Reversible Protection and Reactive Patterning of Amine- and Hydroxyl-Terminated Self-Assembled Monolayers on Gold Surfaces for the Fabrication of Biopolymer Arrays

The reversible protection of amine- and hydroxyl-terminated alkanethiol self-assembled monolayers (SAMs) on gold surfaces using the base-labile protecting group 9-fluorenylmethoxycarbonyl (Fmoc) and the acid-labile protecting group dimethoxytrityl (DMT) is described. When used in combination with UV photopatterning or μ-contact printing techniques, this reversible SAM protection chemistry can be used to control the reactivity and wettability of different portions of the surface. Such spatial control is utilized in the fabrication of DNA arrays. Specifically, gold surfaces modified with the amine-terminated alkanethiol 11-mercaptoundecylamine (MUAM) are reacted with an N-hydroxysuccinimide (NHS) ester derivative of Fmoc, forming a covalent urethane (carbamate) linkage and converting the initially hydrophilic MUAM surface to a hydrophobic, Fmoc-terminated surface. Upon exposure of the Fmoc-modified surface to a solution of a secondary amine, the Fmoc is cleaved, regenerating the original hydrophilic MUAM surface. Fmoc is also utilized as a hydroxyl protecting group, as demonstrated by the reaction of Fmoc-Cl with a monolayer of 11-mercaptoundecanol (MUD). The hydrophobic protecting group DMT is reacted with the ω-hydroxyl groups of both MUD and poly(ethylene glycol)-terminated alkanethiol (PEG-SH) SAMs to form an acid labile ether bond; removal of the DMT group is accomplished by soaking the surface in 3% trifluoracetic acid. A combination of polarization−modulation Fourier transform infrared reflection absorption spectroscopy (PM-FTIRRAS), surface plasmon resonance (SPR), and contact angle measurements is used to characterize the attachment and subsequent removal of the protecting groups. To demonstrate one application of this SAM protection chemistry and reactive patterning, a DNA array is fabricated and used in an SPR imaging measurement of the adsorption of the mismatch binding protein MutS.