High density sulfonated magnetic carbon quantum dots as a photo enhanced, photo-induced proton generation, and photo switchable solid acid catalyst for room temperature one-pot reaction

Article abstract of DOI:10.1007/s11164-019-03829-w

The preparation of photo enhanced and switchable (on/off mode) sulfonated magnetic carbon quantum dots was investigated in the present work. First, the as-synthesized carbon quantum dots (CQDs) were easily separated using a magnet (instead of using a dial

Full text of DOI:10.1007/s11164-019-03829-w

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03829-w
High density sulfonated magnetic carbon quantum dots
as a photo enhanced, photo‑induced proton generation,
and photo switchable solid acid catalyst for room
temperature one‑pot reaction
1
1
2
Negin Sarmasti · Ardeshir Khazaei · Jaber Yousef Seyꢀ
Received: 29 December 2018 / Accepted: 8 April 2019
©
Springer Nature B.V. 2019
Abstract
The preparation of photo enhanced and switchable (on/oꢀ mode) sulfonated mag-
netic carbon quantum dots was investigated in the present work. First, the as-synthe-
sized carbon quantum dots (CQDs) were easily separated using a magnet (instead
of using a dialysis method), then CQDs decorated with high density (9.6 mmol/g)
acid groups (sulfonic+carboxylic) using chlorosulfonic acid. The photo-switchable
activity of the catalyst related to the photoexitation of charge separation which in
turn leads to electron withdrawing property in sulfonic groups. On the other hand,
it was shown that sulfonated magnetic carbon quantum dot (S-M-CQDs) is more
eꢁcient than magnetic carbon quantum dot (M-CQDs). The as-prepared catalyst
was characterized using SEM, TEM, EDX, XRD, IR, VSM and TGA. The catalytic
activity of the as-prepared catalyst was evaluated at room temperature in one-pot
reaction (synthesis of hexahydroquinoline derivatives). The as-synthesized catalyst
easily retrieved using a magnet (than ultracentrifugation) and used in the next cycles.
Keywords Photocatalysis · Quantum dots · One-pot reaction · Chlorosulfonic acid ·
Nano Fe O
3
4
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
-019-03829-w) contains supplementary material, which is available to authorized users.
4
*
Ardeshir Khazaei
khazaei_1326@yahoo.com
1
2
Faculty of Chemistry, Bu-Ali Sina University, Hamedan 6517838683, Iran
Department of Chemical Engineering, Hamedan University of Technology, Hamedan, Iran
Vol.:(012
1
3456789)
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N. Sarmasti et al.
Introduction
Carbon has a broad family of allotropes which includes diamond, graphite, fuller-
ene, carbon nanotubes, single-walled carbon nanohorns (SWNHs), onion-like car-
bon (OLC) spheres, bamboo-like nanotubes, graphene, graphene dots and carbon
dots. Diamond and graphite are the oldest and most known allotropes. C , which is
6
0
known as fullerene is the ꢂrst carbon nanostructure, which was initially reported in
1
985 [1, 2], and 6 years later (in 1991) carbon nanotubes (CNTs) were discovered
by Iijima [3, 4]. In sequence, other carbon nanostructures with unique shapes such
as single-walled carbon nanohorns (SWNHs), onion-like carbon (OLC) spheres,
and bamboo-like nanotubes were discovered. The graphene as the building block
of graphite was ꢂrst isolated and characterized in 2004 by Novoselov et al. [5]. Gra-
phene quantum dots (GQDs) with a dimension of a few nanometers with crystalline
structure are deꢂned as small pieces of graphene monolayer [6]. Finally, in 2004 car-
bon dots (C-dots) were identiꢂed during preparative gel electrophoresis puriꢂcation
of single wall carbon nanotubes (SWNTs), which was generated by the arc-discharge
method [7]. Due to CQDs strong ꢃuorescence property, they have recently attracted
the attention of researchers. Accordingly, several methods have been developed for
the synthesis of C-dots which can be categorized into two main categories, includ-
ing chemical and physical methods [8]. Physical methods include arc-discharge [7],
laser ablation/passivation [9], and plasma treatment [10]. Chemical methods include
the electrochemical synthesis, combustion/thermal/hydrothermal/acidic oxidation,
supported synthesis, microwave/ultrasonic, solution chemistry methods, cage-open-
ing of fullerene, and etc. [11]. C-dots have also been prepared from candle soot fol-
lowing by acid treatments [12]. Because of strong and tunable photoluminescence
(
PL), C-dots have found important and wide applications in chemical sensing, bio-
sensing, bioimaging, nanomedicine, photocatalysis and electrocatalysis [13]. How-
ever, few studies have focused on exploring the surface functionality of CQDs as
catalytic sites in organic transformations. Carbon quantum dots and sulfonated car-
bon quantum dots (through concentrated H SO ) have been used for Aldol conden-
2
4
sations [14, 15], quinazolinone derivatives and aza-Michael adducts [16], esteriꢂca-
tion [15], Beckmann rearrangement [15] and ring opening reaction [17]. Puriꢂcation
of C-dots is achieved through a combination of un-commercial methods, including
ultracentrifugation, ꢂltration, dialysis, and column chromatography or gel electro-
phoresis. On the other hand, ultracentrifugation is used for separation of C-dots
from reaction media, which is not an eꢀective and economical method [14, 17].
With regards to the mentioned issues, in the present work, C-dots were pre-
pared from candle soot, but magnetic nano-Fe O was used for easy separation of
3
4
carbon quantum dots (CQDs). In addition, rapid sulfonation of magnetic C-dots
(
[
M-CQDs) was achieved through chlorosulfonic acid than concentrated H SO
2 4
17] (S-M-CQDs). We demonstrate that S-M-CQDs and M-CQDs act as photo
switchable solid acid catalysts, which can catalyze the one-pot reactions (e.g.
hexahydroquinoline derivatives) under UV irradiation and dark at room tem-
perature. Finally the as-synthesized catalyst magnetically was recovered from the
reaction media, and used in the next cycles.
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3

