sensitive to CrVI. It is necessary either as a pre-treatment on the surface, or by modifying its surface with gold nanostructures to be sensitive to CrVI
5
. Nano-sized materials used currently is very large, one of which is the use of metal nanoparticles in the field of sensors. The use of metal nanoparticles are very knowledgeable in the field of
sensors due to the metal nanoparticles characteristicas having the nature of catalysis and good surface area. The development ofnano-sized materials on analysis of electrochemical techniques, one of which is the nanotube-based
electrode, has shown an increase in high sensitivity. That is the basis of the electrode made of other types of gold nanoparticle-based, considering that the two electrodes are competing for use in electrochemical analysis. Another
advantage of the gold nanoparticles compared to carbon nanotubes is that it can be produced simpler
6
. Surface modification of carbon-based electrodes with AuNPs generate profits in the field of sensor because it can facilitate
electron transfer between the electrode surface and a carbon-based analyte. In this research cyclic voltammetry used on GCE modified withAuNPs to measure CrVI and CrIII. Between CrIII andCrVI there are significant
differences in toxicity, reactivity and bioavailability
7-8
. Therefore, it is important to distinguish between CrIII to CrVI. In addition, sensitive analysis methods are needed to distinguish them.
2. Experimental
2.1 Chemicals Ammonium hydroxide, acetic acid, 37 hydrochloric acid, ethanol, potassium chloride, chromium trichloride
anhydrate, sodium citrate, sodium acetate, sodium tetra hydrobarate, hydrogen tetracloroaurateIII trihydrate of analytical grade and were supplied by Merck.
2.2 Preparation of AuNPs AuNPs colloids were prepared by reducing AuIII using NaBH
4
. A total of 15mL of 0.1mMH AuCl
4
solution was added to 5mL pipette aqua bidest and stirred using a magnetic stirrer for 5 minutes. Then 0.5 mL of 0.1M
sodium citrate solution was added and stirred using a magnetic stirrer for 5 minutes, followed by an addition of 290 mLof 0.1 M NaBH
4
, with a pH of 6.5. The colloid was finally characterized using UV-Vis spectrophotometer at a wavelength of 518 nm and TEM to determine the size and shape of the AuNPs formed.
2.3 Preparation of GCE modified with AuNPs The GCE surface to be modified was rubbed on a 8 nm Al
2
O
3
solids greater than1000 meshs and paper, then sonicated in ethanol and aqua bidest each for 5minutes and then dried. The first modification was conducted by
immersing the GCE, the surface of which had been activated with a solution of AuNPs for 24 hours. After this process had been completed, the AuNPs modified GCE was further characterized by using Scanning Electron
Microscope SEM. The second modification was conducted by a photochemical reaction. Photochemical reaction was made effective by immersing the GCE intoa concentrated ammonium hydroxide solution then irradiated under
UV light at a wavelength of 254 nm for 6 hours. This procedure isknown as self-assembly process
9
. The modified electrode was used as a working electrode.
2.4 Electrochemical measurements. Electrochemical measurements were carried out in a single compartment cell using a potentiostat Metrohm®
ȝAutolab type III 757 VA at room temperature 23ºC. The reference electrode used was AgAgCl and the counter electrode was a Pt electrode. Measurements of CrIII were performed in acetate buffer pH5.0, while the
measurements of CrVI were performed in 0.01M HCl. All potentials given in this work are with respect to the AgAgCl sat. reference electrode 0.2 Vvs.SHE.
3. Resultand Discussion
