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Locally superengineered cascade recognition–
quantification zones in nanochannels for sensitive
enantiomer identification†
Junli Guo,
a
Huijie Xu,
a
Junjian Zhao,
a
Zhida Gao,
a
Zeng-Qiang Wu *
b
and Yan-Yan Song *
a
As an intriguing and intrinsic feature of life, chirality is highly associated with many significant biological
processes. Simultaneous recognition and quantification of enantiomers remains a major challenge. Here,
a sensitive enantiomer identification device is developed on TiO
2
nanochannels via the design of
cascade recognition–quantification zones along the nanochannels. In this system, b-cyclodextrin (b-CD)
is self-assembled on one side of the nanochannels for the selective recognition of enantiomers;
CuMOFs are designed as the target-responsive partners on the other side of the nanochannels for the
quantification of enantiomers that pass through the nanochannels. As a proof-of-principle of the
cascade design, arginine (Arg) enantiomers are tested as the identification targets. The L-Arg molecules
selectively bind in the recognition zone; D-Arg molecules pass through the recognition zone and then
interact with the quantification zone via a specialized reduction reaction. As verified by nanofluidic
simulations, because of the confinement effect of nanoscale channels combined with the condensation
effect of porous structure, the in situ reaction in the quantification zone contributes to an
unprecedented variation in transmembrane K
+
flux, leading to an improved identification signal. This
novel cascade-zone nanochannel membrane provides a smart strategy to design multifunctional
nanofluidic devices.
Introduction
Chiral discrimination is a prominent feature of the living world.
The body is amazingly chiral-selective, exhibiting different
physiological responses to different enantiomers.
1,2
Specically,
some molecules may produce the desired therapeutic activities,
while their isomers may be inactive or produce unwanted
effects. Amino acids are important bioactive substances.
Studies on the enantiomeric recognition of amino acids can
accelerate the understanding of chiral recognition in biological
systems, thus promoting the development of designed molec-
ular devices in biochemical and pharmaceutical elds.
3
Although various strategies such as molecular imprinting,
4,5
ligand exchange,
6,7
and supramolecular interactions
8
have been
proposed for stereospecic molecular discrimination, enantio-
selective recognition of amino acids is still challenging because
of similar physicochemical properties of optical isomers.
9,10
Porous metal–organic frameworks (MOFs) represent a new
class of inorganic–organic supramolecular hybrid materials
comprising ordered networks formed from organic electron-
donor linkers and metal cations.
11
Their tunable pore size and
characteristic functionality that are similar to those of the active
sites in proteins suggest that they may act as promising host
matrices for molecular recognition.
12
In addition, considering
the inherent connement effect within their pores, MOFs can
serve as a preconcentrator to enhance host–guest interactions.
Furthermore, the surface designability of MOFs enables the
incorporation of appropriate specic interaction sites into
a scaffold using strategic organic chemistry techniques.
13
Specic and unique molecular recognition between porous
MOFs and guest substrates is the design criteria for the target
recognition applications.
14
In this case, a combination of signal
transduction pathways and accessible MOF porosity will impart
them with the capability of transducing the host–guest behavior
a
College of Sciences, Northeastern University, Shenyang 110819, China. E-mail:
yysong@mail.neu.edu.cn
b
School of Public Health, Nantong University, Nantong, 226019, China. E-mail:
zqwu@ntu.edu.cn
†Electronic supplementary information (ESI) available: Digital photos of each
step modication; the SEM images, UV-vis absorption spectra and FTIR spectra
of TiO
2
M, CuNPs/TiO
2
M, CuTCA/TiO
2
M and CuTCA/TiO
2
M reaction with NO;
XPS spectra of TiO
2
Mandb-CD/TiO
2
M; EDS mapping of b-CD–CuTCA/TiO
2
M;
zeta potentials of each step modication; XRD patterns of CuTCA and
CuTCA/TiO
2
Maer immersion in acid solution; I–Vcurves of different samples
for Arg detection; I–Vcurves of CuTCA/TiO
2
M for the detection of Arg at
different pHs and KCl concentrations; I–Vcurves of b-CD/TiO
2
Mand
b-CD–CuTCA/TiO
2
M for sensing different enantiomers; current–time response
of b-CD–CuTCA/TiO
2
M; computer domain and boundaries of FEM models;
boundary conditions of FEM models; schematic diagram of the setup for CuNP
modication and chiral Arg detection; comparison of various methods for
L/D-Arg detection. See https://doi.org/10.1039/d2sc03198a
Cite this: Chem. Sci., 2022, 13, 9993
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Received 7th June 2022
Accepted 8th August 2022
DOI: 10.1039/d2sc03198a
rsc.li/chemical-science
© 2022 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2022, 13, 9993–10002 | 9993
Chemical
Science
EDGE ARTICLE
to detectable changes; thus, porous MOFs are postulated as
promising candidates for sensing applications.
15
In general, to
improve the performance of porous MOFs in enantiomer
detection, it not only requires the enhancement of the detection
ability of recognition units via specic host–guest interactions,
but also involves constructing a reliable and sensitive signal
transduction way that can provide information about host–
guest interactions.
