Cellulose based fluorescent macromolecular sensors and their ability in 2, 4, 6-trinitrophenol detection

doi:10.21203/rs.3.rs-269901/v1

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Research Article

Cellulose based fluorescent macromolecular sensors and their ability in 2, 4, 6-trinitrophenol detection

https://doi.org/10.21203/rs.3.rs-269901/v1

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published 30 Nov, 2021

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This article deals with three cellulose-based fluorescent macromolecular sensors by introducing 1,8-naphthimide fluorophore to cellulose. Firstly, through the etherification reaction of cellulose with BrCH₂CH₂NH₂, -NH₂ group bearing cellulose CS 1 was obtained. And then -NH₂ group reacts with 4-bromo-1,8-naphthalic anhydride to synthesize a naphthalimide cellulose derivative (CS 2). Finally, recognition group was introduced by substituting Br atoms, and three cellulose fluorescent probes (CS 3, CS 4, CS 5) were obtained eventually. Structure and fluorescence properties of the macromolecular sensors were characterized and confirmed. Fluorescence detection measurements show that these probes can be used as selective and sensitive fluorescent sensors to 2,4,6-trinitrophenol (TNP). The detection limits are 0.52µM, 0.76µM and 0.81µM, respectively, indicating good detection performance. This work provides a new method for the selective detection of TNP and also a method to enlarge the application scope of cellulose.

Materials Chemistry

Environmental Engineering

Cellulose derivatives

fluorescence

sensor

trinitrophenol (TNP)

Novel cellulose-based fluorescent sensors were prepared.

The as-prepared macromolecular sensors have selectivity and sensitivity to TNP.

As an important reagent in chemical industry such as dyes and pesticides, nitroaromatic compounds are widely used (Dhiman et al. 2020). Due to its toxicity and non-degradability, the excessive use will cause irreversible harms to the soil and water(Cheng et al. 2019). It leads to irritation, skin allergies, anemia, carcinogenicity, liver and kidney damage and a series of health problems to human beings as well (Guo et al. 2020; Wang et al. 2020b; Xu et al. 2020). At the same time, these chemicals are also the main component of explosives and often endanger social security. Therefore, it is necessary to develop and improve rapid real-time monitoring and on-site detection methods for such chemicals (Saini et al. 2020). This requires the detector to have the virtue of high sensitivity, good reliability, low detection limit, convenient carrying and easy operation (Xiao et al. 2020; Xiong et al. 2019).

At present, the common methods for nitroaromatic compounds detection are divided into instrumental analysis and chemical reagent detection (Bora et al. 2019). Compared with traditional instrument detection methods such as mass spectrometry (MS), high performance liquid chromatography (HPLC) and electrochemical analysis, the detection sensitivity of chemical reagents is higher, and the pretreatment methods are simple (Guo et al. 2017; Gupta et al. 2017). It is expected to be used for real-time and rapid monitoring of nitroaromatic compounds and attracts attention. Among them, fluorescence detection technology has the advantages of high sensitivity, strong specificity, fast response, and portability, which has attracted much attention (Peretz et al. 2019; Tripathi et al. 2017; Yang et al. 2019). Fluorescent polymers have been synthesized and used in the detection of trace nitroaromatic chemicals. For example, a triazole-methane-anthracene-functionalized fluorescence quenching polyaromatic chemical sensor was synthesized, which can detect 2,4,6-trinitrophenol (TNP) efficiently and quickly (Qiu et al. 2020). Water-dispersible fluorescent molybdenum disulfide (MoS₂) quantum dots (QDs) capped with 1,4-diaminobutane (DAB) can be used for the selective detection of explosive TNP up to a limit of detection of 5 µM (Sharma and Mehata 2020). A coordination polymer [Pb(L)₂]_n was successfully generated, which can be used as a luminescence sensor for TNP with low detection limit as well (Miao 2019).