High density sulfonated magnetic carbon quantum dots as a photo…
Experimental
Regents and characterization
Ferric chloride hexahydrate (FeCl ·6H O), ammonia (28% by weight), ferrous
3
2
chloride tetrahydrate (FeCl ·4H O), phthalhydrazide, diꢀerent aldehydes, dime-
2
2
done, ethanol, chloroform, acetone, and chlorosulfonic acid were prepared from
Merck company. The reaction progress was monitored using thin layer chroma-
tography (TLC) (silica gel SIL G/UV 254 plates). Perkin Elmer PE-1600-FTIR
was used to determine the Infrared spectra of products. Thermal gravimetric anal-
ysis (TGA) of the catalyst under a nitrogen atmosphere was used to determine the
stability of the as-prepared catalyst. The Field emission scanning electron micros-
copy (FESEM), and Transmission electron microscopy (TEM) images were
recorded with SIGMA VP-500 (ZEISS), Zeiss-EM10C-100 kV, respectively.
Elemental analysis was measured using Energy dispersive X-ray analysis (EDXA)
(
Oxford Instrument). Vibrating sample magnetometer (VSM), model (MDKB),
was used to determine the magnetic property of the as-catalyst.
Synthesis of the catalyst
Synthesis of Fe O nanoparticles
3
4
The co-precipitation approach was used to synthesize the magnetic (Fe O ) nano-
3
4
particles. First 11.3 g FeCl ·6H O and 5.6 g FeCl ·4H O was dissolved in deion-
3
2
2
2
ized water at the 80 °C (600 rpm). After complete dissolution of the FeCl ·6H O
3
2
and FeCl ·4H O in water, 25 ml ammonia (28% by weight) was added at once to
2
2
the solution. The resulting black solution was vigorously stirred for 2 h at 80 °C
under N atmosphere. Then the precipitated magnetic nanoparticles were sepa-
2
rated from the solution using a magnet. Nanoparticles were washed several times
with water and then with acetone. Finally, a stable black magnetic dispersion was
obtained which was dried in an oven.
Preparation of C‑dot nanoparticles from candle soot
The soot was collected by placing an aluminum foil over the ꢃame of a candle
[
12]. The particles in the soot agglomerated, so that micro-particles are formed.
In order to uniform dispersion of C-dot nanoparticles in water, the acid treat-
ment was used, so that 1 g soot was reꢃux in 16 M HNO for 24 h. After cooling
3
the reꢃux media up to room temperature, it was centrifuged (4000 rpm), which
results in a brown solution and a black precipitation. The brown solution decanted
and neutralized with Na CO until the pH 7 (mixture A).
2
3
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N. Sarmasti et al.
Preparation of Fe O @C‑dot nanoparticles
3
4
1
g of the previous synthesized Fe O nanoparticles was dispersed (15 min using
3 4
an ultrasonic bath) in mixture A, and was stirred for 24 h at room temperature.
The Fe O @C-dot nanoparticles were separated using a super magnet and washed
3
4
several times with deionized water, then dried in on oven.
Preparation of Fe O @C‑dot@SO H nanoparticles
3
4
3
1
g Fe O @C-dot nanoparticles (pervious step) was dispersed in dried dichlo-
3 4
romethane (15 min using an ultrasonic bath) and transferred to a bath with 0 °C
controlled temperature (mixture B). Then 1 ml chlorosulfonic acid was mixed
with 10 ml dried dichloromethane, and then it was added dropwise to the mixture
B. Then the solution stirred approximately for 15 min until the all gases exhausted
from the solution. Then the mixture B was stirred for 24 h at room temperature.
Finally, Fe O @C-dot@SiO H was separated using a magnet, washed several
3
4
3
times with dried CH Cl and then dried in an oven. The details of the C-dot nano-
2
2
particles synthesis is given in the literature, so that the HNO was selected as the
3
best oxidant.
Results and discussion
Catalyst characterization
Micrographs obtained from scanning electron microscopy (SEM), and transmission
electron microscopy (TEM) of the S-M-CQDs and M-CQDs are represented in the
Fig. 1. Figure 1a, b show the SEM and TEM images of M-CQDs, respectively. Fig-
ure 1c, d represent the SEM and TEM images of the S-M-CQDs, respectively. The
hydrogen bonds between OSO H groups in S-M-CQDs lead to agglomeration of
3
nanoparticles, which can be seen in Fig. 1c, d. TEM images show that M-CQDs and
S-M-CQD have core/shell structure. The core of the nanoparticles is Fe O and the
3
4
shells are CQDs and S-CQDs.
The elemental analysis [energy-dispersive X-ray spectroscopy (EDX)] conꢂrms
the presence of sulfur (S) atom in the as-synthesized S-M-CQDs nanoparticles
(
Fig. 2). On the other hand, the back titration of S-M-CQDs showed 9.6 mmol/g
acidic groups (SO H+COOH) (Supporting Information) is loaded on the as-pre-
3
pared catalyst, which is much higher than the previous published S-CDQs particles
[
17].
Thermal gravimetric analysis (TGA) was used to investigate the thermal stabil-
ity of the as-synthesized M-CQDs, and S-M-CQDs (Fig. 3). M-CQDs losses 5%
of its weight, which can be attributed to the loaded CQDs on the nano-Fe O sur-
3
4
face. S-M-CQDs has two main losses (region A and B), which correspond to the
losses of loaded SO H groups and CQDs on the nano-Fe O surface. The main loss
3
3
4
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3

