www.acsami.org Research Article Specially Designed Polyaniline/Polypyrrole Ink for a Fully Printed Highly Sensitive pH Microsensor Miguel Zea, Robert Texidó, Rosa Villa, Salvador Borrós, and Gemma Gabriel* Cite This: ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: pH sensing for healthcare applications requires sensors with mechanically stable materials of high sensitivity and high reproducibility combined with low-cost fabrication technologies. This work proposes a fully printed pH sensor based on a specially formulated conducting polymer deposited on a microelectrode in a flexible substrate. A formulation, which combined polyaniline (PANI) and polypyrrole (PPy) with integrated polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS), was specially prepared to be printed by inkjet printing (IJP). The sensor has good sensitivity in the physiological region (pH 7−7.5) key for the healthcare biosensor. This mixture printed over a commercial gold ink, which has a singular chemical functionalization with phthalocyanine (Pc), increased the sensor sensitivity, showing an excellent reproducibility with a linear super-Nernstian response (81.2 ± 0.5 mV/ pH unit) in a wide pH range (pH 3−10). This new ink together with the IJP low-cost technique opens new opportunities for pH sensing in the healthcare field with a single device, which is disposable, highly sensitive, and stable in the whole pH range. KEYWORDS: pH sensor, polymeric ink, polyaniline, polypyrrole, inkjet printing ■ INTRODUCTION pH is a key indicator for many biochemical processes and for reasonable cost. In particular, the healthcare field is growing toward a promising platform for personal wearable electronic this reason, pH sensors have received considerable attention and flexible biochemical sensors, which are envisaged to for monitoring human healthcare due to their versatility, replace bulky and costly medical instruments for healthcare possibility of real-time measurements, and quantitative results.1 monitoring or at least, to complement the laboratory-based Possible areas of interest can be found in the continuous devices, expanding the areas of application. One of the most monitoring of blood2,3 and sweat4,5 and the determination of important digital fabrication techniques in flexible electronics the pH of tumors6 as the chemodynamic therapy heavily for the fabrication of sensors is inkjet printing (IJP). In the last depends on acidic chemical environment pH measurements few years, flexible electronics has grown exponentially in that can determine the effectiveness of this treatment. Even application fields such as healthcare and industry, and the though the pH sensor is a broadly studied one, for all of the market is expected to reach USD 87.21 billion by 2024.8 It is abovementioned applications, integrated pH sensors must an additive manufacturing and noncontact approach that fulfill strict requirements that are technologically unresolved to allows the maskless deposition of functional materials in small date. The most relevant points where the technology still faces drop volumes on a wide range of substrates.9 These singular important challenges to be accomplished are7 (1) flexibility, to features are of significant interest for the fabrication of be adapted to body tissues; (2) the need for miniaturized pH sensors that are minimally invasive; (3) good performance such as good stability, sensitivity, and response time, to allow Received: May 4, 2021 monitoring for definite time; and (4) accuracy at neutral pH, Accepted: June 22, 2021 especially interesting for physiological measurements. Published: July 6, 2021 Most of these pH sensors are designed for single use and for this reason, their fabrication technology must be easy to allow scaling up and production of a wide range of devices and at a © 2021 The Authors. Published by American Chemical Society https://doi.org/10.1021/acsami.1c08043 33524 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 1. (a) Digital design of the pH sensor. (b) Final printed platform with 9 IE and 1 pRE and microscopic image of the printed polymeric electrodes. (c) Inkjet printing steps: (i) printing of Ag RE, tracks, and pads, followed by a dry step; (ii) printing of the Au IE, followed by a dry and thermal sintering of both metallic Au and Ag layers; (iii) printing of the dielectric SU-8, drying, and UV cross-linking; and (iv) printing of CP inks, followed by a drying step. biochemical sensors due to their simple implementation and challenge due to the high precision and resolution required in scalability to high-volume manufacturing. Although IJP cannot this narrow range as well as the difficulty to obtain a stable be considered as a replacement for conventional silicon measure without pronounced potential drifts. The proposed fabrication techniques, it is a reliable technique for the ink displayed a good linearity for the range of pH 3−10, development of sensors with the abovementioned require- solving the common CP drawback of a good performance at a ments, namely, low cost, disposable, robust, and miniaturized10 physiological range of pH. Therefore, the inks and the in novel low-cost flexible substrates such as a broad range of fabrication approach herein described can open new ways for plastics and even paper with real applicability in the future.11 the mass production of miniaturized pH sensors for healthcare As for pH sensors fabricated by IJP reported to date, due to applications fully compatible with large-scale production simplicity and the possibility of miniaturization, all of those methods. reported have been electrochemical sensors based on potentiometric techniques.12 The printed metallic electrodes have been functionalized by depositing or growing some of the ■ EXPERIMENTAL SECTION Materials and Chemicals. For the development of the most representative pH-sensitive materials, usually oxides, microelectrodes, we used four commercially available inks. A low- polymers, and biomolecules. The sensing materials that have a curing gold colloidal ink (Au−Pc) (DryCure Au-J 1010B from pH-dependent potential over printed metallic inks proved until Colloidal Ink Co., Ltd., Japan), low-curing gold nanoparticle ink (Au- now are electrochemically grown iridium oxide,13,14 palladium Np) (Au-LT-20 from Fraunhofer IKTS, Germany), silver nano- oxide grown over a printed palladium ink (Pd/PdO),14,15 and particle ink (Dupont-PE410), and SU-8 ink (2002 from MicroChem). All of the ink formulations were printed with a drop-on-demand electropolymerized polyaniline (PANI) polymers.16 Dimatix Materials Printer. Hydrochloric acid (HCl, 0.1 M) was used Conducting polymers (CPs) are ideally suited for pH- for the chlorination of the printed Ag electrode and potassium sensing applications17,18 because they not only exhibit high chloride (KCl, 0.1 M) for testing the pseudoreference electrode conductivity and electroactivity but can also be used as a (pRE). Polyethylene naphthalate of 125 μm (PEN, TeonexQ65HA general matrix and can be further modified with other DuPont Teijin Films) was used as a substrate to print the compounds to increase the pH range.19 Although many microelectrodes. Commercial