The terminal group of this thiol is known as umbelliferone or 7- hydroxycoumarin, which occurs in many familiar plants such as carrot, coriander and garden angelica. The important property of this group in the context of SAMs is that it absorbs strongly at 300, 305 and 325 nm, with log ? values of 3.9, 3.95 and 4.15 respectively, and it fluoresces blue in both ultraviolet and visible light. Interestingly, the absorption characteristics change in alkaline solution, since the phenolic hydroxyl group is deprotonated (pKa = 7.7). This property makes umbelliferone terminated SAMs suitable as a pH indicators in the range 6.5-8.9. Also, the SAMs of this compound can be used as a fluorescence indicators for metal ions such as copper and calcium.