Universal preparation of cellulose-based colorimetric sensor for heavy metal ion detection

Journal Pre-proof Universal Preparation of Cellulose-based Colorimetric Sensor for Heavy Metal Ion Detection Meng Zhang (Conceptualization) (Methodology) (Formal analysis) (Writing - original draft) (Writing - review and editing), Lina Zhang (Methodology), Huafeng Tian (Conceptualization), Ang Lu (Conceptualization) (Writing - review and editing)

PII:

S0144-8617(20)30211-3

DOI:

https://doi.org/10.1016/j.carbpol.2020.116037

Reference:

CARP 116037

To appear in:

Carbohydrate Polymers

Received Date:

27 November 2019

Revised Date:

3 February 2020

Accepted Date:

18 February 2020

Please cite this article as: Zhang M, Zhang L, Tian H, Lu A, Universal Preparation of Cellulose-based Colorimetric Sensor for Heavy Metal Ion Detection, Carbohydrate Polymers (2020), doi: https://doi.org/10.1016/j.carbpol.2020.116037

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Universal Preparation of Cellulose-based Colorimetric Sensor for Heavy Metal Ion Detection

Meng Zhang,1 Lina Zhang,1 Huafeng Tian,*,2 Ang Lu*,1,2 1 College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, China

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2 Key laboratory of Processing and Quality Evaluation Technology of Green Plastics of China National Light Industry Council, Beijing Technology and Business

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University, Beijing 100048, China

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* To whom correspondence should be addressed. E-mail: [email protected] (A Lu);

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[email protected] (H Tian)

Highlights

The composite films possess good mechanical property and thermal stability.

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Since nano-dyes distribute evenly in cellulose, composite films own uniform color.

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The composite films exhibited excellent fast responsiveness for heavy metal ion.

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The research utilizing the renewable resources is environmentally friendly.

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

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Abstract A simple and universal strategy was developed to prepare cellulose/dye composite film, as colorimetric sensor for heavy metal ions (HMIs) detection. After regenerating cellulose solution in ethanol, the regenerated films were further soaking in dye/ethanol solution followed by hot-pressing, to obtain cellulose/dye composite films. 1-(2pyridylazo)-2-naphthol (PAN) was used as an example, and the resultant cellulose/PAN

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composite films (CPs) possessed robust mechanical property (tensile strength of 52.9 MPa), light transmittance, and thermodynamic stability. PAN distributed uniformly as

nanoparticles of 30 nm on cellulose because of the interaction between N of azo group

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of PAN and cellulose. When used as colorimetric sensor for Zn2+ detection, the

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detection limit of CP was as low as 100 ppb, and the color change was distinguishable after testing with tap water. Moreover, two more dyes including 1-(2-thiazolylazo)-2-

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naphthol (TAN) and dithizone (Dith) were also immobilized successfully on cellulose,

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and the resultant films were effective colorimetric sensor for HMIs like Zn2+ and Cu2+. This work provided a facile and universal method to prepare cellulose-based

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colorimetric sensor or HMI detection, demonstrating great potential in water treatment and natural resources utilization.

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Keywords: cellulose; dye; colorimetric sensor; heavy metal ion

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1. Introduction Water is well-acknowledged as the most essential natural resource on earth(Vörösmarty et al., 2010), yet there has been increasing concerns about the water pollution (Aragay, Pons, & Merkoçi, 2011). Because of the long-term biological and environmental harm, HMI is regarded as one of the most toxic pollutants(Takahashi, 2016; Ware, Whitacre, & toxicology, 2007). Therefore, it is vital to develop methods

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and techniques to detect HMI with low concentration, which is usually found in

environmental water samples. Various methods and techniques for detecting HMIs are developed and widely used in the last decades, such as inductively coupled plasma mass

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spectrometry (ICP-MS)(Ammann, 2007; Michalski, Jabłonska, Szopa, & Łyko, 2011),

