Nanoporous Pt Microelectrode for Neural Stimulation and Recording

Apr 21, 2010 - 3D-nanoporous Pt (L2-ePt) films on multiarrayed microelectrodes electroplated from reverse micelle (L2) phase were evaluated in vitro f...
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J. Phys. Chem. C 2010, 114, 8721–8726

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Nanoporous Pt Microelectrode for Neural Stimulation and Recording: In Vitro Characterization Sejin Park,† Youn Joo Song,‡ Hankil Boo,† and Taek Dong Chung*,‡ Nomadien Corporation, B-206, SK Twintech Tower, Gasan-dong, Geumcheon-gu, Seoul 153-773, Korea, and Department of Chemistry, Seoul National UniVersity, Seoul 151-747, Korea ReceiVed: NoVember 26, 2009; ReVised Manuscript ReceiVed: March 11, 2010

3D-nanoporous Pt (L2-ePt) films on multiarrayed microelectrodes electroplated from reverse micelle (L2) phase were evaluated in vitro for neural cell stimulation and extracellular potential recording. Impedance, capacitance, and charge injection limit were measured for the L2-ePt electrodes with various surface specific areas, i.e., roughness factors. L2-ePt with a roughness factor of 223 showed optimum performances as neural stimulation and recording electrodes, and its electric characteristics were electrode impedance of 0.039 Ω cm2 (1.17 kHz), electrode volume specific capacitance of 43 F cm-3 (1.17 kHz), and charge injection limit of 3 mC cm-2 (400 µs). L2-ePt showed higher mechanical strength than the conventional platinized Pt (Pt black). Introduction Recording of extracellular potential and electrical stimulation of neural tissue have been the key issues in the fields of neural prosthesis (cochlear implant for hearing, electronic retinal prostheses for vision, limb movement restoration), clinical therapies (Parkinson’s disease, epilepsy, depression, dystonia, chronic pain, sleep apnea), and neurophysiology research. The electrodes desirable for recording extracellular potential and stimulation of neural tissue should be as small as possible to achieve high spatial resolution while maintaining low interfacial impedance and high charge injection capability. In general, the small area of an electrode leads to high interfacial impedance when it is exposed to an extracellular medium. The high impedance between the electrode and the biological surface makes it difficult to read the neuronal signals and stimulate effectively. Small electrodes are normally vulnerable to noise. Electronic systems with high impedance suffer from thermal background noise (Vth) known as Johnson noise1 originating from thermal agitation of the conductor. The mean square of thermal voltage j th2) can be expressed as eq 1, where kB is the Boltzmann (V constant, T is the temperature in Kelvin, Rs is the resistance of extracellular solution, Zre is the real part of interfacial impedance, and ∆f is the frequency bandwidth. The high interfacial impedance generates more thermal noise, and low signal-tonoise ratio is a hardly avoidable drawback of neural recording microelectrodes. 2 Vth ) 4kBT(Rs + Zre)∆f

(1)

In addition to the thermal background noise, another noise source is the combination of high electrode impedance and the shunt capacitance distributed between the electrode and the recording amplifier, which reduces the high-frequency response.2 * To whom correspondence should be addressed. E-mail: tdchung@ snu.ac.kr. Phone: +82-2-880-4362. Fax: +82-2-887-4354. † Nomadien Corporation. ‡ Seoul National University.

Thus, it is a reasonable choice to use a material with low impedance (conventionally measured at 1 kHz) as an electrode for extracellular potential recording.3 To suppress the interfacial impedance, materials with high roughness factors (Rf ) Areal/ Aapp; Areal is the real surface area, and Aapp is the apparent surface area) have been utilized as neural signal recording electrodes. In spite of the problems of small electrodes, miniaturization of neural stimulation electrodes is increasingly crucial, particularly for the purpose of implantation. The electrodes to be implanted need to fulfill a few requirements of low power consumption, high efficacy, and safety. First of all, small potential polarization with a given current density allows miniaturized electrodes to consume low power. For safety, the electrodes must be biocompatible so as to do as little harm as possible during the operation. To stimulate efficaciously, they should be able to inject a sufficient amount of charge into the adjacent tissues with only a nonfaradaic reaction if possible or without a irreversible faradaic reaction at least. So far, many electrode materials for neural stimulation have been proposed, but none of them has satisfied all the requirements for size, power consumption, safety, and efficacy. Representative materials that have been suggested for neural electrodes are titanium nitride,3,4 Pt,3,5 iridium oxide, and carbon nanotube (CNT).3,6 Fundamental theory and information on the various neural electrodes are extensively reviewed by Cogan3 and Merrill et al.7 Highly porous titanium nitride, which is obtained by physical vapor deposition (PVD), is chemically stable and passes sufficient electric charge by the capacitive (nonfaradaic) process.3 For example, the porous titanium nitride supplied by Heraeus Co. has a pyramid-like rough surface, which is about 7 µm deep from top to bottom and whose surface-specific accapacitance (Ce) (i.e., capacitance measured by electrochemical impedance spectroscopy, EIS) reaches 18-22 mF cm-2.4 Weilland et al. reported a low charge injection limit (Qinj) of 0.55 mC cm-2 for 200 µs cathodic pulses in the safe voltage range of -0.6 to +0.8 V.8 However, the biocompatibility of titanium nitride has not been confirmed yet and cell death on it was reported.9

