The Effective Charge of Low-Fouling Polybetaine Brushes

Abstract

Polybetaine nanobrushes are widely used as inert platforms for label-free biosensing due to their resistance to nonspecific interactions. Despite being considered cationic or electrically neutral, polybetaines can exhibit a negative zeta potential (ZP) at pHs above their isoelectric point (pI). To clarify whether negative zeta potential effectively contributes to surface interactions, we examined three types of nanobrushes deposited on a planar gold substrate: two polybetaines: poly(carboxybetaine methacrylamide) (pCBMAA) and poly(sulfobetaine methacrylamide) (pSBMAA) and hydrophilic poly[N-(2-hydroxypropyl) methacrylamide] (pHPMAA), which carries no ionic group. All three brushes exhibit a well-defined pI and negative surface ZP at pHs above their pI. The pH dependence of the interactions of these brushes with anionic dextran sulfate (DS) and cationic poly[(N-trimethylammonium)ethyl methacrylate] (PTMAEMA) was monitored by infrared reflection spectroscopies (infrared reflection absorption spectroscopy (IRRAS), grazing angle attenuated total reflectance (GAATR)). DS adsorbs to pCBMAA strongly and only weakly to pSBMAA at pHs below their pI but can adsorb slightly to both polybetaines even at pHs above their pI. This is due to the displacement of their carboxylate or sulfo groups from the interaction with the quaternary ammonium cation by the DS sulfate groups. However, DS does not adsorb to pHPMAA at any pH, and PTMAEMA does not adsorb to any of the brushes, regardless of pH. These findings highlight that zeta potential determinations alone may not be sufficient to predict electrostatic interactions as the apparent negative charge does not necessarily translate into a functional surface charge influencing macromolecular interactions.

1. Chemical Structure of Poly­(carboxybetaine methacrylamide) (pCBMAA), Poly­(sulfobetaine methacrylamide) (pSBMAA), and Poly­[N-(2-hydroxypropyl) methacrylamide] (pHPMAA) Employed in This Study

1

pH dependence of surface zeta potential of the pCBMAA brush measured by the EK method and EPM method.

2

pH dependence of zeta potential of pSBMAA (left) and pHPMAA (right) brushes measured by the EPM method.

3

IRRAS spectra of pCBMAA, pSBMAA, and pHPMAA brushes.

4

IRRAS spectra of DS and PTMAEMA (a drop of water solution evaporated on a gold chip).

5

IRRAS spectra of pH dependence of DS adsorption to the pCBMAA brush.

6

Integral band intensities of the DS sulfate group at 1266 cm^–1 (a), ionized carboxyl groups at 1611 cm^–1 (b), and protonated carboxyl groups at 1739 cm^–1 (c) of pCBMAA vs pH.

7

IRRAS integral band intensities of ionized carboxyl groups at 1611 cm^–1 of pCBMAA in the presence and absence of DS vs pH.

8

IRRAS spectra of pCBMAA at pH 8.5–3.0 in the absence of DS and the pCBMAA spectrum at pH 3.0 in the presence of DS.

9

IRRAS spectra of the initial pCBMAA brush, brush with DS adsorbed at pH 3.0, and brush with DS adsorbed at pH 3.0 and then rinsed with water. There is no sign of pCBMAA degrafting upon DS adsorption and desorption.

10

GAATR spectra of the pCBMAA brush scanned under wet conditions at different pH values.

11

IRRAS spectra of pH dependence of DS adsorption on the pSBMAA brush, indicating a weak DS adsorption at all pH values.

12

IRRAS spectra of pH dependence of DS interaction on the pHPMAA brush, indicating no DS adsorption at any pH.

13

IRRAS spectra of pH dependence of PTMAEMA interaction with gold (a), pCBMAA (b), pSBMAA (c), and pHPMAA (d). There is a weak PTMAEMA adsorption on gold but not on the brushes at any pH.

14

Schematic illustration of surface interactions of pCBMAA, pSBMAA, and pHPMAA brushes with charged macromolecules.