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A Widely Applicable AIE Chemosensor for On-site Fast Detection of Drugs Based on POSS-core Dendrimer with Controlled Self-assembly Mechanism Zhihang An, Si Chen, Xiaoqian Tong, Huiwen He, Jin Han, Meng Ma, Yanqin Shi, and Xu Wang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b03275 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019
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A Widely Applicable AIE Chemosensor for On-site Fast Detection of Drugs Based on POSS-core Dendrimer with Controlled Self-assembly Mechanism Zhihang An,‡ a Si Chen,‡ a* Xiaoqian Tong,a Huiwen He,a Jin Han,a Meng Ma,a Yanqin Shi,a and Xu Wanga* a
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou
310014, China. Phone: 86-0571-88320855, Fax: 86-0571-88320855. * Email:
[email protected] and
[email protected] ‡ Zhihang An and Si Chen contributed equally.
Abstract A novel fluorescence chemosensor quickly detecting synthetic drugs for on-site check and pre-screening is firstly reported. An eight tetraphenylethene (TPE) modified polyhedral oligomeric silsesquioxane (POSS) dendrimer is designed and synthesized as an aggregation-induced emission (AIE) chemosensor, which exhibits great enchancement of unique monomer emission in pure tetrahydrofuran (THF) and AIE emission in THF/water thanks to forming different self-assembly morphologies. And POSS-TPE can sensitively detect methamphetamine and ketamine even in artificial saliva by non-covalent interaction forces. It has great potential to be a new widely applicable AIE chemosensor for aromatics molecules. KEYWORDS: AIE chemosensor, supramolecular self-assembly, monomer emission, synthetic drugs detection
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Introduction Recently, the effect of “aggregation-induced emission” (AIE) has gained increasing academic interest in numerous scientific fields due to the unusual photophysical phenomenon that it is non-luminescent in the solution state but become strongly emissive when aggregated.1 The AIE luminogens (AIEgens) such as tetraphenylethene (TPE) are tremendously potential to be applied in bioprobes, organic light-emitting diodes (OLEDs), chemosensors, et. al.2-4
However, the
working mechanism of most of the AIE chemosensors is to detect target objects through impacting AIEgens aggregations by specific recognition groups, and they are clearly limited in wide applications.5 Because the nitroaromatic compounds have emission quenching response to AIE chemosensors in the previous reports,6 developing a novel widely applicable AIE chemosensor for aromatic molecules by non-covalent interaction forces and supramolecular self-assembly is feasible and meaningful. What's more, some AIE chemosensors simply consisting of AIEgens and specific recognition groups suffer from poor fluorescence intensity.7 In the reported literatures, varied influence factors of improving the AIEgens' photophysical behaviors have been studied, for instance, dendrimers8, self-assembly9, hydrogen bond interactions10, molecular rigidity11, hydrophobicity12, etc. However, the improvement effect of a single factor is often not enough. The previous studies of polyhedral oligomeric silsesquioxane (POSS) dendrimer in our group13-18 indicated that rigid and hydrophobic POSS core had super-strong self-assembly properties by organic
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modification, which could form different morphologies through hydrogen bond interactions. So we designed a POSS dendrimer combined with AIEgens in this paper, not only effectively increasing emission properties due to all the above influence factors, but also developing a widely applicable AIE chemosensor for aromatic molecules which could offer multiple exciton migration channels and diffusion pathways via numerous branches in the three-dimensional (3D) space. Over the recent twenty years, the prevalence of synthetic drugs including methamphetamine and ketamine as the main illicit drugs is clearly increasing.19 The abuse of synthetic drugs in public places and drugged driving seriously threaten social security, which make a quick detection of synthetic drugs and their pre-screening processes highly and urgently demanded. Compared to analytical techniques that require large-scale inspection instruments, AIE chemosensors have unique advantages of their convenience, low cost, high sensitivity as well as on-site workability.6, 20 So the drug molecules that have different functional groups connected on benzene ring can be detected on the spot with naked eyes and pre-screened for accurate detection using the POSS dendrimer with AIE effect. Herein, we reported an eight tetraphenylethene (TPE) modified POSS dendrimer (POSS-TPE) as shown in Scheme 1, whose emission properties were highly improved by ordered supramolecular self-assembly at an extremely low concentration in different solvents. As expected, the POSS-TPE could on-site fast detect methamphetamine and ketamine through quenching aggregate emission because the
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aggregates of POSS-TPE had strong non-covalent interaction forces with drug molecules.
Scheme 1. The synthetic route of POSS-TPE.
