Functionalized π Stacks of Hexabenzoperylenes as a Platform for Chemical and Biological Sensing

Article abstract of DOI:10.1016/j.chempr.2018.03.007

One challenge in tailoring organic semiconductors for sensing applications is that the introduction of reactive groups or binding sites usually impairs π-π interactions. To meet this challenge, this study puts forth an unusual type of π stacking that allo

Full text of DOI:10.1016/j.chempr.2018.03.007

Article
Functionalized p Stacks of
Hexabenzoperylenes as a Platform for
Chemical and Biological Sensing
Changqing Li, Han Wu, Tiankai
Zhang, ..., Jiang Xia, Jianbin Xu,
Qian Miao
miaoqian@cuhk.edu.hk
HIGHLIGHTS
Functionalization of organic
semiconductors without
sacrificing p-p interactions
The self-assembly of
functionalized HBPs provides a
general sensing platform
Silyl ethyl-modified HBP enables
selective detection of fluoride ions
in water
Biotin-modified HBP enables
sensitive and selective detection
of streptavidin
This study puts forth an unusual type of p stacking that allows a variety of functional
groups to be grafted onto organic semiconductors without sacrificing p-p
interactions in the solid state. As a result of this type of p stacking, the self-
assembly of functionalized hexabenzoperylenes (HBPs) provides a general sensing
platform, which has been demonstrated to enable highly sensitive and selective
detection of fluoride ions and streptavidin in water when HBPs are functionalized
with silyl ether and biotin, respectively.
Li et al., Chem 4, 1–11
June 14, 2018 ª 2018 Elsevier Inc.
https://doi.org/10.1016/j.chempr.2018.03.007

Please cite this article in press as: Li et al., Functionalized p Stacks of Hexabenzoperylenes as a Platform for Chemical and Biological Sensing,
Chem (2018), https://doi.org/10.1016/j.chempr.2018.03.007
Article
Functionalized p Stacks
of Hexabenzoperylenes as a Platform
for Chemical and Biological Sensing
Changqing Li,1 Han Wu,1 Tiankai Zhang,2 Yujie Liang,1 Bo Zheng,1 Jiang Xia,1 Jianbin Xu,2
and Qian Miao1,3,
*
The Bigger Picture
SUMMARY
By combining sensory electrical
output with easy device
One challenge in tailoring organic semiconductors for sensing applications is
that the introduction of reactive groups or binding sites usually impairs p-p in-
teractions. To meet this challenge, this study puts forth an unusual type of p
stacking that allows a variety of functional groups to be grafted onto organic
semiconductors without sacrificing p-p interactions in the solid state. As a result
of this type of p stacking, the self-assembly of functionalized hexabenzopery-
lenes (HBPs) provides a general sensing platform for various chemical and bio-
logical species. This supramolecular platform, in a device integrating an
organic field-effect transistor channel and a microfluidic channel, has enabled
highly sensitive and selective detection of fluoride ions and streptavidin in
water when HBPs are equipped with a cleavable Si–O bond and a biotin moiety,
respectively.
fabrication, organic field-effect
transistors (OFETs) promise low-
cost, soft, and biocompatible
sensors for wearable and
implantable electronic devices.
One challenge for tailoring
organic semiconductors in OFET-
based sensors is that the
introduction of reactive or binding
sites usually impairs charge
transport pathways. To meet this
challenge, this study puts forth a
general sensing platform on the
basis of an unusual type of p
stacking, which allows
INTRODUCTION
One of the most promising applications of organic field-effect transistors (OFETs) is
chemical and biological sensing,1–5 because OFET-based sensors not only circum-
vent the need for bulky and expensive equipment by combining the sensory electri-
cal output with easy device fabrication but also promise soft and biocompatible
electronics for wearable and implantable devices.6–8 A unique advantage of OFETs
for sensing applications is that reactive groups or binding sites can be integrated
with organic semiconductors through organic synthesis to provide specific interac-
tions with chosen analytes.9–11 However, introduction of functional groups, particu-
larly those capable of strong supramolecular interactions (e.g., hydrogen bonds),
usually changes the arrangement of p faces in the solid state,12 thus impairing
charge transport.13 Therefore, it is challenging to graft functional groups onto
organic semiconductors without sacrificing p-p interactions. Here, we report the
self-assembly of functionalized hexabenzoperylenes (HBPs) 1a–1e (Figure 1),
which present an unusual type of p stacking for organic semiconductors able to
accommodate a variety of functional groups without sacrificing p-p interactions in
the solid state. As a result of a brickwork arrangement of their twisted p faces, func-
tionalized HBPs provide a general platform for chemical and biological sensing by
self-assembling into a supramolecular nanosheet, which has a two-dimensional p
stack sandwiched between two layers of functional groups. As detailed below, this
unprecedented platform, in a device14,15 integrating a transistor channel and a
microfluidic channel,16 has enabled highly sensitive and selective detection of fluo-
ride ions and streptavidin in water, when HBPs are functionalized with silyl ether (1d)
and biotin (1e), respectively.
