Quantum size effect of poly(o-phenylenediamine) quantum dots: From controllable fabrication to tunable photoluminescence properties

Quantum size effect of poly(o-phenylenediamine) quantum dots: From controllable fabrication to tunable photoluminescence properties

Synthetic Metals 198 (2014) 142–149 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Qua...

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Synthetic Metals 198 (2014) 142–149

Contents lists available at ScienceDirect

Synthetic Metals journal homepage: www.elsevier.com/locate/synmet

Quantum size effect of poly(o-phenylenediamine) quantum dots: From controllable fabrication to tunable photoluminescence properties Shuai Yan a,1, Siwei Yang a,b, **,1, Lin He a , Caichao Ye c , Xun Song a , Fang Liao a, * a Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, College of Chemistry and Chemical Engineering, China West Normal University, Nanchong 637002, PR China b State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Science, Shanghai 20050, PR China c Key Laboratory of Soft Chemistry and Functional Materials of MOE, School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 20 August 2014 Received in revised form 2 October 2014 Accepted 10 October 2014 Available online xxx

In this paper, poly(o-phenylenediamine) quantum dots (PoQDs) with amorphous structures were synthesized via a simple hydrothermal treatment. The size of PoQDs can be easily controlled by changing the reaction time. All these quantum dots show excellent photoluminescent (PL) performance. The p–p* transition between N and conjugated system in PoQDs increased the quantum yield significantly. Further experiments suggested the existence of quantum size effect in amorphous structures for the first time. The controllable PL performance were closely related to the significant quantum size effect. What’s more, these PoQDs were low cost and have high anti-jamming performance, good stability and application perspective of fluorescent bio-imaging. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Conducting polymer Quantum dots Bio-imaging Photoluminescence Quantum size effect

1. Introduction Quantum dots such as: semiconductor quantum dots (usually composed of elements from groups III–V, II–VI, or IV–VI) [1–4], graphene quantum dots [5–7] and carbon quantum dots [8–16] based on their excellent photoluminescent (PL) properties have attracted growing interest due to their great potential application in biological labeling, bio-imaging, fluorescent probes, photodegradation and optoelectronic device. The high quantum yield and small size (mostly smaller than 10 nm) ensure these quantum dots can enter cells easily [3,7,17–19]. However, the significant cytotoxicity of semiconductor quantum dots should not be neglected before their application to cellular or in vivo study [20–22]. Sun et al. reported the carbon quantum dots with sizedependent PL properties via an alkali-assisted electrochemical fabrication progress [23]. Yang et al. reported a novel method for large-scale preparation of heavy doped carbon quantum dots with tunable PL properties [24]. The heavy doped carbon quantum dots

* Corresponding author. Tel.: +86 817 2568067; fax: +86 817 2568067. ** Corresponding author. Tel.: +86 21 62511070 420; fax: +86 21 62511070 420. E-mail addresses: [email protected] (S. Yang), [email protected] (F. Liao). 1 These authors are co-first authors. http://dx.doi.org/10.1016/j.synthmet.2014.10.014 0379-6779/ ã 2014 Elsevier B.V. All rights reserved.

were just several nano-meters, and improved PL with different emission wavelength, high quantum yield, long lifetime and good photostability. Liu et al. reported that N doped CQDs are label-free and sensitive probes for the selective detection of Cu2+ ions [25]. However, the application of quantum dots in bio-imaging is still restricted because of the more or less shortcoming of them. Thus, developing new PL materials with high stability, good antijamming ability and low cytotoxicity for fluorescent bio-imaging is still a huge challenge [26]. On the other hand, conducting polymers (CPs) such as: polyaniline (PANI), polythiophene (PTH), polyparaphenylene (PPP) and polypyridine (PPY) are widely used in fluorescent probes [27–31], adsorbents [32–43], electrode materials [44–47] and photocatalysis [48–53]. Sun et al. synthesized various CPs nano-particles such as PANI, poly(o-phenylenediamine), [email protected] (m-phenylenediamine), poly(m-phenylenediamine), poly(p-phenylenediamine) and polynaphthylamine which were applied in the fluorescence detection of nucleic acid [44–58]. The results showed that these nano-particles can absorb and quench dye-labeled single-stranded DNA (ssDNA) very effectively. As sensitive fluorescent sensing platforms, these CPs nano-particles played an important role in fluorescence detection [57]. Moreover, we also have demonstrated the extensive application prospect of poly(pphenylenediamine) (PpPD) nano-particles in fluorescence detection [59–64]. The PpPD nanofibers show high-efficiency effects in

