Molecular Fluorescence in Citric Acid-Based Carbon Dots

Article pubs.acs.org/JPCC

Molecular Fluorescence in Citric Acid-Based Carbon Dots Julian Schneider,† Claas J. Reckmeier,† Yuan Xiong,† Maximilian von Seckendorff,† Andrei S. Susha,† Peter Kasák,‡ and Andrey L. Rogach*,† †

Department of Physics and Materials Science and Center for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S.A.R. ‡ Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar S Supporting Information *

ABSTRACT: Nitrogen-doped carbon dots synthesized from citric acid as a carbon precursor have recently been considered to contain fluorescent derivatives of citrazinic acid, which contribute to their emission in the blue spectral range. To study the impact of such molecular fluorescent species on the optical properties of carbon dots, we synthesized three samples employing citric acid and three different nitrogen sources: ethylenediamine, hexamethylenetetramine, and triethanolamine. On the basis of the analysis of the nitrogen content and its coordination by X-ray photoelectron spectroscopy, FTIR spectra, and systematically comparing absorption, steady-state emission, and photoluminescence decays of each kind of carbon dot, we derive the influence of the molecular precursors and gain further understanding of the complex structure of carbon dots highlighting the strong impact of molecular fluorescence in the samples produced with ethylenediamine and hexamethylenetetramine.

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INTRODUCTION Carbon dots (CDs) are carbon-based nanoparticles with a strong emission in the blue; their photoluminescence quantum yields (PL QY) in this region often reach 80%.1,2 They possess useful properties such as high chemical stability and low toxicity and are thus considered to be promising materials for a number of biological and optoelectronic applications.3−6 While the optical properties can vary widely for differently synthesized CDs, their common characteristic has been considered to be excitation-wavelength-dependent emission, which has been the subject of a number of recent investigations.7−11 Apart from the surface chemistry of the CDs, which is commonly considered to impact the absorption and emission properties of CDs,8,12−14 Fu et al. explored the presence of polycyclic aromatic hydrocarbons in the CD core.15 The doping of CDs with heteroatoms such as nitrogen, sulfur, and phosphorus has been shown to have a large impact on their optical properties as well, often enhancing the PL QY.1,2,16,17 In this respect, Sarkar et al. reported a theoretical study for nitrogen-functionalized CDs, relating their absorption properties to the presence of different nitrogen coordination sites within the CD framework.18 The synthesis of CDs can be conducted by a vast number of different approaches, both top-down and bottom-up.16 Bottomup syntheses usually involve rather complex chemical reactions, conducted at high temperature and pressure, resulting in a sequence of steps commonly denoted as polymerization and carbonization.1,19 Under such reaction conditions, the formation of molecular fluorophores can be expected, in particular © 2016 American Chemical Society

within the most common synthesis approach toward CDs that employs citric acid as a carbon source and an amine as a nitrogen source.1,20−22 The first reports on the reactions of these precursors date back to Behrman and Hofmann, who studied the reaction of citric acid and ammonia in 1884 and reported the formation of a blue fluorescent molecule, citrazinic acid.23 Sell and Easterfield in 1893 reported the formation of citrazinic acid from the reaction between citric acid and urea at 130 °C.24 Recently, Yang’s group reported the formation of a fluorescent citrazinic acid derivative in the hydrothermal synthesis of CDs with citric acid and ethylenediamine.25 After separation and purification of the reaction products by column chromatography, the main fluorescent phase was analyzed by NMR and mass spectrometry, revealing a molecular organic species of 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-α]pyridine-7carboxylic acid (denoted as IPCA) with a PL QY of 86%. As pointed out by Yang’s group,25 this organic fluorophore possesses strong emission at 440 nm, dominating the optical properties in this spectral range. From yet another perspective, Kasprzyk et al. discussed the formation of this class of organic fluorophores from the reaction of citric acid with α,β-diamines or similar molecules with double functionality (alcohol, thiol), such as cysteine at 180 °C for 1 h.26−29 Although most reported synthesis protocols of CDs employ temperatures of at least 200 °C and longer reaction times (5−10 h) to achieve carbonReceived: December 12, 2016 Published: December 30, 2016 2014

