Mn-Doping-induced hierarchical petal growth of a flower-like 3D MOF assembled with black phosphorous nanosheets as an electrochemical aptasensor of human stress-induced phosphoprotein 1

Magnetic metal-organic frameworks as sensitive aptasensors for coronavirus spike protein

Electrochemical biosensors, known for their low cost, sensitivity, selectivity, and miniaturization capabilities, are ideal for point-of-care devices. The magnetic metal-organic framework (MMOF), synthesized using the in-situ growth method, consists of ferric salt, magnetic nanoparticles, histidine, and benzene tetracarboxylic acid. MMOF was sequentially modified with aptamer-biotin and streptavidin-horseradish peroxidase, serving as a detector for spike protein and a transducer converting electrochemical signals using H2O2-hydroquinone on a screen-printed electrode. MMOF facilitates easy washing and homogeneous deposition on the working electrode with a magnet, enhancing sensitivity and reducing noise. The physical and electrochemical properties of the modified MMOFs were thoroughly characterized using various analytical techniques. The aptasensors' performance achieved a detection limit of 6 pM for voltammetry and 5.12 pM for impedance spectroscopy in human serum samples. This cost-effective, portable MMOF platform is suitable for rapid point-of-care testing for SARS-CoV-2 spike proteins.

Portable self-powered electrochemical aptasensing platform for ratiometric detection of mycotoxins based on multichannel photofuel cell

Self-powered electrochemical sensors based on photofuel cells have attracted considerable research interest because their unique advantage of not requiring an external electric source, but their application in portable and multiplexed targets assay is limited by the inherent mechanism. In this work, a portable self-powered sensor constructed with multichannel photofuel cells was developed for the ratiometric detection of mycotoxins, namely ochratoxin A (OTA) and patulin (PAT). The spatially resolved CdS/Bi2S3-modified photoanodes and a shared Prussian Blue cathode were integrated on an etched indium-tin oxide slide to fabricate the multichannel photofuel cell. The aptamers of OTA and PAT were covalently bonded to individual photoanode regions to build sensitive interfaces, and the specific recognition of analytes impaired the output performance of constructed PFC. Accordingly, ratiometric sensing of OTA and PAT was achieved by utilizing the output performance of a control PFC as a reference signal. This approach effectively eliminates the impact of light intensity on the accuracy of the detection. Under the optimal conditions, the proposed sensing chip exhibited linear ranges of 2.0-1000 nM and 5.0-500 nM for OTA and PAT, respectively. The detection limits (3 S/N) were determined to be 0.25 nM for OTA and 0.27 nM for PAT. The developed ratiometric sensing method demonstrated good selectivity and stability in the simultaneous detection of OTA and PAT. It was successfully utilized for the analysis of OTA and PAT real samples. This work provides a new perspective for construction of portable and ratiometric self-powered sensing platform.

ZIF-8 labelled a new electrochemical aptasensor based on PEI-PrGO/AuNWs for DON detection

In this work, a novel ultrasensitive aptasensor for deoxynivalenol (DON) detection based on the polyethyleneimine-functionalised porous reduced graphene oxide loaded gold nanowires (PEI-PrGO/AuNWs) and methylene blue (MB)-labelled zeolitic imidazolate framework-8 (ZIF-8) signal amplification strategy was proposed. PEI-PrGO/AuNWs with large surface area and excellent conductivity were used as modification materials on bare gold electrodes, which could increase the combining of complementary strand (cDNA) on the electrode substrate and accelerate the electron transfer efficiency. Furthermore, a novel electrochemical signal probe was synthesized using streptavidin-modified zeolitic imidazolate framework-8 (ZIF-8/SA) as a carrier loaded with MB and reverse complementary chain (sDNA). In the presence of DON, the signal probe was introduced to the electrode surface by Watson-Crick base pairing after specific binding of DON to the aptamer (Apt). As expected, under the optimal conditions, the DON concentration was linearly related to the peak current generated by the prepared aptasensor, and the measured data were combined with theoretical calculations to obtain a detection limit of 2.23 × 10-9 mg/mL.

