Synthesis of Au@ZIF-8 nanocomposites for enhanced electrochemical detection of dopamine

An electrochemical sensor based on 2D Zn-MOFs and 2D C-Ti3C2Tx composite materials for rapid and direct detection of various foodborne pathogens

Rapid screening for foodborne pathogens is crucial for food safety. A rapid and one-step electrochemical sensor has been developed for the detection of Escherichia coli (E. coli), Staphylococcus aureus (S. aureus) and Salmonella typhimurium (S. typhimurium). Through the construction of aptamer/two-dimensional carboxylated Ti3C2Tx (2D C-Ti3C2Tx)/two-dimensional Zn-MOF (2D Zn-MOF) composites, the recognition elements, signal tags, and signal amplifiers are integrated on the electrode surface. Pathogens are selectively captured using the aptamer, which increases the impedance of the electrode surface，leads to a decrease in the 2D Zn-MOF current. Bacteria can be rapidly quantified using a one-step detection method and the replacement of aptamers. The detection limits for E. coli, S. aureus, and S. typhimurium are 6, 5, and 5 CFU·mL-1, respectively. The sensor demonstrated reliable detection capabilities in real-sample testing. Therefore, the one-step sensor based on the 2D Zn-MOF and 2D C-Ti3C2Tx has significant application value in the detection of foodborne pathogens.

Au/Ag@ZIF-8 nanocomposite as solid phase extraction adsorbent and SERS substrate for tacrolimus label-free therapeutic drug monitoring in human serum

Surface Enhanced Raman Scattering (SERS) has been extensively utilized in therapeutic drug monitoring (TDM) due to its rapid detection speed, high sensitivity and straightforward sample pretreatment. In this study, Au/AgNPs were obtained through the reduction of AgNO3 on the surface of AuNPs. Subsequently, Au/AgNPs were embedded into the tetrahedral lattice of ZIF-8 MOFs, resulting in the formation of Au/Ag@ZIF-8 nanocomposites. The Au/Ag@ZIF-8 nanocomposites exhibit a robust electromagnetic enhancement of Au/Ag bimetallic nanoparticles and a considerable adsorption capacity of ZIF-8 MOFs. This enables the pre-enrichment of target molecules in the vicinity of the electromagnetic field of the Au/AgNPs, thereby enhancing the sensitivity of SERS detection. The SERS substrate also exhibits high stability and reproducibility, as well as molecular sieving effects, due to the fact that Au/AgNPs are embedded into the tetrahedral lattice of ZIF-8. A TDM method for tacrolimus (FK506) in human serum was developed by using Au/Ag@ZIF-8 nanocomposites as solid phase extraction (SPE) adsorbent and SERS substrates. The results showed that under the optimized conditions, tacrolimus exhibited satisfactory linearity within the concentration range of 10-5-10-11 mol L-1, with a correlation coefficient (R2) of 0.9944, and the limit of detection (LOD) was as low as 6.4 pg mL-1. The recoveries were observed to range between 92 % and 105 %, with an RSD of below 8 %. The method is highly sensitive, exhibiting a sensitivity that is 3-6 orders of magnitude higher than that of existing analytical techniques. It has the potential to be applied in a clinical setting to biological samples.

Magnetic MOF composites for the electrocatalysis and biosensing of dopamine released from living cells

Metal-organic frameworks (MOFs) with fit ligands and metals can be integrated into electrochemical biosensors for the detection of various biomolecules. In this study, we have synthesized novel magnetic MOF composites as electrocatalysts and constructed a novel biosensor for electrochemical detection of dopamine. The composites named Fe3O4@ZIF-8@AuNPs-COOH are synthesized through layer-by-layer assembly. They exhibit excellent stability and cooperative catalytic activity. In addition, green recycling is readily achieved through magnetizing/demagnetizing the electrode. The large specific surface area and ordered porous structures of the magnetic MOFs ensure good dispersion of gold nanoparticles, while the carboxyl group efficiently shields other redox-active interfering substances. The proposed electrochemical biosensor accomplishes the sensitive detection of dopamine in human serums and living cells. This study broadens the application of MOFs in electrochemical biosensing, validates the feasibility of biosensors for in vivo analysis, and provides new insights into green sensing.

Self-assembled ordered AuNRs-modified electrodes for simultaneous determination of dopamine and topotecan with improved data reproducibility

Gold nanomaterials have been widely explored in electrochemical sensors due to their high catalytic property and good stability in multi-medium. In this paper, the reproducibility of the signal among batches of gold nanorods (AuNRs)-modified electrodes was investigated to improve the data stabilization and repeatability. Ordered and random self-assembled AuNRs-modified electrodes were used as electrochemical sensors for the simultaneous determination of dopamine (DA) and topotecan (TPC), with the aim of obtaining an improved signal stability in batches of electrodes and realizing the simultaneous determination of both substances. The morphology and structure of the assemblies were analyzed and characterized by UV-Vis spectra, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray powder diffraction (XRD). Electrochemical studies showed that the ordered AuNRs/ITO electrodes have excellent signal reproducibility among several individuals due to the homogeneous mass transfer in the ordered arrangement of the AuNRs. Under the optimized conditions, the simultaneous detection results of DA and TPC showed good linearity in the ranges 1.75-45 μM and 1.5-40 μM, and the detection limits of DA and TPC were 0.06 μM and 0.17 μM, respectively. The results showed that the prepared ordered AuNR/ITO electrode had high sensitivity, long-term stability, and reproducibility for the simultaneous determination of DA and TPC, and it was expected to be applicable for real sample testing.

