Ti3C2 MXene-based Schottky photocathode for enhanced photoelectrochemical sensing

Article abstract of DOI:10.1016/j.jallcom.2020.157787

Nanomaterials are vital to the realization of photoelectrochemical (PEC) sensing platform that provides the sensitive detection and quantification of low-abundance biological samples. Here, this work reports a Schottky junction-based BiOI/Ti₃C₂ heterostructure, used as a photocathode for PEC bioanalysis. Specially, we realize in situ growth of flower-like BiOI on 2D intrinsically negatively charged Ti₃C₂ MXene nanosheet that endows BiOI/Ti₃C₂ heterostructure with admirably combined merits, noting in particular the generation of built-in electric field and the decrease of contact resistance between BiOI and Ti₃C₂. Under the visible light irradiation, the BiOI/Ti₃C₂ heterostructure-modified PEC platform displays superior cathodic photocurrent signal, while PEC response cuts down with the presence of L-Cysteine (L-Cys) as a representative analyte owing to the metal-S bond formation. The “signal-off” PEC sensing strategy shows good performance in terms of sensitivity, limit of detection (LOD, 0.005 nM) and stability. This research reveals the great potentials of MXene-based heterostructure in the application field of PEC sensor establishment.

Full text of DOI:10.1016/j.jallcom.2020.157787

Journal of Alloys and Compounds xxx (xxxx) xxx
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Ti3C2 MXene-based Schottky photocathode for enhanced
photoelectrochemical sensing
,
,
, ***
Cui Ye a *, Zhen Wu a, Keyi Ma a, Zhuohao Xia a, Jun Pan a **, Minqiang Wang b
,
Changhui Ye a
a College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou, 310014, China
b Division of Engineering and Applied Science, California Institute of Technology, Pasadena, CA, 91125, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanomaterials are vital to the realization of photoelectrochemical (PEC) sensing platform that provides
Received 17 June 2020
Received in revised form
27 October 2020
Accepted 29 October 2020
Available online xxx
the sensitive detection and quantification of low-abundance biological samples. Here, this work reports a
Schottky junction-based BiOI/Ti3C2 heterostructure, used as
a photocathode for PEC bioanalysis.
Specially, we realize in situ growth of flower-like BiOI on 2D intrinsically negatively charged Ti3C2 MXene
nanosheet that endows BiOI/Ti3C2 heterostructure with admirably combined merits, noting in particular
the generation of built-in electric field and the decrease of contact resistance between BiOI and Ti3C2.
Under the visible light irradiation, the BiOI/Ti3C2 heterostructure-modified PEC platform displays su-
Keywords:
Ti3C2 MXene
perior cathodic photocurrent signal, while PEC response cuts down with the presence of L-Cysteine (L-
Cys) as a representative analyte owing to the metal-S bond formation. The “signal-off” PEC sensing
strategy shows good performance in terms of sensitivity, limit of detection (LOD, 0.005 nM) and stability.
This research reveals the great potentials of MXene-based heterostructure in the application field of PEC
sensor establishment.
Photoelectrochemical sensing
Schottky junction
L
-cysteine
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
Recently, research on Ti3C2 MXene is booming, firstly reported
by Gogotsi and his group in 2011 [11]. As a new family of 2D
Photoelectrochemical (PEC) sensing has been a rapidly evolving
analytical technique for trace or ultratrace detection towards
diverse biomolecular in complex samples, since the technique is of
considerate merits of low background signal, high sensitivity, and
simple operating conditions [1e3]. Potential photoelectrode can-
didates should possess highly valid light harvesting, suitable ana-
lytes interfacing, efficient PEC signal response, and stable signal
transduction to exploit ideal PEC sensing platform [4e6]. Up to
now, plentiful efforts are focused on developing photoactive ma-
terials to obtain desirable photoelectrode, such as TiO2-based or
quantum dots-based nanomaterials, as well as porphyrin and its
derivatives [7,8]. As a matter of fact, advanced heterostructure of
various components that formulates Schottky junction can be
conducive to carrier generation, and transfer, thus acquiring
astonishing performance [9,10].
