Fluorogens with aggregation-induced emission (AIEgens) are a unique type of luminescent materials for sensing and imaging applications. The integration of AIE characteristics into metal–organic frameworks (MOFs) structure is an emerging research field that has greatly advanced the optical properties of MOFs. This Review mainly focuses on the development of AIE MOFs with highlights on materials development and their applications in analytical and biomedical fields. The first part discusses the structure design for AIE MOF from the perspective of linkers, metal nodes, and heterogeneous composites. The second part summarizes the major mechanisms on the property modulation of AIE MOFs in response to different stimuli based on fluorescence quenching, in-situ restriction of intramolecular motion, and framework deformation switches. The outlook elaborates on the potential of AIE MOFs, which have opened a new dimension in luminescent material development.

A DNA walker as a new molecular machine can walk on defined tracks to directly generate signal indicators in biosensing and biomedical applications. In this work, a tandem signal amplification strategy was developed on the basis of the DNA-walker-induced conformation switch for bridging palladium nanoparticles/metal−organic framework tags in ultrasensitive electrochemical DNA biosensing. The signal tags were synthesized by in situ reduction of Pd nanocrystals on porphyrinic metal− organic frameworks (PCN-224), followed by conjugation with streptavidin (SA). The as-prepared Pd/PCN-224-SA tag could electrocatalyze the oxidation of NaBH 4 with high efficiency for signal readout. The presence of target DNA released swing arms that were each silenced by a blocker, and then the activated swing arms could hybridize with hairpin DNA. The movement of swing arms was powered by enzymatic cleavage of conjugated oligonucleotides, inducing the allosteric switch from hairpin to SA aptamer. Therefore, Pd/PCN-224-SA tags were brought onto the electrode surface via SA-aptamer biorecognition to generate the enhanced electrochemical signal. The DNA walker-based electrochemical biosensor demonstrated good performance such as 6 orders of magnitude linear range, femtomolar detection limit, and single mismatch differentiation ability. Moreover, the feasibility of the biosensor was identified in serum matrixes. The tandem signal amplification of metal−organic frameworks and DNA walkers provided a new avenue in trace electrochemical biosensing.

Different from conventional DNA walkers, we designed a telomerase-triggered three-dimensional DNA walker consisting of a superhairpin structure with a bulged loop in the stem as the walking strand and dye-labeled tracks for ultrasensitive detection of telomerase activity. In the presence of telomerase, the primers in the stem of the superhairpin structure were elongated and triggered internal strand displacement, thus activating the superhairpin structure. Subsequently, the open superhairpin structure as a swing arm was able to bind with the track, and the swing arm could be released by enzymatic cleavage of the binding duplex domain, resulting in the fluorescence recovery of dye-labeled fragments from the surface of gold nanoparticles. Based on signal amplification of the telomerase-triggered DNA walker, the walking device was further applied to various cancer lines with a low detection limit of telomerase activity equivalent to 90 cells μL −1 for HeLa cells. Moreover, the advantage of this DNA walker strategy was confirmed by calculating telomerase activity in a single cell. This telomerase-triggered DNA walker provides a new concept on signal transduction for telomerase detection and is anticipated to stimulate interest in DNA nanomachine design for bioanalysis.

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Tetrapod DNA quadruplexes were designed for assembly and precise modulation of light emission of an oligonucleotide-grafted fluorogen with aggregation-induced emission.

A pixel counting strategy is designed on the basis of DNA walker-triggered fluorescence spots for ultrasensitive detection of nucleic acid. The two-dimensional DNA walker was constructed by hybridization of two types of capture DNAs, which were covalently modified by click chemistry on a glass slide, and dye-labeled hairpin structure (hDNA) as track and swing strand (sDNA) as DNAzyme, respectively. Introduction of target DNA unlocked the sDNA via strand displacement to form the activated DNAzyme, and the latter cut nearby hDNA with Mn as cofactor, resulting in fluorescence recovery of dye-labeled hDNA on the substrate due to the separation from the quencher. Meanwhile, the DNAzyme sequence of sDNA was released to cut the next hDNA and, thus, initiated autonomous walking of sDNA for signal amplification. The enhanced fluorescence spots were digitalized as pixels on the basis of DNA walker-built compartments and extracted by a homemade program in MATLAB. The association between fluorescent pixel numbers and DNA concentrations was further proved by a mathematical model and led to an ultrasensitive quantification of nucleic acid with a linear range from 100 fM to 10 pM. The designed pixel counting strategy shows a more sensitive and accurate comparison with conventional methods based on fluorescence intensity or spot counts and provides a new dimension in designing next-type biosensors.

Nitrogen-rich heterocyclic compounds (NRHCs) are an emerging type of explosive, and their quantification is important in national security inspection and environmental monitoring. Up until now, designing an efficient NRHCs sensing strategy was still in the early stages. Herein, a new metal–organic framework (MOF) with aggregation-induced emission (AIE) characteristics is synthesized with fluorometric/colorimetric responses for rapid and selective detection of NRHCs. The nonemissive probe is designed with tetraphenylethylene derivative as the linker and Co as the node, quencher, and color-changing agent. Cobalt AIE-MOF exhibits a turn-on emission enhancement due to the competitive coordination substitution between NRHCs and the scaffold as well as the following AIE process of the liberative linkers. Meanwhile, the color appearance of the probe changes from blue to yellow based on the dissociation of the original Co coordinating system. Using this dual-mode probe, single- and dual-ring NRHCs are successfully detected from 5 μM to 7.5 mM within 25 s. The cobalt AIE-MOF exhibits excellent selectivity of NRHCs against a variety of interferences, providing a promising tool for designing a multichannel detection strategy.

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