Colorimetric and fluorescent dual-mode strategy for sensitive detection of sulfide: Target-induced horseradish peroxidase deactivation

Article abstract of DOI:10.1016/j.saa.2020.118296

Environmental pollution caused by sulfide compounds has become a major problem for public health. Hence, accurate detection of sulfide anions (S^2?) level is valuable and vital for environmental monitoring and protection. Here, we report a new colorimetric/fluorescent dual-mode sensor for the determination of S^2? based on the inhibition of enzyme activity and the unique optical properties of produced 2,3-diaminophenazine (DAP), thus making the analytical results more convincing. In this strategy, horseradish peroxidase (HRP) enzyme is used for catalyzing the H₂O₂-mediated oxidation of o-phenylenediamine (OPD) to produce DAP, and the color changed to bright yellow and produced orange yellow fluorescence. But the presence of S^2? could cause the deactivation of HRP, which decreased the amount of DAP and consequently resulted in a substantial SPR band fading and an evident fluorescence quenching simultaneously. The mechanism of S^2? sensor was examined by combining the UV–vis absorption spectra, fluorescence spectra and electrospray ionization mass spectrometry analysis. Under optimal conditions, the colorimetric and fluorescent linear responses of the proposed method exhibited a wide linear range from 2.5 nM–7.5 μM with ultralow detection limits of 1.2 nM and 0.9 nM, respectively. Some potential interferents (such as F^?, Cl^?, Br^?, I^?, SO₄ ^2?, SO₃ ^2?, SCN^?, H₂PO₄ ^?, HPO₄ ^2?, Ac^?, NO₃ ^?, CO₃ ^2?) in real samples showed no interference. Moreover, the proposed method offered advantages of simple, low-cost instruments and rapid assay without the utilization of nanomaterials and has been successfully applied to determine S^2? content in lake water samples with satisfying recoveries over 97.6%. More importantly, the present S^2? sensor not only afforded a new optical sensing pattern for bioanalysis and environment monitoring, but also extends the application field of HRP-catalyzed OPD–H₂O₂ system.

Full text of DOI:10.1016/j.saa.2020.118296

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Colorimetric and fluorescent dual-mode strategy for sensitive
detection of sulfide: Target-induced horseradish peroxidase
deactivation
Jie Bao, Shuxin Xu, Lihua Zhao, Guoyu Peng, Haifeng Lu
PII:
S1386-1425(20)30274-2
https://doi.org/10.1016/j.saa.2020.118296
SAA 118296
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 21 November 2019
Revised date:
20 March 2020
21 March 2020
Accepted
date:
Please cite this article as: J. Bao, S. Xu, L. Zhao, et al., Colorimetric and fluorescent dual-
mode strategy for sensitive detection of sulfide: Target-induced horseradish peroxidase
deactivation, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
(2020), https://doi.org/10.1016/j.saa.2020.118296
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© 2020 Published by Elsevier.

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Colorimetric and fluorescent dual-mode strategy for sensitive detection of
sulfide: Target-induced horseradish peroxidase deactivation
Jie Baoa, Shuxin Xub,Lihua Zhaoa, Guoyu Penga, Haifeng Lua,*
a
Department of Pharmacy, Anhui Provincial Corps Hospital of Chinese People’s Armed
Police Forces, Hefei 230061, PR China.
b
State Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and
Chemical Engineering, Nanjing University, Nanjing 210023, China
* Corresponding author.
Tel: +86-551-64286510, Fax: +86-551-64286531.