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enhancing the PL properties of thiols through the fluorescence resonance energy transfer [63]. 1H NMR study shows that the strong hydrogen bond between poly(p-phenylenediamine) and thiols is the possible media of FRET progress. The theoretical calculated results further conform the obtained data [65]. These reports show that the CPs have favorable property and have great potential application in fluorescent switch or logic gates [65]. In this work, we confirmed the existence of quantum size effect in amorphous structures for the first time. The poly(o-phenylenediamine) quantum dots (PoQDs) with controllable size and tunable PL properties were synthesized via a simple hydrothermal treatment. The size of these quantum dots can be easily controlled by changing the reaction time. The p–p* transition between N and conjugated system in PoQDs increased the PL efficiency significantly. MALDI-TOF-MASS spectra and theoretical calculations proved the tunable PL properties was strongly influenced by the significant quantum size effect. Moreover, these PoQDs with high anti-jamming performance and good stability have application perspective of fluorescent bio-imaging. 2. Materials and methods 2.1. Materials All the chemicals (o-phenylenediamine, (NH4)2S2O8, KCl, NaCl, MgCl2, CaCl2, ZnCl2, Na2SO4, NaNO3, Na2HPO4, NaH2PO4, HCl, NaCl) were purchased from Aladdin. (Shanghai, China) and used as received without further purification. The water used throughout all experiments was purified through a Millipore system. Transmission electron microscopy (TEM) measurements were made on a Hitachi H-8100 EM (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. Fourier transform infrared (FTIR) was performed on a Nicolet 6700 (resolution 0.4 cm1) infrared spectrometer. Samples were dispersed in potassium bromide and compressed into pellets. The UV–vis spectra were obtained on a UV5800 Spectrophotometer. X-ray photoelectron spectra (XPS) were carried out on a PHI Quantera II system (Ulvac-PHI, INC, Japan). MALDI-TOF-MASS spectra were recorded on an ABI Voyager De pro spectrometer. 2.2. Synthesis of PoQDs The typical preparation process of PoQDs (PoQD-1) was as follows: 10.0 mg oPD and 5 mg (NH4)2S2O8 were dissolved in 10.0 mL water and were under ultrasonication for 1 h (200 W, 100 kHz) to obtain homogeneous dispersion system. Then, the mixture was transferred into a 15 mL Teflon-lined autoclave and heated at 180  C for a period of 3 h, and cooling to room temperature naturally. The products were filtered by 0.02 mm microporous membrane and a brown transparent filter solution was contained, and the black precipitates were wasted. The yield was about 40%. The solvent of suspension was removed with the aid of a rotary evaporator. The PoQDs have excellent solubility in many polar organic solvents (such as: DMF, THF, DMSO, CHCl3, CH2Cl2, EtOH, MeOH and acetone) and water with different pH levels (1–14). 2.3. Fluorescence measurements Photoluminescent emission (PLE) and PL spectra were recorded on a PerkinElmer LS55 luminescence spectrometer (PerkinElmer Instruments, U.K.) at room temperature (25  C) in aqueous solution. The slit widths of excitation and emission were 2 nm. The stability of these products was determined via contrast the PL intensity of products aqueous solution under different conservative time at room temperature (25  C). The stability of these

