Article pubs.acs.org/ac

Highly Stable and Luminescent Layered Hybrid Materials for Sensitive Detection of TNT Explosives Fang-Nan Xiao, Kang Wang, Feng-Bin Wang, and Xing-Hua Xia* State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Chemistry for Life Sciences, Nanjing University, Nanjing 210093, China S Supporting Information *

ABSTRACT: Self-assembly is an effective way to fabricate optical molecular materials. However, this strategy usually changes the nanoenvironment surrounding fluorescence molecules, yielding low luminescence efficiency. Herein, we report the intercalation of a ruthenium polypyridine (Ru) complex into the interlayer galleries of layered double hydroxides (LDHs), forming a Ru/LDH hybrid. The Ru complex exists as an ordered monolayer state, and the hybrid exhibits high thermal and photo stability. Its luminescence efficiency and lifetime are increased by ∼1.7 and ∼1 times, respectively, compared to those of free molecules. We constructed a Ru/LDH sensing platform based on a fluorescence quenching effect for highly sensitive detection of TNT with a detection limit of 4.4 μM.

R

formula of LDH maybe expressed as [MII1−xMIIIx(OH)2]x+[Am−x/m]x‑·nH2O (where MII/MIII are divalent/trivalent metal cations and Am− is the interlayer anion of valence m). The expandable interlayer space of LDH can provide a flexible and confined nanoenvironment, and the anion-exchangeable layers meanwhile allow for the intercalation of various guest anions. Moreover, most LDH are transparent and enable direct utilization of the optical properties of the intercalated molecules, particularly dyes. Various reports have focused on understanding the properties of dye/LDH materials.11,15,16 The incorporated molecules often show controlled distribution and may exhibit some unusual physical or chemical behaviors. In this work, we report the synthesis of two-dimensional (2D) LDH/Ru complex supramolecular hybrid materials (Ru/ LDH hybrid) by intercalating the anionic octahedral structure of the derivative Ru complex into the interlayer galleries of Mg−Al layered double hydroxides via an ionic exchange reaction (Figure 1A). The host LDH offers stabilization and protection in the nanoenvironment for the guest Ru complex molecules, resulting in increased thermal stability of the hybrids.17 The guest Ru complex molecules are precisely arranged as individuals within the interlayer galleries of the LDH matrix at the molecular level, which can effectively avoid molecule aggregation and fluorescence self-quenching. Thus, more efficient unit mass luminescence emission of the Ru/ LDH hybrid can be expected. The fluorescence properties of

uthenium polypyridine (Ru) complexes functioning as phosphorescence dyes has received extensive attention for many decades. The excellent stability, strong luminescence emissions, and excited-state redox capacity1 make it an efficient reagent for light-emitting electrochemical devices,2 photocatalysts,3 electrochemiluminescent sensors,4,5 oxygen6 and DNA probes,7 and dye-sensitized solar cells.8 From the viewpoint of practical applications, construction of Ru complexes containing hybrid materials could be a promising approach to manipulate the properties and performance of the Ru complex, which has been an area of great interest over the past few decades.9 It has been reported that glucose oxidase immobilized in nanochannel materials exhibits significantly improved biocatalytic activity, and anionic organic fluorophores assembled into a layered double hydroxide host have reversible piezochromic luminescent properties due to nanoconfinement effects.10,11 In the case of Ru complex-based florescent and electrochemical luminescent sensors, increased complex loading in nanomaterials is usually proposed to enhance the sensitivity of sensors.12,13 However, high loading of Ru complexes will result in inevitable aggregation of the complexes and consequently a self-quenching effect, which will certainly decrease the utility of the fluorescent molecules and sensitivity of the sensors. If all of the Ru complex molecules were to arrange individually in nanomaterials, molecular interactions could be avoided, all of the incorporated fluorescent molecules could then be utilized, and the luminescence efficiency of single molecules could be increased due to the nanoconfinement effect. Layered double hydroxides (LDH) are layered structure compounds with positively charged mixed-metal hydroxides and a compensation anion in the interlayer region.14 The © XXXX American Chemical Society

Received: February 14, 2015 Accepted: March 30, 2015

A

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

Figure 1. Scheme and morphology characterizations: (A) schematic illustration of the preparation procedure for Ru/LDH hybrids and a representation of the driving forces taking part in the assembly of the Ru complex into LDH, including (i) hydrogen bond interactions and (ii) electrostatic interactions. (B) X-ray diffraction patterns of (a) LDH, (b) Ru/LDH (12 h), and (c) Ru/LDH (24 h). (C) LDH and (D) Ru/LDH SEM micrographs. (E) TEM images and (F) HRTEM images of Ru/LDH.

