Article pubs.acs.org/ac
Controlled sp2 Functionalization of Boron Doped Diamond as a Route for the Fabrication of Robust and Nernstian pH Electrodes Zoë J. Ayres,† Alexandra J. Borrill,† Jonathan C. Newland,‡ Mark E. Newton,‡ and Julie. V. Macpherson*,† †
Departments of Chemistry and ‡Physics, University of Warwick, Coventry CV4 7AL, United Kingdom S Supporting Information *
ABSTRACT: The development of a voltammetric boron doped diamond (BDD) pH sensor is described. To obtain pH sensitivity, laser micromachining (ablation) is utilized to introduce controlled regions of sp2 carbon into a high quality polycrystalline BDD electrode. The resulting sp2 carbon is activated to produce electrochemically reducible quinone groups using a high temperature acid treatment, followed by anodic polarization. Once activated, no further treatment is required. The quinone groups show a linear (R2 = 0.999) and Nernstian (59 mV/(pH unit)) pH-dependent reductive current−voltage response over a large analyzable pH range, from pH 2 to pH 12. Using the laser approach, it is possible to optimize sp2 coverage on the BDD surface, such that a measurable pH response is recorded, while minimizing background currents arising from oxygen reduction reactions on sp2 carbon in the potential region of interest. This enables the sensor to be used in aerated solutions, boding well for in situ analysis. The voltammetric response of the electrode is not compromised by the presence of excess metal ions such as Pb2+, Cd2+, Cu2+, and Zn2+. Furthermore, the pH sensor is stable over a 3 month period (the current time period of testing), can be stored in air between measurements, requires no reactivation of the surface between measurements, and can be reproducibly fabricated using the proposed approach. The efficacy of this pH sensor in a real-world sample is demonstrated with pH measurements in U.K. seawater.
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out using voltammetry, where the surface is chemically functionalized with pH sensitive molecules, which undergo proton assisted electron transfer.17,18 These tend to be quinone in nature, where the peak current position for electrolysis of the surface bound quinone groups shows a Nernstian dependence on proton concentration. While this has resulted in the production of robust pH sensors, the derivatization procedures can be time-consuming, complex, and costly due to the number of reagents required.17 To reduce preparation times, it has also been shown that the electroreduction of naturally present quinone groups on the surface of sp2 containing carbon electrodes such as GC, EPPG, and SPE also show a Nernstian pH-dependent current−voltage response.13,15,16 The electrodes all required some form of activation prior to use. For GC13 and EPPG,15 surface mechanical polishing was found to be important, while, for SPE, the surface was chemically oxidized. For all electrodes solution degassing was required,13,15,16 to avoid oxygen reduction reaction (ORR) interferences. This makes measurements at the source, when oxygen is naturally present, challenging.
