INTRODUCTION
Nitrite is an inorganic salt that resembles edible salt in both appearance and taste. In food processing, a moderate amount of nitrite can effectively enhance the color and flavor of food. However, excessive use poses significant safety risks, including nitrite poisoning, which can lead to severe hypoxia, shock, and even death [1]. Therefore, quantitative detection of nitrite in food analysis is of great significance. Common detection methods include spectrophotometry [2], ion chromatography [3], capillary electrophoresis [4], fluorescence spectrometry [5], and electrochemical methods [6]. Compared to other detection methods, electrochemical approaches offer significant potential for nitrite detection due to their ease of operation, good selectivity, and rapid response time [7,8]. Various electrochemical sensors have been developed for nitrite detection, utilizing precious metals [9], non-precious metals [10], and carbon materials [11]. Among these, carbon materials have garnered significant attention due to their unique physical and chemical stability, cost-effectiveness, biocompatibility, heat and corrosion resistance, and high electrical conductivity [12]. Wang et al. deposited Pt on autocatalytically grown Ni-N doped carbon nanotubes [13], Celebi et al. used an electrodeposition technique to prepare Cu@CeO2-rGO nanocomposites on graphite electrodes [14], and Ma et al. prepared NiFe–LDH NSAs on CC substrates [15], all successfully achieving nitrite detection.
Notably, carbon dots (CDs) have garnered significant interest due to their superior optical characteristics, electrical conductivity, favorable biocompatibility, abundance of hydrophilic functional groups, and straightforward synthesis process [16]. CDs, also known as carbon quantum dots or carbon nanodots, were first identified during the electrophoretic purification of single-walled carbon nanotubes [17]. They are typically characterized by a size of less than 10 nm and a zero-dimensional structure [18]. The integration of CDs into electrochemical sensors facilitates electron transfer at the sensing interface and electrodes, thereby enhancing redox reactions [19]. Ferreira et al. [20] have demonstrated that the synergistic interactions between CDs and nanomaterials lead to improved electrocatalytic responses, optimized electron transfer at the electrode-solution interface, and enhanced electrocatalytic efficiency with lower detection limits across various analytes. Additionally, CDs offer the potential to combine with metallic or nonmetallic elements, forming hybrid nanomaterials to detect other substances. For example, the cultivation of CDs/octahedral Cu2O nanocomposites on glassy carbon electrodes via drop-casting has shown remarkable electrocatalytic efficiency for glucose oxidation and H2O2 reduction [21]. Small amounts of CDs-modified multi-walled carbon nanotube composites (CDs/MWCNTs) [22] and AgNPs-modified CQDs nanohybrids (AgNPs/CQDs) [23] both possessed H2O2 reduction properties. Chen et al. employed a hydrothermal synthesis to fabricate nitrogen-doped carbon quantum dots (N–CQDs), which were subsequently integrated with β-cyclodextrins to create Cu–β–CDs/N–CQDs composites. These composites were applied to glassy carbon electrodes via drop-casting, enabling selective detection of Cu2+ [24]. Abazar et al. developed an insulin electrochemical sensor by surface-modifying a glassy carbon electrode with CQDs. The high specific surface area and catalytic activity of the CQDs provided the GCE/CQDs electrode with exceptional sensitivity, selectivity, and stability for insulin oxidation, achieving a detection limit of 2.24 nM [25]. Additionally, hybrid materials such as CD/Au [19], Au@CQDs–MXene [26] and Ag/C NC/GR/GCE [27], prepared using CDs as reducing agents, as well as composites like CQD–PEDOT [28] and PEI–CQDs [29], have all demonstrated strong electrocatalytic oxidizing activity. Li et al. synthesized nitrogen-doped carbon dots (N–CDs) on glassy carbon electrodes, utilizing kiwifruit seeds, white sesame seeds, and black sesame seeds as precursors, and subsequently employed these N–CDs for the detection of nitrites [30]. However, the preparation of most nanocomposites is notably complex, and the commonly used glassy carbon electrodes and active materials often require polymer binders or conductive additives to adhere to the electrode surface. This approach can increase interfacial impedance, reduce stability, decrease the specific surface area of the active material, limit the number of active sites, and hinder the rate of electron transfer, all of which are detrimental to effective contact between the electrode materials and the analyte [31]. The material is also prone to catalyst shedding and potential metal leaching issues. Consequently, this may result in compromised stability and reproducibility [32]. Furthermore, certain metal nanomaterials face multiple constraints, including high costs, diminished catalytic efficacy attributable to a propensity for agglomeration, and the deactivation of metal-based catalysts following extended utilization [33]. Consequently, to address these issues, selecting an appropriate conductive substrate for the electrode and modifying the electrode surface with non-precious metal active materials to develop metal-free nanocatalytic materials with excellent sensing ability and stability toward nitrite is considered an effective solution [34]. Furthermore, the superior water solubility and electrical conductivity of CDs can be leveraged for the fabrication of a variety of electrochemical sensors, through the synthesis of CDs with diverse structures to achieve highly sensitive detection of other substances.
