Ferroelectric Perovskite Oxide@TiO2 Nanorod Heterostructures: Preparation, Characterization, and Application as a Platform for Photoelectrochemical Bioanalysis

Article abstract of DOI:10.1021/acs.analchem.8b01820

This work reports the first synthesis and characterization of a ferroelectric perovskite oxide-based heterostructure as well as its application for photoelectrochemical (PEC) bioanalytical purposes. Specifically, exemplified by [KNbO₃]_1-x[BaNi_1/2Nb_1/2O_3-δ]_x (KBNNO), the ferroelectric perovskite oxides were prepared by solid-state synthesis, while the TiO₂ nanorod (NR) arrays were obtained via a hydrothermal method. Using the technique of pulsed laser deposition (PLD), KBNNO were then deposited on TiO₂ NRs to form KBNNO@TiO₂ NR heterostructures. Various characterization techniques were applied to reveal compositional and structural information on the as-fabricated sample, and favorable alignment existed between the two components as displayed by the PEC test. In the detection of l-cysteine, the as-fabricated KBNNO@TiO₂ NRs demonstrated good performance in terms of sensitivity and selectivity. This work revealed the potential of ferroelectric perovskite oxide and its heterostructures for innovative PEC bioanalytical applications, and we hope it will generate more interest in the development of various ferroelectrics-based heterostructures for advanced PEC bioanalysis.

Full text of DOI:10.1021/acs.analchem.8b01820

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Article
Ferroelectric Perovskite Oxide@TiO2 Nanorods
Heterostructures: Preparation, Characterization and
Application as Platform for Photoelectrochemical Bioanalysis
Li-Min Yu, Yuan-Cheng Zhu, Yi-Li Liu, Peng Qu, Mao-Tian Xu, Qi Shen, and Wei-Wei Zhao
Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b01820 • Publication Date (Web): 21 Aug 2018
Downloaded from http://pubs.acs.org on August 21, 2018
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Analytical Chemistry
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Ferroelectric Perovskite Oxide@TiO2 Nanorods Heterostructures:
Preparation, Characterization and Application as Platform for Pho-
toelectrochemical Bioanalysis
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LiꢀMin Yu,a,b,c,e YuanꢀCheng Zhu,b,e YiꢀLi Liu,a,b,c,e Peng Qu,*a,c MaoꢀTian Xu,a,c Qi Shen,c WeiꢀWei
Zhao*b,d
aHenan Key Laboratory of Biomolecular Recognition and Sensing, College of Chemistry and Chemical Engineering,
Shangqiu Normal University, Shangqiu 476000, China
bState Key Laboratory of Analytical Chemistry for Life Science, School of Chemistry and Chemical Engineering, Nanjing
University, Nanjing 210023, China
cCollege of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, China
dDepartment of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States
eThese authors (L.ꢀM. Y., Y.ꢀC. Z. and Y.ꢀL. L) contributed to this work equally.
* Eꢀmail: qupeng0212@163.com * Eꢀmail: zww@nju.edu.cn; zww@stanford.edu
ABSTRACT: This work reports the first synthesis and characterization of ferroelectric perovskite oxideꢀbased heterostructure as
well as its application for photoelectrochemical (PEC) bioanalytical purpose. Specifically, exemplified by the [KNbO3]1ꢀ
x[BaNi1/2Nb1/2O3ꢀδ]x (KBNNO), the ferroelectric perovskite oxides were prepared by the solidꢀstate synthesis, while the TiO2 nanoꢀ
rods (NRs) arrays were obtained via the hydrothermal method. Using the technique of pulsed laser deposition (PLD), the KBNNO
were then deposit on the TiO2 NRs to form the KBNNO@TiO2 NRs heterostructures. Various characterization techniques were
applied to reveal the compositional and structural information of the asꢀfabricated sample and favorable alignment existed between
the two components as displayed by the PEC test. In the detection of LꢀCysteine, the asꢀfabricated KBNNO@TiO2 NRs demonꢀ
strated good performance in terms of sensitivity and selectivity. This work unveiled the promise of ferroelectric perovskite oxide
and its based heterostructures for innovative PEC bioanalytical application and we hope it could inspire more interests in the develꢀ
opment of various ferroelectricsꢀbased heterostructures for advanced PEC bioanalysis.
