A Stimuli-Responsive Smart Lanthanide Nanocomposite for Multidimensional Optical Recording and Encryption

Article abstract of DOI:10.1002/anie.201700011

A stimuli-responsive lanthanide-based smart nanocomposite has been fabricated by supramolecular assembly and applied as an active material in multidimensional memory materials. Conjugation of the lanthanide complexes with carbon dots provides a stimuli re

Full text of DOI:10.1002/anie.201700011

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201700011
German Edition: DOI: 10.1002/ange.201700011
Supramolecular Assembly
A Stimuli-Responsive Smart Lanthanide Nanocomposite for
Multidimensional Optical Recording and Encryption
Xiang Li, Yujie Xie, Bo Song, Hao-Li Zhang, Hao Chen, Huijuan Cai, Weisheng Liu, and
Yu Tang*
Abstract: A stimuli-responsive lanthanide-based smart nano-
composite has been fabricated by supramolecular assembly
and applied as an active material in multidimensional memory
materials. Conjugation of the lanthanide complexes with
carbon dots provides a stimuli response that is based on the
modulation of the energy level of the ligand and affords
microsecond-to-nanosecond fluorescence lifetimes, giving rise
to intriguing memory performance in the spatial and temporal
dimension. The present study points to a new direction for the
future development of multidimensional memory materials
based on inorganic–organic hybrid nanosystems.
complexes, by supramolecular assembly, which facilitates
multidimensional optical recording and encryption. On the
one hand, fluorescent carbon dots (CDs) are promising
carbon-based nanomaterials that have found applications in
chemical sensing and bioimaging owing to their high stability,
abundant surface functional groups, and environmentally
[
5]
friendly preparation. On the other hand, lanthanide com-
plexes with long lifetimes, line-like emissions, and flexible
coordination modes exhibit distinct advantages in the assem-
[
6]
bly of smart luminescent materials. In our design (Figure 1),
A
s information science rapidly develops, memory devices
with high density, security, and storage ability are urgently
[
1]
required and have attracted tremendous interest. One
approach to increase the data security and storage density
of such devices is to use stimuli-responsive fluorescent
materials that can rapidly switch between different states
as optical data recording and document encryption systems.
[
2]
[3]
Furthermore, the temporal dimension of fluorescence signals
can also be introduced in optical multiplexing, which opens
[4]
more opportunities to improve data storage and security.
We envisioned that the synergistic use of fluorescence color
and lifetime as coding elements will provide a new approach
to enhance document security against forgery and increase
data storage density. In a proof-of-concept study, we herein
report an inorganic–organic hybrid nanosystem that enables
coding in both the spatial and temporal dimension by
manipulating the stimuli-responsive luminescence colors
and decay lifetimes of the individual components.
Figure 1. Operating principle of the stimuli-responsive CDs-Eu-HL
nanocomposite for multidimensional optical recording and encryption.
lanthanide complexes, Eu–HL, were assembled on the surface
of CDs by coordination to give a hybrid nanosystem, CDs-Eu-
Our smart nanocomposite is based on the conjugation of
two different fluorophores, carbon dots and lanthanide
3
+
HL, in which the Eu and CD species emit with microsecond-
to-nanosecond lifetimes, allowing for time-resolved readout.
Meanwhile, the energy levels of the ligand HL can be
[*] X. Li, Y. J. Xie, Prof. H. L. Zhang, H. Chen, H. J. Cai, Prof. W. S. Liu,
modulated with Et N/HCl vapor, providing a chemical
3
Prof. Y. Tang
response to realize secure optical data recording. Moreover,
considering the preparation of environmentally friendly
aqueous fluorescent ink, which can be used in optical
State Key Laboratory of Applied Organic Chemistry
Key Laboratory of Nonferrous Metal Chemistry and Resources
Utilization of Gansu Province
College of Chemistry and Chemical Engineering
Lanzhou University
3
+
recording and encryption, the water solubility of the Eu
complex can be anticipated to be improved by conjugation
[
7]
Lanzhou 730000 (China)
E-mail: tangyu@lzu.edu.cn
with the water-soluble CDs. To the best of our knowledge,
this inorganic–organic hybrid nanosystem is the first example
of a stimuli-responsive smart nanocomposite that simulta-
neously stores information in the spatial and temporal
dimension in a single material. We believe that our multi-
dimensional data storage material will provide new oppor-
tunities in data recording and security.
