All-Diamond Microelectrodes as Solid State Probes for Localized

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All-diamond microelectrodes as solid state probes for localized electrochemical sensing Eduardo L. Silva, Cristol Paiva Gouvea, Marcela C Quevedo, Braulio S. Archanjo, António Jose S. Fernandes, Carlos Alberto Achete, Rui Ferreira e Silva, Mikhail Larionovich Zheludkevich, and Filipe José Oliveira Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b00756 • Publication Date (Web): 09 Jun 2015 Downloaded from http://pubs.acs.org on June 10, 2015

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Analytical Chemistry

All-diamond microelectrodes as solid state probes for localized electrochemical sensing Eduardo L. Silva†, Cristol P. Gouvêa§, Marcela C. Quevedo†, Braulio S. Archanjo§, António J. S. Fernandes‡, Carlos A. Achete§, Rui F. Silva†, Mikhail L. Zheludkevich†, ﬩, Filipe J. Oliveira† †

CICECO – Aveiro Institute of Materials, Dept. of Materials and Ceramic Engineering, University of Aveiro, 3810-193 Aveiro, Portugal § Materials Metrology Division, INMETRO, 25250-020, Duque de Caxias - RJ, Brazil ‡ I3N, Physics Department, University of Aveiro, 3810-193 Aveiro, Portugal ﬩ MagIC, Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck Str. 1, 21502 Geesthacht, Germany ABSTRACT: The fabrication of an all-diamond microprobes demonstrated for the first time. This ME assembly consists on an inner boron doped diamond (BDD) layer and an outer undoped diamond layer. Both layers were grown on a sharp tungsten tip by chemical vapour deposition (CVD) in a stepwise manner within a single deposition run. BDD is a material with proven potential as an electrochemical sensor. Undoped CVD diamond is an insulating material with superior chemical stability in comparison to conventional insulators. Focused ion beam (FIB) cutting of the apex of the ME was used to expose an electroactive BDD disk. By cyclic voltammetry, the redox reaction of ferrocenemethanol was shown to take place at the BDD microdisk surface. In order to ensure that the outer layer was non-electrically conductive, a diffusion barrier for boron atoms was established seeking the formation of boron-hydrogen complexes at the interface between the doped and the undoped diamond layers. The applicability of the microelectrodes in localized corrosion was demonstrated by scanning amperometric measurements of oxygen distribution above an Al-Cu-CFRP galvanic corrosion cell.

In recent years the use of boron doped diamond (BDD) as an electrochemically active and functional material was demonstrated in applications with high technological and socio-economic impact such as the monitoring of neurotransmitters or the identification of tumorous cells, in the microelectrode form 1,2. The interest of the scientific community for this carbon material comes from its unmatched combination of properties: extremely wide potential window, high resistance to fouling and corrosion, high signal-to-noise ratio, multiple functionalization possibilities, among others 3. The application of these small probes has been recently expanded towards the study of corrosion processes, mainly for pH and dissolved oxygen (DO) mapping by using model galvanic cells 4. The use of microelectrodes was pioneered by plant physiologists more than 60 years ago 5. Only later, in the 1980’s, they were adopted by electrochemists mostly by the hands of Wightman 6 and Fleischmann 7. Increased mass transport rates, reduced double layer capacitance, minimal ohmic losses and the possibility of measurements in high resistivity media are some of the reasons that motivated increasing numbers of fabrication techniques of both single microelectrodes and arrays. The construction of a microelectrode and the choice for its geometry will have to account with several aspects such as: the size tolerable for application; the compatibility of the insulating shroud with respect to the electrode material and the stability of the insulator in the electrolyte; the possibility of obtaining rigorous theoretical treatment of current potential curves for a given electrode geometry; and the need for a

