Conductometric discrimination of electro-inactive metal ions using nanoporous electrodes

Electrochimica Acta 56 (2011) 1947–1954

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Conductometric discrimination of electro-inactive metal ions using nanoporous electrodes Je Hyun Bae, Taek Dong Chung ∗,1 Department of Chemistry, Seoul National University, Seoul 151-747, Republic of Korea

a r t i c l e

i n f o

Article history: Received 15 September 2010 Received in revised form 30 November 2010 Accepted 1 December 2010 Available online 8 December 2010 Keywords: Nanoporous Impedance Metal ion Action potential Conductometry

a b s t r a c t We investigated conductometric analysis of electro-inactive metal ions at high concentration based on nanoporous electrodes by electrochemical impedance spectroscopy (EIS). The three dimensional interconnected nanoporous Pt (L2 -ePt) was found to enable significantly sensitive and selective conductometric detection of alkali and alkaline earth metal ions of high concentration at low frequency without any additional surface modification, which can be hardly done by flat Pt. The extremely large surface area of L2 -ePt remarkably suppressed the electrode impedance and the pore effect was additional positive contribution to selective ion sensing by conductometry at low frequency. Importantly, L2 -ePt allowed recognition of fractional ratio reversal of Na+ to K+ ions in a mixed solution at physiological concentration maintaining the constant total ionic strength. The results suggest the possibility of real time extracellular monitoring of instantaneous ion exchange near ion channels of a cell membrane such as action potential propagation along axons in a neuronal system. © 2010 Elsevier Ltd. All rights reserved.

1. Introduction Most of the aqueous media in nature such as sea water, cytosol, blood, and body fluid contain high concentration of ions. Ion concentration and composition in living organisms may instantly vary in the immediate vicinity of the ion channels of cell membranes. For example, the propagation of action potential along the axon in a neuron accompanies asymmetric ion current through ion channels, leading to immediate changes in local ion concentration. In case of mammalian neurons, the intracellular concentrations of K+ and Na+ ions are 140 mM and 5–15 mM at resting states while the extracellular concentrations are 5 mM and 145 mM, respectively [1]. As recent neuroscience research and related technologies increasingly require in situ monitoring of extracellular signals without staining [2–5], prompt recognition of variation in ion composition at high ionic strength becomes more demanding. In this regard, fluorescence techniques and ion selective electrodes (ISEs) are normally considered. Although fluorescence measurements are most widely used and provide tremendous information, they need chemical additives of fluorescent dyes to stain, which may affect the biological system. In addition, it should be noted that fluorescent techniques are too sensitive to detect abundant ions at high concentration like Na+ and K+ ions. Ion selective electrodes (ISEs) are a

∗ Corresponding author. Tel.: +82 2 880 4362; fax: +82 2 887 4354. E-mail address: [email protected] (T.D. Chung). 1 ISE member. 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.12.006

representative approach with attractive features of nondestructive and ion-selective analytical powers, but problematic in terms of sensitivity, dynamic range, response time, etc. More significantly, instantaneous variation of composition of Na+ /K+ /Cl− ions with high spatial resolution is hardly detected by micro or sub micro ISE array, which is not easy to fabricate in practice and does not give stable potentiometric signals either [6,7]. Besides, microfabricated FET array is a reasonable option to take for extracellular signal monitoring [8]. Recently, silicon nano wire-FET array was applied to monitor the electrical neuronal signal propagation from single mammalian neurons [9]. However, these methods have insufficient ion selectivity and the sensitivity is significantly influenced by the buffer electrolyte concentration [10]. Not only for neuronal signal monitoring but for general analysis of electrochemically inactive species in aqueous phase, conductometry has been widely regarded as an inexpensive, nondestructive and simple conventional method [11–13]. As chemical or biological monitoring within a limited volume is increasingly demanding, high resolution conductometric analysis of ions based on miniaturized electrode array attracts keen attention. For instance, microfluidic technology needs smaller and more sensitive conductometric detectors to extend its applications. As a result, electrodes for biological process and analysis are ceaselessly miniaturized and integrated on a chip [14–16]. In conductometry, ions are discerned by equivalent conductance which is a function of intrinsic mobility in the solution [17,18]. As the electrodes for conductometry are reduced in size, the impedance at the electrode–solution interface, electrode

