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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 16, Iss. 17 — Aug. 18, 2008
  • pp: 12607–12617
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A novel, low-cost, high performance dissolved methane sensor for aqueous environments

Cédric Boulart, Matthew C. Mowlem, Douglas P. Connelly, Jean-Pierre Dutasta, and Christopher R. German  »View Author Affiliations


Optics Express, Vol. 16, Issue 17, pp. 12607-12617 (2008)
http://dx.doi.org/10.1364/OE.16.012607


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Abstract

A new method for in-situ detection and measurement of dissolved methane in aqueous media/environments with a limit of detection of 0.2 nM (3σ, and t90~110s) and range (1–300 nM) is presented. The detection method is based on refractive index (RI) modulation of a modified PolyDiMethylSiloxane (PDMS) layer incorporating molecules of cryptophane-A [1] which have a selective and reversible affinity for methane [2]. The refractive index is accurately determined using surface plasmon resonance (SPR) [3]. A prototype sensor has been repeatedly tested, using a dissolved gas calibration system under a range of temperature and salinity regimes. Laboratory-based results show that the technique is specific, sensitive, and reversible. The method is suitable for miniaturization and incorporation into in situ sensor technology.

© 2008 Optical Society of America

1. Introduction

Methane (CH4) has been studied as an important atmospheric component for over 200 years [4

4. W. S. Reeburgh, “Global Methane Biogeochemistry,” Treatise on Geochemistry 4, 1–25 (2003).

], and is thought to be responsible for between 15% and 22% of the greenhouse effect [5

5. D. Amouroux, G. Roberts, S. Rapsomanikis, and M. O. Andreae, “Biogenic gas (CH4, N2O, DMS) Emission to the Atmosphere from Near-Shore of the North-western Black Sea,” Estuar. Coast. Shelf Sci. 54, 575–587 (2002). [CrossRef]

] [6

6. G. Rehder, R. W. Collier, K. Heeschen, P. M. Kosro, J. Barth, and E. Suess, “Enhanced marine CH4 emissions to the atmosphere off Oregon caused by coastal upwelling,” Global Biogeochem. Cycles 16, 10.1029/2000GB001391 (2002). [CrossRef]

] [7

7. E. J. Sauter, S. I. Muyakshin, J.-L. Charlou, M. Schlüter, A. Boetius, K. Jerosch, E. Damm, J.-P. Foucher, and M. Klages, “Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles,” Earth Planet. Sci. Lett. 243, 354–365 (2006). [CrossRef]

]. Aqueous environments, including wetlands and oceans represent important natural components of the global whole [8

8. H. W. Bange, U. H. Bartell, S. Rapsomanikis, and M. O. Andreae, “Methane in the Baltic and North Seas and a reassessment of the marine emission of methane,” Global Biogeochem. Cycles 8, 465–480 (1994). [CrossRef]

] [9

9. S. Houweling, T. Kaminski, F. Dentener, J. Lelieveld, and M. Heimann, “Inverse modeling of methane sources and sinks using the adjoint of a global transport model,” J. Geophys. Res. 106, 26137–26160 (1999). [CrossRef]

] [10

10. W. J. Mitsch and J. G. Gosselink, Wetlands (Wiley, New York, 2000).

] [11

11. E. T. Baker, R. N. Hey, J. E. Lupton, J. A. Resing, R. A. Feely, J. J. Gharib, G. J. Massoth, F. J. Sansone, M. Kleinrock, F. Martinez, D. F. Naar, C. Rodrigo, D. Bohnenstiehl, and D. Pardee, “Hydrothermal venting along Earth’s fastest spreading center: East Pacific Rise, 27.5°-32.3°S,” J. Geophys. Res. 107, 2130, doi:2110.1029/2001JB000651 (2002). [CrossRef]

] and have the potential to become major sources of methane to the atmosphere in a warmer climate [12

12. K. A. Kvenvolden, “Methane hydrate-a major reservoir of carbon in the shallow geosphere?,” Chem. Geol. 71, 41–51 (1988). [CrossRef]

]. However, their contribution to the global methane budget is not precisely known, due to our poor understanding of the different sources and processes that generate methane [8

8. H. W. Bange, U. H. Bartell, S. Rapsomanikis, and M. O. Andreae, “Methane in the Baltic and North Seas and a reassessment of the marine emission of methane,” Global Biogeochem. Cycles 8, 465–480 (1994). [CrossRef]

] [7

7. E. J. Sauter, S. I. Muyakshin, J.-L. Charlou, M. Schlüter, A. Boetius, K. Jerosch, E. Damm, J.-P. Foucher, and M. Klages, “Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles,” Earth Planet. Sci. Lett. 243, 354–365 (2006). [CrossRef]

], the remote location of the sources [13

13. R. W. Collier and M. D. Lilley, “Composition of shelf methane seeps on the Cascadia Continental Margin,” Geophys. Res. Lett. 32, L06609, doi:06610.01029/02004GL022050 (2005). [CrossRef]

], and the lack of accurate and reliable measurements [14

14. S. Kroger and R. J. Law, “Sensing the sea,” Trends in Biotechnology 23, 250–256 (2005). [CrossRef] [PubMed]

]. Information on the concentration and distribution of dissolved methane, in real time, would be of great value, therefore, in understanding the global methane cycle.

Current in-situ dissolved methane sensors are based on gaseous equilibration across a membrane [15

15. J. Bussell, G. Klinkhammer, R. W. Collier, P. Linke, F. Appel, K. Heeschen, E. Suess, M. A. De Angelis, and M. Masson, “Applications of the METS methane sensor to the in situ determination of methane over a range of timescales and environments.,” in EOS Trans. Am. Geophys. Union (1999).

] [16

16. R. T. Short, D. P. Fries, S. K. Toler, C. E. Lembke, and R. H. Byrne, “Development of an underwater mass spectrometry system for in situ chemical analysis,” Meas. Sci. Technol. 10, 1195–1201 (1999). [CrossRef]

] [17

17. S. De Gregorio, S. Gurrieri, and M. Valenza, “A PTFE membrane for the in situ extraction of dissolved gases in natural waters: Theory and applications,” Geochem. Geophys. Geosyst. 6, Q09005, doi:09010.01029/02005GC000947 (2005). [CrossRef]

], with subsequent detection, by semi-conduction [15

15. J. Bussell, G. Klinkhammer, R. W. Collier, P. Linke, F. Appel, K. Heeschen, E. Suess, M. A. De Angelis, and M. Masson, “Applications of the METS methane sensor to the in situ determination of methane over a range of timescales and environments.,” in EOS Trans. Am. Geophys. Union (1999).

