Electrochemical performance of a three-layer electrode based on Bi nanoparticles, multi-walled carbon nanotube composites for simultaneous Hg(II) and Cu(II) detection

Exploring the synchronized effect of MWCNT/X-manganate (X-Cu, Zn) nanocomposite for the sensitive and selective electrochemical detection of Hg(II) and Pb(II) in water

The presence of heavy metal ions in the environment is a long-lasting problem that requires the simultaneous detection of Hg(II) and Pb(II) which is both vital and challenging. This present study examines a simplified and effective approach for synthesizing multi-walled carbon nanotube-copper manganese oxide (MWCNT-CuMn2O4) and multi-walled carbon nanotube-zinc manganese oxide (MWCNT-ZnMn2O4) nanocomposites for electrochemical detection of heavy metal ions. The nanocomposites MWCNT-CuMn2O4 and MWCNT-ZnMn2O4 exceptional electrochemical performance was evaluated using Square Wave Anodic Stripping Voltammetry (SWASV). The fabricated MWCNT-ZnMn2O4 demonstrated lower values of Electrochemical Impedance Spectroscopy (EIS) with charge transfer resistance (Rct) of approximately 34.13 Ω. Remarkably, the MWCNT-ZnMn2O4 electrochemical sensor exhibited the widest linear ranges of 0.5-10 μM with sensitive detection limits (0.011 μM for Hg(II) and 0.014 μM for Pb(II)). Interestingly, the MWCNT-ZnMn2O4 sensor showed excellent capability in detecting Hg(II) and Pb(II) in real water samples with a recovery percentage of 94.1% and 91.3%. Overall, the MWCNT-ZnMn2O4 modified GCE showcased superior selectivity, sensitivity, reproducibility, stability, and repeatability.

Recent advance of nanomaterials modified electrochemical sensors in the detection of heavy metal ions in food and water

As one of the main pollutants, heavy metal ions can accumulate in the human body and cause a cascade of damage. Electrochemical sensors provide great prospects for tracing heavy metal ions because of their properties of high sensitivity, low detection limits and fast response. Electrode surface modification materials play a key role in enhancing the performance of electrochemical sensors. Herein, we summarize in detail the recent work on electrochemical sensors modified by carbon nanomaterials (graphene and its derivatives, carbon nanofibers and carbon nanotubes), metal nanomaterials (gold, silver, bismuth and iron), complexes (MOFs, ZIFs and MXenes) and their composites for the detection of heavy metal ions (mainly include Cd(II), Hg(II), Pb(II), As(III), Cu(II) and Zn(II)) in food and water. The synthetic strategies, mechanisms, innovations, advantages, challenges and prospects of various electrode modification nanomaterials for the detection of heavy metal ions in food and water are discussed.

In-situ integrated electrodes of FeM-MIL-88/CP for simultaneous ultra-sensitive detection of dopamine and acetaminophen based on crystal engineering strategy

Designing and exploiting integrated electrodes is the current inevitable trend to realize the sustainable development of electrochemical sensors. In this work, a series of integrated electrodes prepared by in situ growing the second metal ion-modulated FeM-MIL-88 (M = Mn, Co and Ni) on carbon paper (CP) (FeM-MIL-88/CP) were constructed as the electrochemical sensing platforms for the simultaneous detection of dopamine (DA) and acetaminophen (AC). Among them, FeMn-MIL-88/CP exhibited the best sensing behaviors and achieved the trace detection for DA and AC owing to synergistic catalysis between Fe3+, Mn2+ and CP. The electrochemical sensor based on FeMn-MIL-88/CP showed ultra-high sensitivities of 2.85 and 7.46 μA μM-1 cm-2 and extremely low detection limits of 0.082 and 0.015 μM for DA and AC, respectively. The FeMn-MIL-88/CP also exhibited outstanding anti-interference ability, repeatability and stability, and satisfactory results were also obtained in the detection of actual samples. The mechanism of Mn2+ modulation on the electrocatalytic activity of FeMn-MIL-88/CP towards DA and AC was revealed for the first time through the density functional theory (DFT) calculations. Good adsorption energy and rapid electron transfer worked synergistically to improve the sensing performances of DA and AC. This work not only provided a high-performance integrated electrode for the sensing field, but also demonstrated the influencing factors of electrochemical sensing at the molecular levels, laying a theoretical foundation for the sustainable development of subsequent electrochemical sensing.

