Enhanced Potentiometry by Metallic Nanoparticles - Analytical

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Enhanced Potentiometry by Metallic Nanoparticles T. Noyhouzer, I. Valdinger, and D. Mandler* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel S Supporting Information *

ABSTRACT: Measuring the oxidation−reduction potential (Eh) requires an interface that is not selective toward specific species but exchanges electrons with all redox couples in the solution. Sluggish electron transfer (ET) kinetics with the species will not reflect the “true” Eh of the solution. Here, we present a novel approach by which adsorbed metal nanoparticles (NPs) are used for enhancing ET exchange rates between redox species and electrode surface and therefore affect significantly the measurement of the open circuit potential (OCP) and cyclic voltammetry (CV). The OCP and CV of various organic and inorganic species such as l-dopa, dopac, iron(II), and iodide are measured by bare stainless steel and by stainless steel modified by either Pt or Au NPs. We study the effect of the surface coverage of the stainless steel surface by NPs on the electrochemical response. Moreover, the stainless steel electrode was modified simultaneously by Au and Pt nanoparticles. This improved concurrently the stainless steel response (CV and potentiometry) toward two different species; l-dopa, which shows fast electron transfer on Pt, and catechol, which exhibits fast electron transfer on Au. We believe that this approach could be a first step toward developing a superior electrode for measuring the “true” Eh of complex aquatic systems.

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Nernstian basis.7 Peiffer claimed that the measured potential is a mixed potential, which represents the interaction of all the effective redox couples with the electrode surface, which depends on both the redox species as well as the electrode material.8 In other words, the redox potential of a solution consisting of a few redox couples depends on their exchange rate of ET with the electrode, which affects the time the system reaches equilibrium. Hence, the open circuit potential (OCP) measured by potentiometry does not necessarily reflect the real concentration ratio between the various redox species but is most likely governed by those species that exhibit facile ET with the probe electrode. This issue can be solved by three approaches: choosing an electrode material that exhibits facile electron transfer with all redox species in the solution, modifying the electrode surface (by self-assembled monolayers (SAMs), polymers, etc.)9−11 to facilitate ET or by adding an electron mediator12 which enhances ET between all redox species and the electrode surface. The latter concept was demonstrated by us recently for the determination of the redox potential in milk.13 Adsorbed nanoparticles (NPs) can improve dramatically the potentiometry response. Nanoparticles have been used for potentiometry in the past decade, and although they hold a great promise in this field, their application in ion selective electrodes is still not comprehensive. Compton at el.14,15

otentiometry is among the most common electrochemical measurements used not only in electrochemistry but also in numerous fields such as medicine, biology, and agriculture.1 pH determination2 is by far the most frequent potentiometric measurement, which is applied in all sorts of aqueous media including, for example, blood3 and milk.4 Since no current flows externally, potentiometry almost does not alter the concentration of the species in close proximity of the electrode, which makes it a nondestructive, nonpolluting, and straightforward method. Ion selective electrodes (ISE) are based on potentiometry where the interfacial potential is governed predominantly by specific ions. On the other hand, measuring the oxidation−reduction potential (Eh) requires an interface that is not selective toward specific species but exchanges electrons with all redox couples in the solution. The redox potential of an aquatic system is usually determined by equilibrating the Fermi level of an inert electrode, e.g., platinum, with the reversible Nernst potential of the dissolved species. The latter is controlled by the concentration ratio between the oxidized and reduced states.5 Interpretation of the measured potential is not always straightforward particularly in measuring mixtures of different redox couples. The Eh that is measured should in principle reflect the total contribution of all redox couples. Yet, this is not always the case due to difference in the rates of electron transfer (ET) between the redox species and the electrode surface. This problem is well-known in environmental science.6 For example, of the many Eh measurements reported for natural waters, only a few have been successfully interpreted on a quantitative, © XXXX American Chemical Society