Nitrogen-doped biomass-derived carbon dots for fluorescence determination of sunset yellow

Sunset Yellow (SY) is a widely used food coloring in the food industry. However, exceeding the allowable limit of this dye poses a significant threat to human health. To address this issue, we developed Lycium ruthenicum-derived nitrogen-doped carbon dots (N-CDs) with a stable blue fluorescence through hydrothermal treatment for SY determination. The quantum yield (QY) of these N-CDs was found to be up to 10.63%. Physical characterization of N-CDs was performed using various spectroscopic techniques to confirm their excellent photostability and non-toxic properties. Furthermore, the presence of SY had a substantial quenching effect on the fluorescence intensity (F0/F) of the N-CDs. Leveraging this observation, we developed a fluorescent sensor for the determination of SY in the concentration range of 0.05 to 35.0 μM, with a limit of detection (LOD, 3σ/K) of 17 nM. The excellent fluorescent sensor also showed satisfactory results in the practical drink samples. Moreover, the stability and cytotoxicity of N-CDs as a fluorescent probe were studied. Finally, the N-CDs were applied to cell imaging using A549 cells.

Development of an electrochemical sensitive aptasensor based on a zeolite imidazolate framework-8 and gold nanoparticles for the determination of Staphylococcus aureus bacteria

Staphylococcus aureus (S. aureus) is one of the most important pathogens that cause illness and food poisoning. In this research, using a glassy carbon electrode (GCE) modified with zeolite imidazolate framework-8 (ZIF 8) and gold nanoparticles (AuNPs), a sensitive electrochemical aptasensor has been made for the detection of the S. aureus bacteria. The morphology of the prepared AuNPs-ZIF 8 nanocomposite has been carefully characterized by means of transmission electron microscopy (TEM), field emission scanning electron microscopy (FESEM), and energy-dispersive X-ray spectroscopy (EDS). In the manufacturing process, the S. aureus aptamer is immobilized on the AuNPs-ZIF 8 surface. Electrochemical impedance spectroscopy (EIS) method has been used for quantitative determination of S. aureus bacteria. The changes in the charge transfer resistance (Rct) of the aptamer due to the change in the concentration of bacteria are considered as the analytical signals. The proposed aptasensor has linear response in the concentration range of 1.5 × 101 to 1.5 × 107 CFU mL-1 of S. aureus bacteria. The detection limit of the method is 3.4 CFU mL-1. Using the developed aptasensor, it is possible to determine S. aureus bacteria in water and milk samples.

Tunable growth of a single high-density ZIF nanoshell on a gold nanoparticle isolated in an optical trap

Here, we demonstrate an all-optical method using an optical tweezer to controllably grow high quality zeolitic imidazolate framework (ZIF) nanoshells on the surface of gold nanoparticles (AuNPs) and monitor the growth via darkfield spectroscopy. Our single particle approach allows us to localize an individual NP within a microscope slide chamber containing ZIF precursors at the focus of an optical microscope and initiate growth through localized heating without affecting the bulk system. Darkfield spectroscopy is used to characterize changes to the localized surface plasmon resonance (LSPR) of the AuNP resulting from refractive index changes as the ZIF crystal grows on the surface. We show that the procedure can be generalized to grow various types of ZIF crystals, such as ZIF-8, ZIF-11, and a previously undocumented ZIF variety. Utilizing both computational models and experimental methods, we identify the thickness of ZIF layers to be self-limiting to ∼50 nm or less, depending on the trapping laser power. Critically, the refractive index of the shells here was found to be above 1.6, indicating the formation of high-density crystals, previously accessible only through slow atomic layer deposition and not through a bulk heating process. The single particle method developed here opens the door for bottom-up controllable growth of custom nanostructures with tunable optical properties.

A rapid and sensitive colorimetric sensor for hypochlorite detection based on polyvinylpyrrolidone-stabilized gold nanoparticles

It is important to develop simple and effective approaches for hypochlorite (OCl-) detection at trace levels due to its widespread use as a disinfectant especially for water treatments including drinking water. In this work, a simple, facile colorimetric sensor for the sensitive determination of hypochlorite was developed based on the oxidation of cysteine by OCl-, a process that prohibits the cysteine-triggered aggregation of Au nanoparticles (AuNPs) stabilized by polyvinylpyrrolidone. With this strategy, the concentration of OCl- could be detected with the naked eye and/or ultraviolet-visible spectroscopy, and the limit of detection for OCl- was 1.0 μM and 250 nM, respectively. Additionally, the proposed method shows excellent anti-interference capability against many other interfering ions and real water sample applicability.

