Shearforce-Based Constant-Distance Scanning Electrochemical

Shearforce-Based Constant-Distance Scanning Electrochemical Microscopy as Fabrication Tool for Needle-Type Carbon-Fiber Nanoelectrodes. Emad Mohamed H...
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Anal. Chem. 2010, 82, 5900–5905

Shearforce-Based Constant-Distance Scanning Electrochemical Microscopy as Fabrication Tool for Needle-Type Carbon-Fiber Nanoelectrodes Emad Mohamed Hussien,† Wolfgang Schuhmann,† and Albert Schulte*,‡ Analytische Chemie - Elektroanalytik & Sensorik, Ruhr-University Bochum, D-44780 Bochum, Germany, and Biochemistry-Electrochemistry Research Unit, School of Chemistry and Biochemistry, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand Carbon fiber nanoelectrodes with nanometer radii tip curvatures were fabricated using a shearforce-based constant-distance scanning electrochemical microscope and electrochemically induced polymer deposition. A simple DC etching procedure in alkaline solution provided conically sharpened single carbon fibers with well-formed nanocones at their bottom. Coating the stems but not the end of the tips of the tapered structures with anodic electrodeposition paint was the strategy for limiting the bare carbon to the foremost end and restricting a feasible voltammetry current response to exactly this section. The electrodeposition of the polymer was prevented at the foremost end of the tip using a shearforce-based tip-tosample distance control that allowed approaching the etched tips carefully in just touching distance to a film of a silicone elastomer. Analysis of the steady-state cyclic voltammograms in presence of a reversible redox compound revealed effective radii for the obtained needle-type carbon-fiber nanoelectrodes down to as small as 46 nm. The method offers an alternative pathway toward the fabrication of highly miniaturized carbon electrodes. Voltammetry with ultrasmall working electrodes is nowadays common practice and the recently reviewed utilization of microelectrodes and nanoelectrodes as probes for scanning electrochemical microscopy (SECM),1-5 fast sensors for nanosecond electrochemistry,6 needle-like detectors of the activity of isolated living cells,7-9 and minimal-invasive implanted devices for in vivo brain voltammetry10,11 are examples of the potential of the * Corresponding author. E-mail: [email protected]. Phone: +66 44 22 3968. Fax: +66 44 22 4185. † Analytische Chemie - Elektroanalytik & Sensorik, Ruhr-University Bochum. ‡ Biochemistry-Electrochemistry Research Unit, Suranaree University of Technology. (1) Bard, A. J.; Mirkin, M. V. Scanning Electrochemical Microscopy; Marcel Dekker, Inc.: New York, 2001. (2) Wittstock, G.; Burchardt, M.; Pust, S. E.; Shen, Y.; Zhao, C. Angew. Chem., Int. Ed. 2007, 46, 1584–1617. (3) Sun, P.; Laforge, F. O.; Mirkin, M. V. Phys. Chem. Chem. Phys. 2007, 9, 802–823. (4) Stoica, L.; Neugebauer, S.; Schuhmann, W. Adv. Biochem. Eng./Biotechnol. 2008, 109, 455–492. (5) Amemiya, S.; Bard, A. J.; Fan, F. R. F.; Mirkin, M. V.; Unwin, P. R. Annu. Rev. Anal. Chem. 2008, 1, 95–131. (6) Amatore, C.; Maisonhaute, E. Anal. Chem. 2005, 77, 303A–311A. (7) Schulte, A.; Schuhmann, W. Angew. Chem., Int. Ed. 2007, 46, 8760–8777.

