Change in the level of dopamine (DA) concentration in the human body causes critical diseases such as schizophrenia and Parkinson's disease. Therefore, the determination of DA concentration and monitoring its level in human body fluids is of great importance. An electrochemical sensor based on modification of the gold electrode surface with Nafion (NF), β-cyclodextrin (CD), and gold nanoparticles (AuNPs) was fabricated for the determination of DA in biological fluids. Combined impact of all the modifiers enhances the catalytic activity of the sensor. Gold nanoparticles increase the surface area of the sensor and enhance the electron transfer rate. CD plays a main role in enhancing the accumulation of protonated DA and forming stable complexes via electrostatic interactions and hydrogen bond formation. In addition, extra preconcentration of positively charged DA is achieved through ionic selectivity of NF. High electrocatalytic activity was achieved using the modified sensor for determination of DA in real urine samples in a wide concentration range, 0.05-280 μM with a low detection limit of 0.6 nM in the small linear dynamic range, 0.05-20 μM. Furthermore, common overlapped oxidation peaks of DA in presence of biologically interfering compounds at the gold electrode were resolved by using the modified sensor. Excellent recovery results were obtained using the proposed method for determination of DA in real urine samples.
Change in the level of dopamine (DA) concentration in the human body causes critical diseases such as schizophrenia and Parkinson's disease. Therefore, the determination of DA concentration and monitoring its level in human body fluids is of great importance. An electrochemical sensor based on modification of the gold electrode surface with Nafion (NF), β-cyclodextrin (CD), and gold nanoparticles (AuNPs) was fabricated for the determination of DA in biological fluids. Combined impact of all the modifiers enhances the catalytic activity of the sensor. Gold nanoparticles increase the surface area of the sensor and enhance the electron transfer rate. CD plays a main role in enhancing the accumulation of protonated DA and forming stable complexes via electrostatic interactions and hydrogen bond formation. In addition, extra preconcentration of positively charged DA is achieved through ionic selectivity of NF. High electrocatalytic activity was achieved using the modified sensor for determination of DA in real urine samples in a wide concentration range, 0.05-280 μM with a low detection limit of 0.6 nM in the small linear dynamic range, 0.05-20 μM. Furthermore, common overlapped oxidation peaks of DA in presence of biologically interfering compounds at the gold electrode were resolved by using the modified sensor. Excellent recovery results were obtained using the proposed method for determination of DA in real urine samples.
Dopamine
(3,4-dihydroxy-phenyl-ethyl amine), is a main neurotransmitter
and belongs to catecholamine’s family in the human central
nervous system. Some serious diseases have arisen due to changes in
the level of dopamine (DA) concentration in the human body such as
schizophrenia and Parkinson’s disease.[1] Therefore, the determination of DA concentration and monitoring
its level in human body fluids is of great importance. There is an
increase in the number of elderly people suffering from psychiatric
disorders, sadness, and depression, still the effective treatment
of such diseases remains a clinical challenge.[1,2] The
simultaneous detection of DA, ascorbic acid (AA), and uric acid (UA)
has high impact in the field of diagnostic and pathological research,
and it is of significance in neurochemistry and biomedical chemistry.
One major problem facing the simultaneous determination of DA, AA,
and UA at unmodified electrodes is overlapping of their electrochemical
oxidation responses, besides adsorption of the oxidative products
at the surface of the electrode which causes surface fouling.[3] Thus, there is always need to improve a modified
electrochemical sensors to detect a wide range of biological compounds.[3,4] Clinical diagnosis and medical treatment involving DA require sensitive
sensing with low limit of detection. This can be realized by an adequate
design of surface modification for DA electrochemical determination.
Many electrochemical sensors were applied for the sensitive determination
of DA such as mercapto carboxylic acid monolayer-modified electrode,[5] β-cyclodextrin (CD) graphene-modified glassy
carbon electrode,[6] carbon nanotubes-modified
microelectrode,[7] organic polymer-modified
electrodes,[8,9] and nanoparticles-modified electrodes.[10,11]Nafion (NF) (per-fluorinated sulfonated cation exchanger)
is a
synthetic polymer with ionic properties which are called ionomers.
