Synthesis of 3,4-diaminobenzenethiol and its application in gold nanoparticle-based colorimetric determination of copper ions

Article abstract of DOI:10.1039/c6ra13681h

An improved synthetic method for the synthesis of 3,4-diaminobenzenethiol is presented. The practical application of this reagent as a new ligand for gold nanoparticles for the rapid detection of Cu²⁺ in water is demonstrated. The method is shown to have high sensitivity and selectivity. Well-defined peaks, proportional to the concentration of the corresponding Cu²⁺, were observed from 0.5 μM to 2 μM, and the recovery was in the range of 92-109%. This method provides a facile route for Cu²⁺ analysis.

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Synthesis of 3,4-Diaminobenzenethiol and Its Application in
Gold Nanoparticle-based Colorimetric Determination of Copper
Ions
Chi Zhang,a LiꢀQiang Lu,a* Pan Deng,a XiꢀKe Tian,a* MingꢀYang Liu,b AiꢀMing Lib
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000
An improved synthetic method for synthesis of 3,4ꢀdiaminobenzenethiol is presented. The practical application of this
reagent as a new ligand for gold nanoparticles for the rapid detection of Cu2+ in water is demonstrated. The method is shown
to have high sensitivity and selectivity. Wellꢀdefined peaks, proportional to the concentration of the corresponding Cu2+,
were observed from 0.5 ꢁM to 2 ꢁM, and the recovery was in the range of 92~109%. This method provides a facile route for
Cu2+ analysis.
www.rsc.org/
processing procedures and sufficient sensitivity.18ꢀ27 Recently, a
colorimetric approach for sensing Cu2+ was reported using bare
AuNPs.28 This method can be used without complicated
Introduction
In recent years, the development of economy has led to serious
environmental pollution. Especially, a large quantity of waste water
that contains a huge number of heavy metal ions, has been poured
into rivers, lakes and other water sources. Heavy metal
contamination is known to be very difficult to degrade and be easily
accumulated. Additionally, heavy metals can enter the food chain
readily and thereby harm the health of animals, plants and humans
via biological enrichment.
Copper is an essential nutrient for organism，but the high Cu2+
concentration can cause metabolic disorders, developmental
stagnation and even death.1ꢀ3 It is therefore important to monitor
concentration of Cu2+ in environment.
At present, methods for the detection of Cu2+ mainly include
atomic absorption spectrometry,4,5 inductively coupled plasma
emission spectrometry,6,7 inductively coupled plasma mass
spectrometry8 and Spectrophotometry.9 However, some of these
methods may not be highly sensitive, or some of them are too
complex and expensive. Therefore, a simple, sensitive and rapid
Cu2+ detection method would be of great benefit to the wastewater
monitoring industry.
In recent years, gold nanoparticles (AuNPs) have been widely
used in various research fields because of their high extinction
coefficient and unique optical properties.10ꢀ17 Gold nanoparticlesꢀ
based colorimetric assay is one of these applications and has recently
gained much interest because of its low cost, simple sample
pretreatment but it has relatively high limits of detection. Moreover,
some rapid visual methods were reported for the detection of Cu2+
29, 30
31, 32
based on modified AuNPs
or gold nanorods (AuNRs)
.
These methods could be sensitive. However, the reagents required
could be expensive or difficult to obtain, it is necessary to find
ligands that are inexpensive and technically easy to synthesize.
3,4ꢀDiaminobenzenethiol, which has two amino groups and a thiol
group, was reported by first.33 Its mercapto functional group can
react with AuNPs via a strong covalent AuꢀS bond. The exposed
amino functional groups on the benzene ring from neighboring
AuNPs can bind copper ions, resulting in AuNPs aggregatings
(Scheme1). 3,4ꢀDiaminobenzenethiol can therefore be used as a
ligand to modify gold nanoparticles for the colorimetric detection of
copper ions that can be quantitatively monitored by UV/Vis
spectrophotometry.
