Urethane-Linked Imidazole-Cellulose Microcrystals: Synthesis and

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Urethane-linked Imidazole-Cellulose Microcrystals: Synthesis and their Dual Functions in Adsorption and Naked Eye Sensing with Colorimetric Enhancement of Metal Ions Kewarin Pramual, Varol Intasanta, Suwabun Chirachanchai, Narong Chanlek, Pinit Kidkhunthod, and Autchara Pangon ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04028 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 9, 2018

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ACS Sustainable Chemistry & Engineering

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Urethane-linked Imidazole-Cellulose Microcrystals: Synthesis and their Dual Functions in Adsorption and Naked Eye Sensing with Colorimetric Enhancement of Metal Ions

Kewarin Pramual,† Varol Intasanta,† Suwabun Chirachanchai,‡ Narong Chanlek,§ Pinit Kidkhunthod,§ and Autchara Pangon*,†

†

Nano Functional Textile Laboratory, National Nanotechnology Center (NANOTEC), National Science and Technology Development Agency 111 Thailand Science Park, Phahonyothin Rd., Khlong Nueng, Khlong Luang, Pathumthani 12120, Thailand

‡

The Petroleum and Petrochemical College, Chulalongkorn University, Soi Chula 12, Phyathai Rd., Pathumwan, Bangkok 10330, Thailand

§

Synchrotron Light Research Institute (Public Organization), 111 University Avenue, Muang District, Nakhon Ratchasima 30000, Thailand

* To whom correspondence should be addressed; Tel.: +66 (0) 2564 7100; E-mail: [email protected]

Abstract In this work, we demonstrate a simple approach to synthesize urethane-linked imidazole-cellulose microcrystals (U-ICMs) for simultaneous adsorption and naked eye sensing of metal ions. The synthesis is based on chemical surface modification of cellulose microcrystals

(CMCs) by 1,1-carbonyldiimidazole

and

ACS Paragon Plus Environment

1-(3-aminopropyl)imidazole,


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respectively. The adsorption and sensing of U-ICMs and CMCs in response to diverse metal ions (Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+) are qualitatively and quantitatively elucidated by UV-vis spectroscopy, inductively coupled plasma-optical emission spectroscopy, and X-ray photoelectron spectroscopy (XPS). The U-ICMs exhibit high performance for most metal species adsorption including Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ while CMCs favor Fe3+ adsorption. Comparing the adsorption performance of CMCs and U-ICMs, the presence of urethane-linked imidazole on CMCs leads to an improvement of adsorption capacity for more than 3 times (for Zn2+) up to 23 times (for Ni2+). At the same time, the adsorption of metals on U-ICMs promotes an enrichment of metal colors that can be clearly observed by naked eyes and UV-vis spectroscopic data. XPS and X-ray absorption near-edge structure are investigated to prove how those metal ions transform after exposure to the unmodified and modified CMCs surface. The findings highlight the potential use of U-ICMs together with CMCs as sustainable bioadsorbents and sensing materials for metal separation and water purification. Keywords: Cellulose microcrystal, Metal ion adsorption, Naked eye sensing, Surface modification, Imidazole, Urethane-linked

INTRODUCTION Pollution in water contaminated with heavy metals can cause many severe problems to environment and human health. For example, nickel salts are carcinogen1 and high dose of copper can lead to liver illness, cancer, deficient blood, and neurodegenerative diseases, etc.2 With increasing concerns to those effects, a wide range of adsorbent materials, e.g. activated carbon,3,4 graphene oxide,5-7 silica,8-10 zeolite,11 and bio-adsorbers,12-14 have been developed for sorption of various heavy metals from water. Due to eco-friendly nature, the development


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of adsorbents from bio-derived materials is ideal for contaminants separation and water purification. Microcrystalline cellulose (CMC) is a class of naturally occurring and renewable polymers composed of several hundred units of ß–D-glucose covalently linked together through (1→4) glycocidic bond. CMCs exhibit micro-size dimension with high crystallinity. Considering their molecular structure along with morphology, CMCs possess high surface area with a large number of reactive hydroxyl groups which are suitable for surface chemical modifications. A variety of functional molecules such as poly[2-(dimethylamino)ethyl methacrylate],15

protein,16

polydopamine,17

carboxylate,18-20

phosphate,21

coumarin

fluorophore,22 amines,23,24 rhodamine,25 and long chain alkyl,26 etc. have been reported to chemically modify on crystalline cellulose for different purposes including metal adsorption and metal sensing. Recently, sensing materials have been increasingly demanded as they can provide communication with people and offer safer convenience in daily life. Colorimetric naked eye sensing is one of the most efficient tools for tracing color changes visually in response to target species and concentrations. The colorimetric naked eye sensing allows on-site monitoring for species of interest, with no sophisticated equipment required. Although various kinds of organic and inorganic materials have been reported for metal sensing applications, some particular materials such as dyes, metal nanoparticles and complexes are accompanied by risks of toxicity and non-biodegradability.27-29 In some cases, the removal of those metal sensing materials from water was somehow difficult. This resulted in an increase of toxic contaminants in water. In the meantime, although most adsorbents exhibited high capacity for metal adsorption and removal, their sensing ability was rather limited and/or not mentioned. To our viewpoint, the development of bioadsorbent materials with metal sensing