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J.H. Bae, T.D. Chung / Electrochimica Acta 56 (2011) 1947–1954

impedance called in this report, increases correspondingly. Higher electrode impedance leads to smaller contribution of the solution resistance to the total impedance, which makes less sensitive and selective analysis of ions. This problem becomes worse in the higher ion concentration that results in solution resistance as well [12]. That is why maximum concentration of the dynamic range in conductometric detection does not exceed several tens mM [19]. Although contribution of solution resistance to total impedance can be raised by using a high frequency, conductometry in high ion concentration is inaccurate due to a few problems such as impedance artifacts [20] and interference by the electrodes that are not in contact with the solution [21]. Specifically, miniaturized electrode array on a chip has inevitably thin insulating layer with relatively wide area, which may not be assumed to be perfect insulator. Cross-talk among the electrodes in a cellular medium upon high frequency input signals is a serious problem in practice caused by such a condition. Platinization of electrode surface like a Pt black is a well known treatment to help reduce the electrode impedance with the aid of enlarged electrode surface area and better electrode material, Pt, that allows relatively high exchange current [13,22]. Higher surface-to-volume ratio leads to lower impedance at the electrode surface and larger contribution of the solution resistance to the total impedance [23]. As apparent size of the electrode reduces, porosity of the electrode surface becomes crucial to effectively diminish the impedance [24]. For microelectrodes, only nanoporous electrode surface can make significant contribution. Therefore, the presence of nanopores and high roughness factor of the electrode surface have a great deal of importance, especially for the electrodes on a few micro or sub micro meter scale. In recent years, there has been growing interest in the nanoporous Pt as an electrode material, for which a few novel fabrication methods were developed [25–30]. Particularly, 3 dimensional (3D) interconnected nanoporous Pt, which is denoted by L2 -ePt, possesses the 3D interstitial pores of 1–3 nm in width as well as a high roughness factor [27]. Considering characteristic thickness of the electrical double layer [31], L2 -ePt is regarded as the ultimate electrode with the highest surface-to-volume ratio that can suppress the electrode impedance to the lowest limit. Harnessing extremely high surface roughness and extraordinary spatial environment raising collision frequency inside the pores, L2 -ePt exhibits remarkably enhanced electrocatalytic activity that allows very sensitive detection of electroactive species such as H2 O2 [32]. Therefore, it is no wonder that much attention has been paid to its attractive applications such as electrochemical sensors [33–35], catalyst [36,37], and energy device materials [38,39]. Neural recording and stimulation electrodes are another class of such electrodes with extremely low electrode impedance. Compared with the conventional electrodes for this purpose like titanium nitride, platinized Pt, and carbon nanotube, L2 -ePt is reportedly excellent in electrode impedance, electrode capacitance, and maximum charge injection limit [23]. In this work, we realize conductometric detection of alkali metal and alkaline earth metal ions at low frequency at high concentration by employing a nanoporous Pt of L2 -ePt. Following theoretical impedance modeling of the proposed system, we systematically examine the experimental results of electrochemical impedance spectroscopy (EIS) for conductometric ion analysis in comparison with a flat electrode that has same apparent electrode area. 2. Experimental 2.1. Reagents All chemicals including hydrogen hexachloroplatinate hydrate (KOJIMA chemicals, JAPAN), t-octylphenoxypolyethoxyethanol

