Ensemble and Single Particle Fluorescence Characterization of Dye-Labeled Cellulose Nanocrystals

Characterizing the photoluminescence of fluorescein-labeled cellulose in aqueous and alcohol solutions: influence of the cellulose backbone

Although many dyes have been introduced into cellulose, whether bound to its backbone or within a cellulose matrix, few studies have determined whether the backbone statically or dynamically quenches the photoluminescence of the dye. To advance cellulosic fluorescent films, the influence of the cellulose backbone on photoluminescence must be understood. We determined the fluorescence properties of fluorescein isothiocyanate (FITC) and fluorescein-labeled cellulose (FLC) in water and alcohol, including their quantum yields [Formula: see text], lifetimes [Formula: see text], and rates of radiative [Formula: see text] and nonradiative [Formula: see text] decay. Dissolved FLC had a ~ 30× lower [Formula: see text] than FITC, suggesting that incorporating FITC into the cellulose backbone remarkably reduces the fluorescence efficiency. The FLC solutions had a six-fold lower [Formula: see text] than their FITC counterparts but a 10-20 times higher [Formula: see text]. Presumably, this was because the cellulose backbone interacted weakly with the fluorescein moieties, suggesting a quenching mechanism that can be termed quasi-static, corresponding to static quenching between the fluorescein moieties and cellulose backbone, in addition to the fluorescence quenching caused by the intramolecular nonradiative processes of fluorescein, as observed in conventional molecules. Using the Strickler‒Berg formula, we deduced the analytical radiative decay rate constants [Formula: see text] and eventually estimated the number of very short-lived fluorescein moieties per single fluorescent fluorescein moiety, corresponding well with static quenching.

Cellulose as an Eco-Friendly and Sustainable Material for Optical Anticounterfeiting Applications: An Up-to-Date Appraisal

The falsification of documents, currency, pharmaceuticals, branded goods, clothing, food products, and packaging leads to severe consequences. Counterfeited products can not only pose health risks to consumers but also cause substantial economic losses that can negatively impact the global markets. Unfortunately, most anticounterfeiting strategies are easily duplicated due to rapid technological advancements. Therefore, innovative and cost-effective antiforgery techniques that can offer superior multilevel security features are continuously sought after. Due to the ever-growing global awareness of environmental pollution, renewable and eco-friendly native biopolymers are garnering wide attention in anticounterfeiting applications. This review highlights the potential use of cellulose-based eco-friendly materials to combat the counterfeiting of goods. The initial section of the review focuses on the structure, properties, and chemical modifications of cellulose as a sustainable biomaterial. Further, the topical developments reported on cellulose and nanocellulose-based materials used as fluorescent security inks, films, and papers for achieving protection against counterfeiting are presented. The studies suggest the convenient use of celluose and modified cellulose materials for promising optical antiforgery applications. Furthermore, the scope for future research developments is also discussed based on the current critical challenges in the fabrication of cellulose-based materials and their anticounterfeit applications.

Ferric ion detection mechanism of a dicarboxylic cellulose nanocrystal and a 7-amino-4-methylcoumarin based fluorescent chemosensor

As one of Earth's most widely distributed and abundant elements, iron impacts the natural environment and biological systems. Therefore, developing a simple, rapid, and accurate Fe3+ detection method is vital. Fluorescent dicarboxylic cellulose nanocrystals (FDCN) with selective quenching of Fe3+ were synthesized using 7-amino-4-methylcoumarin (AMC), and dicarboxylic cellulose nanocrystals (DCN) prepared by sequential periodate-chlorite oxidation. The sensing characteristics and detection mechanism of FDCN for Fe3+ were studied by fluorescence spectrophotometry, Fourier-transform infrared spectroscopy (FTIR), the Stern-Volmer equation, Job's plot method, and the Benesi-Hildebrand equation. The results showed that FDCN was highly selective for Fe3+, and other metal ions did not reduce the selectivity. High sensitivity with a detection limit of 0.26 μM and a Stern-Volmer quenching constant of 0.1229 were also achieved. The coordination between Fe3+ and the carboxylic, hydroxyl, and amide groups on the surface of FDCN and the carbonyl of coumarin lactones to form FDCN/Fe3+ complexes prevented the intramolecular charge transfer (ICT) process and led to the fluorescence quenching of FDCN. EDTA restored the fluorescence emission of quenched FDCN. The complexation stoichiometry of Fe3+ to FDCN was 1 : 1, and the association constant was 3.23 × 104 M-1. The high hydrophilicity, sensitivity, and selectivity of FDCN for Fe3+ make the chemosensor suitable for Fe3+ trace detection in drinking water and biology.

