Hg microhemispherical electrodes

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REFERENCES 1. (a) C. Demaille, M. Brust, M. Tsionsky, A. J. Bard, Anal. Chem. 69, 2323 (1997); (b) F.-R. F. Fan, C. Demaille, in Scanning Electrochemical Microscopy, A. J. Bard, M. V. Mirkin, Eds., Marcel Dekker: New York, 2001, pp. 81–91; (c) Ibid, pp. 94–99. 2. (a) M. Brust, D. Bethell, D. J. Schiffrin, C. J. Kiely, Adv. Mater. 7, 795 (1995); (b) D. Bethell, M. Brust, D. J. Schiffrin, C. J. Kiely, J. Electroanal. Chem. 409, 137 (1996). 3. (a) Y.-T. Kim, D. M. Scarnulis, A. G. Ewing, Anal. Chem. 58, 1782 (1986); (b) D. K. Y. Wong, L. Y. F. Xu, Anal. Chem. 67, 4086 (1995). 4. D. A. J. Rand, R. Woods, J. Electroanal. Chem. 31, 29 (1971). 5. (a) M. V. Mirkin, A. J. Bard, Anal. Chem. 64, 2293 (1992); (b) C. Amatore, in Physical Electrochemistry. Principles, Methods and Applications, I. Rubinstein, Ed., Marcel Dekker: New York, 1995, pp. 131–208. 6. A. J. Bard, M. Brust, C. Demaille, M. Tsionsky, Unpublished results (1995). 7. J. Abbou, C. Demaille, M. Druet, J. Moiroux, Anal. Chem. 74, 6355 (2002). 8. J. Abbou, C. Demaille, Unpublished results (2004). 9. J. Abbou, C. Demaille, A. Anne, J. Am. Chem. Soc. 126, 10095 (2004). 10. (a) J. V. Macpherson, P. R. Unwin, Anal. Chem. 72, 276 (2000); (b) J. V. Macpherson, M. A. Webb, P. R. Unwin, Anal. Chem. 73, 550 (2001).

6.3.8

Hg microhemispherical electrodes

Janine Mauzeroll Laboratoire d’Electrochimie Moléculaire, Université Paris, 7-UMR CNRS 7591, France

6.3.8.1

Introduction

The Hg/Pt ultramicroelectrodes (UMEs) described here are hemispherical in shape and can be used in both electrochemical and scanning electrochemical studies (scanning electrochemical microscopy (SECM), Chapter 12 of this handbook) when it is necessary to work in negative potential regions where proton reduction occurs at Pt. Examples include the detection of Tl(I) as a surrogate for K(I) in studies of ion transport through channels in membranes (1, 2) or in studies where a very negative redox couple, like methyl viologen, is needed. This section describes the deposition and characterization of a Hg hemisphere on Pt UMEs (3). Two methods of fabricating hemispherical Hg/Pt UMEs are described: electrodeposition from an inorganic mercury solution or from controlled contact of the Pt UME with a mercury drop. Electrochemical characterization can be performed using linear sweep voltammetry, amperometry (see Chapter 11) and SECM feedback experiments (see Chapter 12). 6.3.8.2

Importance of the choice of substrate

Ideally, the solid support should be easily wet by and have a low solubility in mercury. In the case of glassy carbon, the surface is poorly wet leading to the formation of scattered

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mercury droplets (4). For metals like gold, platinum and silver, the formation of intermetallic compounds at the base metal can occur. Thermal evaporation experiments have shown that the formation of inter-metallic species leads to a potential window that extends to less negative potentials than that of the hanging mercury drop electrode (HMDE) (5). Since the dissolution of Pt is hindered by the presence of surface oxides, Pt supports can still be used in voltammetric studies following the deposition of a sufficiently thick mercury layer (6). Also, in the case of a Hg–Au amalgam UME, Mandler and coworkers (7) have demonstrated the use of such probes in SECM studies of surface reactions catalyzed by Pt. Another possible substrate material is iridium. Osteryoung and coworkers fabricated and studied Ir (8) and Ir/Pt (9) alloy-based mercury UMEs. They are very useful for electrochemical studies but are more brittle than Pt UMEs. 6.3.8.3