], infrared spectroscopy (Contros GmBH, pers. comm.) or mass spectrometry [16

16. R. T. Short, D. P. Fries, S. K. Toler, C. E. Lembke, and R. H. Byrne, “Development of an underwater mass spectrometry system for in situ chemical analysis,” Meas. Sci. Technol. 10, 1195–1201 (1999). [CrossRef]

] [18

18. R. Camilli and H. Hemond, “NEREUS/Kemonaut, a mobile autonomous underwater mass spectrometer,” Trends in analytical chemistry 23, 307–313 (2004). [CrossRef]

]. However, the silicon membranes used are unable to decouple methane concentrations from variabilities in all of: temperature, pressure, and concentrations of longer-chain hydrocarbons [18

18. R. Camilli and H. Hemond, “NEREUS/Kemonaut, a mobile autonomous underwater mass spectrometer,” Trends in analytical chemistry 23, 307–313 (2004). [CrossRef]

] [19

19. R. Collier and G. Klinkhammer, “Applications of the METS Methane Sensor to the In-situ Detection of Methane Over a Range of Time Scales and Environments,” in RIDGE In situ Sensors Workshop(2000).

]. As a result, there is an increasing interest in developing an ability to both detect the presence of, and measure the concentration of dissolved gases in aquatic environments using optical methods [20

20. H.-D. Kronfeldt, H. Schmidt, H. Amann, B. D. MacCraith, M. Lehaitre, M. Leclercq, E. Bernabeu, B. Mizaikoff, and D. Grant, “Technical elements and Potential Application of Spectroscopy for Ocean Monitoring,” in OCEANS’98(1998), pp. 1780–1784.

] [21

21. B. Mizaikoff, “Mid-Infrared evanescent wave sensors - a novel approach for subsea monitoring,” Meas. Sci. Technol. 10, 1185–1194 (1999). [CrossRef]

] [22

22. T. Murphy, S. Lucht, H. Schmidt, and H.-D. Kronfeldt, “Surface-enhanced Raman scattering (SERS) system for continuous measurements of chemicals in sea-water,” J. Raman. Spectrosc. 31, 943–948 (2000). [CrossRef]

] [23

23. P. G. Brewer, G. Malb, J. D. Pasteris, S. N. White, T. Peltzer, B. Wopenka, J. Freeman, and M. O. Brown, “Development of a laser Raman spectrometer for deep-ocean science,” Deep Sea Res. I 51, 739–753 (2004). [CrossRef]

] [24

24. H. Schmidt, N. Bich Ha, J. Pfannkuche, H. Amann, H.-D. Kronfeldt, and G. Kowalewska, “Detection of PAHs in seawater using surface-enhanced Raman scattering (SERS),” Mar. Poll. Bull. 49, 229–234 (2004). [CrossRef]

].

The strategy we have adopted here is to use an indicating polymeric layer whose refractive index (RI) is modified during the absorption of methane. In our method, RI is measured by Surface Plasmon Resonance (SPR) [3

3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

]. SPR offers electrical passivity, light weight, high sensitivity [25

25. K. Ideta and T. Arakawa, “Surface plasmon resonance study for the detection of some chemical species,” Sens. Actuators B 13, 384–386 (1993). [CrossRef]

] and reversibility, allowing continuous, high throughput operations [26

26. E. Mauriz, A. Calle, A. Abad, A. Montoya, A. Hildebrandt, D. Barcelo, and L. M. Lechuga, “Determination of carbaryl in natural water samples by a surface plasmon resonance flow-through immunosensor,” Biosens. Bioelectron. 21, 2129–2136 (2006). [CrossRef]

]. SPR has been used for the detection of gaseous alcohol [27

27. B. C. Sih, M. O. Wolf, D. Jarvis, and J. F. Young, “Surface-plasmon resonance sensing of alcohol with electrodeposited polythiophene and gold nanoparticle-oligothiophene films,” J. Appl. Phys. 98, 10.1063/1061.2138373 (2005). [CrossRef]

] and C1–C4 hydrocarbons [28

28. T. Urashi and T. Arakawa, “Detection of lower hydrocarbons by means of surface plasmon resonance,” Sens. Actuators B 76, 32–35 (2001). [CrossRef]

], for the determination of pesticides in water [26

26. E. Mauriz, A. Calle, A. Abad, A. Montoya, A. Hildebrandt, D. Barcelo, and L. M. Lechuga, “Determination of carbaryl in natural water samples by a surface plasmon resonance flow-through immunosensor,” Biosens. Bioelectron. 21, 2129–2136 (2006). [CrossRef]

] and more generally, environmental pollution monitoring [29

29. C. Nylander, B. Liedberg, and T. Lind, “Gas detection by means of surface plasmon resonance,” Sens. Actuators 3, 79–88 (1982). [CrossRef]

] [28

28. T. Urashi and T. Arakawa, “Detection of lower hydrocarbons by means of surface plasmon resonance,” Sens. Actuators B 76, 32–35 (2001). [CrossRef]

].

Fig. 1. General structure of cryptophane hosts; the cavity volume is determined by Z functionalities, the external properties are dependent on the X and Y groups (a) and molecular structure of cryptophane-A (b).

Here we present the first laboratory results of the detection and measurement of dissolved methane by monitoring the RI of a cryptophane-A loaded polymer-indicating layer, using SPR. These results are the first step towards a new low-cost and high performance in-situ methane sensor for application in freshwater and marine aqueous environments.

2. Experimental section

2.1 Methane sensor

The methane sensor was made of two parts: the reactive layer (sensing layer) and the SPR chip (Fig. 2). The reactive layer was modified PolyDiMethylSiloxane (PDMS – Siloprene K1000 + cross-linker K11 from SigmaAldrich® - refractive index 1.412) loaded with cryptophane-A [33

33. M. Benounis, N. Jaffrezic-Renault, J. P. Dutasta, K. Cherif, and A. Abdelghani, “Study of a new evanescent wave optical fibre sensor for methane detection based on cryptophane molecules,” Proceedings of the 7th European Conference on Optical Chemical Sensors and Biosensors - EUROPT(R)ODE VII 107, 32 (2005).