functionalization of organic
semiconductors without
sacrificing p-p interactions. When
functionalized with silyl ether and
biotin, the self-assembly of
hexabenzoperylenes (HBPs) has
enabled highly sensitive and
selective detection of fluoride ions
and streptavidin in water,
respectively. By grafting different
reactive or binding sites to HBPs,
this general sensing platform can
be utilized to detect various
chemical and biological species
for environmental monitoring and
medical diagnosis.
Chem 4, 1–11, June 14, 2018 ª 2018 Elsevier Inc.
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Figure 1. Structures of Functionalized HBPs 1a–1e and Unfunctionalized HBP 2
RESULTS AND DISCUSSION
Molecular Packing in Crystals
We synthesized functionalized HBPs 1a–1e by modifying the reported synthesis of
alkylated HBPs17 as detailed in the Supplemental Information. Slow evaporation of
solvents from solutions of 1a and 1b afforded red crystals suitable for X-ray crystal-
lography. Similar to the non-functionalized HBPs,18 1a adopts a chiral twisted
conformation in the crystals with its fjord regions distorted with torsion angles of
42.1ꢀ and 40.1ꢀ. The crystal of 1a is racemic, containing P- and M-enantiomers, which
are shown in yellow and light blue, respectively, in Figures 2A and 2B. Each P-enan-
tiomer of 1a stacks with four adjacent M-enantiomers in a brickwork arrangement,
which is essentially the same as the molecular packing of HBPs lacking functional
groups.17 Since the polycyclic p backbone can be regarded as two planar subunits
joined by a twisted benzene ring, the p faces of 1a are stacked in two directions with
˚
a p-to-p distance of about 3.6 A. This packing motif leads to a one-molecule-thick
nanosheet, which has the ester groups of P-enantiomers and M-enantiomers placed
on opposite sites. Figure 2C shows two adjacent nanosheets of 1a (highlighted in
light blue and violet) as viewed along the a axis of the crystal lattice with interdigita-
tion between the substituting butanoate groups.
The crystal structure of 1b reveals a twisted conformation and a brickwork arrange-
ment very similar to those of 1a. As shown in Figure 2D, the brickwork arrangement
of 1b leads to a supramolecular nanosheet, which has a two-dimensional stack of p
faces sandwiched between two layers of hydroxyl groups. Unlike the ester groups
in 1a, the hydroxyl groups of 1b connect the adjacent nanosheets of 1b (Figure S2)
˚
with hydrogen bonds, which have a H/O distance of 2.02 A and a O–H/O angle
of 152.7ꢀ. The hydrogen bonds here, unlike those in the reported hydrogen-
bonding-directed packing of aromatics,19,20 do not significantly change the brick-
work arrangement of twisted HBPs, presumably because the close packing of the
twisted p faces of HBPs can only be achieved with the brickwork arrangement. One
supramolecular nanosheet of 1a and 1b in the crystals is 1.77 nm (Figure 2B) and
1.83 nm (Figure 2D) thick, respectively, as defined by the distance between the
outmost atoms on the opposite surfaces. Alternatively, the nanosheet thickness
of 1a can be measured as 1.51 nm from the d spacing of its (010) plane as shown
in Figure 2C. Similarly, the nanosheet thickness of 1b can be measured as 1.45 nm
from the d spacing of its (002) plane in the crystal. The nanosheet thickness of 1a
and 1b as defined by the surface hydrogen atoms is larger than the corresponding
d spacings because the substituting groups on adjacent nanosheets are interdig-
itated. Besides 1a and 1b, HBPs containing other functional groups, such as chlo-
rine, 2-methoxyethoxyl, and triazolo[4,5-b]pyridine, are found to self-assemble into
very similar nanosheet structures in the crystals. Figure S3 shows the brickwork
1Department of Chemistry, The Chinese
University of Hong Kong, Shatin, New Territories,
Hong Kong, China
2Department of Electronic Engineering, The
Chinese University of Hong Kong, Shatin, New
Territories, Hong Kong, China
3Lead Contact
*Correspondence: miaoqian@cuhk.edu.hk
https://doi.org/10.1016/j.chempr.2018.03.007
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Figure 2. Molecular Packing of 1a and 1b in Crystals
(A) A nanosheet of 1a as viewed along the b axis of the crystal lattice.
