Color-tunable luminescent Ln3+ composite as self-referencing and ratiometric sensor for low-level water detection in ethanol

Accepted Manuscript Title: Color-Tunable Luminescent Ln3+ composite as Self-Referencing and Ratiometric Sensor for Low-Level Water Detection in Ethanol Authors: Tianren Wang, Meiqi Liu, Yige Wang PII: DOI: Reference:

S0025-5408(17)32513-8 http://dx.doi.org/doi:10.1016/j.materresbull.2017.08.022 MRB 9500

To appear in:

MRB

Received date: Revised date: Accepted date:

29-6-2017 9-8-2017 9-8-2017

Please cite this article as: Tianren Wang, Meiqi Liu, Yige Wang, ColorTunable Luminescent Ln3+ composite as Self-Referencing and Ratiometric Sensor for Low-Level Water Detection in Ethanol, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.08.022 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Color-Tunable Luminescent Ln3+ composite as Self-Referencing and Ratiometric Sensor for Low-Level Water Detection in Ethanol Tianren Wang, Meiqi Liu and Yige Wang* Hebei Provincial Key Lab of Green Chemical Technology and High Efficient Energy Saving, School of Chemical Engineering and Technology, Hebei University of Technology, Tianjin, 300130, China, E-mail: [email protected] Graphical abstract

A new color-tunable luminescent Ln3+ (Ln=Eu, Tb or Eu and Tb in different molar ratio) composite was prepared through a simple method, which can be applied as self-referencing and ratiometric sensor for detecting low-level water content (0.010.5 vol%) in ethanol.

Research Highlights: 1. A novel Ln3+ (Ln=Eu, Tb or Eu and Tb in different molar ratio) complex with tunable luminescence from red to green was prepared through a simple procedure; 2. The luminescence color of the Eu/Tb co-doped complex can be tuned by water through the luminescence quenching mechanism as well as the variation of energy transfer efficiency from Tb3+ to Eu3+; 3. The complex with Eu3+:Tb3+=1:1 can be applied as self-referencing and ratiometric luminescent sensor for detecting low-content water (0.01-0.5 vol%) in ethanol.

Abstract: Developing simple, fast and sensitive detection of low-level water in organic solvents is in high demand for academic and industrial applications. In this paper, a novel color-tunable lanthanide composite has been prepared by simply mixing 2,2':6',2''-terpyridine-4'-carboxylic acid (Hctpy) and Ln3+ with a molar ratio of 3:1 in absolute ethanol at room temperature, in which the Ln3+ ions are mainly coordinated with carboxyl moieties rather than with terpyridine moieties of Hctpy. In addition, the emission colors of the composite can also be tuned by water, ascribed to the luminescence quenching mechanism and the variation of Tb-to-Eu energy transfer efficiency. Based on these properties, a selfreferencing and ratiometric luminescent water detector with the Eu/Tb molar ratio of 1:1 has been

developed, showing proportional relationship between the intensity ratio of 5D0→7F2 transition of Eu3+ to 5

D4→7F5 transition of Tb3+ (IEu/ITb) and the water concentration (0.01-0.5 vol%) in ethanol.

Keywords: Lanthanide; Hctpy; Tunable luminescence; Energy transfer; Low-level water detection

1. Introduction Water is generally considered as one of the most significant components in living things and ecosystem, which, on the other hand, has also been regarded as a common impurity in organic solvents and is harmful to many technological applications, such as organic synthesis, food processing and paper production. [1-3] Therefore,

Herein, we present a new color-tunable luminescent Ln3+ composite (named as Hctpy@LnCl3) by simply mixing 2,2':6',2''terpyridine-4'-carboxylic acid (Hctpy) and Ln3+ (Ln=Eu, Tb or Eu and Tb in different molar ratio) in absolute ethanol at room temperature, in which the Ln3+ ions are mainly coordinated with carboxyl moieties rather than with terpyridine moieties of the Hctpy (Scheme 1). The

developing sensors for water content determination in organic

luminescence of the composite can be efficiently quenched by water

solvents is necessary. Recently, optical sensors have gained increasing

molecules in aqueous media, and the luminescence color of the Eu/Tb co-doped composite can also be fine tuned by water due to the

attention due to several advantages like fast response, high sensitivity (ppm level) and simple operation, exhibiting great potential in substitution of the traditional water detection method, i.e., the Karl-

variation of Tb-to-Eu energy transfer efficiency, affording the material potential candidate for simple and fast detection of low water content

Fischer titration. [4-10]

(0.01-0.5 vol%) in organic solvents like ethanol.

