Riboflavin Chelated Luminescent MetalâOrganic Framework

Article pubs.acs.org/crystal

Riboflavin Chelated Luminescent Metal−Organic Framework: Identified by Liquid-Assisted Grinding for Large-Molecule Sensing via Chromaticity Coordinates Tu Lee,* Meng Hsun Tsai, and Hung Lin Lee Department of Chemical and Materials Engineering, National Central University, 300 Jhong-Da Road, Jhong-Li City 320, Taiwan, Republic of China S Supporting Information *

ABSTRACT: Liquid-assisted grinding protocol has been successfully employed to identify (1) the use of riboflavin as a sensing molecule for melamine and acetoguanamine and (2) the chelation of riboflavin on a silver atom. Riboflavin is then chelated onto a visible light-emitting, silver-containing metal− organic framework, AgL {i.e., [AgL] n ·nH 2O (L = 4cyanobenzoate)} to produce (AgL)-(riboflavin) chelation crystals. These chelation crystals can be used for the detection of melamine and acetoguanamine by forming binding crystals of (AgL)-(riboflavin)-(melamine) and (AgL)-(riboflavin)(acetoguanamine). The Commission Internationale de l'Eclairage (CIE) 1931 chromaticity coordinates of AgL crystals, (AgL)-(riboflavin) chelation crystals, (AgL)-(riboflavin)-(melamine) binding crystals, and (AgL)-(riboflavin)-(acetoguanamine) binding crystals based on their solid-state photoluminescence (PL) emission spectra are calculated to be approximately (0.16, 0.16), (0.32, 0.44), (0.25, 0.37), and (0.23, 0.15), respectively. The original PL emission of AgL may be attributed to ligandcentered luminescence. However, the chelation with riboflavin may cause (1) a bathochromatic spectral shift (i.e., red shift), (2) an emission broadening, and (3) a quenching effect through the mode of Förster resonance energy transfer. The binding with the amine analytes (i.e., good electron donors), such as melamine and acetoguanamine, may alter the redox potential of riboflavin inhibiting quenching and enhancing luminescence of binding crystals of (AgL)-(riboflavin)-(melamine) and (AgL)-(riboflavin)(acetoguanamine). π → π* energy in the riboflavin-melamine or riboflavin-acetoguanamine binding complex is then enhanced. Consequently, hypsochromic spectral shifts (i.e., blue shift) are observed in their PL emission responses.

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components.12,13 Despite the growing catalogue of MOF materials, a routine design or identification of MOFs that responds selectively to a target analyte still remains difficult by the methods of high-throughput screening and computational simulations.14,15 Therefore, the aim of this paper is to present a novel approach by chelation of a sensing molecule to the metal center of the already existing luminescent MOF porous framework16,17 similar to the one by surface modification.18−22 Interestingly, this sensing molecule has another binding site specifically for probing a large target analyte. The stereochemical binding event of the target analyte to the sensing molecule will then be transduced through the MOF network to become a photoluminescence (PL) emission color signal. The color then is mathematically transformed into the Commission Internationale de l'Eclairage (CIE) chromaticity coordinate.3,23,24 The search for the candidate of a sensing molecule can be efficiently achieved by liquid-assisted grinding25−28 and without using the

INTRODUCTION Given the many advantages of metal−organic frameworks (MOFs), (1) high porosity, (2) different pore sizes, (3) specific node and linker electronic structures, (4) tunability of linkers and orientations, (5) diversity of framework catenations, (6) various signal transduction methods,1 and (7) thin-film growth techniques,2 MOFs have shown an excellent application potential in chemical sensors1 and noses3 for the detection of a range of organic molecules and ions. However, MOFs' molecular (i.e., analyte) selectivity based on the size exclusion (i.e., molecular sieving) principle has only permitted the adsorption, loading, and encapsulation of atoms or molecules smaller than the MOFs' void dimensions.1,4−6 Conventionally, as the size of the analyte increases, the loading capacities of the porous MOF can only be ameliorated by the expansion of organic links.4,7−9 Yet, the high level of specificity cannot be achieved solely by size or shape selectivity.10 Sophisticated molecular recognition elements may sometimes be installed into MOFs. For example, a chiral amino acid framework was used for enantioselective sorption,11 and coordinately unsaturated metal sites were employed for binding certain gas molecules while remaining blind to other atmospheric © 2012 American Chemical Society

