![Cyclic voltammograms of FTO electrodes modified with (A) pure g‐C3N4 and Cu2O and (B) different nanocomposites in 0.1 M Na2SO4 (pH 6.0). (C) Cyclic voltammograms and (D) Nyquist plot curves of modified electrodes recorded in a 0.1 M KCl containing 5 mM [Fe(CN)6]3−/4−.](https://www.researchgate.net/publication/385212698/figure/fig3/AS:11431281294316477@1733136588567/Cyclic-voltammograms-of-FTO-electrodes-modified-with-A-pure-g-C3N4-and-Cu2O-and-B_Q320.jpg)
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Electrochemical and Photoelectrochemical Bimodal Sensor
Based on Copper Modified g-C3N4for Nitrate Detection
Wafa Aidli,*[a] Daniele Fumagalli,[a] Hanieh Helli,[a] Luigi Falciola,*[a] and Valentina Pifferi[a]
Nitrates (NO3
-) are crucial in agricultural practices and the food
industry, but their excessive presence in water can lead to
adverse health effects. Their leaching into water sources
necessitates regular monitoring. This study introduces a novel
bimodal electrochemical (EC)/photoelectrochemical (PEC) sen-
sor, utilizing copper-modified graphitic carbon nitride (Cu/g-
C3N4), designed for precise nitrate determination. The structural
morphology and chemical composition of the Cu/g-C3N4nano-
composite were meticulously examined using Transmission
Electron Microscopy (TEM) and Fourier-transform infrared
spectroscopy (FTIR). The optimization of copper loading in g-
C3N4was conducted, and the electrochemical behavior and
light irradiation interaction of various Cu/g-C3N4nanocompo-
sites were systematically studied. The investigation revealed
that 20 % Cu/g-C3N4represented the optimal doping ratio,
establishing the most promising candidate for NO3
-. Nitrates
were consistently measured using both EC and PEC techniques,
yielding Limits of Detection (LoD) of 3.75 and 9.60 ppm,
respectively. The sensor‘s robust performance was further
demonstrated in the presence of possible interferents. The
proposed sensors were also successfully used to detect NO3
-in
commercial water. This bimodal sensor presents a promising
approach for accurate nitrate determination, attesting to its
potential for effective cross-validation.
Introduction
Nitrates represent an ubiquitous source of nitrogen naturally
present in ecosystems and an essential part of the nitrogen
cycle.[1] They are also used as key components of synthetic
fertilizers for plant growth and food production for humans and
animals.[2] In normal circumstances, nitrates exhibit beneficial
effects on human health, promoting increased blood flow,
reducing blood pressure, and preventing cardiovascular
diseases.[3] However, due to their intensive and extensive use
not only as fertilizers,[4] their quantity has significantly increased
in water and soil, causing detrimental impacts on both the
environment and human health. From an environmental
perspective, nitrates contribute to the phenomenon of eutro-
phication and play a role in the increase of nitric oxide, one of
the gases responsible for the greenhouse effect.[5] From a health
perspective, a high intake of nitrates from water and food can
result in adverse effects on the endocrine system, potentially
leading to the onset of diseases such as Parkinson’s and
Alzheimer’s,[6,7] Moreover, in the vulnerable digestive systems of
children, it can manifest as specific syndromes, including the
infamous “blue-baby” syndrome.[8] To mitigate these problems,
the World Health Organization (WHO) recommends a maximum
contaminant level of 50 ppm NO3(50 mg L1) in drinking water
to safeguard public health.[9]
Driven by these concerns, researchers have directed their
efforts towards two pivotal areas: the removal of nitrates from
water and their precise analytical quantification.
The removal of nitrates has been a hot research topic, with
various techniques explored, such as adsorption, reverse
osmosis, advanced filtration, and biological denitrification.[10]
However, these methods often come with high operating costs
or the potential to generate secondary pollutants. Recently,
attention has pivoted towards photocatalytic reduction as a
sustainable removal method. Numerous studies have inves-
tigated semiconducting catalysts for NO3photocatalytic reduc-
tion, encompassing metal oxides (e. g., TiO2
[11]), sulfides (e. g.,
CdS,[12] ZnS[13]), and perovskites (e. g., GdCrO3,[14] BaLa4Ti4O15
[15]).
