Due to the Fenton reaction, the presence of Fe and peroxide in electrodes generates free radicals causing serious degradation of the organic ionomer and the membrane. Pt-free and Fe-free cathode catalysts therefore are urgently needed for durable and inexpensive proton exchange membrane fuel cells (PEMFCs). Herein, a high-performance nitrogen-coordinated single Co atom catalyst is derived from Co-doped metal-organic frameworks (MOFs) through a one-step thermal activation. Aberration-corrected electron microscopy combined with X-ray absorption spectroscopy virtually verifies the CoN coordination at an atomic level in the catalysts. Through investigating effects of Co doping contents and thermal activation temperature, an atomically Co site dispersed catalyst with optimal chemical and structural properties has achieved respectable activity and stability for the oxygen reduction reaction (ORR) in challenging acidic media (e.g., half-wave potential of 0.80 V vs reversible hydrogen electrode (RHE). The performance is comparable to Fe-based catalysts and 60 mV lower than Pt/C -60 μg Pt cm ). Fuel cell tests confirm that catalyst activity and stability can translate to high-performance cathodes in PEMFCs. The remarkably enhanced ORR performance is attributed to the presence of well-dispersed CoN active sites embedded in 3D porous MOF-derived carbon particles, omitting any inactive Co aggregates.

Hypoxia, a quite universal feature in most solid tumors, has been considered as the "Achilles' heel" of traditional photodynamic therapy (PDT) and substantially impairs the overall therapeutic efficacy. Herein, we develop a near-infrared (NIR) light-triggered molecular superoxide radical (O 2 −• ) generator (ENBS-B) to surmount this intractable issue, also reveal its detailed O 2 −• action mechanism underlying the antihypoxia effects, and confirm its application for in vivo targeted hypoxic solid tumor ablation. Photomediated radical generation mechanism study shows that, even under severe hypoxic environment (2% O 2 ), ENBS-B can generate considerable O 2 −• through type I photoreactions, and partial O 2 −• is transformed to high toxic OH• through SODmediated cascade reactions. These radicals synergistically damage the intracellular lysosomes, which subsequently trigger cancer cell apoptosis, presenting a robust hypoxic PDT potency. In vitro coculture model shows that, benefiting from biotin ligand, ENBS-B achieves 87-fold higher cellular uptake in cancer cells than normal cells, offering opportunities for personalized medicine. Following intravenous administration, ENBS-B is able to specifically target to neoplastic tissues and completely suppresses the tumor growth at a low light-dose irradiation. As such, we postulated this work will extend the options of excellent agents for clinical cancer therapy.

A selective sensor 1 for the fluorescent imaging of Cd2+ in living cells has been designed and synthesized based on an internal charge transfer (ICT) mechanism. It can distinguish Cd2+ from Zn2+ and can be used in both general fluorescence intensity microscopy and ratiometric fluorescence microscopy.

Compared with fluorescence imaging utilizing fluorophores whose lifetimes are in the order of nanoseconds, time-resolved fluorescence microscopy has more advantages in monitoring target fluorescence. In this work, compound DCF-MPYM, which is based on a fluorescein derivative, showed long-lived luminescence (22.11 μs in deaerated ethanol) and was used in time-resolved fluorescence imaging in living cells. Both nanosecond time-resolved transient difference absorption spectra and time-correlated single-photon counting (TCSPC) were employed to explain the long lifetime of the compound, which is rare in pure organic fluorophores without rare earth metals and heavy atoms. A mechanism of thermally activated delayed fluorescence (TADF) that considers the long wavelength fluorescence, large Stokes shift, and long-lived triplet state of DCF-MPYM was proposed. The energy gap (ΔEST) of DCF-MPYM between the singlet and triplet state was determined to be 28.36 meV by the decay rate of DF as a function of temperature. The ΔE(ST) was small enough to allow efficient intersystem crossing (ISC) and reverse ISC, leading to efficient TADF at room temperature. The straightforward synthesis of DCF-MPYM and wide availability of its starting materials contribute to the excellent potential of the compound to replace luminescent lanthanide complexes in future time-resolved imaging technologies.

