Graphitic Nitrogen Triggers Red Fluorescence in ... - ACS Publications

Graphitic Nitrogen Triggers Red Fluorescence in Carbon Dots Kateřina Holá,† Mária Sudolská,† Sergii Kalytchuk,† Dana Nachtigallová,†,‡ Andrey L. Rogach,§ Michal Otyepka,† and Radek Zbořil*,† †

Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký University Olomouc, Šlechtitelů 27, 78371 Olomouc, Czech Republic ‡ Intitute of Organic Chemistry and Biochemistry, The Czech Academy of Sciences, Flemingovo nám. 2, 16000 Prague 6, Czech Republic § Department of Materials Science and Engineering and Center for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S.A.R. S Supporting Information *

ABSTRACT: Carbon dots (CDs) are a stable and highly biocompatible fluorescent material offering great application potential in cell labeling, optical imaging, LED diodes, and optoelectronic technologies. Because their emission wavelengths provide the best tissue penetration, redemitting CDs are of particular interest for applications in biomedical technologies. Current synthetic strategies enabling red-shifted emission include increasing the CD particle size (sp2 domain) by a proper synthetic strategy and tuning the surface chemistry of CDs with suitable functional groups (e.g., carboxyl). Here we present an elegant route for preparing full-color CDs with well-controllable fluorescence at blue, green, yellow, or red wavelengths. The two-step procedure involves the synthesis of a full-color-emitting mixture of CDs from citric acid and urea in formamide followed by separation of the individual fluorescent fractions by column chromatography based on differences in CD charge. Red-emitting CDs, which had the most negative charge, were separated as the last fraction. The trend in the separation, surface charge, and red-shift of photoluminescence was caused by increasing amount of graphitic nitrogen in the CD structure, as was clearly proved by XPS, FT-IR, Raman spectroscopy, and DFT calculations. Importantly, graphitic nitrogen generates midgap states within the HOMO−LUMO gap of the undoped systems, resulting in significantly red-shifted light absorption that in turn gives rise to fluorescence at the lowenergy end of the visible spectrum. The presented findings identify graphitic nitrogen as another crucial factor that can redshift the CD photoluminescence. KEYWORDS: nitrogen-doped, graphene dots, red fluorescence, fluorescence mechanism, band-gap tuning

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CDs are particularly suitable for biomedical applications because red light shows the best tissue penetration. In addition, red CDs can provide the missing building block for a full-coloremitting spectrum, e.g., for white-light-emitting diodes. Therefore, efficient and reproducible synthetic pathways for the production of red fluorescent CDs can advance applications of CDs in many scientific and industrial areas. Recently, attention has been given to understanding the origin of CD photoluminescence because knowledge in this field may support the rational design of CD optical properties.

arbon dots (CDs) attract broad scientific interest owing to their attractive fluorescence properties. They are resistant to photobleaching akin to traditional semiconductor quantum dots1,2 but still possess a great safety profile.3 Therefore, they have found wide utilization in biomedical fields, e.g., cell labeling, optical imaging, drug delivery, and biosensing.4−9 They have also been applied in various technologies, e.g., light-emitting diodes,10 water splitting,11 and photocatalysis.12 In recent years, many synthetic procedures have been developed to prepare and tune the optical properties of CDs. Since the discovery of this material in 2004, mainly blue, green, and yellow fluorescent CDs have been reported.1,2 However, very few reproducible synthetic strategies for red fluorescent CDs have been published.13,14 Red-emitting © 2017 American Chemical Society

Received: September 8, 2017 Accepted: November 14, 2017 Published: November 14, 2017 12402

DOI: 10.1021/acsnano.7b06399 ACS Nano 2017, 11, 12402−12410

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Figure 1. (A) Reaction scheme used for the preparation of a CD mixture exhibiting all visible colors of fluorescence and (B) the corresponding fluorescence excitation−emission map. (C) Schematic representation of anion-exchange separation of the prepared CD mixture and photographs of the obtained fractions under a 365 nm UV lamp (the fractions are photographed exactly after separation in the concentrated acid). (D−G) Fluorescence excitation−emission map of individual fractions: (D) bCDs, (E) gCDs, (F) yCDs, and (G) rCDs. (H) UV−vis absorption spectra of separated fractions in water.

