Optical Chemosensors and Chemodosimeters for Anion Detection

Article Cite This: ACS Appl. Polym. Mater. 2019, 1, 1757−1768

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Optical Chemosensors and Chemodosimeters for Anion Detection Based on Merrifield Resin Functionalized with Brooker’s Merocyanine Derivatives Rafaela I. Stock,† Juliana P. Dreyer,† Gisele E. Nunes,‡ Ivan H. Bechtold,‡ and Vanderlei G. Machado*,† †

Departamento de Química, Universidade Federal de Santa Catarina, UFSC, CP 476, Florianópolis, SC 88040-900, Brazil Departamento de Física, Universidade Federal de Santa Catarina, UFSC, Florianópolis, SC 88040-900, Brazil

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‡

S Supporting Information *

ABSTRACT: Dyes supported on polymer materials have shown relevance in obtaining optical devices for anionic analyte detection. Thus, in this study, Merrifield resin was functionalized with protonated or silylated perichromic probes, derived from Brooker’s merocyanine to be used in chemosensor and chemodosimeter approaches for the detection of anionic species. The phenolic and silylated functionalized resins were characterized by infrared spectrophotometry, thermogravimetric analysis, scanning electron microscopy, and optical and confocal microscopy techniques. These materials were studied in trichloromethane as chromogenic and fluorogenic systems for H2PO4−, CN−, CH3COO−, and F− detection. Furthermore, the functionalized polymers were analyzed by laser scanning confocal microscopy and showed multifluorescence in all cases, due to the fact that the dye is distributed in different microenvironments in the Merrifield resin. These devices have the potential to be studied through multivariate image analysis to obtain faster and simpler results in the detection of anionic analytes. KEYWORDS: chemosensors, chemodosimeters, functionalized polymers, Merrifield resin, anion sensing, Brooker’s merocyanine, cyanide

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INTRODUCTION The detection of analytes in different situations with the use of chromogenic and fluorogenic molecular and supramolecular optical devices represents a subject of increasing interest in recent years due to the sensitivity, selectivity, and low cost of these strategies.1−7 Anionic species play very important roles in chemical and biological processes, which has led many research groups to focus on the design of optical devices for their detection.8−10 An important target anion in terms of detection is CN−.11,12 The lethality of this species in very low concentrations is due to the fact that it binds strongly to the active site of cytochrome oxidase, which is responsible for a decrease in the oxidative metabolism.11−17 This anion is also used in various industrial activities, such as in the preparation of polymers and in metallurgy and mining activities. In addition, CN− is delivered in the hydrolysis of cyanohydrins in certain seeds and roots, and also in the hydrolysis of some neurotoxic warfare agents.11−13,18 Some optical devices used in the detection of anionic species are chemosensors based on acid−base reactions.19,20 In this © 2019 American Chemical Society

strategy, the molecules, which are colorless in solution, strongly interact with the anionic analyte by proton transfer or more weakly by hydrogen bonding, causing changes in the color and/or fluorescence emission, which can be used to signal the presence of the analyte. Another interesting approach involves the use of chemodosimeters,10,19,21−25 where irreversible reactions are induced by, for instance, a highly nucleophilic anion, such as CN− or F−, at an electrophilic site of a system in order to yield species exhibiting different color or emission fluorescence in comparison with the reactant species. The use of polymeric materials with chromogenic and/or fluorogenic units anchored in their macromolecular structure, in order to achieve the selective detection of a particular analyte, is attracting interest and this type of system has been widely investigated.22,26−36 Received: April 3, 2019 Accepted: May 28, 2019 Published: May 28, 2019 1757

DOI: 10.1021/acsapm.9b00314 ACS Appl. Polym. Mater. 2019, 1, 1757−1768

Article

ACS Applied Polymer Materials

form (BMH) is colorless in solution but can be deprotonated in the presence of basic anionic species, such as CN− or F−, in organic solvents to form the deprotonated form (BM), which is colored in solution (Scheme 2).49,50 Another strategy involves the use of BM as an indicator in displacement assays for the detection of anionic species.47,48,53 A chemodosimeter approach can also be designed, in principle, using BM as a signaling unit, inspired by recent work reported in the literature, where the classical protection reaction was used for the silylation of phenols to yield colorless systems. In this approach, colored phenolate dyes can be generated by breaking the Si−O bond with the use of strongly nucleophilic species, such as F− or CN−.10,54−56 Recently, EHEC was functionalized with BM and the resulting polymer, in the form of a film, was used in an acid− base strategy for anion sensing.52,57 The modified polymer acts as a chromogenic/fluorogenic system for the highly selective detection of CN− in water.52,57 The polymer matrix acts as a support for anchoring the sensing units, resulting in a system with interesting features, such as the ability to detect CN− in a highly selective fashion and in concentrations below the minimum established by the World Health Organization (WHO). Herein, we describe the preparation and characterization of MR polymers decorated with covalently anchored merocyanine units (Scheme 3). The merocyanine systems are based on the BM molecular structure, differing in terms of the substitution, with different substituents in the ortho positions of the phenolic moiety. The systems were characterized and the photophysical properties were studied. The ability of the new systems to detect anionic species in solution by means of an acid−base strategy and chemodosimeter approach was evaluated by means of naked-eye detection and the fluorescence technique.

