**4.1. Platform for biosensing**

Carbon materials have been widely used in both analytical and industrial electrochemistry due to their low cost, wide potential window, relatively inert electrochemistry, and electroca‐ talytic activity for a variety of redox reactions. Recently, graphene has proved to be an excellent nanomaterial for applications in electrochemistry. Graphene-based materials with large surface area are useful as electrodes for electrochemical sensors and biosensors [42-44]. Electrochemical activity of CNW electrode has been investigated by cyclic voltammetry measurements in an aqueous solution of ferrocyanide and a faster electron transfer between the electrolyte and the nanosheet surface has been demonstrated [21-23]. Dopamine (DA) is a hormone and neurotransmitter that plays a very important role in the human brain and body. Since the changes in the concentration of DA are closely linked to a human's health status, its detection has gained significant attention. Ascorbic acid (AA) and uric acid (UA) are also compounds of great biomedical interest, which all are essential biomolecules in our body fluids. Chemically reduced graphene oxide modified glassy carbon electrode was used to detect these neurotransmitters and biological molecules [42]. In these days, researches on the sensing of biological molecules became popular. Figure 17 shows examples of cyclic voltam‐ mogram responses of CNW electrode in the phosphate buffer solution (PBS) with UA, AA, and their mixture at 100 mV/s scan rate. Shang and coworkers demonstrated the excellent electrocatalytic activity of multilayer graphene nanoflakes in simultaneous determination of DA, AA, and UA in PBS [20].

Very recently, electrochemical glutamate biosensor for bioelectronic applications has been demonstrated using platinum (Pt)-functionalized graphene nanoplatelet prepared from graphene oxides [45]. Among the neurotransmitters detected by biosensors, L-glutamate is one of the most important in the mammalian central nervous system, playing a vital role in many physiological processes. The glutamate biosensor is based on the oxidation of glutamate in the presence of glutamate oxidase.

2

In the case of atomic oxygen etching, CNWs are selectively etched from the top edges with almost no change in wall surface morphology, as shown in Figure 14(b). On the other hand, the H2O2 treatment induces the characteristic changes in their morphology with keeping the size of CNWs constant. The nanometer-scale asperities on the CNW surface increase the surface area, which would be useful as a platform for supporting metal nanoparticles and organopollutant degradation devices [24,41]. It is noted that such asperities could be reduced by O radical exposure after H2O2 treatment, resulting in the reduction of the thickness of CNW sheets. These results, including atomic oxygen etching, oxygen RIE, and H2O2 treatment suggest the possibility of realizing etching and thickness control of walls in CNWs, which should be essential for controlling the electrical properties of graphene materials and realizing

17 Fig. 17

18 Fig. 18

21 Fig. 22

**4. Applications of nanoplatform based on vertical nanographene**

Carbon materials have been widely used in both analytical and industrial electrochemistry due to their low cost, wide potential window, relatively inert electrochemistry, and electroca‐ talytic activity for a variety of redox reactions. Recently, graphene has proved to be an excellent nanomaterial for applications in electrochemistry. Graphene-based materials with large surface area are useful as electrodes for electrochemical sensors and biosensors [42-44]. Electrochemical activity of CNW electrode has been investigated by cyclic voltammetry measurements in an aqueous solution of ferrocyanide and a faster electron transfer between the electrolyte and the nanosheet surface has been demonstrated [21-23]. Dopamine (DA) is a hormone and neurotransmitter that plays a very important role in the human brain and body. Since the changes in the concentration of DA are closely linked to a human's health status, its detection has gained significant attention. Ascorbic acid (AA) and uric acid (UA) are also compounds of great biomedical interest, which all are essential biomolecules in our body fluids. Chemically reduced graphene oxide modified glassy carbon electrode was used to detect these neurotransmitters and biological molecules [42]. In these days, researches on the sensing of biological molecules became popular. Figure 17 shows examples of cyclic voltam‐ mogram responses of CNW electrode in the phosphate buffer solution (PBS) with UA, AA, and their mixture at 100 mV/s scan rate. Shang and coworkers demonstrated the excellent electrocatalytic activity of multilayer graphene nanoflakes in simultaneous determination of

Very recently, electrochemical glutamate biosensor for bioelectronic applications has been demonstrated using platinum (Pt)-functionalized graphene nanoplatelet prepared from graphene oxides [45]. Among the neurotransmitters detected by biosensors, L-glutamate is one of the most important in the mammalian central nervous system, playing a vital role in many physiological processes. The glutamate biosensor is based on the oxidation of glutamate

their applications to electronic devices.

