Recyclable Polymeric Thin Films for the Selective Detection and

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Recyclable Polymeric Thin Films for the Selective Detection and Separation of Picric Acid Moumita Gupta, and Hyung-il Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15369 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 8, 2018

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ACS Applied Materials & Interfaces

Recyclable Polymeric Thin Films for the Selective Detection and Separation of Picric Acid Moumita Gupta and Hyung-il Lee Department of Chemistry, University of Ulsan, Ulsan, 680-749, Republic of Korea

Corresponding Author: [email protected]



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ABSTRACT Thin-film probes have been developed for the reversible detection and separation of picric acid (PA) with extreme sensitivity in aqueous media. The free radical copolymerization of dimethylacrylamide (DMA), benzophenone acrylamide (BPAM), and glycidyl methacrylate (GMA) with a feed ratio of 95:1:4 yielded [p(DMA-co-BPAM-co-GMA)] (P1). P1 was transformed to the final polymeric probe, P2, by a subsequent ring-opening reaction between N(pyren-1-ylmethyl)propan-1-amine with the epoxide unit of P1. P2 exhibited rapid and selective sensing properties toward PA in aqueous media via turn-off fluorescence emission. The detection sensitivity was tuned precisely by varying the pH of the solution. After the immobilization of P2 on a quartz slide by spin-coating, followed by exposure to UV light, the resulting film exhibited an attogram-level detection limit toward PA. The photo-induced electron transfer (PET) together with an energy transfer process between PA and the pyrene units of P2 were maximized by the strong π–π stacking of pyrene units of P2, which in turn induced rapid exciton energy diffusion. Furthermore, the separation of PA from the mixture of the various nitroaromatic compounds by the P2 film was achieved. While the detection process of PA was reversible and repeatable over multiple cycles, the P2 film could be recycled.

Keywords: Picric acid; Detection; Fluorometric; Thin film; Pyrene, pH-responsive

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INTRODUCTION Nitroaromatic compounds have been used for the preparation of rocket fuels, leathers, pharmaceutical intermediates, and particularly explosive materials.1 Their explosive nature highlights the need for sensitive and selective detection techniques. While 1,3,5-trinitroperhydro1,3,5-triazine (RDX), and 2,4,6-trinitrotolune (TNT) are used extensively as explosives, 2,4,6trinitro phenol (picric acid (PA)) is another powerful explosive. As PA has a low safety coefficient and high detonation velocity, there is a need to develop sensors to detect PA sensitively and selectively. In addition, trace amounts of PA are harmful to the human body, causing skin and eye irritation, as well as chronic diseases, such as anemia, liver malfunction, and cancer.2 The contamination of groundwater with explosives has increased the need to develop sensors that operate in 100 % aqueous media.3-4 Thus far, detection methods applied to PA sensing include chromatography,5-6 surface-enhanced raman spectroscopy,7 amperometry,8 and energy-dispersive X-ray analysis.9 These techniques, however, have some limitations, such as pre-treatment of the sample, needed for well-trained personnel,

interference

from

other

compounds,

low

sensitivity,

and

sophisticated

instrumentation.10 On the other hand, fluorescence techniques have attracted attention for the detection of PA owing the super-sensitivity, great selectivity, low cost, operation simplicity, portable instrumentation, and facile sample preparation.11-12 To develop a fluorescence sensor, pyrene, a type of organic semi-conductor material, is the most preferred fluorophore, which has a high quantum yield, good chemical stability, and long fluorescence lifetime. This has wide applications in sensing, organic photovoltaic systems, and supramolecular building blocks.13-14 In particular, pyrene derivatives can maintain their high fluorescence efficiency in solution and in film state, highlighting the potential for future fabrication into devices. 15



