Mesoporous ChitosanâSiO2 Nanoparticles: Synthesis

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Mesoporous Chitosan-SiO2 Nanoparticles: synthesis, characterization and CO2 adsorption capacity Sayyid Mahdi Rafigh, and Amir Heydarinasab ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02388 • Publication Date (Web): 24 Sep 2017 Downloaded from http://pubs.acs.org on September 26, 2017

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ACS Sustainable Chemistry & Engineering

Mesoporous Chitosan-SiO2 Nanoparticles: synthesis, characterization and CO2 adsorption capacity

a*

Sayyid Mahdi Rafigh , Amir Heydarinasab

a

a

Department of Chemical Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

*Corresponding author : Tel.: +98 2177914881. E-mail address: [email protected] (S. M. Rafigh). Address: Department of Chemical Engineering, Science and Research Branch, Islamic Azad University, PO Box 14515/775, Tehran, Iran.

1 ACS Paragon Plus Environment


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Abstract Mesoporous chitosan-SiO2 nanoparticles (NPs) were successfully synthesized. The prepared nanoparticles were characterized using TEM, FTIR, XRD, TGA, EDX and (CHN) Elemental Analysis. From TEM micrograph, chitosan-SiO2 NPs were sphere-like, pretty uniformly distributed with coarse surface. Average size of chitosan-SiO2 NPs were determined 211 nm with DLS, which confirmed by TEM. The mesoporous structure of chitosan-SiO2 NPs were characterized with N2 adsorption/desorption measurements. BET surface area was 621 m2 g-1 and the total pore volume was 0.71 m3 g-1. CO2 adsorption was evaluated by a volumetric method. chitosan-SiO2 NPs showed a maximum CO2 adsorption capacity of 4.39 mmol g-1 at 25 °C and high selective separation capacity for CO2-over-N2 (SCO2/N2 =15.46). The influence of amines on carbon dioxide adsorption was discussed. Stable CO2 adsorption/desorption was confirmed after six cycles of experiments. Therefore, chitosan-SiO2 NPs exhibit great potential for CO2 capture.

Keywords: Chitosan-SiO2 Nanoparticles (NPs), CO2 Adsorption, Mesoporous.


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Introduction CO2 emissions as the main source of global warming and climate change is current environmental concern 1. Therefore, more attention has been paid to development of the methods and materials for capturing and sequestration of CO2 1-3. Among them, adsorption could be useful technology because it is simple, low cost and effective with easy operation process. Various solid adsorbents have been studied for CO2 capture such as activated carbons, zeolites, mesoporous silicas, and metal–organic frameworks 4-7. The amine-functionalization of porous materials such as silica and zeolite as sorbents have recently received significant attention by many researchers for high CO2 adsorption 2, 5. Chitosan is pseudo-natural cationic biopolymer that produced through deacetylation of chitin which is the most abundant natural polysaccharide after cellulose

8-9

. Chitosan has one amino

group and two hydroxyl groups in a repeating glucosidic residue, and it has higher proportion of amino groups than chitin9. Chitosan has excellent properties such as good biocompatibility, biodegradability, high adhesion to the surface, high hydrophilicity, expressed chelating properties, nontoxicity, chemically stability and antibacterial activity 9-10. It has been proved that chitosan is an effective adsorbent towards some heavy metals 11, dyes12, and organic pollutants13. The presence of large number of amine groups on the chitosan structure can facilitate the acidic CO2 molecule to get adsorbed on the surface of the adsorbent molecule. Pure chitosan exhibited negligible CO2 adsorption capacity and the maximum CO2 adsorption was reported 0.98 mmol g1

for chitosan based-monolithic silica supports 14-15. The Chitosan market is expected to be USD

4.22 billion in 2020 16.