**4.1. Platform for biosensing**

160 Graphene - New Trends and Developments

DA, AA, and UA in PBS [20].

in the presence of glutamate oxidase.

Intensity (arb. unit) 1.0 0.8 Pt 4f5/2 Pt 4f (b) 7/2 (a) **Figure 17.** Cyclic voltammogram responses of CNW electrode in PBS with AA, UA, and their mixture at 100 mV/s scan rate.

0.4

$$\text{Glutamate} + \text{O}\_2 + \text{H}\_2\text{O} \rightarrow a-\text{ketoglutarate} + \text{NH}\_3 + \text{H}\_2\text{O}\_2\tag{1}$$

The H2O2 produced in this reaction is electroactive at electrodes such as Pt, although it is inactive at many typical carbon-based electrodes. Therefore, it is necessary to add various electrocatalytic materials such as Pt nanoparticles, hydrous iridium oxide, Prussian blue, or peroxidase enzymes on the surface of carbon-based electrodes. As was illustrated in Figure 1, CNW film has many graphene edges on the top, and the CNW sheet itself is com‐ posed of nanodomains of a few tens of nanometers in size [46]. Pt nanoparticles were preferably deposited on the defects such as grain boundaries on the surface of graphite [24]. Therefore, the structure of CNWs can be suitable for the platform of the electrochem‐ ical and biosensing applications. In many cases, Pt nanoparticles have been prepared by the reduction of Pt salt precursors such as H2PtCl6 in solution. As an alternative ap‐ proach to support the metal nanoparticles on the surface of carbon nanostructures, including dense CNTs and CNWs with narrow interspaces, we developed a new method of deposi‐ tion using the supercritical carbon dioxide (sc-CO2) as a solvent of metal-organic com‐ pounds. We demonstrated the preparation of dispersed Pt nanoparticles using the metalorganic chemical fluid deposition (MOCFD) employing supercritical fluid (SCF) [47-49]. TEM image of the surface of the CNW supporting Pt nanoparticles is shown in Figure 18(a). Pt nanoparticles were prepared on the CNWs by the SCF-MOCFD method for 30 min. The pressure and temperature of sc-CO2 were 10 MPa and 130°C, respectively, and the temperature of CNWs was maintained at 180°C. Pt nanoparticles were prepared preferen‐ tially at the domain boundaries of CNW surface, as shown in Figure 18(a). 19 12 the contact angles of water droplets and the water contact angles (WCAs) the water contact angles (WCAs) 19 18 Figures 20(b)-(e). Figures 20(b)-20(e). 19 40 in Figures 20(a) and (b), in Figures 20(a) and 20(b), Binding energy (eV) 0.2 80 78 76 74 72 68 70 10 nm

> Furthermore, nanoparticles of the anatase phase of TiO2 were prepared on the entire surface of CNWs by SCF-MOCFD method at a substrate temperature of 180°C. Figure 19 shows TEM image of the CNWs supporting TiO2 nanoparticles after SCF-MOCFD for 30 min and the statistical distribution of the observed nanoparticle size [41]. For decomposing methylene blue

200

Current (µA)

0

400

600

17 Fig. 17

18 Fig. 18

21 Fig. 22

sensing and biosensing.

the obtained particle size distribution [41].

