Aggregated Molecular Fluorophores in the Ammonothermal Synthesis

Article Cite This: Chem. Mater. 2017, 29, 10352−10361

pubs.acs.org/cm

Aggregated Molecular Fluorophores in the Ammonothermal Synthesis of Carbon Dots Claas J. Reckmeier,† Julian Schneider,† Yuan Xiong,† Jonas Haü sler,‡ Peter Kasák,§ Wolfgang Schnick,‡ and Andrey L. Rogach*,† †

Department of Materials Science and Engineering & Center for Functional Photonics (CFP), City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong S.A.R. ‡ Department of Chemistry, Ludwig-Maximilians-Universität München, Butenandtstrassse 5-13 (D), 81377 Munich, Germany § Center for Advanced Materials, Qatar University, P.O. Box 2713, Doha, Qatar S Supporting Information *

ABSTRACT: Recently, molecular fluorophores were shown to be formed in the bottom-up chemical synthesis, contributing to the emission of carbon dots (CDs), derived from a citric acid precursor. We applied an ammonothermal synthesis toward CDs, employing two reactants citric acid and supercritical ammonia functioning as both solvent and precursor. The resulting nanoparticles are identified as amorphous aggregates of molecular fluorophores based on citrazinic acid derivatives, which resemble many of the emission features typically reported to be characteristic for CDs. The aggregates absorb and emit at short and long wavelengths of the spectrum, a feature prior ascribed to intrinsic CD core and surface states, respectively. We identify three emission states: a high energy and a low energy aggregate state as well as an energy transfer state between both. Energy transfer is triggered only upon excitation within a narrow high energy spectral range, resulting in a characteristic bluegreen double emission. The high energy aggregate state exhibits a trapping mechanism elongating emission lifetime. To further analyze aggregated molecular fluorophores, we studied aqueous solutions and films of citrazinic acid and polyvinylpyrrolidone and demonstrated their concentration dependent optical behavior. Since fluorophore aggregates reproduce the emissive features of CDs, the contribution of sp2/sp3 carbonized products and graphitic domains to the emission features of CDs must be carefully evaluated in future studies.

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temperatures.27 Citrazinic acid can be regarded as the most basic molecular fluorophore of this kind, formed in the reaction of citric acid and ammonia, while different amine precursors or synthetic conditions consequently lead to the formation of citrazinic acid derivatives, such as reported by Kasprzyk et al.18,19 and pointed out in our recent publication.21 Song et al. were able to separate and identify the citrazinic acid derivative 5-oxo-1,2,3,5-tetrahydroimidazo[1,2-α]pyridine-7-carboxylic acid (IPCA) in the CD synthesis using citric acid and ethylenediamine.16,17 When considering the emission mechanism of CDs, it is therefore essential to evaluate the possible contribution of such molecular fluorophores, as well as their interaction with the remaining parts of carbon-based nanoparticles. Fu et al. were among the first to describe the graphitic CD core as layers of stacked polycyclic aromatic hydrocarbons (PAH), which

INTRODUCTION Carbon dots (CDs) are strongly luminescent, carbon-based nanoparticles,1,2 with low toxicity3 and high chemical stability,4 promising for applications such as bioimaging,5−7 biosensing,8 LEDs,9,10 photovoltaics,11 and lasing.12 CDs can conveniently be synthesized by a bottom-up chemical synthesis approach which occurs through polymerization and subsequent carbonization of the molecular precursors.13 Citric acid is by far the most commonly used precursor for the CDs, which is often combined with a nitrogen precursor such as ethylenediamine, urea, or L-cysteine.1,6,7,14,15 A number of studies, however, pointed out that citric acid, and most likely any other organic molecules with a similar chemical structure, can easily react with amines (especially primary amines) forming strongly luminescent molecular fluorophores, mostly emitting in the blue.16−23 The first encounters on such reactions date back to the time between 1884 and 1894 where Behrmann et al. and Sell et al. identified the formation of the fluorophore citrazinic acid in the reaction of citric acid and ammonia or urea,24−26 while urea is able to decompose to ammonia at elevated © 2017 American Chemical Society

Received: August 8, 2017 Revised: November 16, 2017 Published: December 13, 2017 10352

DOI: 10.1021/acs.chemmater.7b03344 Chem. Mater. 2017, 29, 10352−10361

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Chemistry of Materials

Figure 1. (a) Synthetic approach toward CDs. Left: commonly used hydrothermal synthesis from the citric acid and ammonia as precursors in an aqueous solution. Right: ammonothermal synthesis from citric acid and supercritical ammonia serving as both precursor and solvent. On the bottom, the purified reaction products of aq-NH3 (left) and sc-NH3 CDs (right) are shown under ambient light and UV excitation (365 nm). (b) TEM and (c) HRTEM images of sc-NH3 CDs. Due to the low contrast, white lines have been drawn around the poorly recognizable particles in (c). d) Size distribution of sc-NH3 CDs reveals an average particle diameter of 18 nm.

