Electrodeposition of nickel nanoflowers on screen-printed electrodes and their application to non-enzymatic determination of sugars

Article abstract of DOI:10.1039/c6ra15578b

In this work, the electrodeposition of nickel on screen-printed carbon electrodes was carried out. As the main novelty, a galvanostatic electrodeposition methodology (application of a constant current for a specific time) was chosen to perform the electrodeposition from a Ni(ii) solution. Interestingly, these conditions were able to generate nickel nanoflowers of 160 nm all over the surface. The nickel nanoflowers showed a great electrocatalytic effect towards the oxidation of reducing sugars. After the characterization of the electrode surface and the optimization of the experimental conditions, the non-enzymatic electrochemical device was employed for the determination of reducing sugars. A linear range of 25-1000 μM was obtained, showing good performance for the determination of sugars at low concentrations. The reproducibility was 5.5% (intraelectrode) and 6.9% (interelectrode), indicating a high precision using the same or different devices. After fabrication, the electrode is stable at least for 35 days, even using the same device to carry out measurements on different days. Real food samples such as honey and orange juice were also evaluated with the nickel nanoflower electrochemical device.

Full text of DOI:10.1039/c6ra15578b

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Electrodeposition of nickel nanoflowers on screenꢀprinted
electrodes and its application to nonꢀenzymatic determination of
sugars
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Beatriz Pérez-Fernández, Daniel Martín-Yerga* and Agustín Costa-García
Nanobioanalysis group
Department of Physical and Analytical Chemistry
University of Oviedo
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* Corresponding author: Daniel MartínꢀYerga
Nanobioanalysis group
Department of Physical and Analytical Chemistry
University of Oviedo
8 Julián Clavería St., Oviedo 33006 (Spain)
Eꢀmail: martindaniel@uniovi.es
Telephone: (+34) 985103486
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ABSTRACT
In this work, the electrodeposition of nickel on screenꢀprinted carbon electrodes was carried
out. As the main novelty, a galvanostatic electrodeposition methodology (application of a
constant current for a specific time) was chosen to perform the electrodeposition from a Ni(II)
solution. Interestingly, these conditions were able to generate nickel nanoflowers of 160 nm
all over the surface. The nickel nanoflowers showed a great electrocatalytic effect towards the
oxidation of reducing sugars. After the characterization of the electrode surface and the
optimization of the experimental conditions, the nonꢀenzymatic electrochemical device was
employed for the determination of reducing sugars. A linear range of 25ꢀ1000 µM was
obtained, showing good performance for the determination of sugars at low concentrations.
The reproducibility was 5.5% (intraelectrode) and 6.9% (interelectrode), indicating a high
precision using the same or different devices. After the fabrication, the electrode is stable at
least for 35 days, even using the same device to carry out measurements on different days.
Real food samples such as honey and orange juice were also evaluated with the nickel
nanoflowers electrochemical device.
KEYWORDS: Nickel nanoflowers, Sugars, Electrodeposition, Nonꢀenzymatic detection,
Screenꢀprinted electrodes
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1. INTRODUCTION
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Glucose determination is a constant concern for the scientific community due to that the
diagnostic of diabetes mellitus disease has increased in the last years, and over 552 million
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patients are estimated to have this disease in 2030 . Furthermore, the determination of sugars
in food, most importantly, glucose and fructose, has a great interest for the food industry in
order to evaluate the nutritional information or to control the quality of the production. The
most employed techniques for sugar determination are refractometry, densitometry, titration
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and for speciation analysis, high performance liquid chromatography . These techniques have
some disadvantages compared to electrochemical detection such as the bulky and more
expensive instrumentation, large sample consumption and, generally, trained personnel is
necessary to carry out the analyses. Electrochemical sensors have been widely employed for
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,4
glucose determination, specially using enzymeꢀmodified electrodes . This kind of sensors
have some drawbacks such as the need to control exhaustively the experimental conditions
such as pH or temperature. Changes in these conditions may cause a change in the stability of
the enzyme and, therefore, in the performance of the sensor. For these reasons, the
development of nonꢀenzymatic devices for glucose determination is a relentless subject of
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study . Many nanomaterials have shown a strong catalytic effect towards sugars oxidation
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such as Pt, Cu or Au . Currently, these nonꢀenzymatic nanostructured electrodes are widely
used in the food industry, particularly in the area of food safety, traceability and quality
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control, as they provide low detection limits and high stability . Nanostructured electrodes
have several advantages such as providing a more active surface, enhancing the electron
transfer and may show catalytic properties towards different electrochemical reactions. For
instance, different nickel nanomaterials have shown a catalytic effect towards the oxidation of
reducing sugars. Nickel nanoparticles seems to be the most employed for the nonꢀenzymatic
determination of sugars using different strategies such as the addition to a carbon paste
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electrode , embedded in a chitosan membrane or electrodeposited on boronꢀdoped diamond
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electrodes . Nickel nanowires have also shown a strong catalytic effect and have been
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employed for sugar determination coupled to a glassy carbon electrode , to disposable
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electrodes or in a nanowire array strategy . However, nickel nanoflowers (NiNFs) have not
been used as broadly for electroanalytical applications, and instead, they have shown
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interesting features for batteries or supercapacitors . However, the high surface area of
these flowerꢀlike nanoparticles could lead to promising analytical applications. Nonꢀ
enzymatic glucose electrochemical devices based on nickel oxide or hydroxide nanoflowers
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have been previously published , but the complex synthesis of the nanoflowers by a
hydrothermal method and also the complex modification of the electrode surface, does not
allow the fast generation of simple, small and lowꢀcost electrochemical devices. For instance,
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Ibupoto et al. synthesized NiO nanoflowers after growing Ni(OH)
alkaline media for 4ꢀ6 hours at 98 ºC and annealing for 2ꢀ3 hours at 450 ºC. Yang et al.
