Luminescence Tuning of Layered Rare-Earth Hydroxides (LRHs, R

Dec 29, 2018 - School of Basic Medical Sciences, Ningxia Medical University, Yinchuan 750004, People's Republic of China. ∥. School of Materials and...
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Luminescence Tuning of Layered Rare-Earth Hydroxides (LRHs, R = Tb, Y) Composites with 3‑Hydroxy-2-naphthoic Acid and Application to the Fluorescent Detection of Al3+ Rong Guo,†,∇ Feifei Su,†,∇ Hui Wang,† Yuexin Guo,*,‡ Huiqin Yao,*,§ Gailing Huang,∥ Jian Li,*,⊥ Zupei Liang,⊥ Keren Shi,# and Shulan Ma*,†

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†

Beijing Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China ‡ School of Pharmacy, North China University of Science and Technology, Tangshan 063210, People’s Republic of China § School of Basic Medical Sciences, Ningxia Medical University, Yinchuan 750004, People’s Republic of China ∥ School of Materials and Chemical Engineering, Zhengzhou University of Light Industry, Zhengzhou 450001, People’s Republic of China ⊥ Chemistry & Chemical and Environmental Engineering College, Weifang University, Weifang 261061, People’s Republic of China # State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, Ningxia University, Yinchuan 750021, People’s Republic of China S Supporting Information *

ABSTRACT: Tunable luminescence (quenching or blue shift) of HNA/OSLRH composites (HNA is 3-hydroxy-2-naphthoic acid; OS is the anionic surfactant of 1-octanesulfonic acid sodium; LRHs are layered rare-earth hydroxides, R = Tb3+, Y3+) in the solid state and delaminated state is reported, which is utilized as an effective fluorescent probe for detecting metal ions. HNA/OS species are intercalated into LRH layers to generate composites of HNAxOS1−x-LTbH (x = 0.10, 0.15, 0.20 , 0.25) and HNAyOS1−y-LYH (y = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30). In the solid state, LYH composites exhibit green emissions (from 493 to 504 nm) with a large blue shift in comparison to the 542 nm emission of free HNA− anions, while in the delaminated state in formamide (FM), the composites display blue emission (480 nm) relative to the green emission (512 nm) of an HNA soltuion in FM. However, LTbH composites display coquenched luminescence in both the solid state and delaminated state. Also, HNA0.25OS0.75-1:1-LYH, HNA0.25OS0.75-1:2-LYH, and HNA0.05OS0.95-1:1-LYH (1:1 and 1:2 are HNA:NaOH molar ratios) show significantly elongated fluorescence lifetimes of 15.35, 14.37, and 12.72 ns, respectively, in comparison with free HNA-Na (6.44 ns), and their quantum yields of 23.40%, 21.97%, and 22.31%, respectively, are much larger than that of free HNA-Na (4.86%). The LTbH composite (HNA0.25OS0.75-1:1-LTbH) has also a relatively higher quantum yield of 12.46%. The HNA0.25OS0.75-1:1-LYH colloid exhibits excellent recognition selectivity for Al3+ over other metal ions (Mg2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, and Hg2+) with distinct fluorescence sensitization. It shows an intense change in its fluorescence emission when it is bound to Al3+ ions, giving a lower detection limit of 6.32 × 10−6 M. This is novel research on the fluorescence chemosensing of LRH composites.

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INTRODUCTION Up to now, due to multiple applications including selective catalytic oxidation,1 anion exchangers,2 fluorescent sensors,3 and photochemical materials,4 significant attention has been focused on interlayer chemistry. Organic−inorganic hybrids of layered rare-earth hydroxides (LRHs), due to the energy transfer or synergetic effect between the inorganic layered hosts and interlayered organic species, not only improve the stability of organic dyes but also exhibit novel photoluminescence behavior.5−10 There are numerous reports referring to organic− and inorganic−LRH hybrids;5−14 most © XXXX American Chemical Society

of them have focused on the luminescence of the individual components such as layered Eu3+ and Tb3+.5−7,12 Less attention has been given to the luminescence resulting from the interlayer guests or synergetic interactions of the guests with the layered host.9,10,15 The facile delamination method involving the anion surfactant of 1-octanesulfonic acid sodium (abbreviated OS) that we previously reported10 has opened the way to achieve LRH colloidal materials showing particular Received: December 29, 2018

