Homochiral MOF as Circular Dichroism Sensor for Enantioselective

consuming, high expense and sophisticated program sustain- ably hindered simple ..... better synthetic scheme via a few trial experiments has been bui...
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Homochiral MOF as Circular Dichroism Sensor for Enantioselective Recognition on Nature and Chirality of Unmodified Amino Acids Yan-Wu Zhao,† Yan Wang,†,‡ and Xian-Ming Zhang*,† †

School of Chemistry & Material Science, Shanxi Normal University, Linfen 041004, P. R. China Department of Chemistry, Lvliang University, Lishi 033000, P. R. China

‡

S Supporting Information *

ABSTRACT: Self-assembly of zinc salt with rationally designed chiral ligand, (1R,2R)-2-(pyridine-4-ylcarbamoyl) cyclohexanecarboxylic acid (RR-PCCHC) generated 2D homochiral metal−organic framework [Zn(RR-PCCHC)2] (HMOF-1) that is composed of DNA-like right-handed double-helix structure. HMOF-1 shows high solvent and thermal stability and is also stable in neutral, weak acidic and weak basic aqueous solution. Emulsified HMOF-1 shows strong inherent circular dichroism (CD) signal in aqueous solution, which can show regular intensity change by induction of amino acids. On the basis of the measuring of CD signal intensity, a chemosensor for unmodified amino acids is fabricated, which differ from reported those in which CD signal is amplified by a complicated chemical reaction of originally CD-silent molecule with probed amino acids. This chemosensor can be used for rapid, convenient and sensitive detection of micro amount of amino acids. Most remarkably, 3 × 10−8 mol of L-aspartic acid and 4 × 10−8 mol of D-aspartic acid in aqueous solution can completely quench CD signal of emulsified HMOF-1 in H2O. It is found that the difference of recognition ability between D- and L-proline is the largest in all probed amino acids. The LOD (limit of ⎡A − A⎤ detection) of the proposed sensor for the determination of aspartic acid is 13.31 ppm. The recognition efficiency η = ⎣⎢ 0A ⎦⎥ × 0 100% for L-aspartic acid is as high as 92.1%. The interacting mechanism of DNA-like HMOF-1 with probed amino acids is similar to that of groove binding of targeting drug with DNA. KEYWORDS: homochiral, circular dichroism (CD), metal−organic framework, sensor, amino acids

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sensor. Moreover, fluorescence sensor cannot visually reflect stereochemical sign of probed chiral amino acids. CD is the most commonly used method for visual chirality, which has a wide range of application in investigations on the secondary structure of proteins,16,17 charge-transfer transitions,18 and geometric and electronic structure of complexes.19,20 Compared with chromatography, CD optical spectrum has the advantage of fastness.20 Compared with fluorescence, CD could show molecular absolute configuration for identifying chiral molecules.21,22 As far as reported CD sensing for amino acids are concerned, the representative bridged biphenyl probes can determine the absolute configuration by reacting with probed amino acids, which is based on central-to-axial chirality induction.23−25 Because of timeconsuming and elaborate CD sensing for amino acids, researchers have developed a new and rapid method to determine and recognize amino acids, that is, replacement of coordinated water in metal complexes by probed chiral amino acids or direct coordination of probed amino acids to unsaturated metal

INTRODUCTION

In 1806, French chemists discovered the first amino acid (named asparagine),1 and bioscientists subsequently disclosed that amino acids are basic building blocks of proteins.2 Amino acids occur in two enantiomeric configurations,3 and it is interesting to explore their stereoconfigurations. In fact, amino acids in organism and nature occur in L form, and the presence of D form often indicates a negative symptom, aging, or disease. Consequently, chiral recognition and sensing of amino acids in food stuffs and organism are especially important in nutritional analysis and diagnosis of diseases.4 To date, the approaches to enantioselectively recognize and sense amino acids include chromatography and capillary electrophoresis (CE),5−8 fluorescence,9−13 and circular dichroism (CD).14 Conventional chromatography and capillary electrophoresis methods for enantiomeric recognition and separation of a wide range of amino acids are effective, but timeconsuming, high expense and sophisticated program sustainably hindered simple, convenient and rapid sensing for amino acids.8,15 The fluorescence sensor can quickly and effectively sense amino acids,15 but the drawback of this method is that only a strong luminescent substance can be a candidate for the © 2017 American Chemical Society

