A Dual Associated-Functional Fluorescent Switch: From Alternate

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A Dual Associated-Functional Fluorescent Switch: From Alternate Detection Cycle for Fe(III) and pH to Molecular Logic Operations Xue-Song Zhou, Rui-Qing Fan,* Hao-Xin Ye, Kai Xing, A-Ni Wang, Ping Wang, Su-E Hao, and Yu-Lin Yang* MIIT Key Laboratory of Critical Materials Technology for New Energy Conversion and Storage, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin 150001, People’s Republic of China

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ABSTRACT: With the expansion and deepening of scientific research, dualfunctional or multifunctional materials are urgently needed to replace those for single application. Herein, a fluorescence sensing system based on an In(III)-organic complex with in situ Lewis acid sites has been constructed, exhibiting high sensitivity for the detection of Fe(III) ions with a low detection limit of 3.95 μM and a short response time of within 10 s. It is noteworthy that the quenched fluorescence of the Fe(III)-incorporated sample could be reopened linearly with an increase of alkalinity, followed by the reactivation of its functionality to identify Fe(III) ions, forming an alternate detection cycle for Fe(III) and pH with off−on−off fluorescent switch characteristics. Considering its unique molecular recognition capability, an advanced three-input (Fe(III), EDTA, and OH−) and twooutput (B440 and G489) Boolean logic operation comprising BUFF, NOT, OR, and AND logic gates was integrated, possessing potential applications in intelligent multianalyte sensing systems.

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INTRODUCTION As one of the most indispensable elements in biological and environmental systems, iron participates in a greata deal of essential metabolic processes, such as oxygen transport, blood acid−base balance, and organism immunity regulation.1,2 Both deficiency and excess of the normal permissible limit may result in serious disorders.3−5 The deficiency of iron content limits the oxygen delivery to cells, leading to fatigue, poor mental state, and decreased immunity. Conversely, an excess of iron amount can catalyze the production of reactive oxygen species via the Fenton reaction, which can damage lipids, nucleic acids, and proteins. In addition, the cellular toxicity of iron ions has been proved to be related to serious diseases, such as Alzheimer’s, Huntington’s, and Parkinson’s diseases.6 Thus, various analytical techniques have been developed for iron ion monitoring, such as atomic absorption spectroscopy, inductively coupled plasma mass spectrometry, spectrophotometry, and voltammetry.7 However, the interference from other metal ions cannot be ignored, necessitating complicated pretreatment procedures and sophisticated instrumentation. In this regard, sensitive detection of trace iron ions with a resistance to other mixed-metal ions is extremely significant but challenging. Similarly, pH is also one of the key parameters in environmental and biological systems, and probes for accurately monitoring pH values are eagerly desired.8 Recently, fluorescent probes based on metal-organic complexes (MOCs) have emerged as a simple, sensitive, and environmentally friendly technology for the online monitoring © XXXX American Chemical Society

of target analytes without complicated pretreatments, exhibiting great potential in chemical and biological identification.9−15 The tunable compositions, architectures, and functional sites, together with their high crystallinity and suitable porosity, not only make MOCs an optimal platform to design and synthesize tailored functional materials for the real-time detection of target analytes but also make it conducive to further analyze and understand the mechanisms of sensing behavior.16−20 To date, a few fluorescent MOCs have been designed for Fe(III) ions or pH sensing; however, most of them take effect only in organic solvent systems owing to their poor water stability, which limits their practical application.21,22 To design water-stabilized MOC probes, an effective fluorophore, chemically robust structures, suitable pore sizes, and exposed recognition sites are required. On the basis of the above consideration, the organic linker 2,2′-bipyridine-5,5′dicarboxylic acid (H2bpydc) was introduced, of which both pyridine nitrogen atoms and carboxyl oxygen atoms could act as coordination sites for central metals or recognition sites for target analytes. In this work, two new In(III)-based MOCs, namely [In(bpydc)Cl]·CH3CN (In1) and [In(H2bpydc)(H2O)Cl3]·CH3CN·H2O (In2), were successfully designed and synthesized. By precise regulation of the degree of deprotonation, the pyridine nitrogen atoms and carboxyl Received: November 16, 2018

