A breathing A4 paper by in situ growth of green metal–organic frameworks for air freshening and cleaning

Bo Zhang, Hongwen Chen, Liming Jiang, Youqing Shen, Dan Zhao, Zhuxian Zhou,*

1 Key Laboratory of Biomass Chemical Engineering of Ministry of Education and Zhejiang Key Laboratory of Smart Biomaterials, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China

2 Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

3 Department of Chemical & Biomolecular Engineering, National University of Singapore, 117585, Singapore

4 Hangzhou Global Scientific and Technological Innovation Center, Zhejiang University, Hangzhou 311215, China

Keywords:Metal–organic frameworks A4 printing paper In situ synthesis Fragrance encapsulation Formaldehyde removal Carbon dioxide capture

ABSTRACT Surface modification of natural cellulose fibers with nanomaterials is an effective strategy for producing functional textiles for multiple applications.A4-sized printing paper is a commonly used, cheap, and easily acquirable office supply which is mainly made of cellulose fibers.Here,we report green and simple nanofabrication of A4 paper to endow it with high capability for fragrance encapsulation and sustained release, and strong adsorption to indoor air pollutants.The method utilizes the sugar molecule of cellulose for in-situ growth of γ-cyclodextrin (γ-CD) metal–organic frameworks (MOFs) on A4 paper.The obtained γ-CD-MOF/A4 nanocomposites have superior specific surface area and high porous structure.The γ-CD-MOF/A4 nanocomposites can effectively encapsulate fragrant molecules through host–guest interaction.The γ-CD-MOF/A4 nanocomposites also show strong absorption capability to formaldehyde and carbon dioxide through the formation of hydrogen bonding and chemical bonds.These γ-CD-MOF/A4 nanocomposites combine the advantages of both A4 paper and γ-CD-MOF, which can be used in indoor air freshening and cleaning.

1.Introduction

With the industrial revolution and the development of construction technology, synthetic building materials have been widely used, producing a mass of harmful air pollutants.Among these air pollutants,volatile organic compounds(VOCs)are extensively produced from many sources such as building materials,paints, furniture, and cleaning products [1,2].Formaldehyde(HCHO)is one of the most common and harmful indoor VOCs that is released in large quantities during construction and furnishing[3].Exposure to HCHO for a long time will cause serious health problems, such as pneumonia, neurodegenerative diseases, and cancer [4].Carbon dioxide (CO2) is known as the bases of greenhouse gas, and CO2level is an indicator of indoor environmental quality.The massive CO2emission is a global environmental problem, while high levels of indoor CO2will also produce health effects,such as headaches,nausea,fatigue,and dizziness[5].Thus,efficient capture and removal of VOCs and CO2is an important environmental need and essential to improve the indoor air quality for safeguarding life and health.

Cellulose-based fibers have been widely used as chemical filters to remove indoor air pollutants such as particulate matter (PM2.5,aerodynamic diameter ≤ 2.5 μm), HCHO, and CO2[6–8].Cellulose-based materials have many favorable merits, including ease of acquisition, eco-friendly, biocompatible, good flexibility,and high machinability [9–11].However, there are still some demerits limiting their applications,such as low porosity and poor bacterial resistance [9].Surface modification of fibers with nanostructures has become an attractive strategy for developing functional textiles with unique properties such as pollutant removal ability and antibacterial property [12–15].Metal–organic frameworks(MOFs)have shown great potential for gas capture and storage due to their large surface areas and ultrahigh porosity[16,17].MOF/cellulose fiber nanocomposites[18–20],have been developed to combine the advantages of both materials,which greatly extend the application prospects [21,22].MOFs/cellulose fiber nanocomposites can be obtained through various methods, such as selfcrosslinking, pro-chemical modification, forming hydrogen bonds interaction between MOFs and cellulose, stepwise layer-by-layer growth, electrospinning, and so on [23–26].Nevertheless, the fabrication processes are complicated or suffer from heavy loss of MOF crystals from cellulose during vigorous procedures.Continued efforts have been made to develop effective and facile methods for MOFs/cellulose composites, such as biomineralization and pregelation [27–29].