Received: April 1, 2017 Accepted: May 25, 2017 Published: May 25, 2017 20991

DOI: 10.1021/acsami.7b04640 ACS Appl. Mater. Interfaces 2017, 9, 20991−20999

ACS Applied Materials & Interfaces

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complexes. The resulting chiral metal complex can produce CD signs that allow for the successful identity of chiral amino acids.26,27 The above-reported CD sensors, the sensing process was achieved by originally CD-silent and UV-active molecule that can undergo an asymmetric induction to give rise to preferred population of a chiral structure with distinctive CD output. The key to such process is formation of covalent bond or coordinative bond by a complicated chemical reaction between originally CD-silent reporter molecule and probed amino acids. Presently, numerous CD sensors were achieved by originally CD-silent molecules,23,28,29 although very limited example was reported for sensor with initial CD signal to recognize a couple of enantiomers.30 We imagine it is more interesting to develop a simple, convenient, and fast CD method for sensing amino acids without complicated chemical reaction. Metal−organic frameworks (MOFs) have been one of the hot research fields as crystalline porous materials.31 Numerous techniques have been introduced to develop many prominent applications of MOFs, for example, gas storage,32,33 fluorescence,34−42 HPLC and GC for separation of racemates,43−46 and catalysis.47−51 It was rare for chiral MOFs to explore enantioselective recognition and sensing for amino acids.39,41,52 Several enantioselective fluorescent chiral MOFs show recognizing ability for amino acids,39,41 but enantioselective fluorescence itself brings about great challenge. To date, CD has been restricted to simply judging and determining the chirality of synthesized crystalline MOFs,39−41,43−52 and a rare example of chiral MOFs could recognize enantiomers.53 Given that CD is recognized as one of the most powerful techniques for stereochemical analysis,20,21 we aim at developing a CD sensor of chiral MOFs for unmodified amino acids to broaden the application of enantioelective sensing of chiral MOFs. A new strategy toward rapidly, efficiently, and enanioselectively recognizing amino acids could be established by DNAlike homochiral MOF with inherent CD signal interacting with different D/L-amino acids, in which expected recognizing mechanism is similar to groove binding of targeting drugs with DNA.54,55 Nowadays, homochiral MOFs have concentrated on three main ways: (1) by using chiral ligand; (2) by adding uncoordinated chiral induction agents; (3) spontaneous resolution using achiral precursor.56 Compared to the uncertainty of the latter two, selection of chiral ligand guarantees formation of bulky homochiral MOFs. Through a series of comprehensive investigations, we synthesized a chiral ligand (1R,2R)-2-(pyridin-4-ylcarbamoyl) cyclohexanecarboxylic acid (RR-PCCHC) from (1R,2R)-cyclohexane-1,2-dicarboxylic acid as starting materials. First, (1R,2R)-cyclohexane-1,2-dicarboxylic acid is a cheap and easy-to-get material; second, 1,2-substituted cyclohexane derivatives are an important part of many biologically active compounds including alkaloids, pharmaceuticals, and research probes.57 Meanwhile, the formation of amide bond by a condensation reaction of cyclohexane acid and pyridin-4-amine could provide the N- and O-group receptor of the hydrogen bond with amino acids. We report herein a homochiral MOF [Zn(RR-PCCHC)2] (HMOF-1) which shows 2D layered structure constructed by DNA-like righthanded double helical motifs. HMOF-1 itself shows a strong CD signal, which can regularly change upon interacting with unmodified D/L-amino acids.

Research Article

EXPERIMENTAL SECTION

Materials and Methods. All chemicals for the syntheses were commercially available reagents of analytical grade and were used without further purification. The FT-IR spectra were recorded from KBr pellets in range 400−4000 cm−1 on a PerkinElmer Spectrum BX FT-IR spectrometer. UV absorption spectra were recorded in H2O solution and solid with a U-3310 spectrophotometer. Elemental analysis was performed on a Vario EL-II elemental analyzer. Powder X-ray diffraction (PXRD) data were recorded in a Bruker D8 ADVANCE powder X-ray diffractometer. The thermogravimetric analyses (TGA) were carried out in an air atmosphere using SETARAM LABSYS equipment at a heating rate of 10 °C/min. Scanning electron microscopic (SEM) images were obtained with a JSM-7500F operated at beam energy of 25.0 kV. Transmission electron microscopy (TEM) was performed on a JEM-3010 electron microscope. The specific rotation was determined with an WXG-4 Polarimeter (china) using a standard 6 mL cell (path length = 10 cm) at the wavelength of sodium-D (λ = 589 nm) line at 25 °C. The CD spectra were recorded on a JASCO J-815 CD spectrometer (with an accuracy of 0.1−0.3 nm) flushed with dry nitrogen at 25 °C, using a 10 mm quartz cell cuvette with a scanning rate at 100 nm min−1. The baseline correction was performed with the spectrum of corresponding solvents. All spectra were recorded for the wavelength range of 200−400 nm. The solid-state CD spectra were measured on the resulting complexes as crystals (ca. 0.56 mg) in 100 mg of oven-dried KBr. The baseline correction was performed with the spectrum of a pure KBr disk. Synthesis of Organic Ligand (RR-PCCHC). The synthetic route for ligand (1R,2R)-2-(pyridin-4-ylcarbamoyl) cyclohexane-carboxylic acid (RR-PCCHC) is shown in Scheme 1. The detailed procedures for intermediate products 2, 3, 4, 5, and final ligand 6 are described in the Supporting Information.

