Polymeric micelles based on light-responsive block copolymers for the

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Polymeric micelles based on light-responsive block copolymers for the photo-tunable detection of mercury (II) ions modulated by morphological changes Hye-Jin Kim, and Hyung-il Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12441 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 13, 2018

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ACS Applied Materials & Interfaces

Polymeric Micelles Based on Light-Responsive Block Copolymers for the Photo-Tunable Detection of Mercury (II) Ions Modulated by Morphological Changes Hye-Jin Kim and Hyung-il Lee* Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea E-mail: [email protected]

KEYWORDS. Amphiphilic block copolymer, Photo-cleavable polymer, Polymeric micelles, Mercury (II) ions. 1 Environment ACS Paragon Plus

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ABSTRACT Polymeric micelles based on light-responsive block copolymers were prepared and used for the photo-tunable detection of mercury (II) ions. 2-Nitrobenzyl acrylate (NBA) and (E)-2-((4-((4formylphenyl)diazenyl)phenyl)(methyl)amino) ethyl acrylate (FPDEA) were copolymerized from a poly(ethylene oxide) (PEO) macroinitiator via atom transfer radical polymerization (ATRP), leading to a well-defined block copolymer of PEO113-b-[p(NBA10-co-FPDEA3)] with a low polydispersity index (PDI = 1.16). After polymerization, the aldehyde groups of PEO-b-[p(NBA-co-FPDEA)] were converted to aldoxime groups by reacting with hydroxylamine, leading to the formation of a final oxime-containing polymeric probe, PEO-b-[p(NBA-co-HPDEA)], P1. The resulting block copolymer, P1, was self-assembled in water to yield spherical micelles that consist of a PEO block forming a hydrophilic shell and a copolymer of light-responsive NBA and mercury (II) ion-detecting HPDEA block forming a hydrophobic core. Upon the addition of mercury (II) ions to this micellar solution, no detection was observed since water-soluble mercury (II) ions have limited accessability to the oxime units of P1, which are located in the hydrophobic core. After UV light irradiation, however, the photolabile 2-nitrobenzyl moieties were cleaved and hydrophobic PNBA transformed to hydrophilic poly(acrylic acid) (PAA), leading to the photo-induced dissociation of micelles to unimers. As a result, the oxime units of P1 were exposed to a hydrophilic environment and could react with mercury (II) ions to form nitrile groups, resulting in the turn-on detection of mercury (II) ions by UV light irradiation.

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INTRODUCTION Mercury (II) is one of the most toxic heavy metal ions in nature and can cause many severe diseases, even at ppm levels of accumulation in the body.1-2 Accordingly, there is a continuous need to develop rapid, sensitive, and selective sensors for mercury ions.3 Among the many reliable techniques for the detection of mercury ions, atomic absorption spectrometry and inductively coupled plasma mass spectrometry are the most commonly used methods.4-5 On the other hand, they are unsuitable for in-field applications due to the lack of portability and cost effectiveness. In this regard, reaction-based colorimetric chemosensors have attracted considerable attention owing to their high selectivity, sensitivity, and capability of naked eye-detection.6-8 Various kinds of chemosensors derived from small molecular organic/organo-metallic materials have been investigated, but most suffer from structural instability and low solubility in water, limiting their practical applications in environmental and biological conditions.9-10 To overcome these issues, water-soluble polymers with the small incorporation of receptor moieties via covalent attachment or co/block copolymerization have been developed.11-12 Stimuli-responsive polymers show dramatic variations in their chemical and/or physical properties upon exposure to external triggers, such as temperature,13 pH,14-15 and light.16-17 Although stimuliresponsive polymers have numerous potential applications, their use in sensing and detection are still premature.18 The application of stimuli-responsive polymers to the sensing area is attractive because tunable detection sensitivity and adjustable water solubility can be achieved.19-21 Amphiphilic block copolymers can self-assemble into polymeric micelles in aqueous solutions.22-23 If the core-forming hydrophobic block is light-responsive, the micelles can dissociate into unimers by light irradiation.24-25 The light-induced disruption of micelles has many applications, but drug delivery systems are the most studied field,26-27 in which the photocontrolled release of the encapsulated drugs is possible.28-30 In this study, a light-responsive amphiphilic block copolymer, [p(NBA-co-HPDEA)], was designed for the photocontrolled sensing of mercury (II) ions in aqueous solutions. To the best of the authors’ knowledge, this is the first example of the light-triggered turn-on colorimetric detection of mercury (II) 3 Environment ACS Paragon Plus