Received: March 14, 2012 Revised: May 3, 2012 Published: May 8, 2012 3181

dx.doi.org/10.1021/cg3003492 | Cryst. Growth Des. 2012, 12, 3181−3190

Crystal Growth & Design

Article

already existing MOF at all. If both the grinding together of (1) the candidate and the metallic salt used in the MOF's synthesis and (2) the candidate and the target analyte yield new complexes as determined by either their infrared spectra or the color change of those mixtures, the candidate can then be chosen for use as a sensing molecule. The candidate will be grafted to the MOF during its synthesis by chelation as the next step. To demonstrate the feasibility of our concept, we have selected visible light-emitting [AgL]n·nH2O (L = 4-cyanobenzoate) (AgL) as the MOF platform24 and riboflavin (i.e., vitamin B2) as the candidate of a sensing molecule. AgL is chosen because its 10.832 × 6.650 Å2 parallelogram-like nanochannels along the a-direction24 (Figure 1) are apparently

Figure 2. Schematic representation of the chelation of riboflavin (i.e., sensing molecule) on empty AgL (MOF) crystals to form (AgL)(riboflavin) chelation crystals and the immersion of it into the aqueous solution of the analyte (e.g., melamine) to produce (AgL)-(riboflavin)(analyte) binding crystals through the molecular recognition between the riboflavin and the target analyte.

as melamine, acetoguanamine, salicylic acid, and 3,5-dihydroxybenzoic acid, to see if any (riboflavin)-(analyte) binding complex can be generated; (3) the synthesis of the sensing molecule-grafted MOF [i.e., (AgL)-(riboflavin) chelation crystals] is carried out after passing the screening steps of 1 and 2; (4) (AgL)-(riboflavin) chelation crystals were immersed in the aqueous solution of the analyte, which gave a positive result in step 2; and (5) the PL emission responses of the samples of AgL crystals, (AgL)-(riboflavin) chelation crystals, and (AgL)-(riboflavin)-(analyte) binding crystals were measured and transformed to chromaticity coordinates.3,23,24

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Figure 1. Two-dimensional layer of [AgL] n ·nH2 O (L = 4cyanobenzoate) (AgL crystals) with 10.832 Å × 6.650 Å parallelogram-like nanochannels approximately along the a-direction. Lattice water molecules were included in the voids.

MATERIALS AND METHODS

Chemicals. (−)-Riboflavin (anhydrate I) (C17H20N4O6, 99% purity, mp = 290 °C, MW = 376.36, lot 050M1704 V), 4-cyanobenzoic acid (C8H5NO2, 99% purity, mp = 219−221 °C, MW = 147.13, lot S46230 V), melamine (C3H6N6, 99% purity, mp >300 °C, MW = 126.12, lot MKBF0381 V), and 3,5-dihydroxybenzoic acid (C7H6O4, 97% purity, mp = 236−238 °C, MW = 154.12, lot STBB2815 V) were purchased from Sigma-Aldrich (St. Louis, MO). Silver nitrate (AgNO3, 99% purity, mp = 212 °C, MW = 169.87, lot: J49616) was received from J. T. Baker (New Jersey). Acetoguanamine (C4H7N5, 96% purity, mp = 274−276 °C, MW = 125.13, lot: 10168650) was obtained from Alfa Aesar (Lancashire, United Kingdom). Sodium hydroxide (NaOH, ≥96% purity, mp = 318 °C, MW = 40, lot: KU-3238P) and salicylic acid (C7H6O3, 99% purity, mp = 158−161 °C, MW = 138.12, lot: SR2529F) were purchased from Showa Chemical Co. Ltd. (Tokyo, Japan). Solvents. Concentrated hydrochloric acid (HCl, 37 wt % solution, bp = 81.5−110 °C, MW = 36.46, lot: AC0741) was received from Scharlau (Barcelona, Spain). Sulfuric acid (H2SO4, 97% purity, bp = 290 °C) was purchased from Showa Chemical Co. Ltd. (Tokyo, Japan). Reversible osmosis (RO) water was clarified by a water purification system (model Milli-RO Plus) bought from Millipore (Billerica, MA). Liquid-Assisted Grinding. Riboflavin (anhydrate I) and other potential target analytes, such as silver nitrate (Figure 3a), melamine (Figure 3b), acetoguanamine (Figure 3c), 3,5-dihydroxybenzoic acid, and salicylic acid, were physically mixed with a molar ratio of 1:1 and a total weight of less than 60 mg. The mixture was manually ground for 3 min by a mortar and pestle. Twenty microliters of RO water was then added, and the wetted mixture was ground for another 10 min. If the color change was observed, photographs would be taken by a digital camera. The solid-state reaction happening in the mixture was also confirmed by Fourier transform infrared (FTIR) spectroscopy. Synthesis of [AgL]n·nH2O (L = 4-Cyanobenzoate), AgL Crystals. A 740 mg amount of 4-cyanobenzoic acid (5 × 10−3 mol) and 210 mg of NaOH (5 × 10−3 mol) were codissolved in 200 mL of