Photoelectrochemical (PEC) bioanalysis, a rapidly evolving
technique for advanced biomolecular detection, generally neꢀ
cessitates appropriate photoelectrodes as sensing platforms to
accommodate the particular probes and recognition events.1ꢀ6
Since the inception of this technique, analysts have been infatꢀ
uated with developing unique photoelectrodes for innovative
applications.7ꢀ14 Ideal candidates should support efficient light
harvesting, proper biomolecule interfacing, enhanced and staꢀ
ble photocurrent generation, fast responsibility as well as senꢀ
sitive signal transduction when implemented in a bioanalytical
system.15ꢀ21 To this end, previous efforts have exploited diꢀ
verse photoactive materials, mainly including various inorganꢀ
ic nanomaterials and organic semiconductors, such as
CdS/CdTe quantum dots (QDs),14, 22ꢀ24 TiO2 nanomaterials13, 25
and porphyrin as well as its derivatives.26 Recently, advanced
hierarchical heterostructures have experienced soaring popuꢀ
larity due to their combined properties and desirable perforꢀ
mance.27ꢀ30 Well alignment of different components could
contribute to the exciton generation/transfer and thus result in
better photoelectrodes with favorite characteristics. In this
respect, for example, graphitic carbon nitride sensitized with
CdS QDs has been applied for PEC aptasensing of tetracyꢀ
cline.31 CdTeꢀgraphene oxide (GO) was used in a dual channel
NaYF4:Yb,Tm@TiO2 upconversion microrods was studied for
a
nearꢀinfraredꢀtoꢀultraviolet lightꢀmediated PEC apꢀ
tasensing.32 Previously, we have also proposed a plasmonic
scheme consisting of Au nanoparticles (NPs) and a Sillenꢀ
Aurivillius perovskite of Bi4NbO8Cl.33
Ferroelectrics, which enable a spontaneous electric polarizaꢀ
tion that can be reversed by an external electric field, have
ignited tremendous interest as a candidate family of materials
for photovoltaic and related lightꢀenergyꢀharvesting applicaꢀ
tions.34ꢀ36 For example, , an interesting perovskite ferroelectric
solidꢀsolution, [KNbO3]1ꢀx[BaNi1/2Nb1/2O3ꢀδ]x (KBNNO), has
recently been developed with tunable direct bandgap (1.1ꢀ3.8
eV) matching the solar spectrum, rendering it suitable for the
visible light absorption (1.39 eV) and many conversion utilizaꢀ
tions.37ꢀ43 Given their fascinating potentials, surprisingly, hardꢀ
ly any work about these materials has been explored in the
field of PEC bioanalysis. We assume that the ferroelectrics
family could provide a unique opportunity to construct an
innovative hierarchical heterostructure as exquisite PEC platꢀ
form. Unfortunately, such a possibility has not been unveiled.
With this motivation and exemplified by KBNNO, this sceꢀ
nario describes the first synthesis and characterization of the
ferroelectrics@TiO2 nanorods (NRs) arrayed heterojunction as
well as its application as new PEC bioanalytical platform.
selfꢀreference
PEC
biosensor.8
CoreꢀShell
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Specifically, as shown in Scheme 1a, the solidꢀsolution of
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Wuhan Geao science and education instrument Co., Ltd.
Bovine serum albumin (BSA), LꢀGlutathione reduced (GSH),
uric acid (UA), dopamine (DA), ascorbic acid (AA) and
Homocystine (Hcy) were purchased from SigmaꢀAldrich (St.
Louis, MO). Cys, glucose (Glu), βꢀlactose (Lac), Lꢀlysine
(Lys), lactic acid (Lac), sarcosine (Sar), tryptophan (Try) and
urea were purchased from Nanjing Chemical Reagent Co.,
Ltd. (Nanjing, China). Phosphate buffer solution (PBS, pH
7.4) was prepared from Na2HPO4·12H2O (Nanjing Chemical
Reagent Co., Ltd., 99.0%) and NaH2PO4 (Nanjing Chemical
Reagent Co., Ltd., 99.0%). Ultrapure water (18.2 Mꢁ·cm
resistivity at 25 °C, Millier Q) was used in all experiments.