resistivity of 18.2 MΩ cm. Deionized and decarbonated water was used in all experiments. Apparatus. UV−vis absorption spectra were recorded on a UV3600 spectrophotometer (Shimadzu, Japan). Photoluminescence (PL) spectra were carried out on a Shimadzu RF-5301PC spectrophotometer (Shimadzu, Japan). Infrared spectra were recorded on a 6700 FTIR spectrometer (Nicolet, USA). The morphology of the hybrid was examined using a JEM-2100 high-resolution TEM (JEOL, Japan) at an accelerating voltage of 200 kV and an S-4800 scanning electron microscope (Hitachi, Japan). Thermogravimetric analysis was carried out on a Pyris 1 TGA thermoanalyzer (PerkinElmer, USA). X-ray diffraction patterns were collected by a Philips X’pert with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was performed on a PHI5000 Versa Probe spectrometer (ULVACPHI, Japan) in an ultrahigh vacuum chamber. Fluorescence lifetimes spectra were obtained on a FLS920 Fluorescence Lifetime and Steady State Spectrometer (Edinburgh, UK). Synthesis of Mg−Al LDH. Mg(NO3)2·6H2O (30.77 g, 0.12 mol), Al(NO3)3·9H2O (22.6 g, 0.06 mol), and 160 mL of water were added to a 250 mL flask. The solution was adjusted to pH 9 with 2 M NaOH solution at room temperature under N2

confined Ru complex in Ru/LDH hybrids have been studied in detail. The potential application of this hybrid as a fluorescent probe for the sensitive detection of explosive trinitrotoluene (TNT) has been demonstrated based on redox fluorescence quenching between the excited Ru complex and TNT. The incorporation of Ru complex-derived molecules in thin host matrices can provide a new way to tailor fluorescence properties by controlling the molecular geometry structure. The results demonstrate that the resultant Ru/LDH hybrid is a promising building block for constructing sensitive sensors and may find applications in other research areas of photoelectrical chemistry.



EXPERIMENTAL SECTION Reagents. RuCl3·3H2O, 4,4′-dicarboxylato-2,2′-bipyridine, 2,4,6-trinitrotoluene (TNT, 1 mg mL−1 in methanol), and 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA, 97%) were purchased from Aldrich (USA). Mg(NO3)2·6H2O, Al(NO3)3·9H2O, and NaOH were purchased from Nanjing Chemical Reagent Co. Ltd. (Nanjing, China). Other reagents were of analytical reagent grade and used as received. All aqueous solutions were prepared with Millipore water with a B

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

Figure 2. (A) XPS spectra of Ru/LDH for (1) O2s, 24 eV; (2) Mg2p, 50 eV; (3) Al2p, 74 eV; (4) Mg2s, 89 eV; (5) Al2s, 119 eV; (6) Ru3d, 280 eV; (7) C1s, 284 eV; (8, 9) MgKLL, 306, 351, and 385 eV; (10) N1s, 406 eV; (11) Ru3p, 462 and 485 eV; (12) O1s, 532 eV; (13,14) OKLL, 978 and 998 eV; (15) Na1s, 1071 eV; (16) CKLL, 1227 eV; and (17) Mg1s, 1309 eV. High resolution scans in the regions of (B) C1s and Ru3d, (C) Ru3p, and (D) N1s of Ru/LDH.

atmosphere at 75 °C. The white precipitate was aged for 24 h. Then, the precipitate was collected by centrifugation, washed three times with water, and dried at 60 °C for 24 h. Preparation of Tris(4,4′-dicarboxylato-2,2′bipyridine)ruthenium(II) (Ru).29 A solution of 4,4′-dicarboxyl-2,2′-bpy (754 mg, 3.09 mmol) and RuCl3·3H2O (261 mg, 1 mmol) in 60 mL of dimethylformamide (DMF) was refluxed for 3 h under N2 atmosphere. Then, 12 mL of 0.5 M NaOH solution was added and refluxed for 4 h. The solution was concentrated to ∼10 mL under reduced pressure and cooled to room temperature. The dark red precipitate was collected by centrifugation and washed 3 times with 10 mL of DMF. The precipitate was recrystallized from methanol-diethyl ether and dissolved in 20 mL of water. The solution was adjusted to pH 2.5 by 0.1 M HCl and cooled in a refrigerator for 24 h. The resultant precipitate was collected and washed 3 times with pH 2.5 water. Yield: 603.5 mg (64.8%). FTIR (KBr, cm−1): 1610 (νas, COO−), 1375 (νs, COO−). UV−vis: 310, 386, 526 nm. Synthesis of Ru/Mg−Al LDH (Ru/LDH) Hybrid. The Ru/ LDH hybrid was prepared simply using an ion exchange reaction. Typically, 0.3 g of the LDH was added to 25 mL of a 0.036 M Ru complex aqueous solution with water at pH 7−8. The anion exchange reaction was carried out under stirring at 80 °C under N2 atmosphere for 24 h. The resulting precipitate was then collected and washed three times with 10 mL of water. Preparation of Sample Films for Photoluminescence Lifetime and Photo Stability Experiments. The thin films