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Recently, silica nanoparticles were incorporated into the polymers to enhance their mechanical and interfacial properties for several fields of application such as absorbent, reductive degradation and reinforce polymeric materials 17-21. Our work focuses on the following key contributions: (1) synthesis of chitosan-SiO2 nanoparticles (NPs), (2) evaluation of chemical and physical properties of chitosan-SiO2 NPs using TEM, FTIR, XRD, DLS, TGA, EDX and (CHN) Elemental Analysis, (3) N2 and CO2 adsorption capacity and microstructure properties of chitosan-SiO2 NPs were investigated, and (4) cyclic adsorption-desorption were studied to examine the reusability potential of chitosanSiO2 NPs.

Materials and methods Materials Tetraethyl orthosilicate (TEOS 99.9%), Tween 80 (polysorbate 80), absolute ethanol (EtOH 99.5%) and chitosan powder (medium molecular weight Mw = 161 000 g mol-1) were purchased from Sigma Aldrich (St. Louis, MO, USA). All other chemicals used were of analytical grade.

Synthesis of chitosan-SiO2 nanoparticles (NPs) Two-step procedure was used for the sample preparation. The first step consisted of the preparation of silica nanospheres according to a modified method based on the reported protocol22. Briefly, Tween 80 as surface active material (0.3 g) was fully dissolved in an acidic solution containing hydrochloric acid (2 mL) and deionized water (25 mL) in 250 mL Erlenmeyer flask at room temperature. Then followed by slowly addition of TEOS mixture containing TEOS (3 mL) as silica source, EtOH (32 mL) and deionized water (8.5 mL).


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Ammonium hydroxide (3.5 mL) as a catalyst was added dropwise to this mixture. The reaction was allowed to proceed under stirring at 30 °C for 25 min, to give rise to white precipitates. The formed solids were collected by centrifugation at 4000 rpm and dispersed into an acidified ethanol solution to remove the surfactants. The white colored silica nanospheres were then washed by deionised water and finally dried under vacuum at room temperature. In the second step, aqueous 1.5 wt% chitosan solution was prepared by dissolving chitosan (0.18 g) in 1 wt% acetic acid aqueous solution to produce a homogeneous mixture. Then, immerse chitosan solution into SiO2 suspension containing SiO2 nanospheres (1 g) and distilled water (100 mL) under an ultrasonic bath for 20 min. To coat the chitosan on the SiO2 nanospheres, the mixture was kept at 60 °C for 6 h. The resulting suspension was placed in an iced-water bath for 1 h. The as-produced nanoparticles (NPs) were collected by centrifugation at 5000 rpm. The chitosan-SiO2 NPs (0.41 g) that formed during the process were washed to a neutral pH with deionized water for several times and dried at 50°C temperature overnight.

Characterization TEM analysis Transmission electron microscopy (TEM) analysis was performed on a JEOL JSM-2100F electron microscope (Tokyo, Japan, Japan) operated at 200 kV.

FTIR Analysis Fourier transform infrared spectra of the samples were recorded on Perkin-Elmer 1000 spectrometer (Perkin Elmer, Waltham, MA, USA) using spectral range of 4,000-400 cm-1 with a resolution of 4 cm-1 in the transmittance mode employing the KBr disk technique.



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XRD Analysis X-Ray Diffraction spectra were recorded using a XRD 6000 diffractrometer (Shimadzu, Kyoto, Japan) from 2θ = 0 to 70° with a step of 0.05° and integration time of 5 seconds per step, by Cu Kα as a radiation source (λ = 1.5405 Å) operating under a constant current of 30 mA at 40 kV .

TGA Analysis Thermal properties of samples were analyzed using a thermogravimetric analyzer (TGA) Mettler Toledo TGA/SDTA851 (Mettler Toledo Corp., Greifensee, Switzerland). Samples (10.0 mg in a platinum pan) were heated from 25 to 600 °C under nitrogen atmosphere at a heating rate of 20 °C/min with nitrogen flow of 30 mL/min.

Particle Size Analysis The mean diameter of particles was determined by At least five replicate measurements with Dynamic Light Scattering (Zeta Plus; Brookhaven Instruments Corporation, NY, USA) after appropriate dilution with double-distilled Milli-Q water. The mean ± standard deviation (SD) was assessed.

