Article pubs.acs.org/ac
Amperometric Nitric Oxide Sensor Based on Nanoporous Platinum Phthalocyanine Modified Electrodes C. M. Yap, G. Q. Xu,† and S. G. Ang*,† †
Department of Chemistry, National University of Singapore, 3 Science Drive 3, 117543, Singapore S Supporting Information *
ABSTRACT: This article describes the fabrication of electropolymerized Metallo 4′, 4″, 4‴, 4⁗ tetra-amine phthalocyanine (poly-MTAPc) modified electrodes for the detection of nitric oxide (NO) in phosphate-buffered saline (PBS) at pH 7.4. A two-step synthetic protocol using a laboratory microwave reactor was adopted to provide three MTAPc complexes bearing different metal centers (M = Cu2+: CuTAPc, M = Zn2+: ZnTAPc, and M = Pt2+: PtTAPc). The MTAPc complexes and the intermediates were characterized by MALDI-TOF mass spectrometry, UV−vis spectroscopy, 1H NMR spectroscopy, and elemental analysis. The MTAPc products were separately electropolymerized either onto a glassy carbon (GC) electrode as a thin-film or within the pores of Anodisc nanoporous alumina membrane as a densely packed array of poly-MTAPc nanotubes to produce two electrode systems. In the latter system, the surface area enhancement provided by the nanotube-arrayed morphology of the poly-MTAPc enabled a high faradaic (signal) to capacitative (background) current during NO electro-oxidation. Amperometric detection of NO using these two electrode systems shows that the sensitivity and linear ranges were insensitive to the metal centers (M = Cu2+, Zn2+, and Pt2+) of the polyMTAPc material.
N
Scheme 1. Structure of MTAPc
itric oxide (NO) has long been established as a signaling molecule in the cardiovascular system since 1987.1 A large number of reports have demonstrated the involvement of submicromolar amount of NO in a wide range of physiological systems2 and the excessive or impaired production of cellular NO resulted in several diseases.3 The role of NO in the plant signaling network has also been reviewed.4 The demand for rapid and selective detection of the short-lived NO in living cells has been first met by a Nafion-coated porphyrin-modified microelectrode developed by Malinski in 1992.5 Following Malinski, materials used for electrode-modification has expanded to the structurally similar phthalocyanine. To date, various substituted metallophthalocyanine has been exploited for the detection of a wide range of analytes and recent advancement in the understanding of the electrocatalytic mechanisms can be found in the comprehensive review by Zagal, Nyokong, and Bedioui.6 Electrodes modified by electropolymerized Metallo 4′, 4″, 4‴, 4⁗ tetra-amine phthalocyanines (MTAPcs, structure shown in Scheme 1) have been shown to be outstanding electrocatalytic sensors for peroxynitrite,7 hydrogen peroxides,8 glycine,9 L-dopa,10 sulphide,11 glucose,12 hydrazine,13,14 nitrite,15 carbon dioxide,16 peroxides,17 thiols,18 oxygen,19−22 and dopamine. 23 To date, there have been very few reports24−26 on NO sensor electrodes based on electropolymerized MTAPcs. Furthermore, these few studies focused on MTAPcs bearing first-row transition metals Cu2+, Co2+, and Ni2+, as a result of the well-established synthetic procedure of © 2012 American Chemical Society
MTAPcs by Achar in 1987.27 Over the recent decades, other metal centers such as Mn,9 Ti,15 and Cr26 in MTAPcs have been explored by Nyokong et al. that have stimulated considerable interest in the electrocatalytic implications brought about by the variable oxidation states and the coordination environment of these metals. In 2009, Achar reported the preparation of platinum 4′, 4″, 4‴, 4⁗ tetra-amine phthalocyanine (PtTAPc)28 in two steps via the reduction of platinum 4′, 4″, 4‴, 4⁗ tetra-nitro phthalocyanine (PtTNPc). Achar prepared the PtTNPc intermediate in >90% yield by heating a solvent-free solid mixture of PtCl4 and 4-nitro phthalonitrile in a domestic Received: July 23, 2012 Accepted: November 23, 2012 Published: November 23, 2012 107
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Scheme 2. Synthetic Scheme of the Two-Step Synthesis of the Various MTAPc Derivatives via the Reduction of the Corresponding Nitro-Substituted Intermediate
