Accepted Manuscript

Easy synthesis of highly fluorescent carbon quantum dots from gelatin and their
luminescent properties and applications

Qinghua Liang, Wangjing Ma, Yao Shi, Zhi Li, Xinming Yang

PII: S0008-6223(13)00351-5
DOI: http://dx.doi.org/10.1016/j.carbon.2013.04.055
Reference: CARBON 7986

To appear in: Carbon

Received Date: 30 January 2013

Please cite this article as: 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), doi: http://dx.doi.org/
10.1016/j.carbon.2013.04.055

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 Easy synthesis of highly fluorescent carbon quantum dots from

gelatin and their luminescent properties and applications

Qinghua Liang, Wangjing Ma, Yao Shi, Zhi Li* and Xinming Yang

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS), 29

Zhongguancun East Road, Beijing, 100190, PR China.

Abstract

A simple approach for the synthesis of fluorescent carbon dots (CQDs) has been

developed by the hydrothermal treatment of gelatin in the presence only pure water.

The as-synthesized CQDs were found to emit blue photoluminescence (PL) with a

maximum quantum yield of 31.6%. Meanwhile, the CQDs exhibit

excitation-dependent, pH-sensitive and up-converted PL properties. Importantly, these

CQDs are demonstrated to be excellent bioimaging agents and fluorescent ink due to

their stable emission, well dispersibility, low toxicity, long emission life time, and

good compatibility with cells and macromolecules.

*
Corresponding author. Fax: +86 10 62554670

E-mail address: lizhi@mail.ipc.ac.cn (Zhi Li)

1
1. Introduction

During the past decades, due to their unique optical and electronic properties

caused by the quantum confinement and edge effects, luminescent quantum dots (QDs)

have attracted considerable interest for the promising candidates to replace the

conventional phosphor materials [1-7]. However, the traditional semiconductor QDs

based on metallic elements, such as CdS, CdSe, PbSe and Ag2S, more or less suffer

from the problems of toxicity, hydrophobicity and high cost, which limits their

practical application. As a new type of the nanocarbon family, carbon quantum dots

(CQDs) have been found to be an intriguing QDs for numerous applications owing to

their favorable luminescent properties, high chemical stability, low toxicity,

biocompatibility, and easy functionalization [8, 9]. Inspired by these superior

advantages, many works have been carried out on the simple synthesis routes and

intensive studies of the photoluminescence (PL) properties. Since its origin production

from carbon nanotubes during the process of electrophoresis in 2004 [10], a variety of

synthesis approaches including pyrolysis [11], electrochemical exfoliation [6, 12-14],

incomplete combustion oxidation [15], acidic oxidation [16], laser ablation [17],

hydrothermal treatments [18], microwave/ultrasonic passivation [7, 19], and plasma

treatment [20] in recent years have been developed to prepare CQDs with various

precursors (such as graphite oxide [21], citric acid [22], glycerol [23], coffee grounds

[24], soy milk [25], grass [26], egg [20], and so on). Despite many impressive

advances, it is still desirable and urgent to rapidly synthesize high-quality CQDs by an

easy and environmentally benign method with low-cost raw materials.

2
 Recently, as a traditional soft chemical preparation route, the hydrothermal

synthesis route based on water system is considered to be one of the simplest and

most cost-effective methods owing to its cheap apparatus, simple manipulation,

low energy consumption, good selectivity and preparation can be achieved in a

single step without complex control. This method has been used to prepare CQDs by

many researchers. For instance, Liu and co-workers reported the synthesis of CQDs

with a PL quantum yield (QY) of 6.2% by hydrothermal treatment of grass [26].

Zhang et al. produced fluorescent CQDs with a PL quantum yield of 11% by

using bovine serum albumin and as carbon precursor under the passivation of

decanediamine [27]. Sahu and co-workers prepared luminescent CQDs with a PL

quantum yield of 26% from orange juice [18]. It can be seen that, apart from the

synthesis method, an appropriate carbon precursor is another essential factor for

considering. Very recently, Hsu and Chang found that compounds containing

both amino and carboxyl groups are beneficial to synthesize CQDs with high PL

quantum yield [28]. Inspired by this, we assumed that the gelatin that is abundant in

amino and carboxyl groups may be a suitable precursor.

