Z. XIE et al.: LOTUS SEEDPOD-BASED CARBON QUANTUM DOTS: PREPARATION, CHARACTERIZATION ...
141–147
LOTUS SEEDPOD-BASED CARBON QUANTUM DOTS:
PREPARATION, CHARACTERIZATION AND APPLICATION FOR
Fe(III) DETECTION
KVANTNE PIKE OGLJIKA V OBLIKI STROKA LOTUSOVIH
SEMEN: PRIPRAVA, KARAKTERIZACIJA IN UPORABA ZA
DETEKCIJO Fe(III)
Zhouling Xie
1
, Jianfa Sun
2
, Zeijun Zhou
3
, Shibin Xia
1*
1
School of Resources and Environmental Engineering, Wuhan University of Technology, no. 171 Luoshi Street,
Hongshan District, Wuhan 430070, China
2
Key Laboratory of Arable Land Conservation (Middle and Lower Reaches of Yangtze River), Ministry of Agriculture/College of
Resources and Environmental Sciences, Huazhong Agricultural University, Wuhan 430070, China
3
China Petroleum & Chemical Corporation Jianghan Oilfield Branch No. 1 Gas Production Plant, Chongqing 400000, China
Prejem rokopisa – received: 2020-08-13; sprejem za objavo – accepted for publication: 2020-10-05
doi:10.17222/mit.2020.160
Lotus seedpod (LS) was employed as a carbon source for the synthesis of carbon quantum dots (CQDs) using an economical
and facile hydrothermal synthesis method. LS-CQDs were characterized with different techniques, including TEM, FTIR, PL,
XRD, XPS, Raman spectroscopy and Uv-vis. The average particle size of LS-CQDs was found to be 2.1±0.17 nm. The proper-
ties of the excitation-dependent photoluminescence of LS-CQDs were determined, and the quantum yield was calculated to be
1.9 %. The quenching effect of LS-CQDs on Fe(III) ions was also investigated. The normalized linear relationship between the
increasing Fe(III) ion concentration and the fluorescence-emission intensities of LS-CQDs was established. Furthermore, the
quenching mechanism for the reaction between Fe(III) ions and LS-CQDs was elucidated.
Keywords: lotus seedpod, carbon quantum dots, Fe(III) determination
Avtorji so uporabili strok lotusovih semen (LS) kot izvor za sintezo kvantnih pik ogljika (CQD) z uporabo ekonomi~ne in
enostavne metode hidrotermalne sinteze. LS-CQDs so okarakterizirali z razli~nimi tehnikami, vklju~no s TEM, FTIR, PL,
XRD, XPS, Ramanovo spektralno UV-vis analizo. Izmerjena povpre~na velikost delcev LS-CQDs je bila 2,1±0,17 nm. Dolo~ili
so lastnosti povzro~eno odvisne fotoluminescence LS-CQDs in izra~unani kvantni doprinos je bil 1,9 %. Raziskali so tudi
kalilni efekt LS-CQDs na Fe(III). Dolo~ili so normalizirano linearno zvezo med nara{~anjem koncentracije Fe(III) in intenziteto
emisije fluorescence LS-CQDs. Nadalje so avtorji pojasnili {e kalilni mehanizem reakcije med Fe(III) ioni in LS-CQDs.
Klju~ne besede: strok lotusovih semen, kvantne pike ogljika, dolo~evanje Fe(III)
1 INTRODUCTION
Iron, especially Fe(III) ions, is one of the most basic,
richest and essential metal elements in an organism,
which participates in the formation of various enzymes
and proteins as well as regulating many chemical reac-
tions in living organisms.
1,2
Insufficient or excessive
Fe(III) in the human body can disrupt the metabolic ac-
tivity and affect physical health, leading to the develop-
ment of chronic diseases such as anemia, liver and kid-
ney damage and heart failure.
3,4
Besides, an Fe(III) ion
contamination has become an increasingly severe envi-
ronmental problem, attracting much attention in recent
year.
