of 1.5 × 1.5 cm. The sensing measurements were performed in the presence of 680 nm light over 40 min. The properties of the P4VP layer were monitored by tracing the changes in both the refractive index and the optical path with respect to NB gas absorption, as shown in Figs. 3(e) and 3(f), respectively. The insets of Fig. 3(e) show the NB gas chamber and the experimental setup in the chamber. Importantly, the negative refractive index change of the P4VP was not observed to change upon volume expansion. The initial refractive index of the layer was 1.056 with a thickness of 3.00 μm. The changes in the thickness, determined by ellipsometry analysis, were very small due to a very thin initial thickness of the P4VP layer. The P4VP layer displayed compressive stress upon NB absorption, as indicated by the positive refractive index change.

3. Highly sensitive optical gas sensors for nitrobenzene

The end surface of an optical fiber ferrule was cleaned with ethanol and acetone, and 0.2 μl of the prepared P4VP solution was drop-casted onto the cleaned surface using a micro pipet. Solvent was eliminated by drying the sample for 4 h at room temperature, followed by heating at 80°C for 2 h in an oven. The sensor head was inserted into a tube-type gas chamber connected to the NB gas supply. The sensing operation was demonstrated using an erbium-doped fiber amplifier (EDFA) as a broadband light source. A three-port optical circulator was connected to the light source, sensor head, and OSA, thereby guiding the incident broadband light into the FFPI-loaded sensor head. Reflected light from the sensor head was redirected to the OSA and used as a reference in determining the FFPI spectral shift. Quantitatively controlled NB gas was supplied to the chamber using an NB gas bubbler to ensure a constant gas concentration. The sensor head was inserted into the chamber, which was then sealed to create isolated conditions. Figure 4(a)
Fig. 4 The optical gas sensors detected the degree of NB absorption. (a) Magnified views of the spectral shifts produced by the 10 or 13 ppm NB gases. Magnified views of the spectral shifts obtained upon absorption of (b) 10 ppm or (c) 13 ppm NB gas. (d) Modulation characteristics of the sensor upon absorption of 13 ppm NB gas. The sensing and refreshing were operated at room temperature.
shows the wavelength shifts due to the volume expansion of P4VP as a result of NB absorption in the presence of 10 or 13 ppm NB. The response time of the sensor was inversely proportional to the NB concentration because the NB concentration gradient between the surroundings and the P4VP layer determined the conditions of both absorption and diffusion of the NB molecules. Both concentrations of NB gas yielded spectral shifts in the FFPI that exceeded 40 nm. The irregular curves originated from the surface distortions of the P4VP layer that formed during the volume expansion process. These spectral distortions may potentially be reduced by optimizing the device processing conditions. Figures 4(b) and 4(c) show the optical spectral shifts observed within the time window 18–20.5 minutes, as shown in Fig. 4(a), for a gas concentration of 10 or 13 ppm, respectively. Note that a trade-off must be made between sensitivity and the sensing range. This permitted customization of the P4VP layer thickness for a specific sensing frame and with a designed sensitivity. In view of the scanning resolution of the OSA (2 × 10−2 nm) and the detected spectral shift (>40 nm), the maximum sensing resolution may potentially be less than 5 ppb. Modulation characteristics of the sensor in the time domain, are shown in Fig. 4(d). These results indicated that the sensing and refreshing processes were reversible at room temperature. The measurement recording was commenced as soon as the wavelength shift began, upon exposure of the sensor head to the NB gas. The first wavelength shift during the first ~4 min was 0.8 nm. In the absence of external mechanical, thermal, or chemical energy supplies, replacement of the NB gas with pure N2 gas refreshed the sensor head completely within ~3 min. The surviving wavelength subsequently returned almost to the initial levels, verifying the reversible operation of the all-fiber gas sensor. Figure 4 shows the results of the modulation test, in which the iteratively modulated sensor spectral shift was higher than that obtained from an on/off experiment, suggesting good sensitivity. The NB absorption process disrupted P4VP stacking during subsequent iterative modulation steps, thereby reducing the ‘motional barrier’ to the NB molecules. The P4VP layer, therefore, further increased the degree to which it absorbed. After six iterations, the spectral shift was almost twice the value of the first iteration, displaying a trend toward saturation. Because NB saturation was not observed within a single on/off modulation curve, the wavelength shift range may potentially be extended to both higher NB concentrations and to a wider temporal window for sensing.

