Fabrication of Biocompatible, Luminescent Supramolecular Structures

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Fabrication of Biocompatible, Luminescent Supramolecular Structures and Their Application in the Detection of Dopamine Yongxian Guo, Jie Lu, Qi Kang, Ming Fang, and Li Yu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01548 • Publication Date (Web): 12 Jul 2018 Downloaded from http://pubs.acs.org on July 12, 2018

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Fabrication of Biocompatible, Luminescent Supramolecular Structures and Their Applications in the Detection of Dopamine b

c

Yongxian Guo a, 1, Jie Lu a, 1, Qi Kang , Ming Fang , and Li Yu a*

a

Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, P.R. China

b

College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University,

Jinan 250014, P. R. China

c

Department of Chemistry, University of Houston, Houston, 77204, United States

Corresponding author: Prof. Dr. Li Yu Phone number: +86-531-88364807 Fax number: +86-531-88564750 E-mail address: [email protected] 1

Yongxian Guo and Jie Lu contributed equally.

ACS Paragon Plus Environment

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Abstract Supramolecular

materials

N-dodecyl-N'-acetamido

assembled

imidazolium

by

amide-functionalized

bromide

([C12ImCONH2]Br),

SAIL, and

europium-containing polyoxometalates (Eu-POM) were fabricated in aqueous solution by a one-step method via ionic self-assembly (ISA) strategy. The [C12ImCONH2]Br/Eu-POM supramolecular structures exhibit favorable fluorescence properties and represent a 15-fold increase in quantum yield (~13.68 %) compared to Eu-POM. Besides, more fluorescence was quenched obviously with the increasing concentration of dopamine (DA) (within the range of 0~100 µM), based on which DA monitoring could be achieved. The detection limit was identified to be 0.1 µM. The supramolecular nanoparticles is highly specific for the detection of DA. In addition, the hybrid assemblies display not only low cytotoxicity but also excellent biocompatibility to MC3T3-E1 cells. As a result, as-prepared supramolecular materials with these superior properties show the promising application in some fields


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such as biochemistry and biomedical science.

Keywords Ionic self-assembly; Surfactant; Polyoxometalates; Supramolecular nanomaterial； Fluorescence quenching


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Introduction Polyoxometalates (POMs), a class of transition metal oxide clusters, are generally utilized as building blocks to construct nano- or micro- materials. The studies of POMs are of particular interest owing to their relevance in a wide variety of technological functionalities, such as catalysis,1-2 electrochemistry,3-4 biochemistry, materials science,5-6 etc. Thereinto, lanthanide-containing POMs materials display excellent features, such as catalysis, sorption and photoluminescence (long lifetime, tunable emission and narrow emission bands).7-10 However, POMs materials’ applications suffer from the superior water-solubility and toxic side effect on normal cells. For instance, Na9EuW10O3632H2O as one kind of Eu-containing POMs, shows very weak luminescence in aqueous solution because of fluorescence quenching effect induced by water molecules. Therefore, the employment of a biocompatibility organic molecule

to

encapsulate

lanthanide-containing

POMs

with

presenting

a

supramolecular structures is of critical importance for the practical functionality of POMs in materials science and biochemistry.

In recent years, there are two ways to combine organic molecules and POMs, namely via covalent and noncovalent bonds. By contrast, the second way, which supports user-friendly synthetic process and resources conservation, confers several advantages over the first way. Ionic self-assembly (ISA) strategy, as a facile synthetic method based on noncovalent bonds, especially electrostatic interaction, has received numerous attention in terms of its generalizability, simplicity and cheapness.


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Recent studies have reported that the Eu-POM-based hybrids developed via ISA strategy and their applications in the diagnostic and therapeutic fields for tumor, Alzheimer and other diseases.11-12 Wu et al. utilized arginine/lysine-rich peptide of human papillomavirus (HPV L1) and an Eu-containing POM (EuW10) to design and synthesize the hybrid nanospheres via self-assembly approach. The hybrid nanospheres present an identical fluorescence spectral characteristic and outstanding capability of targeted detection for HPV.13 In addition, Yao’s group investigated the luminescence properties of complex composed of Eu-containing decatungstate [EuW10O36]9- and human serum albumin (HSA), which could be used as luminescence labeling agents for some diseases.14 However, studies about hybrids assembled by biocompatibility organic molecule and Eu-POM, which are essential for the application of POMs, are still lacking.

