Article pubs.acs.org/ac
Surface Plasmon Resonance Sensor for Dissolved and Gaseous Carbon Dioxide Thomas Lang, Thomas Hirsch,* Christoph Fenzl, Fabian Brandl, and Otto S. Wolfbeis Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, 93040 Regensburg, Germany S Supporting Information *
ABSTRACT: We describe a novel kind of sensor for carbon dioxide. It is based on surface plasmon resonance (SPR) and a polymer blend that is capable of fully reversibly binding carbon dioxide. The interaction results in a change in the polarity and refractive index that can be detected via SPR. The sensor responds with high specificity. The method is simple and, unlike previous ones, enables continuous sensing over extended periods of time. It can be applied to sense both dissolved and gaseous carbon dioxide. The limits if detection of gaseous CO2 is as low as 10 ppm. (imaging).13 Other fields where sensors for CO2 are employed include integrated enzyme based fuel cells,14 the chemical or enzymatic reduction of CO2,15 in car exhaust16 and bioreactor monitoring,17 and as transducers in clinical biosensors for urea and creatinine.18 A recent article19 describes a scheme for detection (but not continuous sensing) of CO2 in a water/tetrahydrofurane mixture that also requires the presence of substantial quantities of water. It is based on the reaction of liquid dipropylamine to form a carbamate ionic liquid which, in turn, causes a nonfluorescent hexaphenylsilanole to form a fluorescent aggregate. While not specifically indicated in the article, we roughly calculate this method to have a limit of detection of around 0.1% of CO2 in a sample gas. Another recent article claims to present a sensor for CO2 but in essence is a cuvette test, requires the presence of constant quantities of fluoride, and acetonitrile as a solvent.20 This “sensor”, unlike true sensors, does not allow CO2 to be measured or even monitored in samples like seawater or blood. We have previously reported21 on a novel approach toward sensing of CO2 that is based on the use of a hydrophobic amidine capable of reversibly binding gaseous CO2 (gCO2) or dissolved CO2 (dCO2). The strong base N,N,N′-tributylpentan-amidine (first synthesized by Jessop’s group22 and here referred to as TBPAM) has a switchable hydrophilicity and undergoes a large change in hydrophilicity on (reversibly) binding CO2 via an acid−base reaction. This change is detected with the help of a solvatochromic dye, whose color is modulated by changes in the polarity of TB-PAM. The process of binding CO2 is fully reversible and, as will be shown later, accompanied by a change in refractive index (RI). We assumed that this finding may be applied to an RI-based sensing scheme for CO2, and that the
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limate change, a good part due to the emission of CO2, has attracted enormous attention in recent years and is likely to remain of scientific and public interest also the foreseeable future. Its most obvious consequence, that is, global warming, constitutes a threat on climate, present-day forms of life, and thus potentially has a large impact on economics and everyday life. Continuous monitoring (as opposed to single shot testing) of CO2 in the environment therefore has become of utmost importance. This holds for the atmosphere and seawater, the oceans being by far the largest reservoir of CO2. About one-third of emitted CO2 is dissolved there. The capacity of this enormous carbon sink is crucial and mainly driven by its pH value (which is around 8) and temperature.1 Increased quantities of carbon dioxide will increase the fraction of dissolved CO2 and thus will decrease the pH of the hydrosphere.2,3 Sensing of CO2 is, however, not limited to the environment. Safety regulations require CO2 levels to be monitored in the chemical industry, in workplaces, (sub)marine vessels, sewers, wells, tunnels (including subways), and mines. The largest single market for CO2 sensors certainly is in blood gas analysis.4,5 CO2 is an essential part of the blood buffer system and excreted via blood circulation to be finally exhaled via the lungs. The potentiometric method for clinical sensing of CO2 is making use of an electrode that is often referred to as the Severinghaus electrode.4 Conductometric and coulometric types of electrodes also have been reported but are rarely used for sensing CO2.6 Near infrared (NIR) and IR spectroscopy, as well as SPR sensors, have been applied but do not work well in watery solutions and are of limited sensitivity.7 Planar8 and fiber optic9 fluorescent sensors based on the use of pH indicator probe (and following the Severinghaus detection scheme using an internal buffer entrapped in a silicone polymer) also have been reported10 and seem to work well for (continuous) sensing of pCO2 in blood11 and in the detection of bacterial infection.12 Planar sensors for CO2 also enable spatially distributed sensing © 2012 American Chemical Society
Received: June 17, 2012 Accepted: October 8, 2012 Published: October 8, 2012 9085
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Sensor films were then exposed to dCO2 or to humidified gCO2, upon which the amidine (TB-PAM) in the film reacts with CO2 to finally produce the bicarbonate salt of the amidine (Scheme 1). This is accompanied by a substantial change in the
resulting changes can be detected via surface plasmon resonance (SPR)23 which is an established technique to detect minor changes in RI. This was indeed the case and has led to a novel method to continuously sense CO2. We show here that it can be applied over a wide range of concentrations, is fairly specific, fast, and reversible.
