28 _Acta Chim. Slov. 2015, 62, 28-34_DOI: I0.i7344/acsi.20i4.672
Scientific paper
Composite Electrodes With Carbon Supported Ru Nanoparticles For H2O2 Detection
Valdrin Januzaj,1 Vllaznim Mula,1 Graziella L. Turdean2* and Liana Maria Muresan2*
1 Department of Chemistry, University of Pristina, 10000 Pristina, Kosovo
2 Department of Chemical Engineering, Babes-Bolyai University, 400028 Cluj-Napoca, Romania
* Corresponding author: E-mail: limur@chem.ubbcluj.ro; gturdean@chem.ubbcluj.ro Tel: +40264595872; Fax: +40264590818
Received: 26-05-2014
Abstract
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.
Keywords: Carbon supported Ru nanoparticles, H2O2 amperometric detection, carbon paste modified electrodes.
1. Introduction
Ru-based catalysts are well-known for electroca-talytic performances,1 especially in reactions such as oxygen evolution.2-4 These abilities are exploited in some very important practical applications, the main being water electrolysis.3-5
Ru electrodes have shown affinity also for the oxygen reduction in both acid6 and alkaline7 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 solution8 and that additions of up to 3% wt Ru increased the corrosion resistance of the WC-Co alloys.9
Ruthenium oxide composites were reported as efficient catalysts for non-enzymatic glucose oxidation10 and for simultaneous determination of ascorbic acid and dopamine.11 Considerably fewer reports exist on H2O2 detection by using Ru or Ru oxide based electrodes.12,13 Hydrogen peroxide is an efficient oxidizing agent used in textile industry, cleaning products, food industry and environmental protection14 and an essential intermediate product of enzymatic reactions.15 Among these, the electrochemical
tracking of biological targets by way of enzyme-based H2O2 detection is of special interest.
For most electrochemical sensors, the detection of H2O2 was achieved at positive potentials16-17 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.18
In this paper, a novel electrochemical sensor consisting of a carbon paste electrode modified with carbon supported Ru nanoparticles (RuNP) was developed for H2O2 amperometric detection. Cyclic voltammetry and amperometry have been used for the investigation of electrochemical properties and electrocatalytic activity of the nanocomposite modified electrode.
2. Results and Discussions
2. 1. Physico-chemical Characterization of Ru-graphite Nanoparticles
TEM measurements were performed to examine the morphology of carbon supported RuNP. Fig. 1 reveals
Fig. 1. TEM images of carbon supported RuNP catalyst.
images in which a heterogeneous structure consisting of the carbon substrate (light regions) and the catalyst nano-particles (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 literature3 can also be observed.
2. 2. 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 RuO19, Ru(OH)2 or RuO x H2O20, but also to oxides of higher oxidation states (Ru2O3). Moreover, at potentials beyond 1.2 V, Ru oxidation to RuO4 overlaps with oxygen evolu-tion.21 As 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,
E I V vs. Ag/AgCI, KCl
Fig. 2. Cyclic voltammogramms at CPE (thin solid line) and CPE-RuNP modified carbon paste electrodes (a). Influence of the starting potential and scanning domain on the cyclic voltammograms at CPE-RuNP (2.5:1) modified electrode (b). Experimental conditions: electrolyte, 1/15 M phosphate buffer (pH 7); starting potential, -1V vs. Ag/AgCl, KClsat (A, B thin black solid line), 0 V vs. Ag/AgCl, KClsat (B, thick red line); scan rate, 50 mV/s.
a)
0.5 -
« E
b)
« E
0.8-
0.6
0.4
0.2-
0.0
CPE-RuNP (2.5:1) CPE-RuNP (2.5:2) CPE-RuNP (2.5:3)

•	T
•	■

>

-1.0 -0.5 0.0 0.5 1.0 BIM vs. Ag/AgCI, KCl
0.0
0.1
v,a / (V/s)
0.2
m
0.3
Fig. 3. Influence of the scan rate on the electrochemical response of CPE-RuNP (2.5:1) electrode (a); cathodic current vs. v1/2 dependence of the process (IIc) occurring at different CPE-RuNP modified electrodes (b). Experimental conditions: see figure 2A, error bar for 4 similar measurements.
bridged oxygen, OH, and water.22 These results are confirmed also by other researchers.23
It is interesting to note that a potential scan in the negative direction, starting from 0 V, does not reveal any cat-hodic peak because no Ru oxide (that acts as a catalyst for O2 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 = -0.9 V vs. Ag/AgCl, KClsat) than in the case when Ru oxides were formed during the anodic scan (E = -0.5 V vs. Ag/AgCl, KClsat), 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, KClsat 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).24
As expected, at pH 7, irrespective the potential values (at -0.1 V, -0.3 V and -0.5 V vs. Ag/AgCl, KClsat, 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, KClsat (Table 1).
