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pH-Responsive Nanovesicles with Enhanced Emission Co-Assembled by Ag(I) Nanoclusters and Polyethyleneimine as a Superior Sensor for Al

3+

Jinglin Shen, Zhi Wang, Di Sun, Congxin Xia, Shiling Yuan, Panpan Sun, and Xia Xin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b16316 • Publication Date (Web): 10 Jan 2018 Downloaded from http://pubs.acs.org on January 10, 2018

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ACS Applied Materials & Interfaces

pH-Responsive Nanovesicles with Enhanced Emission CoAssembled by Ag(I) Nanoclusters and Polyethyleneimine as a Superior Sensor for Al3+ Jinglin Shen a, Zhi Wang a, Di Sun a *, Congxin Xia b, Shiling Yuan a *, Panpan Sun b, Xia Xin a, b * a

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

b

National Engineering Technology Research Center for Colloidal Materials, Shandong University, Shanda Nanlu No. 27, Jinan, 250100, P. R. China

*

Author to whom correspondence should be addressed, E-mail: [email protected]

Phone: +86-531-88365896. Fax: +86-531-88564750 *


Phone: +86-531-88364218. Fax: +86-531-88364216 *


Phone: +86-531-88363597. Fax: +86-531-88361008

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Abstract Metal nanoclusters (NCs) have been engineered as a new kind of luminescent material. Whereas, the application of metal NCs in aqueous solution was subjected to great limitations owing to their poor solubility, stability and strong luminescence quenching in a single molecule state. Herein, facile supramolecular self-assembly strategy was carried out to enhance the luminescence of Ag(I) nanoclusters (Ag6-NCs) through multiple electrostatic interactions with polyethyleneimine (PEI). Functional colloid aggregates of Ag6-NCs such as nanospheres and nanovesicles were formed along with the enhanced emission due to the formation of compact ordered self-assemblies, which effectively restrict intramolecular vibration (RIV) of the capping ligands on Ag6-NCs to diminish the non-radiative decay. All those could block energy loss and facilitated the radiative relaxation of excited states which ultimately induced an aggregation induced emission (AIE) phenomenon. Furthermore, the luminescent Ag6-NCs/PEI nanovesicles are pH-responsive and show a superior fluorescent sensing behavior for the detection of Al3+ with a limit of detection (LOD) low to 3 µM. This is the first report about AIE of silver NCs with polymer in aqueous solution. This work sheds light on the controlled NCs-based supramolecular self-assembly and the NCs-based functional materials, which will be well-established candidates in controllable drug delivery, biomarker and sensors in aqueous solution. Keywords: Silver (I) nanocluster, self-assembly, AIE, nanovesicle, sensor.


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1. Introduction Atomically precise metal nanoclusters (NCs, such as gold, silver, and copper NCs) have attracted significant attention in the past decades due to their structural aesthetics and excellent physicochemical properties, which have endowed them various applications in the fields of catalysis, chemical sensor, bioscience, optical device and environmental sciences.

1-5

Among abovementioned

properties, luminescence represents one of the most attractive features of these materials. In spite of their intriguing functionality, practical NCs-based materials are still quite scarce because of their poor processability in the solid state and weakened functionality in solution where NCs-based materials usually display luminescence quenching properties in single molecule state. 6-10 Inspired by the aggregation induced emission (AIE) phenomenon that weakly luminescent chromogens emit efficiently during the process of aggregation,

11-13

the AIE methodology has emerged as one of the

most effective methods to lighten the luminescence of metal NCs materials, which provides a good platform for researchers to look into light-emitting processes from luminogen matrixes.

13-17

For

example, Zhu et al. exploited a novel AIE strategy to render non-luminescent Cu(I) complexes strongly luminescent by introducing neutral gold (0) species. The resulting products (Au2Cu6 NCs) showed strong emission at 665 nm with a high quantum yield of 11.7%. 14 Wang et al. developed a facile “green” synthesis route towards to the stable Cu NCs with a quantum yield of 16.6%. In their system, the Cu NCs are non-emissive in solution state, but emissive by adjusting the pH of system with the aggregates of Cu NCs. Thus, the attractive AIE feature allowed the Cu NCs to serve as pH stimuli-responsive functional materials such as biosensing and catalysis applications.

16

Xie et al.

explored methods inducing metal NCs self-assembly by electrostatic interaction with opposite charged polymer or surfactant. 18, 19 In spite of above advances, a detailed investigation of the morphology of aggregatesluminescence relationship is still rare due to the lack of suitable metal NCs . It is well-known that self-assembly of NCs into desirable supramolecular architecture requires the solubility in special



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solvent as well as the enough stability in that solvent, however, only very few metal NCs can concurrently satisfy the above conditions Thus, hitherto, it is a pity that no reports on boosting metal NCs luminescence through building supramolecular amphiphiles by noncovalent interaction which can control dynamic switching of structures (including vesicles, micelles, liquid crystal and so on) towards external stimuli.