Scheme 1. Schematic Route for Synthesis of RR-PCCHC Ligand

Syntheses of Zn(RR-PCCHC)2 Materials (HMOF-1). A mixture of Zn(NO3)2·6H2O (37.2 mg, 0.125 mmol), R,R-PCCHC (24.8 mg, 0.1 mmol) in 3.5 mL of mixed solvent of DMF and deionized water (3:2) was adjusted to pH 5, then placed in a capped vial and heated at 85 °C for 3 days. The colorless block crystals HMOF-1 in 85% yield was isolated upon cooling, then filtrated and dried under atmosphere. Anal. Calcd for C26H30N4O6Zn (%): C, 55.77; H, 5.40; N, 10.01. Found: C, 55.64; H, 5.47; N, 10.13. IR (KBr pellet, cm−1): 3429 (s), 3251 (s), 3162 (s), 3073 (s), 3010 (s), 2933 (s), 2844 (s), 1701 (s), 1600 (s), 1510 (s), 1434 (s), 1311 (s), 1178 (s), 1106 (s), 1029 (s), 914 (s), 838 (s), 767 (s), 716 (s), 615 (s), 538 (s). Single-Crystal X-ray Diffraction Data Collection and Structure Determination. X-ray single-crystal diffraction data were collected on an Agilent Technologies Gemini EOS diffractometer at 298 K using Mo Kα radiation (λ = 0.71073 Å). The program SAINT was used for integration of diffraction profiles, and the program SADABS was used for absorption correction. The structure was solved with the XS structure solution program by direct methods and refined by the full-matrix least-squares technique using Olex2. All nonhydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms of organic group were generated theoretically onto 20992

DOI: 10.1021/acsami.7b04640 ACS Appl. Mater. Interfaces 2017, 9, 20991−20999

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ACS Applied Materials & Interfaces

Figure 1. (a) Representation of the Zn coordination environment in HMOF-1. Symmetry operation: (A) 0.5 + x, 0.5 + y, z; (B) 0.5 − x, 1.5 + y, 2 − z; (C) 0.5 + x, 1.5 + y, −1 + z. (b) Ball and stick view of a single 2D layer of HMOF-1. (c) Space-filling view of a single 2D layer showing right-handed 2-fold helical chains along the b-axis. (d) 2-fold interpenetrated layers. (e) Space-filling view of right-handed double helix. (f) 3D supramolecular array constructed by AB stacking of interpenetrated layers along the a-axis direction. the specific carbon atoms, and refined isotropically with fixed thermal factors. Further details for structural data are summarized in (Tables S1 and S2). CCDC 1536928 contains the supplementary crystallographic data for this paper. CD Spectroscopy. For measurements on CD spectra of free ligand, 2.5 mg of ligand 6 was directly solvated into 50 mL of CH3CN. Due to limited solubility, 1.67 mg of 6 could be solvated into 50 mL of deionized H2O. Before measurements on CD spectra of HMOF-1 and probe amino acids, stock solution of HMOF-1 and 1 × 10−4 M aqueous amino acids was prepared. The mixed HMOF-1 and amino acid stock solution was ultrasounded for 5 min before CD measurements. HMOF-1 stock solutions. Five mg of HMOF-1 crystals were placed in a vial with 3 mL of deionized water, which was mechanically crushed by vigorous stirring overnight. The resulting white emulsion was then diluted with deionized water to 100 mL for CD measurements of various amino acids. Exactly 3 mL of newly prepared HMOF-1 stock solution was added to a quartz cuvette for each CD experiment. Computational Details. Being a polymeric compound, the accurate calculation of HMOF-1 is expensive and would be impossible. So the monomer model Zn(RR-PCCHC)2 was chosen to reveal the origin mechanism of CD spectra. The ground-state geometry optimizations were performed employing the DFT method with B3LYP functional and 6-311G basis set. The oscillator and rotational strengths of 100 lowest singlet excited states were calculated at the same level using TDDFT method. The solvent (water) effects have been included using the polarizable continuum model (PCM).58 All of these were done with Gaussian0959 package. Monte Carlo Simulations of interaction of HMOF-1 with amino acids have been performed by Sorption module of Accelrys Materials Studio61 package. Geometries of amino acids were optimized by forcite module before sorption calculation. Fixed loading of one amino acid was used to simulate van der Waals interactions. At least 1 × 106

