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In the past two decades, the biological and medical fields have seen great advances
in the development of biosensors and biochips for characterizing and quantifying
biomolecules. The biochemistry-based issues may include the characteristics of the
recognition molecules in terms of specificity, binding avidity, reversibility of binding,
stability under conditions of use and storage, functionality after immobilization, signal
generation, biochemical signal amplification, discrimination of specific from
nonspecific binding events, reaction time, availability and cost.
Using bioreceptors from biological organisms or receptors that have been patterned
after biological systems, scientists have developed new methods of biochemical
analysis that exploit the high selectivity of the biological recognition systems [9].
Arrays are being employed for an ever-increasing number of analytes. Ten years
ago, the main focus on sensors was specificity and sensitivity. The need to measure
multiple parameters was solved by bundling several sensors together in order to
multiplex them. Today, arrays with tens of thousands, and even hundreds of
thousands of features are commonplace in the DNA microarray field. Most such
microarrays use fluorescence signals as the transduction mechanism. The ability to
measure so many analytes simultaneously has revolutionized the thinking in sensing
in general and optical sensing in particular. It is no longer sufficient in most
applications to measure a single analyte as such a measurement captures only part
of the information about a complex real-world sample. A variety of interesting and
novel methods for preparing arrays have been developed for a wide variety of
applications. Arrays also offer the ability to make replicate measurements to minimize
false responses and to employ less selective sensing schemes coupled with
intelligent processing to remove the requirement for absolute specificity.
Silicon technology is already pervasive in our everyday lives. Silicon 'chips' often help
us shop, cook and relax. They are involved in many forms of transport and control
most forms of communication at the workplace.
The choice of this thesis for the support material and contemporary for the transducer
element fallen on porous silicon.
• FABRICATION AND CHARACTERIZATION OF POROUS SILICON (PSi)
MATERIALS
The current interest in porous silicon (PSi) results from the demonstration of efficient
visible photoluminescence of this material, first reported by Prof. Canham in 1990.
However, PSi is not a new material: it was first reported over 40 years ago by Uhlir.
During studies of the electropolishing of silicon in aqueous hydrofluoric acid (HF), he
observed that the surface often became black, brown or red. More detailed studies
were performed by Turner and Archer, but these films were not recognized as being
PSi. It was Watanabe et al. who first reported their porous nature. Porous silicon,
then, has been investigated for applications in microelectronics, optoelectronics,
chemical and biological sensors, and biomedical devices.
The in vivo use of porous silicon was first promoted by Leigh Canham, who
demonstrated its resorbability and biocompatibility in the mid 1990s. Subsequently,
PSi or porous SiO2 (prepared from PSi by oxidation) host matrices have been
employed to demonstrate in vitro release of the steroid dexamethasone, ibuprofen
and many other drugs. The first report of drug delivery from PSi across a cellular
barrier was performed with insulin, delivered across monolayers of Caco-2 cells. An
excellent review of the potential for use of PSi in various drug delivery applications
has recently appeared.