Potentiometric Sensors with Carbon Black Supporting Platinum

Oct 4, 2013 - For the first time, a single-piece, all-solid-state ion-selective electrode was fabricated with carbon black supporting platinum nanopar...
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Potentiometric Sensors with Carbon Black Supporting Platinum Nanoparticles Beata Paczosa-Bator,*,† Leszek Cabaj,† Robert Piech,† and Krzysztof Skupień‡ †

AGH-UST University of Science and Technology, Faculty of Material Science and Ceramics, Mickiewicza 30, PL-30059 Cracow, Poland ‡ 3D-nano, Mistrzejowice 5/12, PL-31640 Cracow, Poland ABSTRACT: For the first time, a single-piece, all-solid-state ion-selective electrode was fabricated with carbon black supporting platinum nanoparticles (PtNPs-CB) and a polymeric membrane. The PtNPs-CB, as an intermediate layer, was drop-casted directly on the solid substrate, and then an ionophore-doped solvent polymeric membrane was added in order to form a sensor. The performance of the newly developed electrodes was evaluated on the basis of potassium and nitrate ions. The stability of the electrical potential for the electrodes was examined by performing current-reversal chronopotentiometry, and the influence of the interfacial water film was assessed by the potentiometric aqueous-layer test. Fabricated potassium- and nitrate-selective electrodes displayed a Nernstian slope and several outstanding properties such as high long-term potential stability, potential repeatability, and reproducibility.

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sensors could be easily miniaturized into any given shape.5 On the other hand, the presence of a water layer between ISM and CP cannot be avoided.6 Therefore, other materials are needed for the interlayer in SC-ISE.7,8 Potential candidates are carbonbased materials (CBMs) such as carbon nanotubes, carbon black, graphene, or three-dimensionally ordered macroporous carbon.9−16 When comparing the CP and CBM performance in SC-ISEs, perhaps the most important feature is a limited formation of the interfacial water film due to the hydrophobic character of CBM. In addition, CBMs are insensitive to oxygen or light, which makes them very promising materials for sensor application. This study is the first one presenting the carbon black supporting single Pt nanoparticles (PtNPs-CB) applied to prepare the solid-contact ISE. The potassium-selective electrode was chosen as an exemplary cationic-sensitive system, and the nitrate-selective electrode was chosen as an anionicsensitive one. It has been demonstrated that Pt nanoparticles supported on Vulcan XC-72 (VXC) carbon black (PtNPsVXC) significantly decrease the membrane resistance and contribute to a substantial improvement in the analytical parameters of the sensors, including long-term potential stability, potential repeatability, and reproducibility.

potentiometric sensor can be used to determine an ion concentration in an aqueous sample, including body fluids such as blood or serum. Thanks to its wide range of applications, a small amount of sample needed for analysis and measurement simplicity, the potentiometric sensor has been used for years for chemical analysis of various samples, including biological, geological, and other types of samples. The largest group of potentiometric sensors is represented by the ion-selective electrodes (ISEs). In ISEs, the signal is generated by the charge separation at the interface due to the selective partitioning of the ionic species between the membrane and the solution.1 A new direction in the field of ISEs started in 1970, when Cattrall and Freiser presented a coated wire electrode (CWE) consisting of a metallic conductor covered with an ion-selective membrane (ISM).2 The goal was to entirely eliminate the internal filling solution. However, the measurements performed with CWE are not entirely reliable due to the fluctuation of the electric potential (i.e., potential drift caused by the differences in the type of the electric charge between an electronic conductor and an ion-sensitive membrane).3 Despite these disadvantages, the first stage in developing ISEs with no internal filling solution was achieved. This gave rise to the development of the solid-contact ion-selective electrode (SC-ISE). Briefly, SC-ISE is an asymmetrical sensor, where the membrane on one side is in contact with an aqueous solution sample and on the other with a transducer layer or a solid contact.4 Introducing conducting polymers (CPs) as the transducer layer resulted in improved potential stability owing to their mixed ionic-electronic conductivity. Furthermore, in the absence of the internal reference in the form of an internal reference electrode and internal filling solution, these kind of © 2013 American Chemical Society