he ability to sense pH in aqueous solutions is fundamental to the study of many different chemical environments and is therefore prevalent in many industries including healthcare,1 waste management,2 water and environmental monitoring.3,4 The most common pH sensor to date is the potentiometric glass pH electrode, due to its high sensitivity to protons, large analyzable pH range from pH 0 to pH 12, and fairly rapid response time of 0.1 mM). Oxygen, however, is present at a concentration of ∼0.25 mM,44 and while the ORR signal is pushed out on lasered/activated BDD, compared to, e.g., GC, it could be made less negative due to the presence of interference metals electroplated on the surface. Moreover, the electrochemical signatures due to metal reduction may themselves mask the quinone peak. In this study we investigated the pH response in acidic, neutral, and alkaline aerated solutions (pH 2.6, 6.3, and 9.4, respectively) in the presence of Cu2+, Cd2+, and Zn2+, as shown in SI section S6 and Pb2+ (Figure 6). Also shown is the response with a GC electrode, for comparison. The metal ions were present at much higher concentrations (1 mM) than realistic in real-world environments, to challenge the pH sensor. For all metal ions, over the potential range −0.6 V to +1.0 V, the pH-dependent quinone response is still clearly distinguishable on lasered/activated BDD and the peak potentials show a
4). However, for the heavily BDD employed here, although the Raman signal originates from close to the electrode surface, it is not truly surface sensitive, as with XPS (and electrochemical analysis).41 The data therefore suggests that the lasering procedure has not impacted significantly on the material quality, subsurface. pH Detection. To unequivocally confirm the presence of carbonyl containing quinone groups on the surface of the lasered/activated BDD electrode, SWV was employed to investigate the electrochemical response over the potential range +1 to −0.6 V vs SCE. Quinone reduction has been shown to fall within this range on other carbon electrodes, when they are naturally present.15,13 After lasering and activation of the surface, a clear peak is now observed at +0.41 V in a pH = 2 solution, when scanning from positive to negative potentials (Figure 5a). This contrasts with the response on the bare BDD
Figure 5. (a) Quinone reduction peaks using SWV (frequency = 150 Hz; step potential = 2 mV; amplitude = 200 mV) across the range of pH 2−12 for a lasered/activated BDD electrode. (b) Calibration graphs for three independently fabricated BDD pH sensors all exhibiting an R2 value of 0.999 and Nernstian behavior.
electrode, with the electrode subjected to the same activation procedures as those for the lasered BDD electrode, where a peak is only just discernible at this potential, as shown in SI section 5. To quantify the quinone surface coverage, cyclic voltammograms were recorded in the pH 2 solution on the lasered/activated BDD electrode at a scan rate of 0.1 V s−1 (as shown in SI section 6). The quinone surface coverage of this electrode was calculated to be 1.1 × 10−11 mol cm−2, which is unsurprisingly lower than that calculated for an sp2 carbon electrode, EPPG (5.9 × 10−11 mol cm−2),15 activated by mechanical polishing. The pH dependence of the electrochemical response of surface bound quinones on the lasered/activated BDD electrode was investigated using SWV across a wide pH range (2−12), Figure 5a. As shown, as the pH is increased, the reduction peak position shifts toward more negative potentials. This corresponds well with previous studies with sp2 activated GC and EPPG electrode materials.13,15 Note, for pH 2, at −0.4 V, the current begins to increase again, most likely due to proton reduction. As this current response does not fall within E
DOI: 10.1021/acs.analchem.5b03732 Anal. Chem. XXXX, XXX, XXX−XXX
Article
Analytical Chemistry
− 0.059pH (R2 = 0.999). Using the measured peak potentials, this corresponded to a measured pH of 8.40 ± 0.02 pH units. The seawater pH was independently measured using a commercially available glass pH electrode and found to be 8.39 ± 0.02 pH units, comparing well with that determined using the lasered/activated BDD electrode.
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CONCLUSION A voltammetric pH sensor has been fabricated via the controlled incorporation of sp2 carbon into a BDD electrode using laser micromachining (ablation). Activation of the sp2 carbon to produce pH-dependent, redox active, stable quinone groups on the electrode surface was carried out using a high temperature treatment (>200 °C) in boiling, concentrated acid, followed by anodic oxidation. Laser machining enabled the amount of sp2 carbon added to the surface to be controlled and optimized such that a measurable quinone response could be obtained while maintaining low background currents and minimizing background currents arising from oxygen reduction reactions on sp2 carbon in the potential region of interest. We found that an array of lasered pits of ca. 50 μm diameter, spaced 100 μm apart (center to center) on the BDD surface produced a pH sensor which exhibited excellent Nernstian linearity (R2 = 0.999) over the pH range 2−12, as well as good sensitivity (59 ± 1 mV pH−1). The fabrication process was found to be highly reproducible; three electrodes were produced using this procedure with all returning very similar pH responses. Degassing of the solution to remove oxygen was not necessary with these electrodes, unlike high content sp2 electrodes such as SPE, GC or EPPG, as the ORR signal was shifted to more negative potentials and outside the quinone peak reduction range. This indicates the BDD pH sensor is viable for in situ applications. Furthermore, the sensor could be stored dry and once activated was found to not require further activation between measurements. The pH response was also found to be unaffected by the presence of many different metal ions, deliberately added at high concentration to challenge the electrochemical response. Current electrodes have been used over a time period of 3 months (and are still functioning), producing consistent and Nernstian calibration lines. The BDD electrode was also found to replicate the pH measurement made in seawater using a conventional glass electrode. Finally, future work is directed at investigating BDD pH sensors for in situ applications. In particular, we envisage the sensor to be suitable for scenarios which exploit the material properties of diamond such as high temperature, high pressure, or corrosive environments. The fact that the BDD electrode is exposed to a high temperature acid treatment (>200 °C) as part of the surface activation treatment bodes well for the sensor surviving in high temperature applications. Integration of this pH electrode into multifunctional all-diamond electrochemical devices45 is also possible.
