Article pubs.acs.org/JPCC
Cite This: J. Phys. Chem. C 2018, 122, 12384−12394
Striking Similarities in the Fluorescence Behavior between Carbon Dots and Ionic Liquids: Toward Understanding the Fluorescence Behavior of Carbon Dots Subhasis Roy,† Naupada Preeyanka,† Debashis Majhi,† Sudipta Seth,‡ and Moloy Sarkar*,† †
School of Chemical Sciences, National Institute of Science Education and Research, HBNI, P.O. Jatni, Khurda, Bhubaneswar 752050, Odisha, India ‡ School of Chemistry, University of Hyderabad, Hyderabad 500046, India S Supporting Information *
ABSTRACT: Despite several studies, convincing explanation for the fluorescence of carbon dots (CDs) and its excitation wavelength dependence behavior has not yet emerged. It may be noted that direct structure−property correlation can be misleading based on solely transmission electron microscopy micrographs as it does not fully reveal the possibility of heterogeneous nature of the samples in a sense that it does not fully reveal the possibility of having both carbonaceous nanoparticles as well as small organic molecular-based systems. The present work is undertaken specifically to address this issue. A detailed spectroscopic investigation comprising steady-state absorption, emission, time-resolved fluorescence, and fluorescence correlation spectroscopy (FCS) studies has been carried out on CDs, synthesized from two different sources. Similar investigations have also been carried out on the systems such as aromatic and aliphatic ionic liquids (ILs), which are known to be fluorescent in their neat conditions. Interestingly, the fluorescence behavior of CDs is observed to be very similar to that of neat ILs. Recent studies by Kim and co-workers have categorically demonstrated that fluorescence from neat ILs can originate from associated structures of ILs. In the present work, the excitation wavelength-dependent fluorescence measurements, emission wavelength-dependent radiative recombination and FCS studies on CDs and other systems including ILs have established that the presence of energetically different associated structures (in the ground state) in CD solution is primarily responsible for the fluorescence behavior of CDs. Dynamic light scattering measurements and dilution studies through FCS have also provided evidence in favor of associated structures in CD solution. Excitation wavelength-dependent fluorescence behavior of CDs can also be explained on the basis of energetically different associated structures that are formed in CD solution during its synthesis. Essentially the present investigations have revealed that carbon dots are not inherently fluorescent, rather fluorescence from CD solution arises due to the presence of associated/ networked structures similar to what has been observed in systems such as neat ILs. fluorescence in CDs: (a) the presence of conjugated π-systems in the carbon core,9,10 (b) functional groups present on the main carbon backbone of CDs, also known as surface states,11,12 (c) the presence of fluorescent molecules connected to the surface or inside of CDs,13,14 (d) enhanced emission due to crosslinking,15,16 and (e) electron−hole recombination.17,18 The several propositions with regard to the origin of fluorescence behavior of CDs are self-indicative of the fact that there is no convincing explanation for the fluorescence behavior of CDs. However, of late, several reports have described that inherently CDs are not fluorescent rather a fluorescent polycyclic molecule (produced during the synthesis of CDs) is responsible for the emission of CDs. Please note that many of the above studies depicted that CDs are inherently fluorescent. In majority of these works, the
1. INTRODUCTION In recent times, fluorescent carbon dots (CDs) have attracted considerable attention from the scientific community owing to their simple method of preparations and useful optoelectronic properties.1,2 Interestingly, photoluminescent CDs are also thought to be more attractive candidate in comparison to usual semiconductor quantum dots and organic dyes in terms of high solubility in aqueous medium, chemical inertness, optical stability, easy functionalization, low toxicity, and good biocompatibility.3−5 Even though photoluminescence of CDs are being used extensively in many applications such as bioimaging,6 light harvesting,7 optical materials,8 and so forth, the origin of photoluminescence behavior of CDs is not yet understood fully and thus this issue merits further investigations. Several research groups have made an attempt to unravel the origin of photoluminescence in CDs in recent times. These reports summarize the following possibilities for the origin of © 2018 American Chemical Society
Received: April 24, 2018 Published: April 30, 2018 12384
DOI: 10.1021/acs.jpcc.8b03859 J. Phys. Chem. C 2018, 122, 12384−12394
Article
The Journal of Physical Chemistry C
the existence of energetically different associated structures in the ground state and the inefficiency of the excitation energy transfer process among these structures. These recent important studies unambiguously established that the fluorescence behavior of neat ILs can be best explained on the basis of the associated structure but not just on the fluorescence impurity present within the system. We also note here that while searching for the literature on fluorescence from neat chemical systems we could find that systems having networked structure like dendrimer can fluoresce on its own.36,37 Here, we again note that very recently Baker and co-workers20 have depicted that during the synthesis of CDs, molecular fluorophore, potentially oligomeric and polymeric in nature, may form which predominantly contribute in CD emission. The above literature reports and outcome of the work on the fluorescence behavior of neat ILs demand a re-look at the origin of fluorescence for CDs. Because it is also known that synthetic processes such as hydrothermal and microwave syntheses can give rise to network structures,38 it is highly likely that during the synthesis of CDs through similar synthetic routes, some kind of associated/ networked structures are formed instead of a specific fluorescent molecule, and these associated structures can cause fluorescence similar to what has been observed for ILs. Because the associated structures/molecular aggregates of ILs are known to be primarily responsible species for the emission behavior of ILs, it is expected to be worthwhile objective to investigate the optical behavior of CDs along with neat ILs. Keeping all these in mind, in the present work, we have carefully studied steady-state absorptions, emissions, temperature-dependent emissions, and time-resolved fluorescence decay, in CDs synthesized from different carbon sources along with ILs. Particularly, CDs synthesized from citric acid (CA-CD) and L-tartaric acid (TA-CD), one aromatic IL, 1propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (PMIMTFMSI), and one aliphatic IL, 1-methyl-1-propylpyrollidinium-bis(trifluoromethylsulfonyl)imide (MPPLTFMSI) have been used for this study (Scheme 1). Citric acid and
