Antimonene-Modified Screen-Printed Carbon Nanofibers Electrode for Enhanced Electroanalytical Response of Metal Ions

Abstract

A two-dimensional (2D) Sb-modified screen-printed carbon nanofibers electrode (2D Sb_exf-SPCNFE) was developed to improve the stripping voltammetric determination of Cd(II) and Pb(II), taking advantage of the synergistic effect between the two nanomaterials. The surface morphology of the 2D Sb_exf-SPCNFE was investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, and Raman spectroscopy. The analytical performance of 2D Sb_exf-SPCNFE was compared to those presented by screen-printed carbon electrodes modified with 2D Sb_exf (2D Sb_exf-SPCE) and the corresponding bare electrodes: screen-printed carbon nanofibers electrode (SPCNFE_bare) and screen-printed carbon electrode (SPCE_bare). After optimizing the experimental conditions, the 2D Sb_exf-SPCNFE exhibited much better analytical parameters compared to the other assessed sensors. Analysis in 0.01 mol L⁻¹ HCl (pH = 2) using 2D Sb_exf-SPCNFE showed excellent linear behavior in the concentration range of 2.9 to 85.0 µg L⁻¹ and 0.3 to 82.0 µg L⁻¹ for Cd(II) and Pb(II), respectively. The limits of detection after 240 s deposition time for Cd(II) and Pb(II) were 0.9 and 0.1 µg L⁻¹, and sensitivities between 1.5 and 3 times higher than those displayed by SPCE_bare, SPCNFE_bare, and 2D Sb_exf-SPCE were obtained. Finally, the 2D Sb_exf-SPCNFE was successfully applied to the determination of Cd(II) and Pb(II) traces in a certified estuarine water sample.

1. Introduction

Heavy metal water pollution is a significant environmental hazard that has detrimental consequences for organisms that are exposed to it, including plants, animals, and humans. Although heavy metal contamination of water can be due to both natural processes or anthropogenic activities, the main sources derive from industries, human dwellings, and agricultural activities that release substances into hydrological systems [1,2]. In terms of human health, some metal ions are essential and crucial for biological functions at low concentrations, but many other non-essential heavy metals are hazardous and have a long half-life that leads to the accumulation and contamination of food chains [3,4,5]. In fact, according to the ICH Q3D guidelines [6], metal ions such as arsenic, cadmium, mercury, and lead are considered Class 1 human toxicants. In particular, lead and cadmium are poisonous to humans, causing neurological, endocrine, hematological, renal, cardiovascular, respiratory, and skeletal disorders, miscarriages, and potentially presenting mutagenic and carcinogenic effects [7,8,9]. Cadmium and lead are hazardous at low concentrations, with the maximum limits established by the World Health Organization in drinking water being 3 µg L⁻¹ and 10 µg L⁻¹ for Cd(II) and Pb(II), respectively [2].

Heavy metals can be detected using several analytical techniques such as atomic fluorescence spectrometry (AFS) [10], graphite furnace atomic absorption spectrometry (GFAAS) [11], inductively coupled plasma mass spectrometry (ICP-MS) [12], inductively coupled plasma atomic emission spectrometry (ICP-AES) [13], and advance hyphenated techniques [6,14]. Although these techniques provide low detection limits and high sensitivity, the involved instruments are bulky and costly, and their handling requires skilled operators. In contrast, electroanalytical techniques based on the use of stripping voltammetry represent a powerful alternative to the former analytical techniques due to their fast, simple, selective, sensitive, and portable characteristics [7]. In this context, a wide range of sensors for the striping voltammetric determination of metal ions has been reported in the literature, including electrodes based on mercury, gold, carbon, bismuth, and antimony, among others [15,16]. However, over the past few decades, the manufacture of electrodes by screen-printing technology, which permits the mass fabrication of numerous highly reproducible, disposable, and low-cost screen-printed electrodes (SPEs), has attracted the attention of researchers as it allows to overcome the major drawbacks of solid electrodes. Moreover, the modification of the working electrode of SPEs with nanomaterials is trending because it can provide outstanding properties, such as high surface area, high electron transfer, adsorption capacity, and biocompatibility, thus improving the sensor’s analytical performance [17,18]. In particular, it should be noted that electrodes based on carbon nanomaterials are being used nowadays not only for the determination of metal ions by themselves but also as a support to allow modifications that enhance their analytical performance [19,20,21,22,23].

