Fluorescent Aerogels Based on Chemical Crosslinking between

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Fluorescent aerogels based on chemical crosslinking between nanocellulose and carbon dots for optical sensor Bolang Wu, Ge Zhu, Alain Dufresne, and Ning Lin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b02754 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 12, 2019

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ACS Applied Materials & Interfaces

Fluorescent aerogels based on chemical crosslinking between nanocellulose and carbon dots for optical sensor Bolang Wu, † Ge Zhu, § Alain Dufresne, § and Ning Lin †, ‡ * †

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of

Technology, Wuhan 430070, P. R. China ‡

Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University,

Hefei 230601, P. R. China §

Université Grenoble Alpes, Grenoble INP*, LGP2, F-38000, Grenoble, France (* Institute of

Engineering Université Grenoble Alpes, Grenoble, France)

KEYWORDS. Nanocellulose; fluorescence; aerogel; carbon dot; sensor.

Corresponding

Author:

*

Dr.

Ning

Lin,

Email:

[email protected],

or

[email protected]. Address: #122 Luoshi Road, Wuhan University of Technology, Wuhan 430070, P. R. China. Tel: +86-27-87152611; fax: +86-27-87152611.

ACS Paragon Plus Environment

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ABSTRACT. Generally produced by solvent sublimation via freeze-drying or critical point drying treatment, nanocellulose-based aerogels have attracted considerable interest offering the features of sustainability, available surface reactivity, light weight, high porosity and specific surface area. This study presents a novel strategy for the preparation of fluorescent aerogels based on covalent linkage between the natural skeleton of cellulose nanofibril (CNF) and fluorescent carbon dot (CD). The maximum CD grafting content on the CNF-based aerogel was 113 mg / g, providing bright blue fluorescence under ultraviolet with a high fluorescent quantum yield of 26.2%. Besides improved mechanical properties with 360% increase in compression strength, the covalent-bonded CD nanoparticle further serves as a structural stabilizer to endow the characteristic of shape recovery in water for the fabricated fluorescent aerogel. Finally, this aerogel displays high sensitivity and selection on the recognition of NOx and aldehyde species, which is studied for the detection of glutaraldehyde (GA) at ultralow concentrations (ppm) in water. Taking the innovation of an organic solvent-free route and avoiding the toxic crosslinking reagents or fluorescent sources, the CNF/CD-based fluorescent aerogel developed in this study is a promising functional material for potential optical sensing application.

INTRODUCTION Since the first report in 1931 by Kistler,1 aerogels are defined as a class of materials exhibiting a porous microstructure with three-dimensional and interconnected micro/mesopores. They recently attract much attention in the fields of thermal and acoustic resistance, catalysis, adsorption, biomedical materials and so on.2 Originated from their intrinsic microstructure, aerogels commonly exhibit exceptional properties, for instance an extremely high porosity (generally > 98.0%), low density (approaching 1.2  10-4 g cm-3), enormous active surface area


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(beyond 1000 m2 g-1), and ultralow thermal conductivity (down to 12 mW m-1 K-1).3,4 Considering their optically tunable and porous features, fluorescent aerogels are regarded as functional materials with potential applications as biochip for molecular recognition,5,6 sensor for dust/gas capture,7,8 platform for biocatalysis and bioanalysis (in vivo imaging).9,10 The architecture of fluorescent aerogels is emphasized on the original selection of fluorescent component and aerogel’s skeleton, together with the combination and interaction between both constituents. The conventional sources of fluorescence used in aerogels are heavy metal-based quantum dots and organic dyes,11,12 which inevitably raise safety issue and therefore restricts the practical application of the prepared fluorescent aerogels. The choice of aerogel’s skeleton can be generally divided into two groups, including various inorganic compounds (such as silica, clay, metals)4 and diverse organic polymers (for instance polyurethane,13 polyimide,14 polyurea,15 polyisocyanurate16,17 etc.). Regarding processing, the simple mixture of the skeleton and fluorescent components by physical compounding is a typical approach to fabricate the composite aerogels, after freeze-drying or critical point drying treatment. However, the aerogels processed by this method generally lack sufficient interaction to support the structural stability, which may cause the collapse or close of interconnected micropores in microstructures for the obtained aerogels. Moreover, the weak interaction between aerogel skeleton and fluorescent molecules will induce the aggregation and quenching of fluorescent components, and further results in the loss and unsustainability of fluorescence performance for the aerogels. Derived from the most abundant natural resource, nanocellulose (divided as semi-flexible cellulose nanofibrils and rigid cellulose nanocrystals) is a promising nanomaterial for the development of sustainable aerogels considering its numerous advantages of renewability, nontoxicity, biocompatibility, high strength, low density and tunable surface chemistry.18


