Nile red (also known as Nile blue oxazone) is a

lipophilic Lipophilicity (from Greek language, Greek λίπος "fat" and :wikt:φίλος, φίλος "friendly") is the ability of a chemical compound to dissolve in fats, oils, lipids, and non-polar solvents such as hexane or toluene. Such compounds are c ...

stain. Nile red stains intracellular

lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...

droplets yellow. In most polar solvents, Nile red will not fluoresce; however, when in a lipid-rich environment, it can be intensely fluorescent, with varying colors from deep red (for polar membrane lipid) to strong yellow-gold emission (for neutral lipid in intracellular storages). The dye is highly solvatochromic and its emission and excitation wavelength both shift depending on solvent polarity and in polar media will hardly fluoresce at all. Nile red has applications in cell biology, where it can be used as a membrane dye which can be readily visualized using an epifluorescence microscope with excitation and emission wavelengths usually shared with red fluorescent protein. Nile red has also been used as part of a sensitive detection process for microplastics in bottled water. Additionally, nile red is a remarkable candidate in fabricating membrane for different sensors to detect environmental changes, such as taste, gas, pH, etc. In triglycerides (a neutral lipid), Nile red has an excitation maximum of about 515 nm (green), and emission maximum of about 585 nm (yellow-orange). In contrast, in phospholipids (polar lipids), Nile red has an excitation maximum of about 554 nm (green), and an emission maximum of about 638 nm (red). The diffusion coefficient of Nile red in ethanol has been reported 470 μm²/s.

Synthesis

Nile red can be prepared through an acid hydrolysis by boiling a solution of Nile blue with

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid (English in the Commonwealth of Nations, Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen, ...

. This process replaces an iminium group with a carbonyl group. Alternatively, Nile red and its analogs (naphthooxazine dyes) can be prepared by acid-catalyzed

condensation Condensation is the change of the state of matter from the gas phase into the liquid phase, and is the reverse of vaporization. The word most often refers to the water cycle. It can also be defined as the change in the state of water vapor ...

of corresponding 5-(dialkylamino)-2-nitrosophenols with 2-naphthol. The yields are generally moderate as no co-oxidant is used in this procedure. Since the reaction to generate Nile red does not usually completely exhaust the supply of Nile blue, additional separation steps are required if pure Nile red is needed. File:Nile red 01.jpg, Nile red under visible and ultraviolet (366 nm) light in different solvents. From left to right: 1.

water Water is an inorganic compound with the chemical formula . It is a transparent, tasteless, odorless, and Color of water, nearly colorless chemical substance. It is the main constituent of Earth's hydrosphere and the fluids of all known liv ...

, 2. methanol, 3.

ethanol Ethanol (also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound with the chemical formula . It is an Alcohol (chemistry), alcohol, with its formula also written as , or EtOH, where Et is the ps ...

, 4. acetonitrile, 5. dimethylformamide, 6.

acetone Acetone (2-propanone or dimethyl ketone) is an organic compound with the chemical formula, formula . It is the simplest and smallest ketone (). It is a colorless, highly Volatile organic compound, volatile, and flammable liquid with a charact ...

, 7. ethyl acetate, 8. dichloromethane, 9. n-

hexane Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14. Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as ...

, 10. methyl-''tert''-butylether, 11.

cyclohexane Cyclohexane is a cycloalkane with the molecular formula . Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexan ...

, 12.

toluene Toluene (), also known as toluol (), is a substituted aromatic hydrocarbon with the chemical formula , often abbreviated as , where Ph stands for the phenyl group. It is a colorless, water Water is an inorganic compound with the c ...

. File:Bacillus subtilis stained with Nile Red.jpg, ''

Bacillus subtilis ''Bacillus subtilis'' (), known also as the hay bacillus or grass bacillus, is a gram-positive, catalase-positive bacterium, found in soil and the gastrointestinal tract of ruminants, humans and marine sponges. As a member of the genus ''Bacill ...

'' stained with Nile red as a membrane dye (shown in red). This strain grows partly as cell chains, so a membrane dye may be useful to distinguish internal cell boundaries.

References

{{Use dmy dates, date=March 2019 Oxazine dyes Vital stains Diethylamino compounds