Editorial Feature

Fluorescent Chloride Sensors

Article updated on 09/03/20 by Jo Finchen-Parsons

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A chloride ion sensor is a sensing element combined with a wireless embedded sensor platform to measure the concentration of chloride in a material.

Researchers at The Johns Hopkins University from Applied Physics Laboratory have developed a wireless chloride ion sensor.

The chloride ions are measured by a molecularly imprinted polymer membrane of polypyrrole and inter-digitated AgCl electrodes.

The chloride molecules penetrate the polymer membrane and get trapped. As a result, the electronic characteristics of the polymer are changed and a signal is produced with respect to the amount of chloride present.

The invention of chloride ions in physiological processes stimulates the measurements of intracellular chloride ions in live cells and the development of various fluorescent tools. Fluorescent chloride sensors offer a powerful tool for non-invasive monitoring of intracellular distribution and transmembrane fluxes.

Types of Fluorescent Chloride Sensors

Yellow Fluorescent Protein (YFP)-based Chloride Sensors

YFP-based sensors are mutated versions of a green fluorescent protein They contain four-point mutations and red-shifted excitation and emission spectra when compared to the green fluorescent protein.

YFP fluorescence senses various small anions resulting from ground-state binding near the chromophore. However, these sensors rather exhibit low kinetics of chloride association and dissociation.

Quinoline-based Chloride Sensors

These sensors are operated based on the capability of chloride molecules to quench the fluorescence of heterocyclic organic compounds with quaternary nitrogen. N-(6-methoxyquinolyl)-acetoethyl ester (MQAE), 6-methoxy-N-ethylquinolium Chloride– (MEQ), and 6-methoxy-1-(3-sulfonatopropyl) quinolinium (SPQ) are the most used quinolinium–based chloride–indicators.

Forster Resonance Energy Transfer (FRET)-based Chloride Sensors

These sensors consist of two fluorescent proteins such as cyan fluorescent protein (CFP) and yellow fluorescent protein combined with a polypeptide linker. The FRET-based chloride sensors allow ratiometric chloride measurements based on the chloride sensitivity of YFP and chloride insensitivity of CFP.

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Working Principle

According to Wang J. et al. (2012), the fluorescent chloride sensors are operated based on the principle that if a fluorescent indicator dye molecule binds the to-be-measured ion, the binding of the ion induces a structural change which changes the dye's fluorescent properties thereby changing the fluorescence light intensity.

The intensity of fluorescence light decreases when there is a higher concentration of chloride ions in the bound molecule.

The chloride concentration of material is quantitatively determined using the fluorescence intensity of MQAE, a chloride ion fluorescent indicator. The MQAE solution reacts with the chloride solution.

The solution mixture is then irradiated by UV light, and an optical detector is used to the fluorescence emission. The concentration of chloride ions in the solution is then calculated based on the Stern-Volmer equation.

Applications of Fluorescent Chloride Sensors

Fluorescent chloride sensors are used to measure chloride transport across cell membranes to regulate cell volume, resting potential, charge balance, and membrane excitability. They are also used for the diagnosis of cystic fibrosis.

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