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Miniaturisation and simplification of solid-state proton activity sensors for non-aqueous media and ionic liquids† Orawan Winther-Jensen,* Jessie L. Hamilton, Chun H. Ng, Bartlomiej Kolodziejczyk and Bjorn Winther-Jensen This work is the further development of the previous pH (effective) sensor work where a biologically derived proton-active redox centre – riboflavin (RFN) – was entrapped into a vapour phase polymerised poly(3,4ethylenedioxythiophene) film and ferrocene (Fc) dissolved in the sample solution was used as an internal reference redox couple. Here, we report a disposable solid state pH (effective) sensor where we successfully incorporated both RFN and Fc into a single solid state electrode. The electrodes were then used for pH (effective) sensing where water is not required. The system was further miniaturised and simplified from a 3 electrode to a 2 electrode setup with the working electrode area being as small as 0.09 mm2. The sensors show the ability to measure pH (effective) in both aqueous and non-aqueous
Received 25th August 2014 Accepted 24th November 2014
media including ionic liquids (ILs) regardless of their hydrophobicity. This is an important step towards the ability to customise ILs or non-aqueous media with suitable proton activity (PA) for various
DOI: 10.1039/c4an01556h www.rsc.org/analyst
applications e.g. customised ILs for biotechnological applications such as protein preservation and customised media for PA dependent reactions such as catalytic reactions.
Introduction The measurement of proton activity (PA) in non-aqueous systems has been a challenge and a topic of debate over the past decades. Although recently there have been some commercial pH electrodes for non-aqueous systems coming out in the market, the questions regarding (i) the junction between the electrolyte inside the pH electrode that should be the same medium as that being measured, (ii) cross-contamination between the samples which could easily occur even with the solid state electrode such as the ion sensitive eld effect transistor pH electrode,1 and (iii) the inappropriate use of glass electrodes are still in place. The ability to determine PA (and thereby referred to as pH (effective) throughout the manuscript) in non-aqueous or ionic liquid (IL) systems will assist one in better predicting/designing media for particular reactions (e.g. for fuel cell applications2) and also in better understanding the reaction mechanism in such media as PA can have a great effect on the kinetics of catalytic reactions2–6 and conditions for synthesis.7 In some biological applications such as protein preservation, some
Department of Materials Engineering, Monash University, Clayton, Australia. E-mail: [email protected]; [email protected]; chun.ng@ monash.edu; [email protected]; bjorn.winther-jensen@monash. edu; Fax: +61 3 9905 4940; Tel: +61 3 9905 4939 † Electronic supplementary 10.1039/c4an01556h
information
(ESI)
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proteins can only survive in a quite narrow pH range.8 Therefore, there are several reports on the use of ILs to preserve proteins9–11 as well as on the determination of pKa of ILs.1,2,7,11–23 Recently we reported a solid-state pH (effective) sensor for non-aqueous media including ILs based on the entrapped riboavin (RFN) inside the vapour phase polymerised (VPP) poly(3,4-ethylenedioxythiophene) (PEDOT) lm.24 In that work, RFN was used as the pH (effective) sensor as its redox process requires no water25,26 while ferrocene (Fc) was used as the internal reference redox couple by being dissolved in the tested samples. The system was successfully demonstrated to measure pH (effective) trends in aqueous and non-aqueous solutions, including ILs, covering pH 3–9.24 We have previously used various methods to incorporate the second components into PEDOT in order to make functionalized electrodes to suit the applications.27–29 In this work, we focus on the fabrication of a simplied, miniaturized and disposable all solid state pH (effective) sensor for non-aqueous media and ILs. Therefore, the incorporation method has been slightly modied from ref. 24 so that RFN is mixed with the oxidant and Fc is incorporated by washing it into the PEDOT:RFN lm aer the VPP. Lastly, the changeover from the conventional 3 electrode setup to 2 electrode setup conguration was considered as an important aspect in real applications. A very high capacity PEDOT coated C-paper has been used as the combined reference (RE) and counter (CE) electrode. The high capacity RE & CE is used in combination with a micro-sized, low capacity working
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electrode to minimize the potential dri of the combined RE & CE that could occur from high current ows on the combined RE & CE. Thus the capacitance of the combined RE & CE was designed to be 160 times larger than the capacitance of the working electrode when performing the full redox-cycle. We assembled the miniaturized disposable pH (effective) sensor based on PEDOT:RFN-Fc electrodes. The miniaturized electrode setup was then used with only a single drop of various pH buffers to establish the calibration curve from the difference in RFN and Fc potentials measured using the cyclic voltammetry (CV) technique. The calibration curve was then used to estimate the pH (effective) of ILs (both hydrophilic and hydrophobic) and the mixture of IL and an organic solvent.
Experimental Materials 40% iron(III) p-toluenesulphonate in butanol and 3,4-ethylene dioxythiophene (EDOT) were purchased from Yacoo Chemicals Co., Ltd. RFN, Fc and propylene carbonate (PC) were purchased from Sigma Aldrich. Pyridine was from BDH Chemicals. HCl was purchased from Ajax Finechem Pty Ltd. Di-sodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), ethanol, potassium hydrogenphthalate, 1-ethyl-3methylimidazolium tetracyanoborate (EMIm(CN)4B) and 1ethyl-3-methylimidazolium thiocyanate (EMImSCN) were purchased from Merck. Triisobutyl(methyl)phosphonium tosylate (P1,4,4,4TOS) was supplied by Cytec. 1-Methyl-1-n-propyl pyrrolidinium bis(triuoromethane sulfonyl) imide (P1,3NTf2)30 and ethylammonium nitrate (EAN)31 were prepared in-house according to previously reported procedures. Commercial printed electrodes were obtained from GSI Technologies. Carbon paper (C-paper) TGP-H-090 with 280 mm thickness and 78% porosity was purchased from Toray. Water content determination A Karl Fisher Coulometer (Coulometric 831) ex Metrohm was used to determine the water content of the ILs used in this report. VPP of PEDOT:RFN and incorporation of Fc Approximately 38 mg of RFN was added into the mixture of 24 ml pyridine in 1 ml of 40% Fe(III) p-toluenesulphonate in butanol. VPP PEDOT was then prepared by spin-coating this oxidant– RFN mixture on either Au mylar or commercial printed electrodes, and putting the coated electrode in a polymerisation chamber (with a preheated EDOT monomer) in an oven at 70 C for 35 min. For the printed electrodes, RE and CE were maskedout beforehand using an adhesive tape. The unwashed PEDOT:RFN lm was le to cool to room temperature and used immediately. Approximately 178 mg Fc was dissolved in 4 ml of an ethanol–toluene (1 : 1) mixture. Fc solution was then cast as a droplet over the unwashed PEDOT:RFN lm, le for a few seconds, and drained. This step was repeated twice before rinsing the PEDOT:RFN-Fc electrode with water and drying under room conditions.