Elemental analysis and energy dispersive X-ray spectroscopy Energy Dispersive X-Ray Spectroscopy (EDX) (JEOLJSM-600F) were used to determine the composition of the samples, and further analyzed for the percent of carbon (C), hydrogen (H), nitrogen (N) present in the samples by a 2400 CHNS analyzer (Perkin Elmer, Series II).


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N2 and CO2 Adsorption experiments The N2 adsorption–desorption isotherms at -196 °C and CO2 adsorption isotherms at 0 and 25 °C were obtained under static conditions using ASAP 2460 (Micromeritics) volumetric adsorption analyzer. Before analysis, samples were outgassed under vacuum at 60 °C overnight. Data were measured by admitting or removing a known quantity of adsorbing gas in or out of a sample cell containing chitosan-SiO2 NPs as the solid adsorbent maintained over a wide relative pressure range from 0.01 to 0.99 at a constant temperature. The specific surface area was calculated with the Brunauer– Emmett–Teller (BET) model, from adsorption data in relative pressures range of 0.05 < P/P0 < 0.25. The pore volume versus diameter distribution was derived from analyzing both the adsorption and desorption branches of the isotherm using the Barrett-Joyner-Halenda (BJH) method23. To check the repeatability and reproducibility of CO2 adsorption isotherms, measurements were carried out for several runs under similar operational conditions.

Statistics Statistical analysis was performed using Statistical Analysis System (SAS, version 9.1). A oneway analysis of variance (ANOVA) was conducted with Duncan’s multiple range test at a 95% confidence level (p < 0.05). The data were expressed as mean ± standard deviation for each experiments conducted in triplicates (n = 3).

Results and discussion Structural characteristics of the prepared samples



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Transmission electron microscopy (TEM) micrograph of SiO2 nanospheres was shown in Fig. 1a. It can be observed that the SiO2 nanospheres are monodispersed with good sphericity and fine porous structure, which clearly shows the presence of a hexagonally packed porous structure embedded in the silica spherical particles. The average size of individual silica nanospheres is ≈190 nm. TEM image of SiO2 nanospheres shows clear areas that correspond to the empty pores, and darker regions that correspond to the pore walls. Fig. 1b shows the fine porous structure of chitosan-SiO2 NPs as examined by TEM. The nanoparticles are porous sphere-like, pretty uniformly distributed with coarse surface, with an average size of approximately 200 nm. As shown in Fig 1b, thin layers could be seen at the edge of chitosan-SiO2 NPs as the arrow pointed, which should due to the chitosan coating on SiO2 nanosphers. It is from TEM observed that chitosan is practically filling up the pores as well as covering the outer surface of the particles. Particle size distribution of the chitosan-SiO2 NPs was measured by DLS (Fig. 1b). The hydrodynamic particle size of the SiO2 nanospheres and chitosan-SiO2 NPs are narrowly distributed at 189±21 and 211±39 nm, respectively, which are very consistent with that measured from the TEM images. The FTIR spectrum of pure chitosan were shown in Fig. 2a. The infrared spectrum shows characteristic peaks at 3435 cm -1 (–OH stretch and N–H stretch, overlapped), 2871 cm -1 (C–H stretch), 1656 cm -1 (Amide I), 1596 cm -1 (amide II band (N–H)), 1320 cm -1 (Amide III), 1085 cm

-1

(C–O stretch, secondary hydroxyl group), 1030 cm

group) and an absorption band at 894 cm

-1

-1

(C–O stretch, primary hydroxyl

due to the β-(1,4) glycosidic in chitosan. The same

results for the FTIR spectrum of chitosan has been previously reported 24.