Received: June 11, 2013 Accepted: July 19, 2013

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explained the dramatic influence NPs can have on the field of catalysis and voltametric measurements. They concluded that the size, shape, and material of the NPs will influence ET kinetics in voltammetry. Most of the work in this field involves the incorporation of gold nanoparticals (GNP) in a polymeric membrane or film16−22 or as substrate for SAMs.23−27 Unlike the use of metal nanoparticles to improve the membrane performance toward specific interaction,28−31 carbon nanotubes (CNT) have mainly been used for field effect transistors (FET) or as substrates for aptamers.32−35 Kalanur36 showed that the use of TiO2 nanoparticles improve ET kinetics, while Tratnyek37 found that nano zerovalent iron, which was spontaneously adsorbed on a GCE surface, caused the electrode to behave in the same manner as an iron electrode. Here, we present an approach by which adsorbed metal NPs are used for enhancing ET exchange rates between redox species and the electrode surface. Moreover, we demonstrate the use of an electrode modified simultaneously by two different metal nanoparticles for the potentiometry and voltammetric measurements of two redox systems comprising different kinetics. The concept is schematically shown in Figure 1. The system is based on an electrode that exhibits sluggish ET

assembled by sealing a glassy carbon rod (2 mm diameter, purchased from Atomergic Chemetals, NY, USA) in Teflon sheath under pressure. Pt wire and Ag/AgCl (KCl sat’) were used as a counter and reference electrodes, respectively. Therefore, all potentials are quoted vs this reference electrode. pH measurements were carried out with a Cyberscan 510 pH meter and a pH electrode (Eutech instruments, Singapore). Images of the modified surfaces were obtained by high resolution scanning electron microscopy (HR-SEM Sirion, FEI Company, USA) and ultra high resolution SEM (XHRSEM Magellan, FEI Company, USA). Surface analysis was carried out using graphical software (ImageJ 1.38, National Institutes of Health, USA). Chemicals. Iodine (99.99%), potassium iodide (99%), 3,4dihydroxyphenylactic acid (dopac, 98%), benzoquinone, ferrous sulfate (98%), sodium phosphate monohydrate, and catechol (99+%) were purchased from Sigma-Aldrich. Ascorbic acid, 3(3,4)-dihydroxyphenyl-L-alnine (l-dopa ≥99%), and disodium hydrogen phosphate were ordered from Fluka (Switzerland). Ferric nitrate and citric acid were obtained from BDH, England. Platinum(IV)chloride (99%) and hydroquinone (99.5%) were purchased from Acros organics, and chloroauric acid hydrate was purchased from Alfa Aesar (MA, USA). Sodium hydroxide and ethanol were obtained from J.T. Baker (Deventer, Holland). Solutions were prepared from deionized water (Barnstead Easypure, UV system). Procedures. Electrode Pretreatment. Pt, Au, and GC electrodes were polished with alumina slurry (1 and 0.05 μm, Buehler, IL, USA) and washed with deionized water. Pt and Au electrodes were electrochemically treated in 0.1 M H2SO4 by cycling between the oxidation and reduction of water until a reproducible voltammetry was obtained. The stainless steel surfaces were immersed for 10 min in 1 M NaOH followed by sonication for 10 min in ethanol and water. Potentiometric measurements were conducted under a constant ionic strength which (unless mentioned otherwise) was controlled using a solution that consists of 0.1 M phosphate buffer (pH 7). Electrode Modification. Pt NPs coatings were prepared by spontaneous reduction of a solution of PtCl4 with ascorbic acid following the procedure by Hireo.38 Au coatings were obtained in a modified procedure.39 Namely, the stainless steel surface was immersed into a solution containing 1 mM HAuCl4 and 0.1 M buffer solution (pH 2 prepared by Na2HPO4 and citric acid) followed by electrochemically cycling (30 cycles) between 0.3 and −0.85 V. A combined stainless steel electrode composed of both Au and Pt NPs was prepared using both procedures, where Pt NPs were deposited initially followed by Au NPs electrodeposition.

Figure 1. Schematics of the approach: different redox couples (Red1 and Red2) exhibit sluggish ET on the bare surface (k1 and k2, respectively) as compared with their ET on Pt (k3) and Au (k4) nanoparticles.

(with a rate constants k1 and k2) with both redox systems. One system shows facile ET on Pt (k3) while the other redox system has fast ET on Au (k4). We hypothesize that by adsorbing small amounts of Pt and Au nanoparticles the relatively inert electrode will respond reversibly with both redox species to measure the true redox potential of the solution. Specifically, Pt and Au NPs were deposited onto stainlesssteel (316L and 304L) surfaces. The latter was used (and not, for example, glassy carbon) because its surface can be highly reproducible and flat surfaces, which can later be examined by various techniques, are available. We found that the modified surfaces showed reversible cyclic voltammetry and significantly improved potentiometric response toward a set of organic and inorganic electroactive species, such as l-dopa, catechol, Fe2+, and I−. The dependence of the surface coverage by the NPs on voltammetry and potentiometry was studied. The modified stainless steel electrode showed similar behavior as Pt or Au electrodes providing that the surface coverage of stainless steel exceeded 20% of Pt or 10% of Au nanoparticles.

