Journal of Nanomaterials
Volume 2016 (2016), Article ID 2437873, 11 pages
http://dx.doi.org/10.1155/2016/2437873

Research Article

Real-Time Sensing of Hydrogen Peroxide by ITO/MWCNT/Horseradish Peroxidase Enzyme Electrode

Melinda Magyar,1,2 László Rinyu,³ Róbert Janovics,³ Péter Berki,⁴ Klára Hernádi,⁴ Kata Hajdu,¹ Tibor Szabó,¹ and László Nagy¹

¹Department of Medical Physics and Informatics, University of Szeged, Rerrich B. tér 1, Szeged 6720, Hungary
²Hungarian Academy of Sciences, Biological Research Centre Szeged, Institute of Plant Biology, Temesvári krt. 62, Szeged 6726, Hungary
³Hungarian Academy of Sciences, Institute for Nuclear Research, Hertelendi Laboratory of Environmental Studies, Bem tér 18/c, Debrecen 4026, Hungary
⁴Department of Applied and Environmental Chemistry, University of Szeged, Rerrich B. tér 1, Szeged 6720, Hungary

Received 7 August 2016; Revised 23 October 2016; Accepted 24 October 2016

Academic Editor: Xuping Sun

Copyright © 2016 Melinda Magyar et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

The accurate and sensitive determination of H₂O₂ is very important in many cases because it is a product of reactions catalysed by several oxidase enzymes in living cells and it is essential in environmental and pharmaceutical analyses. The fabrication of enzyme protein activity based biosensors is a very promising way for this purpose because the function of biological molecules is very specific, sensitive, and selective. Horseradish peroxidase (HRP) is the most commonly used enzyme for H₂O₂ detection because it can oxidize hydrogen atoms and, for example, xenobiotics in the presence of H₂O₂. In order to define the limit of detection (LOD) of H₂O₂ we made calibrations with guaiacol and amplex red (AR), which are hydrogen donors of HRP. The accumulation of the reaction products, tetraguaiacol, and resorufin, respectively, then can be easily detected by absorption or emission (fluorescence) spectroscopy. In our experiments an enzyme electrode was fabricated from ITO (indium tin oxide), functionalized multiwalled carbon nanotubes (f-MWCNTs), and HRP. Although the enzyme activity was smaller by about two orders of magnitude when the enzyme was bound to the f-MWCNTs (ca. 10⁻² M H₂O₂/(M HRP·sec) compared to ca. 2 M H₂O₂/(M HRP·sec) and 5 M H₂O₂/(M HRP·sec) with AR and guaiacol in buffer solution), LOD of the H₂O₂ decomposition was about 6 pM H₂O₂/sec and 10 pM H₂O₂/sec in the case of AR and guaiacol, respectively.

1. Introduction

H₂O₂ is one of the reactive oxygen species (ROS) which should be considered in many chemical and biological processes, for example, in food chemistry and biology, physiological, pathological and pharmaceutical procedures, and environmental fields [1, 2]. There is a large interest to reduce the formation of the ROS components because they may react with the components of the systems resulting in their degradation, consequently, reducing the efficiency of the reactions.

Due to the increasing role of free radical mechanisms in air and in aquatic environment, H₂O₂ has an increasing concentration (together with other reactive oxygen species) in the atmosphere and water [3]. In addition, it is a natural product of oxidative processes in metabolism of living cells (like electron transport of biological oxidation and photosynthesis) with a very wide apparent concentration range from few tens of pM to mM [4, 5]. Detection of extracellular H₂O₂ can be an important diagnostic tool for cancer cell lines expressing either transfected dual oxidase (DUOX) or endogenous DUOX (e.g., in thyroid, airway or colonic epithelium, and lung) (see [6], and the references in it).

Although H₂O₂ is a strong two-electron oxidant, many of its two-electron oxidation reactions are too slow to be biologically relevant due to the high activation energy of this process. There are, however, important exceptions. For example, metallocomplexes (e.g., metalloproteins) or specific proteins (thiol or methionine proteins) and lipids (especially unsaturated ones) are extremely sensitive to reactions with H₂O₂ [7, 8].

