Determination of superoxide anion radical by modified CdTe quantum dots

Article abstract of DOI:10.1016/j.jphotochem.2017.08.049

A novel fluorescence probe for determination of superoxide anion radical (O₂[rad]?) was developed with CdTe quantum dots(QDs) modified by thiophene-2-formaldehyde shrinked L-cysteine Schiff base, which was characterized by Fourier transform infrared (FT-IR), thermal gravimetric analysis (TGA), ultraviolet-visible (UV–vis) and fluorescent spectrum. The particle size of CdTe QDs was 3.1 nm as confirmed by X-ray powder diffraction pattern (XRD) and Transmission Electron Microscope (TEM) in presence of β-cysteamine (CA). The fluorescent results showed that the fluorescence intensity of CdTe QDs was decreased with modified Schiff base at 305 nm, but enhanced at 610 nm with O₂[rad]^? and showed an apparent linear dependence in the concentration scope of 2.0 × 10^?5–1.0 × 10^?4 mol L^?1 (r = 0.9934) with a detection limit of 1.8 × 10^?6 mol L^?1, which exhibited high selectivity and stability towards O₂[rad]^?.

Full text of DOI:10.1016/j.jphotochem.2017.08.049

Accepted Manuscript
Title: Determination of <!–<query id="Q1">Please check the
¨
¨
DOC headfor correctness.</query>–>superoxide anion radical
by modified CdTe quantum dots
Authors: Jiaxing Han, Zheng Liu, Yajin Guo, Guo-Cheng
Han, Wei Li, Siyang Chen, Shufen Zhang
PII:
S1010-6030(17)30678-0
DOI:
Reference:
http://dx.doi.org/10.1016/j.jphotochem.2017.08.049
JPC 10824
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
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Accepted date:
22-5-2017
17-8-2017
18-8-2017
Please cite this article as: Jiaxing Han, Zheng Liu, Yajin Guo, Guo-Cheng Han,
Wei Li, Siyang Chen, Shufen Zhang, Determination of superoxide anion radical
by modified CdTe quantum dots, Journal of Photochemistry and Photobiology A:
Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.08.049
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Determination of Superoxide Anion Radical by Modified
CdTe Quantum Dots
Jiaxing Han1, Zheng Liu*,1, Yajin Guo1, Guo-Cheng Han2,*, Wei Li1, Siyang Chen1,
Shufen Zhang1
(1College of Chemical and Biological Engineering, Guilin University of
Technology, Guilin 541004, P.R. China;
2School of Life and Environmental Sciences, Guilin University of Electronic
Technology, Guilin, 541004, P.R. China)
* Corresponding author: lisa4.6@163.com; hangc1981@163.com
Graphical abstract
Fig. 6
Fig. 6 shows the fluorescence spectra of the Schiff base (1.0×10-5 mol·L-1) and
the CdTe QDs (1.678×10-5 mol·L-1). It can be seen from Fig. 6 that the strongest
emission (Fig.6b) wavelength is at 337 nm (λex = 305 nm, Fig.6a), due to the
closed-loop in the formation of Schiff base and the closed-ring structure which
showed excellent fluorescence intensity. And CdTe QDs have significant fluorescence
properties, the strongest emission located at 610 nm (Fig.6d) (λex = 305 nm, Fig.6c),
the emission spectrum is narrow and symmetrical, and the excitation spectrum is
broad.

Highlights
1. A novel fluorescence probe was designed for determination of superoxide
anion radical.
2. The fluorescence probe was based on L-cysteine Schiff base and CdTe
quantum dots.
3. The fluorescence probe has a high selectivity towards O2·-.
4. The Schiff base was served as spacer molecule for fluorescence assay.
Abstract: A novel fluorescence probe for determination of superoxide anion radical
(O2·-) was developed with CdTe quantum dots(QDs) modified by
thiophene-2-formaldehyde shrinked L-cysteine Schiff base, which was characterized
by Fourier transform infrared (FT-IR), thermal gravimetric analysis (TGA),
ultraviolet-visible (UV-Vis) and fluorescent spectrum. The particle size of CdTe QDs
was 3.1 nm as confirmed by X-ray powder diffraction pattern (XRD) and
Transmission Electron Microscope (TEM) in presence of β-cysteamine (CA). The
fluorescent results showed that the fluorescence intensity of CdTe QDs was decreased
with modified Schiff base at 305 nm, but enhanced at 610 nm with O2·- and showed
an apparent linear dependence in the concentration scope of 2.0×10-5~1.0×10-4
mol·L-1 (r=0.9934) with a detection limit of 1.8×10-6 mol·L-1, which exhibited high
selectivity and stability towards O2·-.
Key words: Schiff base; CdTe Quantum Dots; Superoxide anion radical; Fluorescent
Analysis

