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Synthesis of Novel Fluorescent Cellulose Derivatives and Their Applications in Detection of Nitroaromatic Compounds Haoze Hu, Fangyu Wang, Lisha Yu, Kazuki Sugimura, Jinping Zhou, and Yoshiyuki Nishio ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03855 • Publication Date (Web): 04 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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ACS Sustainable Chemistry & Engineering

Synthesis of Novel Fluorescent Cellulose Derivatives and Their Applications in Detection of Nitroaromatic Compounds Haoze Hu1, Fangyu Wang1, Lisha Yu1, Kazuki Sugimura2, Jinping Zhou∗,1, and Yoshiyuki Nishio*,2 1

Department of Chemistry and Key Laboratory of Biomedical Polymers of Ministry of

Education, Wuhan University, Luojia Hill, Wuhan 430072, China 2

Division of Forest and Biomaterials Science, Graduate School of Agriculture, Kyoto University,

Sakyo-ku, Kyoto 606-8502, Japan

∗

Corresponding author, Tel: +86-27-68752977, Fax: +86-27-68754067

E-mail: [email protected] (J. Z.); [email protected] (Y. N.) HU

UH

ACS Paragon Plus Environment

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ABSTRACT: A series of fluorescent cellulose derivatives (AC-AET-DMANMs) with different degree of substitution (DS) were successfully prepared. First, allyl cellulose (AC) was synthesized from cellulose in NaOH/urea aqueous solution, and then 2-aminoethanethiol (AET) was introduced onto the cellulose backbone via thiol-ene click reaction. Finally, the fluorescent groups were introduced by the reaction of the AET modified AC with 4-dimethylamine-1,8naphthalic anhydride (DMANA). The structure and fluorescent properties of AC-AETDMANMs were characterized by elemental analysis, FT-IR, 1H NMR, UV-vis and fluorescence spectroscopy. AC-AET-DMANM with lower DSDMANM (referring to the DS of naphthalimide groups, ≤ 0.25) was soluble in DMSO. AC-AET-DMANM of DSDMANM ≤ 0.09 displayed stable fluorescence in DMSO and even in the solid state. The emission of AC-AET-DMANM in DMSO quantitatively and sensitively responded to 2,4,6-trinitrophenol (TNP) and 2,4dinitrophenylhydrazine (DNH) by fluorescence quenching, and the limits of detection were determined to be 1.4×10-7 and 9.9×10-8 mol/L, respectively. Moreover, water-soluble fluorescent derivative (AC-AET-DMANM-2W) was prepared by further thiol-ene click reaction between AC-AET-DMANM-2 and AET. It can also be applied in the detection of TNP and DNH in aqueous media with the detection limits of 2.5×10-8 and 3.2×10-8 mol/L, respectively. The quenching mechanism is attributed to the photo-induced electron transfer and resonance energy transfer of the fluorescent cellulose derivatives to TNP/DNH molecules. The results illustrate a high applicability of the novel fluorescent cellulose derivatives to the detection of specific chemical entities in aqueous/non-aqueous media. KEYWORDS: Cellulose derivatives, Fluorescence, Thiol-ene click reaction, Detection, Nitroaromatic compounds


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INTRODUCTION It is well known that nitroaromatic compounds are commonly used as explosives. There has been an immense research interest in the rapid and selective detection of nitro-explosives, which is very important to improve public security and pollution problems for human and ecosystem.1-3 In the past few decades, various analytical methods, such as mass spectrometry,4 electrochemical sensors,5 optical sensors,2 and chemosensors,6-9 have been developed for the sensitive detection of nitroaromatic compounds. Among these methods, fluorescent based sensing is one of the most promising approaches owing to its rapid response time, good selectivity, excellent sensitivity, and simplicity.10 Numerous fluorescent polymers have been synthesized and used in the detection of trace amounts of nitroaromatics.1,10,11 For example, the fluorescence of pyrene (Py)doped polystyrene (PS) films with ordered nanopores could be rapidly and selectively quenched by nitroaromatic vapors.12 Nanofibers of Py-functional PS copolymer could be employed for the detection of 2,4,6-trinitrotoluene (TNT) in water with a detection limit of 5 nM.13 Aggregationinduced emission (AIE) active POSS-based copolymer films displayed remarkable fluorescence quenching sensitivity to TNT and 2,4-dinitrotoluene (DNT) vapors.14 The electrospun Pypolyethersulfone nanofibers could be applied for sensitive detection of 2,4,6-trinitrophenol (TNP), TNT, and DNT in aqueous phase with limits of detection of 23, 160, and 400 nM, respectively.15 The conjugated polymer nanoparticles of PFMI displayed a very low detection limit of 30.9 pM for TNP in aqueous environment.16 Water-soluble nonconjugated PEI-glucose nanoparticles showed strong fluorescence emission, and could be used for detection of TNP in aqueous medium with a low detection limit (26 nM).17 As one of the most abundant renewable resources, cellulose is a high-potential polymer in sustainable chemistry.18,19 It can be converted into various derivatives and regenerated


