CoNiTe₂ Nanomaterials as an Efficient Non-Enzymatic Electrochemical Sensing Platform for Detecting Dopamine

Abstract

Dopamine (DA) is an important catecholamine neurotransmitter in the mammalian central nervous system that affects many physiological functions. Hence, a highly sensitive and selective sensing platform is necessary for quantification of DA in the human body. In this study, ternary transition metal tellurides of CoNiTe₂ were successfully synthesized using the hydrothermal method. The proposed CoNiTe₂ nanomaterials were dispersed well in Nafion to form a well-dispersed suspension and, when dropped on a glassy carbon electrode (GCE) as the working electrode (CoNiTe₂/Nafion/GCE) for electrochemical non-enzymatic DA sensing, displayed excellent electrocatalytic activity for dopamine electrooxidation. The morphology and physical/chemical properties of CoNiTe₂ nanomaterials were characterized using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). In order to obtain the best electrochemical response to DA from the fabricated CoNiTe₂/Nafion/GCE, the experimental conditions of electrochemical sensing, including the CoNiTe₂ loading amounts and pH values of the phosphate buffer solution (PBS), were explored to achieve the best electrochemical sensing performance. Under optimal conditions (2 mg of CoNiTe₂ and pH 6.0 of PBS), the fabricated CoNiTe₂/Nafion/GCE showed excellent electrocatalytic activity of DA electrooxidation. The CoNiTe₂/Nafion/GCE sensing platform demonstrated excellent electrochemical performance owing to the optimal structural and electronic characteristics originating from the synergistic interactions of bimetallic Co and Ni, the low electronegativity of Te atoms, and the unique morphology of the CoNiTe₂ nanorod. It exhibited a wide linear range from 0.05 to 100 μM, a high sensitivity of 1.2880 µA µM⁻¹ cm⁻², and a low limit of detection of 0.0380 µM, as well as acceptable selectivity for DA sensing. Therefore, the proposed CoNiTe₂/Nafion/GCE could be considered a promising electrode material for electrochemical non-enzymatic DA sensing.

1. Introduction

Carlsson et al. (1958) reported that dopamine (DA) is a key neurotransmitter in the brain and is used as an immediate precursor in the biosynthesis of noradrenaline [1]. Current data reveal that DA is the predominant neurotransmitter expressed in the mammalian central nervous system (accounting for 80% of the catecholamine content in the brain) [2]. The amount of DA released correlates exclusively to diverse brain functions and is expressed specifically in the ventral tegmental area (VTA) of the midbrain, the substantia nigra pars compacta, and the hypothalamic arcuate nucleus of the human brain [3]. VTA dopaminergic activity controls natural motivation, reward prediction, and contextual learning [4,5]. Parkinson’s disease (PD), one of the most common neurodegenerative disorders, is linked to abnormal dopamine release in the substantia nigra pars compacta. PD is diagnosed by characteristic motor tremors [6,7]. The hypothalamic arcuate nucleus’ dopaminergic neurons are essential in regulating prolactin release. Prolactin is a protein hormone synthesized and secreted by lactotropic cells in the anterior pituitary gland and is involved in the prolactin homeostasis of the body [8,9].

Unquestionably, finding a precise and efficient method to quickly detect DA level deviations, thereby mitigating the societal and economic burdens associated with neurological disorders, is urgent. So far, analytical methods such as fluorescence [10], colorimetry [11], chemiluminescence [12], high-performance liquid chromatography (HPLC) [13], capillary electrophoresis (CE) [14], and electrochemistry [15,16] have been used to detect DA. Electrochemical methods stand out among the conventional analytical techniques because they are affordable, have a short analysis time, give an immediate response, and are user-friendly [17,18]. Electrochemical analytical methods detect DA using two sensing platforms: enzymatic and non-enzymatic. Although enzymatic sensors demonstrate high sensitivity and specificity, they are expensive, are complex to produce, have limited reproducibility, and are susceptible to environmental factors such as pH and temperature [19]. An electrochemical non-enzymatic DA sensor needs to be developed to overcome these drawbacks.