(Triton® X-100, Sigma), sodium chloride (DAE JUNG, Korea), sulfuric acid (Sigma), lithium perchlorate (95+%, A.C.S reagent, Sigma), sodium perchlorate mono-hydrate (98%, A.C.S reagent, Sigma), potassium perchlorate (99+%, A.C.S reagent, Sigma), magnesium perchlorate (A.C.S reagent, Sigma) and calcium perchlorate tetrahydrate (99%, Sigma) were used without further purification. All the aqueous solutions in this experiment were prepared with ultrapure deionized water produced by NANOpure (Barnstead). 2.2. Electrodeposition of L2 -ePt The nanoporous Pt denoted by L2 -ePt was prepared by electroplating of Pt in reverse micelle solution as described in our previous report [27]. Hydrogen hexachloroplatinate hydrate (5 wt%), 0.3 M sodium chloride (45 wt%), and Triton X-100 (50 wt%) were mixed and heated to 60 ◦ C. The mixture as made was transparent and homogeneous. The temperature of the mixture solution was maintained around 40 ◦ C using a thermostat. L2 -ePt was electrochemically deposited on a Pt disk electrode (d = 1 mm, and Ag = 0.00785 cm2 ) at −0.1 V vs. Ag/AgCl. The resulting L2 -ePt electrode was in distilled water for 1 day to extract the Triton X-100 and this procedure was repeated 3–4 times. It follows that the electrode was electrochemically cleaned by cycling potential between +0.68 and −0.72 V vs. Hg/Hg2 SO4 in 1 M sulfuric acid until reproducibly identical cyclic voltammograms were obtained. The surface roughness of the L2 -ePt was determined by measuring the area under the hydrogen adsorption peak of the cyclic voltammogram (scan rate of 0.2 V s−1 ) in 1 M H2 SO4 , using a conversion factor of 210 ␮C cm−2 [40]. 2.3. Electrochemical measurements All electrochemical measurements were performed in a three electrode system, using Model CHI660 (CH Instruments) as electrochemical analyzer. Hg/Hg2 SO4 (saturated K2 SO4 , RE-2C, BAS Inc.) and Pt wire (dia. 0.5 mm, Sigma) were used as reference electrode and counter electrode, respectively. The Pt disk electrode (dia. 1 mm) was used as a flat working electrode to compare with L2 -ePt. All experiments were carried out at room temperature. The solution was purged with high purity nitrogen gas for 15 min prior to use and nitrogen environment was maintained over the solution throughout the experiments. For electrochemical impedance spectroscopy (EIS), a programmed ac input with 10 mV amplitude over a frequency range from 1 Hz to 10 kHz was superimposed on the dc potential, −0.5 V vs. Hg/Hg2 SO4 where no faradaic reaction occurs. 3. Theory In the absence of electroactive species in solution and specific adsorption, equivalent circuit is assumed to be expressed by a serial combination of solution resistance, Rs , and double layer capacitance, Cdl . This is the case for ideal double layer on an electrode. In real systems, however, the curve in a Nyquist plot as a function of frequency may deviate significantly from the ideal vertical linear plot that is simply expressed as 1/jwC. Such a non-ideal behavior, which usually comes from time constant dispersion at solid electrodes, is reportedly caused by roughened surface, non-uniform polycrystallinity, anion adsorption, and so on [41,42]. The derivation from the linear behavior assuming a sing pure capacitance would be empirically represented by a constant phase element (CPE) in equivalent electrical circuit. Considering this factor, the equivalent circuit is normally presented by a solution resistance, Rs , and a CPE in series. The total impedance is given by Z = Rs + Ze ,

Ze =

1 Q (jw)

˛,

w = 2f

(1)

J.H. Bae, T.D. Chung / Electrochimica Acta 56 (2011) 1947–1954

where Rs , Ze , Q, and ˛ stand for solution resistance, electrode impedance, a constant containing the double-layer capacitance, and a function of the phase angle  with the relation of ˛ = /(/2), respectively. The exponent ˛ is mostly in the range 0.9–0.99 depending on the electrode material. Q is a constant which would equal the capacitance at ˛ = 1. When ˛ = / 1, the impedometric behavior is governed by the surface heterogeneity. Ze as a function of the CPE can be expressed as follows. Ze = Zre,e + Zim,e =



1 cos Qw˛

 ˛  2

− j sin

 ˛ 

(2)

2

where Zre,e and Zim,e are the real and imaginary parts of the electrode impedance, respectively. The real part of Ze is attributed to non-ideality of the capacitance in the electrochemical system. The magnitude of Z, that is, |Z| is given by





|Z| =

2 Zre

2 + Zim

Rs2 +

=

2Rs cos Qw˛

 ˛  2

1 Q 2 w2˛

+

(3)

The relationship between CPE parameters and the interfacial capacitance is given by [43] (1−˛) 1/˛

C = [QRs

]

(4)

