Herein, by directly using Watson-Crick base pairing, a highly ordered and field-free three-dimensional (3D) DNA nanostructure is self-assembled by azobenzene (azo)-functionalized DNA nippers in a few minutes, which was applied as a 3D DNA nanomachine with an improved movement efficiency compared to traditional Au-based 3D nanomachines due to the organized and high local concentration of nippers on homogeneous DNA nanostructure. Once microRNA (miRNA) interacts with the 3D nanomachine, the nippers "open" to hybridize with the miRNA. Impressively, photoisomerization of the azo group induces dehybridization/hybridization of the nippers and miRNA under irradiation at different wavelengths, which easily solves one main technical challenge of DNA nanotechnology and biosensing: reversible locomotion in one step within 10 min. As a proof of concept, the described 3D machine is successfully applied in the rapid single-step detection of a biomarker, which gives impetus to the design of new generations of mechanical devices beyond the traditional ones with ultimate applications in sensing analysis and diagnostic technologies.

The development of a sensitive and practical electrochemiluminescence (ECL) bioassay relies on the use of ECL signal tags whose signal intensity is high and stable. In this work, strong ECL emission was achieved from metal organic framework (MOF) accelerator enriched quantum dots (CdTe), which were applied as an efficient ECL signal tag for trace biomarker detection. It is particularly noteworthy that a novel mechanism to drastically enhance the ECL intensity of CdTe is established because isoreticular metal organic framework-3 (IRMOF-3) with 2-amino terephthalic acid (2-NH-BDC) as the organic ligand not only allows for loading a large amount of CdTe via the encapsulating effect and internal/external decoration but also functions as a novel coreactant accelerator for promoting the conversion of coreactant SO into the sulfate radical anion (SO), further boosting the ECL emission of CdTe. On the basis of the simple sandwich immunoreaction approach, cardiac troponin-I antigen (cTnI), a kind of biomarker related with myocardial infarction, was chosen as a detection model using an IRMOF-3-enriched CdTe labeled antibody as the signal probe. This immunosensor demonstrated desirable assay performance for cTnI with a wide response range from 1.1 fg mL to 11 ng mL and a very low detection limit (0.46 fg mL). This suggested that the IRMOF-3-enriched CdTe nanocomposite strategy can integrate the coreactant accelerator and luminophore to significantly enhance the ECL intensity and stability, providing a direction for promising ECL tag preparation with broad applications.

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On the basis of a novel target-cycling synchronized rolling circle amplification (RCA) as a signal amplification strategy and in situ electrochemical generation of silver nanoclusters (Ag NCs) as signal probes, an ultrasensitive and simple electrochemiluminescence (ECL) biosensor was proposed for microRNA (miRNA) detection. It was worth mentioning that the circular template was subtly designed to consist of a guanine-rich (G-rich) region and a binding region for realizing target-cycling synchronized RCA. In the presence of target miR-21, the binding region hybridized with the primer and the target miR-21 to form a ternary "P" junction structure, and then the RCA was triggered from the 3'-end of the primer. Along with the proceeding of RCA, the target miR-21 was released and participated into another trigger of the RCA. On account of the G-rich region in the circular template, the product DNA of the target-cycling synchronized RCA possessed tandem periodic cytosine-rich (C-rich) sequences, which acted as ligands to further in situ electrochemically generate silver nanoclusters (Ag NCs) as ECL signal probes. As expected, the obtained ECL intensity dependent on the amount of the Ag NCs, which was positively related to the concentration of the target miR-21. The ECL assay for miR-21 detection demonstrated excellent linear response to a concentration variation from 100 aM to 100 pM and limit of detection down to 22 aM.

A self-enhanced electrochemiluminescence (ECL) reagent, synthesized by covalently linking bis(2,2'-bipyridyl)(4'-methyl-[2,2']bipyridinyl-4-carboxylicacid) ruthenium(II) (Ru(bpy)2(mcbpy)(2+)) with tris(3-aminopropyl)amine (TAPA), has been chosen as precursor to prepare nanorods ([Ru(bpy)2(mcbpy)(2+)-TAPA]NRs) with high luminous efficiency via a solvent-evaporation-induced self-assembly procedure. Due to the shorter electron-transfer path and less energy loss, the intramolecular reaction between the luminescent Ru(bpy)2(mcbpy)(2+) and coreactive tertiary amine group in TAPA has shown improved luminous efficiency compared with the common intermolecular ECL reactions. Moreover, using the electrochemiluminescent Ru(II)-based complex as precursor to directly prepare a nanostructure with high electro-active surface area is a more effective and convenient method for enhancing the immobilized amount of Ru(II)-based complex in the construction of biosensors compared with the traditional immobilized methods. Meanwhile, the obtained nanorods could be further functionalized easily, owing to their positive electrical property and the amino group on the surface. Here, Pt nanoparticles functionalized [Ru(bpy)2(mcbpy)(2+)-TAPA]NRs are used to load the detection antibody (Ab2). In addition, the Au/Pd dendrimers (DRs) with hierarchically branched structures are synthesized to immobilize capture antibody (Ab1) with increased amount. Based on sandwiched immunoreactions, a simple and sensitive "signal-on" immunosensor is constructed for the detection of N-acetyl-β-d-glucosaminidase (NAG), a biomarker of diabetic nephropathy, with excellent linearity in concentrations from 1 ng mL(-1) to 0.5 pg mL(-1) and a detection limit of 0.17 pg mL(-1).