contribute to the overall emission.28 The presence of such PAHs was recently supported by Righetto et al., who studied both citric acid and arginine based CDs by fluorescence correlation spectroscopy and time-resolved paramagnetic resonance.29 With these techniques, the authors were able to ascribe the major emission characteristics of their samples to small fluorophore molecules in solution, while larger carbon cores were found to emit weakly and at longer wavelengths.29 Based on single particle measurements, Ghosh et al. pointed out on the presence of one single optically active emission center existing in the CDs;30 related to this observation, Sharma et al. and Demchenko et al. assigned the commonly observed excitation dependent emission of CDs in ensemble to their aggregates.31−33 Most recently, Khan et al. investigated the products of the hydrothermal treatment of citric acid only and reported both the existence of particles on the TEM grids and the excitation dependent emission, which were ascribed to the presence of methylsuccinic acid and its aggregation on the TEM grids.34 It is commonly stated in CD related literature that their internal structure and the emission mechanism strongly depend on the synthetic conditions, as their synthesis proceeds through a series of polymerization and carbonization steps, which are influenced by the synthetic temperature. Most of the common syntheses of CDs with the precursors mentioned above are done in aqueous medium. In this work, we have chosen an ammonothermal synthesis approach,35,36 proceeding through the reaction of the two precursors citric acid and supercritical ammonia in an autoclave chamber. Although performed at high pressure and long heating times of up to 10 h, this synthesis is expected to favor molecular fluorophore formation (citrazinic

acid and its derivatives),17,21,24−26 due to the reduced polymerization (cross-linking) ability of the precursors, which limits the carbonization degree of the final product. We found out that this reaction indeed results in the formation of aggregated citrazinic acid based fluorophores. In comparison, we also studied the products of the hydrothermal chemical reaction between citric acid and aqueous ammonia and highlighted that their spectral characteristics are dominated by isolated molecular fluorophores with very weakly emitting low energy states and no visible carbon particle formation. To further analyze the optical properties of aggregated molecular fluorophores, we have embedded pure citrazinic acid into a solid matrix of polyvinylpyrrolidone (PVP) and demonstrated its concentration dependent aggregation.

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EXPERIMENTAL SECTION

Materials. Citric acid monohydrate (purity: ≥ 99.5%) and aqueous ammonia (25%) were purchased from Applichem. Ammonia gas (purity: 99.999%) was purchased from Air Liquide. 2,6-Dihydroxypyridine-4-carboxylic acid (citrazinic acid, purity: 97%) and polyvinylpyrrolidone (PVP40) were purchased from Sigma-Aldrich. Synthesis. The samples denoted as sc-NH3 CDs (sc-NH3 = supercritical-NH3) were synthesized using an ammonothermal approach.35 Citric acid monohydrate (11 mmol) was placed in a PTFE liner, which was then transferred into a high pressure stainless steel autoclave (Parr Instrument Company, vessel type 4740, gage block type 4316). The autoclave was sealed, evacuated, and cooled with ethanol/liquid nitrogen to 198 K. NH3 (20 mL, 0.87 mol) was condensed into the autoclave via a pressure regulating valve to obtain a filling degree of ∼50%. The autoclave body was heated for 10 h at 200 °C (12 MPa). The reaction product was dissolved in 20 mL of Milli-Q water, filtered, and centrifuged. The reference samples denoted as aqNH3 CDs (aq-NH3 = aqueous NH3) were synthesized using a 10353


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Figure 2. a) FTIR spectra of sc-NH3 CDs (black) and aq-NH3 CDs (red). The presence of ammonium and carboxylate absorption bands in sc-NH3 CDs is characteristic for ammonium salts of citrazinic acid, while the amide I and II bands in aq-NH3 CDs are characteristic for the amide of citrazinic acid. b) XPS N 1s spectra for the two species, as indicated. commonly used hydrothermal method. Citric acid monohydrate (11 mmol) and 20 mL of 25% aqueous ammonia solution were mixed in a PTFE liner, which was transferred into a stainless steel autoclave (Parr 4740) and heated for 10 h at 200 °C (1 MPa). The reaction product was filtered and centrifuged to remove larger agglomerates. Fluorophore film samples were prepared by combining aqueous PVP solutions (4 wt %), with different amounts of citrazinic acid (200, 20, 2, and 0.2 mM). The films were drop-casted from freshly made solutions, while the solutions were further aged at ambient conditions for several months. Characterization. X-ray photoelectron spectroscopy (XPS) has been performed on a PHI Model 5802, Fourier Transform Infrared (FTIR) spectroscopy has been performed on a PerkinElmer 16 PC FT-IR spectrophotometer, transmission electron microscopy (TEM) has been performed on a Philips CM-20 microscope, and highresolution transmission electron microscopy (HRTEM) has been performed on a JEOL JEM 2100F microscope. Solutions of CDs for optical measurements were prepared by diluting the highly concentrated stock solutions with Milli-Q water. The concentration was kept constant to ensure comparability between measurements. Absorption spectra were recorded using a Shimadzu UV-3600 UV− vis-NIR absorption spectrometer. Photoluminescence (PL), absolute PL quantum yield (PL QY), photoluminescence excitation (PLE), and PL decay (lifetime) measurements were performed on an Edinburgh

