Clustering-Triggered Emission and Persistent Room Temperature

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Clustering-Triggered Emission and Persistent Room Temperature Phosphorescence of Sodium Alginate Xueyu Dou, Qing Zhou, Xiaohong Chen, Yeqiang Tan, Xin He, Ping Lu, Kunyan Sui, Ben Zhong Tang, Yongming Zhang, and Wang Zhang Yuan Biomacromolecules, Just Accepted Manuscript • DOI: 10.1021/acs.biomac.8b00123 • Publication Date (Web): 20 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Biomacromolecules

Clustering-Triggered Emission and Persistent Room Temperature Phosphorescence of Sodium Alginate Xueyu Dou,†,‡ Qing Zhou, ‡ Xiaohong Chen,‡ Yeqiang Tan,*,† Xin He,¶ Ping Lu,¶ Kunyan Sui, † Ben Zhong Tang, *,& Yongming Zhang, ‡ Wang Zhang Yuan*,‡ †

Collaborative Innovation Center for Marine Biomass Fibers, Materials and Textiles of Shandong

Province, Institute of Marine Biobased Materials, School of Materials Science and Engineering, Qingdao University, Qingdao 266071, China ‡

School of Chemistry and Chemical Engineering, Shanghai Key Laboratory of Electrical

Insulation and Thermal Aging, Shanghai Electrochemical Energy Devices Research Center, Shanghai Jiao Tong University, Shanghai 200240, China ¶

State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun

130012, China &

Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water

Bay, Kowloon, Hong Kong, China KEYWORDS: Sodium alginate; Nonconventional luminogens; Clustering-triggered emission; Rheology; Room temperature phosphorescence; Anti-counterfeiting

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ABSTRACT: Nonconventional biomacromolecular luminogens have attracted extensive interests due to their fundamental importance and potential applications in diverse areas. To explore novel luminogens and moreover to gain deeper insights into their emission mechanism, we study the emission behaviors of sodium alginate (SA), a natural anionic polysaccharide composed of mannuronic (M) and guluronic acids (G). We find that the luminescence from aqueous SA solutions exhibits distinct concentration enhanced emission and aggregation-induced emission (AIE) characteristics. Meanwhile, the ratio of M/G also matters. Rheological measurements reveal the distinct regimes of the solutions, which are consistent with the observed emission, indicative of strong association between the chain entanglement and emission. Moreover, we observe persistent room temperature phosphorescence (RTP) in the amorphous SA solids, which is a rare case even in pure organic aromatic luminogens. Such unique emission can be remarkably enhanced via coordination with Ca2+ ions. These emission behaviors can be well rationalized by the clustering-triggered emission (CTE) mechanism. Namely, the emission is caused by the electron cloud overlap due to the clustering of oxygen atoms and carboxylate units, together with conformation rigidification. Owing to its biocompatibility, intrinsic emission, and moreover persistent RTP, SA shows great potentials for anti-counterfeiting, encryption, intracellular imaging, etc. 

INTRODUCTION

Intrinsically luminescent polymers containing solely nonconventional chromophores, namely aliphatic tertiary amines, carbonyls, esters, cyanos, hydroxyls, amides, imides, etc., are receiving increasing attention because of their fundamental importance and potential applications in biological and biomedical areas, optoelectronic devices, and chemo-sensing.1–21 Unlike conventional conjugated luminophors, such polymers consisting of flexible chains and tunable