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materials.20,21 Recently, many methods have been developed to prepare fluorescent cellulose materials. The plenty of -OH groups make them possible to introduce a diversity of fluorescent functional groups onto the backbone. Through simple and efficient routes, fluorescein-5’isothiocyanate,22

7-hydrazino-4-methylcoumarin,23

7-amino-4-methyl-coumarin,23

5-(4,6-

dichlorotri-azinyl) aminofluorescein,24 fluorescein-substituted lysine25 and other fluorescent groups have been attached onto cellulose nanocrystals (CNCs). L-leucine amino acid could be used as a spacer linker between CNCs and 5 (and 6)-carboxy-2’,7’-dichloro fluorescein to build a pH-sensitive nanomaterial.26 Cellulose nanofibril based graft conjugated polymer film exhibited rapid response toward nitroaromatic vapors.27 Cu2+, Hg2+ and some other heavy ions could be easily detected by the rhodamine modified cellulose materials.28,29 Not only for small molecules or ions detections, fluorescent cellulose materials can also be used as biosensors in the detections of macromolecules like esterase.30 However, attributing to the strong hydrogen bonding and high crystalline of cellulose, preparation of fluorescent cellulose derivatives with good solubility is still a challenge. Furthermore, it is difficult to control the degree of substitution (DS) via the traditional esterification or etherification of cellulose.18 Previously, allyl cellulose (AC) was synthesized from cellulose in NaOH/urea aqueous solution, and thiol–ene click reactions of AC with different thiol compounds were studied.31 It was proved to be an effective route to obtain DS-controllable novel cellulose derivatives. The 1,8-naphthalimide structure, as a highly versatile building unit that absorbs and emits at long wavelengths, has been extensively used in the construction of novel therapeutics and chemical probes.32,33 In the present work, amine groups were introduced onto the cellulosic backbone via allylation that was followed by thiol-ene click reaction, and then a series of fluorescent cellulose derivatives (coded as AC-AET-DMANMs) with different DS were synthesized by introducing of


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4-dimethylamine-1,8-naphthalimide (DMANM) groups. Through further thiol-ene click reaction of AC-AET-DMANM and 2-aminoethanethiol (AET), a water-soluble fluorescent derivative (AC-AET-DMANM-2W) was successfully prepared. The fluorescence properties of AC-AETDMANMs were studied and the selective detections of nitroaromatic compounds in aqueous/non-aqueous media were examined. Moreover, the practical use of AC-AET-DMANMs in real water samples has been studied. The results indicate that the novel fluorescent cellulose derivatives have potential applications in the field of chemosensing.

EXPERIMENTAL SECTION Materials. Microcrystalline cellulose (MCC) was supplied by MERCK (Germany), and the degree of polymerization (DP) was 100-300. Allyl chloride, 2-aminoethanethiol (AET), 2,2dimethoxy-2-phenylacetophenone (DMPA), 4-bromo-1,8-naphthalic anhydride (BNA) and other reagents were of analytical grade and used without further purification. Synthesis of AC-AETs. According to the previous work,31 AC was synthesized from cellulose in NaOH/urea aqueous solutions, and the DSallyl was determined to be 1.25 by 1H NMR measurement. AC (0.1 g) was dissolved in DMSO (20 mL), and then a certain amount of AET, 1 mol/L HCl aqueous solution and DMPA were added at room temperature. The mixture was stirred in a nitrogen atmosphere under UV-radiation (355 nm) for 2 h. The product was precipitated and washed with ethanol and water, and then freeze-dried. By changing the molar ratio of AET to the anhydroglucose unit (AGU) of AC from 0.1:1 to 0.6:1, six AET modified AC (coded as AC-AET) samples were synthesized (Table 1). Synthesis of AC-AET-DMANMs. BNA (2.5 g) was dissolved in 15 mL of DMF at room temperature, then CuSO4 (0.112 g) and dimethylamine solution (50%, 4 mL) were added. The


