Molecular Origin and Self-Assembly of Fluorescent ... - ACS Publications

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Molecular Origin and Self-Assembly of Fluorescent Carbon Nanodots in Polar Solvents Arjun Sharma,†,∥ Trilochan Gadly,‡ Suman Neogy,§ Sunil Kumar Ghosh,‡,∥ and Manoj Kumbhakar*,†,∥ †

Radiation & Photochemistry Division, ‡Bio-organic Division, and §Mechanical Metallurgy Division, Bhabha Atomic Research Center, Mumbai 400085, India ∥ Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India S Supporting Information *

ABSTRACT: Despite numerous efforts, there are several fundamental ambiguities regarding the photoluminescence of carbon dots (CDs). Spectral shift measurements display characteristic of both π−π* and n−π* transitions for the main absorption or excitation band at ∼350 nm, contrary to common assignment of exclusive n−π* transition. Additionally, the generally perceived core-state transition at ∼250 nm, involving sp2-networked carbogenic domains shielded from external environments, needs to be reassessed because it fails to explain the observed fluorescence quenching and spectral shift. These results have been explained based on the molecular origin of PL in CDs invoking the similarity between CD and citrazinic acid. Fluorescent derivatives of the latter are recognized to be produced during citric-acid-based CD synthesis. Concentration-dependent spectral splitting of the main excitation band in combination with the temperature-dependent PL results has been envisioned assuming self-assembly of CDs into various H-aggregates.

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Chart 1. Excitation and Absorption Spectra of CD

ver the past decade numerous efforts have been undertaken to unravel the origin of photoluminescence of carbon dots (CDs) to gain fundamental insights and for better utilization of CDs in various applications including optical sensing,1 tunable color materials,2−4 labels for biological imaging,5,6 anticounterfeiting,7 and so on. The countless variety of CDs prepared through either top-down or bottom-up approach from virtually any precursor makes it hard to correctly evaluate their photoluminescence (PL) behavior from the already available vast literature reports. Although constant deliberation on the actual mechanism of PL in CDs is rife in the research community, a general consensus from various reports highlights four broad possibilities. PL in CDs, as summarized by Zhu et al.,8 originate from (i) the conjugated π-domains of carbon core or the quantum confinement effect;9−11 (ii) the functional groups connected with the carbon backbone, called surface states;12−17 (iii) the fluorescent molecules connected on the surface or interior of the CDs, known as molecular state;18−24 and (iv) the cross-link-enhanced emission (CEE) effect.25,26 However, a uniform explanation, which can address most of the PL behavior of CD, if not all, is yet to emerge. Besides, there are several intriguing aspects that still remain ambiguous in the PL behavior of CDs. Chart 1 depicts the collective explanation for the origin of PL in CDs, invoking core, edge, and surface bands. First, the PL excitation spectra, in most cases, do not resemble the absorption spectra even for the lower energy edge band (i.e., commonly assigned n−π* transition at or above 330 nm).19,27 Additionally, why is this intense excitation band remarkably broad compared with its higher energy core band excitation below 300 nm? Second, are © XXXX American Chemical Society

the weak low-energy absorption bands (>400 nm), apart from the intense n−π* band, really due to low-energy surface/edge states within the n−π* band28 or something else? Recently multiple absorption bands in the visible wavelength range have also been credited to the presence of various CD aggregates apart from in-band heterogeneity of the main excitation band.29 Aside from these low-energy excitation bands, a relatively weak excitation band in the UV region (at ≤280 nm) is Received: January 23, 2017 Accepted: February 15, 2017 Published: February 15, 2017 1044

DOI: 10.1021/acs.jpclett.7b00170 J. Phys. Chem. Lett. 2017, 8, 1044−1052

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The Journal of Physical Chemistry Letters

Figure 1. High-resolution TEM images of CD-f1 (a), CD-f2 (b), and CD-f3 (c) show crystal lattice structure. Scale bar is 10 nm. Raman spectra (d) show the characteristic D and G bands of disordered graphitic structures for all of the CD fractions. Absence of 2D peak indicates multilayer graphitic structure (inset). FTIR spectra (e) display the presence of various functional groups.

