Enhanced Fluorescence in Tetraylnitrilomethylidyne–Hexaphenyl

Article pubs.acs.org/JPCC

Enhanced Fluorescence in Tetraylnitrilomethylidyne−Hexaphenyl Derivative-Functionalized Periodic Mesoporous Organosilicas for Sensitive Detection of Copper(II) Meng Gao,† Shuhua Han,*,† Yongfeng Hu,‡ and Lijuan Zhang§ †

Key Lab of Colloid and Interface Chemistry Ministry of Education, Shandong University, Jinan 250100, P. R. China Canadian Light Source, 44 Innovation Boulevard, Saskatoon, SK S7N 2V3, Canada § Shanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, P. R. China ‡

S Supporting Information *

ABSTRACT: Highly fluorescent and copper(II)-responsive periodic mesoporous organosilicas (TH-PMOs) were successfully obtained by using a (tetraylnitrilomethylidyne−hexaphenyl (TH))-derived tetrasiloxane (TH-Si4) as the organosilica precursor. The TH unit was embedded within the framework of PMOs by four silyl groups without forming associated species, and high fluorescence quantum yields were achieved even for the PMOs prepared from 100% organosilane precursor. The optical studies indicated that the intramolecular rotation of TH was restricted by the framework of THPMOs, resulting in the decline of the nonradiative decay process and enhanced monomeric fluorescence emission. The unique structure of the TH groups not only assured their aggregation-induced enhanced emission (AIEE) characteristics but also provided potential coordinating sites for metal ions. Therefore, the enhanced fluorescence of PMOs showed a highly selective response to copper ions in aqueous solution with the detection sensitivity up to the 10−8 M level. Moreover, the diffusion process of Cu2+ and the competitive effect between Cu2+ and Fe2+ on TH-PMOs were measured by STXM; the results reveal that the specific binding between TH and Cu2+ brings about a relatively high adsorption capacity of the hybrid material toward Cu2+.

■

INTRODUCTION The optical materials in chemosensors have been extensively explored and become one of the fast moving and exciting research directions.1−4 In these sensing systems, the fluorescent receptor and immobilization platform are two important parts to be considered. The receptor part is responsible for the device selectivity, and the immobilization platform is responsible for the device stability as well as the sensitivity.5 Traditional organic fluorophores often suffer from aggregation-caused quenching (ACQ) when they are at high concentration level or in the solid state, which results in severely negative effects on sensitivity and efficiency of chemsensor.6 Promisingly, aggregation induced enhanced emission (AIEE) that emits more efficiently in the aggregated state than the dispersed state has been found in some nonplanar aromatic molecules, originating mainly from restriction of intramolecular rotation (RIR) upon aggregation.7−9 The unique fluorescent emission property of AIEE fluorophores makes them favorable potential candidates in the applications of chemosensors10,11 and bioprobes.12 Recently, many efforts have been made to develop luminescence sensors based on novel AIEE materials for molecular or ionic detection.13,14 Periodic mesoporous organosilicas (PMOs), in which the functional organic groups are densely and covalently embedded within the silica walls,15−17 © 2016 American Chemical Society

are ideal optical support materials for sensitive receptors by incorporation of optically active dye with detective potential applications.18−21 In addition, the density and molecular-scale ordering of the optical bridging groups within pore walls could be controlled by the molecular design of the precursors and the synthesis conditions of materials, which creates a fascinating platform with the AIEE effect. The recognition and detection of metal ions have vital significance in the field of chemical sensing due to their important role in biological and environmental processes.22,23 Copper(II) is an essential trace metal ions for both plants and animals, including human beings, but excess copper(II) may be highly toxic to organisms.24 Therefore, it is crucial to develop an effective chemosensor for sensitive and selective copper(II) sensing.25 Schiff bases are attractive fluorescent chemosensors because of their simple synthetic methods and easily coordinating with various metal ions.26,27 Bearing this in mind, a highly sensitive and selective Cu2+ fluorescence receptor TH with tetraylnitrilomethylidyne (Figure 1) was designed in our work by the Schiff base reaction of pReceived: February 23, 2016 Revised: April 12, 2016 Published: April 13, 2016 9299

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C

Figure 1. Synthesis of tetraylnitrilomethylidyne−hexaphenyl derivative precursor (TH-Si4).

Guangcheng Reagent Company. Acetone, acetonitrile, ethanol, and tetrahydrofuran (THF) were obtained from Tianjin Fuyu Reagent Company. All chemicals were of analytical grade and used without further purification from the commercial sources. Oligomeric cationic surfactant (sym-Ph(1-3-14)3) was synthesized by reference to a reported route.28 Characterization. 1H NMR and 13C NMR analyses were carried out on a Bruker DPX-300 NMR spectrometer. Mass spectral data were acquired on an Agilent 6510 QTOF. Smallangle X-ray scattering (SAXS) measurements were carried out on SAXSess mc2 (Anton Paar). FT-IR spectra were recorded on a Nicolet 700 FT-IR spectrometer (Thermo-Fisher Scientific, Inc., Waltham, MA) with samples prepared as KBr pellets. The solid-state 29Si magic angle spinning nuclear magnetic resonance (MAS NMR) spectra were recorded on a Bruker DSX 300 MHz spectrometer (5000 transients, spin speed 6 kHz, acquisition time 0.02 s, pulse delay 3 s). Nitrogen adsorption/desorption isotherms were measured using a Micromeritics ASAP2020 surface area and porosity analyzer at 77 K. Prior to measurements, samples were outgassed at 140 °C for 8 h. UV−vis spectra were measured using an Agilent HP8453E spectrometer. The steady-state fluorescence spectra were performed on a PerkinElmer LS-55 spectrometer. Using quinine sulfate in 0.1 M H2SO4 (quantum yield = 0.54) as a standard, fluorescence quantum yields were calculated according to the formula Φu = Φs(Fu/Fs)(As/Au), where Φ, F, and A are fluorescence quantum yield, integral fluorescence intensity, and absorbance at fixed wavelength, respectively. Time-resolved fluorescence spectra were obtained using an Edinburgh FLS920 fluorescence spectrophotometer. The instrument was operated with a thyratron-gated flash lamp filled with hydrogen at a pressure of 0.4 bar. The amounts of metal ions in materials were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis using IRIS INTREPID II XSP from Thermo Electron. X-ray absorption experiments were performed on the SXRMB beamline at the Canadian Light Source (CLS). Si(111) crystals were used as the crystal monochromator. Powder samples were mounted using the Kapton tape, and spectra were recorded in fluorescence yield (FY) using a Si(Li) drift detector. EXAFS data were normalized against the