High density sulfonated magnetic carbon quantum dots as a photo…
Fig. 1 Scanning electron microscopy (SEM), and transmission electron microscopy (TEM) of the
M-CQDs (a, b) and S-M-CQDs (c, d)
Fig. 2 The energy-dispersive X-ray spectroscopy (EDX) of M-CQDs (left) and S-M-CQDs (right) MNPs
occurs at 510 °C, which indicate thermal stability of the as-synthesized S-M-CQDs
nanoparticles.
Vibrating sample magnetometer (VSM) of nano-Fe O , M-CQDs, and S-M-
3
4
CQDs (Fig. 4) indicate that CQDs and S-CQDs layers on the nano-Fe O reduce
3
4
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3

N. Sarmasti et al.
1
1
01
00
110
100
90
M-CQDs
S-M-CQDs
A
9
9
9
9
9
9
8
7
6
5
80
0
510
C
C
B
70
60
50
0
200
400
600
800
0
Temperature ( C)
Fig. 3 The thermal gravimetric analysis (TGA) of M-CQDs, and S-M-CQDs
60
40
20
0
Fe O
54
45
3
4
Fe O @CQDs
3
4
Fe O @CQDs@SO H
3
4
3
1
7
-
-
-
20
40
60
-
8000 -6000 -4000 -2000
0
2000
4000
6000
8000
Applied field (Oe)
Fig. 4 Vibrating sample magnetometer (VSM) of nano-Fe O , M-CQDs, and S-M-CQDs
3
4
the saturation magnetization of the nano-Fe O (from 54 emu/g in nano-Fe O to
3
4
3
4
4
5 emu/g, and 17 emu/g in M-CQDs and S-M-CQDs, respectively). But the mag-
netization is enough that M-CQDs and S-M-CQDs nanoparticles could be easily
separated from reaction media using an external magnet.
To further explain the loading of CQDs on the surface of nano-Fe O and sul-
3
4
fonation of hydroxyl group in the CQDs nanoparticles, the FT-IR of nano-Fe O ,
3
4
M-CQDs, and S-M-CQDs were recorded (Fig. 5). The wave numbers around 3400
−1
and 1600 cm in M-CQDs correspond to the vibrations of O–H and C=O bonds,
−
1
respectively. The peaks around 650, 1200, and 3400 cm in S-M-CQDs correspond
to the vibrations of S–O (stretching), S=O (asymmetric stretching), and OH groups,
respectively. Therefore, we consider the changes in the surface groups of the S-M-
CQDs are responsible for the photo-induced proton generation.
1
3

High density sulfonated magnetic carbon quantum dots as a photo…
1
00
C=O of CQDs
-
OH of CQDs
S=O of SO H
-
OH of CQDs and SO3
H
Fe O
3
3
4
Fe O @CQDs
0
4
3
4
Fe O @CQDs@SO H
3
3
4
S-O of SO H
3
000
3500
3000
2500
2000
1500
1000
500
-1
Wave number (cm )
Fig. 5 Infrared Spectra (IR) of nano-Fe O , M-CQDs, and S-M-CQDs
3
4
1
00
Fe O
3
4
Fe O4-M-CQD
3
8
6
4
2
0
0
0
0
0
Fe O4-S-M-CQD
3
10
20
30
40
50
60
70
80
90
2θ
Fig. 6 XRD pattern of Fe O , Fe O -M-CQD, and Fe O -S-M-CQD
3
4
3
4
3
4
The X-ray diꢀraction (XRD) of the nano-Fe O , M-CQDs, and S-M-CQDs
3
4
are shown in the Fig. 6. The presence of sharp peaks at XRD pattern implies the
crystallinity of the as-synthesized nano-particles. The size of the magnetic nano-
particles and the interplaner spacing can be determined from Sherrer’s equation
(
D=kλ/βcosθ) and Bragg equation (d =λ/2sinθ), respectively.
hkl
Catalyst test and evaluation
Reaction between 1 mmol benzaldehyde, 1 mmol ethyl acetoacetate, 1.2 mmol
ammonium acetate, and 1 mmol dimedon was used as benchmark one-pot reac-
tion (hereafter reaction). To elucidate the photo enhanced catalytic abilities of sul-
fonated magnetic carbon quantum dots (S-M-CQDs), the reaction was carried out
in dark, and ultra violet (UV) irradiation (at room temperature), which is shown in
Table 1. To fair comparison of the reaction yield at diꢀerent condition, the reaction
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3