Bi-MOF-Derived Carbon Wrapped Bi Nanoparticles Assembly on Flexible Graphene Paper Electrode for Electrochemical Sensing of Multiple Heavy Metal Ions

The development of nanohybrid with high electrocatalytic activity is of great significance for electrochemical sensing applications. In this work, we develop a novel and facile method to prepare a high-performance flexible nanohybrid paper electrode, based on nitrogen-doped carbon (NC) wrapped Bi nanoparticles (Bi-NPs) assembly derived from Bi-MOF, which are decorated on a flexible and freestanding graphene paper (GP) electrode. The as-obtained Bi-NPs encapsulated by an NC layer are uniform, and the active sites are increased by introducing a nitrogen source while preparing Bi-MOF. Owing to the synergistic effect between the high conductivity of GP electrode and the highly efficient electrocatalytic activity of Bi-NPs, the NC wrapped Bi-NPs (Bi-NPs@NC) modified GP (Bi-NPs@NC/GP) electrode possesses high electrochemically active area, rapid electron-transfer capability, and good electrochemical stability. To demonstrate its outstanding functionality, the Bi-NPs@NC/GP electrode has been integrated into a handheld electrochemical sensor for detecting heavy metal ions. The result shows that Zn²⁺, Cd²⁺, and Pb²⁺ can be detected with extremely low detection limits, wide linear range, high sensitivity, as well as good selectivity. Furthermore, it demonstrates outstanding electrochemical sensing performance in the simultaneous detection of Zn²⁺, Cd²⁺, and Pb²⁺. Finally, the proposed electrochemical sensor has achieved excellent repeatability, reproducibility, stability, and reliability in measuring real water samples, which will have great potential in advanced applications in environmental systems.

A Time-Division Multiplexing Multi-Channel Micro-Electrochemical Workstation with Carbon-Based Material Electrodes for Online L-Trosine Detection

In the background of the rapid development of artificial intelligence, big data, IoT, 5G/6G, and other technologies, electrochemical sensors pose higher requirements for high-throughput detection. In this study, we developed a workstation with up to 10 channels, which supports both parallel signal stimulation and online electrochemical analysis functions. The platform was wired to a highly integrated Bluetooth chip used for wireless data transmission and can be visualized on a smartphone. We used this electrochemical test platform with carbon-graphene oxide/screen-printed carbon electrodes (CB-GO/SPCE) for the online analysis of L-tyrosine (Tyr), and the electrochemical performance and stability of the electrodes were examined by differential pulse voltammetry (DPV). The CB-GO-based screen-printed array electrodes with a multichannel electrochemical platform for Tyr detection showed a low detection limit (20 μM), good interference immunity, and 10-day stability in the range of 20-200 μM. This convenient electrochemical analytical device enables high-throughput detection and has good economic benefits that can contribute to the improvement of the accuracy of electrochemical analysis and the popularization of electrochemical detection methods in a wide range of fields.

Simultaneous Sensing of Cd(II), Pb(II), and Cu(II) Using Gold Nanoparticle-Modified APTES-Functionalized Indium Tin Oxide Electrode: Effect of APTES Concentration

In this work, indium tin oxide (ITO) electrodes were functionalized with varying 3-aminopropyltriethoxysilane (APTES) concentration percentages (0.5, 0.75, 1.0, and 2.0 wt %) to obtain the optimum conditions for the assembly of the as-synthesized gold nanoparticles (AuNPs). The AuNP coverage, wettability, and electrochemical performance of the modified electrodes were evaluated. The AuNP/0.75% APTES-ITO-modified electrode exhibited uniform coverage of AuNPs and high electrochemical performance for the simultaneous detection of Cd(II), Pb(II), and Cu(II) ions. Under the optimum conditions, the AuNP/0.75% APTES-ITO-modified electrode showed a linear detection range of 5-120 ppb and limit of detection of 0.73, 0.90, and 0.49 ppb for the simultaneous detection of Cd(II), Pb(II), and Cu(II) ions, respectively, via square wave anodic stripping voltammetry. The modified electrode demonstrated good anti-interference toward other heavy metal ions, good reproducibility, and suitability for application in environmental sample analysis.