Flexible electrodes offer significant advantages over conventional glassy carbon electrodes, with carbon cloth (CC), composed of an extensive network of interwoven carbon fibers, recognized as an optimal substrate for self-supported flexible electrodes [35]. Nonetheless, commercial CC is limited by its high hydrophobicity, restricted surface area, and diminished electrochemical activity, all of which impede its effectiveness in electrochemical sensing applications [36]. To improve the performance of CC as a nitrite-sensitive electrode, researchers have introduced various oxygenated functional groups on the CC surface through acid oxidation [37] or constant-current electrochemical treatments in a copper sulfate electrolyte [38]. These modifications have significantly increased the hydrophilicity, active specific surface area, active sites, and overall electrochemical performance of CC against nitrite. Additionally, dielectric barrier discharge (DBD) plasma at low temperatures can generate a variety of highly oxidized species in an oxygen-rich environment, promoting the formation of numerous active and functional groups on the surface of carbon-based materials, thereby enhancing surface hydrophilicity [39]. And the use of contaminated reagents is also avoided.
In this study, CC was modified using DBD technology, and CDs were synthesized on the modified CC through direct bonding. Citric acid and ethylenediamine served as precursors in a one-step hydrothermal process, resulting in CDs/CC composite electrodes with tunable surface properties and optimal hydrophilicity, as depicted in Scheme 1. The synthesis conditions were optimized to produce CDs/CC composites with a large electrochemically active surface area, enhanced electrical conductivity, reliable reproducibility, excellent repeatability, and superior electrochemical performance. The sensor was then applied to detect nitrite in ham sausage samples.
MATERIALS AND METHODS
Reagents
CC was procured from Suzhou Shengeruo Technology Co., Ltd. Ethylenediamine, Citric Acid Monohydrate, Sodium Nitrite, Sodium Dihydrogen Phosphate Dihydrate, Disodium Hydrogen Phosphate Dodecahydrate, Potassium Chloride, Urea, Magnesium Sulfate, Iron Nitrate, Sodium Carbonate, and other chemicals was supplied by Chengdu Cologne Chemicals Co., Ltd. Potassium ferricyanide, potassium ferricyanide, glucose was acquired from Shanghai McLean Co., Ltd. All chemicals utilized in the experiments were of analytical grade and were used as received without further purification. Deionized water was employed in all experimental procedures.
Instrumentation
The crystal structure and composition of the materials were analyzed utilizing the X-ray diffractometer (XRD–7000), while the chemical makeup and valence states of the elements on the electrode material surfaces were explored with the X-ray photoelectron spectrometer (XPS) (K-ALPHA). Transmission electron microscope (TEM) (Talos F200S G2) was utilized to observe the morphology of the materials. An energy spectrometer (EDS) was utilized to analyze the structure of each component. The fluorescence signal of the material was observed using an inverted fluorescence microscope (ECLIPSE Ts2-FL), DBD plasma (DBD–50) was employed for hydrophilic treatment of CC, and a contact angle meter (SDC–200S) measured the hydrophilicity of different electrode materials.