Mousavi,

Yamini,

&

Shamsipur,

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inductively coupled plasma atomic emission spectroscopy (ICP-AES)(Karami, 2004),

atomic

absorption

spectroscopy

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(AAS)(Magalhães Padilha, de Melo Gomes, Federici Padilha, Moreira, & Dias Filho,

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1999), and so on(Bansod, Kumar, Thakur, Rana, & Singh, 2017; Gumpu, Sethuraman, Krishnan, & Rayappan, 2015; Quang & Kim, 2010). Though sensitivity, accuracy and

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efficiency were demonstrated, the mentioned methods usually suffer from drawbacks including high expense, sophisticated procedure, professional training, etc(Hadar,

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Bulatov, Dolgin, & Schechter, 2013). Thus, the colorimetric sensor with/without optical detection is a particularly attractive approach, due to its simplicity, low detection limit, portability and low cost(d’Halluin et al., 2017; Danwittayakul, Takahashi, Suzuki, & Thanaboonsombut, 2008; Latt & Takahashi, 2011; Rull-Barrull, d'Halluin, Le Grognec, & Felpin, 2016; Suzuki, Llosa Tanco, Pacheco Tanaka, Hayashi, & Takahashi, 2005; 3

Takahashi, Kasai, Nakanishi, & Suzuki, 2006). A common example of such colorimetric sensor is dye-loaded materials like test strips and kits(Chansuvarn, Tuntulani, & Imyim, 2015; Ding, Wang, Li, & Chen, 2016; Huber & Leopold, 2016; Leopold, Philippe, Wörle, & Schaumann, 2016; Li, Gou, Al-Ogaidi, & Wu, 2013; Oehme & Wolfbeis, 1997; Takahashi et al., 2006).Therefore, it remains significant and important to find a proper carrier to immobilize the dyes. Natural polymers are regarded

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as promising matrix for dyes, attributed to their environmental friendliness and sustainability.

In recent years, the renewable resources have attracted great attention due to their

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renewability, biodegradability, biocompatibility and availability(Klemm, Heublein,

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Fink, & Bohn, 2005). As the most abundant natural polymer, cellulose is a hotspot in green and sustainable chemistry(Liu, Wang, & Lu, 2019; Y. Wang, Yuan, Tian, Zhang,

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& Lu, 2019). Due to the abundant hydrophilic hydroxyl groups, cellulose demonstrates

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tremendous potential applications for water treatment, especially in detection(Kenawy, Hafez, Ismail, & Hashem, 2018; Lin et al., 2016; Pappu et al., 2015) and

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enrichment(Chauhan, Singh, Chauhan, Verma, & Mahajan, 2005; Kanmani, Aravind, Kamaraj, Sureshbabu, & Karthikeyan, 2017; Thakur & Voicu, 2016; J. Wang, Liu,

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Duan, Sun, & Xu, 2019) for HMI, by loading color reagent on filter paper or derived cellulose as matrix. However, due to the high crystallinity and massive inter- and intrainteractions, cellulose is hardly soluble in common solvents. In our previous work, the dissolution of cellulose in the aqueous medium was significantly improved by the introduction of hydrophobic interaction(Y. Wang, Liu, Chen, Zhang, & Lu, 2018), and 4

functional cellulose materials have been fabricated via physical process(Y. Wang, Zhang, & Lu, 2019). Thus, it is vital to fabricate cellulose-based sensor for HMI detection via such aqueous mediums without chemical reaction, which is crucial in the perspective of sustainability. To the best of our knowledge, however, such approach is rarely reported. Herein, a simple and facile process was developed to fabricate dye loaded cellulose

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film as colorimetric sensor for HMI detection. Cellulose was dissolved in

benzyltrimethyl ammonium hydroxide (BzMe3NOH) aqueous solution, and regenerated in ethanol to obtain regenerated cellulose films. The films were immersed

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in the ethanol solution containing organic dyes including PAN, TAN and Dith. After