4-Mercaptopyridine-Modified Sensor for the Sensitive Electrochemical Detection of Mercury Ions

As a highly toxic heavy metal ion, mercury ion (Hg²⁺) pollution has caused serious harm to the environment and human health. In this paper, 4-mercaptopyridine (4-MPY) was selected as the sensing material and decorated on the surface of a gold electrode. Trace Hg²⁺ could be detected by both differential pulse voltammetry (DPV) and electrochemical impedance spectroscopy (EIS) methods. The proposed sensor displayed a wide detection range from 0.01 μg/L to 500 μg/L with a low limit of detection (LOD) of 0.002 μg/L by EIS measurements. Combined with molecular simulations and electrochemical analyses, the chelating mechanism between Hg²⁺ and 4-MPY was explored. Through the analysis of binding energy (BE) values and stability constants, 4-MPY showed an excellent selectivity for Hg²⁺. In the presence of Hg²⁺, the coordination of Hg²⁺ with the pyridine nitrogen of 4-MPY was generated at the sensing region, which caused a change in the electrochemical activity of the electrode surface. Due to the strong specific binding capability, the proposed sensor featured excellent selectivity and an anti-interference capability. Furthermore, the practicality of the sensor for Hg²⁺ detection was validated with the samples of tap water and pond water, which demonstrated its potential application for on-site environmental detection.

Novel Amperometric Mercury-Selective Sensor Based on Organic Chelator Ionophore

A novel amperometric sensor for the direct determination of toxic mercury ions, Hg²⁺, based on the organic chelator ionophore N, N di (2-hydroxy-5-[(4-nitrophenyl)diazenyl]benzaldehyde) benzene-1,2-diamine (NDBD), and multiwalled carbon nanotubes (MWCNT) immobilized on a glassy carbon electrode surface was developed. The parameters influencing sensor performance including the ionophore concentration, the applied potential, and electrolyte pH were optimized. The sensor response to Hg²⁺ was linear between 1-25 µM with a limit of detection of 60 nM. Interferences from other heavy metal ions were evaluated and the sensor showed excellent selectivity towards Hg²⁺. The method was successfully applied to the determination of mercury ions in milk and water samples.

Core-shell architectured NH2-UiO-66@ZIF-8/multi-walled carbon nanotubes nanocomposite-based sensitive electrochemical sensor towards simultaneous determination of Pb2+ and Cu2

Amino functionalized zirconium-based metal-organic framework (NH₂-UiO-66) and zinc-based zeolitic imidazolate framework (ZIF-8) were integrated to develop a core-shell architectured hybrid material (NH₂-UiO-66@ZIF-8, NU66@Z8). The morphology and structure evolutions of core-shell NU6@Z8 were investigated by FE-SEM, XRD, FTIR, and XPS. The NU66@Z8 combined with carboxylated multi-walled carbon nanotubes (CMWCNT) was deposited on a glassy carbon electrode (GCE) for fabricating an electrochemical platform towards detecting Pb²⁺ and Cu²⁺. The NU66@Z8/CMWCNT/GCE revealed significantly improved electrochemical performance for determination of Pb²⁺ and Cu²⁺ compared with the individual components, which can be attributed to the strong adsorption capacity, unique core-shell structure, and large electrochemical active surface area of NU66@Z8/CMWCNT. Under the optimal conditions, the developed sensor exhibited excellent sensing capability with a low limit of detection (Pb²⁺,1 nM; Cu²⁺, 10 nM) and a wide determination range (Pb²⁺,0.003-70 μM; Cu²⁺, 0.03-50 μM). The sensor showed high selectivity towards common interfering ions and good repeatability. The real sample recoveries of proposed sensor were in the range 95.0-103% for Pb²⁺ (RSD ≤ 5.3%) and 94.2-106% for Cu²⁺ (RSD ≤ 5.9%), suggesting that the NU66@Z8/CMWCNT is suitable for examining trace heavy metals in natural environment.

