Method

Visual enantioselective probe based on metal organic framework incorporatingquantum dots

Zhou Long, Jia Jia, Shanling Wang, Lu Kou, Xiandeng Hou, Michael J.Sepaniak

PII: S0026-265X(13)00154-9DOI: doi: 10.1016/j.microc.2013.08.013Reference: MICROC 1818

To appear in: Microchemical Journal

Received date: 20 August 2013Accepted date: 24 August 2013

Please cite this article as: Zhou Long, Jia Jia, Shanling Wang, Lu Kou, Xian-deng Hou, Michael J. Sepaniak, Visual enantioselective probe based on metal or-ganic framework incorporating quantum dots, Microchemical Journal (2013), doi:10.1016/j.microc.2013.08.013

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1

Visual enantioselective probe based on metal organic

framework incorporating quantum dots

Zhou Longa, Jia Jia

a, Shanling Wang

a, Lu Kou

b, Xiandeng Hou*

a,b, Michael J.

Sepaniak*c

a Analytical & Testing Centre, Sichuan University, Chengdu, Sichuan 610064, China.

b College of Chemistry, Key Laboratory of Green Chemistry and Technology of MOE at

Sichuan University, Chengdu, Sichuan 610064, China.

c Department of Chemistry, The University of Tennessee, Knoxville, TN-37996-1600,

U.S.A.

Abstract

A new method was developed for visual enantioselective sensing based on quantum

dots (QDs) doped metal organic framework (MOF), which firstly combined the chiral

selectivity of MOF and sensing of QD quenching. A simple synthesis procedure with

much less reaction time is developed, with small and homogenously-sized QD@MOF

particles obtained, as well as a cheaper dispersant used. The proposed sensing method

is proved to be direct, convenient, and visual. Qualification and preliminary

quantification of enantiomers can be accomplished in a visual fashion. The proposed

method also demonstrates practical utility, and it is expected to be expanded to

enantioselective determination of other enantiomers.

Keywords: Metal organic framework; quantum dots; visual enantioselective sensing.

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1. Introduction

The study of chiral recognition has received compelling attention in recent years, which

is very important to the understanding of rational of asymmetric synthesis [1] or

catalysis [2], and the development of chiral separation [3] and sensing [4]. During the

past few years, considerable efforts have been devoted to the development of simple,

effective and low-cost enantioselective sensors [5], while the design of visual

discrimination of enantiomers, which is more convenient, direct, and low-cost for no

analytical instrument needed, still remains challenging. By present, only a few examples

of visual chiral recognition have been reported, by discernable colour change or

precipitate formation caused by the interactions between target enantiomers and the

sensing platforms using modified nanoparticles [6] or gels [7]. Further exploration of

this research field is thus highly in demand.

In this context, we proposed the first visual chiral fluorescence (FL) sensor employing

metal organic framework (MOF) with quantum dot (QD) caged inside. Since last

decade, FL probes involving QDs have been employed for bio/chemical sensing [8],

following the turn-off mechanism through FL quenching of QDs caused by the

interaction between QDs and target analytes [9]. The broad absorption band in the

visible region of QDs makes it easy to achieve visual sensing [10]. MOFs are extended

crystalline structures wherein metal cations or clusters of cations are connected by

multitopic organic strut or linker ions or molecules [11], and they have already been

used in asymmetric catalysis [12], separation [13] and chromatography [14]. Moreover,

its intrinsic chiral topology, cavity confinement effect and conformational rigidity,

make MOF become an ideal sensing platform for chiral recognition. Up to now, only a

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few examples concerning MOF-based enantioselective sensors have been reported [15],

but the research goal of visual chiral recognition employing MOF has rarely been

realized. Buso et al. [16] lately incorporated QD within MOF for visual molecular

discrimination based on molecular size, but a high level of visual chiral specificity

needs to be achieved by both shape and size selectivity, which will be accomplished by

the work presented herein.

2. Materials and Methods

2.1. Apparatus

The microwave reactor (Uwave-1000) was purchased from Sineo Microwave Chemistry

Technology (Shanghai, China). The microwave working frequency was at 2450 MHz

and the probe ultrasonic working frequency was between 26-28 KHz. The FL data were

collected from an F-7000 FL Spectrometer (Hitachi, Japan) with a 390 nm optical filter.

The powder X-ray diffraction (PXRD) patterns were obtained from an X'Pert Pro MPD

(Philips, Netherlands) using Cuka radiation. The scanning electron microscopy (SEM)

images were obtained from a JEOL model JSM-7500F scanning electron microscope.

