The synthesis of gold nanoparticles via microemulsion method

Nanočestice zlata (AuNČ) sintetizirane su u mikroemulzijskom sustavu Triton X-100/voda/cikloheksan/1-pentanol pri čemu su kod sinteze mijenjani redukcijski uvjeti u mikroemulziji. Istraživane su tri mikroemulzijske sinteze; (i) sinteza AuNČ u mikroemulziji kod jakih redukcijskih uvjeta (dodatak NaBH4), (ii) sinteza AuNČ u mikroemulziji γ-zračenjem (radiolitička sinteza uz umjereno jake redukcijske uvjete) i (iii) sinteza AuNČ u mikroemulziji kod oksidacijskih uvjeta (dodatak vodene otopine NaOH). Sve sinteze odvijale su se na sobnoj temperaturi. Postupak sinteze započinje pripravom dviju mikroemulzija, A i B, od kojih Mikroemulzija A sadrži ione zlata (Au3+), dok Mikroemulzija B sadrži redukcijsko ili oksidacijsko sredstvo. Kada se Mikroemulzije A i B pomiješaju nastaje Mikroemulzija AB gdje dolazi do reakcije između mikroemulzijskih agregata, a time i do redukcije Au3+ iona u Au0 i nastajanja nanočestica zlata. Kada je u mikroemulziji B korišteno jako redukcijsko sredstvo NaBH4, prosječna veličina kristala zlata iznosi 11,7 nm što je utvrđeno iz proširenja difrakcijskih maksimuma. Prosječna veličina čestica na rubovima čestičnih agregata za isti uzorak mjerena elektronskom mikroskopijom iznosila je 7 ± 2 nm. Kod radiolitičke sinteze Au3+ ioni u mikroemulziji bili su reducirani γ-zračenjem. Redukcijska svojstva γ-zračenja bila su podešena propuhivanjem mikroemulzije plinovitim dušikom. Mikroemulzije koje nisu bile propuhane dušikom sadržavale su otopljeni kisik, a time i slabije redukcijske uvjete. γ-zračenjem mikroemulzija zasićenih kisikom dobivene su relativno veće nanočestice zlata (~12 nm) u odnosu na iste uvjete kod dušikom zasićenih mikroemulzija gdje su dobivene nanočestice prosječne veličine čestica 7-10 nm koje su nakon centrifugiranja djelomično agregirale u velike nanočestice promjera oko 150 nm. Dobivene velike nanočestice zlata posjeduju tiksotropna svojstva. Kod mikroemulzijske sinteze na sobnoj temperaturi kod oksidacijskih uvjeta u kiselom području (pH 7, jači oksidacijski uvjeti), nastaju dobro dispergirane nanočestice zlata promjera oko 12 nm. Sinteza nanočestica zlata u mikroemulziji kod alkalnog pH, ali ne i kod kiselog pH, može se objasniti oksidacijom alkoholnih skupina (-C-OH) u karbonilne skupine (>C=O) uz pomoć katalitičkog djelovanja hidroksilnih iona i zlata. Paralelno s katalitičkom oksidacijom alkoholnih skupina u mikroemulziji, ioni zlata (Au3+) reduciraju se u elementarno zlato (Au0) uz nastajanje nanočestica zlata.The gold nanoparticles (AuNPs) were synthesized in a microemulsion Triton X-100/water/cyclohexane/1-pentanol using various reducing agents. Basically, three microemulsion syntheses were studied; (i) the microemulsion synthesis of AuNPs using strong chemical reducing agent (NaBH4), (ii) the microemulsion synthesis of AuNPs using γ-irradiation (radiolytical synthesis at moderate reducing conditions) and (iii) the microemulsion synthesis of AuNPs at oxidising conditions (with the addition of NaOH aqueous solution). The syntheses were performed at room temperature. As the first step, Microemulsion A containing Au3+ ions and Microemulsion B containing reducing or oxidising agent were prepared separately. Then, the Microemulsion B was rapidly poured into Microemulsion A. While obtaining Microemulsion AB, a collision, coalescence and exchange of content between the microemulsion aggregates occurred. As a consequence the Au3+ ions were reduced to Au0 and the gold nanoparticles formed. When the strong chemical reducing agent NaBH4 was used in the microemulsion, the gold nanoparticles of 11,7 nm in size were obtained as determined on the basis of XRD line broadening. The mean particle size at the edges of aggregates for the same sample as measured by electron microscopy was 7 ± 2 nm. In the next experiments the aureate (Au3+) ions in microemulsions were reduced using γ-irradiation. The reducing power of γ-irradiation was tuned by bubbling microemulsions with nitrogen gas. The microemulsions that were not bubbled contained dissolved oxygen and thus more oxidizing conditions in comparison to the nitrogen saturated microemulsions. Basically, the γ-irradiated oxygen-saturated microemulsions produced relatively larger gold nanoparticles (~12 nm) in comparison to the γ-irradiated nitrogen-saturated microemulsions (7-10 nm), which under isolation by centrifugation aggregated in huge nanoparticles of about 150 nm in size. These gold nanoparticles possessed thixotropic properties. In addition, the microemulsion synthesis of gold nanoparticles under oxidising condition was investigated. The microemulsion stirred at room temperature and at pH 7 (oxidising conditions) the well-dispersed gold nanoparticles of 12 nm in size as measured using TEM were formed. Microemulsion synthesis of gold nanoparticles in the alkaline range, but not at an acidic pH, could be explained by the oxidation of alcoholic groups (-C-OH) into carboxylic 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 Au3+ ions reduced to Au0 with subsequently formation of gold nanoparticles