Amongst numerous optical materials, trivalent lanthanide ions (Ln3+)-containing materials with unique luminescent features such as

2. Experimental section

pure and tunable emission color, long excited-state lifetime and low self-absorption, etc., have attracted considerable interest in versatile applications like optoelectronic devices designing or sensing. [11-24] Generally, the luminescence derived from excited state of Ln3+ can be easily quenched by high-frequency O-H oscillators of coordinated water molecules through nonradiative energy transfer process, [2527] making the Ln 3+ luminescent materials suitable for water detection. A series of luminescent Eu3+ complexes have previously been utilized as water detectors. [1, 28, 29] However, these complexes are only based on the intensity of single emission and can be influenced by the sensor concentration and excitation power. [3032] The self-referencing dual-emission composites depending on the intensity ratio of two independent transitions of the same luminophore are not compromised by these problems. So far, several dual-emission composites, such as Eu3+/Tb 3+ co-doped hybrid material and N,S-CDs-encapsulated Eu-MOFs, [33, 34] have been applied for detecting trace amount water in commonly used organic solvents like ethanol or DMF. Nevertheless, developing new dualemission composite as self-referencing luminescent sensor for simple and fast detection of low-level water in organic solvents is still a challengeable task.

Materials. 2,2':6',2''-terpyridine-4'-carboxylic acid (Hctpy, Aldrich, 99%), EuCl3·6H2O (99.9%) and TbCl3·6H2O (99.9%) were used as purchased, without further purification. Preparation of the Ln3+ composite Hctpy@LnCl3. A mixture of 0.0554 g (0.2 mmol) Hctpy and 10 mL absolute ethanol was sonicated for 2 minutes and white suspension was formed, then appropriate amount of 0.1 mol·L-1 LnCl3·6H2O ethanol solution (Ln=Eu, Tb or Eu and Tb in different molar ratio) was added and the mixture was sonicated for 6 hours, resulting in white precipitate. The precipitate was separated by centrifugation (10000 rpm, 5 min), washed by absolute ethanol for three times and dried in air at 80 ℃ for 24 hours. The product was finely ground into a white powder, which we denoted as Hctpy@LnCl3 (including Hctpy@EuCl3, Hctpy@TbCl3 and Hctpy@EuxTbyCl3, where x/y is the molar ratio of the amount of Ln3+ added and x+y=1). Water content detection. [email protected] was dispersed in absolute ethanol in a concentration of 0.5 wt%, then appropriate amount of water was added followed by sonication for 30 seconds, and their photoluminescence properties were measured afterwards. Volume fractions of water: 0, 0.01, 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, 1, 1.5, 2, 3 vol%. Characterization. The scanning electronic microscope (SEM) images and the energy dispersive X-Ray (EDX) spectra of the samples were obtained from a FE-SEM (Hitachi S-4300) at an acceleration voltage of 10 kV. The Fourier transform infrared spectrum (FT-IR) spectra of the powder samples were obtained on a Bruker Vector 22 spectrometer using KBr pellets for the powder samples, from 4000 to 400 cm-1 at a resolution of 4 cm-1 (16 scans collected), about 2 mg of each compound were mixed with KBr (Merck, spectroscopic grade) finely ground and pressed into pellets. The samples for thermo-