hydroxybenzaldehyde with 3,3′-diaminobenzidine. And then, TH tetrapropyltriethoxysilane precursor (TH-Si4, Figure 1), which possesses co-condensation ability on the silica framework, was prepared by the reaction of TH and 3-isocyanatopropyltriethoxysilane. Using the designed TH-Si4 and tetraethoxysilane (TEOS) as the mixture or single precursors, the tetraylnitrilomethylidyne−hexaphenyl derivative bridged PMOs (TH-PMOs), which is a new type of fluorescent chemosensing material with AIEE effect for specific copper ions sensing, were synthesized. In these hybrid materials, the rotating motion of benzene ring in the TH-Si4 is blocked by steric limitation when the triethoxysilane groups of the precursor are anchored into the silica framework, isolating the TH units in the PMOs and giving rise to monomeric fluorescence in the solid state. It is conceivable that the increase of the TH-Si4 loaded in hybrid PMO materials will enhance the ability of restriction of intramolecular rotation of the benzene ring and promote the fluorescence emission. In the aspect of detecting performance of these chemosensors, the fluorophore TH provides the necessary binding site for the metal ions, resulting in the change of fluorescence behavior. Thus, the TH-PMOs hybrid material reserves the same selectivity as that of TH molecule, and the strong fluorescence of TH-PMOs materials can be quenched by Cu2+ via a fluorescence “on−off” mechanism. Besides, the sensitivity of TH-PMOs is better than that of the corresponding free TH molecule, and a detection limit for Cu2+ was obtained as low as 40 nM. To investigate the metal ions on TH-PMOs material in situ, STXM and XANES were employed in our study to provide direct evidence of the diffusion process of Cu2+ and the competitive effect between Cu2+ and Fe2+. The results demonstrate that the specific binding between TH and Cu2+ plays an important role in the adsorption of metal ions.

■

EXPERIMENTAL SECTION Materials and Methods. N,N,N′,N′-Tetramethyl-1,3-propanediamine, 3,3′-diaminobenzidine, 3-(triethoxysilyl)propyl isocyanate, p-hydroxybenzaldehyde, and 1,4diazabicyclo[2.2.2]octane (TEDA) were purchased from Shanghai Jingchun Reagent Company. Tetraethoxyorthosilane (TEOS) and aqueous ammonia were purchased from Tianjin 9300

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C incoming photon flux, and background absorption contributions were removed using Athena.29,30 Cu K-edge k-space (kweight = 2) and Fourier-transform plots were generated using Athena. Soft X-ray Scanning Transmission Microscopy. STXM measurements were performed at the beamline BL08U1 of the Shanghai Synchrotron Radiation Facility. Samples were prepared using TH-PMO-100, which were washed after adsorption of metal ions in aqueous solution (0.5 M). The samples were ultrasonically dispersed in ethanol and deposited on a silicon nitride membrane. First, a dual-energy method was performed to obtain the elements mapping. Photon energies are adjusted just below or above the absorption edge of the chosen element and are 929.0 and 926.4 eV for Cu 709.0 and 705.0 eV for Fe. Then, Cu distribution maps were obtained by analyzing a series of stack-scanned images at energies around the relevant absorption edges (from 926 to 953 eV). They were aligned via a spatial cross correlation analysis method. NEXAFS spectra were extracted from groups of pixels within the image regions of interest using the IDL package aXis2000.31 Synthesis. Synthesis of Tetraylnitrilomethylidyne−Hexaphenyl Derivative (TH). 3,3′-Diaminobenzidine (1.07 g, 5.0 mmol) was dissolved in anhydrous ethanol (160 mL) and heated to 80 °C, then p-hydroxybenzaldehyde (2.93 g, 24.0 mmol) dissolved in anhydrous ethanol (20 mL) was slowly dropped into diaminobenzidine solution with sufficient stirring, and subsequently the glacial acetic acid (4 drops) was added in mixture. The yellow reaction mixture was then stirred for 12 h, and the product was collected by filtration and dried under vacuum for 3 days (yield, 83%). Spectral data for 1H NMR (300 MHz, DMSO-d6, δ) were as follows: 9.97 (2H, PhOH), 9.40 (2H, PhOH), 7.74 (2H, BpH), 7.52 (4H, ArH), 6.89 (4H, ArH), 6.68 (2H, BpH), 6.65 (2H, BpH) and 5.45 (2H, HC N) (Figure S1). 13C NMR (75 MHz, DMSO-d6, δ) 109.54, 116.01, 119.50, 122.02, 127.66, 128.02, 131.04, 137.10, 154.55, 157.14, 159.31(Figure S2). HR-MS (ESI) calcd for TH [M]+, 634.24; found 633.78. Synthesis of TH-Bridged Silsesquioxane Precursor (TH-Si4). A mixture of TH (1.40 g, 2.0 mmol) and TEDA (0.09 g, 0.8 mmol) was dissolved in anhydrous THF (120 mL). Then, 3isocyanatopropyltriethoxysilane (2.47 g, 10.0 mmol) was added as a solution in anhydrous THF (20 mL) followed by 24 h reflux under a nitrogen atmosphere. After cooling to room temperature, the solvent was concentrated under reduced pressure. Finally, the crude product was washed thoroughly with anhydrous n-hexane to afford TH-Si4 as a yellow solid (yield, 74%). Spectral data for 1H NMR (300 MHz, DMSO-d6, δ) were as follows: 7.87 (2H, BpH), 7.75 (4H, ArH), 7.69 (4H, NH), 7.26 (4H, BpH), 7.02 (4H, ArH), 5.65 (2H, HCN), 3.72 (24H, ethoxy CH2), 3.05 (8H, CH2NH), 1.54 (8H, CH2CH2Si), 1.12 (36H, ethoxy CH3), and 0.56 (t, 8H, Si-CH2) (Figure S3). 13C NMR (75 MHz, DMSO-d6, δ) 7.15, 18.16, 22.76, 55.98, 57.64, 115.79, 122.00, 125.26, 127.47, 128.15, 128.84, 130.04, 132.04, 136.60, 142.00, 150.30, 153.12, 153.85 (Figure S4). HR-MS (ESI) calcd for TH-Si4 [M+2H]2+, 810.36; found 810.37. Synthesis of TH-Bridged Periodic Mesoporous Organosilicas (TH-PMOs). Periodic mesoporous organosilicas were prepared using an evaporation-induced self-assembly (EISA) strategy. The mixture precursors of TH-Si4 and TEOS (the total Si amount is 2.0 mmol and the mole fraction of organic silicon of TH-Si4 is 0%, 25%, 50%, 75%, and 100%) and oligomeric cationic surfactant sym-Ph(1-3-14)3 (0.40 g) were