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character is a great challenge, i.e. the materials are able not only to adsorb and remove metals from water but also to preliminarily identify potentially toxic metals contaminated. Herein, we originally propose urethane-linked imidazole-cellulose microcrystals (UICMs) as a novel bioadsorbent with naked eye sensing capability for metal ions. The nitrogen containing molecules, urethane-linked imidazole, acting as ligands to bind with metal ions are introduced on CMCs surface by simple chemical reaction. The synthesis and characterization of U-ICMs are described in details. The role of U-ICMs in adsorption as well as colorimetric enhancement of metal complexes observed by naked eyes is explored in comparison with that of the unmodified CMCs. By modifying CMCs with urethane-linked imidazole, the adsorption capacity and sensing behavior for various metal ions including Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ are altered significantly. After exposure to U-ICMs and CMCs, chemical states of the metals and their transformations are examined by XPS and Xray absorption near-edge structure (XANES).

MATERIALS AND METHODS Materials. Microcrystalline cellulose, 1,1-carbonyldiimidazole (CDI), and 1-(3aminopropyl)imidazole were purchased from Aldrich. Dimethyl sulfoxide (DMSO) and acetone were supplied by Carlo Erba Reagents. Aqueous solutions of metal ions, i.e., Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ were prepared from iron (II) sulfate heptahydrate (FeSO4·7H2O), iron (III) nitrate nonahydrate (Fe(NO3)3·9H2O), cobalt nitrate hexahydrate (Co(NO3)2·6H2O), nickel nitrate hexahydrate (Ni(NO3)2.6H2O), copper nitrate trihydrate (Cu(NO3)2·3H2O), zinc acetate (Zn(CH3COO)2·2H2O), silver nitrate (AgNO3), and cadmium acetate dehydrate (Cd(CH3COO)2·2H2O), respectively. All chemicals were used as received without further purification.


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Synthesis of Urethane-linked Imidazole-Cellulose Microcrystals. The synthesis of urethane-linked imidazole-cellulose microcrystals (U-ICMs) is illustrated in Scheme 1. Cellulose microcrystals (CMCs, 10.0 g) were dispersed in DMSO (120 ml) before adding CDI (20.5 g, 126.6 mmol) into the reaction. The mixtures were stirred at 40 °C for 24 h. The suspension obtained was washed with DMSO for 3 times and collected by centrifugation. The residues were re-dispersed in DMSO (100 ml) and treated with 1-(3-aminopropyl)imidazole (15.0 ml, 126.0 mmol) at 40 °C for 24 h. The suspension was precipitated in acetone and purified by soxhlet extraction. White powder obtained was dried in a vacuum oven at 40 °C for 4 h.

Scheme 1. Synthesis Route for Preparation of U-ICMs N N

HO

N

OH O

HO O

OH

OH O OH

N

N H2N

O O

DMSO 40 oC, 24 h

O N

O

HO

O

HO O

OH N

N

N

OH O OH

N

N

NH O

O

O DMSO 40 oC, 24 h

NH

HO

O OH

HO O

N

OH O OH

NH

O

Metal Adsorption and Sensing Experiments. Scheme 2 displays strategies employed for metal adsorption and sensing of pristine CMCs and U-ICMs. The U-ICMs (0.25 g) were dispersed in eight-selected metal ion (Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+) solutions (50 ml) at concentrations of 0.1 mM, 1.0 mM, and 25.0 mM. The suspensions were stirred at ambient temperature for 72 h before filtration. The solid residues were dried in a vacuum oven at 50 °C for 5 h before characterizations. For a comparison, testing of pristine CMCs with metal ions was performed by using the same manners. The CMCs and U-ICMs after exposure to the above-mentioned metal solutions at a concentration



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of 1.0 mM are named as CMCs/M+ and U-ICMs/M+, respectively, where M+ is a type of metal ions. For example, CMCs/Fe2+ and U-ICMs/Fe2+ are CMCs and U-ICMs after exposure to 1.0 mM solution of Fe2+, respectively.

Scheme 2. Experimental Setup for Studying Metal Adsorption and Sensing

Characterization. Attenuated total reflectance Fourier Transform infrared (ATRFTIR) spectra were collected using a Nicolet 6700 spectrophotometer. The ATR-FTIR spectra were measured in a range of 4000-550 cm-1 at 128 scans with a resolution of 2 cm-1. X-ray photoelectron spectroscopy (XPS) measurement was conducted using a PHI5000 Versa Probe II, ULVAC-PHI, Japan at the SUT-NANOTEC-SLRI Joint Research Facility, Synchrotron Light Research Institute, Thailand. The monochromatic Al Kα X-ray (1486.6 eV) was used as an excitation source. All binding energies of the samples were calibrated with the C1s (C-C bond) peak at binding energy of 285.0 eV. Transmission electron microscopy (TEM) imaging was carried out using a TEM-Hitachi HT7700 at an accelerating voltage of 80 kV. X-ray diffraction (XRD) patterns were obtained from a Bruker D8 Advance with Cu Kα radiation (λ = 0.154 nm) operating at a voltage of 40 kV and a current of 40 mA.