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EXPERIMENTAL SECTION Materials. A colloidal solution of carbon black (Vulcan XC72) supporting platinum nanoparticles was obtained from 3DReceived: July 3, 2013 Accepted: October 4, 2013 Published: October 4, 2013 10255

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Figure 1. SEM images of Vulcan XC-72 layer (a,b) and Vulcan XC-72 supporting platinum nanoparticle layer (c,d) with the EDAX analysis.

solution was prepared by dissolving 315 mg of the membrane components in 2.5 mL of THF. To prepare the electrodes denoted as GCD/VXC/NO3−-ISM and GCD/PtNPs-VXC/ NO3−-ISM, the solid contact layer (GCD/VXC and GCD/ PtNPs-VXC) was subsequently coated with 120 μL of the NO3−-ISM mixture containing 6.5% (w/w) TDMA-NO3, 65.0% (w/w) o-NPOE, and 28.5% (w/w) PVC. The nitrate membrane solution was prepared by dissolving 252 mg of the membrane components in 2 mL of THF. The coated disc electrodes (GCD/K+-ISM and GCD/NO3−ISM) were prepared by covering bare GCD electrodes with above mentioned potassium- or nitrate-selective membranes. All the membrane electrodes were left to dry for 48 h at room temperature. Afterward, the potassium- and nitrate-selective electrodes were conditioned for 24 h in 0.01 M KCl or 0.01 M KNO3 water solutions, respectively. The conditioning step was also repeated before every measurement. For each kind of SCISEs, five identical electrodes were prepared and examined. Potentiometric Measurements. The potentials were measured using a 16-channel mV-meter (Lawson Labs, Inc., Malvern, PA). The reference electrode was a Ag/AgCl electrode with 3 M KCl solution in a bridge cell (type 6.0733.100, Ω Metrohm, Switzerland) or an Ag/AgCl/3 M KCl (type 6.0729.100, Ω Metrohm, Switzerland) with the salt bridge 1 M lithium acetate. Chronopotentiometry. The chronopotentiometry measurements were performed with the use of an Autolab General Purpose Electrochemical System (AUT32N.FRA2-AUTOLAB, Eco Chemie, The Netherlands) connected to a conventional, three-electrode cell. The Ag/AgCl/3 M KCl electrode (type

nano Co., Poland, and was used without any modifications. Carbon black Vulcan XC-72 was obtained from the Cabot Corporation. Potassium ionophore I (valinomycin), potassium tetrakis(4-chlorophenyl)borate (KTpClPB), tridodecylmethylammonium nitrate (TDMA-NO3), o-nitrophenyl octyl ether (oNPOE), poly(vinyl chloride) (PVC) of high molecular weight, and tetrahydrofuran were the selectophore reagents obtained from Fluka. All other chemicals were of analytical-reagent grade. Distilled and deionized water was used to prepare the aqueous solutions. Electrode Preparation. The glassy carbon disc (GCD) electrodes consisting of the GC rods enveloped in the Teflon bodies (GC area = 0.07 cm2) were first polished with 0.3 μm alumina powder, rinsed with water, and finally cleaned ultrasonically with water and methanol. In order to obtain the VXC and PtNPs-VXC-modified GCD electrodes (GCD/VXC and (GCD/PtNPs-VXC), the dropcasting method was used. In the case of GCD/VXC electrodes, the mixture containing 5 mg of VXC dispersed in 1 mL of THF was sonicated for ≈1−1.5 h and then placed on the top of the GCD electrodes. To fabricate the GCD/PtNPs-VXC electrodes, 50 μL of colloidal PtNPs-VXC solution was added on the top of the GCD electrodes. After that, GCD/VXC and GCD/ PtNPs-VXC electrodes were left to dry for 48 h. To prepare the electrodes denoted as GCD/VXC/K+-ISM and GCD/PtNPs-VXC/K+-ISM, a solid contact layer (GCD/ VXC and GCD/PtNPs-VXC) was subsequently coated with 120 μL of a K+-ISM mixture containing 1.1% (w/w) valinomycin, 0.25% (w/w) KTpClPB, 65.65% (w/w) oNPOE, and 33% (w/w) PVC. The potassium membrane 10256

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Figure 2. HR-TEM image of platinum nanoparticles supported on Vulcan XC-72 and the size distribution of Pt nanoparticles for the PtNPs-VXC.

