Using a synthetic receptor to create an optical-sensing ensemble for a class of analytes: a colorimetric assay for the aging of scotch.

One driving force behind the creation of synthetic receptors is to demonstrate that humankind can, by design, create receptors with affinities and selectivities that rival natural receptors such as antibodies and enzymes. 1 Some excellent selectivities can indeed be achieved with synthetic systems. 2 Although this is a very worthwhile endeavor that our group 3 and many others are pursuing, 4 it is still true that the relative simplicity of synthetic receptors render most of these less selective than natural receptors. However, this lack of selectivity can, for some applications, make synthetic receptors more desirable than natural ones. Herein, we report that a receptor with broad recognition properties for a class of analytes can create a sensing system that has a practical application. Fine scotches are required to age for a period of time in oak barrels or casks before they are ready for consumption. This aging process is known to affect the flavor and color of the beverage through extraction of phenolic acids from the wood. 5 Some examples are ellagic acid, protocatechuic acid, caffeic acid, and gallic acid (shown below). The amount of gallic acid present in scotch is considered to be an indication of age, since it is generated through the hydrolysis of tannins over time. 5a However, the concentration of these other “gallate-like” compounds can also act as an indicator of the age of the scotch. Other factors that determine the amount of gallic acid and analogues incorporated into the spirit depend on the type of wood, how many times the casks have been used, and what they were used for. Therefore, the exact level of any specific compound can only be roughly related to age. Yet, gallate is currently the compound most often quantitated. We now report that synthetic receptors can be used when it is desired that a class of structures be collectively analyzed.