Nanozymes from Cu(II) Metal–Organic Gel and Melamine for Highly Active Peroxidase-Like Activity to Detect Alkaline Phosphatase

Enzyme-like catalytic activity and efficiency of carbon-based nanomaterials are closely linked to their size, heteroatom composition, and structure, and hence the heteroatom regulation needs to be further explored. In this work, a simple and efficient strategy was proposed to develop Cu-doped 2D carbon material C3N4 (Cu-C3N4-550) with excellent catalytic performance by pyrolyzing precursors of Cu(II) metal–organic gel (MOG) and melamine directly. Due to its sufficient metal active sites and adequate specific surface areas, the as-prepared Cu-C3N4-550 was endowed with excellent peroxidase-like activity to promote the oxidation of 3,3′,5,5′-tetramethylbenzidine owing to the generation of •OH in the catalytic reaction. It was amazing to find that the peroxidase mimic activity of the prepared Cu-C3N4-550 has enhanced 32.3-fold compared with bare C3N4. High peroxidase-like activity of Cu-C3N4-550 was influenced severely by the addition of antioxidant ascorbic acid (AA), alkaline phosphatase (ALP) as a typical hydrolase could catalyze substrate 2-phospho-l-ascorbic acid into AA, while AA was capable of capturing •OH generated from the catalytic reaction of Cu-C3N4-550. Hence, a sensitive, selective, and colorimetric method for the detection of ALP was established, the linear concentration of ALP in this colorimetric sensor from 0.4 to 20 U/L was acquired with a low detection limit of 0.32 U/L. This work not only provides ideas for designing enhanced peroxidase-like activity nanozymes in practical biological analysis but also broadens the MOG derivatives and carbon-based nanomaterials in colorimetric applications

Nanozyme Rich in Oxygen Vacancies Derived from Mn-Based Metal–Organic Gel for the Determination of Alkaline Phosphatase

Vacancy engineering as an effective strategy has been widely employed to regulate the enzyme–mimic activity of nanomaterials by adjusting the surface, electronic structure, and creating more active sites. Herein, we purposed a facile and simple method to acquire transition metal manganese oxide rich in oxygen vacancies (OVs-Mn2O3-400) by pyrolyzing the precursor of the Mn(II)-based metal–organic gel directly. The as-prepared OVs-Mn2O3-400 exhibited superior oxidase-like activity as oxygen vacancies participated in the generation of O2•–. Besides, steady state kinetic constant (Km) and catalytic kinetic constant (Ea) suggested that OVs-Mn2O3-400 had a stronger affinity toward 3,3′,5,5′-tetramethylbenzidine and possessed prominent catalytic performance. By taking 2-phospho-l-ascorbic acid as the substrate, which can be converted into reducing substance ascorbic acid in the presence of alkaline phosphatase (ALP), OVs-Mn2O3-400 can be applied as an efficient nanozyme for ALP colorimetric analysis without the help of destructive H2O2. The colorimetric sensor established by OVs-Mn2O3-400 for ALP detection showed a good linearity from 0.1 to 12 U/L and a lower limit of detection of 0.054 U/L. Our work paves the way for designing enhanced enzyme-like activity nanozymes, which is of significance in biosensing

Facile in Situ Synthesis of Silver Nanoparticles on the Surface of Metal–Organic Framework for Ultrasensitive Surface-Enhanced Raman Scattering Detection of Dopamine

