The literature is constellated with a wide variety of chemosensors against a plethora of analytes. This seminal library is used to inspire chemists to improve them using chemical synthesis. However, their optimization via chemical synthesis is a difficult task which takes time without the guarantee of final success.We show here that combinatorial chemistry,the use of first and second coordination spheres and the displacement of indicators united within a protein cavity offers an easy-to-assemble colorimetric bio-chemical sensor. It consists only of commercial chemicals. This colorimetric sensor is highly modular, cheap and evolvable. Its X-ray structure reveals the composition of its active site. This allows to design it rationally for the recognition of dopamine with the naked eye. Our bio-sensor therefore resembles a biological receptor for the recognition of neurotransmitters. Its immediate high adaptability and ability to be evolved can be useful for the selective detection of a wide variety of analytes going from small molecules to microorganisms. This discovery therefore makes it possible to dream of new biotechnological or new immunotherapeutic applications.
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