Insight into the hybrid luminescence showed by carbon dots and molecular fluorophores in solution

Article abstract of DOI:10.1039/c9cp03730f

Carbon dots have attracted great attention from the research community given their very attractive luminescent properties. However, the recent discovery that some of these properties may result from fluorescent impurities originating from the synthesis process, and not from the carbon dots themselves, constitute a significant setback to our knowledge of these materials. Herein, we proceeded to the study of carbon dots generated from citric acid and urea via a microwave-assisted synthesis, focusing on their analysis by AFM, HR-TEM, XPS, FT-IR, ESI-MS, UV-Vis and fluorescence spectroscopy. We have found that this synthesis process does generate molecular fluorophores that can mask the luminescence of the carbon dots. More importantly, our data demonstrates that when present in the same solution, the carbon dots and these fluorophores do not behave as separated species with individual emission. Instead, they interact to produce a hybrid luminescence, which excited state properties and reactivity are different from the properties of the individual species. These results indicate the possibility for the development of hybrid materials composed by carbon dots and related molecular fluorophores with new and improved properties.

Full text of DOI:10.1039/c9cp03730f

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Insight into the hybrid luminescence showed by
carbon dots and molecular fluorophores in solution†
Cite this:DOI: 10.1039/c9cp03730f
a
a
a
Ricardo M. S. Sendao, Diana M. A. Crista, Ana Carolina P. Afonso,
˜
b
c
Maria del Valle Martınez de Yuso, Manuel Algarra,
Joaquim C. G. Esteves da Silva and Luıs Pinto da Silva
´
ad
ad
*
´
Carbon dots have attracted great attention from the research community given their very attractive
luminescent properties. However, the recent discovery that some of these properties may result from
fluorescent impurities originating from the synthesis process, and not from the carbon dots themselves,
constitute a significant setback to our knowledge of these materials. Herein, we proceeded to the study
of carbon dots generated from citric acid and urea via a microwave-assisted synthesis, focusing on their
analysis by AFM, HR-TEM, XPS, FT-IR, ESI-MS, UV-Vis and fluorescence spectroscopy. We have found
that this synthesis process does generate molecular fluorophores that can mask the luminescence of
the carbon dots. More importantly, our data demonstrates that when present in the same solution, the
carbon dots and these fluorophores do not behave as separated species with individual emission.
Instead, they interact to produce a hybrid luminescence, which excited state properties and reactivity
are different from the properties of the individual species. These results indicate the possibility for the
development of hybrid materials composed by carbon dots and related molecular fluorophores with
new and improved properties.
Received 2nd July 2019,
Accepted 9th September 2019
DOI: 10.1039/c9cp03730f
rsc.li/pccp
that CDs have gained relevance in several fields, such as in
Introduction
12,13
14
15–17
fabrication of light emission devices,
bioimaging, sensing,
3
18
19
Carbon dots (CDs) are carbon-based nanoparticles with a near photocatalysis, drug delivery and photodynamic therapy.
spherical shape, with a core that might be amorphous or
Despite the high number of studies focusing on the characteriza-
nanocrystalline. The core is thought to be composed mostly tion of CDs and on the development of new applications for them,
1
–3
2
3
7,20
of graphitic carbon (sp carbon) connected by sp carbon atoms the origin of their photoluminescence is still a matter of debate.
4
,5
in between. On the surface can be found different functional In fact, several models and explanations have been presented so far:
2
1,22
23,24
groups (such as carboxylic acids, alcohols and amines), depend- quantum confinement effect,
emission from surface states,
2
5
21,22
ing on the precursors used and the type of synthetic approach self-trapped excitons, band gap emission,
surface dipole
4
,6
26
employed.
emissive centers, and formation of H-aggregate type excitonic
2
7,28
CDs possess an array of desirable properties, such as high states,
among others.
More recently, there has been an increasing focus on the
compability, low toxicity, high photostability and chemical potential role of molecular fluorophores on the photoluminescence
2
,3,7
8
photoluminescence,
a broadband optical absorption, bio-
4
,9
10
6
1
1
2,6
stability, and good water solubility. Thus, there is no surprise of CDs. It has been demonstrated by several authors that bottom-
up synthesis of CDs, arguably the most common route for their
29–33
fabrication, also produces several molecular by-products.