Herein, with a main aim of developing a more sustainable and greener

approach for CQDs production, we report an easy route to synthesize highly

luminescent CQDs by the hydrothermal treatment of gelatin without the

assistance of any chemicals but only pure water. As a relative inexpensive and

water-soluble polymer mainly derived from animal skin and bones, gelatin has

been long used in photographic coating, film-making and food industry. To the

3
best of our knowledge, this is the first time that using the gelatin as the reactants

to fabricate CQDs. The strategy of the synthetic process is illustrated in Fig. 1. A

significant advantage of this method is that neither a strong acid solvent nor a surface

passivation reagent or a complicated post-treatment process is demanded.

Figure 1 A schematic of the synthesis process of photoluminescent CQDs by

hydrothermal treatment of commercial gelatin.

2. Experimental

2.1 Chemicals

Chemical reagents were purchased from the Beijing Chemical Company and

used as received unless otherwise specified. Inertia gelatin (No.55667) was obtained

from Rousselot in France, and was used without further purification. The incubation

media for cells were purchased from Thermo Fisher. Distilled water was used

throughout the experiments.

2.2 Sample preparation

The typical experimental procedure is described as follows: first, 0.8 g gelatin

was added to 40 mL water. Subsequently, the above admixture was poured into a

stainless steel autoclave with a Teflon liner of 50 mL capacity and heated at 200 ℃

4
for 3 h. Finally, the reactor was automatically cooled to room temperature. The

resulting light yellow solution was centrifuged at 16000 rpm for 30 min to remove

weight precipitate and agglomerated particles and then yielded a light brown aqueous

solution of CQDs for further characterization. The yield determined by freeze-dried

method was calculated to be approximate 38.6%.

2.3 Characterization

Transmission electron microscope (TEM) images were taken on a JEOL JEM

2100 TEM at an accelerating voltage of 200 kV. Prior to TEM analysis, the sample

was dropped on a Cu grid coated with an ultrathin amorphous carbon film, and

then dried under ambient condition. UV-Vis absorption was carried out using a Varian

Cary 500 ultraviolet-visible-near infrared (UV-Vis-NIR) spectrophotometer.

Fluorescence spectroscopy was performed using a fluorescence spectrophotometer

(Model Cary eclipse) equipped with a 120 W xenon lamp as the excitation source. The

PL decay curves were recorded on a combined fluorescence lifetime and steady state

spectrometer (F900) using a time-corrected single photon counting system. The

excitation light source for measuring the decay profile of up-converted emission was

the 680 nm line of a laser. Fourier transform infrared spectra (FTIR) investigation was

accomplished by an Excalibur 3100 infrared spectrophotometer using KBr pellets as

the sample matrix in the frequency range 500-4000 cm-1. X-ray photoelectron

spectroscopy (XPS) analysis was characterized by an ESCALab220i-XL electron

spectrometer using 300 W Al Kα as the X-ray source (1486.6 eV), and the energy step

5
size was set as 0.100 eV. Curve fitting of the C1s spectra was achieved by a

Gaussian-Lorentzian peak shape. Powder X-ray diffraction (XRD) was analyzed by a

German Bruker D8 Focus XRD with a graphite monochromatized Cu-Ka radiation

source (λ = 1.5405 Å). The fluorescence microscopy images were taken with an

Olympus BX51TRF microscope. The light source for fluorescence microscopy

observation was a mercury lamp with a fluorescent filter. All measurements were

performed at room temperature.

2.4 Quantum yield measurements

Quantum yield determination was achieved according to previously established

procedure by comparing their integrated PL intensities. Quinine sulfate in 0.1 M

H2SO4 (quantum yield 54%) was chosen as a standard as described [15, 29]. The

absorbance of the aqueous solution of CQDs and reference sample were kept below

0.10 at 350 nm. The calculation of quantum yield was achieved by the following

equation:

( )( ) ( )
φ c = φ q Ac Aq Ic Iq η q η c

Where the subscript “c” and “q” refer to CQDs and quinine sulfate, ф is the PL

quantum yield, A is the absorbance at the excitation wavelength, I is the refractive

index of the solvent.

2.5 Live cell imaging experiments

100 μL suspensions of A549 cells seeded on 96-well plates supplemented with

6
Dulbecco's Modified Eagle Medium (DMEM) that contained 1%

penicillin/streptomycin and 10% fetal bovine serum was carefully cultured in a 5%

CO2 humidified incubator at 37 for 24 h. After removing the incubation media

and then rinsing three times with phosphate buffer solution (PBS), the cells were

further incubated in 1.0 mL fresh DMEM containing 0.3 mL aqueous solution of

CQDs at 37 ℃ for some time. Finally, the cells were washed with PBS for three

times and then imaged using an Olympus BX51TRF microscope.

