Europium (Eu) element has one of the best photoluminescent properties among rare earth elements. Due to the 4f→4f transition, Eu3+
ions can convert ultraviolet light into visible light and have been widely used in fluoroimmunoassay [1
]. The antenna effect induced by the chelation between Eu3+
and negatively charged ligand results in an even stronger photoluminescent performance for ligand/Eu3+
complexes. The complexes of various ligands, e.g., β-diketone, chelated with Eu3+
can be used as photoreceptors, superconductors, magnetic materials, catalysts, fluorescent probes, and light-emitting diodes [3
Complexes of polymer ligands and rare earth elements are considered as high-efficiency light-emitting materials because of their strong photoluminescent ability, easy color mixing, low temperature sensitivity, and high thermal stability [9
]. In the complex of cholesterol-g-poly(N
-isopropylacrylamide) (PNIPAM) and Eu3+
, the oxygen-nitrogen coordination between amide group and Eu3+
provides a strong fluorescent characteristic for Eu3+
]. Those ligands with functional groups containing strong electron-donating groups, e.g., hydroxyls, carboxyls, amides, and nitriles, especially have strong chelation effects with Eu3+
ions or other rare earth elements. Carboxymethyl cellulose (CMC) or cholesterol-g-poly(N-isopropylacrylamide) were used to complex with Eu or Tb, and they showed strong photofluorescent properties [11
]. Hybridized nanofibers were produced between polyacrylonitrile and Eu3+
, which had one-dimensional nanostructure and high specific surface area, showing excellent fluorescent properties [1
]. However, most of these photofluorescent complexes or materials are still facing the problems of stability and durability, i.e., the fluorescence intensity decreases after severe use, due to the leaching or aggregation of fluorescent elements or particles.
When the polymer ligands reduce to nano- or submicron-scale, they provide the materials with special characteristics in mechanical, optical, thermal, or surface properties [15
]. Nanocellulose, with its sustainable nature and nanoscale characteristics, inspires its application in functional materials. So far, only a few studies have used nanocellulose, e.g., cellulose nanocrystal (CNC), cellulose nanofibril (CNF), and bacterial cellulose (BC), as supports or ligands for fluorescent substances. Fluorescein-5′-isothiocyanate (FITC) was used to label CNCs for fluorescence bioassay and bio-imaging applications [16
]. CNCs grafted by poly (N-isopropylacryalamide)(PNIPAAM) brushes had thermo-enhanced fluorescence and can be used in biomedical applications [17
]. The silver nanocluster was loaded on natural CNFs, giving the material fluorescence properties and antimicrobial properties [18
]. A fluorescent film was prepared by chemical modification of CNF film surface with boronate-terminated conjugated polymer chains and used to detect nitro-aromatic vapor [19
Bacterial cellulose (BC) is a type of typical extracellular cellulose produced by acetobacter bacteria. BC was composed of ultrafine and interlaced nanofibers with a width of less than 100 nm to form a network structure. This structural feature provides BC with a large specific surface area and a nanoporous structure [20
]. This nanoporous structure makes BC an ideal support for functional nanoparticles that can be further processed into functional materials. BC membranes were used to support TiO2
nanoparticles and blended with rare earth elements, e.g., lanthanum (La) and cerium (Ce), leading to a high photocatalytic efficiency [22
]. BC and CdSe were combined to prepare composite films which have photoluminescence properties [23
]. BC was used to support nitrogen-doped carbon nanofibers; this composite showed electrode material properties and was used to prepare a flexible all-solid-state high-power supercapacitor [24
]. Pt was supported by BC to prepare a composite material having high catalytic efficiency for methanol fuel battery [25
There are very few studies on the preparation of photofluorescent materials by using BC to support and form complexes with rare earth elements. The possible reason is the high cost of BC and its soft nature upon film forming, which is not able to provide a good stiffness for the materials.
Plant-based cellulosic fibers are well known to be the raw material for paper making and they can also be used as substrates or supporting materials for functional ligands to produce functional paper-based materials. Fluorescent paper has also been produced. Coating or printing method is a common way to composite the fluorescent materials with paper sheets. Fluorescent DNA-based oligodeoxyfluoroside dyes were printed on paper, which can sense food spoilage and ripening in the vapor phase [26
]. 8-hydroxyquinoline aluminum (Alq3)-based bluish green fluorescent composite nanospheres were printed on paper as sensors for nitroaromatic explosive detection [27
]. A ratiometric fluorescent probe-based paper sensor based on paper printed with gold nanoclusters stabilized by bovine serum albumin and fluorescent graphene oxide can be used for the determination of serum blood sugar [28
]. Fluorescent inks such as heterorotaxane and the mixture of triethanolamine and UV absorbent UV-7282 were also used to print on paper for fluorescence [29
]. Fluorescein isothiocyanate (FITC) and thioflavin were absorbed by nanocellulose and that was coated on the surface of paper to prepare fluorescent paper [31
]. Most of the current methods produce fluorescent paper by attaching fluorescent materials to the surface of paper without altering the paper fiber structure. However, common cellulosic fibers do not have nanoporous structure and thus cannot well stabilize the nanosized fluorescent particles, leading to lack of stability, recycling efficiency, and durability.