1. Introduction
Superoxide anion (O2·-) is one of the important free radicals in organism and it's
also the precursor of all reactive oxygen species,[1] which can be converted to the
other active radical, such as H2O2,[2-3] ONOO-[4]. O2·- is closely related to the
occurrence of inflammation, aging and cancer, herein it's of all-important significance
of science for its detection. Currently, the main methods for detecting O2·- include
electron paramagnetic resonance method,[5] SOD enzyme activity assay,[6-8] high
performance liquid chromatography (HPLC),[9-10] chemiluminescence method,[11]
fluorescence analysis method [12] and electrochemical methods[13]
.
With these methods, fluorescenct method[14-16] is not only simple, easy operation,
high sensitivity and selectivity, but also can realize in situ visualization of O2·- within
a living cell[17-18], so as to implement real-time observation in body. Liu and
coworkers [19] established a novel fluorescent probe based on imidazole fluorescent
ionic liquids with anthracene groups, which was supplied in an aqueous system, the
linear range of sensitivity was 1-70 μM with detection limits of 0.1 μM. Meada and
coworkers [20] found that bis(2,4-dinitrobenzenesulfonyl) fluorescein compounds
could be used to detect superoxide radicals, which based on a non-redox mechanism,
the LOD was 1.0 pmol per well, however, it has some limitations, such as high latency,
poor water solubility and high concentration of GSH interference in the biology.
Quantum dots (QDs), also known as semiconductor nano-crystals, is a kind of
nano-particles with a diameter of 10 nm composed of II-VI or III-V subgroup
elements. When the size of nano-crystals is smaller than the exciton Bohr radius, the

small size effect, surface effect, quantum effect and macroscopic quantum tunneling
effect can be presented, which has attracted a wide range of research interests.[21-23]
There have been many reports about Schiff bases for the analysis of the test, but less
in the combination of the Schiff base and the QDs. Studies showed that the selective
analysis and determination of different substances can be carried out after the
cadmium based QDs were modified by different surface modification.[24-28] Schiff
base not only can form stable compounds with most of transition metal elements
under certain conditions, but also can modified on the surface of them to improve
QDs stability and hydrophilicity. Most importantly, the group modified to QDs can
improve the fluorescent recognition to some particular materials.
In this paper, a Schiff base was synthesized and characterized which was derived
from thiophene-2-formaldehyde and L-cysteine and was used to modify the surface of
CdTe QDs stabilized by β-cysteamine (CA) as space interval material between the
CdTe QDs and O2·-, resulting in lower fluorescence intensity at 610 nm. With addition
of O2·-, the fluorescence intensity of QDs was enhanced significantly. Therefore, a
high sensitivity fluorescence analysis method for detecting concentration of O2·- based
on this Schiff base modified CdTe QDs fluorescent probe was developed.
2. Experimental
2.1 Materials and Apparatus
Thiophene-2-formaldehyde, L-cysteine, β-cysteamine (CA), TeO2 and dimethyl
sulfoxide (DMSO) were obtained from Acros Organic Co. (Belgium), Qiangshun