16
Inspired by biological ion channels,
17,18
articial ion nano-
channels with asymmetric structures have been widely con-
structed to mimic biological channels and applied in energy
conversion,
19
biochemical sensing,
20,21
and other elds.
22,23
Asymmetric articial nanochannels with tailorable size and
surface functionality are useful for mimicking ionic transport in
biological ion channels.
24
The changes in ionic transport char-
acteristics can be directly monitored from the current–voltage
(I–V) curves. Specically, varying the asymmetric structure has
been demonstrated as an effective way to induce remarkable
changes in I–Vproperties.
25,26
Recently, heterogeneous articial
nanochannels, in which a composite nanochannel has two or
more chemical compositions, have attracted much attention
because of their multiple functions, novel features, and opera-
tional feasibilities.
27–31
These features are highly attractive for
the design of sensors with a similar goal: to develop large
numbers of low-cost sensors with sensitive performance to
enable extensive application. Depending on the functions of
individual materials, heterogeneous channels are largely
desirable as a promising network for combining the
enantiomer-recognition device and chiral-quantication device
in a system. The development of sensitive enantiomer sensors is
still a great challenge because of the small difference in the
affinities for ligands between the target enantiomers.
Free-standing TiO
2
nanotube/nanochannel arrays provide
a new platform as articial solid-state nanochannels. Particu-
larly, the intrinsic photocatalytic properties of TiO
2
make it
possible to achieve subregional modication with two or more
compositions in TiO
2
nanotubes/nanochannels, thus achieving
asymmetric heterogeneous nanochannels. Here, we investi-
gated designs inspired by biological ion channels to develop
enantioselective recognition sensors based on a free-standing
TiO
2
nanochannel membrane (TiO
2
M). The asymmetric
TiO
2
M contains two different function zones along the
nanochannel-enantiomer recognition zone and quantication
zone. Arginine (Arg), an important functional molecule for cell
division, human brain chemistry, immune responses, blood
vessel dilation, and neurotransmission
32
was applied as the
target enantiomer. On one side of the nanochannels, b-cyclo-
dextrin (b-CD) modication was performed for enantiomer
recognition,
33
which allowed one Arg enantiomer to pass
through. On the other side of the nanochannels, the limit of
light penetration in TiO
2
materials was utilized to trigger the
growth of Cu nanoparticles (CuNPs),
34
which subsequently react
with organic ligand 4,40,400-tricarboxytriphenylamine (H
3
TCA) to
generate CuTCA. When Arg reaches the quantication zone, it
reacts with H
2
O
2
to generate reductive NO,
35,36
which further
induces Cu(II)-nodes on Cu-MOFs to produce Cu(I).
37
This
charge variation reported here was conrmed by a series of
experimental studies from the transmembrane currents as well
as the screening of uorescence recovery of H
3
TCA ligands.
Such a cascade system provides a smart, sensitive, and reliable
strategy to design multifunctional devices.
Results and discussion
Fabrication and characterization of asymmetric membranes
Inspired by biological ion channels, we designed asymmetric
nanochannels composed of cascade recognition and quanti-
cation zones along the TiO
2
nanochannels for the enantiose-
lective detection of Arg enantiomers (Fig. 1a). The recognition
zone was anchored with b-CD, a widely used host molecule for
chiral recognition;
33
the quantication zone was coated with
CuTCA. The target enantiomers were transported from the
recognition zone to the quantication zone. Owing to the larger
specicaffinity of b-CD in the recognition zone with one of the
Arg enantiomers, the other enantiomer can more easily pass
through the recognition zone and reach the quantication
zone. In the quantication zone, these Arg molecules react with
H
2
O
2
to generate NO,
35,36
which subsequently reduces the Cu(II)-
nodes on Cu-MOFs to Cu(I). The charge variation is asymmet-
rically located at the quantication zone of the nanochannels,
which is expected to provide remarkable changes in the trans-
membrane ionic current.
Fig. 1b shows the procedure for preparing asymmetric
nanochannels. TiO
2
M was fabricated by the electrochemical
anodization of Ti foil in a lactic acid-containing glycerol/NH
4
F
electrolyte (details are described in the Experimental
section).
38,39
The formed amorphous TiO
2
M was annealed at
450 C in air for 2 h to remove the remaining organic electrolyte
and meanwhile transform the amorphous TiO
2
into the anatase
phase, which has a better photocatalytic activity than the
amorphous TiO
2
.
40,41
The resulting pale-colour membrane
implies that most of the contaminants inside the TiO
2
nano-
tubes have been burned off(Fig. S1a and b†). As characterized
by scanning electron microscopy (SEM), the as-formed TiO
2
M
was composed of aligned nanochannels with a base entrance of
150 20 nm in diameter (Fig. 1c) and a tip entrance of 40
10 nm (Fig. S2a†). The membrane thickness was estimated to be
35 mm (Fig. S2b†), and the asymmetric structure can be
further conrmed from the cross-section SEM images (Fig. S2c
and d†). The selective decoration of CuNPs was achieved by
combining the intrinsic photocatalytic activity of TiO
2
nano-
channels with a recently reported interfacial growth strategy.
