Voltammetric pH Nanosensor - Analytical Chemistry (ACS Publications)

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Voltammetric pH Nanosensor Magdalena Michalak, Malgorzata Kurel, Justyna Jedraszko, Diana Toczydlowska, Gunther Wittstock, Marcin Opallo, and Wojciech Nogala Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03482 • Publication Date (Web): 30 Oct 2015 Downloaded from http://pubs.acs.org on November 3, 2015

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Analytical Chemistry

Voltammetric pH Nanosensor Magdalena Michalak,† Malgorzata Kurel,† Justyna Jedraszko,† Diana Toczydlowska,†,§ Gunther Wittstock,‡ Marcin Opallo,*,† Wojciech Nogala*,† †

Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, PL-01224 Warsaw, Poland Institute of Chemistry, Center of Interface Science, Faculty of Mathematics and Science, Carl von Ossietzky University of Oldenburg, D-26111 Oldenburg, Germany ‡

ABSTRACT: Nanoscale pH evaluation is prerequisite for understanding of processes and phenomena occurring at solid-liquid, liquid-liquid and liquid-gas interfaces, e.g. heterogeneous catalysis, extraction, partitioning and corrosion. Research on homogeneous processes within small volumes such as intracellular fluids, microdroplets and microfluidic chips also require nanometer scale pH assessment. Due to opacity of numerous systems optical methods are useless and, if applicable, require addition of a pH-sensitive dye. Potentiometric probes suffer from many drawbacks such as potential drift and lack of selectivity. Here we present a voltammetric nanosensor for reliable pH assessment between pH 2-12 with high spatial resolution. It consists of a pyrolytic carbon nanoelectrode obtained by chemical vapor deposition (CVD) inside a quartz nanopipette. The carbon is modified by adsorption of syringaldazine from its ethanolic solution. It exhibits a stable quasi-reversible cyclic voltammogram with nearly Nernstian dependency of mid-peak potentials (-54 mV/pH). This sensor was applied as probe for scanning electrochemical microscopy (SECM) in order to map pH over a platinum ultramicroelectrode (UME) generating hydroxide ions (OH-) by oxygen reduction reaction (ORR) at diffusion-controlled rate in aerated phosphate buffered saline (PBS). The results reveal the alkalization of the electrolyte close to oxygen reducing electrode showing insufficient buffer capacity of PBS to maintain a stable pH at given conditions.

The pH is an important parameter influencing plenty of processes occurring in aqueous phase or at its interfaces. In the case of processes involving protons (hydronium) or hydroxides ions, their mechanisms, kinetics as well as thermodynamics are obviously affected by pH. Even if neither H+ nor OH- ions are directly involved in the process, pH may affect the properties of catalyst or reacting substances. This is the case for biological systems, where reactions occur within confined pH range. Acidity also plays a crucial role in heterogeneous reactions, because pH may influence the structure of the interfaces and heterogeneous catalyst. The pH of bulk aqueous solutions is easily and reliably measured by a commonly used potentiometric glass electrode or a variety of voltammetric,1 colorimetric,2 spectroscopic3 and optical sensors with photoinduced electron transfer signal transduction.4 However, pH measurement inside a femtoliter volume (e.g. biological cells) or locally with micrometric resolution requires appropriate microsensor. Microscale pH evaluation can be done by microspectroscopic measurements5,6 or by potentiometric microsensor.7-10 The former approach can be applied to transparent samples, but the required addition of a pH-sensitive dye may affect the studied process. Mappping of local pH with nanometer resolution by optical methods faces fundamental difficulties due to the Abbe diffraction limit. Potentiometric pH microsensors reported to date require absence of electrochemically active substances in solution. Particularly problematic are compounds that adsorb on the sensor surface and/or exhibit fast electron transfer kinetics. In such cases the open circuit potential (OCP) is a mixed potential, i.e. the sensor is not selective.11,12 Apart from the dependency on matrix composition, miniaturized