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washing and hot-pressing, cellulose/dye composite film was obtained as colorimetric sensor, due to the chelation reaction between empty orbit of HMI and azo group of

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PAN(Kılınçarslan, Erdem, & Kocaokutgen, 2007; Takahashi, 2016). SEM images

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illustrated the uniform distribution of PAN nanoparticles on cellulose, contributing to a low detection limit. The cellulose/dye composite films demonstrated homogeneity in

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structure, robust mechanical property, light transmittance, and relatively low detection limit for HMI. Therefore, the goal of the present work is to provide a facile and effective

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method to fabricate dye-loaded cellulose film as colorimetric sensor for HMI detection via simple dissolution-regeneration process, which is vital for water treatment and utilization of renewable resources.

2. Experimental Section 2.1

Materials 5

Cellulose (cotton linter pulp) containing the α-cellulose content of around 95% was supplied by Hubei Golden Ring Co., Ltd (Xiangfan, China). The viscosity-average molecular weight (Mη) was measured to be 10.8×104 g/mol by viscometer in cadoxen at 25 °C(Brown & Wikström, 1965). Before use, the cellulose sample was vacuumdried at 60 °C for 48 h to eliminate any moisture. BzMe3NOH (40 wt% aqueous solution) was purchased from Sigma-Aldrich Co., Ltd (Shanghai, China). Organic dyes

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including PAN, TAN and Dith were purchased from Macklin Co., Ltd (Shanghai, China). Zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O), copper chloride dihydrate

(CuCl2·2H2O) and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd.

Dissolution of cellulose

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2.2

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All reagents were analytical grade and used without further purification.

Cellulose sample with concentration of cellulose 6 wt% was dispersed in

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BzMe3NOH aqueous solution (1.88 mol/L) at room temperature. The mixture was

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stored in a refrigerator (-24 °C) over 12 h. Subsequently, cellulose dissolved completely after the solid was thawed. The resultant cellulose solution was subjected to

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centrifugation at 6000 rpm for 10 min at 4 °C to eliminate air bubbles, and then directly used for fabricating cellulose/dye composite films. Preparation of cellulose/dye composite films

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2.3

PAN was used here as an example to describe the preparation procedure. Cellulose

solution was cast on glass plate, and soaking in ethanol for regeneration over 12 h. Then, the obtained regenerated cellulose films was further immersed in PAN/ethanol solution for another 12 h. After washing with deionized water to neutral followed by hot-pressed 6

at 110 °C under 20 kPa for 2 h, CP was obtained, ready as sensor. When immersing the regenerated cellulose films in the PAN/ethanol solution, the concentrations of PAN were controlled as 0, 0.2, 0.3, 0.4 and 0.5 wt%, and the resultant CPs were marked as CP-0, CP-1, CP-2, CP-3 and CP-4, respectively. 2.4

Detection for HMI by cellulose based colorimetric sensors

The CP samples with 1 cm width and 2 cm length were soaking in Zn(NO3)2 aqueous

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solution with different concentrations (0, 100, 200, 400, 600, 800, 1000 ppb, 20 mL) at

25 °C for 15 min, and then were subjected for UV-vis scanning test (wavelength from 500 to 800 nm) by UV-vis spectrophotometry (Evolution 201, America). Characterization

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2.5

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The mechanical property of CPs was measured with a universal tensile-compressive tester (CMT 6503, Shenzhen SANS Test Machine, Shenzhen, China). A sample with 8

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cm length and 1 cm width was stretched at a tension speed of 0.5 mm/min at room

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temperature and 60 % humidity. Each sample repeated at least 5 times. The morphology of CPs was characterized by field emission scanning electronic

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microscope (FESEM, FEI Verios460, USA) at an accelerating voltage of 350 V and a current of 6.3 pA.

