Biomedical Engineering Reference
In-Depth Information
formed, it is generally unreactive through the rest of the etch cycle. By varying the etch
current as a function of time, it is therefore possible to produce complex-layered structures
of precisely defined thickness and porosity. As we shall discuss later, these layered struc-
tures are important in the development of optical sensors.
Regardless of the precise device structure, PSi sensors function by transducing a change
in the bulk refractive index of the material effected by infiltration of analytes into the
porous matrix (Figure 11.2). In some respects, this makes PSi sensors somewhat like other
refractive index-based (or “mass sensing”) techniques such as surface plasmon resonance
(SPR)
6
and reflective interferometry (RI).
7
However, the three-dimensional nature of the
transducing surface that is unique to PSi results in both interesting opportunities and chal-
lenges.
The size of the pores produced is just as important as the ability to produce structures
of specific porosity. Generally, PSi is described as falling into one of the three primary cat-
egories:
microporous silicon
(pore diameter <10 nm),
mesoporous silicon
(10-50 nm), and
macroporous silicon
(50-10
m). The bulk of studies on PSi sensors have, till date, focused
on mesoporous silicon; microporous silicon is too small to permit infiltration of most ana-
lytes of interest, and until recently macroporous silicon could not be produced with a suf-
ficient level of control to permit effective optical sensing. Recent work however suggests
that the manufacturing challenges associated with macroporous silicon can be overcome,
as we will discuss later in this chapter.
The seminal discovery that led to much of our early work on PSi sensors was by
Canham, who observed room-temperature photoluminescence from mesoporous mate-
rial.
8
However, the first “proof of concept” demonstrations of PSi-based biosensing relied
on the reflectivity spectra of single-layer PSi devices. Building on initial work toward
understanding the optical reflection spectra of PSi,
9
Sailor, Ghadiri, and coworkers
observed subtle yet detectable binding-dependent shifts in the Fabry-Perot fringes
FIGURE 11.2
Basic operation of a porous silicon sensor: infiltration of an analyte into the porous structure causes a change in
the bulk refractive index of the material, which in turn causes a change (for example, a redshift) in its optical
properties.