Previous reports reveal that electrochemical DA sensors based on transition metal chalcogenides (including transition metal sulfides (TMSs) [20], transition metal selenides (TMSs) [21], and transition metal tellurides (TMTs) [22]) have been successfully used in the development of electrode materials. Transition metal chalcogenides are a promising choice for electrode materials because of their excellent conductivity, abundant active sites, simplified synthesis procedures, and low preparation costs [23]. Furthermore, the electronegativity of Te (2.1) is low compared to S (2.58) and Se (2.55), resulting in weaker bonds between transition metals and Te. This facilitates electrochemical redox reactions [24]. These characteristics make TMTs suitable for extensive applications in the electrochemical field such as in supercapacitors [25], water splitting [26], and CO₂ reduction [27]. From the above-mentioned reports, TMTs have attracted significant attention in the electrochemical field due to the synergic effect caused by multiple redox-active transition metals, the low electronegativity of Te, and their unique structural features. In TMTs, the electronegativity of Te is lower than S and Se, which would tend to weaken covalent bonding between the transition metal and Te. Electronic structure tuning in transition metal chalcogenides should facilitate ion insertion/extraction and electron transport to give an excellent electrochemical performance. Nevertheless, the existing literature provides limited information on TMTs’ use as electrochemical biosensors, especially in DA sensing.

In this study, we prepared CoNiTe₂ ternary transition metal telluride nanomaterials using the hydrothermal method. We fabricated a CoNiTe₂/Nafion/glassy carbon electrode (GCE) sensing platform by drop-casting the prepared CoNiTe₂/Nafion nanocomposite on GCE. The CoNiTe₂/Nafion nanocomposite catalyzed DA electrochemical redox to deliver remarkable electrochemical performance through the synergistic effects of bimetallic Co and Ni, the relatively low electronegativity of Te, and the unique features of nanorod morphology.

2. Materials and Methods

2.1. Reagents

Cobalt (II) chloride hexahydrate (CoCl₂·6H₂O), nickel (II) chloride hexahydrate (NiCl₂·6H₂O), hydrazine monohydrate (N₂H₄·H₂O), and ethylenediamine were obtained from Alfa Aesar (Ward Hill, MA, USA). Tellurium powder (~200 mesh), Nafion solution (5 wt % in mixture of lower aliphatic alcohols and water), sodium phosphate dibasic (Na₂HPO₄), sodium phosphate monobasic (NaH₂PO₄), dopamine hydrochloride (DA), uric acid (UA), and L-ascorbic acid (AA) were purchased from Sigma-Aldrich (St. Louis, MO, USA). Anhydrous ethanol (C₂H₅OH, 99.9%) was purchased from J.T. Baker (Phillipsburg, NJ, USA). The deionized water (DI water) was produced using a Milli-Q water purification system from Millipore Co. (Bedford, MA, USA). All chemicals were analytical grade and were used as received without further purification.

2.2. Synthesis of CoNiTe₂ Nanomaterials

The CoNiTe₂ transition metal tellurides were successfully synthesized via a facile hydrothermal method. As is typical, 0.409 g CoCl₂·6H₂O and 0.411 g NiCl₂·6H₂O were precisely weighed and fully dissolved in 30 mL DI water by stirring for 30 min to form solution (A). At the same time, 0.235 g Te powder was dispersed into 8 mL ethylenediamine and stirred for 30 min to form solution (B). In the next step, solution (B) was added dropwise to solution (A) and stirred for 30 min to make sure all the reactants were totally dissolved. After that, 8 mL N₂H₄·H₂O was selected as the reducing agent and added to the mixture. The mixture was allowed to stir until a black precipitate appeared. Subsequently, the obtained black precipitate containing the Ni–Co telluride precursor was transferred into a 100 mL Teflon-lined stainless autoclave and heated to 180 °C for 12 h in an oven. Under mild hydrothermal conditions, the as-fabricated Ni–Co telluride precursor went through the amorphous–crystalline phase transformation induced by incorporated metal ions. Finally, the resulting CoNiTe₂ nanomaterials were cleaned by with DI water and anhydrous ethanol in a centrifuge 3 times, and then dried in the oven overnight. Then, the dried CoNiTe₂ nanomaterials were collected for subsequent characterization. A schematic illustration of the synthesis of the CoNiTe₂ nanomaterials is shown in Figure 1.

