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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], and is thought to be responsible for between 15% and 22% of the greenhouse effect [5] [6] [7]. Aqueous environments, including wetlands and oceans represent important natural components of the global whole [8] [9] [10] [11] and have the potential to become major sources of methane to the atmosphere in a warmer climate [12]. 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] [7], the remote location of the sources [13], and the lack of accurate and reliable measurements [14]. 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] [16] [17], with subsequent detection, by semi-conduction [15], infrared spectroscopy (Contros GmBH, pers. comm.) or mass spectrometry [16] [18]. 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] [19]. 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] [21] [22] [23] [24].
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]. SPR offers electrical passivity, light weight, high sensitivity [25] and reversibility, allowing continuous, high throughput operations [26]. SPR has been used for the detection of gaseous alcohol [27] and C1–C4 hydrocarbons [28], for the determination of pesticides in water [26] and more generally, environmental pollution monitoring [29] [28].
In our design, dissolved methane detection and measurements are achieved by depositing a sensitive and selective polymeric film on the SPR gold layer. The reactive film consists of cryptophane-A molecules distributed within an optically transparent polymer coating. Cryptophanes are synthetic organic compounds with a cage-like structure defining a lipophilic cavity, which can complex neutral molecular species [30]. The general structure of cryptophanes is given in Fig. 1(a): the cavity volume is determined by Z functionalities, the external properties are dependent on the X and Y groups. One of the smallest of the series, cryptophane-A (Fig. 1(b)), has an internal cavity suitable for accommodating small volatile compounds such as for instance methane, chlorofluorocarbons [2] or xenon [31]. In solution, it exhibits a strong affinity towards methane characterized by an association constant Ka=130 M-1 at 298 K in tetrachloroethane [2]. Once included in the cavity, methane behaves as an integral part of the cryptophane-A, being bond by weak van der Waals forces [32]. The process of inclusion is reversible and dependent of the concentration of guest molecules present in the environment [2] [33]. Previous laboratory studies have shown that the specific absorption of gaseous methane in cryptophane-A, incorporated in an organic transparent cladding, led to an increase of the refractive index of the polymer proportional to the amount of encapsulated methane [33]. This property was used to enhance the performance of evanescent wave optical fibre sensors for the detection of methane in air [33] and the detection of gases in oil-filled transformers [34] but has never been used to detect dissolved methane in water.
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] [34]. 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] following a multistep procedure. A two-step method was then developed starting from vanillyl alcohol and formic acid [36]. A recent procedure using scandium triflate under mild conditions was recently reported [37].
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] 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].
The polymer-cryptophane-A solution was drop-coated and spread over [28] 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].
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].
Accurate calibrations were carried out in a dissolved gas calibration system, adapted from [41] 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:
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] [43]. 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] 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], 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]. 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]. Although the manufactured components have intrinsically low noise [3], 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
Although detection limits should be calculated from blank measurements (i.e. in presence of no methane) [44], 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.
Calibrations show a good linear relationship up to 180 nM (Fig. 6(b)). The response of the sensor to concentrations between 180 and 300 nM is still reproducible despite a lower sensitivity. At methane concentrations greater than 300 nM, the calibration curve plateaus representing possible saturation of the sensor. The encapsulation of methane into cryptophane is a reversible process that depends on the equilibrium between the aqueous phase and the polymer. Saturation of the cryptophanes will therefore be related to the thermodynamic, e.g. the quantity of methane present in the aqueous solution and the association constant (Ka). The quality of the polymer may not have a role on the saturation of the sensing layer but on the diffusion of the gas through the polymer.
The current operation range of the sensor is in accordance with concentrations found in most of oceanic environments [45] and could be used for the investigation of key systems such deep sea hydrothermal plumes [46] [47], gas hydrate plumes [48] [49], continental shelf environments [50] [51] [52] or air-sea interface [8] [53].
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]. 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]. 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]), 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]. 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.
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