Electrochemical sensing platform for hydrogen peroxide using amorphous FeNiPt nanostructures

Abstract

Amorphous ternary FeNiPt nanomaterials with tunable length are reported for constructing the electrochemical sensing platform, in which hydrogen peroxide (H₂O₂) is selected as a model target. It is found that amorphous FeNiPt nanostructures, in particular, the FeNiPt nanorods with large axial-ratio (FeNiPt-NRL) exhibit the enhanced electrocatalytic activity towards both the oxidation and reduction of H₂O₂. Meanwhile, the comparable corrosion potential E_corr and lower corrosion current I_corr are observed at GC electrodes modified with FeNiPt-NRL, revealing the good stability of the FeNiPt-NRL surface. On the basis of these results, H₂O₂ is cathodically determined at glassy carbon (GC) electrodes with FeNiPt-NRL with relatively high selectivity at the appropriate potential of 0 V vs. Ag|AgCl. On the other hand, the sensitivity of the anodic H₂O₂ detection at the same electrode is achieved to be 2.45 mA cm⁻² mM⁻¹, which is 4-fold larger than that of the cathodic detection and those Pt nanoparticles-based H₂O₂ determination, and the detection limit is also developed to 40 nM. The dynamic linear range is broadened from 100 nM to 30 mM, which is wider than those H₂O₂ detection based on Pt nanoparticles and binary Pt alloys. In addition, electrochemical results show the high stability and good reproducibility for the present H₂O₂ sensor. The striking analytical performance combined with the intrinsic properties of amorphous FeNiPt nanomaterials provides a promising technique for the development of non-enzyme H₂O₂ and other molecule sensors with high sensitivity, broad dynamic linear range, long stability, and good reproducibility.

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1. Introduction

Hydrogen peroxide (H₂O₂) is of great interest in connection with the development of fuel cells,^1–5 nanomachines,^6–8 and life sciences^9–11 because H₂O₂ is an important intermediate of the oxygen reduction reaction, new fuel, and a product of several biological enzyme-catalyzed reactions. Therefore, it is of great importance to determine the concentration of H₂O₂ with high sensitivity, low detection limit, long stability, and good reproducibility. Direct electrochemical reduction and oxidation of H₂O₂ at conventional electrodes have been limited by the slow kinetics and surface fouling onto electrochemical devices. In addition, low sensitivity, poor reproducibility, and low stability are other limitations of using the bare and unmodified electrodes for H₂O₂ detection. With the development of biology, numerous electrochemical biosensors based on the use of different enzymes have been developed for the determination of H₂O₂.^12–17 Many of the biosensors show a satisfactory sensitivity for the detection of a relatively low concentration of H₂O₂. However, the limitation of these enzymatic H₂O₂ sensors is lack of long-term stability and good reproducibility originating from the intrinsic characteristic of biomolecules. Therefore, it is desirable to improve the stability and reproducibility for H₂O₂ detection with enhanced sensitivity.

For this purpose, plentiful attempts have been made to develop H₂O₂ sensors without enzymes. In the past couple of decades, there has been a burst of research activities on nanostructured metal particles, particularly noble metal nanoparticles with controlled size and shape, because of their unique physical and chemical properties.^18–20 A number of Pt nanomaterials and nanocomposites have been fabricated and widely used for the electrochemical detection of H₂O₂ because of the excellent stability and striking catalytic ability of Pt. Thus, nanostructured Pt-based H₂O₂ sensors exhibited high sensitivity, long stability, and immunity to poisoning.

However, few papers were concerned with amorphous ternary alloy nanoparticles, despite their potential applications in data storage, high performance permanent magnets, biomedicine, electrocatalytic activities, and electrochemical detection. As a matter of fact, amorphous alloy materials have attracted a lot of attention due to their superior electronic, magnetic, and chemical properties, as well as practical applications in various fields, such as powder metallurgy, magnetic recording media, ferrofluids, composite materials, and catalysis.^21,22 With the development of nanomaterials, great efforts have been paid on preparation of amorphous alloy nanomaterials, which constitutes an overlapping region of two active fields full of challenge, by the combination of the unique properties of amorphous alloys and striking characteristics of nanosized materials.

