The peroxidase‐like activity of nanozymes is promising for chemodynamic therapy by catalyzing H2O2 into .OH. However, for most nanozymes, this activity is optimal just in acidic solutions, while the pH of most physiological systems is beyond 7.0 (even >8.0 in chronic wounds) with inadequate H2O2. We herein communicate an activatable nanozyme with targeting capability to simultaneously break the local pH and H2O2 limitations under physiological conditions. As a proof of concept, aptamer‐functionalized nanozymes, glucose oxidase, and hyaluronic acid constitute an activatable nanocapsule “APGH”, which can be activated by bacteria‐secreted hyaluronidase in infected wounds. Nanozymes bind onto bacteria through aptamer recognition, and glucose oxidation tunes the local pH down and supplements H2O2 for the in‐situ generation of .OH on bacteria surfaces. The activity switching and enhanced antibacterial effect of the nanocapsule were verified in vitro and in diabetic wounds. This strategy for directly regulating local microenvironment is generally accessible for nanozymes, and significant for facilitating biological applications of nanozymes.

An intramolecular catalytic hairpin assembly is implemented on a DNA tetrahedron for mRNA imaging in living cells. The spatial confinement effect enables the acceleration of target-triggered signal generation, with excellent cell permeability and FRET signal stability.

The Au À S bond is the classic way to functionalize gold nanoparticles (AuNPs). However, cleavage of the bond by biothiols and other chemicals is a long-standing problem hindering practical applications, especially in cells. Instead of replacing the thiol by a carbene or selenol for stronger adsorption, it is now shown that the Pt À S bond is much more stable, fully avoiding cleavage by biothiols. AuNPs were deposited with a thin layer of platinum, and an AuNP@Pt-S nanoflare was constructed to detect the miRNA-21 microRNA in living cells. This design retained the optical and cellular uptake properties of DNA-functionalized AuNPs, while showing high-fidelity signaling. It discriminated target cancer cells even in a mixed-cell culture system, where the Au-S based nanoflare was less sensitive. Compared to previous methods of changing the ligand chemistry, coating a Pt shell is more accessible, and previously developed methods for AuNPs can be directly adapted.

DNA-based activatable theranostic nanoprobes are still unmet for in vivo applications. Here, by utilizing the "induced-fit effect", a smart split aptamer-based activatable theranostic probe (SATP) was first designed as "nanodoctor" for cancer-activated in vivo imaging and in situ drug release. The SATP assembled with quenched fluorescence and stable drug loading in its free state. Once binding to target proteins on cell surface, the SATP disassembled due to recognition-triggered reassembly of split aptamers with activated signals and freed drugs. As proof of concept, split Sgc8c against CEM cancer was used for theranostic studies. Benefiting from the design without blocking aptamer sequence, the SATP maintained an excellent recognition ability similar to intact Sgc8c. An "incubate-and-detect" assay showed that the SATP could significantly lower background and improve signal-to-background ratio (∼4.8 times of "always on" probes), thus affording high sensitivity for CEM cell analysis with 46 cells detected. Also, its high selectivity to target cells was demonstrated in analyzing mixed cell samples and serum samples. Then, using doxorubicin as a model, highly specific drug delivery and cell killing was realized with minimized toxicity to nontarget cells. Moreover, in vivo and ex vivo investigations also revealed that the SATP was specifically activated by CEM tumors inside mice. Especially, contrast-enhanced imaging was achieved in as short as 5 min, thus, laying a foundation for rapid diagnosis and timely therapy. As a biocompatible and target-activatable strategy, the SATP may be widely applied in cancer theranostics.

Activatable aptamer probes (AAPs) have emerged as a promising strategy in cancer diagnostics, but existing AAPs remain problematic due to complex design and synthesis, instability in biofluids, or lack of versatility for both in vitro and in vivo applications. Herein, we proposed a novel AAP strategy for cancer cell probing based on fluorophore-labeled aptamer/single-walled carbon nanotube (F-apt/SWNT) ensembles. Through π-stacking interactions and proximity-induced energy transfer, F-apt/SWNT with quenched fluorescence spontaneously formed in its free state and realized signal activation upon targeting surface receptors of living cells. As a demonstration, Sgc8c aptamer was used for in vitro analysis and in vivo imaging of CCRF-CEM cancer cells. It was found that self-assembled Cy5-Sgc8c/SWNT held robust stability for biological applications, including good dispersity in different media and ultralow fluorescence background persistent for 2 h in serum. Flow cytometry assays revealed that Cy5-Sgc8c/SWNT was specifically activated by target cells with dramatic fluorescence elevation and showed improved sensitivity with as low as 12 CCRF-CEM cells detected in mixed samples containing ~100,000 nontarget cells. In vivo studies confirmed that specifically activated fluorescence was imaged in CCRF-CEM tumors, and compared to "always on" probes, Cy5-Sgc8c/SWNT greatly reduced background signals, thus resulting in contrast-enhanced imaging. The general applicability of the strategy was also testified by detecting Ramos cells with aptamer TD05. It was implied that F-apt/SWNT ensembles hold great potential as a simple, stable, sensitive, specific, and versatile activatable platform for both in vitro cancer cell detection and in vivo cancer imaging.