Ni/Mn metal-organic framework decorated bacterial cellulose (Ni/Mn-MOF@BC) and nickel foam (Ni/Mn-MOF@NF) as a visible-light photocatalyst and supercapacitive electrode

Recently, metal-organic frameworks (MOFs) and hybrids with biomaterial are broadly investigated for a variety of applications. In this work, a novel dual-phase MOF has been grown on bacterial cellulose (BC) as a biopolymer nano-fibrous film (Ni/Mn-MOF@BC), and nickel foam (Ni/Mn-MOF@NF) using a simple reflux method to explore their potential for photocatalyst and energy storage applications. The studies showed that the prepared Mn and Ni/Mn-MOFs display different structures. Besides, the growth of MOFs on BC substantially changed the morphology of the samples by reducing their micro sized scales to nanoparticles. The nanosized MOF particles grown on BC served as a visible-light photocatalytic material. Regarding the high surface area of BC and the synergistic effect of two metal ions, Ni/Mn-MOF@BC with a lower band gap demonstrates remarkable photocatalytic degradation efficiency (ca. 84% within 3 h) against methylene blue (MB) dye under visible light, and the catalyst retained 65% of its initial pollutant removal properties after four cycles of irradiation. Besides, MOF powders deposited on nickel foam have been utilized as highly capacitive electrochemical electrodes. There, Ni/Mn-MOF@NF electrode also possesses outstanding electrochemical properties, showing a specific capacitance of 2769 Fg-1 at 0.5 Ag-1, and capacity retention of 94% after 1000 cycles at 10 Ag-1.

Epitaxial growth of metal-organic framework nanosheets into single-crystalline orthogonal arrays

Construction of two-dimensional nanosheets into three-dimensional regular structures facilitates the mass transfer and exploits the maximum potential of two-dimensional building blocks in applications such as catalysis. Here, we report the synthesis of metal-organic frameworks with an orthogonal nanosheet array. The assembly involves the epitaxial growth of single crystalline metal-organic framework nanosheets with a naturally non-preferred facet exposure as the shell on a cubic metal-organic framework as the core. The nanosheets, despite of two typical shapes and crystallographic orientations, also form a single crystalline orthogonally arrayed framework. The density and size of nanosheets in the core-shell-structured composite metal-organic frameworks can be well adjusted. Moreover, metal-organic frameworks with a single composition and hollow orthogonal nanosheet array morphology can be obtained. Benefiting from the unusual facet exposure and macroporous structure, the designed structure exhibits improved electrocatalytic oxygen evolution activity compared to conventional nanosheets.

DNA-directed assembly of nanomaterials and their biomedical applications

In the past decades, DNA has been widely used in the field of nanostructures due to its unique programmable properties. Besides being used to form its own diverse structures such as the assembly of DNA origami, DNA can also be used for the assembly of nanostructures with other materials. In this review, different strategies for the functionalization of DNA on nanoparticle surfaces are listed, and the roles of DNA in the assembly of nanostructures as well as the influencing factors are discussed. Finally, the biomedical applications of DNA-assembled nanostructures were summarized. This review provided new insight into the application of DNA in nanostructure assembly.

Binder-free mechanochemical metal-organic framework nanocrystal coatings

The practical applications of metal-organic frameworks (MOFs) usually require their assembly into mechanically robust structures, usually achieved via coating onto various types of substrates. This paper describes a simple, scalable, and versatile mechanochemical technique for producing MOF nanocrystal coatings on various non-prefunctionalised substrates, including ZrO₂, carbon cloth, porous polymer, nickel foam, titanium foil and fluorine-doped tin oxide glass. We revealed the detailed mechanisms that ensure the coating's stability, and identified the coating can facilitate the interfacial energy transfer, which allowed the electrocatalysis application of the MOF coating on conductive substrates. We further demonstrated that coatings can be directly generated in a one-pot fashion by ball milling MOF precursors with substrates.

Oxygen Vacancies of CeO2 Nanospheres by Mn-Doping: An Efficient Electrocatalyst for N2 Reduction under Ambient Conditions

The electrochemical N₂ reduction reaction (NRR) demonstrates a process of NH₃ synthesis from N₂ molecules under ambient conditions, which is environmentally friendly and recyclable. However, it requires an efficient electrocatalyst to activate inert N₂ molecules, which is still difficult to satisfy. Recently, as an active NRR electrocatalyst and a typical metal oxide, CeO₂ has featured ultrahigh thermal stability and the ability to apply heteroatom doping, which is an imperative approach importing oxygen vacancy by replacing metal ions with selective elements to greatly influence the activity of catalysts. Here, we analyze the unique properties of manganese dopants in modulating the activity of CeO₂ nanospheres for NRR. It attains a larger NH₃ yield of 27.79 μg h^-1 mg_cat^-1 and a higher Faradaic efficiency of 9.1% than pure CeO₂ at -0.30 V in 0.1 M HCl, with high electrochemical and structure stability. With calculations by density functional theory, the performance enhancement of Mn-doped CeO₂ is also proved mathematically.