Here, we present a green and simple method for thein-situgrowth of γ-cyclodextrin-based metal–organic frameworks (γ-CD-MOFs) in A4-sized printing paper (A4 paper).γ-CD-MOFs are biocompatible and low toxic MOFs made by the coordination of potassium ion (K+) and γ-CD [30].γ-CD-MOFs have shown high potential in the application of gas capture, storage, and controlled release [3,31,32].A4 printing paper is a commonly used, low-cost,and readily available office supply, which is mainly composed of cellulose fibers[33,34].We propose the glucose molecules of cellulose fibers can participate in the coordination of γ-CD and K+that will generate γ-CD-MOFs on the surface of A4 paper.We demonstrate the feasibility of thein-situgrowth of γ-CD-MOFs on A4 paper and evaluate the applications of γ-CD-MOF/A4 nanocomposites for indoor air fresh and cleaning, including air pollutant capture, antibacterial, and fragrance encapsulation and release(Fig.1).The green γ-CD-MOF/A4 composites possess the merits of cellulose fibers and MOFs, holding great potential for broad applications in daily life.

2.Experimental

2.1.Materials and methods

Commercial A4 printing paper(70 g?m-2)was manufactured by UPM Copykid.γ-Cyclodextrin (γ-CD), cetyl trimethyl ammonium bromide (CTAB), potassium hydroxide, and all the essential oils were acquired from Aladdin (Shanghai, China).Methanol (MeOH),isopropanol (i-PrOH), formaldehyde (HCHO), and other chemicals(analytical grade) were purchased from Sinopharm Chemical Reagent Co.Ltd.(Shanghai, China) and used without purification.

Scanning electron microscope (SEM) micrographs were recorded with SU-8010 (Hitachi, Japan) equipped with an Oxford EDX analysis system.Formaldehyde concentration was monitored by a formaldehyde detector (Air Detector MEF500, China).The powder X-ray diffractometric (PXRD) patterns were measured on a PANalytical X’Pert diffractometer(PANalytical,Netherlands)with a scan speed of 0.1 s per step, a scanning step size of 0.026° and a 2θ range of 2°–30°.Fourier transform infrared spectroscopic(FT-IR)spectra were recorded between 400 and 4000 cm-1using a Nicolet IS10 spectrophotometer(Bruker TENSOR II,Germany)with 32 successive scans at a resolution of 2 cm-1.The samples were dried,evenly blended with potassium bromide, and pressed into transparent sheet.The N2adsorption/desorption isotherm and specific surface area were recorded by an automatic physical and chemical adsorption instrument(BELSORP-Max,Japan)at -196°C.Samples were activated at 120 °C in the vacuum for 6 h before measurements.The thermal stability was conducted on Pyris 1 TGA(PerkinElmer, USA) at a heating rate of 10 °C?min-1from 50 to 600 °C under air atmosphere.Gas chromatography (GC) analysis was measured on a 7890A gas chromatograph (Agilent Technologies,USA) equipped with a DB-608 capillary column(30 m × 320 μm × 0.5 μm).

2.2.Preparation of γ-CD-MOF/A4 composites

The preparation of γ-CD-MOF/A4 composites is modified from the synthesis of γ-CD-MOF[32].An aqueous solution (20 ml) containing KOH (224 mg, 4 mmol) and γ-CD (648 mg, 0.5 mmol) was added to the centrifugal tube and evenly distributed by ultrasound,then filtered with a syringe filter(0.45 μm PTEE membrane)into a beaker.A4 paper (100 mg) was added into the solution, followed by volatilization of MeOH into the solution at 50 °C for 6 h.Next,A MeOH(20 ml)solution of CTAB(160 mg)was added to the beaker.The mixed solution was incubated overnight at room temperature, and a large number of small-sized crystals appeared on the surface of A4 paper.The γ-CD-MOF/A4 composites were separated and washed with isopropanol three times.The composites were immersed in dichloromethane for one day to exchange the residual moisture and dried in the oven for subsequent usage.