microwave oven.28 Whereas electrodes modified by platinumcentered porphyrins have been reported for use as luminescent oxygen sensors,29−32 to date, the electrocatalytic capability of the electropolymerized MTAPc bearing Pt2+ as the metal center has apparently not been explored. Hence, it will be interesting to investigate the electrocatalytic properties of PtTAPc toward NO detection and compare them to other MTAPcs bearing other metal centers. For this study, three MTAPc complexes bearing different metal centers (M = Cu2+: CuTAPc, M = Zn2+: ZnTAPc, and M = Pt2+: PtTAPc) have been prepared using a two-step synthetic protocol carried out in a CEM Discover-SP microwave reactor via the reduction of the corresponding Metallo 4′, 4″, 4‴, 4⁗ tetra-nitro phthalocyanine (MTNPc) intermediate (Scheme 2). The synthetic procedures and characterization results have been summarized in the Supporting Information. The three asprepared MTAPc complexes were separately electropolymerized either onto a glassy carbon (GC) electrode as a thin-film or within the pores of Pt-coated Anodisc nanoporous membrane forming a high-density nanotube array to provide two electrode systems according to procedures described in our earlier reports.33,34 An additional coating of Nafion results in two amperometric NO sensor electrodes: (1) Nafion/polyMTAPc/GC electrode and (2) Nafion/poly-MTAPc nanotube/AAO/Pt, in phosphate-buffered saline (pH 7.4). Thus, this investigation is a comparison study of the complexes CuTAPc, ZnTAPc, and PtTAPc using sensor electrodes 1 and 2.
conventional 3-electrode cell with an Ag/AgCl (3 M KCl) reference electrode and a platinum foil counter electrode. An FE-SEM (JEOL JSM6700F) system was used to characterize the poly-MTAPc nanotube array. A JEOL, JFC-1600 Auto Fine Coater was used to sputter Pt onto a side of the AAO template. Fabrication of 1: Nafion/Poly-MTAPc/GC Electrode. The electropolymerization of MTAPc on GCE was performed by using degassed DMSO containing 1 mM of MTAPc and 0.1 M TBAP. The GCE was cycled starting from −0.2 V to +0.9 V for 40 cycles at 100 mV/s. The modified GCE was then rinsed well with DMSO, followed by ethanol and D.I. water, and left to airdry. Finally 1 × 20 μL Nafion was deposited over the electroactive area and left to air-dry. Fabrication of 2: Nafion/Poly-MTAPc Nanotube/AAO/Pt Electrode. The fabrication process of the modified nanoporous AAO electrode adopted has been reported previously by Gu et al.34 The Pt coated, disc-shaped AAO template was prepared by sputtering 200 nm of Pt over the filtration surface using JFC1600 Auto Fine Coater. The Pt-coated AAO was connected to an insulated copper wire by soldering. Another layer of masking tape (Sellery PTE LTD, Singapore) was adhered onto the conductive Pt working electrode to prevent contact between the electrochemical solution and this Pt surface. Weak pressure was applied by carefully pinching the periphery of the taped assembly. Additional vacuum treatment of the AAO/Pt assembly was applied to improve adhesion between the tapes and the Pt layer. The handmade AAO/Pt working electrode was soaked in a degassed DMSO solution of 0.1 M TBAP and 0.1 mM MTAPc monomer for 30 min. The working electrode was then cycled from −0.2 to +0.9 V for 200 times at 100 mV/s. The electrode was then rinsed well with DMSO, followed by ethanol and D.I. water, and left to air-dry. Finally 1 × 20 μL Nafion was deposited over the electroactive area (that would be easily identified because of a slight green tinge) and left to air-dry. The masking tape was peeled off and the polymer nanotube array immobilized on the thin Pt layer was characterized using FESEM (JEOL JSM6700F) after etching away the AAO template using 0.1 M NaOH. Sensor Electrode Calibration by DPV and DPA. DPA and DPV calibration parameters were adapted from the work of Gu et al.33 The NO stock solution was prepared and its concentration determined following literature procedures.35