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However, despite the properties of high porosity and specific surface area similar to silica-based aerogels, pure nanocellulose-based aerogels are generally too brittle to be utilized practically. Another limitation derives from the hydrophilic nature of nanocellulose, which tends to make the fabricated aerogels prone to disassembly and structural collapse in water.19 An efficient approach to solve these two issues is based on the chemical reaction between the surface active hydroxyl groups of nanocellulose and diverse crosslinking agents, with the purpose of stabilizing the microstructure and enhancing the mechanical performance of nanocellulose-based aerogels. The reported crosslinking reagents used for the enhancement of nanocellulose aerogels include polyamide-epichlorohydrin

resin,20-22

diisocyanatos,23,24

hydrazide25

and

1,2,3,4-

butanetetracarboxylic acid.26,27 However, most crosslinking reagents are toxic and waterinsoluble (requiring organic solvent for the reaction), which instead adversely affect the biocompatibility and sustainability of the prepared aerogels. To address the present limitations of nanocellulose-based aerogels, we designed in this study a novel fluorescent aerogel using an organic solvent-free fabrication route based on two renewable nanomaterials, carboxylated cellulose nanofibrils and surface-amino carbon dots. In here, cellulose nanofibril (CNF) as a biopolymer is a sustainable precursor and nontoxic skeleton for the construction of aerogels compared to traditional inorganic materials or organic polymers, while carbon dot (CD) is an alternative fluorescent source to replace heavy metal-based quantum dots (QDs) and organic dyes in view of its excellent photostability, biocompatibility and low toxicity.28-30 The structural stability of aerogels can be achieved by covalent bonding from condensation reaction between both nanoparticles involving the carboxyl groups of CNF and amino groups of CD. Ascribed to the covalent anchoring of CD on the surface of CNF, this architecture preserves the microstructure (porous structure) of aerogel skeleton and meanwhile


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mechanical enhancement of aerogel performance based on the formation of a three-dimensional crosslinked network. The introduced CD serves as both fluorescent source to endow the optical function and as rigid crosslinking point to support the stress transfer for the structural recovery and mechanical improvement of ensuing aerogels. The proposed preparation strategy is easy to implement and uses water as the reaction medium, which preserves the original biocompatibility and nontoxicity of both components. The fabricated CNF/CD-based fluorescent aerogel keeps the typical features of the nanocellulose-based aerogels with lightweight (about 0.02 g·cm-3), high porosity (> 98.5%) and specific surface area (> 100 m2·g-1), and also exhibits the additional characteristics of shape recoverability in water, high quantum yield (26.2%) and selective and sensitive detection of NOx and aldehyde molecules as an optical sensor. EXPERIMENTAL SECTION Materials. Whatman filter paper as cellulose source was purchased from Aladdin Corporation (Shanghai, China). 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 98%), hyperbranched polyethyleneimine (PEI, Mw = 1800, 99%), citric acid (CA, 97%), N-(3-dimethylaminopropyl)N-ethylcarbodiimide hydrochloride (EDC, 98%), N-hydroxysuccinimide (NHS, 98%), sodium bromide (NaBr, 99%), sodium hydroxide (NaOH, 96%), sodium hypochlorite (NaClO, 14.5% Cl content), sulfuric acid (H2SO4, 98%) were supplied by Aladdin Corporation and directly used without any treatment. Preparation of cellulose nanofibrils (CNF). CNF derived from Whatman filter paper were prepared by TEMPO-mediated oxidation pretreatment and mechanical disintegration, taking a previous report as reference.31 Briefly, the filter paper (10.0 g) was pulverized using a grinder (LD-T350, China) for 10 min to obtain the cellulose powder, which was then suspended in water