In our previous work, amorphous ternary FeNiPt nanoparticles with different lengths through phase transfer method were fabricated and used for the electroanalysis of thiols.²³ In the present work, the amorphous ternary FeNiPt nanoparticles were employed for constructing an electrochemical sensing platform for determination of H₂O₂ both anodically and cathodically, since FeNiPt nanostructures showed enhanced electrocatalytic activity towards the oxidation and reduction of H₂O₂. The sensitivity of the present H₂O₂ sensor was improved to be 2.45 ± 0.1 mA cm⁻² mM⁻¹ over the broad dynamic detection linear range of 100 nM–30 mM for FeNiPt-NRL electrode, and the detection limit was achieved to be 40 nM.

2. Experimental

2.1 Materials

Dihydrogen hexachloroplatinate (H₂PtCl₆·6H₂O, 99%), FeCl₂·4H₂O (99%), NiCl₂·6H₂O (99%), oleic acid (C₁₈H₃₄O₂, 99%), sodium oleate, propylene glycol (C₃H₈O₂, 99%) were all purchased from Sinopharm Chemical Reagent Co., Ltd. Nafion solution (5 wt%) was received from Sigma and then diluted 50 times with ethanol. Phosphate buffer solution (PBS, pH 7.0) was prepared by mixed stock standard K₂HPO₄ and KH₂PO₄ solutions and pH of PBS was adjusted by pH meter. The freshly prepared H₂O₂ stock solution was stored in an amber glass bottle in a refrigerator (4 °C) prior to experimentation. The serum was separated from the blood by centrifuging at a speed of 4500 rpm for 30 min. Then, the serum was diluted with the same volume of 0.2 M phosphate buffer (pH 7.0) before measurements. All the solutions were prepared with Milli-Q water and were deaerated with high purity nitrogen for 30 min before experiments.

2.2 Synthesis and characterization

Ethyl alcohol absolute solution (2 ml) was mixed with FeCl₂·4H₂O (30 mM), NiCl₂·6H₂O (35 mM), and H₂PtCl₆·6H₂O (1 mM) by ultrasound at room temperature, then added to 60 ml autoclave tube. Sodium oleate, propylene glycol, and oleic acid were added into the solution. The reaction system was sealed and heated at a designed heating rate up to 170 °C. After the reaction was cooled to room temperature, the FeNiPt nanoparticles were collected at the bottom of the container. Scanning electron microscopy (SEM) images were taken by a Quanta 200 FEG (FEI Company). XRD pattern was obtained by a D/max2550VB3+/PC X-ray diffractormeter using Cu (40 kV, 100 mA).

2.3 Electrochemical measurements

All electrochemical experiments were performed with CHI 660 and CHI 832 electrochemical workstations (CH Instrument Company). A conventional three-electrode system was adopted. The working electrode was a modified GC electrode (3 mm in diameter). KCl-saturated Ag|AgCl and a Pt coil were used as the reference and counter electrodes, respectively. All electrode potentials in the present study were referred to the Ag|AgCl reference electrode. A GC disk electrode was first polished with alumina slurries (1 and 0.05 μm) and then cleaned by sonication in Milli-Q water for 10 min. The modification of the working electrode was prepared by dropping dispersed FeNiPt nanoparticles onto the clean GC electrode by a microlitre syringe. Then 4 μL of diluted Nafion solution was dropped to form a thin film over the FeNiPt nanoparticles. The modified electrodes were finally dried in air. The FeNiPt-modified electrode was then rinsed with excessive ethanol and Milli-Q water to remove loosely bound particles. The electrodes modified with FeNiPt nanorods with large axial ration, nanorods, and nanospheres were denoted as FeNiPt-NRL/GC, FeNiPt-NR/GC, and FeNiPt-NS/GC electrodes. Before recording the voltammograms, the FeNiPt-modified surface was cleaned in N₂-saturated 0.5 M H₂SO₄ by cycling between −0.2 and 1.3 V (vs. Ag|AgCl) until the steady CVs were observed. In continuous-flow injection system, the thin-layer radial electrochemical flow cell (BAS) consists of a thin-layer radial flow block with the FeNiPt-modified GC electrode as working electrode, stainless steel as a counter electrode, and KCl-saturated Ag|AgCl electrode as a reference electrode. PBS or H₂O₂/PBS were delivered from gas-impermeable syringes (BAS) and pumped through tetrafluoroethylene hexafluoropropene (FEP) tubing by a microinjection pump (CMA 100, CMA Microdialysis AB, Stockholm, Sweden) to the radial flow cell.

