Distinguishing small structural differences in molecules is a critical and challenging task for applications in biotechnology, environmental monitoring, gas separation and molecular sensor design. In particular, a key principle for the fabrication of chemosensors is that they must be able to detect differences in explosive, harmful or polluting small molecules, and sequentially implement a recognition–transduction protocol (Fig. 1a)1 2 3 4. This recognition should selectively trap specific molecules, and the transduction should convert the recognition event into a detectable signal. A general strategy for increasing selectivity includes enhancing the level of sophistication in the recognition unit for a target molecule by optimizing the host–guest interaction, or simply stated, by arraying various hosts to produce a composite response that is unique to each analyte, known as 'an electronic nose'5. However, these systems require a number of host matrices for the selective sensing of a specific molecule. In this study, we propose a molecular decoding methodology to provide a single host domain that accommodates several chemically diverse analytes that can also differentiate between them by transducing a particular host–guest interaction into a corresponding readout (Fig. 1b). The key to success in this process involved the use of an innovative transduction mechanism that directly visualizes the host–guest interaction itself, which corresponds to the intrinsic nature of the guest analyte. Porous coordination polymers (PCPs)6 7 8 9 10 11 12 13 14 15 16 17 with three-dimensional molecular skeletons that are assembled by organic linkers and inorganic joints can provide host matrices for molecular decoders. This arises from the inherent confinement effect within their nanopores, which enhances the host framework–guest interaction18 19, and the pore surface designability that enables the incorporation of an appropriate decoding unit into a scaffold through the use of strategic organic chemistry20 21. We designed an entangled framework, one of the most characteristic structural features of PCPs, from the elongation of organic linkers that lead to the intergrowth of one framework into the other, resulting in the catenation of the frameworks22 23. The entanglement of two chemically non-interconnected frameworks provides the dynamics from the dislocation of their mutual positions24. This entanglement dynamics of PCPs contribute to a decoding system as a result of the implementation of two processes. The first includes an induced-fit-type structural transformation that enables the entrapment of a variety of guest molecules by changing the pore size and shape to maximize the host–guest interaction (Fig. 2a). The second consists of a cooperative accommodation process due to the ordered structure of the mesoscopic crystal domains, which allows guest molecules simultaneous access to the decoding units and results in a nonlinear sensor response (Fig. 2b). Among several transduction techniques based on host–guest chemistry, such as optical, electrical, quartz crystal microbalance and surface acoustic wave techniques, luminescence is a highly sensitive and simple optical transduction method. This method does not require extensive hardware; the intensity of the luminescence is strong enough to be detected by the naked eye. In particular, luminescence, with a turn-on switching property that is triggered by a guest molecule, is intrinsically more sensitive than the more common luminescence-quenching method because detection occurs relative to a dark background. Although there have been a number of reports on luminescent PCPs25, and a few on guest-dependent luminescence26 27 28 29, none of the approaches thus far have demonstrated a plain distinguishable emission for each guest molecule, turn-on switching properties or a sensing capability for vapourized analytes. In this paper, we report on a 1,4,5,8-naphthalenediimide (NDI)-based entangled PCP that undergoes a dynamic structural transformation to confine a class of aromatic volatile organic compounds (VOCs), or tropospheric air pollutants30 31, and decodes the chemical substitution information of the aromatic species into recognizable photoluminescence in the visible light region. The aromatic species benzene, toluene, xylene, anisole and iodobenzene were indicated as blue, cyan, green, yellow and red, respectively. We show that this unusually wide range of emission arises directly from the enhanced NDI–VOC interaction that generates either exciplex fluorescence with charge transfer (CT) characteristics or phosphorescence stabilized by the heavy atom effect. These emission wavelengths are not only reasonably separated from each other but their intensity is also sufficiently amplified to be detected by the naked eye, whereas the framework without VOCs is not emissive. On the basis of this turn-on switching property, these sensors show a nonlinear luminescence