Design and Fabrication of Highly Photoluminescent Carbon

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General Research

Design and Fabrication of Highly Photoluminescent Carbon-Incorporated Silica from Rice Husk Biomass Zhaofeng Wang, Songshan Zeng, Gaurav N. Joshi, Andrew T. Smith, Huidan Zeng, Zichao Wei, Xiaoyuan Yu, Madhab Pokhrel, Yuanbing Mao, Weixing Wang, and Luyi Sun Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00151 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Industrial & Engineering Chemistry Research

Design and Fabrication of Highly Photoluminescent Carbon-Incorporated Silica from Rice Husk Biomass Zhaofeng Wang,a,b Songshan Zeng,a Gaurav N. Joshi,c,d Andrew T. Smith,a Huidan Zeng,e Zichao Wei,a Xiaoyuan Yu,a,f Madhab Pokhrel,g Yuanbing Mao,g Weixing Wang,h,* and Luyi Suna,* aPolymer

Program, Institute of Materials Science and Department of Chemical & Biomolecular Engineering, University of Connecticut, Storrs, Connecticut 06269, USA b State

Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, Gansu 730000, China

cJohn

B. Little Center for Radiation Sciences, Harvard T.H. Chan School of Public Health, 220 Longwood Avenue, Goldenson 553, Boston, Massachusetts, 02115, USA

dLaboratory

of Systems Pharmacology, Harvard Program in Therapeutic Science, Department of Systems Biology, Harvard Medical School, Boston, Massachusetts, 02115, USA eKey

Laboratory for Ultrafine Materials of Ministry of Education, School of Materials Science and Engineering, East China University of Science and Technology, Shanghai 200237, China fInstitute

of Biomaterials, College of Materials and Energy, South China Agricultural University, Guangzhou, Guangdong 510642, China

gDepartment

of Chemistry and School of Earth, Environmental and Marine Sciences, University of Texas Rio Grande Valley, Edinburg, Texas 78539, USA

hMinistry

of Education Key Laboratory of Enhanced Heat Transfer & Energy Conservation, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong 510640, China

*Authors to whom correspondence should be addressed: Dr. Luyi Sun, Tel: (860) 486-6895; Fax: (860) 486-4745; Email: [email protected] Dr. Weixing Wang, Tel: +86 20 8711 3171; Email: [email protected]

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Abstract In this work, we designed a two-step method to prepare carbon-incorporated silica from rice husk biomass. This two-step method could promote the chemical bonding between carbon and silica that is responsible for the generation of photoluminescence, and effectively remove free carbon in the final product that is responsible for the quenching of photoluminescence. Such a structural optimization of the carbon-incorporated silica derived from rice husks led to a significant improvement in its photoluminescence with the emission band covering the entire visible spectrum. The toxicity of the carbon-incorporated silica from rice husks was two-fold lower compared to commercial crystalline and amorphous spherical silica. Therefore, such rice husk derived carbon-incorporated silica by the two-step method is promising for widespread application. Keywords: Rice husk, carbon-incorporated silica; photoluminescence; two-step method


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1. Introduction Rice husks (RHs) are the byproducts of rice milling, which have low nutritional value and limited applications.1 The conventional disposal of RHs, i.e., open field burning and land filling, results in waste of energy, air pollution, and occupancy of landfill space.2 Therefore, the utilization of RHs in an economic and environmentally friendly way holds great significance.3-6 Since silica is the main component of RHs (ca. 15-28 wt. %),7 the preparation of silicon-based materials from RHs have attracted much attention, which could be widely used in adsorption,8 catalysis,9 energy storage,10,11 optical devices,12,13 etc. For the preparation of high purity silica from RHs (RH silica), it usually requires a reaction temperature of 700 ºC or higher in air.14 It was reported that carbon-incorporated silica (CIS), which exhibited photoluminescence (PL) when excited by an ultraviolet (UV) light, can be obtained from RHs by lowering the thermal treatment temperature.15 This is significant for practical biomedical applications, since both carbon and silica are biocompatible. Although the CIS with PL performance was synthesized from RHs, the reported method by lowering the thermal treatment temperature would create a problem of free carbon formation, which would greatly quench the PL.16 As a result, it is highly desirable to develop a new or improved method to fabricate photoluminescent RH derived CIS (RH-CIS) with high efficiency without the negative impact of free carbon. While the PL of RH-CIS was observed years back, it was not revealed until recently that the carbon structure chemically bonded with silica in the RH-CIS should be the mechanism of PL generation.17 As a result, in order to achieve a high PL performance, one need simultaneously improve the chemical bonding between carbon and silica and remove free carbon in the final product to prevent the PL quenching. Based on the above considerations, in this work, we designed a two-step method for the synthesis of RH-CIS. RHs were first underwent a pretreatment in an oxygen-deficient atmosphere to maintain the carbon content, allowing carbon to chemically bond with silica efficiently. Then, a second thermal treatment was carried out in an oxygen-excessive atmosphere to remove the free carbon. By such a facile two-step calcination method, the contents of free carbon and chemically bonded carbon in RH-CIS can be well controlled, leading to a significant enhancement of PL performance. The obtained emission band covers the entire visible light region with a full width at half maximum (FWHM) of ca. 250 nm, showing promising applications in optical devices, e.g., white light emitting diodes. 3 ACS Paragon Plus Environment