be tuned by doping their structure with different elements, usually nitrogen, boron, or sulfur. Considerable attention has recently been devoted to nitrogen doping.24 It can strongly influence CDs’ applications (e.g., interaction with biological systems),25−27 but it also enhances the nonlinear optical response of this material,28 the ability triggering two-photon excitation, which is preferred in biomedical applications.28−30 Nitrogen doping can also strongly determine their optoelectronic features in light-driven water splitting31,32 or in electrocatalytic oxygen reduction reaction.33 However, the influence of nitrogen on the fluorescence properties is not fully understood because nitrogen can be embedded in CDs in the form of various functional groups (amino, amidic, pyridinic, pyrrolic, graphitic, nitro, etc.).34,35 So far, it has been demonstrated that amino or hydrazine surface groups can cause a wavelength shift from blue to green regions.36−38 A similar observation was also reported for hydrazine-functionalized graphene dots. In addition, surface amino groups can better trap surface electrons for more efficient radiative recombination and hence higher quantum yields.39,40 A similar observation has been reported for pyridinic and pyrrolic groups,

Nevertheless, there is still uncertainty about the origin of photoluminescence in CDs and its control toward red emission. The most discussed feature of CDs is the excitation-dependent emission. Recently, Fu et al. reported a model system comprising a mixture of polycyclic aromatic hydrocarbons mimicking the typical excitation-wavelength-dependent emission of CDs.15 They showed that this typical feature of CDs can originate not only from different surface oxidation as reported earlier16 but also from composition of the aromatic regions of the core. However, the degree of oxidation, which affects the surface charge and hydrophilicity of the system, is still considered a key factor that can be tailored toward red emission.17−19 Another strategy available to red-shift the fluorescence of CDs is to increase their size20 or, to be precise, the size of their isolated sp2 domains.21,22 This effect was clearly demonstrated in a study by Bao et al., who showed that oxidized CDs with sizes of 10−30 kDa emitted at a wavelength of 610 nm, whereas particles with sizes below 3 kDa prepared under the same conditions emitted at just 520 nm.18 So far, this strategy reflects the typical band-gap tuning by size modulation.23 Finally, the fluorescence properties of CDs can 12403


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Figure 2. TEM images of the separated fractions of (A) bCDs, (B) gCDs, (C) yCDs, and (D) rCDs with diameter histogram and HR-TEM images for gCDs and rCDs in the inset. High-resolution C 1s and N 1s XPS spectra for (E, I) bCDs, (F, J) gCDs, (G, K) yCDs, and (H, L) rCDs.

spectra and the relationship between structural and optical properties of carbon dots.

for which it has been shown that the amount of these heterocycles closely correlates with the PL (photoluminescence) quantum yield.41 Nevertheless, it should be noted that the high quantum yields in these syntheses may also be caused by fluorescent molecules such as citrazinic acid and its derivatives, formed from precursors and embedded on the surface of CDs.42−44 Although several studies have reported close correlation between the nitrogen content in CDs and emission shift into the red region,45−47 this effect of nitrogen has not yet been properly addressed. Very recently, a few studies have reported the preparation of nitrogen-doped CDs (N-CDs) with sizes below 4 nm and with full-color emission or fluorescence only in the orange or red regions.48−54 However, the exact structural motif responsible for the emission properties has not yet been determined. In this work, we prepared a mixture of N-containing CDs exhibiting full-color fluorescence emission and separated differently colored fractions according to the different negative charge. Detailed XPS analysis of all the fractions revealed that an increased concentration of graphitic nitrogen is the crucial structural motif causing the transition from blue to red fluorescence in the prepared N-CDs. Several previous studies already suggested that the red-shift in the CDs’ emission can be caused by nitrogen.39,40 To address these suggestions, we have used the separation of a full-color fluorescent mixture as a tool to explain the role and molecular origin of nitrogen related to red emission. Moreover, the role of graphitic nitrogen in the red-shifted fluorescence was in detail explained by theoretical calculations. Besides, the present study represents one of the pioneering works providing theoretical calculations of emission

