2.2. DNA Microarray Fabrication
Thiolated oligonucleotides were grafted on gold-coated prisms (Horiba Scientific-GenOptics, Orsay, France) as Self-Assembled Monolayers (SAM) according to the protocol described in previous work [21,22]. Briefly, the gold surface lying on the prism was first cleaned by plasma treatment (0.6 mbar, 75% Oxygen, 25% Argon, power 40 W, 6 min) in a plasma generator (Femto, Diener Electronic, Ebhausen, Germany). Then, droplets of approximately 4 nL of HK2PO4 buffer (1 M, pH 9.25), containing a mixture of 20 µM of thiol-modified DNA probes and 10 µM of PEG2000 or PEG300 solution, were deposited on the surface using a piezoelectric dispensing system (Siliflow, Valence, France). The deposition was done under a controlled atmosphere of 85% humidity and left at room temperature for 60 min. After overnight drying, prisms were thoroughly rinsed with deionized water and dried under argon stream for few seconds. The size of the spots is approximately 500 µm in diameter, with a 1.3 mm pitch. The probes coverage surface is estimated to 8 pmol/cm² [22]. Prior to the SPR measurements, the functionalized prism surface is blocked with a PEG2000 or PEG300 solution (150 µM) for 60 min incubation at room temperature. Then, surfaces were rinsed with deionized water and dried under argon stream.