Highly Sensitive Titanium-Based MXene-Reduced Graphene Oxide Composite for Efficient Electrochemical Detection of Cadmium and Copper Ions in Water

Abstract

An electrochemically active and promising binary composite that is made up of titanium-based MXene (Ti₃C₂T_x) and rGO is developed to simultaneously detect the Cd²⁺ and Cu²⁺, in water. XRD, FTIR, Raman, XPS, FESEM, elemental mapping, and EDX analysis affirmed the successful formation of the Ti₃C₂T_x-rGO composite. The produced Ti₃C₂T_x-rGO electrode exhibited a homogeneous rGO sheet covering the Ti₃C₂T_x MXene plates with all the detailed Ti2p, C1s, and O1s XPS peaks. The high-performance Ti₃C₂T_x-rGO composite was successfully tested for the Cd²⁺ and Cu²⁺ ions via differential pulse voltammetry (DPV), altering the pH, concentration, and the real water sample’s quality. The electrochemical performances revealed that the proposed Ti₃C₂T_x-rGO composite depicted excellent detection and quantification limits (LOD and LOQ) for both Cd²⁺ (LOD = 0.31 nM, LOQ = 1.02 nM) and Cu²⁺ (LOD = 0.18 nM, LOQ = 0.62 nM) ions, where the result is highly comparable with the reported literature. The Ti₃C₂T_x-rGO was proven highly sensitive towards Cd²⁺ (0.345 μMμA⁻¹) and Cu²⁺ (0.575 μMμA⁻¹) with great repeatability and reproducibility properties. The Ti₃C₂T_x-rGO electrode also exhibited excellent stability over four weeks with a retention of 97.86% and 98.01% for Cd²⁺ and Cu²⁺, respectively. This simple modification of Ti₃C₂T_x with rGO can potentially be advantageous in the development of highly sensitive electrochemical sensors for the simultaneous detection of heavy metal ions.

1. Introduction

“Environmental health hazards” are identified based on the toxicity of the substance and potential exposure to contaminated air, water, soil, and heavy metal ions. They are also classified in the top ten list of the “Agency for Toxic Substances and Disease Registry Priority List of Hazardous Substances” [1]. The most abundant forms of water pollutants that result in negative effects on ecosystems, marine animals, and human health are heavy metals such as copper (Cu), lead (Pb), cadmium (Cd), chromium (Cr), mercury (Hg), and zinc (Zn). Many detection methods have been invented due to the increasing demand for a better evaluation of the quality of water, specifically in respect to heavy metal contamination. The three distinct types of these heavy metal detection approaches are spectroscopic, electrochemical, and optical detection. In comparison, the electrochemical approach is preferred for identifying heavy metals as it requires quick analytical time, requires cheap and easy-to-operate equipment, has great sensitivity, and possesses excellent selectivity [2,3].

MXene consists of a metal carbide, nitride, or carbonitride nanosheet in a two-dimensional (2D) transition material. Meanwhile, Ti₃C₂T_x is a titanium-based MXene, an extensively developed and explored MXene to be employed in the treatment of water [4]. The Ti₃C₂T_x MXene was developed by researchers for identifying heavy metals, specifically for the detection of Cu²⁺, Cr⁷⁺, Ba²⁺, and Pb²⁺ utilizing in situ reductions and adsorption techniques. MXene has strong catalytic activity against a variety of water contaminants in a sensing application, in the presence of -O and -OH functional groups, which provide an abundance of active sites for a direct ion exchange process. Shahzad et al. [5] proposed a 2D Ti₃C₂T_x nanosheet that has active interaction with Cu²⁺ ions to produce an adsorption capacity that is 2.7 times larger than that for typically accessible activated carbon. The introduction of Ti₃C₂T_x MXene nanoribbons drastically enhances the adsorption and reduction properties, where promising and simple electrochemical analysis has demonstrated an excellent LOD of 0.94 nM for Cd²⁺ ions [6]. The alkalinized Ti₃C₂T_x (Ti₃C₂(OH/ONa)_xF_2−x) electrode comprises multiple active Ti-O and Ti-OH sites and has also demonstrated promising signals towards Pb²⁺ purification for environmental remediation [7]. On the other hand, the diverse Ti₃C₂T_x MXene layer has a limited distance within the multiple sheets, which inhibits electrode performance. This is due to only a tiny portion of the electroactive sites being attached during the detection process. Modifying the surface is a viable strategy for improving MXene characteristics for offering potential sensing performance, as this can drastically increase the MXene layer distance. MXene has been altered using numerous electroactive components such as conductive polymers, transition metal oxides, and graphene in order to boost the sensing properties. Xia et al. [8] incorporated carbon black with Ti₃C₂T_x MXene and the result revealed that the aggregation of Ti₃C₂T_x Mxene was successfully prevented, and the electron transfer as well as the electrode surface area had gradually improved via the proposed modification. It has also been proven that the simultaneous detection of heavy metal ions is promisingly high for a nitrogen and phosphorus co-doped Ti₃C₂T_x Mxene electrode as the dopants significantly boost the accessible electroactive region of the electrode in simultaneously detecting Cu²⁺ and Hg²⁺ [9].