synthesized Ni(OH) nanoflowers by heating at 45 ºC for 2 hours an ammoniacal solution of
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on a gold substrate in
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nickel hexaammine. The product was washed and dried for one day. Electrode modification
was carried out by dropꢀcasting using a dispersion obtained after mixing the nickelꢀbased
product with carbon nanotubes in a solution of Nafion in ethanol by ultrasonication.
Therefore, the development of an easier synthesis and modification of nickel nanoflowers on
electrodes would be a very interesting methodology to study the performance of this nickel
nanomaterial for electroanalytical applications.
Screenꢀprinted electrodes are devices comprising a 3ꢀelectrode electrochemical cell on a small
card, which can be mass produced reducing the fabrication cost and generating a reproducible
disposable surface. The lowꢀcost, small size and the integrated electrochemical cell are ideal
characteristics of these electrodes, which result in a very convenient platform for sensing
devices. Nickelꢀbased screenꢀprinted electrodes (SPEs) have also been employed for sugars
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determination. For instance, García et al. employed SPEs modified by dropꢀcasting with
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nickel nanowires and the fabricated devices were used for the determination of total
carbohydrates in several food samples. The good stability of the modification allowed them to
use the devices in a flowꢀinjection analysis system for semiꢀautomatic detection. In a similar
work, the same authors evaluated the possibility to use nickelꢀcopper nanowires, but they
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found that the nickel nanowires were most suitable as the fabrication was simpler . Several
nickelꢀcarbon composites have been reported as useful materials for the modification of the
SPE surface in order to determine glucose in different samples. A Ni/nanoporous carbon
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composite has been employed for the modification of the working screenꢀprinted electrode.
The nanoporous carbon material has a high surface area, which increases significantly the
available electrode area. In other work, a composite formed by graphene oxide, chitosan and
Ni(II) was simultaneously electrodeposited by multiple cathodic cyclic voltammetry on the
SPE surface generating an interesting structure composed by reduced graphene oxide,
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chitosan and nickel nanoparticles . Nickel paste have also been mixed with carbon ink in
order to obtain a material appropriate for screenꢀprinting the working electrode on a ITO
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substrate . This way, nickelꢀbased devices are fabricated directly and the modification of the
electrode surface is not necessary, although the electrochemical activation of the working
electrode is still required. A hybrid NiꢀCo hydroxide material was simultaneously deposited
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on the surface of SPEs as reported in Lien et al. work . The addition of Co seems to decrease
the potential needed for the detection of glucose, although severe interferences by other
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species is found. An interesting device is reported by Niu et al. . Electrodeposition of nickel
is performed in severe conditions (0.2 M Ni(II), 1 M H SO ) applying a high current to the
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electrode (0.1 A for 30 s). In these conditions, nickel is electrodeposited on the electrode
surface while a great amount of hydrogen bubbles is generated and a threeꢀdimensional
porous nickel structure is created on the electrode surface. Although, promising analytical
characteristics are found, only the working electrode is a small, portable screenꢀprinted
electrode, and the system uses a conventional electrochemical cell with conventional auxiliary
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and reference electrodes, decreasing its usefulness for in situ analysis. As SPEs typically have
a solid quasireference electrode, in certain experimental conditions, applying a constant
potential or a potential sweep (potentiostatic/potentiodynamic methods) may not be the best
choice for the electrodeposition of nanomaterials. Electrochemical reactions occurring in the
electrodeꢀsolution interface are processes strongly affected by the electrode surface, and a
small change of the electrode surface may cause a big change in the electrochemical
reactivity. A good alternative is the electrodeposition by a galvanostatic method (application
of a constant current), since, in this case, the electrodeposition is controlled by the current
flowing between the working and counter electrodes. This methodology has already been
successfully applied for gold nanoparticles electrodeposition with a high control of size and
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density or for the reduction of graphene oxides on SPEs
. As far as we know, studies about
the electrodeposition of nickel nanoflowers on screenꢀprinted electrodes using a galvanostatic
method have not been reported.