buffer solutions of pH 3−10 (Panreac), polymers have been studied,20 the most popular polymer-film- HCl and sodium hydroxide (NaOH) were used for the pH calibration coated electrodes reported are the ones using electro- (all reagents from Sigma-Aldrich). polymerized pyrrole and aniline.21,22 For this work, CPs have Polyaniline and Polypyrrole (PPy) Synthesis. The materials used in the synthesis of water-dispersible CP suspensions consisted of been considered as the optimal candidate among all of the aniline (≥99,5%, Sigma-Aldrich, San Louis, MO), pyrrole (≥99%, range of materials used for pH sensors because they allow the Sigma-Aldrich, San Louis, MO), ammonium persulfate (≥98%, Fluka formulation of a suitable ink with adequate rheological Bucharest, Romania), and poly(sodium 4-styrenesulfonate) (PSS) properties to be printed by the IJP technique. solution (Mw 70 000, 30 wt %; Mw 200 000, 30 wt %) purchased from In this work, we propose a printable pH-sensitive CP that Sigma-Aldrich (San Louis, MO). allows the first fully printed polymer-based pH sensor reported PANI and PPy Ink Formulation. PANI:PSS and PPy:PSS were until now. We develop a new water-based ink formulation formulated adding the proper amount of MilliQ water (MQ), Triton based on a mixture of PANI:PSS/PPy:PSS that gives a high X100 as surfactant, and glycerol and dimethyl sulfoxide (DMSO) as the conductor promoter. Details of all of the prepared formulations stability and super-Nernstian sensitivity (81 mV/pH) to the are presented in Table S1. final performance of the pH sensor. This mixture of polymers Ink Characterization: Dynamic Light Scattering (DLS). The pays special attention to neutral pH (pH values 7−7.5), key for particle size distribution of the suspensions was determined by the healthcare biosensor and monitoring applications. To dynamic light scattering in diluted samples (1:10) using a Zetasizer obtain physiological measurements in this pH interval is still a ZEN3600 (Malvern Panalytical, Malvern, U.K.). 33525 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 2. Characterization of CP suspensions. (a) PANI:PSS(II) suspension and (b) PPy:PSS(II) suspension: (i) solutions, (ii) particle size distribution, (iii) FESEM profile image, and (iv) FESEM film image. Fourier Transform Infrared (FTIR) Spectra. PANI:PSS and min to uniformly disperse the solid content. Then, the inks were PPy:PSS IR spectra were obtained using an IR spectrometer (Nicolet filtered through a polar ultrafilter of a 0.45 μm pore size and iS10, Thermo Fisher Scientific). For sample preparation, the introduced into the ink cartridge. To achieve a good pattern definition suspension was lyophilized and then combined with potassium substrate, the cartridge nozzles were heated at 40 and 35 °C. Three bromide (KBr) to be compressed between platens to form pellet for layers of CP inks were printed wet-on-wet on top of the Au IE with a analysis. DS of 10 μm and dried for 15 min at 120 °C. The Ag RE was Microscopic Images. Field-emission scanning electron micros- chlorinated by cyclic voltammetry (detailed in Supporting Informa- copy (FESEM) images were taken to observe the nanostructure with tion (SI) Figure S1) in 0.1 M HCl, scanning the potential from 0 to a field-emission Zeiss Merlin FESEM (Oberkochen, Germany). The 0.2 V versus Ag/AgCl commercial reference electrode at 20 mV/s to film of the samples was prepared depositing a droplet of the obtain a stable Ag/AgCl pseudoreference electrode (pRE). A final suspension on a silicon substrate and dried on a vacuum drier at room printed platform on a PEN substrate with nine IE and one pRE and a temperature. microscopic image of the printed CP electrodes can be observed in Fabrication of Solid-State pH Sensor. The pH sensor structure Figure 1b. was fabricated with IJP technology, using the same approach that we Morphological Characterization of Printed CP. The printed reported in our previous work.13,23 Basically, a drop-on-demand polymeric films were analyzed using an optical microscope Dimatix Material Printer (DMP 2831 for Fujifilm Dimatix, Inc.) was (DM4000M LEICA, Germany), a profilometer (KLA Tencor P-15), used with the 10 pL printhead. The sensor structure was digitally and a scanning electron microscope (SEM, Auriga40 from Carl Zeiss, designed using CleWin 5 software (Figure 1a) and then exported to Germany). BMP to load in the Dimatix Bitmap editor. The printing process was pH Measurements. The open-circuit potential (OCP) of the carried out in standard laboratory conditions without particle filter, inkjet-printed electrodes was measured by a PalmSens3 electro- temperature, and humidity control. Microelectrodes and CP inks chemical station (De Indruk, Netherland). Measurements were (PANI:PSS, PPy:PSS, and PANI:PSS/PPy:PSS) were printed on the carried out in various commercial pH buffer solutions ranging from PEN substrate, as detailed in Figure 1c. The complete fabrication of pH 3 to 10. Sequential immersion in buffer solutions was followed by the pH sensor can be easily understood as two steps: The printing a cleaning step with deionized water to avoid cross-contamination. process of the metallic microelectrodes and the modification of For each test, the electrodes were immersed in the buffer for 30 s and electrodes. In the first step (c-i), the silver (Ag) elements were first the potential between IE and pRE was recorded. The base−acid printed: the reference electrode (RE) of 800 × 800 μm2, tracks, and titration was done from basic (NaOH) to acidic (HCl) standardized pads with a drop spacing (DS) of 20 μm followed by a dry step at 80 solutions with concentrations of 0.01 mM, 0.1 mM, 0.001 M, 0.01 M, °C for 15 min. Then, the Au-Np ink was printed (c-ii) to obtain a 1 0.1 M, and 1 M, which was magnetically stirred. Additionally, the pH mm2 indicator electrode (IE), with a DS of 15 μm, the platen value was recorded with a commercial pH-meter (GLP22, Crison) in temperature was set to 40 °C, and the printhead temperature was set both experiments. ■ to 30 °C. Au elements were dried as before, and both inks were finally sintered in the oven at 140 °C for 30 min. Then, the dielectric ink SU- 8 was printed (c-iii) on Ag tracks to isolate and define the active RESULTS AND DISCUSSION electrode area and pads with a DS of 15 μm. First, a soft bake on a hot Synthesis and Characterization of Printable Poly- plate at 100 °C for 5 min, followed by a UV treatment for 30 s to meric pH-Sensitive Inks. To obtain the CP inks for IJP, polymerize this layer by polymer cross-linking. For the fabrication of microelectrodes with the other Au ink Au−Pc, the DS used was 25 water-dispersible particles of PANI and PPy were synthesized μm and the platen and printhead temperature was set to 35 °C. through the electrostatic interaction suspension method as The second step consisted of the printing process of the pH- described in the Experimental Section. This method is sensitive polymer ink over the Au microelectrode (c-iv) and the especially suitable to obtain CP suspensions with the ability chlorination of the Ag RE. Initially, the CP inks were sonified for 5 to form a continuous conductive film as