Consequently, the rapid, accurate, sensitive, and selective determination of the spatial and temporal variations of these harmful reactive oxygen species is of great significance and became in the focus of many laboratories. It is not the aim of this publication to give a summary; however, it must be mentioned that numerous techniques have been used for the detection of H₂O₂ [9–12]. Besides the classical titrimetry [13], optical [14, 15], and infrared spectroscopy [16], electrochemical techniques coupled with the use of electrodes decorated by redox proteins are more and more promising ways to detect H₂O₂ [17–20]. Natural enzyme-like activity of enzyme mimetics (nanozymes) has been applied to detect various bio- and/or biologically relevant molecules and reviewed by Shin et al. [21]. There are many tasks that should be considered when different methods are used for specific applications. One should optimize the complication and cost of the method itself (the required instrumentation, like vibrational spectroscopies); the size of the device and the sample quantity; the response to be fast, reversible, and reproducible; the possibility of online, real-time detection; the sensitivity, selectivity and, cross-reactions or parallel reactions of the analytes.

The development of H₂O₂ sensors with low detection limit and wide responding range has become a rich research field these days. There are two main directions of strategies for the fabrication of sensitive and specific H₂O₂ detectors. One direction is a nonenzymatic platform which provides very rapid, high sensitivity detection of H₂O₂ and does not suffer from limitations which are inherently present in biological materials, like in enzyme proteins. These are usually related to the reduced stability when isolated from their natural (biological) environment due to high sensitivity to environmental factors and/or to autocatalytic degradation processes [22–25]. In addition, the nonenzymatic biosensors are usually simpler and of low cost, and thanks to recent developments their selectivity and sensitivity are increasing in recent publications [24, 26].

Another direction is the fabrication of enzyme protein activity based biosensors. Besides the difficulties in using biological materials (usually complicated and sometimes high cost immobilization procedures are required) it is a real challenge to use them in new generation technologies (like in optoelectronics, nanobionics, and biosensor technologies) because their function is extremely specific, sensitive, and selective. The definite advantage of enzymatic biosensors is that these are less sensitive to cross-reactions with electrochemical processes, which can modify the electrode surfaces and/or are reflected in kinetic limitations and overpotential. The most commonly used enzyme for the construction of electrochemical H₂O₂ biosensors is horseradish peroxidase (HRP), which catalyses the H₂O₂-dependent one-electron oxidation of suitable hydrogen donors efficiently [27, 28].

Thanks to the intensive research (see, e.g., crystallographic [29, 30] and spectroscopy [31–34] investigations and model calculations [35, 36]) combined with site directed mutagenesis studies [37] we have quite solid knowledge about the molecular mechanism of catalytic processes of peroxidases with different reaction routes and substrates [38]. The structural, kinetic, and energetic details together with the known reaction stoichiometry allow using HRPs in promising applications in biosensor technology.

Transparent conducting metal oxide (TCO) electrodes are widely used in many fields of modern electronics (see [39], and the references in it) because these offer the combination of optical and electric methods in biosensor applications. The oxidized, coloured product of the enzyme reaction is usually very specific and its accumulation can be easily detected by optical spectroscopy, while the electric method offers good sensitivity [40, 41]. The direct electron transfer between the HRP and the electrode is relatively slow and hardly detectable; however, in the presence of a suitable mediator (e.g., carbon nanotubes (CNTs)), it can be facilitated while the enzyme’s bioactivity is largely retained [42].