In recent years, two-dimensional (2D) layered pnictogen materials, namely, phosphorene, arsenene, antimonene, and bismuthene, have arisen due to their exceptional physiochemical properties and functional structures. Liquid phase exfoliation has been extensively used to produce suspensions of many 2D materials mainly because it is the most suitable method to obtain few-layer sheets and it is suitable for industrial scale-up [24]. The amount of energy necessary to overcome the van der Waals forces between the layers of the bulk crystal is usually supplied in the form of ultrasonic waves or shear force. In fact, although some challenges occur for 2D materials that exhibit strong interlayer interactions with a greater covalent character, such as heavier pnictogens (rhombohedral Sb or Bi), there have still been reports on exfoliated pnictogen phases using these methods [25,26,27].

However, despite the remarkable characteristics of these emerging 2D-layered materials that make them suitable to be considered for the design of electrochemical sensors, the number of applications involving 2D-layered pnictogens for sensing is still scarce [26,27,28,29,30,31,32,33,34]. Nevertheless, the few reported applications show promising results. In recent work, bismuthene-based electrodes [27] and antimonene-based electrodes [26,30] demonstrated higher analytical performance for the determination of metal ions than those provided by classic bismuth- and antimony-based electrodes. Thus, the coupling of carbon nanomaterials with 2D-layered pnictogen nanomaterials appears as an attractive and innovative option to further improve the analytical performance of electrochemical sensors for the determination of metal ions. To the best of our knowledge, only a first approach based on the use of a hybrid bismuthene/graphene-modified glassy carbon electrode has been published by Lazanas et al. for the stripping voltammetric determination of Pb(II) and Cd(II) ions [31].

In this work, all aforementioned benefits have been considered in the development of an antimonene-modified electrode via drop-casting on a screen-printed carbon-based nanomaterial substrate leading to a 2D Sb-modified screen-printed carbon nanofibers electrode (2D Sb_exf-SPCNFE). On the one hand, antimonene is a 2D material composed of a single layer of antimony atoms in a rhombohedral structure that has received increasing attention in recent years due to its unique electronic and optical properties, making it a promising material for various applications, including energy storage, electronics, and sensing [34,35,36]. Among all 2D layered pnictogens, antimonene was selected because it has a direct band gap (i.e., it is more conductive and suitable for electronic applications) and exhibits higher stability in a normal atmosphere, which makes it an appealing candidate for electrochemical sensing applications. Furthermore, antimonene has a high surface area compared to its bulk counterpart, gray antimony, which enhances its electrochemical performance [37]. On the other hand, an SPCNFE was chosen as an electrode platform as it was shown that for the development of a classic antimony film electrode, this support provided the best results in terms of sensitivity, repeatability, reproducibility, and detection limits for the determination of metal ions compared to other electrode platforms such as carbon screen-printed electrodes (SPCE), multi-walled carbon nanotubes modified screen-printed electrodes (SPCNTE) and graphene-modified screen-printed electrodes (SPGPE) [23]. The developed 2D Sb_exf-SPCNFE will be compared to a conventional screen-printed carbon-electrode-modified with 2D Sb_exf (2D Sb_exf-SPCE) and their respective bare electrodes (SPCNFE and SPCE) in terms of microscopic characterization and analytical performance in the simultaneous determination of Cd(II) and Pb(II) as a model metal ion system. Taking advantage of the superior analytical performance of 2D Sb_exf-SPCNFE, its applicability has been assessed through the simultaneous determination of Pb(II) and Cd(II) ions in a certified reference estuarine water sample.

2. Materials and Methods

2.1. Liquid-Phase Exfoliation of Gray Antimony

The liquid-phase exfoliation method of bulk gray rhombohedral antimony was adapted from our previously reported method [26] and is schematized in Scheme 1. For a fully detailed explanation of material exfoliation and characterization of the materials and sensors, the reader is referred to the Supplementary Material.