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products was determined via contrast the PL intensity of products aqueous solution under different conservative time at room temperature (25  C). Quantum yield (Ff) was measured according to established procedure (Lakowicz, J.R. rinciples of Fluorescence Spectroscopy, 2nd Ed., 1999, Kluwer Academic/Plenum Publishers, New York). The optical densities were measured by UV–vis spectra which were obtained on a UV5800 Spectrophotometer. Rhodamine 6 G aqueous solution (literature quantum yield 0.98) was chose as a standard. Absolute values are calculated using the standard reference sample that has a fixed and known fluorescence quantum yield value. In order to minimize re-absorption effects, absorbency in the 10 mm fluorescence cuvette was kept under 0.1 at the excitation wavelength. 2.4. Computational procedures Standard DFT were performed using the DMol3 module [65]. All geometries and energies were calculated with the generalized gradient approximation (GGA), using the functional PW91 [66–68] in combination with the double numerical polarized (DNP) basis set. All adsorption geometries were relaxed in three directions of space, based on the geometry optimization criterion (RMS force of 0.002 au/Å and RMS displacement of 0.005 Å). The calculation of total energy and electronic structure was followed by the geometry optimization with SCF tolerance of 1 105 au. A Fermi smearing of 0.005 Hartree. The quality mesh size for numerical integration was chosen. The unrestricted approach was applied that the low-spin state, that is, singlet for systems with even-number electrons and doublet for systems with odd-number electrons. These low-spin states are in general energetically preferable. 3. Results and discussion 3.1. Characterization of PoQDs Typical characterization results of as-prepared PoQDs are shown in Fig. 1. Homogeneous dots with size distribution of 1–2.5 nm were found from the typical TEM image (Fig. 1a). It is noteworthy that the fast Fourier transform (FFT) (Fig. 1a inset) shows on crystalline structure of PoQDs which indicates these quantum dots are amorphous. When the reaction time is longer, the products are larger. Fig 1b–d show the TEM images of PoQDs obtained after 5 h (PoQD-2), 10 h (PoQD-3) and 24 h (PoQD-4). Obviously, the quantum dots are larger when the reaction time was extended. The average size is 2, 3 and 5 nm, respectively (PoQD-2, PoQD-3 and PoQD-4). FFT results show all these quantum dots are amorphous. The FT-IR spectrum of PoQDs is shown in Fig. 2a. It is clear that, the peaks at 758 and 563 cm1, characteristic of C—H out-of-plane bending vibrations of benzene nuclei in the phenazine skeleton [59,61]. The peaks at 902 and 831 cm1 can be due to the out-ofplane deformation of C—H on a 1, 2, 4, 5-tetrasubstituded benzene ring, which implied that the polymers had the basic phenazine skeleton. Moreover, the peaks at 1243 and 1370 cm1 are associated with the C—N stretching in the benzenoid and quinoid imine units [59,63]. The strong peaks at 1602 and 1535 cm1 are assigned to C¼N and C¼C stretching vibrations in phenazine structure, respectively. Moreover, the adsorption peaks at 3321, 3242 and 3203 cm1 corresponding to the N—H stretching mode, and implying the presence of secondary amino groups [64]. XPS data for PoQDs were also investigated. The XPS survey spectrum of PoQDs (Fig. 2b) shows a predominant C 1 s peak at ca. 284 eV, a N 1 s peak at ca. 399 eV. The N/C atomic ratio was 23.1%, which is close to the oPD. The high-resolution C 1 s spectrum (Fig. 2c) confirmed the presence of C—C (284.73 eV), C—N

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Fig. 1. TEM images and corresponding dot size distribution histograms of PoQDs obtained after (a) 3 h (PoQD-1), (b) 5 h (PoQD-2), (c) 10 h (PoQD-3) and (d) 24 h (PoQD-4), inset: the FFT of a single dot.

Fig. 2. (a) FTIR spectra of PoQD-1, (b) XPS survey spectra of PoQD-1. High-resolution XPS spectra of (c) C1s and (d) N1s of PoQD-1.

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Fig. 3. (a) UV–vis absorption spectra of PoQD-1, PoQD-2, PoQD-3 and PoQD-4, (b) PL spectra of PoQD-1, PoQD-2, PoQD-3 and PoQD-4, (c) photograph of PoQD-1, PoQD-2, PoQD-3 and PoQD-4 under visible light, (d) photograph of PoQD-1, PoQD-2, PoQD-3 and PoQD-4 under UV-light (wavelength: 325 nm). The concentrations of all samples are the same (1 106 g L1). (For interpretation of the references to color in text, the reader is referred to the web version of this article.)