] [34

34. M. Benounis, T. Aka-Ngnui, N. Jaffrezic, and J. P. Dutasta, “NIR and optical fiber sensor for gases detection produced by transformation oil degradation,” Sens. Actuators A 141, 76–83 (2008). [CrossRef]

]. PDMS is a readily available and cheap polymer, and only 5mg or cryptophane-A is used per sensors. Though a commercial supply of cryptophane is not yet available, when mass produced this layer should be inexpensive. The synthesis of cryptophane-A was first reported by Gabard and Collet [35

35. J. Gabard and A. Collet, “Synthesis of a (D3)-Bis(cyclotriveratrylenyl) Macrocage by Stereospecific Replication of a (C3)-Subunit,” J.C.S. Chem. Comm. 21, 1137–1139 (1981). [CrossRef]

] following a multistep procedure. A two-step method was then developed starting from vanillyl alcohol and formic acid [36

36. J. Canceill and A. Collet, “Two-step Synthesis of D3 and C3h Cryptophanes,” J.C.S. Chem. Comm. 9, 582–584 (1988). [CrossRef]

]. A recent procedure using scandium triflate under mild conditions was recently reported [37

37. T. Brotin, N. Roy, and J. P. Dutasta, “Improved Synthesis of Functional CTVs and Cryptophanes Using Sc(OTf)3 as Catalyst,” J. Org. Chem. 70, 6187–6195 (2005). [CrossRef] [PubMed]

].

Fig. 2. Cross section of the methane sensor incorporating a Spreeta 2000 chip (adapted from Chinowsky et al., 2003) and the sensing film. A: gold layer (10 mm×4 mm×50 nm); B: reactive layer (PDMS+cryptophane A- 50–100 µm).

The SPR chip used was a Spreeta 2000 (Texas Instruments®) based on the Kretschmann configuration [38

38. E. Kretschmann and H. Raether, “Radiative decay of nonradiative surface plasmons by light,” Naturforsch A 23, 2135–2136 (1963).

] and contained all of the optical components necessary for SPR measurements. This unit costs ~£250, enabling production of a low cost methane sensor. The sensor principle was described in Chinowsky et al. [3

3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

].

The polymer-cryptophane-A solution was drop-coated and spread over [28

28. T. Urashi and T. Arakawa, “Detection of lower hydrocarbons by means of surface plasmon resonance,” Sens. Actuators B 76, 32–35 (2001). [CrossRef]

] four Spreeta gold surfaces. After the solvent was vaporized, a homogeneous and thin layer varying in thickness between 50–100 µm on each of the sensors was obtained. In order to validate the concept of the sensor, a polymeric layer was prepared without cryptophane and coated on a Spreeta chip in the same way as the methane sensor.

The system was adapted to be fully immersed in water. All experiments were performed in artificial seawater, prepared from Q-water (18.2MΩ cm) and sodium chloride (analytical grade).

Data were recorded by the appropriate software (provided with the Spreeta kit), which determines the RI from SPR curves (angle of resonance vs. light intensity). In our setup one data point (RI measurement) was given by one SPR curve. The frequency of measurements was set to be the same as the integration time, i.e. the time to operate the sensor and analyze the curve, automatically chosen by the software during the initialization phase. The frequency varied between 25 and 42.5 ms. The sensor output was treated as a time-series and decomposed by a 30s moving window average method [39

39. P. J. Brockwell and R. A. Davis, The Analysis of Time Series: Theory and Methods (Springer-Verlag, New York, 1986).

].

2.2 Experimental set-up

Initial experiments (detection, reversibility, and calibration of the sensor) were performed at room temperature and atmospheric pressure by dissolving 20 ppm gas methane in artificial seawater (35 mg/L) prepared from degassed Q-water (18.2 MΩ cm) and NaCl (analytical grade). The highest concentration obtained with this set-up was 50 nM after 2h of bubbling. To test the reversibility of the response, the sensor was dipped in N2-purged seawater maintained at equal temperature ([CH4]=5 nM). Seawater samples were taken at regular timed intervals and analysed by headspace gas extraction followed by gas chromatography [40

40. J. W. Swinnerton and V. J. Linnenbom, “Determination of the C1 to C4 hydrocarbons in sea water by gas chromatography,” Journal of Gas Chromatography 5, 570–573 (1967).

].

Accurate calibrations were carried out in a dissolved gas calibration system, adapted from [41

41. M. Sosna, G. Denuault, R. W. Pascal, R. D. Prien, and M. Mowlem, “Development of a reliable microelectrode dissolved oxygen sensor,” Sens. Actuators B 123, 344–351 (2007). [CrossRef]

] and described in Fig. 3. The principle consists of sequential replacement of initially low concentration solution (in closed vessel B) with volumes of concentrated solution from a reservoir (A), and homogenization (via stirring) after each addition for 2 to 3 minutes. After each addition and homogenization, the sensor output values were recorded and averaged for 2 minutes. The calculated concentration in the cell was dependent on the number of replacements (i), the concentration of methane in the reservoir (Csat), the volume exchanged (Vex) and the volume of the measuring cell (Vtot), which remains constant. After the ith addition, the methane concentration is:

ci=ci1+csatVexVtotci1VexVtot
(1)

Equation (1) was used to construct calibration plots allowing a very large number of points due to the small volume displaced by the pump (50 µl). The system was post-calibrated at the end of the experiment by analysing water samples using headspace gas extraction followed by gas chromatography. For all experiments, temperature was recorded independently by accurate temperature probes (0.01°C).

Fig. 3. Schematic representation of the gas calibration system. A: saturated solution reservoir, B: measurement cell, C: outlet connector, D: temperature probe, E: sampling port, F: outlet pump connector, G: valve, H: pure methane gas, I: glass tubing, J: pump, K: SPR sensor, M: magnetic stirrer.

3. Results and discussion

3.1 Sensor characterization

- Temporal response

The sensor was dipped alternately in a degassed (5 nM) and in a 50 nM dissolved CH4 solution. The response to changes in dissolved methane concentration is presented in Fig. 4. Also shown, for comparison, are the equivalent responses of a Spreeta chip coated with a non-sensitive polymeric layer. A significant response can be observed for the methane sensor, whereas the RI of the non-sensitive polymeric layer did not change despite variations of dissolved methane concentration. The response time (t90) between low concentration (5 nM) and high concentration (50 nM) was 1.8 minutes, i.e. 2.4 sec.nM-1. However the response time to decreasing concentration is longer (~6.6 sec.nM-1) but this hysteresis effect is reduced by stirring as shown by the step at the C:D boundary of Fig. 4. Stirring increases the concentration gradient between the sensing layer and the water which would explain the observed response.

