Surfactant-Derived Amphiphilic Carbon Dots with Tunable

Article pubs.acs.org/JPCC

Surfactant-Derived Amphiphilic Carbon Dots with Tunable Photoluminescence Ondřej Kozák, Kasibhatta Kumara Ramanatha Datta, Monika Greplová, Václav Ranc, Josef Kašlík, and Radek Zbořil* Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký University in Olomouc, 17. listopadu 1192/12, 771 46 Olomouc, Czech Republic S Supporting Information *

ABSTRACT: We present a new class of fluorescent carbon dots (CDs) prepared hydrothermally from cationic surfactant cetylpyridinium chloride (CPC). Because of the high carbon content, amphiphilicity, and the presence of a heteroaromatic π system, CPC acts as a carbon source, stabilizing agent, and contributing fluorophore in the prepared CDs-based system. The surfactantderived carbon dots exhibit amphiphilicity, tunable blue−green−yellow photoluminescence dependent upon the solvent polarity, reaction conditions, and excitation wavelength, excellent long-term colloidal and photostability, and a large-scale synthesis potential. The reported findings open the doors for the applicability of surfactants as a carbon source for nanosystems with controllable photoluminescence and amphiphilicity.

1. INTRODUCTION Carbogenic nanoparticles also known as carbon dots (CDs) are one of the fascinating carbon nanomaterials and the scope of many research groups in the past decade.1,2 The intensive study of CDs is conducted with the focus on finding an alternative to their photoluminescent counterparts commonly used for various applications, such as bioimaging,3 solar light harvesting,4 chemical or biochemical analysis,5 photocatalysis,6 optoelectronics,7 etc. Because of their remarkable properties, such as excellent chemical and photostability, biocompatibility, easy functionalization, and upconversion photoluminescence, CDs are promising candidates compared to other fluorescent materials, e.g., organic dyes and inorganic quantum dots.8,9 CDs are generally synthesized by top-down or bottom-up methods. Among many approaches, thermal treatment of different carbon sources provides an effective way to prepare CDs with a wide range of attributes.10−13 The various properties of CDs might be inherited from precursors, achieved by adjusting the carbonization conditions or by subsequent functionalization.1 Hydrothermal carbonization belongs to the most popular synthetic methods for CD production due to its ease, high effectiveness, and inexpensiveness.14−16 Development of new hydrothermal methods using versatile precursors is an attractive strategy to synthesize functional CDs with diverse physical properties. Surfactants in materials science play multiple roles in stabilizing a variety of nanomaterials or formation of various inorganic and nanoporous structures.17,18 Because of the high carbon content, surfactants can also be a good starting compound for the preparation of CDs. However, to the best of our knowledge, there are no reports on using a surfactant © 2013 American Chemical Society

itself as a precursor for the preparation of carbon dots to date. The use of surfactants as a carbon source was motivated by their applicability as nanoparticle stabilizing shells that might be hydrothermally turned into carbon shells, enhancing the photoluminescence (PL) and/or amphiphilicity of the formed materials. With a focus on fundamental physicochemical properties of surfactant-derived nanosystems, carbon dots emerged to be the first step in our research concerns. Mostly, using a combination of precursors,19 severe acidic treatment20 or consequential surface passivation21 are required for obtaining fluorescent and functionalized carbon dots. In this respect, surfactants as multifunctional compounds with a high carbon content and heteroatom-containing functionalities are promising precursors for the synthesis of CDs. Here, we report, for the first time, the synthesis and characterization of photoluminescent CDs prepared hydrothermally from cationic surfactant cetylpyridinium chloride (CPC). Its triple role in the CD preparation is discussed, and the effects of parameters influencing the PL of CDs are examined. Amphiphilicity and tunable photoluminescence are the highlights of the prepared CDs along with the typical multicolor λex-dependent emission and photo and colloidal stability. Moreover, they exhibit a potential of easy and inexpensive large-scale production. Received: April 23, 2013 Revised: August 29, 2013 Published: November 1, 2013 24991