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As shown in Fig. 2a, The SiO2 nanospheres exhibits IR peaks at 3305 cm

-1

(Water and silanol

OH stretching), 1639 cm -1 (Water OH bending), 1107 cm -1 (Si–O stretching), 951 cm -1 (Si–OH stretching), 803 cm

-1

(Si–O–Si symmetric stretching), 451 cm

-1

(Si–O–Si bending). The broad

adsorption peak in the range of 3351 cm-1 are assigned to the stretching vibration of the silanol group and characteristic peak at 1107 cm-1 and 951 cm-1 are belonging to the O-Si-O bonds stretching vibration, which confirm the successful synthesis of SiO2 nanospheres 22.

(a)

(b)

Fig. 1: TEM micrographs and Particles size distribution for (a):SiO2 nanospheres, (b): chitosanSiO2 NPs.



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The FTIR results of chitosan-SiO2 NPs are displayed in Fig. 2a. The FTIR spectrum shows adsorption peaks at 3351 cm -1 (–OH stretch), 2898 cm -1 (C–H stretch), 1632 cm -1 (Water OH bending), 1363 cm bending), 455 cm

-1

-1

(CH2 bending), 1050 cm

-1

(C–O–C bond), 590 cm

-1

(O–H out-of-phase

(Si–O–Si bending), It was indicated that chitosan was combined with SiO2

nanospheres. In addition, As compared with chitosan, the absorption peak of chitosan-SiO2 NPs at 1630 cm−1 was weaker, this result may be mainly attributed to the reduction of the hydrophilicity of chitosan-SiO2 NPs. The XRD patterns of all samples are shown in Fig. 2b. XRD pattern of the prepared silica nanospheres exhibits a typical broad peak associated with the amorphous SiO2 NPs

22

. The

crystallinity index (CrI) of chitosan was calculated according to the reported protocol 25. The Xray diffraction profile of chitosan (Fig. 2b) shows a sharp peak at 2θ=9.3° and a broad peak located approximately at 2θ = 22.5°, that symbolized semi-crystalline chitosan. The same results for the XRD Pattern of chitosan have been previously reported 26. The XRD pattern of chitosan-SiO2 NPs exhibits minor diffraction peak at 2θ = 10.4° and broad diffraction peak at 2θ = 23.6°. XRD Pattern of the chitosan-SiO2 NPs is similar to those for pure chitosan and silica nanospheres. The peak around 2θ = 10.4° which could be resulted from presence of chitosan, with some shift comparing with pure chitosan and the slightly broader and weaker peak around 2θ = 23.6° could be assigned to the amorphous structure (from the SiO2 nanospheres). The decrease in the crystallinity could be due to incorporation of bulky chitosan, which demonstrates that the chitosan molecules were well coated on the silica nanospheres.


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(a)

(b)

(c) Fig. 2: Comparative analysis among samples (a): FTIR spectra, (B): XRD patterns, (b): TGA curves.



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To consider thermal properties and the effect of chitosan attachment to the SiO2 nanospheres, TGA measurements were performed on the SiO2 nanospheres, chitosan and chitosan-SiO2 NPs, and results were shown in Fig. 2c. The weight reduction of chitosan-SiO2 NPs takes place in three steps. The first weight loss (4%) appeared at around 100 °C, which was associated with the evaporation of the absorbed water moisture from the surface due to physical absorption

27

. The

subsequent two weight loss steps were attributed to dehydration and complete decomposition of polymer chains (chitosan backbone) 28. It can be clearly seen in Fig. 2c that the residual weight of chitosan-SiO2 NPs (60.5%) after thermal decomposition is higher than pure chitosan (31.5%) at 600 °C. A rapid weight loss from 220 to 400 °C and a slow degradation from 400 to 600 °C representative of thermal degradation of deposited chitosan on the surface of the SiO2 nanospheres were observed. The significant increase in the weight residues at 600 °C illustrates successful incorporation of higher amounts of silica into the chitosan-SiO2 NPs and ultimately increases in thermal stability. Fig. 3 represents the EDX spectra and quantitative elemental composition of the samples. The results shows the presence of C, N, O and Si in the chitosan-SiO2 NPs, Which confirm attachment of chitosan to the surface of SiO2 nanospheres.


