Interfacial Structure and Function of Nano-Structured Membranes of Newly Synthesized Phosphorylcholine Derivatives on Single Crystal Electrodes
Authors: Sawaguchi Takahiro , Tanaka Mutsuo 
Published: December 2020

Properties of modified surface for biosensing interface are recognized as important subjects to develop methodologies in bio-related analysis and techniques. Therefore, various surface modification materials to fabricate biosensing interface have been synthesized and studied on properties of modified surface of gold,1-4 carbon,5 silicon6 and some polymer7,8 substrates for application to biosensor and micro flow analytical system.
The non-specific adsorption of proteins that occurs during analysis of biomolecules often has a serious detrimental effect on selectivity and detection limit, and research on surface modification materials has attracted great interest with a view to avoiding the interference caused by the non-specific adsorption of proteins. The properties of polyethylene glycol (PEG) such as flexibility, chemical stability, water solubility, and low cytotoxicity make it a versatile surface modification material. The most attractive property of PEG is its suppressive effect on the non-specific adsorption of proteins. While polymers consisting of the ethylene glycol moiety have been studied extensively, it has also been reported that self-assembled monolayers (SAMs) of oligoethylene glycol-alkane thiols suppress the non-specific adsorption of proteins in a similar way to surfaces modified with PEG. Our previous work showed that SAMs of oligoethylene glycol–alkane thiols effectively suppressed the non-specific adsorption of proteins allowing us to realize the high performance detection of proteins.1

On the other hand, phosphorylcholine derivatives are other promising candidates for suppressing the non-specific adsorption of proteins because the phosphorylcholine group is one of the primary lipid components of plasma membranes. Although SAMs of phosphorylcholine-alkane thiols were also reported to suppress the non-specific adsorption of proteins, there have been relatively few studies related to SAMs of phosphorylcholine-alkane thiols compared with those on SAMs of oligoethylene glycol–alkane thiols. Therefore, we are interested in the properties of phosphorylcholine SAMs. It was found that newly synthesized phosphorylcholine-alkane thiols bearing the oligoethylene glycol moiety, namely phosphorylcholine–oligoethylene glycol–alkane thiols, formed SAMs on gold surfaces and suppressed fibrinogen adsorption more effectively than the corresponding oligoethylene glycol–alkane thiols.2 The incorporation of oligoethylene glycol moieties in phosphorylcholine-alkane thiols enhanced the suppressive effect on concanavalin A adsorption but not on fibrinogen adsorption. Furthermore, we studied the newly synthesized zwitterion–alkane thiol SAMs which have phosphorylcholine, inverse phosphorylcholine and sulfobetaine terminal groups as shown in Figure 1, and repot here mainly the results on the interfacial structure and function of the phosphorylcholine-alkane thiol SAMs on a single crystal Au(111) electrode in solution.
An electrochemical scanning tunneling microscope (EC-STM) was used to observe surface feature of phosphorylcholine-dodecanethiol (C12PC) SAMs on Au(111) in 0.1M HClO4, and mostly flat terrace surface with the surface roughness less than 0.3 nm was observed in a wide scan area, indicating that the molecules were closely packed. A typical high-resolution EC-STM images of C12PC SAM on Au(111) was presented in Figure 2. Bright spots aligned toward √3 directions were clearly observed with spacing of 0.5 nm, corresponding to individual C12PC molecules. The molecular arrangement was assigned to (√3 x 3√3)R30° structure with two C12PC molecules in the lattice. The surface coverage of the monolayer was 2/9, which is consistent with that from the charge of electrochemical reductive desorption process with an assumption of one-electron reaction. In the case of the phosphorylcholine–diethylene glycol–alkane thiols (C12O2PC) SAMs, the monolayer exhibited the double-striped (√3 x 3√3)R30° structure with a molecular coverage of 2/9, where every three C12O2PC molecular rows was missing. In comparison with C12PC and C12O2PC SAMs, the molecular arrangement of C12PC would be (√3 x 3√3)R30°-2C12PC missing row structure, which is the same as that of the C12O2PC SAM, because of bulkiness of phosphorylcholine head group. A detailed analysis of the surface structure will be discussed with the function suppressing the non-specific adsorption of proteins.
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Figure 1