and Heptazine-Based Carbon Nitrides - ACS Publications - American

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Cite This: ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Triazine- and Heptazine-Based Carbon Nitrides: Toxicity Qi Dong,† Naziah Mohamad Latiff,† Vlastimil Mazánek,‡ Nur Farhanah Rosli,† Hui Ling Chia,† Zdeněk Sofer,‡ and Martin Pumera*,‡ †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore ‡ Center for Advanced Functional Nanorobots, Dept. Inorganic Chemistry, Faculty of Chemical Technology, University of Chemistry and Technology Prague, Technická 5, 166 28 Dejvice, Prague 6, Czech Republic

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S Supporting Information *

ABSTRACT: Graphitic carbon nitride has attracted extensive interests recently because of its potential in biosensing, photocatalytic, and biomedical applications. Similar to graphene, it is a two-dimensional carbon and nitrogen-based nanomaterial with weak van der Waals forces between each layer. Carbon nitrides can have various structural moieties such as triazine and heptazine. Unlike graphene-substituted nanosheets, the toxicity of graphitic carbon nitrite is largely unknown. In respond to that, toxicological study was carried out to determine its toxicity toward human lung carcinoma epithelial cells (A549). Two formazan-based cell viability assays (water-soluble tetrazolium salt (WST-8) assay and methylthiazolyldiphenyltetrazolium bromide (MTT) assay) were utilized on the A549 cell line to derive the cytotoxicity profile. Both materials demonstrated a dose-dependent toxicological effect with triazine-based carbon nitrides being more cytotoxic than heptazine-based carbon nitrides. These findings are of great importance, and this paves the way for exploring carbon nitride materials in numerous fields such as photocatalysis and electrocatalysis. KEYWORDS: graphitic carbon nitrides, triazine, heptazine, cytotoxicity, cell viability

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INTRODUCTION In recent years, graphitic carbon nitride (g-C3N4) received great attention in the field of material science because of its excellent properties.1 As an analogue of graphite, g-C3N4 is a two-dimensional material that consists of carbon and nitrogen atoms with a general formula of C3N4.2 The discovery of gC3N4 can be traced back to as early as 1830s when Liebig and Gmelin named a yellow residue they discovered melon.3−5 Nonetheless, only after around 200 years, the potential of this material has been recognized. Owing to its structure and the correlated surface properties, g-C3N4 and its derivatives have developed into a hot research topic. Graphitic carbon nitride (g-C3N4) is the most stable allotrope of polymeric carbon nitride.6 This layered structure is held together by weak van der Waals forces.7 However, different stacking of carbon and nitrogen atoms significantly influences its properties. The wideband metal-free semiconductor shows high photocatalytic activities for hydrogen evolution.8 Additionally, g-C3N4 also has wide applications in various other fields such as optics and electronics.9−11 g-C3N4 possesses two basic moieties, which are composed of triazine and tri-s-triazine (heptazine) rings as the basic units.12 The heptazine moiety (h-C3N4) is one of the most common graphitic carbon nitrides, which can be easily prepared from low-cost materials. Numerous methods are available to © XXXX American Chemical Society

synthesize this structural type of carbon nitrides, resulting in various unique physiochemical properties. Additionally, hC3N4 was found to be thermally stable up to 600 °C, making it possible for usage in various applications.13 Presently, possible applications of h-C3N4 include biosensing, bioimaging, and photocatalytic applications.14−20 Another type of graphitic carbon nitride is the triazine moiety (t-C3N4).21 It is predicted by computational studies for potential use as hydrogen purification monolayer membrane.22 It can also be utilized in electrocatalytically switchable CO2 capture.23 The schematic drawing of the two different structural moieties is shown in Scheme 1.25 With the promising future of such materials in various applications, it is important to examine the toxicological profile of these materials to promote awareness of their potential health hazards so that (1) researchers working on these materials could be protected and (2) their application into the biomedical field could be broadened. To date, there are limited toxicological records and research done on t-C3N4 and h-C3N4. One of the studies reported that h-C3N4 is biocompatible with HeLa cells even at a high concentration of 600 μg mL−1.17 In Received: April 27, 2018 Accepted: August 3, 2018

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DOI: 10.1021/acsanm.8b00708 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Scheme 1. Structure of Carbon Nitrides Consisting of Triazine (a,b) and Heptazine (c,d) Motives with a and c Showing Structures of Their Corresponding Basic Units25

