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The highly sensitive detection of low-abundant apurinic/apyrimidinic endonuclease 1 (APE1) activity is of great significance for early diagnosis of disease and pathological research. Many methods for detecting APE1 based on isothermal nucleic acids amplification have been developed for improving its sensitivity. However, some of these methods have certain limitations, such as multiple reaction steps, narrow linear range, and complicated processes for fluorescent labeling. Herein, we develop a highly sensitive and label-free APE1 fluorescence detection method based on rolling circle amplification combined with G-quadruplex (RCA-G4). A hairpin probe (HP) labeled with the AP site can be recognized and cleaved by APE1, leading to the release of the primer sequence, which triggered RCA to produce long chain amplification products with a great amount of repeated sequences. The formed amplicon contains a series G-quadruplex structure, which can be combined with Thioflavin T (ThT) to produce fluorescence and achieve high sensitivity label-free detection of APE1. Benefit from the high amplification efficiency of RCA and the high fluorescence quantum yield of G-quadruplex/ThT, a detection limit as low as 1.52 × 10

Sensitive, specific and rapid methods for detecting microRNAs (miRNAs) play critical roles in disease diagnosis and therapy. Enzyme-free amplification techniques based on DNAzyme assembly have recently been developed for the highly specific miRNA analysis. However, traditional DNAzyme-based assembly (free DNAzyme) amplifiers is mainly dependent on the target-induced split DNAzyme fragments to assemble into activated DNAzyme structures, which have made a compromise between the sensitivity and specificity due to the random diffusion of dissociative probes in a bulk solution with poor kinetics. Herein, based on a rationally designed DNA probe, we developed an intramolecular DNAzyme assembly (intra-DNAzyme) method to overcome these challenges. The miR-373 is used as model analyte for our current proof-of-concept experiments. Compared with the free-DNAzyme method, our method showed significantly improved analytical performance in terms of dynamic range, assay sensitivity and speed. This method can detect miR-373 specifically with a detection limit as low as 4.3 fM, which is about 83.7 times lower than the previous free-DNAzyme method. This intra-DNAzyme strategy would be of great value in both basic research and clinical diagnosis.

From a three-dimensional (3D) metal-organic framework (MOF) of {[Cu(Cmdcp)(phen)(H2O)]2·9H2O}n (1, H3CmdcpBr = N-carboxymethyl-(3,5-dicarboxyl)pyridinium bromide, phen = phenanthroline), a sensitive and selective fluorescence sensor has been developed for the simultaneous detection of ebolavirus conserved RNA sequences and ebolavirus-encoded microRNA-like (miRNA-like) fragment. The results from molecular dynamics simulation confirmed that MOF 1 absorbs carboxyfluorescein (FAM)-tagged and 5(6)-carboxyrhodamine, triethylammonium salt (ROX)-tagged probe ss-DNA (probe DNA, P-DNA) by π…π stacking and hydrogen bonding, as well as additional electrostatic interactions to form a sensing platform of P-DNAs@1 with quenched FAM and ROX fluorescence. In the presence of targeted ebolavirus conserved RNA sequences or ebolavirus-encoded miRNA-like fragment, the fluorophore-labeled P-DNA hybridizes with the analyte to give a P-DNA@RNA duplex and released from MOF 1, triggering a fluorescence recovery. Simultaneous detection of two target RNAs has also been realized by single and synchronous fluorescence analysis. The formed sensing platform shows high sensitivity for ebolavirus conserved RNA sequences and ebolavirus-encoded miRNA-like fragment with detection limits at the picomolar level and high selectivity without cross-reaction between the two probes. MOF 1 thus shows the potential as an effective fluorescent sensing platform for the synchronous detection of two ebolavirus-related sequences, and offer improved diagnostic accuracy of Ebola virus disease.