A

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Schematic Representation of Alternate Detection Cycle toward Fe(III) Ions and pH

sphere. Scanning electron microscope (SEM) images were recorded with a Rili SU 8000HSD Series Hitachi New Generation Cold Field Emission SEM. Dynamic light scattering (DLS) experiments were performed using a Zetasizer Nano-ZS90 instrument (Malvern Instruments Ltd., U.K.). UV−vis absorption spectra were obtained on PerkinElmer Lambda 750 spectrophotometers. X-ray photoelectron spectroscopy (XPS) was carried out on a RBD upgraded PHI-5000C ESCA system (PerkinElmer) with Mg Kα radiation (hν = 1253.6 eV). The density functional theory (DFT) and timedependent density functional theory (TD-DFT) calculations were conducted using the B3LYP/lanl2dz method basis set as implemented in the Gaussian 09 package. Synthesis of [In(bpydc)Cl]·CH3CN (In1). A mixture of H2bpydc (12.21 mg, 0.05 mmol), NaOH (3.00 mg, 0.075 mmol), and InCl3· 4H2O (42.78 mg, 0.15 mmol) in CH3CN/H2O mixed solvent (8.0 mL, 4/4 v/v) was sealed in a 23 mL Teflon-lined stainless-steel autoclave and heated at 120 °C for 120 h. After the mixture was slowly cooled to room temperature and washed with deionized water, yellow block crystals of In1 were collected in 46.3% yield on the basis of the H2bpydc ligand. Anal. Calcd for C14H9N3O4ClIn (433.51): C, 38.75; H, 2.08; N, 9.69. Found: C, 38.64; H, 2.10; N, 9.70. IR (KBr pellet, cm−1, Figure S1a): 3466 (w), 3339 (w), 1688 (m), 1597 (s), 1552 (w), 1397 (s), 1253 (w), 1126 (w), 1027 (m), 881 (s), 781 (s), 708 (m), 654 (w), 536 (m), 454 (s). Synthesis of [In(H2 bpydc)(H2O)Cl3]·CH3CN·H2O (In2). Compound In2 was synthesized by the same procedure used for the preparation of In1, except for a reduction in the amount of NaOH (0.43 mg, 0.011 mmol). After the reaction mixture was slowly cooled to room temperature and washed with deionized water, yellow block crystals of In2 were isolated in 42.1% yield on the basis of the H2bpydc ligand. Anal. Calcd for C14H15N3O6Cl3In (542.46): C, 30.97; H, 2.76; N, 7.74. Found: C, 30.92; H, 2.78; N, 7.75. IR (KBr pellet, cm−1, Figure S1b): 3375 (w), 2876 (w), 1662 (m), 1700 (s), 1579 (m), 1416 (m), 1234 (s), 1134 (m), 1026 (m), 863 (m), 799 (s), 754 (s), 663 (m), 518 (w). X-ray Crystallography. The crystallographic data of In1 and In2 were collected on a Bruker Smart Apex II CCD area-detector diffractometer with graphite-monochromated Mo Kα (λ = 0. 71073 Å) radiation at room temperature using the ω-scan technique. Their structures were solved by direct methods, and all of the non-hydrogen atoms were refined on F2 by full-matrix least squares, using the the SHELXTL-2014 crystallographic software package.23,24 The hydrogen atoms were generated geometrically and refined isotropically using the

oxygen atoms of H2 bpydc in In1 all participated in coordination with the central metal In(III), while the functionality of the carboxyl groups in In2 was reserved as expected. As a result, although both In1 and In2 possess 1D porous channels and typical AIE properties, only In2 exhibits a highly sensitive and selective fluorescence quenching response to trace amounts of Fe(III) ions with a low detection limit of 3.95 μM and short response time of within 10 s, benefiting from the exposed carboxyl group receptors toward the inside of channels. Significantly, the quenched fluorescence owing to the bonding effect between carboxyl groups and Fe(III) ions could be reopened linearly with an increase in alkalinity, acting as a pH sensor, followed by the functional reactivation of carboxyl groups to identify Fe(III) ions, exhibiting an interesting alternate detection cycle (Scheme 1). To our knowledge, this is the first dual-associated-functional MOC-based fluorescent switch for alternately probing Fe(III) and pH with “OFF” and “ON” optical signal response, respectively. To demonstrate the multicomponent identification properties of In2, an advanced integrated analytical circuit based on BUFF, NOT, OR, and AND logic gates is constructed with three signal inputs (Fe3+, EDTA, and OH−) and two signal outputs (B440 and G489), extending the potential application of Boolean logic operations in intelligent multianalyte detection systems.