3. Results and discussion

Fig. 2 (A) shows the TEM image and the diameter distribution of CQDs. It can

be clearly seen that the as-synthesized CQDs are uniform in size and possess a nearly

spherical shape. The Gaussian fitting curve reveals that the average size of the CQDs

is about 1.7 nm, as determined by statistical analysis of more than five hundred

particles by using the ImageJ software (Fig. 2B).

Figure 2 (A) Low and high-magnified (inset) TEM images and (B) the diameter

distribution of the CQDs.

7
 In order to determine the chemical composition of these CQDs, XPS

technique was characterized in detail. Four dominant peaks at 168.2, 284.8, 399.1

and 531.5 eV of the XPS survey spectrum depicted in Fig. 3 (A) were attributed

to S2p, C1s, N1s and O1s [25, 30], suggesting the existence of sulfur, carbon,

nitrogen, and oxygen elements. As shown in Fig. 3 (B), the high resolution of the

C1s spectrum displays the peaks at 284.1, 285.7, 287.7 and 289.6 eV, which

demonstrates that the CQDs were functionalized with C−C, C−N, C=O and O−C

groups [25, 26, 31].

Figure 3 (A) XPS survey spectrum, (B) the high-resolution C1s peaks and the fitting

curves, (C) the FTIR spectrum and (D) the XRD pattern of the as-synthesized CQDs.

Meanwhile, the FTIR spectrum was characterized to obtain further structural

insights about the CQDs, as shown in Fig. 3 (C). The characteristic absorption bands

8
of O−H at 3500 cm-1, the stretching vibration band of C=O at 1700 cm-1, and the

stretching vibration bands of C−O at 1100 and 870 cm-1 indicate the presence of

carboxylic acid and other oxygen-containing functional groups [25]. Furthermore, a

broad absorption around the range of 3050-3250 cm-1 and one sharp peak at 1510 cm-1

are assigned to the vibration and deformation bands of N−H, suggesting the existence

of amino-containing functional groups [19, 28]. In addition, two peaks at 2620 and

680 cm-1 ascribed to stretching vibration band of S−H and S−C, respectively, indicate

the containing of sulfur-containing functional groups [16]. Moreover, three obvious

absorption peaks at 2950, 1420 and 1330 cm-1 are associated with the stretch of

vibration C−H, C=C and C−C, suggesting the presence of alkyl and aryl groups [26].

The FTIR studies are consistent well with the XPS investigation. The XRD profile

depicted in Fig. 3 (D) exhibits a weak broad reflection peak centered at around 22°,

suggesting the interlayer spacing of (002) diffraction peak is 0.41 nm. The larger

interlayer spacing than that of graphite (0.34 nm) is ascribed to the existence of

more functional groups [18]. On the basis of the above analysis, a possible

mechanism for the formation of CQDs from gelatin was proposed, as shown in

Fig.S1. During the hydrothermal treatment process, gelatin was initially pyrolyzed to

small protein that was subsequently hydrolyzed into amino acids with abundant amino

and carboxy groups. With continued hydrothermal processing, the amino acids

can be partially decomposed and polymerized, and then carbonized into the

defect-containing CQDs functionalized with abundant oxygen, sulfur and amino

groups [20, 28]. Meanwhile, the hydrophilic groups make the CQDs freely dispersed

9
in water and other polar solvents, such as methanol, ethanol, dimethylformamide

(DMF) and acetonitrile (see Fig. S2 in the Supplementary data). The good

hydrophilicity of the CQDs is favorable for their functionalization and the

applications in detection and diagnostics.

Figure 4 (A) UV-Vis absorption spectrum and the digital image, (B) the PL emission

and excitation spectrum and (C) the fluorescence decay curve of the aqueous solution

of CQDs. The instrument response function (IRF) is also listed for comparison. (D)

The PL spectra of the CQDs excited at different excitation wavelengths, and the inset

shows the corresponding normalized PL emission. (E-G) Fluorescent images taken

10
under the excitation of UV, blue and green light (scale bars: 50 µm).

To study the optical properties of the CQDs, UV-Vis absorption and PL studies

were carried out in detail. As illustrated in Fig. 4 (A), the absorption spectrum exhibits

a broad peak around the range of 250-290 nm, which is ascribed to the typical

absorption of an aromatic π system or the n–p* transition of the carbonyl [19, 25, 28].

