Nitrogen-Doped Graphene Aerogel for Simultaneous Detection of Dopamine and Ascorbic Acid in Artificial Cerebrospinal Fluid - IOPscience
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Journal of The Electrochemical
Society
OPEN ACCESS
Nitrogen-Doped Graphene Aerogel for Simultaneous Detection of
Dopamine and Ascorbic Acid in Artificial Cerebrospinal Fluid
To cite this article: Veronika Urbanová et al 2020 J. Electrochem. Soc. 167 116521
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Journal of The Electrochemical Society, 2020 167 116521
Nitrogen-Doped Graphene Aerogel for Simultaneous Detection of
Dopamine and Ascorbic Acid in Artificial Cerebrospinal Fluid
Veronika Urbanová,z Štěpán Kment,z and Radek Zbořil
Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacký
University Olomouc, 783 71 Olomouc, Czech Republic
Detection of biological relevant analytes using inexpensive and affordable sensors is of high importance for further development of
personalized healthcare. Combining electrochemistry with sensing platforms based on new emerging nanomaterials is of immense
interest within the scientific community because such connection could bring amelioration in term of sensitivity and possible
miniaturization to traditional sensors. Herein, nitrogen-doped graphene aerogel (N-GA) was utilized for detection of biologically
important analytes, dopamine and ascorbic acid. Both analytes revealed good linear relationship between current peaks and
concentration with detection limit of 0.06 and 0.08 μM for dopamine and ascorbic acid, respectively. Moreover, N-GA was also
applied for simultaneous detection of dopamine along with ascorbic acid in artificial cerebrospinal fluid. In this case, the limit of
detection for dopamine reached value of 0.42 μM.
© 2020 The Author(s). Published on behalf of The Electrochemical Society by IOP Publishing Limited. This is an open access
article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/
by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/
1945-7111/aba6c3]
Manuscript submitted May 6, 2020; revised manuscript received June 29, 2020. Published July 27, 2020. This paper is part of the
JES Focus Issue on 2D Layered Materials: From Fundamental Science to Applications.
Supplementary material for this article is available online
Graphene, a single atom thick layer of sp2-hybridized carbon, has are also desirable for the construction of biosensors and gas-sensing
emerged extensive multidisciplinary research efforts due to its devices.24–27
unique structure and outstanding properties,1–4 i.e. specific large Apart from morphology control, chemical doping is another
area, high speed electron mobility, thermal conductivity and important and effective method to tailor the electrical properties of
electrocatalytic activity. Therefore, graphene and graphene deriva- graphene and thoroughly discussed in review articles.6,9,28,29
tives are considered as promising candidate for various applications, Generally, there are two ways to achieve such doping: (1) surface
including energy conversion and/or storage, electro-catalysis, sen- transfer doping30 that occurs through charge transfer from adsorbed
sors or electronics.5,6 Besides, great efforts are still ongoing to dopant to graphene or (2) substitutional doping28 referring to the
uncover their full potential via controllable tailoring of their substitution of carbon atoms of graphene by atoms with different
properties, composition and structure.7–10 number of valence electrons. Both doping mechanisms can lead to a
The most critical limitation encounter practical use of graphene shift of the Fermi level above or below the Dirac point, called n-type
is the irreversible aggregation or restacking of individual sheets due doping with an excess of electrons or p-type doping with an excess
to the strong van der Waals interactions and/or high inter-sheet of holes.31 Such deliberate introduction of dopants into graphene
junction contact resistance.11 Hence, superior properties of graphene could tailor its electronic band structure, which is of great
(i.e., intrinsically high conductivity, mechanical strength and acces- technological importance for applications in sensor devices, nanoe-
sible surface area) are severely suppress. However, it has been lectronics, nanophononics and green energy technology.32,33
shown that assembling of 2D graphene sheets into three-dimensional Among others, nitrogen belongs to the most studied dopants for
