Derived N-doped carbon through core-shell structured metal-organic frameworks as a novel sorbent for dispersive solid phase extraction of Cr(III) and Pb(II) from water samples followed by quantitation through flame atomic absorption spectrometry

A zirconium metal-organic framework functionalized with a S/N containing carboxylic acid (MOF-808(Zr)-Tz) as an efficient sorbent for the ultrafast and selective dispersive solid phase micro extraction of chromium, silver, and rhodium in water samples

Metal-organic frameworks (MOFs) are good adsorbents for targeted chemicals with their adjustable properties. Herein, we prepared a zirconium based MOF (MOF-808(Zr)) and functionalized it employing 2-mercapto-4-methyl-5-thiazolacetic acid (MOF-808(Zr)-Tz). The prepared MOFs were characterized by XRD, FTIR, SEM-EDX, TGA, N2 sorption, zeta potential measurements, and elemental analysis. The surface area of MOF-808(Zr)-Tz was 1348 m2/g. Dispersive solid-phase micro-extraction (D-SPµE) method based on MOF-808(Zr)-Tz was firstly developed and applied to the extraction of chromium, silver, and rhodium in waters. The determination of the analytes was done by FAAS. The optimal pH and eluent for analytes were 7.0 and 3 mL of 2 mol L-1 HCl, respectively. The contact times were 1 min for adsorption and 3 min for elution. The LOD and PFs of the D-SPμE for analytes were 2.3 μg L-1 and 13.3 for chromium, 2.1 μg L-1 and 13.3 for silver, and 3.1 μg L-1 and 13.3 rhodium, respectively. The D-SPμE method was verified with analyses of NW-TMDA-54.6 Lake water and SPS-WW1 Batch 114 Wastewater and with spiked dam water, river water, well water, sea water, and wastewater. The recoveries of the analytes changed from 89 to 108 %. The results indicated that the method is selective, simple, effective, and rapid for extracting chromium(III), silver(I) and rhodium(III) in waters.

Integrated microfluidic platforms for heavy metal sensing: a comprehensive review

Heavy metals are found naturally; however, anthropogenic activities such as mining, inappropriate disposal of industrial waste, and the use of pesticides and fertilizers containing heavy metals can cause their unwanted release into the environment. Conventionally, detection of heavy metals is performed using atomic absorption spectrometry, electrochemical methods and inductively coupled plasma-mass spectrometry; however, they involve expensive and sophisticated instruments and multistep sample preparation that require expertise for accurate results. In contrast, microfluidic devices involve rapid, cost-efficient, simple, and reliable approaches for in-laboratory and real-time monitoring of heavy metals. The use of inexpensive and environment friendly materials for fabrication of microfluidic devices has increased the manufacturing efficiency of the devices. Different types of techniques used in heavy metal detection include colorimetry, absorbance-based, and electrochemical detection. This review provides insight into the detection of toxic heavy metals such as mercury (Hg), cadmium (Cd), lead (Pb), and arsenic (As). Importance is given to colorimetry, optical, and electrochemical techniques applied for the detection of heavy metals using microfluidics and their modifications to improve the limit of detection (LOD).

Remediation of environmental toxicants using carbonaceous materials: opportunity and challenges

Adsorption and photocatalytic properties of carbonaceous materials, viz., carbon nanotubes (CNTs), fullerene, graphene, graphene oxide, carbon nanofiber nanospheres, and activated carbon, are the legitimate weapons for the remediation of emerging and persistent inorganic/organic contaminants, heavy metals, and radionucleotides from the environment. High surface area, low or non-toxic nature, ease of synthesis, regeneration, and chemical modification of carbonaceous material make them ideal for the removal of toxicants. The research techniques investigated during the last decade for the elimination of environmental toxicants using carbonaceous materials are reviewed to offer comprehensive insight into the mechanism, efficiency, applications, advantages, and shortcomings. Opportunities and challenges associated with carbon materials have been discussed to suggest future perspectives in the remediation of environmental toxicants.