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methodology. Traditional micro- and nanoelectrode precursor materials are metallic microwires or carbon fibers, both of which require an electrical insulation and often shaping for the establishment of electroactive areas with the desired geometry and dimension. In contrast to their metallic counterparts of similar diameters, 5-10 µm diameter carbon fibers are inexpensive and, due to their superior mechanical properties, far better to handle. Importantly, the graphitic nature of carbon fibers provides the often necessary chemical inertness, the option of an intentional generation of acid groups at their surface that promote chemical electrode modification, and, last but not least, a wider working potential window in aqueous electrolytes. On account of these qualities, many methods have been suggested for the fabrication of carbon fiber-based micro- and nanoelectrodes. Disk- and conically shaped carbon fiber microelectrodes (CFMEs) have the conductive graphitic filament usually tightly embedded in glass/ epoxy,12 electrodeposition polymers (EDP),13 electrodeposited polyoxyphenylene,14,15 vapor-grown silica,16 polyethylene17 or Teflon.18 Carbon fiber nanoelectrodes (CFNEs) can be obtained by tapering or thinning of carbon fibers in suitable etching procedures before encapsulation. The combination of an electrochemical cylindrical etching with an anodic EDP coating19 and a flame etching followed by phenol electropolymerization20 allowed the fabrication of disk-shaped CFNEs with active radii of some 100 nm. Moreover, EDP-insulated CFNEs with effective radii as small as 1 nm were accomplished by insulating electrochemically etched and thus sharply tapered carbon fibers completely with a (8) Adams, K. L.; Puchades, M.; Ewing, A. G. Annu. Rev. Anal. Chem. 2008, 1, 329–355. (9) Amatore, C.; Arbault, S.; Guille, M.; Lemaitre, F. Chem. Rev. 2008, 108, 2585–2621. (10) Venton, B. J.; Wightman, R. M. Anal. Chem. 2003, 75, 414A–421A. (11) Robinson, D. L.; Hermans, A.; Seipel, A. T.; Wightman, R. M. Chem. Rev. 2008, 108, 2554–2584. (12) Kelly, R. S.; Wightman, R. M. Anal. Chim. Acta. 1986, 187, 79–87. (13) Schulte, A.; Chow, R. H. Anal. Chem. 1996, 68, 3054–3058. (14) Potje-Kamloth, K.; Janata, P.; Janata, J.; Josowicz, M. Sens. Actuators 1989, 18, 415–425. (15) El-Giar, E. E.-D. M.; Wipf, D. O. Electroanalysis 2006, 18, 2281–2289. (16) Zhao, G.; Giolando, G. M.; Kirchhoff, J. R. Anal. Chem. 1995, 67, 2592– 2598. (17) Chow, R. H.; von Rueden, L. In Single Channel Recording, 2nd ed.; Sakmann, B., Neher, E., Eds.; Plenum Press: New York, 1995; pp 245-275. (18) Liu, B.; Rolland, J. P.; DeSimone, J. M.; Bard, A. J. Anal. Chem. 2005, 77, 3013. (19) Schulte, A.; Chow, R. H. Anal. Chem. 1998, 70, 985–990. (20) Strein, T. G.; Ewing, A. G. Anal. Chem. 1992, 64, 1368–1373. 10.1021/ac100738b  2010 American Chemical Society Published on Web 06/09/2010