∼350 nm has significant contribution of π−π* charge transfer rather than exclusive n−π* character, a colloquial feature in CD literature. Aggregation of surface or molecular states within or among CDs is crucial for understanding the PL behavior, and from the comparison of concentration-dependent spectral measurements of citrazinic acid (2,6-dihydroxypyridine-4carboxylic acid) we also validate the molecular origin of CD fluorescence. CDs were synthesized from citric acid and urea using simple heating mantle under nitrogen atmosphere and were separated through column chromatography based on their polarities. Three main fractions were selected and characterized for the present investigation (cf. Supporting Information). The size of the CDs, as revealed from transmission electron microscopy (TEM) images, was around 6−10 nm (Figure 1a−c), with a uniform height distribution as recorded with atomic force microscopy (Figure S1). The observed lattice spacing was around 0.26, 0.24, and 0.21 nm for the three factions. Raman spectra for all three CDs are overall similar (cf. Figure 1d). The observation of G peak at 1560 cm−1 (due to bond stretching of all pairs of sp2 atoms in both rings and chains) and the D peaks at 1363 cm−1 (due to breathing modes of sp2 atoms only in the rings) indicates stacked nanocrystalline graphene/graphene oxide domains (in the absence of 2D peak) on broad transition bands, possibly due to sp3 amorphous carbon.37 All three CD fractions displayed characteristic IR peaks (cf. Figure 1e) at 1387 cm−1 (COO− stretching), 1567 cm−1 (CN stretching), 1192 cm−1 (C−N stretching), and 1717 cm−1 (CO stretching) along with a broad peak at approximately 2900− 3700 cm−1 (with peaks at 3200, 3347, and 3452 cm−1 for N−H stretching of amide, O−H stretching, and N−H of aromatic amines, respectively). Therefore, the presence of nitrogencontaining pyridine, amide, amino groups and carbonylcontaining functional groups like ketone/aldehyde/carboxyl is

commonly encountered in CD excitation spectra. This band is commonly assigned to π−π* transition arising from CD “core state” of sp2-networked carbogenic domain shielded from external environments.28 Unlike the ∼350 nm excitation band, this weakly emissive band at ≤250 nm has not received serious attention except for a few instances. The third point of contention here is the origin of this commonly assigned π−π* transition band and whether it is truly a “core-state” transition. We will experimentally show that PL for ≤280 nm excitation experiences strong interaction with external quenchers, contradicting the usual core state description. Fourth and the most important argument, can we justify the commonly observed PL in CD invoking similarity with that of regularly used molecular fluorophore and their aggregates? Quite a few seminal reports18,21−24,30,31 have already argued for the molecular origin of fluorescence in CDs (cf. Supporting Information). Demchenko and Dekaliuk32 have proposed, based on advanced single-particle measurements of Ghosh et al.,33 that spontaneous layered stacking of chromophore during the synthesis of CD allows exciton delocalization over the whole particle, leading to its characteristic polarized emission34 by electron−hole recombination. Involvement of different electronic states4,35 and the presence of aggregates29 for the PL behavior in CDs is well known. Therefore, consideration of molecular fluorescence and their aggregation in the context of reported PL behavior of CD across the literature deserves special attention. This has further relevance in the development of tunable color materials,2−4,36 analyte sensing,27 and so on, where CDs are used at higher concentrations. In this contribution, we attempt to address these fundamental questions of PL in CDs, which is critically compared and verified with the experimental results reported by various other groups. We have shown that the core-state band is sensitive to the external media and accessible by quencher molecules. The intense excitation/absorption band at 1045


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solvents like ethanol (EtOH), methanol (MeOH), and water (cf. Figure 2c). On the contrary, in polar aprotic solvents, like acetonitrile (ACN), dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), a bathochromic (red) shift of ∼9 nm has been observed. This observation is similar to the other two CD fractions (cf. Figure S2). The former blue shift indicates n−π* transition (as H bonding with solvent stabilizes the nonbonding electron pair in the ground state relative to that with the excited antibonding π state), while the latter red shift highlights the π−π* character for this excitation band (as the delocalized excited state is expected to have greater energy stabilization with increased polarity). Therefore, we observe contribution from both type of transitions, π−π* and n−π*, leading to the broad absorption band at ∼340 nm. Theoretical investigation of absorption spectra of oxygen-functionalized graphitic CDs by Sudolská et al.38 also suggested that the experimentally observed broad absorption band originates from both n−π* and π−π* charge-transfer transitions. The interlayer chargetransfer transitions between different molecules or fragments with the same molecule of π−π* nature dominate over the commonly weak symmetry restricted n−π* transition. The absence of 2D peak in Raman spectra also points toward multilayer sp2 domains (cf. inset of Figure 1d) as a possible origin of π−π* transition. The recorded excitation spectra in polar protic and aprotic solvents, shown in Figure S3, further substantiate the above unique and distinct observation of spectral blue and red shifts for CDs. These results therefore contradict the general perception of exclusive n−π* transition for this band. The chromophoric groups are possibly located on the surface of CDs expected from the observed solvatochromic shifts and also from the observed fluorescence quenching- and concentration-dependent spectral splitting, as discussed later. The intense absorption band(s) in the UV region (1 mg/mL), the main excitation band at ∼350 nm diminishes with concomitant occurrence of highenergy blue-shifted and low-energy red-shifted excitation bands, albeit with altered intensity of the bands for different CD fractions. The distinct changes in spectral shape indicate the gradual formation of higher order aggregates. Surprising absence of isosbestic points in the excitation spectra indicates nonequilibrium situation. Concentration-dependent excitation spectra for emissions at even higher wavelengths (i.e., at 550 and 600 nm, shown in Figure S6) are qualitatively no different than the above except for different propensity of bands across all the fractions. Spectral measurements with front surface geometry and thickness-dependent lifetime results further substantiate the observation of spectral splitting (cf. Supporting Information). Contrary to this, the changes in emission spectra with concentration are not that dramatic but certainly show gradual evolution of red emissive states (leading to spectral broadening) without altering the shape on the high-energy emissive side (cf. Figure 4d−f). However, at very high concentrations (≥5 mg/mL) the whole spectra move to lower energies. Additionally the PL spectra with blue-shifted excitation reveal additional contribution from high-energy emissive states compared with that at 350 nm excitation, while with the redshifted excitation the PL spectra are noticeably red-shifted (cf. Figure 4g). Hence concentration-dependent PL results are indicative but undeniable evidence toward aggregation, a behavior well known to many molecular fluorophores. The molecular origin of PL in CDs is therefore further strengthened from the above solvent-polarity- and concentration-dependent spectral changes. However, unlike molecular fluorophore, we do not observe significant influence of salts and surfactants on the PL intensity (cf. Figure S9). Perhaps the large number of functional groups (cationic and anionic) coupled to their low zeta potential (−2 to −17 mV) actually limits the role of electrolytes in screening the coulombic repulsions for selfassociation of CDs. To further substantiate the signature of molecular chromophores in CDs, we extended spectroscopic investigations with citrazinic acid (CzA), as its derivatives are linked to the origin of fluorescence in citric-acid-based CDs by Schneider et al.22 Close resemblance of NMR spectra of CD with that of CzA (cf. Figure S10) also supports their argument. It is very interesting to note that within the linear concentration versus absorbance regime there is blue shift for the 350 nm band along with decrease in the visible tail band (cf. Figure 5). The latter

