Seung-Koo Lee†
a,
Min-Jung Song†b,
Jong-Hoon Kimac,
Young-Kyun Lima,
Yoon-Soo Chuna and
Dae-Soon Lim*a
aDept. of Materials Science and Engineering, Korea University, Anam-Dong 5-1, Seoungbuk-gu, Seoul 136-713, Republic of Korea. E-mail: dslim@korea.ac.kr; Fax: +82-2-928-3584; Tel: +82-2-3290-3272
bDept. of Materials Science and Engineering, Yonsei University, 50, Yonsei-ro, Seodaemun-gu, Seoul 120-749, Republic of Korea
cDept. of Technology and Society, The State University of New York, 119 Songdo Moonhwa-ro, Yeonsu-gu, Incheon 406-840, Republic of Korea
First published on 24th February 2015
A method for selective and direct growth of multi-wall carbon nanotubes (MWCNTs) on boron-doped diamond (BDD) was developed using electrostatic self-assembly of catalytic nanoparticles via a conventional photolithography method. As catalysts, stainless steel 316L (SUS316L) nanoparticles consisting of catalytic ions such as Fe and Ni were introduced. As-grown MWCNTs had a porous random-network structure that was suitable for electrochemical biosensor application. The electrochemical properties of the electrodes were characterized using cyclic voltammetry and electrochemical impedance spectroscopy. The effective surface area of the as-developed BDD/MWCNT electrode was larger than that of the BDD electrode and its electron transfer resistance was about 40 Ω, which is around 16 times higher than that of the BDD electrode. To evaluate the biosensing performance, glucose was chosen as a target analyte. The BDD/MWCNT electrode exhibited a higher sensitivity of 7.2 μA mM−1 cm−2 (R = 0.9943) with a wider linear range than did the BDD electrode, because of the synergistic effect between the MWCNTs and BDD.
Recently, boron-doped diamond (BDD) film7–11 has been used as an active electrode for various electrochemical sensors because of its low capacitive current, large potential window, and low adsorption of organic contaminants. These unique properties lead to an enhanced signal-to-noise ratio (S/N) and a low detection limit. In spite of these outstanding properties, however, BDD shows low sensitivity for biosensor applications. To overcome the performance limitation, several researchers performed the modification of the BDD film with various methods, such as nanomaterial decoration, metal ion implantation, plasma treatment, etc.12–15
The CNT/diamond hybrid structure can overcome the limitation of the nanostructure formation of the BDD and exhibit the synergistic properties. Several researchers fabricated the CNTs on diamond structures by various methods.16–20 Varshney et al. showed CNTs on diamond structure exhibits enhanced emission properties and long-term stability as compared CNTs alone.16 Recently, Herbert et al. presented the composite nanomaterial of CNTs on BDD fabricated using partially embedded catalyst in nanocrystalline diamond film and estimated electrochemical properties.20 However, the evaluation of the biosensing performance with CNTs on BDD structure has not yet been explored.
To achieve CNT-based sensor devices, patterning of the CNTs with a three-dimensional (3D) porous network structure is required to provide facile access to the analytes for electrochemical measurement.21 Many researchers have studied patterning of CNTs using a variety of methods, such as template-based synthesis,22 transferring techniques,23 direct etching,24 microcontact printing,25 direct growth on a patterned catalyst,26,27 and so on. Among the diverse CNT patterning methods, direct growth with a patterned catalyst is the most promising technique for fabricating a 3D porous network structure. However, the conventional selective growth techniques reported in the literature require complicated additional processes to make a catalyst layer on the substrate, such as deposition and heat treatment. Thus, development of a simple and cost-effective CNT patterning method is required.
In this study, we describe the method for site-selective and direct growth of the multi-wall CNTs (MWCNTs) on the BDD using electrostatic self-assembly (ESA) of catalytic nanoparticles. In addition, the electrochemical biosensing performances were evaluated with as-developed MWCNTs on BDD electrode pattern. Glucose was chosen as a target analyte. Combining porous random-networked MWCNTs and BDD (BDD/MWCNT) is expected to enhance the electrochemical properties as well as sensing performances.