cathodic EDP. Shrinking of the fresh electrodeposit during an obligatory heat curing treatment exposed a tiny active carbon surface at the apex of the treated fiber tip.21,22 Heat-induced shrinkage of the EDP insulation of nanometric tips and the associated automatic exposure of ultrasmall conductive areas was first reported for the preparation of partially insulated Pt and W tips for electrochemical scanning tunneling microscopy (EC-STM)23-25 and is of course an elegant strategy for the nonmanual establishment of a pointed nanoelectrode geometry. The retraction of EDP paint from the outermost tip region of conductive nanocones upon heat curing depends, however, critically on the proper control of the paint deposition. Success apparently needs suitably thin EDP film formation in particular in the tip region and is thus influenced by process parameter such as the base material and surface roughness of the tapered tips, the type of the employed EDP, the concentration of the EDP, cell arrangement for the deposition, the deposition voltage and time, the temperature of the heat curing of fresh deposits, and the choice between a one-time or multiple-time deposition strategy. An adjustment of the parameter set to optimum is not an easy task, and several reports on the preparation of EC-STM tips,26,27 CFNEs,21,22,28 and Pt nanoelectrodes for combined atomic force microscopy (AFM)-scanning electrochemical microscopy (SECM) measurements29,30 are examples of the many efforts to adapt the originally proposed strategy of EDP retraction to the particular needs of specially designed probes and/or previously not explored EDPs. The idea to exploit SECM as creative tool for micro- and nanostructure fabrication is almost as old as the instrument itself.31 The principle is relying on the fact that an SECM tip micro- or nanoelectrode can be operated as tiny electrochemical machining tool with the exclusive capability to manipulate the surface of a carefully approached substrate on the micrometer and nanometer lateral scale via tip-induced and thus spatially localized electrochemical etching, deposition, or synthesis procedures. Illustrative applications of such a methodology can be found in the literature.32-41 However, as will be confirmed in the following, a SECM not only offers an elegant and efficient pathway to (21) Chen, S.; Kucernak, A. Electrochem. Commun. 2002, 4, 80–85. (22) Chen, S.; Kucernak, A. J. Phys. Chem. 2002, 106, 9396–9404. (23) Schulte, A. Ph.D. Thesis, Westfa¨lische Wilhelms-Universita¨t Mu ¨ nster, Germany, 1994. (24) Bach, C. E.; Nichols, R. E.; Beckmann, W.; Mayer, H.; Schulte, A.; Besenhard, J. O.; Jannakoudakis, P. D. J. Electrochem. Soc. 1993, 140, 1281–1284. (25) Schulte, A. SPIE Proc. Ser. 1998, 3512, 353–357. (26) Guell, A. G.; Die´z-Pe´rez, I.; Gorostiza, P.; Sanz, F. Anal. Chem. 2004, 76, 5218–5222. (27) Thorgaard, S. N.; Bu ¨ hlmann, P. Anal. Chem. 2007, 79, 9224–9228. (28) Wang, C.; Chen, Y.; Wang, F.; Hu, X. Electrochim. Acta 2005, 50, 5588– 5593. (29) MacPherson, M. V.; Unwin, P. R. Anal. Chem. 2000, 72, 276–285. (30) Slevin, C. J.; Gray, N. J.; MacPherson, M. V.; Webb, M. A.; Unwin, P. R. Electrochem. Commun. 1999, 1, 282–288. (31) Mandler, D.; Bard, A. J. J. Electrochem. Soc. 1990, 137, 2468–2472. (32) Ufheil, J.; Boldt, F. M.; Boersch, M.; Borgwarth, K.; Heinze, J. Bioelectrochemistry 2000, 52, 103–110. (33) Kranz, C.; Ludwig, M.; Gaub, H. E.; Schuhmann, W. Adv. Mater. 1995, 7, 568–571. (34) Kranz, C.; Ludwig, M.; Gaub, H. E.; Schuhmann, W. Adv. Mater. 1996, 8, 634–637. (35) Shiku, H.; Uchida, I.; Matsue, T. Langmuir 1997, 13, 7239–7244. (36) Wittstock, G.; Schuhmann, W. Anal. Chem. 1997, 69, 5059–5066. (37) Turyan, I.; Matsue, T.; Mandler, D. Anal. Chem. 2000, 72, 3431–3435. (38) Sauter, S.; Wittstock, G. J. Solid State Electrochem. 2001, 5, 205–211.