It has an ionic selectivity; its permeability is high to cations and
almost nil to anions. NF has intrinsic properties such as chemical
and thermal stability, mechanical strength, chemical inertness, good
electrical conductivity, and proper adhesion on the electrode surface.[12,13] Also, the hydrophilic negatively charged sulfonate groups in the
NF polymer structure helps in the accumulation of the positively charged
molecules via an electrostatic interaction, resulting in enhanced
sensitivity of the measurements.[14−16]The outer surface
of the CD structure is hydrophilic in nature
because of the presence of several hydroxyl groups, whereas its cavity
has hydrophobic character.[6] CDs are cyclic
compounds and belong to the family of oligosaccharides. CDs are used
as inactive fillers in pharmaceutical formulations.[17] Insoluble hydrophobic compounds form water-soluble complexes
with CDs because of their exceptional structure.[18] Therefore, CDs increase the aqueous solubility and the
stability of drugs with their hydrophobic nature.[18] The formation of a complex between CD and specific molecule
is through host–guest inclusion resulting in remarkable molecular
selectivity and enantioselectivity.[19] Therefore,
CDs are used as electrode modifiers to host specific organic and biological
molecules to form stable nanostructures that allows high selectivity
and low detection limits. Also, it is important to increase the bioavailability
of the drugs and minimize their foul smelling.[6,17,18] Atta and co-workers constructed electrochemical
sensors-based β-CD modified electrodes for the sensitive analysis
of some compounds recognized as neurotransmitters as well as medication.[20,21]Nanostructured materials have been widely used as electrode
materials
for electrochemical sensing.[22,23] An important feature
of nanostructured materials is the relative increase in surface area
compared to other surfaces.[23] Among the
widely used nanomaterials in electrode modification are gold nanoparticles.
Au nanoparticles possess several important characteristics such as
size/surface area, biocompatibility in biological environments, conductive
nature, molecular recognition, and high surface activity.[24−28] Thus, gold nanoparticles have been used in applications such as
electrocatalysis, sensing, and biosensing.[22,23]Therefore, it is important to examine the combining effect
of these
materials when applied to a gold substrate for the electrochemical
determination of DA effectively and with considerable simplicity.
In this work, we report for the first time a novel electrochemical
sensor constructed from the following elements: NF, β-CD, and
Au nanoparticles. The current sensor was tested for the determination
of DA in human urine samples. The possible effects of interfering
compounds on the electrochemical signal were also examined.
Results and Discussion
Optimization of Modified
Surface
The modified surface was fabricated and optimized
with respect to
each modifier. The best response was obtained by using concentration
of the NF solution 2% (optimized). CD immobilization was achieved
using electrochemical deposition of CD. The best current response
of DA oxidation was obtained when the film formed by bulk electrolysis
(BE) at potential 1.2 V for 3 min in 10–3 M of CD/0.05
M LiClO4 solution.[21] Furthermore,
two electrochemical methods, namely, cyclic voltammetry (CV) and BE
were employed for the deposition of gold film at the Au/NF/CD electrode
surface. The resulting films were examined in 1 mM DA/0.1 M phosphate-buffered
solution (PBS) at pH 7.40 (Figure A). In case of CV, the potential was cycled between
−0.8 and +0.4 V in (6 mM HAuCl4/0.1 M KNO3) at a scan rate 50 mV s–1 for 10, 15, 20, and
30 cycles, and the best result was obtained when applying 20 cycles
(Figure B). In case
of BE, the applied potential was held constant at −0.4 V for
5, 10, and 15 s, and the current response obtained by this method
was lower than the CV method (Figure C). Thus, CV measurements were recorded with our fabricated
electrode (Au/NF/CD/AuNPs), where AuNPs deposited by the CV method
showed higher current response for oxidation of DA than when the AuNPs
were deposited by the BE method (Figure A).
Figure 1
(A) CVs of 1 mM DA/0.1 M PBS/pH 7.40 at Au/NF/CD/AuNPs
(BE) (black
line), Au/NF/CD/AuNPs (CV) (red line). (B) Relation between the anodic
peak current of DA (μA) and number of CV cycles for electrodeposition
of gold. (C) Relation between the anodic peak current of DA (μA)
and the time of BE for electrodeposition of gold.
(A) CVs of 1 mM DA/0.1 M PBS/pH 7.40 at Au/NF/CD/AuNPs
(BE) (black
line), Au/NF/CD/AuNPs (CV) (red line). (B) Relation between the anodic
peak current of DA (μA) and number of CV cycles for electrodeposition
of gold. (C) Relation between the anodic peak current of DA (μA)
and the time of BE for electrodeposition of gold.