response to VOC vapour concentration, with a higher sensitivity in the VOC lower-vapour-pressure region. Results Structural dynamics in response to guest accommodation The design principle used for the decoding unit involved incorporating a photoactive organic module, NDI 32 33, into the porous framework as a scaffold because of its desirable characteristic photophysical properties. Despite its very low fluorescent quantum yield (Φ f∼0.003), NDI interacts with aromatic VOCs and generates an exciplex emission with CT characteristics, resulting in the production of a new weak fluorescence band (Φ f 2%, which resulted in a nonlinear sensor response. In this report, we have demonstrated a novel sensing method for distinguishing small structural differences in molecules using a sophisticated transduction protocol, molecular decoding. Information regarding different phenyl ring substituents of aromatic VOCs is decoded into a corresponding visible light emission and observed as a readout from the decoding matrix based on an NDI-embedded porous framework. Such a decoding protocol is observed by direct visualization of the host–guest interaction, either by exciplex emission based on the CT characteristics of the NDI and VOC or a phosphorescence-stabilized event based on a heavy atom that is attached to the aromatic guest. The aromatic guest molecules are strongly confined in the nanopores because of a dynamic structural transformation in the framework entanglement, and the resulting host–guest interaction produces an enhanced emission that is detectable by the naked eye. We have demonstrated that the cooperative structural transition induced by the adsorption of a guest molecule provides a nonlinear sensor response that can be used for the amplification of the signal intensity to achieve a high sensitivity. This simple yet sophisticated decoding method of combining a single excitation light wavelength and an RGB detector will enable new novel applications of crystalline porous materials as portable solid-state sensor devices. Methods Materials All of the reagents used, except for dpNDI, were obtained from commercial suppliers and were used without further purification. The dpNDI was synthesized according to a previous report47. Preparation of {[Zn2(bdc)2(dpNDI)]·4(DMF)} n (1a) A mixture containing Zn(NO3)2•6H2O (149 mg, 0.50 mmol), H2bdc (83 mg, 0.50 mmol) and dpNDI (210 mg, 0.50 mmol) was suspended in DMF (50 ml) and heated to 95 °C for a period of 3 days. The slightly yellow crystals of 1a were then collected. Preparation of dried sample [Zn2(bdc)2(dpNDI)] n (1) The crystals of 1a were dried in a vacuum oven at 120 °C for 1 day, resulting in a dried framework, 1. Elemental analysis of 1 calcd (%) for C40H20Zn2N4O12: C 54.63, H 2.29, N 6.37. Found: C 55.26, H 2.40, N 6.48. Preparation of 1 VOC Dried crystals of 1 were immersed in each VOC liquid for 1 week, resulting in the formation of 1 VOC. Measurements The TG analysis was carried out using a Rigaku Thermo plus TG 8120 apparatus (Rigaku) in the temperature range of 303–773 K under flowing N2 gas at a heating rate of 10 K min−1. Elemental analysis was carried out using a ThermoFinnigan EA 1112 apparatus. PXRD data were collected using a Rigaku RINT-2200 Right System (Ultima IV, Rigaku) diffractometer with CuKα radiation and at the BL13XU beamline (SPring-8) using a synchrotron X-ray source (λ=1.55642 Å). The simulated patterns were obtained from single-crystal X-ray analysis. Ultraviolet–vis diffuse reflectance measurements were recorded using a JASCO V-670 spectrometer (JASCO) with an integration sphere attachment. The samples were dispersed in aluminium oxide (5 wt%). The excitation and fluorescence spectra were recorded using a suspension of the dispersed powder in the guest solvent, with a SPEX spectrofluorimeter equipped with a double monochromator in both the excitation and emission channels (HORIBA Fluorolog-3, HORIBA) and 1-cm quartz cuvettes. The emission and excitation spectra were recorded at 450 and 370 nm, respectively. The spectra were corrected for the monochromator wavelength dependence and photomultiplier response functions. Phosphorescence spectra were measured after excitation at 370 nm in a HORIBA Fluorolog-3 apparatus, using a flash lamp with a 50-μs delay after the flash and a gate time of 61 μs, accumulating 100 flash counts per point. The fluorescent and phosphorescent quantum yields were measured using an Absolute photoluminescence quantum yield measurement system (Hamamatsu, C9920-02). The radiative deactivation curves for the fluorescence were recorded using a Time-correlated single-photon counting technique (Edinburgh Instruments, model FL 920) with a time resolution of 30 ps after deconvolution of the excitation pulse. Excitation was performed using Diode lasers (PicoQuant, model LDH 370), using a pulse with a full-width at half-maximum of 150 ps and a repetition rate of 2,500 kHz. The data were recorded