SEM images were performed with a JSMꢀ6701F scanning
electron microscope (Japan). TEM and HRTEM was recorded
by a JEMꢀ1210 microscope (JEOL, Japan). XPS was obtained
from ESCALAB 250Xi (Thermo Scientific, Japan). XRD
spectra were operated at 40 kV, 150 mA with CuꢀKα radiation
(Rigaku D/maxꢀ2400, Japan, λ = 0.15418 nm). The Raman
scattering spectra was determined by a CzemyꢀTumer Labram
HR 800 Raman spectroscopy using a laser light wavelength of
532 nm. PEC measurements were performed from a computer
controlled electrochemical workstation (CH Instruments, CHI
660d) equipped with a xenon lamp with a power density of
100 mW cm−2. The power density of solar simulator was
calibrated with a reference silicon solar cell (CELꢀRCCO,
China). Photocurrent was measured in an electrochemical cell
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KBNNO with various composition ratio were prepared by the
solidꢀstate synthesis and then sintered as KBNNO target,
while the TiO2 NRs arrays were obtained via the hydrothermal
method. Subsequently, with the aid of pulsed laser deposition
(PLD) technique, the condensed KBNNO target were directed
to deposit onto the TiO2 NRs for the first time. The Xꢀray difꢀ
fraction (XRD), scanning electron microscope (SEM), transꢀ
mission electron microscope (TEM), Xꢀray photoelectron
spectroscopy (XPS) and Raman analysis et al. were applied to
reveal the compositional and structural information of the
sample and PEC characterization displayed its excellent light
absorption property. Subsequently, as displayed in Scheme 1b,
this heterostructure was appied to the the PEC detection of Lꢀ
cysteine (Cys), an essential sulfurꢀcontaining amino acid,
which was served as an effective electron donor in present
system. The asꢀfabricated KBNNO@TiO2 NRs demonstrated
good performance in terms of sensitivity and selectivity. This
work features the exploration of ferroelectric perovskite oxide
and its based heterostructures for PEC bioanalysis, which to
our knowledge has not been reported. This work manifested
their attractive potential in future PEC bioanalysis, and we
expect it could inspire more interest in the development of
various ferroelectricsꢀbased heterostructures for advanced
PEC bioanalysis.
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Scheme 1. Schematic Illustrations for (a) the Preparation
of KBNNO@TiO2 NRs Heterostructures and (b) Its
Application for PEC Bioanalysis of Cys
with
a threeꢀelectrode system: a KBNNO@TiO2 NRs
electrode as the working electrode, a Pt plate as the counter
electrode and a saturated mercury electrode (SCE) as the
reference electrode. A 0.05 M Na2S and 0.95 M Na2SO3
solution was used as the supporting electrolyte for
photocurrent measurements. All the PEC measurements were
operated in atmospheres. For Cys detection, the experiments
were processed with a homemade PEC system. The irradiation
source is a 5 W LED lamp which can produce the white light
in front of the electrode. The electrochemical cell consisted of
KBNNO@TiO2 NRs electrode, a Pt wire counter electrode
and an Ag/AgCl reference electrode (with KCl in the saturated
solubility) in 0.1 M PBS solution.
Synthesis of KBNNO: The KBNNO power was prepared
by conventional standard solidꢀstate synthesis method
according to the previous report.37ꢀ38 All components in
different ratios were synthesised by stoichiometric quantities
of K2CO3, BaCO3, Nb2O5 and NiO as starting raw materials.
The powders were ballꢀmilled media in ethanol with Tungsten
carbide (WC) balls for 2 h in Pulverisette 5 Planetary highꢀ
energyꢀmill. The weight ratio of the ball to the powders is
10:1. The milled powders were sieved with 200 mesh sieve
o
after drying at 60 C for 4 h and calcined in an alumina
crucible at 900 oC for 12 h. The KBNNO target was obtained
by vacuum hotꢀpressed sintered technology as follows: the
mixture was cold compacted in a graphite die (Φ 61 mm × 6
mm) and hotꢀsintered under the pressure of 25 MPa with a
o
dynamic vacuum of about 10ꢀ3 Pa at 850 C for 2 h in a
vacuum hotꢀpressing furnace (ZTꢀ45ꢀ20, ShangHai Chen Hua
Electric Furnace Co. Ltd., China). Then, the specimens were
machined to remove surface impurities and polished into fixed
dimension (Φ 60 mm × 5 mm) for later sputtering application.
Synthesis of TiO2 NRs: TiO2 nanorodes (NRs) arrays was
fabricated on FTO glass substrate adopting the hydrothermal
method that has been reported in detail elsewhere.44ꢀ45 One
piece of the FTO glass substrate was firstly ultrasonically
cleaned in a mixture of deionized water, 2ꢀpropanol and
EXPERIMENTAL SECTION
Reagents and Apparatus. Potassium Carbonate (K2CO3),
Barium Carbonate (BaCO3), Niobium Pentaoxide (Nb2O5),
Nickel Oxide (NiO) were supplied from Sinopharm Chemical
Reagent Co., Ltd (China). Titanium tetrachloride (TiCl4) was
perchased from Tianjin Kermel Chemical Reagent Co., Ltd.
