An Efficient Electrochemical Sensor Driven by Hierarchical Hetero-Nanostructures Consisting of RuO2 Nanorods on WO3 Nanofibers for Detecting Biologically Relevant Molecules

By means of electrospinning with the thermal annealing process, we investigate a highly efficient sensing platform driven by a hierarchical hetero-nanostructure for the sensitive detection of biologically relevant molecules, consisting of single crystalline ruthenium dioxide nanorods (RuO2 NRs) directly grown on the surface of electrospun tungsten trioxide nanofibers (WO3 NFs). Electrochemical measurements reveal the enhanced electron transfer kinetics at the prepared RuO2 NRs-WO3 NFs hetero-nanostructures due to the incorporation of conductive RuO2 NRs nanostructures with a high surface area, resulting in improved relevant electrochemical sensing performances for detecting H2O2 and L-ascorbic acid with high sensitivity.


Introduction
Recently, a variety of metal oxide nanostructures have been extensively utilized as efficient electrode substances owing to their outstanding electrocatalytic properties. Among them, ruthenium dioxide (RuO 2 ) has been well described as one of the best electrocatalysts for diverse energy related applications, such as the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and supercapacitors because of its high electric conductivity, catalytic activity, and thermal stability [1][2][3]. Especially, RuO 2 has been used as an efficient electrode system for supercapacitors owing to its excellent charging-discharging behavior [1,[4][5][6][7][8]. Generally, RuO 2 as a promising catalytic material is often used in the forms of hybrid structures or alloys with other abundant transition metals in consideration of the relatively high cost of RuO 2 . Thus, there have been previous reports regarding the use of RuO 2 nanostructures with other metal oxides as supercapacitors [9][10][11][12], and biosensing applications [1][2][3]13,14].
Tungsten trioxide (WO 3 ) nanostructures have been also extensively studied in various applications due to its earth-abundance, high durability, and chemical stabilities in aqueous acid media, as well as good electrochemical conductivity [15][16][17][18]. Thereby it has been developed as a catalyst for the hydrogen evolution reaction (HER) and supercapacitors in an acidic solution [19][20][21][22]. WO 3 also constitutes composites with other novel metals like Pt [23][24][25][26], Ir [17,23,27], and Ru [16,[28][29][30], or supporting materials. Nanostructured catalysts are applied to nonenzymatic electrochemical biosensors. Electrochemical properties can be enhanced from the increase of active surfaces. The detection of hydrogen peroxide (H 2 O 2 ) is important in not only biomedical and environmental applications, but also in the enzymatic system [31]. While ascorbic acid (AA) has an important role in the physiological function of organisms, a deficiency of AA causes several diseases [32,33]. Therefore, the detection and accurate quantification of target material with selectivity is highly required.
In this study, we introduce a facile fabrication of hybrid nanostructures consisting of single crystalline RuO 2 nanorods on eletrospun WO 3 nanofibers by utilizing electrospinning and thermal annealing processes. In addition, the fundamental electrochemical performances of RuO 2 nanorods-WO 3 nanofibers (RuO 2 NRs-WO 3 NFs) are carefully investigated, which confirm their characteristics of fast electron-transfer reactions and possibility as a catalytic sensing platform for detecting l-ascorbic acid (AA) and hydrogen peroxide (H 2 O 2 ) in phosphate buffered solution (PBS).
First, WO 3 nanofibers were synthesized by electrospinning and thermal annealing process according to the reported method [23]. To prepare electrospinning solution, 1.5 g WCl 6 were dissolved in 10.549 mL DMF with 1.25 g PVP and 0.191 mL acetic acid. After being stirred overnight, the solution was loaded into syringe and applied to the needle of the electrospinning system (Nano NC ESR 200R2). The needle was connected to a voltage power supply (applied voltage = 17.5 kV) at a flow rate of 5 µL/min, and the distance from needle tip to aluminum plate to collect as spun NFs was 15 cm. The collected electrospun NFs were calcinated at 500 • C for 1 h under a mixed gas atmosphere of 80 sccm of He and 10 sccm of O 2 with ramping rate of 1 • C/min. Ruthenium hydroxide (Ru(OH) 3 ) precursor was prepared by a precipitation process via the acid-base reaction with controlling pH of aqueous solution. The pH of the final precursor solution at about pH 10 was carefully achieved by slowly dropping 0.1 M NaOH dilute solution into 5 mM RuCl 3 ·xH 2 O aqueous solution [2,13]. After precipitation, the precursor solution was washed five times with deionized water, and then re-dispersed in 2~3 mL pure deionized water again. To grow RuO 2 NRs on WO 3 NFs, 2 mg of WO 3 NFs was dispersed into 1 mL deionized water and then mixed with 1 mL Ru(OH) 3 precursor solution. After sonication for 30 min, the mixed solution was directly dropped on the center of Si wafer. WO 3 nanofibers containing Ru(OH) 3 precursors loaded on the Si wafer was placed into the center of a furnace and calcined at 300°C for 5 h in air. The furnace was then allowed to cool to room temperature.