A review of zeolitic imidazolate frameworks (ZIFs) as electrochemical sensors for important small biomolecules in human body fluids

Small biomolecules play a critical role in the fundamental processes that sustain life and are essential for the proper functioning of the human body. The detection of small biomolecules has garnered significant interest in various fields, including disease diagnosis and medicine. Electrochemical techniques are commonly employed in the detection of critical biomolecules through the principle of redox reactions. It is also a very convenient, cheap, simple, fast, and accurate measurement method in analytical chemistry. Zeolitic imidazolate frameworks (ZIFs) are a unique type of metal-organic framework (MOF) composed of porous crystals with extended three-dimensional structures. These frameworks are made up of metal ions and imidazolate linkers, which form a highly porous and stable structure. In addition to their many advantages in other applications, ZIFs have emerged as promising candidates for electrochemical sensors. Their large surface area, pore diameter, and stability make them ideal for use in sensing applications, particularly in the detection of small molecules and ions. This review summarizes the critical role of small biomolecules in the human body, the standard features of electrochemical analysis, and the utilization of various types of ZIF materials (including carbon composites, metal-based composites, ZIF polymer materials, and ZIF-derived materials) for the detection of important small biomolecules in human body fluids. Lastly, we provide an overview of the current status, challenges, and future outlook for research on ZIF materials.

Metal-organic frameworks/metal nanoparticles as smart nanosensing interfaces for electrochemical sensors applications: a mini-review

Metal-organic frameworks (MOFs) are porous materials with huge specific surface area and abundant active sites, which are composed of metal ions or clusters and organic ligands in the form of coordination bonds. In recent years, MOFs have been successfully applied in many fields due to their excellent physical, chemical, and biological properties. Electrochemical sensors have advantages such as economy, portability, and sensitivity, making them increasingly valued in the field of sensors. Many studies have shown that the electrode materials will affect the performance of electrochemical sensors. Therefore, the research on electrode materials is still one of the hotspots. MOFs are also commonly used to construct electrochemical sensors. However, electrochemical sensors prepared from single MOFs have shortcomings such as insufficient conductivity, low sensitivity, and poor electrochemical catalytic ability. In order to compensate for these defects, a new type of nanocomposite material with very ideal conductivity was formed by adding metal nanoparticles (MNPs) to MOFs. The combination of the two is expected to be widely applied in the field of sensors. This review summarizes the applications of various MNPs/MOFs composites in the field of electrochemical sensors and provides some references for the development of MNPs/MOFs composites-based electrochemical sensors in the future.

A ratiometric electrochemical sensor for detecting lead in fish based on the synergy of semi-complementary aptamer pairs and Ag nanowires@zeolitic imidazolate framework-8

This work describes the synergistic application of semi-complementary aptamer pairs and signals on-off ratio strategies on glassy carbon electrodes (GCE) for detecting lead ions (Pb²⁺) in fish. Gold nanoparticles (AuPNs) as the electrode substrate can provide added binding sites for the aptamers and improve the conductivity of the electrodes. Pb²⁺ aptamers containing ferrocene (Fc) molecules act as molecular recognizers in the sensing system. In the presence of target ions, Fc signals are affected by conformational changes of the aptamer. The "Ag nanowires@zeolitic imidazolate framework-8 with methylene blue (AgNWs@ZIF-8/MB)" can be semi-complementary to the Pb²⁺ aptamer after binding to single-stranded DNA (S₁). However, S₁/AgNWs@ZIF-8/MB self-assembled with Pb²⁺ aptamer (Apt) by hybridization incubation was quickly replaced by Pb²⁺ competitively, resulting in the loss of methylene blue (MB) signaling molecules. Hence, the internal reference signal (MB) and conformation change signal (Fc) comprise the ratio sensing system well. Morphology, spectroscopy, and electrochemistry methods have validated the modification and sensing behaviors. The used Apt has made considerable progress in analytical performance. In interference studies and stability checks, the ratio measurement signal I_Fc/I_MB is a more reliable signal than the single signal readout. Following a log-linear relationship, this sensor provides a wide linear range. Furthermore, the proposed sensor can be used to determine Pb²⁺ in fish samples, and the results agree with those obtained using ICP-MS and recovery tests.

Visual Detection and Sensing of Mercury Ions and Glutathione Using Fluorescent Copper Nanoclusters

Visual detection of mercury ions and glutathione is of great significance to public health and environmental issues. Herein, we developed a fluorescent sensor (l-Cys/CuNCs@ESM) based on the eggshell membrane (ESM) and red-emitting copper nanoclusters (CuNCs) by the in situ strategy via l-cysteine (l-Cys) as the reducing and protective agent for mercury ions and glutathione sensing visually. The as-prepared fluorescent product had good stability, portability, large Stokes shift (250 nm), and long fluorescence lifetime (7.3 μs). Notably, the l-Cys/CuNCs@ESM exhibited a specific fluorescence quenching response toward Hg²⁺. Moreover, the interaction between glutathione (GSH) and Hg²⁺ could subsequently recover the fluorescence effectively. Inspired by this "on-off-on" switch, the l-Cys/CuNCs@ESM was applied as the dual-sensing system for visual detection of mercury ions and glutathione integrating with the portable smartphone. The limit of detection (LOD) of Hg²⁺ is 1.1 μM for visualization and 0.52 μM for the fluorescence spectrometer. The corresponding LODs of GSH are 2.8 and 0.59 μM, respectively. This platform presents significant sensitivity, specificity, and stability, offering a promising potential for real-time/on-site sensing.