Cellulose is considered to be the main raw material of the world's energy and chemical industry due to its abundant sources, non-toxic, non-polluting, easy to modify, and good biocompatibility (Fan et al. 2020; Huang et al. 2020). As a polyhydroxyl linear polymer, cellulose could undergo typical etherification (You et al. 2013), esterification (Kusuma et al. 2019), oxidation (Imamura et al. 2020) and other reactions (Fuchs and Zhang 2019). These reactions endow functional cellulose derivatives being easily prepared, and expanding the application of cellulose (Jasmani et al. 2016). Amino group was introduced by 3-amino-propyltriethoxysilane (APTES) and then grafted to the lignin surface by amidation reaction between the amino group and 1-pyrrolidine butyric acid (PBA), showing obvious fluorescence signal (Xiong et al. 2016). By amidation reaction, Rhodamine B modified with N-hydroxysuccinimide was labeled on cellulose nanofibers (Navarro and Bergstrom 2014). Via combination of fluorescent 1,8-naphthalimide and cellulose nanocrystalline (CNCs), the application in Pb²⁺ sensing was further explored (Song et al. 2019). However, due to the large number of hydrogen bonds and intermolecular interactions in the cellulose chain, it is still a challenge to prepare fluorescent cellulose derivatives with good solubility (Barazzouk and Daneault 2011).

1,8-Naphthimide has the following characteristics: coplanarity; a larger conjugated system; one section of the molecule has a strong electron-donating group, and the other end has a strong electron-withdrawing group (Choi et al. 2016; Li et al. 2020). The electrons in this system are easily irradiated by light to undergo transitions, thereby generating fluorescence. Furthermore, the introduction of different groups on the naphthalene ring can greatly change the fluorescent properties of such fluorophores. They are widely used in chemical sensors, molecular switches, fluorescent probes, lasers and other fields (Bahta and Ahmed 2019).

In this work, BrCH₂CH₂NH₂ was successfully introduced into the cellulose backbone (CS 1) by Williamson synthesis firstly. And then it reacted with 4-bromo-1, 8-naphthalenedicarboxylic acid anhydride to synthesize naphthalimide fluorescent cellulose derivative (CS 2). Finally, three cellulose-based fluorescence sensors (CS 3, CS 4, CS 5) were synthesized by introducing different electron-rich moieties as 2-mercaptobenzothiazole, benzothiazole-2-ylamine and 2-mercaptobenzimidazole. The results show that the new fluorescent cellulose derivatives have potential applications in the field of chemical sensing.

2.1. Materials

4-bromo-1,8-naphthalic anhydride, Cellulose pulp powder (50 µm), Butylamine, 2-Bromoethylamine Hydrobromide, Benzyltriphenylphosphonium hydroxide, 2-Mercaptobenzothiazole, Benzothiazole-2-ylamine, 2-Mercaptobenzimidazole were purchased from J&K China Chemical Ltd.

Nitrobenzene (NB), 4-nitrochlorobenzene (4-NCB), benzoic acid (BA), 3,5-dinitrobenzoic acid (DNBA), aniline (AN), methylbenzene (MB), 4-nitrotoluene (4-NT), phenol (NP), 4-nitrophenol (4-NP), 2,4,6-trinitrophenol (TNP), potassium hydroxide and potassium carbonate are purchased from Aladdin Chemical Co. Ltd. dimethyl sulfoxide (DMSO), N,N-dimethylformamide (DMF), ethanol, acetonitrile, acetone were purified by distillation for further use.

2.2. Instrumentation

Fourier transform infrared spectroscopy (FTIR, 4000–400 cm^− 1, Thermo Fisher, USA) was measured by Nicolet 6700 instrument. The sample was prepared by the standard KBr particle method. ¹H NMR was measured on AVANCE III 600 MHz (Bruker, Switzerland) instrument (in CDCl₃, d₆-DMSO). The elemental, content and binding energy were measured by X-ray photoelectron spectroscopy (XPS, PHI-5300; ESCA PHI China Limited company, Beijing). The microscopic morphologies were obtained by scanning electron microscopy (SEM, JSM-5510LV, Japan). Thermogravimetric analysis (TGA) and differential scanning calorimeter (DSC) were acquired by STA449F3 instrument (NETZSCH, German). UV absorption spectra were measured on a UV-5900PC spectrophotometer (METASH, shanghai, China). Fluorescence spectra were performed on a Shimadzu F-4500 (Shimadzu, Shanghai Global Laboratory Consumables Co., Ltd.) fluorescence spectrophotometer.