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The data were recorded from 5° to 80° 2θ with an increment of 0.02° 2θ per 0.5 s at 25 °C. Degree of crystallinity of CMCs and U-ICMs is calculated from XRD patterns using eq 1:30-31 Degree of crystallinity (%) = Ac/(Ac+Aa)*100

(1)

where Ac and Aa are total areas of crystalline and amorphous phases after background subtraction, respectively. Solid state UV-vis spectroscopy was monitored using an Agilent Cary 5000 Series UV-Vis-NIR-spectrophotometer. Sample powders were loaded in a solid holder and the reflectance spectra were recorded at wavelength from 900 nm to 200 nm. The reflectance was converted into absorbance by using Cary WinUV program. X-ray absorption near-edge structure (XANES) measurement was done at SUT-NANOTEC-SLRI XAS beamline (BL5.2 (XAS)), Synchrotron Light Research Institute, Thailand.32,33 XANES profiles of the samples were acquired in fluorescence mode and compared to those of metal oxide standards in transmission mode. The amount of metals in filtrates was analyzed by using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The measurement was done using a Perkin Elmer ICP-OES optima 7300DV. Metal removal efficiency (%) is calculated from eq 2: Metal removal efficiency (%) = (Ci – Cf)/Ci x 100%

(2)

where Ci is the initial concentration of metals (mg/L) and Cf is the final concentration of metals in the solutions after exposure to CMCs or U-ICMs (mg/L). Adsorption capacity is defined as the amount of metals adsorbed per unit mass of dry sample. The adsorption capacity is calculated by using eq 3: Adsorption capacity (mg/g) = (Ci – Cf)V/M


(3)


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where V is the volume of the metal solution (L) and M is the mass of CMCs or U-ICMs (g).

RESULTS AND DISCUSSION Characterization of Urethane-linked Imidazole-Cellulose Microcrystals (UICMs).

U-ICMs

were

synthesized

by

a

reaction

between

CMCs

and

1-(3-

aminopropyl)imidazole using CDI as a coupling reagent. Structure and surface chemistry of the synthesized U-ICMs compared to the unmodified CMCs were verified by FTIR, XPS, and solid state

13

C nuclear magnetic resonance (NMR) spectroscopy. FTIR spectrum of

CMCs (Figure 1A(a)) displays characteristic peaks of cellulose at 3345 cm-1 (stretching vibration of H-bonded OH), 1649 cm-1 (OH bending of adsorbed water), 1430 cm-1 (crystalline band of CH2 bending), and 1165-1059 cm-1 (stretching vibration of C-O and C-OC).34-36 After modification, absorption bands of imidazole ring are observed at 1650 cm-1 and 1542 cm-1, corresponding to stretching mode of C=C and C=N (Figure 1A(b)). The peaks at 1229 cm-1 and 1284 cm-1 are assigned to C-N and C-C stretching vibration, respectively. Besides, an appearance of absorption bands at 1710 cm-1 (C=O stretching) and 1518 cm-1 (NH bending) indicates a successful grafting of aminopropylimidazole on CMCs through urethane linkage. The FTIR results are consistent with XPS signals displayed in Figure 1B and 1C. XPS survey scan spectrum of CMCs reveals the presence of C and O, while the spectrum of U-ICMs shows the presence of C, O, and N. This result confirmed an existence of C and O together with N elements on the surface of U-ICMs. The high resolution C1s, O1s, and N1s core level spectra of CMCs and U-ICMs with their peak deconvolutions are shown in Figure 1C. For the pristine CMCs (Figure 1C(a)), the deconvolution of C1s peak consists of three main peaks centered at 285.0 eV, 286.7 eV, and 288.1 eV which can be assigned to C-C, C-OH and O-C-O bonds, respectively.37-39 Similar to the previous report,40


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the O1s peak of CMCs can be deconvoluted into two main peaks at the binding energy of 532.9 eV and 533.5 eV corresponding to C-OH and C-OC, respectively. No distinct peak in the N1s region is detected in the CMCs spectrum. After modification (Figure 1C(b)), the C1s core level spectrum becomes broader, and it can be deconvoluted into five component peaks at 285.0 eV, 286.1 eV, 286.7 eV, 287.8 eV, and 289.4 eV which are attributed to C-C/C=C, C-N/C=N, C-OH, O-C-O, and C=O, respectively. In accompany with the findings of CN/C=N and C=O peaks in the C1s spectrum, an additional peak at 531.9 eV which corresponds to O-(C=O)-N is observed in the deconvoluted O1s spectrum of U-ICMs. This clearly proved the formation of urethane group linked between CMCs and propylimidazole. In addition, the N1s peak of U-ICMs can be deconvoluted into three dominant peaks at the binding energies of 399.0 eV, 400.1 eV, and 401.1 eV, corresponding to C=N-, O-(C=O)-N, and C-N, respectively. These results revealed the presence of urethane-linked imidazole on CMCs surface, consistent with the findings of urethane and propylimidazole carbon peaks in the solid state

13

C NMR spectrum of U-ICMs (Figure S1 in the Supporting Information).