it has been previously 33526 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article reported for PANI24,25 and PPy.26−28 The distinctive feature of polymerization.31 It is of interest to note that PANI:PSS this procedure consists of involving electrostatic forces globular shape revealed a smother morphology when produced between monomers and polyelectrolytes to allow compared with PPy:PSS, possibly originated by a coalescence obtaining stable suspensions during the oxidative polymer- phenomenon produced during the drying process. ization. This allows obtaining a controlled size distribution to These four aqueous suspensions were used as the basis for fulfill one of the most restrictive IJP requirements. The IJP ink formulations. The studied suspensions were formulated supporting polyelectrolyte used was PSS solution as the using a surfactant (1, 5% (w/v) of Triton X100) to adjust the charge-balancing dopant during the polymerization. Consider- surface tension of the ink prior to printing. Surface tension data ing the application of the suspensions is an IJP ink, we studied of the evaluated formulation inks are detailed in Table S2 of the influence of the molecular weight of PSS which takes a the Supporting Information. Furthermore, to increase electric critical role both in the properties of the ink (size distribution, conductivity, dimethyl sulfoxide (DMSO) and glycerol (Gly) stability) and in the properties of the printed film. In the were used as a secondary dopant. Gly addition also contributes current literature, it has been reported that high PSS molecular to adjust the viscosity of the CP formulation and the presence weight presents some advantages in terms of superior electrical of DMSO and Gly shows a stabilization of the polymer benzoic and electrochemical performance.29,30 However, there are very structure once the film is formed.32−37 Hence, this intrinsic few references about the effect of PSS molecular weight on the conjugation of the benzoic structure allows a better electron size distribution, suspension stability, and film formation mobility through polymeric chains, which in turn enhances the properties of PANI:PSS and PPy:PSS suspensions. With this electroactivity of CPs. This effect, which is broadly studied for in mind, four different suspensions were prepared, (PANI:PSS- polymers such as the gold standard PEDOT:PSS, is not so (I), PANI:PSS(II), PPy:PPS(I), and PPy:PPS(II)), using PSS widely studied for PANI and PPy formulations and even less with two different molecular weights 70 000 Da for (I) when they are applied as a pH sensor. suspensions and 200 000 Da for (II). After the synthesis, stable Polymeric Ink Printing Process. Once formulated, the suspensions of both PANI (green color, Figure 2a-i) and PPy inks were printed using an inkjet printer to obtain a sensor (black color, Figure 2b-i) were obtained for the two PSS used. array. Printing parameters were adjusted to obtain a Obtaining PANI:PSS and PPy:PSS suspensions through continuous drop ejection for PANI:PSS(I) and (II) and electrostatic interaction synthesis was confirmed through IR PPy:PSS(I) and (II) ink printing. Waveform and printing analysis as detailed in Supporting Information Figure S2a,b. parameters were obtained considering polymeric ink properties None of the suspensions obtained presented signs of to have a reliable printing process of developed polymeric inks precipitation after 6 months at room temperature. Size (Supporting Information Figure S7). As described in the distribution of the PANI:PSS and PPy:PSS formulations was literature, the ink−substrate interaction alters ink deposition, studied through DLS. We observed that PANI:PSS and hence the drop spacing (DS) selection between two PPy:PSS suspensions presented a monodisperse size distribu- consecutive drops and the printer platen temperature tion in all cases, regardless of the monomer used or the PSS determines the quality of the printed pattern on a specific molecular weights. Considering the dependence of PANI substrate.38 The DS determines the resolution and density in polymer on the pH media, the synthesis of PANI:PSS X- and Y-axes of the printed samples; for example, a DS of 15 suspensions was optimized at pH 2 to maximize the number μm represents a print resolution of 1693 dpi (dots per inch). of PANI:PSS particles (data shown in Figure S3). The particle The platen temperature plays a critical role in drop evaporation size average values were around 200 nm, as shown in Figure that it is directly related to line formation, pattern definition, 2a-ii for PANI:PSS(II) and in Figure 2b-ii for PPy:PSS(II) and coffee-ring effect. To evaluate an optimum DS, a line (values for PANI:PSS(I) and PPy:PSS(I) are shown in pattern test was performed with DS from 5 to 140 μm.39 Supporting Information Figure S4). The ζ-potential of the Figure 3 shows the printed line patterns for PANI:PSS(I) suspensions was also measured, obtaining a value around −20 (a) and PPy:PSS(I) (b). Both inks present more or less the mV for PSS of Mw 70 000 and around −40 mV for the PSS of same behavior and no main differences can be observed with 200 000 (data shown in Figure S5). The mean value combined the other formulations using Mw 200 000 PSS (data not with particle size distribution (PDI) values around 0.2 shown). However, depending on the selected DS, different obtained for each suspension fulfilled the requirements of IJP performance can be observed. When the chosen DS is too to avoid the clogging of the printer nozzle. All of the small (5 μm), bulging effects appear due to an excess of suspensions were correctly filtered using a 0.2 μm poly- deposited material. For DS values of 40 μm or higher, the (tetrafluoroethylene) (PTFE) filter before each printing for the formation of isolated drops that are not able to form a elimination of any possible higher particles to prevent the continuous line can be observed. The proposed inks have nozzle clogging. optimized DS values between DS 10 and 35 μm that allow the After filtering, the suspensions were able to form a printing of a homogeneous and continuous line. Apart from continuous film on a silicon substrate after air drying. the line pattern evaluation, it has been observed experimentally FESEM images revealed a continuous structure with a well- that DS values higher than 10 μm do not allow the overlap of defined layer thickness of approximately 5 μm for PANI:PSS contiguous printed lines, as can be observed in Supporting (Figure 2a-iii) and 7 μm for PPy:PSS (Figure 2b-iii) for all of Information Figure S8. This can be explained by a fast the film suspensions studied (Supporting Information Figure evaporation of ink solvents or interaction between the ink and S6). No significant differences were observed in the substrate helped by drop kinetics. Also optimized was the morphology for PANI:PSS or PPy:PSS samples where a substrate temperature, which was set to 40 °C to obtain a different molecular weight PSS was used during their synthesis balance between solvent evaporation, ink spreading, and (Figure 2a-iv,b-iv for Mw 200 000 PSS). As can be seen, both linewidth. Temperatures lower than 40 °C produced an