nanomaterials

Article Spectroscopic Study of the Salicyladazine 2+ Derivative–UO2 Complex and Its Immobilization to Mesoporous Silica

Sujin Park 1, Jaehyeon Park 1, Ji Ha Lee 2,*, Myong Yong Choi 1,* and Jong Hwa Jung 1,*

1 Department of Chemistry and Research Institute of Natural Sciences, Gyeongsang National University, Jinju 52828, Korea; [email protected] (S.P.); [email protected] (J.P.) 2 Department of Chemistry and Biochemistry, The University of Kitakyushu, Hibikino, Kitakyushu 808-0135, Japan * Correspondence: [email protected] (J.H.L.); [email protected] (M.Y.C.); [email protected] (J.H.J.); Tel.: +82-55-772-1488 (J.H.J.)  Received: 2 April 2019; Accepted: 26 April 2019; Published: 2 May 2019 

Abstract: Uranyl ion, the most soluble toxic uranium species, is recognized as an important index for monitoring nuclear wastewater quality. The United States Environmental Protection Agency (US EPA) and the World Health Organization (WHO) prescribed 30 ppb as the allowable concentration of uranyl ion in drinking water. This paper reports on a nanohybrid material that can detect uranyl ions spectroscopically and act as a uranyl ion absorbent in an aqueous system. Compound 1, possessing a salicyladazine core and four acetic acid groups, was synthesized and the spectroscopic properties of 2+ its UO2 complex were studied. Compound 1 had a strong blue emission when irradiated with UV 2+ 2+ light in the absence of UO2 that was quenched in the presence of UO2 . According to the Job’s 2+ plot, Compound 1 formed a 1:2 complex with UO2 . When immobilized onto mesoporous silica, 2+ a small dose (0.3 wt %) of this hybrid material could remove 96% of UO2 from 1 mL of a 100-ppb 2+ UO2 aqueous solution.

2+ Keywords: UO2 ; salicyladazine; fluorescence; mesoporous silica

1. Introduction The development of nuclear technology leads to new environmental concerns, such as radiation 2+ exposure and accidents resulting therefrom. Of special concern is the uranyl cation (UO2 ), a highly toxic neurotoxin that is very mutable in biological systems and can cause radioactive poisoning if proper containment rules are violated [1–4]. Therefore, the development of technologies that can 2+ measure the exact amount of UO2 exposed to the environment is an important safety priority. 2+ Several studies on UO2 sensors have been reported to date [5–10]. L. S. Natrajan et al. reported 2+ a method for detecting UO2 via a unique fluorescence energy transfer process to a water-soluble 2+ europium (III) lanthanide complex triggered by UO2 [11]. Yi Lu et al. developed colorimetric 2+ uranium sensors based on a UO2 -specific DNAzyme and gold nanoparticles using both labeled and label-free methods [12]. Julius Rebek Jr. et al. investigated a tripodal receptor capable of extracting uranyl ion from aqueous solutions. In their system, at a uranyl concentration of 400 ppm, the developed 2+ ligand extracted approximately 59% of the UO2 into the organic phase [13]. 2+ While various detection methods for UO2 have been developed based on fluorogenic and 2+ colorimetric methods, studies on UO2 adsorbents have received much less attention. We aim to synthesize an adsorbent, or uranyl-capture agent based on an organic–inorganic hybrid material because such compounds tend to have higher stability and controllable homogenous pore sizes.

Nanomaterials 2019, 9, 688; doi:10.3390/nano9050688 www.mdpi.com/journal/nanomaterials Nanomaterials 2019, 9, 688 2 of 8

Nanomaterials 2019, 9, x FOR PEER REVIEW 2 of 8 Therefore, we designed and synthesized compound 1 (Figure1). Compound 1 possesses four acetic acid groups as ligands for UO222++. The spectroscopic properties of compound 1 were observed upon acid groups as ligands for UO2 . The spectroscopic properties of compound 1 were observed upon adding UO222++ via fluorometry, IR , and1 1H NMR spectroscopy. Furthermore, compound adding UO2 via fluorometry, IR spectroscopy, and H NMR spectroscopy. Furthermore, compound 1 1 was immobilized to MPS (mesoporous silica nanoparticles) to create an adsorbent for UO22+2+. Herein, was immobilized to MPS (mesoporous silica nanoparticles) to create an adsorbent for UO2 . Herein, we report the spectroscopic properties of the compound 21+–UO22+ complex and the adsorption we report the spectroscopic properties of the compound 1–UO2 complex and the adsorption capacity capacity of mesoporous silica nanoparticles loaded with compound 12+ for UO22+ capture. of mesoporous silica nanoparticles loaded with compound 1 for UO2 capture.