2.3. Fabrication of CoNiTe₂/Nafion/GCE Working Electrode

To prepare the glassy carbon electrode (GCE, diameter 3 mm, Tokai Carbon, Tokyo, Japan) for use, a thorough polishing procedure was performed by using 0.3 μm and 0.05 μm alumina slurries. Subsequently, the electrode was subjected to a rapid ultrasonic cleaning process using deionized (DI) water and anhydrous ethanol, followed by drying in a 70 °C oven. Next, 2 mg CoNiTe₂ nanomaterials were weighed and dispersed in 1 mL 0.5 wt % Nafion solution through ultrasonication for 30 min to form a well-distributed suspension. To fabricate the working electrode, 6 μL of the suspension was dropped on the precleaned GCE (bare GCE) and dried in the oven at 60 °C for 20 min. In this way, CoNiTe₂/Nafion/GCE was obtained for the following electrochemical measurements.

2.4. Characterizations

The morphology was characterized using field emission scanning electron microscopy (FESEM, JSM-7800F, JEOL, Akishima, Japan) and transmission electron microscopy (TEM, JEM-2100F, JEOL, Akishima, Japan). The crystal phase was analyzed by using X-ray diffraction (XRD) (D8 Discover X-ray diffractometer with Cu Kα radiation (Bruker, Karlsruhe, Germany). The chemical structure and composition were determined by using X-ray photoelectron spectroscopy (XPS, PHI-5000 Versaprobe, ULVAC-PHI, Chigasaki, Japan). Electrochemical measurements were performed by using an electrochemical analyzer (Autolab, model PGSTAT 30, Eco Chemie, Utrecht, The Netherlands) in a three-electrode system (the as-prepared samples modified with GCE were used as the working electrode, along with a platinum wire counter electrode and an Ag/AgCl (3 M KCl) reference electrode). All electrochemical measurements were conducted in 0.1 M phosphate-buffered saline (PBS) as the supporting electrolyte in the absence or presence of DA. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) measurements were taken in the working potential window of −0.2~1.0 V and 0~0.5 V. The DPV operational parameters (in 0.1 M PBS at pH 6.0) were optimized as follows: modulation time of 0.050 s, modulation amplitude of 0.050 V, pulse width of 0.050 s, internal time of 0.200 s, and step potential of 0.004 V.

3. Results and Discussion

The FESEM image (Figure 2a) revealed that the CoNiTe₂ surface is composed of a rod-like structure (approximately 700 nm in length) with numerous short nanorods (approximately several nanometers in length) growing on it. The short nanorods provide abundant electrochemical active sites and this boosts electronic/ionic transport during the subsequent electrochemical measurements. The TEM image (Figure 2b) revealed that the CoNiTe₂ nanomaterial’s inner structure concurred with the surface morphology observed by FESEM. Figure 3a–d show the scanning TEM (STEM) images and their corresponding EDS mapping for CoNiTe₂. These affirm that the CoNiTe₂ nanorod structure comprises well-distributed Co, Ni, and Te elements. The EDS spectrum (Figure 3e) of the CoNiTe₂ nanomaterials further revealed that the atomic ratio of Co, Ni, and Te is approximately 0.2:0.2:0.5, which is close to the theoretical atomic ratio of CoNiTe₂. The CoNiTe₂ nanomaterials were then characterized by X-ray diffraction. The standard XRD pattern of CoNiTe₂ (JCPDS No. 65-8961) [28] was obtained.

