An Elaborate Supramolecular Assembly for a Smart Nanodevice for Ratiometric Molecular Recognition and Logic Gates

Article abstract of DOI:10.1002/chem.201505082

Ingenious approaches to supramolecular assembly for fabricating smart nanodevices is one of the more significant topics in nanomaterials research. Herein, by using surface quaternized cationic carbon dots (CDots) as the assembly and fluorescence platform, anionic sulfonatocalix[4]arene with modifiable lower and upper rims as a connector, as well as in situ coordination of Tb³⁺ions, we propose an elaborate supramolecular assembly strategy for the facile fabrication of a multifunctional nanodevice. The dynamic equilibrium characteristics of the supramolecular interaction can eventually endow this nanodevice with functions of fluorescent ratiometric molecular recognition and as a nano-logic gate with two output channels. A smart nanodevice for ratiometric molecular recognition and nano-logic gates has been successfully fabricated by using an elaborate supramolecular assembly strategy that includes the precise assembly of a bifunctional connector sulfonatocalix[4]arene on quaternized cationic carbon dots by inclusion complexation and electrostatic interactions, and the incorporation of Tb³⁺ions by in situ coordination.

Full text of DOI:10.1002/chem.201505082

DOI: 10.1002/chem.201505082
Full Paper
&
Supramolecular Assemblies
An Elaborate Supramolecular Assembly for a Smart Nanodevice
for Ratiometric Molecular Recognition and Logic Gates
Yu-Jie Xie, Wen-Yu Wu, Hao Chen, Xiang Li, Hao-Li Zhang, Liang-Liang Liu, Xing-Xin Shao,
[
a]
Chang-Fu Shan, Wei-Sheng Liu, and Yu Tang*
Abstract: Ingenious approaches to supramolecular assembly
for fabricating smart nanodevices is one of the more signifi-
cant topics in nanomaterials research. Herein, by using sur-
face quaternized cationic carbon dots (CDots) as the assem-
bly and fluorescence platform, anionic sulfonatocalix[4]arene
with modifiable lower and upper rims as a connector, as
elaborate supramolecular assembly strategy for the facile
fabrication of a multifunctional nanodevice. The dynamic
equilibrium characteristics of the supramolecular interaction
can eventually endow this nanodevice with functions of flu-
orescent ratiometric molecular recognition and as a nano-
logic gate with two output channels.
3
+
well as in situ coordination of Tb ions, we propose an
Introduction
thetic conditions and choice of precursors, which include pri-
[
5]
mary amines, carboxylic acids, hydroxyls, etc. Tremendous ef-
forts have been devoted to modifying CDots to achieve
Supramolecular assembly has aroused increasing interest in
surface modifications and functionalization of nanoplatforms
for fabricating various nanodevices with well-controlled struc-
[
6]
different applications, including using different linking strat-
[
7]
[8]
egies such as covalent bonding, coordination, and p–p in-
[1]
[9]
ture and multiple functions. Yang and co-workers reported
carboxylatopillar[5]arene-modified Au nanoparticle (NP)-
based nanodevice prepared by supramolecular assembly for
teractions. However, the available methods often suffer from
a
inefficient reaction yields and complicated purification process-
[
10]
es. Supramolecular assembly strategies likely serve as substi-
tutes for conventional covalent modifications and simplify the
fabrication of multifunctional CDots need to be further stud-
ied.
[2]
sensing the herbicide paraquat. A similar strategy has also
been explored for other nanomaterials including polymer dots,
[
3]
magnetic/silver/silica NPs, quantum dots, graphene, etc. In
these architectures, macrocycles are particularly striking as
supramolecular building units and examples include calixar-
enes, cyclodextrins, cucurbiturils, and pillararenes, which can
Here, we propose an elaborate supramolecular assembly
strategy for the facile fabrication of a CDot-based smart nano-
device for fluorescent ratiometric molecular recognition and
use as a logic gate (Scheme 1). This strategy employs surface
quaternized cationic CDots as the assembly and fluorescence
platform, which allows tedious syntheses and substrate modifi-
cations to be avoided. The CDots can also provide quaternary
ammonium to precisely immobilize functional units through
supramolecular interactions with excellent luminescent proper-
ties.