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DOI: 10.1021/acs.inorgchem.8b03636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry luminescence properties8−10,13,15 and expanded the applications:, for example, luminescence sensing.12,16 Currently, metal ion pollution has caused a serious threat to human health and living organisms. Developing effective recognition systems for detecting these ions is necessary and indispensable.17−21 Aluminum, as one of the most abundant metal elements present in the crust of the earth, plays a key role in various fields, including aluminum foil, drinking bottles, medicines, bleached flour, shipping, automobiles, and food additives.22 Al3+ ions also affect the central nervous system of humans and induce Alzheimer’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis.17−19 Furthermore, Al3+ can stay inside the cells and tissues for a long time before being excreted by the body.23 The above adverse effect of Al3+ on human health and the environment makes it desirable to develop sensitive and specific techniques for Al3+ detection. At present, various analytical methods such as atomic absorption, electrochemical luminescence, and electrochemical have been utilized to detect Al3+.24−26 As discussed, fluorescent chemosensing has the advantage of high sensitivity, easy operation, and rapid response.27−31 In past years, various fluorescent chemosensors which are specific for detecting Al3+ ions have been reported.22,29 Optical chemosensors are designed in such a way that the detection event is easily measurable by observing a photophysical change. Fluorescent chemsensors have been considered as significant and promising approaches to biologically and/or environmentally important analytes, owing to their remarkable application in sensoring.32−34 There has been some research on fluorescent chemsensing using LRH materials in the detection of oxoanionic species. Byeon’s group 35 synthesized terbium-doped layered yttrium hydroxychloride (LYH:xTb) working as a fluorescent detector for Cr(VI) oxoanions in aqueous solution. They also reported europiumdoped layered gadolinium hydroxide (LGdH:Eu) to detect vanadate ions by virtue of the antenna effect.36 They then prepared layered yttrium hydroxide (l-Y(OH)3) as a novel member of LRHs11 to detect and adsorb phosphate from water. Our group demonstrated a MoS4/OS-LEuH composite, whose turn-on luminescence sensing realized feasible detection of traces of water in formamide.12 All of these studies relied on the luminescence change of the RE3+ layer of LRHs, and the recognition seldom involves the metal cations. We reported one example utilizing interlayer fluorescein anions of LRH to discern Fe3+, for which the detection was achieved through fluorescence quenching.16 Indeed, chemosensors showing fluorescence enhancement by interacting with target analytes are more proficient, because of the decreased probability of giving false signals.27,28,30,31,37 Gupta and Kumar wrote a review related to mechanisms for fluorescent “‘turn-on’” probes to detect Al3+.37 However, research progress in fluorescent sensing for Al3+ has been slow-moving,37 possibly due to coordination units for Al3+ being less available. Considering the hard acid nature of Al3+, complexing donors containing hard bases such as O atoms would be preferable. 3-Hydroxy-2-naphthoic acid (HNA, with the structure shown in Scheme S1) is a small fluorescent molecule, having three O donors in its molecular skeleton. HNA has been used for detecting aluminum,38 copper,39 lanthanide,40 and vanadium.41 In addition, the positional isomer of HNA has been used for the spectrofluorimetric determination of beryllium42 and scandium.43 We now select HNA and OS as organic species to cointercalate into LRHs (R = Tb, Y) and

study the luminescence and recognition properties for metal ions. The as-formed HNAyOS1−y-LYH composites present green luminescence (∼500 nm) in the solid state but blue luminescence (480 nm) in the delaminated state in formamide (FM), while the HNAxOS1−x-LTbH composites exhibit nearly no emission in either the solid or colloidal state. More remarkably, the delaminated HNA0.25OS0.75-LYH shows excellent recognition character for Al3+. This work describes a novel example of “turn-on” fluorescent chemosensing for detection of metal cations by means of the interlayer organic chromophores in LRH.

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EXPERIMENTAL SECTION

Preparation of NO3-LRH Precursors. NO3-LTbH and NO3LYH precursors were prepared via a hydrothermal reaction, on the basis of similar procedures described in the literature,9 with details as shown in the Supporting Information. Preparation of Composites of HNA x OS1−x -LTbH and HNAyOS1−y-LYH. The intercalations of HNA/OS guests into LRH were carried out by ion exchange reactions. First, stoichiometric NaOH and HNA (NaOH:HNA molar ratio was 2:1 or 1:1) were added to deionized water to form aqueous solutions of HNA− and HNA2−. Subsequently, HNAxOS1−x-LTbH (x = 0.10, 0.15, 0.20, 0.25) and HNAyOS1−y-LYH (y = 0.05, 0.10, 0.15, 0.20, 0.25, 0.30) were prepared via a facile hydrothermal process. The molar ratio of the organics to LRH was fixed at 3:1. A total amount of 1.29 mmol of OS + HNA was used, corresponding to 0.43 mmol of NO3-LRHs (∼0.076 g of NO3-LYH or ∼0.10 g of NO3-LTbH). The relative molar ratios of OS to HNA were only varied to prepare the composites. The reactions were conducted at 70 °C for 24 h in Teflon-lined autoclaves to give the precipitates, followed by filtration, washing, and vacuum drying.16 For delamination of the LRH composites, 0.05 g of the composite powder was dispersed into 20 mL of formamide (FM) followed by mechanical shaking for 2 h, giving translucent colloidal suspensions. Recognition Experiments of HNA0.25OS0.75-LYH for Metal Ions. To detect the selective recognition of HNA0.25OS0.75-LYH for metal ions (Al3+, Mg2+, Hg2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, and La3+, as obtained from their corresponding nitrate salts), a colloidal suspension of HNA0.25OS0.75-LYH was obtained by dispersing 0.02 g of the powder into 20 mL of FM with mechanical shaking for 1.5 h. Then 3 mL of the colloidal suspension was mixed with 1 mL of an aqueous solution of the metal ion at certain concentrations to measure the luminescence intensity. To determine the desorption behavior of Al3+ from HNA0.25OS0.75LYH, to 4 mL of a mixture of HNA0.25OS0.75-LYH colloid (1 mL) and an aqueous solution of Al 3+ (3 mL, 800 ppm Al3+) was added 1 mL of an aqueous solution of Na2EDTA (30 mM), and then 1 mL of an aqueous solution of Al3+ (800 ppm) was added again. For each step, the luminescence intensity was measured. Characterization Techniques. The powder X-ray diffraction (XRD) measurements were recorded in the range of 4.5−70° (2θ) on a PANalytical X’Pert PRO MPD diffractometer with Cu Kα radiation operated at 40 kV and 40 mA (step size of 0.033°, scan time of 20 s per step). For the small-degree measurements, XRD patterns were measured in the range 0.6−6° (2θ) with a step size of 0.033° and a scan time of 60 s per step. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet-360 Fourier transform infrared spectrometer as KBr pellets. The morphologies of the LRH composites were observed using field-emission scanning electron microscope (FESEM, Hitachi S-4800 microscope), while energydispersive X-ray spectroscopy (EDS) mapping was acquired on Hitachi S-8010 and Bruker XFlash 6160 instruments. Luminescence spectra of samples in the delaminated state were measured with a Cary Eclipse spectrofluorimeter, and the solid luminescence spectra were recorded with an Edinburgh FS5 spectrofluorimeter. The fluorescence lifetimes of samples were monitored in FM solutions of about 10−6 mol/L on an FLS980 spectrofluorimeter. Fluorescence B