A novel utilization of raw sepiolite: preparation of magnetic adsorbent directly based on sol-gel for adsorption of Pb(II)

The constraints of industrial separation technology for low grade sepiolite greatly limit the development and utilization of these potential resources. In this work, a novel sepiolite adsorbent loaded with copper ferrite was prepared by sol-gel method to remove Pb(II) from wastewater. The effects of various factors on Pb(II) removal ratio were investigated. The maximum adsorption capacities at 293, 313, and 333 K were 1285.32, 1325.45, and 1390.54 mg/g, respectively. The adsorption of Pb(II) by magnetic sepiolite was a spontaneous endothermic process. Besides, the adsorption process followed Langmuir isothermal adsorption model and pseudo-second-order kinetic model. The main adsorption mechanism of Pb(II) removal was electrostatic attraction, ion exchange, and surface complexation. The improvement of Pb(II) absorption indicated that the efficient removal of Pb(II) can be realized by phosphate groups introduced in the preparation process and the carbonate groups contained in gangue minerals.

A nucleic acid dye-enhanced electrochemical biosensor for the label-free detection of Hg2+ based on a gold nanoparticle-modified disposable screen-printed electrode

In this paper, a nucleic acid dye-enhanced electrochemical biosensor based on a screen-printed carbon electrode (SPCE) modified with Au nanoparticles (AuNPs) was designed for the detection of Hg²⁺ in water. AuNPs were modified on the surface of the disposable SPCE through the electrodeposition of HAuCl₄. Subsequently, thiolated DNA probes were immobilized on the AuNP-modified electrode surface by Au-S reaction. After Hg²⁺ was bound with a DNA probe by thymine (T)-Hg²⁺-thymine (T) mismatch, the DNA probe was folded into a hairpin structure where positively charged GelRed molecules were embedded into the double-stranded part of the hairpin. Thus, the current of [Fe(CN)₆]^3-/4- increased significantly on account of the decreased electrostatic repulsion at the electrode surface. Under the optimized experimental conditions, the peak current of [Fe(CN)₆]^3-/4- exhibited a good linear relationship with lgC_Hg²⁺ in the concentration of Hg²⁺ linear range of 0.1 nM to 500 nM, and the limit of detection (S/N = 3) was calculated as 0.04 nM. The electrochemical sensor also exhibited excellent selectivity for Hg²⁺ in the presence of nine interfering ions, including Na⁺, Fe³⁺, Ni²⁺, Mg²⁺, Co²⁺, Pb²⁺, K⁺, Al³⁺ and Cu²⁺. Meanwhile, the developed electrochemical sensor was tested in the analysis of Hg²⁺ in tap water and river water samples, and the recoveries ranged from 81.0 to 114%. Therefore, this nucleic acid dye-enhanced electrochemical biosensor provided the advantages of simplicity, sensitivity, and specificity and is expected to be an alternative for Hg²⁺ detection in actual environmental samples.

Verbascoside extracted from Clerodendrum inerme: A natural monomer for the fabrication of a sensitive electrochemical Cu(II) sensor

Verbascoside is isolated from the whole plants of the Verbenaceae family and demonstrates significant levels of bioactivity. In this work, we report on the isolation of verbascoside, a phenolic glycoside, from the stems of Clerodendrum inerme collected in Vietnam and verify its structure by comparing its spectroscopic data with those reported in the literature. Verbascoside is then applied as a monomer for the electrochemical deposition of a layer of polyverbascoside onto glassy carbon electrodes toward Cu2+ sensing. Electrochemical results demonstrate that the polyverbascoside-modified glassy carbon electrode is highly selective for Cu2+ ions compared to other evaluated ions including Ni2+ and Co2+. The developed sensor can detect Cu2+ in a concentration range from 0 to 175 μM with a sensitivity of 0.372 µA µM−1 and a limit of detection of 50 nM Cu2+ ions.

Integration of Metal-Organic Frameworks with Bi-Nanoprobes as Dual-Emissive Ratiometric Sensors for Fast and Highly Sensitive Determination of Food Hazards

Functional nanoprobes which detect specific food hazards quickly and simply are still in high demand in the field of food-safety inspection research. In the present work, a dual-emission metal-organic framework-based ratiometric fluorescence probe was integrated to detect Cu²⁺ and Pb²⁺ with rapidness and ease. Specifically, quantum dots (QDs) and carbon quantum dots (CQDs) were successfully embedded into zeolitic imidazolate framework-67 (ZIF-67) to function as a novel ratiometric fluorescent sensing composite. The ratiometric fluorescence signal of CQDs/QDs@ZIF-67 was significantly aligned with the concentration of metal ions to give an extremely low detection limit of 0.3324 nM. The highly sensitive and selective CQDs/QDs@ZIF-67 composite showed potential for the rapid and cost-effective detection of two metal ions.

