Electropolymerization-Induced Positively Charged Phenothiazine Polymer Photoelectrode for Highly Sensitive Photoelectrochemical Biosensing

Article abstract of DOI:10.1021/acs.analchem.9b03311

Exploring the fabrication of an electrode with high photoelectric conversion efficiency and abundant functional groups for ideal photoelectrochemical (PEC) sensor development is highly urgent but faces a significant challenge. Herein we report an electropolymerization strategy for the preparation of phenothiazine polymeric film on an indium tin oxide (ITO) surface (PPT/ITO), within only a few seconds, and monomers. The fabricated PPT/ITO electrode possessed excellent stability and abundant quaternary ammonium salt groups for developing a highly sensitive PEC sensor through electrostatic binding with negatively charged materials. In this context, a CdS QDs-functionalized PPT/ITO electrode (CdS/PPT/ITO) was proposed and applied to the analysis of chlorpyrifos, used as a model target organophosphorous pesticide (OP). The thiocholine generated from acetylcholinesterase (AChE)-induced catalyzed hydrolysis of acetylthiocholine (ATCh) efficiently directed CdS QDs away from PPT/ITO via electrostatic repulsion, subsequently decreasing PEC current, whereas chlorpyrifos prohibited the generation of thiocholine through inhibiting AChE activity. As compared to the case where chlorpyrifos is absent, significantly enhanced PEC current is determined and is proportional to chlorpyrifos amounts. Thus, the developed CdS/PPT/ITO-based PEC sensor achieved excellent chlorpyrifos biosensing with improved sensitivity down to approximately ng/mL level with good specificity. We envision the proposed strategy will provide a new path to conveniently fabricate photoelectrodes possessing high performance, which will have more useful applications in PEC sensing.

Full text of DOI:10.1021/acs.analchem.9b03311

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Article
Electropolymerization-Induced Positively Charged Phenothiazine Polymer
Photoelectrode for Highly Sensitive Photoelectrochemical Biosensing
Jiao Wang, Wenxin Lv, Jiahui Wu, Haiyin Li, and Feng Li
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b03311 • Publication Date (Web): 27 Sep 2019
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Analytical Chemistry
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Electropolymerization-Induced Positively Charged
Phenothiazine Polymer Photoelectrode for Highly Sensitive
Photoelectrochemical Biosensing
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Jiao Wang, Wenxin Lv, Jiahui Wu, Haiyin Li,* and Feng Li*
College of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University,
Qingdao 266109, People’s Republic of China
* Corresponding author. Tel/Fax: 86-532-86080855;
E-mail: lifeng@qau.edu.cn (F. Li)
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ABSTRACT
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Exploring ingenious strategy to fabricate electrode with high photoelectric conversion
efficiency and abundant functional groups for ideal photoelectrochemical (PEC) sensor
development is highly urgent but faces a significant challenge. Herein, we report an
electropolymerization strategy for preparation of phenothiazine polymeric film on ITO surface
(PPT/ITO) within only a few seconds and monomers. The fabricated PPT/ITO electrode possessed
fabulous stability and abundant quaternary ammonium salt groups for developing highly sensitive
PEC sensor through electrostatic binding with negatively charged materials. In this context, CdS
QDs-functionalized PPT/ITO electrode (CdS/PPT/ITO) was proposed and applied to analysis of
chlorpyrifos, used as the model target organophosphorous pesticide (OP). The thiocholine
generated from acetylcholinesterase (AChE)-induced catalyzed hydrolysis of acetylthiocholine
(ATCh) efficiently made CdS QDs away from PPT/ITO via electrostatic repulsion, subsequently
decreasing PEC current. Whereas, chlorpyrifos prohibited the generation of thiocholine through
inhibiting AChE activity. As compared to the case that chlorpyrifos is absent, significantly
enhanced PEC current is determined, and is proportional to chlorpyrifos amounts. Thus, the
developed CdS/PPT/ITO-based PEC sensor achieved fantastic chlorpyrifos biosensing with
improved sensitivity down to ~ng/mL level in a good specificity-manner. We envision the
proposed strategy will provide a new path to conveniently fabricate photoelectrode with high
performance, and the fabricated photoelectrode will display more useful application in PEC
sensing.