RESULTS AND DISCUSSION The mixture of full-color-emitting CDs was prepared by solvothermal decomposition of urea and citric acid dissolved in formamide (Figure 1A). The presence of urea as well as formamide was crucial for obtaining a mixture of CDs with fullcolor emission from blue to red (Figure 1A,B). The same reaction in water can produce only blue CDs.55 Similarly, the CDs prepared under similar conditions in dimethylformamide emit only at 580 nm.51 The as-prepared mixture was separated on a preparative anion-exchange column (DOWEX 1×8 chloride form) to obtain fractions of CDs exhibiting blue, green, yellow, or red fluorescence alone (Figure 1C). The blue CDs (bCDs; Figure 1D) were eluted from the column with just water, whereas the other fractions were separated using different concentrations of hydrochloric acid: the green fraction (gCDs; Figure 1E) was eluted with a 1 M solution of HCl, the yellow fraction (yCDs; Figure 1F) was eluted with 5 M HCl, and the red fluorescent particles (rCDs; Figure 1G), having the most negative charge, required 10 M HCl. After purification of the obtained mixtures, involving neutralization, desalting via Sephadex (G-25; fine), and dialysis, the particles were fully characterized. The UV−vis absorption spectra of all separated fractions showed a typical π−π* transition in the region 200−250 nm and also a well-resolved n−π* transition at 340 nm typical for nitrogen-doped CDs (Figure 1H).24,56,57 However, yCDs and 12404


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Figure 3. (A) FT-IR and (B) Raman spectra of the separated CD fractions; the AD/AG ratios are specified in the Raman spectra.

these fractions was also identical with the HR-TEM size (Figure S2). XPS spectra revealed that the amount of oxygen in the samples decreased with increasing emission wavelength from blue to red (Figure S3 and Table S1). A similar decrease was observed for COO− in high-resolution C 1s spectra at 288.5 eV, in the region typical for carboxyl groups (Figure 2E−H). This indicates that the red-shifted emission was in this case not caused by the presence of carboxylic groups, as proved for other types of CDs.16,19 Hence, the high-resolution O 1s XPS spectra showed a decreasing amount of hydroxyl groups from blue to red (Figure S4). This trend also reflects a decreasing amount of carboxylic groups because carboxylic groups in XPS are equally present in C−O and CO bands.59 Instead, high-resolution N 1s XPS spectra showed that the most significant parameter responsible for the shift from blue to red photoluminescence was the increasing amount of graphitic nitrogen located around 401.6−401.3 eV (Figure 2I−L, Table S1). Nitrogen originating from surface amide groups (399.7−400.1 eV) dominated in gCDs and bCDs, whereas graphitic nitrogen was the prevailing structural motif in yCDs and rCDs. These findings are in a good agreement with the nonaddressed presence of graphitic nitrogen in red fluorescent CDs used for cell labeling recently reported by Sun et al.60 FT-IR analysis confirmed the presence of CC bonds typical of an aromatic structure at 1600 cm−1 in all fractions (Figure 3A). Carboxylic CO bonds at 1710 cm−1, similarly to surface-related C−N bonds at 1017 cm−1, were more pronounced in the bCD and gCD samples. On the other hand, spectra of yCDs and rCDs displayed more significant peaks at 1360 and 1650 cm−1 ascribed to C−N and CN bonds, thus confirming that these CDs contained an increased amount of graphitic nitrogen, in full accordance with XPS data. 19,61 Raman spectroscopy (Figure 3B) showed a significantly decreasing intensity of the G-band at 1590 cm−1 and increasing intensity of the D-band at 1340 cm−1 from blue to red CDs. This behavior can be explained by a greater number of defects in the rCDs due to the increased number of structurally embedded graphitic nitrogen atoms to the sp2 scaffold.62,63 The broader G-band for yCDs and rCDs can be