Assuming that current density across the surface of a planar electrode is uniform, the solution resistance is given by [17] L Ag

Rs =

(5)

where L is the distance between the electrodes. Ag and  are the geometric area of the electrode and the conductivity of solution, respectively. Capacitance is expressed by C=

εε0 Ar d

(6)

where εε0 and d are the electric permittivity of the medium and the interpolate spacing, respectively. Ar denotes the real area of the electrode assuming that any pore on the surface is larger than the characteristic thickness of the electric double layer [31]. Typical ranges of capacitance values lie between 1 and 50 ␮F cm−2 [42]. By substituting Eqs. (4)–(6) into Eq. (3), we obtain the equation as follows.



|Z| =

L Ag

2

+2

L Ag

2−˛ 

d εε0 Ar

˛ 1 w

cos ˛

significantly different from each other, up to 100 times (roughness factor, fR ) while the apparent geometric areas were the same. The enlarged surface area of L2 -ePt substantially reduced the electrode impedance so as to raise the relative contribution of solution resistance to the total impedance. In addition to the surface area effect, nanoporous electrode provides another factor that should be concerned. Micro- or mesoporous materials like L2 -ePt have numerous pores with nanometer scale in diameter. The pore diameter is comparable to the characteristic thickness of the electrical double layer so called Debye length (−1 ), so that the electrochemically effective area of the electrode surface is not necessarily identical to the real surface area at a low ionic strength [31]. Gouy-Chapman theory states  = (3.29 × 107 )zC*1/2 where C* is the bulk z:z electrolyte concentration in mol L−1 and  is given in cm−1 . Theoretically, the Debye length is about 1.5 nm at the electrolyte concentration of 4.1 × 10−2 M. Since L2 -ePt has narrow interstitial pores of 1–3 nm in diameter [27], the Debye length is similar or longer than the pore radius. This means that the electrochemically effective interface, which corresponds to the equi-potential surface in the electric double layer, does not exactly follow the porous geometry of L2 -ePt electrode. In other words, L2 -ePt may fail to reduce the electrode impedance as much as its roughness factor, that is the surface enlargement, at an ionic strength lower than 0.04 M. Based on this consideration, we can learn that L2 -ePt will be much more effective at a high ion concentration through its low electrode impedance. The sensitive admittance results from L2 -ePt shown in Fig. 1 were ascribed to the low electrode impedance, which leads to correspondingly low total impedance. And the selective responses to various aqueous ions came from the large contribution of solution resistance to the total impedance, as a result of the electrode impedance reduced by not only the enlarged surface area but also high ion concentration ensuring the sufficiently thin electric double layer compared with the pore radius of L2 -ePt. For conductometry, the ion concentrations higher than 0.1 M are often encountered everywhere, as confirmed by noting that sea water and physiological media belong to the samples with this condition. However, such range of ion concentration has been a tough condition for conventional conductometry because higher ionic strength means lower

 ˛  L 2−2˛  d 2˛ 1 2

+

Ag

The experimental results were analyzed and interpreted based on Eq. (7). 4. Results and discussion 4.1. Impedance of aqueous alkali and alkaline earth metal ions Fig. 1 shows the impedometric results from 0.01 to 0.5 M alkali and alkaline earth metal ions. We monitored the admittance that was straightforwardly converted from the measured impedance and proportional to concentration of ions. For this experiment, 1 kHz was selected as the frequency, which is the typically employed for conductometry. As clearly and quickly noted from Fig. 1, the nanoporous Pt (L2 -ePt) was superior to flat Pt in sensitivity as well as selectivity in terms of conductometric recognition of ions. This result is well consistent with what was expected. In conventional conductometry, electro-inactive ions such as alkali and alkaline earth metal ions should be discerned via the differences among their own ion mobilities. Therefore, it is obvious that larger contribution of solution resistance, which is a function of ion mobilities, results in better selectivity. In this experiment, the real surface areas of the nanoporous and flat Pt electrodes were

