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Bioinorganic Chemistry and Applications
Volume 2013, Article ID 198545, 11 pages
Research Article

Characterization of Tibetan Medicine Zuota by Multiple Techniques

Institute of Ethnic Medicine, Southwest University for Nationalities, Chengdu 610041, China

Received 21 April 2013; Accepted 5 August 2013

Academic Editor: Claudio Pettinari

Copyright © 2013 Xiao-Yan Zhao et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


Zuota is regarded as the king of Tibetan medicine. However, due to the confidentiality of this precious medicine, the scientific characterization of Zuota is very scarce, which limits the pharmacology and biosafety studies of Zuota. Herein, we collected four different Zuota samples from Tibet, Qinghai, Gansu, and Sichuan and characterized them by multiple techniques. Our results showed that Zuota was mainly an inorganic mixture of HgS, sulfur, and graphite. Morphologically, Zuota samples were composed of nanoparticles, which further aggregated into microsized particles. Chemically, the majorities of Zuota were S and Hg (in the forms of HgS and pure sulfur). All samples contained pure sulfur with orthorhombic crystalline. Zuota from Qinghai province had different HgS crystalline, namely, hexagonal crystalline. The others were all face-centered cubic crystalline. Carbon in Zuota NPs was in the form of graphite. The implication to future studies of Zuota was discussed.

1. Introduction

Tibetan medicine has a history of over 3800 years [1]. Known as the king of Tibetan medicine, Zuota is the transliteration of Tibetan language “Renqing Ouqu Zuozhu Qinmu.” Here, “Zuo” means refining, and “ta” means grey powder. Thus, Zuota means burning to powder. Zuota has been used as an essential and key component of Renqing series drugs in Tibetan medicine for 1200 years [2, 3]. Zuota cannot be used as medicine alone, but it shows magic effect when used as supplementary material to other medicines [4]. Based on Zuota, many drugs have been developed, such as Qi Shi Wei Zhen Zhu Wan, Er Shi Wu Wei Zhen Zhu Wan, Er Shi Wu Wei Shan Hu Wan, Er Shi Wu Wei Song Shi Wan, Er Shi Wu Wei Er Cha Wan, Renqing Chang Jue, Renqing Mang Jue, Zuo Zhu Da Xi, and Zhi Tuo Jie Bai Wan. These medicines are applied in treating digestive diseases, cardiovascular and cerebrovascular diseases, hepatic and gall diseases, food poisoning, pa-disease, leprosy, and so on [510]. Zuota shows very good performance in treating diseases of skin, knuckle, marrow, and sclerotin [2, 3]. Zuota is also beneficial in replenishing blood, activating blood, and prolonging the life according to Tibetan medicine theory [2, 3, 11, 12]. The proposed effects of Zuota in these medicines are enhancing the pharmaceutical effects and reducing the toxicity [2, 3]. Recently, Zuota has been collected in the Chinese Intangible Cultural Heritage [13].

However, nearly nothing is known about Zuota from the modern scientific perspective, because Zuota is always treated as the top secret of Tibetan medicine. According to the disclosure, Zuota is prepared from Hg by Tibetan special processing method, which takes 300 steps and 40 days [14]. During the processing, many precious animal medicines, 8 heavy metal elements, and 8 mineral substances are added. Despite the mystery of Zuota, the major starting material (Hg) is a toxic element undoubtedly according to modern medicine [15, 16]. Therefore, the biosafety of Zuota has always been concerned [11, 12, 17, 18]. Besides, the pharmacology of Hg in Zuota is still a mystery [1921]. To address the pharmacology and toxicity of Zuota, the first and essential step is to reveal the physicochemical properties of Zuota.

Unfortunately, such information is still scarce to date [2226]. There are only several pilot studies available in literature. Zeng et al. determined the Hg contents in Zuota by high performance liquid chromatography (HPLC) [22]. Lan et al. analyzed the chemical components in Zuota, where the elements were quantified and the morphology was recorded [23]. Similar results were also reported by Yan et al. [24, 25]. In addition, the graphite carbon in Zuota was analyzed by Yan and Ma showing the existence of graphite [26]. However, current studies did not concern the chemical states of elements in Zuota. And the comparison among Zuota samples from different sources has not been performed.