E-mail address: lhfshengeryuan@163.com
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ABSTRACT
Environmental pollution caused by sulfide compounds has become a major problem for
public health. Hence, accurate detection of sulfide anions (S2-) level is valuable and vital for
environmental monitoring and protection. Here, we report a new colorimetric/fluorescent
dual-mode sensor for the determination of S2- based on the inhibition of enzyme activity and
the unique optical properties of produced 2,3-diaminophenazine (DAP), thus making the
analytical results more convincing. In this strategy, horseradish peroxidase (HRP) enzyme is
used for catalyzing the H2O2-mediated oxidation of o-phenylenediamine (OPD) to produce
DAP, and the color changed to bright yellow and produced orange yellow fluorescence. But
the presence of S2- could cause the deactivation of HRP, which decreased the amount of DAP
and consequently resulted in a substantial SPR band fading and an evident fluorescence
quenching simultaneously. The mechanism of S2- sensor was examined by combining the
UV–vis absorption spectra, fluorescence spectra and electrospray ionization mass
spectrometry analysis. Under optimal conditions, the colorimetric and fluorescent linear
responses of the proposed method exhibited a wide linear range from 2.5 nM–7.5 μM with
ultralow detection limits of 1.2 nM and 0.9 nM, respectively. Some potential interferents
2-
2-
-
2-
-
2-
(such as F-, Cl-, Br-, I-, SO4 , SO3 , SCN-, H2PO4 , HPO4 , Ac-, NO3 , CO3 ) in real samples
showed no interference. Moreover, the proposed method offered advantages of simple,
low-cost instruments and rapid assay without the utilization of nanomaterials and has been
successfully applied to determine S2- content in lake water samples with satisfying recoveries
over 97.6 %. More importantly, the present S2- sensor not only afforded a new optical
sensing pattern for bioanalysis and environment monitoring, but also extends the application
field of HRP-catalyzed OPD–H2O2 system.
Keywords: Dual-mode strategy; sulfide; horseradish peroxidase deactivation;
o-phenylenediamine; hydrogen peroxide;
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1. Introduction
Sulfide anions (S2-) are one of the widespread toxic anion pollutants which come from
diverse sources including nature and human activities [1]. The application of
sulfur-containing chemicals in many industrial processes and immobilization of
sulfur-containing minerals by bacteria are responsible for the presence of S2- in wastewater
efuents and natural waters, respectively [2]. As a toxic pollutant, S2- is of great concern for
the environment, and the concentration of S2- is regarded as an important environmental
index for spring, surface and waste water [3]. Physiologically, S2- combines with protons to
form HS− or H2S and chronic exposure to protonated sulfide anions would pose a great threat
to human, such as diabetes, Down's syndrome, Alzheimer's disease and liver cirrhosis [4-7].
Hence, monitoring the level of S2- by using simple and low-cost assays is of great importance
in biological and environmental fields.
Many traditional analytical methods, such as ion chromatography [8], capillary
electrophoresis [9], inductively coupled plasma atomic emission spectroscopy [10] and
electrochemical assays [11,12] have been established for the detection of S2-. Unfortunately,
these methods usually require high costs, time-consuming, complicated pretreatment
processes and specific experts to run the systems, which limit their wide application.
Compared with these methods, optical methods have drawn much attention because of low
cost, high sensitivity and rapidity, and even can be easily monitored by naked eyes. In recent
years, various colorimetric/fluorescent probes including Au@Ag core-shell nanoparticles
NPs (Au@Ag NPs) [13], 3,3′,5,5′-tetramethylbenzidine (TMB) [14], L–DOPA stabilized
AgNPs [15], 4-nitrophenol (4-NP) [16], carbon dots (CDs) [17], yeast extract-stabilized Cu
nanoclusters (Cu NCs) [18], Ce(III) ions functionalized silver nanoclusters (Ce(III)/AgNCs
complex) [19], sulfanilic acid and glutathione functionalized graphene quantum dots
(SSGQDs) [3] have been developed, which provide an effective sensing platform for the
detection of S2-. For example, Deng and co-workers reported a convenient colorimetric
method for the determination of S2- based on the target-induced shielding against the
peroxidase-like activity of GNPs [20]. Barati et al. established two novel metal-ion-mediated
fluorescent probes based on CDs for the indirect detection of S2- [21]. Despite the
importance of above optical sensors, their use still shows drawbacks. On the one hand, most
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of the sensors suffered from the disadvantages of high cost, time-consuming and complex
preparation or modification of nanomaterials, which strongly limits their commercial
applications in many fields. On the other hand, compared with multi-signal sensing
techniques, single mode-based assays offered only one kind of output signal, thus lacking
attractive diversity and improved accuracy [22]. Considering these, developing a new
dual-mode sensing platform with simplicity, sensitivity and efficiency for the detection of S2-
was still highly desired.