microscopic surface modification, but also can be used for the fabrication of nanoelectrodes. We present here an alternative scheme for the preparation of pointed CFNEs based on conically etched carbon fibers thinly covered with the anodic EDP Canguard as insulating matrix everywhere but not on the tip apex. A scanning electrochemical microscope with integrated optical shearforce (SF) positioning mode was applied for placing the to-be-coated carbon fiber tips in close proximity to a soft silicon rubber. After this, with the tip located in the shearforce nearfield, EDP precipitation was triggered by applying a suitable deposition potential to the carbon fiber electrode. Subsequent heat curing produced then the desired needle-type CFNEs. Relevant technical details of the proposed scheme for CFNE construction will be provided along with a characterization of the nanosensors via scanning electron microscopy (SEM) and cyclic voltammetry (CV). The methodology needs only one deposition step and works with an undiluted EDP solution as commercially available. EXPERIMENTAL SECTION Chemicals and Materials. KCl was from J. T. Baker, Deventer, Netherland. [Ru(NH3)6]Cl3 and K3[Fe(CN)6] were from Aldrich, Steinheim, Germany and Riedel-de-Haen, Seelze, Germany, respectively. The anodic electrodeposition paint Canguard was a research sample from BASF Coatings AG, Mu ¨ nster, Germany, and 7.5 µm-diameter carbon fibers of the type PAN E/XAS were a gift from SGL Technik, Meitingen, Germany. Borosilicate glass capillaries (7.5 mm o.d., 1.5 mm i.d.) were from Hilgenberg, Malsfeld, Germany, and the used conductive carbon cement from PLANO, Wetzlar, Germany. The two-component Sylgard 184 silicon elastomer kit is a product of Dow Corning, MI, and was purchased from a German distributor. Solutions were prepared with ultrapure water, which was prepared with a purification system from SG Water, Hamburg, Germany. Preparation of Vibrationable CFNE Precursors. Prior to use, a few centimeter long pieces of carbon fiber bundles were soaked in acetone for 24 h and then thoroughly rinsed first with acetone and then water to get rid of possible organic contaminations at the surface. An individual carbon fiber was withdrawn from a dried bundle with the tip of a syringe needle that was made sticky through contact with a glue stick. The filament was then fixed to one end of a 0.5 mm diameter and about 10 cm long copper wire using a tiny amount of conductive carbon cement. The other end of the copper wire was introduced into a borosilicate glass capillary and threaded through until the fiber/wire junction was located in the middle of the tube and the copper wire extended well beyond the tube end. In this arrangement, the copper wire was fixed relative to the tube wall using the two-component silicon rubber Sylgard 184 at 10:1 ratio of the base to curing agent for fixation. To introduce the necessary amount of an initially liquid and at elevated temperature quickly hardening Sylgard mixture into the capillary, a gentle vacuum suction was applied from the (39) Ufheil, J.; Hess, C.; Borgwarth, K.; Heinze, J. Phys. Chem. Chem. Phys. 2005, 7, 3185–3190. (40) Evans, S.A. G.; Brakha, K.; Billon, M.; Mailley, P.; Denuault, G. Electrochem. Commun. 2005, 7, 135–140. (41) Schwamborn, S.; Stoica, L.; Neugebauer, S.; Reda, T.; Schmidt, H.-L.; Schuhmann, W. Chem. Phys. Chem. 2009, 10, 1066–1070.