The data reported here demonstrates excellent sensor sensitivity, room-temperature sensing operation, and a short sensing time in an all-fiber optical sensing device for detecting the presence of explosive gases.

Acknowledgment

This work was supported by the National Research Foundation (NRF) funded by the Ministry of Education, Science, and Technology (MEST), Republic of Korea (No. 2011-0028978).

References and links

1.

S. H. Lim, L. Feng, J. W. Kemling, C. J. Musto, and K. S. Suslick, “An optoelectronic nose for the detection of toxic gases,” Nat. Chem. 1(7), 562–567 (2009). [CrossRef] [PubMed]

2.

P. C. Chen, S. Sukcharoenchoke, K. Ryu, L. Gomez de Arco, A. Badmaev, C. Wang, and C. Zhou, “2,4,6-Trinitrotoluene (TNT) chemical sensing based on aligned single-walled carbon nanotubes and ZnO nanowires,” Adv. Mater. (Deerfield Beach Fla.) 22(17), 1900–1904 (2010). [CrossRef] [PubMed]

3.

E. Comini, “Metal oxide nano-crystals for gas sensing,” Anal. Chim. Acta 568(1-2), 28–40 (2006). [CrossRef] [PubMed]

4.

Y. C. Lee, H. Huang, O. K. Tan, and M. S. Tse, “Semiconductor gas sensor based on Pd-doped SnO2 nanorod thin films,” Sens. Actuators B Chem. 132(1), 239–242 (2008). [CrossRef]

5.

P. Lin and F. Yan, “Organic thin-film transistors for chemical and biological sensing,” Adv. Mater. (Deerfield Beach Fla.) 24(1), 34–51 (2012). [CrossRef] [PubMed]

6.

G. Lim, U. P. DeSilva, N. R. Quick, and A. Kar, “Laser optical gas sensor by photoexcitation effect on refractive index,” Appl. Opt. 49(9), 1563–1573 (2010). [CrossRef] [PubMed]

7.

S. Roh, T. Chung, and B. Lee, “Overview of the characteristics of micro- and nano-structured surface plasmon resonance sensors,” Sensors (Basel) 11(12), 1565–1588 (2011). [CrossRef] [PubMed]

8.

T. L. Yeo, T. Sun, and K. T. V. Grattan, “Fibre-optic sensor technologies for humidity and moisture measurement,” Sens. Actuators A Phys. 144(2), 280–295 (2008). [CrossRef]

9.

M. Niklès and F. Ravet, “Distributed fibre sensors; depth and sensitivity,” Nat. Photonics 4(7), 431–432 (2010). [CrossRef]

10.

J. Liu, Y. Sun, and X. Fan, “Highly versatile fiber-based optical Fabry-Pérot gas sensor,” Opt. Express 17(4), 2731–2738 (2009). [CrossRef] [PubMed]

11.

Z. Gu, Y. Xu, and K. Gao, “Optical fiber long-period grating with solgel coating for gas sensor,” Opt. Lett. 31(16), 2405–2407 (2006). [CrossRef] [PubMed]

12.

J. Villatoro, D. Luna-Moreno, and D. Monzon-Hernandez, “Optical fiber hydrogen sensor for concentrations below the lower explosive limit,” Sens. Actuators B Chem. 110(1), 23–27 (2005). [CrossRef]

13.

W. E. Tenhaeff, L. D. McIntosh, and K. K. Gleason, “Synthesis of poly(4-vinylpyridine) thin films by initiated chemical vapor deposition (iCVD) for selective nanotrench-based sensing of nitroaromatics,” Adv. Funct. Mater. 20(7), 1144–1151 (2010). [CrossRef]

14.