Among numerous candidates, the electrochemically conductive and mechanically stable reduced graphene oxide (rGO) is an ideal candidate for heavy metal sensing in water. A thermally produced rGO thin film was presented by Maity et al. [10] for rapid Pb²⁺ ion detection in various water sources. Excellent Pb²⁺ detection in a 1 M HCl solution and common water samples was revealed through the employment of an electrochemically developed rGO of graphite-enforced carbon material [11]. The lowest LOD (Pb²⁺ = 0.1 g/L and Cd²⁺ = 1.0 g/L) was observed for simultaneous heavy metal ions, utilizing the micro-patterned rGO, which was effectively fabricated utilizing the lithography approach [12]. Researchers discovered that the restored sp² carbon network in the rGO structure leads to enhanced electro-conductivity [13]. The rGO structure bound with amino groups has improved the electrode in electrically active areas. Hence, rGO can be incorporated with Ti₃C₂T_x to significantly boost the electrochemical performance of the active material in the detection of heavy metal ions in water. This is because the surface area of Ti₃C₂T_x MXene is significantly accessible during the process of detection, where rGO potentially serves as the spacer as well as anti-pile layer, eventually offering greater electroactive sites.

In the present work, a promising binary composite that consists of titanium-based MXene (Ti₃C₂T_x) and rGO was homogeneously prepared via sonication for instantaneous identification of Cu²⁺ and Cd²⁺ in water. The properties of the as-prepared Ti₃C₂T_x-rGO composite were characterized using XRD, FTIR, Raman, FESEM, EDX and XPS. The developed Ti₃C₂T_x-rGO composite was optimized by varying the ratio of Ti₃C₂T_x and rGO. The optimized electroactive material was utilized for a simultaneous detection of Cd²⁺ and Cu²⁺ ions in water. Ti₃C₂T_x-rGO is expected to exhibit a promising limit of detection, an excellent limit of quantification along with a great electrode sensitivity towards the simultaneous heavy metal detection. Ti₃C₂T_x-rGO also illustrated a high peak current retention even after four weeks, signifying an outstanding electrode stability.

2. Materials and Methods

2.1. Materials

Potassium chloride (KCl, 99%) and sulphuric acid (H₂SO₄, 96%) were acquired from Fisher scientific. Meanwhile, dipotassium hydrogen phosphate (K₂HPO₄, 98%), potassium dihydrogen phosphate (KH₂PO₄, 98%) and nitric acid (HNO₃, 65%) were obtained from Merck KGaA. Sigma Aldrich supplied graphene oxide (GO, 4 mg/mL), Titanium aluminum carbide (Ti₃AlC₂, 90%), lithium fluoride (LiF, 97%), hydrochloric acid (HCl, 37%), ethanol (95%), cadmium (II) chloride (CdCl₂, 99.9%), and copper (II) chloride (CuCl₂, 99.9%). Finally, Milli-Q deionized (DI) water was obtained from Millipore (18.5 MΩ·cm, 25 °C).

2.2. Preparation of Ti₃C₂T_x-rGO Nanocomposite

The layered Ti₃C₂T_x MXene was produced via an etching approach of the aluminium phase of MAX Ti₃AlC₂. Firstly, the etching solution was obtained by mixing LiF (1.0 g) in 9 mol/L HCl (20 mL) solution, utilizing magnetic stirring (30 min) approach. Then, 1.0 g of Ti₃AlC₂ was slowly added into the prepared mixture and allowed to stir continuously (24 h) at 35 °C to attain an impure Ti₃C₂T_x MXene. The collected impure Ti₃C₂T_x MXene was washed with DI water and followed by centrifuge (3500 rpm) for 10 min until it reached pH > 6.0. The pure Ti₃C₂T_x MXene nanosheet dispersion was then allowed to dry utilizing a freeze dryer.