Merrifield resin (MR) is based on a copolymer of styrene and chloromethylstyrene, cross-linked with divinylbenzene (Scheme 1).37 The carbons attached to the chlorine atoms Scheme 1. Structure of Merrifield Resin (MR)

correspond to electrophilic centers, which, when attacked by nucleophilic species, lead to functionalized polymer systems. The versatility of the MR is associated with the fact that it is insoluble in water and all organic solvents. This allows reactions to be performed in the solid phase with the easy separation and purification of reagents, intermediates and reaction products. Although the resin is insoluble in organic solvents, it has a high swelling capacity and can increase in volume by up to five times in relation to its initial volume, leading to greater exposure of the chlorine atoms and allowing the desired reaction to occur.38 The resin has particularly good swelling properties in the solvents CH2Cl2, CHCl3, DMF, THF, and dioxane.39 MR has also been used with covalently anchored optical devices in the development of methods for the detection of metals, for instance, K+, Cu2+ and Pb2+,40,41 but no studies related to its application to anion sensing could be found in the literature. Brooker’s merocyanine (BM) is a classical merocyanine which has been extensively used as a perichromic dye.42−46 This dye has been employed in many strategies for the development of optical devices for the detection of anionic species.47−53 The use of this system in an acid−base approach has been demonstrated, since the compound in its protonated

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EXPERIMENTAL SECTION

Materials. All chemicals employed in this study were high-purity commercial reagents. The Merrifield resin (Sigma-Aldrich) used has a size of 50−100 mesh and 2.5−4.0 mmol of chlorine per gram of resin. Trichloromethane (Vetec) was stored on molecular sieves (4 Å;

Scheme 2. Two Possible Strategies, Based on an Acid−base Reaction and with a Chemodosimeter Approach, Using BM as the Signaling Unit