equilibration steps were used in the simulations. Atom based summation method, cubic spline truncation method, cut off distance of 15.5 Å, spline width of 1 Å and buffer width 0.5 Å were used during simulation of van der Waals interaction. Ewald summation was used to account for the periodicity of the lattice in the computation of electrostatic contributions. All LJ parameters to model the framework atoms were taken from the universal force field (UFF).

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RESULTS AND DISCUSSION Syntheses and Structures of Organic Ligand and HMOF-1. After selection of the organic ligand R,R-PCCHC, a better synthetic scheme via a few trial experiments has been built (Scheme 1). Compared with the reported synthesis route of analogues,60 this route has advantages of simple postprocessing, easy-to-handling, mild reaction conditions and high yields. R,R-PCCHC in high yield of 52% was synthesized via five steps from cheap and commercially available (1R,2R)-cyclohexane1,2-dicarboxylic acid. The structure and purity of R,R-PCCHC (compound 6) were confirmed by 1HNMR, 13CNMR (Figure S1), UV absorption spectra (Figure S2), and IR spectra (Figure S3). More importantly, pure and regular crystals HMOF-1 in high yield of 85% could be obtained from homochiral ligand RR-PCCHC. Through continually optimizing experiment conditions such as different solvents, temperature, and adjustment of PH, we were fortunate to acquire wellrepeated vast crystals in completely single phase. These materials are air-stable and insoluble in water or any common organic solvents. The single-crystal X-ray diffraction analysis reveals that HMOF-1 crystallizes in monoclinic chiral space group C2, indicating chiral 20993

DOI: 10.1021/acsami.7b04640 ACS Appl. Mater. Interfaces 2017, 9, 20991−20999

Research Article

ACS Applied Materials & Interfaces introduction from organic ligand to overall complex crystal. As depicted in Figure 1, Zn(II) center in HMOF-1 lies on a crystallographic 2-fold axis and is coordinated to two pyridyl nitrogen atoms and two carboxylate oxygen atoms that come from four different ligands. The Zn(1)−O(1) and Zn(1)−N(1) bond lengths are 1.998(3) Å and 2.028(3) Å, respectively. The L−Zn−L (L = N,O) bond angles range from 93.80(18) to 122.88(19)° and deviates slightly from those of perfect tetrahedron. Each RR-PCCHC ligand is coordinated to Zn atoms in μ2-N,O mode with Zn−C−C−Zn torsion angle of 70.46°, and configuration of RR-PCCHC remained during reactions. Due to nonlinear coordination of RR-PCCHC to Zn atoms, the structure of HMOF-1 is not commonly observed 3D diamond-like frameworks of zinc pyridine-carboxylates but 2D (4,4) topological layer with pore. Interestingly, (4,4) layer can be viewed as fusion of right-handed 2-fold helical chains via sharing Zn atoms. The 2-fold helical chains are propagated along b-axis direction with pitch of 13.4 Å that is twice length of b-axis. Nature hates voids, and two sets of (4,4) layers are interpenetrated into 2-fold interpenetrated strucure. More interstingly, 2-fold interpenetrated layer can be viewed as constructing from right-handed double helical chains, which is similar to the structure of DNA. These double helices are fused via sharing zinc atom to result in a homochiral 2D layer. To the best of our knowledge, coordination network 1 is the first example of perfect double-helix chain-based homochiral (4,4) square grids. The adjacent 2-fold interpenetrated layers are stacked along the a-axis direction in ABAB sequence to finish 3D homochiral supramolecular array. As shown in Figure S5, SEM reveals HMOF-1 sticks together by regular layers, which may be relative to the 2D layer structure. Meanwhile, we prepare the emulsified solution of HMOF-1 in deionized H2O and carefully study its morphology. Transmission electron microscopy (TEM) image of the emulsified solution of HMOF-1 in deionized H2O clearly demonstrates the formation of nanoparticles with average size below 100 nm as presented in Figure 2.

