Colorimetric systems based on DNA oligonucleotides and gold nanoparticles (AuNPs) have attracted much attention in recent years, due to benefits such as ease of operation and naked-eye detection. Many researchers have also explored the unique optical properties of AuNPs, which serve as sensing elements and exploit visual colorimetric changes dependent on inter-particle distance. Incorporating DNA oligonucleotides and AuNPs for colorimetric assays have been successfully developed, however the correlation between aggregation state and catalytic activity has not yet been reported. This work aims to exploit the notable plasmonic and catalytic properties of AuNPs, and provide a better understanding about the peroxidase-like activity of AuNPs (so-called nanozymes), for the oxidation of 3,3’,5,5’-Tetramethylbenzidine (TMB). The results so far suggest that AuNP size and aggregation state have a direct effect on the oxidation of TMB. Larger size or larger aggregated clusters of particles result in a decrease in the nanozyme activity. The preliminary results stimulate the development of a dual colorimetric assay; one is based on particle aggregation and the other is based on the peroxidase-like activity for the detection of DNA oligonucleotides. The assays will be based on one of two formats; DNA desorption or sandwich-based DNA hybridization. Naked-eye detection can be challenging particularly when distinguishing between particle aggregation state, therefore the advantage of this work over previous assays is that by also exploiting the peroxidase-like activity the sensitivity of the assay can be improved. This work also finally aims to improve point-of-care (POC) analysis by integrating assay results into a Smartphone-based diagnostic device, for the on-site detection of DNA oligonucleotides.