Highly Sensitive and Selective Uranium Detection in Natural Water

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Highly Sensitive and Selective Uranium Detection in Natural Water Systems Using a Luminescent Mesoporous Metal-Organic Framework Equipped with Abundant Lewis Basic Sites: A Combined Batch, X-ray Absorption Spectroscopy, and First Principle Simulation Investigation Wei Liu, Xing Dai, Zhuanling Bai, Yanlong Wang, Zaixing Yang, Linjuan Zhang, Lin Xu, Lanhua Chen, Yuxiang Li, Daxiang Gui, Juan Diwu, Jianqiang Wang, Ruhong Zhou, Zhifang Chai, and Shuao Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06305 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 10, 2017

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Highly Sensitive and Selective Uranium Detection in Natural Water Systems

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Using a Luminescent Mesoporous Metal-Organic Framework Equipped with

3

Abundant Lewis Basic Sites: A Combined Batch, X-ray Absorption Spectroscopy,

4

and First Principle Simulation Investigation

5

WEI LIU , XING DAI , ZHUANLING BAI†, YANLONG WANG†, ZAIXING YANG†,

6

LINJUAN ZHANG‡, LIN XU†, LANHUA CHEN†, YUXIANG LI†, DAXIANG GUI†, JUAN

7

DIWU†, JIANQIANG WANG‡, RUHONG ZHOU*†¶ , ZHIFANG CHAI†, SHUAO WANG*†

8

†

9

Center of Radiation Medicine of Jiangsu Higher Education Institutions, Soochow University, 199

†§

†§

School for Radiological and Interdisciplinary Sciences (RAD-X) and Collaborative Innovation

10

Ren’ai Road, Suzhou 215123, China

11

‡

12

Energy Technology, Chinese Academy of Sciences, Shanghai 201800, P. R. China.

13

¶

14

10598; Department of Chemistry, Columbia University, New York, NY 10027, United States.

15

*Corresponding

16

[email protected] (SHUAO WANG); Tel: +86-512-65883945; Fax: +86-512-65883945.

17

§

18

Word count: Main text (5341 words) + 6 small Figures (6 × 300 = 1800 words) + 1

19

big Figure (1 × 600 = 600 words) = 7741 words.

Shanghai Institute of Applied Physics and Key Laboratory of Nuclear Radiation and Nuclear

Computational Biology Center, IBM Thomas J Watson Research Center, Yorktown Heights, NY

authors.

Email:

[email protected]

(RUHONG

ZHOU);

These two authors contributed equally.

20 21 22 23 1


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ABSTRACT: Uranium is not only a strategic resource for the nuclear industry but

25

also a global contaminant with high toxicity. Although several strategies have been

26

established for detecting uranyl ions in water, searching for new uranium sensor

27

material with great sensitivity, selectivity, and stability remains a challenge. We

28

introduce here a hydrolytically stable mesoporous terbium(III)-based MOF material

29

compound 1, whose channels are as large as 27 Å × 23 Å and are equipped with

30

abundant exposed Lewis basic sites, the luminescence intensity of which can be

31

efficiently and selectively quenched by uranyl ions. The detection limit in deionized

32

water reaches 0.9 µg/L, far below the maximum contamination standard of 30 µg/L in

33

drinking water defined by the United States Environmental Protection Agency (EPA),

34

making compound 1 currently the only MOF material that can achieve this goal. More

35

importantly, this material exhibits great capability in detecting uranyl ions in natural

36

water systems such as lake water and seawater with pH being adjusted to 4, where

37

huge excesses of competing ions are present. The uranyl detection limits in Dushu

38

Lake water and in seawater were calculated to be 14.0 and 3.5 µg/L, respectively. This

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great detection capability originates from the selective binding of uranyl ions onto the

40

Lewis basic sites of the MOF material, as demonstrated by synchrotron radiation

41

extended X-ray adsorption fine structure (EXAFS), X-ray adsorption near edge

42

structure (XANES), and first principle calculations, further leading to an effective

43

energy transfer between the uranyl ions and the MOF skeleton.

44 45 2



46

INTRODUCTION

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Uranium is a naturally occurring radioactive and chemically toxic element with an

48

estimated concentration of 2.7 ppm in the Earth’s crust and 3.3 ppb in seawater.1, 2

49

During the past several decades, anthropogenic activities such as uranium mining and

50

processing, improper nuclear waste management, and nuclear safety accidents have

51

resulted in the release of considerable amounts of uranium into the natural

52

environment, mostly through migration in ground or surface water systems as the

53

highly soluble uranyl(VI) ion.3-6 Uranium’s combination of chemotoxicity and

54

radiotoxicity can lead to irreversible kidney damage, urinary system disease, DNA

55

damage or the disruption of biomolecules.7-11 The development of new methods and

56

platforms to accurately detect uranyl ions in natural water systems is therefore critical

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and urgent.

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During the past few decades, various metal ion detection techniques, including

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chemical and instrumental methods, have been developed. Instrumental analytical

60

methods such as inductively coupled plasma mass spectrometry (ICP-MS),12

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inductively coupled plasma atomic emission spectroscopy (ICP-AES),13 atomic

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adsorption spectrometry,1 X-ray fluorescence spectroscopy,14 laser-induced kinetic

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phosphorescence analysis (KPA),15-17 high-performance liquid chromatography

64

(HPLC),18 that based on the intrinsic physical properties of element are the most

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common techniques used to quantify uranium at concentrations from 10-9 to the 10-6

66

g/L level in aqueous solutions. However, these techniques often require a

67

time-consuming pre-treatment procedure, such as the removal of latent competing 3


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metal ions or a solution dilution procedure and can be conducted only in

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well-equipped laboratories.19 Although the well-established KPA method exhibit

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extremely low detection limit at the level of 1 ng/L, it requires many pre-treatment

71

procedures to protect the uranyl luminescence from being quenched (i.e. the detection

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selectivity towards uranium is not excellent).20, 21 Recent developments include the

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introduction of a highly selective DNAzymes system into uranyl fluorescent

74

sensors,22-24 colorimetric sensors,25,

75

methods are based on the interaction between specific catalytic DNA sequences and

76

uranium that shows excellent uranyl detection capability (45 pM detection limit).

