Selectivity by Polarity and Size. Biomimetic Signaling

A very similar colorimetric discrimination of amines was reported for a system that employs a dicyanomethylene-2-chloro-3-amino-indene dye as a signaling reporter and a hydrophobized (trimethylsilylated) mesoporons MCM-41-type solid. The same conceptnal idea was also followed for a hybrid ensemble that contains a styrylpyrylinm derivative anchored at the walls of hydrophobized mesopores. In this case, the sensory material changes its eolor from bine to red by reaction with primary amines. As a remarkable feature, the styrylpyrylium-appended hydrophobized mesoporous solid was able to react with biogenic amines such as histamine from fish extracts and remained silent in the presence of amino acids or long-chain fatty amines.  

Discrimination by size and polarity in combination with colorimetric signaling was also observed for microporous materials—for example, when a pyrylium derivative is anchored to the inner pores of zeolite Beta. Zeolite Beta is characterized by a 3D channel system for which access to the pore voids is limited by 6.8 A aperture that is composed of 12 silicon atoms. A size-based discrimination was observed and only amines smaller than the pore openings induce color modulations. For amines of similar size, an additional discrimination by polarity that is induced by the hydrophobic environment of the inner zeolite was also found (Fig. 11).  

Discrimination by polarity has also been used for the design of robust sensors for vapor detection. For instance, a mesoporous silica material containing a covalently anchored phenoxazinone probe has been used as an optical sensory hybrid for the detection of volatile organic compounds (VOCs). The indication mechanism relies on the property of certain phenoxazinone 

FIGURE 10. The synthesis of a PLA-coated fluorescent sensor material for detection of amine-containing neurotransmitters (dopamine, glutamic acid, and tyrosine). 

FIGURE 11. (a) A chromogenic hybrid material containing a pyrylium dye anchored on the zeolite Beta walls, (b) The color changes observed in this hybrid material upon addition of certain amines. 

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