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Fig. 3 (a) SEM image of the as-deposited BDD thin film (inset: cross-sectional view of the BDD thin film), (b) Raman spectra of the BDD thin film. |
MWCNTs grown selectively on the BDD electrode pattern were prepared to estimate the biosensing performance (Fig. 4). The BDD pattern was fabricated on a quartz substrate using ESAND and HFCVD. Then, a silicon nitride (Si3N4) passivation layer was deposited by photolithography at the edge of the BDD pattern. The as-prepared sensor consisted of the 1 mm-diameter detection part (circle pattern) with the MWCNTs (Fig. 4(b)) and the bonding pad part (Fig. 4(c)). Fig. 4(d) and (e) show SEM images of the synthesized MWCNTs on the detection part. The MWCNTs had a 3D porous network structure and the thickness of MWCNT layer was about 8 μm. As-grown MWCNTs were adhered well to the BDD surface.
The growth phenomenon of the MWCNTs on the catalytic SUS316L nanoparticle were analyzed by SEM (Fig. 5). A bare SUS316L nanoparticle is spherical shape with smooth surface and its average size is about 150 nm (Fig. 5(a)). During the thermal CVD process for MWCNT growth, nano-sized hills were generated on the surface of the SUS316L nanoparticle at 750 °C (Fig. 5(b)). As shown in Fig. 5(c), one SUS316L nanoparticle had several MWCNTs with different diameters of 20–100 nm. Through the generated hills, several MWCNTs from one SUS316L nanoparticle were grown (some-to-one pathway) by diffusion of carbon atoms through C2H2 decomposition, obtaining the 3D porous network MWCNTs with various diameters. It was considered that the diameter of the as-grown MWCNT is related to the size of the generated hills.30
Raman analysis was performed for estimation of the crystalline quality of the MWCNT as shown in Fig. 6. As-grown MWCNTs have two main peaks around 1571 cm−1 (G-band) and 1340 cm−1 (D-band). The G-band as tangential mode indicates the ordered carbon and the D-band characterizes the structural forms of disordered sp2 carbon. The small ratio of D-band/G-band as a good indicator of the quality means low defect level in the atomic carbon structure for the sample.31 This ratio of 0.82 for the as-grown MWCNT indicates a high quality of the structural defects.
For the characterization of the electrochemical properties, the modified electrodes were examined by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in a 0.1 M KCl solution containing 5 mM Fe(CN)63−/4−. Fig. 7(a) shows the CVs of bare BDD and a BDD/MWCNT electrode. For the bare electrode, the anodic peak potential (Epa) was 0.398 V (ipa = 0.435 mA cm−2) and the cathodic peak potential (Epc) was 0.113 V (ipc = −0.587 mA cm−2), yielding a peak-to-peak separation (ΔEp) of 0.285 V. The BDD/MWCNT electrode showed an increase in the oxidation peak current density (ipa = 0.660 mA cm−2 and Epa = 0.337 V) and a decrease in the cathodic peak current density (ipc = −0.570 mA cm−2 and Epc = 0.17 V), with a peak potential separation (ΔEp) of approximately 0.167 V. The improved peak current density includes an increase of the effective surface area and the small ΔEp leads to direct electron transfer between the electrode and redox species. This result confirms that the BDD/MWCNT electrode exhibited an enhanced signal and accelerated electron transfer due to synergistic effects of the MWCNTs and BDD.
An EIS study of the electrodes was also carried out over a frequency range of 10−2 to 105 Hz, with an applied sine wave potential with an amplitude of 5 mV and a formal potential of the system of 0.24 V as shown in Fig. 7(b). A typical Nyquist plot of the impedance spectra consists of the real (x-axis) and imaginary (y-axis) parts of the cell impedance and is divided into a semicircle portion and a linear portion. Generally, the semicircle portion in the high-frequency range and the linear portion in a lower frequency range represent the electron-transfer-limited process and the diffusion-limited process, respectively. The diameter of the semi-circle indicates the electron-transfer resistance (Ret).32 The Ret value of the bare BDD was ca. 650 Ω. After MWCNT growth on BDD electrode, the semicircle domain (corresponding to Ret) was dramatically decreased to ca. 40 Ω (almost a straight line). It can be explained that the BDD/MWCNT electrode has very low Ret at the interface between electrode surface and electrolyte showing excellent electrocatalytic activity.