10.1021/jp911256h  2010 American Chemical Society Published on Web 04/21/2010

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Electrodes modified with carbon nanotube (CNT) are attractive candidates for neural electrodes by virtue of large surface area, high aspect ratio, and low electrical resistance. CNT grown by chemical vapor deposition (CVD) at 900 °C on the microelectrodes (diameter, d ) 80 µm) of a multielectrode array (MEA) reportedly showed a high dc-capacitance (Cdl) (i.e., capacitance measured by cyclic voltammetry, CV) of 10 mF cm-2 (10 F cm-3) and a low electrode impedance (Ze) of 10 kΩ (0.50 Ω µm2) at 1 kHz, and it was used to record extracellular signals from rat cortical cells.6 Vertically aligned MWCNT pillars grown by CVD exhibited surface-specific Ce of 1.6 mF cm-2 and capacitive Qinj of 1-1.6 mC cm-2 after hydrophilic functionalization, and they were used to stimulate hippocampal neurons.10 Iridium oxide injects charge via reversible electrochemical reaction of Ir3+/4+, and its Qinj reaches 4 mC cm-2 for the 200 µs pulse.8 However, iridium oxide electrodes are demolished by high current pulse11 and chronic use,9 and the surrounding tissue is contaminated by the remains. Pt is an ideal material for neural prostheses and clinical therapies especially in terms of biocompatibility.7 However, the Cdl of a smooth Pt is so small (20 µF cm-2) that its Ze is significantly high (557 MΩ µm2, 1 kHz).2 To address these problems, platinized Pt (Pt black) has been used so far, and it has a much higher Ce (6 mF cm-2, 1 kHz)2 and a lower impedance (0.112 Ω µm2, 1 kHz).12 The Qinj of a Pt electrode is in the range of 50-150 µC cm-2 when charge balanced biphasic current pulses with 200 µs duration are exerted.13 Stimulation with Pt electrodes is implemented by charge injection through a reversible faradaic process, which involves not only the electrochemical adsorption and desorption of proton but also the surface oxidation and reduction of Pt. Because smooth Pt normally has high Ze and low Qinj, the conventional and commercial Pt for neural electrodes is platinized Pt (Pt black). However, platinized Pt is rarely used for neural stimulation due to its low abrasion resistance. Nanoporous Pt is a noble candidate to address the deficiency of platinized Pt such as high impedance, low capacitance, and low abrasion resistance. In recent years, a few kinds of nanoporous Pt films received much attention as a new class of porous Pt materials. Such nanoporous Pt films can be deposited in one step on desirable substrate, such as 1D nanoporous Pt14,15 and 3D nanoporous Pt.16 Common features of these nanoporous Pt films are that they are electroplated on a targeted substrate with controllable thicknesses to have bright and robust surfaces while extreme porosity is maintained. Especially, the fabrication process of the 3D nanoporous Pt denoted as L2-ePt16 uses a low viscous electroplating solution (L2 phase) so that it can be micropatterned and electroplated on flexible substrates. Moreover, the 3D porous structure of L2-ePt is expected to enhance mass transfer of electrolyte inside the film and thus lessen the pore resistance, which is the major problem of porous materials for neural recording or stimulation. L2-ePt forms a dense film in comparison with platinized Pt and is expected to show superior physical stability. In this study, in vitro performance of L2-ePt is investigated and discussed to probe how this new candidate works for better recording and stimulation electrodes. Experimental Section Reagents. All chemicals including tert-octylphenoxypolyethoxyethanol (Triton X-100, Sigma), hydrogen hexachloroplatinate hydrate (Aldrich), lead acetate (Aldrich), sodium phosphate monobasic and dibasic (Jusei), sodium chloride (Junsei), and sulfuric acid (Aldrich) were used without further

