Analytical Methods View Article Online PAPER View Journal | View Issue Highly sensitive poisoning-resistant optical carbon dioxide sensors for environmental monitoring Cite this: Anal. Methods,2017,9,55 Eva Fritzsche,a Pia Gruber,a Susanne Schutting,a Jan P. Fischer,b Martin Strobl,a c a a Jens D. Muller,¨ Sergey M. Borisov* and Ingo Klimant A new optical carbon dioxide sensor for environmental monitoring is presented. It combines a robust and long-term stable sensing material with a compact read-out device. The sensing material relies on a NIR pH indicator immobilized into ethyl cellulose along with a quaternary ammonium base. The perfluorinated polymer Hyflon AD 60 used as a protection layer significantly enhances the long-term and mechanical stability of the sensor foils, as well as the robustness against poisoning gases, e.g. hydrogen sulfide. The sensor can be stored under ambient conditions for more than six weeks, whereas sensors covered with silicone rubber deteriorate within one week under the same conditions. The complete sensor device is Received 27th October 2016 applicable after a three-point (re)calibration without a preconditioning step. The carbon dioxide Accepted 24th November 2016 production and consumption of the water plant Egeria densa was measured in the laboratory. Creative Commons Attribution 3.0 Unported Licence. DOI: 10.1039/c6ay02949c Furthermore, results of profiling carbon dioxide measurements during a research cruise on the Baltic Sea www.rsc.org/methods at water depths up to 225 m are presented. 1. Introduction chamber and the direct measurement of the absorption of the analyte. The sensors are very robust but show interference by Carbon dioxide is an analyte with great impact on the marine water vapour,4 condensation of which can present a serious ecosystem. Since the industrialization, started in the late 18th problem. Nevertheless, several IR carbon dioxide sensors have ™ century, the atmospheric level of carbon dioxide increased by been commercialized for marine applications (CO2-Pro CV , This article is licensed under a about 40% due to fossil-fuel burning or deforestation. More Pro-Oceanus Systems Inc., Canada; HydroC® CO2, Kongsberg than a half of the anthropogenically produced CO2 is taken up Maritime Contros GmbH, Germany). by the ocean.1 The rising amount of carbon dioxide in seawater The Severinghaus electrode shows interferences from elec- Open Access Article. Published on 29 November 2016. Downloaded 03/02/2017 13:23:49. causes a reduction of the pH (ocean acidication) and lower tromagnetic elds and is prone to dris due to osmotic pressure carbonate saturation in surface waters.2 The air–sea-exchange is effects.5 A similar design was adapted for use of an optical driven by the difference in the partial pressure of carbon transducer representing a solution of a pH indicator in a bicar- ff dioxide. The dissolved CO2 equilibrates with bicarbonate and bonate bu er. A commercially available sensor for aquatic CO2 carbonate ions. The carbonate system itself is characterized by measurements (SAMI CO2, Sunburst Sensors, LLC, USA) four measurable parameters: pH, TA (total alkalinity), DIC consists mainly of a membrane equilibrator connected to a ber ff (dissolved inorganic carbon) and fCO2 (fugacity of carbon optic ow cell. The ambient seawater carbon dioxide di uses dioxide). The complete system is determined when two of these through a gas-permeable membrane (silicone rubber) and parameters and the equilibrium constants, which are salinity changes the color of the bromothymol blue indicator due to – and temperature dependent, are known.3 a change in the pH of the buffer.6 8 However, the device is bulky Common analytical techniques for measuring carbon since the indicator solution has to be renewed for each dioxide in seawater directly are IR spectroscopy, the Sever- measurement, which can be particularly critical for long-term inghaus electrode and optical chemosensors. The sensors based trials due to the high volume of the reagent needed and the on IR spectroscopy rely on diffusion of the analyte molecules waste which has to be stored or released to the environment. through a gas-permeable membrane into an internal gas Moreover it demonstrates high energy consumption due to pumping and moving parts, which are prone to breakage. On the other hand, the advantage of the approach includes high a Institute of Analytical Chemistry and Food Chemistry, Graz University of Technology, Stremayrgasse 9/II, 8010 Graz, Austria. E-mail: [email protected] resistance to dri due to renewal of the solution. bPyroScience GmbH, Hubertusstraße 35, 52064 Aachen, Germany. E-mail: scher@ In the last decades optical carbon dioxide chemosensors 9–12 pyro-science.com (optodes) became increasingly popular. Although several cDepartment of Marine Chemistry, Leibniz Institute for Baltic Sea Research new concepts of optical carbon dioxide sensors have been Warnemunde,¨ Seestraße 15, 18119 Rostock, Germany. E-mail: jens.mueller@ proposed recently such as using viscosity or polarity-sensitive io-warnemuende.de This journal is © The Royal Society of Chemistry 