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Analytical Letters
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Determination of Dopamine with a Modified Carbon Dot Electrode
Qianhua Li, Zhewen Xu, Wenjie Tang & Ying Wu
To cite this article: Qianhua Li, Zhewen Xu, Wenjie Tang & Ying Wu (2015) Determination of Dopamine with a Modified Carbon Dot Electrode, Analytical Letters, 48:13, 2040-2050, DOI:10.1080/00032719.2015.1015074
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Analytical Letters ,48:2040–2050,2015
Copyright #Taylor &Francis Group,LLC ISSN:0003-2719print/1532-236X online DOI:
10.1080/00032719.2015.1015074
Electrochemistry
DETERMINATION OF DOPAMINE WITH A MODIFIED CARBON DOT ELECTRODE
Qianhua Li,1Zhewen Xu,1Wenjie Tang,1and Ying Wu 1,2
1
College of Chemistry,Chemical Engineering and Materials Science,Soochow University,Suzhou,China 2
The Key Lab of Health Chemistry and Molecular Diagnosis of Suzhou,Suzhou,China
A facile and green synthesis was employed to synthesize water-soluble carbon dots that were shown to be uniform monodisperse spheres with an average diameter of approximately 4nanometers.Because of their strong adsorption,the carbon dots were assembled on a glassy carbon electrode without a bridging agent.The modified electrode was employed for the sensitive determination of dopamine.In 0.1mole per liter phosphate buffer at pH 6.0,the peak currents of dopamine increased at the modified electrode.A calibration curve for dopamine was obtained from 1.5×10à7to 1.5×10à4mole per liter with a limit of detection of 2.6×10à8mole per liter.The sensor was employed to determine dopamine in human plasma without interferences from uric acid and ascorbic acid.The carbon dot modified electrode also exhibited high stability and excellent precision.The results demonstrate the facile fabrication of a carbon dot-based sensor and a sensitive method for the determination of dopamine.
Keywords:Carbon dots;Cyclic voltammetry;Differential pulse voltammetry;Dopamine;Electrochemical behavior;Modified electrode
INTRODUCTION
Carbon nanomaterials with sizes from 1to 100nanometers in one or more dimensions are relevant due to their exceptional electrical,thermal,chemical,and mechanical properties.Consequently,they have been used as composite materials,for energy storage and conversion,sensors,drug delivery,field emission devices,and electronic components.Many studies on carbon nanomaterials have focused on the development of sensors,which are characterized by high electrocatalytic activity and good electrical conductivity (Liu et al.2009;Asadian et al.2014;Goornavar et al.2014).In recent years,new nanocarbon materials,namely,fluorescent carbon dots,have been widely studied because of high emission quantum
Received 25October 2014;accepted 25January 2015.
Address correspondence to Ying Wu,College of Chemistry,Chemical Engineering and Materials
Science,Soochow University,Suzhou 215123,P.R.China.E-mail:yingwu@https://www.doczj.com/doc/2d16282505.html,
Color versions of one or more of the figures in this article can be found online at www.tandfonline.
com/lanl .
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yields,size-tunable emission,chemical and physical stability,narrow spectral bands,and ready surface modification for specific sensing applications (Algar,Tavares,and Krull 2010).Compared to semiconductor quantum dots,carbon dots are superior in terms of eco-friendliness,ease of preparation,low cytotoxicity (Han et al.2012;Mewada et al.2013;Jiang et al.2014),high electrochemical activity,and conductivity (Dai et al.2012;Gao,Han,and Ma 2013).Moreover,carbon dots are usually coated with hydroxyl and carboxyl groups that enhance water solubility and biocompatibility (Li et al.2015).To date,carbon dots were successfully employed in photocatalysis (Zhang et al.2011;Zhang et al.2012),chemiluminescence (Lin et al.2011;Amjadi,Manzoori,and Hallaj 2014)and benign optical probes and labels for bioimaging (Yang et al.2009a ,2009b ;Han et al.2012).Owing to the high surface area,excellent electron transfer ability,and low cytotoxicity,applications of carbon dots as sensors are advantageous.For example,due to the high conductivity and electrochemical activity,a chitosan and carbon dot modified electrode showed high sensitivity for triclosan (Dai et al.2012).Based on biocompatibility and rapid electron transfer,an electrochemical immunosensor was fabricated by using a polyamidoamine dendrimer capped carbon dot –gold nanocrystal nanocomposite for alpha-fetoprotein (Gao,Han,and Ma 2013).
