Unravelling the Multiple Emissive States in Citric ... - ACS Publications

Subscriber access provided by UNIV OF NEBRASKA - LINCOLN

Article

Unravelling the Multiple Emissive States in Citric Acid-Derived Carbon-Dots Namasivayam Dhenadhayalan, King-Chuen Lin, Raghupathy Suresh, and Perumal Ramamurthy J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b08516 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on November 25, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Unravelling the Multiple Emissive States in Citric Acid-Derived Carbon-Dots

Namasivayam Dhenadhayalan,a King-Chuen Lin*,a Raghupathy Suresh,b and Perumal Ramamurthy b

a

Department of Chemistry, National Taiwan University, and

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan. *Corresponding author. Tel: +886-2-3366-1162. E-mail: [email protected] (King-Chuen Lin)

b

National Centre for Ultrafast Processes, University of Madras, Chennai - 600113, Tamil nadu, India.

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

Abstract Steady-state and time-resolved fluorescence spectroscopy techniques were used to probe multi-fluorescence resulting from citric acid-derived carbon-dots (C-dots). Commonly, both carboxyl/amine functionalized C-dots exhibit three distinct emissive states corresponding to the carbon-core and surface domain. The shorter wavelength fluorescence (below 400 nm) originates from carbon-core absorption band at ~290 nm, whereas the fluorescence (above 400 nm) is caused by two surface states at ~350 and 385 nm. In addition to three emissive states, a molecular state was also found in amine functionalized C-dots. Time-resolved emission spectra (TRES) and time-resolved area normalized emission spectra (TRANES) were analyzed to confirm the origin of excitation wavelength dependent fluorescence of C-dots. The surface functional groups on the C-dots are capable of regulating the electron transfer to affect the multifluorescence behavior. The electron transfer takes place from carbon-core to surface domain by the presence of ‒COOH on the surface and vice-versa for the case of ‒NH2 present on the surface. To the best of our knowledge, this is the first report that the multi-emissive states is probed in C-dots system using TRES and TRANES analyses and related fluorescence mechanism are verified clearly.

Keywords: TRES, TRANES, Electron transfer, Carbon-core state, Surface state


2

Page 3 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Introduction To understand the origin and mechanism of carbon-dots (C-dots) multi-fluorescence behavior has been an attractive theme in recent years.1-11 Many research groups have studied the fluorescence characteristic of C-dots and proposed the mechanism in a different manner such as the recombination of electron-hole pairs, quantum effect, surface functional groups, surface states, molecular state and fluorophores with different degrees of π-conjugation.12-27 In general, C-dots is composed of carbon-core and surface domains. Carbon-core domain is characterized by sp2 carbon structure similar to the graphene, whereas surface domain contains abundant functional groups. These two domains are mainly responsible for the fluorescence characteristic of C-dots. Krysmann et al. reported that the C-dots fluorescence originates from both carbogenic core and surface states.28 Wen et al. investigated the fluorescence origin by using ultrafast spectroscopy and proposed that the fluorescence with two spectral-band overlaps are ascribed to intrinsic and extrinsic fluorescence associated with localized sp2 carbon domain and surface states.29 Recently Wang et al. suggested that common green luminescence in C-dots assigned to special edge states is related to carbon backbone and functional groups.30 It was also reported that the fluorescence comes from free zigzag sites.31,32 According to temperature-dependent fluorescence studies, Yu et al. suggested that relatively strong electron-electron scattering and weak electron-phonon interactions in C-dots could be attributed to large amounts of mobile πelectron carriers.33 Moreover, Dekaliuk et al. reported the C-dots fluorescence response represents a composition of individual emitters, but not a collective effect.34 They further claimed that these individual emitters do not exchange their excited-state energies via the fluorescence resonance energy transfer (FRET) mechanism.


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 37

The radiative recombination of excitons (electron–hole pairs) in C-dots has been a broadly accepted mechanism for their photoluminescence. Though, the absorption and fluorescence properties of C-dots were influenced by different size of particles and distribution of different surface energy traps.35,36 Sun et al. reported that the fluorescence from C-dots was attributed to the presence of surface energy traps that became emissive upon surface passivation due to the quantum confinement effect of emissive energy traps on their surface.37 On the other hand, Zhao et al. reported that the excitation wavelength dependent fluorescence was caused by size differences rather than different emissive trap sites.38 Further, Li et al. removed the surface oxygen of C-dots by hydrogen plasma, and the C-dots fluorescence results showed no change before and after the hydrogen plasma. They then concluded that fluorescence should arise from quantum-confinement effects and size-dependent properties of C-dots.39 Thus far, the detailed mechanism and origin of excitation wavelength dependent fluorescence of C-dots still remain a mystery and are a subject of debate. In this work, carboxyl- and amine functionalized C-dots (for convenience, C-dots/COOH and C-dots/NH2 named as C-dots1 and C-dots2, respectively, thereafter) were synthesized. Steady-state and time-resolved fluorescence measurements were carried out to understand the fundamental mechanism of fluorescence as a function of excitation wavelength. Upon irradiation at 280 and 400 nm, time-resolved fluorescence decays of both C-dots were measured at different emission wavelengths, and for the first time, time-resolved emission spectrum (TRES) and timeresolved area normalized emission spectrum (TRANES) analyses were carried out in order to characterize the emissive states of C-dots. The surface passivation was adopted to increase fluorescence quantum yield of C-dots.12-14 It is also used to affect the fluorescence characteristics of carbon-core such as intensity, band gap and band shape. But how surface functional groups


4

Page 5 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


affect the carbon-core emission is yet to be known. In this work, we demonstrate that the electron-withdrawing and electron-donating properties of the functional groups may play a key role to affect the carbon-core fluorescence.

Experimental section Materials Citric acid and 1,2-ethylenediamine (EDA) were purchased from ACROS. Milli-Q ultrapure water was used throughout the experiments.

