All-Biomass Fluorescent Hydrogels Based on Biomass Carbon Dots

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All-biomass fluorescent hydrogels based on biomass carbon dots and alginate/nanocellulose for biosensing Yuyuan Wang, Zicheng Liang, Zhiping Su, Kai Zhang, Jun-li Ren, Run-Cang Sun, and Xiaohui Wang ACS Appl. Bio Mater., Just Accepted Manuscript • DOI: 10.1021/acsabm.8b00348 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on September 30, 2018

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ACS Applied Bio Materials

All-biomass fluorescent hydrogels based on biomass carbon dots and alginate/nanocellulose for biosensing Yuyuan Wang,† Zicheng Liang,† Zhiping Su,† Kai Zhang,‡ Junli Ren,† Runcang Sun,§ Xiaohui Wang*,† †

State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,

Guangzhou 510640, China. ‡

Wood Technology and Wood Chemistry, Georg-August-University of Goettingen, Büsgenweg

4, 37077 Göttingen, Germany §

Centre for Lignocellulose Science and Engineering and Liaoning Key Laboratory Pulp and

Paper Engineering, Dalian Polytechnic University, Dalian 116034, China.

*

Corresponding author. E-mail address: X. H. Wang, Email: [email protected] 1

ACS Paragon Plus Environment

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ABSTRACT This work describes an all-biomass fluorescent hydrogel fabricated by functionalizing alginate (ALg) and cellulose nanofibers (CNF) hydrogels with fluorescent biomass carbon dots (CQDs) derived from glucose, xylose, and glucosamine. The biomass CQDs played dual functions in the composite hydrogels: first, endowing hydrogels with good fluorescent characters; secondly, enhancing the the mechanical properties of hydrogels due to the crosslinking effect of the abundant oxygen-containing groups or amino groups on surface with ALg or CNF. The elastic modulus of ALg hydrogel and CNF hydrogel was increased by 4.7 times and 1.5 times, respectively, by the adding CQDs. As a proof of concept, ALg/CQDs-3 hydrogel and CNF/CQDs-3 hydrogel were used to detect Fe3+ ions and gold nanoparticle (AuNPs) in aqueous solution, showing high sensitivity. The prepared all-biomass fluorescent hydrogels hold great potentials in biological imaging, biosensing, and biological monitoring fields.

KEYWORDS: fluorescent hydrogel, biomass carbon dots, alginate, cellulose nanofibers, fluorescent sensing

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INTRODUCTION Hydrogels are three-dimensional networks formed by crosslinking hydrophilic polymers. The highly

crosslinked

polymer

chains

in

hydrogel

structure

formed

unique

extracellular-matrix-like cells, which led to fascinating performances of hydrogels including high water retention capacity and good tunability of mechanical, chemical, and biological properties.1,2 Given the excellent properties, numerous researchers have paid attention on the applications of hydrogels in drug delivery,3,4 tissue engineering,5 and cell engineering2,6 fields. The traditional hydrogels are based on synthetic polymers such as polyacrylamide,7,8 poly(N-isopropyl-acrylamide) (PNIPAM),9,10 and Poly (vinyl alcohol).11,12 However, the applications of these synthetic polymer-based hydrogels are limited owing to their non-biodegradability, weak biocompatibility, and inferior interaction with cells. Compared with synthetic polymers, biomass polysaccharides, such as cellulose,13,14 chitosan15 and alginate16,17 are advantageous in their good renewability and biocompatibility, and thus have been regarded as the promising candidates for preparing high-performance hydrogel materials. In addition, for broadening the applications of hydrogels, many functionalized hydrogels, for example, thermos-,18,19 pH-,20 pressure-,21 or electricity-21,22 responsive and fluorescent hydrogels23–25 have been developed. Among above members, fluorescent hydrogels are attracting increasing attention due to their great potential for biological imaging,26 biosensing,27,28 and biological monitoring.29

