Short Communication
Taciana Holanda Kunst
Taciana Holanda Kunst
Universidade Federal de Pernambuco, Centro de Ciências Exatas e da Natureza, Departamento de Química Fundamental, Av. Jornalista Aníbal Fernandes, s/n – 50740-560 Recife, PE, Brazil.
E mail: taciana.kunst@ufpe.br
Sergio Gonçalves Batista Passos
Sergio Gonçalves Batista Passos
Universidade Federal de Pernambuco, Centro de Ciências Exatas e da Natureza, Departamento de Química Fundamental, Av. Jornalista Aníbal Fernandes, s/n – 50740-560 Recife, PE, Brazil.
E mail: sergiogbp@yahoo.com.br
Ana Paula Silveira Paim
Ana Paula Silveira Paim
Corresponding
Author
Universidade
Federal de Pernambuco, Centro de Ciências Exatas e da Natureza, Departamento de
Química Fundamental, Av. Jornalista Aníbal Fernandes, s/n – 50740-560 Recife,
PE, Brazil.
E-mail address: ana.paim@ufpe.br.
Tel/fax.: +55-81-2126-7469
Abstract Keywords
Green carbon
source, microwave heating, probe, quenching, fennel tea, carbon dots.
1.
Introduction
Quercetin is a
flavonoid that has biological activities including antioxidant, anticancer,
anti-allergic, and also contributes to the prevention of cataracts and coronary
diseases. It is present in natural products, such as fruit and vegetables,
e.g., in onion, broccoli, apple and teas [1]. Thus,
monitoring the amount of quercetin in these foods is important as a
contribution to a healthier diet. Quercetin has been quantified by methods
involving chromatography [2] and
electrochemistry [3]. However, these
methodologies can be expensive, time-consuming and require prior knowledge of equipment
operation and data processing. In contrast, methods involving fluorescent
nanoparticles are easy to apply, economical and quick. In this study, we
synthesized carbon quantum dots (CQDs) from waste office paper and water via
microwave heating. This represents a faster, cheaper and simpler CQDs
synthesis, yielding fluorescent nanoparticles solution which could be used as
quercetin sensor. Several aspects of the CQDs synthesis were evaluated, some
characterizations of the obtained CQDs were achieved and an attempt to apply it
as quercetin sensor was performed.
2.
Materials and methods
2.1 Materials
Solutions were
prepared using ultrapure water (18.0 MΩ cm) from a Milli-Q system (Millipore
Inc., Bedford, MA). Methanol (HPLC grade) was purchased from Sigma-Aldrich (St.
Louis, MO, USA). A 1000 mg L-1 quercetin stock solution was prepared
from HPLC grade quercetin (≥95%, Sigma-Aldrich, St. Louis, MO, USA) in
methanol. Other reagents were of analytical grade. All buffer solutions were
prepared according to Gomori [4], adapting
the final volumes. Quinine sulfate (53%) in an aqueous H2SO4
solution (0.05 M) was used as the reference for calculating quantum yield with
excitation at 366 nm [5]. The paper used
in the syntheses were A4 computer
printout sheets which were no
longer being used. These sheets were cut into strips by an office paper shredder
and then cut with scissors, resulting in small rectangles of about 8 by 1 mm.
2.2 Apparatus
The Microwave
oven was a Start D model (Milestone, Sorisole, Italy) with cavity of ten
Teflon® bottles capacity. Centrifuge was a Hettich EBA 200 with speed of 6000
rpm. Spectrofluorometers Horiba-Jobin Yvon Fluorolog-3 with xenon lamp, Perkin
Elmer model LS 55, and Shimadzu model RF-5301PC were used according to their
availability. Absorption spectrophotometer was an Agilent model 8453. TGA was a
Shimadzu model 50WS, and DSC was a Shimadzu model DSC-60, both operating in
nitrogen flow (50 mL min-1; heating rate: 10 °C min-1).
Rotator evaporator was a Büchi R-210 with B-491 bath and Vario PC 3001 vacuum
pump. Nuclear magnetic resonance (NMR) device was a 400 MHz VNMRS from Varian
(solvent: D2O). DLS device was a NanoBrook Omni. Zeta potentiometer
was a Zetasizer. The X-ray diffractometer was a Bruker D8 Advance with Kα
radiation of Cu (λ = 1.5418 Å). Microscopy images were acquired from a Delong
LVEM5 with 5 kV acceleration using a grid covered with ultrafine carbon film as
substrate.