(3D) architectures might overcome this limitation.12,13 Maintaining graphene.6,34 When a nitrogen atom is doped into graphene, it
intrinsic properties of graphene in the bulk thus enhance its usually has three common bonding configurations (i.e. pyrrolic N,
applicability for practical applications. Moreover, such 3D structures quaternary N and pyridinic N) within the two C atoms at the edges or
possess also rich pore network as well as multidimensional electron defects of graphene.6,35 So far, nitrogen doped graphene showed
transport pathway.14 enhanced electrocatalytic properties toward oxygen reduction reac-
Graphene gels are usually produced using hydrothermal and tion (ORR)36 and improved sensing performance37 for H2O2 and
solvothermal reduction strategies without introducing any other glucose,38 nicotine,39 bisphenol A,40 methyl parathion41 or hydro-
chemicals or further purification treatment. In addition, these quinone together with catechol.42
processes are compatible with synthesis of many functional mate- In this work, we report on exploration of nitrogen doped
rials allowing convenient incorporation of a variety of secondary graphene aerogel (N-GA) for simultaneous determination of dopa-
components into the 3D graphene framework.12,15 Among them, mine and ascorbic acid. N-GA was prepared by hydrothermal
graphene aerogels (GAs) have received considerable attention since synthesis using graphene oxide and ethylenediamine with subse-
they have great potential in many fields.16,17 They are usually quent freeze-drying in order to obtain graphene-based aerogel. As
produced via sol-gel chemistry which involves reducing graphene prepared material was then used for modification of glassy carbon
oxide (GO) to form a highly cross-linked graphene hydrogel (GH), electrode that were employed for determination of dopamine (DA)
followed by freeze-drying or supercritical drying in order to remove and ascorbic acid (AA) as they both have significant relevance for
adsorbed water.18 Such GAs resulting in materials with large pore human body. Dopamine is known as a neurotransmitter in the human
volumes that enable fast mass transfer and higher electric conduc- brain that is responsible for motor and cognitive control. Among
tivities facilitating faster charge transport of the redox species across other, dopamine is responsible for our mood or attention. Ascorbic
the surface.15,19 These features can be advantageously employed in acid, i.e. vitamin C is an antioxidant that is involved in maintenance
the design of highly efficient counter electrodes for dye-sensitized of diverse neuro-physiological processes in human body including
solar cells (DSSCs)20 and energy storage devices.17,21–23 Alongside, for example synthesis of catecholamine or wound healing. In
GAs determined by ultralow-mass densities and large surface areas addition, ascorbic acid is significant interferent for dopamine
determination in human body. Dopamine level in human body could
also be taken as clue indicator of many neurological diseases/
z
E-mail: urbanova.sci@gmail.com; stepan.kment@upol.cz disorders (e.g. schizophrenia, Parkinson´s disease or ADHD) and
Journal of The Electrochemical Society, 2020 167 116521
thus development of cheap and selective sensors is of high recorded on a DXR Raman microscope (Thermo Scientific) using
importance for personal healthcare. Our proposed electrochemical the 532 nm excitation line of a diode laser.
sensor based on N-GA revealed good analytical performance with
low detection limits and capability of high selectivity even within the Electrochemical measurements.—Glassy carbon electrodes
complex matrix. (GCEs, 3 mm in diameter, 2Theta, Czech Republic) were polished
on wet silicon carbide paper using alumina 1 and 0.05 μm Al2O3
Experimental powder sequentially and then washed in ethanol followed by
distilled water. The GCEs were then modified with GA or N-GA
Chemicals.—Graphite flakes, NaNO3, H2SO4, KMnO4, H2O2,
aerogel samples by drop coating: 10 μl of water dispersion
HCl, monobasic potassium phosphate, dibasic potassium phosphate,
(1.5 mg ml−1) was drop onto GCE surface and allowed to dry at
ascorbic acid, dopamine, NaCl, KCl, MgSO4, KH2PO4, NaHCO3,
room temperature.
glucose, sucrose, CaCl2 and ethylenediamine were purchased
All electrochemical measurements were performed using a
from Sigma-Aldrich and used without further purification. The
PGSTAT128N potentiostat (Metrohm Autolab B.V.) monitored by
conductivity of deionized water used within this work was below
NOVA software. A conventional three-electrode cell configuration
15 μS cm−1.