too small for molecules like melamine with a size of 5.4−6.2 Å29 to be adsorbed directly. However, the analyte such as melamine may be bound to riboflavin because hydrogels of riboflavin at a 1:1 molar ratio with melamine, acetoguanamine, salicylic acid, and 3,5-dihydroxybenzoic acid were known to exist.30 Also, it is very likely for riboflavin to be grafted on the silver ions in AgL because riboflavin was known to chelate with metal ions.31−33 Consequently, riboflavin34−36 is an ideal candidate to serve as a sensing molecule for the analyte such as melamine by molecular recognition on one hand and to be connected to the silver atom in AgL by chelation on the other, to form a (AgL)-(riboflavin)-(analyte) binding system (Figure 2). This is analogous to the (Fc fragment)-(Fab fragment)(antigen) in the immune system.37 Therefore, we present (1) the screening steps and (2) the working logics for making the functional (MOF)-(sensing molecule)-(analyte) binding platform. We will use the (AgL)(riboflavin)-(melamine) binding system and the like for a demonstration. Moreover, our (AgL)-(riboflavin) chelation crystals may serve as a simple alternative toolkit for rapid detection of melamine because it is unethically used by milk manufacturers in adulterating milk to make it appear more protein-rich.38 There are five key steps to follow: (1) the possible coupling of AgL with riboflavin in (AgL)-(riboflavin) chelation complex is tested by liquid-assisted grinding25−28 riboflavin with silver nitrate.24 AgNO3 is the metallic salt used in the production of AgL crystals, and (2) the possible binding of riboflavin to some of the potential analytes37 is screened by liquid-assisted grinding25−28 riboflavin with those analytes, such 3182



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Figure 3. Complexes were produced by liquid-assisted grinding together: (a) riboflavin with silver nitrate, (b) riboflavin with melamine, and (c) riboflavin with acetoguanamine. (d) The chelation complex was generated by adding riboflavin into the reaction medium of AgL, and binding crystals were produced by immersing (AgL)-(riboflavin) chelation crystals in the aqueous (e) melamine solution and (f) acetoguanamine solution. Red dashed lines represent the hydrogen bonds (i.e., molecular recognition), and the blue arrow denotes the chelating bond (i.e., chelation). stirring. The resulting solution turned dark-red, which was filtered off after 24 h. The filtrate was stored in the dark for 2−3 weeks to yield colorless needles.

water at 80 °C to form solution A. A 840 mg amount of AgNO3 (5 × 10−3 mol) was dissolved in 100 mL of water to form solution B. Solution B was gradually added into solution A upon continuous 3183