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(E)-2,6-Dimethyl-4-(2-(pyridin-4-yl)vinyl)phenol (2). 4-Methylpyridine (0.82 g, 8.75 mmol), 3,5-dimethyl-4-hydroxybenzaldehyde (0.88 g, 5.83 mmol), and acetic anhydride (1.19 g, 11.7 mmol) were refluxed (120−127 °C) for 44 h. The mixture was poured into 30 mL of ice water and stirred for 30 min. The aqueous phase was separated from the oil formed, and the oil was then refluxed (85−95 °C) with 14 mL of KOH ethanolic solution (c = 0.75 mol L−1) for 4.5 h. The mixture was cooled at room temperature, transferred to a beaker and acetic acid was added until pH = 6.0. The compound was purified by column chromatography using n-hexane/ethyl acetate (2:3 vol/vol) as the eluent. The product is a yellow solid (0.65 g, 49.5% yield) with a melting point of 208−209 °C. IR (KBr, ν̅max/ cm−1): 3026 (C−H), 2916 (−C−H), 1627, 1586, and 1548 (CC). 1H NMR (200 MHz, DMSO-d6) δ/ppm: 8.57 (1H, s), 8.48 (2H, d, J = 5.1 Hz), 7.47 (2H, d, J = 5.1 Hz), 7.36 (1H, d, J = 16.5 Hz), 7.23 (2H, s), 6.98 (1H, d, J = 16.5 Hz), 2.18 (6H, s). (E)-2,6-Dimethoxy-4-(2-(pyridin-4-yl)vinyl)phenol (3). 4-Methylpyridine (0.61 g, 6.58 mmol), 3,5-dimethoxy-4-hydroxybenzaldehyde (0.80 g, 4.39 mmol), and acetic anhydride (0.90 g, 8.79 mmol) were refluxed (115−120 °C) for 50 h. The mixture was poured into 25 mL of ice water and stirred for 30 min. The aqueous phase was separated from the oil formed, and the oil was then refluxed (85−95 °C) with 25 mL of KOH ethanolic solution (c = 0.75 mol L−1) for 19 h. The mixture was cooled at room temperature and transferred to a beaker, and acetic acid was added until pH = 6.0. The compound was purified through column chromatography using n-hexane/ethyl acetate (1:1; vol/vol) as the eluent. The product is a yellow solid (0.55 g, 48.7% yield) with a melting point of 150−152 °C. IR (KBr, ν̅max/ cm−1): 3353 (O−H), 3028 (C−H), 2998 (−C−H), 1633, 1584, and 1513 (CC). 1H NMR (200 MHz, acetone-d6) δ/ppm: 8.50 (2H, dd, J = 1.6 Hz and J = 4.7 Hz), 7.46−7.41 (3H, m), 7.11 (1H, d, J = 16.4 Hz), 6.99 (2H, s), 3.88 (6H, s). (E)-2,6-Diphenyl-4-(2-(pyridin-4-yl)vinyl)phenol (4). 4-Methylpyridine (0.27 g, 2.90 mmol), 3,5-diphenyl-4-hydroxybenzaldehyde (0.40 g, 1.45 mmol), and acetic anhydride (0.29 g, 2.82 mmol) were refluxed (115−120 °C) for 52 h. The mixture was poured into 15 mL of ice water and stirred for 30 min. The solid was filtered and refluxed (85−95 °C) with 8 mL of KOH ethanolic solution (c = 0.75 mol L−1) for 19 h. The mixture was cooled at room temperature, transferred to a beaker and acetic acid was added until pH = 6.0. The product was purified by column chromatography using n-hexane/ethyl acetate (1:4; vol/vol) as the eluent, yielding a yellow solid (0.20 g, 36% yield) with a melting point of 231−233 °C. IR (KBr, νm̅ ax/ cm−1): 3440 (O−H), 3026 (C−H), 2922 (−C−H), 1633, 1590, and 1495 (C C); 1H NMR (400 MHz, DMSO-d6) δ/ppm: 8.64 (1H, s), 8.50 (2H, dd, J = 1.6 Hz and J = 4.7 Hz), 7.59−7.44 (13H, m), 7.38−7.35 (2H, m), 7.20 (1H, d, J = 16.4 Hz). 13C NMR (100 MHz, DMSO-d6) δ/ ppm: 151.4, 150.0, 144.9, 138.6, 132.8, 131.4, 129.6, 128.9, 128.8, 128.4, 127.2, 124.0, and 120.7. ESI-MS: m/z: 348.1383 (calcd), 348.1381 (exptl). (E)-2,6-Dibromo-4-(2-(pyridin-4-yl)vinyl)phenol (5). 4-Methylpyridine (0.53 g, 5.72 mmol), 3,5-dibromo-4-hydroxybenzaldehyde (2.0 g, 7.14 mmol), and acetic anhydride (1.17 g, 11.43 mmol) were refluxed (115−120 °C) for 24 h. The mixture was poured into 130 mL of ice water and stirred for 60 min. The precipitate was washed in cold water, recrystallized in ethanol/acetone, and then refluxed (85− 95 °C) with 25 mL of KOH ethanolic solution (c = 0.75 mol L−1) for 90 min. The mixture was cooled at room temperature and transferred to a beaker, and acetic acid was added until pH = 6.0. The product is an orange solid (1.01 g, 49.9% yield) with a melting point of 252−254 °C. IR (KBr, ν̅max/ cm−1): 3432 (O−H), 3024 (C−H), 1633, 1605, and 1596 (CC); 1H NMR (400 MHz, DMSO-d6) δ/ppm: 8.52 (2H, d, J = 5.7 Hz), 7.85 (2H, s), 7.48 (2H, d, J = 5.7 Hz), 7.40 (1H, d, J = 16.4 Hz), 7.19 (1H, d, J = 16.4 Hz). Preparation of Functionalized Merrifield Resins. Functionalized resins MR1OH−MR5OH were prepared according with the following general procedure: 1.2 g of MR was swollen in 12 mL of DMF for 3 h. After this, a certain amount of compounds 1−5 was added so that its concentration in the reaction was 4.0 × 10−2 mol L−1 (1−3 and 5) or 1.25 × 10−2 mol L−1 (4). The mixture was stirred for