A petal-shaped MOF assembled with a gold nanocage and urate oxidase used as an artificial enzyme nanohybrid for tandem catalysis and dual-channel biosensing

A facile one-pot precipitation method was employed to prepare a petal-shaped hybrid under mild conditions. The hybrid is composed of urate oxidase (UOx) encapsulated into a zeolite-like metal-organic framework (MOF) with the doping of a hollow gold nanocage (AuNC). As one of the MOF-enzyme composites, a UOx@MOF(AuNC) hybrid with the features of artificial nanoenzymes was developed as a novel dual-channel biosensing platform for fluorescence (FL) and electrochemical detection of uric acid (UA). As for FL biosensing, enzymatic catalysis of the hybrid in the presence of UA triggered tandem catalysis and oxidation reactions to cause FL quenching. UA was linearly detected in the 0.1-10 μM and 10-300 μM ranges, with the limit of detection (LOD) of 20 nM. As for electrochemical biosensing, the hybrid was dropped on a glassy carbon electrode (GCE) surface to construct a hybrid/GCE platform. Based on the redox reaction of UA on the platform surface, UA was linearly detected in the 0.05-55 μM range, with a LOD of 15 nM. Experimental results confirmed that the hybrid-based dual-channel biosensing platform enabled selective and sensitive responses to UA over potential interferents. The platform has an excellent detection capability in physiological samples. The dual-channel biosensing platform facilitates the exploration of new bioanalysis techniques for early clinical diagnosis of diseases.

Flexible paper-based Ni-MOF composite/AuNPs/CNTs film electrode for HIV DNA detection

It is very important to develop a rapid, simple, low cost point-of-care (POC) method for the early diagnosis of pathogens. In this work, a flexible paper-based electrode based on nickel metal-organic framework (Ni-MOF) composite/Au nanoparticles/carbon nanotubes/polyvinyl alcohol (Ni-Au composite/CNT/PVA) was constructed to detect target human immunodeficiency virus (HIV) DNA by DNA hybridization using methylene blue (MB) as a redox indicator. The CNT/PVA and Ni-Au composite were deposited on the cellulose membrane by vacuum filtration and drop-coating method in turn to obtain Ni-Au composite/CNT/PVA (CCP) film electrode. Compared to the CNT/PVA film electrode, CCP film electrode makes a higher loading of the probe DNA for its large specific surface area and conjugated π-electron system that can provide hydrogen bond sources to achieve interactions between MOF and single-stranded DNA, which improves the sensitivity for detecting target DNA. The variation of peak current for MB molecules adsorbed onto DNA before and after hybridization with HIV DNA was monitored. Electrochemical results proved that the CCP film maintained stable electrochemical property even after bending 200 times or stretching under different strains from 0% to 20%. The flexible paper electrode showed excellent sensing performance with a linear range of 10 nM-1 μM and a low detection limit of 0.13 nM. The target HIV DNA was successfully detected even in complex serum samples using the flexible CCP film electrode. Therefore, the simple and inexpensive flexible paper-based MOF composite film electrode can also be utilized for other pathogens POC diagnosis.

Recent advances in black phosphorus-based electrochemical sensors: A review

Since the discovery of liquid-phase-exfoliated black phosphorus (BP) as a field-effect transistor in 2014, BP, with its 2D layered structure, has attracted significant attention, owing to its anisotropic electroconductivity, tunable direct bandgap, extraordinary surface activity, moderate switching ratio, high hole mobility, good biocompatibility, and biodegradability. Several pioneering research efforts have explored the application of BP in different types of electrochemical sensors. This review summarizes the latest synthesis methods, protection strategies, and electrochemical sensing applications of BP and its derivatives. The typical synthesis methods for BP-based crystals, nanosheets, and quantum dots are discussed in detail; the degradation of BP under ambient conditions is introduced; and state-of-the-art protection methodologies for enhancing BP stability are explored. Various electrochemical sensing applications, including chemically modified electrodes, electrochemiluminescence sensors, enzyme electrodes, electrochemical aptasensors, electrochemical immunosensors, and ion-selective electrodes are discussed in detail, along with the mechanisms of BP functionalization, sensing strategies, and sensing properties. Finally, the major challenges in this field are outlined and future research avenues for BP-based electrochemical sensors are highlighted.

