engleski

Synthesis of gold nanoparticles under highly oxidizing conditions

Gold nanoparticles (AuNPs) were synthesized in a microemulsion water/Triton X-100/1- pentanol/cyclohexane using various reducing agents. Basically, three microemulsion syntheses of AuNPs were studied: (i) one using a strong chemical reducing agent (NaBH4), (ii) another using γ- irradiation under moderately strong reducing/oxidizing conditions, and (iii) yet another under highly oxidizing conditions (with the addition of NaOH aqueous solution). All the three were performed at room temperature. When a strong chemical reducing agent NaBH4 was used in the microemulsion, gold crystallites 11.7 nm in size were obtained, as determined on the basis of X-ray powder diffraction line broadening. The γ-irradiation of nitrogen- saturated microemulsion at the acidic pH produced AuNPs about 12 nm in size, which under the isolation by centrifugation aggregated into large preconcentrated AuNPs about 150 nm in size. These AuNPs possess thixotropic properties. The microemulsion stirred at room temperature and at the pH < 7 under oxidizing conditions did not produce gold nanoparticles. Under the identical experimental conditions and at the pH > 7 (stronger oxidizing conditions), well-dispersed AuNPs 12 nm in size were formed. The microemulsion synthesis of AuNPs in the alkaline range but not at an acidic pH was explained by the oxidation of alcohol groups (–OH) into carbonyl groups (>C=O) due to the catalytic action of hydroxyl ions and gold. In parallel with the catalytic oxidation of alcohol groups in microemulsion, the Au(III) were reduced with the subsequent formation of gold nanoparticles. The synthesis of AuNPs in 1-pentanol by adding the aqueous NaOH solution at room temperature without using microemulsions confirmed the role of the base catalyzed oxidation of alcohols in the formation of AuNPs. Based on the findings in this study, we propose the base catalyzed alcohol oxidation at room temperature as a new, simple, and versatile synthesis route for obtaining gold nanoparticles. The results of this study suggest that the classical approach of using a reducing agent for the synthesis of AuNPs is not a determining factor, since a diametrically opposite approach to the synthesis of AuNPs can be used, namely, stimulating the oxidation of the functional organic groups in close proximity to gold ions.

Gold nanoparticles ; Microemulsion ; Alcohol ; Oxidizing conditions ; Base catalyzed ; Cloud point extraction

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