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EXPERIMENTAL SECTION

Materials and Measurements. All general reagents and solvents (AR grade) were commercially available and were used without further purification. Elemental analyses (EA) for C, H, and N were carried out on a PerkinElmer 2400 element analyzer. FT-IR spectra (4000−400 cm−1) were recorded with a Nicolet Impact 410 FT-IR spectrometer. Powder X-ray diffraction (PXRD) patterns were recorded in the 2θ range of 5−50° using Cu Kα (λ = 1.5418 Å) radiation with a Shimadzu XRD-6000 X-ray diffractometer. Thermogravimetric analyses (TGA) was performed on a ZRY-2P thermal analyzer with a heating rate of 10 °C min−1 under a flow of air. The fluorescence excitation, emission, and lifetime spectra were recorded on an Edinburgh FLS920 luminescence spectrometer at 298 K. Solid-state fluorescence quantum yields were obtained on a FluoroLog UltraFast spectrofluorometric analyzer with an integrating B

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry riding model except for those of water molecules. Due to the highly disordered solvent molecules in In1, the SQUEEZE routine of PLATON was applied to model their contribution to the densities.25 The chemical formulas were determined by the combination of X-ray diffraction data, elemental analysis, and TGA results. The CCDC file numbers 1853651 and 1853652 contain crystallographic data for In1 and In2. These data can be obtained free of charge at www.ccdc.cam. ac.uk/deposit. The details of crystal structure parameters for In1 and In2 are summarized in Table 1, selected bond lengths and bond angles can be found in Tables S1 and S2, and hydrogen-bonding data of In2 are given in Table S3.

create a single helical chain as the sheath of a 1D tubular-like channel along the b axis (Figure 1b), with a rhombic section of 10.782 × 9.380 Å2. Furthermore, every tubular-like channel stacks closely with four neighboring symmetrically equivalent ones, sharing bpydc2− ligands as the wall, to form a three-dimensional (3D) framework (Figure 1d). The total solvent-accessible volume in the framework of In1 is determined to be 38.9% of its unit cell, as estimated by PLATON. Topologically, the overall structure can be simplified to a binodal cdt network with the point symbol (103)(103) (Figure S2f) regarding the bpydc2− organic linkers and SBUs as two different 3-connected nodes (Figure S2c,e), respectively. Structural Description for In2. When a smaller dosage of NaOH was used, In2 crystallizing in the triclinic space group P1 was successfully obtained. The central metal In(III) is sixcoordinated by three chlorine atoms (Cl1, Cl2, and Cl3), one oxygen atom (O5) from a water molecule, and two nitrogen atoms in the form of chelation from the same H2bpydc ligand, giving a distorted-quadrangular-bipyramidal configuration (Figure 3b). Significantly, neither carboxyl group of the H2bpydc ligand was deprotonated, owing to the precise regulation of NaOH. The detailed coordination environment of In(III) is illustrated in a form of a dimer (Figure 2a), which is connected through C−H···Cl hydrogen bonds (Cl2···H9A = 2.866 Å) between two asymmetric units. Under-distance accumulation (3.343 Å) of face-to-face pyridine rings as well as the large overlap area of the π-electron atmosphere are thought to yield robust π···π stacking interactions within each dimer (Figure S4). Moreover, adjacent dimers are connected to each other via three kinds of hydrogen bonds (Cl3···H4A = 2.970 Å, O4···H5A = 2.041 Å, and O1···H5C = 2.639 Å, respectively), giving the final 3D supramolecular framework with 1D tubularlike channels of 9.221 × 6.678 Å2 (Figure 2b). It is noteworthy that the reserved −COOH groups possess applicable orientation toward the inside of the channels, which play vital roles in the unique sensing properties of In2 as discussed below. The total solvent-accessible volume in framework of In2 is determined to be 28.3% of its unit cell, as estimated by PLATON. From a topological viewpoint, the dimer can be seen as six-connected parallelepiped nodes (Figure 2c); the 3D structure of In2 can thus be simplified as an uninodal net with the point symbol 412.63, which is classified as the pcu topology (Figure 2d). Powder X-ray Diffraction (PXRD) and Thermal Stability. The phase purity of In1 and In2 was confirmed by PXRD measurements, as each PXRD pattern of the assynthesized samples is consistent with the simulated pattern (Figure S5). Moreover, their thermal stability was further investigated by thermal gravimetric analysis (TGA) in an air atmosphere in the temperature range 30−600 °C (Figure S6). In1 loses 9.29% of weight in the temperature range of 30−170 °C, which is thought to correspond with the removal of free CH3CN molecules (calcd 9.42%), and the framework can remain stable up to about 310 °C. In2 shows a weight loss of 10.41% in the range of 30−135 °C, which is attributed to the departure of guest H2O and CH3CN molecules (calculated 10.88%), followed by a further weight loss at about 200 °C, corresponding to the collapse of the framework. The higher thermal stability of compound In1 in comparison to that of In2 should be attributed to the stronger rigidity of coordinate bonds in comparison to the hydrogen bond interactions. Finally, the In2O3 phase should be left over for both In1 and

