J Solid State Electrochem (2018) 22:51–60 DOI 10.1007/s10008-017-3721-1
ORIGINAL PAPER
Development of an iridium-based pH sensor for bioanalytical applications S. Bause 1,2 & M. Decker 1 & F. Gerlach 1 & J. Näther 3 & F. Köster 3 & P. Neubauer 2 & W. Vonau 1
Received: 24 May 2017 / Revised: 26 July 2017 / Accepted: 30 July 2017 / Published online: 7 August 2017 # Springer-Verlag GmbH Germany 2017
Abstract A new iridium-based planar pH sensor for bioanalytical purposes is introduced. The fabrication of the sensor was carried out by a two-stage coating process of different iridium solutions on a platinum thick film surface. The pH response behaviour and the Nernstian characteristics of the double-layer electrode exhibited better results than the single iridium depositions. An almost theoretical Nernstian slope could be obtained as well as a pH response time of about 3 to 5 min in a pH range of 4.01 to 9.18. Furthermore, a biofilm growth of different microorganisms onto the iridium-coated electrodes could be achieved. Afterwards, the viability of the microorganisms was demonstrated via cell plating studies. Keywords Iridium oxide . Electrodeposition . pH sensor . Biocompatibility . Bioanalytical application
Introduction The concept of pH was introduced by Arrhenius in the nineteenth century with its ionic theory which provides the relationship between hydrogen and hydroxyl ions. Sørensen defined 1909 the pH scale from 0 to 14 inferred from the ionic * S. Bause
[email protected]
1
Kurt-Schwabe-Institut für Mess- und Sensortechnik e.V. Meinsberg, Kurt-Schwabe-Straße 4, 04736 Waldheim, Germany
2
Technische Universität Berlin, Chair of Bioprocess Engineering, Ackerstraße 76, 13355 Berlin, Germany
3
Hochschule Mittweida, Fachgruppe Fertigungs- und Werkstofftechnik, University of Applied Sciences, Technikumplatz 17, 09648 Mittweida, Germany
product of water. He also invented the term pH as Bpondus Hydrogenii^ which he derived from the attempt to take the negative decadic logarithm of the hydrogen ion concentration. Together with Lindstrøm-Lang, they submitted the definition of pH, which is based on the activity of the hydrogen ions aH in solution [1]. pH ¼ −log ðaHþ Þ
ð1Þ
The pH value is an important biotechnological process parameter and plays a crucial role in biological systems. Microorganisms are thereby sensitive to pH fluctuation wherefore the acidity of the medium surrounding the microorganisms should be monitored [2–4]. A superior way for online detection in fluid cultures is the use of conventional glass electrodes. It has occupied a specific place between the pH measuring electrodes because of their excellent electrode performance such as long lifetime, detection limit and good selectivity. Instead of this, manifold commercial alternatives are present for example optical pH measurements or microsensors [5–10]. In the same way, all-solid-state electrodes with similar glass electrode characteristics offer to be a good alternative and therefore low-priced as well as biocompatible metal oxide electrodes like RuO2 or IrOx were developed [11]. Functionalized carbon electrodes were introduced for pH determination with advantages in temperature range, inexpensiveness, miniaturisation and mechanical robustness, and therefore they are suitable for harsh environments [12]. Further carbon paste electrodes incorporated with sulphatemodified nanosized α-Fe2O3 show a response time of 10 s, a linear response range from pH = 1.5 to pH = 12.5 with a slope of −58.5 ± 0.6 mV/pH at 25 °C and no hysteresis effects [13]. In a pH range from 0 to 6, solid-contact electrodes with a membrane made of polyvinyl chloride plasticized with bis(2ethylhexyl)phthalate and containing neutral pH-selective
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ionophore hexabutyltriamidophosphate and potassium tetrakis-p-Cloro-phenylborate cation exchanger were devised [14]. Li- and Na-molybdenum-oxide bronzes are a different type of metal-oxide pH ion-sensitive electrodes which show near-Nerstian behaviour in a range from pH = 4 to pH = 9 [15]. Another material for the fabrication of highly sensitive potentiometric pH electrodes is thin film PbO2. The electrodeposition of PbO2 onto carbon ceramic electrodes shows a near-Nernstian slope in a pH range from 1.5 to 12.5 [16]. For thin biofilms with a complex interfacial chemistry, the commercial available sensors