Composite Electrodes With Carbon Supported Ru Nanoparticles For H 2 O 2 Detection

A new carbon paste electrode (CPE) incorporating Ru nanoparticles (RuNP) stabilized on graphite powder was developed for H2O2 amperometric detection. Cyclic voltammetric measurements, performed in phosphate buffer solutions at different potential scan rates and different potential ranges were carried out in order to evaluate the electrochemical behavior of the CPE-RuNP modified electrodes. From cyclic voltammetry, at –0.1 V vs. Ag/AgCl, KClsat, the relative increase of the H2O2 reduction current varies in the following order: 28.47% (CPE) < 94.81% (CPE-RuNP (2.5:1)) < 118.19% (CPE-RuNP (2.5:3)) < 152.43% (CPE-RuNP (2.5:2), recommending the new electrodes as a promising sensors for hydrogen peroxide detection.


Introduction
][5] Ru electrodes have shown affinity also for the oxygen reduction in both acid 6 and alkaline 7 electrolytes.The electrocatalytic reduction of oxygen plays a major role in several industrial processes and in corrosion protection.Thus, it was reported that Ru has a beneficial effect on the passivity of duplex stainless steel corrosion in sodium chloride solution 8 and that additions of up to 3% wt Ru increased the corrosion resistance of the WC-Co alloys. 9uthenium oxide composites were reported as efficient catalysts for non-enzymatic glucose oxidation 10 and for simultaneous determination of ascorbic acid and dopamine. 11onsiderably fewer reports exist on H 2 O 2 detection by using Ru or Ru oxide based electrodes. 12,13Hydrogen peroxide is an efficient oxidizing agent used in textile industry, cleaning products, food industry and environmental protection 14 and an essential intermediate product of enzymatic reactions. 15Among these, the electrochemical tracking of biological targets by way of enzyme-based H 2 O 2 detection is of special interest.
For most electrochemical sensors, the detection of H 2 O 2 was achieved at positive potentials [16][17] where the results may be affected by the presence of interferences, (e.g., ascorbic and uric acid).Therefore, decreasing the oxidation potential or performing analysis at its reduction potential is essential for effective detection. 18n this paper, a novel electrochemical sensor consisting of a carbon paste electrode modified with carbon supported Ru nanoparticles (RuNP) was developed for H 2 O 2 amperometric detection.Cyclic voltammetry and amperometry have been used for the investigation of electrochemical properties and electrocatalytic activity of the nanocomposite modified electrode.

1. Physico-chemical Characterization of Ru-graphite Nanoparticles
TEM measurements were performed to examine the morphology of carbon supported RuNP.Fig. 1 reveals images in which a heterogeneous structure consisting of the carbon substrate (light regions) and the catalyst nanoparticles (dark regions) can be noticed.The carbon supported Ru nanoparticles are highly dispersed and very small.Nevertheless, agglomerates of different size which are similar in morphology to other Ru-based catalysts reported in the literature 3 can also be observed.

