Regulating peroxidase-like activity of Pd nanocubes through surface inactivation and its application for sulfide detection

Article abstract of DOI:10.1039/c8nj05138k

A new analytical method is described for the colorimetric detection of sulfide. This approach involves the use of Pd nanocubes (PdNCs) as artificial peroxidases that can effectively generate a colored signal by catalyzing the oxidation of peroxidase substrates. In the presence of sulfide, the colored signal is greatly diminished because of the specific and efficient inhibition of the peroxidase-like activity of the PdNCs through their surface inactivation by the formation of PdS. Based on this finding, sulfide assay can be achieved in the 0.125-2.75 μM concentration range with a detection limit of 80 nM. This method shows high selectivity to sulfide besides other ions, and has been applied for the determination of sulfide in spiked real samples. It also features simplicity and rapidness since the assay can be conducted at room temperature, in aqueous solution, and requires only ～5 min.

Full text of DOI:10.1039/c8nj05138k

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Regulating peroxidase-like activity of Pd
nanocubes through surface inactivation and its
application for sulfide detection†
Cite this: New J. Chem., 2019,
43, 371
Yi Wang, *a Pu Zhang,b Lei Liu,a Fei Xue,c Maochang Liu,*c Ling Lia and
Wensheng Fu*a
A new analytical method is described for the colorimetric detection of sulfide. This approach involves
the use of Pd nanocubes (PdNCs) as artificial peroxidases that can effectively generate a colored signal
by catalyzing the oxidation of peroxidase substrates. In the presence of sulfide, the colored signal is
greatly diminished because of the specific and efficient inhibition of the peroxidase-like activity of the
PdNCs through their surface inactivation by the formation of PdS. Based on this finding, sulfide assay
can be achieved in the 0.125–2.75 mM concentration range with a detection limit of 80 nM. This method
shows high selectivity to sulfide besides other ions, and has been applied for the determination of sulfide
in spiked real samples. It also features simplicity and rapidness since the assay can be conducted at
room temperature, in aqueous solution, and requires only B5 min.
Received 10th October 2018,
Accepted 19th November 2018
DOI: 10.1039/c8nj05138k
rsc.li/njc
In addition, Pt nanocubes and Ru nanoframes were also found
to have high enzyme-mimic catalytic efficiencies.13–15 However,
Introduction
Peroxidases are a family of enzymes that catalyze the oxidation of most applications of the peroxdiase-like activity of metal nano-
certain compounds with peroxides (H2O2 in most cases). They particles for sensing are limited to H2O2 or oxidase substrates
are known to play vital roles in biological systems such as (e.g., glucose, xanthine, and uric acid) which can generate H2O2
detoxifying H2O2 in human bodies or defending against patho- in the presence of relevant oxidases.16–18 Therefore, design and
gens in plants.1,2 Unlike natural peroxidases, peroxidase mimics exploration of new sensing strategies by means of the unique
made of nanoscale particles are easy to isolate and purify, and peroxidase-like catalytic activity of noble metal nanostructures is
are also more stable under different harsh conditions.3 A wide essential for expanding their applications.
range of nanomaterials including metal oxides,4,5 noble metals,6
As a traditional environmental contaminant, sulfide is widely
carbon materials,7 metal–organic frameworks,8,9 and hybrid distributed in natural water and wastewater. It is either used as a
materials10 were reported to show similar peroxidase-like proper- reactant or generated as a byproduct in petrochemical, leather, and
ties over the past decade. Among different nanomaterials, noble paper industries.19 Excessive exposure to sulfide may cause a series
metal nanocrystals with anisotropic shapes or enclosed by a of problems such as dizziness, unconsciousness, irritation in
specific facet usually show high catalytic efficiencies. For mucous membranes, permanent brain damage or even asphyxia-
instance, Zheng et al. synthesized 2D Pd-based nanostructures tion.20 Therefore, quantitative detection of sulfide is important in
and found that their enzyme-mimic catalytic activities are struc- the assessment of environmental quality and protection of human
ture- and composition-dependent.11 Xia et al. prepared a novel health. To this end, a variety of different analytical methods
structure of Pd–Ir core–shell nanocubes enclosed by {100} facets, have been developed for sulfide assay, including titration,21
which show a catalytic constant (Kcat) as high as 1.9 ꢀ 106 sꢁ1 12
.
chromatography,22 electrochemistry,23–25 fluorimetry,26 capillary
electrophoresis,27 and inductively coupled plasma-atomic emis-
sion spectroscopy.28 However, many of the available methods for
sulfide detection are limited in practical applications due to their
intrinsic shortcomings of being time consuming, having poor
selectivity, and requiring expensive instruments or complicated
procedures. Therefore, development of rapid, simple and con-
venient approaches for sulfide detection is still highly desired.