77

However, the poor stability of these materials in the presence of high-ionic-strength

78

samples greatly limits their applications under environmentally relevant conditions

79

(e.g., lake water or seawater).

26

and electrochemical biosensors.27 All these

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Recently, luminescent MOFs constructed through lanthanides as metal nodes

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have emerged as a promising fluorescent sensing platforms. These materials possess

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inherent advantages over traditional porous materials in selective adsorption and

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chemical sensing because of their controllable pore size/shape and facile

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functionalization.28-31 Through the elaborate arrangement of the organic or inorganic

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moieties, one can precisely control the spatial arrangements of donor and acceptor

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chromophores or introduce chemically active sites to functionalize the skeleton to

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achieve the desired luminescence properties.32 These tunable architectures with

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extended porosity or channels offer numerous opportunities for guest molecules to

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enter and interact with the framework, establishing a relationship between guest 4



90

concentration and luminescence intensity. On the other hand, the capture of guest

91

molecules within the pores or channels gives rise to guest-molecule pre-concentration,

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which is responsible for improving the detection sensitivity. In addition, the chemical

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and thermal stability of some MOF materials make them viable for real applications.33

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Numerous MOFs have already been developed to detect various substances,

95

including small molecules,34-38 heavy metal ions,39-44 and toxic gases,45-47 via the

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aforementioned mechanism. However, this strategy has not previously been

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well-developed for uranium detection. The two examples reported thus far is the

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utilization of MOFs to detect uranyl ions in solution conducted by Sun et al. and Xing

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et al., with relatively poor detection limits of ca. 23.8 mg/L and 0.42 mg/L

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respectively, precluding practical applications for the precise evaluation of uranium

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contamination in a natural water system.48 The limited detection capabilities likely

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stem from the uranyl ions being sorbed by MOF-76 through a simple ion-exchange

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process so that only weak interactions, such as electrostatic interaction or van der

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Waals interaction, occur, resulting in an ineffective energy transfer between the uranyl

105

ions and the luminescent framework. In addition, a high detection selectivity can not

106

be expected for these materials.

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There are several objectives for this work. First, we introduce a hydrolytically

108

stable luminescent MOF material with abundant soft nitrogen donors that can

109

effectively chelate uranyl ions in aqueous solutions, resulting in an effective energy

110

transfer from the framework to the uranyl ion. Second, we demonstrate that this

111

effective energy transfer gives rise to a promising detection sensitivity and selectivity 5


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for detecting uranyl ions in drinking water and natural water system including

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seawater. The detection limits are far below the uranium contamination concentration

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standard defined by the United States Environmental Protection Agency (EPA).49

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Finally, the detection mechanism originates from the selective and efficient

116

enrichment of uranium into the solid material, as elucidated by a combination of

117

uranium

118

characterizations on the uranium sorbed sample. This is further confirmed by the first

119

principle Density Functional Theory (DFT) calculations, which well unravels the

120

binding sites of uranyl ions in the MOF material.

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Synthesis of compound 1. A mixture of 0.1 mmol (0.0435 g) of Tb(NO3)3•6H2O,

123

0.05

124

(1,3,5-triazine-2,4,6-triyltriimino)tris-benzoic acid), 2 mL of H2O and 3 mL of DMF

125

(N,N-dimethylformamide) was added to a Teflon-lined stainless steel reactor and

126

heated at 100 °C for 72 h, followed by slow cooling to room temperature at a rate of

127

0.6 °C min-1.

128

X-ray crystallography studies. Single crystal X-ray diffraction data collection was

129

accomplished on a Bruker D8-Venture diffractometer with a Turbo X-ray Source

130

(Mo–Kα radiation, λ = 0.71073 Å) adopting the direct-drive rotating anode technique

131

and a CMOS detector at 273 K. The data collection was carried out using the program

132

APEX3 and processed using SAINT routine in APEX3. The structure of compound 1

133

was solved by direct methods and refined by the full-matrix least squares on F2 using

uptake

batch

experiment

and

X-ray

absorption

spectroscopic

EXPERIMENTAL SECTION

mmol

(0.025

mg)

of

6


H3TATAB

(4,4′,4″-


134

the SHELXTL-2014 program.

135

Uranium concentration dependent luminescence spectra. The crystalline material

136

of compound 1 was finely ground before used. Then 3 mg of compound 1 was

137

dispersed in 2 mL of uranium solution with different concentrations from 0 to 400

138

mg/L and ultrasonic treatment for about 10 minutes to form a homogeneous

139

suspension, aging for 1 hour, then the spectra were collected immediately after

140

ultrasonic treatment for 5 minutes. To investigate the influence of competing metal

141

ions, 3 mg of compound 1 was dispersed into 2 mL of 500 ppm M(NO3)x·n(H2O) (X=

142

U6+, Th4+, Eu3+, Sr2+, Cs+, Al3+, Ca2+, K+) aqueous solution. 3 mg of compound 1 was

143

dispersed into 2 mL deionezed water without uranyl were prepared as a further control

144

experiment and the solutions were aged for half an hour. Then the spectra were

145

collected immediately after ultrasonic treatment for 5 minutes. In order to eliminate

146

the error induced by the suspension change or instrument fluctuation, all spectra were

147

collected in parallel for three times, and the average data were used for plotting.