was employed—modified glassy carbon electrodes (GCE) were used
as working electrodes, with a saturated Ag/AgCl (2Theta, Czech
Synthesis of nitrogen doped graphene aerogel (N-GA).—
Republic) and a platinum wire as reference and counter electrode,
Graphene oxide (GO) was prepared by chemical oxidation and
respectively. All experiments were performed at room temperature.
exfoliation of graphite under acidic condition according to modified
Hummers’ method.43 Nitrogen doped graphene aerogel (N-GA) was
Results and Discussion
then prepared by hydrothermal assembly of GO and ethylenediamine
subsequently combining with freeze-drying. In a typical experiment, Characterization and morphology.—In this work, a hydro-
68 mg GO was dispersed in deionized water (34 ml) and exposed thermal assembly of graphene oxide (GO) and ethylenediamine
to ultrasound for 45 min in order to obtain a uniform suspension. with subsequent freeze-drying was adopted for nitrogen doped
Then 200 μl ethylenediamine (EDA) was added to as-prepared GO aerogel (N-GA) preparation. As shown in Fig. 1A, the XRD pattern
dispersion and stirred thoroughly. Stable mixture was sealed in a of GO (red curve) exhibits a strong characteristic peak at 13.1°
Teflon autoclave and hydrothermally treated at 180 °C for 12 h to assign to the introduction of oxygen-containing groups along with its
form N-doped graphene hydrogel. Then, the as-prepared hydrogel vanishing after reduction process and hydrothermal treatment as
was lyophilized to gain aerogel (N-GA). For comparison, graphene shown for GA (black curve) and N-GA (blue curve). Contrary, one
aerogel (GA) without nitrogen was also prepared following the can observe new broad peak emerged at 28.4° for both GA and N-
same synthetic procedure mentioned above without addition of GA corresponding to the reduction of GO and recovery of graphitic
ethylenediamine. structure. Raman spectroscopy is another conventional way to
characterize the structural changes of GO during the hydrothermal
Characterization techniques.—Morphology of the samples were process by comparing G and D bands since G band corresponds to
investigated by scanning electron microscopy (SEM, HITACHI SU in-plane bond-stretching motion of the pairs of sp2 carbon atoms,
6600 microscope) and transmission electron microscopy (TEM, whereas D band stems from the breathing mode of the sp2 ring of the
JEOL 2010F microscope operated at 200 kV). X-ray diffraction graphene layer, which is relative to the defects such as bond angle
(XRD) patterns were measured on a powder X-ray diffractometer disorder, bond length disorder and hybridization.43 In Fig. 1B one
PANalytical X’Pert PRO MPD (PANalytical, The Netherlands) can observe similar position of D bands whereas G band was slightly
diffractometer in the Bragg-Brentano geometry, Co-Kα radiation shifted comparing GO, GA and N-GA samples. ID/IG ratios were
(40 kV, 30 mA, λ = 0.1789 nm) equipped with an X’Celerator estimated to be 0.96, 1.11 and 1.10 for GO, GA and N-GA,
detector and programmable divergence and diffracted beam anti- respectively. Increasing ID/IG ration accompanied with upper G-
scatter slits. X-ray photoelectron spectroscopy (XPS) was carried out band shift when going from GO to N-GA indicating, as expected,
using a PHI VersaProbe II spectrometer using an Al Kα source restoration of sp2 graphitic sheets as a result of nitrogen
(15 kV, 50 W). All spectra were measured in a vacuum of 1.4 × functionalization.44
10−7 Pa and at room temperature. The XPs spectra were evaluated Further, X-ray photon spectroscopy (XPS) analysis was per-
with MultiPak (Ulvac-PHI, Inc.) software. All binding energies were formed in order to understand the nitrogen bonding within the N-GA
referenced to the C 1s peak at 284.80 eV. Raman spectra were sample. The survey spectrum (see Fig. 2A) revealed presence of C
Figure 1 (A) X-ray diffraction patterns and (B) Raman spectroscopy for GO (red curve), GA (black curve) and N-GA (blue curve).