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(1000 mL) for TEMPO-mediated oxidation pretreatment. After the addition of sodium bromide (1.0 g) and TEMPO (0.16 g) in the suspension, the oxidation of cellulose was initiated by dropping the NaClO solution (14.5%, 51.3 g) with pH regulation at 10.0 using diluted NaOH (1.0 M). The pretreatment was performed under mechanical stirring at room temperature for 5 h until reaching stable pH equilibrium. After the introduction of ethanol (50 mL) and HCl solution (0.1 M) to stop the reaction, the treated slurry was further homogenized by strong mechanical disintegration using a Waring Blendor (8010S, U.S.A.) for 30 min. It should be noted that to avoid thermal degradation of cellulose the disintegration process was conducted in 5 phases with duration control at 6 min for each treatment. Finally, the suspension containing the carboxylated CNF was purified in dialysis bag (cutoff Mn = 14 kDa) successive against distilled water for one week. Synthesis of amino-modified carbon dots (CD) by hydrothermal treatment. Organic citric acid was used as the carbon source for the preparation of CD together with the surface aminomodification by the introduction of branched PEI.32 The mixture of PEI (0.945 g) and citric acid (0.1 g) was added to distilled water (30 mL) under 1 h magnetic stirring for complete dissolution. The temperature of the oven was controlled as 180 C and kept for 30 min before starting the reaction. Then, a Teflon-lined stainless steel hydrothermal reactor (50 mL) containing all the reactants was placed in the oven for 20 h reaction duration. After the hydrothermal reaction, the transparent liquid with yellow color containing CD was carefully put into dialysis bag (cutoff Mn = 3.5 kDa) in distilled water for 3 days to remove the unreacted reagents. Preparation of fluorescent aerogels CNF/CD-x. The pure CNF aerogel was fabricated by freeze-drying, and further placed into the 10 ml amino-modified CD aqueous suspension at varied concentrations (0.5, 1.0, 2.0, 3.0, 3.5, 4.0 wt%). The condensation reaction between


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carboxyl groups of CNF-based skeleton and amino groups of CD was initiated by the addition of NHS (8 mg) and EDC (10 mg) catalysts for 1 h. To promote the complete reaction from the external to internal aerogels, the reactor was sealed and transferred to a shaking incubator (TS100C, China) for further reaction (12 h). Purification of the aerogels was performed by repeated washings against water five times to remove any free CD or small molecules, until the washing raffinate was colorless of and without any fluorescence under ultraviolet radiation. Finally, the obtained products were freeze-dried again to release the fluorescent aerogels, which were labeled as CNF/CD-x (x represents the treated CD concentration of the aerogel). Characterization. Structure and properties of cellulose nanofibrils and amino-modified carbon dots. The morphology of CNF and CD was observed by transmission electron microscopy (TEM). A drop of diluted suspension containing 0.01 wt% CNF or CD nanoparticles was observed on a Tecnai G2 F30 instrument (FEI, U.S.A.) at 300 kV. The CNF suspension was negatively stained with 2% (w/v) uranyl acetate before the observation. The crystalline properties of freeze-dried CD were analyzed by X-ray diffraction (XRD) on a D8 Advance X-ray diffractometer (Bruker), with diffraction angles (2θ) ranging from 5° to 60°. The zeta potential of CNF and amino modified CD were measured using a Malvern Zetasizer Nano ZS (Malvern Instruments Co., UK). The fluorescent features of CNF and CD suspensions were recorded on a UV spectrophotometer (UV-2600, Japan) at a concentration of 0.01 wt% with a scanning range of 200-800 nm. The fluorescent characteristic of CD suspension (5 mg mL-1) was further detected by a fluorescence spectrophotometer (F-7000, HITACHI, Japan) at the excitation wavelengths ranging from 280 nm to 360 nm. Covalent grafting of amino-modified CD on CNF-based aerogels.


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The covalent reaction between CNF-based aerogels and CD was proved by Fourier transform infrared spectroscopy (FTIR), which was performed on a FTIR iS5 spectrometer (Nicolet, Madison, U.S.A.) in the range of 4000-400 cm-1. To achieve the maximum CD grafting for fluorescent aerogels, the pure CNF aerogels were immersed into CD suspensions with gradient concentrations ranging from 0.5 wt% to 4.0 wt%. All the raffinates (including washing residues) were collected and diluted 20 times for UV absorbance measurements at the characteristic wavelength of 365 nm for CD, corresponding to the calculation of unreacted CD concentration. The CD grafting values (GCD) for fluorescent aerogels can be determined by the following equation:

𝐺𝐶𝐷(𝑚𝑔/𝑔) =

𝑉𝑖𝐶𝑖 ― 𝑉𝑟𝐶𝑟 𝑚𝐶𝑁𝐹

(1)

where, Vi and Ci are the volume and concentration, respectively, of the initial CD suspension before reaction; Vr and Cr represent the volume and concentration, respectively, of the raffinate CD suspension after reaction; and mCNF is the weight of pure CNF aerogel. In the designed experiments, Vi and mCNF were controlled as 10 mL and about 0.063 g, while Ci ranged from 0.5 wt% to 4 wt%. The values of Vr were measured after the raffinate collection, and Cr was calculated according to the UV absorbance and standard curve of CD. Porosity, specific surface area and microstructure of fluorescent aerogels. The porosity of obtained aerogels was calculated according to the following equation: porosity(%) = 1 ―