The XRD pattern identified the structure and phase composition of the CoNiTe₂ nanomaterials based on the standard patterns in the crystalline phase of CoNiTe₂ (JCPDS No. 65-8961) [28], as shown in Figure 4. In the XRD pattern of the CoNiTe₂ nanomaterials, the diffraction peaks located at 2θ equaling 31.1°, 43.2°, 45.9°, 56.5°, and 58.9° are attributed to the (101), (102), (110), (201), and (103) planes, respectively (indexed to the crystalline phase of CoNiTe₂). These characterizations confirm that the CoNiTe₂ ternary transition metal tellurides were successfully synthesized via a facile hydrothermal method.

The surface elemental composition and valence states of the CoNiTe₂ nanomaterials were characterized by XPS, as shown in Figure 5. Figure 5a shows the high-resolution Co 2p XPS spectra of the CoNiTe₂ nanomaterials. Two spin-orbit doublets corresponding to Co 2p_3/2 and Co 2p_1/2 were found. Co²⁺ was observed at 780.8 eV and 797.2 eV (binding energies), while Co³⁺ was observed at 778.8 eV and 796 eV. In addition to the two spin-orbit doublets, shake-up satellites (marked as Sat.) were observed adjacent to each doublet, located at 785.4 eV and 802.6 eV, respectively. Figure 5b shows the high-resolution Ni 2p XPS spectra of the CoNiTe₂ nanomaterials. Two spin-orbit doublets were ascribed to Ni 2p_3/2 and Ni 2p_1/2. Ni³⁺ was located at binding energies 855.4 eV and 873.4 eV, while Ni²⁺ was observed at binding energies 852.4 eV and 870.2 eV. Likewise, two shake-up satellites (marked as Sat.) were observed adjacent to the two spin-orbit doublets, which were situated at 861.4 eV and 879.0 eV, respectively. Figure 5c shows the high-resolution Te 3d XPS spectra of the CoNiTe₂ nanomaterials. The spin-orbit doublets of Te 3d_5/2 and Te 3d_3/2 appeared at 573 eV and 583.4 eV, respectively, and are attributable to Te²⁻. The other two peaks at 576 eV and 586.4 eV are shake-up satellites (marked as Sat.) [29].

The electrochemical characteristics of CoNiTe₂ nanomaterials for DA detection were performed by cyclic voltammetry (CV). Figure 6 shows the CV curves of bare GCE (black lines), Nafion/GCE (blue lines), and CoNiTe₂/Nafion/GCE (red lines) in 0.1 M PBS (pH 7.0) in the absence (dashed lines) and presence (solid lines) of 1 mM DA at a 50 mV s⁻¹ scan rate within the 0–1.0 V potential window. In the absence of DA, all electrodes show no redox peaks due to the absence of redox-active species. With the addition of 1 mM DA, all electrodes exhibited well-defined peaks (especially the CoNiTe₂/Nafion/GCE electrode) related to the electrochemical DA redox reaction mechanism. The possible electrochemical DA redox reaction mechanism is described in the previous literature [30]. During the electrochemical oxidation process, DA was oxidized to dopaminoquinone (DAQ), and during the electrochemical reduction process, DAQ was reduced to DA. The electrochemical DA redox reaction is a reversible process involving two electron/two proton transfers. Notably, CoNiTe₂/Nafion/GCE possesses the highest electrocatalytic activity in DA sensing. These results demonstrate that the synergistic interaction of bimetallic Co and Ni, the low electronegativity of Te, and the unique morphology of CoNiTe₂ nanorods improved electrochemical performance. According to a previous report [31], bimetallic Ni–Co transition metal chalcogenides generally exhibited satisfactory electrochemical performance. In Ni–Co tellurides, the electronegativity of Te is lower than those of S and Se. This results in weakened covalent bonding between the transition metal and Te, further accelerating electron transfer and modulating electronic structure in the electrochemical process. This enhanced electrochemical performance.