technically simple method of construction, among others 8. The completion of this survey points out the limits of microelectrode fabrication for any given material/composite. Although CVD diamond coatings can range from a few nanometers to a few micrometers in thickness, the microelectrode size is ultimately defined by the size of the insulating shroud. Additionally the behavior of the microelectrode might deviate from theory due to the existence of a defective insulation 9. A well-defined geometry is always required if an adequate theoretical treatment is to be made. Therefore, the chemical stability of the insulator is of utmost importance, as it may compromise the response of the electroactive BDD surface if it decomposes. Similarly, the technical difficulties associated to the application of the insulating layer are a frequent source of electrochemical data misinterpretation. In a recent work 10 from Joseph et al. a solution for these issues is presented, by reporting a laser micromachining based route for the fabrication of co-planar all-diamond microelectrodes with a critical dimension of 50 µm (imposed by technical limitations). In what concerns needle-like microelectrodes, or microprobes, used in localized scanning electrochemical measurements, a significant fabrication improvement has been demonstrated by Hu et al 11. In their work, electrophoretic paint was used as insulation sheath, with complete coverage of the diamond microprobe. Afterwards, FIB (Focused Ion Beam) was used for cutting the rear end of this assembly, yielding a well-defined diamond microdisk, which they used for SECM measurements.

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In this work we present a simple route for fabricating microprobes entirely made of CVD diamond, by synthesizing the whole microelectrode body in a single deposition run. The final result is an all-diamond microelectrode, shaped by FIB (Focused Ion Beam) cutting in order to meet the requirement of a well-defined characteristic dimension, a microdisk in this case. Such a fabrication approach might contribute for improving the performance and applicability of BDD microprobes in the sense that these will no longer be stranded by the inferior chemical resistance of the insulation material. The applicability potential of the developed MEs is demonstrated in the scope of local oxygen measurements on the Al-Cu-CFRP system, which is of high relevance for the development of novel composite materials for the aeronautics industry. EXPERIMENTAL SECTION Reagents and Solutions All solutions were prepared with deionized water (18.3 Ω, Barnstead EASYpure RF). A 50 mM NaCl (p.a., SigmaAldrich) solution was used for determination of the water stability window. performed in an electrolyte solution of 50 mM NaCl and with a 1 mM ferrocenemethanol in 50 mM NaCl solution. Materials Bi-layer polycrystalline diamond films were grown by HFCVD (Hot Filament Chemical Vapor Deposition), using sharp tungsten wires (99.95%, Goodfellow) as substrates, prepared by electrochemical etching according to a procedure reported elsewhere 12. The sharp wires where then “seeded” in a nanodiamond powder (98+ %, ABCR) suspension by dipcoating, at a rate of 1mm/s. After positioning the seeded wires horizontally on the substrate holder, the inner BDD conductive layer was grown during 30 minutes in a thermally activated methane–hydrogen atmosphere (CH4 99.9995%, H2 99.9998%, Airliquide) with a CH4/H2 ratio of 0.05, a pressure of 50 mbar and a substrate temperature of 800ºC. A mixture of boron oxide+ethanol (B2O3 99.98%, Alpha-Aesar, ethanol p.a., Carlo Erba) with a B/C ratio of 14000 ppm was used for doping the films by being dragged by argon gas (99.9995%, Airliquide) through a gas-washing bottle into the reaction chamber. The outer diamond layer was grown with a CH4/H2 ratio of 0.07 and the system pressure was raised to 150 mbar in order to grow an electrically insulating diamond layer during 45 minutes. The system was then cooled down at an average rate of 16ºC/min in a hydrogen-rich environment in order to remove adventitious sp2 carbon from the surface. A focused ion beam system (FEI SEM/FIB Nova NanoLab 600), operating at 30 keV, was used to cut the extremity of the coated wires, exposing a conductive BDD disk surrounded by the insulating diamond layer. The crystalline quality of both diamond layers was confirmed by µ-Raman spectroscopy (HORIBA JOBIN YVON HR800UV) using a laser 532 nm wavelength. In order to perform the scanning amperometric measurements of oxygen concentration, the microelectrodes were submitted to surface modification by CF4 RF-plasma treatment during 5 minutes (EMITECH K1050X, Quorum Tech., UK), according to our previous work13. Electrochemical Measurements The electrochemical characterization of the microelectrodes was evaluated in terms of solvent window and redox response