Graphitic carbon nitride (g-C3N4), a metal-free semiconductor,
has lately regained popularity owing to its exceptional
efficiency, cost-effectiveness, chemical stability, and ease of
modification. Successful nitrate photocatalytic reduction using
g-C3N4has been demonstrated, marking a promising advance-
ment in sustainable removal methodologies.[16]
Regarding nitrate detection, traditional quantification meth-
ods such as spectrophotometric,[17] chromatographic,[18]
electrophoretic,[19] colorimetric[20] and fluorescence[21] techniques
have long been employed. Despite their effectiveness, these
methods present challenges, including high costs, prolonged
analysis times, complexity, and the need for sample pre-
treatments, making them less suitable for on-site applications.
In the pursuit of developing assays that offer rapid, cost-
effective, and user-friendly solutions, electrochemical techni-
ques have emerged as promising tools for nitrate sensing.
Given the electroactive nature of nitrates,[22] various electro-
[a] W. Aidli, D. Fumagalli, H. Helli, L. Falciola, V. Pifferi
Electroanalytical Chemistry Group, Department of Chemistry
Università degli Studi di Milano
Via Golgi 19, 20133, Milano, Italy
E-mail: wafa.aidli@unimi.it
luigi.falciola@unimi.it
Homepage: sites.unimi.it/ELAN
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/celc.202300557
© 2024 The Authors. ChemElectroChem published by Wiley-VCH GmbH. This
is an open access article under the terms of the Creative Commons Attri-
bution License, which permits use, distribution and reproduction in any
medium, provided the original work is properly cited.
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chemical methods have been devised, relying on the electro-
reduction process.[23,24] Researchers have explored a spectrum of
nanomaterials, with a particular emphasis on noble metals (e. g.,
Au, Pd, Ag)[25–27] and transitional metals (e. g., Zn, Cu)[28,29] in
crafting sensitive and selective nitrate electrochemical sensors.
Copper, in particular, stands out due to its exceptional
catalytic activity in nitrate reduction, coupled with high
conductivity, all while maintaining cost-effectiveness and
affordability. Furthermore, the versatility of copper extends to
its capacity for easy integration with various other materials,
allowing for the creation of different structures that contribute
to enhanced performance.[30–32]
However, despite the undeniable advantages of electro-
chemical methods in terms of ease and speed of use, they, like
all other techniques, are susceptible to interferents and matrix
effects present in real samples, leading to a reduction in
accuracy during practical applications.[33]
To face these challenges, a growing trend in sensors design
towards the integration of different transduction modes into
one sensor. These sensors, often referred to as ‘bimodal’ or
‘dual-mode’ sensors, leverage the versatile properties of nano-
materials to combine various sensing techniques such as
electrochemical-colorimetric,[34] electrochemical-electro-
chemiluminescent,[35] electrochemical-photoelectrochemical,[36]
fluorescent-electrochemical,[37] fluorescent-colorimetric[38] meth-
odologies.
The adoption of this dual-mode approach, was found to
provide a cross-validation of analytical results, thereby enhanc-
ing the reliability of the sensor.[39]
In this context, we introduced a novel bimodal electro-
chemical-photoelectrochemical (EC-PEC) sensor for the detec-
tion of nitrates based on copper modified graphitic carbon
nitride (Cu/g-C3N4). The choice of the nanomaterial is driven by
the dual electrocatalytic and photocatalytic properties of
copper and graphitic carbon nitride, respectively, towards
nitrate reduction. The primary analytical focus of this sensor is
the determination of NO3at neutral pH through electro-
chemical reduction. Operating in PEC mode involves conduct-
ing measurements under UV LED irradiation, ensuring optimal
photoactivation. The EC and PEC outputs mutually reinforce
each other, providing more reliable results than single-mode
sensing. While nitrate electrochemical sensing has been exten-
sively studied, to the best of our knowledge, this work marks
the first reported instance of photoelectrochemical nitrate
sensing.
Results and Discussion
Materials Characterization
The synthesis process of the copper-modified graphitic carbon
nitride (Cu-g-C3N4) active material and its subsequent immobili-
zation on a conductive Fluorine-doped Tin Oxide (FTO)
electrode are elucidated in Figure 1. Comprehensive material
characterization was conducted at each stage of the electrode
preparation.