Until now, various fluorescent systems have been developed for the turn-off detection of PA, such as polymeric films,16-17 conjugate polymers,10, 18-19 quantum dots,20 micro/nano-aggregates,21 and metal-organic frameworks (MOF).11, 22 On the other hand, the development of fluorescence sensors for PA with super-sensitivity and high selectivity in 100% aqueous medium has been highly demanding and challenging in recent times. Recently, a pyrene-based small molecular probe was reported to detect PA at the µM level,23 a supramolecular assembly was developed to detect PA at the femtogram level.24 In this study, polymeric thin-films were developed for the detection and separation of PA in aqueous media. Here, the polymeric probes consisted of pyrene fluorophores as a sensing unit for the detection of PA, tertiary amine groups attached to the pyrene fluorophores as a receptor to detect PA by turn-off fluorescence quenching, and cross-linkable benzophenone moieties to immobilize the polymeric film onto quartz slides. The electrostatic interactions between the positively charged polymeric probe and picrate in aqueous solution (after dissociation of PA in water) quenched the fluorescence emission intensity of pyrene, which in turn made the separation of PA possible. In addition, the detection sensitivity can be tuned by changing the pH of the solution.

EXPERIMENTAL SECTION Materials Glycidyl methacrylate (GMA, Aldrich, 99.0%) and dimethylacrylamide (DMA, Aldrich, 99.0%) were passed through a column filled with basic alumina prior to polymerization. 2,2′-Azobis (isobutyronitrile) (AIBN, Aldrich, 98%) was recrystallized from EtOH before use. 2dodecylsulfanylthiocarbonylsulfanyl-2-methylpropionic acid (CTA), 2,4-Dinitrophenol (2,4-


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DNP), 4-nitrophenol (4-NP), 4-nitrotoluene (4-NT), 2,4-dinitrotoluene (2,4-DNT), 2,6dinitrotoluene (2,6-DNT), 1,3-dinitrobenzene (1,3-DNB), 1,4-dinitrobenzene (1,4-DNB) and dimethylnitrobenzene (DMNB) were purchased from Aldrich at the highest purity available and used as received. The solvents were obtained from commercial suppliers and used without further purification. Instrumentation The 1H nuclear magnetic resonance (1H NMR) spectroscopy was conducted with a Bruker Avance 300 MHz. Gel permeation chromatography (GPC, Agilent Technologies 1200 series) was conducted in DMF using a polymethyl methachrylate (PMMA) standard at 30 °C with a flow rate of 1.00 mL/min. The emission spectra were obtained with a HORIBA FluoroMax-4Pm spectrophotometer. The time-correlated single-photon counting (TCSPC) method on a FS5 spectrophotometer (Edinburgh Instruments) was equipped with an EPLED-330 ps pulsed LED laser at 298 K. A 125-W medium-pressure Hg lamp with the wavelength filters (365 nm) was used as the light source for the immobilization of polymer films on a quartz slide. Extraction of PA was monitored by high performance liquid chromatography (HPLC, Agilent Technologies 1200 series, having XTERRA RP 18, 5 µM column). The mixture of acetonitrile and water (70:30, v/v) was used as a mobile phase with a constant flow rate of 1.0 mL/min and 50 µL was injected for each study. Absolute photoluminescence quantum yields (PLQYs, ΦpH3 and ΦpH11) of P2 solution at pH 3 and pH 11 were measured on an absolute PL quantum yield spectrophotometer (Quantaurus-QY C11347-11, Hamamatsu Photonics) equipped with a 3.3 inch integrating sphere.

Synthesis of N-(pyren-1-ylmethyl) propan-1-amine Propylamine (1.5 mg, 26.0 mmol) was added to a solution of 1-Pyrenecarboxaldehyde (2g, 8.69 mmol) in MeOH and refluxed at 60 °C