Table 1. Crystal Data and Structure Refinement Details for In1 and In2 In1 empirical formula formula wt cryst syst space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z calcd density (mg/m3) μ(Mo Kα) (mm−1) F(000) θ range (deg) limiting indices no. of data/restraints/params goodness of fit on F2 final R indices (I > 2σ(I)) R1a wR2b R indices (all data) R1 wR2 largest diff peak, hole (e Å−3) CCDC

C14H15N3O6Cl3In 542.46 triclinic P1̅ 8.380(5) 10.200(6) 13.832(8) 83.95(6) 89.98(7) 66.13(6) 1074.0(1) 2 1.677 1.506 536 2.51−26.00 −10 ≤ h ≤ 9, −12≤ k ≤ 12, −17 ≤ l ≤ 17 4009/0/244

1.089

1.227

0.0409 0.0364

0.1615 0.0567

0.1351 0.1241 0.691, −0.492

0.1818 0.1796 3.737, −5.678

1853651

1853652

R1 = ∑||Fo| − |F c||/∑|F o|. ∑[w(Fo2)2]]1/2. a

In2

C14H9N3O4ClIn 433.51 orthorhombic Pna21 18.199(1) 9.261(7) 10.782(8) 90.00 90.00 90.00 1817.2(2) 4 1.435 1.457 760 2.34−27.45 −23 ≤ h ≤ 23, −12 ≤ k ≤ 12, −13 ≤ l ≤ 13 4146/1/182

b

wR2 = [∑[w(F o2 − Fc2)2]/

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RESULTS AND DISCUSSION Structural Description for In1. A crystal structure analysis revealed that In1 crystallized in the orthorhombic space group Pna21. Benefiting from the entirely deprotonated bpydc2− ligand with (μ2(O)-η1:η1:η1:η1)-(μ1(N)-η1:η1) coordination mode, the central metal In(III) adopts an In(C2N2)(CO2)2Cl secondary building unit (SBU) distorted-decahedral geometry (Figures S2d and S3a), which comprises two pyridine nitrogen atoms (N1 and N2) from a bpydc2− ligand, four chelating carboxylate oxygen atoms (O1, O2, O3, and O4) from two different bpydc2− ligands, and one terminal chlorine atom (Cl1).26 These SBUs are further linked by bpydc2− ligands in a form of counterclockwise rotation, to C

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Coordination environment of In(III) in In1 with ellipsoids drawn at the 30% probability level. (b) 1D tubular-like channel along the b axis. (c) Simulated diagram of the single-helical chain. (d) Central projection view of the 3D framework. (e) Simulated diagram of the 3D framework.

Figure 2. (a) Coordination environment of In(III) in In2 in a form of a dimer. (b) Central projection view of the 3D supramolecular framework. (c) Six-connected parallelepiped nodes. (d) pcu topology network. (e) Simulated diagram of the 3D supramolecular framework with exposed carboxyl sites.

systems, have been investigated as promising candidates for potential fluorescent materials.27−32 The solid-state fluorescence properties of the ligand H2bpydc and as-synthesized In1 and In2 were tested at room temperature (Figure S7). Almost the same excitation peaks for all of them at around 350 nm indicate that the fluorescence emissions of In1 and In2 were

In2 with the theoretical values 32.02% for In1 and 25.59% for In2, which are in agreement with the experimental results (found: 31.43% for In1 and 26.03% for In2). Solid/Liquid State Fluorescence Performance Tests. Metal−organic complexes, especially those constructed from d10 metal centers and organic linkers with large conjugate D

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 3. (a) Fluorescence emission spectra and (b) relative intensity histograms of In2 in aqueous suspension with different metal ions and corresponding fluorescent images under (c) daylight and (d) 365 nm UV light, respectively.