Characterization of both pristine and Cu-modified g-C3N4
was performed using FTIR, as reported in Figure 2A. The
presence of CN heterocycles is confirmed by the trigonal
CN(C)C and bridging CNHC stretching signals between
952 cm1and 1601 cm1. The sharp peak at 792 cm1confirms
the presence of the typical g-C3N4structure, attributed to the
breathing mode of the tri-s-triazine units,[40] Additionally, the
broad absorption band at 3074 cm1validates the composition
of g-C3N4, representing the stretching vibration of NH/
HNH.[41]
Upon modification with copper, all characteristic vibrational
peaks related to pristine g-C3N4are retained, suggesting that
the structural integrity of graphitic carbon nitride remained
intact. Although the three IR spectra appear very similar, small
differences can be seen upon careful examination. This includes
the appearance of a new weak absorption band at 2176 cm1
attributed to the formation of CN triple bonds, a common
defect site originating from the opening of triazine rings. This
suggests that copper is chemically coordinated to the g-C3N4
host, most likely in the form of CuN bonds within the in-plane
cavities of g-C3N4. These observations are typical in metal doped
g- and align with findings reported in previous studies on metal
doped g-C3N4.[42,43]
As a comparative analysis we examined the as-prepared
pure copper and observed a characteristic peak at 597 cm1,
corresponding to the CuO bonds in the Cu2O structure.[44] The
absence of this peak in the Cu/g-C3N4spectrum confirms the
presence of Cu in its most stable, single-atom form.
Exfoliated g-C3N4was studied with TEM imaging (Figure 2C),
confirming the presence of 2D porous layered nanosheets. The
structure was amorphous, and the single sheet size was in the
range of hundreds of nanometers. TEM analysis of the Cu-g-
C3N4material reveals similar graphitic stacking structures,
indicating that the fundamental morphology remains largely
unaffected after copper doping. The nanosheets appeared
more densely packed, featuring increased pore density and
smaller sheet sizes. Such a behavior is common when single
atom metal is introduced into the structure.[45,46] The absence of
clusters further supports the proposition that atomic copper is
incorporated into the g-C3N4structure, rather than the
formation of discrete copper or copper oxide particles.[47]
The electrochemical characterization was conducted follow-
ing the immobilization of the prepared materials on Fluorine-
doped Tin Oxide (FTO) glass slides. Initially, cyclic voltammetry
was performed in Na2SO4. Figure 3A and 3B illustrate the results,
where Cyclic Voltammetry reveals no peaks for bare g-C3N4. In
contrast, distinct Cu oxidation and reduction peaks are
observable in the copper-doped g-C3N4, providing clear con-
firmation of successful copper incorporation.
Next Electrochemical Impedance Spectroscopy (Figure 3D,
the fitted parameters obtained with the inset circuit are
reported in Table S1) in the presence of 5 mM [Fe(CN)6]3-/4- as
redox probe.
Nyquist spectra showed that the charge transfer resistance
of g-C3N4, which corresponds to the diameter of the semicircle,
decreased when the Cu percentage was raised from 5 % to 30 %
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in comparison to pristine g-C3N4. This indicates that Cu doping
increases the conductivity of g-C3N4significantly.
Increasing the percentage to 40 % led to excessive loading,
leading to higher resistance again. This effect is attributed to
the shielding of the surface’s active sites, suggesting that there
exists an optimal Cu percentage for maximizing conductivity
without causing excessive loading.
Interestingly, the resistance of both 20 %wt and 30 %wt Cu/
g-C3N4was lower than that of pure Cu2O. This finding implies
that g-C3N4provides a high specific surface area with abundant
accessible CuN active sites in comparison to bulk copper,
highlighting the synergistic effect between copper and graph-
itic carbon nitride.
These results were further confirmed by cyclic voltammetry
recorded in the presence of the ferrocyanide probe (Figure 3C).
The high peak-to-peak separation of 0.45 V in bare g-C3N4was
successfully reduced to 0.14 V, and the current intensity
followed the same pattern observed for the charge transfer
resistance, with the best value obtained at 20 %wt Cu/g-C3N4,
while a decrease was observed at higher concentrations.
