Electrochemical Detection of Multiple Heavy Metal Ions Using a Hybrid Nanocomposite of a Metal–Organic Framework and Cellulose Nanocrystal-PEDOT:PSS

Abstract

A novel hybrid nanocomposite based on a metal–organic framework (MOF) and electroconductive cellulose nanocrystals (CNCs) was developed for the individual and simultaneous detection of heavy metal ions Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺. The reported modified electrode was based on a thin layer of the hybrid nanocomposite formed on a glassy carbon electrode via a drop-casting method. The MOF, DUT-67, provided binding sites for heavy metal ions due to putative interactions between organic linkers and metal cation analytes. The electroconductive matrix was composed of PEDOT:PSS and CNC, which not only served as an effective template for the DUT-67 MOF but also enhanced electron transfer of the hybrid nanocomposites through the interconnected network of conductive CNC/PEDOT:PSS. Following optimization of the solution pH, accumulation time, and accumulation potential, the developed hybrid nanocomposite-based electrochemical sensor was successfully applied for individual and simultaneous detection of target heavy metal ions in aqueous solutions with high sensitivity and selectivity. Under the optimized conditions, linear detection ranges of simultaneous detection of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ were found to be 20–163, 20–163, 20–84, and 20–112 μg L^–1, respectively, with detection limits of 2.5, 1.78, 0.226, and 0.294 μg L^–1, respectively. No major interference effects were found from possible coexisting, interfering ions. This allows the practical use of the sensor to examine a variety of different aqueous environments. This study demonstrates the feasibility of our hybrid nanocomposite as a novel sensing platform for the electrochemical detection of multiple heavy metal ions.

1. Introduction

Heavy metal ions are extremely dangerous pollutants because they are distributed ubiquitously and cannot be easily detoxified. Heavy metal ions have deleterious effects on both human health and the environment. , An increase in the toxicity of heavy metal ions is caused by localization of a large amount of the metals. Exposure to cadmium ions (Cd²⁺) can result in toxicity in virtually any system of the body, including the skeletal, nervous, and cardiovascular systems, with long-term exposure increasing cancer risk. The U.S. Environmental Protection Agency (EPA) has set the drinking water maximum contaminant level (MCL) for Cd²⁺ to 5 μg L^–1. Lead ion (Pb²⁺) exposure (MCL = 15 μg L^–1) has been shown to damage the renal, nervous, and reproductive systems. Copper ion (Cu²⁺) ingestion (MCL = 1.3 mg L^–1) can result in Alzheimer’s disease and general loss of cognition. Mercury ion (Hg²⁺) ingestion (MCL = 2 μg L^–1) can result in fatigue, behavioral shifts, hallucinations, and death with prolonged exposure. Moreover, environmental samples often contain multiple heavy metal ion species. Although it is important to detect individual heavy metal ions, multiple different heavy metal ions are generally present in the same sample under real conditions. ,

A variety of conventional detection methods have been applied for the detection of heavy metal ions, such as inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectroscopy (AAS), gas chromatography/mass spectrometry (GC/MS), liquid chromatography/mass spectrometry (LC/MS), and X-ray fluorescence spectrometry. Although these standard techniques can accurately detect heavy metal ions, costly equipment and complicated operations are involved. They often require sophisticated analytical procedures, which are relatively time-consuming and expensive. Moreover, these techniques are unsuitable for on-site detection. As such, efforts to develop simpler methodologies for the effective detection of heavy metal ions are ongoing. Electrochemical detection is favorable due to its low cost, reliability, high sensitivity, low detection limit, and accumulation potential for on-site testing. ,

Metal–organic frameworks (MOFs) have been considered ideal candidates for various analytical platforms due to their abundant accessible metal sites, large surface areas, and unique structural morphologies. , MOF-based modified electrodes have played an emerging role in developing advanced analytical methods. These methods are mainly based on two approaches: optical detection using fluorescent or luminescent MOFs and electrochemical detection, which employs methods such as square wave anodic stripping voltammetry (SWASV) − and differential pulse voltammetry. For electrochemical detection, one of the most crucial objectives for MOF-based modified electrodes is to achieve an improved conductivity with enhanced electron transfer. Due to the poor electrical conductivity of MOFs in an aqueous suspension, however, the development of MOF-based electrochemical sensors remains a challenge. This obstacle can be overcome by immobilizing the MOFs with conductive nanomaterials. Examples of such nanomaterials include conducting polymers, metal nanoparticles, carbon nanotubes, graphite, graphene, graphene oxide, and quantum dots. , Several conducting polymers such as polyaniline (PANI) and PEDOT:PSS have been incorporated to improve the conductivity of MOF-based electrochemical sensing devices. For instance, in the construction of a modified electrode for cadmium ion detection, a PANI and UiO-66-NH₂ MOF nanocomposite was prepared by loading PANI onto the surface of the MOF. Compared with existing modified electrodes for the detection of cadmium ions, the PANI and UiO-66-NH₂ MOF nanocomposite-modified electrode exhibited a lower limit of detection, a wider linear range, and better reproducibility due to its enhanced conductivity. , Similarly, a composite consisting of UiO-66-NH₂ and PEDOT:PSS was developed for the detection of toluene. The combination of the large surface area of UiO-66-NH₂ and the high conductivity of PEDOT:PSS in the composite electrode afforded improved electrical conductivity with good sensitivity and selectivity. Thus, conducting polymers have been successfully applied in MOF-based electrochemical sensing devices. However, some drawbacks of the application of conducting polymers include weak mechanical and thermal properties, low solubility, and low porosity. Conducting polymers have been blended with nanomaterials for electrochemical applications, where the nanomaterials improve hierarchical flexibility, mechanical property, biocompatibility, electrical conductivity, and processability, resulting in synergistic effects of hybrid nanocomposites.

In recent years, cellulose nanocrystals (CNCs) have received significant attention in the fabrication of conductive nanocomposites used in electrochemical applications due to their large surface area, dispersion stability, and excellent mechanical properties. Although CNCs themselves are electrically insulating, they can be used to form porous matrices, providing a template that strengthens conductive nanocomposites. For example, CNCs have been proven to enhance the stability and binding performance of conducting polymers. One study demonstrated that a lightweight, porous, and flexible hybrid aerogel using polypyrrole (PPy) and CNCs possessed high porosity and flexibility anticipated to improve the electrochemical performance by offering enhanced electrolyte diffusion. Another study showed that CNC/PEDOT:PSS conductive nanocomposite produced through a simple blending method improves the electrical conductivity and enhances its mechanical stability. , Similarly, the CNC/PEDOT:PSS nanocomposite obtained by attaching PEDOT:PSS nanostructures to the surface of CNC increases electrical conductivity. The CNC was shown to be an excellent template for structuring PEDOT:PSS. Thus, the CNC was shown to be beneficial not only for the structuring of the conducting polymers but also for electroanalytical performance enhanced with the porous structure of the CNC-based conductive nanocomposites.