Figure 1. Chemical structure of compound 1. Figure 1. Chemical structure of compound 1. 2. Materials and Methods

2.1.2. Materials Reagents and InstrumentsMethods

2.1. ReagentsAll reagents and Instruments were purchased from Sigma-Aldrich (Buchs, Switzerland) and Tokyo chemical industry (Fukaya, Japan). The solvent was purchased from Samchun Pure Chemicals (Pyeongkaek, All reagents were purchased from Sigma-Aldrich (Buchs, Switzerland) and Tokyo chemical Korea) and used with further purification. 1H and 13C NMR spectra were obtained with a Bruker DRX industry (Fukaya, Japan). The solvent was purchased from Samchun Pure Chemicals (Pyeongkaek, 300 apparatus (Rheinstetten, Germany). The IR spectra were measured on a Shimadzu FT-IR 8400S 1 13 Korea) and used with further purification. H and C NMR spectra were obtained1 with a Bruker DRX instrument (Kyoto, Japan) by KBr pellet method in the range of 4000 1000 cm− . A JEOL JMS-700 300 apparatus (Rheinstetten, Germany). The IR spectra were measured− on a Shimadzu FT-IR 8400S mass spectrometer (Kyoto, Japan) was used to obtain the mass spectra. The UV vis absorption and instrument (Kyoto, Japan) by KBr pellet method in the range of 4000−1000 cm−1. A− JEOL JMS-700 mass fluorescence spectra were obtained at 298 K with a Thermo Evolution 600 spectrophotometer (Waltham, spectrometer (Kyoto, Japan) was used to obtain the mass spectra. The UV−vis absorption and MA, USA) and a RF-5301PC spectrophotometer (Kyoto, Japan), respectively. A PerkinElmer 2400 fluorescence spectra were obtained at 298 K with a Thermo Evolution 600 spectrophotometer series (Waltham, MA, USA) was employed for the elemental analyses. The quantitative analysis was (Waltham, MA, USA) and a RF-5301PC spectrophotometer (Kyoto, Japan), respectively. A PerkinElmer performed using ICP-DRC-MS (ELAN DRC II, PerkinElmer, Waltham, MA, USA). The morphological 2400 series (Waltham, MA, USA) was employed for the elemental analyses. The quantitative analysis images were observed using a TEM (TECNAI G2 F30, FEI, Hillsboro, OR, USA). was performed using ICP-DRC-MS (ELAN DRC II, PerkinElmer, Waltham, MA, USA). The 2.2.morphological Synthesis of images Compound were1 observed using a TEM (TECNAI G2 F30, FEI, Hillsboro, OR, USA).