𝑎𝑒𝑟𝑜𝑔𝑒𝑙 𝑠

(2)

aerogel and s represent the apparent density and skeletal density, respectively, of the aerogel, which can be calculated by the following equations:


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aerogel = 𝑠 =

𝑚 𝑉

1 𝐶𝑁𝐹

(3)

(4)

where, m and V are the weight and volume, respectively, of the fluorescent aerogel. CNF represents the density of CNF as 1.600 g cm-3 according to a previous report.33 The specific surface area of aerogels was measured at the relative pressure of P/Po = 0.294 by Brunauer−Emmett−Teller (BET) method on a specific surface and pore analyzer (Tristar II 3020, USA). The nitrogen isothermal adsorption and desorption volumes were measured at the relative pressure of P/Po = 0.991 using Barrett-Joyner-Halenda (BJH) method. The microstructure of aerogels was observed by scanning electron microscopy (SEM, Hitachi S4800 instrument) at an accelerating voltage of 10 kV. In order to preserve the original morphologies, aerogels were treated in liquid nitrogen, immediately cut in the transverse direction, and coated with gold using a sputter coater for SEM observation. Mechanical properties and structural recoverability of fluorescent aerogels. Cylindrical aerogel samples with a size of 15 mm (diameter) × 17 mm (height) were compressed at a speed of 1 mm min-1 on a CMT6503 universal testing machine (SANS, China) in both air and water. To investigate the structural recoverability, the reciprocating compression in water was performed for the fluorescent aerogel, with 10%, 20% and 30% compression strains. Meanwhile, the experiment of multiple cycles involving repeated loading-unloading compression (10 times) was carried out for the fluorescent aerogel in water.34,35


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The reusability of the aerogels (CNF and CNF/CD-3%) was evaluated by the measurement of water absorption during the water absorbing−discharging process for 15 times. The discharging treatment was performed in an oven at 45 C overnight for the complete drying of aerogels. Fluorescent behavior and optical sensing of aerogels. The fluorescent behavior of all aerogels was photographed under UV irradiation in a UV spectroscopy analyzer (WFH-203B, China). The fluorescence quantum yield of fluorescent aerogels was measured using a steady-state transient fluorescence spectrometer (FLS-1000, UK) with an excitation wavelength of 310 nm. The optical sensing applications of fabricated fluorescent aerogels were evaluated in different model gases and liquids to investigate their selective and sensitive fluorescent behavior. Typically, the experiments were conducted in various pure gases including N2, CO2, SO2 and NOx, and pure liquids consisting of water, toluene, ethanol, tetrahydrofuran (THF), acetone and diluted formaldehyde (FA, 40 wt%) and glutaraldehyde (GA, 1 wt%) aqueous solutions. To investigate the correlation between the fluorescence property of aerogels and the concentration of detected liquid, the selected fluorescent aerogel was immersed in the GA solutions at gradient concentrations (1 ppm, 5 ppm, 10ppm, 50 ppm, 100 ppm) for 24 h for the measurement of the fluorescence quantum yield of treated aerogels. RESULTS AND DISCUSSION Characterization of cellulose nanofibrils (CNF) and carbon dots (CD). The schematic illustration of the preparation route for fluorescent aerogels is shown in Figure 1, including the preparation of CNF-based aerogel’s skeleton, synthesis of surface amino-modified CD and covalent bonding between CNF and CD based on carboxyamine condensation. It is


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worth noting that the whole preparation is an organic solvent-free route (in water), with the preservation of renewability and low-toxicity merit of the raw materials (cellulose for CNF and organic citric acid for CD) for the fabricated aerogels. The CDs are expected to be grafted on the CNF-based skeleton and serve as both the fluorescent source and rigid crosslinking agent for the support of structural stability.

Figure 1. Preparation route for CNF/CD-based fluorescent aerogels (SL: photographs under sunlight, UV: photographs under ultraviolet). The resultant CDs exhibit spherical and monodisperse nanoparticle morphology with average diameter of 3.6 nm, as shown in Figure 2A. High resolution TEM (HRTEM) imaging for CD with higher resolution reveals the typical graphite facet (100) with a lattice spacing of 0.21 nm


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(as shown in the insert in Figure 2A).36 The semi-flexible CNF with entangled fibrils can be observed in Figure 2B, with 10-20 nm in diameter and several m length.

