BODIPY-Derived Polymeric Chemosensor Appended with

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BODIPY-derived Polymeric Chemosensor Appended with Thiosemicarbazone Units for the Simultaneous Detection and Separation of Hg(II) Ions in Pure Aqueous Media Ujjal Haldar, and Hyung-il Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00408 • Publication Date (Web): 18 Mar 2019 Downloaded from http://pubs.acs.org on March 18, 2019

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ACS Applied Materials & Interfaces

BODIPY-derived Polymeric Chemosensor Appended with Thiosemicarbazone Units for the Simultaneous Detection and Separation of Hg(II) Ions in Pure Aqueous Media Ujjal Haldar and Hyung-il Lee* Department of Chemistry, University of Ulsan, Ulsan 680-749, Republic of Korea

*Corresponding Author: E-mail: [email protected]

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ABSTRACT Developing a simple and cheap analytical method for the selective detection and quantitative separation of toxic ions present in aqueous media is the biggest challenge faced by the chemosensing research community. Here, a 5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5Hdipyrrolo-diazaborinine

(BODIPY)-derived

water-soluble

polymer

integrated

with

thiosemicarbazone units was rationally designed and synthesized for the simultaneous detection and separation of Hg(II) ions in pure aqueous solution. The water-soluble polymer scaffold poly(N,N´-dimethyl

acrylamide-co-5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5H-dipyrrolo-

diazaborinine-2-carbaldehyde) was synthesized by reversible addition-fragmentation chain transfer (RAFT) polymerization, followed by post-polymerization modification with thiosemicarbazide, leading to the formation of the target probe, P1. The non-emitting P1 exhibited bright yellow emission upon exposure to Hg(II) ions, with a limit of detection as low as 0.37 µM. This turn-on emission behavior triggered by Hg(II) ions might originate from the suppression of isomerization around the C=N bond of the thiosemicarbazone moiety caused by the formation of a coordination complex between P1 and Hg(II) ions. In addition, P1 displayed excellent selectivity toward Hg(II) ions over other metal cations. Finally, the selective removal of Hg(II) ions from an aqueous solution containing various metal ions was achieved by precipitation, probably caused to the fact that coordination complexes whereby Hg(II) ions acted as bridgeheads between P1 molecules had formed. KEYWORDS: water-soluble copolymer, chemosensor, mercury (II), BODIPY, metal ion removal


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INTRODUCTION In recent years, the development of low cost, selective, and sensitive fluorescent materials for the efficient tracking of toxic heavy transition metal ions with efficient separation ability has drawn significant attention, owing to the diverse impact that these metallic species have on the eco-system and the human organism.1-5 Among the various heavy transition metals, both the ionic and non-ionic form of mercury are extremely toxic and non-biodegradable pollutants, which can be easily accumulated in the human body and in the bodies of other living organisms through the food chain.6-8 Mercury accumulation in the human body, even in the parts per million (ppm) ranges, can lead to serious health consequences.9-12 Moreover, due to rapid industrialization and urbanization, the contamination level of waterways by Hg(II) ions has been increasing steadily.13-14 A prime challenge for researchers is, therefore, to develop systems that efficiently realize the recognition and separation of mercury ions in aqueous media. To date, several analytical techniques have been established and exploited for the efficient detection of Hg(II) ions; these techniques are based, for instance, on electrochemistry,15 atomic absorption,16 inductively coupled plasma-atomic emission/mass spectrometry (ICPAES/ICPMS),17-18 and chromatograph.19 Implementation of such traditional techniques, however, often requires expensive instrumentation setups, complex sample treatments, and long processing times. Hence, an urgent need remains to develop a simple, highly sensitive, selective, real-time, and cheap analytical method for monitoring mercury contamination both in water and the environment. In this regard, fluorescence-based techniques are the most promising and the most widely used for the detection of Hg(II) ions, owing to their outstanding naked eye visualization capability, portability, high selectivity, sensitivity, and easy implementation.20-23 However, although fluorescence-based analytical techniques relying on traditional chemosensors have been employed for the quantitative detection of Hg(II) ions, they have not been used to separate these ions from the aqueous medium they contaminate. Thus far, various separation techniques, such as membrane filtration, complexation and precipitation, adsorption, reverse osmosis, and ion exchange have been exploited.24-27 Among them, due to its low cost and high efficacy, the complexation and precipitation technique is the simplest and most promising approach to the removal of low-concentration toxic heavy metal ions.28-29