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Figure 4. Polymorphic flowchart showing the preparation methods for six modifications of riboflavin where anhydrate I → dihydrate ↔ monohydrate ↔ anhydrate II and anhydrate I ↔ tetrahydrate ↔ anhydrate III were based on refs 34, 35, and 36, respectively (scale bar = 20 μm). Synthesis of Riboflavin Chelated [AgL]n·nH2O (L = 4Cyanobenzoate), (AgL)-(Riboflavin) Chelation Crystals. The procedure was identical to the synthesis of [AgL]n·nH2O (L = 4cyanobenzoate) above except for the addition of 70 mg of riboflavin (2 × 10−4 mol) in solution A (Figure 3d). The filtrate was then stored in the dark for 2−3 weeks to yield orange-tainted needles. Large-Molecule Sensing by Riboflavin Chelated [AgL]n·nH2O (L = 4-Cyanobenzoate), (AgL)-(Riboflavin)-(Analyte) Binding Crystals. A 0.01 M aqueous solution of the analyte (i.e., 12.6 mg of melamine or 12.5 mg of acetoguanamine dissolved in 10 mL of RO water) was prepared. Ten milligrams of (AgL)-(riboflavin) chelation crystals was immersed in 2 mL of 0.01 M aqueous solution of the analyte for 6 h. (AgL)-(riboflavin)-(analyte) binding crystals were then filtered and dried at 40 °C for 5 h (Figure 3e,f). Instrumentations. Polarized Optical Microscopy. The interference color caused by birefringence of all sample crystals and crystal habits was examined by an Olympus BX51 Microscope (Olympus., Tokyo, Japan) equipped with an Olympus U-PO3 polarizer (Olympus., Tokyo, Japan), a MotiCam 2000 2.0 Megapixel CCD camera (Motic China Group Co. Ltd., Xiamen, People's Republic of China), and a Motic Images Plus 2.0 ML digital camera software. Transmission FTIR Spectroscopy. Transmission FTIR spectroscopy was utilized to measure purity, detect bond formation, and verify chemical identity. Transmission FTIR spectra were recorded on a Perkin-Elmer Spectrum One spectrometer (Perkin-Elmer Instruments LLC, Shelton, CT). The KBr sample disk was scanned with a scan number of 8 from 400 to 4000 cm−1 having a resolution of 2 cm−1. Powder X-ray Diffraction (PXRD). PXRD diffractograph at 25 °C provided another piece of information for identification, polymorphism, and crystallinity of solids. PXRD diffractograms were detected by Bruker D8 Advance (Germany). The source of PXRD was Cu Kα (1.542 Å), and the diffractometer was operated at 40 kV and 41 mA. The X-ray was passed through a 1 mm slit and the signal a 1 mm slit, a nickel filter, and another 0.1 mm slit. The detector type was a scintillation counter. The scanning rate was set at 0.05° 2θ/s ranging from 5 to 60°. The quantity of sample used was around 20−30 mg. Differential Scanning Calorimetry (DSC). DSC analysis was mainly used to identify the enthalpy of fusion and solid−liquid (melting) temperature. Thermal analytical data of 3−5 mg of samples in perforated aluminum sample pans (60 μL) were collected on a PerkinElmer DSC-7 calorimeter (Perkin-Elmer Instruments LLC) with a temperature scanning rate of 10 °C/min from 50 to 550 °C under a

constant nitrogen 99.990% purge. The instrument was calibrated with indium and zinc 99.999% with reference temperatures of 156.6 and 419.47 °C, respectively (Perkin-Elmer Instruments LLC). Thermal Gravimetric Analysis (TGA). TGA analysis was carried out by TGA 7 (Perkin-Elmer, Norwalk, CT) to monitor sample weight loss as a function of temperature. The heating rate was 10 °C/min ranging from 50 to 600 °C. Weight loss was usually associated with solvent evaporation close to the boiling point of a solvent as in the case of solvates or sample decomposition. The open platinum pan and stirrup were washed by ethanol and burned by spirit lamp to remove all impurities. All samples were heated under nitrogen atmosphere to avoid oxidization. About 3 mg of sample was placed on the open platinum pan suspended in a heating furnace. PL. PL measurements were conducted on a Perkin-Elmer LS-55 PL spectrometer (Perkin-Elmer Instruments LLC) equipped with a pulsed xenon lamp at 60 Hz. The excitation and emission slits were set to 10 nm during the operation. The scanning speed was 100 nm/ min, and the scanning range was from 200 to 700 nm. A 1% attenuator was used to reduce the intensity to correspond to the scale. The excitation monitoring wavelength was 330 nm with a 390 nm emission cutoff filter. About 5 mg of sample microcrystals was used. Each experiment and measurement were repeated at least three times.

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RESULTS AND DISCUSSION

The phase transformation of riboflavin is complicated as shown in Figure 4. The PXRD, FTIR, TGA, and DSC characterizations of all modifications34−36 of riboflavin and solubility curves of some of them in water are illustrated as Figures S1− S11 in the Supporting Information. The characteristic IR assignments of riboflavin, melamine, and acetoguanamine are also given in Table 1.24,33,39−41 The FTIR spectrum of (riboflavin)-(Ag) complex is compared with the ones of riboflavin and silver nitrate in Figure 5. The bands at 1730 and 1548 cm−1 are characteristic of the stretching modes of CO (amide carbonyl) and CN conjugated system present in the free riboflavin ligand, respectively (Figure 5a). The infrared spectra of the (riboflavin)-(Ag) complex reveal the broadening around 3400 cm−1 standing for the characteristic O−H stretching modes overlapping four hydroxyl groups exhibited in one riboflavin ligand moiety chelated to the silver 3184