Electrochemical Behavior of the Modified Electrodes
Cyclic voltammetry (CV) experiments were carried out to investigate the electrochemical behavior of the composite electrode material, in different experimental conditions (variable CPE:RuNP ratios, different potential scan windows) and the results are depicted in Fig. 2A-B.
While CV on CPE presents no peaks, in the presence of carbon supported RuNP in the CPE, the CV features (Fig. 2A) exhibit one anodic (Ia) and one cathodic peak (IIc).The anodic peak could be attributed to ruthenium oxidation and the cathodic one, to dissolved oxygen reduction.Previously it was reported that, depending on the potential, ruthenium can be oxidized to hydrated RuO 19 , Ru(OH) 2 or RuO x H 2 O 20 , but also to oxides of higher oxidation states (Ru 2 O 3 ).Moreover, at potentials beyond 1.2 V, Ru oxidation to RuO 4 overlaps with oxygen evolution. 21As expected, the anodic peak intensity increases proportionally with the Ru amount in the carbon paste.The lack of a peak corresponding to Ru oxides reduction suggests the irreversibility of the formation of these oxides, which are composed of tridimensional aggregates consisting of a structure including various Ru oxides, bridged oxygen, OH, and water. 22These results are confirmed also by other researchers. 23t is interesting to note that a potential scan in the negative direction, starting from 0 V, does not reveal any cathodic peak because no Ru oxide (that acts as a catalyst for O 2 reduction) is formed during the anodic scan.As can be seen from Fig. 2B, the height of the cathodic peak (IIc) attributed to oxygen reduction is placed at a much more negative value of the potential (E = -0.9V vs. Ag/AgCl, KCl sat ) than in the case when Ru oxides were formed during the anodic scan (E = -0.5 V vs. Ag/AgCl, KCl sat ), proving clearly the electrocatalytic properties of Ru oxides.
As expected for a diffusion-controlled reaction, the current intensity of the oxygen reduction (peak IIc) depends on the potential scan rate in the range 5-250 mV/s (Fig. 3A), the slope of log I -log v plot at -0.5 V vs. Ag/AgCl, KCl sat being 0.500 ± 0.022 for CPE-RuNP (2.5:1), 0.456 ± 0.042 for CPE-RuNP(2.5:2) and 0.360 ± 0.008 for CPE-RuNP(2.5:3),respectively, with R = 0.990, n = 5).The slope values of the linear dependence of the cathodic current on the square root of the scan rate (Fig. 3B), close to 0.5, certify the diffusion control of the oxygen mass transport to the CPE-RuNP electrodes.Also, at high scan rate (> 100 mV/s), a deviation from linearity is observed, indicating that an insufficient solute quantity reaches the electrode surface (results not shown). 24s expected, at pH 7, irrespective the potential values (at -0.1 V, -0.3 V and -0.5 V vs. Ag/AgCl, KCl sat , respectively), the currents recorded during the cathodic potential scan on CPE-RuNP modified electrodes (peak IIc) increase with the amount of RuNP, respectively with the quantity of Ru oxides formed during the anodic scan until +1.3 V vs. Ag/AgCl, KCl sat (Table 1).
Although the current values obtained at -0.5 V or -0.3 V vs. Ag/AgCl, KCl sat are greater than those recorded at -0.1 V vs. Ag/AgCl, KCl sat , in view to avoid interferences, an applied potential of -0.1 V vs. Ag/AgCl, KCl sat was used for further amperometric measurements of H 2 O 2 reduction.
In can be noticed that at concentrations of 3 mM H 2 O 2 , both the anodic and cathodic currents increase with the RuNP amounts present in CPE-RuNP (Fig. 4A).
The influence of the H 2 O 2 concentrations on the voltammetric currents recorded at -0.5 V vs. Ag/AgCl, KCl sat is depicted in Fig. 5A and the corresponding calibration curves in Fig. 5B.
The linear dependence between the currents and H 2 O 2 concentration allows the determination of the CPE-RuNP modified electrodes sensitivity (Table 2), which, as expected increase with the amount of RuNP present in the electrode matrix.
The catalytic current, observed in the presence of H 2 O 2 (Fig. 5A), varied linearly with its concentration in the range between 1-5 mM, disregarding the RuNP amount existing in the carbon paste matrix (Fig. 5B).The

3. Amperometry
Batch amperometric calibration for H 2 O 2 using the different modified electrodes was performed at a constant potential of -0.1 V vs. Ag/AgCl, KCl sat .The obtained calibration curves are linear in the range up to 0.1 mM H 2 O 2 (Fig. 6), with a sensitivity increasing with the amount of RuNP included in the carbon paste matrix (Table 2).The sensitivity of CPE-RuNP(2.5:1) electrode determined by amperometric measurements increases 35 times comparing to the unmodified CPE.The increasing values of the sensitivities observed for both investigation techniques is related to (i) the increase of the electron transfer rate of the H 2 O 2 to the RuNP, and to (ii) the improved accessibility and reversibility of the electron-transfer process on increasing amounts of RuNP present in the electrode composite matrix. 25The linear domain and the sensitivity of the electrodes are in agreement with the values reported in the literature for other Ru oxide based electrodes for H 2 O 2 reduction (e.g.3.5 *10 -5 A/mM for nano-ruthenium oxide/riboflavin modified glassy carbon). 26he response time, estimated as t 95% , was less than 1 min.The best LOD value (signal/noise ratio of 3) is obtained for the CPE-RuNP(2.5:1)electrode which is almost half than in the case of unmodified CPE.For other electrodes having amounts of RuNP approaching or exceeding the amount of graphite, despite the fact that an enhancement of the sensitivity is observed, the LOD value is affected by the increasing values of experimental errors (i.e. S a ).
As expected, working in amperometric mode allowed using lower H 2 O 2 concentrations and the sensors sen-sitivity estimated from the obtained calibration curves is higher than in cyclic voltammetric method.