In addition, CNCs have been incorporated with a variety of MOFs to form novel hybrid nanocomposites used in a wide range of applications such as water treatment, , air purification, energy storage, and drug delivery. , One study demonstrated porous aerogels obtained by combining CNCs and MOFs (e.g., ZIF-8, UiO-66, and MIL-100) used as absorbents for water purification with a hierarchical porous structure and controllable MOF loading up to 50 wt %. Another study showed a fluorescence hybrid nanocomposite based on cellulose nanofibers or CNCs and a MOF for the detection of Cu²⁺ and lactic acid. Despite the extensive applications of the hybrid nanocomposites containing a MOF and CNCs, there have been a few recent studies of the MOF- and nanocellulose-based nanocomposites for their potential use in electrochemical sensing platforms. ,

Here, we report a novel strategy for the production of a hybrid nanocomposite consisting of a MOF and CNC/PEDOT:PSS deposited on an electrode via a facile drop-cast method for the detection of multiple heavy metal ions (Figure ). The DUT-67 MOF was selected due to its previously reported affinity for heavy metal ions, such as Pb²⁺. In our previous works, we demonstrated that DUT-67 could adsorb Pb²⁺ via putative cation−π interactions; it has also been shown that DUT-67 can adsorb Hg²⁺ by a similar mechanism. , Although to the best of our knowledge the adsorption of Cd²⁺ and Cu²⁺ has not been demonstrated with DUT-67, thiophene-containing materials have been used to adsorb these metal ions, most likely through direct Lewis base S–M²⁺ coordination. − Moreover, the CNC/PEDOT:PSS matrix may effectively increase the conductivity of the hybrid nanocomposite by enhancing both electron transfer and metal ion diffusion. , To the best of our knowledge, this is the first report in which a hybrid nanocomposite composed of DUT-67 and CNC/PEDOT:PSS has been used for electrochemical detection of multiple heavy metal ions.

1.

Schematic illustration of the preparation of the hybrid nanocomposite of the MOF and CNC/PEDOT:PSS. (A) Hybrid nanocomposite component that includes CNCs, PEDOT:PSS, and the MOF formed via suspension of these components in an ethanol solution. (B) Schematic representation of the modification of the electrode through drop-casting. SEM images show that the DUT-67 MOFs are well cross-linked through the interconnected network of conductive CNC/PEDOT:PSS.

2. Experimental Section

2.1. Materials

All of the reagents employed in the study were commercially obtained and utilized without additional purification. A 1 wt % solution of poly­(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) in water, cadmium nitrate tetrahydrate (Cd­(NO₃)₂·4H₂O), lead­(II) nitrate (Pb­(NO₃)₂), copper­(II) chloride dihydrate (CuCl₂·2H₂O), mercury­(II) chloride (HgCl₂), N,N-dimethylformamide (DMF), N-methyl-2-pyrrolidone (NMP), potassium ferrocyanide (K₄Fe­(CN)₆), potassium ferricyanide (K₃Fe­(CN)₆), and potassium chloride (KCl) were purchased from Sigma-Aldrich (St. Louis, MO). Ethanol (EtOH) and a 70% nitric acid solution were received from Thermo Fisher Scientific (Waltham, MA). 2,5-Thiophenedicarboxylic acid was purchased from TCI America (Portland, OR). Zirconium chloride was purchased from Alfa Aesar. Glacial acetic acid was purchased from Pharmco-Aaper. A 0.1 M acetate buffer solution was prepared at pH 5 by mixing appropriate amounts of sodium acetate and acetic acid. A 5 mM Fe­(CN)₆ ^3–/4– solution with 150 mM supporting KCl electrolyte was also prepared by mixing the appropriate amounts of ferricyanide and potassium chloride. All other chemicals were analytical grade and prepared using ultrapure deionized (DI) water from a Direct-Q UV water purification system (18.2 MΩ cm, Millipore Milli-Q Purification System).

2.2. Preparation and Purification of CNCs

CNCs were obtained by performing hydrolysis, centrifugation, and dialysis in sequence. , In brief, Whatman No. 1 filter paper (4 g) was treated with 65% (w/w) sulfuric acid (50 mL) at 45 °C for 30 min and quenched by 10-fold dilution with chilled DI water for 6 h. The resulting suspension was washed by centrifugation at 13000g for 10 min and resuspended in DI water followed by ultrasonication for 10 min. The washing steps were repeated three times. The acidity of the suspension was neutralized by a dialysis membrane with a 10 kDa molecular weight cutoff (Thermo-Fisher Scientific) while changing the bath DI water every 10 h for 5 days. The bundled cellulose was further removed via syringe filtration (Uniflo Syringe Filter, 0.45 μm PVDF, Cytiva Whatman). The CNC suspension was freeze-dried and stored in a vacuum desiccator until further use.

2.3. Synthesis of DUT-67

DUT-67 was synthesized according to a modified version of a previously published procedure. Zirconium chloride was dried under high vacuum at room temperature for 16 h prior to use. Zirconium chloride (0.466 g, 2.00 mmol, 1 equiv) was weighed into a 125 mL Erlenmeyer flask. A 1:1 DMF/NMP solution was prepared by mixing 25 mL of DMF and 25 mL of NMP, and this solution was added to the zirconium chloride in the flask. The mixture was sonicated for 10 min. Glacial acetic acid (20 mL, 348 mmol, 174 equiv) was added, and the mixture was sonicated for 10 min. 2,5-Thiophenedicarboxylic acid (0.229 g, 1.33 mmol, 0.7 equiv) was added, and the mixture was sonicated for 10 min. The resulting solution was filtered through a GE 25 mm PVDF syringe filter (0.45 μm) in 5 mL portions into 20 mL glass vials. The vials were sealed with Teflon-lined caps and incubated in an oven at 120 °C for 48 h. After cooling to room temperature, the resulting powders were combined in two 50 mL centrifuge tubes and centrifuged for 10 min at 5000g. The reaction liquid was decanted. Three washes were completed in which the powder in each centrifuge tube was suspended in 20 mL of fresh DMF, and the tubes were centrifuged for 5 min at 5000g. Three additional washes were completed, in which the powder in each centrifuge tube was suspended in 20 mL of fresh 200-proof ethanol, and the tubes were centrifuged for 5 min at 5000g. The powders were dried under high vacuum at room temperature for 16 h. The powder was characterized by PXRD, and the data were in agreement with those previously reported for DUT-67.