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ions by light-responsive block copolymer micelles. Combining the light-responsive nature of PNBA with the sensitive and selective detection ability of PHPDEA leads to smart block copolymer micelles that are capable of the photo-tunable detection of mercury (II) ions.

EXPERIMENTAL Materials 2,2′-Azobisisobutyronitrile (AIBN, Aldrich, 98%) was recrystallized from ethanol prior to use. N,N-Dimethylformamide (DMF, 99.8%), N-(2-hydroxyethyl)piperazine-N′-(2-ethanesulfonic acid) (HEPES), 4-nitrobenzaldehyde, acrylic acid, 4-(dimethylamino)pyridine (DMAP), sodium nitrite, poly(ethylene glycol) methyl ether (PEO, average Mn ~5000), acryloyl chloride (98%), N-(3dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC), 2-dimethylaminoethanol, and dichloromethane (DCM, 99.9%) were purchased from Sigma-Aldrich and used as received. Ethyl 2bromoisobutylate

(EBiB),

nickel

(II)

chloride,

anisol

(99%),

and

N,N,N’,N’’,N’’-

pentamethyldiethylenetriamine (PMDETA) were purchased from TCI. Triethylamine (99%), 2nitrobenzyl alcohol (97%), and tin (II) chloride were purchased from Alfa Aesar. Cadmium (II) chloride, copper (II) perchlorate hexahydrate, zinc (II) perchlorate hexahydrate, cobalt (II) perchlorate hexahydrate, manganese (II) perchlorate hydrate (99%), and CuBr (98%) were of the highest purity supplied by Aldrich and used as received. Instrumentation 1H NMR spectroscopy (Bruker Avance 300 MHz, Varian) was employed with CDCl3 and DMSO-d6 as solvents. The apparent molecular weights and molecular weight distributions were measured by gel permeation chromatography (GPC, Agilent technologies 1200 series) using a poly(methyl methacrylate) (PMMA) standard and DMF as the eluent at 30 oC with a flow rate of 1.00 mL/min. The UV-vis spectra were recorded using a Varian Cary-100 UV-vis spectrophotometer. For the irradiation experiments, the micelles were irradiated with UV light at 365 nm using a UV lamp (VILBER LOURMAT, VL-4LC, 4 W) operated at 365 nm. The hydrodynamic diameters were measured by dynamic light scattering (DLS, Nano ZS, Malvern, UK) using zetasizer software 7.01. Polymer 4 Environment ACS Paragon Plus