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EXPERIMENTAL SECTION Fabrication Methods of poly-MTAPc Modified Electrodes. Tetrabutylammonium perchlorate (TBAP) (electrochemical grade) and DMSO (HPLC grade) were purchased from Alfa Aesar and from Lab Scan respectively and used without any purification. Nafion perfluorinated resin solution (5 wt % in mixture of lower aliphatic alcohols and 45% water) was purchased from Sigma Aldrich. The cylindrical-shaped GCE was housed in a Teflon casing revealing a disk-shaped area of 0.07 cm2 (3 mm diameter). PK-3 Electrode Polishing Kit from BASF: polishing pad mounted on a glass plate and polishing alumina (0.05 μm) was used for GCE polishing. AAO templates were Anodisc 25 membrane filters with a thickness of 60 μm and a quoted pore diameter of 200 nm were purchased from Whatman. The AAO filters templates were packed with the filtration surface (200 nm pore size) uppermost. Cyclic voltammetric electropolymerization was performed with an Autolab TYPE II Potentiostat connected to a
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RESULTS AND DISCUSSION Although the synthesis of PtTAPc has previously been reported,28 to the best of our knowledge no study on the 108
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electropolymerization and the electrocatalytic properties of PtTAPc has been reported. Hence, we report our results with PtTAPc as the example. Electropolymerization of PtTAPc. A dark-greenish polyPtTAPc film coats the bare GC electrode, after subjecting the latter to 40 cyclic voltammetry (CV) scans between −0.2 to +0.9 V at 100 mV/s in DMSO containing 1 mM PtTAPc monomer and 0.1 M TBAP. Figure 1 shows the growing CV
Figure 2. DPV responses of (A) unmodified GC electrode, in pure PBS (absence of NO); (B) unmodified GC electrode; (C) polyPtTAPc/GC electrode; and (D) Nafion/poly-PtTAPc/GC electrode to 2 μM of NO.
porphyrin42 modified electrodes eliminates NO2− interferences effectively based on Donnan exclusion.47 Nafion, a perfluorinated polymer network bearing sulfonate groups (−SO3−)48 blocks the transfer of anionic NO2− by charge-exclusion49 while allowing the transfer of cations or neutral species such as NO. In this case, only neutral NO molecules diffuses freely across the Nafion layer as the oxidizable species generating anodic current proportional to the NO concentration. Furthermore, the efficacy of the Nafion layer to eliminate NO2− interferences has been verified in our previous report.33 Hence, an extra layer of Nafion was applied over the electroactive poly-MTAPc layer, and the entire assembly is denoted as 1: Nafion/poly-MTAPc/ GC electrode. Plot D shows that the additional Nafion coating over polyPtTAPc/GCE resulted in suppressed sensitivity and a +0.01 V overpotential of the NO oxidation agreeing well with previous results.33,42,50 In addition, compared to the DPV responses from the Nafion-free electrodes (plot B and C), plot D also shows a positive shift in the baseline implying an increase in the capacitative (background) current possibly due to a diffusional barrier contributed by the Nafion layer. SEM Characterization Poly-PtTAPc Nanotubes. On the basis of an earlier report,34 commercially obtained disk-shaped AAO membrane filter (from Whatman Co.) with a thickness of 60 μm and a reported pore diameter of 200 nm were coated by Pt on the filtration surface by sputtering. We have verified that the filtration surface clearly shows a more densely packed array of pores than what was observed for the opposite side (FESEM image not shown). Hence, this filtration surface was judiciously coated by Pt to ensure maximal density of the anticipated poly-MTAPc nanotubes. The resultant Pt-coated AAO membrane filter was subsequently insulated by masking tape (over the Pt layer) and denoted as AAO/Pt. Part A of Figure 3 shows a photograph of the AAO/Pt electrode focusing on the masking tape covering the sputtered Pt. The tape ensures the MTAPc solution accesses the conductive Pt layer exclusively through the pores of the AAO membrane (part B of Figure 3). Dark-colored poly-MTAPc film sandwiched between the masking tape and the Pt occurs mainly due to poor adhesion between the masking tape and the Pt, especially along the periphery of the disc-shaped AAO/Pt that allows the electrochemical solution to seep in. The gentle pinching of the taped AAO/Pt followed by a brief vacuum
Figure 1. Cyclic voltammograms obtained at GCE in DMSO solution of 1 mM PtTAPc containing 0.1 M TBAP at scan rate of 100 mV/s. GCE area = 0.07 cm2. Eighteen scans are shown.