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PEDOT coated C-paper electrodes A solution of 24 ml pyridine in 1 ml of 40% Fe(III) p-toluenesulphonate in butanol was diluted in butanol to a 1 : 1 (v/v) ratio. The oxidant solution was spin-coated on the C-paper and vapor phase polymerised for 30 min. The VPP procedure was repeated twice before washing the electrode with ethanol three times. Buffer preparation Various aqueous buffers were prepared across a pH range of 2.3–11.0. Details of the buffers used are as follows: potassium hydrogen phthalate (C8H5KO4)/HCl buffers: pH 2.3 and 3.0; sodium phosphate buffers (Na2HPO4/NaH2PO4): pH 5.0, pH 6.9 or pH 7.0, pH 9.0; and sodium phosphate/sodium hydroxide (Na2HPO4/NaOH) pH 11.0. The pH of these buffers was measured using a Mettler Toledo pH meter. Electrochemical experiments For the 3 electrode setup, the electrolyte was purged with N2 for at least 30 min before the CV scans were started. Ag/AgCl (3 M NaCl) and Pt wire were used as a reference and counter electrode, respectively. N2 bubbling was omitted when the commercial printed electrodes or the 2 electrode setup was used. A drop of each of P1,3NTf2, EMIm(CN)4B, EMImSCN, 67% P1,4,4,4TOS in PC or EAN was cast over the commercial printed or the 2 electrode sensor and CVs were scanned. The CV scan rate was 10 mV s1 for all experiments. In all CV gures, the Y-axis represents current. The axis labels were omitted to enhance the readability of the gures. In all CV gures, the Ehalf of Fc was zeroed, therefore all X-axes were labelled as normalised potentials. All reported CVs were obtained from cycle # 2 unless otherwise stated. pH (effective) calculations CVs from various pH buffers, using PEDOT:RFN-Fc as the working electrode, were used to construct a calibration curve. The RFN and Fc peak potentials (Eox and Ered) and Ehalf, which was obtained from the mid-point between Eox and Ered, were used to determine DEhalf between RFN and Fc (see Fig. S1† for more details). The calibration curve was then established from the DEhalf and pH of the solution. The DEhalf obtained from the CVs of each IL was then substituted in the equation from the respective calibration curve and the pH (effective) was calculated. Scanning electron microscopy The samples were sputter-coated with a thin gold layer and SEM was performed using a JEOL 7100F Field Emission Gun Scanning Electron Microscope at 5 kV. Energy-dispersive X-ray analysis (EDX) was performed at 15 kV for elemental analysis. Raman spectroscopy Raman spectroscopy was performed using a Horiba Jobin Yvon T64000 Raman system. A blue 488 nm diode laser was used for
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excitation with a power of 0.65 mW. The spectral resolution of this Raman system and setup parameters is 5.94 cm1. The wavelength precision of the instrument was studied and RMS variation in peak position from 5 repetitive measurements is determined to be only 0.87 cm1. For PEDOT samples, scans were performed using an acquisition time of 5 s averaged over 5 accumulations at 100% laser power. For the PEDOT:RFN samples, a 20 s accumulation time averaged over 10 accumulations at 100% laser power was used; this was due to the much lower Raman signal exhibited by these lms. Fourier transform infrared (FTIR) spectroscopy FTIR spectra were obtained using a Perkin Elmer Spectrum100 FTIR with diamond ATR on dry lms or powdered samples. Laser etching The combined RE & CE area was maintained to be about 160 times larger than the WE area in the 2 electrode setup using laser etching to dene the working areas. A VersaLaser VLS2.30 laser engraver has been used to precisely cut ‘electrode windows’ in the laminating sheet allowing for a very small working electrode area with high reproducibility. A VersaLaser VLS2.30 is equipped with a 10.6 mm CO2 laser. Cutting parameters used to produce laminated patterns were 4.4 W laser power and 6100 mm min1 cutting speed.