3. Results and discussion

3.1 Surface properties of FeNiPt nanostructures

The amorphous FeNiPt nanostructures with different lengths were prepared as previously reported.²³ The morphology of as-synthesized FeNiPt nanomaterials is strongly dependent on the heating rates in this solid–liquid–solution system. As mentioned as in Experimental, the FeNiPt nanoparticles dispersion was dropped onto the carbon electrode and was confined in Nafion. Fairly dispersed large axial-ratio nanorods with the length of ∼700 nm and the diameter of ∼40 nm demonstrated in Fig. 1(A) were obtained at the heating rate of 3–8 °C min⁻¹, while short nanorods with the length of ∼200 nm and the average diameter of ∼20 nm (Fig. 1(B)) were produced at the heating rate of 10–12 °C min⁻¹. On the other hand, when the heating rate reached to 16–18 °C per minute it produced FeNiPt nanoparticles with the average diameter of ∼60 nm as shown in Fig. 1(C).

Fig. 1 SEM images of (A) FeNiPt-NRL/GC, (B) FeNiPt-NR/GC, (C) FeNiPt-NS/GC surfaces; (D) X-ray diffraction patterns of (a) FeNiPt-NRL/GC, (b) FeNiPt-NR/GC, (c) FeNiPt-NS/GC surfaces.

X-ray diffraction (XRD) pattern of FeNiPt-NRL (Fig. 1(D)) (a), FeNiPt-NR (b), and FeNiPt-NS (c) confined on GC surfaces shows the strong (111) peak at ∼40.0° and a (200) peak at ∼46.5°, corresponding to the standard face-centered cubic (fcc) structure with a random crystalline orientation.²⁴

For testing the stability of these amorphous FeNiPt nanostructures-modified electrodes, linear sweep voltammetry was carried out for four types of electrodes: bare GC (a), FeNiPt-NRL/GC (b), FeNiPt-NR/GC (c), and FeNiPt-NS/GC (d), as demonstrated in Fig. 2. The corrosion potential E_corr for bare GC electrodes was observed at −230 mV, E_corr for FeNiPt-NS/GC and FeNiPt-NR/GC was observed at more negative potentials of −252 mV and −243 mV, respectively. However, the presence of FeNiPt-NRL redirected E_corr in the opposite direction (back to −230 mV) due to the noble electrochemical characteristics of FeNiPt-NRL.

Fig. 2 Polarization curves for (a) bare GC, (b) FeNiPt-NRL/GC, (c) FeNiPt-NS/GC, and (d) FeNiPt-NR/GC electrodes in 0.5 M H₂SO₄ at 1 mV s⁻¹ with nitrogen bubbling.

The electroactive surface areas of all the electrodes could be estimated according to the Randles-Sevcik equation.²⁵

I_p = 2.69 × 10⁵AD^1/2n^3/2γ^1/2C

Where n is the number of electrons participating in the redox reaction, A is the area of the electrode (cm²), D is the diffusion coefficient of the molecule in solution (cm² s⁻¹), C is the concentration of the probe molecule in the bulk solution (mol cm⁻³), and γ is the potential scan rate (V s⁻¹). The surface areas of FeNiPt-NRL/GC, FeNiPt-NR/GC, FeNiPt-NS/GC electrodes employed in the present work were calculated using Fe(CN)₆⁴⁻ solution. The roughness factor of FeNiPt-NRL/GC, FeNiPt-NR/GC, FeNiPt-NS/GC electrodes was estimated from the electroactive surface area compared to the geometric one to be 2.53, 2.28, and 1.67, respectively, indicating the increased surface area of FeNiPt-modified electrodes.

