Preparation And Characterization Of APTES SAM

Amino-terminated Self-assembled Monolayers (SAMs) have been widely used in the fundamental researches and industrial applications. Especially, the 3-amino-propyl-triethoxy-silane (APTES) is the mostly used monomer to prepare the amino-terminated SAMs. The amino groups on the APTES SAM surface are of high reactivity and easy to react with the carboxyl, acyl-chloride, acid anhydride, halogen, and so on, which could promote adhesion of other molecules on the glass surfaces, or as a coupling layer to produce polymer and other functional films on silica. Since the amine groups on the SAM surface are easy to be oxidated under the electron beam, APTES SAM also has the application in lithography. Moreover, APTES SAM has been found to be able to immobilize DNA and protein onto the substrate surface, and increase the cell adhesion without losing their bioactivity. Due to its wide application, APTES SAM has been investigated by many research groups. These researches included the studies of APTES SAM structure, surface composition, surface wetting property, and surface potential. Most of the researches showed the interests in the studies of the surface properties of APTES SAM. Contact angle, AFM, XPS and Ellipsometry were usually employed in the researches. The contact angle measurement was the most commonly used characteristic technique, however the reported contact angles ranged from 26°to 80°, which implied the complexity of the surface property of the APTES SAM. In this paper, we mainly used contact angle measurements to investigate the kinetic formation process and surface wetting property of APTES SAM. Our experiment results show that the contact angle is remarkably influenced by the droplet residence time on the APTES SAM surface, and a great contact angle hysteresis is found on APTES SAM surface in advancing and receding contact angle measurements. We also found the receding contact angle behaves a good linear relationship with the droplet volume. AFM, XPS and Ellipsometry were also used to characterize the surface topography, composition and film thickness of APTES SAM. The following conclusions were obtained:(1) After about 30 min reaction, a well-organized APTES SAM could completely formed from 1×10^-3mol/L APTES-toluene solution at 18.1±0.1℃. The initial contact angle on the APTES SAM sample surface is 50±2°, and advancing contact angle 45±2°. The ellipsometry measurement showed that the thickness of the APTES SAM is about 0.5±0.1nm, and the XPS analysis indicated that the ratio of the C:N of the APTES SAM is about 6:1, which implies at least 1 or 2 ethoxyl-groups were hydrolyzed for each APTES molecule. The topography image captured by AFM revealed that, the structure inside the APTES SAM is not closely packed, and the surface of the APTES SAM is of a loosely fuzzy feature with a surfaceroughness, RMS, about 0.131 nm/2μm×2μm.(2) A great hysteresis was found in the contact angle measurement, which implied that the APTES SAM surface is a non-ideal surface. Linear relationship had been found between the contact angle and droplet residence time on the sample surface, meanwhile linear relation also existed between the receding contact and the droplet volume. We attributed these phenomena to the hydrogen bones between the H₂O molecule and the amino group on the APTES SAM surface, which resulted in the extra strong interaction between the droplet and the sample surface.(3) Since the initial contact angle on the APTES SAM surface was in a metastable state, and the receding contact angle was dramatically influenced by the droplet volume, contrastively, the advancing contact angle was relatively stable during the measurement. We believe that, to characterize the surface wetting property of APTES SAM, the advancing contact angle was much more credible and accurate.