Scheme 1. a) The predicted 3D structure of Hctpy@LnCl3, which

gravimetric analysis (TGA) was transferred to open platinum crucibles and analyzed by using an SDT-TG Q 600 TA instrument at a heating

reveals the interactions between Hctpy and Ln3+; b) digital photos of

rate of 58 ℃·min-1. The luminescence spectra and the lifetimes of the

ethanol dispersions (2 wt%) and powders of Hctpy@LnCl3 (under 302 nm UV lamp illumination).

samples were measured on an Edinburgh Instrument FS920P spectrometer, with a 450 W xenon lamp as the steady-state excitation source, a double excitation monochromator (1800 lines mm-1), an

emission monochromator (600 lines mm -1), a semiconductor cooled

moieties rather than terpyridine moieties, in contrast to the

Hamamatsu RMP928 photomultiplier tube. A microsecond flash lamp (pulse length: 2 μs) was used as the excitation source for the lifetime

previously reported Eu3+ composite Hcptpy@EuCl3. [29] In addition, the bands at 3435 and 1383 cm-1 have shifted to 3380 and 1413 cm-1,

measurements. Photons were collected up to 10 ms until maximum of 104 counts. The absolute luminescence quantum yields of the

respectively, suggesting that the coordination of Ln3+ to O atoms of Hctpy in which the protons of carboxylic acid are still present and lead

samples were determined by an absolute method on the above-

to hydrogen-bonding within the Ln3+-coordinated Hctpy (see Scheme

mentioned Edinburgh Instruments FLS920P using a BaSO 4-coated integrating sphere (diameter: 150 mm).

1). Similar coordination features have also been reported previously. [36, 38] Unfortunately, no corresponding crystals of the composites were obtained, thus their specific structure cannot be determined.

3. Results and discussion 100

b) 1555

90

1471

80

3380 O-H

1710 absent

70

Weight / %

Transmittance / %

H-bond C=N 1413 O-H

a)

60 50

1471

40

1555

30

H-bond

C=N

Tb4O7

Hctpy@EuCl3

20

1710 C=O

Hctpy@TbCl3

3435 O-H

Eu2O3

10

1383 O-H

100

200

300

400

500

600

700

800

900

1000

Temperature / C 3600

3300

1800

1500

1200

900

600

-1

Wavenumber / cm

Fig. 1. FT-IR spectra of a) Hctpy and b) Hctpy@EuCl3. The Ln 3+ composite Hctpy@LnCl3 was obtained through a simple onestep method by just mixing Hctpy and Ln3+ in absolute ethanol at room temperature, without deprotonation of carboxyl groups of the Hctpy. The morphology of Hctpy@EuCl3 was characterized by SEM images, as shown in Fig. S1a (see supporting information), a number of amorphous nanoparticles with an average diameter of 50-500 nm were observed, and similar morphology with smaller size nanoparticles was also observed in the SEM image of Hctpy@TbCl3 (Fig. S1b). The elements in the material can be verified by EDX spectra of Hctpy@EuCl3 and Hctpy@TbCl3 shown in Fig. S1, indicating that the Ln3+ ions were successfully reacted with Hctpy molecules. The interactions between the Ln3+ ions and Hctpy molecules can be confirmed by the FT-IR spectra (see Fig. 1). The absorption bands peaking at 3435 (1383) and 1710 cm-1 observed in the spectrum of Hctpy can be ascribed to the vibrations of O-H and C=O groups, respectively, while two sharp peaks at 1555 and 1471 cm -1 are attributed to the C=N stretching of the pyridine rings [29, 35-37]. The absence of the bands at 3502 (O-H) and 1720 (C=O) cm-1 assigned to the free carboxylic acid indicates that the carboxyl groups are mainly connected through hydrogen bonds. The coordination between Hctpy and Eu3+ results in the disappearance of absorption band of C=O stretching at 1710 cm-1 (see Fig. 1b), and no obvious changes for absorption bands of C=N stretching can be observed, indicating that the Ln3+ in Hctpy@LnCl3 are mainly coordinated with carboxyl