Figure 3. Thermogravimetric analysis of the as-synthesized HMOF-1.

Figure 4. Powder X-ray diffraction patterns of (a) simulated HMOF-1, (b) as-synthesized HMOF-1, (c) HMOF-1 after immersing in CH3CN, (d) water, (e) hexane, (f) MeOH, (g) EtOH, (h) THF, (i) DMF, and (j) CHCl3 for 12 months.

to as-synthesized HMOF-1 (Figure 5). What’s more, the recovered sample after emulsifing, recognizing, and sensing for D/L-amino acids retains the original structure, as confirmed by PXRD (Figure S26). As can be seen from above, HMOF-1 shows not only high thermal stability but also solvent stability, which lays a solid foundation for recognition of amino acids in solution.

Figure 2. TEM image of HMOF-1 micelles (left) and amplified image (right). The inset (in left) presents the emulsified solution of 5 mg of HMOF-1 in 3 mL of deionized H2O, and the inset (in right) does diluted HMOF-1 stock solution.

Stability Measurements. TGA curve reveals that HMOF-1 has high thermal stability up to 300 °C in air atmosphere (Figure 3 and Figure S25). HMOF-1 is also stable in neutral, weak acidic and basic solutions. After immersion of HMOF-1 in water and other organic solvents for near a year, the powder X-ray diffraction patterns (PXRD) indicate that the host framework completely matches those of the pristine samples (Figure 4). Meanwhile, HMOF-1 crystals were immersed in acidic D/L-aspartic acid, alkaline D/L-histidine and weakly basic water solution for a month, PXRD patterns are almost identical

Figure 5. Powder X-ray diffraction patterns of (a) simulated HMOF-1, (b) as-synthesized HMOF-1, and after immersion in (c) D-aspartic acid, (d) L-aspartic acid, (e) D-histidine, (f) L-histidine, and (g) weakly basicity of pH 8 water solutions for a month. 20994

DOI: 10.1021/acsami.7b04640 ACS Appl. Mater. Interfaces 2017, 9, 20991−20999

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ACS Applied Materials & Interfaces CD Recognition and Sensing for D/L-Amino Acids. The CD and corresponding UV−vis spectra for free RR-PCCHC and HMOF-1 in different solvents were carefully studied. UV absorption maxima of RR-PCCHC and HMOF-1 in various solvents lie in the near-ultraviolet region of 250 and 270 nm (Figures S6−S10), respectively. The UV signals are slightly shifted with the polarity change of solvents, which may be related to solvent effect. However, CD signals of HMOF-1 in different solvents have nearly no change in the range of 240−300 nm except for intensity (Figures S8−S10). Generally, positive Cotton effect near 280 nm and negative Cotton effect near 250 nm were observed, and CD signals of RR-PCCHC in H2O have larger changes due to the worse solubility of RR-PCCHC in H2O than in CH3CN. Compared with RR-PCCHC, CD signals of HMOF-1 have a large shift toward long wavelength, which may be caused by chiral propagation from single stereogenic-center-based chirality of free ligand to double-helix chirality of HMOF-1 with periodic polymeric structure. RR-PCCHC and HMOF-1 contain aromatic ring, carboxylate, and amide groups as chromophores, which exhibit characteristic CD peaks above 200 nm due to ππ* and nπ* transitions. Absorption maxima of RR-PCCHC around 250 nm and HMOF-1 around 270 nm are assigned to ππ* transitions of amino-pyridine and nπ* transitions of COOH and CONH group, respectively. Molar absorption coefficients ε at 243 and 251 nm for RR-PCCHC in CH3CN and H2O are 0.37 × 104 L mol−1 cm−1 and 0.42 × 104 L mol−1 cm−1, respectively (Figures S6 and S7). HMOF-1 show different absorption bands in H2O, EtOH and 1,2-dicholoroethane at 270 nm as ε = 0.54 × 104 L mol−1 cm−1, at 275 nm as ε = 0.26 × 104 L mol−1 cm−1 (204 nm is absorption peak of ethanol, ε = 2.97 × 104 L mol−1 cm−1), and at 276 nm as ε = 0.98 × 104 L mol−1 cm−1, respectively (Figures S8−S10). In addition, CD spectra of solid HMOF-1 show similar spectra and slight shift toward long wavelength, compared with that of HMOF-1 solution (Figure S 11). For RR-PCCHC in water solution, the strongest CD signal appears at 250 nm, which completely matches with UV absorption. For HMOF-1 in water solution, the strongest CD signal appears at 280 nm, a normal 10 nm shift compared with UV absorption maximum.62 Amino acids commonly contain COO− and NH3+ chromophores that exhibit certain characteristic far-ultraviolet region CD peaks below 200 nm due to the presence of nπ* and ππ* transitions, which also is confirmed by our measurement of D/L-aspartic acid (Figure S15). These far-ultraviolet region CD signals are difficult to be detected by commercial instrument.63,64 CD and corresponding UV spectra of several couples of amino acids with aromatic group are shown in Figures S12−S14, in which the observed absorption maxima at 210−220 nm are assigned to the π−π* transition of these amino acids. Molar absorption coefficients ε are in the range of 3−6 × 104 L mol−1 cm−1 for D/L-phenylalanine, D/L-tyrosine, and D/L-histidine. Besides, except for absorption of D/L-tyrosine in 278 nm and D/L-phenylalanine in 259 nm, absorption in 220−300 nm for dilute aqueous solution of amino acids does not appear. Interestingly, significant interaction between HMOF-1 with eight couples of probed D/L-amino acids (1 × 10−4 mol/L) in deionized water solution can be detected by signal changes in CD spectra (Scheme 2). The different extent of intensity decrease in the CD signal for HMOF-1 in the region of 240−300 nm was observed by adding different amino acids, the same amino acids with different amounts, or the same amino