DOI: 10.1021/acs.inorgchem.8b03636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry quantum yields were determined on an Edinburgh FLS-980 instrument with an integrating sphere system. The metal ion contents were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Jarrel-ASH, ICAP9000). C, H, and N elemental analyses were determined by using an Elementar vario EL elemental analyzer. The chemical formulas of the products were obtained on the basis of the data of ICP-AES, CHN content, and charge balance considerations.

that the HNA anions with larger size act as the dominant interlayer guests. For HNAyOS1−y-1:1-LYH (y = 0.05−0.30) with LYH layers but varied HNA/OS ratios in the interlayers (Figure 1B), no matter whether the HNA contents were more or less, there appeared similar diffraction peak positions and close dbasal values of ∼2.0 nm, illustrating that the interlayer space was supported mainly by the OS. Though a large amount of HNA existed in the interlayer, the close dbasal values (∼2.0 nm) show that HNA adopted a close-packed arrangement in the gallery and OS mainly supported the interlayer space (Scheme 1). In

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RESULTS AND DISCUSSION Structure and Morphology of the Composites. The XRD patterns of the precursors of NO3-LTbH and NO3-LYH and the composites are shown in Figure 1. Figure 1A-a shows a

Scheme 1. Arrangements of Gallery Species in (a) NO3LTbH Precursor and (b) HNA/OS-LTbH and (c) HNA/ OS-LYH Composites

addition, the HNA0.25OS0.75-1:2-LYH composite (Figure 1Bg), displayed diffractions similar to those of HNAyOS1−y-1:1LYH, implying that the structure underwent no large change with the deprotonation degree. On the basis of the layer thickness of LRH (0.65 nm)6 and the sizes of HNA (∼0.81 × 0.51 × 0.18 nm) and OS (∼1.04 × 0.37 × 0.26 nm), the gallery heights of ∼1.52 nm (=2.17 − 0.65) and ∼1.37 nm (=2.02 − 0.65) denote a bilayered antiparallel arrangement between HNA and the adjacent OS (Scheme 1b,c), resulting from the electrostatic interactions of −COO− and/or −O− (phenol) of HNA and the positively charged LRH layers. Meanwhile, the π−π stacking interactions of adjoining benzene rings further ensure the stability of the structure. In addition, all samples had a nearly coincident peak at ∼0.31 nm, ascribed to the nonbasal (220) reflection of the LRH host layers.45,46 The chemical formulas of all of the LRH composites were calculated on the basis of the results of ICP and CHN data and charge balance considerations (Table 1). The calculated elemental contents are very close to the experimental values in the estimated chemical formulas, indicating the accuracy of the chemical compositions. The HNA/OS molar ratios measured in the as-prepared composites agree well with those added in the starting reactants, which means that both HNA and OS were easily soluble in water and they reacted nearly completely with the LRH layers. Figure 2 describes the FT-IR spectra of the LRH precursors and composites. For HNA (Figure 2a), the O−H stretching vibrations of the C−OH group appeared at 3284 cm−1.47 A very strong band at 1665 cm−1 corresponded to the CO stretching vibrations of −COOH. The peaks at 1516 and 1467 cm−1 were assigned to the naphthalene ring C−C stretching vibrations. For OS (Figure 2b), the SO3− absorptions appeared at 1199 and 1067 cm−1,48 and the bands at 2921 and 2852 cm−1 were the νas and νs stretching vibrations of −CH2 and −CH3 groups, respectively. For NO3-LTbH (Figure 2c), the sharp and strong absorption peak at 1384 cm−1 was characteristic of the ν3 vibration mode of uncoordinated

Figure 1. (A) XRD patterns of the precursors NO3-LTbH (a) and NO3-LYH (b) and composites HNAxOS1−x-1:1-LTbH (x = 0.10 (c, c′), 0.15 (d, d′), 0.20 (e, e′), 0.25 (f, f′)). (B) XRD patterns of HNAyOS1−y-1:1-LYH (y = 0.05 (a, a′), 0.10 (b, b′), 0.15 (c, c′), 0.20 (d, d′), 0.25 (e, e′), 0.30 (f, f′)) and HNA0.25OS0.75-1:2-LYH (g, g′). The d values are given in nanometers.