Fig. 4. Response of the sensor due to changes of dissolved methane concentration (solid line) compared with the response of non-sensing layer, i.e. without cryptophane (dashed line). Filled circles are the concentration of methane measured in control samples by gas chromatography. A: in degassed solution, B: in 50 nM CH4-solution, C: in degassed solution, D: in degassed solution with mixing.

- Water absorption

The sensing layer exhibits strong water absorption, as shown in Fig. 5, which is a common characteristic of PDMS-like polymers [42

42. R. Schirrer, P. Thepin, and G. Torres, “Water absorption, swelling, rupture and salt release in salt-silicone rubber compounds,” J. Mat. Sci. 27, 3424–3434 (1992). [CrossRef]

] [43

43. T. Shioda, N. Takamatsu, K. Suzuki, and S. Shichijyo, “Influence of water sorption on refractive index of fluorinated polyimide,” Polymer 44, 137–142 (2003). [CrossRef]

]. The sensing layer required immersion for ~10 minutes to stabilize the signal before methane measurements could be made.

Fig. 5. Water absorption in the polymer (filled circles). 10 minutes are necessary to obtain a stable baseline at room temperature. The presence of cryptophane-A in the polymer (grey squares) does not influence the water absorption.

- Sensitivity

Figure 6(a) shows the results of sensor calibration using the gas calibration system. The system was soaked for 15 minutes in the water before the first measurement was performed to take the water absorption effect into account. A good relationship was observed between methane concentration and the RI of the sensing layer (R2=0.9885). Previously Benounis et al. [33

33. M. Benounis, N. Jaffrezic-Renault, J. P. Dutasta, K. Cherif, and A. Abdelghani, “Study of a new evanescent wave optical fibre sensor for methane detection based on cryptophane molecules,” Proceedings of the 7th European Conference on Optical Chemical Sensors and Biosensors - EUROPT(R)ODE VII 107, 32 (2005).

] reported that variations of the RI were dependent on the quantity of methane present in the media. The sensitivity calculated from our calibration curve is 5.5 10-6 RIU/nM. Several calibration curves (Fig. 6(b)) were obtained under varying experimental conditions (direct bubbling or dissolved calibration systems), with an averaged R2 of 0.95, highlighting the reproducibility of the sensor output. Table 1 gives a comparison of the sensitivities obtained for a number of different sensors of the same design but with subtly varying sensing layer properties (due to manufacturing tolerances). With the exception of “sensor 1” and “sensor 2 – 30days”, calculated sensitivities were all in the same range (3.2 to 5.5 RIU/nM), emphasizing the reproducibility of the coating process. Reduced sensitivity could be the result of the lower quality of the sensing film due to, either the process of coating, or, the degradation of the layer over time. Contrary to Benounis et al. [33

33. M. Benounis, N. Jaffrezic-Renault, J. P. Dutasta, K. Cherif, and A. Abdelghani, “Study of a new evanescent wave optical fibre sensor for methane detection based on cryptophane molecules,” Proceedings of the 7th European Conference on Optical Chemical Sensors and Biosensors - EUROPT(R)ODE VII 107, 32 (2005).

], where the sensing film was stable for several months, it was noted that the sensing film detaches from the Au surface overtime when used in seawater. To counter this effect, the sensing layer was replaced every two weeks.

Fig. 6. Calibration curve obtained with “sensor 4” using the gas calibration rig (filled circles). Data were linearly fitted (R2=0.9885). Errors bars are 2 times the standard deviation on RI measurement. Samples (opened circles) were taken for control (a). Calibration curves obtained in different experimental conditions (direct bubbling and gas calibration rig). Filled circles: sensor 2 (direct bubbling); filled triangles: sensor 2–30 days (direct bubbling); grey squares: sensor 3 (direct bubbling); opened circles: sensor 3 (gas rig); opened triangles: sensor 4 (gas rig) (b and c).

- Noise, detection limits and operation range

Sensor noise places a limit on the concentration of dissolved methane detectable by the sensor [3

3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

]. To correct for this the noise for each sensor was calculated from the standard deviation of the measurement (3σ) during stable conditions (lowest concentration of methane and temperature). Results are given in Table 1. Three factors contribute to the noise observed in the Spreeta: detector noise, shot noise and LED fluctuations [3

3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

]. Although the manufactured components have intrinsically low noise [3

3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

], it is necessary to optimize the noise performance by choosing an appropriate integration of signals as well as the RI measurement-time. In our experiments, the integration time of SPR measurements was automatically chosen by the software during the initialization of the sensor and the RI measurement frequency was adapted to be the same as the integration time. The lowest noise level measured with our sensor was found to be 9 10-7 RIU (3σ – sensors 3 and 4). The noise level depends also on the experimental conditions as highlighted in the Table 1. The highest noise levels were obtained using the direct bubbling calibration system, which produced strong concentration gradients and agitation of the sensing layer and hence a high noise level.

Table 1. Sensitivity, noise and detection limits

table-icon
View This Table

Although detection limits should be calculated from blank measurements (i.e. in presence of no methane) [44

44. L. A. Currie, “Detection: International update, and some emerging dilemmas involving calibration, the blank and multiple detection decisions,” Chemom. Intell. Lab. Systems 37, 151–181 (1997). [CrossRef]

], they were obtained here from the noise levels measured in the sample with the lowest methane concentration, i.e. stable conditions. An estimate of the detection limits is given in Table 1 for each sensor. With the last set-up, which reduced the experimental noise level, the detection limit was 0.16 nM. To our knowledge, this is the first time that such low detection limits were measured for a dissolved methane sensor.

The current operation range of the sensor is in accordance with concentrations found in most of oceanic environments [45

45. W. S. Reeburgh, “Oceanic Methane Biogeochemistry,” Chem. Rev. 107, 486–513 (2007). [CrossRef] [PubMed]

] and could be used for the investigation of key systems such deep sea hydrothermal plumes [46

46. J.-L. Charlou and J.-P. Donval, “Hydrothermal methane venting between 12°N and 26°N along the Mid-Atlantic Ridge,” J. Geophys. Res. 98, 9625–9642 (1993). [CrossRef]

] [47

47. J. P. Cowen, X. Wen, and B. N. Popp, “Methane in aging hydrothermal plumes,” Geochim. Cosmoch. Acta 66, 3563–3571 (2002). [CrossRef]

], gas hydrate plumes [48

48. C. K. Paull, W. Ussler III, W. S. Borowski, and F. N. Spiess, “Methane-rich plumes on the Carolina continental rise: Associations with gas hydrates,” Geology 23, 89–92 (1995). [CrossRef]

] [49

49. J. L. Charlou, J. P. Donval, T. Zitter, N. Roy, P. Jean-Baptiste, J. P. Foucher, and J. Woodside, “Evidence of methane venting and geochemistry of brines on mud volcanoes of the eastern Mediterranean Sea,” Deep Sea Res. I 50, 941–958 (2003). [CrossRef]

], continental shelf environments [50

50. J. J. Middelburg, J. Nieuwenhuize, N. Iversen, N. Høgh, H. de Wilde, W. Helder, R. Seifert, and O. Christof, “Methane distribution in European tidal estuaries,” Biogeochemistry 59, 95–119 (2002). [CrossRef]

] [51

51. N. Shakova, I. Semiletov, and G. Panteleev, “The distribution f methane on the Siberian Arctic shelves: Implications for the marine methane cycle,” Geophys. Res. Lett. 32 (2005).