3.2 Electrocatalytic ability towards H₂O₂

Films of FeNiPt nanostructures modified on GC electrodes exhibited electrocatalytic activity for both reduction and oxidation of H₂O₂. Fig. 3 displays CVs obtained at FeNiPt-NRL/GC (a), FeNiPt-NR/GC (b), and FeNiPt-NS/GC (c) in the presence of 1 mM H₂O₂ (Fig. 3(A)) and in the absence (Fig. 3(B)) in PBS. For comparison, CV of H₂O₂ at bare GC electrode was also given curve d in Fig. 3(A). Only the small anodic and cathodic peaks corresponding to Pt oxide and reduction formation were observed at +600 mV and the range of 50–100 mV at FeNiPt-modified electrodes in the absence of H₂O₂, respectively (Fig. 3B), which are in good agreement with the previous reports.²³ Meanwhile, no obvious currents were obtained at bare GC electrode in the presence of H₂O₂. However, the reduction and oxidation currents were obviously increased at FeNiPt-modified GC electrodes in the presence of H₂O₂ as shown in Fig. 3(A), indicating redox reaction of H₂O₂ was enhanced both in anodic and cathodic directions by using FeNiPt nanostructures. The reduction peak of H₂O₂ was obtained at around 100 mV with a peak current density of 251 μA cm⁻² at the FeNiPt-NRL/GC surface, while at FeNiPt-NR/GC and FeNiPt-NS/GC films, the peak potentials were observed for H₂O₂ reduction at about 85 mV and 56 mV, respectively, and the peak current densities are a little smaller than that obtained at the FeNiPt-NRL/GC surface. On the anodic sweep, H₂O₂ oxidation occurred with an onset at around 200 mV and reached almost a plateau at 600 mV. The oxidation current increased from 52 μA cm⁻² at FeNiPt-NS/GC surface to 90 μA cm⁻² at FeNiPt-NRL/GC surface, revealing that the electrocatalytic ability of the present nanostructured FeNiPt surfaces towards H₂O₂ redox reaction increases in the order: FeNiPt-NS/GC < FeNiPt-NR/GC < FeNiPt-NRL/GC.

Fig. 3 CVs obtained at (a) FeNiPt-NRL/GC, (b) FeNiPt-NR/GC, (c) FeNiPt-NS/GC, and (d) bare GC electrodes in 0.1 M PBS solution in the presence of H₂O₂ (A) and in the absence of (B) 1 mM H₂O₂. Potential scan rate: 20 mV s⁻¹.

The enhanced electrocatalytic ability of the FeNiPt nanostructures was proposed to be attributed to the combined properties of amorphous alloys and the nanosized materials. For further understanding of the mechanism, the phase transformation from fcc to long-range chemically ordered fct was carried out by annealing the amorphous FeNiPt-NRL at 823 K for 1 h under Ar atmosphere. The fct phase of the resulted annealed FeNiPt-NRL was clearly observed from the XRD pattern (inset of Fig. S1, ESI†). The electrocatalytic ability of the annealed FeNiPt-NRL were also examined towards H₂O₂, compared with that of the amorphous one. From Fig. S1, ESI,† it is clear that electrocatalytic ability of the annealed FeNiPt-NRL/GC towards H₂O₂ (Fig. S1b, ESI†) is obviously weaker either in potential or in current density than those of the amorphous FeNiPt-NRL/GC (Fig. S1a, ESI†). These results suggested that the unique properties of amorphous alloys, i.e. the short-range order and long-range disorder in structure, together with the striking characteristic of nanosized materials, especially, the 1D nanosized materials may play the important role in the electrochemical catalysis.