1. Materials
Ru nanoparticles (RuNP) stabilized on carbon powder were prepared by controlled reduction of RuCl 3 in polyols followed by slow addition of carbon powder 27 and were a kind gift from Dr. D. Goia (Clarkson University, USA).For preparing carbon paste electrodes (CPE), graphite powder (99.9% purity) and paraffin oil were purcha- All reagents were of analytical degree and were used without further purification.Distilled water was used for preparing all solutions.

2. Preparation of the CPE and CPE-RuNP Electrodes
Unmodified carbon paste (CPE) was prepared by mixing 0.04 g of graphite powder with 0.02 ml of paraffin oil.The RuNP modified carbon paste electrodes (CPE-RuNP) were prepared by thoroughly mixing 0.04 g of graphite powder, 0.02 ml of paraffin oil and 0.02 g RuNP, (CPE-RuNP (2.5:1)), 0.04 g RuNP (CPE-RuNP (2.5:2)) or, 0.06 g carbon supported RuNP (CPE-RuNP (2.5:3)), respectively.The un/modified carbon paste was placed into a 3 mm diameter cavity of a Teflon tip (geometric surface area of 0.07 cm 2 ), the electric contact being assured by a copper piece placed on the holder surface.
The obtained electrode surface was smoothed manually using a clean filter paper.When necessary, a new electrode surface was obtained by removing a 2 mm thick layer from the outer paste layer, or adding freshly modified paste.

Characterization Methods
For the electron microscopic illustration of the carbon supported RuNP, a transmission electron microscopy TEM was used (Hitachi Automatic TEM H7650, accelerating voltage 40-120 kV, zoom 200×-600000×).
All electrochemical measurements (cyclic voltammetry and amperometry) were performed using a PC controlled electrochemical analyzer (AUTOLAB PG-STAT302N EcoChemie, Utrecht, Netherlands) into a conventional undivided three-electrodes cell equipped with a Pt wire, as counter electrode, and a Ag/AgCl, KCl sat reference electrode.As working electrode the above described tip containing un/modified carbon paste (CPE, CPE -Ru-NP (w:w)) was fixed on an immobile holder (for unstirred cyclic voltammetry experiments) or on a rotating disc electrode holder (EDI-10K, Radiometer Analytical, France) for controlling the stirring rate of the solution in amperometric experiments.
Batch amperometric measurements were carried out at an applied potential of -0.1 V vs. Ag/AgCl, KCl sat by addition of increasing volumes of 0.01 M H 2 O 2 solution into a 1/15 M phosphate buffer (pH 7).
All experiments were carried out in aerated solution at ambient temperature.

Conclusions
A new carbon paste electrode (CPE) incorporating carbon supported Ru nanoparticles (RuNP) for H 2 O 2 amperometric detection was developed and characterized.
The investigation by electrochemical methods of the CPE-RuNP modified electrodes reveals the formation of the Ru oxides at +1.0 ÷ +1.1 V vs. Ag/AgCl, KCl sat and the reduction of oxygen at -0.5 V vs. Ag/AgCl, KCl sat , value much more positive than those obtained in the absence of RuNP as electrocatalyst.The reduction current of the oxygen is much higher than in the case of the unmodified electrode and is dependent on the scan rate, proving the diffusion control of the redox process involved at the electrode surface.

Table 2 .
Analytical parameters for CPE-RuNPs modified electrodes.Experimental conditions: see Fig. 6 the detection limit was calculated as the ratio between the 3S a /b where: S a is the standard deviation of the intercept of the linear regression, and b is the slope of the linear regression (I = a+ b [H 2 O 2 ]), when the signal/noise ratio is 3. Calibration curves for H 2 O 2 electroreduction at CPE and CPE-RuNP modified electrodes.Inset: I vs. time dependence for additions of 0.01 mM H 2 O 2 at CPE-RuNP (2.5:1) modified electrode.Experimental conditions: electrolyte, 1/15 M phosphate buffer (pH 7); applied potential, -0.1 V vs. Ag/AgCl, KCl sat ; rotation speed, 500 rpm. *