Molecular Origin and Self-Assembly of Fluorescent Carbon Nanodots ...

Addition/Correction Cite This: J. Phys. Chem. Lett. 2017, 8, 5861-5864

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Addition to “Molecular Origin and Self-Assembly of Fluorescent Carbon Nanodots in Polar Solvents” Arjun Sharma, Trilochan Gadly, Suman Neogy, Sunil Kumar Ghosh, and Manoj Kumbhakar* J. Phys. Chem. Lett. 2017, 8, 1044−1052. DOI: 10.1021/acs.jpclett.7b00170 fluorophores,1 without emphasizing the self-assembly of CD particles. In fact, the observed very fast rotational depolarization than expected from particles of over nanometer dimension8 can also be realized from these FCS results, an acknowledgment of the presence of small and free fluorophore moieties. Additionally, excitation wavelength resolved FCS measurements also hint at the presence of larger hydrodynamic radii particles at excitation wavelengths over 440 nm, which is consistent with the dimensions of CDs found from transmission electron microscopy (TEM) measurements. Following their results, we also recorded FCS curves with 488 nm excitation, as shown in Figure 1. The presence of slow diffusing species with hydrodynamic radius (rh) of 4.5 nm (∼23%), similar to earlier TEM results,1 further reaffirms the presence of emissive CD particles. However, even with 488 nm excitation, PL contribution from subnanometer species is quite significant in our CD sample. Righetto et al.4 further argued from time-resolved electron paramagnetic resonance (TREPR) measurements that carbon sp2 domains are embedded within carbon sp3 scaffolds of carbon cores, similar to the inferences made from investigations with Raman spectroscopy.1 Single particle imaging and nanocavity-based quantum yield measurements with similar excitation wavelengths by Ghosh et al.11 have conclusively demonstrated bright emission from single CD particles, and their estimated hydrodynamic dimensions match high-resolution TEM and atomic force microscopy (AFM) results, besides the unique structural insight of CDs and their correlation with observed PL.

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e provide here additional new results in the context of molecular origin and particle structure1 to further support our arguments of heterogeneity for the fascinating photoluminescence (PL) behavior of carbon nanodot (CD) samples. Our spectroscopic investigations have established in-band heterogeneity in the main excitation−emission band2 and molecule-like PL behavior of CD samples, especially quenching- and concentration-dependent measurements.1 In spite of emphasizing the molecular origin for PL in a CD sample, in the absence of direct measurements like fluorescence correlation spectroscopy (FCS), it is very challenging to identify the luminescence moiety as free molecular fluorophore or chromophore embedded CD particles. Although it is debatable whether molecular fluorophores are embedded into carbonized nanoparticles (CDs) or otherwise,3 recent FCS results by Righetto et al. clearly prove that the main excitation−emission band is exclusively due to small molecule-like species,4 as was earlier pointed out by Krysmann et al.5 and later isolated by Song et al.6,7 Now we have also recorded FCS curves for these previous CD samples in water with 405 nm excitation wavelength (see Figure 1), which reiterates diffusion of subnanometer molecular species similar to coumarin 503 (C503). Hence, in view of the exclusive FCS results by Righetto et al.4 and the present one, earlier reported concentration-dependent broadening of excitation spectra and splitting at very high concentrations along with other molecular aspects of PL is also certainly attributed to aggregation of these free molecular species similar to other

Figure 1. FCS curves with three-dimensional diffusion fits (smooth lines) for C503 (blue), Atto488 (green), and CD-f2 (red) in water. The present FCS setup and analysis were reported earlier,9 except for the additional excitation line with CW 405 nm diode laser. FCS curve for CD-f2 with 488 nm excitation was best fitted with two diffusion times (τd). Diffusion coefficients for standard dyes C5034 and Atto48810 are 6.72 × 10−10 m2 s−1 and 4.0 × 10−10 m2 s−1. Overall results are also similar for other CD fractions.