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solution (0.01 wt. %, 1.2 mL) was filtered through a 0.50 µm syringe filter prior to measurement. Average size and % volume values were calculated from the three replicate measurements and reported as mean diameter ± standard deviation. The morphology of the polymer aggregates was investigated by atomic force microscopy (AFM, Park Systems NX10). For AFM study, an aliquot of polymer solution (0.005 wt. %, 6µL) was spin-coated (1500 rpm, 60s) on to a mica substrate prior to measurement. Synthesis (E)-2-((4-((4-Formyl phenyl)diazenyl)phenyl)(methyl)amino) ethyl acrylate (FPDEA)31, 2nitrobenzyl acrylate (NBA)15, and PEO113-Br32 were prepared as previously reported. PEO113-b-[p(NBA10-co-FPDEA3)] NBA (1.16g, 5.6 mmol), FPDEA (0.8g, 2.4 mmol), PEO-Br (0.4g, 0.08 mmol), PMDETA (16.7 µL, 0.08 mmol), and anisole (3.50 mL) were added to a 10 mL Schlenk flask equipped with a magnetic stir bar. Oxygen was removed by three freeze-pump-thaw cycles, which was followed by the addition of CuBr (0.0114 g, 0.08 mmol) under argon. Polymerization was conducted at 80 °C for 27 h. The reaction was quenched by opening the flask to air. The catalyst was removed by passing the solution through a neutral alumina column. The polymer was precipitated by adding the solution to cold diethyl ether, at which point the product was dried overnight under vacuum at room temperature. (DP of NBA = 10, DP of FPDEA = 3, as determined by 1H-NMR spectroscopy). Mn = 18 000 g/mol, Mw/Mn = 1.16. 1H NMR (300 MHz, CDCl3, δ in ppm): 10.10-9.90 (1H, -OH); 8.207.70 (6H, ArH, FPDEA part); 7.70-7.20 (4H, ArH, NBA part); 6.91 - 6.57 (2H, ArH, FPDEA part); 5.85.0 (2H, -CH2-O-(C=O)); 4.5-4.0 (4H, -CH2-CH2-O-(C=O)); 3.98-3.29 (113H, PEO part). PEO113-b-[p(NBA10-co-HPDEA3)] (P1). PEO113-b-[p(NBA10-co-FPDEA3)] (0.1 g, 46.24 µmol per HPDEA repeating unit), and hydroxylamine-hydrochloride (14.8 mg, 0.185 mmol) were heated in dry ethanol (10 mL) for 12 h under reflux. After evaporating the solvent, the resulting polymer was dissolved in THF and precipitated to hexane. The polymer was dried overnight under vacuum at room temperature to give P1 as an orange powder. Mn= 18 100 g/mol, Mw/Mn= 1.19. 1H NMR (300 MHz, CDCl3, δ in ppm): 8.27-7.70 (6H, ArH, HPDEA part); 7.69-7.26 (4H, ArH, NBA part); 6.91-6.56 (2H, ArH, HPDEA part); 5.67 - 5.03 (2H, -CH2-O-(C=O)); 4.92 - 4.78 (1H, -CH =N-OH); 4.3 – 3.95 (4H, 5 Environment ACS Paragon Plus


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CH2-CH2-O-(C=O)); 3.94-3.33 (113H, PEO part).

RESULTS AND DISCUSSION

Scheme 1. Synthesis of an amphiphilic block copolymer with photocleavable nitrobenzyl moieties and oxime groups attached to the azo chromophores as a receptor to detect mercury (II) ions.

Figure 1. Overlaid GPC traces of PEO, PEO-Br, PEO-b-[p(NBA-co-FPDEA)], and PEO-b-[p(NBA-coHPDEA)] (P1).


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Figure 2. 1H-NMR spectra of (a) PEO-b-[p(NBA-co-FPDEA)] and (b) PEO-b-[p(NBA-co-HPDEA)] (P1). Scheme 1 shows the synthetic strategy used in this study. The hydrophobic copolymer block of NBA and FPDEA was extended from a hydrophilic PEO-Br macroinitiator via ATRP.33 Anisole was used as a solvent, and the CuBr/PMDETA catalyst system was employed for block copolymerization. The initial ratio of NBA, FPDEA, and PEO-Br macroinitiator was 70:30:1, and polymerization was stopped when the conversion of NBA and FPDEA reached 20 % and 10 %, respectively (DP, theory of PNBA = 14 and DP, theory of FPDEA = 3). We have restricted the monomer conversion of NBA up to 20 % because NBA monomer is difficult to polymerize due to retardation effect caused by the nitro-aromatic group.34 The molecular weight and molecular weight distribution of PEO-b-[p(NBA-co-FPDEA)] were determined 7 Environment ACS Paragon Plus


on a GPC DMF line using PMMA standards (Mn = 18 000 g/mol, Mw/Mn = 1.16) (Figure 1), showing a clear shift to a high molecular weight region. The experimental DP of P1 was obtained by 1H NMR spectroscopy by calculating the integration area of the protons next to the ester group (j) of NBA at 5.3 ppm, the aldehyde protons (a) of FPDEA at 10.1 ppm, and the signals (k) at 3.6 ppm of the PEO repeat unit (-OCH2CH2-) (DP,