O-phenylenediamine (OPD) could be oxidized by hydrogen peroxide (H2O2) to obtain
2,3-diaminophenazine (DAP) [23-25]. DAP is bright yellow colored and exhibits an orange
yellow fluorescence under the irradiation of ultraviolet light. Thus, DAP is a kind of
promising chromophore and organic fluorophore. The chromogenic response has already
been utilized to develop a series of assays for relevant reactants in the previous literatures
[26-29]. However, to the best of our knowledge, no literature reported about the
target-induced horseradish peroxidase deactivation for dual-mode sensing of S2-.
In the study, we developed a new, convenient and dual-mode sensor with both
colorimetric and fluorometric readout for detecting S2- by using OPD as the substrate.
Previous reports have shown that H2O2 could oxidize OPD via the catalysis of horseradish
peroxidase (HRP) to produce DAP, showing a bright yellow color as well as an orange
yellow fluorescence when irradiated by ultraviolet light [25]. More interestingly, the
dual-mode response of the OPD/ H2O2/HRP sensing system can be inhibited by introducing
the S2-, revealing the deactivation of HRP by S2-. Based on the above facts, a rapid and
sensitive method for S2- detection was established and successfully applied to analyze S2- in
lake water samples. This is the first method in which OPD/H2O2/HRP system has been used
for the dual-mode sensing of S2-.
2. Experimental
2.1. Reagent and chemicals
All chemicals were of analytical grade and used without further purification.
O-phenylenediamine (OPD) and 2,3-diaminophenazine (DAP) were purchased from Tianjin
Zhiyuan Chemical Reagent Co., Ltd. (Tianjin, China). Horseradish peroxidase (HRP, 300
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units/mg) was obtained from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China).
Sodium sulfide (Na2S·9H2O, 98%), hydrogen peroxide (30 wt% H2O2), sodium acetate
(NaAc), acetic acid (HAc), potassium iodide (KI), sodium nitrate (NaNO3), sodium
hydroxide (NaOH) and sodium carbonate (Na2CO3) were achieved from Sinopharm
Chemical Reagent Co., Ltd. (Shanghai, China). Sodium fluoride (NaF), sodium chloride
(NaCl), sodium bromide (NaBr), sodium sulphate (Na2SO4), sodium sulfite (Na2SO3),
potassium thiocyanate (KSCN), Disodium hydrogen phosphate (Na2HPO4), sodium
dihydrogen phosphate (NaH2PO4) acquired from Nanjing Nanshi Chemical Reagent Co., Ltd.
(Nanjing, China). Ultrapure water (18.2 M cm-1) prepared by a Millipore system (Millipore,
USA) was used throughout the whole experiments.
2.2. Instruments.
UV–visible absorption spectra were recorded using an Agilent 8453 UV–vis
spectrometer (Agilent, USA). Fluorescence measurements were performed on a Shimadzu
RF-5301 PC fluorescence spectrophotometer. Electrospray ionization mass spectrometry
(ESI-MS) was carried out on a Finnigan LCQ ion trap mass spectrometer (Waters MS
Technologies, Manchester, UK). The pH value of buffer was adjusted by titrating with HAc
and controlled by a PHS-3C pH meter (Leici, Shanghai).
2.3. Procedure for dual-mode sensing assay
A typical dual-mode sensing assay for S2- was carried out as follows. Firstly, 50 μL of
0.15 μg mL-1 HRP and 100 μL of S2- standard solution with various concentrations were
mixed in 1500 μL of HAc/NaAc buffer (50 mM, pH 6.0). Secondly, 200 μL of 0.2 mM OPD
solution and 100 μL of 5 mM H2O2 were successively added to each of the mixture solutions,
which were shaken and equilibrated at incubation temperature of 20 °C for 15 min. Finally,
NaOH (50 μL, 0.5 M) was added to the mixture solutions to end the enzymatic reaction.