20-24

Especially, vesicles, similar to the phospholipid bilayer in biological

system, are usually used for drug delivery, nanoreactors, gene-transport, and biological functional models.

25-28

Thus, NCs-based self-assemblies with enhanced luminescent property will provide a

versatile performance and satisfy the urgent demands in bioscience. Herein, positively charged polyethyleneimine (PEI) was employed to control self-assembly of (NH4)6[Ag6(mna)6] (H2mna = 2-mercaptonicotinic acid, Ag6-NCs). Diverse Ag6-NCs/PEI supramolecular architectures (nanospheres and nanovesicles) were obtained and the luminescence was also boosted during the formation of Ag6-NCs/PEI aggregates. Besides, the luminescent nanovesicles exhibit pH-responsive behavior that with the addition of HCl, the protonation degree of PEI was increased, enhancing the hydrophilcity, which further break the structure of nanovesicles along with luminescent quenching. In contrast, with the addition of NaOH, luminescent nanovesicles were recovered. At the same time, the luminescent nanovesicles could selectively detected Al3+ with a limit of detection (LOD) of 3 µM. The presented NCs-polymer self-assembly system, together with the versatile architecture and AIE property opens a new avenue for designing NCs-based advanced functional materials in aqueous solution. 2. Experiment section 2.1 Materials. Branched-polyethyleneimine

and

pentaethyle

were

purchased

from

Sigma

Aldrich.

Polyethyleneimine was named as different molecular weights that PEI-1 (Mw=750000), PEI-2 (Mw=250000), PEI-3 (Mw=2000), PEI-4 (pentaethyle, Mw=200). NaNO3, Ba(NO3)2, Zn(NO3)2, Pb(NO3)2, Al(NO3)3, Ni(NO3)2, Mg(NO3)2, Co(NO3)2, Cu(NO3)2, and Fe(NO3)3 were purchased from


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Sinopharm Chemical Reagent Co. All the solvents were used without further purification. Ultra-pure water used in the experiments was triply distilled by a quartz water purification system. 2.2 Synthesis of Ag6-NCs: the Ag6-NCs was synthesized according to our published literature.

29

The molecular structure of Ag6-NCs is shown in Figure 1a and S1. 2.3 pH-responsibility of nanovesicles: 40 µL HCl (pH = 2) and 40 µL NaOH (pH = 12) was added into 2mL nanovesicles solution (cAg6-NCs/cPEI-1 = 50 µM/0.055 µM) alternately to study the pHresponsivity of nanovesicles. 2.4 Detection of Al3+: 50 µL metal salts (cmetal = 2 mM) was added into 2 mL nanovesicles solution ( cAg6-NCs/cPEI-1 = 50 µM/0.055 µM) to study the detection for metal ion. 50 µL Al3+ salts (cmetal= 2 mM) was added into 2 mL nanovesicles solution (cAg6-NCs/cPEI-1 = 50 µM/0.055 µM) containing other kinds of metal ions (V = 50 µL, cmetal = 2 mM) to study the selectivity of nanovesicles toward Al3+ ions. 2.5 Methods and characterizations. A drop of solution was putted on a transmission electron microscopy (TEM) grid and dried for 30mins with infrared lamp. Then the samples were observed on a JEOL JEM-100 CXII TEM under an accelerating voltage of 100 kV. The high-resolution transmission electron microscopy (HRTEM) images were obtained on a HRTEM JEOL 2100 under 200 kV. Field emission scanning electron microscopy (FE-SEM) observations were carried out on Hitachi SU8010 under 10 kV. Sample was placed on a silica wafer and The wafers were dried for 30mis with an infrared lamp. For Atomic force microscopy (AFM) observations, a drop of solution was putted on a silica wafer and dried with an infrared lamp for 1 hours. And the samples was operated on Dimension Icon (American) with scan asyst. Confocal laser scan microscopy (CLSM) observations were performed using an inverted microscope (model IX81, Olympus, Tokyo, Japan) equipped with a high-numerical-aperture 60 oilimmersed objective lens.