Surface Morphology
Scanning electron
microscopy (SEM) was used to investigate the surface morphology of
different modified surfaces. Figure A,B shows the SEM of Au/NF/CD and Au/NF/CD/AuNPs, respectively.
The morphology of Au/NF/CD (Figure A) exhibits a distinctive morphology with porous surface
resulting in greater surface area and enhancement of the contact area
with the analyte. The SEM image of Au/NF/CD/AuNPs (Figure B) shows that gold nanoparticles
are distributed over the Au/NF/CD surface with the formation of some
clusters exhibiting a large surface area (Figure C shows higher magnification of the surface).
The combining effect of modifiers, NF, CD, and gold nanoparticles,
affected the conductivity level of the composite and increased the
electrocatalytic activity of the resulting composite. Figure D shows the energy-dispersive
X-ray analysis of Au/NF/CD/AuNPs confirming the presence of AuNPs.
Figure 2
SEM micrograph
of (A) Au/NF/CD, (B) Au/NF/CD/AuNPs, (C) Au/NF/CD/AuNPs
(higher magnification), and (D) EDX of Au/NF/CD/AuNPs.
SEM micrograph
of (A) Au/NF/CD, (B) Au/NF/CD/AuNPs, (C) Au/NF/CD/AuNPs
(higher magnification), and (D) EDX of Au/NF/CD/AuNPs.
Comparison between Different Modified Surfaces
Figure shows cyclic
voltammograms (CVs) of electro-oxidation of 1 mM DA/0.1 M PBS at pH
7.40, recorded at five different surfaces, namely, bare Au, Au/AuNPs,
Au/NF/AuNPs, Au/CD/AuNPs, and Au/NF/CD/AuNPs. A summary of the oxidation
potential and current values of DA at all of the studied surfaces
is presented in Table . A poorly defined oxidation peak was obtained at the bare gold electrode
at 442 mV. For all surfaces studied, the current response increases
upon inclusion of gold nanoparticles. A high current response (90.3
μA) is observed at Au/NF/CD/AuNPs at 210 mV compared to the
other studied surfaces (Table ). The oxidation current of DA increased by 11.2, 3.7, 12.6,
and 20.9 folds at Au/AuNPs, Au/NF/AuNPs, Au/CD/AuNPs, and Au/NF/CD/AuNPs,
respectively, compared to bare Au electrode which confirmed the catalytic
DA oxidation at the sensor surface. Therefore, the proposed sensor
showed remarkable improvement in both the current response and the
oxidation potential towards the electro-oxidation of DA compared to
the bare Au electrode. Improvement in the catalytic activity for the
proposed sensor was observed. NF film increases the electrical conductivity
of the composite and enhances the surface preconcentration of DA by
ion selectivity and accumulation of DA in the hydrophilic regions.[24,29] Stable inclusion complex was formed between CD and DA because of
hydrogen bonds formation.[30−34] Besides, extra advantages were offered by gold nanoparticles because
of their unique properties that results in improving the electron-transfer
process and current response.[24,31] Surface roughness also
contributed in enhancing the current signal as the surface area of
the electrode increases upon modification. We used a redox probe,
K3[Fe(CN)6], and the Randles–Ševćik
equation to estimate the surface area for each electrode. The estimated
electroactive surface areas are 0.0435, 0.109, and 0.917 cm2 for Au/NF, Au/NF/CD, and Au/NF/CD/AuNPs, respectively, compared
to 0.0177 cm2 for the Au electrode.
Figure 3
CVs of 1 mM DA/0.1 M
PBS/pH 7.40 recorded at different working
electrodes; bare Au, Au/AuNPs, Au/CD/AuNPs, Au/NF/AuNPs, and Au/NF/CD/AuNPs,
scan rate 50 mV s–1.
Table 1
Values of Anodic Peak Current Ipa, Anodic Peak Potential Epa,
Potential Peak Separation ΔE, and Diffusion
Coefficient Dapp (1 mM
DA/0.1 mol L–1 PBS/pH 7.4, Scan Rate 50 mV s–1) at Different Electrodes
electrode
Epa (mV)
Ipa (μA)
ΔE (mV)
Dox (cm2/s)
bare
Au
442
4.33
404
0.213 × 10–5
Au/AuNPs
203
48.5
58
26.7 × 10–5
Au/NF/AuNPs
549
16.0
592
2.91 × 10–5
Au/CD/AuNPs
206
54.6
63
33.9 × 10–5
Au/NF/CD/AuNPs
210
90.3
74
92.7 × 10–5
CVs of 1 mM DA/0.1 M
PBS/pH 7.40 recorded at different working
electrodes; bare Au, Au/AuNPs, Au/CD/AuNPs, Au/NF/AuNPs, and Au/NF/CD/AuNPs,
scan rate 50 mV s–1.