2.3 CQDs
synthesis
Aiming to
evaluate the best conditions of CQDs synthesis the independent variables such
as microwave power (P), microwave irradiation time (t) and mass of paper (m)
were varied at two levels on a 23 factorial design. The upper (+)
and lower (-) levels for P were respectively 200 and 100 W. The levels for t
were 30 and 15 min, respectively. And these levels for m were 1.0 (+) and 0.5 g
(-). For each combination of variable levels one synthesis using two microwave
reactors was executed. The monitored responses were absorption and emission
spectra. Statistica software (StatSoft, Inc., version 12.0) was used to
generate the matrix of experiments.
In a typical
synthesis, for each reactor, 1 g of paper was weighed in a beaker, 10 mL of
ultrapure water was added and the beaker was stirred until every piece of paper
was wet; then the mixture was transferred to the reactor. The reactors were
exposed to 200 W of power for 30 min. After reaching room temperature the
reactors were opened. Both products were transferred to a single 50 mL Falcon
tube, using water to help the transfer. Then, the tube was topped up to 45 mL.
The liquid and solid remaining in the tube were equally divided among four 15
mL Falcon tubes and centrifuged for 10 min. The supernatant was equally divided
into two other 15 mL Falcon tubes and centrifuged for further 5 min, then the supernatant
was transferred to a glass and stored at room temperature.
2.4 Quercetin
measurement
To a quartz
cuvette, 0.5 mL of 5% (v/v) CQDs solution of the as-synthesized CQDs, 2 mL of
TRIS-HCl buffer solution at pH 8 and 0.5 mL of 10 to 1000 mg L-1
standard solution were added. At the time of adding the standard solution, a
timer was triggered; the mixture was stirred and the fluorescence spectrum was
obtained within 3 minutes.
2.5 Study of chemical parameters in quercetin analysis
2.6 Tea sample
Fennel tea bags were acquired at a
local supermarket and stocked at room temperature. The tea was prepared in
infusion following the package instructions (1 bag of 2 g in 200 mL of hot
water for 5 min) and left to cool down to room temperature. The brewed tea was
then diluted to 10% (v/v), by taking 1 mL and completing the volume of 10 mL
with methanol, subsequently, the diluted solution was stored in a refrigerator. Before analysis,
the 10% solution of brewed tea was left to reach room temperature, when it was
added to the cuvette containing 0.5 mL of 5% (v/v) CQDs solution and 2 mL of
TRIS-HCl buffer solution at pH 8. The fluorescence spectrum was obtained after
3 minutes from the sample addition.
3. Results
and discussion
Since 2004 [6] CQDs have been
synthesized from different carbon sources, including expensive reagents like
ionic liquids [7] or low-value materials
like waste paper [8]. CQDs synthesis usually
needs a heating source such as autoclave, microwave radiation, or plasma [9–13]. The methodologies so far developed for the
synthesis of CQDs from office printing paper, paper filters, brown paper
tissues, or newspaper are time consuming, multi-step or use chemical reagents [8, 14–21]. Then, we investigated the conditions
of CQDs synthesis from waste office paper and water via microwave heating.
First, a 23 factorial design was executed to assess the synthesis
conditions. This factorial design variated the microwave irradiation power, P
(200 W (+), 100 W (-)), the heating time, t (30 min (+), 15 min (-)), and the
mass of paper, m (1.0 g (+), 0.5 g (-)). The best condition was expected to
show higher intensities in absorption and emission spectra. As seen in
absorption (Fig. 1(a)) and
emission spectra (Fig. 1(b)) of
the different CQDs suspensions obtained through the factorial design, the best
synthesis condition used power of 200 W, 30 min of heating and 1.0 g of paper.
Besides, a synthesis with one hour of heating was run; but no significant
improvement in spectra intensities was observed.
Fig. 1 Absorption (a) and emission (λexc = 400 nm) (b)
spectra of the CQDs synthesized with different conditions of power (P: 200 W
(+), 100 W (-)), time (t: 30 min (+), 15 min (-)) and mass of paper (m: 1.0 g
(+), 0.5 g (-)).
At the end of the synthesis, a considerable amount of paper remained. This would be usually discarded, but it could be used again to produce more CQDs. Using such residue in a new synthesis procedure, exhibits the advantage of producing a solution with higher fluorescence intensity (Fig. S1). This indicates that a greater number of nanoparticles were formed. Therefore, the usage of synthesis residue as starting material should be further investigated.