Synthesis of C-dots Carboxyl functionalized C-dots were synthesized from citric acid by using microwaveassisted method. The 1 g of citric acid dissolved in 10 mL of water and the solution was heated in a domestic microwave oven (Panasonic, NN-ST342) at 550 W power for 7 min. During microwave heating, the solution color was changed from colorless to pale brown which indicates the formation of C-dots. The obtained product was dispersed in Millipore water and then centrifuged (10000 rpm, 15 min) to remove large size particles. The supernatant solution was dialyzed against Millipore water using a dialysis membrane with MWCO of 3.5 – 5.0 kD (Spectra/Por Float-A-Lyzer G2) for two days. For the synthesis of amine functionalized C-dots, 500 µL EDA was added into 10 mL of prepared C-dots/COOH solution (20 µg/mL). The mixture was stirred well and then was refluxed at 120 °C for 15 h. The obtained solution was cooled down to room temperature and then centrifuged and dialyzed. Finally the obtained pure C-dots were used for characterization and experimental studies. The pH value of the C-dots


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 37

solution was tuned by concentrated hydrochloric acid/sodium hydroxide solution and monitored by a pH meter.

Instrumentations High-resolution transmission electron microscopy (HR-TEM) observations were conducted on a Philips/FEI Tecnai 20 G2 S-Twin transmission electron microscope (200 KV). Wide-angle X-ray powder diffraction (XRD) patterns were recorded on a PANalytical (X'Pert PRO) diffractometer using Cu Kα radiation (wavelength λ = 0.1541 nm). Infrared spectra were recorded on a Thermo Scientific Nicolet iS5 FT-IR Spectrometer. Elemental analysis was conducted on Elementar Vario EL-III (for NCH) and EL cube (for O). The absorption spectra of the samples were recorded on a Thermo Scientific evolution 220 UV-visible spectrophotometer. The fluorescence spectral measurements were recorded on a Perkin-Elmer LS-45 spectrometer. Time-resolved fluorescence decays were recorded on excitation at 280 and 400 nm by the timecorrelated single-photon counting (TCSPC) technique with microchannel plate photomultiplier tube (Hamamatsu, R3809U) as a detector.40 The second and third harmonics output from the mode-locked femtosecond laser (Tsunami, Spectra Physics) was used as the excitation source. The instrument response time for TCSPC system is ∼50 ps. The data analysis was carried out by the software provided by IBH (DAS-6) which is based on deconvolution technique using nonlinear least squares methods. All measurements were recorded at ambient temperature.

TRES and TRANES analysis TRES were constructed from the time-resolved fluorescence decays measured at a number of wavelengths (with a 10 nm interval) across the entire emission spectrum.41 The


6

Page 7 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


fluorescence decays were then fitted by using a multi-exponential decay function to deconvolute the instrumental response, and the impulse response function at each wavelength calculated. To construct the TRES, the impulse response functions at different wavelength are normalized to make the intensity integrated at each wavelength equal to the steady-state intensity at that wavelength. A set of H(λ) was calculated as follows41 (1) where ISS, τi and αi are steady-state fluorescence intensity, fluorescence decay lifetime and preexponential factor, respectively. Then the normalized impulse response functions, denoted as IN(λ,t), are multiplied with H(λ) to obtain the normalized intensity decay function I’(λ,t), as given by41 (2) TRES, calculated based on I'(λ,t), are treated as normalized profile that allows visualization of the time-dependent spectral shifts. TRANES were constructed by normalizing the area of each TRES spectrum such that the area of the spectrum at time t is equal to the area of the spectrum at the shortest t. Area normalized spectra were performed using the Peakfit v4.12 software.

Quantum yield calculation Quinine sulphate (0.1 M H2SO4 as solvent, Φ = 0.54) was chosen as a standard to measure the quantum yield (Φ) of the C-dots. The quantum yield was determined at an excitation wavelength of 360 nm by the equation,41

Φ‫ = ݔ‬Φ‫ݐݏ‬

‫ݔߟ ݐݏܣ ݔܫ‬2 ൬ ൰൬ ൰ቆ 2 ቇ ‫ݐݏߟ ݔܣ ݐݏܫ‬

(3)

where Φ is the quantum yield, I the integrated fluorescence intensity, A the absorbance at


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 37

excitation wavelength, and η the refractive index of the solvent. The subscript st and x refer to standard with known quantum yield and the sample, respectively. The absorbance of standard and carbon dots solutions were kept below 0.05.

Results and discussion Physical and optical characterization of C-dots1 and C-dots2 The size and morphology of C-dots were observed by transmission electron microscopy. The TEM image clearly indicates that the formed C-dots were spherical in shape with average diameter of 3.5±0.3 nm and well dispersed from each other (Figure 1a and Figure S1, Supporting Information). The XRD pattern (Figure 1b) showed a broad diffraction peak at ~20.7° (d = 0.44 nm) and 20.1° (d = 0.45 nm) for C-dots1 and C-dots2, respectively and their calculated interlayer distance is larger than that of graphite (d = 0.34 nm). The FTIR spectra were recorded to determine the surface functional groups of C-dots (Figure 1c). Both C-dots exhibit a broad peak with the maxima at ~3400 cm−1 corresponding to the O-H and N-H stretching vibration of carboxylic acid and amine groups, respectively. Both C-dots display C-H vibration at ~2920 cm−1 and C=C vibration at ~1650 cm−1. IR spectrum of C-dots1 shows a peak at 1721 cm−1 due to the stretching of C=O (carbonyl) in addition with O-H band, confirming that the carboxylic functional group exists on the surface of C-dots1. In C-dots2, strong absorption peaks observed at 1544 and 1427 cm−1 indicate the presence of amine N-H bending and amide C-N stretching bands, respectively, thereby revealing that EDA reacted with carboxylic groups on the surface forming the amide group.33 We expect that the EDA residual in the C-dots solution are negligible, because the free EDA molecules were separated from the C-dots solution to the most extent while conducting the dialysis for two days against Millipore water. FTIR results reveal