Dopamine,a well-known neurotransmitter,plays a crucial role in the renal,cardiovascular,central nervous,and hormonal systems.The determination of dopa-mine in the human body is significant for clinical and medical applications.For this purpose,a variety of methods have been employed,including liquid chromatography (Muzzi et al.2008),ultraviolet-visible spectrometry (Barreto et al.2008),capillary electrophoresis (Li et al.2010),fluorescence spectrometry (Liu et al.2013),and electrochemical analysis (Li et al.2011;Tsierkezos et al.2013).Among these,electrochemical analysis has attracted much attention owing to its high sensitivity,rapid response,low cost,and simple instrumentation.Carbon nanomaterial-modified electrodes are commonly used for the selective determination of dopamine.For example (Raghu et al.2014),studied the oxidation of dopamine by immobilizing multiwalled carbon nanotubes on a poly(glycine)modified carbon paste electrode.The resulting sensor successfully determined dopamine in the presence of various interferences and in clinical preparations.Bu et al.(2013)investigated the electro-chemical behavior of dopamine on graphene modified electrodes,which exhibited remarkable catalytic activity toward the oxidation of dopamine.
In this study,a facile,economical,and green synthesis of water-soluble carbon dots is reported.The carbon dots were assembled on the glassy carbon electrode without a bridging agent because of the large number of carboxyl groups on the surface produced strong adsorption.The modified electrode provided fast electron transfer,good sensitivity,and good stability for the determination of dopamine,demonstrating favorable performance compared to quantum dot composite modified electrodes reported in the literature (Sun et al.2012;Huang et al.2014).EXPERIMENTAL
Reagents and Chemicals
Dopamine,uric acid,and ascorbic acid were purchased from Sigma-Aldrich.Glucose and glycol were purchased from the National Pharmaceutical Group
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(China).Na 2HPO 4·12H 2O,NaH 2PO 4·2H 2O,K 3Fe(CN)6,and K 4Fe(CN)6were obtained from Shanghai Reagent (Shanghai,China)and used without further purification.All other reagents were of analytical reagent grade.Double distilled water was used throughout the work.
Apparatus
An electric thermostatic drying oven was purchased from Shanghai Kang Lu (China).Transmission electron micrographs (TEM)were obtained on a Tecnai G2F20S-TWIN from FEI (US).Molecular absorption spectra were recorded using a Cary 60spectrometer (Agilent,Australia).The Fourier transform infrared spectra and X-ray photoelectron spectra (XPS)were obtained using a Prostar LC240(Japan)and an Axis Ultra DLD,(USA)respectively.Electrochemical experiments were conducted in a conventional three-electrode system using a bare or modified glassy carbon electrode (GCE)as the working electrode,a platinum disk electrode as the counter electrode,and a saturated calomel electrode (SCE)as the reference electrode.Cyclic voltammetry (CV)and differential pulse voltammetry (DPV)were carried out on a RST 3100D electrochemical workstation (Suzhou Ruisite Instruments Co.China).All measurements were performed at room temperature.
Synthesis of Carbon Dots
Carbon dots were synthesized from glucose following a modification of the method reported by Han et al.(2012).The 0.4gram of glucose was added to a 20milliliter glycol/distilled water (v:v/1:1)mixture and ultrasonicated to obtain a clear solution.The mixture was added to a high pressure nitrifying pot (45milliliters)and heated to 190degree Celsius in an oven for three hours to form the product as the solution color changed from colorless to clear yellow.The carbon dots were stored at room temperature until use.