Herein, we utilized an amide-functionalized surface active ionic liquid (SAIL), N-dodecyl-N'-acetamido europium-containing

imidazolium polyoxometalates

bromide

([C12ImCONH2]Br),

(Eu-POM)

to

build

and the

[C12ImCONH2]Br/Eu-POM supramolecular nanostructures in aqueous solution via ISA approach and applied them to DA detection (Scheme 1).15,16 The as-prepared hybrids exhibit excellent photoluminescent properties and favorable biocompatibility. Interestingly, the [C12ImCONH2]Br/Eu-POM supramolecular structures demonstrates highly sensitive fluorescence response to the variation of DA concentration in solution. It is concluded that the lower detection limit for DA is 0.1 µM. Therefore, this work enables the implementation of a simple, slight and label-free detection assay for the


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DA detection as a promising tool for the diagnosis of Huntington’s diseases and Parkinson’s diseases, etc.

Scheme 1. Schematic diagram of the chemical structures of Eu-POM and [C12ImCONH2]Br, the assembling mechanism of [C12ImCONH2]Br/Eu-POM supramolecular materials and the working process for the DA detection mechanism. (The Eu-POM structure and chemical structure of [C12ImCONH2]Br molecule obtained by Gaussian software in B3LYP/6-31G (d, p) level and schematic drawing of self-assembly pattern for the hybrids.)

Experimental Section

Materials Europium

nitrate

hexahydrate

(99%),

sodium

tungstate


dehydrate

(99%),

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L-Phenylalanine (99%), L-Serine (99%) and L-Histidine (98%) were obtained from J&K Chemical Technology, China. Imidazole (99%), bromododecane (98%), bromocelamide (98%), dopamine hydrochloride (98%) and L-Alanine (99%) were purchased from Aladdin Chemistry Co., Ltd. of China. L-Cysteine (99%) were obtained from Shanghai Shifeng Biological Technology Co., Ltd. of China. Nor-epinephrine (98%) was purchased from Co., Ltd. of China. Epinephrine (98%) was bought from Shanghai Macklin. Serotonin (98%) was obtained from Sigma-Aldrich. Glycine was bought from Tianjin Bodi Chemical Reagent Company of China. Acetylcholine was purchased from TCI. Acrylonitrile, calcium chloride, potassium chloride, magnesium chloride, sodium chloride, methanol, ethanol, isopropanol, sodium hydroxide, chloroform and glacial acetic acid were obtained from Sinopharm Chemical Reagent Co., Ltd. All the reagents were used as received without any further purification. Triply distilled water (specific conductivity approximately 1.2 mS·cm-1) were used throughout the experimental work.

Synthesis of Eu-POM and [C12ImCONH2]Br Eu-POM was synthesized according to the reported literature.17

The synthetic method of [C12ImCONH2]Br is similar with that of [N-C12, N'-COOH-Im]Br reported previously,18-20 and the detailed steps are as follows:

(i) Imidazole (0.15 mol), acrylonitrile (0.24 mol) and methanol (16 mL) were added into a flask. The mixture was stirred at 55 °C for 8 h. The resultant solution was evaporated to remove the residual methanol and acrylonitrile.


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(ii) The product of the first step was mixed with 0.12 mol bromododecane and 20 mL isopropyl alcohol under stirring for 24 h at 60 °C. (iii) The resultant solution was mixed with 60 mL NaOH solution (m/m: 15%) and 35 mL chloroform. After stirring for 3 h at room temperature, the solution was used for extraction. Repeated this step at least 8 times and chloroform was evaporated completely.

(iv) The solid obtained by evaporation was dissolved in 100 mL acetonitrile, and 0.1 mol bromoacetamide was added into the mixing solution, which was stirred for 12 h at 78 °C. There were some precipitates generated with prolonging the reaction time. The crude product was obtained by filtration, and then washed with acetonitrile. The final product was collected after recrystallization and drying in vacuum atmosphere.