Scheme 2. Eight Pairs of Probed D/L-Amino Acids

acids with different configuration (i.e., D and L). This indicates HMOF-1 can recognize and sense different amino acids. Under the optimized condition, the emulsified solution of HMOF-1 is titrated with the accurate volume of the standard solution of D- or L-amino acids, and all CD spectra are recorded as shown in Figure 6 and Figures S16−S22. Using D- and L-Asp as an

Figure 6. CD spectra of HMOF-1 upon titration with 1 × 10−4 M concentration of (a) L-Asp and (b) D-Asp, (c, d) the corresponding UV absorption spectra, and (e, f) the corresponding calibration curve (at 280 nm).

example, the quantitative analysis is studied by constructing the calibration curve. From Figure 6, a gradual decrease in CD signal in emulsified HMOF-1 aqueous solution was observed with an increasing amount of D/L-aspartic acids, and remarkably, CD signal almost disappeared when 300 μL (3 × 10−8 mol) of L-aspartic acids and 400 μL (4 × 10−8 mol) of D-aspartic acids were added into emulsified HMOF-1, which indicated HMOF-1 could effectively and sensitively detect trace amounts of D/L-Asp by the change of CD signal. To further comprehend the detected efficiency of HMOF-1 for Asp, the calibration curve of D- or L-Asp can be obtained by plotting the CD signal as a function of the volume of D- or L-Asp as 20995

DOI: 10.1021/acsami.7b04640 ACS Appl. Mater. Interfaces 2017, 9, 20991−20999

Research Article

ACS Applied Materials & Interfaces shown in Figure 6e, f. The working linear volume range of the proposed sensor for detecting D- or L-Asp is found in the range of 0−400 μL or 0−300 μL, respectively. The regression equation is found to be ΔCD (medg) = 0.0133 V (μL) + 0.5341 (for D-Asp) or ΔCD (medg) = 0.0205 V (μL) + 0.7624 (ΔCD = CD0 − CD) (for L-Asp) with the correlation coefficient (r2) of 0.9856 or 0.9772, respectively. The limit of detection (LOD) has been calculated as the volume of L-Asp or D-Asp giving the ΔCD (medg) equal to 3× standard deviation of CD0, where CD0 is the CD signal of emulsion HMOF-1 in the absence of L-Asp or D-Asp at 280 nm. The LOD of the proposed sensor for the determination of L-Asp or D-Asp are 13.31 ppm. The repeatability of the present method is also evaluated. The relative standard deviation from the detection of 50 μL L-Asp or D-Asp for three replicates is less than 1%. Meanwhile, their corresponding UV absorption spectra produce interesting change. As shown in Figure 6c and 6d, after the respective interaction of 20−150 μL of L-Asp and 30−200 μL of D-Asp with emulsified HMOF-1, their UV absorption intensity slightly increase. However, a large augment of UV absorption intensity is observed in the course of interaction of 200 μL of L-Asp and 300 μL of D-Asp with emulsified HMOF-1. Until that of 300 μL of L-Asp and 400 μL of D-Asp with emulsified HMOF-1, their UV absorption intensity is about 2 times and 4 times as many as original one, respectively. Similar phenomenon is not observed for the other seven couples of amino acids. More interestingly, 6 mg of HMOF-1 crystal was soaked into 20 mL of D/L-aspartic acid stock solution for one month, and the filtrate after filtration of the crystal shows no evident CD signal in the region of 240−300 nm, whereas the corresponding UV absorption spectra of filtrate show strong signal near 270 nm, which should be caused by the interaction of HMOF-1 crystal and aspartic acids (Figure 7). However, the