Chemical Co., Ltd (Shanghai, China), Jingchun Biochemical Polytron Technologies
Inc. (Shanghai, China), Guangfu Fine Chemical Research Institute (Tianjin, China)
and Kelong Chemical Reagent Factory (Chengdu, China), respectively. KBH4,
N,N'-dicyclohexyl carbodiimide (DCC) and N-hydroxyl succinimide were obtained
from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All of the reagents
used in the experiments were analytical grade. All solutions were prepared with
Milli-Q water.
Excitation and emission spectra were recorded on a FL-4600 fluorescence
spectrofluorimeter with 5 nm bandwidths and ground state electronic absorption
spectra were recorded on a Shimadzu UV-Vis 2450 spectrophotometer. FT-IR spectra
and thermal gravimetric analysis (TGA) were obtained on Shimadzu FTIR-8400 and
STA-499, respectively. The X-ray diffraction (XRD) data analysis was carried out
using Bruker SMARD CCD.
2.2 Synthesis of thiophene-2-formaldehyde shrinked L-cysteine Schiff base
Thiophene-2-formaldehyde (2 mmol, 0.224 g) was dissolved in 20 mL absolute
methanol and were dropped slowly into the water-soluble L-cysteine (2 mmol, 0.242 g)
under magnetic stirring at 25℃for 12 h in a three-necked flask. Ocher-yellow
powders were obtained, washed with water several times and dried out for further use.
Yields: 68.3%, m.p. 151~154℃. The synthesis route (Scheme 1) is as follows:
(Scheme 1)
2.3 Preparation of Schiff base modified CdTe QDs
CdCl2 (1 mmol, 0.2284g) and β-cysteamine (CA) (3mmol, 0.2315g) were mixed

into three-necked flask with 100 mL H2O as the precursor solutions of QDs. TeO2 (0.5
mmol, 0.0798g) and KBH4 (0.5 mmol, 0.027g) were added when the temperature
heated up to 200℃in protection of N2. The solution quickly changed from colorless to
orange and finally turned into wine red and quickly removed. The solution was cooled
down to 3℃and sealed placed in the refrigerator and for further use.
(Scheme 2)
The Schiff base (1mmol, 0.04 g) was dissolved in the N,N'-dimethyl formamide
(DMF) and was adjusted to pH=9 with triethylamine in a three-necked flask.
N-hydroxyl succinic diamide (NHS) (0.238mmol, 0.049g) were added into the above
solution under stirring. After 30 min, N,N'-dicyclohexyl carbodiimide (DDC) (0.239
mmol, 0.04g) were added and activated for 24 h at 25℃.[29] Different amount of
as-prepared CdTe QDs were added and continued reacting for 24 h to make sure that
CdTe QDs were combined with Schiff base steadily.
(Scheme 3 )
The CdTe QDs-Schiff base complexes were obtained at 85℃by rotary
evaporation and the base solution was recycled with same volume of anhydrous
ethanol for 12 h. The precipitate was washed further with anhydrous ethanol to
remove unreacted CdTe QDs and Schiff base. The composite solid was obtained after
another rotary evaporation, recycled base solution, centrifuged and dried. The
obtained complexes were sonicated in DMF for further applications.
2.4 Method for recognition of O2·-
O2·- was generated in alkaline DMSO according to literature methods.[30-31]

Briefly, 100 μL 0.5 mol·L-1 NaOH was dropped into 10 mL DMSO. The reaction was
lasted 30 min under magnetic stirring. The concentration of O2·- could be calculated
according to Beer’s law with its molar absorptivity (20.061 L/(mol·cm)-1 at 271 nm)
in DMSO.[32-33] The DMSO was reacted in alkaline as follows:
2 DMSO +2 OH- + 3 O2 → 2 O2·- + 2 CH3SOOH + CH3OOCH3 (1)
The CdTe QDs-Schiff base complexes dissolved in DMF were added to various
concentration of O2·- which were diluted by DMF and ultrasonicated for 15 min.
Hence, the mixture was allowed to equilibrate for 30 min before recording the
fluorescence signal for each concentration of O2·-. All measurements were recorded at
room temperature.
3. Results and discussion
3.1 Characterization of modified CdTe QDs
3.1.1 Spectra characteristic of CdTe QDs
Fig. 1 shows the X-ray powder diffraction (XRD) pattern which was employed
to elucidate the crystal nature and size of the QDs with the Scheler formula. The 2θ
values for CA-CdTe QDs were 24.0o, 39.7o and 47.0o corresponds to the diffraction
pattern with planes at [110], [220] and [311], respectively. They were coincided with
the position of diffraction peak, compared with the standard spectra (PDF
No.01-075-2083) and the diffraction peaks were narrow as shown in the XRD pattern
of CdTe QDs. It can be seen from Fig. 2 that quantum dots are uniform in particle size
and have good dispersibility. The diameter of CdTe QDs showed in TEM (Fig. 2) is
3.87 nm, a little higher than the calculated value of 3.1 nm according to Scherrer