Preparation of [C12ImCONH2]Br/Eu-POM Supramolecular Materials The [C12ImCONH2]Br/Eu-POM supramolecular structures were prepared in aqueous solution by ionic self-assembly strategy. Both 9 mL [C12ImCONH2]Br solution (0.5 mM) and 1 mL Eu-POM solution (0.5 mM) were added into a 50 mL beaker under stirring for 5 min. The mixed solution became turbid. The solids of supramolecular structures were obtained by filtrating the turbid solution with filterable membrane (220 µm). Then the solids were washed with water three times and dried in vacuum atmosphere for 24 h.

Characterizations of the Supramolecular Materials


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For the TEM measurements, samples were prepared as follows: dropping colloidal solution onto copper grids with carbon films, siphoning off the residual solution with filter paper and drying in air. Then microcosmic structures of the materials were identified by TEM measurements (TEM, JEM-100CX II (JEOL)). 1

H NMR spectroscopy measurements were carried out on a Bruker Avance 300MHz

NMR spectrometer at 25 ℃. [C12ImCONH2]Br was dissolved in deuterated dimethyl sulfoxide (DMSO) with tetramethylsilane as an internal reference.

Fluorescence spectroscopy measurements were conducted with a Hitachi F-4500 fluorospectro photometer. The fluorescence signals (excitation at 280 nm with a Xe lamp) were recorded in the range of 570~670 nm. 5 and 10 nm slits were used for excitation and emission, respectively. The concentration of the supramolecular solution used for the fluorescence experiments was 0.5 mM.

The diameter and zeta potential of the supramolecular nanoparticles were determined on a Laser particle analyzer (Malvern, Zetasizer Nano ZS). (Malvern Instruments). Fourier-transform infrared (FTIR) spectra between 4000 and 400 cm−1 were collected using a VERTEX-70/70v FITR spectrometer (Bruker Optics, Germany) on pressed thin KBr disks of samples.

Small-angle X-ray scattering (SAXS) characterizations were performed using an Anton-paar SAX Sess mc2 system with a Ni-filtered Cu Kα radiation (1.5406 Å), operating at 50 kV and 40 mA.


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Isothermal Titration Calorimetry curve was measured by a Microcal ITC200TM at 25 ℃. There are two cells in the calorimeter, the reference cell filled with water and the sample cell filled with 200 µL Eu-POM solution (0.05 mM), respectively. [C12ImCONH2]Br solution (5 mM, 40 µL) was added into the syringe. During the entire recording process, 20 times injection was produced under stirring with 1000 rpm. Thereinto, the injection volume for the first time is 0.4 µL, and 2 µL for the rest with a period of 120 s. Thermodynamic parameters were resolved by fitting the data via one-site model. Software of Origin 7.0 modified by ITC-200 instrument was used for data processing.

MTT Assay. MC3T3-E1 cells were cultured in 96-well microtiter plates and incubated at 37 ℃ in 5% CO2 for 12 h. After removing the original medium, the cells were individually incubated with various concentrations (5, 10, 25, 50, 100, 200 µM) of supramolecular solutions for 24 h. Next, 200 µL α-MEM culture solution and 20 µL MTT solution were successively added to the sample well. After removing the remaining MTT solution, 150 µL DMSO was added into each well to dissolve the precipitations. The absorbance was measured at 490 nm with the RT 6000 plate reader.

Results and Discussion Characterizations of [C12ImCONH2]Br/Eu-POM Supramolecular Materials Based on ISA strategy, the [C12ImCONH2]Br/Eu-POM supramolecular structures were prepared in aqueous solution by one-step method. After mixing, the clear


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solution of [C12ImCONH2]Br and Eu-POM turned colloidal, and concomitantly Tyndall effect was observed when irradiated by a laser pointer (Figure S1). The TEM image demonstrates that the diameter of formed dispersive nanoparticles is around 200 nm on average (Figure 1a). From the selected area electron diffraction (SAED) pattern (Figure S2), there are several diffuse rings with two concentric rings of maximum intensity, indicating slightly broad spatial distribution of the aggregates and short-range order. The particle size distribution curve (Figure 1b) determined by laser particle analyzer also indicates that the average diameter of particles in colloidal solution is approximately 200 nm, consistent with the TEM result. Figure S3 represents that the zeta potential value of nanoparticles is about -12.2 mV. Hence the favorable stability of [C12ImCONH2]Br/Eu-POM hybrids in aqueous solution can be ascribed to the negatively-charged supramolecular nanoparticles.

