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Cite this: Chem. Commun., 2014, 50, 15983 Received 27th August 2014, Accepted 4th November 2014 DOI: 10.1039/c4cc06756h
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An improved organic/inorganic solid receptor for colorimetric cyanide-chemosensing in water: towards new mechanism aspects, simplistic use and portability† Arash Mouradzadegun* and Fatemeh Abadast
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A heterogeneous reaction-based colorimetric chemodosimeter toward cyanide anions was designed based on a hybrid pyrylium/ alumina probe which possesses a great selectivity for CN over other common anions even at significantly higher concentrations. The synergistic incorporation of the inorganic matrix and the organic receptor can play a pivotal role in the convenient field-usable determination of this toxic anion in water. Moreover, the catalytic properties coming from the inorganic matrix can greatly amplify the sensor performance.
The development of chromogenic probes for the detection of anions that can be potentially harmful to the environment or human health is an area of emerging interest.1 Among the anions, the cyanide ion is well-known to be one of the most toxic materials to mammals, leading to vomiting, loss of consciousness and eventually death.2 Since cyanide does not easily decompose in the environment, even very small amounts of this toxic chemical generated by industrial plants or biological sources can contaminate drinking waters and become a serious threat to living creatures. In this regard, colorimetric sensors for cyanide have received a great deal of attention in recent years because the color change can easily be observed by the naked-eye, thus requiring less labor and no complicated spectroscopic equipment.3 However, most of them display sensing features only in organic solvents or mixtures of organic solvents and water which limit their application to the analysis of real samples. Furthermore, in most of the current methods the molecular receptors have to be in solution to perform sensing. This defect prevents quick and in situ sensing of cyanide for environmental testing inside or outside the laboratory. Despite these limitations, so far, only a few studies of heterogeneous anion sensors or probes Department of Chemistry, Faculty of Science, Shahid Chamran Uni., Ahvaz, Iran. E-mail:
[email protected]; Fax: +98(61)33337009; Tel: +98(61)33331042 † Electronic supplementary information (ESI) available: Full experimental details and characterization data IR, 1H, 13C NMR, HRMS, X-ray diffraction. CCDC 995679. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4cc06756h
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(e.g. films, membranes, or supported surfaces) for cyanide in water have been reported.4 Consequently, the development of easy and affordable protocols with potential use as solid systems or practical kits for naked-eye cyanide determination in water is especially appealing. Triaryl substituted pyrylium derivatives could be ideal candidates for the development of naked-eye sensory materials due to both their simple synthesis and the unique delocalization of the triarylpyrylium scaffold, which leads to the preparation of colored compounds. Unfortunately, the insolubility and poor response of these compounds in water limit their applications as chromogenic reagents. To overcome this problem, some examples have recently been reported relating to solid-supported triarylpyrylium receptors, in which the pyrylium derivatives are chemically grafted onto polymers,4c zeolite4d and silica.5 However, the preparation of such dye-sensitised solid-supports suffer from several drawbacks such as the need for specific functional groups on the receptor for binding to a solid-support, multiple synthetic steps, long reaction times, complicated and often laborious workup and purification steps, limited loading of the receptor and harsh reaction conditions. Nevertheless, as far as we know, there has been no report based on physical impregnation of these compounds into solid-supports. This type of incorporation of receptors, which are prepared by simple impregnation of the chemical receptors into the porous supports, remain of interest because they have advantages in terms of material costs, high loading of organic receptors and the wide variety of possible combinations of receptors and supports. Under this light, as a part of our ongoing interest in the development of practical and improved synthetic methodologies for the organic transformation of pyrylium and thiopyrylium salts,6 and also our recent interest in the investigation of the nucleophilic reaction of cyanide with triarylpyrylium derivatives,7 herein, we have attempted to design a new type of highly selective solid-state chemosensor based on the incorporation of a pyrylium derivative into neutral alumina. We demonstrate that this approach provides a convenient solid-state system with a unique interaction with cyanide.