42
A
bias (+1.0 V) was applied to drive the migration of Cu
2+
ions
from the tip entrance to the base entrance of TiO
2
M, and an
LED (365 nm) was used to irradiate the base entrance side to
trigger the photocatalytic reduction to form CuNPs. Because the
light attenuation increases with the optical path length through
the light absorber,
43
CuNPs were mainly formed on the base
entrance and the closed channel wall (Fig. S3†). To ensure the
growth of CuNPs only on the base entrance side, experimental
conditions such as the irradiation time (Fig. S4†) and Cu
2+
concentration (Fig. S5a–d†) were optimized. Under the optimal
conditions of 5.0 mM Cu
2+
and 120 min LED irradiation, CuNPs
were mainly found to be distributed on the base entrance and
9994 |Chem. Sci., 2022, 13, 9993–10002 © 2022 The Author(s). Published by the Royal Society of Chemistry
Chemical Science Edge Article
wall with a high density, and the depth of the CuNP layer was
determined as 5.0 mm from the SEM image (Fig. S5e and f†).
Using these CuNPs as the precursor of Cu
2+
, uniform and well-
dispersed CuTCA nanocrystals were further fabricated on the
base entrance side via a hydrothermal process in H
3
TCA.
37
The
resulting sample exhibits a green color (Fig. S1d†). As shown in
Fig. 1e, the CuTCA nanocrystals appear on the tube wall close to
the base entrance with a high density. No CuTCA was found on
the tip entrance side (Fig. S6†), indicating successful asym-
metric decoration. The transformation of CuNPs/TiO
2
Mto
CuTCA/TiO
2
M was conrmed from the X-ray diffraction (XRD)
patterns and Fourier transform infrared (FTIR) spectra. In the
XRD patterns, the presence of the characteristic peak of CuTCA
at 7.5and the disappearance of the characteristic peak of
CuNPs at 43.5also show the successful transformation of
CuNPs to CuTCA (Fig. 2a). The FTIR bands at 778, 1172, 1273,
and 1321 cm
1
are consistent with the absorption bands
recorded for bulk CuTCA (Fig. S7†). In Fig. 2b, the transmission
electron microscopy (TEM) images show the morphology of
CuTCA/TiO
2
M, maintaining a typical nanochannel structure. In
Fig. 2c–e, the high-resolution TEM (HRTEM) images show that
the wall of the nanochannel is covered with a layer of small
nanocrystals, and the characteristic spacing of 0.35 nm can be
indexed to the (101) lattice plane of anatase TiO
2
(JCPDS #21-
1272).
To prepare the recognition zone, b-CD was used as the host
molecule for Arg enantiomers and anchored onto TiO
2
nano-
channels from the tip side of the TiO
2
M using sodium phenyl
phosphate (PP) as the connecting bridge. As shown in Fig. 1b,
PP modication was rst performed on the nanochannel wall
via the well-known affinity interaction between phosphoric acid
and Ti–OH groups.
44–46
Then, b-CD was introduced onto the
nanochannel wall through the host–guest interaction between
b-CD molecules and phenyl on PP.
33
The PP loading amount was
found to depend on the pH of the PP solution (Fig. S8a†). The
solution pH was optimized as pH 4 (Fig. S8b†), where the
highest PP loading amount was achieved. The PP modication
leads to the appearance of absorption bands at 1166 and
1740 cm
1
in the FTIR spectra (Fig. S9†), which are indexed to
the P–O stretching and bending modes, respectively.
45
X-ray
photoelectron spectroscopy (XPS) analysis was carried out to
conrm b-CD loading onto PP–TiO
2
M (Fig. S10†). Because of the
presence of hydrocarbon groups on the b-CD molecule, the
enhancement of C 1s signals (Fig. S10b†) and the decrease in
the intensity of P 2p signals (Fig. S10c†) indicate the successful
loading of b-CD onto PP–TiO
2
M.
To clearly show the spatial arrangement of b-CD and CuTCA
on different sides of the nanochannels, b-CD/TiO
2
M and
CuTCA/TiO
2
M zones were selectively dyed with two different
uorescent probes (Fig. 2f–h). Riboavin sodium phosphate
(RFMP), a green uorescent probe, was labeled onto b-CD/
TiO
2
M through the host–guest interaction between b-CD and
thymine-like groups on RFMP.
33
Rhodamine B (RhB, 0.56 nm
1.18 nm 1.59 nm), a typical red uorescent probe, was
encapsulated in the pores of CuTCA (the pore radius is about 1
nm).
37
Upon excitation with a light of 460–495 nm, as shown in
Fig. 2f, one side of the membrane shows brilliant green uo-
rescence, indicating the b-CD/TiO
2
M zone. Under an excitation
wavelength of 530–550 nm, as shown in Fig. 2g, the other side of
the membrane exhibits red uorescence, indicating the CuTCA/
TiO
2
M zone. Apparently, clearly divided regions with different
colors are located on either side of the membrane (Fig. 2h). In
Fig. 1 (a) Schematic illustration of b-CD–CuTCA/TiO
2
M and the principle of the recognition–quantification membrane. (b) Illustration of the
preparation of b-CD–CuTCA/TiO
2
M from TiO
2
M. SEM images of the base entrance side and cross-section for (c) TiO
2
M and (d) CuTCA/TiO
2
M.