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RESULTS AND DISCUSSION Measuring the redox potential of a solution consisting of a redox couple is usually carried out using a Pt electrode. For redox species, which undergo outer-sphere electron transfer, their redox potential can often be measured by other electrode materials as well. Figure 2 shows the OCP measured in solutions containing different ratios of hexacyanoferrate(III)/ hexacyanoferrate(II). This redox couple is characterized by relatively fast ET kinetics on various materials, and therefore, the potentiometry response (Figure 2) nicely follows the Nernst equation. The slope that is obtained for Pt, Au, and GC electrodes equals 58 ± 1, 58 ± 3, and 56 ± 3 mV, respectively. Yet, most redox couples do not show such behavior due to a more complex ET mechanism, usually of inner-sphere type. A

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EXPERIMENTAL SECTION Instrumentation. Cyclic voltammetry (CV) and potentiometric measurements were preformed with an Autolab PGSTAT10 potentiostat using GPES software, version 4.9 (EcoChemie, Utrecht, The Netherlands) or CHI-630B (CH Instruments Inc., TX, USA). Measurements were conducted with either commercial Pt or Au disk electrode (2 mm diameter, CH instruments, USA) or stainless steel (316L and 304L) and glassy carbon electrode (GCE). The latter was B

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observed by scanning with a Pt electrode (although the CV is not reversible), they are not seen and presumably shifted to more positive and negative potentials, respectively, when scanning with stainless steel electrodes. This indicates that the rate of ET of l-dopa and dopac on stainless steel is substantially more sluggish than on Pt. The oxidation processes are attributed to the oxidation of l-dopa or dopac to the open chain quinone.45−47 The CV is consistent with the OCP shown in Figure 3, namely, the redox potential estimated from the average of the oxidation and reduction waves (using the Pt electrode) is in agreement with the potentiometric measurement. This suggests that by increasing the rate of ET on conductive materials the potentiometric response will better reflect the true redox potential of the solution and will enable using these electrodes for analytical applications. The rate of ET should be enhanced by introducing catalytic sites (which are characterized by fast ET) on these electrode materials (Figure 1). Such sites can be formed upon depositing NPs. Platinum NPs can be grown on metallic surface by different approaches, such as galvanic displacement,48 electroless deposition,38 and electrochemical deposition or adsorption.49,50 We found that fairly uniform and controllable Pt NPs can be formed on stainless steel by electroless deposition. Specifically, the stainless steel surfaces were immersed for different durations (1−1000 min) in a Pt electroless plating solution, which contained PtCl4 and ascorbic acid.38 Figure 5 shows the linear correlation between the surface that is covered with Pt NPs (calculated from the SEM images using ImageJ software51) and the time the surface was immersed in the deposition solution. It is worth mentioning that shaking the solutions during the deposition process led to aggregation. Figure S1, Supporting Information, shows the HR-SEM image of a SS-304L electrode after immersion for 1000 min in the electroless solution. The surface is covered by uniform NPs where the coverage varies between 20 and 30%. The next step was to employ the stainless steel modified surface for potentiometry. Figure 6 shows the effect of the Pt deposition time on the potentiometric response of a SS-304L surface toward l-dopa. As before, the solution consisted of only the reduced state of this redox species. The response of a bare SS-304L is shown as well. It can be seen that the immersion time has a dramatic effect on the OCP measurement. More specifically, the bare surface and that treated for 1 and 10 min are insensitive to the concentration of l-dopa. Namely, a 4-fold change in the l-dopa concentration had a negligible effect on the OCP. On the other hand, the stainless steel that was treated

Figure 2. OCP of solutions containing different ratios of hexacyanoferrate(II/III) measured by different electrodes: ■, Pt; red ●, Au; blue ▲, GC.

