Rapid microwave synthesis of fluorescent hydrophobic carbon dots

Abstract

Fluorescent hydrophobic carbon dots (HCDs) have been fabricated from a poloxamer by a simple microwave assisted process within a few minutes for the first time. Small and distinct spherical HCDs emitted bright blue and green fluorescent light depending upon the excitation wavelength and were dispersed easily in a wide variety of organic solvents. HCDs exhibited excellent water resistant behavior with a contact angle of ∼122° enforcing good hydrophobic character which will be quite useful in potential surface applications.

---

Carbon dots (CDs) constitute an important class of carbon nano-structures owing to their good photoluminescence (PL) properties, high stability without photobleaching, tunable PL emission depending on the excitation wavelength and obviously low toxicity in comparison to other quantum dots. Significant efforts have been paid towards the synthesis of various functionalized and non-functionalized CDs, which include microwave irradiation of sucrose¹ and poly(ethylene glycol);^2,3 combustion of carbon soot,⁴ activated carbon⁵ and carbon xerogel;⁶ laser ablation of graphite⁷ and degradation of polysaccharides.^8,9 Strikingly the CDs reported so far are hydrophilic in nature and therefore suitable for bioimaging,^3,10 biosensing,¹¹ homogeneous catalysis² and photocatalyst design.¹² The unique functional properties prompted researchers to design hydrophobic or organophilic CDs.^13–17 Of late nature inspired hydrophobic surface tailoring is being hotly pursued for industrial applications,¹⁸ self-cleaning behavior¹⁹ and suppression of chemical reactions on the surface. In particular, a hydrophobic fluorescent probe can be useful in labelling hydrophobic microdomains²⁰ and at the same time exhibiting water resistance. So far Nile red is the widely used hydrophobic fluorescent probe to mark hydrophobic microdomains in bacteria.²⁰ However Nile red can undergo photobleaching,²¹ therefore hydrophobic quantum dots will be an exclusive and suitable alternative to organic fluorescent probes, capable of exhibiting fluorescent behavior without photobleaching and resistant to removal in the presence of moisture or water. Bourlinos et al. have reported organophilic CDs made by using long chain amines with citric acid¹³ or citrate salts;¹⁴ a similar methodology was addressed by Wang et al.¹⁵ and later on their follow-up study reported organosilane functionalized CDs for designing hydrophobic films and monoliths.¹⁶ Long times and difficult synthesis routes^13–15 limit their applications, while the surface silane groups with high absorptive properties may be solely responsible for organophilic behavior in the latter¹⁶ as described in the literature.²² We have designed fluorescent hydrophobic carbon dots (HCDs) emitting multiple ‘nano-lights’ under different excitation wavelengths with strong water repellent behavior within a few minutes by microwave assisted fabrication for the first time. Herein, we report a simple microwave assisted synthesis of HCDs from a poloxamer, polyoxyethylene-polyoxypropylene-polyoxyethylene (PEO-PPO-PEO) block co-polymer pluronic F-68 (PF-68). Use of multistep functionalization and expensive precursors can be avoided; to the best of our knowledge straightforward microwave synthesis has been introduced for the first time for the production of HCDs.

HCDs were synthesized by microwave pyrolysis of PF-68 in the presence of o-phosphoric acid in a domestic microwave oven for 4 min at 450 W. The product was extracted using toluene (detailed in ESI†) and isolated accordingly. Complete removal of phosphoric acid was confirmed from ³¹P NMR spectra, and no phosphate was detected; therefore HCDs were isolated in pure form from the reaction mixture. Fig. 1a showed the normalized UV-Vis absorbance and PL spectra of HCDs in toluene. A peak at 285 nm in the UV-Vis indicated the formation of carbon dots which corroborated the other experimental results.¹ Meanwhile the PL spectrum was recorded at 380 nm excitation wavelength exhibiting a broad peak with emission maximum centred at 442 nm. Fig. 1b shows the PL spectra of HCDs at different excitation wavelengths starting from 330 nm to 430 nm in which maximum PL intensity was observed at 380 nm excitation wavelength. A gradual red shifted emission was observed with decreasing PL intensity. Corresponding normalized PL spectra of HCDs in toluene are shown in Fig. S1, ESI.† Similar to toluene, HCDs were highly dispersed in chloroform as well and the corresponding PL spectrum at different excitation wavelengths is shown in Fig. S2, ESI† with a red shifted pattern. Therefore λ_ex-dependent λ_em was noted in both the cases, which is a characteristic feature of semiconductor CDs as previously reported by Sun et al. and others.^23,24 The excitation wavelength dependent emission wavelength and intensity were attributed to quantum effects and/or emissive traps present on their surface owing to the presence of functional groups.⁹ At certain excitation wavelengths, some emissive sites would be excited and fluoresce, giving rise to excitation-dependent emission spectra.²⁵

