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Supplementary information

Oxygen accelerated scalable synthesis of highly fluorescent sulfur

quantum dots

Yiheng Song,a Jisuan Tan,a Guan Wang,b Pengxiang Gao,a Jiehao Leia and Li Zhou*a

a Key laboratory of New Processing Technology for Nonferrous Metal and Materials

(Ministry of Education), Guangxi Key Laboratory of Optical and Electronic

Materials and Devices, and College of Materials Science and Engineering, Guilin

University of Technology, Guilin 541004, China

b Institute of Materials Research and Engineering, A*STAR, Singapore 138634,

Singapore

* Corresponding author. E-mail: zhouli@glut.edu.cn

Electronic Supplementary Material (ESI) for Chemical Science.This journal is © The Royal Society of Chemistry 2019

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Materials. Sublimed sulfur, NaOH, polyethylene glycol (PEG, Mn = 400 Da),

poly(vinyl alcohol) (PVA, Mw = 90-98 kDa, 90+% hydrolyzed), acrylamide (AM),

methylene-bis-acrylamide (MBA), tetramethylenediamine (TEMED), and potassium

persulfate (K2S2O8) were purchased from Beijing Innochem Technology Co. Ltd.

(Beijing, China) and used as received. High purity oxygen (O2, 99.999%) and argon

(Ar, 99.999%) were purchased from Huaao Gas Co., Ltd. (Liuzhou, China). All other

reagents were analytical grade and used as received. Milli-Q water (18.2 MΩ) was

used throughout the experiments.

Characterizations. Photoluminescent spectra were collected using a Varian Cary 100

spectrometer. Transmission electron microscopy (TEM) was carried out on a

JEOL-2010 TEM at 200 kV. Samples were prepared by placing a drop of dilute

aqueous dispersion of SQDs on the surface of a copper grid. X-ray photoelectron

spectroscopy (XPS) measurements were made on Kratos AXIS UltraDLD (Kratos

Analytical Ltd.) with mono Al Kα radiation (hν = 1487.71 eV) at a power of 75 W.

Fourier transform infrared (FT-IR) spectra were recorded using a PE Paragon 1000

spectrometer (KBr disk). Absorption spectra were recorded on a UV-3600 UV-vis

spectrophotometer (Shimadzu). Powder X-ray diffraction (XRD) spectra were

collected on a Holland PANalytical X’Pert PRO X-ray diffractometer with Cu Kα

radiation. Confocal laser-scanning microscopy (CLSM) images were recorded on a

Zeiss LSM 510 (Jena, Germany) CLSM with imaging software (Fluoview FV1000).

Absolute photoluminescence quantum yields of the SQDs samples in aqueous

solution (0.1 mg/mL) were determined on a FluoroMax-4 (HORIBA)

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photoluminescence spectrometer with an integrating sphere, and the excitation

wavelength is 400 nm. The elementary analysis measurement was performed on a

Perkin-Elmer 240C Elemental Analytical Instrument. Raman spectra were recorded

on a LabRam-1B Raman spectroscope with 532.05 nm incident radiation and a 50×

aperture.

Preparation of fluorescent SQDs. In a typical procedure, 4 g of NaOH and 3 g of

PEG were dissolved in 50 mL of water before adding 1.6 g of sublimed sulfur powder.

The mixture was stirred at 90 oC for 10 h under pure O2 atmosphere. The resulted

SQDs were purified by dialysis against water (molecular weight cut off, 3500 Da) for

at least three days and followed by freeze-drying. Elemental analysis result of the

SQDs: S 38.78 wt%, H5.82 wt%, C 32.36 wt%, and O 23.04 wt% (calculated).

The product yield of SQDs was defined as follows:

100%Product yield = SQDs

sublimed sulfur

SM W

M

where MSQDs is the mass of freeze-dried SQDs, WS is the weight content of S element

according to the elemental analysis result, and Msublimed sulfur is the mass of used

sublimed sulfur. For comparison, the preparation of SQDs under air and argon

atmosphere was also preformed using the same procedure as that under O2

atmosphere.

Preparation of fluorescent PVA-SQDs composite film. The fluorescent and

transparent PVA-SQDs nanocomposite film could be easily fabricated by the

following procedure. Firstly, 0.98 g of PVA was dissolved in 10 mL of water to form a

homogeneous solution. Then 0.02 g of SQDs was added and the mixture was stirred

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for 10 min. Finally, the resulted mixture was casted into a petri-dish and followed by

water evaporation at 50 oC.

Preparation of fluorescent polyacrylamide (PAM)-SQDs composite hydrogel. For

preparing fluorescent PAM-SQDs composite hydrogel, two solutions were prepared.

Solution A contains SQDs (25 mg), AM (1 g), MBA (0.2 g), TEMED (30 mg), and 9

mL of water. Solution B contains K2S2O8 (15 mg) and 1 mL of water. Then solution A

and solution B were mixed and the mixture was quickly cast to a glass beaker. After

keeping the system at 45 oC for 8 h, fluorescent PAM-SQDs composite hydrogel was

obtained.

