Synthesis of luminescent 3D microstructures formed by carbon quantum dots and their self-assembly properties

Article abstract of DOI:10.1039/c4cc02496f

We report in this communication the synthesis of star-shaped carbon quantum dots-(poly-γ-benzyl-l-glutamate) conjugates that self-assemble into microstructures and retain the characteristic emission properties of the native dots. Dots were used either as an initiator to give a daisy-like peptide-polymer structure or as capping agents towards more elaborated hybrid nanostructures. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02496f

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Synthesis of luminescent 3D microstructures
formed by carbon quantum dots and their
self-assembly properties†
Cite this: Chem. Commun., 2014,
0, 6592
5
Received 4th April 2014,
Accepted 1st May 2014
D. Mazzier, M. Favaro, S. Agnoli, S. Silvestrini, G. Granozzi, M. Maggini and
A. Moretto*
DOI: 10.1039/c4cc02496f
www.rsc.org/chemcomm
11
We report in this communication the synthesis of star-shaped organic polymers. In this context, we have explored the synthesis
carbon quantum dots–(poly-c-benzyl-L-glutamate) conjugates that of CQDs–polypeptide hybrids, presenting here a novel CQD–poly-g-
self-assemble into microstructures and retain the characteristic benzyl-L-glutamate conjugated system (CQD–PBLG) where several
emission properties of the native dots. Dots were used either as polymer chains are grown in a star shaped fashion over a discrete
an initiator to give a daisy-like peptide–polymer structure or as CQD core. We further modified this structure by capping the loose
capping agents towards more elaborated hybrid nanostructures.
end of the polymer with additional CQDs, thus obtaining functional
nanostructures composed by the CQD-core, PBLG-spacers and an
1
Since their discovery, carbon quantum dots (CQDs) have repre- outer CQDs-shell. In this case, the morphology of the resulting
sented a fascinating class of water-soluble carbon nanostructures, material was assayed by transmission electron microscopy (TEM)
which are characterized by a discrete, quasi circular shape with after growing silver nanoparticles (AgNPs) directly on the CQD shell.
2
3
diameters of up to 10 nm. CQDs are non-toxic carbon nanostruc- Interestingly, the CQD–PBLG and CQD–PBLG–CQDs nanosystems
tures that show peculiar photoluminescence (PL) properties, such as self-assemble in solution to give spherical aggregates that retain the
4
multi-colour emission that varies with the excitation wavelength.
peculiar emission properties of the starting CQD material. We
They have also been proposed as interesting candidates for sensing, envisioned the use of N-carboxyanhydride ring opening polymeriza-
bioimaging and, in general, for applications where the size, cost, tion (NCA-ROP) to grow polymeric structures starting from amino
5
and biocompatibility of the label are critical issues. PL properties of groups present on the surface of CQDs.
CQDs have been attributed to the presence of surface energy traps
We therefore synthesized the starting CQDs by using arginine
that become emissive upon stabilization as a result of surface and 1,2-ethylendiamine precursors, as schematically reported in
4
passivation. This requirement, in order for CQDs to become PL Fig. 1A, in order to maximize the number of available amine
active, is reportedly shared by silicon nanocrystals, for which a moieties. TEM analysis of the products (Fig. 1C) revealed round
widely accepted PL mechanism is the radiative recombination of particles with uniform sizes and a diameter of 1.4 Æ 0.2 nm. High
6
excitons. Several procedures to prepare CQDs have been reported, resolution TEM analysis (Fig. 1D) and Fast Fourier Transform (FFT)
7
8
according to both bottom-up and top-down approaches. Recently, (Fig. 1D, inset) highlight an interplanar distance of 0.21 nm,
nitrogen-rich organic molecules and natural precursors (e.g. carbo- compatible with the (0001) graphite basal plane. The elemental
hydrates, peanut skin, soy milk) have been used to prepare highly analysis yield was 71.47% for C, 5.05% for H, 17.09% for N and
PL active CQDs by hydrothermal treatment at high temperature for 5.39% for residual elements, while MALDI analysis (ESI,† Fig. S1)
9
several hours, without further surface passivation. Also, microwave- displayed a MW dispersion, as expected from the random combi-
5c
assisted hydrothermal transformation of amino acids has been nation of different atoms, centred at 1150 Da. Solid state FT-IR
proposed as a direct route for the production of CQDs starting data (ESI,† Fig. S2) are consistent with the presence of amines. XPS
10
from abundant and inexpensive precursors. CQDs have been used measurements were carried out to provide a deeper insight into the
either in their pristine form or blended with inorganic materials. On different chemical species introduced by the synthesis. The XPS
the other hand, they have been studied rarely in combination with survey spectrum reported in Fig. 2a shows the C 1s, N 1s and O 1s
photoemission lines to be centred at binding energies (BEs) of
2
84.9 eV, 399.8 eV and 531.6 eV, respectively.