] [52

52. V. Kitidis, L. H. Tizzard, G. Uher, A. G. Judd, R. C. Upstill-Goddard, I. M. Head, N. D. Gray, G. Taylor, R. Duran, J. Iglesias, and S. Garcia-Gil, “The biogeochemical Cycling of Methane in Ria de Vigo, NW Spain: sediment Processing and Sea-Air exchange,” J. Mar. Syst. 66, 258–271 (2006). [CrossRef]

] or air-sea interface [8

8. H. W. Bange, U. H. Bartell, S. Rapsomanikis, and M. O. Andreae, “Methane in the Baltic and North Seas and a reassessment of the marine emission of methane,” Global Biogeochem. Cycles 8, 465–480 (1994). [CrossRef]

] [53

53. H. W. Bange, R. Ramesh, S. Rapsomanikis, and M. O. Andreae, “Methane in surface waters of the Arabian Sea,” Geophys. Res. Lett. 25, 3547–3550 (1998). [CrossRef]

].

3.2 Influence of environmental parameters

- Temperature

Figure 7 displays the effect of temperature on RI measurements for the polymeric layer (i.e. not loaded with Cryptophane-A) and the sensitive layer: the RI is inversely proportional to temperature. From these results it is evident that a high variation of temperature strongly influences the signal (2.35 10-5 RIU/°C) but it is unclear whether temperature influences the density of the polymer or the sensor performance itself. Changes in temperature modify the density of the polymer (i.e., number of C-H bonds per volume unit) and therefore the RI [54

54. G. L. Klunder, J. Bürck, H.-J. Ache, R. J. Silva, and R. E. Russo, “Temperature Effects on a Fiber-Optic Evanescent Wave Absorption Sensor,” Appl. Spectrosc. 48, 387–393 (1994). [CrossRef]

]. Spreeta chips are also dependent on temperature variations as they have no active temperature control and need to be calibrated for the temperature effect [3

3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

]. Future developments will require a detailed characterization of the temperature effect on the operating device. Once these effects have been characterized measurement of water temperature, and the temperature of the SPR chip will enable compensation and hence improved sensor accuracy. For the majority of applications in the environment one could argue that temperature changes are not expected to be rapid, and/or widely varying. However, further experiments are required to study the sensor output when operated in different temperature range, as it might affect the Ka of the encapsulation.

Fig. 7. Temperature effect on the sensor response (opened squares) compared with the response of a non-sensing layer (filled circles).

- Salinity

As the future sensor will operate in various conditions (from fresh water to marine environments) the effect of salinity on the sensor operability and response must also be tested. The salinity effect was tested by adding increasing concentrations of NaCl to the test apparatus to obtain varying salinities (0, 5, 10, 20, 25, 30, 35 and 40 PSU) whilst measuring the RI of the polymer. The salinity does not influence the RI of the coating (Fig. 8) as no significant RI changes were recorded.

Fig. 8. Salinity effect on the sensor response (filled circles and opened squares) compared with the response of a non-sensing layer (grey triangles).

4. Conclusions and perspectives

A novel, low-cost method for the in-situ determination of dissolved methane concentrations is presented, based on refractive index modulation of a PDMS layer loaded with cryptophane-A molecules. The sensor described is low-cost as it utilizes a low-cost SPR sensor (SPREETA [3

3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

]), uses a cheap polymer substrate, and extremely small quantities of the cryptophane (~5mg per sensor). We have developed a laboratory system to test the principle of detection and to calibrate the response to methane concentration changes.

Results showed that the method was suitable for the detection of methane at low concentrations (1–300 nM), typical of open ocean environments, with detection limits lower than 0.2 nM. Cryptophane-A promises specificity to methane and is insensitive to larger hydrocarbons [33

33. M. Benounis, N. Jaffrezic-Renault, J. P. Dutasta, K. Cherif, and A. Abdelghani, “Study of a new evanescent wave optical fibre sensor for methane detection based on cryptophane molecules,” Proceedings of the 7th European Conference on Optical Chemical Sensors and Biosensors - EUROPT(R)ODE VII 107, 32 (2005).

]. Our results show that the sensor is insensitive to the concentration of dissolved salts. Further investigation is required to evaluate cross-sensitivity effects from molecules of the same size as methane and an affinity constant suitable for encapsulation (ammonia, halomethanes).

Preliminary results obtained with a 50–100 µm thick membrane showed a quick response to increasing concentration (2.4 seconds per nM) but a hysteresis effect was observed when concentration was decreased. The hysteresis was reduced by maintaining a permanent concentration gradient between the sensing layer and the aqueous environment. Further development will include the optimization of the sensing layer: by reducing the thickness of the layer we expect a quicker diffusion of methane through the polymer and an optimization of the surface plasmon resonance measurement, as it is measured only within a few hundreds nanometers above the gold layer.

Laboratory experiments showed a degradation of the polymeric sensing film over time when the sensor was left in the water for more than two weeks. The degradation was both physical and chemical. It is possible to modify the chemical composition of the current polymer and to provide surface treatments of the gold to aid bonding. This will be undertaken in future research.

The calibration of the sensor was performed in the laboratory at room temperature (20°C). However, temperature influences dramatically the RI measurement. Further work will calibrate the sensor for different combinations of temperature and methane concentration.

Acknowledgments

This work has been supported and funded by EU-FP6-MoMAR network (MRTN-CT-2004-505026), NERC Technology Innovation Funds (NOCS), and NERC Core Strategic/Oceans 2025.

References and links

1.