2.3. Synthesis

Cellulose derivatives (CS 1, CS 2, CS 3, CS 4 and CS 5, their structures could be seen in Scheme 1) containing functional monomer naphthalimide groups were prepared, and the synthesis routes were shown in scheme 1. In order to further prove that the partial structure of the above-mentioned fluorophores can be synthesized readily, as reference molecules, n-butylamine instead of compound CS 1 was used to synthesize small fluorescent probe molecules (AC 2, AC 3, AC 4 and AC 5, their structures could be seen in Scheme 2).

2.3.1. Synthesis of CS 1

CS 1 was synthesized according to the literature (Abe et al. 2017). Firstly, cellulose powder (0.1 g) was dissolved in [P_4,4,4,4]OH aq. solution and stirred at 25℃ in an atmosphere of nitrogen for 30 minutes. Then 2-Bromoethylamine Hydrobromide (0.55 g) were added to the cellulose solution and reacted at 60°C for 24 h under nitrogen atmosphere. Afterwards, the reaction product was precipitated, washed with water and ethanol, and then vacuum dried at 35°C for 24 hours to obtain the product (0.08 g; 80% yield).

¹H NMR (600 MHz, DMSO, ppm): δ 5.74 (s, bromoethylamine H-9), 3.02 (s, cellulose H-1, 2, 3), 2.65 (s, cellulose H-4, 5), 2.53 (s, cellulose H-6).

2.3.2. Synthesis of CS 2

CS 1 (0.1 g) was suspended in 10 mL dry DMF at 25℃, followed by several drops of 4-bromo-1,8-naphthalic anhydride (0.2 g). The mixture was recycled at nitrogen gas at 80℃ for 14 hours, cooled to room temperature (Hu et al. 2018). The solids were precipitated and washed with DMF and acetone to get 0.09 g brownish powder CS 2 (yield 90%).

¹H NMR (600 MHz, DMSO, ppm): δ 8.29 (s, naphthalimide H-9,10,11,12,13), 4.56 (s, ethylamine H-7, 14), 4.22 (s, ethylamine H-8), 3.00 (s, cellulose H-1, 2, 3, 4, 5), 2.53 (s, cellulose H-6).

2.3.3. Synthesis of CS 3, CS 4 and CS 5.

CS 2 (0.1 g), 2-Mercaptobenzothiazole (0.7 g) and potassium carbonate (0.4 g) were suspended in dry DMF and stirred at 85℃ for 14 h (Ruiz-Sanchez et al. 2020). Yellow precipitated material was filtered and the residual 2-mercaptobenzothiazole on the solid was washed away by using an excess of acetone. The final residue was dried in vacuum to obtain 0.085 g yellow solid of CS 3. Similarly, we successfully synthesized polymers CS 4 and CS 5 using the same method. CS 4 is a brown solid with a yield of 87%, and CS 5 is a yellow solid with a yield of 90%.

CS 3: ¹H NMR (600 MHz, DMSO, ppm): δ 8.10 (s, naphthalimide H-9,10,11,12,13), 7.93 (s, mercaptobenzothiazole H-14,15,16,17), 4.67 (s, ethylamine H-8, 18) 3.85(s, ethylamine H-7), 3.02 (s, cellulose H-1, 2, 3), 2.65 (s, cellulose H-4, 5), 2.53 (s, cellulose H-6).

CS 4: ¹H NMR (600 MHz, DMSO, ppm): δ 8.29 (s, naphthalimide H-9,10,11,12,13), 7.93 (s, aminobenzothiazole H-14,15,16,17), 4.65 (s, ethylamine H-8, 18) 3.76 (s, ethylamine H-7), 2.86 (s, cellulose H-1, 2, 3), 2.69 (s, cellulose H-4, 5), 2.62 (s, cellulose H-6).