Herein, we collected four different Zuota samples and characterized them by various techniques. The morphology of Zuota samples was characterized by scanning electron microscopy (SEM). The element contents were analyzed by X-ray fluorescence (XRF). The inorganic components were further analyzed by X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, X-ray diffraction (XRD), and Thermogravimetric analysis (TGA). The infrared (IR) spectroscopy was adopted to provide the preliminary information on the chemical groups of Zuota samples. Our results showed that Zuota was mainly an inorganic mixture of HgS, sulfur, and graphite, forming nanoparticles (NPs). The implication to future studies of Zuota as nanomedicine was discussed.

2. Materials and Methods

2.1. Materials

The Zuota samples were kindly provided by four institutes in China. The sources were listed in Table 1. The main protocols in preparing these samples were well documented in the literature [14]. The samples were sealed in plastic tubes and stored in desiccator before characterization.

Table 1: Sources of Zuota samples.
2.2. Measurements

The morphology of Zuota was characterized by SEM (Quanta 200FEG, FEI, the Netherlands) at different magnifications. The elemental compositions of Zuota samples were characterized by XRF (S4-Explorer, Bruker, Germany). The element compositions of Zuota samples were analyzed by XPS (Axis Ultra, Kratos, UK) using an Al anode. During collecting the whole spectra, the scanning step was set as 1000 meV and the power was 150 W. For analyzing each element, the scanning step was set as 100 meV and the power was 150 W. Raman spectra of Zuota samples were recorded on a Renishaw micro-Raman instrument equipped with a Melles-Griot 35 mW He : Ne laser (laser excitation at 633 nm, 1.75 mW power, ×50 objective, laser spot size ~2 μm × 2 μm, and 50 s collection time). The crystallization information of Zuota samples was obtained by XRD (D8 Advance, Bruker, Germany). The scanning was performed under 26°C from the 2θ of 5° to 80° with a step size of 0.02° and a step time of 17.7 s. The weight loss of Zuota samples in air was analyzed by TGA (SDT 2900, Thermal, USA). Around 5 mg Zuota sample was analyzed at a ramp of 10°C/min. The balance gas was nitrogen (40 mL/min) and the sample gas was air (60 mL/min). The samples were extracted by ethanol for 24 h and the supernatant was analyzed on HPLC (Agilent technologies 1200, USA). The chemical groups of Zuota samples were analyzed by IR spectra (Magna-IR 750, Nicolet, USA).

3. Results and Discussion

3.1. Morphology

Zuota samples displayed different morphologies under SEM (Figure 1). Generally, Zuota samples were majorly composed of very small NPs with other morphologies [24, 25]. The diameters of these NPs varied among the four samples; in the range of 10–50 nm. Zuota NPs further assembled into larger particles. The aggregation of NPs was the characteristics of Zuota samples, thus, we did not disperse the samples before analyses. Interestingly, in S2 and S4, there was another morphology observed, seeming like sludge. Zuota NPs were decorated on top of the sludge. This might indicate that the crystallization of S2 and S4 was not completed yet.

Figure 1: SEM images of Zuota samples (×300). (a) S1; (b) S2; (c) S3; (d) S4. The sects were images of higher magnification (×50,000).

The finding of NPs in Zuota samples was kind of surprise. We did not suggest that Tibetan had developed this “nanomedicine” in purpose. However, this nanomorphology might provide a possible route to explain the magic effects of Zuota, since NPs have been proven to have different pharmaceutical effect comparing to the bulk ones [2729]. The nanomorphology might also provide a simple route to prepare analogues of Zuota. Scientists have developed various techniques to prepare NPs. The modern techniques hold great potential in facilitating the preparation of Zuota-like NPs, which will definitely promoting the production and applications of Zuota. Thus, the chemical components of Zuota need to be distinguished and quantified.

3.2. Element Concentrations

The compositions from XRF were expressed as the percentages of elements. As shown in Table 2, the most abundant component was S and Hg. There were about 50 wt% of S and 50% of Hg in the samples. S2 had the lowest S content, namely, 42.4%. S2 and S4 had higher Hg contents (55.3 wt% for S2 and 50.3 wt% for S4). Other elements, including Mg, Al, Si, K, Ca, Fe, Cu, Zn, Rb, Sn, and Pb, were also detected with much lower contents. It is worth to note that XRF provides the result of deep scanning, which stands for the average information of sample. When converting the weight ratio to atom ratio, Zuota samples showed very high S : Hg ratios (6.8, 4.8, 7.1, and 5.8 for S1–4, resp.). For the compounds of Hg and S, the common forms were HgS and Hg2S. Thus, there was excess S existing in Zuota samples, which might be in the form of pure sulfur. Actually, when performing the Brunauer-Emmett-Teller (BET) analyses, the pure sulfur evaporated during the pretreatment. Light yellow powders formed during the evaporation, which indicated the existence of pure sulfur. The abundant Hg and S detected in Zuota samples were consistent with the chemical composition of starting materials, mercury, and sulfur.