2.2. SynthesisThe compounds of Compound1–4 were1 synthesized according to the reported method (Scheme S1) [14,15]. Compound 2 (2.56 mg, 0.4 mmol) was dissolved in THF (10 mL), followed by the addition of sodium hydroxideThe compounds solution (10 mL,1–4 0.8were M). synthesized The mixture according was stirred to at the room reported temperature method for 2(Scheme h. After completionS1) [14,15]. ofCompound the reaction, 2 (2.56 the mixturemg, 0.4 mmol) was added was todissolved aq HCl solutionin THF (10 (1 wtmL), %) followed to give yellow by the precipitation, addition of sodium which hydroxide solution (10 mL, 0.8 M). The mixture was stirred at room temperature for 2 h.1 After was filtered off and dried under vacuum to yield compound 1 (121 mg, 57%). IR (KBr pellet) cm− 3424, completion of the reaction, the mixture1 was added to aq HCl solution (1 wt %) to give yellow 3014, 1728, 1495 1629, 1389, 1277, 1164; H NMR (300 MHz, DMSO-d6) δ 12.25 (s, 4H), 11.07 (s, 2H), 8.98 (s, 2H),precipitation, 7.63 (d, J = which2.2 Hz, was 2H), filtered 7.40 (dd, off Jand= 8.5, dried 2.2 Hz,under 2H), vacuum 6.96 (d, toJ = yield8.4 Hz, compound 2H), 3.78 1 (s, (121 4H), mg, 3.42 57%). (s, 8H); IR 13(KBr pellet) cm−1 3424, 3014, 1728, 1495 1629, 1389, 1277, 1164; 1H NMR (300 MHz, DMSO-d6) δ 12.25 C NMR (75 MHz, DMSO-d6) δ 172.74, 162.82, 158.30, 134.37, 131.08, 130.12, 118.33, 116.99, 56.80, 53.96.; (s, 4H), 11.07 (s, 2H), 8.98 (s, 2H), 7.63 (d, J = 2.2 Hz, 2H), 7.40 (dd, J = 8.5, 2.2 Hz, 2H), 6.96 (d, J = 8.4 ESI-MS: calculated for C24H26N4O10, [M H]− 529.16; found, 529.15; Anal. calcd for C24H26N4O10: C, 13 − 54.34;Hz, 2H), H, 4.94;3.78 (s, N, 4H), 10.56; 3.42 found: (s, 8H); C 54.31, C NMR H 4.97, (75 NMHz, 10.51. DMSO-d6) δ 172.74, 162.82, 158.30, 134.37, 131.08, 130.12, 118.33, 116.99, 56.80, 53.96.; ESI-MS: calculated for C24H26N4O10, [M − H]− 529.16; found, 529.15; 2.3.Anal. Preparation calcd for ofC24MPSH26N4O10: C, 54.34; H, 4.94; N, 10.56; found: C 54.31, H 4.97, N 10.51. The MPS was synthesized according to the reported method [16]. 8.2 g of (1-Hexadecyl) 2.3. Preparation of MPS trimethyl-ammonium bromide was dissolved in H2O (600 mL). After stirring for 10 min, 1.6 mL of triethanolamineThe MPS was was synthesized added and theaccording reaction to mixture the report wased heated. method When [16]. the 8.2 reaction g of (1-Hexadecyl) temperature reachedtrimethyl-ammonium 80 ◦C, TEOS (tetra bromide ethyl was ortho dissolved silicate) (60in H mL)2O (600 was mL). added After and stirring stirredfor for 1 10 h. min, The solvent1.6 mL ofof thetriethanolamine reaction was removedwas added by and rotary the evaporator reaction mixture and the was resulting heated. solid When (including the reaction some temperature water) was heatedreached at 80 500 °C,◦C TEOS for 5 (tetra h by usingethyl aortho furnace. silicate) (60 mL) was added and stirred for 1 h. The solvent of the reaction was removed by rotary evaporator and the resulting solid (including some water) was heated at 500 °C for 5 h by using a furnace.

2.4. Preparation of MPS-1

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2.4. Preparation of MPS-1 0.1 g of compound 1 and 1 g of MPS in of acetonitrile (20 mL) were stirred for 10 min. The reaction mixture was refluxed at 80 ◦C for 24 h. After the reaction mixture was cooled to room temperature, the solid product was filtered and washed with 200 mL acetonitrile.

2.5. Photophysical Studies The UV vis absorption and fluorescence spectra were determined over the range 200 800 nm. − − 2+ The samples were prepared by dispersion in H2O solution. The concentration of standard UO2 solution was 100 ppb.

2.6. NMR Measurement Compound 1 (2.65 mg, 0.005 mmol) and uranyl acetate (8.48 mg, 0.02 mmol) were dissolved in 0.5 mL and 0.1 mL of of DMSO-d6 (0.5 mL) in DMSO-d6 (0.1 mL), respectively. To NMR , the different amount of the uranyl acetate solution (12.5 µL, 25 µL, 37.5 µL, 50 µL) was added to compound 1 of DMSO-d6 (0.5 mL).

2.7. ICP-MS

2+ + 2+ MPS-1 (1 mg, 3 mg, and 5 mg) were dispersed in aqueous solution containing UO2 , Na , Mg , 2+ 2+ + 2+ 2+ 2+ 2+ Ca , Cu , Ag , Co , Ni , Mn and Pb (100 ppb) for 10 min. Mixture was added to H2O (4 mL). The mixture solution was centrifuged and the supernatant solution was filtrated with syringe filter (PTFE, 0.45 µm). The collected solution was measured 3 times by ICP-MS.