In this work, we carried out the electrodeposition of nickel on the surface of screenꢀprinted
carbon electrodes. As a novelty, a galvanostatic electrodeposition method was employed,
which allows a fast and simple generation of nickel nanoflowers on screenꢀprinted electrodes.
The screenꢀprinted electrodes modified with nickel nanoflowers (NiNFSPEs) were employed
for the nonꢀenzymatic determination of reducing sugars.
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2.1. Apparatus and electrodes
Electrochemical measurements were performed with an Autolab PGSTAT10
potenciostat/galvanostat controlled by GPES 4.9 software. All measurements were carried out
at room temperature. Screenꢀprinted carbon cards (Ref. DRPꢀ110) were purchased from
DropSens (Spain). Each card is formed by a 3ꢀelectrode electrochemical cell with carbonꢀ
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based working and counter electrodes, whereas quasireference electrode and electric contacts
are fabricated in silver. The diameter of the working electrode was 4 mm. Screenꢀprinted
electrodes were connected to the potentiostat through a specific connector, DRPꢀDSC. A
JEOL 6610LV scanning electron microscope was used to characterize the electrodes modified
with the nickel nanoflowers. Xꢀray photoelectron measurements were performed on a SPCEs
Phoibos 150/MCDꢀ5 spectrometer, using monochromatic Al Kα excitation source with an
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energy of 1486.74 eV. The survey and highꢀresolution spectra were collected with 90 eV and 30 eV of
pass energy and 1 eV and 0.1 eV of step energy, respectively.
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2.2. Reagents and solutions
Glucose, absolute ethanol, sodium hydroxide and sodium chloride were purchased from
Merck. Fructose, xylose, arabinose, mannose, galactose, glycerol, ascorbic acid, lactic acid,
citric acid and nickel(II) sulfate hexahydrate were purchased from SigmaꢀAldrich. Ultrapure
water obtained with a Millipore Direct Q5 purification system from Millipore Ibérica was
used throughout this work. All other reagents were of analytical grade. Working solutions of
Ni(II) were prepared in 0.1 M NaCl. Working solutions of sugars and interfering species were
prepared in 0.1 M NaOH.
2.3. Electrode modification with nickel nanoflowers
Nickel nanoflowers were electrodeposited on screenꢀprinted carbon electrodes by a
galvanostatic method. 40 µL of a 10 mM Ni(II) solution was dropped in the electrochemical
cell and a constant current of ꢀ25 µA was applied for 60 s. Ni(II) is reduced to Ni(0), which is
deposited on the carbon surface. In contact with air, the surface of the deposited Ni(0) is
spontaneously oxidized (passivated) to Ni(II) oxides and hydroxides. In order to stabilize the
nickelꢀmodified surface, a pretreatment applying 50 cycles of cyclic voltammetry from +0.2
V to +0.7 V in a 0.1 M NaOH solution was performed (100 mV/s).
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2.4. Chronoamperometric measurements
Chronoamperometric measurements for the determination of sugars were carried out by
applying a constant potential of +0.6 V for 100 s. The current measured at 100 s was chosen
as the analytical signal.
2.5. Sample preparation
For the orange juice samples, 1 µL of sample was diluted in 1 mL of 0.1 M NaOH solution.
Several samples with different added amounts of glucose were prepared in order to carry out
the determination by the standard additions method.
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For the honey samples, 1.0020 ± 0.0001 g of honey is diluted in 50 mL of H O. Then, 100 µL
of this solution is diluted 1:400 in 0.1 M NaOH. Several samples with different added
amounts of glucose were prepared in order to carry out the determination by the standard
additions method.