Such solutions were transferred separately into 1 cm quartz cuvette and directly monitored
by UV–vis spectrometer and fluorescence spectrophotometer, respectively. The ΔA
417nm
signal (ΔA = A0–A, where A0 and A are the absorbance at 417 nm in the absence and
presence of S2-) and fluorescence quenching efficiency (Effq = (F0–F)/F0, where F0 and F are
the fluorescence intensity at 560 nm in the absence and presence of S2-) were used as a
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criterion to appraise the performance of colorimetric and fluorescent sensor, respectively.
2.4. Specificity of the sensor
Selectivity is another key parameter in sensing assay. To evaluate the selectivity of
dual-mode sensor for detecting S2-, several potential interfering substances (F-, Cl-, Br-, I-,
2-
2-
-
2-
-
2-
SO4 , SO3 , SCN-, H2PO4 , HPO4 , Ac-, NO3 , CO3 ) coexisting in real water samples were
selected and pre-mixed with 100 μL of 25 μM S2-. Afterwards, they were separately added to
the HRP-catalyzed OPD–H2O2 sensing system. All experiments were carried out under the
optimized conditions.
2.5. Analysis of S2- in real samples
For real samples analysis, lake water samples were collected from Chaohu Lake (Hefei,
China), and the analysis was conducted after the sample collection. Firstly, the water samples
were filtered with 0.22 μm Millipore filter to remove the insoluble components. Then, a
series of spiked samples was prepared by adding standard solutions containing various
concentrations of S2- in water samples. Following that, the resulting solutions were added
into the OPD/ H2O2/HRP sensing system as mentioned. Finally, content of S2- in lake water
samples was calculated according to linear equation obtained from a standard solution of S2-.
3. Results and Discussion
3.1. Design and principle of the sensing system
The working principle of the designed dual-mode method for sensing the S2- is
schematically shown in Scheme 1. Previous report has shown that OPD as a substrate could
be oxidized by H2O2 via the catalysis of HRP to form DAP, exhibiting a bright yellow color
as well as an orange yellow fluorescence when irradiated by ultraviolet light [25]. However,
the introduction of S2- can cause the deactivation of HRP and, hence, further affects the
amount of DAP produced, which is accompanied by a substantial SPR band fading and an
evident fluorescence quenching simultaneously. Thus, a new dual-mode sensor for the
detection of S2- was established by coupling three reactions, which can be expressed by the
following equations:
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To demonstrate the proposed sensing mechanism, several control tests were
implemented under the different conditions by using visual color readout, UV–vis absorption
and fluorescence spectrophotometer. As indicated from Fig. 1A and B, the control groups
(pure OPD and OPD/H2O2) always remained colorless and exhibited negligible fluorescence
at incubation temperature of 20 °C for 15 min (curve a, b). In contrast, as a result of adding
HRP into the OPD/H2O2 solution, an obvious absorption peak at 417 nm and corresponding
fluorescence emission peak at 560 nm were observed ((curve c), which is consistent with the
results that of pure DAP aqueous solution (Fig.S1). Furthermore, the molecular ion peak at
+
211.1 (m/z, [M+H] ) ascribed to DAP can be obviously observed in the mass spectrometry
(Fig. S2). These results indicated HRP essentially catalyzes the oxidative reaction of
OPD/H2O2 system to form DAP. However, the introduction of S2- into the OPD/H2O2/HRP
system resulted in a substantial SPR band fading and an evident fluorescence quenching
simultaneously (curve d), indicating the presence of S2- can cause the deactivation of HRP
which affects the amount of DAP produced. Meanwhile, the corresponding
color/fluorescence change of different test solutions were shown in Fig. 1C and D, which
provide a convenient and instrument-free mean for the readout of S2- sensing.
3.2. Optimization of detection conditions
Based on HRP manufacturer suggestion (https:// www.aladdin-e.com), the sensing
system should be better carried out at an effective working pH of 6.0 and an effective
incubation temperature of 20 °C. Under these conditions, other possible experimental
parameters including HRP concentration, incubation time, OPD concentration, H2O2
concentration and ending agent were investigated in detail. ΔF was used as a criterion to
optimize the detection conditions and defined as F0–F, where F0 and F are the fluorescence of
sensing system in the absence and presence of 1.25 μM S2-, respectively.