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INTRODUCTION
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Advance in photoelectrochemical (PEC) sensing system over the past few years has created a
significant affect on the diagnosis of various diseases due to their advantages of electrochemistry
and photochemistry.1-5 In PEC system, electrode can convert light into current, and then is
regarded as the most critical component, thus attracting substantial attention to explore new
manufacturing technology and architecture. Despite recent progresses, the design and
development of electrodes are still depended on drop coating method,6-8 which easily leads to
unrepeatability and instability, subsequently precluding their PEC sensing applications. In addition,
diverse materials such as TiO2, 9,10 ZnO,11,12 and CdS13,14 have been used as photoactive elements
for development of photoelectrodes, but their applications are quite astricted as a result of lack of
functional groups. Therefore, PEC electrodes are currently facing two great challenges: poor
manufacturing technology and lack of functionality, which force us to conquer them for PEC
sensing applications.
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Conjugated polymers, which have broad light absorption range, tunable redox active properties
and abundant functional groups, are recognized as ideal candidates to fabricate the
photoelectrodes.15-18 For example, 1,4-diethynylbenzene polymer has been used as functional
material for photoelectrochemical water reduction.17 In spite of the wide investigations, these
aromatic polymers are currently prepared through the chemical polymerizations,19 such as Suzuki
reaction,20 Heck reaction,21 Wittig-Horner reaction,22 and oxidative reaction,23 and are limited by
the following obstacles: 1) Expensive precious metal catalysts are required in the polymerization
process;20 2) The prepared polymers possess poor repeatability and solubility because of the rigid
chains and strong interaction force, making difficult to forming film or layer on electrode
surface;16 3) The process is both tedious and complicated, and produces large numbers of waste.
From this context, it is highly desirable to propose an effective and practical film-forming
approach for conjugated polymers for fabricating the photoelectrodes.
Recent studies demonstrate that electropolymerization can be used as an alternative tool for
fabricating conjugated polymers-modified electrodes without the need of any catalyst and
template.24,25 Regarding the exceptional processability of conducting polymers obtained from
electropolymerization, substantial progress has been made towards the development of electrodes
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from various aromatic monomers via electropolymerization.26,27 Among these monomers,
phenothiazine and its derivatives, comprising of electron-rich N and S, are recognized as the
highly popular building units for fabricating the electrodes due to the reversible radical cation
formation and tunable redox property.28-31 The inherent features enable people to develop
polyphenothiazine-modified electrode via electropolymerization for applications in solar cells.32-34
If we introduce phenothiazine polymers into photoelectrodes for developing PEC sensors via
electropolymerization, such developed ones could efficiently address the challenge of film
formation for sensitive and accurate determination of various target biomolecules. However, as far
as we are concerned, these fabricated electrodes still lack functional groups, making adverse to
modification with functional substances, and no effort has been devoted to exploring such
electrode for PEC sensing.
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Herein, by making full use of organic coupling and electropolymerization reactions, a
phenothiazine polymer-modified electrode (PPT/ITO) was successfully fabricated with abundant
quaternary ammonium salt groups on the surface. On the basis of positive groups of the fabricated
PPT/ITO, a facile conjugation strategy for devising PEC sensor was elaborately proposed through
the electrostatic interaction. In the present study, the conjugation was applied for negatively
charged CdS QDs, used as the model conjugates. Scanning electron microscopy (SEM), energy
dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS), PEC, and other
characterizations depict that CdS QDs were effectively decorated on PPT/ITO surface, and the
amount of CdS QDs produced a direct impact on PEC current. We display that CdS/PPT/ITO is a
promising acetylcholinesterase (AChE)-responsive PEC sensor for organophosphorous pesticides
(OPs) assay. CdS/PPT/ITO displays high PEC current, and contrarily, exhibits relatively low PEC
current in the presence of AChE. This decrease is caused by the far separation of PPT/ITO and
CdS QDs induced by thiocholine deriving from AChE-catalyzed hydrolysis of acetylthiocholine
(ATCh). When OP was present in the system, AChE activity would be severely inhibited, and then
no thiocholine generated. As a result, the CdS QDs would be also compactly anchored on
PPT/ITO surface, subsequently hampering the PEC current decrease. Based on the variation of
PEC current, sensitive detection of OP can be readily achieved.
