Preparation and Application of Bismuth/MXene Nano-Composite as Electrochemical Sensor for Heavy Metal Ions Detection

Abstract

A nano-form composite of MXenes (Ti₃C₂T_x, T_x = -O, -OH, -F) was synthesized through depositing bismuth-nanoparticle (BiNPs) onto Ti₃C₂T_x sheets. Because of the preventive effect of the two-dimensional layered structure of Ti₃C₂T_x, the nanoparticles of Bi were uniform and well attached on the Ti₃C₂T_x. The obtained BiNPs/Ti₃C₂T_x nano-composite was applied for sensors construction of electrochemical detecting of Pb²⁺ and Cd²⁺ heavy metal ions. The produced BiNPs@Ti₃C₂T_x-based sensor showed high effective surface area and excellent conductivity. Also, the BiNPs were efficient for anodic-stripping voltammetric to detect heavy metal ions. After conditions optimization, the BiNPs@Ti₃C₂T_x nano-sensor could detect Pb²⁺ and Cd²⁺ simultaneously and the detection limits were 10.8 nM for Pb²⁺ and 12.4 nM for Cd²⁺. The BiNPs@Ti₃C₂T_x was promising for detecting heavy metal ions due to their high surface area, fast electron-transfer ability, environmental friendliness, and facial preparation.

1. Introduction

Heavy metal ions and non-degradable ions, such as lead (Pb), cadmium (Cd), mercury (Hg), and chromium (Cr), do not decompose for decades or even centuries. This can produce various diseases for human health and other living environments [1]. Also, these ions in the environment is very dangerous and we need to remove it before accumulation. The development of sensitive approaches for detecting and determining toxic heavy metal ions is an urgent need [2,3,4]. Several analytical methods including Inductively Coupled Plasma-Mass Spectrometry (ICP-MS), Atomic Absorption Spectroscopy (AAS), X-ray Fluorescence Spectrometer (XRF), and Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) are used for testing of these ions, but they do not have widespread applications due to their high cost and they require trained people to have contact with the equipment [5,6]. Thus, developing simple and cheap methods for sensitive detection of heavy metal ions is required. Electrochemical technique plays a critical role in detecting heavy metal ions due to its low cost, high sensitivity, rapid performance, and portability [7,8,9].

Recently, bismuth-based electrodes have become an alternative approach to mercury-based ones with some advantages like their environmental friendliness and excellent resolution of neighboring peaks. Because of having a highly specific surface and plentiful active sites, bismuth nanoparticles (BiNPs) have higher electro-analytical and catalytic activity than the traditional Bi films [10,11]. Until now, different BiNPs have been used to construct bismuth-based electrodes for electrochemical measurements. After modifying BiNPs, the behavior of the electrodes will be improved due to the newly forming multicomponent alloys and the combination of the nanomaterials properties [12]. However, due to a high surface free energy, BiNPs easily tend to aggregate and result in reduced original availability and decreased performance. A credible strategy to address this problem is anchoring BiNPs on specific supports like carbon nanomaterials such as graphene, carbon nanotube, graphite nanofiber, and Metal–organic frameworks (MOFs)-derived porous carbons [13].

As a kind of novel two-dimensional materials, MXenes have gained growing attention in supercapacitors, batteries, transparent electrodes, sensors, and catalysts. It has many positive properties such as excellent electrical conductivities, good structure, large hydrophilic surface area, and chemical stability [14]. In our previous report, acetylcholinesterase biosensor modified with Ag@MXene nano-composite was prepared for malathion detection and the obtained sensor exhibited good sensitivity at low levels and acceptable stability [15]. However, as far as we know, there are no reports regarding anchoring BiNPs onto Ti₃C₂T_x MXene nano-sheets for constructing electrochemical sensors for detecting heavy metal ions. Thus, in this work, the BiNPs/Ti₃C₂T_x nano-composite was designed, synthesized and applied for glassy carbon electrode (GCE) development as an electrochemical sensor for assaying of heavy metal ions. Also, the Pb²⁺ and Cd²⁺ ions in different water kinds were determined by the obtained electrochemical sensor and the sensitivity, selectivity and repeatability properties were studied.