The Ti₃C₂T_x-rGO nanocomposite was fabricated via sonication and followed by electrochemical reduction (Figure 1a). First, the Ti₃C₂T_x dispersion (3 mg/mL) was prepared by magnetically stirring Ti₃C₂T_x powder (15 mg) with DI water (5 mL) for 30 min. The 3 mg/mL GO solution that sonicated for 1 h was then mixed with the 5 mL Ti₃C₂T_x dispersion and proceed with ultrasonic treatment for 1 h. The prepared dispersion (5 μL) was drop-casted on a clean glassy carbon electrode (GCE) surface and allowed to dry at room temperature. GCE was polished on the polishing cloth, employing 0.5 μm of alumina slurry. The electrochemical activation of GCE was performed in 0.1 M H₂SO₄ via cyclic voltammetry (CV), applying potential from +1.5 to −0.4 V for 15 cycles. The GCE was later sonicated for 10 min each in HNO₃ and the DI water to obtain a clean electrode surface. The digital pictures in Figure 1b clearly differentiate the surface of GCE via electrode modification. The dried Ti₃C₂T_x-GO modified GCE was electrochemically treated in the phosphate buffer solution (PBS) (pH 7) to successfully transform GO into rGO. The chronoamperometry method (−0.8 V) was performed on the electrode for 3 min [14,15], utilized three-electrode configuration where the Ti₃C₂T_x-GO coated GCE, platinum (Pt) wire and silver/silver chloride (Ag/AgCl) acted as the working electrode, counter electrode and reference electrode, respectively. The produced Ti₃C₂T_x-rGO nanocomposite was labelled as the working electrode in this application.

2.3. Ti₃C₂T_x-rGO Nanocomposite Characterization

X-ray diffraction analysis was conducted to examine the phase composition of the synthesized samples using Bruker X-ray diffractometer D8 advance. The vibration modes and functional groups signals of the materials were retrieved from Raman spectroscopy (Thermo Scientfific Raman spectrometer, 488 nm) and Fourier Transform Infrared Spectrometer (FTIR, Perkin-Elmer Spectrum100), respectively. The field emission scanning microscopy (FESEM, ZEISS MERLIN) and X-ray photoelectron spectroscopy (XPS, XSAMHS Kratos Analytical) were performed to determine the morphological and the chemical compositions of the composite surfaces, respectively.

2.4. Electrochemical Detection of Heavy Metals

The prepared binary Ti₃C₂T_x-rGO composite was investigated for simultaneous heavy metal detection, namely, Cd²⁺, and Cu²⁺ ions in water sample. Various analysis was conducted to investigate the performance of Ti₃C₂T_x-rGO on the detection of analytes. All electrochemical analyses were performed via potentiostat (Autolab PGSTAT204), utilizing electroactive material coated GCE (working electrode), Pt wire (counter electrode) and Ag/AgCl (reference electrode); in a three-electrode configuration). Differential pulse voltammetry (DPV) assessments were conducted for the proposed Ti₃C₂T_x-rGO electrode at a potential ranging from −0.95 to −0.05 V for pH study to determine the optimum pH of PBS for the simultaneous copper (Cu²⁺) and cadmium (Cd²⁺) ions detection. The potential range was then widened from −0.95 to +0.1 V for the concentration study, real sample study, interference study, reproducibility test, repeatability test and stability test in this sensor application. The simultaneous heavy metal ions detection process began with a pre-electroreduction step, where a potential of –0.95 V (vs Ag/AgCl) was applied using DPV. In this work, the detection of Cu²⁺ and Cd²⁺ ions using a MXene/rGO composite electrode via DPV is typically carried out under careful optimized experimental conditions to achieve high sensitivity and selectivity towards the heavy metal ions detection. The actual experimental conditions of this work were properly developed following strict and standard procedures. The effect of supporting electrolyte pH on the voltametric response of the mixture of Cd²⁺, and Cu²⁺ on the prepared electrode was evaluated in the PBS (pH 4–6.5 (Figure 1c)). The DPV signal of the proposed electrode was recorded for various pH of PBS containing heavy metal ions.

The concentration study was carried out by increasing the concentration of both analyte (Cd²⁺ and Cu²⁺) in the optimized condition. A calibration curve was obtained from the relationship between the same analyte concentration against the produced oxidation peak current and the error bars (relative standard deviation), where it was generated for each concentration of analyte. This analysis was conducted via DPV method at 50 mV pulse amplitude, 50 ms pulse width and 20 mV/s scan rate. The reproducibility of electrochemical sensor was examined by measuring the analytes using five different electrodes and the relative standard deviation (RSD) was calculated. Repeatability of the sensor was evaluated by recording ten successive measurements using the same electrode. Stability of the electrochemical sensor was studied by preparing different electrodes and storing it at room temperature for a period of time. The current response of the stored electrodes was recorded after 1 week, 2 weeks, 3 weeks and 4 weeks via DPV analysis. The percentage for signal change was calculated and compared in continuous 4 weeks.