Hg/Pt hemispherical UME fabrication by electrodeposition

The solid support for the Hg hemisphere is a conventional Pt UME as reported in detail in Section 6.3.1 of this Handbook and other published work (10). (a) Solutions To deposit mercury onto a Pt UME, a 10 mM Hg2(NO3)2 (J.T. Baker Chem. Co., Phillipsburg, NJ) in 0.1 M KNO3 solution acidified to 0.5% with HNO3 is used. Following deposition, cyclic voltammetry and SECM characterization experiments use 1 mM in cobalt sepulchrate trichloride (Aldrich), hexamineruthenium (III) chloride (Strem Chem., Newbury Port, MA), or methyl viologen (Aldrich) redox couples. The supporting electrolyte used is formed by 0.1 M KCl solutions buffered by a 1:1 molar ratio of NaH2PO4/Na2HPO4 of total concentration of 0.01 M at pH 7. All solutions are prepared with Milli-Q (Millipore Corp.) reagent water and degassed with Ar for 30 min prior to all experiments. (b) Instruments A CHI Model 900 scanning electrochemical microscope (CH Instruments, Austin, TX) can be used to control UME tip potentials, obtain approach curves and monitor the tip to underlying substrate distance. Voltammetric and SECM experiments can be performed either with the SECM head in a glove bag under positive pressure or the SECM cell covered with parafilm and in the presence of an argon blanket. Where SECM characterization is not possible, any potentiostat can be used to deposit and characterize the Hg/Pt UME. (c) Reaction cell To monitor mercury deposition in situ, a microscope reaction cell can be used. The working Pt UME and counter electrode are inserted through a hole at the base of the cell while the reference electrode is positioned in a side compartment as shown in Figure 6.3.8.1. Once mounted on an optical microscope equipped with a water immersion objective (Olympus FLxw40), a camera and personal computer can then be used to record images of the mercury deposition. (d) In-situ mercury deposition Mercury is deposited onto a Pt UME from a Hg2(NO3)2 solution in a three-electrode setup and controlled by a potentiostat. A 1 mm Pt wire serves as a counter electrode and a fritted

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Underwater Objective

Fritted Ag/AgCl ref. electrode

PT counter Pt UME

Figure 6.3.8.1 Experimental setup for the formation of Hg/Pt hemispherical UME by electrodeposition from a Hg2(NO3)2 solution with 0.1 M KNO3 acidified to 0.5% with HNO3 as supporting electrolyte. This glass cell was joined at the base with a microscope slide. The underwater objective was lowered into solution and the deposition curve recorded during a 300 sec potential step of ⫺0.1 V vs. Ag/AgCl. Reprinted with permission from reference (3). Copyright, the American Chemical Society.

Ag/AgCl electrode serves as a reference electrode. The deposition curve is recorded during a 300 sec potential step of ⫺0.1 V vs. Ag/AgCl. In the first stages of deposition, a very thin layer of inter-metallic species (Pt2Hg) is formed (5). This is followed by the spontaneous formation of mercury nuclei (Figure 6.3.8.2a) close to the edge of the Pt/glass interface where the current density is the highest (11). With time, the nuclei coalesce until a full hemisphere is formed. This coalescence alters the surface area of the electrode and leads to indentations in the current deposition curves (Figure 6.3.8.2b). These results are consistent with previously reported work (7). (e) Hg/Pt hemishperical UME characterization: voltammetry The electrochemical behavior and stability of a Hg UME is evaluated using linear sweep voltammetry. After mercury deposition, proton reduction shifts to more negative potentials from that seen at bare Pt by about 800 mV (Figure 6.3.8.2c). Dirty or damaged electrodes only shift the potential by about 200 mV and show prewaves characteristic of Pt micro arrays. Clean UMEs with a thick mercury deposit, however, are well behaved. Proton reduction at Pt is catalyzed by methyl viologen. Electrochemical studies of this couple in aqueous media must, therefore, be performed at a mercury electrode. The methyl viologen voltammogram at the Hg UME is well behaved (Figure 6.3.8.3a) and confirms good coverage of the Pt disk. The change in geometry from a disk to a hemisphere can be observed by the change in limiting current in voltammograms of Ru(NH3)63+ (Figure 6.3.8.3a). This follows theoretical equations of the steady-state current at microelectrodes (see Section 6.1 in Chapter 6 of this handbook) where the ratio of the limiting current of a disk UME (12)