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EXPERIMENTAL SECTION
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Preparation of Phenothiazine Monomer PT. Phenothiazine (3.00 g, 0.015 mol), 1,4-
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dibromobutane (9.72 g, 0.045 mol), and NaH (0.72 g, 0.03 mol) were added into 30 mL of N,N-
dimethylformamide (DMF), and the solution was stirred for 12 h. Subsequently, the reaction
solution was poured into ice water and extracted with dichloromethane three times. The organic
phase was dried using MgSO4 and concentrated through rotary evaporation. Then, intermediate P-
Br was purified by column chromatography (silica gel, CH2Cl2/n-hexane 1/6), and dried in a
vacuum at room temperature overnight.
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Subsequently, the intermediate P-Br was added into 30 mL of acetonitrile, and the solution
was stirred for 10 min to make P-Br fully dissolving. 2.0 mL of triethylamine was added into the
mixture, and the resulting solution was permitted to react for 8.0 h at 60 oC under N2 atmosphere.
Then the end-product PT was gotten with the yield of 84.3 % via recrystallization in toluene
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solvent. H NMR (500 MHz, CDCl3) δ (ppm): 1.25 (m, 9H), 1.52 (d, 2H), 1.73 (d, 2H), 3.06 (d,
2H), 3.24 (d, 2H), 3.28 (m, 6H), 6.68 (d, 2H), 6.75 (d, 2H), 6.91 (d, 2H), 6.99 (d, 2H). FT-IR (KBr)
ν (cm−1): 3453, 2972, 1465, 1268, 1146, 1032, 758.
Fabrication of PPT/ITO Electrode via Electropolymerization. The acetonitrile
solvent comprising of 0.01 M PT and 0.2 M sodium perchlorate was used as electrolyte solution.
The electropolymerization experiment was carried out in a 5.0 mL of electrolyte solution with the
potential of 0-1.5 V and scan rate of 0.1 V/s through using ITO (0.25 cm2), Ag/AgCl, and Pt wire
as working electrode, reference electrode, and counter electrode, respectively. After different
number of CV scans, the electrode was cleaned with acetonitrile to remove the adsorbed PT and
sodium perchlorate, and subsequently dried in a vacuum to afford PPT/ITO electrode.
Fabrication of CdS/PPT/ITO Electrode. 20 µL of CdS QDs solution with different
concentrations was dripped on PPT/ITO electrode, and incubated for 30 min. After that, the
electrode was rinsed with ultrapure water three times to remove the unreacted CdS QDs. The
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fabricated CdS/PPT/ITO electrode was placed in a humid condition at 4 C for the subsequent
determination.
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Detection of Chlorpyrifos Using CdS/PPT/ITO-Based PEC Sensor. The
chlorpyrifos-triggered inhibition reaction was performed in a 40 μL of 0.1 M PBS buffer (5 mM
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KCl, pH 7.4) containing 70 mU/mL AChE and chlorpyrifos with varying concentrations at 37 C
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for 30 min. Subsequently, 160 μL of ATCh solution (0.375 mM) was added. The fabricated
CdS/PPT/ITO electrode was immersed in this 200 μL reaction solution for 35 min. After being
rinsed with ultrapure water three times, the photoelectrode was put into PBS buffer containing