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by using a potentiostat (CompactStat potentiostat/galvanostat, Ivium, The Netherlands). A three electrode configuration was used, with a Pt counter-electrode and an Ag|AgCl reference electrode.The electrical connection of the diamond microelectrodes was made by attachment to a metal pin with silver conductive paint (RS). A model system consisting of an Al-Cu-CFRP three wireelectrode cell was used to evaluate the applicability of the microelectrodes for localized mapping of the O2 concentration near active corroding metallic surfaces. This cell was probed in the coupled and uncoupled condition when immersed in solutions of 50 mM NaCl and 50 mM NaCl + 2.5mM BTA + 2.5mM Ce(NO3)3. Microamperometry measurements were performed with an IPA2 amplifier (Applicable Electronics Inc., USA) in the voltammetric/amperometric mode, using a 2 electrode arrangement, with a Ag/AgCl electrode as counter and reference electrode. 2D X-Y scans were carried out using a microstepping motor driver (USDIGITAL, USA) at a distance of 100 µm between the diamond ME and the surface. XZ scans permitted obtaining the normal distribution of species above the electrodes. Maps were recorded at -1 V (vs. Ag/AgCl) for simultaneous detection of dissolved oxygen and Al3+. pH measurements were also performed and are presented in Section II of the supporting information. RESULTS AND DISCUSSION First probe design Fig. 1 provides a brief description of bi-layered CVD growth of diamond MEs. By introducing boron in the usual methane-hydrogen gas chemistry of CVD diamond, a first layer of this material is grown on top of a tungsten sharp wire. The choice for tungsten relies in its ability to form an electrically conductive carbide interlayer that precedes the nucleation of diamond crystals 14. Moreover, being the metal with highest melting point it is perfectly suitable for the relatively high temperatures used for the CVD technique, usually in the range of 500-1000 ºC. A second layer of undoped diamond is then required to be grown on top of the first one with the purpose of being electrically insulating. Consequently, after this step, the entire assembly is electrochemically unresponsive, which leads us to the need of exposing an electroactive area.

Figure 1. Schematic representation of the CVD deposition process for obtaining all-diamond microelectrodes.

This is done by controllably cutting the apex of the probe by FIB (Fig. 2a). A clear view of the outcome of this process with


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clear distinction of each layer is depicted in Fig. 2b. In this case a thin tungsten oxide layer was exceptionally evaporated in-between both diamond layers only to provide a clear view of the interfacial contour (later converting to WC). A description of this procedure can be found in reference 15. After cutting a few slices of the microprobe the tungsten substrate becomes exposed (Figs. 2c and d). The interest of doing so is merely for demonstration of the multi-layered structure in this case. In order to obtain a typical BDD electrochemical behaviour, it is required that no tungsten is in contact with the media. Since both diamond layers can be hard to distinguish, the length of the FIB cut can be either too short, not reaching the BDD layer, or too long reaching the tungsten substrate. Therefore cutting in the right length (Figs. 3a and 3b) will depend on knowing the thickness of each layer. This can be ensured by rigorous control of the CVD parameters and constancy of the growth and cooling periods.

Figure 2. Focused Ion Beam cutting sequence of an all-diamond microprobe progressively showing the distinction between doped and undoped diamond layers and the tungsten substrate for deeper cuts.

Figure 3. Fabrication of bi-layer diamond microelectrodes showing: a) CVD diamond layers on the tungsten sharp substrate; b)

SEM micrograph of the diamond microdisk after FIB cutting; c) the voltammetric response in 1 mM ferrocenemethanol + 50mM NaCl.