Photoactivity of the nanomaterials was evaluated using
irradiation with a 395 nm LED. As shown in Figure 4A, Bare g-
Figure 1. Schematic representation of the (A) synthesis of g-C3N4nanosheets, (B) its modification with copper and (C) the immobilization on a conductive
glass slide to be used as electrode.
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C3N4showed a low photocurrent of only 1.023 μA, due to low
efficiency of the electron-hole separation. Cu2O, thanks to its
lower band gap and better electron transfer kinetic, reached
2μA of intensity. The best results were achieved with the
copper modified g-C3N4, in particular with 20 %wt Cu/g-C3N4
whose intensity was 39 times higher than that of pure g-C3N4.
This can be explained with the formation of a p-n hetero-
junction which improves carriers’ separation and electron
transfer kinetics. Notably, the signal remained stable for multi-
ple cycles, indicating the stability of the nanomaterial. Given
the results obtained from both photoactivity and electro-
chemical measurements, the 20 wt% Cu/g-C3N4sample was
selected for further analytical detection experiments.
Nitrate Studies
Nitrate reduction with the prepared electrodes was initially
evaluated using CV. Figure 4B the characteristic cathodic peak
at 0.8 V, corresponding to the reduction reaction:
NO
3þ2Hþþ2e!NO
2þH2O
Figure 2. FTIR specta of Cu2O, g-C3N4and the 20 % Cu/g-C3N4(A) and of the composite material with different percentages of copper (B). TEM images of
exfoliated g-C3N4(C) and Cu/g-C3N4(D).
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The peak intensity was found to be significantly influenced
by the copper loading ratio, with the 20 %wt Cu/g-C3N4
composite exhibiting the highest current response.
This trend was consistently observed in photoelectrochem-
ical (PEC) analysis, where various modified electrodes exhibited
a cathodic photocurrent signal. Notably, a substantial
enhancement was observed in the presence of nitrate ions,
with the 20 % doping ratio demonstrating the highest intensity
(Figure 4C).
Nitrate detection was performed using the 20 %wt Cu/g-
C3N4modified FTO electrode. The bimodal nature of the sensor
allowed us to establish a calibration line using both electro-
chemical methods, Linear Sweep Voltammetry (LSV), and PEC.
In Figure 5A, the LSV results exhibit a clear increase in the
reduction peak at 0.78 V, revealing two linear ranges for
concentrations between 10–100 ppm and 100–200 ppm. The
Limit of Detection (LoD) was calculated as 3.75 ppm (S/N =3.29)
(Figure 5A, inset). For PEC analysis, a single linear range was
observed between 10 and 200 ppm, and the corresponding
Limit of Detection was calculated as 9.60 ppm (S/N =3.29)
(Figure 5B and inset). The obtained values are compared with
other literature works in Table 1. Notably, the preparation of the
Figure 3. Cyclic voltammograms of FTO electrodes modified with (A) pure g-C3N4and Cu2O and (B) different nanocomposites in 0.1 M Na2SO4(pH 6.0). (C)
Cyclic voltammograms and (D) Nyquist plot curves of modified electrodes recorded in a 0.1 M KCl containing 5 mM [Fe(CN)6]3/4.
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sensor presented in this work is easier than that of most of
other works, the cost of the components is lower, and the
working conditions are at neutral pH, while the sensors based
on copper are usually optimized for very acidic pH. Further-
more, according to our knowledge, no prior reports were found
about PEC sensing of nitrates. As shown in Figure 5CD and S1,
the presence of interferents such as NO2, SO4
2, Ca2+, Cu2+, Cl,
Na+, Mg2+and HCO3was negligible, as expected for copper-
modified sensors.[48,49] Along interference effects, the stability of
the device was evaluated (Figure S2) as a key parameter for
environmental monitoring applications. Overall, this sensor
proved to be robust and reliable. The sensor’s response was
tested with real samples, analyzing commercial bottled water.
As reported in Table 2, EC and PEC measurements were very
consistent, reporting values which deviated less than 3.10 %
from the labeled ones. The matrix effects of the real samples
did not affect the performances of the sensor, with apparent
recovery factors ranging from 98.16 % to 101.69 % and from
98.03 % to 100.77 % for EC and PEC measurements, respectively.