Fig. 2. TGA curves of Hctpy@EuCl3 (red line) and Hctpy@TbCl3 (green line). The thermal behaviors of Hctpy@LnCl3 were studied and shown in Fig. 2. The TGA curve of Hctpy@EuCl3 exhibits three main degradation steps, the first weight loss (ca. 8%) might be attributed to the decomposition of the unreacted Hctpy molecules, and the remaining steps with about 75% weight loss from 420 to 1000 ℃ can be attributed to the decomposition of Hctpy ligand. The final plateau is corresponding to the formation of stable compound Eu2O3 (found: 15.31%, calcd: 16.15%). Similar TGA curve was also observed for Hctpy@TbCl3 in Fig. 2 (final product: Tb4O 7, found: 16.85%, calcd: 16.98%). The contents of other elements in the composite were determined by elemental analysis (Hctpy@EuCl3: found: C 52.28, H 2.99, N 11.33%; calcd: C 52.87, H 3.03, N 11.56%. Hctpy@TbCl3: found: C 51.80, H 2.97, N 11.21%; calcd: C 52.53, H 3.01, N 11.49%). These data suggest that the Hctpy and Eu3+ in the material are 3:1 in molar ratio, which can also be supported by the EDTA titration experiments. The Hctpy@LnCl3 with different molar ratio of Eu3+ to Tb3+ can be prepared through the same method. Both the powders and ethanol dispersions of Hctpy@LnCl3 can emit bright and tunable luminescence from red to green under 302 nm UV lamp illumination (see Scheme 1). The luminescent properties of Hctpy@LnCl3, as well as the luminescence quenching behaviors of Hctpy@LnCl3 in organic solvents with the existence of water will be discussed in detail by the following luminescence data.

The luminescent excitation spectrum of Hctpy@EuCl3 was shown

ascribed to the 5D0→7FJ (J=0, 1, 2, 3, 4) transitions of Eu3+, with the

in Fig. 3a. A broad band from 240 to 380 nm centered at 340 nm can be observed, which is due to the absorption of the Hctpy ligand.

5D

0→

7F

2

Besides, two much weaker sharp peaks at 395 and 465 nm ascribed to the intra-4f6 transitions of Eu3+ between 7F0 and the 5L6 and 5D2 can

5D

4→

7F

J

also be observed, implying that the Eu3+ are essentially excited via

the 5D4→7F5 transition. The 5D0 lifetime of Eu3+ (τEu) in Hctpy@EuCl3

ligand-to-metal energy transfer process rather than by direct

and The 5D4 lifetime of Tb3+ (τTb) in Hctpy@TbCl3 was measured to be

population of the Eu3+ absorption level. [12] The excitation spectrum of Hctpy@TbCl3 shown in Fig. 3b also exhibits a broad absorption

0.25 (±0.01) ms and 0.63 (±0.01) ms through the corresponding decay curves (see Fig. S2, supporting information), respectively, and the

band from 240 to 420 nm with maximum at 350 nm, and no obvious f-f transitions of Tb 3+ can be observed, indicating that energy transfer

absolute luminescence quantum yield of Hctpy@EuCl3 (ΦEu) and Hctpy@TbCl3 (ΦTb) was determined to be 6.63% (±1.26%) and 13.18%

from the ligand to Tb3+ is more efficient than that to Eu3+. [37] Upon excitation at 345 nm, the emission spectrum of Hctpy@EuCl3 (Fig. 3a)

(±1.87%) by using the integrating sphere method, respectively. [39, 40]

transition as the prominent feature, while four typical emission bands at 489, 546, 583 and 618 nm corresponding to the transitions (J=6, 5, 4, 3) of Tb3+ can also be observed in the emission spectrum of Hctpy@TbCl3 (Fig. 3b), which is dominated by

exhibits five characteristic peaks at 579, 592, 614, 650 and 697 nm

Fig. 3. a) Excitation (black, λmon =614 nm) and emission (red, λex=345 nm) spectra of Hctpy@EuCl3; b) excitation (black, λmon=546 nm) and emission (green, λex=345 nm) spectra of Hctpy@TbCl3; c) emission spectra of Hctpy@LnCl3 (λex=345 nm); d) CIE 1931 chromaticity diagram within the coordinates of Hctpy@LnCl3 excited at 345 nm. Tunable emission color of lanthanide luminescent materials plays