Figure 8. Recognition efficiency of different D/L-amino acids upon addition to an aqueous emulsion of HMOF-1.

initial and final CD signal intensity. According to recognition efficiency in Figure 8, very high recognition efficiency up to 92.1% is observed in 300 μL L-aspartic acid, which is the highest among all amino acids. The next is L-proline with up to 65.7% recognition efficiency. The difference of recognition ability between D- and L-proline is the largest among measured eight couples of amino acids. In addition, polar R group of amino acids can be better recognized than nonpolar one, i.e., polar threonine and tyrosine. To explore the universality of CD sensor application, we also tested α-hydroxyl carboxylic acids, (R,R)/(S,S)-tartaric acid, and D/L-lactic acid (Scheme S1). Interestingly, the similar intensity change of CD signal for emulsified HMOF-1 in the region of 240−300 nm are observed by adding different volume of α-hydroxyl carboxylic acids. (R,R)/(S,S)-tartaric acid can remarkably decrease CD signal of emulsified HMOF-1 but D/L-lactic acid produced little effect. As shown in Figure S23, (S,S)-tartaric acid cause more intensity decrease of CD signal for emulsified HMOF-1 than (R,R)-tartaric acid. Recognition efficiency (η) of (S,S)-tartaric acid can reach 53.2% but (R,R)tartaric acid only has 37.5%. These results indicate HMOF-1 aqueous emulsions as CD sensor can sensitively recognize and sense enantiomeric carboxylic acids. To further explore whether the N-protected amino acids can still work, D/L-N-Boc proline and N-Boc glycine are made into 1 × 10−4 mol/L CH3CN solution and tested (Scheme S2). It is found that L-N-Boc proline has a larger decrease in CD signal than D-N-Boc proline (Figure S24). In constrast, N-Boc glycine can hardly make intensity change of CD signal for HMOF-1. Recognition efficiency (η) of D- and L-N-Boc proline are 17.2 and 34.5%, respectively. Given that HMOF-1 displays a stronger CD signal, especially in H2O, which creates the conditions for probing and recognizing D/L-amino acids, it is very necessary for us to understand the origin and attribution of CD signal of HMOF-1. On the basis of our previous work,65 the CD and UV spectra were generated as a sum of Gaussians, centered at the calculated wavelengths (λcalcd) with integral intensities proportional to the rotational (for CD) or oscillator (for UV) strengths of corresponding transitions. The half bandwidth Γ at εmax/e is 12 in 200−237 nm, 13 in 237−243 nm, 18 in 243−300 nm respectively, to best reproduce the experimental spectra. The resulting CD and UV spectra, shown in Figure 9, exhibit the following characteristics. In the region of 200−300 nm, the

Figure 7. CD spectra of filtrate for soaking HMOF-1 in (a) L-Asp and (b) D-Asp and (c, d) the corresponding UV absorption spectra.

other seven couples of amino acids cannot attain such effect, and CD signals are just gradually reduced (Figure S16−S22). Actually, when above 120 μL of amino acids is added to emulsified HMOF-1, CD amplitude is almost unaffected by seven couples of amino acids even at volume as high as 400 μL except D/L-proline. In light of change of CD signal of HMOF-1 caused by these amino acids, recognition efficiency η is preliminarily established to intuitively judge the recognition or sensing ability of HMOF-1 for amino acids. Recognition ⎡A − A⎤ efficiency is defined as η = ⎢⎣ 0A ⎥⎦ × 100%, where A0 and A are 0 20996

