Facile visual colorimetric sensor based on iron carbide nanoparticles encapsulated in porous nitrogen-rich graphene

Development of a dual-sensing colorimetric probe for total antioxidant capacity measurement using iron(III)-o-phenanthroline reagent

In this study, a dual-mode sensing system was developed for the determination of total antioxidant capacity (TAC) using the Fe(III)-phenanthroline (Fe(III)-phen) reagent. The first detection mechanism of the system is based on the reduction of the Fe(III)-phen reagent by antioxidants, leading to the formation of the orange-red Fe(II)-phen chelate, which is quantified by the absorbance change at 510 nm. The second mechanism exploits the oxidase-like activity of the Fe(III)-phen complex. This complex generates superoxide anion radicals that oxidize 3,3',5,5'-tetramethylbenzidine (TMB) to produce a blue-colored oxidized TMB (ox-TMB) charge-transfer complex. In the presence of antioxidants, this reaction is inhibited, resulting in a decrease in ox-TMB formation, and the absorbance change at 652 nm correlates with the TAC of the tested sample. The proposed system was successfully applied to standard antioxidants, synthetic antioxidant mixtures, and real food extracts, demonstrating its applicability and sensitivity for TAC analysis. The linear equation of the calibration graphs obtained for different trolox (TR) concentrations were found to be A510 = 0.0221CTR + 0.0223 (A: absorbance and C: concentration in μM) and ΔA = 0.0301CTR + 0.0583 (ΔA: the difference of absorbance resulting from decreasing ox-TMB formation in the presence of TR, and C: concentration in μM) for the reduction-based Fe(III)-phen method and the TMB-based Fe(III)-phen method, respectively. The limits of detection (LOD) for the reduction based Fe(III)-phen method and the TMB-based Fe(III)-phen method were found to be 0.45 and 0.87 μM, respectively, for trolox. The LOD was calculated using the equation; LOD = 3 sbl/m (sbl: standard deviation of a blank, m: slope of the calibration line). This study presents an innovative approach by utilizing the same probe, Fe(III)-phen, through two distinct mechanisms for the simple, rapid, and sensitive determination of TAC.

A review on synthesis of MOF-derived carbon composites: innovations in electrochemical, environmental and electrocatalytic technologies

Carbon composites derived from Metal-Organic Frameworks (MOFs) have shown great promise as multipurpose materials for a range of electrochemical and environmental applications. Since carbon-based nanomaterials exhibit intriguing features, they have been widely exploited as catalysts or catalysts supports in the chemical industry or for energy or environmental applications. To improve the catalytic performance of carbon-based materials, high surface areas, variable porosity, and functionalization are thought to be essential. This study offers a thorough summary of the most recent developments in MOF-derived carbon composite synthesis techniques, emphasizing innovative approaches that improve the structural and functional characteristics of the materials. Their uses in electrochemical technologies, such as energy conversion and storage, and their function in environmental electrocatalysis for water splitting and pollutant degradation are also included in the debate. This review seeks to clarify the revolutionary effect of carbon composites formed from MOFs on sustainable technology solutions by analyzing current research trends and innovations, opening the door for further advancements in this rapidly evolving sector.

Facile Fabrication of Wood-Derived Porous Fe3C/Nitrogen-Doped Carbon Membrane for Colorimetric Sensing of Ascorbic Acid

Fe3C nanoparticles hold promise as catalysts and nanozymes, but their low activity and complex preparation have hindered their use. Herein, this study presents a synthetic alternative toward efficient, durable, and recyclable, Fe3C-nanoparticle-encapsulated nitrogen-doped hierarchically porous carbon membranes (Fe3C/N-C). By employing a simple one-step synthetic method, we utilized wood as a renewable and environmentally friendly carbon precursor, coupled with poly(ionic liquids) as a nitrogen and iron source. This innovative strategy offers sustainable, high-performance catalysts with improved stability and reusability. The Fe3C/N-C exhibits an outstanding peroxidase-like catalytic activity toward the oxidation of 3,3',5,5'-tetramethylbenzidine in the presence of hydrogen peroxide, which stems from well-dispersed, small Fe3C nanoparticles jointly with the structurally unique micro-/macroporous N-C membrane. Owing to the remarkable catalytic activity for mimicking peroxidase, an efficient and sensitive colorimetric method for detecting ascorbic acid over a broad concentration range with a low limit of detection (~2.64 µM), as well as superior selectivity, and anti-interference capability has been developed. This study offers a widely adaptable and sustainable way to synthesize an Fe3C/N-C membrane as an easy-to-handle, convenient, and recoverable biomimetic enzyme with excellent catalytic performance, providing a convenient and sensitive colorimetric technique for potential applications in medicine, biosensing, and environmental fields.