3. Results and Discussion

2+ 3.1. Spectroscopic Properties of Complex 1 with UO2 2+ Binding between UO2 and specific ligands, such as cyclic peptides [17,18], porphyrins [19,20], and naphthobipyrrole [21,22], is well known. We prepared a salicyladazine derivative as a ligand 2+ for UO2 . The salicyladazine derivative was synthesized starting from 2-hydroxybenzaldehyde. The diethyl 2,20-azanediyldiacetate groups were designed on both sides of the compound to create a symmetric structure. As the final step, hydrochloric acid treatment of the precursor yielded the desired compound 1. Compound 1 contains the acetic acid end group (–CH2COOH) to form the binding 2+ 1 13 site for UO2 and was characterized via FT IR, H and C NMR, , and elemental analysis (Figures S1, S2 and S3 in Supplementary Materials). UV–vis spectroscopy was performed to confirm that a coordination bond between compound 2+ 1 and UO2 resulted in a colorimetric change. Compound 1 was dissolved in an aqueous solution containing 1% of DMSO, and the UV–vis absorption spectrum was measured (Figure2A). Before adding 2+ 2+ UO2 , the π–π * absorption band of compound 1 appeared at around 300–350 nm. When UO2 (from 0.5 to 3 equivalents in 0.5 equivalent steps) was added to compound 1, the absorption peak intensities 2+ at 300 and 350 nm decreased until up to 2 equivalents of UO2 were added. At 2.5 or more equivalents 2+ 2+ of UO2 , the ligand-to-metal charge transfer absorption wavelength between compound 1 and UO2 was observed at around 365–380 nm [23]. 2+ Figure2B shows the photographs of the cuvettes used when UO 2 was added to compound 1 2+ and irradiated under UV light. The change in fluorescence after more than 2 equivalents of UO2 were added was visible to the naked eye. Compound 1 yielded an emission wavelength around 560 nm 2+ (excitation = 365 nm). When 0.5 equivalents of UO2 were added to compound 1, the fluorescence 2+ intensity decreased. The decrease in fluorescence was noticeable from 0.5 to 2 equivalents of UO2 ; however, the fluorescence intensity remained constant thereafter (Figure2C). Figure2D presents a 2+ plot of the fluorescence intensity vs. amount of UO2 added. To determine the stoichiometric ratio Nanomaterials 2019, 9, 688 4 of 8

2+ between compound 1 and UO2 , we constructed a Job’s plot using the fluorescence data and found a 2+ 1:2 binding ratio for compound 1:UO2 (Figure S4 ). 2+ NanomaterialsNanomaterialsTo investigate 20192019,, 99,, the xx FORFOR chemical PEERPEER REVIEWREVIEW interactions between compound 1 and UO2 , we used nuclear magnetic44 ofof 88 1 resonance (NMR) spectroscopy. We measured the H NMR signal of compound 1 in DMSO-d6 while 2+ 2+ dd66 whilewhile increasingincreasing2+ thethe UOUO222+ contentcontent (Figure(Figure 3).3). WhenWhen 0.50.5 equivalentequivalent2+ ofof UOUO222+ waswas addedadded toto increasing the UO2 content (Figure3). When 0.5 equivalent of UO 2 was added to compound 1 in compoundcompound 11 inin DMSO-DMSO-dd66,, thethe protonproton peakpeak (aromatic(aromatic OH:OH: 11.0811.08 ppm)ppm) ofof compoundcompound 11 decreaseddecreased andand DMSO-d6, the proton peak (aromatic OH: 11.08 ppm) of compound 1 decreased and multiple new peaks 2+2+ multiplemultiple newnew peakspeaks werewere observed.observed. WhenWhen wewe addedadded 211+ equivalentequivalent ofof UOUO22 ,, thethe ratioratio ofof thethe protonproton were observed. When we added 1 equivalent of UO2 , the ratio of the proton peaks of compound peakspeaks ofof compoundcompound 11 andand thethe resultingresulting complexcomplex waswas 1:1.1:1. BecauseBecause compoundcompound 11 hashas C2C2 symmetry,symmetry, 2+ 1 and the resulting complex2+ was 1:1. Because compound 1 has C2 symmetry, coordination of UO2 coordinationcoordination ofof UOUO222+ occursoccurs onon oneone sideside ofof compoundcompound 11 (bound(bound toto twotwo aceticacetic acidacid ligands)ligands) andand nono occurs on one side of compound 1 (bound to two acetic acid ligands) and no coordination2+ occurs on the coordinationcoordination occursoccurs onon thethe otherother sideside ofof compoundcompound 11.. UponUpon additionaladditional UOUO222+ inputinput (over(over 22 other side of compound 1. Upon additional UO 2+ input (over 2 equivalents), all the peaks representing equivalents),equivalents), allall thethe peakspeaks representingrepresenting freefree compoundcompound2 11 disappeareddisappeared entirely.entirely. Therefore,Therefore, compoundcompound 2+ free compound 1 disappeared entirely.2+ Therefore, compound 1 has a binding capacity of two UO 11 hashas aa bindingbinding capacitycapacity ofof twotwo UOUO222+ moleculesmolecules inin DMSO-DMSO-dd66 (forms(forms aa 1:21:2 complex).complex). ThisThis resultresult isis 2 moleculesconsistentconsistent in withwith DMSO- thethe dJob’sJob’s6 (forms plot.plot. a 1:2 complex). This result is consistent with the Job’s plot.