Hydrochloric acid (HCl, 36.5%ꢀ38% by weight) was supplied
from Xilong Chemical Reagent Co., Ltd. The Fluorineꢀdoped
tin oxide (FTO) glass (14 ohm per square) was obtained from
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acetone with volume ratio of 1:1:1 for 60 min, subsequently
decreased distinctly compared with KNbO3 (KNO, x = 0.0).
This phenomenon can be ascribed to the inner lattice strains on
account of different ionic radius of Ni/Nb and K/Ba. In
addition, Ba2+ at Aꢀsite leaded to the generation of K+
vacancies, which reduced growth velocity of crystal and
resulted in the decline of particle size ultimately.48
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rinsed extensively with deionized water, then dried with N2
stream and finally was placed at an certain angle against the
wall of Teflonꢀlined reactor with the conducting side facing
down. 40 mL of HCl was mixed with 40 mL deionized water.
The mixture was stirred for 5 min before 0.6 mL TiCl4 was
added and then poured into Teflonꢀlined reactor after being
stirred for another 5 min. The hydrothermal synthesis was
operated at 180 oC for 3 h in oven. After synthesis, the
hydrothermal synthesis reactor was cooled to room
temperature under flowing cold water. Subsequently, the
sample was taken out, rinsed with ultrapure water and finally
dried in an electric oven at 60 oC.
Synthesis of KBNNO@TiO2 NRs: KBNNO@TiO2 NRs
was fabricated by PLD technique (COMPexPro 205) with a
KrF excimer laser (λL = 248 nm, pulse duration τL= 25 ns
(FWHM), repetition rate 5 Hz).46ꢀ47 The KBNNO target was
fixed in target holder while the FTOꢀTiO2 NRs substrate was
fixed in sample supporter. Prior to ablating the KBNNO
target, the deposition chamber was evacuated by the coalition
of mechanical pump and molecular pump to 2.4 × 10ꢀ4 Pa.
Both the KBNNO target and FTOꢀTiO2 NRs substrate were
cleaned by oxygen plasma for 5 min before ablating the target
in O2 atmosphere. The target was placed parallel to FTOꢀTiO2
NRs substrate with a distance of 55 mm. In the pulse
sputtering, parameters were interceded as follows: laser energy
was set in 300 mJ and pulse frequence was set as 5 Hz as well
as 6000 number of pulses was presented. Futhermore, a speed
of 15 rpm of the target rotational speed was applied during the
laser ablating for avoiding the deep craters’ formation, and the
sample supporter ratated at the speed of 10 rpm in order to
achieve uniform films.
XRD patterns of the different KBNNO target components
were recorded with the results shown in Figure 1c. The
appearance of NiO diffraction peak came from the NiO
additive after doping with Ni/Ba elements. All the XRD
analysis related KBNNO matched well with the International
Centre for Diffraction (ICDD) data: The diffraction peaks for
pure KNO could be indexed as the orthorhombic crystal lattice
in the space group of Amm2 (JCPDS no. 71ꢀ2171), while the
patterns of Ni/Ba doped KBNNO material could be fully
indexed as cubic perovskite crystal of the Pm3m space group
(JCPDS no. 52ꢀ1517). Furthermore, it can be evidently noticed
that peaks of (211) and (112) have been merged into one
single peak for KBNNO compositions according to the
enlargenment of area marked with gray bar, which was
magnified in Figure 1d. Therefore, it can be inferred that the
composition in the KBNNO underwent a wide phase
evolution: i.e., from orthorhombic lattice to tetragonal system
and tetragonal system to cubic phase. Besides, the diffraction
patterns of KBNNO gradually shifted to lower diffraction
angels with the increasing Ni/Ba content, which was mainly
due to the introduction of larger ionic radius Ni2+ (0.69 Å)
based on the Bragg equation.
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PEC Detection: Solutions with different concentrations of
Cys were prepared before analyzing and then PEC
measurements were performed on the assembled electrodes at
room temperature. The specificity experiments were then
evaluated with different biological analytes including Glu,
Lac, GSH, Lys, Lac, Sar, Try and BSA in the electrolyte of 0.1
mol L−1 PBS.