images present a globular morphology due to the interaction excessive spreading and formation of inconsistent lines due to between the oxidant and the structuring agent during the slower evaporation of the ink, and temperatures higher 33527 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article than 40 °C showed thinner lines and exaggerated coffee-ring effects as a result of rapid solvent evaporation. Finally, a DS of 10 μm at 40 °C was determined as the optimal point to obtain the most homogeneous lines with a controlled coffee-ring effect to print the polymeric suspensions onto the Au microelectrodes. Furthermore, different number of CP layers were printed wet-on-wet and evaluated later in the profilometer (one (1L), two (2L), and three layers (3L)). A desired thickness of around 1−3 μm should be achieved to decrease the CP film electrical resistance, decrease the pH behavior hysteresis, increase the pH sensor lifetime, and ensure a good repeatability as previously reported.40−42 Four layers (4L) were also tested; however, these conditions were not under consideration because the films were completely detached when being immersed in an aqueous solution, as shown in Figure S9, due to an excess of material over the metallic electrode. Profilometer measurements are shown in Figure 3c for PANI:PSS(I) and in Figure 3d for PPy:PSS(II); the well- known coffee-ring effect was observed for all of the printed layers, 3L being necessary for achieving a nonuniform thickness of around 3 μm in the central part of the pattern. Figure 3. Line patterns printed on the PEN substrate for (a) After printing, PANI and PPy films with PSS of Mw 70 000 PANI:PSS(I) and (b) PPy:PSS(I) formulated inks. Cross-sectional revealed a good stability in aqueous media, indicating its profile for (c) PANI:PSS(I) and (d) PPy:PSS(II) for 1L, 2L, or 3L. suitability as a sensor. However, when a PSS of Mw 200 000 was used, the polymeric film was completely detached after a few minutes of being in contact with the aqueous media. Thus, Figure 4. Plots of potentials against pH value reading from pH-meter for (a) PPy:PSS + DMSO 3L, (b) structure of PANI macromolecule changes due to protonation/deprotonation, (c) PANI:PSS 2L, (d) PANI:PSS 3L, (e) PANI:PSS + Gly 2L, (f) PANI:PSS + Gly 3L, (g) PANI:PSS + DMSO 2L, (h) PANI:PSS + DMSO 3L, and (i) PANI:PSS/PPy:PSS + DMSO 3L (n = 3, variation coefficient below 5%). 33528 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 5. Potential interfering ion test for PANI:PSS/PPy:PSS sensors: (a) Na+ and (b) K+. Determination of pH value for a set of mediums used in the bioengineering field: (c) Comparison between PANI:PSS/PPy:PSS sensors (n = 3) with a laboratory pH-meter and (d) compared using a linear correlation. we selected the PANI:PSS(I) and PPy:PSS(I) inks for its higher slope values, indicating better electron mobility and evaluation and validation as a pH sensor. high electroactive behavior of the polymer. A result of this Evaluation of the Printed CP Inks as pH Sensors. For behavior can be seen in the printed sensors of PANI:PSS(I) the evaluation of the polymeric printed films, potentiometric without conductivity promoters, where two linear regions can pH measurements were carried out by measuring the open- be observed in Figure 4c,d, corresponding to a film of 2L or circuit potential at room temperature. Sensors were placed 3L. The linear regions present in PANI:PSS(I) 2L and 3L films three times in the same buffer solution to evaluate the pH presented similar slopes (28.3 mV/pH for the pH 3−7 region response and repeatability of the polymeric film. Considering and 30.3 mV/pH for the pH 7−10 region). These changes the applicability of this sensor in biological applications, in may be attributed to the presence of sulfonate groups in PSS, addition to evaluating its sensitivity and linearity, special which generates local acidic regions that allow the regulation of attention was paid to the physiological pH range (around pH PANI protonation. Although PANI:PSS films reduce the effect 7.5). of the change in the structure by improving the linearity of the Figure 4 shows the potentiometric pH measurements of the sensor response, the neutral pH zone still presents a poorly evaluated formulations of PANI:PSS(I) and PPy:PSS(I). defined signal. PANI:PSS(I) films formulated with a PPy:PSS(I) formulation revealed a poor linearity in the conductivity promoter such as glycerol (Figure 4e,f) or selected pH range even when a secondary dopant (DMSO) DMSO (Figure 4g,h) only magnified the effect of structure was used to enhance the polymer electroactivity (Figure 4a). protonation, presenting higher differences between the This behavior of PPy as a pH sensor is described in the potential output signals of the two regions. This effect is literature for similar devices.43 clearly visible for PANI:PSS samples with DMSO, where the For PANI:PSS(I), the potential decreases with the increase measurement performed at pH 7 presented a high deviation. of the pH, revealing two good linear regions similar to the Increasing the number of layers of printed PANI:PSS film from behavior of electrodeposited PANI pH sensors but with less 2 to 3 did not produce any significant improvement in this pronounced changes in the slopes.44 The presence of these sense. regions is ascribed to the pH-dependent emeraldine salt (ES)− To improve the measurements in the physiological region as emeraldine base (EB) transition of PANI macromolecules, well as the linearity in the studied range of pH, we used a where, depending on the pH, there is a different level of formulation that combines PANI:PSS(I) and PPy:PSS(I) protonation of the EB imine groups (Figure 4b). This (PANI:PSS/PPy:PSS). We hypothesized that when electron protonation−deprotonation of PANI structure provides its mobility cannot be carried out through the PANI chain due to electroactive properties allowing changes in the potential the EB/ES structure change, the PPy chain supports electron output signal, which reveals two different slopes corresponding mobility. First, we optimized the IJP conditions of the two to the protonation levels, where the acid pH region allows polymers together. It was observed that mixing PANI:PSS(I) 33529 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 6. Schematic illustration of π-junction of Au−Pc ink modified with phthalocyanine and PANI:PSS/PPy:PSS polymeric chain. with PPy:PSS(I) (1:1 v/v) allowed the printability of a stable solutions of unknown pH used in the field of bioengineering: polymeric sensor. Figure S10 of the Supporting Information simulated body fluid (SBF), Dulbecco’s modified Eagle shows the line pattern of this new ink and the corresponding medium (DMEM), phosphate buffered saline (PBS), artificial cross-sectional profile for 1L to 3L, as previously done for urine, Luria-Bertani broth (LB-broth), and commercial PANI and PPy base inks. In this case, the DS optimized is also Corning SF medium. Figure 5c shows the pH value of the 10 μm. However, the cross-sectional profiles show how the ink solutions measured using the PANI:PSS/PPy:PSS sensor accumulates in the central part, noted by the distribution