NMR

of PNBA = 10 and DP,

NMR

of FPDEA = 3) (Figure 2a). After

polymerization, the aldehyde groups of PEO-b-[p(NBA-co-FPDEA)] were converted to aldoxime groups by reacting with an excess of hydroxylamine to ensure 100% conversion, leading to the formation of a final oxime-containing polymeric probe, PEO-b-[p(NBA-co-HPDEA)], P1. The 1H NMR spectra provided evidence for quantitative post-polymerization modification. A new peak (a’) representing the –CH=N-OH protons of HPDEA appeared at 4.9 ppm (Figure 2b), whereas the aldehyde peaks (a) of FPDEA disappeared completely (Figure 2a). In addition, the ratio of the integration area of the protons next to the ester group (j) of NBA at 5.3 ppm and –CH=N-OH protons (a’) of HPDEA at 4.9 ppm was 20:3 (Figure 2b), indicating that post-polymerization modification was almost quantitative. The GPC traces in Figure 1 also showed that there were no significant changes in the apparent molecular weight for P1 compared to PEO-b-[p(NBA-co-FPDEA)].

40

Volume %

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(b)

30

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Hydrodynamic Diameter (nm) 8 Environment ACS Paragon Plus

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10

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Figure 3. Hydrodynamic size distributions of dynamic light scattering (DLS) analysis of P1 (a) before (■) and (b) after light irradiation (●) at 25 oC in 0.01 wt. % aqueous micelles.

When designed appropriately, the amphiphilic block copolymer (P1) would form self-assembled aggregates, such as micelles, in an aqueous solution. Here, a hydrophobic p(NBA-co-HPDEA) block forms the inner core of the aggregates, whereas the outer shell consists of hydrophilic PEO. UV light irradiation (365 nm) leads to the dissociation of 2-nitrobenzyl groups and transforms a hydrophobic PNBA block to the hydrophilic PAA block.35 Because the DP of PNBA and PHPDEA is 10 and 3, respectively, the hydrophobic block is rendered sufficiently hydrophilic via this light-triggered transformation. As a result, polymeric micelles can be dissociated completely to molecularly dissolved unimers. An aqueous micellar solution (0.01 wt. %) was prepared by adding water to a solution of P1 in THF. Briefly, a 1.0 mg sample of P1 was dissolved in 1 mL of THF, and 10 mL of water was then added dropwise overnight to form micelles while the THF was allowed to evaporate completely. The solution was filtered through a 0.22-mm filter prior to use. The resulting solution was then irradiated with UV light for 2 min. DLS was performed to observe the light-induced decrease in the hydrodynamic size resulting from micellar disruption. Figure 3 shows the size distributions obtained from DLS before and after UV irradiation for P1. The average hydrodynamic diameter of the original micelles was 49.6 nm, which decreased to 3.0 nm after UV irradiation. This confirmed the successful transformation of polymeric micelles to unimers.



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Figure 4. Representative AFM phase images of P1 solutions spin-coated on mica (a) before and (b) after UV light irradiation.

The photoinduced disruption of micelles was further confirmed by atomic force microscopy (AFM), which showed that the micelles had undergone morphological changes. AFM images of the samples (0.005 wt. %) deposited directly on mica after micellization revealed the presence of uniform, welldispersed individual globular micelles with a mean diameter of 50 nm (Figure 4a). The micelle solution of P1 was then exposed to 365 nm UV light for 2 min and spin-coated quickly onto a mica substrate for AFM analysis. Previous well-defined globular micelles gave way to molecularly resolved individual polymer chains (Figure 4b), indicating that the light-triggered cleavage of 2-nitrobenzyl groups induced micellar disruption.


