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Porous carbon-modified electrodes as highly selective and sensitive sensors for detection of dopamine† Pitchaimani Veerakumar,‡a Rajesh Madhu,‡b Shen-Ming Chen,*b Chin-Te Hung,a Pi-Hsi Tang,ac Chen-Bin Wangc and Shang-Bin Liu*ad Carbon porous materials (CPMs) with high surface areas up to 2660 m2 g1, directly fabricated by a facile microwave-assisted route, were applied to the electrochemical detection of dopamine (DA). The CPM-
Received 16th June 2014 Accepted 16th July 2014
modified electrodes exhibited excellent selectivity, a desirable detection limit (2.9 nM), and extraordinary sensitivity (2.56 mA mM1 cm2) for detection of DA, even in the presence of large amounts of foreign
DOI: 10.1039/c4an01083c
species, such as ascorbic acid (AA) and uric acid (UA), making feasible the practical applications of these
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electrodes as DA sensors.
1. Introduction Carbon porous materials (CPMs) possess numerous unique physicochemical properties, such as high surface areas, tailorable pore size, good electrical conductivity, biocompatibility, and desirable thermal, mechanical, and electrical properties, and they have attracted considerable R&D attention.1 It is these unique properties that make CPMs promising supports for electrodes and/or for electrode materials themselves in many important applications, such as in adsorption/separation, purication, catalysis, fuel cells, supercapacitors, sensors, and bioanalysis.2–9 Numerous methods have been developed for the synthesis of CPMs by various hard or so templates.1,10–15 For example, it has been shown that ordered mesoporous carbons may be fabricated via a facile self-assembly route using low-cost phenolic resin and commercial surfactants, followed by a carbonization treatment.16 Compared to conventional methods, CPM fabrication via microwave-assisted heating has drawn considerable R&D attention owing to its advantages, such as facile control of porosity and surface area, increased product yield and purity, reduced reaction time and operation cost, and feasibility for mass production.17 In fact, CPMs prepared under microwave irradiation have been extensively exploited as a
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. E-mail: [email protected]
b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan. E-mail: [email protected]
c Department of Chemical and Materials Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 33449, Taiwan d
Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
† Electronic supplementary information (ESI) available: Schemes, electrochemical oxidation mechanism, calculation details, Raman, SEM, TEM, pore size distribution, CV and DPV results. See DOI: 10.1039/c4an01083c ‡ These authors contributed equally.
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supports for electrodes in fuel cells.18–22 CPMs have also been widely utilized as electrodes for disposable electrochemical sensors, owing to their high surface area, excellent conductivity, chemical inertness, bio-compatibility, and low cost.5,23–28 It has also been shown that CPMs readily provide favourable sites for electron transfer in biomolecules; hence, CPMs can greatly improve the sensitivity of electrochemical detection.23–26 Dopamine (DA), one of the major catecholamines, plays many critical roles in the human central nervous and cardiovascular systems.29,30 For instance, dopaminergic neurotransmission is found to be closely related to diseases such as Parkinson's and schizophrenia.31 In recent decades, many techniques have been developed for sensitive detection of DA, including uorimetry,32 spectrophotometry,33 and electrochemical methods.34,35 Among them, electrochemical detection of DA was found to be the most simple, cost-effective, and userfriendly method. The electrochemical method also exhibits the desirable sensitivity and selectivity. However, scant reports on the selective detection of DA and/or simultaneous detection of DA, ascorbic acid (AA), and uric acid (UA) based on CPMmodied electrodes are available.36–40 In this study, we report a facile microwave-assisted synthesis route for fabricating CPMs with high surface area and pore volume. The CPMs so prepared were utilized for electrochemical detection of DA. It is shown that such CPM-modied, glassy carbon electrodes (GCEs) exhibit excellent sensitivity and selectivity and should be highly feasible as practical and cost-effective DA sensors.
2. 2.1
Experimental Chemicals and solutions
Resorcinol (ACS, 99.98%, Acros), formaldehyde (37% in water, Acros), anhydrous sodium carbonate (ACS, Fisher), triblock copolymer Pluronic F-127 (EO106PO70EO106, Mw ¼ 12 600, Analyst
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Sigma-Aldrich), ethanol (C2H5OH, 99%), and dopamine hydrochloride (Sigma-Aldrich) were obtained commercially and used without further purication. The phosphate buffer solution (PBS) at pH 7.0 was prepared by using 0.05 M Na2HPO4 and NaH2PO4 solutions. The pH of the solutions was adjusted with 0.5 M H2SO4 and 2.0 M NaOH. All other chemicals used were analytical grade and all solutions were prepared using ultrapure water (Millipore).
(900 or 1000 C), and then being kept for 1–2 h before it was eventually brought down to RT under a N2 gas ow. These CPMs activated with CO2 at 900 C for 1 h, 1000 C for 1 h, and 1000 C for 2 h are denoted as CPM-x-A1, CPM-x-A2, and CPM-x-A3, respectively, where x represents the original carbonization temperature.
2.2
To prepare carbon electrodes for electrochemical sensors, typically 5.0 mg CPM was rst dispersed in 1.0 mL water and sonicated for 2 h; then 6.0 mL of the substrate was withdrawn and placed onto the pre-cleaned GCE. The CPM-modied electrode was allowed to dry in air at 30 C for 2 h, followed by gently rinsing with doubly distilled water several times to remove the loosely bound carbon prior to further electrochemical measurements, which were carried out at RT under inert (N2) atmosphere.