Activatable aptamer probes (AAPs) are promising in molecular imaging of tumors, but the reported shape-switchingdependent AAPs are still challenged by unsatisfied noise suppression, poor stability, and sophisticated sequence design. To address the problem, we constructed a pH-activatable aptamer probe (pH-AAP) by utilizing an acid-labile acetal linker as the responsive element to be fused with a tumor-targeted aptamer. Specifically, a Cy5-labeled aptamer was connected with the quencher BHQ2 through the acetal group, thus generating pH-AAP with quenched fluorescence. Due to the stable proximity of Cy5 to BHQ2, pH-AAP was found to have ultralow background with a quenching efficiency as high as 98%. In comparison with shape-switching-dependent AAPs, the noise suppression of pH-AAP was well maintained for a much longer time in both serum and mouse body, thus showing a robust fluorescence stability. By a combination of the fluorescence recovery induced by acid hydrolysis of acetal linkers and the tumor-targeted recognition of aptamers, pH-AAP could either specifically anchor the extracellular pH-activated signals on the target cell surface in an acidic tumor microenvironment or be activated by acidic lysosomes after it was internalized into target cells. As proof of concept, in vitro evaluation and in vivo imaging of A549 lung cancer cells were performed by using S6 aptamer as a demonstration. It was indicated that pH-AAP realized washing-free, bispecific, and contrast-enhanced tumor imaging. The strategy is simple and free of sequence modification, which promises to provide a universal platform for sensitive and precise tumor diagnosis.

Activatable aptamers have emerged as promising molecular tools for cancer theranostics, but reported monovalent activatable aptamer probes remain problematic due to their unsatisfactory affinity and poor stability. To address this problem, we designed a novel theranostic strategy of DNA nanotriangle-scaffolded multivalent split activatable aptamer probe (NTri-SAAP), which combines advantages of programmable self-assembly, multivalent effect and target-activatable architecture.Methods: NTri-SAAP was assembled by conjugating multiple split activatable aptamer probes (SAAPs) on a planar DNA nanotriangle scaffold (NTri). Leukemia CCRF-CEM cell line was used as the model to investigate its detection, imaging and therapeutic effect both in vitro and in vivo. Binding affinity and stability were evaluated using flow cytometry and nuclease resistance assays.Results: In the free state, NTri-SAAP was stable with quenched signals and loaded doxorubicin, while upon binding to target cells, it underwent a conformation change with fluorescence activation and drug release after internalization. Compared to monovalent SAAP, NTri-SAAP displayed greatly-improved target binding affinity, ultralow nonspecific background and robust stability in harsh conditions, thus affording contrast-enhanced tumor imaging within an extended time window of 8 h. Additionally, NTri-SAAP increased doxorubicin loading capacity by ~5 times, which further realized a high anti-tumor efficacy in vivo with 81.95% inhibition but no obvious body weight loss.Conclusion: These results strongly suggest that the biocompatible NTri-SAAP strategy would provide a promising platform for precise and high-quality theranostics.

Intracellular pH (pHi) is an important parameter associated with cellular behaviors and pathological conditions. Sensing pHi and monitoring its changes are essential but challenging due to the lack of high-sensitive probes. Herein, a ratiometric fluorescent probe with ultra pH-sensitivity is developed based on hairpin-contained i-motif strand (I-strand, labeled with Rhodamine Green and BHQ at two termini) and complementary strand (C-strand, labeled with Rhodamine Red at its 5'-end). At neutral pH, both I-strand and C-strand hybridize into a rigid duplex (I-C), which holds the Rhodamine Red and the BHQ in close proximity. As a result, the fluorescence emission (F) of the Rhodamine Red is strongly suppressed, while the Rhodamine Green (F) is in a "signal on" state. However, the slightly acidic pH enforced the I-strand to form an intramolecular i-motif and initiated the dehybridization of I-C duplex, leading to Rhodamine Red in a "signal on" state and a decreased fluorescence of Rhodamine Green. The ratio (F/F) can be used as a signal for pH sensing. Due to the rational internal hairpin design of I-C duplex probe, almost 70-fold change in the ratio was observed in the physiological pH range (6.50-7.40). This probe possesses efficient stability, fast response, and reversible pH measurement capabilities. Furthermore, intracellular application of the ratiometric probe was demonstrated on the example of SMMC-7721 cells. With different recognition elements in engineering of i-motif based platforms, the design might hold great potential to become a versatile strategy for intracellular pH sensing.