potentiometric sensors suffer from potential drift caused by the finite input impedance of voltage followers used for its measurement and resulting parasitic non-zero current. For instance 1 pA of parasitic current (1 TΩ input resistance, 1V) for a 1 µm diameter disk microsensor corresponds to a current density of ca. 130 µA cm-2 which is incompatible with the measurement of an OCP. Moreover, an important disadvantage of potentiometric sensors is their long response time determined by the time for the OCP equilibration. This substantially prolongs SECM experiments.13 Herein, we present a simple voltammetric pH nanosensor (50 nm diameter) which is deprived of the drawbacks of potentiometric sensors. Contrary to optical methods, it can be used in opaque solutions and suspensions. Its response time is limited by the time required for recording of a voltammogram. Potential drift problem is eliminated by imposing a potential by an external amplifier (voltage clamp, potentiostat). The sensor consists of a carbon nanoelectrode (CNE)7,14-17 with preadsorbed syringaldazine (4-hydroxy-3,5dimethoxybenzaldehyde azine, Syr). This water-insoluble compound has been earlier applied as a pH sensitive voltammetric probe adsorbed on carbon ceramic electrodes.17 Here, Syr-modified carbon nanoelectrode (Syr-CNE) exhibits stable cyclic voltammamograms (CVs). Its mid-peak potential is linearly dependent on the pH, because protons are involved in the electrode reaction of Syr (Scheme 1).18 As a case study, it was applied as an SECM probe for pH mapping over a 10 µm diameter Pt microelectrode at which ORR was carried out. Results reveal local pH changes in the vicinity of the sample, even in buffered solution.

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Scheme 1. The syringaldazine.

mechanism

of

electrode

reaction

of

EXPERIMENTAL SECTION Fabrication and characterization of nanoelectrodes. CNEs were prepared in a similar manner as described elsewhere.7,14-17 A quartz capillary (o.d. 1.0 mm, i.d. 0.7 mm, Sutter Instruments) was pulled by a laser puller (P-2000, Sutter Instruments) using the following program: line 1: HEAT=700, FIL=4, VEL=55, DEL=130, PUL=55; line 2: HEAT=700, FIL=4, VEL=45, DEL=132, PUL=150. The back side of the obtained nanopipette was connected to a silicone tube fed with petroleum gas (ca. 60 vol% butane, ca. 40 vol% propane) under 1.5 bar. Using a micromanipulator (PT3/M, Thorlabs), the tip of the nanopipette was inserted into a bigger quartz tube (o.d. 3.0 mm, i.d. 2.0 mm), which opposite side was fed with a 15 mL min-1 argon stream (N5.0, Multax) maintained using gas flow controller (SmartTrak 2, Sierra Instruments). The petroleum gas inside the argon-shielded nanopipette was pyrolysed 3 times by directing a butane-air gas torch (Miniset 1450°C, Rothenberger Industrial) flame at the nanopipete tip for ca.5 sec (video in Supporting Information S1). Between the pyrolysis steps, the silicone tube with 1.5 bar petroleum gas was disconnected and connected to the pipette several times in order to remove gaseous products of the pyrolysis and to deliver fresh propane-butane reactant gas. An 0.5 mm diameter bended copper wire was inserted into the pipette with carbonized interior to provide electrical connection. Such prepared nanoelectrodes were examined by CV in aqueous 1 mM ferrocenemethanol (FcMeOH) and 0.1 M KCl with Axon Multiclamp 700B patch-clamp amplifier with automatic cell capacitance compensation controlled by DigiData 1440 acquisition system (Molecular Devices). A 0.5 mm diameter silver wire (99.999 %, Mint of Poland) immersed ca. 1 cm into the electrolyte served as quasireference and counter electrode. Its potential was assumed to be stable during the experiment when sub-nanoampere currents were recorded. CNEs were accepted for further experiments only when exhibited a stable steady-state voltammogram at 10 V s-1 (ca. 80 % of all fabricated). Scanning electron microscopy (SEM) was performed with a FEI Nova NanoSEM 450 without additional sample coating. Carbon nanoelectrodes were modified with Syr by immersion in 0.5 mg mL-1 solution of Syr (99%, Sigma-Aldrich) in ethanol (99.8 %, POCH) for 20 seconds and dried under ambient conditions. Nanosensor calibration and pH mapping protocol. CVs of Syr-CNEs were recorded in two-electrode configuration with a Ag|AgCl|NaCl(3M) reference and counter electrode