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3.1. Characterization of nickel nanoflowersꢀmodified screenꢀprinted electrodes
The electrodeposition of nickel on screenꢀprinted carbon electrodes was studied using cyclic
voltammetry. Figure 1 shows the voltammograms for different concentration of Ni(II) (0, 10
and 20 mM) in a 0.1 M NaCl solution. For all cases, two cathodic processes are observed, a
process with a peak potential at about ꢀ1.0 V, which it is attributed to the oxygen reduction
(also observed in the blank solution), and a process with a peak potential at ꢀ1.25 V, attributed
to the Ni(II) to Ni(0) reduction. The backward curve crossed the forward curve as it is
typically found for an electrodeposition process. Therefore, the nickel electrodeposition can
be carried out under these experimental conditions using screenꢀprinted electrodes. However,
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as mentioned in the introduction, the screenꢀprinted cards have a silver quasireference
electrode, and the applied potential could be slightly different for different electrodes (or it
could shift in the same measurement). Surface processes such as electrodeposition can be very
sensitive to these potential changes, and therefore, different amounts/density of metal may be
electrodeposited using a potentiostatic/potentiodynamic method. In order to minimize these
issues, a galvanostatic method was chosen for the electrodeposition of nickel on the screenꢀ
printed electrode surface. A comparison between the galvanostatic and the potentiostatic
methods is presented in the Supporting Information, under similar electrodeposition
conditions. Although a comparable electrocatalytic effect observed, a slightly better
reproducibility is found for blank solution using the galvanostatic method, even carrying out
the electrodeposition in mild conditions for the silver quasireference electrode (0.1 M NaCl
aqueous solution).
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[FIGURE 1]
Therefore, the nickel electrodeposition was performed by applying a constant current of ꢀ25
µA for 60 s on a 10 mM Ni(II) solution in 0.1 M NaCl, and the modified electrode was
characterized. The chronopotentiogram obtained for the galvanostatic electrodeposition is
shown in the Figure S1. The potential taken by the electrode at the beginning is about ꢀ0.75
V. This potential is not kept constant and varies quickly until a value of ꢀ0.94 V at 10 s. As
described previously, the first process occurring could be due to the oxygen reduction.
However, the nickel reduction could also be produced at these potentials. When the oxygen
concentration decreases, the potential reaches ꢀ0.94 V, which is likely due only to the
reduction of Ni(II) to Ni(0). From 10 s to the end of the current application, the potential
remains practically constant (varying only from ꢀ0.94 to ꢀ0.91 V), suggesting that not all the
Ni(II) in solution is electrodeposited under these conditions. If this were the case, a new
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decrement in the potential should happen. As the Figure 2 shows, the cyclic voltammetry of
the modified electrode in 0.1 M NaOH showed an anodic process at +0.60 V and a cathodic
process at +0.34 V (Figure 2A). NaOH is an electrolytic medium widely employed for the
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nonꢀenzymatic electrochemical detection of sugars because it has been demonstrated that OH
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ions in the solution play a crucial role in the reaction . The observed processes are assigned
to the oxidation of Ni(II) to Ni(III) and its correspondent reduction (equation 1). The
generation of Ni(II) on the electrode surface from the electrodeposited Ni(0) could happen by
two ways: on the one hand, the application of positive potentials (by the cyclic voltammetry)
in a NaOH medium could easily oxidize Ni(0) to oxygenated Ni(II), and on the other hand,
the spontaneous oxidation of Ni(0) to Ni(II) by atmospheric oxygen has been proposed
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previously
.
ꢅ
ꢅ
ꢀꢁ(ꢂꢃ) + ꢂꢃ ꢆ⇄ ꢀꢁꢂ(ꢂꢃ) +ꢆꢃ ꢂ +ꢆꢇ
(1)
ꢄ
ꢄ
[FIGURE 2]
The Ni(II)/Ni(III) redox process in 0.1 M NaOH was studied at different scan rates. Figure 2B
shows the electrochemical response with increasing scan rates (10, 25, 50, 75, 100, 250, 500
mV/s). An increment in the peak potential difference is observed with increasing scan rates as
expected for a quasireversible electrochemical process. Furthermore, a linear relationship
between the peak currents and the scan rate up to 50 mV/s is found (Figure S2), indicating a
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surfaceꢀconfined process for the reaction of adsorbed Ni(OH) /NiO(OH). However, a linear
relationship between the peak currents and the square root of the scan rate is found at higher
scan rates (Figure S3). This fact could indicate that the reaction at these scan rates is
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controlled by the diffusion of OH (involved in the reaction as indicated in equation 1) to/from
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flow of OH to the surface is high enough to observe the characteristics of the adsorbed nickel
film. Therefore, with the data obtained at low scan rates, the adsorbed concentration of nickel
was estimated using the following equation 2:
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∗
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(ꢎ)
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p
where i is the peak current, n is the number of electrons exchanged in the electrochemical
reaction, A is the electrode area, Γ* is the surface concentration, R is the gas constant, T is the
absolute temperature and v is the scan rate of the cyclic voltammetry. The adsorbed amount of
ꢀ9
nickel on the screenꢀprinted surface was 7.9x10 moles and considering the geometric area of
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the electrode the surface concentration was 6.6x10 mol/cm . The expected nickel amount
deposited in the electrodeposition step can be estimated considering the transferred charge (Q
= i t), which in these conditions (ꢀ25 µA for 60 s) was 1.5 mC. Then, the amount of nickel can
be calculated by the Faraday laws using equation 3, where m is the amount of nickel (g), M is
the molar mass (g/mol), Q is the charge transferred (C), n is the number of electrons
exchanged and F is the Faraday constant (96480 C/mol). The estimated amount of nickel
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electrodeposited was of 7.8x10 moles. This value is very close to the estimated with the
voltammetric peak, suggesting that all the applied current is employed for the
electrodeposition of nickel on the electrode surface and, besides, the electrodeposited nickel is
stable and stick to the electrode surface. The surface concentration found in our device for the
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optimal conditions (660 nmol/cm ) seems a higher value than other published works where
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this concentration was estimated. For instance, Hutton et al. reported a boronꢀdoped
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diamond electrode modified with 20 nmol/cm of Ni(OH)
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nanoparticles as the optimal
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surface concentration or Sharifi et al. reported a glassy carbon electrode modified with about
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40 nmol/cm of NiO nanoparticles. In our case, a higher amount of nickel is necessary to
obtain the best electrocatalytic effect than for these different electrodes.