RESULTS AND DISCUSSION
Structural and Compositional Characterization. Figure 1
showed the SEM images of asꢀprepared TiO2 NRs and
KBNNO powders, as well as the corresponding XRD patterns
and Raman spectroscopy of KBNNO powders. Figure 1a
presented the typical topꢀview SEM of TiO2 NRs, which
exhibited a selfꢀorganized NRs structure with good conformity
on the FTO substrate. Figure 1a inset of the crossꢀsectional
image indicates these NRs were approximately 2.67 ꢂm in
length and vertically aligned with smooth surface. Obviously,
such a wellꢀaligned structure could not only provide large
surface area for the following PLD deposition of guest
material (KBNNO), but also be advantageous to the increased
light absorption and efficient directional charge transportation
within the arrayed NRs. The morphology of asꢀprepared
KBNNO powder (x = 0.3) was shown in Figure 1b, which
manifested that the particles were spherical with a size of ca.
100 nm, and a lot of particles spliced with each other to form a
cohesive body. The EDS mapping in Figure S1 recorded the
corresponding elemental distribution of KBNNO powder (x =
0.3). Figure S2 demonstrated the morphologies of asꢀprepared
KBNNO powders with variable component ratio. As shown,
the radius of KBNNO (x = 0.1, 0.2, 0.3, 0.4) powders
Figure 1. (a) Topꢀview FESEM of TiO2 NRs. Inset: the
corresponding crossꢀsectional view. (b) The SEM image of
KBNNO nanoparticles (x = 0.3). (c) XRD patterns of the asꢀ
prepared KBNNO (x = 0, 0.1, 0.2, 0.3, 0.4) targets,
respectively. (d) XRD peaks of the enlargenment of area
marked with gray bar in Figure 1c. (e) Raman spectroscopy of
the different KBNNO components (x = 0, 0.1, 0.2, 0.3, 0.4).
Raman spectroscopy, a competent examination method to
analyze the phase evolution and stucture change, was then
applied to investigate the asꢀsynthesised KBNNO samples on
account of its sensitivity to small distortions of crystal lattice.
Figure 1e depicted the Raman spectra data of KNbO3 and
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Ni/Baꢀdoped KBNNO samples. The band at 129 cmꢀ1 can be
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assigned to AꢀO (KꢀO or BaꢀO) vibrations, suggesting the
existence of K+ and Ba2+ cations. The peaks at 188 cmꢀ1 and
275 cmꢀ1 can belong to a frigerprint for the longꢀrange polar
order,49 and their sharp dip demonstated that the longꢀrange
polar order weaken languishingly with the increase of Ni/Ba
content. Furthermore, the peaks for pure KNbO3 appeared at
275 cmꢀ1 can be assigned to NbꢀO6 bending mode and 531 cmꢀ
1
and 600 cmꢀ1 can be ascribed to NbꢀO6 stretching mode,
while peaks located at 263 cmꢀ1 for KBNNO shifted to a lower
wavenumber by 12 cmꢀ1. The reason was that the OꢀNiꢀO
vibration had substituted part of OꢀNbꢀO vibration, indicating
the successful Niꢀdoping in KBNNO. Normally, a lower
Raman stretching frequency implied a longer length of the
metalꢀoxygen bond. The Raman data shift towarded left,
especially at the band of 600 cmꢀ1 and 831 cmꢀ1, signified the
significantly increasment of OꢀNiꢀO bond, which was
corresponding to the pattern of XRD.
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Using the PLD technique, the KBNNO were then deposited
onto the TiO2 NRs to form the asꢀdesigned KBNNO@TiO2
NRs heterostructures. Figure 2a showed the crossꢀsectional
SEM image of the asꢀsynthesized KBNNO@TiO2 NRs arrays
(x = 0.3). It is clear that the KBNNO@TiO2 NRs kept similar
morphology to that of TiO2 NRs after sputtering KBNNO,
indicating the covering of KBNNO on the TiO2 surface will
not affect their NRs shape. TEM was then used to disclose
more structural information about KBNNO@TiO2 NRs.
Figure 2b presented the TEM image of a truncated single
KBNNO@TiO2 NR, which possessed a rough surface which
might be caused by the KBNNO deposition. The HRTEM
image in Figure 2c demonstrated that the TiO2 NRs was
covered by an irregular granule KBNNO film of ca. 2.66 nm.