DOI: 10.1021/acs.jpcc.6b12519 J. Phys. Chem. C 2017, 121, 2014−2022

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The Journal of Physical Chemistry C

Figure 1. Synthesis conditions of citric acid-based CDs using three different nitrogen-containing precursors. (a) Reaction of citric acid and ethylenediamine, resulting in e-CDs and the fluorophore IPCA, as previously reported.25 (b) Reaction of citric acid with hexamethylenetetramine, producing h-CDs and citrazinic acid and/or 3,5 derivatives (marked by −X), due to the decomposition of hexamethylenetetramine to ammonia and formaldehyde at temperatures exceeding 96 °C. (c) Reaction of citric acid and triethanolamine, resulting in t-CDs and no derivatives of citrazinic acid since the tertiary amine prohibits their formation. (a−c) Images of the purified reaction products under ambient light and corresponding diluted solutions under UV light excitation, which reveal blue emission with PL QYs as labeled on the graph.

fluorescence spectrometer (Edinburgh Instruments), and VG Scientific ESCALAB 5 XPS spectrometer. PL decays were recorded using picosecond pulsed diode lasers (Edinburgh Instruments EPLED-320 320 nm, 5 mW and EPL-405 405 nm, 5 mW). The decay data were deconvoluted with the instrument response function and fitted with a multiexponential fit to derive fluorescence lifetimes by taking the weighted average of exponential components. An integrating sphere from Edinburgh Instruments was used to measure the absolute PL QYs. X-ray photoelectron spectroscopy (XPS) measurements have been conducted on a PHI model 5802, Fourier transform infrared (FTIR) spectroscopy was conducted on a PerkinElmer 16 PC FT-IR spectrophotometer, and transmission electron microscopy (TEM) was conducted on a Philips CM-20.

ization, molecular fluorescent species can still be formed in parallel and can notably influence the optical properties of the resulting reaction products.1,2,30−34 Taking these considerations into account, we have studied here three CD materials produced by hydrothermal reactions of citric acid with three different amine precursors: ethylenediamine (reaction product denoted as e-CDs), hexamethylenetetramine (h-CDs), and triethanolamine (t-CDs). After discussing the synthesis conditions, we analyze the nitrogen content and its coordination in the three samples by X-ray photoelectron spectroscopy (XPS). We then focus on the optical properties of the three CD samples and systematically compare them to pure citrazinic acid. With an emphasis on excitation-dependent and time-resolved PL spectroscopy, we derive the influence of molecular fluorophores on the properties of CDs and confirm their presence in e-CD and h-CDs.

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RESULTS AND DISCUSSION Three kinds of CDs studied here (denoted as e-CDs, h-CDs, and t-CDs) were synthesized by reactions of citric acid with three different nitrogen-containing precursors, namely, ethylenediamine, hexamethylenetetramine, and triethanolamine, respectively, as shown in Figure 1. CD formation occurs through carbonization of the precursors at 200 °C19 and yields small particles, as shown in the TEM images in Figure S1. While t-CDs could be clearly resolved as isolated particles, eand h-CDs appeared as larger agglomerates. Apart from the carbonization, reactions at 200 °C are expected to produce molecularly fluorescent species in the synthesis of e-CDs and hCDs (Figure 1a,b). In particular, as previously reported by Song et al., the reaction of citric acid and ethylenediamine yields strongly fluorescent IPCA (PL QY 86%) (Figure 1a),25 which contributes to the high PL QY (53% at 320 nm excitation) of these CDs. For the reaction of citric acid and hexamethylenetetramine (Figure 1b), the purified product (h-CDs) shows a similar dark color under ambient light as the e-CDs, but PL QY is only 17% (320 nm excitation). From the reaction of citric acid and triethanolamine (Figure 1c), a pale-yellow solution with a PL QY of 7% (320 nm excitation) is obtained. As reported previously,27 citric acid, which contains three carboxyl