148

Uranium uptake kinetics studies. Uptake kinetics experiments were carried out at

149

room temperature, and the solid/liquid ratio was 1 g/L. The pH values were adjusted

150

to 4 with 0.1 M HNO3 and NaOH solutions. The adsorption kinetics experiment was

151

conducted by immersing 60 mg of compound 1 into 60 ml 5 mg/L uranium solution

152

and the mixture was then stirred for certain time before sampling. The sampling

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solutions were filtered with a 0.22 µm nylon membrane filter and the concentrations

154

of uranium were measured by inductively coupled plasma mass spectrometry

155

(ICP-MS, Thermo Scientific). 7


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Uranium adsorption isotherm experiments. Isotherm experiments were carried out

157

at room temperature, and the solid/liquid ratio was 1 g/L. The pH values were

158

adjusted to 4 with 0.1 M HNO3 and NaOH solution. 10 mg of compound 1 was added

159

to 10 mL aqueous solutions containing various amounts of uranium. The mixtures

160

were then stirred for desired contact time. The resulting solutions were filtered with a

161

0.22 µm nylon membrane filter and the concentrations of uranium were measured by

162

ICP-MS.

163

Influence of competing cations. The influence of the competing ions was studied

164

using the mixed cation solutions (M(NO3)x•n(H2O), M = U6+, Th4+, Eu3+, Sr2+, Cs+,

165

Al3+, Ca2+, and K+) and mixed anion solution (anion source is NaCl, Na2CO3 and

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humic acid) with four different concentrations of 0.6, 1, 3, 5, and 10 mg/L for each

167

ions, respectively. 10 mg of the compound 1 was added to 10 mL the above mixed

168

solution and stiring for 24 hours and the sampling solutions were filtered with a 0.22

169

µm nylon membrane filter and the concentrations of uranium were measured by

170

ICP-MS.

171

Desorption experiment. 40 mg of compound 1 was dispersed into 40 mL of 1 mg/L

172

UO2(NO3)2•6(H2O) solution then stired for approximately 20 minutes at room

173

temperature to prepare the uranium-loaded sample. After separation by centrifugation

174

and dried in the air the loaded sample was dispersed into 40 mL of HNO3 solution at

175

pH 2 for another 20 minutes to desorb the uranium. The sampling solutions were

176

filtered with a 0.22 µm nylon membrane filter and the concentrations of uranium were

177

measured by ICP-MS. 8



178

EXAFS and XANES data collection. Samples for X-ray absorption spectroscopy

179

analysis were prepared by immersing compound 1 in aqueous solutions containing

180

three concentrations of UO2(NO3)2•6(H2O) (100, 200, and 300 mg/L) for 5 h,

181

followed by washing with ethanol 3 to 5 times separated by centrifugation and drying

182

in the air. X-ray adsorption spectra were collected at the beamline 14W1 in Shanghai

183

Synchrotron Radiation Facility using a Si(111) double-crystal monochromator in

184

transmission mode for the uranium L3-edge spectra. The electron beam energy of the

185

storage ring was 3.5 GeV, and the maximum stored current was approximately 210

186

mA. The uranium L3-edge EXAFS data were analyzed using the standard procedures

187

described by Demeter.50 Theoretical EXAFS data were calculated using the FEFF 9.0

188

software.51 The fitting procedure was performed on the k3-weighted FT-EXAFS data

189

from 3 to 12.5 Å-1. An R window of 1–2.5 Å was used for the fitting.

190

Computational models and methods. Density functional theory (DFT) calculations

191

were performed using Gaussian 09 program52 to explain the interaction mechanism

192

between the hydrate uranyl ion and compound 1. As the adsorbent, one or two

193

TATAB fragments (abbreviated as L) were extracted from the single crystal analytical

194

crystalline structure and were then partially optimized by fixing three or six O atoms

195

of the carboxylate terminals. Each of these fixed O atoms is simultaneously

196

coordinated by two Tb atoms and hence was not expected to have a large movement.

197

Other O atoms of the TATAB carboxylate terminals were saturated by H atoms

198

(COO-→COOH). Considering the possible deprotonation of N atoms in compound 1,

199

both deprotonated and neutral TATAB models were used in calculations. Since the 9


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nitrate uranyl is mainly in the form of hydrate uranyl in aqueous solution,53 we only

201

considered the UO22+(H2O)5 molecule as the adsorbate in calculations. For the

202

complexes, we fully considered various possible binding sites and obtained six

203

representational structures geometry optimizations for analyzing.

204

Geometry optimizations and frequency calculations of the each reactant and product

205

were performed in gas phase at B3LYP-D3/RECP~6-31G(d) level. In this level, the

206

Becke three parameters hybrid exchange-correlation functional54, 55 (B3LYP) with the

207

combination of the Grimme’s third generation dispersion correction56 (D3) were

208

employed. The relativistic effective small-core potential57 (RECP, containing 60 core

209

electrons) and its corresponding (14s13p10d8f6g)/[10s9p5d4f3g] valence basis set58

210

(32 valence electrons), which can introduce a certain degree of the scalar relativistic

211

effect, were used for U atom. The standard Gaussian-type basis sets 6-31G(d)59 was

212

used for C, N, O and H atoms. All of the optimized structures were then used to

213

calculate single-point energies in liquid phase at a more precise level of

214

M062X-D3/RECP~6-311++G(d, p). In this level, the M062X functional60 was used

215

and the basis sets for describing other atoms was replaced by 6-311++G(d, p)61, 62.

216

Solvation effects were included by using SMD63 solvation model with water as the

217

solvent.