The well-dispersed aqueous suspension of CQDs exhibits bright blue emission under

UV light, which could be easily observed with the naked eye and taken with a digital

camera (inset in Fig. 4B). Furthermore, the suspensions of the CQDs in other polar

solvents also exhibited intense blue fluorescence under UV light (Fig. S3). A broad

emission centered at 430 nm was observed in the emission spectrum with an

excitation wavelength at 350 nm. Two peaks at 248 and 350 nm appeared in the

excitation spectrum reveals that the emission may be related to two kinds of

transitions. The quantum yield measured by using quinine bisulfate as standard

sample was 31.6%, which is higher than the previous report (typically < 20%) [11, 23,

26, 27]. The higher quantum yield may be ascribed to the chemical nature and

abundant surface defects of the as-synthesized CQDs. The time-resolved fluorescence

decay curve measured by time-correlated single photon counting method is illustrated

in Fig. 4 (C). The decay curve can be very well fitted to a triple-exponential function.

The lifetimes are τ1 = 10.68, τ2 = 4.15 and τ3 = 0.81 ns, and the mean lifetime is

calculated to be 6.76 ns, which is longer than previous reports [10, 21]. Meanwhile,

almost no bleaching was observed after continuous irradiation with UV light for 8 h,

suggesting the emission was quite stable (Fig. S4). Moreover, the PL spectra also

11
display the characteristic feature that the maximum emission moves to a longer

wavelength with reduced intensity as the excitation wavelength increased. As shown

in Fig. 4 (D), the strongest emission peak shifts from 430 nm to 580 nm and the

intensity decrease gradually as the excitation alters from 340 nm to 500 nm. Thus, the

CQDs exhibited different emission colors including blue, green and red when they

were excited by different excitation wavelengths, as investigated under a fluorescent

microscope (Fig. 4E-4G). This phenomenon has been widely observed in the

luminescent carbon nanomaterials, which may be caused by the optical selection

of different surface defect states near the Fermi level of CQDs [17]. To further

investigate the chemical environment dependence PL behavior, the qualitative

relationship between pH values and emission was studied. Interestingly, the maximum

intensity of the emission spectra obviously decreases under overly acidic or basic

environment (Fig. S4). The reason may be that too many H+ or OH– ions induce the

significant change of functional groups, and then the electronic transition of some

defects would be disrupted or even prohibited.

Significantly, in addition to the down-converted PL properties, the CQDs exhibit

a distinct up-converted characteristic. Inset in Fig. 5 (A) displays a typical PL

spectrum of CQDs excited by 680 nm light with a maximum peak located at 430 nm.

The up-converted emission of the CQDs should be ascribed to the multi-photon active

process, in which the simultaneous absorption of two or more photons leads to the

emission of light at a shorter wavelength than that of the excitation [17]. The

up-converted emission decay profile was also recorded by monitoring at 430 nm with

12
the excitation wavelength at 680 nm. As illustrated in Fig. 5 (A), the lifetime decay

curve is close, but not identical to the instrument response function (IRF), which can

be very well fitted to a double-exponential function, the lifetimes are τ1 = 0.382 and τ2

= 2.57 ns, and the average lifetime is determined to be 0.49 ns that is much shorter

that that of down-converted emission. Most interestingly, similar to the

down-converted emission, the up-converted PL emission exhibits an

excitation-dependent feature as well. Fig. 5 (B) demonstrates the up-converted

luminescence spectra of the CQDs excited by long wavelength ranging from 550 to

1100 nm with the emission located in the range from 330 to 610 nm. The maximum

up-converted emission and the chromaticity coordinates that were calculated from the

corresponding PL spectra were summarized in Table S1. The results imply that the

up-converted emission color of the CQDs can be finely tuned by adjusting the

excitation wavelength, as illustrated in the inset in Fig. 5 (B).

Figure 5 (A) The PL decay profile of the solution of CQDs measured by monitoring

the emission at 440 nm with 680 nm excitation, and the inset exhibits the lifetime data

and the parameter generated by the exponential fitting. The instrument response

function (IRF) is also listed for comparison. (B) The up-converted emission of CQDs

13
excited by different excitation wavelengths and their corresponding color coordinates.

To evaluate the potential of using the highly luminescent CQDs as optical

labels for in vitro cell imaging, A549 cells were chosen as an example, and were

cultivated in the presence of the as-prepared CQDs. As depicted in Fig. 6, strong

blue fluorescence of the A549 cells can be seen after incubation with CQDs for 6

h. Moreover, no autofluorescence emitting from the cells indicates the CQDs

remain exhibited intense PL emission after being internalized into the cells (Fig.