2.2. Preparation of 2D Sb Modified Screen-Printed Electrodes

The accurate weight of the exfoliated layered antimony (antimonene) was suspended in deoxygenated water to obtain modifier suspensions at different concentrations: 0.75, 1.12, 2.50, and 5.00 mg mL⁻¹. The prepared suspensions were sonicated for 30 min while keeping the temperature lower than 20 °C and then stored in the freezer until further use. The obtained suspensions could be stored for over one month without signs of degradation.

Unless otherwise indicated, 2D Sb-modified screen-printed carbon electrode (2D Sb_exf-SPCE) and 2D Sb-modified screen-printed carbon nanofibers electrode (2D Sb_exf-SPCNFE) were prepared as shown in Scheme 2, by drop-casting 5 µL of exfoliated antimony suspension (1.12 mg mL⁻¹) on the working electrode surface (4 mm diameter) of a carbon screen-printed electrode (SPCE) and a carbon nanofibers screen-printed electrode (SPCNFE) and drying them in the oven for 30 min at 25 °C.

2.3. Voltammetric Measurements

All electrochemical measurements were carried out in a glass vessel at room temperature and without deaeration with a three-electrode configuration: Ag|AgCl|KCl (3 mol L⁻¹) as reference electrode, a platinum wire as a counter electrode, and a bare SPCE/SPCNFE or a modified SPCE/SPCNFE as working electrode.

DPASV measurements were conducted in 0.01 mol L⁻¹ HCl (pH = 2) applying the following voltammetric parameters: deposition potential (E_d) of −1.4 V, deposition time (t_d) of 240 s with stirring, rest time (t_r) of 5 s, step potential of 8 mV, modulation time of 50 ms, pulse amplitude of 50 mV, and potential sweep ranging from −1.4 to −0.5 V. A conditioning/cleaning step was set before each DPASV measurement by applying −0.5 V for 30 s. Repeated scans of the blank solution were performed before starting the measurements until a constant baseline signal was achieved. A new bare or modified sensor unit was used for each set of measurements.

Calibration curves for the simultaneous DPASV determination of Pb(II) and Cd(II) were obtained by measuring increasing concentrations of the considered metal ions from 0.15 to about 130 µg L⁻¹ using the following electrodes: SPCE_bare, SPCNFE_bare, 2D Sb_exf-SPCE, and 2D Sb_exf-SPCNFE at the above-described experimental conditions.

Certified estuarine water sample analysis was performed in triplicate following the standard addition method. For this purpose, a volume of the estuarine water sample was placed in the vessel with 0.01 mol L⁻¹ HCl (pH = 2) (dilution factor 6/26), and the voltammogram was recorded at the above-established conditions. Then, three aliquots of Cd(II) and Pb(II) standard solutions were successively added, and the subsequent voltammograms were recorded.

3. Results and Discussion

3.1. Characterization

Scanning electron microscopy (SEM) was used to confirm the effective modification of the SPCE with 2D Sb and fibrillar carbon nanofibers (CNF). The as-received SPCE_bare (Figure 1a) showed a surface predominantly composed of carbon (C), consistent with its role as a working electrode in which C is the active element. SPCNFE_bare (Figure 1b) showed a surface that was better covered by C due to the presence of CNF, whereas in 2D Sb_exf-SPCE (Figure 1c), the distribution of bulk 2D Sb was visible. The SEM image of 2D Sb_exf-SPCNFE (Figure 1d) showed a good distribution of Sb within CNF, preserving the morphology of the starting materials. Elemental mapping of the materials was further evaluated using energy-dispersive X-ray spectroscopy (EDX). As expected, the EDX results (Figure S1) showed a higher carbon coverage of the electrode surface in SPCNFE_bare (Figure S1b) than in SPCE_bare (Figure S1a). Moreover, Sb distribution onto 2D Sb_exf-SPCE can be observed in Figure S1c and, with respect to 2D Sb_exf-SPCNFE (Figure S1d), both Sb and C were well-distributed and present in the sample, indicating that the 2D Sb_exf-SPCNFE sample had a well-balanced elemental composition.

