A new method for fluoride determination by using fluorophores and dyes anchored onto MCM-41Electronic supplementary information (ESI) available: IR spectra, SEM images, X-ray diffraction patterns and TG/TD analysis. See http://www.rsc.org/suppdata/cc/b1/b111128k/

Abstract

A new colourimetric and fluorimetric method for fluoride determination in aqueous samples based on the specific reaction between fluoride and silica has been developed and applied on real samples.There has been an increasing interest during the last years in the development and study of new anion chemosensors showing an optical output signal. In the presence of target anions these spectroscopic chemosensors respond by means of a variation on fluorescence (fluoroionophores) 1 or absorbance (chromoionophores), 2 the latter having the advantage of responding to the presence of the target anion with an easily observable colour change. Different approaches have been followed in the design of chemosensors most of them involving the coupling of anion binding sites with chromogenic or fluorogenic signalling subunits. However in most cases colour changes are only observed in non-aqueous solvents such as chloroform or acetonitrile and there are relatively few examples of chemosensors for anion sensing that work in aqueous solution. An alternative method for anion sensing to the anion recognition approach is the use of specific reactions produced by target anions adequately coupled to a signalling event. Following this approach we describe an optical method for fluoride determination in aqueous samples in which we take advantage of the specific attack of fluoride onto silica at acidic pH. The silica is used as support for covalent anchoring of chromophores or fluorophores, then, the presence of fluoride in solution implies the destruction of the silica support and the liberation of the organic molecule to the solution.Some colourimetric reactions for fluoride determination have been reported involving a recognition approach 3 or displacement reactions 4 , for instance the SPADNS colorimetric procedure in which fluoride is reacted with the Zr-SPADNS dye to form the colourless complex [ZrF 6 ] 22 . 5 The main inconvenience of these methods is the presence of important interferences as many anions compete with fluoride for the cation coordination and also many cations compete for the fluoride coordination.The success of our approach is based on the coupling between a silica matrix and a sensing molecule. The former has been chosen from the mesoporous solids of the MCM-41 6 family due to its optimal characteristics. Thus, its large surface area (ca. 1000 m 2 g 21 ) allows a high degree of functionalization which, in turn, will be translated to a higher response of the final solid. At the same time, the MCM-41 silica porous system may provide proper protection for the signalling molecules. In addition, the high degree of control previously reached in the synthesis of these materials 7 has allowed us to use an MCM-41 silica which presents a bimodal pore system. In this way, our inorganic matrix is structured as mesoporous nanoparticles joined together in microsized conglomerates giving rise to a second porous system which is in the range of large-meso to macro in size. This second porous system allows a better accessibility of t...

Highly sensitive and selective fluoride detection in water through fluorophore release from a metal-organic framework

“…The sensitivity is also comparable to other optical fluoride detection methods recently discussed in the literature, which can detect fluoride concentrations in the low ppm1112 or also in the ppb101314 range. However, these methods typically require a solvent mixture of water and an organic solvent such as chloroform10, methanol11, acetonitrile13 or DMSO1214, or need to be carried out in a buffer at acidic pH111213 or in the presence of other compounds such as CTAB12. Moreover, in some cases the use of organic solvent mixtures made it necessary to use tetraalkylammonium salts instead of the naturally more abundant and relevant sodium or potassium salts when studying the selectivity and sensitivity1014.…”

“…The anions that were examined for interference in this work were chosen to reflect species that are typically present in natural water and that are also usually investigated when assessing the performance of a fluoride detection method12131433. Our investigations show that there is virtually no interference by other halides (see Fig.…”

“…Several strategies have been reported on fluoride detection using recognition-transduction schemes based on the optical read-out of a luminescent or colorimetric signal. Examples include molecular recognition protocols using boron5691011 or stibonium12-based anion receptors, or mesoporous silica host materials13 that are equipped with a reporter molecule to monitor either luminescence quenching or turn-on processes, whereas recently a sensitive colorimetric fluoride sensor based on supramolecular interactions such as charge or electron transfer, respectively, was described14. However, most of the molecule-based analytical methods suffer from interference with other anions or are incompatible with aqueous media, which greatly limits the scope of their use1516.…”

A Colorimetric Chemosensor Based on a Nozoe Azulene That Detects Fluoride in Aqueous/Alcoholic Media

“…inducing cleavage of an Si-O or Si-C bond (For a review, see: Chen et al, 2019. See also: Yamaguchi et al, 2000Descalzo et al, 2002;Kim and Swager, 2003;Zhu et al, 2005;Bozdemir et al, 2010;Hu et al, 2010;Lu et al, 2011;Baker and Phillips, 2012;Li et al, 2014;Turan and Akkaya, 2014;Zou et al, 2014;Chavali et al, 2015;Mahapatra et al, 2015;Gabrielli and Mancin, 2016;Chansaenpak et al, 2018). Strategies for chemosensor design include coordination of fluoride to trivalent boron (For a review, see: Wade et al, 2010.…”

Azo Dyes Functionalized with Alkoxysilyl Ethers as Chemodosimeters for the Chromogenic Detection of the Fluoride Anion

“…A colorimetric sensory material for fluoride detection based on the use of a mesoporous silica solid, MCM-41, [ functionalized with different dyes has also been reported. [14] In this case, the addition of fluoride to the acidic acetonitrile/water 7:3 v/v mixtures of the functionalized solid resulted in the hydrolysis of the solid support, with a subsequent dye release and chromogenic response. Given our interest in the development of chromo-fluorogenic chemosensors for anions [15] and the very few examples described that are able to signal fluoride in the presence of water, we report herein the synthesis, characterization, and sensing studies of a new fluoride-selective chemodosimeter based on a pyridine azo dye functionalized with a tert-butyldimethylsilyl moiety which displays a selective chromogenic response to fluoride anion in acetonitrile/water mixtures.…”