In this paper, a simply and optimalizing approach to synthesis of
3,4ꢀdiaminobenzenethiol using a threeꢀstep synthetic process with oꢀ
nitroaniline as a starting material was developed. The intermediates
and final products were characterized by FTIR, NMR and GCꢀMS.
A gold nanoparticleꢀbased colorimetric method to detect copper ions
using 3,4ꢀdiaminobenzenethiol as functioning ligand was established.
The experimental conditions were optimized and the test results
were compared with ICPꢀOES, and the feasibility of this method was
verified.
a Nano-Mineral Materials and Application Engineering Research Center of
Ministry of Education, Faculty of Materials Science and Chemistry, China
University of Geosciences, Wuhan 430074, P. R. China. E-mail:
llqdick@cug.edu.cn
bHubei Environment Monitoring Center, Wuhan 430072, P. R. China
†
Electronic supplementary information (ESI) available: FTIR, 1H NMR and GCꢀ
MS spectra.
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Oꢀnitroaniline (8.25 g, 60 mmol) and sodium thiocyanate (18 g, 0.22
mol) were dissolved in 100 ml glacial acetic acid（98%）and then
cooled to 5℃. Bromine (9.6 g, 60 mmol) was dropped into 10 mL
glacial acetic acid（98%）and then cooled to 5℃. Above two kinds
of solution were mixed and reacted for 72 h, then the mixture was
warmed to 15℃ and poured into 500 ml ddH2O, resulting in an
orange turbid solution. The solution was filtered. The solid material
acquired was washed with ddH2O, and then dissolved in 100 ml
acetone. The resulting solution was filtered and evaporated. A
yellow solid product was gained.
Scheme 1 Schematic representation of the sensing mechanism of the AuNPsꢀbased
colorimetric determination of Cu2+
.
(2) Synthesis of 4ꢀaminoꢀ3ꢀnitrobenzenethiol
Experimental section
The first step product was added multiple times to 100 ml potassium
hydroxide (6 g) ethanol solution, stirred for 1 h at 5℃. Sulfuric acid
ethanol solution (5%) was added slowly until the color of the
mixture changed from dark violet to orange. The orange mixture was
then poured into 400 ml ddH2O and extracted three times by 100 ml
ethyl acetate. The combined organic extracts were washed with brine,
dried over magnesium sulfate, and evaporated. In the end, red solid
was obtained.
Chemicals and materials
Ligand: 3,4ꢀDiaminobenzenethiol was synthesized in a threeꢀstep
procedure, sealed in a brown reagent bottle at 5℃.
10 mM Cu2+ solution: 0.4262 g of copper(II) chloride dihydrate
(CuCl2·2H2O) is accurately weighed and dissolved with ddH2O, and
then stored at 5℃. The stock solution was then diluted to the desired
concentrations with ddH2O.
Buffers: Phthalate buffer, mixed phosphate buffer and sodium
tetraborate buffer were purchased from Shanghai Leici Instrument
Co., Ltd. All buffers were stored at 5℃. Prior to use, pH of the
buffers were adjusted again.
Sodium chloride, triꢀsodium citrate and other chemical agents were
of analytical reagent grade (Sinopharm Chemical Reagent Co., Ltd.).
All solutions were prepared by ddH2O (18 Mohm cm resistance),
unless noted specially. All glasswares were cleaned with aqua regia
(3:1 (v/v) HClꢀHNO3) thoroughly and rinsed with ddH2O prior to
use.
(3) Synthesis of 3,4ꢀdiaminobenzenethiol
The product of the second step was dissolved in ethanolꢀwater
solution (300 ml, 1:1) and then sodium dithionite (13.4 g, 80 mmol)
was added over a period of 20 min. After that, the pH was adjusted
to 3.0 by adding sodium hydroxide. The stirred solution was
refluxed for 1 h at 105℃ and then extracted with chloroform after
cooled to room temperature. A crude solid resulting from vacuum
drying, was subjected to silica gel column chromatography using
MeOH–EtOAc (1:5) to acquire a yellow solid product.