S5). A more careful observation found that the luminescent spots widely

appeared in the membrane and cytoplasmic area of the A549 cell, whereas

fluorescence at the central region involved with nucleus was very weak. In this

case, genetic disruption of the cells may be avoided, which agrees well with the

results of previous investigation on the interaction of living cells with CQDs and

other QDs [8, 32, 33]. And most of the available cell imaging studies of CQDs

generally suggests a possible endocytosis mechanism accounting for the

inhomogeneous distribution of CQDs inside the cell [8, 28, 34]. Elucidating the

exact mechanism of C-dot uptake by cells is still under way. Furthermore, to

evaluate their biocompatibility, the cytotoxicity of the CQDs was subsequently

assessed through the MTT assay, which is described in the Supplementary data

in detail. As can be seen from Fig. S6 and S7, more than 90% of the A549 cells

were remaining alive and exhibited intense blue fluorescence even they were

incubated with the as-prepared CQDs at a high dose of ~240 µg mL-1 for 24 h.

These results firmly demonstrate that the as-synthesized CQDs are of low

14
cytotoxicity. Although the exact mechanism for the endocytosis of CQDs into the

cell is still not clear at this stage, these results safely indicate that such CQDs

possess good biocompatibility and can be effectively applied for in vivo cell

imaging and biological labeling [18, 34, 35].

Figure 6 (A) Bright field image, (B) fluorescence microscopy image obtained

through a band-pass filter of 420 nm under irradiation of UV light (340 nm), (C)

their merged image of A549 cells incubated in the presence of CQDs for 6 h

(scale bar: 50 µm). Objective used was 50× /0.75 NA. The exposure time is 150

ms. Digital photograph of the gelatin-CQDs composites (D, E) and dip pen

writing pattern of the gelatin-CQDs composites on a microscope slide (F, G)

under visible and UV light excitations.

Furthermore, CQDs has been recently pursued for potential application in optical

detectors by dispersing the CQDs in polymer matrixes. For this purpose, an attempt

that 5 mL aqueous solution of CQDs (~0.8 mg mL-1) was added into the solution of

gelatin (50 wt %) at 45 under agitation was conducted to investigate the

15
compatibility with macromolecules without further chemical modifications. Fig. 6 (D,

E) depicts the photograph of the resultant composites under the irradiation of

ordinary and UV light after gelling process. It can be clearly seen that the

composites still exhibited bright blue emission under the irradiation of UV light,

suggesting the CQDs retained their fluorescent properties upon integration into gelatin

matrices. Meanwhile, there is not any evident change of their PL emission spectra

(Fig. S8), indicating no significant surface deterioration or aggregation of the

CQDs occurred. Meanwhile, due to the high transmittance of composites, it can be

hardly seen any information under visible light when the composite was written on a

glass slide through a dip pen (Fig. 6F). However, three characters read “ABC” can be

clearly seen under the irradiation of UV light (Fig. 6G). These results safely

demonstrate that the as-synthesized CQDs exhibit nice compatibility with

polymers, which enables them possible incorporation into solid matrices for

potential applications as optoelectronic devices, anti-counterfeiting ink and

gelatin-based optical waveguides [9, 20, 35, 36]. By the way, highly luminescent

CQDs were also obtained by exploiting protease as the reactants and using

ethanol as the solvent (the reaction temperature was kept at 180 ℃). The TEM

images and luminescence properties of them were shown in Fig. S9 and S10.

Further investigation is still under way.

4. Conclusions

The present work describes a green, cheap and simple approach to synthesize

16
high quality fluorescent CQDs by hydrothermal treatment of commercial gelatin

without any additive and complicated post-treatment. The as-synthesized CQDs were

found to emit strong blue emission with a maximum quantum yield of 31.6% at the

excitation wavelength of 350 nm. Meanwhile, these CQDs exhibit typical

excitation-dependent, pH-sensitive and up-converted PL properties. The results

demonstrate that, coupling with the excellent dispersibility, stable emission, relative

long emission life time, pH-sensibility, low toxicity, good compatibility with cells and

macromolecules, and the distinctive up-converted emission, such CQDs may provide

promising applications in the fields of bioscience, optical device, information

encryption and energy conversion.

Acknowledgments

This work was financially supported by the National Natural Science

Foundation of China (Grant No. 20873169) and the Laboratory of Special

Photographic Materials, Technical Institute of Physics and Chemistry, Chinese

Academy of Sciences (CAS).

Appendix A. Supplementary data

Supplementary data associated with this article can be found in the online version.

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