1. National Metrology Institute of South Africa, Private Bag X34, Lynnwood Ridge, 0040, Pretoria, South Africa
2. Department of Applied Chemistry, Faculty of Science, University of Johannesburg, Doornfontein Campus, P.O. Box 17011, Johannesburg, 2028, South Africa
3. College of Science Engineering and Technology, Nanotechnology and Water Sustainability Research Unit, University of South Africa, UNISA Science Campus, P.O. Box 392 UNISA 0003, Florida, 1709 Johannesburg, South Africa