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Figure 1. (A) MEA employed in this study (microelectrodes electroplated with L2-ePt appear as bright spots, and bare ones as dark gray), (B) SEM image of L2-ePt microelectrode (Rf ) 223), and (C) vertical profiles of MEA and L2-ePt microelectrodes (Rf ) 223, 654, 1570). One grid on the y-axis corresponds to 1 µm.

purification. All the aqueous solutions were prepared with ultrapure deionized water produced by NANOpure (Barnstead). Electrodeposition of Nanoporous Pt. A nanoporous Pt denoted by L2-ePt was prepared by electroplating Pt in a reverse micelle solution following the procedure described in our previous report.16 A mixture of Triton X-100 (50 wt %), 0.3 M NaCl aqueous solution (45 wt %), and hexachloroplatinic acid (HCPA) (5 wt %) was prepared and equilibrated at 40 °C. L2-ePt was electrochemically deposited on a microelectrode (d ) 45 µm, and Aapp ) 1.6 × 10-5 cm2) of multielectrode array (MEA60 200Pt GND, Ayanda Biosystems) shown in Figure 1A by applying -0.2 V vs Ag/AgCl. The surface roughness of the L2-ePt was determined by measuring the area under the hydrogen adsorption peak in the cyclic voltammogram (V ) 0.2 V s-1) in 1 M H2SO4, using a conversion factor of 210 µC cm-2.17 The surface profile of L2-ePt on each microelectrode of MEA was measured by a surface profiler (XP-1, Ambios Technology) by a scanning probe at 10 µm s-1. Surface

Characterization of 3D-Nanoporous Pt Films morphology of L2-ePt was observed with a field emission scanning electron microscope (S-48000, Hitachi). Electrochemical Characterization. Electrochemical measurements were performed in a three-electrode system, employing a Model CHI660 (CH Instruments) as potentiostat and a Ag/AgCl (3 M KCl) as a reference electrode. Electrochemical impedance spectroscopy (EIS) was performed in phosphate buffered saline (PBS) solutions at open-circuit potential (Eoc ) +0.35 V vs Ag/AgCl) by applying ac-potential (Eac) of 10 mV amplitude using a Pt foil with sufficient area as a counter electrode. The Cdl was determined from cyclic voltammograms of L2-ePt in deaerated PBS solutions (V ) 50, 100, 150, 200, 250, and 300 mV s-1) by using the equation (icc + ica) ) 2CdlV, where icc and ica are the cathodic and anodic charging currents, respectively, and V is the potential sweeping rate. To measure Qinj, biphasic current pulses with 400 µs width and 20 µs interpulse delay in open circuit condition were applied between L2-ePt and Ag/AgCl, using a homemade pulse generating circuit. The maximum charge injected within the potential range from -0.6 to +0.8 V vs Ag/AgCl was chosen as Qinj. Mechanical Stability Test. To compare the mechanical stabilities, L2-ePt (Rf ) 210) and platinized Pt (Rf ) 180) were electroplated on Au films sputtered on Si wafer with Ti adhesive layers, and tested with a conventional scratch tester (Nano Scratch Tester, CSM Instrument, no. NST50-149) equipped with a spheroconical diamond indentor (d ) 4 µm). Results and Discussion Electroplating of L2-ePt on Microelectrodes. L2-ePt was electroplated on Pt microelectrodes of a MEA shown in Figure 1A immersed in a mixture solution of HCPA (5 wt %) + 0.3 M NaCl (45 wt %) + Triton X-100 (50 wt %). The microelectrodes were located at the bottom of a microwell of SU80 (height ) 4.5 µm, d ) 45 µm). L2-ePts with a variety of thicknesses (i.e., various roughness factors, Rf) were electroplated on each microelectrode of a MEA. The SEM image of a typical example in Figure 1B shows the surface morphology of L2-ePt with Rf ) 223. In microscale view, the L2-ePt is a dense uniform film on which there are some cracks with submicrometer width. If required, crack-free L2-ePt can be prepared by changing the electroplating potential to a less negative potential than -0.16 V versus Ag/AgCl.18 Reportedly, L2-ePt has a 3D nanoporous structure with pore size of 1-2 nm, and the nanoscopic structure of the 3D nanoporous L2-ePt was described in detail in our previous report.16 Figure 1C shows the surface profiles of L2-ePts with a few Rf values (223, 654, 1570) in comparison with a bare Pt surface of MEA, revealing that the thickness of L2-ePt with Rf ) 223 was about 900 nm. L2-ePt grows inside the microwell at the early stage until Rf reaches near 654, and then extrudes over the well as a result of subsequent electroplating. EIS of L2-ePt on Microelectrode. The EIS results on porous electrodes are usually discussed on the basis of the transmission line model. For example, Elliott et al.19 interpreted the EIS data of a nanoporous Pt with hexagonally arranged linear nanopores (i.e., H1-ePt) using the transmission line model. According to the transmission line model, the interfacial impedance (Ze) is expressed as eq 2. In eq 2, r is electrolyte resistance per unit length in the pore (Ω cm-1), and l is film thickness (cm). The nonideal capacitive behaviors are expressed as a constant phase element (cpe) with K, a constant analogous to Ce (F), and R, a dimensionless constant. Particularly, eq 2 is simply approximated into eq 3 in the