Scheme 3. Structures of the Functionalized Merrifield Resins (MR) Studied Herein

Sigma-Aldrich). All anions (HSO 4 − , H 2 PO 4 − , NO 3 − , CN − , CH3COO−, F−, Cl−, Br−, I− and BF4−) were used as tetra-nbutylammonium salts with purity greater than 95−99%. The anions were purchased from Fluka (Cl−, >98%; NO3−, > 97%), Vetec (Br−, >99%; I−, >99%; HSO4−, >99%) and Sigma−Aldrich (H2PO4−, >99%; CN−, >95%; CH3COO−, >97%; F−, 98%) and were dried over P4O10 under vacuum before use. Methods. The melting points were uncorrected. Infrared (IR) spectra were obtained with KBr pellets. The NMR spectra were recorded using DMSO-d6 or acetone-d6 as the solvent, with 200 and 400 MHz spectrometers. Chemical shifts were recorded in ppm with the solvent resonance as the internal standard and data are reported as follows: chemical shift, multiplicity (s = singlet, d = doublet, t = triplet, dd = double doublet, m = multiplet), coupling constants (Hz), and integration. Thermogravimetric analysis (Shimadzu, TGA-50) was carried out in a platinum cell, heated from 20 to 600 °C at a rate of 10 °C min−1 and under nitrogen atmosphere (50 mL min−1). The morphology of the spheres was studied using a scanning electron microscope (HITACHI TM3030) with acceleration voltage of 5 kV and 15 kV. Diameters were measured using optical images (Motic, SMZ-168 with a BestScope tablet, BLC-250) and ImageJ software (public domain). The spheres were observed with a laser scanner confocal microscope (LSCM) using a DMI6000 B (Leica Microsystems) apparatus with a 10 objective lens and numerical aperture of 0.40, at room temperature (23 °C). The LSCM was used to collect fluorescence confocal images and scanning spectra. To obtain the images, a diode laser with a wavelength (λ) of 405 nm and an Ar laser with λ values of 488 and 514 nm were used to excite the spheres. The scanning spectra were collected employing a diode laser with a λ of 405 nm (with power rating (PR) of 20−60%), an Ar laser with λ values of 458 nm (PR 15−60%), 476 nm (PR 15−80%), 488 nm (PR 8−80%), 496 nm (PR 15−80%), and 514 nm (PR 15−80%), and a He−Ne laser with λ values of 543 nm (PR 20−80%), 594 nm (PR 20−80%), and 633 nm (PR 20−80%). Surface area analysis was carried out using an Autosorb-1 analyzer (Quantachrome Instruments) with nitrogen as the adsorbate. Fluorescence emission spectra were obtained using an Ocean Optics USB4000 spectrophotometer and a UV lamp as the excitation source (λexc = 365 nm). A Nikon Coolpix p510 camera was used to capture the images. Synthesis of Compounds 1−5. Compound 1, (E)-4-(2(pyridin-4-yl)vinyl)phenol, was synthesized according to a method described in the literature,58 with a melting point of 259−260 °C (literature:52 258−260 °C). IR (KBr, νm̅ ax/ cm−1): 3440 (O−H), 3069 (C−H), 1633, 1584, and 1513 (CC); 1H NMR (400 MHz, DMSO-d6) δ/ppm: 9.78 (1H, s), 8.49 (2H, d, J = 5.9 Hz), 7.48 (4H, d, J = 7.5 Hz), 7.45 (1H, d, J = 16.4 Hz), 7.01 (1H, d, J = 16.4 Hz), 6.80 (2H, d, J = 8.6 Hz). Compounds 2−5 were prepared based on the synthesis of compound 1, with some modifications. 1759


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ACS Applied Polymer Materials Scheme 4. Synthesis of Compounds 1−5 and Functionalized Merrifield Resins

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50 min at 50−53 °C. The functionalized resins were filtered and washed with methanol, ethanol, ethyl acetate, acetone, and trichloromethane. The resins were then placed in a beaker with trichloromethane, and five drops of acetic acid were added followed by stirring for 5 min. Lastly, the resins were filtered, washed with ethanol, and stored in a stove (50 °C) for 3 days. Silylation of Functionalized Merrifield Resins. In this procedure, 0.6 g of functionalized resins MR1OH−MR5OH was swollen in 6 mL of DMF for 3 h. In the next step, 50 μL of tetra-n-butylammonium hydroxide were added and the mixture was stirred for 30 min, followed by filtration. The resins were placed in a round-bottomed flask with 6 mL of DMF and chlorotriisopropylsilane (TIPSCl, 0.058 g, 3.04 × 10−4 mol), and the mixture was stirred for 150 min at room temperature. Lastly, the resins were filtered, washed with ethanol, and stored in a stove (50 °C) for 3 days. Determination of the Concentration of Compounds 1−5 Anchored in Merrifield Resin. The concentration of compounds 1−5 anchored in MR was determined from calibration curves, obtained from plots of absorbance at λmax as a function of the concentration of each compound (1−5) in DMF (Figures S33−S37). A linear fit for each plot provided an equation for each calibration curve. At the end of each functionalization reaction, an aliquot was withdrawn and diluted, and its absorbance at the corresponding λmax was measured. From this experimental data and the corresponding equation from the calibration curve, the final concentration of each compound in the reaction mixture was calculated. The difference between the initial and final mass of the compound provided the amount of 1-5 anchored. Functionalized Resins in the Assays with the Anions. First, 10 mg of functionalized resin was placed in a 2 mL vial, 1 mL of trichloromethane was added, and the resin was left to swell for 75 min. Subsequently, the studied anion was added and after 5 min an image was recorded. This procedure was performed for each anion studied. In order to obtain the fluorescence images the vials were placed on a UV lamp (λexc = 365 nm). Study on Fluorescence of Resins Functionalized with the Anions. In this procedure, 10 mg of functionalized resin was placed in a 2 mL vial, 1 mL of trichloromethane was added, and the resin was left to swell for 75 min. The studied anion was then added and after 5 min the resin was placed on a glass slide and the emission spectrum was obtained. Titration Experiments. The titrations were carried out as previously described; however, for each titration, several flasks were prepared, with each containing different and increasing concentrations of the anion to be analyzed.