Intracellular viscosity strongly influences transportation of mass and signal, interactions between the biomacromolecules, and diffusion of reactive metabolites in live cells. Fluorescent molecular rotors are recently developed reagents used to determine the viscosity in solutions or biological fluid. Due to the complexity of live cells, it is important to carry out the viscosity determinations in multimode for high reliability and accuracy. The first molecular rotor (RY3) capable of dual mode fluorescence imaging (ratiometry imaging and fluorescence lifetime imaging) of intracellular viscosity is reported. RY3 is a pentamethine cyanine dye substituted at the central (meso-) position with an aldehyde group (CHO). In nonviscous media, rotation of the CHO group gives rise to internal conversion by a nonradiative process. The restraining of rotation in viscous or low-temperature media results in strong fluorescence (6-fold increase) and lengthens the fluorescence lifetime (from 200 to 1450 ps). The specially designed molecular sensor has two absorption maxima (λ(abs) 400 and 613 nm in ethanol) and two emission maxima (in blue, λ(em) 456 nm and red, 650 nm in ethanol). However it is only the red emission which is markedly sensitive to viscosity or temperature changes, providing a ratiometric response (12-fold) as well as a large pseudo-Stokes shift (250 nm). A mechanism is proposed, based on quantum chemical calculations and (1)H NMR spectra at low-temperature. Inside cells the viscosity changes, showing some regional differences, can be clearly observed by both ratiometry imaging and fluorescence lifetime imaging (FLIM). Although living cells are complex the correlation observed between the two imaging procedures offers the possibility of previously unavailable reliability and accuracy when determining intracellular viscosity.

Identifying cancer cells and quantifying cancer-related events in particular organelles in a rapid and sensitive fashion are important for early diagnosis and for studies on pathology and therapeutics of cancers. Herein a smart "off-on" cyclooxygenase-2-specific fluorescence probe (ANQ-IMC-6), able to report the presence of cancer cells and to image Golgi-related events, has been designed and evaluated. Cyclooxygenase-2 (COX-2) has been used as imaging target in the probe design, since this enzyme is a biomarker of virtually all cancer cell lines. In the free state in aqueous solution, ANQ-IMC-6 mainly exists in a folded conformation where probe fluorescence is quenched through photoinduced electron transfer between the fluorophore acenaphtho[1,2-b]quinoxaline (ANQ) and the recognition group, indomethacin (IMC). Fluorescence is turned on, by restraining the photoinduced electron transfer, when ANQ-IMC-6 is forced to adopt the unfolded state following binding to COX-2 in the Golgi apparatus of cancer cells. ANQ-IMC-6 provides high signal-to-background staining and has been successfully used to rapidly differentiate cancer cells from normal cells when using flow cytometry and one- and two-photon fluorescence microscopic imaging. Furthermore, ANQ-IMC-6 may be able to visualize dynamic changes of the Golgi apparatus during cancer cell apoptosis, with possible application to early diagnosis.

Reactive oxygen species (ROS) and cellular oxidant stress have long been associated with cancer. Unfortunately, the role of HClO in tumor biology is much less clear than for other ROS. Herein, we report a BODIPY-based HClO probe (BClO) with ultrasensitivity, fast response (within 1 s), and high selectivity, in which the pyrrole group at the meso position has an "enhanced PET" effect on the BODIPY fluorophore. The detection limit is as low as 0.56 nM, which is the highest sensitivity achieved to date. BClO can be facilely synthesized by a Michael addition reaction of acryloyl chloride with 2,4-dimethylpyrrole and applied to image the basal HClO in cancer cells for the first time and the time-dependent HClO generation in MCF-7 cells stimulated by elesclomol, an effective experimental ROS-generating anticancer agent.

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