Fig. 1 (a) UV-Vis absorbance and normalized PL spectra of HCDs at 380 nm excitation wavelength; (b) PL spectra of HCDs at different excitation wavelengths.

Fig. 2a and 2b shows the low and high resolution transmission electron microscopy (TEM) images of HCDs, which indicate that the particle sizes were small ranging from 5 nm to 20 nm, and they were completely spherical in nature. Partial self-passivation would lead to variation in sizes however a distinct spherical morphology was maintained throughout. A height profile of HCDs was obtained from atomic force microscopy (AFM) which also showed distinct spherical particles with an average height of ∼5 nm as shown in Fig. 2c. Surface functionality of HCDs was clarified from the FTIR spectra (Fig. S3, ESI†) which suggested that the carbon dots were comprised of –OH groups (3423 cm⁻¹), C–H groups (2916 cm⁻¹ and 2852 cm⁻¹, asymmetric and symmetric stretching), carbonyl (1638 cm⁻¹), C–O and C–O–C (1210 cm⁻¹)¹ groups. An X-ray diffraction (XRD) pattern of HCDs (Fig. S4, ESI†) showed a single broad peak centred at 2θ = 23.7° indicating the [002] plane of carbon with poor crystallinity or nearly amorphous nature.^2,25 The selected area electron diffraction pattern (SAED) pattern (Fig. S5, ESI†) of HCDs is in agreement with these results.

Fig. 2 (a) Low and (b) high resolution TEM images of HCDs; (c) AFM image of HCDs with the associated height profile.

Although the synthesized CDs appeared as a brownish-black precipitate in water, they were immediately extracted into organic solvents and therefore the hydrophobicity of such CDs was illustrated simply with the ease of dispersion in different organic solvents. Fig. 3a displays the digital images of HCDs in toluene–water and chloroform–water systems. Under normal light both the toluene (1) and chloroform (2) layers exhibited brown color but under exposure to UV light the layers turned bright green (3, 4). However no significant green coloration was observed from the aqueous layers. Even simple water droplets appeared to be quasi-spherical when they were placed on a glass slide drop-cast with HCDs providing a hydrophobic surface. A digital image of water droplets on a HCD-treated glass surface is shown in Fig. 3b. HCDs were highly dispersed in a wide variety of organic solvents including acetone, ethanol, chloroform, toluene, THF, NMP, hexane, cyclohexane, DMF and acetonitrile. The dispersions were stable for a long time without any precipitation, except in acetonitrile where HCDs were partially precipitated. Fig. S6, ESI† shows the digital image of HCDs dispersed in different organic solvents and corresponding normalized PL spectra are shown in Fig. S7, ESI† at 380 nm excitation wavelength. The mechanism of formation and the origin of PL had been matters of debate; it was speculated that radiative recombination of excitons,^24,26 emissive traps,²⁴ free zig-zag sites,²⁶ quantum confinement²⁷ and defect sites during incorporation of functionality⁹ were responsible for the PL properties of such carbogenic dots. In this case, HCDs were probably formed by fragmentation of PF-68 forming the carbon core, followed by mild partial passivation via condensation with hydroxyl groups of fragmented or un-fragmented PF-68 under microwave heating conditions forming C–O–C bonds, which is known to occur with active hydroxyl end groups.²⁸ Remaining marginal hydroxyl and carbonyl groups on the surface of those HCDs resulted in the formation of defect sites⁹ and emissive traps²³ which contributed to its PL properties. Radiative recombination of excitons trapped within the defects^9,23 produced the most intense PL band and consequently its strong PL. On the other hand, reduced surface hydroxyl groups along with hydrophobic long chains of fragmented or un-fragmented PF-68²⁸ resulted in its hydrophobic character. This speculated mechanism is significantly similar to the other organophilic carbon dots where long chain amines contributed to passivation after amide bond formation on the carbon dot.^14,16 However our microwave-assisted approach was undoubtedly convenient to produce hydrophobic dots and unique as well in comparison to the methods reported so far (see Table 1, ESI†). Once more selection of precursor confirmed that the good PL of HCDs was solely due to the formation of carbon dots, not due to organic fluorophoric compounds such as pyrene.¹⁴ HCDs exhibited bright blue and green fluorescence under a fluorescence microscope at two different excitation wavelengths, as shown in Fig. S8, ESI.† The fluorescence decay profile (Fig. S9, ESI†) of HCDs was measured at the emission maximum. The average lifetime was found to be 2.13 ns with a tri-exponentially fitting decay profile.²⁹ Such typical lifetime decay may be explained by the localized conduction band electrons in shallow trap states,³⁰ fast band gap transition and oxygen related emission.²⁵ Short lifetime of HCDs⁸ also indicated radiative recombination of excitons^23,24 giving rise to fluorescence during passivation. All the data are shown in ESI (Table 2†). Photo-stability measurements (Fig. S10, ESI†) revealed that HCDs were exceptionally stable without photobleaching. Fluorescence intensity of HCDs was almost unperturbed for about 420 min during the experiment under photo-irradiation conditions. PL quantum yield (QY) of HCQDs was found to be 7% at 380 nm excitation wavelength in comparison to quinine sulfate chosen as the standard. A plot of PL QY is shown in Fig. S11, ESI† and detailed calculations are summarized in ESI.†