E. Souteyrand, D. Nicolas, J. R. Martin, J. P. Chauvet, and H. Perez, “Behaviour of cryptophane molecules in gas media,” Sens. Actuators B 33, 182–187 (1996). [CrossRef]

2.

L. Garel, J. P. Dutasta, and A. Collet, “Complexation of methane and chlorofluorocarbons by cryptophane-A in organic solution,” Angew.Chem. Int. Ed. Engl. 32, 1169–1171 (1993). [CrossRef]

3.

T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, “Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor,” Sens. Actuators B 91, 266–274 (2003). [CrossRef]

4.

W. S. Reeburgh, “Global Methane Biogeochemistry,” Treatise on Geochemistry 4, 1–25 (2003).

5.

D. Amouroux, G. Roberts, S. Rapsomanikis, and M. O. Andreae, “Biogenic gas (CH4, N2O, DMS) Emission to the Atmosphere from Near-Shore of the North-western Black Sea,” Estuar. Coast. Shelf Sci. 54, 575–587 (2002). [CrossRef]

6.

G. Rehder, R. W. Collier, K. Heeschen, P. M. Kosro, J. Barth, and E. Suess, “Enhanced marine CH4 emissions to the atmosphere off Oregon caused by coastal upwelling,” Global Biogeochem. Cycles 16, 10.1029/2000GB001391 (2002). [CrossRef]

7.

E. J. Sauter, S. I. Muyakshin, J.-L. Charlou, M. Schlüter, A. Boetius, K. Jerosch, E. Damm, J.-P. Foucher, and M. Klages, “Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles,” Earth Planet. Sci. Lett. 243, 354–365 (2006). [CrossRef]

8.

H. W. Bange, U. H. Bartell, S. Rapsomanikis, and M. O. Andreae, “Methane in the Baltic and North Seas and a reassessment of the marine emission of methane,” Global Biogeochem. Cycles 8, 465–480 (1994). [CrossRef]

9.

S. Houweling, T. Kaminski, F. Dentener, J. Lelieveld, and M. Heimann, “Inverse modeling of methane sources and sinks using the adjoint of a global transport model,” J. Geophys. Res. 106, 26137–26160 (1999). [CrossRef]

10.

W. J. Mitsch and J. G. Gosselink, Wetlands (Wiley, New York, 2000).

11.

E. T. Baker, R. N. Hey, J. E. Lupton, J. A. Resing, R. A. Feely, J. J. Gharib, G. J. Massoth, F. J. Sansone, M. Kleinrock, F. Martinez, D. F. Naar, C. Rodrigo, D. Bohnenstiehl, and D. Pardee, “Hydrothermal venting along Earth’s fastest spreading center: East Pacific Rise, 27.5°-32.3°S,” J. Geophys. Res. 107, 2130, doi:2110.1029/2001JB000651 (2002). [CrossRef]

12.

K. A. Kvenvolden, “Methane hydrate-a major reservoir of carbon in the shallow geosphere?,” Chem. Geol. 71, 41–51 (1988). [CrossRef]

13.

R. W. Collier and M. D. Lilley, “Composition of shelf methane seeps on the Cascadia Continental Margin,” Geophys. Res. Lett. 32, L06609, doi:06610.01029/02004GL022050 (2005). [CrossRef]

14.

S. Kroger and R. J. Law, “Sensing the sea,” Trends in Biotechnology 23, 250–256 (2005). [CrossRef] [PubMed]

15.

J. Bussell, G. Klinkhammer, R. W. Collier, P. Linke, F. Appel, K. Heeschen, E. Suess, M. A. De Angelis, and M. Masson, “Applications of the METS methane sensor to the in situ determination of methane over a range of timescales and environments.,” in EOS Trans. Am. Geophys. Union (1999).

16.

R. T. Short, D. P. Fries, S. K. Toler, C. E. Lembke, and R. H. Byrne, “Development of an underwater mass spectrometry system for in situ chemical analysis,” Meas. Sci. Technol. 10, 1195–1201 (1999). [CrossRef]

17.

S. De Gregorio, S. Gurrieri, and M. Valenza, “A PTFE membrane for the in situ extraction of dissolved gases in natural waters: Theory and applications,” Geochem. Geophys. Geosyst. 6, Q09005, doi:09010.01029/02005GC000947 (2005). [CrossRef]

18.

R. Camilli and H. Hemond, “NEREUS/Kemonaut, a mobile autonomous underwater mass spectrometer,” Trends in analytical chemistry 23, 307–313 (2004). [CrossRef]

19.

R. Collier and G. Klinkhammer, “Applications of the METS Methane Sensor to the In-situ Detection of Methane Over a Range of Time Scales and Environments,” in RIDGE In situ Sensors Workshop(2000).

20.

H.-D. Kronfeldt, H. Schmidt, H. Amann, B. D. MacCraith, M. Lehaitre, M. Leclercq, E. Bernabeu, B. Mizaikoff, and D. Grant, “Technical elements and Potential Application of Spectroscopy for Ocean Monitoring,” in OCEANS’98(1998), pp. 1780–1784.

21.

B. Mizaikoff, “Mid-Infrared evanescent wave sensors - a novel approach for subsea monitoring,” Meas. Sci. Technol. 10, 1185–1194 (1999). [CrossRef]

22.

T. Murphy, S. Lucht, H. Schmidt, and H.-D. Kronfeldt, “Surface-enhanced Raman scattering (SERS) system for continuous measurements of chemicals in sea-water,” J. Raman. Spectrosc. 31, 943–948 (2000). [CrossRef]

23.

P. G. Brewer, G. Malb, J. D. Pasteris, S. N. White, T. Peltzer, B. Wopenka, J. Freeman, and M. O. Brown, “Development of a laser Raman spectrometer for deep-ocean science,” Deep Sea Res. I 51, 739–753 (2004). [CrossRef]

24.

H. Schmidt, N. Bich Ha, J. Pfannkuche, H. Amann, H.-D. Kronfeldt, and G. Kowalewska, “Detection of PAHs in seawater using surface-enhanced Raman scattering (SERS),” Mar. Poll. Bull. 49, 229–234 (2004). [CrossRef]

25.

K. Ideta and T. Arakawa, “Surface plasmon resonance study for the detection of some chemical species,” Sens. Actuators B 13, 384–386 (1993). [CrossRef]

26.

E. Mauriz, A. Calle, A. Abad, A. Montoya, A. Hildebrandt, D. Barcelo, and L. M. Lechuga, “Determination of carbaryl in natural water samples by a surface plasmon resonance flow-through immunosensor,” Biosens. Bioelectron. 21, 2129–2136 (2006). [CrossRef]

27.