8

Page 9 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that the C-dots have various surface functional groups such as hydroxyl, carbonyl, epoxy, carboxylic and amine groups. The elemental analysis shows that the C-dots1 is composed of C 55.4, H 5.3, N 0.03 and O 40.5%, whereas the C-dots2 is composed of C 54.9, H 6.9, N 18.4 and O 28.3%. A decrease in an oxygen content of C-dots2 further confirms the reaction occurrence between EDA and -COOH on the C-dots surface. The quantum yield of C-dots1 and C-dots2 is estimated to be 7.2 and 21.8%, respectively, in reference to quinine sulphate as the standard. The C-dots1 shows optical absorption in the UV region (Figure 1d). After surface passivation the C-dots2 shows a strong absorption above 300 nm with respect to C-dots1. In both C-dots, the absorption band below 300 nm is attributed to the π-π* transition of the conjugated C=C units corresponding to carbon-core, whereas the band above 300 nm is assigned to the n-π* transition corresponding to the carbonyl/amine functional groups on the surface.12-14, 42 Fluorescence spectra of C-dots1 and C-dots2 were recorded at different excitation wavelength as shown in Figures 2a and 2b, respectively. The resultant fluorescence spectra reveal that C-dots exhibit multi-emission behavior, and the fluorescence maximum and intensity were found to depend on the excitation wavelengths. In both C-dots, the higher fluorescence intensity was observed at 340 nm excitation and the fluorescence maximum was red shifted with increasing excitation wavelength. It is interesting to note that the fluorescence spectrum of Cdots1 exhibits the peak at ~385 nm with a shoulder at ~440 nm on excitation at 280 nm (Figure 2a). This fact suggests that the peak observed at ~385 nm may arise from the carbon-core, whereas the shoulder at ~440 nm arises from the surface domain. The fluorescence resulting from the carbon-core by the direct electron-hole recombination dominates over the fluorescence from the surface domain. On the contrary, the fluorescence spectrum of C-dots2 shows a strong


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 37

peak at ~440 nm (Figure 2b), suggesting that the fluorescence emitting from carbon-core diminishes, while the fluorescence resulting from the surface domain dominates in C-dots2. When the excitation wavelength is tuned to above 340 nm, the fluorescence is initiated from the surface domain for both C-dots, in which the fluorescence maximum with a larger extent of red shift is observed in C-dots2. Such red-shifted phenomena is caused by the surface functionalization by amine groups. In this manner, the increase of electron density by the presence of lone paired electrons in nitrogen of amine functional groups reduces the energy gap between HOMO and LUMO for the surface domain in C-dots2, and consequently the fluorescence maximum shifts to the longer wavelength. Jin et al. have observed similarly red shifted fluorescence for amine functionalized graphene quantum dots and considered as the contribution of lone paired electrons from nitrogen to the conjugated graphene layers.43 From the steady-state absorption and fluorescence results, we believe that the fluorescence peaking at 385 nm and above 440 nm correspond to the carbon-core and surface domains, respectively. The excitation spectra of C-dots1 and C-dots2 were found to vary with different emission wavelength in the range of 360 to 540 nm as shown in Figures 3a and 3b, respectively. It was noticed that three distinct excitation transitions at ~285, 350 and 385 nm are verified. While monitored at a fixed emission wavelength of 380 nm, the excitation spectrum of C-dots1 exhibits a peak at ~288 nm with narrower bandwidth (Figure 3a), and C-dots2 yields a peak at ~290 nm with relatively smaller intensity (Figure 3b). This indicates that the fluorescence peaking at 385 nm is initiated from the absorption of ~280 nm in the carbon-core. When emission spectrum is scanned from 440 to 480 nm, the excitation spectra for both C-dots exhibit a peak at ~350 nm. But a small shoulder appears at ~385 nm while further scanning the emission wavelength range from 490 to 540 nm. Especially in C-dots2, the relative intensity at 350 nm was found to


10

Page 11 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


decrease, whereas the intensity at 385 nm increases with increasing the emission wavelength (Figure 3b). This indicates that the two different surface states (at 350 and 385 nm, named as SSI and SS-II respectively) are responsible for the origin of C-dots fluorescence in the range of 440 to 540 nm. The SS-I is closely associated with the fluorescence observed at shorter wavelength (~440 to 490 nm), while the SS-II plays a more significant role for the fluorescence at longer wavelength (above 490 nm). From the above optical characterization of C-dots, the observed multi-fluorescence bands mainly originate from three distinct states corresponding to the carboncore (280 nm) and surface domain (350 and 385 nm).

pH dependence of fluorescence The pH dependence of C-dots fluorescence was studied at different excitation wavelengths (280 and 400 nm) to understand the influence of surface functional groups on the fluorescence of C-dots. It is well known that the carboxylic (‒COOH) and amine (‒NH2) functional groups attached to the C-dots1 and C-dots2 have electron withdrawing and donating properties, respectively.9,42,43 The absorption spectra of both C-dots were acquired as a function of pH from 2 to 12 and the resultant spectra show not much change in the absorption band (Figure S2, Supporting Information). The fluorescence spectra of C-dots1 with varying pH are shown in Figure 4. On excitation at 280 nm, the fluorescence peak intensity at 385 nm was found to disappear and the shoulder at ~440 nm becomes predominant with decreasing pH from 7 to 2 (Figure 4a). At acidic pH, the electron withdrawing nature of ‒COOH groups on the C-dots1 surface becomes stronger, because of less probability to form deprotonated type (‒COO−). Accordingly, the ‒COOH group on the surface tends to withdraw the electrons from conduction band of carbon-core and prevent the subsequent electron‒hole recombination, such that the