Effect of Scan Rate
The scan rate
was changed between 10 and 100 mV s–1, and its effect
on the current response was examined for 1 mM DA/0.1 M PBS/pH 7.4
at the Au/NF/CD/AuNPs using the CV technique (Figure ). The results indicate a linear relation
between Ip and ν1/2 (Figure ; inset), illustrating
a diffusion-controlled process. The corresponding eq is
Figure 4
CVs of 1 mM DA/0.1 M
PBS/pH 7.40 at Au/NF/CD/AuNPs at different
scan rates (10–100 mV s–1). Inset: Linear
relation between the anodic peak current of DA (μA) and the
square root of the scan rate (mV s–1)1/2.
CVs of 1 mM DA/0.1 M
PBS/pH 7.40 at Au/NF/CD/AuNPs at different
scan rates (10–100 mV s–1). Inset: Linear
relation between the anodic peak current of DA (μA) and the
square root of the scan rate (mV s–1)1/2.The correlation coefficient is r2 =
0.995.Diffusion coefficient (D, cm2 s–1) values were calculated using Randles–Ševćik
equation for a quasireversible process[22] (eq ), where (n = 2), (A = 0.0177 cm2), and
(C = 1 × 10–6 mol/cm3)Dapp values
were
0.213 × 10–5 and 92.7 × 10–5 cm2·s–1 at the gold electrode
and Au/NF/CD/AuNPs, respectively, displaying the highest value at
Au/NF/CD/AuNPs.
Effect of Solution pH
The influence
of pH of the supporting electrolyte on the electrochemical response
of Au/NF/CD/AuNPs toward DA compound is investigated (Figure A). The study showed that the
oxidation potential of DA shifted to less positive values by the increase
in solution pH (from 2.0 to 11) and the oxidation potential versus
the pH displayed linear relation (Figure B). This study indicated protonation/deprotonation
steps for the DA oxidation at Au/NF/CD/AuNPs. The corresponding eq is
Figure 5
(A) CVs of 1 mM DA/0.1 M PBS with different pH values
(2–11)
at Au/NF/CD/AuNPs. (B) Linear relation between the anodic peak potential
of DA (mV) and the pH value, scan rate, 50 mV s–1. (C) Relation between anodic peak current and pH values.
(A) CVs of 1 mM DA/0.1 M PBS with different pH values
(2–11)
at Au/NF/CD/AuNPs. (B) Linear relation between the anodic peak potential
of DA (mV) and the pH value, scan rate, 50 mV s–1. (C) Relation between anodic peak current and pH values.With a correlation coefficient of 0.997.The slope
value of −58 mV/pH over the pH range (2.0–11)
was close to the Nernstian theoretical slope value of −59 mV,
indicating that there is an equal number of electrons and protons
involved in the DA oxidation process. The protons number involved
was predicted to be 2, as the oxidation of DA is a two-electron process.
This indicates a 2e–/2H+ process (Scheme ).
Scheme 1
Oxidation Mechanism
of DA
The present study was performed
at pH 7.4 as the maximum current
value was obtained at pH 7.0 (Figure C).
Characteristics of Au/NF/CD/AuNPs
Sensor
Stability, Repeatability, and Reproducibility
of the Au/NF/CD/AuNPs
In order to investigate the stability
of Au/NF/CD/AuNP-modified electrode, the cyclic voltammetry of 1 mM
DA/0.1 M PBS at pH 7.4 was studied for 30 repeating cycles (Figure ). Excellent current
stability was obtained with a slight decrease in the current response.
Thus, the electrode has fouling resistance. Repeatability was examined
by performing four runs in 1 mM DA/0.1 M PBS/pH 7.4 using the same
Au/NF/CD/AuNPs electrode. Also, reproducibility was investigated by
applying three independent measurements in 1 mM DA/0.1 M PBS/pH 7.4
using three similarly prepared Au/NF/CD/AuNPs electrodes. Low RSD
values were obtained; 0.75 and 0.69%, respectively, indicating that
the sensor has good repeatability and reproducibility.