(286.09 eV) and C¼N bonds (288.77 eV). What’s more, the highresolution N 1 s spectrum of (Fig. 2d) reveals the presence of amidogen N (398.45 eV) and C—N¼C (400.50 eV) [25,69,70]. 3.2. Optical properties of PoQDs It is clear that, there are no obvious difference in chemical constitution between these PoQDs with different size. However, there are significant differences in optical properties (both UV–vis spectra, and PL spectra). Fig. 3a shows the UV–vis spectra of PoQDs. The UV–vis spectra are red shifted with the increased size. The UV–vis spectrum of PoQD-1 shows tow peaks, the absorption peak at 233 nm indicates the p–p* transition of conjugated system. The absorption peak at 373 nm indicates the p–p* transition between N and conjugated system. The absorption peak of p–p* transition slightly red shifted, Table 1 The peak position of p–p* transition, and p–p* transition in UV–vis spectra, PL excited (lex), emitted (lem) wavelength, Stokes shift (DnSt), quantum yield (Ff), fluorescence lifetime (t ) of fluorescence lifetime of PoQDs with different size. PoQDs

PoQD1 PoQD2 PoQD3 PoQD4 a b

UV–vis

PL

p–p* (nm)

p–p* (nm)

lex

lem

DnSt

(nm)

(nm)

(nm)

233

373

380

430

247

414

430

251

450

460

255

485

500

Ff

t (ns)

50

0.61a

0.97

500

70

0.55a

1.21

545

85

0.43a

1.33

90

b

1.57

590

0.27

Quininesulfate as a standard (Ff = 0.55 in 50 mM H2SO4 solution). Rhodamine 6 G as standard (Ff = 0.95, ethanol).

the peak located at 247, 251 and 255 nm when the size is 2, 3 and 5 nm (PoQD-2, PoQD-3 and PoQD-4), respectively. However, the absorption peak of p–p* transition red shifted obviously, the peak located at 414, 450 and 485 nm when the size is 2, 3 and 5 nm (PoQD-2, PoQD-3 and PoQD-4), respectively. It is clear that, the size of PoQDs have a strong effect on the p–p* transition between N and conjugated system [24,61]. Fig. 3b shows the digital photograph of PoQD-1, PoQD-2, PoQD-3 and PoQD-4 aqueous solution with same

Table 2 Comparison of different semiconductor quantum dots or carbon based “quantum dots” for quantum yield (Ff), fluorescence lifetime (t ) of PoQDs.

PoQD-1 PoQD-2 PoQD-3 PoQD-4 Oxidized CQDs N-CQDs S-CQDs Se-CQDs CQDs CQDs CQDs N-GQDs P-GQDs B-GQDs N-GQDs GQDs Si QD/SiO2 CdS CdS CdS CdSe–CdS a b

Ff

t (ns)

Refs.

0.61a 0.55a 0.43a 0.27b 0.06a 0.39a 0.24a 0.19b 0.12a 0.04–0.1b 0.15b 0.18a 0.117a 0.048a 0.022a 0.0256a 0.031–0.035 0.9 0.5 – 0.19

0.97 1.21 1.33 1.57 1.31 1.79 4.85 3.27 – – – – – – – – – – 1.22 2.4 10.5

This This This This [23] [23] [23] [23] [71] [72] [73] [74] [75] [75] [75] [76] [77] [78] [79] [80] [81]

Quininesulfate as a standard (Ff = 0.55 in 50 mM H2SO4 solution). Rhodamine 6 G as standard (Ff = 0.95, ethanol).

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Fig. 4. (a) PL spectra of PoQD-1 with different excitation wavelengths at room temperature. (b) The relationship between excitation wavelengths and emission wavelengths of PoQD-1, PoQD-2, PoQD-3 and PoQD-4. The concentrations of all samples are the same (1 106 g L1).