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structures without any conventional chromophores have also been found to strongly emit under certain conditions.6–14 Owing to their good hydrophilicity, facile preparation, environmental friendliness, and outstanding biocompatibility, such nonconventional luminogens promise their diverse practical applications, and even more suitable for biological and medical applications when compared with the conventional emitters.11,15,16 Despite the attractive advantages of nonconventional luminogens, a thorough understanding of their emission mechanism is still under debate.1–22 Even for the representative systems, such as poly(amino amides) (PAMAMs) and polyethylenimines (PEIs), a sole consensus is difficult to reach due to the various mechanisms proposed for their emissions, including oxidation, architecture effect, terminal group effect, and pH influence.12,17–22 In 2013, we observed bright blue light emission in rice, a mixture of natural components. Further examination of its major component, namely starch, and other natural products like cellulose, glucose, and dextran illustrated their similar aggregation-induced emission (AIE) features.23,24 Based on these observations, we proposed the clustering-triggered emission (CTE) mechanism, namely the clustering of nonconventional chromospheres and subsequent electron cloud overlap to form extend conjugation with simultaneously conformation rigidification to rationalize the emission behaviors of these luminogens.23,25 Such mechanism was supported by further measurements of synthetic compounds and natural biomolecules, including polyacrylonitrile (PAN),25 poly(Nhydroxysuccinimide methacrylate) (PNHSMA),26 nonaromatic amino acids, and poly(amino acids).27 Although more detailed photophysical processes and underlying insights of the nonconventional luminogens remain to be elucidated, current findings highly suggest that CTE mechanism does not only coincide with our observations, but can also apply to the other systems.1,6,28–40


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Unexpected emission of starch and cellulose prompted us to find out if luminescence is a common phenomenon among natural macromolecules, which can be meaningful for both mechanism study and technical exploration. In this contribution, sodium alginate (SA), a plantderived anionic polysaccharide possessing similar composition to starch (Figure 1), was focused. It consists of 1,4-linked β-D-mannuronic acid (M) and its C-5 epimer α-L-guluronic acid (G) residues, which are arranged in an irregular block wise pattern with various proportions of GG (Gblock), MG, and MM (M-block) units (Figure 1).41–44 The G block is stiffer and more extended than the M block due to its higher degree of hindered rotation around the glycosidic linkages. 45 Herein, SA with M/G ratios of 0.69 (GrM) and 1.53 (GMr) (Figure 1) were investigated. Both GrM and GMr are practically nonluminescent in dilute solutions, but get emissive upon aggregation. Amazingly, persistent room temperature phosphorescence (RTP) hardly observed even in pure organic aromatic molecules is also noticed.26,46–51 Notably, their emission can be adjusted by Ca2+ ions due to the high ion-binding selectivity of G-rich regions.44 Rheological measurements indicate the increase of overlap between neighboring SA chains when the concentration changes from dilute to concentrated regimes,52 which turns on the emission, thus further validating the CTE mechanism.

Figure 1. Chemical structure of G and M units and typical chain sequence of SA. The information of GrM and GMr is also given.


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

EXPERIMENTAL SECTION

Materials. GMr (~1.53 M/G blocks, 430 mPa·s for 1% aqueous solution at 25 °C) and GrM (~0.69 M/G blocks, 460 mPa·s for 1% aqueous solution at 25 °C) were obtained from Qingdao Hyzlin Biology Development Co., Ltd. (Qingdao, China). Acid ion exchange resin (Amberlite IR 120) was purchased from Nankai Hecheng Technology Co., Ltd. (Tianjin, China). Sodium hydroxide was provide by Sinopharm Chemical reagent Co., Ltd. (Shanghai, China). Sodium chloride, calcium chloride, and ethanol were supplied by Shanghai Lingfeng Chemical reagent Co., Ltd. (Shanghai, China). The pure water was bought from Hangzhou Wahaha Group Co., Ltd. (Zhejiang, China) for the preparation of SA aqueous solutions. Purification of SA. 10 g of SA was added into 1000 mL of pure water to obtain an aqueous solution (10 g L–1). Then 100 g of strong acid ion exchange resin was added to the solution. The mixture was stirred for 24 h and filtered through a sintered G2 glass filter. Afterwards, pH of the solution was adjusted to 8 with 25 wt% NaOH aqueous solution. The polymer (200 mL SA aqueous solution) was precipitated in ethanol (mixed with NaCl equivalent to nSA). The solid was collected by filtration through a sintered G2 glass filter and successively washed with 80 vol%, 90 vol% ethanol/water mixtures (1000 mL), and absolute ethanol (500 mL) three times. The solids were dried in a vacuum oven at 30 °C to remove any remaining solvent. Preparation of SA Films. Stock solution was prepared by dissolving 0.4 g of SA powders in 4.6 g of pure water and subsequent stirring for 6 h. After removal of the air bubbles by standing overnight, the resultant stock solution was cast into moulds with various shapes and dried under ambient conditions to evaporate most of the solvent. The resultant films were further dried in a vacuum oven at 30 °C. The Ca2+ crosslinked films were fabricated by immersing the cast films in a 4 wt% calcium chloride solution for 24 h, followed by drying in vacuum oven at 30 °C.