are unfeasible as well as the glass electrodes, the last one especially because of their unsuitable bioanalytical application due to their non-planarity of the sensor membranes, pressure and temperature dependence and their adverse cleansing and miniaturisation properties. Herein, the pH environment must be detected beneath the biofilm directly. In many cases, iridium oxide electrodes were used for biomedical applications [11, 17, 18]. In comparison to other materials, e.g. electrodes based on antimony, titanium, ruthenium or palladium, iridium oxide films exhibit some crucial advantages such as a faster response over a wide pH range and an excellent biocompatibility [17, 19–21]. Iridium oxide films (IROFs) establish miscellaneous application areas like the use as implantable neural stimulating electrodes [21–24] or in electrochromic devices [25, 26] and pH sensors [17, 27–32]. Possible methods for the fabrication of iridium oxide films comprise sputtering techniques [28, 30, 33], atomic layer deposition [34], sol-gel process [35–38], melt oxidation [39], electrochemical growth [31] or electrodeposition [17, 25, 29, 40] to produce iridium layers with different characteristics. The Nernstian response obviously depends on the manufacturing process. While sputtering techniques and melt oxidation result in sensors with sensitivities close to theoretical Nernstian response (59 mV/pH) at 25 °C, electrochemical growth and electrodeposition exhibit super-Nernstian characteristics (> 59 mV/pH). The divergent Nernstian behaviour is attributed to iridium layer composition of various oxides and hydroxides with numerous iridium oxidation states from Ir3+ to Ir5+ associated to a transfer of more than one proton per electron in the potential generating reaction [41–43]. Prats-Alfonso et al. [17] used anodic electrochemical grown iridium oxide films (AEIROF) by the description of Cruz et al. for the fabrication of microelectrodes applied for the urea detection. In this paper, the manufacturing of a pH sensitive AEIROF electrode onto a platinum thick film electrode covered with metallic iridium located on an aluminium oxide chip is described. Therefore, the best electrochemical properties of Nernstian slope and response time of the different deposition methods could be achieved. This planar sensor is intended to be applied for
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the monitoring of pH changes in liquid cultures as well as beneath biofilms due to their distinguished biocompatibility.
Experimental Reagents, microorganisms, media, buffer solutions Iridium trichloride hydrate (IrCl3 ∙ xH2O, 99.9%), Agar for microbiology and yeast extract were purchased from Sigma-Aldrich (München, Germany). Sodium chloride (NaCl, 99%) was received from VWR International GmbH (Darmstadt, Germany). Potassium carbonate (K2CO3, 98%), calcium sulphate (CaSO4 ∙ 2H2O, 98%), tryptone/peptone ex casein and meat extract were obtained from Carl Roth (Karlsruhe, Germany). Potassium hydrogen phthalate (C 8 H 5 KO 4 , 99.9%) was received from Polskie Odczynniki Chemiczne (Gliwice, Poland). Potassium dihydrogen phosphate (KH2PO4) was obtained from Reanal (Budapest, Hungary). Oxalic Acid (H2C2O4, 97%), disodium hydrogen phosphate (Na2HPO4 ∙ 2H2O, 99.5%) and sodium hydrogen carbonate (NaHCO3, 97%) were purchased from Laborchemie Apolda (Apolda, Germany). Sodium tetraborate (Na 2 B 4 O 7 ∙ 10H 2 O, 99. 5%) was rec eived from Bola b GmbH (Bon n, Germany). Sodium hexabromoiridate(IV) (Na 2 [IrBr 6 ], 99.9%) was purchased from METAKEM (Usingen, Germany). Escherichia coli S17-1 λpir was obtained from DECHEMA Forschungsinstitut (Frankfurt, Germany). Iridium oxide electrode fabrication As basis for the iridium oxide electrodeposition, an 8 mm in diameter circular platinum thick film electrode screen printed on aluminium oxide (KSI, Germany) was used. For the AEIROF thin film deposition, a solution containing 0.2 mM of IrCl3 ∙ xH2O, 1 mM of H2C2O4 ∙ 2H2O and 5 mM of K2CO3 dissolved in deionised water was aged for 4 days at 37 °C. The electrodeposition was performed with a Gamry Instruments Interface 1000 Potentiostat/Galvanostat (Warminster, USA) on either platinum thick film electrodes previously immersed for 1 h in concentrated nitric acid or on dry and equilibrated iridium thin film electrodes. As counter electrode a platinum sheet with a surface of 2 cm2 and as pseudoreference electrode a platinum wire were used [21]. Working electrode and counter electrode were placed in a parallel arrangement at a reproducible distance of 1.5 cm with the pseudoreference electrode nearby the working electrode. After immersion into the iridium solution, the electrodeposition was executed by a linear sweep voltammetry