CS 5: ¹H NMR (600 MHz, DMSO, ppm): δ 8.37 (s, naphthalimide H-9,10,11,12,13), 7.93 (s, mercaptobenzimidazole H-14,15,16,17), 4.25 (s, ethylamine H-8, 18), 3.87 (s, ethylamine H-7), 2.87 (s, cellulose H-1, 2), 2.71 (s, cellulose H-3), 2.65 (s, cellulose H-4, 5), 2.53 (s, cellulose H-6).

3.1. Design and synthesis

1,8-Naphthimide fluorescent dyes are a very important class of fluorescent reagent, which exhibit good colorimetric and fluorescent reactions in biological assays and other specific photophysical studies, and have high light stability and quantum yield. There are a large number of hydrogen bonds in cellulose molecules, which are easy to form complex intermolecular and intramolecular bonds, with high crystallinity. Once naphthalimide was attached to the cellulose chain for modification and functionalization, cellulose-based fluorescent macromolecular sensor could be synthesized. Scheme 1 shows the synthesis steps of cellulose fluorescent probe. The cellulose-based fluorescent polymer derivatives were characterized by FTIR and XPS, and further characterized by SEM, TGA, DSC and fluorescence spectroscopy. Similarly, small fluorescent probe molecules (AC 2, AC 3, AC 4 and AC 5) were characterized by FTIR, ¹H NMR, MS, seeing the supporting information. (Fig. S1-S5).

3.2. Structure of fluorescent celluloses

3.2.1. FTIR analysis

The FTIR spectra of cellulose and their derivatives are shown in Fig. 1(a). It can be seen that original CS has a strong peak at about 3439 cm^− 1, which is related to the large amount of -OH on the cellulose chain. The tensile vibration of C-H is around 2910 cm^− 1. For CS 1, the peak at 1254 cm^− 1 is attributed to the vibration of the C-N bond, and the wider peak at 894 cm^− 1 to 707 cm^− 1 belongs to the out-of-plane bending vibration of the amino group, which indicates that the amino group has been successfully grafted onto cellulose. When naphthalimide moiety was introduced, a new peak of CS 2 appeared at 521 cm^− 1, which was caused by C-Br stretching; at the same time, the absorption peaks at 1374 cm ^− 1 and 790 cm ^− 1 indicated the presence of a naphthalene ring. The peak at 1630 cm^− 1represented the stretching vibration of carbonyl groups (-C = O). Since the benzothiazole nucleophiles substituted for Br on the naphthalimide are similar, the peaks appearing in the FTIR spectra are not particularly obvious. For CS 3, CS 4, and CS 5, the absorption peak around 1066 cm^− 1, which is an absorption peak of the C-S-C stretching vibration. These phenomena confirm that the compounds CS 3, CS 4, CS 5 have been synthesized.

3.2.2. ¹H NMR analysis

Cellulose is difficult to dissolve in DMSO, but through a series of modification reactions, its condensed state is destroyed, making it more soluble in the polar solvent DMSO. Therefore, ¹H NMR spectrum have been made to analysis its structure. Figure 1 displays the ¹H NMR spectra of cellulose derivatives in DMSO-d₆. As can be seen, typical peaks at 2.53–3.02 ppm could be assigned as the unit of cellulose (H1, H2, H3, H4, H5, H6), peaks at 4.22 ppm are responsible for amino (H18). after reacted with naphthalimide, peaks at 8.37 ppm could be assigned to naphthalimide. After substitution reaction, peaks at 7.93 ppm could be assigned to benzothiazole groups (H14, H15, H16, H17).