(B) A nanosheet of 1a as viewed along the c axis of the crystal lattice.
(C) Two nanosheets of 1a as viewed along the a axis of the crystal lattice.
(D) A nanosheet of 1b as viewed along the a axis of the crystal lattice.
In (A), (B), and (D), the P- and M-enantiomers, except their ester and hydroxyl groups, are shown in
yellow and light blue, respectively; in (C), the hydrogen atoms are removed, and carbon and oxygen
atoms are shown as gray and red ellipsoids, respectively, set at 50% probability.
arrangement of triazolo[4,5-b]pyridine-functionalized HBP in crystals, where the
larger functional groups, also placed at the exterior of the nanosheet, do not
change the molecular packing motif. With such an unprecedented ability to graft
a variety of functional groups to p stacks without sacrificing p-p interactions, these
functionalized HBPs also differentiate themselves from structurally related planar
hexa-peri-hexabenzocoronene and contorted hexabenzo[a,d,g,j,m,p]coronene,
which organize into columnar liquid crystals,21,22 nano-cables,23 or nanotubes,24,25
depending on the structure and number of flexible chains attached to the conju-
gated backbones.
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Thin-Film Transistors and Supramolecular Assemblies in Thin Films
As estimated from the first oxidation potentials in the cyclic voltammograms (Fig-
ure S1), HBPs 1a–1e have the highest occupied molecular orbital (HOMO) energy
levels of À5.32 to À5.27 eV. In agreement with the HOMO energy levels and
the above crystal structures, dip-coated films of 1a–1e were found to behave as
p-type semiconductors in top-contact bottom-gate OFETs exhibiting field-effect
mobilities in the range of 9.7 3 10À3 to 8.2 3 10À2 cm2/Vs, as detailed in the Sup-
plemental Information.
To study the supramolecular assemblies of 1a–1e in the transistors, we characterized
their dip-coated films with X-ray diffraction (XRD) and UV-vis absorption spectros-
copy. The XRD from the dip-coated films of 1a exhibited an intense diffraction
peak at 2q = 5.81ꢀ (d spacing of 1.52 nm) accompanied by a higher-order peak at
2q = 17.51ꢀ (d spacing of 0.507 nm). The two peaks are in accordance with the
(010) and (030) diffractions derived from the crystal structure of 1a. Similarly, the
XRD from the film of 1b exhibited a diffraction peak at 2q = 5.90ꢀ (d spacing of
1.50 nm) corresponding to the (002) diffraction. These diffraction peaks indicate
that the dip-coated films of 1a and 1b contain nanosheets parallel to the substrate
surface. Similarly, the dip-coated films of 1c, 1d, and 1e exhibited diffraction peaks
with d spacings of 1.50, 1.77, and 2.18 nm, respectively, indicating that their films
also comprised nanosheets parallel to the substrate surface. The films of 1a–1e
on glass exhibited the longest-wavelength absorption maxima in the range
of 562–577 nm, whereas solutions of 1a–1e in CH2Cl2 at the same concentration
(1 3 10À5 M) exhibited almost identical UV-vis absorption spectra with the
longest-wavelength peak at 512 nm, as shown in Table S3 and Figure S7. Relative
to those in the solutions, the red-shifted absorption of 1a–1e in the films was similar
to that of previously reported non-functionalized HBPs.17 This red shift can be attrib-
uted to electronic delocalization between p-stacked molecules and is thus an indi-
cator of strong p-p interactions.26,27 The XRD and UV-vis absorption spectra of
1c–1e suggested the formation of the same type of p-stacked nanosheets as 1a
and 1b, in agreement with the similar field-effect mobilities of 1a–1e in the dip-
coated films. Moreover, the film of 1d exhibited a contact angle of 100ꢀ, whereas
the films of 1c and 1e exhibited smaller contact angles of 68ꢀ and 77ꢀ, respectively,
with water. These contact angles are in agreement with the p-stacked nanosheets of
1c–1e, where the functional groups (the nonpolar trimethylsilyl group in 1d, the polar
hydroxyl group in 1c, and the polar biotin group in 1e) are placed on the surface.