0.001 M AA solution to carry out the PEC experiment under the 430 nm light excitation.
RESULTS AND DISCUSSION
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Design and Characterization of PPT/ITO Electrode. To guarantee the successful
fabrication of the modified electrode with ideal characteristics, monomer PT was elaborately
devised, which contained phenothiazine moiety and quaternary ammonium salt groups, providing
the units for electropolymerization and positively charged groups on the surface, respectively. The
synthetic route for PT was illustrated in Figure 1A, and mainly comprised of organic coupling
reaction and quaterisation with the yield of 84.3%. 1H NMR and FT-IR spectra were employed to
confirm PT’s molecular structure (Figure S1). Further, PT was used as raw material to fabricate
phenothiazine polymer modified electrode (PPT/ITO) using cyclic voltammetry (CV) technique in
acetonitrile solution comprising of 0.20 M sodium perchlorate (Figure 1B). Figure 1C gives the
CV information of PT up to 20 cycles. In the first CV scan, the onset potential of PT located at
0.58 V, corresponding to the oxidation of phenothiazine moiety. Along with the increased
potential, the current elevated rapidly, strongly justifying the oxidation of more PT. Meanwhile, it
should be noted that the current does not obviously increase accompanied with the increase in
number of CV scans, which may be due to the poor conductivity induced by the increase in film’s
thickness. Thus, one CV scan was applied in preparation of PPT/ITO electrode. This approach for
direct fabrication of PPT/ITO electrode is quite excellent due to the fact that it only requires some
seconds and very few monomers.
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Figure 1. (A) Synthetic route for monomer PT. (B) Diagram illustration of fabrication of PPT/ITO
via CV electropolymerization. (C) CV curves (1st to 20th cycles) of PT solution in acetonitrile in
the presence of NaClO4 electrolyte at 25 oC with a scan rate of 0.1 V s-1.
To confirm the successful fabrication of the proposed electrode, CV characterizations were
conducted in PBS buffer (Figure S2A). In comparison with bare ITO electrode, PPT/ITO
displayed a pair of well-defined redox peaks, with the potentials appeared at 0.60 V and 0.55 V,
respectively, corresponding well to the characteristic of phenothiazine polymer that was coated on
ITO surface. Meanwhile, PPT/ITO depicted a significant DPV peak current at 0.58 V compared
with that of ITO (Figure S2B). Furthermore, from the electrochemical impedance spectroscopy
(EIS) measurements, it can be clearly seen that phenothiazine polymer made the electron transfer
resistance (Ret) significantly increasing due to the weak conductivity of phenothiazine polymer
(Figure S2C). In addition, some peaks in EDS and XPS curves appeared, corresponding to C, N,
and S elements deriving from phenothiazine (Figure S2D and Figure S2E). These information
firmly implied the effective deposition of phenothiazine polymeric film on ITO electrode.
Subsequently, the electronic characteristics of phenothiazine polymer film was studied via the
UV-vis absorption spectra (Figure S3). The phenothiazine film obtained from spin coating
demonstrated two absorption peaks located at 254 and 300 nm, respectively, which were
attributable to different π-π* transitions in monomer. By contrast, phenothiazine polymer film
presented red-shift peaks located at 277 nm and 307 nm, respectively, ascribed to the linkage of
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phenothiazine units. Under the radiation of 365 nm light, PPT/ITO exported PEC current with
intensity of 33.17 nA. Changing the excitation wavelength into 430 nm, the current greatly
reduced (Figure S4), corresponding well to the UV-vis spectrum. Moreover, PPT/ITO presented a
smooth and compact surface morphology, which is imperatively important for utilization in
development of PEC sensors (Figure S5). Furthermore, the chemical stability of PPT/ITO
electrode was studied by dispersing it in highly concentrated solution comprising of Na2SO4, KCl,
Mg(NO3)2, and ZnCl2 for 12 h. CV and EIS characterizations revealed that PPT/ITO retained the
peaks and conductivity that are the same as those of untreated PPT/ITO (Figure S6).