Dr. B. Song
State Key Laboratory of Fine Chemicals
School of Chemistry
Dalian University of Technology
Dalian 116024 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201700011.
Herein, CDs were prepared by a microwave-assisted
pyrolysis method using citric acid as the carbon source and
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[
8]
urea as a surface-passivation agent. The chemical structure
of the CDs was characterized by transmission electron
microscopy (TEM), X-ray photoelectron spectroscopy
the characteristic absorption bands of CDs and the Eu-HL
complexes in the UV/Vis absorption spectra (Figure S12).
Furthermore, a full-scan XPS plot (Figure S13) confirmed
that the CDs-Eu-HL composite was mainly composed of C, N,
and O. The corresponding high-resolution C 1s, N 1s, and O 1s
XPS spectra revealed the presence of carbonyl, carboxyl, and
amine groups, which was also confirmed by FTIR spectros-
copy. Furthermore, we also observed a weak peak corre-
sponding to Eu 3d (Figure S13a). The high-resolution Eu 3d
XPS spectrum (Figure S14) shows two peaks, corresponding
(
XPS), and Fourier transform infrared spectroscopy (FTIR;
see the Supporting Information, Figures S1–S3). We thus
confirmed the presence of abundant carboxyl and amide
groups on the surface of the as-obtained CDs. These func-
tional groups provide plenty of coordination sites that can
directly chelate Eu ions. The fluorescence spectra of the
CDs reveal optimal excitation and emission wavelengths of
3
+
3
+ [12]
3
49 nm and 448 nm, respectively (Figure S1d). Additionally,
to the 3d5/2 (1134.7 eV) and 3d3/2 (1164.8 eV) states of Eu ,
which further indicated that the surface of the CDs has been
the CDs show excellent fluorescence stability over a broad
pH range (3–11) and under continuous excitation for one
hour (Figure S4).
Interestingly, the fluorescence of the CDs undergoes
a significant red shift from 449 to 491 nm upon an increase in
the solute concentration from 10 to 300 mgmL (Figure S5).
The fluorescence color changes from blue to green, which is
consistent with the corresponding chromaticity coordinates
3
+
successfully functionalized with Eu-HL. The Eu content of
the CDs-Eu-HL composites was determined to be 22.51 wt%
by thermogravimetric analysis (TGA). Similarly, the as-
obtained CDs-Eu-HL composites also display typical excita-
tion-wavelength-dependent fluorescence owing to the pres-
ence of the CDs. As shown in Figure S15, the emission peak is
gradually red-shifted from 443 to 552 nm when the excitation
wavelength is increased from 340 to 500 nm.
ꢀ
1
(
Figure S6). This red shift is best attributed to re-absorption in
view of the spectral overlap between absorption and emission
To study the stimulus response of the nanodevice, we
investigated the energy levels of the ligand HL and depro-
[9]
(
Figure S7), which can occur at small interparticle distances.
ꢀ
3+
This was further confirmed by the fact that the average
lifetime of the fluorescence of the CDs changed from 9.42 to
tonated ligand L in the Gd complexes on the CD platform.
The corresponding energy levels of the triplet excited states
3
7
.72 ns as the concentration was increased (Figure S8 and
( pp*) of the ligand were calculated from the low-temper-
Table S1) because re-absorption processes compete with
ature (77 K) phosphorescence spectra of the corresponding
Gd complexes in a 1:1 methanol/ethanol mixture. Accord-
[10]
3+
radiative transitions and then shorten the lifetime.
Fur-
thermore, the significant shift in wavelength that is described
above is somewhat similar to the excitation-dependent
ing to Latvaꢀs empirical rule, an optimal ligand-to-metal
3
+
3
5
energy transfer process for Ln needs DE = pp*ꢀ D =
J
[
9b,c]
ꢀ1
3+ [13]
fluorescence properties of the as-obtained CDs.