J. Phys. Chem. C, Vol. 114, No. 19, 2010 8723 low and intermediate frequency region. The Bode plot of eq 3 features a short plateau of log Zre due to pore resistance lr/3 at intermediate frequencies (eq 4a) and parallel log Zre and log Zim lines with a slope of -R and the separation of log(tan(Rπ/2)) at low frequencies (from eqs 4b and 5). At f ) 1/2π, log Zim ) log(sin(Rπ/2)) - log K (eq 5). Thus, fitting EIS data according to eq 3 may provide useful information about K, R, and r.

Ze ) (r/K)1/2(j2πf)-R/2 coth(l(rK)1/2(j2πf)R/2)

(2)

Ze ) lr/3 + K-1(2πf)-R cos(Rπ/2) jK-1(2πf)-R sin(Rπ/2) (both intermediate and low frequency) (3) log Zre ≈ log(lr/3)

(intermediate frequency)

(4a)

log Zre ≈ -R log(2πf) + log(cos(Rπ/2)) - logK (low frequency) (4b) log Zim ) -Rlog(2πf) + log(sin(Rπ/2)) - logK (both intermediate and low frequency) (5) Ce ) (2πfZim)-1

(6)

EIS was conducted to investigate the interfacial characteristics of L2-ePt microelectrodes as a function of Rf in PBS at Eoc (+0.35 V vs Ag/AgCl). Panels A and B of Figure 2 exhibit the representative EIS results obtained from a L2-ePt microelectrode with Rf ) 223. It should be noted that the Bode plot was modified by subtracting the bulk solution resistance (Rs) from the original one. As expected, the Nyquist and Bode plots show typical characteristics that are normally observed in porous electrodes. The impedance data were fitted based on eq 3, and the fitting results provide impedance information to be obtained below 1.17 kHz (solid lines in Figure 2B). In a low-frequency range, cpe appears in a nonvertical line in the Nyquist plot (Figure 2A) and two parallel lines with a slope of -R ) -0.85 and a line separation of 0.62 (≈log(tan(0.85 × π/2))) in the Bode plot (Figure 2B). The K value per unit area, which is a constant analogous to area specific Ce, was determined to be 15.7 mF cm-2. At intermediate frequencies, the real impedance branch in the Bode plot does not show an obvious plateau but still depends on frequency. This is because the pore resistance (lr/3) is not overwhelmingly large compared to K-1(2πf)-R cos(Rπ/2) so that eq 3 cannot be assumed to be equal to eq 4a. The pore resistance (lr/3) per unit area was roughly estimated to be 0.015 Ω cm2 from the fitting result based on eq 3. Reportedly, the 3D porous structure is responsible for the low pore resistance of L2-ePt.18 The previous work reported Warburg behavior in the high-frequency region, whose characteristic features were the unit slope curve in the Nyquist plot and the merged curve with R/2 slope in the Bode plot.19 However the presence of Warburg behavior was not clearly found in the frequency range applied in this study (Figure 2, panel A (inset) and panel B). At an ac-frequency of 1.17 kHz, the Ze value of the L2-ePt microelectrode (Rf ) 223) was 2.4 kΩ (0.039 Ω cm2). Most of Ze comes from Zim of 0.035 Ω cm2 and the electrode Ce at 1.17

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Figure 3. (A) Cyclic voltammograms of L2-ePt microelectrode (Aapp ) 1.6 × 10-5 cm2, Rf ) 223) in N2-purged PBS measured at various scan rates of 50, 100, 150, 200, 250, and 300 mV s-1. (B) Linear plot of charging current at -0.15 V vs Ag/AgCl. The Cdl was estimated to be 5.1 mF cm-2 from the slope. (C) Cdl vs Rf.