2.4. Preparation of the MOF/CNC/PEDOT:PSS Hybrid Nanocomposite

A MOF/CNC/PEDOT:PSS hybrid nanocomposite was fabricated by using appropriate volumes from micropipettes. An aqueous mixture of 1.0 wt % PEDOT:PSS in water was pipetted into EtOH in a 9:1 ratio to yield a 1 mg mL^–1 PEDOT:PSS in EtOH solution. A MOF/CNC suspension was prepared by adding 5 mg of MOF to 390 μL of EtOH and injecting 100 μL of a 10 mg mL^–1 aqueous CNC suspension into the same suspension for 10 mg mL^–1 MOF and 2 mg mL^–1 CNC. The MOF/CNC/PEDOT:PSS nanocomposite was obtained after a 10 μL injection of the previously prepared stock PEDOT:PSS solution into the MOF/CNC/EtOH mixture to obtain a PEDOT:PSS concentration of 0.02 mg/mL in the final composite. The MOF/CNC/PEDOT:PSS nanocomposite mixture was ultrasonicated for 30 s using an ultrasonic cleaner (Cole-Parmer) to achieve homogenization. It should be noted that these compositions were determined by performing SWASV for the detection of the Pb²⁺ ion and analyzing changes in the oxidation peak current, while varying the amounts of CNC and PEDOT:PSS (1, 5, and 10 mg mL^–1 MOF and 0.002, 0.02, and 0.2 mg mL^–1 PEDOT:PSS) with 2 mg mL^–1 CNC (Table S1 and Figure S1). The oxidative peak current increased with the amount of MOF or PEDOT:PSS. However, the largest amount of conducting polymer PEDOT:PSS with 10 mg mL^–1 MOF resulted in a slight decrease in the oxidative peak current, which may be attributed to less effective electron transfer throughout the composites due to an excess of PEDOT:PSS. Thus, the resulting masses of the MOF/CNC/PEDOT:PSS nanocomposite drop-casted on the surface (area of 7.07 mm²) of the GCE were 4 μg of CNC, 0.4 μg of PEDOT:PSS, and 0.1 mg of MOF. Moreover, a 4:1 volume ratio of the ethanol/water mixture was used to effectively disperse CNCs and to improve the conductivity of PEDOT:PSS by solvating the hydrophilic PSS and hydrophobic PEDOT chains. , The film-forming ability and morphology of PEDOT:PSS also improve due to PSS agglomeration being hindered by EtOH.

2.5. Modification of a Glassy Carbon Electrode

Glassy carbon electrodes (GCE) with a diameter of 3 mm (CH Instruments) were used. The GCE was polished using a polishing pad with 1.0 μm and 0.05 μm diameter alumina slurries and cleaned successively with EtOH and DI water. The electrode surface was then dried with a nitrogen gas stream. The GCE surface was modified by drop-casting the MOF/CNC/PEDOT:PSS nanocomposite on the surface of the GCE immediately after sonication to avoid aggregates. The modified electrode was then dried at room temperature for 1 h. A schematic illustration of the surface modification process is shown in Figure .

2.6. Characterization

Scanning electron microscopy (SEM) micrographs of MOF, PEDOT:PSS, CNC/PEDOT:PSS, and MOF/CNC/PEDOT:PSS samples were obtained by using a field emission scanning electron microscope (FESEM, JSM-7600F, JEOL) equipped with an energy dispersive X-ray spectrometer (EDS). An Octane Plus silicon drift detector (EDAX AMETEK, Inc.) was used for the elemental analysis. The samples were prepared with a Polaron E5100 sputter coater (Quorum Technologies) by using a gold–palladium sputtering target. Powder X-ray diffraction (PXRD) patterns were collected on a Bruker AXS X8 Prospector CCD single-crystal diffractometer using the “pilot” plugin for the collection of multicrystalline XRD patterns. The instrument was equipped with a copper IμS microsource with a laterally graded multilayer (Goebel) mirror for monochromatization (λ = 1.54178 Å; beam size of 0.1–0.2 mm) and an ApexII CCD area detector. Powder samples were thoroughly ground to ensure that a representative number of crystallites were present in the X-ray beam. Powder samples were mixed with small amounts of mineral oil and mounted on a 0.4 mm diameter Mitegen micromesh mount for data collection. Samples were centered in the beam using the instrument’s mounting microscope video camera. Data were collected in an emulated θ–2θ setup using the Apex2 software package of Bruker AXS. The sample mount was aligned horizontally (χ = 0°), and θ angles were set to eight different angles between 12° and 96° to cover a range equivalent to a 0–110° range of a powder X-ray diffractometer operated in Debye–Scherrer mode (ω angles of each run were set to half of the θ values). Samples were rotated around the mount’s spindle axis during measurement (360° rotation around ϕ), and typical exposure times were 30 s per frame collected. The eight individual patterns taken were corrected for an unequal sample–detector surface distance (“unwarped”) and were combined into one continuous pattern using the “pilot plugin” software embedded in the Apex2 software package. Data were integrated over 2θ, converted into powder XRD patterns in Bruker “raw” format, and further processed with standard powder XRD software packages.

2.7. Electrochemical Measurements

Electrochemical characterizations were conducted using a Gamry Interface 1010E potentiostat (Gamry Instruments, Warminster, PA), employing a three-electrode cell configuration.

