Self-assembled monolayers (SAMs) of gold nanoparticles (AuNPs) have many applications including imaging, lithography, sensing, and catalysis. A simple and effective way to form this type of SAMs is to deposit sessile drops containing AuNPs onto desired substrates. However, colloids in solution typically form non-uniform deposit patterns upon solvent evaporation, commonly known as  “coffee rings.” While many studies exploring drying dynamics have been performed, little attention has been paid to ensure the chemical and physical homogeneity of the substrates. This devalues the results, as even the slightest surface heterogeneities cause localized droplet pinning and distort final deposit patterns. We have adapted a system for creating molecularly smooth and chemically homogeneous substrates. Plasma oxidation was used to remove surface contaminants from silicon wafers, creating superhydrophilic surfaces functionalized with silanol groups. The wafers were then modified through silanization using tris(trimethylsiloxy)chlorosilane for varying amounts of time in the vapor phase resulting in substrates with different degrees of hydrophobicity. Steric stabilization of AuNPs was accomplished by conjugation with poly(vinyl alcohol) (PVOH) in aqueous solution. Microliter-sized droplets containing PVOH-AuNPs were cast and dried on clean and modified silicon wafers. Macroscopic deposit patterns were characterized using optical microscopy and nanoparticle packing within each deposit pattern was analyzed using atomic force microscopy (AFM). The deposit pattern on superhydrophilic clean silicon wafer is extremely uniform, consisting of a monolayer of closely packed AuNPs. On the other hand, deposit patterns on modified substrates all consist of “rings” whose size and width vary as a function of substrate receding contact angle. This is the first time that particle packing pattern within dried deposit is correlated to substrate dynamic contact angles, especially receding contact angle, using AFM analysis.

Role of substrate receding contact angle in deposit patterns of sessile droplets containing stabilized gold nanoparticles

249th National Meeting (2015)

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