2. Experimental Section

2.1. Materials

Analytical grade Pb(NO₃)₂, Bi(NO₃)₃, CdCl₂, FeCl₃, MgCl₂, CoCl₂, ZnCl₂, MnCl₂, CuSO₄, Al(NO₃)₃, Ni(NO₃)₂, H₂SO₄, HNO₃, C₆H₅Na₃O₇, NaBH₄, CH₃COOH, chitosan(CS), absolute ethanol, and polyethylene glycol were purchased from Fengchuan Chemical Reagent Co. Ltd., Tianjin, China. Acetate buffer (0.1 M) solution with different pH levels was prepared from stock solutions of 0.1 M sodium acetate and acetic acid and then applied as a supporting electrolyte agent.

2.1.1. Synthesis of BiNPs@Ti₃C₂T_x Nano-Composite

Ti₃C₂T_x was prepared through selective etching of Al from Ti₃AlC₂ according to our previous report [16], and the detailed synthesis procedures were displayed on the supplementary materials. The BiNPs@Ti₃C₂T_x nano-composite was obtained by mixing Ti₃C₂T_x nano-sheets with Bi as follows: 2 mg of Ti₃C₂T_x was dispersed into 5 mL of ethylene glycol by ultra-sonication for 1 h, and then 5 mg of C₆H₅Na₃O₇ was dissolved into the dispersed solution; 5 mL of Bi(NO₃)₃·5H₂O (16.3 mg) was added into the mixture. After stirring for 12 h, 0.5 mL of 0.1 M NaBH₄ was added with shaking for 1 h. The solid products were precipitated using centrifugation, and then washed three times by distilled water and then absolute ethanol. The obtained nano-composite was dried in a vacuum oven at 50 °C for 24 h.

2.1.2. Preparation of BiNPs@Ti₃C₂T_x/GCE Sensor

For preparing sensors, GCE was polished with alumina slurries mechanically and then sequentially sonicated with absolute ethanol, 0.05 M HNO₃ and distilled water separately and sequentially. The refined GCE was activated by H₂SO₄ (0.5 M). The BiNPs@Ti₃C₂T_x nano-composite was added into 0.20% CS solution to obtain homogeneous suspension. A suspension aliquot was drop-cast on the refined GCE and dried in air. The obtained BiNPs@Ti₃C₂T_x/GCE sensor was stored until used.

2.2. Electrochemical Measurements

Electrochemical performance of the produced nano-sensor was evaluated by a CHI660E electrochemical workstation (Shanghai Chenhua, China). The electrochemical measurements were conducted on a three electrode system; BiNPs@Ti₃C₂T_x/GCE (prepared electrode), platinum wire (counter electrode) and saturated calomel electrode (SCE, reference electrode). The tested electrochemical techniques were cyclic voltammograms (CV), electrochemical impedance spectra (EIS) and chronocoulometry (CC). CV was carried out at a scan rate of 50 mV s⁻¹, and EIS was performed with afrequency that changed from 10⁻² to 10⁵ Hz with a signal amplitude of 5 mV. The electrochemical properties of the produced sensor were determined according to previous studies [11].

The active area of BiNPs@Ti₃C₂T_x nano-composite was calculated using the following equation:

$$Q\left(t\right)=\frac{2nFAc{D}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}{t}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}{{\pi }^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}+Q_{dl}+Q_{ads}$$

where Q is the absolute value of the reduction charge; n is the number of electrons transfer; F is the Faraday constant (96,485 C/mol); A is the effective area; c is the substrate concentration; D is the diffusion coefficient (7.6 × 10⁻⁶ cm²/s); t is the time; Q_dl is the double-layer charge; and Q_ads is the Faradaic charge consumed by adsorbed species.

2.3. Characterization

The morphology of Ti₃C₂T_x nano-sheets and BiNPs@Ti₃C₂T_x nano-composite was characterized using Scanning Electron Microscope (SEM) (NanoSEM45011, FEI, Portland, OR, USA) and Transmission Electron Microscope (TEM) (JEM-2100, JEOL Inc., Tokyo, Japan). Elemental compositions of the samples were analyzed by Energy Dispersive X-Ray Spectroscopy (EDX) spectroscopy (EDAX Inc., Mahwah, NJ, USA). The crystallinity of Ti₃C₂T_x nano-sheets and BiNPs@Ti₃C₂T_x nano-composites was determined by XRD diffractometer (D8 Advance, Bruker, karlsruhe, Germany). The chemical state of the samples was demonstrated using XPS (Da Vinci, Bruker, karlsruhe, Germany).

