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

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. It is 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 emphasized for identifying heavy metal as it requires quick analytical time and cheap and easy equipment/operation great sensitivity and possesses excellent selectivity [2, 3].

MXene consists of 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]. Ti₃C₂T_x MXene has been developed by researchers for identifying heavy metals, specifically for the detection of Cu²⁺, Cr⁷⁺, Ba²⁺, and Pb²⁺ utilizing in-situ reductions and adsorption technique. MXene has strong catalytic activity against a variety of water contaminants in the 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 adsorption capacity of 2.7 times bigger than the typically accessible activated carbon. The introduction of Ti₃C₂T_x MXene nanoribbons drastically enhance the adsorption and reduction properties, where promising and simple electrochemical analysis demonstrate an excellent LOD of 0.94 nM for Cd²⁺ ion [6]. The alkalinized Ti₃C₂T_x (Ti₃C₂(OH/ONa)_xF_2−x) electrode comprises multiple active Ti-O and Ti-OH sites also demonstrated promising signal towards the Pb²⁺ purification for environmental remediation [7]. On the other hand, the diverse Ti₃C₂T_x MXene layer possesses a restricted distance within the multiple sheets inhibits electrode performance. This is due to only a tiny portion of the electroactive sites are attached in the detecting process. Modification of the surface is a viable strategy for improving MXene characteristics for offering potential sensing performance as this can drastically increase MXene layer distance. MXene has been altered using numerous electroactive components such as conductive polymers, transition metal oxide, 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 has been successfully prevented and the electron transfer as well as the electrode surface area has been gradually improved via the proposed modification. It is also proven that the simultaneous detection heavy metal ions is promisingly high for 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. An excellent Pb²⁺ detection in a 1 M HCl solution and common water samples were revealed employing an electrochemically developed rGO of graphite enforced carbon material. [11]. The lowest LOD (Pb²⁺ = 0.1 g/L and Cd²⁺ = 1.0 g/L) were observed for simultaneous heavy metal ions, utilizing the micro-patterned rGO, which was effectively fabricated utilizing 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 of the electrically active area. Hence, rGO can be incorporated with the Ti₃C₂T_x to significantly boost the electrochemical performance of the electroactive material for identifying the presence of heavy metals 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 approach for instantaneous identification of Cu²⁺ and Cd²⁺ in water. The properties of the as-prepared Ti₃C₂T_x-rGO composite morphological structure was characterized using XRD, FTIR, Raman, FESEM, EDX and XPS. The developed Ti₃C₂T_x-rGO 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 of heavy metals and an excellent limit of quantification of them, with great electrode sensitivity. Ti₃C₂T_x-rGO is also illustrates a still high peak current retention after a long period of usage, signifying an outstanding electrode stability.

2. Materials and Methods

2.1. Materials

Potassium chloride (KCl, 99%) as well as sulphuric acid (H₂SO₄, 96%) were acquired from Fisher scientific. Meanwhile, dikalium hydrogen phosphate (K₂HPO₄, 98%), kalium dihydrogen phosphate (KH₂PO₄, 98%) and nitric acid (HNO₃, 65%) were obtained from Merck KGaA. Sigma Aldrich supplied graphene oxide (GO, 4 mg/mL) and polyvinylidene fluoride (PVDF), 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%). 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 from the 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 solvent. The collected impure Ti₃C₂T_x MXene was washed with the 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 1(a)). 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 (5μL) 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 GCE was later sonicated for 10 minutes with HNO₃ and deionized water to obtain a clean electrode surface. The dried Ti₃C₂T_x-GO modified GCE was electrochemically treated in the PBS solution (pH 7), and performed a chronoamperometry method (-0.8 V) for 3 min [14, 15]. The produced Ti₃C₂T_x-rGO nanocomposite was labelled as the working electrode in this application. The digital pictures in Figure 1(b) clearly differentiates the surface of GCE via electrode modification.