Electrode Modification
A glassy carbon electrode was polished to a mirror-like finish with 1.0,0.3,and 0.05micrometer alumina slurries on fine abrasive paper and washed ultrasonically with 0.5mole per liter sodium hydroxide,anhydrous ethanol,and double distilled water.The bare electrode was scanned in 0.5mole per liter H 2SO 4between à0.2and 1.5volts until a reproducible cyclic voltammogram was obtained.The bare elec-trode was then rinsed thoroughly with double distilled water and allowed to dry under nitrogen.An aliquot of 15microliters of the carbon dots solution was hand-cast on the surface of a clean glassy carbon electrode.The modified electrodes were dried overnight at room temperature before use.RESULTS AND DISCUSSION
Characterization of the Prepared Carbon Dots
Figure 1shows a transmission electron micrograph (TEM)of the synthesized carbon dots,showing uniform,monodispersed spheres with an approximate average
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diameter of 4nanometers (Figure 1).A typical photoluminescence spectrum of carbon dots was obtained.When the carbon dots were excited at 352nanometers,the fluorescence spectrum showed symmetrical emission at 444nanometers.A weak absorption peak was observed at 350nanometers which corresponded to the fluorescence excitation.These phenomena are consistent with a previous report (Han et al.2012).
Fourier transform infrared spectroscopy (FTIR)and X-ray photoelectron spectroscopy (XPS)were employed to characterize the carbon dots.Figure 2demonstrates the presence of O –H,C –H,C =O,C =C,and C –O groups on the surface.Characteristic stretching of C =O at 1630per centimeter and of O –H at 3385per centimeter were present,demonstrating carboxyl groups on the surface of carbon dots (Han et al.2012).This result was also verified by the X-ray photoelectron spectra.The C 1s spectrum showed peaks at 284.8,286.8,and 288.8electron volts,attributed to C –C,C –O,and C =O.The carbon dots also contained C =O at 531.3electron volts and C –O –C at 532.9electron volts as shown in O 1s spectrum.These results were consistent with a previous report (López et al.2015
).
Figure 1.Transmission electron micrograph of the carbon
dots.
Figure 2.Fourier transform infrared spectrum of the carbon dots.
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Electrochemistry of Dopamine at the Carbon Dot Modified Glassy Carbon Electrode
Figure 3shows cyclic voltammograms of 0.1millimole per liter dopamine at a bare glassy carbon electrode and the carbon dot modified glassy carbon electrode in phosphate buffer at pH 6.0.A weak response from dopamine was observed at the bare electrode (curve a),while the response was enhanced at the modified electrode (curve b).The oxidation and reduction peak potentials (E pa and E pc )of dopamine were at 221and 172millivolts at the bare electrode and 213and 182millivolts at the modified electrode.The 4E p value was 49millivolts and the anodic and cathodic peak current ratio (I pa /I pc )was 1.13at the bare glassy carbon electrode,indicating the reaction was quasireversible.For the carbon dot modified glassy carbon electrode,the 4E p value was 31millivolts,and I pa /I pc was1.25.The oxidation response I pa was 10.5microampere,which was enhanced by 2.2fold compared to the value at the unmodi-fied electrode.These results suggest that the enhanced performance and larger surface area resulting from the carbon dots significantly enhanced the electrochemical activity and facilitated the electron transfer for dopamine.The electroactive area of the carbon dot modified glassy carbon electrode was calculated to be 0.121square centimeter using the Randles-Sevcik equation based on a cyclic voltammogram of the [Fe(CN)6]3à/[Fe(CN)6]4àredox couple,which is almost 1.7times larger than the geometric area of the bare electrode.