More
a
b
c
Chemistry Research Unit (CIQUP), Faculty of Sciences of University of Porto,
R. Campo Alegre 697, 4169-007 Porto, Portugal
importantly, these by-products were shown to be quite fluorescent.
In fact, efficient separation of the CDs and molecular fluorophore
fractions revealed the former to be weakly fluorescent and the latter
X-Ray Photoelectron Spectroscopy Lab, Central Service to Support Research
Bulding (SCAI), University of M a´ laga, 29071 M a´ laga, Spain
29–33
strongly fluorescent.
This evidence suggest that the fluores-
CQM-Centro de Qu ´ı mica da Madeira, Universidade da Madeira,
Campus da Penteada, 9020-105 Funchal, Portugal
cence typically associated with CDs may result instead from these
fluorescent impurities, which constitute a significant setback to our
knowledge of these nanomaterials.
Given this, it is essential to have a better understanding of
the relationship between the fluorescence of the CDs and these
fluorescent impurities. More specifically, we intend to understand
d
LACOMEPHI, GreenUPorto, Department of Geosciences, Environment and
Territorial Planning, Faculty of Sciences of University of Porto, R. Campo Alegre
6
97, 4169-007 Porto, Portugal. E-mail: luis.silva@fc.up.pt
†
Electronic supplementary information (ESI) available: FT-IR spectra, XPS
spectra, ESI-MS spectra, fluorescence spectra in different conditions. See DOI:
0.1039/c9cp03730f
1
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s
FT-IR analysis was performed using a PerkinElmer Spec-
if when present in the same solution, the carbon dot and the
fluorescent impurities are well separated and behave as individual trum Two FT-IR spectrometer.
TM
species with well-defined fluorescent behavior. Or if upon synthesis,
Direct injection ESI-MS was made using a Thermo Finnigan
TM
the CDs and the fluorescent impurities interact to generate a LCQ
Deca XP Max (Thermo Electron Corporation, Waltham,
synergistic effect. It should be noted that has been demonstrated USA) mass spectrometer. This device consists on electrospray
n
that the fluorescent moiety of the CDs is present inside the interface as ionization source and a quadruple ion trap for MS
34
nanoparticle in a flexible environment.
While that would not negate the need for a through and capillary voltage, Æ15 V; capillary temperature, 300 1C.
efficient sample purification and characterization, if correct the X-ray photoelectron spectroscopy (XPS) measurements were
experiments. It was operated as follows: spray voltage, 5 kV;
latter hypothesis could lead to future fabrication of hybrid recorded on a Physical Electronic PHI VersaProbe II spectro-
materials with different properties. To this end, we have meter utilizing Al-K with a hemispherical multichannel detec-
a
proceeded to the microwave-assisted treatment of citric acid tor (53.6 W, 15 kV and 1486.6 eV). Spectra were recorded using a
and urea in aqueous solution. Characterization by AFM, XPS, 200 mm diameter circular analysis area, with a constant pass
FT-IR, ESI-MS, UV-Vis and fluorescence spectroscopy revealed the energy value at 29.35 eV. PHI SmartSoft software was employed
existence of both CDs and fluorescent impurities. Subsequent for results analysis, further processed using MultiPak Version
fractioning of the mixture allowed us to obtain three samples: 9.6 package. Carbon C 1s signal (284.8 eV) was used as
CDs, the fluorescent impurities, and a mixture containing both reference to determine the binding energy values, using Shirley
the nanoparticle and the fluorescent impurities. Study of the type background and Gauss–Lorentz curves.
photochemical reactivity of the three samples towards different
AFM analysis was carried out using a Veeco Metrology
electron-withdrawing/-donating probes demonstrated that when Multimode/Nanoscope IVA by tapping. A silica plate was used
present in the same solution, the fluorescent impurities and CDs to deposit the sample for analysis and an AFM RTESP cantilever
do not present individual photoluminescent properties. On the was used.
contrary, they interact to generate synergistic effects, different
Suspensions of nanoparticles were characterized by high-
than their individual responses. This finding indicates that: the resolution transmission electron microscopy (HR-TEM) and
efficient purification of CDs samples is required because existent examined under a FEI Talos F200X.
fluorescent impurities not only mask their fluorescent signal,
but alter it; there is the potential for the development of carbon
dot – molecular fluorophores with hybrid properties.