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2. EXPERIMENTAL SECTION 2.1. Synthesis of CP-CDs. Typically, 0.716 g of cetylpyridinium chloride monohydrate (Sigma-Aldrich) was dissolved in 50 mL of deionized water (Aqual, Czech Republic). The pH was adjusted to ∼11.5 with 1 M NaOH (Lachema, Czech Republic). The surfactant solution was transferred into a stirred Teflon-lined stainless steel autoclave (Parr 4566 mini Reactor, 300 mL) and maintained at 150 °C for 2 h. After cooling the reaction mixture down to room temperature, a brown-colored colloid was obtained. The asprepared sample, denoted as CP-CDs, was directly characterized by TEM, IR, UV−vis, PL, and ζ-potential measurements. For XRD and TGA studies, the as-prepared dispersion was dried at 65 °C to obtain a brown-colored powder. To get rid of the excess surfactant and other water-soluble ions, the prepared carbon dots were also separated and purified twice by a phase transfer to a mixture of acetone and chloroform, details of which are described in the Supporting Information. The purified CP-CDs were then redispersed in different solvents and characterized. 2.2. Characterization of CP-CDs. Transmission electron microscopy (TEM) observations were carried out on a JEOL 2010F microscope operated at 200 kV (LaB6 cathode, resolution 0.19 nm) using a carbon-coated copper grid. X-ray diffraction (XRD) patterns were recorded on an X’Pert PRO MPD diffractometer (PANalytical, Netherlands) using ironfiltered Co Kα radiation (λ = 0.178901 nm, 40 kV, 30 mA). Spectroscopic studies were performed on a Nicolet iS5 infrared spectrometer (Thermo Scientific, U.S.A.), a Specord S 600 UV−vis diode array spectrophotometer (Analytik Jena, Germany), and QuantaMaster 40 and LaserStrobe spectrofluorometers (PTI, U.S.A.), respectively. ζ-potential was measured on a ZEN3600 Zetasizer Nano particle analyzer (Malvern Instruments, U.K.). Thermal gravimetric analysis (TGA) was performed in an argon atmosphere at a heating rate of 2 °C min−1 using an STA 449 C Jupiter analyzer (Netzsch). Elemental analysis was performed using a Flash 2000 Elemental Analyzer (Thermo Fisher Scientific) and BBOT (2,5-bis-(5tert-butyl-benzoxazol-2-yl)-thiophen) as a reference material.

Figure 1. Representative TEM image of CP-CDs with particle size histogram.

Information). They did not show any signs of precipitation even after storing them for a period of 6 months, indicating their long-term colloidal stability. The as-prepared CP-CDs were spectrally compared with the original surfactant (CPC) as well as with the purified CP-CDs (after the removal of the surfactant excess). Infrared spectral characterization (shown in Figure S3 in the Supporting Information) uncovered the presence of functional groups containing N−H bonds, reflected by the spectral bands at 1325 and 1371 cm−1, respectively. Also, the intensive band at 3381 cm−1 is probably due to the presence of N−H bonds. Moreover, nitrogen-containing bonds interpreted as CN are indicated by the spectral bands at around 1641 cm−1. Similar positions of peaks in the spectra of CPC and CP-CDs indicate the presence of unmodified surfactant molecules. This hypothesis was tested by purifying the carbon dots by phase transfer. It was found that the intensity of bands assigned to CN and C−N bonds increased relatively to C−H bands’ intensity, confirming not only the presence of pyridinium rings on CDs but also their different spatial arrangement since a decrease in aliphatic carbon proportion and also a shift of C−N bond-related bands are observed (see Figure S3 in the Supporting Information). Namely, at least some of the pyridinium groups are no longer bound in CPC molecules after carbonization but rather are attached to the CDs’ surface directly or by means of a shorter aliphatic fragment. Importantly, no new distinct absorption bands were detected after carbonization and purification, pointing out that no other surface groups affect the observed optical properties. To obtain further information about the CP-CDs’ composition, thermal gravimetric analysis (TGA) of the dried sample was performed. TGA points out the presence of unbound surfactant (weight loss of 78% in the range of 150−240 °C) and firmly attached surface groups (weight loss of 11% in the range of 240−500 °C); see Figure S4 in the Supporting Information. Moreover, the retention of a heteroaromatic-related absorption band at about 260 nm as well as the high ζ-potential of the purified CPCDs redispersed in water (Figures S5 and S2 in the Supporting Information) shows that inherited pyridinium groups still remain attached to the surface of CP-CDs even after the purifying process. Consequently, further washing procedures were tested, but they led to unwanted changes of the prepared CDs, i.e., excessive aggregation associated with the loss of