Sensing of Fluoride
Unlike the reported OFET-based chemical sensors with binding sites physically ad-
sorbed on organic semiconductors,28,29 OFETs of functionalized HBPs allow reactive
groups and binding sites to be covalently linked to organic semiconductors for
sensing applications. Equipped with a Si–O bond selectively cleavable by fluoride,
1d is designed for selective detection of fluoride ions in water, which is of great
importance because of the duplicitous nature of fluoride but is still a challenge.
Whereas low doses of fluoride ions are beneficial for dental health, exposure to
high doses of fluoride can lead to serious health hazards, such as dental or skeletal
fluorosis.30 Although many fluorescent and colorimetric molecular probes have
been developed for sensing fluoride ions, most of them only work in organic solvents
or aqueous and organic mixtures.31,32 This cleavable Si–O bond has been widely
used in the design of fluorescent and colorimetric sensors for fluoride ions33 because
the Si–O bond reacts with fluoride ions selectively, resulting in the formation of a
Si–F bond and a hydroxyl group. However, to the best of our knowledge, it has
not been reported as a reactive site bonded to organic semiconductors. To fabricate
4
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Figure 3. An OFET-Based Sensor and Performance of Transistors under Water
(A) Schematic drawing of the sensor combining a microchannel and a transistor channel of
functionalized HBPs.
(B) Transfer I-V curves for the thin-film transistor of 1d and 1c with VDS = À4 V as measured in air and
under deionized water for a certain period of time.
electrical sensors based on the transistors of 1d, we carefully bonded a polydime-
thylsiloxane (PDMS) slab containing open microchannels onto the top-contact thin
film transistors of 1d under a microscope. As shown in Figure 3A, the resulting device
has a microchannel 70 mm wide on the top of a transistor channel 150 mm long, such
that the ‘‘length’’ of the fluid channel was oriented in the same direction as the
‘‘width’’ of the transistor channel.34 As a result, water flowed across the organic semi-
conductor layer perpendicular to the current flow direction without touching the
gold electrodes. In order to operate with low voltages to avoid electrolysis of water
and high ionic conduction through the analyte solution,14 the transistors here had
AlOx/TiOy, a solution-processed high-k metal oxide layer, as the dielectric to
achieve high capacitance per unit area. During operation of this device under water,
no phenomena (e.g., bubbles or change of pH) suggesting electrolysis of water were
observed. The surface of AlOx was modified with a self-assembled monolayer of 12-
methoxydodecylphosphonic acid (MODPA), which provided an ordered dielectric
surface wettable by common organic solvents for high-performance OFETs.35 As
found from atomic force microscopy (AFM) section analysis (Figure S9), these dip-
coated films were about 23.5–36.6 nm thick and thus consisted of 13–20 nanosheet
layers. In this device, the transistor of 1d exhibited very good stability when the
microchannel was filled with deionized water. As shown in the transfer I-V curves
(Figure 3B), the drain current slightly decreased in the first 10 min and then became
stable. In contrast, the same device with 1c, which had a hydroxyl group replacing
the trimethylsilyloxyl group in 1d, had its drain current dramatically decreased after
the transistor channel was exposed to water, as shown in Figure 3B. The drain current
decreased by one order of magnitude after the transistor channel was exposed to
water for 30 min, and the transistor channel of 1c finally lost the gate effect after
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Figure 4. Detection of Fluoride Ion with the Transistor of 1d
(A) Transfer I-V curves for the thin-film transistor of 1d as measured under deionized water and
under an aqueous solution of KF (1 mM) for a certain period of time.
(B) Change in the drain current of 1d in the transfer I-V curves as a result of exposure to different
anions with the same concentration (1 mM) in deionized water (the solution of KH2PO4/K2HPO4 had
a total concentration of anions of 1 mM and pH 7) for 5 min.
(C) Real-time change in the drain current of the transistor of 1d as measured with the drain and gate
voltages both fixed at À4 V in response to 1 mM fluoride in deionized water.