Preparation of Modified Electrodes
Pretreatment of CC
Initially, the CC was divided into 1×1 cm² sections and sequentially cleaned in anhydrous ethanol, acetone, and deionized water utilizing an ultrasonic bath. The cloth was dried at 70°C for subsequent use. Subsequently, the CC was subjected to the preliminary hydrophilic modification within the DBD plasma, where the process was carried out under conditions of a 10 kV discharge voltage, 8 kHz frequency, a 14-minute treatment duration, and an airflow rate of 150 mL/min.
Preparation of CDs/CC electrodes
Accurately, 0.48 g of ethylenediamine and 0.46 g of citric acid monohydrate were measured in a 25 mL reaction vessel. Then, 10 mL of deionized water was added, and the mixture was ultrasonically dispersed and dissolved for 10 minutes. The DBD plasma-treated CC was subsequently immersed in the reaction vessel and placed in an oven at 220°C for 5 hours. After the reaction, the CC was repeatedly rinsed with distilled water and dried. To determine the optimal conditions for the synthesis of CDs, experiments were conducted with varying reaction times (4–7, and 8 hours) (Fig. S1a–b) and temperatures (160, 180, 200, and 220°C) (Fig. S1 c–d).
Electrochemical experiments
This study employed the Shanghai Chenhua CHI 760E electrochemical workstation for the execution of all electrochemical experiments, employing a three-electrode system for the electrochemical tests, utilizing Ag/AgCl as the reference electrode, and a platinum sheet (1 × 1 cm²) as the counter electrode, CC and modified CC materials as the working electrodes. The electrochemical detection of nitrite at various material electrodes was conducted via cyclic voltammetry (CV), utilizing a 0.1 M phosphate buffer solution (PBS) as the electrolyte, with a potential range of 0.6–1.0 V and a scan rate of 50 mV/s. The unmodified CC electrode and modified working electrodes with diverse materials were immersed in a 5.0 mM [Fe(CN)6]3–/4– solution in 0.1 M KCl electrolyte for electrochemical alternating current impedance (EIS) analysis, with the frequency range spanning from 0.01 to 105 Hz. Optimal detection voltage and sensitivity assays were executed employing the current-time curve method (i–t), utilizing 0.1 M PBS (pH=7.0) as the electrolyte and incorporating slow magnetic stirring within the electrolytic cell.
Actual sample pretreatment
Initially, 3 g of commercially procured ham sausage was homogenized using a mortar and pestle, followed by the transfer of the homogenized sample to a 250 mL volumetric flask. Subsequently, 6 mL of 50 g/L saturated borax solution was introduced, and approximately 150 mL of water preheated to 70°C was incorporated; the mixture was then subjected to heating in a boiling water bath for 15 minutes, followed by ultrasonication for 10 minutes, and subsequently removed. Next, 5 mL of 106 g/L potassium ferricyanide solution was added with gentle rotation, ensure thorough mixing, followed by the addition of 5 mL of 220 g/L zinc acetate solution to facilitate protein precipitation. Ultimately, the volume was adjusted to the calibration mark with the addition of water, allowed to stand for 30 minutes, the mixture was subjected to filtration to eliminate the upper layer of oil and grease, and the resulting filtrate was retained as the actual sample for subsequent analysis.
RESULTS AND DISCUSSION
Characterization of materials
To assess the alterations in the hydrophilic and hydrophobic characteristics of materials throughout the modification process, water contact angle measurements were conducted. As depicted in Fig. 1a, the contact angle for the pristine CC was 130.2°, indicating hydrophobicity. This hydrophobic nature is attributed to numerous nonpolar C–C bonds on the CC surface, which are less likely to interact with water molecules. The contact angle for the DBD-treated carbon cloth was reduced to 45.1° (Fig. 1b), indicating hydrophilicity, as the DBD plasma in an air atmosphere produces reactive groups on the carbon cloth surface, altering its surface properties to be hydrophilic [40]. Nonetheless, the contact angle for CDs/CC materials was less than 5° (Fig. 1c), suggesting superhydrophilicity. The significant enhancement in the wettability of the CDs/CC electrode materials is due to the proliferation of CDs on the surface, introducing hydrophilic functional groups such as C–OH, COOH, and –NH2 [41]. The enhanced hydrophilicity allows for full contact between the electrode and the electrolyte, which expands the electrochemically active surface area and improves the efficiency of nitrite electrocatalytic oxidation [42].