N and P co-doped MXenes nanoribbons for electrodeposition-free stripping analysis of Cu(II) and Hg(II)

The present electrochemical stripping analysis (ESA) for multiple heavy metal ions (HMI) generally requires an electrodeposition process at a very low potential below -1.0 V, which inevitably makes the sensing procedures more complex, inefficient and power-wasting. Meanwhile, the emerging MXenes rising-star materials have been studied in various fields recently. While there are only few reports focusing on the heteroatom doping of MXenes, especially no doping-MXenes for electroanalysis. Based on these issues, a novel multifunctional heteroatoms-doped MXenes nanomaterial, N and P co-doped Ti₃C₂T_x MXenes nanoribbons (N,P-Ti₃C₂T_xR), was prepared herein for the first time, and then N,P-Ti₃C₂T_xR was used as electrode material to propose an electrodeposition-free ESA strategy for multiple HMI (Cu²⁺, Hg²⁺). Owing to the unique spontaneous adsorption and reducing capacities of N,P-Ti₃C₂T_xR towards Cu²⁺ and Hg²⁺ coupled with the excellent sensing performances, Cu²⁺ and Hg²⁺ can undergo self-reduction to be preconcentrated on N,P-Ti₃C₂T_xR surface with the form of Cu⁰ and Hg⁰, thus a simple and ultrasensitive electrodeposition-free ESA platform was developed successfully for the simultaneous detection of Cu²⁺and Hg²⁺. This work opened a new pathway for the detection for multiple HMI and the preparation/application of heteroatoms doping MXenes.

Electrical and Electrochemical Sensors Based on Carbon Nanotubes for the Monitoring of Chemicals in Water-A Review

Carbon nanotubes (CNTs) combine high electrical conductivity with high surface area and chemical stability, which makes them very promising for chemical sensing. While water quality monitoring has particularly strong societal and environmental impacts, a lot of critical sensing needs remain unmet by commercial technologies. In the present review, we show across 20 water monitoring analytes and 90 references that carbon nanotube-based electrochemical sensors, chemistors and field-effect transistors (chemFET) can meet these needs. A set of 126 additional references provide context and supporting information. After introducing water quality monitoring challenges, the general operation and fabrication principles of CNT water quality sensors are summarized. They are sorted by target analytes (pH, micronutrients and metal ions, nitrogen, hardness, dissolved oxygen, disinfectants, sulfur and miscellaneous) and compared in terms of performances (limit of detection, sensitivity and detection range) and functionalization strategies. For each analyte, the references with best performances are discussed. Overall, the most frequently investigated analytes are H+ (pH) and lead (with 18% of references each), then cadmium (14%) and nitrite (11%). Micronutrients and toxic metals cover 40% of all references. Electrochemical sensors (73%) have been more investigated than chemistors (14%) or FETs (12%). Limits of detection in the ppt range have been reached, for instance Cu(II) detection with a liquid-gated chemFET using SWCNT functionalized with peptide-enhanced polyaniline or Pb(II) detection with stripping voltammetry using MWCNT functionalized with ionic liquid-dithizone based bucky-gel. The large majority of reports address functionalized CNTs (82%) instead of pristine or carboxyl-functionalized CNTs. For analytes where comparison is possible, FET-based and electrochemical transduction yield better performances than chemistors (Cu(II), Hg(II), Ca(II), H2O2); non-functionalized CNTs may yield better performances than functionalized ones (Zn(II), pH and chlorine).

Recent Advancement in Disposable Electrode Modified with Nanomaterials for Electrochemical Heavy Metal Sensors

Heavy metal pollution has gained global attention due to its high toxicity and non-biodegradability, even at a low level of exposure. Therefore, the development of a disposable electrode that is sensitive, simple, portable, rapid, and cost-effective as the sensor platform in electrochemical heavy metal detection is vital. Disposable electrodes have been modified with nanomaterials so that excellent electrochemical properties can be obtained. This review highlights the recent progress in the development of numerous types of disposable electrodes modified with nanomaterials for electrochemical heavy metal detection. The disposable electrodes made from carbon-based, glass-based, and paper-based electrodes are reviewed. In particular, the analytical performance, fabrication technique, and integration design of disposable electrodes modified with metal (such as gold, tin and bismuth), carbon (such as carbon nanotube and graphene), and metal oxide (such as iron oxide and zinc oxide) nanomaterials are summarized. In addition, the role of the nanomaterials in improving the electrochemical performance of the modified disposable electrodes is discussed. Finally, the current challenges and future prospect of the disposable electrode modified with nanomaterials are summarized.