Although the current values obtained at -0.5 V or -0.3 V vs. Ag/AgCl, KClsat are greater than those recorded at -0.1 V vs. Ag/AgCl, KClsat, in view to avoid interferences, an applied potential of -0.1 V vs. Ag/AgCl, KClsat was used for further amperometric measurements of H2O2 reduction.
2. 3. Electrocatalytic Activity for H2O2 Reduction
2. 3. 1. Cyclic Voltammetry
In order to investigate the electrocatalytic activity of the CPE-RuNP modified electrodes toward H2O2 reduction, CVs have been recorded at constant H2O2 and increa-
Table 1. Cathodic current intensity dependence on the RuNP amounts in the modified electrodes at fixed values of potential. Experimental conditions: see Fig. 2A.
Electrode		Ec = -0.1 V	Ic / A Ec = -0.3 V	Ec = -0.5 V
CPE-RuNP(2.5	: 1)*	-0.48 10-4 ± 0.19 10-4	-1.26 10-4 ± 0.22 10-4	-2.54 10-4 ± 0.38 10-4
CPE-RuNP(2.5	: 2)*	-2.00 10-4 ± 0.57 10-4	-3.17 10-4 ± 0.49 10-4	-4.71 10-4 ± 0.68 10-4
CPE-RuNP(2.5	3)**	-2.45 10-4 ± 1.24 10-4	-3.69 10-4 ± 1.18 10-4	-5.71 10-4 ± 1.16 10-4
where: * values are mean of 4 measurements; ** values are mean of 3 measurements; Ec is expressed as V vs. Ag/AgCl, KClsa
E I V vs. Ag/AgCI, KCl
E I V vs. Ag/AgCI, KCl
Fig. 4. Electroreduction of 3 mM H2O2 at CPE-RuNP modified electrode (a) and CPE (b). Experimental conditions: electrolyte, 1/15 M phosphate buffer (pH 7) (A, B solid line); starting potential, -1V v.?. Ag/AgCl, KClsat (A); scan rate, 50 mV/s; graphite:RuNP ratio, see inset legend.
sing RuNP concentration (Fig. 4A) and at unmodified CPE (Fig. 4B) electrodes.
In can be noticed that at concentrations of 3 mM H2O2, both the anodic and cathodic currents increase with the RuNP amounts present in CPE-RuNP (Fig. 4A).
The influence of the H2O2 concentrations on the vol-tammetric currents recorded at -0.5 V vs. Ag/AgCl, KClsat is depicted in Fig. 5A and the corresponding calibration curves in Fig. 5B.
The linear dependence between the currents and H2O2 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 H2O2 (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
E ! M vs. Ag/AgCI, KCl
[H О 1 I mM
2 2
Fig. 5. Cyclic voltammograms at CPE-RuNP (2.5:1) modified electrode at different concentrations of H2O2 (a). Calibration curve of CPE and CPE-RuNP modified electrodes (b). Experimental conditions: see Fig. 4.
relative increase of the H2O2 reduction current (I%), calculated as (I-I0)*100/I0 (where: I is the current intensity at 3 mM H2O2 and I0 is the current in the absence of H2O2 concentration), increases as follows: 28.47% (CPE) < 942.81% (CPE-RuNP (2.5:1)) < 118.19% (CPE-RuNP (2.5:3)) < 152.43% (CPE-RuNP (2.5:2)).
2. 3. 2. Amperometry
Batch amperometric calibration for H2O2 using the different modified electrodes was performed at a constant potential of -0.1 V vs. Ag/AgCl, KClsat. The obtained calibration curves are linear in the range up to 0.1 mM H2O2 (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 H2O2 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.25 The 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 H2O2 reduction (e.g. 3.5 *10-5 A/mM for nano-ruthenium oxi-de/riboflavin modified glassy carbon).26
The response time, estimated as t95%, 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. Sa).
As expected, working in amperometric mode allowed using lower H2O2 concentrations and the sensors sen-
Fig. 6. Calibration curves for H2O2 electroreduction at CPE and CPE-RuNP modified electrodes. Inset: I vs. time dependence for additions of 0.01 mM H2O2 at CPE-RuNP (2.5:1) modified electrode. Experimental conditions: electrolyte, 1/15 M phosphate buffer
(pH 7); applied potential, speed, 500 rpm.