were fabricated by taking 1.5 mg of the samples in 0.1 mL of distilled water solvent, coated on a glass plate, and air dried. The fabricated films on the plate were kept facing the excitation light source directly with data collection. Quenching Experiments with TNT. The addition of quencher (TNT) was made by injection to the sample cuvette under Ar atmosphere. The concentration of the stock quencher solution and Ru/LDH were 4.4 mM and 10 μM, respectively. Small volume addition was made to avoid altering the initial Ru complex concentration.



RESULTS Structural Analysis. LDH was synthesized by coprecipitation at constant pH under a nitrogen atmosphere to exclude CO2.14 The interlayer structure of the synthesized LDH was characterized by the X-ray diffraction method. As shown in Figure 1B (curve a), the diffractogram for pristine LDH consists of the basal plane (00l) and (hkl) reflections. The basal plane (003) shows a characteristic peak at 2θ = 9.96° (d003 spacing close to 8.8 Å). The d003 spacing is equivalent to the thickness of the unit layers. The thickness of the LDH sheet is 4.8 Å. Therefore, the gallery height is estimated to be 4.0 Å, which coincides with that of LDH with NO3− ions in the interlayer.18 The peak of the doublet close to 2θ = 60.08° is due to diffraction of the (110) and (111) planes, which is the closest metal−metal distance in the layer. After the anionexchange reaction for 12 h (curve b) and 24 h (curve c), the XRD patterns for the Ru/LDH hybrid show the consistent (hkl) reflection peaks at (101), (015), and (110), but the basal C

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

Figure 3. (A) TGA curves for (a) LDH, (b) the Ru complex, and (c) Ru/LDH. (B) UV−vis spectra of (a) the Ru complex and (b) Ru/LDH. (C) Fluorescence emission spectra of (a) LDH, (b) the Ru complex, and (c) Ru/LDH at the same Ru complex concentration at an excitation wavelength of 465 nm in water. (D) Photoluminescence lifetime of (a) the Ru complex and (b) a Ru/LDH solid film at room temperature. (E) Photobleaching experiment of (a) free Ru complex and (b) a Ru/LDH solid film at room temperature with excitation from a focused laser beam of a Mercury lamp. Luminescence signal was collected by an ICCD camera coupled to a fluorescence microscope.

approximately a few dozen nanometers and has a high aspect ratio. In comparison with the LDH, the plane dimension of the Ru/LDH particle (Figure 1D) is reduced to smaller size that is less than 100 nm, and the particle shape becomes more irregular. The change in morphology indicates that the ion exchange process influences stacking and corrosion of the hydroxide layers, resulting in the loss of crystallinity (as confirmed from the broadening of XRD reflections in Figure 1B). Figure 1E shows the typical transmission electron microscopy (TEM) image of the Ru/LDH hybrid. The particle-like structure of Ru/LDH is similar to that of the SEM image. A high-resolution TEM image of the particle (Figure 1F) clearly shows the multilayered packing structure, which further confirms the maintenance of layered structure after intercalation. Highly dense black dots (∼2−3 nm) dispersed in the interlayers can be observed, demonstrating

plane (00l) reflections shift to a lower 2θ value. The presence of the (hkl) planes reflections indicate that the 2D structure of LDH remains in the ionic exchange process. The appearance of the (003) diffraction peak at 6.61° (spacing close to 13.4 Å) indicates the formation of a new ordered structure. The gallery height of the Ru/LDH hybrid is calculated as 8.6 Å, representing the average thickness of intercalated Ru complex molecules. It has been reported that Ru complex takes ∼8.0 Å.18−20 These results indicate that Ru complex anions have been successfully intercalated into the interlayer galleries of the LDH matrix taking a monolayer arrangement. The surface morphology of the LDH and Ru/LDH (with a 24 h ionic exchange reaction) was characterized by scanning electron microscopy (SEM, Figure 1C, D). The synthesized LDH (Figure 1C) is composed of a plate-like material and exhibits a smooth surface. The thickness of platelets is D