Results and discussion PEDOT:RFN-Fc electrodes were fabricated as described in the Experimental section. Both RFN and Fc are physically entrapped into the PEDOT lm. To conrm the presence of RFN and Fc in the PEDOT lm, SEM, Raman and FTIR were performed (Fig. 1). Excess of RFN appears as needles in the size of about 1 5 mm all over the lm (Fig. 1a). Raman spectra of PEDOT:RFN showed a slight shi of the Ca ¼ Cb symmetric stretching peak32 to 1441 cm1 compared to 1433 cm1 in the PEDOT spectrum (Fig. 1b) indicating that there is some interaction at the molecular level between PEDOT and RFN leading to a slight reduction of the PEDOT.33 RFN by itself is highly uorescent, using the 488 nm laser and Raman measurement setup as described in the Experimental section, causing off-limit
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intensity (not shown). The spectra in Fig. 1b are not base-line corrected hence a slightly higher uorescence of PEDOT:RFN lms is observable compared to the PEDOT lm indicating the effect of RFN incorporation (see full Raman spectra and band assignments for PEDOT and PEDOT:RFN lms in the ESI Fig. S2†). Fc was neither stable under the operating conditions (high vacuum) for SEM nor under Raman laser penetration (subliming in both cases) therefore FTIR was performed. The presence of RFN and Fc was clearly observed as various peaks from the PEDOT:RFN-Fc FTIR spectrum. However, the main characteristic peaks for PEDOT (at around 1555 and 1438 cm1 (assigned to C]C ring stretching34), 1265 cm1 (due to Ar–O stretching), and bands around 1000–1160 cm1 (assigned to the C–O stretching of ethylenedioxy group35)) were overlapping with some of the RFN and Fc peaks. The C–S bond in the thiophene ring at around 950 cm1 (ref. 35) only appeared as a shoulder for the PEDOT:RFN-Fc sample. Therefore, no interaction can be observed from FTIR spectra. Peak assignments for RFN and Fc can be found in the ESI.† The electrodes were then tested under N2 in various pH buffers in the 3 electrode setup and the CVs are shown in Fig. 2a. The Ehalf of the Fc redox couples ranges from 0.24 V to 0.28 V and it was normalized (Fig. 2a) whereas RFN redox couples shied linearly with pH. The DEhalf between Fc and RFN ranged from 0.44 V (pH 2.3) to 0.85 V (pH 9). This result is in good agreement with the previous results when Fc was dissolved in the sample solution.24 The oxidation peak at about 40– 80 mV below the Ehalf of Fc is a relaxation peak from PEDOT oxidation which is oen seen in PEDOT composites.36 To test the effect of miniaturisation, PEDOT:RFN-Fc was fabricated over the working electrode area of the commercial printed microelectrodes. Various pH buffers (3.0, 5.0 and 9.0) were scanned using these electrodes and CVs are shown in Fig. 2b. For real application, it is not practical if oxygen has to be completely removed from a single drop of sample before the measurement. N2 bubbling was therefore omitted from these experiments. PEDOT is a good catalyst for oxygen reduction37 hence, without N2 bubbling, the oxygen reduction reaction takes place and can be seen from the waves in the negative potential region shiing down especially at high pH (pH 9.0). Nonetheless, a calibration curve with R2 0.998 was established from the difference in Ehalf of Fc and RFN. A drop of EMImSCN was tested and the pH (effective) was calculated from the DEhalf
Fig. 1 (a) SEM images of the PEDOT:RFN film (scale bar 100 mm), (b) Raman spectra of PEDOT and PEDOT:RFN-Fc films (red vertical line scale bar equals 2000 counts), and (c) FTIR spectra of RFN (dotted red trace), PEDOT (thin blue trace), Fc (dashed grey trace) and PEDOT:RFN-Fc (thick black trace) films.
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Fig. 2 CVs (cycle # 2) of PEDOT:RFN-Fc scanning at 10 mV s1 in various buffers (a) with N2 bubbling in a traditional 3 electrode setup (inset) where Ag/AgCl (3 M NaCl) and Pt were used as counter and reference electrodes respectively and (b) using the commercial printed microelectrodes (inset) without N2 bubbling. Scale bars for the peak currents (presented as red vertical lines below the CVs) are 20 mA.
between RFN and Fc and found to be 3.80 using this miniaturised 3 electrode setup. To simplify the system, the capacity of PEDOT-coated C-paper was studied to evaluate whether it can be used as the combined RE & CE. SEM and EDX were rst performed to observe the PEDOT layer on the C-paper and it was conrmed that PEDOT coating was not blocking the C-paper pores (Fig. S3†). The PEDOT coated C-paper electrode was then scanned in 0.1 M phosphate buffer (PB) pH 5 in comparison to the bare C-paper (Fig. S4†). The capacity of the PEDOT-coated C-paper electrode, calculated as DImax multiplied by the scan rate, was much higher than the C-paper alone and calculated to be approximately 10 mF cm2. In comparison, the capacity generally observed on the PEDOT:RFN-Fc electrode during the redox cycling is in the same range (11 mF cm2). By designing the setup with asymmetrical electrode areas, the capacity of the combined RE & CE is about 160 times that of the WE which is expected to be sufficient for the combined RE & CE electrode to prevent potential dri that could interfere with the measurement. The WE area was then miniaturised to be as small as 0.09 mm2 using the laser engraver technique as described in the Experimental section. The laminating sheet was laser-etched leaving windows in the size of 0.09 mm2 for the WE and 15 mm2 for the combined RE & CE (Fig. 3a). This allows the combined RE & CE electrode area to be signicantly higher (geometrically)
than the WE area for the consideration mentioned above. Various pH buffers were scanned using this microelectrode setup and the calibration curves were established (Fig. 3). The results from two different PEDOT:RFN-Fc lms gave two almost identical calibration curves (Fig. 3b) with very good linearity of 0.9995 and 0.9924 for pH 3–11. The obtained DEhalf from PB pH 5 using the 2 electrode setup in three separate experiments was 0.644, 0.639 and 0.647 V which has given the average of 0.643 0.004 V. This has proven the good reproducibility of the lm preparation and measurement methods. The slope of the calibration curve from this 2 electrode setup is higher than the theoretical 59 mV whereas the slopes measured with the 3 electrode setup are close to this theoretical value.24 This could probably be explained by the fact that the open circuit potential of PEDOT on the combined RE & CE systematically shied with pH in the range of interest (pH 3–11) as previously reported to be 30 mV per pH unit in this pH range.38 To investigate the ability of the sensor to detect the pH (effective) of ILs and non-aqueous media, two hydrophobic ILs (P1,3NTf2 and EMIm(CN)4B), a hydrophilic IL (EMImSCN), one protic IL (EAN) and a mixture of P1,4,4,4TOS in PC (67% v/v) were then scanned using only 1 drop of each and their CVs are shown in Fig. 4. The pH (effective) of P1,3NTf2, EMIm(CN)4B, EAN, EMImSCN and 67% P1,4,4,4TOS was then calculated using the respective calibration curve in Fig. 3b. The values of Eox, Ered of
Using the miniaturised 2 electrode setup (a) CVs (cycle # 2) of PEDOT:RFN-Fc scanning at 10 mV s1 in various buffers pH 3 to 11 and (b) calibration curves established from the CVs in (a). Fig. 3
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Fig. 4 CVs (cycle # 2) of PEDOT:RFN-Fc scanning at 10 mV s1 in P1,3NTf2, EMIm(CN)4B, EMImSCN, 67% P1,4,4,4TOS in PC, and EAN. Scale bars (presented as thick red vertical lines next to the graphs) are 5 mA for P1,3NTf2, EMImSCN, and 67% P1,4,4,4TOS in PC, and 1 mA for EMIm(CN)4B and EAN.