transition metal carbides/carbonitrides/nitrides, MXene is obtained
by selective etching away A layers from Mnþ1AXn (MAX) phases (M
represents transition metal, A represents group IIIA/IVA element, X
represents C/N element and n ¼ 1e3). Ti3C2, one of the most widely
used MXene, possesses lots of superior merits including electronic
conductivity, mechanical property, huge surface area, especially
compatibility with other nanomaterials. The mentioned merits
make Ti3C2 MXene be widely used in electrocatalysis [12], energy
storage batteries [13], electrochemical sensor [14,15] and so on, yet
Ti3C2 MXene-based photoelectrochemical (PEC) sensing has
seldom been exploited so far. It is worth noting that MXene,
particularly OH-functionalized Ti3C2, can be efficiently conductive
to separate and transmit the photogenerated carriers in a Ti3C2/
semiconductor heterostructure, since there is difference between
valence band and Fermi levels (EF) to generate Schottky junction,
and then give rise to the built-in electric field between Ti3C2 and
photoactive materials such as semiconductor [16,17]. Besides, in-
tegrated with the unique properties of intrinsically negatively
charged surfaces and active OHeTi sites on accordion-like multi-
layer architecture, as well as the Schottky barrier near to zero by
* Corresponding author.
** Corresponding author.
*** Corresponding author.
E-mail address: ye0702@zjut.edu.cn (C. Ye).
https://doi.org/10.1016/j.jallcom.2020.157787
0925-8388/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: C. Ye, Z. Wu, K. Ma et al., Ti3C2 MXene-based Schottky photocathode for enhanced photoelectrochemical sensing,
Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2020.157787

C. Ye, Z. Wu, K. Ma et al.
Journal of Alloys and Compounds xxx (xxxx) xxx
first principle calculation [18], Ti3C2-based hybrid composite is
potentially motivated to fabricate functional PEC loading platform,
thus exploiting it in PEC sensing domain.
method [11,25]. Briefly, LiF (0.5 g) was dissolved in HCl (9 M, 10 mL)
under stirring at room temperature to obtain the LiF/HCl solution.
Then, Ti3AlC2 (MAX phase, 0.5 g) was slowly added to the above LiF/
HCl solution within 5 min. The suspension was stirred at 35 ꢀC for
24 h. The product was washed with ultrapure water under centri-
fugation (3500 rpm, 5 min) until the pH value of the filtrate reached
6.0. The precipitate was taken out and added to ultrapure water to
obtain the Ti3C2 suspension (10 mg mLꢁ1, 30 mL). After that, the
Ti3C2 suspension was ultrasonicated for 30 min under the nitrogen
atmosphere. Finally, the supernatant of Ti3C2 nanosheets was
gathered via centrifugation at 7500 rpm for 20 min.
Bismuth oxyiodide (BiOI), a typical p-type semiconductor with a
narrow band gap (about 1.8 eV), has been widely used as an
economical and promising visible-light-driven (VLD) photocathode
materials [19,20]. Since pure BiOI suffers from quick recombination
of photogenerated electron-hole pairs and poor conductivity, en-
gineering BiOI by compounding with Ti3C2 is undoubtedly logical.
Differing from previous widely reported photoactive materials,
BiOI/Ti3C2 heterostructure not only equipped with efficient pho-
toconversion efficiency, but also diminish the contact resistance
due to their combined properties. In this respect, we propose the
novel BiOI/Ti3C2 heterostructure regarding as photocathode to
construct PEC sensing platform. And we adopt a typical thiol-
2.2. Preparation of BiOI/Ti3C2 heterostructure
For synthesis of BiOI/Ti3C2 heterostructure, (Bi(NO3)3$5H2O)
(0.25 mmol) was added to the supernatant of Ti3C2 nanosheets
(0.5 mg mLꢁ1, 5 mL) and stirred for 30 min to obtain a homoge-
neous solution (referred to as Solution A). Meanwhile, KI
(0.25 mmol) was dissolved in ethylene glycol (5 mL) (referred to as
Solution B). Then, Solution B was added dropwise to Solution An
under stirred constantly. The resultant product obtained after
washing several times with ultrapure water and drying under
vacuum at 80 ꢀC for 6 h. For comparison, the pure BiOI was pre-
pared in the absence of Ti3C2 nanosheets under the same condition.
containing amino L-Cysteine (L-Cys) to explore the PEC sensing
performance, which is broadly used in clinical diagnoses because it
is concerned with lots of syndromes such as hypoglycemic brain
and liver damage, skin lesions, as well as hair depigmentation, and
can regulate the function of a biological system [21e23].