equation,[34] which may due to the fact that the surface atoms are chemically more
active and caused precipitation.[35] At the same time, the diffraction peak slightly
shifts towards CdS cubic, the S-H is hydrolyzed in the formation of QDs because little
sulfur is coordinated with Cd2+ and developed CdS.
(Fig.1 and 2)
3.1.2 FT-IR analysis
Fig. 3 shows the IR spectra of thiophene-2-formaldehyde shrinked L-cysteine
Schiff base, CdTe QDs-Schiff base and CdTe QDs in the range of 4000~400 cm-1. For
the Schiff base (Fig. 3a), the broad band around 3100~2400 cm-1 may be assigned to
free -COOH groups and the band at 3432 cm-1 may be ascribed to O-H stretching
vibration. For CdTe QDs (Fig. 3c), the band between 3437~3135 cm-1 may be
assigned to -NH2 stretching which may due to the stabilizer of β-cysteamine (CA)
coated on the surface of CdTe QDs. Fig. 3b shows the IR spectra of CdTe QDs-Schiff
base. It was observed that the broad band around 3100~2400 cm-1 was diminished and
a new band assigned to the -CO-NH- stretch was observed at 1619 cm-1. Such band
could also be proved by the group of -NH- at 3438 cm-1 and was not observed in pure
Schiff base and CdTe QDs alone (Fig. 3a and c). Therefore, the IR spectra
demonstrate clearly that the -COOH of the Schiff base had been successfully shrinked
with -NH2 of CA-CdTe QDs.
(Fig. 3)
3.1.3 Thermal properties
Fig. 4 shows the Schiff base and CdTe QDs-Schiff base thermal curves measured

under a nitrogen atmosphere. It can be seen from Fig. 4 that when the temperature is
increased, the thermal decomposition of the Schiff base and CdTe QDs-Schiff base
occurs at different decomposition steps.[36]For them, the weight loss steps were in the
range of 135~390℃(curve a) and 263~496℃(curve b), respectively. It concluded that
the CdTe QDs-Schiff base had better thermal stability than that of Schiff base alone.
(Fig.4)
3.1.4 Spectrophotometric properties
Fig. 5 shows the UV-vis absorption of the Schiff base (1.0×10-5 mol·L-1) in DMF.
The maximum absorption wavelength value of the Schiff base is at 305 nm, which
can be assigned to conjugated C=N π~π* transition. Fig. 6 shows the fluorescence
spectra of the Schiff base (1.0×10-5 mol·L-1) and the CdTe QDs (1.678×10-5 mol·L-1).
It can be seen from Fig. 6 that the strongest emission (Fig.6b) wavelength is at 337
nm (λex = 305 nm, Fig.6a), due to the closed-loop in the formation of Schiff base and
the closed-ring structure which showed excellent fluorescence intensity. And CdTe
QDs have significant fluorescence properties, the strongest emission located at 610
nm (Fig.6d) (λex = 305 nm, Fig.6c), the emission spectrum is narrow and symmetrical,
and the excitation spectrum is broad.
Using Rhodamine B as the standard sample, the quantum yield of CA-CdTe
quantum dots before the modification was calculated by the equation (2):
2
퐹∙퐴 ∙푛
푆푡푑
∅퐹=∅퐹(푆푡푑)
(2)
2
퐹
푆푡푑
∙퐴∙푛
푆푡푑
In the formula: 퐴 and 퐴푆푡푑 are the absorbance of the sample to be tested and the
standard sample at each excitation wavelength respectively (퐴 = 0.017, 퐴푆푡푑
=