Colorimetric detection of Cu2+
A volume of 1 ml of 3,4ꢀdiaminobenzenethiol solution (0.2 mg/L)
and 0.1 ml of Cu2+ with different concentrations were added to 0.9
ml phosphate buffer solution (pH 6.0). The reaction proceeded at
Apparatus
ACYꢀ600ꢀU ultraꢀpure water system (Chongqing Yi Yang
Enterprise Development Co., Ltd., China); TUꢀ1901 double beams
UVꢀVis spectrophotometer (Beijing Purkinje General Instrument Co., room temperature for 5 min, then 1 ml of AuNPs suspension were
Ltd., China); CM12 transmission electron microscope (Philips Ltd., added and the mixture was incubated for 15 min. The UV
Netherlands); UBꢀ7 digital pH meter (Denver Instrument, USA); measurements were carried out by UV/Vis photometer (TUꢀ1901) in
ME204E analytical balance (Mettler Toledo Ltd., Switzerland); ICPꢀ Faculty of Materials Science and Chemistry, China University of
Geosciences.
5000 inductively coupled plasma emission spectrometer (Hangzhou
Juguang Technology Co., Ltd., China); Agilent 7890ꢀ5975C gas
chromatographyꢀmass spectrometer (Agilent Co., Ltd., USA);
Nicolet 6700 Fourier transform infrared spectrometer (Thermo
Fisher Co., Ltd., USA); and VNMR 400 nuclear magnetic resonance
spectrometer (Varian Co., Ltd., USA).
Fourier transform infrared spectroscopy (FTIR)
After vacuum drying, the sample with dry KBr was ground. The
FTIR measurements were carried out by Nicolet 6700 in Faculty of
Materials Science and Chemistry, China University of Geosciences.
Nuclear magnetic resonance spectroscopy (NMR)
Synthesis of AuNPs
These samples were dissolved with chloroformꢀd solution, and the
nuclear magnetic resonance measurements were carried out by
VNMR 400 at Central China Normal University.
Citrateꢀcapped AuNPs were prepared by method of the chemical
reduction of HAuCl4 in the liquid phase.34 Briefly, 35 ml of
deionized water and 0.6 ml of HAuCl4 (25 mM) were mixed in a
conical flask. After boiling for several minutes, 1 ml of triꢀsodium
citrate solution (38.8 mM) was added quickly with vigorous stirring.
The color of the solution changed into red in a few seconds. After
boiling for 5 min, the solution was cooled to room temperature and
then stored at 5℃.
Gas chromatographic mass spectrometry (GC-MS)
The samples were dissolved with ethyl acetate, and the GCꢀMS
measurements were carried out by full scanning (Agilent 7890ꢀ
5975C) at the Hubei Environmental Monitoring Center.
Transmission electron microscope characterization (TEM)
Synthesis and purification of 3,4-diaminobenzenethiol33, 35-39
(1) Synthesis of 2ꢀnitroꢀ4ꢀthiocyanoaniline
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The samples were prepared on copper grids. The TEM
characterization was carried out in Faculty of Materials Science and
Chemistry, China University of Geosciences.
Detection mechanism
AuNPs could be capped by 3,4ꢀdiaminobenzenethiol via thiol groups
due to their strong binding ability to sulphur. The AuꢀS bond is
stable in high concentration salt solutions and can resist the attack of
amino groups. The amino groups on the 3,4ꢀdiaminobenzenethiol
have strong binding ability to copper ions, and the stability constant
of the aminoꢀCu complex is higher than most of other interfering
ions. Therefore, the 3,4ꢀdiaminobenzenethiolꢀcapped AuNPs could
bind copper ions to form stable complexes, and subsequently
aggregate, resulting in a color change. This mechanism is consistent
with some previous reports.40ꢀ42 This color change can be
quantitatively analyzed by UVꢀVis spectrophotometry.