Recently, we have fabricated an ITO/f-/HRP electrode, which was able to reduce H₂O₂, as proved by electrochemical (cyclic voltammetry) and optical (absorption change and fluorescence) measurements. The results showed that electron transfer can be facilitated between the active sites of the enzyme and the surface of the electrode and that the enzyme remains accessible for the substrate. The enzyme reaction was followed by measuring the fluorescence change of the coloured product of the guaiacol [43]. After optimizing the enzyme electrode preparation protocol and the fluorescence detection system the sensitivity of our measurement increased by several orders of magnitude which allowed monitoring the H₂O₂ concentration in real time. Introducing a sensitive C¹⁴ radioassay in determining the amount of bound enzyme the absolute enzyme activity was determined and compared to the one measured in solution. In addition, besides the guaiacol, another sensitive fluorescence probe, amplex red, was introduced and the enzyme activity and LOD of H₂O₂ detection are introduced.

2. Materials and Methods

2.1. Sample Preparation

The functional groups of carboxyl-functionalized multiwalled carbon nanotubes (f-) (0.14 mg/mL) were activated by using crosslinkers N-hydroxysuccinimide (NHS, Sigma-Aldrich) and 1-[3-dimethylaminopropyl]-3-ethyl-carbodiimide (EDC, Sigma-Aldrich). After the activation procedure, the mixture was dialysed in potassium phosphate buffer (PBS, 0.1 M, pH 7.0) to remove the excess amount of the crosslinkers. Then the HRP enzyme solution (1 μM, salt free, Reanal) was added to the activated f- suspension and it was stirred intensively at 4°C for 2 h. At the end, the sample was washed until the supernatant did not show enzyme activity and then separated by ultracentrifuge.

2.2. Absorption Kinetics and Enzyme Activity

Absorption change was measured at 470 nm (guaiacol, Sigma-Aldrich) and 570 nm (amplex red (AR), Sigma-Aldrich) by a single-beam kinetic spectrophotometer of local design [44]. The good signal/noise ratio made it possible to measure even 10⁻⁴–10⁻⁵ optical density change. By following the accumulation of the coloured products the amount and the rate of the H₂O₂ decomposition was calculated from the reaction stoichiometry. The enzyme reactions follow the reaction stoichiometry according to the following equations:

The enzyme activity was determined from the absorption and fluorescence kinetics. By measuring the concentration change of the enzyme reaction’s coloured product, the concentration change of H₂O₂ can be determined by the known molar extinction coefficients ( = 26611 M−1 cm⁻¹ and = 58000 M−1 cm⁻¹). The absorption change was calculated by measuring the voltage of the detector after current/voltage conversion, which is proportional with the light intensity:

Here is the DC level of the detector (typically 100 mV) and is the voltage change in time accompanying the enzyme reaction and is the amplification. Applying the Beer-Lambert’s law the amount of the reduced H₂O₂ can be calculated from the measured .

Here is the optical path and refers to the stoichiometric ratio of H₂O₂ and tetraguaiacol or resorufin, which was 4 : 1 or 1 : 1, respectively.

2.3. Fluorescence Measurements

Fluorescence was measured by a spectrofluorimeter (Perkin Elmer MPF44A) in a sensitive way with one hundred times amplification. The reaction was initiated by the addition of H₂O₂ to the mixture of the f-/HRP bionanocomposite and the guaiacol/amplex red hydrogen donors whose reagents can be simply oxidized and theirs oxidation results in coloured product (tetraguaiacol/resorufin). The excitation and the emission wavelengths were 300 nm and 355 nm or 545 nm and 585 nm in the case of guaiacol and amplex red, respectively.

On the one hand, measuring fluorescence change assures a more sensitive way of detection than measuring absorption change; on the other hand, the presence of carbon nanotubes makes the analysis more difficult because of the light scattering characteristics of the nanotubes.

The change in the fluorescence can be approximated by the following equation:

Here is the incident light intensity; is the fluorescence intensity as a function of time; is the concentration change of the coloured product as a function of time; is the extinction coefficient; is the optical path; is the quantum yield of fluorescence; and is the instrumental constant [43]. By knowing the amount of the enzyme, the enzyme activity can be determined in /(·s) unit. Measurement of the fluorescence with high sensitivity allows us to determine the H₂O₂ concentration in real time without large incubation and integration time. The integration time for the measurement was typically one second.