3.2.3. XPS analysis

X-ray photoelectron spectroscopy (XPS) was used to further measure the chemical composition of cellulose derivatives. As we all know, CS is composed of three elements: H, C, and O, combining with the naphthalimide, other elements will be introduced. The element content of each component is shown in Table 1. Peak signal for each element could be seen in SI (Table S1). Combining Table 1 and Table S1, the cellulose spectra have two peaks near 285 eV and 532 eV, corresponding to C1 and O1, respectively. For CS 1, a new peak appears at 70 eV, indicating that the amino group has been successfully introduced. When naphthalimide was introduced, CS 2 exhibits a new bromine peak (Br 3d). When 2-mercaptobenzothiazole was connected, the Br peak (Br 3d) disappeared, a new sulfur peak (S 2p) appeared, and the O and C content in the molecule has also changed. For the other two samples CS 4 and CS 5, new sulfur peaks (Br 3d) also appeared, which can prove the successful substitution reaction on the cellulose surface. The degree of substitution (DS) refers to the amounts of substances in which the active hydroxyl groups on each glucose unit of cellulose are substituted. Since 3 hydroxyl groups on each dehydrated glucose unit in the cellulose molecular chain can be substituted at most, the degree of substitution is between 0 to 3. According to the data in Table 1, it is roughly determined that there is one naphthalimide group in 14 glucose units, and one recognition group in 25 glucose units (such as benzothiazole groups). Therefore, by definition, the DS value of the cellulose substituent is calculated by the following equation:

DS = \(\frac{{\text{n}}_{\text{substitution group}}}{{\text{n}}_{\text{glucose unit}}}\) (1)

The DS of naphthalimide groups (DS_{CS 2}) was calculated to be 0.07 and the DS of benzothiazole groups (DS_{CS 3}) was calculated to be 0.04.

Table 1

The content of elemental components in cellulose samples by XPS.
Atomic %	CS	CS 1	CS 2	CS 3	CS 4	CS 5
C	57.65	60.66	70.27	61.68	57.78	59.37
O	42.35	38.25	26.65	37.2	41.29	39.5
N	-	1.09	2.59	0.8	0.73	1.02
Br	-	-	0.49	-	-	-
S	-	-	-	0.33	0.19	0.11

3.2.4. SEM analysis

Scanning electron microscope (SEM) was used to analyze the microstructure of cellulose and determine the structural changes of cellulose. It can be clearly seen that the original CS is long rod-shaped and its surface is smooth (Fig. 2(a)). Because CS dissolves in [P_4,4,4,4]OH aq. solution, it will cause surface changes. It can be seen from Fig. 2(b) that after the reaction, the surface becomes rough and loose (Luo et al. 2019). The SEM of CS 2 and CS 3 are shown in Fig. 2(c) and Fig. 2(d). Because cellulose undergoes a series of reactions, fluorescent groups are introduced, which destroyed the regular structure of the cellulose chain and reduces the crystallinity degree of cellulose. Therefore, cellulose derivatives are not as easy to crystallize as before, and the aggregate structure becomes loose. It can be seen that the cellulose gradually became rough from the original smooth surface, leaving many pores on the surface, which increases the specific surface area of the cellulose and makes it easier to combine with analytes. The SEM of CS 4 and CS 5 are shown in the supporting information. (Fig. S6). Many physical and chemical properties of the modified cellulose molecules have been improved by this way (Wang et al. 2020a).

3.2.5. TGA and DSC analysis

The TGA curves are shown in Fig. 3(a), and the thermal analysis data is summarized as well. As shown in the figure, the weight of the four samples decreased at around 100°C, which was caused by the loss of adsorbed water and volatile substances. Because there are many intramolecular hydrogen bonds on cellulose macromolecules, cellulose has high heat resistance (de Faria et al. 2017), and the sample CS loses weight rapidly at 320°C. When fluorescent monomers are attached to cellulose, the decomposition temperature of cellulose decreases, sample CS 3, CS 4, and CS 5 begin to lose weight at 280°C. This may be due to the fact that the inserted substituents have destroyed part of the original rigid structure. It’s about 350°C, most of the sample has been decomposed. When the temperature exceeds 400°C, the biomass of the cellulose derivative decreases uniformly, and at the same time, it can be seen from the table in Fig. 3(a) that the cellulose-modified derivative shows a higher residual mass than the cellulose raw material. This also shows that the organic compound is successfully grafted onto the cellulose chain.