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Table 1. IR Assignments of Riboflavin, Melamine, Acetoguanamine, AgL Crystals, (AgL)-(Riboflavin) Chelation Crystals, (AgL)-(Riboflavin)-(Melamine) Binding Crystals, and (AgL)-(Riboflavin)-(Acetaguanamine) Binding Crystals wavelength (cm−1)

functional groups

riboflavin 3550−3230 O−H, stretching 3212 (3500−3300) N−H, stretching 1732 (1745−1700) CO, stretching 1548 CN, stretching melamine 3470, 3419 (3500−3100) NH2, symmetric stretching 1653 NH2, deformation 1552 CN, stretching acetoguanamine 3506, 3408 (3500−3100) NH2, stretching 1680−1640 NH2, deformation AgL crystal 3574−3200 O−H, stretching 3062, 1946 C−H, stretching 2241 −CN, stretching 1591−1388, 694 −COO−, stretching (AgL)-(riboflavin) chelation crystal (new) 3527 O−H, stretching from riboflavin (AgL)-(riboflavin)-(melamine) binding crystal (new) 3471, 3460 NH2, stretching from melamine (new) 3135 N−H, stretching from melamine (new) 1474, 1447 C−NH2, stretching from melamine (AgL)-(riboflavin)-(acetoguanamine) binding crystal (new) 3431, NH2, stretching from acetoguanamine (new) 3135 N−H, stretching from acetoguanamine (new) 1475 C−NH2, stretching from acetoguanamine

ref 40 40 40 40 39 39 39

Figure 6. FTIR spectra of (a) riboflavin, (b) melamine, and (c) riboflavin ground with melamine. The characteristic −NH2 peaks at 3470 and 3418 cm−1 and 3000−3500 cm−1 of melamine disappeared and broadened, respectively, and the >CO bond of riboflavin was shifted from 1730 to 1713 cm−1 in c.

39 40 24 24 24 24

broadened, respectively, after liquid-assisted grinding riboflavin with melamine (Figure 6c). These results suggest that the molecular recognition through the hydrogen bonds between the >CO groups of riboflavin and the amino hydrogen atoms of melamine within the (riboflavin)-(melamine) binding complex does occur.42 The FTIR spectra in Figure 7 of

40 40 39 41 40 39 41

Figure 7. FTIR spectra of (a) riboflavin, (b) acetoguanamine, and (c) riboflavin ground with acetoguanamine. The vibration peaks of −NH2 group in the 3000−3500 cm−1 region of acetoguanamine were broadened due to hydrogen bond formation, and the >CO bond of riboflavin was shifted from 1730 to 1713 cm−1. New peaks at 1699 and 1605 cm−1 appeared.

riboflavin, acetoguanamine, and riboflavin ground with acetoguanamine exhibit a similar shift of >CO group of riboflavin from 1730 to 1713 cm−1. The vibration peaks of the −NH2 group in the 3000−3500 cm−1 region of acetoguanamine are broadened due to hydrogen bond formation. The new peaks at 1699 and 1605 cm−1 also appeared in the sample of riboflavin ground with acetoguanamine (Figure 7c). These results confirm the formation of the (riboflavin)-(acetoguanamine) binding complex.39 However, the FTIR spectra of the samples obtained by liquid-assisted grinding riboflavin with 3,5-dihydroxybenzoic acid, and riboflavin with salicylic acid, do not show any peak

Figure 5. FTIR spectra of (a) riboflavin, (b) silver nitrate, and (c) riboflavin ground with silver nitrate. The −OH peaks at around 3400 cm−1 and the >CO peak at 1732 cm−1 of riboflavin were broadened and shifted to 1690 cm−1, respectively, in c.

atom (Figure 5c). In addition, the peak originally at 1730 cm−1 of the riboflavin free ligand (Figure 5a) is shifted to 1690 cm−1, showing that the (riboflavin)-(Ag) complex is generated (Figure 5c).33 The FTIR spectra in Figure 6 show that (1) the >CO peak of riboflavin has shifted from 1730 to 1710 cm−1 and (2) the characteristic −NH2 peaks at 3470 and 3418 cm−1 and 3000−3500 cm−1 of melamine disappeared and are 3185