dissolved in 10.0 mL of a 9:1 (v/v) mixture of THF and demonized water, followed by the addition of 1 M hydrochloric acid (40 μL). The mixture was poured into a Petri dish after being stirred at room temperature for 24 h, and then the solvent was evaporated at room temperature for 48 h. In order to fully condense the silica units and remove the surfactant, the films were heated in NH3·H2O and kept at 50 °C for 12 h and finally refluxed in appropriate water for 2 h. The resulting samples are denoted as TH-PMO-X, where X represents the molar fraction of organic silicon to the total Si.

■

RESULTS AND DISCUSSION Structural Properties. Evidence for the incorporation of TH groups to the frameworks of PMOs is provided by both FT-IR spectra and solid-state 29Si NMR spectra. The FT-IR spectrum of TH shows characteristic CN stretching vibration of imide group at 1612 cm−1, and the CC stretching vibration of the phenyl rings at 1500−1600 cm−1 (Figure S6). Two new characteristic bands are observed in the spectra of TH-PMOs, the bands at 1350−870 cm−1 correspond to the stretching vibrations of the Si−OH and Si−O−Si frameworks, and another band at 1632 cm−1 is ascribed to the CO stretching vibration from propylamide that linked the siloxane. The characteristic bands of the TH moieties remain, and the absorption intensity increases with increasing amount of TH precursor after the extraction, which suggested that the TH groups are stably incorporated into the pore walls of the hybrid materials. The covalent interaction between TH precursor and the silica framework is further confirmed by solid-state 29Si NMR spectra (Figure 2). The spectrum of the TH-PMO-50 displays two regions of major peaks, centered at about −85 to −120 and −45 to −75 ppm, which correspond to Si(−O−)4 (Qn sites) and RSi(−O−)3 (Tn sites) species, respectively. In

Figure 2. 29Si MAS NMR spectra of TH-PMO-50 (a) and TH-PMO100 (b) after extraction of surfactant. 9301

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C addition, only Tn sites are shown in the spectrum of the THPMO-100 because the PMOs formed with only addition of TH-Si4. There is no detection of uncondensed silicon species (T0: RSi(OH)3; Q0: Si(OH)4), suggesting that both sides of the silyl groups of the TH-Si4 are attached on the frameworks of PMOs. Small-angle X-ray scattering (SAXS) patterns of as-prepared TH-PMOs are shown in Figure 3. A sharp scattering peak at q

photograph of TH-PMO-25 reveals the presence of mesochannels with center distance about 3.8 nm. For samples with higher content of organic groups (Figure S8), disordered pore channels are observed in the HRTEM images. It is very credible that the higher loading of TH could disrupt the ability for the surfactant to form micelles and corresponding mesophases, resulting in the assembly of more disordered materials. In addition, the mesoporous framework contained more TH groups with large size and less rigidity would be more likely to collapse after extraction of the template. The nitrogen adsorption−desorption isotherm of TH-PMOs is classified as type IV curve, which suggests the successful formation of mesopores in materials (Figure S9). The pore size distribution of TH-PMO-25 calculated with the BJH method is centered at 2.54 nm, and the BET surface area and the total pore volume are calculated to be 300.2 m2 g−1 and 0.39 cm3 g−1, respectively. As indicates shown in Table S1, the surface areas and pore volumes of TH-PMOs decrease with the increase of amount of TH groups, indicating that the incorporation of the TH moieties into the pore walls has a negative effect on the surface properties of the hybrid materials. Optical Properties. As shown in Figure 5, the UV−vis spectrum of TH-Si4 in solution exhibits two absorption bands

Figure 3. SAXS patterns of as-prepared TH-PMO-X. From top being X = 0, 25, 50, 75, and 100, respectively.