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Fig. 3: EDX spectra of samples.

Mesoporosity of the SiO2 nanospheres and chitosan-SiO2 NPs The N2 adsorption–desorption isotherm for the SiO2 nanospheres and chitosan-SiO2 NPs are displayed in Fig. 4a and b, respectively.



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The N2 adsorption/desorption isotherms of all samples correspond to isotherms of type IV (IUPAC classification International Union of Pure and Applied Chemistry), and exhibit obvious hysteresis loops in the high relative pressure region, which suggesting the mesoporosity is maintained in the adsorbent system. As shown in Fig. 4a, the mesoporous structure of the particles is also confirmed by TEM image. The steepness of the capillary condensation step demonstrates uniformity of their mesopores. The sharpness of the isotherm indicates the narrow pore size distribution. Compared with the SiO2 nanospheres, although the adsorbed amount of nitrogen reduced, the shape of the hysteresis loop remained unchanged, which reveals that the pore shape was not changed much. The morphology and structure of mesoporous particles were characterized. According to N2 sorption measurement, the total pore volume is 1.21 cm3 g-1, and the BET specific surface area is 1032 m2 g-1, indicative a high porosity of the SiO2 nanospheres. For chitosan-SiO2 NPs, the estimated BET surface area is 630 m2 g-1 and the total pore volume is 0.72 m3 g-1. Fig. 4b illustrates the pore size distribution (PSD) plots of the samples calculated by the BJH method showing that, the pore size distribution of the spherical particles are relatively narrow with uniform size around 2.54±0.26 and 1.41±0.15 nm for the SiO2 nanospheres and chitosanSiO2 NPs, respectively. The pore size uniformity is also confirmed by TEM images. The reduction of BET specific surface, pore size and pore volume of the chitosan-SiO2 NPs, are attributed to the filling up the pores by chitosan.


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(b)

(a)

Fig. 4: (a): N2 adsorption adsorption-desorption isotherms (at 77 K) and (b): Pore size distributions.

CO2 adsorption The CO2 adsorption experiments were performed at 0 and 25 °C under 1 bar pressure. The CO2 adsorption capacity are about 1.22 and 4.35 mmol g-1 adsorbent for the SiO2 nanospheres and chitosan-SiO2 NPs, respectively. The temperature dependence of the CO2 isotherm was considered, as shown in Fig. 5. The slope of the initial increases as well as the maximum uptake became smaller with increasing temperature. The CO2 uptake isotherms illustrates that an increase in the temperature of adsorption resulted in a markedly decrease in the CO2 uptake of the chitosan-SiO2 NPs as adsorbent, indicating that exothermic physical adsorption is the underlying process. It could be due to an increase in the thermal energy of the CO2 molecules at higher temperatures, leading to lower adsorption uptakes 29.



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Fig. 5: CO2 adsorption isotherms at 0 and 25 °C.

Effect of chitosan content on CO2 adsorption The CO2 adsorption capacity of pure chitosan has been reported very low, because of its nonporous structure and low specific surface area (SBET= 0.31 m2g-1)14. In addition, it is believed that the amino groups could increase the affinity toward CO2 adsorption 30. In order to elucidate the effect of chitosan content on CO2 adsorption, the parameters of textural properties were quantitatively determined for chitosan-SiO2 NPs with adding different amount of chitosan (Table 1). It is observed, increase in chitosan/silica ratio significantly decreases the pore size, pore volume and BET surfaces. When the concentration of chitosan varied from 0.06 to 0.33 g, the CO2 adsorption increased rapidly, and these could occur due to the high number of amine groups that were present in chitosan-SiO2 NPs. However, higher composition of chitosan caused a decrease in CO2 adsorption. This behavior, which seems to be contradictory at first glance, which can be explained by the filled or plugged mesopores with chitosan and decrease in the surface sites of the chitosan-SiO2 NPs for CO2 molecules.