Applications of water-stable metal-organic frameworks in the removal of water pollutants: A review

Because the pollutants produced by human activities have destroyed the ecological balance of natural water environment, and caused severe impact on human life safety and environmental security. Hence the task of water environment restoration is imminent. Metal-organic frameworks (MOFs), structured from organic ligands and inorganic metal ions, are notable for their outstanding crystallinity, diverse structures, large surface areas, adsorption performance, and excellent component tunability. The water stability of MOFs is a key requisite for their possible actual applications in separation, catalysis, adsorption, and other water environment remediation areas because it is necessary to safeguard the integrity of the material structure during utilization. In this article, we comprehensively review state-of-the-art research progress on the promising potential of MOFs as excellent nanomaterials to remove contaminants from the water environment. Firstly, the fundamental characteristics and preparation methods of several typical water-stable MOFs include UiO, MIL, and ZIF are introduced. Then, the removal property and mechanism of heavy metal ions, radionuclide contaminants, drugs, and organic dyes by different MOFs were compared. Finally, the application prospect of MOFs in pollutant remediation prospected. In this review, the synthesis methods and application in water pollutant removal are explored, which provide ways toward the effective use of water-stable MOFs in materials design and environmental remediation.

Preparation of melon peel biochar/CoFe2O4 as a new adsorbent for the separation and preconcentration of Cu(II), Cd(II), and Pb(II) ions by solid-phase extraction in water and vegetable samples

The present research describes the successful preparation of melon peel biochar modified with CoFe₂O₄ (MPBC/CoFe₂O₄) followed by its usage as a new sorbent to separate, preconcentrate, and determine the toxic heavy metal ions by magnetic solid-phase extraction. The metal ion desorption was performed by 0.1 M HCl solution with a volume of 5.0 mL. Flame atomic absorption spectrometry (FAAS) was utilized for detection of the analyte levels. SEM-EDX, TEM, XRD, and FTIR techniques were carried out to illuminate the structure of MPBC/CoFe₂O₄. The fundamental variables affecting the adsorption and elution efficiencies of the analyte ions including solution pH, MPBC/CoFe₂O₄ amount, type and concentration of eluent, adsorption and desorption equilibrium time, and sample volume were optimized. The detection limits were calculated as 0.41, 1.82, and 3.16 µg L^-1 for Cu²⁺, Cd²⁺, and Pb²⁺ ions, respectively, with the relative standard deviation of lower than 4.2%. There were no substantial interference effects on the analyte ion recovery due to the presence of foreign ions at high levels. Five minutes of contact time was adequate to attain the adsorption equilibrium. The adsorption capacity of MPBC/CoFe₂O₄ was obtained as 106.4, 65.4, and 188.7 mg g^-1 for Cu²⁺, Cd²⁺, and Pb²⁺ ions, respectively, by utilizing Langmuir isotherm model. The pseudo-second order model is favorable to identify the adsorption kinetics. The method was validated by spike/recovery test, and then, it was successfully implemented to determine the aforementioned analyte levels in sea and stream water, pepper, black cabbage, eggplant, and tomato samples.

[Research progress in application of metal-organic framework-derived materials to sample pretreatment]

