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A facile colorimetric method for rapid detection of putrescine (Put) and cadaverine (Cad) based on the peroxidase-like activity of (Fe,Co) codoped-CDs was developed. With the enzymatic hydrolysis by diamine oxidase (EC 1.4.3.6, DAO), Biogenic amines (BAs) were decomposed to produce H2O2 which oxidizes 3,3',5,5'-tetramethyl-benzidine with the catalysis of (Fe,Co)-codoped CDs. By means of principal component analysis, Put and Cad were differentiated from BAs and their total amounts were detected by measuring the spectral absorption at 652 nm with a linear range from 0.25 to 10 mg kg−1. The colorimetric method was applied to the detection of Put and Cad in various fish samples with a detection limit ((3σ/k, n = 9)) of 0.06 mg kg−1. Besides verifying with HPLC, the recoveries of the colorimetric method were also tested by spiking standards at three levels with the range of 98.2%–115.7%, indicating that the established colorimetric method was accurate and sensitive.

Rapid detection of hypoxanthine (Hx), an important freshness indicator of aquatic products, is still urgent and challenging. In the present study, a colorimetric method for the rapid detection of Hx in aquatic products was established based on the peroxidase-like activity of cobalt-doped graphite phase carbon nitride (Co-doped-g-C3N4). After calcination of the mixture of melamine, cobalt chloride, potassium chloride and sodium chloride, Co-doped-g-C3N4 was obtained with strong peroxidase-like activity and good dispersity. Through the decomposition of Hx by xanthine oxidase, the generated hydrogen peroxide (H2O2) was detected based on the oxidation reaction of H2O2 and 3,3′,5,5′-tetramethylbenzidine catalyzed by Co-doped-g-C3N4. The content of Hx could be directly measured by monitoring the spectral absorbance at 652 nm with a linear range of 2.50–153.1 mg kg−1 and a detection limit of 1.84 mg kg−1 calculated based on 3σ/K (n = 9). The established method was applied for the detection of Hx content in various aquatic products at different storage times after execution, and the data were validated by the HPLC method. The results showed that the Hx measurement by the established method was sensitive, accurate and reliable, and could be used for the rapid determination of the freshness of aquatic products at an early stage.

Functional DNAs-functionalized magnetic beads (MBs) offer great potential in bioanalysis field because of their target recognition and magnetic separation functions. However, the recognition capability and hybridization affinity of DNA probes often suffer from limited available space, poor probe conformation and non-selective adsorption. To overcome these limitations, we herein used aptamer-pendant DNA tetrahedron nanostructure-functionalized MBs (TETapt-tet MBs) to develop a target-response fluorescence method with tetracycline (TET) as a model. In the absence of TET, 6-carboxy-X-rhodamine-labeled complementary DNAs (ROX-cDNAs) were assembled on the surface of MBs. Upon the addition of target TET, the ROX-cDNAs were separated and released from the MBs to generate fluorescence signal. The limit of detection and limit of quantification for TET were found to be 6 pg mL−1 and 20 pg mL−1, respectively. Compared with ssDNA-functionalized MBs surface, the designed DNA tetrahedron nanostructure-based surface could decrease the hybridization time and reduce false positives, ensuring the accuracy of TET detection in complex samples. The presented method was successfully employed for TET detection in honey samples. Moreover, this functionalization strategy could be extended to detect multiple antibiotics by simply substituting different aptamer sequences. Therefore, the proposed method has great potential in the field of food safety and public health.

Summary Using spherically directed reduction of metal ions, a facile and universal method for in situ crosslinking of DNA micelles is described. By incorporating a series of specific template domains into lipid-DNA monomers, copper-, silver-, and gold-crosslinked DNA micelles are formed. By combining characterization results and simulation algorithms, accurate structural profiling of DNA micelles is achieved for the first time. The prepared metal-crosslinked DNA micelles show obvious merits, including one-step formation of core structure and ligand corona, facile size tunability, monodispersity, stability against salt-induced aggregation, high utilization of DNA ligand, mild synthesis conditions, and less time consumption and better internalization. Based on our spherically directed synthesis, metal-crosslinked DNA micelle flares were further prepared for effective intracellular imaging by integrating an aptamer-toehold strategy. In addition, we extended the strategy to cholesterol-based double-stranded DNA micelles, giving strong evidence that our method is not only universal but also flexible.