1949

εε0 Ar

w2˛

(7)

contribution of solution resistance to the total impedance. That is why higher frequency is required to obtain reasonable ion sensitivity and selectivity for conductometry at high ionic strength. In these respects, the results in Fig. 1 suggest nanoporous electrode like L2 -ePt as a promising way to enable both sensitive and selective conductometry in high ion concentration without increasing frequency. The dc offset in Fig. 1 was −0.5 V vs. Hg/Hg2 SO4 . 4.2. ac frequency and roughness factor Suppose that there are two different solutions of alkali or alkaline earth metal ions and we apply ac inputs with certain frequency to measure the total impedance of each sample. The ratio of the difference between the two absolute total impedances to the average, ı|Z|/[|Z|] ≡ , is a good index of how discriminative the ion ac conductometry is. At a given frequency, higher  value indicates better resolution in distinguishing one ion to the other.  values for two different solutions, for example aqueous NaClO4 and KClO4 , can be calculated from Eq. (7) and plotted as a function of the input ac frequency as shown in Fig. 2a. As the frequency increases, the respective curve in Fig. 2a undergoes a transition to the upper

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plateau at its own characteristic frequency regime. The plateau in the high frequency regime can be quantitatively estimated from Eq. (7), which is reduced to |Z| = L/Ag = Rs . As a consequence,  is expressed by =

2 − 1 ıRs =2 1 + 2 [Rs ]

(8)

In this ultimate condition, the total impedance is completely determined by the solution resistance so that the resolution, iondiscriminating power, reaches a maximum. However, exceedingly high frequency is not favorable for ion conductometry. Negligible electrode impedance due to high frequency significantly reduces the total impedance, which may become too low to be reliably measured. In particular, the total impedance is seriously lowered in high ion concentration because the solution resistance substantially drops as well. Moreover, high frequency condition imposes another practical restriction that electronic cross-talk should be concerned during the design and fabrication of the conductometric devices. This may be more important concern when such a conductometer should be integrated with other electroanalytical cells for advanced systems, e.g. multiplex detection or sequential processes on a chip-based analyzer. On the other hand, the total impedance at sufficiently low frequency is overwhelmingly regulated by the electrode impedance rather than the solution resistance. Eq. (7) reduces to

|Z| =

L Ag

1−˛ 

 

d εε0 Ar

˛ 1 w˛

=

1 = |Ze | Qw˛

(9)

where Ze  is the magnitude of electrode impedance. In Eq. (9), there is no parameter dependent on ion concentration and composition but the solution conductivity, . Actually, ion concentration and composition may modify the capacitance, in which dielectric constant is involved. However, dielectric constant is known to be almost independent of ion concentration and composition as long as concentration is lower than 1 M [17,44]. Therefore,  is expressed by Fig. 1. Admittance, Y, as a function of concentration for a comparison between the nanoporous and flat electrode. Impedometric results of (a) alkali metal and (b) alkaline earth metal ions. The admittances for K+ ion were measured up to 0.1 M due to the limited solubility of KClO4 . The frequency was 1 kHz which is a typical frequency for conductometry. The dc offset potential and the amplitude of ac input signals were −0.5 V (vs. Hg/Hg2 SO4 ) and 10 mV, respectively. All measurements were repeated 5 times under identical condition and average values were plotted with error bars of corresponding standard deviations.

=

˛−1 − 2˛−1 ı|Ze | = 2 1˛−1 [|Ze |]  + ˛−1 1

(10)

2

In ideal electrode system, ˛ equal to one so that Eq. (10) should be zero. A value of Eq. (8) is always larger than that of Eq. (10) because ˛ is usually in range 0.9–0.99. The geometric area (Ag ) and real area (Ar ) of a flat electrode should be almost same. On the other hand, porous electrodes in this study have fR values higher than 30. fR can be easily controlled by experimental parameters such as electroplating time. Fig. 2 shows the theoretical (Eq. (7)) and

Fig. 2. Relationship between ı|Z|/[|Z|] and frequency for a variety of roughness factors. Theoretical (a) and experimental (b) results in 0.1 M NaClO4 and KClO4 , respectively. At lower frequency, larger fR provides better selectivity for recognizing metal ions. The dc offset potential and the amplitude of ac input signals were −0.5 V (vs. Hg/Hg2 SO4 ) and 10 mV, respectively.