Results and discussion
Given our aim of understanding the possible interactions
Experimental methods
Synthesis of carbon dots
between CDs and fluorescent impurities resulting from their
synthesis, three samples needed to be obtained. One would be a
sample where both species co-exist in solution, while the other
two would be samples of the individual samples. The first
sample is the one that results after centrifugation, now termed
CDscentrifuged, given that this purification method is only able to
CDs were prepared by microwave treatment (5 minutes at
700 W in a domestic microwave) of citric acid (0.5 g) and urea
(0.5 g) in aqueous solution (5 mL), which was placed in a glass
beaker. In the end, 5 mL of deionized water were used to
re-suspend the resulting product, yielding a solution of CDs.
This solution was subsequently subjected to centrifugation
eliminate suspended impurities and not low-weight fluorescent
15,16,29
compounds.
Individual samples of the CDs and fluores-
29,33
cent impurities were obtained via dialysis.
This process
(10.000 rpm for 10 minutes), being this a common method of
generates two water fractions, one inside and one outside the
1
5,16
removing suspended impurities from the solution of CDs.
dialysis bag. The inside fraction consists on high-weight species,
However, centrifugation is not able by itself to separate the CDs
from molecular impurities that originate from the bottom-up
synthesis. To this end, CDs were further purified by dialysis.
29,33
which are expected to be the CDs themselves,
being this
sample termed CDsdialyzed. The outside fraction consists on low-
weight species, which are expected to be the fluorescent
s
This process was carried out using a Float-A-Lyzer G2 Dialysis
29,33
impurities,
being this sample now termed WaterFI.
s
Device SPECTRUM (molecular weight cut-off of B1000 Da).
The first step of this study was to perform a structural
analysis of the CDs, both in the presence (CDscentrifuged sample)
and in the absence (CDsdialyzed sample) of fluorescent impu-
rities. The size of the CDs was determined by AFM analysis of
the CDsdialyzed sample (Fig. S1, ESI†), which were found to have
The dialysis process ran continuously for 3 days with regular
changes in the dialysis wash waters.
Samples analysis and characterization
Fluorescence was measured in standard 10 mm fluorescence a size of 23.1 nm. It should be noted that while CDs are typically
quartz cells by using a Horiba Jovin Yvon Fluoromax-4 spectro- sized bellow 10 nm, it is not uncommon to find such nano-
3
5,36
fluorimeter. The spectra were obtained with a 1 nm interval and particles with bigger sizes (up to 30 nm).
nm slit widths. morphology of CDsdialyzed was analysed by HR-TEM microscopy
Absorption spectra were obtained with a VWR UV-3100PC (Fig. 1), showing well dispersed nanoparticles with a medium
Nevertheless, the
5
s
spectrophotometer, by using quartz cells.
diameter size of 6.5 nm, with uniform spherical shapes.
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Fig. 1 HR-TEM image for CDsdialyzed
.
Thus, we can conclude that some aggregation occurred during
AFM experiments.
XPS analysis was performed to characterize the surface
composition of CDscentrifuged and CDsdialyzed (Fig. S2–S4,
3
7
ESI†). The XPS mole fraction of CDcentrifuged is of 59.70% C,
6.75% N and 23.55% O. A detailed scan for the internal levels
1
of C 1s, O 1s and N 1s was made, towards deconvolution and
chemical state and the quantitative analysis. The C 1s splits
into three peaks: at binding energies of 284.8 eV (54.53%),
288.3 eV (33.50%) and 286.6 (11.95%). These peaks can be
attributed to C–C/C–H/adventitious carbon (284.8 eV), C–O/C–N
(
(
286.6–286.9 eV) and CQO/O–CQO carbonyl/carboxylic groups
288.3–288.6 eV), respectively. The core level spectrum of N 1s
revealed a dominant peak at 400 eV (88.9%) and a shoulder at
01.5 eV (11.1%). These can be attributed to amine/amide
4
groups and to protonated amines, respectively. Finally, the
deconvolution of O 1s spectrum revealed two peaks: a dominant
one at 531.5 eV (80.9%) due to CQO linkage; a smaller one at
532.7 eV (18.1%), which results from C–O/C–O–C groups. The
results for CDsdialyzed are quite similar, with just a slight increase
in the XPS mole fraction of C (from 59.70 to 62.57%), with the
subsequent decrease of the mole fractions of N (from 16.75 to
15.47%) and O (from the 23.55 to 21.97%). Thus, these results
indicate that the surface composition of the CDscentrifuged
and CDsdialyzed are identical, and that purification by dialysis
apparently has a limited effect on the CDs sample.