Table 2: Element concentrations in Zuota samples. The data were obtained from XRF and expressed as percentage of weight.

3.3. Inorganic Components

According to XRF, Zuota samples were mostly inorganic. We further utilized several techniques to characterize the inorganic components of Zuota samples. Unlike XRF, XPS only detects the surface (several nanometers in depth) and provides the surface compositions (except H). As shown in Figure 2 and Table 3, several light elements were detected (C, N, and F). Other elements, such as O, Hg, Se, and S, were identified in Figure 2. Obviously, the element species in the surface were different from those in the whole sample. Again, Zuota samples showed high S : Hg ratios (2.6, 2.4, 2.2, and 2.5 for S1–4, resp.). These values were much lower than those from XRF. Nevertheless, S was still excess, implying the existence of pure sulfur.

Table 3: Element contents in Zuota samples. The data were obtained from XPS and expressed as percentage of weight.
Figure 2: Full XPS spectra of Zuota samples. (a) S1; (b) S2; (c) S3; (d) S4.

XPS is powerful in characterizing the chemical state of element. As shown in Figure 3, two typical peaks were observed in the Hg4f spectra. The peak around 100 eV indicated the Hg4f5/2. The peak around 104.5 eV was the Hg4f7/2 XPS peak. The intensities of the four samples have nearly identical patterns. According to the literature, the enhanced doublet indicated that Hg was oxidized [30]. Therefore, we inferred that Hg was in the form of HgS, which was consistent with the XRD results.

Figure 3: Hg4f spectra of Zuota samples. (a) S1; (b) S2; (c) S3; (d) S4.

Two groups of S2p peaks were observed in the S2p spectra (Figure 4). In each group, the area of the bigger one was set as twice of that of the smaller one. The binding energy of the bigger one was set as 1.2 eV lower than that of the smaller one [31]. The peaks around 161.5 and 162.7 eV indicated the S2−, for example, HgS. The peaks around 163.0 and 164.2 eV stood for pure sulfur. The components of the four samples showed similar patterns. S2 had the lowest pure sulfur, while S1 had the highest. The content of S2− was higher than that of Hg, which implied that there might be other sulfides in Zuota samples.

Figure 4: S2p spectra of Zuota samples. (a) S1; (b) S2; (c) S3; (d) S4.

Another element detected in XPS was C, which might be introduced by heating and carbonizing of the organic starting materials. Clearly, some C atoms were oxidized (Figure 5). S3 had the highest oxidative degree. 40.96% of C was oxidized to form C–O bonds. 12.01% of C atoms in S3 was further oxidized to form C=O bonds. S2 and S4 were less oxidized. More than 60% of C in S2 and S4 remained at 0 valent. Apparently, the C1s XPS spectra of Zuota samples were very similar to those of sp2 carbon materials, such as graphene oxide and carbon NPs [32, 33]. Hence, we adopted Raman spectroscopy to characterize the graphite structure in Zuota samples.

Figure 5: C1s spectra of Zuota samples. (a) S1; (b) S2; (c) S3; (d) S4.

Typically, the signal around 1590 cm−1 (G-band) indicates the graphite structure of carbon materials. As shown in Figure 6, there were meaningful signals around 1590 cm−1 in all Zuota samples. S3 had the highest C content and showed more distinguishable G-band in Raman spectrum. The G-band signals were recognized in S1, S2, and S4, but the relative intensities were lower. Another typical signal of graphite is D-band. D-band was observed in all Zuota samples. It is well known that D : G ratio is an indicator for the integrity of graphite structure. S1 had the highest D : G ratio, indicating that the graphite structure was the most disordered one. There might be more defects in S1. S3 showed the lowest D : G ratio, suggesting that graphite structure was more integrated in S3.