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for 5 h. After cooling the solution to room temperature, NaBH4 was added and the resulting solution was stirred for 24 h. A 5 M HCl solution was then added to neutralize the superfluous NaBH4. After the filtration, followed by evaporation of MeOH, the resulting solution was extracted with ethyl acetate and concentrated to afford the product.25 1H NMR (CDCl3, 300 MHz, δ in ppm): 8.31-7.97 (9H, m, Py-H), 4.5 (2H, S, -CH2-Py), 2.75 (2H, s, N-CH2), 1.61 (2H, m, N-CH2CH2), 0.94 (3H, t, N-CH2CH2CH3). Synthesis of PA: PA was synthesized as previously reported (Figure S1)26. Synthesis

of

poly(glycidyl

methacrylate-co-benzophenone

acrylamide–co-

dimethylacrylamide) [p(DMA-co-BPAM-co-GMA)], P1 In a Schlenk flask, GMA (0.12 g, 0.8 mmol), DMA (2.00 g, 20.17 mmol), BPAM (0.052 g, 0.2 mmol), and AIBN (0.17 mg, 0.001 mmol) were placed into 10 mL of anhydrous DMF. After 0.5 h purging with argon, the reaction mixture was heated to 65°C for 12 h. The polymerization was stopped by exposing the solution to air. The solution was concentrated under vacuum, and precipitated twice into cold ether, dried under vacuum at room temperature for 24 h.27 1H NMR (300 MHz, δ in ppm) in CDCl3): 7.77-7.47 (9H, m, Ar-H), 4.35−3.87 (2H, dd, COOCH2), 3.030.84 (17H, m, -O-CH2, aliphatic protons, and -N(CH3)2). GPC: Mn = 39000, Mw = 93000, and PDI = 2.3. Post-polymerization modification reaction of P1 to yield P2 (0.20 g, 1.4 mmol, assuming 5% incorporation of GMA in the backbone) was placed in 10 mL acetonitrile and N-(pyren-1ylmethyl) propan-1-amine (0.67 g, 2.1 mmol) was then added to the solution. The reaction mixture was heated to 90 °C. After 48 h, the reaction mixture was concentrated under vacuum, precipitated twice into cold ether, and dried under vacuum at room temperature for 24 h.25 1H NMR (300 MHz, δ in ppm) in CDCl3): 8.21-8.01 (9H, m, Py-H), 7.77-7.47 (9H, m, Ar-H), 4.32 (4H, S, -CH2-Py


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and -CHCH2CH3), 3.84 (1H, s, -COOCH2), 3.13-0.89 (17H, m, aliphatic protons, and -N(CH3)2). GPC: Mn = 42000, Mw = 95000, and PDI = 2.3. HPLC Analysis The number of moles of PA was calculated by using anthranone as an internal standard (IS) with a ratio of 8:2(v/v) (solution of analytes/anthranone solution). The percentage of mole fraction was calculated by using the standard formula: Amount of PA= [1-(xcyn / xcyo)] × 100 Where, xcyo= no. of moles of PA in solution of analytes before dipping the film and xcyn= no. of moles of PA in solution of analytes after dipping the film in corresponding cycles (n= no. of cycles).

RESULTS AND DISCUSSION

Scheme 1. Synthesis of pyrene-derived polymeric probe P2.



Figure 1. 1H NMR spectra of (a) P1 and (b) P2.

Pyrene-derived pH-responsive polymeric probe, P2, was synthesized, as shown in scheme 1. GMA, DMA and BPAM were copolymerized by free radical polymerization with a feed ratio 95:1:4 (DMA:BPAM:GMA) to obtain P1. 1H NMR spectroscopy was used to confirm the successful synthesis of P1 and calculate the final incorporation ratio (Figure 1a). The appearance of the diastereotopic –OCH2- protons of GMA at 4.35 and 3.87 ppm, aromatic protons of BPAM at 7.47-7.77 ppm, and the dimethyl protons of DMA at 2.3-3.3 ppm confirmed the successful synthesis of P1 (Figure 1a). From the ratio of these peak areas, the final incorporation ratio of DMA:BPAM:GMA was calculated to be 94: 1:5, which was similar to the initial feed ratio. The


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number average molecular weight of polymer P1 was 39,900 with a polydispersity (Mw/Mn) of 2.3. P1 was subjected to a post-polymerization modification reaction with PyPA (Figure S2) to obtain P2. The appearance of the aromatic protons of the pyrene units of P2 at 8.37-8.05 ppm confirmed the successful incorporation of the pyrene chromophore into the polymer backbone (Figure 1b). The integral area of the diastereotropic protons from the GMA unit was compared with those of the pyrene unit to calculate the degree of the post-modification reaction (Figure S3). The postmodification reaction was almost quantitative. GPC analysis revealed P2 to have a number average molecular weight of 42,000 [Mw/Mn) = 2.3] (Figure S4).