3



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Typically, fluorescent hydrogels can be obtained by introducing luminescent materials, such as organic fluorescent dyes (OFDs)30–32 and rare elements (REs)23,24,33 into the structure of hydrogels. However, the inclusion of OFDs in hydrogels generally results in impaired fluorescence performance of OFDs and reduced mechanical properties of hydrogels due to their hydrophobic nature.34 Although REs-based fluorescent hydrogels possess better fluorescence properties, higher transparency and mechanical properties,35,36 the potential bio-toxicity of REs may limit their biological applications.37,38 Hence, it is necessary to search for a natural, environment-friendly, biocompatible and hydrophilic luminescent material to endow hydrogel materials with fluorescence. CQDs are a new type of carbon nanomaterials. They are generally referred to as monodisperse spherical carbon nanomaterials with a size less than 10nm consisting sp2/sp3 carbon core and outer oxygen/nitrogen functional groups.39,40 CQDs are considered to be an ideal fluorescent light emitting materials, due to their excited/tunable emission wavelength, two-photon excitation, and attractive electrochemical properties. Nowadays, CQDs have been widely used in many fields, such as biological imaging, disease treatment, photoelectric devices, sensing, catalysis and printing.41–44 In our previous efforts, we have demonstrated biomass CQDs that derived from biomass resources, like cellulose, hemicellulose, chitosan and lignin, are nontoxic, biocompatible, and biodegradable carbon nanomaterials.45–47 Compared to the traditional fluorescent agents, biomass CQDs are superior in green preparation, non-toxicity, biocompatibility, water 4


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solubility and excellent optical stability.45,46 With those advantages, biomass CQDs have been used in live cells imaging, sensing explosives, pigments and metal ions. In addition, the biomass CQDs have rich hydroxyl, carboxyl and epoxy groups on the surface,45,46 which makes them a potential physical crosslinker for biopolymers. Thus, we envision that the introduction of biomass CQDs to biomass polysaccharide hydrogels networks would probably result in an enhanced mechanical properties due to the crosslinking effect of CQDs and endow them fluorescence in the meantime. In this study, we describe a series of all-biomass fluorescent hydrogels produced by functionalizing the hydrogels of alginate and cellulose nanofibers (CNFs) with biomass CQDs prepared from glucose, xylose and glucosamine, respectively. The composite hydrogels demonstrated good fluorescent properties and clearly improved mechanical properties due to the cross-linking effect of CQDs. For the first time, the all-biomass fluorescent hydrogel was used as a solid sensing platform to detect Fe3+ and Au nanoparticles (NPs) in aqueous solution. This study provides a potential functional biomass hydrogel materials for biological applications such as biological imaging, biosensing, and biological monitoring.

MATERIALS AND METHODS Materials. Anhydrous FeCl3 was used as-received from Sigma-Aldrich. Glucose, xylose, glucosamine, alginate, anhydrous CaCl2, and other reagents were provided by local suppliers. Cellulose nanofibers (CNFs) were and nanogold prepared following literature methods.48,49 5



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Synthesis of Biomass CQDs. Three kinds of CQDs were prepared from glucose, xylose, and glucosamine, respectively, by using our previous method.46 Firstly, the monosaccharide precursor was dissolved in deionized water to prepare a solution with a mass fraction of 5%. Then the solution was poured into a 100 mL Teflon-autoclave. After reacting at 200 °C for 12 h in a muffle furnace, the synthesized product was cooled down at room temperature and dispersed for 1h by ultrasound. The unreacted residues were separated by centrifuging at 21000 g/min for 45 min. Then the supernatant was further filtered by a 0.22 µm filtration membrane for removing the insoluble impurities. The prepared CQDs were purified by dialyzing the filtrate in deionized water for 2 days, and then freeze-dried at -65 °C for 2 days. For simplicity, the CQDs derived from glucose, xylose, and glucosamine were named as CQDs-1, CQDs-2, and CQDs-3, respectively. Preparation of CQDs-Alginate and CQDs-CNF hydrogels. The preparation process of CQDs-polysaccharide hydrogels is shown in Fig.1. Firstly, 5 mL 2% sodium alginate (ALg) solution or 5 mL 0.5% (w/v) CNF solution were poured into a 20 mL beaker. Secondly, 1% (biomass CQDs:ALg) CQDs were added to the alginate or CNF solution at ultrasonic condition (40 W). Then CaCl2 solution (mass fraction of 10%) was added to the reaction system (mCaCl2:mALg=1:2; mCaCl2:mCNF=1:10). After ultrasounding for 1 h, the CQDspolysaccharide hydrogel was obtained. Then the resultant product was further washed with deionized water to remove excess reactants. In this system, ALg and CNF played the roles to construct the 3D frameworks of hydrogels. CQDs acted as fluorescent donor and 6