To calculate the effective surface area of the modified electrodes using the Randles–Sevcik equation, CVs were measured in a 3 M KCl solution containing 10 mM K3Fe(CN)63−/4− at various scan rates. The peak currents (Ip) showed a linear relation with the square root of the scan rate (v1/2) for both electrodes, as shown in Fig. 7(c). This means that the reaction on each electrode is almost reversible and mainly diffusion-controlled with respect to the mass-transfer phenomenon in the double-layer region of the electrode. The Randles slopes of the bare BDD and BDD/MWCNT electrodes were 0.00115 (R = 0.99892) and 0.00168 (R = 0.99885), respectively. From these results, the effective surface area (A) of the BDD/MWCNT electrode was calculated as ca. 0.007158 cm2, which is approximately 1.46 times larger than that of the bare BDD electrode because of the 3D-network porous structure of MWCNTs.
Fig. 8(a) and (b) show calibrated current versus glucose concentration curves for the bare BDD and BDD/MWCNT electrodes, along with the full-range data. For the bare BDD electrode (Fig. 8(c)), the sensitivity was 0.35 μA mM−1 cm−2 (R = 0.9989) with a linear range of 0.00565–0.0565 mM. The BDD/MWCNT electrode had a sensitivity of 7.2 μA mM−1 cm−2 (R = 0.9943) in the linear range of 0.00565–1.1567 mM, as shown in Fig. 8(d).
The reproducibility of the as-developed BDD/MWCNT electrode was measured at five different glucose concentrations as shown in Table 1. The relative standard deviation (R.S.D.) according to each concentration was below 4% indicating the good reproducibility.
Concentration [mM] | Current density [mA cm−2] | R.S.D. [%] | |||
---|---|---|---|---|---|
Sensor #1 | Sensor #2 | Sensor #3 | Average | ||
1.1567 | 0.02295 | 0.02274 | 0.02363 | 0.02311 | 2.01 |
0.5141 | 0.01809 | 0.01775 | 0.01749 | 0.01777 | 1.69 |
0.22853 | 0.01617 | 0.0167 | 0.01744 | 0.01677 | 3.80 |
0.05079 | 0.01519 | 0.01441 | 0.01558 | 0.01506 | 3.96 |
0.00565 | 0.01469 | 0.01444 | 0.01395 | 0.01436 | 2.62 |
Table 2 shows the electrochemical biosensing performances for the amperometric glucose detection based on the BDD electrode modified with various nanomaterials.33–36 Comparing the results, as-developed BDD/MWCNT electrode exhibits higher sensitivity with wider linear range and low detection limit than other modified electrodes. The combination of the MWCNTs having the large effective surface area and high electron transfer efficiency and the BDD having the low capacitive current and large potential window provided enhanced sensitivity over a wide linear range.
Electrode | Linear ranges [mM] | Detection limit [μM] | Sensitivity | R | Ref. |
---|---|---|---|---|---|
a Polyaniline.b Platinum-nanoparticles.c Screen printed boron-doped diamond powder.d Cobalt phthalocyanine.e 3-Aminopropyltriethoxysilane. | |||||
BDD/GOx | 0.00565–0.0565 | 0.424 | 0.35 [μA mM−1 cm−2] | 0.9989 | This work |
BDD/PANIa/GOx | 0.347–13.33 | 68.5 | 1.39 [μA mM−1] | 0.9975 | 33 |
BDD/Pt-NPsb/GOx | 0.177–8.9 | 77.3 | 3.21 [μA mM−1 cm−2] | 0.9916 | 34 |
BDD/PANI/Pt-NPs/GOx | 0.00594–0.514 | 0.102 | 5.54 [μA mM−1] | 0.9947 | 33 |
BDDPc/CoPcd/GOx | 0.2–1 | — | 3.9 [μA mM−1] | — | 35 |
BDD/X-APTESe–GOx/Nafion | 0.035–8 | 30 | — | 0.9983 | 36 |
BDD/MWCNT/GOx | 0.00565–1.1567 | 0.0696 | 7.2 [μA mM−1 cm−2] | 0.9943 | This work |
Recently, non-invasive measurement of the glucose in human body using biofluids (i.e. saliva, tears, and sweat) has much attention for the indirect diagnosis of diabetes.37 These sources include the glucose of which the concentration is lower than that of blood (micro molar range) and correlates with glucose concentration in blood. Although the linear range of the BDD/MWCNT electrode is not adequate for human body concentration (3–10 mM), it can be used for non-invasive measurement of the glucose concentration in saliva (∼70 μM), tears (∼200 μM), and sweat (∼120 μM).38 Consequently, the BDD/MWCNT electrode shows promise for biosensor application to detect the low concentration of biomolecules as well as non-invasive measurement of the glucose in living beings.
Footnote |
† These authors are contributed equal to this work. |
This journal is © The Royal Society of Chemistry 2015 |