© 2022 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2022, 13, 9993–10002 | 9995
Edge Article Chemical Science
addition, the energy-dispersive X-ray spectroscopy (EDS) images
also verify that Cu and C elements mainly appear on one side of
the as-proposed membrane, while the P element appears on the
other side of the membrane (Fig. S11†). These results show the
successful fabrication of the two functional zones on the
different sides of TiO
2
M. The content of the Cu element in the
resulting b-CD–CuTCA/TiO
2
M sample was further determined
as 10.5 wt% by inductively coupled plasma-optical emission
spectroscopy (ICP-OES) analysis.
Enantioselective characterization of the membrane
As a well-known enantioselective guest molecule, b-CD has
a strong affinity with the enantiomer through the interaction of
the polar amino and carboxyl groups of the target enantiomer
with the hydroxyl groups of b-CD, thus forming hydrogen bonds
at the mouth of the b-CD.
33,47
To estimate the b-CD
enantioselectivity-induced enantiomer transport difference in
the as-proposed membrane, directional diffusion experiments
were performed. A schematic setup of the diffusion experiment
is shown in Fig. 3a. The b-CD/TiO
2
M (0.38 cm
2
in area) was xed
in the middle of the two cells. The lecell was lled with 1.0 mL
of 50 mMD-Arg or L-Arg solution, which served as the feed
solution. The right cell only contained deionized water. Fig. 3b
shows the circular dichroism (CD) spectra of the initial L/D-Arg
in the lecell (solid lines) and the L/D-Arg in the right cell aer
a 24 h-diffusion experiment (dashed lines). Based on the
absorption peak intensities, the crossed D-Arg is nearly 16 times
the crossed L-Arg, verifying that D-Arg has much higher trans-
port ability than L-Arg when passing through the recognition
zone. This difference can be attributed to the different feasi-
bility for hydrogen bonds because of the stereoselectivity (the
steric hindrance at a chiral site), thus endowing b-CD-modied
channels with the enantioselective ability.
47
The I–Vcurves of the membranes were recorded using
a home-made electrochemical cell (Fig. S12†). The as-prepared
membrane was placed in the middle of two half cells, and two
Ag/AgCl electrodes were inserted into the cell chamber con-
taining 1.0 mM KCl. Fig. 3c shows the I–Vcurves of the
membrane recorded at each step, and the corresponding ionic
current changes at +1.0 V are shown in Fig. 3d. The trans-
membrane ionic current is enhanced aer CuTCA growth,
which can be attributed to the increased surface charge
Fig. 2 (a) XRD patterns of TiO
2
M, CuNPs/TiO
2
M, CuTCA/TiO
2
M, and CuTCA. (b–e) TEM images of b-CD–CuTCA/TiO
2
M. Fluorescence images
of (f) the recognition zone (b-CD/TiO
2
M) labeled with RFMP (excitation wavelength 460–495 nm), (g) quantification zone (CuTCA/TiO
2
M)
labeled with RhB (excitation wavelength 530–550 nm), and (h) whole asymmetric membrane (b-CD–CuTCA/TiO
2
M) under bright field.
9996 |Chem. Sci., 2022, 13, 9993–10002 © 2022 The Author(s). Published by the Royal Society of Chemistry
Chemical Science Edge Article
densities induced by the organic ligand H
3
TCA. Notably, aer
the subsequent PP and b-CD modication, the decrease in the
ionic current can be observed in two steps. To gain a clear
insight on the contribution of CuTCA, PP, and b-CD to the ionic
current, zeta potentials of the membrane were measured at each
step (Fig. 3e and S13†). TiO
2
M has a negatively charged surface
with a zeta potential of 37.8 mV. CuNPs/TiO
2
M shows a posi-
tively charged surface (+14.9 mV), which can be attributed to the
remaining Cu
2+
adsorbed on the surface. Owing to the presence
of plenty of –COOH groups on the H
3
TCA ligand, the growth of
CuTCA results in a negatively charged surface (22.2 mV). It
should be noted that the zeta potential exhibits ignorable
changes aer modication with PP and b-CD. Therefore, a clear
decrease in the ion ux of PP–CuTCA/TiO
2
M and b-CD–CuTCA/
TiO
2
M could be related to the hydrophobicity and the steric
hindrance of the phenyl group and b-CD.