The Raman spectroscopy characterization of the studied samples (Figure 2a) presented differences in their vibrations and structural characteristics. The SPCNFE_bare sample showed the typical D- and G-bands of carbon structures, and the 2D Sb_exf-SPCE sample exhibited the phonon frequencies of the E_g and A_1g modes related to 2D Sb at 112.2 and 150.0 cm⁻¹ (Figure S2), respectively [9]. These frequencies were in agreement with those observed for the starting gray-Sb (Figure S2) and were also observed at 109.6 and 150.0 cm⁻¹ in the Raman spectrum of 2D Sb_exf-SPCNFE, which also presented D- and G-bands at 1340.7 and 1574.2 cm⁻¹, respectively. For easier inspection, a more detailed spectrum of 2D Sb_exf-SPCNFE is shown in Figure 2b. Overall, these results show that there is a well-integrated combination of Sb and CNF in the 2D Sb_exf-SPCNFE sample.

TEM characterization was performed to confirm that the bulk Sb was effectively converted into 2D Sb_exf through the liquid exfoliation process (Figure S3). The TEM micrograph in Figure S3a shows a representative view of the few-layered 2D Sb_exf with well-defined regular lattice fringes (Figure S3b) with a lattice spacing of 0.313 and 0.315 nm. The selected area electron diffraction (SAED) pattern (Figure S3c) indicates that the Sb_exf is crystalline with a rhombohedral lattice. The STEM micrograph in Figure S3d shows the elemental mapping, which confirms a homogeneous distribution of Sb on the material surface.

3.2. Differential Pulse Anodic Stripping Voltammetry (DPASV) of Cd (II) and Pb (II)

The DPASV response corresponding to the determination of Cd(II) and Pb(II) using a 2D Sb_exf-SPCNFE was assessed and then compared to those provided by 2D Sb_exf-SPCE and their respective bare electrodes (SPCE and SPCNFE).

First, the experimental conditions for the DPASV determination of Cd(II) and Pb(II) were optimized using 2D Sb_exf-SPCNFE. 0.01 mol L⁻¹ hydrochloric acid (pH = 2) and acetate buffer 0.1 mol L⁻¹ (pH = 4.5) were evaluated for the determination of Pb(II) and Cd(II) using 2D Sb_exf-SPCNFE. Hydrochloric acid 0.01 mol L⁻¹ (pH = 2) was selected as the optimal supporting electrolyte since it provided better-defined peaks for Cd(II) and Pb(II) compared to those attained using acetate buffer (pH = 4.5), especially at lower concentrations.

Deposition potential (E_d) and deposition time (t_d) were evaluated considering ranges from −0.9 V to −1.4 V and from 30 s to 240 s, respectively, in a solution containing 50 µg L⁻¹ Cd(II) and Pb(II) in hydrochloric acid 0.01 mol L⁻¹ (pH = 2) and using a 2D Sb_exf-SPCNFE prepared from a 2D Sb_exf suspension of 1.12 mg mL⁻¹. The selected compromise conditions were an E_d of −1.4 V, applied with stirring during a t_d of 240 s.

Once the voltammetric conditions were established, the concentration of the exfoliated layered antimony (antimonene, 2D Sb_exf) suspension employed in the drop-casting procedure was optimized. In the case of 2D Sb_exf-SPCE, it was reported in a previous work that the best voltammetric response was achieved by depositing a 5 µL drop of 1.12 mg mL⁻¹ 2D Sb_exf suspension on the SPCE surface [26] and therefore, this concentration was considered optimal. Nevertheless, this value cannot directly be assumed as optimal for 2D Sb_exf-SPCNFE because, as seen in the previous section, the morphological characteristics of the substrate are different and could influence the drop-casting of the 2D Sb_exf suspension. Therefore, different SPCNFE units were modified with 2D Sb_exf suspensions of four different concentrations (0.75, 1.12, 2.50, and 5.00 mg mL⁻¹) following the modification protocol established in Section 2.2 and simultaneous voltammetric measurements of a solution containing 20 µg L⁻¹ of Cd(II) and Pb(II) were performed with each 2D Sb_exf based- SPCNFE. As shown in Figure 3, the highest and best-defined Cd(II) and Pb(II) voltammetric peaks were obtained by modifying the SPCNFE with 1.12 mg mL⁻¹ 2D Sb_exf suspension. Therefore, for both 2D Sb_exf-SPCE and 2D Sb_exf-SPCNFE, the 2D Sb_exf suspension of 1.12 mg mL⁻¹ was considered for further modification procedures.