In this work, the peroxidase-like activity of Pd nanocubes
(PdNCs) is precisely regulated through surface poisoning by sulfide.
a Chongqing Key Laboratory of Green Synthesis and Applications, and College of
Chemistry, Chongqing Normal University, Chongqing 401331, China.
E-mail: ywang@cqnu.edu.cn, fuwensheng@cqnu.edu.cn
b College of Pharmacy, Chongqing Medical University, Chongqing 400016, China
c International Research Center for Renewable Energy, State Key Laboratory of
Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an,
Shanxi 710049, China. E-mail: maochangliu@mail.xjtu.edu.cn
† Electronic supplementary information (ESI) available: Experimental details,
additional figures and results. See DOI: 10.1039/c8nj05138k
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concentrations were added into 685 mL phosphate buffer
(pH 4.5). The solution was mixed thoroughly and incubated
for 5 min at room temperature, and then used for absorption
spectroscopy measurements.
Results and discussion
Strategy design for sulfide assay
The detection method relies on the high peroxidase-like activity
of PdNCs and specific inhibition of their catalytic activity by
sulfide. It has been demonstrated that the PdNCs enclosed by
{100} facets exhibit excellent peroxidase-like activity.12 In this
case, the oxidation of TMB by H2O2 can be efficiently catalyzed
and thus generates an intense blue-colored signal (i.e., from
oxidized TMB with a maximum absorbance at 652 nm). In
the presence of sulfide, some of the surface Pd atoms on the
nanocubes would transform into PdS immediately since the
solubility product constant (Ksp, 25 1C) of PdS is as low as
2.03 ꢀ 10ꢁ58. Namely, the surfaces of the PdNCs are specifically
and strongly passivated, resulting in partial or total deactiva-
tion of the PdNCs for catalysis. Thus, the generation of the
blue-colored signal will be inhibited. The more sulfide in the
detection system, the weaker the blue-colored signal generated
by the catalytic reaction. Based on the precise regulation of
sulfide concentration, sulfide assay can be conveniently achieved
through measuring the absorbance at 652 nm of the solution
using an UV-Vis spectrometer or observing the solution color
with the naked eye.
Scheme 1 Schematic illustration of the regulating peroxidase-like activity
of Pd nanocubes through surface inactivation and its application for sulfide
assay.
PdNCs of 18 nm in size were prepared by a one-pot synthesis
method according to a previously reported procedure.29 It was
found that PdNCs have excellent catalytic activity toward the
oxidation of peroxidase substrate 3,30,5,50-tetramethylbenzidine
(TMB) in the presence of H2O2. Nevertheless, their peroxidase-
like activity was significantly inhibited in the presence of NaHS,
and the activity decrease was correlated with the concentration of
HSꢁ. Based on this finding, a colorimetric method for sensitive
and selective detection of sulfide is developed (Scheme 1).
Experimental
Chemicals and characterization
The details of all chemicals and instrumentation used in this
work are shown in the ESI.†
Preparation of PdNCs
Feasibility demonstration of the strategy for sulfide assay
In a typical synthesis, 8.0 mL of an aqueous solution containing
105 mg of poly(vinyl pyrrolidone) (PVP), 60 mg of ascorbic acid
(AA) and 600 mg of KBr was hosted in a vial and preheated
to 80 1C in an oil bath under magnetic stirring for 10 min.
Subsequently, 3.0 mL of an aqueous solution containing 57 mg
of Na2PdCl4 was added using a pipette. After the vial had been
capped, the reaction was allowed to continue at 80 1C for 3 h.
The cubic Pd product was collected by centrifugation, washed
three times with water and re-dispersed in 11 mL water.