218

The complexation processes were designed as the following hypothetical reaction:

219

L + U H O → L U H O + nH O (m=0,1,2, while n=2, 3, 4)

220

Where m indicates the number of deprotonation and the quantity of negative charges,

221

and n represents the number of dehydration after adsorptions. 10


(1)


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The thermodynamic feasibility was evaluated by calculating the liquidus Gibbs free

223

energy changes (∆G, at 298.15 K, 1 atm) of the complexation reaction, which was

224

defined as:

225

∆G = ∆ε + ∆G + ZPE + ∆G→

226

Where, εele, ∆Gsolv, ZPE and ∆G0→T indicate the electronic energy, the solvation

227

energy, the zore-point energy correction and the thermal correction to Gibbs free

228

energy, respectively. The terms of εele + ∆Gsol were calculated at M062X-D3 level in

229

liquid phase, while ZPE + ∆G0→T were calculated at B3LYP-D3 level in gas phase.

230

The binding strength between the Uranyl and the MOF was assessed by the binding

231

energy Eb:

232

E = E!"# − E %&

233

(3)

234

Where, the Ecomplex represents the total energy of the complex, the last two terms

235

represent the total energies of the UO22+ fragment of the complex and remaining

236

fragments of the complex. All the three terms of Eb were calculated at M062X-D3

237

level in liquid phase. Detailed geometric parameters of uranyl hydrate and

238

computational models (d-i) are shown in the Table S3.

'(

(2)

− E)*+,

239 240

RESULTS AND DISCUSSION

241

Material synthesis, crystal structure, and characterizations. The synthesis of

242

compound 1 was initially reported by Cheng et. al.64 The size of the rod-like crystals

243

ranges from 5 to 25 µm (see EDS images in the part VIII of supporting information) 11


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and the surface area was calculated form N2 sorption data as 525 cm2g-1 based on the

245

Brunauer–Emmett–Teller method.64 Powder X-ray diffraction analysis confirmed the

246

phase purity (Figure S1). The crystal structure of compound 1 was characterized by

247

single-crystal X-ray diffraction analysis and the results are depicted in Figure 1.

248

Compound 1 crystallizes in the monoclinic space group P21/c and can be best

249

described as a non-interpenetrated mesoporous 3D framework based on binuclear Tb

250

carboxylate clusters as the metal nodes and deprotonated TATAB ligands as the

251

linkers. The asymmetric unit consists of two eight-coordinated Tb3+ ions and two

252

TATAB ligands adopting similar coordination modes. Each TATAB ligand offers three

253

carboxylate groups with two different coordination modes binding to Tb3+ ions,

254

forming a dinuclear cluster of [Tb2(CO2)4] as the secondary building unit. One of

255

these groups is in both chelating/bridging tridentate modes, whereas the other two are

256

in the syn-syn bridging bidentate mode. The dinuclear clusters are further connected

257

by additional carboxylate groups to form terbium-carboxylate chains. The chains are

258

supported and linked by the rigid phenyl rings to form a 3D framework with a series

259

of 1D rhombic channels with dimensions of 27 Å × 23 Å along c axis based on the

260

distances between opposite metal centers, these channels are sufficiently large to

261

accommodate hydrated uranyl cations. Substantial amounts of coordination-available

262

amine (6 moles per mole of compound 1) and triazine groups (6 moles per mole of

263

compound 1) face toward the inner side of the channels, ready to chelate uranyl ions.

264

Before the investigation of uranyl detection in different aqueous media, the

265

hydrolytic stability of compound 1 was checked by soaking the material in aqueous 12



266

solutions with various pH values (2-10), 500 mg/L concentrations of multiple types of

267

metal salt solutions, natural lake water and seawater. As shown in Figure S1,

268

compound 1 can maintain its crystallinity under all these experimental conditions

269

within 24 hours. Solubility test under the selected above conditions also indicates

270

great stability of compound 1 (Figure S2). The solubility of compound 1 was also

271

investigated to be quite low in various water media which is shown in part III of

272

Supporting information. During the test, only minor portion of Tb ions (1.15% to

273

5.35%) are released into the water, Dushu lake water and simulated seawater at pH 4.

274

In addition, under these conditions the mass recovery ratio ranges from 94.2% to

275

98.0%, which is crucial for the real applications under environmentally relevant

276

conditions. The Tb ions release ratio and mass recovery of compound 1 under high

277

ionic strength conditions were also investigated and are shown in Figure S2. In

278

addition, compound 1 also exhibits excellent radiation resistance towards high dose γ

279

radiation (Figure S1), making it possible for potential applications in high level

280

nuclear waste solutions.

281

General fluorescence-based uranyl detection experiments. Under excitation at 365

282

nm, compound 1 exhibits the characteristic transitions of Tb3+ at 488, 545, 581 and

283

620 nm, which are ascribed to the 4f-4f transitions of

284

7

285

44.8% (Figure S3). We speculate that after the uranyl and the host interact, energy

286

transfer from the framework to the uranyl may result in change of luminescence

287

intensity. To verify this speculation, we investigated the luminescence evolution of

5

D4 → 7F6, 5D4 → 7F5, 5D4 →

F4, and 5D4 → 7F3, respectively.65 The quantum yield was determined to be as high as

13


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compound 1 treated with different concentrations of UO2(NO3)2·6(H2O) in deionized

289

water solutions (0.2 to 350 mg/L, pH = 4, solid/liquid ratio = 1.5 mg/mL). The uranyl

290

concentration-dependent luminescent intensity was clearly resolved by monitoring the

291

strongest transition of the Tb3+ ions, 5D4 → 7F4 centered at 545 nm. The distinct

292

luminescence quenching can even be observed with the naked eye (Figure 2a). As

293

shown in the inset of Figure 2a, the intensity decreased to 37.5% of the original level

294

at approximately 10 mg/L and was almost completely quenched at approximately 200

295

mg/L. The Langmuir model was successfully used to fit the luminescence quenching

296

ratio (denoted by (I0-I)/I0%) plotted as a function of uranium concentration (R2 =

297

0.9911), (Figure 2b). Therefore, the luminescence quenching phenomenon may be

298

directly correlated with the concentration-dependent uranyl adsorption behavior of

299

compound 1, which is often described by the Langmuir model. When the simulated

300

Langmuir equation was transformed into the correlation between uranyl concentration

301

and C[/(I0-I)/I0], where C is the uranyl concentration (Figure 2b inset), a nearly linear

302

correlation with R2 = 0.9999 was established. This method can therefore be used for

303

the highly accurate quantitative detection of uranium over a wide concentration range

304

(0.2 to 350 mg/L).