2. Experimental 2.1 Pretreatment of RHs RHs (obtained from Rice Hull Specialty Products, Inc., Stuttgart, Arkansas, USA) were first treated by deionized (DI) water as reported in previous work1,2,5,6 and then were ground to powders (100 mesh). The RH powders were washed by DI water and dried in an oven at 90 ºC for 24 h. Next, the DI water rinsed RH powders were refluxed in a round bottom flask with a 5.0 wt. % HCl solution at 95 ºC for 2 h. The acid treatment was conducted mainly to remove metal cations in RHs, particularly K+, which were reported to catalyze the melting of silica to generate aggregations.1 The acid treated RH powders were subsequently washed by DI water to remove the residual HCl and dried at 90 ºC for 24 h. 2.2 Synthesis of RH-CIS First, 3.0 g of the above acid treated RH powders were transferred in a corundum crucible. Then, the RH powders were calcined in a tube furnace (OTF-1200X-III, MTI Corp., Richmond, California) at 600 ºC for 2 h under a predetermined air flow rate (20, 50, or 100 scc). Then, the resultant samples were further treated at 600 ºC for 20 min in an oxygen-excessive atmosphere by using a cubic furnace (LINDBERG BLUE M-1100, Asheville, North Carolina, USA), to obtain the final products. 2.3 Characterization All PL excitation and emission spectra were collected on an Edinburgh Instruments FLS980 fluorometer, and corrected for the spectral sensitivity of the system and detector, as well as intensity variation in a Xe light-source using a reference diode.18 Powder samples of the as-prepared RH-CIS were held in a home-designed sample holder. Samples were excited at 365 nm using a Xenon Lamp source to take the emission spectra. Excitation spectra of the RH-CIS samples were collected by monitoring emission at 525 nm. All the emission and excitation spectra measurements were performed at room temperature. The entire system was controlled though the Edinburgh Instruments F980 data acquisition software. X-ray diffraction (XRD) patterns were recorded on a Bruker D5 diffractometer with Bragg-Brentano θ-2θ geometry using a graphite monochromator with Cu Kα (λ = 0.1540 nm) radiation. X-ray photoelectron spectra (XPS) of the samples were recorded using an ESCALAB 250XT spectrometer (Thermo Fisher). The X-ray source was Al Kα (1486.6 eV). 4 ACS Paragon Plus Environment

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Elemental analysis (EA) was conducted in an Elementar vario MICRO cube analyzer. 2.4 Cell viability assay MH-S alveolar macrophages (CRL-2019; American Type Culture Collection, Manassas, VA) were maintained at 37°C in an RPMI-1640 medium supplemented with 10% heat-inactivated fetal bovine serum (FBS), 2 mM L-glutamine, and 100 μg/mL penicllin/streptomycin (complete medium) at 37°C in a 5% CO2 incubator. For measuring cell viability, CellTiter-Glo (Promega) was used.19 Cells were plated at 1×104 cells per well of a white (opaque) walled, transparent bottom 96-well dish (Corning 3610) over-night. The next day, complete medium was changed to CO2 independent medium twice. A final change was done to have a total of 200 µL CO2 independent medium and the cells were exposed to the sample particles at different concentrations. The cells were incubated in a humidified ambient air incubator for 24 h at 37°C. A sample of 100 µL of medium was aspirated from each well, followed by the addition of 50 µL CellTiter-Glo solution. The plates were incubated in dark for 15 minutes on a shaker. The luminescence was measured on a Synergy H1 microplate reader (BioTek Instruments). 2.5 Endolysosomal leakage assay MH-S alveolar macrophages were plated at 1×104 cells per well using a complete medium in a 96-well plate compatible with a high-throughput imaging system (Corning 3603). The cells were incubated overnight at 37°C in a 5% CO2 incubator. The next day, the complete medium was replaced with a complete medium containing 1 mg/mL 4 kD FITC-Dextran (Sigma) and incubated for 2.5 h following which they were washed 3 times. The media was replaced with CO2 independent medium, and the sample particles were added at appropriate concentrations.20,21 The plate was imaged using InCell 6000 (GE HealthCare Life Sciences) in real time. 3. Results and discussion The RH-CIS samples prepared by heating the acid-treated RHs at 600 ºC for 2 h under a air flow rate of 20, 50, and 100 scc were named as RH-CIS-20-1 (A), RH-CIS-50-1 (B), and RH-CIS-100-1 (C), respectively (Figure 1a). These samples (A-C) were further treated at 600 ºC for 20 min in an oxygen-excessive atmosphere, and the resulting products were named as RH-CIS-20-2 (D), RH-CIS-50-2 (E), and RH-CIS-100-2 (F), respectively (Figure 1a). The 5 ACS Paragon Plus Environment