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The design is based on the use of triaryl substituted pyrylium derivatives and on the well-known reactivity of the pyrylium cycle.8 We have recently reported that the triarylpyrylium derivatives 1 could be easily converted into corresponding cyanodienone derivatives 2 upon treatment with sodium cyanide in acetonitrile.7a The nucleophilic attack of the cyanide anion on the ortho position of the pyrylium cation is responsible for this conversion (Scheme S1, ESI†). Most of the triarylpyrylium derivatives are yellow as are also the cyanodienone analogues that cannot be used in a potential colorimetric reaction. On the basis of our experience in the design of new triarylpyrylium derivatives containing electrondonating and withdrawing groups in the para position of substituted phenyl rings, we became interested in the application of a deep red pyrylium derivative 1a bearing a methoxy group at the para position of the 2,6-aryl groups as a receptor that react with cyanide anions. The heteroaromatic core of this compound can be obtained readily via a one-pot cyclization reaction between two equivalents of p-methoxy acetophenone and one equivalent of benzaldehyde (Scheme S2, ESI†). The structure of receptor 1a was characterized using IR, 1 H NMR and 13C NMR analyses (see ESI†). The chromoreactand 1a is highly colored with an intense absorption centered at 469 nm which should most likely be ascribed to a charge transfer (CT) band due to the presence of the electron donor methoxy substituted phenyl ring and the electron acceptor pyrylium moiety. Addition of cyanide anions to an acetonitrile solution of 1a caused a dramatic change in color from deep red to light yellow after 25 minutes as shown in Fig. 1. The color change is clearly ascribed to the formation of the corresponding cyanodienone 2a, which is accompanied by decrease of the p-conjugation and intramolecular charge transfer (the hypsochromic shift). The structure of product 2a was characterized unambiguously using IR, 1H NMR, 13C NMR, HRMS and single crystal X-ray analyses (see ESI†). In the FT-IR spectra, there are new peaks at 2217 and 1638 cm 1, suggesting the presence of the cyanide and carbonyl groups in the product 2a, respectively. The mass spectra showed molecular ions and fragmentation patterns consistent with the proposed structure (see ESI†). To gain more realistic insight about the structure of the product 2a, a shiny yellow single crystal 2a with dimensions of 0.26 0.18 0.10 mm was chosen for the X-ray diffraction study. This compound was crystallized in an orthorhombic crystal system with the space group Pbca and Z = 8. The molecular
Fig. 1 Conversion of receptor 1a into the corresponding cyanodienone 2a in acetonitrile at room temperature.
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Fig. 2 (a) The single crystal X-ray structure of 2a and (b) the packing structure.
structure of 2a together with its packing structure is shown in Fig. 2 (more details in the ESI†). As shown in Fig. S1 in the ESI,† the color shift from deep red (lmax = 469 nm) to light yellow (lmax = 280 nm) in acetonitrile follows that this conversion can be a possible operating mechanism for the colorimetric detection of cyanide. Interestingly, in the emission spectra, a significant decrease of the fluorescence intensity at 554 nm was observed upon excitation at 490 nm (Fig. S2 in the ESI†). As stated earlier, the poor response of this receptor in water limits its application to the analysis of real samples. In this stage, we hypothesized that the combination of the chromoreactand 1a with a suitable solid-support could offer the possibility of the preparation of a new type of heterogeneous chemosensor material to detect the desired anion in aqueous solutions. In our previous study, we demonstrated that neutral alumina is a promising catalyst for the expeditious synthesis of cyanodienones from the corresponding pyrylium salts.7c With this background and according to the widespread applications of g-alumina as an inorganic solid-support due to its several attractive features namely, thermal and mechanical stability, inexpensiveness, large surface area and highly porous exteriors available to substrates,4b,9 it seemed interesting to us to examine its performance as a solid-support in this conversion. Therefore, in an attempt to enhance the response of 1a in terms of both sensitivity and stability in water, we designed a route to impregnate the chromogenic probe into neutral alumina. Neutral alumina chosen for this study was commercially available and was used as obtained.10 The initial investigations showed that if g-alumina were simply transferred into a dichloromethane solution of receptor 1a, after removing the solvent under reduced pressure, the dye would be completely impregnated into the alumina which would then adopt the characteristic deep red color of the dye itself without leaching of the impregnated dye into the environment (Fig. 3). In a typical assay, this heterogeneous system (1a/g-alumina) was soaked with various concentrations of cyanide ions in water. The sensing was successful and a light yellow colored solid was obtained (Fig. 3). This may be rationalized by considering the fact that 1a/g-alumina could provide a water-rich environment to the sensing motif, permitting the solvated cyanide ions to reach the chromoreactand 1a, giving rise to the sensing phenomenon, resulting in cyanide detection. It is noteworthy that the preparation of such hybrid organic/ inorganic receptor 1a/g-alumina in this simple way without
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Fig. 4 Photographs of TLC strips (a) only TLC strip, 1a/g-alumina in the presence of CN (mol L 1): (b) 0, (c) 1 10 4, (d) 6 10 4, (e) 3 10 3, (f) 6 10 3. Fig. 3 Naked-eye and solid-state detection of CN ions with 1a/g-alumina (a) only g-alumina (b) before and (c) after immersion into aqueous solution with CN without leaching of the impregnated dye into the environment.