4 44 FigureFigureFigure 2. 2.2.(A (()AA UV–vis)) UV–visUV–vis spectra spectraspectra of compoundofof compoundcompound1 (2.65 11 (2.65(2.6510 ×× ppb)1010 ppb)ppb) in DMSO inin DMSO/HDMSO/H/H2O (1:9922OO (1:99(1:99v/v) containingv/vv/v)) containingcontaining various 2+ 2+2+ × amountsvariousvarious of amountsamounts UO2 .( ofofB )UOUO Photographed22 .. ((BB)) PhotographedPhotographed cuvettes fromcuvettescuvettes (A) underfromfrom (( UVAA)) underunder light irradiation. UVUV lightlight irradiation.irradiation. (C) Fluorescence ((CC)) 4 4 4 22 2+ spectraFluorescenceFluorescence of compound spectraspectra1 ofof(2.65 compoundcompound10 ppb) 11 (2.65(2.65 in DMSO ×× 1010/ Hppb)ppb)2O (1:99inin DMSO/HDMSO/Hv/v) containingOO (1:99(1:99 variousv/vv/v)) containingcontaining amounts variousvarious of UO 2 2+ × 2+ amounts of UO222+ (0–5 equivalents). (D) Plot of fluorescence intensity of (C) vs.2 amount+ of UO222+ . (0–5amounts equivalents). of UO ((0–5D) Plot equivalents). of fluorescence (D) Plot intensity of fluorescence of (C) vs. intensity amount of of (C UO) vs.2 amount. of UO .

1 FigureFigure 3.3. ((AA)1) 1HH nuclearnuclear magneticmagnetic resonanceresonance (NMR)(NMR) spectraspectra ofof compoundcompound 11 (10(10 mM)mM) inin DMSO-DMSO-dd66 Figure 3. (A) H nuclear magnetic resonance (NMR) spectra of compound 1 (10 mM) in DMSO-d6 2+ containingcontaining variousvarious equivaleequivalentsnts ofof uranyluranyl acetate.acetate. ((BB)) ProposedProposed structurestructure ofof complexcomplex 11 withwith UOUO222+.. 2+ containing various equivalents of uranyl acetate. (B) Proposed structure of complex 1 with UO2 .

TheThe carbonylcarbonyl oxygenoxygen (C=O)(C=O) ofof thethe endend groupgroup ofof aceticacetic acidsacids (–CH(–CH22COOH)COOH) inin compoundcompound 11 isis wellwell 2+ knownknown toto bindbind stronglystrongly toto aa radionuclideradionuclide ionion suchsuch asas UOUO222+ byby supplyingsupplying electronselectrons [23,24].[23,24]. IRIR 2+ spectroscopyspectroscopy waswas usedused toto classifyclassify thethe complexcomplex formationformation ofof compoundcompound 11 withwith UOUO222+.. AsAs shownshown inin