Scheme 1. Schematic of the Reaction of CO2 with TB-PAM to Result in a Change in Refractive Indexa
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EXPERIMENTAL SECTION The sensing chip consists of a glass slide with high refractive index (nD = 1.6100) coated first with a thin film of gold (50 nm) on top of a 5 nm chromium adhesion layer and then with a cocktail that consists of a solution of ethyl cellulose (4.9 mg), TB-PAM (1.5 μL; 3.9 mM), and Nile Red (0.7 mg, 2.1 mM) in 1.0 g of a toluene/ethanol mixture (10:1; v/v). Ethyl cellulose was chosen as a matrix polymer because it is highly permeable for CO2 but rejects protons and other ions.24 A spin coater (WS-400-6NPP-Lite, Laurell Technologies Corp.; www.laurell. com) was used to create a thin (230 ± 110 nm) and homogeneous sensor layer on the gold film of the glass support by placing 100 μL of the cocktail on the center of the glass slide and rotating it for 1 s at 1000 rpm and then for 15 s at 4500 rpm. The homogeneity of the film on the slides can be inspected by optical microscopy because the Nile Red added acts as an optical probe. (Note: Nile Red may as well be omitted because it is not involved in the SPR based sensing process). SPR measurements were performed with a 2-channel device (Biosuplar 6; from Mivitec GmbH; www.biosuplar.com) as schematically shown in Figure S1 in the Supporting Information. The flow cell (with a total volume of 40 μL) divides the chip into two equally sized channels, namely, a reference channel (exposed to nitrogen only), and a sensor channel. Both are connected to a peristaltic pump (ISM-930, from Ismatec; www.ismatec.com) that was operated at a flow rate of 40 μL/min. SPR signals (as the intensity of the reflected light at a constant angle) were recorded over time. To compare individual measurements in aqueous phase, the signals were calibrated and displayed as refractive index units (RIU; see Supporting Information (Figure S2). As calibration in terms of refractive index is not possible in case of gCO2, all values given for gCO2 have to be considered as relative units of refractive index (RU).
a
CO2 and water first form hydrocarbonic acid as an intermediate.
refractive index and, in turn, in the resonance angle of the surface plasmons. Ethyl cellulose was chosen as a matrix material because it is hydrophobic and prevents the penetration of water into the sensor layer. Yet, there is enough residual water trapped in the polymer matrix (because of the presence of TB-PAM) that reacts with CO2 to form carbonic acid and, finally, the hydrogen carbonate salt of protonated TB-PAM. Signal changes caused by dCO2 were monitored by passing bicarbonate (in concentrations between 228 μM and 570 mM) over the sensor layer. Figure 1 displays a typical increase in the
Figure 1. Change in refractive index of a sensor layer on exposure to varying concentrations of dCO2 (a = 14.3 mM, b = 28.5 mM, c = 42.8 mM, d = 57.0 mM, e = 114 mM) and to the reference solution (f).
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RESULTS AND DISCUSSION The response of the sensor to dissolved CO2 was tested with aqueous bicarbonate solutions in concentrations ranging from 250 μM to 550 mM. The ionic strength of all solutions was kept constant at 2.0 M by adding respective quantities of sodium chloride warrant that the change in refractive index was not caused by different concentrations of bicarbonate. A solution of sodium chloride with an ionic strength of 2.0 M was used as a reference and for recording a baseline before each addition of analyte. The partial pressure of carbon dioxide (the pCO2) of such solutions was calculated as described in the Supporting Information. The response to gCO2 was tested with mixtures of air and CO2. The single gases were obtained from Linde AG (www.linde.de) and mixed by computer-regulated mass flow controllers (MKS Inc.; www.mksinst.com) in order to obtain concentrations of gCO2 in air in the range from 0 to 3000 ppm (0−0.3%). The reference channel was flushed with nitrogen gas at the same flow rate. All gases were humidified by bubbling them through water.