-0.1 V vs. Ag/AgCl, KClsat; rotation
sitivity estimated from the obtained calibration curves is higher than in cyclic voltammetric method.
3. Materials and Methods
3. 1. Materials
Ru nanoparticles (RuNP) stabilized on carbon powder were prepared by controlled reduction of RuCl3 in polyols followed by slow addition of carbon powder27 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-
Table 2. Analytical parameters for CPE-RuNPs modified electrodes. Experimental conditions: see Fig. 6
Electrodes	Cyclic voltammetry [H2O2]-01V 1- 5 mM Sensitivity (A/mM) R/n	Amperometry [H2O2]-01V 0.01- 0.1 mM Sensitivity (A0mM) R/n LOD*/M	
CPE	1.2 10-6 ± 8.3 10-8	1.3 10-6 ± 8.1 10-8	
	0.9846/4	0.9923/6	5.43
CPE-RuNP(2.5:1)	50.2 10-6 ± 4.3 10-6	45.9 10-6± 1.1 10-6	
	0.9640/6	0.9976/11	3.78
CPE-RuNP(2.5:2)	107.9 10-6 ± 9.5 10-6	31.6 10-6± 1.7 10-6	
	0.9626/6	0.9878/11	8.62
CPE-RuNP(2.5:3)	123.8 10-6 ± 10.9 10-6	78.8 10-6 ± 3.5 10-6	
	0.9626/6	0.9912/11	7.37
* the detection limit was calculated as the ratio between the 3S /b where: S is the standard deviation of
a	a
the intercept of the linear regression, and b is the slope of the linear regression (I = a+ b [H2O2]), when the signal/noise ratio is 3.
sed from Fluka (Germany). Hydrogen peroxide (30% H2O2) (Merck, Germany) was used for daily preparing of 0.01 M standard solutions.
The supporting electrolyte was a 1/15 M phosphate buffer (pH 7) prepared by dissolving the appropriate amounts of Na2HPO4.12H2O (Reactivul-Bucuresti, Romania) and KH2PO4 (Sigma, Germany) in distilled water. The pH of the buffer solutions was adjusted to the desired values by adding H3PO4 or KOH solutions (Merck, Germany).
All reagents were of analytical degree and were used without further purification. Distilled water was used for preparing all solutions.
3. 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 cm2), 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.
3. 3. 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 200x-600000x).
All electrochemical measurements (cyclic voltam-metry 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, KClsat 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 ampe-rometric experiments.
Batch amperometric measurements were carried out at an applied potential of -0.1 V vs. Ag/AgCl, KClsat by
addition of increasing volumes of 0.01 M H2O2 solution into a 1/15 M phosphate buffer (pH 7).
All experiments were carried out in aerated solution at ambient temperature.
4. Conclusions
A new carbon paste electrode (CPE) incorporating carbon supported Ru nanoparticles (RuNP) for H2O2 am-perometric 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, KClsat and the reduction of oxygen at -0.5 V vs. Ag/AgCl, KClsat, 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.
The relative increase of the H2O2 reduction current at CPE-RuNP electrodes was evaluated from cyclic voltam-metry measurements at -0.1 V vs. Ag/AgCl, KClsat, and it varies in the range: 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 electrode as a promising sensor for hydrogen peroxide detection.
5. Acknowledgments
The authors are grateful to Prof. D. Goia (Clarkson University, USA) for the kind gift of RuNP and to Dr. G. Katona (Babes-Bolyai University, Romania) for performing the TEM measurements.
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Povzetek
Za amperometrično detekcijo H2O2 smo razvili novo ogljikovo elektrodo (CPE), stabilizirano z vgrajenimi Ru nanodel-ci (RuNP). Njene elektrokemične lastnosti smo preverili z meritvami ciklične voltametrije, izveden v fosftanem pufru napram Ag/AgCl, KClsat elektrodi pri različnih spremembah hitrosti in območja potenciala. Pri napetosti -0.1 V napram Ag/AgCl, KClsat, relativni porast redukcijskega toka za H2O2 sledi v naslednjem zaporedju: 28.47% (CPE) < 94.81% (CPE-RuNP (25:1)) < 118.19% (CPE-RuNP (2.5:3)) < 152.43% (CPE-RuNP (2.5:2). Dobljene vrednosti kažejo, da nova elektroda predstavlja obetajoč senzor za detekcijo peroksida.