The quantication ability of the CuTCA zone was evaluated
from the I–Vcurves of CuTCA/TiO
2
M in a 1.0 mM KCl solution
at room temperature using different concentrations of L-Arg as
the target. The ionic currents were found to strongly decrease
with the concentration of L-Arg in the presence of 1.0 mM H
2
O
2
(Fig. S14a†), indicating that the amount of Cu(I) was dependent
on the Arg concentration in the quantication zone. For
comparison, the I–Vcurves of CuTCA/TiO
2
M were also recorded
in the absence of H
2
O
2
(Fig. S14b†)orL-Arg (Fig. S14c†), which
showed ignorable changes in the ionic currents. This indicates
that the reduction reaction was limited. In addition, negligible
changes in the ionic currents were found for bare TiO
2
M in the
presence of both L-Arg and H
2
O
2
(Fig. S14d†), indicating that
CuTCA was the key component for achieving a sensitive
response in ion ux. To obtain satisfactory sensitivity in Arg
quantication, the effect of solution pH and KCl concentration
on transmembrane ionic currents was also optimized. For the
Arg–H
2
O
2
reaction, the generated NO is related to the pH of
electrolyte (Fig. S15†) with a remarkable drop at pH 4. Addi-
tionally, larger ionic currents were recorded in higher concen-
trations of KCl solution (Fig. S16†). Therefore, the following I–V
measurements were performed in 0.5 mM KCl (pH 4).
Signal magnication in I–Vcurves for chiral recognition
In the resulting nanochannel-based sensing device, the enan-
tioselectivity is believed to originate from the b-CD based
recognition zone. As shown in Fig. 4a, b-CD/TiO
2
M also exhibits
a change in the ionic current in the presence of 10 mML-/D-Arg
with a larger current change for L-Arg. However, it is difficult to
obtain a distinct response for the ionic ux when the Arg
concentration is lower than this concentration (10 mM). Fig. 4b
shows the ionic currents of b-CD–CuTCA/TiO
2
M for sensing 0.1
mML/D-Arg. Obviously, CuTCA modication is helpful for
amplifying the ionic current response. Compared to the ionic
currents monitored on b-CD/TiO
2
M, it should be noted that b-
CD–CuTCA/TiO
2
M exhibits current responses to D-Arg and L-Arg
in a reverse direction mode. For example, a larger current
decrease was observed for D-Arg when b-CD–CuTCA/TiO
2
M was
used. In this case, this current decrease originates from the
Cu(I) generation by Cu(II) reduction. The larger affinity between
L-Arg and b-CD enables more L-Arg molecules to be captured in
the recognition zone than D-Arg. As a result, more D-Arg mole-
cules arrived in the CuTCA based quantication zone and
reduced Cu(II) to Cu(I), thus leading to a more substantial
decrease in the ionic current. In contrast, when more L-Arg
molecules were captured in b-CD/TiO
2
M, the greater steric
hindrance thus resulted in a larger decrease of ionic current.
FTIR spectroscopy was employed to investigate the reaction
between CuTCA and NO. The FTIR spectra display two new
Fig. 3 (a) Schematic illustration of Arg transport through the b-CD/TiO
2
M. (b) CD spectrum of 50 mMD/L-Arg (solid line) and D/L-Arg permeated
solution after 24 h for b-CD/TiO
2
M (dashed line). (c) I–Vcurves for each step of modification. (d) The current values of each step of modification
at +1.0 V. (e) Corresponding zeta potential of each step of modification. (f) Ionic current changes (DI) at +1.0 V of CuTCA/TiO
2
MinL-Arg with
H
2
O
2
(gray), L-Arg (red), and H
2
O
2
(green), and DIat +1.0 V of TiO
2
MinL-Arg with H
2
O
2
(yellow).
© 2022 The Author(s). Published by the Royal Society of Chemistry Chem. Sci., 2022, 13, 9993–10002 | 9997
Edge Article Chemical Science
bands at 1683 and 1763 cm
1
(Fig. S17†), which can be ascribed
to the anti-symmetric and symmetric N–O stretching of Cu(I)–
NO adducts.
48
These results indicate that some of the Cu(II)
centers were reduced to Cu(I) by NO.
The sensing performance of the as-proposed b-CD–CuTCA/
TiO
2
M was further evaluated from the I–Vcurves in the presence
of Arg enantiomers. Specically, on the CuTCA side of the
membrane (quantication zone), the half-cell was lled with
0.5 mM KCl solution containing 1.0 mM H
2
O
2
; on the b-CD side
of the membrane (recognition zone), the half-cell was lled with
0.5 mM KCl solution containing the target enantiomers.
Fig. S18a and b†show the transmembrane ionic currents of b-
CD–CuTCA/TiO
2
M in the presence of different concentrations
of D-Arg and L-Arg (2.5–100 nM), respectively. The ionic current
changes (DI,dened as DI¼I
0
I, where I
0
and Iare dened as
the ionic current at +1.0 V, derived from the I–Vcurves recorded
in an electrolyte containing H
2
O
2
aer 30 min of reaction) at
different D/L-Arg concentrations are shown in Fig. 4c. Although
the DIvalues increased with Arg concentration from 2.5 to
100 nM, the current changes induced by D-Arg are clearly larger
than that of L-Arg at the same concentrations, suggesting that
fewer L-Arg molecules reached the quantication zone. These
results are consistent with the directional diffusion results
(Fig. 3b), verifying that the b-CD based recognition zone has
a stronger affinity with L-Arg than D-Arg. The resulting b-CD–
CuTCA/TiO
2
M exhibits a good linear response to Arg sensing
over the range of 2.5–100 nM (Fig. 4c). The limit of detection
(LOD) was estimated to be 0.7 nM using a 3SD/Lmethod (SD is
the standard deviation of control, and Lis the slope of the
calibration curve). Notably, when a high concentration of target
enantiomers (0.25–10 mM) was applied, the DIvalues of D-orL-
Arg exhibit smaller differences at the same concentration
(Fig. 4d and S18†), which can be attributed to a saturated
adsorption state of D- and L-Arg in the recognition zone. In this
case, most of the recognition sites provided by the b-CD zone
were rapidly occupied. Compared with most of the recent
reports on L/D-Arg recognition (Table S1†), this enantiomer
sensing device showed obvious advantages, such as easy oper-
ation, low cost, and a lower LOD value.