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

and the hydroxyl layers. These would restrict thermal motion of the Ru complex and affect its structure, thereby hindering the diffusion of volatile decomposition products and oxygen in the interlayer galleries,30 which definitely enhances the thermal stability of the incorporated Ru complex. Optical Properties. Ultraviolet−vis spectra reflect the state of valence electrons in the Ru complex. As shown in Figure 3B (curve a), the characteristic absorbance peaks of Ru complex at 303.5 nm, 326.5 nm (π → π* ligand-centered transition), 354.5 nm (metal-centered transition), 437.5, and 468.5 nm (metal-toligand charge transfer (d → π*) transition, MLCT) can be observed, which are consistent with previous results.28,29 Upon Ru complex being intercalated within the LDH layers (curve b), the ligand-centered absorption peak of the Ru complex at 303.5 nm exhibits a decrease in absorption cross section, and the shoulder at 286.5 nm disappears, indicating that the electronic state of 4,4′-dicarboxylato-2,2′-bipyridine ligands has been perturbed. Other studies on Ru complex adsorbed on montmorillonite and kaolin have indicated that the probable cause of the changes is distortion of the bipyridine ligands in restricted space.31 Meanwhile, the MLCT band of the Ru complex is slightly blue-shifted by 4 nm. The result indicates that its transition energy (d → π*) increases, which may be due to change of the d orbital energy level. It has been reported that spectroscopic properties of the Ru complex are sensitive to the microenvironment.32 Under the confinement imposed by 2D interlayer structures, the microenvironment surrounding Ru complex molecules is quite different, which may be the cause of the blue-shift in the MLCT band. Similar luminescence blueshifts have also been observed for LDH-CdSe quantum dot, Ru-Zeolite Y composites.33,34 These changes suggest the presence of strong interactions between the Ru complex and LDH, including electrostatic interactions (negatively charged layers and Ru complex anions) and hydrogen bonding (hydroxyl group of LDH and carboxyl group of the Ru complex). In addition, the deprotonated Ru complex is much more soluble, and its absorbance peak is blue-shifted.28,29 Figure 3C shows the fluorescence emission spectra of aqueous solutions of LDH, Ru complex, and Ru/LDH. LDH does not exhibit fluorescence emission peaks (curve a) as expected. This indicates that the LDH are inert to visible light. Compared to the Ru complex (curve b), the Ru/LDH fluorescence emission (curve c) shows a significant increase in intensity and a slight blue-shift in peak position, which demonstrates that the LDH does not quench the fluorescence emission of the incorporated Ru complex. Similar results for Ru complex in polyelectrolyte and silicate have been attributed to protection of the Ru complex against quenching by molecular oxygen and the decreased rate of nonradiative decay of the excited state of the Ru complex.12,35 Blue-shift of the luminescence peak and enhanced intensity have also been observed and explained by the rigid nature of the microenvironment surrounding the Ru complex.34,36,37 In the case of Ru/LDH hybrids, the intermolecular interaction of the Ru complex is disturbed by electrostatic and hydrogen bond interactions as mentioned within the interlayer galleries of the LDH matrix, resulting in transfer of the random dispersion into the monomolecular layer status. We assume that the octahedral structure and high density charge of Ru complexions makes it so that they cannot closely stack like planar dye molecules, suppressing the quenching effect that arises from aggregation (Figure S2A, Supporting Information). To prove this, we prepared 3,4,9,10-perylenetetracarboxylic acid/LDH (PTCA/