X. Wang, X. Wang, R. Fernandez, L. Ocola, M. Yan, and A. La Rosa, “Electric-field-assisted dip-pen nanolithography on poly(4-vinylpyridine) (P4VP) thin films,” ACS Appl. Mater. Interfaces 2(10), 2904–2909 (2010). [CrossRef]

15.

K. J. Albert, N. S. Lewis, C. L. Schauer, G. A. Sotzing, S. E. Stitzel, T. P. Vaid, and D. R. Walt, “Cross-reactive chemical sensor arrays,” Chem. Rev. 100(7), 2595–2626 (2000). [CrossRef] [PubMed]

16.

Y. Cong, Z. Zhang, J. Fu, J. Li, and Y. Han, “Water-induced morphology evolution of block copolymer micellar thin films,” Polymer (Guildf.) 46(14), 5377–5384 (2005). [CrossRef]

17.

S. Park, J. Y. Wang, B. Kim, W. Chen, and T. P. Russell, “Solvent-induced transition from micelles in solution to cylindrical microdomains in diblock copolymer thin films,” Macromolecules 40(25), 9059–9063 (2007). [CrossRef]

18.

M. Goodarzi, P. R. Duchowicz, M. P. Freitas, and F. M. Fernandez, “Prediction of the Hildebrand parameter of various solvents using linear and nonlinear approaches,” Fluid Phase Equilib. 293(2), 130–136 (2010). [CrossRef]

19.

P. Bustamante, M. A. Pena, and J. Barra, “The modified extended Hansen method to determine partial solubility parameters of drugs containing a single hydrogen bonding group and their sodium derivatives: benzoic acid/Na and ibuprofen/Na,” Int. J. Pharm. 194(1), 117–124 (2000). [CrossRef] [PubMed]

20.

N. Schuld and B. A. Wolf, “Polymer‐solvent interaction parameters” in Polymer Handbook, 4th ed., J. Brandrup, E. H. Immergut, and E. A. Grulke, eds. (Wiley, New York, 2003) p. 247.

OCIS Codes
(050.2230) Diffraction and gratings : Fabry-Perot
(060.0060) Fiber optics and optical communications : Fiber optics and optical communications
(060.2370) Fiber optics and optical communications : Fiber optics sensors
(160.5470) Materials : Polymers

ToC Category:
Sensors

History
Original Manuscript: December 3, 2012
Revised Manuscript: January 10, 2013
Manuscript Accepted: January 10, 2013
Published: January 17, 2013

Citation
Mi-Kyung Bae, Jung Ah Lim, Sangsig Kim, and Yong-Won Song, "Ultra-highly sensitive optical gas sensors based on chemomechanical polymer-incorporated fiber interferometer," Opt. Express 21, 2018-2023 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-2-2018