[
1c,4]
serve as responders, modulators, or even building blocks.
In particular, calix[4]arenes have emerged as molecular scaf-
folds for building appropriate binding cores owing to their
wide range of topological features. Notably, the ingenious
design of supramolecular assembly processes towards smart
nanodevices with enhanced functions remains challenging.
Carbon dots (CDots) have attracted considerable attention
because of their superior properties including tunable fluores-
cence, excellent water solubility, low toxicity, etc. Diverse sur-
face groups can be readily introduced onto CDots through var-
ious synthesis methodologies. This is controlled by the syn-
The anionic sulfonatocalix[4]arene (SC4A) as an important
macrocyclic receptor displays strong binding ability and high
[
11]
selectivity toward various analytes.
It was chosen as the
supramolecular functional block because it not only serves as
the supramolecular connector on the surface of the CDots, but
also generates an outer functional layer through coordination
of the hydroxyl groups with metal cations. Owing to the
strong driving forces for the inclusion of quaternary ammoni-
um groups in the cavity, as well as the electrostatic interactions
with the sulfonate groups, bifunctional SC4A can be precisely
assembled on the surface of the quaternized CDots. In addi-
[
a] Y.-J. Xie, W.-Y. Wu, H. Chen, X. Li, Prof. H.-L. Zhang, L.-L. Liu, X.-X. Shao,
C.-F. Shan, Prof. W.-S. Liu, Prof. Y. Tang
State Key Laboratory of Applied Organic Chemistry, Key Laboratory of Non-
ferrous Metal Chemistry and Resources Utilization of Gansu Province, Col-
lege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
7
30000 (P. R. China)
Fax: (+86)931-891-2582
E-mail: tangyu@lzu.edu.cn
3
+
tion, luminescent Tb ions, with tunable fluorescence signals
towards the analytes in the exchangeable coordination sphere,
can be incorporated in this architecture by coordination with
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201505082.
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3
+
4À
Scheme 1. Schematic illustration of the supramolecular assembly of CDots@SC4A-Tb nanodevice and the ratiometric sensing mechanism for H
2
PO detec-
tion.
the hydroxyl groups on SC4A, which also serves as a sensitizer
for the coordinated Tb ions.
cate the successfully preparation of surface quaternized CDots
(see the Supporting Information). The highest emission wave-
length was 440 nm when the CDots were excited at 350 nm.
Figure S4 (in the Supporting Information) shows the yellow
aqueous solution of the CDots emitting strong blue fluores-
cence under the 365 nm lamp. Once the CDots are further
functionalized to enable use in practical sensing applications,
they must remain stable toward ambient environments. To in-
vestigate the stability, the fluorescence intensity of the CDots
with interferences in solution was measured. Remarkably, no
significant change in the signal was observed in the presence
of various metal cations or anions (Figure S5 in the Supporting
Information).
3
+
This system takes advantage of the unique spectroscopic
3
+
characteristics of the Tb ions, including long fluorescence
lifetimes, large Stokes shifts, and sharp line-like emission bands
[12]
arising from partially forbidden f–f transitions.
Meanwhile,
the CDots can be employed as the fluorescence interior refer-
ence to improve detection accuracy and sensitivity. This can
also be used to fabricate a logic gate. Based on this elaborate
supramolecular assembly, the nanodevice can be used as a flu-
orescent ratiometric probe for phosphates. Also, it provides an
opportunity to be employed as a nano-logic gate with two
output channels.
The formation of CDots@SC4A was easily achieved by
mixing the surface quaternized CDots with SC4A at room tem-
perature and stirring the resulting solution for 12 h. Unbound
SC4A was then removed by using dialysis bags for 72 h. The
resulting adduct, CDots@SC4A, was further functionalized by
coordination of the hydroxyl groups of the immobilized SC4A
Results and Discussion
The rationally designed nanodevice was constructed with con-
secutive supramolecular loading of SC4A and coordinative in-
3
+
corporation of Tb ions on the quaternized CDots (Scheme 1).