Figure 5. Absorption spectra of CzA at different concentrations indicated by the colors in the absorbance versus PL intensity plot in the inset. Corresponding excitation spectra shows broadening with increase in CzA concentration.

observation is similar to CDs, although the main absorption band does not show shift (cf. Figure S11), but surprisingly the CzA excitation spectra display the beginning of spectral splitting (cf. Figure 5 inset), an observation similar to CDs. A closer inspection of the absorption spectra (cf. Figure S12) also indicates the presence of additional band in the 370−410 nm regions at high CzA concentration, similar to its excitation spectra. Furthermore, the recorded absorption spectra in different polar solvents (cf. Figure S13) illustrate bathochromic shift for polar aprotic solvent, while hypsochromic shift is witnessed in polar protic solvents. Therefore, although CDs are a much more complex and larger system, their spectral features bear a close resemblance to CzA. On the contrary, comparison of lifetime decays (cf. Figure S12c) discerns complex and altered photophysics in CDs than basic CzA unit. It is important to mention here that this molecular state, as described by Choi et al.,28 is different from the edge state, which is related to the boundary between sp2- and sp3hybridized carbon and the surface-exposed functional groups, although they undergo similar n−π* transition with a similar energy gap. Further, reaction temperature in hydrothermal synthesis imparts influence on the extent of carbonization and the formation of fluorophore units in CD samples.18,22,40−42 Zhang et al.40 has shown that formation of CDs starts at or above 180 °C from the carbonization of fluorescent polymer chains generated by the condensation of initially produced small fluorescent molecules. Although quantum yield exhibits significant change, the general spectral features of CDs prepared at different reaction temperature are quite similar, especially excitation-dependent emission behavior.18,40−42 Spectroscopic investigations of our CD samples prepared at different temperatures also substantiate the above observations (cf. Supporting Information). Another astonishing observation from Figure 4 is the gradual shift of 240 nm excitation band to lower energies, emphasizing the extended delocalization at higher concentrations. This, in return, points to the already demonstrated fact that it is not a true core-state transition but is susceptible to external solutes. On the contrary, the similarity of this excitation band with that 1048


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Figure 6. Effect of increasing temperature on the emission spectra of CD-f1 measured with different excitation wavelengths. The concentration of CD is 0.5 mg/mL.