an important role in optical device applications, which can be easily

fulfilled in this case by varying the molar ratio between Eu 3+ and Tb3+

difference after contact with 2 vol% H2O with naked eyes (see Fig. S5,

in Hctpy@EuxTbyCl3, and the corresponding emission spectra as well as their CIE (Commission Internationale de L'Eclairage) 1931

supporting information). Therefore, we select [email protected] 0.5Cl3 as luminescent probe for detecting trace amount water in organic

coordinates were displayed in Fig. 3c and 3d, respectively. The emission intensity at 546 nm decreases remarkably when increasing

solvents. The luminescence performance of [email protected] as a

the doping concentration of Eu3+, therefore the intensity ratio of

function of the volume fraction of water (V w) in ethanol was

5D

transition of Eu3+ to 5D4→7F5 transition of Tb3+ (IEu/ITb)

researched, as shown in Fig. 4. Obviously, even a very low content of

increases (Table 1), resulting in various emission colors (see Scheme 1b). The increased IEu/ITb can be attributed to the increased

water (0.01 vol%) can lead to about 2% decrease in the IEu/ITb, which decreases gradually when the Vw is further increased up to 0.75 vol%,

concentration of Eu3+ and the possible occurrence of Tb-to-Eu energy transfer in the co-doped complexes. The energy transfer efficiency

resulting in the variation of emission colors of [email protected] from red to yellow under 302 nm UV lamp (see Fig. 4b). The value of

from Tb3+ to Eu3+ (η Tb→Eu) can be calculated by using the following equation: 22

the IEu/ITb maintained a constant in the range of Vw=0.75-3 vol%, and the luminescence of the material has almost been quenched

0→

7F 2

ηTb→Eu = 1 – τ/τ0

(1)

absolutely at a Vw of 3 vol%. The corresponding τTb in [email protected] were also measured, thus the variation tendency

where the τ0 (τTb , 0.63 ms) and τ is the 5D4 lifetime of Tb3+ in

of ηTb→Eu versus Vw was obtained (see Fig. S6, supporting information)

Hctpy@TbCl3 and Hctpy@EuxTbyCl3, respectively. The values of τ and

according to the equation (1), which is consistent with that of the IEu/ITb. Clearly, the η Tb→Eu in [email protected] is also dependent on

η Tb→Eu of Hctpy@EuxTbyCl3 were listed in Table 1, indicating that the energy transfer efficiency from Tb3+ to Eu3+ increases as the concentration of the Eu3+ is increased, which is consistent with the results reported elsewhere. [23, 41, 42] This is possibly due to the

the change of water concentration, and the luminescence behavior mentioned above can therefore be well explained.

proportional relationships between the energy transfer probability

Table 1. The intensity ratio of 5D0→7F2 transition of Eu3+ to 5D4→7F5

from Tb3+ to Eu3+ and R-6, where R is the average distance between Eu3+ and Tb 3+, [43] the reduced R resulted from the incremental molar

transition of Tb3+ (IEu/ITb), 5D 4 lifetimes of Tb3+ (τTb) and the corresponding energy transfer efficiency from Tb3+ to Eu3+ (ηTb→Eu) in

ratio of Eu3+ to Tb3+ leads to the increase of ηTb→Eu.

Hctpy@EuxTbyCl3.

