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 spectroscopy, 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.