Figure 1. The TEM image (a) and laser particle analyzer curve (b) of [C12ImCONH2]Br/Eu-POM hybrid solution.


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Theoretically, Eu-POM would trap nine counterions to form organic/inorganic hybrids. Based on previous studies, it is technically difficult to obtain the exact binding site number in aqueous solution since they are dynamically affected by varying steric effect and other interactions among those trapped cationic surfactants.21 In the present study, Isothermal Titration Calorimetry (ITC) was employed to estimate the binding site

number

and

thermodynamic

parameters

of

interactions

between

[C12ImCONH2]Br and Eu-POM.22 Figure 2 depicts the ITC titration curves of injecting [C12ImCONH2]Br solution into Eu-POM solution. In Figure 2 (top), each peak denotes a single injection. It can be deduced that the self-assembly process is a strong exothermic reaction. With the addition of surfactant solution, the exothermic heat increases dramatically at first, but approaches zero eventually. The integrated data was fitted using a proper model and the corresponding interaction parameters, namely ∆H = -4.82 ± 0.22 kcal/mol, K = (2.67 ± 0.50) × 104 M-1, ∆S = 4.09 cal/(mol deg) and n = 5.95 ± 0.20, were obtained. The calculated binding molecular number (n) is around 6. In other words, each Eu-POM molecule interacts with six [C12ImCONH2]Br molecules on average. Therefore, the formula of the supramolecule can be written as [C12ImCONH2]6Na3[EuW10O36]. Moreover, based on the thermodynamic equation ∆G = ∆H – T∆S, the Gibbs free energy change (∆G) was calculated to be -6.04 ± 0.22 kcal/mol. The results indicate that the formation of [C12ImCONH2]Br/Eu-POM supramolecular hybrids is a process driven by both enthalpy and entropy.13


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Figure 2. ITC data for the interaction between Eu-POM and [C12ImCONH2]Br. Top frame describes the calorimetric process by injecting [C12ImCONH2]Br solution (40 µL, 5 mM, in syringe) into Eu-POM solution (200 µL, 0.05mM, in the cell); bottom frame is the fitted line based on the one-site binding model for the [C12ImCONH2]Br/Eu-POM supramolecular nanostructures.

To further clarify the arrangement of hydrophobic chains in the supramolecular structures and the driving forces during the generation of supramolecular nanoparticles, FTIR measurements were performed. The observation of four peaks


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located at 706, 783, 843 and 945 cm-1 in the FTIR spectra (Figure 3) is attributed to the

characteristic

bands

of

Eu-POM.

For

the

[C12ImCONH2]Br/Eu-POM

supramolecular materials, existence of the peaks at 724, 789, 839 and 937 cm-1 suggests the well-maintaining structural integrity of Eu-POM in the hybrids, only with slight shifts induced by the electrostatic interaction between [C12ImCONH2]Br and Eu-POM. Besides, the bands at 2922 and 2853 cm-1 in the FTIR spectrum of [C12ImCONH2]Br/Eu-POM hybrids were assigned to the asymmetrical stretching mode νas(CH2) and symmetrical stretching mode νs(CH2) of methylene, respectively. These spectral features known as gauche conformation indicate that the hydrophobic chains in the supramolecular structures are not well ordered.23 The stack style of hydrophobic chains hints that hydrophobic interaction plays a vital role in the generation of C12ImCONH2]Br/Eu-POM nanoparticles and supplies a hydrophobic environment for the emission of Eu3+.


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Figure 3. FTIR spectra of Eu-POM and [C12ImCONH2]Br/Eu-POM.

To obtain a comprehensive understanding of internal molecular arrangement in the supramolecular structures, small angel X-ray scattering (SAXS) spectroscopy was conducted. In Figure S4, there are three reflection peaks at 1.70, 3.00 and 3.35 nm-1, respectively. The corresponding q values in the ratios of 1: 3 : 4 , imply the existence of a hexagonal columnar structure. Based on the first scattering peak (q100), the repeated lattice parameter (a0, the distance between the centers of two adjacent columns) calculated by the equation a 0 = 4π

3q100 is about 42.6 Å. Density

functional theory (DFT) calculations were conducted by using the Gaussian 09 package at the level of B3LPY/6-31G(d, p) and the hydrocarbon chain length of [C12ImCONH2]Br was evaluated to be about 15.3 Å (Scheme 1).24,25 It is well known


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that the length of the Eu-POM molecule is about 8 Å. Based on this, it is estimated that the maximum semidiameter of a hexagonal columnar structure is about 38.6 Å, which is higher than half of a0, but less than the a0 value. Hence, the molecular orientation was speculated and illustrated in Scheme 1.