Fig. 3 HCDs in toluene (1) and chloroform (2) under normal light exhibiting brown color, and bright green fluorescence of toluene (3) and chloroform (4) layers under UV light; (b) water droplets on a HCD-treated glass slide.

HCDs found potential applications in fabrication of fluorescent hydrophobic surfaces. A HCD layer on a glass slide altered its properties, where water droplets appeared to be quasi-spherical. The wettability of the surface was governed by the contact angle measurement as shown in Fig. 4. A regular hydrophilic glass slide exhibited a low contact angle with spreading of water droplets on it; however a HCD drop-cast glass slide exhibited a remarkable enhancement in water contact angle up to ∼122°, resulting in good hydrophobic character³¹ and excellent water resistance (Fig. S12, ESI†). Detailed contact angle measurements are shown in Table 4 (ESI†).

Fig. 4 Water contact angle measurement on HCD-treated glass slide.

In summary, hydrophobic carbon dots were fabricated from a poloxamer, which exhibited bright blue and green fluorescent light under different excitation wavelengths. Small spherical HCDs evolved nanolight with high QY and could be used to fabricate fluorescent hydrophobic surfaces with excellent water resistance. So far biocompatible hydrophilic CDs have been used for applications; however HCDs are in early stages and might require suitable modifications for biology based applications, which are in progress. Nevertheless, the unique hydrophobicity together with good PL would open the door for potential surface applications.

The author would like to acknowledge DBT-GOI, ICAR-NAIP, ICAR-National Fund, ISI plan project and TIFAC for financial help. SM and TK are thankful to CSIR (New Delhi) for financial help throughout the work.