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shift of >CO group of riboflavin at 1730 cm−1 (data not shown). In other words, these phenomena predict that (AgL)(riboflavin) chelation crystals will only be able to detect the presence of melamine and acetoguanamine but not to the presence of 3,5-dihydroxybenzoic acid and salicylic acid. This hypothesis will be verified later by immersing (AgL)(riboflavin) chelation crystals in the 0.01 M aqueous solutions of melamine, acetoguanamine, 3,5-dihydroxybenzoic acid, and salicyclic acid experimentally. Although hydrogels of riboflavin with 3,5-dihydroxybenzoic acid and riboflavin with salicylic acid were known to exist,39 the probability of hydrogen bond formation, Pm, for melamine detection or acetoguanamine detection by (AgL)-(riboflavin) chelation crystals is much higher (i.e., Pm = 97%). As a matter of fact, the Pm of O···H−O and N−H···O within the hydrogels of riboflavin with 3,5dihydroxybenzoic acid and riboflavin with salicylic acid is lower (i.e., Pm = 10%) than the Pm of O···H−N and N−H···N within both the hydrogels of riboflavin with melamine and riboflavin with acetoguanamine.39,43 In addition to the FTIR spectra, the color change of riboflavin appeared to the naked eye during the liquid-assisted grinding experiments can serve as another piece of reliable evidence for the chelation of riboflavin with silver, the binding of riboflavin to melamine, and the binding of riboflavin to acetoguanamine. As demonstrated by the photographs in Figure 8, the original yellow color of riboflavin44 (Figure 8a)

(Figure 10c).24 The PXRD pattern has the characteristic 2θ = 8.68°, 15.30°, 17.41°, 26.25°, and 31.50° corresponding to the (011), (013), (022), (033), and (042) reflections. The IR absorption frequencies for −OH stretching of lattice water of 3574−3200 cm−1, C−H stretching of aromatic ring of 3062 and 1946 cm−1, −CN stretching of cyano group of 2241 cm−1 and −COO− stretching of 1591−1388 cm−1 and 694 cm−1 of AgL crystals24 are clearly illustrated in Figure 11a and Table 1. The TGA scan shows that AgL crystals contain 1.2 mol of H2O per mol of AgL crystals (Figure S10 in the Supporting Information). The SXD analysis reveals that the crystal structure of AgL consists of wavelike 2D MOFs and lattice water molecules, having 10.832 × 6.650 Å2 parallelogram-like nanochannels (Figure 1).24 The Ag atom is threecoordinated by two carboxylate O atoms and one cyano N atom with the Ag···Ag separation of 2.8303(4)Å in the centrosymmetric dimeric [Ag2(O2C)2] cluster (Figure 9a).24 After the synthetic techniques of the AgL crystals are harnessed, the next is to chelate riboflavin to AgL. This is probably done by the interaction between the nitrogen atom connected to the ribitol side chain (Figure 9c)33 and the Ag atom in the AgL framework. (AgL)-(riboflavin) chelated crystals are then characterized by POM, PXRD, FTIR spectroscopy, and TGA. The POM image shows that (AgL)(riboflavin) chelated crystals are needlelike and tainted with an orange color (Figure 9d). However, the PXRD diffractogram of the chelated crystals (Figure 10b) looks the same as the one of the AgL crystals (Figure 10a), indicating that the lattice framework of AgL is not altered significantly by the chelation of 1.48 mol % of riboflavin as determined by TGA (Figure S11 in the Supporting Information). Moreover, the crystallographic information of AgL crystals can be obtained from the literature,24 and there is no need to perform single-crystal determination on the chelation crystals. The FTIR spectrum of the chelated crystals further reveals a new peak appearing at 3527 cm−1 designated for the −OH stretching of the ribitol side chain of riboflavin (Figure 11b and Table 1). The large-molecule sensing feasibility for target analytes of (AgL)-(riboflavin) chelated crystals is fully tested by immersing the chelated crystals into 0.01 M aqueous solutions of melamine, acetoguanamine, 3,5-dihydroxybenzoic acid, and salicyclic acid. The lateral surfaces of the orange needlelike (AgL)-(riboflavin) chelation crystals (Figure 9d) immersed into 0.01 M aqueous solutions of melamine and acetoguanamine are bristled with many small crystalline needles forming a dark spiky coating (Figure 9f,h). It is speculated that the binding between the riboflavin dangling out of the chelation crystal surfaces (Figure 2) and the free analyte (i.e., melamine or acetoguanamine) in the aqueous solution (Figure 9e,g) create a film of supersaturation of the analyte and induce nucleation and crystal growth of the analyte near the surface of the chelation crystals. The POM observations are further confirmed by FTIR spectroscopy. The IR characteristic bands of 3471 and 3460 cm−1 for −NH2 stretching, 3135 cm−1 for N−H stretching, and 1474 and 1447 cm−1 for C−NH2 stretching from melamine appear for the (AgL)-(riboflavin)-(melamine) binding crystal sample (Figure 11c and Table 1), and the IR characteristic bands of 3431 cm−1 for −NH2 stretching, 3135 cm−1 for N−H stretching, and 1475 cm−1 for C−NH2 stretching from acetoguanamine become visible for the (AgL)-(riboflavin)(acetoguanamine) binding crystal sample (Figure 11d and Table 1). The TGA scans and PXRD patterns for binding crystals of (AgL)-(riboflavin)-(melamine) and (AgL)-(ribo-