Potential vs Ag/AgCl (V)

of CNWs by SCF-MOCFD method at a substrate temperature of 180C. Figure 19 shows TEM image of the CNWs supporting TiO2 nanoparticles after SCF-MOCFD for 30 min and 19 12 the contact angles of water droplets and the water the water contact angles (WCAs) **Figure 18.** (a) TEM image of the surface of the CNW supporting Pt nanoparticles after the SCF-MOCFD for 30 min, and (b) XPS profile of the Pt 4f region of the Pt-supported CNW after SCF-MOCFD [50].

the statistical distribution of the observed nanoparticle size [41]. For decomposing

under ultraviolet irradiation, a high photocatalytic decomposition rate of 6 mg/h was obtained for 1 mg of TiO2 supported on CNWs [41]. CNW-based electrochemical platform, which possesses large surface area with edges and electrochemical activity, offers great promise for providing a new class of nanostructured electrodes for electrochemical sensing and biosensing. methylene blue under ultraviolet irradiation, a high photocatalytic decomposition rate of 6 mg/h was obtained for 1 mg of TiO2 supported on CNWs [41]. CNW-based electrochemical platform, which possesses large surface area with edges and electrochemical activity, offers great promise for providing a new class of nanostructured electrodes for electrochemical contact angles (WCAs) 19 18 Figures 20(b)-(e). Figures 20(b)-20(e). 19 40 in Figures 20(a) and (b), in Figures 20(a) and 20(b),

**Figure 19.** (a) TEM image of CNWs supporting TiO2 nanoparticles after SCF-MOCFD for 30 min and (b) **Figure 19.** (a) TEM image of CNWs supporting TiO2 nanoparticles after SCF-MOCFD for 30 min and (b) the obtained particle size distribution [41].

#### **4.2. Surface chemical modification of carbon nanowalls for the wide range control of 4.2. Surface chemical modification of carbon nanowalls for the wide range control of surface wettability**

**surface wettability**  In view of the practical applications using CNWs in sensors and platforms for cell culturing, the effects of morphologies of CNWs and their surface properties should be considered systematically. We investigated the surface wettability of CNWs with emphasis on the chemisorption effect by postprocessing using plasma treatments [51,52]. Here, CNW In view of the practical applications using CNWs in sensors and platforms for cell culturing, the effects of morphologies of CNWs and their surface properties should be considered systematically. We investigated the surface wettability of CNWs with emphasis on the chemisorption effect by postprocessing using plasma treatments [51,52]. Here, CNW samples grown from CH4/H2 mixtures on the Si substrate were chosen as primary forms.

The surface of as-grown CNWs from CH4/H2 mixture was terminated with H atoms. After the preparation of CNW film sample using RI-PECVD for 15 min, its surface was oxidized using Ar atmospheric pressure plasma for 1 to 30 s [51,52]. The distance between the CNW film

2

sample and the atmospheric pressure plasma source was 5 mm. We expect that soft oxidation by oxygen radicals was realized, while the effect of ion bombardment on the surface was negligible during the exposure to atmospheric pressure plasma due to the very short mean free path of ions at the atmospheric pressure. For comparison, the surface of CNW sample was fluorinated for 5 s to add hydrophobic properties to the CNWs. For the fluorination treatment, the CNW sample was exposed to CF4 plasma generated in the VHF-CCP region of RI-PECVD chamber without using H2 SWP [51,52].

After the plasma surface treatments using the Ar atmospheric pressure plasma for oxidation and the CF4 plasma for fluorination in short duration, no noticeable change was observed in the morphology of CNW samples. These results indicate that such short-duration plasma treatments would induce surface chemical functionalization without etching or deposition. Figure 20(a) shows the water contact angles (WCAs) on the CNWs before and after the Ar atmospheric pressure plasma treatment, as a function of plasma treatment duration, together with the WCA after the CF4 plasma treatment for 5 s. The inset shows SEM image of CNW film sample after Ar atmospheric pressure plasma treatment for 5 s. CNW film samples examined in this experiment have all the same morphology. The side view photographic images of the water droplets on the CNWs before and after the plasma treatments are shown in Figures 20(b)-20(e). The WCAs in the case of diamond films are reported to be approximately 75˚ on the H-terminated surface and 65˚ on the O-terminated surface [26]. In contrast, the WCA on the surface of as-grown CNWs prepared employing CH4/H2 mixture was 51˚ [Figure 20(b)]. The surface of as-grown CNWs prepared with CH4/H2, of which edges and defects would be partially H-terminated, was rather hydrophilic. After the Ar atmospheric pressure plasma treatment for just 1 s, the WCA was reduced drastically to 11˚. Then the WCAs decreased gradually with further increase of the Ar atmospheric pressure plasma treatment duration. As a result of Ar atmospheric pressure plasma treatment for 30 s, the WCA on the CNWs was 5˚, indicating that the CNW surface was completely superhydrophilic [Figure 20(d)]. On the other hand, after the CNW sample was exposed to CF4 plasma for 5 s, the WCA on the CNWs increased significantly to 147˚, indicating that the surface of fluorinated CNWs was superhy‐ drophobic [Figure 20(e)]. From these experiments, it was found that the surface wettability of CNW films could be controlled from superhydrophilic to superhydrophobic by the postplas‐ ma treatments without changing morphology.