Furthermore, the elemental composition of the pristine bare CC electrode was examined via EDS. As illustrated in Fig. 1d, the surface of the unmodified bare CC primarily consists of carbon (C), with only a sparse presence of oxygen (O) elements. A more detailed analysis of the oxygen distribution (Fig. 1e) revealed numerous bare black regions on the CC surface, suggesting a sporadic and scattered distribution of oxygen elements, likely due to adsorption on the CC surface [38]. The morphology of the CDs/CC was delineated through TEM, as depicted in Fig. 1f, revealing spherical and uniformly dispersed CDs with an approximate diameter of 4.30 nm. However, particles of varying morphologies and sizes were also observed, which deviated from the typical dimensions of CDs, potentially due to the presence of copolymers of citric acid and ethylenediamine on the CDs/CC electrode surface [43]. Figs. 1g–j demonstrate a more uniform distribution of constituents C, N, and O across the CC, confirming the successful synthesis of the CDs/CC material. Fig. 1k shows an inverted fluorescence microscopy image of unmodified bare CC, indicating that the bare CC is non-fluorescent under UV irradiation. Fig. 1l shows the inverted fluorescence microscope image of the CDs/CC material. Under UV irradiation, significant blue fluorescence signals are visible, indicating the successful growth and uniform distribution of CDs on the CC. These findings align with the TEM results presented earlier.
The elemental and atomic composition of the unmodified bare CC surface is presented in Fig. 1m, showing that the electrode is composed of C and O, with atomic percentages (At%) of 97.64% for C, 2.36% for O, and 0% for N. Following DBD treatment, the atomic percentage of O increased slightly to 2.65% (Fig. 1n). In contrast, Fig. 1o shows the elemental and atomic composition of the CC functionalized with CDs, with the atomic percentage of C at 91%, O increased to 6.92%, and N at 2.08%. Compared to the unmodified and DBD-treated CC, the CDs-modified surface displayed a significant increase in N and O content, suggesting the binding of numerous oxygenated functional groups (C–OH, COOH) to the C and their anchoring to the CC surface. The presence of the –NH2 group further confirmed the successful preparation of the CDs/CC.
The crystalline phases of the unmodified CC, DBD-treated CC, and CDs/CC electrodes were characterized by X-ray diffraction spectroscopy. As shown in Fig. 2a, all three electrode materials exhibited two broad peaks near 25.58° and 43.76°, corresponding to the diffraction from the (002) and (101) planes of a typical hexagonal crystalline graphitic carbon structure (PDF#75–1621) [44]. Additionally, no extra diffraction peaks were observed for the CDs/CC electrode, suggesting that no other crystalline phases or impurities were introduced during the preparation of the CDs/CC electrode other than the introduction of functional groups.
To elucidate the surface elemental composition and identify the functional groups of the electrode materials, XPS analysis was conducted on the unmodified CC, DBD-treated CC, and CDs/CC electrodes. The comprehensive spectral data for the three electrodes are depicted in Fig. 2b. The unmodified CC electrode exhibited prominent carbon characteristic peaks and minimal oxygen characteristic peaks. However, a significant increase in oxygen content was noted for the DBD-treated bare CC, indicating that the DBD technique effectively introduces oxygenated functional groups, consistent with the EDS results. A faint N 1s peak was also detected, likely due to the interaction of atmospheric nitrogenous compounds with the carbon surface during the DBD plasma treatment. Additionally, C, O, and N characteristic peaks were identified on the CDs/CC electrode. Fig. 2c presents the C1s core-level spectra for the three electrode materials. For the unmodified CC, the peaks observed at binding energies of 284.3 eV, 285.97 eV, and 289.56 eV correspond to C–C, C–OH, and COOH bonds, respectively [45]. The broad and low-intensity nature of the latter peaks indicates a limited oxygenated functional groups. Following DBD treatment, three distinct peaks were detected at approximately 284.1 eV, 285.6 eV, and 288.5 eV, associated with C–C, C–OH, and COOH bonds, respectively [46]. Furthermore, the C1s spectra of the CDs/CC electrode materials exhibited four distinct peaks at 284.2 eV, 285.2 eV, 286.3 eV, and 290.6 eV, corresponding to C–C, C–N, C–OH, and COOH, respectively [47]. The increased intensities of these peaks suggest that the growth of CDs promotes the formation of bonds between C–OH, COOH, and –NH2 functional groups with carbon atoms.