Over the past few decades, a significant number of fluorescent chemosensors based on small molecules have been investigated as probes to monitor trace amounts of Hg(II) ions in aqueous media.30-32 In fact, although most fluorescent sensors based on small organic and organometallic molecules are highly sensitive and selective, they have a few limitations, including low solubility in pure water, structural instability, and fluorescence self-quenching in aqueous solution. In this context, water-soluble polymeric probes integrated with a small amount of a metal-ionrecognizing motif are promising candidates for addressing the described shortcomings, since these structurally stable and biocompatible species can be used in sensing studies conducted in pure water.33-37 Various fluorophore units (e.g., rhodamine, coumarin, and naphthalene) have been employed as a signal-transducing agents to monitor the presence of trace amounts of Hg(II) ions in aqueous and semi-aqueous media.38 Among these units, 5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5Hdipyrrolo-diazaborinine (BODIPY) offers several advantages, such as a high extinction coefficient, a high fluorescence quantum yield, and a high thermal and photolytic stability.39-40 Use of BODIPY based chemosensors is quite common in small-molecule-based systems, but relatively rare in macromolecular systems.41 In fact, much room still exists for the improvement of macromolecular sensors with respect to their usability in pure aqueous media, their ease of synthesis, their sensing selectivity, their anti-interference ability, and their analyte detection limit. Against this background, herein we report the design and synthesis of novel BODIPY-derived small-molecule and macromolecular chemosensors comprising thiosemicarbazone moieties, whose fluorometric detection performance toward Hg(II) ions we have studied in semi-aqueous and pure aqueous media, respectively, at physiological pH. The BODIPY-based small-molecule probe we synthesized, which we named M1 (see Scheme 1), showed excellent selectivity, sensitivity, and reversible detection ability toward the Hg(II) ion, but it could not separate this ion from a mixture of various alkali and transition metal cations. On the other hand, use of the water-soluble polymeric probe we developed, which we named P1 (see Scheme 2), enabled us to detect and even separate Hg(II) ions from other metal cations present in pure aqueous media.


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EXPERIMENTAL Materials N, N´-dimethylacrylamide (DMA, 97.0%) was purchased from TCI, and it was purified prior to the polymerization reaction by making it pass through a basic alumina column. 2,2′-Azobisisobutyronitrile (Aldrich, 98%) was recrystallized twice from methanol. N-(2Hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES), 2-(dodecylthiocarbonothioylthio)2-methylpropionic acid, and all metal salts at the highest available purity were purchased from Sigma Aldrich and used as received. All other chemicals were of analytical grade and used without further purification, unless specified otherwise. The aldehyde-terminated BODIPY monomer was synthesized according to the procedure detailed in our previous report.41 CDCl3 (99.8% D), D2O (99% D) and DMSO-d6 (99.8% D) for nuclear magnetic resonance (NMR) spectroscopy experiments were purchased from Cambridge Isotope Laboratories, Inc., USA. Solvents such as hexanes (mixture of isomers), ethyl acetate (EtOAc), tetrahydrofuran (THF), dichloromethane (DCM), ethanol (EtOH), methanol (MeOH), dimethyl formamide (DMF), and diethyl ether were purchased from local chemical suppliers. All spectroscopic measurements were performed in aqueous HEPES buffer (50 mM, pH 7.4) solutions. Instrumentation 1H

NMR spectra were recorded on a Bruker Avance 300 MHz NMR spectrometer operating at

298 K. The number averaged molecular weight (Mn) and molecular weight distribution (MWD) were measured by gel permeation chromatography (Agilent Technologies 1200 series) performed at 30 °C at a flowrate of 1.00 mL/min using a polystyrene standard with dimethylformamide (DMF) as the eluent. The UV–vis and fluorescence spectra were recorded on a Varian Cary 100 spectrometer and a HORIBA FluoroMax-4Pm spectrophotometer, respectively. The concentration of toxic analytes was estimated employing an instrument for performing inductively coupled plasma optical emission spectroscopy (ICP-OES) (SPECTRO Analytical Instruments GmbH). This piece of equipment was calibrated using certified standard solutions from Merck (Inorganic Ventures’ Quality Control Standard 26).