The working electrodes consisted of either bare or modified GCE, while a silver/silver chloride (Ag/AgCl) electrode and platinum wire coil were applied as the reference and counter electrodes, respectively (Figure S2). The platinum wire was incubated in a 10% nitric acid solution for 5 min and rinsed with DI water prior to testing to remove inorganic impurities. Both cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses were performed using a 5 mM [Fe­(CN)₆]^3–/[Fe­(CN)₆]^4– (1:1) solution containing 150 mM KCl for characterization of the electrodes. CV was conducted with a scan rate of 100 mV s^–1 and a step size of 5 mV with a potential range of −1.0 to 1.0 V, unless stated otherwise. A total of five cycles were conducted for each experiment, and cycle 3 was reported for the results. EIS scans were taken at the appropriate DC voltage for a constant phase element (CPE) with a diffusion study and performed over a frequency range from 1.0 Hz to 1.0 MHz. For the detection of heavy metals, SWASV was carried out in the range from −1.0 to 0.5 V. All electrochemical experiments were carried out in 15 mL of a 0.1 M acetate buffer solution (pH 5) with a step size of 5 mV, a frequency of 25 Hz, and a pulse size of 20 mV unless stated otherwise. Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ heavy metal ion solutions were prepared by dissolving Cd­(NO₃)₂·4H₂O, Pb­(NO₃)₂, CuCl₂·2H₂O, and HgCl₂, respectively, in DI water.

3. Results and Discussion

3.1. Characterization of the MOF/CNC/PEDOT:PSS/GCE

Powder X-ray diffraction (PXRD) was used to characterize the crystallinities of the constituent composite materials for comparison, and the results are displayed in Figure . Amorphous PEDOT:PSS exhibited a broad diffractogram, while the highly crystalline MOF displayed sharp diffraction peaks. The diffractogram of the CNC/PEDOT:PSS composite contained CNC reflections at 15°, 22°, and 34°. The MOF/CNC/PEDOT:PSS composite displayed an XRD pattern similar to that of the MOF powder, exhibiting reflections from both the MOF and CNC at previously mentioned CNC reflection points as well as 4°, 7°, 8°, 9°, 13°, 14°, 19°, and 20°. CNC and MOF materials display diffraction peaks in the same positions as previously recorded XRD data when combined with PEDOT:PSS. , Notably, the reflections associated with the MOF are unchanged, indicating that the structure and crystallinity of the MOF material are intact following the addition of CNCs and PEDOT:PSS. ,

2.

Powder X-ray diffraction patterns of the PEDOT:PSS, MOF, CNC/PEDOT:PSS, and MOF/CNC/PEDOT:PSS modification layers.

Scanning electron micrographs of the MOF, PEDOT/PSS, CNC/PEDOT:PSS, and MOF/CNC/PEDOT:PSS materials are displayed in Figure . Polyhedral microcrystals characteristic of DUT-67 were observed for the MOF sample (Figure A,E). In the case of PEDOT:PSS, a compact and homogeneous bulk structure was observed (Figure B,F). The surface of the PEDOT:PSS material contains grains with a good film-forming capability. The CNC/PEDOT:PSS nanocomposite exhibits a uniform network-like wrinkled surface morphology and appears to be rougher than pure PEDOT:PSS (Figure C,G). Interestingly, the surface morphology of CNC/PEDOT:PSS is affected by evenly packed CNCs. Moreover, it is likely that the hierarchical porosity of its microstructure of CNC/PEDOT:PSS may provide a large specific surface area and improve mass transfer. , The MOF/CNC/PEDOT:PSS nanocomposite displays the layer of conductive CNC/PEDOT:PSS formed on the surface of the MOF (Figure D,H) that may be beneficial in providing more electrochemically active sites of the MOF and enhancing the electron transfer and metal ion diffusion in the matrix. Moreover, the MOF/CNC/PEDOT:PSS nanocomposite shows that the DUT-67 MOFs are well cross-linked through the interconnected network of conductive CNC/PEDOT:PSS, which not only served as an effective template for the DUT-67 MOF but also enhanced the electron transfer of the hybrid nanocomposites (Figure S3). Thus, the incorporation of DUT-67 MOF with the CNC/PEDOT:PSS matrix can effectively enhance the electrochemical detection performance of MOF/CNC/PEDOT:PSS/GCE.

3.

Secondary electron micrographs of (A and E) the MOF, (B and F) PEDOT:PSS, (C and G) CNC/PEDOT:PSS, and (D and H) the MOF/CNC/PEDOT:PSS.

3.2. Electrochemical Characterization of MOF/CNC/PEDOT:PSS/GCE

CV and EIS were used as electrochemical characterization techniques to evaluate the electrochemical behavior of the electrodes. Both techniques were performed in a 5 mM [Fe­(CN)₆]^3–/[Fe­(CN)₆]^4– (1:1) solution containing 150 mM KCl. The GCE, PEDOT:PSS/GCE, CNC/PEDOT:PSS/GCE, and MOF/CNC/PEDOT:PSS/GCE were evaluated, as shown in Figure . The resulting cyclic voltammogram shows reversible anodic and cathodic peak currents (I _pa/I _pc = 1). The anodic peak currents (I _pa) were found to be 91 μA for GCE, 115 μA for PEDOT:PSS/GCE, 89 μA for CNC/PEDOT:PSS/GCE, and 169 μA for MOF/CNC/PEDOT:PSS/GCE (Figure A). The peak current of CNC/PEDOT:PSS/GCE decreased after the addition of CNC. The decrease in the peak current is mainly due to the inherently insulating characteristics of the CNCs, resulting in hindered electron transfer on the surface of the electrode. , After the addition of CNC together with MOF and PEDOT:PSS, however, the peak current of MOF/CNC/PEDOT:PSS/GCE was found to be greater than those of GCE, PEDOT:PSS/GCE, and CNC/PEDOT:PSS/GCE, confirming the enhanced electrical conductivity with the hybrid nanocomposite. Although PEDOT:PSS is a major contributor of enhanced electrical conductivity, the MOF/CNC/PEDOT:PSS hybrid nanocomposite’s larger electroactive area and porous structure may also be key factors in the enhanced conductivity. In comparison to PEDOT:PSS, it is possible that the electron transfer kinetics are faster in the network-like matrix of MOF/CNC/PEDOT:PSS. Moreover, the peak-to-peak separation (ΔE _p) values were found to be 0.59 V for GCE, 0.155 V for PEDOT:PSS/GCE, 0.5 V for CNC/PEDOT:PSS/GCE, and 0.12 V for MOF/CNC/PEDOT:PSS/GCE. The ΔE _p decreased for MOF/CNC/PEDOT:PSS/GCE, which may be attributed to improved electron transfer kinetics and an increase in the electroactive surface area. Furthermore, EIS was conducted to further investigate the electrochemical properties of the electrodes. EIS is a nondestructive quantitative method for characterizing the interfacial electrical properties of the surface of electrodes. This method can provide information such as solution and electron transfer resistances and the capacitance of an electrode through an equivalent circuit. , The Nyquist plots of the electrodes were compared and fitted with the equivalent circuit (Figure B) containing the solution resistance (R _s), constant phase element (CPE), charge transfer resistance (R _ct), and Warburg element (Z _w) (inset of Figure B). The Nyquist plots displayed clear semicircular and linear regions, where the diameter of the semicircle corresponded to R _ct. In the presence of the [Fe­(CN)₆]^3–/[Fe­(CN)₆]^4– redox couple, R _ct represents the resistance of the electrons transferred through the GCE surface/modified composite layer/electrolyte interface. GCE displayed a large R _ct of 2250 ± 21.78 Ω, which is a characteristic of the diffusion-limited electron transfer process. The electron transfer resistance of PEDOT:PSS/GCE noticeably decreased to 250 ± 2.45 Ω. The addition of CNCs caused the R _ct to increase to 1300 ± 5.69 Ω. The changes in R _ct indicate that the base electrode GCE becomes successfully modified with the materials. R _ct increases due to a decrease in the electroactive surface area from insulative material. In cooperation with the MOF, the R _ct value decreased to 100 ± 1.97 Ω, which is even lower than that of PEDOT:PSS/GCE. The decrease in the R _ct of MOF/CNC/PEDOT:PSS/GCE demonstrates that the addition of the MOF may increase the hierarchical porosity and improve the electron transfer through the network-like microstructure of the MOF/CNC/PEDOT:PSS hybrid nanocomposite. These results are consistent with the results of the peak current and ΔE _p from CV analysis, which are electrochemical parameters directly related to the rate of electron transfer at the electrode surface. ,