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 morphology and the chemical composition of the composite surfaces, respectively.

2.4. Electrochemical Detection of Heavy Metals

The prepared binary Ti₃C₂T_x-rGO was investigated for simultaneous heavy metal detection namely, Cd²⁺, and Cu²⁺ ions. 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), platinum (Pt) wire (counter electrode) and silver/silver chloride (Ag/AgCl, reference electrode); in a three-electrode configuration). Differential pulse voltammetry (DPV) assessments were conducted for the proposed Ti₃C₂T_x-rGO electrode at the potential ranging from -0.95 to -0.05 V for all the analysis (pH study, concentration study, real sample study, interference study, reproducibility test, repeatability test, stability test) in the sensor application. In this work, the detection of copper (Cu²⁺) and cadmium (Cd²⁺) ions using a MXene/rGO composite electrode via Differential Pulse Voltammetry (DPV) is typically carried out under careful optimized experimental conditions to achieve high sensitivity and selectivity towards the heavy metal detection. The actual experimental conditions of this work were properly developed following strict and standard procedures that are commonly conducted. The effect of supporting electrolyte pH on the voltammetric response of the mixture of Cd²⁺, and Cu²⁺ on the prepared electrode was evaluated in the phosphate buffer solution (PBS) in pH 4-6.5 (Figure 1(c)). DPV was performed to record the pH effect for the detection.

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 relation between same analyte concentration against the produced oxidation peak current and the error bars (relative standard deviation) were 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 same electrode. Stability of the electrochemical sensor was studied by preparing different electrodes and store it at the room temperature for a period. The current response of the stored electrodes was recorded after 1 week, 2 weeks, 3 weeks and 4 weeks via DPV analysis. The percentage of signal change was calculated and compared for all 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°. The diminished (104) plane of T₃C₂T_x and the (002) plane of Ti₃C₂T_x MXene is noticeably lower in intensity and broader 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]. The effective electrochemical reduction procedure results in a wide rGO diffraction peak of 2θ = 25.3 (002), implying 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 3(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 initially presence at 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 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 3(b) 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. 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) stretching mode of the carboxyl group of rGO is noticeably smaller than the GO (3301 cm^-1), validating successful electrochemical reduction process. The deformation vibration of Ti-O bond and C=O of Ti₃C₂T_x MXene is detected at the respective peaks of 667 and 1640 cm^-1. 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(i)) and rGO (Figure 4a(ii)) illustrate wrinkle-like morphology. The inset of Figure 4a(i-ii) denotes that the rGO has a more pronounce wrinkle-like morphology compared to the GO, which is the result of the electrochemical reduction process [19, 30, 31]. This statement is in good agreement with the XRD, FTIR and Raman results. Ti₃C₂T_x (Figure 4a(iii)) depicts a multi layered MXene flakes morphology after a successful chemical etching. Whereas, the Ti₃C₂T_x-rGO composite which 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 elemental mapping as depicted in Figure 5(a). Titanium (Ti), Carbon (C), Oxygen (O) and Aluminum (Al) are noticed from the analysis, and it can be clearly spotted that the 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, which indicate that there is incomplete etching process at the inner layers of Ti₃AlC₂ [32]. From the EDX analysis (Figure 5(b)) 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 is 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 6(a)). Ti2p signal originated 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 6(b). The deconvolution of Ti2p spectrum depicts seven different peaks, which appear 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) [33,34,35,36]. The C1s spectrum presented in Figure 6(c) illustrates four deconvoluted XPS peaks, which indicate the C=C/C–C, C–O (epoxy and hydroxy), C=O and O–C=O interactions happened 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 and it also proves that the rGO within the composite still consist of several oxygen-containing functional groups [37]. The O1s spectrum (Figure 6(d)) 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) [33]. XPS result affirms that the Ti₃C₂T_x MXene is successfully obtained via the 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 7(a) 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 solution (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 noticibly higher than the Cu²⁺ ion, indicating the difference in sensitivity of electrode for both 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 electrochemical surface area. On the other hands, the pristine GCE, Ti₃C₂T_x MXene demonstrates prominent absorbtion 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 has resulted in greater peak currents as the rGO increased the interlayer spacing of Ti₃C₂T_x, created a greater surface area for the better interaction of Cd²⁺ and Cu²⁺, with Ti₃C₂T_x-rGO composite [38]. 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 improves the detection of Cu²⁺ and Cd²⁺ ions electrochemically. The synergistic effect within the Ti₃C₂T_x-rGO electrode lead to outstanding oxidation signals towards Cd²⁺ and Cu²⁺ ions.