Fig. 2d presents the O1s X-ray photoelectron spectra for three distinct types of electrode materials. The unmodified CC electrode exhibited two peaks at 532.9 eV and 531.8 eV, corresponding to C–OH and COOH, respectively [48]. After DBD modification, there was a noticeable increase in the peak intensities of these functional groups, with binding energies of 531.8 eV for C–OH and 533.2 eV for COOH [49]. The CDs/CC electrode displayed two prominent peaks attributed to COOH (531.0 eV) and C–OH (532.4 eV), highlighting the presence of oxygenated functional groups on the surface of the CDs/CC material [50]. Compared to the unmodified and DBD-treated carbon cloths, the O1s peak intensities for the CDs/CC material are higher, suggesting an increased content of oxygenated functional groups on the surface. As depicted in Fig. 2e, the peak signals at 398.8 eV and 400.0 eV for the CDs/CC material are attributed to the C–N–C and N–H groups [51], exhibiting greater intensities indicative of the presence of amino functional groups. In summary, these findings demonstrate the successful introduction of oxygenated and amino functional groups onto the CC surface following the hydrothermal grafting of CDs.
Electrochemical performance of CDs/CC electrode
Fig. 3a illustrates the cyclic voltammetric current responses for the unmodified bare CC, DBD-treated CC, and the CDs/CC electrodes. All three electrodes displayed reversible redox peaks. Notably, the CDs/CC electrode demonstrated a current response intensity of approximately 2.35 and 1.24 times greater than that of the unmodified and DBD-treated CC electrodes. This increased current response is likely due to the formation of CD nanoparticles on the CC and the introduction of oxygenated and amino functional groups, which imparted superhydrophilicity to the CC. This superhydrophilicity facilitated contact with nitrite, thereby enhancing the electron transfer rate. The electrical conductivity of the modified electrodes was further evaluated using EIS. Fig. 3b displays the Nyquist plots for the various electrodes in a 0.1 M KCl solution with 5 mM [Fe(CN)6]3–/4–. The electron transfer resistance (Rct) serves as an indicator of the electrode materials’ conductive capability, with the semicircle diameter indicating the charge transfer resistance of the electrodes [52,53]. A smaller semicircle diameter in the impedance spectrum indicates a lower Rct and higher charge transfer efficiency. The Rct values for the unmodified CC, DBD-treated CC, and CDs/CC electrodes were determined to be 24.14 , 15.85 , and 11.46 , respectively, based on the fitted Nyquist plots. These findings suggest that the nucleation of CD nanoparticles enhances the electrode's conductivity, increases the electrochemically active surface area, and improves the electrocatalytic and interfacial characteristics, making the CDs/CC electrode more effective for detecting electrode current signals.