3.3 Cathodic detection of H₂O₂

As demonstrated above, amorphous FeNiPt nanostructures provided a platform for sensing H₂O₂ both anodically and cathodically. Fig. 4 demonstrates amperometric responses for addition of H₂O₂ with various concentrations at the applied potential of 0 V vs. Ag|AgCl using FeNiPt-NRL/GC electrode. The cathodic steady-state currents were obtained upon the successive addition of H₂O₂ from 0.6 μM to 60 mM, and the calibration curve was plotted in the inset of Fig. 4. The detection limit was developed to 200 nM and the sensitivity was estimated to be 0.13 ± 0.01 mA cm⁻² mM⁻¹. To test the selectivity of the H₂O₂ sensor, the interferences that may exist in food and biological samples were examined, such as ethanol, cysteine, xanthine, glucose, urine acid, ascorbic acid, dopamine, Ca²⁺, Mg²⁺, Cd²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cl⁻, SO₄²⁻, SO₃²⁻, NO₃⁻, NO₂⁻, and CO₃²⁻. As shown in Fig. S2, ESI,† almost no cathodic current response (< 1%) of the interferences was observed at FeNiPt-NRL/GC electrode at 0 V vs. Ag|AgCl, compared to the cathodic current of H₂O₂ with the same concentrations, indicating the good selectivity of this sensor at the appropriate working potential of 0 V vs. Ag|AgCl.

Fig. 4 Amperometric responses of FeNiPt-NRL/GC electrode in 0.1 M PBS with successive addition of (a) 4 μM, (b) 8 μM, (c) 12 μM, (d) 16 μM, (e) 20 μM, (f) 40 μM, (g) 60 μM, (h) 80 μM, (i) 100 μM, (j) 120 μM, (k) 150 μM, (l) 300 μM H₂O₂. The solution was stirred with a magnetic stirrer at 600 rpm. Potential was kept at 0 V vs. Ag|AgCl.

3.4 Anodic detection of H₂O₂

Although the present cathodic H₂O₂ sensing shows good selectivity and broad detection linear range, the sensitivity is still lower than those anodic H₂O₂ sensors based on Pt nanoparticles reported previously.²³ For this purpose, the anodic amperometric responses were examined at the positive potential of 0.45 V vs. Ag|AgCl. Fig. 5(A) shows continuous steady-state anodic currents at FeNiPt-NRL/GC electrodes polarized at 0.45 V in response to H₂O₂ additions in N₂-saturated 0.1 M PBS (pH 7.0), and Fig. 5(B) depicts the linear calibration plots. The addition of H₂O₂ resulted in an apparent increase of the oxidation current. The 95% response of the steady-state current was obtained within 4 s. The current response remained constant until a new sample was injected into the electrochemical cell.

Fig. 5 (A) Amperometric responses of FeNiPt-NRL/GC electrode in 0.1 M PBS with successive addition of (a) 100 nM, (b) 200 nM, (c) 500 nM, (d) 1 μM, (e) 2 μM, (f) 4 μM, (g) 6 μM, (h) 8 μM, (i) 15 μM, (j) 25 μM, (k) 35 μM, (l) 50 μM, (m) 70 μM, (n) 120 μM H₂O₂. Potential was kept at 0.45 V. Inset: Current responses to addition of 100 nM H₂O₂ in 0.1 M PBS (a) and the same volume of 0.1 M PBS without the analyte (a′); (B) Calibration plots of anodic currents against the concentration of H₂O₂ at FeNiPt-NRL/GC electrode.

An average sensitivity of 2.45 ± 0.1 mA cm⁻² mM⁻¹ with r² = 0.9966 (n = 10) was measured over the range of 100 nM–30 mM for FeNiPt-NRL/GC electrode. The detection limit was achieved to be 40 nM estimated at an S/N ratio of 3, and for confirming this result, the same volume of 0.1 M PBS (10 μL without the analyte) was injected and caused no measurable response above background current (inset of Fig. 5a). The sensitivity over the broad detection linear range of 100 nM–30 mM is much higher than those obtained at 2.5 nm Pt nanoparticles²⁸ estimated at 0.056 A M⁻¹ cm⁻² and at low surface density assemblies of Pt nanoparticles,²⁹ and is comparable with that achieved at mesoporous Pt ultra-microelectrode (25 μm diameter).³⁰ In addition, the detection linear range of the present H₂O₂ sensor is wider than those obtained at 2.5 nm Pt nanoparticles,²⁸ 2.5 nm polyacrylate-capped Pt nanoparticles,²⁹ mesoporous Pt ultramicroelectrode (25 μm diameter),³⁰ Pt nanoparticles-modified single-wall carbon nanotubes (SWCNTs).³¹

3.5 Reproducibility and stability

Reproducibility of the current response of the electrode was investigated in N₂-saturated 0.1 M PBS (pH 7.0) solution containing 2 mM H₂O₂ by measuring the CVs five times every 30 min after holding the electrode in the solution. The relative standard deviation (RSD) was 1.1% (n = 5). For the interelectrode reproducibility of four electrodes in the same batch, the RSD was 3%. The stability of the electrode was investigated by storing the electrode in air for 4 weeks and then measuring the current response of 2 mM H₂O₂ in N₂-saturated 0.1 M PBS (pH 7.0) solution. Current responses within a variation of 5% were achieved.