The difference in fluorescence response to organic solvent molecules for In1 and In2 can, to a great extent, be attributed to the more robust framework of In1 in comparison to that of In2; however, intermolecular energy transfer seems unconvincing to explain their abnormal fluorescence enhancement in aqueous solution. Considering the fact that both In1 and In2 are soluble in common organic solvents but insoluble in water, their liquid-state fluorescence emission spectra in H2O/ C2H5OH mixed solvents with the same concentration of 0.15 mM but different water contents from 0% to 100% (volume/volume percentages) have been recorded. As shown in Figure S9, there is a clear trend that the fluorescence intensity of In1 and In2 increases with an increase in water content. The subsequent dynamic light scattering (DLS) measurements provided more direct evidence for particles of In1 and In2 in H2O/C2H5OH mixed solvents becoming obviously aggregated (Figures S10 and S11) with an increase in water content, indicating that the strong fluorescence emission of the aqueous suspensions can be attributed to aggregation-induced emission (AIE) effects. Fe(III) Ion Detection. Considering the uncoordinated carboxyl groups as electron-donating sites with reserved bonding ability, the application of In2 as a fluorescent probe for sensing metal ions was explored. As a contrast, the same operations were conducted for In1. The intense fluorescence emission of In1 and In2 in aqueous solution inspired us to choose water as the dispersion medium. Uniformly ground samples of In1 and In2 were immersed in 5 mL of deionized water and ultrasonically dispersed to form stable suspensions containing different MClx (5 mM, M = Na+, Mg2+, Al3+, K+, Ca2+, Mn2+, Fe2+, Fe3+, Ni2+, Cu2+, Zn2+, Sn4+, Ba+, Eu3+, Hg2+) for subsequent spectroscopic measurements. It is noteworthy that the fluorescence intensities of In2 exhibit an obvious dependence on the species of metal ions; however, the fluorescence of In1 is almost unaffected, as shown in Figure 3a and Figure S12. Moreover, only Sn4+ ions have a slightly enhanced effect on the fluorescence emissions, while all of the other metal ions possess different degrees of quenching effects,

both ligand-based. In comparison with the pure ligand H2bpydc, In1 and In2 show obviously stronger fluorescence intensity (emission bands at 479 nm for In1 and 454 nm for In2, respectively), implying that the formation of MOCs could effectively stabilize the radiative transition pathway and ensure the transformation of excited state energy to intense fluorescence emission. In addition, the small changes in the position and shape of their emission peaks might result from the different electronic structures of In1 and In2. Notably, the fluorescence quantum yield (φ) of In1 (13.9%) is slightly lower than that of In2 (15.3%), which might be attributed to the robust π···π stacking interactions in In2, providing greater restriction of the intramolecular rotation to reduce the nonradiative decay process. Moreover, the solid-state fluorescence lifetimes (τ) of In1 and In2 were also measured to be 7.91 and 8.63 ns, respectively. According to the definitions of kr = φ/τ and knr = (1 − φ)/τ, radiative (kr) and nonradiative (knr) decay rate constants were estimated (kr = 1.76 × 107 s−1 for In1 and 1.77 × 107 s−1 for In2; knr = 1.09 × 108 s−1 for In1 and 9.81 × 107 s−1 for In2; Table S4).8,33 The higher kr/knr value for In2 in comparison to that of In1 further affirmed the benign effect of robust π···π stacking interactions on intense fluorescence emission. Obviously, such fluorescence dynamics provide very favorable conditions for a deep understanding of the photophysical properties of fluorescent molecules. For such ligand-based emission systems, the energy absorbed by organic ligands from the excitation may decay in the presence of solvent molecules via intermolecular energy transfer.34,35 This prompted us to study the emission stability of In1 and In2 in suspensions by dispersing fresh samples in different solvents. Significantly, in comparison to the fluorescence emissions in the solid state, the intensities of In1 and In2 were reduced drastically in suspensions of various solvents with the same concentration of 0.15 mM. In addition, the fluorescence emissions of In2 are largely dependent on solvents, while those of In1 show good stability in most of the prepared suspensions, except for the distinct enhancement in aqueous solution for both In1 and In2, as shown in Figure S8. E

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. (a) Emission spectra for In2 in an aqueous suspension upon incremental addition of Fe(III) ions. (b) Corresponding SV plots and fluorescent images at low Fe(III) concentrations. (c, d) Selective detection of Fe(III) ions for In2 in the presence of other metal ions.

10 s and complete quenching after 40 s, further demonstrating its sensitivity (Figure S13). Usually, many metal ions coexist in a practical biological and environmental system. Considering the high sensing sensitivity of In2 toward Fe(III), we further tested its detection selectivity in the presence of other metal ions by fluorescence spectra. In the control experiment, aqueous suspensions of In2 with MClx (5 mM, M = Na+, Mg2+, Al3+, K+, Ca2+, Mn2+, Fe2+, Ni2+, Cu2+, Zn2+, Sn4+, Ba+, Eu3+, Hg2+) were initially prepared, followed by the addition of Fe(III) ions. With increasing Fe(III) concentration, the fluorescence emissions of these aqueous suspensions were quenched efficiently, easily visualized by plotting the percentage fluorescence intensity versus the rise of Fe(III) concentration (Figure 4c,d), clearly demonstrating the high selectivity of In2 for Fe(III). Mechanism for Probing Fe(III) Ions. Generally, the quenching effect of analytes on fluorescent molecules can be ascribed to three different mechanisms: (1) the collapse or transformation of the framework caused by the analytes, (2) molecule or ion exchange between the probe molecules and analytes, (3) strong interaction between the active sites of probe molecules and analytes.40,41 To identify the mechanism in this case, further research was carried out. As shown in the PXRD patterns of In2 and Fe(III)@In2 (Figure S14), the similarity in peaks indicated that the main framework of In2 remained stable during the quenching process. Scanning electron microscope (SEM) measurements provided more direct evidence to support the maintained crystallinity of Fe(III)@In2 (Figure S15). Meanwhile, the corresponding elemental mapping images revealed that the Fe(III) ions were distributed homogeneously in the framework of In2. On the other hand, the results of an inductively coupled plasma (ICP) analysis for pure In2 and Fe(III)@In2 (Table S6) further indicated that there was no ion exchange between the center metal In(III) of the probe and analyte Fe(III) ions. Thus, the collapse or transformation of the framework and ion exchange