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References

  1. S. H. Lim, L. Feng, J. W. Kemling, C. J. Musto, and K. S. Suslick, “An optoelectronic nose for the detection of toxic gases,” Nat. Chem.1(7), 562–567 (2009). [CrossRef] [PubMed]
  2. P. C. Chen, S. Sukcharoenchoke, K. Ryu, L. Gomez de Arco, A. Badmaev, C. Wang, and C. Zhou, “2,4,6-Trinitrotoluene (TNT) chemical sensing based on aligned single-walled carbon nanotubes and ZnO nanowires,” Adv. Mater. (Deerfield Beach Fla.)22(17), 1900–1904 (2010). [CrossRef] [PubMed]
  3. E. Comini, “Metal oxide nano-crystals for gas sensing,” Anal. Chim. Acta568(1-2), 28–40 (2006). [CrossRef] [PubMed]
  4. Y. C. Lee, H. Huang, O. K. Tan, and M. S. Tse, “Semiconductor gas sensor based on Pd-doped SnO2 nanorod thin films,” Sens. Actuators B Chem.132(1), 239–242 (2008). [CrossRef]
  5. P. Lin and F. Yan, “Organic thin-film transistors for chemical and biological sensing,” Adv. Mater. (Deerfield Beach Fla.)24(1), 34–51 (2012). [CrossRef] [PubMed]
  6. G. Lim, U. P. DeSilva, N. R. Quick, and A. Kar, “Laser optical gas sensor by photoexcitation effect on refractive index,” Appl. Opt.49(9), 1563–1573 (2010). [CrossRef] [PubMed]
  7. S. Roh, T. Chung, and B. Lee, “Overview of the characteristics of micro- and nano-structured surface plasmon resonance sensors,” Sensors (Basel)11(12), 1565–1588 (2011). [CrossRef] [PubMed]
  8. T. L. Yeo, T. Sun, and K. T. V. Grattan, “Fibre-optic sensor technologies for humidity and moisture measurement,” Sens. Actuators A Phys.144(2), 280–295 (2008). [CrossRef]
  9. M. Niklès and F. Ravet, “Distributed fibre sensors; depth and sensitivity,” Nat. Photonics4(7), 431–432 (2010). [CrossRef]
  10. J. Liu, Y. Sun, and X. Fan, “Highly versatile fiber-based optical Fabry-Pérot gas sensor,” Opt. Express17(4), 2731–2738 (2009). [CrossRef] [PubMed]
  11. Z. Gu, Y. Xu, and K. Gao, “Optical fiber long-period grating with solgel coating for gas sensor,” Opt. Lett.31(16), 2405–2407 (2006). [CrossRef] [PubMed]
  12. J. Villatoro, D. Luna-Moreno, and D. Monzon-Hernandez, “Optical fiber hydrogen sensor for concentrations below the lower explosive limit,” Sens. Actuators B Chem.110(1), 23–27 (2005). [CrossRef]
  13. W. E. Tenhaeff, L. D. McIntosh, and K. K. Gleason, “Synthesis of poly(4-vinylpyridine) thin films by initiated chemical vapor deposition (iCVD) for selective nanotrench-based sensing of nitroaromatics,” Adv. Funct. Mater.20(7), 1144–1151 (2010). [CrossRef]
  14. X. Wang, X. Wang, R. Fernandez, L. Ocola, M. Yan, and A. La Rosa, “Electric-field-assisted dip-pen nanolithography on poly(4-vinylpyridine) (P4VP) thin films,” ACS Appl. Mater. Interfaces2(10), 2904–2909 (2010). [CrossRef]
  15. K. J. Albert, N. S. Lewis, C. L. Schauer, G. A. Sotzing, S. E. Stitzel, T. P. Vaid, and D. R. Walt, “Cross-reactive chemical sensor arrays,” Chem. Rev.100(7), 2595–2626 (2000). [CrossRef] [PubMed]
  16. Y. Cong, Z. Zhang, J. Fu, J. Li, and Y. Han, “Water-induced morphology evolution of block copolymer micellar thin films,” Polymer (Guildf.)46(14), 5377–5384 (2005). [CrossRef]
  17. S. Park, J. Y. Wang, B. Kim, W. Chen, and T. P. Russell, “Solvent-induced transition from micelles in solution to cylindrical microdomains in diblock copolymer thin films,” Macromolecules40(25), 9059–9063 (2007). [CrossRef]
  18. M. Goodarzi, P. R. Duchowicz, M. P. Freitas, and F. M. Fernandez, “Prediction of the Hildebrand parameter of various solvents using linear and nonlinear approaches,” Fluid Phase Equilib.293(2), 130–136 (2010). [CrossRef]
  19. P. Bustamante, M. A. Pena, and J. Barra, “The modified extended Hansen method to determine partial solubility parameters of drugs containing a single hydrogen bonding group and their sodium derivatives: benzoic acid/Na and ibuprofen/Na,” Int. J. Pharm.194(1), 117–124 (2000). [CrossRef] [PubMed]
  20. N. Schuld and B. A. Wolf, “Polymer‐solvent interaction parameters” in Polymer Handbook, 4th ed., J. Brandrup, E. H. Immergut, and E. A. Grulke, eds. (Wiley, New York, 2003) p. 247.

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