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opposite tube end via a plastic syringe. Sylgard curing was facilitated by a heat treatment in an oven at 190 °C for 10 min. The capillaries containing the carbon fibers were pulled with a homemade vertical pipet puller to produce an about 2 cm long and thinly tapered glass tip that sealed tightly to the protruding carbon fiber and was flexible enough for being used with shearforce-based SECM. Projecting carbon fiber were trimmed to about 2 mm and carefully rinsed with streams of 70% ethanol and water. Electrochemical Etching of Carbon Fibers. Carbon fibers extending beyond the tips of the tapered glass pipettes were electrochemically sharpened following a known procedure.42 In brief, the etching arrangement consisted of the carbon fiber to be etched (anode), a Pt ring (cathode), and 0.1 M NaOH as etchant. The carbon fiber anode was mounted on a manual XYZ micromanipulator. Placed in the center of the cathode ring, the carbon fiber was lowered into the etching solution until about onehalf to two-thirds of the 2 mm long filament was immersed. With a DC laboratory power supply, the etching potential of 3 V was applied and the development of the etching current was followed through the reading of a digital multimeter. Typically, the current declined initially slowly and at some point in time more rapidly from its initial values of a few tenths of microampere toward 0. The power supply was immediately switched off when zero current was attained. Freshly etched carbon fibers were rinsed several times with water and then inspected with a dissection stereomicroscope. Only carbon fibers with visibly sharp tapers were further processed for the further nanoelectrode preparation. Anodic EDP Insulation of Etched Carbon-Fiber Tips. The anodic EDP insulation of etched carbon fiber tips was carried out in a scanning electrochemical microscope with integrated shearforce positioning43-45 (Sensolytics GmbH, Bochum, Germany). The shearforce signal was read out with a tip-to-sample distance control unit with an optical detection of shearforces between the vibrating SECM tip (here: the etched carbon fiber tip) and a surface (here: a film of a soft silicon rubber). The system comprised a piezoelectric tube (PSt 150/4/20, Piezomechanik Pickelmann, Mu¨nchen, Germany) for probe agitation, a function generator (HP 33120A, Hewlett-Packard) as supply of the agitation voltage, a Laser (LDM-5-635-1, Optronics, Kehl, Germany) emitting a precisely focused beam, a split photodiode (Spot 4D, laser2000, Wessling, Germany) for optoelectronic signal detection, and a lock-in amplifier (Model 7280, Signal Recovery, Wokingham, U.K.) for tip vibration amplitude and phase quantification. The photodiode and the Laser source were attached to manual course micromanipulators to aid their alignment. The vibrating carbon fiber tip was fixed in space and the electrochemical cell for the EDP deposition was moved through a stage of three joined stepper motors (SPI Robot Systems, Oppenheim, Germany) with a nominal resolution in x-, y-, and z-direction of 10 nm per microstep. The entire setup was placed on an active vibration damping table and located in a Faraday cage for noise elimination. Software in Visual Basic 6.0 (Microsoft, Unterschleissheim, Germany) con(42) Mousa, M. S. Appl. Surf. Sci. 1996, 94/95, 129–135. (43) Ludwig, M.; Kranz, C.; Schuhmann, W.; Gaub, H. E. Rev. Sci. Instrum. 1995, 66, 2857–2860. (44) Hengstenberg, A.; Kranz, C.; Schuhmann, W. Chem.sEur. J. 2000, 6, 1547– 1554. (45) Pitta Bauermann, L.; Schuhmann, W.; Schulte, A. Phys. Chem. Chem. Phys. 2004, 6, 4003–4008.

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Figure 1. Scanning electron micrographs (SEM) of an electrochemically etched carbon fiber with a sharply pointed nanometric tip. Lowmagnification overview of the entire etched carbon fiber (A) and the close-up view of the front region of the cone structure (B).