Rapid detection of malachite green (MG) by fluorescence probes of CdTe quantum dots (QDs) coated with molecularly imprinted polymer (MIP) has been reported, but low sensitivity, slow response and narrow linear range were the main disadvantages for these MIP@QDs probes. Here, an ultra-sensitive ratiometric fluorescence probe based on self-assembly functionalized polystyrene (PS) and core-shell structured MIP was developed for the detection of MG in fish. After modified with poly(diallyldimethylammonium chloride) and hexadecyl trimethyl ammonium bromide, CdTe QDs and silica encapsulated carbon quantum dots (CDs@SiO2) with negative charges, used as detection and reference fluorophores, were respectively self-assembled on the surface of PS to form PS-QDs&CDs. And a core-shell structured MIP layer was fabricated on the surface of PS-QDs&CDs to form a ratiometric fluorescence probe of MIP@PS-QDs&CDs with sol-gel method using MG, (3-aminopropyl)triethoxy-silan and tetraethyl orthosilicate as template, functional monomer and cross-linker, respectively. With intensive fluorescence densities of QDs&CDs in nucleus, the fluorescence emission of probe at λem of 630 nm could be linearly quenched by MG, while the fluorescence emission at λem of 430 nm remained unchanged. The probe of MIP@PS-QDs&CDs owned good fluorescence stability (RSD ≤2.3%), fast response time (5 min), wide linear range (0.1–32 μmol L−1) and low detection limit (5.6 μg kg−1), and has been applied to the detection of MG in fish samples with recoveries of 94.3–110.2%. The result validated that the as-prepared probe of MIP@PS-QDs&CDs had an excellent performance for MG detection in fish samples.

Fluorescence-based aptasensors possess high sensitivity but are complicated and usually require multistep labeling and modification in method design, which severely limit the practical applications. Here, a label-free fluorescence-based aptasensor, consisting of aptamer, gold nanoparticles (AuNPs), and cadmium telluride (CdTe) quantum dots (QDs), was developed for the detection of sulfadimethoxine (SDM) in water and fish based on the specific recognition of SDM-aptamer and the inner filter effect of QDs and AuNPs. In the absence of a target, AuNPs dispersed in salt solution because of the aptamer protection, which could effectively quench the fluorescence emission of QDs, while in the presence of SDM, AuNPs aggregated due to the specific recognition of SDM-aptamer to SDM, which resulted in fluorescence recovery. A linear response of SDM concentrations in the range of 10–250 ng mL−1 ( R2 = 0.99) was obtained, and the detection limit was 1.54 ng mL−1 (3σ, n = 9), far below the maximum residue limit (100 ng mL−1) of SDM in edible animal tissues regulated by China and the European Commission. The fluorescence-based aptasensor was applied to the detection of SDM in aquaculture water and fish samples with high accuracy, excellent precision, and ideal selectivity. The results indicated that the developed aptasensor was simple in design, easy to operate, and could be used to detect rapidly and accurately SDM in water and fish samples.

Tetracycline (TET) is a broad-spectrum antibiotic, which is frequently used in the prevention and treatment of animal diseases, feed additives, and so on. However, its residue and accumulation in animal-derived foods could cause several side effects to the human body. Herein, we fabricated TET aptamer-pendant DNA tetrahedral nanostructure-functionalized magnetic beads (Apt-tet MBs) as a probe to detect TET. In the presence of target TET, DNA primer was released from Apt-tet MBs since the TET aptamer could specifically bind TET. Next, the separated DNA primer could effectively initiate rolling circle amplification (RCA) reaction and generate a long tandem single-stranded sequence. Finally, with SYBR Green I as the fluorescence dye, the fluorescence signal could be detected by detection probes through hybridizing the RCA product. Under optimal conditions, the fluorescent signal increased with the increasing target TET concentration within the 5 orders of magnitude dynamic range from 0.001 to 10 ng mL-1. The detection limit was calculated to be 0.724 pg mL-1 and the method showed high selectivity toward TET among different antibiotics. More impressively, this method was employed for TET determination in fish and honey samples. The as-obtained results were consistent with those of ELISA kits, holding great potential in the field of food analysis.