Herein, we develop BiOI/Ti3C2 heterostructure-based photo-
cathode using L-Cys as a model molecule to demonstrate the PEC
sensing strategy. Detailedly, 2D Ti3C2 nanosheet is obtained by
exfoliating bulk Ti3AlC2 in a mixture of LiF and HCl, and is taken to
mediates the in situ BiOI generation on account of the admirable
adsorption affinity to metal ions, which endows BiOI/Ti3C2 heter-
ostructure with Schottky junction. In the PEC sensing strategy,
enormous cathodic photocurrent derived from VLD BiOI/Ti3C2
heterostructure was generated, and obvious decrease of PEC
response was observed because of interaction of metal-S bond [24],
thus realizing the cathodic “signal-off” pattern-based PEC selective
sensing of L-Cys (Scheme 1). This work is benefit to inspire more
rational designs of Ti3C2-based heterostructure with Schottky
junction for further diverse establishment of PEC sensing platform.
2.3. Preparation of PEC sensing platform
A glassy carbon electrode (GCE) was used to study the PEC
performance of BiOI/Ti3C2 heterostructure. Prior to modification,
the GCE surface was polished with alumina slurry of 0.3
mm and
0.05 m and sonicated in ethanol and ultrapure water. Then, BiOI/
m
Ti3C2 heterostructure was sonicated in ultrapure water for 1 min to
obtain uniformly distributed suspension with a concentration of
2 mg mLꢁ1 10
mL of the above solution was dripped onto GCE and
2. Experimental
evaporated at room temperature, the modified photoelectrode was
referred to as BiOI/Ti3C2/GCE. Furthermore, the BiOI/Ti3C2/GCE was
2.1. Synthesis of Ti3C2 nanosheets
incubated with L-Cys (10 mL) for 30 min, followed by washing and
drying. Finally, the BiOI/Ti3C2/GCE was stored in a refrigerator for
further studies.
Ti3C2 nanosheets were prepared via a previously reported
Scheme 1. (A) Synthesis of BiOI/Ti3C2 heterostructure; (B) schematic illustration of fabricating PEC sensing L-Cys detection.
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2.4. PEC detection for L-Cys
transformed to Ti3C2. In particular, the shift of (002) peak to lower
angles indicates the intercalation of Liþ among the layers of Ti3C2
[25]. Besides, the overall peaks located at 29.5ꢀ, 31.6ꢀ, 55.2ꢀ of BiOI
are observed, which can be ascribed to tetragonal BiOI (JCPDS
10e0445). For BiOI/Ti3C2 heterostructure, the characteristic
diffraction peaks exsit, and no diffraction peak of Ti3C2 is observed,
which might be due to the low content and homogeneous disper-
sion of Ti3C2 in the heterostructure.
The morphology of the samples were studied by scanning
electron microscope (SEM). Figs. S1A and B shows the SEM image of
Ti3AlC2 and Ti3C2 nanosheets, respectively. As seen in Fig. 1B,
atomic force microscopy (AFM) image shows the ultra thin foil, and
the corresponding height profile displays the thickness of Ti3C2
nanosheets is about 3.2 nm (Fig. 1C), demonstrating the ultrathin
structure of the Ti3C2 nanosheets. As seen in Fig. 1D, a thin 2D
layered structure of Ti3C2 can be further observed from the trans-
mission electron microscopy (TEM) image, especially because of
the high atomic number of Ti [12]. As depicted in Figs. S1C and D,
The PEC experiments were recorded on a CHI760 E electro-
chemical work station using a three-electrode system with the
modified GCE as the working electrode, platinum wire as the
counter electrode, and Ag/AgCl as the reference electrode. For L-Cys
monitoring, the photocurrent response of the BiOI/Ti3C2/GCE
incubated with various concentrations of L-Cys was measured in
0.1 M phosphate buffer (PH 7.4) by applying a bias potential
of ꢁ0.2 V under visible light irradiation (300 W Xenon lamp) at
room temperature.
3. Results and discussion
3.1. Morphology and structure characterization of BiOI and BiOI/
Ti3C2 heterostructure
The crystal structure of the synthesized sample was analyzed by
X-ray diffraction (XRD). As shown in Fig. 1A, after two-step treat-
ments of the etching and delaminating, the typical diffraction peak
assigned to Ti3AlC2 at 39ꢀ disappear, and the intense peak ascribed
to the (002) crystal plane shifts from 9.4ꢀ to 6.3ꢀ, certifying the
aluminum layer was successfully removed, and Ti3AlC2 has
BiOI has a flower-like structure with an average grain size of 4 mm.