Figure 2. Fluorescence spectra of P2 alone (2.1 × 10-7 M of pyrene units) in water at various pH.

The photophysical responses of P2 (2.1 × 10-7 M; assuming the 5% incorporation of pyrene units in the P2 backbone) were examined by emission spectroscopy in various pH solutions (Figure 2). 8 ACS Paragon Plus Environment


The emission intensity of the pyrene peak at 395 nm decreased gradually as pH of the medium increased. The trialkylamine groups of P2 can be protonated at low pH to form quaternary ammonium cations, resulting in an increase in the emission intensity via a suppression of the PET process. The availability of the lone pair electron on the N-alkyl position tended to increase with increasing pH of the solution, leading to a decrease in the emission intensity by the PET process 25, 28-32.

This trend was further verified by calculating quantum yield of P2 at pH 3 (ΦpH3 = 0.23)

and at pH 11 (ΦpH11 = 0.01), respectively. These pH-responsive properties allow P2 to behave as a fluorescent pH indicator.

Figure 3. (a) Emission spectral changes of P2 (2.1× 10-7 M of pyrene units) upon the gradual addition of PA (0 to 0.168 mM) in aqueous media at pH 3 and (b) selectivity of P2 with other structurally similar analytes in water (where I0 and I indicate the fluorescence emission of P2 in the absence and presence of PA, respectively).

PA-sensing studies were carried out in aqueous media at pH 3 where the probe showed the highest intensity. Marked quenching in fluorescence at 395 nm was observed as PA was gradually added 9 ACS Paragon Plus Environment

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into the solution of P2 (2.1× 10-7 M) (Figure 3a). This turn-off fluorescence detection can be observed by the naked eye under a UV lamp (See inset in Figure 3a). In an aqueous solution, PA dissociates to H+ and picrate immediately. The tertiary N-alkyl amine attached to the pyrene unit in P2 can be protonated by PA and an electrostatic interaction between picrate and the in-situgenerated quaternary ammonium cation of P2 leads to fluorescence quenching. The limit of detection (LOD) was found to be 56 µM from the linear regression curve (Figure S10a), indicating that P2 could be an excellent sensor for PA. To evaluate the selectivity of P2, other nitroaromatics, such as 4-NT, 2,4-DNT, 2,6-DNT, DMNB, 1,3-DNB, 1,4-DNB, 4-NP, and 2,4-DNP, various cations and anions were tested in aqueous media. As most nitroaromatics have a similar electron deficiency, it is very challenging to develop selective chemosensors for PA (Figure 3b and S5-S6). There was no interference found from the cations and anions (Figure S7-S8). While P2 exhibited the highest selectivity for PA compared to other nitroaromatics, nitroaromatics with a phenolic group, such as 4-NP, and 2,4-DNP also expressed slight fluorescence quenching. The reason for the highest selectivity for PA over 4-NP, and 2,4-DNP could be that the electrostatic interaction, between picrate and the in-situ generated quaternary ammonium cation of P2 is highest because PA is the strongest acid (pKa ~ 0.38).



Figure 4. (a) Extent of emission quenching upon the addition of PA (0.168 mM) at various pH (inset: photograph of P2 (2.1× 10-7 M of pyrene units) before and after the addition of PA) and (b) schematic representation for the mechanism of pH-tunable detection sensitivity of P2 upon the addition of PA in H2O.