Efficient Labeling of Nanocellulose for High-Resolution Fluorescence Microscopy Applications

The visualization of naturally derived cellulose nanofibrils (CNFs) and nanocrystals (CNCs) within nanocomposite materials is key to the development of packaging materials, tissue culture scaffolds, and emulsifying agents, among many other applications. In this work, we develop a versatile and efficient two-step approach based on triazine and azide-alkyne click-chemistry to fluorescently label nanocelluloses with a variety of commercially available dyes. We show that this method can be used to label bacterial cellulose fibrils, plant-derived CNFs, carboxymethylated CNFs, and CNCs with Cy5 and fluorescein derivatives to high degrees of labeling using minimal amounts of dye while preserving their native morphology and crystalline structure. The ability to tune the labeling density with this method allowed us to prepare optimized samples that were used to visualize nanostructural features of cellulose through super-resolution microscopy. The efficiency, cost-effectiveness, and versatility of this method make it ideal for labeling nanocelluloses and imaging them through advanced microscopy techniques for a broad range of applications.

Covalent immobilization of bromocresol purple on cellulose nanocrystals for use in pH-responsive indicator films

This study developed pH-indicator films by combining esterified cellulose nanocrystals (e-CNCs) with activated bromocresol purple (a-BCP) via covalent bonding for pH-sensitive color-changing applications. The e-CNC/a-BCP particles were incorporated into cellulose acetate polymer to prepare pH-sensitive color changing films. Binding of a-BCP to e-CNCs was proven by attenuated total reflection infrared (ATR-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Colorimetric analysis showed that films containing 10% or 15% e-CNC/a-BCP particles had critical color changes either at pH 4-5, or pH 7-8. The films with 10% e-CNC/a-BCP particles also revealed excellent leaching resistance under acidic conditions. Color changes were reversible between pH 2 and 10. These pH-indicator films had visible color changes in response to pH variations, color reversibility, leaching resistance, and sufficient rigidity even though mechanical properties decreased as the e-CNC/a-BCP content increased from 0% to 15%.

Additive printing of recyclable anti-counterfeiting patterns with sol-gel cellulose nanocrystal inks

The assembly of cellulose nanocrystals (CNCs) that produce attractive structural color shows great potential in anti-counterfeiting application, but their processability and recyclability remain unsatisfactory due to the strong hydrogen bonds between CNCs. For the first time, optical anti-counterfeiting patterns are obtained by additive printing of surface-functionalized CNC inks (CNC-DC5700-NPES). The surface-functionalized CNC inks are prepared by sequential modification of CNCs with organosilane (DC5700) and polyoxyethylene ether (NPES), which show good flowability under shearing force and transform into a gel-like phase rapidly after printing, making possible ink-jet printing without additives. The printed patterns are transparent under natural light but show vivid interference color, showing anti-counterfeiting features between crossed polarizers. The texture and optical properties of the printed patterns can be facilely controlled by tuning the printing parameters, such as nozzle diameter, writing angle, and filling width. Moreover, the CNC-DC5700-NPES patterns with a core-shell structure could be collected in various solvents and reprinted after removing solvents. This work provided a new pathway for the preparation of optical anti-counterfeiting patterns from biomass resources.

Fluorescent labeling and characterization of dicarboxylic cellulose nanocrystals prepared by sequential periodate-chlorite oxidation

High-performance fluorescent composites are key to the development and improvement of fluorescent molecular probe technology. In this study, cellulose nanocrystals (CNC) with high carboxyl concentrations were prepared via sequential periodate-chlorite oxidation. Then, fluorescent cellulose nanocrystals (FCNC) were prepared by attaching 7-amino-4-methylcoumarin (AMC) onto CNC under 4-(4,6-dimethoxy-1,3,5-triazin-2-yl)-4-methylmorpholinium chloride (DMTMM) catalysis. The morphology and fluorescence properties of FCNC were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, transmission electron microscopy, elemental analysis, ultraviolet-visible absorbance, fluorescence spectrophotometry, and fluorescence spectroscopy. The results showed that AMC was grafted onto the CNC surface by an amidation reaction, and the absorption and emission maxima for FCNC were blue-shifted from 350 nm and 445 nm of AMC to 335 nm and 440 nm, respectively. FCNC retained the crystallinity and nano-topography size of the CNC. The fluorescence intensity, quantum yield, and fluorescence lifetime of FCNC showed the same change law; it first increased and then decreased with an increase in the graft density of AMC from 0.201 to 0.453 AMC molecules per nm2. The FCNC prepared in this study have good optical properties and can be used in the fields of fluorescent molecular probes and biological imaging.