3. Result and Discussion

3.1. Characterization

Figure 2 demonstrates XRD diffraction peaks of various samples. This analysis was conducted to determine the sample phase compositions. The Ti₃AlC₂ illustrates XRD diffraction peaks at 9.5° (002), 18.9° (004), 34.0° (101), 38.9° (104), 41.8° (105), 48.4° (107), 56.5° (109) and 60.7° (110), which matches well with the JCPDS pattern 052-0875 of Ti₃AlC₂ (hexagonal lattice) [16]. The Ti₃C₂T_x MXene produced through etching process demonstrates (002), (004), (101), (104), (105), (107), (109) and (110) planes at 8.3°, 19.1°, 34.0°, 38.7°, 41.8°, 48.4°, 56.4° and 60.5°, respectively. The diminished (104) plane of T₃C₂T_x and the (002) plane of Ti₃C₂T_x MXene is noticeably lower in intensity and broader in peak at 8.3°, signifying the successful Al-etching of Ti₃AlC₂ [17,18]. GO illustrates a diffraction peak at 10.2°, indicating the lattice plane (001) [19,20,21]. An effective electrochemical reduction procedure results in a wide rGO diffraction peak at 2θ = 25.3° (002), indicating the presence of graphite-like sheets [14,22,23]. The Ti₃C₂T_x-rGO illustrates all XRD signals of Ti₃C₂T_x and rGO, validating a successful formation of the sample.

The vibration modes within the as-prepared materials were investigated via Raman spectroscopy (Figure 3a. A strong D band (sp³-hybridized carbon) and G band (sp²-hybridized carbon) are observed for GO (D band = 1355 cm⁻¹ and G band = 1594 cm⁻¹) and rGO (D band = 1354 cm⁻¹ and G band = 1594 cm⁻¹) samples. The band intensity ratio of D over G (I_D/I_G) can be adopted to estimate the degree of disorder in the graphite structure. The ratio value of I_D/I_G larger than 1 signifies that the sample comprises more sp³-hybridized carbon atoms than sp²-hybridized carbons [24,25,26]. The measured I_D/I_G ratio of GO is 0.94, whereas the I_D/I_G ratio of rGO (1.24) and Ti₃C₂T_x-rGO (1.32) confirm that the proposed electrochemical reduction process diminished oxygenated functional groups that was originally appear on the GO layer [27]. Ti₃C₂T_x MXene depicts Raman peaks at 147.8, 279.4, 391.5, 591.0 cm⁻¹ correspond to the low levels of anatase TiO₂ on the outermost surface of Ti₃C₂T_x MXene [28]. The Raman signal at 721.8 cm⁻¹ shows the A_1g symmetrical out-of-plane vibration of Ti and C atoms [29]. The D band and G band of Ti₃C₂T_x MXene are observed at 1325.2 and 1559.8 cm⁻¹, where the D band represents disorder induction within the structure. The synthesized Ti₃C₂T_x-rGO illustrates all the characteristic peaks of Ti₃C₂T_x and rGO.

Figure 3b represents the FTIR spectra of GO, rGO, Ti₃C₂T_x MXene, and Ti₃C₂T_x-rGO electrodes. GO demonstrates C-O-C, C=C, C=O and O-H functional groups at 1057, 1387, 1621 and 3301 cm⁻¹, respectively. After reduction reaction, rGO illustrates peaks at 1047 cm⁻¹ (C-OH), 1363 cm⁻¹ (C=C), 1597 cm⁻¹ (C=O) and 3307 cm⁻¹ (O-H). The intensity of O-H (3307 cm⁻¹) carboxyl stretching mode of rGO is noticeably smaller than the GO (3301 cm⁻¹), validating successful electrochemical reduction process. The Ti-O (667 cm⁻¹)and C=O (1640 cm⁻¹) vibration modes are clearly seen from Ti₃C₂T_x MXene. The existence of hydroxyl groups is verified by the absorption signals at 3301 and 1640 cm⁻¹, which are ascribed to the absorbed external water and highly hydrogen-bonded OH or exceptionally strong coordinated H₂O in the Ti₃C₂T_x MXene. The detected FTIR signal of Ti₃C₂T_x-rGO further affirms the formation of the composite.