rCDs also exhibited lower energy absorption bands at around 430 and 550 nm, respectively. In accordance with the increased intensity of these bands, the n−π* transition at 340 nm decreased. Such low-energy bands are typically connected with narrowing of the electronic band gap and are often seen for red fluorescent CDs.19,47,58 The position of the lower energy absorption bands is also related to the wavelength region of fluorescence excitation (Figure 1D,E). According to the colors observed under the UV lamp (Figure 1C), rCDs had emission maxima at 630 nm, yCDs at 550 nm, and bCDs at 460 nm (Figure S1). Only the gCD sample was partly a fluorescent mixture of different fractions (blue and yellow). However, green emission at 510 nm was also clearly identified (Figure S1B). The quantum yields (QY) for the single fractions after purification in water were 13.3% for bCDs, 10.0% for gCDs, 11.6% for yCDs, and 4.0% for rCDs. It is important to note that these generally lower values manifest that the fluorescence is not related to the highly fluorescent molecules formed from precursors as can be typically seen in the literature.44 Similarly, any organic fluorescent molecule would not survive such an acidic environment as 10 M hydrochloric acid (QY for rCDs in the CD mixture and after purification was almost identical). This fact confirms the principal role of graphitic nitrogen as an “intrinsic parameter” allowing the red-shift of photoluminescence in CDs developed in this study. Individual color fractions were thoroughly analyzed by highresolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR), Raman spectroscopy, and zeta-potential measurements to identify differences responsible for the fluorescence red-shift. Importantly, the size of individual CDs was 2−3 nm and the separated fractions did not exhibit any significant differences in their size distribution according to TEM images (Figure 2A−D and histograms in the inset). Furthermore, according to the comparison of HR-TEM images of gCDs and rCDs (see insets in Figure 2), there is no significant difference in the size of their isolated sp2 domains; both fractions also exhibited a lattice spacing of 0.22 nm typical for (100) graphitic carbon. The atomic force microscopy (AFM) height profile for 12405


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ACS Nano a consequence of increasing the defect-induced D′-band.64 However, the AD/AG ratios (Figure 3B) were very similar. To elucidate the influence of nitrogen doping on CD absorption and fluorescence, the electronic and optical properties of computationally tractable models of CD chromophores containing graphitic nitrogen atoms were evaluated using time-dependent density functional theory (TD-DFT) (for computational details, see the Supporting Information). We used pyrene-based models, which were successfully applied as models in previous theoretical and experimental studies.15,35,65 It should be noted that the used models were smaller (∼1 nm) than the prepared dots (2−3 nm), and hence the absolute values were not directly comparable. However, relative comparison among the models enabled identification of trends and suggested explanations for the observed findings.35,65−67 The pyrene-based models contained carboxyl and hydroxyl groups on the surface and also two graphitic nitrogens inside the pyrene structure. Detailed analysis of the UV−vis absorption and fluorescence properties was conducted for the four most stable model structures (Figure 4, NP1−NP4), and the optical features of these models were compared to the undoped system of the same size (Figure 4, P0). The calculated long-wavelength absorption of the most stable nitrogen-doped structure NP1 featured a forbidden S0 → S1 transition at 684 nm and an intense S0 → S2 transition at 456 nm with an oscillator strength (f) of 0.15 au (Figure 4A, Table S2 in the Supporting Information). Whereas the transition into the S1 state was clearly dominated by the HOMO → LUMO contribution (henceforth denoted H and L for brevity), the transition to the S2 state had major contributions from H → L+2 and H → L+1 configurations (Table S3 and Figure S5). The combination of extremely weak S0 → S1 and strong S0 → S2 may indicate antiKasha fluorescence, i.e., emission from a higher excited state such as S2.68 In addition, a large energy difference between S1 and S2 can further promote this phenomenon, as characteristic for conjugated compounds, including polycyclic aromatic hydrocarbons. Therefore, S2 → S0 fluorescence was assumed for the NP1 system, with a calculated wavelength of this deexcitation pathway of 602 nm. In contrast, the calculated lowenergy absorption (S0 → S1) of the nitrogen-free system P0 occurred at 420 nm (f = 0.16) and the corresponding fluorescence at 460 nm (Table S3, S4), indicating that the fluorescence of NP1 was red-shifted with respect to P0 by more than 140 nm. The isomeric structures NP2, NP3, and NP4 behaved similarly to NP1. Their longest wavelength S0 → S1 transitions were forbidden, and intense S0 → S2 transitions occurred in the range 500−580 nm (Figure 4A, Table S2). Fluorescence emission maxima were expected at 688, 676, and 789 nm for NP2, NP3, and NP4, respectively (Table S4), reflecting an even more pronounced red-shift with respect to P0. In summary, the introduction of graphitic nitrogen into the carbon lattice resulted in red-shifted vis absorption from 420 nm in P0 to ∼450−580 nm and even to the near-infrared spectral range in the doped systems. This red-shift originated from H−L gap narrowing due to the presence of graphitic nitrogen in the structure (Figure 4B). Calculations using a larger aromatic system based on coronene derivatives (Figure S6) documented that the red-shift increased with the increasing graphitic nitrogen concentration (Table S5 in the Supporting Information). The calculations show that the graphitic nitrogen doping creates midgap states within the original H−L gap of an