4.

(A) Cyclic voltammograms and (B) Nyquist diagrams of the GCE, PEDOT:PSS/GCE, CNC/PEDOT:PSS/GCE, and MOF/CNC/PEDOT:PSS/GCE. The inset shows the equivalent circuit applied to fit the data.

Moreover, the mass transfer process on MOF/CNC/PEDOT:PSS/GCE was confirmed by performing CV at various scan rates, shown in Figure . Both anodic and cathodic current responses increased linearly and in proportion to the square root of the scan rate (v ^1/2) in the experimental range of 25–175 mV s^–1 in Figure A. This was in accordance with the Randle–Sevick equation:

$$I=(2.69\times {10}^5)n^{3/2}AC{D}^{1/2}{v}^{1/2}$$

where n is the number of transferred electrons (n = 1), A is the electrode’s active surface area (square centimeters), C is the concentration of the electroactive species (moles per cubic centimeter), D is the diffusion coefficient of Fe­(CN)₆ ^3–/4– (6.7 × 10^–7 cm² s^–1), and v is the scan rate (volts per second). As shown in Figure B, the linearity of peak currents versus the square root of the scan rate with correlation coefficients (R ²) of 0.9992 and 0.9945 was achieved for both anodic and cathodic peak currents. The electroactive surface areas of GCE and MOF/CNC/PEDOT:PSS/GCE were calculated to be 0.053 and 0.243 cm², respectively. The linear increase in current peaks indicates that diffusion-controlled mass transfer occurs on MOF/CNC/PEDOT:PSS/GCE. The surface area of the modified GCE increased nearly 5-fold.

5.

(A) Cyclic voltammograms of MOF/CNC/PEDOT:PSS/GCE in a 5 mM [Fe­(CN)₆]^3–/[Fe­(CN)₆]^4– (1:1) solution containing 150 mM KCl at scan rates of 25, 50, 75, 100, 125, and 175 mV s^–1. (B) Plots of the anodic peak current (I _pa) and cathodic peak current (I _pc) over the square root of the scan rate.

To evaluate the detection efficacy of MOF/CNC/PEDOT:PSS/GCE, we compared the SWASV responses of the modified electrodes for the detection of heavy metal ions. The electrochemical responses of 200 μg L^–1 Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in a 0.1 M acetate buffer solution (pH 5) of the different electrodes are shown in Figure A. The oxidation peak currents of MOF/CNC/PEDOT:PSS/GCE were found to be 0.7 μA for Cd²⁺, 3.7 μA for Pb²⁺, 25.0 μA for Cu²⁺, and 13.7 μA for Hg²⁺, which are higher than those of other GCE, PEDOT:PSS/GCE, and CNC/PEDOT:PSS/GCE electrodes. A comparison of the anodic current peaks between GCE and MOF/CNC/PEDOT:PSS/GCE is displayed in Table S2. The stronger electrochemical response of MOF/CNC/PEDOT:PSS/GCE can be ascribed to the synergy between the MOF and CNC/PEDOT:PSS. Thus, MOF/CNC/PEDOT:PSS/GCE provides high conductivity and many electroactive sites for the detection of the heavy metal ions. PEDOT:PSS provides conduction pathways that improve electron transfer between the solution and the working electrode. As is evident with the Cu²⁺ ion data, the cooperative relationship among MOF, CNC, and PEDOT:PSS increases the peak intensity observed by more than 10-fold when compared to that of unmodified GCE. It should be noted that, due to the interference of coexisting metal ions, mutual interference was present for MOF/CNC/PEDOT:PSS/GCE. An additional peak located at −0.45 V can be observed between the Pb²⁺ and Cu²⁺ response signals, which is due to the formation of the Pb–Cu intermetallic compound during the accumulation step. It can also be attributed to the reduced level of binding of heavy metal ions to themselves (Cu to Cu²⁺ and Pb to Pb²⁺). Also, it is likely that competitive interactions between Cd²⁺ and Cu²⁺ or Hg²⁺ may cause the Cd²⁺ anodic peak current to decrease. , When compared to those of GCE, our results indicate that multiple heavy metal ions can be more effectively detected using MOF/CNC/PEDOT:PSS/GCE, due to its increased sensitivity, surface area, and electron transfer kinetics.

6.

