Fig. 3.

Left: an illustration of the photolithography process for RNA array fabrication. Incoming UV light (purple arrows) reflects onto the surface only when the corresponding micromirrors are properly tilted in the “ON” position. The reflected UV light triggers the deprotection of the NPPOC group (black ball) at the 5′ end of the growing oligonucleotide strand. Right: chemical coupling cycle of an RNA phosphoramidite. After UV light deprotection as shown in the left part of the figure, the amidite couples to the 5′-OH oligonucleotides using dicyanoimidazole as a coupling agent, followed by drying of the surface, and oxidation of the phosphite triesters into phosphotriesters. Microarray synthesis then enters a new cycle with the photodeprotection of selected NPPOC groups. After synthesis, RNA oligonucleotides are deprotected in two steps under mildly basic conditions. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)