Conclusions
In this work, a bimodal electrochemical-photoelectrochemical
sensor based on copper-modified g-C3N4was designed specifi-
cally for the determination of nitrates in an aqueous environ-
ment and was extensively compared with pristine g-C3N4and
Cu2O. Both electrochemical and photoelectrochemical charac-
terizations demonstrated superior performance with the Cu
doped g-C3N4, particularly at a 20 % copper loading. The 20 %wt
Cu/g-C3N4exhibited the best analytical performance for nitrate
determination, achieving Limit of Detection (LoD) values of 3.75
and 9.60 ppm with EC and PEC methods, respectively, without
interference from major matrix species. Real water sample
analyses demonstrated excellent apparent recovery factors and
an efficient cross-validation method, highlighting the efficacy of
the bimodal sensor.
Figure 4. (A) Photocurrent responses of different nanocomposites at 0 V in 0.1 M Na2SO4under intermittent 395 nm LED irradiation. (B) EC (cyclic voltammetry,
50 mVs1) and (C) PEC (chronoamperometry, 0 V vs Ag/AgCl) measurements recorded in Na2SO4in the presence of 100 μM NaNO3.
Table 1. Summary of the performances of other nitrate sensors present in the literature.
Method Sensing matrix pH/Cell purging Linear Range (ppm) LOD (ppm) Reference
LSV Copper nanowire 2.0/N2purging 1.24–155 1.48 [50]
CV Ag NPs/Au electrode 5.0/N2purging
5.0/N2purging
0.62–620 1.48
0.62
[27]
LSV Cu nanowires 2.0/N2purging 0.5–363 0.08 [48]
CA Pd NPs/epoxyCu 7.0/No purging 2–35 N.A [51]
LSV PdCu/SWCN SPE 2.0/No purging 6.2–124 3.23 [52]
CA CNTs/PPYs/NiR 7.5/N2purging 27–90 10 [53]
DPV Pd NCsPPy 7.0/No purging 6.2–49
55–86
0.05
0.03
[26]
LSV Inkjet-printed Ag 8.0/No purging 1–200 1.40 [54]
LSV-PEC Cu/g-C3N46.0/No purging 10–200 3.75 and 9.60 This work
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Experimental Section
Reagents
Melamine (99 %), Sulfuric Acid (98 %), Copper(II) nitrate
hemi(pentahydrate) (98 %), Acetic Acid (glacial, 99.7 %), Sodium
hydroxide (97 %), Poly(diallyldimethylammonium chloride) solution
(20wt. % in H2O), Sodium hydroxide (97.0 %), Ethanol (99.8 %),
Acetone (99.5 %), Sodium Sulfate (99.0 %), Sodium Nitrate ((
99.0 %), Potassium hexacyanoferrate(II) trihydrate (98.5–102.0 %)
and Potassium chloride (99.0–100.5 %) were purchased from Sigma-
Aldrich and used without further purification. Water for the
preparation of the solutions was doubly distilled (conductivity:
0.055 μS cm1) purified by a MilliQ apparatus.
Synthesis of g-C3N4Nanosheets
To synthesize bulk g-C3N4, thermal condensation of melamine was
used.[55] Briefly, 6 g of melamine were heated at 550 °C for 4 hours
with a temperature ramp of 3 °C min1. The obtained yellow
powder (1 g) was later exfoliated:[42] after one night of stirring in
30 mL of H2SO4, the solution was poured in 100 mL of water and
sonicated for 2 hours. The obtained white powder was separated
by centrifugation (9000 rpm, 10 min) rinsed with water to neutral
pH and dried at 60°overnight.
Figure 5. EC (Linear Sweep Voltammetry, 20 mV s1) and PEC (Chronoamperometry, 0 V vs Ag/AgCl) recorded with consecutive additions of NaNO3. The
corresponding calibration lines are reported as inset. (C) EC and (D) PEC response in presence of interferents, chronoamperometry response reported as inset.
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Synthesis of Copper Doped g-C3N4Nanosheets
Cu/g-C3N4nanosheets were prepared via in-situ precipitation
method. In a 100 mL flask, 0.1 g of the previously synthesized g-
C3N4was mixed with the proper amount of Cu(NO3)2·2.5H2O in an
aqueous solution and sonicated for half an hour. After the addition
of 100 μL of CH3COOH, the solution was heated to its boiling point.