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The crystal structure of CPs was determined by X-ray diffraction patterns (XRD,

PANalytical X'Pert PRO Diffraction, Co-Kα radiation, reflection geometry). Fourier transfer infrared spectroscopy (FT-IR) spectra was performed by a Thermo Nicklet 5700 FTIR spectroscopy, following the KBr pellet method. X-ray photoelectron spectroscopy (XPS) was analyzed on a Thermo Fisher ESCALAB 250Xi spectrometer, 7

which used Al Kα radiation as the excitation resource. Elemental analysis (EA) of CPs was characterized by vario EL Ⅲ Element Analyzer (German, elementar Anaysensysteme GmbH). Before these tests, the CP samples were cut into powders, and vacuum-dried for 48 h at 60 °C to remove any moisture. Thermogravimetric

(TG)

measurement

was

recorded

on

TGA

Q500

thermogravimetric analyzer (TA Instruments, USA) in air. Initially, the temperature was

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elevated from room temperature to 80 °C to remove moisture in the samples, and then further elevated from 80 °C to 800 °C with a scan rate of 10 °C/min.

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3. Result and Discussion

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A simple and physical procedure was developed to fabricate the CP sample as colorimetric sensor for the detection of HMI, as shown in Scheme S1. The chemical

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structure of the dyes is displayed in Fig. S2. In the following part, PAN was used as an

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example of dyes. Cellulose solution was obtained by dissolving in BzMe3NOH solvent. After regeneration in ethanol, the obtained cellulose films were further immersed in

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PAN/ethanol solution, during which PAN molecules were immobilized on the films, leading to the colorless film turned to scarlet, as shown in Fig. S1. During the

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fabrication process no chemical reaction occurred, benefit to sustainability. The morphology of CPs was shown in Fig. 1 and S3. The regenerated cellulose films demonstrated a relatively dense structure, due to the hot-press process, while the morphology of the cellulose film substrate hardly changed after loading PAN. The immobilized PAN on CP existed as spherical nanoparticles with size of around 30 nm, 8

and dispersed uniformly on the surface of film, contributing to the uniform scarlet color of CP. With increasing the PAN concentration in ethanol solution, more PAN nanoparticles were loaded on the surface of CPs, indicating the increased content of PAN immobilized on cellulose. Therefore, CPs could be easily fabricated via the

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physical process, ready for HMI detection.

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Fig. 1. Surface SEM images of CP-1 (a), CP-2 (b), CP-3 (c), and CP-4 (d).

The mechanical property is important to CPs as sensors. Fig. 2 displays the tensile

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strength and elongation at break of CPs. The tensile strength and elongation at break of

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the pristine cellulose film were about 52.9 MPa and 6.95 %, respectively. After loading PAN nanoparticles, both of tensile strength and elongation at break of CPs enhanced slightly. The overall influence of PAN on the mechanical property of the cellulose substrate was not obvious. The phenomenon could be explained that when regenerating cellulose in ethanol, the aggregate structure of the cellulose films was constructed and fixed. When further soaking the regenerated film in PAN/ethanol solution, the 9

incorporated PAN nanoparticles hardly changed the cellulose structure, as shown in Fig. 1. It was noted that the average tensile strength of CPs was about 57.8 MPa, which

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guaranteed the potential application as colorimetric sensor for HMI detection.

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Fig. 2. Tensile strength and elongation at break of CPs.

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The crystalline structure of PAN and CPs was shown in Fig. 3a. For all the CP samples with different PAN contents, three typical peaks were observed at 2θ = 12.1,

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20.3 and 21.5 °, assigned to the (1-10), (110), and (200) planes, suggesting a typical cellulose Ⅱ structure. According to the previous study(Chang, Zhang, Zhou, Zhang, &