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mixture was refluxed at 135 °C under a nitrogen atmosphere for 3 h to obtain 4-dimethylamino1,8-naphthalic anhydride (DMANA). DMF was removed by using an evaporator, and the purified DMANA (1.40 g; 68% yield) was obtained by recrystallization with ethanol. AC-AET (0.1g) was dissolved in 10 mL of DMSO at 25 °C, and then absolute ethanol (50 mL) and triethyl amine (0.5 mL) were added dropwise into the solution. After further addition of DMANA, the mixture was refluxed at 80 °C under a nitrogen atmosphere for 4 h. Following this, the mixture was cooled to room temperature to afford a yellow precipitate. The precipitate was washed with acetone and distilled water, and then freeze-dried to obtain AC-AET-DMANM. As listed in Table 1, six preparations were made from the six AC-AETs with different DSAET (referring to the AET moiety). Table 1. Preparation Conditions and Molecular Parameters of AC-AET-DMANMs Sample

m AC

m AET

a

DS

S%

AET

b

DS AET

m

DS

DMANA

Solubility in e

a

(g)

Yield

(g)

(g)

AC-AET-DMANM-1

0.1

0.004

0.1

1.32

0.09

0.03

0.03c

+

0.38

AC-AET-DMANM-2

0.1

0.008

0.2

2.69

0.20

0.06

0.09d

+

0.36

AC-AET-DMANM-3

0.1

0.012

0.3

3.63

0.28

0.09

0.17d

+

0.21

AC-AET-DMANM-4

0.1

0.016

0.4

4.67

0.37

0.12

0.25d

+

0.01

AC-AET-DMANM-5

0.1

0.020

0.5

5.59

0.46

0.15

-

〇

-

AC-AET-DMANM-6

0.1

0.024

0.6

6.65

0.57

0.18

-

-

-

DMSO

Theoretical value

b

c

Quantum

DMANM

Determined by elemental analysis

Calculated from the UV-Vis spectral data

d

Determined by 1H NMR measurement

e

(+) soluble; (-) insoluble; (〇) soluble at 80 °C


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Synthesis of Water-Soluble AC-AET-DMANM. AC-AET-DMANM-2 (0.1 g) (see Table 1) was dissolved in 20 mL of DMSO. Then, 0.2 g AET, 2 mL 1 mol/L HCl aqueous solution, and 0.05 g DMPA were added at room temperature, and the mixture was stirred in a nitrogen atmosphere under the UV-radiation (355 nm) for 3 h. The product was dialyzed with a regenerated cellulose tube (Mw cutoff 8000, USA.) against distilled water for 3 days, and then freeze-dried to obtain a water-soluble derivative, which was coded as AC-AET-DMANM-2W. Characterizations. FT-IR spectra were performed on a Shimadzu Perestige-21 Fourier transform infrared spectrometer. The test specimens were prepared by a standard KBr pellet method. UV-vis spectra were recorded with a UV-vis spectrophotometer (Hitachi U-4100). Elemental analysis was carried out with a CHNS elemental analyzer, Vario EL cube (Elementar, Germany). DSAET values of the AC-AET derivatives were calculated from the sulfur content (S%) by the following equation:

DSAET =

(161 + DSAC × 40) × S % 3200 − S % ×112.5

(1)

where, DSAC is 1.25 in common. NMR measurements of the derivatives in DMSO-d6 were carried on a Varian INOVA300 spectrometer (1H frequency = 300.1 MHz) at 22 °C with a standard 5 mm probe. The polymer concentration was 3 mg/mL. The chemical shift was referenced to signals of the solvent and tetramethylsilane (TMS). The molecular weight of AC-AET-DMANM-2W was determined by size exclusion chromatography combined with laser light scattering (DAWN EOS, Wyatt Technology, USA). The fluent was 0.1 mol/L NaCl aq., and the flow rate was 0.6 mL/min. Fluorescence measurements were performed on a spectrofluorophotometer (Shimadzu RF5300PC) at room temperature. Fluorescence quantum yields (Φ) of the fluorescent derivatives


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were evaluated with reference to fluorescein in 0.1 mol/L NaOH aq. (Φ = 0.93),34 by comparing areas under the corrected luminescence spectra by using the equation: ΦX =ΦST

kX kST

(

n2X n2ST

)

(2)

where, ΦX and ΦST are the fluorescence quantum yields of the test and standard samples, respectively; kX and kST are the gradients from the plots of the integrated fluorescence intensity vs absorbance of the test and standard solutions, respectively; nX and nST are the refractive index of the test and standard solutions, respectively.35,36 The Stern-Volmer equation was used to quantify the quenching efficiency:1 I0⁄I =(1+Ksv×c)