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method consisting of 100 cycles between the open circuit potential and 0.55 V vs. open circuit potential with a scan rate of 10 mV/s and a step size of 10 mV. The prepared electrodes were stored in a NBS buffer solution with pH 6.86. Previous works [21] described that this fabrication method leads to a sponge-like quasimorphous open structure with a reproducible stoichiometry of an iridium oxohydroxide with an oxidation state of Ir4.9+, derived from X-ray diffraction (EDX) analysis and X-ray photoelectron spectroscopy (XPS) studies. For iridium thin film plating (metallic iridium), a solution of 26 mmol/L sodium hexabromoiridate(IV) was used for the galvanic deposition. The platinum substrates were etched in nitric acid to remove contaminations. An inert anode with iridium/ruthenium mixed oxide coating (Metakem GmbH, Germany) was used. The cathodic current densities were adjusted to 0.5 mA/cm2, while an anode-cathode-surface-ratio of 2:1 or more is needed. The electrode distance amounted to 1.5 cm. Deposition current was controlled by PS-2000 (Sensortechnik Meinsberg GmbH, Germany). The deposition required a temperature of 80 °C. The process time was set to 20 min which resulted in iridium layer thicknesses of 250 nm. Energy-dispersive X-ray spectroscopy analysis with FEI Fig. 1 Electrode surfaces (from top left to bottom right) without iridium coating (platinum thick film), iridium oxohydroxide deposited, with a metallic iridium layer and with the double coated iridium (platinum—metallic iridium—iridium oxohydroxide). (The brown colouring of the electrode surface is caused due to image acquisition and the miscolouration of the blue covering layer due to the frequent application of the iridium electrodes)
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Helios NanoLab 660 DualBeam system (Eindhoven, Netherlands) showed a uniform distribution of iridium at the entire electrode surface. Sensor calibration and pH measurement The pH measurements were carried out with a Knick pHMeter 764 Multi-Calimatic (Berlin, Germany) by recording the potentials against a conventional external saturated Ag/ AgCl reference electrode (SSE) (KSI, Germany) and compared with a glass electrode (KSI, Germany) in standard buffer solutions of pH = 4.01 … pH = 9.18. To test the functionality of the deposition procedures, platinum thick film, iridium oxohydroxide, metallic iridium thin film electrodes and double coated iridium electrodes, all based on a platinum thick film electrode (KSI, Germany) were fabricated independently and the pH responses and Nernstian slopes of these electrodes have been compared. Subsequent for equilibration of the iridium thin film electrode surface the sensors were stored in a buffer solution of pH = 6.86 for 7 days because of the unsatisfying electrochemical properties direct after fabrication. Thereafter, the pH response characteristics were also measured in a range between pH = 4.01 and pH = 9.18.
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Fig. 2 SEM of the different iridium coating techniques onto platinum thick film. (Large iridium oxohydroxide regions are circled in red)
Microorganism, bioreactor preparation and cell plating study The biocompatibility test of Escherichia coli on the iridium oxide electrode was accomplished by set on a culture of E. coli in LB medium (5 g/L yeast extract, 10 g/L tryptone and 10 g/L NaCl). Culturing in a shake flask was executed at 37 °C and 150 rpm for 7 days. The electrode was fixed at the bottleneck
and the iridium oxohydroxide structure immersed into the culture broth. Afterwards, the electrodes were removed and either immersed into sterile distilled water and once laid down on a LB agar plate or in fresh LB medium for microscopic analysis. For cell plating study, the LB agar plates were incubated at 37 °C for 1 day. The scanning electron microscopy was performed with the FEI Helios NanoLab 660 DualBeam system followed after 1 day of drying of the electrode at room temperature.