Instruments FLSP920 spectrometer with a PMT detector (R928P, Hamamatsu). Excitation source for PL and PLE measurements was a 450 W Xe900 continuous xenon arc lamp. All PLE spectra were excitation corrected. The PL QY was measured by an absolute method using an integrating sphere (Edinburgh Instruments). For timeresolved measurements, an EPL-405 405 nm, 5 mW picosecond pulsed diode laser and an EPLED-320 320 nm, 5 mW picosecond pulsed diode laser (Edinburgh Instruments) were used. The fluorescence decays were separated from the instrument response by deconvolution and fitted with appropriate multiexponential functions. Fluorescence lifetime was calculated by taking a weighted average of the exponential components.

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RESULTS AND DISCUSSION Synthesis and Structural Characterization. Figure 1a illustrates the ammonothermal synthesis employed here, as compared to the commonly used hydrothermal approach. While aqueous solutions of ammonia have been reported to react with citric acid under both atmospheric24−26 and high pressure,37 determining the impact of ammonia (100%) on the reaction is a subject of study here. We have used supercritical ammonia as a precursor and solvent at the same time, which reacted with citric acid under otherwise comparable conditions 10354


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Figure 3. (a) Absorption of aq-NH3 CDs and sc-NH3 CDs in comparison with citrazinic acid fluorophore in solution. All spectra have been normalized to the most pronounced peak at around 340 nm, with numbers indicating the respective peak position. (b) PLE spectra of sc-NH3 CDs and citrazinic acid at different detection wavelengths. (c) Excitation dependence of the emission (tracked at the respective emission peak) for sc-NH3 (red), aq-NH3 (blue) CDs, and citrazinic acid (green). (d) Emission spectra of sc-NH3 CDs at different excitation wavelengths showcasing the double peak emission belonging to different aggregation states.

absorption signals indicate aromatic sp2 C−H bending vibrations, as well as N−H out-of-plane bending vibrations, predominantly in sc-NH3 CDs. At 1580 cm−1 strong sp2 CC vibrations are present, belonging to stretching of the aromatic ring structure, while C−N stretching at 1400 cm−1 belongs to the pyridone unit of the fluorophore. Apart from these similarities, the two samples show one significant difference. Sc-NH3 CDs possess strong ammonium and carboxylate vibrations, indicating the presence of the ammonium salt of citrazinic acid, while aq-NH3 CDs possess amide I and II vibrations. This is further supported by X-ray photoelectron spectroscopy (XPS) data (Figure S2), which show distinct differences between the two species in the O 1s and C 1S spectra (Figure S3), but especially in the N 1s spectra, presented in Figure 2b. While the N 1S spectrum of aq-NH3 CDs follows the typical shape for the nitrogen doped CDs,21 showing the presence of pyridinic (399.3 eV), pyrrolic (400 eV), and quaternary (401.2 eV) nitrogen with a decreasing intensity in the same order, sc-NH3 CDs show an overall mirrored peak. The structural motives of pyridinic and pyrrolic nitrogen can be ascribed to the presence of different mesomeric structures of citrazinic acid derivatives. The strongest contribution emerges from quaternary nitrogen species (402.2 eV, note the general shift to higher binding energies in sc-NH3 CDs), highlighting the strong presence of the ammonium cation in sc-NH3 CDs.39 Ammonium salts of citrazinic acid are hydrophilic in their carboxylate functionality and relatively hydrophobic at the aromatic unit, which gives the molecule a surfactant-like character and promotes its aggregation.