References

1. H. Zhu, X. Wang, Y. Li, Z. Wang, F. Yang and X. Yang, Chem. Commun., 2009, 5118.
2. S. Mitra, S. Chandra, P. Patra, P. Pramanik and A. Goswami, J. Mater. Chem., 2011, 21, 17638.
3. A. Jaiswal, S. S. Ghosh and A. Chattopadhyay, Chem. Commun., 2012, 48, 407.
4. H. Liu, T. Ye and C. Mao, Angew. Chem., Int. Ed., 2007, 46, 6473.
5. Y. Dong, N. Zhou, X. Lin, J. Lin, Y. Chi and G. Chen, Chem. Mater., 2010, 22, 5895.
6. S. Chandra, S. Mitra, D. Laha, S. Bag, P. Das, A. Goswami and P. Pramanik, Chem. Commun., 2011, 47, 8587.
7. L. Gao, X. Wang, M. J. Meziani, F. Lu, H. Wang, P. G. Luo, Y. Lin, B. A. Harruff, L. M. Veca, D. Murray, S. Y. Xie and Y. P. Sun, J. Am. Chem. Soc., 2007, 129, 11318.
8. S. Chandra, S. H. Pathan, S. Mitra, B. H. Modha, A. Goswami and P. Pramanik, RSC Adv., 2012, 2, 3602.
9. Y. Yang, J. Cui, M. Zheng, C. Hu, S. Tan, Y. Xiao, Q. Yang and Y. Liu, Chem. Commun., 2012, 48, 380.
10. S. Chandra, P. Das, S. Bag, D. Laha and P. Pramanik, Nanoscale, 2011, 3, 1533.
11. W. Shi, Q. Wang, Y. Long, Z. Cheng, S. Chen, H. Zheng and Y. Huang, Chem. Commun., 2011, 47, 6695.
12. H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C. H. A. Tsang, X. Yang and S. T. Lee, Angew. Chem., Int. Ed., 2010, 49, 4430.
13. A. B. Bourlinos, A. Stassinopoulos, D. Anglos, R. Zboril, V. Georgakilas and E. P. Giannelis, Chem. Mater., 2008, 20, 4539.
14. A. B. Bourlinos, A. Stassinopoulos, D. Anglos, R. Zboril, M. Karakassides and E. P. Giannelis, Small, 2008, 4, 455.
15. F. Wang, S. Pang, l. Wang, Q. Li, M. Kreiter and C. Y. Liu, Chem. Mater., 2010, 22, 4528.
16. F. Wang, Z. Xie, H. Zhang, C. Liu and Y. Zhang, Adv. Funct. Mater., 2011, 21, 1027.
17. A. Rahy, C. Zhou, J. Zheng, S. Y. Park, M. J. Kim, I. Jang, S. J. Cho and D. J. Yang, Carbon, 2012, 50, 1298.
18. T. Kako, A. Nakajima, H. Irie, Z. Kato, K. Uematsu, T. Watanabe and K. Hashimoto, J. Mater. Sci., 2004, 39, 547.
19. X. Feng and L. Jiang, Adv. Mater., 2006, 18, 3063.
20. F. Aldeek, C. Mustin, L. Balan, T. Roques-Carmes, M. P. Fontaine-Aupart and R. Schneider, Biomaterials, 2011, 32, 5459.
21. A. L. McIntosh, B. P. Atshaves, H. Huang, A. M. Gallegos, A. B. Kier, F. Schroeder, H. Xu, W. Zhang, S. Wang and J. C. Liu, Methods in Molecular Biology, 2007, 398, Humana Press Inc., Totowa, NJ.
22. H. Sayilkan, S. Erdemoglu, S. Sener, F. Sayilkan, M. Akarsu and M. Erdemoglu, J. Colloid Interface Sci., 2004, 275, 530.
23. Y. P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. Wang, P. G. Luo, H. Yang, M. E. Kose, B. Chen, L. M. Veca and S. Y. Xie, J. Am. Chem. Soc., 2006, 128, 7756.
24. S. N. Baker and G. A. Baker, Angew. Chem., Int. Ed., 2010, 49, 6726.
25. L. Bao, Z. L. Zhang, Z. Q. Tian, L. Zhang, C. Liu, Y. Lin, B. Qi and D. W. Pang, Adv. Mater., 2011, 23, 5801.
26. J. Zhang, W. Shen, D. Pan, Z. Jhang, Y. Fang and M. Wu, New J. Chem., 2010, 34, 591.
27. S. Liu, J. Tian, L. Wang, Y. Luo and X. Sun, RSC Adv., 2012, 2, 411.
28. T. Wang, Y. Wu and A. J. Zeng, J. Appl. Polym. Sci., 2010, 117, 604.
29. H. X. Zhao, L. Q. Liu, Z. D. Liu, Y. Wang, X. J. Zhao and C. Z. Huang, Chem. Commun., 2011, 47, 2604.
30. I. S. Liu, H. H. Lo, C. T. Chien, Y. Y. Lin, C. W. Chen, Y. F. Chen, W. F. Su and S. C. Liou, J. Mater. Chem., 2008, 18, 675.
31. P. F. Rios, H. Dodiuk, S. Kenig, S. Mccarthy and A. Dotan, J. Adhes. Sci. Technol., 2007, 21, 399.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c2ra21048g

---