These results are supported by further analysis of the surface
functional groups of both CDscentrifuged and CDsdialyzed, this
time made by FT-IR spectroscopy (Fig. S5, ESI†). FT-IR analysis
revealed quite similar spectra for both samples. They show
Fig. 2 (A) Normalized UV-Vis spectra for CDscentrifuged, CDSdialyzed and WaterFI
B) normalized fluorescence spectra for CDscentrifuged, CDSdialyzed and WaterFI
The excitation wavelength maxima used were 410 nm for CDscentrifuged and
;
(
.
À1
the presence of a band at 3300 cm , indicating the presence
of O–H/N–H groups. Also, both samples displayed peaks at WaterFI, and 380 nm for CDsdialyzed; (C) emission wavelength as a function of
À1
excitation wavelength for CDscentrifuged and CDsdialyzed
.
1
655 cm (generally attributed to CQC groups or primary
À1
amides), 1575 cm (N–H bending vibrations for secondary
À1
À1
amides), 1350 cm (O–H bending vibrations) and at 1185 cm
generally attributed to C–N stretching vibrations from amines). (CQC/CQN bonds) transitions, respectively.
However, optical analysis does not support this similarity less well-resolved and small band at 410 nm. By its turn, the UV-
shoulder at 245 nm. These can be attributed to n–p* and p–p*
3
8,39
(
There is also
between CDscentrifuged and CDsdialyzed. The UV-Vis spectrum Vis spectrum of CDcentrifuged presents not just one but two
of CDdialyzed (Fig. 2A) revealed a main band at 340 nm and a shoulders, at 245 and 275 nm. Furthermore, the band at
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4
3
10 nm is now well-resolved and is higher than the one at 350–500 m/z region. By their turn, the mass spectrum of
40 nm. The most significant optical difference between the CDsdialyzed is dominated by peaks in the 350–500 m/z region,
two CD samples is relative to their emission profiles (Fig. 2B). while the mass spectrum of WaterFI has no significant peaks in
While the emission maximum of CDcentrifuged is at 540 nm, the that region but presents in the same peaks as CDscentrifuged in
emission of CDsdialyzed is significantly blue-shifted, with a peak the 50–350 m/z region.
at 475 nm. The excitation wavelength maxima of the samples
This analysis is not enough to properly identify the fluor-
are also difference, with CDcentrifuged being excited at 410 nm escent impurities (which is outside the scope of this study) but
and CDsdialyzed being excited at 380 nm. This indicates that demonstrates that the bottom-up synthesis of CDs does pro-
purification by dialysis can produce a significant effect on the duce a mixture of fluorescent species that needs to be subjected
photoluminescent properties of the CDs samples. Nevertheless, to further purification steps (like dialysis) to separate the lower-
2
9
it should be noted that both CDscentrifuged and CDsdialyzed weight fluorescent impurities from the desired nanoparticles.
present excitation-dependent emission, with both samples Nevertheless, regarding the possible identity of the fluores-
reaching similar emission wavelengths at higher excitation cent impurities, we wish to point out that in the negative
wavelengths (Fig. 2C). It should be noted that what is plotted ionization ESI-MS analysis (Fig. S8, ESI†), the predominant
on Fig. 2C is the centre of the emission peak.
peak for CDscentrifuged and WaterFI is at 179.27 m/z. This peak
The differences between the optical properties of CDscentrifuged could be attributed to 4-hydroxy-1H-pyrrolo[3,4-c]pyridine
À1
and CDsdialyzed can be attributed to the presence of fluorescent 1,3,6(2H,5H)-trione (HPPT, mol. wt. of 180 g mol ), which
29
33
impurities.
was identified by Kasprzyk et al. as a fluorescent by-product of
In fact, optical analysis by both UV-Vis (Fig. 2A) and fluores- the microwave-assisted synthesis of CDs. Moreover, HPPT was
cence spectroscopy (Fig. 2B) of the WaterFI sample revealed an found to have a bright green emission and an absorption peak
identical spectrum to CDcentrifuged, but different from CDdialyzed
.