The transmission electron microscopy (TEM) images were obtained from an FEI Tecnai

G2 F20 S-TWIN transmission electron microscope.

2.2. Reagents

All of the chemicals used are AR grade. Ultrapure water (18.25 Mcm) produced with

a purification water machine (PCWJ-10, Pure Technology Co. Ltd, Chengdu, China)

was used throughout this work. Sodium tellurite, mercaptopropionic acid, D- and L-

tartaric acid, D- and L-dimethyl tartrate, D- and L-mandelic acid, were purchased from

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Aladdin Reagents Co.,Ltd. (Shanghai, China), CdCl22.5H2O, trisodium citrate

dehydrate, NaBH4, N, N-Dimethylformamide (DMF), Zn(NO3)26H2O and ethanol were

obtained from Kelong Chemical Reagent Co. Ltd. (Chengdu, China); D-camphoric acid

and 4, 4-dipyridyl were ordered from Juhui Chemical Reagent Co. Ltd. (Chengdu,

China). All chemicals and standards were kept at 4 oC in a refrigerator until use.

2.3. Synthesis of CdTe QDs (QDs)

The QDs were prepared with a procedure similar to the one-pot synthetic method

reported previously [17]. 0.5 mmol CdCl22.5H2O and 200 mg trisodium citrate

dehydrate were dissolved in 50 mL water, followed by instant addition of 52 L

mercaptopropionic acid (MPA). The pH of the solution was adjusted to 10.5, followed

by the addition of 0.1 mmol Na2TeO3 and 50 mg NaBH4. The solution was then placed

in a microwave reactor set at 100 oC to produce the QDs capped with MPA, with no

extra pressure applied. With the increase of reaction time, the FL emission was red

shifted (Fig. 1). The obtained solution containing the QDs was then concentrated by

reducing the volume to 10 mL, mixed with the same volume of ethanol and then

centrifuged at 8000 rpm for 15 min. Dark red powder was obtained at the bottom of the

solution, which was then collected and dissolved in water.

Fig.1

2.4. Synthesis of QD@MOF

For our method, CdTe QDs wrapped inside Zn2camph2bipy[18] (MOF) structures

synthesized with Zn(NO3)2 and D-camphoric acid were employed as an example to

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demonstrate our idea. The intrinsic structure of D-camphoric acid leads to its easy

bending, which is conducive to forming an enantioselective interface due to an easily

distorted pattern of secondary building unit (SBU) [19] (Fig. 2). The MOF precursors

were prepared by mixing 0.2 mmol Zn(NO3)26H2O, 0.2 mmol D-camphoric acid and

0.1 mmol 4, 4-dipyridyl in 40 mL DMF. Subsequently, 1 mL of 0.1 mM QDs was

added in and the solution was placed in the microwave reactor, and heated at 110 oC for

150 min. Then, light red powder crystals could be observed. After cool down to room

temperature, the obtained red powder was washed several times with DMF and

ultrapure water.

Fig. 2

3. Results and discussion

Compared with previous work about the synthesis of QD@MOF composites [16, 20], a

simple synthesis procedure with less time was developed. Small and homogenously-

sized QD@MOF particles were obtained, and cheaper dispersant of

dimethylformamide (DMF) instead of N,N-diethylformamide [16] was used.

Three pairs of enantiomers were added into the same QD@MOF suspension,

respectively, with only L-tartaric acid causing complete QD quenching, which

demonstrates both size and shape selectivity of the MOF. Furthermore, different

mixtures composed of D- and L-tartaric acids were tested as well, with quenching extent

increasing with the enantiomeric excess percentage (e.e.%). Moreover, it has been

reported that possible interferents such as some metal ions [21] or small molecules [22]

could not cause any obvious QD quenching, although some of them can diffuse into the

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MOF cavities based on their size, so it can be expected that the proposed method would

have practical utility.

The QD@MOF started to grow due to high affinity between the carboxyl group of

MPA and the Zn-rich metal centres, which was conducive to organized adsorption of D-

camphoric acid, formation of SBUs, and the ultimate embedding of the QDs within the

formed MOF framework [23]. The obtained QD@MOF particles appeared light red,

which were thoroughly rinsed with DMF and water. Moreover, no obvious change in

FL signals of the QDs was seen upon the addition of any reactant, which demonstrates

that the QDs kept stable all through the synthesis process of the QD@MOF.