The concentration of HRP has great influence on the performance of the proposed assay,
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which has been optimized first. Fig. 2A shows that ΔF increased with the increment of HRP
concentration up to 3.75 ng/mL, above which it decreased. So, 3.75 ng/mL of HRP was
selected for the following study. Different incubation time ranging from 3 min to 21 min was
investigated to study its effect on the ΔF. The result shows that the highest ΔF is obtained at
15 min, and then the ΔF decreases with the increase of incubation time (Fig. 2B), which has
been chosen as the optimal time for the following study. The effect of the concentration of
OPD and H2O2 on the ΔF was displayed in Fig. 2C and D, respectively. It can be obviously
seen that the concentration of OPD and H2O2 being too high or too low is not suitable for S2-
detection. The experiments show that the highest ΔF is obtained when the OPD
concentration is 20 μM and the H2O2 concentration is 0.25 mM. Therefore, 20 μM of OPD
and 0.25 mM of H2O2 were used for the dual-mode assay. Fig. S3A and B indicate the
fluorescence spectrum of OPD/H2O2/HRP/S2- system with adding 12.5 mM of NaOH is
more stable than that in the absence of NaOH. Thus, 12.5 mM of NaOH in this study was
used to end the enzymatic reaction.
3.3. Linearity and detection limit
Under the optimal conditions employed here, the sensitivity and dynamic measurement
range of colorimetric and fluorescent dual-mode sensor for detecting S2- was evaluated. Fig.
3A depicts the UV–vis absorption spectra of the OPD/H2O2/HRP sensing system with
different concentrations of S2- ranging from 0 to 7.5 μM. It is clearly observed that the
absorption intensity decreased gradually with increasing S2- concentration, which was
attributed to the deactivation of HRP by S2-. Fig. 3B shows a typical S2- concentration–
response curve, and the ΔA 417nm increased linearly with the logarithm of the concentration of
S2- ranging from 2.5 nM to 7.5 μΜ (Y1 = 0.1071 Log (C) + 0.2805, R2 = 0.9931) (inset of Fig.
3B). Likewise, the fluorescence spectrum of S2--treated OPD/H2O2/HRP system with
different concentrations from 0 to 7.5 μM was also observed, as shown in Fig. 3C. Fig. 3D
shows a typical S2- concentration–response curve, and a satisfying linear relationship
between the (F0-F)/F0 signal and the logarithm of S2- concentrations in the range from 2.5
nM to 7.5 μM, Y2 = 0.2959 Log (C) + 0.7488, was shown in the inset of Fig. 3D. Meanwhile,
the photographs of OPD/H2O2/HRP system with different concentrations of S2- under
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ambient light and a 365 nm UV lamp were taken. A dramatic color change from bright
yellow to pale yellow and an obvious fluorescence intensity transformed from strong to weak
can be observed (Fig. 3E). On the basis of dual-mode signal response, the S2- level could be
simply visualized by the naked eye and further quantified by UV-vis or fluorescence spectra.
The detection limits of colorimetric and fluorescent sensor were calculated by a
signal-to-noise ratio of 3 (3σ/B). Here, σ represents the standard deviation of blank
measurements, and B means the slope of linear equation. Thus, the colorimetric and
fluorescent sensor can be used to detect S2- with a detection limit of 1.2 and 0.9 nM,
respectively. A comparison of characteristics between the proposed method and other S2-
sensors reported elsewhere was summarized in Table 1 [20, 30-36]. The detection limit and
linear range of S2- in this work were comparable or even better than those of reported sensors.
In addition, the repeatability of the dual-mode biosensor was also evaluated by five repeated
measurements of 1.25 uM of S2- under optimal experiment conditions, and the relative
standard deviation (RSD) was 1.0 % and 1.2 %, demonstrating the reliability of the proposed
method (Fig. S4).