= 1.54 nm−1 in the low-angle region corresponding to the dspacing value of 4.1 nm for TH-PMOs samples is observed, indicating appearance of ordered mesoporous structure in all the TH-PMOs samples. The intensity of the scattering peak deceases with increasing amounts of the TH-Si4 precursor, suggesting that more TH organic groups could disrupt the ordering of the mesostructure.32,33 After extraction of the template, the intensity of scattering peaks decreases, demonstrating the mesophase partly collapse without the support of surfactant (Figure S7). The formation of mesopore structure is further confirmed by HRTEM images of the extracted TH-PMO-25 (Figure 4). The

Figure 5. UV−vis absorption spectra of (a) TH-Si4 (5 × 10−5 M) and TH-PMO-100 (2 × 10−5 g mL−1) in acetonitrile; (b) TH-PMOs in acetonitrile/water (1:1 v/v, 2 × 10−5 g mL−1).

at 251 and 322 nm. The band at 251 nm could be due to the π−π* transition, and the second band at 322 nm could be assigned to the transition of n−π* in TH molecule structure.34 A slight red-shift (3 nm) occurs for the π−π* transition of THPMOs compared with that of TH-Si4, which might be due to higher conjugation or coplanarity degree of the benzene rings in the TH-PMOs.35 In addition, the absorption intensities of TH-PMOs increase with increasing the amount of TH, whereas the spectral shape is unchanged and similar to that of the THSi4 solution (Figure 5b), indicating that the weak interaction between TH groups is reserved in the ground state after the attachment of TH into the framework of PMOs. The optical properties of TH-Si4 and TH-PMOs were further investigated by the fluorescence spectra (Figure 6). An emission peak with the λem around 393 nm is observed for the TH-Si4 precursor in acetonitrile solution. However, the positions of the fluorescence emission peaks of the TH-PMOs blue-shift to 387 nm compared with that of the TH-Si4 precursor. The possible explanation for this phenomenon is that the effect of the steric hindrance induces the formation of more rigid structure.36 Additionally, although the emission intensities of TH-PMOs increase with the increase of the THSi4 content, the spectral shapes are symmetrical and positions of the emission peaks are unchanged (Figure 6b), which suggests

Figure 4. HRTEM images of TH-PMO-25 after extraction of the template. 9302

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C

Figure 6. Fluorescence emission spectra (a) TH-Si4 and TH-PMO-100 in acetonitrile (5 × 10−7 M−1); (b) TH-PMOs in acetonitrile/water (1:1 v/v, 10−6 g mL−1), from bottom being TH-PMO-25, TH-PMO-50, TH-PMO-75, and TH-PMO-100, respectively.

fluorescence decay curves of both TH and TH-PMOs (Figure 7) are fitted to monoexponential profiles, which reflect the

that weak or no interaction between TH units is preserved in the excited state.37 TH units embedded in the frameworks are isolated without forming associated species, which might be caused by the electrostatic repulsion interactions of precursors with the positive charge in acidic hydrolysis−condensation process. Moreover, the intramolecular rotational motion of the TH groups could be restricted in TH-PMOs because of the fixation of four Si−C bonds; therefore, the interaction between the TH groups in the excited state could be suppressed.38 It is noteworthy that the TH units in the framework have not formed quenching sites, even in the densely accumulated state from 100% organosiliane precursor. After the unity of the concentration of TH, the enhanced emission is observed from TH-PMO-25 to TH-PMO-100 (Figure S10), which could be attributed to the fact that more severe restriction of the rotation motion is preserved for the samples with the higher TH content. To understand the origin of the fluorescence behaviors of TH in the PMOs, the relative fluorescence quantum yields of TH and TH-PMOs were measured using quinine sulfate in 0.1 M H2SO4 (quantum yield = 0.54) as a standard. Table 1 Table 1. Quantum Yields Lifetimes and Radiative/ Nonradiative Rate Constants of TH and TH-PMOs

TH TH-PMO-25 TH-PMO-50 TH-PMO-75 TH-PMO-100 a

quantum yield Φa

lifetime τb (ns)

radiative rate kf (× 108 s−1)

nonradiative rate knr (× 108 s−1)

0.46 0.52 0.68 0.75 0.84

1.49 1.61 1.59 1.57 1.58

3.09 3.23 4.28 4.77 5.32

3.62 2.98 2.01 1.60 1.01

Figure 7. Fluorescence decay curves (λex 320 nm) of TH and THPMOs.

absence of associated species in PMOs. The fluorescence lifetimes (τ) of TH-PMOs are extended after the TH groups are anchored into the hybrid frameworks. Furthermore, the radiative (kf) and nonradiative (knr) rate constants could be calculated from both of the quantum yield and fluorescence lifetime, according to eqs 1 and 2 (Table 1). In comparison with that of TH, the larger kf and the smaller knr are obtained for all TH-PMOs samples, which should be beneficial for emission enhancement. Besides, the kf is increased and the knr is decreased gradually with the increase of the TH percentage in the hybrid materials. Importantly, TH-PMO-100 exhibits the highest kf and the lowest knr value, and the ratio value of kf/knr is up to 5.27-fold, which is much higher than the value of TH in solution (0.85-fold). The above results indicate that the fluorescence emission enhancement of TH-PMOs could be