drastic decrease in intensity with a rise in temperature compared with that with isolated chromophore or monomer emission. Figure 6c also displays gradual increase in relative higher energy emissions with temperature, expected from Haggregates. Thus the red-shifted excitation band does not seem to be of J-aggregate; rather, we call it weakly H-aggregate. The unusual observation of low-energy H-aggregate excitation is perhaps a culmination of both weak coupling and structural distortions due to larger separation among CD particles and their nonideal mutual spatial configuration, which relaxes selection rule for lower energy excitonic transitions, that is, red-shifted excitation band. Radiant red-shifted excitation for weakly coupled H-aggregates has been reported for carbocyanin dyes by Berlepsch et al.53 They have also shown that weakly coupled H-aggregates are organized in well-ordered, extended monolayer sheets, whereas the strongly coupled H-aggregates appear to consist of particles of only a few nanometers in size. Although detailed structural investigation is required to determine CD aggregates, preliminary TEM images of concentrated and matured CD samples also display sheet-like structures layered one above the other, sheets with curved layers, and so on (cf. Figure S14−S16). In summary, the origin of excitation and emission bands is considerably more complex and heterogeneous than the simple interpretations prevalent in literature. We have shown simple but definite evidence that directly contradicts the general corestate proposition. Apart from the demonstration of heterogeneity for the edge band, our experiments also for the first time recognize aggregation-induced spectral splitting like molecular fluorophore for CDs. Although additional evidence is required to exactly explain the titillating PL behavior of CDs at high concentrations, based on the temperature-dependent PL studies we tentatively argue for the simultaneous presence of weak and strong H-aggregates. In addition to it, our investigation also reveals the possibility of different origin of near-similar emission spectra for the core and edge band excitation. These highly significant and new results will certainly instigate researchers to reassess the PL behavior of CDs, an essential not only for fundamental understanding but also for various applications from bioimaging to white-light materials. Although CDs have been used in various superresolution imaging techniques like stimulated emission depletion (STED),54 super-resolution optical fluctuation imaging (SOFI),5 and localization-based super-resolution microscopy,55 the dearth of clarity in several issues starting from synthesis of vast CD samples to systematic investigation for the origin of complex fluorescence behavior actually limits its wide applicability. Moreover, CDs can also be potentially

of CzA (cf. Figure S12b) also highlights the possible role of molecular states. This argument could potentially explain the fluorescence quenching and spectral measurements for this high-energy band, unlike the core state model. Further investigations are on to characterize this high-energy excitation band. Despite attempts to rationalize PL behavior by exploiting aggregation,3,21,27,29,32,34,43 the general mechanism for PL is insufficiently characterized, and here, based on these above results, we attempt to frame the concentration-dependent spectral features. According to exciton theory of Kasha et al.,44,45 weakly emissive H-aggregates are characterized by blueshifted excitation/absorption band, whereas for highly emissive J-aggregates, red-shifted excitation/absorption band is observed (cf. Supporting Information). On the basis of these models we may assume that the blue-shifted excitation/absorption band arises due to H-aggregates, while the red-shifted band is for Jaggregates. It is quite possible that J- and H-aggregates coexists.46−48 Observation of rod- or needle-shaped structure in TEM images for CDs possibly indicates J-aggregates, although the red-shifted excitation band is not surprisingly narrowed, unlike observed with other fluorophore aggregates.27,29,36 Furthermore, mesoscopic ribbon-like or tubular H-aggregate structure of fluorophores is also reported.48,49 For more complex aggregate structures, simple description of pure H- and J-aggregates is inadequate to differentiate the aggregates as excitation/absorption and emission spectra display vibronic structures (cf. Supporting Information). On the basis of the excitonic coupling strength, Spano50,51 has provided invaluable insights to distinguish between the two types of aggregates (cf. Supporting Information). In the present case, extending the argument of Spano to distinguish H- and Jaggregates, we recorded temperature-dependent PL with excitation at blue- and red-shifted excitation maxima, shown in Figure 6. The excitations show either an almost unaltered or a small decrease in emission intensity with temperature at the lower energy emissions, around 530 nm (cf. Figure 6a,c), compared with drastic decrease at 450 nm (cf. Figure 6b). Differential temperature dependence for the low- and highenergy emission spectra of CD has also been reported by Gan et al.52 In the case of weakly emissive H-aggregate, an increase in intensity with increase in temperature is predictable, but this has to be exceedingly sufficient to overcome the strong decrease in isolated chromophore intensity due to its increase in nonradiative deactivation, even though the isolated chromophores have a small population in the blue- or redshifted excitation wavelength. However, with red-shifted excitation, assuming it to be J-aggregate, one would expect a 1049