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cross-linking agent. The Ca2+ ions were used for further enhancing the cross-linking degree of hydrogels. In order to study the effects of CQDs contents on the mechanical properties of biomass hydrogels, CQDs-ALg hydrogels with 0.5%, 1%, 2%, 4% CQDs (biomass CQDs:ALg) were prepared by adding different amounts of CQDs into 5 mL 4% ALg solution and denoted as ACm, where A was Alg, C for CQDs, and the m stands for the weight ratio of CQDs/Alg.

Figure 1. The preparation process of CQDs-biopolymer hydrogels.

Characterization

of

biomass

CQDs

and

CQDs-ALg/CNF

hydrogels.

The

micromorphology of CQDs was characterized by a VFEI TECNAI G20 F20 field emission scanning electron microscope (FEI, USA). The AFM imagines of CQDs were obtained by Bruker Nano Surface Division Multimode 8 atomic force microscope (AFM). The X-ray diffraction (XRD) patterns were recorded by a Rigaku D/max-3A X-ray diffractometer equipped with Cu Kα radiation (λ = 1.54051 Å). The chemical structures and states of CQDs 7



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were analyzed by a Bruker TENSOR 27 infrared spectrometer and an AMICUS X-ray photoelectron spectroscopy equipped with a X-ray source Mg Kα (1253.6 eV) (Shimadzu). The surface ζ-potential of sample was measured by Malvern Nano ZS analyzer. The UV absorption spectras of samples were obtained from a UV-3600 UV spectrophotometer (Shimadz). The fluorescence spectras (PL) were determined by Hitachi F-7000 and Horiba FluoroMax-4 fluorescence spectrophotometer with the slit width of 5 nm. The quantum efficiency (QY) was calculated according to eq.(1),50 using quinine sulfate as a reference (QS, 360 nm excitation, QY = 54%). 2 Φ = n 2 A ref F Φ ref / n ref AF ref

(1)

where, Φ and Φref are quantum efficiency of biomass CQDs and quinine sulfate, respectively.

A and F are the absorbance and integrated emission intensity of the sample at 360 nm. n represents the refractive index of solution. The mechanical properties of the hydrogels were measured by an Instron 5565 tensile compression testing machine. The fluorescence spectras of hydrogel samples were obtained by Horiba FluoroMax-4 fluorescence spectroscope with slit width of 2.5 nm. The microstructures of prepared hydrogels were characterized by a Zeiss EVO 18 scanning electron microscope. Before testing, the hydrogels were freeze-dried to obtain an aerogels. The swelling properties of aerogels were measured by determining their weight changes, after aerogel samples were immersed in deionized water for different time, until the hydrogels reached the equilibrium of water absorption. 8


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Sensing of Fe3+ and Au NPs by CQDs-3 hydrogels. CQDs-3-ALg hydrogel and CQDs-3-CNF hydrogel were used as sensing systems for detecting Fe3+ and AuNPs, respectively. For Fe3+ sensing, 0.05 M Fe3+ solution was gradually added to a cuvette containing a CQDs-3-ALg hydrogel patch (2.5×2.5 cm) and 2.5 ml deionized water. Then the fluorescence spectra (PL) of above mixture system was obtained by a Horiba FluoroMax-4 fluorescence spectrophotometer using 350 nm excitation source with the slit width of 2.5 nm. For AuNPs sensing, Fe3+ solution and CQDs-3-ALg hydrogel were replaced by AgNPs solution (0.5 mg/ml) and CQDs-3-CNF hydrogel.

