Optical probe for the analysis of trace indole in shrimp.

Indole is a chemical from the decomposition of shrimp and is used extensively to indicate seafood freshness. US Food and Drug Administration (FDA) sets its concentration of <25 µg/100 g shrimp as the threshold for Class I (fresh shrimp). A novel optical probe is reported to quantitatively analyze trace indole in shrimp, including the Class I threshold concentration. Based on an Ehrlich-type reaction, visible spectroscopic analysis of indole in petroleum ether gives a limit of detection (LoD) and quantification (LoQ) of 0.05 and 0.16 µg mL-1, respectively. For 25 µg indole/100 g shrimp extracted into petroleum ether, the probe successfully detects it and the color change is visible to the naked eye. Analysis of the probe response by a visible spectrometer leads to quantification of ≤25 µg indole/100 g shrimp, when recovery is accounted for. When a handheld colorimeter, based on the CIELAB color space, and a smartphone with Bluetooth connectivity are used, the probe demonstrates similar sensitivity for indole in shrimp. The current probe is made of 4-(dimethylamino)benzaldehyde (DMAB) and catalyst p-toluenesulfonic acid (PTSA) in thin films. Indole in shrimp samples after extraction reacts with DMAB to give red ß-bis(indolyl)methane.

Product in indole detection by Ehrlich's reagent.

Ehrlich's reagent (p-dimethylaminobenzaldehyde [DMAB, 1] in 95% EtOH with HCl as catalyst) was employed in spot tests of indoles, providing a diagnosis of, for example, liver diseases, hemolytic processes, occlusion of the common bile duct, and carcinoid syndrome. Although the reagent has been widely used for more than a century, it is not clear how many indole molecules react with a DMAB molecule and whether the reaction takes place at the α- or ß-position of the indole molecule. Research here shows that the reaction of DMAB (1) with indole (2) in a 1:2 ratio gives ß-bis(indolyl)methane (3). The reaction occurs at the ß-position of indole under the conditions of the Ehrlich test, as confirmed by the crystal structure of 3.