49–52
To demonstrate the chiral selectivity of the as-proposed
asymmetric membrane for identifying Arg enantiomers, the
recognition performance for other enantiomers, i.e.,L/D-gluta-
mic acid (L/D-Glu), L/D-phenylalanine (L/D-Phe), and L/D-histidine
(L/D-His) was evaluated. For comparison, the chiral recognition
ability of b-CD/TiO
2
M was also tested (Fig. 4e, S19 and S20†).
The I–Vcurves of b-CD/TiO
2
M showed ignorable variation when
the concentration of these enantiomers was 0.1 mM (Fig. S19†),
indicating poor sensitivity. When the enantiomer concentration
was increased to 10 mM, b-CD/TiO
2
M samples exhibited similar
recognition ability for all the enantiomer groups (Fig. S20†)
a larger ionic current variation was found for all the lechiral
enantiomers (Fig. 4e). In contrast, the b-CD–CuTCA/TiO
2
M
exhibited a remarkable difference response while sensing L/D-
Arg (Fig. S21†). As shown in Fig. 4f, compared with L/D-Arg, the
ionic currents recorded for Glu, Phe, and His enantiomers show
smaller changes. Furthermore, while applying for 0.1 mM Arg
enantiomer recognition, the DIvalue recorded on b-CD–CuTCA/
TiO
2
M (Fig. 4f) is 147 times higher than that of b-CD/TiO
2
M
(Fig. S19†). These results veried the excellent enantioselectivity
and sensitivity of b-CD–CuTCA/TiO
2
M upon the recognition of
L/D-Arg, and the remarkable sensing performance can be
ascribed to the cascade of the recognition zone and quanti-
cation zone along the TiO
2
nanochannels. Another important
criterion to evaluate the enantiomer sensing platform is the
stability. The tolerance of CuTCA under the experimental
conditions was investigated by XRD patterns (Fig. S22†).
Compared with the freshly prepared CuTCA and CuTCA/TiO
2
M,
the samples show satisfactory stability aer immersion in an
aqueous KCl (0.5 mM, pH 4) solution for 12 h.
Nanouidic simulations to obtain insight into the mechanism
of signal magnication
It is well known that the ion transport behavior can be regulated
by the surface charge of the nanochannels.
53,54
To explore the
sensing mechanism, i–tcurves of b-CD–CuTCA/TiO
2
M at +1.0 V
(the evolution of ionic current with reaction time, Fig. S23†)in
the presence of 0.1 mML-/D-Arg and the corresponding zeta
potentials of the membranes before and aer the recognition
reaction (Fig. S24†) were measured. According to these data,
Fig. 4 (a) I–Vcurves for sensing 10 mML/D-Arg of b-CD/TiO
2
M. (b) I–V
curves for sensing 0.1 mML/D-Arg of b-CD–CuTCA/TiO
2
M. (c) The
ionic current changes (DI) at +1.0 V for sensing different concentra-
tions (2.5–100 nM) of L/D-Arg. (d) The ionic current changes (DI)at
+1.0 V for sensing different concentrations (0.25–10 mM) of L/D-Arg. (e)
Ionic current changes (DI)ofb-CD/TiO
2
M at +1.0 V towards different
chiral molecules. The concentration of each chiral molecule is 10 mM.
(f) Ionic current changes (DI)ofb-CD–CuTCA/TiO
2
M at +1.0 V
towards different chiral molecules. The concentration of each chiral
molecule is 0.1 mM. The electrolyte for electrochemical measurements
contains 0.5 mM KCl and 1.0 mM H
2
O
2
at pH 4. Error bars indicate the
standard deviation of triplicate tests.
9998 |Chem. Sci., 2022, 13, 9993–10002 © 2022 The Author(s). Published by the Royal Society of Chemistry
Chemical Science Edge Article
a theoretical model (Fig. 5a, S25 and Table S2†) based on the
nite element method (FEM) combined with Poisson and
Nernst–Planck (PNP) equations
55,56
(please see the Experimental
section in the ESI†) was employed to simulate the Arg sensing
process in b-CD–CuTCA/TiO
2
M. To calculate the ion distribu-
tion, the Comsol Multiphysics 5.5 was used with the “electro-
statics (Poisson equation)”and “Nernst–Planck without
electroneutrality”modules. As schematically presented in
Fig. 5a, the CuTCA nanocrystal-based recognition zone is
located at the base-entrance side of TiO
2
M. The pore size of
CuTCA is 2.0 nm
37
and the spacing between the holes of
CuTCA is 0.7 nm.