under ultraviolet irradiation, a high photocatalytic decomposition rate of 6 mg/h was obtained for 1 mg of TiO2 supported on CNWs [41]. CNW-based electrochemical platform, which possesses large surface area with edges and electrochemical activity, offers great promise for providing a new class of nanostructured electrodes for electrochemical sensing and biosensing.

**Figure 18.** (a) TEM image of the surface of the CNW supporting Pt nanoparticles after the SCF-MOCFD for 30 min, and

**Figure 19.** (a) TEM image of CNWs supporting TiO2 nanoparticles after SCF-MOCFD for 30 min and (b)

**Figure 19.** (a) TEM image of CNWs supporting TiO2 nanoparticles after SCF-MOCFD for 30 min and (b) the obtained

**4.2. Surface chemical modification of carbon nanowalls for the wide range control of surface**

In view of the practical applications using CNWs in sensors and platforms for cell culturing, the effects of morphologies of CNWs and their surface properties should be considered systematically. We investigated the surface wettability of CNWs with emphasis on the chemisorption effect by postprocessing using plasma treatments [51,52]. Here, CNW samples

The surface of as-grown CNWs from CH4/H2 mixture was terminated with H atoms. After the preparation of CNW film sample using RI-PECVD for 15 min, its surface was oxidized using Ar atmospheric pressure plasma for 1 to 30 s [51,52]. The distance between the CNW film

2

In view of the practical applications using CNWs in sensors and platforms for cell culturing, the effects of morphologies of CNWs and their surface properties should be considered systematically. We investigated the surface wettability of CNWs with emphasis on the chemisorption effect by postprocessing using plasma treatments [51,52]. Here, CNW

**4.2. Surface chemical modification of carbon nanowalls for the wide range control of** 

grown from CH4/H2 mixtures on the Si substrate were chosen as primary forms.

sensing and biosensing.

19 40 in Figures 20(a) and (b), in Figures 20(a) and 20(b),

19 18 Figures 20(b)-(e). Figures 20(b)-20(e).

19 12 the contact angles of water

droplets and the water contact angles (WCAs)

17 Fig. 17

18 Fig. 18

21 Fig. 22

the obtained particle size distribution [41].

particle size distribution [41].

**surface wettability** 

**wettability**

**Figure 18. (a)** TEM image of the surface of the CNW supporting Pt nanoparticles after the SCF-MOCFD for 30 min, and (b) XPS profile of the Pt 4f region of the Pt-supported CNW after SCF-MOCFD [50].

Intensity (arb. unit)


1.0 0.8 0.6 0.4 0.2

Potential vs Ag/AgCl (V)

Binding energy (eV)

80 78 76 74 72 68 70

Pt 4f5/2 Pt 4f (b) 7/2

Furthermore, nanoparticles of the anatase phase of TiO2 were prepared on the entire surface of CNWs by SCF-MOCFD method at a substrate temperature of 180C. Figure 19 shows TEM image of the CNWs supporting TiO2 nanoparticles after SCF-MOCFD for 30 min and the statistical distribution of the observed nanoparticle size [41]. For decomposing methylene blue under ultraviolet irradiation, a high photocatalytic decomposition rate of 6 mg/h was obtained for 1 mg of TiO2 supported on CNWs [41]. CNW-based electrochemical platform, which possesses large surface area with edges and electrochemical activity, offers great promise for providing a new class of nanostructured electrodes for electrochemical

the water contact angles (WCAs)

(b) XPS profile of the Pt 4f region of the Pt-supported CNW after SCF-MOCFD [50].