Figure 4. (A) Calculated UV−vis absorption spectra (in the range 200−600 nm) for the low-energy nitrogen-doped models (NP1, NP2, NP3, NP4) and nitrogen-free system P0 of the same size: carbon (green), hydrogen (white), oxygen (red), nitrogen (blue). (B) The long-wavelength absorption in the nitrogen-doped systems is due to the transition into the second singlet excited state (S0 → S2). The envelope functions assume Gaussian broadening of the peaks with σ = 10 nm. Relative energy levels of the occupied (blue) and unoccupied (red) molecular orbitals of the same structures. Differences between the HOMO and LUMO (H−L gaps) are shown and clearly indicate narrowing of the H−L gap in the nitrogen-doped systems. (C) Model fluorescence spectra with indicated wavelengths (in nm) of the emission maxima of the nitrogen-doped systems NP1 (red), NP2 (orange), NP3 (blue), and NP4 (green) and nitrogen-free system P0 (black) document the red-shifted fluorescence in the nitrogen-doped systems. 12406


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CONCLUSIONS In summary, a mixture of full-color fluorescent CDs was prepared by the simple solvothermal decomposition of urea and citric acid in formamide. The individual fluorescent components were separated by column chromatography based on differences in the CD surface charge. The blue CDs exhibited almost neutral charge and were first to be eluted. Conversely, the red CDs having the most negative charge were eluted with 10 M HCl as the last fraction. Both experimental and theoretical data confirmed that the red fluorescence emitted by rCDs was caused by the increased amount of graphitic nitrogen in the CD structure. DFT calculations suggested that the red fluorescence originated from narrowing of the band gap due to the doping with graphitic nitrogen, which is an electrondonating element and creates midgap states in the original gap of the undoped system. We believe that the present data considerably extend current knowledge on the origin/control of photoluminescence of CDs. In particular, the presence of graphitic nitrogen represents an intrinsic variable allowing to achieve the red-shift in CDs in addition to already developed strategies controlling the size of sp2 domains and types of surface molecular fluorophores. Finally, the developed synthetic/separation procedure represents an easy way to prepare red fluorescent CDs, offering great application potential in biomedical technologies.