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ꢏ
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Scanning electron microscopy micrographs were obtained in order to study the morphological
and structural aspects of the electrodeposited nickel. Figure 3A shows different micrographs
for the nickelꢀmodified screenꢀprinted electrode. Ni was electrodeposited applying a constant
current of ꢀ25 µA for 60 s on a solution of 10 mM of Ni(II) in 0.1 M NaCl. In these
conditions, as can be seen in the micrographs, the electrode surface is completely coated with
flowerꢀshaped nickel nanoparticles. The approximate size of the nanoparticles was 160±24
nm and an uniform distribution is observed across the electrode surface. Furthermore, some
porous structure is observed between the nanoflowers, which could lead to larger surface area
for the electrochemical reactions. Figure 3B shows a SEM image of the NiNFSPEs after
performing the electrochemical activation (explained in the following sections). It seems that
the size and morphology of the nanoflowers did not change after the electrochemical
activation, and therefore, the improvement observed in the measurements is probably due to
some surface process, as could be the generation of Ni(OH) from NiO. In a previous
2
unpublished study, we performed the electrodeposition of Ni(II) under the same conditions
3 3
but using a 0.1 M H BO /NaCl solution (see Supporting Information). In that case, nonꢀ
flower shaped spherical nickel nanoparticles can be found with a size of around 130 nm
(Figure 3C). Therefore, the solution in which the electrodeposition is carried out is crucial to
determine the shape of the generated nanoparticles.
[FIGURE 3]
The oxidation state of the electrodeposited nickel was studied by XPS. Figure 4 shows the Ni
2p and O 1s regions of the spectrum. The peaks with binding energies of 873.2 eV and 855.8
eV can be tentatively assigned to Ni 2p1/2 and Ni 2p3/2 of Ni(II), respectively, and
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characteristic of Ni(OH) . The peak with binding energy of 861.6 eV could be assigned to a
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multielectron excitation of Ni , but difficult to be assigned to NiO or Ni(OH ) . The
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spectrum for O 1s shows peaks with binding energies of 528.3 and 530.2 eV, which can be
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tentatively assigned to O (NiO) and OH (Ni(OH)
2
), respectively
. These results confirm
the presence of NiO and Ni(OH) species on the electrode surface. Comparing the O1s spectra
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against the obtained previously for the bare electrode , significant differences can be
observed. Two XPS peaks appear for the O1s spectra for the bare electrode at around 534.2
eV (smaller intensity) and 531.8 eV (higher intensity), assigned to different CꢀO bonds. For
the nickelꢀmodified electrodes, no peaks appeared at these binding energies, suggesting that
the response is completely due to the different NiꢀO bonds.
[FIGURE 4]
3.2. Electrocatalytic effect of nickel nanoflowers towards sugars oxidation
The electrocatalytic effect of nickel nanoflowers towards the oxidation of reducing sugars was
studied by cyclic voltammetry. Figure 5A shows the electrochemical response of NiNFSPEs
in presence of 5 mM glucose, 5 mM fructose and a 0.1 M NaOH solution (blank). Cyclic
voltammetries for other reducing sugars such as arabinose, galactose, mannose and xylose are
shown in the Figure S4. It is easily detected as the Ni(II) to Ni(III) oxidation process is
enhanced in presence of the reducing sugar in the solution. However, for the Ni(III) to Ni(II)
reduction process a significant decrease in the peak current is observed. The catalytic
34
oxidation of sugars is attributed to the following reactions (specified for the glucose case):
ꢅ
ꢅ
ꢀꢁ(ꢂꢃ) +ꢆꢂꢃ ⟶ ꢀꢁꢂ(ꢂꢃ) +ꢆꢃ ꢂ +ꢆꢇ
(4)
ꢄ
ꢄ
ꢀꢁꢂ(ꢂꢃ) + ꢆꢓꢔꢕꢖꢗꢘꢇꢆ ⟶ ꢀꢁ(ꢂꢃ) + ꢓꢔꢕꢖꢗꢉꢗꢔꢙꢖꢚꢗꢉꢇ (5)
ꢄ
This mechanism is consistent with the observed electrochemical response. The NiO(OH)
species reacts chemically with the sugar, and, therefore, the electrochemical oxidation of
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Ni(OH) to NiO(OH) will increase to regenerate the NiO(OH) consumed by the coupled
chemical reaction. For this reason, an increased current flow is expected to carry out the
oxidation of Ni(II) to Ni(III). For the reduction process (Ni(III) to Ni(II)) and due to that the
sugar oxidation regenerates the Ni(II) chemically, the intensity of the electrochemical process
decreases as less Ni(III) will be available. Although in some works where nickelꢀmodified
3
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electrodes are used, the catalytic effect is only observed for glucose
, in our case, the
electrodeposited nickel nanoflowers catalyze the oxidation of several reducing sugars. This is
36,12,22
also observed in other nickelꢀbased electrodes previously described in the literature
.