Estimation of the analyte detection limit To evaluate the detection limit of probes, fluorometric titration experiments were carried out at a fixed concentration of probe in solution in the presence of progressively increasing concentrations of Hg(II) ions, which were obtained by adding 5.0 µL aliquots of the analyte solution in sequence using a micro-pipette. To determine the standard deviation of the blank samples, the fluorescence emission spectra of the solutions of the probes were measured three times. To find the slope of linear regression curve, the fluorescence emission at 540 nm (in the case of M1) and 545 nm (in the case of P1) of the solutions containing probe and analyte were plotted against the concentration of Hg(II) ions. Finally, the limit of detection (LOD) of the analyte was calculated using the following equation:36 LOD = 3σ/K………….(1), where σ is the standard deviation of the measurements performed on the blank samples, and K denotes the slope of the curve. Quantification of the probe–Hg(II) binding association constant The value of the binding association constant (Ka) of the reaction between Hg(II) and probe was calculated based on fluorescence emission intensity data using the Benesi–Hildebrand equation:42 1/∆F = 1/∆Fmax+(1/K[C]) (1/∆Fmax). Here ∆F = F – Fmin and ∆Fmax = Fmax – Fmin; Fmin, F, and Fmax are the emission intensities of the probe measured in the absence of Hg(II), at an intermediate Hg(II)-ion concentration, and at a concentration of complete probe saturation, respectively. Furthermore, K is Ka and [C] is the concentration of Hg(II). By plotting (Fmax – Fmin)/(F – Fmin) against [C]-1, we were able to determine the value of Ka based on the intercept and slope of the straight line thus obtained. Determination of the probe quantum yields (ΦF) The fluorescence quantum yield (ΦF) of the probes in the absence and presence of Hg(II) ions were calculated using rhodamine 6G (ΦF = 0.95 in EtOH) as reference. The value of ΦF was estimated using the following equation:


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ΦF = Φref × IProbe/Iref × Aref/AProbe × Probe2/ref2, where, ΦF and Φref are the quantum yields of the probe and of rhodamine 6G, respectively; Iprobe and Iref are the areas of the integrated emission peaks of the probe and of rhodamine 6G, respectively; AProb, and Aref are the absorbance at the excitation wavelength of the probe and of rhodamine 6G, respectively; probe and ref are the refractive indices of solvents, respectively. (probe,water = 1.33 and ref,EtOH = 1.36). Synthesis

of

5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5H-dipyrrolo-diazaborinine

(BODIPY). BODIPY was synthesized according to the previously mentioned protocol.43 1H NMR (Figure S1, 300 MHz, in CDCl3) δ (ppm): 7.47–7.51 (3H, m), 7.27–7.29 (2H, m), 5.98 (2H, s), 2.56 (6H, s), 1.37 (6H, s). Note: CDCl3 and partial phenyl proton signals merge together. Synthesis

of

5,5-difluoro-1,3,7,9-tetramethyl-10-phenyl-5H-dipyrrolo-diazaborinine-2-

carbaldehyde (BODIPY-CHO). The selective monoformylation of the BODIPY dye was carried out implementing the relevant literature procedure.43 1H NMR (Figure S2, 300 MHz, in CDCl3) δ (ppm): 10.01 (1H, s), 7.52–7.54 (3H, m), 7.26–7.30 (2H, m), 6.15 (1H, s), 2.83 (3H, s), 2.62 (3H, s), 1.65 (3H, s), 1.39 (3H, s). Synthesis of M1. An ethanolic solution of thiosemicarbazide (0.09 g, 1 mmol) was carefully added to a BODIPY-CHO solution (0.19 g, 1.1 mmol) prepared in anhydrous EtOH (3 mL). 2–3 drops of acetic acid were added to the reaction mixture, which was then stirred at 70 °C for 24 h. The mixture solvent was subsequently completely removed by rotary evaporation, and the residue thus obtained was purified by column chromatography using silica gel (100–200 mesh) as the stationary phase and hexanes/EtOAc (7:3, v/v) as an eluent to obtain a solid material (yield: 78%). 1H NMR (Figure S3, 300 MHz, in CDCl3) δ (ppm): 7.91 (1H, s, -NH), 7.59-7.53 (3H, s), 7.31-7.27 (2H, m), 6.12 (1H, s), 2.75 (3H, s), 2.62 (3H, s), 1.51 (3H, s), 1.42 (3H, s). Synthesis of the copolymer poly(N,N´-dimethyl acrylamide-co-5,5-difluoro-1,3,7,9tetramethyl-10-phenyl-5H-dipyrrolo-diazaborinine-2-carbaldehyde)

[p(DMA-co-

BODIPYCHO)] via reversible addition-fragmentation chain transfer polymerization.