Instrumentation
All powdered X-ray diffraction (XRD) patterns were recorded on a PANalytical (X'Pert PRO) diffractometer using CuKa radiation (wavelength l ¼ 0.1541 nm). Textural properties of CPMs, including Brunauer–Emmett–Teller (BET)-specic surface areas, pore volumes, and pore sizes were analyzed by nitrogen adsorption/desorption isotherm data measured on a Quantachrome Autosorb-1 apparatus (at 77 K). The pore size distribution of each CPM was derived from the adsorption branch of the isotherm using the Barrett–Joyner–Halanda (BJH) method. The morphology of various samples was monitored by eld emission-transmission electron microscopy (FE-TEM; JEOL JEM-2100F). All Raman spectra were recorded at ambient temperature on a WITeck CRM200 confocal microscopy system equipped with a laser (l ¼ 488 nm). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies were performed on an electrochemical analyzer (CH instruments; CHI 900). 2.3
Material synthesis
CPMs were synthesized under conditions based on procedures reported previously.41–44 Typically, 0.0056 g of Na2CO3 was rst dissolved in carbon precursors consisting of formaldehyde (1.13 g) solution (37 wt%) and resorcinol (1.10 g) at RT for 1 h in a stirring water bath (see Step I, Scheme S1 in the ESI†). The resultant polymeric RF resol (light brown) was then mixed with a solvent mixture (ethanol–water ¼ 7 : 2) together with the amphiphilic Pluronic F-127 surfactant polyol (0.8 g; as the structural directing agent or so template). To this solution, 1.0 mL of 2.0 M HCl were added dropwise under continuous stirring. Subsequently, the precipitated mixture was then subjected to microwave irradiation (typically for 1–2 h), followed by curing (at 100 C overnight), washing and drying of the polymeric gel (Step II, Scheme S1; ESI†). Finally, the dried sample was subjected to a carbonization procedure in a tube furnace under a continuous ow of N2 gas. Typically, the carbonization procedure was carried out by slowly ramping from RT to 350 C with a rate 1 C min1, then, by further liing the temperature to 600–900 C at a rate of 2 C min1, and kept at the nal temperature for 5 h before being cooled to RT (Step III, Scheme S1; ESI†). The samples so prepared are denoted as CPM-x, where x denotes the carbonization temperature (Tc). Additional activation of CPMs with CO2 were also performed.45 This was done by loading the pristine CPM (1 g) in a quartz tube placed inside the furnace. Subsequently, the substrate was rst heated under a N2 environment at a rate 10 C min1 until reaching a temperature of 900 C, followed by switching to a CO2 gas stream (ow rate 30 cm3 min1) at a designated temperature
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2.4
3. 3.1
Fabrication of CPM-modied electrode
Results and discussion Structural and textural properties of CPMs
Fig. 1 shows the large-angle powdered XRD patterns of assorted pristine and activated CPM samples. Regardless of the postsynthesis activation treatment, all CPMs exhibit two main characteristic peaks associated with turbostratic carbons with graphitized structures.46–48 More specically, the diffraction peaks at 23.5 and 43.5 may be attributed to (002) and (100) index planes, respectively. The degree of graphitization of various CPMs was also examined by Raman spectroscopy. Raman spectra obtained from CPMs revealed two characteristic peaks centering at ca. 1360 (D-band) and 1600 (G-band) cm1, which may be attributed to disorder or defects located at the edges of graphite platelets and sp2-bonded graphitized carbons, respectively (Fig. S1; ESI†).49,50 Accordingly, the D- to G-band
Fig. 1 Large-angle XRD patterns of (a) CPM-350, (b) CPM-600, (c) CPM-900, (d) CPM-900-A1, (e) CPM-900-A2, and (f) CPM-900-A3 materials.
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peak intensity ratio (ID/IG) deduced for the CPM-900 sample is 1.1, revealing the presence of graphitized carbon structure. Based on SEM results, the as-synthesized CPMs exhibited morphology of micron-size crystalline aggregates with irregular shapes (see Fig. S2; ESI†). Additional TEM studies revealed that the pristine CPMs thus fabricated via the microwave-assisted synthesis procedure showed mesosopic pores with short-range ordering (Fig. 2a and S3; ESI†), indicating the presence of multidimensional wormhole-like pore structure,51,52 which is consistent with the results obtained from N2 adsorption/ desorption isotherm measurements. As displayed in Fig. 3A, the isotherms obtained from pristine CPM-x (x ¼ 350, 600, and 900 C) samples exhibit typical Type-IV curves with indicated capillary condensation steps at P/P0 0.4–0.7 and well-dened H1-type hysteresis loop anticipated for mesoporous materials.53 The sharp increase of the isotherm curve at P/P0 < 0.05 also reveals the presence of microporosity in these CPMs. Moreover, a notable increase in BET surface area (SBET) and total pore volume (Vo) with increasing carbonization temperature (Tc) may be inferred for the pristine CPMs. As shown in Table 1, the SBET observed for CPMs prepared with Tc from 350 to 900 C increased from 243 to 744 m2 g1, while the corresponding Vo also increased from 0.20 to 0.53 cm3 g1. On the other hand, a consistent decrease in average pore size (dBJH) with Tc was observed for these CPMs, as revealed by results deduced from their BJH pore-size distributions (Table 1 and Fig. S4; ESI†). For example, a notable decrease in pore size was observed for the CPM-900 sample (dBJH ¼ 5.9 nm) compared to CPM-600 (dBJH ¼ 6.9 nm). It is noteworthy that a narrower pore-size distribution prole was observed for CPMs graphitized at higher Tc, indicating that carbon framework shrinkage is accompanied by improved structural ordering, which coincides with results
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Fig. 3 N2 adsorption/desorption (77 K) isotherms of various CPMs (A) before and (B) after activation treatment by CO2. (a) CPM-350, (b) CPM-600, (c) CPM-900, (d) CPM-900-A1, (e) CPM-900-A2, (f) CPM900-A3.
obtained from TEM (Fig. 2a and S3; ESI†) and isotherm measurements (Fig. 3A). The presence of microporosity was further analyzed by using the t-plot method. The results of the microporous surface area and pore volume derived from t-plot analyses are given in Table 1. Upon activation with CO2, notable increases in structural and absorptive properties were observed for the activated CPMs as compared to the as-synthesized CPM-900 sample. This may be readily seen by the variations in TEM proles (Fig. 2) and isotherm curves (Fig. 3B). As a result, substantial increases in SBET and Vo were also found. This phenomenon, which became more pronounced with increasing severity of the activation treatment, was accompanied by simultaneous increase in microporosity as well as a decrease in dBJH (Table 1). For example, upon activation of the pristine CPM-900 at 1000 C for 2 h under the CO2 environment, signicant increases in SBET (from 744 to 2661 cm2 g1) and Vo (from 0.53 to 2.05 cm3 g1) were observed for the resultant CPM-900-A3 (Table 1). Meanwhile, the corresponding microporous surface area and pore volume were also found to increase from 411 to 510 cm2 g1 and 0.18 to 0.28 cm3 g1, respectively, while dBJH decreased from 5.9 to 5.4 nm. However, a slight degradation of mesostructure was observed for carbon samples subjected to prolonged treatment at elevated activation temperature (Ta $ 1000 C), as evidenced by the broadening of characteristic diffraction peaks (Fig. 1). Nonetheless, a similar ID/IG ratio (1.1) was deduced for CPM-900 sample before and aer activation treatment (Fig. S1; ESI†). Similar behaviours were also found for CO2-activated mesoporous carbons prepared from phenol–formaldehyde resol.54 3.2
Fig. 2 TEM images of (a) as-synthesized CPM-900, (b) CPM-900-A1, (c) CPM-900-A2 and (d) CPM-900-A3 obtained after various CO2activation treatments (see text).