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immersed into a 0.1 M phosphate buffer solutions of various pH. The pH was determined by a glass-electrode pH-meter (CG 837, Schott) calibrated using standard buffer solutions (Sigma-Aldrich).The calibration curve was constructed from the mid-peak potential vs. pH. The measurements were performed with VA-10X patch-clamp amplifier (npi Electronic Instruments), operated under SECMx software19 through PCI-DAS1602/16 analog-digital (AD) and digitalanalog (DA) and PCI-DDA04 DA cards (Measurement Computing). In order to evaluate mid-peak potential accurately from noisy CVs they were smoothed using Savitzky-Golay algorithm implemented in OriginPro8 software (Supporting Information S2). A pH-nanoprobe was positioned by a piezo positioner of CHI900B SECM system (CH Instruments) controlled by the same DA card and SECMx software. A model sample for pH mapping was a disk microelectrode prepared by sealing a 10 µm diameter Pt wire into borosilicate glass capillary and polished finally with 50 nm grade alumina lapping tape (Buehler). A pH-nanoprobe was initially positioned ca. 10 µm over the Pt UME using CHI900B motors and an optical microscope (Specwell 10x30) in air. After retracting the probe electrode by 1 cm (above further electrolyte level), the phosphate-buffered saline (PBS) was gentle poured into the cell and the Faraday cage was closed and the probe returned to the position optimized in air. It was then polarized at a potential of -0.7 V sufficiently low for oxygen reduction reaction (ORR). The approach with 9.77 nm steps was interrupted automatically when an ORR current dropped down by 20 % of its initial value (Supporting Information S3). Next, the probe was withdrawn 2.5 µm from the sample surface. A pH imaging was performed at constant height with 0.66 V s-1 scan rate of a single CV recorded in every grid point of scanning raster when the sample was polarized at -0.8 V using the second VA-10X patch-clamp amplifier. From the obtained CVs the map of mid-peak potentials were extracted and rearranged to a map of local pH using MIRA software.20

RESULTS AND DISCUSSION Carbon nanoelectrodes. CV provides fast and reliable screening of CNEs for further modification. Any peak-shaped voltammograms obtained in the presence of the used redox mediator indicates a leaky or recessed electrode unsuitable for further use.17,21 It is important to exclude electrodes that allow a penetration of the electrolyte because the diffusion of solution components in and out of the pipette cavity through its tiny orifice prolongs its response time. For this application a short response time is more important than the potential advantage from larger amounts of redox-active adsorbate of the sensors when using inner surfaces of the pipettes.17 Diffusion limiting current at disk UME is given by the following equation:22 ∗

(1)

where n = 1 is the number of electrons exchanged with FcMeOH, F is the faraday constant, D (7.6×10-6 cm2 s-1)23 is the diffusion coefficient of FcMeOH, rT is the radius of active UME area, c* is the bulk concentration of mediator and β is a parameter dependent on the radius of glass sheath of the UME. For a 50 nm diameter UME which a glass sheath thickness of 25 nm, the calculated steady-state current of 1 mM FcCH2OH oxidation is ca. 8 pA. This is in accordance


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with the measured current and SEM micrograph of CNE (Fig. 1 inset).

pH

.

11.27

(2)

with a Pearson correlation coefficient R2 = 0.99 (Fig. 3b).

Figure 1. Cyclic voltammograms of carbon nanoelectrode recorded in 1 mM ferrocenemethanol in 0.1 M KCl at various scan rates: 0.1 V s-1 (black) and 10 V s-1 (red). Inset: SEM micrograph of analyzed electrode.

The absence of peak-shaped CVs even at scan rates as high as 10 V s-1 proves that the nanopipette is filled completely with carbon and the aqueous electrolyte does not penetrate into its interior.17,21 The agreement between calculated and measured diffusion-limited current means that no electrically connected carbon layer is deposited on the exterior of the pipette. In such a case the measured current would exceed the calculated value for the UME geometry derived from SEM micrographs.

