Prime importance of the supporting electrolyte in the realization of molecularly imprinted polymers

This study concerns the highlighting of the role of supporting electrolyte in the realization of stable and reproducible molecular imprinted polymer (MIP), as it isone of the key element which can affect the MIP’s morphology, and thus the corresponding electrochemical’s response. Here we have designed a MIP with pyrrole, as functional monomer, and prostate specific antigen (PSA) as template. Several electrolytes were tested at a fixed concentration of 0,16 M: phosphate buffer solution (PBS), NaCl, KCl and LiClO4in PBS solutions. Square wave voltammetry was investigated to get information about the most appropriate electrolyte.The follow up of variations of PSA peak currents indicates that the MIPs realized in LiClO4 give the better electrochemical response. The morphological analyses of the electropolymerized MIPs,investigated with atomic force microscopy, confirm the electrochemical results and permitted to highlightthe role ofanions and cations in MIP’s design.


Introduction
Molecularly imprinted polymers (MIPs) are attracting considerable attention nowadays, due to their "relative ease" of concept and the possibility of their use in several applications varying from drug release, protein and DNA sensing, to quenchers in SERS experiments. Molecular imprinting principle is based on the creation of specific and selective cavities in a 3D-polymeric network, which are complementary in size and shape of target species, providing thus interaction points and a coordination sphere around the template molecule.

SciForum
Mol2Net, 2018, 1(Section A, B, C, etc.), 1-x, type of paper, doi: xxx-xxxx 2 Among the large panel of functional monomers, we have chosen pyrrole, due to its biocompatibility, the possibility of its use at neutral pH in either aqueous or organic solvents, and mainly therelative ease of its polymerization. Because of the rapidity of polypyrrole (PPy)electropolymerization, several synthesis mechanisms were proposed in literature [1].The most consensual one is that presented by Genies et al [2]: the electropolmerization is initiated by the generation of radical cations monomers at the surface of the electrode and then followed by a rapid growth of the PPy chain. However, there are many factors that can interfere in the polymer synthesis such as solvent nature, pH, temperature and mainly supporting electrolyte [3]. This study is devoted to highlighting the role of the supporting electrolyte in the design of PPy molecular imprinted polymer. Prostate Specific Antigen was investigated as template molecule.

Results and Discussion
Previous studies have demonstrated that the supporting electrolyte play an important role in the PPy electropolymerization process [4][5]. The most appropriate one corresponds generally to a compromise between several parameters, mainly the solubility, the electrochemical stability and the interaction with reaction intermediate. Here, we have chosen to investigate four electrolytes at a fixed concentration of 0,16 M: NaCl, KCl and LiClO4(in PBS solution) and PBS. Square wave voltammetry was carried out to follow up the oxidation peak of PSA. Square wave voltammograms presented in Figure. 1 indicate that the oxidation peak of PSA is more intense in a PBS solution, but it is less resolved than that of LiClO4 in PBS. Broad peaks were recorded in KCl/PBS and NaCl/PBS supporting electrolytes.This result is in good agreement with the previous publications relating the role of counter ionsin the process of electropolymerization and electrical conductivity of the PPy [4][5][6][7].LiClO4 in PBS was thus chosen to realize the PSA/MIPs. NaCl, KCl and LiClO4molecularly imprinted polymers topographies were investigated by means of atomic force microscopy. Figure. 2 shows an obvious difference between the realized MIPs.In fact, MIPs realized with ClO4ions are more uniform than those made with Clsmall ion. These results confirm the role of counterion in the structure regularity and morphology of PPyfilms [8][9]. The comparison between Figure 2a and 2b indicates that the cations can also play a role in MIPs morphology. In fact, and despite the fact that NaCl and KCl based MIPs present "rings like structures", those of NaCl are more regular and are regularly distributed on the surface. Note that this is not a salt effect accumulated on surfaces, as the electrodes were copiously rinsed with DI water prior to AFM analysis. This results very interesting, as electrochemical studies concerns generally only the influence of the nature of the counter ion.
Prior to any measurement, a drop of a piranha solution (98% H2SO4 / 30% H2O2 1:1 V/V) was deposited, during 20 min, on the gold sensing areas of the gold electrodes. The substrates were then copiously rinsed with deionized double distilled water, then with ethanol before being dried under ambient air.
Before MIP deposition, a thin barrier PPy layer was electroplymerized by CA at a constant potential of 1.05 V vs SCE during 2 s. Chronoamperometry and Square wave voltammetry measurements were carried out with a CHI 650E potentiostat with the following parameters: increment = 5 mV, amplitude = 25 mV, frequency = 25 Hz, and sensitivity = 1.10 -4 .A three-electrodes configuration was used: a gold electrode, a saturated calomel electrode (SCE) and a platinum wire as the working, the reference and the auxiliary electrodes respectively.

Conclusions
This study permits to highlight the role of the supporting electrolyte on both electrochemical response and structural morphology of a molecular imprinted polymer. Square wave voltammetry results indicate that LiClO4 (in a PBS solution) is the most appropriate supporting electrolyte. AFM topographic studies showed thatvarying the counterion (ClO4vs Cl -)modify the MIPs morphology, and that ClO4anions permitted to obtain more uniform microscopic structure than Clones. Further investigations are necessary to establish the cations effects on the electrical conductivity of PPy based MIPs.