6.0733.100, Ω Metrohm, Switzerland) was used as a reference electrode, and a glassy carbon rod was used as an auxiliary electrode. Scanning Electron Microscopy. The morphologies and chemical analysis of the carbon black (Vulcan XC-72) layer before and after Pt nanoparticles deposition were examined using a scanning electron microscope (SEM), model LEO 1530 from LEO Electron Microscopy Ltd. equipped with the Image and X-ray Analysis system, model Vantage, from ThermoNoran for energy- dispersive x-ray analysis (EDAX). Transmission Electron Microscope. High-resolution transmission electron microscope (HR-TEM) measurements were performed with a JEOL 2100 F (Japan) with a field gun emission of 200 kV. Samples diluted in ethanol were prepared for the HR-TEM study by placing a small drop of the PtNPsVXC colloidal solution on the conducting substrate and drying in air.

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RESULTS AND DISCUSSION The results of the chemical analysis of the VXC and the PtNPsVXC together with SEM images showing the morphology of the layers are presented in Figure 1. After VXC with platinum was deposited, the Pt peaks appear in the EDAX spectrum, confirming the presence of Pt nanoparticles in the electrodes. Figure 2 presents a typical HR-TEM image of the PtNPsVXC. The dark spots represent the very uniform platinum nanoparticles with a mean particle size of about 5 nm. Figure 3 shows schematically the structure of the PtNPsVXC electrode immersed in an aqueous solution containing the analyte ions (X) (Figure 3a) and the close-up of the membrane (Figure 3b). Typically, the membrane is made of poly(vinyl chloride) impregnated with the plasticizer, which dissolves the ion-selective ionophore (L), the complex (LX), and hydrophobic ions (R). The response times of the GDC/PtNPs-VX/K+-ISM and GCD/PtNPs-VXC/NO3− electrodes for the progressive addition of different amounts of potassium and nitrate ions in the concentration range from 10−7 to 10−1 M are shown in Figure 4. Interestingly, even at a low concentration, the

Figure 3. (a) Schematic representation of the GCD/PtNPs-VXC/XISM electrode and (b) a close-up of the active part of the GCD/ PtNPs-VXC/X-ISM electrode.

response time was very short (about 5 s) and better than the values obtained for potassium electrodes with nanotubes, graphene, or carbon black and platinum nanoparticles used separately.7,12−16 The response reversibility of the studied electrodes recorded in the main ion solutions for concentrations of 10−5 and 10−4 M, respectively, is shown in the inset in Figure 4. The calibration curve slope values measured for the developed electrodes with PtNPs-VXC used as an intermediate layer are close to the Nernstian value. Figure 5 shows the dependency of log aK+ and log aNO3‑ versus the electromotive force (EMF) for the GCD/PtNPs-VXC/K+-ISM and GCD/ PtNPs-VXC/NO3−-ISM electrodes. The plots were obtained by the recording of the EMF after storing the electrodes in the 10257