Photoluminescent Property of [C12ImCONH2]Br/Eu-POM Supramolecular Materials The emission spectra of Eu-POM and [C12ImCONH2]Br/Eu-POM solutions were manifested in Figure 5a. The excitation wavelength is 280 nm, associated with the intramolecular energy transfer from the ligand-to-metal charge transfer (LMCT) band of O→W to the photoluminescent Eu3+ core.26 Upon the excitation, the O→W LMCT band induces the hoping of d1 electron, with the following deactivated recombination between d1 electron and hole in the lattice induces energy releasing in a radiation way. Next, the energy transfers from O→W LMCT state to the 5D0 emitting state of Eu3+. Lastly, the electron originating from the 5D0 excited state relaxes to the 7Fj (j=0, 1, 2, 3, 4) ground state and triggers the emission.27 Evidently, the emission property of [C12ImCONH2]Br/Eu-POM solution is more conspicuous than that of Eu-POM solution (Figure 5a). The reason is that the emission of Eu3+ is highly dependent on the coordinated water. Therefore the nonradiative deactivation process of the 5D0 state, through weak coupling with the vibrational states of high-frequency O-H oscillators in water ligands, is absent.17 As a consequence, compared to Eu-POM solution, such enhancement in the fluorescence intensity of [C12ImCONH2]Br/Eu-POM system (illustrated in Figure 4a) could be explained by the formation of hydrophobic


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environment. The cationic surfactant molecules interact with Eu-POM and replace the coordinated water molecules surrounding Eu-POM. Additionally, hydrophobic interaction between the hydrophobic chains of [C12ImCONH2]Br molecules around Eu-POM also prevents water molecules from recombining with Eu-POM.

Figure 4. Fluorescence spectra (a) and time-resolved fluorescence spectra (b) of Eu-POM solution and [C12ImCONH2]Br/Eu-POM hybrids solution.

Moreover, the time-resolved fluorescence spectra (Figure 4b) of Eu-POM and [C12ImCONH2]Br/Eu-POM systems were measured, respectively. Consequently, the quantum yields and fluorescence lifetimes are estimated and listed in Table 1. As seen from Table 1, compared with Eu-POM, the supramolecular structures receive a tremendous improvement in both quantum yield and fluorescence lifetime. The original quantum yield of Eu-POM solution is 0.91 %, whereas it increases by 15-fold to 13.68 % for [C12ImCONH2]Br/Eu-POM system. In addition, the fluorescence


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lifetime of [C12ImCONH2]Br/Eu-POM is around 6 times longer than that of Eu-POM. Based on the results above, the [C12ImCONH2]Br/Eu-POM supramolecular structures possess favorable fluorescence properties, which is promising to be used in some fields, e.g. chemical substance detection and biochemistry, etc.

Table 1 The fluorescence lifetimes and quantum yields of Eu-POM and [C12ImCONH2]Br/Eu-POM aqueous solutions.

τ1 [ms]

τ2 [ms]

Total τ [ms]

Quantum yield η

Eu-POM

0.15

0.20

0.35

0.91 %


0.07

2.09

2.16

13.68 %

In addition, we investigated the stability of the [C12ImCONH2]Br/Eu-POM supramolecular materials. The Tyndall effect of the hybrid solution evolution with time was observed when irradiated by a laser pointer. From the digital photographs of the [C12ImCONH2]Br/Eu-POM (Figure S5), we cannot find obvious change with prolonging the time (the observation time is 10 min, 30 min, 1 h, 5 h and 3 days, respectively.). The fluorescence intensity at 620 nm for the supramolecular materials was observed with time (Figure S6). Obviously, there was no apparent alteration over


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time for the [C12ImCONH2]Br/Eu-POM hybrids. Figure S7 depicts that fluorescence intensity of the hybrids could be almost equally quenched by the same concentration of DA (1 µM) at different time. Hence, the [C12ImCONH2]Br/Eu-POM supramolecular nanostructures in aqueous solution exhibit favorable stability for the detection of DA.