3. RESULTS AND DISCUSSION 3.1. Physicochemical Characterization. The TEM image of the as-prepared carbon dots (CP-CDs) shows nearly spherical particles with a mean size of 2 nm (standard deviation of 0.8 nm) (see Figure 1). The dots are finely dispersed with no trace of agglomeration. We found that the hydrothermal treatment of CPC carried out at specified conditions (2 h, 150 °C) yields a mixture of both CP-CDs and unreacted surfactant molecules, as confirmed by several methods (XRD, IR, TGA). On the basis of the TEM results, we are convinced that extra surfactant molecules act as surface stabilizing agents (shells) to prevent carbon dots from being aggregated. Hence, an excess of surfactant is necessary to provide the surface passivation of formed carbon dots. Severe hydrothermal conditions (e.g., 250 °C, 16 h) cause the formation of a reddish precipitate with no PL, implying large-sized and poorly stabilized particles. The purified CP-CDs exhibit broad diffraction peaks of very low intensity in the range of 15−25°, suggesting the presence of rather amorphous carbon (Figure S1 in the Supporting Information). The as-prepared carbon dots showed a high ζpotential of +57.5 mV due to the presence of a positively charged surfactant shell (Figure S2 in the Supporting 24992

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colloidal stability and fluorescence. The elemental analysis of purified CP-CDs showed the following composition (w/w): C, 71.34%; H, 10.91%; N, 3.91%; sum of other elements: 13.84%. The DFT calculations performed for dialkylpyridinium salts by Gupta et al.22 show that the deprotonation in the presence of base takes place preferentially on the α-carbon of the alkyl chain, which is also assumed here; see Figure 2. The process

The blue peak at 475 nm can be assigned to the radiative recombination of localized electron−hole pairs.1,27 This λexdependent emission is predominant when CP-CDs are transferred to chloroform (Figure S6 in the Supporting Information). On the other hand, the green emission centered at 550 nm is inherited from the deprotonated CPC that was demonstrated by measuring the PL spectra of CPC aqueous solution with an excessive amount of NaOH (not shown). This correlates with almost no λex dependency of this band, which generally can be expected for the CD-related emission.1 This emission band is not observed when CP-CDs are transferred into chloroform, suggesting its strong susceptibility to the solvent polarity. It might also be overlapped by the increased blue emission band that is probably partially quenched in water by unreacted CPC molecules. Not surprisingly, the presence of unreacted surfactant molecules proved to be essential for the colloidal stability and fluorescence, both of which decreased considerably after redispersing the purified CP-CDs in water. On the contrary, the blue emission in the organic phase remains unaffected by the standard purifying procedure. Using benzene in hexane as a reference,28,29 the fluorescence quantum yield of the asprepared CP-CDs in water was calculated to be 1.2%, and it increased to 3.5% after transferring them to chloroform. Besides water and chloroform, CP-CDs were redispersed in a range of solvents of different polarity, namely, toluene, cyclohexanone, acetone, ethanol, and glycerine. The carbon dots proved to be soluble in all of these solvents due to the presence of both hydrophobic and hydrophilic regions on the particle surface and/or unreacted surfactant molecules. More interestingly, PL properties of CP-CDs are preserved in all solvents, varying in the spectral shape and intensity, as shown in Figure 4. As can be seen, the PL properties also strongly depend upon the excitation wavelength (compare panels A and B in Figure 4).

Figure 2. Schematic depiction of CP-CD formation.

generally described as hydrothermal carbonization may include several chemical reactions, such as hydrolysis, dehydration, decarboxylation, polymerization, aromatization, etc.23 Though the exact bottom-up mechanism of the carbon dots’ origin is not clearly understood yet, here we suppose that basic conditions and elevated temperature cause dehydrogenation of alkyl chains, followed by the formation of aromatic structures within the carbonaceous core. The resulting sp2 domains embedded in the sp3 matrix are considered to be the “intrinsic” luminescence centers.24 3.2. Photoluminescence Study. The most attractive feature of CP-CDs relates to their emissive qualities. Generally, cationic surfactants, such as CPC, are referred to as fluorescence quenchers;25,26 however, the carbon dots prepared from CPC exhibit significant photoluminescence (PL). The excitation−emission spectra of the as-prepared CP-CDs measured in water are shown in Figure 3. The visible emission from CP-CDs comprises two convoluting bands at 475 and 550 nm, while only the latter one remains when the excitation wavelength exceeds 420 nm.