0.007). 퐹 and 퐹
are the integral areas of the fluorescence curve of the quantum
푆푡푑
dots and the standard samples. 푛 and 푛푆푡푑 are the refractive indices (푛 = 1.33, 푛푆푡푑
= 1.36) of the solvent used for the sample to be tested and the standard sample. The
calculated value of ∅퐹 of CA-CdTe quantum dots is 0.15.
(Fig.5 and 6)
3.2 Effect of Schiff base yields on the fluorescence of CdTe QDs
Effect of Schiff base yields on the fluorescence of CdTe QDs was studied. The
fluorescence spectra of the CdTe QDs complexed with different yields of Schiff base
(Fig. 7) were measured at room temperature. It indicates that CdTe QDs showed
broad emission spectra as expected.[37~38] The Fig. 7 curve (a) represents the
fluorescence spectra of CdTe QDs alone and Fig. 7 curve b-g represent different ratio
of QDs to Schiff base. It is noteworthy that the fluorescence intensity decreased
significantly with addition of Schiff base. It can be seen from illustration that when
the concentration of Schiff base is 3333 μmol·L-1, the curve is almost parallel to the X
axis. It indicated that the fluorescence of the quantum dot no longer changes with the
addition of Schiff base. So 3333 μmol·L-1 is the most suitable concentration of Schiff
base, in which case, the modified quantum dots yield reaches maximum.
(Fig.7)
The fluorescence quantum yields of the CdTe QDs modified before and after were
calculated according to literature methods.[39] The fluorescence yield of the modified
CA-CdTe quantum dots is calculated by Equation 3:
푚표푑푖푓푦
퐹
∅
푚표푑푖푓푦=∅퐹(푄퐷)
(3)
푄퐷
퐹(푄퐷)
퐹
푄퐷

In the formula: ∅퐹(푄퐷) is the fluorescence quantum yield of CA-CdTe quantum dots
before modification, and is used as a standard sample. 퐹(푄퐷) is the fluorescence
intensity of the CA-CdTe quantum dots before modification, and 퐹푚표푑푖푓푦 is the
푄퐷
fluorescence intensity after quantum dot modification.
The results show that the fluorescence quantum yields of QDs complexes are
much lower than the values for the QDs individual. The fluorescence quantum yields
of CA-CdTe quantum dots and modified thiophene-2-form- aldehyde Schiff base are
shown in Table 1.
(Table1)
3.3 Fluorescence recognition of O2·-
Fluorescence response of the CdTe QDs-Schiff base in the presence of varying
concentration of O2·- was examined. The original concentration of Schiff base is 2000
mol·L-1 and the fluorescence intensity is 4458 a.u. with addition of different
concentrations of O2·-. As shown in Fig. 8, with an increase of O2·- concentration, the
fluorescence intensity was markedly enhanced and thus gives a linear relationship in
the range of 2.0×10-5~1.0×10-4 mol·L-1. The linear equation is y=56.27c (10-4 mol·L-1)
+ 4362.810 with r=0.9934 (RSD=0.156%).[40] The corresponding limit of detection
(LOD) was calculated following the 3 δ/S criteria where δ is the standard deviation of
blank measurement and S is the slope of the calibration graph (inserts in Fig. 8). The
LOD value for CdTe QDs-Schiff base was 1.8×10-6 mol·L-1.
The covalent bond of the Schiff base may has an effect on the QDs to set up a
new trap state on the surface of them, which leads to the attenuation of the

fluorescence emission. After the addition of O2·-, the space and distance between the
Schiff base and QDs increase when the Schiff base is served as spacer molecule, and
enhance the fluorescence intensity.
(Fig.8)
3.4 Interference study on the recognition of O2·-
The interference of the CdTe QDs-Schiff base of various possible co-existing
species (8-hydroxyquinoline, lithium perchlorate, sodium nitrite, potassium nitrate,
L-cysteine and β-thiol) was carried out to investigate the selectivity of the proposed
QDs. Before adding the interfering substance, the fluorescence intensity of each group
is the same. Fig. 9 shows the fluorescence intensity of QDs-Schiff-O2·- when the
interference ion concentration is 1×10-3 mol·L-1 and the superoxide anion
concentration is 3.3×10-6 mol·L-1. Blank is a control group with no interfering
substance. It can be seen from Fig. 9 that there were almost no change in fluorescence
intensities in the presence of the above interferences, which means it is possible to
discriminate between O2·- and other species. We can conclude that the CdTe
QDs-Schiff base system is highly selective for O2·-
(Fig.9)
4. Conclusion
In this work, CdTe QDs were successfully conjugated to the
thiophene-2-formaldehyde shrinked L-cysteine Schiff base to form a CdTe
QDs-Schiff base complex. The complex was characterized. The synthesized complex
was successfully recognized O2·- in aqueous solution. Interference study illustrates