2017 Anal. Methods,2017,9,55–65 | 55 View Article Online Analytical Methods Paper dyes as transducers,11,13,14 the so-called “plastic type” sensors (viscosity 1000 cSt), methylhydrosiloxane–dimethylsiloxane remain most popular.15–19 These sensors are based on a pH- copolymer (25–35 cSt), 1,3,5,7-tetravinyl-1,3,5,7-tetramethyl- sensitive indicator dye embedded in a polymer matrix along cyclotetrasiloxane (97%) (delayer) and platinum–divinylte- with a lipophilic quaternary ammonium base. The dye changes tramethyldisiloxane complex in vinyl terminated poly- its spectral properties according to the degree of protonation dimethylsiloxane (3–3.5% Pt) (catalyst) were obtained from induced by carbon dioxide. A wide range of indicators have been ABCR GmbH. Hyon AD 60 was acquired from Solvay GmbH, reported but only a few show favorable optical properties, high Teon AF 1600 from DuPont de Nemours GmbH and Cytop 809 photostability and sensitivity sufficient for measuring accu- A from AGC Chemicals. Anhydrous ethanol was purchased from rately at atmospheric levels of CO2. Fluorescent sensors also Merck. Sodium bicarbonate and cyclohexane were received from require a reference luminophore to obtain reliable results. In VWR. A poly(ethylene naphthalate) (PEN) support Teonex Q51 most cases an analyte-insensitive reference dye which possesses and poly(ethylene terephthalate) (PET) support Melinex 505 were ff adierent emission spectrum (ratiometric 2-wavelength received from Putz¨ GmbH + Co. Folien KG. The FireStingO2 and measurement) or a different luminescence decay time (Dual an oxygen sensor (OXR 230-0, oxygen retractable microsensor) Lifetime Referencing) is used.20–23 By far, hydroxypyrene trisul- were acquired from PyroScience GmbH. Optical plastic bres fonate (HPTS) has been the most popular uorescent indi- were obtained from Ratioplast-Optoelectronics GmbH. Per- cator.9,22,24,25 Unfortunately, the brightness, photostability and uorodecalin (PFD, 98%; cis and trans,ABCR)waswashedwitha1 0 the sensitivity of these sensors are far from being optimal. M aqueous solution of K2CO3 prior to use. Synthesis of 4,4 -(5,5- l4 l4 0 0 Recently reported BF2-chelated tetraarylazadipyrromethene di uoro-1,9-diphenyl-5H-4 ,5 -dipyrrolo-[1,2-c:2 ,1 -f][1,3,5,2]tri- indicators (aza-BODIPYs) represent a promising alternative.26–28 azaborinine-3,7-diyl)diphenol (di-OH-aza-BODIPY) and staining These highly photostable dyes absorb and emit in the near- of polystyrene-microparticles (PS-particles) with 3,7-bis(4-butox- infrared region of the electromagnetic spectrum which is yphenol)-5,5-diuoro-1,9-diphenyl-5H-4l4,5l4-dipyrrolo-[1,2-c:20,10-f] benecial due to low levels of autouorescence and availability [1,3,5,2]triazaborinine (di-butoxy-complex) were performed 28 Creative Commons Attribution 3.0 Unported Licence. of low cost excitation sources and photodetectors. Colorimetric according to Schutting et al. 2015. Silanized Egyptian blue was aza-BODIPY indicators were demonstrated to be very promising produced analogously to the literature procedure29 but using for the design of carbon dioxide sensors with a tunable dynamic 1H,1H,2H,2H-peruorooctyldimethylchlorosilane instead of tri- range.28 methylchlorosilane. The engraving pen used was a MICROMOT These (and other) promising indicators published recently 50/E with a 2 mm mounted point made of fused aluminium enable greater exibility of choice with respect to optical prop- oxide from PROXXON GmbH. The temperature control was erties, photostability and sensitivity. However, comparably little performed with a cryostat F12 from Julabo GmbH. The pH was effort has so far been put into improving the long-term stability determined with a pH meter SevenEasy combined with a pH of the CO2 optodes. Low stability of the sensing materials electrode InLab Routine Pro from METTLER-TOLEDO GmbH. This article is licensed under a hinders application of this promising technology in oceanog- raphy and many other elds. For example, the optode presented by Atamanchuk et al. 2.2. Preparation of sensor foils 2014 (ref. 23) showed sufficient stability only aer pre- 2.2.1. Planar optodes (model system) for dynamic Open Access Article. Published on 29 November 2016. Downloaded 03/02/2017 13:23:49. conditioning over several months during which the indicator response, poisoning and stability tests. 100 mg ethyl cellulose lost most of the original signal. Moreover, the calibration was and 1 mg m-cresol purple were dissolved in 2.4 g of tolue- time consuming and complicated, the sensor has to be stored in ne : ethanol mixture (6 : 4 w/w). The viscous solution was an aqueous solution and complete poisoning by hydrogen ushed with carbon dioxide and 100 mL of tetraoctylammonium ¼ m À1 sul de (cH2S 175 mol L ) was observed within 3 hours. hydroxide solution (20% w/w TOAOH in methanol) was added. In this contribution we present a chemically and photo- The “cocktail” was knife coated onto a PET foil to obtain chemically robust, long-term stable sensing material, based on a sensing lm with a thickness of 3 mmaer evaporation of a highly sensitive di-OH-aza-BODIPY indicator dye.