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Nanomaterials 2019, 9, x FOR PEER REVIEW 5 of 8 The carbonyl oxygen (C=O) of the end group of acetic acids (–CH2COOH) in compound 1 is 2+ well known to bind strongly to a radionuclide ion such as UO2 by supplying electrons [23,24]. Figure S5, the oxygen of the carboxylic acid carbonyl (C=O) in compound 1 before 2the+ addition of IR spectroscopy was used to classify the complex formation of compound 1 with UO2 . As shown in UO22+ produced a peak at 1728 cm−1, whereas the C=O peak after the addition of UO22+ was shifted to Figure S5, the oxygen of the carboxylic acid carbonyl (C=O) in compound 1 before the addition of 1557 2+cm−1. This shift to a lower wavenumber1 is indicative of the C=O in compound2+ 1 providing UO2 produced a peak at 1728 cm− , whereas the C=O peak after the addition of UO2 was shifted electrons in 1a dative bond to UO22+ and confirms that the carboxylic acid groups are employed in to 1557 cm− . This shift to a lower wavenumber is indicative of the C=O in compound 1 providing complex formation with UO22+. 2+ electrons in a dative bond to UO2 and confirms that the carboxylic acid groups are employed in complex formation with UO 2+. 3.2. Immobilization of Compound2 1 to Mesoporous Silica Nanoparticles 3.2. ImmobilizationThe morphology of Compound of mesoporous1 to Mesoporous silica nanoparticle Silica Nanoparticles immobilized with 1 (MPS-1) was observed via transmissionThe morphology electron of mesoporous microscopy silica (TEM). nanoparticle The TEM image immobilized of MPS-1 with revealed1 (MPS-1 a spherical) was observed structure via withtransmission a narrow electron size distribution microscopy (circa (TEM). 40 The nm) TEM (Figure image 4A). of MPS-1 Thermogravimetryrevealed a spherical analysis structure (TGA) withwas performeda narrow size to distributiondetermine (circathe amount 40 nm) (Figureof compound4A). Thermogravimetry 1 immobilized analysisonto MPS-1 (TGA) (Figure was performed 4B). At approximatelyto determine the 150 amount °C, the ofweight compound of MPS-11 immobilized decreased by onto 4.2%.MPS-1 This (Figuremass reduction4B). Atapproximately was attributed to moisture. At ~500 °C, compound 1 was pyrolyzed and the weight of MPS-1 decreased to 86.8% 150 ◦C, the weight of MPS-1 decreased by 4.2%. This mass reduction was attributed to moisture. (Figure S6). Thus, the amount of compound 1 introduced into MPS-1 was 9% by weight (Figure 4B). At ~500 ◦C, compound 1 was pyrolyzed and the weight of MPS-1 decreased to 86.8% (Figure S6). Thus, Figurethe amount S7 presents of compound the IR1 introducedspectra of MPS into MPS-1 and MPS-1was 9%; a by C-H weight vibration (Figure peak4B). Figureof 2940 S7 cm presents−1 was confirmed. This further supported the presence of compound 1 on the1 surface of the mesoporous the IR spectra of MPS and MPS-1; a C-H vibration peak of 2940 cm− was confirmed. This further silicasupported nanoparticles. the presence Under of compound UV light 1irradiation,on the surface the offiltered the mesoporous silica nanoparticles silica nanoparticles. fluoresced Under blue, indicatingUV light irradiation, that compound the filtered1 was present silica nanoparticleson the mesoporous fluoresced silica blue,nanoparticle indicating surface. that Fluorescence compound 1 wasspectra present of MPS-1 on the (2 mesoporous mg) in water silica (2 mL) nanoparticle and MPS-1 surface. (2 mg) Fluorescence in 100 ppb UO spectra22+ solution of MPS-1 (2 mL)(2 mg) were in also measured. (Figure 4C). In 3.5% NaCl aqueous2+ solution of 2mL, we measured the fluorescence water (2 mL) and MPS-1 (2 mg) in 100 ppb UO2 solution (2 mL) were also measured. (Figure4C). spectraIn 3.5% of NaCl MPS-1 aqueous (2 mg) solution and MPS-1 of 2mL, (2 we mg) measured in 100 ppb the fluorescenceUO22+ solution, spectra respectively of MPS-1 (Figure(2 mg) andS8). Fluorescence changes of MPS-12+ in the present of 100 ppb UO22+ in water or 3.5% NaCl aqueous MPS-1 (2 mg) in 100 ppb UO2 solution, respectively (Figure S8). Fluorescence changes of MPS-1 solution were shown similar results.2+ This means that MPS-1 can bind UO22+ not only in water but in the present of 100 ppb UO2 in water or 3.5% NaCl aqueous solution were shown similar results. also NaCl aqueous solution. 2+ This means that MPS-1 can bind UO2 not only in water but also NaCl aqueous solution.

Figure 4. (A(A) )Transmission Transmission electron electron microscopy microscopy (TEM) (TEM) image image of mesoporous of mesoporous silica silica (MPS-1 (MPS-1) and) and(B) thermogravimetry(B) thermogravimetry analysis analysis (TGA) (TGA) thermogram thermogram of of MPSMPS (black)(black) and and MPS-1MPS-1 (red).(red). ( (CC)) The The photograph photograph and the Fluorescence spectra of of ( (aa)) MPS-1 (2 mg) in water (2 mL) and ( b)) MPS-1 (2 mg) in 100 ppb 2+ UO22+ solutionsolution (2 (2 mL). mL).