Subsequently, 2 mL of a 1 mol.L1NaBH4solution was quickly
added to the suspension and reacted for 30 min, accompanied by a
large amount of dark precipitate. The resultant precipitates were
centrifuged (9,000 rpm, 10 min) and purified with water and
ethanol 3 times to get rid of the unbonded Copper. The materials
obtained were labelled as X wt.% Cu/g-C3N4, with X =5,10, 20, 30
and 40 where X denotes the nominal metal loading of copper. For
comparison, cuprous oxide was prepared by carrying out the same
method without adding g-C3N4.
Preparation of Cu/g-C3N4Modified FTO Electrode
As shown in Figure 1C, the sensor was prepared on a clean
Fluorine-doped Tin Oxide (FTO) glass slide (from Sigma-Aldrich).
The conductive glass slides (2×3 cm) were cleaned by sonication in
a acetone, ethanol, water and 10 % NaOH solution for 10 min each.
The supports were then dried with N2flux and the nanomaterial,
added in 100 mm3of PDDA 0.5 % solution, was spray-coated on
top. The sensor was later dried at 100 °C and stored at room
temperature.
Structural and Morphological Characterization
The shown FTIR spectra are the results of 64 accumulation recorded
in the range 4000–400 cm1with a resolution of 4 cm1. The
measurements were conducted on 1 mm2samples using a Nicolet
380 spectrometer with a Smart Orbit ATR accessory with a diamond
crystal.
TEM images were collected with a JEOL JEM 3010UHR microscope
operating at 300 kV. Before the analysis, the material was dispersed
in ethanol and deposited on 200 mesh Cu “holey” carbon grids.
Electrochemical Measurements
All the electrochemical measurements were recorded at room
temperature with a PGstat30 potentiostat/galvanostat (Autolab,
The Netherlands) equipped with FRA module and controlled with
NOVA 2.0 software. A standard 3 electrode setup was used, with
the modified FTO as WE (immersed area 2 cm2), saturated KCl Ag/
AgCl as RE and Pt wire as CE). Degassing, when working with
nitrates, was achieved bubbling N2for 15 min.
CV and LSV were conducted in 0.1 M Na2SO4as supporting
electrolyte, with a scan rate of 50 and 20 mV s1, respectively. EIS
measurements were conducted at 0.2 V between 0.1 Hz and
100 kHz, with 0.01 V amplitude. 5 mM [Fe (CN)6]3/4was added as
redox probe and KCl 0.1 M was selected as supporting electrolyte.
The obtained data was processed using Z-View 3.1.
PEC measurements were recorded at 0 V, irradiation was achieved
using a Thorlabs LED lamp (M395 L4) powered by a LEDD1B power
supply.
Acknowledgements
The authors acknowledge the Italian Ministry of Foreign Affairs
and International Cooperation (MAECI) for the grant awarded to
Wafa Aidli.
Conflict of Interests
The authors declare no conflict of interest.
Data Availability Statement
The data that support the findings of this study are available
from the corresponding author upon reasonable request.
Keywords: Sensor ·Nitrates ·Photochemistry ·Graphitic carbon
nitride ·Bimodal sensor
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Table 2. Nitrate detection in commercial bottled water using the proposed bimodal sensor.
Sample Spiked
Nitrate
(ppm)
LSV PEC
Found
(ppm)
Apparent
recovery
factor (%)
Relative
error %
Found
(ppm)
Apparent
recovery
factor (%)
Relative
error %
Levissima
1.3 ppm
0 – – – – – –
10 11.32 101.69 3.02 11.28 98.62 1.58
20 21.28 98.16 2.80 21.28 98.03 2.80
30 31.29 99.23 2.87 31.29 99.23 2.87
Uliveto
6.5 ppm
0 6.46 99.37 4.80 6.44 99.09 2.53
10 16.60 101.54 2.33 16.55 100.77 1.79
20 26.48 99.70 2.89 26.48 99.64 2.33
30 36.43 98.94 1.59 36.43 98.94 3.10
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Manuscript received: October 16, 2023
Revised manuscript received: December 21, 2023
Version of record online: October 24, 2024
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