Identification of the surface morphology of the as-prepared samples was performed using FESEM analysis and presented in Figure 4. Both GO (Figure 4a) and rGO (Figure 4b illustrate wrinkle-like morphology. The inset of Figure 4b denotes that the rGO has a more pronounced wrinkle-like morphology compared to the GO, which is the result of the successful electrochemical reduction process [19,30]. This statement is in good agreement with the XRD, FTIR and Raman results. Ti₃C₂T_x (Figure 4c depicts a multi layered MXene flakes morphology after a successful chemical etching. Whereas, the Ti₃C₂T_x-rGO composite (Figure 4d) which is prepared through simple sonication method, shows that the rGO sheet uniformly covers the multi layered MXene flakes.

Ti₃C₂T_x-rGO composite was further evaluated through an elemental mapping as depicted in Figure 5a. Titanium (Ti), Carbon (C), Oxygen (O) and Aluminum (Al) are noticed from the analysis, and it can be clearly spotted that all the elements are distributed evenly on the composite, confirming homogeneous formation of the composite. The Al signal still can be observed in the Ti₃C₂T_x-rGO composite even after the Al-etching indicates that there is incomplete etching process at the inner layers of Ti₃AlC₂ [31]. From the EDX analysis (Figure 5b) of Ti₃C₂T_x-rGO composite, Ti, C, O, and Al are successfully recorded with the respective weight percentage of 88.1, 10.3, 1.5 and 0.1%. EDX result revealed that only minimal amount of Al (0.1%) present within the composite, confirming a successful etching of Al and there are still few unetched Al within the inner structure of Ti₃C₂T_x MXene.

The chemical composition of the as-prepared Ti₃C₂T_x-rGO composite was investigated via XPS analysis (Figure 6). Ti2p, C1s and O1s signals are obtained at the binding energy of 458, 285 and 529 eV, respectively (Figure 6a). Ti2p signal originates from Ti₃C₂T_x MXene, while C1s and O1s are produced by both Ti₃C₂T_x and rGO. The Ti2p_1/2 and Ti2p_3/2 characteristics are observed from Figure 6b. The deconvolution of Ti2p spectrum depicts seven different peaks, which appears at the binding energy of 454.7 (Ti-C 2p_3/2), 455.2 (Ti(II)), 456.5 (Ti-O 2p_3/2), 459.2 (TiO₂), 461.1 (Ti-C 2p_1/2), 461.9 (Ti(III)), 463.3 eV (Ti-O 2p_1/2) [32,33,34,35]. The C1s spectrum presented in Figure 6c illustrates four deconvoluted XPS peaks, which indicates the C=C/C–C, C–O (epoxy and hydroxy), C=O and O–C=O interactions happen at specific binding energies of 281.4, 282.1, 284.6 and 286.2 eV, respectively. From the result, it can be clearly seen that the intensity of C-C/C=C signal is relatively higher than the C-O (hydroxy and epoxy), revealing a successful reduction of GO. It also proves that the rGO within the composite still consist of several oxygen-containing functional groups [36]. The O1s spectrum (Figure 6d) is deconvoluted into four peaks that are clearly noticed at the binding energy of 529.7 eV (O-Ti), 530.6 eV (C-Ti-O_x), 531.4 eV (C-Ti-OH_x) and 532.7 eV (H₂O-Ti) [32]. The XPS result affirms that the Ti₃C₂T_x MXene is successfully obtained via a chemical synthesis route. The electrochemical reduction effectively reduced GO to rGO without disturbing the structure of Ti₃C₂T_x MXene. The XPS signal is also in full alignment with the XRD, FTIR, Raman, FESEM, EDX and elemental mapping results presented earlier.