N. Sarmasti et al.
Table 1 Synthesis of hexahydroquinoline catalyzed by S-M-CQDs, M-CQD and other catalysts at room
temperature
Entry
Yield (%)a
Dark
t (min)
UVb
1
2
3
4
5
6
7
8
Fe O @CQD-SO H
35
21
66
40
46
39
31
18
75
48
68
41
48
40
32
20
10
10
10
10
10
10
10
10
3
4
3
Fe O @CQD
3
4
p-toluene sulfonic acid
Benzoic acid
Acetic acid
Phenol
Nano Fe O
3
4
Without catalyst
The temperature was kept constant (25 °C) using a water bath circulator
a
The yields was obtained through crystallization method in ethyl acetate till 6 °C
Ultraviolet irradiation at 320 nm
b
was ceased after 10 min. When there was no any light source (dark condition), the
achieved yield is 35%. The diꢀerence in the reaction yield under the UV irradia-
tion and dark condition is 40%, indicating that the UV irradiation has considerable
eꢀect on the reaction yield. On the other hand, the reaction was carried out using
magnetic carbon quantum dots (M-CQDs), as the control reaction. After 10 min, the
achieved reaction yield in the dark, and UV irradiation condition was 21%, and 48%,
respectively. These results imply that S-M-CQDs is more eꢁcient than M-CQDs.
To ꢂgure out the eꢀect of active sites (-SO H) on the S-M-CQDs, further control
3
experiments in the presence of phenol, p-toluene sulfonic acid, benzoic acid, and
acetic acid were carried out. Obtained reaction yield (in the dark, and UV irradia-
tion condition) in the presence of benzoic acid, acetic acid, and phenol implies that
the active site aren’t hydroxyl group and carboxylic acid groups. Achieved yield in
the presence of p-toluene sulfonic acid, indicate that the sulfonic acid group is the
active site. In addition, the reaction was carried out in the presence of nano-Fe O ,
3
4
so that the obtained reaction yield (Table 1, entry 7) shows that nano-Fe O isn’t an
3
4
eꢁcient catalyst in dark, and UV irradiation condition. Lowest yield in the entry 8
shows that the catalyst has a vital role in the progression of the reaction.
To further clarify the eꢀect of light on the M-S-CQDs (we call it M-CQD-SO H
3
to explain the mechanism), pH of the water solution in dark and UV irradiation was
measured. The pH value at diꢀerent times is shown in Fig. 7. Water circulating bath
was used to control the temperature at 25 °C, both in dark and UV irradiation. The
reduced pH in the presence of UV irradiation is due to photon enhance dissociation
1
3