2.4. Application of BiNPs@Ti₃C₂T_x-Based Sensorfor Heavy Metal Ions Detection

To investigate the applicability of the prepared sensor, two kinds of water samples (tap and lake water) were used. Standard solutions of Pb²⁺ and Cd²⁺ were added. BiNPs@Ti₃C₂T_x-based sensor determined Pb²⁺ and Cd²⁺ ions through the prestigious square wave anodic stripping voltammetry (SWASV) technique [2]. SWASV was applied at a deposition potential of −1.0 V for 300 s in 0.1 M NaAc/HAc buffer (pH 5.0), and performed in the potential range of −0.95 to −0.35 V with a frequency of 15 Hz, an amplitude of 25 mV, and an increment potential of 4 mV. For comparison, ICP-MS standard method was used to confirm the concentrations of heavy metal ions found in tap and lake waters.

3. Results and Discussion

3.1. Preparation and Characterization of BiNPs@Ti₃C₂T_x

Heavy metal ions are environmental dangerous ions due to their non-degradability properties, causing harmful diseases for human and not decomposability. For that, researchers need to detect these metals by low levels in the environment; the suitable technology for that is sensors. In the current work, a nano-composite-based sensor was designed and produced. Ti₃C₂T_x nano-sheets were prepared using selective etching method [16] and then characterized by SEM, TEM and EDX instruments. As shown in Figure 1A, the prepared Ti₃C₂T_x nano-sheets displayed typical two-dimensional layered structures. It appears as homogeneous, continuous, obtains a large area, and facilitates the mass transfer [17]. When Bi(NO₃)₃ was added to the reaction, Bi³⁺ ions could be adsorbed and attached on Ti₃C₂T_x nano-sheets through electrostatic interaction [18] and then the Ti₃C₂T_x nano-sheets served the nucleation sites of BiNPs. Because of the reduction action by NaBH₄, BiNPs was grown on the surface of Ti₃C₂T_x nano-sheets [13]. Figure 1B indicated that ununiform Bi nanoparticles were dispersed on the surface or interlayers of Ti₃C₂T_x nano-sheets. To further investigate the microstructure of BiNPs@Ti₃C₂T_x, TEM examination was conducted and the resulted picture indicated that the BiNPs (dark dots) were distributed on the Ti₃C₂T_x nano-sheets (Figure 1C) and un-agglomerated nanoparticles could be observed. In the case of elemental analyses, EDX mapping revealed that the BiNPs@Ti₃C₂T_x nano-composite involved Bi and confirmed the attaching of BiNPs onto Ti₃C₂T_x nano-sheets (Figure 1D). In addition, the layered morphology of Ti₃C₂T_x displayed a few changes after the formation of Bi nanoparticles. This may be due to the attachment of Bi on the surface of Ti₃C₂T_x nano-sheets and making some changes in the sheets surface [19]. The elemental analysis confirmed that the elements Ti, Al, Si, C, F, O, and Bi were detected by deferent percentages.

For the crystal structure of Ti₃C₂T_x and BiNPs@Ti₃C₂T_x, XRD pattern of pristine Ti₃C₂T_x nano-sheets as presented in Figure 2A(b) showed three peaks of projecting diffraction corresponding to (002), (004) and (110) intensity, respectively. Meanwhile, the XRD pattern of BiNPs@Ti₃C₂T_x (Figure 2A(a)) showed obvious peaks at 27.2, 38.1 and 39.7°, which were assigned to (012), (104), and (110) planes of the rhombohedra crystal structure of Bi. The successful deposition of BiNPs on Ti₃C₂T_x is demonstrated. For XPS spectrum (Figure 2B), it is shown that both Ti₃C₂T_x nano-sheets and BiNPs@Ti₃C₂T_x were composed of Ti, C, O, and F. It is in agreement with previous reports [20]. Moreover, the XPS spectra of BiNPs@Ti₃C₂T_x showed the presence of Bi, suggesting that BiNPs@Ti₃C₂T_x had been synthesized successfully. As shown in Figure 2C, the peaks of Bi 4f (4f_7/2 and 4f_5/2) were qualified to Bi (0), which indicated that nearly all bismuth in the BiNPs@Ti₃C₂T_x was Bi (0). In brief, all these results confirmed the successful preparation of BiNPs@Ti₃C₂T_x.