Fig. 3 pH response behaviour (left) and Nernstian slope (right) of the platinum thick film electrode (n = 3)
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Fig. 4 pH response behaviour (left) and Nernstian slope (right) of the iridium oxohydroxide directly deposited onto platinum thick film electrodes (n = 3)
Results and discussion The utilisation of various iridium deposition techniques onto platinum thick film electrodes resulted in distinct surface colours as it can be seen in Fig. 1 where platinum thick film shows a uniform bright colour. The metallic iridium thin film fabrication technique also shows a uniform colouration. The pure and subsequent iridium oxohydroxide deposition leads to a characteristic discolouration along the edge of the electrode surface. In Fig. 2, a closer observation on the surface characteristics after iridium deposition is shown which has been executed with scanning electron microscopy (SEM) with FEI Helios system. Platinum thick film (light grey) is presented as a heterogeneous and deep fissured structure onto aluminium oxide (dark background). The platinum paste consists of various ingredients, like additionally bismuth oxide as fluxing agent, wherefore dark grey spots can be identified on the surface of the platinum matrix. The iridium oxohydroxide surface is nearly identical compared with the platinum thick film electrode.
However, on the entire surface, there can be seen bright grey spotted, nearly circular regions of iridium oxohydroxide. Larger locations of the uneven allocated iridium are marked by red circles. Depositions of metallic iridium are homogenous distributed over the whole surface. The heterogeneous coloured structure of platinum is entirely overlaid with iridium and none of the dark grey spots can be observed anymore. An additional deposition of iridium oxohydroxide on metallic iridium (double coating) leads to a rough and less structured surface. As described in the BExperimental^ section, a circular platinum thick film electrode provides the basis for subsequent iridium oxide electrode fabrication. For the avoidance of misinterpretation of the iridium coating efficiency, the pH responding characteristics of platinum itself were examined, shown in Fig. 3. As expected, platinum shows a quite decent pH response. An adjustment of the sensor to the current pH value will be assumed if the potential changes are less than 0.1 mV in an interval of 10 s. Unconditioned platinum thick film electrodes take four times longer in acidic and twice the time in neutral
Fig. 5 pH response behaviour (left) and Nernstian slope (right) of metallic iridium deposition and the dual layer iridium coating onto the platinum thick film electrodes (n = 9)
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sponge-like open structure with a possible unit formula of IrO1.1(OH)2.7⋅0.4H2O and a corresponding oxidation state of Ir4.9+, as Cruz et al. [21] mentioned. Olthuis et al. [31] stated that the exact stoichiometry for hydrous electrochemically grown iridium films is difficult to define which is caused due to many possible forms of iridium oxides and oxohydroxides and therefore various possible reaction mechanisms can occur. The hydroxylated oxides which develop during AIROF deposition can be considered as amphoteric sites and thus can take up or release a proton dependent on the pH of the solution which is exemplarily depicted for the trivalent oxidation state of iridium [31]. Fig. 6 pH drift measurements in different buffer solutions over 12 h of the dual coated iridium electrode
buffer solutions for stabilisation of the potential than the conditioned electrodes. However, in alkaline buffer solution, the pH adjustment of the untreated electrodes lasts just 200 s instead of more than 900 s as with conditioned electrodes. Unconditioned as well as conditioned electrodes exhibit a strong sub-Nernstian characteristic with −44.9 and −37.6 mV/pH at 25 °C which is not appropriate for the application in biological media. The standard deviation of the slope averages ±0.4 mV/pH for the unconditioned electrodes and ±5.1 mV/pH for the conditioned electrodes. The calibration curves of the unconditioned electrodes are ±10 mV parallelly shifted wherefore the standard deviations are apparent. In the case of the conditioned electrodes, the slope deviation of ±5.1 mV/pH indicates that there exists no parallel displacement and the calibration lines are widely different particularly with regard to the slope and the ordinate at the origin. At the beginning of the dual coating fabrication, both deposition methods of iridium were separately applied onto the platinum thick film electrodes for the evaluation of the Nernstian quality and response characteristics of each fabrication process. Iridium oxohydroxide (see Fig. 4) was generated at the platinum electrode via electrodeposition and resulted in a