Figure 2. TEM images for CD (A) and CNF (B) as well as the observation at higher resolution for lattice spacing and size distribution for prepared CD nanoparticles (inserted images). The crystalline property and fluorescent behavior of CD have been further characterized by XRD, UV and photoluminescence (PL) spectroscopy. As shown in Figure 3A, the synthesized CD shows a broad peak located at 19-23, ascribed to the (002) lattice spacing for the carbon nanomaterial.36 Apparent blue fluorescence can be observed for the CD aqueous suspension under UV irradiation (Figure 3B), which displays two characteristic peaks located at 240 nm and 360 nm ascribed to the –* transition of aromatic ring and n–* transition of carbonyl group from amino-modified CD on the UV spectrum.37 As shown in Figure 3C, the characteristic excitation wavelength of CD is confirmed as 310 nm by the results of PL spectroscopy, which will be used for subsequent measurement of fluorescence quantum yield for the obtained fluorescent aerogels. Finally, the possible covalent linkage between CNF-based aerogels and CD nanoparticles was investigated by FTIR. As shown in Figure 3D, the features of –NH2 and –CO–


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NH– stretching located to 1557 cm-1 and 1650 cm-1 can be observed on the spectrum for the prepared CD, while the typical cellulose characteristics are attributed to the pure CNF aerogel besides the presence of carboxyl stretching (–COOH) at 1730 cm-1 formed during the TEMPOoxidization pretreatment. The disappearance of carboxyl peaks on the spectrum for CNF/CD-3% aerogel can prove the complete chemical reaction between CNF and CD. The characterization of surface charges for CNF and CD was performed as shown in Figure S1 in Supporting Information. The CNF exhibits a typical negatively-charged surface with the -potential of -62.3 eV due to the TEMPO-pretreatment (selective oxidization to carboxyl groups). As the comparison, the weak charged surface with -potential of -4.5 eV is observed for the prepared CD in alkaline condition (pH=9). The results of -potentials reflects the fact that the interaction between CNF and amino-modified CD is strong covalent linkage (-COOH and –NH2 reaction) but not electrostatic adsorption, which is in consistent with the FTIR results.


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Figure 3. XRD pattern (A) and UV spectrum (B) for CD; fluorescence spectra for 0.5 wt% CD aqueous suspension at the excitation wavelengths from 280 nm to 360 nm (C); and FTIR spectra for (a) CD, (b) CNF aerogel, (c) CNF/CD-3% aerogel (D). UV absorption and gravimetric measurement have been performed for fluorescent aerogels to determine their CD grafting content. The raffinate and washing solution for the removal of free CD were collected during the fabrication of fluorescent aerogels, and then analyzed by UV absorption for the different CD concentrations (Figure 4A and Table S1). Based on the UV absorption and gravimetric measurement, the fluorescent aerogel CNF/CD-3% has the maximum CD grafting content (113 mg for 1 g CNF aerogel), as shown in Figure 4B. The high CD grafting content is a critical factor to determine its fluorescent behavior and application of the resultant aerogel.

Figure 4. UV absorption spectra for the collected raffinate (diluted 20 times) from the washing of fluorescent aerogels (A); and calculated CD grafting content for fluorescent aerogels based on UV absorption results and gravimetric measurement (B). (Inserted image in A is the standard curve and equation from CD concentration in water vs. absorbency) Fluorescent behavior and microstructure of fluorescent aerogels.


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The appearance and fluorescent behavior of aerogel are shown in Figure 5. As shown in panel A, the pure CNF aerogel does not exhibit fluorescence when exposed to both sunlight and ultraviolet. After the introduction of CD component, all the aerogels exhibit apparent blue fluorescence, which proved the maintenance of original fluorescence during the covalent linkage between CD and CNF components. To check the reactive integrity and CD dispersibility, the fluorescent aerogel CNF/CD-3% was vertically and transversely cut for cross-sectional observation. As shown in panel B, all the cross-sections of the aerogel display the same fluorescence as its surface, indicating the uniform grafting of CD from the external to internal of the aerogel attributed to the experimental design for this reaction in the shaking incubator. The crushed aerogel CNF/CD-3% was immersed in water for 3 h to observe the possible fluorescence quenching and any leakage of free CD component. As shown in panel C, the prepared aerogel retains its stable fluorescent behavior in water; meanwhile non-fluorescence is observed in the supernatant therefore demonstrating the absence of any CD leakage from the aerogel.


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Figure 5. Appearances and fluorescent behavior for the prepared aerogels (A), cross-sectional observation for vertically and transversally cut aerogel CNF/CD-3% (B), and crushed powder from aerogel CNF/CD-3% in water (C) under sunlight and ultraviolet. The fluorescent properties of aerogels were further evaluated by measuring the fluorescent quantum yield (Yf, %), as shown in Figure 6. The values of Yf for all the obtained aerogels are higher than 10%, which is consistent with the bright and blue fluorescence images observed in Figure 5. In addition, the fluorescent aerogel CNF/CD-3% has the maximum Yf (26.2%) and therefore should exhibit the strongest fluorescence performance, attributed to the maximum CD grafting content as discussed before.