of the compared with a conventional pH-meter with a glass electrode. material. We observed that the most homogeneous film was The pH values of these real solutions were determined achieved with 3L, with a thickness of about 5 μm. revealing no significant differences between the measurements Second, we studied the sensor response at different pH carried out with the PANI:PSS/PPy:PSS sensor and with the values. Figure 4i shows potential against pH plot for the laboratory pH-meter. From a qualitative point of view, Figure PANI:PSS/PPy:PSS sensor. The potentiometric pH measure- 5d shows the correlation of the pH values of the two electrodes ments revealed a good linearity in the range of pH 3−10. for the six evaluated solutions. As shown, the linear regression These results confirm our hypothesis because not only did we presents a coefficient of determination of 0.988, indicating the solve the drawbacks of PANI:PSS at the physiological range of good agreement between values and demonstrating that the pH but we also increased the stability of the PPy:PSS films. developed sensors present the same behavior as a laboratory Furthermore, the sub-Nernstian response was in good device. agreement with other reported sensitivities for polymeric pH- Effect of the Gold Substrate on the Nernstian based sensors.18,44−47 All measurements subsequently per- Behavior of the pH Sensor Response. In this section, we formed were done with the PANI:PSS/PPy:PSS ink printed explore the utilization of an Au-based printed electrode over the Au microelectrodes with a platen temperature of 40 modified with phthalocyanine (Pc) as a method to increase °C, DS of 10 μm, and 3L. the sensitivity of the polymeric-based pH sensor. The Selectivity is one of the most important factors in commercial Au−Pc metallic ink used to print the Au electrodes potentiometric sensors and defines the ability to specifically was modified with a derivative metal-free phthalocyanine, measure hydronium ions in the presence of other ions. For this which presented a delocalized planar structure, promoting the reason, we evaluated the potentiometric selectivity perform- formation of large π-conjugated regions. These regions, ance of the developed pH sensor against major potential directly in contact with the surface of the Au nanoparticle, interfering sodium (Na+) and potassium (K+) ions, observing improved the electrical pathway among them,50 allowing good the voltage drifts at different pH values for constant ion conduction characteristics equivalent to those of the conven- concentrations (Figure 5a,b). Potential against pH plots of tional nanoinks (Au-Np) with a minimum heat treatment buffered solution in the presence of Na+ and K+ revealed small (usually, sintering at 100−120 °C for several minutes gives voltage drifts similar to other similar devices described in the well-conducted patterns). As previously described, the literature.48,49 Based on these results, the presence of Na+ and incorporation of phthalocyanine into PPy or PANI chains K+ does not interfere with the pH measurements of real results in the increase of the delocalized regions, enhancing the unknown solutions. In this context, the performance of the doping level of the CP, the electron mobility, and the PANI:PSS/PPy:PSS sensor was evaluated using six standard electroactivity.51,52 33530 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 7. Calibration curves of PANI:PSS/PPy:PSS. (a) Sensor sensitivity of PANI:PSS/PPy:PSS on Au-Np and Au−Pc microelectrodes in the pH range of 3−10 (n = 3, variation coefficient below 3% for Au−Pc and below 4% for Au-Np). (b) Repeatability of the electrode of PANI:PSS/ PPy:PSS on Au−Pc at different pH values from 3 to 10 (Inset: midterm pH sensor stability at pH 6 for 10 h). Table 1. Comparison of Selected Works from the Literature, Highlighting the Conducting Polymers Used for pH Sensing, the Fabrication Technique Used for Deposition of the Conducting Polymer, pH Range, and Sensitivitya conducting polymer technique pH range sensitivity (mV/pH) refs PPy electropolymerization 2−12 54.67 ± 0.7 53 PANI nanofibers polymerization 3.9−10.1 62.4 54 PANI nanopillar soft lithography 2.38−11.61 60.3 55 PANI electropolymerization 5−7 21 PANI electropolymerization 5.5−8 59.2 22 PPy + CNT, PANI + CNT electrodeposition 1−13 59 56 PANI laser carbonized 4−10 51 57 PANI coating 4−10 50 41 PANI coating 4−8 54 ± 0.51 58 PANI + CNT electrodeposition 1−13 58 59 PPy electropolymerization 3−10 46 60 PANI + DBSA spin coating 5.4−8.6 58.57 61 PANI + MWCNT screen printing 2−11 20.63 62 PANI + PU electrospinning 2− 7 60 63 PANI electrodeposition 4−8 60.6 64 PANI + PPy + PSS inkjet printing 3−10 81.2 ± 0.5 this work a Abbreviations: CNT, carbon nanotubes; MWCNT, multiwalled carbon nanotubes; DBSA, dodecyl benzene sulfonic acid; PU, polyurethane; PSS, sodium 4-styrenesulfonate. Figure 6 shows the interactions of the Au−Pc ink together delocalized π-region between the electrode and the PAN- with the CPs. Once the PANI:PSS/PPy:PSS ink was printed I:PSS/PPy:PSS printed polymeric film is not only interesting onto the Au−Pc microelectrode, the π-conjugated regions of in terms of obtaining a highly sensitive pH sensor but it also the PANI, PPy, and PSS interacted with the π-conjugated describes a strategy that could be implemented in other CPs to regions of Au−Pc, enhancing the electron mobility between increase electroactivity, thus unlocking new applications. the printed Au−Pc substrate and the polymeric film in The complete evaluation of the pH-sensing characteristics comparison with the bare Au-Np ink. required a reproducibility study of the PANI:PSS/PPy:PSS Figure 7a shows the effect of Pc in direct contact with the printed on top of the Au−Pc microelectrode. Three PANI:PSS/PPy:PSS-based sensor. pH calibration was per- independent microelectrodes, made from the same fabrication formed for PANI:PSS/PPy:PSS printed on the nonmodified batch, were measured by OCP and the base-to-acid and acid- Au substrate (Au-Np) revealing a Nernstian sensitivity of 48.5 to-base changes were studied. Figure 7b shows the response of ± 0.5 mV/pH with a high correlation coefficient r2 of 0.990 in each individual microelectrode, and as can be observed, the all of the pH ranges (3−10), in good agreement with other potential responses for each one almost overlap, indicating an previously reported CP-based pH sensors.41 When the excellent reproducibility. The average sensitivity of the three PANI:PSS/PPy:PSS polymeric film was printed onto the electrodes is 81.2 mV/pH with an average variation coefficient Au−Pc, a super-Nernstian response of 81.2 ± 0.5 mV/pH with for each pH level under 0.5% (Table S3), confirming a good high linearity was achieved. Table 1 summarizes the most repeatability of PANI:PSS/PPy:PSS sensor with a correlation recent papers on conducting polymers used for pH sensing. As coefficient larger than 0.996 for all measurements. The can be observed, our work has the highest sensitivities achieved midterm stability of the pH sensor near protonation and for a polymeric-based sensor.17 As stated before, this increase deprotonation regions was further investigated by measuring