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Development of CdS/PPT/ITO Electrode. The abovementioned experimental
information promoted us to explore the development of functional heterojunction photoelectrode
from PPT/ITO, which was conductive to excellent performance. As is well known to all, CdS QDs
are one of the most used n-type semiconductors in preparation of photoelectrode with high
photoelectric conversion efficiency due to low cost, excellent photophysical characters, uniform
morphology, and large surface area, and thus were applied as model conjugates to fabricate the
photoelectrode. In the present study, CdS QDs with Cd2+ rich surface were prepared via a
hydrothermal method by using sodium citrate as stabilizing agent.35 TEM and zeta measurements
demonstrated that the prepared CdS QDs depicted sphere morphology with the size of 4 nm in
diameter (Figure S7A), and possessed a significant negative zeta potential (-20.10 mV), justifying
the distinguished water dispersibility and strong electrostatic interaction toward positively charged
substances. In addition, CdS QDs show two absorption bands at 363 nm and 423 nm with the
molar absorptivity of 367000 M-1cm-1 and 323300 M-1cm-1, respectively, benefitting the highly
efficient visible light photo-excitation (Figure S7B).
The diagram illustration of interaction of PPT/ITO and CdS QDs was manifested in Figure 2A.
PPT/ITO has quaternary ammonium salt groups on the surface, and thus can adsorb negatively
charged CdS QDs to fabricate the heterojunction photoelectrode. As expected, PEC current
gradually increased with the elevating of CdS QDs amount from 0 to 2.0 μM under 430 nm light
irradiation due to the electrostatic adsorption (Figure 2B). However, when CdS QDs amount
exceeded 2.0 μM, no evident change in PEC current was observed, arising from the saturated
adsorption of CdS QDs on PPT/ITO surface. Consequently, 2.0 μM was used as the optimized
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concentration for CdS QDs to fabricate the electrode. In addition, reaction time was also
investigated to make CdS QDs sufficiently decorated on PPT/ITO surface, and 30 min was chosen
as the optimum time obtained from the unchanged PEC currents (Figure S8). To shed new light on
decoration of CdS QDs on PPT/ITO surface, as illustrated in Figure S2E and Figure S5, XPS,
SEM and EDS characterizations of CdS/PPT/ITO were carried out, respectively. It is obvious that
the image, XPS curve and EDS curve of CdS/PPT/ITO displayed new nanoparticles (deemed as
CdS QDs) and peaks (3.3 keV in EDS curve, and 405.24 eV and 412.46 eV in XPS curve,
corresponding to Cd element) compared with that of PPT/ITO, implying the fact that CdS QDs
was successfully anchored on PPT/ITO surface.
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To gain more insight into the fabrication of CdS/PPT/ITO, monomer PT, poly dimethyl diallyl
ammonium chloride (PDDA), and phenothiazine polymeric films prepared from different CV
scans (2, 4, 6, and 8) were applied for fabricating the photoelectrodes to explore the PEC
performance (Figure 2C). After the immobilization of CdS QDs, the spin-coated phenothiazine
film possessed a PEC current of only 208.9 nA. In contrast, the phenothiazine polymeric films
exhibited significantly enhanced PEC currents of 480 nA, 470.6 nA, 460.5 nA, 425.6 nA, and
402.9 nA, respectively, which are far greater than that of phenothiazine monomer in the spin-
coated film state. In the phenothiazine polymeric film, the phenothiazine units are linked into film
by C-C covalent bonds, and these linkages made more quaternary ammonium salt groups exposed
on the surface, which are conductive to immobilization of more CdS QDs, subsequently
increasing the PEC current. In addition, the trend of reducing PEC current from 1 to 8 CV scans
implies that film deposition thickness is also crucial for enhancing PEC current, either to permit
for positive groups on the surface or influence of conductivity. PDDA is one kind of positively
charged polymer, and enjoys high solubility in water solution. When PDDA was used to replace
phenothiazine polymer to immobilize CdS QDs, the fabricated electrode just demonstrated a very
low PEC current of 105.1 nA. This firmly verified that the phenothiazine polymeric film not only
provides abundant positive groups to adsorb CdS QDs, but also works as an effective charge
acceptor and transporter to enhance the separation of photo-generated charges, consequently
improving the generation efficiency of PEC current.