Once the concentration of the modifier suspension was optimized, the electroanalytical performance of 2D Sb_exf-SPCNFE was studied in terms of sensitivity, linear range, the limit of detection (LOD), and the limit of quantification (LOQ). For this purpose, calibration curves were carried out for the simultaneous determination of Cd(II) and Pb(II) ions in 0.01 mol L⁻¹ HCl (pH = 2). DP stripping voltammograms were recorded with 2D Sb_exf-SPCNFE at the above-stated experimental conditions using sequential additions of metal ions within a range of concentrations between 0.05 and 130 µg L⁻¹, as shown in Figure 4. Well-shaped voltammetric peaks appeared at ca. −0.95 and −0.70 V for different concentrations of Cd(II) and Pb(II) ions, respectively, as shown in Figure 4A. The analytical parameters calculated are summarized in

Table 1. Comparison of the analytical performance of SPCE_bare, SPCNFE_bare, 2D Sb_exf-SPCE, and 2D Sb_exf-SPCNFE for the simultaneous determination of Pb(II) and Cd(II) by DPASV at pH 2, E_d of −1.4 V and t_d of 240 s. The standard deviations are shown within brackets.

	SPCEbare		SPCNFEbare		2D Sbexf-SPCE		2D Sbexf-SPCNFE
	Cd(II)	Pb(II)	Cd(II)	Pb(II)	Cd(II)	Pb(II)	Cd(II)	Pb(II)
Sensitivity (nA V µg−1 L)	2.9 (0.1)	2.6 (0.1)	4.69 (0.09)	5.6 (0.1)	4.09 (0.06)	4.50 (0.01)	6.19 (0.05)	6.597 (0.008)
Intercept (µg L−1)	−62 (6)	−1 (1)	−59 (5)	−36 (5)	−12 (4)	−0.5 (0.5)	−28 (2)	−0.1 (0.2)
1st Linear range (µg L−1) a R2	20.3–104.5 0.987	6.9–29.6 0.980	10.8–108.7 0.994	9.8–105.1 0.993	9.1–132.7 0.994	1.1–128.3 0.999	2.9–85.0 0.999	0.3–82.0 0.999
2nd Linear range (µg L−1) a R2	-	29.6–101.1 0.988	-	-	-	-	-	-
LOD (µg L−1)	6.1	2.1	3.2	2.9	2.7	0.3	0.9	0.1

^a The lowest value of the linear range was calculated from the LOQ.

. The results revealed that there was a good linear relationship up to a concentration of ca. 85 µg L⁻¹ with a regression coefficient (R²) of 0.999 for both metal ions (Figure 4B). The sensitivity established from the slope of the calibration plots was 6.19 (0.05) and 6.597 (0.008) nA V µg⁻¹ L for Cd(II) and Pb(II) ions, respectively. LODs were calculated as 0.9 and 0.1 µg L⁻¹, and LOQs were calculated as 2.9 and 0.3 for Cd(II) and Pb(II), respectively, using the equation ‘3 σ/s’ for LOD and ‘10 σ/s’ for the LOQ (where ‘σ’ is the standard deviation of the intercept and ‘s’ is the slope of the calibration plot). This finding indicates that there is a high sensitivity of the novel 2D Sb_exf-SPCNFE regarding the determination of Cd(II) and Pb(II) ions.