High density sulfonated magnetic carbon quantum dots as a photo…
Fig. 7 pH value of water+M-S-
CQDs solution in dark and UV
irradiation
8
7
6
5
4
3
2
UV irradiation
in dark
0
10
20
30
40
50
60
Time (min)
Fig. 8 The sulfonation and proposed light-induced proton generation of S-M-CQDs under light irradia-
tion
−
+
of the M-CQD-SO H (M-CQD-SO H+H O ↔ M-CQD-SO + H O ). These
3
3
2
3
3
behaviors conꢂrm the increase rate of the reactions in the presence of M-S-CQDs.
+
Released H O activates the carbonyl group in benzaldehyde, which initiate the
3
reaction.
Based on the above results and discussions, the proposed mechanism of photon
induced proton generation under visible light and UV irradiation is shown in Fig. 8.
Due to the acidic properties of the sulfonic groups, it is assumed that the light causes
the generation of sulfonic protons [15].
In order to clarify the eꢀect of light on the S-M-CQDs, the following experiments
were done. Under UV irradiation, 10 experiments were carried out and the reactions
ceased after 1, 2, 3, 4, 5, 6, 8, 10, 12, and 14 min, respectively. In addition, the next
5
experiments lasted for 5 min under UV condition and immediately transferred to
darkness and the reactions stopped after 6, 8, 10, 12, and 14 min, respectively. To
ꢂgure out the eꢀect of sulfonic groups, the same procedure was followed by unsul-
fonated M-CQDs. The results are given in the Fig. 9 for M-CQDs and S-M-CQDs.
The circle and square markers show the reaction yield in the presence of S-M-CQDs
and M-CQDs catalyst, respectively. As is shown, after cutting oꢀ the light source,
the reactions progress has stopped, which conꢂrm the photo switchable property of
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N. Sarmasti et al.
Fig. 9 Yield versus time in
the presence of M-CQDs and
S-M-CQDs
80
60
40
20
0
M-CQDs
UV
M-CQDs
dark
S-M-CQDs
UV
S-M-CQDs
dark
0
5
10
15
20
Time (min)
Fig. 10 FT-IR spectrum of
benzaldehyde in the presence of
S-M-CQDs and M-CQDs in UV
and dark conditions
the as-synthesized catalysts. The separation point (5 min for S-M-CQDs and 10 min
for M-CQDs) in the Fig. 9 shows that the catalyst is switchable, so that switches the
conversion to on/oꢀ mode. In addition, the yield of the reaction is higher in the pres-
ence of S-M-CQDs than M-CQDs, in both UV and dark conditions, which implies
dissociation of sulfonic protons. UV light enhances the reaction yield by hydrogen
bonding [14] (for M-CQDs) and dissociation of sulfonic protons [15] (S-M-CQDs)
mechanisms.
FT-IR technique was used to clarify the reaction yield using M-CQDs and S-M-
CQDs (in UV and dark condition). It has been shown that acidic catalysts activate
carbonyl bond of benzaldehyde in the benchmark reaction [18]. Accordingly, a