3.2. Electrochemical Detection

Figure 7a depicts the DPV curve of bare GCE, rGO, Ti₃C₂T_x MXene, and Ti₃C₂T_x-rGO electrodes for the detection of 1 mM Cd²⁺ and Cu²⁺ in PBS (pH = 5.0). An obvious and low intensity Cd²⁺ signal and a broad and weak Cu²⁺ peak is obtained for bare GCE at the respective values of −0.74 and −0.16 V. Comparatively, the Cd²⁺ ion peak current is noticeably higher than the Cu²⁺ ion, indicating the difference in sensitivity of electrode for both the heavy metal ions. The introduction of Ti₃C₂T_x or rGO on a bare GCE illustrates an evident spike in peak currents and increase in the electrochemical signal through instantaneous ions detection, caused by the higher electrocatalytic activity and enhanced electrochemical surface area of the electrode. On the other hand, the pristine GCE, Ti₃C₂T_x MXene demonstrates prominent absorption peaks at the respective −0.75 and −0.17 V that indicate the peak of Cd²⁺ and Cu²⁺. Meanwhile, the Cu²⁺ signal is found weak for rGO. Therefore, the integration of Ti₃C₂T_x and rGO to form Ti₃C₂T_x-rGO composite has resulted in greater peak currents as the rGO increased the interlayer spacing of Ti₃C₂T_x, creating vast surface area for a better interaction of Cd²⁺ and Cu²⁺ [37]. Result implies that the Ti₃C₂T_x-rGO displays high intensity peak current than the Ti₃C₂T_x, rGO and bare GCE. Interestingly, Ti₃C₂T_x-rGO shows completely separated and intense peak currents that improve the detection of Cu²⁺ and Cd²⁺ ions electrochemically. The synergistic effect within the Ti₃C₂T_x-rGO electrode leads to outstanding oxidation signals towards Cd²⁺ and Cu²⁺ ions.

Figure 7b demonstrates the DPV of Ti₃C₂T_x-rGO composite immersed in the PBS consisting of 1mM Cd²⁺-Cu²⁺ within the pH range of 4.0 to 6.5. The redox reactions under the proton influence causes slight shifting in peak potentials of Cd²⁺-Cu²⁺ at the negative potential as the pH of the PBS rises [38,39]. This is because the presence of protons in the PBS reduces as the pH of the solvent rises. The formation of Cd²⁺ and Cu²⁺ ions from its metallic forms (Cu⁰ and Cd⁰) is rapid in the high pH PBS. The transformed heavy metal ions develop an electrostatic repulsion within Cd²⁺, Cu²⁺ as well as Ti₃C₂T_x-rGO composite, causing difficulty for electrochemical reaction to occur at high pH with low peak currents. Figure 7c illustrates the peak current versus pH of Cd²⁺ and Cu²⁺. The peak currents for Cd²⁺ and Cu²⁺ intensified when the pH elevated from 4 to 5, potentially due to the competition between the targeted heavy metal ions and protons for the binding sites on the electrode surface [40]. This phenomenon is due to the increase of pH of the PBS, which has resulted in the amount of proton present in the analyte solution to decrease. The Cd²⁺ and Cu²⁺ signals from pH 5.5 to 6.5 are observed with low peak currents, which is due to the hydrolysis of heavy metal ions [41,42]. The ideal pH used for this task is pH 5 as it illustrates highest peak current of 51.4 and 3.47 μA for Cd²⁺ and Cu²⁺, respectively. The relationship of the peak potential (E_p) of Cd²⁺ and Cu²⁺ versus pH is demonstrated in Figure 7d–e. The E_p of both Cd²⁺ and Cu²⁺ are noticeably proportional to the PBS pH in accordance with the regression equation of E_p (V) = −0.046 pH−0.637 (R² = 0.989) for Cd²⁺ and E_p (V) = −0.042 pH + 0.018 (R² = 0.967) for Cu²⁺, respectively.

The simultaneous detection of Cd²⁺ and Cu²⁺ was performed via DPV analysis (Figure 8a) utilizing Ti₃C₂T_x-rGO. Figure 8a depicts the Cd²⁺ and Cu²⁺ ions detection in PBS (pH 5), varying the concentration of Cd²⁺ (7.5–150 nM) and Cu²⁺ (1–150 nM). Figure 8b,d displays differential pulse voltammograms that focus on Ti₃C₂T_x-rGO composite in various Cd²⁺ and Cu²⁺ concentrations ranging from 7.5 to 150 nM and 1 to 150 nM, respectively. Result implies that the peak current of Cd²⁺ and Cu²⁺ increases with increasing concentration [43]. The plot of peak current against concentration of Cd²⁺ and Cu²⁺ is exhibited in Figure 8c,e, respectively. The Cd²⁺ peak currents rise gradually with the concentration of Cd²⁺ and the correlation between peak current with Cd²⁺ concentration shall be potentially represented in the form of I_pa (μM) = 0.345 Cd²⁺ (μM) + 0.010 with R² = 0.999. The sensitivity of Ti₃C₂T_x-rGO against Cd²⁺ is 0.345 μMμA⁻¹, which is attained from the slope of the equation. Similarly, the peak currents of Cu²⁺ constantly increase as the concentration of Cu²⁺ rises. Cu²⁺ also shows a straight-line curve of peak current and Cu²⁺ concentration, that is presented as I_pa (μM) = 0.575 Cu²⁺ (μM) + 0.158 where R² = 0.993. The achieved sensitivity of Ti₃C₂T_x-rGO towards the detection of Cu²⁺ is 0.575 μMμA⁻¹. It can be concluded that the modified Ti₃C₂T_x-rGO electrode is capable to demonstrate a complete-separation of oxidation peak and the electrochemical detection of Cd²⁺ and Cu²⁺ that does not interfere with each other. Limit of detection (LOD) and limit of quantification (LOQ) are measured via Equations (1) and (2), where σ and s are standard deviation and slope of the calibration curve, respectively. The LOD of Ti₃C₂T_x-rGO modified electrode for Cd²⁺ and Cu²⁺ are 0.31 and 0.18 nM, respectively. Whereas the LOQ discovered for Cd²⁺ and Cu²⁺ are 1.02 and 0.62 nM, respectively. The performance of the suggested Ti₃C₂T_x-rGO composite and the other modified electrodes in detecting Cd²⁺ and Cu²⁺ is tabulated in