typical example is shown in Figure 3 where the potentiometry of 3-(3,4)-dihydroxyphenyl-L-alnine (l-dopa) and 3,4-dihydroxyphenylactic acid (dopac) is plotted for different electrode materials. We employed Pt, Au, and stainless steel (SS-304L and SS-316L). The concentrations of the redox species span over 4 orders of magnitude. The OCP measured for the different electrode materials is evident. While the Pt electrode shows good linearity of the OCP as a function of log[redox species] with a slope of −54 ± 5 mV for l-dopa (Figure 3A), Au electrode exhibits a slope of −17 ± 1 mV while SS-304L and SS-316L show poor response (slope ca. −2 and −4 mV, respectively) and linearity. Interestingly, the slope of Pt correlates with one electron transfer. Previous reports on dopamine40−42 and other organic compounds such as ascorbic acid,43,44 which undergo a two-electron oxidation−reduction process, also showed a slope that corresponded with a oneelectron transfer process. At the same time, similar experiments conducted by us with mixtures of benzoquinone and hydroquinone show a linear two-electron transfer slope (not shown). Figure 3B shows the results obtained for dopac. A trend similar to that of l-dopa is observed, whereby the Pt electrode yielded good linearity and a similar slope to that of l-dopa, while the other electrode materials showed poor response. It is evident that the nature of ET affects very much the OCP that is measured. It is very likely that the rate of ET of these two species is significantly slower on SS-304L and SS-316L as compared with Pt. Figure 4 shows the CV of l-dopa (Figure 4A) and dopac (Figure 4B) in phosphate buffer (pH 7, 0.1 M). It is clear that, while oxidation and reduction peaks are

Figure 3. OCP measurements of solutions containing different concentrations of: (A) l-dopa and (B) dopac. Solutions were measured for 30 s with Pt (■), Au (blue ▼), stainless steel 304L (red ●), and 316L (green ▲) electrodes. C

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Figure 4. CV of: (A) l-dopa (2.5 mM) and (B) dopac (2.5 mM) in a solution containing 0.1 M phosphate buffer (pH 7.0) using Pt (black), SS-304L (red), and SS-316L (blue) electrodes. Scan rate equals 50 mV·s−1.

electrodes underwent treatment with Pt NPs for 1000 min. The modified electrodes that were used had a surface coverage of 21−30%. In both solutions (Figure S2A,B, Supporting Information), it can be easily seen that the electrodes modified with Pt NPs showed almost the same Nernstian behavior as the standard Pt electrode. The slopes were between 50 and 60 mV, and the coefficient of determination, R2, varied between 0.912 and 0.977. Clearly, the Pt NPs acted as an electron transfer mediator, hence, improving the electrode potentiometric response. These findings evidently prove our hypothesis that, by modification of the electrode surface with NPs, we can significantly increase the rate of ET. Moreover, this also suggests that ET is channeled via the catalytic sites. Covering the stainless steel surface with Pt NPs clearly affects the rate of electron transfer and therefore should influence also the CV. Figure 7A shows the CV of a solution containing 10 mM FeSO4 and 0.1 M HCl recorded with Pt, SS-304L, and Pt NPs modified SS-304L. Fe2+/3+ has facile ET on Pt while sluggish kinetics on iron surfaces. This is clearly seen in the CV. While the CV on a Pt surface (Figure 7A) is quasi-reversible (ΔEpK = 90 mV), the reduction−oxidation waves on bare SS304L are hardly seen. The electrode that was treated for 10 min or less responded similarly to a bare stainless steel electrode. Increasing the deposition time increases the oxidation− reduction currents of the iron species. This increase is due to increasing the active area of the electrode upon modification by Pt NPs. Initially and under low coverage of Pt, the diffusion of Fe2+ should be radial, and therefore, the CV should attain a sigmoid shape. This is indeed the case for short, i.e., 10 min or less, deposition times. Increasing the surface coverage transforms the diffusion of the electroactive species into linear. Under these conditions, the flux of the reduction peak, j, should be linearly dependent on the diffusion surface coverage, θdiff (eq 1), where j0 equals the flux measured with a Pt electrode and θdiff is ascribed to the area of the electrode surface that is overlapped by the diffusion layer.

Figure 5. Percent calculated surface coverage of stainless steel by Pt NPs as a function of the immersion time in the electroless plating solution.

