MIP Sensors on the Way to Real-World Applications - Designing Receptors for the Next Generation of Biosensors - page 166

mixture. Furthermore, it can be extended to larger template species and hence
allows for facile synthesis of receptors, e.g., against biopolymers, viruses, or
cells. Generally speaking, MIPs combine three highly advantageous features [ 12 ]:

• First, they are highly affine and selective and in some cases may even compete
with natural receptors in these features.
• Second,

their synthesis procedure is very straightforward and usually

inexpensive.
• Third, they usually are thermally and mechanically stable and thus have appre-
ciable ruggedness.

Their interactions with the respective analytes are somewhat similar to those
observed in natural antibody-antigen systems. However, in contrast to them, MIPs
as sensor coatings usually yield reversible responses, which enables researchers to
synthesize a new class of materials referred to as plastic antibodies or plastibodies
[ 13 - 16 ]. In applications outside the scope of sensors, MIPs can, e.g., be optimized
to catalyze a range of chemical reactions thus mimicking naturally occurring
enzymes [ 17 , 18 ]. Therefore it is even possible to synthesize an MIP for a com-
pound or reaction for which no natural antibody or enzyme is known.

2 Molecular Imprinting Strategies

Depending on the nature of the analyte, two different imprinting protocols are
employed to generate sensor layers, namely bulk imprinting and surface imprinting.
Furthermore, synthesizing MIP nanoparticles is also receiving increasing attention.

2.1 Bulk Imprinting

Bulk imprinting is usually employed for molecules with masses of up to a few
hundred atomic mass units, such as volatile organics, polycyclic aromatic
hydrocarbons, pharmaceutically active compounds, environmental contaminants,
and others. During polymerization, the respective template is directly added to the
monomer mixture. Therefore, interaction sites are not only present on the surface of
the respective sensor material, but they are also distributed within the whole bulk of
the matrix, hence the term “bulk imprinting.”
Consequently, the respective sensitive layer can accommodate a comparably
large number of interaction sites, even though the majority of them may only be
accessible via diffusion pathways within the polymeric matrix. Albeit sometimes
leading to increased sensor response times (depending on layer height), in the case
of small analyte molecules this guarantees the necessary sensitivity. This is of
fundamental interest, since many of these templates are non-immunogenic and
therefore not directly accessible for immunoassays due to the lack of suitable
antibodies. A schematic protocol for bulk imprinting is shown in Fig. 2 .