both RFN and Fc, and the DEhalf between the two redox couples are also tabulated (Table 1). It is worth noting that the pH (effective) of EMImSCN, scanned using this 2 electrode setup, was found to be 3.68 which is very close to the value obtained (3.80) when the commercial microelectrode (3 electrode setup) was used indicating the general viability of the concept. Multiple pH (effective) measurements were also performed for P1,3NTf2, using electrodes from different batches, and the reproducibility of the measurement is within 3% (pH 9.14 0.25). The stability of RFN and Fc peaks over the successive scans has been studied and the %RSD of peak current upon three repetitive CV scans in most ILs was in the range of 2 to 23% (Table S1†). The exception is for 67% P1,4,4,4TOS in PC where % RSD from Fc peaks was as high as 54%. This is due to the swelling properties of PEDOT in this particular IL as reported in ref. 39 causing the detachment of Fc during scanning. Nonetheless, the peaks at cycle 2 (Fig. 4) are still denitely distinguishable at this rate of decrease.
Table 1
CVs of EAN (Fig. 4) systematically shi toward more positive potentials over the cycles, which is probably due to the slow change in the oxidation state of PEDOT in this IL. However, in the adjacent cycle i.e. cycle # 8 and cycle # 9, it was obvious that both RFN and Fc peaks shied at the same distance hence the DEhalf was constant at 0.83 V for both cycles. The DEhalf between RFN and Fc increased over time e.g. from 0.69 V in cycle # 2 to 0.83 V in cycle # 8 causing the pH (effective) to increase from 6.08 to 7.71, respectively. This could be due to moisture absorption over time into the single droplet of the IL where the experiment was performed under room conditions. The effect of water content has been observed previously where the water content caused the difference in the dissociation of choline dihydrogenphosphate IL and consequently its pH (effective).24,40 Nonetheless, the pH between the two cycles, i.e. cycle # 8 and cycle # 9, was constant at 7.71 indicating that the method (scanning at 10 mV s1) is valid for determining the pH value of the IL despite the moisture absorption issue. P1,4,4,4TOS and EMImSCN are hydrophilic and can be dissolved in water. 67% of these ILs in water give a pH of 1.56 and 6.42, respectively, using a traditional glass pH electrode. The reading for P1,4,4,4TOS conrms that the IL is acidic both in water and in PC. The reading for EMImSCN was not stable and kept increasing, indicating that the AgCl junction was disturbed and conrming the problem in using this kind of traditional glass electrode to measure the pH of materials with high ionic strength. P1,3NTf2 and EMIm(CN)4B are hydrophobic and hence it was not possible to read the pH in aqueous solutions of these two ILs. Although the purpose of this work is not necessarily answering the economical question but rather nding the reliable system for use in non-aqueous samples and reducing the complexity of the operation, the material cost of the combined RE & CE and WE used in this work is calculated to be approximately $0.03 US per test – a price that may be worth considering especially in comparison to the hydrogen electrode system.
Conclusion Miniaturised all solid state pH (effective) sensors were successfully fabricated based on the PEDOT:RFN-Fc electrode. The working electrode area can be as small as 0.09 mm2 and only a single drop of the sample is needed. The electrode responds linearly with the pH change over the range of pH 3 to 11. The miniaturised and simplied 2 electrode setup has
Eox, Ered of RFN and Fc read from CVs in Fig. 4, DEhalf between the two redox couples and the calculated pH (effective) RFN (V)
Fc (V)
ILs (%H2O)
Eox
Ered
Eox
Ered
DEhalf (V)
pH (effective)
P1,3NTf2 (0.04) EMIm(CN)4B (0.02) EMImSCN (0.34) 67% P1,4,4,4TOS in PC (1.30) EAN (3.31)
0.69 0.43 0.12 0.44 0.47
0.84 0.53 0.02 0.51 0.57
0.30 0.47 0.69 0.22 0.21
0.16 0.40 0.46 0.16 0.14
0.99 0.91 0.52 0.51 0.69
8.96 8.06 3.68 3.81 6.08
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proven its ability to determine the pH (effective) of various ILs and non-aqueous media. Further testing of the solid state pH (effective) sensor in various non-aqueous solvents and in the vast amounts of different ILs will be an ongoing process. However, this report has potentially solved various problems currently existing regarding the measurement of PA in nonaqueous media. In addition, this simplied and miniaturised system is transportable which made it convenient for use in the eld.
Acknowledgements The authors thank Ciaran McDonnell-Worth and Kevin J Fraser for supplying a few drops of EAN and P1,3NTf2 respectively. The authors acknowledge the use of facilities under the Australian Research Council's Centres of Excellence funding scheme (COE for Design in Light Metals) within the Monash Centre for Electron Microscopy.
