Contact-Angle Titrations of Mixed .omega.-Mercaptoalkanoic Acid/Alkanethiol Monolayers on Gold. Reactive vs Nonreactive Spreading, and Chain Length Effects on Surface pKa Values

Creager, Stephen E.; Clarke, James Stanier

doi:10.1021/la00022a048

Cited by 233 publications

(262 citation statements)

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“…It is known that carboxylic acid-terminated SAMs change their surface energy in response to the pH of the surrounding medium (18,26,40). These SAMs can be titrated, with the range and position of the pH-dependent change in AW varying with the surface concentration of COOH-containing thiolates.…”

Section: Discussionmentioning

confidence: 99%

Effect of Substratum Surface Chemistry and Surface Energy on Attachment of Marine Bacteria and Algal Spores

Ista

Callow

Finlay

et al. 2004

Appl Environ Microbiol

187

139

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Two series of self-assembled monolayers (SAMs) of -substituted alkanethiolates on gold were used to systematically examine the effects of varying substratum surface chemistry and energy on the attachment of two model organisms of interest to the study of marine biofouling, the bacterium Cobetia marina (formerly Halomonas marina) and zoospores of the alga Ulva linza (formerly Enteromorpha linza). SAMs were formed on gold-coated glass slides from solutions containing mixtures of methyl-and carboxylic acid-terminated alkanethiols and mixtures of methyl-and hydroxyl-terminated alkanethiols. C. marina attached in increasing numbers to SAMs with decreasing advancing water contact angles ( AW ), in accordance with equation-of-state models of colloidal attachment. Previous studies of Ulva zoospore attachment to a series of mixed methyl-and hydroxyl-terminated SAMs showed a similar correlation between substratum AW and zoospore attachment. When the hydrophilic component of the SAMs was changed to carboxylate, however, the profile of attachment of Ulva was significantly different, suggesting that a more complex model of interfacial energetics is required.Upon submersion in a nonsterile aqueous liquid, most surfaces become rapidly colonized by collections of bacteria and other microorganisms. These attached cells, along with extracellular material they produce and other organic compounds adsorbed to the surface, comprise a structure referred to as a biofilm (17). Biofilms are ubiquitous in natural aqueous milieus and are increasingly considered to represent a separate developmental form of microorganisms (29). Similarly, a number of macroorganisms, such as algae, exploit a unicellular form for attachment and colonization of surfaces. Surface coverage by both macro-and microorganisms, therefore, depends initially on the ability of single cells to adsorb and adhere to the attachment substratum.The tendency (driving force) for a microorganism to attach to a given surface is given by the free energy of adhesion (⌬G adh ), which can be expressed by the following equation as a thermodynamic energy balance between the interfacial energies between the substratum, the organism, and the surrounding liquid (1): ⌬G adh ϭ ␥ BS Ϫ ␥ BL Ϫ ␥ SL , where ␥ BS is the interfacial tension between the organism (e.g., a bacterium) and substratum, ␥ BL is the interfacial tension between the organism and the liquid, and ␥ SL is the interfacial tension between the substratum and the liquid.Experimental determination of the interfacial energy values for the above equation is controversial and has led to three different, yet complementary, models (8). All rely on estimation of interfacial energies by contact angles as indicated by Young's equation (2), which states that ␥ SV (the vapor interfacial tensions [surface tensions] of the substratum) is related to the contact angle () formed by a drop of liquid on the substratum such that ␥ SV ϭ ␥ SL ϩ ␥ LV cos, where ␥ SL is the interfacial tension between the surface and the liquid and ␥ LV is the interfac...

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Section: Discussionmentioning

confidence: 99%

Effect of Substratum Surface Chemistry and Surface Energy on Attachment of Marine Bacteria and Algal Spores

Ista

Callow

Finlay

et al. 2004

Appl Environ Microbiol

187

139

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“…For determination of surface pK a of SAMs, to our best knowledge, few methods, such as contact angle titration [13,14], chemical force titration using atomic force microscopy (AFM) [15,16], quartz crystal microbalance (QCM) [17], surface potential [18], spectroscopy [19] and electrochemical method [20] have been adopted previously. Among them electrochemical method is a more simple and convenient approach than others.…”

Section: Introductionmentioning

confidence: 99%

Determination of Surface pKa of SAM Using an Electrochemical Titration Method

et al. 1999

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A self-assembled monolayer (SAM) of 3-mercaptopropionic acid (HSCH 2 CH 2 COOH) was formed on a gold electrode. The effect of the charge of the end group on the electrochemical response of Fe(CN) 6 37 at the SAM modi®ed electrode was studied by using cyclic voltammetry. At high pH, when the ±COOH groups are dissociated, Fe(CN) 6 37 current is suppressed; as the solution pH is lowered, Fe(CN) 6 37 current increases. The electrochemical titration curve was obtained by correlating the currents to the different electrolyte pH values, from which the surface pK a was obtained to be 5.2 AE 0.1. Furthermore, a calculation equation was presented to simulate the electrochemical titration. As comparison, the surface pK a was also measured by contact angle titration as 5.6 AE 0.1. The surface pK a values determined by the two methods in our work are consistent and accurate.