Provided that the inner BDD layer is sensitive to the target redox couple, the outer layer must be highly resistive, in order to avoid the electrochemical reaction to proceed on its surface. For this purpose, the growth conditions were adjusted to grow microcrystalline boron doped diamond (B-MCD) on the inner layer and nanocrystalline diamond (NCD) on the external layer. Although B-MCD exhibits higher grain size, it can easily be made conductive by substitutional boron doping; as for NCD, it can be grown in much thinner between a few tenths to a few hundred nm. Additional figures can be found on the supporting information, showing the interface between layers and the NCD morphology of the external layer. Nevertheless, with this first probe design the entire assembly became electrically conductive. This could be observed in Fig. 3c where the voltammetric response of such all-diamond microelectrode in a solution of 1 mM FcOH0/+ in 50 mM NaCl is shown. It can be observed that the magnitude of the current signal varies according to the immersion depth, with a typical macroelectrode response, i.e., although the outer layer was grown without added boron, it is electrically conductive allowing the redox reaction to proceed. Therefore, a modification on the probe fabrication was acomplished as further described. Final design and electrochemical behavior of the alldiamond microelectrode The role of boron impurities in CVD diamond is known to be dependent on microstructural effects and crystallographic orientation 16. In the case of polycrystalline diamond with small grain size, a higher abundance of boron impurities occurs at grain boundaries and grain edge regions 16,17. The diffusion of hydrogen in diamond adds up to define the electronic properties of this material. In p-type diamond, H+ is the dominant hydrogen diffusing species 18. It has been shown that the interaction between boron and hydrogen in diamond gives origin to (B, H) pairs 17–19. The most relevant consequence deriving from the formation of these complexes is the passivation of the boron impurities, i.e., the reduction of hole concentration. In a study by Uzan-Saguy et al. 20 the authors provide a clear demonstration of hydrogen-boron interactions in diamond by comparing samples before and after deuteration treatments. The increase of the hole concentration activation energy from 0.27 eV before deuteration to 0.32 eV after deuteration verifies the phenomenon. Moreover these observations were accompanied with an increase in hole mobility, which is an indication that charge carrier migration is less affected by ionized impurity scattering. This is an experimental confirmation that rather than compensate boron acceptors, these are indeed passivated in the form of (B, H) pairs. According to these observations, hydrogen was used to hinder boron diffusion in an alternative fabrication route for the microprobes. Considering the growth scheme depicted in Fig. 4a, the creation of a barrier between the doped and the undoped layer was used in order to “lock” boron impurities in the former. Hence, after the growth of the BDD layer the CVD chamber was evacuated and refilled afterwards with a 100% hydrogen atmosphere. The coated wires were exposed to this atmosphere during 1 hour. The purpose of this step was the creation of a hydrogen rich region at the surface and near-



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surface of BDD. The proposed effect of this region is to passivate boron atoms, as they diffuse outwards, impeding them of reaching the insulating layer. After the formation of this diffusion barrier, the outer non-doped diamond layer was grown, within the same deposition, Fig. 4b. Fig. 4c shows the Raman spectra for both layers, where the diamond signature could be calculated at 1338 cm-1 for the inner layer and at 1332 cm-1 for the outter one. The latter coincides with the normal position of the diamond peak, while the value of 1338 cm-1 measured at the inner diamond disk is indicative of a compressive stress state. This can be explained by the nature of the measured areas: the spectrum for the inner layer was recorded from the cut area, while the spectrum of the outer layer was measured on the lateral surface near the apex. The cut area corresponds to the true diamond bulk, where the atomic bonding is less relaxed comparatively to the surface layers and it is also closer to the tungsten substrate, which presents different thermal expansion. For the inner layer it can also be observed that the intensity of the TPA (traspolyacetylene), and the graphite D and G bands is slightly higher. These carbonic phases are sp2 bonded and are generally removed from the surface of diamond films by activated hydrogen. Their electrochemical contribution can be observed by redox peaks usually around 300 mV, as observed for sp2 carbon electrodes with considerable fraction of exposed edge plane where carbon-oxygen groups can form 21.