We
2500–4000 cm for Eu . Based on the low-temperature
phosphorescence spectrum of the CDs-Gd-HL nanocompo-
sites, we calculated the energy level of the triplet excited state
believe that these interesting properties of the CDs should
be helpful in anti-counterfeiting. First, printing patterns can
only be observed under a UV lamp when CDs are utilized as
the fluorescent ink (Figure S9). Moreover, when the concen-
tration of the CDs in the ink changes, the fluorescence color
also changes. As shown in Figure S10, the part of the image
printed with an ink with a low CD concentration was
completely invisible in daylight unlike the area printed at
a higher CD concentration, which can make anti-counter-
feiting marks difficult to duplicate.
ꢀ
1
of the ligand HL to be 22026 cm , which is not very suitable
5
3+
ꢀ1
for energy transfer to the D level of Eu (17500 cm ;
0
Figure S16a). However, the energy level of the triplet excited
state of HL can be tuned by deprotonation on the CDs. As
shown in Figure S16b, the corresponding energy level of the
ꢀ
ꢀ1
ligand L in the CDs-Gd-L nanocomposite (20790 cm ) was
ꢀ1
5
3+
located approximately 3290 cm above the D level of Eu .
0
Thus efficient intramolecular energy transfer is possible. Not
surprisingly, we observed the emissions characteristic of the
As the energy levels of the amide-type b-diketone ligand
N-(2-pyridinyl)benzoylacetamide (HL) can be modulated by
3
+
Eu ion in the fluorescence spectrum of the EuL complex,
[
11]
5
7
acid/base vapor, we further designed a stimuli-responsive
nanocomposite that combines the advantages of the CDs and
which were attributed to the D ! F (J = 0–4) transitions
0
J
upon excitation at l = 370 nm (Figure S17).
ex
3
+
3+
Eu complexes by coordination effects. The Eu ions act as
bridges to join the CDs and HL ligands. The successful
supramolecular assembly of Eu-HL on the surface of the CDs
was investigated by FTIR, UV/Vis absorption, and XPS
spectroscopy. As shown in Figure S11, the HL ligands
exhibited characteristic CꢀH in-plane and out-of-plane
To test potential applications of the CDs-Eu-HL nano-
composites, we further studied the fluorescence properties of
the nanocomposites after deposition on filter paper (with no
background UV fluorescence). The paper was first dipped
into an aqueous solution of CDs-Eu-HL and then dried in air.
The broad emission peak at 450 nm was attributed to the CDs,
and the emission peak at 612 nm was assigned to the
bending and stretching vibrations of the pyridine group at
ꢀ
1
3+
1
157 and 908 cm , respectively. The absorption peaks in the
characteristic transitions of Eu ions. Interestingly, the
fluorescence quickly changed from blue to blue-violet upon
ꢀ1
1
435–1547 cm region can also be attributed to n(C=C) and
n(C=N) bands of the pyridine group. Enol-type C=O stretch-
exposure to Et N vapor (Figures 2a and S18). The average bi-
exponential fluorescence lifetime of Eu in the CDs-Eu-HL
is 390.4 ms (Figure 2b and Table S2), which increases remark-
3
3
+
ing vibrations of the b-diketone give rise to the peak at
ꢀ
1
1
652 cm . These peaks can also be found in the spectra of
CDs-Eu-HL and illustrate that Eu-HL is successfully assem-
bled on CDs through supramolecular coordination to form
the smart nanosystem. The CDs-Eu-HL composites display
ably to 562.22 ms after exposure to the Et N vapor and
decreases again to 396.62 ms upon exposure to HCl gas. Good
reversibility was observed even after six cycles (Figure 2c).
3
2
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5
Figure 2. a) Fluorescence emission spectra and b) fluorescence lifetime decay profiles from the D energy level of the CDs-Eu-HL nano-
0
composites: Changes after exposure to Et N vapor and HCl gas (l =350 nm, l =612 nm). c) Changes in the ratio of the emission intensities
3
ex
em
at 612/450 nm of the CDs-Eu-HL nanocomposites during cycles of exposure to Et N/HCl vapor. d) Fluorescence intensities of the CDs-Eu-HL
3
nanocomposites and EuL complexes on filter paper during continuous excitation with a UV beam (l =350 nm). Irradiation time: 0 to 60 min
ex
(lem =612 and 450 nm, respectively).