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 37

fluorescence originating from carbon-core (385 nm) may disappear in pH 2. When the condition was changed to be basic from pH 7 to 12, the fluorescence intensity was successively enhanced for excitation of 280 nm as shown in Figure 4b. The carboxylic group on the surface becomes deprotonated (‒COO−) as the pH is increased, thereby reducing the electron withdrawing efficiency. Thus, the electron‒hole recombination becomes feasible resulting in the fluorescence enhancement upon excitation at 280 nm. On the other hand, on the excitation at 400 nm the fluorescence quenching was observed with decreasing pH from 7 to 2 (Figure 4c) whereas the fluorescence enhancement was observed with increasing basic pH (Figure 4d) from pH 7. It seems difficult to interpret the fluorescence behavior as a function of pH from the viewpoint of electron transfer, as excited at 400 nm from the surface domain. In fact, the (n, π*)CO band of the –COOH functional group is excited at 400 nm; that is, a nonbonding electron of O in C=O is promoted to the antibonding-orbital of C=O. According to Norrish reaction, the (n, π*)CO band of –COOH group can easily couple with the adjecent (n, σ*) band of OH and cause the single bond rupture if the (n, σ*) bond is a repulsive configuration. Thus, the fluorescence intensity of C-dots/COOH will be suppressed to some extent by the adjecent bond coupling. In contrast, the (n, π*)CO band in the –COO- functional group lacks the diabatic coupling with its adjecent single bond such that the resulting fluorescence should be stronger than that in the –COOH group. Accordingly, when the pH value is adjusted to increase from pH 2 (acidic) to pH 12 (basic), the amount ratio of –COOH to –COO- becomes decreasing. That is why the fluorescence intensity shows to decrease when pH 7 is reduced to pH 2 (Figure 4c), whereas the fluorescence intensity is enhanced with increasing pH 7 to pH 12 (Figure 4d). For C-dots2 under either acidic or basic condition, the fluorescence intensity was found to decrease on both 280 and 400 nm excitation, as compared with that at pH 7 (Figure S3,


12

Page 13 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Supporting Information). The electron donating nature of ‒NH2 groups on the C-dots2 surface becomes stronger in basic pH, whereas the protonated form (NH3+) in acidic solution may reduce the electron donating efficiency. As shown in Figure S3 (Supporting Information), the carboncore fluorescence is quenched at 280 nm excitation under either acidic or basic conditions. The fluorescence intensity is expected to diminish under basic pH, because the sufficient electron donation from surface HOMO to valence band of carbon-core. Filling of partial holes by the surface electron donation should diminish the radiative recombination between the electrons of conduction band and the holes of valence band. Nevertheless, the carbon-core fluorescence did not show any enhancement as expected under acidic condition that decreases the electron donating efficiency of the -NH3+ protonated group. On the other hand, at excitation of 400 nm, the surface fluorescence is successively quenched under the acidic condition (pH 7 to 2). The fact is caused by less capability of the electron donation from surface domain to carbon-core. In other words, partial electrons flow back to the surface domain with respect to the condition at pH 7. However, the fluorescence is not enhanced at basic pH as expected from the viewpoint of the electron transfer. Apart from the electron transfer effect, we expect that the slight fluorescence quenching for both conditions such as at 400 nm excitation in basic solution and at 280 nm excitation in acidic solution might be caused by the presence of molecular state in C-dots2. Recently, Yang and co-workers have investigated the photoluminescent mechanism of C-dots which were prepared by using citric acid and ethylenediamine.19,44,45 They found that a fluorescent molecule (imidazo[1,2-a]pyridine-7-carboxylicacid, 1,2,3,5-tetrahydro-5-oxo-, IPCA) is produced in the C-dots during the heating process of CA and EDA mixture solution.45 The higher quantum yield of C-dots is mainly due to the formation of this molecular state along with carbon core. The


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 37

IPCA molecule has the absorption at 240 and 350 nm, and fluorescence at 442 nm.45 The photoluminescence properties of the C-dots is strongly affected by IPCA with the molecular state.45 In characterizing the pH dependent fluorescence behavior of C-dots prepared by CA and EDA, Zhu et al. have similarly found that the surface state/molecular state (molecular groups) are strongly affected by the low and high pH.19 Accordingly, we expect that the IPCA molecule was formed in C-dots2 during the reflux of the solution of C-dots1 with EDA.19,23,44 Thus, IPCA existence in C-dots2 explains why the fluorescence quenching was observed in acidic and basic condition at the excitation of both 280 and 340 nm. The fluorescence quenching or enhancement observed in C-dots1 may be reasonably interpreted by the electron transfer between carbon-core and surface domain that is regulated by the pH adjustment. Nevertheless, apart from influence by the electron transfer, the fluorescence behavior related to C-dots2 is also sensitive to the molecule state.

Characterization by TRES and TRANES In order to confirm the multi-emissive states and to understand the interplay between carbon-core and surface domain, time-resolved fluorescence measurements were acquired at different emission wavelength for both C-dots that were excited at 280 and 400 nm (Figure S4, Supporting Information). The resulting fluorescence decay can be characterized by a multiexponential function at either excitation wavelength. The fluorescence lifetimes were found to differ from each other at different emission wavelength. The fluorescence lifetimes of C-dots1 monitored at 390 nm (λexc 280 nm) yielded 1.02 and 3.91 ns, whereas the lifetimes of 0.52, 2.41 and 6.41 ns were obtained at 470 nm (λexc 400 nm). Similarly, the fluorescence lifetimes of Cdots2 monitored at 450 nm (λexc 280 nm) were found to be 0.19, 1.67 and 6.52 ns, whereas the