Figure 6. Fluorescent quantum yield for the fabricated aerogels. The microstructure of the fabricated aerogels was accessed by observing their cross-section using SEM, as shown in Figure 7. As for pure CNF aerogel (panel A), all fluorescent aerogels containing CD nanoparticles exhibit a porous microstructure (panels B to E), with the presence of numerous interconnected micropores of different sizes. The covalently-bonded CD on the


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CNF-based skeleton do not seriously induce a structural collapse or micropores’ closure for the aerogels, which is essential for their potential sensing application.

Figure 7. SEM images of cross-sectional observation for aerogels CNF (A), CNF/CD-1% (B), CNF/CD-2% (C), CNF/CD-3% (D), CNF/CD-4% (E). The nitrogen isothermal adsorption and desorption is a conventional method to analyze the pore volume and pore width of porous materials. As shown in Figure 8A, the pure CNF aerogel presents the exactly overlapping adsorption-desorption behavior, indicating weak adsorption of N2 molecules in the skeleton of cellulose-based aerogel. As a contrast, a typical hysteresis loop can be observed on the adsorption-desorption curve for fluorescent aerogel (in the case of CNF/CD-3%), demonstrating the possible adsorption of gaseous molecules by the introduced CD nanoparticles. Furthermore, the measured pore volumes of aerogels increased attributed to the presence of nanosized CDs (Figure 8B), corresponding to the gradual increase of specific surface area (as summarized in Table S2). In accordance with a previous report,38 the BET specific surface area for pure CNF aerogel prepared by freeze-drying treatment was measured as 63.5


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m2·g-1 in this study. However, because of additional CD nanoparticles, the BET specific surface area of fluorescent aerogels gradually increases to > 100 m2·g-1 for CNF/CD-3%, 3.5% and 4%. Regardless of the increase in surface area, the apparent density (𝜌aerogel) and calculated porosity of fluorescent aerogels retain a value of about 0.02 g·cm-3 and 98.5%, respectively, preserving the light-weight and porous advantages of CNF-based aerogel.

Figure 8. N2 adsorption-desorption curves for the CNF and CNF/CD-3% aerogels (A); and BET curves of pore volume vs. pore width for the prepared aerogels (B). Structural stability and mechanical performance of fluorescent aerogels. The water absorption was measured to reflect the integrity of the porous structure for the aerogels. As shown in Figure S2, all fluorescent aerogels have similar water absorption values to that of the pure CNF aerogel, which indicates the preservation of the original porous structure of CNF-based skeleton when introducing CD nanoparticles. The structural stability of aerogels can be evaluated from their reusability by measuring the water absorption during repeated water absorbing−discharging processes.39 One drawback of hydrophilic cellulose-based aerogels is their redispersion and therefore easy disassembly in water. As shown in Figure 9A, the pure CNF aerogel can only experience four times the water absorbing−discharging treatment, and then


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exhibits complete structural collapse (shown in the inserted image). In fact, the structural disintegration appears after the first treatment for the pure CNF aerogel even with a careful manipulation (Video 1 in Supporting Information). The fluorescent aerogel CNF/CD-3%, by contrast, retains a stable structure (Video 2 in Supporting Information) together with a high level of water absorption at about 3700% even after 15 times of continuous water absorbing−discharging (Figure 9B). The enhanced structural stability for fluorescent aerogels can be attributed to the support of covalently-linked CD nanoparticles serving as crosslinking agent for the porous skeleton of CNF-based aerogel. Is there CD leakage for compression of the fluorescent aerogels during the successive compressions? An experiment was performed to investigate the influence of mechanical compression to the fluorescent behavior of aerogels. Briefly, the fluorescent aerogel CNF/CD-3% was placed in 10 ml of water and suffered 1, 5, 10 cycles of compression tests at the fixed strain of 30%. After taking out the aerogel sample, the residual liquids were collected for the photograph under ultraviolet and characterized by UV spectroscopy. As shown in Figure S3, no fluorescence appears in the residual liquids and meanwhile no absorption peak at the feature of 360 nm (CD fluorescence) is observed, which proves the strong linkage of CDs on the skeleton of CNF aerogel and therefore without any fluorescence and free CDs leakage under the compression tests.