(pressure, temperature, and reaction time) in the autoclave. Since the syntheses were initially designed to produce nitrogen doped citric acid based CDs, we will keep the “CD” annotation for the products of the reaction in this study, even though the formation of the molecular fluorophores will be emphasized in the following discussion. Figure 1a also shows photographs of highly concentrated solutions of aq-NH3 and sc-NH3 CDs after purification. AqNH3 CDs appear dark red-brown, resembling the color of citrazinic acid at high concentrations,25,38 while sc-NH3 CDs appear as pale brownish-yellow solution. In most of the reported syntheses of CDs, citric acid derived nanoparticles typically appear brownish as-synthesized and pale-yellow after dilution and purification.6,14,19−21 TEM images of sc-NH3 CDs in Figure 1b,c reveal the presence of amorphous particles, with an average diameter of 18 nm. No lattice fringes could be resolved in the high-resolution images, indicating that the particles are noncrystalline. In the case of aq-NH3 CDs, no regular particle formation on similar scale was observed by TEM. To further analyze the chemical structure of the products, which are expected to form via the reaction mechanism, illustrated in Figure S1, we identified the characteristic absorption bands of distinct functional groups from FTIR spectra (Figure 2a). Both samples show vibrations which can be ascribed to citrazinic acid and its derivatives. From 3500 to 2800 cm−1, the stretching vibrations of C−H, N−H2, O−H, and carboxylic acid COO−H contribute to the strong and broad absorption. From 900 to 600 cm−1, multiple sharp 10355


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Chemistry of Materials Aggregation of such molecules in the sc-NH3 CDs sample is further facilitated due to intermolecular hydrogen bonding and π−π stacking of the pyridone unit,40,41 which is in agreement with the TEM images (Figure 1), showing relatively large (18 nm) amorphous particles. Besides, another factor favoring aggregation might be fast evaporation of the solvent after opening the autoclave in the ammonothermal synthesis. Optical Properties of Fluorophores in Solution. Figure 3a shows the absorption spectra of the two CD samples in comparison with the diluted fluorophore citrazinic acid in solution. The absorption spectra of aq-NH3 CDs (blue line) and citrazinic acid (dashed green line) are almost overlapping (Figure 3a), pointing out that aq-NH3 CDs mostly consist of citrazinic acid and its derivatives. In contrast, the absorption spectrum of sc-NH3 CDs shows more distinct features. The main absorption peak of sc-NH3 CDs is blue-shifted to 333 nm as compared to aq-NH3 CDs (341 nm) and citrazinic acid (345 nm). Sc-NH3 CDs have an overall increased UV absorption and the valley at 280 nm features a significantly higher absorption strength. The main peak shows a tail toward longer wavelengths with a clear shoulder at 380 nm and absorption extending up to 550 nm. Consequently, we expect sc-NH3 CDs to feature a high amount of low energy states, originating from aggregated fluorophores.42 The high energy absorption peak around 235 nm is assigned to π−π* transitions, emerging from conjugated π electrons in the aromatic unit of the fluorophore, while the peak around 345 nm is assigned to n−π* transitions,19,43 which is in good agreement considering pyridine as a structural base unit.44 However, such an assignment does not hold up when aggregated aromatic structures are involved. In this case, π−π* transitions of intermolecular charge transfer contribute to the high energy part of the 345 nm peak,45 while aggregation induced overlap of molecular orbitals results in the formation of additional low energy states. The blueshift of the absorption peak of sc-NH3 CDs compared to citrazinic acid in solution is furthermore in agreement with the existence of molecular (π−π-stacked) H-type aggregates.32,46 To illustrate the excitation dependent properties of the samples under study, Figure 3b shows PLE spectra of sc-NH3 CDs in comparison with citrazinic acid at short (440 nm, 460 nm) and long (560 nm) detection wavelengths. The PLE spectra reveal several contributing emission centers for sc-NH3 CDs, especially at low energy. With the detection wavelength set to the emission range of citrazinic acid (440 nm/460 nm), the PLE spectrum is of similar shape as for citrazinic acid, but the main emission peak is significantly red-shifted (Figure 3b, dashed vs solid curve). The emission peak around 370 nm is thus attributed to aggregated fluorophores and denoted as the high energy aggregate state in Figure 3b. At low emission energy (560 nm, solid red curve), two additional emissive bands appear at 295 and 435 nm excitation. Since both bands contribute exclusively to the low energy emission, an energy transfer pathway between them is likely. The high energy donor state is denoted as energy transfer state in Figure 3b (295 nm), while the acceptor state is denoted as the low energy aggregate state (435 nm). Charge transfer between π−π* orbitals in stacked aromatic domains at high energies is a competing process and explains the small excitation range of the energy transfer state and the low PL QY of only 4% of this sample at high energy excitation. In comparison with the absorption spectra (Figure 3a), it is noted that the PLE peak at 295 nm falls into a valley of overall increased absorption. The peak at 435 nm is located within the extended absorption tail of sc-NH3