The surface morphology of as-grown products was examined by field emission scanning electron microscopy (FE-SEM; JEOL JSM-6700F). The detailed crystal structures were also investigated by a high-resolution transmission electron microscopy (HRTEM, Cs-corrected STEM, JEOL JEM-2100F) instrument equipped with selected area electron diffraction (SAED) micrographs and elemental EDX mapping with a Tecnai-F20 system operated at 200 kV. Additionally, high resolution X-ray diffraction measurement (XRD; Bruker D8 DISCOVER, Cu Kα radiation), and X-ray photoelectron spectroscopy (XPS; Theta Probe AR-XPS System. Al Kα radiation) were performed to investigate the crystal structure and surface binding energies of as-grown RuO 2 NR-WO 3 NFs.
For electrochemical measurements, a three-electrode system was used with a modified glassy carbon (GC) electrode (3 mm in diameter), a saturated calomel electrode (S.C.E.), and a coiled Pt wire (1 mm in diameter, length immersed in a solution~10 cm) as the working electrode, the reference electrode, and the counter electrode, respectively. All electrochemical experiments conducted with CHI 650E workstation (CH Instruments) and BAS100B (BAS Inc.). To modify the surface of a GC electrode with synthesized nanomaterials, 2 mg of RuO 2 NR-WO 3 NFs was suspended in 1.0 mL deionized water. Subsequently, 10 µL of the solution were dropped onto the GC electrode surface three times. Then, 10 µL of 0.05 wt% Nafion solution were loaded onto the modified GC electrode surface. Cyclic voltammetry (CV) measurements was used for analyze the capacitive behavior in 1 M H 2 SO 4 . For sensing experiments, linear sweep voltammetry (LSV) was also used with rotating disk electrode (RDE) at a scan rate of 5 mV s −1 with rotating speed of 1600 rpm, and amperometry measurements were used in 0.1 M phosphate buffered saline (PBS) at physiological condition pH (7.4).

Results and Discussion
3.1. Synthesis of Hybrid Nanostructures of RuO 2 Nanorods on Electrospun WO 3 Nanofibers Figure 1A,B show FE-SEM images of WO 3 NFs annealed at 500 • C. The calcined WO 3 NFs revealed a very fine structure and the diameter of the fibers was around 200 nm. On the other hand, after the heat treatment of the mixed solution composed of Ru(OH) 3 precursors and WO 3 NFs at 300°C for 5 h, it is readily identified that RuO 2 NRs were directly grown on the electrospun WO 3 NFs as shown in Figure 1C,D. Figure 1D represents the as grown RuO 2 NRs covering the entire surface of WO 3 NFs. The lateral dimension of RuO 2 NRs is estimated to be about 40 nm and the length up to 300 nm. Careful EDS measurements indicate that the atomic ratio of Ru to W is confirmed as 45:55. According to our previous real-time study by in situ synchrotron XRD, a simple recrystallization process by thermal annealing might be responsible for the growth mechanism of RuO 2 NRs. It was carefully suggested that Ru diffusion to the amorphous nanoparticles followed by diffusion to the growing surface of the nanorod plays an essential role in the growth of RuO 2 NRs in oxygen ambient, which is supported by the nucleation theory [34]. electrode with synthesized nanomaterials, 2 mg of RuO2 NR-WO3 NFs was suspended in 1.0 mL deionized water. Subsequently, 10 μL of the solution were dropped onto the GC electrode surface three times. Then, 10 μL of 0.05 wt% Nafion solution were loaded onto the modified GC electrode surface. Cyclic voltammetry (CV) measurements was used for analyze the capacitive behavior in 1 M H2SO4.
