Chemistry

Zooming in on pH

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Science  21 Aug 2009:
Vol. 325, Issue 5943, pp. 921
DOI: 10.1126/science.325_921d
CREDIT: HARBUZARU ET AL., ANGEW. CHEM. INT. ED. 48, 6476 (2009)

Local acidity plays a central role in many chemical and physiological processes. Molecular-scale tools that can sense or alter the pH of their environment are therefore eagerly sought. Harbuzaru et al. present a miniaturized pH sensor operative in the physiologically relevant pH 5 to 7.5 range. The sensor is based on a metal-organic framework material (optical micrograph shown below) that contains strongly photoluminescent trivalent europium ions in two distinct coordination environments. The authors combined this material with a commercial optical fiber in their prototype design. Because only one of the two ion sites is sensitive to pH, the sensor is self-calibrating.

In a similarly directed study, Alibrandi et al. show that [1,1,1]cryptand—a cagelike molecule that gradually captures protons from aqueous solution—can be used as a molecular-scale automatic titrator to smoothly vary the pH of a solution through nearly two units over time. Beyond pKa determinations (which the authors demonstrate), this system may find use in studying phenomena such as the pH-dependent unfolding of proteins.

Angew. Chem. Int. Ed. 48, 6476; 6332 (2009).

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