The observed elemental composition (C 74.3%, N 18.8% and O
Department of Chemical Sciences, University of Padova, Via Marzolo 1,
3
5131 Padova, Italy. E-mail: alessandro.moretto.1@unipd.it
6
.9%) is in good agreement with that of the previous elemental
analysis. The detailed scans of C 1s and N 1s lines were decon-
voluted into single chemically-shifted components, as reported in
†
Electronic supplementary information (ESI) available: Synthetic details, SEC
traces, UV-Vis absorption and emission spectra of polymers (1 and 2) are reported
in the ESI. See DOI: 10.1039/c4cc02496f
6592 | Chem. Commun., 2014, 50, 6592--6595
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Fig. 1 (A) Schematic representation of CQDs synthesis. Reagents and
À1
conditions: (i) arginine and 1,2-ethylendiamine (1 : 1 mol mol ) in ultrapure
water; (ii) domestic microwave, 700 watt; (iii) dissolution in ultrapure water
and centrifugation; (iv) lyophilisation; (v) dissolution in ultrapure water (left:
under irradiation, visible-light; right: under irradiation, UV-light at 365 nm).
Fig. 3 Left part: hypothetic representation for the chemical structure of a
single CQD, and ROP of BLG-NCA initiated by CQDs which yield 1. Right
part: schematic illustration of 2.
(
B) Normalized emission spectra under different lex. (C) and (D) CQDs TEM
and HRTEM images, respectively.
Fig. 2b and c. The multicomponent analysis of the C 1s line dependence of the emission maxima on the excitation wavelength
confirms the presence of C–N bonds (285.2 eV) as well as that of (for non-normalized spectra, see Fig. S4, ESI†). We evaluated the
several oxygen-bearing functional groups, such as C–O (286.5 eV), emission quantum yield upon 360 nm excitation as 30.1% using
12
CQO (288.0 eV) and O–CQO (288.9 eV). The weak component on quinine sulphate as a reference, in line with data reported earlier
2
,11
the low BE slope of the main peak can be attributed to C atoms using similar synthetic methods.
13
around vacancy sites (283.8 eV). The fit of the N 1s line, on the
Primary amines can be used to promote controlled NCA-ROP
14
other hand, shows five different chemical components centred at processes. Thus, BLG-NCA was polymerized using CQDs as the
BEs of 398.0 eV, 399.1 eV, 400.2 eV, 401.3 eV and 402.2 eV, amino-group-bearing initiator. NMR, FT-IR and circular dichroism
corresponding to pyridinic, amine, pyrrolic, graphitic and pyridine (CD) spectra of the hybrid material 1 (schematically shown in Fig. 3)
oxide groups, respectively.
In particular, the ratios between these last functional groups Fig. S5–S7), the CD data being related to the secondary, rod-like,
suggest that, on average, each CQD contains 8–10 –NH
groups a-helical, structure of PBLG. Thermogravimetric analysis (TGA)
either aliphatic or aromatic, see Fig. 3 for a schematic representa- yields a BLG:CQDs weight ratio of 97 : 3 (ESI,† Fig. S8). While we
tion). The UV-Vis absorption spectrum of the as-prepared CQDs were not able to detect the MW distribution for 1 by MALDI
ESI,† Fig. S3) shows an absorption shoulder around 350 nm, typical experiments, size exclusion chromatography (SEC) showed an aver-
of aromatic structures. Normalized emission spectra are shown in age MW of around 45 KDa (ESI,† Fig. S9).