Figure 8. Photographs of (a) yellow riboflavin powder sample, (b) red powder sample of riboflavin ground with silver nitrate, (c) orange powder sample of riboflavin ground with melamine, and (d) orange powder sample of riboflavin ground with acetoguanamine.

changes from yellow to red45 (Figure 8b), from yellow to orange (Figure 8c), and from yellow to orange (Figure 8d), respectively. The color change may be due to the occurrence of electron transfer from silver to riboflavin,45 the stiffness of the structure,46 and the donor−acceptor charge transfer after the coupling between the riboflavin and the analyte.47 AgL crystals are successfully reproduced by us.24 They are fully characterized by POM (Figure 9b), PXRD (Figure 10a), FTIR spectroscopy (Figure 11a), and TGA (Figure S10 in the Supporting Information). The results are confirmed with the literature data.24 POM images show that AgL colorless needlelike crystals have an average length of about 350 μm (Figure 9b). The PXRD diffractogram of AgL crystals (Figure 10a) closely resembles the calculated PXRD pattern from the single-crystal X-ray diffraction (SXD) data in the literature 3186



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Figure 9. Transformation of AgL crystal to (AgL)-(riboflavin) chelation crystal by chelation with riboflavin and the transformation of (AgL)(riboflavin) chelation crystal to (AgL)-(riboflavin)-(melamine) binding crystal and (AgL)-(riboflavin)-(acetoguanamine) binding crystal by immersion (AgL)-(riboflavin) chelation crystal in the aqueous solution of analyte. For the AgL crystal, (a) the Ag atom was three-coordinated by two carboxylate O atoms and one cyano N atom yielding a near T-shaped configuration (Figure 3d) and (b) polarized optical micrograph (scale bar = 50 μm). For the (AgL)-(riboflavin) chelation crystal, (c) (AgL)-(riboflavin) chelation complex (Figure 3e) and (d) polarized optical micrograph (scale bar = 200 μm). For the (AgL)-(riboflavin)-(melamine) binding crystal, (e) (AgL)-(riboflavin)-(melamine) binding complex (Figure 3e) and (f) polarized optical micrograph (scale bar = 100 μm). For the (AgL)-(riboflavin)-(acetoguanamine) binding crystal, (g) (AgL)-(riboflavin)(acetoguanamine) binding complex (Figure 3f) and (h) polarized optical micrograph (scale bar = 100 μm).

flavin)-(acetoguanamine) are included in Figures S12 and S13 in the Supporting Information. Because the binding crystals of (AgL)-(riboflavin)-(melamine) and (AgL)-(riboflavin)-(acetoguanamine) were fabricated by simply immersing the already existing chelation crystals in the analyte solutions, we do not expect any significant changes in the crystal lattice of chelation crystals. Therefore, the differences of the PXRD patterns in Figure S13 in the Supporting Information and the relatively high weight loss in the TGA scans in Figure S12 in the Supporting Information are thought to be simply due to the presence of the coating of either melamine crystals (Figure 9f) or acetaminoguanamine crystals (Figure 9h).

The solid-state PL emission spectra of AgL crystals, (AgL)(riboflavin) chelation crystals, (AgL)-(riboflavin)-(melamine) binding crystals, and (AgL)-(riboflavin)-(acetoguanamine) binding crystals are studied at room temperature (Figures 12 and 14). Their corresponding CIE 1931 chromaticity coordinates of the visible emissions excited by 330 nm light are determined46 to be (0.16, 0.16), (0.32, 0.44), (0.25, 0.37), and (0.23, 0.15), respectively (Figures 13 and 15). Because the features of the PL emission spectrum of AgL crystals (Figure 12a) look very similar to the ones of the linker of 4-cyanobenzoic acid (not shown), the original PL emission of AgL framework may be attributed to ligand-centered luminescence.16,24 However, molecular interactions between 3187



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Figure 10. PXRD patterns of (a) AgL crystal, (b) (AgL)-(riboflavin) chelation crystal, and (c) theoretical PXRD pattern calculated from SXD of AgL. Figure 13. CIE 1931 chromaticity coordinates: (a) (0.16, 0.16) for AgL crystals and (b) (0.32, 0.44) for (AgL)-(riboflavin) chelation crystals. The excitation wavelength was 330 nm with a 390 nm emission cutoff.