Table 1. Performance of various MXene-based electrodes for heavy metal detection.

No.	Material	Heavy MetalDetected	LOD (nM)	Linear Range of Detection (μM)	Reference
1	alk-Ti3C2	Cu2+	39.00	0.1–1.4 μM	[44]
		Cd2+	82.00	0.1–1.4 μM
2	H–C3N4/Ti3C2Tx	Cd2+	1.00	0.5–1.5 μM	[45]
		Pb2+	0.60	0.5–1.5 μM
3	Ti3C2@N-C	Cd2+	2.25	0.1–4 μM	[46]
		Pb2+	1.10	0.05–2 μM
4	BiNPs/Ti3C2Tx	Cd2+	12.4	0.08–0.8 μM	[47]
		Pb2+	10.8	0.06–0.6 μM
5	Ti3C2Tx-rGO	Cd2+	0.31	7.5–150 nM	This work
		Cu2+	0.18	1–150 nM

alk-Ti₃C₂: alkaline intercalation of Ti₃C₂, H–C₃N₄/Ti₃C₂T_x: protonated carbon nitride/Ti₃C₂T_x, Ti₃C₂@N-C: nitrogen-doped carbon-coated Ti₃C₂-MXene, BiNPs/Ti₃C₂T_x: bismuth-nanoparticles/Ti₃C₂T_x.

. The Ti₃C₂T_x-rGO composite result is found comparable with the reported literature. The proposed electroactive material in this work also demonstrates an outstanding LOD for simultaneous heavy metals detection, which is significantly lower than the other reported MXene based composites.

$$LOD={\displaystyle \frac{3\mathsf{\sigma }}{s}}$$

(1)

$$LOQ={\displaystyle \frac{10\mathsf{\sigma }}{s}}$$

(2)

The reproducibility of Ti₃C₂T_x-rGO was determined by testing 0.5 mM Cd²⁺ and Cu²⁺ using five distinct electrodes and the calculated relative standard deviation (RSD) of 2.42% and 2.36% are attained for Cd²⁺ and Cu²⁺, respectively. The repeatability of Ti₃C₂T_x-rGO, is evaluated at 10 DPV signal using a similar electrode and this test is performed in the 0.5 mM solution of Cd²⁺ and Cu²⁺. The calculated RSD are 1.93% and 3.58% for Cd²⁺ and Cu²⁺, respectively, signifying outstanding repeatability of the proposed material. The Ti₃C₂T_x-rGO sensor constancy was determined upon testing 0.1 mM Cd²⁺ and 0.1 mM Cu²⁺ in the pH 5 PBS. Although the approximate concentration of dissolved oxygen in water at room temperature and 1 atm pressure is around 0.25 mM, even nanomolar concentrations of metal ions can significantly suppress the oxygen signal observed in DPV. This seemingly disproportionate effect arises from several electrochemical and chemical interactions. Certain metal ions, such as Cu²⁺, Fe²⁺, or Mn²⁺, can catalyze the oxygen reduction reaction (ORR), to alter the kinetics and mechanisms of oxygen’s electrochemical behaviors. These ions can form transient complexes with oxygen or its reduction intermediates, thereby modifying the redox potential and diminishing the distinct oxygen peak in DPV. Additionally, metal ions can adsorb onto the electrode surface and alter its electrochemical properties, including electron transfer rates and surface reactivity. This surface modification can hinder the reduction of oxygen or shift its peak, leading to apparent suppression. Despite their low concentration, these ions can exert a catalytic or a surface-blocking effect that disrupts the sensitivity and resolution of DPV, which is a highly sensitive technique designed to detect subtle changes in current. Thus, the suppression of oxygen signals by trace metal ions highlights the importance of understanding both direct and indirect interactions in electrochemical analyses.