(3)

where, I0 and I are the fluorescence intensities of the AC-AET-DMANM solutions at 530 nm in the absence and presence the analytes, respectively. c is the concentration of the added analyte, and KSV is the Stern-Volmer constant. The PL lifetimes were obtained from a single photon counting spectrometer on Edinburgh Instruments (FLS920) with a Picosecond Pulsed UVLASTER (LASTER377) as the excitation light source. Cyclic voltammetry measurements were conducted on a three-electrode cell with platinum wire as counter electrode, saturated Ag/AgNO3 electrode as reference electrode and a glassy carbon as working electrode. Tetrabutylammoniumhexafluoro-phosphate (0.1 M) in acetonitrile was used as supporting electrolyte. The Fc+/Fc couple was employed as internal reference and all the measurements were performed at room temperature under inert atmosphere. Single oxidation peak was observed for AC-AET-DMANM-2W. The higher occupied molecular orbital (HOMO) level was calculated from the onset method [EHOMO = −(E(onset,ox vs Fc+/Fc) + 4.8) (eV)].16 Band gap of AC-AET-DMANM-2W was determined from the onset of UV-visible spectrum to calculate its lower unoccupied molecular orbital (LUMO) level.


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Scheme 1. Synthesis of the Fluorescent Cellulose Derivatives AC-AET-DMANMs. OH O

O

HO

NaOH/urea aq.

OH

OR2

OR1

35 oC, 72h

UV light, 2h

O

O

R1O

O

AET, DMPA, DMSO, HCl

OR2

OR1

CH2=CHCH2Cl

O

R2O

S

R2 = H or

R1 = H or

NH2

or Allyl Cellulose (AC)

Cellulose

CH3

H3 C

O Br

AC-AET

DMF, O

N H

H3 C

O

BNA

DMANA

OR4

OR3

O

N

S or

O

R3 = H or

H3C

TEA, 80oC, 4h S O

or

N

S

N NH2

OR3

AET, DMPA, DMSO, HCl S O

DMSO, EtOH

O

R3O

OR4 or

O

UV light, 3h

O

R4O

R4 = H

O

H3 C

135oC, reflux, 3h

O

O N

, CuSO4 5H2O

or

CH3

Water-soluble AC-AET-DMANM

O

N NH2

H3 C

CH3

AC-AET-DMANM

RESULTS AND DISCUSSION Synthesis and Structure of AC-AET-DMANMs. Scheme 1 illustrates the synthesis of ACAET-DMANMs, and the reaction parameters and results are summarized in Table 1. With an increase in the in-fed amount of AET, the sulfur content (S%) of AC-AET increased. The DSAET calculated from the sulfur content was in good accordance with the theoretically expected DSAET values. Therefore, DSAET of AC-AET can be easily controlled attributing to the high selectivity and efficiency of thiol-ene click reaction. All the AC-AET samples were well dissolved in DMSO. After addition of absolute ethanol, AC-AET solution turned into dispersion. As the


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reaction of AC-AET with newly added DMANA proceeded, the dispersion became turbid and an insoluble product appeared. Ultimately, a yellowish solid product was obtained after the process of purification and freeze-drying. AC-AET-DMANM-1–4 can be dissolved in DMSO at room temperature. AC-AET-DMANM-5 was soluble in DMSO at 80 °C at a low concentration, but precipitated when the solution was cooled down to the room temperature. AC-AET-DMANM-6 cannot be dissolved in DMSO and other solvents, attributing to a comparatively higher DS of DMANM. AC-AET-DMANM-2W was prepared by further thiol-ene click reaction between ACAET-DMANM-2 and AET in DMSO, which can be well dissolved in both DMSO and water to obtain clear orange solutions. The weight-average molecular weight (Mw) and number-average molecular weight (Mn) of AC-AET-DMANM-2W were determined to be 3.17×104 and 2.49×104 g/mol, respectively.

AC-AET-DMANM-2

AC-AET-DMANM-4

AC-AET-DMANM-5 AC-AET-DMANM-6

1379 1682

790

1640 1583

4000 3500 3000 2500 2000 1500 1000

500

-1

Wavenumber (cm

)

Figure 1 Figure 1 shows the IR spectra of the selected AC-AET-DMANMs. The presence of a naphthalic ring was indicated by the absorption peaks at 1583 cm-1 (skeleton vibration) and 790 cm-1 (out-of-plane bending vibration). The peaks at 1682 and 1640 cm-1 represented the


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stretching vibration of carbonyl groups (C=O). A signal of C-N bond was observed at 1379 cm-1. With increasing DSAET of AC-AET as precursor, the peaks associated with naphthalimide significantly enhanced, suggesting the increasing DS of DMANM in these derivatives.