5
Therefore, the research on the Fe(III) detection is
of great importance to both the environment and human
health.
At present, the methods used for detecting Fe(III) in-
clude spectrophotometry, atomic spectroscopy, the elec-
trochemical method and mass spectrometry.
6–11
These an-
alytical methods usually have a broad detection range,
high sensitivity and excellent repeatability. However,
there are also shortcomings such as a long detection
time, cumbersome sample-preparation process, compli-
cated operation, possible interference with another ion
and expensive equipment. Thus, it is necessary to design
a simple, fast and accurate method for the Fe(III) detec-
tion. In comparison with the other methods, the fluores-
cent-probe method is highly sensitive, relatively simple
and more rapid, exhibiting fewer background signals and
spanning over a wider linear dynamic range. It has be-
come an important analytical method for the determina-
tion of metal ions.
12–17
As a fluorescent probe, carbon
quantum dot (CQD) has received considerable attention
due to its advantages including a facile synthesis, and
cost efficiency and adjustable fluorescence emission.
18
A lotus seedpod (LS) is the receptacle surface of a lo-
tus with many honeycomb holes. LS is usually treated as
a waste and low-calorific fuel, but it is also used as food
or a medicinal substance.
19
Previous studies showed that
LSs can be used as adsorbents or functional materials
Materiali in tehnologije / Materials and technology 55 (2021) 1, 141–147 141
UDK 620.1:582.724.4:620.1 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 55(1)141(2021)
*Corresponding author's e-mail:
xiashibin@126.com (Shibin Xia)

due to their high specific surface area after pyrolysis ac-
tivation.
20,21
Hence, there is a great potential for LSs to
be applied in the development of novel CQDs. Compared
with the other carbon sources, lotus seedpod exhibits ob-
vious advantages including a low cost and an easy ac-
cess.
In this study, LS was employed as s carbon source for
synthesizing CQDs (hereinafter referred to as LS-CQDs)
via the hydrothermal-synthesis method. The synthesized
LS-CQDs were characterized with transmission electron
microscopy (TEM), Raman spectroscopy, X-ray diffrac-
tion (XRD), X-ray photoelectron spectroscopy (XPS)
and Fourier transform infrared (FTIR). The effect of the
quenching mechanism of Fe(III) on the photolumi-
nescence (PL) intensity of LS-CQDs was also eluci-
dated.
2 EXPERIMENTAL PART
2.1 Instruments and materials
A homegrown mature lotus seedpod was purchased at
a local supermarket (Wuhan, Hubei, in the summer). The
reagents used in this study included quinoline sulfate,
calcium chloride, ferric chloride, chromium tri-chloride,
sodium chloride, aluminum chloride, potassium chloride,
magnesium chloride and zinc chloride. All the reagents
were of analytical grade and were used as received. Dis-
tilled water was used to prepare the Fe(III) ion solution.
TEM measurement was carried out using a high-reso-
lution JEM-2100F transmission electron microscope.
Raman spectral measurement was conducted using an
InVia Raman spectrometer. The PL intensity was mea-
sured with spectrofluorometric detection (Sigma-
Aldrich). The particle-size distribution of CQDs was an-
alyzed using dynamic laser-light scattering. Spectropho-
tometric measurement was performed using an
UNICOWFUV-2 UV-Vis spectrometer. XRD patterns
were assessed with a D8 Advance X-ray diffractometer.
XPS measurement was conducted using an ESCALAB
Xi+ X-ray photoelectron spectrum analyzer. FTIR as-
sessment was carried out using a Nexus FTIR spectrome-
ter. Other instruments such as an electronic balance
(BS223S), drying box (DGG-9123A), PTFE-lined hy-
drothermal synthesis reactor (LSRP-25), centrifuge
(TG16-II) and lyophilizer (JL-D10N-50C) were also
used in this study.