For sensing experiments, linear sweep voltammetry (LSV) was also used with rotating disk electrode (RDE) at a scan rate of 5 mV s -1 with rotating speed of 1600 rpm, and amperometry measurements were used in 0.1 M phosphate buffered saline (PBS) at physiological condition pH (7.4). Figure 1A and 1B show FE-SEM images of WO3 NFs annealed at 500 °C. The calcined WO3 NFs revealed a very fine structure and the diameter of the fibers was around 200 nm. On the other hand, after the heat treatment of the mixed solution composed of Ru(OH)3 precursors and WO3 NFs at 300 ℃ for 5 h, it is readily identified that RuO2 NRs were directly grown on the electrospun WO3 NFs as shown in Figure 1C,D. Figure 1D represents the as grown RuO2 NRs covering the entire surface of WO3 NFs. The lateral dimension of RuO2 NRs is estimated to be about 40 nm and the length up to 300 nm. Careful EDS measurements indicate that the atomic ratio of Ru to W is confirmed as 45:55. According to our previous real-time study by in situ synchrotron XRD, a simple recrystallization process by thermal annealing might be responsible for the growth mechanism of RuO2 NRs. It was carefully suggested that Ru diffusion to the amorphous nanoparticles followed by diffusion to the growing surface of the nanorod plays an essential role in the growth of RuO2 NRs in oxygen ambient, which is supported by the nucleation theory [34].   Figure 2B demonstrates that all peaks are closely matched with the monoclinic phase of WO3 [19,35]. On the other hand, XRD spectrum of composite RuO2 NR-WO3 NFs confirms the same monoclinic phase WO3 peaks including two major peaks at 27.1° and 34.8° corresponding to (110) and (101) crystallographic planes of tetragonal RuO2 structure as displayed in Figure 2A [2,13]. To investigate the oxidation states of Ru, W, and O atoms, XPS measurements were performed. In Figure 2C, two separated binding energies at 35.1 eV and 37.3 eV are clearly identified as two spin-orbit states of W 4f5/2 and W 4f7/2, respectively, which indicates the oxidation state of +6 for W in WO3 NFs [16,36]. Both high resolution Ru 3d and Ru 3p spectra were shown in Figure 2E, F. Although the peak position of Ru 3d3/2 is overlapped with C 1s [16,37], the oxidation state of Ru species is readily identified to Ru 4+ based on the binding energies of 280.7 eV   Figure 2B demonstrates that all peaks are closely matched with the monoclinic phase of WO 3 [19,35]. On the other hand, XRD spectrum of composite RuO 2 NR-WO 3 NFs confirms the same monoclinic phase WO 3 peaks including two major peaks at 27.1 • and 34.8 • corresponding to (110) and (101) crystallographic planes of tetragonal RuO 2 structure as displayed in Figure 2A [2,13]. To investigate the oxidation states of Ru, W, and O atoms, XPS measurements were performed. In Figure 2C, two separated binding energies at 35.1 eV and 37.3 eV are clearly identified as two spin-orbit states of W 4f 5/2 and W 4f 7/2 , respectively, which indicates the oxidation state of +6 for W in WO 3 NFs [16,36]. Both high resolution Ru 3d and Ru 3p spectra were shown in Figure 2E,F. Although the peak position of Ru 3d 3/2 is overlapped with C 1s [16,37], the oxidation state of Ru species is readily identified to Ru 4+ based on the binding energies of 280.7 eV and 462.8 eV, indexed to Ru 3d 5/2 and Ru 3p 3/2 , respectively [37,38]. In addition, the peak at 530.5 eV of O 1s is associated with O 2− in RuO 2 and WO 3 metal oxides as shown in Figure 2D.  [37,38]. In addition, the peak at 530.5 eV of O 1s is associated with O 2-in RuO2 and WO3 metal oxides as shown in Figure 2D.   Figure 3E reveals the existence of many different crystalline phases in a WO3 nanofiber which confirms the polycrystalline nature of a WO3 nanofiber. On the contrary, the fast Fourier transform (FFT) of the lattice-resolved image for a RuO2 nanorod in Figure 3F represents highly ordered lattice fringes with a single crystal nature. The values of lattice spacing of adjacent planes are estimated by about 0.318 nm and 0.263 nm, corresponding to those of between the (110) planes and (101) for the tetragonal RuO2, respectively. Furthermore, TEM-EDS element mapping analysis from the highangle annular dark field (HAADF) STEM image shown in Figure S1 confirms the homogenous distribution of Ru, W, and O in distinct regions in the hierarchical nanostructure. W atoms exist on the backbone of the nanofibers, whereas Ru atoms exclusively exist on the branched nanorods. Oxygen atoms exist both on the backbone of the nanofibers and branched nanorods. Thus, we successfully fabricate the high density of single-crystalline RuO2 nanorods on WO3 nanofibers by using a combination of an electrospinning process and a thermal annealing process. Our growth process thus provides a simple methodology for the fabrication of highly efficient electrocatalysts.

