pH-Dependent plasmonic catalysis of 4-nitrobenzenethiol in aqueous environment

Article abstract of DOI:10.1016/j.saa.2015.09.033

Plasmon-driven chemical reaction of 4-nitrobenzenethiol (4-NBT) dimerizing to p,p′-dimercaptoazobenzene (DMAB) has been successfully monitored under different pH solutions. The experimental results indicated that the chemical reactions were dependent on the pH values, and they proceeded more completely under acidic conditions than those under neutral and alkaline conditions. Furthermore, the rate of the chemical reactions was effectively controlled by exposure time and incident laser power. The stable pH dependence demonstrated the plasmon-driven chemical reactions of 4-NBT to DMAB are sensitive to the surrounding environment, and have wider applications.

Full text of DOI:10.1016/j.saa.2015.09.033

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 542–545
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
journal homepage: www.elsevier.com/locate/saa
pH-Dependent plasmonic catalysis of 4-nitrobenzenethiol in
aqueous environment
Jingang Wang a,b,1, Yong Dong a,1, Yuanzuo Li , Zhenglong Zhang ⁎, Fengcai Ma ⁎
c
d,
a,
a
Department of chemistry and Physics, Liaoning University, Shenyang 110036, PR China
b
Department of Physics, Shenyang Aerospace University, 110036, PR China
College of Science, Northeast Forestry University, Harbin 150040, PR China
c
d
Leibniz Institute of Photonic Technology, Albert-Einstein-Str. 9, 07745 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 May 2015
Received in revised form 9 June 2015
Accepted 27 September 2015
Available online 30 September 2015
Plasmon-driven chemical reaction of 4-nitrobenzenethiol (4-NBT) dimerizing to p,p′-dimercaptoazobenzene
(DMAB) has been successfully monitored under different pH solutions. The experimental results indicated that
the chemical reactions were dependent on the pH values, and they proceeded more completely under acidic con-
ditions than those under neutral and alkaline conditions. Furthermore, the rate of the chemical reactions was ef-
fectively controlled by exposure time and incident laser power. The stable pH dependence demonstrated the
plasmon-driven chemical reactions of 4-NBT to DMAB are sensitive to the surrounding environment, and have
wider applications.
Keywords:
pH-Dependent
Plasmon-driven chemical reaction
© 2015 Elsevier B.V. All rights reserved.
4-NBT
Aqueous environment
1
. Introduction
can be selectively dimmerized to DMAB when absorbed on plasmonic
nanostructures [24–27]. The reactions are strongly influenced by the re-
Azobenzene compounds are high-value and important chemicals
widely used in chemical industry as dyes, food additives, and drugs
1]. They are synthesized either by oxidation of anilines or by reduction
of nitroaromatics. In 2008, an environmentally friendly gold-nanoparti-
cle-catalyzed synthetic method was proposed to selectively form
azobenzene compounds from aromatic anilines [2]. It was reported
that DMAB, an azobenzene derivative, can be produced from the selec-
tive catalytic coupling reaction of typical aromatic compounds PATP or
action timescales, substrate materials, laser wavelength and power, and
the surrounding atmosphere [28–30]. In particular, the plasmon-driven
oxidation reaction of PATP to DMAB could be effectively controlled by
pH values [31–34], which would occur under neutral and alkaline con-
dition, while not happen under acidic condition. However, how to con-
trol the plasmon-driven chemical reaction of 4-NBT to DMAB by
different pH values, is still not clear.
With the help of surface plasmon, two 4-NBT molecules dimerize to
a DMAB molecule, and can this plasmon catalyzed reaction be con-
trolled by pH values? If possible, the dependence of pH on the catalytic
reaction of 4-NBT to DMAB can be similar to that of PATP to DMAB? If
not, the reactions can occur under all pH conditions, which need
fewer constraints and then present a wider application. Herein, the dy-
namic process of the plasmon-driven conversion of 4-NBT into DMAB
has been monitored via time-dependent and laser power-dependent
surface-enhanced Raman scattering (SERS) spectra. Above all, we dem-
onstrated the plasmon-driven catalytic reaction of 4-NBT to DMAB is
dependent of pH values.
[
4
-NBT on rough noble metal surfaces [3,4].
Recently, a novel application of plasmonics on chemical reactions
has been widely investigated, such as photochemical isomerization
5], photodissociation [6,7], and catalytic reactions [8–19]. In this
[
paper, we focused on plasmon-driven surface-catalyzed reactions. In
plasmon-driven chemical reactions, plasmonic nanostructures are
used to enhance the surface-catalyzed reactions [20,21]. Because they
generate a significantly enhanced local electromagnetic field caused
by local surface plasmon polaritons (SPPs), and noble metal nanoparti-
cles exhibit high catalytic activity in many catalytic reactions [22,23].
Extensive experimental and theoretical studies have demonstrated
that the plasmon-driven chemical reactions of both PATP and 4-NBT
2. Experimental
The SRES active Ag colloid was prepared by sodium citrate reduction
of AgNO [35]. 30 mg AgNO was dissolved in 150 mL deionized water.
3 3
This solution was well-sealed and brought to heat with the microwave
oven (595 W) for three minutes. Then 4 mL sodium citrate solution
⁎
Corresponding authors.
E-mail addresses: Zhenglong.Zhang@ipht-jena.de (Z. Zhang), mafengcai@lnu.edu.cn
(
F. Ma).
1
Contributed equally.
http://dx.doi.org/10.1016/j.saa.2015.09.033
386-1425/© 2015 Elsevier B.V. All rights reserved.
1

J. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 542–545
543
(
1%) was added. After stirring enough, the mixed solution was heated in
the microwave oven for 5 min, and then made it cool at room tempera-
ture. The final Ag nanoparticles were mixed with 4-NBT ethanol solu-
−
5
tion (1 × 10 M) for 24 h, and were introduced into capillary, sealing
the both ends up, for SERS measurement. On the other hand, the pH
value effects on the plasmon-driven chemical reaction were also inves-
tigated. Diluted HCl and NaOH solution were added to the mixed solu-
tion, to control pH = 3 and 10, respectively.
The surface morphology of Ag nanoparticles was characterized by
SEM (Hitachi S-4800). Fig. 1a shows the SEM image of the coupling of
Ag nanoparticles, and Fig. 1b is the SEM image of Ag nanoparticles
mixed with 4-NBT molecule. Combining Fig. 1a and b, it was found
that there were many three-dimensional (3D) nanogaps (“hot spots”)
in Fig. 1b. 3D hot spots can produce stronger localized surface plasmon
resonance (LSPR) than 2D roughened substrate, thus further enhancing
the plasmon-driven catalytic reactions. Furthermore, we measured the
extinction spectra of Ag solutions and Ag solutions mixed with 4-NBT
−
5
ethanol solutions (1 × 10
M) in the same volume under different
pH values using absorption spectroscopy (HP8453, USA) (see Fig. 2a).
From the extinction spectra, it was found that the absorption peak of
Ag nanoparticles is about 410 nm, and there is a red shift of SPR peak
of Ag nanoparticles after being mixed with 4-NBT molecule. We sug-
gested this shift presumably resulted from the interaction of 4-NBT mol-
ecule and Ag nanoparticles and the effect of ethanol solvent.
The normal Raman spectrum of 4-NBT and SERS spectra of 4-NBT on
the Ag nanoparticles were measured with the Renishaw invia spectrom-
eter, and a 632.8 nm He–Ne laser (3.28 mW) was used as the exciting
light. The all spectra were recorded with an accumulation time of 10 s.
Fig. 2. (a) The extinction spectra of Ag sol and 4-NBT in Ag sol mixed with 4-NBT molecule
under different pH values, and (b) normal Raman spectrum of 4-NBT powder.
completely, even though the exposure time further added. Therefore,
the varying Raman peaks provided direct and strong evidence for the
plasmon-driven chemical reaction of DMAB produced from 4-NBT.
Meanwhile, the time-varying Raman intensity demonstrated this reac-
tion is time-dependent.
The same work was done under alkaline condition. All measure-
ments were at the same exposure time and incident laser power
(100%). Fig. 3b (the red lines) showed an increasing intensity of
3
. Results and discussion
−
1
−1
−1
Raman peaks centered at 1143 cm , 1390 cm , 1439 cm , which
clearly distinguished from the spectrum in Fig. 3b, indicating an in-
creased extent of DMAB formation. We concluded that the plasmon-
driven catalytic reaction of DMAB from 4-NBT also could occur under al-
kaline condition, and the reaction was time-dependent.
Fig. 2b shows the normal Raman spectra of 4-NBT powder, and there
−
1
−1
−1
are three strong Raman peaks at 1100 cm , 1332 cm , 1576 cm
.
−
1
Additionally, the strongest Raman peak at 1332 cm is attributed to
(NO ) [26]. The SERS spectra of 4-NBT absorbed on the Ag nanoparti-
v
s
2
cles were measured under 100% laser power (~3.28 mW) under neutral
condition (see Fig. 3a). When laser irradiated at the first time of 10 s, it
was found that the SERS spectrum was nearly identical to the Raman
spectrum of 4-NBT powder (see Fig. 2b). However, as the exposure
The above studies have suggested that the plasmon-driven chemical
reaction of 4-NBT to DMAB could occur under neutral and alkaline con-
dition. Then, will it occur in acidic environment? Therefore, we studied
this reaction under acidic condition using the same method. Fig. 3c illus-
trated that the plasmon-driven catalytic reaction of 4-NBT to DMAB had
not happened before 50 s. As the exposure time added to 50 s, the reac-
−
1
time increasing to 50 s, there occurred three weak peak at 1143 cm
,
−
1
−1
1
390 cm , 1439 cm (the red dash line in Fig. 3a), which were slight-
−
1
ly different from the Raman spectrum of 4-NBT powder. With the expo-
sure time adding to 100 s, a very significant difference was observed. As
the previously reported [4], the appearance of Raman peaks at
tion begun to occur, and the intensity of Raman peak at 1439 cm had
−
1
exceeded that at 1332 cm (see Fig. 3c). When the radiation time in-
creased to 200 s, the yield of DMAB kept growing. Until 350 s, the extent
of this reaction came to the maximum. It can conclude that the plas-
mon-driven catalytic reaction of 4-NBT to DMAB can take place under
acidic condition, and was time-dependent, having similar behaviors as
those under neutral and alkaline condition. It was worth mentioning
that the reactions were more completely at pH = 3 than those under
other two pH environments (see the spectra of 350 s in Fig. 3a, b and c).
−
1
−1
1
390 cm , 1439 cm were related to –N = N– stretching vibration
of DMAB, which demonstrated the formation of DMAB. Further, as the
laser irradiation time increased, the intensity of Raman peak at
−
1
1
439 cm
Raman peak at 1439 cm
Raman peak of v (NO ) of 4-NBT at 1332 cm
continued to enhance. Until 200 s, the intensity of the
−
1
−1
exceeded that at 1332 cm , but the
−
1
s
2
had not disappeared
Fig. 1. (a) The SEM imaging of Ag nanoparticles without 4-NBT molecule, and (b) the morphology of Ag nanoparticles mixed with 4-NBT molecule.

