**5. Synthesis and characterization of nanomaterials**

Silver nanoparticles were, characterized and applied in the receptor layer to enhance the sensitivity of the biosensor. The synthesis method and materials, and the characterization procedures and results are discussed in the following sections.

#### **5.1 Synthesis procedure and materials**

Silver nanoparticles were prepared by chemical reduction method, using silver nitrate (AgNO3) as the silver precursor. The preparation procedure of silver nanoparticles is adopted from Muzamil et al., [38] with minor modification. All reagents were of analytical grade and used without any further purification. The reagents include Silver nitrate (AgNO3, ≥99%), Trisodium citrate (Na3C6H5O7, 1%), Polyethylene glycol (C2nH4n + 2On + 1, ≥99%), and Sodium hydroxide (NaOH, ≥99%), and were all purchased from Sigma Aldrich. The experiment was conducted at room temperature (23°C) unless otherwise stated. To synthesize silver nanoparticles, 50 ml of 1 mM silver nitrate solution and 5 ml of polyethylene glycol were taken and heated till boiling on a magnetic stirrer. When the solution started to boil, 1% trisodium citrate solution of 4 ml was quickly added with constant stirring. A few minutes later, the solution turned into yellow color, hinting the formation of silver nanoparticles. The pH of the nanoparticles was kept at 7.6 by adding NaOH. Afterward, it was cooled to room temperature and stored at 4°C before use.