Highly transparent, writable and photoluminescent foldable polymer film: When fluorescent dyes or pigments join cellulose-based microgel

We prepared a self-dispersed cellulose-based microgel via chemically bonding hydrophilic gelatin peptide chain onto cellulose glucose chain. Following, a variety of highly transparent, foldable, and writable photoluminescent polymer films was obtained through loading organic dyes and inorganic pigments onto cellulose-based microgel matrix, respectively. Benefiting from the coupling sites and network effect of microgel as well as the abundant hydroxyl, amino, and imino groups in its structure, the microgel containing organic dyes and inorganic pigments exhibit good dispersion and stability, and the resultant photoluminescent films emit bright yellow, orange, yellow-green, and blue-green fluorescence under UV light, respectively, especially the cellulose-based microgel stabilized inorganic alkaline earth aluminate hybrids exhibit continuous luminescence for a long time in the dark. Compared with the existing regenerated cellulose or CNCs-based fluorescent films, the cellulose-based microgel fluorescent films present higher transmittance and good biodegradability. This study can bring new ideas for the development of flexible luminescent devices.

Recent Progress on Cellulose-Based Ionic Compounds for Biomaterials

Glycans play important roles in all major kingdoms of organisms, such as archea, bacteria, fungi, plants, and animals. Cellulose, the most abundant polysaccharide on the Earth, plays a predominant role for mechanical stability in plants, and finds a plethora of applications by humans. Beyond traditional use, biomedical application of cellulose becomes feasible with advances of soluble cellulose derivatives with diverse functional moieties along the backbone and modified nanocellulose with versatile functional groups on the surface due to the native features of cellulose as both cellulose chains and supramolecular ordered domains as extractable nanocellulose. With the focus on ionic cellulose-based compounds involving both these groups primarily for biomedical applications, a brief introduction about glycoscience and especially native biologically active glycosaminoglycans with specific biomedical application areas on humans is given, which inspires further development of bioactive compounds from glycans. Then, both polymeric cellulose derivatives and nanocellulose-based compounds synthesized as versatile biomaterials for a large variety of biomedical applications, such as for wound dressings, controlled release, encapsulation of cells and enzymes, and tissue engineering, are separately described, regarding the diverse routes of synthesis and the established and suggested applications for these highly interesting materials.

Functionalized Cellulose Nanocrystals for Cellular Labeling and Bioimaging

Cellulose nanocrystals (CNCs) are unique and promising natural nanomaterials that can be extracted from native cellulose fibers by acid hydrolysis. In this study, we developed chemically modified CNC derivatives by covalent tethering of PEGylated biotin and perylenediimide (PDI)-based near-infrared organic dye and evaluated their suitability for labeling and imaging of different cell lines including J774A.1 macrophages, NIH-3T3 fibroblasts, HeLa adenocarcinoma cells, and primary murine dendritic cells. PDI-labeled CNCs showed a superior photostability compared to similar commercially available dyes under long periods of constant and high-intensity illumination. All CNC derivatives displayed excellent cytocompatibility toward all cell types and efficiently labeled cells in a dose-dependent manner. Moreover, CNCs were effectively internalized and localized in the cytoplasm around perinuclear areas. Thus, our findings demonstrate the suitability of these new CNC derivatives for labeling, imaging, and long-time tracking of a variety of cell lines and primary cells.

Stimuli-Responsive Naphthalene Diimide as Invisible Ink: A Rewritable Fluorescent Platform for Anti-Counterfeiting

Herein, we report an approach to combat counterfeiting and storage of valuable information based on the solid-state fluorescence switching behavior of isoniazid functionalized naphthalene diimide (ISO_NDI) in response to an external stimuli (i. e., HCl vapor). The unique feature of ISO_NDI is further utilized to develop an invisible ink (ISO_NDI-PVA) with commercial polymer polyvinyl alcohol (PVA). A solid-state fluorescence recovery was observed while loading with HCl vapors. This exclusive property of the material could be applied directly as a security ink for confidential data storage purpose. Based on above strategy, we successfully realized the rewritable application by using ISO_NDI-PVA ink and confirm its practical efficacy on various substrates by creating different patterns. The solid-state fluorescence switching behavior of ISO_NDI-PVA ink exhibited reversible on/off signal for multiple cycles under the influence of HCl/NH₃ vapors. Mechanistic investigation supports a clear participation of intermolecular charge transfer (ICT) phenomenon in the solid-state fluorescence switching property. The ease of fabricating the ink with invisible to visible characteristics in response to HCl vapors provides new opportunities for exploring the application of ISO_NDI-PVA as invisible ink for targeted security applications.