Figure 2 Figure 2 displays the 1H NMR spectra of AC-AET-DMANM-1–4 in DMSO-d6. The resonance peaks at 4.95-5.35 and 5.9 ppm are assigned to H9a and H8a, respectively,31,37 i.e., proton signals of the rest –C=C groups which did not react with AET. The peaks appearing from 4.0 to 4.5 ppm are assigned to H7a, overlapping with a signal of H1 (4.4 ppm) of cellulose. The decrease in intensity of the H7a, H8a and H9a peaks reflects the increasing rate of the reacted C=C groups in order of the code number of the AC-AET-DMANM derivatives. The peak at 3.05 ppm is assigned to H17/H18 (dimethylamino group) and H11 (N-methylene), which overlapped with the peak of H2O. The signal at 1.8 ppm is assigned to H10c. The peaks at 8.4-8.5, 7.8, and 7.2 ppm are assigned to H12/H14/H16, H13 and H15 on the naphthalic ring, respectively.38 The


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enhancement of these peaks signifies an increase of DSDMANM that refers to the fluorescent DMANM groups. The rest of protons in the cellulosic backbone were signalized between 3.0 and 3.8 ppm. For AC-AET-DMANM-2–4, DSDMANM values were readily determined from the 1H NMR spectral data and the results are listed in Table 1. Figure S1a shows the UV-Vis spectra of AC-AET-DMANM-1–4 in DMSO. Four samples displayed a maximum absorption peak at 428 nm. Moreover, the DSDMANM of the derivatives displayed an excellent linear relationship with the peak intensity at 428 nm (Figure S1b). From the linear regression, the DSDMANM of AC-AET-DMANM-1 was estimated to be 0.03. The results demonstrate the successful synthesis of AC-AET-DMANMs with different DSDMANM values by the reaction of DS-controlled AC-AET and DMANA. The DSDMANM of DMSOsoluble AC-AET-DMANM-1–4 increased from 0.03 to 0.25 with increasing DSAET of AC-AET from 0.1 to 0.4. Because of the poor solubility in DMSO and other solvent, the DSDMANM values of AC-AET-DMANM-5 and 6 could not be determined from the 1H NMR and UV-vis measurements, which were qualitatively estimated to be higher than 0.25 from the IR spectra. 1

H NMR and IR spectra of AC-AET-DMANM-2W are shown in Figure 3. In the NMR

spectrum (Figure 3a), the proton signals of –C=C disappear, indicating that the rest –C=C groups of AC-AET-DMANM-2 (see data in Figure 2b) completely reacted with the thiol groups from AET. In comparison with AC-AET-DMANM-2, the clearer peak of H10b at 2.0 ppm evidences a definitely high DS of amine groups. Moreover, the peaks at 8.4-8.5, 7.8, 7.2 ppm, which are assigned to the protons of naphthalic ring, hardly changed. In the IR spectrum (Figure 3b), a sharp absorption peak associated with –NH2 groups is prominent at 1632 cm-1, differing from the situation in AC-AET-DMANM-2 (see Figure 1). This result also suggests a higher DS of the amine group in AC-AET-DMANM-2W.


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Figure 3 Fluorescent Properties of AC-AET-DMANMs. Usually, the emission of naphthalimide derivatives is utterly quenched in the solid state because the luminogens tend to aggregate together so as to trigger off the so-called aggregation-caused quenching (ACQ).39 Interestingly, AC-AET-DMANMs with lower DSDMANM remained fluorescent even in the solid state. As demonstrated in Figure 4a, AC-AET-DMANM-1 and 2 showed strong fluorescence in their solid states under UV irradiation at 365 nm. When DSDMANM was higher (AC-AET-DMANM-3–5), the fluorescence was obviously quenched and the extent became noticeable with increasing DSDMANM. The fluorescence of AC-AET-DMANM-6 was hardly identified from the photograph. From these observations, it suggested that the semi-rigid cellulose skeleton hinders the DMANM groups from approaching closer to cause ACQ at DS lower than a certain value.40 The excellent fluorescent property of AC-AET-DMANM-1 and 2 make them possible to be used in the solid states including blends with other materials.


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Figure 4 Fluorescent spectra of AC-AET-DMANM-1–4 in DMSO are shown in Figure 4b. It was found for all the samples that the peak maxima of the fluorescent excitation and emission spectra were situated at 453 nm and 530 nm, respectively. The Φ values of AC-AET-DMANMs are listed in Table 1. With an increase of DSDMANM, the Φ values of the derivatives decreased, because of the stronger aggregation tendency of DMANM groups on the cellulose chains.38 Figure 4c displays fluorescent emission spectra of AC-AET-DMANM-2 in DMSO with various concentrations. The fluorescent intensities at 530 nm on the concentration of AC-AETDMANMs are shown in Figure 4d. For AC-AET-DMANM-1 and 2, the fluorescent intensity increased proportionally with increasing polymer concentration in the explored range (≤ 3.0 mg/mL). This linear correlation between the fluorescent intensity and concentration indicates that these two samples never form aggregates of the attached fluorophores at the concentration of 3 mg/mL or lower. However, AC-AET-DMANM-3 and 4 offered a non-linear correlation. As for