Φ = kf/(kf + knr) (eq 1). τ = 1/(kf + knr) (eq 2). b

summarizes the quantum yields Φ of TH molecule and the four TH-PMO materials with different contents of TH. Specifically, the fluorescence quantum yield of TH in solution is 0.46, whereas the fluorescence quantum yields of TH-PMOs increase from 0.52 to 0.84 when the TH contents of PMOs are increased from 25 to 100% accordingly. It is remarkable that such a high quantum yield is achieved for the PMOs prepared from 100% organosiliane precursor TH-Si4. The fluorescence efficiency for the TH-PMOs would be enhanced with the increase of TH fraction, which is consistent with the result obtained from the fluorescence spectra. Moreover, the 9303

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C

ions in the fluorescence response of TH-PMO-100 to Cu2+ were tested. It is noted that the addition of the mixture of Cu2+ and other metal ions mentioned above show almost identical fluorescence intensity compared with the sample with Cu2+ only (Figure 8b). These findings indicate that the designed THPMOs have an outstanding selectivity for Cu2+ probing in aqueous solution. The fluorescence intensity of TH-PMO-100 exhibited a gradual decrease upon the addition of Cu2+ (Figure 9). Good

mainly originated from the reduced nonradiative decay when the intramolecular rotation of TH groups become restricted due to the fixed Si−C bonds and steric hindrance of silica framework.39,40 In addition, the higher content of TH units in the backbones could create much more probability of the rotational restriction due to the increased density of the organic groups in the pore walls, making the AIEE effect much more obvious.41 The observed AIEE effect was also supported by the fluorescence studies of TH in H2O/THF mixtures with different contents of water (Figure S11). When the poor solvent water was added into the solution of TH in THF, the aggregation occurred and the fluorescence intensity of TH increased. Sensitive and Selective Probe toward Cu2+. The responses of the TH-PMO-100 toward Cu2+ and various metal ions were monitored by fluorescence spectroscopy. As depicted in Figure 8a, upon the addition of Cu2+ ions, the

Figure 9. Fluorescence spectra of TH-PMO-100 in acetonitrile/water (v/v = 1/1) with different concentrations of Cu2+ (0 to 1 × 10−5 M). Inset: linear calibration plot for Cu2+ in low concentration (1 × 10−7− 1 × 10−6 M).

linearity of the fluorescence intensity as a function of the Cu2+ concentration between 1 × 10−7 and 1 × 10−6 M was established (R2 = 0.980). The limit of detection (LOD) is determined from the equation LOD = 3S0/k, where S0 is the standard deviation of the blank solution and k is the slope of the calibration curve. The obtained result is 4.0 × 10−8 M for Cu2+, indicating that TH-PMO-100 is suitable to be a highly sensitive Cu2+ chemosensor. It is conceivable that the THPMO-100 bring out the relatively low LOD than the TH molecule in solution (Figure S12), which is due to the stability and AIEE effect of the TH-PMOs. Additionally, the calculating binding constant (K) that TH molecule coordinate with Cu2+ ion is about 1.0 × 108 M−1 (Figure S13). Furthermore, the Xray absorption near-edge spectroscopy (XANES) analysis (Figures S14 and S15) is used to confirm the coordination of TH units with Cu2+, which is the origin of fluorescence quenching. The reproducibility and recycling are the outstanding advantages of solid sensors. Figure S16 shows the regeneration potential of TH-PMO-100 toward Cu2+ in aqueous solutions. The quenched fluorescence of TH by Cu2+ could be recovered with the addition of EDTA to the suspensions. Moreover, after washing the EDTA added before by deionized water, the regenerated solid hybrid material still shows highly fluorescence response to Cu 2+ . These results imply the excellent reproducibility of TH-PMOs. STXM is an effective technique to map the chemical composition in situ and enables characterization of the elemental distribution of adsorbed metal ions. Figure 10 presents optical density image (a), dual energy image (b), and

Figure 8. (a) Fluorescence spectra of TH-PMO-100 (5 × 10−7 g mL−1) upon addition of various metal ions (10−5 M) in acetonitrile/ water (1:1 v/v). (b) Fluorescence emission intensity of TH-PMO-100 (5 × 10−7 g mL−1) in the presence of a single metal ion (red bars) and in the mixture of Cu2+ and other metal ions (black bars). Solvent: acetonitrile/water = 1/1; [metal] = 10−5 M.

fluorescence of TH-PMO-100 is dramatically quenched by Cu2+ ions, while no discernible influence is observed from other alkali earth and transition metal ions (including Ag+, K+, Na+, Cu2+, Fe2+, Co2+, Ni2+, Zn2+, Cd2+, Hg2+, Mn2+, Pb2+, Mg2+, Ca2+, Ba2+, Al3+, Cr3+, and Fe3+ ions). Additionally, in the competition experiments, the influences of interfering metal 9304

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C

Figure 10. Distribution of Cu in the TH-PMO-100 material determined by STXM. X-ray optical density image (a), dual energy image (b), and stack-scanned image (c) obtained at the energies around Cu L-edge; the stack mappings are decomposed into blue, green, red, and yellow regions, and the corresponding XANES spectra are shown in (d).