The analytical performance obtained for 2D Sb_exf-SPCNFE was compared with those achieved by Sb_exf-SPCE and their corresponding bare electrodes (i.e., SPCE and SPCNFE). Figure 5 compares the DPASV responses for 55 µg L⁻¹ Pb(II) and Cd(II) in HCl 0.01 mol L⁻¹ (pH = 2) recorded using SPCE_bare, SPCNFE_bare, 2D Sb_exf-SPCE, and 2D Sb_exf-SPCNFE. As expected, the bare electrodes provided the lowest voltammetric peaks, being the Cd(II) and Pb(II) peaks attained by SPCNFE_bare, which were higher than those obtained by SPCE_bare, particularly in the case of Cd(II). This is due to the increased effective surface area and improved electron-transfer kinetics provided by the presence of CNF. By comparing the DPASV voltammograms obtained by SPCNFE_bare and 2D Sb_exf-SPCE, it can be concluded that a slightly larger response was accomplished by the modification with 2D Sb_exf instead of using CNF as a modifier. However, the highest and most well-defined Cd(II) and Pb(II) peaks were achieved by Sb_exf-SPCNFE, in which the excellent properties of both nanomaterials were merged. These findings are supported by the analytical parameters summarized in Table 1. As can be seen in Table 1, the sensitivity provided by 2D Sb_exf-SPCNFE was between 1.2 and 2.5 times higher, depending on the considered metal ion, than those provided by SPCE_bare, SPCNFE_bare, and 2D Sb_exf-SPCE, while still maintaining a reasonably linear range. However, it should be noted that CNF-based sensors yield higher sensitivity than those based on carbon. Regarding the LODs, the lowest LODs were also achieved using 2D Sb_exf-SPCNFE, whose values were between 3 and 29 times lower, depending on the considered metal ion, than those attained by the other assessed sensors. On the view of the obtained results, it can be concluded that the excellent analytical performance exhibited by 2D Sb_exf-SPCNFE can be attributed to a synergistic effect between both nanomaterials: the high electrical conductivity and large surface area provided by CNF together with the high surface volume ratio and the high charge carrier mobility added by 2D Sb_exf.

The repeatability of 2D Sb_exf-SPCNFE was evaluated under optimized conditions using 25 µg L⁻¹ of Cd(II) and Pb(II). Five repetitive measurements were made using the same 2D Sb_exf-SPCNFE unit. The calculated relative standard deviations (RSD) were 1.7% and 1.3% for Cd(II) and Pb(II), respectively. Moreover, the reproducibility study of the 2D Sb_exf-SPCNFE involved the preparation of three 2D Sb_exf-SPCNFE units, which were then applied to the determination of 25 µg L⁻¹ of Cd(II) and Pb(II). The RSDs of 2D Sb_exf-SPCNFE were 6.9% and 2.7% for Cd(II) and Pb(II), respectively. These results indicated that the developed 2D Sb_exf-SPCNFE showed great repeatability and reproducibility, similar to those achieved by 2D Sb_exf-SPCE [26].

Regarding the previous findings, it should be noted that, to the best of our knowledge, apart from the above-discussed approach based on the use of 2D Sb_exf-SPCE [26], there are no more works dealing with the use of antimonene or 2D Sb_exf for the voltammetric determination of metal ions. Compared to other developed electrodes based on the use of antimony and carbon nanomaterials for the simultaneous determination of Cd(II) and Pb(II) (Table S1), the developed 2D Sb_exf-SPCNFE provides, in general terms, better analytical parameters [15,22,23,38,39]. For example, the LODs obtained by an antimony nanoparticle-multiwalled carbon nanotubes composite immobilized on a carbon paste electrode were 0.77 µg L⁻¹ for Cd(II) and 0.65 µg L⁻¹ for Pb(II) [22]. Much higher LODs were also achieved, for example, by an antimony film electrode prepared from CNF modified screen-printed electrode (2.1 and 1.1 µg L⁻¹ for Cd(II) and Pb(II), respectively) [23], by a graphene oxide-modified GC electrode coated with in situ antimony film (6.07 and 5.39 µg L⁻¹ for Cd(II) and Pb(II), respectively) [38], and by a multiwall carbon nanotube modified by antimony oxide–carbon paste electrode (16.77 and 6.12 µg L⁻¹ for Cd(II) and Pb(II), respectively) [39]. Compared to the LODs obtained by the hybrid bismuthene/graphene-modified GC electrode (0.3 µg L⁻¹ for both Cd(II) and Pb(II)) [31], 2D Sb_exf-SPCE provided a better LOD for Pb(II) and a somewhat higher LOD for Cd(II). Moreover, it should be underlined that the developed 2D Sb_exf-SPCNFE has advantages, such as the relatively higher stability in the normal atmosphere of 2D Sb_exf, the easy and very fast modification procedure based on the drop-casting approach, and the use of low-cost, disposable and reproducible platforms. On the other hand, the durability of the 2D Sb_exf immobilization on every screen-printed platform for a large set of measurements (more than 20) without loss of sensitivity enables the voltammetric determination of metal ions with the same 2D Sb_exf-SPCNFE unit.