especially for Fe(III) ions, indicating the potential application prospects of In2 for Fe(III) sensing as a chemodosimeter. As one of the most indispensable elements in biological systems, Fe(III) participates in a great number of essential metabolic processes, such as oxygen metabolism, blood acid−base balance, and organism immunity regulation.36−38 In this regard, selective and sensitive detection of trace amounts of Fe(III) ions has been regarded to have great significance. Encouraged by the highest quenching efficiency up to 99% of Fe(III) ions for the fluorescence emission of In2, we performed quantitative titration experiments of the suspensions upon incremental addition of Fe(III) ions (5 mM, 5 μL addition each time) to assess the sensing sensitivity of In2. Obviously, the fluorescence intensity of In2 is heavily dependent on the concentration of Fe(III) ions. As shown in Figure 4a, with an increase i Fe3+ concentration from 0 to 0.40 mM, the fluorescence emission decreased monotonically. The quenching efficiency can be quantitatively explained by the Stern−Volmer (SV) equation:39 I0/I = 1 + KSV[M], where I0 and I are the fluorescence intensities of pure In2 and Fe(III)incorporated In2 (Fe(III)@In2) suspensions, respectively, Ksv is the quenching constant (M−1), and [M] is the molar concentration of the Fe(III) ions. It was found that the SV plot reveals a great linear correlation (R2 = 0.9915) in the low concentration range from 0 to 0.10 mM (Figure 4b) but subsequently deviates from linearity with an upward bend as the concentration is increased and tends to be horizontal over 0.35 mM. On the basis of the experimental data, the KSV value was calculated to be 1.17 × 104 M−1 for In2. According to the KSV values and the standard error (σ) from three repeated fluorescence measurements of blank solutions, the detection limit (3σ/KSV) was calculated to be 3.95 μM. In comparison with reported MOC sensors, the sensing sensitivity of In2 toward Fe(III) is relatively good under the same conditions, as shown in Table S5. Moreover, In2 exhibits an extremely short response time with a nearly 70% quenching of emission within F

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. (a) Absorption spectra of In2 in aqueous suspension with the presence of different metal ions. (b) Job plot of In2-Fe3+. (c) XPS spectra and (d) O 1s spectra of In2 and Fe(III)@In2.

Figure 6. (a) Fluorescence lifetime curves for In2 toward Fe(III) ions of different concentrations at 298 K. (b) Corresponding linear fitting for the calculation of dynamic quenching constant.

provided more direct evidence to support the strong bonding effects between Fe(III) and the −COOH groups (Figure 5d). The Job plot for the fluorescence emission was drawn with the same total concentration (0.15 mM) but different molar ratios of In2 and Fe(III) ions (Figure 5b), to determine the stoichiometry.42 The resulting 1:2 stoichiometry was further verified by the Benesi−Hildebrand (B−H) equation:43−45

mechanisms were both ruled out, while the strong interaction between the −COOH sites of In2 and Fe(III) ions seems to be most reasonable to explain its sensing property toward Fe(III) ions. In order to verify the above conclusions, UV/vis absorption spectra of In2 aqueous suspensions with different metal ions were recorded. Similar absorption peaks were observed for most as-selected metal ions, except for the new peak appearing around 360 nm upon the addition of Fe(III) ions (Figure 5a). This can be ascribed to the strong interaction between framework of In2 and Fe(III) ions, through which energy transfer occurred to decrease or quench the fluorescence. Subsequently, the X-ray photoelectron spectra (XPS) of In2 and Fe(III)@In2 were recorded to further determine the position of bonding behavior, as shown in Figure 5c. The peaks appearing at 723 and 711 eV for Fe(III)@In2 were assigned to the electron binding energies of Fe 2p1/2 and Fe 2p3/2, respectively, confirming the presence of Fe(III). Moreover, the chemical shift of the O 1s peak from 531.4 to 532.2 eV