undoped system, which is a consequence of donated excess electrons into the unoccupied π* orbitals of a conjugated system.35 Narrowing of the H−L gap was pronounced in the fluorescence properties of the graphitic nitrogen-doped systems, which exhibited significantly red-shifted emission with respect to the undoped system. These findings provide a plausible hypothesis for the mechanism of red-shifting fluorescence in graphitic-nitrogen-doped CDs. For the sake of completeness, we note that other chemical forms of nitrogen, namely, pyrrolic and pyridinic, can cause the opposite effect, i.e., blue-shifted emission,69 which indicates that not only the doping element but also its chemical form control the final doping effect. The models of CD chromophores were also used to explain the trends observed in anion-exchange chromatography. Separation of the mixture of CDs with emission spanning the whole range of visible spectra was based on fractionalization by a negative charge. This was clearly demonstrated by the measured ζ-potential values at pH 7, which indicated gradually increasing negative surface charge from bCDs to rCDs (bCDs +2 mV, gCDs −22 mV, yCDs −24 mV, and rCDs −29 mV). Surprisingly, this trend was not explainable by an increasing amount of negatively charged carboxyl groups (see XPS spectra in Figure 2 and the corresponding discussion). For instance, the most negative fraction (rCDs) did not contain the highest amount of negatively charged carboxyl groups. This also demonstrates the important role of doped nitrogen on the acidity of carboxyl groups. Indeed, the computed electrostatic potential (ESP) map of the nitrogen-doped NP1 model indicated that graphitic nitrogen doping resulted in a higher electrostatic potential on carboxyl oxygens, which causes easier deprotonation and hence increased acidity of carboxyl groups with respect to the nitrogen-free model P0 (Figure 5). DFT

METHODS All of the used chemicals, solvents, and the resin have been obtained from Sigma-Aldrich. The mixture of full-color-emitting CDs was prepared according to the following procedures. Citric acid (1 g) and urea (1 g) were dissolved by mild ultrasonication in 15 mL of formamide. The clear solution was afterward transferred into a 20 mL Teflon-lined stainless-steel autoclave and placed into an oven heated at 180 °C. After 12 h, the autoclave was cooled to laboratory temperature, and the dark red mixture was directly used for the separation. The glass column was filled with 30 mL of the DOWEX 1×8 chloride form (100−200 mesh), equilibrated with 0.5 M hydrochloric acid, and washed many times with deionized water to a neutral pH. Afterward, 1 mL of the prepared mixture of CDs was carefully loaded on the column, and deionized water was used for washing of the unbound fractions. A UV lamp (365 nm) was used for controlling the fluorescence of eluted fractions. The blue luminescence material (bCDs) was eluted by water. After elution with a 1 M solution of HCl a green fraction (gCDs) was obtained. The yellow fraction was obtained by elution with 5 M HCl (yCDs), and the red one with 10 M HCl (rCDs). The solution was neutralized by solid NaOH immediately after separation (to pH 5−6). The excess NaCl was afterward removed by passing the sample through a Sephadex column (Sephadex G-25, fine). The collected fluorescent fraction was then dialyzed in a 2 kDa cut-off dialysis membrane against deionized water. Characterization Techniques. The UV−vis spectra were measured on a Record S 600 UV−vis spectrometer. The fluorescence excitation−emission maps of the separated fractions were measured on an FLS980 fluorescence spectrometer (Edinburgh Instruments). PL quantum yields were determined by an absolute method using the same fluorescence spectrometer equipped with an integrating sphere with its inner surface coated with BENFLEC. The apparatus was tested against reference dyes with known QY: for rhodamine 100 in ethanol (reference QY of 100%) the PL QY was measured as 100%; for 2aminopyridine (reference QY of 84%) the PL QY obtained was 84%. The microscopic images were measured by TEM JEOL 2010 with an LaB6-type emission gun, operating at 160 kV. The high-resolution TEM images were obtained by a FEI Titan electron microscope operating at 80 kV. The size of particles and the histograms were determined by ImageJ software by manual counting of 1000 particles,

Figure 5. Electron density maps of NP1 and P0 and their respective carboxylate anions. The red color in the electrostatic map represents the most negatively charged and blue the most positively charged regions. The pKa of the NP1 carboxylate was lowered by 4.3 with respect to P0.