at 410 nm, which is in line with the optical properties of
3
3
More specifically, WaterFI is also photoexcited at 410 nm and CDscentrifuged and WaterFI (Fig. 2A and B). Furthermore, the
has an emission wavelength maximum at 540 nm, besides peak at B179 m/z is not relevant in the mass spectrum of
presenting a UV-Vis spectrum with the same bands and CDsdialyzed. Thus, these results point to HPPT being a main
shoulders as CDscentrifuged. Thus, these measurements demon- responsible for masking the fluorescence of the carbon dot in
strate that the optical signals observed from CDscentrifuged result the CDscentrifuged sample, being removed from the solution after
not from the CDs itself, but from lower-weight fluorescent dialysis.
impurities. We have also compared the fluorescence intensity
The view that the bottom-up synthesis can generate fluor-
of CDsdialyzed, CDscentrifuged and WaterFI in aqueous solution escent impurities that, if not removed, can mask the signal of
(
(
Fig. S6, ESI†), with the same concentration for each sample the carbon dot was further supported by measuring the fluores-
1 mg mL ). These results showed that while CDscentrifuged cence of the three samples (CDscentrifuged, CDsdialyzed and
À1
and WaterFI present intensities of the same magnitude, they WaterFI) at different pH values (Fig. S9, ESI†). This was achieved
present intensities 6 and 8 times higher than that presented by by measuring the fluorescence of these samples in neat water,
CDsdialyzed. This is in line with recent literature in which it is in aqueous solution acidified with HCl (0.1 M), and in aqueous
indicated that fluorescent impurities are strongly fluorescent, while solution basified with NaOH (0.1 M). As expected, the emission
2
9–33
the CDs are only weakly fluorescent.
the attribution of the fluorescence emitted by CDscentrifuged to the conditions. Namely, both these samples show a green emission
fluorescent impurities, which mask the signal of the CDs. in neat and acidified water, while there is a significantly blue-
This analysis supports profiles of CDscentrifuged and WaterFI are identical in these
Mass spectroscopic analysis was also made by subjecting the shift at basic pH. As for CDsdialyzed, there are no significant
CDscentrifuged, CDsdialyzed and WaterFI to direct-injection ESI-MS. shifts in the emission peak with changes in the pH.
The resulting mass spectra (in the positive and negative ioniza-
A similar study was made by measuring the fluorescence
tion modes) can be found on Fig. S7 and S8 (ESI†). In the spectra of CDscentrifuged, CDsdialyzed and WaterFI in different
positive ionization mode (Fig. S7, ESI†), the mass spectrum of solvents: methanol (MeOH), acetonitrile (ACN), dimethylforma-
CDscentrifuged (scanned between 50.0 m/z and 500.0 m/z) pre- mide (DMF) and dimethyl sulfoxide (DMSO). These results
sents three predominant peaks at 173.13 m/z, 190.20 m/z and can be found on Fig. 3. Despite their different properties,
397.53 m/z followed by several medium sized ones. However, these solvents had only a negligible effect on the fluorescent
the mass spectrum of CDdialyzed in positive ionization mode spectrum of CDsdialyzed. This could point to the fluorescent
presents just a dominant peak at 397.73 m/z, followed by small moieties of CDsdialyzed to be inside the nanoparticle, and so, not
3
4,40
peaks at 291.47 m/z and 374.53 m/z. By its turn, the mass exposed to the external microenvironment.
By its turn, the
spectrum in the positive ionization mode of WaterFI is quite fluorescent spectra of WaterFI undergo relevant blue-shifts
like that of CDscentrifuged, with the difference of a now reduced when comparing with the 540 nm in aqueous solution:
contribution by the peak at 397.73 m/z. Similar differences 515 nm in MeOH, 510 nm in ACN, and 500 nm in DMSO and
and relationships can be seen on the mass spectra in the DMF. That would indicate that the fluorescent moieties are
34,40
negative ionization mode (Fig. S8, ESI†). The mass spectrum exposed to the external medium,
of CDscentrifuged is composed by several predominant peaks with between the WaterFI and CDsdialyzed samples. Interestingly, the
m/z lower than 400.0 m/z, and by several moderate peaks in the solvents exert a somewhat intermediate effect on CDscentrifuged
which indicates a difference
.