To demonstrate the performance of our hybrid nano-
fabricated according to the binary codes of the standard 8 bit
ASCII characters, with blue and blue-violet spots represent-
ing “0” and “1”, respectively. According to the ASCII binary
codes, the null character is translated into “00000000” while
the capital letter “L” can be translated into “01001100”. We
carried out this encoding–reading experiment using a desktop
inkjet printer (Canon ip2780), and solutions of the CDs-Eu-
HL nanocomposite and the original CDs were used as the red
and blue inks, respectively. Next, we printed patterns on filter
paper substrates and then obtained fluorescence images
under UV (365 nm) illumination. In this experiment, the
inkjet printer acts just like an encryptor, which can be used to
encoded the raw information (Figure 3a). Therefore, as
shown in Figure 3b, all blue fluorescent signals were collected
as binary coding and can merely be translated into null
characters according to the ASCII binary codes. However,
system as a secure memory device, the acronym of Lanzhou
University (LZU) was handwritten on a filter paper with a gel
pen loaded with the CDs-Eu-HL aqueous solution as
fluorescent ink (Figure S18). These three letters were
almost invisible in daylight. Upon irradiation with a UV
lamp (365 nm), bright blue letters became visible. Upon
exposure to Et N vapor, the fluorescence changed immedi-
3
ately from blue to blue-violet in 10 s. Remarkably, the blue
fluorescence reappeared when the filter paper was exposed to
HCl gas. Moreover, excitation-wavelength-dependent fluo-
rescence was also observed (Figure S19). Furthermore, the
3
+
photoluminescence stability of the Eu complexes clearly
improved when conjugated to the CDs (Figure 2d). Specif-
3
+
ically, the fluorescence emission intensity of Eu in the CDs-
Eu-HL nanocomposite upon exposure to Et N vapor showed
3
a slight decrease after continuous excitation for one hour
after exposure to Et N vapor, the blue fluorescent signals that
3
(
Figure 2d, red curve), whereas the fluorescence intensity of
the free complex EuL at 612 nm declined more strongly
Figure 2d, black curve). In contrast, the intensity of the
were due to the CDs-Eu-HL nanocomposite rapidly changed
to blue-violet. After this “chemical decoding” step, the
fluorescent dot matrix can be easily read and translated into
“LZU”, the acronym of Lanzhou University. Moreover, the
information can be encrypted again by exposure to HCl gas
owing to the good fluorescence reversibility of the CDs-Eu-
HL nanocomposite as mentioned above. Owing to these
remarkable properties, the security of this binary coding
system has been significantly improved to avoid information
leakage.
(
fluorescence emission at 450 nm of the CDs-Eu-HL nano-
hybrid (Figure 2d, blue curve) remained almost constant,
similar to that of the original CDs (Figure S4b).
Based on the remarkable fluorescence properties of the
CDs-Eu-HL nanocomposite, we next tried to expand its
potential utility to multidimensional optical recording and
encryption. A two-color microarray data storage pattern was
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Figure 3. a) Schematic illustration of the information coding, reading,
encryption, and decryption process. Digital photographs of b) binary-
coded and c) Morse-coded two-color microarray data storage chips.
As an information coding standard, the Morse code has
been used for longer than any other encoding program.
Originally, the Morse code represents characters (including
letters, numbers, and symbols) by a variety of specific
combinations of short and long signals called “dots” and
Figure 4. a) Time-resolved emission spectra of CDs-Eu-HL nanocom-
posites after exposure to Et N vapor for delay times from 0 to 500 ms.
3
b) Simulation of the lifetime-encoded document security and data
storage process. c) Steady-state and time-gated fluorescence images of
a glass cover slip coated with CDs-Eu-HL and CDs (A), after exposure
“
dashes”. In consideration of the dual-channel fluorescence
to Et N vapor (B), and after exposure to HCl gas (C). Delay time:
3
output of the CDs-Eu-HL nanocomposite, we can encrypt
and transmit information by using the Morse code, in which
blue and blue-violet spots represent “dashes” and “dots”,
respectively. As shown in Figure 3c, we obtained raw
information that had been encrypted by the inkjet printer.
Initially, we only observed some blue fluorescent spots.
33 ms.