Surface-enhanced Raman scattering (SERS) signals are intensively dominated by the Raman hot spots and distance between analyte molecules and metallic nanostructures. Herein, an efficient SERS substrate was developed by in situ synthesis of silver nanoparticles (AgNPs) on the surface of MIL-101 (Fe), a typical metal–organic framework (MOF). The as-prepared SERS substrate combines the numerous Raman hot spots between the high-density Ag NPs and the excellent adsorption performance of MOFs, making it an excellent SERS substrate for highly sensitive SERS detection by effectively concentrating analytes in close proximity to the Raman hot spots domains between the adjacent AgNPs. The resulting hybrid material was used for ultrasensitive SERS detection of dopamine based on the peroxidase-like activity of MIL-101 (Fe) by utilizing the enzyme-linked immunosorbent assay (ELISA) colorimetric substrate, 2,2′-azino-bis­(3-ethylbenzthiazoline-6-sulfonic acid) diammonium salt (ABTS) as a SERS marker. This new developed method showed good linearity in the range from 1.054 pM to 210.8 nM for dopamine with the correlation coefficient of 0.992, detection limit of approximately 0.32 pM [signal-to-noise ratio (S/N) = 3], and acceptable recoveries ranging from 99.8% to 108.0% in human urine. These results predict that the proposed SERS system may open up a new opportunity for chemical and biological assay applications

Dynamically Long-Term Imaging of Cellular RNA by Fluorescent Carbon Dots with Surface Isoquinoline Moieties and Amines

Cellular RNA dynamics are closely associated with a vast range of physiological processes that are mostly long-lasting. To uncover the association between RNA dynamics and these processes, fluorescent RNA probes with high specificity, photostability, and biocompatibility are compulsory. Herein, a series of fluorescent carbon dots (CDs) have been prepared by one-pot hydrothermal treatment of o-, m-, or p-phenylenediamines with triethylenetetramine. Only CDs derived from the meta precursor (m-CDs) with excellent photostability and biocompatibility can specifically bind to cellular RNA, allowing successfully long-term (up to 3 days) monitoring of RNA dynamics during cell apoptosis, mitosis, and proliferation. This RNA affinity can be attributed to the isoquinoline moieties and amines on the surface of m-CDs, which can bind to RNA through π–π stacking and electrostatic bonding, respectively. The cellular internalization of m-CDs is time-, temperature-, ATP-, caveolar, and microtubule-dependent. Additionally, investigations on the in vivo behavior of m-CD suggest that they can be efficiently and rapidly excreted from the zebrafish larvae body after 48 h. Our results provide a powerful tool for clarifying complex relationships between RNA dynamics and basic biological processes, disease development, or drug interactions

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Electrochemiluminescence Resonance Energy Transfer System Based on Silver Metal–Organic Frameworks as a Double-Amplified Emitter for Sensitive Detection of miRNA-107

As a class of electrochemiluminescence (ECL) enhancers, silver-based materials have broad application prospects. In this work, a novel silver metal–organic framework (AgMOF) was developed as a self-enhanced ECL emitter by one-step mixing and standing at room temperature. The AgMOF could produce strong and stable ECL emissions based on a double-amplification method, which originated from the aggregation-induced ECL emission of ligands and catalyzing S2O82– to produce more SO4•– by silver. Moreover, an ECL resonance energy transfer (ECL-RET) biosensor with AgMOF as a donor and BHQ2 as an acceptor was fabricated by duplex-specific nuclease (DSN)-assisted target recycling amplification to detect miRNA-107. The biosensor exhibited a strong ECL-RET effect due to the higher ECL emission of the AgMOF and perfect match of spectra between the AgMOF and BHQ2. Upon the introduction of DSN and target miRNAs, the specific DNA–RNA binding and nuclease cleaving could trigger the detachment of BHQ2, resulting in an increased ECL signal of AgMOF. Benefiting from the ECL-RET and DSN-assisted target recycling amplification methods, this biosensor achieved a wide linear relationship range from 20 to 120 fM with a low limit of detection (4.33 fM). This research presents an effective emitter for self-enhanced ECL systems, which broadens the potential ECL applications of silver-based nanomaterials

Self-Targeting Carbon Quantum Dots for Peroxynitrite Detection and Imaging in Live Cells