Apart from
articles which make CQDs from other carbon sources [7,
9–13, 22, 23], the methodologies in literature that synthesize CQDs from
paper may need several steps [15, 19, 21], take
up to 15 hours of heating [16–19], or use
reagents such as sodium hydroxide [16], sulfuric
acid [14], urea [14,
19], or ionic liquids [15]. The
synthesis proposed herein offers the advantage of using only waste paper and
water, instead of reagents. Other advantages are the demand for only 30 min of
heating, the use of a controllable heating system, and the presence of only one
step for nanoparticles formation.
One feature that
allowed the present synthesis methodology to be faster was the exclusion of a
dialysis step usually found in many methodologies. Although dialysis is used as
a purification step, for example to remove inorganic and molecular impurities,
it takes a very long time. Thus, with a view to the target application, the
obtained CQDs suspension after centrifugation required no further purification.
Observing the
optical characteristics of the synthesized CQDs (Fig. 2) is possible to affirm the quality of CQDs and the
feasibility of the method is very satisfactory. As shown in Fig. 2(a), there is a band in the
excitation spectrum around 375 nm and a shoulder around 400 nm. The
fluorescence emission spectrum (excited at 400 nm) has a band at 496 nm,
showing a Stokes shift of 96 nm and a full width at half maximum (FWHM) of 137
nm. The fluorescence also presented a very common property to carbon dots
reported in the literature: the excitation-dependent emission (Fig. 2(b) and 2(c)). The quantum yield was calculated to be 9%, relative to
quinine, at 366 nm [5]. This 9% is
acceptable when taking into account the range of quantum yields of other CQDs
prepared from paper which varies from 0.27% to 20% [8,
15–18, 20]; although, it can appear to be very low when compared to
other reported quantum yields [10]. The
UV-Vis spectrum (Fig. 2(a)),
showed an obvious shoulder around 263 nm associated to π-π* transition and some
not so distinctive shoulders in the range of 270-300 nm and around 320 nm that
are associated to n-π* and π-π* transitions of C═O, respectively [8, 15–17].
Fig. 2 Absorption, excitation and emission spectra of CQDs produced
after improvement of synthesis condition (a). Emission spectra of CQDs
excited at different wavelengths (250 - 600 nm) (b). Normalized emission
spectra of CQDs excited at different wavelengths (300 - 600 nm) (c).
The obtained
emission spectra were suspected to be composed of three components: the fluorescence
from CQDs, the fluorescence from the paper fluorophore and the water
autofluorescence. To verify that, the paper fluorophore was extracted from the
paper by adding some pieces of paper into a beaker with water for a few minutes
in a sonication bath and then blue fluorescent solution was obtained. The
fluorescence spectra of the extracted fluorophore were acquired (Fig. S2(a)), these spectra
were also acquired for pure water (Fig. S2(b)). Paper
fluorophore emission is fixed at 438 nm and varied its intensity with
wavelength excitation. This fluorophore is added to paper during its
fabrication and is composed mainly of TiO2 [25].
On the other hand, water emission wavelength is variable with
excitation. Both emissions are present together in the
emission spectrum of the solution produced in the CQDs synthesis. In order to
view the CQDs fluorescence and avoid water and fluorophore emissions,
excitation at 400 nm was selected for further experiments.
Thermal analyses of
TGA and DSC were performed with the as-prepared CQDs after evaporation under vacuum
and drying under acetone (Fig. S3). TGA shows that
some water is lost in the beginning, some other losses of weight happened and
above 600 °C more than 40% of mass remained, demonstrating the presence of
inorganic residue. This residue is attributed to the oxides added during
fabrication for paper conditioning such as Al2O3, SiO2
and TiO2 [25]. In DSC, various
endothermic signals around 117 and 160 °C can be seen, indicating CQDs undergoe
some reactions and these reactions might spoil the CQDs luminescent properties.
Thus the thermal stability of CQDs was verified to be up to 115 °C.
The 1H
NMR of CQDs is shown in Fig. S4 where it is possible to see signals of H from: sp3
carbons, hydroxyls, ethers, carboxyls or aldehydes [27].
No signals of H from aromatic or sp2 carbon can be seen
between 6 and 8 ppm. This indicates that the synthesized CQDs were not
graphitic quantum dots (GQDs). The X-rays diffraction pattern of CQDs showed broad bands around 10° and 23° indicating a very
amorphous carbon structure (Fig. S5) which
corroborates to the deduction the CQDs were not GQDs.