(A) Comparison of the SWASV results of GCE, PEDOT:PSS/GCE, CNC/PEDOT:PSS/GCE, and MOF/CNC/PEDOT:PSS/GCE for simultaneous detection of 200 μg/L Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in a 0.1 M acetate buffer solution at pH 5 at an accumulation potential of −1.0 V with an accumulation time of 300 s. (B) Optimization of the pH for MOF/CNC/PEDOT:PSS/GCE in 150 μg L^–1 Cd²⁺, Pb²⁺, and Cu²⁺ and 250 μg L^–1 Hg²⁺ with an accumulation potential of −1.0 V and an accumulation time of 300 s. (C) Accumulation potentials for MOF/CNC/PEDOT:PSS/GCE in 150 μg L^–1 Cd²⁺, Pb²⁺, and Cu²⁺ and 250 μg L^–1 Hg²⁺ in a 0.1 M acetate buffer solution (pH 5) with an accumulation time of 300 s. (D) Accumulation times for MOF/CNC/PEDOT:PSS/GCE in 150 μg L^–1 Cd²⁺, Pb²⁺, and Cu²⁺ and 250 μg L^–1 Hg²⁺ in a 0.1 M acetate buffer solution (pH 5) with an accumulation potential of −1.0 V.

3.3. Optimization of the Experimental Conditions

To achieve the most efficient detection of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ using our MOF/CNC/PEDOT:PSS/GCE composite, the experimental parameters of pH, accumulation potential, and accumulation time were optimized (n = 3). The pH of the aqueous solution is a crucial parameter influencing the anodic peak currents generated by the heavy metal ions on the surface of the electrode. The effect of the pH on the anodic peak currents was examined in the experimental range of 3.0–7.0. The results are shown in Figure B. The anodic peak currents of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ increased when the pH increased from 3.0 to 5.0, and the maximum peak currents occurred at pH 5.0 for Pb²⁺ and Cu²⁺. When the pH exceeded 5.0, the anodic peak current of each ion decreased. A pH range of 3.0–7.0 was targeted because it has been previously demonstrated that at pH <3.0, DUT-67 exhibits a decrease in adsorption of heavy metal ions, including Pb²⁺, due to the protonation of the thiophene. At pH >5.0, the anodic peak current can also decrease due to hydrolysis of the heavy metal ions, which would inhibit accumulation. ,,, Thus, an acetate buffer solution at pH 5.0 was chosen as the optimal condition for electrochemical detection of heavy metals using MOF/CNC/PEDOT:PSS/GCE.

The accumulation potential and time also influence the sensitivity of the modified electrode. The effects of accumulation potential were investigated from −0.6 to −1.1 V. As shown in Figure C, the anodic peak currents increased to −1.0 V for each heavy metal ion. It is likely that the reduction of heavy metal ions is facilitated, which causes the anodic peaks to increase. , Therefore, an accumulation potential of −1.0 V was chosen as the optimal accumulation potential for further experiments. Additionally, the accumulation time affects the amount of metal ions that accumulate on the modified electrode’s surface, and its optimization improves sensitivity. , In this study, the accumulation time was optimized for successful heavy metal ion detection using MOF/CNC/PEDOT:PSS/GCE. The effects of accumulation time on the anodic peak currents of each heavy metal ion individually are shown in Figure D. The peak currents of the heavy metal ions increased from 0 to 300 s. As the accumulation time increased, the peak current of each heavy metal ion increased due to the amount of the metal ions accumulating on the electrode surface. However, the anodic peak currents of Cd²⁺ and Hg²⁺ decreased after 300 s, indicating saturation of MOF/CNC/PEDOT:PSS/GCE. Precipitation of heavy metal ions is also possible, which would decrease anodic peak currents. Thus, an optimal accumulation time of 300 s was employed in further experiments. Overall, the optimized experimental conditions for MOF/CNC/PEDOT:PSS/GCE were a pH of 5, a potential of −1.0 V, and an accumulation time of 300 s.

3.4. Individual Detection of Heavy Metal Ions by MOF/CNC/PEDOT:PSS/GCE

The electrochemical performance of MOF/CNC/PEDOT:PSS/GCE for the determination of individual heavy metal ions, Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, was evaluated by using SWASV under the optimized conditions. The peak currents determined from the anodic scan are displayed in Figure . The linear concentration ranges were 10–210 μg L^–1 for Cd²⁺, 10–210 μg L^–1 for Pb²⁺, 15–85 μg L^–1 for Cu²⁺, and 250–550 μg L^–1 for Hg²⁺ and are displayed in Figure S4. The linear regressions were calculated by plotting the peak currents against the ion concentrations. The correlation coefficients (R ²) of the linear regression were found to be 0.9837 for Cd²⁺, 0.9808 for Pb²⁺, 0.9760 for Cu²⁺, and 0.9713 for Hg²⁺. The sensitivities for each individual ion detected were 0.0651 μA L μg^–1 for Cd²⁺, 0.0078 μA L μg^–1 for Pb²⁺, 0.0515 μA L μg^–1 for Cu²⁺, and 0.0024 μA L μg^–1 for Hg²⁺. The limit of detection (LOD) was obtained with the 3σ LOD calculated from 3 × SD/S, where SD is the standard deviation of the measurements and S is the slope of the calibration plot. The LOD of each individual ion was found to be 0.497 μg L^–1 for Cd²⁺, 5.74 μg L^–1 for Pb²⁺, 0.693 μg L^–1 for Cu²⁺, and 1.33 μg L^–1 for Hg²⁺. The EPA’s MCL values for these ions in drinking water are 5.0 μg L^–1 for Cd²⁺, 15.0 μg L^–1 for Pb²⁺, 1.3 mg L^–1 for Cu²⁺, and 2.0 μg L^–1 for Hg²⁺. ,− The analytical performance of MOF/CNC/PEDOT:PSS/GCE was compared with those of previously reported MOF-based electrochemical sensing platforms for detection of heavy metals (Table ). ,,,,,, The results show that the developed electrochemical sensing platform exhibits a relatively wider linear range and low detection limits, and it is worth noting that the LODs are lower than the standard MCLs set by the EPA with the LODs obtained from individual detection (Table ).

7.

SWASV response of MOF/CNC/PEDOT:PSS/GCE for individual heavy metal ion detection in a 0.1 M acetate buffer solution (pH 5) containing (A) Cd²⁺, (B) Pb²⁺, (C) Cu²⁺, and (D) Hg²⁺ over linear concentration ranges of 10–210 μg L^–1 (Cd²⁺), 5–250 μg L^–1 (Pb²⁺), 15–85 μg L^–1 (Cu²⁺), and 250–550 μg L^–1 (Hg²⁺), respectively.