Figure 6
Repeated cycle stability
of 1 mM DA/0.1 M PBS/pH 7.40 at Au/NF/CD/AuNPs,
30 repeated cycles, scan rate 50 mV s–1.
Repeated cycle stability
of 1 mM DA/0.1 M PBS/pH 7.40 at Au/NF/CD/AuNPs,
30 repeated cycles, scan rate 50 mV s–1.
Robustness
The stability of the
current response upon small variations in the experimental parameters
was examined. Low RSD values 0.62 and 0.76% were obtained for minor
changes in the NF content and pH value, respectively, indicating the
robustness of the suggested method.
Precision
Also, the proposed method
for DA determination at the Au/NF/CD/AuNP-modified electrode was examined
by intraday and interday precisions. The intraday precision represented
the investigation of the same surface in 1 mM DA/0.1 M PBS/pH 7.4
solution four times while the interday precision represented the investigation
of four similar sensors made independently in four separated DA/0.1
M PBS/pH 7.4 solutions with the same concentration four times. Low
RSD values were obtained; 0.91 and 0.95%, respectively, approving
good precisions of the suggested method.
Analytical
Performance
Calibration Curve for
DA in Real Samples
Standard addition method was applied for
the analysis of DA at
Au/NF/CD/AuNPs sensor in real urine sample. In this respect a stock
DA solution of 1 mM was prepared by dissolving DA in 0.1 mM phosphate
buffer solution (PBS)/(pH 7.4) and standard additions were carried
out from DA solution in 10 mL of diluted urine (the urine sample was
diluted 20 times by PBS). Differential pulse voltammograms (DPVs)
of DA (0.05–280 μM) at Au/NF/CD/AuNPs sensor are shown
in Figure A. The oxidation
current increased linearly with increasing concentration of DA in
the range of (0.05–280 μM). Figure B shows the calibration curve of oxidation
current values of DA at the sensor in the linear dynamic range of
(0.05–20 μM) with linear regression equation (eq )
Figure 7
(A) DPVs of DA in urine
at Au/NF/CD/AuNPs for concentrations from
(0.05 to 280 μM). (B) Calibration curve of DA in urine in the
linear range (0.05 → 20 μM) at Au/NF/CD/AuNPs. Inset:
Calibration curve of DA in the linear range (40 → 280 μM)
at Au/NF/CD/AuNPs.
(A) DPVs of DA in urine
at Au/NF/CD/AuNPs for concentrations from
(0.05 to 280 μM). (B) Calibration curve of DA in urine in the
linear range (0.05 → 20 μM) at Au/NF/CD/AuNPs. Inset:
Calibration curve of DA in the linear range (40 → 280 μM)
at Au/NF/CD/AuNPs.Figures of merit were
as follows: sensitivity of 0.886 μA/μM,
detection limit (DL) of 0.6 nM, and quantification limit (QL) of 2.0
nM.Inset of (Figure ) shows the calibration curve of oxidation current values
of DA at
the Au/NF/CD/AuNPs sensor in the linear dynamic range from (40 to
280 μmol L–1) with linear regression equation
(eq )(DL) and (QL)[20,22] were calculated using the following eqs and 7, respectively)where “s” is
the standard deviation and “b” is the
slope of the calibration curve.Table illustrates
a comparison for the determination of DA at Au/NF/CD/AuNPs with other
modified electrodes mentioned in the literature.[30,36−44] Reasonable sensitivity, linear concentration range, and lower DL
were the achievements of the proposed electrode.
Table 2
Comparison of Au/NF/CD/AuNPs Electrode
with Different Modified Electrodes Mentioned in Literature for DA
Determinationa
To evaluate
the accuracy and precision of the proposed method for the determination
of DA in urine sample, four different concentrations were repeated
five times (Table ). Suitable recovery results were obtained in the range of 99.9–101.3%
with low SD values in the range of 0.229 × 10–7 to 5.0 × 10–7. Good agreement was realized
between the obtained results and those obtained using other reported
methods indicating that the validation for DA quality control analysis
was achieved by this method.[35]
Table 3
Evaluation of the Accuracy and Precision
of the Proposed Method for Determination of DA in Urine Sample
sample
concentration
of DA added (μM)
concentration
of DA found (μM)a
recovery
(%)
standard
deviation ×10–7
standard
error ×10–7
1
0.6
0.608
101.3
0.710
0.410
2
20
19.98
99.9
0.229
0.132
3
140
141
100.7
5.0
2.89
4
280
280.5
100.2
1.53
0.882
Average of five determinations.