1 1 1 = + 3+ 2 I0 − I I0 − Imin K (I0 − Imin)[Fe ]

Imin is the minimum fluorescence intensity in the presence of Fe(III) ions and K is the association constant. As shown in Figure S16, the good linear fitting (R = 0.9977) for the plot of 1/(I0 − I) against 1/[Fe3+]2 indicated that the duty ratio of In2 and Fe(III) ions certainly was 1:2. This stoichiometry was also consistent with the molar content of the −COOH groups in In2, further demonstrating that the bonding effects surely occurred between the −COOH sites and Fe(III) ions. As reported in the literature, fluorescence quenching can be divided into dynamic and static quenching.46 The dynamic G

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 7. (a) Fluorescence intensities of In1, In2, and Fe(III)@In2 as a function of pH value. (b) Emission spectra of In2 in aqueous suspension at pH 7 and 8 with corresponding fluorescent photographs taken under an irradiation of 365 nm UV light.

those around 1600 and 1400 cm−1, as shown in the IR spectral data of In2 before and after the addition of NaOH (Figure S17). Moreover, the molecular frontier orbitals provided more direct evidence that the narrowed energy gaps between the LUMO and HUMO (ΔEL‑H) of In2 with deprotonated carboxyl groups should be responsible for the red shift of the fluorescence peak, as shown in Figure S18. Benefiting from the basement of In2, although Fe(III)@In2 showed no fluorescence emission under acid conditions owing to the quenching effect of Fe(III) ions, significant linear pHdependent fluorescence enhancement was observed under alkaline conditions between pH 7.0 and 9.0, which can be convincingly explained by the formation of Fe(OH)3 as the alkalinity increased to reopen the quenched fluorescence at 489 nm. Notably, in comparison with In2, Fe(III)@In2 possesses a larger titrate jump and changes in intensity ranges; on the other hand, Fe(III)@In2 exhibits a short response time for an alkaline environment (Figure S19), further indicating its high sensitivity as a pH-sensing material. Alternate Detection Cycle for Fe(III) Ions and pH. From the perspective of practical application, reusability is a significant parameter for chemosensors. Therefore, the alternate detection properties of In2 have been investigated repeatedly. Gratifyingly, the reactivated fluorescence excitation of In2 under the influence of OH− can still be quenched by the readdition of Fe(III) ions gradually, exhibiting an alternate detection cycle for Fe(III) ions and pH with off−on−off fluorescent switch characteristics. In addition, the quenching efficiencies of regenerated In2 for Fe(III) ions are basically maintained after five cycles, demonstrating its recyclability and potential as a chemosensor (Figure 8). Molecular Logic Operations with Three Inputs and Two Outputs. Recently, molecular logic operations, especially those based on multiple chemical signal inputs and optical signal outputs, have been regarded as a frontier technology, which can work on the molecular level.49−51 The fluorescence response properties of the probe In2 inspired us to explore its application in multiple logic operations, with the quantitative addition of analytes Fe3+, EDTA, and OH− (40, 80, and 80 equiv of In2, respectively) as input signals and the corresponding fluorescence emission intensity at 440 nm (B440) and 489 nm (G489) as output signals. In order to standardize binary signals, the presence and absence of the analytes were defined as 1 and 0 inputs, while the higher and lower fluorescent emission intensities (threshold value 700), were defined as 1 and 0 outputs, respectively.

part refers to the quenching caused by the collision between the analytes and excited fluorescence molecules, along with the loss of excitation energy in the form of energy transfer or charge transfer. The static part refers to the quenching due to the formation of nonfluorescent ground-state complexes based on the analytes and fluorescent molecules. According to the aforementioned results of UV/vis absorption spectroscopy, it is necessary to determine the formation of nonfluorescent ground-state complexes in the quenching process, pointing to static quenching. On the other hand, the fluorescence lifetimes of probe In2 suspensions with different Fe(III) ion concentrations were recorded in the range of 0−0.10 mM (Figure 6a), and there was an obvious trend of shortened lifetime with an increase in Fe(III) ion concentrations, corresponding to dynamic quenching. That is, the quenching effect of Fe(III) ions on the probe In2 involved both dynamic and static quenching. Thus, the S−V equation should be modified as I0/I = (K1[M] + 1)(K2[M] + 1), where K1 and K2 are the dynamic and static quenching constants, respectively. Considering the low concentration range of Fe(III) ions, the K1K2[M]2 term could be neglected, followed by simplification of the above S−V equation to I0/I = (K1 + K2)[M] + 1. Herein, the K1 value of 2.92 × 103 M−1 was obtained from a linear fitting (R2 = 0.9947) of fluorescence lifetime data according to the equation τ0/τ = 1 + K1[M] (Figure 6b), where τ0 and τ are the lifetimes before and after the addition of Fe(III) ions, respectively. With the known KSV and K1 values, K2 was subsequently determined to be 8.78 × 103 M−1. Obviously, static quenching acted as the predominant mechanism in this case. pH Value Detection. pH is one of the key indexes in environmental or biological systems.47,48 Encouraged by the exposed −COOH sites in In2, which can undergo deprotonation under alkaline conditions for pH sensing, the effect of pH on fluorescence emission for In1, In2, and Fe(III)@In2 was investigated in their aqueous suspensions with pH values from 2.0 to 12.0, as shown in Figure 7a. For In1, intense fluorescent emission was well maintained within the pH range of the test, suggesting that In1 exhibits excellent pH stability, which may be attributed to the absence of pH-dependent groups and the conformational rigidity of its framework. Unlike In1, In2 showed an obvious intensity drop between pH 7.0 and 8.0 with a red shift of the fluorescence peak from 440 to 489 nm (Figure 7b). This may be attributed to the new pathways for electron or energy transitions resulting from the deprotonation of carboxyl groups, which is supported by the reduction of peaks at ca. 1700 cm−1 and the enhancement of H