Colorimetric Sensors for Chemical and Biological Sensing Applications

Colorimetric sensors have been widely used to detect numerous analytes due to their cost-effectiveness, high sensitivity and specificity, and clear visibility, even with the naked eye. In recent years, the emergence of advanced nanomaterials has greatly improved the development of colorimetric sensors. This review focuses on the recent (from the years 2015 to 2022) advances in the design, fabrication, and applications of colorimetric sensors. First, the classification and sensing mechanisms of colorimetric sensors are briefly described, and the design of colorimetric sensors based on several typical nanomaterials, including graphene and its derivatives, metal and metal oxide nanoparticles, DNA nanomaterials, quantum dots, and some other materials are discussed. Then the applications, especially for the detection of metallic and non-metallic ions, proteins, small molecules, gas, virus and bacteria, and DNA/RNA are summarized. Finally, the remaining challenges and future trends in the development of colorimetric sensors are also discussed.

An iron-based metal-organic framework as a novel dispersive solid-phase extraction sorbent for the efficient adsorption of tetrabromobisphenol A from environmental water samples

For environmental safety, it is important to establish a simple, rapid, and sensitive method for emerging pollutants. Here, a dispersive solid-phase extraction (d-SPE) method based on an iron-based metal-organic framework (Fe-MIL-88-NH₂) combined with high-performance liquid chromatography (HPLC) was developed for tetrabromobisphenol A (TBBPA) in water samples. Fe-MIL-88-NH₂ was synthesized using a solvothermal method and completely characterized. Fe-MIL-88-NH₂ had good water stability and gave a maximum adsorption capacity of 40.97 mg g^-1 for TBBPA. The adsorption of TBBPA on Fe-MIL-88-NH₂ followed Langmuir adsorption models and a pseudo-second-order kinetic model. The bromine ion and the hydroxyl group of TBBPA could form strong hydrogen bond interactions with the amino protons around the cavity of Fe-MIL-88-NH₂, which was in accord with the molecular simulation calculations. Furthermore, several important d-SPE parameters were optimized, such as the amount of materials, extraction time, pH, ionic strength, elution solvent type, and volume. The established method showed good linearity in the concentration range of 0.005-100 μg g^-1 (r² ≥ 0.9996). This method's limits of detection (LOD) and quantification (LOQ) were 0.001 μg g^-1 and 0.005 μg g^-1, respectively. The recoveries in spiked water samples ranged from 87.5% to 104.9%. The proposed method was applied successfully to detect TBBPA in environmental water samples.

Recent advances on preparation and environmental applications of MOF-derived carbons in catalysis

Carbon materials derived from metal organic frameworks (MOFs) have excellent properties of high surface area, high porosity, adjustable pore size, high conductivity and stability, and their applications in catalysis have become a rapidly expanding research field. In this review, we have summarized the synthesis strategies of MOF-derived carbons with different physical and chemical properties, obtained through direct carbonization, co-pyrolysis and post-treatment. The potential applications of derived carbons, especially monometal-, bimetal-, nonmetal-doped and metal-free carbons in organo-catalysis, photocatalysis and electrocatalysis are analyzed in detail from the environmental perspective. In addition, the improvement of catalytic efficiency is also considered from the aspects of increasing active sites, enhancing the activity of reactants and promoting free electron transfer. The function and synergy of various species of the composites in the catalytic reaction are summarized. The reaction paths and mechanisms are analyzed, and research ideas or trends are proposed for further development.

3D NiCo-Layered double Hydroxide@Ni nanotube networks as integrated free-standing electrodes for nonenzymatic glucose sensing

Nickel cobalt layered double hydroxide (NiCo-LDH)-based materials have recently emerged as catalysts for important electrochemical applications. However, they frequently suffer from low electrical conductivity and agglomeration, which in turn impairs their performance. Herein, we present a catalyst design based on integrated, self-supported nickel nanotube networks (Ni-NTNWs) loaded with NiCo-LDH nanosheets, which represents a binder-free, hierarchically nanostructured electrode architecture combining continuous conduction paths and openly accessible macropores of low tortuosity with an ultrahigh density of active sites. Similar to macroscale metallic foams, the NTNWs serve as three-dimensionally interconnected, robust frameworks for the deposition of active material, but are structured in the submicron range. Our synthesis is solely based on scalable approaches, namely templating with commercial track-etched membranes, electroless plating, and electrodeposition. Morphological and compositional characterization proved the successful decoration of the inner and outer nanotube surfaces with a conformal NiCo-LDH layer. Ni-NTNW electrodes and hydroxide-decorated variants showed excellent performance in glucose sensing. The highest activity was achieved for the catalyst augmented with NiCo-LDH nanosheets, which surpassed the modification with pure Ni(OH)₂. Despite its low thickness of 20 µm, the optimized catalyst layer provided an outstanding sensitivity of 4.6 mA mM^-1 cm^-2, a low detection limit of 0.2 µM, a fast response time of 5.3 s, high selectivity and stability, and two linear ranges covering four orders of magnitude, up to 2.5 mM analyte. As such, derivatized interconnected metal nano-networks represent a promising design paradigm for highly miniaturized yet effective catalyst electrodes and electrochemical sensors.