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the MEs were again cycled in the electrolyte solution alone. Anodic and cathodic current increase is observed due to water electrolysis with a ~3V gap in-between from approximately 1.5 V to +1.5 V, which is typical for BDD electrodes (Fig. 5b). Furthermore, in the inset of this curve, no Faradaic response is detectable from -1 V to +2 V indicating the absence of any significant electrochemical contribution from sp2 carbon. When the redox couple is added in solution, a typical microelectrode sigmoidal response corresponding to the redox reaction of FcOH0/+ with the slope indicating relatively facile electron transfer kinetics, Fig. 5c. Additionally the current signal is independent from the immersion depth. In opposition to the first probe design, these results put in evidence that the current response is solely due to the BDD microdisk, without contribution from the insulating diamond layer, confirming the effectiveness of the hydrogen rich barrier in this fabrication process. A demonstration of the applicability of the microprobes is shown in Fig. 6. A model galvanic cell consisting of Al-CuCFRP coupled wires in 50 mM NaCl (Fig. 6a) was probed in the near surface of the electrodes, 100µm above.

Figure 5. Voltammetric response of the ME: a) before FIB cutting in 50 mM NaCl (black line: diamond ME; red line: glass, for comparison); b) after FIB cutting in 50 mM NaCl; c) after FIB cutting in 1 mM ferrocenemethanol + 50mM NaCl (black line: 2 mm immersion; red line: 5 mm immersion). Inset in b): absence of sp2 carbon response.

Figure 4. Alternative fabrication route of bi-layer diamond microelectrodes showing: a) the CVD growth sequence including the hydrogen rich interface and b) SEM micrograph of the diamond microdisk after FIB cutting. c) Raman spectra of the BDD (black line) and undoped diamond (upper red line) layers showing the respective diamond peak and the TPA, D and G bands.

Figs. 5a and 5b show the voltammetric response of the ME in Fig. 4b, fabricated by the above described procedure. The electroactive BDD microdisk is ~1.5 µm in diameter, with a nanocrystalline diamond insulation of 800 nm in thickness. Voltammetry was performed before and after FIB cutting, in solutions of 50 mM NaCl and 1 mM FcOH0/+ in 50 mM NaCl, respectively. Before the cut, the probe exhibits total absence of electrochemical response from -2 V to +2V in NaCl, which is a first indicator of high electrical resistivity and imperviousness of the outer diamond layer (Fig. 5a). After the FIB cut,

For this purpose the microelectrodes undergone surface modification in CF4 plasma, in order to become sensitive to dissolved oxygen (DO)13. The MEs were able to monitor the progress of corrosion in a Al-Cu-CFRP galvanic cell by following the variation of local DO concentration (Fig. 6a). The measurements were performed using a SPET (Scanning Polarographic Electrode Technique) system in the amperometric mode, with a polarization of -1 V vs Ag|AgCl. Each amperometric map was recorded in ~30 minutes, covering an area of 36 mm2, with 100 µm between measurements. Because the standard electrode potential of aluminum is much lower than that of copper and the carbon fibers in CFRP, aluminum will act as the anode when coupled to Cu and CFRP, while DO is expected to be reduced at the surface of both copper and CFRP, according to equation 1. O2 (g) + 2H2O (l) + 4e−→ 4OH− (aq)


(1)

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As a result of this depletion, lower current are expected to be measured by the diamond microelectrode when scanning above the Cu and CFRP cathodes. Accordingly, Fig. 6b shows the amperometric mapping after 1 hour immersion time, where oxygen depletion above the cathodic areas could be observed. The current increase above the anodic area is most likely due to hydrogen reduction, as shown by the pH profile in Section 2 of the supporting information. Fig. 6c shows the activity of the coupled electrodes when a corrosion inhibitor is added (50 mM NaCl + 2.5mM BTA + 2.5mM Ce(NO3)3). In this case, both the cathodic and anodic activities are lower due to the synergistic effect of both inhibitors, with Ce3+ precipitation as an insoluble hydroxide (2) preferentially at cathodic sites, and BTA blocking the anodic sites by an adsorption mechanism22. Accordingly the current magnitude registered by the ME is higher above the entire measured area, especially above CFRP, indicating higher DO concentration. Ce3+(aq) + 3OH-(aq) → Ce(OH)3 (s)