Figure 6: Raman spectra of Zuota samples. The position of 1590 cm−1 was indicated by the dash line.

The abundant inorganic components in Zuota samples were evidenced in the XRF and XPS analyses. Such components might be in the form of crystals. Therefore, we collected the crystallization information of Zuota samples by XRD (Figure 7). Despite the detailed information, there were two distinct characteristics reflected by the XRD analyses. Firstly, XRD confirmed the existence of pure sulfur in Zuota samples. All samples contained pure sulfur with orthorhombic crystalline. The sulfur crystals were all face-centered. The cell parameters were as follows: , , , and . Secondly, S2 had different HgS crystalline, namely, hexagonal crystalline. HgS in S2 was primitive hexagonal crystal. The cell parameters were as follows: , , , and . The others (S1, S3, and S4) were all face-centered cubic crystalline. The cell parameters were as follows: and . The results here suggested that although all samples were the so-called Zuota, they did have significant difference in physiochemical properties. The different crystalline structures might come from the various processing protocols, because each institute has its own tricks in processing Zuota beyond sharing the same major steps. The influence of such difference on the bioactivity of Zuota samples is worthwhile to investigate in the future.

Figure 7: XRD spectra of Zuota samples.

The existence of HgS and pure sulfur was complementarily reflected by TGA. As shown in Figure 8, all samples had two stages of weight loss. The first stage occurred at about 225°C and the second stage occurred at 350°C. It is very hard to make an accurate conclusion on what were burned during the TGA analyses. Probably, S and C were burned at the first stage, where the temperature was quite close to the burning point of S (about 232°C). Hg was evaporated at the second stage (close to the boiling point of Hg). The ratio of the two stages was very close to the weight ratio of S : Hg in XRF analyses (Table 2). Some other metals were left in the ash (less than 10%).

Figure 8: TGA curves of Zuota samples. (a) S1; (b) S2; (c) S3; (d) S4.
3.4. IR Spectra

Although the majority of Zuota samples were inorganic substances, we did observe some organic substances in the HPLC (Figure 9). The existence of organic substances in Zuota was reasonable, because many animal and plant drugs are added during the production of Zuota [14, 23]. However, the system was too complicated to analyze the organic substances in detail [23]. As a preliminary characterization, we used IR spectroscopy to explore the potential organic components (Figure 10). There were very tiny peaks observed at 2923 cm−1. This indicated that there were C–H bonds in Zuota samples. Interestingly, S3 had a broad band at 3363 cm−1, which suggested the existence of –OH or –COOH. This was consistent with the C1s XPS spectrum, that S3 showed the highest oxidation degree. It seems to us that S3 had more H atoms, since the stronger band at 3363 cm−1 and peak at 2923 cm−1 were presented. The peaks around 1620 cm−1 were assigned to C=O bonds. The signals around 1000 cm−1 might indicate the C–O bonds. The peaks at about 717 cm−1 (S1, S2, and S4) and at 840 cm−1 (S2 and S3) might be the C–H bonds on aromatic rings. Indeed, such characterization was very preliminary. For future studies, detailed characterizations of the organic components of Zuota samples are highly encouraged.

Figure 9: HPLC analyses of organic matters in Zuota samples.
Figure 10: IR spectra of Zuota samples.

4. Conclusion

Overall, we characterized four Zuota samples from different sources. Zuota samples were found to be inorganic NPs. The existence of graphite, sulfur, and HgS was confirmed by multiple techniques. Referring to IR spectra, there are also some functional groups containing oxygen atoms and hydrogen atoms in Zuota samples. We believe that our results will benefit the understanding of Zuota from modern scientific perspective. The nanoeffect of Zuota NPs and the analogues of Zuota NPs by nanotechnology are highly encouraged to investigate in future.


The authors thank Dr. Xiangwei He at Tsinghua University for his help in instruments and data analyses. They thank the financial support from China Natural Science Foundation (no. 81173653), the China Ministry of Science and Technology (no. 2012BAI27B07), the National Major Scientific and Technological Special Project of the State Ministry of Science and Technology (2011ZX09307-002-01), and the Research Funds of Southwest University for Nationalities (13NLY01).