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EXPERIMENTAL SECTION Materials and Methods. Citric acid (anhydrous, cell culture tested), hexamethylenetetramine (≥99.5%), triethanolamine (reagent grade), and citrazinic acid (97%) were purchased from Sigma-Aldrich. Ethylenediamine (≥99%) was purchased from Accuchem. Milli-Q water was used in all syntheses and spectroscopic measurements. Synthesis of CDs. CDs were synthesized following a standard hydrothermal procedure.1 Citric acid (1.05 g, 5.5 mmol) and an amine precursor (5 mmol) were dissolved in 10 mL of water in a Teflon liner. The Teflon liner was placed in a stainless steel autoclave and held at 200 °C for 5 h. After the autoclave was cooled to room temperature, the reaction products were filtered and centrifuged at 5000 rpm for 10 min to separate them from agglomerated larger particles and redissolved in water for further measurements. Experiments conducted in parallel, which included additional sample purification by dialysis (24 h), showed negligible changes in the overall trends in the photophysical properties of CDs. Characterization. Sample characterization was carried out on a Shimadzu UV-3600 spectrophotometer, FLS920P 2015


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Figure 2. XPS spectra (upper row) and high-resolution C 1s (middle row) and N 1s (bottom row) spectra of (a) e-CDs, (b) h-CDs, and (c) t-CDs.

functionalities, can undergo intramolecular condensation and cyclization with α,β-diamines, β-amino thiols, and β-amino alcohols to form derivatives of citrazinic acid, such as IPCA, alongside the carbonization reaction (Figure 1a). For the reaction of citric acid and hexamethylenetetramine, the parallel formation of the fluorophore can happen through the thermal decomposition of hexamethylenetetramine to ammonia and formaldehyde, which starts at temperatures above 96 °C in acidic media.35,36 Under hydrothermal conditions, ammonia reacts with citric acid to form the fluorophore citrazinic acid.24,28,29,37 Further derivatization of this fluorophore in the 3,5 position can be expected, considering the presence of reactive formaldehyde. However, because of the slow process of decomposition and the lower nucleophilic strength of ammonia, the yield of this fluorophore formation is lower than for the reaction of citric acid and ethylenediamine. The reaction of citric acid and triethanolamine gives no possibility to form similar molecular fluorophores because of the tertiary amine in the reactant (Figure 1c). More likely, carboxylic functionalities from citric acid may react with the terminal alcohols from triethanolamine under hydrothermal conditions, resulting in the formation of networks linked by ester bonds. To study the composition of the three CD species, XPS measurements (Figure 2) with a particular emphasis on highresolution C 1s and N 1s spectra were conducted. The ratio of carbon to nitrogen was determined for all CDs, namely, 83:17 for e-CDs, 86:14 for h-CDs, and 96:4 for t-CDs. It has been previously observed that high nitrogen content typically correlates with high PL QYs of CDs.38 The e-CDs have the greatest amount of nitrogen and the highest PL QY of 53%, while t-CDs have the smallest amount of nitrogen and the lowest PL QY of 7%. The nitrogen content in h-CDs is only slightly lower as compared to that in e-CDs, but the PL QY experiences a significant drop to 17%.