3.6 Continuous-flow injection

The high sensitivity of the FeNiPt-NRL/GC electrode towards the oxidation and determination of H₂O₂ was further confirmed by amperometric measurements in a continuous-flow system, as shown in Fig. 6. When the electrode was poised, the addition of H₂O₂ resulted in an apparent increase of the oxidation current. The sensitivity was 2.80 ± 0.05 mA cm⁻² mM⁻¹. When the PBS containing no analyte was pumped into the electrochemical flow cell, the current was quickly declined to the background current.

Fig. 6 Typical amperometric responses of H₂O₂ (a 1 μM; b 10 μM) in a continuous-flow cell with the FeNiPt-NRL/GC electrode. The electrode was poised at +0.7 V. Flow rate, 3 μl min⁻¹.

3.7 Real sample detection

Determination of H₂O₂ at the FeNiPt-NRL/GC electrode in real sample, such as serum was also tested. The human serum sample was diluted with phosphate buffer solution (pH 7.0) before the measurement. The results obtained for H₂O₂ in real sample are listed in Table 1. To ascertain the correctness of the results, the real samples were added with certain amounts of H₂O₂ at levels similar to those found in the samples themselves. The recovery rate of 3 samples was determined to be 97– 99%. The total concentrations of H₂O₂ in the serum samples were found to be in good agreement with previously reported results.³²

Table 1 Determination of H₂O₂ in 3 real serum samples with the FeNiPt-NRL/GC electrode

Sample	Detected concentration (μM)	Added concentration (μM)	Total concentration (μM)	Recovery rate (%)
1	39.2±3.0	50	88.0±7.5	98.7
2	45.4±3.3	50	93.1±7.9	97.6
3	43.0±3.2	50	90.7±7.7	97.5

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4. Conclusion

A facile and effective electrochemical sensing technique was first developed by using amorphous FeNiPt nanoparticles with tunable length. The FeNiPt nanoparticles showed enhanced electrocatalytic activities towards both the oxidation and reduction of H₂O₂, and electrocatalytic activity increased in the order: FeNiPt-NS < FeNiPt-NR < FeNiPt-NRL. Meanwhile, the comparable corrosion potential E_corr and lower corrosion current I_corr were observed at FeNiPt-NRL/GC surface, compared with those obtained at bare GC electrodes, revealing the good stability of FeNiPt-NRL surface. Subsequently, H₂O₂ was determined cathodically at GC electrode modified with amorphous FeNiPt-NRL with high selectivity at the appreciative potential of 0 V vs. Ag|AgCl. On the other hand, for the anodic detection of H₂O₂ at the same electrode, the sensitivity was achieved to 2.45 ± 0.1 mA cm⁻² mM⁻¹ in the broad dynamic linear range of 100 nM–30 mM, and the detection limit was improved to be 40 nM on the basis of the signal-to-noise ratio of 3. Meanwhile, the present FeNiPt-based H₂O₂ sensor also exhibited high stability and good reproducibility. This investigation opened an alternative way for determination of H₂O₂ and other molecules based on amorphous FeNiPt nanoparticles, as well as extending the applications of amorphous nanomaterials, especially the amorphous ternary nanomaterials to electrocatalysis, chemical sensors and biosensors, and other electrochemical purposes.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (20975057 and 20771085), the Program for New Century Excellent Talents in University (NCET-06-0380), and the project sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars from State Education Ministry, China, and Nanometer Science Foundation of Shanghai (0952nm04900).

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Footnote

† Electronic supplementary information (ESI) available: CV of H₂O₂ at annealed FeNiPt-NRL and the affect of some unusual interferences at 0 V. See DOI: 10.1039/b9ay00209j

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