Figure 2. Cyclic voltammograms of carbon nanoelectrode with preadsorbed syringaldazine recorded in 0.1 M phosphate buffer pH 7.05 at various scan rates: 10, 40, 60, 80, 100 mV s-1. Inset: plot of cathodic peak current vs. scan rate.

pH nanosensor. Carbon nanoelectrodes with physisorbed Syr exhibit stable quasi-reversible voltammetric behavior with peaks characteristic of exhaustive electrolysis of the adsorbate (Fig. 2). A linear relation between peak current and scan rate also confirms the adsorption of Syr. The Faradaic charge estimated by peak integration is ca. 10 pC corresponding to ca. 3 × 107 molecules, i.e. around 15,000 molecules per square nanometer of geometric area of the nanoelectrode (Supporting Information S4). Such a high adsorbate loading is probably caused by the microporosity of pyrolytic carbon24 and possible multilayer adsorption. Moreover, some excess of Syr could precipitate upon evaporation of ethanol after removing the nanoelectrodes from the adsorption solution. Such precipitated nanocrystals can undergo complete electrooxidation/electroreduction as previously found for bulk electrolysis of solid-state Syr microcrystals.25 We noticed that further immersion of Syr-CNE in ethanolic Syr solution may cause depletion of redox-active adsorbate, i.e. partial dissolution of previously deposited Syr. The electrode reaction of Syr is a two electron/two proton process (Scheme 1). The formal potential E°’ is proportional to pH with a theoretical slope of -59 mV pH-1.18 This is in agreement with the found dependence of the mid-peak potentials on pH dependency (Fig. 3). A slight difference between theoretical and experimental (-54 mV pH-1) slopes can be caused by hindered access of protons to redox-active Syr molecules adsorbed inside the micropores of carbon or inside precipitated nanoctystal. A least square fit of a linear calibration function yielded

Figure 3. Selected CVs (a) of carbon nanoelectrodes with preadsorbed syringaldazine recorded in various pH (labeled) 0.1 M phosphate solutions. Current is normalized versus anodic peak current. (b) Plot of mid-peak potential of CVs at various pH.



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where Emp is the mid-peak potential vs. Ag|AgCl|NaCl(3M) reference electrode. It is worth to mention that filtering of the current signal with the cut off frequency of the analog low-pass filter set to a value below the data acquisition rate causes distortion of the voltammetric signal, especially an enlargement of the peak-topeak separation. However, the mid-peak potential is unaffected by such ‘overfiltering’ providing a convenient way for its accurate evaluation. The proposed nanosensor as well as other modified nanoelectrodes require careful handling during their use. Especially important is the elimination of electromagnetic noise when the nanoelectrode is in contact with the electrolyte including the moments of its immersion and removal. The Faraday cage has to be closed all the time when a nanoelectrode is in contact with electrolyte. Otherwise, external alternating electromagnetic field induces parasitic currents, which shifts the nanoelectrode potential to extreme values and may damage them irreversibly. To keep the nanosensor intact one has to control its insertion into electrolyte by means of remotely controlled positioners mounted inside the Faraday cage. Although we did not perform Syr-CNE voltammetry in redox probe solutions, its presence at millimolar concentrations should not prevent pH probing, unless any electroactive substance, which redox potential is close to that of syringaldazine, adsorbs on Syr-CNE. Non-adsorbed mediator undergoing electrooxidation/electroreduction within a voltage scanning range would cause steady-state current offset (Fig. 1), which does not affect the determination of the mid-peak potential of Syr or is easy to deconvolute. In addition, a matrix dependent calibration can be performed in a solution containing the redox-active compound.

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distance of ca. 10 times the microelectrode radius.26 Here the bulk pH is found at a distance of ca. 50 µm. Mapping of a whole diffusion profile would require large scanning area and corresponding long imaging times. In order to present the ability of Syr-CNE for high resolution imaging of low local pH changes we used a buffered solution. A PBS was selected due to its common application in the biological studies. Its considerable, but finite buffer capacity causes depression of pH changes around the OH- source. However, the buffer capacity was selected such that it is not able to completely consume all OH- at the point probed by the Syr-CNE. This causes a diffusion-limited transport of buffer components towards the OH- source where a sharp reaction zone is formed.27-29 A pH map in Fig. 4a reveals that pH reaches ca. 8 over the center of UME. This means that even in buffered solution the pH at an ORR catalyst can be significantly shifted locally. This may influence reaction mechanisms,30 or cause deactivation of enzymatic catalyst.31 One needs to take into account the fact that during SECM study of ORR catalyst in redox competition32 or generation-collection mode33 the pH at the sample is also affected by the SECM probe, especially for shorter probe-to-sample distances. 30 µm aside from the sample microelectrode center, the measured OH- concentration is only about 6.5 × 10-8 M higher than in bulk PBS (pH 7.5 vs. pH 7.4).