High-resolution C 1s XPS spectra shown in the middle row of Figure 2 demonstrate obvious differences for the three CD species. Peak fitting reveals the main C−C feature at 284.7 eV in all materials. Other notable features originate from the C−N bond at 286.0 eV and CN/CO coordination at 287.8 eV, especially for e- and h-CDs. Because of the lower nitrogen content in t-CDs, the C−N feature is consequently less pronounced. Instead, the strong signal of C−C coordination is ascribed to the predominantly carbonized character of this material. In contrast to that, additional strong signals from C N/CO in h-CDs and both C−O and CN/CO in e-CDs indicate the presence of species with high heteroatom content, such as the fluorophore molecules (Figure 1). More details can be observed in the N 1s spectra shown in the bottom row of Figure 2, which also show significant differences, especially between e-CDs and h-CDs on one side and t-CDs on the other. While e-CDs and h-CDs have a similar overall peak shape, which is dominated by the C−N−C signal at 399.6 eV, the peak of e-CDs is broader, indicating a larger contribution of nitrogen in different coordination states. In e-CDs, the feature of nitrogen in N−H coordination at 400.4 eV, which is also referred to as pyrrolic nitrogen,18 is highly pronounced and can be related to the molecular structure of IPCA. The N−H signal is also present in the h-CDs; however, the ratio to C−N−C, which is also referred to as pyridinic nitrogen,18 is much smaller because of the presence of citrazinic acid (or a 3,5 derivative, as depicted in Figure 1b). For t-CDs, the total intensities of the peaks in the N 1s spectrum are much smaller and the ratio of the features is very different. The C−N−C coordination drops, while at the same time the N−(C)3 signal at 401.8 eV gains significance. This pronounced feature in t-CDs can be linked to the structure of the original nitrogen precursor and also confirms that molecular structural units from the precursors are still observable in the resulting product. 2016


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Figure 3. (a) Absorption and (b) photoluminescence excitation (PLE) spectra of three CD species, compared to the spectra of pure citrazinic acid. Vertical lines show the corresponding excitation wavelengths of the time-resolved PL measurements. At 320 nm, the carbon edge and fluorophore’s specific features are excited, while mostly lower-energy surface states of CDs are excited at 405 nm.

likely to originate from transitions in molecular fluorophores. At wavelengths longer than 400 nm, CDs show a characteristic broad, low-strength absorption extending up to 550 nm. This absorption is commonly assigned to surface states related to functional surface groups in CDs, which form low-energy subband gaps within the n−π* band gap.12,40 We note from Figure 3a that pure citrazinic acid has no absorption in this energy range, as expected. The photoluminescence excitation spectra (PLE) shown in Figure 3b reveal further details about the existence of fluorophores in CDs. The main emissive contribution of CDs usually originates from the excitation region around 330 nm (Figure 3b) and has been assigned to n−π* transitions at the CD edge.12,39 The same features can also be found in citrazinic acid (Figure 3b). In fact, comparing e-CDs and citrazinic acid shows an almost exact superposition of the peaks at 340 nm, pointing out a strong emissive contribution from molecular fluorophores. Similar observations are made for the high-energy peak at around 240 nm excitation, which has been assigned to π−π* transitions in the CD core39 but can also originate from the aromatic structure of citrazinic acid or its derivatives. In contrast to the PLE spectrum of pure citrazinic acid, which shows two discrete transitions, our CDs show, besides broader signals, generally higher PLE intensities in the spectral region between the two features at 240 and 340 nm, indicating a broader range of the excited states contributing to the main emission. In agreement with expected lower molecular fluorophore contributions in h- and t-CDs, the molecular character of the PLE spectra appears to decrease from e-CDs to h-CDs and t-CDs. Especially for t-CDs we note that the high energy peak is red shifted to 255 nm demonstrating a nonfluorophore origin. Additional excitation spectra at different emission wavelengths are provided in Figures S4−S6 for e-, h-, and t-CDs, respectively. At longer emission wavelengths, the intensities of the main excitation peaks at 240 and 340 nm continuously decrease, whereas a small shoulder develops at around 450 nm. This is in agreement with the broad signal in the absorpion spectra and is assigned to the excitation of lowenergy surface states. Two vertical lines in Figure 3 mark the positions of the two excitation wavelengths (320 and 405 nm) and the optical densities, chosen for the time-resolved PL measurements. At 320 nm excitation, we expect to excite the fluorophore molecules as well as the CD core and edge states, which are responsible for the brightest emission in CDs. At 405 nm,