3.2. Electrochemical Characterization of the BiNPs@Ti₃C₂T_x/GCE

The basic electrochemical techniques like cyclic voltammograms (CV), electrochemical impedance spectra (EIS) and chronocoulometry (CC) were conducted to study the properties of BiNPs@Ti₃C₂T_x/GCE sensor. CV and EIS were realized in a 5 mM [Fe(CN)₆]^3−/4− with 0.1 M KCl. Three peak currents of increased oxidation are shown in Figure 3A, which reached 53.6 μA (GCE bare), 130.5 μA (Ti₃C₂T_x/GCE) and 162.9 μA (BiNPs@Ti₃C₂T_x/GCE). These results showed that BiNPs@Ti₃C₂T_x exhibited an excellent electrical conductivity and influenced a large area. In addition, EIS was used for studying the interfacial electron transfer resistance (R_et). The data in Figure 3B showed three R_et of 1500 U on GCE bare, 720 U on Ti₃C₂T_x/GCE and 450 U on BiNPs@Ti₃C₂T_x/GCE, respectively, were obtained. The reduced R_et was attributed to the distended high electrical conductivity and specific area, which can improve the transfer of electron and mass exchange of electro-active indicators on the surface [21]. BiNPs@Ti₃C₂T_x nano-composite’s effective area was measured by CC tests in 0.1 mM K₃Fe(CN)₆ solution, and the result is illustrated in Figure 3C.

As illustrated in Figure 3D, the effective area of BiNPs@Ti₃C₂T_x/GCE reached 0.04857 cm², which is bigger than that of GCE bare (0.01677 cm²) and Ti₃C₂T_x/GCE (0.02681 cm²). All the above results demonstrated that the excellent properties including excellent electrical conductivity, high structural stability and large active area of Ti₃C₂T_x and BiNPs were integrated into the BiNPs@Ti₃C₂T_x nano-composite. This is closely in agreement with the results obtained by L. Shi, et al. [13].

3.3. Optimization of Experimental Conditions

Benefitting from the high active area of BiNPs@Ti₃C₂T_x/GCE, highly active electrochemical sensors were built for the simultaneous detection of Pb²⁺ and Cd²⁺ with SWASV. As presented in Figure 4, different electrodes were used to detect Pb²⁺ and Cd²⁺. Comparing with the responses of GCE bare and Ti₃C₂T_x/GCE, the BiNPs@Ti₃C₂T_x/GCE composite showed well-defined two individual voltammetric peaks at −0.57 V (Pb²⁺) and −0.78 V (Cd²⁺) with higher signals, representative of better detection choosiness and exactness of BiNPs@Ti₃C₂T_x/GCE toward the Pb²⁺ and Cd²⁺. This phenomenon could be attributed to the integration and synergic effect of Ti₃C₂T_x and BiNPs, which was a benefit of the higher ion adsorption, resulting in a higher response of Pb²⁺ and Cd²⁺ detection [22].

The pH level is a crucial factor for heavy metal ions analysis, as well as the behavior of electrochemical sensor responding. The effect of pH levels (ranging from 3.0 to 6.0) on responses to the obtained electrochemical sensor was investigated in the present work. Figure 5A shows that increasing signals when pH increased from 3.0 to 5.0, and decreased sharply when pH changed from 5.0 to 6.0. This can be explained as follows: at lower pH values, the protonation on the BiNPs@Ti₃C₂T_x/GCE composite resulted in weak signals; when the pH was higher than 5.0, the hydrolysis occurred and then induced a decrease of signals [23]. In the case of the deposition potential, the effect of deposition potential on response of signals was investigated and the stripping peak currents exhibited increased value obviously at −1.0 V, while deposition potentials were more negative than −1.0 V (Figure 5B). The good hydrogen evolution was enhanced, leading to the failure of signals response [24]. Thus, −1.0 V was selected to be the optimum admission potential. Moreover, as shown in Figure 5C, the enlarging of signals response from 100 to 500 s was observed. The time of deposition (300 s) was chosen for the following experiments due to the fact that a higher response signal under shorter deposition time was expected in applications.