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AC-AET-DMANM-3, the fluorescent intensity kept rising with increasing concentration. While for AC-AET-DMANM-4, an apparently fluorescent self-quenching was observed, and the fluorescent intensity decreased with an increase of the polymer concentration higher than 1 mg/mL. The time resolved photoluminescence of AC-AET-DMANM-4 in DMSO with various concentrations are shown in Figure S2. The PL lifetimes were in the range of 5.9-6.1 ns when the concentration of AC-AET-DMANM-4 was lower than 1.0 mg/mL, and then decreased with further increasing of polymer concentration. The phenomena can be explained that the distance of DMANM groups in the cellulose derivatives should be appropriate to photoluminescence at lower concentration. As the concentration increased, the distance became shorter between the chromophores because of the interactions of polymer chains to decrease the intensity of fluorescence as well as the Φ values.

Figure 5


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Figure 5 displays the fluorescent properties of AC-AET-DMANM-2W in aqueous solutions. The maximum peaks of excitation and emission spectra of this derivative were observed at 437 and 530 nm, respectively (Figure 5a). Compared with the excitation spectrum of AC-AETDMANM-2 in DMSO, there is a blue shift of the maximum peak-position; this might be attributed to the difference in the solution behavior. Moreover, the Φ value of AC-AETDMANM-2W was determined to be 0.12. Figure 5b and 5c show variations of the fluorescent intensity for AC-AET-DMANM-2W as a function of the concentration in aqueous solutions (pH=7.12). Like the fluorescent spectra of AC-AET-DMANM-2 in DMSO, the linear correlation found between the intensity at 530 nm and the polymer concentration. This suggested that the DMANM groups of AC-AET-DMANM-2W never aggregated at the concentration of 1 mg/mL or lower. Figure 5d shows the dependence of the fluorescent intensity at 530 nm on the pH of the solution. It was taken that, in the pH range of 2–9, the protonated amino groups avoided the fluorophore aggregation by electrostatic repulsion, and therefore the fluorescent intensity was kept stable. With a pH value lower than 2, we found an obvious decrease in fluorescent intensity. Presumably, the excessive hydrogen ions protonated the dimethylamine groups of DMANM and changed the energetics of the fluorophore in the protonation process. When the pH of the solution exceeded 9, the electrostatic repulsion of amino groups was shielded and the fluorophore aggregation easily occurred. Actually, the derivative was flocculated in 10 min at pH ≥ 10 and the fluorescent intensity of the solution dropped immediately. The results indicate that the fluorescent properties of AC-AET-DMANM-2W are stable in the environment with varying pH from weak acid to weak base.


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Figure 6 Detection of Niroaromatic Compounds. Herein, aromatic compounds, such as benzene, methylbenzene (Mb), 1,4-dimethylbenzene (DMB), nitrobenzene (Nb), p-nitrotoluene (p-NT), DNT, TNT, 3-nitrophenol (3-NP), p-nitrophenol (p-NP), 2,4-dinitrophenol (DNP), TNP, and 2,4dinitrophenylhydrazine (DNH), were used to study the selective detection of fluorescent cellulose derivatives. Figure 6a and 6b present the fluorescent emission spectra and the relative quenching efficiency of AC-AET-DMANM-2 in DMSO in the presence of aromatic compounds (10 mM). The fluorescent intensity remarkably reduced with the addition of DNH or TNP. The fluorescence quenching efficiencies caused by benzene, Mb, DMB, Nb, p-NT, DNT, 3-NP, p-NP, TNP and DNH were 1.61, 5.76, 1.80, 7.28, 4.70, 5.04, 2.25, 3.70, 78.5 and 99.8%, respectively (Figure 6b), when the efficiency was calculated by (1−I/I0)×100%. Thus AC-AET-DMANM-2 imparted the higher selectivity in quenching to DNH and TNP over the other aromatic


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compounds explored. Figure 6c and 6d displays the fluorescent intensities at 530 nm as a function of TNP/DNH concentration, and the Stern-Volmer plots are shown in the inset. As soon as TNP or DNH was added to the polymer solution, the fluorescent intensity dropped immediately and significantly. The fluorescent intensities decreased linearly with an increase in the concentration of added TNP and DNH up to 60 µM. Therefore, these two compounds could be quantitatively detected by the fluorescence quenching of AC-AET-DMANM-2 in DMSO. The detection limits (S/N=3) and Ksv values were determined to be 9.9×10-8 and 1.4×10-7 mol/L, and 1.26×104 and 8.67×103 M-1 for DNH and TNP, respectively. Compared with TNP, the higher KSV value of DNH indicates a better quenching efficiency for AC-AET-DMANM-2.