the successful intercalation of Ru complex into the LDH matrix. The black dot is much larger than the diameter of the Ru complex, which suggests decomposition of Ru complex under strong electron beam irradiation during TEM characterizations to form metallic ruthenium clusters.21,22 X-ray photoelectron spectroscopy (XPS) characterizations were performed to analyze the elemental composition of the Ru/LDH hybrid. As shown in Figure 2A, the presenting peaks can be assigned to Mg, Al, Ru, C, O, and N in the hybrid. The Mg/Al molar ratio is 2.07, which corresponds roughly to the expected value of 2.00. The high resolution scans in the regions of C1s and Ru3d, Ru3p, and N1s are shown in Figures 2B−D, respectively. The peaks at 484.8 eV (Ru3p1/2), 462.1 eV (Ru3p3/2), and 280.4 eV indicate the presence of Ru2+ in Ru/ LDH, but the binding energy for Ru3d5/2 is lower than that of the free Ru complex.23,24 The peak at ∼285.7 eV is assigned to the CN (pyridine moiety) bonding state, and the peaks at 287.8 and 289.0 eV are assigned to CO and O−CO bonds, respectively. The N1s peak contains two major components corresponding to the functional groups: CN bonds at 399.5 eV and NO bonds at 406.5 eV. The binding energy of nitrogen in free pyridine is lower than 399.0 eV and the peak appearing at 399.5 eV, which shows that the nitrogen atom is coordinated to the Ru atom.25 These results demonstrate that no decomposition of th Ru complex occurs under the measurement conditions. However, the orbital energy level of the Ru complex has been changed upon intercalation in the interlayer galleries of the LDH matrix. In addition, a large amount of unexchanged NO3− ions remain in the hybrid. Figure 3A shows the TGA curves of the LDH, Ru complex, and Ru/LDH. LDH (curve a) shows three major stages of weight loss. The first step (from 25 to 170 °C) comes from the loss of physically absorbed and interlayered water. The second step (from 170 to 360 °C) is attributed to partial dehydroxylation of the host layers. The third step (from 360 to 650 °C) is attributed to total dehydroxylation of the layers. The thermo-decomposition process of free Ru complex (curve b) was studied as a reference to compare with Ru/LDH. Weight loss below 240 °C corresponds to the loss of physically absorbed and crystalline water. The sharp weight loss from 240 to 410 °C is attributed to the decomposition of Ru complex. The thermal decomposition process of Ru/LDH (curve c) shows three steps in the temperature range of 25−650 °C. The first step (from 25 to 300 °C) contains the removal of surface adsorbed and interlayered water molecules and crystalline water and partial dehydroxylation of the hydroxide layers. The second step (from 310 to 500 °C) includes the decomposition of the Ru complex and the dehydroxylation of the layers. The third step (from 500 to 650 °C) corresponds to collapse of the LDH layers. Hence, the decomposition temperature of the intercalated Ru complex is ∼70 °C, which is higher than its pristine form. The enhanced thermal stability of the Ru complex may be due to the nanoconfinement effect of host− guest interactions. The Ru complex molecules are homogeneously dispersed in the LDH matrix, resulting in an enormous interfacial area and a strong interaction between the Ru complex and LDH.26 In addition, the FTIR spectra (Figure S1, Supporting Information) show that the band of Ru/LDH shifts to 1674 cm−1 (deprotonated carboxyl groups, −COO−) compared to 1720 cm−1 (protonated carboxyl groups, −COOH) for the pristine Ru complex,27−29 indicating a strong interaction between the carboxylate of the Ru complex E

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

that LDH can effectively protect the excited state and increase the luminescence lifetime of the Ru complex. To evaluate the photo stability of the hybrids, we measured the Ru complex and Ru/LDH films in glass against photo bleaching using a mercury lamp as an excitation source. The results of the photo stability analysis are normalized and summarized in Figure 3E. Similar to the lifetime analysis, the photo stability of the Ru complex increases when intercalated into the LDH layers. The emission intensity of the free Ru complex decays much faster than that of the intercalated Ru complex in LDH. It is well-known that reducing the dynamic interactions with adjacent molecules helps to limit the relaxation process, resulting in improved photo stability of the Ru complex in a confined system. The observed higher thermal stability of Ru/LDH indicates that Ru complex motion is severely constrained in layered nanoconfinement. Moreover, hydrogen bonding may also help anchor Ru complexes directly to the LDH surface and further discourages random molecular motions. With molecular motions considerably subdued, dynamic interactions that lead to photo degradation are less likely to occur.1 TNT Assay. Recently, fluorescent nanomaterials and metal complexes have been used for detecting chemically active species, such as TNT and nitric oxide.42,43 Because of the redox capacity of the excited-state of the Ru complex, prepared Ru/ LDH can be used to detect various redox molecules. In this work, detection of TNT has been demonstrated.28 The mechanism of quenching the fluorescence by TNT is shown in eqs 3 and 4.28,44−46

LDH) nanocomposites. A PTCA molecule has a typical planar structure, and its molecular orientation has often been used for understanding intermolecular interactions.38 Compared with the strong fluorescence emission of free PTCA, PTCA/LDH (Figure S3C, Supporting Information) shows almost complete fluorescence quenching. The fluorescence quenching and interlayer distance of PTCA/LDH imply the formation of Jtype aggregates due to the strong π−π interaction between PTCA molecules.39 This is similar to the N,N′-bis(phenyl-3,5disulfonic acid)perylene-3,4,9,10-tetracarboxydiimide and N,N′bis(4-benzosulfonic acid)perylene-3,4,9,10-tetracarboxydiimide incorporated into LDH.15,40 Moreover, a multiply charged molecule, such as the Ru complex, has a large dipole moment in the excited state, which results in the effect of significant solvent reorganization. In the case of the Ru complex confined within a rigid layer, solvent reorganization cannot take place freely, which means that thermal relaxation of the Franck−Condon 1 MLCT excited state does not occur and that nonradiative decay is decreased. In addition, many co-intercalated NO3− ions play an important role in enlarging the distance between adjacent Ru complex molecules, further decreasing the possible self-quenching. Thus, LDH provides an interesting nanoenvironment for achieving enhanced and blue-shifted fluorescence emission of the Ru complex. Picosecond-to-nanosecond time-correlated single photon counting was used to measure the solid luminescence decay of the Ru complex and Ru/LDH hybrid at room temperature (Figure 3D).The decay curves are fitted by a double exponential model, which is expressed as I(t ) = A1exp( −t /τ1) + A 2 exp(−t /τ2)

