Water-Soluble Polymeric Probes for the Selective Sensing of Mercury

Article pubs.acs.org/Macromolecules

Water-Soluble Polymeric Probes for the Selective Sensing of Mercury Ion: pH-Driven Controllable Detection Sensitivity and Time A. Balamurugan and Hyung-il Lee* Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea S Supporting Information *

ABSTRACT: Polymeric probes with dithioacetal units in the side chains were designed and synthesized for the selective and efficient colorimetric sensing of Hg2+ ions in aqueous solutions. These polymeric sensors were prepared by a reaction between aldehyde groups of the side chain in the polymer and ethanethiol or 3-mercaptopropionic acid using BF3 as a Lewis acid. In aqueous solution, they exhibited a 30−40 nm red-shift in their absorption maxima upon the addition of Hg2+ ions, accompanied by a change in the color of the solution, from pale yellow to dark red. These results clearly demonstrated that the sensitive signaling behaviors originated from the Hg2+-promoted deprotection reaction of dithioacetal groups to form aldehyde functionalities. The sensors have excellent selectivity toward Hg2+ ions over other alkali and transition metal ions. The detection time for Hg2+ ion was finely tuned by a change in the pH of the solution. In particular, it took less than 1 min to complete Hg2+ ion detection at low pH. Given with fast and pH-tunable Hg2+ ion detection abilities, these polymeric probes are expected to offer unique potential platforms for integrating stimuli-responsive water-soluble polymers with tunable sensing behaviors.

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INTRODUCTION In recent years, there has been growing interest in the development of selective chemosensors for the sensing and recognition of toxic heavy-metal ions because of their impact on health and the environment.1−4 Among them, mercury is one of the most toxic and dangerous elements and it can cause many severe diseases.5−9 Both elemental and ionic forms of mercury can be converted into dangerous methylmercury species (CH3HgX) by certain anaerobic bacteria, which can then be readily bioaccumulated through the food chain.10−14 Moreover, even ppm levels of mercury accumulation in the body can cause various diseases and affect the nervous system, causing symptoms including prenatal brain damage, cognitive and motion disorders, vision and hearing loss, and Minamata disease.15,16 Thus, there is a need for new molecular probes for the fast, efficient, and selective sensing of mercury. To date, several molecular design strategies have been exploited to develop chemosensors and chemodesimeters for mercury ion detection.17−24 Among the many molecular probes, specific chemical reaction-based systems have attracted attention because of their ion-specific selectivity and high sensitivity.25,26 Indeed, many molecular probes with high Hg2+ © XXXX American Chemical Society

ion selectivity have been designed on the basis of desulfurization reactions or Hg2+-promoted deprotection reactions of dithioacetal groups.27−39 Commonly, these probes have been found to suffer from limited solubility in water and the sensing studies needed to be conducted in pure organic solvents or mixture of organic solvents/water.1,18,25 This can seriously affect their real practical use in environmental and biological conditions. To address this, water-soluble synthetic polymers or polymeric assemblies to which appropriate receptor moieties are covalently attached have been developed recently.40 Stimuli-responsive polymers respond to small changes driven by external triggers such as temperature, pH, and light in the environment, resulting in dramatic property variations.41−47 Although they have been developed for uses in various fields, sensing and detection systems based on stimuli-responsive polymers have been relatively less exploited. Compared with other conventional sensing systems, based on water-soluble Received: November 20, 2014 Revised: January 13, 2015

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Macromolecules Scheme 1. Synthesis of Water-Soluble Polymeric Probes for the Sensing of Mercury Ions