2+ 3.3. The Adsorption Capacity of MPS-1 for UO2 3.3. The Adsorption Capacity of MPS-1 for UO22+ The United States Environmental Protection Agency (US EPA) and the World Health Organization The United States Environmental Protection Agency (US EPA) and the World Health 2+ (WHO) have prescribed safe limits of UO2 in drinking water at 30 ppb [25,26]. The adsorption Organization (WHO) have prescribed safe limits of UO22+ in drinking water at 30 ppb [25,26]. The 2+ capacity of MPS-1 was tested by adding 1, 3, and 5 mg of MPS-1 to 1 mL of 100 ppb UO2 solution. adsorption capacity of MPS-1 was tested by adding 1, 3, and 5 mg of MPS-1 to 1 mL of 100 ppb UO22+ After standing for 10 min, the solution was filtered through a 0.45-µm syringe filler and the UO 2+ solution. After standing for 10 min, the solution was filtered through a 0.45-μm syringe filler and the2 levels were determined by ICP-MS (experiment performed in triplicate). Calibration curves were UO22+ levels were determined by ICP-MS (experiment performed in triplicate). Calibration curves 2+ obtained with dilute UO2 solutions (0.1, 1, 10, 50, and 100 ng/L), and the linearity of the calibration were obtained with dilute UO22+ solutions (0.1, 1, 10, 50, and 100 ng/L), and the linearity of the curve was confirmed (correlation coefficient was 0.9974) (Figure S9). Figure5 and Table S1 present was confirmed (correlation coefficient was 0.9974) (Figure S9). Figure 5 and Table 2+ the results of the UO2 adsorption experiment using various amounts of MPS-1. The percentage of S1 present the results of the UO22+ adsorption experiment using various amounts of MPS-1. The percentage of UO22+ removed was 70%, 96%, and 95% for 1, 3, and 5 mg of MPS-1, respectively (RSD values all less than 15%). 1 mg of MPS-1 was not sufficient for absorbing 100 ppb UO22+. Using 3 or 5 mg of MPS-1 removed 95% or more of the UO22+; there was no statistically significant difference

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2+ UObetween2 removed the two was 70%,absorbent 96%, anddose 95% amounts. for 1, 3, In and conclusion, 5 mg of MPS-1 MPS-1, respectively (even a small (RSD amount: values all0.3 lesswt %) 2+ thancould 15%). reduce 1 mg 100 of MPS-1 ppb ofwas UO2 not2+ <5 su ppbfficient of UO for22+ absorbing; this result 100 would ppb UOsatisfy2 . both Using EPA 3 or and 5 mg WHO of MPS-1 drinking 2+ removedwater 95%standards. or more of the UO2 ; there was no statistically significant difference between the two absorbent dose amounts. In conclusion, MPS-1 (even a small amount: 0.3 wt %) could reduce 100 ppb 2+ 2+ of UO3.4.2 Adsorption<5 ppb ofof UOUO222+ and; this Other result Cations would onto satisfy MPS-1 both EPA and WHO drinking water standards.

The elemental2 +analysis of MPS, MPS-1, and UO22+-adsorbed MPS-1 was performed via TEM 3.4. Adsorption of UO2 and Other Cations onto MPS-1 dispersive X-ray spectroscopy (EDX) (Figure S10). Nitrogen was observed in the EDX spectrum of 2+ MPS-1The elemental, thus providing analysis evidence of MPS ,forMPS-1 the presence, and UO of2 compound-adsorbed 1MPS-1 in MPS-1was. In performed the EDX spectrum via TEM of dispersiveUO22+-adsorbed X-ray spectroscopy MPS-1, uranium (EDX) (Figurewas detected. S10). Nitrogen This confirms was observed our rationale in the EDX in spectrumdesigning of this MPS-1adsorbent:, thus providing UO22+ was evidence bound to for the the compound presence 1 ofattached compound onto 1thein surfaceMPS-1 .of In MPS the. EDX spectrum 2+ + of UO2 We-adsorbed also confirmedMPS-1 ,the uranium adsorption was capacity detected. of This MPS-1 confirms (5 mg) our for rationaleother metal in ions, designing such as this Na , + 2+ + + + adsorbent:Mg2+, Ca UO2 , 2Cu2was, Ag bound, Co2+ to, Ni the2+, compoundMn2+, and Pb1 attached2 (100 ppb) onto under the surface the same of MPS conditions.. Among the + metalWe alsoions confirmed tested, 42.3% the adsorptionof Ca2+ was capacity adsorbed of ontoMPS-1 the(5 surface mg) for of other MPS-1 metal; for ions,the remaining such as Na metal, 2+ 2+ 2+ + 2+ 2+ 2+ 2+ Mgions,, Ca <30%, Cu were, Ag adsorbed, Co , Ni (Table, Mn S2)., andThese Pb findings(100 ppb) suggest under that the sameMPS-1 conditions. would be Among useful theas an 2+ metaladsorbent ions tested, for UO 42.3%22+. of Ca was adsorbed onto the surface of MPS-1; for the remaining metal ions, <30% were adsorbed (Table S2). These findings suggest that MPS-1 would be useful as an adsorbent 2+ for UO2 .