  1. E. Žuškin, J. Lipozenčić, J. Pucarin-Cvetković et al., “Ancient medicine A,” Acta Dermatovenerologica Croatica, vol. 16, no. 3, pp. 149–157, 2008. View at Google Scholar · View at Scopus
  2. Y. Yutuo, Medical Canon in Four Sections (Si Bu Yi Dian, Chinese Version), translated by Y. Li, People’s Publishing House, Beijing, China, 1983.
  3. D. Luo, New Repair Jingzhu Materia Medica (Jing Zhu Ben Cao, Chinese Version), translated by J. Mao D. Luo, Z. Wang, and S. Ma, Shanghai Science and Technology Press, Shanghai, China, 1986.
  4. S. Wang, “Clinical observation of Tibetan drug with Songshi-25 pill for chronic hepatitis B,” China Journal of Chinese Materia Medica, vol. 5, no. 4, p. 10, 1999. View at Google Scholar
  5. B. Yang, Y. Zhang, T. Huang et al., “Clinical observation of chronic superficial gastritis (Syndrome differentiation of TCM Being hepatic Qi attacking stomach) Tibetan drug Zuo-ta-de-zi-ma, attaching with clinical report of 149 cases,” Journal of Chengdu University of Traditional Chinese Medicine, vol. 24, no. 1, pp. 14–16, 2001. View at Google Scholar
  6. M.-C. Wan, J.-S. Lou, and Y.-W. Qin, “Application of Rannasangpei in the treatment of cardiovascular and cerebrovascular diseases,” Pharmaceutical Care and Research, vol. 9, no. 4, pp. 283–285, 2009. View at Google Scholar · View at Scopus
  7. C. Zhang, X. Pan, W. Qiu, B. Shi, J. Chang, and Y. Ni, “59 clinical reports of Tibetan drug with Songshi-25 pill for chronic hepatitis B,” Chinese Journal of Integrated Traditional and Western Medicine on Liver Diseases, vol. 9, no. 3, pp. 28–29, 1999. View at Google Scholar
  8. C. Limao and J. Qie, “62 clinical reports of dispersing stagnated liver qi for regulating stomach and detoxification for chronic gastritis treatment,” Chinese Medicine Modern Distance Education of China, vol. 7, no. 11, pp. 88–89, 2009. View at Google Scholar
  9. J. Yang, J. Deng, D. Li, and J. Li, “Jintang village investigation: one consideration about the cause of Pa-disease,” Chinese Journal of Endemiology, vol. 19, no. 1, pp. 36–40, 2000. View at Google Scholar
  10. Y. Zhang and Y. Xiang, “To analyze the usage of toxicant in traditional Tibetan medicine based on 64 dermatological formulas by oral administration,” World Science and Technology/Modernization of Traditional Chinese Medicine and Materia Medica, vol. 7, pp. 64–66, 2005. View at Google Scholar
  11. B. Yang, “The pharmacology and safety of mercury in Tibetan medicine Zuota,” Tibet Research, no. 1, pp. 74–80, 2004. View at Google Scholar
  12. Z. Chen, X. Pu, W. Li, K. Wu, G. Lan, and J. Cui, “Influence of Tibetan medicine Zuota on the life of fruit fly,” Lishizhen Medicine and Materia Medica Research, vol. 22, no. 2, pp. 422–423, 2011. View at Google Scholar
  14. L. Suo, “Processing of Zuota,” Journal of Medicine & Pharmacy of Chinese Minorities, no. 5, p. 40, 2007. View at Google Scholar
  15. B. A. Bowman and R. M. Russell, Present Knowledge in Nutrition, The International Life Sciences Institute, Washington, DC, USA, 9th edition, 2006.
  16. J. M. Llobet, G. Falcó, C. Casas, A. Teixidó, and J. L. Domingo, “Concentrations of arsenic, cadmium, mercury, and lead in common foods and estimated daily intake by children, adolescents, adults, and seniors of Catalonia, Spain,” Journal of Agricultural and Food Chemistry, vol. 51, no. 3, pp. 838–842, 2003. View at Publisher · View at Google Scholar · View at Scopus
  17. G. Dou, “Analysing the toxicity of Tibetan pharmacist-prepared medicine in “Zuo Tai” as well as its compatibility,” Journal of Qinghai Junior Teachers’ College, no. 4, pp. 72–75, 2005. View at Google Scholar