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DOI: 10.1021/acs.jpclett.7b02991 J. Phys. Chem. Lett. 2017, 8, 5861−5864

The Journal of Physical Chemistry Letters

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Figure 2. TEM images of CD agglomerates of spherical particles (a,b), which shows both compact (d) as well as hollow particles (c) marked with blue arrows of well-ordered graphene-like lamellae structure (c−i). Yellow arrows indicate the hexagonal lamellar arrangement. Particles with deformed lamellae structure at boundaries (h) are also evident along with regular lamellar structure (g). Multicore particles (f, red arrows) were also frequently observed other than generally reported regular crystalline CD structure (j). All the CD fractions show similar internal structure.

particles. Insight of CD particles and its comparative assessment with spherical carbon soots and carbonaceous particles is highly imperative, especially in the context of designing CD particles with improved PL and other characteristics as a luminescent probe or marker for use with visible excitation wavelengths. High-resolution TEM images were recorded for the same citric acid-based samples earlier prepared for exploring the molecular origin and self-assembly of CDs and also used for present FCS measurements. This allows us to directly disentangle the structural aspect and heterogeneity in these CD samples. Other than spherical particles with regular crystal lattices, TEM images also reveal an array of agglomerate structures with hundreds of spherical primary particles, which we generally avoid considering in our analysis (see Figures S14−S16 in ref 1). Such a representative agglomerate structure is shown in Figure 2a. A closer inspection of these spherical primary structures reveals a striking morphology of curved lattice arrangement with occasional not so defined boundaries, shown in Figure 2b,c. Distinct primary particles (Figure 2d,e,g,i) indicate that they

Further stability against photobleaching for this longer wavelength emission has been attributed to the protection offered by carbon matrix to the incorporated chromophore by Xiong et al.3 These reports suggest that the higher wavelength excitation/emission is predominantly due to CD particles, and PL results nicely corroborate with TEM measurements. Generally, the internal structure of these carbogenic CD particles have received minuscule attention except for the regular lattice spacing of around 0.22 nm analogous to graphite. The G and D bands, comparable to stacked graphene/graphene oxides, further pipe into the layered structure of CDs. However, such regular crystal lattice structure under electron microscopy is also probable due to molecular aggregates. So, are the observed nanometer-sized particles with regular lattice structure in electron micrographs due to aggregates of molecular species (induced by drying on TEM grids)12 or due to true CD particles? Here we further explored the structure of CD particles for insight into their formation from these molecular precursors and its resemblance with other reported naturally occurring or man-made carbon 5862



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excitation wavelengths leading to excitation of different aromatic domains in the carboneous structure by Rightteo et al.4 Based on these results and all other relevant reports, we conclude that primary heterogeneity in a CD sample, responsible for its fascinating PL behavior, is due to the presence of both molecular fluorophores and CD particles, compounded with the abundance of functional groups, size, and structural distribution.4,5,11,20,21 The occurrence of prototropic equilibrium of functional groups like carboxyl, hydroxyl, etc. further enriches electronic state heterogeneity in prepared CD samples.20,22 Observed composite spectral behavior for CDs is additionally complex due to the possibility of self-assembly of these emissive units and alteration of involved electronic states.1 The huge excitation-dependent emission spectral shift in CD samples, which apparently challenges the classical Kasha−Vavilov rule, is primarily due to the involvement of multiple electronic states arising from heterogeneity in samples.2