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Figure 2. (A) Diagram illustration of interaction of PPT/ITO and negatively charged CdS QDs. (B)
PEC curves of PPT/ITO electrodes corresponding to different CdS QDs amounts. (C) PEC current
intensities of different electrodes toward 2.0 μM CdS QDs: (a) PDDA/ITO, (b) PT/ITO, (c)
PPT/ITO fabricated from one CV scan, (d) PPT/ITO fabricated from two CV scans, (e) PPT/ITO
fabricated from four CV scans, (f) PPT/ITO fabricated from six CV scans, and (g) PPT/ITO
fabricated from eight CV scan.
To study such a CdS-guided PEC current improvement phenomenon, the electrostatic
adsorption of PPT/ITO toward CdS QDs was evaluated through the pre-incubation of negatively
charged ssDNA with PPT/ITO, and subsequent addition of CdS QDs. The drawing scheme was
depicted in Figure 3A. Evidently, ssDNA could cover the surface positive charges of PPT/ITO
electrode via the electrostatic adsorption, thus making the electrode transforming from positive
charge to negative charge. The surface negative charge helps ssDNA/PPT/ITO to prohibit the
anchorage of CdS QDs on PPT/ITO electrode due to electrostatic repulsion. As a result, the PEC
current was not increased due to immobilization of no CdS QDs. The detailed experimental results
are shown in Figure 3B. In comparison with the case that ssDNA was absent, pre-incubation of
ssDNA with PPT/ITO led to a relatively low PEC current similar to that of PPT/ITO only,
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ascribed to the strong electrostatic adsorption of ssDNA and PPT/ITO, and strong electrostatic
repulsion of ssDNA/PPT/ITO and CdS QDs.
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Figure 3. (A) Diagram illustration of influence of ssDNA on the preparation of CdS/PPT/ITO
electrode. (B) PEC current intensities of PPT/ITO electrodes under different conditions: (a)
PPT/ITO, (b) ssDNA/PPT/ITO + CdS QDs, and (c) CdS/PPT/ITO.
Analysis of OPs based on CdS/PPT/ITO-Based PEC Sensor. Taking account of
the aforementioned features, it was expected that CdS/PPT/ITO electrode can be used as a popular
PEC sensor to detect chemicals or biologics in a high-sensitivity manner. We depicted this
hypothesis by soaking the photoelectrode in PBS buffer comprising of OPs, ATCh, and AChE,
followed by PEC measurement using AA as sacrificial electron donor. OPs act an essential role in
pest and disease control, but the toxicities of OPs on humans and animals easily make AChE
activity dysfunctional, subsequently resulting in neurodegenerative disorders. As a result,
exploring a reliable assay for OPs sensitive biosensing is in high demand. The sensing mechanism
for PEC detection of OPs is depicted in Figure 4. AChE catalyzed hydrolysis of ATCh into
thiocholine, which competitively interacted with CdS QDs in CdS/PPT/ITO via the stronger Cd-S
bonds than electrostatic binding, subsequently forming the thiocholine-modified CdS QDs with
abundant quaternary ammonium salt groups on the surface. As a consequence, CdS QDs fell down
from PPT/ITO electrode due to electrostatic repulsion, and PEC current was significantly reduced.
Upon the pre-incubation of OPs with AChE, ATCh cannot be hydrolyzed into thiocholine because
of the depressed AChE activity. Whereas, ATCh only has the thioether bond and quaternary
ammonium salt group, and consequently cannot wrest the CdS QDs from CdS/PPT/ITO via the
formation of stronger chemical bonds.36-38 From this context, PEC current would not be decreased.