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solution of 10−2 M KCl or KNO3, respectively, for 1 h (Figure 5a) and 1 month (Figure 5b). Every measurement was repeated three times. The slopes calculated from the linear range of the calibration plots after 1 h storage in solution were 57.98 ± 1.02 mV/decade (GCD/PtNPs-VXC/K+-ISM) and −58.63 ± 0.24 mV/decade (GCD/PtNPs-VXC/NO3−-ISM). After 1 month of storage in 10−2 M KCl or KNO3 solution, the slopes were 58.07 ± 0.48 mV/decade for GCD/PtNPs-VXC/K+-ISM and −58.58 ± 0.06 mV/decade for GCD/PtNPs-VXC/NO3−-ISM. Under the same experimental conditions, the sensors based on the GCD/VXC layer showed calibration curves with slopes equal to 58.25 ± 0.50 mV/decade (GCD/VXC/K+-ISM) and −58.99 ± 0.06 mV/decade (GCD/VXC/NO3−-ISM). The detection limit calculated as the intersection of the two slope lines is 10−6.1 and 10−6.3 M for the GCD/PtNPs-VXC/ K+-ISM and GCD/PtNPs-VXC/NO3−-ISM electrodes, respectively (Figure 6). In the case of the GCD/VXC/K+-ISM and GCD/VXC/NO3−-ISM electrodes, the detection limit was 10−5.9 and 10−6.5 M, respectively.

Figure 4. Potentiometric response of the studied electrodes vs time determined in the KNO3 or KCl solutions.

Figure 6. Detection limit for GCD/PtNPs-VXC/K+-ISM and GCD/ PtNPs-VXC/NO3−-ISM electrodes.

The fabricated electrodes show a very stable response over time. Even after a long conditioning time in 0.01 M solution with the main ions (6−7 weeks), the electrodes still showed a linear response in the same range of the K+ or NO3− activity. After 24 h of conditioning, the standard potential values of the GCD/PtNPs-VXC/K+-ISM and GCD/PtNPs-VXC/NO3−ISM electrodes were equal to 515.1 and 370.7 mV, respectively, and after 1 month, the values changed slightly by 4.9 and 3.9 mV. The standard potential values of the GCD/VXC/K+-ISM and GCD/VXC/NO3−-ISM electrodes were 421.2 and 189.4 mV after 48 h of conditioning; after 1 month of conditioning in the 0.01 M solution of the main ion, the electrode potential changed by 6.7 and 5.1 mV, respectively. The reproducibility of standard potential value for five different GCD/PtNPs-VXC/K+-ISM (and GCD/VXC/K+ISM) and GCD/PtNPs-VXC/NO3−-ISM (and GCD/VXC/ NO3−-ISM) electrodes measured after 72h of conditioning was 521.2 ± 1.4 mV (422 ± 2.3 mV) and 371 ± 0.2 mV (189.8 ± 0.9 mV), respectively.

Figure 5. EMF dependence on K+ activities for GCD/PtNPs-VXC/ K+-ISM and NO3− activities for GCD/PtNPs-VXC/NO3−-ISM after 1 h (a) and 1 month (b) of conditioning in a solution of 10−2 M KCl or KNO3.

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Table 1. Comparison of the Potentiometric Selectivity Coefficients of the Proposed GCD/PtNPs-VXC/K+-ISM and GCD/ PtNPs-VXC/NO3−-ISM Electrodes logKpot K,X X:

graphene-based SC-ISE15

PtNPs-VXC/K+-ISM −4.6 −4.4 −2.1 −6.5 −5.7

+

Na Li+ NH4+ Mg2+ Ca2+

± ± ± ± ±

−4.5 −3.5 −2.0 −4.5 −3.6

0.2 0.2 0.3 0.2 0.2

± ± ± ± ±

nanotube-based SC-ISE12 −4.6 −3.6 −2.1 −4.6 −3.9

0.3 0.2 0.4 0.3 0.2

± ± ± ± ±

0.2 0.3 0.4 0.4 0.3

pot logKNO 3,Y

Y: −

Cl ClO4− HCO3− PO43‑ acetate NO2− salicylate SO42‑

PtNPs-VXC/NO3−-ISM −2.2 2.5 −3.1 −2.6 −3.2 −1.3 1.4 −2.8

± ± ± ± ± ± ± ±

0.2 0.2 0.3 0.2 0.2 0.1 0.1 0.1

Beckman 39618 −2 2

graphene-based SC-ISE19

conventional ISE20

−1.9 ± 0.1

−1.46 −1.5 −2.9 −1.2 −1.63

−2.2 −1.2 −3.1 ± 0.1

The potential drift of the GCD/PtNPs-VXC/K+-ISM electrode over 172 h was only 8.2 ± 1.4 μV/h (and 17.8 ± 1.9 μV/h for the GCD/VXC/K+-ISM electrode). With regard to the nitrate-selective electrodes, the potential drift over 172 h was 6.3 ± 1.2 μV/h (GCD/PtNPs-VXC/NO3−-ISM) and 9.7 ± 1.0 μV/h (GCD/VXC/NO3−-ISM). The obtained values are better than those yielded for carbon black or platinum nanoparticles used separately as an intermediate layer.7,16 The potentiometric selectivity coefficients of all studied electrodes were obtained with the separate solution method according to the traditional procedure18 (n = 3) using chloride salts of different cations (in the case of potassium-selective electrodes) and sodium salts of different anions (for nitrateselective electrodes). Similar results were obtained when comparing VXC- and PtNPs-VXC-based electrodes. Exemplary potentiometric selectivity coefficient values for the GCD/ PtNPs-VXC/K+-ISM and GCD/PtNPs-VXC/NO3−-ISM electrodes are presented in Table 1. The redox sensitivity measurements were carried out for the GCD/PtNPs-VXC, as well as for the GCD/PtNPs-VXC/K+ISM and GCD/PtNPs-VXC/NO3−-ISM electrodes. The solution used contained a FeCl3 and FeCl2 redox couple (1 mM) with the log of Fe3+/Fe2+ ratio equal to −1, −0.5, 0, 0.5, and 1 (Figure 7). The GCD/PtNPs-VXC electrode shows a clear redox response. However, after covering the PtNPs-VXC layer with ISM (GCD/PtNPs-VXC/K+-ISM and GCD/PtNPsVXC/NO 3 − -ISM electrodes), no redox sensitivity was observed. Similar behavior was observed for the electrodes with graphene, carbon black, or the PtNPs intermediate layer.7,12−16 The stability of the solid-contact sensors was investigated by recording the EMF versus time dependencies in the solution of primary ions followed by the solution of interfering ions and again in the solution of primary ions, as suggested by Fibbioli et al.6 The studied electrodes were initially conditioned in the primary ion solution (0.01 M of KCl or KNO3, respectively). After 24 h, the solution was changed to 0.01 M NaCl or KCl (an interfering ion solution). Finally, the solution was changed again into 0.01 M KCl or KNO3 (see Figure 8). The dynamic EMF response was analyzed in terms of the potential drifts

−1.43

Figure 7. Redox sensitivity of the electrodes: GCD/PtNPs-VXC/K+ISM (Δ), GCD/PtNPs-VXC (□), and GCD/PtNPs-VXC/NO3−-ISM (○).

upon the primary ions being replaced by the interfering ions. The observed potential drift could be attributed to the formation of a water layer between the ion-selective membrane and the solid contact.6 Unlike the coated disc electrodes, the GCD/PtNPs-VXC/K+-ISM and GCD/PtNPs-VXC/NO3−ISM electrodes are stable when the sample is changed, despite being in contact with the conditioning solution for a longer period of time (5 weeks). This demonstrates that the water layer was reduced in the developed SC(PtNPs)-ISEs. Contrary to the electrodes, in which the Pt nanoparticles were added to the ion-selective membrane,21 and similar to those with carbon black or platinum nanoparticles,7,16 the GCD/PtNPs-VXC/ISM electrodes do not show any significant sensitivity to O2 and CO2. In addition, the electrodes are also insensitive to H+. This was confirmed by changing the pH value of the main ion solution from 3 to 9 while measuring the electrode potential. Current-reversal chronopotentiometry was used to evaluate the electric capacity of the solid contact and the potential stability of the electrodes.22 Figure 9 shows a typical change in 10259