Journal of The Electrochemical Society, 2020 167 116521
Figure 2. XPS of N-doped graphene aerogel (N-GA). (A) Survey spectrum, (B) high resolution C 1s spectrum, (C) high resolution N 1 s spectrum and (D) high
resolution O 1 s spectrum.
1s, N 1s and O 1s as illustrated by the main peaks at 286, 399.5 as observed in cyclic voltammetry, i.e. the highest for N-GA (3.72 ×
and 532 eV, respectively. The high-resolution N 1s spectrum 10–3 cm s−1) followed by bare GCE (1.91 × 10−3 cm s−1) and GA
(Fig. 2C) confirmed the presence of pyridinic (398.76 eV), pyrrolic (1.83 × 10−3 cm s−1). Hence, own to the interesting inherent
(400.06 eV) and graphitic (401.53 eV) nitrogen and thus one can electrochemical properties, nitrogen doped graphene aerogel met
assume successful incorporation of nitrogen into the graphitic essential criteria to be applied as platform for further analytical
structure. The C 1s spectrum (Fig. 2B) showed indicative peaks at application.
284.76, 285.61, 286.36 and 287.93 eV corresponding to C–C sp2 Since the main target of this work was employment of N-GA for
hybridization, C–C sp3 hybridization, C–N and C=O, respectively. simultaneous electrochemical detection of dopamine and ascorbic
The oxygen functionalities found in O 1s spectrum (Fig. 2D) were acid, first of all several optimization steps were performed in order to
assigned to C=O (533.09 eV) and C–O (531.31 eV). XPS analysis obtain best condition for this purpose as shown in Fig. 5. The
of graphene oxide used as starting material for the N-GA synthesis is influence of pH was studied regarding the current peak and potential
shown in Fig. S1 (available online at stacks.iop.org/JES/167/116521/ of dopamine (DA) and ascorbic acids (AA). As shown in Fig. 5A,
mmedia). The survey spectrum revealed mainly presence of carbon the highest current intensity for dopamine was observed when using
and oxygen as expected. Trace amount of sulphur that can be phosphate buffer of pH 6 whereas ascorbic acid revealed better
observed in the survey is the contamination assign to the synthetic current value in pH 5. Since the main issue of determination of
process. dopamine in complex matrices such as blood arising from the
Finally, the morphology of nitrogen-doped graphene aerogel after presence of other interfering, in particular ascorbic acid, it is of the
freeze-drying process was studied using SEM and TEM (Fig. 3). As highest importance to find sensing platform that enable simultaneous
can be seen from SEM images (Figs. 3A, 3B), N-GA evinced 3D electrochemical detection of both analytes with their sufficient
framework consists of interconnected open pores as is typical for separation. For this reason, next step in optimization was to find
graphene aerogels. TEM images (Figs. 3C,3D) showed presence of out how the pH affected the separation of DA and AA using N-GA.
transparent and thin graphene nanosheets that eventually overlap- The measurements revealed (Fig. 5B) that reasonable separation
ping and resulting in wrinkled structure. could be reached at pH 6, 7 and 8 when the difference between
potential was ca. 200 mV. Considering both current intensity and
Electrochemical behavior.—Electrochemical properties of ni- separation of the DA and AA, pH 6 was optimal for sensing. On the
trogen doped graphene aerogel (N-GA) were first studied by mean of other hand, the situation could change in the case of simultaneous
cyclic voltammetry in order to determine its electron transfer detection, because as already mentioned, both analytes were inter-
properties (Fig. 4). Measurement in 0.1 M KCl containing fered with each other and so their potentials could differ. The
[Fe(CN)6]3−/4− redox probe revealed peak-to peak separation of simultaneous detection is shown in Fig. 5C clearly demonstrated
112 mV that was better than those obtained with bare glassy carbon well separation in all pH values while the highest current intensity of
electrode (GCE, 161 mV) or undoped graphene aerogel (GA, both analytes was observed for pH 6 and thus it was considered as
164 mV). These results indicated improvement in electron transfer optimal for further experiments. Figure 5D demonstrates possibility
behavior after nitrogen introduction within the graphene aerogel of detection of increasing DA concentration (0, 90 and 210 μM) in
structure as also confirmed by calculation of heterogenous electron the buffer solution containing 1 mM AA illustrating thus common
0
transfer (HET) rate constant (kobs ) by adoption of Nicholson’s situation for the detection in real medical samples. One can clearly
equation.45 Estimated HET rate constants followed the same trend observed two distinguish peaks of analytes with proportionally
Journal of The Electrochemical Society, 2020 167 116521
Figure 3. (A), (B) SEM images and (C), (D) TEM images of nitrogen doped graphene aerogel (N-GA).