RESULTS AND DISCUSSION Microstructures of CQDs. The AFM images of prepared CQDs are shown in Figure 2. As shown in Figure 2(a, d, g), CQDs-1, CQDs-2, and CQDs-3 present the similar morphology of monodisperse spheres. The AFM height images in Figure 2(b, e, h) indicate that the particle heights of prepared CQDs are 1~2 nm. Figure 2(c, f, i) reveal the average particle sizes of CQDs-1, CQDs-2, and CQDs-3 are 21 nm, 19 nm, and 31 nm, respectively. The anisotropic sizes of CQDs demonstrate CQDs are flat shape. This result is consistent with the result in literature.51

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Figure 2. AFM analysis of (a, b, c) CQDs-1, (d, e, f) CQDs-2, and (g, h, i) CQDs-3.

Chemical structures and compositions of CQDs. The FT-IR spectrums and XPS results of prepared CQDs are shown in Figure 3. It can be found from Figure 3(a, d, h) that compared with original monosaccharides the intensities of C-H stretching vibration at 2934 cm-1 and C-C tretching vibration at 1043 cm-1 are weakened in CQDs. This result indicates that the 10


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sugar ring structures of monosaccharides were completely destroyed during the hydrothermal synthesis process. The intensities of -OH blending effects at 1700-1620 cm-1 in CQDs-1 and CQDs-2, and -NH peaks at 1587 cm-1 in CQDs-3 are strengthened. This can be assigned to that numerous -OH groups were formed, while the amino groups in glucosamine was preserved in hydrothermal reaction process. The XPS element analysis results in Figure 3(b, e, h) reveal that the elementary compositions of three types of CQDs are different. Compared with CQDs-3, nitrogen element is absent in CQDs-1 and CQDs-2. The contents of C element in CQDs-1, CQDs-2, and CQDs-3 are 57.23%, 74.79%, and 85.46%, respectively. Meanwhile, the O element contents of CQDs-1, CQDs-2, and CQDs-3, are 42.77%, 25.21%, and 12.85%, respectively. The resolved C1s peaks of CQDs-1 in Figure 3(c) shows that the chemical states of C element in CQDs-1 consist of C-C/C=C bonds (284.7 eV) and C-O bond (286.3 eV). Figure 3(i) reveals that the presence of abundant C-C/C=C bonds, and comparably much less C-N, C-O, and C=O bonds in CQDs-3. These results indicate that CQDs-3 had more regular carbon core structure and the highest carbonization degree. The carbonization degree of CQDs-1 was the least, while that of CQDs-2 was in the middle. The surface Zeta potential of CQDs-1, CQDs-2 CQDs-3 are -15.7 mV, -11.9 mV, and -17.5 mV, suggesting all of CQDs were electronegative. The surface Zeta potentials of the obtained biomass CQDs are correlated with their carbonization degree possibly due to the the serious graphitization or aromatic cyclization tend to push the oxygen-containing groups to the surface. Another possible reason 11



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for the low Zeta potential of CQDs-3 may be attributed to the amino groups of CQDs-3 were transformed into pyridine nitrogen structures.52–54

Figure 3. FTIR and XPS characterization of three monosaccharides CQDs: (a)-(c) FTIR spectras, XPS

spectra and C1s peaks of CQDs-1; (d)-(f) CQDs-2; (g)-(i) CQDs-3.