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similar conditions (ΔEdc/Δt = 19.9 μV/s for GCD/VXC/K+ISM and 0.8 mV/s for GCD/K+-ISM or 6.1 μV/s for GCD/ VXC/NO 3 −-ISM and 6.49 mV/s for GCD/NO 3 −-ISM measured when the current of 1 nA was used). From the equation ΔEdc/Δt = I/C, the value of C for the GCD/PtNPs-VXC/K+-ISM electrode is calculated to be 217 μF. In the case of the GCD/PtNPs-VXC/NO3−-ISM electrode, C was equal to 1666 μF. These values are much better than those measured for CPs-CNT (83 μF14), graphene (83 μF15), Printex XE-2 carbon black (51 μF16), or platinum nanoparticles (82 μF7) based on the solid-contact transducers (in the case of the GCD/K+-ISM electrode, C = 1.3 μF, and for GCD/NO3−ISM, C = 0.16 μF).

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CONCLUSIONS We have successfully introduced the carbon black-supported Pt nanoparticles (PtNPs-CB) as an intermediate layer in solidcontact anion- and cation-selective electrodes. The presence of PtNPs-CB significantly decreases the membrane resistance and improves the analytical parameters of sensors such as the longterm potential stability, the potential repeatability and the reproducibility. These novel sensors, fabricated in a simple drop-casting procedure provide a good Nernstian response to potassium or nitrate ions. The resulting detection limit for nitrate- and potassium-selective electrodes is 10−6.3 and 10−6.1 M, respectively. No significant water layer between the ionselective membrane and the solid-contact layer was detected. Interestingly, the developed electrode shows a greatly improved long-term potential stability as compared to the one observed for the sensors where carbon black or platinum nanoparticles were used separately as an interlayer.

Figure 8. Water layer test of the GCD/PtNPs-VXC/K+-ISM and GCD/PtNPs-VXC/NO3−-ISM electrodes performed for the GCD/ PtNPs-VXC/K+-ISM in 10−2 M KCl, 10−2 M NaCl, and again in 10−2 M KCl and for the GCD/PtNPs-VXC/NO3−-ISM electrode in 10−2 M KNO3, 10−2 M KCl, and again in 10−2 M KNO3.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +480126341201. Tel.: +480126175021. Notes

The authors declare no competing financial interest.

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Figure 9. Chronopotentiograms for the GCD/PtNPs-VXC/NO3−ISM electrode recorded in 0.1 M KNO3.

ACKNOWLEDGMENTS This work was supported by NCBiR (No. LIDER/31/7/L-2/ 10/NCBiR/2011).

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the potential versus time recorded for the GCD/PtNPs-VXC/ NO3−-ISM electrode when currents of +1,+5, and +10 nA are applied to the working electrodes for 100 s. Subsequently, currents of −1, −5, and −10 nA are applied for the same time interval. The potential jump observed in the response after current polarity change (ΔEdc) was used to calculate the total resistance of the electrode: Rtotal = ΔEdc/2I, where the I is the current applied. The estimated values of Rtotal are 0.16 MΩ for GCD/ PtNPs-VXC/K+-ISM, 1.3 MΩ for GCD/VXC/K+-ISM. In the case of nitrate-selective electrodes, the total resistance values were 14.5 kΩ for GCD/PtNPs-VXC/NO3−-ISM and 19.9 kΩ for GCD/CB-VXC/NO3−-ISM. The potential drift of the electrodes was derived from the ΔEdc/Δt ratio.20 The received values of 4.6 μV/s calculated for the GCD/PtNPs-VXC/K+-ISM electrode (when the current of 1 nA was applied) and 0.6, 2.6, and 5.2 μV/s for the GCD/ PtNPs-VXC/NO3−-ISM electrode (when the current of 1, 5, and 10 nA was applied) are lower than those obtained from the VXC-based sensors and coated disc electrodes developed under

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