(1)

where I(t) is the luminescence intensity, τ1 and τ2 represent the rate constants, and A1 and A2 represent the amplitudes of the components, respectively. The average lifetime τ value is calculated by the expression in eq 2 τ = (A1τ12 + A 2 τ2 2)/(A1τ1 + A 2 τ2)

The fitting parameters of A1, τ1, A2, τ2, and τ are summarized in Table 1. Table 1. Fitting Parameters of the Ru Complex and Ru/LDH sample

τ1 (ns)

A1 (%)

τ2 (ns)

A2 (%)

τ

36.86 115.34

25.73 13.24

282.05 552.04

74.27 86.76

271.2 538.5

(3)

*Ru 2 + + ArNO2 → Ru 3 + + ArNO2−

(4)

The fluorescence intensity decreases as the TNT concentration increases, ranging from 4.4 μM to 22 μM, due to quenching effects (Figure 4A). The typical calibration curve is shown in Figure 4B in which each point represents three repeated measurements (n = 3) with standard deviations (s.d.) ranging from 4 to 6%. The plot shows a good line fit with a correlation coefficient of 0.99. The detection limit for TNT is 4.4 × 10−6 M (S/N = 3), which is 2 orders of magnitude lower than that of free Ru complex.45,46 The results indicate that the prepared sensor is promising for sensitively monitoring trace amounts of TNT.

(2)

Ru complex Ru/LDH

Ru 2 + + hv → *Ru 2 +



DISCUSSION Factors influencing the luminescence efficiency of optical molecular materials mainly include two aspects: impurities (chemical purity) and molecular packing mode. LDH is of high crystallinity and optical inertia and is thus avoided as a fluorescence quenching center. On the other hand, the layered structure of LDH leads to orderly arrangement of fluorescent molecules, presenting a kind of crystal structure that can isolate interlayer fluorescence molecules. In addition, LDH with its nanoconfinement effect blocks fluorescent molecular motion (similar to the low temperature fluorescence effect), reducing radiation energy losses and improving luminescence efficiency. However, two kinds of fluorescent molecules, Ru complex and PTCA, restricted in the interlayer galleries of LDH have different fluorescence responses. One reason is attributable to differences in their luminescence mechanisms. Luminescent Ru complex is triplet emission, and its lifetime is much longer than that of PTCA. Excited Ru complex molecules are more likely to

The lifetime of the Ru complex (curve a) consists of two time-decay components, a short-lived minor component followed by a long-lived major one. The fast component is due to protonation−deprotonation of the excited Ru complex.28 When Ru complex intercalated into the LDH layers (curve b), the fast component weight decreases from 25.73% to 13.24%. The restricted rotation of the bpy ligand binding to LDH could be considered the cause of the decreased weight. The average lifetimes of the Ru complex and Ru/LDH are 271.2 and 538.5 ns, respectively. Changes in the excited state lifetimes parallel the emission intensity of the Ru complex. The results are consistent with dodecylbenzene/LDH, which indicates that the changes are due to decreasing motion of the Ru complex in restricted space.41 These results demonstrate F

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry

Figure 4. TNT detection through (A) fluorescence emission spectra of Ru/LDH quenched by varying concentrations of TNT in a pH 7 solution. The concentrations (mol L−1) of TNT were (a) 0, (b) 8.8 × 10−7, (c) 4.4 × 10−6, (d) 8.8 × 10−6, (e) 1.32 × 10−5, (f) 1.76 × 10−5, (g) 2.2 × 10−5, (h) 4.4 × 10−5, and (i) 1.32 × 10−4. (B) Plot of the relative intensity of (F0/F) vs TNT concentration.

Notes

be affected by their surrounding environment. Therefore, our results demonstrate that the Ru complex experiences a more significant nanoconfinement effect of LDH than PTCA. Moreover, the PTCA molecules with planar structure intercalated within the interlayer galleries of LDH are parallel stacked (J-aggregate) with the molecular long axis parallel to each other and the molecular dipole along the molecular long axis direction. Although the isolated layer between molecules might not be fully parallel stacked, strong molecular dipole interactions exist among the intercalated PTCAs. Forbidden photochemical transition levels are formed, and lower luminescence efficiency of confined PTCA is obtained. In the case of the Ru complex, the molecular dipole of the excited species with octahedral structure, however, is along ligand to Ru atom direction (LUMO on ligand and HOMO on Ru center) and not parallel to each other (less than 1/3). Thus, the molecular dipole interaction is lower, and higher luminescence efficiency of the intercalated Ru complex is observed.