However, the KBNNO film was in amorphous structure, while
the TiO2 NRs are of its singleꢀcrystalline nature. Besides, no
interspace can be seen, indicating the strong contacts between
TiO2 NRs and KBNNO shell. To further validate the
formation of KBNNO@TiO2 NRs, elemental mapping was
performed with the results shown in Figure 2d. Obviously,
each element has been detected by the energy dispersive Xꢀray
spectroscopy (EDS), the homogenous distrubution of these
elements verified the successful deposition of KBNNO shell
onto TiO2 NRs. XPS was further utilized to determine the
surface chemical compositions and valence states of the asꢀ
fabricated KBNNO@TiO2 NRs, with the binding energy of
contaminated carbon (C 1s = 284.8 eV) as the internal marked
standard. Figure 2e depicted the survey spectrum of the
sample with the relevant elements of Ni, O, K, Ba and Nb,
while the corresponding highꢀresolution XPS spectra of Ni 2p
and Ba 3d were illustrated in Figure 2f. Ni 2p signal located at
about 855.08 eV can be assigned to the unreacted NiO and the
peak at 856.49 eV was ascribed to the Ni atom in KBNNO
crystal, which confirmed the substitution of Ni2+ with Nb5+ in
the Niꢀdoped KBNNO system. As shown in Figure 2f inset,
the Ba 3d peaks located at 794.57 eV and 779.19 eV with a
peak separation of 15.38 eV.
Figure 2. (a) The crossꢀsectional SEM of the asꢀprepared
KBNNO@TiO2 NRs (x = 0.3). (b) TEM image of single
KBNNO@TiO2 NR (x = 0.3). (c) The HRTEM image of
KBNNO@TiO2 NR. (d) The EDS mapping of single
KBNNO@TiO2 nanorod (x = 0.3). (e) The fullꢀscan XPS
spectrum of the asꢀfabricated KBNNO@TiO2 NRs. (f) The
highꢀresolution XPS spectra of Ni 2p. Inset: The highꢀ
resolution XPS spectra of Ba 3d.
Electrochemical Characterization. To acquire more
information on the PEC properties of the asꢀprepared
heterojunctions, the chronoamperometric iꢀt curves were recꢀ
orded from the stepwise transient photocurrent responses upon
the intermittent light irradiation. As shown in Figure 3a, all the
composite KBNNO@TiO2 NRs (x = 0, 0.1, 0.2, 0.3, 0.4)
electrodes with varying contents exhibited higher photocurrent
density compared with pristine TiO2 NRs electrode, the results
of which also indicated the successful combination of the
KBNNO with the TiO2 NRs. Significantly, the numerical
value of the composite electrodes increased with the doping
content and reached to maximum of 0.37 mA cmꢀ2 (when x =
0.3), which is about 4.9 times as high as that of pristine TiO2
NRs (0.075 mA cmꢀ2). Moreover, there is no obvious decline
under illumination for 4000 s in electrolyte, as shown in
Figure S3, indicating the excellent structural and chemical
stability of the asꢀdeveloped KBNNO@TiO2 NRs electrodes.
These properties can be attributed to the following reasons: (1)
Essentially, the excitation through bandgap was a charge from
the O 2p states at the maximum of valence band (VB) to the
Nb 4d states at the minimum of the conduction band (CB) for
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KNO. However, for KBNNO, photoelectrons would be
the electrode/electrolyte interface after deposition of KBNNO
shell. In all, with balanced property, the KBNNO@TiO2 NRs
(x = 0.3) was selected for subsequent utilization.
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excited from the hybridized Ni 3d states and O 2p states to Nb
4d states, leading to the reduction of the bandgap value.37ꢀ38
Therefore, the NiꢀdopedꢀKBNNO shell, with appropriate
bandgap, could absorb broad light and served as an effective
photosensitizer in the heterostructures. (2) The photoelectrons
would transfer to the oxygen vacancies states after being
excited to Nb 4d states from the hybridized states, followed by
transfer to TiO2 NRs. The experimental phenomenon caused
by oxygen vacancies had been presented in Figure S4. Note
that the existence of oxygen vacancies in KBNNO was
extremely beneficial to lengthen life spans of photoinduced
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carriers and decrease the recombination of photo electrons and
51
holes.50,
Firstly, the oxygen vacancies states could be
inserted between the CB of KBNNO and CB of TiO2, which
could act as capture center to acquire the electrons transferred
from CB of KBNNO, inducing the photogenerated electrons
prefer to inject into oxygen vacancies states rather than return
to the valence band of KBNNO. Secondly, the oxygen
vacancy, served as an active electron trap, could effectively
delay the recombination of electrons and holes between
oxygen vacancies states and the ground state. Thirdly, the
lifetimes of photo electrons in electron traps are multiple
expansions longer than that in the CB. These characteristics
provide a big advantage for reduce the charge recombination
losses and allow the fast electron injection into the CB of
TiO2, followed by collected in FTO substrate through the wellꢀ
aligned crystalline TiO2 NRs. (3) As an excellent ferroelectric,
Ni/Baꢀdoped KBNNO with inernal fields induced spontaneous
polarization band bending, is also beneficial to the spatial
separation of photoꢀgenerated charge carriers, and thereby
reducing the recombination of photoꢀinduced electronsꢀholes
pairs.