First, the quaternized CDots were synthesized by carbonization
of the salt precursor containing betaine hydrochloride and
tris(hydroxymethyl)aminomethane (Tris). The carbonization was
conducted at a moderate temperature (2508C) to obtain CDots
with exposed ammonium units (Scheme S2 in the Supporting
3
+
3+
with Tb to form a SC4A-Tb complex on the surface of the
nanomatrix. The excitation energy is transferred from the excit-
3
+
ed triplet state level (T ) of the ligand SC4A to Tb , sensitizing
1
3
+ [15]
the emission of Tb
.
The successful assembly of SC4A on
the surface of CDots was investigated by transmission electron
microscopy (TEM), zeta potential measurements, X-ray photo-
electron spectroscopy (XPS), and Fourier transform infra-red
(FTIR) spectra. From a typical TEM image of CDots@SC4A (Fig-
ure 1a), we see that the CDots@SC4A were well dispersed with
an average size of 5 nm. The assembly of SC4A on the surface
of CDots was confirmed by a change in zeta potential from
+11.5 mV (bare CDots) to À15.1 mV (after interaction with
SC4A). It is well known that SC4A is negatively charged be-
[
13]
Information). The as-prepared CDots exhibited blue fluores-
cence and were highly soluble in water and polar organic sol-
vents. They were used as building blocks for the functionaliza-
tion by controlled assembly with SC4A through the multiple
binding ability between the quaternary ammonium moieties
[14]
and hydrophobic cavity of SC4A.
The chemical structure of the CDots has been investigated
by different methods, such as transmission electron microsco-
py (TEM), Fourier transform infrared spectroscopy (FTIR), and
X-ray photoelectron spectroscopy (XPS). These method indi-
[
14a]
cause of its four sulfonic groups.
The final negative value of
the zeta potential indicates that the surface of the CDots has
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Figure 1. a) TEM images and b) HR-TEM images of CDots@SC4A. c) XPS spectrum scan of CDots@SC4A (light line) and CDots (dark line). d) Emission spectra
3
+
3+
upon titration of Tb into CDots@SC4A solution in 10 mm HEPES buffer pH 7.4, ranging from 0 to 90 mm Tb , lex =275 nm.
À1
been successfully functionalized with SC4A based on supra-
molecular interactions (Figure S7 in the Supporting Informa-
tion).
The CDots exhibited absorption peaks for NÀH (3391 cm ), C=
À1 À1
H (2929 cm ), and CÀO (1449 and 1056 cm ). These peaks
are also found in the spectra of CDots@SC4A and illustrate
that the CDots serve as the matrix without undergoing struc-
tural change during the formation of the CDots@SC4A nano-
The XPS spectra were recorded to explore the chemical
composition and chemical bonds of CDots@SC4A. As shown in
Figure 1c, the CDots@SC4A show four peaks at 283.6, 400.1,
[
17]
À
hybrid. Strong absorbance peaks for SO3 are observed in
À1
5
30.6, and 167.0 eV in the XPS spectrum. These are attributed
both SC4A and CDots@SC4A at 1189 and 1049 cm . This indi-
to C1s, N1s, O1s, and S2p, respectively. The high-resolution
XPS spectra of C1s, N1s, O1s (Figure S3 in the Supporting In-
formation) further confirmed that the CDots@SC4A nanohybrid
has hydroxyl, carbonyl, carboxylic, and amine groups. The ap-
pearance of new peaks at 167.0 eV (S2p) and 231.2 eV (S2s) in
the XPS spectrum (Figure 1c) is assigned to the sulfonic group
of SC4A and confirms the successful surface modification of
the CDots by SC4A. The high-resolution XPS S2p spectrum can
be devolved into two sets of components with binding ener-
gies of 168.0, 167.9 eV and 166.2, 165.7 eV. These peaks corre-
spond to oxidized forms of sulfur (Figure S8 in the Supporting
Information). According to the spectra, the 168.0 and 167.9 eV
cates the attachment of SC4A to the CDots. In addition, the
À1
peak at 1639 cm in the SC4A spectrum is slightly shifted to
À1
1649 cm in the CDots@SC4A spectrum. This represents the
[
18]
characteristic absorption of a phenyl unit and confirms that
SC4A is successfully assembled on the CDots through supra-
molecular interactions to form the CDots@SC4A nanohybrid.