10 nm

200

Current (µA)

(a)

162 Graphene - New Trends and Developments

0

400

600

X-ray photoelectron spectroscopy (XPS) measurements were carried out ex situ to analyze the CNW surface exposed to Ar atmospheric pressure plasma for oxidation. Figure 21 shows the composition ratio of O to C (O/C) at the surface of the CNWs as a function of plasma treatment duration. The composition ratio O/C was calculated from the peak intensity ratio of O 1s to C 1s corrected using the relative intensity factors. O content was detected even for the as-grown CNWs without plasma treatment, as shown in Figure 21. Because of ex situ XPS measurements, CNW surface was oxidized when exposed to the atmosphere. Due of the slight existence of oxygen at the surface of CNWs, the as-grown CNWs prepared from CH4/H2 would exhibit hydrophilic property as shown in Figures 20(a) and 20(b), in contrast to the hydrophobic surface of H-terminated diamond [53]. As the duration of Ar atmospheric pressure plasma treatment increased, the composition ratio O/C at the surface of CNWs increased rapidly at by the postplasma treatments without changing morphology

samples grown from CH4/H2 mixtures on the Si substrate were chosen as primary forms.

VHF-CCP region of RI-PECVD chamber without using H2 SWP [51,52].

The surface of as-grown CNWs from CH4/H2 mixture was terminated with H atoms. After the preparation of CNW film sample using RI-PECVD for 15 min, its surface was oxidized using Ar atmospheric pressure plasma for 1 to 30 s [51,52]. The distance between the CNW film sample and the atmospheric pressure plasma source was 5 mm. We expect that soft oxidation by oxygen radicals was realized, while the effect of ion bombardment on the surface was negligible during the exposure to atmospheric pressure plasma due to the very short mean free path of ions at the atmospheric pressure. For comparison, the surface of CNW sample was fluorinated for 5 s to add hydrophobic properties to the CNWs. For the fluorination treatment, the CNW sample was exposed to CF4 plasma generated in the

After the plasma surface treatments using the Ar atmospheric pressure plasma for oxidation and the CF4 plasma for fluorination in short duration, no noticeable change was observed in the morphology of CNW samples. These results indicate that such short-duration plasma treatments would induce surface chemical functionalization without etching or deposition. Figure 20(a) shows the contact angles of water droplets and the water contact angles (WCAs) on the CNWs before and after the Ar atmospheric pressure plasma treatment, as a function of plasma treatment duration, together with the WCA after the CF4 plasma treatment for 5 s. The inset shows SEM image of CNW film sample after Ar atmospheric pressure plasma treatment for 5 s. CNW film samples examined in this experiment have all the same morphology. The side view photographic images of the water droplets on the CNWs before and after the plasma treatments are shown in Figures 20(b)–(e). The WCAs in the case of diamond films are reported to be approximately 75 on the H-terminated surface and 65 on the O-terminated surface [26]. In contrast, the WCA on the surface of as-grown CNWs prepared employing CH4/H2 mixture was 51 [Figure 20(b)]. The surface of as-grown CNWs prepared with CH4/H2, of which edges and defects would be partially H-terminated, was rather hydrophilic. After the Ar atmospheric pressure plasma treatment for just 1 s, the WCA was reduced drastically to 11. Then the WCAs decreased gradually with further increase of the Ar atmospheric pressure plasma treatment duration. As a result of Ar atmospheric pressure plasma treatment for 30 s, the WCA on the CNWs was 5, indicating that the CNW surface was completely superhydrophilic [Figure 20(d)]. On the other hand, after the CNW sample was exposed to CF4 plasma for 5 s, the WCA on the CNWs increased significantly to 147, indicating that the surface of fluorinated CNWs was

**Figure 20.** (a) WCAs on CNW films as a function of treatment duration using Ar atmospheric pressure plasma, togeth‐ er with WCA after CF4 plasma treatment for 5 s. Inset shows SEM image of CNW sample after Ar atmospheric pres‐ sure plasma treatment for 5 s. Photos of water droplets on (b) as-grown CNWs, CNWs after Ar atmospheric pressure plasma treatment for (c) 5 s and (d) 30 s, and (e) CNWs after CF4 plasma treatment for 5 s [52].

first, then very slowly from 5 s, and became almost constant after 15 s. C 1s photoelectron spectra after the plasma treatment were recorded (data not shown). There were various types of oxygen-related components in the CNWs after the Ar atmospheric pressure plasma surface treatment, although components related to the oxidized graphene were small. Therefore, the oxidation occurred only at the edges or surface defects, while the primary structure of CNWs has hardly been changed by the Ar atmospheric pressure plasma exposure.

