Background and Originality Content
Supramolecular materials are dynamic materials with its components
connected by a variety of non-covalent interactions, such as hydrophobic
interactions, hydrogen bonds, π-π stacking, metal ligand coordination,
and host/guest complexation [1-2]. The material
could be imparted with intriguing properties such as self-repairing and
chirality via incorporation of supramolecular recognition motifs.
2-ureido-4[1H]-pyrimidinone (UPy) is a well-explored supramolecular
building block that can form robust dimer through quadruple hydrogen
bonds. Owing to the high binding constant and ease of synthesis, UPy has
been widely used to construct supramolecular polymers with self-healing,
shape memory properties as well as enhanced mechanical performances[3-4]. However, there are less examples to use UPy
units for achieving room-temperature phosphorescence (RTP), which
requires the well-organization of recognition motifs and emission
centers.
Carbon dots (CDs) [5], a new type of carbon
nanomaterial, have attracted extensive attentions due to its unique
optical properties as well as inherent advantages of superior stability,
good solubility and low toxicity[6-9]. CDs has widely used in sensing, in vitro/in
vivo imaging, cancer therapy, catalysts and optoelectronic devices[10-17]. Despite the well documented fluorescent
properties, CDs with RTP have also been reported for several groups[18]. Lin and co-workers reported the production
of effective RTP by aggregation-induced of CDs in trimellitic acid[19]. Shan and co-workers achieved UV
phosphorescence by restriction of CDs in NaCNO crystals[20]. Yang and co-workers synthesized a series of
CDs based RTP materials via incorporation of polymers[21]. Qiu and co-workers synthesized full-color
long lifetime RTP by pyrolysis of the citric acid and boric acid
precursors with various mass ratios at different temperatures[22]. Despite the progress achieved, matrices are
commonly required in these works to produce effective RTP for the CDs by
stabilization of the triplet state [23-25].
However, the use of matrices not only complicates the preparation
process,
but also reduces the emission stability and uniformity. Hence,
development of matrix-free RTP CDs materials is highly desirable[26,27].
In this work, o -phenylenediamine (o -PD) and
ethylenediamine tetraacetic acid (EDTA) were employed as precursors to
synthesize EDTA-CDs with abundance of surface functional groups (e.g.,
-NH2, -OH). Then, UPy was covalent attached to the
surface of EDTA-CDs via isocyanate reaction to obtain UPy-CDs.
Thermogravimetric and Fourier Transform Infrared Spectrometer (FT-IR)
characterizations demonstrated the successful decoration of UPy on the
surface of UPy-CDs. The strong hydrogen bonds between UPy moieties
causes the self-assembly of UPy-CDs, as revealed by the obvious
aggregation from Transmission Electron Microscope (TEM) images. Owing to
the rigidifying effect induced by interparticle restriction, UPy-CDs not
only inherent the fluorescence of EDTA-CDs, but also exhibit RTP without
the aid of matrix. The afterglow emission after remove of UV light
source could be observed by naked eyes, with a phosphorescence lifetime
of 33.4 ms. On this basis, the application potential of UPy-CDs in the
fields of anti-counterfeiting and information encryption were further
exploited.
Results and Discussion
Results
Firstly, EDTA-CDs was prepared
via hydrothermal method with o -PD and EDTA as precursors[28]. During the synthesis of EDTA-CDs, the
polymerization and carbonization of o -PD were suggested to form
the carbon core, while various functional groups (such as hydroxyl and
amino groups) derived from the residue of precursors were decorated on
the surface. These surface groups could react with isocyanate
functionalized UPy (UPy-NCO, molecule 1 )[29], enabling the attachment of UPy to the
surface of EDTA-CDs to obtain UPy-CDs (Scheme. 1). The as-