2.2 Preparation of a dried LS
Lotus seeds were removed from a fresh LS and the
remaining LS was dried at a ventilated place for 30 days
to prepare a dried LS. Then, the obtained dried LS was
ground into powder and filtered through a 200-mesh
sieve.
2.3 Preparation of LS-CQDs
Two grams of dried LS powder and 50 mL of pure
water were mixed and placed into the PTFE-lined hydro-
thermal synthesis reactor, followed by heating for3hat
180 °C. After the reaction, the obtained solution was fil-
tered through a 0.45-μm polyether sulfone membrane.
The filtrate was then centrifuged at 10,000 min
–1
for
15 min and the supernatant was filtered again using a
0.22-μm syringe filter. The resulting filtrate was dialyzed
for 2 days and the dialysis fluid was changed daily. After
completing the dialysis, the solution was freeze-dried to
obtain the desired solid LS-CQDs.
2.4 Determination of the fluorescence quantum yield
(QY) and lifetime
The QY of LS-CQDs was measured with a standard
procedure. Quinoline sulfate (QS) with 55 % QY was
employed as the reference standard. The integrated fluo-
rescent intensity and absorbance (A) values of LS-CQDs
and QS solution were determined at the same excitation
wavelength and the percentage of QY were calculated in
accordance with Equation (1):
QY QY
I
I
A
A
n
n
CQD QS
CQD
QS
QS
CQD
CQD
QS
=⋅⋅⋅
⎛ ⎝ ⎜ ⎜ ⎞ ⎠ ⎟ ⎟ ⋅
2
100 % (1)
where QY represents the fluorescence quantum yield
(%), I refers to the integrated fluorescent intensity, A in-
dicates the absorbance value and n stands for the refrac-
tive index of the solvent.
To determine the fluorescence lifetime of LS-CQDs,
Equation (2) was used for the exponential-decay-curve
fitting:
22
IA e
n
n
t
n
t
=⋅
∑
−
1
(2)
where I represents the fluorescence intensity, A stands
for the weight coefficient and  represents the fluores-
cence lifetime.
Determination of Fe(III) ions with LS-CQDs
Solid LS-CQDs were dissolved and diluted to a
1 mg/L solution with ultrapure water. For the determina-
tion of Fe(III) ions, 2 mL of different concentrations of
the Fe (III) ion solution were mixed with a CQD solution
(ratio = 1:1, v/v). After a thorough mixing, the fluores-
cence intensity of the LS-CQDs solution was deter-
mined.
3 RESULTS
3.1 Morphological characterization
As illustrated in Figure 1a, the morphology of
LS-CQDs exhibited a spherical shape with a uniform
dispersion, which confirmed a successful preparation of
LS-CQDs.
23
Figure 1b demonstrates the crystal lattices
Z. XIE et al.: LOTUS SEEDPOD-BASED CARBON QUANTUM DOTS: PREPARATION, CHARACTERIZATION ...
142 Materiali in tehnologije / Materials and technology 55 (2021) 1, 141–147

of LS-CQDs (lattice spacing = 0.22 nm), which are
slightly smaller than those of the CQDs reported in other
studies.
24,25
Figure 1c indicates that the average particle
size of LS-CQDs is 2.1 nm (ranging from 0.5 nm to
4 nm). Figure 1d reveals the selected-area electron-dif-
fraction (SAED) pattern of LS-CQDs, indicating the
amorphous phase of LS-CQDs.
26
Photoluminescence and spectroscopic characterization
Figure 2a indicates the UV-visible spectrum of
LS-CQDs. The existence of the maximum absorption
peak at around 204 nm is due to the  –  * transition of
the aromatic C=O bond.
27
In the inset, the LS-CQD solu-
tion is brown under the visible-light illumination, while
emitting bright green under the UV-light illumination.