scans for the electropolymerization process. The red-colored plot represents the first cycle where the irreversible oxidation wave between 0.7 and 0.85 V (I) was attributed to the single electron oxidation of the amine substituent (−NH2) that initiated the polymerization.36 The reversible wave between 0.4 and 0.45 V (II) can be assigned as a single electron process of the phthalocyanine ligand. Following the first scan, signals I and II diminishes and were progressively replaced by a quasi-reversible redox wave with a maxima of 0.55 V for the forward anodic scan (III) and a minima of 0.35 V for the reverse cathodic scan (III′). The gradual increase in both anodic and cathodic currents was a result of the addition of new PtTAPc from the solution to the existing polymer on the GCE surface.37 Apart from the charge transport involved in the oxidation/reduction of existing polymer film, PtTAPc cross-linking reaction also added to the total current.38 Similar trends in the CV scans have also been observed for CuTAPc39,40 and ZnTAPc41 in this work. Sensitivity of Poly-PtTAPc to NO Oxidation. The viability of poly-PtTAPc/GC as a NO sensor electrode was confirmed by comparing the DPV current response to that of a bare GC electrode in PBS (pH 7.4) containing 2 μM of NO. In plot B of Figure 2, the bare GCE shows no detectable signal to 2 μM of NO, whereas the current response of the polyPtTAPc/GC electrode is characterized by a well-defined anodic peak at 0.775 V (relative to Ag/AgCl). The anodic current enhancement observed in the electrochemical oxidation of NO agrees well with a huge pool of reported electrodes modified by metallophthalocyanine complexes.6 In aqueous medium or in biological media, NO radical molecule readily oxidizes into nitrite (NO2−). The oxidation potential of NO2− is typically equal to or 60−80 mV higher33,42−45 than that of NO and hence NO2− is a major interferent in the electro-oxidation of NO. An additional coating of Nafion over metal-phthalocyanine 24,25,46 or 109
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Figure 3. Photograph of the AAO/Pt electrode (A) with the general schematic shown in (B). FE-SEM of the AAO/Pt after dissolution of the AAO membrane filter shows the annular morphology of the sputtered Pt (C).
treatment are effective measures to create adequate insulation. After removal of the tape, treatment of the AAO/Pt in 0.1 M NaOH reveals the annular morphology34,51 of the sputtered Pt (part C of Figure 3). During Pt sputtering on the nanoporous AAO membrane, the Pt particles enter the nanosized pores and travel until they are attracted by the inside walls of the channel, therefore giving rise to the annular morphology.52 The AAO/Pt electrode was subjected to 200 repetitive cycling between −0.2 to +0.9 V in DMSO containing 0.1 mM PtTAPc monomer and 0.1 M TBAP, at a scan rate of 100 mV/ s. The choice of the monomer concentration, scan rate and number of CV scans was adapted from previous findings.34 Before the cyclic voltammetric electropolymerization began, immersing AAO/Pt in the MTAPc solution for 30 min allows complete infiltration of the nanosized channels.34,53 After drying, the immersed portion of the AAO/Pt shows a faint greenish coloration which indicated successful formation of poly-PtTAPc within the pores of the AAO membrane. Part A of Figure 4 depicts the poly-MTAPc nanotubes array entrenched within the pores of the AAO membrane filter. The resultant poly-PtTAPc nanotube array immobilized on the annularshaped Pt layer show an average length of 1 μm and forms clusters due to the surface tension effects of the evaporating solvent between the nanotubes54 (part B of Figure 4). On the basis of the mechanism of polymer tube formation proposed by Gu et al.34 and Cho et al.,51 the main reason for the hollow poly-PtTAPc nanotube formation within the AAO pores has been the annular morphology (Figure 3) of the sputtered Pt onto the filtration surface of the AAO membrane coupled with the low monomer concentration (0.1 mM). Electrode 1 versus Electrode 2: DPA Calibration of NO concentration between 0.1−1.0 μM. Figure 5 shows the stepwise DPA plots obtained using electrodes 1 and 2 (fabricated with electropolymerized PtTAPc) in PBS (pH 7.4) containing NO concentration ranging from 0.1 to 1.0 μM. Compared to electrode 1, the steeper steps of the current response from the nanoporous electrode 2 visually indicated a major sensitivity improvement.