**Figure 21.** Composition ratio of O to C at the surface of CNWs evaluated from XPS results as a function of plasma treatment duration using the Ar atmospheric pressure plasma [52].

Binding energy (eV)

80 78 76 74 72 68 70

Pt 4f5/2 Pt 4f (b) 7/2

the water contact angles (WCAs)

10 nm


1.0 0.8 0.6 0.4 0.2

Intensity (arb. unit)

Potential vs Ag/AgCl (V)

200

Current (µA)

(a)

0

400

600

**Figure 22.** XPS C 1s spectrum of CNWs exposed to CF4 plasma for 5 s [52].

first, then very slowly from 5 s, and became almost constant after 15 s. C 1s photoelectron spectra after the plasma treatment were recorded (data not shown). There were various types of oxygen-related components in the CNWs after the Ar atmospheric pressure plasma surface treatment, although components related to the oxidized graphene were small. Therefore, the oxidation occurred only at the edges or surface defects, while the primary structure of CNWs

**Figure 20.** (a) WCAs on CNW films as a function of treatment duration using Ar atmospheric pressure plasma, togeth‐ er with WCA after CF4 plasma treatment for 5 s. Inset shows SEM image of CNW sample after Ar atmospheric pres‐ sure plasma treatment for 5 s. Photos of water droplets on (b) as-grown CNWs, CNWs after Ar atmospheric pressure

samples grown from CH4/H2 mixtures on the Si substrate were chosen as primary forms.

VHF-CCP region of RI-PECVD chamber without using H2 SWP [51,52].

by the postplasma treatments without changing morphology

164 Graphene - New Trends and Developments

The surface of as-grown CNWs from CH4/H2 mixture was terminated with H atoms. After the preparation of CNW film sample using RI-PECVD for 15 min, its surface was oxidized using Ar atmospheric pressure plasma for 1 to 30 s [51,52]. The distance between the CNW film sample and the atmospheric pressure plasma source was 5 mm. We expect that soft oxidation by oxygen radicals was realized, while the effect of ion bombardment on the surface was negligible during the exposure to atmospheric pressure plasma due to the very short mean free path of ions at the atmospheric pressure. For comparison, the surface of CNW sample was fluorinated for 5 s to add hydrophobic properties to the CNWs. For the fluorination treatment, the CNW sample was exposed to CF4 plasma generated in the

17 Fig. 17

18 Fig. 18

21 Fig. 22

19 12 the contact angles of water

droplets and the water contact angles (WCAs)

19 18 Figures 20(b)-(e). Figures 20(b)-20(e).

19 40 in Figures 20(a) and (b), in Figures 20(a) and 20(b),

After the plasma surface treatments using the Ar atmospheric pressure plasma for oxidation and the CF4 plasma for fluorination in short duration, no noticeable change was observed in the morphology of CNW samples. These results indicate that such short-duration plasma treatments would induce surface chemical functionalization without etching or deposition. Figure 20(a) shows the contact angles of water droplets and the water contact angles (WCAs) on the CNWs before and after the Ar atmospheric pressure plasma treatment, as a function of plasma treatment duration, together with the WCA after the CF4 plasma treatment for 5 s. The inset shows SEM image of CNW film sample after Ar atmospheric pressure plasma treatment for 5 s. CNW film samples examined in this experiment have all the same morphology. The side view photographic images of the water droplets on the CNWs before and after the plasma treatments are shown in Figures 20(b)–(e). The WCAs in the case of diamond films are reported to be approximately 75 on the H-terminated surface and 65 on the O-terminated surface [26]. In contrast, the WCA on the surface of as-grown CNWs prepared employing CH4/H2 mixture was 51 [Figure 20(b)]. The surface of as-grown CNWs prepared with CH4/H2, of which edges and defects would be partially H-terminated, was rather hydrophilic. After the Ar atmospheric pressure plasma treatment for just 1 s, the WCA was reduced drastically to 11. Then the WCAs decreased gradually with further increase of the Ar atmospheric pressure plasma treatment duration. As a result of Ar atmospheric pressure plasma treatment for 30 s, the WCA on the CNWs was 5, indicating that the CNW surface was completely superhydrophilic [Figure 20(d)]. On the other hand, after the CNW sample was exposed to CF4 plasma for 5 s, the WCA on the CNWs increased significantly to 147, indicating that the surface of fluorinated CNWs was superhydrophobic [Figure 20(e)]. From these experiments, it was found that the surface wettability of CNW films could be controlled from superhydrophilic to superhydrophobic