Electrochemical Properties for Capacitive Behaviors of RuO2 NRs-WO3 NFs
The general electrochemical activities of RuO2 NRs-WO3 NFs and WO3 NFs were examined by CV in 10 mM [Fe(CN)6] 3-aqueous solution containing 1 M KCl. Figure S2 displays CV curves of RuO2 NRs-WO3 NFs and WO3 NFs at a scan rate 100 mV s -1 . Voltammetric current peaks at RuO2 NRs-WO3 NFs are reversible, while those of WO3 NFs are quasi-reversible. It seems to be ascribed to that RuO2 NRs-WO3 NFs allow very facile heterogeneous electron transfer kinetics with high electric conductivities in contrast to WO3 NFs. Moreover, RuO2 NRs-WO3 NFs show a much larger charging current in CV than WO3 NFs.
To characterize the charging behavior of the synthesized materials, CV was measured for a potential range from 0.1 V to 0.9 V (vs. S.C.E.) in 1 M H2SO4 as seen in Figure 4. Figure 4A shows CV   Figure 3A,B, low-magnification TEM images show the high density of RuO 2 nanorods directly grown on the porous surface of WO 3 nanofiber. The SAED pattern shown in Figure 3E reveals the existence of many different crystalline phases in a WO 3 nanofiber which confirms the polycrystalline nature of a WO 3 nanofiber. On the contrary, the fast Fourier transform (FFT) of the lattice-resolved image for a RuO 2 nanorod in Figure 3F represents highly ordered lattice fringes with a single crystal nature. The values of lattice spacing of adjacent planes are estimated by about 0.318 nm and 0.263 nm, corresponding to those of between the (110) planes and (101) for the tetragonal RuO 2 , respectively. Furthermore, TEM-EDS element mapping analysis from the high-angle annular dark field (HAADF) STEM image shown in Figure S1 confirms the homogenous distribution of Ru, W, and O in distinct regions in the hierarchical nanostructure. W atoms exist on the backbone of the nanofibers, whereas Ru atoms exclusively exist on the branched nanorods. Oxygen atoms exist both on the backbone of the nanofibers and branched nanorods. Thus, we successfully fabricate the high density of single-crystalline RuO 2 nanorods on WO 3 nanofibers by using a combination of an electrospinning process and a thermal annealing process. Our growth process thus provides a simple methodology for the fabrication of highly efficient electrocatalysts.

Electrochemical Properties for Capacitive Behaviors of RuO 2 NRs-WO 3 NFs
The general electrochemical activities of RuO 2 NRs-WO 3 NFs and WO 3 NFs were examined by CV in 10 mM [Fe(CN) 6 ] 3− aqueous solution containing 1 M KCl. Figure S2 displays CV curves of RuO 2 NRs-WO 3 NFs and WO 3 NFs at a scan rate 100 mV s −1 . Voltammetric current peaks at RuO 2 NRs-WO 3 NFs are reversible, while those of WO 3 NFs are quasi-reversible. It seems to be ascribed to that RuO 2 NRs-WO 3 NFs allow very facile heterogeneous electron transfer kinetics with high electric conductivities in contrast to WO 3 NFs. Moreover, RuO 2 NRs-WO 3 NFs show a much larger charging current in CV than WO 3 NFs.
