A traditional painkiller as a probe for microheterogeneity in 1-propanol-water mixtures

► Spectral properties of indomethacin can sense solvent clusters in mixtures of 1-propanol-water. ► The drug forms fluorescent complex with alcohol self-association clusters at <40-60 wt. % water. ► It forms non-fluorescent complex with hydrogen-bonded water clusters at >40-60 wt. % water. ► Increasing water converts drug from fluorescent to non-fluorescent complex. ► This conversion makes the drug a sensor for microheterogeneity in alcohol-water mixtures.