(D) Change in the drain current of 1d after exposure to fluoride in deionized water for 100 s versus
the concentration of fluoride (each value of DI/I0 with error bars is an average of three independent
channels) as measured with the drain and gate voltages both fixed at À4 V.
1 hr. The different performance of the thin films of 1d and 1c under water can be
attributed to the different functional groups. With the two-dimensional p stack sand-
wiched between two layers of trimethylsilyloxyl groups, the nanosheets of 1d have a
contact angle of 95ꢀ–100ꢀ with water. This hydrophobic surface blocks diffusion of
water molecules into the film underneath. In contrast, with hydroxyl groups attached
to HBP, the nanosheets of 1c have a hydrophilic surface with a contact angle of 68ꢀ
with water. As a result, water molecules can easily diffuse into the film of 1c and
finally reach the semiconductor-dielectric interface, where the first few molecular
layers of organic semiconductors are the locus of charge transport and responsible
for current modulation. When the electrical current goes through the transistor
channel, 1c, as a p-type semiconductor, is oxidized forming cations, which react
with water and thus reduce the drain current of the transistor.
In order to test the sensing ability of the transistors of 1d for fluoride, we filled
the microchannel with deionized water and then an aqueous solution of KF
(1 mM). Figure 4A shows the transfer I-V curves of 1d as measured under deionized
water and under an aqueous solution of KF (1 mM) for a certain period of time. We
6
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found that the drain current (VDS = À4 V and VGS = À4 V) decreased by 43% after the
transistor channel was exposed to fluoride for 1 min and decreased by 99.7% with
the absence of a gate effect after 5 min. Such dramatic reduction of drain current
can be regarded a result of the cleavage of Si–O bonds by fluoride accompanied
by a change in the contact angle from 95ꢀ to 73ꢀ (Figure S11) and the consequent
diffusion of water into the active layers of semiconductors to reduce the drain cur-
rent. This is in agreement with the low drain current of 1c as measured under water.
The response of the transistor of 1d toward aqueous solution of fluoride is even
faster than that of 1c toward water, most likely because the dip-coated films of 1d
and 1c, as shown in Figures S5 and S13, have different morphologies, which lead
to different diffusion speed of water. To test the selectivity of the sensor of 1d toward
different anions, we measured the transfer curves after the transistor channel was
exposed to aqueous solutions of different salts with the same concentration
(1 mM), including KCl, KBr, KI, K2SO4, KNO3, and KH2PO4/K2HPO4 (total concentra-
tion of anions of 1 mM and pH 7), for 5 min. All these solutions were neutral to
avoid the effect of hydroxide, which can also cleave the Si–O bond. As shown in
Figure 4B, all anions, except SO42À, led to a reduction of the drain current. Among
them, ClÀ decreased the current by 11.5%; other anions decreased the current by
2.4%–6.0%. In contrast, exposure to SO42À led to a negligible increase in the drain
current by 1.0%, which is not yet understood. These results show that the transistor
of 1d has a very good selectivity for fluoride in agreement with the selective cleavage
of the Si–O bond by fluoride. The sensing ability of 1d was also tested with tap water,
which contained 9–34 mM fluoride ion according to the report of the Water Supplies
Department of the Government of Hong Kong SAR.36 As shown in Figure S15, expo-
sure of the film of 1d to tap water for 5 min led to a decrease in the drain current by
14.1%, and extra 10 mM, 100 mM, and 1 mM KF in the tap water led to a decrease in
the drain current by 13.0%, 18.2%, and 88.4%, respectively, as measured from the
drain current (VDS = À4 V and VGS = À4 V) in the transfer curves.
Another way to test the sensing ability of OFETs is real-time measurement of the
drain current with fixed drain and gate voltages. The real-time change of drain cur-
rent measured in this way is usually not the same as that measured from the transfer
curves as a result of gate-bias stress of OFETs.37,38 To examine the sensing ability of
the transistors of 1d for fluoride of different concentrations in water, we measured
the real-time change in drain current with the drain and gate voltages both fixed
at À4 V when a flow of aqueous solution of KF was injected into the above device
of 1d at a speed of 30 mL/min with a syringe pump. Before injecting the solution
of KF, we filled the microchannel with deionized water in order to exclude the
response of the drain current toward water and applied a gate bias of À4 V to the
transistor channel for about 5 min until the drain current stabilized. The device of
1d was able to detect fluoride down to a concentration of 1 mM, which is the same
as the detection limit of the fluoride-selective electrode.39 Figure 4C shows the
real-time responses of the drain current of 1d toward 1 mM of KF in water, exhibiting
fast reduction of the drain current. After exposure to 1 mM fluoride for 100 s, the drain
current decreased by about 8%. As shown in Figure 4D, the relative change of
the drain current (DI/I0) of 1d increased as the concentration of fluoride ion increased
from 1 mM to 1 mM. However, a quantitative relationship between the value of DI/I0
and the concentration of fluoride has not been established on the basis of these
data.