14

Page 15 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


lifetimes became 0.92, 3.15 and 8.83 ns at 470 nm (λexc 400 nm). At excitation of 280 nm, the acquired excitation and emission processes were closely associated with both carbon-core and surface domains, whereas the surface domain was involved substantially at 400 nm excitation. The TRES of C-dots were constructed using steady-state emission intensity and fluorescence decay parameters (lifetime and pre-exponential factor) following the procedure described by Lakowicz.41 Figure 5 shows the TRES results of C-dots1 and C-dots2 at 280 nm excitation. In the TRES of C-dots1, the fluorescence maximum gradually shifted to the smaller wavenumber with increasing delay time, but the spectral shape did not change significantly. In contrast, C-dots2 shows relatively less spectral shifts in TRES. In general, the red shifted fluorescence spectra in TRES was reported to be due to the excited state relaxation process and/or multiple emissive species or states.41 Upon excitation at 280 nm, the fluorescence maxima were found to shift from ~373 to 410 nm, when delay time is prolonged from 0 to 10 ns (Figure 5a). The peak at 373 nm originated from the carbon-core, while the peak at 410 nm came from the surface domain. Accordingly, we conclude that the spectral shifts of C-dots1 in TRES are ascribed to the contribution of excited electron relaxation from carbon-core to surface domain. But such a relaxation process was not observed in C-dots2 (Figure 5b). Note that C-dots1 yielded a fluorescence peak at 385 nm, but this peak became very weak in C-dots2. When pH is adjusted to 2, the TRES of C-dots1 immediately following excitation at 280 nm shows a fluorescence peaking at ~371 nm with a shoulder at ~425 nm (Figure 6a). Then, with increasing the delay time, the fluorescence maximum at ~371 nm shifted to a smaller wavenumber with gradual decrease in the intensity. As the delay time was further extended, the fluorescence at ~371 nm diminished completely and a new fluorescence peaking at ~445 nm appeared. The acidic condition strengthens the electron withdrawing efficiency of the –COOH


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 37

functional group and thus results in decrease in the carbon-core fluorescence with varying time. The results are consistent with the steady-state fluorescence and pH dependent studies, confirming that electron transfer occurs from carbon-core to surface domain for C-dots1. In the pH 2 result of C-dots2 at 280 nm, TRES shows larger fluorescence spectral shifts with time (Figure 6b), as compared to that at pH 7. Note that the fluorescence at 25470 cm-1 (392 nm) appearing at 0 ns initiated from the carbon-core was observed under acidic condition, but such spectral shift does not appear at pH 7. The acidic condition apparently prolongs the lifetime of the carbon-core fluorescence for the C-dots2. The electron donating efficiency of amine group is reduced at pH 2 due to the protonated amine (NH3+) formation which retards the HOMO electron donation to the valence band of carbon-core. Thus, the slow diminishing of carbon-core fluorescence should prolong its lifetime. To evidence involvement of multi-emissive states, TRANES of C-dots were constructed as shown in Figure 7. The TRANES of C-dots1 at 280 nm excitation exhibits an isoemissive point at 26000 cm-1 (385 nm) in the time window of 3 to 10 ns (Figure 7a). Generally, an isoemissive point in TRANES indicates existence of two emissive states or species present in the system.46,47 The finding of an isoemissive point in TRANES analysis along with the TRES results in the C-dots1 confirms that two emissive states are associated with the carbon-core and surface state. After 10 ns, the TRANES shows that the fluorescence maximum shifts successively but without appearance of additional isoemissive point (Figure S5, Supporting information). On the other hand, the TRANES of C-dots2 exhibits similarly one isoemissive point at 24120 cm-1 (414 nm) in the time window of 0 to 4 ns (Figure 7b), confirming that the observed fluorescence originates from two emissive states as ascribed to the carbon-core and surface state.


16

Page 17 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures 8a and 8b show the TRES of C-dots1 and C-dots2 in the time scales of 0 to 50 ns at 400 nm excitation wavelength, respectively. TRES spectra of C-dots1 and C-dots2 gradually shifted to the red side with increase of delay time; however, the spectral bandwidth reduces with increasing time. This indicates that emission spectrum originates from more than one emissive state. Further, TRANES of C-dots1 exhibits an isoemissive point at 464 nm (Figure 9a), whereas C-dots2 exhibits two isoemissive points at 450 and 507 nm (Figure 9b). In general, multiple isoemissive points exist only when the fluorescence lifetimes of different emissive species must differ distinctly from each other.46,47 In the present systems, the distinct lifetimes of C-dots are responsible for the multi-isoemissive points in TRANES. The TRANES analysis concludes that the observation of one isoemissive point for C-dots1 is ascribed to the surface states composed of the SS-I (350 nm) and SS-II (385 nm). On the other hand, two isoemissive points for C-dots2 should be indicative of the involvement of three emissive states which contain SS-I and SS-II, and the third emissive state that might be caused by the molecular state. According to the results as described above, the proposed origin of multi-fluorescence and excited state process of C-dots is summarized as follows. (1) The absorption transition of Cdots occurs from both carbon-core at below 300 nm and surface domains with a broad region at above 300 nm. (2) Upon irradiation of the carbon-core, the electron promoted from the valence band to conduction band results in the formation of electron and hole charges. In contrast, the n‒ π* absorption transition occurs at above 300 nm in the surface states involving the lone paired electron present in the functional groups. Two surface states SS-I and SS-II at ~350 and 385 nm, respectively, have been verified. (3) The observed multi-fluorescence bands of C-dots mainly originate from carbon-core and surface domain. The carbon-core fluorescence at 385 nm is due to the radiative recombination of electron‒hole pairs, while the surface state fluorescence is