Fig. 1a shows a representative transmission electron micro-
scopy (TEM) image of the prepared PdNCs from a standard
synthesis. It was found that the particles displayed well-defined
cubic shape, sharp corners and smooth surfaces. The average
edge length of the cubes was measured to be 18.2 ꢂ 1.5 nm.
The high-resolution TEM image of an individual cube clearly
shows a lattice fringe spacing of 1.9 Å (Fig. 1b), which can be
indexed to the {200} reflections of face centered-cubic Pd.29
Interestingly, obvious morphological changes of the PdNCs were
observed after their interaction with NaHS.
Preparation of Pd octahedrons
Specifically, the surfaces of the cubes became rugged and
the corners protruded out of the cubes (Fig. 1c). The same
phenomenon was also observed by high-angle annular dark-
field scanning TEM (HAADF-STEM) imaging after their reaction
(see the white arrows, Fig. 1d). Moreover, energy dispersive
X-ray (EDX) elemental analysis further intuitively demonstrated
the formation of a thin layer of PdS on the surfaces of the
PdNCs (Fig. 1e). The EDX spectrum clearly showed that Pd and
S were both involved in the sample after the reaction of PdNCs
with HSꢁ, and the atomic ratio of Pd to S was B9 : 1 (Fig. 1f). In
addition, X-ray photoelectron spectroscopy (XPS) and dynamic
light scattering (DLS) measurements were also carried out, and
the results further confirmed that PdS was formed on the
Pd octahedrons (B20 nm) were prepared using a seeded growth
method. Firstly, Pd seeds of B10 nm in size were synthesized
using the same method as that for the preparation of PdNCs,
except for the addition of 300 mg of KBr. Then, 0.3 mL of the Pd
seeds was mixed with 105 mg PVP, 100 mL HCHO and 7.6 mL
water in a vial at 60 1C. After that, 3 mL of an aqueous solution
containing 28.5 mg of Na2PdCl4 was rapidly injected into the
vial. The reaction was allowed to continue at 60 1C for 3 h. The
octahedral Pd nanocrystals were collected by centrifugation,
washed three times with water and re-dispersed in water.
General procedure for sulfide detection
Typically, 100 mL TMB (1.0 mM), 100 mL H2O2 (100 mM), surfaces of PdNCs (Fig. S1 and S2, ESI†). Our previous work
15 mL PdNCs (1.5 mg mLꢁ1), and 100 mL NaHS at different has indicated that sulfuration of noble metal nanoparticles on
372 | New J. Chem., 2019, 43, 371--376
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Fig. 2 Typical UV-Vis spectra and corresponding photographs of solu-
tions containing (a) TMB–H2O2, (b) PdNCs + TMB–H2O2, and (c) NaHS +
PdNCs + TMB–H2O2. Concentrations: TMB, 0.1 mM; H2O2, 10 mM; NaHS,
1 mM; and PdNCs, 22.5 mg mLꢁ1
.
surface poisoning, which is able to be used for colorimetric
detection of sulfide.
Colorimetric assay of sulfide
Based on the continual decrease of the absorbance at 652 nm
along with the increase of HSꢁ concentration (Fig. 3a), a new
analytical method for quantitative detection of sulfide was
established. The effects of solution pH as well as the amounts
of introduced PdNCs and TMB on the inhibition of the
peroxidase-like activity of PdNCs were investigated. To obtain
absolute concentrations of PdNCs, the nanoparticles were firstly
dissolved in aqua regia and then measured using a UV-Vis
spectrometer. The concentration of the PdNCs was calculated
according to the calibration curve that was obtained by recording
Fig. 1 Structural and composition characterizations of the PdNCs before
(a and b) and after (c–f) reaction with NaHS: (a) TEM image of the PdNCs;
(b) HRTEM of a typical PdNC; (c) TEM image of the PdNCs in the presence
of NaHS; (d) HAADF-STEM image of the PdNCs after reaction with NaHS;
(e) EDX elemental mapping image and (f) EDX spectrum of the PdNCs in
the presence of NaHS.
2ꢁ
the absorbance of PdCl4 at 279 nm (Fig. S3, ESI†), which had
been proved to be an effective approach for calculating the
concentration of Pd nanoparticles in our previous work.32
It was found that the maximum inhibition of the catalytic
activity occurred when the pH was around 4.5, and 22.5 mg mLꢁ1
PdNCs and 10 mM TMB were introduced into the system
(Fig. S4–S6, ESI†).