Interestingly, when dispersed in water, both the Hctpy@EuCl3 and Hctpy@TbCl3 suffered from rapid and absolute luminescence quenching (see Fig. S3, supporting information). Such luminescence quenching behaviors of the composites toward water were studied, as shown in Fig. S4 (supporting information). Upon treatment with water, the characteristic emission intensity of Ln3+ ions decreased remarkably, and both their luminescence lifetimes declined to be about 0.01 ms. Considering the coordination sphere of Ln3+ in the material mentioned above, we speculate that the coordination bond

Eu:Tb

1:1

1:9

1:24

1:99

IEu/ITb

6.55

2.74

0.53

0.10

τTb/ms

0.13

0.31

0.43

0.61

ηTb→Eu

79.4%

50.8%

31.7%

3.2%

The quantitative relationship between the normalized IEu /ITb and Vw in ethanol was also established, as shown in Fig. 6b. The curve can be linearly fitted in the range of Vw=0.01-0.5 vol% (R2=0.996), with the

between Ln3+ and Hctpy is not stable. As a consequence, the first coordination sphere of Ln3+ in Hctpy@LnCl3 can be easily occupied by

slope and intercept of -1.7331 and 0.9905, respectively. Such dualemission luminescent sensor is fast-response, sensitive and self-

the O-H oscillators of water, resulting in efficient luminescence quenching. Since the luminescence of Ln3+ in Hctpy@LnCl3 can be

calibrating, without using any other parameters. Actually we have also tested several frequently used neutral organic solvents (see Fig.

quenched by water and the Tb-to-Eu energy transfer process exists in

S7, supporting information), unfortunately, the [email protected]

Hctpy@EuxTbyCl3, the emission colors of the Eu/Tb co-doped composites may therefore be tuned by water molecules, [33, 44, 45]

exhibits very poor dispersibility in THF, MeCN, acetone, diethyl ether,

which means that a colorimetric sensor for water content detection in organic solvents may be established based on the luminescence properties. Simultaneously, we dispersed the co-doped composites ([email protected] 0.5Cl3, [email protected], [email protected] and [email protected]) into absolute ethanol (0.5 wt%), and we observed that the [email protected] exhibits more obvious

methylbenzene andethyl acetate. Although the material can be well dissolved in MeOH, DMF and DMSO, the luminescence of the material can be changed or efficiently quenched by the solvents. Therefore, we conclude that the [email protected] can be used for detecting low water content (0.01-0.5 vol%) in ethanol.

Fig. 4. a) Water-dependent emission spectra of [email protected] in EtOH (0.5 wt%) with increasing amount of water (0-3 vol%), λex=345 nm; b) the variation of IEu/ITb versus Vw, and the red line is the fitted curve (each data point was measured for three times); the inset shows the digital photos of [email protected] in EtOH upon treatment with various amount of water (from top to bottom: 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.75, 1, 1.5, 2, 3 vol%, under 302 nm UV lamp illumination).

4. Conclusions In summary, a new color-tunable luminescent Ln3+ composite (Hctpy@LnCl3) in which the Ln3+ ions are mainly coordinated with carboxyl moieties rather than with terpyridine moieties of the organic ligand, has been successfully prepared by simply mixing Hctpy and LnCl3·6H2O (Hctpy:Ln3+=3:1 in molar ratio) in absolute ethanol at room temperature. The lifetime and absolute quantum yield were measured to be 0.25 ms and 6.63% for Hctpy@EuCl3 as well as 0.63 ms and 13.18% for Hctpy@TbCl3, respectively. In addition, the luminescence of the Eu 3+/Tb3+ co-doped composite ([email protected] 0.5Cl3) can be fine tuned by low-level water (0.010.75 vol%) in ethanol on the basis of the luminescence quenching mechanism and variation of Tb-to-Eu energy transfer efficiency, with linear relationship between the intensity ratio of 5D 0→7F2 transition of Eu3+ to 5D 4→7F5 transition of Tb3+ (IEu /ITb ) and volume fraction of water (Vw, 0.01-0.5 vol%), and the sensing process is simple and fast. The good luminescence behaviors afford the Hctpy@LnCl3 ideal candidate in developing optical devices and self-referencing sensor for low-content water detection in ethanol.

Acknowledgements Financial support by the National Natural Science Foundation of China (21171046, 21271060, and 21236001), the Tianjin Natural Science Foundation (13JCYBJC18400), the Hebei Natural Science Foundation (B2016202147), and Educational Committee of Hebei

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