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Electrochemical oxidation of DA
We applied cyclic voltammetry (CV) technique to assess the performance of electrochemical oxidation of DA over the CPMmodied glassy carbon electrode (GCE). Over the unmodied GCE, the DA oxidation peak recorded in the presence of N2saturated phosphate buffer solution (PBS) with 2.0 mM DA was found to locate at 0.453 V (Fig. S5a; ESI†). Moreover, as a blank test, no peak was observed for the bare CPM-900-A3-modied GCE in PBS without the presence of DA (Fig. S5b; ESI†). On the other hand, an enhanced DA oxidation peak at 0.209 V was
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Table 1 Textural properties of as-synthesized and activated CPM samples, electrochemical activities, and relevant kinetic parameters obtained from corresponding CPM-modified GCE during DA detectiona
CO2 activation
SBET (m2 g1)
Vo (cm3 g1)
Sample
Tc ( C)
Ta ( C)
Time (h)
Total
Micropore
Total
Micropore
dBJH (nm)
Ipab (mA)
Dc (cm2 s1)
Kad (cm s1)
CPM-350 CPM-600 CPM-900 CPM-900-A1 CPM-900-A2 CPM-900-A3
350 600 900 900 900 900
— — — 900 1000 1000
— — — 1.0 1.0 2.0
243 541 744 1116 1349 2661
120 381 411 449 486 510
0.20 0.37 0.53 0.69 0.84 2.05
0.31 0.15 0.18 0.19 0.22 0.28
7.9 6.9 5.9 5.8 5.9 5.4
2.3 4.7 8.9 2.7 2.8 27.0
0.007 0.031 0.110 0.010 0.011 1.012
0.01 0.04 0.59 0.66 0.69 0.88
a SBET ¼ BET surface area; Vo ¼ pore volume; dBJH ¼ BJH pore size; Ipa ¼ anodic peak current; D ¼ diffusion coefficient; Ka ¼ apparent electron transfer rate constants. b Determined with a scan rate of 50 mV s1. c Calculated from Randles–Sevcik equation (ESI). d Derived from Laviron equation (ESI).
observed over the CPM-900-A3-modied GCE at a scan rate of 50 mV s1 (Fig. S5c; ESI†). Interestingly, the corresponding peak current increased 27-fold compared to that of the pristine GCE; this may be attributed to the faster diffusion and electron transfer rate observed for the CPM-900-A3-modied GCE compared to the other CPMs (Table 1; more details in ESI†).55,56 For comparison, CV curves recorded from various modied electrodes are also depicted (Fig. S6; ESI†). Evidently, electrodes modied with CPM-900-A3 exhibited the best electrochemical performance towards the detection of DA. This may be attributed to the enriched porosities and higher surface possessed by CPM-900-A3 (vide supra). It is also noteworthy that the DA oxidation potential so observed over the CPM-modied GCE is much lower than that over other carbon modied electrodes, such as carbon nanotube (CNT)-modied ionic liquid gel (ILG),57 graphene,58 or thionine-naon supported on multi-wall carbon nanotubes (MWCNTs),59 as summarized in Table 2. This is attributed to the excellent conductivity and high surface areas possessed by the CO2-activated CPM.60 Moreover, the CPM-900A3-modied GCE also exhibited a well-dened oxidation peak of DA with peak to peak separation (DEp) of 41 mV. Such a small DEp value is favourable for fast electron transfer of DA at the
Table 2
electrode surface.61 These results reveal that CPM-900-A3 pores act as reservoirs, which effectively reduce the diffusion length of ions from the electrolyte. This was further veried by varying the scan rate of CV measurement, as illustrated in Fig. 4a for the CPM-900-A3-modied GCE in N2-saturated PBS with 2 mM DA. The fact that the oxidation peak current (Ipa) showing a linear dependence with the scan rate between 50 and 500 mV s1 (Fig. 4b) indicates that oxidation of DA at the CPM-modied electrode is indeed a surface-controlled process.62 A mechanism invoking the reversible electron transfer between DA and dopamine-o-quninone over the CPM-modied electrode is illustrated in Scheme S2 (ESI†). Next, we studied the effects of pH in the buffer solution on electro-oxidation of DA over CPM-modied GCE. This was carried out by varying the pH (3.0–11.0) of N2-saturated PBS containing 2.0 mM DA. The CV curves recorded with a scan rate of 50 mV s1 under varied buffer solution pH are displayed in Fig. 5. Upon increasing pH of the buffer solution containing DA, the CV curve of CPM-900-A3-modied GCE tends to shi toward more negative anodic and cathodic peak potentials, and vice versa for descending pH. On the basis of linear correlation between the anodic peak potential and pH of the buffer
Types of modified electrodes and performance as DA sensors
Modied electrodesa
DA detection limit (mM)
Sensitivity (mA mM1 cm2)
Detection methoda
Oxidation potential (V)
Reference
MWCNT/GONR MPEG polymer Degraded dopamine CCINPs-CS GNPs/graphene Pt/carbon CNTs-ILG Graphene Thionine-naon/MWNT Calix[4]arene crown-4 ether CPM
0.08 0.05 0.04 0.83 0.04 0.12 0.10 — 0.20 3.40 0.002
3.14 198.4 1.20 2.20 2.27 — — — — — 2560
Amperometry DPV SWV DPV DPV CV DPV DPV DPV CV DPV
0.20 0.25 0.19 0.24 0.27 0.21 0.27 0.25 0.52 0.44 0.17
33 34 35 36 37 38 57 58 59 63 This work
Abbreviations: MWCNT ¼ multi-wall carbon nanotube; GONR ¼ graphene oxide nanoribbon; MPEG ¼ methoxypolyethylene glycol; CCINPs-CS ¼ carbon-coated iron nanoparticles-chitosan; GNPs ¼ gold nanoparticles; ILG ¼ ionic liquid gel; CPM ¼ carbon porous material; DPV ¼ differential pulse voltammetry; SWV ¼ square-wave voltammetry; CV ¼ cyclic voltammetry.
