Before the anodization process, the silicon CHIR 99021 substrate was immersed in HF solution for 2 min to remove the native oxide

layer. Since the PSi fabrication process is self-stopping, it is possible to obtain adjacent layers with different porosities by changing the current density during the electrochemical etching [4]. A current density of 200 mA/cm2 for 1.2 s was applied to obtain low refractive index layers (n L = 1.542; d L = 125 nm) while a current density of 100 mA/cm2 was applied for 1.4 s for high refractive index layers (n H = 1.784; d H = 108 nm). After the electrochemical process, the pore dimension was increased to favour the infiltration of biological matter by rinsing the fresh-made PSi microcavities in a KOH ethanol solution (1.5 mM) for 15 min [5]. The structures were then thermally oxidized against uncontrolled environmental

aging and corrosion in alkaline solutions. The thermal oxidation has been performed in pure O2 by a two-step process: pre-oxidation at 400°C for Torin 1 30 min followed by oxidation at 900°C for 15 min. Silane surface modifications Eight oxidized PSi microcavities (PSi-Ma-h) were immersed in piranha solution (H2O2:H2SO4 1:4) at RT for 30 min to generate Si-OH groups on the PSi surface. After that, the samples were extensively washed in Milli-Q® water flow (Millipore, Billerica, MA, USA) and dried with nitrogen gas. Structures were then silanized by immersion in different 5% aminosilane solutions, (3-aminopropyl)triethoxysilane

(APTES) or (3-aminopropyl)-dimethyl-ethoxysilane (APDMES), in dry toluene for 30 min at RT. Samples PSi-Ma,c,e,g were silanized by APTES and samples PSi-Mb,d,f,h by APDMES. The reaction conditions were optimized on a crystalline silicon-varying solvent for silane dissolution and incubation time [12]. The PSi-silanized samples were rinsed three times in the solvent used for the process so as to remove the ungrafted else silanes. The last step of silanization is curing at 100°C for 10 min. Oligonucleotide synthesis Chemicals and solvents were purchased from Sigma-Aldrich (St. Louis, MO, USA). Reagents and phosphoramidites for DNA synthesis were purchased from Glen Research (Sterling, VA, USA). Solid-phase ON syntheses were performed on a PerSeptive Biosystem Expedite 8909 DNA automated synthesizer (Framingham, MA, USA). The 19-mer mixed-sequence oligonucleotide 5′-GATTGATGTGGTTGATTTT-3′ was assembled on two different aminosilane-modified microcavities, following phosphoramidite chemistry by 19 growing cycles [13]. PSi structures, PSi-Mg,h-NH2 (Mg = APTES, Mh = APDMES), were introduced in a suitable column reactor to be used in the automated synthesizer; the syntheses were performed according to the scheme reported in Figure 1. In all cases, the first reaction step involved the attachment of the 3′-ending nucleobase to the amino group of PSi-bound APTES or APDMES.