Results and Discussion Synthesis: The synthesis of POSS-TPE was fulfilled according to the synthetic route of Scheme S1-S3 and Figure S1 in the supplement information. The signals at 7.11, 6.95, 6.43 and 1.31 with an integration ratio of 15: 2: 2: 2 in its 1H NMR spectrum were assigned to the protons of TPE and POSS core, suggesting that the eight TPE groups was connected to POSS moiety (Figure S2). Photophysical Properties: The photophysical properties of POSS-TPE were screened using UV-vis absorption and fluorescence spectroscopy in different solvents as shown in Figure S3-S4 and Figure 1. POSS-TPE was soluble in some common organic
solvents,
such
as
chloroform,
tetrahydrofuran
(THF),
and
N,
N-dimethylformamide (DMF), but was insoluble in water. The UV–vis absorption spectrum of POSS-TPE in THF (1 M) exhibited π-π* transition with absorption
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maxima at 335 nm (Figure S3). The AIE behaviors of POSS-TPE were investigated in aqueous THF solution with different THF–water ratios. No spectral changes were observed in photoluminescent (PL) spectrum when the water fraction in dilute THF solution (1M) was increased from 0% to 80% v. The photoluminescent intensity enhanced rapidly at addition of 90% v water fractions in dilute THF solution (1M). A new peak at 470 nm emerged in 90% v aqueous solution and it was a 374-fold enhancement of emission as compared to that in 100% THF. Like many other TPE derivatives reported previously,21 this phenomenon resulted from that POSS-TPE molecules aggregated into packed aggregates in THF/H2O and intramolecular rotations of TPE units were highly hindered, which blocked the nonradiative decay and led an aggregation-induced emission effect. What’s more, POSS-TPE had a strong fluorescence emission at 407 nm and 430 nm in pure THF at 0.1 M (Figure 1b), corresponding to monomer characteristic emission. Similar monomer emission behaviors of TPE derivatives has been reported before.22 The two peaks of monomer emission decreased rapidly upon the increase of water fractions from 0% to 50% v in THF/H2O solvent mixture with a small (5 nm) red-shift in the emission maximum caused by increased π-π stacking interactions,23 and showed the obvious AIE behavior when the H2O content reached 90% v. Because the POSS-TPE aggregate formation in aqueous mixtures would restrict intramolecular motions and increase π−π stacking interactions. The former enhanced emission while the latter shifted the emission
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spectrum to the long wavelength region and quenched the emission. The competition between the both of the above determined the emission behavior.23 In the pure THF solvent at 0.1 M, POSS-TPE formed possible small multimers and steric hindrance restricted the intramolecular rotation of phenyl rings in TPE, resulting in the emission of POSS-TPE monomer.24 As the concentration was increased from 0.1 M to 1 M or water fraction in THF increased to 50% v at 0.1 M, the increased aggregation would lead to a decrease in the monomer concentration of POSS-TPE and π-π stacking interactions were enhanced, quenching the monomer emission.25 Finally the aggregate emission at 470 nm was appeared in THF/H2O (1:9 v/v). The above results indicated that the POSS-TPE is a very rare AIE compound, which not only displayed an AIE effect but also exhibited monomer emission. Compared to the solutions of TPE-NH2 containing the same molar content of TPE, the 6-fold enhancement of aggregate emission and the 9-fold enhancement of monomer emission were observed when POSS-TPE dissolved separately in THF/H2O (1:9 v/v) at 1 M and 100% THF at 0.1 M (Figure S5,S6). And the fluorescence quantum yield ( f) of POSS-TPE in THF/H2O (1:9 v/v) solution is 26.6%, which was 1.8-fold to the TPE-NH2's due to the molecular structure rigidity of POSS core26 and the effect of hydrogen bond interactions. In comparison with TPE-NH2, the POSS-TPE displays more excellent aggregate emission, monomer emission and quantum yields in the aggregate state. The above results indicate POSS-TPE can form possible small multimers and aggregation structures in different solvents at different
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concentration thanks to the perfect self-assembly property introduced by POSS core and hydrogen bond interactions. These especial self-assembly structures further restrict the intramolecular rotations of phenyl rings in TPE or multi-TPE terminal groups, forcefully block the non-radiative path and activate radiative decay. Consequently, POSS-TPE shows excellent emission properties at an extremely low concentration because of its special molecular structure.
Figure 1. The PL spectra of POSS-TPE with different H2O fractions in THF, λex = 335 nm, ex/em slit widths = 5/5 nm. (a) [C] = 1 M and (b) [C] = 0.1 M.
Self-assembly Properties: The self-assembly structures of POSS-TPE solvents in 100% THF and THF/H2O (1:9 v/v) at different concentration were observed to study the self-assembly behaviors. All solid samples were prepared by freeze-drying the original solutions. The POSS-TPE in pure THF at 0.1 M produced oval multimers with a width of about 50 nm, which were found a tendency to form a single nanofiber (Figure 2a). But when the concentration of POSS-TPE in pure THF increased to 1 M (Figure 2b), the individual nanofibres with tens of nanometers in diameter combined
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together to form the fiber bundles whose diameter was about 150 nm. A large number of these fiber bundles were also observed by SEM as showed in Figure S7a. Increasing water fractions to 90%, the self-assembly structures of POSS-TPE at 1 M were totally different. It formed nano-strips structures about 317 nm in width and 137 nm in thickness according to statistical results (Figure 2c, S8b). The above two kinds of different self-assembly methods of POSS-TPE in pure THF and THF/H2O (1:9 v/v) at 1 M were completely consistent with the self-assembly morphologies of the organic/inorganic hybrid POSS-based dendrimer gelator as reported before.14 Thus the results show three quite different self-assembly models of POSS-TPE in different solvents at different concentration as shown in the schematic representation (Figure 2d). When POSS-TPE dissolves in pure THF at 0.1 M, the slightly self-assembled small multimers restrict the intramolecular rotation of phenyl rings in TPE and block the non-radiative annihilation process causing the stronger monomer emission.25 But the monomer emission is quenched due to the aggregation with increasing of water fraction ratio or concentration. Until the H2O content reached 90% v, POSS-TPE forms regular self-assembled nano-strips and causes the aggregation-induced restriction of the intramolecular rotation of TPE units, which effectively enhance the aggregate emission.