confirmed the formation of PBLG arms from the CQDs core (ESI,†
2
(
(
Fig. 1B and display the distinctive features of CQDs, such as the
Like pristine CQD, 1 shows a dependence of its emission
spectra on lex (ESI,† Fig. S10). The CQD–PBLG–NH structure of 1
2
was further modified at its N-terminal position via reaction with a
large excess of succinic anhydride in order to obtain the corre-
sponding carboxylic derivative, CQD–PBLG–COOH. A negative Kaiser
test (ninhydrin) confirmed the absence of primary amine groups
upon reaction with the anhydride. Interestingly, CQD–PBLG–COOH
did not display significant emission spectra, hinting at the possible
modification of the amino groups of the CQD cores themselves,
besides those capping PBLG chains. The carboxylic groups of this
intermediate were further derivatized, by activation with diphenyl-
phosphoryl azide and reaction with a large excess of CQDs. The SEC
trace of the resulting material 2 confirmed the addition of several
CQDs per core, by showing an increase of the MW, in comparison
to 1, of ca. 10 KDa (ESI,† Fig. S9). TGA analysis corroborated this
finding, indicating a 22% increase of the CQD abundance with
respect to PBLG in comparison to hybrid 1. By assuming a MW of
1150 Da for CQDs, we estimate a covalent attachment of an average
of nine peripheral CQDs per core for hybrid 2. Another indirect
confirmation of the attachment of new CQDs is the restoration of
the typical CQD multicolour emission that had been shut off by the
modification of the amino groups in 1 with succinic anhydride
Fig. 2 (a) XPS survey scan. (b) Multipeak analysis for the carbon 1s and
(c) nitrogen 1s photoemission lines.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 6592--6595 | 6593

View Article Online
ChemComm
ESI,† Fig. S9). In order to observe the structure of 2, we exploited
Communication
(
the electron-donating capabilities of photoexcited CQDs, which
enable the reduction of silver salts to the corresponding AgNPs
1
5
on the surface of the CQDs themselves. To this end, a THF
solution of 2 was photoexcited using ultraviolet irradiation in the
presence of silver nitrate (Experimental details in the ESI†). The
formation of AgNPs brought about a change in hue from yellow
to brown (Fig. 4, upper part). AgNPs are expected to form on the
surface of the external layer of 2, which is composed by CQDs. In
Fig. 4 (lower part), the TEM analysis of the resulting organic–
inorganic hybrid 3 shows the formation of star-shaped nano-
structures with sizes in the 30–60 nm range, carrying several
1
5
5
nm AgNPs. According to previous studies, the templating and
clustering effect of the CQD supports has a dramatic effect on the
plasmonic properties of the NPs. This is demonstrated by a broad
surface plasmon resonance peak in the UV-Vis absorption spec-
trum of 3, between 400 and 520 nm, reported in Fig. 4. The
remarkable broadening of the UV-Vis spectrum of 3 compared to
that of free-standing AgNPs in a similar size range (with a sharp
absorption peak at 400 nm), is due to the compact arrangement Fig. 5 (A) and (B) TEM and SEM images of microstructures generated by
self-assembling of 1 and 2, respectively. Upper part: emission colours
occurring from 2 microstructures.
of the AgNPs in 3, which results in strong plasmonic coupling
1
6
interactions.
Finally, we made use of the self-aggregation properties of PBLG
to generate self-assembled microstructures from hybrids 1 and 2. To
17
this end, we solubilized 1 or 2 in a THF:DMF mixture and dialyzed emission characteristics described previously for CQDs and hybrids
against water for 48 h. The resulting milky, light-brown (1) and dark- 1 and 2. We were able to picture it directly in the fluorimeter by
brown (2) solutions were submitted to a size exclusion gel-filtration using a digital camera, as shown on the top of Fig. 5 (see also ESI,†
(water as the eluant) and the main fractions were collected and Fig. S11, ESI†).
examined by TEM (without uranyl-acetate treatment) and scanning In conclusion, in this work we reported the synthesis and
electron microscopy (SEM), as shown in Fig. 5. In both cases characterization of carbon quantum dots bearing amino groups at
spherical aggregates with diameters ranging from 100 to 300 nm their edges, as demonstrated by elemental and XPS analyses.
were observed. When 1 was used, the surfaces of the aggregates Functionalization of the dots with poly-g-benzyl-L-glutamate gave
showed a smooth profile (Fig. 5A), whereas in the case of 2 the the first reported CQD–polypeptide polymeric systems in a star
surfaces appeared rough, with limited flat areas (Fig. 5B). This is shaped fashion that were further decorated with an outer shell of
compatible with the exposition of the hydrophilic CQDs on the CQDs. We tracked the spectroscopic properties of the materials
surface of the aggregate formed by 2. Furthermore, the aggregated throughout the syntheses, and exploited the electron-donating
structures obtained in this last case, still displayed the multicolour capabilities of photoexcited CQDs to grow silver nanoparticles.
