Let it shine: New hypoxia-sensitive fluorescent probes were developed; they consist of a rhodamine moiety with an azo group directly conjugated to the fluorophore. Because of an ultrafast conformational change around the NN bond, the compounds are nonfluorescent under normoxia. However, under hypoxia, the azo group is reduced, and a strongly fluorescent rhodamine derivative is released.
Fluorescence imaging is one of the most powerful techniques for visualizing temporal and spatial changes of biological phenomena in living cells, and many fluorescent probes have been developed. In particular, xanthene dyes such as fluorescein and rhodamines have favorable characteristics, such as high water solubility, high fluorescence quantum yield and high molar extinction coefficient, and they have been utilized as fluorescent cores for fluorescent probes working in the green to red wavelength region. Recently, silicon-substituted xanthene dyes such as 2,7-N,N,N',N'-tetramethyl-9-dimethyl-10-hydro-9-silaanthracene (TMDHS), Si-rhodamines and TokyoMagentas, in which the O atom at the 10-position of xanthene is replaced with a Si atom, have been developed as novel far-red to near-infrared fluorescent cores that retain the key advantages of the parent structures. Fluorescent probes based on them have opened up new possibilities for imaging biological processes in living cells. This minireview covers recent progress in silicon-substituted xanthene dyes, including representative applications for in vivo tumor imaging, triple-color imaging of neuronal activity, and super-resolution microscopy.
f Photocatalysis describes the excitation of titanium dioxide nanoparticles (a wide-band gap semiconductor) by UVA light to produce reactive oxygen species (ROS) that can destroy many organic molecules. This photocatalysis process is used for environmental remediation, while antimicrobial photocatalysis can kill many classes of microorganisms and can be used to sterilize water and surfaces and possibly to treat infections. Here we show that addition of the nontoxic inorganic salt potassium iodide to TiO 2 (P25) excited by UVA potentiated the killing of Gram-positive bacteria, Gram-negative bacteria, and fungi by up to 6 logs. The microbial killing depended on the concentration of TiO 2 , the fluence of UVA light, and the concentration of KI (the best effect was at 100 mM). There was formation of long-lived antimicrobial species (probably hypoiodite and iodine) in the reaction mixture (detected by adding bacteria after light), but short-lived antibacterial reactive species (bacteria present during light) produced more killing. Fluorescent probes for ROS (hydroxyl radical and singlet oxygen) were quenched by iodide. Tri-iodide (which has a peak at 350 nm and a blue product with starch) was produced by TiO 2 -UVA-KI but was much reduced when methicillin-resistant Staphylococcus aureus (MRSA) cells were also present. The model tyrosine substrate N-acetyl tyrosine ethyl ester was iodinated in a light dose-dependent manner. We conclude that UVA-excited TiO 2 in the presence of iodide produces reactive iodine intermediates during illumination that kill microbial cells and long-lived oxidized iodine products that kill after light has ended. Heterogeneous photocatalysis is the use of photoactivated titanium dioxide to produce reactive oxygen species in order to destroy organic pollutants and to kill different classes of microorganisms (1). TiO 2 is a wide-band gap semiconductor, and when a photon of sufficient energy is absorbed, an electron is excited from the valence band to the conduction band, generating a positive hole in the valence band (2). Since energy levels are not available to promote ready recombination of the electron and the hole (as they are in metallic conductors), the electrons and holes survive long enough to carry out reactions. The electrons can reduce oxygen to superoxide, while at the same time the holes can oxidize water to hydroxyl radicals (3). Hydrogen peroxide and singlet oxygen are also produced (4).Although TiO 2 can occur in several different crystalline forms (anatase, rutile, and brookite), the anatase form is usually employed for applications in photocatalysis (5). Because the process is heterogeneous, TiO 2 nanoparticles that have the maximum surface area/mass ratio are optimum for efficient catalytic activity. The maximum absorption wavelength of the TiO 2 nanoparticle is about 360 nm (6), equivalent to the semiconductor band gap of 3.35 eV (7). The preparation of TiO 2 known as Degussa or Aeroxide (Evonik) P25 is composed of 21-nm-diameter nanoparticles, which are about 75% anatase, 1...
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