calculations confirmed a shift in the carboxyl group pKa of the NP1−NP4 models to lower values with respect to P0 (ΔpKa for NP1 and P0 = 4.3). The balanced charge distribution in P0 was disturbed by the introduction of graphitic nitrogen atoms bringing two excess electrons into the conjugated system, which were pulled toward the more electronegative oxygen atoms of the carboxyl groups. In turn, the anion-exchange resin binding ability of graphitic nitrogen rich CDs was significantly higher than that of CDs with lower levels of graphitic nitrogen. This explains why the particles with red fluorescence and the highest amount of graphitic nitrogen were eluted by concentrated HCl as the very last fraction. 12407


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ACS Nano and the size distribution was fitted with a log-normal function. The AFM images were collected on an NTegra Spectra from NT-MDT (Moscow, Russia). Elemental analyses (C, H, and N) were performed using an EA1108 CHN analyzer (Fissons Instruments). The XPS analysis was carried out by a PHI VersaProbe II (Physical Electronics) spectrometer using an Al Kα source (15 kV, 50 W). The obtained data were fitted by the MultiPak (Ulvac-PHI, Inc.) software. All of the binding energies were referred to the C 1s peak at 284.80 eV. Fourier transform infrared spectra were measured on an iS5 Thermo Nicolet FT-IR spectrometer using the Smart Orbit ZnSe ATR technique. Raman spectra were recorded on a DXR Raman microscope using the 780 nm excitation line of a diode laser. The zeta potential of the fraction was recorded at neutral pH on a Zetasizer Nano Zs instrument (Malvern, UK). Computational Details. To represent characteristic CD chromophores consisting of conjugated sp2 carbon domains,15 we opted for pyrene-based systems. In the graphitic nitrogen-doped models, two nitrogen atoms replaced two inner carbon atoms; that is, each nitrogen atom was bound to three carbon atoms. Carboxy (−COOH) and hydroxy (−OH) groups substituted for the total of four hydrogen atoms at the pyrene edges of all models to mimic the nature of the CDs’ surface (Figure 4A). Introduction of the two graphitic nitrogen atoms into the pyrene conjugated system gives nine isomers with different mutual positions of doping atoms. Based on the isomers’ electronic energies related to the most stable structure (denoted as NP1), the four most favorable systems were selected for a detailed analysis of their UV−vis absorption and fluorescence and compared to the reference nitrogen-free system (P0). The isomer NP5 was also used for calculation of the electron density map. In this study, we adopted a previously tested approach to model the optical properties of oxygen- and nitrogen-functionalized and/or -doped CDs.35,65 Full geometry optimizations and frequency analyses of all considered models were performed using density functional theory (DFT) with the Becke three-parameter hybrid density functional (B3LYP)70 with the D3 empirical dispersion correction by Grimme et al.71 and the 6-31++G(d,p) Pople basis set.72,73 Subsequently, tests to confirm the stability of the singlet ground state wave function were performed. Absorption spectra were computed within the time-dependent DFT framework using the range-separated hybrid ωB97xD exchange−correlation functional74 and 6-31+G(d) basis set.75 The chosen functional represents a reasonable balance between accuracy and computer efficiency for low-lying electronic transitions of large aromatic molecules,76 although the energies of π → π* transitions tend to be consistently overestimated by ∼0.30 eV.77 However, we note that the discussion is based solely on relative comparisons of absorption and fluorescence properties. Next, based on the analysis of the vertical electronic transitions of the individual model structures, the geometries of the first and second singlet excited states (S1 and S2) have been fully optimized with the TD-ωB97xD functional and 6-31+G(d) basis set, followed by frequency analyses to confirm the nature of the optimized structures (genuine minima). All calculations have been performed in a vacuum because the effects of water environment (common dispersion environment for CDs) have been shown to be small and uniform for the differently functionalized CDs.35,65 The Gaussian 09.D01 software package has been used throughout this work.78