Note that the purity of the synthesized U-ICMs was traced by thin layer chromatography (TLC). It was found that there was no imidazole by-product detected under iodine vapor. This result indicated that there was two possibilities, i.e. the unbound imidazole was completely removed from U-ICMs by soxhlet extraction or the imidazole was adsorbed strongly on UICMs via hydrogen bond network.



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10

1059

3345

1113 1165 1430

(a)

3800

1649

3200

2600 1800

1500

1200

900

600

N1s

(a)

1400 1200 1000 800

Wavenumber / cm-1

C

600

400

200

0

Binding Energy / eV C-OH

O1s

C1s

C1s

O1s

(b) O1s

Intensity / a.u.

Intensity / a.u.

1518 1542 1650

(b)

B

1284 1229

1710

C1s

3345

A

O-(C=O)-N

N1s

C-N/C=N C=N

C-N

C-OH C-OC

C-C/C=C

O-(C=O)-N

O-C-O

Intensity / a.u.

O-(C=O)-N

(b)

(b)

(b) C-OH

C-OH

(a)

C-OC O-C-O

C-C

(a)

291

(a)

288

285

282

537

535

533

531

403

401

399

397

Binding Energy / eV

D (a)

(b)

22.6°

E Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b)

15.1° 16.5° 34.6°

(a)

10

20

30

40

50

60

2θ΅/ °

Figure 1. (A) FTIR spectra, (B) XPS survey spectra, (C) high resolution XPS spectra of C1s, O1s, and N1s core levels, (D) TEM images and (E) XRD patterns of (a) pristine CMCs and (b) U-ICMs.

Figure 1D displays TEM images of CMCs and U-ICMs. It has clearly revealed that U-ICMs has maintained the rod-like morphology as CMCs with 150-300 nm in length and 10-20 nm in width. This observation suggested that the grafting of urethane-linked imidazole


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did not change the microstructure of the cellulose. XRD was used to study crystallinity of the modified CMCs. Similar to that of pristine CMCs in Figure 1E(a), crystalline patterns of UICMs (Figure 1E(b)) appear at 15.1°, 16.5°, 22.6°, and 34.6° 2θ, corresponding to (1 1 0) (110) (200) (004) planes of cellulose I structure. With no changing in diffraction pattern of UICMs, we believed that the structural modification occurred at the surface of the microcrystals. It should be noted that the modification of CMCs with urethane-linked imidazole by this approach did not significantly affect crystallinity of the microcrystals. As revealed by XRD results, the degree of crystallinity of CMCs and U-ICMs was 65.7±3.5% and 63.0±2.8%, respectively. Likewise, the calculation from

13

C NMR spectra (see

supporting information) indicated that the degree of crystallinity of CMCs and U-ICMs was 57.5% and 58.0%, respectively.

Adsorption and Colorimetric Sensing of Metal Ions. The adsorption and colorimetric testing of CMCs and U-ICMs in response to metal ions was performed by simply dispersing the samples in different metal ion solutions. Here, a series of metal ions, i.e., Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ was chosen as a model study. As shown in Figure 2A(a), the metal ion solutions at a concentration of 1.0 mM are clear and their colors are not visible by naked eyes, except Fe2+ and Fe3+ solutions. The colors of Fe2+ and Fe3+ solutions are very pale yellow and deep yellow, respectively. Originally, CMCs and UICMs are white powder (see inset in Figure 4A). Figures 2A(b) and 2A(c) show photographs of CMCs and U-ICMs dispersing in different metal solutions, respectively. When adding CMCs in the metal ion solutions, the colors of the suspensions were identical to those metal solutions, except Ag+ suspension. The suspension of CMCs in Ag+ solution is very pale brown in color. When U-ICMs were added into metal solutions, significant color changes of the certain metal solutions, i.e. Fe2+, Co2+, Ni2+, and Cu2+ were observed by naked eyes. The



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Fe2+ solution with U-ICMs are deep yellow while the Co2+, Ni2+, and Cu2+ solutions with UICMs are pale pink, pale green, and blue, respectively (Figure 2A(c)). These revealed the role of U-ICMs on color enhancement of metal ions, especially for Fe2+ and Cu2+ solutions. Figures 2B(b) and 2B(c) display photographs of CMCs and U-ICMs sedimenting in metal solutions after leaving the suspensions at room temperature for a while without stirring. It was found that the difference in colors of the samples was clearer detected. This was due to the agglomeration of the dispersed solid phases.

Figure 2. Photographs of metal ion solutions at concentration of 1.0 mM (a) before and after adding (b) CMCs, and (c) U-ICMs; (A) the samples dispersing in metal solutions and (B) zoom images of the samples sedimenting in metal solutions.