57
The thickness of the recognition zone is
determined as 10 mm based on the SEM image in Fig. 1d.
Fig. 5b–e show the distribution of K
+
and Cl
concentrations at
the surface of the sensing zone of b-CD–CuTCA/TiO
2
M for 0.1
mML-/D-Arg sensing. Since the CuTCA-based sensing zone
carries negative surface charges, the strong electrostatic inter-
actions will attract more K
+
ions into the CuTCA/TiO
2
M nano-
channel, resulting in the accumulation of K
+
ions in the CuTCA/
TiO
2
M nanochannel at a bias of +1.0 V (Fig. 5b) and 1.0 V
(Fig. 5c). The accumulated K
+
ions in the nanochannels of
CuTCA/TiO
2
M result in an increased ion conductance of
CuTCA/TiO
2
M. Fig. 5d shows the concentration proles of K
+
ions at 1.0 V in the CuTCA/TiO
2
M nanochannel for L-/D-Arg
sensing and the blank solution. Compared to the blank solution
and L-Arg, the K
+
ux in the sensing zone shows an obviously
high intensity for D-Arg sensing, which can be attributed to the
lower affinity between the b-CD based recognition zone and D-
Arg. These results are consistent with the experimental ionic
current changes in Fig. 4 (the DIvalues induced by D-Arg are
larger than those of L-Arg at the same concentrations). Owing to
the strong electrostatic repulsion, it is difficult for Cl
ions to
enter the negatively charged CuTCA/TiO
2
M. As a result, the Cl
concentration in the sensing zone is very low for L-Arg and D-Arg
sensing, and the ionic ux intensity is similar to that for the
blank solution (Fig. 5e).
Fluorescence sensing ability for L/D-Arg discrimination
Besides the electrochemical signal from the I–Vcurves of
nanochannels, the appearance of a solid-state uorescence
emission in Arg sensing provides a visual method for quanti-
cation. Owing to the p–p*transition of the triphenylamine
group,
37
the TCA ligand has a strong photoluminescence at
430 nm when excited at 350 nm. Attributed to the quenching
effect of the paramagnetic center, the uorescence emission of
TCA disappeared by the coordinating reaction with Cu(II)to
form CuTCA. It has been discovered that the diamagnetic
species Cu(I) can alleviate uorescence quenching caused by
paramagnetic Cu(II) ions.
37
Therefore, a recovery of uorescence
is expected to be observed on the as-prepared asymmetric
membrane aer the Cu(II) in CuTCA was reduced to Cu(I) using
NO. To estimate the visual sensing possibility, the solid-state
uorescence spectra of b-CD–CuTCA/TiO
2
M in the presence of
different concentrations of D-Arg and L-Arg were recorded. As
shown in Fig. S26a and b,†the uorescence emission appeared
in the presence of D/L-Arg enantiomers, and the intensity
increased with increasing Arg concentration. Fig. S26c†shows
the relationship between L/D-Arg concentrations and uores-
cence intensities. The membranes show distinct differences in
uorescence intensities when sensing Arg enantiomers from 0.1
Fig. 5 (a) Calculation model of the 2D computation domain for the heterochannels. Note that the figure is not drawn to scale. Simulated ionic
concentration profiles in the b-CD–CuTCA/TiO
2
M membrane in the absence of Arg and the presence of 0.1 mML-Arg or D-Arg when the applied
voltage is +1.0 V (b) and 1.0 V (c). Simulated ionic flux images of (d) K
+
and (e) Cl
ions in the CuTCA based sensing zone for sensing 0.1 mML-/D-
Arg (the transmembrane voltage was set as 1.0 V).
© 2022 The Author(s). Published by the Royal Society of Chemistry Chem. Sci.,2022,13,9993–10002 | 9999
Edge Article Chemical Science
mMto10mM. Compared to L-Arg, larger uorescence emissions
were observed for the same concentration of D-Arg, which can be
explained by the high affinity between b-CD and L-Arg inducing
less L-Arg in the quantication zone. To study the possibility of
application as a visual screening platform, the uorescence
images of the membrane for sensing a series of concentrations
of Arg enantiomers were recorded (Fig. S26d†). Apparently, the
uorescence became more obvious with increasing Arg
concentration. Notably, the lowest D-Arg concentration for
a visual uorescence is only 0.1 mM, whereas a visual uores-
cence requires 2.5 mML-Arg. The solid-state uorescence
recovery of the as-proposed b-CD–CuTCA/TiO
2
M can also serve
as a visual platform for the qualitative discrimination of Arg
enantiomers.
Experimental
Preparation of TiO
2
M
TiO
2
nanochannel membranes were grown from Ti foils (15 mm
15 mm 0.1 mm) by electrochemical anodization. For this
purpose, the Ti foils were rst sequentially rinsed with acetone,
ethanol, and deionized water and then dried in air. Anodization
was carried out in a mixture of ethylene glycol/lactic acid/water
electrolyte containing 0.1 M NH
4
F at 120 V and 150 V for 20 min
and 2 min, respectively. To obtain an open-ended TiO
2
nano-
channel membrane, the obtained samples were dipped in H
2
O
2
(30%) until the titanium substrates were separated from the
membrane. The prepared samples were annealed at 450 C for
2 h in air with a heating rate of 3 C min
1
.