Having examined the turn-off fluorescence detection of PA, the pH-dependent sensing behaviors were studied. The relative fluorescence intensity at 395 nm was measured before and after the addition of PA in various pH solutions. The extent of fluorescence quenching was maximized at low pH because the emission intensity is highest at low pH. The detection sensitivity decreased with increasing pH of the solution (Figure 4a and S9). The pH-dependent difference in sensitivity could be observed clearly by the naked eye (Figure 4a inset). The representative schematic diagram of pH-driven control in PA detection sensitivity is shown in Figure 4b. To gain more insight into the mechanism for the fluorescence quenching of P2 by PA, the SternVolmer (S-V) constant was evaluated. The S-V constant is the slope of I0/I vs. [Q] plot, (where I0 and I indicate the fluorescence emission of the donor in the absence and presence of PA, respectively, and [Q] represents the various concentrations of the picric acid (Figure S10b). The S-V constant of P2 for PA was calculated to be 7.75× 104 (calculated by linear fitting of S-V plot, at low concentration of PA upto 84 µM), which is quite high, explaining the high sensitivity. The non-linear nature of the S-V plot implies that the fluorescence quenching was induced by electron transfer together with an energy transfer process from the pyrene units of P2 to PA due to the strong electrostatic interaction between them. The PET-triggered fluorescence quenching of P2 by PA could be explained by an energy level diagram, as shown in Figure S10c. As reported previously,33 the LUMO of the pyrene unit is higher


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than that of PA, facilitating PET from P2 to PA along with the subsequent participation in nonradiative decay. As a consequence, the fluorescence of pyrene was quenched during the addition of PA. In addition to the PET process, energy transfer was also expected to participate in fluorescence quenching. In general, the energy transfer process from the donor (pyrene) to acceptor (PA) could be verified exclusively by the spectral overlap between the emission spectra of the donor and the absorbance spectra of the acceptor. The significant spectral overlap between the emission spectra of the donor pyrene of P2 and the absorbance spectra of the acceptor PA suggests that energy transfer between the donor and acceptor is significant (Figure S10d). The fluorescence quenching phenomenon of P2 was examined further by the time-resolved excited state lifetime decay (Figure S11). The exponential fluorescence decay profiles showed that the lifetime of P2 alone was 77 ns at 395 nm (λex = 345 nm), but that of P2 in the presence of PA (84 µM) was 66 ns. This substantial difference in lifetime was attributed mainly to the dynamic interaction of PA with the pyrene unit of P2.



Figure 5. (a) Schematic diagram of the film casting and the mechanism for the attogram level detection of PA, (b) fluorescence spectra of the P2 film with various concentration of PA (inset: the photograph of P2 and P2 + PA under UV light), and (c) photograph of the P2 film upon the addition of various concentrations of PA under UV light.

The instant surface detection of toxic nitro-explosives, such as PA, is necessary for in-field applications. To achieve this task, P2 was spin-coated onto the quartz slide.27 The dried P2 film was exposed to UV light (365 nm) for 10 min with a mask on. The resulting P2-immobilized film was washed several times with THF to remove the unbound free polymers, leading to a crosslinked thin film (Figure 5a). When PA was added gradually, the fluorescence of the P2 film was quenched, observed in the solution (Figure 5b). It should be noted that the lowest amount of PA


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required for fluorescence quenching was found to be 11.4 attogram (10-14 M) (Figure 5b). The bright blue emission of the P2 film was quenched rapidly upon the addition of of PA under UV light exposure (Figure 5c and S12). The reason for this amplified fluorescence quenching of P2 film, even for an extremely small amount of PA, could be the π-π stacking of the pyrene units, which offered an exciton hoping process via intermolecular electronic coupling, facilitating the long range exciton diffusion in the well-assembled solid state (Figure 5a).23, 34 To support our proposed mechanism for the attogram-level detection of PA via highly packed pyrene chromophore, the optical density (OD) of an absorption maximum of pyrene units of P2 at 345 nm in the solution (THF) and the film was fixed at 0.1 and fluorescence spectra of P2 in both states were compared (Figure S13). The emission intensity of P2 solution was higher than that of P2 film due to the molecular aggregation of pyrene molecules via the π-π stacking in the film state. The selectivity of the P2 film towards PA was examined over 2,4-DNP and 4-NP (nitroaromatics with phenolic group). The P2 film showed better selectivity than P2 solution (Figure S14).