2.4. Scanning Electron Microscopy

In order to visualize the surface coverage of ITO by the carbon nanotubes scanning electron microscopy (SEM) investigations were performed. SEM images were taken by a Hitachi S-4700 type II FE-SEM operating in the range of 3–5 kV. Prior to the measurement the samples were mounted on a conductive carbon tape.

2.5. Cyclic Voltammetry

The electrochemical activity of the enzyme electrode was checked by cyclic voltammetry as well. Cyclic voltammograms were measured using a PGSTAT10 potentiostat/galvanostat at ambient temperature. A three-electrode cell containing 50 mL potassium phosphate buffer (0.1 M, pH 7.0) and 20 mM KCl was used. The cell was purged with high purity N₂ prior to each measurement. The working electrode was the enzyme electrode fabricated from ITO, carboxyl-functionalized MWCNTs, and HRP. The counter and reference electrodes were platinum and Ag/AgCl, respectively. The scan rate was 50 mV s⁻¹ [40].

2.6. Preparation of the ITO/f-MWCNTCOOH/HRP Enzyme Electrode

First the surface of ITO covered glass (CEC020B, Praezisions Glas & Optik GmbH, Iserlohn, Germany) was cleaned by oxygen plasma cleaner and then silanized ((3-aminopropyl) triethoxysilane) to create amino-functional groups on it. After silanization, the carboxyl groups of the functionalized MWCNTs (0.14 mg/mL) were activated by the addition of crosslinkers NHS (0.15 mg/mL) and EDC (0.5 mg/mL) solved in distilled water and deposited on the top of the ITO for 2 h and then washed intensively.

HRP was bound to the ITO/f- electrode by the same crosslinkers as used before by depositing the activated enzyme on the surface for 2 h at 4°C. Then the electrode was washed intensively with phosphate buffer (0.1 M, pH 7.0) and distilled water several times [45]. Schematic arrangement of the multilayer structure of the ITO/f-/HRP electrode is presented by Figure 1.

Figure 1: Schematic representation of the multilayer structure of the ITO/f-/HRP electrode. The structure of the enzyme was downloaded from the Brookhaven Protein Data Bank (http://www.rcsb.org/) by using the 1H58.pdb data file and prepared by Jsmol program.

2.7. Measuring the Carbon Quantity

In order to determine the enzyme activity, the amount of the enzyme bound to the f- was determined. Since, the contribution of modern carbon (percent of modern carbon, pMC; [46]) should be different in MWCNTs and HRP, measuring the pMC of different samples offers sensitive detection of the sample quantity. The procedure was carried out shortly as follows.

The carbon content of the samples was liberated by sealed tube combustion method. The sample and the MnO₂ oxidant were weighted into a glass tube. The tubes were evacuated to < mbar and sealed by a torch. The samples were combusted to CO₂ in a muffle furnace at 550°C for 24 hours. The gained CO₂ was purified using a dedicated gas handling system equipped with cryogenic traps in order to remove the other combusted gas components [47]. The quantity of the pure CO₂ was determined in a known volume by high-precision pressure sensor [48]. The yield of the carbon extraction can be calculated from the quantity of the pure CO₂ gases. The trapped and cleaned CO₂ gases have been converted to graphite by zinc reduction sealed tube graphitization method [49].

The measurements of the radiocarbon contents were carried out on a MICADAS (MIni CArbon DAting System [50, 51] type accelerator mass spectrometer in Institute for Nuclear Research, Debrecen, Hungary) [52]. In order to monitor and to take the possible modern carbon contamination during the pretreatment and combustion process into account, we have extracted chemical standards with well-known radiocarbon activity (IAEA C7 and C8, [53]) on the same treatment line and measured them together with the samples in the same magazines. The pMC (5) unit was used to compare the radiocarbon contents of the samples. All of the radiocarbon results were processed using the BATS AMS data evaluation software developed by Wacker et al., ETH Zurich [54].