More structural information on the CD samples was obtained from FTIR spectra, which are presented in Figure S2 and revealed the presence of characteristic surface chemical groups of CDs. The spectra show broad absorption features at around 3500 cm−1, confirming the presence of alcohol (−OH) and amine groups (−NH2) in all CDs. Strong signals in the range of 1800−1600 cm−1 are associated with CO stretching vibrations and reveal several carbonyl and carboxyl functionalities, especially in e- and h-CDs. The strong signal at 1710 cm−1 in e- and h-CDs is ascribed to the CO stretching vibration of α,β unsaturated carboxylic acid moieties, found in citrazinic acid and its derivatives. Additional signals at low wavenumbers observed for e- and h-CDs can be linked to the presence of aromatic structures, which is in agreement with previously discussed results. To highlight the contribution of molecular fluorophores in eand h-CDs, we now focus on the spectroscopic studies of the three different CD species. Figure 3a shows the steady-state absorption spectra of three citric acid-based CDs (e-CDs, hCDs, t-CDs). We also include molecularly pure (commercially available) citrazinic acid in our measurements in order to compare the optical properties of CDs with the most basic unit from the presumed class of fluorophores. Citrazinic acid shows a sharp high-energy UV peak at 234 nm and a broad peak at 345 nm. Similar features are visible in the spectra of e-CDs, which show a shoulder at the exact same high-energy position (234 nm) and a slightly broader peak at 340 nm (Figure 3a). While the shoulder at 234 nm is not recognizable in the h-CDs spectrum (not shown), the main absorption peak of h-CDs is shifted to shorter wavelengths (330 nm), maintaining a similar shape compared to that of e-CDs and citrazinic acid. The blue shift in comparison to citrazinic acid, which is observed in all spectroscopic measurements of h-CDs, points out the possible further derivatization of citrazinic acid by formaldehyde, which is formed in the synthesis of h-CDs. In contrast to both other CD species, t-CDs show none of these specific features, except for a strong high-energy UV absorption. In particular, the absence of any features around 340 nm confirms the substantial difference between t-CDs and e-/h-CDs. For the previously reported CDs, high-energy UV absorption (at around 234 nm; visible here only for e-CDs and citrazinic acid in Figure 3a) has been commonly assigned to π−π* transitions of sp2-hybridized carbon, while the absorption region around 340 nm has been assigned to n−π* transitions at the edge of the carbon lattice.39 However, our data show that such distinct features are more 2017


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Figure 4. Average PL lifetimes of three CD species and citrazinic acid as a function of the detection wavelength at (a) 320 nm and (b) 405 nm excitation. Dashed lines show the corresponding PL spectra. At 320 nm excitation, fluorophores and higher-energy CD edge states are excited, resulting in blue PL, while PL is red-shifted at 405 nm excitation due to the excitation of lower-energy CD surface states. Citrazinic acid shows no excitation dependence, and its PL lifetime does not change with the excitation wavelength, as expected for a molecular fluorophore.

