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Individual and simultaneous electrochemical detection of hazardous Pb(II) and Cu(II) trace ions using Ni-doped ZIF-67 framework

This study presents a highly sensitive and selective electrochemical sensor based on a nickel-doped ZIF-67 framework that enables the ultrasensitive individual and simultaneous detection of trace Pb(II) and Cu(II) ions through enhanced electron transfer and synergistic catalytic effects.

Original authors: Kamlesh B. Deore, Nilkanth B. Devkar, Harshal S. Gavale, Mahendra D. Shirsat

Published 2026-10-15

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CC BY 4.0

Original authors: Kamlesh B. Deore, Nilkanth B. Devkar, Harshal S. Gavale, Mahendra D. Shirsat

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Water is the foundation of life, yet it remains vulnerable to invisible threats that accumulate silently over time. Among the most persistent of these threats are heavy metal ions, atoms of elements like lead and copper that, once released into the environment, do not break down or disappear. Instead, they linger in soil and water, eventually finding their way into the food chain and posing serious risks to human health, including damage to the nervous system and kidneys. Detecting these contaminants at the tiny concentrations where they first become dangerous is a critical challenge for public safety. Traditional laboratory methods can identify these metals with high precision, but they often require expensive equipment and skilled operators, making them difficult to use for quick, on-site testing. This has driven scientists to develop simpler, more sensitive tools that can act as early warning systems, capable of spotting trace amounts of pollution before it spreads.

In a recent study, researchers from India have engineered a new type of sensor designed to catch two of the most common hazardous metals, lead and copper, with remarkable sensitivity. The team focused on a class of materials known as metal-organic frameworks, which are essentially sponge-like structures built from metal atoms connected by organic links. These materials are prized for their vast internal surface area, offering countless spots where contaminants can stick. However, a specific framework made of cobalt, known as ZIF-67, while structurally excellent, suffers from poor electrical conductivity, which limits its ability to send a clear signal when it captures a metal ion. To solve this, the researchers introduced nickel atoms into the cobalt framework, creating a hybrid material that combines the porous structure of the original sponge with the improved electrical properties of the new metal.

The team synthesized this nickel-doped material and applied a thin layer of it onto a standard glassy carbon electrode, a common tool in electrochemical testing. They then submerged this modified sensor into water samples containing the target metals. When an electrical current was applied, the sensor attracted the lead and copper ions, holding them on its surface. By measuring the electrical response as these ions were released back into the solution, the sensor produced distinct signals that indicated exactly how much metal was present. The results showed that the nickel-doped material performed significantly better than the original cobalt-only version. It could detect lead and copper at concentrations as low as less than one billionth of a mole per liter, a level of sensitivity that allows for the identification of pollutants long before they reach dangerous levels.

What makes this development particularly notable is its ability to handle both metals at the same time. In many real-world scenarios, lead and copper appear together in water sources, and distinguishing between them can be difficult for standard sensors. The new sensor, however, produced two separate and clear signals for the two metals, allowing them to be measured simultaneously without confusion. The researchers found that the presence of nickel not only improved the speed at which electrons moved through the material but also created a synergistic effect that enhanced the sensor's ability to grab onto the specific metal ions. This combination of high speed, high sensitivity, and the ability to distinguish between different contaminants suggests that this material could serve as a robust platform for monitoring water quality in the field.

The study also confirmed that the sensor is reliable over time. When tested repeatedly, the device produced consistent results with very little variation, and it maintained its performance over a period of thirty days. This stability is crucial for any tool intended for practical use, as it ensures that the readings remain trustworthy day after day. While the researchers noted that the simultaneous detection of the two metals showed slightly more complexity than detecting them individually, the overall performance remained strong and well within the limits required for environmental safety standards. By successfully integrating nickel into the framework, the team has overcome the electrical limitations of the original material, creating a sensor that is not only highly sensitive but also capable of providing clear, actionable data for protecting human health and aquatic ecosystems.

Technical Summary: Individual and Simultaneous Electrochemical Detection of Pb(II) and Cu(II) Using Ni-Doped ZIF-67

Problem Statement
The presence of hazardous heavy metal ions (HMIs), specifically lead [Pb(II)] and copper [Cu(II)], poses significant risks to human health and aquatic ecosystems due to their environmental persistence and bioaccumulation. While traditional analytical techniques such as ICP-OES, AAS, and ICP-MS offer high sensitivity, they are often expensive, require sophisticated instrumentation, and lack portability for real-time, on-site monitoring. Electrochemical sensing offers a cost-effective and portable alternative; however, standard glassy carbon electrodes (GCEs) suffer from poor sensitivity, low charge transfer rates, and limited selectivity. Furthermore, pristine Metal-Organic Frameworks (MOFs), such as Zeolitic Imidazolate Framework-67 (ZIF-67), possess high surface areas but are hindered by intrinsically poor electrical conductivity, limiting their effectiveness in trace-level sensing applications.