DOI: 10.1021/acsami.7b04640 ACS Appl. Mater. Interfaces 2017, 9, 20991−20999

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ACS Applied Materials & Interfaces

dominated by nσ → πampy*, resulting from the 257th KS orbital to the 285th, with the 274th to the 282nd. The calculated UV spectra (Figure 10) display a bell-shaped dominant band between 220 and 280 nm, with some transition at about 240− 250 nm, corresponding to the strong CD band in Figure 9. Amino acids exist in zwitter ionic forms, and thus nitrogen and oxygen rich HMOF-1 easily form additive by hydrogen bond, coulomb force and/or dipole−dipole with amino acids of various forms including neutral, cationic and anionic species.66 This speculation is supported by changes in UV absorption strength in Figure 6c, d. To get microscopic insight into the interaction of HMOF-1 and probed D/L-amino acids, molecular simulations were conducted by Materials Studio software. Taking aspartic acid as an example, Figure 11 illustrates the interaction sites of

Figure 9. Calculated CD and UV spectra of the Zn(RR-PCCH)2 model in water solution.

CD spectra mainly showed a strong positive band and a negative one with almost the same amplitude, whereas a very weak positive tail emerged in the shorter wavelength. All of these bands are dominated by the transition n → π*and π → π*, just belonging to different groups. The Kohn−Sham (KS) orbitals mainly related to transitions are illustrated in Figure 10.

Figure 11. Simulated binding interactions between HMOF-1 and (left)/L (right)-aspartic acid.

D

HMOF-1 with amino acids. As can be seen, the enantiomers of amino acids are preferentially located in groove formed by cyclohexane acid, Zn and pyridine groups. However, the orientation and potential map of D-enantiomer are distinctly different from those of L-enantiomer. Consequently, the specific binding energies are −12.810 and −13.721 kcal/mol for D- and L-Asp, respectively. This indicates that L-amino acids have a stronger interaction with HMOF-1 than its D-counterpart. The differences in orientation and binding energy are expected to cause enantioselective adsorption. L-Amino acids more easily form a stable complex with HMOF-1 than their D-counterpart, which is consistent with the experiment. Interaction between HMOF-1 and amino acid herein is reminiscent of interaction between DNA and targeted drug, where noncovalent binding also occurs in a groove interaction.

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CONCLUSIONS In conclusion, we have successfully developed a CD sensor that can fast, conveniently and sensitively recognize micro amount of unmodified amino acids, in which recognition ability of emulsified HMOF-1 on D/L-Asp is highest and the difference of recognition ability of one for D/L-Pro is largest. In addition, polar R group of amino acids can be better recognized than nonpolar one. The origin mechanism of CD signal based on HMOF-1 in H2O is calculated using the TDDFT method, which is assigned to ππ* transitions and nπ* transitions of different group from HMOF-1. Experimental and simulated results illustrate the sensing ability of HMOF-1 for amino acids is attributed to different interaction site and binding energies of D- or L-configuration amino acids within microenvironment of HMOF-1. Importantly, this CD sensor not only avoids the time-consuming, expensive, and sophisticated program of the conventional identification method for amino acids (such as chromatography and capillary electrophoresis) but also solves the detecting problem of weak fluorescence MOFs for enantiomers, which further extends applications of MOFs.

Figure 10. Corresponding KS orbitals of Zn(RR-PCCH)2 in water solution.

The positive band around 250 nm is due to nσ→ πampy* (ampy refers to amino pyridine) and πampy → πampy*, originating from the 271st KS orbital to the 279th (LUMO) and the 278th (HOMO) to the 281st. The negative band at about 240 nm is resulted from πampy → πampy*, which arises from the 276th KS orbital to the 281st, mixed with the 275th to the 281st. The weak tail around 220 nm is also a mixed absorption band 20997

DOI: 10.1021/acsami.7b04640 ACS Appl. Mater. Interfaces 2017, 9, 20991−20999

Research Article

ACS Applied Materials & Interfaces

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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/acsami.7b04640. Synthesis, 1HNMR, 13CNMR, UV and IR spectra of free RR-PCCHC; structure details, IR spectra, and SEM image of HMOF-1; CD spectra of free RR-PCCHC; HMOF-1 in different solvents and amino acids in deionized H2O; CD spectra of HMOF-1 with amino acids; PXRD of HMOF-1 (PDF) Crystallographic information file for C26H30N4O6Zn (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xian-Ming Zhang: 0000-0002-8809-3402 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Ministry of Education of China (Grant IRT1156), the National Science Fund for distinguished Young Scholars (NSFC 20925101), the Plan for 10 000 Talents in China, and Sanjin Scholar is greatly appreciated.

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