fluorophore absorption and emission are very low, while surface-state transitions of CDs dominate. Figure 4a,b presents the data on average PL lifetimes of the three CD species and pure citrazinic acid (colored symbols) at these two excitation wavelengths of 320 and 405 nm, respectively, over a broad emission detection range. PL spectra of the three CD species and citrazinic acid are also provided in Figure 4; such a presentation format is chosen to illustrate the trends in the average PL lifetimes for the different CDs and link them to the corresponding PL spectra. All corresponding decay curves are displayed in Figure S3. At 320 nm excitation (Figure 4a), the emission peak of citrazinic acid (green) is located at 440 nm, and PL lifetimes of this molecular fluorophore remain constant at 6.4 ns over the full emission range, as expected. The emission peak of e-CDs (black, Figure 4a) is located at the same position (440 nm), but the PL lifetimes show a very different trend. After a sharp increase from 7.3 ns at 380 nm to 10.4 ns at the emission maximum at 440 nm, the longest PL lifetime of 10.8 ns is found at 490 nm, followed by a slow decrease toward the surface-state emission (9.7 ns at 590 nm). For h-CDs (red), the PL lifetime exhibits a similar trend but is considerably shorter. It increases from 6.5 ns (365 nm) to 7.9 ns (390 nm) with a maximum of 8.6 ns at 440 nm. The longest PL lifetime of hCDs is found at shorter wavelengths compared to those for eCDs, which is in agreement with the blue shift of the emission peak to 415 nm. For longer emission wavelengths, the average PL lifetime remains constant before decreasing to 7.5 ns at 565 nm. In contrast, t-CDs (blue, Figure 4a), which according to our previous discussion contain no molecular fluorescent species, show a clearly different trend of average lifetime over the emission range. A gradual increase in the PL lifetimes from 4.8 ns at 360 nm to 6.3 ns at 520 nm is observed. The maximum lifetime is located within the emission range attributed to surface-state emission of CDs (note the low emission intensity of the blue dashed emission spectrum of tCDs in Figure 4a). The emission peak of t-CDs at 415 nm is similarly blue-shifted compared to that of e-CDs but has a significantly broader tail compared to those of e- and h-CDs. This is characteristic of CDs with a strong emissive contribution from surface states. The trends in the average PL lifetimes presented in Figure 4a can thus be used to identify the presence of fluorophores in CDs: A sharp increase in the PL lifetime within the emission range of the molecular

fluorophores is a characteristic footprint of their presence within the CDs. In contrast to free fluorophore molecules in solution, varying PL lifetimes at different emission wavelengths indicate the attachement of the fluorophores to the CDs. Small variations in the lifetime are ascribed to changes in the local environment of the fluorophore. Toward longer emission wavelengths, the fluorophore contribution diminishes, as observable by the decreasing PL lifetimes. To probe the low-energy surface states, all CD species were further excited at 405 nm (Figure 4b). At this wavelength, the absorption of molecular citrazinic acid is weak but still present (cf. Figure 3a). Consequently, the fluorophore emission is still centered at 440 nm, and a constant PL lifetime of 6.4 ns over the full emission range is detected. In contrast, the emission spectra of CDs appear to be broadened and are subject to the typical red shift under surface-state emission, as a broad ensemble of low-energy sub-band gaps (surface states) is excited. Notably, the emission peak of each CD species is different (h-CDs at 480 nm, e-CDs at 486 nm, and t-CDs at 495 nm), highlighting their different internal structure and energy sub-band landscape. By analyzing the PL lifetime trends (Figure 4b), we find the clear footprint of molecular fluorophores in e-CDs only. The PL lifetime of e-CDs is elongated within the emission range of the fluorophore (11.3 ns at 430 nm), here as observed from the emission spectrum of citrazinic acid, but follows the trend of the other CDs at longer emission wavelengths where the influence of fluorophores is negligible. For h- and t-CDs, the PL lifetimes gradually increase until about 540 nm, which is in agreement with a fluorophorefree emission. While we observed a fluorophore contribution in h-CDs at 320 nm excitation (Figure 4a), excitation at 405 nm is likely to be out of the excitation range of the fluorophore in hCDs. (Note the blue shift in the main emission.) Beyond 550 nm emission wavelength, the average PL lifetimes of all CD species decrease. Since the fluorophore contribution can be neglected at these wavelengths, emission likely originates from the low-energy surface states of carbonized CDs. The faster PL lifetimes suggest an energy-transfer mechanism from higherenergy states rather than (slow) hopping between states of decreasing energy. We further addressed the excitation-dependent emission properties of the three CD species as shown in Figure 5 by comparing the PL peak positions (symbols ●) and normalized 2018


