Electrochemical sensors modified with ion-imprinted polymers for metal ion detection
Introduction
Monitoring of metal ions is a particularly important topic as they can have a negative impact on both the environment and human health [1,2]. Even trace amounts of some metal ions can lead to disastrous consequences so it is essential to develop analysis methods that can reach low detection limits and be efficient even in the case of complex matrices. Though conventional techniques, such as Atomic Absorption Spectroscopy (AAS) and Inductively Coupled Plasma Mass Spectrometry (ICP-MS), have great accuracy and widespread applications for the measurement of the total amount of metals, they suffer from expensive equipment and time-consuming preparation and analysis processes. Thus, in order to easily and quickly monitor the levels of trace metal ions, the development of affordable sensors has become a hot research topic.
Biomimetic electrochemical sensors based on the molecular imprinting technology have already proved their high efficiency for the quantification of organic pollutants [[3], [4], [5], [6]]. In such a case, Molecularly Imprinted Polymers (MIPs) act as efficient and highly selective receptors that will considerably improve the sensitivity of the associated electrodes [7,8]. The development of Ion-Imprinted Polymers (IIPs) for the selective extraction of ion species came after MIPs [[9], [10], [11]]. The growing interest in the preparation of specific electrodes incorporating IIPs for the quantification of metal ions is also more recent [[12], [13], [14], [15]].
Like MIPs for organic targets, IIPs are specially designed to selectively recognize a target ion, called template ion, which is used for their synthesis. The general procedure to prepare IIPs is based on the initial formation of a complex between the template ion and a molecule bearing at least one chelating group, the ligand. The structure of the so-formed complex is then frozen by crosslinking in order to form a ‘rigid’ three-dimensional network around the complex that will maintain the shape and size of the binding cavities after the template removal (Fig. 1). The properties of selective recognition of IIPs first lead to their use as sorbents for solid-phase extraction [16]. This is still a large area of applications of IIPs that encompasses preconcentration, speciation and removal of metal ions [[17], [18], [19], [20]]. Similarly to MIPs, the selectivity of IIPs makes them particularly interesting as receptors for sensing applications. For optical sensors, this is not straightforward because most metal ions cannot be directly quantified by the measurement of an optical signal. Thus the design of such sensors based on IIPs requires the incorporation of a chromophore [21,22] or a fluoroionophore [[23], [24], [25]] inside the polymer matrix to generate or quench an optical signal (absorbance or fluorescence). Another option is to use fluorescent particles like quantum dots [[26], [27], [28], [29]]. On the other hand, most metal ions can be quantified by electrochemical detection methods, such as voltammetry or potentiometry [[30], [31], [32], [33]]. Therefore, no specific requirements are needed for the conception of IIPs for electrochemical sensors, in which they will act as selective receptors for the binding of the target ion to improve the sensitivity and selectivity. This is of particular interest for applications in various complex samples such as environmental or waste waters, human biological fluids or solid samples in which the analytical difficulties can come from the presence of interfering ions or from the complexity of the matrix. Moreover, coupling the ion-imprinting technology with electrochemical detection can provide IIP-modified electrochemical sensors (IIPECS) which are relatively inexpensive to develop, as a general feature of the IIP preparation.
This review aims to provide insight into how IIPs can be used in electrochemical sensors to improve their performances and to show how effective they can be regarding selectivity. In particular, one of the objectives of the present survey is to help scientists in their conception of IIPECS by giving general indications on the design of suitable IIPs and electrodes. For that purpose, the different routes that can be implemented to prepare IIPs are detailed before developing the various strategies to prepare IIP-modified electrodes for potentiometric and voltammetric applications. A focus on the IIPECS characterization and applications is also included.
Section snippets
Free radical polymerization
Free Radical Polymerization (FRP) is the most common method to synthesize IIPs as the process is well-established and easy to implement [37,38]. FRP involves the use of vinylated monomers. For that purpose, commercial vinylated crosslinkers such as ethylene glycol dimethacrylate (EGDMA), divinylbenzene (DVB), N,N′-methylenebis(acrylamide) (MBA), or trimethylolpropane trimethacrylate (TRIM) are generally employed.
The formation of the selective binding cavities of imprinted polymers is a crucial
Preparation of IIP-modified electrochemical sensors
The determination of metal ions by IIPECS is mainly based on classical potentiometry and voltammetry by taking advantage of the selectivity properties of IIPs. In the case of potentiometry, IIP (usually in a particle format) replace the classical ionophores used in ISE. For voltammetry, the role of IIP is to selectively accumulate the metallic ion.
Fig. 3 gives a brief overview of the different strategies that can be implemented to prepare IIP-modified electrodes based on the use of IIP either
Optimization of analytical parameters
In section 3, the optimization of the composition of the IIP-modified electrodes for potentiometry (amount of IIP, type and amount of plasticizer, …) and voltammetry (amount of IIP, electroconducting additives, …) was discussed. This sub-section is devoted to the optimization of the experimental analytical conditions under which the measurements are performed. As was previously stated, the advantage of using IIPECS is that they can improve the selective adsorption of the target ion. This is why
Conclusions and perspectives
From the reviewed literature, it appears that IIPs show great promise as selective receptors to prepare different types of electrodes for IIP-modified electrochemical sensors. The flexibility of their preparation makes IIPs excellent candidates for the design of selective biomimetic electrochemical sensors, for which they could be in competition with bioreceptors such as peptides, enzymes and functional nucleic acids. Nevertheless, in comparison, IIPs present the advantage of being quite easy
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Alexandre SALA reports financial support was provided by Regional Council Provence-Alpes-Cote d’Azur. Catherine BRANGER reports financial support was provided by Interreg Marittimo.
Acknowledgements
This work was financially supported by the European Interreg Italy-France Maritime 2014-2020 Project «GEREMIA» (Gestione dei reflui per il miglioramento delle acque portuali) and by the Regional Council of Provence Alpes Côte d’Azur (France).
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