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Miniaturisation and simplification of solid-state proton activity sensors for non-aqueous media and ionic liquids† Orawan Winther-Jensen,* Jessie L. Hamilton, Chun H. Ng, Bartlomiej Kolodziejczyk and Bjorn Winther-Jensen This work is the further development of the previous pH (effective) sensor work where a biologically derived proton-active redox centre – riboflavin (RFN) – was entrapped into a vapour phase polymerised poly(3,4ethylenedioxythiophene) film and ferrocene (Fc) dissolved in the sample solution was used as an internal reference redox couple. Here, we report a disposable solid state pH (effective) sensor where we successfully incorporated both RFN and Fc into a single solid state electrode. The electrodes were then used for pH (effective) sensing where water is not required. The system was further miniaturised and simplified from a 3 electrode to a 2 electrode setup with the working electrode area being as small as 0.09 mm2. The sensors show the ability to measure pH (effective) in both aqueous and non-aqueous
Received 25th August 2014 Accepted 24th November 2014
media including ionic liquids (ILs) regardless of their hydrophobicity. This is an important step towards the ability to customise ILs or non-aqueous media with suitable proton activity (PA) for various
DOI: 10.1039/c4an01556h www.rsc.org/analyst
applications e.g. customised ILs for biotechnological applications such as protein preservation and customised media for PA dependent reactions such as catalytic reactions.
Introduction The measurement of proton activity (PA) in non-aqueous systems has been a challenge and a topic of debate over the past decades. Although recently there have been some commercial pH electrodes for non-aqueous systems coming out in the market, the questions regarding (i) the junction between the electrolyte inside the pH electrode that should be the same medium as that being measured, (ii) cross-contamination between the samples which could easily occur even with the solid state electrode such as the ion sensitive eld effect transistor pH electrode,1 and (iii) the inappropriate use of glass electrodes are still in place. The ability to determine PA (and thereby referred to as pH (effective) throughout the manuscript) in non-aqueous or ionic liquid (IL) systems will assist one in better predicting/designing media for particular reactions (e.g. for fuel cell applications2) and also in better understanding the reaction mechanism in such media as PA can have a great effect on the kinetics of catalytic reactions2–6 and conditions for synthesis.7 In some biological applications such as protein preservation, some
Department of Materials Engineering, Monash University, Clayton, Australia. E-mail: [email protected]; [email protected]; chun.ng@ monash.edu; [email protected]; bjorn.winther-jensen@monash. edu; Fax: +61 3 9905 4940; Tel: +61 3 9905 4939 † Electronic supplementary 10.1039/c4an01556h
information
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proteins can only survive in a quite narrow pH range.8 Therefore, there are several reports on the use of ILs to preserve proteins9–11 as well as on the determination of pKa of ILs.1,2,7,11–23 Recently we reported a solid-state pH (effective) sensor for non-aqueous media including ILs based on the entrapped riboavin (RFN) inside the vapour phase polymerised (VPP) poly(3,4-ethylenedioxythiophene) (PEDOT) lm.24 In that work, RFN was used as the pH (effective) sensor as its redox process requires no water25,26 while ferrocene (Fc) was used as the internal reference redox couple by being dissolved in the tested samples. The system was successfully demonstrated to measure pH (effective) trends in aqueous and non-aqueous solutions, including ILs, covering pH 3–9.24 We have previously used various methods to incorporate the second components into PEDOT in order to make functionalized electrodes to suit the applications.27–29 In this work, we focus on the fabrication of a simplied, miniaturized and disposable all solid state pH (effective) sensor for non-aqueous media and ILs. Therefore, the incorporation method has been slightly modied from ref. 24 so that RFN is mixed with the oxidant and Fc is incorporated by washing it into the PEDOT:RFN lm aer the VPP. Lastly, the changeover from the conventional 3 electrode setup to 2 electrode setup conguration was considered as an important aspect in real applications. A very high capacity PEDOT coated C-paper has been used as the combined reference (RE) and counter (CE) electrode. The high capacity RE & CE is used in combination with a micro-sized, low capacity working
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electrode to minimize the potential dri of the combined RE & CE that could occur from high current ows on the combined RE & CE. Thus the capacitance of the combined RE & CE was designed to be 160 times larger than the capacitance of the working electrode when performing the full redox-cycle. We assembled the miniaturized disposable pH (effective) sensor based on PEDOT:RFN-Fc electrodes. The miniaturized electrode setup was then used with only a single drop of various pH buffers to establish the calibration curve from the difference in RFN and Fc potentials measured using the cyclic voltammetry (CV) technique. The calibration curve was then used to estimate the pH (effective) of ILs (both hydrophilic and hydrophobic) and the mixture of IL and an organic solvent.
Experimental Materials 40% iron(III) p-toluenesulphonate in butanol and 3,4-ethylene dioxythiophene (EDOT) were purchased from Yacoo Chemicals Co., Ltd. RFN, Fc and propylene carbonate (PC) were purchased from Sigma Aldrich. Pyridine was from BDH Chemicals. HCl was purchased from Ajax Finechem Pty Ltd. Di-sodium hydrogen phosphate (Na2HPO4), sodium dihydrogen phosphate (NaH2PO4), ethanol, potassium hydrogenphthalate, 1-ethyl-3methylimidazolium tetracyanoborate (EMIm(CN)4B) and 1ethyl-3-methylimidazolium thiocyanate (EMImSCN) were purchased from Merck. Triisobutyl(methyl)phosphonium tosylate (P1,4,4,4TOS) was supplied by Cytec. 1-Methyl-1-n-propyl pyrrolidinium bis(triuoromethane sulfonyl) imide (P1,3NTf2)30 and ethylammonium nitrate (EAN)31 were prepared in-house according to previously reported procedures. Commercial printed electrodes were obtained from GSI Technologies. Carbon paper (C-paper) TGP-H-090 with 280 mm thickness and 78% porosity was purchased from Toray. Water content determination A Karl Fisher Coulometer (Coulometric 831) ex Metrohm was used to determine the water content of the ILs used in this report. VPP of PEDOT:RFN and incorporation of Fc Approximately 38 mg of RFN was added into the mixture of 24 ml pyridine in 1 ml of 40% Fe(III) p-toluenesulphonate in butanol. VPP PEDOT was then prepared by spin-coating this oxidant– RFN mixture on either Au mylar or commercial printed electrodes, and putting the coated electrode in a polymerisation chamber (with a preheated EDOT monomer) in an oven at 70 C for 35 min. For the printed electrodes, RE and CE were maskedout beforehand using an adhesive tape. The unwashed PEDOT:RFN lm was le to cool to room temperature and used immediately. Approximately 178 mg Fc was dissolved in 4 ml of an ethanol–toluene (1 : 1) mixture. Fc solution was then cast as a droplet over the unwashed PEDOT:RFN lm, le for a few seconds, and drained. This step was repeated twice before rinsing the PEDOT:RFN-Fc electrode with water and drying under room conditions.
