An Electrochemical Sensor for Hydrazine Based on In Situ Grown Cobalt Hexacyanoferrate Nanostructured Film

2. 1. Materials and methods

N-[3-(trimethoxysilyl) propyl]diethylenetriamine (the silane monomer used to prepare the SSG) was received from Sigma-Aldrich. K₃[Fe(CN)₆], CoCl₂, and N₂H₄ were obtained from Dae-Jung Chemicals. All the chemicals were of analytical grade and were used as received. Field emission scanning electron microscopy (SEM) images were recorded using a JEOL JSM-7800F instrument. X-ray diffraction (XRD) profiles were recorded using a SmartLab (Rigaku) instrument. X-ray photoelectron spectroscopy (XPS) spectra were recorded using an ULVAC-PHI 5000 Versa Probe instrument. ITO (dimension: 2 × 1 cm²) and its modified forms were used as a working electrode. A Pt wire served as a counter electrode, and Ag/AgCl (in 3 M NaCl solution) was used as a reference electrode. All the electrochemical experiments were conducted in a single-compartment three-electrode cell using an Ivium Technologies electrochemical work station. Nitrogen gas (N₂) was bubbled for 30 min before each experiment.

2.2 Synthesis of SSG solution

A homogeneous SSG solution was prepared [25] by adding 5 μL of 1 M silane monomer to 5 mL of aqueous solution under vigorous stirring, and the stirring was continued for another 60 min.

2.3 Fabrication of ITO/SSG-Cohcf electrode

A known volume (50 μL) of SSG solution was drop-cast on an ITO electrode that had been pretreated with piranha solution (3:1, H₂SO₄:H₂O₂; extreme care should be taken while handling) and dried at 37°C. The dried ITO/SSG electrode was soaked in an acidic solution (0.01 M HCl) of 0.001 M [Fe(CN)₆]³⁻ ions for 30 min, taken out, and rinsed in double-distilled water. Next, it was soaked in a solution containing Co²⁺ ions (0.001 M) for another 30 min, taken out, rinsed in double-distilled water, and allowed to dry. The process is illustrated schematically in Fig. 1.

Fig. 1.

Schematic representation of SSG-Cohcf electrode fabrication.

2.4 Electrocatalysis and sensor

N₂H₄ electrooxidation at the ITO/SSG-Cohcf electrode was studied by recording cyclic voltammograms. During the measurements, 1 mM N₂H₄ was added to the electrolyte, which consisted of 0.1 M phosphate buffer solution (PBS) (pH 7.0). The experiments were performed at room temperature (25°C) in a potential window of −0.2 to 1.2 V. The N₂H₄ sensors were experimentally examined using linear sweep voltammograms (LSVs).

3.1 Surface characterization of the modified electrode

The surface modification process offers a platform to alter the basic structure of underlying substrates; therefore, the physical and chemical properties of materials can be tuned for use as electrodes with applications in sensing and catalysis. Fig. 2A-F illustrates that a stable electroactive Cohcf thin film was successfully fabricated by electrostatic adsorption of Co²⁺ on the ITO/SSG electrode pre adsorbed with [Fe(CN)₆]³⁻ ions as an in situ chemical precipitant. The morphology of the Cohcf NSs shown in Fig. 2F demonstrates that the NSs are highly porous and interconnected. In addition, Cohcf is a polynuclear mixed-valence coordination compound, and its stoichiometry may vary depending on the preparation methodology, which could affect its electrochemical behavior. Fig. 2G-I summarizes the SEM-energy-dispersive X-ray (EDX) analysis of the ITO/SSG-Cohcf electrode and confirms the elemental composition and successful fabrication of Cohcf NSs by the electroless method. Furthermore, XRD (Fig. S1) and XPS (Fig. 3) analyses were performed to determine the crystallinity and identify the chemical components, respectively. As can be seen in Fig. S1, the peaks at 26.9, 35.7, 38.9, 53.8, and 56.2° were assigned to the (2 2 0), (4 0 0), (4 2 0), (6 0 0), and (6 2 0) planes, respectively, of the face-centered cubic Cohcf crystallographic structures [26]. As shown in Fig. 3, the core-level high-resolution scan mode revealed three metal ions (Co, Fe, and K) and two elements (N and Si). In addition, it can be inferred that Cohcf was interconnected and stabilized by the SSG.

Fig. 2.

(A-F) SEM images and (G-I) EDX analysis of SSG-Cohcf electrode.

Fig. 3.

XPS analysis of the ITO/SSG-Cohcf electrode.