RESULTS AND DISCUSSION Synthesis. The functionalized Merrifield resins were obtained as shown in Scheme 4. First, compounds 1−5 were Table 1. Degradation Temperatures, Obtained from DTG Analysis, of Compounds 1−5, MR, and the Modified Resins 1 2 3 4 5 MR MR1OH MR2OH MR3OH MR4OH MR5OH MR1OTIPS MR2OTIPS MR3OTIPS MR4OTIPS MR5OTIPS

TD1 (°C)

TD2 (°C)

TD3 (°C)

310.9 291.0 174.8 202.4 253.4 331.6 151.7 168.7 90.3 172.2 149.0 124.3 160.7 136.0 71.8 146.9

295.7 373.1 362.2 451.8 397.6 390.4 144.0 396.7 401.7 229.0 234.8 235.9 235.8 405.3

414.7 426.9 528.3 439.3 441.8 402.3 444.4 437.2 401.0 397.0 401.9 385.6 439.2

TD4 (°C)

551.2

441.6

442.2 442.6 437.6 442.2

prepared according to the methodology described by Koopmans and Ritter58 by means of the condensation of 4methylpyridine with the corresponding substituted hydroxybenzaldehyde. The chlorine atoms bonded to the aliphatic carbons present in MR were then substituted with the pyridyl nucleophilic groups present in compounds 1−5 to yield the functionalized resins MR1OH−MR5OH. MR beads are colorless, but after the functionalization reactions the color had changed to yellow in all cases. Each functionalized resin was washed several times with solvent to remove the unreacted organic compound. This procedure revealed that the beads changed color, indicating the deprotonation of the dye. Therefore, the resulting MRs were protonated with acetic acid, followed by filtration, washing and drying. The phenol groups in the functionalized resins could be easily silylated. In the first reaction step, the resin was swelled in DMF and deprotonated with a small amount of tetra-n-butylammonium 1760


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Characterization of Phenolic and Silylated Functionalized Resins. Several characterization techniques were used to verify the functionalization of MR. First, the amounts of 1− 5 anchored in MR were calculated using the calibration curve for each compound in DMF (Figures S33−S37). A linear fit of λmax (334 nm) as a function of c(2), for example, provided the equation: absorbance = 23140.1 c(2) + 0.089 (R2 = 0.99975). Thus, the mass of 2 remaining in the reaction medium could be calculated at the end of its reaction with MR. Since the initial mass of 2 added to the reaction medium was known, the amount of compound 2 anchored in MR could be calculated (0.022 g of 2 per gram of MR). The amounts (g) of each anchored compound per gram of MR are shown in Table S1. The MR functionalization was also confirmed by the IR results (Figure S12−S16). The starting resin (spectrum A in Figure S15) exhibits stretching bands centered at 3080 and 3018 cm−1 (H−C); 2937 and 2846 cm−1 (H−C−); 1607, 1408, and 1421 cm−1 (CC), results which are similar to data reported in the literature.41,59 When the resin was functionalized with compound 4 (spectrum B in Figure S15) a band centered at 1712 cm−1 appeared, associated with CC stretching of the anchored compound, which is also consistent with data reported in literature.52,60 With the silylation of MR4OH, MR4OTIPS is formed (Figure S15, spectrum C), followed by the disappearance of the band at 1712 cm−1 and

Figure 1. DTG curves for MR (black), MR1OH (blue), and MR1OTIPS (red) up to 600 °C in nitrogen atmosphere.

hydroxide (TBAOH), which resulted in a change in the color of the beads from yellow to blue, green or violet, depending on the functionalized material used. After the filtration, the deprotonated resin was reacted with TIPSCl in DMF, followed by filtration, washing, and drying. The beads had then become yellow again, but with a brighter aspect.