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Metal removal efficiency and adsorption capacity of CMCs and U-ICMs were quantitatively analyzed in filtrated solutions using ICP-OES. From the individual metal ion solutions at a concentration of 1.0 mM, U-ICMs exhibit higher efficiency than the pristine CMCs for Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ removal (Figure 3(a)). In other words, this revealed capability of U-ICMs for those metal ions capture. As shown in Figure 3(b), UICMs have higher adsorption capacity toward most of the metal ions than CMCs, except for Fe3+. The adsorption capacity of CMCs/Fe3+ was higher than that of U-ICMs/Fe3+. The metal adsorption capacity of U-ICMs was in the order of Cu2+ and Ag+ > Zn2+ > Fe2+ > Ni2+ > Co2+ > Cd2+ > Fe3+. At the initial concentration of metal ions for 1.0 mM, it was found that the adsorption capacity of Cu2+ and Ag+ was more than 100 mg/g, and that of Zn2+, Fe2+ , Ni2+, Co2+, and Cd2+ was in the range of 51-100 mg/g. Although CMCs exhibit some adsorption capacity toward Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+, the uptake of those metal ions from the solutions containing CMCs was relatively poor (less than 28 mg/g), compared to that from the solutions containing U-ICMs. Therefore, the improvement of adsorption capability of Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ was attributed to urethane-linked imidazole units modified on CMCs surface.



14

25

Removal Efficiency / %

(a)

20

CMCs U-ICMs

15 10 5 0 Fe2+ Fe3+ Co2+ Ni2+ Cu2+ Zn2+ Ag+ Cd2+

(b) Adsorption Capacity / mg.g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

120 100

CMCs U-ICMs

80 60 40 20 0 Fe2+ Fe3+ Co2+ Ni2+ Cu2+ Zn2+ Ag+ Cd2+

Figure 3. (a) Removal efficiency and (b) adsorption capacity of CMCs and U-ICMs for the uptake of different metal ions at 25 °C (0.25 g of the samples in 50 ml of metal ion solutions with an initial concentration of 1.0 mM, adsorption time 72 h).

To verify the presence of metals on CMCs and U-ICMs, after exposure to metal ion solutions, all solid samples were separated from the solutions by filtration, and dried before characterized by UV-vis spectroscopy, XPS, and XANES. Figure 4 displays photographs of CMCs and U-ICMs before and after subjected to 1.0 mM solutions of metal ions and their resulting solid state UV-vis spectra. CMCs exhibit color changes in response to Fe2+, Fe3+, and Ag+ while the color of U-ICMs are sensitive to Fe2+, Fe3+, Co2+, Ni2+, and Cu2+. The CMCs subjected to Fe2+ and Fe3+ solutions are very pale yellow and deep yellow, respectively. In contrast, the color of U-ICMs has changed to deep yellow and very pale yellow after exposure to Fe2+ and Fe3+ solutions, respectively. CMCs/Ag+ is brown whereas


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U-ICMs/Ag+ is white. Similar to the results of the suspensions in Figure 2A(c), UICMs/Co2+, U-ICMs/Ni2+, and U-ICMs/Cu2+ are pale pink, pale green, and blue, respectively. In Figure 4A, pure CMCs exhibit broad absorption bands in ultraviolet region (~200-400 nm), and the major absorption band of pure U-ICMs is evident at 223 nm. Consistent with the observation by naked eyes, after exposure to different metal ions at concentration of 1.0 mM, CMCs/Fe2+, CMCs/Fe3+, CMCs/Ag+, U-ICMs/Fe2+, U-ICMs/Fe3+, U-ICMs/Co2+, UICMs/Ni2+, and U-ICMs/Cu2+ show new distinct absorption bands in visible wavelength (~400-700 nm) of UV-vis spectra. In fact, the colors of metal complexes are commonly originated from a split of incompletely filled d-orbitals of metal when interacting with electron cloud of ligands.41 When light passes through the metal ion complex, it is able to absorb a particular energy in the visible light for promoting electron transition from lower energy d-orbital to higher energy d-orbital, and appears colors of the emitted wavelength. As a result, it was possible that the colors of Fe2+, Fe3+, Co2+, Ni2+, and Cu2+ complexes with d6,

d5, d7, d8, and d9 electron configurations, respectively, were visible due to d-d transitions. As Zn2+, Ag+, and Cd2+ contain completely filled d-orbitals (d10 electron configuration), their complexes are colorless. Note that the metal complex containing metal ion with d10 electron configuration can appear color by charge transfer.



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Figure 4. Solid state UV-vis spectra and photographs (inset images) of (a) CMCs (dash line) and (b) U-ICMs (solid line) (A) before and after exposure to 1.0 mM solutions of (B) Fe2+, (C) Fe3+, (D) Co2+, (E) Ni2+, (F) Cu2+, (G) Zn2+, (H) Ag+, and (I) Cd2+.