Figure 4. (A) Schematic diagram of the poly-MTAPc nanotube array entrenched within the AAO membrane filter after electropolymerization. (B) FE-SEM images of the densely packed poly-PtTAPc nanotube array attached to the Pt substrate after dissolution of the AAO membrane.
Figure 6 summarizes the sensitivities of electrodes 1 and 2 in the form of 3D plots with data presented for all three MTAPcs. DPA calibration in the NO concentration range of 0.1−1.0 μM shows minor sensitivity differences for the three MTAPcs for both electrode systems. The results summarized in Figures 5 and 6 clearly show the superior sensitivity of the nanoporous design of electrode 2 simply due to the enhanced surface area provided by the polyMTAPc nanotube array. Adopting the theory derived from the study of nanoelectrode ensemble (NEE) by Ugo and coworkers,55,56 Faradaic-to-capacitive currents at the nanoporous electrode 2 and the flat electrode 1 of the same geometric area are related by the following relationship: 110
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(diffusion-controlled).58 Walcarius have explained further that for a diffusion-controlled reaction the capacitive current of a highly porous electrode is expected to increase parallel to the Faradaic current when the active surface area is increased.57 The sensitivity improvement of nanoporous electrode 2 over the flat electrode 1 observed in Figure 5 and 6 strongly suggests that the MTAPc-mediated NO oxidation process likely involves a rate-limiting step involving adduct formation between NO and the MTAPc unit, in good agreement with the proposed mechanism by Jin et al. in 199925 and Bedioui et al. in 2003.59 Other studies conducted to investigate the influence of different metal centers in MPcs on the electrocatalytic reduction of SOCl2,60 O25 and electro-oxidation of OH−60 have concluded that the redox reaction occurred through a metal-based redox process. These studies6,60,61 have asserted that central metal ion serves as the active site for the ligation of SOCl2, O2, and OH− prior to further reaction. Interestingly, switching the central metal ions (only the first row transition metals have been explored) of the MPc derivatives effectuated significant changes to the catalytic activity. On the basis of the examples provided for SOCl2,60 O25, and OH−,61 the three authors invoked the coordination preference of the central metal ion, the number of d electrons, the energy of d orbitals, the strength of the bonding between the central metal ion, and the nature of the analyte/adsorbate to explain the differences in electrocatalytic activity in relation to the metal center of the MPcs. Our results in Figure 6 clearly rule out the possibility of NO interaction with the metal center and a ligand-based redox process possibly dictates the oxidation of NO molecules accounting for low sensitivity differences of the three different metal centers. In view of the enhanced sensitivity displayed by its nanoporous design, electrode 2 has demonstrated an impressive detection limit of 10nM, which was of an order lower than that of the flat electrode 1 (0.1 μM). Figure 7 shows the DPA calibration of electrode 2 (based on electropolymerized PtTAPc) obtained by 10 successive injections of 10nM equivalent of NO at a regular interval of 30 s. The corresponding linear calibration plot shows an excellent sensitivity of 31.5 nA/nM. Table 1 lists the sensitivity of electrode 2 for NO concentration range of 10nM to 0.1 μM, for the three MTAPcs. As seen in Table 1, irrespective of the metal center, the three MTAPcs again show similar sensitivities (of the same order). Whereas the impressive sensitivity and a linear range of the Nafion-coated electrode 2 has been demonstrated using DPA, this electrode can also show good repeatability up to a month when stored in deoxygenated PBS solution (pH 7.4). With an additional layer of Nafion, good selectivity against NO2− has been demonstrated in our previous investigation.33 However in biological media, ascorbate, uric acid, hydrogen peroxide, carbon monoxide, dopamine, norepinephrine, serotonin, 3,4dihydroxyphenylacetic acid (DOPA), and 5-hydroxyindole-3acid (5-HIAA) are known interferents of NO.62 A study by Bedioui63 have shown that the Nafion and o-phenylenediamine ad-layers over nickel tetrasulfonated phthalocyanine (NiTSPc) modified electrodes imparts decent selectivity against the above-mentioned interferents. Similar study on the permeability of different electropolymerized polymer films to uric acid, dopamine, acetaminophene and L-ascorbic acid has also been presented by Lee.64 Further work should focus on the selectivity of the reported electrodes.