series of strong (00l) reflections, showing a basal spacing (dbasal) of 0.83 nm.44 The NO3-LYH (Figure 1A-b) had a dbasal value of 0.89 nm, in accordance with a previous report.45 The sharp and symmetric diffractions suggest the formation of wellcrystallized layered compounds. As shown in Figure 1A-c−f, different proportions of HNA/OS were intercalated into NO3LTbH to form the LTbH composites, with dbasal values of 1.96−2.17 nm. For the composite with less HNA and more OS (x = 0.10, Figure 1A-c), there was a series of peaks at 1.99, 1.00, 0.67, 0.50, and 0.40 nm, indicating a pure single phase with OS as the main component supporting the layered structures. In Figure 1A-e,f, when HNA compositions were increased (x = 0.20, 0.25), in addition to the 1.96/1.98 nm dbasal value (attributed to the OS intercalated phase), there appeared an enlarged dbasal of 2.15/2.17 nm, revealing the presence of double phases. The enlarged dbasal values indicate C

DOI: 10.1021/acs.inorgchem.8b03636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Chemical Compositions of the LRH Composites found (calcd), wt % sample

chemical formula

Y

HNA0.05OS0.95-1:1-LYH HNA0.10OS0.90-1:1-LYH HNA0.15OS0.85-1:1-LYH HNA0.20OS0.80-1:1-LYH HNA0.25OS0.75-1:1-LYH HNA0.30OS0.70-1:1-LYH HNA0.25OS0.75-1:2-LYH HNA0.10OS0.90-1:1-LTbH HNA0.15OS0.85-1:1-LTbH HNA0.20OS0.80-1:1-LTbH HNA0.25OS0.75-1:1-LTbH HNA0.25OS0.75-1:2-LTbH

Y(OH)2.5(C11H7O3)0.024(C8H17O3S)0.476·1.44H2O Y(OH)2.5(C11H7O3)0.052(C8H17O3S)0.448·1.13H2O Y(OH)2.5(C11H7O3)0.074(C8H17O3S)0.426·1.09H2O Y(OH)2.5(C11H7O3)0.10(C8H17O3S)0.40·1.27H2O Y(OH)2.5(C11H7O3)0.124(C8H17O3S)0.376·1.65H2O Y(OH)2.5(C11H7O3)0.148(C8H17O3S)0.352·0.98H2O Y(OH)2.5(C11H7O3)0.11(C8H17O3S)0.29·0.80H2O Tb(OH)2.5(C11H7O3)0.05(C8H17O3S)0.45·0.59H2O Tb(OH)2.5(C11H7O3)0.06(C8H17O3S)0.425·1.36H2O Tb(OH)2.5(C11H7O3)0.12(C8H17O3S)0.39·0.65H2O Tb(OH)2.5(C11H7O3)0.125(C8H17O3S)0.375·2.34H2O Tb(OH)2.5(C11H6O3)0.10(C8H17O3S)0.30·0.43H2O

34.29 35.89 36.45 35.58 33.59 36.71 39.98

Tb

(35.05) (35.86) (35.97) (35.54) (34.62) (36.33) (38.98) 51.55 49.79 51.12 47.27 56.07

(51.76) (49.14) (50.23) (46.84) (55.65)

C 18.85 20.13 20.78 20.65 19.83 22.03 19.05 16.22 15.27 17.14 15.61 14.82

(19.27) (20.11) (20.50) (20.63) (20.43) (21.80) (20.01) (15.44) (14.53) (17.94) (15.46) (14.70)

H 5.26 5.15 5.10 5.02 4.93 4.74 3.69 3.78 4.03 3.63 4.29 3.19

(5.38) (5.14) (5.04) (5.01) (5.08) (4.69) (3.59) (3.76) (3.83) (3.63) (4.25) (3.17)

Figure 3. SEM images of NO3-LTbH (a, a′), NO3-LYH (b, b′), HNAxOS1−x-1:1-LTbH (x = 0.20 (c, c′), 0.25 (d, d′)), and HNAyOS1−y-1:1-LYH (y = 0.15 (e, e′), 0.20 (f, f′), 0.25 (g, g′), 0.30 (h, h′)).

platelike morphology with a rough surface. It can also be seen that the composites were composed of microcrystalline particles. The NO3-LTbH (Figure 3a,a′) crystals mainly crystallized into elongated hexahedral platelets, and some of them grew into columnar or flowerlike aggregates, which are consistent with a previous report.34 In comparison with NO3LTbH, NO3-LYH (Figure 3b,b′) presented a nearly regular hexagonal shape. The HNAxOS1−x-LTbH composites (Figure 3c,c′,d,d′) maintained the configuration of the NO3-LTbH precursor, and the HNAyOS1−y-LYH composites (Figures 3e− h,e′−h′) kept the morphology of the NO3-LYH precursor. The similarity of the morphology of the composites to that of their precursors implied a topotactic intercalation when the interlayer species came into the LRH layers. The elemental mappings of Tb, Y, C, and S and EDS in the two composites HNA0.25OS0.75-LYH and HNA0.25OS0.75-LTbH are displayed in Figure 4. They clearly showed the presence of different layer elements (LYH/LTbH layers) and the interlayer organic species. The relative brightness of the element represents their corresponding content. Luminescence Properties of Composites in the Solid State. Powder emission spectra of HNA-Na and LTbH/LYH composites were measured, as shown in Figure 5 and Figure S1. For NO3-LTbH, measured with the 368 nm excitation

Figure 2. FT-IR spectra of HNA (a), OS (b), NO3-LTbH (c), NO3LYH (d), HNA0.25OS0.75-1:1-LTbH (e), HNA0.20OS0.80-1:1-LYH (f), HNA0.25OS0.75-1:1-LYH (g), and HNA0.25OS0.75-1:2-LYH (h).