Figure 7 AC-AET-DMANM-2W can also be applied in the detection of DNH, DNP and TNP in aqueous media. Similar to AC-AET-DMANM-2, the fluorescent intensity of AC-AETDMANM-2W aqueous solutions significantly decreased with the addition of DNP, TNP and


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DNH (Figure 7a). The fluorescence quenching efficiencies caused by benzene, Mb, DMB, Nb, pNT, DNT, TNT, 3-NP, p-NP, DNP, TNP and DNH were 1.95, 2.56, 3.48, 2.38, 2.05, 20.98, 6.46, 5.45, 1.51, 90.69, 89.27 and 79.21%, respectively (Figure 7b). The fluorescent intensities at 530 nm as a function of TNP/DNH concentrations and the Stern-Volmer plots are shown in Figure 7c and 7d. The fluorescent intensities decreased linearly with increasing concentration of the added TNP and DNH up to 20 µM. The detection limits (S/N=3) and Ksv values were determined to be 2.5×10-8 and 3.2×10-8 mol/L, and 4.16×104 and 2.61×104 M-1 for TNP and DNH, respectively. Compared with AC-AET-DMAN-2 in DMSO, AC-AET-DMANM-2W displayed relative low detection limits in aqueous solutions, though their DSDMANM values were the same. Moreover, Table S1 summarizes the detection limits of TNP with various fluorescent materials. AC-AETDMANM-2W shows a relative low detection limit for TNP in the aqueous environment. The quenching of the original emission might be attributed to the high affinity of the quencher molecules (TNP and DNH) to the sensor moiety, DMANM.41 To investigate the possible quenching mechanism, the LUMO level (-1.98 eV) and the energy band gap (3.27 eV) of AC-AET-DMANM-2W was obtained via cyclic voltammetry (Figure 8a) and UV-vis (Figure 8b) measurements, respectively. Hence, the HOMO level of AC-AET-DMANM-2W was calculated as -5.25 eV. When the electron deficient molecule (DNH and TNP as analytes) is present in the solution concerned, an electron-transfer quenching would occur from the excited DMANM to the LUMO level of the analyte (Figure 8c).42 However, the electron transfer is not the only mechanism involved in the quenching process. Figure 8d shows the normalized emission spectrum of AC-AET-DMANM-2W and UV-vis absorption spectra of nitroaromatic compounds in aqueous solutions. A spectral overlap was observed between the emission spectrum of AC-AET-DMANM-2W and UV-vis absorption spectra of DNH and TNP. There is a


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strong possibility of resonance energy transfer (RET) from the fluorophore AC-AET-DMANM2W (donor) to the nonemissive DNH/TNP (acceptor).16,43,44 Figure S3 displays the time-resolved decay curves of AC-AET-DMANM-2W in the absence and presence of DNH and TNP. The PL lifetime AC-AET-DMANM-2W was determined to be 5.343 ns, which decreased with the addition of DNH/TNP. The results further support the possibility of energy transfer from ACAET-DMANM-2W to DNH/TNP, and suggest a dynamic quenching mechanism.16

Figure 8 The detections of TNP and DNH in real water samples were studied to investigate the practical use of the proposed approach. The real water samples were collected from the East Lake near our campus and the tap water in the lab. Then, a series of water samples were prepared by spiking them with the standard TNP (5 µM) and DNH (5 µM) solutions, respectively. As shown in Table S2, AC-AET-DMANM-2W displayed stable fluorescence intensity in both lake


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water and tap water, and the intensity significantly decreased with the addition of TNP and DNH. The results obtained in real water were similar to those in distilled water, and the recovery was quite close to 100%. Moreover, AC-AET-DMANM-2 films were prepared by casting method. As shown in Figure S4, the dry AC-AET-DMANM-2 film displayed strong fluorescence as its solid powder. When immersed in the saturated TNP vapor and 1 mM TNP aqueous solution, the fluorescent intensities of the films dropped significantly. The results indicated that AC-AETDMANMs were suitable for the detection of nitroaromatic compounds in real samples by fluorescence quenching.