Figure 11. FTIR spectra of (a) AgL crystal, (b) (AgL)-(riboflavin) chelation crystal, (c) (AgL)-(riboflavin)-(melamine) binding crystal, and (d) (AgL)-(riboflavin)-(acetoguanamine) binding crystal. Figure 14. PL emission spectra of (a) (AgL)-(riboflavin) chelation crystals, (b) (AgL)-(riboflavin)-(melamine) binding crystals, and (c) (AgL)-(riboflavin)-(acetoguanamine) binding crystals. The excitation wavelength was 330 nm with a 390 nm emission cutoff.

the riboflavin and the linker of 4-cyanobenzoic acid enable electronic interactions between two different lumophores through the mode of Förster resonance energy transfer,48 which can cause a bathochromatic spectral shift (i.e., red shift), broadening in the emission, and a quenching effect as observed for the PL emission spectrum of the AgL crystals (Figure 12a) chelated with riboflavin (Figure 12b). Interestingly, subsequent binding of (AgL)-(riboflavin) chelation crystals with the aminecontaining analytes (i.e., good electron donors) such as melamine and acetoguanamine may alter the redox potential of riboflavin, inhibiting quenching and enhancing luminescence of the resultant (AgL)-(riboflavin)-(melamine) or (AgL)(riboflavin)-(acetoguanamine) binding crystals (Figure 14b,c). Hypsochromic spectral shifts (i.e., blue shift) are also observed for those crystals. Those phenomena may be attributed to the

Figure 12. PL emission spectra of (a) AgL crystals and (b) (AgL)(riboflavin) chelation crystals. The excitation wavelength was 330 nm with a 390 nm emission cutoff. 3188



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alternative toolkit for rapid detection of melamine, which is unethically used by milk manufacturers in adulterating milk to make it appear more protein-rich.38

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of riboflavin modifications, solubility curves, and Figures S1−S15. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +886-3-422-7151 ext. 34204. Fax: + 886-3-425-2296. Email: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the grants from the National Science Council of Taiwan, Republic of China (NSC 101-3113E-007-001). We are greatly indebted to Prof. Yu-Wen Chen at the Department of Chemical and Materials Engineering for his generous support and the characterization assistance from JuiMei Huang and Ching-Tien Lin at the Precision Instrument Center in National Central University, Taiwan, Republic of China.

Figure 15. CIE 1931 chromaticity coordinates: (a) (0.32, 0.44) for (AgL)-(riboflavin) chelation crystals, (b) (0.25, 0.37) for (AgL)(riboflavin)-(melamine) binding crystals, and (c) (0.23, 0.15) for (AgL)-(riboflavin)-(acetoguanamine) binding crystals. The excitation wavelength was 330 nm with a 390 nm emission cutoff.

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enhancement of the intraligand (i.e., riboflavin−analyte complex) π → π* energy.3 The necessity of using (AgL)-(riboflavin) chelation crystals to bind the analyte has also been justified by the unchanged PL emission responses of AgL before and after its immersion in a 0.01 M aqueous solution of melamine (data not shown). To test further that (1) the carbonyl groups that interacted with the analyte are actually on the riboflavin molecules chelated on the AgL framework, instead of on those free riboflavin molecules or on riboflavin molecules attached on the AgL crystal surface due to the nonpreferred interactions, and (2) the stability of (AgL)-(riboflavin) chelation crystals, additional experiments are performed: (AgL)-(riboflavin) chelation crystals are rinsed with copious amount of water, immersed in water at 25 °C for 12 h, and sonicated in an ice bath for 10 min. The FTIR spectra and PL emission spectra of (AgL)(riboflavin) chelation crystals before and after this special water treatment remain almost the same (Figures S14 and S15 in the Supporting Information), indicating that riboflavin molecules are chelated on the AgL framework with great stability.

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CONCLUSIONS The chelation of a sensing molecule (i.e., riboflavin) to a visible light-emitting MOF (i.e., AgL) has successfully turned porous MOF materials into a chemical sensor. This sensor targets specially for an analyte (i.e., melamine and acetoguanamine) in a liquid medium whose relatively large molecular size will normally exclude it from entering into MOF's smaller voids. The procedures for screening the appropriate sensing molecule by liquid-assisted grinding have also been established, which will speed up the development of the chemical and biological sensing platform and open a new doorway for biomimetic tongue, drug discovery, catalysis design, immunology, and enzymology in the future. However, for the time being, our (AgL)-(riboflavin) chelation crystals may serve as a simple 3189



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