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Kennedy, 2010), the crystalline peaks of CPs (regenerated cellulose) were different from that of native cellulose, whose main peaks located at 2θ = 14.8, 16.3, and 22.6°, assigned to the (110), (1-10), and (200) planes of cellulose Ⅰcrystalline(Isogai, Usuda, Kato, Uryu, & Atalla, 1989). The result indicated the transition from cellulose I to cellulose II during regeneration in ethanol, and further soaking in PAN/ethanol solution 10

made no obvious influences. No obvious PAN peak was observed in the CP samples, possibly due to the very low content of PAN(Liu et al., 2019; Xu et al., 2010). The FTIR spectra of CPs and PAN are displayed in Fig. 3b. The absorption band at 1431 cm-1 was observed in all the CP samples, assigning to the scissoring motion of CH2 of cellulose(L. Zhang, Ruan, & Zhou, 2001). The absorption band at 1436 cm-1 was assigned to the stretching motion of N=N of PAN molecules(Taher, Jarelnabbi, Al-

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Sehemi, El-Medani, & Ramadan, 2009), which was not well identified in the CP samples, possibly due to the low amount of PAN immobilized on cellulose. No new evident absorption peak was observed after the PAN loading, indicating the

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immobilization of PAN on cellulose was mainly based on physical interaction rather

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than chemical reactions. In addition, the PAN content in CPs was determined. With increasing the PAN concentration in ethanol, an increasing proportion of PAN in CPs

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was observed, as shown in Fig. 3c. When the PAN concentration increased from 0.2 to

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0.5 wt%, the N proportion of CPs increased monotonously from 0.34 to 0.44 wt%, in

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accordance with the SEM observation.

Fig. 3. XRD patterns (a) of CPs and PAN. FT-IR spectra (b) of CPs and PAN. Proportion of N in CPs as a function (c) of PAN concentration in ethanol.

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The interaction between PAN and cellulose was further analyzed by XPS. Full scan of CP-0 and CP-3 is displayed in Fig. 4a. Both of C and O existed in CP-0 and CP-3, whereas N presented only in CP-3, suggesting the immobilization of PAN on cellulose. High resolution spectra of C 1s region of CP-0 and CP-3 is shown in Fig. 4b, and no obvious difference was observed. High resolution spectra of N 1s region of PAN and CP-3 is presented in Fig. 4c. The peaks of N 1s of PAN appeared at 400.5 eV and 399.0

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eV, assigning to nitrogen atom of azo group and pyridinic group, respectively(Yoshida, 1980; H.-Y. Zhang et al., 2017). After PAN loading on cellulose, the binding energy of N 1s of pyridinic group remained at 399.0 eV, whereas the binding energy of N 1s of

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azo group shifted to lower binding energy of about 399.7 eV, suggesting interaction

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between cellulose and azo group of PAN.

Fig. 4. Survey scan spectra (a) of CP-0 and CP-3. High resolution C 1s spectra (b) of

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CP-0 and CP-3. High resolution N 1s spectra (c) of PAN and CP-3.

The thermal stability of CPs was also investigated. TGA and differential thermogravimetric (DTG) curves of CPs are shown in Fig. 5a and b, respectively. CP samples with different PAN loading demonstrated no obvious difference in the thermal 12

stability, indicating the slight impact of PAN on cellulose matrix, possibly due to the low content of PAN on cellulose substrate, and the structure of cellulose was not influenced obviously. As shown in Fig. 5c, the transmittance of CPs was recorded. The CP-0 demonstrated transmittance of about 70.1 % (550 nm) in the visible light region. With PAN loading, the transmittance decreased, due to the scarlet color of PAN. The transmittance at 550 nm of CP-1 was about 35.5 %, and further increasing the PAN

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content the transmittance of the resultant composite film stayed at about 20.7 %. The transmittance of CPs in wet state was also shown in Fig. S4, and a similar behavior was

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observed.

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Fig. 5. TG thermograms (a) and differential thermogravimetric curves (b) of CPs. Light

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transmittance (c) of CPs from 300 to 800 nm in dry state.

