**2. Materials and Methods**

#### *2.1. Materials*

Retinol-binding protein aptamer (RBP-A) 5 -ATA CCA GCT TAT TCA ATT ACA GTA GTG AGG GGT CC GTC GTG GGG TAG TTG GGT CGT GGA GAT AGT AAG TGC AAT CT-3 [21], was synthesized by Biomers.net GmbH (Ulm, Germany). RBP4, alpha-2-macroglobulin (A2MG), leptin and bovine serum albumin (BSA) were purchased from Sino Biologicals (Beijing, China), ProsPec (Ness-Ziona, Israel), R&D Systems (Minneapolis, MN, USA) and Merck (Pty) Ltd. (Rahway, NJ, USA), respectively. Chloroauric acid (HAuCl4·3H2O), sodium chloride (NaCl), and sodium citrate were purchased from Sigma-Aldrich (St Louis, MO, USA). Purified Milli-Q water was used in all experiments (Millipore, MA, USA). All the chemicals were of analytical grade.