a
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Fig. 4 (a) Cyclic voltammogram of CPM-900-A3-modified GCE in N2-saturated PBS with 2.0 mM DA recorded at different potential scan rates (50500 mV s1). Inset (b): Ipa and Ipc vs. scan rate.
solution, a slope of Epa (V) 45.9 mV pH1 was derived from the regression tting equation shown in Fig. 5b (inset). This value is very close to the theoretical value of 59 mV pH1 at 25 C for the transport process with an equal number of protons (H+) and electrons (e).63 Thus, we may conclude that electro-oxidation of DA over the CPM-modied GCE invokes an equal number of protons and electrons during the electrochemical transfer process. Moreover, from the variation of anodic peak current (Ipa) with pH, a maximum peak current at pH ¼ 7.0 may be inferred for the presence of DA. 3.3
Electrocatalytic performance of CPM-modied electrode
Fig. 6a shows the typical differential pulse voltammetry (DPV) curves for electrooxidation of DA over CPM-900-A3-modied GCE in N2-saturated PBS (pH ¼ 7.0) with different DA concentrations (from 9.0 nM to 0.3 mM). While a maximum oxidation
Fig. 5 (a) CV curves of CPM-900-A3-modified GCE in N2-saturated PBS of varied pH (3–11) with 2.0 mM DA recorded at a scan rate of 50 mV s1. Insets: (b) E0 vs. pH, and (c) Ipa vs. pH plots.
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Fig. 6 (a) DPV curves of CPM-900-A3-modified GCE in N2-saturated PBS with varied concentrations of DA (9.0 nm–0.3 mM). Inset (b): correlation of peak current with DA concentration.
peak at 0.172 V was observed for DA, a linear correlation between the peak current and DA concentration was also observed, as shown in Fig. 6b. The linear correlation may be expressed as Ipa ¼ 202.37 (0.36) [DA] + 0.6529 (0.054), with an excellent regression coefficient of R2 ¼ 0.9929. Accordingly, an excellent DA detection limit of 2.9 nM. An extraordinary sensitivity of 2.56 mA mM1 cm2 may be derived, which surpasses other modied electrode materials previously reported for electrochemical detection of DA (Table 2). The superior performance is attributed to the large surface area, good electrical conductivity, higher energy adsorption sites of the CO2activated CPM, and the lower over-potential and the enormous anodic peak current observed for the proposed DA sensor using the CPM-modied electrode. Thus, the unique CPM-modied electrode report herein is highly desirable for sensitive detection of DA. 3.4
Stability, reproducibility and selectivity in DA detection
The stability of the CPM-900-A3-modied GCE was investigated by monitoring the variation of CV curve over a period of 10 days. Again, modied electrodes in 0.1 M PBS containing 2.0 mM DA were employed to record the CV curve daily for 10 days. The 10 CV curves so collected readily overlapped on top of each other, as shown in Fig. 7a. The anodic peak response current declined only slightly over the 10 day period (Fig. 7b; inset), revealing a satisfactory stability for the detection of DA. Moreover, the reproducibility of the CPM-modied GCE during DA detection was accessed by performing CV measurement consecutively for 10 cycles. The resultant CV curves nearly overlapped on top of one another (Fig. 7c; inset). Thus, our device offers excellent reproducibility as a DA sensor. To examine the relative reactivity and selectivity of the CPM900-A3-modied electrode toward detection of DA, DPV curves were performed with varied concentrations of DA while in the presence of two other electroactive biomolecules, namely ascorbic acid (AA) and uric acid (UA). Since AA and UA have rather similar electrochemical properties with DA (see Scheme 3; ESI†), they tend to oxidize at nearly the same potential over
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may be inferred, indicating the excellent electrocatalytic performances of the CPM-modied GCE for sensitive and selective detection of DA, even in the presence of high concentrations of foreign species.
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4. Conclusions
Fig. 7 (a) Ten overlapped CV curves of CPM-900-A3-modified GCE in N2-saturated PBS with 2.0 mM DA recorded (scan rate of 50 mV s1) daily over a period of 10 days. Insets: (b) variation of response peak current with time, and (c) CV curves recorded over 10 consecutive cycles.
most conventional electrodes.64 Thus, it is highly desirable to develop sensitive and selective methods for detection of DA in the presence of UA and AA.65,66 Interestingly, linear dependence of the observed peak current with DA concentration was also observed over a wide range (0.05–0.45 mM) in the presence of large quantities of AA (10 mM) and UA (1 mM), as shown in Fig. 8. The linear correlation may be expressed as Ipa ¼ 175.35 [DA] + 4.157, with R2 ¼ 0.9963. Accordingly, the calculated value of the detection limit and sensitivity were determined as 3.2 nM and 2.21 mA mM1 cm2, respectively. Similar conclusions may be drawn based on results obtained from separate experiments carried out with even higher concentrations of AA (20 mM), and UA (2 mM), as shown in Fig. S7 (ESI†). In this case, a detection limit of 2.5 nM and a sensitivity value of 2.37 mA mM1 cm2
Fig. 8 (a) DPV curves of CPM-900-A3-modified GCE in N2-saturated PBS with varied concentrations of DA (0.05–0.45 mM) in presence of 10 mM ascorbic acid (AA), and 1 mM uric acid (UA). Inset (b): correlation of peak current with DA concentration.
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Carbon porous materials (CPMs) were successfully synthesized by a microwave irradiation route and were characterized by XRD, N2 adsorption/desorption isotherm measurements, SEM, FE-TEM, and Raman spectroscopy. These CPMs possess a high surface area (up to 2660 m2 g1) aer activation treatment by CO2. We assessed their electrocatalytic performances via cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CPM-modied sensor exhibits a linear correlation between peak current and DA concentration (9.0 nM–0.3 mM), with an excellent detection limit of 2.9 nM and ultrahigh sensitivity of 2.56 mA mM1 cm2 for detecting DA, surpassing the conventional GCE and other modied electrodes. Moreover, we report superior stability, reproducibility, and selectivity of the new CPMmodied electrode, which should have important practical application as a DA sensor in biological sample systems, even in the presence of foreign species.
Acknowledgements Financial support for this work by the Ministry of Science and Technology, Taiwan (NSC101-2113-M-001-020-MY3 to SBL) is gratefully acknowledged. The authors thank Prof. Tom T. S. Lin for helpful discussions.