To ascertain the specific electrochemical active area (EASA) for various electrode materials, DBD-treated carbon cloth electrodes (Fig. S2c–d) and CDs/CC electrodes (Fig. 3c) were evaluated using CV in a 0.1 M KCl solution with 5 mM potassium ferricyanide/ferrocyanide ([Fe(CN)6]3–/4–) at varying scan rates. The performance of these electrodes was assessed based on the peak current density. As shown in Fig. 3d, the anodic peak current in the CDs/CC electrode showed a good linear relationship with the square root of the sweep rate, R2=0.998. In this process, the EASA can be determined utilizing the Randles-Sevcik equation, as shown in Eq. (1) [54]
Where Ip denotes the peak anodic current in the cyclic voltammogram of the potassium ferricyanide solution, A is the effective active surface area, D represents the diffusion coefficient of [Fe(CN)6]3–/4– (7.6 × 10–6 cm–2 s–1), n is the number of electrons transferred, ʋ is the scan rate (V/s), and C denotes the concentration of [Fe(CN)6]3–/4– (mol cm–3). Based on the slopes of the Ip versus ʋ1/2 curves, the specific EASA values for the CDs/CC, the DBD-treated CC, and the unmodified CC electrodes were determined to be 7.46 cm², 5.82 cm², and 2.62 cm², respectively. The EASA of the CDs/CC electrode was 2.85 times higher than that of the unmodified bare CC and 1.28 times higher than that of the DBD-treated bare CC. From an electrochemical perspective, introducing hydrophilic groups (C–OH, COOH, –NH2) on the CC surface after modification by CD materials significantly increased the electrochemically active surface area. This enhancement improved the adsorption capacity, optimized the electrocatalytic active sites, and expanded the charge transport pathways, resulting in greater nitrite participation in the electrochemical reaction.
Effect of electrolyte pH on the electrochemical performance of CDs/CC
Hydrogen ions play a crucial role in the electrochemical oxidation of nitrite, and since the pH of a solution is the negative logarithm of the hydrogen ion concentration, pH has a significant impact on nitrite determination. As depicted in Fig. 4a, CV tests were conducted in a 0.1 M PBS solution containing 1 mM nitrite at varying pH levels. Furthermore, the oxidation peak currents of NO2-, as detected by CDs/CC electrodes in electrolytes of distinct pH values, are illustrated in Fig. 4b. The oxidation peak current gradually increases as the pH rises from 5.0 to 6.0, likely because nitrite is unstable in acidic solutions and readily decomposes into NO and NO3- [55]. Conversely, the oxidation peak current gradually decreases when the pH reaches 8 and 9, possibly due to the reduction in hydrogen protons as pH increases, making the electrocatalytic oxidation of NO2- more challenging and reducing the electrical signal [56]. The peak current reached its maximum value at pH 7. Therefore, pH=7 was chosen as the optimal pH for subsequent studies in this experiment.
Electrochemical determination of nitrite
Unmodified bare CC, DBD-treated bare CC, electrodes for the direct growth of CDs on CC without DBD modification and CDs/CC material electrodes were tested for their ability to catalyze nitrite decomposition using the CV method. The CV plots for the four electrode materials, immersed in a 0.1 M PBS solution with 1 mM nitrite, are depicted in Fig. 5a. The data reveal that the electrode fabricated from CDs/CC demonstrates the highest oxidation peak current value, indicating that the CDs/CC material possesses superior catalytic activity for nitrite oxidation. Furthermore, it is evident that modification via DBD enhances the electrocatalytic performance of the electrode materials. By comparison, no notable redox peaks were observed for the electrodes composed of the four materials in a 0.1 M PBS solution devoid of 1 mM nitrite, as illustrated in Fig. 5b. Highlighting that the CDs/CC electrode exhibits a specific catalytic activity toward nitrite. Compared to the unmodified bare CC, DBD-treated bare CC and electrodes for the direct growth of CDs on CC without DBD modification, the CDs/CC electrodes showed a higher current response signal for nitrite. This difference is mainly attributed to two factors: first, the CDs/CC electrode has good catalytic activity for NO2-, and second, the CDs/CC material exhibits excellent conductivity, which reduces the diffusion barrier and promotes electron transport. Consequently, the CDs/CC electrode prepared by growing CDs can accelerate the conversion of NO2- to NO3- and enhance the response signal.
To investigate the reaction kinetics and electron transfer mechanism of nitrite on CDs/CC electrodes, the redox of CDs/CC materials in 1 mM NO2- at different scan rates (10–100 mV/s) was tested (Fig. 5c). Fig. 5d demonstrates the relationship between the oxidation current signal and the square root of the scan rate, with the corresponding linearized equation: y(mA) = 2.4233 – 0.0799x(V/s), R2 = 0.991, highlighting that the nitrite oxidation reaction on the CDs/CC electrode is a diffusion-controlled process [57]. As illustrated in Fig. 5e, the linear regression equation of the oxidation peak potential versus the logarithm of the scanning rate is y(V) = 0.0768x (lnV/s) + 1.134 (R2=0.994), which corresponds to the theoretical model of Laviron shown in Eq. (2). This model allows the determination of the number of electrons involved in the reaction [58].