Figure 3b presents a set of linear sweep voltammograms
(LSV) that were conducted in dark and under light
illumination. All the pristine TiO2 NRs and KBNNO@TiO2
NRs (x = 0, 0.1, 0.2, 0.3, 0.4) photoelectrodes exhibited low
dark current compared with their corresponding photocurrents,
manifesting no drastic electrocatalytic waterꢀsplitting happens.
Apparently, the photocurrent density of KBNNO@TiO2 NRs
photoelectrodes under illumination also increased with the
doping content when x ≤ 0.3 and decreased when x = 0.4, and
all the heterojunction photoelectrodes exhibited higher current
than that of pristine TiO2 NRs electrode, which was
corresponding to the results of iꢀt tests. In addition, the onset
potential of KBNNO@TiO2 NRs (x = 0.3) photoelectrode
exhibits ꢀ0.983 V vs. SCE, which appeared a slightly shift
compared with ꢀ0.886 V vs. SCE for pristine TiO2 NRs. The
lower onset potential value would make charge separation and
transportation more efficiently in KBNNO@TiO2 NRs (x =
0.3) photoelectrode. The electrochemical impedance
spectroscopy (EIS) was then tested on these photoelectrodes
under one light irradiation covering the frequency interval of
105 to 10ꢀ2 Hz with an amplitude of 5 mV. As shown in Figure
3c, obviously, only one semicircle was obtained in the EIS
Nyquist plot. However, a larger radius resistanceꢀcircle
obtained for pure KNbO3@TiO2 NRs compound than pristine
TiO2 NRs, which was mainly due to low electronic
conductivity of KNbO3 material. It was interesting to see that
the Ni/Baꢀdoped KBNNO@TiO2 NRs photoelectrodes
achieved smaller resistanceꢀcircles than pristine TiO2 NRs
with the increasing of Ni/Ba content, suggesting the
dramatically reducement of the chargeꢀtransfer resistance at
Figure 3. (a) The transient photocurrent responses (iꢀt) of
pristine TiO2 NRs and KBNNO@TiO2 NRs (x = 0, 0.1, 0.2,
0.3, 0.4) photoelectrodes at the potential of 0.3 V vs. SCE
under 100 mW cmꢀ2 illumination. (b) The linear sweep
voltammograms of pristine TiO2 NRs and KBNNO@TiO2
NRs (x = 0, 0.1, 0.2, 0.3, 0.4) photoelectrodes collected in
dark and under 100 mW cmꢀ2 exposure with scan rate of 0.1
mV sꢀ1. (c) EIS Nyquist of TiO2 NRs and KBNNO@TiO2 NRs
(x = 0, 0.1, 0.2, 0.3, 0.4) photoelectrodes measured under light
irradiation (100 mW cmꢀ2).
PEC detection. The KBNNO@TiO2 NRs was then applied
for the quantitative determination of Cys, an important
indicator of many biological processes. Specifically, the
responses of the KBNNO@TiO2 NRs (x = 0.3) electrode to
Cys was studied via the timeꢀdependent current measurements
at 0.0 V applied voltage. As illustrated in Scheme 1b, the
consumption of the photoexcited holes in KBNNO would
contribute to efficient photoinduced charge separation and
thus lead to the enhancement of photocurrent. In other words,
with the increase of Cys concentration, more photoexcited
holes on the VB of KBNNO were steered to oxidize Cys,
therefore the response of photocurrent was increased
correspondingly as observed in Figure 4a and 4b. Figure 4c
depicted the corresponding derived calibration curve with a
wide linear range from 5.0 × 10−6 to 1.0 × 10−2 M and its
detection limit was found as 1.3×10ꢀ6 mol L−1, which was
comparable to those obtained by cyclic voltammetry,
colorimetry,
amperometry,
electrogenerated
chemiluminescence (ECL) method and other PEC methods, as
shown in Table S1. Since the concentration of Cys in human
serum varies from 165.1 to 335.3 ꢂmol L−1, the results
indicated the developed system processed great potential for
Cys detection in biological applications. The reproducibility of
this PEC bioanalysis was then assessed by relative standard
deviation (RSD), and the RSD of 6.4% was obtained by
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testing three electrodes at the concentration of 1.0×10ꢀ4 mol
CONCLUSIONS
L−1 independently, suggesting favorable reproducibility.