mass concentration (2 mg mL1) under visible light. The color of these aqueous solution become more darken with the increased size. The aqueous solution of PoQD-1 is colorless and turn light yellow, yellow and orange when the size is 2, 3 and 5 nm (PoQD-2, PoQD-3 and PoQD-4), respectively. The different color can be due to the absorption center shifts of p–p* transition [24,61]. On the other hand, the PL properties of these PoQDs also show obvious changes when the size was changed. Fig. 3c shows the PL spectra of these PoQDs. The emitted wavelength (lem) depends strongly on the size of PoQDs. When the size of PoQDs is 1 nm (PoQD-1), the lem is 430 nm. Fig. 3d shows the aqueous solution of PoQD-1 emitted intensive blue light under UV irradiation. The optimum excitation condition (lex) of PoQD-1 is 380 nm, which corresponds to the UV–vis peak at 373 nm. The quantum yield (Ff) of PoQDs (Ff = 0.61, Table 1) is higher than most carbon based ‘quantum dots’ (Table 2). The high PL efficiency can be attributed to the p–p* transition between N and conjugated system [61–63,69]. The high Ff of PoQD-1 indicates it can be applied to fluorescent bio-imaging. What’s more, like carbon dots and graphene quantum

dots, the PoQD-1 exhibit a maxima emission wavelength shift of 150 nm when the excitation wavelength increases from 320 to 450 nm (Fig. 4). When the quantum dots is larger, the lem red shifted. The lem is 500, 545 and 590 nm when the size is 2, 3 and 5 nm, respectively. The corresponding digital photograph of these quantum dots shows they emitted luminescent with different color under UV irradiation. The lex of these PoQDs with different size is also corresponds to the p–p* transition in UV–vis peak. Moreover, the Ff of PoQD-2, PoQD-3 and PoQD-4 is 0.55, 0.43 and 0.27, respectively (Table 1). The decrease of Ff is closely related to the stronger self-absorption effect and higher energy loss in larger PoQDs [61]. The increased stokes shift (Table 1) also indicates the stronger self-absorption effect and higher energy loss in larger PoQDs. Furthermore, PoQD-2, PoQD-3 and PoQD-4 also exhibit a maxima emission wavelength shift when the excitation wavelength changed (Fig. 4b). The time-resolved PL curves of PoQD-1, PoQD-2, PoQD-3 and PoQD-4 are well fitted with a single exponential decay, and the

Fig. 5. Typical decay profiles from (a) PoQD-1, (b) PoQD-2, (c) PoQD-3 and (d) PoQD-4 at room temperature. The concentrations of all samples are the same (1 106 g L1). The PL intensity values of different samples cannot be compared directly.

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Fig. 6. PL lifetimes of (a) PoQD-1, (b) PoQD-2, (c) PoQD-3 and (d) PoQD-4 with different excitation wavelengths at room temperature.

Fig. 7. (a) MALDI-TOF-MASS spectrum of PoQD-1, PoQD-2, PoQD-3 and PoQD-4, (b) band gap of PoQD-1, PoQD-2, PoQD-3 and PoQD-4 (which was calculated from UV–vis absorption spectra) and the band gap of the oligomers of oPD (which was calculated by theoretic calculation).

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Fig. 8. (a) Stability of PoQD-1, PoQD-2, PoQD-3 and PoQD-4 under ultraviolet radiation. The stability under ultraviolet radiation is measured under ultraviolet (wavelength: 365 nm) at 25  C. (b) PL intensity of PoQD-1, PoQD-2, PoQD-3 and PoQD-4 in different pH (25  C). The concentrations of all samples are both 2 mg mL1. F0 and F are PL intensities when pH is 7.26 and specify pH, respectively. (c) Fluorescence response of PoQD-1, PoQD-2, PoQD-3 and PoQD-4. F0 and F are PL intensities without and with the presence of ions and molecules at 25  C and pH 7.0, respectively. The concentration of the copolymer is 2  106 g L1. Bars represent the ratio of F/F0. The ions are K+, Na+, Mg2 + , Ca2+, Zn2+, Cl, SO42, NO3, HPO42, H2PO4 and the concentrations are all 1 103 M.