3
+
After introducing Tb ions to generate an ionic outer layer
3
+
and obtain the nanoprobe CDots@SC4A-Tb , the outer layer
3
+
interaction between SC4A and Tb in aqueous solution was
verified with fluorescence, UV/Vis spectra, and XPS measure-
3
+
ments. When different concentrations of Tb ions were intro-
duced into CDots@SC4A aqueous system, the emission intensi-
À
3+
are SO3 states and the 166.2 and 165.7 eV are attributed to
ties of the Tb ions were enhanced as a result of efficient
3
+
the partial presence of sulfur in oxidized forms. The lower
binding energy can be explained by electron transfers from
the CDots, in particular from the surface positively charged
energy transfer from the SC4A ligands to the Tb ions (Fig-
ure 1d). The observed spectrum patterns correspond to the
5
7
5
7
5
7
D ! F (494 nm), D ! F (545 nm), D ! F (587 nm), and
4
6
4
5
4
4
[16]
5
7
3+
amine groups to sulfonic groups. FTIR spectra (Figure S9 in
the Supporting Information) were further used to identify the
surface modification process of the CDots@SC4A nanohybrid.
D ! F (625 nm) transitions. These are particular to the Tb
4
3
-centered luminescence, sensitized by the SC4A ligand by the
[
15a,b,19]
antenna effect.
8341
The enhancement of the fluorescence in-
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3
+
tensity should be attributed to the coordination of Tb with
Phosphates play pivotal roles in a variety of fundamental
physiological processes such as genetic storage and regulation,
energy metabolism, signal processing, DNA replication and
the phenolic hydroxyl group of SC4A and the formation of the
CDots@SC4A-Tb nanohybrid. Similarly, the blue–green fluo-
rescence may be attributed to the synergistic effect of the sur-
3
+
[
21]
transcription, etc.
Developing selective and cost-effective
3
+
face SC4A-Tb complex and CDots core.
phosphate recognition probes that work under physiological
To rule out the possibility of nonspecific binding between
conditions is important for phosphate-related biomedical diag-
3
+
[22]
the CDots and Tb , a control experiment was performed in
nosis and environmental remediation.
In bio-systems
3
+
À
2À
which the CDots were mixed with Tb in HEPES buffer. No
significant signal change was observed in the control experi-
ment (Figure S11 in the Supporting Information). The chemical
composition was also characterized by XPS spectra to verify
(pH 7.4), H PO4 and HPO4 commonly coexisted and are at
2
À
a dynamic equilibrium state. Thus, we chose H PO4 as an ex-
2
ample to illustrate the detecting ability of our nanoprobe. Fig-
3
+
ure 3a shows that the green fluorescence of Tb is quenched
3
+
À
the formation of CDots@SC4A-Tb . A weak peak at 150.9 eV
in the presence of H PO4 , but the blue fluorescence of the
2
could be assigned to Tb4d (Figure 2a). This confirms the suc-
CDots (l =440 nm) remains constant. The other anions
em
À
À
À
À
2À
À
À
2À
À
(
AcO , BF , PF6 , Br , C O , CF SO , ClO4 , CO3 , F ,
4
2
4
3
2À
3
À
À
À
À
HCO3 , HSO4 , I , SCN , and SO , 25 mm) tested in this
4
system do not exert significant quenching effects on the fluo-
rescence intensity of the nanoprobe (Figure 3b, dark-colored
bars).
Interference experiments were also carried out by compar-
ing the fluorescence ratios of the nanoprobe solution in the
presence of these anions (Figure 3b, light-colored bars). The
apparent decrease in the emission intensity of the probe solu-
À
tion upon addition of the H PO4 was not affected by the co-
2
existence of other anions. Moreover, the presence of common
+
+
2+
2+
2+
3+
2+
2+
2+
cations like Na , K , Mg , Ca , Zn , Fe , Mn , Co , Ni
,
+
2+
Ag , and Cd did not create interference for the detection of
À
2+
H PO4 ; only Cu showed minor quenching (Figure S12 in the
2
Supporting Information).