Studies are scarce explaining why certain nickelꢀbased electrode materials show a catalytic
effect only for glucose and in other cases, it occurs for several reducing sugars. Oxidation of
sugars is produced as a dehydrogenation reaction, although in all reducing sugars the group
37
involved in the reaction is a hemiacetal group. Compton et al. suggested that the
electrocatalysis process is generally observed to occur via the adsorption of the analyte to the
electrode surface, probably involving dꢀelectrons and empty dꢀorbitals of the metallic
substrate (in this case, the Ni(III) species). It is, therefore, probable that the adsorption of the
sugars on the catalyst is crucial in order to achieve the oxidation reaction. Although they have
similar structures, there are some differences in the reducing sugars such as the number of
carbons in pentoses and hexoses, or especially, the configurational differences in hydroxide
groups. Therefore, it is likely that these small differences in the structure play a significant
role in the adsorption of the sugars over the electrode materials, process which appears to be
highly important for the catalysis. For this reason, it seems that in some materials only
glucose adsorption would occur, being selective to this sugar instead of other also oxidizable
sugars. This is a possible explanation for the different selectivity of the various materials
described in the literature, however, it seems clear that more studies are needed to clarify
these processes and the influence of the nickel structures, although this is not within the scope
of this article.
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[FIGURE 5]
The catalytic process of the glucose oxidation (500 µM) was studied at different scan rates
(10ꢀ500 mV/s). Figure 5B shows the electrochemical response with increasing scan rates. The
anodic peak current obtained is linearly proportional to the root of the scan rate (Figure S5),
indicating that the limiting step of the electrochemical reaction is the diffusion of the glucose
to the electrode surface.
3.3. Analytical performance of NiNFSPEs for sugar determination
The optimization of the different parameters affecting the analytical signal was carried out.
Firstly, the optimization of the experimental conditions for the nickel electrodeposition such
as the electrodeposition current and time or the nickel concentration was performed. Several
conditions were used for the Ni(II) concentration (0.1, 0.5, 1, 5, 10 and 15 mM), for the
electrodeposition current (ꢀ5, ꢀ25, ꢀ75, ꢀ100 µA) and for the electrodeposition time (30, 60, 90
and 120 s). The highest signal/background ratio was obtained by applying ꢀ25 µA for 60 s to a
Ni(II) solution of 10 mM.
Secondly, it was necessary to perform an activation to the nickel electrodeposited surface in
order to improve the stability of the formed film, as shown in the Figure 6A. In this figure, the
electrochemical response of several cycles of cyclic voltammetry using a nonꢀactivated
electrode is shown. In Figure 6B, a higher stability on the response is observed after
performing an electrochemical activation to the electrode surface. This fact could be due to
that different Ni(II) species are presented in the surface, which shows a different
electrochemical behavior. After the activation in a NaOH solution, it is probably that the most
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stable Ni(OH) species are generated all over the surface, preventing Ni(II) mixed processes
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and improving the stability . It is possible to apply a wide number of activation methods in
order to improve the surface stability. In our case, we chose to carry out several cycles of
cyclic voltammetry from +0.2 to +0.7 V (100 mV/s). The number of cycles was optimized
with the aim to improve the stability of the surface and enhance the signal/background ratio. It
was achieved after the application of 50 cycles.
[FIGURE 6]
As chronoamperometry was used to carry out the determination of sugars, the effect of the
applied potential was also evaluated. In order to perform a measurement, 40 µL of the glucose
solution in 0.1 M NaOH was added to the device and a potential able to oxidize the Ni(II)
film is applied for a certain time. The chronoamperometric current obtained at 100 s was
considered as the analytical signal. Therefore, several potentials were tested in order to
improve the signal/noise ratio using a 250 µM glucose solution. 0.6 V was chosen as the most
appropriate potential to carry out the detection of sugars.