XRD patterns of the samples (Figure 1c) show a broad diffraction band around 22.5˚, suggesting an amorphous silica phase. As shown in Figure 1a, with an increase of air flow rate, the color of the powder samples gradually changed from black, to gray, and to white for samples A, B, and C, indicating a decrease of free carbon content. After the second step calcination (600 ºC for 20 min) under an oxygen-excessive atmosphere, the free carbon was further removed, and the samples (D, E, and F) exhibit white color.

Figure 1. Photographs (a and b) and XRD patterns (c) of the RH-CIS samples prepared at different conditions: (a) without UV irradiation; (b) under UV irradiation. The samples are: (A) RH-CIS-20-1, (B) RH-CIS-50-1, (C) RH-CIS-100-1, (D) RH-CIS-20-2, (E) RH-CIS-50-2, and (F) RH-CIS-100-2. Figure 1b shows the photoluminescent digital photographs of the RH-CIS samples under the irradiation of 365 nm. It is obvious that all white powders exhibit intense PL, which should be originated from the chemically bonded carbon in the structure of silica.17,22 The quenched PL for the black samples is due to the existence of a large amount of free carbon.16 Apparently, the second calcination of the samples under excessive oxygen could lead to a much brighter PL, suggesting the advantage of the two-step method. Figure 2a shows the PL spectra of the RH-CIS samples under the excitation of 365 nm. The corresponding variations of the PL intensity and the FWHM are presented in Figure 2b. It is clearly observed that after the second step of calcination, the PL intensity of RH-CIS was significantly improved. In particular, for sample RH-CIS-20-1, its PL intensity was improved more than 500 times. In addition, the FWHM of the RH-CIS samples were also greatly increased after the second step of calcination. For example, the emission bands of RH-CIS-20-2 and RH-CIS-50-2 cover the entire visible spectrum (400-750 nm) with broad absorptions from 300 to 400 nm region as indicated by the excitation spectra shown in Figure 2c, promising for applications in optical devices, such as white light emitting diodes.23 Because the samples were composed of carbon, silicon, and oxygen elements, they are 6 ACS Paragon Plus Environment

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expected to exhibit excellent biocompatibility and could potentially be employed for bio-probes and photo-thermal therapy. Note that the air flow rate in the first step exhibited a critical influence on the PL performance of the RH-CIS samples, and sample RH-CIS-50-2 exhibited the most intensive PL.

Figure 2. (a) PL spectra of the RH-CIS samples synthesized under different conditions; (b) variations of FWHM and integrated PL intensity, and (c) excitation spectra monitored at 525 nm of the RH-CIS samples: (A) RH-CIS-20-1; (B) RH-CIS-50-1; (C) RH-CIS-100-1; (D) RH-CIS-20-2; (E) RH-CIS-50-2; (F) RH-CIS-100-2. To further investigate the PL mechanism as well as the influence of air flow rate on the PL features, the structure of the RH-CIS samples was investigated. Figure 3 presents the XPS C 1s spectra of the RH-CIS samples under different synthesis conditions. For each sample, there are sp2 C, C-O, and O-C=O groups, locating at 284.8, 286.5, and 288.5 eV, respectively.24 Since the RH-CIS samples underwent high temperature calcination (600 ºC) for over 2 h, regular C-O and O-C=O groups were decomposed.25 Therefore, the C-O and O-C=O groups in the RH-CIS samples should be the functional groups chemically bonded with silica, which were reported to possess a high thermal stability.26,27 To further confirm the chemical bonding between carbon and silicon, XPS Si 2p spectrum of RH-CIS-20-1 was acquired, which could be resolved into two components at 103.4 and 105.1 eV, corresponding to Si-O-Si, and Si-O-C, respectively (Figure 3g).28 7 ACS Paragon Plus Environment