Detection of DA using [C12ImCONH2]Br/Eu-POM Supramolecular Materials Fluorescent materials have attracted enormous scientific and industrial attention in the past decades due to their potential applications in multifarious fields, including sensing and optoelectronic devices.28 Dopamine (DA), a significant neurotransmitter in the central and peripheral nervous system, plays a critical role in the behavioral responses and hormonal functions of human body.29 Normal level of DA in brain is closely related to human health. Superabundant excretion of DA often affects human health and causes a lot of physical problems (e.g. Huntington’s illness), whereas deficiency of DA probably leads to various symptoms, including Parkinson’s illness and schizophrenia.30 Therefore, some detection methods of DA have been reported, mainly electrochemistry and fluorometry assays. Although electrochemistry approach has some advantages, in the meanwhile it suffers from several drawbacks. For example, it depends on the clean electrode which is generally contaminated by proteins in the human body.

Herein, we attempted to identify DA based on the photoluminescent properties of the


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as-prepared [C12ImCONH2]Br/Eu-POM supramolecular structures. Figure 5a depicts that fluorescence intensity of the hybrids at 620 nm could be dramatically quenched by DA under 280 nm excitation. Within the observed concentration range of DA (0~100 µM), higher concentration has a stronger capability to quench the fluorescence of supramolecular solution. The relationship between the changed fluorescence intensity versus the initial intensity ((I0-I)/I) at 620 nm and the concentration of DA, which can be elucidated by the Stern-Volmer equation.34 I0 = 1 + Kqτ 0  DA    I

(1)

where I0 and I are the fluorescence intensities in the absence and presence of the quencher (DA), respectively. Kq is the quenching constant, τ 0 is the lifetime of the [C12ImCONH2]Br/Eu-POM in the absence of DA, which is determined to be 2.64×10-3 s. Obviously, there exists a good linear relationship between (I0-I)/I0 and DA concentration (Figure 5b). Both the regression equation (y= 0.003x+0.01) and coefficient square (R2=0.96) in the linear relationship could be obtained. Kq value was calculated to be 2.14×106 L/mol·s. As reported,35 the quenching constant for dynamic quenching is less than 1.0 × 1011 L/mol·s, therefore fluorescence quenching of [C12ImCONH2]Br/Eu-POM hybrids caused by DA is probable dynamic quenching. The detectable concentration of DA was evaluated to be as low as 0.1 µM, which proves that the [C12ImCONH2]Br/Eu-POM supramolecular structures are sensitive enough to detect DA efficiently. For ease of comparison, the detection limit of the [C12ImCONH2]Br/Eu-POM utilized in this work and luminescent supramolecular


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structures about DA detection reported earlier are summarized together in Table 2. The lowest detection concentration of DA did not research the minimum, but the method of [C12ImCONH2]Br/Eu-POM applied to the detection of DA has some advantages such as briefness and low-cost.

Table 2 Comparison of analytical property for the [C12ImCONH2]Br/Eu-POM in this work and the other DA detective approaches reported previously. Detective type

Detection limit/µM

Reference

DiSC3(5)/LCA

0.4×10-3

36

tyloxapol

2.2

37

CB–Fe2+

10

38

β-CD-AuNC

2×10-3

39


0.1

This work

Previous studies have proven that hydrogen bonding between Eu-POM and other acceptors could block intramolecular luminescent resonant energy transfer between WO6 octahedron and Eu3+ ion and cause fluorescence quenching.9 The time-resolved


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fluorescence spectra of [C12ImCONH2]Br/Eu-POM hybrids solution in the presence of different concentration of DA were determined and shown in Figure 5c. To observe the variation trend clearly, the relation curve of fluorescence lifetime with DA concentration was presented in Figure 5d. The fluorescence lifetime value undergoes an obvious decay when the DA concentration is below 50 µM. After that, a near-plateau region is obtained. This observation implies that at 50 µM, the binding of DA with [C12ImCONH2]Br/Eu-POM hybrids reaches maximum. Based on these results, we speculate that the fluorescence quenching of DA may belong to lifetime-altering dynamic quenching which corresponds to a possible collisional quenching model. This result is in agreement with that of Stern-Volmer equation mentioned above.


