3D Co-doped Ni-based conductive MOFs modified electrochemical sensor for highly sensitive detection of l-tryptophan

Trends in conductive MOFs for sensing: A review

Metal-organic frameworks (MOFs) are porous, ordered arrays formed by coordination bonds between organic ligands and metal ions or clusters. The highly tunable properties of the MOF structure and performance make it possible to meet the needs of many applications. Conductive MOFs are essential in the domain of sensing due to their electrical conductivity, porosity, and catalytic properties, offering an effective platform for detection. Numerous sensing devices that utilize conductive MOFs have been created. This text presents a thorough overview of the diverse applications of conductive MOFs within the sensing field. The results of this work provide insights into the properties and synthesis methods of conductive MOFs and the working mechanisms of sensors based on conductive MOFs, which will help to deepen the study of such materials, provide a new vision for the design and production of novel functional materials, and promote the development and application of sensors based on conductive MOFs.

A polyoxometalate/chitosan-Ti3C2Tx MXene nanocomposite constructed by electrostatically mediated strategy for electrochemical detecting L-tryptophan in milk

L-tryptophan (L-Trp) is crucial for human metabolism, and its imbalance or deficiency can lead to certain diseases, such as insomnia, depression, and heart disease. Since the body cannot synthesize L-Trp and must obtain it from external sources, accurately monitoring L-Trp levels in food is essential. Herein, a nanocomposite film based on polyoxometalate (P2Mo17V), Ti3C2Tx MXene, and chitosan (Cs) was developed through a green electrostatically mediated layer-by-layer self-assembly strategy for electrochemical detection of L-Trp. The composite film exhibits fast electron transfer and remarkable electrocatalytic performance for L-Trp with a wide linear range (0.1-103 μM), low limit of detection (0.08 μM, S/N = 3), good selectivity, reproducibility, and repeatability. In milk sample, the recoveries of L-Trp were from 95.78% and 104.31%. The P2Mo17V/Cs-Ti3C2Tx electrochemical sensor not only provides exceptional recognition and detection capabilities for L-Trp but also shows significant potential for practical applications, particularly in food safety and quality control.

Cobalt-copper bimetallic selenides embedded in nitrogen-doped porous carbon nanocubes for diclofenac electrochemical sensing

Diclofenac (DCF) is a non-steroidal anti-inflammatory drug (NSAID), and its overuse poses a potential threat to human health and the aquatic environment, designing high-efficiency electrocatalysts for DCF detection is urgent. Herein, cobalt-copper bimetallic selenides embedded in nitrogen-doped porous carbon nanocubes (CoCuSe@NC) were elaborately designed via one-step in situ selenization of bimetallic CoCu-MOF. The chemical constituents and micromorphology of CoCuSe@NC composites can be further optimized by precisely regulating the selenization process and the doping ratio of bimetal in MOF precursor. As an electrocatalyst, CoCuSe@NC was proved to be highly efficient in electrochemical sensing of DCF with a broad linear range of 0.1-400 µmol/L and a detection limit of 0.024 µmol/L. This was attributed to the synergistic advantages between the heterogeneous structures, which produced more electrochemically active sites, effectively shortened the electron transport path, and improved electrocatalytic performance. Consequently, the constructed sensor exhibits high sensitivity, remarkable stability and applicability, and in particular can selectively detect DCF from other structurally similar coexisting analogs, resulting from the unique metal chelation ability. This work paves the way for designing effective bimetallic selenide electrocatalysts and exploring their applications in DCF electrochemical sensing.

Hollow-like three-dimensional structure of methyl orange-delaminated Ti3C2 MXene nanocomposite for high-performance electrochemical sensing of tryptophan

Tryptophan(Trp) is being explored as a potential biomarker for various diseases associated with decreased tryptophan levels; however, metabolomic methods are expensive and time-consuming and require extensive sample analysis, making them urgently needed for trace detection. To exploit the properties of Ti3C2 MXenes a rational porous methyl orange (MO)-delaminated Ti3C2 MXene was prepared via a facile mixing process for the electrocatalytic oxidation of Trp. The hollow-like 3D structure with a more open structure and the synergistic effect of MO and conductive Ti3C2 MXene enhanced its electrochemical catalytic capability toward Trp biosensing. More importantly, MO can stabilize Ti3C2 MXene nanosheets through noncovalent π-π interactions and hydrogen bonding. Compared with covalent attachment, these non-covalent interactions preserve the electronic conductivity of the Ti3C2 MXene nanosheets. Finally, the addition of MO-derived nitrogen (N) and sulfur (S) atoms to Ti3C2 MXene enhanced the electronegativity and improved its affinity for specific molecules, resulting in high-performance electrocatalytic activity. The proposed biosensor exhibited a wide linear response in concentration ranges of 0.01-0.3 µM and 0.5-120 µM, with a low detection limit of 15 nM for tryptophan detection, and high anti-interference ability in complex media of human urine and egg white matrices. The exceptional abilities of the MO/Ti3C2 nanocatalyst make it a promising electrode material for the detection of important biomolecules.