Vein-Like Ni-BTC@Ni3S4 with Sulfur Vacancy and Ni3+ Fabricated In Situ Etching Vulcanization Strategy for an Electrochemical Sensor of Dopamine

In this study, a novel Ni-BTC@Ni₃S₄ composite was fabricated by solvothermal reaction using an in situ etching vulcanization strategy and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), and Brunauer-Emmett-Teller (BET) analyses. The existence of a sulfur vacancy and Ni³⁺ in the as-prepared vein-like Ni-BTC@Ni₃S₄ greatly promoted the electrochemical sensing activity of the materials. Herein, a simple electrochemical sensor (Ni-BTC@Ni₃S₄/CPE) has been fabricated and used for the detection of dopamine (DA). The current signal of the Ni-BTC@Ni₃S₄/CPE-modified electrode was linear with the concentration of DA in the range of 0.05-750 μM (R² = 0.9995) with a sensitivity of 560.27 μA·mM^-1·cm^-2 and a detection limit of 0.016 μM. At the same time, the sensor has good stability and anti-interference ability. This study could provide a new idea and strategy for the structural regulation of composite electrode-modified materials and sensitive sensing detection of small biological molecules.

Growth Control of Metal-Organic Framework Films on Marine Biological Carbon and Their Potential-Dependent Dopamine Sensing

Ever-evolving advancements in films have fueled many of the developments in the field of electrochemical sensors. For biosensor application platforms, the fabrication of metal-organic framework (MOF) films on microscopically structured substrates is of tremendous importance. However, fabrication of MOF film-based electrodes always exhibits unsatisfactory performance, and the mechanisms of the fabrication and sensing application of the corresponding composites also need to be explored. Here, we report the fabrication of conformal MIL-53 (Fe) films on carbonized natural seaweed with the assistance of an oxide nanomembrane and a potential-dependent electrochemical dopamine (DA) sensor. The geometry and structure of the composite can be conveniently tuned by the experimental parameters, while the sensing performance is significantly influenced by the applied potential. The obtained sensor demonstrates ultrahigh sensitivity, a wide linear range, a low limit of detection, and a good distinction between DA and ascorbic acid at an optimized potential of 0.3 V. The underneath mechanism is investigated in detail with the help of theoretical calculations. This work bridges the natural material and MOF films and is promising for future biosensing applications.

Degradation of contaminants of emerging concern in a secondary effluent using synthesized MOF-derived photoanodes: A comparative study between photo-, electro- and photoelectrocatalysis

Three metal-organic framework (MOF)-based photoanodes were prepared by deposition on TiO₂ nanotubes using Ti as substrate (Ti/TiO₂NT): i) Ti/TiO₂NT-Au@ZIF-8, ii) Ti/TiO₂NT-Ru₃(BTC)₂, iii) Ti/TiO₂NT-UiO-66(Zr)NH₂. These photoanodes were characterized by FEG-SEM, EDX and DRX. The analyses showed a successful modification and a high homogeneity of the different MOFs on the Ti/TiO₂NT surface. The photoanodes were studied in the degradation of Contaminants of Emerging Concern (CECs) in a spiked secondary effluent from a Municipal Wastewater Treatment Plant (MWWTP). Sodium diclofenac (DCF), sulfamethazine (SMT) and carbamazepine (CBZ) were used as CECs at low concentration (200 μg/L each CEC). The samples were preconcentrated using Solid Phase Extraction (SPE) and analyzed by a HPLC-DAD system. The MOF-based photoanodes exhibited a high photoelectrochemical (PEC) activity towards the oxidation of CECs, achieving up to 50%, 70% and 80% of removal using Ti/TiO₂NT-Au@ZIF-8, Ti/TiO₂NT-UiO-66(Zr)NH₂, Ti/TiO₂NT-Ru₃(BTC), respectively. The influence of the generation of hydroxyl radical was then studied. The results indicate that PEC degradation using Ti/TiO₂NT-Ru₃(BTC)₂ and Ti/TiO₂NT-UiO-66(Zr)NH₂ is more affected by the concentration of the radical.

A dual-recognition MIP-ECL sensor based on boric acid functional carbon dots for detection of dopamine

We report a molecularly imprinted polymer electrochemiluminescence (MIP-ECL) sensor with dual recognition effects on dopamine (DA). Boric-acid-functionalized carbon dots (B-CDs) with good ECL performance at - 2.0 V (vs. Ag/AgCl) were prepared and immobilized on a glassy carbon electrode (GCE). The MIP was then introduced via electropolymerization using o-phenylenediamine (OPD) as a functional monomer and DA as a template molecule to fabricate the MIP-ECL sensor. The cavities in the MIP after elution were used to capture the target molecular DA. The affinity of boric acid of B-CDs to the cis-diol of DA, as well as the special recognition of MIP, provides dual recognition effects on DA. The selective readsorption of DA onto the sensor leads to the ECL quenching of B-CDs. The quenching effect was used to detect DA from 1.0 × 10^-9 to 1.0 × 10^-5 mol·L^-1 with a detection limit of 2.1 × 10^-10 mol·L^-1. The dual recognition caused the MIP-ECL sensor exhibiting excellent selectivity and sensitivity toward  DA. The sensor was successfully used to determine DA in real samples.