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PEDOT coated C-paper electrodes A solution of 24 ml pyridine in 1 ml of 40% Fe(III) p-toluenesulphonate in butanol was diluted in butanol to a 1 : 1 (v/v) ratio. The oxidant solution was spin-coated on the C-paper and vapor phase polymerised for 30 min. The VPP procedure was repeated twice before washing the electrode with ethanol three times. Buffer preparation Various aqueous buffers were prepared across a pH range of 2.3–11.0. Details of the buffers used are as follows: potassium hydrogen phthalate (C8H5KO4)/HCl buffers: pH 2.3 and 3.0; sodium phosphate buffers (Na2HPO4/NaH2PO4): pH 5.0, pH 6.9 or pH 7.0, pH 9.0; and sodium phosphate/sodium hydroxide (Na2HPO4/NaOH) pH 11.0. The pH of these buffers was measured using a Mettler Toledo pH meter. Electrochemical experiments For the 3 electrode setup, the electrolyte was purged with N2 for at least 30 min before the CV scans were started. Ag/AgCl (3 M NaCl) and Pt wire were used as a reference and counter electrode, respectively. N2 bubbling was omitted when the commercial printed electrodes or the 2 electrode setup was used. A drop of each of P1,3NTf2, EMIm(CN)4B, EMImSCN, 67% P1,4,4,4TOS in PC or EAN was cast over the commercial printed or the 2 electrode sensor and CVs were scanned. The CV scan rate was 10 mV s1 for all experiments. In all CV gures, the Y-axis represents current. The axis labels were omitted to enhance the readability of the gures. In all CV gures, the Ehalf of Fc was zeroed, therefore all X-axes were labelled as normalised potentials. All reported CVs were obtained from cycle # 2 unless otherwise stated. pH (effective) calculations CVs from various pH buffers, using PEDOT:RFN-Fc as the working electrode, were used to construct a calibration curve. The RFN and Fc peak potentials (Eox and Ered) and Ehalf, which was obtained from the mid-point between Eox and Ered, were used to determine DEhalf between RFN and Fc (see Fig. S1† for more details). The calibration curve was then established from the DEhalf and pH of the solution. The DEhalf obtained from the CVs of each IL was then substituted in the equation from the respective calibration curve and the pH (effective) was calculated. Scanning electron microscopy The samples were sputter-coated with a thin gold layer and SEM was performed using a JEOL 7100F Field Emission Gun Scanning Electron Microscope at 5 kV. Energy-dispersive X-ray analysis (EDX) was performed at 15 kV for elemental analysis. Raman spectroscopy Raman spectroscopy was performed using a Horiba Jobin Yvon T64000 Raman system. A blue 488 nm diode laser was used for
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excitation with a power of 0.65 mW. The spectral resolution of this Raman system and setup parameters is 5.94 cm1. The wavelength precision of the instrument was studied and RMS variation in peak position from 5 repetitive measurements is determined to be only 0.87 cm1. For PEDOT samples, scans were performed using an acquisition time of 5 s averaged over 5 accumulations at 100% laser power. For the PEDOT:RFN samples, a 20 s accumulation time averaged over 10 accumulations at 100% laser power was used; this was due to the much lower Raman signal exhibited by these lms. Fourier transform infrared (FTIR) spectroscopy FTIR spectra were obtained using a Perkin Elmer Spectrum100 FTIR with diamond ATR on dry lms or powdered samples. Laser etching The combined RE & CE area was maintained to be about 160 times larger than the WE area in the 2 electrode setup using laser etching to dene the working areas. A VersaLaser VLS2.30 laser engraver has been used to precisely cut ‘electrode windows’ in the laminating sheet allowing for a very small working electrode area with high reproducibility. A VersaLaser VLS2.30 is equipped with a 10.6 mm CO2 laser. Cutting parameters used to produce laminated patterns were 4.4 W laser power and 6100 mm min1 cutting speed.