Figure 6. (a) Separation of PA from the mixture of all other competitve analytes by HPLC technique, (b) reversible and repeatable detection of PA (10-3 M) by the P2 film up to four cycles: relative difference in the emission intensity at 395 nm in the presence and absence of PA, and (c) schematic representation of HPLC analysis.

The P2 film was able to separate PA from the mixture of different nitroaromatics (Figure 6a and 6c). The mixture of nitroaromatic compounds such as PA, 4-NT, 2,4-DNT, 2,6-DNT, DMNB, 1,3DNB, 1,4-DNB, 4-NP, and 2,4-DNP, were prepared in acetonitrile (10-3 M of each analyte). Every peak in the chromatogram was assigned by the retention times obtained from the individual chromatogram of each analyte (Figure S15). Initially, the P2 film was dipped into the solution mixture and stirred for 10 min to ensure the uniform absorption of the analytes. While the P2 film 15 ACS Paragon Plus Environment

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was taken out from the solution, washed completely with water, dried in a vacuum oven after each exposure to PA, an aliquot from the remaining solution mixture was subjected for HPLC analysis (Figure 6c). This process was repeated for four times, above which no significant separation of PA was observed. After four cycles, about 63 % of PA was removed from the mixture of the nitroaromatics (Figure 6a, Figure S15, see also experimental section for the calculation of % amount of PA remained after the treatment with the P2 film). The reusability of the P2 film was confirmed by fluorescence analysis (Figure 6b). After each cycle of separation process, the complete recovery of fluorescence was achieved by washing the P2 film with water. The electrostatic interaction between picrate and quaternary ammonium cation of P2 was readily broken by thorough washing with water, allowing P2 film to be reused. This stable and reusable film can be used for the fabrication of protective jackets and portable devices for real-life applications.

CONCLUSIONS This paper reported the reversible and selective detection of PA at the attogram level using a thin film of pyrene-derived polymeric probe, P2. This lowest detection level was attributed to the cooperative effect of an electrostatic interaction, electron transfer from the donor (fluorophore) to acceptor (PA), and importantly, to the presence of π-π stacking, which promotes rapid exciton hopping. P2 also showed the highest selectivity towards PA over other nitroaromatics due to the strong acidic behavior of PA. The fluorescence quenching phenomena of the probe P2 could also be tuned by varying the pH of the solution. While the P2 film was able to separate PA from the mixture of different nitroaromatics, it can be recycled for the detection and separation of PA by simple washing with water. Given the naked-eye detection capability at the attogram level with



the good stability and reusability, these P2 films may be used for the future fabrication of protective clothes and devices.

ASSOCIATED CONTENT Supporting Information. Supplementary data includes fluorescence spectra of P2 over the addition of various nitroaromatics, life-time decay profiles, pH-dependent property of P2, GPC traces, HPLC chromatogram, 1H NMR. “This material is available free of charge via the Internet at http://pubs.acs.org.”

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

ACKNOWLEDGMENTS 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) Sun, X.; Wang, Y.; Lei, Y. Fluorescence Based Explosive Detection: From Mechanisms to Sensory Materials. Chem. Soc. Rev. 2015, 44, 8019-8061. 17 ACS Paragon Plus Environment

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Graphical abstract

x O HO

O O

y N O

O2 N

z NH

OH

NO2

NO2

N

H2O

O

P2

Turn OFF

Crosslinked film

PA Detection

PA

0M

H2O

10-14 M 10-13 M

10-12 M 10-5 M 10-4 M 10-2 M


Fluorescence quenching

Title: Recyclable Polymeric Thin Films for the Selective Detection and Separation of Picric Acid

=

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Recyclable Polymeric Thin Films for the Selective Detection and

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