Peroxynitrite (ONOO–), a highly reactive nitrogen species (RNS) generated mainly in mitochondria, has been identified to be associated with numerous pathophysiological processes, and thus accurate ONOO– imaging with superior sensitivity and selectivity is highly desirable. Herein, we prepared a new type of carbon quantum dots (CQDs) with mitochondria-targeting function without the aid of any targeting molecules via a simple one-step hydrothermal route. The as-prepared CQDs not only displayed relatively uniform size distribution, few surface defects, high photostability, and excellent biocompatibility but also exhibited good selective fluorescence turn-off response toward ONOO–, owing to the oxidation of amino groups on the surface of carbon dots. A great linear correlation between the quenching efficiency and ONOO– concentration in the range from 0.15 to 1.0 μM with a detection limit of 38.9 nM is shown. Moreover, the as-prepared CQDs acting as a functional optical probe through a self-targeting mechanism were successfully applied for in situ visualization of endogenous ONOO– generated in the mitochondria of live cells, providing great promise for elucidating the complex biological roles of ONOO– in related pathological processes

Endogenous Adenosine Triphosphate-Assisted Three-Dimensional DNA Walker Assembled on Soft Nanoparticles for Intracellular MicroRNA Imaging

DNA walkers, a sophisticated type of nanomachines, exhibit intelligent application in biosensing with high programmability and flexibility but usually need additional auxiliary driving force, particularly when walking on hard surfaces. Herein, we construct a three-dimensional (3D) DNA walker on the soft surface of DNA nanospheres (DSs) by using a single-stranded DNA (ssDNA), which is powered by endogenous adenosine triphosphate (ATP) of live cells, so as to sensitively image microRNA (miRNA) in the tumor microenvironment. When the DS walker enters into live cells, miR-21, a general overexpressed biomarker in cancer cells, binds with the blocking strand (B), releasing the walking strand (W) and triggering an ATP-propelled walking reaction. The walking of the DS walker then generates an increasing Cy3 fluorescence signal that indicates the content of miR-21 with about 2.73-fold increase in sensitivity and about 157-fold decrease in the detection limit. Notably, the assembly of the DS walker on soft nanoparticles needs just an easy hybridization process, which facilitates the operation. Meanwhile, this endogenous ATP-powered 3D DNA walker walking on the soft surface performs real-time in situ imaging of miR-21 in live cells, which not only avoids the complex cell treatment and signal error induced by additional auxiliary factors, but also shows high promise of designing programmable DNA nanomachines

Zinc–Metal Organic Frameworks: A Coreactant-free Electrochemiluminescence Luminophore for Ratiometric Detection of miRNA-133a

Developing a coreactant-free ratiometric electrochemiluminescence (ECL) strategy based on a single luminophore to achieve more accurate and sensitive microRNA (miRNA) detection is highly desired. Herein, utilizing zinc–metal organic frameworks (Zn-MOFs) as the single luminophore, a novel dual-potential ratiometric ECL biosensor was constructed for ultrasensitive detection of miRNA-133a. The as-prepared Zn-MOFs exhibited simultaneous cathode and anode ECL emission. Furthermore, the Zn-MOFs were confirmed to be a multichannel ECL sensing platform with excellent annihilation and coreactant ECL emission. The corresponding ECL behaviors were investigated in detail. Benefiting from the hybridization chain reaction (HCR) amplification technology, N,N-diethylethylenediamine (DEAEA) was modified on hairpin DNA, and the gained products loaded with quantities of DEAEA enhanced the anodic ECL intensity of Zn-MOFs. In the presence of miRNA-133a, the ECL intensity ratio of anode to cathode (Ia/Ic) was significantly increased, which realized the ultrasensitive ratiometric detection of miRNA-133a. In addition, without an exogenous coreactant, the biosensor revealed superb accuracy and stability. Under optimal conditions, the detection linearity of miRNA-133a was from 50 aM to 50 fM with a low detection limit of 35.8 aM (S/N = 3). This is the first work to use Zn-MOFs as a single emitter for reliable ratiometric ECL bioanalysis, which provides a new perspective for fabricating a ratiometric ECL biosensor platform