Inspired by the enormous importance attributed to the biological function of miRNA, we pour our attention into the design and synthesis of four ruthenium(II) complexes and evaluate their applications as miR-185 detection agents by spectroscopic measurements. It was found that all complexes can form sensing platform for the detection of the complementary target miR-185 through the introduction of carboxyfluorescein (FAM) labeled single stranded DNA (P-DNA), giving the detection limits of 0.42nM for Ru 1, 0.28nM for Ru 2, 0.32nM for Ru 3, 0.85nM for Ru 4, all with instantaneous detection time in 1min. The results of the binding constant, fluorescence anisotropy (FA) and polyacrylamide gel electrophoresis experiments (PAGE) revealed that the ruthenium(II) complexes prefer to bind P-DNA other than hybrid duplexes DNA@RNA upon recognition, resulting in the detection of miR-185. These results provide useful suggestions in the new type of metal-based miRNA detection agents.

Herein, we present a facile strategy for dopamine (DA) sensing by a water-stable MOF of {[Tb(Cmdcp)(H2O)3]2(NO3)2·5H2O}n (1, H3CmdcpBr = N-carboxymethyl-(3,5-dicarboxyl)pyridinium bromide). Without any post-modification, MOF 1 functions as an effective fluorescent sensor for the label-free detection of DA with the detection limit of 0.41 μM (S/N = 3). Under the optimum condition of 80 °C, pH 9 for 80 min in Tris-HCl with natural ambient oxygen, DA polymerizes to give polydopamine (pDA), which adheres to the surface of MOF 1 and quenched its green luminescence thoroughly. The sensing process is visible to naked eyes under 365 nm UV light irradiation due to the partial overlap of its excitation spectrum with the absorption spectrum of pDA. The sensing process is not interfered by coexisting of bio-related organic substances, such as glucose (Glu), 5-hydroxytryptamine (5-HT), homocysteine (Hcy), ascorbic acid (AA), uric acid (UA), cysteine (Cys), glutathione (GSH), as well as the presence of metal ions, including Zn2+, Ca2+, Mg2+, Ni2+ and Co2+. The sensing process is also adaptable in biological fluids of serum and urine with satisfactory recoveries ranging from 96.14% to 104.32%.

A zwitterionic three-dimensional (3D) metal−organic framework (MOF) of {[Cu(Cdcbp)(bipy)]·4H2O}n (1) has been synthesized and characterized (H3CdcbpBr = 3-carboxyl-(3,5-dicarboxybenzyl)-pyridinium bromide; bipy = 4,4′-bipyridine). MOF 1 exhibits a variety of structural traits, such as ligand conjugated, positively charged pyridinium center, and Cu(II) cations that collectively enable its efficient hybridization with the flexible, negatively charged, single-stranded, and thymine-rich (T-rich) DNA. The T-rich DNA is labeled with carboxyfluorescein (FAM) fluorescent probe (characterized as P-DNA), but the resultant MOF 1 – P-DNA hybrid (characterized as P-DNA@1) is non-emissive (off-state) because of the fluorescent quenching by MOF 1. The P-DNA@1 hybrid functions as an effective and selective sensor for Hg2+ due to the formation of rigid hairpin-like T-Hg2+-T double-stranded DNA (ds-DNA@Hg2+) which is subsequently ejected by MOF 1, triggering a recovery of the P-DNA fluorescence (on-state). Subsequent addition of biothiols further sequestrates Hg2+ from the ds-DNA@Hg2+ duplex driven by the stronger Hg–S coordination, thus release the P-DNA and, in turn, resorbed by MOF 1 to regain the initial hybrid (off-state). P-DNA@1 hybrid thus detects Hg2+ and biothiols sequentially via a fluorescence “off-on-off” mechanism. The limits of detection (LOD) for Hg2+, biothiols, including cysteine (Cys), glutathione (GSH) and homocysteine (Hcy) are 3.0, 14.2, 15.1 and 8.0 nM, respectively, with the detection time of 60 min for Hg2+, and instantaneous detection for all the three biothiols. The detection mechanism is further confirmed by circular dichroism (CD), fluorescence anisotropy (FA), binding constant and molecular simulation. This sequential detection of Hg2+ and biothiols counter-proofs the presence of each other and may shed light to the occurrence of related diseases, such as neurodegenerative disorders of Parkinson's disease (PD), and Alzheimer's disease (AD).

A zwitterionic three-dimensional (3D) metal-organic framework (MOF) of {[Cu(Cdcbp)(bipy)]·4H