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Inorganic Chemistry

logic operation are given in the truth table (Figure 9a), followed by the build of a corresponding combinatorial logic gate circuit for a better description, as shown in Figure 9d. Considering the decisive effect of the OH− anion on the fluorescence emission at 489 nm, a single BUFF gate could be competent for G489 output, while NOT, OR, and AND gate operations were all necessary for B440 output.

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CONCLUSION In summary, we designed and synthesized two In(III)-based metal−organic complexes based on the same pyridinecarboxylate ligand. By precise regulation of the dosage of NaOH, the −COOH groups were retained exposed in In2 as expected, to be good candidates as recognition sites for Fe(III) ions and pH sensing. The rapid response, low detection limit, and excellent anti-interference ability against other metal ions make In2 a potential fluorescent sensing material for Fe(III) ions with high sensitivity and selectivity. Moreover, the quenched fluorescence of In2 during the Fe(III) sensing could be reopened linearly with an increase in alkalinity, followed by the reactivation of its ability to identify Fe(III) ions, exhibiting a fascinating alternate detection cycle with off−on−off fluorescent switch characteristics. According to the different responses of In2 to specific analytes, an advanced three-input and two-output analytical circuit has been constructed, instilling new vitality into the application of molecular logic devices in the sensing fields.

Figure 8. Reproducibility of alternate detection cycles for In2 toward Fe(III) ions and pH. The purple and orange bars represent the fluorescence intensity with the addition of Fe(III) ions and NaOH, respectively.

As shown in Figure 9c, without the addition of analytes Fe3+, EDTA, and OH−, the original aqueous suspension of In2 showed strong fluorescence emission at 440 nm, which can be simplified to be [(0,0,0)|(1,0)] in a binary form of [inputs (Fe3+, EDTA, OH−)|outputs (B440, G489)]. When Fe3+ existed as the only addition, the fluorescence at 440 nm was quenched, but if EDTA or OH− was added to the suspension along with the Fe3+ ions, strong fluorescence emission could be maintained owing to the formation of complexes such as EDTA-Fe(III) and Fe(OH)3, eliminating the quenching effect of Fe3+ ions. Notably, in comparison with EDTA ([(1,1,0)| (1,0)]), which had no influence on the supramolecular framework, excess OH− could further induces the movement of the fluorescence peak from 440 to 489 nm by changing the hydrogen-bonding network of In2, corresponding to a different signal of [(1,0,1)|(0,1)]. Moreover, in the presence of all three analytes Fe3+, EDTA and OH−, the effect of OH− anion was still significant with a system default value of [(1,1,1)|(0,1)]. All the possible binary data for this three-input and two-output

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03209. IR, PXRD, TGA, and additional structural data (PDF)

Figure 9. (a) Truth table of the three-input and two-output logic gate circuit. (b) Column diagram of the normalized fluorescence intensities of B440 and G489 (blue for B440 and green for G489, respectively). (c) Fluorescence response spectra with different combinations of analytes with a threshold of 700. (d) Schematic representation of the logic gate circuit. I

DOI: 10.1021/acs.inorgchem.8b03209 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Accession Codes

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CCDC 1853651−1853652 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*R.-Q.F.: fax, +86-451-86413710; e-mail, [email protected]. cn. *Y.-L.Y.: e-mail, [email protected]. ORCID

Rui-Qing Fan: 0000-0002-5461-9672 Yu-Lin Yang: 0000-0002-2108-662X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant 21571042).

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