Figure 2b indicates the fluorescence emission and
excitation of LS-CQDs. The maximum adsorption peak
of the fluorescence intensity was located at 360 nm in
the excitation spectra. Figure 2c indicates the excita-
tion-dependent property of LS-CQDs, suggesting the oc-
currence of a redshift with the increasing excitation
wavelengths. The PL is generated from a pre-existing
surface defect on LS-CQDs, playing a major role in trap-
ping the excitation energy and emitting light at a particu-
lar excitation wavelength, thus leading to an excita-
tion-dependent property.
As shown in Figure 2d, different fluorescence-life-
time values were observed and an average fluores-
cence-decay lifetime of 5.50 ns was determined. The
quantum yield of LS-CQDs was calculated to be 1.9 %,
which is relatively low compared to the CQDs synthe-
sized from other carbon sources.
28,29
Z. XIE et al.: LOTUS SEEDPOD-BASED CARBON QUANTUM DOTS: PREPARATION, CHARACTERIZATION ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 141–147 143
Figure 2: a) UV-visible spectrum (inset: photographs of LS-CQD solution under visible light (left) and UV light (right)), b) fluorescence emis-
sion and excitation spectra of LS-CQDs, c) emission spectra of LS-CQDs at excitation wavelengths varying from 300 nm to 540 nm, d) fluores-
cence-decay lifetime of LS-CQDs at a 360-nm excitation wavelength
Figure 1: a) TEM image, b) high-resolution TEM image, c) parti-
cle-size distribution, d) SAED pattern of LS-CQDs

3.2 Surface function group and structure characteriza-
tion
The functional groups on the surfaces of LS-CQDs
were characterized with an FTIR analysis. As shown in
Figure 3a, a broad peak was observed at 3388 cm
–1
,
which corresponded to the -OH stretching vibration
peak.
30
The peak at 2931 cm
–1
was ascribed to the sym-
metric -C-H stretching vibration group, while the peaks
at 1633 cm
–1
and 1410 cm
–1
were attributed to the pres-
ence of a –COO group. The peaks at 1256 cm
–1
and 1048
cm
–1
were corresponded to C–O–C and –C-N stretching
vibration groups, respectively. The peak at 611 cm
–1
was
ascribed to the C–C stretching vibration group.
Figure 3b indicates the XRD pattern of LS-CQDs.
There is an obvious absorption peak at around 27.6°,
which demonstrates the amorphous form of LS-CQDs.
This result is similar to a previous study.
31
Figure 3c indicates the Raman spectrum of
LS-CQDs. Typically, the fluorescence intensities of the
D and G peaks in the Raman spectrum were used to cal-
culate the graphitization degrees of LS-CQDs. As shown
in Figure 3c, both the G peak (representing crystallinity)
and D peak (representing the degree of disorder) are sta-
tistically insignificant. It is deduced that the fluorescence
interference from LS-CQDs may impede the
Raman-spectrum signal.
32,33
Z. XIE et al.: LOTUS SEEDPOD-BASED CARBON QUANTUM DOTS: PREPARATION, CHARACTERIZATION ...
144 Materiali in tehnologije / Materials and technology 55 (2021) 1, 141–147
Figure 4: XPS spectra of LS-CQDs: a) XPS survey spectrum and
binding-energy spectra of b) C1s and c) O1s Figure 3: a) FTIR, b) XRD and c) Raman spectra of LS-CQDs

3.3 XPS analysis
X-ray photoelectron spectroscopy was employed to
assess the surface composition and oxidation state of
LS-CQDs. Figure 4a shows an XPS survey spectrum
with two obvious shoulder peaks at 285 eV and 532 eV,
which correspond to the characteristic peaks of C1s and
O1s, respectively. Figure 4b indicates the high-resolu-
tion spectrum of C1s with a major peak at 284.8 eV and
a weak peak at 285.8 eV, which correspond with the C-C
and C-N bond formation on sp
2
carbon, respectively.
34
Figure 4c indicates the high-resolution spectrum of O1s
with a shoulder peak at 532 eV, which is ascribed to the
C-OH/C-O-C groups.