3.4. Selectivity
Selectivity is another key parameter in sensing assay. To evaluate the selectivity of the
dual-mode sensor for detecting S2-, several potential interfering compounds coexisting in real
2-
2-
-
2-
-
sample, including F-, Cl-, Br-, I-, SO4 , SO3 , SCN-, H2PO4 , HPO4 , Ac-, NO3 , CO32- were
examined. As shown in Fig. 4A, the ΔA 417nm and the (F0-F)/F0 signal in the presence of S2-
with single other ions were almost similar to those in the presence of S2- without interfering
substances. Moreover, the color/fluorescence changes of different tested solutions in Fig. 4B
and C were in good accordance with the above corresponding ΔA 417nm and (F0-F)/F0 signal,
respectively. These results demonstrate that only target S2- could result in the deactivation of
HRP, and the potential interfering substances had no evident effect on the
absorbance/fluorescence intensity and the color of tested solutions.
3.5. Real sample assay
To test the feasibility of the developed sensor for practical application, we applied it to
determine the concentration of S2- in lake water samples. The recovery experiments with
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adding different concentrations (100, 300 and 500 nM) of S2- standard solutions were carried
out and the results are shown in Fig. 5. According to above linear equation, the average
concentration of S2- in lake water samples (1 and 2) were calculated to be 108.4±3.6,
297.5±10.8, 514.8±16.1, 101.5±4.1, 292.9±9.7, 510.1±18.2 nM as well as 103.8±4.4,
306.1±11.5, 509.7±18.7, 99.3±4.3, 302.1±12.5, 508.2±17.9 nM by colorimetric and
fluorometric measurements, respectively. The recoveries of known amount of S2- in lake
water samples were between 97.6% and 108.4% with the RSD ranging from 3.1% to 4.3%.
These results reveal that the proposed sensor was of high accurate and could be applied for
rapid detection of S2- in real samples in a simple way.
4. Conclusions
In summary, we constructed a label-free and reliable colorimetric and fluorescent
dual-mode sensor for the quantitative detection of S2- based on the OPD/H2O2/HRP system.
In the study, OPD could oxidized by H2O2 via the catalysis of HRP to produce DAP, while
the introduction of S2- induced the deactivation of HRP, which decreased the amount of DAP
and consequently resulted in a substantial SPR band fading and an evident fluorescence
quenching simultaneously. Under the optimal conditions, the dual-mode sensor has a good
dynamic range and a relatively low detection limit of 1.2 nM and 0.9 nM by colorimetric and
fluorometric measurements, respectively. More importantly, this method is successfully
applied to the detection of S2- in lake water samples with high simplicity, feasibility and
reproducibility, illustrating its great potential in biological and environmental fields.
Compared with the previously reported method [37], the present sensing platform has several
extraordinary features as following: (1) the developed method does not require complicated
synthetic procedures and specific modification approach, making it more convenient and
easy operations; (2) The sensing assay can present colorimetric and fluorescent dual-readout
signal, making the experimental results more convincing.
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Conflicts of interest
There are no conflicts of interest to declare.
Acknowledgments
This research did not receive any specific grant from funding agencies in the public,
commercial, or not-for-profit sectors.
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Figure captions
Scheme 1. (A) Schematic illustration of the dual-mode sensor for the detection of S2- based
on target-induced deactivation of horseradish peroxidase. (B) Photographs and UV−vis
absorption/fluorescence spectra in the presence (1) and absence (2) of S2-.
Fig. 1. (A) Absorbance and (B) fluorescence emission spectra of (a) OPD, (b) OPD + H2O2,
(c) OPD + H2O2 + HRP, and (d) OPD + H2O + HRP + S2- in 50 mM HAc/NaAc buffer at pH
6.0. The corresponding photographs under (C) visible light and (D) 365 nm UV light. The
final concentrations of the OPD, HRP, H2O2 and S2-, are 20 μM, 3.75 ng/mL, 0.25 mM and
1.25 μM respectively.