structure−property correlation, in particular, structure−optical property correlation, was based on the standard characterization technique such as transmission electron microscopy (TEM) and spectroscopic data from the spectrometer in a separate manner. We would like to recall the recent investigation by Ferrante and co-workers19 where they have categorically mentioned that direct structure−property correlation can be misleading based on solely TEM micrographs as it does not fully reveal the possibility of heterogeneous nature of the samples in a sense that it does not fully reveal the possibility of having both carbonaceous nanoparticles as well as small organic molecular-based systems. Please also note that TEM results will not be able to tell whether the emission is originating from carbonaceous particles or small or oligomeric molecular-based system. In this context, we would also like to note that very recently Baker and co-workers20 have shown that during the synthesis of CDs, molecular fluorophores (potentially oligomeric and polymeric in nature) may form which predominantly contribute in CD emission. One more interesting aspect related to the same issue that is observed is the excitation wavelength-dependent emission of CDs. As per Kasha’s rule,21,22 the emission maximum of a given fluorophore should be independent of the excitation wavelength. However, in the case of some CDs, upon increasing the excitation wavelength, the emission maximum is observed to shift toward the longer (red) wavelength region.14,23 This aspect also needs to be understood in conjunction with the usual fluorescence behavior of CDs. In this context, it is noteworthy to mention that a recent work by Kumbhakar and co-workers23 have shown the presence of various types of aggregates even at very dilute solution of CDs that are found to be responsible for the formation of various multiple discrete electronic states which causes the excitation wavelengthdependent emission in CDs. While discussing the fluorescence behavior of CDs, we would like to take a serious note on the recent observations that have emerged in explaining the fluorescence behavior of neat ionic liquids (ILs).24−26 It has been recently demonstrated that the associated structures of molecules of ILs can be responsible for the fluorescence behavior of a given sample.27−32 It has also been shown that π-conjugate systems are not essential, rather non-π-conjugated systems such as pyrrolidinium ILs can also exhibit fluorescence.31 At this point, it is interesting to note that some of the earlier reports on neat ILs,33,34 suggested that neat ILs itself cannot be fluorescent, and their fluorescence originates from the chemical impurities. However, recent work by Kim and co-workers through fluorescence correlation spectroscopy (FCS) studies has demonstrated that fluorescence from neat ILs originates from molecular aggregates in the neat ILs.27,28 This particular study by Kim’s group and earlier work by Samanta and coworkers29,30 have categorically demonstrated that the fluorescence from neat ILs is not due to any fluorescence impurity rather associated structures of the IL. This is a very interesting observation in the context of research regarding CDs’ fluorescence where the presence of fluorescence impurity is thought to be one of the main reasons for emission of CDs. It may be noted here that the excitation wavelength-dependent fluorescence behavior of neat ILs are also explained on the basis of associated structures that are formed in neat ILs. In fact, it has been demonstrated by Paul and co-workers30,35 that excitation wavelength-dependent fluorescence which is also known as “red-edge effect (REE)” in neat ILs can arise due to
Scheme 1. Molecular Structures of Different ILs
tartaric acid-based CDs are used in the present study as these CDs are explored quite extensively for several works. The outcome of the present work is interesting in a sense suggests that the associated structure of molecules produced during the synthesis of CDs can cause fluorescence of CDs similar to what has been observed for ILs. The present study is expected to provide a new pathway toward understanding the origin of fluorescence in CDs. 12385
DOI: 10.1021/acs.jpcc.8b03859 J. Phys. Chem. C 2018, 122, 12384−12394
Article
The Journal of Physical Chemistry C
Eclipse fluorescence spectrophotometer, respectively. Temperature-dependent emission spectra were recorded with a Edinburgh FS5 spectrofluorimeter. The fluorescence spectra were corrected for the spectral sensitivity of the instrument, and the samples were excited at different wavelengths. Timeresolved fluorescence measurements were carried out using a time-correlated single-photon counting (TCSPC) spectrometer (Edinburgh, OB920) using a 375 nm picoseconds pulse diode laser (EPL), and the signals were collected at the magic angle (54.7°) using a Hamamatsu microchannel plate photomultiplier tube (R3809U-50). The instrument response function (IRF) was recorded by a scatterer (dilute ludox solution in water) in place of the sample. IRF of our TCSPC setup is 75 ps for 375 nm picoseconds pulse diode laser which was measured by determining the full width half-maxima of the IRF profile. Nonlinear least-squares iteration procedure was used for decay analysis using F900 decay analysis software. The qualities of the fit were determined by judgments of the chi square (χ2) values and by visual inspection of the residuals which were obtained by fitting. All the experiments were done at room temperature (298 K), and the temperature was controlled by circulating water through the cell holder using a Quantum, North West (TC 125) temperature controller with ±0.2 °C accuracy bands. The FCS measurements of the CDs, ILs were determined by using a time-resolved confocal fluorescence microscope, MicroTime 200 (PicoQuant).
2. EXPERIMENTAL SECTION 2.1. Materials. 1-Propyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide and 1-propyl-1-methylpyrrolidinium-bis(trifluoromethyl-L-sulfonyl)imide were obtained from Merck, Germany (>99% purity) and used as received. The water and halide contents of the ILs were