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Figure 2. TEM images for the solid formed by POSS-TPE molecules in pure THF solutions, (a) [C] = 0.1uM, (b) [C] = 1 uM, and (c) in THF/H2O = 1/9 (v/v) , [C] =1 uM. The scale bar is 500 nm. (d) Schematic representation of self-assembly ways of POSS-TPE in different solvents at different concentration.
Detect Synthetic Drugs: The intriguing AIE effect of POSS-TPE prompts us to explore its potential application as a chemosensor for synthetic drugs. N-benzylisopropylamine and N-(4-chlorophenyl) cyclohexyl carboxamide are the commercially available isomers of methamphetamine and ketamine and employed as model molecules to do simulation experiments, which results are validated by detecting methamphetamine and ketamine in artificial saliva. The PL intensity of POSS-TPE in THF/H2O (1:9 v/v) at 1 M was progressively weakened as an increasing amount of N-benzylisopropylamine was added into solution. When the
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N-benzylisopropylamine concentration is increased to 1.25 mM, virtually no light was emitted and the Stern–Volmer plot of I0/I - 1 of POSS-TPE with the addition of N-benzylisopropylamine were shown in Figure S9 to further quantify the quenching efficiency. But the quenching concentration of N-(4-chlorophenyl) cyclohexyl carboxamide decreased to 0.15 mM (Figure S10) because carbonyl formed more hydrogen bond interactions and cyclohexyl had good hydrophobicity, both causing a closer combination with POSS-TPE and higher quenching sensitivity. Then we detected methamphetamine and ketamine by using artificial saliva as the H2O component and the results were exactly the same as simulation experiments', which were clearly observed with naked eyes as shown in Figure 3d. The unique phenomenon was attributed to that the small drug molecules enter the 3-dimensional self-assembled structures of the aggregates through non-covalent interaction forces and offered more diffusion channels for the excitons to migrate, allowing aggregates to be non-radiatively annihilated by the drug quenchers.6 This process was shown briefly in the schematic representation (Figure 2d). Besides, we detected the hippuric acid which was the human metabolite of methamphetamine following the same method in artificial urine (Figure S11). Although hippuric acid could form hydrogen bond interactions, there were hydrophilic carboxyl groups resulting in the quenching concentration was 1 mM. Thus, quenching aggregation emission of POSS-TPE is more sensitive to ketamine than methamphetamine, which help we infer the selectivities of POSS-TPE as a novel AIE chemosensor toward to aromatic molecules.
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Depending on the above experimental results, the aromatic molecules that can form hydrogen bond interactions and contain hydrophobic functional groups have lower quenching concentration and higher selectivity because of a stronger capacity to assemble with the aggregates of POSS-TPE.
Figure 3. The molecular structures of (a) methamphetamine and (b) ketamine. The PL spectra of POSS-TPE in THF/artificial saliva=1/9 (v/v) with the addition of (c) methamphetamine and (b) ketamine, λex =335 nm, [C] = 1 M. Insets: images showing the fluorescence change of POSS-TPE as AIE chemosensor in response to ketamine (0 M and 1.5×10-4 M).
Conclusion In summary, an eight tetraphenylethene modified POSS core dendrimer with AIE effect was synthesized via an amide condensation reaction. The TPE-modified nano-sized POSS dendrimer can enhance monomer emission, aggregate emission and quantum yields in the aggregate state thanks to the different self-assembly methods in different solvents at different concentration. These results are helpful to develop new AIE molecules which improve the AIEgens' photophysical behaviors by
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supramolecular self-assembly and study the relation of AIE mechanism and self-assembly mechanism of POSS dendrimer. In addition, the POSS-TPE as a new widely applicable AIE chemosensor for aromatic molecules by non-covalent interaction forces can on-site fast detect methamphetamine and ketamine in artificial saliva through quenching aggregate emission, which is crucial to punish drug crime and maintain social security. Therefore, more potential applications of this AIE chemosensor will be explored in our future work.
†Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI:
Acknowledgment Financial support from the National Natural Science Foundation of China (Grant No: 51773180) are gratefully acknowledged. Thanks Gan Chen for making pictures for this paper.
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