that the complex has a high selectivity towards O2·-. Hence, the proposed method
presents great promise in the recognition of O2·- at 610 nm in the concentration scope
of 2.0×10-5~1.0×10-4 mol·L-1 (r=0.9934) with a detection limit of 1.8×10-6 mol·L-1.
Acknowledgments
This work was supported by the National Nature Science Foundation of China
(No. 21266006, 61661014, 61301038), the Nature Science Foundation of Guangxi
Province (No. 2016 GXNSFAA3 80109, 2015GXNSFBA139041,
2013GXNSFAA019224) and Guangxi Key Laboratory of Electro chemical and
Magneto-chemical Functional Materials, Collaborative Innovation Center for
Exploration of Hidden Nonferrous Metal Deposits and Development of New
Materials in Guangxi.

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Figure Captions
Fig. 1 XRD pattern of CdTe QDs
Fig. 2 TEM image of CdTe QDs
Fig. 3 FT-IR spectrum of Schiff base (a), CdTe QDs-Schiff base (b) and CdTe QDs (c)
Fig. 4 TG curves of thiophene-2-formaldehyde shrinked L-cysteine Schiff base (a)
and CdTe QDs-Schiff base (b)
Fig. 5 UV-Vis spectrum of the Schiff base.
Fig.6 Fluorescence excitation (a) and emission spectra (b) of Schiff base and
fluorescence excitation (c) and emission spectra (d) of CA-CdTe QDs.
Fig.7 Effect of addition of different concentrations of Schiff base on the fluorescence
of CdTe QDs. Ratios of QDs and the Schiff base: (a) 0, (b) 1:1667, (c) 1:2000, (d)
1:2500, (e) 1:3333, (f) 1:5000, (g) 1:10000; Insert: Relationship between
concentration of schiff base and fluorescence intensity.
Fig.8 Effect of addition of different concentrations of O2·- on the fluorescence of CdTe
QDs-Schiff base. Concentration of O2- : (a) 0, (b) 2.0×10-5, (c) 4.0×10-5, (d) 6.0×10-5,
(e) 8.0×10-5, (f) 1.0×10-4 (mol·L-1); Insert: Linear relationship plot.
Fig. 9 Effect of different interfering species on CdTe QDs-Schiff base system.
Scheme 1. Synthesis route of thiophene-2-formaldehyde shrinked L-cysteine
Schiff-base
Scheme 2. Synthesis route of modified CdTe QDs.
Scheme 3. Synthesis route of CdTe QDs-Schiff base.

Fig. 1
Fig. 2
10nm
Fig. 3

Fig. 4
Fig. 5
Fig.6

Fig. 7
Fig.8
Fig. 9

Scheme 1
Scheme 2
Scheme 3

Table1: Fluorescence quantum yield of CA-CdTe quantum dots and modified
thiophene-2-form- aldehyde Schiff base.
(Table1)
Number
The proportion of
modification
(QDs: schiff base)
CA-QDS
fluorescence
intensity (F)
Fluorescence quantum
yield (ΦF)
a
b
c
d
e
f
8525
6313
5082
2797
396.1
321.3
282.5
0.150*
1:1667
8.998×10-2
7.244×10-2
3.987×10-2
5.646×10-3
4.580×10-3
4.027×10-3
1:2000
1:2500
1:3333
1:5000
g
1:10000
* is the fluorescence quantum yield of CA-CdTe quantum dots before modification,
calculated by equation (2)，used as the standard value of equation (3).