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“…The disorder of the SAM could make the surface less hydrophilic, leading to such a large contact angle of water on MHA-SAM. 45 This observation indicates that each component in our MHA/HT mixed SAM may be less ordered, and therefore be very similar to the liquid state.…”

Section: Composition Of Mixed Samsmentioning

confidence: 81%

“…[39][40][41][42] The COOH-terminated SAMs, such as thiol and silane, have been analyzed by FT-IR spectroscopy. 44 Especially, in the case of COOH-terminated SAMs, the effect of the intramonolayer hydrogen bonding between the neighboring carboxyl groups in the SAMs had been considered with respect to their surface property, such as wettability 45 and surface acidity (pKa) of the carboxyl groups. [41][42][43] In the present paper, we report an AFM force spectroscopic study concerning the effect of intramonolayer hydrogen bonding in COOH-terminated SAMs of 6-mercaptohexanoic acid (MHA) on the adhesion force observed between a SAM of MHA and a phenylurea derivative attached to the substrate and probe tip, respectively.…”

Section: Introductionmentioning

confidence: 99%

Effect of Intramonolayer Hydrogen Bonding of Carboxyl Groups in Self-assembled Monolayers on a Single Force with Phenylurea on an AFM Probe Tip

2006

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IntroductionDuring the recent decade, atomic force microscopy (AFM) has attracted much attention as a promising analytical tool for precisely measuring intermolecular forces.1-5 AFM can be used to detect the force between its probe tip and substrate surfaces, even at the pN level. It is thus of great importance that there is almost no limitation of the environment and conditions where the AFM force measurements are carried out. Thus, if the AFM probe tip and the substrate are modified chemically with selfassembled monolayers (SAMs) 6,7 terminating with target functional groups, the intermolecular force in their molecular pairs can be evaluated by force-curve measurements in appropriate solvents by AFM. [8][9][10][11][12][13][14][15][16][17][18][19]36 Furthermore, the individual force for a variety of intermolecular forces at a single-molecule level can also be evaluated by statistical analyses of the forces obtained by repetitive adhesion force measurements. [20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35]37,38 In our previous studies, we measured specific intermolecular forces attributable to the characteristic molecular functionalities of the functional molecules, such as crown ether 36,37 and spirobenzopyran 38 derivatives, attached covalently on the probe tips. These functionalized AFM probe tips, which possess a powerful capability of detecting supramolecular interactions, can be potentially utilized for developing a novel analytical tool for surface analysis based on force sensing by AFM.In such AFM force measurements, SAMs are often used for conveniently established covalent bonding of the target molecules onto the surfaces of the tip and substrate. Then, the intramonolayer interaction between the components in the SAMs can be expected to affect the force observed by the AFM technique. In other words, the AFM technique could detect such effects as the intramonolayer interaction on the interaction between the tip and the substrate. Intramonolayer hydrogen bonding is crucial in the SAMs of thiols or silanes bearing a terminal functional group, which can form hydrogen bondings between themselves. For example, terminal carboxyl groups in the SAMs of ω-carboxyalkanethiol can easily form hydrogen bonding within the monolayers. 39-42The COOH-terminated SAMs, such as thiol and silane, have been analyzed by FT-IR spectroscopy. 44 Especially, in the case of COOH-terminated SAMs, the effect of the intramonolayer hydrogen bonding between the neighboring carboxyl groups in the SAMs had been considered with respect to their surface property, such as wettability 45 and surface acidity (pKa) of the carboxyl groups. [41][42][43] In the present paper, we report an AFM force spectroscopic study concerning the effect of intramonolayer hydrogen bonding in COOH-terminated SAMs of 6-mercaptohexanoic acid (MHA) on the adhesion force observed between a SAM of MHA and a phenylurea derivative attached to the substrate and probe tip, respectively.Urea derivatives have a powerful ability to form hydrogen bondings, sin...

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Contact-Angle Titrations of Mixed .omega.-Mercaptoalkanoic Acid/Alkanethiol Monolayers on Gold. Reactive vs Nonreactive Spreading, and Chain Length Effects on Surface pKa Values

Cited by 233 publications

References 0 publications

Effect of Substratum Surface Chemistry and Surface Energy on Attachment of Marine Bacteria and Algal Spores

Effect of Substratum Surface Chemistry and Surface Energy on Attachment of Marine Bacteria and Algal Spores

Determination of Surface pKa of SAM Using an Electrochemical Titration Method

Effect of Intramonolayer Hydrogen Bonding of Carboxyl Groups in Self-assembled Monolayers on a Single Force with Phenylurea on an AFM Probe Tip

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