NO3− located between LRH layers.49 The absorption peaks of NO3-LYH (Figure 2d) were very similar to those of NO3LTbH, showing the similar structures of LRHs intercalated by NO3−. After ion exchange, the NO3− bands in all of the composites (Figure 2e−h) became invisible, showing that there was nearly complete substitution of the NO3− by the organic guests. The bands around 2925 and 2855 cm−1 were related to aliphatic C−H stretching vibrations, and the vibrations at 1171 and 1049 cm−1 verified the existence of − SO3− of OS into the LRH galleries. The νas and νs stretching vibrations of −COO− observed at 1596 and 1464 cm−1 indicataed the entrance of the HNA anions, and in addition the bathochromic shift suggested the deprotonation of −COOH and its interactions with the layer hydroxides. The structures and morphologies of the synthesized samples are depicted in Figure 3 via SEM observations, which showed a D

DOI: 10.1021/acs.inorgchem.8b03636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 4. SEM images of HNA0.25OS0.75-LYH (a) and HNA0.25OS0.75-LTbH (b): (a-1)−(a-3) corresponding elemental distribution maps of Y, C, and S; (b-1)−(b-3) corresponding elemental distribution maps of Tb, C, and S; (a′) and (b′) related EDS of (a) and (b).

OS and LYH host layers prevent the intermolecular aggregation of HNA anions,51 thus enhancing the luminescence. A CIE 1931 chromaticity diagram displays the photoluminescence color of the composites located in the green region (Figure 5A′). All of the chromaticity coordinates are in accordance with the emissions of the corresponding samples. Luminescence Performance of Composites under Delaminated State in FM. The luminescence behaviors of the HNA-Na solution in FM and the delaminated colloidal suspensions of the composites in FM were investigated systematically. The delaminated HNAyOS1−y-LYH composites (the 1:1 HNA:NaOH ratio was omitted for clarity) presented blue emissions (∼480 nm) (Figure 6A), different from the

Figure 5. (A) Emission spectra of HNA-Na (a), HNAxOS1−x-1:1LTbH (x = 0.10 (b), 0.15 (c), 0.20 (d), 0.25 (e)), and HNAyOS1−y1:1-LYH (y = 0.05 (f), 0.10 (g) 0.15 (h), 0.20 (i), 0.25 (j), 0.30 (k)) in the solid state. (A′) CIE1931 chromaticity diagrams of the corresponding samples showing the photoluminescence color.

wavelength of Tb3+, there were characteristic Tb3+ emissions at 488, 545, 589, and 621 nm (Figure S1B), respectively assigned to 5D4−7FJ (J = 6, 5, 4, 3) radiative-relaxational transitions of Tb3+.50 For HNA-Na, at an excitation wavelength of 394 nm, an emission peak at 542 nm assigned to green luminescence was observed (Figure 5A-a and Figure S1D). The composites of HNAxOS1−x-LTbH (with pure Tb3+ layers and changed HNA/OS ratios) displayed no emission, meaning that the Tb3+ emissions were fully quenched (with only some weak peaks, Figure 5A-b-e), no matter which excitation wavelength (368 nm for Tb3+ or 394 nm for HNA-Na) was used. Figure S2 depicts the excitation spectra of the composites in the solid state. The coquenching of the emissions of the layer Tb3+ and interlayer HNA might be ascribed to an energy transfer between them. Meanwhile, the high-energy vibration of −OH in the HNA anions would also compete with the provided energy to generate nonradiative relaxation channels. The LYH composites (Figure 5A-f-k), however, present luminescence much stronger than that of free HNA-Na, and the intensity was enhanced gradually with the increasing HNA:OS molar ratios. At the lowest HNA content, the HNA0.05OS0.95-1:1-LYH presented the weakest and the greatest blue-shift emission (493 nm, Figure 5A-f), while at the nearly highest HNA content, the HNA0.25OS0.75-1:1-LYH revealed the strongest and the least blue-shift emission (502 nm, Figure 5A-k), most proximal to the emission position of free HNA-Na. Further increased HNA content (HNA0.30OS0.70-1:1-LYH) caused almost no change in the emission intensity and position. This indicated that the anionic surfactant OS played an important role in the blue shift of the emissions. Meanwhile, the LYH layers also contribute to the blue shift. The interlayer

Figure 6. (A) Emission spectra of samples in FM: free HNA-Na (a) and HNAyOS1−y-1:1-LYH (λex = 390 nm) (y = 0.05 (b), 0.10 (c), 0.15 (d), 0.20 (e), 0.25 (f), 0.30 (g)). (A′) CIE1931 chromaticity diagrams of the corresponding samples showing the photoluminescence color. Insets in (A′) are the corresponding photographs of (a) and (e) under 365 nm UV irradiation.