Figure 6. OCP of different concentrations of l-dopa in 0.1 M phosphate buffer (pH 7.0). The solutions were measured for 30 s with Pt electrode (■) and 304L stainless steel electrodes that were treated with Pt electroless solution for 0 (purple, left pointing triangle), 1 (red ●), 10 (blue ▲), 100 (green ▼), and 1000 (pink, right pointing triangle) min.

for 100 and 1000 min gave a slope of −37 ± 1 and −60 ± 5 mV, respectively, which is comparable with the response of a Pt electrode (−54 ± 5 mV). The same behavior was observed for dopac (not shown). Furthermore, the other alloy, i.e., SS-316L, responded similarly for both redox couples. Figure S2, Supporting Information, shows the potentiometric results in solutions that contained different concentrations of l-dopa (Figure S2A, Supporting Information) and dopac (Figure S2B, Supporting Information). The difference between Figure 3 and Figure S2, Supporting Information, is that the stainless steel

j = j0 θdiff

(1)

Figure 7B shows the relations between θdiff and the immersion time of the surface in the electroless plating solution. The sigmoid shape that is obtained is due to change in the nature of the diffusion from radial to linear. Similar CVs were obtained with SS 316L electrode. Moreover, other redox species, such as iodide (in 0.1 M HCl), showed also the same trend of increasing the current, D

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Figure 7. (A) CV of 10 mM FeSO4 in 0.1 M HCl with Pt (black) electrode and 304L stainless steel electrode treated with Pt NPs for different durations. (B) The relation between θdiff (see eq 1) and the immersion time of the surface in the electroless plating solution.

Figure 8. CV of 1 mM catechol in a 0.1 M H2SO4 solution using an Au modified stainless steel electrode with: (A) low and (B) fast scan rates. The inlets show the ratio between log(jpa) and log(scan rate).

with linear diffusion.55 On the other hand, Figure 8B shows the behavior of the same electrode at higher scan rates (500−4000 mV·s−1) where the peak-shaped voltammogram is replaced by a sigmoid shaped arising from radial diffusion. Also, for this range of scan rates, a linear dependence between the logarithm of the current density and the logarithm of the scan rate is obtained; however, the slope decreases to 0.2 (Figure 8B, inset). This behavior was previously reported for nanoelectrode ensembles (NEEs).56,57 The change of the voltamogram as a function of scan rate as well as the change in the slope can be explained by the effect of the scan rate on the overlap between the diffusion layers. At low scan rates, the radial diffusion layers that are formed due to the oxidation of catechol at the Au NPs overlap resulting in linear diffusion, where the electrode behaves as a macroelectrode. At higher scan rates, the diffusion layers are reduced, which causes the electrode to behave as a collection of individual nanoelectrodes. Figure 9 shows the HR-SEM image of SS-304L that was covered with Au and Pt NPs. The Pt NPs were initially deposited via an electroless process followed by the electrodeposition of the Au NPs. The two different NP populations can easily be seen. EDS analysis indicates that the relatively large NPs (ca. 80 ± 20 nm) are made of Au, while the smaller NPs (ca. 40 ± 20 nm) are made of Pt. To prove our hypothesis, we examined the voltammetry and potentiometry response of this bimetallic electrode toward catechol and l-dopa. We recall that, while catechol exhibits faster ET on Au than on Pt, l-dopa shows the reverse behavior, where its ET is faster on Pt as compared with Au. Figure 10A shows the potentiometric response of l-dopa using three SS-

approaching that of a Pt electrode, by employing Pt NPs modified 304L or 316L stainless steel surfaces. The potentiometric results (Figure 6) are not restricted to organic species. Figure S3, Supporting Information, shows the potentiometric response of solutions containing iron(II) and iron(III) measured by Pt and stainless steel electrodes before (Figure S3A, Supporting Information) and after (Figure S3B, Supporting Information) modification with Pt NPs. Our next step was to fabricate an electrode that was made of a surface, which exhibited sluggish ET kinetics such as stainless steel, and modify it with two different nanoparticles materials. Pt NPs were one choice, while we chose Au NPs as the second component based on its known catalytic activity in various ET processes. Figure S4, Supporting Information, shows an HRSEM image of 304L stainless steel that was cycled for 30 times in a HAuCl4 solution.39 We found that this protocol yielded better control over the deposited NPs compared with other approaches. The surface coverage was ca. 27 ± 1%. The electrochemical oxidation of catechol is reversible on Au52,53 whereas quasi-reversible on Pt.54 Figure 8 shows the CV of catechol at different scan rates in a solution containing 0.1 M H2SO4 using SS-304L modified with Au NPs (surface coverage ca. 10%). Well-defined reversible peak-shaped voltammograms can be seen in Figure 8A, which were carried out at relatively low scan rates (5−100 mV·s−1). The peak currents vary linearly with the root of the scan rate (Figure S5, Supporting Information) signifying that the current is controlled by linear diffusion. The inlet shows the linear dependence obtained between the logarithm of the peak current density, jpa, and that of the scan rate. The slope of the plot equals 0.5 in accordance E

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behavior. Hence, it is evident that ET is dramatically enhanced, this time, by the Au NPs.