Remote magnetically stimulated xanthan-biochar-Fe3O4-molecularly imprinted biopolymer hydrogel toward electrochemical enantioselection of l-tryptophan

Monitoring the levels of L-Tryptophan (L-Trp) in body fluids is crucial due to its significant role in metabolism and protein synthesis, which ultimately affects neurological health. Herein, we have developed a novel magneto-responsive electrochemical enantioselective sensor for the recognition of L-Trp based on oriented biochar derived from Loofah, Fe3O4 nanoparticles, and molecularly imprinted polydopamine (MIPDA) in xanthan hydrogel. The successful synthesis of these materials has been confirmed through physicochemical and electrochemical characterization. Various operational factors such as pH, response time, loading sample volume, and loading of active materials were optimized. As a result, the sensor exhibited an affordable linear range of 1.0-60.0 μM, with a desirable limit of detection of 0.44 μM. Furthermore, the proposed electrochemical sensor demonstrated good reproducibility and desirable selectivity for the determination of L-Trp, making it suitable for analyzing L-Trp levels in human plasma and serum samples. The development presented offers an appealing, easily accessible, and efficient strategy. It utilizes xanthan hydrogel to improve mass transfer and adhesion, biochar-stabilized Fe3O4 to facilitate magnetic orientation and accelerate mass transfer and sensitivity, and polydopamine MIP to enhance selectivity. This approach enables on-site evaluation of L-Trp levels, which holds significant value for healthcare monitoring and early detection of related conditions.

Ultrasensitive detection and photocatalytic degradation of polyketide drug tetracycline in environment and food samples using dual-functional Ag doped zinc ferrite embedded functionalized carbon nanofibers

The efficient degradation and accurate quantification of tetracycline in environment and food samples is pivotal for ensuring public health and safety by monitoring potential contamination and maintaining regulatory standards. Hence, in this study, photocatalytic degradation of tetracycline and its electrochemical detection in environment and food samples based on dual-functional silver-doped zinc ferrite nanoparticles embedded chitosan-functionalized carbon nanofibers fabricated on a screen-printed carbon electrode (AgZFO/CHIT-CNF/SPCE) is presented. A hydrothermal method was used in the synthesis of Ag-doped ZFO, and chitosan was functionalized on the CNF surface using a swift and cost-effective chemical modification process of carboxyl groups. Various techniques, such as XRD, HRTEM, elemental mapping, EIS, XPS, FTIR, VSM, BET, UV-Vis DRS, and Raman spectroscopy were used to analyze the characteristics of the prepared nanocomposite. Cyclic voltammetry and differential pulse voltammetry were used to evaluate the surface-controlled electrocatalytic properties of AgZFO/CHIT-CNF towards tetracycline. Electrochemical tests revealed that the proposed electrode exhibited excellent sensitivity for detecting tetracycline. The fabricated electrode had a low detection limit of 1 nM and a wide linear range (0.2-53.2 μM). The sensor also demonstrated exceptional selectivity, stability, and reusability. The practical feasibility evaluated with real samples, including chicken feed, shrimp, milk, soil, and wastewater, resulted in high recovery values.

Nitrogen Defect-Rich Graphitic Carbon Nitride for Highly Sensitive Voltammetric Determination of Tryptophan

Here, a highly sensitive electrochemical sensor for detection of tryptophan (Trp) using a nitrogen defect graphitic carbon nitride-modified glassy carbon electrode (ND-CN/GCE) was introduced. ND-CN/GCE showed a higher oxidation current for Trp than the graphitic carbon nitride-modified glassy carbon electrode (g-CN/GCE) and bare glassy carbon electrode (BGCE). The synthesized nitrogen defect-rich graphitic carbon nitride (ND-CN) was characterized using X-ray photoelectron spectroscopy, X-ray diffraction spectroscopy, Fourier-transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. Electrochemical impedance spectroscopy and cyclic voltammetry were used to further analyze the electrochemical properties of BGCE, g-CN/GCE, and ND-CN/GCE. The oxidation of Trp at ND-CN/GCE is a diffusion-controlled process at pH 3.0. It was calculated that the transfer coefficient, rate constant, and diffusion coefficient of Trp were 0.53, 2.24 × 103 M-1 s-1, and 8.3 × 10-3 cm2 s-1, respectively, at ND-CN/GCE. Trp was detected using square wave voltammetry, which had a linear range from 0.01 to 40 μM at pH 3.0 and a limit of detection of about 0.0034 μM (3σ/m). Analyzing the presence of Trp in a milk and multivitamin tablet sample with a percentage recovery in the range of 97.0-108% satisfactorily demonstrated the practical usability of the electrochemical sensor. The ND-CN/GCE additionally displays good repeatability and reproducibility and satisfactory selectivity.

