Determination of the Surface pK of Carboxylic- and Amine-Terminated Alkanethiols Using Surface Plasmon Resonance Spectroscopy

Abstract

When using self-assembled monolayers (SAMs) with ionizable functional groups, such as COOH and NH2, the dissociation constant (pKd) of the surface is an important property to know, since it defines the charge density of the surface for a given bulk solution pH. In this study, we developed a method using surface plasmon resonance (SPR) spectroscopy for the direct measurement of the pKd of a SAM surface by combining the ability of SPR to detect the change in mass concentration close to a surface and the shift in ion concentration over the surface as a function of surface charge density. This method was then applied to measure the pKd values of both COOH- and NH2-functionalized SAM surfaces using solutions of CsCl and NaBr salts, respectively, which provided pKd values of 7.4 and 6.5, respectively, based on the bulk solution pH. An analytical study was also performed to theoretically predict the shape of the SPR plots by calculating the excess mass of salt ions over a surface as a function of the difference between the solution pH and surface pKd. The analytical relationships show that the state of surface charge also influences the local hydrogen ion concentration, thus resulting in a substantial local shift in pH at the surface compared to the bulk solution as a function of the difference between the bulk solution pH and the pKd of the surface.

Kelvin-probe force microscopy of the pH-dependent charge of functional groups

“…A pK a of 6.5 was reported for NH 2 -(CH 2 ) 11 -SH monolayers on Au and a pK a of 7.4 for COOH-(CH 2 ) 11 -SH monolayers on Au by surface plasmon resonance spectroscopy. 24 For the shorter chain length we used in the present work, Marmisolle et al recently reported a pK a of NH 2 -thiol (6-amino-1-hexanethiol) of approximately 7 for monolayers on Au measured by titration and impedance spectroscopy. 25 Thus, we can reasonably assume that our patterns have similar pK a 's, which means that, in terms of their ionization state, we would expect predominantly positively charged NH 3 þ -groups at low (acidic) pH and predominantly neutral NH 2 -groups at high (basic) pH.…”

Comparison Between Empirical Protein Force Fields for the Simulation of the Adsorption Behavior of Structured LK Peptides on Functionalized Surfaces

“…The two SAM surfaces used in this study were represented by a hydrophobic, methyl-terminated SAM (CH 3 -SAM) and a hydrophilic, negatively charged carboxylic acid-terminated SAM (COOH-SAM). These surfaces were constructed using methods that Latour and coworkers [16][17][18][19][20][21][22] and others [23][24][25][26][27][28] have used in several previous simulation studies involving SAMs. The SAM surfaces were each represented as consisting of functionalized alkyl chains of 10 carbons, including the functional group carbon.…”

“…The SAM surfaces were each represented as consisting of functionalized alkyl chains of 10 carbons, including the functional group carbon. 50% of the carboxylic acid groups of the COOH-SAM were deprotonated, as appropriate for the experimentally determined pK a value of 7.4 [22]. To maintain neutrality of the systems with the COOH-SAM, 60 additional Na ?…”

Force fields for simulating the interaction of surfaces with biological molecules

“…For this reason, in cases where there are charged groups on the surface, atomic force microscopy has been used to determine the surface potential and through this, the area density of surface charges [68]. Similarly, for SAMs where the composition is known a priori, but can also be confirmed with XPS, surface plasmon resonance (SPR) can measure the pKa of the surface functional groups and help in modelling the correct area density of charges under different pH conditions [69].…”