544
J. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 542–545
Fig. 3. Exposure time manipulated dynamics of plasmon-driven catalytic reactions under (a) neutral, (b) alkaline and (c) acidic. SERS spectra of 4-NBT with increasing exposure time from
50 s to 350 s respectively.
+
To further monitor the chemical reaction over exposure time, the
especially in acidic condition. It indicate that the H and OH− is helpful
+
dynamics of the reaction of 4-NBT converting into DMAB was compara-
tively analyzed under various pH conditions. According to the chemical
equation [36]:
to this reaction, and H in acidic condition make it much easier to cleave
oxygen for 4-NBT to DMAB.
To better understand the chemical reactions happening under acidic
conditions, we measured the SERS spectra of 4-NBT in pH = 3 environ-
ment with different incident laser power. Fig. 5a is the SERS of 4-NBT at
low laser power (1%). the three main Raman peaks are the same as the
normal Raman spectrum of 4-NBT powder (see Fig. 2b), which indicated
the plasmon-driven chemical reaction of 4-NBT to DMAB had not occur.
With the increase of the incident laser power, three new weak Raman
peaks at 1143 cm , 1390 cm , 1439 cm appeared at 5% (see the
red lines in Fig. 5b), which indicates that the catalyzed reaction began
to take place. With the laser power further increasing, the SERS spectra
À
Á
−
1
1
I 1439 cm
¼
kt
−
Ið1345 cm
Þ
here the intensity of the bands at 1439 cm−1 and 1345 cm−1 represent
the reactant 4-NBT and product DMAB, respectively. The apparent rate
constant k is proportional to the chemical reaction rate constant. Fig. 4
−
1
−1
−1
−
1
−1
shows the corresponding plot of I(1439 cm )/I(1345 cm ) versus ex-
posure time t, where curves are the records at pH = 7, 10, 3, respective-
ly. The analysis shows that, the apparent rate constants of surface-
catalyzed reactions in pH = 3 environment is somehow bigger than
those obtained under neutral and alkaline conditions, giving kpH = 3
=
,
8.810 ± 0.756) × 10−3
s
−1
, kpH = 10 = (6.100 ± 0.738) × 10
−3 −1
s
(
−
3
−1
and kpH = 7 = (4.520 ± 1.340) × 10
s
. Same to the results in
Fig. 3, the reaction is more complete in acidic and alkaline condition,
Fig. 4. Determination of reaction rate constant for plasmon-driven reduction reactions of
Fig. 5. Laser power controlled dynamics of plasmon-driven catalytic reactions under acidic
4-NBT dimerizing into DMAB under various pH conditions.
condition. SERS spectra of 4-NBT with laser power at 1%, 5%, 10%, 50%, 100%, respectively.