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Fluorescence properties of biomass-CQDs. The UV absorption spectrums and fluorescence spectrums (PL) of prepared CQDs are presented in Figure 4. Figure 4(a, c, e) show that the UV absorption peaks of CQDs-1, CQDs-2, and CQDs-3 are at 255 nm, 261 nm, and 260 nm, respectively. The PL spectrums in Figure 4(b, d, f) reveal that the maximum fluorescence emissions of CQDs-1, CQDs-2, and CQDs-3 are at 455 nm, 437 nm, and 442 nm with the excitation wavelengths of 360 nm, 350 nm, and 370 nm, respectively. In addition, it can be found that the half peak width (FWHM) of CQDs-2 is only about 50 nm which is lower than that of CQDs-1 and CQDs-3. This can be attributed to the uniform size distribution of the CQDs-2. The quantum efficiencies (QY) of CQDs-1, CQDs-2, and CQDs-3 are determined to be 3.38%, 3.27%, and 16.55%, respectively. The remarkable increase in QY of CQDs-3 is due to the inherent nitrogen doping of the aminoglucose.

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Figure 4. UV and PL spectras of (a, b) CQDs-1, (c, d) CQDs-2, and (e, f) CQDs-3.

The micromorphology of CQDs-ALg/ CNF hydrogels. The SEM images of CQDs-ALg and CQDs-CNF hydrogels are shown in Figure 5. It can be seen that ALg-hydrogel has a 14


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loosely lamellar structure with the smooth and thick cell walls, and presents some pores with size of 100-200 nm. The similar lamellar structures are also observed in CNF-hydrogel. In addition, it shows that the addition of CQDs has negligible effects on the microstructures of composite hydrogels.

Figure 5. SEM image of (a) ALg hydrogels, (b) ALg+CQDs-1 hydrogels, (c) ALg+CQDs-2 hydrogels,and (d) ALg+CQDs-3 hydrogels.), (e)-(h) CQDs-CNF hydrogels (Mag=150X).

Mechanical and swelling properties of fluorescent hydrogels. The compressive stress-strain curves of the composite hydrogels are shown in Figure 6. As shown in the Figure 6(a, b) and Table 1 that after adding biomass CQDs, the mechanical properties of composite hydrogels are improved. Particularly, after adding CQDs-3, the elastic modulus of ALg hydrogel and CNF hydrogel are increased by 4.7 times and 1.5 times, respectively. The role of CQDs in the improvement of mechanical properties can be ascribed to that the abundant oxygen-containing groups or amino groups on the surface of the CQDs form hydrogen bonds with ALg or CNF, thus promoting to form more stable and robust crosslinking structures in 15



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composite hydrogels.55 It was also found the mechanical properties of the composite hydrogels were correlated with the doping amount of CQDs. According to Figure 6(c, d) the compression strength and elastic modulus of ALg hydrogel reached maximum with 1% CQDs. Further increasing of CQDs contents resulted in declined mechanical performance probably due to the reduction of physical crosslinking density caused by the partial agglomeration of CQDs.55 In addition, it also reveals that the reinforcement ability of CQDs for CNF hydrogel is lower than that for ALg hydrogel. According to Figure 6(b), the 3D structure of CNF hydrogel can be easily destroyed once the compressive strain exceeds 40%, suggesting the CNF hydrogel has a loose network due to the dilute concentration, and thus has week interactions with CQDs. On the other hand, the ALg hydrogel has a more compact network due to the strong interaction between alginate chains with Ca2+ ions56,57 and this further favors their interaction with CQDs.

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Figure 6. The compressive stress-strain curves biomass CQDs-hydrogels (a)-(c): (a) ALg hydrogels; (b) CNF hydrogel; (c) ACm hydrogels. And elasticity modulus of ACm hydrogels at 50% strain (d).