To confirm the feasibility of CdS/PPT/ITO-based PEC sensor for OPs detection, several
experiments in the absence/presence of OPs were carried out by using chlorpyrifos as model OP
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(Figure 5A). Evidently, AChE or ATCh or chlorpyrifos alone made slight change in PEC current
compared with that of CdS/PPT/ITO. In contrast, when both AChE and ATCh are present, a
remarkably decreased PEC current was recorded with the value of 63.04 nA. It came as no
surprise to us that hydrolysis product efficiently leads to the separation of CdS QDs and PPT/ITO,
which can be also confirmed by the decreased XPS peak located at 405.24 eV and 412.46 eV
corresponding to Cd element (Figure S2E). In another validation experiment, the pre-incubation of
chlopyrifos with AChE brings about a significantly enhanced PEC response as compared with that
of AChE alone. Besides, PEC current value elevates along with the increased chlopyrifos amounts,
corresponding well to the principle that more chlopyrifos would inhibit more AChE activity to
generate thiocholine. On the basis of the PEC current change, it is completely feasible to detect
OPs using the fabricated CdS/PPT/ITO photoelectrode.
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Figure 4. Diagram illustration of the working principle for sensitive detection of OPs based on
CdS/PPT/ITO-based PEC sensor.
Analytical Performance of CdS/PPT/ITO-Based PEC Sensor. For the sake of
getting a preeminent performance for OPs assay, AChE concentrations, ATCh concentrations,
reaction time of AChE, ATCh and CdS/PPT/ITO, and pre-incubation time of OPs and AChE that
are directly related with the generation of thiocholine were investigated (Figure S9). Under the
optimized conditions (AChE 14 mU/mL, ATCh 30 μM, reaction time 35 min, and pre-incubation
time 30 min), the analytical performance of CdS/PPT/ITO-based PEC sensor was evaluated
through testing PEC currents of the system comprising of various chlorpyrifos amounts, and the
experimental data was presented in Figure 5B. The PEC current kept increasing with the growing
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chlorpyrifos amounts from 1.5 μg/L to 200 μg/L, demonstrating an obviously positive relationship.
To quantitatively assess the ability of CdS/PPT/ITO-based PEC sensor for chlorpyrifos analysis,
working curve was plotted by employing PEC current (I) and chlorpyrifos concentrations (Cchl) as
vertical and horizontal coordinates, respectively. The curve in Figure 5C implied that I was
linearly relevant to chlorpyrifos amounts ranging from 1.5 μg/L to 200 μg/L. The linear equation
was described as I=1.606 Cchl+73.05, with a coefficient of 0.9899 and detection limit (LOD) of
0.63 μg/L. The assay performances of our proposed sensor and other approaches for OPs were
summarized in Table S1. The results demonstrated that the LOD was comparable or lower than
that of previously reported methods, confirming the sufficiency of our sensor for OPs biosensing.
Furthermore, five different CdS/PPT/ITO electrodes were employed to detect chlorpyrifos (200
μg/L), respectively. The test results suggested that the relative standard deviations (RSDs) were
calculated to be 1.78%, 2.03%, 1.52%, 2.75%, and 2.66%, respectively, which are within an
acceptable range, implying the good repeatability.
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Selectivity was investigated due to its key role in assuring the diagnosis accuracy and reducing
misdiagnosis. According to the sensing principle, CdS/PPT/ITO-based PEC sensor is only
supposed to be responsive for AChE inhibitors. Thus, vitamin B, citric acid, glucose, dibrom, and
omethoate were applied as interferences to verify the specificity and universality of the established
PEC sensor for OPs (Figure 5D). Vitamin B or citric acid or glucose made little influence on the
PEC current compared with that of the solution comprising of AChE and ATCh due to their weak
or no depression on AChE activity. In contrast, both dibrom and omethoate caused significant
PEC currents enhancement, corresponding well to the fact that dibrom and omethoate belong to
OPs, and can inhibit AChE activity with high efficiency. All of these results suggest that the
CdS/PPT/ITO-based PEC sensor possesses good versatility for OPs analysis, and splendid
selectivity for distinguishing OPs against other interfering biomolecules, providing a significant
potential for application in real samples.