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Figure 5. (a) Schematic diagram of the light-responsive block copolymer micelles for the light-induced turn-on colorimetric detection of mercury (II) ions and (b) light-induced disruption of micelles, followed by the detection of mercury (II) ions via the formation of strong electron-withdrawing nitrile groups promoted by mercury (II) ions.



0.6

0.6

P1 micelles Before UV irradiation After UV irradiation 2+ [Hg ] 1 mM

0.4

2 mM 3 mM 4 mM 5 mM 6 mM 7 mM 8 mM 9 mM 10 mM

[Hg ] 0 mM 1 mM 2 mM 3 mM 4 mM 5 mM 6 mM 7 mM 8 mM 9 mM 10 mM

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Absorbance

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Wavelength (nm)

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10 mM of Hg [UV irradiation time] 30 s 60 s 90 s 120 s 150 s

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(d)

(c) 300

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∆λmax

Absorbance

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0 400

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2+

Hg

2+

Cd

Wavelength (nm)

2+

Cu

2+

Co

2+

Zn

Ni

2+

2+

Mg

Metal Ions

Figure 6. UV-vis absorption spectra of 0.005 wt. % micellar solution of P1 (22 µM of oxime units) with various concentrations of mercury (II) ions in an aqueous solution at 25 oC (a) before and (b) after light irradiation (150 s). (c) UV-vis absorption spectra of P1 micelles with the addition of 10 mM mercury (II) ions, followed by UV light irradiation for 150 s. (d) Selectivity bar diagram of P1 micelles after light irradiation with various metal ions (∆λmax = λχ-λ0, where λ0 is the absorption maximum of P1 and λχ is absorption maximum after the addition of 8 mM of various cations).

Having demonstrated the light-induced transitions of micelles to unimers, an attempt was made to utilize this system for the photo-tunable detection of mercury (II) ions (Figure 5a). After micellization, oxime-containing HPDEA units are located in the inner hydrophobic core, where mercury (II) ions have no access. The subsequent UV irradiation converts a p(NBA-co-HPDEA) block to a p(AA-co-HPDEA) 12 Environment ACS Paragon Plus

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block. Although PHPDEA units with DP 3 retain some hydrophobic character, the p(AA-co-HPDEA) block becomes hydrophilic due to PAA with DP 10 being dominant. As a result, the oxime groups of PHPDEA are exposed to the hydrophilic environment, where mercury (II) ions are present, enabling the detection of mercury (II) ions via the transformation of oxime to a strong electron-withdrawing nitrile group (Figure 5b). For this, an aqueous micellar solution (0.005 wt. %) was prepared in a HEPES buffer (pH = 5), where the detection of mercury (II) ions is optimized.31 The mercury (II) ion-sensing properties of P1 micelles (22 µM of oxime moieties) were examined by UV-Vis absorption spectroscopy. As shown in Figure 6a, there was no shift in the absorption maximum of P1 at 437 nm observed upon the gradual addition of mercury (II) ions up to 10.0 mM. The red shift to 525 nm was not achieved until 220.0 mM mercury (II) ions had been added, above which no further shift was observed (Figure S1). The lowest limit of detection (LOD) of a P1 micellar solution before UV irradiation for the detection of mercury (II) ions was obtained by linear regression36-37 and was 20.0 mM (Figure S2). The same micellar solution (0.005 wt. %) in a HEPES buffer (pH = 5) was irradiated with UV light for 150 s. The UV-Vis absorption spectrum of the solution after UV irradiation was similar to that before UV irradiation (red line in Figure 6b), indicating that the absorbance at 435 nm had originated from HPDEA units. Upon the gradual addition of mercury (II) ions up to 10.0 mM, the irradiated solution exhibited an 85 nm red-shift in the absorption maximum with an isobestic point at 485 nm (Figure 6b). As a consequence, an obvious yellow to purple color change in the solution occurred, which could be observed easily by the naked eye (Figure 5b, photograph). The lowest LOD of the solution after UV irradiation for the detection of mercury (II) ions was found to be 0.20 mM (Figure S3), which is almost one hundred times lower than that before UV irradiation. This remarkable enhancement in detection sensitivity was attributed to the ready exposure of receptor units, oxime groups of PHPDEA, to mercury (II) ions after UV irradiation. The light-induced morphological changes presented here can be used for the latent detecting system. To assess this concept, the UV-irradiation time-dependent detection of mercury (II) ions was evaluated. After the micellization of P1, 10.0 mM of mercury (II) ions was added to the solution. In this study, 10.0 13 Environment ACS Paragon Plus