3. Results and Discussions

3.1. Preparation of the Enzyme Electrode

Figure 2 shows the SEM image of f- bound to the surface of ITO which serves as a base for the immobilization of the HRP enzyme. The image indicates that the coverage is not homogenous, but dense enough with f- for the further steps of the preparation. Conducting mechanisms of the transparent conducting oxides (TCOs) also of the ITO depend strongly on the grain and layer structure, consequently, on the deposition technique, on the quality, and on prehistory of the oxide film [55–58]. The SEM image of the bare ITO shows that the surface morphology (i.e., the homogenous surface coverage and the organization of the rain structure) of the ITO in the biohybrid sample is essentially the same as before the treatment (cf. with insert of Figure 2).

Figure 2: Scanning electron microscopy image of carboxyl-functionalized MWCNT attached to the surface of ITO. Insert shows the SEM image of the bare ITO, for comparison.

3.2. Enzyme Activity in Solution

Our aim was to design an ITO/f-/HRP electrode in order to determine the concentration of H₂O₂ in real time in a sensitive way. Measuring fluorescence instead of light absorption of the product of the enzyme reaction is more advantageous in this system because of two reasons. On the one hand, fluorescence measurement is more sensitive and, on the other hand, because of the light scattering of the f-/HRP complex Beer-Lambert’s law of absorption cannot be applied directly.

First, the kinetics of the absorption change of tetraguaiacol and resorufin due to the enzyme reaction (see Materials and Methods) was measured and the rate of the H₂O₂ decomposition was calculated at different enzyme concentrations without MWCNT in PBS buffer solution; then the initial rate of the decomposition was determined in each case. Figure 3 shows results of typical measurements at the concentration of 20 nM HRP after the addition of guaiacol (3.5 mM) and amplex red (6.5 μM), as indicated.

Figure 3: The amount of H₂O₂ reduced by 20 nM HRP (with guaiacol and amplex red as indicated) as a function of time. Figure was made by the absorption change of guaiacol and AR in PBS buffer solution (0.1 M, pH 7.0). Figure shows the initial slopes and theirs straight-line equations with the goodness-of-fits ().

In the set of second experiment the conversion of guaiacol to tetraguaiacol and amplex red to resorufin was followed by measuring the fluorescence change with different enzyme concentrations (same as used for measuring the absorption change) and the initial rate of the fluorescence change was calculated (Figure 4). These instrumental conditions were used for determining the enzyme activity of the f-/HRP complex.

Figure 4: The initial rate of the fluorescence change as a function of the enzyme concentration. Figure was made by the fluorescence change of guaiacol (3.5 mM) and AR (6.5 μM) in PBS buffer solution (0.1 M, pH 7.0). Figure shows the straight-line equations with the goodness-of-fits ().

The initial slopes of the fluorescence changes were calibrated to the initial rates of the absorption measurements for H₂O₂ decomposition (Figure 5). This way, by using the same instrumental conditions, the rate of the H₂O₂ decomposition can be determined from the fluorescence change measurements belonging to certain complexes and electrodes containing the HRP enzyme.

Figure 5: The initial rate of the H₂O₂ decomposition as a function of the initial slope of the fluorescence change of tetraguaiacol and resorufin. Figure shows the straight-line equations with the goodness-of-fits ().

Using the initial slope of the fluorescence change the rate of the decomposition of H₂O₂, consequently, the enzyme activity of 5 M H₂O₂/(M HRP·sec) () and 2 M H₂O₂/(M HRP·sec) () was determined and used for further calibrations of the fluorescence measurement in case of guaiacol and AR, respectively. By using the calibrations the limit of detection of H₂O₂ by the HRP itself was calculated in both cases and it was found to be 124 nM H₂O₂ s⁻¹ in case of guaiacol and 59 nM H₂O₂ s⁻¹ in case of AR, in good agreement with the values published earlier [43].