CDs, underlining the significance of aggregate related emission centers in sc-NH3 CDs. For aq-NH3 CDs (Figure S4), the PLE spectrum closely follows the spectrum of citrazinic acid, with the main peaks separated by just a few nanometers. In contrast to the spectrum of sc-NH3 CDs, only a very weak emission at longer wavelengths is detected (Figure S4, tail from 420 nm onward). In general, this is attributed to CD surface states20,28,43,47−49 but can also originate from fluorophore derivatives or their dimers or trimers.42 A small amount of aggregated fluorophores is also in agreement with the small blueshift of the aq-NH3 CDs absorption peak (Figure 3a) and the weak intensity of the excitation dependent emission, which will be discussed in the following.46 Figure 3c summarizes the excitation dependence of the CD samples and citrazinic acid as a function of the emission peak (spectra are presented in Figure S5). Aq-NH3 CDs (blue line) behave similar to the other previously reported citric acid based CDs, with weak or no excitation dependence until about 400 nm, followed by a step-like excitation dependent redshift.2,6,17,20,28,50 This excitation independent emission of aqNH3 CDs until 400 nm excitation is identical to citrazinic acid fluorophore (green stars), pointing out its presence in the sample. Furthermore, absolute PL QY of aq-NH3 CDs is 32% at 345 nm excitation, which is the same value as observed for citrazinic acid (32%, within pH 6−11) at 348 nm excitation. The emission around 441 nm is therefore labeled as f luorophore state in Figure 3c. The excitation dependence at longer wavelengths, where single fluorophores no longer emit, can be ascribed to fluorophore dimers, as per previous discussion. As the emission intensity of aq-NH3 CDs under excitation wavelengths longer than 400 nm is very weak, only a small degree of dimerization is assumed. While these states are generally characterized as surface states in the CD related literature,20,43,47−49 it is important to note that the characteristics of surface states can be reproduced in a fluorophore system with little or no graphitic CD formation. We will keep the surface state notation in the following discussion but explicitly include low energy emission from such fluorophore based systems. The emission of sc-NH3 CDs is red-shifted compared to aqNH3 CDs and centered at 460 nm (Figure 3c). Furthermore, sc-NH3 CDs have a PL QY of 14% at 380 nm excitation and only 4% at 335 nm excitation, which both can be explained by the aggregation of fluorophore molecules and related fluorescence quenching.53 In the excitation window from 280 to 320 nm, a double emission peak, consisting of a blue and green component, appears (Figure 3c,d). As discussed before, we located an energy transfer state at around 295 nm (Figure 3c, blue circle). Within its range, emission shifts from 460 nm toward the fluorophore state energy level (441 nm) as blue emission from the aggregate donor state is quenched, and only blue emission from nonaggregated, molecular-like fluorophores within the sc-NH3 CDs sample remains. The energy from the aggregate energy transfer state is transferred to the low energy aggregate state (acceptor) and appears as the second emission peak around 540 nm (Figure 3c, left green circle). The emission energy is similar to the emission observed when the low energy aggregate state is directly excited, as seen in the first emission plateau at around 435 nm excitation wavelength (Figure 3c, right green circle). While the high energy blue emission of CDs is often assigned to core or edge states in the literature,20,51,52 we note that it is possible to reproduce such emission behavior with a noncarbonized fluorophore aggregate 10356