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 37

caused by the radiative relaxation from excited state to ground state. The excitation of SS-I (350 nm) and SS-II (385 nm) results in the fluorescence observed predominately in the range of 440 to 490 nm and 500 to 540 nm, respectively. (4) The ‒COOH and ‒NH2 functional groups may affect the fluorescence characterization via electron transfer based on the electron withdrawing and electron donating behavior of functional groups, as demonstrated in the pH dependence of steady-state fluorescence and TRES measurements. (5) The existence of molecular state in Cdots2 due to the IPCA formation was confirmed by TRANES analysis, and this molecular state was sensitive to the low and high pH conditions. Apart from multi-emissive states, the C-dots exhibit different fluorescence maximum at different excitation wavelengths. For example, the fluorescence maxima of C-dots2 were found to be at 489, 494, 497 and 500 nm while excited at 420, 430, 440 and 450 nm, respectively. It was reported previously that the fluorescence red-shift observed at excitation of longer wavelength is due to the presence of different size of particles.35,48,49 Due to the quantum confinement effect, the energy gap decreases with increasing size of the C-dots. As a consequence, the larger size of C-dots might be excited at the longer wavelength and a smaller red-shift in the fluorescence maximum is found at each excitation wavelength. Moreover, the size distribution obtained from the TEM image reveals that different size of carbon dots are present in this system. Accordingly, we anticipate that the red-shift of fluorescence maximum with varying excitation wavelength is caused by different size of particles in conjunction with multi-emissive states.


18

Page 19 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Conclusions By using steady-state and time-resolved fluorescence spectroscopy, the multifluorescence in citric acid-derived C-dots has been verified to mainly originate from three different emissive states, one from carbon-core and the other two from surface domain. However, the fluorescence behavior of C-dots2 is significantly dependent on an additional emissive molecular state of IPCA co-generated in the synthesis process. The TRES and TRANES analyses further confirm these emissive states. The fluorescence resulting from either carbon-core or surface domain is significantly affected by the nature of the functional groups on the surface. Due to the electron withdrawing and donating properties of functional groups, the electron transfer may take place to regulate the fluorescence intensity and lifetime. Apart from the three emissive states, the size effect should be considered to understand the details of excitation wavelength dependent fluorescence of C-dots. We believe that this work may gain more insight into the nature of C-dots.

Acknowledgment This work was supported by Ministry of Science and Technology of Taiwan, Republic of China under contract number NSC 102-2113-M-002-009-MY3. N.D. thanks the National Taiwan University for a postdoctoral fellowship under the contract number 104-R-4000.

Supporting Information TEM and HRTEM image of C-dots1; the absorption spectra of C-dots with varying pH; fluorescence spectra of C-dots2 with varying pH at excitation of 280 and 400 nm; fluorescence decays of C-dots1 (a) and C-dots2 (b) recorded at different emission wavelengths; TRANES of


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

C-dots1 at different time scales; references (1, 9, 18, 30, 37 and 48). This information is available free of charge via the internet at http://pubs.acs.org.

Author information Corresponding Author * King-Chuen Lin Department of Chemistry, National Taiwan University, and Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan. E-mail: [email protected]; Fax: +886-2-2362-1483; Tel: +886-2-3366-1162.

References (1)

LeCroy, G. E.; Sonkar, S. K.; Yang, F.; Veca, L. M.; Wang, P.; Tackett, K. N.; Yu, J.-J.; Vasile, E.; Qian, H.; Liu, Y.; et al. Toward Structurally Defined Carbon Dots as Ultracompact Fluorescent Probes. ACS Nano 2014, 8, 4522–4529.

(2)

Zhu, S.; Song, Y.; Zhao, X.; Shao, J.; Zhang, J.; Yang, B. The Photoluminescence Mechanism in Carbon Dots (Graphene Quantum Dots, Carbon Nanodots and Polymer Dots): Current State and Future Perspective. Nano Res. 2015, 8, 355-381.

(3)

Gao, M. X.; Liu, C. F.; Wu, Z. L.; Zeng, Q. L.; Yang, X. X.; Wu, W. B.; Lia, Y. F.; Huang, C.

Z.

A

Surfactant-Assisted

Photoluminescent

Carbon

Redox

Quantum

Hydrothermal Dots

with

Route

to

Prepare

Aggregation-Induced

Highly Emission

Enhancement Properties. Chem. Commun. 2013, 49, 8015–8017.


20

Page 21 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(4)

Nie, H.; Li, M.; Li, Q.; Liang, S.; Tan, Y.; Sheng, L.; Shi, W.; Zhang, S. X.-A. Carbon Dots with Continuously Tunable Full-Color Emission and Their Application in Ratiometric pH Sensing. Chem. Mater. 2014, 26, 3104–3112.

(5)

Hu, C.; Yu, C.; Li, M.; Wang, X.; Yang, J.; Zhao, Z.; Eychmüller, A.; Sun, Y.-P.; Qiu, J. Chemically Tailoring Coal to Fluorescent Carbon Dots with Tuned Size and Their Capacity for Cu(II) Detection. Small 2014, 10, 4926–4933.

(6)

Li1, X.; Zhang, S.; Kulinich, S. A.; Liu, Y.; Zeng, H. Engineering Surface States of Carbon Dots to Achieve Controllable Luminescence for Solid–Luminescent Composites and Sensitive Be2+ Detection. Sci. Rep. 2014, 4, 4976.

(7)

Barman, M. K.; Bhattacharyya, S.; Patra, A. Steady State and Time Resolved Spectroscopic Study of C-dots-MEH-PPV Polymer Nanoparticles Composites. Phys. Chem. Chem. Phys. 2013, 15, 16834–16840.

(8)

Liang, Q.; Ma, W.; Shi, Y.; Li, Z.; Yang, X. Easy Synthesis of Highly Fluorescent Carbon Quantum Dots from Gelatin and Their Luminescent Properties and Applications. Carbon 2013, 60, 421–428.

(9)

Hola, K.; Bourlinos, A. B.; Kozak, O.; Berka, K.; Siskova, K. M.; Havrdova, M.; Tucek, J.; Safarova, K.; Otyepka, M.; Giannelis, E. P.; et al. Photoluminescence Effects of Graphitic Core Size and Surface Functional Groups in Carbon Dots: COO− Induced Red-Shift Emission. Carbon 2014, 70, 279–286.