To characterize the charging behavior of the synthesized materials, CV was measured for a potential range from 0.1 V to 0.9 V (vs. S.C.E.) in 1 M H 2 SO 4 as seen in Figure 4. Figure 4A shows CV results comparing RuO 2 NRs-WO 3 NFs and WO 3 NFs at a scan rate 100 mV s −1 . It supports the enhanced capacity of RuO 2 NRs-WO 3 NFs as the RuO 2 NRs were grown on WO 3 NFs. To examine the charging performance, the average specific capacitance values (C sp , F g −1 ) were calculated with the following Equation (1) using CV curves shown in Figure 4B. results comparing RuO2 NRs-WO3 NFs and WO3 NFs at a scan rate 100 mV s -1 . It supports the enhanced capacity of RuO2 NRs-WO3 NFs as the RuO2 NRs were grown on WO3 NFs. To examine the charging performance, the average specific capacitance values (Csp, F g -1 ) were calculated with the following Equation (1) using CV curves shown in Figure 4B.
where v is the scan rate (V s -1 ), ∆ is the weight of electrode materials, ∆ is the potential range, and is the area under CV curve [39]. At the scan rate of 10 mV s -1 , the Csp values of the synthesized materials, RuO2 NRs-WO3 NFs and WO3 NFs, are 98.15 F g -1 and 0.95 F g -1 , respectively. The Csp of RuO2 NRs-WO3 NFs is obviously 103-fold higher than that of WO3 NFs as shown in Figure 4C. As the scan rate increases, Csp becomes smaller and the Csp of RuO2 NRs-WO3 NFs and WO3 NFs decreased down to 57% and 42%, respectively, while increasing the scan rate from 10 mV s -1 to 200 mV s -1 . This additionally indicates the successful decoration of WO3 NFs with RuO2 NRs forming the hierarchical hetero-nanostructures.
Electrochemical impedance spectroscopy (EIS) was also employed to examine the electrochemical where v is the scan rate (V s −1 ), ∆m is the weight of electrode materials, ∆V is the potential range, and IdV is the area under CV curve [39]. At the scan rate of 10 mV s −1 , the C sp values of the synthesized materials, RuO 2 NRs-WO 3 NFs and WO 3 NFs, are 98.15 F g −1 and 0.95 F g −1 , respectively. The C sp of RuO 2 NRs-WO 3 NFs is obviously 103-fold higher than that of WO 3 NFs as shown in Figure 4C.
As the scan rate increases, C sp becomes smaller and the C sp of RuO 2 NRs-WO 3 NFs and WO 3 NFs decreased down to 57% and 42%, respectively, while increasing the scan rate from 10 mV s −1 to 200 mV s −1 . This additionally indicates the successful decoration of WO 3 NFs with RuO 2 NRs forming the hierarchical hetero-nanostructures. Electrochemical impedance spectroscopy (EIS) was also employed to examine the electrochemical behavior of RuO 2 NRs-WO 3 NFs and WO 3 NFs. EIS measurement was carried out at 0.5 V (vs. S.C.E.) under the same condition of CV experiments with a frequency range of 0.1 Hz-1000 kHz as shown in Figure S3. The Nyquist plot of RuO 2 NRs-WO 3 NFs was closer to a vertical line than that of WO 3 NFs, exhibiting nearly pure capacitive behavior of RuO 2 NR-WO 3 NFs [1,40]. The stability of RuO 2 NRs-WO 3 NFs for capacitance was demonstrated by monitoring the change of C sp during repeated CV cycles as depicted in Figure 4D. RuO 2 NRs-WO 3 NFs excellently maintained about 96% of its original C sp for the 1000 CV cycles at a scan rate of 100 mV s −1 .  Figure S3. The Nyquist plot of RuO2 NRs-WO3 NFs was closer to a vertical line than that of WO3 NFs, exhibiting nearly pure capacitive behavior of RuO2 NR-WO3 NFs [1,40]. The stability of RuO2 NRs-WO3 NFs for capacitance was demonstrated by monitoring the change of Csp during repeated CV cycles as depicted in Figure 4D. RuO2 NRs-WO3 NFs excellently maintained about 96% of its original Csp for the 1000 CV cycles at a scan rate of 100 mV s −1 .

Applications to Electrochemical Sensing of AA and H2O2
The electrochemical properties of RuO2 NRs-WO3 NFs for AA oxidation were also studied. LSV measurements in 0.1 M PBS were used for examining the oxidations of various biomaterials such as AA, DA, UA, AP, and glucose. The chosen concentrations are slightly above the physiological concentrations. As shown in Figure 5A, AA oxidation started to occur from the most negative potential compared with other biomaterials. Amperometric measurements of RuO2 NRs-WO3 NFs and WO3 NFs were conducted at 0 V (vs. S.C.E.) which possibly allow for the oxidation of AA only, excepting for the other tested biomolecules as seen in the LSV results of Figure 5A.

