Reactive oxygen species (ROS) have captured the interest of many researchers in the chemical, biological, and medical fields since they are thought to be associated with various pathological conditions. Fluorescent probes for the detection of ROS are promising tools with which to enhance our understanding of the physiological roles of ROS, because they provide spatial and temporal information about target biomolecules in in vivo cellular systems. ROS probes, designed to detect specific ROS with a high selectivity, would be desirable, since it is now becoming clear that each ROS has its own unique physiological activity. However, dihydro-compounds such as 2',7'-dichlorodihydrofluorescein (DCFH), which have traditionally been used for detecting ROS, tend to react with a wide variety of ROS and are not completely photostable. Some attractive fluorescent probes that exhibit a high degree of selectivity toward specific ROS have recently been reported, and these selective probes are expected to have great potential for elucidating unknown physiological mechanisms associated with their target ROS. This review focuses on the design, detection mechanism, and performance of fluorescent probes for the detection of singlet oxygen ((1)O(2)), hydrogen peroxide (H(2)O(2)), hydroxyl radicals ((.)OH), or superoxide anion (O(2) (-.)), a field in which remarkable progress has been achieved in the last few years.

Site-specific chemical labeling utilizing small fluorescent molecules is a powerful and attractive technique for in vivo and in vitro analysis of cellular proteins, which can circumvent some problems in genetic encoding labeling by large fluorescent proteins. In particular, affinity labeling based on metal-chelation, advantageous due to the high selectivity/simplicity and the small tag-size, is promising, as well as enzymatic covalent labeling, thereby a variety of novel methods have been studied in recent years. This review describes the advances in chemical labeling of proteins, especially highlighting the metal-chelation methodology.

A fluorescent photochromic compound, composed of diarylethene, fluorescein and succinimidyl ester units, was developed for the controllable fluorescent labeling of biomolecules based on a small molecule.

A fluorescent probe using a novel 'spin exchange' concept was developed for monitoring nitric oxide (NO) production. The probe is composed of 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) labeled with acridine and N-dithiocarboxysarcosine (DTCS)-Fe(II) complex. When the non-fluorescent acridine-TEMPO was incubated with DTCS-Fe(II) complex in buffer solution, the nitroxide radical in the acridine-TEMPO interacted with the Fe(II) through a redox interaction. This interaction recovered the fluorescence based on the acridine moiety. The addition of an NO-releasing reagent caused a fluorescent decrease of the probe due to the irreversible binding of NO to the Fe(II), and the amount of the fluorescent decrease strictly corresponded to that of released NO. Using this probe, less than 100 nM of NO can be detected. This probe system is not only useful for monitoring direct production of NO in an aqueous solution, but is also interesting as a basic concept by which to construct new types of NO fluorescent probes.

A swallow-tailed perylene derivative including a triphenylphosphine moiety was synthesized and applied to the detection and the live-cell imaging of lipid hydroperoxides. The novel probe, named Spy-LHP, reacted rapidly and quantitatively with lipid hydroperoxides to form the corresponding oxide, Spy-LHPOx, which emits extremely strong fluorescence (Phi approximately 1) in the visible range (lambda(em) = 535 nm, 574 nm). Spy-LHP was highly selective for lipid hydroperoxides, and the addition of other reactive oxygen species (ROS) including hydrogen peroxides, hydroxyl radical, superoxide anion, nitric oxide, peroxynitrite, and alkylperoxyl radical, caused no significant increase in the fluorescence intensity. The probe exhibited good localization to cellular membranes and was successfully applied to the confocal laser scanning microscopy (CLSM) imaging of lipid hydroperoxides in live J774A.1 cells, in which lipid peroxidation was proceeded by the stimulation of 2,2-azobis(2-amidinopropane)dihydrochloride (AAPH). These findings establish Spy-LHP as a promising new tool for investigating the physiology of lipid hydroperoxides.