in sensitivity can be explained due to enhanced electron its potential drift over 10 h at room temperature. For this mobility between the Au−Pc substrate and PANI:PSS/ purpose, the platform was immersed in pH 6 buffered solution PPy:PSS. The utilization of Au−Pc ink to generate a high (Figure 7b inset) and continuously measured. Although some 33531 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article Figure 8. Titration of strong base with strong acid. (a) Potentiometric pH titration curves for NaOH and HCl at the indicated molarity, (b) representation of the equivalence point for each NaOHmolarity. The red dotted line represents the theoretical values, and (c) stability of the pH sensor in acid conditions, acetic, and acetic glacial acid (i) and buffer solutions at pH 7 and 7.5 (ii). slight potential fluctuations can be explained by temperature affected after performing measurements in nonaqueous acid changes, the sensor shows an excellent stability of continuous media. Additionally, since the substrate proposed is flexible, reading over a 10 h test resulting in a potential drift of 0.2 mV/ the pH sensor evaluation was completed, verifying that the h, an average reading of 228 mV, and a standard deviation of sensor response remained unaltered after repeated bending 2.3 mV, thus providing reliable pH read-outs for long tests. For this, a proof of concept was carried out to verify how measurements. sensor sensitivity evolves after being subjected to different Validation of the PANI:PSS/PPy:PSS Film as pH bending cycles (Figure S12 of the Supporting Information). It Sensor. To evaluate if the PANI:PSS/PPy:PSS sensor is a was found that after 50 bending cycles of 90°, the sensitivity of good candidate for local pH determination, pH titration was the sensors remained stable around 77 and 78 mV/pH, studied. Titration was performed over a range of molarities demonstrating that the sensor flexibility did not affect the pH (from 0.01 mM to 1 M) from NaOH to HCl by addition of a sensor response. ■ specific volume of acid. Figure 8a shows the titration plot of aqueous NaOH with HCl. Results obtained for monoprotic titration of strong alkalies with strong acids are comparable CONCLUSIONS with results obtained with a commercial glass electrode. The We have presented a novel approach for fabricating a stable pH PANI:PSS/PPy:PSS sensor presented had a reasonable sensor using a highly rough printed Au ink as a substrate metal detection limit (0.01 mM), a concentration range (from 1 M electrode. The pH sensors were completely fabricated with IJP to 0.01 mM), and a pH range (from 3 to 10) compared with based on PANI:PSS/PPy:PSS inks deposited on a gold previous studies.17,65 The theoretical equivalence point for this microelectrode printed on a flexible substrate. The different titration is 10 mL at pH 7, due to pKa of NaOH and HCl. approaches allowed the optimization of pH sensor properties Figure 8b shows the experimental calculated equivalence and, in particular, IJP ink formulation allowed the improve- points for all concentrations and their standard deviation. The ment of the pH sensor sensitivity. Furthermore, the sensors present good performance and accuracy, allowing combination of PANI:PSS and PPy:PSS particles improved detection of pH changes even ±0.01 pH units with a high the linearity of pH against potential as well as expanding the sensitivity of 81.05 ± 0.08 mV/pH. The same behavior was pH range (from pH 3 to 10) and resolving problems of PANI observed when the titration was performed from the acidic to sensors regarding physiological pH, thereby promoting basic range as shown in Figure S11. An equivalence point utilization in biological applications. The use of a gold ink determined at pH 7 of 10 mL and a detection limit of 0.01 mM modified with Pc for the gold microelectrode substrate was obtained. Figure 8c-i shows the stability of PANI:PSS/PPy:PSS sensor revealed an enhancement of electron mobility between the for 1 h in acetic acid and acetic glacial acid showing a conducting polymer chains and the gold nanoparticles, stabilization time of 300 s for these low pH values, a small drift generating a delocalized π-region that improved the electro- of 1 mV/h for acetic glacial, and 6 mV/h for acetic acid. This activity. In this regard, the sensor reached a linear super- proves the good behavior of the sensor in extreme pH Nernstian response (81.2 ± 0.5 mV/pH unit), one of the conditions, allowing its utilization on nonaqueous media. The highest sensitivity values for a polymeric pH sensor reported to stability was also measured in aqueous buffer solutions at pH 7 date. and 7.5 (Figure 8c-ii) to demonstrate how the sensors met the The characterization of the sensor also revealed a wide range proposed benchmarks, showing that the response time was of versatile properties that extends the range of possible achieved within 7 s and a small drift of 1.5 mV/h for pH 7 and applications. Furthermore, the sensor presented high stability 1.8 mV/h for pH 7.5. The sensitivity did not alter after 2 h in in aqueous and nonaqueous media (acetic acid), confirming its extreme conditions, obtaining values of 80.7 mV/pH of the ability to perform highly accurate titration measurements by PANI:PSS/PPy:PSS sensor (data not shown). This result detecting small changes in the concentration of strong demonstrates that PANI:PSS/PPy:PSS polymeric film is not monoprotic alkalies with strong acid (0.01 M). 33532 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces ■ www.acsami.org Research Article ASSOCIATED CONTENT CNM (CSIC) of ICTS “NANBIOSIS.” This work has also * sı Supporting Information made use of the Spanish ICTS Network MICRONANOFABS The Supporting Information is available free of charge at partially supported by MEINCOM. https://pubs.acs.org/doi/10.1021/acsami.1c08043. Pseudoreference electrode chlorination and stability characterization; ink formulations; surface tension ink ■ REFERENCES (1) Bandodkar, A. J.; Jeerapan, I.; Wang, J. Wearable Chemical formulations; IR spectra characterization; CP suspen- Sensors: Present Challenges and Future Prospects. ACS Sens. 2016, 1, sionspH dependence; DLS characterization; ζ-poten- 464−482. tial characterization; FESEM images; parameters of the (2) Jin, W.; Wu, L.; Song, Y.; Jiang, J.; Zhu, X.; Yang, D.; Bai, C. printing process; images of printing test and non- Continuous Intra-Arterial Blood PH Monitoring by a Fiber-Optic adherence of polymeric film; PANI:PSS/PPy:PSS line Fluorosensor. IEEE Trans. Biomed. Eng. 2011, 58, 1232−1238. (3) Chaisiwamongkhol, K.; Batchelor-McAuley, C.; Compton, R. G. pattern study; variation coefficient; titration of strong Optimising Amperometric PH Sensing in Blood Samples: An Iridium acid with strong base; and pH sensor sensitivity after Oxide Electrode for Blood PH Sensing. Analyst 2019, 144, 1386− bending (PDF) 1393. ■ AUTHOR INFORMATION Corresponding Author (4) Dang, W.; Manjakkal, L.; Navaraj, W. T.; Lorenzelli, L.; Vinciguerra, V.; Dahiya, R. Stretchable Wireless System for Sweat PH Monitoring. Biosens. Bioelectron. 