To obtain CoNiTe₂ nanomaterials with optimal electrochemical performance for DA sensing, the CoNiTe₂ loading amounts and pH values of the phosphate buffer solution were varied to understand electrochemical parameter optimization. To do this, 1~4 mg of CoNiTe₂ was weighed and dispersed into 1 mL 0.5 wt % Nafion to form a homogenous dispersion, then immobilized on the surface of GCE by a drop-casting method. Figure 7a shows the CV curves of CoNiTe₂/Nafion/GCE at different CoNiTe₂ loading amounts in 0.1 M PBS (pH 7.0) in the presence of 1 mM DA at a scan rate of 50 mV s⁻¹. It was observed that the oxidation peak current dramatically increased as the CoNiTe₂ loading amount varied from 1 to 2 mg. However, when the CoNiTe₂ loading amount exceeded 2 mg, the oxidation peak current decreased, indicating that the presence of the excess CoNiTe₂ hindered the mass transfer of dopamine [32]. Figure 7b displays the CoNiTe₂/Nafion/GCE CV curves at different phosphate buffer solution pH values (from pH 5 to 8) in the presence of 1 mM DA. The oxidation peak potential (E_pa) decreased with increasing pH value from 5 to 8, revealing that DA electrochemical behavior is associated with the pH dependence of interfacial electron–proton transfer. The corresponding linear relationship between oxidation peak potential and pH was calculated as follows: E_pa (V) = 0.846–0.059 pH (R² = 0.9771) (see the inset of Figure 7b). The results indicate a 0.059 V/pH linear slope. This is close to the theoretical value, implying that the electrochemical DA redox reaction mechanism involves two electron/two proton transfer steps, as governed by the Nernst equation [33]. Notably, the oxidation peak current increased slightly as the pH increased from 5 to 6, and then decreased rapidly from pH 6 to 8. The maximum DA oxidation peak current was at pH 6. Consequently, 2 mg of CoNiTe₂ (loading amount) and a phosphate buffer solution pH of 6.0 (pH value) were selected as the optimal parameters for DA sensing.

To further examine the electrochemical behavior of the CoNiTe₂ nanomaterials, CoNiTe₂/Nafion/GCE was studied through CV performed in 0.1 M PBS (pH 6.0) in the presence of 1 mM DA at 50 to 300 mV s⁻¹ scan rates (Figure 8a). It was observed that the redox peak current increased with increasing scan rate. Figure 8b reveals that both the oxidation peak current (I_pa) and reduction peak current (I_pc) were linear with the square root of the scan rate (v^1/2). The linear regression equation is expressed as I_pa (μA) = 34.8800 + 3.2760 v^1/2 ((mV s⁻¹)^1/2) (R² = 0.9856) and I_pc (μA) = −22.7890–5.0160 v^1/2 ((mV s⁻¹)^1/2) (R² = 0.9897). These results establish that the electrochemical DA redox reaction is controlled by diffusion according to the Randles–Sevcik equation [34].

We measured the electrochemical performance of CoNiTe₂/Nafion/GCE for DA electrooxidation using differential pulse voltammetry (DPV) in 0.1 M PBS (pH 6.0) with increasing DA concentrations (0~100 μM) under optimal experimental parameters to evaluate the feasibility of the proposed electrochemical non-enzymatic DA sensing. Figure 9a shows the CoNiTe₂/Nafion/GCE DPV responses against various DA concentrations from 0 to 100 μM. It was observed that the DPV response increased with increasing DA concentration. The DPV responses corresponding to DA concentrations ranging from 0.05 to 100 μM were plotted to obtain the corresponding calibration curve (Figure 9b). The linear regression equation for the oxidation peak current (I_pa) against the DA concentration is expressed as I_pa (μA) = 1.0030 + 0.0910 C (μM). The DA sensor calibration curve was linear from 0.05 to 100 μM (R² = 0.9928), and the slope and intercept were 0.0910 µA µM⁻¹ and 1.0030 μA, respectively. The sensitivity (according to the geometric area of GCE (0.0707 cm²) and the slope of the calibration curve), limit of detection (LOD) based on 3 S_b/m, and limit of quantification (LOQ) based on 10 S_b/m (S_b is the blank signal standard deviation for n = 3, and m is the calibration plot slope) were estimated as 1.2880 µA µM⁻¹ cm⁻², 0.0380 µM, and 0.1270 mM, respectively.