2.3.Loading content of γ-CD-MOF on A4 paper

The mass ratio of γ-CD-MOF on the A4 paper,w0(g?g-1,γ-CDMOF/A4), was determined by thermogravimetric (TGA) curves through the calculation of the remaining mass percentages of γ-CD-MOF (w1, %), the free A4 paper (w2, %) and the composites(w3, %).The calculation formula is as follows:

2.4.Loading of essential oils in γ-CD-MOF/A4

Essential oils were loaded in pristine A4 paper and γ-CD-MOF/A4 composites by the simple soaking method [32,35], A4 and γ-CD-MOF/A4 (10 mg) were soaking into various essential oils,including cinnamaldehyde (CA), geraniol (Ger), ethyl propionate(EP), 5-methyl-2-isopropylphenol (MI), myrcene (Myr), and linalool (Lin).After oscillating for 12 h, the A4/fragrances (FG) and γ-CD-MOF/A4/FG in the centrifuge tube were washed with hexane and dried in the vacuum at 25 °C for 6 h.Fragrance loading contents were calculated by comparing the mass changes of A4 paper and γ-CD-MOF/A4 composites before and after encapsulation.The release percentage of fragrance was monitored by static head space gas chromatography according to the reported literature [32,35].

2.5.Antibacterial activity

Antibacterial activity of the A4,A4/cinnamaldehyde(CA),γ-CDMOF/A4, and γ-CD-MOF/A4/CA were evaluated againstE.coli(ATCC 47107).The A4/CA and γ-CD-MOF/A4/CA were prepared by the immersing method.

Diskdiffusionmethod.A disc-diffusion assay was used to determine the growth inhibition of bacteria.First,A4,A4/CA,γ-CD-MOF/A4,and γ-CD-MOF/A4/CA were added to DMSO solution(2 ml)for the antibacterial experiment.For the disc diffusion assay, approximately 109CFU?ml-1E.coliwas uniformly spread over the agar plates by applying a 100 μl culture medium, and the OD600value was around 0.8,A4,A4/CA,γ-CD-MOF/A4,γ-CD-MOF/A4/CA round paper were added to the plates and cultured at 37°C for 24 h.The agar plate was photographed, and the diameters of the inhibition zones were measured.

Opticaldensity(OD600)measurements.The OD600of bacteria suspensions ofE.coliwas adjusted to around 0.18 in LB broth.A4,A4/CA,γ-CD-MOF/A4, and γ-CD-MOF/A4/CA round paper were added to 3 ml bacteria suspensions.The mixed suspension was kept in an incubator with continuous shaking,and the OD value was measured at 600 nm at different periods(0,2,4,6,8,10,12,and 24 h)to examine the growth of bacteria.

Fig.1. Illustration of the preparation of γ-CD-MOF/A4 nanocomposites for air freshening and cleaning, including air pollutant capture, antibacterial, and fragrance encapsulation and release.

2.6.Formaldehyde adsorption by activated carbon,γ-CD-MOF,A4,and γ-CD-MOF/A4

Activated carbon, γ-CD-MOF, A4, and γ-CD-MOF/A4 (50 mg)were put into a tightly sealed container(50 L),and a formaldehyde solution (～1.136 mmol?L-1) was added.An electrical heater was used to heat and turn formaldehyde into gas.A formaldehyde detector meter was used to monitor the changes of formaldehyde concentration at 25 °C and 101.325 kPa.

2.7.CO2 capture

An adsorption experiment of CO2was performed according to the reported literature[3].γ-CD-MOF/A4 was cut into a quadrilateral shape and put into a CH2Cl2solution of methyl red(1 mmol?L-1).After soaking for 12 h, it was washed three times with CH2Cl2and dried in a vacuum oven at room temperature.The treated γ-CD-MOF/A4 was photographed.The methyl redloaded γ-CD-MOF/A4 was transferred to scintillation and exposed to CO2(from sublimed dry ice).γ-CD-MOF/A4 was photographed after 15 min.CO2adsorption isotherms were analyzed on an adsorption apparatus (ASAP 2460, Micromeritics, USA).