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a Flexible Carbon Dot Ionogel. J. Phys. Chem. Lett. 2014, 5, 1412− 1420. (5) Chizhik, A. M.; Stein, S.; Dekaliuk, M. O.; Battle, C.; Li, W.; Huss, A.; Platen, M.; Schaap, I. A. T.; Gregor, I.; Demchenko, A. P.; et al. Super-Resolution Optical Fluctuation Bio-Imaging with DualColor Carbon Nanodots. Nano Lett. 2016, 16, 237−242. (6) Ding, H.; Yu, S.-B.; Wei, J.-S.; Xiong, H.-M. Full-Color LightEmitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism. ACS Nano 2016, 10, 484−491. (7) Jiang, K.; Zhang, L.; Lu, J.; Xu, C.; Cai, C.; Lin, H. Triple-Mode Emission of Carbon Dots: Applications for Advanced Anti-Counterfeiting. Angew. Chem., Int. Ed. 2016, 55, 7231−7235. (8) Zhu, S.; Song, Y.; Zhao, X.; Shao, J.; Zhang, J.; Yang, B. The Photoluminescence Mechanism in Carbon Dots (Graphene Quantum Dots, Carbon Nanodots, and Polymer Dots): Current State and Future Perspective. Nano Res. 2015, 8, 355−381. (9) Thiyagarajan, S. K.; Raghupathy, S.; Palanivel, D.; Raji, K.; Ramamurthy, P. Fluorescent Carbon Nano Dots from Lignite: Unveiling the Impeccable Evidence for Quantum Confinement. Phys. Chem. Chem. Phys. 2016, 18, 12065−12073. (10) Li, H.; He, X.; Kang, Z.; Huang, H.; Liu, Y.; Liu, J.; Lian, S.; Tsang, C. H. A.; Yang, X.; Lee, S.-T. Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem., Int. Ed. 2010, 49, 4430−4434. (11) Bhattacharya, A.; Chatterjee, S.; Prajapati, R.; Mukherjee, T. K. Size-dependent Penetration of Carbon Dots Inside the Ferritin Nanocages: Evidence for the Quantum Confinement Effect in Carbon Dots. Phys. Chem. Chem. Phys. 2015, 17, 12833−12840. (12) Liu, H.; Ye, T.; Mao, C. Fluorescent Carbon Nanoparticles Derived from Candle Soot. Angew. Chem., Int. Ed. 2007, 46, 6473− 6475. (13) Zheng, H.; Wang, Q.; Long, Y.; Zhang, H.; Huang, X.; Zhu, R. Enhancing the Luminescence of Carbon Dots With a Reduction Pathway. Chem. Commun. 2011, 47, 10650−10652. (14) Das, S. K.; Liu, Y.; Yeom, S.; Kim, D. Y.; Richards, C. I. SingleParticle Fluorescence Intensity Fluctuations of Carbon Nanodots. Nano Lett. 2014, 14, 620−625. (15) Yu, P.; Wen, X.; Toh, Y.-R.; Tang, J. Temperature-Dependent Fluorescence in Carbon Dots. J. Phys. Chem. C 2012, 116, 25552− 25557. (16) Wang, L.; Zhu, S.-J.; Wang, H.-Y.; Qu, S.-N.; Zhang, Y.-L.; Zhang, J.-H.; Chen, Q.-D.; Xu, H.-L.; Han, W.; Yang, B.; et al. Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots. ACS Nano 2014, 8, 2541−2547. (17) Wen, X.; Yu, P.; Toh, Y.-R.; Hao, X.; Tang, J. Intrinsic and Extrinsic Fluorescence in Carbon Nanodots: Ultrafast Time-Resolved Fluorescence and Carrier Dynamics. Adv. Opt. Mater. 2013, 1, 173− 178. (18) Krysmann, M. J.; Kelarakis, A.; Dallas, P.; Giannelis, E. P. Formation Mechanism of Carbogenic Nanoparticles with Dual Photoluminescence Emission. J. Am. Chem. Soc. 2012, 134, 747−750. (19) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem., Int. Ed. 2013, 52, 3953−3957. (20) Song, Y.; Zhu, S.; Xiang, S.; Zhao, X.; Zhang, J.; Zhang, H.; Fu, Y.; Yang, B. Investigation Into the Fluorescence Quenching Behaviors and Applications of Carbon Dots. Nanoscale 2014, 6, 4676−4682. (21) Gude, V.; Das, A.; Chatterjee, T.; Mandal, P. K. Molecular Origin of Photoluminescence of Carbon Dots: Aggregation-Induced Orange-Red Emission. Phys. Chem. Chem. Phys. 2016, 18, 28274− 28280. (22) Schneider, J.; Reckmeier, C. J.; Xiong, Y.; von Seckendorff, M.; Susha, A. S.; Kasak, P.; Rogach, A. L. Molecular Fluorescence in Citric Acid Based Carbon Dots. J. Phys. Chem. C 2017, 121, 2014−2022. (23) Song, Y.; Zhu, S.; Zhang, S.; Fu, Y.; Wang, L.; Zhao, X.; Yang, B. Investigation from Chemical Structure to Photoluminescent Mechanism: A Type of Carbon Dots from the Pyrolysis of Citric Acid and an Amine. J. Mater. Chem. C 2015, 3, 5976−5984.