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Figure 9. Water absorption of aerogels after repeated absorbing−discharging water treatments for the pure CNF aerogel (A), and the fluorescent CNF/CD-3% aerogel (B). The mechanical performance and structural recovery of aerogels have been investigated by compression tests, as shown in Figure 10. Due to covalent crosslinking and stress transferring from CD nanoparticles, the mechanical properties for all fluorescent aerogels are enhanced when increasing the grafted CD content, as shown from the compression stress-strain curves of aerogels in Figure 10A. Specifically, the fluorescent aerogel CNF/CD-3% exhibits a remarkable enhancement in the compression strength whose value is 21.52 kPa, compared to only 4.68 kPa for the pure CNF aerogel (Figure S4 in Supporting Information). A similar improvement can also be observed for the aerogels in the wet state (water) as shown in Figure 10B, despite the relatively low compression stress measured during the tests due to water resistance. Consistent with the previous discussion, the fluorescent aerogels can retain the structural integrity in water due to the covalent bonding between aerogel’s skeleton and rigid CD nanoparticles, while the pure CNF aerogel seems to suffer from structural disintegration (with the reduction of compression stress to about 0) in water, particularly under high compression strains (> 30%). In fact, the fluorescent aerogels present a promising structural-recovery performance in water with


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the repeated load-unload processes (Video 3 in Supporting Information). To further investigate this flexibility property, the fluorescent aerogel CNF/CD-3% was submitted to cyclic compression tests under two different conditions. As shown in Figure 10C, the CNF/CD-3% aerogel can recover its initial mechanical status after the stress unloading during the first compression cycle. This structural recovery of the fluorescent aerogel CNF/CD-3% is also preserved after 10 compression cycles (Figure 10D), indicating the stability and resilience of the microstructure for this porous material with the incorporation of reactive CD nanoparticles. It should be noted that the slight reduction in compression stress under high strains (30%) may result from the hysteresis of the structural recovery for the aerogel during the tests.

Figure 10. Mechanical performance of the fabricated aerogels: compression stress-strain curves in air (A) and in water (B); successive compression tests for the fluorescent aerogel CNF/CD-3% at strains of 10%, 20% and 30% (C); compression stress-strain curves for the fluorescent aerogel CNF/CD-3% after 1, 5, 10 cycles of compression at the fixed strain of 30% (D).


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Optical sensing of fluorescent aerogels on selective detection of gases and liquids. Fluorescent aerogels can be used in optical sensing to detect a specific gas or liquid, depending on the molecular adsorption and fluorescence quenching of targeted species in the porous microstructure of aerogels. The selective detection of the prepared fluorescent aerogel CNF/CD3% to diverse gases and liquids is shown in Figure 11. Compared with the weak effect on the fluorescent property for N2, CO2, and SO2 gases, the nitric oxide gases (NOx) exhibit a rapid and complete fluorescence quenching to the CNF/CD-3% aerogel (image A). The strong fluorescence quenching for CNF/CD-3% aerogel by the nitric oxide gases can be explained by the prevention of radiative recombination of electrons from the interaction between electrondonating CD nanoparticles (branched PEI on the surface) and electron-withdrawing NOx molecules,40 which are adsorbed and trapped in the porous structure of the aerogel. Regarding the liquid detection, the CNF/CD-3% aerogel exhibits a high selection to aldehyde species (image B), for instance the observation of fluorescence quenching in 40 wt% FA and 1 wt% GA aqueous solutions. The possible explanation may result from the covalent reactions between aldehyde groups and surface amino groups of CDs that therefore induce the restriction of fluorescent excitation by the adsorption coverage. In view of its rapid fluorescence quenching, the GA aqueous solution (1 wt%) was used as model to investigate the detective sensitivity of the aerogel as a function of time. Over time, the effect of fluorescence quenching from GA molecules to the CNF/CD-3% aerogel reveals gradually, exhibiting visible fluorescence decay during the first 10-30 min and complete fluorescence disappearance after 4 h.


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Figure 11. Fluorescence behavior for the fabricated aerogel CNF/CD-3% under ultraviolet before and after gas adsorption (pure N2, CO2, SO2, NOx gases) (A), before and after liquid adsorption (pure water, toluene, ethanol, THF, acetone liquids and 40 wt% FA aqueous solution, 1 wt% GA aqueous solution) (B), and GA adsorption as a function of time (C). (THF: tetrahydrofuran, FA: formaldehyde, GA: glutaraldehyde) The sensitivity of the prepared fluorescent aerogel towards the detection of GA concentration was further analyzed, and we proposed a mathematical equation between the concentration of GA (CGA) and fluorescence quantum yield (Yf) of the aerogel. As shown in Figure 12, the CNF/CD-3% aerogel has a high sensitivity to GA at the ppm level, exhibiting a gradient fluorescence decay and quenching in the range 1 to 100 ppm of GA solutions. Typically, in comparison with that of 26.2% for untreated CNF/CD-3% aerogel, the Yf value for the aerogel gradually decreases to 22.6%, 13.4% and 1.5% after exposure to 1 ppm, 10 ppm and 100 ppm


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GA solutions. The established equation reveals a logarithmic relationship between CGA and Yf, which demonstrates the high selection and sensitivity to aldehyde species and potential use for water/air quality monitoring.