coalesced with other particles in the agglomerates after complete formation of particles. Concentric nanostructures confirm that these particles were formed during high-temperature synthesis from organic materials. These primary structures with varying aspect ratios often have a hollow fullerenic (Figure 2c) or hairpin-like core (Figure 2f) with concentric multiple layers of molecular precursors, which are possibly similar to graphene oxides. The later possibility is supported by the presence of oxygen as determined by energy dispersive X-ray (EDX) measurements and earlier reported1 G and D bands in the Raman spectrum for the same CD sample. Primary particles of hexagonal shape with round corners (yellow arrows in Figure 2b) were also observed. The interlamellar distance in these local crystallites were around 0.362 nm, which is little larger than the interlamellar distance of 0.335 nm for the ⟨002⟩ plane of graphite. The spacing of 0.21−0.26 nm for earlier reported regular lattice structure (Figure 2j) corresponds to the ⟨100⟩ plane of graphite. To be noted, similar CD structures were earlier reported by Ghosh et al.11 and were also identified in fluorescence confocal images. Recently, similar structure was also reported by Li et al.13 for graphene quantum dots under highly acidic conditions. In addition, deformed lamellae at the boundary with anisotropic arrangements were also evident, though less frequent (Figure 2h). All these structures are very commonly encountered with carbon soot aggregates, irrespective of their origin from combustion of wood, diesel engine emission, or dark pigment layers covering speleothems.14−18 Primary soot particles are composed of ordered domains of small parallel carbon layers analogous to graphite. The structural similarity of CD particles with carbon soot particles further highlight the comparable combustion process. Following the depiction of carbon soot particles by Heidenreich et al.,19 we schematically represent the internal structure of CD in Chart 1. The layers of molecular precursors

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AUTHOR INFORMATION

ORCID

Sunil Kumar Ghosh: 0000-0003-2508-6181 Manoj Kumbhakar: 0000-0001-9076-8045 Notes

The authors declare no competing financial interests.

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REFERENCES

(1) Sharma, A.; Gadly, T.; Neogy, S.; Ghosh, S. K.; Kumbhakar, M. Molecular Origin and Self-Assembly of Fluorescent Carbon Nanodots in Polar Solvents. J. Phys. Chem. Lett. 2017, 8, 1044−1052. (2) 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. (3) Xiong, Y.; Schneider, J.; Reckmeier, C. J.; Huang, H.; Kasák, P.; Rogach, A. L. Carbonization Conditions Influence the Emission Characteristics and the Stability Against Photobleaching of Nitrogen Doped Carbon Dots. Nanoscale 2017, 9, 11730−11738. (4) Righetto, M.; Privitera, A.; Fortunati, I.; Mosconi, D.; Zerbetto, M.; Curri, M. L.; Corricelli, M.; Moretto, A.; Agnoli, S.; Franco, L.; et al. Spectroscopic Insights into Carbon Dot Systems. J. Phys. Chem. Lett. 2017, 8, 2236−2242. (5) 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. (6) 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. (7) Shi, L.; Yang, J. H.; Zeng, H. B.; Chen, Y. M.; Yang, S. C.; Wu, C.; Zeng, H.; Yoshihito, O.; Zhang, Q. Carbon Dots with High Fluorescence Quantum Yield: The Fluorescence Originates from Organic Fluorophores. Nanoscale 2016, 8, 14374−14378. (8) 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. (9) Sharma, A.; Enderlein, J.; Kumbhakar, M. Photon Antibunching in Complex Intermolecular Fluorescence Quenching Kinetics. J. Phys. Chem. Lett. 2016, 7, 3137−3141. (10) Zhang, X.; Poniewierski, A.; Jelińska, A.; Zagożdżon, A.; Wisniewska, A.; Hou, S.; Hołyst, R. Determination of Equilibrium and Rate Constants for Complex Formation by Fluorescence Correlation Spectroscopy Supplemented by Dynamic Light Scattering and Taylor Dispersion Analysis. Soft Matter 2016, 12, 8186−8194. (11) 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

Chart 1. Internal Structure of Spherical Primary Particles in CD Agglomerates

stack with certain orderness to form spherical particles. Such stacking and layering of luminescent molecular precursors (LMP) within the curved domains of CD particle is indicative of a range of possible aggregate states. The dipoles of these LMPs inside CD particles may be organized in any geometric arrangements from head-to-head to head-to-tail and in any oblique association. Considerably reduced fluorescence of CD particles than its molecular precursors is further indicative of predominant H-type aggregation. These structural insights exceedingly corroborate the proposal by Demchenko et al. of H-aggregate-type excitonic states of stacked luminescent molecular precursors with their variable coherence spreading over the nanoparticles. This further supports the argument of different 5863



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