assigned to better intrinsic electrochemical properties, i.e. faster
electron transfer, of N-GA over GA. The peak potential position of
both analytes also changed and they found to be at less positive
potential values in the case of N-GA (Fig. S2 B). When investigating
the ability of simultaneous detection of DA and AA by the mean of
cyclic voltammetry using GA (Fig. S2D), one observed rather broad,
not well-defined peaks. Moreover, when increasing DA concentra-
tion, both peaks are increasing that strongly indicates that the
voltammetric separation of analytes was poor and somehow over-
lapping during simultaneous detection.
The calibration study for both, DA and AA using square wave
voltammetry (SWV) is shown in Figs. 6A, 6C and clearly demon-
strates good linear response for both target molecules. The linear
regression equation for dopamine was IDA = 3.48 + 0.43cDA with
correlation coefficient R2 = 0.992 (Fig. 6B) in the concentration
range of 1–100 μM, while for the ascorbic acid it was equal to IAA =
1.65 + 0.01cAA with correlation coefficient R2 = 0.997 (Fig. 6D) in
the range of 100–1000 μM. The limit of detection (LoD) were
Figure 4. Typical cyclic voltammetry responses recorded with bare GCE estimated to be 0.06 μM and 0.08 μM for dopamine and ascorbic
(black) and GCE modified with graphene aerogel GA (blue) or nitrogen- acid, respectively. LoD were calculated from calibration curves on
doped graphene aerogel N-GA (red). All measurements were performed in the basis of 3.3xSD/S, where S is slope of calibration curve and SD
0.1 M KCl containing 5 mM [Fe(CN)6]3−/4− at scan rate 100 mV s−1. is standard deviation of the response. For the practical point of view,
limit of quantification (LoQ) were also estimated to be 0.2 μM for
increasing current intensity for increasing concentration of added dopamine and 0.24 μM for ascorbic acid. Table I summarized some
dopamine while current peak of AA stayed stable since its of the sensors for dopamine and/or ascorbic acid based on different
concentration was not changed within the experiment. For further carbonaceous sensing platforms, including graphene aerogel, gra-
emphasis of detection sensitivity of N-GA, same tests were phene oxide or carbon fibres. The detection limits in these cases
performed and evaluate with undoped graphene aerogel (GA) as varied from nM to μM range and one can assume that these
shown in Fig. S2. On can clearly conclude that GA provide lower variations would arise mainly from the porosity and thus surface
current peaks for both, DA and AA, under the same experimental area of such sensing platforms.
condition compared to N-GA (Fig. S2A). In the pH 6 that was Finally, in order to underline utility of N-GA as platform for
considered as optimal, N-GA provided seven times higher current detection of dopamine in the presence of ascorbic acid in complex
response toward dopamine while response toward ascorbic acid was matrices, artificial cerebrospinal fluid (aCSF) was used to mimic
five time higher (see Fig. S2C). Such higher sensitivity could be vital environment—aCSF is complex buffer solution containing
Journal of The Electrochemical Society, 2020 167 116521
Figure 5. Peak current (A) and peak potential position (B) of 1 mM dopamine (DA) and 1 mM ascorbic acid (AA) in 0.1 M phosphate buffer of different pH
values. (C) Square wave voltammetry (SWV) curves of simultaneous determination of 1 mM DA and AA at different pH: pH 5 (black), pH 6 (red), pH 7(blue)
and pH 8 (magenta). (D) Cyclic voltammetry responses in 0.1 M phosphate buffer (pH = 6) containing 1 mM AA and increasing concentration of DA. All
measurements were recorded using GCE modified with N-GA.