5
3+
Moreover, the D0 excited-state lifetime of Eu was
prolonged to 562.22 ms after exposure to Et N vapor. Thus,
3
according to the decoding rule that we have stipulated, the
authenticity and accuracy of encoded information can be
validated through a further processing step. As shown in
Figure S22, the encoded information (LZU) inscribed with
the CDs-Eu-HL nanocomposite can only be read during 10–
However, after exposure to Et N vapor, we can easily retrieve
3
the valuable information, which was decrypted into “LZU”
according to the rules of the Morse code. The information can
also be concealed again by exposure to HCl vapor.
As the fluorescence lifetime of the CDs-Eu-HL nano-
300 ms. Upon exposure to Et N vapor, LZU can also be
3
3
+
composite ranges from microseconds (Eu : 390.40 ms,
Table S2) to nanoseconds (CDs: 7.78 ns, Figure S20),
improvements in information security and data storage
domains can be realized with a new optical coding and
decoding dimension. To confirm the temporal/dimensional
optical coding, time-resolved emission spectra of CDs-Eu-HL
observed during 400–600 ms. This result provides a new option
for a more secure way of document encryption so that only
well-trained and authorized personnel who know the correct
decoding rule and thus the fluorescence decay lifetimes are
able to access the confidential information.
In summary, a novel vapor-responsive lanthanide optical
material (CDs-Eu-HL) has been fabricated by supramolec-
ular assembly, and applied in a multidimensional information
memory device. The smart nanocomposite showed strong and
stable fluorescence as well as microsecond-to-nanosecond
fluorescence lifetimes and remarkable changes in the fluo-
rescence upon exposure to acidic or basic environments. The
conjugation of CDs and Eu-HL improved not only the water
solubility but also the stability of the Eu-HL photolumines-
cence. Optical recording and encryption in the spatial and
temporal dimension was enabled by the stimuli-response
energy-level modulation of the ligand and the various
fluorescence lifetimes of the lanthanide ions and CDs. This
work opens new opportunities for future memory devices
carrying more codes through a combination of color, intensity,
and lifetime, revealing the potential of luminescence as
a powerful tool to deal with the challenges in high-density
data storage as well as document security against forgery.
and the nanocomposite after exposure to Et N vapor and HCl
3
gas were collected (Figures 4a and S21). Obvious changes in
the fluorescence intensities at 450 nm and 613 nm were seen.
As shown in Figure 4a, the peak at 450 nm disappeared when
the delay was set to 100 ms while the peak at 613 nm gradually
decreased in intensity when the delay time became much
longer. Figure 4b shows a simulation diagram of lifetime-
encoded document security and data storage as the oval was
printed with CDs while the letters (LZU) were printed with
CDs-Eu-HL. First, the blue fluorescence ascribed to the CDs
can be easily observed, while the blanks show a word, namely
“
Chemistry”. After exposure of the pattern to Et N vapor,
3
along with an extension of the delay time, the blue fluores-
cence of the CDs disappears, revealing the background color.
At that time, the acronym of Lanzhou University, “LZU”,
3
+
emerges owing to the long fluorescence lifetime of the Eu
ions (Figure 4b). This proposition was confirmed to some
extent by steady-state and time-gated fluorescence images
(
Figure 4c). The changes in the fluorescence color are
consistent with the time-resolved emission spectra.
4
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Manuscript received: January 1, 2017
Revised: January 15, 2017
Final Article published: && &&, &&&&
[
4] Y. Q. Lu, J. B. Zhao, R. Zhang, Y. J. Liu, D. M. Liu, E. M.
Goldys, X. Yang, P. Xi, A. Sunna, J. Lu, Y. Shi, R. C. Leif, Y. J.
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Supramolecular Assembly
Hidden talents: A stimuli-responsive
lanthanide nanocomposite was fabri-
cated by functionalization of carbon dots
with lanthanide complexes and applied in
a multidimensional memory device. The
fluorescence lifetime and color can be
changed by modulating the energy levels
of the lanthanide ligand.
X. Li, Y. J. Xie, B. Song, H. L. Zhang,
H. Chen, H. J. Cai, W. S. Liu,
Y. Tang*
&&& — &&&
A Stimuli-Responsive Smart Lanthanide
Nanocomposite for Multidimensional
Optical Recording and Encryption
6
www.angewandte.org
ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!