B. C. Sih, M. O. Wolf, D. Jarvis, and J. F. Young, “Surface-plasmon resonance sensing of alcohol with electrodeposited polythiophene and gold nanoparticle-oligothiophene films,” J. Appl. Phys. 98, 10.1063/1061.2138373 (2005). [CrossRef]

28.

T. Urashi and T. Arakawa, “Detection of lower hydrocarbons by means of surface plasmon resonance,” Sens. Actuators B 76, 32–35 (2001). [CrossRef]

29.

C. Nylander, B. Liedberg, and T. Lind, “Gas detection by means of surface plasmon resonance,” Sens. Actuators 3, 79–88 (1982). [CrossRef]

30.

A. Collet, J.-P. Dutasta, B. Lozach, and J. Canceill, “Cyclotriveratrylenes and cryptophanes: Their synthesis and applications to host-guest chemistry and to the design of new materials,” in Supraolecular Chemistry I — Directed Synthesis and Molecular Recognition(1993), pp. 103–129.

31.

K. Bartik, M. Luhmer, J. P. Dutasta, A. Collet, and J. Reisse, “129Xe and 1H NMR Study of the Reversible Trapping of Xenon by Cryptophane-A in Organic Solution,” J. Am. Chem. Soc. 120, 784–791 (1998). [CrossRef]

32.

Z. Tosner, O. Petrov, S. V. Dvinskikh, J. Kowalewski, and D. Sandstrom, “A 13C solid-state NMR study of cryptophane-E:chloromethane inclusion complexes,” Chem. Phys. Lett. 388, 208–211 (2004). [CrossRef]

33.

M. Benounis, N. Jaffrezic-Renault, J. P. Dutasta, K. Cherif, and A. Abdelghani, “Study of a new evanescent wave optical fibre sensor for methane detection based on cryptophane molecules,” Proceedings of the 7th European Conference on Optical Chemical Sensors and Biosensors - EUROPT(R)ODE VII 107, 32 (2005).

34.

M. Benounis, T. Aka-Ngnui, N. Jaffrezic, and J. P. Dutasta, “NIR and optical fiber sensor for gases detection produced by transformation oil degradation,” Sens. Actuators A 141, 76–83 (2008). [CrossRef]

35.

J. Gabard and A. Collet, “Synthesis of a (D3)-Bis(cyclotriveratrylenyl) Macrocage by Stereospecific Replication of a (C3)-Subunit,” J.C.S. Chem. Comm. 21, 1137–1139 (1981). [CrossRef]

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46.

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47.

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49.

J. L. Charlou, J. P. Donval, T. Zitter, N. Roy, P. Jean-Baptiste, J. P. Foucher, and J. Woodside, “Evidence of methane venting and geochemistry of brines on mud volcanoes of the eastern Mediterranean Sea,” Deep Sea Res. I 50, 941–958 (2003). [CrossRef]

50.

J. J. Middelburg, J. Nieuwenhuize, N. Iversen, N. Høgh, H. de Wilde, W. Helder, R. Seifert, and O. Christof, “Methane distribution in European tidal estuaries,” Biogeochemistry 59, 95–119 (2002). [CrossRef]

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52.

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53.

H. W. Bange, R. Ramesh, S. Rapsomanikis, and M. O. Andreae, “Methane in surface waters of the Arabian Sea,” Geophys. Res. Lett. 25, 3547–3550 (1998). [CrossRef]

54.

G. L. Klunder, J. Bürck, H.-J. Ache, R. J. Silva, and R. E. Russo, “Temperature Effects on a Fiber-Optic Evanescent Wave Absorption Sensor,” Appl. Spectrosc. 48, 387–393 (1994). [CrossRef]

OCIS Codes
(010.4450) Atmospheric and oceanic optics : Oceanic optics
(130.6010) Integrated optics : Sensors
(280.4788) Remote sensing and sensors : Optical sensing and sensors
(010.0280) Atmospheric and oceanic optics : Remote sensing and sensors

ToC Category:
Atmospheric and oceanic optics

History
Original Manuscript: April 29, 2008
Revised Manuscript: May 28, 2008
Manuscript Accepted: May 28, 2008
Published: August 6, 2008

Citation
Cédric Boulart, Matthew C. Mowlem, Douglas P. Connelly, Jean-Pierre Dutasta, and Christopher R. German, "A novel, low-cost, high performance dissolved methane sensor for aqueous environments," Opt. Express 16, 12607-12617 (2008)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-17-12607


