Ultralong phosphorescence cellulose with excellent anti-bacterial, water-resistant and ease-to-process performance

Phosphorescent materials offer long emission lifetimes, high signal-to-noise ratios, and large Stokes shifts, enabling applications in bioimaging, encryption, anticounterfeiting, LEDs, and lasers. Organic room-temperature phosphorescence (RTP) materials are attractive over metal-based systems due to low toxicity, cost, and flexibility, but achieving RTP requires promoting intersystem crossing (ISC) and suppressing non-radiative transitions. Conventional strategies (heavy atoms, crystals, supramolecular assemblies, encapsulation, crosslinking) often compromise processability. While printable or water-soluble RTP polymers have emerged, their biodegradability is limited, which is problematic for small, unrecyclable labels and coatings. This study uses natural cellulose—biodegradable, sustainable, and rich in hydroxyl groups—to create a cationic derivative by introducing cyanomethylimidazolium cations and chloride anions. The design promotes ISC and stabilizes triplet states via hydrogen bonding and electrostatic interactions, yielding easily water-processed RTP films, fibers, coatings, and patterns; with glutaraldehyde crosslinking, the materials also become water-resistant and antibacterial.

The paper reviews key requirements for organic RTP: (1) enhancing ISC, commonly via heavy atoms to increase spin–orbit coupling; and (2) suppressing non-radiative decay through restricting molecular motion and isolating oxygen, often by crystalline packing, supramolecular assemblies, encapsulation, or crosslinked networks. However, such rigid or complex architectures hinder processing. Recent advances include printable and water-soluble RTP polymers that improve formability, but most synthetic polymers are not fully biodegradable, a growing priority for sustainable materials. Prior works have also explored hydrogen-bonding networks to achieve RTP and noted weak inherent phosphorescence in cellulose. The authors position cellulose as an ideal, modifiable, hydrogen-bonding-rich, biodegradable platform to overcome processability and environmental limitations in RTP materials.

- Materials: Microcrystalline cellulose (DP ~220); ionic liquid 1-allyl-3-methylimidazolium chloride (AmimCl) synthesized from 1-methylimidazole and allyl chloride (12 h, 50 °C), purified to 99.7% with <0.3 wt% water. - Synthesis of cellulose 2-chloropropionate (Cell-Cl): Dissolve cellulose (3 g) in AmimCl (114 g) at 80 °C; add ultradry DMF (10 mL); introduce 2-chloropropionyl chloride (stoichiometry varied to tune degree of substitution, DSc), react at 40 °C for 0.5–2 h; precipitate in ethanol/water, wash, reprecipitate from DMSO, dry. - Synthesis of cellulose 1-cyanomethylimidazolium chloride (Cell-ImCNCl): Dissolve Cell-Cl (e.g., DSc 1.24) in DMF; add 1-(cyanomethyl)imidazole (stoichiometry varied) and react 24–48 h at 80 °C; precipitate in acetone, wash, dry. Degrees of substitution for imidazolium (DS_CN) tuned by feed; overall DS (DS_C + DS_CN) controlled via Cell-Cl preparation. - Anion exchange (Cell-ImCNX): Dissolve Cell-ImCNCl in water; add salts (LiBF4, NaPF6, LiTf2N, NaC(CN)3, NaN(CN)2, NaSCN), stir 30 min; isolate solids by centrifugation, wash, dry, yielding Cell-ImCNX with different counteranions. - Control materials: (i) Cell-BimCl via reacting Cell-Cl with 1-butylimidazole (80 °C, 24 h); (ii) Cell-Br via acylation with 2-bromopropionyl chloride; (iii) Cell-ImCNBr via quaternization of Cell-Br with 1-(cyanomethyl)imidazole. - Processing/forming: Owing to water solubility, fabricate materials via aqueous processes: doctor blade coating to films; dip coating to PVA fibers and cellulose films; screen printing, inkjet printing, and mask casting to produce patterns on paper, glass, ceramics, plastics, stainless steel, and aluminum. - Water-resistant, antibacterial patterning: Add 200 µL of 10% glutaraldehyde to 1 mL of Cell-ImCNCl aqueous solution (100 mg/mL); spray through a stencil (e.g., plane shape) onto glass; dry to form patterns with chemical crosslinks (with residual cellulose hydroxyls) in addition to physical interactions, imparting water resistance and antibacterial activity. - Characterization: 1H NMR (Bruker AV-400) to quantify DS; FTIR (Nicolet 6700); XPS (Thermo ESCALab 250Xi); XRD (Rigaku D/MAX-2500). Photoluminescence spectra (Hitachi F-7000); quantum yield and phosphorescence lifetime (Edinburgh FLS980 with integrating sphere and microsecond lamp; lifetime via multi-channel single photon counting). Imaging under 365 nm UV with digital camera. Antibacterial test: agar diffusion (S. aureus and E. coli, 10^6–10^7 cfu/mL), incubated 20 h at 37 °C, inhibition rings observed. - Mechanistic probes: Synthesized CNMImCl (small-molecule analog) to test low-temperature phosphorescence; compared Cell-BimCl to assess role of cyano vs butyl; varied DS_CN, DS_C, and anions to study effects on RTP.