Fig. 1E exhibits the SEM image of BiOI/Ti3C2 heterostructure, in
which the flower-like BiOI was wrapped with Ti3C2 tightly. The
corresponding energy-dispersive X-ray (EDX) spectroscopy map-
ping results confirm the presence and uniform distribution of Ti, C,
Fig. 1. (A) XRD patterns. (B) AFM image and (C) corresponding height profile of Ti3C2. (D) TEM image of Ti3C2. (E) SEM image and (F) the corresponding mapping images of BiOI/
Ti3C2 heterostructure. (G, H) TEM and (I, J) HRTEM images of BiOI/Ti3C2 heterostructure.
3

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Journal of Alloys and Compounds xxx (xxxx) xxx
Bi, O, and I (Fig. 1F). Moreover, the surface of the flower-like BiOI
was covered with an ultrathin Ti3C2 nanosheet to form a 2D het-
erojunction (Fig. 1G and H). Fig. 1HeJ exhibit the high-resolution
TEM (HRTEM) images of BiOI/Ti3C2 heterostructure. Fig. 1I and J
clearly shows the lattice spacing of 0.232 nm and 0.301 nm,
which are attributed to the (008) crystal plane of Ti3C2 and the
(102) crystal plane of BiOI, respectively. These results demonstrate
the BiOI/Ti3C2 heterostructure with favourable interface interac-
tion, which is beneficial to promote the separation of photo-
generated carrier under light illumination, and thus acquire the
satisfactory PEC response.
The surface chemical compositions and valence states of BiOI/
Ti3C2 heterostructure were analyzed by X-ray photoelectron spec-
troscopy (XPS) spectra. The XPS survey spectrum (Fig. S2A) clearly
certified the existence of all the elements of BiOI/Ti3C2 hetero-
structure, and the result is consistent with that of elemental
mapping in Fig. 1F. Fig. 2A shows high resolution Bi 4f XPS spec-
trum. Two peaks located at 164.2 eV and 158.8 eV are separately
ascribed to Bi 4f5/2 and Bi 4f7/2, which corresponds to Bi3þ of BiOI
[26]. The peaks at binding energy of 529.7 eV and 530.9 are ascribed
to the BieO bonds of BiOI, and the peak ascribed to 532.4 eV in BiOI/
Ti3C2 heterostructure is assigned to OeC]O functional groups in
Ti3C2 (Fig. 2B) [27]. As for I 3d spectrum (Fig. S2B), two peaks
centered at 630.3 eV and 618.8 eV can be assigned to I 3d3/2 and I
3d5/2, respectively. In Fig. 2C, the spectrum of C 1s exhibits four
typical peaks at 281.8, 284.5, 285.9 eV and 287.9 eV. In detail, the C
1s peak located at 284.5 eV belongs to the adventitious carbon
(CeC), while the other three peaks at 281.7 eV and 285.8 eV are
respectively assigned to CeTi and CeO bonds [28]. For Ti 2p region,
the peaks at 457.2 eV and 460.7 eV can be ascribed to TieC bonds,
and two peaks appeared at 458.9 eV and 465.8 eV attributed to
TieO bonds (Fig. 2D). Noteworthily, the fact that the strongest peak
located at 465.8 eV is assigned toTieO bonds, and the characteristic
peak belonging to the TieF bonds cannot be observed powerfully
demonstrates terminal eOH or eO substitute terminal eF. The
result not only further confirms the strong interface interaction
between BiOI and Ti3C2, but also indicates the massive possibility
for Ti3C2 to capture photogenerated electrons from BiOI [29,30].
3.2. Electrochemical analysis of modified electrodes
Optical characterization of the prepared BiOI and BiOI/Ti3C2
heterostructure was researched using the UVevis diffuse spectra.
As can be seen in Fig. 3A, BiOI presents strong absorption in the
visible light range. For BiOI/Ti3C2 heterostructure, an apparent red
shift appears in comparison with BiOI, which benefits the augment
of the visible light absorption. Furthermore, the Tauc plot is fitted,
and then the band gap can be obtained according to the Tauc
equation. As shown in Fig. 3A, the band gap energies (Eg) of BiOI
and BiOI/Ti3C2 heterostructure are 1.9 eV and 1.7 eV, respectively.