Figure 2. SEM micrographs of MR (A, B), MR4OH (C, D), and MR4OTIPS (E, F). 1761


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Figure 3. Dispersion of bead sizes for MR (A), MR4OH (B), and MR4OTIPS (C) obtained from optical microscopy images.

Figure 4. Confocal microscopy images of MR (A−C), MR2OH (D−F), and MR2OTIPS (G−I) upon laser excitation at 405 nm (A, D, and G), 488 nm (B, E, and H), and 514 nm (C, F, and I). Bar shows scale (100 μm).

the simultaneous appearance of a band centered at 1689 cm−1. A band at 890 cm−1 is also verified after the silylation reaction, corresponding to Si−O stretching. The thermal stability of the materials was evaluated based on the thermogravimetric analysis results. The maximum degradation temperatures obtained for compounds 1−5 and the modified resins are reported in Table 1. Figure 1 provides the derivative thermogravimetry (DTG) curves for MR, MR1OH, and MR1OTIPS. MR shows mass losses at 331.6, 451.8, 528.3, and 551.2 °C, with the greatest loss being at 451.8 °C. MR1OH shows solvent loss at 151.7 °C, related to the DMF

used in the reaction,61,62 and two mass losses at 397.6 and 439.3 °C, which decrease the thermal stability of the material.63 RM1OTIPS shows mass losses at 124.3 and 229.0 °C, corresponding, respectively, to losses of the solvent and TIPS-Cl used in the reaction, and two mass losses at 401.0 and 442.2 °C. This differs from the RM1OH degradation behavior, indicating that silylation occurred. Thus, the data show that the thermal stability decreased for all modified resins and the silylated resins exhibited mass losses that were distinct from those of their phenolic analogues. 1762


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MR5OTIPS revealed no morphological changes, having a homogeneous surface (Figure S41), as in the case of the other silylated resins. The bead size distributions could also be determined from the optical microscopy images. Figure 3A shows a graph of the frequency vs diameter of MR, and diameters of 180−250 μm were observed. The functionalized resins MR4OH (Figure 3B) and MR4OTIPS (Figure 3C) had diameters of 170−240 and 170−230 μm, respectively. Thus, the functionalization did not alter the size of the beads. The other materials prepared had similar bead diameters (Figures S42−S45). Fluorescence Measurements. Based on the fluorescence of the BM system,64 the emission of the resins was investigated using the laser scanner confocal microscopy (LSCM) technique. Figure 4 shows fluorescence images for MR, MR2OH, and MR2OTIPS, which were excited at 405 nm (A, D, and G), 488 nm (B, E, and H), and 514 nm (C, F, and I), respectively. MR showed very low intensity emission, with a blue color, when excited at 405 nm, and no emission was verified with excitation of the sample at 488 and 514 nm. On the other hand, MR2OH and MR2OTIPS emitted blue, green, and red fluorescence when excited at 405, 488, and 514 nm, respectively. All other modified resins showed diversified fluorescence (Figures S52−S55). The data suggest that this multifluorescent emission, verified using different excitation values, may be related to the fact that the signaling units are anchored in internal domains in the resin, which are not homogeneous, having distinct micropolarity. Emission spectra were also obtained for the resins using the confocal microscopy technique. The samples were excited at nine wavelengths, as shown in Figure 5. Data for the maximum emission wavelengths corresponding to each excitation wavelength used are given in Table S2. MR (Figure 5A) only exhibited emission at λexc = 458 nm, with low intensity. On the other hand, MR2OH showed fluorescence emission when excited at six wavelengths out of the nine studied (Figure 5B). For MR2OTIPS (Figure 5C), emission was also verified at the λexc values cited above, but at different maximum wavelengths compared with MR2OH. The other resins exhibited similar behavior (Figures S56−S59). Study on Fluorescence of Resins Functionalized with Anions. Figure 6 shows the behavior of the functionalized resins, swollen in trichloromethane, in the absence and presence of several anions, as tetra-n-butylammonium salts. The anion assays were performed with concentrations in the range of 4.6 × 10−5−7.3 × 10−4 mol L−1 (Figures S60−S64). The beads are yellow and a variety of colors was verified after the addition of H2PO4−, CN−, CH3COO−, and F−. For instance, MR2OH is dark green in the presence of H2PO4− while MR4OH and MR5OTIPS are cyan and purple, respectively. The ability of the functionalized MRs to detect the anionic species is related to the capacity of the analyte to act as a base, abstracting the proton of the phenolic sensing units (Scheme 5) and generating the respective colored phenolate species. In the case of the silyl-protected chemodosimeters, the nucleophilic species act by cleaving the silylether bond, also generating the respective colored phenolates. The anion assays could not be performed with MR5OH, since it does not remain protonated in the solvent. The phenolic functionalized resins were placed in the presence of hydroxide in order to use the resulting color related to the phenolate species to compare with the respective chemosensors and chemodosimeters. However, the phenolic units