Next, the prepared sensor was stored for 30 days at atmospheric temperature and the detailed peak current retention (%) of Ti₃C₂T_x-rGO is tabulated in

Table 2. Stability study of Ti₃C₂T_x-rGO electrode for the detection of Cd²⁺ and Cu²⁺.

Stability Period	Peak Current Retention (%)
	Cd2+	Cu2+
1 Week	98.19%	99.81%
2 Week	98.61%	99.48%
3 Week	99.89%	98.49%
4 Week	97.86%	98.01%

. Result shows that Ti₃C₂T_x-rGO electrode retained 97.86% (Cd²⁺) and 98.01% (Cu²⁺) of its initial peak current responses, implying excellent stability of Ti₃C₂T_x-rGO towards simultaneous detection Cd²⁺ and Cu²⁺.

The impact of various interference ions in the PBS containing 1 mM Cd²⁺ and Cu²⁺ were investigated using Ti₃C₂T_x-rGO. The 100-fold and 1000-fold concentration of the interference ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Cl⁻, SO₄²⁻) were tested and the result shows that the injected ions do not interfere in a simultaneous detection of Cd²⁺ and Cu²⁺ ions in PBS (pH = 5) where the signal change is less than 5% [39]. An excellent interference resistance disclosed that the Ti₃C₂T_x-rGO is reliable even under ambient conditions. The practical effectiveness of Ti₃C₂T_x-rGO for simultaneous Cd²⁺ and Cu²⁺ detection has been explored by employing lake water and tap water. A predetermined quantity of Cd²⁺ and Cu²⁺ was injected into the solution for the purpose of the recovery experiment, which was carried out using DPV analysis. The quantity of Cd²⁺ and Cu²⁺ found in lake and supplied drinking water was identified using the traditional addition technique, and the recovery of Cd²⁺ and Cu²⁺ in percentage ranged between 96% and 99.5% (

Table 3. Recover data on concurrent detection of Cd²⁺ and Cu²⁺ in lake water (n = 3).

Sample	Added (nM)		Obtained (nM)		Recovery (%)
	Cd2+	Cu2+	Cd2+	Cu2+	Cd2+	Cu2+
1	60	60	58.4	58.9	97.3%	98.2%
2	80	80	78.1	79.3	97.6%	99.1%
3	100	100	98.9	99.5	98.9%	99.5%

and

Table 4. Recover data on concurrent detection of Cd²⁺ and Cu²⁺ in tap water (n = 3).

Sample	Added (nM)		Obtained (nM)		Recovery (%)
	Cd2+	Cu2+	Cd2+	Cu2+	Cd2+	Cu2+
1	60	60	58.7	57.6	97.8%	96.0%
2	80	80	78.2	78.3	97.8%	97.9%
3	100	100	99	98.8	99.0%	98.8%

). The results show that the Ti₃C₂T_x-rGO composite can detect Cd²⁺ and Cu²⁺ simultaneously using actual water samples.

4. Conclusions

A promising Ti₃C₂T_x-rGO sensor for Cd²⁺ and Cu²⁺ detection was successfully developed employing chemically synthesized Ti₃C₂T_x and electrochemically produced rGO by demonstrating obvious and intense Cd²⁺ and Cu²⁺ oxidation peaks via DPV analysis. Ti₃C₂T_x-rGO composite revealed significant electro-chemical-catalytic activity with respect to the Cd²⁺ and Cu²⁺ oxidation. It is also found that the Ti₃C₂T_x-rGO composite with improved electron transfer characteristics in comparison to the bare GCE, Ti₃C₂T_x and rGO. The results demonstrate a significantly low LOD and LOQ for concurrent detection of Cd²⁺ (LOD = 0.31 nM, LOQ = 1.02 nM) and Cu²⁺ (LOD = 0.18 nM, LOQ = 0.62 nM) ions in water. The promising Ti₃C₂T_x-rGO electrode illustrates an excellent sensitivity of 0.345 and 0.575 μMμA⁻¹ for Cd²⁺ and Cu²⁺ ions, respectively. Ti₃C₂T_x-rGO composite also disclose promising duplicability, repeatability, and consistency of Cd²⁺ and Cu²⁺ detection. Thus, Ti₃C₂T_x-rGO is proven as an outstanding electrochemical sensor for identifying Cd²⁺ and Cu²⁺ successfully.