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Figure 5. (a) The fluorescence spectra of [C12ImCONH2]Br/Eu-POM supramolecular solution at room temperature upon adding different concentrations of DA (concentration: 0, 0.1, 0.5, 1, 10, 25, 50, 75 and 100 µM; excitation wavelength: 280 nm). (b) The linear relationship between (I0-I)/I0 and DA concentration (0.1-100 µM). Error bar indicates standard deviation (SD, n=3). (c) Time-resolved luminescence decay

lines

and

(d)

decay

trend

of

fluorescence

lifetime

for

[C12ImCONH2]Br/Eu-POM hybrids with different concentration of DA.

Selectivity of [C12ImCONH2]Br/Eu-POM for DA Detection In recent years, detection of different neurotransmitters such as many monoamines by fluorescent methods have been considered as a significant task. These methods contian direct multiphoton excitation,37,38 false neurotransmitters,39 fluorogenic detection,40 and fluorescent tagging.41 Although these tools exhibit satisfactory response to the detectable objects, application of [C12ImCONH2]Br/Eu-POM hybrids to the detection of DA could avoid complex synthesis pathway and only need low doses.

It is of significance that [C12ImCONH2]Br/Eu-POM supramolecular structures for the identification of DA has a highly specificity on account of the similar structures among

monoamines.

Hence,

we

measured

the

fluorescence

spectra

of

[C12ImCONH2]Br/Eu-POM supramolecular materials in the presence of relevant substances such as epinephrine, nor-epinephrine and serotonin (Figure S8). For comparison, the result of DA detection is also shown in Figure S8. As can be seen,


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only DA could trigger the remarkable decrease in the fluorescence intensity of [C12ImCONH2]Br/Eu-POM hybrids while other substances were unable to get a dramatical fluorescence quenching effect. The results confirm that the supramolecular materials have favorable selectivity for DA detection in the practical application.

To further elucidate whether the [C12ImCONH2]Br/Eu-POM supramolecular structures could detect DA specifically, the influence of other interfering species were analyzed, such as L-amino acids (L-Cysteine, L-Phenylalanine, L-Alanine, L-Serine, and L-Histidine), neurotransmitters (Glycine and Acetylcholine), and other metabolites (CaCl2, KCl, MgCl2, and NaCl). Figure S9 demonstrates that compared with 0.1 µM DA, 1 mM of these molecules almost do not quench the fluorescence of the

supramolecular

material

solution

at

all,

which

proves

that

[C12ImCONH2]Br/Eu-POM system was able to applied for the selective determination of DA.

MTT Assay

To evaluate the biocompatibility or cytotoxicity of [C12ImCONH2]Br/Eu-POM supramolecular structures constructed in this work, MTT assay with human normal cell MC3T3-E1 was performed. The absorbance of MTT at 490 nm is considered to associated with the activation degree of cell. Therefore, the cellular activity was represented by the ratio of absorbance between treated (incubated with supramolecular structures) and untreated cells. As illustrated in Figure S10, the cell viability was still strong (≥90%) after incubated by different concentrations of


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supramolecular solution, which suggests that the fabricated supramolecular structures exhibit benign biocompatibility or low cytotoxicity.

Conclusions In summary, [C12ImCONH2]Br/Eu-POM supramolecular structures assembled by amide-functionalized SAIL and Eu-POM have been constructed by ionic self-assembly strategy. The estimated binding molecular number (n) is 6. Electrostatic interaction, hydrophobic interaction, and hydrogen bond were identified as the main driving forces during the formation of [C12ImCONH2]Br/Eu-POM nanoarchitectures. The fluorescence property of [C12ImCONH2]Br/Eu-POM hybrid solution was enhanced obviously compared with pure Eu-POM solution. Furthermore, fluorescence quenching of the supramolecular nanoparticles could be evoked immediately upon the addition of DA solution, which could be utilized to detect DA. MTT array shows that the supramolecular materials have favorable biocompatibility to MC3T3-E1 cells. The reported supramolecular materials provide new clues for the detection of DA, which has a broad range of practical interest including the diagnosis of Parkinson’s illness and schizophrenia.

Acknowledgments This work was supported by Natural Science Foundation of China (No. 21373128), Scientific and Technological Projects of Shandong Province of China (No. 2018GSF121024).


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TOC


Fabrication of Biocompatible, Luminescent Supramolecular Structures

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