The corresponding conduction band (CB) and valence band (VB)
can be calculated by following empirical formula [31]:
ECB
¼
c
e E0 e 0.5 Eg,
(1)
(2)
EVB ¼ ECB þ Eg
Where ECB and EVB denote the conduction band (CB) and valence
band (VB) edge potentials, respectively; represents the absolute
electronegativity of the semiconductor, that is the geometric mean
of the electronegativity, and the value of BiOI is 6.2 eV; E0 in-
c
c
dicates the energy of free electrons on the hydrogen scale
(approximately 4.5 eV). And ECB and EVB of BiOI are calculated to be
0.85 eV and 2.75 eV. Besides, Fig. 3B shows the cathodic PEC
response of the prepared photoelectrodes under visible light illu-
mination. During the cathodic process, it is the photogenerated
holes that is substantially determined for the cathodic photocur-
rent generation and augment. In the meanwhile, the oxygen
Fig. 2. High-resolution XPS spectra of BiOI/Ti3C2 heterostructure for (A) Bi 4f, (B) O 1s, (C) C 1s and (D) Ti 2p.
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Journal of Alloys and Compounds xxx (xxxx) xxx
Fig. 3. (A) UVevis diffuse reflectance spectra and Tauc’s plots of the BiOI and BiOI/Ti3C2 heterostructure. (B) Photocurrent response for the designed sensor of Ti3C2/GCE, BiOI/GCE,
and BiOI/Ti3C2/GCE. (C) MottꢁSchottky plot of BiOI and BiOI/Ti3C2 heterostructure at 1 kHz under dark conditions. (D) OCP response of BiOI and BiOI/Ti3C2 heterostructure under
dark and visible light irradiation. (E) EIS plots of (a) bare GCE, (b) Ti3C2/GCE, (c) BiOI/GCE, and (d) BiOI/Ti3C2/GCE in 0.1 M KCl solution containing 5.0 mM [Fe(CN)6]3-/4- recorded in
the frequency range from 0.1 Hz to 100 kHz. The inset shows the corresponding equivalent circuits. (F) CV curves of different electrodes in 0.1 M KCl solution containing 5.0 mM
[Fe(CN)6]3-/4-
.
dissolved in the electrolyte acts as an acceptor to capture and
consume the photogenerated electrons. Hence, the PEC tests were
executed in an air-saturated electrolyte. The pure BiOI display a
weaker photocurrent intensity (about 51 nA), while the BiOI/Ti3C2
heterostructure shows an enhanced photocurrent intensity (about
334 nA), which is six times as larger as that of BiOI, convincingly
confirming the improvement of charge transfer ability of BiOI/Ti3C2
heterostructure with respect to pure BiOI. Additionally, the
photocurrent intensity of Ti3C2 is extremely weak, verifying that
the introduction of Ti3C2 is functioned to capture the photo-
generated electron in BiOI/Ti3C2 heterostructure. Owing to its
excellent electrical conductivity, Ti3C2 can promote photogenerated
electrons transfer, greatly improving the photoelectric conversion
efficiency of BiOI/Ti3C2 heterostructure.
Fig. 3C shows the Mott-Schottky (M ꢁ S) plots of BiOI and BiOI/
Ti3C2 heterostructure, which can verdict the semiconductor type.
The slope of linear part in M ꢁ S plots is positive for n-type semi-
conductor and negative for p-type semiconductor [32]. Obviously,
the BiOI and the BiOI/Ti3C2 heterostructure display the properties
of p-type semiconductor. Additionally, the carrier density is studied
based on the slopes of the M ꢁ S plots via the following equation
[33]:
Integrated the fact that the higher
D
OCP (OCP ¼ OCPlight - OCPdark
)
implied the larger band bending degree and the faster charge
transfer at the electrode-electrolyte interface, BiOI/Ti3C2 hetero-
structure are confirmed to have better PEC activity [33,34].
Fig. 3E presents the electrochemical impedance spectroscopy
(EIS) plots recorded on various electrodes, and the inset shows the
corresponding equivalent circuits. The diameter of the semicircle in
the high frequency region represents the charge transfer resistance
(Rct). Herein, the Rct value for bare GCE is evaluated to be 90.09
After the electrode is modified with BiOI, the Rct value is up to
331.7 , while the Rct value of BiOI/Ti3C2 heterostructure modified
electrode decreased to be 225.8 . This result indicates that Ti3C2 is
U.