3.Results and Discussion

3.1.Preparation and characterization of γ-CD-MOF/A4 paper nanocomposites

γ-CD-MOF is formed by coordinating glucose units of γ-CD with K+.A4 paper is mainly composed of cellulose fibers which contain similar glucose units with γ-CD.Therefore, we hypothesize that cellulose substrate can participate in the coordination of γ-CD and K+to generatein-situgrowth of γ-CD-MOF on A4 paper, as illustrated in Fig.2(a).The obtained γ-CD-MOF/A4 paper nanocomposites were firstly observed by SEM(Fig.2(b)).The paper’s surface is covered by dense cubic crystals with an average particle size of around 200 nm.The elemental mapping images of γ-CD-MOF/A4 were collected to further confirm the structure (Fig.2(c)).There are apparent signals of potassium atom from γ-CD-MOFs apart from the carbon and oxygen atoms.The signal of the calcium atom is due to the addition of reinforcing agents (calcium carbonate) in the production process of A4 paper.

Fig.2. (a) Illustration of the preparation of the γ-CD-MOF/A4 paper.(b) SEM images.(c) EDX elemental analysis.

We further characterized the nanocomposites by PXRD (Fig.3(a)) and nitrogen adsorption–desorption isotherms (Fig.3(b)) to study the crystallinity and porosity.The γ-CD-MOF/A4 exhibits broaden peaks (2θ=3.90°, 5.45°, and 6.85°) with slight shifts to lower 2θ values compared with the simulated pattern of γ-CDMOF.Such change maybe due to the interaction between MOF crystals and cellulose [30].The N2adsorption–desorption isotherms show that A4 paper is nonporous material.In contrast,the isotherm of γ-CD-MOF/A4 suddenly increasesat the lower relative pressure,representing the existence of micropores [36].The surface area of γ-CD-MOF/A4 is calculated as 237 m2?g-1, much higher than A4 paper.The FT-IR spectra (Fig.3(c)) of γ-CD-MOF, A4, and γ-CDMOF/A4 all show broad absorption peaks in the region of 2890–2920 and 3200–3600 cm-1, which are attributing to the —CH2—and —OH stretching vibrations in the glucose units, respectively[37].The peaks at 1645 and 1413 cm-1are corresponding to hydrate water vibration and—OH group bending vibration,respectively[38].Compared to A4 paper,the relative intensity of peaks at 1020 cm-1of γ-CD-MOF/A4 is increased, which is related to the C—O stretching vibration, further proving the successful growth of γ-CD-MOF on A4 paper.To study the loading content of γ-CDMOF and thermal stability, we performed TGA of γ-CD-MOF/A4 and compared with A4 paper and γ-CD-MOF (Fig.3(d)).From the TGA traces,γ-CD-MOF,γ-CD-MOF/A4 and A4 paper begin to decompose at about 230, 250, and 300 °C, respectively [39].The final remaining mass ratio of A4 paper is 26.90%, higher than that of γ-CD-MOF(13.05%).This result is probably due to reinforcing agents such as calcium carbonate in the A4 paper[34].The remaining mass percentage of γ-CD-MOF/A4 is 23.83%, lower than A4 papers,attributing to the loss of γ-CD-MOF on the substrate [24].Accordingly,the mass ratio of γ-CD-MOF on the A4 paper is calculated as(0.285±0.03)g?g-1by the equation described in Section 2.5,which is very close to the value(0.297±0.01)g?g-1measured by the simple weighing method.Afterward, we evaluated the stability of γ-CD-MOF/A4 by bending experiment and determined the remaining γ-CD-MOF by TGA(Fig.S1 in Supplementary Material).The remaining mass percentage of γ-CD-MOF/A4 increases from 23.582% to 23.656%, which indicate the mass ratio of γ-CD-MOF on the A4 paper decreases from 0.305 to 0.296 g?g-1.Therefore,no significant MOFs fall off from the A4 paper,furtherly confirming the stability of γ-CD-MOF/A4.