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(a)

t=0s

t=5s

t = 25 s

(b)

(c)

50

100

150 200 Time (s)

Pt

Hg/Pt

0.2 µA

0.01 µA

0

t = 300 s

250

300 0

-0.5

-1 -1.5 E (V vs. Ag/AgCl)

-2

Figure 6.3.8.2 Characterization of a 25 m Hg/Pt hemispherial UME. (a) In-situ micrographs of mercury deposition (0–300 sec) from a 10 mM Hg2(NO3)2 solution with 0.1 M KNO3 supporting electrolyte acidified to 0.5% with HNO3. (b) The deposition curve recorded during a 300 sec potential step of ⫺0.1 V vs. Ag/AgCl. A 1 mm Pt wire served as the counter electrode and a fritted Ag/AgCl electrode served as the reference electrode. (c) Current potential curves at Pt and Hg/Pt UMEs in 0.1 M KNO3. Reprinted with permission from reference (3). Copyright, the American Chemical Society.

and that of a hemispherical UME is close to /2. The observed change of steady-state current (ih/id = 1.47) from the Pt UME to the Hg UME is close to this value for Ru(NH3)63+, confirming the near hemispherical geometry of the UME. A similar increase in the steady-state ratio (ih/id = 1.57) was observed for cobalt sepulchrate trichloride (Figure 6.3.8.3a). Thus, both the optical and voltammetric analyses confirm the hemispherical geometry of the UME. ( f ) Hg/Pt hemispherical UME characterization: SECM Theory (13, 14) and applications of SECM are discussed in Chapter 12. Relevant here is the effect of UME tip geometry on SECM approach curves as shown in Figure 6.3.8.3b–d. Approach curves to a Hg/Pt hemispherical UME are compared with those at a Pt disk for three redox couples and the comparison demonstrates the reduced sensitivity of the Hg/Pt hemispherical UMEs relative to Pt disk electrodes in terms of SECM feedback experiments. The Hg/Pt experimental approach curves are consistent with an analytical approximation

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Figure 6.3.8.3 (a) Voltammetric behavior of the 25 m (--) Pt UME and (solid line) Hg/Pt hemispherical UMEs in 1 mM Ru(NH3)6Cl3, cobalt sepulchrate trichloride, and methyl viologen in 0.1 M KNO3. (b) Positive feedback SECM fitting of the () finite disk theory, (♦) hemispherical disk theory, and (line) experimental results for the 1 mM Ru(NH3)6Cl3. (c) Positive feedback SECM fitting of the () finite disk theory, (♦) hemispherical disk theory, and (line) experimental results for the 1 mM cobalt sepulchrate trichloride in 0.1 M KNO3. (d) Positive feedback SECM fitting of the () finite disk theory, (♦) hemispherical disk theory, and (line) experimental results for the 1 mM methyl viologen. Reprinted with permission from reference (3). Copyright, the American Chemical Society.

for positive feedback (⫾1%) (normalized current vs. distance) for a hemispherical UME developed by Selzer and Mandler (15):

iT iT,⬀

 L − 0.1  = 0.873 + ln(1 + L−1 ) − 0.20986 exp  −   0.55032 

(6.3.8.1)

where iT is the tip current, iT,⬁ is the steady-state current when the tip is far from the substrate and L is the ratio of the tip to substrate spacing (d) and the active electrode radius (12.5 m) (i.e., L = d/r0). The response is significantly different from that reported for a disk UME (16):

iT iT,⬁

= 0.68 +

0.7838  1.0672  + 0.3315 exp  − L L  

(6.3.8.2)