The electrochemical response for different concentrations of glucose, fructose and a 1:1
mixture in 0.1 M NaOH was evaluated. Figure 7 shows the chronoamperograms for different
concentrations of glucose (Figure 7A) and the calibration plots obtained with a linear range
from 25 to 1000 µM for all cases (Figure 7B). Similar results were found for other reducing
sugars such as arabinose, galactose, mannose and xylose (see Figure S6). The slope of the
calibration plots was similar for all the reducing sugars evaluated. The reproducibility
obtained for the slopes of the calibration plots was in all cases under 8% (RSD, n=3). The
ꢀ1
ꢀ2
sensitivity obtained was between 0.21ꢀ0.23 µA µM cm and a detection limit between 8ꢀ20
µM was estimated. The limit of detection was calculated as the concentration corresponding
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to three times the standard deviation of the estimate, as proposed by Miller . A quantitative
comparison of several devices for nonꢀenzymatic detection of sugar using nickelꢀmodified
screenꢀprinted electrodes is shown in Table 1. The device fabricated with 3D nickel
nanoporous structures stand out over the other devices in terms of linear range and limit of
detection. This fact is due to that the screenꢀprinted electrode is used in a stirred highꢀvolume
conventional cell, with a Pt wire and Ag/AgCl conventional electrodes. A higher volume of
the sample and the improved mass transfer due to the stirring of the solution allows to achieve
a lower limit of detection. It could be interesting to evaluate this electrode in a quiescent
solution, as generally employed for screenꢀprinted electrodes. Comparing the other nickelꢀ
based screenꢀprinted electrodes, our device is highly competitive in terms of the linear range
and limit of detection. Furthermore, the high stability shown by the nickel nanoflowers and
the simplicity for the fabrication of the nanostructured surface are really interesting
characteristics in order to apply this device in a real world application, such as the
determination of sugars in food.
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[FIGURE 7]
[TABLE 1]
3.4. Stability and precision studies
Besides the estimation of the precision of the nonꢀenzymatic electrode by evaluating the
residual standard deviation of the calibration slopes as described in the previous paragraph, a
study of intraꢀ and interelectrode precision was performed. In order to do so, a solution of 500
µM of glucose was measured using the optimized experimental conditions. The RSD obtained
for intraelectrode precision was 5.5 % (n=10), showing that the device is very precise even
reusing the same electrode, which can be extremely useful to save costs in an industrial
environment. The RSD obtained for the interelectrode precision study was 6.9 % (n=3),
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showing a high precision using different devices.
Several devices were fabricated and activated (50 voltammetric cycles) on the same day and
were stored at room temperature until the day of use. The results show that the NiNFSPE
device is stable at least up to 35 days (Figure 8), considering the signals obtained for different
electrodes (interelectrode stability) as for signals obtained using the same electrode
(intraelectrode stability). In all cases, the electrode surface was rinsed with ultrapure water
before the measurements. No reꢀactivation of the electrode surface was necessary in order to
obtain a reproducible response. The fact that the same device can be used in different days
and keep its electrochemical response is a great advantage compared to other previously
published devices.
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[FIGURE 8]
3.5. Selectivity study
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The effect of different species that could interfere with the determination of sugars due to the
proximity of its oxidation processes was evaluated. The interfering species studied were
ascorbic acid, citric acid, lactic acid, ethanol and glycerol, as they are species that could be
found in food samples, typically at lower concentrations than sugars. Solutions of these
species were prepared at different increasing concentrations (from 0.1 mM to 10 mM) and a
constant glucose concentration of 1 mM was used. These solutions were measured by
chronoamperometry with the optimized experimental conditions. Table 2 shows the lower
concentration of the interfering species that influenced the analytical signal and the variation
in this signal. No significant interference effect was found for citric and lactic acids up to 10
mM, showing that the device does not respond to these species even at high concentrations. A
concentration of 1 mM of ascorbic acid increased the analytical signal by 23% (in order to
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know the influence of ascorbic acid, it is worth to mention that typical concentrations of
sugars/ascorbic acid in orange juice could be at least in a 100:1 ratio). An ethanol
concentration of 5 mM increased the analytical signal by 13%, suggesting that the device
could have issues to determine the sugar content in highꢀconcentration alcoholic beverages.
The higher interfering effect was found for glycerol as a concentration of 0.25 mM was
enough to increase the analytical signal by 31%. However, when it is added to sugarꢀ
containing food, the glycerol concentration is typically lower than sugar concentration.