These, in turn, allowed visualization of the structure of the resulting
hybrid materials by TEM imaging, thanks to their contrasting effect.
Finally, we assembled supramolecular aggregates of spherical shape
with evidence of rough surface profiles in the case of CQD–capped
hybrid materials, which may indicate the presence of aggregated
areas of CQDs on the external layer. These microstructures are able
to retain the characteristic emission properties of the native CQDs.
The approach developed in this study is a facile, but highly efficient
approach in the synthesis of fluorescent, peptide-based materials,
which are attractive for many applications including optical sensing,
biolabelling, imaging, targeted drug delivery/tracking and even UV
protection for optical devices. We note that this synthetic approach
is also highly versatile, since both a-amino acids and non-natural
amino acids can be used, resulting in quantum dots with tailored
surface chemistries that can be quantitatively assayed.
Financial support from the University of Padova (PRAT
CPDA114475 and HELIOS STPD08RCX5) and MIUR (FIRB
Fig. 4 Upper part: schematic representation of the photoreaction to
RBPAP1158Y, FIRB RBFR128BEC) is gratefully acknowledged.
M. F. acknowledges Fondazione Cariparo for financial support.
generate 3. Bottom part left: UV-Vis absorption spectra for 2, 3 and single
AgNPs. Bottom part right: two TEM images of single units of 3.
6594 | Chem. Commun., 2014, 50, 6592--6595
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Notes and references
(c) Y. P. Sun, X. Wang, F. S. Lu, L. Cao, M. J. Meziani, P. J. G. Luo,
L. R. Gu and L. M. Veca, J. Phys. Chem. C, 2008, 112, 18295;
(d) X. Wang, L. Cao, F. S. Lu, M. J. Meziani, H. Li, G. Qi, B. Zhou,
B. A. Harruff, F. Kermarrec and Y. P. Sun, Chem. Commun., 2009,
3774; (e) J. Peng, W. Gao, B. K. Gupta, Z. Liu, R. Romero-Aburto,
L. Ge, L. Song, L. B. Alemany, X. Zhan, G. Gao, S. A. Vithayathil,
B. A. Kaipparettu, A. A. Marti, T. Hayashi, J.-J. Zhu and P. M. Ajayan,
Nano Lett., 2012, 12, 844.
9 (a) C. Hsu and H. T. Chang, Chem. Commun., 2012, 48, 3984;
(b) Y. Yang, J. Cui, M. Zheng, C. Hu, S. Tan, Y. Xiao, Q. Yang and
Y. Liu, Chem. Commun., 2012, 48, 380; (c) Z. Lian Wu, P. Zhang,
M. X. Gao, C. F. Liu, W. Wang, F. Lenga and C. Z. Huang, J. Mater.
Chem. B, 2013, 1, 2868; (d) Y. Dong, R. Wang, H. Li, J. Shao, Y. Chi,
X. Lin and G. Chen, Carbon, 2012, 50, 2810; (e) S. Liu, J. Tian,
L. Wang, Y. Zhang, X. Qin, Y. Luo, A. M. Asiri, A. O. Al-Youbi and
X. Sun, Adv. Mater., 2012, 24, 2037; ( f ) S. Manav and S. Sabyasachi,
Diamond Relat. Mater., 2012, 24, 11; (g) C. Zhu, J. Zhaia and S. Dong,
Chem. Commun., 2012, 48, 9367; (h) H. Peng and J. Travas-Sejdic,
Chem. Mater., 2009, 21, 5563.
1
(a) X. Y. Xu, R. Ray, Y. L. Gu, H. J. Ploehn, L. Gearheart, K. Raker and
W. A. Scrivens, J. Am. Chem. Soc., 2004, 126, 12736; (b) H. Liu, T. Ye
and C. Mao, Angew. Chem., Int. Ed., 2007, 46, 6473.
2
3
4
For a recent review see: S. N. Baker and G. A. Baker, Angew. Chem.,
Int. Ed., 2010, 49, 6726.
P. M. Sk, A. Jaiswal, A. Paul, S. S. Ghosh and A. Chattopadhyay,
Sci. Rep., 2012, 2, 383.