The CQDs
microscopy images were obtained from low voltage electron microscopy (LVEM). Fig. 3 presents a LVEM image where
several well-dispersed nanoparticles are seen. In the inset, the histogram of
these particles shows size dispersion around 11 nm. The zeta potential of CQDs was measured at
-5.92 mV, due to its hydroxyl, carbonyl or carboxyl surface groups, and this
negative charge promotes electrostatic repulsions that can indicate a stable
CQDs dispersion [8, 11].
Fig. 3 LVEM image of the as-prepared CQDs. Inset: Histogram of size
distribution of CQDs (n = 200).
In order to
assess the possibility of separating CQDs populations obtained together in the
synthesis, a column was prepared with powder cellulose as stationary phase and
water as mobile phase. During cellulose packaging luminescence was observed in
the water that run-off before the CQDs were added, so the column was washed
until the fluorescence was minimized. The CQDs solution was passed through the
column and fractions of the output solution were collected according to
visualized color or fluorescence changes. Fluorescence spectra of these
fractions were obtained and shown in Fig. S6. Some intensities variation is observed in 3 regions of the
spectrum: 455, 500 and 560 nm, indicating that there are at least 3 majority
populations of CQDs and that these populations are separable. It might be
possible to obtain each population separately if a column with better
separation or a preparative HPLC column were used. This is still ongoing.
Perhaps, it would be advantageous to add this post-synthesis procedure in the
future, in order to obtain different solutions of CQDs emitting in different
regions of the spectrum with a narrow emission profile.
With a view to
its application, CQDs can be used to different purposes including analytical
determinations of neutral, positively and negatively charged, organic or
inorganic species [27]. These sensors may
quantify species at the nanomolar level with great selectivity [22]. Benefic compounds in foods or drinks [7, 23, 28] and toxic species like mercury II [29] have been targets of their determinations.
Herein, the as-prepared CQDs were used in an attempt to propose a fast and
practical quercetin sensor, regarding the mentioned importance to determine
quercetin.
Initially, the
experimental conditions including pH, concentration of CQDs and reaction time
were optimized in order to obtain the highest sensitivity, i.e., the greatest
difference between the CQDs emission intensity in the presence (F) and in the
absence (F0) of the analyte. The pH could influence the reactivity
and stability of the CQDs, affecting the fluorescence intensity or sensor
sensitivity [7]. The study of the pH was
performed using buffer solutions at different levels of pH (from 5 to 8) so its
influence on quenching of CQDs fluorescence could be evaluated. As can be seen
in Fig. 4(a), the pH which gives
greater quenching is 8, therefore pH 8 was selected for further studies.
Fig. 4 Effect of pH (a), CQDs concentration (b) and
reaction time (c) to quenching of CQDs.
High
concentrations of CQDs can result in self-quenching while low concentrations
can lead to poor sensitivity, hence is necessary to establish the best
concentration of CQDs to be used in the analysis [28].
With the pH fixed at 8 as determined above, the CQDs concentration was
varied between 1% and 5% (v/v) and the fluorescence signal quenching (F0/F)
was obtained (Fig. 4(b)). The
results showed a quenching slightly better for the concentration of 5% (v/v),
so this concentration was chosen. The evaluation of the reaction time between
CQDs and analyte is important to make sure the measured signal is stable.
Furthermore, is convenient to evaluate the shortest time in which the quenching
can be measured. The quenching (Fig.
4(c)) was measured up to 20 min. Less variation was observed between 1
and 9 min. Consequently, in order to have higher analytical frequency and a
safety margin for variations in quenching signal, the selected measurement time
was 3 min.
Given that, the
following analyses were conducted using a buffer solution at pH 8 (TRIS-HCl), 5% (v/v) CQDs solution and 3 min of
reaction time. Standard solutions with concentrations from 10 to 1000 mg L-1
of quercetin were analysed through the sensor proposed (Fig. 5). A linear
relationship was observed in the range of 10 to 100 mg L-1 quercetin
and a regression was determined to be F0/F = 0.01236[Q] + 0.9245
(where [Q] is the concentration of quercetin, in mg L-1) with a
determination coefficient (R2) of 0.9811 and relative standard
deviation (RSD) of 1.6% (n = 9; 50 mg L-1). According to IUPAC
criteria, the detection and quantification limits were 0.8 and 2.6 mg L-1
quercetin, respectively. This detection limit is not as low as most of the
limits reported for other quercetin analysis methodologies [3, 7, 28, 30–33], however,
the sensor proposed here is more advantageous because of its low cost
and simple preparation. Moreover, the present detection limit is slightly lower
than the limit reported by de Paula et al. [23] (0.85
mg L-1) and quite lower than the reported by Jeevika et al. [34] (2.03 mg L‑1). These authors
also used carbon dots-based sensor for quercetin analysis, but their carbon
dots were prepared with longer heating time and from sodium citrate or garlic
peel as carbon source.