Table I. Electrochemical detection of dopamine and ascorbic acid based on different sensing platforms.
Sensing platform Target molecule Linear range LoD References
N-doped graphene aerogel DA 1–250 M 0.1 μ M 46
N-doped graphene DA 5 × 10–7–1.7 × 10–4 M 2.2 × 10–7 M 47
AA 5 × 10–6–1.3 × 10−3 M 2.2 × 10–6 M
3D N-doped graphene DA 3 × 10−6–1 × 10−4 M 1 nM 48
N-doped rGO DA 1–60 μM 0.1 μ M 49
AA 0.1–4 mM 9.6 μM
MWCNTs spaced graphene aerogel DA 5 nM to 20.0 μM 1.67 nM 50
N-doped carbon fibres AA 50–3000 μM 50 μM 51
DA 1–10 μM, 10–200 μM 0.5 μM
graphene DA 4−100 μM 2.64 μM 52
graphene oxide DA 1–15 μM 0.27 μM 53
N-doped graphene aerogel DA 1–100 μM 0.06 μM this work
AA 100–1000 μM 0.08 μM
119 mM NaCl, 26.2 mM NaHCO3, 2.5 mM KCl, 1 mM NaH2PO4, 0.42 μM with corresponding LoQ of 1.3 μM. These data clearly
1.3 mM MgCl2 and 10 mM glucose. Typical behavior of such demonstrated capability of N-GA for sensitive dopamine detection
measurements is shown in Fig. 7. The measurements were recorded even in the presence of different interferences.
with GCE modified with N-GA in aCSF containing fixed concentra-
tion of ascorbic acid (250 μM) with subsequent additions of
Conclusions
dopamine in the concentration range from 12.5 μM to 200 μM.
One can observe two well defined peaks at ca. 40 and 200 mV Herein, nitrogen-doped graphene aerogel (N-GA) was prepared
representing peaks of ascorbic acid and dopamine, respectively. via hydrothermal assembly of graphene oxide (GO) and nitrogen
Under the continuous dopamine addition, the current peak assigned precursor represented by ethylenediamine (EDA) with subsequent
to dopamine linearly increased while the current peak of ascorbic freeze-drying process in order to obtain porous aerogel like structure
acid stayed unchanged. In this case, the linear regression equation of final material. Successful nitrogen incorporation within the
for dopamine was IDA = 11.3 + 0.26 cDA with correlation graphene lattice was confirmed by X-ray photon spectroscopy,
coefficient R2 = 0.988. LoD for dopamine determination according X-ray diffraction and Raman spectroscopy. Further N-GA revealed
to the observed calibration curve (see Fig. S3) was found to be better electrochemical behaviour compare to its undoped counterpartJournal of The Electrochemical Society, 2020 167 116521
Figure 6. Square wave voltammetry (SWV) recorded with GCE modified with N-GA for increasing concentration of dopamine (A) and ascorbic acid (C) with
respective calibration curves (B), (D). All measurements were performed in phosphate buffer (pH 6.0). SWV conditions: step potential 5 mV, modulation
amplitude 20 mV, frequency 25 Hz.
potential interest of such nanomaterial for healthcare practice is
demonstrated by its operational ability of dopamine determination in
matrix containing usual interferences that occur in physiological
environment.
Acknowledgments
V.U. acknowledge the financial support from Czech Science
Foundation (Project GACR no. 17-22194Y). A. Stuchlá (Palacky
University) is gratefully acknowledge for the material preparation.
ORCID
Veronika Urbanová https://orcid.org/0000-0003-4499-2235
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