The DSC curve was used to evaluate the modified cellulose macromolecules, and shown in Fig. 3(b). The DSC of the original material cellulose CS showed that the glass transition temperature (T_g) was about 339°C. After modification, the T_g of cellulose derivative samples CS 3, CS 4, and CS 5 were 340℃, 337℃ and 327℃, respectively. Due to the influence of the substituents, part of the hydrogen bonds and intermolecular interactions are destroyed, which reduces the degree of cross-linking of the cellulose molecular chain, but the proportion of fluorescent molecules introduced is not large, so the Tg is slightly reduced. In this study, the measured DSC data is close to the TGA data, indicating that the two systems have good consistency.

3.3. Detection of aromatic and aromatic nitro compounds

Using the sensing properties of cellulose naphthalimide derivative macromolecular fluorescent probes, 10 kinds of aromatic and aromatic nitro compounds, nitrobenzene (NB), 4-nitrochlorobenzene (4-NCB), benzoic acid (BA), 3,5-dinitrobenzoic acid (DNBA), aniline (AN), methylbenzene (MB), 4-nitrotoluene (4-NT), phenol (NP), 4-nitrophenol (4-NP), 2,4,6-trinitrophenol (TNP) were tested. Cellulose is difficult to dissolve in common solvents such as acetonitrile and ethanol. Therefore, we choose DMSO with greater polarity as the solvent, grind the product into a fine powder, and dissolve the product after ultrasonic mixing. 10 aromatic and aromatic nitro compounds were added to the cellulose fluorescent probe solution, and the UV-vis spectra and fluorescence spectra of the mixed solution were recorded. After testing, in addition to polymer CS 4 has the ability to recognize 4-NP and TNP, CS 3, CS 5 have specific recognition ability to TNP.

Figure 4(a) shows the fluorescence emission spectra of CS 3 in the presence of nitro-aromatic compounds (50µM). It can be seen from Fig. 4(a) that the fluorescence intensity is significantly reduced after adding TNP, which is about two-thirds reduction, while the fluorescence intensity is basically unchanged when other aromatic compounds are added. Therefore, the selectivity of quenching by TNP is higher than that of other aromatic compounds. This indicates that the fluorescent probe CS 3 has good selectivity for TNP. The UV-Vis spectra of Fig. S8(a) includes naphthalimide units, which are characterized by a distinct absorption band at 365 nm. Since the low-concentration TNP solution is yellow-green, when TNP is added, the absorbance increases, the absorption peak also changes, and the color of the solution changes from colorless to yellow-green. When 4-NP was added to the CS 3 solution, the maximum absorption wavelength blue shifted from 365 nm to 320 nm, and the absorbance also increased slightly. But when other nitro compounds were added, the UV-Vis spectra of CS 3 remained basically unchanged, and the color of the solution didn’t change significantly.

Figure 4(b) shows the change of the fluorescence intensity of compound CS 3 vs the concentration of TNP. After adding TNP to the polymer solution, the fluorescence of the solution decreased significantly. As the concentration of TNP increased, the fluorescence intensity first showed a linear downward trend and then became slower. The photo of the sensor CS 3 performing the fluorescence titration on all the test objects is shown in Fig. S11, and the change trend is consistent with the actual measured data. Therefore, the fluorescence quenching of CS 3 in DMSO solution can quantitatively detect TNP. The standard slope is obtained from the linear relationship diagram of CS 3 in Fig. 4(c), and the detection limit of the sensor is obtained according to the limit detection (LOD) calculation formula. As a result, the detection limit was calculated to be 0.52 µM.

It can be seen from Fig. 5(a) that the maximum emission peak of CS 4 is 453nm, and fluorescnece quenched by 4-NP and TNP after adding 10 different aromatic nitro compounds. Among them, 4-NP quenched half of it, and the addition of TNP quenched up to 75%. Similar to CS 3, when CS 4 encounters nitroaromatic compounds, the UV-vis spectra of TNP shifts from 343 nm to 356 nm, and the UV absorption is significantly enhanced. The spectral absorption peak of 4-NP moves to 310nm, and the ultraviolet absorption is also slightly enhanced. (Fig. S8).