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Cite this: DOI: 10.1039/c4an01083c
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Porous carbon-modified electrodes as highly selective and sensitive sensors for detection of dopamine† Pitchaimani Veerakumar,‡a Rajesh Madhu,‡b Shen-Ming Chen,*b Chin-Te Hung,a Pi-Hsi Tang,ac Chen-Bin Wangc and Shang-Bin Liu*ad Carbon porous materials (CPMs) with high surface areas up to 2660 m2 g1, directly fabricated by a facile microwave-assisted route, were applied to the electrochemical detection of dopamine (DA). The CPM-
Received 16th June 2014 Accepted 16th July 2014
modified electrodes exhibited excellent selectivity, a desirable detection limit (2.9 nM), and extraordinary sensitivity (2.56 mA mM1 cm2) for detection of DA, even in the presence of large amounts of foreign
DOI: 10.1039/c4an01083c
species, such as ascorbic acid (AA) and uric acid (UA), making feasible the practical applications of these
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electrodes as DA sensors.
1. Introduction Carbon porous materials (CPMs) possess numerous unique physicochemical properties, such as high surface areas, tailorable pore size, good electrical conductivity, biocompatibility, and desirable thermal, mechanical, and electrical properties, and they have attracted considerable R&D attention.1 It is these unique properties that make CPMs promising supports for electrodes and/or for electrode materials themselves in many important applications, such as in adsorption/separation, purication, catalysis, fuel cells, supercapacitors, sensors, and bioanalysis.2–9 Numerous methods have been developed for the synthesis of CPMs by various hard or so templates.1,10–15 For example, it has been shown that ordered mesoporous carbons may be fabricated via a facile self-assembly route using low-cost phenolic resin and commercial surfactants, followed by a carbonization treatment.16 Compared to conventional methods, CPM fabrication via microwave-assisted heating has drawn considerable R&D attention owing to its advantages, such as facile control of porosity and surface area, increased product yield and purity, reduced reaction time and operation cost, and feasibility for mass production.17 In fact, CPMs prepared under microwave irradiation have been extensively exploited as a
Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. E-mail: [email protected]
b
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei 10608, Taiwan. E-mail: [email protected]
c Department of Chemical and Materials Engineering, Chung Cheng Institute of Technology, National Defense University, Taoyuan 33449, Taiwan d
Department of Chemistry, National Taiwan Normal University, Taipei 11677, Taiwan
† Electronic supplementary information (ESI) available: Schemes, electrochemical oxidation mechanism, calculation details, Raman, SEM, TEM, pore size distribution, CV and DPV results. See DOI: 10.1039/c4an01083c ‡ These authors contributed equally.
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supports for electrodes in fuel cells.18–22 CPMs have also been widely utilized as electrodes for disposable electrochemical sensors, owing to their high surface area, excellent conductivity, chemical inertness, bio-compatibility, and low cost.5,23–28 It has also been shown that CPMs readily provide favourable sites for electron transfer in biomolecules; hence, CPMs can greatly improve the sensitivity of electrochemical detection.23–26 Dopamine (DA), one of the major catecholamines, plays many critical roles in the human central nervous and cardiovascular systems.29,30 For instance, dopaminergic neurotransmission is found to be closely related to diseases such as Parkinson's and schizophrenia.31 In recent decades, many techniques have been developed for sensitive detection of DA, including uorimetry,32 spectrophotometry,33 and electrochemical methods.34,35 Among them, electrochemical detection of DA was found to be the most simple, cost-effective, and userfriendly method. The electrochemical method also exhibits the desirable sensitivity and selectivity. However, scant reports on the selective detection of DA and/or simultaneous detection of DA, ascorbic acid (AA), and uric acid (UA) based on CPMmodied electrodes are available.36–40 In this study, we report a facile microwave-assisted synthesis route for fabricating CPMs with high surface area and pore volume. The CPMs so prepared were utilized for electrochemical detection of DA. It is shown that such CPM-modied, glassy carbon electrodes (GCEs) exhibit excellent sensitivity and selectivity and should be highly feasible as practical and cost-effective DA sensors.
2. 2.1
Experimental Chemicals and solutions
Resorcinol (ACS, 99.98%, Acros), formaldehyde (37% in water, Acros), anhydrous sodium carbonate (ACS, Fisher), triblock copolymer Pluronic F-127 (EO106PO70EO106, Mw ¼ 12 600, Analyst
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Sigma-Aldrich), ethanol (C2H5OH, 99%), and dopamine hydrochloride (Sigma-Aldrich) were obtained commercially and used without further purication. The phosphate buffer solution (PBS) at pH 7.0 was prepared by using 0.05 M Na2HPO4 and NaH2PO4 solutions. The pH of the solutions was adjusted with 0.5 M H2SO4 and 2.0 M NaOH. All other chemicals used were analytical grade and all solutions were prepared using ultrapure water (Millipore).
(900 or 1000 C), and then being kept for 1–2 h before it was eventually brought down to RT under a N2 gas ow. These CPMs activated with CO2 at 900 C for 1 h, 1000 C for 1 h, and 1000 C for 2 h are denoted as CPM-x-A1, CPM-x-A2, and CPM-x-A3, respectively, where x represents the original carbonization temperature.
2.2
To prepare carbon electrodes for electrochemical sensors, typically 5.0 mg CPM was rst dispersed in 1.0 mL water and sonicated for 2 h; then 6.0 mL of the substrate was withdrawn and placed onto the pre-cleaned GCE. The CPM-modied electrode was allowed to dry in air at 30 C for 2 h, followed by gently rinsing with doubly distilled water several times to remove the loosely bound carbon prior to further electrochemical measurements, which were carried out at RT under inert (N2) atmosphere.