PAN was previously applicable for the detection of some HMIs like Zn2+, Ni2+ and

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Co3+ (Takahashi et al., 2006).To demonstrate the potential application of HMI detection, CP-3 was used as colorimetric sensor for the detection of Zn2+, as an example. Due to the formation of chelate [Zn(PAN)2]2+(Takahashi et al., 2006), the color of CP-3 would change after reacting with Zn2+. The color change and the relative color intensity was measured by UV-vis absorption spectra in the range from 500 to 800 nm, with the Zn2+ 13

concentration varied. As shown in Fig. 6a, the new peak at 565 nm was assigned to the characteristic absorption of chelate [Zn(PAN)2]2+ (Takahashi et al., 2006). The increased peak intensity with increasing the Zn2+ concentration enabled us to determine quantitatively the concentration of Zn2+ in the tested water sample, and the intensity of the characteristic absorption peak at 565 nm as a function of Zn2+ concentration is depicted in Fig. 6b. The absorbance intensity of CPs at 565 nm increased gradually with

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increasing Zn2+ concentration, suggesting an increase of [Zn(PAN)2]2+ content in CP-3.

The limit of detection of CP-3 is comparable to literatures in the field of test strips or kits for HMIs, as shown in Table S1, and at the same time the preparation process

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demonstrated simplicity, low cost and sustainability. Optical images of CP-3 samples

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after interacting with Zn2+ are shown in Fig. 6c. The samples displayed distinguishable color changes upon increasing Zn2+ concentration, which could be clearly identified

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with the naked eyes, indicating the potential of CPs for the detection of HMI. For very

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low concentration of HMI, however, UV-vis spectrophotometer was still needed to

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measure accurately the concentration.

Fig. 6. UV-vis absorption spectra (a) of CP-3 after interacting with various 14

concentrations of Zn2+. Calibration curves (b) of CP-3 by plotting the intensity at 565 nm as a function of Zn2+ concentrations. Optical images (c) of CP-3 after interacting with various concentrations of Zn2+.

The morphology of CP-3 before and after chelating with Zn2+ was further recorded in Fig. 7a and b, respectively. Obviously, the introduction of Zn2+ led to increased

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particle size of the immobilized PAN, due to the chelation between Zn2+ and PAN, allowing the UV measurement and even identification by naked eye. To study the

crystalline structure of CP-3 before and after interacting with Zn2+, the XRD patterns

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were recorded in Fig. 7c. No evident difference was observed between CP-3 reacting

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with and without Zn2+, indicating the interaction mainly occurred between Zn2+ and PAN, rather than cellulose, attributed to the inertness of cellulose. Furthermore, no

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obvious crystalline peak of PAN and [Zn(PAN)2]2+ was observed, possibly due to low

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content of PAN on cellulose.

Fig. 7. Surface SEM images of CP-3 before (a) and after (b) interacting with Zn2+. XRD patterns (c) of CP-3 before and after interacting with Zn2+.

Except PAN, the developed strategy can also be applied to other dyes, demonstrating 15

its universality and applicability to prepare cellulose/dye films as colorimetric sensors, expanded its potential in HMIs detection. In this work, two more organic dyes of TAN and Dith were used to prepare composite films as HMI sensors. Following the similar preparation process, cellulose/TAN (CT) and cellulose/Dith (CD) composite films were prepared. The photographs of CT and CD are depicted in Fig. S1. The incorporation of TAN and Dith turned the transparent regenerated cellulose film to red and brown color,

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respectively. The interactions between dyes and cellulose were also investigated. The survey scan spectra of CT and CD is displayed in Fig. 8a. The presence of N 1s at about

400 eV of both CT and CD suggested TAN and Dith were loaded on cellulose films. In

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detail, the high-resolution spectra of N 1s of CT and TAN was illustrated in Fig. 8b.