305

To explore the inherent relation between these unique luminescence evolution

306

characteristics and uranyl adsorption behavior, we investigated the uranyl adsorption

307

kinetics and isotherm. The adsorption kinetics were studied as a function of time

308

using 5 and 50 mg/L initial UO2(NO3)2•6(H2O) solution at pH 4 in deionized water.

309

Removal percentages as high as 98% at both 5 and 50 mg/L further confirm the strong 14



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310

uranyl enrichment capability of compound 1. As shown in Figure 3a and Figure S14a,

311

the uptake of uranium by compound 1 increased rapidly with increasing contact time

312

and reached 80% (at 5 mg/L) and 60% (at 50 mg/L) within only 10 minutes. These

313

results demonstrate the fast kinetics of uranyl adsorption by compound 1, which is a

314

critical factor for the quick response of a uranyl sensor to uranium in aqueous

315

solutions. The fast uranyl removal rate at the beginning is attributed to the rapid

316

diffusion of uranyl from the aqueous solution to the external surface of compound 1.

317

The subsequent slow process is attributed to the diffusion of uranyl into the inner

318

surface of compound 1.66, 67 However, the sorption isotherm data, which are well

319

fitted by the Langmuir model (R2 = 0.98), reveal a monolayer sorption mechanism

320

(Figure S14b), consistent with the presence of coordination-available amine groups as

321

the binding sites. And the sorption capacity was calculated as 179.08 mg (U)/g. Based

322

on the above experiment, the luminescence evolution characteristic is strictly

323

correlated with the uranyl sorption behavior, which can be precisely simulated with

324

the Langmuir model. Using these data, the correlation between the quenching ratio

325

and the sorbed uranium is also established, which can be fitted in the following

326

equation:

327

y = 31.64x/(1 + 0.35x)

328

where y is the quenching factor of luminescence intensity ((I0-I)/I0%) monitored at

329

545 nm, and x is the mass of sorbed uranium per gram of compound 1.

330

Because the chemical stability of compound 1 has been confirmed, the sorbed uranyl

331

ion on compound 1 can be desorbed under acidic conditions, which is critical for the

(4)

15


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recyclable performance of compound 1. Figure 3b shows the ratio of adsorption and

333

desorption of uranium. As measured, the ratio of uranium adsorption are higher than

334

99.5% and the desorption ratio are higher than 87.6% for at least three rounds. This

335

indicates that the uranium sorption onto compound 1 is reversible and therefore

336

compound 1 can be facilely regenerated.

337

Uranyl detection limit and selectivity. As shown in Figure 2a, the sharp change in

338

luminescence intensity for the low uranyl concentration region, especially between 0

339

and 0.2 mg/L, indicates the substantially low detection limit of compound 1. Given

340

the high quantum yield of the material, if the solid-liquid ratio is reduced to a lower

341

extent but still at a level where the luminescence signal can be clearly recorded, the

342

detection resolution will be greatly improved. Indeed, when the solid-to-liquid ratio

343

was reduced to a much lower extent at 0.05 g/L, compound 1 responded more

344

sensitively to uranyl at extremely low concentrations, even 5 µg/L of uranyl induced a

345

clear reduction of the luminescence intensity (Figure S4). Therefore, we investigated

346

the luminescence behavior under relatively low concentrations (＜10 mg/L) in detail

347

to determine the detection limit (Figure S17 and Figure S18). The detection limit was

348

calculated as:

349

DT=3σ/slope

(5)

350

σ=100×(ISE/I0)

(6)

351

(DT is the detection limit, ISE is the standard error of the luminescence intensity

352

measurement, as determined by the baseline measurement of blank samples

353

monitored at 545 nm (Figure S23), and I0 is the measured luminescence intensity of 16



354

compound 1 in deionized water. The slope was obtained from the linear fit of the

355

uranyl concentration-dependent luminescence intensity curve in the 0-3 mg/L region

356

(Figure S18 inset).68 Using these data, we determined the detection limit to be 0.9

357

µg/L, clearly demonstrating the potential application of compound 1 as a sensitive and

358

quantitative uranyl sensor.

359

To further explore the luminescence behavior of compound 1 in solutions with other

360

metal cations, compound 1 was immersed in highly concentrated (500 mg/L)

361

solutions of M(NO3)x·n(H2O) (M = Th4+, Eu3+, Sr2+, Al3+, Ca2+, Cs+, K+ as

362

representative monovalent, divalent, trivalent, and tetravalent cations). Impressively,

363

all these metal cations negligibly influenced the emission intensity of compound 1,

364

with the exception of Eu3+ (Figure 4a), which exhibits a slight quenching effect, but

365

still much weaker than that of UO22+ (Figure S5 and Table S1). After the addition of

366

uranyl to the above solutions, the luminescence was again completely quenched,

367

further confirming the uranyl-specific quenching process. This detection selectivity

368

likely originates from the electronic structure of the uranyl ion, which can lead to

369

more efficient resonance energy transfer than other common cations. This effect can

370

be further amplified by the soft nitrogen donor ligand, which also enables selective

371

enrichment of the uranyl ion in the presence of large excesses of competing cations.