1. Comparison of the Performance of Various MOF-Based Electrochemical Sensing Platforms for the Detection of Heavy Metals.

electrode	method	analyte	linear range (μg L–1)	limit of detection (μg L–1)	ref
GAs-UiO-66-NH2/GCE	DPSV	Cd2+	1.13–169	1.01
		Pb2+	0.21–414.5	0.21
		Cu2+	0.64–102	0.51
		Hg2+	0.202–442	0.18
Yb-MOF/GCE	SWASV	Cd2+	0–50	3.00
		Pb2+	0–50	1.60
UiO-66-NH2@PANI/GCE	DPV	Cd2+	0.5–600	0.30
Ln-MOF/GCE	SWASV	Cd2+	56.205–224.82	0.19
		Pb2+	103.5–414	0.23
Ni-MOF/GCE	SWASV	Pb2+	103.5–1242	103.50
trGNO/Fc-NH2-UiO-66/GCE	DPASV	Cd2+	1.12–224.82	0.96
		Pb2+	20.7–414	0.12
		Cu2+	6.36–127.1	0.16
MOF/CoFe2O4@C-600/GCE	DPV	Pb2+	0.33–19872	0.30
MOF/CNC/PEDOT:PSS/GCE	SWASV	Cd2+	10–210	0.497	this work
		Pb2+	5–250	5.74
		Cu2+	15–85	0.693
		Hg2+	250–550	1.33

^aAbbreviations: GA, graphene aerosol; Yb, ytterbium; PANI, polyaniline; Ln, lanthanide; Ni, nickel; trGNO, thermally reduced graphene oxide; Fc, ferrocene; CoFe₂O₄, cobalt ferrite; GCE, glassy carbon electrode; DPV, differential pulse voltammetry; DPSV, differential pulse stripping voltammetry; DPASV, differential pulse anodic stripping voltammetry; SWASV, square wave anodic stripping voltammetry.

2. Comparison between the Drinking Water Maximum Contaminant Levels (MCLs) of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in Drinking Water Set by the EPA and the LODs Obtained during Individual and Simultaneous Detections.

condition	Cd2+ (μg L–1)	Pb2+ (μg L–1)	Cu2+ (μg L–1)	Hg2+ (μg L–1)
EPA	5	15	1330	2
individual	0.497	5.74	0.693	1.33
simultaneous	2.5	1.78	0.226	0.294

3.5. Simultaneous Detection of Heavy Metal Ions by MOF/CNC/PEDOT:PSS/GCE

In actual environmental samples, various metal ions can coexist. Mutual interference among each of the metal ions poses a great challenge for the detection and calibration of the target metal ions with high sensitivity. The efficacy of MOF/CNC/PEDOT:PSS/GCE during the simultaneous detection of the target analytes was evaluated. The simultaneous detection of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ was performed in the potential range of −1.0 to 0.5 V by increasing the concentrations of different heavy metal ions synchronously under the optimized experimental conditions (Figure A). As shown in Figure S5, the linear ranges were 20–163 μg L^–1 for Cd²⁺, 20–163 μg L^–1 for Pb²⁺, 20–84 μg L^–1 for Cu²⁺, and 20–112 μg L^–1 for Hg²⁺. The R ² values of the linear regression were 0.9104 for Cd²⁺, 0.9515 for Pb²⁺, 0.9343 for Cu²⁺, and 0.9108 for Hg²⁺. The sensitivities were found to be 0.0139 μA L μg^–1 for Cd²⁺, 0.0257 μA L μg^–1 for Pb²⁺, 0.1581 μA L μg^–1 for Cu²⁺, and 0.0889 μA L μg^–1 for Hg²⁺. The LODs for each ion were found to be 2.5 μg L^–1 for Cd²⁺, 1.78 μg L^–1 for Pb²⁺, 0.226 μg L^–1 for Cu²⁺, and 0.294 μg L^–1 for Hg²⁺. Compared to individual detection, there was noticeable intermetallic interference during the simultaneous detection of coexisting Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺. The results showed narrower linear ranges, lower R ² values, and higher LODs. However, there were noticeable increases in the sensitivities from 0.0078 to 0.0257 μA L μg^–1 for Pb²⁺, from 0.0515 to 0.1581 μA L μg^–1 for Cu²⁺, and from 0.0024 to 0.0889 μA L μg^–1 for Hg²⁺, with the sensitivity decreasing from 0.0651 to 0.0139 μA L μg^–1 for Cd²⁺. The suppression of the sensing performance for simultaneous detection may be due to the competitive deposition and formation of the intermetallic compound of Pb–Cu on active sites of the modified electrode. ,,, Despite the interaction among the four heavy metal ions, the current peaks were clearly separated in the presence of multiple heavy metal ions, and excellent linear ranges and high sensitivity were exhibited, demonstrating that MOF/CNC/PEDOT:PSS/GCE could serve as a promising sensing platform for the simultaneous detection of multiple heavy metal ions. A comparison of the standard MCLs set by the EPA with the LODs obtained from simultaneous detection showed that MOF/CNC/PEDOT:PSS/GCE is capable of detecting coexisting Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, simultaneously, with excellent electrochemical performance (Table ).

8.

(A) SWASV responses of MOF/CNC/PEDOT:PSS/GCE for simultaneous detection of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in the potential range of −1.0 to 0.5 V in a 0.1 M acetate buffer solution (pH 5). The ranges for each ion were 20–163 μg L^–1 for Cd²⁺, 20–163 μg L^–1 for Pb²⁺, 20–84 μg L^–1 for Cu²⁺, and 20–120 μg L^–1 for Hg²⁺. (B) Reproducibility and (C) stability of MOF/CNC/PEDOT:PSS/GCE in the simultaneous detection at 150 μg L^–1 Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ under the optimized experimental conditions (n = 4). (D) SWASV results for MOF/CNC/PEDOT:PSS/GCE at 100 μg L^–1 Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ under the optimized experimental conditions with the addition of each interfering ion (1000 μg L^–1).