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The results obtained so far show that the bottom-up synthesis
of the present CDs also generate fluorescent by-products that can
mask the fluorescent signal of the nanoparticle itself, which is in
2
9–33
line with recent literature.
However, these results do not
indicate if when present in the same solution, the CDs and the
fluorescent impurities interact to generate synergistic effects.
To this end, we proceeded to study the excited state reactivity of
the CDscentrifuged, CDsdialyzed and WaterFI samples. This was
made by measuring the response of the three samples toward
electron-donating (diphenylamine) and electron-withdrawing
(nitromethane) molecules. By comparing the responses of the
individual particles (either CDsdialyzed and Water ) with that of
FI
the samples where both co-exist (CDscentrifuged), we can assess if
the CDs and the fluorescent impurities do interact to generate a
different signal than just the one resulting from their individual
responses.
The next step of this study was then to assess the photo-
chemical responses of the three samples (CDscentrifuged, CDsdialyzed
and Water ) toward nitromethane (Fig. 4). This is an electron-
FI
withdrawing molecule, and so, can be a useful non-ionic electron-
acceptor probe for the study of photoinduced electron transfer
(PET) reactions involving the present samples. Nitromethane
induces quenching for both samples, indicating that all samples
might be capable of PET reactions as an electron-donor. Further-
more, all emission profiles follow a Stern–Volmer relationship.
However, nitromethane is a significantly more efficient quencher
À1
of CDsdialyzed (Stern–Volmer constant, K , of 23.7 Æ 0.1 mM
)
SV
than of the other samples. Moreover, the K for CDs
SV
centrifuged
À1
À1
(
5.4 Æ 0.2 mM ) and WaterFI (6.1 Æ 0.4 mM ) are quite similar.
Once again indicating that the responses observed for CDs
samples only subjected to centrifugation might result only from
fluorescent impurities.
We have also analysed the effect exerted by diphenylamine
on the fluorescence of CDscentrifuged, CDsdialyzed and WaterFI
(Fig. 5). Diphenylamine is a known redox indicator and a strong
electron-donor. Thus, it can also be used to study the potential
of the three samples in PET reaction, in this case as electron-
acceptors. Given the results so far, we were expecting somewhat
different emission profiles between CDscentrifuged/WaterFI and
CDsdialyzed. To our surprise, that was not the case. There were
indeed observed differences between CDscentrifuged and CDsdialyzed
.
Fig. 3 Normalized emission spectra for CDscentrifuged (a), CDsdialyzed (b)
and WaterFI (c) in the presence of acetonitrile (ACN), dimethylformamide
(
DMF), dimethyl sulfoxide (DMSO) and methanol (MeOH).
The solvents do induce a blue-shift to the emission maxima, the
same as Water , but to a lesser extent (between 525 nm and
FI
530 nm). This would indicate that the fluorescent moieties of
CDscentrifuged are also exposed to the solvent, the same as WaterFI.
However, the smaller blue-shift indicates that the fluorescent moi-
0
Fig. 4 (a) F /F values in the presence of different concentrations of nitro-
eties are also more shielded than the moieties present in WaterFI
.
methane (0–50 mM); (b) represents the variation of F
0
/F values of CDsdialyzed
Thus, the presence on the nanoparticles on the CDscentrifuged sample
affects the fluorescence properties of the fluorescent impurities.
À1
samples (0.04 mg mL ) in the presence of nitromethane (45 mM), to
À1
which were added different amounts of WaterFI (0.02–0.08 mg mL ).
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fluorescent impurities and the nanoparticles can interact to
generate a synergistic effect. That is, they produce an effect
different than the one originating from their separated forms.
A synergistic effect can also be seen if we re-evaluate the
results obtained during the nitromethane-related assays
(Fig. 4A). More specifically, by using a B380 nm excitation
instead of the excitation maximum of 410 nm, the fluorescence
spectra of CDscentrifuged presents a shoulder at B460 nm, which
can be attributed to the emission of the nanoparticle itself
(Fig. S10, ESI†). Thus, we have measured the response of
CDscentrifuged towards nitromethane by using a B380 nm excita-
tion, and by measuring the fluorescence intensity at B460 nm
(Fig. 4). This should allow us to monitor the nanoparticles itself
but in the presence of fluorescent impurities. The resulting
emission profile (Fig. 4A) also shows a significant quenching
effect that follows a Stern–Volmer relationship, which indicates a
qualitative agreement with the results of CDsdialyzed. However,
À1
the determined KSV (38.0 Æ 2.5 mM ) was now almost the
À1
double of that determined for CDs
(23.7 Æ 0.1 mM ).
dialyzed
Thus, it appears that the presence of fluorescent impurities
enhances the nitromethane-related quenching process of the
nanoparticle itself. Given that the fluorescent impurities by
themselves do not suffer significant quenching (KSV of 6.1 Æ
À1
0
.4 mM ), this enhancement must result from a synergistic
effect between the carbon dot and the fluorescent impurities,
and not just from an additive phenomenon.
