1. Photonics Centre Council for Scientific and Industrial Research PO Box 395 Pretoria 0001 South Africa
2. Centre for Nanostructures and Advanced Materials DSI‐CSIR Nanotechnology Innovation Centre Council for Scientific and Industrial Research Pretoria 0001 South Africa
3. School of Chemistry and Physics University of KwaZulu‐Natal Pietermaritzburg Campus, King Edward Avenue Pietermaritzburg 3201 South Africa
4. Department of Human Biology Division of Biomedical Engineering University of Cape Town Cape Town 7925 South Africa