fluorescence, and molecular orbitals of the modeled systems (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Sergii Kalytchuk: 0000-0002-6371-8795 Dana Nachtigallová: 0000-0002-9588-8625 Andrey L. Rogach: 0000-0002-8263-8141 Michal Otyepka: 0000-0002-1066-5677 Radek Zbořil: 0000-0002-3147-2196 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge support from the Ministry of Education, Youth and Sports of the Czech Republic (LO1305 and CZ.1.05/2.1.00/19.0377), student project IGA_PrF_2017_025 of Palacký University Olomouc, and the Czech Science Foundation (P208/12/G016). Assistance provided by the Research Infrastructure NanoEnviCz, supported by the Ministry of Education, Youth and Sports of the Czech Republic under Project No. LM2015073, is also gratefully acknowledged. A.L.R. acknowledges the financial support from NPRP Grant No. 8-878-1-172 from the Qatar National Research Fund (a member of the Qatar Foundation). The authors acknowledge the help of Dr. Petr, Dr. Opletalová, Ms. Stráská, Dr. Froning, and Ms. Šedajová with material characterization of the samples. REFERENCES (1) Xu, X.; Ray, R.; Gu, Y.; Ploehn, H. J.; Gearheart, L.; Raker, K.; Scrivens, W. A. Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736−12737. (2) Baker, S. N.; Baker, G. A. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem., Int. Ed. 2010, 49, 6726−6744. (3) Wang, K.; Gao, Z.; Gao, G.; Wo, Y.; Wang, Y.; Shen, G.; Cui, D. Systematic Safety Evaluation on Photoluminescent Carbon Dots. Nanoscale Res. Lett. 2013, 8, 122−130. (4) Yang, S.-T.; Cao, L.; Luo, P. G.; Lu, F.; Wang, X.; Wang, H.; Meziani, M. J.; Liu, Y.; Qi, G.; Sun, Y.-P. Carbon Dots for Optical Imaging in Vivo. J. Am. Chem. Soc. 2009, 131, 11308−11309. (5) Zheng, M.; Liu, S.; Li, J.; Qu, D.; Zhao, H.; Guan, X.; Hu, X.; Xie, Z.; Jing, X.; Sun, Z. Integrating Oxaliplatin with Highly Luminescent Carbon Dots: An Unprecedented Theranostic Agent for Personalized Medicine. Adv. Mater. 2014, 26, 3554−3560. (6) Hola, K.; Zhang, Y.; Wang, Y.; Giannelis, E. P.; Zboril, R.; Rogach, A. L. Carbon dotsEmerging Light Emitters for Bioimaging, Cancer Therapy and Optoelectronics. Nano Today 2014, 9, 590−603. (7) Shan, X.; Chai, L.; Ma, J.; Qian, Z.; Chen, J.; Feng, H. B-Doped Carbon Quantum Dots as a Sensitive Fluorescence Probe for Hydrogen Peroxide and Glucose Detection. Analyst 2014, 139, 2322−2325. (8) Loo, A. H.; Sofer, Z.; Bouša, D.; Ulbrich, P.; Bonanni, A.; Pumera, M. Carboxylic Carbon Quantum Dots as a Fluorescent Sensing Platform for DNA Detection. ACS Appl. Mater. Interfaces 2016, 8, 1951−1957. (9) Yew, Y. T.; Loo, A. H.; Sofer, Z.; Klímová, K.; Pumera, M. CokeDerived Graphene Quantum Dots as Fluorescence Nanoquencher in DNA Detection. Appl. Mater. Today 2017, 7, 138−143. (10) Yu, H.; Zhao, Y.; Zhou, C.; Shang, L.; Peng, Y.; Cao, Y.; Wu, L.Z.; Tung, C.-H.; Zhang, T. Carbon Quantum Dots/TiO2 Composites

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b06399. Detailed information on the preparation and characterization of the discussed material, theoretical calculations, emission spectra of prepared CDs, XPS survey spectra and elemental composition, optimized structures of the chromophore models, calculated vis absorption and 12408


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