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Figure 5. (A) PEC curves of CdS/PPT/ITO-based sensor under different conditions: (a)
CdS/PPT/ITO, (b) CdS/PPT/ITO + AChE, (c) CdS/PPT/ITO + ATCh, (d) CdS/PPT/ITO +
chlorpyrifos, (e) CdS/PPT/ITO + AChE + ATCh+ 200 μg/L chlorpyrifos, (f) CdS/PPT/ITO +
AChE + ATCh + 80 μg/L chlorpyrifos, (g) CdS/PPT/ITO + AChE + ATCh, and (h) PPT/ITO. (B)
PEC curves of CdS/PPT/ITO-based sensor subjected to different chlorpyrifos concentrations: (a)
200，(b) 160, (c) 120, (d) 80, (e) 40, (f) 10, (g) 1.5, (h) 0 μg/L. (C) Linear curve of PEC current
intensities and chlorpyrifos concentrations. (D) PEC current intensities of CdS/PPT/ITO-based
sensor corresponding to different substances: (a) vitamin B, (b) citric acid, (c) glucose, (d) dibrom,
(e) omethoate, (f) chlorpyrifos.
Further, CdS/PPT/ITO-based PEC sensor was applied in analysis of chlorpyrifos in cabbage
extract and lake water obtained from Qingdao Agriculture University through the spiking method
to explore its practical performance. The experiment was conducted by adding chlorpyrifos into
cabbage extract and lake water with the concentrations of 30 μg/L, 60 μg/L, 100 μg/L, followed
by PEC measurement, and the results are summarized in Table S2. The CdS/PPT/ITO-based PEC
sensor depicted recoveries in the range of 96.20%-107.00% and low RSDs ranging from 2.78% to
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6.86%. Moreover, the diagnosis results were also in accord with HPLC data. Therefore, the
figures of merit implied that the CdS/PPT/ITO-based PEC sensor has been prosperously
developed with excellent practical ability for OPs assay in real samples.
CONCLUSIONS
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In summary, we have reported a versatile strategy to fabricate phenothiazine polymeric film on
ITO electrode (PPT/ITO) with fabulous stability and abundant quaternary ammonium salt groups
that permit for electrostatic binding with functional materials, and we also presented their
utilization in the development of PEC sensor for target analyte detection. Due to the strong
electrostatic adsorption of PPT/ITO electrode toward CdS QDs, the CdS/PPT/ITO-based PEC
sensor was successfully developed with high PEC current under visible light irradiation. This
character rendered it able to assay OPs present at ~ng/mL level in sensitivity and specificity-
manners. The applicability of the CdS/PPT/ITO-based PEC sensor for real sample determination
was displayed by assaying chlorpyrifos spiked in cabbage extract and lake water. This strategy is
widely applicable to many other substances with negative charges. Highly sensitive detection of
other analytes can be readily achieved by varying the negatively charged substances, thus
depicting a general PEC sensor for diverse biomolecules assay. We envisage that this work will
provide a new idea to fabricate photoelectrode with high photoelectric conversion efficiency, and
introduce a new era in PEC sensing.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGEMENTS
This work was funded by the National Natural Science Foundation of China (21605093 and
21775082), the Special Foundation for Distinguished Taishan Scholar of Shandong Province
(ts201511052), and the Major Program of Shandong Province Natural Science Foundation
(ZR2018ZC0127).
SUPPORTING INFORMATION
Additional information as noted in the text. OPs assay performance of the present strategy with
other methods; recoveries for chlorpyrifos; Figure S1-S10 as described in the text.
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