The fluorescence sensing experiments demonstrated that
À
the nanoprobe is highly selective for H PO4 . This can be ex-
2
À
3+
plained by stronger interactions between H PO4 and Tb
2
versus the interactions between the phenolic hydroxyl groups
3
+ [15c,22a]
in SC4A with Tb
.
To prove the different interaction
strengths, we calculated the association constants between
À
3+
3+
H PO4 and Tb as well as the Tb and CDots@SC4A. The as-
2
À
3+
sociation constant between H PO and Tb was estimated to
2
4
5
À1
be 7.85”10 m from the linear regression using a Stern–
Volmer plot (Figure S15 in the Supporting Information). The as-
3
+
sociation constant (K ) between the CDots@SC4A and Tb
s
3
+
can be found by titrating Tb into the CDots@SC4A solution
Figure S16 in the Supporting Information). This K value was
(
s
4
À1
estimated to be 3.9218”10 m , which is far smaller than the
K value for H PO and Tb . Therefore, the observation of
À
3+
s
2
4
Figure 2. a) High-resolution Tb4d XPS spectrum. b) UV/Vis absorption spec-
tral responses of CDots@SC4A in aqueous solution upon the addition of 0–
fluorescence quenching at 545 or 490 nm of the CDots@SC4A-
3
+
À
3
+
Tb
may be due to the substitution of SC4A by H PO
2
25 mm Tb (inset: enlarged area).
4
3
+
anions. Phosphates have no antennae effect on the Tb -cen-
tered emission. Similarly, the other anions have much poorer
3
+
3+
cessful coordination of Tb with SC4A on the surface of the
bonding ability with Tb than SC4A, and thus they cannot re-
place the SC4A on the CDots@SC4A-Tb
The ratiometric sensing behavior of the CDots@SC4A-Tb
3
+
CDots. The high-resolution XPS spectrum of Tb3d also shows
.
3
+
two peaks corresponding to 3d
(1240.1 eV) and 3d3/2
5
/2
3
+
(
1275.9 eV) of Tb
(Figure S10 in the Supporting Informa-
toward phosphates was further investigated in HEPES buffer
[
20]
3+
À
tion).
Evidence for the formation of CDots@SC4A-Tb
is
(10 mm, pH 7.4). Upon addition of H PO4 , there is a gradual
2
also seen in the UV/Vis absorption spectra. Upon addition of
decrease in the fluorescence intensity at 490 and 545 nm (Fig-
ure 3c, Figure S9 in the Supporting Information). The highest
fluorescence intensity ratios are at 490/545 nm and 440 nm
3
+
Tb , the UV/Vis absorption spectrum of the CDots@SC4A so-
lution blueshifts and is slightly enhanced (Figure 2b). This illus-
3
+
trates that the Tb ions bind to the surface of the CDots@S-
C4A through a coordinative interaction.
(I490/I440 and I /I440). Figure 3d plots the I545/I440 signal as a func-
545
À
tion of H PO4 concentration. The ratio of I545/I440 gradually de-
2
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3
+
4À
3+
Figure 3. a) Fluorescence intensity ratio of the CDots@SC4A-Tb to H
2
PO and other relevant species. b) Effect of different anions (1: CDots@SC4A-Tb , 2:
À
À
2À
À
À
À
2À
À
À
À
À
À
À
2À
AcO , 3: Br , 4: C
2
O
4
, 5: CF
3
SO
3
, 6: Cl , 7: ClO
4
, 8: CO
3
, 9: F , 10: HCO
3
, 11: HSO
4
, 12: I , 13: NO
3
, 14: SCN , 15: SO
4
3+
) on the fluorescence proper-
3
+
ties of CDots@SC4A-Tb . The dark-colored bars represent the addition of anions (25 mm) to the solution of CDots@SC4A-Tb (pH 7.4). The light-colored
À
3+
bars represent the subsequent addition of 25 mm H
2
PO
4
to the solution. c) Fluorescence spectra of CDots@SC4A-Tb in HEPES (10 mm, pH 7.4) obtained
from 0 to 25 mm, under excitation at 275 nm. d) The plot of I545/I440 as a function of H
À
4À
upon titration with H
2
PO
4
2
PO concentration obtained with CDots@S-
3
+
C4A-Tb (pH 7.4).