employed in subdiffraction resolution imaging with super resolution by polarization demodulation (SPoD),56 as it displays anisotropic PL from the electric dipole of CDs established by scanning of azimuthally polarized laser beam (APLB) at the focal region.33 Although different imaging techniques exploit various parameters of CD as fluorescent marker (i.e., photostability, blinking, polarization, etc.), the trickiest of them is to have excitation-independent emission (detrimental in selecting/designing donor−acceptor pairs for energy-transfer experiments). Although a general consensus for the control of PL mechanism is yet to emerge, it is primarily linked to surface passivation and homogeneous surface/ molecular states structure. Recently, several studies have come up with excitation-independent (or very weakly dependent) CD samples either by engineering reaction schemes21,57−63 or by suitable functionalization64 or doping of CDs,65 signifying a brighter prospect of CD as an inexpensive, stable, and biocompatible marker.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.7b00170. Information regarding preparation of CND, experimental methods and analysis, AFM and TEM measurements, steady-state and time-resolved fluorescence results, and other supplementary figures. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Manoj Kumbhakar: 0000-0001-9076-8045 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by DAE, India. A.S. acknowledges DAEBARC for his Ph.D. fellowship. Encouragement and support from Drs. D. K. Palit, Head, RPC Division, BARC and Dr. H. Pal, Head, MP Section, RPCD, BARC are gratefully acknowledged. We thank Mr. A. Das and Dr. S. Kapoor of RPCD, BARC for AFM and Raman measurements. Support from Dr. A. Gupta and A. Ballal of MBD, BARC for the TEM measurements is also gratefully acknowledged.

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REFERENCES

(1) Shen, P.; Xia, Y. Synthesis-Modification Integration: One-Step Fabrication of Boronic Acid Functionalized Carbon Dots for Fluorescent Blood Sugar Sensing. Anal. Chem. 2014, 86, 5323−5329. (2) Lu, S.; Cong, R.; Zhu, S.; Zhao, X.; Liu, J.; S.Tse, J.; Meng, S.; Yang, B. pH-Dependent Synthesis of Novel Structure-Controllable Polymer-Carbon NanoDots with High Acidophilic Luminescence and Super Carbon Dots Assembly for White Light-Emitting Diodes. ACS Appl. Mater. Interfaces 2016, 8, 4062−4068. (3) Chen, Y.; Zheng, M.; Xiao, Y.; Dong, H.; Zhang, H.; Zhuang, J.; Hu, H.; Lei, B.; Liu, Y. A Self-Quenching-Resistant Carbon-Dot Powder with Tunable Solid-State Fluorescence and Construction of Dual-Fluorescence Morphologies for White Light-Emission. Adv. Mater. 2016, 28, 312−318. (4) Wang, Y.; Kalytchuk, S.; Zhang, Y.; Shi, H.; Kershaw, S. V.; Rogach, A. L. Thickness-Dependent Full-Color Emission Tunability in 1050