In Fig. 3a&3b, it can be obviously seen that the QDs (dark spots with similar size)

homogeneously disperse inside the framework based on the transmission electron

microscope (TEM) images of the QD@MOF, while no similar dark spots were

observed in the TEM image of the MOF (Fig. 3c). It should also be mentioned that

Yang et al. [24] lately showed the TEM images of ZnO QDs inside porous carbon,

which looked very similar to those in Fig. 3d, while QDs (Fig. 3e) on the surface look

different, showing clear lattice lines that images of the QD@MOF do not have.

Moreover, according to the spectra of powder X-ray diffraction (PXRD) (Fig. 4), the

dominating peaks of the MOF were consistent with previously reported [13], which

also remained in the spectra of the QD@MOF regardless of some emerging peaks of

the QDs. This demonstrates that the chiral topology structure of the MOF was well

retained even with the QDs caged inside.

Fig. 3

Fig. 4

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The obtained QD@MOF particles were homogeneously dispersed in DMF to prepare a

suspension with a density of 60 g/mL, which appeared red (Fig. 5a). For the

suspension, with excitation at 365 nm, two emission bands were obtained at 463 nm

(MOF) and 615 nm (QD), which are significantly blue shifted compared to that of

MOF (472 nm) and QD (643 nm), respectively (Fig. 6). This blue shift relates to the

increase of the band gap correlated to QD particle size (quantum-size effect) [25]. 10

L of 5 mg/mL of each of six enantiomers (D- and L-tartaric acid, D- and L-dimethyl

tartrate, D- and L-mandelic acid) in DMF was added into 1 mL of the afore-mentioned

suspension. After 4 h, only L-tartaric acid caused the colour change from red to blue

(Fig. 5a), while the others did not cause any obvious colour change at all. The

phenomena highly suggest that the QD@MOF can be employed for enantioselective

sensing of tartaric acid. Furthermore, eleven tartaric acid solutions of varied e.e.% were

added into eleven same QD@MOF suspension, one for each. After 4 h, it can be

observed that the colour of the suspensions changed from red to blue (inset of Fig. 7) as

e. e.% increased from -100% to 100% due to different QD quenching extents, and each

specific tartaric acid gave a unique appearance. Moreover, several different dispersants

were tested, with DMF giving the best result. The phenomena indicate that the proposed

QD@MOF-based visual enantioselective sensing method can not only accomplish fast

qualification of specific enantiomer but also provide useful information for preliminary

quantification.

Fig. 5

Fig. 6

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Fig. 7

In order to validate the chiral recognition of the QD@MOF, several experiments based

on FL measurements were performed. It should be noted that once molecules with

similar size to MOF cavity diffuse in, they are very easy to be stuck inside the

framework. Accordingly, motions (torsional displacements, vibrations, etc.) of SBUs

are inhibited, nonradiative decay was slowed and the fraction of excited species for

radiative decay was increased, all of which can cause FL intensity of MOF increased

[26]. Moreover, from Fig. 8, it can be seen that even 12 h after the addition of L-tartaric

acid, the MOF structure was well maintained because the FL signal of the MOF stayed

high and the wavelength of the MOF peak did not change. Or else, FL would be

quenched if the MOF structure were destroyed. As far as our experiments are concerned,

DMF might act as a collisional quencher [27], and upon the replacement of DMF by

any enantiomer diffusing in, the quenching effect might be eliminated, which can also

result in the increase of FL intensity of the MOF. For the FL experiments performed,

firstly, the QD quenching was studied by adding each enantiomer into 1 mL of 0.1 mM

QD solution (without the MOF). The QDs were quenched immediately and the FL

intensity of the QD solution decreased by about 95% (Fig. 5b) except D- and L-

dimethyl tartrate. Secondly, D- and L-tartaric acid, and D- and L-mandelic acid was

added into the QD@MOF suspension, respectively. On one hand, the addition of D-

and L-mandelic acid did not cause obvious change in FL intensity of even after 12 h

(Fig. 8), most probably because it was difficult for them to diffuse into the framework

based on size selectivity of the MOF. On the other hand, significant difference was

observed in the FL signal change after the addition of D- and L-tartaric acid,

respectively. FL intensity of the QDs decreased sharply less than 0.5 h after the addition

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of L-tartaric acid (Fig. 5c), while the decrease of FL intensity of the QDs was much less

even 4 h after the addition of D-tartaric acid (Fig. 5d). The mostly possible explanation

is that L-tartaric acid could diffuse faster and more easily into the MOF and quench the

QDs caged in the frameworks than D-tartaric acid based on chiral selectivity of the

MOF. Thirdly, with the increase of L-tartaric acid concentration (Fig. 9) or e.e.% of

tartaric acid (Fig. 7), FL intensity of the MOF increased and QD decreased in a linear

fashion, respectively, which illustrates that the proposed method is applicable for

quantification of the enatiomers, even without preliminary separation. Fourthly, D-

dimethyl tartrate and L-dimethyl tartrate, the according ester enantiomers of D- and L-

tartaric acid, were tested as well. The change of FL intensity of the MOF caused by the

addition of L-dimethyl tartrate was much more than D-dimethyl tartrate (Fig. 8) with

the same concentration, which further validates the enantioselectivity of the MOF.