P1 consisting of 94 mol% of DMA and 6 mol% of BODIPY-CHO units was synthesized according to the procedure detailed in our previous report.41 For this species, Mn = 8300 g/mol and PDI = 1.2. The final incorporation ratio was calculated based on 1H NMR spectroscopy data. Post-polymerization modification of P1 p(DMA-co-BODIPYCHO), the polymer characterized by an aldehyde terminus, (100 mg) (assuming a 6% BODIPY-CHO incorporation into the polymer chain) was dissolved in 15 mL of anhydrous EtOH in a 25 mL round-bottom flask equipped with small magnetic bar. An excess amount of thiosemicarbazide (an at least 10-fold excess with respect to p(DMA-coBODIPYCHO)) was then added to the reaction flask, which was subsequently heated at 80 °C for 12 h. Finally, the polymer solution was concentrated slightly under vacuum and purified by dialysis against MeOH, changing this solvent every 2 h. This process was repeated at least 5–6 times. Afterwards, the resulting polymer was dried under high vacuum at 40 °C overnight to obtain the desired product, P1.

RESULTS AND DISCUSSION


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Scheme 1. (a) Synthesis of M1 and (b) mechanism of Hg(II) ion reversible sensing by M1. Fluorescence images of M1 and M1-Hg(II) complex recorded under UV light (365 nm). AcOH: acetic acid; Anh.: anhydrous; EtOH: ethanol; FL.: fluorescence. The synthetic route for the preparation of the small-molecule probe M1 is illustrated in Scheme 1a. The key intermediate (i.e. BODIPY-CHO) for M1 synthesis was prepared according to the previously reported procedure. BODIPY-CHO was then treated with an excess amount of thiosemicarbazide in anhydrous EtOH at 80 °C, resulting in the formation of the target probe, M1. The successful synthesis of M1 was confirmed by 1H NMR spectroscopy (Figure S3). BODIPY was chosen as the signal reporter due to its outstanding photophysical properties, whereas thiosemicarbazone units were elected to act as receptors owing to their strong binding affinity toward Hg(II) ions.44-45 M1 was observed to be non-emissive (ΦF, M1 = 0.02), presumably as a consequence of a non-radiative deactivation process of the excited state via isomerization around the C=N bond of the thiosemicarbazone moiety.46-47 Restricting the rapid isomerization around this C=N bond as a consequence of the metal coordination by the thiosemicarbazone moiety of M1 could be a very simple and practical strategy to cause the BODIPY fluorophore to recover its intrinsic high-intensity fluorescence (Scheme 1b).48-49 M1 displayed good solubility in the most commonly used organic solvents, including THF, DCM, MeOH, acetone, and DMF, but it proved insoluble in water. To optimize the pH range for Hg(II) sensing by M1, a pH titration was carried out over a wide pH range (4–10) in EtOH–H2O (9/1; v/v) HEPES buffer. The fluorescence intensity of M1 was very weak and remained unaffected by changes in pH (Figure S4). After the addition of Hg(II) ions (5 equivalents) into the M1 solution at pH > 6, the emission intensity at 540 nm increased markedly. At pH < 6, on the other hand, the increase in emission intensity was relatively low due to the protonation of the imino nitrogen of the thiosemicarbazone unit, a protonation that interferes with the formation of the M1–Hg(II) complex. Hence, the physiological pH 7.4 was chosen as optimal for our sensing studies.



0.8

M1 M1+100M Hg(II)

(a)

FL. Intensity

Absorbance

0.4

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100  Hg(II)

Hg(II)

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FL. Intensity

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0  Hg(II)

Hg(II)

0.0

450 500 550 Wavelength (nm)

600

500

650

550

600

Wavelength (nm)

650

700

(d)

M1+ Hg(II)

M1 + various metal cations M1+ Cu(II)

0.0 500


700

750

Figure 1. (a) Absorption spectra of pure M1 (2 × 10-5 M) in ethanol–H2O (9:1; v/v) HEPES buffer and of the solution obtained upon adding Hg(II) ions (100 µM, 5 equivalents) to the solution of pure M1. (b) Fluorescence spectra of pure M1 (2 × 10-5 M) in ethanol–H2O (9:1; v/v) HEPES buffer and of the solutions obtained upon the progressive addition of Hg(II) ions (0–100 µM, 0–5 equivalents) to the solution of pure M1. Inset: fluorescence images of M1 and M1Hg(II) complex recorded under UV light (365 nm). (c) Fluorescence spectra of an M1 solution (2×10-5 M) in ethanol–H2O (9:1; v/v) HEPES buffer recorded following the addition of various metal cations (1.0 mM). (d) Partial 1H NMR spectra of (1) BODIPY-CHO, (2) M1 right after dissolution and (3) M1 after 12 h incubation in DMSO–d6/D2O (9/1; v/v) at room temperature. The sensing behavior of M1 (2 × 10-5 M) toward Hg(II) ions (0–100 µM, 0–5 equivalents) was investigated by recording UV–vis absorption and fluorescence spectra in EtOH–H2O (9/1; v/v) HEPES buffers at pH 7.4. The UV–vis spectrum of M1, which comprises a single BODIPY unit, was characterized by a distinct absorption band at 529 nm (Figure 1a). Upon the addition of