Cytotoxicity evaluation. The cytotoxicity of SQDs was evaluated using the

methyl-thiazolyldiphenyl-tetrazolium (MTT) assay. 293T human embryo kidney cells

and MCF-7 breast cancer cells were seeded in 96-well plates at a density of 1 × 104

cells/mL, respectively. After 24 h of incubation, the medium was replaced by the

aqueous dispersion of SQDs with concentrations of 100, 75, 50, 25, and 0 μg/mL and

further incubated for 24 h. Subsequently, the wells were washed thrice with PBS

buffer, and 100 μL of freshly prepared MTT (0.5 mg/mL) solution in culture medium

was added to each well. The MTT medium solution was carefully removed after 3 h

of incubation. Dimethyl sulfoxide (100μL) was then added into each well, and the

plate was gently shaken for 20 min. The absorbance of MTT at 570 nm was recorded

by the microplate reader. Cell viability was expressed by the ratio of absorbance of

the cells incubated with SQDs to that of the cells incubated with culture medium only.

Cell imaging. MCF-7 cancer cells were cultured in the chambers at 37 oC. After 80%

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confluence, the medium was removed and the adherent cells were washed twice with

1 × PBS buffer. The SQDs suspension (20 µg/mL, 0.4 mL) was then added to the

chamber. After incubation for 2 h, cells were washed three times with 1 × PBS buffer

and then fixed by 75% ethanol for 20 min, which was further washed twice with 1 ×

PBS buffer and imaged by CLSM (Zeiss LSM 510, Jena, Germany) with imaging

software (Fluoview FV1000).

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Fig. S1. Emission spectra of the mixture of PEG and NaOH before (a) and after (b)

stirring at 90 oC for 10 h under O2 atmosphere. Insets: photographs of the mixture of

PEG and NaOH before (a) and after (b) stirring at 90 oC for 10 h under O2 atmosphere

under daylight (left) and 365 nm UV lamp (right).

320 360 400 440 480 520 560 600

0

200

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600

800

Inte

nsit

y (

a.u

.)

Wavelength (nm)

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340 nm

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(b)

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Inte

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ity

(a

.u.)

Wavelength (nm)

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(a)

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Fig. S2. Emission (a-g) and excitation (λem = 490 nm) (h) spectra of the reaction

mixture after stirring for different time under O2 atmosphere.

360 400 440 480 520 560 6000

200

400

600

800Under O

2

Wavelength (nm)

Inte

ns

ity

(a

.u.)

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(e)

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.)

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(f)

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(a

.u.)

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(g)

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Under O2

In

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(h)

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Under O2

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.u.)

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(c)

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Under O2

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Wavelength (nm)

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.u.)

4 h(d)

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.u.)

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(b) Under O2

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Wavelength (nm)

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(a) Under O2

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10 h(a) Under air

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(b) Under air

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.u.)

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Under air

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Under air

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.u.)

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Wavelength (nm)

Fig. S3. Emission spectra of the reaction mixture after stirring for different time under

air atmosphere.

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Fig. S4. (a) Photographs of the reaction mixture after stirring at 90 oC for 20 h under

air atmosphere under daylight (left) and 365 nm UV lamp (right). (b) Photographs of

the reaction mixture after stirring at 90 oC for 60 h under Ar atmosphere under

daylight (left) and 365 nm UV lamp (right).

(a)

(b)

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10 20 30 40 50 60 70

2 Theta (degree)

O2 - 10 h

O2 - 2 h

Inte

nsit

y (

a.u

.)

(a)

4000 3500 3000 2500 2000 1500 1000 500

SQDs

PEG

C-H

C-O

C-H

Tra

nsm

itta

nce (

%)

Wavenumber (cm-1)

(b)

Fig. S5. (a) XRD patterns of SQDs prepared with different reaction time under O2

atmosphere. (b) FTIR spectra of SQDs and pure PEG.

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200 220 240 260

Ab

so

rba

nce (

a.u

.)

Wavelength (nm)

Before purification

After purification

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(a)

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Ab

so

rba

nc

e (

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Before purification

After purification

327

(b)

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Inte

ns

ity

(a

.u.)

Wavelength (nm)

Before purification

After purification

(c)

Fig. S6. UV-vis absorption (a,b) and emission (c) spectra of the SQDs before and after

purification.

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Fig. S7 The effects of solution pH (a) and concentration of NaCl (b) on the

fluorescent intensity of the aqueous dispersion of SQDs.

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Inte

ns

ity

(a

.u.)

Wavelength (nm)

pH=4

pH=5

pH=6

pH=7

pH=8

pH=9

pH=10

(a)

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Inte

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y (

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Wavelength (nm)

0 M

0.1 M

0.2 M

0.5 M

0.8 M

1.0 M

(b)

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Fig. S8. Confocal fluorescence image by excitation at 458 nm (a) and 514 nm (b), and

merged image of MCF-7 cells without incubation of SQDs.

(a) (b) (c)

20 μm 20 μm 20 μm

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Fig. S9. (a) Photographs of SQDs in various solvents under daylight (top) and 365 nm

UV lamp (bottom). (b) Emission spectra of SQDs in various solvents (λex = 400 nm).

H2O

Ethanol DMF

THF DCM

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Inte

nsit

y (

a.u

.)

Wavelength (nm)

Water

Ethanol

DCM

Chloroform

THF

DMF

(a)

(b)

Chloroform

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Fig. S10. Emission spectra of PVA-SQDs composite film with diverse excitation

wavelength. Inset: photograph of PVA-SQDs composite film under 365 nm UV lamp.

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y (

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