Methodology
To address these limitations, the authors developed a nickel-doped ZIF-67 (Ni_ZIF-67) composite as an advanced electrochemical sensing platform.

Synthesis: Ni_ZIF-67 was synthesized via a solvothermal method by incorporating nickel nitrate hexahydrate into the standard ZIF-67 synthesis (using cobalt nitrate and 2-methylimidazole in methanol). The resulting composite was characterized using Powder X-ray Diffraction (P-XRD), Fourier-Transform Infrared Spectroscopy (FT-IR), Brunauer–Emmett–Teller (BET) analysis, Field Emission Scanning Electron Microscopy (FE-SEM) with Energy Dispersive X-ray Spectroscopy (EDS), and Atomic Force Microscopy (AFM).
Electrode Fabrication: A glassy carbon electrode (GCE) was modified by drop-casting a suspension of the synthesized Ni_ZIF-67 material.
Electrochemical Analysis: The sensing performance was evaluated using a standard three-electrode system in a 0.2 M acetate buffer solution (pH 5). Cyclic Voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) were used to assess electron transfer kinetics. The primary detection method was Differential Pulse Voltammetry (DPV) for both individual and simultaneous determination of Pb(II) and Cu(II).

Key Contributions and Results
The study demonstrates that the strategic incorporation of nickel into the ZIF-67 framework significantly enhances electrochemical performance through synergistic interactions.

Material Characterization: P-XRD and FT-IR confirmed that the ZIF-67 crystalline structure was preserved upon Ni doping, with only slight lattice distortions. FE-SEM and AFM revealed that the Ni_ZIF-67 retained the characteristic rhombic dodecahedral morphology of ZIF-67 but exhibited increased surface roughness and granular texture, indicative of successful Ni incorporation. BET analysis showed a high surface area of approximately 588 m²/g.
Electrochemical Enhancement: EIS and CV results indicated that Ni_ZIF-67 possessed significantly lower charge transfer resistance ( $R_{ct}$ ) and higher peak current densities compared to pristine ZIF-67, attributed to improved electron transport facilitated by the Ni species.
Sensing Performance:
- Selectivity: The Ni_ZIF-67/GCE sensor exhibited high selectivity for Pb(II) and Cu(II) over interfering ions such as Hg(II), Fe(II), Ni(II), and Cd(II).
- Individual Detection: The sensor achieved ultra-low limits of detection (LOD) of 0.96 nM for Pb(II) and 0.79 nM for Cu(II) with linear ranges of 1–9 nM and 1–7 nM, respectively. Sensitivities were recorded at 43.5 µA/M for Pb(II) and 7.66 µA/M for Cu(II).
- Simultaneous Detection: The sensor successfully resolved distinct stripping peaks at -0.504 V (Pb) and 0.176 V (Cu). The simultaneous LODs were 1.3 nM for Pb(II) and 1.6 nM for Cu(II). Notably, the sensitivity for Cu(II) increased during simultaneous detection (17.09 µA/M), which the authors attribute to the potential formation of a Ni–Cu alloy on the electrode surface.
Stability and Reliability: The sensor demonstrated excellent repeatability (RSD < 2%), reproducibility (RSD < 6%), and long-term stability over 30 days.

Significance and Claims
The paper claims that the Ni_ZIF-67/GCE sensor represents a robust and reliable platform for the ultrasensitive monitoring of hazardous heavy metal contaminants. The authors attribute the superior performance to the synergistic interaction between Ni species and the porous ZIF-67 matrix, which promotes efficient ion adsorption, accelerates electron transfer, and improves electrocatalytic activity.

The achieved detection limits surpass regulatory thresholds set by the U.S. Environmental Protection Agency (US-EPA) and the World Health Organization (WHO). The study positions this work as a significant advancement in electrochemical sensing, offering a cost-effective, sensitive, and stable solution for real-time and on-site environmental monitoring of Pb(II) and Cu(II) without the need for complex instrumentation. The authors conclude that the material's high porosity, efficient ion chelation, and superior electron transfer capabilities make it a promising candidate for safeguarding public health and ecological systems against heavy metal pollution.

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