2018, 107, 192−202. (5) Nakata, S.; Shiomi, M.; Fujita, Y.; Arie, T.; Akita, S.; Takei, K. A Gemma Gabriel − Instituto de Microelectrónica de Barcelona Wearable PH Sensor with High Sensitivity Based on a Flexible IMB-CNM (CSIC), 08193 Cerdanyola del Vallès, Charge-Coupled Device. Nat. Electron. 2018, 1, 596−603. Barcelona, Spain; CIBER de Bioingeniería, Biomateriales y (6) Zhao, P.; Tang, Z.; Chen, X.; He, Z.; He, X.; Zhang, M.; Liu, Y.; Nanomedicina (CIBER-BBN), Zaragoza, Spain; Ren, D.; Zhao, K.; Bu, W. Ferrous-Cysteine−Phosphotungstate orcid.org/0000-0003-2140-6299; Nanoagent with Neutral PH Fenton Reaction Activity for Enhanced Email: gemma.gabriel@imb-cnm.csic.es Cancer Chemodynamic Therapy. Mater. Horiz. 2019, 6, 369−374. (7) Ray, T. R.; Choi, J.; Bandodkar, A. J.; Krishnan, S.; Gutruf, P.; Authors Tian, L.; Ghaffari, R.; Rogers, J. A. Bio-Integrated Wearable Systems: Miguel Zea − Instituto de Microelectrónica de Barcelona IMB- A Comprehensive Review. Chem. Rev. 2019, 119, 5461−5533. CNM (CSIC), 08193 Cerdanyola del Vallès, Barcelona, (8) Flexible Electronics Market Size Growth | Industry Forecast Spain; PhD in Electrical and Telecommunication Report 2024. https://www.grandviewresearch.com/industry-analysis/ Engineering, Universitat Autonoma de Barcelona (UAB), flexible-electronics-market/toc (accessed Jun 4 2020). Barcelona, Spain (9) Yin, Z.; Huang, Y.; Bu, N.; Wang, X.; Xiong, Y. Inkjet Printing Robert Texidó − Grup d’Enginyeria de Materials, Institut for Flexible Electronics: Materials, Processes and Equipments. Chin. Químic de Sarrià-Universitat Ramon Llull, 08017 Barcelona, Sci. Bull. 2010, 55, 3383−3407. (10) Sui, Y.; Zorman, C. A. ReviewInkjet Printing of Metal Spain Structures for Electrochemical Sensor Applications. J. Electrochem. Soc. Rosa Villa − Instituto de Microelectrónica de Barcelona IMB- 2020, 167, No. 037571. CNM (CSIC), 08193 Cerdanyola del Vallès, Barcelona, (11) Sundriyal, P.; Bhattacharya, S. Inkjet-Printed Sensors on Spain; CIBER de Bioingeniería, Biomateriales y Flexible Substrates. In Energy, Environment, and Sustainability, Nanomedicina (CIBER-BBN), Zaragoza, Spain Bhattacharya, S.; Agarwal, A. K.; Chanda, N.; Pandey, A.; Sen, A. Salvador Borrós − Grup d’Enginyeria de Materials, Institut K., Eds.; Springer: Singapore, 2018; pp 89−113. Químic de Sarrià-Universitat Ramon Llull, 08017 Barcelona, (12) Qin, Y.; Kwon, H.-J.; Howlader, M. M. R.; Deen, M. J. Spain; orcid.org/0000-0002-4003-0381 Microfabricated Electrochemical PH and Free Chlorine Sensors for Water Quality Monitoring: Recent Advances and Research Complete contact information is available at: Challenges. RSC Adv. 2015, 5, 69086−69109. https://pubs.acs.org/10.1021/acsami.1c08043 (13) Zea, M.; Moya, A.; Fritsch, M.; Ramon, E.; Villa, R.; Gabriel, G. Enhanced Performance Stability of Iridium Oxide Based PH Sensors Author Contributions Fabricated on Rough Inkjet-Printed Platinum. ACS Appl. Mater. M.Z. and R.T. contributed equally to this work. The Interfaces 2019, 11, 15160−15169. manuscript was written through contributions of all authors. (14) Xu, Z.; Dong, Q.; Otieno, B.; Liu, Y.; Williams, I.; Cai, D.; Li, All authors have given approval to the final version of the Y.; Lei, Y.; Li, B. Real-Time in Situ Sensing of Multiple Water Quality manuscript. Related Parameters Using Micro-Electrode Array (MEA) Fabricated by Inkjet-Printing Technology (IPT). Sens. Actuators, B 2016, 237, Notes 1108−1119. The authors declare no competing financial interest. ■ (15) Qin, Y.; Alam, A. U.; Howlader, M. M. R.; Hu, N.-X.; Deen, M. J. Inkjet Printing of a Highly Loaded Palladium Ink for Integrated, ACKNOWLEDGMENTS Low-Cost PH Sensors. Adv. Funct. Mater. 2016, 26, 4923−4933. M.Z. acknowledges his financial support covered by Fundación (16) Määttänen, A.; Vanamo, U.; Ihalainen, P.; Pulkkinen, P.; Centro de Estudios Interdisciplinarios Básicos y Aplicados Tenhu, H.; Bobacka, J.; Peltonen, J. A Low-Cost Paper-Based Inkjet- (CEIBA)-Gobernación de Bolivar (Colombia). This work was Printed Platform for Electrochemical Analyses. Sens. Actuators, B supported by the Spanish government-funded projects 2013, 177, 153−162. (17) Alam, A. U.; Qin, Y.; Nambiar, S.; Yeow, J. T. W.; Howlader, RTI2018-096786-B-I00 and RTI2018-102070-B-C21 by Min- M. M. R.; Hu, N. X.; Deen, M. J. Polymers and Organic Materials- isterio de Ciencia, Innovación y Universidades (MINECO/ Based PH Sensors for Healthcare Applications. Prog. Mater. Sci. 2018, FEDER, EU). The authors also thank the support of the 96, 174−216. Generalitat de Catalunya to 2017-SGR-988 and to 2017-SGR (18) Korostynska, O.; Arshak, K.; Gill, E.; Arshak, A. Review on 1559 and the SU-8 Unit of the CIBER in Bioengineering, State-of-the-Art in Polymer Based PH Sensors. Sensors 2007, 7, Biomaterials, and Nanomedicine (CIBER-BBN) at the IMB- 3027−3042. 33533 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article (19) Binag, C. A.; Bartolome, A. J.; Tongol, B.; Santiago, K. S. Dielectric with Ink Jet for Source and Drain Electrode of OTFTs. Org. Electronically Synthesized Polymer-Based PH Sensors. Philipp. J. Sci. Electron. 2010, 11, 854−859. 1999, 128, 247−252. (38) Oh, Y.; Kim, J.; Yoon, Y. J.; Kim, H.; Yoon, H. G.; Lee, S.-N.; (20) Kocak, G.; Tuncer, C.; Bütün, V. PH-Responsive Polymers. Kim, J. Inkjet Printing of Al2O3 Dots, Lines, and Films: From Polym. Chem. 2017, 8, 144−176. Uniform Dots to Uniform Films. Curr. Appl. Phys. 2011, 11, S359− (21) Fanzio, P.; Chang, C.-T.; Skolimowski, M.; Tanzi, S.; Sasso, L. S363. Fully-Polymeric PH Sensor Realized by Means of a Single-Step Soft (39) Moya, A.; Sowade, E.; del Campo, F. J.; Mitra, K. Y.; Ramon, Embossing Technique. Sensors 2017, 17, No. 1169. E.; Villa, R.; Baumann, R. R.; Gabriel, G. All-Inkjet-Printed Dissolved (22) Guinovart, T.; Valdés-Ramírez, G.; Windmiller, J. R.; Andrade, Oxygen Sensors on Flexible Plastic Substrates. Org. Electron. 2016, 39, F. J.; Wang, J. Bandage-Based Wearable Potentiometric Sensor for 168−176. Monitoring Wound PH. Electroanalysis 2014, 26, 1345−1353. (40) Lopez Aldaba, A.; González-Vila, Á .; Debliquy, M.; Lopez-Amo, (23) Zea, M.; Moya, A.; Abrao-Nemeir, I.; Gallardo-Gonzalez, J.; M.; Caucheteur, C.; Lahem, D. Polyaniline-Coated Tilted Fiber Bragg Zine, N.; Errachid, A.; Villa, R.; Gabriel, G. In All Inkjet Printing Sensor Gratings for PH Sensing. Sens. Actuators, B 2018, 254, 1087−1093. Device on Paper: For Immunosensors Applications, 2019 20th (41) Rahimi, R.; Ochoa, M.; Parupudi, T.; Zhao, X.; Yazdi, I. K.; International Conference on Solid-State Sensors, Actuators and Dokmeci, M. R.; Tamayol, A.; Khademhosseini, A.; Ziaie, B. A Low- Microsystems Eurosensors XXXIII (TRANSDUCERS EUROSEN- Cost Flexible PH Sensor Array for Wound Assessment. Sens. SORS XXXIII), 2019; pp 2472−2475. Actuators, B 2016, 229, 609−617. (24) Park, Y. R.; Doh, J. H.; Shin, K.; Seo, Y. S.; Kim, Y. S.; Kim, S. (42) Lindfors, T.; Ivaska, A. PH Sensitivity of Polyaniline and Its Y.; Choi, W. K.; Hong, Y. J. Solution-Processed Quantum Dot Light- Substituted Derivatives. J. Electroanal. Chem. 2002, 531, 43−52. Emitting Diodes with PANI:PSS Hole-Transport Interlayers. Org. (43) Lakard, B.; Segut, O.; Lakard, S.; Herlem, G.; Gharbi, T. Electron. 