Table 1. Comparison of electrochemical DA sensor performance based on the different transition metal chalcogenides.

Electrode Materials	Linear Range (mM)	Sensitivity (μA μM−1 cm−2)	LOD (μM)	Reference
NiCo2S4/NF	0.50~100	14.9920	0.200	[35]
Sb2S3/GO/GCE	1.55~15.55	0.3077	0.800	[36]
fGO-Ga0.7Fe0.3Se2/GCE	2~170	0.2188	0.110	[21]
Bi2Te3/rGO/GCE	10~1000	0.2229	0.060	[37]
CoNiTe2/GCE	0.05~100	1.2880	0.038	This Work

summarizes previous reports regarding electrochemical non-enzymatic DA sensors based on the different transition metal chalcogenides [21,35,36,37]. It is noted that the CoNiTe₂ electrode material is comparable to the other electrochemical non-enzymatic DA sensors based on the different transition metal chalcogenides.

In a physiological environment, the electrochemical similarities between dopamine (DA), uric acid (UA), and ascorbic acid (AA) biomolecules can cause overlapping detection signals, potentially impacting DA sensing accuracy and reliability. Therefore, it is necessary to perform an interference study to evaluate the sensing ability and distinguish the interfering species. This ensures the accurate determination of the target molecules. To evaluate the selectivity of the CoNiTe₂ nanomaterials in the presence of the interfering molecules for DA sensing, Figure 10 displays the interference effect results for CoNiTe₂/Nafion/GCE in 0.1 M PBS (pH 6.0) in the presence of 200 μM AA and 10 μM UA (both are reasonable concentrations in the human body) and various DA concentrations (0.05, 0.5, 1, 3, 5, 7.5, and 10 μM). As observed, the DPV responses of the interfering molecules AA and UA were found at 0.15 V and 0.35 V, respectively. However, the DPV response of DA was observed at about 0.22 V, and its response increased with increasing DA concentration. In the experimental results, no significant interference effect was observed for DA determination in the presence of AA and UA. The linear regression equation for the oxidation peak current (I_pa) against the DA concentration in the presence of AA and UA (interference effect) is expressed as I_pa (μA) = 2.3262 + 0.1401 C (μM). The calibration curve was linear from 0.05 to 10 μM (R² = 0.9946), and the slope and intercept were 0.1401 µA µM⁻¹ and 2.3262 μA, respectively. For interference effect evaluation, a comparison of slope differences in the solvent and interference addition calibration curves showed that there was a small increase in slope in the presence of interfering molecules, indicating a slight interference effect. To minimize interference effects, a quantitative analysis was further performed using matrix-matched calibration curves to reduce interference-induced effects. As discussed above, it can be deduced that CoNiTe₂/Nafion/GCE has acceptable selectivity at DA concentrations ranging from 0.05 to 10 μM when AA and UA were also present, demonstrating the excellent anti-interference ability of the CoNiTe₂ nanomaterials.

4. Conclusions

In this study, a hydrothermally synthesized CoNiTe₂ nanomaterial-fabricated sensing platform (CoNiTe₂/Nafion/GCE) demonstrated prominent electrochemical performance for DA sensing owing to its excellent properties caused by bimetallic Co and Ni, the low electronegativity of Te, and its unique structural features (nanorods). In CoNiTe₂ nanomaterials, the lower electronegativity of Te leads to a weakened covalent bonding between transition metals and Te, resulting in the modulation of the electronic structure, which could propose an effective way to accelerate electron/ion transportation between the interface of the electroactive materials and electrolyte solution, exhibiting remarkable electrochemical performance for DA sensing. The electrochemical non-enzymatic DA sensor based on a CoNiTe₂ nanomaterials sensing platform exhibits excellent performance for the linear range from 0.05 to 100 μM, a sensitivity of 1.2880 µA µM⁻¹ cm⁻², and a limit of detection (LOD) of 0.0380 µM. Furthermore, the acceptable selectivity of the presented CoNiTe₂/Nafion/GCE in the presence of interfering species makes it a promising DA sensing platform for it practical use.