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Optical Sensors Based on Molecularly Imprinted Nanomaterials Smart Nanomaterials for Sensor Application 61
into measurable signals. So far there are generally four types of signal transduction ways for the rebinding
of MIPs to the target analytes [10, 21].
2.1. Application of Fluorescent Templates and Analogues
For a target analyte that has a special optical property, such as fluorescence, it can be directly used for
detection. For example, a MIP fluorimetric sensor for monoamine naphthalene compounds was developed
by Valero-Navarro et al. [22] using non-covalent molecular imprinting techniques and naphthalene as
template. The system is based on the measurement of the native fluorescence signals of monoamine
naphthalene compounds when they are adsorbed on-line on the MIP. It can be used for simultaneous
determination of 1-naphthylamine and 2-naphthylamine at ng mL −1 level with a response time of 2 min.
A potential problem with above method is that residual template molecules in the polymer matrix may lead to a
high background signal and result in decreased sensitivity. A remedy could be to imprint the polymer with a
nonfluorescent analyte analogue. On the other hand, when the analyte does not display optical properties for the
spectroscopic analysis, it can be determined using a labeled template or analogue derivative in a displacement
or competitive assay [23-25]. As an example, a chloramphenicol MIP fluorescent sensor was developed based
on monitoring the competition of chloramphenicol and its dansylated derivative in binding to the imprinted sites
[25]. As an alternative approach, Haupt [26] used non-related fluorescent probes for the detection of the
herbicide and synthetic auxin 2, 4-dichlorophenoxyacetic acid.
Benito-Peňa et al. [27] developed a fluorescence competitive assay for penicillin G analysis using
pyrenemethylacetamidopenicillanic acid as the labeled competitor and successfully applied it to a
pharmaceutical formulation analysis. An automated molecularly imprinted sorbent based assay for the rapid
and sensitive analysis of penicillintype -lactam antibiotics was proposed by Urraca et al. using penicillin G
procaine salt as template and a stoichiometric quantity of a urea-based functional monomer [28]. Highly
fluorescent competitors containing pyrene labels while keeping intact the 6-aminopenicillanic acid moiety for
efficient recognition by the cross-linked polymers were tested as analyte analogues in the competitive assay.
Pyrenemethy Lacetamido Penicillanic Acid (PAAP) was the tagged antibiotic providing for the highest
selectivity when competing with penicillin G for the specific binding sites in the MIP. Upon desorption from
the MIP, the emission signal generated by the PAAP was related to the antibiotic concentration in the sample.
Recently, González et al. [29] described a flow-injection optical sensor for digoxin by combination of
sensor technology with MIP as the recognition phase. The MIP was packed into a flow cell and placed in a
spectrofluorimeter to integrate the reaction and detection systems. The new fluorosensor showed high
selectivity and sensitivity with a detection limit of 17 ng l -1 . The method was successfully applied for the
determination of digoxin concentration of human serum samples. A fluorescent indicator-displacement
molecular imprinting sensor array based on phenylboronic acid functionalized mesoporous silica was
developed for discriminating saccharides [30].
2.2. Incorporation of Fluorescent Reporter within MIP Structures
A more widely applicable approach for generation of optical signals in MIP binding is to incorporate
responsive chromophores or fluorophores into the polymer matrix [31-35]. When the analytes bind to the
imprinted cavities, the microenvironment (e.g. polarity, pH) around the fluoro/luminophore is altered,
resulting in quenching or enhancement of the fluorescence or energy transfer [36-40].
Turkewitsch et al. developed a MIP sensor for cyclic adenosine 3′, 5′-monophosphate (cAMP) by using a
fluorescent functional monomer trans-4-[p-(N,N-dimethylamino) styryl]-N-vinylbenzylpyridinium chloride
together with a conventional functional monomer [31]. Upon binding to the imprinted sites, the analyte
interacts with the fluorescent groups and quenches their fluorescence, allowing the analyte to be quantified.
One of the limitations of this strategy is the high background signal of the MIP. The fluorescence signal
only changed by 20% on binding cAMP. A possible reason for this is that many of the fluorescent
monomers were not incorporated into binding sites and were unresponsive to the bound analytes.