À
creased with increasing H PO concentration and reached
nel at 440 nm in the absence and presence of CDots@SC4A
can be represented by a “YES” logic gate (Figure 4). Only the
presence of CDots@SC4A can implement the change in fluo-
2
4
À
a plateau when the concentration of H PO was higher than
2
4
2
0 mm. There is good linearity between 50–350 nm with a corre-
lation coefficient of 0.997 (Figure 3d, inset). The detection limit
[23]
was 33.86 nm based on a signal-to-noise ratio of 3. This is
better than or comparable to those obtained with other fluo-
À
3À
rescent probes for H PO . Other phosphates such as PO4
,
2
4
2
À
HPO4 , and PPi are also able to quench the fluorescence of
3
+
the CDots@SC4A-Tb effectively. The different quenching abil-
ity towards the common phosphates are shown in the Sup-
porting Information (Figure S14, Table S1). In addition, the re-
versibility and reusability of this nanodevice were evaluated by
3
+
À
repeated complex/stripping cycles by adding Tb /H PO4 ; the
2
fluorescence spectrum was recorded after each step. The re-
sults demonstrate that the procedure can be repeated at least
five times (Figure S17 in the Supporting Information).
Formation of logic gates using changes in optical responses
is important because they can be used to solve complex prob-
[24]
3+
lems on the molecular level. The CDots@SC4A-Tb nanode-
vice can be used as a fluorescence logic gate by using
3
+
À
CDots@SC4A, Tb , and H PO as the chemical inputs. As
2
4
shown in Figure 4a, the fluorescent change can be described
and manipulated as a logic gate by a threshold of CDots@S-
3
+
À
C4A, Tb and H PO . The absence and presence of these
2
4
ions were defined as the 0 and 1 states, respectively. The eight
possible input combinations are shown in the truth table in
Figure 4b. This system can form two different patterns of logic
gate at the two different wavelengths. The fluorescence chan-
Figure 4. a) Schematic illustration of the fabrication procedures for the novel
3
+
nano-logic gate based on the CDots@SC4A-Tb assay. b) Fluorescence re-
sponse of CDots@SC4A at 440 nm and 545 nm in the absence and presence
3
+
À
of Tb and H
2
PO
4
and truth table for the logic gate.
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rescence at 440 nm. This provides an output signal of 1. The
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3
+
À
the absence of both Tb and H PO (100), the CDots@SC4A
2
4
has no significant emission at 490 or 545 nm (considered as
3
+
output signal “1”). However, upon addition of Tb
(input
signal 110), a new emission at 490 or 545 nm emerges, owing
to the antenna effect of SC4A on the CDots surface due to
3
+
À
Tb . When the hybrid is treated with H PO (input signal
2
4
1
01), no fluorescence change is seen at 545 nm. This results in
a fluorescence signal pattern of 0.
3
+
À
However, in the presence of CDots@SC4A, Tb , and H PO4
2
À
(
input signal 111), the quenching effect of H PO results in an
2
4
emission decrease at 490 or 545 nm. Thus, the fluorescence
signal pattern at 490/545 nm in the presence of CDots@SC4A,
3
+
À
Tb , and H PO4 (111) is 0. Therefore, only the input combi-
2
nations (110) led to an output of 1, or the on state; all other
input combinations correspond to an output of 0, or the off
state. With two different channels of fluorescence signals, this
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Conclusion
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We have described an elaborate supramolecular assembly
strategy to fabricate a CDot-based nanodevice for ratiometric
molecular recognition and use as a nano-logic gate. By taking
advantage of the dynamic equilibrium characteristics of supra-
molecular interactions, the ingeniously designed nanodevice
can be endowed with enhanced functions. The in-depth inves-
tigations of the devised architecture indicate that it can be
successfully applied for selective and sensitive phosphate de-
tection as well as a nanoscale logic gate. This strategy provides
a convenient methodology to fabricate nanodevices with con-
trollable properties by supramolecular assembly. This can be
potentially applied in wide-ranging applications such as biop-
robes, nanoreactors, and medical diagnostics.
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Acknowledgments
The research leading to these results has received funding
from the National Natural Science Foundation of China (Proj-
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim