Formation mechanism of carbogenic nanoparticles with dual photoluminescence emission

Article abstract of DOI:10.1021/ja204661r

We present a systematic investigation of the formation mechanism of carbogenic nanoparticles (CNPs), otherwise referred to as C-dots, by following the pyrolysis of citric acid (CA)-ethanolamine (EA) precursor at different temperatures. Pyrolysis at 180 °C leads to a CNP molecular precursor with a strongly intense photoluminescence (PL) spectrum and high quantum yield formed by dehydration of CA-EA. At higher temperatures (230 °C) a carbogenic core starts forming and the PL is due to the presence of both molecular fluorophores and the carbogenic core. CNPs that exhibit mostly or exclusively PL arising from carbogenic cores are obtained at even higher temperatures (300 and 400 °C, respectively). Since the molecular fluorophores predominate at low pyrolysis temperatures while the carbogenic core starts forming at higher temperatures, the PL behavior of CNPs strongly depends on the conditions used for their synthesis.

Full text of DOI:10.1021/ja204661r

Communication
pubs.acs.org/JACS
Formation Mechanism of Carbogenic Nanoparticles with Dual
Photoluminescence Emission
†
†
Marta J. Krysmann, Antonios Kelarakis, Panagiotis Dallas, and Emmanuel P. Giannelis*
Department of Materials Science and Engineering, Cornell University, Ithaca, New York 14853, United States
*
S Supporting Information
strate that their photoluminescence (PL) can arise from two
different sources. Following pyrolytic decomposition of CA and
ethanolamine (EA) (1:3 molar ratio), we isolate three distinct
photoluminescent species (which correspond to three different
stages of the pyrolysis) and show that each exhibits a distinct
emission behavior. At low temperatures pyrolysis leads to a
CNP precursor whose intense PL is due to organic
fluorophores. As the pyrolysis proceeds to higher temperatures,
the CNP precursor is gradually consumed in favor of the CNPs.
At high pyrolysis temperatures, CNPs are formed and their
carbogenic cores are the main source of their PL, with the PL
intensity and Φ much diminished. At intermediate temper-
atures the PL can be resolved into two components, one
stemming directly from their intrinsic carbogenic core and the
other from the presence of amide-containing fluorophores. The
results presented here rationalize the disparate and often
confusing behavior reported in terms of PL intensity and Φ.
They also provide a straightforward strategy to fine-tune the PL
properties of CNPs and optimize them for different
applications.
ABSTRACT: We present a systematic investigation of the
formation mechanism of carbogenic nanoparticles
(
CNPs), otherwise referred to as C-dots, by following
the pyrolysis of citric acid (CA)−ethanolamine (EA)
precursor at different temperatures. Pyrolysis at 180 °C
leads to a CNP molecular precursor with a strongly intense
photoluminescence (PL) spectrum and high quantum
yield formed by dehydration of CA−EA. At higher
temperatures (230 °C) a carbogenic core starts forming
and the PL is due to the presence of both molecular
fluorophores and the carbogenic core. CNPs that exhibit
mostly or exclusively PL arising from carbogenic cores are
obtained at even higher temperatures (300 and 400 °C,
respectively). Since the molecular fluorophores predom-
inate at low pyrolysis temperatures while the carbogenic
core starts forming at higher temperatures, the PL
behavior of CNPs strongly depends on the conditions
used for their synthesis.
The synthetic approach follows the controlled pyrolysis of
CA and EA at different temperatures (above the melting point
hotoluminescent compounds are widely used in everyday
P
life as well as in highly specialized applications such as
optical analysis, photonics, chemical and biological sensing,
of CA, T = 153 °C) in the absence of solvent. After pyrolysis
m
at 180 °C for 30 min, no NPs are detectable by either DLS or
TEM. Instead, a fluid that is bright yellow, highly photo-
luminescent, and readily soluble in water is obtained, referred
to as CNP180. The UV−vis spectrum of the sample in water is
1,2
molecular tracing, and cellular imaging.
For the most
demanding applications, the fluorescent materials should
exhibit structural and photochemical stability in aggressive
environments while showing high absorption coefficient and
quantum yield (Φ) with minimal toxicity. To meet these
requirements, the library of available fluorescent materials
1
5
shown in Supporting Information (SI) Figure 1a. The
N
21
22
NMR (SI Figure 2), XPS (SI Figure 3), and FTIR spectra
(SI Figure 4c) suggest the presence of amide functional groups
in the product formed by a simple intermolecular condensation
reaction accompanied by loss of water molecules (Scheme
(
conventional dyes, polymers, or proteins) has been expanded
to include colloidal semiconductor nanocrystals (so-called
3
4
23−25
quantum dots), lanthanide chelates, core−shell silica nano-
1).
5
particles (NPs), and carbogenic nanoparticles (CNPs, known
To further elucidate the molecular character of the
product(s) obtained after pyrolysis at 180 °C, the fluid was
fractionated by flash chromatography over silica gel (70−230
mesh) using DCM/methanol (6:4 v:v) as the mobile phase.
The highly fluorescent fraction (referred to as PL-CNP180)
6
−20
also as C-dots).
By virtue of their biocompatibility, low
toxicity, and ease of preparation, CNPs are environmentally
benign alternatives to quantum dots comprised of heavy
7
,8
metals.
fractionation of suitable carbon sources followed by surface
CNPs are often obtained by fragmentation/
1
13
was characterized by H and C NMR, FTIR, XPS, ESI-MS,
and MS/MS.
9
−11
passivation.
Alternatively, they can be prepared by bottom-
up strategies utilizing molecular precursors and simple
techniques such as combustion, thermal, or microwave
The FTIR spectrum of PL-CNP180 (SI Figure 4d) confirms
the presence of amide functional groups, as evidenced by their
vibrational fingerprints centered at 1550 (N−H in-plane
bending), 1636 (CO stretching of the amide bond), and
3300 cm− (N−H stretching). The XPS spectra (SI Figure 5)
1
2−20
treatments.
Our group has reported a one-step thermal
decomposition of citric acid (CA)-based molecular precursors
to synthesize surface-passivated CNPs with hydrophilic or
1
1
4,15
hydrophobic surface groups.
In this report, we present a systematic study of the formation
Received: May 21, 2011
mechanism of CNPs from molecular precursors and demon-
Published: December 25, 2011
©
2011 American Chemical Society
747
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Journal of the American Chemical Society
Communication
Scheme 1. Reaction Scheme of EA and CA toward the
Formation of PL-CNP180
22
also confirm the presence of amide groups. Specifically, amide
carbonyl groups (C 1s at 288.2 eV), amide nitrogen (N 1s at
401 eV), carbonyl oxygen (O 1s 533.5 eV), and aliphatic
carbons (C 1s at 286.3 eV) can be seen in the spectra.
1
13
The H and C NMR spectra of the PL-CNP180 are shown
1
in SI Figure 6. With respect to H NMR, the upfield (2.6 ppm)
and downfield (2.8 ppm) doublet peaks can be attributed to
methylene protons of the CA, the triplet at 3.3 ppm to the
protons of the hydroxyethyl group adjacent to the amide bond,
and the triplet at 3.7 ppm to the protons on the hydroxyethyl
group next to the hydroxyl group. With respect to 13C NMR,
the peak at 171.7 ppm is due to carbons in the amide bonds,
the peaks at 74.9 and 59.8 ppm to carbons adjacent to hydroxyl
groups, the peak at 43.6 ppm to methylene carbons, and the
peak at 41.2 ppm to carbon near the amide bond.
Figure 1. PL spectra of aqueous dispersions of (a) CNP180, (b)
CNP230, and (c) CNP300. The sample concentration was 0.1 mg/
mL. The excitation wavelength was varied from 275 to 600 nm with a
fixed increment of 25 nm. Colors correspond to different excitation
wavelengths: red, green, blue, cyan, magenta, black, dark yellow, navy,
purple, wine, dark cyan, royal, and orange correspond to 275, 300, 325,
350, 375, 400, 425, 450, 475, 500, 525, 550, 575, and 600 nm.
More importantly, the ESI-MS spectrum (SI Figure 7)
suggests a native molecule (M) with m/z = 321, which is
identical to the product in Scheme 1. The assignment is based
−
on the presence of M at m/z = 320 (negative spectra) and the
+
detection of the adduct ions with m/z =322 (M−H ), 344 (M−
+
+
+
Na ), 360 (M−K ), and 665 (2M−Na ) (positive spectra).
(
Our analysis was further supported by MS/MS data listed in
SI Figure 7.)
Together, the experimental evidence unambiguously suggests
that pyrolysis at 180 °C follows the reaction shown in Scheme
1
and results in a molecular precursor containing amide groups.
The strong PL of model amides (maximum at λem = 455 ± 15
nm) has been assigned to a direct singlet-to-triplet transition.
26
To further confirm the mechanism, an alternative synthesis
approach (SI Figure 8a) was also used. EA and CA were
dissolved in water and kept in an ice bath to allow the
formation of tris(2-hydroxyethyl)ammonium citrate salt (SI
Salt Synthesis). Upon heating, the citrate salt undergoes
dehydration at 140 °C under vacuum to form an amide that is
identical to the PL-CNP180 molecule (SI Figure 8b,c). Given
that the photophysical properties of PL-CNP180 closely
resemble those of the CNP180, we focus below on the PL
behavior of the latter system (for comparison the PL spectra of
PL-CNP180 are shown in SI Figure 9, while its UV−vis
spectrum is shown in SI Figure 1). Particularly, the excitation of
an aqueous solution of the CNP180 between 200 and 400 nm
shows a single, excitation-independent PL (Figure 1a) that is
characterized by a remarkably high Φ ≈ 50%. The emission
Figure 2. pH dependence of the maximum PL intensity of aqueous
dispersions of (a) CNP180, (b) CNP230, and (d) CNP300. Spectra
recorded at λex = 375 nm (red circles, decreasing pH) and (blue
squares, increasing pH), λ = 450 nm (green triangles). (c) Maximum
ex
PL intensity of aqueous dispersions of CNP230 in the presence of
various metal ions (λex = 375 nm).
Pyrolysis at higher temperatures (e.g., at 230 °C for 30 min)
produces spherical NPs (abbreviated as CNP230), and the
sample appears to be light brown. As described in the
experimental details in the SI, an essential step of our protocol
is the extensive purification of CNP230 through prolonged
dialysis to eliminate the presence of the free CNP180. Despite
the simplicity and the template-free nature of the preparation
method, CNP230 shows spherical symmetry and relatively
narrow size distribution, with an average diameter 19 nm
band centers at λ = 455 nm and shows maximum intensity at
em
λex = 375 nm.
The emission intensity exhibits a plateau within the pH range
−12, but decreases abruptly at lower pH (Figure 2a). The
(
Figure 3a). Buildup of NPs is attributed to extensive cross-
4
linking reactions and formation of interchain imide bonds, as
revealed by the band centered at 1776 cm− (stretching CO
of the imide group) in the FTIR spectrum (SI Figure 4e).
Formation of imide derivatives during pyrolysis has been
reversible effects of pH can be understood in terms of extensive
protonation−deprotonation of the amide group given that the
deprotonated amide groups have much higher electron-
donating efficiency. Finally, we note that excitation above λex
1
27
>
400 nm fails to induce any measurable emission (Figure 1a).
previously reported. The CNP230 is easily dispersible in
7
48
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Journal of the American Chemical Society
Communication
accounts for the significant shrinkage of CNPs. Elemental
analysis (50.5% C, 3.7% H, 13.1% N) indicated an increased
amount of carbon with a concomitant decrease in hydrogen
compared to CNP230, suggesting that CNP230 underwent
further carbonization. In addition, thermogravimetric analysis
(
TGA) measurements (SI Figure 10) indicate a lower organic
content compared to CNP230. Nevertheless, the NPs are well
dispersed in water, suggesting the presence of a number of
functional groups on their surface. The FTIR spectra (SI Figure
4
f) confirm the presence of a certain (although considerably
reduced) amount of amide and imide groups that remain stable
even after such a high-temperature treatment.
Figure 3. TEM images and size histograms (each plotted on the basis
The Raman spectrum of CNP300 is dominated by two peaks
centered at approximately 1355 and 1595 cm− that can be
identified as the D and G bands of the amorphous carbon,
respectively (SI Figure 11a). (For CNP230 strong PL radiation
masks the Raman spectra.) The G peak arises from the E2g
of 150 fields) of (a) CNP230 and (b) CNP300.
1
water and other solvents (e.g., DMSO, DMF, ACN, methanol),
confirming the high content of surface polar organic groups.
Elemental analysis indicated an increased stoichiometric
amount of carbon in CNP230 with an associated decrease in
hydrogen (44.85% C, 5.75% H, 10.85% N) compared to
CNP180 (41% C, 7.8% H, 11.7% N). From the TEM images,
the absence of multiple nuclei embedded within individual
particles suggests that pyrolysis at 230 °C leads to single
carbogenic domains (cores) with surface organic groups that
impart dispersibility.
2
vibration mode of the sp bonded carbon, while the D peak is
considered the signature of the disorder in the graphite lattice,
since it is assigned to the A1g (zone-edge) breathing vibration
phonon that is activated only in the presence of a neighboring
3
30,31
sp defect.
2
3
Isolated sp domains dispersed within a sp matrix can
function as PL centers, an effect that is governed by the π states
2
32
The luminescence spectra of CNP230 in water between 200
of sp sites. In particular, the PL behavior of amorphous/
2
3
<
λex < 400 nm show a maximum at 455 nm with the strongest
disordered graphites containing a mixture of sp and sp
intensity recorded for λ = 375 nm (Figure 1b). In that sense,
carbons has been attributed to the photogeneration of
ex
the emission mode of CNP230 closely resembles that of the
precursor. However, the Φ for CNP230 is only ∼15%. This
value is significantly lower compared to CNP180 (Φ ≈ 50%)
and suggests that a large amount of the fluorophores are used as
the building blocks of the carbogenic domains. On the other
hand, excitation with longer wavelengths (λex > 400 nm)
induces an excitation-dependent emission, given that as λex
increases the emission spectra are systematically displaced
toward longer wavelengths and the intensity decreases (Figure
electron−hole pairs that can induce radiative recombination
2
of the trap carriers localized within small sp carbon clusters,
3
which are surrounded by sp defects, a mechanism that can
3
2,33
remain active in the presence of heteroatoms.
Evidently,
this photophysical mechanism can function in parallel with
other emission modes (induced from the organic fluoro-
phores).
The dual emissive mode of the CNP300 in water is clearly
seen in Figure 1c that displays a series of emission spectra
recorded at various excitation wavelengths, λex. Between 200 <
1
b inset). The dependence of emission wavelength and
intensity on λex has been identified as a generic feature of
various types of CNPs.
λ
ex < 400 nm, the PL band is composed of two largely
overlapping peaks, one that has a maximum at a fixed position
λem = 455 nm with Φ ≈ 4% (for which the stronger signal is
6
As shown in Figure 2b, the emission maximum at λ = 375
nm is relatively stable within the pH range 4−12, but falls
dramatically below pH < 4. The maximum intensity (λ = 450
ex
recorded for λex = 375 nm) and another that monotonically
red-shifts with increasing λex. The latter dominates the spectra
at λex > 400 nm. As in the case of CNP230, we attribute the
excitation-independent emission to the presence of organic
fluorophores and the excitation-dependent emission to the
carbogenic cores. Due to the limited organic content of
CNP300, the Φ and the pH dependence of the organo-
fluorophore emission are much suppressed compared to
CNP230 (Figure 2d). In addition, the emission due to the
carbogenic domains remains virtually unchanged with pH
(Figure 2d).
ex
nm) of the excitation-wavelength-dependent emission is
essentially unchanged over a broad pH range (Figure 2b).
The PL emission is significantly reduced in the presence of 3d
metal ions Cr(III) and Co(II) (but not in the presence of other
metals shown in Figure 2c), due to a selective metal−
28
fluorophore complexation. These preliminary results suggest
that sensor platforms based on CNPs can in principle respond
to complex environments, serving for example as probes for the
detection of analytically important metals that is of vital
importance for environmental monitoring and wastewater
Further pyrolysis (at 400 °C for 1 h) gives rise to clustering
and the formation of larger and nonuniform carbogenic
particles (abbreviated as CNP400) with diameters in the
order of a few hundred nanometers, presumably due to
aggregation (SI Figure 12). The FTIR spectrum (SI Figure 2g)
indicates the absence of any functional groups, while elemental
analysis suggests almost zero nitrogen content. The Raman
spectrum of CNP400 shows both G and D peaks (SI Figure
11b). The dispersibility of CNP400 in water is relatively
limited. Nevertheless, the dispersion exhibits excitation-depend-
ent emission, although the intensity is low (SI Figure 13a). The
dispersibility of the particles is significantly enhanced after
2
9
management.
Further pyrolysis (at 300 °C for 1 h) results in much more
darkening of the sample and the formation of CNPs
(
(
abbreviated as CNP300) with an average diameter of 8 nm
Figure 3b), consistent with earlier reports of CNPs with an
11−15
average diameter of 4−10 nm by various pyrolytic routes.
(Direct heating of CNP180 at 300 °C yields similar results.)
The significantly smaller size of the NPs (compared to 19 ± 1
nm found for CNP230) is consistent with major chemical
changes. TGA-MS experiments indicate that during pyrolysis at
3
00 °C a massive amount of CO escapes from the system that
2
7
49
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Journal of the American Chemical Society
Communication
treatment with HNO that allows surface oxidation of the
fluorophore-free CNP400 (abbreviated as ox-CNP400). The
ox-CNP400 exhibit excitation-dependent emission PL within
A.K. dedicate this Communication to Prof. Colin Booth
(Manchester University) on the occasion of his 80th birthday.
3
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■
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dx.doi.org/10.1021/ja204661r | J. Am. Chem.Soc. 2012, 134, 747−750