Applications to Electrochemical Sensing of AA and H 2 O 2
The electrochemical properties of RuO 2 NRs-WO 3 NFs for AA oxidation were also studied. LSV measurements in 0.1 M PBS were used for examining the oxidations of various biomaterials such as AA, DA, UA, AP, and glucose. The chosen concentrations are slightly above the physiological concentrations. As shown in Figure 5A, AA oxidation started to occur from the most negative potential compared with other biomaterials. Amperometric measurements of RuO 2 NRs-WO 3 NFs and WO 3 NFs were conducted at 0 V (vs. S.C.E.) which possibly allow for the oxidation of AA only, excepting for the other tested biomolecules as seen in the LSV results of Figure 5A. As observed in Figure 5B, the anodic currents of both electrodes were increased linearly with the concentration of AA increased from 5 μM to 2 mM. Also, the calibration curves based on the amperometric data were depicted in inset of Figure 5B. The sensitivity of RuO2 NRs-WO3 NFs (171.7 μA mM -1 cm -2 , R 2 = 0.9990, normalized to GC substrate electrode area, 0.072 cm 2 ) were surprisingly increased by 244 times compared to that of WO3 NFs (0.704 μA mM -1 cm -2 , R 2 = 0.9990). Most of the typical biological samples are complex, having various oxidizable species, so selectivity to a targeted analyte is an essential requirement for any sensor. In Figure 6A, current responses for AA oxidation were stable against the additions of 0.1 mM AP, 0.1 mM UA, 0.1 μM DA and 5 mM glucose at 0 V. Additionally, the stability of RuO2 NRs-WO3 NFs was measured by monitoring the change of current at 0 V in 0.1 M PBS containing 0.3 mM AA. The amperometric response of RuO2 NRs-WO3 NFs retained 96% of the initial current level during over a 4200s measurement in Figure 6B, supporting its excellent stability. Table 1 summarizes the properties of RuO2 NRs-WO3 NFs in comparison with other Ru-based materials used as AA sensors.  As observed in Figure 5B, the anodic currents of both electrodes were increased linearly with the concentration of AA increased from 5 µM to 2 mM. Also, the calibration curves based on the amperometric data were depicted in inset of Figure 5B. The sensitivity of RuO 2 NRs-WO 3 NFs (171.7 µA mM −1 cm −2 , R 2 = 0.9990, normalized to GC substrate electrode area, 0.072 cm 2 ) were surprisingly increased by 244 times compared to that of WO 3 NFs (0.704 µA mM −1 cm −2 , R 2 = 0.9990). Most of the typical biological samples are complex, having various oxidizable species, so selectivity to a targeted analyte is an essential requirement for any sensor. In Figure 6A, current responses for AA oxidation were stable against the additions of 0.1 mM AP, 0.1 mM UA, 0.1 µM DA and 5 mM glucose at 0 V. Additionally, the stability of RuO 2 NRs-WO 3 NFs was measured by monitoring the change of current at 0 V in 0.1 M PBS containing 0.3 mM AA. The amperometric response of RuO 2 NRs-WO 3 NFs retained 96% of the initial current level during over a 4200-s measurement in Figure 6B, supporting its excellent stability. Table 1  As observed in Figure 5B, the anodic currents of both electrodes were increased linearly with the concentration of AA increased from 5 μM to 2 mM. Also, the calibration curves based on the amperometric data were depicted in inset of Figure 5B. The sensitivity of RuO2 NRs-WO3 NFs (171.7 μA mM -1 cm -2 , R 2 = 0.9990, normalized to GC substrate electrode area, 0.072 cm 2 ) were surprisingly increased by 244 times compared to that of WO3 NFs (0.704 μA mM -1 cm -2 , R 2 = 0.9990). Most of the typical biological samples are complex, having various oxidizable species, so selectivity to a targeted analyte is an essential requirement for any sensor. In Figure 6A, current responses for AA oxidation were stable against the additions of 0.1 mM AP, 0.1 mM UA, 0.1 μM DA and 5 mM glucose at 0 V. Additionally, the stability of RuO2 NRs-WO3 NFs was measured by monitoring the change of current at 0 V in 0.1 M PBS containing 0.3 mM AA. The amperometric response of RuO2 NRs-WO3 NFs retained 96% of the initial current level during over a 4200s measurement in Figure 6B, supporting its excellent stability. Table 1 summarizes the properties of RuO2 NRs-WO3 NFs in comparison with other Ru-based materials used as AA sensors.    3 Ref. [13], 4 Ref. [41], 5 Ref. [42], 6 Ref, [43].