  18. X. Li, W. Feng, Q. Ma, J. Zhu, C. Nima, and R. Ge, “Preliminary study on the absorption and excretion of mercury in Tibetan medicine Zuotai in rats,” Lishizhen Medicine and Materia Medica Research, vol. 21, no. 2, pp. 290–292, 2010. View at Google Scholar
  19. A. Jiang, C. Zhang, J. Wang et al., “Pharmacodynamics research of Tibetan medicine Zuotai,” Lishizhen Medicine and Materia Medica Research, vol. 20, no. 8, pp. F3–F4, 2009. View at Google Scholar
  20. X.-Y. Li, W.-L. Feng, J.-B. Zhu, C.-R. Nima, and R.-L. Ge, “Effect of Tibetan medicine Zuotai on in vivo pharmacokinetics of crocin-1 in rats,” Chinese Traditional and Herbal Drugs, vol. 40, no. 9, pp. 1425–1428, 2009. View at Google Scholar · View at Scopus
  21. Y. Zeng, Y. He, Y. Liu, Z. Wang, and Y. Zhang, “Pharmacological effect of Tibetan medicine Zuota to central nervous system,” Journal of Sichuan of Traditional Chinese Medicine, no. 11, pp. 36–37, 2005. View at Google Scholar
  22. Y. Zeng, P. Wang, Y. He, and Y. Zhang, “Determination of Hg in Zuota (a Tibetan medicine) by HPLC,” Central South Pharmacy, vol. 6, no. 1, pp. 60–63, 2008. View at Google Scholar
  23. G. Lan, Z. Chen, W. Li, K. Wu, J. Cui, and L. Piao, “Analysis on chemical constituents of Tibetan medcine Zogta,” Lishizhen Medicine and Mataeria Medica Research, vol. 21, no. 12, pp. 3209–3211, 2010. View at Google Scholar
  24. L. Yan, “Microstructure and component of Tibetan medicine Zuotai,” China Tibetology, no. 3, pp. 150–152, 2007. View at Google Scholar
  25. L. Yan, X. Ma, and Q. Zhu, “Inorganic components analysis of Tibetan medicine Zuotai,” China Journal of Chinese Materia Medica, vol. 32, no. 2, pp. 159–160, 2007. View at Google Scholar
  26. L. Yan and X. Ma, “The chemical and structural analysis of carbon in Ttibetan medicine Zuotai,” Chinese Journal of Ethnomedicine and Ethnopharmacy, vol. 19, no. 5, pp. 1–2, 2010. View at Google Scholar
  27. T. L. Doane and C. Burda, “The unique role of nanoparticles in nanomedicine: Imaging, drug delivery and therapy,” Chemical Society Reviews, vol. 41, no. 7, pp. 2885–2911, 2012. View at Publisher · View at Google Scholar · View at Scopus
  28. M. E. Lobatto, V. Fuster, Z. A. Fayad, and W. J. M. Mulder, “Perspectives and opportunities for nanomedicine in the management of atherosclerosis,” Nature Reviews Drug Discovery, vol. 10, no. 11, pp. 835–852, 2011. View at Publisher · View at Google Scholar · View at Scopus
  29. R. Mout, D. F. Moyano, S. Rana, and V. M. Rotello, “Surface functionalization of nanoparticles for nanomedicine,” Chemical Society Reviews, vol. 41, no. 7, pp. 2539–2544, 2012. View at Publisher · View at Google Scholar · View at Scopus
  30. E. Sasmaz, Fundamental understanding of mercury removal from coal combustion [Ph.D. dissertation], Stanford University, 2011.
  31. G. Hota, S. B. Idage, and K. C. Khilar, “Characterization of nano-sized CdS-Ag2S core-shell nanoparticles using XPS technique,” Colloids and Surfaces A, vol. 293, no. 1-3, pp. 5–12, 2007. View at Publisher · View at Google Scholar · View at Scopus
  32. S.-T. Yang, S. Chen, Y. Chang, A. Cao, Y. Liu, and H. Wang, “Removal of methylene blue from aqueous solution by graphene oxide,” Journal of Colloid and Interface Science, vol. 359, no. 1, pp. 24–29, 2011. View at Publisher · View at Google Scholar · View at Scopus
  33. S. Yang, J. Luo, J. Liu et al., “Carbon nanoparticles for cationic dye (methylene blue) removal from aqueous solution,” Nanoscience and Nanotechnology Letters, vol. 4, no. 8, pp. 839–842, 2012. View at Google Scholar