Engineering the ZIF-8 Pore for Electrochemical Sensor Applications-A Mini Review

Zinc imidazole framework-8, abbreviated as ZIF-8, is a member of the metal organic framework (MOF) family. The chemical architecture of ZIF-8 consists of zinc metal duly coordinated with an organic ligand/fragment, resulting in a cagelike three-dimensional network with unique porosity. Because of such a unique architecture and physicochemical property, ZIF-8 has recently been explored in various applications such as gas storage, catalysis, electrochemical sensing, drug delivery, etc. Electrochemical sensors are currently a hot topic in scientific advances, where small, portable, Internet of Things (IoT)-enabled devices powered by electrochemical output show a newer path toward chemo and biosensor applications. The unique electrochemical property of ZIF-8 is hence explored widely for possible electrochemical sensor applications. The application and synthesis of the bare ZIF-8 have been widely reported for more than a decade. However, new scientific advancements depict tailoring the bare ZIF-8 structure to achieve smart hybrid ZIF-8 materials that show more advanced properties compared to bare ZIF-8. The framework is formed by joining inorganic (metal-containing) units with organic linkers by reticular synthesis, which results in the formation of a cross-linked crystalline network with permanent porosity. This unique porosity of ZIF-8 has recently been utilized for the encapsulation of suitable guest species to enhance the native physicochemical activity of ZIF-8. These engineered ZIF-8 materials show excellent results, especially for electrochemical sensing application. This review is intended to describe the research, including the one done by our group, where the ZIF-8 pore size is used for encapsulating nanoparticles, enzymes, and organic compounds to avail suitable sensor applications.

AuNP@ZeNose (ZIF-based electrochemical nose) for detection of flu biomarker in breath

A novel electrochemical sensor is reported for the detection of isoprene levels in breath using a ZIF-based electrochemical nose. This sensor incorporates a hybrid detection system using gold nanoparticles encapsulated inside the ZIF-8 moiety. Breath-based analysis is widely being used for monitoring the metabolic state of the body. It is associated with the change in the concentration of volatile organic compounds and inorganic gases released endogenously and can be tracked using breath as the sample. One such volatile organic compound, isoprene, has been correlated to the presence of influenza virus or respiratory inflammation. Analytical techniques such as powder X-ray diffraction, scanning electron microscopy, atomic force microscopy, Fourier transform infrared spectroscopy, and tunneling electron microscopy were used to understand the structural features of the composite. The electrochemical nose system uses chronoamperometry as the transduction mechanism to monitor the diffusion kinetics of the target analyte across the electrode-electrolyte interface. The presented work demonstrates isoprene sensing with high sensitivity and specificity and a detection limit of 10 parts per billion in air. We successfully demonstrate the functionality of the ZIF-based electrochemical nose for point-of-care screening of isoprene levels by developing a prototype device using a commercially available development board. We foresee that the developed sensing platform can help in early screening for the presence of influenza virus and help control the infection rate.

Molecularly imprinted electrochemiluminescence sensor based on ZIF-8 doped with CdSe quantum dots for the detection of trace estriol

A composite of the metal-organic framework compound ZIF-8 doped with CdSe quantum dots (QDs) with sensitive and stable luminescence was synthesized, and a molecularly imprinted electrochemiluminescence (ECL) sensor was constructed on the basis of this composite. The ZIF-8@CdSe molecularly imprinted ECL sensor combines the high sensitivity of ECL and the high selectivity of molecular imprinting to realize the sensitive and specific detection of estriol. CdSe QDs and gold nanoparticles were encapsulated within ZIF-8 to obtain the ZIF-8@CdSe QDs/GNP (ZIF@CdSe/GNP) composite. Subsequently, the GNPs were further loaded on the surface of this composite to obtain the GNP/ZIF@CdSe/GNP composite. L-cysteine was used to immobilize the GNP/ZIF@CdSe/GNP composite on the surface of a gold electrode to obtain the GNP/ZIF@CdSe/GNP-modified gold electrode. A molecularly imprinted polymer (MIP) film was prepared on the surface of the modified electrode via electropolymerization with o-phenylenediamine as the functional monomer and estriol as the template molecule. After elution, estriol could be specifically recognized by the cavities. The re-adsoption of estriol by the MIP can prevent the coreactant from reaching the electrode surface through the cavities, thereby weakening ECL. A good linear relationship existed between the ∆ECL and lg C of estriol concentrations of 1 × 10^-14 - 1 × 10^-9 mol·L^-1 . The detection limit was as low as 8.9 × 10^-16 mol·L^-1 . The sensor was applied in the determination of estriol in serum samples with a recovery of 97.0 % - 102 %.

Yeast powder derived carbon quantum dots for dopamine detection and living cell imaging

Dopamine (DA) is an important neurotransmitter in the brain of mammals. There is a critical need for fast and sensitive determination approaches to monitor these potential diseases due to various weaknesses in clinical trials of the existing methods for DA detection. DA can effectively quench the fluorescence of carbon quantum dots (CDs) through the inner filter effect and static quenching. In this work, fluorescent yeast CDs (Y-CDs) were prepared via a simple hydrothermal approach of using yeast powder and regarded as the fluorescent nanoprobe to directly monitor the DA concentration. The as-prepared detection platform exhibited excellent sensitivity and selectivity toward DA with a low detection limit of 30 nM and a wide linear range of 0.05-150 μM. Benefiting from these outstanding features, the developed label-free method has been successfully applied for fast DA detection in human serum samples with satisfactory recoveries. Furthermore, it demonstrated that the Y-CDs were well suitable for live cell imaging and showed low toxicity toward MCF-7 cells. Consequently, this work will facilitate the great potential of the versatile Y-CDs in developing biosensors for clinical diagnosis and other biological applications.