The fluorescence measurements were measured on a LS-55 spectrofluorometer (PerkinElmer, Waltham, MA, USA). UV−vis spectra was recorded on a Hitachi UV−vis 4100 spectrophotometer. X-Ray photoelectron spectroscopy (XPS) was operated on ESCALAB 250 X-ray photoelectron spectrometer with a monochromatized Al Ka X-ray source (1486.71 eV). The sample was freezedried to obtain the solid powder for test. The size and zeta potential of samples were collected on Malvern Zetasizer Nano ZS ZEN3600. 3. Results and discussion 3.1 Phase Behavior of Ag6-NCs/PEI. Firstly, from the HRTEM observations, it can be seen that Ag6-NCs are well dissolved in water with a diameter about 1.2±0.2 nm meaning Ag6-NCs does not capable of self-assembling into nanostructures in water (Figure 1b). The size is smaller than that gotten from X-ray single-crystal diffraction data (1.5 nm), which attributes to that the contrast of ligand is low and can’t be observed by HRTEM (Figure S1, S2, Table S1-S3). Then, different molecular weights of oppositely-charged PEI were chosen to induce Ag6-NCs aggregate. The concentration of Ag6-NCs is fixed at 50 µmol L1

, while that of PEI (cPEI) is gradually increased. Photos of typical samples are given in Figure 1c and

1d and the phase behavior of Ag6-NCs/PEI mixed systems as a function of cPEI is shown in Figure 1e. It was found that PEI-1 and PEI-2 with higher molecular weights share the same phase sequence with increasing cPEI that the clear, isotropic Ag6-NCs aqueous solution gradually turns turbid and the precipitates formed finally. When cPEI is further increased, the precipitates disappeared, and the turbid solutions re-formed. After that, the turbidity of the solution decreases continuously and finally the solution becomes clear again. The rich phase behaviors mentioned above are probably induced by the electrostatic attraction between the oppositely-charged Ag6-NCs and PEI. However, differences were observed for PEI-3 and PEI-4 with lower molecular weights that no stable turbid area was formed, only clear solution or precipitate occurred (Figure 1e and S5). This phase behavior


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indicated that only PEI with higher molecular weights could successfully stabilize Ag6-NCs in aqueous solution owing to the better flexibility and hydrophility of Ag6-NCs/PEI.

Figure 1. (a) X-ray structure of Ag6-NCs by removing the H atoms and NH4+ for clarity. (b) HRTEM result of Ag6-NCs (c = 50 µM). (c) Typical photos of samples with different cPEI (cPEI-1 = 0.001, 0.005, 0.02, 0.025, 0.03, 0.04, 0.045, 0.05, 0.055, 0.06 µM). (d) Typical photos of samples with different cPEI (cPEI-2 = 0.3, 0.6, 0.7, 0.8, 0.9, 1.2, 1.3, 1.4, 0.5, 1.6 µM). (e) Phase behaviors of Ag6NCs/PEI system as a function of cPEI at 50 µM Ag6-NCs for different molecular weight of PEI. Filled stars: transparent solutions. Open circles: turbid solutions. (left lower concentration: Tur-I, right higher concentration: Tur-II) Filled rhombus: precipitates. The turbidity of solutions at λ = 500 nm as a function of cPEI was shown in Figure S3 and S4 and turbid samples with the Tyndall effect indicated the formation of supramolecular aggregates. 31

30,

Electron microscopical studies were performed to study the morphologies of the self-assemblies in

Tur-II region. The sample composed of cAg6-NCs/cPEI-1 = 50 µM/0.055 µΜ was chosen as a typical example. From SEM and TEM, discrete spheres can be observed and the diameters of spherical assemblies were polydisperse, varying from 70 to 500 nm (Figure 2a and 2b). The enlarged TEM image in Figure 2b suggested that the spherical assemblies were nanovesicles in nature: a clear boundary between interior part and the membrane was exhibited (Figure S6). The strong electronic



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contrast of nanovesicles is originated from the electron-dense Ag6-NCs as shown in HRTEM (Figure 2c). In AFM (Figure 2e), the nanovesicle was 30 nm in height and 250 nm in diameter which indicated that the nanovesicles in dried state have a high aspect ratio of ca. 8 with an oblate shape. All above results further confirmed the spherical aggregates is hollow nanovesicles.

32, 33

The DLS

result revealed that the average size of vesicle is 190 nm, which is consistent with the results observed by electron microscope. Similarly, the morphologies of the solution composed of cAg6NCs/cPEI-1

= 50 µM/0.06 µM and cAg6-NCs/cPEI-2 = 50 µM/1.5 µM are also nanovesicles with the

diameters in the range of 100~250 nm (Figure 2d and 2f). The energy-dispersive X-ray (EDX) spectra (Figure S7) and SEM mapping gives the elements of O, C, N, Ag and S for all of these nanovesicles (Figure 2g), confirming the successful hybridation of Ag6-NCs into nanovesicles. From the UV-vis absorption spectrum (Figure S8), it can be observed that Ag6-NCs exhibited a absorption peak at 270 nm due to π−π* transition of ligands. This peak red-shifts to 275 nm with the addition of PEI-1 owing to the electrostatic interaction between –COO- and –NH2, but no characteristic surface plasmon band of larger Ag nanoparticles at around 380–500 nm confirming the stability of Ag6-NCs in colloidal solution.