Figure 5 demonstrates the effect of metal ion concentrations (0.1 mM, 1.0 mM, and 25 mM) on color changes of U-ICMs. When increasing the concentration of metal ions, the colors of U-ICMs/Fe2+, U-ICMs/Fe3+, U-ICMs/Co2+, U-ICMs/Ni2+, and U-ICMs/Cu2+ become more intense. As expected, the solid state UV-vis spectra of those metals with UICMs exhibited an increase of absorption intensity with increasing metal ion concentration (Figure S2 in the Supporting Information). It should be noted that the addition of U-ICMs in 25.0 mM solution of Ag+ resulted in a color change of U-ICMs from white to pale brown. At 0.1 mM of metal ions, only pale yellow of U-ICMs/Fe2+ and pale blue of U-ICMs/Cu2+ are visible. The difference in colors at 0.1 mM of metal ions could be seen clearer from the


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sediments presented in the Supporting Information (Figure S3). These results indicated that U-ICMs could be used for naked eye detection of i) Fe2+ and Cu2+ with a detection limit of ~10-4 M, ii) Fe3+, Co2+, and Ni2+ with a detection limit of ~10-3 M, and iii) Ag+ with a detection limit of ~10-2 M.

Figure 5. Photographs of U-ICMs after exposure to various metal ion solutions at concentrations of (a) 0.1 mM, (b) 1.0 mM, and (c) 25.0 mM.

Surface compositions of CMCs and U-ICMs after subjected to various metal ions were examined by XPS technique. For CMCs, Fe2p and Ag3d signals are observed in the high resolution XPS spectra of CMCs/Fe3+ (Figure 6B(a)) and CMCs/Ag+ (Figure 6G(a)), respectively, suggesting the presence of Fe3+ and Ag0 on CMCs surface. In agreement with the UV-vis spectroscopy results, brown color of CMCs after subjected to Ag+ solution arose from a transformation of Ag+ to Ag0 nanoparticles. The peak assignment of metals at different binding energies of CMCs and U-ICMs after exposure to metal ion solutions is



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summarized in Table 1. For U-ICMs, all high resolution XPS spectra show the metal signals at different binding energies, except U-ICMs/Fe3+. This finding was in good agreement with the ICP-OES and UV-Vis spectroscopy results. In Fe3+ solution, the uptake of Fe3+ by UICMs was relatively low, suggesting that binding affinity of U-ICMs toward Fe3+ was limited. This is due to the fact that Fe3+ is a hard acid.42 It prefers binding to OH as hard base rather than imidazole as borderline base. In contrast, Fe2+ is a borderline acid, preferable to bind with imidazole rather than OH. Unexpectedly, similar to CMCs/Fe3+, U-ICMs/Fe2+ exhibits two peaks at binding energy of 711.7 eV and 725.1 eV, corresponding to 2p3/2 and 2p1/2 of Fe3+, respectively. The appearance of Co2p, Ni2p, Cu2p, Zn2p, Ag3d, and Cd3d signals in the XPS spectra of U-ICMs (Figures 6C(b)-6H(b)) indicates the adsorption of cobalt, nickel, copper, zinc, silver, and cadmium on U-ICMs surface. Based on XPS results, the valence states of the those metals were accordingly assigned to Co2+, Ni2+, Cu+, Zn2+, Ag+, and Cd2+. It should be mentioned that, as revealed by FTIR and XPS results (Figures S4 and S5, Supporting Information), the adsorption of metals on CMCs and U-ICMs was associated with the presence of their counter ions.


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Figure 6. High resolution XPS spectra of (a) CMCs and (b) U-ICMs after exposure to 1.0 mM solutions of (A) Fe2+, (B) Fe3+, (C) Co2+, (D) Ni2+, (E) Cu2+, (F) Zn2+, (G) Ag+, and (H) Cd2+ solutions.



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Table 1. XPS results revealing peak assignment of metals at different binding energies of CMCs and U-ICMs after exposure to various metal solutions

Peak assignment of metals at different binding Initial Metal

energies (eV)

Reference

Solution CMCs

U-ICMs Fe2p3/2 711.7 eV and

FeSO4

N.D.

43-44

Fe2p1/2 725.1 eV Fe2p3/2 711.7 eV and Fe(NO3)3

N.D.

43-44

Fe2p1/2 725.1 eV Co(NO3)2

N.D.

Co2p3/2 781.3 eV

45

Ni(NO3)2

N.D.

Ni2p3/2 855.8 eV

46

Cu(NO3)2

N.D.

Cu2p3/2 932.4 eV and

47,48

Cu2p1/2 952.3 eV Zn2p3/2 1022.3 eV and Zn(CH3COO)2

N.D.

49

Zn2p1/2 1045.3 eV Ag3d5/2 368.4 eV and

Ag3d5/2 368.0 eV and

Ag3d3/2 374.4 eV

Ag3d3/2 374.0 eV

N.D.

Cd3d5/2 405.5 eV

(AgNO3),

Cd(CH3COO)2

*N.D. = Metal signals are not detected.