Results and discussion PEDOT:RFN-Fc electrodes were fabricated as described in the Experimental section. Both RFN and Fc are physically entrapped into the PEDOT lm. To conrm the presence of RFN and Fc in the PEDOT lm, SEM, Raman and FTIR were performed (Fig. 1). Excess of RFN appears as needles in the size of about 1 5 mm all over the lm (Fig. 1a). Raman spectra of PEDOT:RFN showed a slight shi of the Ca ¼ Cb symmetric stretching peak32 to 1441 cm1 compared to 1433 cm1 in the PEDOT spectrum (Fig. 1b) indicating that there is some interaction at the molecular level between PEDOT and RFN leading to a slight reduction of the PEDOT.33 RFN by itself is highly uorescent, using the 488 nm laser and Raman measurement setup as described in the Experimental section, causing off-limit
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intensity (not shown). The spectra in Fig. 1b are not base-line corrected hence a slightly higher uorescence of PEDOT:RFN lms is observable compared to the PEDOT lm indicating the effect of RFN incorporation (see full Raman spectra and band assignments for PEDOT and PEDOT:RFN lms in the ESI Fig. S2†). Fc was neither stable under the operating conditions (high vacuum) for SEM nor under Raman laser penetration (subliming in both cases) therefore FTIR was performed. The presence of RFN and Fc was clearly observed as various peaks from the PEDOT:RFN-Fc FTIR spectrum. However, the main characteristic peaks for PEDOT (at around 1555 and 1438 cm1 (assigned to C]C ring stretching34), 1265 cm1 (due to Ar–O stretching), and bands around 1000–1160 cm1 (assigned to the C–O stretching of ethylenedioxy group35)) were overlapping with some of the RFN and Fc peaks. The C–S bond in the thiophene ring at around 950 cm1 (ref. 35) only appeared as a shoulder for the PEDOT:RFN-Fc sample. Therefore, no interaction can be observed from FTIR spectra. Peak assignments for RFN and Fc can be found in the ESI.† The electrodes were then tested under N2 in various pH buffers in the 3 electrode setup and the CVs are shown in Fig. 2a. The Ehalf of the Fc redox couples ranges from 0.24 V to 0.28 V and it was normalized (Fig. 2a) whereas RFN redox couples shied linearly with pH. The DEhalf between Fc and RFN ranged from 0.44 V (pH 2.3) to 0.85 V (pH 9). This result is in good agreement with the previous results when Fc was dissolved in the sample solution.24 The oxidation peak at about 40– 80 mV below the Ehalf of Fc is a relaxation peak from PEDOT oxidation which is oen seen in PEDOT composites.36 To test the effect of miniaturisation, PEDOT:RFN-Fc was fabricated over the working electrode area of the commercial printed microelectrodes. Various pH buffers (3.0, 5.0 and 9.0) were scanned using these electrodes and CVs are shown in Fig. 2b. For real application, it is not practical if oxygen has to be completely removed from a single drop of sample before the measurement. N2 bubbling was therefore omitted from these experiments. PEDOT is a good catalyst for oxygen reduction37 hence, without N2 bubbling, the oxygen reduction reaction takes place and can be seen from the waves in the negative potential region shiing down especially at high pH (pH 9.0). Nonetheless, a calibration curve with R2 0.998 was established from the difference in Ehalf of Fc and RFN. A drop of EMImSCN was tested and the pH (effective) was calculated from the DEhalf
Fig. 1 (a) SEM images of the PEDOT:RFN film (scale bar 100 mm), (b) Raman spectra of PEDOT and PEDOT:RFN-Fc films (red vertical line scale bar equals 2000 counts), and (c) FTIR spectra of RFN (dotted red trace), PEDOT (thin blue trace), Fc (dashed grey trace) and PEDOT:RFN-Fc (thick black trace) films.
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Fig. 2 CVs (cycle # 2) of PEDOT:RFN-Fc scanning at 10 mV s1 in various buffers (a) with N2 bubbling in a traditional 3 electrode setup (inset) where Ag/AgCl (3 M NaCl) and Pt were used as counter and reference electrodes respectively and (b) using the commercial printed microelectrodes (inset) without N2 bubbling. Scale bars for the peak currents (presented as red vertical lines below the CVs) are 20 mA.
between RFN and Fc and found to be 3.80 using this miniaturised 3 electrode setup. To simplify the system, the capacity of PEDOT-coated C-paper was studied to evaluate whether it can be used as the combined RE & CE. SEM and EDX were rst performed to observe the PEDOT layer on the C-paper and it was conrmed that PEDOT coating was not blocking the C-paper pores (Fig. S3†). The PEDOT coated C-paper electrode was then scanned in 0.1 M phosphate buffer (PB) pH 5 in comparison to the bare C-paper (Fig. S4†). The capacity of the PEDOT-coated C-paper electrode, calculated as DImax multiplied by the scan rate, was much higher than the C-paper alone and calculated to be approximately 10 mF cm2. In comparison, the capacity generally observed on the PEDOT:RFN-Fc electrode during the redox cycling is in the same range (11 mF cm2). By designing the setup with asymmetrical electrode areas, the capacity of the combined RE & CE is about 160 times that of the WE which is expected to be sufficient for the combined RE & CE electrode to prevent potential dri that could interfere with the measurement. The WE area was then miniaturised to be as small as 0.09 mm2 using the laser engraver technique as described in the Experimental section. The laminating sheet was laser-etched leaving windows in the size of 0.09 mm2 for the WE and 15 mm2 for the combined RE & CE (Fig. 3a). This allows the combined RE & CE electrode area to be signicantly higher (geometrically)
than the WE area for the consideration mentioned above. Various pH buffers were scanned using this microelectrode setup and the calibration curves were established (Fig. 3). The results from two different PEDOT:RFN-Fc lms gave two almost identical calibration curves (Fig. 3b) with very good linearity of 0.9995 and 0.9924 for pH 3–11. The obtained DEhalf from PB pH 5 using the 2 electrode setup in three separate experiments was 0.644, 0.639 and 0.647 V which has given the average of 0.643 0.004 V. This has proven the good reproducibility of the lm preparation and measurement methods. The slope of the calibration curve from this 2 electrode setup is higher than the theoretical 59 mV whereas the slopes measured with the 3 electrode setup are close to this theoretical value.24 This could probably be explained by the fact that the open circuit potential of PEDOT on the combined RE & CE systematically shied with pH in the range of interest (pH 3–11) as previously reported to be 30 mV per pH unit in this pH range.38 To investigate the ability of the sensor to detect the pH (effective) of ILs and non-aqueous media, two hydrophobic ILs (P1,3NTf2 and EMIm(CN)4B), a hydrophilic IL (EMImSCN), one protic IL (EAN) and a mixture of P1,4,4,4TOS in PC (67% v/v) were then scanned using only 1 drop of each and their CVs are shown in Fig. 4. The pH (effective) of P1,3NTf2, EMIm(CN)4B, EAN, EMImSCN and 67% P1,4,4,4TOS was then calculated using the respective calibration curve in Fig. 3b. The values of Eox, Ered of
Using the miniaturised 2 electrode setup (a) CVs (cycle # 2) of PEDOT:RFN-Fc scanning at 10 mV s1 in various buffers pH 3 to 11 and (b) calibration curves established from the CVs in (a). Fig. 3
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Fig. 4 CVs (cycle # 2) of PEDOT:RFN-Fc scanning at 10 mV s1 in P1,3NTf2, EMIm(CN)4B, EMImSCN, 67% P1,4,4,4TOS in PC, and EAN. Scale bars (presented as thick red vertical lines next to the graphs) are 5 mA for P1,3NTf2, EMImSCN, and 67% P1,4,4,4TOS in PC, and 1 mA for EMIm(CN)4B and EAN.