Instrumentation
All powdered X-ray diffraction (XRD) patterns were recorded on a PANalytical (X'Pert PRO) diffractometer using CuKa radiation (wavelength l ¼ 0.1541 nm). Textural properties of CPMs, including Brunauer–Emmett–Teller (BET)-specic surface areas, pore volumes, and pore sizes were analyzed by nitrogen adsorption/desorption isotherm data measured on a Quantachrome Autosorb-1 apparatus (at 77 K). The pore size distribution of each CPM was derived from the adsorption branch of the isotherm using the Barrett–Joyner–Halanda (BJH) method. The morphology of various samples was monitored by eld emission-transmission electron microscopy (FE-TEM; JEOL JEM-2100F). All Raman spectra were recorded at ambient temperature on a WITeck CRM200 confocal microscopy system equipped with a laser (l ¼ 488 nm). Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) studies were performed on an electrochemical analyzer (CH instruments; CHI 900). 2.3
Material synthesis
CPMs were synthesized under conditions based on procedures reported previously.41–44 Typically, 0.0056 g of Na2CO3 was rst dissolved in carbon precursors consisting of formaldehyde (1.13 g) solution (37 wt%) and resorcinol (1.10 g) at RT for 1 h in a stirring water bath (see Step I, Scheme S1 in the ESI†). The resultant polymeric RF resol (light brown) was then mixed with a solvent mixture (ethanol–water ¼ 7 : 2) together with the amphiphilic Pluronic F-127 surfactant polyol (0.8 g; as the structural directing agent or so template). To this solution, 1.0 mL of 2.0 M HCl were added dropwise under continuous stirring. Subsequently, the precipitated mixture was then subjected to microwave irradiation (typically for 1–2 h), followed by curing (at 100 C overnight), washing and drying of the polymeric gel (Step II, Scheme S1; ESI†). Finally, the dried sample was subjected to a carbonization procedure in a tube furnace under a continuous ow of N2 gas. Typically, the carbonization procedure was carried out by slowly ramping from RT to 350 C with a rate 1 C min1, then, by further liing the temperature to 600–900 C at a rate of 2 C min1, and kept at the nal temperature for 5 h before being cooled to RT (Step III, Scheme S1; ESI†). The samples so prepared are denoted as CPM-x, where x denotes the carbonization temperature (Tc). Additional activation of CPMs with CO2 were also performed.45 This was done by loading the pristine CPM (1 g) in a quartz tube placed inside the furnace. Subsequently, the substrate was rst heated under a N2 environment at a rate 10 C min1 until reaching a temperature of 900 C, followed by switching to a CO2 gas stream (ow rate 30 cm3 min1) at a designated temperature
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2.4
3. 3.1
Fabrication of CPM-modied electrode
Results and discussion Structural and textural properties of CPMs
Fig. 1 shows the large-angle powdered XRD patterns of assorted pristine and activated CPM samples. Regardless of the postsynthesis activation treatment, all CPMs exhibit two main characteristic peaks associated with turbostratic carbons with graphitized structures.46–48 More specically, the diffraction peaks at 23.5 and 43.5 may be attributed to (002) and (100) index planes, respectively. The degree of graphitization of various CPMs was also examined by Raman spectroscopy. Raman spectra obtained from CPMs revealed two characteristic peaks centering at ca. 1360 (D-band) and 1600 (G-band) cm1, which may be attributed to disorder or defects located at the edges of graphite platelets and sp2-bonded graphitized carbons, respectively (Fig. S1; ESI†).49,50 Accordingly, the D- to G-band
Fig. 1 Large-angle XRD patterns of (a) CPM-350, (b) CPM-600, (c) CPM-900, (d) CPM-900-A1, (e) CPM-900-A2, and (f) CPM-900-A3 materials.
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peak intensity ratio (ID/IG) deduced for the CPM-900 sample is 1.1, revealing the presence of graphitized carbon structure. Based on SEM results, the as-synthesized CPMs exhibited morphology of micron-size crystalline aggregates with irregular shapes (see Fig. S2; ESI†). Additional TEM studies revealed that the pristine CPMs thus fabricated via the microwave-assisted synthesis procedure showed mesosopic pores with short-range ordering (Fig. 2a and S3; ESI†), indicating the presence of multidimensional wormhole-like pore structure,51,52 which is consistent with the results obtained from N2 adsorption/ desorption isotherm measurements. As displayed in Fig. 3A, the isotherms obtained from pristine CPM-x (x ¼ 350, 600, and 900 C) samples exhibit typical Type-IV curves with indicated capillary condensation steps at P/P0 0.4–0.7 and well-dened H1-type hysteresis loop anticipated for mesoporous materials.53 The sharp increase of the isotherm curve at P/P0 < 0.05 also reveals the presence of microporosity in these CPMs. Moreover, a notable increase in BET surface area (SBET) and total pore volume (Vo) with increasing carbonization temperature (Tc) may be inferred for the pristine CPMs. As shown in Table 1, the SBET observed for CPMs prepared with Tc from 350 to 900 C increased from 243 to 744 m2 g1, while the corresponding Vo also increased from 0.20 to 0.53 cm3 g1. On the other hand, a consistent decrease in average pore size (dBJH) with Tc was observed for these CPMs, as revealed by results deduced from their BJH pore-size distributions (Table 1 and Fig. S4; ESI†). For example, a notable decrease in pore size was observed for the CPM-900 sample (dBJH ¼ 5.9 nm) compared to CPM-600 (dBJH ¼ 6.9 nm). It is noteworthy that a narrower pore-size distribution prole was observed for CPMs graphitized at higher Tc, indicating that carbon framework shrinkage is accompanied by improved structural ordering, which coincides with results
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Fig. 3 N2 adsorption/desorption (77 K) isotherms of various CPMs (A) before and (B) after activation treatment by CO2. (a) CPM-350, (b) CPM-600, (c) CPM-900, (d) CPM-900-A1, (e) CPM-900-A2, (f) CPM900-A3.
obtained from TEM (Fig. 2a and S3; ESI†) and isotherm measurements (Fig. 3A). The presence of microporosity was further analyzed by using the t-plot method. The results of the microporous surface area and pore volume derived from t-plot analyses are given in Table 1. Upon activation with CO2, notable increases in structural and absorptive properties were observed for the activated CPMs as compared to the as-synthesized CPM-900 sample. This may be readily seen by the variations in TEM proles (Fig. 2) and isotherm curves (Fig. 3B). As a result, substantial increases in SBET and Vo were also found. This phenomenon, which became more pronounced with increasing severity of the activation treatment, was accompanied by simultaneous increase in microporosity as well as a decrease in dBJH (Table 1). For example, upon activation of the pristine CPM-900 at 1000 C for 2 h under the CO2 environment, signicant increases in SBET (from 744 to 2661 cm2 g1) and Vo (from 0.53 to 2.05 cm3 g1) were observed for the resultant CPM-900-A3 (Table 1). Meanwhile, the corresponding microporous surface area and pore volume were also found to increase from 411 to 510 cm2 g1 and 0.18 to 0.28 cm3 g1, respectively, while dBJH decreased from 5.9 to 5.4 nm. However, a slight degradation of mesostructure was observed for carbon samples subjected to prolonged treatment at elevated activation temperature (Ta $ 1000 C), as evidenced by the broadening of characteristic diffraction peaks (Fig. 1). Nonetheless, a similar ID/IG ratio (1.1) was deduced for CPM-900 sample before and aer activation treatment (Fig. S1; ESI†). Similar behaviours were also found for CO2-activated mesoporous carbons prepared from phenol–formaldehyde resol.54 3.2
Fig. 2 TEM images of (a) as-synthesized CPM-900, (b) CPM-900-A1, (c) CPM-900-A2 and (d) CPM-900-A3 obtained after various CO2activation treatments (see text).