trolled the SECM setup and data acquisition and storage was done using a 16bit AD/DA board (PCI-2517, Measurement Computing Corporation, Norton, MA). With the etched carbon fiber tips either positioned far away from or in the shearforce regime of the silicon rubber substrate at just touching distance, the formation of the anodic EDP film was induced with a deposition voltage of 3.8 V that was applied between the carbon fiber anode and a Pt ring counter electrode for 4 min. Freshly EDP coated carbon fibers were then cured at 190 °C for 20 min to bake the fresh electrodeposit and form the insulation layer. As a routine, the glass tip/carbon fiber junction was thinly covered with Sylgard, which was cured at 190 °C for 5 min. This avoided undesired leakage currents at this critical section. Characterization of EDP Insulated CFNEs. Completed CFNEs were inspected with a stereomicroscope, scanning electron microscope (SEM) and cyclic voltammetry (CV). The latter used a computer-controlled VA10 potentiostat (npi electronic GmbH, Tamm, Germany) and a special module in the software of the SECM device. A Ag/AgCl served as pseudo reference electrode and a Pt wire as counter electrode. As electrolyte, a 0.1 M KCl containing 5 mM of either K3[Fe(CN)6] or [Ru(NH3)6]Cl3 was used. RESULTS AND DISCUSSION Starting point of the proposed SECM-assisted nanoelectrode fabrication are electrochemically etched carbon fibers with sharply pointed tips as displayed in Figure 1A. High-resolution SEM inspections of the tapered structures revealed smooth tip surfaces and tip apertures down to only some tenths of nanometers (Figure 1B). It is worth mentioning that the employed DC etching procedure in 0.1 M NaOH had a good success rate in terms of tip sharpening and typically 6 to 7 out of 10 etched tips qualified for further use as nanoelectrode precursors. Processing selected graphitic nanocones into the envisioned voltammetric nanoelectrodes requires the electrical insulation of all but not the extreme front of the conductive cones. Using the anodic electrodeposition polymer Canguard, the establishment of the envisaged nanometric electroactive carbon area at the cone front of etched carbon fibers did not work with a single EDP

Figure 2. Schematical representation of the constant-distance mode SECM setup used for the insulation of the very end of the conical carbon tip with an EDP. An agitation piezo tube is used to vibrate the flexible glass pipet/carbon fiber assembly at its resonance frequency. Simultaneously, a laser beam is focused on the vibrating electrode and the generated diffraction pattern is focused on a split photodiode. The difference of the output current at both parts of the photodiode scales with the vibration amplitude of the swinging tip and is amplified with a lock-in-amplifier. Monitoring the lock-in output which is either the magnitude or the phase-shift of the signal with respect to the reference provides the means of a controlled tip positioning through utilization of the surface-near shearforces that obstruct free tip vibration. The z-approach was automatically stopped when the etched fiber tips were just touching a film of a silicon elastomer.

deposition and the above-mentioned heat curing and conventional paint retraction method alone. Success with a tip-excluding encapsulation was attained with an anodic EDP coating performed while the carbon fiber tip was in soft physical contact with the surface of the silicon rubber film at the bottom of the deposition chamber. This tip position was reproducibly established with the SECM setup sketched schematically in Figure 2 in combination with a shearforce-based constant-distance mode z-approach curve following previously described routines.43-45 For performing the computer-controlled feedback distance control, it was essential to vibrate the flexible glass housing of the carbon fibers at a predetermined resonance frequency, focus the Laser beam properly onto the oscillating tapered glass structure, and guide the resultant diffraction pattern straight to the split photodiode. In this configuration, the output voltage which is a measure for the difference current of the left minus the right section of the split photodiode is related to the vibration amplitude of the oscillating tip. Monitored with a phase-sensitive lock-in amplifier (LIA) in terms of magnitude and phase shift with respect to the voltage of the agitation piezo, changes in the photodiodes response reflected the onset (approach) or loss (withdrawal) of the interfacial shearforce interaction between the carbon tip and soft silicone rubber. Figure 3 shows a typical plot of the magnitude of the lock-in signal reflecting the tip vibration amplitude in arbitrary units versus the tip-to-sample distance, d in micrometers as obtained when moving an etched carbon fiber tip from far above to a just touching distance to the silicon rubber surface. At distances above about 1 µm, which is out of the reach of the shorter-ranged

Figure 3. Polymer insulation of the very end of a conical carbon finer tip using the constant-distance scanning electrochemical microscope with optical shearforce detection: Shearforce-assisted positioning of the etched carbon fiber tip in just contact with a soft rubber substrate by means of z-approach curve measurements. The magnitude of the optical signal detected with the lock-in amplifier is recorded as a function of the tip-to-sample distance while bringing the conical tip of etched carbon fibers at slow traveling speed of 200 nm s-1 from clearly above into touching distance with a soft silicon rubber film. (1) The tip is freely vibrating without being influenced by the presence of the substrate. (2) Shearforce interaction impedes free tip vibration. Here, a distance-dependent gradual decrease is monitored in the vibration amplitude and LIA output. (3) The point of the first contact appears as a clearly visible deflection of the approach curve. The tip approach is automatically stopped at this position and the gentle tip-to-substrate contact is used for preventing the deposition of the EDP at the tip area which is in contact with the silicon rubber.