Figure 6. SWV of quinone reduction peaks in three different pH solutions (2.6, 6.3, and 9.4) in the presence of 1 mM Pb2+ for (a) the lasered/activated BDD electrode and (b) a mechanically polished GC electrode.
linear pH−voltage gradient over the three pH solutions investigated. However, as shown in Figure 6 for Pb2+ and in SI Figure S6 for Cd2+, with GC, the ORR background currents begin to mask the quinone reduction response especially as the pH is made more alkaline and the peak shifts in the negative potential direction. Extraction of a peak potential at pH 9.4 is not possible with GC. For Cu2+ which is the most easily reduced of all the metal ions investigated, on both electrodes in acid solutions, a second peak which could be due to Cu2+ reduction, or electrocatalyzed ORR on electrodeposited Cu, is observed at −0.122 V on BDD and −0.078 V on GC. However, for both electrodes the peaks were sufficiently separated from the quinone response to enable quantification of the pH response. Real-World Analysis. The pH of seawater collected from Poldhu Beach, Mullion, Cornwall, U.K. was analyzed (at T = 298 K) in order to test the capabilities of the BDD pH sensor in a complex sample matrix. The SWV response of the lasered/ activated BDD electrode in seawater (aerated and unfiltered, and with no additional salts added) is shown in Figure 7. The seawater had a measured solution conductivity of 54.3 mS cm−1. The electrode recorded three consecutive SWVs, returning peak potentials of 0.023, 0.019, and 0.019 V, respectively. The electrode was previously calibrated, as shown in Figure 5b, returning a calibration line of y = 0.521
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b03732. (1) Electrochemical and interferometric analysis of the lasered/activated BDD electrode, (2) oxygen reduction reaction, (3) XPS survey spectra, (4) angle-resolved XPS, (5) background quinone response on a bare BDD
Figure 7. Replicate SWVs corresponding to the first (black), second (blue), and third (red) repeat scans recorded in seawater using a lasered/activated BDD electrode. F
DOI: 10.1021/acs.analchem.5b03732 Anal. Chem. XXXX, XXX, XXX−XXX
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electrode, (6) quinone surface coverage determination, (6) stability testing of the electrochemical response of the BDD pH electrode, and (7) investigation of metal contaminant interferences on pH detection (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS J.V.M. thanks the Royal Society for an Industry Fellowship. Z.J.A. acknowledges the EPSRC and Element Six (E6) Ltd. for funding and we all thank E6 for providing the polished freestanding polycrystalline boron doped diamond material used herein (Dr. Tim Mollart, Dr. Richard Bodkin, and Nicola Palmer). We thank Lingcong Meng for taking the image in Figure 1d, Dr. Marc Walker and Dr. Claire Hurley for XPS advice, the Warwick Photoemission Facility for access to XPS instrumentation, Mr. Roy Meyler (Warwick Chemistry) for useful discussions, and Dr. Max Joseph for help with FEM simulations (Supporting Information section S1).
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