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For better illustration, the composition of the SiO2 nanospheres, pure chitosan and chitosan-SiO2 NPs-4 were determined by a qualitative detection of the of (CHN) elements. Elemental analysis data (Table 2) showed that the content of N increased to 1.22% in chitosan-SiO2 NPs-4, compared with the SiO2 nanospheres. Therefore, chitosan-SiO2 NPs-4 as absorbents with the optimum chitosan/silica ratio were chosen for further investigations.

Table1: Compositions and textural properties of different adsorbents Mass of Chitosan loaded to MSNs (1 g) (g)

Adsorbent SiO2 nanospheres chitosan-SiO2 NPs-1 chitosan-SiO2 NPs-2 chitosan-SiO2 NPs-3 chitosan-SiO2 NPs-4 chitosan-SiO2 NPs-5 chitosan-SiO2 NPs-6

0 0.06 0.12 0.18 0.33 0.42 0.51

Pore size (nm)

2.54 1.93 1.66 1.41 1.37 1.30 1.21

Pore volume (cm3/g)

1.21 0.88 0.86 0.72 0.71 0.66 0.63

BET surfaces (m2g-1)

CO2 Adsorption (mmol g-1)

1032 931 844 630 621 609 556

1.22 2.84 3.45 4.35 4.39 4.11 3.82

Table2 : Elemental analysis of samples Sample SiO2 nanospheres Chitosan Chitosan-SiO2 NPs-4

C (%) 0.75 45.27 17.62

H (%) 1.93 7.26 2.54

N (%) 11.41 1.63

N/C Ratio 0.25 0.93

Effect of initial CO2 concentration The effect of the initial CO2 loading was investigated for different concentrations of CO2 (20% and 100%) and other operating conditions were the same as the basic operation conditions. The CO2 adsorption capacity of chitosan-SiO2 NPs-4 reached faster to the maximum value (4.39 mmol g-1) at a higher concentration (100%) of CO2. The chitosan-SiO2 NPs-4 reached its



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saturation capacity at higher concentrations quickly, because of more molecules were in contact with the adsorbent surface and saturation of opened pores of the chitosan-SiO2 NPs-4 occurred rapidly.

The evaluation of CO2 adsorption capacity To examine the adsorption capacity of chitosan-SiO2 NPs-4, the CO2 capture capacities of various previously reported CO2 adsorbents were summarized in Table 3

4, 6, 31-35

. The

mesoporous chitosan-SiO2 NPs have a large surface area to volume ratio than monolithic materials and the results show that a significant increase in the mass transfer has occurred because more contact area has been available for CO2 adsorption. The prepared chitosan-SiO2 NPs-4 demonstrates excellent adsorption ability for CO2 adsorption. In order to evaluate selectivity factor for separation of CO2-over-N2, adsorption kinetics of CO2 and N2 were measured for chitosan-SiO2 NPs-4 in same operational condition at 25 °C under 1 bar pressure, as shown in Fig. 6a. The rate of the CO2 capture was fast and reached to the maximum adsorption value (4.39 mmol CO2 g-1) in ≈4 min. The adsorbed amount of N2 was 0.28 mmol N2 g-1 and significantly lower than CO2. So, adsorption selectivity for CO2 over nitrogen N2 (S CO2/N2) was determined 15.46 in this study and it was more than most previous studies.


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Table 3: Comparison of the CO2 adsorption capacity of different adsorbents (at 25 °C and 1 bar). Adsorbent

Pore size (nm)

Pore volume (cm3 g-1)

BET surfaces (m2 g-1)

S CO2/N2

Capacity (mmol g-1)

Ref.

Acid treated bentonitic clay Chitosan-Silica Support Zeolite-like MOF (sodalite) Mesoporous MgO Microwave activated carbon SAPO-34 molecular sieve Activated template carbon Sawdust based porous carbon Mesoporous SiO2 nanospheres Mesoporous Chitosan-SiO2 NPs NR :Not reported

NR 0.23 NR 2.20 1.98 0.38

Mesoporous ChitosanâSiO2 Nanoparticles: Synthesis

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