3.3. Enzyme Activity of the f-MWCNTCOOH/HRP Complex

As an introductory experiment it should be proved that (a) the HRP binds to the CNT, (b) the active centre of the enzyme remains accessible to the substrates (i.e., the protein keeps the enzyme activity), and (c) there are no substantial changes in the protein conformation after the binding which affect the enzyme reaction. For this reason we have bound HRP to carboxyl- and amine-functionalized MWCNT in suspension and the enzyme reaction was followed by fluorescence measurements using guaiacol as e⁻-donor substrate.

3.3.1. Rate of H₂O₂ Decomposition by the f-MWCNTCOOH/HRP Complex

Figure 6 shows that HRP bound either to amine- or to carboxyl-functionalized MWCNTs showed enzyme activity. The smaller yield of the amine-functionalized sample can probably be explained by the smaller amount of enzyme bound to the MWCNT. This can be due to the glutaraldehyde (GTA) crosslinker, which crosslinks the amine groups. In order to avoid this inherent crosslinkings of the MWCNTs in the sample we used carboxyl-functionalized MWCNTs for further experiments.

Figure 6: The fluorescence change of the tetraguaiacol produced by the enzyme reaction of HRP bound to the /and f- as a function of time.

Figure 7 shows the emission spectra of the tetraguaiacol produced by the f-/HRP complex added at different concentrations, at the rate of the enzyme reaction’s saturation (after fifteen minutes of starting the reaction, when the reaction rate is maximal). The excitation wavelength was 300 nm. The increase of the fluorescence clearly indicates that the f-/HRP complex possesses considerable enzyme activity; that is, the HRP was bound to f-s and the active centre remained accessible to the substrates after the binding. f- suspension alone did not show fluorescence increase indicating that the illumination of the carbon nanotube without the enzyme does not contribute to the guaiacol oxidation. The f-/HRP complex in the presence of guaiacol—and in the absence of H₂O₂—also did not show change in the fluorescence indicating that HRP alone does not oxidize the guaiacol.

Figure 7: The emission spectra of tetraguaiacol produced by the enzyme reaction of f-/HRP complex before (guaiacol) and after the reaction at different HRP incubation concentrations (as indicated) and the carboxyl-functionalized MWCNT as a reference. Measurement was done as described in Materials and Methods.

After measuring the fluorescence change accompanying the enzyme reaction of the guaiacol or amplex red added to the f-/HRP complex, the initial rate of the enzyme reaction was determined (Figure 8). Under our experimental conditions, based on the calibrations (see Section 3.2), our complex was able to reduce H₂O₂ with the rate of 9.6 pM H₂O₂ s⁻¹ and 12 pM H₂O₂ s⁻¹ in the case of guaiacol and AR, respectively. These resolutions were better than the ones measured in solution by about 6–4 orders of magnitudes (cf. Section 3.2).

Figure 8: The fluorescence change of the coloured products of guaiacol and amplex red substrates oxidized by 0.5 μM f-/HRP complex as a function of time. Figure also shows the best fits of the initial changes of the signals and their equations. , the goodness-of-fit, is also indicated.

3.3.2. Enzyme Activity of the f-MWCNTCOOH/HRP Complex

In order to determine the enzyme activity of the f-/HRP complex the amount of the protein bound to the MWCNT was determined. For this reason the pMC (5) unit was determined for the HRP, f-, and f-/HRP samples.

The percent of modern carbon is defined as

where is the specific activity of the sample normalized for fractionation and is the specific activity of NIST-SRM-4990c oxalic acid standard normalized for fractionation.

The HRP fraction of the f-/HRP complex is obtained by ¹⁴C balance equation. Radiocarbon content of the two-component mixture is defined as

where , , and are the measured radiocarbon content of the HRP, f-, and f-/HRP complex, respectively. is the HRP fraction of the radiocarbon content of the f-/HRP complex. The HRP fraction of the mixture is obtained after the rearrangement of (6):

Finally, the total HRP/f- ratio and the amount of HRP by the known molecular weight (MW: 44 kDa, 45% carbon content) can be calculated using the HRP fraction constant and the carbon extraction yields of the appropriate components. The data obtained during the determination of radiocarbon quantity are summarized in Table 1.