Figure 4. Emission spectra of CP-CDs redispersed in different solvents: (A) λex = 290 nm, (B) λex = 400 nm.

With a focus on the hydrothermal preparation of CP-CDs, the crucial precondition for obtaining fluorescent CP-CDs was found to be the pH of the initial surfactant solution. Generally, the pH has to be in the range of 11−12; otherwise, there is no emission in the visible region (Figure S7 in the Supporting Information). At lower pH, a small amount of carbon dots was also formed (proved by TEM) but fluorescence was dominated by the UV emission with two convoluting peaks at 334 and 349 nm. We expect that this emission arises from an unsaturated organic compound formed during the hydrothermal process as

Figure 3. Emission (full line) and excitation (dotted line) fluorescence spectra of CP-CDs measured at given excitation/emission wavelengths. 24993

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a side-product. The corresponding emission was not observed when the CP-CDs were transferred to an organic solvent probably due to reabsorption, as suggested by the peak at about 350 nm in the excitation spectra (Figure S6 in the Supporting Information). At higher initial pH (11−12), CPC gets partly deprotonated and a brown-colored CP-CDs solution originates under the hydrothermal treatment. Besides, a larger amount of luminescent CDs formed; the UV emission from the prepared sample might be suppressed due to a longer mean lifetime of the related fluorophore and thus a lower probability of transition to the ground state compared to the ones causing the emissions in VIS (Figure S8 in the Supporting Information). Finally, excessive NaOH addition to the CPC solution, followed by hydrothermal treatment, gives rise to a black hydrophobic precipitate adhering to the reaction apparatus and a dark, nontransparent suspension with no fluorescence and a lot of light scattering. Apparently, in this case, there is a lack of stabilizing surfactant due to excessive carbonization. However, the black precipitate exhibits the typical blue fluorescence with a maximum at 475 nm if dissolved in chloroform. More precisely, the shape and intensity of CP-CDs PL spectra are tunable by several parameters. Among them, the initial CPC concentration and the reaction time in relation to the exact amount of NaOH added are of crucial importance. The effect of these factors is depicted in Figure 5. Obviously, the portion of deprotonated (thus fluorescent) CPC can be controlled by its initial concentration as well as by the amount of NaOH. The elevated temperature speeds up the first step of the reaction depicted in Figure 2 and thus augments the extent