Table 1. Elasticity modulus of ALg and CNF hydrogels before/after adding CQDs

Elasticity modulus(MPa) Hydrogel sample

ALg hydrogels

CNF hydrogels

Blank

0.01887

0.00552

Adding CQDs-1

0.05662

0.00819

Adding CQDs-2

0.04445

0.00703

Adding CQDs-3

0.08629

0.00834

17



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Table 2 shows the effects of CQDs on the swelling properties of ALg and CNF hydrogels. It indicates that after adding CQDs-1 ALg hydrogel still maintains its good swelling properties. However, the addition of CQDs-2 and CQDs-3 result in a significantly decrease in the swelling ability of ALg hydrogel. This is attributed to the addition of CQDs-2 and CQDs-3 is favorable for forming compact crosslinking structures in hydrogels, thus reducing their water retention capacity. In addition, it can be found that the addition of CQDs-1 and CQDs-2 has negligible effect on the swelling property of CNF hydrogel and a slight decrease in swelling ability is obtained with the CQDs-3 reinforced CNF hydrogel. Table 2. Swelling properties of ALg and CNF hydrogels before/after adding three monosaccharides CQDs Swelling ratio (%) Hydrogel sample

ALg

CNF

1h

3h

6h

18 h

24 h

Blank

1205

1519

2359

3050

3094

CQDs-1

1158

1284

2289

2784

2971

CQDs-2

744

836

875

910

942

CQDs-3

839

841

859

847

872

Blank

3103

3184

3193

2981

2760

CQDs-1

2906

3070

2957

3084

2709

CQDs-2

3114

3007

2944

2862

2870

CQDs-3

2475

2521

2390

2496

2420

Fluorescence properties of composite hydrogels. The photographs of composite hydrogels and their fluorescent responses are present in Figure 7(a)-(d). Figure 7(a)-(c) show both ALg-CQDs and CNF-CQDs hydrogels have excellent shape stability. Meanwhile, Figure 7(b, 18


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d) reveal that the CQDs-1 and CQDs-2 functionalized hydrogels show faint blue fluorescence, and the CQDs-3 functionalized hydrogels present brightly blue fluorescence. The PL spectrums in Figure 7(e)-(g) further prove the fluorescence properties of CQDs were well kept in the Alg and CNF hydrogels with almost overlapping emission curves. Figure 7(f) reveals that the half peak width (FWHM) of the emission peaks of CQDs-2 functionalized hydrogels are increased, which may be attributed to the hydrogen bonding between the CQDs-2 and ALg/CNF resulting in the broadening of the particle size distribution.

Figure 7. Photograph of three biomass CQDs-hydrogels (a)-(d): (a) ALg hydrogels observed under daylight; (b) ALg hydrogels observed under UV lamp radiation; (c) CNF hydrogels observed under daylight; (d) CNF hydrogels observed under UV lamp radiation. And PL spectra of the three biomass CQDs in aqueous solution, ALg hydrogels and CNF hydrogels (e)-(g) (excited at 350 nm): (e) CQDs-1; (f) CQDs-2; (g) CQDs-3.

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Sensing ability of composite hydrogels. Biomass CQDs can be quenched in aqueous solution by some bioactive molecules, such as Fe3+ ions. So it is expected to build such a hydrogel system for biosensing. Here, the ALg-CQDs and CNF-CQDs hydrogels were studied as a 3D solid-phase fluorescence detection platform to sensitively determine low concentration of Fe3+ metal ions and Au nanoparticles in aqueous solution. To eliminate the effect hydrogel size and thickness, the hydrogels were cut into 1 × 1 cm with thickness of 0.5 cm. The response of the fluorescence hydrogels to Fe3+ ions was studied first. As illustrated in Figure 8 (a, b), clear fluorescence quenching can be observed upon the addition of Fe3+ ions. When the Fe3+ concentration was increased to 5 mM, 80% of the fluorescence could be quenched. The fluorescence quenching results was also evaluated with the Stern-Volmer equation: I଴ /I = 1 + K ௦௩ [Q],46 where I0 and I are the fluorescence intensities in the absence and in the presence of the quenching agents [Q]. The S-V quenching constant (K ௦௩ ) was calculated with the linear portion of the S-V plot. According to Figure 8(b), the fluorescence titration of CQDs-3-ALg with Fe3+ yields a linear relationship, and the Ksv constant was calculated to be 846.37 M-1, the detection limit was determined to be 1.42 µM. It was demonstrated in our previous study that the biomass CQDs can selectively and sensitively detect Fe3+ ions through a photoinduced electron transfer (PET) quenching process. In this study, results prove the sensing properties of CQDs towards Fe3+ ions was preserved in the