Herein, we report a facile method for cholesterol detection by coupling the peroxidase-like activity of polypyrrole nanoparticles (PPy NPs) and cholesterol oxidase (ChOx). ChOx can catalyze the oxidation of cholesterol to produce H2O2. Subsequently, PPy NPs, as a nanozyme, induce the reaction between H2O2 and 3,3',5,5'-tetramethylbenzidine (TMB). Under optimal conditions, the increase is proportional to cholesterol with concentrations from 10 to 800 μM in absorbance of TMB at 652 nm. The linear range for cholesterol is 10-100 μM, with a detection limit of 3.5 μM. This reported method is successfully employed for detection of cholesterol in human serum. The recovery percentage is ranged within 96-106.9%. Furthermore, we designed a facile and simple portable assay kit using the proposed system, realizing the on-site semiquantitative and visual detection of cholesterol in human serum. The cholesterol content detected from the portable assay kit were closely matching those obtained results from solution-based assays, thereby holding great potential in clinical diagnosis and health management.

A method for the aptamer-based determination of chloramphenicol (CAP) was developed by exploiting the peroxidase mimicking activity of hemin. The method includes two hemin-modified DNA probes termed P1 and P2. P1, which was modified at its 5′ end with one hemin monomer, contains the CAP-binding sequence. The hybridization between P1 and P2 brings the two hemin monomers in close proximity, resulting in the formation of a hemin dimer with low peroxidase mimicking activity. The duplex structure was dehybridized in the presence of CAP. The formed hemin monomer featured a strong peroxidase mimicking activity and catalyzed the conversion of non-fluorescent tyramine into fluorescent dityramine by hydrogen peroxide. Fluorescence (with an excitation/emission maxima at 320 and 410 nm, respectively) increased linearly in the 0.1 ng mL−1 to 10 ng mL−1 CAP concentration range. The detection limit based on the 3σ/k criterion reached 0.07 ng mL−1. The proposed assay was successfully employed for CAP detection in (spiked) honey samples with recoveries of 94.3–117.2%. Given its high sensitivity and good stability, this method shows potential in providing a platform for antibiotic detection.

Due to complex matrixes and specific reagent deficiency, the rapid detection of histamine is still a challenge to date. Based on the high peroxidase-like activity of iron-cobalt co-doped carbon dots, an indirect competitive enzyme-linked immunosorbent assay (ic-ELISA) was established for histamine detection using the mimic enzyme labeled with histamine antibody (His-Ab). Through the competitive binding of the labeled His-Ab to solid-phase and sample antigens, histamine content was detected with a linear range of 2.5–150 μg mL−1. The detection limit based on 3σ/K was 0.50 mg kg−1, which was much lower than those of commercial His-kit and HPLC methods. The ic-ELISA method was applied to histamine detection in fish samples with the recovery of (103.4 ± 0.5)%, which was in accord with those of commercial His-kit and HPLC methods. The results indicated that the established ic-ELISA method was suitable for rapid detection of histamine in fish samples with high accuracy, sensitivity and stability.

Organophosphorus pesticides can prevent or eliminate various pathogenic bacteria, insects, and weeds, and thus they are widely applied in agricultural production. However, illegal use and issues with organophosphorus pesticide residues contribute to global environmental pollution and pose a threat to public health safety. In this study, we developed a sensitive glyphosate (Glyp) fluorescence detection method using papain-stabilized gold nanoclusters (papain-AuNCs) as the fluorescence probe and a tyrosinase (TYR)/dopamine (DA) fluorescence-quenching system. The TYR catalyzed the oxidized conversion of DA into DA chrome, which served as an electron acceptor to quench the fluorescence of papain-AuNCs. However, Glyp inhibited the activity of TYR, thereby preventing DA oxidization and leading to the fluorescence recovery of papain-AuNCs. Under the optimum conditions, the fluorescence intensities of papain-AuNCs exhibited a good linear relationship with the concentration of Glyp in the range of 0.04–0.4 ng·mL−1, and the limit of detection for Glyp was 0.035 ng·mL−1. Furthermore, a paper-based sensor was constructed using the proposed system, which enabled on-site visual and semiquantitative detection of Glyp residues in tap-water samples. Overall, our strategy provides new opportunities for detection of organophosphorus pesticides and evaluation of environmental security.