IrOðOH Þ⇌IrOO− þ Hþ
ð2Þ
IrOðOH Þ þ Hþ ⇌IrOðOH 2 Þþ
ð3Þ
Based on this, the sensors indicated a response time of more than 600 s in all buffer solutions which is quite inconvenient. The Nernstian slope averages −56.6 mV/pH at 25 °C, which is almost conform to the theoretical Nernstian slope. The large error bars develop due to the concentrated nitric acid treatment. Therefore, the calibration lines are parallelly shifted about ±110 mVand the slope of the calibration plots varies in a range between −54.6 mV/pH and −59.0 mV/pH with a standard deviation of ±1.8 mV/pH. The deposition of metallic iridium onto platinum causes an improvement of responding time but particularly the dual coating process exhibits positive effects on the potentiometric electrode properties which are demonstrated in Fig. 5. Untreated electrodes require more than 600 s in acidic buffer solutions for pH equilibration. Neutral and alkaline solutions lead to a faster responding behaviour with up to 200 s. The electrode function is quite similar to the unconditioned platinum thick film electrodes and the standard deviation of
Fig. 7 pH response behaviour (left) and Nernstian slope (right) of the dual coated iridium electrode at different temperatures (n = 3)
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Fig. 8 Biofilm growth Escherichia coli onto a dual coated iridium electrode
the slope amounts ±3.5 mV/pH in a range of −41.6 mV/pH up to −52.1 mV/pH. A parallel displacement of ±25 mV between the calibration lines exist and a wider diversity in alkaline region is noticeable. A cause for the pH-sensitivity of the unconditioned electrodes may be the gradually formation of an Ir/Ir(OH)3 metal oxide during the immersion into the aqueous buffer solutions [42]. Ir þ 3H2 O⇌IrðOH Þ3 þ 3Hþ þ 3e−
ð4Þ
Through the conditioning, a slight but still unsatisfactorily enhancement in the electrode function can be admittedly acquired. It is stated by Juodkazyte et al. that Ir(OH)3 is unstable Fig. 9 Cell plating study of Escherichia coli after biofilm growth onto dual coated iridium electrodes
in oxygen-containing solutions and prone to be oxidised to constitute Ir(IV) oxide, whose hydrated sites can cause the improved measurement properties [42]. 4IrðOHÞ3 þ O2 ⇌4IrO2 þ 6H2 O
ð5Þ
Burke and Whelan described that these amphoteric characteristic in combination with the redox properties may be the reason for the deviation of the pH sensitivity from −59 mV/pH [31, 44]. A distinct enhancement of the slope standard deviation referring to the untreated metallic iridium electrodes of about ±1.6 mV/pH (mean of the slope, −47.7 mV/pH) could be
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received. So, too, were the iridium oxohydroxide electrodes a parallel shift admittedly with a definite decrease of about ±20 mV is the actuator for the error bars of the adjustment graph. The response times decrease in acidic solutions to 390 s for pH = 4.01 and 230 s for pH = 5.40. However, in neutral and alkaline range, the reaction time ascend to 300 and 490 s. Optimum results are preserved with the double coating fabrication technique. From pH = 4.01 to pH = 5.40, a response under 3 min can be reached. In neutral pH range, equilibration lasts a little more than 4 min (pH = 6.86) and 5 min (pH = 7.60). Only alkaline pH adjustment has taken more than 5 min. Especially remarkable is the super-Nernstian slope with −63.3 mV/pH at 25 °C. Calibration slopes of −60.1 mV/pH up to −65.2 mV/pH with a standard deviation of ±1.6 mV/pH could be gained. The progressively rise of the error bars with further treatment of the electrodes is likewise recognisable. Through the funding of multiple iridium layers, the chemical constitution of the electrodes varied resulting in increasing error bars while fabrication process. A parallel shift of ±60 mV causes the increasing error bars, ideally. It can be assumed, that the single iridium oxohydroxide deposition on platinum causes a longer responding time and possesses a flatter electrode function than the deposition onto metallic iridium due to the formation of a mix-potential. Thus, the dual coating technique leads to an entirely by iridium generated potential. For the verification of the electrode drift behaviour (see Fig. 6), pH measurements in different buffer solutions were carried out. After 10 min of equilibration in the pH buffer systems, the reached value was chosen as the basis of monitoring of the potential changes over time. A faster adjustment of the pH value is not that important because in biological media the pH alterations occur rather slowly. As it can be identified a