3.2.γ-CD-MOF/A4 nanocomposites for fragrance encapsulation and sustain-release

Fragrance is widely used indoors to provide an aroma and comfortable environment,e.g., air freshener [32,40,41].Scented paper is a popular choice for indoor refreshing.We speculate that γ-CD-MOF/A4 nanocomposites with increased surface area and high porosity may be excellent carriers for fragrance encapsulation and sustained release.To test the fragrance loading efficiency, we choose six commonly used fragrant molecules (cinnamaldehyde(CA), geraniol (Ger), ethyl propionate (EP), 5-methyl-2-isopropylphenol (MI), myrcene (Myr), and linalool (Lin)) (Fig.4(a)).The encapsulation of fragrance in porous materials is highly affected by the fragrance’s physicochemical properties, such as molecular size,polarity,hydrophobicity,and saturated vapor pressure [32,35,42,43].The loading capacity of γ-CD-MOF/A4 to those fragrance is in the range of 55–540 mg?g-1, which is about 10–200 times higher than those of pristine A4 paper (Fig.4(b)).The increased fragrance encapsulation capability of γ-CD-MOF/A4 is probably benefiting from the host–guest interaction between the MOF pore and guest molecules.

Fig.3. (a) XRD patterns.(b) N2 adsorption–desorption isotherms.(c) FT-IR spectra.(d) TGA curves.

Fig.4. (a)The chemical structures of fragrant molecules,cinnamaldehyde(CA),geraniol(Ger),ethyl propionate(EP),5-methyl-2-isopropylphenol(MI),myrcene(Myr),and linalool(Lin).(b)The fragrances encapsulation capacity in A4 and γ-CD-MOF/A4.(c)FT-IR spectra of CA and CA-loaded in A4 and γ-CD-MOF/A4.(d)Release kinetics of CA and CA-loaded A4 papers.

We studied the capability of γ-CD-MOF/A4 for sustained release using CA as a representative fragrance.After CA encapsulation,the FT-IR spectrum of γ-CD-MOF/A4/CA shows a new peak at 1682 cm-1,which is corresponding to the C=O stretch of the aldehyde in cinnamaldehyde, suggesting the successful encapsulation(Fig.4(c)).The release of CA was monitored by the GC-SPME method.As shown in Fig.4(d),free CA and A4/CA were completely released in around 10 and 21 days, respectively, while γ-CD-MOF/A4/CA only released about 30% in 21 days.It should be noted that the pristine A4 paper can only trap a little fragrance.All these results suggest γ-CD-MOF/A4 is a superior carrier for fragrance encapsulation that can be used for indoor air refreshing.

Fig.5. (a) Results of antibacterial test disk (1: A4, 2:γ-CD-MOF/A4, 3: A4/CA and 4:γ-CD-MOF/A4/CA) to E.coli.(b) Real-time OD600 values of the samples against E.coli.

3.3.Antibacterial activity of CA-loaded γ-CD-MOF/A4 paper composites

Natural cellulose fibers,including paper products,are not resistant to bacteria,limiting their daily usage.γ-CD-MOF/A4 with porous structure allow the encapsulation of antibacterial agents,including some specific fragrances with antibacterial activity,such as CA [44].We evaluated the antibacterial activity of γ-CD-MOF/A4/CA onE.coliby antibacterial disk test and chose raw A4 paper(A4), CA-loaded paper (A4/CA), and γ-CD-MOF/A4 as control groups (Fig.5(a)).γ-CD-MOF/A4/CA exhibit high antibacterial activity againstE.coli, producing apparent non-growing halos,while the other groups show no obvious inhibition zone.The high antibacterial activity of γ-CD-MOF/A4/CA was confirmed by monitoring the OD600of bacterial after each treatment (Fig.5(b)).It shows the growth ofE.coliwas substantially inhibited by γ-CDMOF/A4/CA but not by the other groups.This result indicates that γ-CD-MOF/A4 has a strong bactericidal effect onE.coli, while A4/CA has no apparent impact, probably due to the low loading capacity.