Sample pretreatment technology plays a vital role throughout the analysis of complex samples. Sample pretreatment can not only increase the concentration of trace targets in the sample, but also effectively eliminate interference from the sample matrix in instrumental analysis. Adsorbent materials are a key component of sample pretreatment technology. Therefore, the development of efficient and stable new adsorbent materials has acquired significance in research on pretreatment technology. Porous materials are advantageous for use in diverse applications, such as in adsorbents, when they possess controllable nanostructures, a tailored pore surface chemistry, and abundant porosity, and are inexpensive. Particularly in recent years, porous materials derived from metal-organic frameworks (MOFs) feature excellent properties, such as diverse morphology and structure, adjustable pore size, high specific surface area, good thermal stability, and chemical resistance. MOF-derived materials, when used as adsorbents for sample pretreatment, offer the following advantages: (1) The porous materials derived from MOFs typically possess a larger specific surface area than other porous materials. This characteristic is beneficial to improve the extraction capacity and extraction efficiency via an increase in the contact area between the materials and targets; (2) The microscopic porous structure of MOF-derived materials can be easily tuned (by controlling the temperature and time during pyrolysis, gas atmosphere, and heating rate), which is conducive to improve the selectivity of sample pretreatment methods; (3) The metal active sites can be evenly distributed. Owing to the ordered distribution of metal ions in the precursor MOFs and a good periodic framework structure, the metal active sites of the derivatives formed can still maintain a corresponding distance. These metal active sites will not form agglomerates and affect the extraction performance; conversely, other porous materials often require extremely complicated processes to achieve a uniform distribution; (4) Heteroatoms such as nitrogen and sulfur can be easily doped on the framework of MOF-derived porous materials. This doping enables the materials to induce additional interactions such as hydrogen bonding and π-π stacking for adsorbing target analytes. The excellent properties of MOF-derived materials make them promising for use in sample pretreatment. Novel sample pretreatment methods that use MOF-derived materials are constantly being developed. However, the use of MOF-derived materials is limited by the complex preparation process and high production cost of MOF precursors, along with difficulties in mass production. Further, the precise design or functionalization of MOF-derived materials according to the characteristics of targets is a new direction with immense challenges as well as application potential. This review summarizes the application of MOF-derived materials in sample pretreatment methods, including dispersive solid phase extraction (dSPE), magnetic solid phase extraction (MSPE), solid phase microextraction (SPME), stir bar sorptive extraction (SBSE), and dispersive micro solid phase extraction (DMSPE). The preparation methods, functional control, and enrichment efficiencies of various MOF-derived materials are also reviewed. Finally, the application prospects of MOF-derived materials in sample pretreatment are discussed to provide a clear outlook and reference for further related research.

Hierarchical zeolitic imidazolate framework-67 derived from in-situ synthesized CO-Al layered double hydroxide embedded within porous-anodized aluminum foil for thin film microextraction of caffeine followed by its high performance liquid chromatography-ultraviolet detection

In this study, the in-situ conversion of the synthesized Co-Al layered double hydroxide (Co-Al LDH) nanosheets to three dimensional hierarchical zeolitic imidazolate framework-67 (3D HZIF-67) was presented as a cost-effective, highly efficient, flexible and robust sorbent to carry out the microextraction process. In the first stage, the anodized aluminum foil was prepared electrochemically. Then, the Co-Al LDH precursor was constructed on the surface of the previously-prepared anodized Al foil applying in-situ formation approach. The procedure is followed by the conversion of the prepared Co-Al LDH film to 3D HZIF-67 film via a facile solvothermal method without adding cobalt salt. The in-situ prepared 3D HZIF-67-anodized Al was used for the thin film microextraction (TFME) of caffeine. The effective factors in TFME procedure were investigated and optimized through applying Central Composite Design (CCD). In the obtained optimal condition, the calibration curves for TFME-HPLC-UV of caffeine were linear in the range of 1-200 µg L^-1 with the coefficient of determination (r²) higher than 0.9915. The limits of detection were 0.33 and 0.38 µg L^-1, in water and urine matrices, respectively. Moreover, the enrichment factors (EFs) and absolute recoveries (%AR) were also calculated as 173-198 and 57.1%-65.3%, respectively. The inter-day relative standard deviations (RSDs) were evaluated as the method precision for 20 and 200 µg L^-1 of spiked sample and were between 4.9-6.1%. The repeatability of the preparation step was investigated as batch-to-batch reproducibility and it was found to be 4.9%; as a result, the reproducibility of the presented film was approved. Finally, the proposed method was utilized to determine caffeine (as the model analyte) from different types of real samples including urine, coffee, beverage (Pepsi) and shampoo. The obtained recoveries (higher than 88%) confirmed the capability of the method for real sample analysis.