Figure 11. Distribution of Cu and Fe elements in the TH-PMO-100 sample determined by STXM: (a) Cu element; (b) Fe element; (c) merged image of Cu and Fe elements.

edge, respectively. The Cu L3 absorption edge in spectra exhibit an absorption maximum at 931.0 eV (CuCl2) with a shoulder at about 931.9 eV that is ascribed to the energy of Cu(II) bonding with N atoms in TH groups.42,43 It is worth noting that the absorption intensity of the peak at 931.9 eV increases gradually from the surface to the inner layer (Table S2), indicating more Cu2+ are coordinated with TH in the inner space. This result reveals that the specific binding between TH and Cu2+ could promote diffusion of ions to the interior of the material. STXM is an ideal approach to distinguish the distribution of competitive elements based on the energy differences of various elements in NEXAFS spectra. The Cu2+ and Fe2+ distributions in TH-PMO-100 were obtained by dual energy analysis from the same sample region after exposure to the mixture solution with same concentration of Cu2+ and Fe2+. As shown in Figure 11, Cu2+ ions exhibit more widely dispersion compared with that of Fe2+ ions, especially in the inner region of the materials, because there is no specific binding sites between TH and Fe2+, which make it relatively hard to permeate into the internal part of the material. To further measure the metal ions contained in this material, ICP-AES analysis shows that Cu and Fe contents are 0.25 and 0.16 mmol/g, respectively, which agreed with the STXM results.

stack-scanned image (c) of the spatial distribution of adsorbed Cu2+ ions in the TH-PMO-100 material. The Cu element exhibited a heterogeneous distribution in the dual energy analysis, and the concentration of Cu element is relatively low in the inner region of materials, which has been controlled by the diffusion process. Stack analyses are then applied on this Cu2+ distributing region and presented as a color-coded map (Figure 10c). The STXM stack mapping shows four distinct regions, colored with blue (background), green (surface of the materials), red (intermediate in materials), and yellow (inner in materials), which are divided by the spectroscopic differences of Cu L-edge absorption. The highest content of Cu2+ is exhibited in the intermediate space of the materials (red), which is corresponding to the result of dual energy analysis. The possible explanation is that copper ions enter into the material by diffusion control which could induce a concentration gradient: surface > intermediate > inner. However, the reduction of copper ions in the surface layer could be ascribed to the washing action in the preparation process. Furthermore, the corresponding near-edge X-ray absorption fine structure (XANES) spectra (Figure 10d) extracted from the color-coded map reveal that the peaks around 930.0−932.0 and 950.0− 952.0 eV are corresponding to the Cu L3 and L2 absorption 9305

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C

■

(4) Lei, J.; Wang, L.; Zhang, J. Ratiometric pH Sensor Based on Mesoporous Silica Nanoparticles and Förster Resonance Energy Transfer. Chem. Commun. 2010, 46, 8445−8447. (5) Aragay, G.; Pons, J.; Merkoçi, A. Recent Trends in Macro-, Micro-, and Nanomaterial-Based Tools and Strategies for Heavy-Metal Detection. Chem. Rev. 2011, 111, 3433−3458. (6) Birks, J. B. Photophysics of Aromatic Molecules; Wiley: London, 1970. (7) Hong, Y. N.; Lam, J. W. Y.; Tang, B. Z. Aggregation-Induced Emission. Chem. Soc. Rev. 2011, 40, 5361−5388. (8) Tang, B. Z.; Zhan, X. W.; Yu, G.; Lee, P. P. S.; Liu, Y. Q.; Zhu, D. B. Efficient Blue Emission from Siloles. J. Mater. Chem. 2001, 11, 2974−2978. (9) Okazawa, Y.; Kondo, K.; Akita, M.; Yoshizawa, M. Polyaromatic Nanocapsules Displaying Aggregation-Induced Enhanced Emissions in Water. J. Am. Chem. Soc. 2015, 137, 98−101. (10) Shellaiah, M.; Wu, Y. H.; Singh, A.; Ramakrishnam Raju, M. V.; Lin, H. C. Novel Pyrene- and Anthracene-based Schiff Base Derivatives as Cu2+ and Fe3+ Fluorescence Turn-On Sensors and for Aggregation Induced Emissions. J. Mater. Chem. A 2013, 1, 1310− 1318. (11) Samanta, S.; Manna, U.; Ray, T.; Das, G. An aggregationinduced emission (AIE) active probe for multiple targets: a fluorescent sensor for Zn2+ and Al3+ & a colorimetric sensor for Cu2+ and F−. Dalton Trans. 2015, 44, 18902−18910. (12) Yuan, W. Z.; Lu, P.; Chen, S.; Lam, J. W. Y.; Wang, Z.; Liu, Y.; Kwok, H. S.; Ma, Y.; Tang, B. Z. Changing the Behavior of Chromophores from Aggregation-Caused Quenching to AggregationInduced Emission: Development of Highly Efficient Light Emitters in the Solid State. Adv. Mater. 2010, 22, 2159−2163. (13) Zhao, J.; Yang, D.; Zhao, Y.; Yang, X. J.; Wang, Y. Y.; Wu, B. Anion-Coordination-Induced Turn-On Fluorescence of an Oligourea Functionalized Tetraphenylethene in a Wide Concentration Range. Angew. Chem., Int. Ed. 2014, 53, 6632−6636. (14) Wang, X. R.; Hu, J. M.; Zhang, G. Y.; Liu, S. Y. Highly Selective Fluorogenic Multianalyte Biosensors Constructed via EnzymeCatalyzed Coupling and Aggregation-Induced Emission. J. Am. Chem. Soc. 2014, 136, 9890−9893. (15) Inagaki, S.; Guan, S.; Fukushima, Y.; Ohsuna, T.; Terasaki, O. Novel Mesoporous Materials with a Uniform Distribution of Organic Groups and Inorganic Oxide in Their Frameworks. J. Am. Chem. Soc. 1999, 121, 9611−9614. (16) Melde, B. J.; Holland, B. T.; Blanford, C. F.; Stein, A. Mesoporous Sieves with Unified Hybrid Inorganic/Organic Frameworks. Chem. Mater. 1999, 11, 3302−3308. (17) Asefa, T.; MacLachlan, M. J.; Coombs, N.; Ozin, G. A. Periodic Mesoporous Organosilicas With Organic Groups Inside the Channel Walls. Nature 1999, 402, 867−871. (18) Waki, M.; Mizoshita, N.; Maegawa, Y.; Hasegawa, T.; Tani, T.; Shimada, T.; Inagaki, S. Enhanced Fluorescence Detection of Metal Ions Using Light-Harvesting Mesoporous Organosilica. Chem. - Eur. J. 2012, 18, 1992−1998. (19) Chandra, D.; Yokoi, T.; Tatsumi, T.; Bhaumik, A. Highly Luminescent Organic-Inorganic Hybrid Mesoporous Silicas Containing Tunable Chemosensor inside the Pore Wall. Chem. Mater. 2007, 19, 5347−5354. (20) Lei, J.; Yang, L.; Lu, D.; Yan, X.; Cheng, C.; Liu, Y.; Wang, L.; Zhang, J. Carbon Dot-Incorporated PMO Nanoparticles as Versatile Platforms for the Design of Ratiometric Sensors, Multichannel Traceable Drug Delivery Vehicles, and Efficient Photocatalysts. Adv. Opt. Mater. 2015, 3, 57−63. (21) Lu, D.; Chen, H.; Yan, X.; Wang, L.; Zhang, J. Ratiometric Hg2+ Sensor Based on Periodic Mesoporous Organosilica Nanoparticles and Förster Resonance Energy Transfer. J. Photochem. Photobiol., A 2015, 299, 125−130. (22) Quang, D. T.; Kim, J. S. Fluoro- and Chromogenic Chemodosimeters for Heavy Metal Ion Detection in Solution and Biospecimens. Chem. Rev. 2010, 110, 6280−6301.