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all-biomass hydrogels, indicating the 3D porous network structure of biomass hydrogel can provide an efficient mass transfer enabling the rapid fluorescence response. Au NPs is a kind of nano metal particles that can combine with a variety of biological molecules. For example, the multi-response sensor system was built by Au NPs and CQDs to detect thiocyanate ions and biological sulfhydryl compounds.58,59 The sensing property of CQDs-3-CNF composite hydrogel towards Au NPs was studied and shown in Figure 9. Figure 9(a) shows that there are overlapping spectra between CQDs-3 and Au NPs, which indicates Au NPs may induce fluorescence quenching for CQDs-3 via fluorescence resonance energy transfer (FRET) mechanism.45,46 Figure 9(b, c) show that the adding Au NPs concentration resulted in the gradual decrease of fluorescence emission intensity in CQDs-3-CNF hydrogel, demonstrating our envision that the fluorescence ability of CQDs can be quenched by Au NPs. When the concentration of Au NPs was reduced to 10 µg/mL the fluorescence intensity was decreased by ~30%, indicating CQDs-3-CNF hydrogel is sensitive to Au NPs. These results demonstrate that the all-biomass fluorescent composite hydrogels reported in this study owns the potential for being used as a fluorescent sensor in drug delivery and disease diagnosis.

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3+

Figure 8. Quenching behaviors of Glucosamine CQDs in Alg hydrogels by Fe : (a) Quencing

spectra (excited at 350 nm); (b) Corresponding quenching plot.

Figure 9. Quenching behaviors of Glucosamine CQDs in CNF hydrogels by AuNPs: (a)

Spectral overlap between glucosamine CQDs and AuNPs; (b) Quencing spectra (excited at 350 nm); (c) Corresponding quenching plot.

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CONCLUSIONS In summary, we synthesized different biomass CQDs using glucose, xylose and glucosamine as raw materials and prepared CQDs/ALg and CQDs/CNF fluorescent hydrogels. All of the prepared CQDs showed excellent fluorescence properties and owned richly active surface groups such as oxygen-containing groups or amino groups. The introduction of CQDs could improve the crosslinking degree of the 3D networks of hydrogels, thus improving the elastic modulus of ALg hydrogel and CNF hydrogel by 4.7 times and 1.5 times, respectively. In addition, the presence of CQDs resulted in excellent fluorescence properties and fluorescence quenching behaviors, which is of vital importance to bio-sensing. Compared with CQDs-1 and CQDs-2, CQDs-3 doped hydrogels presented the strongest fluorescent emission. The CQDs-3/ALg composite hydrogel presented a linear fluorescence response towards Fe3+ with high sensitivity. In addition, it was also demonstrated that the emission of CQDs-3/CNF could be efficiently quenched by AuNPs. The results showed that the prepared all-biomass fluorescent hydrogels owned the great potential to be used as biodegradable and biocompatible fluorescent sensors in biological field.

ACKNOLEDGEMENTS This work was financially supported by the National Science Foundation of China (51673072), the Independent Study Projects of the State Key Laboratory of Pulp and Paper Engineering (2017C04), the Fundamental Research Funds for the Central 23



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Universities, SCUT (2017ZD077) and the National Program for Support of Top-notch Young Professionals of China.

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Graphical Abstract

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All-Biomass Fluorescent Hydrogels Based on Biomass Carbon Dots

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