Figure 7(b) demonstrates the DPV of Ti₃C₂T_x-rGO composite immersed in the PBS solution consisting of 1mM Cd²⁺ - Cu²⁺ within the pH range of 4.0 to 6.5. The peak potentials of Cd²⁺ - Cu²⁺ have shown slightly deviations to the negative potential as the pH of the PBS rises, which validated the redox reactions under the proton influence [39, 40]. This is because the presence of proton in the PBS solution reduces as the pH of the solvent rises. Cd²⁺ and Cu²⁺ ions rapidly form anion in the high pH PBS solution. The produced anion develops an electrostatic repulsion within Cd²⁺, Cu²⁺ as well as Ti₃C₂T_x-rGO, causing difficulty for electrochemical reaction to occur at high pH with low peak currents. Figure 7(c) 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 [41]. This phenomenon is due to the increase of pH of the PBS, which have resulted in the amount of proton present in the analyte solution to decrease. This later caused the Cd²⁺ and Cu²⁺ ions to be easily oxidized and form anion easily at higher pH. The presence of these anions will cause an electrostatic repulsion between the heavy metal ions and the Ti₃C₂T_x-rGO composite, resulting low peak current [40]. 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 [42, 43]. 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 7(d-e). The E_p of both Cd²⁺ and Cu²⁺are noticibly 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 8(a)) utilizing Ti₃C₂T_x-rGO. Figure 8(a) depicts the Cd²⁺ and Cu²⁺ ions detection in PBS solution (pH 5), varying the concentration of Cd²⁺ (7.5-150 nM) and Cu²⁺ (1-150 nM). Figure 8 (b) and (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 [44]. The plot of peak current against concentration of Cd²⁺ and Cu²⁺ is exhibited in Figure 8 (c) and (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 increases as the concentration of Cu²⁺ is 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 each other. Limit of detection (LOD) and limit of quantification (LOQ) is measured via Eq. (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 composite and the other modified electrodes in detecting Cd²⁺ and Cu²⁺ is tabulated in Table 1. The Ti₃C₂T_x-rGO composite result is comparable with the reported literature. The proposed electroactive material in this work also demonstrated an outstanding LOD for simultanuous 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 for 10 DPV signal of an electrode and analysed 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 0.1mM Cu²⁺ in the pH 5 PBS solution. 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 Differential Pulse Voltammetry (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), altering 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 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. 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 the simultaneous detection of Cd²⁺ and Cu²⁺ ions in PBS (pH = 5) where the signal change is less than 5% [40]. The excellent interference resistance disclosed that 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 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 were ranged between 96% and 99.5% (Table 3 and Table 4). The results show that the Ti₃C₂T_x-rGO composite is capable of detecting Cd²⁺ and Cu²⁺ simultaneously using actual water samples.

4. Conclusions

A promising Ti₃C₂T_x-rGO sensor for Cd²⁺ and Cu²⁺ deterction 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 a significant electro-chemical-catalytic activity with respect to the Cd²⁺ and Cu²⁺ oxidation. It is also found that improved electron transfer characteristics in comparison to the bare GCE, Ti₃C₂T_x and rGO. Ti₃C₂T_x-rGO sensor were obtained. The results demonstrated 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 illustrated 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.