dichloromethane (25 mL), and the organic layer was concentrated and precipitated in diethyl ether. The polymer obtained was further purified by a reprecipitation process. 1H NMR (CDCl3, 300 MHz, δ in ppm): 7.79−7.86 (4H, m, Ar−H), 7.59−7.57 (2H, m, Ar−H), 7.68− 7.62 (2H, m, Ar−H), 5.0 (1H, s, Ar−CH−S), 4.21 (2H, t, O−CH2−), 3.72 (2H, t, N−CH2), 3.14−0.84 (12H, s, N−(CH3)2 and aliphatic H). GPC: Mn = 14 000 and PDI = 1.1. P3 was synthesized by following procedure as shown in P2. P1 (0.050 g, 0.147 mmol), mercaptopropionic acid (0.093 g, 0.882 mmol), and Lewis acid BF3·Et2O (0.146 g (0.13 mL), 1.029 mmol) were taken in dry dichloromethane (5 mL) at 0 °C. 1H NMR (CDCl3, 300 MHz, δ in ppm): 7.81−7.86 (4H, m, Ar−H), 7.58−7.55 (2H, m, Ar−H), 7.68−7.64 (2H, m, Ar−H), 4.90 (1H, s, Ar−CH−S), 3.98 (2H, t, O−CH2−), 3.73 (2H, t, N−CH2), 3.15−0.84 (12H, s, N− (CH3)2 and aliphatic H). GPC: Mn = 15 000 and PDI = 1.1. Sensing Studies. The P2 and P3 stock solutions were prepared (according to 5% incorporation of dithioacetal moieties along the polymer chain) in a 1.5 × 10−4 M concentration of dithioacetal moieties. The solutions of metal ions (1 × 10−3 M) were prepared in HEPES buffer solution. The sensing studies were carried out by adding the solution of metal ions into P2 and P3 in HEPES buffer in a 0.7 mL cuvette.

polymers, the construction of stimuli-responsive polymers with molecular receptor moieties or sensing units can bring extra advantages, such as adjustable water solubility, multifunctional sensing capability, and tunable detection sensitivity.40,48−51 Among the various of stimuli-responsive polymers, pHresponsive polymers have drawn attention because they can achieve reversible conformational changes between the extended and collapsed states in aqueous solution through their functional groups capable of donating or accepting protons upon pH changes.50 For the detection or extraction of toxic metal ions from industrial and commercial pollutants, it is very important to design new sensing tools that can selectively detect toxic metal ions by slight changes in the pH of the aqueous solution. In this regard, the development of watersoluble polymeric probes for the tunable detection of Hg2+ ions driven by changes in the pH of the aqueous solution is an attractive and challenging approach to be addressed. In the present study, water-soluble colorimetric polymeric probes containing dithioacetal groups in the side chains were synthesized and used for the rapid and selective detection of Hg2+ ion with pH-tunable sensitivity. High selectivity and sensitivity were achieved by using the Hg2+-promoted deprotection reaction of the dithioacetal group. The Hg2+ ion sensing time was controlled by varying the pH of the aqueous solutions.

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RESULTS AND DISCUSSION Two kinds of polymeric probes for the sensitive sensing of mercury ions were synthesized as shown in Scheme 1. The azoderived polymer (P1) with aldehyde functionalities in the side chains was prepared as reported previously.52 The aldehyde groups of P1 were readily converted into dithioacetal groups by reacting with ethanethiol or 3-mercaptopropionic acid in the presence of BF3, leading to P2 or P3, respectively.27−29 The resulting dithioacetal protected polymers (P2 and P3) were characterized by 1H NMR spectroscopy. The complete disappearance of the aldehyde proton at 10.02 ppm (peak a) in the 1H NMR spectrum of P1 and the generation of peak e of dithioacetal groups in the NMR spectrum of P2 and P3 confirmed the successful formation of P2 and P3 (Figure 1a,b and Figure S1). A model compound containing the dithioacetal groups was also synthesized and characterized by 1H NMR spectroscopy to further support the purity of P2 (Figure 1c). The number-averaged molecular weight (Mn) of P2 and P3 was 14 000 and 15 000 with low molecular weight distributions (MWD = 1.14 and 1.17, respectively), as determined by gel permeation chromatography (GPC) (Figure S2). Hg2+ ion-sensing studies with P2 and P3 were carried out in HEPES buffer (pH = 7.4). Changes in UV−vis absorption spectral responses of P2 and P3 were monitored with the gradual addition of Hg2+ ions. As expected, there was a marked red-shift from 425 to 458 nm with an isosbestic point at 430