Sensing of Streptavidin
Equipped with biotin, 1e is designed for selective detection of biotin-binding pro-
teins, such as streptavidin, a tetrameric protein with an astonishingly high affinity
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to bind biotin.40,41 Grafting biotin onto different sensing platforms42–45 has been re-
ported for a variety of bioanalytical applications. However, functionalization of
organic semiconductors with biotin is challenging because the hydrogen bonds
from biotin can change the molecular packing of organic semiconductors impairing
p-p interactions. To the best of our knowledge, only two biotin-functionalized semi-
conducting polymers have been documented46,47 to exhibit field-effect mobility
lower than that of 1e, most likely because of poor p-p interactions. AFM revealed
that thin films of 1e dip coated on the MODPA-modified AlOx/TiOy surface were
107–125 nm thick and thus consisted of about 50 layers of nanosheets (Figure S10).
In the sensing device depicted in Figure 3A, the transistor of 1e exhibited similar sta-
bility to 1d when the microchannel was filled with phosphate-buffered saline (PBS;
1.0 mM, pH 7.4), as shown in Figure S17. The film of 1e exhibited a contact angle
of 77ꢀ with water, which indicated a less hydrophilic surface than that of 1c. The tran-
sistor stability of 1e can be attributed to the less hydrophilic surface and its long alkyl
chains, which can form a densely packed layer blocking diffusion of water into the p
stacks. The sensing ability of 1e was tested with a solution of streptavidin in PBS with
pH 7.4. In order to exclude the response of the drain current toward water, we filled
the microchannel with the PBS before injecting streptavidin at a speed of 30 mL/min
by using a syringe pump for 5 min. We then rinsed the microchannel with the PBS to
remove unbound proteins and performed electrical measurements in the PBS. Fig-
ure 5A compares the transfer I-V curves of the device of 1e operated under PBS
without and with streptavidin. The drain current (VDS = À4 V and VGS = À4 V)
increased by 80% after the transistor channel was exposed to 3.6 nM streptavidin
in the PBS. This increase in the current is attributable to streptavidin bound to the
surface of the HBP nanosheet through the biotin-streptavidin interaction. With an
isoelectric point (pI) of 6.08,48 streptavidin is negatively charged in the buffer solu-
tion (pH 7.4). As a result, the bound streptavidin acts as an extra gate, inducing
more holes in the transistor channel of 1e.42,48,49 In agreement with this sensing
mechanism, when the pH of the buffer solution was changed to 6.0, which is very
close to the pI of streptavidin, the drain current was essentially unchanged with a
slight decrease, as shown in Figure S18A. When the pH of the buffer solution was
changed to 5.6, the drain current (VDS = À4 V and VGS = À4 V) decreased by 28%
accompanied by the threshold voltage shifting to a more negative value after the
transistor channel of 1e was exposed to streptavidin of the same concentration
(3.6 nM), as shown in Figure S18B. This phenomenon is attributable to the bound
streptavidin, which, at a pH of 5.6, is positively charged and thus acts as an extra
gate to decrease the concentration of mobile holes in the transistor channel of 1e.
To study the selectivity of the device of 1e, we conducted a few control experiments. As
shown in Figure 5B, the device of 1e exhibited essentially the same transfer curves when
the transistor channel was exposed to PBS and 3 mM bovine serum albumin (BSA), a pro-
tein that non-specifically adheres to surfaces with a strong tendency.50 This demon-
strates that the device of 1e has a high selectivity for streptavidin over other proteins.