To assess if the fluorescent impurities can indeed modulate
the photochemical reactivity of the CDs, we have measured the
response of CDsdialyzed towards nitromethane in the presence of
increasing concentrations of WaterFI (Fig. 4B). In fact, we can
see that addition of fluorescent impurities to CDsdialyzed
increases the nitromethane-induced quenching. However, this
quenching enhancement decreases with increasing amounts of
the impurities. This can be explained as follows: the fluorescent
impurities interact with the carbon dot, which leads to the
formation of a sort of hybrid material with synergistic properties,
such as the enhancement of the nitromethane-induced quenching.
Increasing the quantity of the fluorescent impurities to a certain
threshold, however, could mask the surface of the carbon dot and
prevent its interaction with the quencher. Thereby, offsetting the
enhancement effect generated by the fluorescent impurities –
carbon dot synergy.
Fig. 5
0
F /F values of CDscentrifuged (a), CDdialyzed (b) and WaterFI (c) in the
presence of diphenylamine.
The former sample was subjected to quenching induced
by diphenylamine that followed a Stern–Volmer relationship
Conclusions
À1
(K
SV of 0.0054 mM ). By its turn, diphenylamine induces an
opposite effect (fluorescence enhancement) on the emission of In conclusion, we have characterized the structural and optical
CDsdialyzed, a difference expected given the findings described properties of citric acid, urea-derived CDs by using a diverse set
above. What surprised us was that diphenylamine had no effect of experimental techniques: AFM, HR-TEM, XPS, FT-IR, ESI-MS,
on the fluorescence of Water . That indicates that the UV-Vis and steady-state fluorescence spectroscopy. We have
FI
diphenylamine-induced quenching observed for CDscentrifuged demonstrated this microwave-assisted synthesis produces
did not originate from fluorescent impurities. However, given green-emitting molecular fluorophores that can mask the
that diphenylamine only induced fluorescence enhancement luminescence of the blue-emitting CDs. More importantly,
for CDsdialyzed, that diphenylamine-induced quenching could our data show that when present in the same solution, these
not have originated from the CDs itself. Thus, this data fluorophores and the carbon dots do not behave as separated
indicates that when combined in the same solution, the species with individual emission. Instead, they interact to
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generate a hybrid luminescence, which excited state properties
and reactivity are different than just the sum of their individual
properties. These results indicate the possibility for the devel-
opment of hybrid materials composed by CDs and related
molecular fluorophores with improved properties.
8 Z. Peng, X. Han, S. Li, A. O. Al-Youbi, A. S. Bashammakh,
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This work was made in the framework Sustainable Advanced
Materials (NORTE-01-00145-FEDER-000028), funded by ‘‘Fundo
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‘Programa Operacional do Norte’’ (NORTE2020). Acknowledg-
ment to projects POCI-01-0145-FEDER-006980, PTDC/QEQ-QFI/
289/2014 and PTDC/QEQ-QAN/5955/2014 are also acknowledged.
1
5 D. M. A. Crista, G. P. C. Mello, O. Shevchuk, R. M. S. Sendao,
E. F. C. Simoes, J. M. M. Leitao, L. P. da Silva and J. C. G.
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0
The first project is funded by FEDER through COMPETE2020,
while the latter two are co-funded by FCT/MEC (PIDDAC) and by
FEDER through COMPETE-POFC. Lu ´ı s Pinto da Silva acknowledges
funding from FCT under the Scientific Employment Stimulus
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CEECIND/01425/2017). The Laboratory for Computational
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0 M. K. Barman and A. Patra, J. Photochem. Photobiol., C, 2018,
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Infraestruturas Cient ´ı ficas na RAM (M1420-01-0145-FEDER-000008)
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