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References

  1. E. Souteyrand, D. Nicolas, J. R. Martin, J. P. Chauvet, and H. Perez, "Behaviour of cryptophane molecules in gas media," Sens. Actuators B 33, 182-187 (1996). [CrossRef]
  2. L. Garel, J. P. Dutasta, and A. Collet, "Complexation of methane and chlorofluorocarbons by cryptophane-A in organic solution," Angew.Chem. Int. Ed. Engl. 32, 1169-1171 (1993). [CrossRef]
  3. T. M. Chinowsky, J. G. Quinn, D. U. Bartholomew, R. Kaiser, and J. L. Elkind, "Performance of the Spreeta 2000 integrated surface plasmon resonance affinity sensor," Sens. Actuators B 91, 266-274 (2003). [CrossRef]
  4. W. S. Reeburgh, "Global Methane Biogeochemistry," Treatise on Geochemistry 4, 1-25 (2003).
  5. D. Amouroux, G. Roberts, S. Rapsomanikis, and M. O. Andreae, "Biogenic gas (CH4, N2O, DMS) Emission to the Atmosphere from Near-Shore of the North-western Black Sea," Estuar. Coast. Shelf Sci. 54, 575-587 (2002). [CrossRef]
  6. G. Rehder, R. W. Collier, K. Heeschen, P. M. Kosro, J. Barth, and E. Suess, "Enhanced marine CH4 emissions to the atmosphere off Oregon caused by coastal upwelling," Global Biogeochem. Cycles 16, 10.1029/2000GB001391 (2002). [CrossRef]
  7. E. J. Sauter, S. I. Muyakshin, J.-L. Charlou, M. Schlüter, A. Boetius, K. Jerosch, E. Damm, J.-P. Foucher, and M. Klages, "Methane discharge from a deep-sea submarine mud volcano into the upper water column by gas hydrate-coated methane bubbles," Earth Planet. Sci. Lett. 243, 354-365 (2006). [CrossRef]
  8. H. W. Bange, U. H. Bartell, S. Rapsomanikis, and M. O. Andreae, "Methane in the Baltic and North Seas and a reassessment of the marine emission of methane," Global Biogeochem. Cycles 8, 465-480 (1994). [CrossRef]
  9. S. Houweling, T. Kaminski, F. Dentener, J. Lelieveld, and M. Heimann, "Inverse modeling of methane sources and sinks using the adjoint of a global transport model," J. Geophys. Res. 106, 26137-26160 (1999). [CrossRef]
  10. W. J. Mitsch and J. G. Gosselink, Wetlands (Wiley, New York, 2000).
  11. E. T. Baker, R. N. Hey, J. E. Lupton, J. A. Resing, R. A. Feely, J. J. Gharib, G. J. Massoth, F. J. Sansone, M. Kleinrock, F. Martinez, D. F. Naar, C. Rodrigo, D. Bohnenstiehl, and D. Pardee, "Hydrothermal venting along Earth's fastest spreading center: East Pacific Rise, 27.5o-32.3oS," J. Geophys. Res. 107, 2130 doi:2110.1029/2001JB000651 (2002). [CrossRef]
  12. K. A. Kvenvolden, "Methane hydrate-a major reservoir of carbon in the shallow geosphere?," Chem. Geol. 71, 41-51 (1988). [CrossRef]
  13. R. W. Collier and M. D. Lilley, "Composition of shelf methane seeps on the Cascadia Continental Margin," Geophys. Res. Lett. 32, L06609, doi:06610.01029/02004GL022050 (2005). [CrossRef]
  14. S. Kroger and R. J. Law, "Sensing the sea," Trends in Biotechnology 23, 250-256 (2005). [CrossRef] [PubMed]
  15. J. Bussell, G. Klinkhammer, R. W. Collier, P. Linke, F. Appel, K. Heeschen, E. Suess, M. A. De Angelis, and M. Masson, "Applications of the METS methane sensor to the in situ determination of methane over a range of timescales and environments," in EOS Trans. Am. Geophys. Union(1999).
  16. R. T. Short, D. P. Fries, S. K. Toler, C. E. Lembke, and R. H. Byrne, "Development of an underwater mass-spectrometry system for in situ chemical analysis," Meas. Sci. Technol. 10, 1195-1201 (1999). [CrossRef]
  17. 1. S. De Gregorio, S. Gurrieri, and M. Valenza, "A PTFE membrane for the in situ extraction of dissolved gases in natural waters: Theory and applications," Geochem. Geophys. Geosyst. 6, Q09005, doi:09010.01029/02005GC000947 (2005). [CrossRef]
  18. R. Camilli and H. Hemond, "NEREUS/Kemonaut, a mobile autonomous underwater mass spectrometer," Trends in analytical chemistry 23, 307-313 (2004). [CrossRef]
  19. R. Collier, and G. Klinkhammer, "Applications of the METS Methane Sensor to the In-situ Detection of Methane Over a Range of Time Scales and Environments," in RIDGE In situ Sensors Workshop(2000).
  20. H.-D. Kronfeldt, H. Schmidt, H. Amann, B. D. MacCraith, M. Lehaitre, M. Leclercq, E. Bernabeu, B. Mizaikoff, and D. Grant, "Technical elements and Potential Application of Spectroscopy for Ocean Monitoring," in OCEANS'98(1998), pp. 1780-1784.
  21. B. Mizaikoff, "Mid-Infrared evanescent wave sensors - a novel approach for subsea monitoring," Meas. Sci. Technol. 10, 1185-1194 (1999). [CrossRef]
  22. T. Murphy, S. Lucht, H. Schmidt, and H.-D. Kronfeldt, "Surface-enhanced Raman scattering (SERS) system for continuous measurements of chemicals in sea-water," J. Raman. Spectrosc. 31, 943-948 (2000). [CrossRef]
  23. P. G. Brewer, G. Malb, J. D. Pasteris, S. N. White, T. Peltzer, B. Wopenka, J. Freeman, and M. O. Brown, "Development of a laser Raman spectrometer for deep-ocean science," Deep Sea Res. I 51, 739-753 (2004). [CrossRef]
  24. H. Schmidt, N. Bich Ha, J. Pfannkuche, H. Amann, H.-D. Kronfeldt, and G. Kowalewska, "Detection of PAHs in seawater using surface-enhanced Raman scattering (SERS)," Mar. Poll. Bull. 49, 229-234 (2004). [CrossRef]
  25. K. Ideta and T. Arakawa, "Surface plasmon resonance study for the detection of some chemical species," Sens. Actuators B 13, 384-386 (1993). [CrossRef]
  26. E. Mauriz, A. Calle, A. Abad, A. Montoya, A. Hildebrandt, D. Barcelo, and L. M. Lechuga, "Determination of carbaryl in natural water samples by a surface plasmon resonance flow-through immunosensor," Biosens. Bioelectron. 21, 2129-2136 (2006). [CrossRef]
  27. B. C. Sih, M. O. Wolf, D. Jarvis, and J. F. Young, "Surface-plasmon resonance sensing of alcohol with electrodeposited polythiophene and gold nanoparticle-oligothiophene films," J. Appl. Phys. 98, 10.1063/1061.2138373 (2005). [CrossRef]
  28. T. Urashi and T. Arakawa, "Detection of lower hydrocarbons by means of surface plasmon resonance," Sens. Actuators B 76, 32-35 (2001). [CrossRef]
  29. C. Nylander, B. Liedberg, and T. Lind, "Gas detection by means of surface plasmon resonance," Sens. Actuators 3, 79-88 (1982). [CrossRef]
  30. A. Collet, J.-P. Dutasta, B. Lozach, and J. Canceill, "Cyclotriveratrylenes and cryptophanes: Their synthesis and applications to host-guest chemistry and to the design of new materials," in Supramolecular Chemistry I ?? Directed Synthesis and Molecular Recognition(1993), pp. 103-129.
  31. K. Bartik, M. Luhmer, J. P. Dutasta, A. Collet, and J. Reisse, "129Xe and 1H NMR Study of the Reversible Trapping of Xenon by Cryptophane-A in Organic Solution," J. Am. Chem. Soc. 120, 784-791 (1998). [CrossRef]
  32. Z. Tosner, O. Petrov, S. V. Dvinskikh, J. Kowalewski, and D. Sandstrom, "A 13C solid-state NMR study of cryptophane-E:chloromethane inclusion complexes," Chem. Phys. Lett. 388, 208-211 (2004). [CrossRef]
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