2015, 19, 131−139. Potentiometric Miniaturized PH Sensors Based on Polypyrrole Films. (25) Jang, J.; Ha, J.; Cho, J. Fabrication of Water-Dispersible Sens. Actuators, B 2007, 122, 101−108. Polyaniline-Poly(4-Styrenesulfonate) Nanoparticles For Inkjet- (44) Gao, W.; Song, J. Polyaniline Film Based Amperometric PH Printed Chemical-Sensor Applications. Adv. Mater. 2007, 19, 1772− Sensor Using a Novel Electrochemical Measurement System. 1775. Electroanalysis 2009, 21, 973−978. (26) Han, H.; Lee, J. S.; Cho, S. Comparative Studies on Two- (45) Lakard, B.; Segut, O.; Lakard, S.; Herlem, G.; Gharbi, T. Electrode Symmetric Supercapacitors Based on Polypyrrole: Poly(4- Potentiometric Miniaturized PH Sensors Based on Polypyrrole Films. Styrenesulfonate) with Different Molecular Weights of Poly(4- Sens. Actuators, B 2007, 122, 101−108. Styrenesulfonate). Polymers 2019, 11, No. 232. (46) Lindfors, T.; Ivaska, A. PH Sensitivity of Polyaniline and Its (27) Texidó, R.; Borrós, S. Allylamine PECVD Modification of Substituted Derivatives. J. Electroanal. Chem. 2002, 531, 43−52. PDMS as Simple Method to Obtain Conductive Flexible Polypyrrole (47) Lakard, B.; Segut, O.; Lakard, S.; Herlem, G.; Gharbi, T. Thin Films. Polymers 2019, 11, No. 2108. Potentiometric Miniaturized PH Sensors Based on Polypyrrole Films. (28) Texidó, R.; Orgaz, A.; Ramos, V.; Borrós, S. Stretchable Sens. Actuators, B 2007, 122, 101−108. Conductive Polypyrrole Films Modified with Dopaminated Hyalur- (48) Masalles, C.; Borrós, S.; Viñas, C.; Teixidor, F. Simple PVC- onic Acid. Mater. Sci. Eng., C 2017, 76, 295−300. PPy Electrode for PH Measurement and Titrations. Anal. Bioanal. (29) Nardes, A. M.; Kemerink, M.; Janssen, R. A. J.; Bastiaansen, J. Chem. 2002, 372, 513−518. A. M.; Kiggen, N. M. M.; Langeveld, B. M. W.; Van Breemen, A. J. J. (49) Dang, W.; Manjakkal, L.; Navaraj, W. T.; Lorenzelli, L.; M.; De Kok, M. M. Microscopic Understanding of the Anisotropic Vinciguerra, V.; Dahiya, R. Stretchable Wireless System for Sweat PH Conductivity of PEDOT:PSS Thin Films. Adv. Mater. 2007, 19, Monitoring. Biosens. Bioelectron. 2018, 107, 192−202. 1196−1200. (50) Minari, T.; Kanehara, Y.; Liu, C.; Sakamoto, K.; Yasuda, T.; (30) Lang, U.; Muller, E.; Naujoks, N.; Dual, J. Microscopical Yaguchi, A.; Tsukada, S.; Kashizaki, K.; Kanehara, M. Room- Investigations of PEDOT:PSS Thin Films. Adv. Funct. Mater. 2009, Temperature Printing of Organic Thin-Film Transistors with π- 19, 1215−1220. Junction Gold Nanoparticles. Adv. Funct. Mater. 2014, 24, 4886− (31) Yan, W.; Han, J. Synthesis and Formation Mechanism Study of 4892. Rectangular-Sectioned Polypyrrole Micro/Nanotubules. Polymer (51) Radhakrishnan, S.; Deshpande, S. D. Electrical Properties of 2007, 48, 6782−6790. Conducting Polypyrrole Films Functionalized with Phthalocyanine. (32) Zhang, J.; Gao, L.; Sun, J.; Liu, Y.; Wang, Y.; Wang, J. Mater. Lett. 2001, 48, 144−150. Incorporation of Single-Walled Carbon Nanotubes with PEDOT/PSS (52) Panwar, V.; Kumar, P.; Ray, S. S.; Jain, S. L. Organic Inorganic in DMSO for the Production of Transparent Conducting Films. Hybrid Cobalt Phthalocyanine/Polyaniline as Efficient Catalyst for Diamond Relat. Mater. 2012, 22, 82−87. Aerobic Oxidation of Alcohols in Liquid Phase. Tetrahedron Lett. (33) Eom, S. H.; Senthilarasu, S.; Uthirakumar, P.; Yoon, S. C.; Lim, 2015, 56, 3948−3953. J.; Lee, C.; Lim, H. S.; Lee, J.; Lee, S. H. Polymer Solar Cells Based on (53) Prissanaroon-Ouajai, W.; Pigram, P. J.; Jones, R.; Sirivat, A. A Inkjet-Printed PEDOT:PSS Layer. Org. Electron. 2009, 10, 536−542. Sensitive and Highly Stable Polypyrrole-Based PH Sensor with (34) Savagatrup, S.; Chan, E.; Renteria-Garcia, S. M.; Printz, A. D.; Hydroquinone Monosulfonate and Oxalate Co-Doping. Sens. Zaretski, A. V.; O’Connor, T. F.; Rodriquez, D.; Valle, E.; Lipomi, D. Actuators, B 2009, 138, 504−511. J. Plasticization of PEDOT:PSS by Common Additives for (54) Park, H. J.; Yoon, J. H.; Lee, K. G.; Choi, B. G. Potentiometric Mechanically Robust Organic Solar Cells and Wearable Sensors. Performance of Flexible PH Sensor Based on Polyaniline Nanofiber Adv. Funct. Mater. 2015, 25, 427−436. Arrays. Nano Convergence 2019, 6, No. 9. (35) Latonen, R. M.; Määttänen, A.; Ihalainen, P.; Xu, W.; Pesonen, (55) Yoon, J. H.; Hong, S. B.; Yun, S.-O.; Lee, S. J.; Lee, T. J.; Lee, M.; Nurmi, M.; Xu, C. Conducting Ink Based on Cellulose K. G.; Choi, B. G. High Performance Flexible PH Sensor Based on Nanocrystals and Polyaniline for Flexographical Printing. J. Mater. Polyaniline Nanopillar Array Electrode. J. Colloid Interface Sci. 2017, Chem. C 2017, 5, 12172−12181. 490, 53−58. (36) Ghamouss, F.; Brugère, A.; Anbalagan, A. C.; Schmaltz, B.; (56) Ferrer-Anglada, N.; Kaempgen, M.; Roth, S. Transparent and Luais, E.; Tran-Van, F. Novel Glycerol Assisted Synthesis of Flexible Carbon Nanotube/Polypyrrole and Carbon Nanotube/ Polypyrrole Nanospheres and Its Electrochemical Properties. Synth. Polyaniline PH Sensors. Phys. Status Solidi B 2006, 243, 3519−3523. Met. 2013, 168, 9−15. (57) Rahimi, R.; Ochoa, M.; Yu, W.; Ziaie, B. In A Highly Stretchable (37) Lee, M. W.; Lee, M. Y.; Choi, J. C.; Park, J. S.; Song, C. K. Fine PH Sensor Array Using Elastomer-Embedded Laser Carbonized Patterns, Patterning of Glycerol-Doped PEDOT:PSS on Hydrophobic PVP 2015 Transducers - 2015 18th International Conference on Solid- 33534 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535 ACS Applied Materials & Interfaces www.acsami.org Research Article State Sensors, Actuators and Microsystems (TRANSDUCERS), 2015; pp 1897−1900. (58) Punjiya, M.; Rezaei, H.; Zeeshan, M. A.; Sonkusale, S. In A Flexible PH Sensing Smart Bandage with Wireless CMOS Readout for Chronic Wound Monitoring, 2017 19th International Conference on Solid-State Sensors, Actuators and Microsystems (TRANSDUCERS), 2017; pp 1700−1702. (59) Kaempgen, M.; Roth, S. Transparent and Flexible Carbon Nanotube/Polyaniline PH Sensors. J. Electroanal. Chem. 2006, 586, 72−76. (60) Aquino-Binag, C. N.; Kumar, N.; Lamb, R. N.; Pigram, P. J. Fabrication and Characterization of a Hydroquinone-Functionalized Polypyrrole Thin-Film PH Sensor. Chem. Mater. 1996, 8, 2579−2585. (61) Li, Y.; Mao, Y.; Xiao, C.; Xu, X.; Li, X. Flexible PH Sensor Based on a Conductive PANI Membrane for PH Monitoring. RSC Adv. 2020, 10, 21−28. (62) Bao, Q.; Yang, Z.; Song, Y.; Fan, M.; Pan, P.; Liu, J.; Liao, Z.; Wei, J. Printed Flexible Bifunctional Electrochemical Urea-PH Sensor Based on Multiwalled Carbon Nanotube/Polyaniline Electronic Ink. J. Mater. Sci.: Mater. Electron. 2019, 30, 1751−1759. (63) Hou, X.; Zhou, Y.; Liu, Y.; Wang, L.; Wang, J. Coaxial Electrospun Flexible PANI//PU Fibers as Highly Sensitive PH Wearable Sensor. J. Mater. Sci. 2020, 55, 16033−16047. (64) Wang, R.; Zhai, Q.; Zhao, Y.; An, T.; Gong, S.; Guo, Z.; Shi, Q.; Yong, Z.; Cheng, W. Stretchable Gold Fiber-Based Wearable Electrochemical Sensor toward PH Monitoring. J. Mater. Chem. B 2020, 8, 3655−3660. (65) Yu, K.; He, N.; Kumar, N.; Wang, N.; Bobacka, J.; Ivaska, A. Electrosynthesized Polypyrrole/Zeolite Composites as Solid Contact in Potassium Ion-Selective Electrode. Electrochim. Acta 2017, 228, 66−75. 33535 https://doi.org/10.1021/acsami.1c08043 ACS Appl. Mater. Interfaces 2021, 13, 33524−33535