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mM was chosen because this concentration is too low for the detection before UV irradiation, but sufficient for the significant detection after UV irradiation of the probe solution (Figure 6a and 6b, S1). As expected, no detection was monitored. Upon UV irradiation, however, the turn-on detection of mercury (II) ions occurred. The photostationary state with a entire red shift was reached within 150 s, which was accompanied by an obvious color change (Figure 6c, S4). Overall, the detection of mercury (II) ions by a dormant P1 micellar probe was activated by UV irradiation, demonstrating the phototunable turn-on detection capabality. To evaluate the selectivity of the probe (22 µM) toward mercury (II) ions, a range of metal cations (8 mM), such as Cd2+, Cu2+, Co2+, Zn2+, Ni+, and Mg2+, were screened after UV irradiation of P1 micellar solution. None of the other metal cations showed appreciable shifts in the absorption maxima, suggesting the excellent selectivity of P1 toward Hg (II) ions (Figure 6d, S5)

CONCLUSIONS A well-defined block copolymer of PEO-b-[p(NBA-co-FPDEA)] with a low polydispersity index was synthesized by ATRP and converted to PEO-b-[p-(NBA-co-HPDEA)], P1. P1 self-assembled into spherical micellar aggregated in water. No detection of mercury (II) ions was observed initially, but micellar disruption by light irradiation resulted in turn-on detection. Therefore, the light-triggered turnon colorimetric detection of mercury (II) ions was achieved by light-responsive block copolymer micelles. The general strategy presented herein can potentially be used for the preparation of polymeric micelles capable of the light-induced tunable detection of various kinds of water-soluble analytes.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website.


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UV-vis absorption spectra of P1 over the addition of various metal cations and Hg (II) ions; kinetic plots as a function of irradiation time of P1; linear regression curve (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.L.). ORCID Hyung-il Lee: 0000-0001-9965-7333

ACKNOWLEDGMENT This work was supported by the Basic Science Research Program (NRF-2017R1A2B4003861) administered by the National Research Foundation of Korea, funded by the Ministry of Science, ICT, and Future Planning of Korea.

REFERENCES (1) Hutton, M. Human Health Concerns of Lead, Mercury, Cadmium and Arsenic. Lead, mercury, cadmium and arsenic in the environment 1987, 31, 53-68. (2) Carter, K. P.; Young, A. M.; Palmer, A. E. Fluorescent Sensors for Measuring Metal Ions in Living Systems. Chem. Rev. 2014, 114 (8), 4564-4601. (3) Hu, J.; Dai, L.; Liu, S. Analyte-Reactive Amphiphilic Thermoresponsive Diblock Copolymer Micelles-Based Multifunctional Ratiometric Fluorescent Chemosensors. Macromolecules 2011, 44 (12), 4699-4710. (4) Narin, Đ.; Soylak, M.; Elçi, L.; Doğan, M. Determination of Trace Metal Ions by AAS in Natural Water Samples After Preconcentration of Pyrocatechol Violet Complexes on an Activated Carbon Column. Talanta 2000, 52, 1041-1046. 15 Environment ACS Paragon Plus


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Graphical abstract

Title: Polymeric Micelles Based on Light-Responsive Block Copolymers for the Photo-Tunable Detection of mercury (II) Ions Modulated by Morphological Changes


Polymeric micelles based on light-responsive block copolymers for the

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