Table 1: Summary of data obtained during determination of the radiocarbon quantity.

Sample
Sample quantity (mg)
Measured carbon (mg)
Yield (%)
pMC

HRP
3.0
1.3
43.1
145.2 ± 0.4

f-MWCNT COOH
1.1
1.0
88.6
46.5 ± 8.3

f-MWCNT COOH/HRP
3.2
0.5
16.4
100.1 ± 0.5

54.0 ± 2.0

By using the (5)–(7) under oversaturating enzyme concentration during the preparation procedure the w/w ratio of the HRP/f- is 2.7. Using the rates of H₂O₂ decompositions (cf. Figures 5 and 8) the enzyme activity of 1.3 10⁻² M H₂O₂/(M HRP·sec) and 1.0 10⁻² M H₂O₂/(HRP·sec) was calculated for AR and guaiacol, respectively. It is interesting to note that these values after the binding in the composite complex are about two orders of magnitudes smaller than the ones found in solution (cf. Section 3.2); however, the LOD is reduced to the concentration of pM. This probably is due to the fact that the reduced accessibility/functionality of the active centre is compensated by the larger local concentration of the enzyme after the binding. The reduced catalytic activity of the enzymes after the binding to CNTs is known in the literature and is already demonstrated by other groups as well [59–61]. However, we do not have experimental evidence until now for the larger sensitivity for LOD. If it is related to the larger local concentration of the enzyme after the binding it should be sensitive to the concentration of the reactants, which requires systematic sets of experiments. The investigation of this possibility is under progress in our laboratory.

3.4. Rate of H₂O₂ Decomposition by the ITO/f-MWCNTCOOH/HRP Electrode

First, the fluorescence of the substrates oxidized by the HRP was measured in the well-known system (Figure 9). The ITO/f-/HRP electrode was placed in a 1 cm cuvette containing the guaiacol (or AR) in PBS buffer solution (0.1 M; pH 7.0); then the H₂O₂ was added to initiate the enzyme reaction. Based on the calibrations, the rate of the H₂O₂ decomposition can be calculated from the slope of the fluorescence change of our samples. The initial rate of the H₂O₂ decomposition was found to be 10 pM H₂O₂/sec and 6 pM H₂O₂/sec for guaiacol and AR, respectively.

Figure 9: The fluorescence change of the coloured products of guaiacol and amplex red substrates oxidized by the ITO/f-/HRP electrode as a function of time. Figure also shows the best fits of the initial changes of the signals and their equations. , the goodness-of-fit, is also indicated.

3.5. Cyclic Voltammetry

Our principal aim is to design a device which can be a base for the detection of H₂O₂ in a sensitive way. One classical way is the detection of luminescence (fluorescence) of the coloured product of the enzyme reaction; however, the sensitive detection usually requires sophisticated instrumentation (photomultiplier, high voltage power supply, amplifiers, etc). However, measuring the electric signal accompanying the redox transitions of the enzyme reactions offers large sensitivity and relatively simple instrumentation. It should be noted that the interpretation of the data is sometimes difficult and requires careful considerations. It can be a successful strategy to combine measurements of the optical and electric signals accompanying the enzyme reaction parallel.

In order to do this we have bound the f-/HRP complex to ITO which can be applied as a working electrode in a classical electrochemical cell. After the construction of this special enzyme electrode the cyclic voltammograms were recorded in the absence and in the presence of the H₂O₂ substrate. Figure 10 shows that there is a catalytic transition at around −350 mV [62] when the enzyme electrode is used in the electrochemical cell in the presence of H₂O₂, indicating that the electrode is capable of catalysing the decomposition of H₂O₂. Neither the bare ITO electrode nor the silanized ITO or the f- alone facilitates electrochemical transition at this potential in the presence of H₂O₂ [43].