EXPERIMENTAL SECTION

Materials. N-(2-Hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES), ethanethiol, BF3·Et2O, 3-mercaptopropionic acid, and all metal perchlorate salts were purchased from Aldrich at the highest available purity and were used as received, with no further purification. Instrumentation. 1H NMR spectra of the monomer and polymer were collected in CDCl3 on a Bruker Avance 300 MHz NMR spectrometer. The apparent molecular weight and molecular weight distributions were measured by GPC (Agilent Technologies 1200 series) using a polystyrene standard, with DMF as the eluent at 30 °C and a flow rate of 1.00 mL/min. UV−vis absorption was recorded by Varian Cary 100 spectrophotometer in HEPES buffer solution. Synthesis of Polymers. P1 was synthesized as previously reported.52 Mn = 13 000 and PDI = 1.1. P2: Under a N2 atmosphere, BF3·Et2O, as a Lewis acid (0.17 g (0.15 mL), 1.235 mmol) was added into a mixture of P1 (0.06 g, 0.176 mmol) and ethanethiol (0.065 g (80 μL), 1.058 mmol) in dry dichloromethane (5 mL) at 0 °C. After stirring at 0 °C for 5 h, aqueous NaHCO3 (0.01 M, 3.0 mL) was added into the purple reaction mixture to adjust the pH to neutralize excess of Lewis acid. The neutralization of Lewis acid was carefully noticed by turning of color from purple to yellow. The resulting mixture was extracted with B

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was observed before saturation was reached. In addition to their high sensitivity, P2 and P3 exhibited good selectivity toward Hg2+ ion over several alkali and transition metal cations. As shown in Figure 3b and Figures S4−S8, the addition of other metal cations such as Pb2+, Cu2+, Zn2+, Cd2+, Co2+, K+, Na+, and Ag+ to the P2 and P3 solutions in HEPES buffer (pH =7.4) did not lead to any appreciable absorption spectral changes. This unique sensing behavior can be detected directly with the naked eye in the photograph as shown in Figure 3b. Having demonstrated the selective and sensitive detection of Hg2+ ions with these polymers, we attempted pH-induced control of Hg2+ ion detection time. The detection time profile of P2 and P3 with dithioacetal groups (24 μM) toward Hg2+ ion (24 μM) (Figure 4 and Figure S9) in various pH solutions was measured. It was found that time required for the complete sensing of Hg2+ ions with P2 and P3 was ∼60 and ∼15 min at pH = 7.4 (approximately same pKa), respectively. Interestingly, the sensing time of P3 toward the Hg2+ ion was almost 4 times faster than that of P2. These results indicated that the COOHbased dithioacetal groups of P3 can react with the Hg2+ ion faster than the CH3CH2-based dithioacetal groups of P2. Variations in the pH of the solution of P2 did not lead to significant changes in sensing time profile whereas the detection time of P3 toward Hg2+ ions varied depending on the pH value. For example, it took less than 1 min for P3 at pH = 3 to detect Hg2+ ions completely, 15 min at pH = 7, and 60 min at pH = 11. The influence of variations in pH of the solution of P2 and P3 on the detection time of Hg2+ ion could be explained as shown in Figure 4c. The different chemical structure of dithioacetal groups of P2 and P3 might be the reason for the different sensing kinetics. It is known that the −COOH group has some binding affinity for Hg2+ ions.53−56 Thus, −COOH groups (3-mercaptopropionic acid) of P3 are anticipated to form CO···Hg interaction and further activate the C···S−Hg cleavage, leading to fast detection of Hg2+ ions. The CH3CH2-based dithioacetal groups of P2, without carboxyl groups, would not be expected to show such an interaction and activation with Hg2+ ions, resulting in the relatively slower detection of Hg2+ ions. For a more fundamental examination of the pH-induced control of Hg2+ ion detection time based on C···S−Hg cleavage activated by the CO···Hg coordination interaction in the P3 polymeric probe, concentration-dependent UV−vis absorption measurements were carried out (Figure 5 and Figure S10) in various pH solutions. When hydrophobic alkyl, aromatic, or basic dimethylamino groups are incorporated into polymers

Figure 1. 1H NMR spectra of (a) P1, (b) P2, and (c) a model compound in CDCl3.