Designing Highly Luminescent Cellulose Nanocrystals with Modulated Morphology for Multifunctional Bioimaging Materials

Spherical cellulose nanocrystals (SCNs) and rod-shaped cellulose nanocrystals (RCNs) were extracted from different cellulose materials. The two shape forms of cellulose nanocrystals (CNs) were designed with a combination of isothiocyanate (FITC), and both the obtained FITC-SCNs and FITC-RCNs exhibited high fluorescence brightness. The surfaces of SCNs and RCNs were subjected to a secondary imino group by a Schiff reaction and then covalently bonded to the isothiocyanate group of FITC through a secondary imino group to obtain fluorescent cellulose nanocrystals (FITC-CNs). The absolute ζ-potential and dispersion stability of FITC-CNs (FITC-SCNs and FITC-RCNs) were improved, which also promoted the increase in the fluorescence quantum yield. FITC-RCNs had a fluorescence quantum yield of 30.7%, and FITC-SCNs had a morphological advantage (better dispersion, etc.), resulting in a higher fluorescence quantum yield of 35.9%. Cell cytotoxicity experiments demonstrated that the process of FITC-CNs entering mouse osteoblasts (MC3T3) did not destroy the cell membrane, showing good biocompatibility. On the other hand, FITC-CNs with good dispersibility can significantly enhance poly(vinyl alcohol) (PVA) and poly(lactic acid) (PLA); their mechanical properties were improved (the highest sample reached to 243%) and their fluorescent properties were imparted. This study provides a simple surface functionalization method to produce high-luminance fluorescent materials for bioimaging, multifunctional nanoenhancement/dispersion marking, and anticounterfeiting materials.

Rapid Determination of the Distribution of Cellulose Nanomaterial Aggregates in Composites Enabled by Multi-Channel Spectral Confocal Microscopy

There is increased interest in the use of cellulose nanomaterials for the mechanical reinforcement of composites due to their high stiffness and strength. However, challenges remain in accurately determining their distribution within composite microstructures. We report the use of a range of techniques used to image aggregates of cellulose nanocrystals (CNCs) greater than 10 µm2 within a model thermoplastic polymer. While Raman imaging accurately determines CNC aggregate size, it requires extended periods of analysis and the limited observable area results in poor reproducibility. In contrast, staining the CNCs with a fluorophore enables rapid acquisition with high reproducibility, but overestimates the aggregate size as CNC content increases. Multi-channel spectral confocal laser scanning microscopy is presented as an alternative technique that combines the accuracy of Raman imaging with the speed and reproducibility of conventional confocal laser scanning microscopy, enabling the rapid determination of CNC aggregate distribution within composites.

Aggregation-Induced Emission (AIE)-Labeled Cellulose Nanocrystals for the Detection of Nitrophenolic Explosives in Aqueous Solutions

Aggregation-induced emission (AIE) active cellulose nanocrystals (TPE-CNCs) were synthesized by attaching tetraphenylethylene (TPE) to cellulose nanocrystals (CNCs). The structure and morphology of TPE-CNCs were characterized by FT-IR, XRD, ζ-potential measurements, elemental analysis, TEM, atomic force microscopy (AFM), and dynamic laser light scattering (DLS). Fluorescent properties of TPE-CNCs were also further studied. Unlike aggregation-caused quenching (ACQ), TPE-CNCs emitted weak fluorescence in the dilute suspensions, while emitting efficiently in the aggregated states. The AIE mechanism of TPE-CNCs was attributed to the restriction of an intramolecular rotation (RIR) process in the aggregated states. TPE-CNCs displayed good dispersity in water and stable fluorescence, which was reported through the specific detection of nitrophenolic explosives in aqueous solutions by a fluorescence quenching assay. The fluorescence emissions of TPE-CNCs showed quantitative and sensitive responses to picric acid (PA), 2,4-dinitro-phenol (DNP), and 4-nitrophenol (NP), and the detection limits were 220, 250, and 520 nM, respectively. Fluorescence quenching occurred through a static mechanism via the formation of a nonfluorescent complex between TPE-CNCs and nitrophenolic analytes. A fluorescence lifetime measurement revealed that the quenching was a static process. The results demonstrated that TPE-CNCs were excellent sensors for the detection of nitrophenolic explosives in aqueous systems, which has great potential applications in chemosensing and bioimaging.

Current characterization methods for cellulose nanomaterials

A new family of materials comprised of cellulose, cellulose nanomaterials (CNMs), having properties and functionalities distinct from molecular cellulose and wood pulp, is being developed for applications that were once thought impossible for cellulosic materials. Commercialization, paralleled by research in this field, is fueled by the unique combination of characteristics, such as high on-axis stiffness, sustainability, scalability, and mechanical reinforcement of a wide variety of materials, leading to their utility across a broad spectrum of high-performance material applications. However, with this exponential growth in interest/activity, the development of measurement protocols necessary for consistent, reliable and accurate materials characterization has been outpaced. These protocols, developed in the broader research community, are critical for the advancement in understanding, process optimization, and utilization of CNMs in materials development. This review establishes detailed best practices, methods and techniques for characterizing CNM particle morphology, surface chemistry, surface charge, purity, crystallinity, rheological properties, mechanical properties, and toxicity for two distinct forms of CNMs: cellulose nanocrystals and cellulose nanofibrils.