Where E0 represents the standard electrode potential (V), α indicates the transfer coefficient, k0 is the heterogeneous electron transfer rate constant (s–1), T denotes the temperature (298.15 K), R indicates the ideal gas constant (8.314 J/(mol·K)), and F represents the Faraday constant (96485 C/mol). According to the slope of the linear correlation between Epa and lnʋ, αn was determinded to be 0.334, using equation (3):[59]
In this equation Ep/2 is the half-peak potential, α is calculated to be 0.476. Thus, the number of electrons transferred during the rate-determining step of nitrite electrocatalytic oxidation at the CDs/CC electrode n is estimated to be 0.7≈1. Consequently, the mechanism of nitrite electrocatalytic oxidation at the CDs/CC electrode can be inferred as shown in Eq. (4–7)
First, nitrite adsorption on the surface of CDs/CC electrodes to form [CDs CC NO2–)] (Eq. (4)). Then, NO2 is obtained by the loss of an electron through [CDs CC NO2–)] (Eq. (5)). Then, NO2 undergoes a disproportionation reaction to generate NO3- and NO2- (Eq. (6)). Eventually, NO3- becomes a single product of electrocatalytic oxidation of nitrite at the CDs/CC electrode (Eq. (7)).
Fig. 5f presents the CV plots obtained from the CDs/CC electrode as the nitrite concentration varies from 0 mM to 10 mM. Notably, the peak potential shifts positively with increasing nitrite concentration, and the corresponding anodic peak current also increases, displaying a good linear relationship (Fig. 5g). The linear correlation coefficient of R2=0.996 indicates that the CDs/CC electrode exhibits distinct current responses to varying nitrite concentrations, demonstrating its sensitivity to nitrite detection.
Methodological validation
Sensitivity, linear range and detection limit
Optimizing the test voltage before assessing the sensor's sensitivity is essential. Given the potential for discrepancies between the i–t and CV detection methods, an optimal applied voltage close to the CV results was selected to achieve the best conditions for nitrite detection using the i–t method. As shown in Fig. 6a, with constant stirring, 0.1 mM nitrite was continuously added dropwise to 0.1 M PBS solution at applied voltages ranging from 0.75 to 0.90 V. As the applied voltage increased, the overall current response also increased; however, at 0.9 V, a significant background disturbance was observed. The highest steady-state current response, coupled with a minimal background current, was achieved at 0.85 V, while the current response was relatively low at voltages below 0.85 V. As a result, 0.85 V was chosen as the optimal applied voltage. Under this optimized condition, the amperometric response of unmodified bare CC, DBD-treated bare CC and CDs/CC electrode materials to continuous dropwise addition of 0.1 mM nitrite was tested (Fig. 6b). The findings clearly demonstrated that the CDs/CC electrode generated a higher current response to nitrite, aligning with the results obtained from the CV measurements.
To assess the nitrite detection capabilities of the CDs/CC electrode, the current response was measured at 0.85 V following the incremental addition of varying nitrite concentrations in a 0.1 M PBS solution (Fig. 6c). The response current gradually increased with rising nitrite concentrations, and the interpolated plot (Fig. 6c) demonstrated that the constructed sensor was capable of detecting low concentrations of nitrite. Further analysis of the sensor’s performance was conducted by applying a linear fit to the data in Fig. 6c, which revealed three distinct linear ranges: a sensitivity of 4620.0 μAmM–1 cm–2 at 0.5–7 μM (y= 4620.0x + 20.4) (Fig. 6d), a sensitivity of 1458.6 μA mM–1 cm–2 at 8-1000 μM (y= 1458.6x + 85.3) (Fig. 6e), and a sensitivity of 386.8 μA mM–1 cm–2 at 2000–7000 μM (y= 386.8x + 1675.8) (Fig. 6f). Using the formula LOD = 3 /s, LOD = 0.14 μM (S/N = 3) was calculated. These findings indicate that the CD s/CC sensor has high sensitivity, a low detection limit, and a broad linear range for detecting nitrite. Table 1 presents a comparative analysis of the CDs/CC electrodes fabricated in the current study with other nitrite sensors documented in the existing literature, and the data indicate that the CDs/CC sensors exhibit a broader linear response range and lower detection limits, compared with sensors that demonstrate comparable or superior analytical performance.