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Chemistry of Materials system. The emission at longer excitation wavelengths of scNH3 CDs is similar to the aq-NH3 CDs, and both converge toward a “lowest emission state” (Figure 3c, orange circle). The negligible difference of aq- and sc-NH3 CDs shows that already small fluorophore aggregates can produce low energy emissive states. Figure 3d shows the emission spectra of sc-NH3 CDs. The first double peak is observed at 280 nm excitation (Figure 3d, red line) with the strongest emission component in the green close to 535 nm, while the blue peak component is already slightly blueshifted from the high energy aggregate state. At 300 nm excitation (Figure 3d, green line), the intensity of the blue component decreases and is identical to citrazinic acid at 441 nm (fluorophore state). The corresponding green component has its peak at 540 nm, which is identical to the emission observed at the excitation of the low energy aggregate state at 420 and 440 nm (pink and orange line). In the excitation range outside the resonant energy transfer band, which is located at 295 nm, emission does not show double peaks and occurs within the high energy aggregate state emission band (260 nm, black line; 400 nm, cyan line). Therefore, excitation between 280 and 320 nm results in either energy transfer to the low energy aggregate state or in a blue emission by excited, nonaggregated molecular fluorophores. In the following, we will therefore look closer at the aggregate related emission of citrazinic acid fluorophore. Optical Properties of Aggregated Citrazinic Acid. In order to gain a better understanding of the optical properties of fluorophore aggregates, we mixed varying concentrations of citrazinic acid (0.2 mM to 200 mM) with a fixed amount of PVP (4 wt %) in solution to fabricate films and induce aggregation of the fluorophores in the solid state. Even in solutions, just after 1 h of the mixing the components, we already observed color changes in the samples with the highest citrazinic acid concentrations. Control samples without PVP showed no visible changes in the same time range. After further storage for several months to ensure a stable system, we examined the optical properties of the three solutions, which are presented in the inset of Figure 4a. Normalized absorption spectra in Figure 4a exhibit a significant tail in the long wavelength absorption beyond 400 nm. Since all samples were diluted to the same low optical density range for the measurements, the spectra clearly show that it is the initial concentration of citrazinic acid that correlates with the degree of long wavelength absorption. An additional shoulder appears at around 430 nm, and a tail becomes visible up to 700 nm, which is in contrast to highly concentrated solutions of citrazinic acid, where long wavelength absorption is simply caused by the overall high intensity of the absorption peak (Figure S6). Besides the absorption, we also investigated the excitation dependent PL properties, which are presented in Figure 4b. The comparison of the three samples, in particular to freshly prepared citrazinic acid fluorophores in solution (Figure 3), shows a clear transition from excitation independent into excitation dependent emission. While the excitation dependent PL of the 2 mM solution resembles the spectral behavior of typical CDs, higher initial concentrations of citrazinic acid enhance the emission at longer wavelengths, while the overall PL intensity decreases. Another feature that arises at the increasing concentration of citrazinic acid is the double emission peak at around 430 and 540 nm, when excited at 280 and 300 nm, similar as reported above for the sc-NH3 CDs. The PLE spectrum, recorded for the 200 mM sample (Figure

Figure 4. (a) Normalized absorption spectra of aqueous solutions of citrazinic acid (2, 20, 200 mM) containing 4 wt % PVP, measured after several months of storage. The inset shows a photograph of the solutions with different concentrations, as indicated. (b) PL spectra recorded at different excitation wavelengths showcasing the double peak emission belonging to different aggregation states.

S7), also shows additional features, in particular for the emission at 540 nm, where the main peak is located around 440 nm. Apart from the optical characterization of the aged solution samples, we have also studied them by TEM and observed rather regular shaped particles with a size of 4.5 ± 1.2 nm (Figure S8). Such particle formation can be understood in terms of polymer induced aggregation of citrazinic acid, yielding overall very similar features, in comparison to scNH3 CDs. After drop casting the solutions on glass substrates, similar investigation of the corresponding films has been carried out, with the data presented in Figure S9. We observe the color changes for the films with increasing concentrations − from transparent (0.2 mM), to light green (2 mM), brown-purple (20 mM), and brown-black (200 mM), indicating the formation of citrazinic acid aggregates.38 Furthermore, PL intensity after excitation at 365 nm decreases with increasing concentrations, in agreement with the widely reported aggregation induced emission quenching, with no PL detectable from the 200 mM film sample.53 10357


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Figure 5. Average PL lifetime over emission wavelength of (a) the 0.2 mM PVP/citrazinic acid film and (b) the sc-NH3 and aq-NH3 CD samples in solution (solid lines) at 320 and 405 nm excitation. The dashed lines show representative PL spectra at the indicated excitation wavelengths. (c) Illustration of a single citrazinic acid fluorophore and a fluorophore aggregate, highlighting the differences in the PL lifetimes when probed at different emission wavelengths. The single fluorophore inherits a constant PL lifetime, while fluorophore aggregates exhibit a high energy aggregate state, which is characterized by a longer, trap-like emission lifetime and a shorter emission lifetime from the low energy aggregate state. The aq-NH3 CDs show a similar lifetime as citrazinic acid in solution.

Figure S9a shows the absorption spectra of PVP films with increasing concentrations of citrazinic acid. In agreement with the color change, there are clear signs of additional long wavelength absorption features. However, a detailed analysis of the spectra is difficult due to the occurrence of thin film interference, as revealed by measuring PVP-only films against glass substrates (Figure S10). In relation to the previous discussion on CDs, it is important to realize that an additional set of states emerges at long wavelengths after aggregation of citrazinic acid both in solution and in films and without the presence of any carbonized (graphitic) domains.22,40−42 This is a hint that the long wavelength absorption, observed for aqNH3 CDs (weak) and sc-NH3 CDs (strong, see Figure 3a) can be explained not only by the presence CD surface states but also by aggregated fluorophores.20,28 Figure S9b shows PL spectra of the 0.2 mM PVP/citrazinic acid film at selected excitation wavelengths. Similar to sc-NH3 CDs (as in Figure 3), a double peak emission is observed within a limited excitation range (300−340 nm). At high energy excitation in the range of 280−300 nm, a strong green component at 505 nm and a weak blue component around 395 nm are observed indicating energy transfer to a low energy aggregate state. At 320 nm excitation, however, exactly within the high energy absorption peak, the blue emission component becomes stronger (395 nm), and the green component is only visible as a shoulder at 505 nm (Figure S9b, blue line). The blue emission is assigned to a high energy aggregate state, comparable to sc-NH3 CDs, as a further increase of the