minor potential change of the dual coated iridium electrode is obvious at pH = 4.01, where no further drift can be monitored after 3 h of equilibration. The neutral and alkaline pH range displays a major variance over time, especially the buffer solution of pH = 9.18 shows a slight and prolonged drift even after 12 h. In Fig. 5 after fabrication of the double-coated iridium electrode, a super-Nernstian slope of −63.3 mV/pH is achieved. During drift measurement, the Nernstian slope decreased to an almost theoretical value of −57.3 mV/pH. Further severe descendent tendencies could not be observed. Microorganism cultivation is commonly performed from 30 to 37 °C. E. coli can be cultured at these temperatures, too. The results of temperature alteration in buffer solutions are depicted in Fig. 7. A temperature rise results in a gradual potential decrease. At room temperature (25 °C), a fast response behaviour of 100 s up to 290 s can be preserved. The duration of the pH adjustment in acidic and neutral range lasts approximately 300 s, in alkaline solutions more than 600 s at 30 °C. While pH response in acidic and alkaline buffer solutions at 37 °C
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endures 410 s and more than 600 s, the adjustment in neutral solutions is decisively faster and continues almost 100 s. The Nernstian slopes are approximately constant and just a minor linear shift of the curves between the constituent temperatures is detectable. Therefore, it is depictable that the dual layer deposition performs proficient at elevated temperatures, too. Surprisingly, no increase in Nernstian slope can be detected. Apparently, changes in iridium oxide structure modify the slope characteristics with increasing temperature. Before application of the iridium electrodes in bioanalytical topics, growth conditions of different microorganisms on this metal have been monitored. Exemplarily, the model organism E. coli was selected. Cultivation was realised as it was stated in experimental section and afterwards scanning microscopy images were recorded (see Fig. 8). The basis is formed by the dual iridium coated platinum thick film which is represented in pale. The grown microorganisms are displayed darker than the iridium coated platinum thick film and are visible as filamentous structures or as rod shaped bacteria. They are not that precise delimited like the iridium surface and appear to be blurred and indistinct. The pictures indicate that microorganism growth on the dual coated iridium electrode is possible. Further investigations proved the viability of the electrode grown microorganisms through cell plating studies (Fig. 9). The cell plating study exhibits that the grown E. coli biofilm onto the iridium electrodes encloses viable microorganisms which are still able to divide and can form colonies on the LB agar plates. The colonies are of the same shape, texture and colour in each agar plate. This evidences the iridium coating as a biocompatible surface which can be overgrown by microorganisms and represents a possible alternative for pH measurements in biological media and underneath the biofilm.
Conclusions The obtained results demonstrate the applicability of planar iridium coated electrodes for bioanalytical applications. Sensor materials like PbO2 and molybdenum bronzes are meanwhile toxic and therefore unsuitable for microbial pH control [16, 17]. Platinum thick film electrodes showed inadequate pH response characteristics. The single iridium coatings did not achieve acceptable pH response qualities and Nernstian slopes. The funding of metallic iridium onto a platinum thick film with subsequent iridium oxohydroxide electrodeposition depicts the best combined results in Nernstian behaviour and pH adjustment time. An almost theoretical Nernstian slope could be obtained and an acceptable pH response between 3 and 5 min from pH = 4.01 to pH = 9.18 was achievable. In comparison to other materials, the electrode is biocompatible which is of high interest in bioreactor measurements and a miniaturisation of the system is possible.
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Furthermore, an assumable biofilm growth of E. coli onto the iridium-coated electrode is identifiable and their viability could be confirmed. Further investigations will examine the pH response characteristics of the electrodes with artificial and natural biofilm coatings, especially for electroactive and industry-relevant microorganisms like C. necator. Acknowledgements Financial support by the Bundesministerium für Wirtschaft und Energie (BMWi) through the federation for industrial research (AiF) (IGF-project: 18150BG) is gratefully acknowledged.
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