PH-Dependent Stability of Self-Assembled Monolayers on Gold

Abstract

Self-assembled monolayers (SAMs) have attracted considerable attention because of their wide applications to biosensors, optoelectronic devices, control of wettability and biocompatibility, corrosion resistance, etc. Among various types of SAMs formed on numerous kinds of substrates, the SAMs of alkanethiols on gold have extensively been studied, primarily because it is relatively easy and simple to form well-organized monolayers on gold compared with the methods used for other substrates. In addition, the gold surface enables us to effortlessly use analytical techniques, including surface plasmon resonace spectroscopy, quartz crystal microbalance, reflection absorption infrared spectroscopy and ellisometry, in the mechanistic studies and potential applications of SAMs. Especially, the SAMs of long-chain alkanethiols with ω-functional groups, such as OH, COOH, and NH2, have attracted a great deal of interest, because the use of these alkanethiols would yield more interesting properties of surfaces through post-modification reactions, such as nucleophilic substitution, nucleophilic addition, esterification, and acylation, in addition to their own chemical properties. For example, the surface modifications of OH-, COOH-, and NH2-terminated SAMs have been used for the attachment of biopolymers, such as oligonucleotides and peptides, onto gold surfaces, which is the essential step for the application to biotechnology. Since the post-modifications and applications of SAMs are performed under various conditions of temperature, pH and solvents, studies on the stability of SAMs against potentially destructive conditions of monolayers are important for wider applications of SAMs as well as design of post-modification reactions. The air-stability of alkanethiolbased monolayers on gold was studied with long-chain alkanethiols for one week to several months. Their longterm stability was also investigated in biological media, because biomaterials and biological devices, which use SAMs as a platform, necessitate prolonged exposure to the biological media. Recently, the effect of storage conditions, such as air, N2, ethanol, phosphate buffer and H2O, on the stability of mixed poly(ethylene oxide)-thiol SAMs was studied to ensure the long-term preservation of biosensing property. In addition to these studies on the stability of SAMs, the pH dependency of the stability of SAMs in aqueous solutions is to be established, since the numerous surface modification reactions and applications of SAMs are performed at various pH values. As a related work, the dissociation phenomena of thiolate ligands from the surface of cadmium chalcogenide nanocrystals have been examined by varying the pH values. In this work, we investigated pHdependent stability of alkanethiol SAMs with various functional groups at their terminals, in order to give a basic but crucial information for post-modifications and other SAMbased reactions. The SAMs of long-chain alkanethiols with CH3, OH, COOH, or NH2 group were prepared on gold, and their stability was studied for one week at pH values from 1 to 14 by measuring the changes in the ellipsometric thickness. For the preparation of the SAMs on gold, a clean gold substrate was immersed for 20 h in each alkanethiol solution of HS-(CH2)15-CH3, HS-(CH2)16-OH, HS-(CH2)15-COOH, or HS-(CH2)16-NH2. The SAMs were characterized by FTIR spectroscopy (Figure 1). Figure 1a shows the IR peaks