excitation wavelength triggers an excitation dependent emission redshift toward the low energy aggregate state. At 380 nm excitation, no contribution from the original blue component can be seen, but the emission peak at 470 nm and the red tail include contributions from the 505 nm component. At longer excitation wavelengths, the low energy aggregate state is excited directly, the emission shifts back to 505 nm, and no further shifts are detected. This demonstrates that the broad high energy absorption of aggregated citrazinic acid includes all three aggregate states which we have previously discussed for sc-NH3 CDs: a high energy band favoring energy transfer; a high energy aggregate state with blue emission; and a low energy aggregate state with green emission which acts as acceptor for energy transfer and can as well be excited directly. A similar double peak emission is also observed for the higher concentrated film samples shown in Figure S11. As expected, the intensity of the PL components and positions of the low energy peaks belonging to aggregates are concentration dependent. Overall, the strong similarity to the emission observed in sc-NH3 CDs leads us to the assumption that indeed small aggregates of citrazinic acid can be seen as the origin of the emission in scNH3 CDs. Photoluminescence Lifetime Characteristics of Fluorophores and CDs in the Solid State. In the following, we will use the 0.2 mM citrazinic acid film sample as an underlying model to analyze the emission decay characteristics of citrazinic acid aggregates in sc-NH3 CDs. Figure 5a shows the PL lifetimes of the film sample excited at the high (320 nm) and 10358


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trapping and a short PL lifetime due to the intrinsic emission of the low energy aggregate state. For long wavelength emission of sc-NH3 CDs (>600 nm), the PL lifetime is shorter than for aqNH3 CDs and citrazinic acid (6.4 ns) at both excitation wavelengths: 6 ns at 605 nm emission under 320 nm excitation and 6.1 ns at 650 nm under 405 nm excitation. This underlines that the low energy emission originates from an aggregated state and not from single fluorophores. Furthermore, PL lifetime under 320 nm excitation is slightly shorter than that under 405 nm excitation. This difference is present in both film and CD samples and shows that if the low energy aggregate state is excited via energy transfer, only a limited subset of states emit compared to direct excitation of the low energy aggregate state.

low (405 nm) energy aggregate state. PL lifetimes are presented in dependence on the corresponding emission wavelength, providing information on the different emission processes. In addition, representative PL spectra are shown as dashed lines, in order to connect the forthcoming discussion of the emission lifetimes at a specific wavelength to the corresponding part of the PL spectrum. At 320 nm excitation of the film sample (Figure 5a), a strong emission peak from the high energy aggregate state is detected, in addition to a weaker green peak created by energy transfer to the low energy aggregate state. Emission lifetime within the high energy aggregate state is noticeably long, with 23 ns at 400 nm emission. It significantly drops in the low energy aggregate state to 12 ns at 490 nm emission and further decreases to 7.5 ns at 580 nm in the long wavelength emission tail. Under direct excitation of the low energy state (405 nm), the PL lifetime is much shorter, ranging from 9.6 at 450 nm emission to 9 ns at 510 nm and 7.6 ns at 610 nm (Figure 5a, red squares). Overall, PL lifetime decreases with increasing emission wavelength, while its slight increase at 450 nm for intrinsic low energy state emission is likely due to the contribution of high energy states with longer PL lifetimes. The long emission lifetime in the high energy aggregate state indicates a trapping mechanism that elongates emissive decay of the blue emission. The second, green emissive peak at 320 nm excitation has a PL lifetime comparable to the lifetime of the intrinsic low energy state emission. Since excitation occurs at high energy (320 nm) and no trapping is observed, emission takes place via an energy transfer into the low energy state, as previously emphasized.54 At long emission wavelengths, the PL lifetime measured at high energy excitation (320 nm) drops below the lifetime measured under 405 nm excitation. Similar trends have been observed for the film samples at higher concentrations (Figure S12), although emission characteristics and PL lifetimes slightly vary due to the larger degree of aggregation (quenching). The appearance of double PL peaks and PL lifetime elongation due to self-trapping as well as fast energy transfer mechanisms are in agreement with other studies on aggregated fluorophores.54−56 Figure 5b shows the PL lifetime of both aq- and sc-NH3 CDs at 320 and 405 nm excitation. For aq-NH3 CDs, PL lifetime remains on the same level of about 6.5 ns over the entire emission range for both excitation wavelengths. This is a very characteristic behavior of molecular fluorophores53 and thus similar to citrazinic acid, which shows a constant emission lifetime of 6.4 ns (Figure S13). We therefore denote the emission of aq-NH3 CDs as f luorophore-like in Figure 5b. In contrast, the PL lifetime of the emission of sc-NH3 CDs (Figure 5b, purple curve) at 320 nm excitation increases sharply toward the main emission around 450 nm, with a maximum PL lifetime of 8 ns in the high energy aggregate state (abbreviated with “high energy”). Toward the low energy aggregate state (abbreviated with “low energy”), PL lifetime drops to 6.9 ns at 530 nm. As shown in Figure 5a for the film samples, this change in the PL lifetimes is characteristic for the emission mechanism of aggregated fluorophores (Figure 5c), including the trapping in the high energy aggregate state. Excitation at 405 nm (Figure 5b, red curve) excites both high and low energy aggregate states in the case of sc-NH3 CDs, as can be seen by the initially long PL lifetime within the high energy state. However, the 405 nm excitation cannot emit via energy transfer, as the spectral excitation window is located at higher energies (around 300 nm, see Figure 3). Thus, at 405 nm excitation we observe a long PL lifetime in the high energy aggregate state due to