CONCLUSION In summary, the copper ion sensing TH-PMOs with different content of TH groups were successfully prepared through the co-condensation of the TH bridged precursors and TEOS. The TH units fixed within the framework by four silyl groups were isolated and favorable as highly fluorescent bridging grounps in the hybrid materials. The solid PMO materials displayed fluorescence enhancement characteristic with the increase of the amount of TH-Si4 precursor, which could be ascribed to the restriction of intramolecular rotation of TH groups by the steric hindrance of framework of the TH-PMOs. Meanwhile, such enhanced fluorescence could be selectively quenched by Cu2+ due to the specific binding between TH and Cu2+ with a LOD value of 40 nM. The STXM measurements suggest that TH moieties in TH-PMOs could serve as the binding sites for Cu2+, and this would promote the adsorption capacity toward Cu2+ of the hybrid material. Such hybrid materials that combine with highly fluorescence and detection-sensitive characteristics are promising candidates for various luminescence applications such as solid-state optical emitters and fluorescence sensing.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b01837. 1 H NMR spectra of TH and TH-Si4; FT-IR spectra of TH and TH-PMOs; SAXS patterns, HRTEM images, N2 adsorption/desorption isotherms, and the normalization fluorescence emission intensity of TH-PMOs; fluorescent tests of TH; XANES studies and the reproductive test of TH-PMOs; comparative data of Cu2+ detection (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +86-531-88365450 (S.H.). Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This research was financially supported by the Natural Science Foundation of China (No. 50572057). Soft X-ray absorption experiment was conducted at Beamline BL08U of Shanghai Synchrotron Radiation Facility. Hard X-ray absorption experiment was conducted at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Econmic Diversification Canada, and the University of Saskatchewan.

■

REFERENCES

(1) Zhu, H.; Fan, J. L.; Wang, B. H.; Peng, X. J. Fluorescent, MRI, and Colorimetric Chemical Sensors for the First-row d-block Metal ions. Chem. Soc. Rev. 2015, 44, 4337−4366. (2) Kim, H. N.; Ren, W. X.; Kim, J. S.; Yoon, J. Fluorescent and Colorimetric Sensors for Detection of Lead, Cadmium, and Mercury ions. Chem. Soc. Rev. 2012, 41, 3210−3244. (3) Singh, K.; Sareen, D.; Kaur, P.; Miyake, H.; Tsukube, H. Materials-Based Receptors: Design Principle and Applications. Chem. Eur. J. 2013, 19, 6914−6936. 9306

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307

Article

The Journal of Physical Chemistry C

ture to Tune Fluorescence Efficiency. J. Phys. Chem. B 2008, 112, 16382−16392. (41) Mu, G. Y.; Zhang, W. Z.; Xu, P.; Wang, H. F.; Wang, Y. X.; Wang, L.; Zhuang, S. Q.; Zhu, X. J. Constructing New n-Type, Ambipolar, and p-Type Aggregation-Induced Blue Luminogens by Gradually Tuning the Proportion of Tetrahphenylethene and Diphenylphophine Oxide. J. Phys. Chem. C 2014, 118, 8610−8616. (42) Shimizu, K.; Maeshima, H.; Yoshida, H. Satsuma, A.; Hattori, T. Ligand Field Effect on the Chemical Shift in XANES Spectra of Cu(II) Compounds. Phys. Chem. Chem. Phys. 2001, 3, 862−866. (43) Yang, J. J.; Liu, J.; Dynes, J. J.; Peak, D.; Regier, T.; Wang, J.; Zhu, S. H.; Shi, J. Y.; Tse, J. S. Speciation and Distribution of Copper in a Mining Soil Using Multiple Synchrotron-Based Bulk and Microscopic Techniques. Environ. Sci. Pollut. Res. 2014, 21, 2943− 2954.