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Previously reported studies of gold spherical UMEs prepared by self-assembly of gold nanoparticles (17) show similar behavior (see Section 6.3.7). 6.3.8.4

Hg/Pt hemispherical UME fabrication by contact to a mercury pool

(a) Solution The supporting electrolyte used is identical to that in the first protocol. The redox couples used for electrochemical characterization are 2 mM hexamineruthenium (III) chloride (Strem Chem., Newburyport, MA) and 0.1 mM Tl(I) nitrate (Aldrich). All solutions are prepared using Milli-Q (Millipore Corp.) reagent water and degassed with Ar for 30 min prior to all experiments. (b) Instruments A CHI Model 900 scanning electrochemical microscope (CH Instruments, Austin, TX) can be used to control UME tip potentials, obtain approach curves and to approach a Pt UME to a mercury pool. Similar results can be achieved with a micromanipulator and an independent potentiostat. (c) In-situ mercury deposition A Hg/Pt UME can be formed by applying –1.1 V vs. Hg/Hg2SO4 (Radiometer, Copenhagen, Denmark)) at a Pt UME and contacting it with the mercury (Bethlehem Instr., Hellertown, PA) of an HDME (Metrohm Instr., Herisau, Switzerland) or a Hg pool in phosphate buffer (Figure 6.3.8.4a). (d) Hg/Pt hemispherial UME characterization: voltammetry As shown in Figure 6.3.8.4b, a 0.5 V overpotential for hydrogen evolution is observed in deaerated phosphate buffer (pH = 7) following Hg deposition onto Pt. The response shown in Figure 6.3.8.4b suggests full coverage of Pt by Hg. The extension of the potential window allows the detection of Tl(I) electrochemistry at the Hg/Pt UME (Figure 6.3.8.4c). The voltammogram shows a stable steady-state current for the Tl(I) reduction and a characteristic stripping peak for the oxidation of the Tl amalgam. (e) Hg/Pt hemispherical UME characterization: SECM Hg/Pt hemispherical UME show positive feedback with Ru(NH3)62+ when approaching an HMDE (Figure 6.3.8.5) or a Hg pool. The experimental approach curves fit theory developed by Selzer and Mandler (15) for a hemispherical UME as described in Section 6.3.8.2. Reproduction of the voltammetric and SECM characterization confirms the equivalency of the two methods used to form Hg/Pt hemispherical UMEs. In SECM experiments, the close approach of the disk UME is often hampered by the insulating sheath, which may strike the substrate due to misalignment of the tip. The protrusion of the active electrode area as in the mercury hemisphere permits an uninhibited approach and a better estimation of the true zero distance. 6.3.8.5

Microelectrode maintenance and storage

Once formed, the mercury hemisphere is firmly attached to the Pt substrate and can withstand washing; however, it cannot be stored in air and left to dry. When dry, the hemisphere

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Figure 6.3.8.4 Formation and characterization of the Hg/Pt hemispherical submarine UME. (a) The Pt submarine electrode in phosphate buffer (pH = 7) as it approached the HMDE while poised at ⫺1.1 V vs. Hg/Hg2SO4. Upon contact with the HMDE, a hemispherical mercury layer is deposited onto the Pt UME. (b) Hydrogen evolution at Pt and Hg/Pt submarine UME in phosphate buffer. (c) Voltammogram of the 10⫺4 M Tl(I) at the Hg/Pt submarine electrode in phosphate buffer. Reprinted with permission from reference (2). Copyright the American Chemical Society.