Methylthiazolyldiphenyltetrazolium bromide (MTT) and watersoluble tetrazolium salt (WST-8) were procured from Sigma-Aldrich and Dojindo, respectively. Other reagents used in the experiment include: Dulbecco’s modified eagle medium (DMEM) and phosphate buffer solution (PBS, pH 7.2) from Gibco, dimethyl sulfoxide (DMSO) from Tedia, and fetal bovine serum and 1% penicillin− streptomycin liquid from PAA Laboratories. Trypsin was obtained from Life Technologies, Singapore. Synthesis of Nanomaterials. For h-C3N4, dicyandiamide was placed in a quartz glass crucible placed in a quartz reactor and then repeatedly evacuated and refilled with nitrogen. Subsequently, the precursor was heated at 550 °C for 5 h in a nitrogen atmosphere with a heating and cooling rate of 10 °C/min. For t-C3N4, dicyandiamide (0.5 g) was heated with 15 g of a eutectic mixture of KBr/LiBr (52 wt % LiBr/48 wt % KBr) in a quartz glass ampule. The homogenized mixture of precursor and bromides was filled in quartz glass ampule (30 × 130 mm; wall thickness = 2 mm) in an argon filled glovebox and subsequently sealed under vacuum (1 × 10−3 Pa) by an oxygen/hydrogen welding torch. The ampule was heated at 400 °C for 5 h followed by heating at 600 °C for 60 h. The heating and cooling rate was 20 °C/min. The heating procedure was performed in a steel protective tube, since a high pressure of ammonia is present in ampule. The ampule was cooled below −40 °C before it was opened, and the reaction mixture was dissolved in water and separated by suction filtration and repeatedly washed with water and ethanol. Finally, the reaction product was dried in vacuum oven for 48 h at 50 °C. Material Characterization. Scanning electron microscopy (SEM) images were obtained with a Jeol 7600F SEM (Jeol, Japan), which operates at 5 kV. Energy-dispersive X-ray spectroscopy (EDX) was conducted on a Jeol 7600F (Jeol, Japan) at 15 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed

contrast, many toxicological studies have been carried out to investigate the biocompatibility of graphene oxide (GO). To fill up this gap in the research, we investigated the cytotoxicity of these two groups of nanomaterials (t-C3N4 and h-C3N4) on the A549 cell line (lung epithelial carcinoma cell from human). This is achieved by incubating the nanomaterials with A549 cells for 24 h and subsequently conducting in vitro cell viability studies. This particular cell line was selected because of the high probabilities of airborne particles being inhaled into our bodies through the lungs. Various concentrations of t-C3N4 and h-C3N4 were incubated with A549 cells, and the cell viabilities were tested using the watersoluble tetrazolium salt (WST-8) and methylthiazolyldiphenyltetrazolium bromide (MTT) assays. Upon contact with viable cells, these two assays produced colored formazan dyes. Cell viability is directly proportional to the color intensity of the dyes and could be measured using a spectrophotometer; thereby, a toxicological profile with different concentrations of nanomaterial exposure can be determined. To minimize the possibility of precursor residues influencing the toxicity and to achieve an overall fair comparison, t-C3N4 and h-C3N4 synthesized from the same precursor (dicyandiamide) were used.

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EXPERIMENTAL SECTION

Reagents Used. Dicyandiamide, lithium bromide, and potassium bromide were obtained from Sigma-Aldrich, Czech Republic. Nitrogen of 99.9999% purity was obtained from SIAD, Czech Republic. The human lung carcinoma epithelial cell line (A549) used during the experiment was obtained from Bio-REV, Singapore. B


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ACS Applied Nano Materials using a Phoibos 100 spectrometer and a monochromatic Mg X-ray radiation source (SPECS, Germany). Survey (wide scan) spectra and high-resolution spectra of C 1s and N 1s were taken for each sample. C/O ratios were determined from the survey spectra by using relative sensitivity factors. For SEM, EDX, and XPS measurements, sample preparation was done by coating a uniform layer of the materials investigated on carbon conductive tape. X-ray powder diffraction data were collected at room temperature on a Bruker D8 Discoverer (Bruker, Germany) powder diffractometer with parafocusing Bragg− Brentano geometry using Cu Kα radiation (λ = 0.15418 nm, U = 40 kV, I = 40 mA). Data were scanned over the angular range of 10−80° (2θ) with a step size of 0.019° (2θ). Data evaluation was performed in the software package EVA. Combustible elemental analysis (CHNSO) was performed using a PE 2400 Series II CHNS/O Analyzer (PerkinElmer, USA). The instrument was used in CHN operating mode (the most robust and interference-free mode) to convert the sample elements to simple gases (CO2, H2O, and N2). The PE 2400 analyzer automatically performed combustion, reduction, and homogenization of product gases as well as separation and detection. An MX5 microbalance (Mettler Toledo) was used for precise weighing of the samples (1.5−2.5 mg per single sample analysis). Using this procedure, the accuracy of CHN determination is better than 0.30% abs. Internal calibration was performed using an N-fenyl urea. For analysis of Br, the sample was combusted by the Schöniger method. Formed Br− was titrated by a solution of Hg(NO3)2 for sodium nitroprusside as an indicator. FTIR spectra were obtained via an FTIR spectrometer iS50R (Thermo Scientific, USA) using diamond ATR and DLaTGS detector. Raman spectroscopy at 514 nm was measured using a LabRam HR Instrument from HoribaScientific. UV−Raman spectroscopy using a He−Cd laser (325 nm, 22 mW) with a 40× NUV objective was conducted with an inVia Raman microscope (Renishaw, England) with a CCD detector. For both analyses, instruments were calibrated with a Si reference, which gives a peak center at 520 cm−1 and a resolution h-C3N4 according to the WST-8 results. In the case of h-C3N4, at 25 μg/mL, the cell viability percentage was slightly higher than 100%. This could be due to the uneven cell seeding that resulted in more cells being added to the wells, thus increasing the cell viability. A similar toxicity trend was also observed using the MTT assay (Figure 7) where t-C3N4 demonstrated higher toxicity as compared to h-C3N4 for all testing concentrations. A F


