Experimenting with Plasmonic Copper Nanoparticles To Demonstrate Color Changes and Reactivity at the Nanoscale

Abstract

The authors describe a series of simple experiments related to the synthesis, oxidation, and aggregation of plasmonic copper nanoparticles (CuNPs) stabilized by iodide ions. These experiments can help with (i) substituting noble-metal-based plasmonic nanoparticles for nanotechnology-oriented lessons, (ii) demonstrating high reactivity of nanosized objects, and (iii) attracting students’ attention by colorful experiments. The main compounds required for the experiments are CuSO4, KI, NaBH4, and diluted H2SO4. These compounds are relatively safe, available, cost-effective, and do not require special recycling. The color of the reaction mixture rapidly changes from colorless (diluted CuSO4) to wine red (fresh CuNPs) and then to yellow (oxidized CuNPs) or dark green and blue (agglomerated CuNPs), making the reactions more visible to the students. The fast oxidation of CuNPs by the oxygen in air was proposed to demonstrate the high reactivity of nanosized copper and to enrich the nanotechnology lessons with chemical reactions. The authors also highlight and discuss the importance of accounting for potential side reactions (e.g., hydrolysis) for the synthesis and oxidation of CuNPs. The important role of iodide ions in providing colloidal stability of CuNPs is discussed, and several comparative experiments with other stabilizers are proposed. In addition, the authors compare the optical properties of CuNPs with those of gold- and silver-based nanoparticles. The experiments are designed to be completed in less than 45 min and have been regularly used at chemistry club lessons for secondary school students in 2017 and 2018. The efficiency of the experiments has also been tested several times with undergraduate students at Saratov State University.