A facile and practical ratiometric fluorescence probe based on two CdTe quantum dots (QDs) coated with molecularly imprinted polymers (MIPs) was prepared for the detection of trace malachite green (MG) in fish. Two CdTe QDs coated with MIPs were fabricated by a one-pot method using MG, (3-aminopropyl) triethoxysilane (APTES) and tetraethyl orthosilicate (TEOS) as template, functional monomer, and cross-linker, respectively. CdTe QDs with λem 530 nm (gQDs) and 630 nm (rQDs) were used as the referential fluorophore and target sensitive fluorophore, respectively. The fluorescence intensity of gQDs remained unchanged in the presence of MG, while the fluorescence of rQDs could be quantitatively quenched by MG based on the strategy of fluorescence resonance energy transfer. The ratiometric fluorescence probe (MIPs@gQDs&rQDs) was characterized by transmission electron microscopy and Fourier transform infrared spectroscopy. The linear range of MG detection was 0.1–32 μmol L−1 with a detection limit of 8.8 μg kg−1. The constructed probe has been successfully applied to the detection of MG in fish with the recoveries of 92.3–109.1%, which were validated by the method of HPLC. The result indicated that the probe possessed rapid response, wide linear range, high sensitivity, and relatively high selectivity, and was low-cost and easy in operation in the detection of MG in fish samples.

The organization of nanoparticles (NPs) with controlled chemical composition and size distribution into well-defined sheets will find many practical applications, but the chemistry remains problema...

The crustal structure of collisional orogen is critical for deciphering the crustal evolution and geodynamics of an orogen. Metamorphic rocks and post-collisional magmatic rocks are the key subjects to probe the crustal structure. In this study, we conducted an integrated study of zircon U Pb age, Hf and O isotopes for migmatites and post-collisional granites in the southern segment of the Tongbai orogen. Zircon U−Pb dating yields protolith ages of 755 ± 55 to 713 ± 6 Ma and metamorphic ages of 162 ± 11 Ma to 138 ± 2 Ma for the migmatites and intrusion ages of 137 ± 2 Ma for the post-collisional granites. The inherited magmatic cores in the migmatites exhibit similar Hf isotopic compositions to the metamorphic zircons. Their eHf(t) values range from 13.5 to −9.5 with crust Hf model ages of 787 Ma to 2233 Ma, indicating that the protoliths were derived from reworking of Paleoproterozoic crust with the addition of juvenile components during the Neoproterozoic. Zircon δ18O of inherited magmatic cores are of −1.45‰ to 5.90‰, signifying the protoliths were formed by remelting of hydrothermally altered low δ18O rocks. By comparison, the post-collisional granites mostly show normal mantle-like O zircon isotope compositions and have lower eHf(t) and eNd(t) values than the migmatite rocks. These features suggest that the granites were formed by fusion of the ancient crust that might have suffered insufficient water-rock interaction. The discrepancies in O–Hf–Nd isotopic compositions of the studied migmatites and granites might be inherited from their precursors. It is thus inferred that the precursors of the granites might lie at the shoulder of the Neoproterozoic rift and thus might have experienced slight water-rock hydrothermal alteration and crust-mantle interaction; whereas the protoliths of the metamorphic rocks might be located at or near the rift center and thus had suffered intensive water-rock hydrothermal alteration and crust-mantle interaction. Prior to the late Mesozoic magmatism, the Tongbai orogen would have an uncoupled double-layered crust, where the upper layer consists of the metamorphic rocks and the lower is the sources of the post-collisional granites. They were stacked by the Permian-Triassic orogeny. The final architecture of the Tongbai orogen might be constituted by the emplacement of the voluminous granites from the deeper crust and the formation of the migmatites in the Tongbai complex. This configuration might widespread for other collisional orogens.