Figure 5. Fluorescence emission spectra for MR (A), MR2OH (B), and MR12OTIPS (C).

In order to evaluate the morphology of the materials, scanning electron microscopy (SEM) analysis was carried out. Figure 2 shows the SEM micrographs for MR, MR4OH, and MR4OTIPS. The unmodified resin beads have a smooth surface, but in the case of MR4OH the bead surface was wrinkled. The data suggest that this change in the morphology occurs due to the interaction between the chlorine atoms of the resin and the hydrogens of the anchored hydroxyl phenolic groups. When the dye undergoes the silylation reaction, forming MR4OTIPS, the surface becomes smooth again, since the bulky triisopropylsilyl groups become attached to the dye, causing the expansion of the bead surface. This behavior was also verified for MR1OH, MR2OH, and MR3OH and for their respective silylated resins (Figures S38−S40). However, MR5OH, with bromine groups in its structure, did not present a wrinkled surface (Figure S41). In this case, the data suggest that bromine is a bulky atom, which is responsible for repulsion from the chlorine atoms present in the resin, hindering the interaction between the hydrogens of the phenolic hydroxyls and the remaining chlorine groups. 1763


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Figure 6. Images of MR1OH−MR4OH and MR1OTIPS−MR5OTIPS in the presence of 5.5 × 10−4 mol L−1 of several anions in trichloromethane.

S65) was analyzed in the presence of anions. Using the UV lamp as the excitation source (λexc = 365 nm), suppression of the emission intensity occurs in the presence of increasing concentrations of H2PO4−, CN−, CH3COO−, and F−. In the case of MR3OH, suppression of the emission intensity occurs in the presence of lower anion concentrations compared with MR3OTIPS, which may be due to the greater facility of the anion in abstracting a proton than causing silyl-ether bond cleavage. MR3OH has an emission maximum at 534 nm and when placed in the presence of H2PO4−, CN−, CH3COO−, and F− suppression of the emission intensity is verified (Figure 8A). It was also observed that CH3COO− and F− shift the maximum emission intensity to 521 nm and for CN− the maximum emission occurs at 504 nm. MR3OTIPS, with a fluorescence emission maximum at 534 nm, shows a similar behavior with the addition of H2PO4−, CN−, CH3COO−, and F−, but in this case for H2PO4− and CN− the maximum emissions are shifted to 537 and 514 nm, respectively (Figure 8B). This could be because each anion has a distinct size and geometry and interacts differently with the sensing units, since the latter are located in distinct regions in the resin. MR3OH and MR3OTIPS Titrations with CN− and F−. Figure 9 shows the emission spectra for MR3OH in the presence of increasing concentrations of CN− (A) and F− (B). For these two species, an increase in c (anion) caused suppression of the emission intensity for the band at λmax = 534 nm followed by the appearance of other emission bands with lower intensity, that is, λmax = 504 nm in the presence of CN− and 521 nm for F−. The titration curves obtained in the form of emission intensities at 534 nm as a function of c (anion) are shown in Figure 9 for CN− (C) and F− (D). In the titration

Scheme 5. Deprotonation of MR1OH−MR5OH and Reaction of MR1OTIPS−MR5OTIPS with an Anion To Generate MR1O−−MR5O−

were not fully deprotonated because the TBAOH used has 30 coordinating water molecules, preventing HO− from deprotonating the sensing units located deep within the resin, since the MRs are hydrophobic materials. By combining the test results reported above, for cases that showed chemosensor/chemodosimeter and analyte interactions, the anion being analyzed can be determined, since each result has a different color. It is possible to determine CH3COO−, for example, because the combination of the results for this anion differs from the other responses. Since the functionalized resins were fluorescent, the fluorescence of MR3OH (Figure 7) and MR3OTIPS (Figure 1764


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Figure 7. Fluorescence images of functionalized resin MR3OH excited at 365 nm in the absence (A) of anions and in the presence of increasing concentrations of anions in trichloromethane: 4.6 × 10−5 (B), 9.1 × 10−5 (C), 1.8 × 10−4 (D), 3.6 × 10−4 (E), 5.5 × 10−4 (F), and 7.3 × 10−4 (G) mol L−1.