Average of five determinations.
Study of the Inclusion Complex of DA with
β-CD by Infrared
The IR spectrum of DA reveals the
presence of two peaks at 3344.93 and 3215.72 cm–1 assigned to NH2 symmetric and antisymmetric stretching
vibration modes[45]Figure S1A. The spectrum of β-CD is characterized by intense
peak at 3423.99 due to O–H stretching vibration, whereas the
vibration of the −CH and −CH2– groups
appears in the region (2800–3000 cm–1)[46]Figure S1B. Upon
complexation, the DA peaks at 3344.93 and 3215.72 cm–1 were not identified any more in the IR spectrum and the OH band
of β-CD was slightly shifted toward a lower wave number 3391.21
because of the presence of host–guest interactions as shown
in Figure S1C. These suggest the possibility
of formation of hydrogen bonds between the hydroxyl groups of the
host cavities β-CD and the DAhydroxyl groups.
Interference Study
Simultaneous Detection
of DA with Common
Interfering Compounds
Determination of DA in the presence
of AA and ST (interfering compounds) is very important for patients
under medical treatment from anxiety and depression.[3,4,47] Acetaminophen (APAP) is a common
drug for treating pain and reducing high temperature of the body.[22,24] Also, if APAP is taken in high doses, it can cause serious liver
damage.[48] Therefore,
the detection of DA in presence of these compounds was investigated.Figure A shows
the DPV of 0.5 mM DA, 3 mM AA, and 2 mM UA/0.1 M PBS/pH 7.4 at Au/NF/CD/AuNPs.
Three defined peaks were obtained at 160, −42, and 431 mV respectively. Figure B shows the DPV of
0.5 mM DA, 3 mM AA, and 2 mM APAP/0.1 M PBS/pH 7.4 at Au/NF/CD/AuNPs.
Three defined peaks were obtained at 164, −40, and 428 mV,
respectively. A comparable trend was obtained in a binary mixture
of DA and ST. Figure C shows the simultaneous determination of 1 mM DA, and 1 mM ST/0.1
M PBS/pH 7.4 at Au/NF/CD/AuNPs. Two well-defined oxidation peaks were
obtained at 152 and 324 mV with current values 48 and 42 μA
for DA and ST, respectively. Thus, high current response and good
potential peak separation were achieved for simultaneous determination
of DA in the presence of interfering compounds.
Figure 8
(A) DPV of 0.5 mM DA
in the presence of 2 mM UA and 3 mM AA at
Au/NF/CD/AuNPs. (B) DPV of 0.5 mM DA in the presence of 3 mM AA and
2 mM APAP at Au/NF/CD/AuNPs. (C) DPV of 1 mM DA and 1 mM ST at Au/NF/CD/AuNPs.
(A) DPV of 0.5 mM DA
in the presence of 2 mM UA and 3 mM AA at
Au/NF/CD/AuNPs. (B) DPV of 0.5 mM DA in the presence of 3 mM AA and
2 mM APAP at Au/NF/CD/AuNPs. (C) DPV of 1 mM DA and 1 mM ST at Au/NF/CD/AuNPs.
Calibration Curves of
(DA and APAP) and
(DA, ST, and TY) in Real Urine Samples
Further study was
achieved by changing the concentration of DA and APAP in real urine
samples in the following ranges (0.05 → 15 μM) and (0.05
→ 15 μM), respectively (Figure A). The linear relations for DA and APAP
(insets) are represented by the following equations
Figure 9
(A) DPVs for binary mixture DA and APAP/diluted
urine with increasing
concentrations of DA (0.05 → 15 μM) and APAP (0.05 →
15 μM). (Insets) Plots of peak current vs the concentration
for DA and APAP. (B) DPVs for the ternary mixture of DA, ST, and TY/diluted
urine with increasing concentrations of DA (0.2 → 15 μM),
ST (0.5 → 30 μM) and TY (0.5 → 30 μM) using
Au/NF/CD/AuNPs. (Insets) Plots of peak current vs the concentration
for DA, ST, and TY.