The Cohcf NSs were successfully grown in situ at the ITO/SSG electrode via the electrostatic interaction between the irreversibly adsorbed [Fe(CN)₆]³⁻ and Co²⁺ ions. Furthermore, the Cohcf NSs ranged in size from 100 to 200 nm (Fig. 2F) and were found to be densely packed throughout the ITO/SSG electrode, indicating a high amount Cohcf NS loading on the electrode surface. In the present approach, the amine groups of the SSG essentially served as nucleation sites [27] for the growth of Cohcf NSs. In addition, the highly porous 3D SSG acts as a host for accommodating the in situ grown Cohcf NSs. The electrostatic interaction between the positively charged SSG (due to the protonated NH₃⁺) and the negatively charged [Fe(CN)₆]³⁻ ions present in the acidic medium facilitates the irreversible adsorption of [Fe(CN)₆]³⁻ ions at the ITO/SSG electrode [23]. The NH₂ groups of the SSG were protonated when the electrode was dipped in an acidic solution containing [Fe(CN)₆]³⁻ ions. Next, the ITO/SSG-[Fe(CN)₆]³⁻ electrode was dipped in a solution containing Co²⁺ ions, which led to in situ growth of Cohcf NSs at the SSG; the growth reached saturation within 30 min.

3.2 Growth and stability of the modified electrode

The electrochemical characteristics of Cohcf are well known, and it is widely assumed to be a suitable material for chemically modified electrodes for various applications. Fig. 4A shows the cyclic voltammogram of the modified electrode at a scan rate of 50 mV/s within a potential window of −0.2 to 1.2 V vs. a Ag/AgCl electrode. Two pronounced redox peaks are located at ~0.5 V. They correspond to the redox reaction of Fe^II/III in Cohcf, which is accompanied by K⁺ ions entering and exiting the cyanobridged metallic framework to maintain local charge neutrality [28]. The symmetry of each peak indicates high reaction reversibility. Equation (1) describes the in situ growth of Cohcf NSs on the ITO/SSG electrode during the electrostatic interaction between the irreversibly adsorbed [Fe(CN)₆]³⁻ and Co²⁺ ions. Cohcf can exhibit two different stoichiometric forms [15] [Eqs. (2) and (3)]; hence, in its solid state, it can accept K⁺ ions from the electrolyte to maintain electrical neutrality. This tendency toward K⁺ ion insertion led to the appearance of the two redox peaks for the Cohcf NSs (Peak I: Co_1.5^(II)[Fe^(III)(CN)₆] and Peak II: KCo^(II)[Fe^(III)(CN)₆]).

Fig. 4.

(A, B) CV curves of ITO/SSG-Cohcf electrode in 0.1 M KCl obtained at a scan rate of 50 mV/s. (A) Cycle 1 and (B) (a-e) cycles 1-100 at intervals of 25 cycles, from outside to inside). (C) CV curves and (D) corresponding calibration plots of ITO/SSG-Cohcf electrode in 0.1 M KCl at various scan rates (30, 50, 80, 160, 200, 250, and 300 mV/s, from inside to outside).

Furthermore, the operational stability of the modified electrode is observed by recording 100 potential cyclic voltammetry (CV) cycles at a scan rate of 50 mV/s (Fig. 4B). Clearly, the current of peak I remains almost the same, whereas peak II exhibits a considerable current decrease in the forward scan and a negative shift in the backward scan. This finding can be explained as follows: the SSG offers good protection in terms of encapsulating the Cohcf NSs, and, during the potential CV cycles, insertion of K⁺ ions leads to this typical redox behavior of peak II. To apply this material in device fabrication, good performance stability is the key factor to obtaining the desired efficiency; thus, the cycling stability of the modified electrode was examined by repetitive potential CV cycling. After 100 potential cycles, 92% of the original peak current (peak I) was retained, demonstrating the high chemical and structural robustness of the Cohcf NSs on the modified electrode. This improved operational stability, combined with the ease of preparation, makes it a highly promising candidate for device fabrication.

The effect of the scan rate (ν) on the peak currents (i_pa and i_pc) was investigated in the range of 30-300 mV/s (Fig. 4C). As the scan rate increased, two anodic and cathodic peaks merged completely, and the peak-to-peak separation of the redox pairs tended to increase, suggesting a limitation in the chargetransfer kinetics. In addition, the overlap between the two oxidative and reductive peaks was probably due to chemical interactions between the electrolyte species with cobalt ions at higher scan rates. The corresponding calibration plots (Fig. 4D) between the peak currents and the scan rates (ν) for the redox reactions are linear. This observation clearly suggests that Cohcf NSs on the modified electrode, which nucleated throughout the 3D SSG and grew in the pores of the SSG, became SSG-encapsulated and interconnected. In addition, Cohcf NSs acted as charge transport carriers between the substrate and the solution interface.