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CONCLUSIONS We conducted the synthesis of nitrogen doped citric acid based CDs by treating citric acid with supercritical (ammonothermal) (sc-NH3) and aqueous (hydrothermal) ammonia (aq-NH3) and compared the products which showed distinct differences in their structural and optical properties. The synthesis in aqNH3 yielded a product, which resembles most of the properties of citrazinic acid fluorophore, such as a weak long-range absorption, overlapping emission spectra, and similar PL lifetimes. On the other hand, ammonothermal synthesis yielded amorphous nanoparticles (sc-NH3 CDs), which showed clear evidence of fluorophore aggregates. As indicated by FTIR and XPS measurements, aggregation of the fluorophore can be explained by the formation of the ammonium salt of citrazinic acid, which may act as an aggregation promoter due to the induced surfactant-like character of the resulting molecule. To further highlight these results, we studied the optical properties of citrazinic acid at increasing concentrations mixed with aqueous solutions of PVP and found concentration dependent fluorophore aggregation by aging at ambient conditions. This effect is further enhanced after fabricating films from such mixtures, which serve in this study as a model system for aggregated fluorophores. Analysis of the optical properties of the solutions and films showed a significant increase in the long-wavelength absorption. Furthermore, the average emission lifetimes of both films and sc-NH3 CDs highlight the presence of three emission mechanisms: a trapping mechanism elongating PL lifetimes in a high energy aggregate state, an energy transfer process from a narrow high energy transfer band into a low energy aggregate state and intrinsic emission from the low energy aggregate state. It is found that amorphous fluorophore aggregates share many common optical properties, which were ascribed to the nitrogen doped graphitic CDs in the previously published literature. For future research on CDs it is therefore essential to pay close attention to the precursor system and chemical reaction used to evaluate the impact of fluorophores and their aggregates on the emission of CDs. Overall, our results challenge the commonly used CD models based purely on carbonized graphitic particles. Organic fluorophores similarly possess sp2/sp3 carbon domains, and their aggregation likely involves stacking interaction that allows for a certain degree of self-arrangement, possibly leading to the appearance of crystalline domains in TEM images. Our results show that certain CDs may ultimately be hybrid particles, built from fluorophore aggregates within or attached to carbonized material offering protection from the environment. Carbonization may act as a protective matrix or seed for fluorophore 10359


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aggregation, while the aggregation state shapes the individual spectral properties of CDs.

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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.chemmater.7b03344. Additional characterization by X-ray photoelectron spectroscopy, photoluminescence excitation spectroscopy, steady state and time-resolved photoluminescence spectroscopy for sc- and aq-NH3 CDs and citrazinic acid in solutions and PVP films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Peter Kasák: 0000-0003-4557-1408 Wolfgang Schnick: 0000-0003-4571-8035 Andrey L. Rogach: 0000-0002-8263-8141 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Klaus Speck (Ludwig-MaximiliansUniversität, Munich) for valuable discussions regarding the chemistry of citric acid. This work was funded by NPRP grant no. 8-878-1-172 from the Qatar National Research Fund (a member of the Qatar Foundation).

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Aggregated Molecular Fluorophores in the Ammonothermal Synthesis

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