All solution preparations and syntheses were carried out under a fume hood. Phenol is harmful upon contact with tissue or ingested. Upon mixing the ZrNO3 the following precautions should be followed during synthesis of the modified electrode: Protective garment and gloves were worn at all times. For preparation of the modified sonogel-carbon zirconium oxide, all steps were carried out under a hood and care was taken not to inhale the ZrNO3 or graphite powder. All MSDS information for chemicals involved can be found at www.sigmaaldrich.com
Construction of the unmodified (bare) electrodes
A 0.50 mm O.D., 12.5 cm long copper wire was inserted into a capillary tube (Sutter Instrument, 0.69 mm I.D., 1.2 mm O.D., 10 cm length, borosilicate glass both ends open) so as the wire was exposed by 2.5 cm only on one end of the tube; the wire functioned as an electrical contact component. 500 μL MTMOS and 100 μL 0.2 M HCl was added to a 40 mL borosilicate glass vial. 1.0 g of graphite powder (Alfa Aesar 99.9%, 2-15 micrometer) was added and homogenously dispersed via sonication in the same vial containing the MTMOS and HCl solution. The vial contained the mixture was ultrasonicated (2510R-DH, Bransonic) (with cap on) for 15 seconds. The sonicated graphite mixture was manually packed. Previously made capillary tube with copper wire core was dipped firmly into the graphite packing so as about 0.5 cm of the dipping end of the tube was filled with the packing mixture.
Construction of the modified electrodes
The zirconium nitrate material was utilized to prepare the zirconium oxide coating solution as follows: 2.62 grams of Tween 80 (Aldrich) were dissolved in isopropanol (99%, Fisher), followed by the addition 0.68 mL of concentrated acetic acid (Aldrich) and 4.6 grams of zirconium nitrate under vigorous stirring. A bare electrode (as the one made above) was dipped into the zirconium oxide coating solution for 5 seconds. After coating, the electrode was heated at 500οC for 20 minutes, and cooled naturally.
All electrochemical measurements were carried out on a Bioanalytical System Epsilon, in a three electrodes set up (sonogel carbon zirconium oxide working electrode, Ag/AgCl /3 M NaCl reference electrode and platinum auxiliary electrode wire). The phenol was purchased from Sigma-Aldrich (99+% purity). All phenol solutions were made with a phosphate buffer (0.1 mol/L NaH2PO4 and 0.1 M NaCl, pH ~4) solvent. 0.1 M NaOH solution was used to adjust pH level. Cyclic voltammetry at a scan rate of 100mV/s was the electrochemical technique applied to study the oxidation behavior of phenol. The oxidation peak of the zirconium oxide modified electrode appeared at approximately 620 mV in a phenol concentration of 0.005 M. Noreduction peak was observed. The sensitivity of this electrode is notable as Figure 3 shows the limit of phenol detection to be a at 10-5M. The optimum response pH of 7.5 was utilized to observe a phenol oxidation peak with this modifed electrode. At the lower and the higher pH values of solutions with the same phenol concentrations, no peak was observed or detected. The electrode without the zirconium oxide coating (bare carbon) did not produce an oxidation peak when a cyclic voltammogram in phenol solution was carried out.
Figure 1: Cyclic Voltammogram of ZrO2 electrode utilized to detect 0.005 M phenol in phosphate buffer (pH=7.5), (100mV/s) vs. Ag/AgCl/3M NaCl.
Figure 2: Cyclic voltammogram of bare carbon electrode to detect 0.005 M phenol in phosphate buffer (pH=7.5), (100 mV/s) versus Ag/AgCl/ 3M NaCl
Figure 3: Cyclic voltammogram of ZrO2 electrode to detect 5 X10-5 M phenol in phosphate buffer (pH=7.5), (100mV/s) versus Ag/AgCl/3M NaCl.