(2)

2.5mM BTA + 2.5mM Ce(NO3)3 after 1 hour c) electrodes coupled; d) electrodes uncoupled. Afterwards the electrodes were uncoupled. Fig. 6d shows the resulting oxygen distribution, mostly even across the entire measured area with still a slight DO decrease above CFRP. Furthermore, when comparing this ME fabrication method with our previous work on BDD microelectrodes for DO detection13, the SEM imagery shows a very significant dimensional improvement. Although the use of varnish is a faster way of shrouding a microelectrode, the exposed length of the ME ranged from 10-30 µm, which represents a significant limitation in terms of resolution. The present report shows that microdisks with radius in the range of 1-2 µm can be fabricated with an RG (ratio between ME+insulation diameter and ME diameter) of approximately 2, which points out the capability for high resolution measurements and low damage to the surrounding environment. However, with such a small RG establishing a current-concentration relation is not straightforward since the BDD microdisk is insulated by a finite layer with similar dimension. Therefore the premise of a microdisk insulated by an infinite plane does not apply and diffusion from the back side of the ME needs to be considered 23. Additionally achieving a perfect microdisk shape is not always possible due to substrate related imperfections. As an alternative to mathematical formulation, determining a calibration curve can be a more practical procedure with the advantage of better representing the real experimental conditions, in cases where analyte quantification is pretended (see Section 3 in supporting information). These results are an indication of the feasibility of this novel microelectrode fabrication method, which consists on using the superior chemical inertness of diamond to shroud BDD, instead of using other substances with weaker stability. This improvement in insulation quality and thickness is a promising contribution for improving the response time and miniaturization of diamond microprobes, thereby enabling measurements with higher resolution. CONCLUSIONS For the first time the fabrication of a needle-like microelectrode based on an all-diamond body is reported. The twolayered structure consists of an inner, electroactive, boron doped diamond layer and an outer undoped diamond layer. In order to impede the migration of boron from the inner to the outer layer, a hydrogen enrichment of the interface was performed in order to trap and passivate these doping impurities. Cyclic voltammetry in 50 mM NaCl confirmed the insulating character of the undoped diamond layer. After exposing an electroactive BDD disk by FIB cutting, voltammetry in 1 mM FcOH0/+ + 50mM NaCl, showed the independence between the redox reaction current signal and microprobe immersion depth, proving the feasibility of the fabrication method. Furthermore the applicability of the all-diamond microprobes for localized corrosion was demonstrated by amperometric oxygen concentration measurements, following the progress of corrosion in Al-Cu-CFRP galvanic cell.

Figure 6. Scanning amperometric DO concentration measurements: a) Al-Cu-CFRP triplet immersed in 50 mM NaCl and b) DO distribution after 1 hour, coupled. DO distribution above Al-Cu-CFRP triplet immersed in 50 mM NaCl +

ASSOCIATED CONTENT



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Supporting Information Available (1) SEM figures of all-diamond microelectrodes; (2) pH measurements on the Al-Cu-CFRP cell; (3) dissolved oxygen calibration curve. This information is available free of charge via the Internet at http://pubs.acs.org/.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]

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(19) Goss, J.; Briddon, P.; Jones, R.; Teukam, Z.; Ballutaud, D.; Jomard, F.; Chevallier, J.; Bernard, M.; Deneuville, a. Phys. Rev. B 2003, 68, 235209. (20) Uzan-Saguy, C.; Reznik, A.; Cytermann, C.; Brener, R.; Kalish, R.; Bustarret, E.; Bernard, M.; Deneuville, a.; Gheeraert, E.; Chevallier, J. Diam. Relat. Mater. 2001, 10, 453–458. (21) Fischer, A. E.; Swain, G. M. J. Electrochem. Soc. 2005, 152, B369. (22) Serdechnova, M.; Kallip, S.; Ferreira, M. G. S.; Zheludkevich, M. L. Electrochem. commun. 2014, 41, 51–54. (23) Mirkin, M. V.; Bard, A. J. J. Electroanal. Chem. 1992, 323, 29–51.