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The binding energy of N 1s of azo group of CT (399.9 eV) was lower than that of TAN (400.0 eV), demonstrating the chemical environment of N of azo group varied, and

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interactions between the cellulose and azo group of TAN contributed to the

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immobilization of TAN on cellulose. Analogically, the high-resolution spectra of N 1s of Dith and CD was also investigated in Fig. 8c. Due to the interaction between

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cellulose and azo group of Dith, the high resolution of N 1s of CD (400.6 eV) was about 0.7 eV higher than that of Dith (399.9 eV), leading to immobilization of Dith on

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cellulose. The successful preparation of CP, CT and CD composite films indicated that the physical approach could be applied for the fabrication of cellulose based colorimetric sensors with various dyes.

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Fig. 8. Survey scan spectra (a) of regenerated cellulose film, CT and CD. High resolution N 1s spectra of TAN and CT (b). High resolution N 1s spectra (c) of Dith

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and CD.

The prepared CT and CD composite films were also tested as colorimetric sensor for

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HMI detection. Similar to PAN, TAN was applicable for Cu2+, Ni2+ and Pb2+, and Dith

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was applicable for Ag+, Zn2+ and Hg2+ (Takahashi et al., 2006), due to the chelation between lone pair electrons of azo group of dyes and empty orbits of HMIs(Kılınçarslan

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et al., 2007). As shown in Fig. S5, CT and CD were interacted with various

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concentrations of Cu2+ and Zn2+, respectively. The color change of CT and CD was not evident for very low HMI concentration. With increasing the concentration of HMI, the

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color change of CT and CD was observed easily, even by naked eye, demonstrating the potential of the composite films as colorimetric sensor and the potential application of

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the preparation strategy. In addition, in order to demonstrate the daily use of the prepared sensor, water samples were tested by CP-3. The HMI detection of deionized water, tap water and east lake water by CP-3 is displayed in Fig. S6, respectively. Compared with deionized water, darker color was observed for both the tap water and east lake water, indicating the existence of HMIs with higher concentration. Obviously, 17

CP-3 sample interacted with tap water displayed the darkest color, indicating highest concentration of HMI, which might be caused by the long-term utilization of the transport pipeline of the tap water. The result suggested potential application of the composite films as HMI colorimetric sensor in daily life.

4. Conclusion

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A simple and universal strategy to fabricate cellulose-based colorimetric sensor for HMI detection was developed. Cellulose was first dissolved in BzMe3NOH aqueous solution at low temperature, and cellulose films were prepared by regeneration in

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ethanol. The regenerated films were further soaking in dyes/ethanol solution followed

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by washing/hot-pressing, to get cellulose/dye composite films which were ready for HMI detection. PAN was used as an example, and the CPs demonstrated robust

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mechanical property, optical transmittance and thermal stability up to about 270 °C.

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The introduction of PAN hardly changed the properties of the cellulose substrate, possibly due to the low content of PAN in the CPs and the cellulose structure was fixed

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during regeneration. Physical interaction between cellulose and azo group contributed to the immobilization of PAN on the substrate, and PAN distributed uniformly as

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nanoparticles on cellulose. The detection limit of CP for Zn2+ was as low as 100 ppb, exhibiting great potential in colorimetric sensor of on-site detection for HMI. More importantly, dyes including TAN and Dith were also loaded on cellulose following the developed approach, and the resultant films were proved as colorimetric sensor for HMI detection, even with naked eye. Therefore, the strategy developed herein provide a 18

simple and universal approach to fabricate cellulose-based sensor with a low detection limit for HMI detection, exhibiting great potential in water treatment.

Supporting Information

Acknowledgements

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This work was supported by the National Natural Science Foundation of China

(51973166 and 51573143), the Fundamental Research Funds for the Central Universities (2042018kf0042), the Opening Project of Key laboratory of Processing

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and Quality Evaluation Technology of Green Plastics of China National Light Industry

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council (Beijing Technology and Business University) (Grant No. PQETGP2019005), the Major International (Regional) Joint Research Project (21620102004), and the

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Funds for International Cooperation and Exchange of the National Natural Science

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Foundation of China (21811530006).

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Conflict of Interest

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The authors declare no conflict of interest.

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