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Hg(II) ions (100 µM) to the solution of M1, the wavelength of this absorption maximum shifted from 529 to 520 nm, a change associated with a slight color change from pink to light orange, indicating the possibility of structural changes in the BODIPY unit. The optical detection of Hg(II) ions was further surveyed by conducting fluorescence spectroscopy experiments in the same conditions employed for recording the UV–vis spectra. M1 was non-emissive (ΦF = 0.02) in the absence of Hg(II) ions due to the mechanism described above. Following the gradual addition of Hg(II) ions to the M1 solution, however, the emission intensity at 540 nm (λexc = 485 nm) increased dramatically, reaching a maximum of ~83-fold fluorescence enhancement (ΦF = 0.29) (Figure 1b) when the concentration of added Hg(II) was ~80 µM. Instantaneously, the turnon yellow fluorescence response of M1 toward Hg(II) ions was also observed by the naked eye under UV light irradiation (365 nm). As already hypothesized above, such enhancement in emission intensity could originate from the restriction of the isomerization process around the C=N bond brought about by the formation of strong coordination bonds between the thiosemicarbazone moieties of M1 molecules and Hg(II) ions (Scheme 1b). Notably, the formation of a M1–Hg(II) complex was investigated recording the 1H NMR spectra of M1 in DMSO-d6–D2O (9:1; v/v), both in the presence and in the absence of Hg(II) ions (Figure S5). After the addition of 2 equivalents of Hg(II) ions, the resonance signals due to the imino protons (-CH=N-) of the thiosemicarbazone units (δ: 8.13 ppm) and to the methyne protons of the BODIPY units (δ: 6.30 ppm) shifted slightly upfield, indicating the formation of the M1–Hg(II) complex. The rest of the resonance signals were left unchanged by the binding process. The stoichiometry of the M1–Hg(II) complex was determined by the Job’s plot. In detail, as the total (additive) concentration of M1 and Hg(II) ions was kept constant at 2.0 × 10−5 M, changes in fluorescence emission intensity were recorded as a function of the (varying) mole fraction of Hg(II) ions in the sample solution. The curve of the fluorescence emission intensity thus drawn exhibited a maximum when the mole fraction of Hg(II) ions was ~0.4, indicating the formation of a 2:1 complex between M1 and Hg(II) (Figure S6). Based on the fluorescence titration curve (Figure 1b), and using the Benesi–Hildebrand equation, the Ka of the M1–Hg(II) complex was calculated to be 1.15 × 104 M-2 (Figure S7). The Hg(II) ion detection limit of M1 was determined to be 0.12 µM based on equation 1, which means that this probe can be effectively used for the detection of even low concentrations of Hg(II) ions (Figure S8). In addition to a high detection sensitivity, M1 exhibits excellent selectivity toward Hg(II) ions over other alkali and transition



metal ions. Among all the metallic species tested in the presence of M1, only Hg(II) ions caused significant spectral changes (Figures 1c and S9). To probe further the selectivity of M1, a solution of M1 was treated with Hg(II) ions in the presence of other competing metal ion species (Figure S10). No significant spectral changes were noticed with respect to the case when M1 had been treated exclusively with Hg(II) ions, suggesting that M1 has a strong anti-interference ability. Along with high selectivity and low LOD value, a short sensing time is also a crucial characteristic to have for the practical on-spot application of a Hg(II)-ion probe. For this reason, experiments for the time-dependent detection of Hg(II) ions by M1 were carried out. Interestingly, the maximum in emission intensity was reached within 5 s after an M1 2 × 10-5 M solution was exposed to an 80 µM concentration of Hg(II) ions, which corroborated the idea that this probe could be utilized for real on-field applications (Figure S11). The stability of a chemosensor in solution is crucial for the sensor’s long-term use. To determine the stability of M1 in solution, we kept an M1 solution (EtOH–H2O-9/1; v/v - HEPES buffer) at pH 7.4 and monitored its emission intensity for a total of 48 h (Figure S12). The emission maxima centered at 510 nm appeared gradually as time passed upon excitation at 485 nm. Consequently, the initially non-fluorescent M1 solution became green fluorescent, a color change that could be noticed by the naked eye under the irradiation of a UV lamp (365 nm). Twelve hours after dissolving M1 in DMSO-d6–D2O (9/1; v/v), the 1H NMR spectrum of the resulting solution displayed the presence of a resonance at 10.02 ppm due to the aldehyde protons, indicating the formation of BODIPY-CHO as a consequence of the hydrolysis of the imine bond of M1 (Figure 1d).41 Evidence indicates, therefore, that the small-molecule probe M1 is vulnerable to hydrolysis and special caution should be taken for its long-term use.