of deprotonation. This was proved by a brief heating of the CPC solution (pH ≈ 11.5) in a microwave, resulting in a redbrown solution exhibiting only the green emission at 550 nm. A similar solution can be obtained by letting NaOH react with CPC for a long period of time, hours to days. On the other hand, the length of the hydrothermal reaction determines the portion of carbonized CPC and thus affects the relative intensity of the blue emission at 475 nm (Figure 5A,B). We compare here the emission properties of CP-CDs prepared with 40 and 50 mmol of NaOH, respectively, sampled discontinuously after indicated time intervals. First, both the blue and the green emission intensities increase continuously with time as a result of ongoing CPC carbonization and deprotonation, respectively. We suppose that both processes continue simultaneously to a certain extent, which is dependent upon the amount of NaOH. The unreacted and deprotonated surfactant then gives rise to more CP-CDs, which is reflected in the increase of the blue emission intensity. After that, further development of fluorescent carbon dots is not feasible under the defined conditions. Instead, a slight intensity decrease was observed with increasing the carbonization time, likely due to the aggregation of particles. Obviously, NaOH as a base plays a key role in the synthesis of CP-CDs because it greatly facilitates the carbonization and it gives rise to the green emission from deprotonated CPC. As can be seen in Figure 5C, the shape of the emission spectra of CP-CDs in water is also tunable by varying the initial CPC concentration when the other parameters remain unchanged. Here, the temperature, pH, and reaction time were kept constant for all experiments. There is only the blue, core-related λex-dependent emission until the initial concentration of CPC reaches ∼10 mM. Above that, the second emission peak at 550 nm rises with increasing the initial CPC concentration. The exact CPC concentration threshold above which the green emission appears is sensitive to the precise pH adjustment since the amount of base influences the portion of deprotonated (thus luminescent) CPC, as described above. No considerable change in emission spectra was observed when the concentration of CPC was decreased below its critical micellar concentration (0.887−1.24 mM),30 indicating that the formation of micelles is not required for obtaining CP-CDs. There is a red-shift of the blue emission maximum with increasing initial CPC concentration from ∼425 to 475 nm, suggesting the corresponding growth of the CP-CDs’ mean size, with respect to the quantum size effect.31,32 Another option for tuning the ratio between blue and green emission of the CP-CD system is the ex-post pH adjustment. The effect of acid and base addition to the as-prepared CP-CDs is represented in Figure 5D. Generally, the gradual addition of acid causes an increase in the blue emission intensity, whereas base causes its decrease. It is widely thought that surface states play an important role in the photoluminescence of carbon dots as sites for charge carrier localization, followed by radiative recombination. The observed responsive behavior of the prepared carbon dots indicates the presence of pH-sensitive surface groups and lays the foundations for their prospective sensoric applications.9 The photostability of CP-CDs was examined by several independent time-based intensity measurements within 30 min as well as recording the fluorescence spectra after 17 h of continuous UV irradiation, which was carried out utilizing the PL instrument (excitation monochromator set to 350 nm, slits wide open, lamp power of 75 W). There was no photo-

Figure 5. Depiction of factors influencing the emission spectra of the CP-CDs: (A, B) The effect of carbonization time at different amounts of added NaOH (A: 40 mmol; B: 50 mmol); λex = 400 nm. (C) The effect of the initial CPC concentration (0.5−40 mM) at fixed reaction conditions (150 °C, 2 h) and NaOH amount (40 mmol); λex = 340 nm. (D) The effect of gradual ex-post addition of HCl (dashed line) and NaOH (dotted line) to a water dispersion of CP-CDs (full line); λex = 380 nm. 24994

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bleaching observed, and the overall intensity even increased almost twice after long-term illumination (see Figure S9 in the Supporting Information) probably due to passivation of the surface energy traps responsible for nonradiative transitions.33

4. CONCLUSION As a first stage of a complex research in the field of surfactantderived carbon nanosystems, we have presented here the synthesis of photoluminescent carbon dots prepared by hydrothermal treatment of cetylpyridinium chloride as an alternative and promising carbon source. The prepared nanoparticles exhibit excitation- and pH-dependent emission in the blue−green−yellow range, amphiphilicity, long-term photo- and colloidal stability, and a potential of inexpensive and large-scale production. The emission properties of CP-CDs can be easily tuned by controlling the CPC concentration, the amount of NaOH added to the initial CPC solution, and the duration of carbonization. With respect to the desired colloidal and optical properties, CPC as a single precursor plays a triple role in carbon dot synthesis. It provides carbon for the CD assembly, contributes to the emission by employing its heteroaromatic π system, and stabilizes the formed carbon dots owing to its amphiphilic nature. On the basis of the described features, the presented carbon dots appear to be a suitable alternative for conventionally used semiconductor nanosystems in a range of applications.

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ASSOCIATED CONTENT

S Supporting Information *

XRD patterns, ζ-potential, IR spectra, TGA, UV−vis, and complementary PL data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +420 58-563-4947. Fax: +420 58-563-4958. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the Operational Program Research and Development for Innovations − European Regional Development Fund (CZ.1.05/2.1.00/03.0058) and by the Operational Program Education for Competitiveness − European Social Fund (CZ.1.07/2.3.00/20.0155). K.K.R.D. acknowledges financial support by the Operational Program Education for Competitiveness − European Social Fund (project CZ.1.07/2.3.00/30.0041 of the Ministry of Education, Youth and Sports of the Czech Republic). The authors thank MSc. Zdeněk Marušaḱ and MSc. Tomás ̌ Šilha from Palacky University, Olomouc, Czech Republic, for performing the thermal gravimetric analysis and elemental analysis, respectively.

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REFERENCES

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