Carbon dots, which are less than 10 nm in diameter, have been widely investigated because of their unique luminescence properties and potential for use in bioimaging. In the present work, natural carbon dots (L-CDs) were obtained by molecular aggregation, using ethanol-extracted cellulolytic enzyme lignin. The whole process for the preparation of L-CDs was green and simple to operate and did not use toxic chemical reagents or harsh conditions. The newly prepared L-CDs emitted multicolor photoluminescence following one- and two-photon excitation. The L-CDs also showed good cellular biocompatibility, which is crucial for biological applications. One- and two-photon cell-imaging studies demonstrated the potential of L-CDs for bioimaging.

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-2phenylindole (DAPI) dye. The colloidal size of CEL-NPs could be tuned from 80 to 600 nm via changing CEL concentrations and solvent exchange. CEL-NPs showed nice colloidal stability in acidic environment and at low 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.

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The effect of hydrothermal conditions on enzymatic hydrolysis lignin (EHL) degradation in water-isopropyl alcohol co-solvent and optimal conditions were investigated. The yields and reactivity toward formaldehyde of degraded enzymatic hydrolysis lignin (DEL) were determined. The optimal conditions of temperature, time, and ratio of solids to liquids were 250 °C, 60 min, and 1:10 (w/v), respectively. The EHL and DEL were characterized by gel permeation chromatography (GPC), Fourier transform infrared spectroscopy (FT-IR), 1 H nuclear magnetic resonance ( 1 H NMR), thermal gravity (TG), and differential scanning calorimetry (DSC) analyses. The results revealed that the molecular weight and polydispersity of DEL were lower than that of EHL. Although the fundamental structure of lignin before and after hydrothermal degradation was retained, the ether (β-O-4, α-O-4, etc.) content decreased, while that of hydroxyl (phenolic and aliphatic) increased. The DTGmax and Tg values shifted from 334 and 117 °C to 304 and 105 °C, respectively.

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