50,51,52,49

53-54

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XANES analysis was conducted to prove predominant oxidation states of the adsorbed metals on U-ICMs comparing to the unmodified CMCs. Figures 7(a) and 7(b) illustrate Fe K-edge XANES spectra of CMCs and U-ICMs after exposure to Fe2+ and Fe3+ solutions. It was found that all XANES spectra of the unmodified and modified samples with Fe2+ and Fe3+ match well with that of Fe2O3 used as a reference. From the XANES results, local structure of Fe ions with CMCs and U-ICMs was rather identical to α-Fe2O3. The αFe2O3 is thermodynamically stable form of Fe atoms with +3 oxidation state.55 It was also confirmed by XPS deconvolution in the Fe2p region, i.e., the satellite peak appeared at 718.6719.3 eV revealed the changes from initial Fe2+/Fe3+ ions to Fe2O3 (Figure S6 in the Supporting Information). It should be mentioned that XANES profiles of all samples show the presence of metals in different forms, but metal signals do not appear in XPS spectra of CMCs/Fe2+, CMCs/Co2+, CMCs/Ni2+, CMCs/Cu2+, CMCs/Zn2+, CMCs/Cd2+, and UICMs/Fe3+. It was due to the fact that the amount of these particular metals on CMCs and UICMs is relatively small, i.e. less than detection limit of the XPS instrument. Therefore, the existence of Fe2+, Co2+, Ni2+, Cu2+, Zn2+, and Cd2+ associated with CMCs as well as Fe3+ associated with U-ICMs was mainly resulted from physisorption of metals on the microcrystal surfaces. The background-corrected and normalized XANES spectra at Co K-edge, Ni K-edge, and Zn K-edge for metal oxide standards and the samples with cobalt, nickel, and zinc ions are shown in Figures 7(c), 7(d), and 7(f), respectively. The absorption edge positions of CMCs and U-ICMs subjected to Co2+, Ni2+, and Zn2+ ions correspond to those of CoO (at 7720 eV), NiO (at 8344 eV), and ZnO (at 9661 eV) standard compounds, respectively.56-58 Consistent with the XPS findings, the results suggested that the adsorbed cobalt, nickel, and zinc in U-ICMs were in the forms of +2 oxidation state. Therefore, there were no significant



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changes in oxidation state of Co, Ni, and Zn after exposure to the microcrystalline samples. Similar to that of CuO standard compound illustrated in Figure 7(e), the Cu K-edge of CMCs/Cu2+ and U-ICMs/Cu2+ at 8891 eV reflects a dominant presence of Cu2+ ions.59 This result was different from the finding from XPS observation for U-ICMs/Cu2+. XPS is a surface-sensitive technique. It is possible that Cu2+ was the majority of copper ions in UICMs while Cu+ was a minor species presented on the sample surface. For U-ICMs/Cu2+, an appearance of a shoulder in rising-edge at ~8988 eV ascribed to shakedown (1s→4pz) transition, denotes ligand-to-metal charge transfer.60 Figure 7(g) displays Ag L3-edge XANES spectra of the samples and Ag2O and Ag nanoparticle (AgNP) references for Ag+ and Ag0, respectively. Ag2O exhibits edge peak at 3349 eV whereas AgNP presents Ag L3-edge at 3351 eV.61 It was found that the edge position of U-ICMs/Ag+ and CMCs/Ag+ is very close to that of AgNP. At the same time, edge peak at Ag L3-edge is also observed in the XANES profiles of U-ICMs/Ag+ and CMCs/Ag+ samples. These results revealed the presence of mixed Ag+ and Ag0 in the samples. After exposure to CMCs and U-ICMs, the initial Ag+ ions were partially reduced and transformed into Ag0. Similar to findings by Koteľnikova et al.,62 the reduction of Ag+ to Ag0 was observed in the presence of cellulose matrix. This was due to electron transfer from oxygen-containing groups of cellulose to Ag+ ions. It is evident that, compared to that of Ag2O, the edge peak of U-ICMs shifts to higher energy, and decreases in intensity. This observation becomes more pronounced for CMCs/Ag+. These indicated that the fraction of Ag+ in U-ICMs is higher than that in CMCs. Most silver atoms in CMCs were in the form of Ag0. The Cd L3-edge XANES spectra (Figure 7(h)) of CMCs/Cd2+ and UICMs/Cd2+ display a pre-edge feature at ~3539 eV, similar to those of CdO standard, indicative of +2 oxidation state of Cd atoms.63


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23

1.5

1.8

(a)

CMCs/Fe2+ U-ICM/Fe2+ Fe2O3

1.2 0.9 0.6 0.3 0.0 7100

7120

7140

7160

Normalized xµ µ(E)


1.8

1.5

(b)

1.2 0.9 0.6 0.3 0.0 7100

7180

CMCs/Fe3+ U-ICMs/Fe3+ Fe2O3

7120

Energy / eV CMCs/Co2+ U-ICMs/Co2+ CoO

7180

1.5 1.0 0.5

7715

7735

7755

8335

8355

8375

0.9 0.6 0.3

9035

(f) Normalized xµ µ(E)

CMCs/Cu2+ U-ICMs/Cu2+ CuO

8995 9015 Energy / eV

0.5

8395

8415

Energy / eV

1.2

8975

1.0

2.0

1.8

0.0 8955

1.5

0.0 8315

7775

Energy / eV

(e)

CMCs/Ni2+ U-ICMs/Ni2+ NiO

(d) Normalized xµ µ(E)


(c)