3.3. Analysis of an Estuarine Water Sample Using a 2D Sb_exf-SPCNFE

A certified estuarine water reference material (LGC6016) was chosen to evaluate the feasibility of a 2D Sb_exf-SPCNFE sensor for the simultaneous determination of Cd(II) and Pb(II) in a natural water sample. The determination of the studied metal ions was performed by the standard addition method. Then, DPASV measurements, including the analyzed sample and three additions of Cd(II) and Pb(II), were conducted in triplicate at the above-stated conditions. Figure 6A shows representative voltammograms with very well-defined peaks for both metal ions attained in the analysis of the estuarine water samples using 2D Sb_exf-SPCNFE. Figure 6B illustrates the calibration plot for Cd(II) and Pb(II) with a notorious correlation between peak areas and the added concentrations for both considered metal ions.

Table 2. Simultaneous determination of Pb(II) and Cd(II) by DPASV in a certified estuarine water sample (LGC6016) on 2D Sb_exf-SPCNFE at pH 2, E_d of −1.4 V, and t_d 240 s.

	Cd(II)			Pb(II)
	c (µg L−1)	RSDa (%)	Relative Error (%)	c (µg L−1)	RSDa (%)	Relative Error (%)
2D Sbexf-SPCNFE	99.1	3.18	1.88	196.3	2.52	0.17
Certified metal value	101.0	2.00	-	196.0	1.50	-

^an = 3 was considered for RSD (%) calculation.

reports the Cd(II) and Pb(II) concentration data obtained from the DP stripping voltammetric analysis of three replicates of the estuarine water sample performed using the 2D Sb_exf-SPCNFE. An outstanding agreement was attained between all replicates as well as with the Cd(II)- and Pb(II)-certified values provided in the estuarine water reference material sample. Furthermore, it should be highlighted that the certified estuarine water sample contains more constituents, such as Cu, Mn, Ni, Ca, Mg, K, Na, and Zn, which did not disturb the determination of Cd(II) and Pb(II) in the sample.

Considering the excellent reproducibility and trueness achieved in the simultaneous determination of Cd(II) and Pb(II) ions in the estuarine water sample, we can state that the developed sensor based on the combination of two nanomaterials (CNF and 2D Sb_exf) is fully suitable for voltammetric stripping measurements of metal ions at low trace levels in real water sample levels, even in the presence of other elements at similar or higher concentrations than Pb(II) and Cd(II).

4. Conclusions

A 2D Sb_exf-SPCNFE was developed for the simultaneous DPASV determination of Pb(II) and Cd(II) as a model metal ion system and morphologically characterized by SEM, EDX, and Raman spectroscopy. The developed 2D Sb_exf-SPCNFE takes advantage of the merging of antimonene (2D Sb_exf) and CNF within the same platform. 2D Sb_exf-SPCNFE showed well-defined and separated stripping peaks for Cd(II) and Pb(II) with great repeatability and reproducibility. The analytical performance provided by 2D Sb_exf-SPCNFE was much better than those achieved by 2D Sb_exf-SPCE and the respective bare electrodes (SPCNFE and SPCE) in terms of linear range, LODs, LOQs, and sensitivities. Compared to other reported sensors based on the use of antimony and carbon nanomaterials, the analytical parameters exhibited by 2D Sb_exf-SPCNFE were also generally superior. The excellent analytical performance demonstrated by 2D Sb_exf-SPCNFE can be associated with a synergistic effect between the excellent properties attributed to both merged nanomaterials, i.e., the high surface-volume ratio and the high charge carrier mobility provided by 2D Sb_exf, and the high electrical conductivity and the large surface area added using CNF.

The applicability of the developed sensor was evaluated by detecting trace levels of Pb(II) and Cd(II) in a certified reference estuarine water sample, obtaining good reproducibility and trueness without the interference of constituents at similar or higher concentrations than Pb(II) and Cd(II).

Thus, the remarkable analytical performance displayed by 2D Sb_exf-SPCNFE coupled with the simple modification procedure involved and the use of low-cost, disposable, and reproducible platforms suggests that 2D Sb_exf-SPCNFE is a great alternative to other reported sensors for the voltammetric determination of metal ions in environmental samples.