at the emission maximum of each sample. The samples were measured in a suspension of dispersed powder in the guest solvent. The radiative deactivation curve of the phosphorescence was recorded using a HORIBA FluoroCube (HORIBA). Excitation was carried out using a SpectraLED-370. Data were recorded at the emission maximum of 635 nm. Electron spin resonance spectra were recorded at 77 K using a JEOL JES-RE2X spectrometer (JEOL) operating in the X-band. Resonance frequency was measured using an ADVANTEST R5372 microwave frequency counter (ADVANTEST). Sorption isotherm measurements were recorded using an automatic volumetric adsorption apparatus (BELSORP-max; BEL Japan). The as-synthesized samples were evacuated under high vacuum (<10−2 Pa) at 393 K overnight to remove guest molecules. The adsorbate was placed into the sample tube and, before measurement, evacuated again using the degas function of the analyser for 2 h at 393 K. The change in pressure was monitored, and the degree of adsorption was determined by the decrease in pressure at the equilibrium state. SCXRD measurements of 1a––1c Single crystals of 1a–1c were mounted in a loop. Measurements were taken using a Rigaku AFC10 diffractometer using a Rigaku Saturn CCD system (Rigaku) equipped with a rotating anode X-ray generator that produced multilayer mirror monochromated MoKα radiation. In all cases, the structure was elucidated using direct methods and refined using full-matrix least-squares techniques on F 2 (SHELXL-97). All of the non-hydrogen atoms were anisotropically refined, whereas all hydrogen atoms were placed geometrically and refined using a riding model with Uiso constrained to be 1.2 times Ueq of the carrier atom. The crystal of 1c was a pseudomerohedral twin, and the refinement against the intensities was carried out using the SHELXL-97 software package, including the twinning operator (k, h, l) (TWIN) as the twin fraction (batch scale factor, initially set to 0.5). The twin component of the final material comprised 66.18% of the crystal. The number of toluene molecules was determined by thermogravimetric measurements: 2.5 toluene molecules per two Zn atoms. The two guest toluene molecules were initially generated by fixing the bond length to 1.35 Å, the bond angle to 120° and the torsion angle to 0°. The remaining 0.5 toluene molecule was highly disordered; further refinement was unsuccessful. All parameters were refined by restraining these values and the thermal factors. Crystallographic data for 1a, 1b and 1c (Supplementary Data S1, 2, 3) have been deposited in the Cambridge Crystallographic Data Center under deposition numbers CCDC 743922, 773586 and 743923, respectively. Sensor response as a function of toluene vapour pressure Sensor response measurements were taken using a Fluorescent microscope (BX 51, Olympus) with a 100-W mercury lamp (USH-1030L, Olympus) connected to a BEL-Flow vapour control system (BEL Japan). A schematic illustration of this system is shown in Supplementary Fig. S17. The light source was filtered using an excitation filter that had a light transmission bandwidth between 330 and 385 nm (U-MWU2, Olympus), as well as neutral density filters (U-25ND25 and U-25ND6, Olympus). The fluorescence above 400 nm was detected using a CCD camera (DP72, Olympus) with a 30 ms exposure time. The relative toluene vapour pressure in a helium gas stream was controlled using three independent mass flow controllers in the range of 0.0–90.0%. The toluene vapour saturated carrier gas was generated by passing it through toluene (28 °C) and cooling it to 25 °C using a condenser. The toluene-saturated gas was then mixed with dry helium gas to obtain the desired relative pressure. The vapour gas flowed into a quartz capillary that contained the sample for a period of 10 min at a flow rate of 100 cm3 min−1. Author contributions S.F. and S.K. conceived and designed the experiments. Y.T. conducted all synthetic and characterization experiments. V.M.M., Y.T. and S.F. performed luminescent spectroscopy and luminescent lifetime measurements. M.K., Y.T. and K.S. performed single crystal X-ray crystallography. Y.T. and S.S. performed electronic paramagnetic resonance measurements. Y.T., H.U., M.N. and S.F. performed sensor response experiments. S.F., Y.T. and V.M.M. analysed the data and co-wrote the manuscript. All authors discussed the results and commented on the manuscript. Additional information How to cite this article: Takashima, Y. et al. Molecular decoding using luminescence from an entangled porous framework. Nat. Commun. 2:168 doi: 10.1038/ncomms1170 (2011). Supplementary Material Supplementary Figures, Tables and References Supplementary Figures S1-S18, Supplementary Tables S1-S2 and Supplementary References Supplementary Data 1 Crystallographic data for 1a Supplementary Data 2 Crystallographic data for 1b Supplementary Data 3 Crystallographic data for 1c
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