U
U
beneficial to promote electron transfer and ensure better PEC per-
formance for BiOI/Ti3C2 heterostructure. Furthermore, the Cyclic
Voltammetry (CV) curves of various modified electrodes were
determined in 1 M KCl solution containing 5.0 mM [Fe(CN)6]3-/4-
(Fig. 3F). The value of anode peak current (ip) for BiOI/GCE is
77.8
mA, lower than 102.5
mA of bare GCE. After modified with BiOI/
Ti3C2 heterostructure, the ip value increases to 87.4
is due to the good conductivity of Ti3C2, which improves the surface
activity of the electrode and leads to an increase in peak current.
mA. The increase
3.3. Optimization of experimental conditions
Nd ¼ (2/eεε0) [d (1/C2)/dV]ꢁ1
,
(3)
To obtain a better PEC performance, the influences of concen-
tration of BiOI/Ti3C2 heterostructure, pH value of phosphate buffer
and incubation time of target during PEC detection toward L-Cys is
studied. As shown in Fig. S3A, the photocurrent intensity increases
with a concentration increase of BiOI/Ti3C2 heterostructure from
0.5 to 2 mg mLꢁ1. Since the further augment of BiOI/Ti3C2 hetero-
structure, the photocurrent intensity decreases slightly, probably
due to the thicker coating hindering electron transfer. Moreover,
the incubation time of L-Cys has a significant effect on the perfor-
mance of the PEC sensor. As can be seen in Fig. S3B, with the
extension of incubation time, the photocurrent intensity decreases
rapidly up and then levels off after 30 min. Hence, 2 mg mLꢁ1 for
where e (e ¼ 1.602 ꢂ 10ꢁ19 C) represents electronic charge unit, ε is
the dielectric constant of material, ε0 (ε0 ¼ 8.85 ꢂ 10ꢁ14 F cmꢁ1) is
the permittivity of vacuum, and V denotes applied potential. As
shown in Fig. 3C, the slope of BiOI/Ti3C2 heterostructure is much
smaller compared to the pure BiOI, implying a higher carrier den-
sity that is closely relevant to the photocurrent intensity. Besides,
we executed the open circuit potential (OCP) test to further
investigate the property of BiOI/Ti3C2 heterostructure. Fig. 3D
shows that OCP of the two samples dramatically shifts to much
more positive potential with irradiation, and then slowly decreases
without irradiation, indicating the effective hole transfer ability.
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Journal of Alloys and Compounds xxx (xxxx) xxx
the optimal concentration of BiOI/Ti3C2 heterostructure and 30 min
for the optimal incubation time of L-Cys are applied for further
experiment.
The photocurrent intensity decreases in the case of target existing
or target and potential interferences coexisting, since only the
group of eSH in the structure of L-Cys can forcefully and effectively
interact with the surface of the modified photoelectrode. The re-
sults indicate that the proposed PEC platform possesses good
selectivity towards L-Cys sensing. By the way, as far as we know
that L-Cys detection is prone to be affected by the possible oxida-
tion reactions happened in the interface of PEC platform, the pro-
posed PEC biosensor on the basis of the cathodic photocurrent
signal can skillfully evade the issue, and then achieve more accurate
PEC performance. Moreover, the stability of the PEC biosensor was
studied. Fig. 4D shows the photocurrent intensity of prepared
3.4. Photoelectrochemical sensing of L-Cys
Under the optimal experimental conditions, the proposed PEC
biosensor was applied to the quantitative determination of L-Cys.
Fig. 4A displays the relationship between the concentration of L-Cys
and photocurrent intensity. The larger the concentration of L-Cys,
the weaker the photocurrent intensity. The results may be caused
by the steric hindrance effect, inducing the photocurrent response
decreases spontaneously for the established PEC platform. More-
over, the photocurrent intensity is well linear to the logarithm of L-
photoelectrode containing 1 mM L-Cys. After 15 times testing based
on the off-on irradiation cycle, the photocurrent intensity remained
95% of the initial one, which shows accredited stability. Besides, the
reproducibility of the sensor was also examined via contrasting six
individual modified photoelectrodes. As depicted in Fig. S4, a
relative standard deviation (RSD) of 3.4% was obtained, indicating
good reproducibility of the proposed PEC sensing platform.