J.H. Bae, T.D. Chung / Electrochimica Acta 56 (2011) 1947–1954

1951

Fig. 3. Bode plots from a L2 -ePt in 0.1 M various alkali metal ions without correction (a) and with correction by the solution resistances (b). The data in (b) were fitted based on Eq. (12) (solid line). Bode plots from a flat Pt in 0.1 M various alkali metal ions without correction (c) and with correction by the solution resistances (d). The dc offset potential and the amplitude of ac input signals were −0.5 V (vs. Hg/Hg2 SO4 ) and 10 mV, respectively.

experimental influences of fR in 0.1 M NaClO4 and KClO4 solutions. Solution conductivity, , is expressed by  = F/1000

|zi |ui Ci

i

where F is the Faraday constant (96,485 C mol−1 ), |zi | is a charge of species i (1), ui is mobility of species i (Na+ : 5.193 × 10−4 , K+ : 7.619 × 10−4 , ClO4 − : 7.05 × 10−4 (cm2 s−1 V−1 )) and Ci is concentration of species i (0.1 M). We set other parameters for theoretical estimation as follows; L (1 cm), ˛ (0.95), εε0 /d (10 ␮F cm−2 ), Ag (0.00785 cm2 ) and Ar (fR × Ag ). The experimental data of  were converted from the impedance data that were obtained in respective NaClO4 and KClO4 solutions. The calculated results in Fig. 2a show that the frequency where  undergoes transition from the lower limit to upper plateau keeps shifting to lower region as fR rises. This is consistent with what is expected. Larger fR brings about more contribution of the solution resistance to the total impedance through enlarged Ar . In this regard, the experimental results in Fig. 2b are largely in accordance with theoretical prediction. However, the  values at the low frequency region significantly deviate from the calculated. And the transition frequencies of the experimental curves were found in notably higher frequency region than theoretical prediction. This behavior is not surprising but what was expected. First of all, the parameters employed for calculation were not exactly relevant to the experimental system. For example, the ui value is the information assuming infinitely diluted aqueous solution at 25 ◦ C. More significantly, the electrochemical cell employed in this study is a conventional three electrode system that has a working electrode with the small area and a counter electrode with the excessively wide area. This is appropriate for examination of what is happening at the nanoporous electrode, but provides ill-defined cross sectional area of the solution that should be substituted for the area in the term for solution resistance. The experimental cross sectional area of the solution is predicted to be much larger than the working electrode, resulting in the differ-

ences observed in Fig. 2a and b. Nonetheless, the overall trend can be compared and discussed between the theoretical and experimental impedance spectra in Fig. 2, which is enough to probe the effect of nanoporous electrode structure. At nanoporous Pt, the negative values of  appeared in low frequency region, indicating that the electrode impedance for Na+ ion was lower than that for K+ ion. This is interesting behavior in that the relative electrode impedance for the two ions is opposite to that in solution resistance. Such an impedance reversal contrary to ion mobility order is unusual phenomenon that is observed only in extremely narrow nanopores. The origin of the negative values of  in porous Pt has not been clearly understood yet and the detailed study is in progress. 4.3. Geometric effect of porous electrodes The nanopore effect on selective ion detection was investigated by electrochemical impedance spectroscopy (EIS). Transmission line model for the porous electrode was previously applied to nanoporous Pt electrode [45,46]. According to this work, the electrode impedance of the porous electrode is expressed by



Ze =

r Q (jw)

˛ coth(l

˛

rQ (jw) )

(11)

where r is electrolyte resistance per unit length in the pores, l is the film thickness, Q and ˛ are the same as the notations in the present report. In high frequency region, real and imaginary parts are merged into a line with a slope of −˛. Approximate expression of Eq. (11) in the low frequency region is as follows: Ze =

lr 1 cos + 3 Qw˛

 ˛  2

−

j sin Qw˛

 ˛  2

(12)

Although this equation results from approximation in low frequency limit, a short plateau due to pore resistance as the constant

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J.H. Bae, T.D. Chung / Electrochimica Acta 56 (2011) 1947–1954