Moreover, the device of unfunctionalized HBP 2 (Figure 1)17 exhibited only a small
decrease in drain current when the transistor channel was exposed to streptavidin, as
shown in Figure 5C. The results in the above two control experiments clearly show that
the increase in drain current of 1e as a response to streptavidin (Figure 5A) is a result
of the specific biotin-streptavidin binding interaction. As found from the experiments
exploring the sensitivity limit, the device of 1e is able to detect streptavidin down to a
concentration of 36 pM, which is comparable with that of biotin-modified silicon nano-
wires (10 pM).42 As shown in Figure 5D, the relative change of the drain current (DI/I0,
measured from the transfer curve at VDS = À4 V and VGS = À4 V) for the device of 1e in-
creases as the concentration of streptavidin increases from 36 pM to 360 nM with a linear
8
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Figure 5. Detection of Streptavidin with the Transistor of 1e
(A) Transfer I-V curves for the transistor of 1e as measured under PBS and a solution of streptavidin
(3.6 nM) in PBS.
(B) Transfer I-V curves for the transistor of 1e as measured under PBS and a solution of BSA (3 mM)
in PBS.
(C) Transfer I-V curves for the transistor of 2 as measured under PBS and a solution of streptavidin
(3.6 nM) in PBS.
(D) Change in the drain current of 1e (DI/I0, measured from the transfer curve at VDS = À4 V and
VGS = À4 V) versus the concentration of streptavidin (each value of DI/I0 is an average of three
measurements).
relationship between DI/I0 and the logarithm of the streptavidin concentration. This rela-
tionship indicates that the amount of streptavidin bound to the surface of the films of 1e
depends on the logarithm of the streptavidin concentration in solution, suggesting that
the binding of streptavidin onto biotin-functionalized HBPs is a first-order dynamic pro-
cess (surface binding rate = k 3 d[streptavidin]/dt) under the conditions for the above
measurement.
Conclusion
In summary, the crystal structures of functionalized HBPs reveal an unusual type of p
stacking that is able to accommodate a variety of functional groups without sacri-
ficing p-p interactions in the solid state. This molecular packing motif provides a
general sensing platform for the detection of a variety of chemical and biological
species by allowing functionalization of organic semiconductors without impairing
charge transport. Integration of a microchannel on the thin-film transistors of
functionalized HBPs has enabled highly sensitive and selective detection of
fluoride ions and streptavidin in water when HBPs are equipped with a Si–O bond
cleavable by fluoride and a biotin moiety to bind streptavidin, respectively. Studies
on the nanosheets of HBPs equipped with other reactive groups and binding sites for
sensing different chemical and biological species are in progress in our laboratory.
Chem 4, 1–11, June 14, 2018
9

Please cite this article in press as: Li et al., Functionalized p Stacks of Hexabenzoperylenes as a Platform for Chemical and Biological Sensing,
Chem (2018), https://doi.org/10.1016/j.chempr.2018.03.007
EXPERIMENTAL PROCEDURES
Full experimental procedures are provided in the Supplemental Information.
DATA AND SOFTWARE AVAILABILITY
The accession numbers for the crystallographic data for 1a, 1b, and triazolo[4,5-b]
pyridine-functionalized HBP reported in this paper are CCDC: 1548513, 1548514,
and 1817810, respectively.
SUPPLEMENTAL INFORMATION
Supplemental Information includes Supplemental Experimental Procedures, 46 fig-
ures, 3 schemes, 4 tables, and 3 data files and can be found with this article online at
https://doi.org/10.1016/j.chempr.2018.03.007.
ACKNOWLEDGMENTS
We thank Ms. Hoi Shan Chan (the Chinese University of Hong Kong) for the single-
crystal crystallography. This work was supported by the Research Grants Council
of Hong Kong (project numbers GRF14303614, GRF14302315, and CRF C4030-
14G). H.W. thanks a Hong Kong PhD Fellowship for financial support.
AUTHOR CONTRIBUTIONS
Q.M. conceived and directed the project. C.L. performed most of the experiments.
H.W. and B.Z. contributed to the fabrication and operation of the microchannels.
T.Z. and J. Xu contributed to the study with AFM. Y.L. and J. Xia provided strepta-
vidin and contributed to the experiments for sensing of streptavidin. Q.M. and
C.L. wrote the manuscript, and all authors checked the manuscript.
DECLARATION OF INTERESTS
The authors declare no competing interests.
Received: November 2, 2017
Revised: January 18, 2018
Accepted: March 10, 2018
Published: April 19, 2018
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