Figure 2. (A, B) Results of impedance analysis performed with L2ePt microelectrode (Rf ) 223) in N2-purged PBS (pH 7.4) at Eoc (+0.35 V vs Ag/AgCl) and ac-amplitude of 10 mV: (A) Nyquist plot and (B) Bode plot corrected by subtracting Rs of 2900 Ω (0.046 Ω cm2) (open circles, Zre; open squares, Zim) with fitting curves based on the transmission line model (solid lines). (C, D) Impedances and capacitance at 1.17 kHz vs Rf of L2-ePt microelectrodes: (C) Ze (open circles) and Zre (open squares) and (D) Zim (open diamonds) and Ce (filled triangles).

kHz was 3.9 mF cm-2 from eq 6. Considering that the thickness of such a high capacitive electrode is ∼0.9 µm, its volume specific Ce reaches 43 (1.17 kHz). Interfacial Impedance and Capacitance of L2-ePt Microelectrode. Panels C and D of Figure 2 show the effects of Rf on the interfacial impedance and capacitance of L2-ePt micro-

electrodes measured at 1.17 kHz. In Figure 2C, the total electrode impedance (Ze) decreases until Rf reaches ca. 200. Then at higher Rf, the Ze slightly increases with Rf, as the real part of electrode impedance (Zre) becomes the dominant factor in determining Ze. Figure 2C shows that Zre augments for the L2-ePt microelectrode with Rf higher than 200, which is ascribed to the increase in pore resistance. The imaginary part of electrode impedance (Zim) steeply decreases with increasing Rf in the range less than 200, and the decreasing rate slows down in the high Rf range (Figure 2D). The electrode capacitance (Ce) of L2-ePt microelectrodes can be determined from eq 6 (Figure 2D). The Ce value increases at Rf less than 200 and the slope becomes slow at Rf higher than 200. The EIS results in Figure 2C,D indicate that thicker L2-ePt films with higher Rf values do not effectively reduce Ze or raise Ce, and may result in higher Zre. Rf around 223 is an optimum of the L2-ePt film in terms of Ze, Ce, and Zre. Determination of the Cdl of the L2-ePt surface was carried out by dc-voltammetry based on the relation (icc + ica) ) 2CdlV. Figure 3A shows cyclic voltammograms of the L2ePt microelectrode (Rf ) 223) measured at various scan rates in a PBS solution. The potential was cycled between -0.2 and -0.1 V, where only charging current exists. When charging currents were sampled at -0.15 V, a linear relation between charging current and scan rate gave a slope of 0.16 nA mV-1 s and the Cdl was determined to be 5.1 mF cm-2

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TABLE 1: Comparison of L2-ePt with Other Representative Neural Electrode Materials Ze(1 kHz), Ω cm2 titanium nitride CNT 0.50b,6 iridium oxide platinized Pt 1.6b,21 0.112b,12 L2-ePt 0.039b

Ce, mF cm-2 (F cm-3) 18-22 (26-31)a,4 1.6a,10

Cdl, mF cm-2 Qinj(pulse time), (F cm-3) mC cm-2 (µs) 10 (10)6

0.55 (200)8 1-1.6 (1000)10 4 (200)8

5.1 (57)

3 (400)

6(6)b,2 15.7 (174);a 3.9 (43)b

a K value which was determined by fitting of impedance data and supposed to be the same as capacitance. b Capacitance value measured near 1 kHz.