shearforces, the vibrating tip swings freely in resonance and a stable maximum amplitude of oscillation is seen in the approach curve in Figure 3. Once the oscillating carbon tip gets closer to the substrate than the interaction distance of shearforces, the onset of tip-to-surface shearforce communication starts to impede the free tip vibration and a distance-dependent gradual decrease of the signal was observed. Most important for the tip positioning is, however, the final stage of the approach. Actually, the sharp bend of the signal that reproducibly occurred at a certain distance between the vibrating tip and sample could be used as precise indicator for the beginning tip-to-substrate contact. At this point, the downward movement of the tip was stopped and tip vibration switched off. With the carbon tip in slight surface contact, the Petri dish used as electrochemical cell was slowly filled with the Canguard EDP solution to a level surely above the glass/carbon fiber junction. The electrodeposition of a thin polymer film onto the carbon fiber was then triggered by applying the deposition potential (3.8 V; 4 min) for inducing water electrolysis, liberation of protons, and neutralization of the polymer micelles, which finally is leading to the precipitation of a uniform and thin EDP coating. The coated fibers were withdrawn from the silicon rubber substrate after the deposition potential was switched off, removed from the SECM setup, rinsed with water, and then heat-cured at 190 °C for 10 min. Heat curing changes the fresh and porous anodic EDP film into a dry, dense, pinhole free, and insulating EDP coverage. The morphological appearance of the EDP-insulated carbon fiber tips was inspected using high-resolution SEM. Cyclic Analytical Chemistry, Vol. 82, No. 13, July 1, 2010

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Figure 4. Scanning electron micrographs (SEMs) of (A) a bare electrochemically etched carbon fiber, (B-D) an electrochemically etched carbon fiber tip after EDP insulation leaving the very end of the tip free by contact to a silicone rubber. (E) An electrochemically etched carbon fiber tip with a thicker EDP coating obtained after EDP deposition with the tip in bulk solution rather than touching the rubber surface.

Figure 5. Cyclic voltammogram of (A) a CFNE tip fabricated following the proposed strategy in 5 mM [Fe(CN)6]3-/0.1 M KCl solution and (B) a similarly etched but completely insulated carbon fiber in 5 mM [Ru(NH3)6]3+/0.1 M KCl solution (50 mV/s).

voltammetry was used to evaluate the quality of the obtained electrical insulation and prove the completion of the CFNEs. Figure 4A is the SEM image of a bare electrochemically etched carbon fiber which underlines the almost perfect needle-type and smooth contour of the tip without any visible (or “with no visible”) split-offs, cracks, or pits. Sharp carbon fiber tips after EDP insulation using the aforementioned shearforce-based positioning strategy are shown in Figure 4B-D as a series of SEM images of increasing magnification. In agreement with previous work involving Canguard for carbon fiber and metal STM tip insulation,13,19,25 the EDP paint coatings that were obtained under the conditions used here consistently were uniform, thin, and defect-free. Also, they aligned well to the shape of the underlying carbon cone. Occasionally, modest bulges as in Figure 4B appeared on the EDP insulated tips, however, at distances from the apex where their presence did not disturb. Because of the very thin EDP films and the fact that the carbon fiber and the polymer coating scatter the secondary electrons, which are responsible for image formation and contrast, similarly, a border between the polymer-covered upper and bare tip regions could not be visualized by SEM. Cyclic voltammetry using the [Fe(CN)6]4-/3- redox couple was, however, possible with the tips and revealed the sigmoidal-shaped I/V curves that are typical for highly miniaturized electrodes (Figure 5A). Etched carbon fiber tips were EDP insulated in control experiments with a deposition potential and time as stated before (3.8 V; 4 min) but kept in the bulk of the polymer solution rather than placed in proximity to the silicon rubber. SEM inspections revealed that in the case of this control the resulting coatings at the tip regions were always thicker (see Figure 4E) and unsurprisingly voltammetry with such tips did not work (see Figure 5B). 5904