Solid-State Nanopore/Nanochannel Sensing of Single Entities

Solid-state nanopores/nanochannels, with their high stability, tunable geometry, and controllable surface chemistry, have recently become an important tool for constructing biosensors. Compared with traditional biosensors, biosensors constructed with solid-state nanopores/nanochannels exhibit significant advantages of high sensitivity, high specificity, and high spatiotemporal resolution in the detection single entities (such as single molecules, single particles, and single cells) due to their unique nanoconfined space-induced target enrichment effect. Generally, the solid-state nanopore/nanochannel modification method is the inner wall modification, and the detection principles are the resistive pulse method and the steady-state ion current method. During the detection process, solid-state nanopore/nanochannel is easily blocked by single entities, and interfering substances easily enter the solid-state nanopore/nanochannel to generate interference signals, resulting in inaccurate measurement results. In addition, the problem of low flux in the detection process of solid-state nanopore/nanochannel, these defects limit the application of solid-state nanopore/nanochannel. In this review, we introduce the preparation and functionalization of solid-state nanopore/nanochannel, the research progress in the field of single entities sensing, and the novel sensing strategies on solving the above problems in solid-state nanopore/nanochannel single-entity sensing. At the same time, the challenges and prospects of solid-state nanopore/nanochannel for single-entity electrochemical sensing are also discussed.

Nanomaterial-based electrochemical sensors for detection of amino acids

Amino acids are the building blocks of proteins and muscle tissue. They also play a significant role in physiological processes related to energy, recovery, mood, muscle and brain function, fat burning and stimulating growth hormone or insulin secretion. Accurate determination of amino acids in biological fluids is necessary because any changes in their normal ranges in the body warn diseases like kidney disease, liver disease, type 2 diabetes and cancer. To date, many methods such as liquid chromatography, fluorescence mass spectrometry, etc. have been used for the determination of amino acids. Compared with the above techniques, electrochemical systems using modified electrodes offer a rapid, accurate, cheap, real-time analytical path through simple operations with high selectivity and sensitivity. Nanomaterials have found many interests to create smart electrochemical sensors in different application fields e.g. biomedical, environmental, and food analysis because of their exceptional properties. This review summarizes recent advances in the development of nanomaterial-based electrochemical sensors in 2017-2022 for the detection of amino acids in various matrices such as serum, urine, blood and pharmaceuticals.

A Cu-functionalized MOF and multi-walled carbon nanotube composite modified electrode for the simultaneous determination of hydroquinone and catechol

Designing MOF-based materials with desired electrochemical activity and high electron conductivity may yield a novel electrochemical sensor that effectively detects various organic pollutants and conducts health monitoring. This study developed a facile and versatile electrochemical sensor for simultaneously monitoring the environmental pollutants hydroquinone (HQ) and catechol (CT). The electrodes are fabricated by modifying a GCE with a Cu-functionalized MOF (UiO-bpydc-Cu) and multi-walled carbon nanotubes (MWCNTs). The Cu-functionalized MOF effectively improved the electronic conductivity by metalating the 2,2'-bipyridyl-derived UiO-bpydc with Cu²⁺ ions. Moreover, due to the synergic effect, the composite electrode exhibits a significant voltammetric response to HQ's and CT's electro-redox. A rapid and sensitive method of synchronously detecting HQ and CT has been established by differential pulse voltammetry (DPV). The experiments reveal that the linear response ranges were 0.5-565 μM and 1-1350 μM for HQ and CT, respectively, with low detection limits of 0.361 μM and 0.245 μM. The proposed UiO-bpydc-Cu/MWCNTs/GCE electrochemical sensor shows high sensitivity, good anti-interference, reproducibility, and stability. It can also be applied for detecting HQ and CT in actual samples.

Design and Application of Electrochemical Sensors with Metal-Organic Frameworks as the Electrode Materials or Signal Tags

Metal-organic frameworks (MOFs) with fascinating chemical and physical properties have attracted immense interest from researchers regarding the construction of electrochemical sensors. In this work, we review the most recent advancements of MOF-based electrochemical sensors for the detection of electroactive small molecules and biological macromolecules (e.g., DNA, proteins, and enzymes). The types and functions of MOF-based nanomaterials in terms of the design of electrochemical sensors are also discussed. Furthermore, the limitations and challenges of MOF-based electrochemical sensing devices are explored. This work should be invaluable for the development of MOF-based advanced sensing platforms.