(23) Kim, J. H.; Gensch, T.; Zhao, D. B.; Stegemann, L.; Strassert, C. A.; Glorius, F. RhIII-Catalyzed C-H Activation with Pyridotriazoles: Direct Access to Fluorophores for Metal-Ion Detection. Angew. Chem., Int. Ed. 2015, 54, 10975−10979. (24) Que, E. L.; Domaille, D. W.; Chang, C. J. Metals in Neurobiology: Probing Their Chemistry and Biology with Molecular Imaging. Chem. Rev. 2008, 108, 1517−1549. (25) Lu, D.; Lei, J.; Tian, Z.; Wang, L.; Zhang, J. Cu2+ Fluorescent Sensor Based on Mesoporous Silica Nanosphere. Dyes Pigm. 2012, 94, 239−246. (26) Vigato, P. A.; Tamburini, S. The Challenge of Cyclic and Acyclic Schiff Bases and Related Derivatives. Coord. Chem. Rev. 2004, 248, 1717−2128. (27) Cheng, J. H.; Ma, X. F.; Zhang, Y. H.; Liu, J. Y.; Zhou, X. G.; Xiang, H. F. Optical Chemosensors Based on Transmetalation of Salen-Based Schiff Base Complexes. Inorg. Chem. 2014, 53, 3210− 3219. (28) Gao, M.; Han, S. H.; Qiu, X. Y.; Wang, H. Biphenyl-bridged Periodic Mesoporous Organosilicas: Synthesis and in Situ ChargeTransfer Properties. Microporous Mesoporous Mater. 2014, 198, 92− 100. (29) Hu, Y. F.; Coulthard, I.; Chevrier, D.; Wright, G.; Igarashi, R.; Sitnikov, A.; Yates, B. W.; Hallin, E. L.; Sham, T. K.; Reininger, R. The 10th International Conference on Synchrotron Radiation Instrumentation, Melbourne, Australia, Sept 2009; pp 343−346. (30) Ravel, B.; Newville, M. ATHENA, ARTEMIS, HEPHAESTUS: data analysis for X-ray absorption spectroscopy using IFEFFIT. J. Synchrotron Radiat. 2005, 12, 537−541. (31) Zhang, L. J.; Xu, Z. J.; Zhang, X. Z.; Yu, H. N.; Zou, Y.; Guo, Z.; Zhen, X. J.; Cao, J. F.; Meng, X. Y.; Li, J. Q.; et al. Latest Advances in Soft X-ray Spectromicroscopy at SSRF. Nucl. Sci. Tech. 2015, 26, 040101. (32) Wahab, M. A.; Hussain, H.; He, C. Photoactive Perylenediimide-Bridged Silsesquioxane Functionalized Periodic Mesoporous Organosilica Thin Films (PMO-SBA15): Synthesis, Self-Assembly, and Photoluminescent and Enhanced Mechanical Properties. Langmuir 2009, 25, 4743−4750. (33) Mizoshita, N.; Goto, Y.; Maegawa, Y.; Tani, T.; Inagaki, S. Tetraphenylpyrene-bridged Periodic Mesostructured Organosilica Films with Efficient Visible-Light Emission. Chem. Mater. 2010, 22, 2548−2554. (34) Fang, B. Y.; Liang, Y.; Chen, F. Highly Sensitive and Selective Determination of Cupric ions by Using N,N′-bis(salicylidene)-ophenylenediamine as Fluorescent Chemosensor and Related Applications. Talanta 2014, 119, 601−605. (35) Dong, Y. F.; Wang, W. L.; Zhong, C. W.; Shi, J. B.; Tong, B.; Feng, X.; Zhi, J. G.; Dong, Y. P. Investigating the Effects of Side Chain Length on the AIE Properties of Water-Soluble TPE Derivatives. Tetrahedron Lett. 2014, 55, 1496−1500. (36) He, J. T.; Xu, B.; Chen, F. P.; Xia, H. J.; Li, K. P.; Ye, L.; Tian, W. J. Aggregation-Induced Emission in the Crystals of 9,10Distyrylanthracene Derivatives: The Essential Role of Restricted Intramolecular Torsion. J. Phys. Chem. C 2009, 113, 9892−9899. (37) Mizoshita, N.; Goto, Y.; Kapoor, M. P.; Shimada, T.; Tani, T.; Inagaki, S. Fluorescence Emission from 2,6-Naphthylene-Bridged Mesoporous Organosilicas with an Amorphous or Crystal-Like Framework. Chem. - Eur. J. 2009, 15, 219−226. (38) Mizoshita, N.; Goto, Y.; Tani, T.; Inagaki, S. Highly Fluorescent Mesostructured Films that consist of Oligo(phenylenevinylene)-Silica Hybrid Frameworks. Adv. Funct. Mater. 2008, 18, 3699−3705. (39) Shi, J. B.; Wu, Y. M.; Tong, B.; Zhi, J. G.; Dong, Y. P. Tunable Fluorescence upon Aggregation: Photophysical Properties of Cationic Conjugated Polyelectrolytes Containing AIE and ACQ Units and Their Use in the Dual-channel Quantification of Heparin. Sens. Actuators, B 2014, 197, 334−341. (40) André, P.; Cheng, G.; Ruseckas, A.; van Mourik, T.; Früchtl, H.; Crayston, J. A.; Morris, R. E.; Cole-Hamilton, D.; Samuel, I. D. W. Hybrid Dendritic Molecules with Confined Chromophore Architec9307

DOI: 10.1021/acs.jpcc.6b01837 J. Phys. Chem. C 2016, 120, 9299−9307