372

Another reason of this selective quenching effect may partly stem from the selective

373

absorption property of compound 1 toward uranyl. To verify this hypothesis, we

374

conducted uranium uptake selectivity experiments. Specifically, finely ground powder

375

of compound 1 was immersed into 0.6, 1, 3, 5, and 10 mg/L of mixed metal (UO22+, 17


Page 18 of 36

Page 19 of 36


376

Th4+, Eu3+, Sr2+, Cs+, Al3+, Ca2+, and K+) nitrate salt solutions as well as mixed NaCl,

377

Na2CO3, and humic acid solutions. The uranyl sorption percentage was measured to

378

range from 93.9% to 99.3% in the mixed nitrate salt solutions and from 98.7% to 99.6%

379

in the mixed NaCl, Na2CO3, and humic acid solutions, respectively, as shown in

380

Figure 4b and Figure S15. These results demonstrate that the excellent detection

381

selectivity toward uranyl may indeed originates from the capability of the efficient

382

and selective enrichment of uranium by compound 1, providing opportunities for

383

applications under environmentally relevant conditions involving high ionic strength

384

values.

385

To test this capability, we investigated uranyl detection in two different natural water

386

systems: fresh lake water from Dushu Lake in Jiangsu Province, China, simulated

387

seawater and the actual seawater from Bohai Sea, China. The emission spectra of

388

compound 1 in these solutions were initially collected without additional uranium as

389

shown in Figure S6. Impressively, seawater negligibly affected the luminescence

390

intensity even at such a high ionic strength. We then performed spectroscopic

391

measurements in Dushu Lake water and in simulated seawater with different uranyl

392

concentration (0 to 400 mg/L) using the same experimental procedure as that used for

393

the measurements in deionized water. As shown in Figure 5a and b, similar trends

394

regarding the variation of luminescence intensity were obtained, and these data were

395

also successfully fitted by the Langmuir model (Figure 5c and d). Using these data,

396

we calculated the detection limit as 14.0 and 3.5 µg/L in Dushu Lake water and

397

seawater, respectively. The latter value is similar to the average uranium concentration 18



398

in seawater. In order to test the performance of compound 1 in real environment water

399

for uranium concentration determination, the concentration of uranium of an artificial

400

sample simulating the uranium contaminated Bohai seawater was determined by

401

ICP-MS and this method respectively as a comparison. Interestingly, the conentration

402

was determained to be 114.6 µg/L by ICP-MS and 106.7 µg/L by the reported method

403

(Figure S24).

404

Mechanism at the molecular level. As deduced from the PXRD analysis, the

405

luminescence quenching was not induced by framework collapse. Therefore, the

406

luminescent response toward the uranyl ion can be attributed to only the strong

407

interaction between uranyl and the skeleton of compound 1. The Fourier Transform

408

Infrared Spectroscopy (FT-IR) (Figure S7) and Energy Disperse Spectroscopy (EDS)

409

measurements (Figures S8-S12) of the sample treated with uranium aqueous solution

410

fully support the successful adsorption of uranium onto compound 1. We speculate

411

that the convergent orientation of the nitrogen sites with suitable distances in the

412

channel of the MOF provide compatible binding sites for the adsorbed uranium and

413

further impair the energy transfer from the TATAB ligand to the Tb3+ ions. Hence,

414

extended X-ray adsorption fine structure (EXAFS) and X-ray absorption near edge

415

structure (XANES) analyses were carried out to investigate the local coordination

416

environment of uranium uptake by compound 1 (Figure 6a and Table S2). The

417

samples were prepared by immersing the material in 100, 200 and 300 mg/L

418

UO2(NO3)2•6(H2O) solutions for 5 h (denoted by a, b and c, respectively). The

419

uranium concentrations in these samples were calculated as 1.3%, 1.7% and 5.1% 19


Page 20 of 36

Page 21 of 36


420

respectively. As shown for all three samples, the best fit on the coordination of

421

uranium is a combination of two axial uranyl oxygen atoms at distances from 1.78 to

422

1.79 Å and 5.8–5.9 equatorial donor atoms at distances from 2.45 to 2.47 Å, affording

423

hexagonal bipyramid coordination geometry. This contrasts sharply with the case of

424

uranyl hydrate compounds containing five equatorial oxygen atoms, providing a solid

425

evidence for chelation of uranyl ion by compound 1, given that all uranyl compounds

426

in hexagonal bipyramid coordination contain ligand chelation.69 In addition, these

427

equatorial bond distances are clearly longer than those in uranyl hydrate compounds

428

but are similar to the U-N interatomic distance of 2.44 Å observed in nitride fuels,70

429

further indicating that the amine donors play a key role in the inner-sphere

430

coordination mechanism. Given these nitrogen sites are in large molar excess when

431

the concentration of uranium is low, the uranyl coordination environment derived

432

from the relatively high uranium loading samples is also informative and likely the

433

same with cases of low uranium loading samples. This can be also confirmed by the

434

theoretic interpretation (see discussion below) unraveling the uranium binding sites

435

with large energetic preferences. Moreover, the formation of the U-N bond is also

436

supported by the XANES analysis. As shown in Figure 6b, peak A is considered to be

437

related to the bond length of equatorial-plane oxygen atoms. Compared to the

438

corresponding peak in the XANES spectrum of the uranyl hydrate compound, peak A

439

is shifted to a lower-energy position in the spectra of all three samples, indicating

440

elongation of the equatorial uranium bond distances.71

441

Uranyl binding sites analyzed by the first principle DFT calculations. In order to 20