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Figure 6.3.8.5 Positive feedback SECM curve fitting of (line) Hg/Pt hemispherical submarine UME approach curve to (♦) hemispherical SECM theory and () disk SECM theory for a 2 mM hexamineruthenium chloride solution in phosphate buffer (pH = 7). The Hg/Pt hemispherical UME approached an HMDE. Reprinted with permission from reference (2). Copyright the American Chemical Society.

shrinks and sometimes exposes Pt as a result of surface tension changes. Hg UMEs should therefore be stored in a degassed potassium nitrate solution. The main limitation of Hg/Pt UMEs is their reusability. For studies where the redox couples do not adhere to the mercury or form an amalgam, electrochemical cycling and storage in PBS is sufficient. In cases where amalgam formation is important, no amount of cycling is able to completely remove all traces of amalgam. Many times, it is easier to mechanically polish the UME to remove the mercury and expose a fresh Pt surface. In SECM studies, where mechanical polishing implies an increase in RG, this cleaning method can become cumbersome. 6.3.8.6

Conclusions, limitations and prospects

Hg/Pt hemispherical UMEs can be fabricated by two methods which produce identical UMEs and characterized using optical and electrochemical instrumentation. Voltammograms are well behaved and the extension of the potential window to more negative potentials facilitates the use of mediators such as methyl viologen. Approach curves recorded over conductors for different redox couples show good agreement with hemispherical theory (3). These electrodes can be used in voltammetric, amperometric, and SECM studies in negative potential regions where proton reduction occurs at Pt. They are also very smooth and

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easily adaptable to stripping voltammetry studies. Their positive potential region is limited by stripping (oxidation) of the mercury deposit.

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

M. Rueda, I. Navarro, G. Ramirez, F. Prieto, C. Prado, A. Nelson, Langmuir 15, 3672 (1999). J. Mauzeroll, M. Buda, A. J. Bard, F. Prieto, M. Rueda, Langmuir 18, 9453 (2002). J. Mauzeroll, E. A. Hueske, A. J. Bard, Anal. Chem. 75, 3880 (2003). H. P. Wu, Anal. Chem. 66, 3151 (1994). Z. Yoshima, Bull. Chem. Soc. Jpn. 1981, 556 (1981). K. R. Wehmeyer, R. M. Wightman, Anal. Chem. 57, 1989 (1985). Y. Selzer, I. Turyan, D. Mandler, J. Phys. Chem. B 103, 1509 (1999). J. Golas, Z. Galus, J. Osteryoung, Anal. Chem. 59, 389 (1987). C. Wechter, J. Osteryoung, Anal. Chim. Acta 234, 275 (1990). F.-R. F. Fan, C. Demaille, in Scanning Electrochemical Microscopy, A. J. Bard, Ed., Marcel Dekker: New York, 2001, p. 75. B. Sharifer, G. J. Hills, J. Electroanal. Chem. 130, 81 (1981). R. M. Wightman, D. O. Wipf, Electroanal. Chem. 15, 267 (1988). A. J. Bard, F.-R. F. Fan, J. Kwak, O. Lev, Anal. Chem. 61, 132 (1989). J. M. Davis, F.-R. F. Fan, A. J. Bard, J. Electroanal. Chem. Interfacial Electrochem. 238, 9 (1987). Y. Selzer, D. Mandler, Anal. Chem. 72, 2383 (2000). M. Arca, A. J. Bard, B. R. Horrocks, T. C. Richards, D. A. Treichel, Analyst 119, 719 (1994). C. Demaille, M. Brust, M. Tsionsky, A. J. Bard, Anal. Chem. 69, 2323 (1997).

6.3.9

Clarke oxygen microelectrode

Katherine B. Holt Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin, Texas 78712-0165, USA

6.3.9.1

Introduction

Platinum and carbon ultramicroelectrodes (UMEs) are commonly used to determine the concentration of dissolved oxygen in solution, by measuring the current for the reduction of oxygen (1, 2). However, difficulties with signal stability are often encountered, associated with interference by other electroactive species, or poisoning of the electrode surface by adsorbed impurities. These problems can be overcome by covering the electrode with a membrane that is permeable to oxygen but not to other solution species, as first proposed by Clarke in his design for the membrane oxygen electrode (3–6). If both working and counter electrodes are placed behind the membrane, with electrochemical contact maintained through a thin layer of immobilized electrolyte, then measurements of oxygen concentration in the gas phase are possible. The use of an UME tip as the basis for a Clarke