**3. Results and Discussion**

AuNPs-based aptasensors have been widely applied in the detection of various targets including chemical compounds, proteins, pesticides, toxins, and viruses [36,38–40,48–50]. Their simple preparation, rapid detection, and the fact that they do not require special instrumentation have made them the preferred choice for PoCT [51]. The sensors rely on the salt-induced aggregation of bare AuNPs, leading to a change of colour from ruby red to purple/blue. In the absence of the target or biomarker molecule, the detecting molecule or MRE stabilizes the AuNPs against salt-induced aggregation and no colour change is observed [52]. Similarly, this study investigated the feasibility of the RBP-A-aptasensor for detection of RBP4. Although at least two other studies demonstrated the development of aptasensors for RBP4, neither of these assays can be used at the PoCT as both of these assays require specialised instruments and must therefore be carried out in a laboratory setting. In contrast, the assay described herein does not require any specialised instruments and can be carried out in the field.