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by grants from the National 973 Basic Research Program (2012CB933800), the National Natural Science Foundation of China (21275070, 21275071, 21205059, 21327902), and the National Science Fund for Creative Research Groups (21121091).





CONCLUSIONS In summary, we have successfully synthesized Ru/LHD hybrids with all of the Ru complexes taking the monolayer mode within the interlayer galleries of the LDH matrix. Because of the layered confinement effect, this Ru/LDH hybrid exhibits greater thermal and photo stability, more efficient unit mass luminescence emission, and a longer photoluminescence lifetime. In combination with good adsorption characteristics and high surface area of the Ru/LDH hybrid, a platform for highly sensitive detection of TNT has been developed. The present work promises an effective means of arranging fluorescent chromophores in a monolayered structure to form highly efficient fluorescence materials, which can be utilized for a variety of applications in sensing, photoelectronics, and photocatalysts.



ASSOCIATED CONTENT

S Supporting Information *

Preparation, characterization, and schematic illustration of PTCA/LDH; FTIR spectra; and photo images of Ru/LDH. This material is available free of charge via the Internet at http://pubs.acs.org.



REFERENCES

(1) Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. Top. Curr. Chem. 2007, 280, 117−214. (2) Lyons, C. H.; Abbas, E. D.; Lee, J. K.; Rubner, M. F. J. Am. Chem. Soc. 1998, 120, 12100−12107. (3) Ozawa, H.; Haga, M. A.; Sakai, K. J. Am. Chem. Soc. 2006, 128, 4926−4927. (4) Yin, X. B.; Dong, S. J.; Wang, E. K. Trends Anal. Chem. 2004, 23, 432−441. (5) Hu, L. Z.; Xu, G. B. Chem. Soc. Rev. 2010, 39, 3275−3304. (6) Carraway, E. R.; Demas, J. N.; DeGraff, B. A.; Bacon, J. R. Anal. Chem. 1991, 63, 337−342. (7) Friedman, A. E.; Chambron, J. C.; Sauvage, J. P.; Turro, N. J.; Barton, J. K. J. Am. Chem. Soc. 1990, 112, 4960−4962. (8) O’Regan, B.; Grätzel, M. Nature 1991, 335, 737−740. (9) Sanchez, C.; Shea, K. J.; Kitagawa, S. Chem. Soc. Rev. 2011, 40, 471−472. (10) Wang, C.; Ye, D. K.; Wang, Y. Y.; Lu, T.; Xia, X. H. Lab Chip 2013, 13, 1546−1553. (11) Yan, D. P.; Lu, J.; Ma, J.; Qin, S. H.; Wei, M.; Evans, D. G.; Duan, X. Angew. Chem., Int. Ed. 2011, 50, 7037−7040. (12) Li, J.; Guo, L. R.; Gao, W.; Xia, X. H.; Zheng, L. M. Chem. Commun. 2009, 48, 7545−7547. (13) Qian, L.; Yang, X. R. Adv. Funct. Mater. 2007, 17, 1353−1358. (14) Wang, Q.; O’Hare, D. Chem. Rev. 2012, 112, 4124−4155. (15) Chakraborty, C.; Dana, K.; Malik, S. J. Phys. Chem. C 2011, 115, 1996−2004. (16) Yan, D. P.; Lu, J.; Wei, M.; Evans, D. G.; Duan, X. J. Mater. Chem. 2011, 21, 13128−13139. (17) Yang, Q. Z.; Sun, D. J.; Zhang, C. G.; Wang, X. J.; Zhao, W. A. Langmuir 2003, 19, 5570−5574. (18) Ogawa, M. Langmuir 2000, 16, 4202−4206. (19) Traynor, M. F.; Mortland, M. M.; Pinnavaia, T. J. Clays Clay Miner. 1978, 26, 318−326. (20) Hagerman, M. E.; Salamone, S. J.; Herbst, R. W.; Payeur, A. L. Chem. Mater. 2003, 15, 443−450. (21) Ding, Y.; Zhang, D.; Xia, X. H. J. Nanosci. Nanotechnol. 2008, 8, 1512−1517.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] G