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Electrochemical oxidation of DA
We applied cyclic voltammetry (CV) technique to assess the performance of electrochemical oxidation of DA over the CPMmodied glassy carbon electrode (GCE). Over the unmodied GCE, the DA oxidation peak recorded in the presence of N2saturated phosphate buffer solution (PBS) with 2.0 mM DA was found to locate at 0.453 V (Fig. S5a; ESI†). Moreover, as a blank test, no peak was observed for the bare CPM-900-A3-modied GCE in PBS without the presence of DA (Fig. S5b; ESI†). On the other hand, an enhanced DA oxidation peak at 0.209 V was
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Table 1 Textural properties of as-synthesized and activated CPM samples, electrochemical activities, and relevant kinetic parameters obtained from corresponding CPM-modified GCE during DA detectiona
CO2 activation
SBET (m2 g1)
Vo (cm3 g1)
Sample
Tc ( C)
Ta ( C)
Time (h)
Total
Micropore
Total
Micropore
dBJH (nm)
Ipab (mA)
Dc (cm2 s1)
Kad (cm s1)
CPM-350 CPM-600 CPM-900 CPM-900-A1 CPM-900-A2 CPM-900-A3
350 600 900 900 900 900
— — — 900 1000 1000
— — — 1.0 1.0 2.0
243 541 744 1116 1349 2661
120 381 411 449 486 510
0.20 0.37 0.53 0.69 0.84 2.05
0.31 0.15 0.18 0.19 0.22 0.28
7.9 6.9 5.9 5.8 5.9 5.4
2.3 4.7 8.9 2.7 2.8 27.0
0.007 0.031 0.110 0.010 0.011 1.012
0.01 0.04 0.59 0.66 0.69 0.88
a SBET ¼ BET surface area; Vo ¼ pore volume; dBJH ¼ BJH pore size; Ipa ¼ anodic peak current; D ¼ diffusion coefficient; Ka ¼ apparent electron transfer rate constants. b Determined with a scan rate of 50 mV s1. c Calculated from Randles–Sevcik equation (ESI). d Derived from Laviron equation (ESI).
observed over the CPM-900-A3-modied GCE at a scan rate of 50 mV s1 (Fig. S5c; ESI†). Interestingly, the corresponding peak current increased 27-fold compared to that of the pristine GCE; this may be attributed to the faster diffusion and electron transfer rate observed for the CPM-900-A3-modied GCE compared to the other CPMs (Table 1; more details in ESI†).55,56 For comparison, CV curves recorded from various modied electrodes are also depicted (Fig. S6; ESI†). Evidently, electrodes modied with CPM-900-A3 exhibited the best electrochemical performance towards the detection of DA. This may be attributed to the enriched porosities and higher surface possessed by CPM-900-A3 (vide supra). It is also noteworthy that the DA oxidation potential so observed over the CPM-modied GCE is much lower than that over other carbon modied electrodes, such as carbon nanotube (CNT)-modied ionic liquid gel (ILG),57 graphene,58 or thionine-naon supported on multi-wall carbon nanotubes (MWCNTs),59 as summarized in Table 2. This is attributed to the excellent conductivity and high surface areas possessed by the CO2-activated CPM.60 Moreover, the CPM-900A3-modied GCE also exhibited a well-dened oxidation peak of DA with peak to peak separation (DEp) of 41 mV. Such a small DEp value is favourable for fast electron transfer of DA at the
Table 2
electrode surface.61 These results reveal that CPM-900-A3 pores act as reservoirs, which effectively reduce the diffusion length of ions from the electrolyte. This was further veried by varying the scan rate of CV measurement, as illustrated in Fig. 4a for the CPM-900-A3-modied GCE in N2-saturated PBS with 2 mM DA. The fact that the oxidation peak current (Ipa) showing a linear dependence with the scan rate between 50 and 500 mV s1 (Fig. 4b) indicates that oxidation of DA at the CPM-modied electrode is indeed a surface-controlled process.62 A mechanism invoking the reversible electron transfer between DA and dopamine-o-quninone over the CPM-modied electrode is illustrated in Scheme S2 (ESI†). Next, we studied the effects of pH in the buffer solution on electro-oxidation of DA over CPM-modied GCE. This was carried out by varying the pH (3.0–11.0) of N2-saturated PBS containing 2.0 mM DA. The CV curves recorded with a scan rate of 50 mV s1 under varied buffer solution pH are displayed in Fig. 5. Upon increasing pH of the buffer solution containing DA, the CV curve of CPM-900-A3-modied GCE tends to shi toward more negative anodic and cathodic peak potentials, and vice versa for descending pH. On the basis of linear correlation between the anodic peak potential and pH of the buffer
Types of modified electrodes and performance as DA sensors
Modied electrodesa
DA detection limit (mM)
Sensitivity (mA mM1 cm2)
Detection methoda
Oxidation potential (V)
Reference
MWCNT/GONR MPEG polymer Degraded dopamine CCINPs-CS GNPs/graphene Pt/carbon CNTs-ILG Graphene Thionine-naon/MWNT Calix[4]arene crown-4 ether CPM
0.08 0.05 0.04 0.83 0.04 0.12 0.10 — 0.20 3.40 0.002
3.14 198.4 1.20 2.20 2.27 — — — — — 2560
Amperometry DPV SWV DPV DPV CV DPV DPV DPV CV DPV
0.20 0.25 0.19 0.24 0.27 0.21 0.27 0.25 0.52 0.44 0.17
33 34 35 36 37 38 57 58 59 63 This work
Abbreviations: MWCNT ¼ multi-wall carbon nanotube; GONR ¼ graphene oxide nanoribbon; MPEG ¼ methoxypolyethylene glycol; CCINPs-CS ¼ carbon-coated iron nanoparticles-chitosan; GNPs ¼ gold nanoparticles; ILG ¼ ionic liquid gel; CPM ¼ carbon porous material; DPV ¼ differential pulse voltammetry; SWV ¼ square-wave voltammetry; CV ¼ cyclic voltammetry.