Assembly of ZIF-67 nanoparticles and in situ grown Cu(OH)2 nanowires serves as an effective electrocatalyst for oxygen evolution

Due to the slow kinetics of oxygen evolution at the anode, the efficiency of electrocatalytic water decomposition is critically reduced, and its large-scale application is severely restricted. Therefore, it is urgent to develop electrocatalysts with excellent performance and stability to accelerate the oxygen evolution reaction (OER) reaction kinetics. Herein, a self-supporting binder-free electrocatalyst was successfully prepared using in situ grown Cu(OH)2 nanowires on CF as the carrier to grow ZIF-67 via a room temperature immersion method. The combination of Cu(OH)2 nanowires and the unique structure of ZIF-67 forms a three-dimensional nanostructured catalyst, in which the unique structure and the existence of synergy may contribute to a larger electrochemical active surface area, expose more electrochemically active sites, adjust the electronic structure, and accelerate the rate of electron transfer, thus greatly improving the electrocatalytic activity and durability for OER. The as-prepared Cu(OH)2@ZIF-67/CF exhibited excellent OER performance under alkaline conditions and required overpotentials of 205 mV and 276 mV to drive current densities of 10 mA cm-2 and 100 mA cm-2, respectively, with a small Tafel slope of 70.5 mV dec-1 for OER. The stability test of Cu(OH)2@ZIF-67/CF at the current density of 10 mA cm-2 displayed excellent stability for 22 h. This study provides a feasible strategy for the rapid preparation of low-cost and efficient electrocatalysts in alkaline media.

Electrochemical Aptasensors Based on Hybrid Metal-Organic Frameworks

Metal-organic frameworks (MOFs) offer a unique variety of properties and morphology of the structure that make it possible to extend the performance of existing and design new electrochemical biosensors. High porosity, variable size and morphology, compatibility with common components of electrochemical sensors, and easy combination with bioreceptors make MOFs very attractive for application in the assembly of electrochemical aptasensors. In this review, the progress in the synthesis and application of the MOFs in electrochemical aptasensors are considered with an emphasis on the role of the MOF materials in aptamer immobilization and signal generation. The literature information of the use of MOFs in electrochemical aptasensors is classified in accordance with the nature and role of MOFs and a signal mode. In conclusion, future trends in the application of MOFs in electrochemical aptasensors are briefly discussed.

Emerging metal ion-coordinated black phosphorus nanosheets and black phosphorus quantum dots with excellent stabilities

In this work, emerging metal ion-coordinated black phosphorus nanosheets (M@BPNSs) and quantum dots (M@BPQDs) were prepared via the sonication-assisted liquid-phase exfoliation of bulk black phosphorus (BP) crystals in the presence of a metal ion (M) and solvothermal reaction of the exfoliated few-layer M@BP nanosheets. Based on theoretical calculations, a bonding mode exists between M and BP. Consequently, the adsorption energies of M on BP via the bonding mode are lower than that of M on BP via the non-bonding mode. Under the bonding mode, the adsorption energy of Zn2+ (-2.04 eV) on BP is lower than other M. Zn2+, serves as the preferred M and can be easily adsorbed on the surface of BP. We experimentally prepared emerging M@BPNSs and M@BPQDs, characterized, and compared various morphologies, microstructures and spectra under different conditions. It is verified, that the surface coordination of M with BP protects BP from oxidization and degradation of its nanostructures upon exposure to O2 and H2O. In comparison to the bare BPNSs, Zn@BPNSs showed high microstructural stability. Moreover, in comparison to bare BPQDs, Zn@BPQDs exhibited high colloidal stability and excellent stabilities with fluorescence and photothermal conversion performances. The long-term stabilities are due to the M-coordination with BP through P-M bonding on BP nanostructures. Thus, the excellent long-term stabilities in microstructure, fluorescence and photothermal conversion levels endow the emerging two-dimensional M@BPNSs and zero-dimensional M@BPQDs with great prospects towards promising applications, especially in electronics, optoelectronics, optical and biomedical fields.