Figure 5. Amperometric current−time curve response obtained for successive addition of 0.1 μM of NO with Nafion/poly-PtTAPc nanotube/AAO/Pt (red line) and, Nafion/poly-PtTAPc/GCE (black line) for NO concentration of 0.1−1.0 μM in PBS (pH 7.4). The reported sensitivity of the two sensor electrodes were 8.84 μA/μM and 0.57 μA/μM respectively.
Figure 6. Comparison of the sensitivity of planar electrode 1 and nanoporous electrode 2 based on DPA calibration for NO concentration range of 0.1 to 1.0 μM (data presented for the MTAPcs with Cu, Zn, and Pt metal centers).
() () IF IC
2
IF IC
1
=
A2 A1 (1)
Where IF is the faradaic current, IC is the double layer charging current, A2 is the total surface area of the poly-MTAPc nanotube array in electrode 2, and A1 is the total surface area of the poly-MTAPc film on planar electrode 1. Eq 1 implies that the improvement in the signal-to-noise ratio of electrode 2 over electrode 1 can be approximated by the surface area enhancement provided by the poly-MTAPc nanotubes. On the basis of the above equation, assuming a pore density of 109cm−2 for the AAO membrane, 1 μm length and 200 nm diameter for the poly-MTAPc nanotubes, electrode 2 should be 6 times more sensitive than electrode 1, which is close to the 4 to 13 times as calculated from the data in Figure 6. On the basis of several examples cited in the review by Walcarius,57 the nanoporous design of electrode 2 offers improvement in sensitivity over its flat counterpart (electrode 1) only when the analytes adsorb onto the electrode surface (surface confined species) as the rate-determining step but not for those reactions that are limited by transport of the analyte 111
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ASSOCIATED CONTENT
S Supporting Information *
Additional details on the synthetic procedures of all complexes and the corresponding characterization data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Funding Source
National University of Singapore (Grant number: R143- 000− 369−112) Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful to Dr. Gu Feng for guidance in the electroanalysis.
Figure 7. Amperometric current−time curve response obtained using electrode 2 (based on poly-PtTAPc) in PBS (pH 7.4) with additions of NO concentration from 10 nM to 0.1 μM. (B) Linear plot of the current change versus [NO]/nM.
Table 1. DPA Sensitivities of the Electrode 2 (For Different Metal Centers) in PBS (pH 7.4) in NO Concentration Range of 10nM to 0.1μM M
sensitivity (nA/nM)
R2
Cu Zn Pt
20 18 31.54
0.992 0.9882 0.9903
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CONCLUSIONS An AAO-templated method was employed to create an array of hollow nanotube structures of electropolymerized MTAPc entrenched within the nanosized pore channels, and immobilized onto the sputtered Pt at the base. Due to the unique nanotube-arrayed morphology and the high electroactive area of the electrocatalytic poly-MTAPc redox mediator, an enhanced sensitivity and a lower detection limit was observed for each system when compared to a thin film of the same polymer immobilized on GC electrode of equal plane area. Optimizing the length and morphology of the poly-MTAPc nanotubes as well as the AAO pore sizes toward NO sensing performance are important questions at present. However, biocompatibility, selectivity, and resistance to fouling are more urgent issues. We envisage that more novel methods and electrode design can be developed to maximize the sensitivity and selectivity of such nanoporous modified sensor electrodes. 112
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Analytical Chemistry
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dx.doi.org/10.1021/ac302081h | Anal. Chem. 2013, 85, 107−113