A highly selective and sensitive electrochemical sensor for dopamine based on a functionalized multi-walled carbon nanotube and poly(N-methylaniline) composite

Dopamine (DA) is an important neurotransmitter used for diagnosing various diseases from its abnormal concentrations in human fluids. Herein, an electrochemical sensor based on a composite of re-doped poly(N-methylaniline) (rePNMA) and modified multi-walled carbon nanotubes (fMWCNTs), termed fMWCNT-rePNMA, was developed to measure DA concentration. The successful modification of the fMWCNT surface was confirmed by Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Cyclic voltammetry (CV) displayed an excellent electrocatalytic activity of the fMWCNTs-rePNMA composite towards the oxidation of DA. The developed fMWCNTs-rePNMA composite demonstrated a broad linear range from 5 to 90 μmol L^-1 with a low limit of detection (LOD) value of 2.23 μmol L^-1, and a fast response with a high sensitivity of 251.5 nA μmol^-1 L as determined from the calibration curve of the DA determination. In addition, the fMWCNTs-rePNMA composite selectively identified and quantified DA in the presence of ascorbic acid (AA) and uric acid (UA). Therefore, the fMWCNTs-rePNMA composite sensor shows potential to determine the level of DA in human urine.

2D leaf-like ZIF-L decorated with multi-walled carbon nanotubes as electrochemical sensing platform for sensitively detecting thiabendazole pesticide residues in fruit samples

Excessive use of pesticides in modern agriculture results in large amounts of pesticide residues in agricultural production, greatly threatening human health. Herein, novel two-dimensional leaf-like zeolitic imidazolate framework-L decorated with multi-walled carbon nanotubes (MWCNTs/ZIF-L) was prepared by a facile solvent way and exploited as electrode material for sensitive electrochemical sensing of thiabendazole (TBZ). Two-dimensional ZIF-L presents high surface area, large pore volume, and abundant active sites, which exhibits high enrichment ability towards TBZ molecules, while the MWCNTs interspersed on ZIF-L can prominently enhance the electron transport capability and improve the electrocatalytic activity for TBZ oxidation. Due to the intriguing synergy between the components, the MWCNTs/ZIF-L-based electrochemical sensor reveals a limit of detection (LOD) of 6.0 nmol·L^-1, which is lower than that reported in most literatures. Additionally, satisfactory reproducibility and repeatability, long-term stability, and excellent selectivity are acquired. The proposed method was also applied for the detection of TBZ in apple and orange samples with acceptable recoveries.

Metal-organic frameworks for chemical sensing devices

Metal-organic frameworks (MOFs) are exceptionally large surface area materials with organized porous cages that have been investigated for nearly three decades. Due to the flexibility in their design and predisposition toward functionalization, they have shown promise in many areas of application, including chemical sensing. Consequently, they are identified as advanced materials with potential for deployment in analytical devices for chemical and biochemical sensing applications, where high sensitivity is desirable, for example, in environmental monitoring and to advance personal diagnostics. To keep abreast of new research, which signposts the future directions in the development of MOF-based chemical sensors, this review examines studies since 2015 that focus on the applications of MOF films and devices in chemical sensing. Various examples that use MOF films in solid-state sensing applications were drawn from recent studies based on electronic, electrochemical, electromechanical and optical sensing methods. These examples underscore the readiness of MOFs to be integrated in optical and electronic analytical devices. Also, preliminary demonstrations of future sensors are indicated in the performances of MOF-based wearables and smartphone sensors. This review will inspire collaborative efforts between scientists and engineers working within the field of MOFs, leading to greater innovations and accelerating the development of MOF-based analytical devices for chemical and biochemical sensing applications.

Electrochemical characterization of and theoretical insight into a series of 2D MOFs, [M(bipy)(C4O4)(H2O)2]·3H2O (M = Mn (1), Fe (2), Co (3) and Zn (4)), for chemical sensing applications

The electrochemical sensing applications of a series of water-stable 2D metal–organic framework (MOF)-modified screen-printed carbon electrodes (SPCEs) are reported. The MOF materials in this study are [M(bipy)(C₄O₄)(H₂O)₂]·3H₂O, in which bipy = 4,4′-bipyridine and M = Mn, Fe, Co and Zn. The MOF materials are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), showing that the MOFs have a layer-by-layer rod structure with a smooth surface. We use the nitrofurazone molecule as a probe to investigate the influence of the metal ions of MOFs on electrochemical sensing ability. Cyclic voltammetry demonstrated that the Mn-MOF electrode of interest delivered stronger signals than that of other electrodes. Through first-principles calculations, we also revealed that the change in the spin polarization of divalent metal ions passing from the free ion state to the MOF environment appeared to be significantly correlated with the enhancement in the peak response current. The theoretical and experimental results consistently indicate that Mn-MOF has the smallest bandgap and good sensitivity among these MOF materials. Accordingly, we proposed a simple model to illustrate this observation and disclosed the importance of the electron configuration of the transition metal constructing the MOF materials used in improving electrochemical sensing applications.