CONCLUSIONS In summary, novel fluorescent cellulose derivatives were successfully synthesized by using 4dimethylamino naphthalimide as the fluorescent group. The DSDMANM of the DMSO-soluble derivatives ranged from 0.03 to 0.25 by changing DSAET of the AC-AET precursor. Fluorescent properties of AC-AET-DMANM were strongly influenced by DSDMANM, and the quantum yields decreased with increasing DSDMANM. AC-AET-DMANM with lower DSDMANM (≤ 0.09) showed strong fluorescence indicating no aggregation of DMANM groups at the polymer concentration of 3 mg/mL or lower. When DSDMANM was between 0.09 and 0.25, AC-AET-DMANM showed the fluorophore aggregation and, concomitantly, the concentration quenching was noticeable. When DSDMANM was higher than 0.25, the product hardly dissolved in DMSO and other solvent at room temperature. Water-soluble AC-AET-DMANM-2W was derived by further thiol-ene click reaction of AC-AET-DMANM (DSDMANM = 0.09) and AET. AC-AET-DMANM-2W showed strong fluorescence in aqueous solution at the concentrations lower than 1 mg/mL, and the fluorescent intensity remained stable in the pH range of 2–9. AC-AET-DMANM was used to


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detect TNP and DNH in DMSO, and the detection limits were 1.4×10-7 and 9.9×10-8 mol/L, respectively. Moreover, AC-AET-DMANM-2W was applied in the detection of TNP and DNH in aqueous media with the detection limits of 2.5×10-8 and 3.2×10-8 mol/L, respectively. It was also proved to be effective for the detection of nitroaromatic compounds in real samples. The selective detection of the fluorescent cellulose derivatives for TNP/DNH is attributed to photoinduced electron transfer and resonance energy transfer. Considering their controllable DS parameters and excellent selectivity to the analytes, the fluorescent cellulose derivatives were expected to be applied as environmental surveillance and biological monitoring sensors in various fields.

ASSOCIATED CONTENT Supporting Information Table S1 and S2, Figures S1–S4. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (J. Z.); [email protected] (Y. N.) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT


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This work was financially supported by the National Natural Science Foundation of China (51473128) and the Foundation of China Scholarship Council.

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For Table of Contents Use Only

Novel fluorescent cellulose derivatives were prepared, and were used for the selective and sensitive detection of nitroaromatic compounds.


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Figure Captions Figure 1. FT-IR spectra of AC-AET-DMANM derivatives. Figure 2. 1H NMR spectra of (a-d) AC-AET-DMANM-1–4 in DMSO-d6 at 22 °C. Figure 3. (a) 1H NMR spectrum of AC-AET-DMANM-2W in DMSO-d6 at 22 °C, and (b) FT-IR spectrum of AC-AET-DMANM-2W. Figure 4. Fluorescent properties of AC-AET-DMANMs: (a) photographs of the solid samples under visible light (top) and UV irradiation at 365 nm (bottom); (b) excitation (λEx=453 nm) and emission (λEm=530 nm) spectra in DMSO (3 mg/mL); (c) emission spectra of AC-AETDMANM-2 in DMSO with various concentrations; (d) dependence of the fluorescent intensity at 530 nm on the concentration of the derivatives in DMSO. The numbers 1–6 refer to AC-AETDMANM-1–6. Figure 5. Fluorescent properties of AC-AET-DMANM-2W in aqueous solutions: (a) excitation (λEx=437 nm) and emission (λEm=530 nm) spectra (1 mg/mL), (b) emission spectra at various concentrations, (c) dependence of the fluorescent intensity at 530 nm on the polymer concentration, (d) pH dependence of the fluorescent intensity at 530 nm. Figure 6. Detection of nitroaromatic compounds in DMSO by AC-AET-DMANM-2 (3 mg/mL): (a) fluorescence emission spectra, (b) the relative fluorescent intensities (IF/I0 at 530 nm) in the presence of aromatic compounds (10 mM). Fluorescent intensities at 530 nm in the presence of (c) TNP and (d) DNH with various concentrations, inset of (c, d) are the Stern-Volmer plots. Figure 7. Detection of nitroaromatic compounds in aqueous solutions by AC-AET-DMANM2W (1 mg/mL): (a) fluorescence emission spectra, (b) the relative fluorescent intensities (IF/I0 at 530 nm) in the presence of aromatic compounds (10 mM). Fluorescence intensity at 530 nm in the presence of (c) TNP and (d) DNH with various concentrations, insets of (c, d) are the Stern-


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Volmer plots. Figure 8. (a) Cyclic voltammogram and (b) energy band gap of AC-AET-DMANM-2W. (c) Pictorial representation of electron transfer from LUMO of AC-AET-DMANM-2W to the LUMO of DNH/TNP. (d) Spectral overlap between emission spectrum of AC-AET-DMANM2W and UV-vis absorption spectra of aromatic compounds in aqueous solution.


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Synthesis of Novel Fluorescent Cellulose ... - ACS Publications

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