- Successful synthesis of a cationic cellulose derivative (Cell-ImCNCl) bearing cyanomethylimidazolium cations and chloride anions confirmed by 1H NMR and FTIR (e.g., CN stretch ~2065–2075 cm−1; imidazolium peaks ~1562–1568 and ~1174–1176 cm−1). - Room-temperature phosphorescence (RTP) achieved in amorphous Cell-ImCNCl via synergistic effects: imidazolium cyano groups promote ISC; strong hydrogen-bonding (cellulose OH with Cl− and CN) and electrostatic attractions confine motion and suppress non-radiative decay. - Visual afterglow: Cell-ImCNCl powder exhibits phosphorescence persisting for ~4 s after 365 nm excitation is removed. - Optimized composition: At total DS ≈ 1.24 and DS_CN ≈ 0.60, Cell-ImCNCl shows maximum performance with photoluminescence quantum yield 11.81% and average phosphorescence lifetime 158 ms. - Structure–property relationships: - Varying DS_CN: phosphorescence intensity, QY, and lifetime increase then decrease; excessive ionic content introduces electrostatic repulsion, loosening chain packing and increasing non-radiative decay. - Varying DS_C (chloropropionate content) at fixed DS_CN ≈ 0.60: performance increases then decreases; increased Cl− enhances H-bonding until hydroxyl donors become too scarce, weakening the network. - Counteranion effects: Strong H-bond acceptor anions (Cl−, F−) support high RTP; weaker H-bonding anions (BF4−, PF6−, Tf2N−, C(CN)3−, N(CN)2−, SCN−) reduce RTP. Heavy atom effect of anions (e.g., Br−) is less important than hydrogen-bonding basicity; Cell-ImCNBr shows significantly decreased intensity, QY, and lifetime. - Controls: Cell-BimCl (butylimidazolium) exhibits poor RTP (average lifetime ~5 ms), indicating the necessity of cyano functionality and strong H-bonding; native cellulose has weak phosphorescence (QY 3.76%, lifetime ~5 ms). Small-molecule CNMImCl shows phosphorescence in the solid and in aqueous solution only at 77–137 K, supporting the need for polymeric confinement and H-bonding at room temperature. - Processability: Cell-ImCNCl is water-soluble and readily forms films, fibers (including woven PVA fiber knots), microspheres, and printed patterns on diverse substrates via doctor blade coating, dip coating, screen printing, inkjet printing, and mask casting. - Water resistance and antibacterial activity: Incorporating a small amount of glutaraldehyde during deposition creates chemical crosslinks, yielding water-resistant phosphorescent patterns that retain emission after 24 h water immersion and exhibit inhibition zones against E. coli and S. aureus. - Application demonstration: Combined use of phosphorescent Cell-ImCNCl and fluorescent Cell-BimCl enables multi-state anticounterfeiting patterns that display different images under UV on vs. afterglow (e.g., “88” under UV becomes “23” after UV-off).

The study demonstrates that integrating cyanomethylimidazolium cations and chloride anions into cellulose leverages two mechanisms essential for organic RTP: enhanced ISC through the cyano-functionalized imidazolium and suppression of non-radiative decay via dense hydrogen-bonding and electrostatic interactions among cellulose OH groups, Cl−, and CN groups. Systematic variation of cation content (DS_CN), ester content (DS_C), and counteranion identity confirms that the strength of hydrogen-bonding networks dominates RTP performance over heavy-atom effects of anions. The optimized Cell-ImCNCl achieves a balanced ionic content that maximizes confinement without excessive electrostatic repulsion, producing an average phosphorescence lifetime of 158 ms and QY of 11.81% at room temperature. The material’s intrinsic water solubility enables environmentally benign aqueous processing into multiple formats and substrates. Introducing glutaraldehyde during deposition yields double crosslinking (physical and chemical), imparting water resistance and antibacterial properties, broadening applicability to anticounterfeiting, information encryption, and smart labeling. These findings address the need for RTP materials that combine strong emission, processability, sustainability, and added functionality.

This work presents a simple, scalable strategy to create biodegradable, water-processable organic RTP materials by cationizing cellulose with cyanomethylimidazolium chloride. The designed ionic interactions and hydrogen-bonding networks enhance ISC and stabilize triplet states, enabling room-temperature afterglow with an average lifetime up to 158 ms and quantum yield of 11.81% at optimized DS values. The material can be fabricated into films, fibers, coatings, and intricate patterns via aqueous methods, and with minimal glutaraldehyde it becomes water-resistant and antibacterial. The eco-friendly, multifunctional phosphorescent cellulose platform is promising for advanced anticounterfeiting, information security, disposable smart labels, and storage/monitoring applications in food and medicine.