Figure 3. XPS C 1s spectra of the RH-CIS samples synthesized at different conditions: (a) RH-CIS-20-1; (b) RH-CIS-50-1; (c) RH-CIS-100-1; (d) RH-CIS-20-2; (e) RH-CIS-50-2; (f) RH-CIS-100-2, and (g) XPS Si 2p spectrum of RH-CIS-20-1. It is known that pure silica cannot exhibit PL because of its wide bandgap (ca. 9 eV).29 The chemical incorporation of carbon on the structure of silica is suggested to be responsible for the generation of PL. This is because the incorporated carbon can produce a series of localized energy levels within the bandgap of silica, allowing the acceptance of the excited electrons and the subsequent recombination with holes.30 While elemental carbon substance 8 ACS Paragon Plus Environment

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typically shows a strong absorption from UV to visible light through non-radiative transitions, it quenches luminescence.31 Figure 4a shows the variation of the mass fractions of the free carbon and chemically bonded carbon in the RH-CIS samples, which can reflect the PL quenching and generation, respectively. The above fractions were calculated based on the total carbon contents in the RH-CIS samples determined by elemental analysis, as well as the ratios of the free carbon and the chemically bonded carbon revealed by XPS. It is found that the treatment of RHs in the first step at a low air rate (20 scc) would be beneficial to covalently incorporate carbon onto silica (RH-CIS-20-1). However, the lower air flow rate would also result in a higher free carbon content (RH-CIS-20-1), which is difficult to be thoroughly removed in the second step (RH-CIS-20-2), as some of the free carbon is encapsulated by the silica particles. Therefore, based on the combined effects of free carbon and chemically bonded carbon, sample RH-CIS-50-2 with 0.44 wt. % of chemically bonded carbon and 1.09 wt. % of free carbon exhibited the highest PL performance. Its color coordinate was calculated to be (0.311, 0.345), which locates in the white light region in the 1931 CIE diagram as shown in Figure 4b. Therefore, our two-step method is effective in preparing RH-CIS samples with significantly enhanced PL, which might be adopted for large-scale production of highly photoluminescent RH-CIS for various practical applications.

Figure 4. (a) Mass fractions of free carbon and chemically bonded carbon in RH-CIS samples; (b) CIE color coordinates of the as-prepared RH-CIS samples: (A) RH-CIS-20-1; (B) RH-CIS-50-1; (C) RH-CIS-100-1; (D) RH-CIS-20-2; (E) RH-CIS-50-2; (F) RH-CIS-100-2. Considering RH-CIS-50-2 exhibited the highest luminescence and is promising for biomedical applications, its potential toxicity was investigated and compared to that of crystalline silica particles (Min-U-Sil 5 alpha-quartz crystalline silica, US Silica Corporation) which is responsible for causing silicosis and 3 µm amorphous spherical silica particles (Alltech Allsphere, now a part of Grace-Davison) that has been reported to be toxic.21 Cell viability was measured using CellTiter-Glo in MH-S macrophages exposed to different 9 ACS Paragon Plus Environment


particle types at varying concentrations. CellTiter-Glo measures Adenosine Triphosphate (ATP) in the cells and is therefore indicative of the metabolically active cells. With increasing cell death, a decrease in ATP is expected. There was virtually no difference in cell viability among the three particle types and neither particle was toxic up to 50 µg/cm2. Upon exposure to 75 µg/cm2 crystalline and spherical silica particles, there was a greater reduction in cell viability compared to RH-CIS-50-2, which indicates that RH-CIS-50-2 particles were much less toxic compared to crystalline and spherical silica particles (Figure 5).