As shown in Fig. 1, AuNPs show a strong SPR peak at 520 nm
(black line). When the ligand 3,4ꢀdiaminobenzenethiol was added,
this characteristic peak only slightly decreased (red line) and the
color of the solution remained red (left inset). However, upon the
addition of Cu2+, the color of the solution changed from red to blue
(right inset). While the original SPR peak at 520 nm significantly
decreased, a new distinct SPR peak at 700 nm appeared (blue line),
indicating the Cu2+ꢀinduced aggregation of 3,4ꢀdiaminobenzenethiolꢀ
capped AuNPs.
Results and discussion
Chemical structure confirmation of synthetic products
Fig. S1 shows the FTIR spectra of synthetic products. In the
spectrum of 2ꢀnitroꢀ4ꢀthiocyanoaniline (Fig. S1A), the characteristic
bands of primary amine (NH2) are 3475 cmꢀ1 and 3357 cmꢀ1 for
asymmetric and symmetric stretching, respectively, the thiocyano
moiety’s symmetric stretching shows an absorption band at 2150 cmꢀ1
(C≡N), and the nitro aromatic moiety’s symmetric stretching band
appears at 1552 cmꢀ1 (NꢀO), confirming the presence of expected
functional groups in the synthesized 2ꢀnitroꢀ4ꢀthiocyanoaniline. The
1H NMR spectrum of 2ꢀnitroꢀ4ꢀthiocyanoaniline (Fig. S2A) also
indicates the presence of ꢀNH, where the signal at
attributed to the protons of ꢀNH.
δ = 6.2 ppm is
In the FTIR spectrum of 4ꢀaminoꢀ3ꢀnitrobenzenethiol (Fig. S1B),
while the NH2 asymmetric and symmetric stretching bands (3463
cmꢀ1 and 3350 cmꢀ1, respectively) and the NꢀO symmetric stretching
band (1554 cmꢀ1) appear, there is no absorption band at 2150 cmꢀ1,
indicating that the thiocyano moiety has been hydrolyzed. The 1H
NMR spectrum of 4ꢀaminoꢀ3ꢀnitrobenzenethiol (Fig. S2B) confirms
the presence of ꢀSH, where the signal at
the protons of ꢀSH.
δ = 3.4 ppm is attributed to
The FTIR spectrum of 3,4ꢀdiaminobenzenethiol presented in Fig.
S1C shows that the characteristic bands of NH2 are at 3476 cmꢀ1 and
3361 cmꢀ1 for asymmetric and symmetric stretching, respectively,
and at 1619 cmꢀ1 for NꢀH, and the primary aromatic amine
symmetric stretching band appears at 1247 cmꢀ1 (CꢀN). These results
show that a portion of nitro groups in the sample was reduced to
amino groups.
GCꢀMS measurements were further performed to confirm the
chemical structure of synthetic products. The total ion flow
chromatogram of 2ꢀnitroꢀ4ꢀthiocyanoaniline (Fig. S3A) shows that
2ꢀnitroꢀ4ꢀthiocyanoaniline (molecular weight 195) is eluted at 39.75
min, other peaks were fragment ions whose corresponding molecular
weights are 149, 122, 105, 95, 63 and 52. These characteristic peaks
were consistent with those of 2ꢀnitroꢀ4ꢀthiocyanoaniline in the NIST
library.
Fig. S3B shows the total ion flow chromatogram of 4ꢀaminoꢀ3ꢀ
nitrobenzenethiol. The NIST library does not contain this material,
therefore there is no direct standard. Despite this, some thiol
compound is detected at 34.61 min, and its m/z is close to molecular
weight of 4ꢀaminoꢀ3ꢀnitrobenzenethiol. Fragment ions of m/z 149,
122, 105, 95, 63 and 52 are also detected. The characteristic peaks
indicate that thiocyano group has been hydrolyzed to thiol group,
which is consistent with the reported 4ꢀaminoꢀ3ꢀnitrobenzenethiol.33
The total ion flow chromatogram of 3,4ꢀdiaminobenzenethiol is
depicted in Fig. S3C. Because the NIST library does not contain this
material, it cannot be directly matched. However, some compound
which has same relative molecular weight as target product is
detected at 38.46 min. Besides m/z 95, 80 and 52, fragment ion of
m/z 107 that could be ꢀC6H3NH2NH2 is detected, suggesting that
nitro group has been reduced to amino group. The result is consistent
with the reported 3,4ꢀdiaminobenzenethiol.33
Fig. 1 UVꢀVis spectra of AuNPs in the absence and presence of the
ligand 3,4ꢀdiaminobenzenethiol and Cu2+. The insets show the
optical photographs of AuNPs solutions in the absence (left) and
presence (right) of Cu2+.