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CONCLUSIONS

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ASSOCIATED CONTENT

Measuring the “true” open circuit potential of various redox couples depends on their rate of electron transfer with the electrode material. We have shown that sluggish ET can be dramatically enhanced by modifying a conducting surface by Pt and Au nanoparticles. A variety of species (organic and inorganic) which exhibits sluggish electron transfer on stainless steel was examined. Covering 10−20% of the stainless steel surface with nanoparticles significantly improved electron transfer and therefore the cyclic voltammetry and the potentiometry response, resulting in a stainless steel electrode that behaved as if it were made of the nanoparticle materials. Moreover, the stainless steel could be modified simultaneously by Au and Pt nanoparticles. This was used for improving the stainless steel electrode response (CV and potentiometry) toward two different species: l-dopa, which shows fast electron transfer on Pt, and catechol, which exhibits fast electron transfer on Au. Evidently, electron transfer is channeled through the nanoparticles. Yet, we do not aim to provide mechanistic aspects, which are beyond this first manuscript. Nevertheless, we believe that the mechanism of the redox reactions that were examined is not different than that on the same bulk material, i.e., on either Pt or Au. Furthermore, adsorption can also influence electron transfer processes. Yet, this approach can be portrayed as an array of resistors, where the current (and therefore also the potentiometric and voltammetric responses) will be channeled via the lowest resistance. This means that, even if the surface is contaminated as a result of adsorption, as long as the nanoparticles will not be fouled, the potentiometry will be Nernstian. Hence, we believe that this approach could be a first step toward developing a superior electrode for measuring the “true” Eh of complex aquatic systems.

Figure 9. HR-SEM image of SS-304L surface that was coated with Pt and Au NPs. The Pt NPs were deposited for 1000 min (as described above). This was followed by Au NPs electrodeposition (as described above).

304L surfaces modified with NPs: Au (26% surface coverage), Pt (25% surface coverage), and Pt/Au (ratio 1:1 by EDS). It is clear that, while the stainless steel surface that was modified with only Au NPs is insensitive to the changes in the l-dopa concentration, both stainless steel surfaces modified either with Pt NPs or with the bimetallic NPs show a Nernstian behavior. The slopes for the Pt and the Pt/Au modified surfaces are −46 ± 2 and −47 ± 3 mV, respectively. Moreover, the fact that the electrode modified with both Au and Pt NPs behaved similarly to the Pt NPs modified electrode and not to that modified with Au NPs is evidence for the path of ET through the Pt NPs, which is not affected by the presence of Au NPs. To complete this last stage, we show (Figure 10B) the CV of 1 mM of catechol in 0.1 M H2SO4 using a Pt, Au, stainless steel, and the bimetallic modified surface. Clearly, ET is almost blocked on the bare stainless steel electrode. The bare Pt electrode shows sluggish kinetics and a quasi-reversible voltamogram. On the other hand, both the Au and the bimetallic modified stainless steel electrodes show a reversible

* Supporting Information S

Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 10. (A) OCP measured in solutions containing different concentrations of l-dopa using SS-304L modified with Au NPs (■), Pt NPs (red ●), and Pt/Au NPs (blue ▲). (B) CV of 1 mM catechol in a 0.1 M H2SO4 solution using an Au (black), Pt (red), SS-304L (green), and SS-304L modified with Pt/Au NPs (blue) electrodes. F

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AUTHOR INFORMATION

Corresponding Author

*Tel: +972 2 6585831. Fax: +972 2 6585319. E-mail: daniel. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the European project (HYDRONET, http://www.hydronet-project.eu) and the Israel Science Foundation (1150/11). Teva pharmaceuticals LTD is acknowledged for sponsoring T.N. We are indebted to the Harvey M. Krueger Family Center for Nanoscience and Nanotechnology of the Hebrew University.

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dx.doi.org/10.1021/ac401744w | Anal. Chem. XXXX, XXX, XXX−XXX