3.4. Analytical Performance of the Produced Electrochemical Sensor

As shown in Figure 6, the sensor’s sensitivity and linear range for Pb²⁺ and Cd²⁺ were evaluated in the current work. With the increase of ion concentrations, the response of signals was enhanced. Both Pb²⁺ and Cd²⁺ stripping peak currents were increased proportionally with the increasing concentrations of Pb²⁺ and Cd²⁺ ions. The calibration curves of Pb²⁺ and Cd²⁺ were linear from 0.06 to 0.6 μM and 0.08 to 0.8 μM, respectively. The linear equations were evaluated as $\frac{\mathrm{I}}{\mathsf{\mu }\mathrm{A}}=1.531+42.85\mathrm{C}/\mathsf{\mu }\mathrm{M}$ and $\frac{\mathrm{I}}{\mathsf{\mu }\mathrm{A}}=1.895+35.08\mathrm{C}/\mathsf{\mu }\mathrm{M}$ for Pb²⁺ and Cd²⁺, with correlation coefficients of 0.9995 and 0.9976, respectively. The detection limits were recorded to be 10.8 nm for Pb²⁺ and 12.4 nm for Cd²⁺, and it is much lower than the recommended concentrations in drinking water as noted by the World Health Organization [25].

The joint interfering between Cd²⁺ and Pb²⁺ on BiNPs@Ti₃C₂T_x/GCE was explored to evaluate the problem of mutual interference between heavy metal ions in immediate detection. The concentration of Cd²⁺ was fixed, while the concentration of Pb²⁺ was increased (Figure 7A). Also, the peak current of Pb²⁺ increased while the concentration of Cd²⁺ remained the same. However, the data in Figure 7B represented that when the concentration of Pb²⁺ is fixed, and increasing the concentration of Cd²⁺, the peak current of Pb²⁺ fluctuated narrowly while the peak current of Cd²⁺ increased. The calibration plots of the peak present against concentration exhibited good linearity with the ranges of 0.06 to 0.6 μM for Pb²⁺ and 0.08 to 0.6 μM for Cd²⁺. The linear equations are $\frac{\mathrm{I}}{\mathsf{\mu }\mathrm{A}}=-0.4056+\frac{39.64\mathrm{C}}{\mathsf{\mu }\mathrm{M}}$ and $\frac{\mathrm{I}}{\mathsf{\mu }\mathrm{A}}=-1.584+\frac{34.39\mathrm{C}}{\mathsf{\mu }\mathrm{M}}$ for Pb²⁺ and Cd²⁺, with correlation coefficients of 0.9951 and 0.9992, respectively. The detection slopes of both Pb²⁺ and Cd²⁺ were similar to data illustrated in Figure 6B (39.64 versus 42.85 for Pb²⁺ and 34.39 versus 35.08 for Cd²⁺), and thus, designated that no joint interference between Pb²⁺ and Cd²⁺ occurred at BiNPs@Ti₃C₂T_x/GCE.

3.5. Selectivity, Reusability and Stability of the Produced Sensor

Selectivity is the main factor in the detection of heavy metal ions with the SWASV technique. Samples of Mg²⁺, Fe³⁺, Co²⁺, Ni²⁺, Mn²⁺, Zn²⁺, Al³⁺, and Cu²⁺ ions with a concentration of 10 μM were added to explore the ions interference in an immediate detection of Pb²⁺ and Cd²⁺. The result in Figure 8 shows little changes in response signals except for Cu²⁺. However, in the state of adding Cu²⁺, the signals were significantly reduced because of the formation of intermetallic compounds. This phenomenon could be avoided through adding ferrocyanide to exclude the interference of Cu²⁺.