green fluorescence (∼500 nm) in the solid state (Figure 5A). This was also markedly different from the green luminescence (512 nm) of an FM solution of free HNA−, as seen in Figure 6A-a. The blue shift of the emission from 542 nm in the solid state to the 512 nm in FM solution indicates the effect of FM solvents. Furthermore, the much larger blue shift to 480 nm (from 512 nm) of the LYH composites verifies the function of LYH layers and the surfactant of OS. Here the delamination of the composites provides free environments of the HNA− surrounding the LYH layers (no excessively tight combination with the positively charged layers), giving a chance for OS to develop its ability. The delaminated composites prefer blue emission. The blue-shift phenomenon was also observed in hydrothermally treated DDS-AQS/LDH.52 It is known that surfactants can alter the aggregation of photoactive species.53 E

DOI: 10.1021/acs.inorgchem.8b03636 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Here the OS as a surfactant may dilute/isolate the HNA− anions, thus preventing their aggregation that weakens the luminescence.51,54 Upon an increase in the HNA content from 0.05 to 0.10, the ∼480 nm emission had a sharp enhancement (Figure 6A-b,c), but further increase in the HNA content did not give an obvious increase in the intensity (Figure 6A-d,e). This suggests that suitable HNA content (such as an HNA:OS ratio of 0.15:0.85) contributes to the strongest intensity of the favorable blue emission. No matter whether the HNA:NaOH molar ratios were 1:1 or 1:2, the LYH composites exhibited blue emission at 480 nm (Figure 7A-b,c), but the emission intensity was decreased with

Figure 7. (A) Emission spectra of HNA-Na (a), HNA0.25OS0.75-1:1LYH (b), HNA0.25OS0.75-1:2-LYH (c), HNA0.20OS0.80-1:1-LTbH (d), HNA0.25OS0.75-1:1-LTbH (e), and HNA0.25OS0.75-1:2-LTbH (f) on delamination in FM. Insets in (A) are the corresponding photographs of (a)−(c) under 365 nm UV irradiation. (A′) is CIE1931 chromaticity diagrams of the corresponding samples showing the photoluminescence color.

Figure 8. Photoluminescence decay curves of (a) HNA-Na, (b) HNA 0.25 OS 0.75 -1:1-LYH, (c) HNA 0.25 OS 0.75 -1:2-LYH, (d) HNA0.05OS0.95-1:1-LYH, and (e) HNA0.25OS0.75-1:1-LTbH in FM.

fitted by the single-exponential equation55 I(t) = I0 exp(−t/τ), where I(t) is the fluorescence intensity dependence with time, I0 is the fluorescence intensity at time zero, and τ is the lifetime for each component. The τ values of HNA-Na, HNA0.25OS0.751:1-LYH, HNA0.25OS0.75-1:2-LYH, HNA0.05OS0.95-1:1-LYH, and HNA0.25OS0.75-1:1-LTbH were determined to be 6.44, 15.35, 14.37, 12.72, and 6.34 ns, respectively (Table 2). The

the increasing deprotonation degree, indicating that the 1:1 deprotonation state (HNA ) could give the optimal fluorescence intensity. LTbH composites (Figure 7A-d−f) still presented a quenching effect of the emissions of HNA and Tb3+, in agreement with the results in the solid state. In the delaminated state, LTbH composites displayed faint emission, but it was slightly stronger than that in the solid state, indicating weakened interactions between the HNA and delaminated LRH layers giving the HNA some chance to emit. In comparison with the weak emissions of Tb3+ that the LTbH composites displayed in the solid state (Figure 5A-b− e), in the delaminated state, fluorescence of HNA̅ was present (Figure 7A-d−f), which suggested a difference in the luminescence for different states. The inset diagrams show photographs of colloidal composites under 365 nm ultraviolet light. The color of the samples was in accord with the emission spectrum. From the CIE 1931 chromaticity diagrams (Figure 7A′), there appeared a distinct transformation of the luminescence color, indicating that pure blue emission was achieved in FM. Fluorescence Lifetimes and Quantum Yields of Composites. To explore the excited-state information for the samples, fluorescence decay times were measured for all samples in the colloid state with different excitation wavelengths (400 nm for HNA-Na, 390 nm for HNA0.25OS0.75-1:1LYH, HNA0.25OS0.75-1:2-LYH, and HNA0.05OS0.95-1:1-LYH, and 355 nm for HNA0.25OS0.75-1:1-LTbH) and collected at the emission wavelengths of 512, 480, 480, 480, and 512 nm, respectively. Logarithmic decay graphs (Figure 8) of HNA-Na, H N A 0 . 2 5 O S 0 . 7 5 - 1 : 1 -L Y H , H N A 0 . 2 5 O S 0 . 7 5 - 1 : 2 - L YH , HNA0.05OS0.95-1:1-LYH, and HNA0.25OS0.75-1:1-LTbH were