0.0 7695


7160

2.0

2.0

1.5

7140

Energy / eV

2.5

1.0

0.5

0.0 9635

9055

CMCs/Zn2+ U-ICMs/Zn2+ ZnO

1.5

9655

9675

9695

9715

9735

Energy / eV

1.2 1.0

(g)

1.2

0.8 0.6 0.4 CMCs/Ag+ U-ICMs/Ag+ Ag2O Ag NP

0.2 0.0 3325

3345

3365

3385

Energy / eV

3405

3425



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(h)

0.9 0.6 0.3 0.0 3515

CMCs/Cd2+ U-ICMs/Cd2+ CdO

3535

3555

3575

3595

3615

Energy / eV

Figure 7. (a), (b) Fe K-edge, (c) Co K-edge, (d) Ni K-edge, (e) Cu K-edge, (f) Zn K-edge, (g) Ag L3-edge, and (h) Cd L3-edge XANES spectra of metal references and CMCs and U-ICMs with different metals.

To demonstrate the promising potential for metal filtration with sensing capability, the CMCs and U-ICMs are packed into glass columns prior to adding solution of metal (Figure 8(a)). Figure 8(b) shows photographs of CMCs and U-ICMs after adding Cu2+ solution into the columns as a function of Cu2+ concentration. When the solution passed through the



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samples, the color of CMCs was unchanged while the U-ICMs turned blue. The increase of Cu2+ concentration results in the more copper adsorption of U-ICMS as observed an expansion of the blue area until saturated.

Figure 8. (a) Schematic illustration of column filled with sample and (b) photographs showing filtration of Cu2+ solution through CMCs and U-ICMs as a function of Cu2+ concentration.

CONCLUSIONS In the present study, U-ICMs were introduced as a bioadsorbent for metal adsorption with naked eye sensing. The U-ICMs were synthesized under mind condition using two steps, i.e. modification of CMCs with CDI and 1-(3-aminopropyl)imidazole, respectively. The success of the reaction was monitored by determination of structural changes and surface chemistry of U-ICMs compared to the pristine CMCs using FTIR, solid state 13C NMR, and XPS. After modification, U-ICMs maintained the CMCs’ morphology in the form of rod-like microstructure with high crystallinity. The findings from UV-Vis spectroscopy, ICP-OES, and XPS revealed that, after modification of CMCs with urethane-linked imidazole, the


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adsorption capacity toward Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+ was improved by forming metal complexes with U-ICMs ligands. This allowed visual detection of metal (Fe2+, Co2+, Ni2+, Cu2+) colors in the presence of U-ICMs. In contrast, CMCs were sensitive to Fe3+ and Ag+ ions. The studies by XPS and XANES indicated that, in the presence of CMCs, the pristine Fe3+ and Ag+ ions were mostly transformed into Fe2O3 and Ag0, respectively. With U-ICMs, Fe2+ ions were oxidized to form Fe2O3 while the states of Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+ were predominantly preserved. Combining with CMCs, U-ICMs could be used not only for adsorption of different metal ions but also for colorimetric naked eye sensing of some particular metals, for example, Fe2+, Fe3+, Co2+, Ni2+, Cu2+, and Ag+. The U-ICMs obtained are potentially applied for removal of metals from waste water and colorimetric reporting of metal species.

ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge on ACS publications website. Solid state 13C NMR spectra of CMCs and U-ICMs; Solid state UV-vis spectra of UICMs after exposure to 0.1 mM, 1.0 mM, and 25.0 mM solutions of Fe2+, Fe3+, Co2+, Ni2+, Cu2+, Zn2+, Ag+, and Cd2+; Photographs of U-ICMs sedimenting in different metal solutions at a concentration of 0.1 mM; FTIR spectra of CMCs and U-ICMs before and after exposure to aqueous metal solutions at concentration of 1.0 mM; High resolution S2p XPS spectra of CMCs and U-ICMs after exposure to aqueous solution of 1.0 mM FeSO4·7H2O; Fe2p XPS spectra and peak deconvolution of CMCs



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after exposure to 1.0 mM solution of Fe2+ and U-ICMs after exposure to 1.0 mM solution of Fe3+ (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (A.P.). ORCID Autchara Pangon: 0000-0001-5486-2720

ACKNOWLEDGEMENTS This work is co-supported by The Thailand Research Fund (TRF) with a grant number TRG5880261 and National Nanotechnology Center, Thailand. We are grateful to SUT-NANOTEC-SLRI (BL5.2 and BL5.3) beamline’s members for XPS and XANES measurement.

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For Table of Contents Use Only

Fe2+

Fe3+

Co2+ Ni2+ Cu2+ Zn2+

Ag+ Cd2+

Fe2+

Fe3+

Co2+ Ni2+ Cu2+ Zn2+

Ag+ Cd2+

M+

M+

Synopsis: The cellulose microcrystals chemically-tailored with urethane-linked imidazole are promising bioadsorbent with naked eye sensing capability for sustainable development of metal-separating and water purifying biomaterials.


Urethane-Linked Imidazole-Cellulose Microcrystals: Synthesis and

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