MIP Sensors on the Way to Real-World Applications - Designing Receptors for the Next Generation of Biosensors - page 176

vegetables and fruits [ 47 ]. This has led to attempts to design suitable sensor
materials: as early as 1999, Sergeyeva et al. [ 48 ] developed a conductometric
sensor for the detection of atrazine in aqueous solutions, using MIP membranes
prepared by photopolymerization of methacrylic acid as the functional monomer
and tri(ethylene glycol) dimethacrylate as the cross-linker. First of all, the imprinted
polymer exhibited very high selectivity toward atrazine, with signals an order of
magnitude higher than structurally related compounds, such as simazine,
prometryn, and others. Furthermore, the sensor response is almost independent of
environmental parameters, such as pH or salt load. Therefore the approach, as such,
is highly suitable for real-life applications, especially since the lower limit of
detection was 5 nM. Similarly Pardieu et al. [ 49 ] reported an electrochemical sensor
for the detection of atrazine, in which a molecularly imprinted conducting polymer
(MICP)—poly (3,4-ethylenedioxythiophene-co-thiophene-acetic acid)—was elec-
trochemically synthesized on a platinum electrode in the presence of atrazine as
template. This MICP-based sensor showed selectivity toward the triazine family
with a wide dynamic range (10 7 mol L 1 to 1.5 10 2 mol L 1 atrazine) and low

detection limit (10 7 mol L 1 ). Detection in this case took place via cyclic
voltammetry, an approach that had already proven useful in an earlier paper
targeting 2,4-dichlorophenoxyacetic acid (2,4-D) [ 41 ]; however this did not include
assessment of the sensor response in real-life samples.
Substantial efforts have been made for the fabrication of MIP-based sensors for
the detection of these pesticides down to ppb levels. All these papers led to highly
appreciable sensitivity that at least approaches the levels that are realistically
required in environmental sensing. They do have electrochemical detection in
common, which of course clearly indicates that the overall sensor properties are
not governed only by the MIP, but by the entire sensor setup. Hence, the interplay
between layer and device plays a crucial role for these applications.
A different approach to atrazine sensing has been published by Schirhagl et al.
[ 43 ], who used a double imprinting strategy to generate “artificial antibodies.” First,
nanoparticles were templated with natural anti-atrazine immunoglobulins. In a
second step these nanoparticles were used as a stamp for surface imprinting of
polymer layers to produce antibody replicas in a copolymer system based on
methacrylic acid, vinylpyrrolidone, and dihydroxyethylene-bisacrylamide
(DHEBA). These layers exhibit nearly four times higher sensor response—and
hence sensitivity—than their natural counterparts with the limit of detection
being 0.04

gmL 1 . Such a comparison is of course of academic interest, but
also strongly influences the potential applications, as it allows for assessing the
quality of artificial recognition materials and how they compare to the
corresponding natural systems. There is one thing to be said about the atrazine-
selective polymer: it is not surprising that this analyte yields outstandingly appre-
ciable results. First, a heteroaromatic system that is inherently rigid dominates the
structure. Second, triazine compounds have very pronounced functionalities that
are able to undergo hydrogen bonding. Most of the polymer systems applied
provide such hydrogen bonding between analyte and layer, leading to the apprecia-
ble sensor effects mentioned. This is worth mentioning when comparing MIP with
antibody-antigen interactions—MIP are sometimes referred to as “artificial

m