(a)

(b)

Figure 10: Cyclic voltammograms of ITO, silanized ITO (ITO/silane), ITO/f- (as references), and the enzyme electrode fabricated with HRP immobilized on f- deposited on ITO (ITO/MWCNT/HRP), before (a) and after the addition 5 mM H₂O₂ (b).

3.6. Reliability, Reproducibility, and Specificity

When new biohybrid material is designed for a successful application (e.g., using them for biosensors) special attention should be paid to the reliability and reproducibility of the measurement and the specificity and stability of the device. The reliability and reproducibility in our case are highly dependent on the successful enzyme binding procedure, which can be monitored essentially by SEM. The sample which did not show sufficient coverage (by visual observation) was not used for further experiment. However, after finding the most appropriate preparation procedure (physico-/chemical parameters and incubation times) the reliability and sensitivity of the measurements increased significantly.

The investigation of specificity is a difficult, however, important issue and should be a topic for further measurements. There can be alternative substrates for HRP (like ascorbate, SH-substrates, etc.) or other enzymes (like catalases) which can be in competition with HRP, specially, under real tissue conditions. The possibilities for cross-reactions, for over- or underestimation of determination of H₂O₂ and advantages or disadvantages of using HRP electrodes are summarized by Maghzal et al. and Grisham [63, 64]. These investigations are under progress in our laboratory.

4. Summary

Our aim was to create a system suitable for detecting H₂O₂ in a sensitive way. We applied two kinds of substrates (guaiacol and amplex red), whose reagents can be oxidized in a multistep cation radical mechanism and theirs oxidation results in coloured products (tetraguaiacol and resorufin). The accumulation of these products can then be easily detected by specific light absorption or emission (fluorescence) spectroscopy. By determining the concentration of the products, the concentration change of the hydrogen peroxide can be calculated in absolute value. By using the calibration made from the absorption kinetic and fluorescence measurements belonging to the same enzyme concentrations, we determined the enzyme activity of horseradish peroxidase, which were 5 M H₂O₂/(M HRP·sec) and 2 M H₂O₂/(M HRP·sec) for guaiacol and amplex red, respectively, and the limit of detection of H₂O₂, which was 124 nM H₂O₂ s⁻¹ and 59 nM H₂O₂ s⁻¹ for guaiacol and amplex red, respectively. We bound the enzyme to carboxyl-functionalized carbon nanotubes, and then an electrode was prepared by immobilizing the complex on the surface of the silanized ITO. Although the enzyme activity was smaller by about two orders of magnitude when the enzyme was bound to the f-MWCNTs (1.3 10⁻² M H₂O₂/(M HRP·sec) and 1.0 10⁻² M H₂O₂/(M HRP·sec) for AR and guaiacol, resp.) the LOD of the H₂O₂ decomposition was in the range of picomole. Morphological characterization (SEM) of the electrode showed that the binding was successful and it was able to reduce H₂O₂ as proved by electrochemical (cyclic voltammetry) measurements. By using the calibration made from the absorption kinetic and fluorescence measurements we determined the limit of detection of H₂O₂ of the complex and the electrode: f-/HRP with guaiacol 9.6 pM H₂O₂ s⁻¹; with amplex red 12 pM H₂O₂ s⁻¹; ITO/f-/HRP with guaiacol 10 pM H₂O₂ s⁻¹; with amplex red 6 pM H₂O₂ s⁻¹.

Competing Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

This work was supported by the grants from the Hungarian Scientific Research Fund OTKA K112688, PD116739, and PD121225, from Switzerland through the Swiss Contribution (SH/7/2/20), and from GINOP-2.3.2-15-2016-00009. Tibor Szabó acknowledges the support by the ÚNKP 16-3 New National Excellence Program of the Ministry of Human Capacities in Hungary. Thanks are due to Ms. Judit Tóth for the valuable technical assistance.

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