exact fluorescence lifetimes (t ) for these samples are indicated in Fig. 5. The t of PoQD-1 is 0.97 ns and is close to other kinds of quantum dots (such as semiconductor quantum dots and carbon based 'quantum dots') (Table 2). The t of PoQD-2, PoQD-3 and PoQD-4 is 1.21, 1.33 and 1.57 ns, respectively. The increasing of fluorescence lifetimes can be due to the relaxation in larger PoQDs. To understand PL mechanism of these PoQDs, the t of PoQDs were measured at different lem under optimum excitation condition. When the lem changed from 260 nm to 580 nm, the t of PoQD-1 did not change significantly (Fig. 6). The longest t (0.97 ns) of PoQD-1 can be found at optimum emitted wavelength (430 nm) under optimum excitation condition (380 nm). What’s more, the t of PoQD-1 is little shorter when the lem changed. Moreover, the t of PoQD-2 and PoQD-3 also doesn't changed obviously at different lem under optimum excitation condition. This indicates PoQD-1, PoQD-2 and PoQD-3 have a single illuminate center and the p–p* transition between N and conjugated system is the key factor in PL progress. However, the t of PoQD-4 obviously changed when the lem changed. The t is 1.57 ns when the lem is 590 nm, but the t is 0.71 ns when the lem is 740 nm. The evident difference of t can be due to the diverse surface activity groups of PoQD-4 which have the largest size. In order to further understand the mechanism of photoluminescence and the relationship between the band gap and size of these PoQDs, the MALDI-TOF-MASS spectra and theoretical calculations were carried out. Fig. 7a shows the MALDI-TOF-MASS spectra of PoQD-1, PoQD-2, PoQD-3 and PoQD-4. The spectra of these 4 kinds of quantum dots all show 3 main peaks. The peak which located at m/z = 201.013 can be attributed to the dimer of oPD. The peak located at m/z = 313.114 and m/z = 416.131 can be attributed to the trimer and tetramer of oPD, respectively. Thus, all these 4 kinds of quantum dots are composed of oligomers (dimer, trimer and tetramer). What’s more, the component ratio of these

oligomers in these 4 kinds of quantum dots are almost exactly the same. This indicates these quantum dots have same chemical component. Fig. 7b shows the band gap of these quantum dots. The band gap is 2.90 eV when the average size of PoQDs is 1.5 nm. The band gap is 2.47, 2.28 and 2.11 eV when the average size of PoQDs is 2, 3 and 5 nm, respectively. Moreover, the band gap of dimer, trimer and tetramer is 2.03, 1.21 and 0.74 eV, respectively. It deserved to note that the band gap of all these quantum dots are larger than oligomers. The change of band gap can be due to the quantum size effect in these quantum dots like semiconductor quantum dots [1–4,75–77]. All the above results indicated that these quantum dots have excellent PL properties. The p–p* transition between N and conjugated system increased the PL efficiency significantly. Furthermore, the tunable PL properties was strongly influenced by the significant quantum size effect in these quantum dots. Moreover, the high Ff of PoQD-1 is remarkable and indicates its application perspective of fluorescent bio-imaging. For a practical bio-imaging application, the stability and anti-jamming performance of PoQDs are very important. Fig. 8a shows the stability of PoQDs under ultraviolet radiation. The PoQDs showed excellent stability after continuous excitation under ultraviolet radiation (365 nm) for 24 h. Moreover, the PL intensity stayed at 80% of its original PL intensity after under ultraviolet radiation for 72 h. On the other hand, the PoQDs show good stability in near-neutral pH solution (pH 5–9, Fig. 8b) which indicates these quantum dots can be applied in gentle condition (such as cell). The anti-jamming performance of PoQDs are shown in Fig. 8c. The anti-jamming performance of PoQDs was evaluated by screening common ions such as K+, Na+, Mg2+, Ca2+, Zn2+, Cl, SO42, NO3, HPO42 and H2PO4 which are the common ions in cells or cell culture media, the PL intensity data indicate the strong anti-interference ability for ions.

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4. Conclusions In summary, we synthesized PoQDs via a simple hydrothermal treatment. The excellent PL performance is remarkable. The p–p* transition between N and conjugated system in PoQDs increased the PL efficiency significantly. Further experiments suggested the existence of quantum size effect in amorphous structures for the first time. The tunable PL properties was strongly influenced by the significant quantum size effect. This work is helpful for synthesizing new bio-imaging materials with low cost, high anti-jamming performance and good stability. Acknowledgments This work was supported by China West Normal University Students Research Fund (42713070) and China West Normal University Fund (12B017). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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