The dispersed and aggregated AuNPs were also confirmed by
TEM images (Fig. 2). While Fig. 2A shows that AuNPs have nearly
spherical shape and uniformly disperse, Fig. 2B displays significant
aggregation of AuNPs after the addition of the ligand and Cu2+.
A
B
Fig. 2 TEM images of AuNPs in the (A) absence and (B) presence of
3,4ꢀdiaminobenzenethiol and Cu2+.
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Optimum concentration of ligand for the aggregation of AuNPs
Ligand concentration has a high impact on the efficacy of this
system. The citrateꢀstabilized surface of AuNPs is negatively
charged and a high concentration of positively charged ligand can
cause aggregation of the AuNPs. Alternatively, a low concentration
of ligand can lead to isolated AuNPs, but this latter condition may
not be conducive to highly sensitive detection of Cu2+. The ligand
was therefore diluted to different concentrations and the absorption
spectra were recorded.
As shown in Fig. 3, when the ligand concentration was 2 mg/L,
the absorption peak of AuNPs at 520 nm was significantly lower
than the absorption peak with no ligand added (0 mg/L) and a new
absorption peak at 670 nm appeared, demonstrating that high
concentration of the ligand causes AuNPs aggregation. When the
concentration of ligands was 0.2 mg/L, ligands did not induce
significant aggregation of AuNPs as evidenced by the absorption
peak being not dramatically decreased and the absence of AuNP
solution color change. Therefore, to gain highest sensitivity, 0.2
mg/L of 3,4ꢀdiaminobenzenethiol was chosen for subsequent
experiments.
Fig. 4 Effect of reaction time on the determination
Optimum pH
The pH value of solution not only has influence on the stability of
AuNPs probes, it also affects the stability of the amineꢀCu complex.
According to previous experimental methods, a range of pH values
from 4 to 9 of buffer solution was tested without changing other
conditions, and the absorbance ratio (A650/A520) was determined. Fig.
5 depicts how the pH of solution influences aggregation of AuNPs.
At pH values ranging from 4 to 8, the ratio of A650/A520 remains
stable, indicating this pH region can be chosen for subsequent
experiments. When the pH was greater than 8, the ratio of A650/A520
decreased dramatically. This indicates that pH value influences the
stability of the complexes likely through the formation of metal
hydroxides at higher pH. The formation pH of copper hydroxide for
micro molarity of Cu2+ estimated from the corresponding solubility
product (Ksp = 2.2×10ꢀ20) is about 6.8. Considering the coordination
of the ligand, the actual formation pH would be higher than 6.8.
Based on these results, a pH of 6 was chosen as the optimum pH
condition in our experiments.
Fig. 3 Effect of ligand concentration on the aggregation of AuNPs
Optimum reaction time
A concentration of 10 µM of Cu2+ was reacted with ligand modified
AuNPs at room temperature and the absorbance of the solution was
determined at different times. The effect of reaction time is shown in
Fig. 4, where the ordinate denotes A650/A520, the ratio of the
extinction coefficients at 650 nm and 520 nm, which is on the behalf
of the molar ratio of the aggregated to the dispersed AuNPs. It is
shown that the ratio of A650/A520 was stable when reacting after 15
min, and therefore in order to ensure the convenience of
experimental operation, these experimental parameters were chosen
as optimum for analysis.