In order to ensure the reusability of the prepared sensor, it was determined at a concentration of 0.2 μM Pb²⁺ and 0.4 μM Cd²⁺, and the acceptable relative standard deviations (RSDs) were achieved with 3.7% Pb²⁺ and 2.7% Cd²⁺. Fortunately, after ensuring the reproducibility of the obtained sensor, similar responses were obtained by each electrode and the RSDs of 4.0% Pb²⁺ and 4.6% Cd²⁺. These results indicated that the obtained sensor has excellent reproducibility characteristics. Furthermore, the stability of sensor was examined for storing it in the refrigerator in dry state (4 °C). After 6 weeks of storing, 95.2% and 94.5% of the initial response sensitivity can be retained for Pb²⁺ and Cd²⁺, respectively. The high stability character was ascribed to the structure stability nature and binding strength of BiNPs@Ti₃C₂T_x nano-composite on GCE [26].

3.6. Application of BiNPs@Ti₃C₂T_x Sensor forHeavy Metal Ions Detection in Tap and Lake Water

Heavy metal ions such as Pb²⁺ and Cd²⁺ are considered more dangerous for humans and other living organisms, because it causes many diseases. For this reason, it should be removed from all environments. In this work, BiNPs@Ti₃C₂T_x sensor was produced for heavy metal ions detection at the lowest levels. Two heavy metal ions, Pb²⁺ and Cd²⁺, were tested for detection in two deferent water types (tap and lake water). The concentrations of Cd²⁺ and Pb²⁺ were 150 nM. As illustrated in

Table 1. Determination of Pb²⁺ and Cd²⁺ in water samples with the BiNPs@Ti₃C₂T_x-based electrode.

Samples		Added (nM)	ICP-MS Result (nM)	Result (nM)	Recovery (%)
Tap water	Pb2+	150	149.6	151.8 ± 0.85	101.2
	Cd2+	150	150.8	147.4 ± 1.12	98.3
Lake water	Pb2+	150	151.6	159.4 ± 0.57	106.3
	Cd2+	150	151.2	152.2 ± 0.84	101.5

, the recoveries of the detected heavy metalions were reached at 98.3% and 101.2% for Cd²⁺ and Pb²⁺, respectively in tap water, while it was 101.5% and 106.3% for Cd²⁺ and Pb²⁺, respectively, in lake water. In addition, the results of ICP-MS and the electrochemical analysis are in reasonably good agreement. These results confirmed the reliability and efficiency of the produced sensors for detection of Pb²⁺ and Cd²⁺ ions.

In another case, the analytical performance of the obtained electrochemical sensor for Pb²⁺ and Cd²⁺ detection is presented in

Table 2. List of various electrochemical sensors for Pb²⁺ and Cd²⁺ detection.

Electrochemical Platform	Technique	Analyte (HMI)	LOD (ppb)
Bi-C nano-composite [27]	SWASV	Pb2+	0.65
		Cd2+	0.81
RGO/Bi nano-composite [28]	ASV	Pb2+	0.55
		Cd2+	2.80
BiNPs [29]	SWASV	Pb2+	2.00
		Cd2+	5.00
Nano-Bi [30]	SWASV	Pb2+	1.97
		Cd2+	2.54
NPCGS/Bi nano-composite [11]	SWASV	Pb2+	0.66
		Cd2+	0.46
Ti3C2Tx/BiNPs nano-composite	SWASV	Pb2+	2.24
		Cd2+	1.39

. The limits of detection (LOD) of the sensor described in this work are lower than some of the previous works. Thus, the optimized BiNPs@Ti₃C₂T_x showed acceptable results. Considering the acceptable LOD and the high recovery values, the developed sensor has a great potential for detecting Pb²⁺ and Cd²⁺ in practical water samples.

4. Conclusions

The BiNPs@Ti₃C₂T_x nano-composite that integrated the advantages of BiNPs and Ti₃C₂T_x with high surface area and good conductivity was prepared. BiNPs@Ti₃C₂T_x was used to modify the electrochemical electrode for heavy metal ions detection as sensing technology. The obtained sensor can simultaneously detect Pb²⁺ and Cd²⁺ with high sensitivity and good exactness. However, Cu²⁺ can cause interference in this detection process. Considering the high effective surface area and excellent conductivity of the BiNPs@Ti₃C₂T_x, the nano-composite can be applied as an excellent electrode-modification material for fast and convenient determination of heavy metal ions in environmental systems.