Figure 5. Cell viability measurements in MH-S alveolar macrophages upon exposure to different silica particles. MH-S alveolar macrophages were exposed to different concentrations (0, 5, 25, 50, and 75 µg/cm2) of RH-CIS-50-2, crystalline silica, and amorphous spherical silica for 24 h. Data are representative from three individual experiments. The earliest event upon silica particle uptake by a cell is the damage to endolysosomal and phagolysosomal compartments.20,32,33 This results in the release of cathepsin proteases that has been shown to activate intrinsic apoptosis pathway.34,35 Particles were therefore tested for their ability to cause the release of 4 kD FITC-Dextran from the endolysosomal compartments in MH-S macrophages. FITC-Dextran is endocytosed by the cells and because of acidic environment in the endosomes and lysosomes, the fluorescence of FITC is quenched and it appears dimly fluorescent. Upon leakage of endosomes and lysosomes, FITC-Dextran is released into the cytoplasm and its fluorescence is unquenched making it appear bright. The untreated cells showed punctate looking endolysosomal vesicles (Figure 6A, FITC-Dextran) and the cells were healthy till 11.5 h (Figure 6A, DIC). A gradual decrease in FITC fluorescence was due to photo-bleaching. Few cells showed an increase in FITC fluorescence upon exposure to RH-CIS-50-2 particles (Figure 6B, FITC-Dextran, green 10 ACS Paragon Plus Environment

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triangle), whereas all cells showed leakage of FITC-Dextran exposed to crystalline and spherical silica particles (Figures 6C and 6D, FITC-Dextran, green triangles). Some cells exposed to the crystalline and spherical silica particles also underwent apoptosis as seen by morphological blebbing (Figures 6C and 6D, DIC, red triangles). The latter is consistent with the previously published reports and the toxicity associated with these particles.36 While we did not do uptake assays, previous studies have shown effective uptake of silica particles of different sizes.37 Thus, RH-CIS-50-2 particles are much less toxic compared to other types of silica particles and this decreased toxicity could be due to a reduced endosomal and lysosomal vesicle damage.

DIC

FITCDextran

DIC

A Control

FITCDextran

DIC

B RH-CIS-50-2

FITCDextran

DIC

C Crystalline Silica

DIC

D Spherical Silica

450

FITCDextran

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Time (hours)

0

6

1

9

11.5

Figure 6. Endo- and phagolysosomal leakage upon exposure to different particle types. MH-S alveolar macrophages loaded with 4 kD FITC-Dextran were exposed to different silica particles (RH-CIS-50-2, crystalline silica, and amorphous spherical silica) at 75 µg/cm2 and imaged every 30 minutes. The control cells show intact endolysosomes as evidenced by 11 ACS Paragon Plus Environment


punctae in FITC-Dextran channel. The cells exposed to RH-CIS-50-2 particles do not show an evidence of endolysosomal leakage except for the one labeled with green triangle. The cells exposed to crystalline silica particles showed leakage as early as one hour (green and red triangles) with the cells marked by red triangles showing blebs (DIC channel) and dying as early as 6 h. The cells exposed to spherical silica particles also showed leakage as early as one hour (green and red triangles) with the cell marked by red triangles showing blebs (DIC channel) and dying as early as 9 h. While all of the cells exposed to crystalline and spherical silica particles show phagolysosomal leakage only selected cells are labeled with green triangles for the purposes of clarity. Conclusions In summary, a two-step calcination method was developed to synthesize highly photoluminescent carbon-incorporated silica, whose emission band covers the entire visible spectrum (400-750 nm). Compared to the one-step method, the enhanced PL of the optimized RH-CIS could be ascribed to the high content of chemically bonded carbon (0.44 wt. %) in the structure of silica as well as the effective removal of free carbon. The carbon-incorporated silica synthesized by the two-step calcination method exhibited virtually no toxicity and thus is promising for applications in the fields of optics, photonics, and biomedicine. Acknowledgements We would like to thank the financial support by the U.S. Environmental Protection Agency (P3 Award, SU-83529201), the USDA National Institute of Food and Agriculture, HSI Collaboration: Integrating Food Science/Engineering and Education Network (IFSEEN, No. 2015-38422-24059), and the Guangzhou Science and Technology Planning Project (No. 201704030022). The authors would also like to thank Dr. Kristopher A. Sarosiek at the Harvard School of Public Health for the generous use of laboratory materials and instruments, the Harvard Program in Therapeutics Sciences (HiTS) at Harvard Medical School for access to InCell 6000, and Dr. David A. Knecht at the University of Connecticut for a kind gift of crystalline and amorphous spherical silica particles. Z.W. thanks the support from the CAS Pioneer Hundred Talents Program.

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Design and Fabrication of Highly Photoluminescent Carbon

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