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Seeking Brightness from Nature: J-Aggregation-Induced Emission in Cellulolytic Enzyme Lignin Nanoparticles Zhuoming Ma, Chen Liu, Na Niu, Zhijun Chen, Shujun Li, Shou-Xin Liu, and Jian Li ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03265 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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Seeking Brightness from Nature: J-Aggregation-Induced Emission in Cellulolytic Enzyme Lignin Nanoparticles Zhuoming Ma, +a Chen Liu, +a Na Niu,a,b Zhijun Chen,*,a Shujun Li,*,a Shouxin Liu, *,a and Jian Lia a.
Key Laboratory of Bio-based Material Science and Technology of Ministry of Education.,
Northeast Forestry University, Hexing Road 26, Harbin 150040, P.R. China b.
Colledge of Science, Northeast Forestry University, Hexing Road 26, Harbin 150040, P.R. China
* Corresponding author. E-mail address:
[email protected] (Z. Chen),
[email protected] (S. Li),
[email protected] (S. Liu). Key words: Cellulolytic enzyme lignin, AIE, J-aggregates, nanoparticles, fluorescent sensor
Abstract Nanomaterials that show aggregation-induced emission (AIE) have tremendous potential in sensors, bioimaging, phototherapy and organic light-emitting diodes. Although big progress have been achieved in developing AIE nanomaterials and their applications, one downside of most previously described AIE nanomaterials is that they required the complicated organic synthesis of precursor molecules and several preparative steps. Here, a biomass material, cellulolytic enzyme lignin (CEL), was used to prepare AIE nanoparticles (CEL-NPs) by a simple one-step self-assembly method. The J-aggregates were formed in CEL-NPs, which were shown to be the reason for fluorescence emission. The fluorescence of CEL-NPs demonstrated temperature-dependent property and better resistance to photobleaching than that of commercially-available 4',6-diamidino-2-phenylindole (DAPI) dye. The colloidal size of CEL-NPs could be tuned from 80 nm to 600 nm via changing CEL concentrations and solvent exchange. CEL-NPs showed nice colloidal stability in acidic environment and at low 1
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temperature. CEL-NPs and a poly(vinyl alcohol) (PVA)/CEL-NPs composite film demonstrated good fluorescent responses to formaldehyde (FA) solution and vapor, respectively. This work opens up new possibilities for preparation of AIE nanomaterials and also provides a new high value-added routing for utilization of CEL. Introduction Fluorescence techniques have applications in sensing, tracking and understanding complex processes in many different fields.1 Most recently, many fluorescent organic dyes/nanoparticles2-4 and inorganic nanoparticles5 have been investigated for potential use as sensors, organic light-emitting diodes.
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phototherapy reagents
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The disadvantage of inorganic nanoparticles, such as semiconducting
quantum dots and upconverting nanoparticles is their poor biocompatibility5 and the disadvantages of fluorescent dyes are that they tend to be easily photobleached and their emission is weakened when the molecules aggregate. The latter problem has been solved by the aggregation-induced emission (AIE) technology developed by Tang’s group.9 Unlike traditional organic dyes, which suffer from aggregation-induced fluorescence quench, molecules that undergo AIE generate even stronger fluorescence emission when they aggregate.10 These AIE nanomaterials can have high fluorescence quantum efficiency, superb cytocompatibility and good resistance to photobleaching.10 Generally, AIE is achieved by restriction of intramolecular motion, either by random molecular aggregation,1 J-aggregation11 or excited-state intramolecular proton transfer.12 Because of their favorable properties, several groups subsequently developed AIE nanomaterials for biological applications,13-15 as organic light-emitting diodes16-18 and as sensors.19-20 Most AIE nanomaterials have been fabricated by nanoprecipitation of precursor molecules that were prepared by organic synthesis, but this process involves complicated organic synthesis and preparation steps. If AIE nanomaterials could be fabricated 2
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in one step from biomass materials, it should be possible to explore more applications because of the low cost and good biocompatibility of biomass materials. To this end, attention has been paid to lignin as a starting material for AIE nanomaterials. Lignin, a natural biomass that is rich in aromatic rings, has strong fluorescence and is thought to be the main contributor to fluorescence emission in wood cell walls.21 In addition to its fluorescent properties, lignin also shows self-assembly properties in both organic and aqueous solvents22 and this ability to self-assemble has been exploited for the preparation of nanoparticles in many previous studies.23-27 Our group has also described the preparation of cellulolytic enzyme lignin nanoparticles (CEL-NPs), using the self-assembly method.28 To our surprise, the CEL-NPs emitted strong fluorescence emission at about 430 nm. The fluorescence was unexpected since CEL only contains limited conjugated structures and such structures would not be expected to give rise to fluorescence emission. Alkali lignin and lignosulfonate were reported to have AIE active fluorescence, which was due to the clustering of carbonyl groups and restriction of intramolecular rotation.29 Consideration of these previous reports29-30 led us to hypothesize that the emission might be caused by aggregation of the CEL. If this hypothesis is correct, CEL would be an ideal precursor for the preparation of AIE nanomaterials. Firstly, CEL can be obtained easily and cheaply and, secondly, CEL can be used to prepare CEL-NPs in ethanol using a simple self-assembly method. These considerations prompted us to explore the AIE and fluorescence properties of CEL-NPs (Figure 1a, b, c). After demonstration AIE active fluorescence of CEL-NPs, CEL-NPs and a CEL-NPs/PVA composite film were investigated as fluorescent sensors for FA as exposure to exogenous FA via inhalation or ingestion poses a significant threat to human health (Figure 1d).31-32
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Figure 1. a) Schematic illustration of CEL-NPs preparation; b) TEM image of CEL-NPs, scale bars = 500 nm; c) Fluorescent image of the composite film of PVA and CEL-NPs upon excitation with light at 365 nm in the presence of logo mask. d) Schematic illustration of sensing FA using a PVA/CEL-NPs composite film. Experimental Section Materials The CEL used in this work was prepared in our previous report.29 Coniferyl alcohol (99%) was purchased from Sigma-Aldrich. Guaiacylglycerol-β-guaiacyl ether (> 97%) was purchased from TCI. The PVA (No.1750) was purchased from Sigma-Aldrich. All other solvents were purchased from Sigma-Aldrich or Fisher Scientific. Methods Preparation of CEL-NPs The raw fermentation residue was ground to a powder (