Fig. 5 Effect of pH on the determination
Detection of Cu2+
Under the optimized conditions, different concentrations of Cu2+
were mixed with 0.2 mg/L 3,4ꢀdiaminobenzenethiol ligand and
AuNPs for 15 min, and then UVꢀVis spectroscopy was used to
quantitatively determine the content of Cu2+ in aqueous solution. As
shown in Fig. 6A, when Cu2+ concentration increased, more ligands
would bond to Cu2+, and AuNPs were aggregated. Concentration of
Cu2+ was determined by measuring A650/A520. Linear equation, A =
0.296 C−0.127 (R2 = 0.998), was generated over the range of 0.5ꢀ2
µM Cu2+ (Fig. 6B).
4 | J. Name., 2016, 00, 1ꢀ3
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A
Fig. 7 Influence of various cations on the determination of Cu2+.
Cu2+ (2 µM); K+, Ca2+, Na+, Mg2+, Al3+, Fe3+ (100 µM); Mn2+, Cr6+
(40 µM); Pb2+, Cd2+ (20 µM); Zn2+(10 µM). The insets are the
corresponding optical photographs.
B
Real water samples analysis
To demonstrate the viability of the assay in measuring Cu2+ in real
water samples, we measured the amount of Cu2+ in electroplating
wastewater. As shown in Table 1, the method provided recoveries of
92 to 109% of Cu2+ in electroplating wastewater samples. The
concentration of Cu2+ in electroplating wastewater was found to be
49 µM. To compare the data with wellꢀestablished techniques, we
also measured the Cu2+ content using ICPꢀOES. The amount of Cu2+
in electroplating wastewater was found to be 52 µM.
0.5
1.0
1.5
2.0
Fig. 6 (A) UVꢀVis spectra of different concentration gradient of
Cu2+ and (B) Working curve
Conclusions
Selectivity of the method
In summary, we adopted an improved threeꢀsteps synthesis method
To evaluate the selectivity of the colorimetric detection of Cu2+,
some other environmentally related metal ions were chosen for the
investigation, including Na+, K+, Mg2+, Al3+, Ca2+, Zn2+, Fe3+, Cr6+,
Mn2+, Pb2+ and Cd2+. Fig. 7 shows that only Cu2+ lead to a
significant increase in the value of A650/A520 compared to blank,
while that of other metal ions exhibitted no significant changes.
Additionally, the color of the solution in the presence of Cu2+
changed into blue (inset), while that of solutions containing other
metal ions remained red, also indicating that this colorimetric assay
method exhibits high selectivity for Cu2+ detection.
for synthesis of 3,4ꢀdiaminobenzenethiol,
a novel ligand for
modifying AuNPs. Further, we established a sensitive and selective
colorimetric assay for Cu2+ in water. And satisfied results were
obtained in detecting the actual samples. The operational simplicity
of the method may facilitate the analysis of Cu2+ in industrial waste
water. This method would be valuable for colorimetric detection in
environmental monitoring.
Acknowledgements
This work was supported by geological survey project of China
Geological Survey (12120113015300), the Foundation for
Innovative Research Groups of the National Natural Science
Foundation of China (NSFC, No. 41521001) and the Fundamental
Research Funds for the Central Universities, China University of
Geosciences (Wuhan) (No. 2014029).
Table 1 Results of detection and recovery of Cu2+ in electroplating wastewater (n=3)
Added
(ꢁg/L)
Detected
(ꢁg/L)
Recovery
(%)
RSD
(%, n=3)
Sample
1
2
3
3.2
6.4
13.18
16.38
22.78
92
95
3.2
2. 8
2.3
12.8
109
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RSC Advances
View Article Online
DOI: 10.1039/C6RA13681H
Graphic Abstract
3,4-Diaminobenzenethiol can react with AuNPs via a strong covalent Au-S bond. The
exposed amino functional groups on the benzene ring from neighboring AuNPs can
bind copper ions, resulting in AuNPs aggregating.