Fig. 2. Optimization of the reaction conditions for the detection of S2-. Plots of fluorescence
intensity in the absence and presence of S2- and plots of ΔF signal versus the (A) HRP
concentrations (1.25–6.25 ng/mL), (B) reaction time (from 3 to 21 min), (C) OPD
concentrations (5–37.5 μM) and (D) H2O2 (0.05–0.5 mM). The final concentrations of the
OPD, HRP, H2O2 and S2-, are 20 μM, 3.75 ng/mL, 0.25 mM and 1.25 μM, respectively.
Fig. 3. (A) UV-vis absorption spectra of the OPD/H2O2/HRP sensing system upon addition
of various concentrations of S2- (from 0 to 7.5 μM). (B) A dose–response curve between S2-
concentration and the ΔA 417nm signal under optimum conditions (inset: linear calibration plot
versus the logarithm of S2- concentrations). (C) Typical fluorescence emission spectra of the
OPD/H2O2/HRP sensing system in the presence of varying S2- concentrations (from 0 to 7.5
μM). (D) A dose–response curve between S2- concentration and the (F0-F)/F0 signal under
optimum conditions (inset: linear calibration plot versus the logarithm of S2- concentrations).
(E) Photographs of the tested solutions under (1) ambient light and (2) 365 nm UV lamp.
Error bars represent the standard deviations (n = 3).
Fig. 4. (A) Selectivity of the dual-mode sensing system toward various potential interfering
substances. (B) Photographs (ambient light and UV light) of the solutions from 1 to 14
2-
represent (1) control, (2) S2- + F-, (3) S2- + Cl-, (4) S2- + Br-, (5) S2- + I-, (6) S2- + SO4 , (7)
2-
-
2-
-
S2- + SO3 , (8) S2- + SCN-, (9) S2- + H2PO4 , (10) S2- + HPO4 , (11) S2- +Ac-, (12) S2- + NO3 ,
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2-
(13) S2- + CO3 and (14) S2-, correspondingly. The concentration of other potential
interfering substances and S2- was 1.25 μM.
Fig. 5. Dual-mode detection of the content of S2- for the lake water samples spiked with
different concentrations (100, 300 and 500 nM) of S2- standard solutions. The error bars
represent the standard deviation (n=3).
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Table 1 Comparison of different sensors for S2- detection.
Linear range
System
Signal output
LOD (nM)
References
(μM)
iodide/Cu@Au nanoparticle
ABTS–Au (III) system
Colorimetry
Colorimetry
0 – 10
300
280
[30]
[31]
0.5 – 15
β-FeOOH) nanorods/TMB/H2O2
Colorimetry
Colorimetry
5 – 30
2190
80
[32]
[20]
bare Au nanoparticles
Cyclen-capped FPNsa–Cu2+
complex
0.5 – 10
Fluorescence
Fluorescence
0 – 80
2100
60
[33]
[34]
CDs/PEI/NBb/Cu2+
0.1 – 8.0
MTTc-AuNDs
Fluorescence
Fluorescence
0.87 – 16
0 – 9
2
[35]
[36]
DNA-Au/Ag NCs
0.83
Colorimetry/fluores
cence
OPD/H2O2/HRP
0.0025 – 7.5
1.2/0.9
this work
a fluorescent polymeric nanoparticles
b polyethyleneimine and Nile Blue
g 6-mercapto-striazolo(4,3-b)-s-tetrazine
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Declaration of competing interests
☒ The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
☐The authors declare the following financial interests/personal relationships which may be
considered as potential competing interests:
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Credit Author Statement
Jie Bao conceived the project and prepared writing-original draft. Mass
spectrometry experiment was done by Shu-Xin Xu. Lihua Zhao and Guoyu
Peng collected and processed the water samples included in this study. Haifeng
Lu contributed to review, editing and funding acquisition.
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Graphical abstract
Highlights
 A new and label-free colorimetric and fluorescent dual-mode sensor was designed to
detect S2-.
 The sensing mechanism is designed by target-induced horseradish peroxidase
deactivation.
 The detection limits of S2- are as low as 1.2 nM and 0.9 nM by colorimetric and
fluorometric method, respectively.
 The proposed method is successfully applied to the detection of S2- in real samples.
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