mixture of benzaldehyde+S-M-CQDs, and benzaldehyde+M-CQDs stirred under
UV and dark condition, after separation of catalysts using a magnet, the FT-IR
of benzaldehyde was recorded immediately. The results are given in the Fig. 10.
The frequency of carbonyl group is shifted to lower wave numbers. S-M-CQDs,
−
1
which reduces carbonyl wave number from 1687 to 1653 cm , and from 1687 to
−
1
1
672 cm at UV and dark condition, respectively. M-CQDs is less eꢁcient than
−
1
S-M-CQDs so that carbonyl wave number shift from 1687 to 1664 cm , and from
−1
1
687 to 1679 cm at UV and dark condition, respectively. These ꢂndings conꢂrm
the benchmark reaction yield using the as-synthesized photo switchable catalyst
in UV and dark conditions. It should be noted that the M-CQDs in UV condition
is more eꢁcient than S-M-CQDs in dark condition, which is seen in both reaction
yields (Fig. 9) and FT-IR spectra (Fig. 10).
1
3

High density sulfonated magnetic carbon quantum dots as a photo…
Table 2 Synthesis of hexahydroquinoline derivatives catalyzed by M-CQDs, and S-M-CQD in dark and
UV irradiation condition
R
UV
Dark
M-CQDs
ta
S-M-CQDs
Yield
M-CQDs
S-M-CQDs
Yieldb
t
t
Yield
t
Yield
3
2
2
4
3
4
2
2
3
-OMe
9
15
17
12
12
14
10
8
75
69
74
78
63
68
83
83
67
6
80
71
79
81
65
75
86
90
69
23
29
32
21
22
29
19
20
26
74
65
72
74
62
53
80
81
65
20
25
27
20
16
25
18
18
20
78
70
78
79
62
82
85
87
65
,3-diCl
-NO2
-Br
-OH
-Me
-F
12
15
9
10
10
7
-Cl
-F
6
11
15
a
The time that the reaction has been completed
The yield that was obtained through the crystallization in the ethyl acetate till 6 °C
b
To generalize the application of the photo driven property of the as synthesized
catalysts, namely M-CQDs, and S-M-CQDs, diꢀerent derivatives of hexahydro-
quinoline were synthesized in dark and UV irradiation condition. The results are
given in the Table 2. As one can see, the reaction time (to complete the reaction)
using the M-CQDs is higher than the S-M-CQDs for both in dark and UV irradia-
tion condition, which shows that the –OSO H group has vital role to promote the
3
reaction. In addition, for all derivatives using S-M-CQDs catalyst in dark condi-
tion, the reaction times are approximately two or three times than UV irradiation
condition.
Proposed catalytic mechanism
The plausible catalytic mechanism is given in the Fig. 11. The heterogeneous
solid acid catalyst activates the aldehyde through –SO H groups on the catalyst
3
surface. Then, the enol-keto form of dimedone attacks to the activated aldehyde
(
intermediate 1). On the other hand, solid acid catalyst activates the carbonyl
group of the ethyl acetoacetate which is attacked by ammonia (intermediate 2).
Finally, the intermediate 1 and 2 react to form the ꢂnal product.
1
3