Table 2. Fluorescence Decay Times and Quantum Yield of the As-Prepared Samples sample

fluorescence decay time (τ), ns

quantum yield (Φ), %

HNA-Na HNA0.25OS0.75-1:1-LYH HNA0.25OS0.75-1:2-LYH HNA0.05OS0.95-1:1-LYH HNA0.25OS0.75-1:1-LTbH

6.44 15.35 14.37 12.72 6.34

4.86 23.40 21.97 22.31 12.46

LYH composites had enormously elongated fluorescence lifetimes (>12 ns), while the LTbH composite (6.34 ns) had almost no change relative to that of the HNA-Na salt itself (6.44 ns). This is in accord with the tendency of the luminescence emission. The longer lifetimes of the LYH composites in comparison to free HNA-Na indicate a certain energy transfer from Y3+ to HNA anions exists. PL quantum yields (Φ) of the composites were obtained using an integrating sphere technique.56 As seen in Table 2, the Φ values were consistent with the tendency of the decay lifetime, and HNA0.25OS0.75-1:1-LYH had the largest Φ value. In addition, the LYH composites had quantum yields >21%, significantly larger than that (4.86%) of the HNA-Na; the LTbH composites had quantum yields of 12.46%, also being larger than that (4.86%) of HNA-Na. The increased quantum F

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addition of different metal cations (Mg2+, Co2+, Ni2+, Cu2+, Zn2+, Pb2+, Cd2+, Hg2+, and the lanthanide ion La3+), all mixtures exhibit slightly changed emissions except for Al3+. A significant fluorescence enhancement and blue shift was solely observed when Al3+ ions were added, suggesting that the composite exhibited good selectivity for Al3+ over other metal ions. Figure 9B displays a bar diagram of the fluorescence intensity of the HNA0.25OS0.75-LYH colloid mixed with various metal ions at optimal emission. The intensity is almost invariable when Mg2+, Cd2+, Co2+, Hg2+, Ni2+, Pb2+, and Zn2+ are added. In addition, Cu2+ gave some slightly increased intensity. The enhanced fluorescence with addition of Al3+ may be attributed to the coordination interaction between Al3+ and the oxygen atom sites of HNA− ions in the composites.37 The absorption band at 525 nm was blue-shifted to 460 nm when Al3+ was added to the colloidal suspension of HNA0.25OS0.75LYH. Both the enhanced fluorescence and the blue shift may be attributed to the inhibition of an intramolecular charge transfer (ICT) process. In this system, the HOMO of the sensor has electron density near the electron-donating group while the LUMO of the sensor has electron density close to the electron-acceptor unit upon excitation.59 The interaction of Al3+ with the oxygen atom results in a decrease in the electrondonating character of the oxygen atom; hence, there is a blue shift in the emission spectrum.37 The results may be due to the inhibition of ICT from the oxygen atom of hydroxyl directly linked to the naphthalene ring to the carbonyl acceptors.37,60−62 When solutions of Al3+ were mixed with other concomitant ions, as shown in Figure 10A, the fluorescence

yields of both LYH and LTbH composites showed the significant function of the LRH layers. Overall, the prolonged luminescence lifetimes and the raised quantum yields especially found in LYH composites demonstrated the improved luminescence properties of HNA on interaction with the LRH layers. Luminescence Mechanism Analysis. As mentioned above, LYH composites always presented strong emission intensity in both the solid state and colloidal state, while the LTbH composites showed insignificant emissions no matter what the state. There existed a significant coquenching effect between HNA and LTbH layers. Normally, a sensitization luminescence effect needs an effective energy transfer from the activators to the acceptors,6 which means a perfect fit of the energy gap between the excited state energy levels of RE3+ and the triplet state energy levels of the organics. If the energy difference is too large, then the probability of electron conversion will be small, thus decreasing the chance of transition. If the energy difference is too small, inverse or mutual transition might occur, while most of the energy would be inactivated by vibration rather than luminescence. An energy gap of 3500 cm−1 or higher is necessary to facilitate efficient and irreversible energy transfer.57 The triplet energy level of the HNA− anions is 21413 cm−1,57 and the energy gap between the triplet energy level of HNA and the 5D4 level of Tb3+ (20540 cm−1)58 is only 873 cm−1, which is not optimum for effective energy transfer. The mutual or two-way mutual transition between HNA− and layer Tb3+ may result in emission quenching. For LYH composites, the enhanced luminescence behavior is thanks to the LYH layers, which dilute/separate the organic anions to allow their aggregation and give them freedom to generate effective emissions. As discussed above, the surfactant OS is used in the preparation of LRH composites, for the following three reasons: (1) the flexible long-chain structure of the OS may achieve a preintercalation into the LRHs, resulting in the facile entrance of the bulk HNA̅ anions into the gallery; (2) the presence of the long chain of OS makes it easier to delaminate the composites; (3) as an anionic surfactant, the OS is helpful in diluting and isolating the interlayer fluorophores so as to reduce their aggregation, thus changing or improving the fluorescence performance of the composites. Ion Recognition of HNA0.25OS0.75-LYH. Emission spectra of HNA0.25OS0.75-LYH colloid mixed with various metal ion solutions are depicted in Figure 9 and Figure S3. Note that, on

Figure 10. (A) Emission spectra of HNA0.25OS0.75-LYH mixed with Al3+ and concomitant ions (3 mL of composite colloid was mixed with 1 mL of aqueous solutions of metal ions (90 ppm concentration for each ion)). For the “blank” as a control experiment, 3 mL of composite colloid was mixed with 1 mL of water. (B) Fluorescence spectra before and after HNA0.25OS0.75-LYH colloid was added to Al3+ and Na2EDTA and then further addition of Al3+. For all of the cases, λex is 353 nm.