Framework-to-metal charge transfer of the MOF materials results in enhancing electrochemical sensing ability to nitrofurazone.

Gold nanoparticle decorated rGO-encapsulated metal-organic framework composite sensor for the detection of dopamine

In this study, by encapsulation of reduced graphene oxide (rGO) into Ni-based metal–organic framework (Ni-MOF) structure, the composite rGO@Ni-MOF was first prepared. Then, gold nanoparticle (AuNP) decorated rGO@Ni-MOF (rGO@Ni-MOF/AuNP) were obtained through the electrodeposition. The morphology and structure of rGO@Ni-MOF/AuNP were characterized by SEM, FTIR, and XRD. The rGO@Ni-MOF/AuNP modified electrode was used for the detection of dopamine. Combining the catalysis from Ni-MOF and AuNP with the conductivity of rGO endowed rGO@Ni-MOF/AuNP with synergetic high catalytic activity to the electrochemical oxidation of dopamine. The developed modified electrode had a good linear relationship with dopamine in the concentration range of 0.5∼120 μM, and the detection limit was 0.33 μM (S/N = 3). Additionally, the potential interferents, electrode stability, reproducibility, and practical applications were also studied and satisfactory results were obtained.

Electrochemical/visual dual-readout aptasensor for Ochratoxin A detection integrated into a miniaturized paper-based analytical device

Development of portable, sensitive and reliable devices for Ochratoxin A (OTA) detection is highly demanded, especially for resource-limited regions. Herein, a novel paper-based analytical device (PAD) is designed through wax printing and screen-printed technologies, which integrates sample flowing, electrode modification, cleaning and electrochemical (EC)/colorimetric signal output. To greatly enhance the detection sensitivity, we synthesized a chitosan functionalized MoS₂-Au@Pt (Ch-MoS₂-Au@Pt) via electrostatic self-assembly, and used it to immobilize the label aptamer (apta₂) for signal regulation and amplification. Concretely, with the addition of analytes, the Ch-MoS₂-Au@Pt-apta₂ could be combined on the sensing interface by specific biorecognition and catalyzed reduction of H₂O₂, resulting in a remarkable EC response. Meanwhile, the released hydroxyl radicals (·OH) flowed to the visualization zone and promoted the oxidation of 3,3',5,5'-tetramethylbenzidine for colorimetric detection. Consequently, the dual-mode PAD achieved acceptable prediction and accurate analysis in the range of 0.1-200 ng mL^-1 and 1 × 10^-4-200 ng mL^-1 by matching the visual and EC signal intensity, respectively. Compared with traditional single-mode sensor for OTA, the proposed dual-mode aptasensor featuring independent signal conversion and readout, not only avoided the false-positive signal associated with detection condition and operation, but also enlarged the detection ranges and improved the sensitivity. Furthermore, the consistency of EC/colorimetric assay was validated in real OTA samples. Overall, this work provided a portable, cost-effective, sensitive and visualized aptasensor platform, which could be extended to various other mycotoxins in the field of food safety.

Magnetic molecularly imprinting polymers, reduced graphene oxide, and zeolitic imidazolate frameworks modified electrochemical sensor for the selective and sensitive detection of catechin

A glassy carbon electrode (GCE) was modified with magnetic molecularly imprinted polymers (mMIPs) using catechin as a template, reduced graphene oxide (rGO), and zeolitic imidazolate frameworks-8 (ZIF-8) for the sensitive detection of catechin (mMIPs/rGO-ZIF-8/GCE). The prepared rGO, ZIF-8, and mMIPs exhibited typical structures and properties determined by various characterizations. The mMIPs showed good selectivity for catechin among several structural analogs. The mMIPs/rGO-ZIF-8/GCE showed a higher maximum peak current for catechin than that of a single component modified GCE. After the optimization of the material ratio, coating amounts, pH, and scan rate, the mMIPs/rGO-ZIF-8/GCE exhibited good selectivity, good linearity, and a low detection limit (LOD) for catechin. The linear range was 0.01 nmol/L-10 μmol/L and the LOD was 0.003 nmol/L (S/N = 3). The relative standard deviations for reproducibility and stability tests (n = 6) were 5.2% and 6.1%, respectively. A recovery between 99.1 and 101.3% was obtained in the detection of catechin in spiked samples. Based on these findings, the proposed mMIPs/rGO-ZIF-8/GCE could be developed further, and future research could be conducted on alternate fabrication strategies and methods to create more portable and practical electrochemical sensors. Graphical Abstract.

Two-dimensional conductive phthalocyanine-based metal-organic frameworks for electrochemical nitrite sensing

2D nickel phthalocyanine based MOFs (NiPc-MOFs) with excellent conductivity were synthesized through a solvothermal approach. Benefiting from excellent conductivity and a large surface area, 2D NiPc-MOF nanosheets present excellent electrocatalytic activity for nitrite sensing, with an ultra-wide linear concentration from 0.01 mM to 11 500 mM and a low detection limit of 2.3 μM, better than most reported electrochemical nitrite sensors. Significantly, this work reports the synthesis of 2D conductive NiPc-MOFs and develops them as electrochemical biosensors for non-enzymatic nitrite determination for the first time.