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[TABLE 2]
3.6. Determination of sugars in real samples
The performance of the nonꢀenzymatic electrode for the determination of reducing sugars in
food samples such as orange juice and honey was evaluated. Standard additions method was
employed in order to minimize the matrix effects and a known amount of glucose (50, 100
and 200 µM) was added to the different samples diluted in 0.1 M NaOH. For the orange juice
samples, a concentration of 0.49±0.04 g/L of sugar was estimated (value statistically similar
to the 0.49 g/L specified in the nutritional information). For the honey sample, a
concentration of 6.8±0.9 g/kg of sugar was found (compared to the 7.8 g/kg specified in the
nutritional information and 6.3±0.1 g/kg obtained with glucose and fructose sensors
4
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previously published by our group
). These results show that the NiNFSPE
electrochemical device is able to determine with good accuracy the concentration of reducing
sugars in complex food samples.
4. CONCLUSIONS
In this work, we were able to generate in situ nickel nanoflowers on screenꢀprinted carbon
electrodes by a galvanostatic electrodeposition methodology. Nickel nanoflowers have a
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quasiꢀspherical geometry but with different edges that increase its surface area compared to
typical nanoparticles. Nickel nanoflowers electrodeposited on screenꢀprinted electrodes
showed a great electrocatalytic effect towards the oxidation of reducing sugars. The nonꢀ
enzymatic device is very promising for the determination of reducing sugars in food samples,
even at low µM concentrations, in a short analysis time and with low sample consumption.
The excellent stability presented by this nanostructured device, even being able to reuse the
same device on different days without loss of the electrochemical response, could mean
notable cost savings if the device were to be implanted in the food industry.
ACKNOWLEDGEMENTS
This work has been supported by the FCꢀ15ꢀGRUPINꢀ021 project from the Asturias Regional
Government and the CTQ2014ꢀ58826ꢀR project from the Spanish Ministry of Economy and
Competitiveness (MEC). Daniel MartínꢀYerga thanks the MEC for the award of a FPI grant
(BESꢀ2012ꢀ054408). Authors thank the Spectroscopy Unit of the Scientific and Technical
Services of the University of Oviedo for the help with the XPS measurements.
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J. Biscay, E. Costa Rama, M. B. González García, J. M. Pingarrón Carrazón, and A. Costaꢀ
García, Electroanalysis, 2011, 23, 209–214.
42.
J. Biscay, E. Costa Rama, M. B. González García, A. Julio Reviejo, J. M. Pingarrón Carrazón,
and A. C. García, Talanta, 2012, 88, 432–8.
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5
73
5
5
5
74
75
76
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5
5
77
78
TABLES AND FIGURES
Electrode material
Ni nanoflowers
NiCu nanowires
Ni nanowires
Linear Range (µM)
25ꢀ1000
Detection Limit (µM)
Reference
This work
8
40
ꢀ
1
1
8
2
50ꢀ1000
50ꢀ1000
25ꢀ3700
20ꢀ240
0.5ꢀ4000
2
2
NiCo
ꢀ
1
2
2
2
9
3
0
1
Niꢀdoped nanoporous carbon
10
0.07
4.1
3
Dꢀporous Ni nanostructures
NiNPꢀchitosanꢀrGO
Ni/C composite
200ꢀ9000
1000ꢀ10000
400
5
5
5
79
80
81
Table 1. Analytical characteristics of different nickelꢀmodified screenꢀprinted electrodes for
the determination of sugars.
5
82
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Concentration
Interfering species
(
%variation)
Citric acid
Lactic acid
Ascorbic acid
Ethanol
ꢀ
ꢀ
1 mM (+22%)
5 mM (+13%)
Glycerol
0.25 mM (+31%)
5
5
5
5
83
84
85
86
Table 2. Lower concentration of interfering species that influences the analytical signal for 1
mM of glucose and the variation of the analytical signal produced.
5
87
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88
89
5
Figure 1. Cyclic voltammograms of several concentrations of Ni(II) and blank in a 0.1 M
5
90
91
NaCl solution.
5
2
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5
5
5
5
92
A)
B)
93
94
95
96
Figure 2. A) Cyclic voltammogram of the nickelꢀmodified electrode in 0.1 M NaOH. B)
Cyclic voltammograms of the nickelꢀmodified electrode in 0.1 M NaOH at several scan rates
(10, 25, 50, 75, 100, 250, 500 mV/s).
2
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597
A)
598
B)
5
6
99
00
C)
Figure 3. A) SEM micrograph of the nickelꢀmodified electrode. B) SEM micrograph
601
602
603
of the nickelꢀmodified electrode after the activation with 50 CV cycles. C) SEM
3 3
micrograph of the nickelꢀmodified electrode using a H BO /NaCl solution.
2
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604
605
6
06
07
6
608
Figure 4. XPS spectrum of the NiNFSPE electrode: A) Ni 2p region and B) O 1s region.
2
9