Naked‐Eye Heterogeneous Sensing of Fluoride Ions by Co‐Polymeric Nanosponge Systems Comprising Aromatic‐Imide‐Functionalized Nanocellulose and Branched Polyethyleneimine

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Abstract
Heterogeneous colorimetric sensors for fluoride ions were obtained by cross‐linking TEMPO‐oxidized cellulose nanofibers (TOCNF) with chemically modified branched polyethyleneimine 25 kDa (bPEI). Functionalization of bPEI primary amino groups with aromatic anhydrides led to the formation of the corresponding mono‐ and bis‐imides on the grafted polymers (f‐bPEI). A microwave‐assisted procedure allowed the optimization of the synthetic protocol by reducing reaction time from 17 h to 30 minutes. Hydrogels obtained by mixing different ratios of TOCNF, bPEI and f‐bPEI were lyophilized and thermally treated at about 100 °C to promote the formation of amide bonds between the amino groups of poly‐cationic polymers and the carboxylic groups of cellulose nanofibers. This approach generated a series of cellulose nanosponges S1‐S3 which were characterized by FT‐IR and by solid state 13C CPMAS NMR. These sponge materials can act as colorimetric sensors for the selective naked‐eye recognition of fluoride ions over chloride, phosphate and acetate ions at concentrations of up to 0.05 M in DMSO. Moreover, when the sponges were functionalized with perylene tetracarboxylic diimide, successful naked‐eye detection was achieved with only 0.02 % w/w of chromophore units per gram of material.

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