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Figure 5. PL peak position (●) and normalized PL intensity (▲) as a function of the excitation wavelength for citrazinic acid (a) and the three CD species (b−d). Emission pathways are illustrated in the simplified Jablonski diagrams: arrow colors represent the shift in the emission wavelengths from blue to green (high to low energies), and arrow thicknesses indicate the relative intensities of different transitions. Schematic structures for each CD species are drawn on the right. The attachment of molecular fluorophores is depicted by the hexagons for both e- and h-CDs.

PL intensities (symbols ▲) of pure citrazinic acid, e-CDs, hCDs, and t-CDs at excitation wavelengths ranging from 300 to 500 nm, which covers the full excitation range of possible fluorophores, CD core/edge, and surface states. The excitationdependent emission has been a widely reported phenomenon for CDs1,9,10 and holds important information about different transitions and emission centers. In contrast to CDs, molecular fluorophores do not show, according to the Kasha−Vavilov rule, any excitation-dependent emission peak shifts and are typically characterized by a rather narrow range of excitation wavelengths (as indicated by the relative PL intensities). Both features can indeed be observed in the corresponding plot of pure citrazinic acid (Figure 5a). The emission peak position of this molecular fluorophore remains constant at 440 nm over the entire excitation range. While the highest PL intensity is observed at an excitation wavelength of 340 nm, it drops

quickly to zero at excitation wavelengths beyond 420 nm. We illustrate these trends by a simplified Jablonski diagram (Figure 5a), which shows the emissive transitions and their intensities in correlation to the excitation wavelength. While the emission intensity is indicated by the arrow thickness, the emission wavelength of a transition is roughly illustrated by its color. For citrazinic acid, the experimental observations translate into a single transition, which is typical for a molecular fluorophore. On the basis of citrazinic acid as a reference, we find similar characteristics in the plots of e-CDs and h-CDs (Figure 5b,c). Especially for e-CDs, the PL emission is clearly divided into two separate regions. Under excitation in the range of 300−390 nm, the emission at 440 nm is excitation-independent and reveals the dominance of a single emissive transition. Entering the excitation range beyond 400 nm results in an emission red shift of the main emission peak to 500−520 nm, at much lower 2019


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comprising carbonized components and molecular fluorophores are currently underway.