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Lastly, more research could be done on various types of cell lines other than carcinogenic cells to enrich the toxicological profile and provide a more complete guide on usage of such materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b00708. Figure 8. Total glutathione content in A549 cells after 24 h of exposure to 50 and 100 μg/mL t-C3N4 and h-C3N4. Data is normalized to a negative control where cells were not treated with any test materials.

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Generally, both nanomaterials (t-C3N4 and h-C3N4) show a dose-dependent cytotoxicity effect, and t-C3N4 is generally more toxic than h-C3N4 according to WST-8 and MTT assays. A background subtraction of results from control experiments (performed under cell-free conditions) was made to minimize any potential nanomaterial-induced interference effects. The variation in the toxicology profile could potentially be a result of inherent differences between the two materials such as the N/C ratio, bonding structures, and compositions. We attempted to study the effect of KBr on the synthesis of tC3N4 but found it was not a contributing factor to the higher toxicity of the material relative to h-C3N4. We found that tC3N4 is capable of producing more oxidative stress toward A549 cells compared to h-C3N4. In comparison to GO in a previous report performed under similar conditions, t-C3N4 and h-C3N4 show much lower toxicity effects.35 Both t-C3N4 and h-C3N4 show low toxicity effects at low concentrations (0 to 50 μg mL−1). These results show that triazine- and heptazine-based carbon nitrides can be good candidates for potential biomedical applications with moderate usage. More cell lines should be explored in further studies to expand on this scope. In addition, carbon nitrides can also be synthesized using copolymerization to derive different molecular structures.36,37 This could potentially influence the toxicological profile and will be intriguing to explore for future studies.

High-resolution TEM images of the triazine- and heptazine-based carbon nitrides under study, ATR spectrum and XRD of dicyandiamide, and toxicity assessment of KBr (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected], pumera.research@gmail. com. ORCID

Zdeněk Sofer: 0000-0002-1391-4448 Martin Pumera: 0000-0001-5846-2951 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.M.L. and N.F.R. were funded by Singapore’s National Research Foundation (NRF) under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. Z.S. acknowledges the Czech Science Foundation (GACR No. 16-05167S) as well as specific university research (MSMT No 20-SVV/2016). This work was supported by the project Advanced Functional Nanorobots (reg. No. CZ.02.1.01/0.0/0.0/15_003/0000444 financed by the EFRR).

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REFERENCES

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CONCLUSION Two-dimensional graphitic nanomaterials had gained increasing attention in recent years due to their remarkable properties. Howeer, little is known about the toxicological properties of the new members of the graphitic family, namely, heptazine and triazine-based carbon nitrides. To fill in this gap in the current research work, we conducted in vitro cytotoxicity studies on them. Seven concentrations of the nanomaterials were incubated for 24 h with A549 cells to test their toxicity. The results were quantified through WST-8 assays and verified with an MTT assay. The toxicity effects were found to be dosedependent for both materials, and t-C3N4 was observed to be more toxic than h-C3N4. The higher toxicological effect of tC3N4 than h-C3N4 could be attributed to the inherent differences such as the N/C ratio, bonding structures, and compositions. It was founded that KBr does not increase the toxicity of the nanomaterials, and t-C3N4 exerts more oxidative stress toward A549 cells compared to h-C3N4. As compared to graphene oxide (GO), t-C3N4 and h-C3N4 demonstrated lower toxicity.35 These findings could potentially open up more opportunities for carbon nitride in biomedical applications. G


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and Heptazine-Based Carbon Nitrides - ACS Publications - American

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