Fig. 4. Relationship between ı|Z|/[|Z|] and frequency as a function of concentration for Na+ and K+ ions with ClO4 − . Theoretical (a) and experimental (b) results were obtained from a porous Pt (squares) and a flat Pt (triangle). Arrows indicate the parallel shifts as the concentration augments ten times (from 0.1 M to 1 M for theoretical analysis and from 0.01 M to 0.1 M for experimental data). The dc offset potential and the amplitude of ac input signals were −0.5 V (vs. Hg/Hg2 SO4 ) and 10 mV, respectively.

term, lr/3 appears in the intermediate frequency region. In the low frequency range, the real and imaginary curves are in parallel with a slope of −˛ and separated by log(tan(˛/2)). In Eq. (12), solution resistance is omitted so that measured impedance including the solution resistance can be written as Z = Rs +

lr 1 cos + 3 Qw˛

 ˛  2

−

j sin Qw˛

 ˛ 

(13)

2

Fig. 3a and b shows the Bode plots obtained from L2 -ePt for a series of alkali metal cations. Regardless of the sort of metal ions, the curves in the Bode plots exhibit similar behavior, which are a plateau in intermediate frequency region and two parallel lines between log Zre and log Zim with −˛ slope in the low frequency regime. The short plateau in Fig. 3b is characteristic aspect that is observed at nanoporous electrodes [45,46]. The corrected Zre by subtracting Rs , that is Zre,e , corresponds to the component of constant phase element (CPE) and resistance independent of frequency in the Zre , real part of the electrode impedance. Unlike the nanoporous electrode, the plateau of log Zre is even shorter at the flat electrode due to large electrode impedance as shown in Fig. 3c. Subtracting Rs yields the curves in Fig. 3d, in which the plateau completely disappears. This critical difference comes from the pore resistance in the nanoporous electrode. The pore resistance and ˛ were determined by fitting the experimental data on the basis of the transmission line model. The solution resistance was obtained from the extrapolation in the high frequency data of flat Pt. Table 1 presents that pore resistance of nanoporous electrode is another factor for ion discrimination, in addition to solution resistance. Li+ , Na+ , and K+ ions exhibit characteristic pore resistances in decreasing order. This tendency is consistent with the ion mobilities in the bulk solution, and thus in good accordance with the solution resistance. Alkaline earth metal ions exhibit the same trend. 4.4. Effect of ion concentration and mixed solutions From the hitherto study, we got the lesson that alkali and alkaline earth metal ions at high concentration (>0.1 M) can be selectively detected by conductometry at low frequency by Table 1 Impedometric parameters measured from L2 -ePt.

2

Solution resistance, Rs ( cm ) Pore resistance ( cm2 ) CPE exponent (˛)

Li+

Na+

K+

Mg2+

Ca2+

4.22 0.21 0.94

3.99 0.19 0.94

3.16 0.11 0.95

2.38 0.12 0.91

1.99 0.09 0.94

employing nanoporous electrode. This is interesting in that conductometric ion detection in high ionic strength at low frequency suggests the way to monitor instantaneous change in ion concentration and composition in non-destructive manner in natural environments such as physiological fluids and cell media. For extracellular monitoring propagation of axon potential, for instance, very small electrode is required to be placed as closely the ion channels as possible and to quickly detect composition reversal of Na+ and K+ ions. Smaller geometric area of the electrode makes larger contribution of the electrode impedance to the total impedance, which becomes less sensitive to the local change in ion composition near the electrode surface. Nanoporous electrode provides huge surface area that remarkably reduces the total impedance. The pore resistance helps further us to discriminate the ions. As a result, it is expected that even very small nanoporous electrode would be available for conductometric ion detection at a sufficiently low frequency that corresponds to typical range of frequency adopted in conventional measurements. Importantly, patterned and miniaturized electrode array on a chip must be thin and relatively wide area of insulating cover layer that may give rise to serious problems such as imperfect insulation of passivated electrode [21] and cross-talk in a cellular medium when a high frequency is applied. Considering the recent trend that electrodes are ceaselessly miniaturized and integrated on a chip for biological process and analysis, this issue is increasingly crucial. Therefore, nanoporous electrode, which allows conductometry at substantially lower frequency, proposes new exciting applications based on conventional conductometry by just switching the electrode material. Fig. 4 shows theoretical and experimental data of the difference in total impedance, ı|Z|/[|Z|], as the concentrations of Na+ and K+ ions with ClO4 − were increased to ten times (from 0.1 M to 1 M for theoretical analysis and from 0.01 M to 0.1 M for experimental data). As expected from the general behavior in conventional conductometry, the transition to saturated maximum plateau shifted to higher frequency range when the concentrations of Na+ and K+ ions were augmented. This means that higher frequency is needed to detect any change in fractional ratio of Na+ and K+ ions at higher concentrations. As we have seen through ı|Z|/[|Z|], the difference in the total impedance successfully allows us to discriminate Na+ and K+ ions of the same concentration. This result indicates that the total impedance would be sensitive to the fractional ratio of Na+ and K+ ions under the condition of a constant total concentration of the two ions (the sum of the ions is maintained to be constant). We checked if changes in the ratio of Na+ to K+ ions in a mixed solution at the same ionic strength can be detected. The results in Fig. 5 were obtained from mixed solutions of 0.1 M NaClO4 and 0.01 M

