4. Sensors for basic and acidic gases

pH indicators are not only useful for preparation of pH sensors but also for sensing of acidic or basic gases. Thus, indicators with high pKa values were immobilized into hydrophobic gas-permeable ethylcellulose matrix along with quaternary ammonium base (“plastic” or Mills type sensor). The fluorescent diketo-pyrrolo-pyrroles [25] were found to be very useful for ratiometric sensing and imaging of carbon dioxide (Fig. 4.1) including imaging with an RGB camera, [26] however the sensors possessed rather low photostability. In contrast, the colorimetric aza-BODIPY dyes featured unmatched photostability but required a rather sophisticated scheme (addition of two phosphor materials) to convert the colorimetric signal into referenced read-out via luminescence phase shift. [27]
Figure 4.1. Photographic image of the carbon dioxide sensor based on diketo-pyrrolo-pyrrole dye embedded into ethylcellulose under UV excitation. Reprodcued from [25].
Since the plastic CO2 sensors require base to function, their operational stability is a critical issue. In fact, the sensors are irreversibly poisoned by acidic gases present in environment, particularly SO2 or H2S (especially abundant in some marine sediments). These species can be oxidized to yield very strong non-volatile sulphuric acid, thus irreversibly protonating the pH indicator. Silicone rubber represents the most commonly used material which protects the carbon dioxide sensors from diffusion of ionic species, however it does not protect the sensor from poisoning. We demonstrated that using various perfluorinated polymers (Teflon AF, Hyflon AD, Cytop) as a protective layer dramatically improves the sensor lifetime at ambient conditions and in H2S-rich environments due to significantly lower permeability of these materials to such gases as H2S or SO2 (Fig. 4.2). [28] The new materials were shown to be suitable for long-term deployment (over 1 month) in marine environment. Unfortunately, the dynamic response of the sensors is also slower compared to those based on silicone rubber, but Hyflon AD represented a good compromise between the response times and protective properties.
Figure 4.2. Protective properties of silicone rubber and Hyflon AD 60. (A) Photographic images of the planar optode based on m-cresol purple and TOAOH in ethyl cellulose covered with the protective layers; (B) and (C) corresponding UV-VIS absorption spectra for the Hyflon AD 60 and silicone rubber, respectively. Reprodcued from [28].
In contrast to the carbon dioxide sensors, optical ammonia sensors rely on indicators having comparably low pKa values (< 5 for highly sensitive sensors). In these sensors, ammonia diffuses via a gas-permeable membrane into the sensing material and deprotonates the indicator. Our group reported highly sensitive optical ammonia sensors and microsensors based on fluorescent indicators fulfilling these requirements, namely ethyleosin and 2´,7´-dichlorofluorescein methylester. [29-31] The low pKa value of the indicators and the high solubility of ammonia in the cellulose polymers resulted into detection limits below 1 µg/L and a dynamic range between 5 and 1000 µg/L which makes the sensors promising for fish-farming applications. In the further work, a trace ammonia sensor based on bromophenol blue indicator (pKa 4.1) and fluorescent light-harvesting system was realized. [32] Combination of two coumarin dyes not only efficiently increased the effective Stokes shift but also resulted in high sensor brightness which enabled preparation of thinner sensing layer which faster response. In this work, we also demonstrated that substitution of cellulose acetate via polyurethane hydrogels significantly improves the sensitivity of the sensors. Fast response times were achieved by applying porous Teflon filter membrane as a protective layer. In fact, the t90 response time and the recovery response time between 10 μg/l and 100 μg/l of ammonia were determined to be 60 and 50 seconds, respectively (Fig. 4.3). Finally, referenced phase-fluorometric read-out via Dual Lifetime Referencing scheme was realized. For this purpose, reference microparticles containing a Ru(II) complex were added. Unfortunately, the stability of the sensors in water was not very high, and they showed noticeable drift after several days of operation.
Figure 4.3. Up: calibration plots for ammonia sensors ammonia sensors based on cellulose acetate (CA) and different polyurethane hydrogels with the indication of the ammonia toxicity level in fish of 25 μg/l (grey line); below: response and recovery of sensor B between 10 and 100 μg/l ammonia in 100 mM phosphate buffer at pH 7.2.
Reprodcued from [32].
Within the palette of aza-BODIPY pH indicators synthesized in our group several representatives bearing 2,6-dichlorophenol receptor possessed particularly low pKa values making them suitable for design of ammonia sensors. The spectral properties of the dyes and the used DLR reference (Egyptian blue) are fully compatible with the optoelectronics of a commercially available oxygen meter Firesting (Pyro Science) which made preparation of compact fiber-optic sensors possible (Fig. 4.4). [33] The stability of the sensors was very high (no drift within 1 week of operation in water) due incorporation of the indicator-containing hydrogel in form of emulsion in silicone rubber. As a trade-off, the sensors featured slow response and recovery times above 1 h.  
Figure 4.4. Response of the NH3-sensor based on aza-BODIPY dye and reference Egyptian blue phosphor. The insert shows the fiber optic sensors together with the read-out device from Pyro Science. Reprodcued from [33].

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References:

[25] Schutting, S.; Borisov, S. M.; Klimant, I. Diketo-Pyrrolo-Pyrrole Dyes as New Colorimetric and Fluorescent PH Indicators for Optical Carbon Dioxide Sensors. Anal. Chem. 2013, 85 (6), 3271–3279. [26] Schutting, S.; Klimant, I.; Beer, D. de; Borisov, S. M. New Highly Fluorescent PH Indicator for Ratiometric RGB Imaging of PCO2. Methods Appl. Fluoresc. 2014, 2 (2), 024001. [27] Schutting, S.; Jokic, T.; Strobl, M.; Borisov, S. M.; Beer, D. de; Klimant, I. NIR Optical Carbon Dioxide Sensors Based on Highly Photostable Dihydroxy-Aza-BODIPY Dyes. J. Mater. Chem. C 2015, 3, 5474–5483. [28] Fritzsche, E.; Gruber, P.; Schutting, S.; Fischer, J. P.; Strobl, M.; Müller, J. D.; Borisov, S. M.; Klimant, I. Highly Sensitive Poisoning-Resistant Optical Carbon Dioxide Sensors for Environmental Monitoring. Anal. Methods 2016, 9 (1), 55–65. [29] Waich, K.; Mayr, T.; Klimant, I. Fluorescence Sensors for Trace Monitoring of Dissolved Ammonia. Talanta 2008, 77 (1), 66–72. [30] Waich, K.; Mayr, T.; Klimant, I. Microsensors for Detection of Ammonia at Ppb-Concentration Levels. Meas. Sci. Technol. 2007, 18 (10), 3195–3201. [31] Waich, K.; Borisov, S.; Mayr, T.; Klimant, I. Dual Lifetime Referenced Trace Ammonia Sensors. Sensors and Actuators B: Chemical 2009, 139 (1), 132–138. [32] Abel, T.; Ungerböck, B.; Klimant, I.; Mayr, T. Fast Responsive, Optical Trace Level Ammonia Sensor for Environmental Monitoring. Chemistry Central Journal 2012, 6, 124. [33] Strobl, M.; Walcher, A.; Mayr, T.; Klimant, I.; Borisov, S. M. Trace Ammonia Sensors Based on Fluorescent Near-Infrared-Emitting Aza-BODIPY Dyes. Anal. Chem. 2017, 89 (5), 2859–2865.

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