3.4.γ-CD-MOF/A4 paper nanocomposites for HCHO and CO2 capture

We evaluated the capability of γ-CD-MOF/A4 paper nanocomposites for the removal of indoor HCHO.It is reported γ-CD-MOF can effectively adsorb HCHO by forming hydrogen bonding between γ-CD-MOF and hydroxyl groups of CD-MOF (Fig.6(a))[3].Thus, γ-CD-MOF/A4 paper may inherit the specific properties from γ-CD-MOF and maintain cellulose materials’merits.We compared the HCHO capture capability of γ-CD-MOF/A4 with pristine A4 paper, activated carbon, and γ-CD-MOF (Fig.6(b)).The experiment was conducted in a 50 L volume of container with HCHO concentration of 1.136 mmol?L-1and 50 mg of various sorbents.The result shows γ-CD-MOF/A4 (28.5% (mass)γ-CD-MOF) has comparable HCHO capture capacity to activated carbon and is much higher than plain A4 paper.In addition,we also carried out regenerate experiments to evaluate its recyclability.The used MOFs were degassed at 120 °C under vacuum for 12 h before the next adsorption.As shown in Fig.S2 in Supplementary Material, the adsorption ability shows that the adsorption ability of γ-CDMOF/A4 is relatively stable during the three subsequent cycles and can be recycled efficiently.Thus, γ-CD-MOF/A4 may be used for HCHO removal to improve indoor air quality.

We next tested the CO2capture capability of γ-CD-MOF/A4.It is reported that γ-CD-MOF is highly selective for the absorption of CO2at low pressures by the formation of carbonic acid (Fig.7(a))[31].Methyl red was used as a CO2adsorption indicator that turns yellow to orange after forming carbonate ester with CO2.After encapsulation of methyl red, the color of γ-CD-MOF/A4 changed from gray to yellow (Fig.7(b)), while pristine A4 paper could not load the dye, and no color change was observed.The methyl redloaded γ-CD-MOF/A4 was transferred to a scintillation vial and exposed to CO2(from sublimed dry ice).It was found γ-CD-MOF/A4 quickly turned from yellow to orange, indicating the capture of CO2that produced an acid environment and protonated the dye.The CO2adsorption isotherms (Fig.7(c)) show that the CO2uptake content of the γ-CD-MOF/A4 (γ-CD-MOFs mass content～28.5%) and A4 paper are around 9.28 cm3?g-1and 0.05 cm3?g-1,respectively, at the low pressure (P/P0=0.001), which confirm the CO2adsorption capability of γ-CD-MOF.Meanwhile, these observations also indicate that covalent bonds preferentially occur at low pressures and physisorption occur at elevated pressures for γ-CD-MOF[31].Thus,γ-CD-MOF/A4 is a type of promising material for indoor air refreshing and HCOH and CO2removal.

Fig.6. (a) Adsorption of HCHO over γ-CD-MOF by hydrogen bonding interactions.(b) HCHO adsorption curves of A4, activated carbon,γ-CD-MOF, and γ-CD-MOF/A4 at 25 °C and 101.325 kPa.

Fig.7. (a) Schematic diagram illustrating the adsorption of CO2 by γ-CD-MOF/A4.(b)The photographs of methyl red-loaded γ-CD-MOF/A4 before and after CO2 adsorption.(c) CO2 adsorption isotherms for γ-CD-MOF/A4 and A4 paper at 25 °C.

4.Conclusions

In this work, we first reported a green and simple method for nanofabrication of γ-CD-MOF/A4 paper nanocomposites throughin-situgrowth of γ-CD-MOF on A4 paper.The surface of A4 paper is covered with dense nanosized γ-CD-MOF.γ-CD-MOF/A4 combines the advantages of both γ-CD-MOF and A4 paper, with high adsorption, porous structure, and flexibility.We demonstrate that γ-CD-MOF/A4 is excellent in fragrance encapsulation and sustained release, HCHO removal, and CO2capture, which can be applied for indoor air freshening and cleaning.Thus, we believe the γ-CD-MOF/A4 has a great promise for a broad application in daily life.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work is supported by the National Key Research and Development Program of China(2016YFA0200301),the National Natural Science Foundation of China(21875211,52073249,51833008,and 51603181),and the Zhejiang Provincial Key Research and Development Program (2020C01123).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.12.003.