Polyurea Molecular Layer Deposition Using Low-Melting-Point Precursors for Use in Biosensor Design

Molecular layer deposition (MLD) is a promising tool that could replace silane-based self-assembled monolayers (SAMs) in some surface functionalization applications, such as in biosensor design, where a multilayer film is acceptable and a precise, conformal film thickness is desirable. This work presents a polyurea MLD chemistry using 2,4-toluene diisocyanate (TDIC) and ethylenediamine (ED). This process has some benefits over the similar and more common 1,4-phenylene diisocyanate (PDIC) precursor, such as reduced cost and a lower melting point (and thus higher vapor pressure at room temperature). Film characterization suggests that this process is similarly well-behaved to PDIC-based polyurea: in situ ellipsometry shows that the process is both linear up to 57 cycles and self-saturating, X-ray photoelectron spectroscopy (XPS) analysis shows the expected atomic ratio of oxygen and nitrogen in the film, and FTIR shows amide I and II peaks characteristic of polyurea. In addition, the films were analyzed using zeta potential analysis, fluorescence tagging, and water contact angle (WCA) goniometry to characterize their suitability for use in biosensor design.