(10) Hu, S.; Tian, R.; Dong, Y.; Yang, J.; Liua, J.; Chang, Q. Modulation and Effects of Surface Groups on Photoluminescence and Photocatalytic Activity of Carbon Dots. Nanoscale 2013, 5, 11665–11671.


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 37

(11) Linehan, K.; Doyle, H. Size Controlled Synthesis of Carbon Quantum Dots Using Hydride Reducing Agents. J. Mater. Chem. C 2014, 2, 6025–6031. (12) Baker, S. N.; Baker, G. A. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem. Int. Ed. 2010, 49, 6726–6744. (13) Li, H.; Kang, Z.; Liu, Y.; Lee, S.-T. Carbon Nanodots: Synthesis, Properties and Applications. J. Mater. Chem. 2012, 22, 24230–24253. (14) Wang, Y.; Hu, A. Carbon Quantum Dots: Synthesis, Properties and Applications. J. Mater. Chem. C 2014, 2, 6921–6939. (15) Hu, S.-L.; Niu, K.-Y.; Sun, J.; Yang, J.; Zhao N.-Q.; Du, X.-W. One-Step Synthesis of Fluorescent Carbon Nanoparticles by Laser Irradiation. J. Mater. Chem. 2009, 19, 484– 488. (16) Srivastava, S.; Gajbhiye, N. S. Carbogenic Nanodots: Photoluminescence and RoomTemperature Ferromagnetism. ChemPhysChem 2011, 12, 2624–2632. (17) Fang, Y.; Guo, S.; Li, D.; Zhu, C.; Ren, W.; Dong, S.; Wang, E. Easy Synthesis and Imaging Applications of Cross-Linked Green Fluorescent Hollow Carbon Nanoparticles. ACS Nano 2012, 6, 400–409. (18) Cao, L.; Wang, X.; Meziani, M. J.; Lu, F.; Wang, H.; Luo, P. G.; Lin, Y.; Harruff, B. A.; Veca, L. M.; Murray, D.; et al. Carbon Dots for Multiphoton Bioimaging. J. Am. Chem. Soc. 2007, 129, 11318–11319. (19) Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly Photoluminescent Carbon Dots for Multicolor Patterning, Sensors, and Bioimaging. Angew. Chem. Int. Ed. 2013, 52, 3953–3957.


22

Page 23 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(20) Bourlinos, A. B.; Zboril, R.; Petr, J.; Bakandritsos, A.; Krysmann M.; Giannelis, E. P. Luminescent Surface Quaternized Carbon Dots. Chem. Mater. 2011, 24, 6–8. (21) Yang, S.-T.; Cao, L.; Luo, P. G.; Lu, F.; Wang, X.; Wang, H.; Meziani, M. J.; Liu, Y.; Qi, G.; Sun, Y.-P. Carbon Dots for Optical Imaging in Vivo. J. Am. Chem. Soc. 2009, 131, 11308–11309. (22) Mao, X.-J.; Zheng, H.-Z.; Long, Y.-J.; Du, J.; Hao, J.-Y.; Wang, L.-L.; Zhou, D.-B. Study on the Fluorescence Characteristics of Carbon Dots. Spectrochim. Acta, A 2010, 75, 553– 557. (23) Zhai, X.; Zhang, P.; Liu, C.; Bai, T.; Li, W.; Dai, L.; Liu, W. Highly Luminescent Carbon Nanodots by Microwave-Assisted Pyrolysis. Chem. Commun. 2012, 48, 7955–7957. (24) Huang, J. J.; Zhong, Z. F.; Rong, M. Z.; Zhou, X.; Chen, X. D.; Zhang, M. Q. An Easy Approach of Preparing Strongly Luminescent Carbon Dots and Their Polymer Based Composites for Enhancing Solar Cell Efficiency. Carbon 2014, 70, 190–198. (25) Lin, Z.; Xue, W.; Chen, H.; Lin, J.-M. Classical Oxidant Induced Chemiluminescence of Fluorescent Carbon Dots. Chem. Commun. 2012, 48, 1051–1053. (26) Zhang, S.; He, Q.; Li, R.; Wang, Q.; Hu, Z.; Liu, X.; Chang, X. Study on the Fluorescence Carbon Nanoparticles. Mater. Lett. 2011, 65, 2371–2373. (27) Hsu, P.-C.; Chang, H.-T. Synthesis of High-Quality Carbon Nanodots from Hydrophilic Compounds: Role of Functional Groups. Chem. Commun. 2012, 48, 3984–3986. (28) 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.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 37

(29) Wen, X. M.; Yu, P.; Toh, Y. R.; Hao, X. T.; Tang, J. Intrinsic and Extrinsic Fluorescence in Carbon Nanodots: Ultrafast Time-Resolved Fluorescence and Carrier Dynamics. Adv. Opt. Mater. 2013, 1, 173–178. (30) Wang, L.; Zhu, S.-J.; Wang, H.-Y.; Qu, S.-N.; Zhang, Y.-L.; Zhang, J.-H.; Chen, Q.-D.; Xu, H.-L.; Han, W.; Yang, B.; et al. Common Origin of Green Luminescence in Carbon Nanodots and Graphene Quantum Dots. ACS Nano 2014, 8, 2541–2547. (31) Pan, D.; Zhang, J.; Li, Z.; Wu, M. Hydrothermal Route for Cutting Graphene Sheets into Blue-Luminescent Graphene Quantum Dots. Adv. Mater. 2010, 22, 734–738. (32) Zhu, S.; Zhang, J.; Liu, X.; Li, B.; Wang, X.; Tang, S.; Meng, Q.; Li, Y.; Shi, C.; Hu, R. Graphene Quantum Dots with Controllable Surface Oxidation, Tunable Fluorescence and Up-Conversion Emission. RSC Adv. 2012, 2, 2717–2720. (33) Yu, P.; Wen, X.; Toh, Y.-R.; Tang, J. Temperature-Dependent Fluorescence in Carbon Dots. J. Phys. Chem. C 2012, 116, 25552–25557. (34) Dekaliuk, M. O.; Viagin, O.; Malyukin, Y. V.; Demchenko, A. P. Fluorescent Carbon Nanomaterials: Quantum Dots or Nanoclusters? Phys. Chem. Chem. Phys. 2014, 16, 16075–16084. (35) Sahu, S.; Behera, B.; Maiti, T. K.; Mohapatra, S. Simple One-Step Synthesis of Highly Luminescent Carbon Dots from Orange Juice: Application as Excellent Bio-Imaging Agents. Chem. Commun. 2012, 48, 8835–8837. (36) De, B.; Karak, N. Green and Facile Approach for the Synthesis of Water Soluble Fluorescent Carbon Dots from Banana Juice. RSC Adv. 2013, 3, 8286–8290.