442

elucidate the binding site of adsorbed uranyl ions in compound 1 and confirm the

443

inner-sphere coordination mechanism, the first principle Density Functional Theory

444

(DFT) calculations were performed. The large one-dimensional channels provide

445

sufficient space for efficient diffuse of hydrated uranyl cation into compound 1

446

without any orientation limitation. The mechanism of uranyl adsorption is generally

447

interpreted as uranyl complexation with the functional groups containing lone-pair

448

electrons.72 Moreover, the deprotonation of functional groups (may derive from the

449

complexation competition of uranyl ions) can further promote the adsorption. Based

450

on the crystal structure of compound 1, it can be expected that the uranyl ions should

451

be chelated by the exposed N donors in the TATAB ligand rather than the O atoms in

452

the carboxylate unit, which are saturated by Tb frames and mostly blocked by the

453

hydrophobic benzene rings (Figure 7b). Several potential uranyl binding sites were

454

identified and analyzed. Figures 7d, e and f show three embedded binding modes with

455

different levels of deprotonation of the TATAB ligand. In these cases, the uranyl ion

456

was encapsulated inside the interlayer between two TATAB ligands. In the first case,

457

the uranyl ion was chelated by two deprotonated amine group, two N donors from

458

triazine group from two different TATAB ligands, and two coordinating water

459

molecules (Figure 7d). The ∆G value of the uranyl complexation is negative (-6.48

460

kcal/mol) while binding energy Eb (-86.99 kcal/mol) is significantly stronger than that

461

of isolated hydrate uranyl cation (-63.64 kcal/mol, see Figure 7c), indicating uranyl

462

complexation on this site is highly thermodynamically favorable. The calculated

463

average equatorial coordination distance is ca. 2.58 Å, which is slightly longer than 21


Page 22 of 36

Page 23 of 36


464

that in isolated hydrate uranyl (ca. 2.48 Å). Both the coordination number and bond

465

distance are consistent with the EXAFS analysis result (see Table S2). Analysis on the

466

cases of single-deprotonated or neutral MOF shown in Figures 7e and f result in

467

positive ∆G values, suggesting uranyl complexations under these situations are not

468

preferential.

469

Figures 7g and h show that the uranyl ions were chelated by a single-deprotonated

470

TATAB fragment in the forms of 6- and 5-coordination (including 4 or 3 coordinating

471

water molecules, 1 triazine N atom and 1 deprotonated amine N- atom), respectively.

472

Although the calculated ∆G of binding mode (h) is more negative than that of mode

473

(g), the Eb of mode (g) is larger than that of both mode (h) and the isolated hydrate

474

uranyl mainly because of the increasing number of ligands. Meanwhile, the

475

coordination number and bond distance of (g) also agree with the EXAFS data. Figure

476

7i indicates that the binding of uranyl ion to the edge of compound 1 is unfavorable.

477

Therefore, we have obtained three possible binding modes (d, g and h) with two of

478

them (d and g)) consistent with the experimental analysis on the uranyl coordination,

479

providing direct evidence on strong interaction between the uranyl ion and compound

480

1, further leading to the efficient uptake as well as sensitive and selective detection

481

capabilities.

482

Environmental Implications. We successfully introduced a mesoporous luminescent

483

MOF material into the field of uranium detection. High selectivity over competing

484

environmentally and nuclear-fission relevant metal ions, together with the high

485

sensitivity of compound 1, indicate that this porous material can perform as a highly 22



486

desired uranyl fluorescence sensor. The detection limits in deionized water, natural

487

fresh water, and seawater were calculated as 0.9 µg/L, 14.0 µg/L and 3.5 µg/L,

488

respectively. More importantly, this material provides an opportunity for uranium

489

detection in a wider range than the traditional materials, and it could be used to

490

directly detect uranyl ion in both severely polluted areas (e.g., around mining and

491

processing sites) and less-polluted sites. To our knowledge, compound 1 is the first

492

luminescent MOF sensor that can respond to uranium specifically in natural fresh

493

water and seawater and the only MOF material that can detect uranium below the

494

standard of 30 µg/L in drinking water defined by the United States EPA. The

495

inner-sphere coordination mechanism accounting for these superior capabilities was

496

confirmed by EXAFS and XANES analysis. Furthermore, the arrangement of

497

coordination-available nitrogen donors in the crystal structure of compound 1

498

provides suitable binding sites for preferential uranyl complexation, as illustrated by

499

the DFT calculations. Finally, we have demonstrated that this superior detection

500

capability can be directly correlated with the adsorption behaviors of uranyl onto the

501

solid material, which sheds light on material design for heavy-metal detection with

502

enhanced sensitivity and selectivity.

503 504

ASSOCIATED CONTENT

505

●S Supporting Information. This information is available free of charge via the

506

Internet at http://pubs.acs.org.

507

ASSOCIATED CONTENT 23


Page 24 of 36

Page 25 of 36


508

Corresponding Authors

509

*Corresponding

510

[email protected] (SHUAO WANG); Tel: +86-512-65883945; Fax: +86-512-65883945.

511

Notes

512

The authors declare no competing financial interest.

513

§

authors.

Email:

[email protected]

(RUHONG

ZHOU);

These two authors contributed equally.

514 515

516

This work was supported by grants from the National Science Foundation of China

517

(21422704,

518

(JCKY2016212A504), the Science Foundation of Jiangsu Province (BK20140007,

519

BK20160312), the General Financial Grant from the China Postdoctoral Science

520

Foundation (2016M590493, 2016M591901), a Project Funded by the Priority

521

Academic Program Development of Jiangsu Higher Education Institutions (PAPD),

522

and "Young Thousand Talented Program" in China.