The effect of scan rate (v)on the peak currents of dopamine at the carbon dot modified glassy carbon electrode was investigated between 20and 700millivolts per second.The logarithm of the peak current was linearly proportional to the logarithm of the scan rate.The regression equations were:
log I pa ?0:6763à0:8862log v millivolts per second eTR 2?0:9979e1Tlog I pc ?à0:7323t0:8788log v millivolts per second eTR 2?0:9983
e2T
This behavior is expected for surface adsorbed species.The enhancement of dopamine at the carbon dot modified glassy carbon electrode may be due
to
Figure 3.Cyclic voltammograms of 0.1millimole per liter dopamine in pH 6.0phosphate buffer at the (a)bare glassy carbon electrode and (b)carbon dot modified glassy carbon electrode.Scan rate:50millivolts per second.
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adsorption on the surface by hydrogen bonds,physical adsorption,and the electrostatic interactions (Huang et al.2011).
Effect of pH
The voltammetric behavior of dopamine at the carbon dot modified glassy car-bon electrode was investigated as a function pH from 4.0to 8.0.Figure 4shows that the anodic and cathodic peak potentials shifted towards negative values as the pH increased,indicating that protons are involved in the electrode reaction.A linear relationship between E pa ,E pc ,and pH was observed.The slopes of à59.8and à62.2millivolts per pH showed that the electron transfer was accompanied by the reaction of an equal number of protons.This behavior nearly followed the Nernst equation for a reaction with equal numbers of electrons and protons (Kang and Lin 2006).When the pH was set to 6.0,the oxidation and reduction peak currents reached a maximum value.Thus,the optimal pH of phosphate buffer was selected to be 6.0.
Analytical Figures of Merit
To determine the linearity of dopamine at the carbon dot modified electrode,a series of standard dopamine solutions were determined by differential pulse voltam-metry (Figure 5).Under optimal conditions,the oxidation peak current of dopamine increased with concentration from 1.5×10à7to 1.5×10à4mole per liter.The linear regression equation was:
I p microampere eT?12:60t3:74C dopamine micromole per liter eTeR 2?0:9989T
e3T
The detection limit was determined to be 2.6×10à8mole per liter based on the standard deviation of the response and the slope of the calibration curve.Table 1compares the performance of the carbon dot modified glassy carbon electrode to modified electrodes reported in the literature.The carbon dots provided enhanced better sensitivity than most of the previous
approaches.
Figure 4.Cyclic voltammograms of 0.2millimole per liter dopamine at the carbon dot modified glassy carbon electrode at pH values of (a)4.0,(b)4.6,(c)5.0,(d)5.6,(e)6.0,(f)6.6,(g)7.0,(h)7.6,and (i)8.0.Scan rate:50millivolts per second.Inset:anodic and cathodic peak currents as a function of pH.
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Validiation of the Sensor
In order to demonstrate the selectivity of the modified electrode,an interference investigation was performed in phosphate buffer containing 0.01millimole per liter dopamine with ascorbic acid and uric acid,two common interferences for
dopamine.
Figure 5.Differential pulse voltammograms as a function of dopamine concentration:(a)1.5×10à7,(b) 2.5×10à7,(c)6×10à7,(d)1×10à6,(e) 2.55×10à6,(f) 3.95×10à6,(g)7×10à6,(h)1×10à5,(i)1.5×10à5,(j)6×10à5,(k)1×10à4,and (l)1.5×10à4mole per liter.Inset:peak currents as a function of dopamine concentration.