nm upon the addition of aliquots of Hg2+ ions into the solution of P2 (Figure 2a). P3 also showed similar absorption characteristics with a 30 nm red-shift toward Hg2+ ions (Figure 2b). The yellow color of the solution rapidly turned to dark red with the addition of Hg2+ ions, which could easily be seen with the naked eye. The appearance of the dark red color with a strong absorption band centered at 458 nm in the UV−vis absorption spectra was attributed to intramolecular charge transfer (ICT) via the push−pull effect of the aldehyde group of P1.27,29,52 These results indicated that Hg2+ ion-facilitated deprotection reaction of dithioacetal groups in P2 and P3 occurred rapidly and P1, with aldehyde groups in the side chain, was formed subsequently. The titration curves were plotted with shifts in the absorption maximum against the amount of Hg2+ ions added to the solution. As shown in Figure 3a and Figure S3, a steady red-shift in the absorption maximum

Figure 2. UV−vis absorption spectra of (a) P2 (24 μM) and (b) P3 (24 μM) with various amounts of Hg2+ ions in HEPES buffer (pH 7.4). C

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Figure 3. (a) Plot of λx − λ0 versus Hg2+ ion concentration for the P2 solution [λ0 is the absorption maximum at 0 μM of Hg2+ ions, and λx is absorption maximum at x μM of Hg2+ ions]. (b) Selectivity bar diagram and photograph of polymers P2 and P3 (24 μM) with various alkali and transition metal ions in HEPES buffer.

Figure 4. Plot of shift in λmax of (a) P2 and (b) P3 (24 μM) versus reaction time in various pH solutions with the addition of an equimolar amount (24 μM) of Hg2+ ion. (c) Schematic representation of fast sensing properties of P3 as compared to P2. The amount of dithioacetal moiety and Hg2+ ion was taken according to the amount needed for a complete dithioacetal deprotection reaction.

with multiple carboxylic acid groups, the pKa value of these polymers usually becomes higher than that of a single carboxyl acid.57−59 For example, the pKa of monomeric acrylic acid is 4.76, but the pKa of poly(acrylic acid) (PAA) ranged from 6.0 to 7.0, depending on the molecular weight.60 Thus, it is

expected that the approximate pKa value of P3 with azobenzene-based carboxylic acid groups could be 6.5−7.0. Aqueous solutions of P3 with a fixed amount of dithioacetal groups (24 μM) at various pH values (pH = 3, 5, 7, 9, and 11) were prepared. To these solutions, aqueous solutions of Hg2+ D

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Figure 5. Plot of shift in λmax of P3 (24 μM) versus reaction time with various amounts of Hg2+ ion in various pH solutions: (a) pH = 3, (b) pH = 7, and (c) pH = 11.

Figure 6. Schematic representation of possible mechanism for pH-tunable Hg2+ ion detection time.

ions at different concentrations (6, 12, 24, 48, 72, and 96 μM) were added. As shown in Figure 5a, P3 showed an incomplete sensing behavior when the concentration of Hg2+ ions was less than 24 μM at pH = 3 (below pKa). This was likely due to a deficiency in the amount of Hg2+ ions needed to achieve complete deprotection of the dithioacetal moiety. With further increases in the amount of Hg2+ ions to 24 μM ([dithioacetal]: [Hg2+ ion] = 1:1), the reactivity curve quickly reached a plateau, within 1 min. Even with the addition of higher amounts of Hg2+ ions (48−96 μM) at pH = 3, the detection time of P3 was the same as that for 24 μM. On the other hand, when the concentration of Hg2+ ion was less than 24 μM at pH = 11 (above pKa), the absorption spectra of P3 showed a minimal

red-shift (∼4−10 nm), and it took longer (>15 min) to reach saturation (Figure 5c). Even with a 4-fold increase in the amount of Hg2+ ions, P3 showed a sluggish increase in the redshift of the absorption maximum along with a still long detection time. It should be noted that even after the addition of excess amount of Hg2+ ions in pH = 11 solutions, the detection time was slow as compared to other low-pH solutions. Heavy metal ion salts have a tendency to form metal hydroxides under high pH conditions (above pH = 10). Thus, the possible reason for the relatively slow detection time at pH = 11 even with high concentrations of Hg2+ ions may be due to the formation of metal hydroxides, which decreased the amount of Hg2+ ions available for the detection.61 In Figure 5b, E