BC can be tightly bound to cellulosic fibers due to their abundant surface hydroxyl groups [20
], whose features can be used to prepare functional paper-based materials if BC is properly functionalized. In cellulosic fiber/BC composite functional materials, cellulosic fibers can reduce the cost of pure BC materials and provide high stiffness, while BC can still provide the materials with high toughness, high tearing, good stability, and durability. Catalytic paper sheets were produced by loading Pt or Au onto BC surface and then compositing with cellulosic fibers, which showed excellent catalytic efficiency, stability, and reusability [32
]. In this study, we prepared fluorescent paper by making BC/Eu complex first and then processing the complex and cellulosic fibers into paper sheets. This method incorporates the fluorescent complexes as part of the cellulosic fiber matrix and thus may improve the stability and durability of fluorescent paper.
2. Materials and Methods
ATCC23767 was obtained from Nanjing High Tech University Biological Technology Research Institute Co., Ltd. (Nanjing, China) and used to produce the bacterial cellulose (BC) pellicles according to the static fermentation method [20
]. The degree of polymerization (DP) and Segal crystallinity index (CI) of BC was predetermined as 1081 and 96.0%, respectively [20
Europium trioxide (Eu2O3), hydrochloric acid, and sodium hydroxide were purchased from Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). All other chemicals are analytical grade chemicals.
2.2. Preparation of EuCl3·6H2O
Eu2O3 (0.727 g) was completely dissolved in 6.0 mol/L HCl solution. The mixture was placed in boiling water bath for 30 min to evaporate the unreacted HCl. The solution was then diluted with deionized water to volume 100 mL to adjust the concentration to 0.0416 mol/L.
2.3. Preparation of Eu-BC
Approximately 20 g of wet BC pellicles (moisture content = 98.5%) was disintegrated by a lab homogenizer (SKG 1246, Foshan, China) for 20 s in 130 mL of water. The water was then filtered to obtain the disintegrated BC. The pH of the EuCl3·6H2O solution was adjusted to 5–6 with 1 M NaOH. The disintegrated BC was then added slowly to the EuCl3·6H2O solution at 45 °C under a magnetic stirring (400 rpm). The pH of mixture was adjusted to about 7 with 1 M NaOH. Finally, the mixture was refluxed at 70 °C for 1 h 30 min to prepare Eu-BC.
2.4. Manufacture of Fluorescent Paper
A vacuum filtering method was used to prepare the fluorescent paper because at high EuBC addition it would take a very long time at the water drainage step for the standard TAPPI method. In detail, bleached sugarcane bagasse pulps (BSBP) and Eu-BC were composited with different ratios, i.e., 1%, 2%, 5%, 10%, and 20% (the proportion of Eu-BC based on total paper dry weight). The mixture was homogenizer for 20 s and the mixture was filtered by a sand core filter device. After water was drained, the paper sheet was formed. Finally, the paper was dried in an oven at 9 °C for 50 min.
2.5. Characterization of Eu-BC and Fluorescent Paper
Nitrogen adsorption–desorption isotherm measurements were performed on an ASAP 2460 (Micromeritics, Norcross, GA, USA) volumetric adsorption analyzer at 77 K. The Brunauer–Emmett–Teller (BET) method was utilized to calculate the specific surface area of each sample. All samples were degassed at 120 °C at 0.05 atm for 6 h before the measurement began.
Fluorescent paper sheets or Eu-BC were taken under an AGL-9406 UV lamp for excitation at 256 nm. The ultraviolet light absorption of the samples was measured by solid-state ultraviolet spectrophotometer (UV-2450, SHIMADZU, Kyoto, Japan). The fluorescence spectra of the samples were recorded by a steady-state transient fluorescence spectrometer (FluoroMax-4, HORIBA Jobin Yvon, Longjumeau, France).
The Eu content of the paper was measured by an atomic absorption spectrometer (Z-2000, Hitachi, Tokyo, Japan). The surface morphologies of unmodified BC, Eu-BC, Eu-BC fluorescent papers were observed by a scanning electron microscope (SEM) (Merlin, Zeiss, Munich, Germany); X-ray photoelectron spectroscopy (XPS) was conducted on the fluorescent paper by a X-ray photoelectron spectrometer (Kratos Axis Ulra DLD, Kratos Analytical, Manchester, UK) to analyze the valence and binding energy of the Eu ions.