(A) DPVs for binary mixture DA and APAP/diluted
urine with increasing
concentrations of DA (0.05 → 15 μM) and APAP (0.05 →
15 μM). (Insets) Plots of peak current vs the concentration
for DA and APAP. (B) DPVs for the ternary mixture of DA, ST, and TY/diluted
urine with increasing concentrations of DA (0.2 → 15 μM),
ST (0.5 → 30 μM) and TY (0.5 → 30 μM) using
Au/NF/CD/AuNPs. (Insets) Plots of peak current vs the concentration
for DA, ST, and TY.The sensitivity of the
sensor toward the electro-oxidation of DA
was very close to the value mentioned in Section . This ascertained the independent electrochemical
oxidation behaviors of these compounds at the Au/NF/CD/AuNPs sensor.Considering the coexistence of DA, ST, and tyrosine (TY) in biological
fluids, thus developing an effective method that can separate and
simultaneously determine DA, ST, and TY in real urine samples, is
useful for diagnosis and treatment purposes (Figure B). The calibration curves for DA, ST, and
TY in real urine sample (insets) are represented by the following
equationsThe sensitivity of
the sensor for electro-oxidation of DA was very
close to the value mentioned earlier in Section . Three well-defined oxidation peaks
were separated. Thus, the Au/NF/CD/AuNPs sensor possessed excellent
anti-interference capability for simultaneous determination of DA,
ST, and TY.
Conclusions
A novel
electrochemical sensor based on modification of a gold
electrode with two successive layers of β-CD and gold nanoparticles
over a thin film of NF; Au/NF/CD/AuNPs was fabricated. The sensor
showed good electrocatalytic activity for the determination of DA
compared to other studied electrodes. The synergism existed between
gold nanoparticles, β-CD and NF, resulting in formation of a
conductive matrix, large surface area, and enhancement of the catalytic
process. Furthermore, the NF film facilitated the surface preconcentration
of DA by ion selectivity and accumulation of DA in the hydrophilic
regions, and CD formed a stable host–guest inclusion complex
with DA and enhanced the electron-transfer kinetics. The calibration
curves for DA in urine were linear in the ranges of (0.05–20
μM) and (40–280 μM) with R2 of 0.995 and DL of 0.6 nM in low concentration range. The
proposed method demonstrated that it is possible to discriminate DA
from AA and UA or AA and APAP at physiological pH. Furthermore, the
sensor possessed excellent anti-interference capability for simultaneous
determination of DA, ST, and TY in biological fluids.
Experimental Part
Chemicals and Solutions
Chemicals,
namely, β-CD, acetonitrile, lithium perchlorate (LiClO4), NF, ethyl alcohol, gold chloride (AuCl2), potassium
nitrate (KNO3), DA, UA, AA, TY, APAP, and serotonin (ST)
were supplied by Aldrich Chem. Co. (Milwaukee, WI, USA). 0.1 M PBS
(1 M K2HPO4 and 1 M KH2PO4) was used as the supporting electrolyte.
Apparatus
The voltammetry measurements
were carried out with a BAS Epsilon electrochemical analyzer in a
three-electrodes cell. The electrodes were Ag/AgCl, a platinum wire,
and a gold electrode. A Quanta 250 FEG instrument and Gamry-750 system
was used for SEM measurements and electrochemical impedance spectroscopy.[21]
Fabrication of the Au/NF/CD/AuNPs
Sensor
Preparation of the Au/NF/CD/AuNPs sensor was performed
as follows:
In the first step, a layer of 10 μL from 2% NF was casted on
a Au-electrode surface, and then, the electrode was left to dry in
open air for 15 min. In the second step, a layer of CD was electrodeposited
from a solution containing 10–3 M CD/0.05 M LiClO4 by BE method at +1.2 V for 3 min. The last layer of gold
nanoparticles was electrodeposited from a solution of (6 mM HAuCl4/0.1 M KNO3), over the Au/NF/CD surface by the
CV technique, and the potential was cycled between −0.8 and
+0.4 V for 20 cycles at a scan rate 50 mV s–1. The
electrode was represented as Au/NF/CD/AuNPs (Scheme ).
Scheme 2
Schematic Representation of the Modified
Electrode; Au/NF/CD/AuNPs
Used for the Electrochemical Oxidation of DA
Authors: Selvakumar Palanisamy; Vijaylakshmi Velusamy; Sukanya Ramaraj; Shih-Wen Chen; Thomas C K Yang; Sridharan Balu; Craig E Banks Journal: Mater Sci Eng C Mater Biol Appl Date: 2018-12-29 Impact factor: 7.328
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Scheme 1
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Scheme 2