Colorimetric detection of sulfide was subsequently achieved
under the optimal conditions. All measurements were perfor-
med in triplicate, and the data were recorded as a mean. As
shown in Fig. 3b, the value of absorbance decrease at 652 nm was
linearly correlated with the increasing concentration of NaHS
from 0.125 to 2.75 mM, and the limit of detection (LOD) was
80 nM (3s). Compared with most previous studies for colorimetric
sulfide assay using metal nanoparticles as nanoprobes,30,33–41 this
method shows higher sensitivity and a comparable detectable
range (Table 1).
their surface easily occurs when trace amounts of sulfide
exist.30 In addition, it has also been demonstrated that the
sulfuration preferentially occurs at the corners of the metal
nanoparticles when they are in anisotropic shapes (e.g., tri-
angular and cubic shapes).31 Herein, the preferential sulfura-
tion at the corners of the PdNCs leads to the formation of
slightly concaved surfaces on the cubes.
Based on this finding, we started to demonstrate the feasi-
bility of the PdNC-catalyzed oxidation of TMB by H2O2 and its
application for colorimetric sulfide assay. As shown in Fig. 2,
the solution of a TMB–H2O2 mixture (trace a) is colorless,
indicating that TMB cannot be oxidized by H2O2 in the absence
of a catalyst. In the presence of PdNCs in the system, the
TMB–H2O2 solution exhibits a distinguishable blue color
with a maximum absorbance at 652 nm (trace b). However,
when PdNCs were incubated with 1 mM NaHS before adding
to the TMB–H2O2 solution, an obvious decrease of absor-
bance at 652 nm as well as a change in the solution color was
Selectivity and real sample detection
observed (trace c). This result implies that HSꢁ can effectively The absorbance responses of this method toward common ions
inhibit the peroxidase-like activity of the PdNCs through in environmental water were measured. As shown in Fig. 4,
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Fig. 4 Selectivity of this colorimetric method for sulfide assay. The absor-
bance decrease at 652 nm (DA652) in the absence and presence of various
ions was recorded using the standard procedure for sulfide detection
(1–20: Fꢁ, Clꢁ, SO42ꢁ, NO3ꢁ, CH3COOꢁ, HCO3ꢁ, SO32ꢁ, Iꢁ, CO32ꢁ, Brꢁ,
Fe3+ Fe2+ Cu2+ Pb2+
, , , ,
2ꢁ
2ꢁ
2ꢁ
ꢁ
2ꢁ
SiO3
,
S2O3
,
HPO4
,
H2PO4
,
S2O8 ,
and Hg2+). The concentration of HSꢁ is 2.5 mM, and the concentration of
other ions is 25 mM. Error bars represent the standard deviations of three
replicate measurements.
Table 2 Results for the detection of sulfide in tap water and river water
(n = 3)
Found
(mM)
Recovery
(%)
Found (mM) Recovery (%)
Spiked This
This
Standard
Standard
method
Sample
(mM)
method method method
Tap water
0.5
1.0
2.0
0.482
1.025
1.958
0.465
0.988
1.890
96.4
102.5
97.9
93.0
98.8
94.5
0.495
0.942
1.914
0.506
0.965
1.908
99.0
94.2
95.7
101.2
96.5
95.4
River water 0.5
1.0
2.0
Fig. 3 (a) Typical UV-Vis spectra of the PdNCs/TMB/H2O2 system in the
presence of different concentrations of NaHS (from a to e: 0, 0.5, 1, 2, and
3 mM). (b) Plot of the absorbance decrease (DA652) versus the concentration
of added NaHS in a linear range of 0.125–2.75 mM. Error bars represent the
standard deviations of three replicate determinations.
and thus high selectivity of this method for sulfide assay. We
speculated that the high selectivity was attributed to the strong
interaction between the Pd2+ on the surfaces of the PdNCs and
sulfide (Ksp of PdS at 25 1C is 2.03 ꢀ 10ꢁ58).