Cys concentration in the range from 0.01 nM to 10 mM (Fig. 4B). The
corresponding linear regression equation is I (
m
A) ¼ ꢁ0.034 log c
(nM) þ 0.002 (correlation coefficient R2 ¼ 0.998). Besides, the limit
of detection (LOD) is estimated to be 0.005 nM based on the
analytical function of LOD ¼ K
s/S, where K is used as 3, s is the
standard deviation of the blank solution (n ¼ 10), S is the slope of
regression line. Compared with the various method for L-Cys
determination reported in the previous reports (Table S1), the
proposed PEC sensor shows a wider linear range and lower
detection limit.
4. Conclusion
In summary, BiOI/Ti3C2 heterostructure has been successfully
synthesized, and applied to PEC analysis. Highly sensitive, stability
and selective L-Cys monitoring was achieved via BiOI/Ti3C2 heter-
ostructure serving as a photocathode. Considering the Schottky
junction-based PEC sensing platform, we demonstrate that the
combined merits of proposed photocathode of built-in electric field
generation and contact resistance diminution can efficiently boost
the PEC performance with “signal-off” pattern. Given those unique
properties, the LOD of L-Cys sensing based on the PEC analytic
technique is as low as 0.005 nM. The study unveils the immense
potential of Ti3C2 MXene-based photoactive heterostructure in the
establishment of PEC sensing platform, and paves the road for
3.5. Selectivity, stability and reproducibility of the PEC sensor
Since the selectivity is a vital factor for the PEC performance of
proposed biosensor, interference measurement was carried out by
evaluating the PEC response (1
10 M UA, 10 M DA, 10 M Glu, 10
and 20
M Clꢁ in 0.1 M phosphate buffer). As shown in Fig. 4C, the
m
M L-Cys, 10
m
m
M L-Lys, 10
m
m
M L-Arg,
m
m
m
m
M Cu2þ,10
M SO24ꢁ,10
M Ca2þ
m
PEC response signal has almost no change before and after the
existence of above mentioned interferences instead of the target.
Fig. 4. (A) PEC responses of the fabricated sensor in 0.1 M phosphate buffer containing 0, 0.01, 0.1, 1, 10, 100, 1000 and 10,000 nM L-Cys at ꢁ0.2 V. (B) The corresponding calibration
curve for the determination of L-Cys. (C) The evaluation of specificity of the proposed PEC sensor to the interfering substances (photocurrent intensity of 1 M L-Cys, 10 M L-Lys,
10 M L-Arg, 10 M UA, 10 M DA, 10 M Glu, 10
M Cu2þ,10 M SO42ꢁ,10 M Ca2þ and 20 M Clꢁ in 0.1 M phosphate buffer). (D) Stability of PEC sensor in 0.1 M phosphate buffer
containing 1 M L-Cys.
m
m
m
m
m
m
m
m
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6

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Journal of Alloys and Compounds xxx (xxxx) xxx
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Cui Ye: Writing - review & editing, Funding acquisition, Super-
vision. Zhen Wu: Writing - original draft, Data curation. Keyi Ma:
Writing - original draft, Data curation. Zhuohao Xia: Writing -
original draft, Data curation. Jun Pan: Formal analysis. Minqiang
Wang: Writing - review & editing, Formal analysis. Changhui Ye:
Funding acquisition.
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Ti3C2 and TiO2 exposing {001} facets toward enhanced photocatalytic activ-
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Declaration of competing interest
[17] Z. Kang, Y. Ma, X. Tan, M. Zhu, Z. Zheng, N. Liu, L. Li, Z. Zou, X. Jiang, T. Zhai,
Y. Gao, MXeneesilicon van der waals heterostructures for high-speed self-
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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.
Acknowledgement
[20] Y. Zhu, K. Yan, Z. Xu, J. Wu, J. Zhang, Cathodic “signal-on” photo-
electrochemical aptasensor for chloramphenicol detection using hierarchical
porous flower-like Bi-BiOI@C composite, Biosens. Bioelectron. 131 (2019)
79e87.
This work was supported by the National Natural Science
Foundation of China (Grant No. 21904116). The authors are grateful
to Prof. Changhui Ye (Deceased on Nov 1 2019) for assistance.
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