Electrochemical chemosensors

PZ transducers coupled to MIP recognition elements are very effective in improving LOD. Determination of atrazine can serve as an illustrative example [159,160]. For the MIP-PZ chemosensor, LOD for atrazine was as low as 2 pM, being six times lower than that for an electrochemical chemosensor (see Sect. 4.4 below). 

Trojanowicz, M., T.K. vel Krawczyk, and P.W. Alexander. 1997. Organic conductings as active materials in electrochemical chemosensors and biosensors. Chem Anal 42 199. 

The present chapter critically encompasses developments and achievements reached in MIP-based selective sensing combined with optical, piezoelectric (PZ) and electrochemical signal transduction. General procedures of MIP preparation along with methods of MIP immobilization for chemosensor fabrication are presented. Protocols of analyte determination involving measurement complexities, like template presence or absence, have been addressed in detail. Moreover, analytical parameters, such as detectability, sensitivity, selectivity, linear dynamic... 

In situ polymerization, and electrochemical polymerization in particular [22], is an elegant procedure to form an ultra thin MIP film directly on the transducer surface. Electrochemical polymerization involves redox monomers that can be polymerized under galvanostatic, potentiostatic or potentiodynamic conditions that allow control of the properties of the MIP film being prepared. That is, the polymer thickness and its porosity can easily be adjusted with the amount of charge transferred as well as by selection of solvent and counter ions of suitable sizes, respectively. Except for template removal, this polymerization does not require any further film treatment and, in fact, the film can be applied directly. Formation of an ultrathin film of MIP is one of the attractive ways of chemosensor fabrication that avoids introduction of an excessive diffusion barrier for the analyte, thus improving chemosensor performance. This type of MIP is used to fabricate not only electrochemical [114] but also optical [59] and PZ [28] chemosensors. 

Electrochemical MIP-based chemosensors have gained particular attention owing to robustness of the material used for their fabrication, appreciable detectability, sensitivity and promising possibility of miniaturization. 

Electroanalytical techniques, such as conductometry [174], potentiometry [22], voltammetry [6], chronoamperometry [25] and EIS [175], have been used extensively for transduction of the detection signal in the MIP-based chemosensors. The chemosensor response may be due to different interfacial phenomena occurring at the electrode-electrolyte interface [16], which will be discussed below in the respective sections. The electrochemical transduction scheme can be devised for accurate measurements tailored to the analytes exhibiting either faradic or non-faradic electrode behaviour. In many instances, the detection medium is an inert buffer solution [24]. In order to enhance the chemosensor response, some of the... 

Commonly, the transduction mechanisms characteristic for electrochemical MIP chemosensors can fall into two categories, as shown in Scheme 4. For some transductions, like conductometric, impedimetric or potentiometric, sole presence of the target analyte in the MIP film is sufficient to produce an appreciable detection signal. However, this presence is insufficient for application of other techniques like chronoamperometry or voltammetry. As mentioned above in Sects. 4.4.4 and 4.4.5, proper electrode reaction is necessary in the latter techniques to generate the detection signal. Moreover, in the case of chronoamperometry or voltammetry, electrogenerated products may often foul the electrode surface. That is, these... 

This procedure can be applied irrespectively of whether the analyte is electroactive or electroinactive (Case b in Scheme 4). Here, the template is removed from the MIP film with a suitable solvent solution. Next, the chemosensor with template-free MIP is immersed in the test solution for analyte preconcentration. Analyte determination can be carried out with the chemosensor in the same test solution. Alternately, the determination can be performed in another solution. For an electroactive analyte, the chemosensor is removed from the test solution (after preconcentration) and transferred to a blank electrolyte solution followed by the analyte determination. In the case of an electroinactive analyte, the chemosensor is transferred to solution of an electroactive competitor for displacement of this analyte from the MIP film. All the electrochemical transductions, i.e. conductometric, impedimetric, potentiometric, chronoamperometric and voltammetric, are operative under this scheme. In particular, voltammetry and chronoamperometry can be used for determination of both electroactive and electroinactive analytes. This approach as well as the relevant experimental design will be elaborately discussed below in the same section. 

The detection techniques vary with the commercial insh ument they include reaction with different regents (i.e, dessicants to remove water, NaOH to remove carbon dioxide), chemiluminescence, coulometry, or electrochemically-based chemosensors. The... 

While much of the surveyed research exhibits promising vapor-phase sensing performance, many of the technologies remain experimental and bound to a laboratory setting. Most of the commercial gas sensors available today utilize older, more mature technologies such as electrochemical cells, catalytic beads, photoionization detectors (PID), SAW, metal oxide semiconductors (MOS), and QCM. The dearth of viable organic solid-state vapor-phase chemosensors indicates that there is much work still to be done (in terms of material stability, selectivity, etc.) before commercialization becomes commonplace for organic sensors. 

Nanocable chemosensors have been formed in which an inner core fiber filament is further modified by polymerization of the conducting polymer on its surface. This was first described for sensing by Zhang et al. in which a carbon fiber was used as the template for the electrochemical polymerization of a thin film of PANI [27]. The resulting nanoelectrode sensor was used to detect changes in pH resulting from the level of protonation in the polymer backbone. PPy nanofibers have been formed by the electrospinning of nylon fibers. 

Pearson. A.J. Hwang. J.-J. Crown-annelated p-phenyl-enediamine derivatives as electrochemical and fluorescence-responsive chemosensors Cyclic voltammetry studies. Tetrahedron Lett. 2001. 3541-3543. 

An interesting aspect of the receptor 12 is that the presence of lithium ions can be detected electrochemically. For the free complex 12, three irreversible oxidations were observed at 683,963 and 1150 mV (CH3CN/CH2CI2, against Ag/AgCl). In the presence of LiCl, the first peak potential was shifted by more than 350 mV toward anodic potential [ 14]. This offers the possibility to use receptor 12 as the recognition unit of an amperometric chemosensor for lithium ions. 

For the establishment of the receptor selectivity, different chemical interactions can be used. For example, the output of potentiometric sensors is strongly influenced by the equilibrium of the analyte with the sensitive layer of the chemosensor. Polymeric matrix membrane-based ion-selective electrodes are utilizing the concentration-dependent extraction of the analyte in the organic layer, while the analyte-dependent shift of potential of ion-selective sensors based on electrodes of the second kind can be described by the solubility product of the hardly soluble salt and the resulting Nemst equation of the electrochemical base reaction. Further equilibria can be predicated on ion exchanges, complexation, or adsorption effects. The interplay of analyte and receptor is determined by... 

---