Table https://www.doczj.com/doc/2d16282505.html,parison of the analytical performance of modified electrodes for the determination of dopamine
Modified electrode Method Linear dynamic range (10à6mole per liter)Detection limit (10à6mole per liter)Reference Horseradish peroxidase-multiwalled carbon nanotubes-silica sol-gel/poly(glycine)Differential
pulsevoltammetry
15–165
0.60
Raghu et al.2014
Fe 3O 4nanoparticles Differential
pulsevoltammetry 6.0–600.10Xu et al.2012Graphene-chitosan.Differential
pulsevoltammetry 5–200–Wang et al.2009Chitosan-ZnO/polyanilne
nanocomposite Differential
pulsevoltammetry 20–180
0.21
Pandiselvi and Thambidural 2014
Graphene/poly
(p-aminobenzoic acid)Differential
pulsevoltammetry 0.03–1.160.01Huang et al.2011Graphene Cyclic voltammetry 0.5–100.50Ma,Chao,and Wang 2012
Overoxidized polypyrrole/graphene Cyclic voltammetry
25–1000
0.10
Zhuang et al.2011
Carbon dots
Differential
pulsevoltammetry
0.15–1500.026This work
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No significant interferences were observed by differential pulse voltammetry for 0.01millimole per liter of dopamine in the presence of 1millimole per liter of ascorbic acid and 1millimole per liter of ascorbic acid and uric acid.The favorable selectivity of the modified electrode may because the carbon dot film is in its anionic form in pH 6.0phosphate buffer.Under these conditions,ascorbic acid (pK a ?4.1)and uric acid (pK a ?5.75)are negatively charged,but dopamine (pK a ?8.89)is positively charged (Wang et al.2009).Consequently,the carbon dots act as cationic exchangers at the electrode surface,selectively attracting dopamine and allowing it to pass to the electrode surface.As a result,100fold excess of ascorbic acid and 100fold excess of uric acid did not react on the electrode,providing acceptable selectivity for dopamine.
The reproducibility of the method for the determination of dopamine was inves-tigated by five replicate measurements at one carbon dot-based sensor.The relative standard deviation of the five slopes was 1.98percent,indicating that the reproduci-bility of the sensor was satisfactory.Five carbon dot modified glassy carbon electro-des were fabricated independently and used to determine dopamine.The relative standard deviation of the slopes was 5.74percent,demonstrating that the precision of the sensor was acceptable.The long-term stability of the modified electrode was monitored by measuring the current response of dopamine two weeks after preparation with storage at 4degree Celsius.The relative standard deviation of the slopes was 4.09percent after two weeks.In addition,stable signals were obtained from the carbon dot modified electrode under continuous cyclic potential scanning for 200cycles for 0.2millimole per liter dopamine.These findings demonstrate that the sensor has acceptable stability.
Application of the Carbon Dot Modified Glassy Carbon Electrode The carbon dot-based sensor was employed to determine dopamine in human plasma.For this purpose,the plasma was diluted twenty times with 0.1mole per liter phosphate buffer at pH 6.0.The differential pulse voltammetric signal associated with dopamine was not detected in untreated plasma,showing the dopamine concen-tration was below 5.2×10à7mole per liter.Human plasma was then fortified with dopamine and the response at carbon dot modified glassy carbon electrode was recorded.The recovery values were between 98.5and 102.7percent with a relative standard deviation of 2.8percent,demonstrating practical application of the modified electrode (Table 2).
Table 2.Determination of dopamine in human plasma Sample Initial concentration
Added concentration (10à6mole per liter)
Measured concentration (10à6mole per liter)
Recovery (percent)Relative Standard Deviation (n ?3)
(percent)
1à0.500.49599.0?1.62à 1.000.98598.5?2.33
à
1.50
1.540
102.7
?2.8
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CONCLUSIONS
A facile,economical,and green method was employed to synthesize water-soluble carbon dots.Strong adsorption of the carbon dots allowed preparation of a modified electrode without a bridging agent.The carbon dot-modified electrode showed good electrochemical behavior in terms of accelerated electron transfer and excellent stability.As a further application,the sensor was used for the determination of dopamine.The results demonstrated the favorable performance of the sensor.The results demonstrate facile fabrication of carbon dot-based sensors that offer good sensitivity,simplicity and stability,and rapid measurements for the determination of dopamine.
FUNDING
We gratefully acknowledge the financial support extended by the National Natural Science Foundation of China (No.21475092),the Natural Science Foundation of Jiangsu Province (BK2011273),the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD)Project of Scientific and Technologic Infrastructure of Suzhou (SZS201207),and the Open Research Project of the Battery Technology Innovation Center for Public Services of Jiangsu Province (HDP201206).REFERENCES
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D o w n l o a d e d b y [S h a n g h a i J i a o t o n g U n i v e r s i t y ] a t 00:03 14 N o v e m b e r 2015