2+ 2+ Figure 5. 2+ MPS-1 2+ FigureResult 5. Result of UO of2 UOadsorption2 adsorption of of MPS-1in 100 in 100 ppb ppb UO 2UO2solution. solution. 4. Conclusions 4. Conclusions We synthesized the salicyladazine-based compound 1, designed to be a uranyl ion capture ligand. We synthesized the salicyladazine-based2+ compound 1, designed to be a uranyl ion capture Compound 1 formed a 1:2 complex with UO2 as confirmed by the Job’s plot. A fluorescence change ligand. Compound 1 formed a 1:2 complex with UO22+ as confirmed by the Job’s plot. A fluorescence was observed when UO 2+ was bound to compound 1. IR and NMR measurements were performed 2 2+ change was observed when UO2 was2+ bound to compound 1. IR and NMR measurements were to identify compound 1 and the two UO2 coordination sites. Compound 1 was immobilized into performed to identify compound 1 and the two UO22+ coordination sites. Compound 1 was mesoporous silica (MPS-1); the resulting sorbent could remove 96% of the UO 2+ from 1 mL of 2 2+ immobilized2 +into mesoporous silica (MPS-1); the resulting sorbent could remove 96% of the UO2 a 100-ppb UO2 aqueous solution. A material was successfully developed that was capable of from 1 mL of a 100-ppb UO22+ aqueous solution. A material was successfully developed that was simultaneously absorbing uranyl ions and detecting their presence by fluorescence. We believe that capable of simultaneously absorbing uranyl ions and detecting2+ their presence by fluorescence. We this organic–inorganic hybrid material paradigm for detecting UO2 will have a broad impact for the believe that this organic–inorganic hybrid material paradigm for detecting UO22+ will have a broad study on porous materials and their application. impact for the study on porous materials and their application.

SupplementarySupplementary Materials: Materials:The The following following are are available available online online at athttp: www.mdpi.com/xxx/s1.//www.mdpi.com/2079-4991 Scheme/9 S1./5/ 688Synthesis/s1. 1 Schemeroute S1. of compound Synthesis route1; Figure of compound S1. FT-IR spectrum1; Figure of S1. compound FT-IR spectrum 1; Figure of compoundS2. 1H NMR1 ;spectrum Figure S2. of compoundH NMR 1 13 spectrum of compound 1 in13 DMSO-d6; Figure S3. C NMR spectrum of compound 1 in DMSO-d6; Figure S4. Job’s in DMSO-d6; Figure S3. C NMR spectrum of compound2+ 1 in DMSO-d6; Figure S4. Job’s plot for complex2+ formed plot for complex formed between compound 1 and UO2 ; Figure S5. FT-IR spectra of 1 and 1 with UO2 ; Figure between compound 1 and UO22+; Figure S5. FT-IR spectra of 1 and 1 with UO22+; Figure S6. TGA thermogram of S6. TGA thermogram of compound 1; Figure S7. FT-IR spectra of (A) MPS and (B) MPS-1; Figure S8. Fluorescence compound 1; Figure S7. FT-IR spectra of (A) MPS and (B) MPS-1; Figure S8. Fluorescence2+ spectra of (a) MPS-1 spectra of (a) MPS-1 (2 mg) in 3.5% NaCl solution (2mL) and (b) MPS-1 (2 mg) with UO2 solution (100 ppb) in (2 mg) in 3.5% NaCl solution (2mL) and (b) MPS-1 (2 mg) with UO22+ solution2+ (100 ppb) in 3.5% NaCl (2 mL); 3.5% NaCl (2 mL); Figure S9. Linear equation of various concentrations of UO2 ; Figure S10. TEM EDX mapping Figure S9. Linear equation of various concentrations of UO22+; Figure S10. TEM EDX mapping of (A) MPS (B)

Nanomaterials 2019, 9, 688 7 of 8

2+ 2+ of (A) MPS (B) MPS-1 and (C) MPS-1 with UO2 ; Table S1. Adsorption Capacities of MPS-1 for UO2 (100 ppb) solution; Table S2. Adsorption Capacities of MPS-1 (5 mg) with various metal ions (100 ppb) solution. Author Contributions: Conceptualization, S.P. and J.P.; Data curation, S.P.; Formal analysis, S.P. and J.P.; Investigation, S.P. and J.P.; Methodology, J.P.; Project administration, J.H.L. and J.H.J.; Supervision, J.H.L., M.Y.C. and J.H.J.; Writing—Original draft, J.H.L.; Writing—Review & editing, J.H.L., M.Y.C. and J.H.J. Funding: This research was supported by the NRF (2018R1A2B2003637, 2017M2B2A9A02049940 and 2017R1A4A1014595) supported by the Ministry of Education, Science and Technology, Korea. In addition, this work was partially supported by a grant from the Next-Generation BioGreen 21 Program (SSAC, Grant no. PJ013186052019), Rural Development Administration, Korea. Conflicts of Interest: The authors declare no conflict of interest.

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