J. Wang et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 153 (2016) 542–545
545
in Fig. 5c and d showed significantly different from that in Fig. 5a, which
Acknowledgments
indicates that more 4-NBT molecules react to DMAB. When the laser
power increased to the maximum (100%), the extent of this reaction
came to the maximum (see Fig. 5e). The SERS spectrum at 100% of
laser power exhibited the 4-NBT and DMAB coexisted, which indicated
the plasmon-driven chemical reaction in Ag sol was a partial reaction.
Besides, the reactions in pH = 3 environment proceeded more
completely.
This work was supported by the National Natural Science Founda-
tion of China (Grant Nos. 91436102, 11374353, 11374045 and
11404055), the Program of Liaoning Key Laboratory of Semiconductor
Light Emitting and Photocatalytic Materials, and the Heilongjiang Pro-
vincial Youth Science Foundation (Grant No. QC2013C006).
It was known that both PATP and 4-NBT can convert into DMAB.
However, the plasmon-driven chemical reaction of PATP to DMAB was
strongly dependent on the pH values [31–33]. The difference in pH de-
pendence of two reactions can be explained from the point of molecular
structures, the amino and nitro group (−NH and –NO ). The amino
2 2
group owns a long-pair electron, and easily combines a hydrogen ion
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Herein we investigated the influence of pH values on the plasmon-
[
driven chemical reaction of 4-NBT dimerizing into DMAB absorbed on
Ag nanoparticles using time-dependent and laser power-dependent
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