In order to determine the detection limit of the cellulose-based probes for TNP and 4-NP, Fig. 5(c) and Fig. 5(d) record the fluorescence spectra under different concentrations of 4-NP and TNP. As could be seen, the fluorescence intensity of CS 4 decreases with the increase of the concentration of nitro compounds. The lowest concentrations of TNP and 4-NP that can be recognized by CS 4 are 5µM and 10µM, respectively. When CS 4 meets nitro compounds at this concentration, the fluorescence change can still be seen (Fig. S11). When 60µM TNP was added, the fluorescence intensity of the sensor solution was quenched by about 80%; when 110µM 4-NP was added, the fluorescence reached the complete quenched state. It can be seen from the three aspects of fluorescence spectra, UV-vis spectrum and color change that this probe is a better choice for detecting 4-NP and TNP. As shown in Fig. 5(d) and 5(e), the standard curve of TNP and 4-NP are obtained. The limit of detection (LOD) of probe CS 4 for TNP and 4-NP were calculated as 0.76µM and 1.5µM, respectively. This result indicates that CS 4 is more sensitive to TNP.

By adding a series of nitro-aromatic compounds, the fluorescence emission spectra and UV-vis spectra are obtained and shown in Fig. 6(a) and Fig. S8(c). Compound CS 5 emits fluorescence with a maximum wavelength of 455 nm under excitation of 365 nm. It can be seen that upon adding of TNP, the fluorescence intensity of CS 5 is about 70% quenched. Except TNP, the presence of other aromatic nitro compounds didn’t cause notable changes. It shows that CS 5 has a good response to TNP. When adding 20µM TNP, the fluorescence intensity of CS 5 solution has been quenched by 70%, which also shows that CS 5 is very sensitive to TNP. The UV-Vis spectra also prove this. UV-Vis spectra of the probe solution added with TNP red-shifted from 372nm to 378nm, and the absorbance increased greatly. Although the spectra of the probe solution with 4-NP blue shifted from 372nm to 316nm, and the absorbance also changed slightly, the fluorescence spectra of the solution didn’t change much, so CS 5 can still be used as a fluorescent probe for detecting TNP.

The fluorescence response of probe CS 5 upon adding TNP was discussed by titration spectra analysis. As illustrated in Fig. 6, as the concentration of TNP increases, the fluorescence intensity of the solution decreases. This can also be proved by photographs under the illumination of the ultraviolet lamp observed by the naked eye (Fig. S11). As expected, there is a good linear relationship between fluorescence intensity and TNP concentration in the range of 0 ~ 18µM. Subsequently, according to the formula, the LOD of TNP was calculated to be 0.81µM. Therefore, CS 5 can also be used as a macromolecular sensor to detect the presence of TNP.

3.4. Detection mechanism

Due to the poor solubility of cellulose derivatives, as a reference molecule, small fluorescent probe molecules (AC 2, AC 3, AC 4 and AC 5) were synthesized using n-butylamine instead of compound CS 1. Small molecule reference probes AC 3 and AC 4 were used to study the selective detection mechanism of aromatic nitro compounds. As shown in Fig. 7, when different analytes were added, AC 3 and AC 4 only showed obvious quenching reaction to TNP. Although small molecule probes also have specific detection ability for TNP, the sensitivity of small molecule probes is not enough. When the TNP concentration is 7 times the equivalent of the sensor (7×10^− 5 mol/L), the obvious quenching effect appears. In addition, the fluorescence of the AC 3 solution is very weak, and it is almost invisible under ultraviolet light (Fig. S11). Therefore, grafting fluorescent probes onto the cellulose molecular chain increases fluorescence by aggregate induced emission (AIE) (Zhang et al. 2013). The more fluorescent units introduced to cellulose, the greater the sensitivity of the probe. This also proves that the cellulose fluorescent derivative does have the function of selectively identifying TNP, and further improves the sensitivity, which is conducive to the application and development of the detection of nitro-aromatic compounds.

