Experimental and theoretical studies on electropolymerization of polar amino acids on platinum electrode
Graphical abstract
Introduction
Electropolymerization process can lead to conducting or insulating polymers which are used in many applications including pH sensor and biosensors [1], [2], [3], [4]. In this context, amino acids are good candidates since they are the elementary components of polypeptides, and can have several functional groups such as COOH, NH2, OH, and/or -SH on the same molecule. The presence of various functional groups is an asset for amino acids and proteins to interact with surfaces. In addition, amino acids are mainly water soluble and can be used for water-processable electrodes which represent and advance toward green and sustainable electrochemistry. As a result, many applications were developed in the field of energy storage [5], [6], [7], biosensors for medical applications and also to investigate fouling processes in industrial developments [8], [9]. Amino acids can be classified by different methods according to functional groups, polarity, and pH for instance. Amongst them, l-serine, l-threonine, l-asparagine, and l-glutamine belong to polar amino acids because of the presence of hydroxyl group in l-serine and l-threonine [10]. l-asparagine and l-glutamine have amide group in their sidechain [11].
In recent twenty years, there has been a renewed interest in studying the oxidation and adsorption of amino acids on a surface. For instance, the oxidation and adsorption of l-glycine, l-serine and l-threonine on Pt and Au electrodes in alkaline electrolyte was investigated by cyclic voltammetry and Raman spectroscopy [12]. It was shown that these amino acids are adsorbed via hydrogen bonding, and this strong adsorption is increased with the increase of the electrode potential. Gold surfaces with (100) and (111) orientations were used to study by cyclic voltammetry and ATR-IR the adsorption and oxidation of l-alanine and l-serine in acidic electrolytes [13]. Comparison to theoretical harmonic vibration frequency calculations of the zwitterion adsorbed on Au (111) was carried out. Platinum single crystal electrode as working electrode was used to adsorb serine in sulfuric acid-based electrolyte [14]. Modified platinum electrode by carbon nanotube and glutamine was used to develop a sensor for As(III) titration [15].
More recently, adsorbed threonine on carbon paste electrode was used to detect tyrosine [16]. The observed peak current decreases with the increasing number of cycles, indicating a polymeric film growth on the electrode surface. The electrochemical behavior of electrodeposited threonine showed that polythreonine was electrosynthesized. The influence of α-alanine and asparagine acid on copper anodic processes was investigated in alkaline electrolytes [17]. They found that these two amino acids stimulate anodic processes at passive region and cause metal local activation. The adsorption of asparagine on gold electrode was studied by cyclic voltammetry and IRAS spectroscopy [18].
We have shown recently that anodic oxidation of amino acid-based electrolytes (case of l-glycine and l-alanine) can lead to polypeptide-like compounds poly(glycine) and poly(alanine), respectively [19], [20]. In this study, we generalized this electrochemical behavior to polar amino acids with uncharged R group (serine, threonine, asparagine and glutamine) on smooth platinum electrode. Cyclic voltammetry and spectroscopic techniques (ATR-FTIR and XPS) were used to characterize the resulting polymeric thin films as well as scanning electron microscope (SEM) to image coating morphologies. Less common with thin film coating, it was possible to estimate the resulting length chain by MALDI-TOF MS technique. As a potential application, the polymeric thin films on Pt electrode were tested as a transducing membrane having proton affinity in solid state pH measurements.
Section snippets
Chemicals
l-serine, l-threonine, l-asparagine and l-glutamine, NaOH, and NaBF4 were from Sigma-Aldrich (France) and ultra-pure water (Milli-Q-Millipore) was used.
Electrochemical measurements
The cyclic voltammetry measurements were performed on a μAutolab (ECOChemie, The Netherlands) coupled to a quartz crystal microbalance (QCM) PM710 frequency meter (Maxtek, USA), either on a smooth platinum electrode disk (1 mm diameter) or a polished platinum coated quartz crystal (5 MHz resonance frequency, area: 1.37 cm2) mounted on a PTFE
Electrochemical behavior of polar amino acids
The cyclic voltammograms of the anodic oxidation of l-serine in zwitterionic state on smooth platinum electrode are shown in Fig. 1. There is an irreversible anodic peak at about 1.70 V (Fig. 1a). This is consistent with anodic oxidation of amino acids found in the literature [19] and can be related to primary amine oxidation. There is no peak on the reverse scans. From EQCM measurements coupled to cyclic voltammetry, there is an increasing mass deposition especially beyond the anodic peak (
Conclusions
Electropolymerization of four l-amino acids including l-serine, l-threonine, l-asparagine and l-glutamine on smooth platinum electrode using an anodic oxidation method by cyclic voltammetry has been used to electrosynthesize poly-l-serine, poly-l-threonine, poly-l-asparagine and poly-l-glutamine coating thin films. The anodic oxidation of l-serine, l-threonine, l-asparagine and l-glutamine gives a good electrodeposition of polymer in zwitterion state which was characterized by ATR-FTIR and XPS
Acknowledgments
We gratefully acknowledge the Iraqi Ministry Higher Education and the Scientific Research and Cultural Bureau/Embassy of the Republic of Iraq in Paris – France for financial support. Computations have been performed on the supercomputer facilities of the “Mesocentre de calcul de Franche-Comte”, France. This work was also partly supported by the French RENATECH network.
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