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Figure 2. (a) SEM, (b) TEM (c) HR-TEM and (e) AFM images of cAg6-NCs/cPEI-1 = 50 µM/0.055 µM vesicle. (d) TEM image of cAg6-NCs/cPEI-1 = 50 µM/0.06 µM. (f) TEM image of cAg6-NCs/cPEI-2 = 50 µM/1.5 µM. (g) SEM element mapping analyses of nanovesicle. The scale bar for all the magnified images is the same, which is 100 nm. How about the self-assembly behavior in Tur-I region? Thus, the morphologies of the turbid samples in Tur-I region were also studied. We take the sample (cAg6-NCs/cPEI-1 = 50 µM/0.02 µM) as an example to below studies. In SEM and TEM, spheroidal structures with 50~80 nm adhered to each other closely were observed (Figure 3a, 3c). In AFM analyses, the height is 58 nm and the diameter is 80 nm which means that the ratio of height to diameter is close to 1, indicating solid nanospheres were formed in this concentration ratio (Figure 3d). In order to study the stability of Ag6-NCs in the mixed system, EDX and XPS measurement were carried out to analyze the surface compositions and chemical states of Ag6-NCs/PEI-1 (Figure 3e and S9). EDX spectra confirm the successful hybridation of Ag6-NCs into nanospheres and as observed from the high-resolution XPS spectrum, in the case of Ag 3d spectra, the strong peaks at 373.5 and 367.5eV corresponding to Ag 3d3/2 and Ag



3d5/2 are the characteristics of Ag+. 34, 35 This is similar to the spectra of native Ag6-NCs , suggesting the Ag6-NCs is stable in the colloidal solution (Figure S10). The schematic model of the formation of nanospheres and nanovesicles for Ag6-NCs/PEI system with the increasing of cPEI-1 or cPEI-2 was shown in Scheme 1.

Figure 3 (a) TEM (b) HRTEM (c) SEM (d) AFM images and (e) XPS spectra of cAg6-NCs/cPEI-1 = 50 µM/0.02 µM nanospheres. (f) zeta potential of aqueous solution of with the increasing of cPEI-1 at cAg6-NCs = 50 µM.


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Scheme 1. Schematic model of the formation of nanospheres and nanovesicles for Ag6-NCs/PEI system with the increasing of cPEI-1 or cPEI-2 In order to further detect the evolution of the aggregation behavior and molecular configuration of Ag6-NCs/PEI-1 system, zeta potential (ξ) experiment was done. As shown in Figure 3f, upon addition of the cationic PEI-1, a continuous increase of ξ is observed (turbid occurs) and reached to 0 mV (precipitates occurs) and finally, ξ changes to become positive (precipitates re-dissolves). The ξ of nanovesicles (cAg6-NCs/cPEI-1 = 50 µM/0.055 µM) and nanospheres (cAg6-NCs/cPEI-1 = 50 µM/0.02 µM) are 44.5 mV and -22.2 mV, respectively. In nanospheres structure, the negative ξ indicates the Ag6NCs were located in the shell of nanospheres. While in nanovesicles structure, the ξ is positive, indicating PEI-1 settled in the outside of bilayer vesicles. The formation of different self-assembled morphologies with the increasing of cPEI-1 can be reasoned as follow. First, the structure of Ag6-NCs is similar to particles, the surface free energy of them is high, thus the nanospheres with Ag6-NCs at the shell have a high free energy. The nanospheres adhere to each other to deminish free energy (Figure 3c, 3d). In another way, to minimize the free energy of nanospheres, the equilibrium spherical morphologies will favor larger aggregates to reduce charge density.

36

Therefore,

nanospheres are pushed to transfer to nanovesicles with increasing the PEI-1 concentration. The morphology of precipitates also confirmed the assumption (Figure S11). Second, the pH value of system increased with the increasing of cPEI-1, which restricted the degree of ionization of the amino



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groups. The number of Ag6-NCs molecules bounded to per PEI-1 by electrostatic interaction is decreased and the arrangement of Ag6-NCs is changed from dense to looser. The packing parameter (P), estimated by P=V/al (where a is the mean effective surface area of head group area, V is the volume of the hydrophobic chain, and l is the maximum effective length of hydrophobic chain), often was used to predict the microstructure of aggregation that spherical structures are formed at P

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