Cationic self-assembled monolayers composed of gemini-structured sulfur compounds on gold : A new approach to control ionic functions on surfaces and chiral detection.

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Cationic self-assembled monolayers (SAMs) composed of quaternary ammonium (QA) sulfur derivatives have been synthesized to control the distance between charged head groups on gold substrates. Two molecules bearing resembling molecular structures, "gemini"-structured didodecyl dithiol (HS-gQA-SH) and didodecyl disulfide (QA-SS-QA) or dodecyl thiol (QA-SH), were utilized in this study, and the formation and structure of the SAMs were characterized by surface plasmon resonance spectroscopy (SPR), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared-reflection adsorption spectroscopy (FTIR-RAS) and Kelvin Probe method. In the HS-gQA-SH SAM, the orientation and distance between QA groups are specified by the covalent bonding with ethylene spacer, while those of the QA-SS-QA SAM and QA-SH are determined by the electric repulsion between charged head groups, that is, QA groups in the QA-SS-QA SAM and QA-SH are more randomly located, being more distant than with those in the HS-gQA-SH SAM. Contact potential difference data obtained by KP suggested the existence of much larger dipoles on HS-gQA-SH SAM compared with on QA-SH SAM as a result of higher density of the functional group.We found that L-tartaric acid (TA), a probe molecule with two carboxyl groups having the distance of an ethylene unit, exhibits a strong affinity on the HS-gQA-SH SAM. In contrast, a specific binding was little observed on the QA-SS-QA SAM and QA-SH SAM. We demonstrate the crystal growth of L-TA on TA enantiomers (L or D-TA) immobilized QA-SH SAM, and found selective crystal growth on the same chiral surfaces (L-TA on L-TA). We have also tried one-step synthesis of gold nanoparticles capped by the QA-SS-QA SAM in this study.