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Figure 5A is a representative example of a CV in a solution containing 5 mM [Fe(CN)6]3- recorded at a tip-exposed but otherwise well EDP-insulated carbon fiber cone. In this particular case, the diffusion-limited steady-state current, ilim, was as about 100 pA. Figure 5B is a CV that was recorded with an etched carbon fiber tip that was insulated in bulk solution. As expected for a fully insulated cone, no Faraday current was detected caused by the reduction of [Ru(NH3)6]3+. As previously has been assumed for cathodic EDP-insulated CFNE analogues of a similar tapered shape and encapsulation,21,22 the active electrode area gained at the end of conically etched carbon fibers using the proposed EDP insulation was considered to be hemispheroidal. Thus, the dimensions of the fabricated CFNEs in terms of their effective radius, reff, was calculated from the experimental value of ilim following the equation ilim ) KnFDC∞reff where K is the valid geometric accessibility factor (here: 2π),21,22 n the number of transferred electrons in the electrode reaction (here: 1), F the Faraday constant (96485 C), D the diffusion coefficient of the relevant electroactive species in the chosen supporting electrolyte (here: 5 mM [Fe(CN)6]3- in 0.1 M KCl; 7.2 × 10-6 cm2 s-1),46 and C∞ the bulk concentration of the redox mediator (here: 5 mM). Using these values and the above formula leads to an reff of about 46 nm for the tip that had ilim of 100 pA as its steady-state current in 5 mM [Fe(CN)6]3- solution. In general, about 60% of the prepared CFNEs displayed sigmoidal voltammograms. However, the determined experimental diffusion-limited current values varied randomly between 100 and 600 pA. Accordingly, the corresponding calculated effective CFNE radii covered about an 46-275 nm range. Apparently, the proposed shearforce-based SECM strategy for EDP insulation cannot produce a CFNE with a predefined effective radius. However, with a passable success rate of slightly more than one out of two, the method is valuable and worth trying for the manufacturing of carbon electrodes with clearly submicrometer effective dimension. CONCLUSION A procedure for the fabrication of needle-like carbon-fiber nanoelectrodes was proposed based on a sequence of electro(46) Konopka, S. J.; McDuffie, B. Anal. Chem. 1970, 42, 1741–1746.

chemical etching of individual carbon microfibers followed by insulating the cylindrical stem but not the very end of etched carbon fibers with a dense thin film of an anodic electrodeposition polymer. The apparent effective radii were as small as 46 nm. The microscopically small uncoated cone front was achieved with the aid of the tip-to-sample distance control unit of a constant-distance SECM together with holding the etched carbon fiber tips in a controlled manner in a just touching distance to the surface of a soft silicone elastomer film. Because of their contour and their very small active areas, these CFNEs are ideal tips for in situ electrochemical probe microscopy techniques such as electrochemical STM and constant-distance mode SECM. Particular potentials are seen for probing the local redox activities and constitution of tiny living cells, corrosion pits, membrane pores and for measurements in small-volume droplets, vials and the

outlets of capillary electrophoresis tubings, and future work will address these applications. ACKNOWLEDGMENT E.M.H. and W.S. are grateful to the DFG in the framework of the special research area (SFB459; A5). A.S. expresses his gratitude to the Thailand Research Fund (TRF) for sponsoring this work through a personal grant and thanks the Suranaree University of Technology for the financial support in favor of the Biochemistry - Electrochemistry Research Unit.

Received for review March 23, 2010. Accepted May 24, 2010. AC100738B

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