emission intensities. As illustrated in the simplified Jablonski diagram of Figure 5b, this is the emission range attributed to surface states (green transitions), which are created by an ensemble of low-energy sub-band gaps and are present in all three CDs.12,40 For h-CDs (Figure 5c), the main emission is, as previously mentioned, blue-shifted compared to that of e-CDs and also shows a range of excitation independence: when excited between 300 and 340 nm, h-CDs emit at 420 nm with the highest relative PL intensities. For excitation wavelengths above 340 nm, the emission gradually shifts to longer wavelengths, along with a slow decrease in the PL intensity toward the low-energy emission. For t-CDs (Figure 5d), the emission is strongly excitation-dependent, characterized by a continuous emission peak shift toward longer wavelengths. This shift indicates the presence of multiple emissive transitions at low energy, while low emission intensities in the range of 320− 340 nm confirm the absence of any molecular fluorophores. The PL data obtained for the samples with a higher (∼20fold) concentration of CDs (Figures S4−S7) demonstrate a peak shift for all CDs to longer emission wavelengths when the concentration increases. Such shifts have been previously observed and explained by reabsorption from lower-energy states, which is amplified at high optical densities.41 However, large differences were observed for the emission intensities at different excitation wavelengths, which are also reflected in the PLE spectra presented in Figures S4−S6. The main emission peak at 320 nm excitation for h-CDs and 340 nm for e-CDs is significantly quenched, especially in comparison to their emission at longer excitation wavelengths (380−440 nm). While t-CDs show a similar shift for the main emission peak to 380 nm excitation, the quenching at the main emission (340 nm excitation) is less pronounced in comparison to that of eand h-CDs (340 and 320 nm excitation, respectively). Aside from the reabsorption, concentration-dependent quenching of the main emission in e- and h-CDs can be explained by the formation of CD aggregates. Aggregates, which we also observed in the TEM images of these two samples (Figure S1), show strongly decreased emission intensity, in agreement with previously reported results.42,43 All of these observations provide a strong indication of the absence of fluorophore emission in t-CDs compared to e-CDs and h-CDs. Emission in t-CDs originates solely from different sizes of sp2-hybridized areas in the core (sp2) and edges (sp3) of CDs and surface-related states. At 320 nm, mainly high-energy n−π* transitions are excited. The resulting emission has its maximum in the short-wavelength region, but pathways to lower excited states do exist and contribute to the overall broad emission. A larger number of low-energy transitions are in agreement with the PL lifetime analysis. In contrast, e- and hCDs inherit derivatives of citrazinic acid, which dominate their optical properties in the spectral region of core and edge states of CDs and contribute to their high PL QYs. As illustrated in Figure 5b,c, we expect the fluorophores to be attached to the CDs instead of just free in the solution, which seems likely given the carboxylic functionality of these molecules (Figure 1). In support of this, we observed a fundamentally different character for the PL lifetimes of h- and e-CDs, in comparison to that of free citrazinic acid. In addition, our earlier studies on highly luminescent CDs showed significant quenching behavior for CDs in different solvents, pointing out direct interactions between the fluorophores and the environment.40 Further studies on the composition of these hybrid structures

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CONCLUSIONS The origin of strong blue emission in citric acid-based CDs has recently been explained by the presence of derivatives of citrazinic acid, a molecular fluorophore from the family of pyridines.23 We have examined the potential impact of molecular fluorophores on the optical properties of three different species of CDs, based on citric acid and three nitrogen precursors: ethylenediamine (e-CDs), hexamethylenetetramine (h-CDs), and triethanolamine (t-CDs). We pointed out the differences in the reactivities of different molecular precursor, resulting in a variety of simultaneous chemical reactions under hydrothermal conditions. These conditions facilitate the parallel formation of carbonized nanoparticles (CDs) and molecular species, which are likely to be attached to the CDs. Related structural differences in the three species have been addressed in terms of the nitrogen coordination (from XPS data) and functional surface groups (from FTIR spectra). We further focused on the absorption and emission properties of the three CD species, which clearly demonstrated the significant differences between samples predominantly composed of carbon dots (t-CDs) and those containing molecular derivatives of citrazinic acid (e-CDs and h-CDs). PL lifetimes were measured at two distinct excitation wavelenghts of 320 nm (addressing CD core/fluorophores) and 405 nm (addressing CD surface states). All three CD species showed similar trends in the surface-related emission lifetimes, while differences were revealed for e-/h-CDs and t-CDs in the core/fluorophorerelated emission region. By comparing their PL lifetimes to the lifetime of pure citrazinic acid, we obtained additional proof of fluorophore attachement to e-CDs and h-CDs. Excitationdependent emission characteristics pinpoint the substantial differences in the CD species and can be used as a reliable indicator to reveal the contribution of distinct molecular fluorescence.

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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.jpcc.6b12519. TEM, FTIR, and photoluminescence spectroscopy data on excitation-dependent emission measurements and PLE spectra recorded for different emission wavelengths. PL and PLE characterization of CD samples at low and high concentrations. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Andrey L. Rogach: 0000-0002-8263-8141 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was funded by NPRP grant no. 8-878-1-172 from the Qatar National Research Fund (a member of the Qatar Foundation). 2020


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2022


Molecular Fluorescence in Citric Acid-Based Carbon Dots

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