(Figure 3B). In Figure 3C, the Cdl measured for a few L2ePt microelectrodes is linearly dependent on Rf even in the range higher than 200, which is different behavior from what was observed for Ce in Figure 2D. In terms of Ce and Cdl, L2-ePt with Rf ) 223 is superior to any neural electrode materials that have been reported. As aforementioned and summarized in Table 1, L2-ePt (Rf ) 223) shows a Ce (1.17 kHz) of 3.9 mF cm-2 (43 F cm-3) and a Cdl of 5.1 mF cm-2 (57 F cm-3). The Ce of L2-ePt is higher than the 1.6 mF cm-2 reported for MWCNT.10 The volume specific Cdl of L2-ePt is higher than the 10 F cm-3 reported for CNT.6 Although porous titanium nitride produces an extreme Ce of 22 mF cm-2,4 its volume specific Ce is estimated to be only 31 F cm-3 considering its known thickness (7 µm). The Ce of platinized Pt was reported to be 6 mF cm-2,2 which was obtained with 10 µm thick film, and the volume specific Ce of platinized Pt is estimated to be 6 F cm-3. The Ze of L2-ePt (Rf ) 223) is measured to be 2.4 kΩ (0.039 Ω cm2), which is also lower than the reported values, such as 10 kΩ (0.50 Ω cm2) of the CNT microelectrode whose diameter was 80 µm 6 and 112 kΩ (0.112 Ω µm2) of the platinized Pt microelectrode (10 × 10 µm2).12 Charge Injection of L2-ePt Microelectrode. Charge injection limit (Qinj), which is a key characterization standard for stimulation electrodes, was measured by applying biphasic current pulse and monitoring potential response. Figure 4A shows a typical current pulse (height of 40 µA and width of 400 µs) and interpulse delay (20 µs) in open circuit condition. Raw data of potential response for the current pulse indicate the presence of access voltage (Va) in every head and tail of the pulses due to ohmic voltage drop (Figure 4B). The potential excursion curve was corrected by eliminating Va, revealing whether the applied current pulse causes irreversible faradaic reaction or not (Figure 4C). For the biased current pulse of 40 µA and 400 µs, maximum cathodic and anodic potential were -0.20 and +0.57 V vs Ag/AgCl, respectively, which are within the safe stimulating potential range between -0.6 and +0.8 V in PBS. To evaluate the Qinj of the L2-ePt microelectrode, potential response to current pulse was measured with increasing pulse height. In Figure 4D, the potential approaches the safety limit when the height of the current pulse (400 µs) is 120 µA, and Qinj is measured to be 3 mC cm-2. This result tells that the Qinj of the L2-ePt microelectrode is clearly higher than that of titanium nitride (0.55 mC cm-2)8 or CNT (1-1.6 mC cm-2),10 and is comparable with the highest values reported for iridium oxide (4 mC cm-2)8 as summarized in Table 1. Mechanical Stability of L2-ePt. It is well-known that the mechanical instability is the main drawback of platinized Pt as a neural electrode material. To demonstrate how robust the L2ePt electrode is, the platinized Pt and L2-ePt were tested by a

Figure 4. Current pulse applied to a L2-ePt microelectrode (Aapp ) 1.6 × 10-5 cm2, Rf ) 223) in PBS (pH 7.4) and potential responses for Qinj measurement. (A) Cathodic first symmetric biphasic current pulses (40 µA with 400 µs of time duration and 20 µs of delay). Potential excursion responding to the current pulse without (B) and with (C) correction for iR drop. (D) Potential responses of L2-ePt electrode to current pulses of 40, 80, 120, and 160 µA in increasing order. The Qinj is estimated to be 3 mC cm-2.

commercially available scratch tester. Platinized Pt was electroplated by cycling potential between -0.4 and +0.6 V versus Ag/AgCl in HCPA (3.5 wt %) + lead acetate (0.005 wt %) according to a well-known procedure.20 L2-ePt was prepared by electroplating at -0.2 V versus Ag/AgCl. The mechanical stabilities of platinized Pt and L2-ePt were compared by scratching them with diamond indentor under the same normal

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Figure 5. SEM images of platinized Pt (A) and L2-ePt (B) before being scratched and platinized Pt (C) and L2-ePt (D) after being scratched by a normal force of 12.6 mN with a diamond indentor (d ) 4 µm).