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Macromolecules the sensing characteristics of P3 toward Hg2+ ions in neutral pH = 7 (approximately same pKa) showed intermediate behavior between those at pH = 3 and pH = 11 due to the presence of 50:50 ratio of carboxylic acid and carboxylate ions. For example, the Hg2+ ion detection time of P3 with 24 μM Hg2+ was estimated to be 15 min, whereas the detection time for pH = 3 was 1 min and for pH = 11 was 60 min in the same concentration of Hg2+ ion. In pH = 5 and 9 solutions, the sensing behaviors of P3 followed a similar trend to that at pH = 3 and pH = 11, respectively (Figure 5 and Figure S10). The possible reason that we observed the similar sensing behaviors for pH = 3 and pH = 5 solutions could be that the concentration of carboxylic acid could be almost the same since these pH values are well below the pKa value of P3. The similar sensing behaviors for pH = 9 and pH = 11 solutions were also observed due to these pH values being well above the pKa value of P3. These results indicate important features of the pH-dependent sensing behaviors, as follows: (i) at low pH, less time and less Hg2+ were sufficient for the complete detection of Hg2+ ions, and (ii) at higher pH, more Hg2+ and a longer time were required for the complete sensing of Hg2+ ions. A possible explanation for these results is shown schematically in Figure 6. The carboxylic acid of mercaptopropionic acid would be expected to be fully protonated at low pH below the pKa value of P3 (6.5−7.0). Similarly, the carboxylic acid has a tendency to be converted into a carboxylate (sodium salt) at high pH above the pKa value of P3. If transition or lanthanide metal cations are added to this solution, they will replace sodium and form metal complexes because the carboxylate anion has more affinity toward transition or lanthanide metal cations than the sodium cation.62−66 As shown in Figure 6, during the addition of Hg2+ ion into low pH (below pKa) solutions, the dithioacetal group can easily be deprotected by Hg2+ ion with the assistance of the CO···Hg coordination interaction, leading to fast sensing of Hg2+ ions. In the case of high pH (above pKa) solutions, mercaptopropionic acid would be expected to be fully ionized and exist as a carboxylate anion with which Hg2+ ions can preferentially form −COO− Hg2+− OOC− ion complexes in the beginning. Because only a small amount of Hg2+ ions of those added initially can be used to form C−O···Hg coordination complexes, further addition of excess Hg2+ ions is required to accomplish the complete sensing even with the slow detection time. Although there was no direct evidence for this competitive phenomenon of C−O··· Hg coordination and S−Hg interaction (cleavge) at high pH, it has been observed that the presence of multiple carboxylic acid groups in 2,2-(arylmethylene)bis(sulfanediyl)diacetic acids intervene the formation of stable S−Hg bonds.67,68 On the basis of these reports, we assume that the formation of −COO− Hg2+−OOC− ion complexes interrupted the C···S−Hg cleavage effectively and slowed down the detection time.

P3 toward the Hg2+ ion was almost 4 times faster than that of P2. Detailed UV−vis absorption studies of P3 in various pH solutions with different amount of Hg2+ ions suggested that the detection time of P3 for Hg2+ ions can be tunable by varying the pH values. Additionally, these studies evidenced that the CO···Hg coordination interaction and −COO− Hg2+− OOC− ion complexation playing vital roles in the pH-tunable detection time of Hg2+ ion. These results suggest that the polymeric probes presented here may be potential candidates for the tunable detection of Hg2+ ions in water.

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ASSOCIATED CONTENT

S Supporting Information *

NMR, GPC chromatogram, UV−vis absorption spectra of P2 and P3 over addition of various metal ions and various pH solutions. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.-i.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Priority Research Center Program (2009-0093818) and the Basic Science Research Program (2012R1A1A2039730) through the National Research Foundation of Korea, funded by the Ministry of Education, Science, and Technology of Korea.

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CONCLUSIONS Two new polymeric probes, P2 and P3, with dithioacetal units in the side chains were developed for the selective and efficient colorimetric sensing of Hg2+ ions in aqueous solutions. The Hg2+-promoted deprotection reaction of dithioacetal groups was responsible for the colorimetric detection of Hg2+ ions. Both probes showed high sensitivity and selectivity for the detection of Hg2+ among several common alkali and transition metal ions. The sensing kinetics of these probes was examined in various pH solutions to investigate the detection time of probes for Hg2+ ions. The results indicated that sensing time of F

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DOI: 10.1021/ma502350p Macromolecules XXXX, XXX, XXX−XXX