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In conclusion, this work has explored and exhibited the
The selectivity was then evaluated through the detection of
some common interferents. Among the common potential
interferents in human serum, the thiolꢀcontaining biomolecules
such as GSH and Hcy, which process the similar structures
with Cys, should be considered primarily. Specifically, it was
documented that the normal concentration of GSH is at the
level of 14.0 ± 7 ꢂmol L−1, which is nearly 10 times lower
potential
of
ferroelectric
for PEC
perovskite
bioanalytical
oxideꢀbased
application.
heterostructures
Exemplified by KBNNO@TiO2 NRs, the sample was prepared
by the combined use of solid state synthesis, hydrothermal
method and PLD, and then systematically characterized by
various techniques, which revealed the desired integration of
KBNNO with TiO2 NRs. As shown in the
chronoamperometric I-t results, PEC tests also manifested the
favorable alignment between the KBNNO and TiO2 NRs. For
the detection of Cys, the PEC sensor demonstrated desirable
performance in terms of good sensitivity and stability. These
results clearly indicated that the KBNNO@TiO2 NRs
heterojunction could be a competitive candidate platform in
the development of PEC bioanalysis. This work not only
featured the first use of a unique KBNNO@TiO2 NRs in PEC
bioanalysis but also offered a new perspective for the general
development of numerous ferroelectricsꢀbased heterostructure
for innovative applications in this field. We further expect the
ingenious implementation of these materials would lead to
advanced PEC biosensors and devices with superior analytical
performance.
53
than the concentration of Cys (165.1ꢀ335.3 ꢂmol L−1).52,
Therefore, the interfering GSH was set at the level of 1.0× 10ꢀ5
mol L−1 in this selectivity test and no obvious response was
produced compared with the responce of Cys. To some extent,
it was due to the distinction of electronꢀdonating ability
between GSH and Cys, which has provided particular
explanation in previous study.15 Likewise, the discriminability
of system to Hcy was investigated. Although Hcy has a very
similar structure to cysteine, it is present in a much lower
concentration (5ꢀ15 µM) than Cys in healthy human
serum.54,55 As a consequence, at the concentration of 1.0× 10ꢀ5
mol L−1, the signal of Hcy was rather low which could hardly
impact the performance of the asꢀfabricated electrode.
Besides, considering the normal physiological glucose level in
human blood was 3ꢀ8 mM, which is nearly 30 times higher
than those of many of the interfering species.56 The level of
glucose was chose as 1.0×10ꢀ2 mol L−1. Other interferences
including Lys, Lac, Sar, Try, BSA, DA, AA, UA and Urea
were set at the level of 1.0×10ꢀ3 mol L−1 which remained the
same concentration with Cys. As shown in Figure 4d, it was
demonstrated that none of these interference factors could
cause obvious photocurrent response except the target Cys,
demonstrating the favorable selectivity of the system.
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ASSOCIATED CONTENT
Supporting Information
EDS mapping of asꢀprepared KBNNO powder (x = 0.3), SEM
images of KBNNO nanoparticles at varying doping content,
The photocurrent responses upon light irradiation for 4000 s,
Typical decay and rise curve for KBNNO@TiO2 NRs (x =
0.3) photoelectrode under light off and on, Comparison of
Different Methods for LꢀCys Determination (PDF).
AUTHOR INFORMATION
Corresponding Author
* Eꢀmail: qupeng0212@163.com
* Eꢀmail: zww@nju.edu.cn; zww@stanford.edu
Author Contributions
L.ꢀM.Y., Y.ꢀC.Z. and Y.ꢀL. L. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank the National Natural Science Foundation of China
(Grant Nos. 21605100, 21505091, 21405103 and 21675080),
Henan
Joint
International
Research
Laboratory
of
Chemo/Biosensing and Early Diagnosis of Major Diseases and
Natural Science Foundation of Jiangsu Province (Grant
BK20170073) for support.
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