This study attempted to fabricate a novel colorimetric method for the quantitative detection of malachite green (MG) residues in fish and water samples. Based on the recognition of the detection target and the enhancement of the peroxidase-like activities of Fe3O4NPs by MG-aptamer, a colorimetric method was established for MG detection. The detection limit (3α/κ, n = 9) was 16.7 μg kg–1, and the detection linear range was 0.06–2.38 μmol L–1. The recovery rates by standard addition in fish and water ranged from 90.1% to 119% with standard deviations (n = 3) of less than 6.6 %, and was well validated by a HPLC method. The detection procedure could be completed within 1 h. Therefore, the detection method was simple, stable, and reliable, and could be used to detect MG residues rapidly and accurately in fish and aquaculture water.

The development of functional DNA-based nanosensors in living cells has experienced some design challenges, including, for example, poor cellular uptake, rapid nuclease degradation, and high false positives. Herein, we designed selectively permeable poly(methacrylic acid) (PMA) nanocapsules to encapsulate functional DNAs for metal ions and small-molecules sensing in living cells. Since functional DNAs are concentrated in the nanocapsules, an increasing reaction rate is obtained in vitro. During endocytosis, polymeric capsules simultaneously improve cellular uptake of functional DNAs and preserve their structural integrity inside the confined capsule space. More importantly, selective shell permeability allows for the free diffusion of small molecular targets through capsule shells but limits the diffusion of large biomolecules, such as nuclease and nonspecific protein. Compared to the free DNAzyme, PMA nanocapsules could reduce false positives and enhance detection accuracy. Furthermore, PMA nanocapsules are biocompatible and biodegradable. Through the controllability of wall thickness, permeability, and size distribution, these nanocapsules could be expanded easily to other targets, such as microRNAs, small peptides, and metabolites. These nanocapsules will pave the way for in situ monitoring of various biological processes in living cells and in vivo.

Exosomes are extracellular vesicles (50-100 nm) circulating in biofluids as intercellular signal transmitters. Although the potential of cancerous exosomes as tumor biomarkers is promising, sensitive and rapid detection of exosomes remains challenging. Herein, we combined the strengths of advanced aptamer technology, DNA-based nanostructure, and portable electrochemical devices to develop a nanotetrahedron (NTH)-assisted aptasensor for direct capture and detection of hepatocellular exosomes. The oriented immobilization of aptamers significantly improved the accessibility of an artificial nucleobase-containing aptamer to suspended exosomes, and the NTH-assisted aptasensor could detect exosomes with 100-fold higher sensitivity when compared to the single-stranded aptamer-functionalized aptasensor. The present study provides a proof-of-concept for sensitive and efficient quantification of tumor-derived exosomes. We thus expect the NTH-assisted electrochemical aptasensor to become a powerful tool for comprehensive exosome studies.

A dichromatic label-free aptasensor was described for sulfadimethoxine (SDM) detection. Compared with the binding of SDM-aptamer to SDM, the higher affinity of aptamer to cDNA may result in the hybridization of dsDNA. In the presence of SDM, the aptamer specifically binds to SDM, leading to a blue color of AuNPs in deposit and fluorescence at 530 nm in supernatant after adding cDNA and SGI. With no target of SDM, AuNPs protected with the aptamer re-disperse in PBS with a red color, and no fluorescence occurs in supernatant. Based on the principle, SDM can be quantitatively detected through both fluorescent emission and AuNPs color changes with recoveries ranging from 99.2% to 102.0% for fish and from 99.5% to 100.5% for water samples. An analytical linear range of 2–300 ng mL−1 was achieved with the detection limits of 3.41 ng mL−1 for water and 4.41 ng g−1 for fish samples (3σ, n = 9).