(a) Y.-P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, P. Pathak,
M. J. Meziani, B. A. Harruff, X. Wang, H. Wang, P. G. Luo, H. Yang,
M. E. Kose, B. Chen, L. M. Veca and S.-Y. Xie, J. Am. Chem. Soc., 2006,
1
28, 7756; (b) L. Wang, S.-J. Zhu, H.-Y. Wang, S.-N. Qu, Y.-L. Zhang,
J.-H. Zhang, Q.-D. Chen, H.-L. Xu, W. Han, B. Yang and H.-B. Sun,
ACS Nano, 2014, 8, 2541.
(a) S. Zhu, Q. Meng, L. Wang, J. Zhang, Y. Song, H. Jin, K. Zhang,
H. Sun, H. Wang and B. Yang, Angew. Chem., Int. Ed., 2013, 52, 3953;
5
(
b) H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C. Him,
A. Tsang, X. Yang and S.-T. Lee, Angew. Chem., Int. Ed., 2010,
4
9, 4430; (c) S. Zhu, J. Zhang, S. Tang, C. Qiao, L. Wang, H. Wang, 10 J. Jiang, Y. He, S. Li and H. Cui, Chem. Commun., 2012, 48, 9634.
X. Liu, B. Li, Y. Li, W. Yu, X. Wang, H. Sun and B. Yang, Adv. Funct. 11 P. Zhang, W. Li, X. Zhai, C. Liu, L. Dai and W. Liu, Chem. Commun.,
Mater., 2012, 22, 4732.
2012, 48, 10431.
W. L. Wilson, P. F. Szajowski and L. E. Brus, Science, 1993, 262, 1242. 12 (a) Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai and L. Qu, J. Am.
6
7
(a) H. P. Liu, T. Ye and C. D. Mao, Angew. Chem., Int. Ed., 2007,
6, 6473; (b) K. Linehan and H. Doyle, RSC Adv., 2014, 4, 18;
Chem. Soc., 2012, 134, 15; (b) L. Li, G. Wu, G. Yang, J. Peng, J. Zhao
and J.-J. Zhu, Nanoscale, 2013, 5, 4015.
4
(
c) L. Tian, D. Ghosh, W. Chen, S. Pradhan, X. Chang and 13 M. Favaro, S. Agnoli, C. Di Valentin, C. Mattevi, M. Cattelan,
S. Chen, Chem. Mater., 2009, 21, 2803; (d) J. Hou, J. Yan, Q. Zhao,
Y. Li, H. Ding and L. Ding, Nanoscale, 2013, 5, 9558; (e) R. L. Liu,
L. Artiglia, E. Magnano, F. Bondino, S. Nappini and G. Granozzi,
Carbon, 2014, 68, 319.
D. Q. Wu, S. H. Liu, K. Koynov, W. Knoll and Q. Li, Angew. Chem., 14 (a) H. R. Kricheldorf, Angew. Chem., Int. Ed., 2006, 45, 5752;
2
009, 121, 4668 (Angew. Chem., Int. Ed., 2009, 48, 4598); ( f ) W. Kwon,
(b) H. Lu, J. Wang, Z. Song, L. Yin, Y. Zhang, H. Tang, C. Tu,
Y. Lin and J. Cheng, Chem. Commun., 2014, 50, 139.
J. Lim, J. Lee, T. Park and S.-W. Rhee, J. Mater. Chem. C, 2013, 1, 2002;
(
g) J. Zhou, X. Shan, J. Ma, Y. Gu, Z. Qian, J. Chena and H. Feng, 15 H. Choi, S.-J. Ko, Y. Choi, P. Joo, T. Kim, B. R. Lee, J.-W. Jung,
RSC Adv., 2014, 4, 5465.
H. J. Choi, M. Cha, J.-R. Jeong, I.-W. Hwang, M. H. Song, B.-S. Kim
8
(a) L. Cao, X. Wang, M. J. Meziani, F. S. Lu, H. F. Wang, P. J. G. Luo,
Y. Lin, B. A. Harruff, L. M. Veca, D. Murray, S. Y. Xie and Y. P. Sun, 16 P. K. Jaina and M. A. El-Sayed, Chem. Phys. Lett., 2010, 487, 153.
J. Am. Chem. Soc., 2007, 129, 11318; (b) S. L. Hu, K. Y. Niu, J. Sun, 17 P. Papadopoulos, G. Floudas, H. A. Klok, I. Schnell and T. Pakula,
and J. Y. Kim, Nat. Photonics, 2013, 7, 732.
J. Yang, N. Q. Zhao and X. W. Du, J. Mater. Chem., 2009, 19, 484;
Biomacromolecules, 2004, 5, 81.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 6592--6595 | 6595