Fig. 5 Fluorescence spectra of CQDs in the absence and presence of
quercetin. The inset (F0/F of these spectra at 485 nm) is an
analytical curve with quercetin concentrations up to 1000 mg L-1 (a).
Analytical curve for quercetin in the linear range (b).
Considering the
linear Stern-Volmer equation (Equation
1) and the linear regression found for the present sensor, a
Stern-Volmer constant (Ksv) of 0.01236 can be established [23, 34].
The CQDs-based
sensor proposed here exhibited great potential to be used for quercetin
monitoring in natural products and drinks. Although there are several features
to be assessed about the present sensor, e.g., selectivity or interference
tests, an exemplification of the sensor used for quercetin analysis in the real
sample was performed. Once the amount of quercetin found in teas is usually
high [1], this kind of sample does not
require very low detection limits from the sensor neither large amounts from
the sample. Therefore, brewed fennel tea was utilized in the exemplification.
Because the tea has its fluorescence, it was necessary to be subtracted from
the emission of the mixture of the CQDs with the tea. Then a mixture prepared
without the CQDs was also recorded. Once the sample was properly analysed and
submitted to the analytical curve, the result showed a concentration of 145.6
mg L-1 quercetin in the brewed tea. The same sample was spiked with
30 mg L-1 quercetin and a recovery of 124% was found.
4.
Conclusions
Considering the
acquired results, we assume the aims of the present work were mostly reached.
The CQDs synthesis conditions were established at 200 W of microwave power, 30
min heating and 1 g of paper; signaling a greener, faster, and more feasible
method than in the literature. The obtained CQDs showed good quality and a
quantum yield of 9%, which appears to be low; however, is acceptable compared
to other nanoparticles prepared from similar sources. And the application
attempt as quercetin sensor was studied, determining the best analysis
condition as pH 8, 5% (v/v) CQDs solution and reaction time of 3 min. The
sensor showed a wide linear range from 10 to 100 mg L‑1, limits
of detection and quantification of 0.8 and 2.6 mg L‑1,
respectively, and RSD of 1.6% (n = 9), indicating its applicability. Finally, a
real sample of brewed tea was successfully analysed and submitted to a recovery
test, suggesting the potential application of this sensor. However, much study
is still ongoing to improve and better explore the sensor performance.
Authors’ contributions
Conceptualization,
T.H.K.; Methodology, T.H.K., S.G.B.P. and A.P.S.P.; Validation, A.P.S.P.;
Formal Analysis, S.G.B.P. and A.P.S.P.; Investigation, T.H.K., S.G.B.P. and
A.P.S.P.; Resources, A.P.S.P.; Data Curation, T.H.K. and S.G.B.P.; Writing –
Original Draft Preparation, T.H.K.; Writing – Review & Editing, A.P.S.P.;
Visualization, T.H.K.; Supervision, S.G.B.P. and A.P.S.P.; Project
Administration, A.P.S.P.; Funding Acquisition, A.P.S.P.
Acknowledgements
The
English text of this paper has been revised by Sidney Pratt, Canadian, MAT (The
Johns Hopkins University), RSAdip-TESL (Cambridge University), and by Roderick
M. Mackenzie, teacher at Britanic® school in Pernambuco,
Brazil.
Funding
The authors thank
the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) for
the scholarship granted for T.H.K. (process no. 145834/2015-8); FACEPE/NUQAAPE (APQ-0346-1.06/14); FACEPE
(APQ-0557-1.06/15); and the Instituto Nacional de Tecnologias Analíticas Avançadas
- INCTAA (CNPq grant 465768/2014-8, and FAPESP grant 2014/50951-4) for
supporting this work.
Conflicts of interest
The authors
declare no conflict of interest.
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Abstract Keywords
Green carbon
source, microwave heating, probe, quenching, fennel tea, carbon dots.
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