**Figure 21.** Composition ratio of O to C at the surface of CNWs evaluated from XPS results as a function of plasma

treatment duration using the Ar atmospheric pressure plasma [52].

has hardly been changed by the Ar atmospheric pressure plasma exposure.

plasma treatment for (c) 5 s and (d) 30 s, and (e) CNWs after CF4 plasma treatment for 5 s [52].

2 As mentioned before, by exposure to CF4 plasma for 5 s, the superhydrophobic surface of CNWs could be easily obtained from the as-grown H-terminated CNWs without changing the morphology of CNWs. The effect of CF4 plasma treatment for surface fluorination was also investigated using XPS analysis. The composition ratio of F to C at the surface of the CNWs after CF4 plasma treatment for 5 s was approximately 2.1. The composition ratio F/C was calculated from the ratio of the intensities of F 1s and C 1s peaks, corrected using the relative intensity factors. Figure 22 shows C 1s photoelectron spectrum of CNWs after the surface treatment using the CF4 plasma for 5 s. The binding energy of 284.6 eV in the XPS spectrum of CNWs is attributed to the C—C (sp2 ) bonds. The peaks at 289.1, 291.4, and 293.5 eV in the XPS spectrum shown in Figure 22 are assigned to the CF, CF2, and CF3 functional groups, respec‐ tively [54]. These three peaks in the XPS spectrum indicate that the F-terminated surface of CNWs was obtained by the CF4 plasma surface treatment for 5 s, resulting in the realization of superhydrophobic surface.

### **4.3. Detection of protein using surface-modified carbon nanowalls as electrodes**

Surface-oxidized CNW films were used as electrodes to detect bovine serum albumin (BSA) in phosphate-buffered solution (PBS). BSA, a serum albumin protein derived from cows, is often used as a protein concentration standard. CNWs were grown on SiO2 substrates using RI-PECVD employing C2F6/H2 mixture [5]. For the application of CNWs as an electrode of biosensor, the surface of CNW film was exposed to the Ar atmospheric pressure plasma for obtaining superhydrophilic surface. Electrochemical measurements were conducted using a standard three-electrode setup with an Ag/AgCl reference electrode and a Pt wire counter electrode. The cyclic voltammogram (CV) profiles of as-grown (bare) CNWs (500 nm in height), oxidized CNWs (500 nm), and oxidized CNWs of low height (350 nm) were recorded at scan rate of 100 mVs-1. Figure 23 shows the CV profiles using these CNW electrodes in PBS containing BSA. In the CV profile measured using bare CNW electrode without the Ar atmospheric pressure plasma treatment, which had slightly hydrophilic surface, weak oxidation and reduction peaks were observed in anode peak potential at 0.2 V and cathodic peak potential at -0.3 V, respectively. In the CV profile using the typical oxidized CNW electrode, on the other hand, a broad oxidation and a high peak reduction currents were observed in anode peak potential of 0.2 V and cathodic peak potential at -0.75 V, respectively. The surface of as-grown CNW electrode could be easily modified into superhydrophilic one by the surface oxidation using the Ar atmospheric pressure plasma. In the case of oxidized CNW electrode with low height, the CV profile exhibited small peak currents due to the small surface area. The results in Figure 23 indicate that superhydrophilic surfaces of CNWs with large surface areas were useful as electrodes for biosensor.

**Figure 23.** CV profiles of as-grown (bare) CNWs (500 nm in height), oxidized CNWs (500 nm), and oxidized CNWs of low height (350 nm) in PBS containing BSA. Scan rate: 100 mVs-1 [52].