Selectivity, repeatability, reproducibility and stability
To evaluate the specificity of the constructed sensor for detecting nitrite in real samples, interfering ions at a concentration of 1 mM and nitrite solution at a concentration of 10 μM were added separately to a 0.1 M PBS electrolyte under uniform stirring. The current-time curves were then obtained by chronoamperometry, as shown in Fig. 7a. The first addition of nitrite resulted in a significant fluctuation in the current response, indicating a strong catalytic reaction. In contrast, the sensor showed no significant catalytic oxidation effect when interfering ions were added, but a rapid current signal reappeared when the 10 μM nitrite solution was added again. This response highlighted the specificity and good selectivity of the CDs/CC electrochemical sensor for nitrite, demonstrating its suitability for detecting nitrite in real samples.
The repeatability of the electrode materials was evaluated utilizing the CV method by measuring the current response of six identically prepared electrodes to 1 mM nitrite. As shown in Fig. 7b, the oxidation peak current values of the six CDs/CC electrodes exhibited minimal variation, resulting in a relative standard deviation (RSD) of 1.76%. To assess reproducibility, six consecutive tests were conducted on a single electrode, which produced an RSD of 0.74% (Fig. 7c). These results demonstrated the excellent repeatability and reproducibility of the nitrite sensor constructed from CDs/CC material.
To examine the stability of the CDs/CC electrodes, the prepared electrodes were stored in a self-sealing bag at room temperature under dry conditions. The current response to 1 mM nitrite was recorded every six days. After 24 days, the current signals retained 96.47% of the initial response (Fig. 7d), indicating that the sensor possesses good long-term stability.
Application: Determination of nitrite in food samples
The ability to detect actual samples is a crucial technical criterion for evaluating the feasibility and accuracy of a sensor. Therefore, the sensor was used to detect ham sausage samples containing a certain amount of nitrite. As shown in Table 2, the spiked recoveries for the ham sausage ranged from 97.72% to 101.40%, with an RSD of less than 2.70%. Additionally, nitrite content in t he samples was measured using the spectrophotometric method (naphthalene ethylenediamine hydrochloride method) according to the national standard GB5009.33–2016. The results obtained with this method were consistent with those from the electrochemical sensor, confirming the accuracy of the experimental results. This verification demonstrates that the electrochemical sensor based on CDs/CC is effective for detecting nitrite in ham sausage.
CONCLUSION
In summary, this study developed a non-enzymatic nitrite sensor using CDs, known for their hydrophilic functional groups, as the sensing material. Material characterizations, including TEM, XRD, XPS, EDS, and inverted fluorescence microscopy, were conducted to confirm the successful preparation of CDs/CC materials. CV and EIS analyses demonstrated the excellent electrocatalytic activity and electrical conductivity of the CDs/CC materials, validating their suitability for electrochemical sensing. Nitrite response experiments revealed a wide linear range of detection with a limit of detection of 0.14 μM. Additionally, the sensor showed superior selectivity, reproducibility, repeatability, and stability. The results were satisfactory for determining nitrite content in real samples, with good recoveries observed. The sensor shows promise for practical applications in food safety detection. The electrochemical sensors developed in this study do not depend on elaborate synthesis procedures or the incorporation of metal-based materials during their fabrication. The sensing performance can be substantially improved, through the grafting of CDs onto the CC surface and the introduction of additional hydrophilic functional groups. This provides a novel perspective for the development of practical electrodes in electrochemical applications.
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