Fig. 8

Fig. 9

Conclusion and Perspective

A new method was proposed herein to accomplish visual enantioselective determination

by employing QD@MOF as the sensing platform for the first time. Zn2camph2bipy

MOF and CdTe QD were chosen as the model MOF and QD, respectively. A simple

synthesis procedure was used, with small and homogeneously sized QD@MOF

particles obtained. No further modification or functionalization of sensing particles was

needed as previously reported concerning visual enantioselective sensing. The proposed

method is proved to be applicable for fast and facile qualification and preliminary

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quantification of enantiomers, without any analytical instrument needed. Moreover, the

application of this method could be expanded to enantioselective determination of other

enantiomers. First, one specific MOF has been proved to be able to accomplish the

chiral separation for quite a few pairs of enantiomers, with varied mobile phase

employed [14b]. Accordingly, it is expected to be feasible that by using varied

dispersant for our proposed method, one specific MOF could be enabled to recognize

more than one pair of enatiomers which can cause QD quenching. Second, the catalogue

of MOF materials with various chiral topology is growing fast, and some of them have

demonstrated chiral recognition to specific enantiomer. This makes it highly possible

that more pairs of enantiomers which can cause QD quenching would be distinguished,

as long as the right MOF is found or prepared. In short, the proposed strategy herein

will have good perspective for visual enantioselective sensing in the future.

Acknowledgements

We acknowledge the financial support from National Natural Science Foundation of

China (No. 21205083), Sichuan Bureau of Science and Technology (11DXYB353SF-

027) and Sichuan University (No. 2011SCU11070), and assistance from our Analytical

& Testing Centre for SEM, TEM and PXRD data.

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Figures

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Fig.1 Normalized CdTe QD (QD) FL emission spectra at different reaction time ranging

from 1 min to 60 min.

Fig.2 The secondary building units of the Zn2camph2bipy (MOF). The pink balls stand

for Zn atoms, the sky blue balls stand for O atoms, and the gray balls stand for C atoms.

H atoms are omitted. The figure is drawn with the Diamond software (version 3.2i,

Crystal Impact GbR, Bonn, Germany).

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Fig. 3 TEM image of the QD@MOF (a, b and d), the MOF (c) and the QDs (e), as well

as scanning electron microscope image of the QD@MOF (f). Scale bar: 20 nm for (a),

10 nm for (b) and (c), 2 nm for (d) and (e), and 100 nm for (f).

Fig. 4 PXRD spectra of the synthesized MOF (black) and the QD@MOF (green). The

red bars were stimulated based on the according CheckCIF file of the MOF.

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Fig. 5 (a) Time evolution of UV-excited (292 nm) QD@MOF fluorescent photographs

with injection of L-tartaric acid; (b) Time evolution of QD percentage (%) FL intensity

with the injection of tartaric acid; (c, d) 3D time-resolved evolution of normalized FL

intensity of the QD@MOF after the injection of L-tartaric acid (c) and D-tartaric acid

(d).

Fig.6 FL emission spectra of the MOF (blue), the QD (red) and the QD@MOF (black), excited at

365 nm.

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Fig. 7 FL spectra, FL intensity (inset above) and appearance (inset below) of

QD@MOF suspension 4 h after adding different mixtures of D- and L-tartaric acid.

Fig. 8 FL spectra of the QD@MOF 12 h after the addition of D- and L-mandelic acid;

D- and L-tartaric acid; and D- and L-dimethyl tartrate.

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Fig. 9 FL spectra of the QD@MOF suspension 4 h after adding L-tartaric acid with

different concentrations (top), and the calibration plot of the FL intensity of the MOF

(middle) and the QD (bottom) changing with the increase of L-tartaric acid

concentration.

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Highlights

Combining size and shape selectivity of MOF and sensing of QD quenching for visual

enantioselective sensing;

A simple synthesis procedure with much less reaction time for small and

homogenously-sized QD@MOF particles;

Direct and convenient for visual enantioselective sensing.

Method

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