ACKNOWLEDGMENTS

U1532259,

21601131),

the

Science Challenge

Project

523 524 525 526 527 528 529 530 531 532 533 534

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K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; J. A. Montgomery, J.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Keith, T.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, O.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01 Gaussian, Inc., Wallingford CT,. 2013. (53) Thompson, H. A.; Brown, G. E.; Parks, G. A. XAFS spectroscopic study of uranyl coordination in solids and aqueous solution. Am. Mineral. 1997, 82, 5-6, 483-496. (54) Becke, A. D. Density-Functional Thermochemistry .3. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 7, 5648-5652. (55) Lee, C. T.; Yang, W. T.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron-Density. Phys. Rev. B 1988, 37, 2, 785-789. (56) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104/1-154104/19. (57) Kuchle, W.; Dolg, M.; Stoll, H.; Preuss, H. Energy-Adjusted Pseudopotentials for the Actinides - Parameter Sets and Test Calculations for Thorium and Thorium Monoxide. J. Chem. Phys. 1994, 100, 10, 7535-7542. (58) Cao, X. Y.; Dolg, M. Segmented contraction scheme for small-core actinide pseudopotential basis sets. J. Mol. Struc-Theochem. 2004, 673, 1-3, 203-209. (59) Hehre, W. J.; Ditchfield, R.; Pople, J. A. Self-Consistent Molecular-Orbital Methods .12. Further Extensions of Gaussian-Type Basis Sets for Use in Molecular-Orbital Studies of Organic-Molecules. J. Chem. Phys. 1972, 56, 5, 2257-2261. (60) Zhao, Y.; Truhlar, D. G. The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals. Theor. Chem. Acc. 2008, 120, 1-3, 215-241. (61) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. V. Efficient Diffuse Function-Augmented Basis Sets for Anion Calculations. Iii. The 3-21+G Basis Set for First-Row Elements, Li-F. J. Comput. Chem. 1983, 4, 3, 294-301. (62) Mclean, A. D.; Chandler, G. S. Contracted Gaussian-Basis Sets for Molecular Calculations .1. 2nd Row Atoms, Z=11-18. J. Chem. Phys. 1980, 72, 10, 5639-5648. (63) RSC AdvancesMarenich, A. V.; Cramer, C. J.; Truhlar, D. G. Universal Solvation Model Based on Solute Electron Density and on a Continuum Model of the Solvent Defined by the Bulk Dielectric Constant and Atomic Surface Tensions. J. Phys. Chem. B. 2009, 113, 18, 6378-6396. (64) Zhang, H.; Chen, D.; Ma, H.; Cheng, P. Real-Time Detection of Traces of Benzaldehyde in Benzyl Alcohol as a Solvent by a Flexible Lanthanide Microporous Metal-Organic Framework. Chemistry. 2015, 21, 44, 15854-15859. (65) Zhao, J.; Wang, Y. N.; Dong, W. W.; Wu, Y. P.; Li, D. S.; Zhang, Q. C. A Robust Luminescent Tb(III)-MOF with Lewis Basic Pyridyl Sites for the Highly Sensitive Detection of Metal Ions and Small Molecules. Inorg. Chem. 2016, 55, 7, 3265-3271. (66) Yang, S.; Zhao, D.; Zhang, H.; Lu, S.; Chen, L.; Yu, X. Impact of environmental conditions on 28



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Figure Captions

732

Figure 1 Schematic of the synthesis procedure and the crystal structure of compound 1.

733

Figure 2 (a) Emission spectra of compound 1 in UO2(NO3)2•6(H2O) solution; the inset is the

734

correlation between luminescence intensity and UO2(NO3)2•6(H2O) concentration, the picture

735

below is the corresponding luminescence photograph of compound 1 after immersed in different

736

concentration of UO2(NO3)2•6(H2O) solution. (b) Simulated correlation between (I0-I)/I0 and

737

UO2(NO3)2•6(H2O) concentration using the Langmuir model. The inset is the correlation between

738

uranyl concentration and C/[(I0-I)/I0] %.

739

Figure 3 (a) Adsorption kinetics measured using 5 mg/L initial UO2(NO3)2•6(H2O) solution at pH

740

4. (b) Adsorption and desorption ratios for three rounds. 29


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741

Figure 4 (a) Luminescence intensity of compound 1 in different metal salt solutions with/without

742

uranium, below is the corresponding luminescence photograph. (b) The uranyl adsorption ratio

743

measured in 0.6, 1, 3, 5, and 10 mg/L mixed competing metal-ion solutions.

744

Figure 5 (a), (b) Emission spectra of compound 1 in Dushu Lake water and in seawater,

745

respectively, with a wide range of uranium concentrations from 0 to 400 mg/L. (c), (d) Simulated

746

correlation between (I0-I)/I0 and concentration using the Langmuir model in Dushu Lake water

747

and in seawater, respectively. The insets are the correlations between uranyl concentration and

748

C/[(I0-I)/I0]%.

749

Figure 6 X-ray absorption spectroscopy results: (a) Radial structural functions obtained from the

750

EXAFS analysis and (b) near-edge region of the raw U L3 adsorption spectra for uranyl-sorbed

751

samples a, b and c, and uranyl hydrate compounds.

752

Figure 7 (a) Crystal structure of compound 1 along Z axis. (b) Fragment of compound 1. (c)

753

Structure of uranyl hydrate. (d-f) Embedded adsorption modes with different levels of

754

deprotonation of the TATAB ligand. (g-i) Edge adsorption modes with different levels of

755

deprotonation of the TATAB ligand. Eb: Binding strength between the uranyl ion and compound

756

1. ∆G: Gibbs free energy changes (at 298.15 K, 1 atm) of the complexation reaction.

757 30



758

Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Table of contents

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Highly Sensitive and Selective Uranium Detection in Natural Water

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