Electrochemiluminescence (ECL) integrates the advantages of electrochemical detection and chemiluminescent techniques. The method has received particular attention because it is highly sensitive and selective, has a wide linear range but low reagent costs. The use of nanomaterials with their unique physical and chemical properties has led to new kinds of biosensors that exhibit high sensitivity and stability. Compared to other nanomaterials, DNA nanostructures are more biocompatible, more hydrophilic, and thus less prone to nonspecific adsorption onto the electrode surface. We describe here a label-free and ultrasensitive ECL biosensor for detecting a cancer-associated microRNA at a femtomolar level. We have designed two auxiliary probes that cause the formation of a long-range self-assembly in the form of a μm-long 1-dimensional DNA concatamer. These can be used as carriers for signal amplification. The intercalation of the ECL probe Ru(phen)3 2+ into the grooves of the concatamers leads to a substantial increase in ECL intensity. This amplified sensor shows high selectivity for discriminating complementary target and other mismatched RNAs. The biosensor enables the quantification of the expression of microRNA-21 in MCF-7 cells. It also displays very low limits of detection and provides an alternative approach for the detection of RNA or DNA detection in diagnostics and gene analysis.

Hydrophobic nanoparticles have shown substantial potential for bioanalysis and biomedical applications. However, their use is hindered by complex phase transfer and inefficient surface modification. This paper reports a facile and universal strategy for phase transfer and surface biofunctionalization of hydrophobic nanomaterials using aptamer-pendant DNA tetrahedron nanostructures (Apt-tet). The Janus DNA tetrahedron nanostructures are constructed by three carboxyl group modified DNA strands and one aptamer sequence. Each tetrahedron edge is an 18-base-pair double helix, making the tetrahedral edges about 5.8 nm in length. The pendant linear sequence is an aptamer, in this case AS1411, known to specifically bind nucleolin, typically overexpressed on the plasma membranes of tumor cells. The incorporation of the aptamers adds targeting ability and also enhances intracellular uptake. Phase-transfer efficiency using Apt-tet is much higher than that achieved using single-stranded DNA. In addition, the DNA tetrahedron nanostructures can be programmed to permit the incorporation of other functional nucleic acids, such as DNAzymes, siRNA, or antisense DNA, allowing, in turn, the construction of promising theranostic nanoagents for bioanalysis and biomedical applications. Given these unique features, we believe that our strategy of surface modification and functionalization may become a new paradigm in phase-transfer-agent design and further expand biomedical applications of hydrophobic nanomaterials.

We have developed a simple method for direct detection of circulating microRNAs in serum by using the p19 protein-functionalized magnetic beads (PFMBs) for specific enrichment and rolling circle amplification (RCA). The detection limit for microRNAs is 1 fM. Therefore, the proposed method has the potential of being used in the analysis of circulating microRNAs and clinical diagnosis.

MicroRNAs (miRNAs), a kind of endogenous, noncoding RNAs (19-24 nucleotides), play vital roles in regulating gene expression and cellular processes. In recent years, it has been found that circulating miRNAs are differentially expressed in patients and healthy controls. This leads to the suggestion that circulating miRNAs are promising biomarkers for cancer classification and prognosis. However, it is still difficult to detect circulating miRNAs directly from real samples such as human serum without prior extraction and purification. In this work, we developed an ultrasensitive electrochemical biosensor for detection of cancer-associated circulating miRNAs based on DNA concatamers amplification. The proposed biosensor showed a high sensitivity for target miRNA-21 in a concentration range from 100 aM to 100 pM with a detection limit of 100 aM. Furthermore, the biosensor was successfully employed for direct detection of circulating miRNAs in human serum. Due to the high sensitivity, good selectivity and stability, the proposed electrochemical biosensor might have potential clinical application for circulating miRNAs in relation to diagnosis and prognosis.

Biosensors based on nanomaterials have been used for detection of various biological molecules with high sensitivity and selectivity. Herein, we developed a simple and ultrasensitive electrochemical DNA biosensor using long-range self-assembled DNA nanostructures as carriers for signal amplification, which can achieve an impressive detection limit of 5 aM human immunodeficiency virus (HIV) DNA even in complex biological samples. In this study, we designed two auxiliary probes. A cascade of hybridization events between the two auxiliary probes can lead to long-range self-assembly and form micrometer-long one-dimensional DNA nanostructures. In the presence of target DNA, each copy of the target can act as a trigger to connect a DNA nanostructure to a capture probe on the electrode surface. Then, a great amount of redox indicator [Ru(NH(3))(6)](3+) can be electrostatically bound to the DNA nanostructures and eventually result in significantly amplified electrochemical signals.