intensity displayed hardly any change in comparison to those with only Al3+, demonstrating that the other cations have no interference with the detection of Al3+ by using this composite. To further study the sensing performance of the HNA0.20OS0.80-LYH composite toward Al3+ ions, luminescence measurements were performed with the addition of various concentrations of Al3+ to the composite colloid. Figure 11 shows the change in emission wavelength and enhanced fluorescence intensity with increasing concentrations of Al3+. Following the increase in Al3+ concentration, the emission wavelength shifted gradually from 525 to 460 nm in the first range (0−30 ppm), with some increase in the fluorescence intensity. At Al3+ concentrations higher than 30 ppm, the intensity kept on strengthening and finally remained unchanged at >800 ppm, reaching an enhancement exceeding 5-fold. The column diagram (Figure 11B) further exhibits the

Figure 9. (A) Emission spectra of HNA0.25OS0.75-LYH in FM with various metal ions (λex 355 nm). For measurements, 3 mL of the composite colloid was mixed with 1 mL of aqueous solutions of metal ions with 90 ppm concentration. For the “blank” case, as a control experiment, 3 mL of the composite colloid was mixed with 1 mL of pure water. (B) Bar diagram showing the fluorescence intensity of HNA0.25OS0.75-LYH mixed with Al3+ (460 nm) and other metal ions (525 nm) (λex = 355 nm). G

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state, blue emissions (480 nm) are achieved, thanks to the dilution function of the LYH layers and intermolecular interactions between HNA and OS. For LTbH composites, coquenched luminescence is displayed in the solid state as well as in the delaminated state, probably due to the mutual energy transition between the interlayer HNA and layer Tb3+. Also, the delaminated HNA0.25OS0.75-LYH colloid is selective to detection of Al3+ over other metal ions. The colloid is an effective turn-on chemosensor for Al3+ with a detection limit of 6.32 × 10−6 M. In the presence of Al3+, the emission wavelength undergoes a blue shift from 525 to 460 nm. The reason for the high selectivity of the colloidal composite might be due to the complexation of Al3+ and HNA̅ that gives rise to the inhibition of intramolecular charge transfer (CT) from the oxygen atom directly linked to the naphthalene ring to carbonyl acceptors. This work offers a new approach for the design of LRH materials applied to fluorescence chemosensing.

Figure 11. (A) Emission spectra of HNA0.25OS0.75-LYH in FM with the addition of various concentrations of Al3+ (0−800 ppm) (3 mL of colloid in FM + 1 mL of aqueous solutions). (B) Bar diagram showing the fluorescence intensity of HNA0.25OS0.75-LYH at optimal wavelengths and with the presence of Al3+ at various concentrations.

obvious change trend of the fluorescence intensity at optimal wavelength. The fluorescence intensity of the suspension was plotted as a function of Al3+ concentration, as shown in Figure 12. The detection limit for Al3+ calculated on the basis of 3σ/

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03636.

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Figure 12. Linear relationship between the Al3+ concentration and fluorescence intensity of the colloids in the concentration range of 0− 100 ppm (0−3.7 mM).

Synthesis of NO3-LRH precursors, chemical structure of 3-hydroxy-2-naphthoic acid, excitation and emission spectra of NO3-LTbH, HNA-Na, and all composites in the solid state, and emission spectra of a mixture of HNA0.25OS0.75-LYH in FM with La3+ (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: *E-mail: *E-mail: *E-mail:

S63 is 6.32 × 10−6 M, which is lower than the highest limit of Al3+ in drinking water recommended by the WHO (7.41 μM),64 representing a rare example of a reported fluorescent probe for Al3+. The reversibility of metal binding with the HNA/OS-LYH composite will be meaningful information concerning practical applications. The desorption behavior of HNA0.25OS0.75-LYH was studied using EDTA as a ligand, because it is available in abundance and at relatively low cost. As shown in Figure 10B, after EDTA was added, the emission intensity of the mixture was diminished significantly, and after the sample was mixed with Al3+, the intensity was increased, which means that EDTA has a certain binding ability to Al3+. In addition, when we added an excessive amount Al3+, the fluorescence intensity was increased, which was slightly lower than that of the starting mixture. Thus, HNA0.25/OS0.75-LYH had good reversibility and was reusable.

[email protected]. [email protected]. [email protected]. [email protected].

ORCID

Shulan Ma: 0000-0002-8326-3134 Author Contributions ∇

R.G. and F.S. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation of China (No. U1832152 and 21665021), Beijing Natural Science Foundation (No. 2182029), Shandong Provincial Natural Science Foundation (No. ZR2015BL024), Natural Science Foundation of Ningxia (NZ17052), and Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (No. 2017-K07).

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CONCLUSIONS In conclusion, the organic molecule 3-hydroxy-2-naphthoic acid (HNA) and the anionic surfactant 1-octanesulfonic acid (OS) were cointercalated into LRH (R = Y, Tb) via ion exchange, giving rise to the composites HNAxOS1−x-LTbH and HNAyOS1−y-LYH, exhibiting tunable luminescence (from green to blue emission) and effective ion recognition capability. In the solid state, LYH composites display green emissions (from 493 to 502 nm), while in the delaminated

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DOI: 10.1021/acs.inorgchem.8b03636 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03636 Inorg. Chem. XXXX, XXX, XXX−XXX