Table
1
Comparison of the present approach with previous metal
To demonstrate the reliability and potential applications of
this strategy for sulfide assay, the concentrations of sulfide in
environmental samples including tap water (collected from our
lab) and river water (collected from the Yangtze River) were
detected. First, the concentrations of sulfide in tap water and river
water were measured by a conventional methylene blue standard
method. However, no sulfide was detected in either of the two real
samples. Then, the standard addition method was used to achieve
the sulfide assay in real samples. Satisfactory quantitative spike
recoveries of 96.4–102.5% and 93.0–98.8% in tap water and river
water were obtained, respectively (Table 2). The results of this
method are also comparable with those of the conventional
methylene blue standard method. All of these results demonstrate
nanoparticle-based colorimetric methods for sulfide assay
Analytical probe
Linear range (mM)
LOD (mM)
Ref.
Ag nanoparticles
Ag nanoparticles
Au nanoparticles
Au nanoparticles
Au nanoparticles
Au/Ag nanorods
Au/AgI nanoparticles
Au@Pt nanoparticles
Cu nanoclusters
Cu nanoparticles
Pd nanocrystals
0.5–7.5
0.8–6.4
5–15
3–10
0.5–10
0.5–5
0.15
0.35
3
0.2
0.08
0.5
0.5
0.0075
0.5
8.1
0.08
30
33
34
35
36
37
38
39
0.5–5
0.01–0.1
0.5–20
12.5–50
0.125–2.75
40
41
This work
the absorbance change at 652 nm reached B0.5 when 2.5 mM that the present method has great potential applications for the
HSꢁ was introduced. In contrast, the values were B0.1 or less detection of sulfide in environmental samples.
when other common ions of the same concentration were
Universality of this method for sulfide assay
added (i.e., one fifth of the signal that was observed in the
presence of HSꢁ). These results indicate that common ions In addition, the universality of peroxidase-like activity and
have a negligible effect on the absorbance values of the system its application for sulfide assay was further studied using Pd
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nanocrystals with an octahedral shape as the nanozyme. In other words, Pd nanocrystals with different shapes and
As shown in Fig. 5, the as-prepared truncated Pd octahedrons relatively small sizes are able to be used as a universal nano-
are uniform in both size and shape, with an edge length of zyme and thus applied for sulfide detection.
B20 nm. Similar to PdNCs, the Pd octahedrons also have the
ability to catalyze the oxidation of TMB in the presence of H2O2,
and their peroxidase-like activity is also able to be inhibited in
Conclusions
the presence of NaHS. The detectable range and sensitivity for
sulfide assay are the same as those of PdNCs (Fig. S7, ESI†), and
the method involved in Pd octahedrons also shows good selec-
tivity toward sulfide besides other common ions (Fig. S8, ESI†).
In summary, the peroxidase-like activity of Pd nanocrystals was
precisely regulated through surface inactivation by sulfide.
Based on this mechanism, a reliable colorimetric method for
sulfide assay was successfully developed. This method is simple,
fast (complete within 5 min), and highly sensitive (LOD: 80 nM).
The high detection sensitivity relies on the ultrahigh peroxidase-
It is known that different shapes or facets of Pd nanocrystals
usually show distinct catalytic activity;42 however, herein the Pd
nanocrystals with cubic and truncated octahedral shapes dis-
like activity of Pd nanocrystals and the efficient inhibition toward
played almost the same peroxidase-like activity and thus the
performance in the detection of sulfide. This result might be
with Pd nanocrystals and thus the formation of PdS layers allow
attributed to the relatively small sizes of the Pd nanocrystals
that were used in the present system, where the shape/facet-
the present method will find widespread use in quantifying sulfide
dependent catalytic activity and selectivity are negligible.
this catalytic activity by sulfide. The specific interaction of sulfide
selective sulfide assay besides other common ions. We believe that
ions or hydrogen sulfide in environmental scenarios or biological
processes. Moreover, Pd nanocrystals with superior peroxidase-like
activity can also be extended to other systems for sensing.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science Founda-
tion of China (No. 21775014, 21505012, and 81501837), Natural
Science Foundation of Chongqing, China (No. cstc2017jcyjAX0368),
and Achievement Transfer Program of Institutions of Higher
Education in Chongqing (No. KJZH17112). Y. W. was also sup-
ported by the Program for Top-Notch Young Innovative Talents
of Chongqing Normal University (No. 02030307-00026).
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