covalent anchoring could be of interest in the search for convenient methodologies for cyanide recognition and sensing. However, a closer inspection of the system showed that in the presence of g-Al2O3, not only was the color variation performed successfully in pure water, but also the response time decreased to 4 minutes. This indicates that g-Al2O3 is of particular interest in this conversion because it can play the dual role of the solid-support as well as the promoter in the synthesis of cyanodienone from pyrylium salt. This result is in good agreement with our previous background.7c The response of this organic/inorganic solid receptor, visible to the naked-eye after 4 minutes from deep red to light yellow in pure water suggests that 1a/g-alumina is a promising optical sensor for the rapid screening of the toxic cyanide anion in the environment. As the pH value of a system is often considered to be a significant influencing factor on interactions, the effect of pH was investigated over a wide range of values. A pH of 9 was selected in order to overcome the competition of the OH anion in the ring-opening process, but at the same time remaining at a pH where the nucleophilic species CN can still occur. Lower pH values result in the formation of the protonated HCN derivatives. To establish the sensitivity of 1a/g-alumina to cyanide and the detection limit of the system, the absorbance of the 1a/g-alumina at different cyanide concentrations was recorded (Fig. S3 in the ESI†). From the plots of absorbance at 469 nm versus increasing quantities of cyanide added to aqueous solutions of the chromoreactand 1a/g-alumina, a detection limit of ca. 0.23 ppm of cyanide was determined which is slightly above the MCL11 (see ESI†). To assess the specificity of our chemodosimetric sensor toward cyanide, various anions were examined in parallel under the same conditions. As shown in Fig. S4 in the ESI,† the reaction of chromoreactand 1a/g-alumina with CN provided a strong absorbance change, whereas the presence of other anions such as NO3 , I , Br , NO2 , SO42 , Cl , SCN , HSO4 , S2O32 , S2O52 , H2PO4 , AcO , CO32 , F and S2 caused no obvious changes, even the higher concentrations, indicating that the reaction with cyanide is highly selective in water. Bearing in mind these favorable characteristics in water, to extend its performance to a portable chemosensor kit, we
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soaked TLC strips of neutral alumina (1 3 cm2) into the dichloromethane solution of receptor 1a for 12 h and then dried them in air (see the video in the ESI†). The dye-sensitised Al2O3 strips were immersed into the aqueous solutions of CN with different concentrations. The remarkable color change from deep red to light yellow was observed (Fig. 4). The development of such ‘‘dip-sticks’’ is extremely attractive as instant qualitative information without resorting to any additional equipment. Since recent observations have led to the realization that the conversion of pyrylium to cyanodienone occurs along with hypsochromic (blue) shift because of decrease of the p-conjugation and intramolecular charge transfer (ICT), we questioned the previous report that suggests a reversible colorimetric sensing of cyanide in water using a pyrylium derivative anchored into a suitable hydrophilic polymer.4c In the mentioned report the authors propose a large bathochromism in the absorbance (yellow to red) for the conversion. Furthermore, exposing the isolated cyanodienone to an acidic solution indicates that the process is irreversible and the back reaction cannot occur. It means that all efforts for transforming of cyanodienone to former pyrylium in acidic conditions are unsuccessful and only cis–trans isomerization happens. This is completely in accordance with that obtained by Balaban et al.12 Therefore our results reject the results of the previously published paper.4c However, based on our experiences in the chemistry of pyrylium and thiopyrylium salts, we surmise that at pH 11, the yellow to red color change in previously published paper can be ascribed to the formation of a red-colored pseudobase anion derivative 3, typical of the species generated in the presence of the OH nucleophile.13 Anion 3 was readily converted to the former pyrylium cation in the presence of acidic solutions (Scheme 1). In summary, we have combined the different research fields such as chromogenic sensing and solid-state chemistry in the quest for developing new chromogenic devices with enhanced sensing properties. The designed organic/inorganic solid receptor
Scheme 1
The conversion of pyrylium salts into the pseudobase anion 3.
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based on a triarylpyrylium derivative incorporated into g-alumina displayed a highly selective colorimetric chemodosimeter toward the cyanide anion. This is one of the very few colorimetric probes for cyanide detection in pure water. The simplicity and portability of the analysis coupled with the low cost of the starting materials suggests that this new method may find application in a variety of different environments where easy and rapid determination of the cyanide anion might be required. From an alternative point of view, the results suggest that physically incorporation of colored pyrylium derivatives into porous solids could be applied as a suitable general approach for the design and development of new chemosensors towards a broad range of target species.
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