In situ detection of heavy metal ions in sewage with screen-printed electrode-based portable electrochemical sensors

The rapid development of industrial technologies continuously increases the heavy metal pollution of water resources. Recently, portable electrochemical analysis-based devices for detecting heavy metal ions have attracted much attention due to their excellent performance and low fabrication costs. However, it has proven difficult to accommodate complex testing needs in a cost-effective manner. To address these limitations, we propose a new system for the in situ detection of heavy metals in wastewater using an organic light-emitting diode-based panel to display data in real time and Bluetooth to transmit data to a smartphone for rapid analysis. The fabricated device integrates an in situ signal analysis circuit, a Bluetooth chip, a photocured 3D-printed shell, and an electrode sleeve interface. In addition, a fully screen-printed functional electrode plate containing chitosan/PANi-Bi nanoparticle@graphene oxide multi-walled carbon nanotubes is utilized for the rapid detection of heavy metal ions. This device can perform wireless data transmission and analysis and in situ signal acquisition and processing. The sensor exhibits a high sensitivity (Hg²⁺: 88.34 μA ppm^-1 cm^-2; Cu²⁺: 0.956 μA ppm^-1 cm^-2), low limit of detection (Hg²⁺: 10 ppb, Cu²⁺: 0.998 ppm) and high selectivity during the detection of copper and mercury ions in tap water under non-laboratory conditions, and the results of real-time tests reveal that parameters measured in the field and laboratory environments are identical. Hence, this small, portable, electrochemical sensor with a screen-printed electrode can be effectively used for the real-time detection of copper and mercury ions in complex water environments.

Facile and ultra-sensitive voltammetric electrodetection of Hg2+ in aqueous media using electrodeposited AuPtNPs/ITO

In this study, we have investigated the use of electrodeposited Au-Pt nanoparticles (AuPtNPs) on indium tin oxide (ITO) for the detection of Hg²⁺ heavy ions in water samples. The mechanism of AuPtNP electrocrystallization on ITO glass in an aqueous solution containing 0.5 mM HAuCl₄ + 0.5 mM H₂PtCl₆ is described for the first time. The nucleation mechanism of monometallic AuNPs on ITO was found to be progressive; however, a transition from progressive to instantaneous was observed for bimetallic AuPtNPs at elevated overpotentials. The modified ITOs were then assessed for the electrodetection of Hg²⁺ in aqueous media. It was shown by differential pulse voltammetry (DPV) that the sensitivity of the constructed AuPtNPs/ITO electrode toward Hg²⁺ was about 2.08 μA nM^-1. An approximate detection limit of 4.03 nM Hg²⁺ was achieved, which is below the permissible level of 30.00 nM Hg²⁺ in drinking water, according to the World Health Organization (WHO). Characterization of AuPt nanostructures was carried out by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), and different electrochemical techniques (cyclic voltammetry (CV), chronoamperometry, and electrochemical impedance spectroscopy (EIS)). Our results indicate a good potential of a facile and robust electrochemical assembly for on-site detection of heavy metals in water samples.

Simple, rapid, and sensitive on-site detection of Hg2+ in water samples through combining portable evanescent wave optofluidic biosensor and fluorescence resonance energy transfer principle

Rapid and sensitive detection of Hg²⁺ in the environment and drinking water is vital because of its non-degradability, bioaccumulation, and high toxicity. Herein, we report a portable evanescent wave optofluidic biosensor (EWOB) for simple sensitive detection of Hg²⁺ using fluorescence labeled poly-A DNA strand (CY-A14) and quencher labeled poly-T DNA strand (BQ-T14) as signal reporter and biorecognition element, respectively. Both CY-A14 and Hg²⁺ can competitively bind with BQ-T14 based on DNA hybridization and the specifical binding of Hg²⁺ and T bases of DNA to form T-Hg²⁺-T mismatch structure, respectively. Higher concentration of Hg²⁺ lead to less CY-A14 bound to BQ-T14 and thus a higher fluorescence intensity. The influence of several key environmental factors on Hg²⁺ biosensor, such as pH, temperature, and ionic strength, was investigated in details because they were essential for practical applications of Hg²⁺ biosensor. Under optimal conditions, a detection cycle for a single sample, including the measurement and regeneration, was less than 10 min with a Hg²⁺ detection limit of 8.5 nM. The high selectivity of the biosensor was showed by evaluating its response to various potentially interfering metal ions. Our results clearly demonstrated that the portable EWOB could serve as a powerful tool for rapid and sensitive on-site detection of Hg²⁺ in real water samples. The EWOB is also potentially applicable to detect other heavy metal ions or small molecule targets for which DNA/aptamers could be applied as specific biosensing probes.