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P2 P2+5 eqv. Hg(II) P2+5 eq. Hg(II)+ 5eq. Na2S

80 (c)

(b) S N F

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N NH

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Figure 2. (a) Fluorescence spectra of M1, M1–Hg(II) complex, and M1–Hg(II) + Na2S in ethanol–H2O (9:1; v/v) HEPES solution at pH 7.4. (where [M1] = 20 µM, [Hg(II) ions] = 100 µM, [Na2S] = 100 µM, and λexc = 485 nm). (b) Reversible reaction between M1 and M1-Hg(II) complex. (c) Normalized fluorescence intensities based on the emission maximum at 540 nm; the reversible sensing of Hg(II) ions by M1 for up to five cycles modulated by the alternating addition of Hg(II) ions and Na2S. FL.: fluorescence; HEPES: hydroxyethyl)piperazine-N′-(2ethanesulfonic acid). The reversibility of the reaction between sensor and analyte and the possibility for the sensor to be regenerated are important traits for a chemosensor to have. To assess whether the binding process between Hg(II) ions and M1 is reversible, the changes in the fluorescence emission maxima at 540 nm were measured as a function of the presence of 5 equivalents of Na2S in an EtOH–H2O (9:1; v/v) HEPES buffer containing 1 equivalent of M1 and 5 equivalents of Hg(II) ions (Figure 2a). Upon addition of 100 µM Na2S to a solution of the M1–Hg(II) complex, a rapid decrease in the intensity of the emission spectrum was observed. The ‘original’ emission intensity of the M1–Hg(II) solution could, however, be recovered simply by introducing additional Hg(II) ions. As was suggested by the relatively low Ka value (1.15 × 104 M-2) of the M1–Hg(II) complex, the complexation and dissociation processes proved to be reversible (Figure 2b). This entire process was repeated for five consecutive cycles with no substantial deviations observed (Figure 2c).

Scheme 2. Synthetic scheme for the preparation of P1, and P1’s probable Hg(II)-ion sensing mechanism. Fluorescence images of P1 and P1-Hg(II) complex recorded under UV light (365 nm). AcOH: acetic acid; Anh.: anhydrous; EtOH: ethanol; FL.: fluorescence.



Although M1 displayed excellent sensitivity, selectivity, and reversibility as a sensor for the detection of Hg(II) ions, M1’s poor solubility in pure water and its weak hydrolytic stability made this chemosensor not very attractive for real-world applications. To overcome these issues, the BODIPY fluorophore with a semicarbazone moiety attached to it was integrated into the backbone of a water-soluble polymer (Scheme 2). A random copolymer consisting of DMA and BODIPY-CHO units was prepared as previously reported. Importantly, the DMA units were introduced into the polymer backbone to make the resulting polymer water-soluble. The incorporation ratio between DMA and BODIPY-CHO was 94:6. Notably, only 6% BODIPY units were introduced into the polymer backbone to avoid the self-aggregation of BODIPY fluorophores and to maintain the hydrophilicity of the polymer at a sufficiently high level. Next, the aldehyde groups of the BODIPY-CHO units were made to react with an excess of thiosemicarbazide, resulting in the formation of the desired polymeric probe, P1. This probe was then characterized by 1H NMR spectroscopy (Figure 3). The successful post-polymerization modification was confirmed by the complete disappearance of the aldehyde proton resonances at 10.02 ppm and the appearance of new signals due to -CH=N- protons at 8.13 ppm. Notably, P1 proved fully soluble in most organic solvents and water.


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Figure 3. 1H NMR spectra of (a) p(DMA-co-BODIPYCHO) and (b) P1 in CDCl3.