35
The XPS results demonstrate the
presence of a water-soluble oxygen-containing function
group on the surface of LS-CQDs.
3.4 Detection of Fe(III) ions
Figure 5a displays the emission spectra of LS-CQDs
with Fe(III) concentrations varying from 0 μmol/L to
100 μmol/L. An obviously decreased fluorescence inten-
sity at a peak of 449 nm was observed, indicating the flu-
orescence-quenching effect of Fe(III) on LS-CQDs.
Figure 5b shows a calibration graph of F
0
/F versus
Fe(III) concentrations where F
0
and F are the fluores-
cence intensities of LS-CQDs at 449 nm without and
with varying Fe(III) concentrations, respectively. An R
2
value of 0.98 was obtained, suggesting a positive correla-
tion between the F
0
/F of LS-CQDs and the concentra-
tions of Fe(III). In addition, the results also show the
suitability of LS-CQDs for the determination of Fe(III)
ions.
Figure 5c shows the fluorescence quenching effi-
ciency (F
0
/F) of the other metal ions (i.e., Al
3+
,C a
2+
,
Cr
3+
,K
+
,Mg
2+
,Na
+
and Zn
2+
) on LS-CQDs at the same
concentration (100 μmol/L). Except for Cr(III), all the
metal ions exhibited no obvious effect on the fluores-
cence intensity of LS-CQDs, confirming the high selec-
tivity of LS-CQDs for the Fe(III) ion detection.
In this study, no remarkable shift occurs at the emis-
sion peak around the center at 449 nm, indicating that
the fluorescence-quenching mechanism should probably
be attributed to electron transfer.
36
Further, the kinetic
mechanisms of the fluorescence-quenching effect were
elucidated.
Typically, fluorescence quenching can be classified
with regard to the static and/or dynamic quenching ef-
fects. The ground-state complex formation model of the
static or dynamic quenching effect was determined in ac-
cordance with the Stern-Volmer relationship Equation
(3):
F
0
/F=1+K
SV
C
q
=1+K
q
 0
C
q
(3)
where K
SV
stands for the quenching constant;  0
repre-
sents the average lifetime of LS-CQDs (5.5 ns); and K
q
refers to the quenching-rate constant. Notably, K
q
was
determined to be 4.27 × 10
11
M
–1
s
–1
, which is markedly
greater than the maximum scatter collision quenching
constant (~1.0×10
10
M
–1
s
–1
). These results indicate the
static quenching effect of Fe(III) on LS-CQDs.
4 CONCLUSIONS
CQDs were prepared with a hydrothermal synthesis
using LS as the carbon source. The synthesized
LS-CQDs exhibited a uniform particle-size distribution
(the mean = 2.1 nm) and were slightly smaller than the
previously reported CQDs. The synthesized LS-CQDs
demonstrated wavelength-dependent excitation and emit-
ted a green light at 365 nm. In addition, LS-CQDs were
Z. XIE et al.: LOTUS SEEDPOD-BASED CARBON QUANTUM DOTS: PREPARATION, CHARACTERIZATION ...
Materiali in tehnologije / Materials and technology 55 (2021) 1, 141–147 145
Figure 5: a) Emission spectra of different concentrations of Fe(III)
ions, b) the standard curve of F
0
/F at a 360 nm excitation wavelength,
c) the quenching efficiency (F
0
/F) of other metal ions against Fe(III)
ions

employed as a fluorescent probe for the Fe(III) ion detec-
tion. With the increasing Fe(III) ion concentration, the
fluorescence quenching rate of LS-CQDs on Fe(III) ions
was increased. However, the synthesized LS-CQDs still
possess the disadvantage of a low QY, which should be
improved in future studies.
Acknowledgment
The authors sincerely thank for the funding from
Study on Comprehensive Control of Rocky Desertifica-
tion and Ecological Service Function Improvement in
Karst Peaks (No. 2016YFC0502402).
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