N. Sarmasti et al.
Fig. 11 The plausible catalytic mechanism in the synthesis of hexahydroquinoline derivatives
Catalyst stability
To check the catalyst stability in the one-pot reaction, the IR technique was use.
Accordingly, after the catalyst preparation the IR spectrum of the catalyst was
recorded. Then, after the completion of the one-pot reaction, the as-prepared catalyst
was recovered and the IR spectrum was recorded. The IR spectra were identical,
which imply the stability of the as-prepared catalyst in the one-pot reaction.
In the present work, the synthesized carbon quantum dots was separated using
an external magnet, so that the brown water solution turned into clear water solu-
tion. This method is very economical in terms of time and cost than the dialysis
method [7, 14, 17, 19]. Although sulfonated carbon quantum dots have been used for
ring opening reaction [17], and carbon quantum dots for Aldol condensations [14],
but separation of the carbon quantum dots is done by centrifugation (approximately
2
2,000–23,000 rpm) [14, 17]. Separation is not very common by this method, and
the catalyst may remain in the reaction media. On the other hand, concentrated sul-
furic acid (98% H SO ) has been used to sulfunation of the hydroxyl group in the
2
4
1
3

High density sulfonated magnetic carbon quantum dots as a photo…
carbon quantum dots [17], which is not an eꢀective method to achieve a high density
sulfonated functional group. Accordingly, in the present work, chlorosulfonic acid
was used to convert –OH into –OSO H groups (SN mechanism), which is a simple
3
2
approach with high yield. Magnetic carbon quantum dots modiꢂed with palladium
[
15], and magnetic core–shell carbon dots@MFe2O4 with application in Suzuki
reaction and reduction of p-nitrophenol [18], respectively, have been reported. Use
of carbon quantum dots as catalyst in organic chemistry is in the early stages and
needs more research, so that in this work magnetic carbon quantum dots was used in
the synthesis of hexahydroquinoline derivatives as a representative of multicompo-
nent one-pot reactions.
Conclusion
In summary, we demonstrated that separation of CQDs can be easily achieved
through magnetic Fe O than ultracentrifugation and dialysis methods. Anchoring
3
4
high density stronger acid groups such as –OSO H on the surface of magnetic CQDs
3
can be obtained using chlorosulfonic acid. Magnetic CQDs (M-CQDs) and sul-
fonated S-M-CQDs was synthesized and photo proton generation property of S-M-
CQDs was shown. The preparation of photo enhanced and switchable (on/oꢀ mode)
sulfonated magnetic carbon quantum dots is described and used in the one-pot mul-
ticomponent reaction at room temperature. Hope these ꢂndings motivate researchers
to develop more photo responsive catalysts in organic chemistry.
Acknowledgements The authors gratefully acknowledge the Bu-Ali Sina University Research Council
and Center of Excellence in Development of Environmentally Friendly Methods for Chemical Synthesis
(
CEDEFMCS), Iran National Science Foundation (INSF) for providing support to this work. This work
was supported by Key Laboratory of Advanced Control and Optimization for Chemical Processes (CN)
Grant No. 32-1716).
(
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