Preparation of CuNPs/TiO
2
M
CuNPs/TiO
2
M was assembled in a home-made H-type cell. The
membrane was placed between the two cells of the homemade
electrolyte cell, and a quartz window was set on one side of the
cell (base side of TiO
2
M) for allowing UV light to pass through
and reach the TiO
2
M surface. One half of the cell (tip side of
TiO
2
M) was lled with 5.0 mM Cu(CH
3
COO)
2
$H
2
O. Another half
of the cell (base side of TiO
2
M) was lled with pure water. When
driven under +1.0 V for 120 min, the CuNPs migrated and then
deposited on the base side of the nanochannel exposed to UV
light (3 W LED, 365 nm).
Preparation of CuTCA/TiO
2
M
The Cu-MOF-modied TiO
2
M was prepared using H
3
TCA as the
organic ligand and the anchored CuNPs as the precursor of
Cu
2+
.
37,58
Briey, 10 mg of H
3
TCA was dissolved in 10 mL of
a mixture of DMF and CH
3
OH (V
DMF
:V
CH
3
OH
¼1 : 1). The
solution was sonicated for 30 min and then transferred to
a50mLTeon-lined autoclave. The as-formed CuNPs/TiO
2
M
(diameter 10 mm) was added to the aforementioned solution.
The sealed vessel was then held at 120 C for 48 h to grow
CuTCA in TiO
2
M. The resulting CuTCA/TiO
2
M sample was
carefully washed with DMF and CH
3
OH to remove the unreac-
ted ligands in the nanochannels and then dried in an 80 C
vacuum oven to remove the remaining solvent.
Preparation of bulk CuTCA
A mixture of H
3
TCA (94 mg, 0.25 mmol) and Cu(NO
3
)
2
6H
2
O
(242 mg, 1 mmol) was dissolved in 15 mL of a mixture of DMF
and CH
3
OH (V
DMF
:V
CH3OH
¼1 : 1). The solution was sonicated
for 30 min and then transferred to a 50 mL Teon-lined auto-
clave. The sealed vessel was kept in an oven at 120 C for 48 h.
The resulting green crystals were carefully washed with DMF
and CH
3
OH to remove the unreacted ligands and then dried in
an 80 C vacuum oven to remove the remaining solvent.
Preparation of b-CD–CuTCA/TiO
2
M
The diffusion method was used for b-CD modication in
CuTCA/TiO
2
M in a home-made H-type cell. One half of the cell
(tip side of CuTCA/TiO
2
M) was lled with 1.0 mM PP. Another
half of the cell (base side of TiO
2
M) was lled with pure water to
diffuse for 24 h. Then, 1.0 mM b-CD solution was used for self-
assembly with PP.
Detection of chiral arginine by electrochemical measurements
Chiral arginine was detected in a home-made H-type cell. A pair
of homemade Ag/AgCl electrodes was used to measure the
resulting ionic current. The membrane was mounted between
two halves of the conductance cell. One half of the cell was lled
with 0.5 mM KCl and chiral arginine solutions with different
concentrations. Another half of the cell was lled with 0.5 mM
KCl and 1.0 mM H
2
O
2
. The effective membrane area for I–V
property measurements is 3.14 mm
2
. Linear sweep voltammetry
(LSV) was carried out from 1.0 V to 1.0 V at a scan rate of
50 mV s
1
.
Conclusions
In summary, an enantioselective sensing platform for Arg
enantiomers with high sensitivity was constructed based on
asymmetric TiO
2
M. Beneting from the I–Vproperties of the
nanochannel structure and the cascade recognition–quanti-
cation zone design along the TiO
2
nanochannels, the resulting
b-CD–CuTCA/TiO
2
M exhibited a sensitive and selective perfor-
mance for the discrimination of Arg enantiomers. Besides
providing the I–Vsignal, the as-proposed membrane can also
act as a visual uorescence platform for a convenient and rapid
discrimination of Arg enantiomers. This work not only paves
a new way to design asymmetric nanochannels, but also veries
that the cascade of recognition–quantication zones is an
effective strategy to achieve selective and sensitive recognition
of enantiomers.
Data availability
The data supporting the ndings of this study are available
within the article and in the ESI.†
Author contributions
Y.-Y. Song conceived the concept and directed the project. J. L.
Guo, X. J. Xu, and J. J. Zhao performed the experiments. Z.-Q.
10000 |Chem. Sci., 2022, 13, 9993–10002 © 2022 The Author(s). Published by the Royal Society of Chemistry
Chemical Science Edge Article
Wu carried out the theoretical study. Z. D. Gao collected and
analyzed the data. J. L. Guo prepared the rst draof this
manuscript, and all the authors modied the manuscript.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21874013, 22074013, 21775066, and
21974058), and the Fundamental Research Funds for the
Central Universities (No. N2105018 and N2005027). Special
thanks are due to the instrumental or data analysis from
Analytical and Testing Center, Northeastern University.
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