J.H. Bae, T.D. Chung / Electrochimica Acta 56 (2011) 1947–1954

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iting electrode impedance imposed by nanoporous Pt suggests an opportunity to miniaturize electrode array sufficiently, even down to the nano scale, for non-destructive real time monitoring of extracellular neuronal signals. Fabrication and biological applications of nanoporous Pt nano array for in situ extracellular monitoring of action potential in a living neuronal system are in due course. Acknowledgements

Fig. 5. Admittance, Y, that detected the changes in the ratio of Na+ to K+ ions in the mixed solution at the same ionic strength. The dc offset potential, the frequency, and the amplitude of ac input signals were −0.5 V (vs. Hg/Hg2 SO4 ), 1 kHz, and 10 mV, respectively. K+ :Na+ = 1:10 means the mixed solution composed of KClO4 0.01 M and NaClO4 0.1 M. Unlike bare electrode, L2 -ePt sensitively detected a change in the ion composition. All measurements were repeated 5 times under identical condition and average values were plotted with error bars of corresponding standard deviations.

KClO4 or 0.01 M NaClO4 and 0.1 M KClO4 . At a typical frequency for conductometry (1 kHz), L2 -ePt sensitively detects change of the ion composition while flat Pt cannot does. This sensitivity of nanoporous Pt to fractional ratio of ions benefits from enlarged surface area and pore resistance as discussed above. 5. Conclusions We demonstrated that nanoporous Pt is absolutely superior to flat Pt in terms of both sensitivity and selectivity through theoretical and experimental approaches. The total impedance, or the admittance, from a nanoporous Pt, L2 -ePt in this work, allows us to discriminate between alkali metal and alkaline earth metal ions in solution, at low frequency and high ionic strength. The principal origins underlying this behavior are summarized into extremely enlarged surface area and pore resistance inside the nanoporous electrode. The former markedly reduces electrode impedance and thereby the total impedance or admittance responds more sensitively to ion mobility and concentration. The roughness factor of L2 -ePt employed in the present study is close to the maximum taking account of the thickness of electric double layer that is directly linked to electrochemically effective surface area. Therefore, L2 -ePt provides the lowest electrode impedance that could be achieved by enlarging the surface area of electrode. The latter is additional contribution to discriminative ion sensing, leading to successful conductometric detection of ion composition. Based on the transmission line model, we confirmed that the pore resistance played a role of electrochemical inactive ion recognition and its tendency was consistent with what had been expected from the order of ion mobility. The key achievement of this work is that the ion-selective conductometric detection at high ion concentration was shown to be enabled in the range of conventional frequency, which is typically adopted for conductometry at low ion concentration, by just replacing flat electrode with nanoporous one, L2 -ePt with extremely miniaturized and 3-dimensionally interconnected nanopores. The proposed technique provides significant advantages in terms of in situ neuro signal monitoring. Compared with the conventional methods, it suggests direct, nondestructive, and inexpensive way to look into neural systems. The nanoporous Pt electrode allowed us to detect fractional ratio reversal in Na+ and K+ ions at the constant ionic strength, which is the condition similar to what happens near ion channels during action potential propagation along the axon of a neuron. The lim-

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