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Characterization. Molecular weights (Mw and Mn) and polydispersity indexes (PDI, Mw/Mn) of SA were measured on a GPC max (Malvern Viscotek, UK). Solid-state

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C-nuclear magnetic

resonance (13C-NMR) spectra of the samples were measured on a 400 MHz Bruker AVANCE III HD (Switzerland) to determine the M/G ratios. 1H NMR spectra were measured on a Bruker ARX 400 NMR spectrometer using D2O as solvent and tetramethylsilane (TMS) as internal standard. Absorption spectra were taken on a Lambda 35 UV/Vis spectrometer (Perkin Elmer, USA). Excitation and emission spectra were determined on a LS 55 fluorescence spectrometer (PerkinElmer, USA), on which the recorded emission spectra with a delay time (td) ≥ 0.1 ms can exclude all nanosecond signals. Emission spectra of various aqueous GMr and GrM solutions were measured simultaneously under parallel conditions. Fluorescent and phosphorescent lifetimes (f and p) were measured on a QM/TM/IM steady-transient time-resolved spectroscopy (PTI, USA) and a FLS 980 spectrometer (Edinburgh Instruments, UK), respectively. Quantum yields (Φ) were determined on a spectrophotometer (PTI, USA) equipped with SPEKTRON-R98 coated integrating sphere (φ 80 mm) (Everfine, China), with an excitation wavelength (λex) of 320 nm. Wide angle X-ray scatteing (WAXS) measurements were carried out on a Xeuss 2.0 SAXS/WAXS system. Cu Kα X-ray source (GeniX3D Cu ULD), generated at 50 kV and 0.6 mA, was utilized to produce X-ray radiation with a wavelength of 1.5418 Å. A semiconductor detector (Pilatus 300 K, DECTRIS, Swiss) with a resolution of 487 × 619 pixels (pixel size = 172 × 172 μm2) was used to collect the scattering signals. Each WAXS pattern was collected with an exposure time of 20 minutes. The one-dimensional intensity profiles were integrated from background corrected 2D WAXS patterns. All photographs were taken by a digital camera (Canon EOS 70D, Japan). Photographs of SA solids excited by different lights of UV (330-385 nm), blue (460-495 nm), and green (530-550 nm) were recorded on a Reflected Fluorescence System (Olympus BX61, Japan).


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Biomacromolecules

Cell Culture and Confocal Imaging. HeLa cells were seeded in a 4-chamber glass bottom dish (35 mm dish with 20 mm bottom well) with Dulbecco’s modified Eagle’s medium (DMEM, high glucose, 1% L-glutamine) containing 10% fetal bovine serum and 1% penicillin-streptomycin. The dish was cultured in a humidified incubator containing 5% CO2 at 37 °C for 24 h, then the cells were stained with GMr/DMEM solution (500 μL, 0.5 wt%) for 1.5 h. Afterwards, they were imaged using a laser scanning confocal microscopy (Leica SP8 STED 3X) at a λex of 405 nm. 

RESULTS AND DISCUSSION

Not only do G and M contents in SA impact on the chain flexibility but also lead to different interchain association, which may greatly exert the emission. To have a better understanding, the G-rich GrM (Mw = 310 000, PDI = 2.1, M/G = 0.69) and M-rich GMr (Mw = 320 000, PDI = 2.1, M/G = 1.53) were investigated (Figure 1, S1, and S2). Both of them are nonemissive in dilute solutions (i.e. 0.005 wt%, Φ ≈ 0), with extremely low photoluminescence (PL) signals almost parallel to the abscissa recorded (Figure 2a-c, S3a-c). Such emission remains until the concentration increases to 0.5 wt%, at which rather faint but visible emission of GMr is observed with maximum at ~352 nm (λex = 286 nm). GrM acts similarly, but with even weaker PL peaking at ~358 nm (λex = 290 nm) at the same concentration. Further enhanced emission is detected as SA concentration increases. Notably, for concentrated solutions (i.e. 8 wt%), bright grey white emissions are observed upon 365 nm UV excitation with the Φ values of 3.9%/3.1%, along with multiple peaks at 352/417/484 and 358/419/486 nm for GMr/GrM, respectively (Table 1). Contrary to the ACQ luminogens, both GMr and GrM exhibit concentration enhanced emission properties. Meanwhile, the emission maxima (λem) of GMr/GrM vary from 352/358 to 580/588 nm while their λexs change from 286/290 to 440 nm (Figure 2d and S3d). These λex dependent emissions might be ascribed to the formation of heterogeneous clusters, which is supported by their different f