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Article

Analytical Chemistry (22) Ding, Y.; Jin, B.; Gu, G.; Xia, X. H. J. Mater. Chem. 2009, 19, 9141−9146. (23) Qiu, D. F.; Zhao, Q.; Bao, X. Y.; Liu, K. C.; Wang, H. W.; Guo, Y. C.; Zhang, L. F.; Zeng, J. L.; Wang, H. Inorg. Chem. Commun. 2011, 14, 296−299. (24) Abdo, S.; Canessan, P.; Cruz, M.; Fripiat, J. J.; Van Damme, H. J. Phys. Chem. 1981, 85, 797−809. (25) Lei, P. X.; Hedlund, M.; Lomoth, R.; Rensmo, H.; Johansson, O.; Hammarström, L. J. Am. Chem. Soc. 2008, 130, 26−27. (26) Liu, J.; Chen, G.; Yang, J. Polymer 2008, 49, 3923−3927. (27) Nazeeruddin, Md. K.; Zakeeruddin, S. M.; Humphry-Baker, R.; Jirousek, M.; Liska, P.; Vlachopoulos, N.; Shklover, V.; Fischer, C. H.; Grätzel, M. Inorg. Chem. 1999, 38, 6298−6305. (28) Park, J.; Ahn, J.; Lee, C. J. Photochem. Photobiol., A 1995, 86, 89−95. (29) Nazeeruddin, Md. K.; Kalyanasundaram, K.; Grätzel, M.; Sullivan, B. P.; Kevin, M. Inorganic Syntheses; Wiley: New York, 1998; Vol 32, pp 181−186. (30) Wang, J.; Wei, M.; Rao, G. Y.; Evans, D. G.; Duan, X. J. Solid State Chem. 2004, 177, 366−371. (31) Della Guardia, R. A.; Thomas, J. K. J. Phys. Chem. 1983, 87, 990−998. (32) Kumar, C.; Williams, Z. J. J. Phys. Chem. 1995, 99, 17632− 17639. (33) Venugopal, B. R.; Ravishankar, N.; Perrey, C. R.; Shivakumara, C.; Rajamathi, M. J. Phys. Chem. B 2006, 110, 772−776. (34) Mori, K.; Kawashima, M.; Kagohara, K.; Yamashita, H. J. Phys. Chem. C 2008, 112, 19449−19455. (35) Park, J. W.; Kim, M. H.; Ko, S. H.; Paik, Y. H. J. Phys. Chem. 1993, 97, 5424−5429. (36) Wheeler, J.; Thomas, J. K. J. Phys. Chem. 1982, 86, 4540−4544. (37) Meisel, D.; Matheson, M. S.; Rabani, J. J. Am. Chem. Soc. 1978, 100, 117−122. (38) Yamane, H.; Kera, S.; Okudaira, K. K.; Yoshimura, D.; Seki, K.; Nobuo, U. Phys. Rev. B 2003, 68, 033102/1−033102/4. (39) Yan, D. P.; Lu, J.; Wei, M.; Ma, J.; Evans, D. G.; Duan, X. Phys. Chem. Chem. Phys. 2009, 11, 9200−9209. (40) Bauer, J.; Behrens, P.; Speckbacher, M.; Langhals, H. Adv. Funct. Mater. 2003, 13, 241−248. (41) Liu, X. L.; Wei, M.; Wang, Z. L.; Evans, D. G.; Duan, X. J. Phys. Chem. C 2008, 112, 17517−17524. (42) Zhang, K.; Zhou, H. B.; Mei, Q. S.; Wang, S. H.; Guan, G. J.; Liu, R. Y.; Zhang, J.; Zhang, Z. P. J. Am. Chem. Soc. 2011, 133, 8424− 8427. (43) Sun, J.; Yan, Y. H.; Sun, M. T.; Yu, H.; Zhang, K.; Huang, D. J.; Wang, S. H. Anal. Chem. 2014, 86, 5628−5632. (44) Bock, C. R.; Connor, J. A.; Gutierrez, A. R.; Meyer, T. J.; Whitten, D. G.; Sullivan, B. P.; Nagle, J. K. J. Am. Chem. Soc. 1979, 101, 4815−4824. (45) Glazier, S.; Barron, J. A.; Morales, N.; Ruschak, A. M.; Houston, P. L.; Abruña, H. D. Macromolecules 2003, 36, 1272−1278. (46) Kim, H. B.; Kitamura, N.; Kawanishi, Y.; Tazuke, S. J. Phys. Chem. 1989, 93, 5757−5764.

H

DOI: 10.1021/acs.analchem.5b00630 Anal. Chem. XXXX, XXX, XXX−XXX

Highly stable and luminescent layered hybrid materials for sensitive detection of TNT explosives.

Self-assembly is an effective way to fabricate optical molecular materials. However, this strategy usually changes the nanoenvironment surrounding flu...
448KB Sizes 4 Downloads 9 Views