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Hydrothermal Route to Prepare Highly Photoluminescent Carbon Quantum Dots With Aggregation-Induced Emission Enhancement Properties. Chem. Commun. 2013, 49, 8015−8017. (44) Kasha, M. Energy Transfer Mechanisms and the Molecular Exciton Model for Molecular Aggregates. Radiat. Res. 1963, 20, 55−71. (45) Kasha, M.; Rawls, H. R.; El-Bayoumi, M. A. The Excitonic Model in Molecular Spectroscopy. Pure Appl. Chem. 1965, 11, 371− 392. (46) Eisfeld, A.; Briggs, J. S. The J- and H-Bands of Organic Dye Aggregates. Chem. Phys. 2006, 324, 376−384. (47) Deng, Y.; Yuan, W.; Jia, Z.; Liu, G. H- and J-Aggregation of Fluorene-Based Chromophores. J. Phys. Chem. B 2014, 118, 14536− 14545. (48) Berlepsch, H. v.; Ludwig, K.; Böttcher, C. Pinacyanol Chloride forms Mesoscopic H- and J-Aggregates in Aqueous Solution − A Spectroscopic and Cryo-Transmission Electron Microscopy Study. Phys. Chem. Chem. Phys. 2014, 16, 10659−10668. (49) Berlepsch, H. v.; Brandenburg, E.; Koksch, B.; Bottcher, C. Peptide Adsorption to Cyanine Dye Aggregates Revealed by CryoTransmission Electron Microscopy. Langmuir 2010, 26, 11452− 11460. (50) Spano, F. C.; Silva, C. H- and J-Aggregate Behavior in Polymeric Semiconductors. Annu. Rev. Phys. Chem. 2014, 65, 477−500. (51) Spano, F. C. The Spectral Signatures of Frenkel Polarons in Hand J-Aggregates. Acc. Chem. Res. 2010, 43, 429−439. (52) Gan, Z.; Hao, Y.; Shan, Y. Temperature-Dependent Dual Emission from Sucrose-Derived Carbon Nanodots: A Ratiometric Fluorescent Thermometer. Chem. Nano. Mater. 2016, 2, 171−175. (53) Berlepsch, H. v.; Böttcher, C. H-Aggregates of an Indocyanine Cy5 Dye: Transition from Strong to Weak Molecular Coupling. J. Phys. Chem. B 2015, 119, 11900−11909. (54) Leménager, G.; De Luca, E.; Sun, Y.-P.; Pompa, P. P. SuperResolution Fluorescence Imaging of Biocompatible Carbon Dots. Nanoscale 2014, 6, 8617−8623. (55) Verma, N. C.; Khan, S.; Nandi, C. K. Single-Molecule Analysis of Fluorescent Carbon Dots Towards Localization-Based SuperResolution Microscopy. Methods Appl. Fluoresc. 2016, 4, 044006. (56) Hafi, N.; Grunwald, M.; van den Heuvel, L. S.; Aspelmeier, T.; Chen, J.-H.; Zagrebelsky, M.; Schütte, O. M.; Steinem, C.; Korte, M.; Munk, A.; et al. Fluorescence Nanoscopy by Polarization Modulation and Polarization Angle Narrowing. Nat. Methods 2014, 11, 579−584. (57) Li, X.; Zhang, S.; Kulinich, S. A.; Liu, Y.; Zeng, H. Engineering Surface States of Carbon Dots to Achieve Controllable Luminescence for Solid-Luminescent Composites and Sensitive Be2+ Detection. Sci. Rep. 2014, 4, 4976. (58) Dong, Y.; Pang, H.; Yang, H. B.; Guo, C.; Shao, J.; Chi, Y.; Li, C. M.; Yu, T. Carbon-Based Dots Co-Doped with Nitrogen and Sulfur for High Quantum Yield and Excitation-Independent Emission. Angew. Chem., Int. Ed. 2013, 52, 7800−7804. (59) Liu, Y.; Zhou, L.; Li, Y.; Deng, R.; Zhang, H. Facile Synthesis of Nitrogen-Doped Carbon Dots with Robust Fluorescence in a Strongly Alkaline Solution and a Reversible Fluorescence ‘Off−On’ Switch Between Strongly Acidic and Alkaline Solutions. RSC Adv. 2016, 6, 108203−108208. (60) Zhang, Y.; He, J. Facile Synthesis of S, N Co-Doped Carbon Dots and Investigation of their Photoluminescence Properties. Phys. Chem. Chem. Phys. 2015, 17, 20154−20159. (61) Yuan, Y. H.; Liu, Z. X.; Li, R. S.; Zou, H. Y.; Lin, M.; Liu, H.; Huang, C. Z. Synthesis of Nitrogen-Doping Carbon Dots with Different Photoluminescence Properties by Controlling the Surface States. Nanoscale 2016, 8, 6770−6776. (62) Hou, J.; Wang, L.; Zhang, P.; Xu, Y.; Ding, L. Facile synthesis of carbon dots in an immiscible system with excitation-independent emission and thermally activated delayed fluorescence. Chem. Commun. 2015, 51, 17768−17771. (63) Wen, Z.-H.; Yin, X.-B. Excitation-Independent Carbon Dots, from Photoluminescence Mechanism to Single-Color Application. RSC Adv. 2016, 6, 27829−27835.