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values of 5.4/6.4 and 6.2/3.8 ns at 424/484 (GMr) 422/482 (GrM), respectively (Figure S4). Their dilute solutions show negligible absorptions, whereas those of concentrated solutions progressively enhanced with increasing concentration, accompanied by the appearance of a shoulder at ~270 nm (Figure 2e, S3e). Meanwhile, the remarkable deviation from the base line caused by the Mie effect and obvious Tyndall effect (Figure S5) in concentrated solutions strongly suggest the formation of aggregates, which is consistent with above results.

Figure 2. (a) Photographs taken under 365 nm UV light and (b) emission spectra (λex = 286 nm) of varying aqueous GMr solutions. (c) Net emission intensity increase (I/I0–1) of varying aqueous GMr solutions. (d) Emission spectra of 8 wt% aqueous GMr with different λexs. (e) Absorption spectra of varying aqueous GMr solutions.


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Table 1. Emission peaks and quantum yields of GMr and GrM aqueous solutions and solids solution (wt%) 0.005

powder

film

8

2+

film-Ca

GMr

 (%)

0

3.9

6.4

3.2

3.5

em

–

352, 417, 484

360, 429, 450, 494, 534

415, 422, 488, 516, 536, 584

381, 420, 484, 527, 584

GrM

 (%)

0

3.1

5.3

2.0

4.2

em

–

358, 419, 486

386, 425, 490, 530, 610

388, 422, 447, 488, 516, 582

418, 446, 487, 516, 586

Rheological property of polymer solutions is sensitive to the chain association. To well correlate the photophysical processes with chain interactions (aggregation), shear viscosity as well as storage (G′) and loss (G′′) moduli of varying SA solutions were measured. As shown in Figure 3a and S7a, a typical Newtonian liquid behavior is observed within a wide range of shear rates, demonstrating more apparent shear-thinning feature with increasing concentration. Meanwhile, the crossover point (at which G′ = G″) is observed, which moves towards lower frequency at higher concentrations, indicating the networks are brought about by chain entanglement (Figure S6 and S7b).53,54 The plots of specific viscosity (ηsp) versus concentration (Figure 3b and S7c) are further derived from the zero shear viscosity obtained from Figure 3a and S7a. The ηsp~C plots establish different concentration regimes, wherein C*, Ce, and CD are the intersections successively between dilute, semidilute unentangled, semidilute entangled, and concentrated regimes, respectively. Ce is characterized as the stage at which significant overlap of the polymer chains topologically constrains the macromolecular chain motion, whereas CD is the onset of the concentrated regime. The viscosity scaling relationships of GMr/GrM are thus obtained as ηsp ~ C/ηsp ~ C7/10, ηsp ~ C1/2/ηsp ~ C1/2, ηsp ~ C3/2/ηsp ~ C2, and ηsp ~ C33/10/ηsp ~ C7/2 for the dilute, semidilute unentangled, semidilute entangled, and concentrated regions (Figure 3b and S7c), respectively, which agree well with the theoretical predictions for polyelectrolyte systems.52,55,56


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Figure 3. (a) Viscosity of varying GMr aqueous solutions at different shear rates. (b) The plot of ηsp versus the concentration of GMr solutions. (c) Schematic illustration of the chain conformations of GMr in varying solutions. Aforementioned results reveal the chain interactions in varying SA solutions, which should be helpful for the further revelation the emission mechanism. In dilute solutions (C

Clustering-Triggered Emission and Persistent Room Temperature

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