MIP Sensors on the Way to Real-World Applications - Designing Receptors for the Next Generation of Biosensors - page 168

from the matrix. And third, even if that should prove possible, diffusion into
cavities lying further below the surface would be substantially sterically hindered.
To overcome these limitations, surface-imprinting approaches have been devel-
oped as shown in Fig. 3 . In this case the respective template species are self-
organized on a stamp surface which is then pressed onto the pre-polymerized
oligomeric mixture coated onto the surface of the respective transducer. This
again is followed by hardening the material and washing out the template. It has
turned out that the selectivity of such “artificial antibodies” is determined by shape,
size, and surface chemistry of the respective template species in the same way as for
bulk approaches.

2.3 Imprinted Nanoparticles

One of the continuous major efforts in chemical or biochemical sensing is to
achieve optimum sensitivity and selectivity of a sensor toward its corresponding
analyte. The former can be achieved in different ways, e.g., by increasing the
fundamental device frequency in the case of mass-sensitive transducers [ 19 ].
However, from the material point of view, either increasing the number of interac-
tion sites within the respective recognition layer or increasing their accessibility is
another feasible approach. Out of these two possibilities, the latter turned out to be
more practicable [ 20 ], because increasing the number of recognition sites would
require higher amounts of template, which can inhibit polymerization. Improved
accessibility can be realized by applying nanoparticles rather than thin films as
recognition materials.
Such MIP nanoparticles provide substantially increased surface area for
analyte-receptor interactions as compared to thin films. Figure 4a shows a typical
AFM image of glass surface coated with imprinted nanoparticles and Fig. 4b
represents the mass-sensitive sensor responses of quartz crystal microbalances
(QCM) coated with a thin film and MIP nanoparticles, respectively, toward folic
acid. Evidently, the nanoparticle-MIP coated surface yields higher response
because of shorter diffusion pathways and therefore an increased number of avail-
able interaction sites as compared to a thin film consisting of the same amount of
selective material.

3 Molecular Imprinting in Chemical Sensing

MIPs are especially suitable for recognition in chemical sensing, because they
combine high selectivity with comparably low cost per layer. Among others, this
leads to constantly increasing number of publications in this field. Generally
speaking, chemical sensors are miniaturized devices capable of continuously