force (12.6 mN). Panels A and B of Figure 5 show the SEM images of platinized Pt and L2-ePt as prepared. Platinized Pt showed a powdery surface whereas L2-ePt was electroplated in a compact film. A SEM image of platinized Pt film after it was scratched with a spheroconical diamond indentor (d ) 4 µm) during the scratch test (Figure 5C) shows a drain with irregular edge, which shows that the platinized Pt film was demolished and peeled off along the drain while it was scratched. However, L2-ePt shows smooth drains without a damaged surface (Figure 5D). This reveals that L2-ePt is stable enough to endure the mechanical stress imposed by the indentor during the scratching process. Conclusions 3D nanoporous Pt (L2-ePt) electroplated on each microelectrode of a multielectrode array was tested in vitro for future applications as neural stimulation electrodes and extracellular potential recording electrodes. Compared with representative stimulation or recording electrodes such as titanium nitride, iridium oxide, platinized Pt, and carbon nanotube, the L2-ePt microelectrode with an optimized roughness is excellent in all of the common evaluation characteristics such as electrode impedance, electrode capacitance, and maximum charge injection limit. L2-ePt, which can be easily electroplated in ambient environment and at moderate temperature, has a great advantage over CNT or titanium nitride fabricated by chemical vapor deposition (CVD) or physical vapor deposition (PVD), respectively. L2-ePt remarkably improved the mechanical stability, which is the problem with platinized Pt, indicating L2-ePt is a promising alternative to platinized Pt. Acknowledgment. This research was supported by the Nano/ Bio Science&Technology Program (M10536090001-05N360900110) and by the National Research Foundation of Korea

(NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of the Korea government (No. 20100001637). References and Notes (1) Johnson, J. B. Phys. ReV. 1928, 32, 97. (2) Robinson, D. A. Proc. IEEE 1968, 56, 1065. (3) Cogan, S. F. Annu. ReV. Biomed. Eng. 2008, 10, 275. (4) Norlin, A.; Pan, J.; Leygraf, C. J. Elecrochem. Soc. 2005, 152, J7. (5) James, C. D.; Spence, A. J. H.; Dowell-Mesfin, N. M.; Hussain, R. J.; Smith, K. L.; Craghead, H. G.; Isaacson, M. S.; Shain, W.; Turner, J. N. IEEE Trans. Biomed. Eng. 2004, 51, 1640. (6) Gabay, T.; Ben-David, M.; Kalifa, I.; Sorkin, R.; Abrams, Z. R.; Ben-Jacob, E.; Hanein, Y. Nanotechnology 2007, 18, 035201. (7) Merrill, D. R.; Bikson, M.; Jefferys, J. G. R. J. Neurosci. Methods 2005, 141, 171. (8) Weiland, J. D.; Anderson, D. J. IEEE Trans. Biomed. Eng. 2002, 49, 1574. (9) Weiland, J. D.; Anderson, D. J. IEEE Trans. Biomed. Eng. 2000, 47, 911. (10) Wang, K.; Fishman, H. A.; Dai, H.; Harris, J. S. Nano Lett. 2006, 6, 2043. (11) Cogan, S. F.; Guzelian, A. A.; Agnew, W. F.; Yuen, T. G.; McCreery, D. B. J. Neurosci. Methods 2004, 137, 141. (12) Jun, S. B.; Hynd, M. R.; Smith, K. L.; Song, J. K.; Turner, J. N.; Shain, W.; Kim, S. J. Med. Biol. Eng. Comput. 2007, 45, 1015. (13) Rose, T.; Robblee, L. S. IEEE Trans. Biomed. Eng. 1990, 37, 1118. (14) Attard, G. S.; Bartlett, P. N.; Coleman, N. R. B.; Elliott, J. M.; Owen, J. R.; Wang, J. H. Science 1997, 278, 838. (15) Choi, K.-S.; McFarland, E. W.; Stucky, G. D. AdV. Mater. 2003, 15, 2018. (16) Park, S.; Lee, S. Y.; Boo, H.; Kim, H.-M.; Kim, K.-B.; Kim, H. C.; Song, Y. J.; Chung, T. D. Chem. Mater. 2007, 19, 3373. (17) Trasatti, S.; Petrii, O. A. J. Electroanal. Chem. 1992, 327, 353. (18) Park, S.; Song, Y. J.; Han, J.-H.; Boo, H.; Chung, T. D. Electrochim. Acta 2010, 55, 2029. (19) Elliott, J. M.; Owen, J. R. Phys. Chem. Chem. Phys. 2000, 2, 5653. (20) Feltham, A. M.; Spiro, M. Chem. ReV. 1971, 71, 1770193. (21) Watanabe, T.; Kobayashi, R.; Momiya, K.; Fukushima, T.; Tomita, H.; Sugano, E.; Kurino, H.; Tanaka, T.; Tamai, M.; Koyanagi, M. Jpn. J. Appl. Phys. 2007, 46, 2785.

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