Biomedical Engineering Reference
In-Depth Information
(a)
(b)
FIGURE 8.13
Contact angle of (a) untreated and (b) Teflon-treated ZnO film.
also been used to anisotropically etch ZnO films using HBr/Ar plasma with photoresist
as the etch mask (Min et al., 2008). The etching of ZnO films in the HBr/Ar gas mixture is
controlled by chemically assisted sputter etching.
For microfluidic applications, ZnO has the useful property that it is hydrophilic with
a contact angle typically ranging from 50° to about 80° (see Figure 8.13a), with the value
depending on the surface conditions and light exposure (Kenanakis
et al., 2008). Ultraviolet
irradiation of ZnO films results in a superhydrophilic surface (Sun et al., 2001). For efficient
droplet pumping, a hydrophobic surface is normally needed, and methods for improving
the hydrophobic properties of the ZnO films include (Hu et al., 2007; Badre et al., 2007; Mei
et al., 2003): (1) spin coating PTFE (Teflon), (2) a monolayer of octadecyl thiol (ODT), (3)
a monolayer of octadecylesilane (ODS, 97% Aldrich), and (4) an octadecyltrichrorosilane
(OTS) self-assembled monolayer. After such treatment, the contact angle can be as high as
100° to 120° (see Figure 8.13b).
Functionalization of ZnO Surface for Biosensing
A critical issue in developing a high-performance biosensor is to determine a simple and
reliable process for the functionalization of the ZnO surface through a covalent method
to form a robust immobilization of appropriate probe molecules. Normally, Au is prede-
posited on the ZnO surface, and a cystamine surface atomic monolayer (SAM) forms on
the Au surface to which antibodies are to be attached. Currently, little work has been per-
formed on the direct surface functionalization of the ZnO films. Initial studies for immo-
bilization of antibodies on the ZnO film surface have been realized using (Corso et al.,
2008; Krishnamoorthy, 2006) (1) amine-terminated silane, 3-aminopropltryiethoxysilane,
and glutaraldehyde as the secondary crosslinker to bind a protein; (2) 3-mercaptopropyl-
trimethoxysilane in dry toluene; or (3) trimethoxysilane in dry toluene to immobilize the
antibody. Although Au is not recognized as a good CMOS-compatible material that can be
fully integrated into the fabrication process, the direct immobilzation on the ZnO film has
its advantages in biosensing applications.
AlNFilms
Currently, there is a concern that ZnO film is very reactive and unstable even in air or
moisture. Therefore, the stability and reliability is potentially a major problem. Compared
with ZnO, AlN shows a slightly lower PE coupling. The Rayleigh wave phase velocity in
AlN is much higher than that in ZnO, which suggests that AlN is better for high-frequency