24

Page 25 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(37) Sun, Y. P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K. A. S.; Pathak, P.; Meziani, M. J.; Harruff, B. A.; Wang, X.; Wang, H.; et al. Quantum-Sized Carbon Dots for Bright and Colorful Photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756–7757. (38) Zhao, Q. L.; Zhang, Z. L.; Huang, B. H.; Peng, J.; Zhang M.; Pang, D. W. Facile Preparation of Low Cytotoxicity Fluorescent Carbon Nanocrystals by Electrooxidation of Graphite. Chem. Commun. 2008, 5116–5118. (39) Li, H. T.; He, X. D.; Kang, Z. H.; Huang, H.; Liu, Y.; Liu, J. L.; Lian, S. Y.; Tsang, C. H. A.; Yang X. B.; Lee, S. T. Water-Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design. Angew. Chem. Int. Ed. 2010, 49, 4430–4434. (40) Dhenadhayalan, N.; Selvaraju, C.; Ramamurthy, P. Photoionization and Time-Dependent Stokes Shift of Coumarin 307 in Soft Matter: Solvation and Radical-Ion Pair Recombination Dynamics. J. Phys. Chem. B 2011, 115, 10892−10902. (41) Lakowicz, J. R. Principles of Fluorescence Spectroscopy. 3rd ed.; Springer Science: New York, 2006. (42) Dhenadhayalan N.; Lin, K.-C. Chemically Induced Fluorescence Switching of Carbon-Dots and Its Multiple Logic Gate Implementation. Sci. Rep. 2015, 5, 10012. (43) Jin, S. H.; Kim, D. H.; Jun, G. H.; Hong, S. H.; Jeon, S. Tuning the Photoluminescence of Graphene Quantum Dots Through the Charge Transfer Effect of Functional Groups. ACS Nano 2013, 7, 1239–1245. (44) Song, Y.; Zhu, S.; Xiang, S.; Zhao, X.; Zhang, J.; Zhang, H.; Fu, Y.; Yang, B. Investigation into the Fluorescence Quenching Behaviors and Applications of Carbon Dots. Nanoscale, 2014, 6, 4676–4682.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 37

(45) 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. (46) Koti, A. S. R.; Krishna, M. M. G.; Periasamy, N. Time-Resolved Area-Normalized Emission Spectroscopy (TRANES): A Novel Method for Confirming Emission from Two Excited States. J. Phys. Chem. A 2001, 105, 1767–1771. (47) Koti, A. S. R.; Periasamy, N. Application of Time Resolved Area Normalized Emission Spectroscopy to Multicomponent Systems. J. Chem. Phys. 2001, 115, 7094–7099. (48) Peng, J.; Gao, W.; Gupta, B. K.; Liu, Z.; Romero-Aburto, R.; Ge, L.; Song, L.; Alemany, L. B.; Zhan, X.; Gao, G.; et al. Graphene Quantum Dots Derived from Carbon Fibers. Nano Lett. 2012, 12, 844–849. (49) Melnikov, D. V.; Chelikowsky, J. R. Quantum Confinement in Phosphorus-Doped Silicon Nanocrystals. Phys. Rev. Lett. 2004, 92, 046802.


26

Page 27 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions Figure 1. TEM image (a) of C-dots2, inset shows size distributions; XRD patterns (b), FTIR spectra (c) and absorption spectra (d) of C-dots. Figure 2. Fluorescence spectra of C-dots1 (a) and C-dots2 (b) monitored at different excitation wavelengths in the range of 280 to 460 nm. Figure 3. Excitation spectra of C-dots1 (a) and C-dots2 (b) monitored at different emission wavelengths in the range of 360 to 540 nm. Figure 4. Fluorescence spectra of C-dots1 with varying pH at excitation of 280 (a, b) and 400 nm (c, d). Figure 5. Normalized TRES of C-dots1 (a) and C-dots2 (b) at different time scales. Excitation at 280 nm. Figure 6. Normalized TRES of C-dots1 (a) and C-dots2 (b) in pH 2 at different time scales. Excitation at 280 nm. Figure 7. The TRANES of C-dots1 (a) and C-dots2 (b) at different time scales. Excitation at 280 nm. Figure 8. Normalized TRES of C-dots1 (a) and C-dots2 (b) at different time scales in the range of 0 to 50 ns. Excitation at 400 nm. Figure 9. The TRANES of C-dots1 (a) and C-dots2 (b) at different time scales. Excitation at 400 nm.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

Figure 1


28

Page 29 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 37

Figure 3


30

Page 31 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4


31


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

Figure 5


32

Page 33 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

Figure 7


34

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8


35


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

Figure 9


36

Page 37 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table of Content (TOC)


37

Unravelling the Multiple Emissive States in Citric ... - ACS Publications

Recommend Documents