The catalytic effect of RuO 2 NRs-WO 3 NFs for H 2 O 2 reduction was also measured. Figure 7A shows overlaid LSV results of RuO 2 NRs-WO 3 NFs and WO 3 NFs. It presents clearly that H 2 O 2 reduction at RuO 2 NRs-WO 3 NFs starts from a much less negative potential with much greater reduction current level than that at WO 3 NFs. In fact, the cathodic current level measured at −0.2 V (vs. S.C.E.) was more greatly increased for RuO 2 NRs-WO 3 NFs than WO 3 NFs in response to the successive increase of H 2 O 2 concentration ( Figure 7B). Inset of Figure 7B shows the calibrated current vs concentration with good linearity. Obtained sensitivities from the calibration curves are 619.7 µA mM −1 cm −2 (R 2 = 0.9960), and 5.5 µA mM −1 cm −2 (R 2 = 0.9384) for RuO 2 NRs-WO 3 NFs and WO 3 NFs, respectively. Sensitivity of RuO 2 NR-WO 3 NFs is 112-fold higher than the value of WO 3 NFs, and therefore it supports the enhanced activities of RuO 2 NRs-WO 3 NFs toward H 2 O 2 reduction. The H 2 O 2 reduction current instead of the oxidation current was monitored to sense H 2 O 2 in order to avoid the interference from many oxidizable species generally present in biological systems. Figure S4 represents the selectivity of RuO 2 NRs-WO 3 Table 2. RuO 2 NRs-WO 3 NFs for measuring H 2 O 2 reduction current was less stable than that for AA oxidation. In fact, H 2 O 2 reduction current measured at −0.2 V was decreased to~60% of the initial current level after 4200-s continuous measurement (data not shown).

Conclusions
We report the successful fabrication of single crystalline RuO2 nanorods on WO3 nanofibers by electrospinning and calcination. Microscopic and spectroscopic measurements such as SEM with EDS, XRD, and XPS were used to characterize the structure and composition of RuO2 NRs-WO3 NFs. The RuO2 NRs-WO3 NFs showed improved electrocatalytic activities over WO3 NFs through a series of electrochemical measurements. In 1 M H2SO4 solution, RuO2 NRs-WO3 NFs represent a higher Csp, 98.15 F g -1 , by 103-fold with good stability and a sharper slope than pure WO3 NFs. Additionally, the RuO2 NRs-WO3 NFs have dramatically enhanced sensing abilities, in accordance with 224 times (171.7 μA mM -1 cm -2 ) sensitivity for AA oxidation, and 112 times (619.7 μA mM -1 cm -2 ) sensitivity for H2O2 reduction, respectively, compared to those of pure WO3 NFs. These results thus suggest that RuO2 NRs-WO3 NFs could be a promising candidate electrocatalyst for the fabrication of an efficient electrochemical sensor due to its highly effective electrochemical performance.

Conclusions
We report the successful fabrication of single crystalline RuO 2 nanorods on WO 3 nanofibers by electrospinning and calcination. Microscopic and spectroscopic measurements such as SEM with EDS, XRD, and XPS were used to characterize the structure and composition of RuO 2 NRs-WO 3 NFs. The RuO 2 NRs-WO 3 NFs showed improved electrocatalytic activities over WO 3 NFs through a series of electrochemical measurements. In 1 M H 2 SO 4 solution, RuO 2 NRs-WO 3 NFs represent a higher C sp , 98.15 F g −1 , by 103-fold with good stability and a sharper slope than pure WO 3 NFs. Additionally, the RuO 2 NRs-WO 3 NFs have dramatically enhanced sensing abilities, in accordance with 224 times (171.7 µA mM −1 cm −2 ) sensitivity for AA oxidation, and 112 times (619.7 µA mM −1 cm −2 ) sensitivity for H 2 O 2 reduction, respectively, compared to those of pure WO 3 NFs. These results thus suggest that RuO 2 NRs-WO 3 NFs could be a promising candidate electrocatalyst for the fabrication of an efficient electrochemical sensor due to its highly effective electrochemical performance.