Fig. 5A and B compares the SEM images before and after 100 potential CV cycles; structural deformation due to the insertion of K⁺ ions from the electrolyte into the Cohcf NSs is clearly visible. The SEM-EDX analysis confirms the elemental composition after 100 CV cycles. Furthermore, from the cyclic voltammogram presented in Fig. 4A, the concentration of Cohcf NSs at the electrode surface (Γ) was calculated using Eq. (4) [23]:

Fig. 5.

(A,B) SEM images and (C) EDX analysis of SSG-Cohcf electrode (A) before and (B,C) after 100 potential CV cycles.

where Q is the charge equivalent to the reduction peak (around 0.5 V), n is the number of electrons involved in the redox process, F is the Faraday constant, and A is the electrode area. The surface concentration of incorporated Cohcf was calculated as 2.0314 × 10⁻⁸ mol/cm². This larger surface coverage can provide more electrocatalytic sites.

3.3 Electrooxidation of N₂H₄

N₂H₄ is a precise analyte that has high oxidation overpotential; hence, it cannot be oxidized on bare electrodes. In this study, we investigated the electrochemical features of N₂H₄ on the fabricated modified electrodes and the bare electrode (see summary in Figs. 6-8). Fig. 6A shows the CV curves of the Cohcf-NS-modified electrodes in 0.1 M PBS (pH 7.0) in the absence (a) and in the presence (b) of N₂H₄. A comparison of these two CV curves indicates the vital role of Cohcf NSs in the film, i.e., electrochemically generation (at 0.52 V) upon oxidation of N₂H₄. An increase of ca. 23.10 μA in the current in the anodic region corresponding to oxidation of FeII to Fe^III (first peak at 0.52 V) on the ITO/SSG-Cohcf electrodes is observed when N₂H₄ is added. The increased current response due to the addition of N₂H₄ in the anodic region can be attributed to diffusion of N₂H₄ toward the electrode surface, and the current electrochemically reduces the produced Fe^III. In addition, the Fe^II is regenerated by N₂H₄ during the potential sweep; consequently, the anodic current increases. As shown in Fig. 6B, the electrochemical responses toward N₂H₄ are almost negligible on bare ITO (a) and ITO/SSG (b) electrodes. In contrast, a well-behaved anodic wave formed at a peak potential of 0.52 V on the ITO/SSG-Cohcf electrode (Fig. 6A). In addition, in the absence of the Cohcf NSs, N₂H₄ was oxidized around 0.8-0.9 V on the bare ITO and ITO/SSG electrodes, whereas a large decrease in the overpotential can be observed on the ITO/SSG-Cohcf electrode. This distinct electrochemical response (“b” in Fig. 6A) at the reduced overpotential for N₂H₄ is facilitated by the highly porous and interconnected Cohcf NSs on the modified electrode. We further investigated the electrochemical features of N₂H₄ on the ITO/SSG-Cohcf electrode by recording CV curves at various scan rates (30-300 mV/s) (Fig. 7). With increasing scan rate, the oxidation peak potential shifted in the positive direction, and the oxidation current was also enhanced. A plot of the anodic peak current versus the square root of the scan rate (ν^1/2) shows a linear relationship, revealing a diffusioncontrolled process [29]. In addition, the peak potential was directly proportional to the natural logarithm of the scan rate (ln ν), revealing a kinetic limitation in the reactions between the redox sites of the electron transfer mediator (Cohcf) and N₂H₄ [30]. LSVs were obtained for the ITO/SSG-Cohcf electrode by adding 100 μM N₂H₄; the results are summarized in Fig. 8. At the sensor electrode, the Cohcf NSs exhibited a stable response, and the catalytic current was proportional to the concentration of N₂H₄. A linear dependence of the oxidation peak current due to N₂H₄ addition was obtained with a correlation coefficient (R²) of 0.971, and the sensitivity was 0.187 μA/μM.

Fig. 6.

(A) Comparison of CV curves at ITO/SSG-Cohcf electrode in the (a) absence and (b) presence of 1 mM N₂H₄. (B) CV curves obtained on bare ITO, ITO/SSG, and ITO/SSG-Cohcf electrodes in the presence of 1 mM N₂H₄. Conditions: supporting electrolyte, 0.1 M PBS (pH 7.0); scan rate, 50 mV/s.

Fig. 8.

(A) LSVs recorded on the ITO/SSG-Cohcf electrode in 0.1 M PBS (pH 7.0) for (a) no N2H4 and (b) 100 μM, (c) 200 μM, (d) 300 μM, (e) 400 μM, (f) 500 μM, (g) 600 μM, and (h) 700 μM N₂H₄. (B) Corresponding calibration plot.

Fig. 7.

(A) Electrochemical response of ITO/SSG-Cohcf electrode to 1 mM N₂H₄ at scan rates of 30, 50, 80, 120, 160, 200, 250, and 300 mV/s. (B) Peak current as a function of the square root of the scan rate, and (C) linear relationship of peak potential against the natural logarithm of the scan rate.