Amperometric sensor based on ZIF/g-C3N4/RGO heterojunction nanocomposite for hydrazine detection

A dense  zeolitic imidazolate framework (ZIF) nanosheet is for the first time molded by reduced graphite oxide (RGO) and graphitic carbon nitride (g-C₃N₄) to fabricate an original 2D/2D/2D heterojunction (ZIF/g-C₃N₄/RGO nanohybrid), which is pipetted onto carbon cloth electrode (CCE) (ZIF/g-C₃N₄/RGO/CCE) as an electrochemical sensor. Profiting from the renowned synergistic and coupling effects, the resulting nanohybrid endows excellent electrocatalytic activity towards hydrazine. Amperometric detection reveals that the hybrid sensor possesses a low detection limit of 32 nM (S/N = 3) in a monitoring range of 0.0001 to 1.0386 mM, along with a high sensitivity 93.71 μA mM^-1 cm^-2. Importantly, the minimum detection concentration of hydrazine in the actual sample is lower than the maximum allowable limit of the World Health Organization (WHO) and has high reproducibility (RSD = 4.82%). As expected, the high sensing capability  of ZIF/g-C₃N₄/RGO combines the advantages of abundant surface-active sites and high conductivity along with 2D interfaces between ZIF, g-C₃N₄, and RGO nanosheets. This study provides a promising to expand 2D-based ternary nanojunction as a bridge for promoting sensing performance.Graphical abstract.

3D Graphene Materials: From Understanding to Design and Synthesis Control

Carbon materials, with their diverse allotropes, have played significant roles in our daily life and the development of material science. Following 0D C₆₀ and 1D carbon nanotube, 2D graphene materials, with their distinctively fascinating properties, have been receiving tremendous attention since 2004. To fulfill the efficient utilization of 2D graphene sheets in applications such as energy storage and conversion, electrochemical catalysis, and environmental remediation, 3D structures constructed by graphene sheets have been attempted over the past decade, giving birth to a new generation of graphene materials called 3D graphene materials. This review starts with the definition, classifications, brief history, and basic synthesis chemistries of 3D graphene materials. Then a critical discussion on the design considerations of 3D graphene materials for diverse applications is provided. Subsequently, after emphasizing the importance of normalized property characterization for the 3D structures, approaches for 3D graphene material synthesis from three major types of carbon sources (GO, hydrocarbons and inorganic carbon compounds) based on GO chemistry, hydrocarbon chemistry, and new alkali-metal chemistry, respectively, are comprehensively reviewed with a focus on their synthesis mechanisms, controllable aspects, and scalability. At last, current challenges and future perspectives for the development of 3D graphene materials are addressed.

Bifunctional MIL-53(Fe) with pyrophosphate-mediated peroxidase-like activity and oxidation-stimulated fluorescence switching for alkaline phosphatase detection

Alkaline phosphatase (ALP) is extensively used as a clinical biomarker because of its close relevance with a variety of diseases. Thus, exploring reliable and practical methods for ALP analysis is of great significance. In the present work, we explored MIL-53(Fe) as a bifunctional platform with pyrophosphate (PPi)-mediated peroxidase-like activity and oxidation-stimulated fluorescence switching for ALP sensing. The proposed MIL-53(Fe) could exhibit favorable peroxidase-mimicking activity to catalytically decompose H2O2 to hydroxyl radicals, which had strong oxidizing ability to oxidize the terephthalic acid bridging ligand, resulting in the oxidation-stimulated turn-on fluorescence of MIL-53(Fe) itself. Due to the strong coordination interaction between PPi and Fe3+, the former with a relatively large molecular structure was able to inhibit the catalytic activity of MIL-53(Fe) via capping active Fe3+ sites, leading to the suppression of its self-fluorescence response. When ALP was present, it could hydrolyze the PPi inhibitor and restore the dual functions of MIL-53(Fe) to provide fluorescence again. With the above principle, highly sensitive and selective determination of ALP with a linear scope of 2-80 U L-1 and a detection limit down to 0.7 U L-1 was achieved. The MIL-53(Fe) was also demonstrated to be very reliable in measuring the target in human serum, indicating its great promise as an integrated tool for ALP detection in clinical practice.

Nanomaterials with enzyme-like characteristics (nanozymes): next-generation artificial enzymes (II)

An updated comprehensive review to help researchers understand nanozymes better and in turn to advance the field.