We managed to obtain Syr-CNE based stable pH nanosensor after failed attempts using 1-aminoanhtraquinonediazonium cation grafting on CNEs and self-assembly of 11mercaptoundecylhydroquinone on gold nanoelectrodes.

pH mapping. Polarization of Syr-CNE at -0.7 V enables ORR and allows its controlled approach to the sample surface (Supporting Information S3) without losing its pH-sensing function. The CVs at more positive potentials shows stable Syr voltammograms as before the approach procedure. First, pH mapping over an oxygen reducing Pt-UME (diameter 10 µm) was performed with non-buffered 0.1 M KCl solution. The pH values from 9.1 to 9.7 were detected close to the Pt substrate electrode. However, no sharp peak appeared in a pH map of (70 × 70) µm2 scanning area that would reveal the dimension of the active sample. This is due to the local alkalization of the electrolyte caused by hydroxide anions generation at the sample: O2 + 4 e- + 2 H2O → 4 OH-

(3)

In non-buffered solution OH- ions generated by reaction (3) at diffusion-limited rate at the sample, diffuse towards the bulk electrolyte. In hemispherical diffusion regime, the product concentration is close to its bulk concentration already at a

Figure 4. (a) pH map recorded ca. 2.5 µm above a 10 µm diameter Pt disc electrode polarized at -0.8 V to reduce oxygen in aerated phosphate buffer saline (pH 7.4); (b) CVs recorded at the marked points within the pH map at a scan rate of 0.66 V s-1.


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The time for recording a pH map depends on the scan rate and the potential range of CV experiment, probe translation rate and the total number of image points. In our case the scan rate is restricted by specifications of the digital-analog and analog-digital converters. Recording CVs with 2 mV potential step is possible at a maximum scan rate of 0.66 V s-1. The scan rate could be increased with increased potential step on the expense of lower precision of mid-peak potential and pH evaluation. An increase of scan rate requires higher acquisition rates of the current signal, which usually causes noise enhancement. Because faradaic currents corresponding to adsorbed Syr oxidation/reduction are proportional to scan rate (Fig. 2) current noise would not be a problem. This enables further improvement of presented technique by faster CV acquisition. The potential range of CV has to be adjusted according to expected pH values over the analyzed sample with additional overpotentials required to complete oxidation/reduction of the adsorbate (Fig. 4b). In the example presented in Fig. 4 each point of the pH map was calculated from the mid-peak potential of CV recorded within the potential range 0 to 0.32 V. The time required for an individual CV is 0.97 s. Recording of all CVs of the image took 20 min. Another 20 min. was spent on probe translation. One important advantage of the nanosensors even for imaging of larger sample features is the avoidance of hindered diffusion of reactants to the sample in the samplegeneration/tip-collection mode.34,35 In addition, there is a prospect for a substantial improvement in lateral resolution because the probe size ultimately limits the resolution of any scanning probe techniques, CNE as small as 10 nm in diameter could be prepared16 and modified with syringaldazine. Such electrodes can become applicable for scanning a samples with nanostructured topographic features when combined with a vertical position control based on a distance-dependent signal.

CONCLUSIONS In summary, we presented a nanoprobe for pH imaging with fast response and high spatial resolution. It is based on the determination of the mid-peak potential of a cyclic voltammogram of an adsorbed redox-active organic compound with pH dependent electrode potential. The fabricated nanoprobes were carbon nanoelectrodes with preadsorbed, water-insoluble syringaldazine exhibiting stable quasireversible voltammetric response within the pH range 2-12. It was successfully applied to map local alkalization of phosphate buffered saline in the vicinity of a sample microelectrode during oxygen reduction reaction.

ASSOCIATED CONTENT Supporting Information Details of nanoelectrode preparation, CV curves smoothing, probe approaching, amount of adsorbate evaluation and sample voltammetry. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], [email protected], Phone: +48 22 343 3375

Present Addresses § Laboratory of Bioinformatics and Protein Engineering, International Institute of Molecular and Cell Biology in Warsaw, PL-02109 Warsaw, Poland

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. All authors contributed equally.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the Ministry of Science and Higher Education (Poland) for funding (grant no. IP2012 048872).

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Voltammetric pH Nanosensor - Analytical Chemistry (ACS Publications)

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