(24) Dhenadhayalan, N.; Lin, K.-C.; Suresh, R.; Ramamurthy, P. Unravelling the Multiple Emissive States in Citric-Acid-Derived Carbon Dots. J. Phys. Chem. C 2016, 120, 1252−1261. (25) Zhu, S.; Wang, L.; Zhou, N.; Zhao, X.; Song, Y.; Maharjan, S.; Zhang, J.; Lu, L.; Wang, H.; Yang, B. The Crosslink Enhanced Emission (CEE) in Non-conjugated Polymer Dots: From the Photoluminescence Mechanism to the Cellular Uptake Mechanism And Internalization. Chem. Commun. 2014, 50, 13845−1384. (26) Sun, Y.; Cao, W.; Li, S.; Jin, S.; Hu, K.; Hu, L.; Huang, Y.; Gao, X.; Wu, Y.; Liang, X.-J. Ultrabright and Multicolorful Fluorescence of Amphiphilic Polyethyleneimine Polymer Dots for Efficiently Combined Imaging and Therapy. Sci. Rep. 2013, 3, 3036. (27) Liu, Z. X.; Wu, Z. L.; Gao, M. X.; Liu, H.; Huang, C. Z. Carbon Dots with Aggregation Induced Emission Enhancement for Visual Permittivity Detection. Chem. Commun. 2016, 52, 2063−2066. (28) Choi, Y.; Kang, B.; Lee, J.; Kim, S.; Kim, G. T.; Kang, H.; Lee, B. R.; Kim, H.; Shim, S.-H.; Lee, G.; et al. Integrative Approach toward Uncovering the Origin of Photoluminescence in Dual HeteroatomDoped Carbon Nanodots. Chem. Mater. 2016, 28, 6840−6847. (29) Sharma, A.; Gadly, T.; Gupta, A.; Ballal, A.; Ghosh, S. K.; Kumbhakar, M. Origin of Excitation Dependent Fluorescence in Carbon Nanodots. J. Phys. Chem. Lett. 2016, 7, 3695−3702. (30) Kasprzyk, W.; Bednarz, S.; Bogdal, D. Luminescence Phenomena of Biodegradable Photoluminescent Poly(Diol Citrates). Chem. Commun. 2013, 49, 6445−6447. (31) Kasprzyk, W.; Bednarz, S.; Ż mudzki, P.; Galica, M.; Bogdał, D. Novel Efficient Fluorophores Synthesized from Citric Acid. RSC Adv. 2015, 5, 34795−34799. (32) Demchenko, A. P.; Dekaliuk, M. O. The Origin of Emissive States of Carbon Nanoparticles Derived from Ensemble-Averaged and Single-Molecular Studies. Nanoscale 2016, 8, 14057−14069. (33) Ghosh, S.; Chizhik, A. M.; Karedla, N.; Dekaliuk, M. O.; Gregor, I.; Schuhmann, H.; Seibt, M.; Bodensiek, K.; Schaap, I. A. T.; Schulz, O.; et al. Photoluminescence of Carbon Nanodots: Dipole Emission Centers and Electron−Phonon Coupling. Nano Lett. 2014, 14, 5656− 5661. (34) Dekaliuk, M. O.; Viagin, O.; Malyukin, Y. V.; Demchenko, A. P. Fluorescent Carbon Nanomaterials: ‘‘Quantum Dots’’ or Nanoclusters? Phys. Chem. Chem. Phys. 2014, 16, 16075−16084. (35) Fu, M.; Ehrat, F.; Wang, Y.; Milowska, K. Z.; Reckmeier, C.; Rogach, A. L.; Stolarczyk, J. K.; Urban, A. S.; Feldmann, J. Carbon Dots: A Unique Fluorescent Cocktail of Polycyclic Aromatic Hydrocarbons. Nano Lett. 2015, 15, 6030−6035. (36) Jeong, C. J.; Lee, G.; In, I.; Park, S. Y. Concentration-Mediated Multicolor Fluorescence Polymer Carbon Dots. Luminescence 2016, 31, 897−904. (37) Ferrari, A. C. Raman Spectroscopy of Graphene and Graphite: Disorder, Electron−Phonon Coupling, Doping and Nonadiabatic Effects. Solid State Commun. 2007, 143, 47−57. (38) Sudolská, M. r.; Dubecký, M. s.; Sarkar, S.; Reckmeier, C. J.; Zbořil, R.; Rogach, A. L.; Otyepka, M. Nature of Absorption Bands in Oxygen-Functionalized Graphitic Carbon Dots. J. Phys. Chem. C 2015, 119, 13369−13373. (39) Reckmeier, C. J.; Wang, Y.; Zboril, R.; Rogach, A. L. Influence of Doping and Temperature on Solvatochromic Shifts in Optical Spectra of Carbon Dots. J. Phys. Chem. C 2016, 120, 10591−10604. (40) Zhang, Y.; Wang, Y.; Feng, X.; Zhang, F.; Yang, Y.; Liu, X. Effect of Reaction Temperature on Structure and Fluorescence Properties of Nitrogen-Doped Carbon Dots. Appl. Surf. Sci. 2016, 387, 1236−1246. (41) Wu, Q.; Li, W.; Wu, P.; Li, J.; Liu, S.; Jin, C.; Zhan, X. Effect of Reaction Temperature on Properties of Carbon Nanodots and Their Visible-light Photocatalytic Degradation of Tetracyline. RSC Adv. 2015, 5, 75711−75721. (42) Zhang, J.; Yang, L.; Yuan, Y.; Jiang, J.; Yu, S.-H. One-Pot GramScale Synthesis of Nitrogen and Sulfur Embedded Organic Dots with Distinctive Fluorescence Behaviors in Free and Aggregated States. Chem. Mater. 2016, 28, 4367−4374. (43) Gao, M. X.; Liu, C. F.; Wu, Z. L.; Zeng, Q. L.; Yang, X. X.; Wu, W. B.; Li, Y. F.; Huang, C. Z. A Surfactant-Assisted Redox 1051


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The Journal of Physical Chemistry Letters (64) Chen, H.; Wang, L.; Fu, H.; Wang, Z.; Xie, Y.; Zhang, Z.; Tang, Y. Gadolinium Functionalized Carbon Dots for Fluorescence/ Magnetic Resonance Dual-Modality Imaging of Mesenchymal Stem Cells. J. Mater. Chem. B 2016, 4, 7472−7480. (65) Cheng, J.; Wang, C.-F.; Zhang, Y.; Yang, S.; Chen, S. Zinc IonDoped Carbon Dots with Strong Yellow Photoluminescence. RSC Adv. 2016, 6, 37189−37194.

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