Biosensors for Diagnostic Applications - Molecular Diagnostics - page 117

Biosensors based on electrochemical detection have already found their way into
POCT devices, with most of them being used as glucose meters [ 2 , 8 ]. However,
compared with the numerous publications and patents available, there is a huge
gap between scientific output and commercial success, the latter implying avail-
ability and acceptance in clinical routine [ 13 , 14 ].
One reason for this discrepancy may result from the sample matrix, which is more
complex in clinical samples (e.g., serum, containing numerous proteins) than in
laboratory samples prepared to demonstrate the system's performance (e.g., buffer
solution, containing only the target analyte). The complex sample matrix can
interfere with the biosensor signal response, which therefore has to be investigated in
addition to the system's performance shown with buffer solutions [ 13 , 14 ].
Furthermore, considering commercialization, a powerful biosensor instrument
requires both a high-performance biosensor component and a user-friendly and
efficient instrumental housing [ 15 ]. However, in many cases only individual aspects
of biosensor instruments were thoroughly investigated, i.e., only single components
were optimized, disregarding potential system integration. Finally, not least owing to
biomarker profiles still under investigation, there is a need for versatile sensing
technologies which can easily be adapted to the respective application, including
multiplex sensing as well as miniaturization and low costs [ 13 , 14 ].
In the following we will describe biosensor transduction principles and bio-
recognition layers as well as test formats with regard to their potential use in
diagnostic applications. We will outline relevant aspects concerning biosensor
integration in an efficient analytical device. This will be followed by an overview
of biosensor applications in diagnostics focusing on serum biomarkers in cardio-
vascular diseases (CVDs), cancer, and autoimmune diseases as well as biomarkers
in cerebrospinal fluid (CSF) for neurodegenerative diseases. We will describe the
latest developments and give an overview of future trends of biosensors in this
field.
2 Biosensors and Biosensor Systems
A biosensor is an analytical device which combines a biorecognition element with
a transducer [ 12 ]. Whereas the biorecognition element determines the degree of
selectivity or specificity of the biosensor, the biosensor's ability to detect low
concentrations is mainly influenced by the transducer, as it transforms the bio-
logical or biochemical response into a quantifiable output signal [ 16 ]. Biosensors
can be classified by the transduction principle, i.e., the detection, or by the type of
biorecognition layer, which defines the type of specific analyte binding [ 12 ].
Table 1 gives an overview of the most common types of biosensors following
these classifications. It is obvious that, for instance, immunosensors can be
designed using optical or electrochemical detectors, acoustic biosensors can be
based on antibody or aptamer coating, etc. Terms such as ''micromachined''