both RFN and Fc, and the DEhalf between the two redox couples are also tabulated (Table 1). It is worth noting that the pH (effective) of EMImSCN, scanned using this 2 electrode setup, was found to be 3.68 which is very close to the value obtained (3.80) when the commercial microelectrode (3 electrode setup) was used indicating the general viability of the concept. Multiple pH (effective) measurements were also performed for P1,3NTf2, using electrodes from different batches, and the reproducibility of the measurement is within 3% (pH 9.14 0.25). The stability of RFN and Fc peaks over the successive scans has been studied and the %RSD of peak current upon three repetitive CV scans in most ILs was in the range of 2 to 23% (Table S1†). The exception is for 67% P1,4,4,4TOS in PC where % RSD from Fc peaks was as high as 54%. This is due to the swelling properties of PEDOT in this particular IL as reported in ref. 39 causing the detachment of Fc during scanning. Nonetheless, the peaks at cycle 2 (Fig. 4) are still denitely distinguishable at this rate of decrease.
Table 1
CVs of EAN (Fig. 4) systematically shi toward more positive potentials over the cycles, which is probably due to the slow change in the oxidation state of PEDOT in this IL. However, in the adjacent cycle i.e. cycle # 8 and cycle # 9, it was obvious that both RFN and Fc peaks shied at the same distance hence the DEhalf was constant at 0.83 V for both cycles. The DEhalf between RFN and Fc increased over time e.g. from 0.69 V in cycle # 2 to 0.83 V in cycle # 8 causing the pH (effective) to increase from 6.08 to 7.71, respectively. This could be due to moisture absorption over time into the single droplet of the IL where the experiment was performed under room conditions. The effect of water content has been observed previously where the water content caused the difference in the dissociation of choline dihydrogenphosphate IL and consequently its pH (effective).24,40 Nonetheless, the pH between the two cycles, i.e. cycle # 8 and cycle # 9, was constant at 7.71 indicating that the method (scanning at 10 mV s1) is valid for determining the pH value of the IL despite the moisture absorption issue. P1,4,4,4TOS and EMImSCN are hydrophilic and can be dissolved in water. 67% of these ILs in water give a pH of 1.56 and 6.42, respectively, using a traditional glass pH electrode. The reading for P1,4,4,4TOS conrms that the IL is acidic both in water and in PC. The reading for EMImSCN was not stable and kept increasing, indicating that the AgCl junction was disturbed and conrming the problem in using this kind of traditional glass electrode to measure the pH of materials with high ionic strength. P1,3NTf2 and EMIm(CN)4B are hydrophobic and hence it was not possible to read the pH in aqueous solutions of these two ILs. Although the purpose of this work is not necessarily answering the economical question but rather nding the reliable system for use in non-aqueous samples and reducing the complexity of the operation, the material cost of the combined RE & CE and WE used in this work is calculated to be approximately $0.03 US per test – a price that may be worth considering especially in comparison to the hydrogen electrode system.
Conclusion Miniaturised all solid state pH (effective) sensors were successfully fabricated based on the PEDOT:RFN-Fc electrode. The working electrode area can be as small as 0.09 mm2 and only a single drop of the sample is needed. The electrode responds linearly with the pH change over the range of pH 3 to 11. The miniaturised and simplied 2 electrode setup has
Eox, Ered of RFN and Fc read from CVs in Fig. 4, DEhalf between the two redox couples and the calculated pH (effective) RFN (V)
Fc (V)
ILs (%H2O)
Eox
Ered
Eox
Ered
DEhalf (V)
pH (effective)
P1,3NTf2 (0.04) EMIm(CN)4B (0.02) EMImSCN (0.34) 67% P1,4,4,4TOS in PC (1.30) EAN (3.31)
0.69 0.43 0.12 0.44 0.47
0.84 0.53 0.02 0.51 0.57
0.30 0.47 0.69 0.22 0.21
0.16 0.40 0.46 0.16 0.14
0.99 0.91 0.52 0.51 0.69
8.96 8.06 3.68 3.81 6.08
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proven its ability to determine the pH (effective) of various ILs and non-aqueous media. Further testing of the solid state pH (effective) sensor in various non-aqueous solvents and in the vast amounts of different ILs will be an ongoing process. However, this report has potentially solved various problems currently existing regarding the measurement of PA in nonaqueous media. In addition, this simplied and miniaturised system is transportable which made it convenient for use in the eld.
Acknowledgements The authors thank Ciaran McDonnell-Worth and Kevin J Fraser for supplying a few drops of EAN and P1,3NTf2 respectively. The authors acknowledge the use of facilities under the Australian Research Council's Centres of Excellence funding scheme (COE for Design in Light Metals) within the Monash Centre for Electron Microscopy.
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