In order to further reveal the process of TNP binding to the macromolecular sensors, we refer to the binding process of TNP and the small molecule probe AC 3. As shown in Fig. 7, ¹H NMR spectra of probe AC 3 were taken in DMSO-d₆ in the presence of TNP. It can be seen that the peak at 3.3 ppm moved to the low field and peak broadening occurs, which may be the reason for the interaction with water. According to the literature, the chemical shift of Hp on TNP is above 9 ppm (Zhang et al. 2013). Once the interaction between TNP and the sensor occurred, the proton signal on the aromatic ring of TNP will shift to the upper field. In Fig. 7, the chemical signal of TNP is at 8.55ppm, so it can be proved that TNP does interact with the sensor. Secondly, slight upfield shifts were observed for the proton signals of the benzene ring. These results indicate that after adding TNP, the naphthalimide probe interacts with TNP to a certain extent, resulting in fluorescence quenching. A reasonable explanation may be the use of photo-induced electron transfer (PET) mechanism to interact electron-deficient TNP compounds with electron-rich naphthalimide small molecules to generate electron transfer, thereby causing field shift and fluorescence quenching (Cao et al. 2017; Kundu et al. 2019; Ma et al. 2016). Similarly, the same is true for other small molecule sensors. Therefore, when a large number of naphthalimide electron-donating groups are connected to the cellulose chain, the macromolecular sensor can effectively combine with the electron-deficient nitrophenol to achieve the purpose of detection.

In summary, using naphthalimide as the fluorophore, cellulose based fluorescent probes were prepared. The as-prepared cellulose sensors have highly selective and sensitive ability to TNP. The best detection limit of three cellulose-based probes reaches 0.52µM. The selective detection of TNP by fluorescent cellulose derivatives is mainly due to the mechanism of photoinduced electron transfer (PET) and resonance energy transfer. By introducing electron-rich moieties of 2-mercaptobenzothiazole, benzothiazole-2-ylamine, 2-mercaptobenzimidazole, the polymeric sensors combine the electron-poor TNP easily. The cellulose-based fluorescent sensors exhibit selective and sensitive ability to TNP accordingly. By this modification of cellulose, the application scope of cellulose was enlarged.

This work is approved by the Ethics Committee of Wuhan Institute of Technology; there are not any animal test in this work. The authors declare no competing financial interest.

Acknowledgement

The authors would like to thank the financial support of National Natural Science Foundation of China (NSFC, No. 41573106).

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GraphicalAbstract.png
Scheme1.png
Scheme 1. Synthetic route of the cellulose macromolecular fluorescent probes
Scheme2.png
Scheme 2. Synthetic route of the reference molecular fluorescent probes
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Cellulose based fluorescent macromolecular sensors and their ability in 2, 4, 6-trinitrophenol detection

Status:

Journal Publication

Version 1

Abstract

Figures

Highlights

1. Introduction

2. Experimental

2.1. Materials

2.2. Instrumentation

2.3. Synthesis

2.3.1. Synthesis of CS 1

2.3.2. Synthesis of CS 2

2.3.3. Synthesis of CS 3, CS 4 and CS 5.

3. Results And Discussion

3.1. Design and synthesis

3.2. Structure of fluorescent celluloses

3.2.1. FTIR analysis

3.2.2. ¹H NMR analysis

3.2.3. XPS analysis

3.2.4. SEM analysis

3.2.5. TGA and DSC analysis

3.3. Detection of aromatic and aromatic nitro compounds

3.4. Detection mechanism

4. Conclusion

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

Cellulose based fluorescent macromolecular sensors and their ability in 2, 4, 6-trinitrophenol detection

Status:

Journal Publication

Version 1

Abstract

Figures

Highlights

1. Introduction

2. Experimental

2.1. Materials

2.2. Instrumentation

2.3. Synthesis

2.3.1. Synthesis of CS 1

2.3.2. Synthesis of CS 2

2.3.3. Synthesis of CS 3, CS 4 and CS 5.

3. Results And Discussion

3.1. Design and synthesis

3.2. Structure of fluorescent celluloses

3.2.1. FTIR analysis

3.2.2. 1H NMR analysis

3.2.3. XPS analysis

3.2.4. SEM analysis

3.2.5. TGA and DSC analysis

3.3. Detection of aromatic and aromatic nitro compounds

3.4. Detection mechanism

4. Conclusion

Declarations

References

Supplementary Files

Status:

Journal Publication

Version 1

3.2.2. ¹H NMR analysis