Bi-metallic zeolite imidazole framework nanofibers for the selective determination of Cd2+ ions

Cobalt zinc-zeolite imidazole framework (Co/Zn-ZIF) nanofibers are made via an electrospinning (ES) approach and tested for the detection of heavy metal cadmium ions. Electrostatically attracted cobalt and zinc ions are bound regularly on the surface of the ZIF network. The cobalt and zinc ions are organized with the ZIF network, which provides the sturdily bonded tetrahedral structure of Co/Zn-ZIF, giving essential steadiness to the composite material. Cyclic voltammetry revealed that the observed profile is reversible, and the catalytic behavior of the electrodes provided evidence of interfacial electron transfer between the nanofiber-modified GCE surface and the metal ions. Interestingly, a careful determination of Cd2+ ions within the range of 100 nM to 1 mM with a low limit detection of 27.27 nM was undertaken. The established heavy metal ion detector shows excellent anti-interference abilities toward the observed electroactive species, and it was successfully employed using a tap water sample for Cd2+ ion detection, where good results were observed.

In Situ Laser-Induced Fabrication of a Ruthenium-Based Microelectrode for Non-Enzymatic Dopamine Sensing

In this paper, we propose a fast and simple approach for the fabrication of the electrocatalytically active ruthenium-containing microstructures using a laser-induced metal deposition technique. The results of scanning electron microscopy and electrical impedance spectroscopy (EIS) demonstrate that the fabricated ruthenium-based microelectrode had a highly developed surface composed of 10 μm pores and 10 nm zigzag cracks. The fabricated material exhibited excellent electrochemical properties toward non-enzymatic dopamine sensing, including high sensitivity (858.5 and 509.1 μA mM⁻¹ cm⁻²), a low detection limit (0.13 and 0.15 μM), as well as good selectivity and stability.

Stretchable Electrochemical Biosensing Platform Based on Ni-MOF Composite/Au Nanoparticle-Coated Carbon Nanotubes for Real-Time Monitoring of Dopamine Released from Living Cells

Existing electrochemical biosensing platforms, using traditional rigid and unstretchable electrodes, cannot monitor the biological signaling molecules released by cells in a mechanically deformed state in real time. Here, a stretchable and flexible electrochemical sensor was developed based on nickel metal-organic framework composite/Au nanoparticle-coated carbon nanotubes (Ni-MOF composite/AuNPs/CNTs) for sensitive detection of dopamine (DA) released by C6 living cells in real time. A Ni-MOF composite was obtained by introducing Ni, NiO, and a carbon frame onto the surface of two-dimensional (2D) Ni-MOF nanosheets using an efficient one-step calcination method. The hybrid of Ni-MOF composite/AuNPs/CNTs that deposited on the poly(dimethylsiloxane) (PDMS) film endowed the sensor with excellent electrochemical performance with a wide linear range of 50 nM to 15 μM and a high sensitivity of 1250 mA/(cm² M) and also provided the sensor with desirable stability against mechanical deformation. Furthermore, the stretchable electrode also displayed good cellular compatibility while C6 living cells can be cultured and proliferated on it with strong adhesion. Then, the DA released by C6 living cells with chemical induction in both natural and stretched states was monitored using our stretchable and flexible electrochemical sensor in real time. This indicates that our new design of flexible Ni-MOF composite/AuNPs/CNTs/PDMS (NACP) film electrodes provides more opportunities for the detection of chemical signals released from cells and soft living organisms even under mechanically deformed states.

A DNA Based Biosensor Amplified With ZIF-8/Ionic Liquid Composite for Determination of Mitoxantrone Anticancer Drug: An Experimental/Docking Investigation

An ultrasensitive DNA electrochemical biosensor based on the carbon paste electrode (CPE) amplified with ZIF-8 and 1-butyl-3-methylimidazolium methanesulfonate (BMIMS) was fabricated in this research. The DNA/BMIMS/ZIF-8/CPE was used for the selective determination of a mitoxantrone anticancer drug in aqueous solution, resulting in a good catalytic effect and a powerful ability for determining mitoxantrone. Also, the interaction of the mitoxantrone anticancer drug with guanine bases of ds-DNA was used as a powerful strategy in the suggested biosensor, which was confirmed with docking investigation. Docking study of mitoxantrone into the ds-DNA sequence showed the intercalative binding mode of mitoxantrone into the nitrogenous-based pairs of ds-DNA. The effective factors such as ds-DNA concentration, temperature, buffer types, and incubation time were also optimized for the fabricated mitoxantrone biosensor. The results showed that, under optimum conditions (T = 25°C; incubation time=12 min; pH= 4.8 acetate buffer solution and [DNA] = 50 mg/L), the DNA/BMIMS/ZIF-8/CPE could be used in mitoxantrone assay in a concentration ranging from 8.0 nM to 110 μM with a detection limit of 3.0 nM. In addition, recovery data between 99.18 and 102.08% were obtained for the determination of mitoxantrone in the injection samples using DNA/ZIF-8/BMIMF/CPE as powerful biosensors.