Figure 4. (a) Fluorescence spectra of P1 (2.0 × 10-5 M) in the presence of various concentration of Hg(II) ions upon excitation at λexc = 485 nm. Inset: Fluorescence images of P1 and P1-Hg(II) complex recorded under UV light (365 nm). (b) Combined linear regression curves of M1 (2.0 × 10-5 M) in ethanol–H2O (9:1; v/v) HEPES buffer solution and P1 (2.0 × 10-5 M) in aqueous HEPES buffer solution at pH 7.4. (c) Plot of the maximum fluorescence emission intensity of M1 and P1 solutions versus the seconds after the addition to the said solutions of Hg(II) ions to reach a concentration of 80 µM. (d) Selectivity bar diagram of fluorescence responses of P1 (2.0 × 10-5 M) in aqueous HEPES buffer with various metal cations (1 mM concentration) at pH 7.4. Inset: photograph of solutions containing P1 and various alkali and transition metal ions when illuminated by UV light at 365 nm. 1: P1 + Na(I); 2: P1 + Ca(II); 3: P1 + Mg(II); 4: P1 + Ag(I); 5: P1 + Fe(II); 6: P1 + Cu(II); 7: P1 + Pb(II); 8: P1 + Co(II); 9: P1 + Zn(II); 10: P1 + Ni(II); 11:


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P1 + Cd(II); 12: P1 + Hg(II). FL.: fluorescence; HEPES: hydroxyethyl)piperazine-N′-(2ethanesulfonic acid) . The sensing behavior of P1 toward various concentrations of Hg(II) ions in aqueous HEPES buffer solution at pH 7.4 was investigated by UV–vis absorption and fluorescence spectroscopies. The absorption maximum of a P1 solution (0.04 mg of P1/1.0 mL of HEPES buffer, 2.0 × 10-5 M of BODIPY receptors) displayed a negligible blue-shift (~4 nm) upon addition to it of Hg(II) ions to a final concentration of 100 µM (Figure S13). However, a new emission band that caused the solution to acquire a bright yellow color appeared at 545 nm (ΦF = 0.17) when the solution was excited with light at a wavelength of 485 nm; the emission intensity of this band, furthermore, increased linearly with the concentration of Hg(II) ions, up to a concentration of 0.08 mM, above which no further increase in emission intensity was observed (Figure 4a). Based on the linear regression curve (Figure 4b), the detection limit of P1 was calculated to be 0.37 µM, which is about three times higher than that of M1, but still comparable with the other polymeric sensors.50-52 The time-dependent profiles of the intensities of the fluorescence emission maxima of solutions of M1 and P1 to which had been added Hg(II) ions to a concentration of 80 µM were also measured. The times needed for the complete detection of the Hg(II) ions using M1 and P1 as chemosensors were 5 and 72 s, respectively (Figure 4c). Although it is difficult to determine the precise reason for the relatively low sensitivity of P1 with respect to M1, it is reasonable to assume that the low extent of exposure of the hydrophobic BODIPY receptors to the hydrophilic Hg(II) ions in pure aqueous phase and the low mobility of BODIPY receptors associated with the complex architecture of the P1 polymer could be important factors. No discernible difference in Hg(II) ion selectivity was observed between M1 and P1. In fact, P1 has good selectivity toward Hg(II) ions, as well as strong anti-interference ability (Figures 4d, S14, and S15). Whereas M1 displayed vulnerability to hydrolysis, P1 showed remarkable stability in aqueous solution. No changes in the fluorescence (Figure S16) and 1H NMR (Figure S17) spectra were observed for P1 after 7 days of incubation in aqueous HEPES buffer solution, confirming the high hydrolytic stability of this polymer. Similarly to what was done in the case of M1, experiments to probe the reversibility of the binding between P1 and Hg(II) ions were conducted; these experiments consisted in the alternate and consecutive addition of solutions of Hg(II) ions



and Na2S to the P1 solution (aqueous HEPES buffer, pH 7.4, Figure S18). In particular, the intensity of the emission peak at a wavelength of 545 nm, whose presence had been induced by the addition of the Hg(II)-ion solution to the solution of P1, decreased significantly as a consequence of the addition of the Na2S solution. Furthermore, this addition was accompanied by the formation of HgS. Due to low solubility of HgS in aqueous media, the repeated detection was not achieved.