Figure 8. Emission spectra for MR3OH (A) and MR3OTIPS (B) in the absence and in the presence of different anions, as tetra-n-butylammonium salts (λexc = 365 nm) in trichloromethane (canion = 7.4 × 10−4 mol L−1).

with CN−, the emission intensity decreased with the addition of the anion and became constant above 4.5 × 10−4 mol L−1, whereas for F− the emission intensity became constant above c(F−) = 6.0 × 10−5 mol L−1. The limits of detection (LOD) and quantification (LOQ) were estimated from the linear region of each titration curve, with LOD = 6.48 × 10−5 mol L−1 and LOQ = 2.16 × 10−4 mol L−1 for CN− and LOD = 9.19 × 10−5 mol L−1 and LOQ = 3.06 × 10−4 mol L−1 for F−. Figure S66 shows the influence of the addition of increasing amounts of CN− (A) and F− (B) on the emission spectrum for MR3OTIPS. At low anion concentrations (5.2 × 10−5−9.1 × 10−5 mol L−1), the emission intensity values increase slightly followed by a decrease. Figure S66 shows the corresponding calibration curves constructed from the emission intensity at 532 nm as a function of c (anion) for CN− (C) and F− (D). In

the titration with CN−, the emission intensity becomes constant after 6.0 × 10−4 mol L−1, whereas in the titration with F− the emission intensity continues to decrease until the maximum concentration used (9.3 × 10−4 mol L−1). LOD and LOQ values could not be calculated because of the lack of linearity verified at the beginning of the titrations. Since the sensing units are located in a hydrophobic microenvironment and the solvent used is of a nonpolar nature, the data suggest that the anion interacts electrostatically with the positive charge of the pyridinium moiety of the sensing units before the nucleophilic attack on the silyl center.

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CONCLUSIONS MR was functionalized with BM and its derivatives and then silylated, with all modifications being carried out through 1765


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Figure 9. Influence of the addition of increasing amounts of CN− (A) and F− (B) on the fluorescence emission spectra for MR3OH. Curve showing the variation in the fluorescence at 534 nm with the addition of increasing amounts of CN− (C) and F− (D) in trichloromethane (λexc= 365 nm).

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nucleophilic substitution reactions, with simple and practical procedures. The phenolic and silylated functionalized resins were characterized by IR spectroscopy, TGA, SEM, and optical and confocal microscopy techniques. The confocal microscopy technique showed that the functionalized resins present multifluorescence and this phenomenon can be explained by the fact that the dye is distributed in different microenvironments in the functionalized material. The phenolic and silylated functionalized resins were exposed to several concentrations of the anions studied. The results showed a change in the fluorescence and color in the presence of H2PO4−, CH3COO−, F−, and CN−, indicating phenol deprotonation in the case of chemosensors and cleavage of the silyl-ether bond for the chemodosimeters, exhibiting a combination of specific results in each case. Thus, it would be of interest to evaluate these optical devices applying chemometric techniques, such as multivariate image analysis, in order to obtain faster and simpler results using a single photograph. However, the optical devices reported herein exhibit limitations regarding their use for analytical purposes and the design of other systems based on less hydrophobic functionalized resins is required to allow the application of these devices in aqueous systems. In addition, MR combined with BM derivatives can also be used in the construction of optical devices for the detection of cationic species or in the development of supramolecular lightconverting devices.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsapm.9b00314. 1 H NMR, 13C NMR, and IR spectra; TGA and DrTGA curves; UV−vis results; SEM micrographs; optical microscopy images; confocal microscopy images; emission spectra; and images of the functionalized resins in the presence of anions (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax +55 48 3721 6852. Tel. +55 48 3721 4542. ORCID

Ivan H. Bechtold: 0000-0001-6393-7245 Vanderlei G. Machado: 0000-0002-6995-6591 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for the financial support of the Brazilian governmental agencies Conselho Nacional de ́ Desenvolvimento Cientifico e Tecnológico (CNPq), Fundaçaõ de Amparo à Pesquisa e Inovaçaõ do Estado de Santa Catarina (FAPESC), and Coordenaçaõ de Aperfeiçoamento de Pessoal ́ Superior (CAPES − Finance Code 001), as well as de Nivel the Laboratório Central de Biologia Molecular (CEBIME/ 1766


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Optical Chemosensors and Chemodosimeters for Anion Detection

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