SPR signal (expressed as refractive index units; RIU) exposure to increasing levels of dCO2. Figure S3 in Supporting Information shows how the angle of resonance changes on exposure of the sensor to a solution containing 23 μmol/L of bicarbonate which is equivalent to a pCO2 of 58 ×10−3 hPa. On switching back to the reference solution, the signal (Figure 1) almost completely returns to the baseline. The figure also shows an upward trend (+0.03% over 150 min), which is ascribed to an increase in temperature. Refractive indices, and thus SPR signals, are very sensitive to changes in temperature. The measurements described here were performed at room temperature without thermostatting the flow-through cell, and this was causing the drift. In parallel, experiments were performed with chips coated with a polymer matrix without TB-PAM. These yielded no change of the signal if exposed to different concentrations of dCO2. A calibration plot is given in Figure S3 in the Supporting Information and shows the response to dCO2 to be linear, with 9086
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a regression coefficient (r2) of 0.996 and within the concentration range from 228 μM to 570 mM of bicarbonate (corresponding to 0.06−145 hPa of CO2). The limit of detection (defined as 3·S/N) is 6.8 hPa. The (potentially interfering) effect of the pH value on the sensor was tested by exposing it to a rather strong (1 M) solution of hydrochloric acid. However, no signal change is detectable which demonstrates the beneficial effect of using ethyl cellulose (which is impermeable to protons but well permeable to CO2) as the main matrix material of the sensing film. We presume that other strong acids (formed in aqueous solution from acidic gases) also remain inert, but that weak acids with a measurable vapor pressure (such as acetic acid) in their nondissociated form will interfere. It is a fortunate incidence that the pH values of blood range from 6.9 to 7.6 and that seawater has an average pH of 8 where weak acids are (mainly) present in the anionic (i.e., charged) form and thus cannot enter the sensor film. As with other sensors for CO2, ammonia and volatile amines also are likely to interfere because they will react with carbonic acid and/or bicarbonate. The response to gaseous CO2 was studied next. If exposed to dry mixtures of synthetic air with CO2, the sensor chip does not give a signal change. If, however, the gases are first bubbled through water, a linear dependency between the change of the SPR signal and the concentration of gCO2 is found (see Figure 2). The sensor is capable of detecting as little as 10 ppm of
obtained for three cycles are given in Figure 3. Signal changes occur within typically 1−2 min both in the forward and the
Figure 3. Reversibility of the SPR sensor on cycling between humidified air and humidified air containing 3000 ppm of gCO2. The graph also indicates the relative signal change and the fast response time in both the forward and back direction.
back direction. This is much faster than the response to dCO2, and probably because of the slow diffusion of CO2 in water. Most notably, signal changes are fully reversible, which makes the system a reversible (and thus true) sensor for continuous monitoring of CO2 in flowing samples, such as air, seawater, or blood. As in all CO2 sensors based on acid−base chemistries, acidic gases and vapors, if capable of penetrating the sensor matrix, are likely to interfere. Typical species include NOx, SO2, gaseous hydrochloric acid, and volatile carboxylic acids. This was confirmed by exposing the sensor to a sample gas containing 60 ppm of NOx. The resulting signal change is ∼200 RU (which is higher than that for CO2) and attributed to the higher polarity of NOx compared to CO2. Also, it takes 20 times longer to establish the equilibrium, mainly because ethyl cellulose is much less permeable to NOx than to CO2. It shall be reminded here, however, that such species do not occur in samples where CO2 is most often sensed, that is, in seawater (pH 7.5−8.6) and whole blood (pH 6.9−7.6).
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Figure 2. Change in the SPR signal of a 230-nm film of ethyl cellulose doped with TB-PAM on exposure to varying concentrations of gaseous CO2.
CONCLUSIONS In summary, we have developed a fairly simple sensing scheme for CO2. It allows both dCO2 and gCO2 to be determined over a wide range of concentration and represents an attractive and fast alternative to classical sensors. Sample volumes can be as small as 20 μL which is important in clinical assays but of course plays no role when monitoring seawater or ambient air. It represents an indicator-free sensing scheme and there is no need for sample pretreatment. The method is likely to be interfered, however, by the same species that compromise the Severinghaus methods. Unlike single shot and organic solutions-based methods,19 it enables continuous sensing of CO2 both in gaseous and fluidic samples. The scheme also represents the first approach toward sensing changes in the polarity of a polymer21 by the SPR technique. In our estimation, it can be applied to continuously sense numerous other species undergoing a (reversible) chemical reaction for
gCO2, and response is linear in the concentrations range between 150 and 1500 ppm. In contrast to the situation with dCO2, the sensor can detect very low concentrations of gCO2. The presence of humidity obviously is mandatory, and its effect cannot be neglected. If the polymer film is made from ethyl cellulose only without adding any TB-PAM, the change in the signal for 1500 ppm gCO2 is of the same magnitude as the noise (5 RU). The signal does not change on switching from dry air to humidified air. If the gases are humidified, the signal change on exposure to 1500 ppm of gCO2 is about 20 RU. This can be explained by the swelling of ethyl cellulose when exposed to water. The reversibility of the sensor was tested by alternatively flushing the sensor chips with humidified air and with humidified air containing 3000 ppm of gCO2. Results 9087
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which an appropriate (so-called stimulus-responsive) polymer is available.
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ASSOCIATED CONTENT
S Supporting Information *
Synthesis of N,N,N′-tributyl-pentanamidine (TB-PAM), evaluation of the thickness of the sensitive polymer film, scheme of the SPR setup, calibration of the SPR signal, relationship between the molarity of bicarbonate solutions and the partial pressure of CO2, change in SPR angle on exposure to dissolved CO2, and calibration plots for dCO2. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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