Figure 5. (a) Precipitation of the P1–Hg(II) coordination complexes when Hg(II) ions were added to a final concentration of 100 µM to a highly concentrated aqueous solution of P1 (50 mg/mL, 2.5 × 10-2 M of BODIPY receptors). (b) Selectivity bar diagram representing the Hg(II)ion removal efficiency by P1 in aqueous medium in the presence of competing metal ions. The black and red bars represent the concentrations of the relevant metal ions before and after


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complex precipitation, respectively. (c) Schematic representation of the Hg(II)-ion removal process by P1. Having successfully demonstrated the selectivity and sensitivity of the optical detection of Hg(II) ions by P1 in aqueous medium, we proceeded to focus our attention on removing Hg(II) ions from aqueous media in the presence of other metal cations. At low concentration (e.g., 0.04 mg/mL), P1 merely serves as a fluorescent detector of Hg(II) ions. At higher concentration, however, a sufficient number of Hg(II)-ion receptors (i.e. the thiosemicarbazone moieties) of P1 end up binding Hg(II) ions in an intermolecular fashion to cause the formation of intermolecular crosslinks that prompt the precipitation of the P1–Hg(II) coordination complexes from the aqueous solution, hence facilitating the separation of Hg(II) ions from other metal cations (Figure 5a). The Hg(II) ion removal efficacy by way of precipitation of the P1–Hg(II) coordination complexes was quantitatively analyzed by ICP-OES.53 In detail, an aqueous solution of P1 (50 mg in 1.0 mL of deionized water, 2.5 × 10-2 M of BODIPY receptors) was first prepared that contained the following metal cations, each at 1 ppm concentration: Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Pb(II), Cd(II), and Hg(II). An ICP-OES measurement was then performed on this solution to determine the initial concentration of each metal ion (see black columns in Figure 5b). A pink precipitate formed after the mixture thus prepared was vigorously stirred for 30 min. The remaining solution was then separated from the precipitate by filtration and analyzed by ICP-OES (see red columns in Figure 5b). Only a concentration of 0.12 ppm of Hg(II) ions (86% removal) was detected in the filtrate, whereas no noticeable reduction in the concentrations of the other metal ions was observed (Figure 5b, S19). The entire separation process is pictorially represented in Figure 5c.

CONCLUSIONS We have developed a novel water-soluble, BODIPY-appended, and semicarbazone-based polymeric chemosensor (P1) for the selective detection and separation of Hg(II) ions, and the Hg(II)-ion sensing performance of this species was compared with that of M1, its small-molecule analog. Both M1 and P1 showed a turn-on fluorescence response to being exposed to Hg(II) ions, as well as good Hg(II)-ion selectivity and anti-interference ability. Such turn-on



fluorescence response triggered by the presence of Hg(II) ions was the result of the formation of coordination complexes between the chemosensors’ thiosemicarbazone moieties and Hg(II) ions. The formation of the relevant coordination bonds restricted, in turn, the isomerization process around the thiosemicarbazone moiety’s C=N bond, ultimately causing BODIPY’s natural fluorescence to be restored. Even though M1 displayed higher Hg(II)-ion sensitivity (with LOD = 0.12 µM) and faster detection time (~5 s) than P1 (LOD = 0.37 µM; detection time ~72 s), M1’s poor hydrolytic stability, its requirement for semi-aqueous working media, and its lack of Hg(II)-ion separation ability made it less attractive than P1. In fact, Hg(II)-ion-induced turn-on fluorescence was easily achieved in the presence of P1 in pure aqueous media at pH 7.4, in conditions whereby P1 displayed excellent hydrolytic stability. Furthermore, P1 proved able to effect the selective removal Hg(II) ions (~86%) from an aqueous solution containing other metal cations by way of precipitation. In summary, alongside high sensitivity, selectivity, and rapid detection response, the novel polymeric Hg(II)-ion probe we developed possessed high hydrolytic stability and a remarkable ability to separate toxic Hg(II) ions from other potentially competing metal ions, widening the scope of environmental remediation and pollution control.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. 1H NMR spectra of BODIPY, BODIPY-CHO, M1; the effect of pH; 1H NMR spectra of M1 in the absence/presence of Hg(II); LOD determination of M1; Job’s plot and binding association constant (Ka) measurements for M1; UV-vis spectrum of P1 before/after the treatment of Hg(II); selectivity and anti-interference measurements, and hydrolytic stability of both M1 and P1; raw data of ICP-OES data (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.L.).


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ORCID Hyung-il Lee: 0000-0001-9965-7333

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Graphical abstract Title: BODIPY-derived Polymeric Chemosensor Appended with Thiosemicarbazone Units for the Simultaneous Detection and Separation of Hg(II) Ions in Pure Aqueous Media


BODIPY-Derived Polymeric Chemosensor Appended with

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