3.6. Reproducibility, Stability, and Anti-interference Tests

For the practical application of MOF/CNC/PEDOT:PSS/GCE, reproducibility and stability were evaluated by the relative standard deviation (RSD). The reproducibility of MOF/CNC/PEDOT:PSS/GCE was investigated with six independently prepared electrodes for the four target heavy metal ions in the simultaneous detection of 150 μg L^–1 Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ under the optimized experimental conditions (Figure B). The SWASV responses for Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ were reproducible with RSD values of 4.25%, 5.16%, 7.93%, and 7.88%, respectively. The stability of MOF/CNC/PEDOT:PSS/GCE was further evaluated for 10 days (Figure C). The oxidative peak currents of MOF/CNC/PEDOT:PSS/GCE were retained at 90.6% for Cd²⁺, 92.3% for Pb²⁺, 87.6% for Cu²⁺, and 91.7% for Hg²⁺ of their initial values at the end of 10 days. These results demonstrate the acceptable reproducibility and stability of MOF/CNC/PEDOT:PSS/GCE. In addition, the presence of interfering species in samples may inhibit the accurate detection of target analytes. To investigate how the presence of nontarget ions interferes with the detection of the target heavy metal ions, an interference study was conducted with a solution that contained 100 μg L^–1 Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ with concentrations of the selected nontarget ions, Na⁺, K⁺, Ca²⁺, Mg²⁺, Zn²⁺, Mn²⁺, Fe²⁺, Al³⁺, Co²⁺, Ni²⁺, and Fe³⁺, each 10-fold greater in concentration (Figure D). The addition of the nontarget species showed a minimal effect on the simultaneous detection of the target analytes. No obvious interference from the selected nontarget ions was observed in the potential range of −1.0 to 0.5 V. The inability of the MOF/CNC/PEDOT:PSS hybrid nanocomposite to detect Fe²⁺ is likely due to the fact that thiophene or thiophene-containing ligands coordinating to Fe²⁺ are scarce. They are often limited to η4-coordinating motifs through the two CC π-bonds. , Additionally, it has been shown that Fe²⁺ can prefer solvato ligands over thiophene coordination, even if the thiophene moiety is incorporated into a ligand already bound to Fe²⁺, overcoming the formation of a thiophene-containing chelate. Thus, the results indicate that MOF/CNC/PEDOT:PSS/GCE provides excellent anti-interference against nontarget species and possesses selectivity toward Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ ions during simultaneous detection.

3.7. Real Sample Analysis

The applicability of MOF/CNC/PEDOT:PSS/GCE for the simultaneous detection of trace amounts of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ was assessed in real samples under the optimized experimental conditions. The samples included tap water, Mahoning River water, East Palestine Lake water, and soil. Each sample was filtered with a 25 mm sterile syringe filter (0.2 μm of cellulose acetate, VWR) to remove the suspended solids. Then, solutions of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ at 100 μg L^–1 were spiked into each real sample solution for recovery testing in 0.1 M acetate buffer. The analytical results from the samples using MOF/CNC/PEDOT:PSS/GCE showed no heavy metal ions were detected in the samples before spiking the samples (Table S3), indicating that the target heavy metals in the solution either were not detectable or lower than the LOD. The highest RSD among all of the measurements was 9.70%, and the recoveries were 88.63–91.94% for Cd²⁺, 81.82–105.35% for Pb²⁺, 92.03–103.33% for Cu²⁺, and 84.31–102.40% for Hg²⁺. All of the results indicated that the developed MOF/CNC/PEDOT:PSS/GCE composite provides a platform that is accurate for the simultaneous detection of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in real samples. Despite the successful detection of trace amounts of Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺, it should be noted that the mutual interference may be present as we observed the shoulder peak of the Pb–Cu intermetallic compound formed during the simultaneous detection. Therefore, the development of functional material layers and novel designs for electrode modifications is still necessary to reduce the interference.

4. Conclusion

In summary, we have developed a novel hybrid nanocomposite based on the MOF DUT-67 and electroconductive CNC/PEDOT:PSS for the detection of multiple heavy metal ions (Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺), individually and simultaneously. The modified electrode (MOF/CNC/PEDOT:PSS/GCE) was successfully fabricated by using a simple and effective drop-casting method. The hybrid nanocomposite composed of a conducting polymer and cellulose nanocrystals not only served as an effective template for the DUT-67 MOF but also enhanced the electron transfer of the hybrid nanocomposite through the interconnected network of conductive CNC/PEDOT:PSS. DUT-67 functioned to bind the heavy metal analytes of interest. The developed MOF/CNC/PEDOT:PSS/GCE composite was successfully applied to detect Cd²⁺, Pb²⁺, Cu²⁺, and Hg²⁺ in aqueous solutions. The LODs for individual detection of target ions were 0.497 μg L^–1 for Cd²⁺, 5.74 μg L^–1 for Pb²⁺, 0.693 μg L^–1 for Cu²⁺, and 1.33 μg L^–1 for Hg²⁺, and those for simultaneous detection were 2.5 μg L^–1 for Cd²⁺, 1.78 μg L^–1 for Pb²⁺, 0.226 μg L^–1 for Cu²⁺, and 0.294 μg L^–1 for Hg²⁺. Due to its enhanced electrochemical properties, such as an increase in the electroactive surface area, the developed electrode showed a low detection limit, a wide dynamic range, and good reproducibility. Furthermore, the developed electrode displayed excellent selectivity in the presence of interfering ions and in real sample analysis. Practical applications in real samples also demonstrated that the developed method can be used to simultaneously detect multiple metal ions with high accuracy and reliability. The highest RSD among all of the measurements was 9.70%, and the recoveries were 88.63–91.94% for Cd²⁺, 81.82–105.35% for Pb²⁺, 92.03–103.33% for Cu²⁺, and 84.31–102.40% for Hg²⁺. The results suggest that this hybrid nanocomposite has great potential to be used as an alternative sensing platform for the electrochemical detection of multiple heavy metal ions in practical settings. While the focus of the present study was to develop a novel hybrid nanocomposite for electrochemical detection of heavy metal ions, future research will be directed toward the application of the hybrid nanocomposite as a flexible electrode and biohybrid materials. It may be important to investigate how the hybrid nanocomposite functions as a mechanically stable and electrochemically effective template for wearable bioelectronics.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.langmuir.5c00611.

• Tables S1–S3 and Figures S1–S5 (PDF)

D.T.G. and B.-W.P. conceptualized the study. J.M.M. and D.T.G. synthesized and characterized the MOF DUT-67. K.P., K.D., P.D., C.V.S., and B.-W.P. designed, fabricated, and characterized the resulting electrochemical sensing platform and performed the experiments. All authors analyzed the data and wrote the manuscript.

The authors declare no competing financial interest.