Figure 12. Experimental data and fitted equation between GA aqueous concentration (CGA, ppm) vs. fluorescence quantum yield (Yf, %) for the prepared CNF/CD-3% aerogel. Discussion Using an organic solvent-free process, a novel fluorescent aerogel has been developed in this study based on the skeleton of cellulose nanofibrils (CNF) and chemical crosslinking of carbon dots (CD). As illustrated in Figure 13, the covalent linkage between CNF and CD components significantly promotes the enhancement of the mechanical properties, resulting in structural stability and fluorescent sensing application for the obtained aerogels. On the one hand, the introduction of CD nanoparticles in the CNF-based aerogel serves as rigid crosslinking agent to enhance the compression strength and fix the porous microstructure to avoid its disintegration in water. This phenomenon is similar to the effect of screw fasteners in wooden furniture.


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Furthermore, the obtained CNF/CD-based aerogels exhibit a special performance of waterinduced shape recovery after immersion of compressed aerogels in water. This behavior can be explained by the repulsion of the molecules towards water absorption by numerous micropores and therefore by the expansion of the pores of this hydrophilic material, just like the absorbing process of some dehydrated plants in water. On the other hand, the obtained aerogels exhibit sensitive and selective detection towards nitric oxide and aldehyde species derived from the fluorescence quenching of CD, which results from the surface shielding and electron transfer blocking to fluorescent excitation of CD nanoparticles. This fluorescence quenching is also supposed to be effective for solid particles (e.g. dust), and therefore the fabricated fluorescent aerogels could be used as air optical sensing as anticipated in our original experiment design, such as for the detection of PM2.5 in the air. It is unfortunate that this part has not been performed in this study because of current instrumental limitations, which will be realized in future work. However, the developed CNF/CD aerogel still displays promising features of improved mechanical performance and structural stabilization together with functional fluorescence, which endow this sustainable and porous material potential for new optical sensing technologies.


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Figure 13. Proposed mechanisms for the structural enhancement and fluorescent sensing for the fabricated CNF/CD aerogels. CONCLUSIONS In this study, surface amino-modified carbon dot (CD) was used as fluorescence source and covalently bonded onto cellulose nanofibril (CNF)-based aerogel via carboxyamine condensation to produce fluorescent aerogels. From the chemical reaction between CNF aerogel’s skeleton and 3 wt% CD suspension, the obtained aerogel held maximum content of grafted CD nanoparticles, and therefore achieved the highest fluorescent quantum yield (Yf). The developed fluorescent aerogel met the need for non-toxicity, low density, high surface area and porosity from cellulosebased porous materials, and meanwhile exhibited improved mechanical properties and structural stability (shape recovery) attributed to the introduction of reactive CD nanoparticles. Moreover, a significant selection of fluorescent quenching was observed for the fluorescent aerogel towards specific gaseous and liquid molecules. In the case of glutaraldehyde (GA) detection, the


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CNF/CD-based aerogel displayed high sensitivity to trap the GA molecules at ppm concentrations, serving as an excellent material in the function of optical sensing. ASSOCIATED CONTENT Supporting Information. Data of diluted raffinates for UV spectroscopy analysis to determine the CD grafting contents on the aerogels in Table S1; Physical and macroscopic parameters of aerogels in Table S2; -potentials of prepared CNF and CD in Figure S1; water absorption of aerogels in Figure S2; UV spectrum of collected residual liquids for the fluorescent aerogel after successive compression tests in Figure S3; compression strength of aerogels in Figure S4 and captions of videos 1-3. AUTHOR INFORMATION Corresponding

Author:

*

Dr.

Ning

Lin,

Email:

[email protected],

or

[email protected]. Address: Luoshi Road #122, Wuhan University of Technology, Wuhan 430070, P. R. China. Tel: +86-27-87152611; fax: +86-27-87152611. ACKNOWLEDGMENT This study was supported by the National Natural Science Foundation of China (51603159) and Natural Science Foundation of Hubei Province (2017CFB490). The authors also wish to acknowledge the financial support of Anhui Province Key Laboratory of Environment-friendly Polymer Materials (KF2019004). REFERENCES (1) Kistler, S. S. Coherent Expanded Aerogels and Jellies. Nature 1931, 127, 741–741. DOI: 10.1038/127741a0.


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Fluorescent Aerogels Based on Chemical Crosslinking between

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