a
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Fig. 4 (a) Cyclic voltammogram of CPM-900-A3-modified GCE in N2-saturated PBS with 2.0 mM DA recorded at different potential scan rates (50500 mV s1). Inset (b): Ipa and Ipc vs. scan rate.
solution, a slope of Epa (V) 45.9 mV pH1 was derived from the regression tting equation shown in Fig. 5b (inset). This value is very close to the theoretical value of 59 mV pH1 at 25 C for the transport process with an equal number of protons (H+) and electrons (e).63 Thus, we may conclude that electro-oxidation of DA over the CPM-modied GCE invokes an equal number of protons and electrons during the electrochemical transfer process. Moreover, from the variation of anodic peak current (Ipa) with pH, a maximum peak current at pH ¼ 7.0 may be inferred for the presence of DA. 3.3
Electrocatalytic performance of CPM-modied electrode
Fig. 6a shows the typical differential pulse voltammetry (DPV) curves for electrooxidation of DA over CPM-900-A3-modied GCE in N2-saturated PBS (pH ¼ 7.0) with different DA concentrations (from 9.0 nM to 0.3 mM). While a maximum oxidation
Fig. 5 (a) CV curves of CPM-900-A3-modified GCE in N2-saturated PBS of varied pH (3–11) with 2.0 mM DA recorded at a scan rate of 50 mV s1. Insets: (b) E0 vs. pH, and (c) Ipa vs. pH plots.
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Fig. 6 (a) DPV curves of CPM-900-A3-modified GCE in N2-saturated PBS with varied concentrations of DA (9.0 nm–0.3 mM). Inset (b): correlation of peak current with DA concentration.
peak at 0.172 V was observed for DA, a linear correlation between the peak current and DA concentration was also observed, as shown in Fig. 6b. The linear correlation may be expressed as Ipa ¼ 202.37 (0.36) [DA] + 0.6529 (0.054), with an excellent regression coefficient of R2 ¼ 0.9929. Accordingly, an excellent DA detection limit of 2.9 nM. An extraordinary sensitivity of 2.56 mA mM1 cm2 may be derived, which surpasses other modied electrode materials previously reported for electrochemical detection of DA (Table 2). The superior performance is attributed to the large surface area, good electrical conductivity, higher energy adsorption sites of the CO2activated CPM, and the lower over-potential and the enormous anodic peak current observed for the proposed DA sensor using the CPM-modied electrode. Thus, the unique CPM-modied electrode report herein is highly desirable for sensitive detection of DA. 3.4
Stability, reproducibility and selectivity in DA detection
The stability of the CPM-900-A3-modied GCE was investigated by monitoring the variation of CV curve over a period of 10 days. Again, modied electrodes in 0.1 M PBS containing 2.0 mM DA were employed to record the CV curve daily for 10 days. The 10 CV curves so collected readily overlapped on top of each other, as shown in Fig. 7a. The anodic peak response current declined only slightly over the 10 day period (Fig. 7b; inset), revealing a satisfactory stability for the detection of DA. Moreover, the reproducibility of the CPM-modied GCE during DA detection was accessed by performing CV measurement consecutively for 10 cycles. The resultant CV curves nearly overlapped on top of one another (Fig. 7c; inset). Thus, our device offers excellent reproducibility as a DA sensor. To examine the relative reactivity and selectivity of the CPM900-A3-modied electrode toward detection of DA, DPV curves were performed with varied concentrations of DA while in the presence of two other electroactive biomolecules, namely ascorbic acid (AA) and uric acid (UA). Since AA and UA have rather similar electrochemical properties with DA (see Scheme 3; ESI†), they tend to oxidize at nearly the same potential over
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may be inferred, indicating the excellent electrocatalytic performances of the CPM-modied GCE for sensitive and selective detection of DA, even in the presence of high concentrations of foreign species.
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4. Conclusions
Fig. 7 (a) Ten overlapped CV curves of CPM-900-A3-modified GCE in N2-saturated PBS with 2.0 mM DA recorded (scan rate of 50 mV s1) daily over a period of 10 days. Insets: (b) variation of response peak current with time, and (c) CV curves recorded over 10 consecutive cycles.
most conventional electrodes.64 Thus, it is highly desirable to develop sensitive and selective methods for detection of DA in the presence of UA and AA.65,66 Interestingly, linear dependence of the observed peak current with DA concentration was also observed over a wide range (0.05–0.45 mM) in the presence of large quantities of AA (10 mM) and UA (1 mM), as shown in Fig. 8. The linear correlation may be expressed as Ipa ¼ 175.35 [DA] + 4.157, with R2 ¼ 0.9963. Accordingly, the calculated value of the detection limit and sensitivity were determined as 3.2 nM and 2.21 mA mM1 cm2, respectively. Similar conclusions may be drawn based on results obtained from separate experiments carried out with even higher concentrations of AA (20 mM), and UA (2 mM), as shown in Fig. S7 (ESI†). In this case, a detection limit of 2.5 nM and a sensitivity value of 2.37 mA mM1 cm2
Fig. 8 (a) DPV curves of CPM-900-A3-modified GCE in N2-saturated PBS with varied concentrations of DA (0.05–0.45 mM) in presence of 10 mM ascorbic acid (AA), and 1 mM uric acid (UA). Inset (b): correlation of peak current with DA concentration.
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Carbon porous materials (CPMs) were successfully synthesized by a microwave irradiation route and were characterized by XRD, N2 adsorption/desorption isotherm measurements, SEM, FE-TEM, and Raman spectroscopy. These CPMs possess a high surface area (up to 2660 m2 g1) aer activation treatment by CO2. We assessed their electrocatalytic performances via cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The CPM-modied sensor exhibits a linear correlation between peak current and DA concentration (9.0 nM–0.3 mM), with an excellent detection limit of 2.9 nM and ultrahigh sensitivity of 2.56 mA mM1 cm2 for detecting DA, surpassing the conventional GCE and other modied electrodes. Moreover, we report superior stability, reproducibility, and selectivity of the new CPMmodied electrode, which should have important practical application as a DA sensor in biological sample systems, even in the presence of foreign species.
Acknowledgements Financial support for this work by the Ministry of Science and Technology, Taiwan (NSC101-2113-M-001-020-MY3 to SBL) is gratefully acknowledged. The authors thank Prof. Tom T. S. Lin for helpful discussions.
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