ACKNOWLEDGMENT E.L. Silva would like to acknowledge FCT (Fundação para a Ciencia e a Tecnologia) for the grant SFRH/BD/61675/2009. R.F. Silva acknowledges the research grant from CNPq “Ciência sem Fronteiras” project. 402251/2012-1 “Nanostructured carbon allotropes”. This work was supported by projects PTDC/CTM/108446/2008, PTDC/CTM-MET/113645/2009, funded by FEDER through COMPETE programmme-Operacional Factors for Competitivity and by national funds through FCT – Portuguese Science and Technology Foundation. The FP7 MarieCurie Programe is also gratefully acknowledged for the support provided in frame of SISET (FP7-PEOPLE-2010-IRSES Reference 269282) and PROAIR (FP7-PEOPLE-2013-IAPP Reference 612415) projects. This work was developed in the scope of the project CICECO-Aveiro Institute of Materials (Ref. FCT UID /CTM /50011/2013), financed by national funds through the FCT/MEC and when applicable co-financed by FEDER under the PT2020 Partnership Agreement.

REFERENCES (1) Fierro, S.; Yoshikawa, M.; Nagano, O.; Yoshimi, K.; Saya, H.; Einaga, Y. Sci. Rep. 2012, 2. (2) Suzuki, A.; Ivandini, T. A; Yoshimi, K.; Fujishima, A.; Oyama, G.; Nakazato, T.; Hattori, N.; Kitazawa, S.; Einaga, Y. Anal. Chem. 2007, 79, 8608–8615. (3) Kraft, A. Int. J. Electrochem. Sci 2007, 2, 355–385. (4) Silva, E. L.; Bastos, A. C.; Neto, M. A.; Silva, R. F.; Ferreira, M. G. S.; Zheludkevich, M. L.; Oliveira, F. J. Electrochem. commun. 2014, 40, 31–34. (5) Bond, A. M. Analyst 1994, 119, 1–21. (6) Wightman, R. M. Anal. Chem. 1981, 53, 1125–1134. (7) Fleischmann, M. Anal. Chem. 1987, 59, 1391–1399. (8) Fleischmann, M.; Pons, S.; Rolison, D. R.; Schmidt, P. Ultramicroelectrodes; Datatech Systems, 1987. (9) Zoski, C. G. Handbook of Electrochemistry; Elsevier B.V.: Amsterdam, 2007. (10) Joseph, M. B.; Bitziou, E.; Read, T. L.; Meng, L.; Palmer, N. L.; Mollart, T. P.; Newton, M. E.; Macpherson, J. V. Anal. Chem. 2014, 86, 5238–5244. (11) Hu, J.; Holt, K. B.; Foord, J. S. Anal. Chem. 2009, 81, 5663–5670. (12) Silva, E. L.; Silva, R. F.; Zheludkevich, M.; Oliveira, F. J. Rev. Sci. Instrum. 2014, 85, 095109. (13) Silva, E.; Bastos, A. C.; Neto, M.; Fernandes, A. J.; Silva, R.; Ferreira, M. G. S.; Zheludkevich, M.; Oliveira, F. Sensors Actuators B Chem. 2014, 204, 544–551. (14) Hu, J.; Foord, J. S.; Holt, K. B. Phys. Chem. Chem. Phys. 2007, 9, 5469–5475. (15) Neto, M. A.; Silva, E. L.; Fernandes, A. J. S.; Oliveira, F. J.; Silva, R. F. Surf. Coatings Technol. 2011, 206, 103–106. (16) Pleskov, Y. V. Prot. Met. 2006, 42, 103–118. (17) Goss, J. P.; Eyre, R. J.; Briddon, P. R. Phys. Status Solidi 2008, 245, 1679–1700. (18) Nebel, C. E.; Ristein, J. Thin-Film Diamond II; Elsevier B.V.: Amsterdam, 2004.


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All-Diamond Microelectrodes as Solid State Probes for Localized

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