Direct Quantification of Proteins Modified on a Polystyrene Microparticle Surface Based on ζ Potential Change

The zeta potential (zeta) of a particle is a surface charge density (sigma)-dependent parameter. If a change in sigma can be induced by surface modification, the number of molecules modified on the particle can be detected as a measurable change in zeta. In this study, we demonstrate protein detection at zmol or pM levels (bovine serum albumin (BSA), myoglobin (Mb), and lysozyme (Lyz)) on carboxy-functionalized polystyrene (PS) microparticles using the zeta change. Protein modification of the PS particles changes sigma because the negatively charged carboxy group is used for protein binding, and proteins also have charged amino acids. The pH dependence of zeta for the protein-modified particles at 4 < pH < 10 is well-explained using the acid dissociation of the acidic and basic amino acids and the Gouy-Chapman-Stern model. An increase in the binding number of proteins per single PS particle (n(pro/PS)) leads to a decrease in zeta, which is consistent with the results estimated by the proposed model. The detection limits of n(BSA/PS), n(Mb/PS), and n(Lyz/PS) are 1.17 X 10(4), 1.22 X 10(4), and 1.20 X 10(4) at pH 8.52, respectively, which means that the concentration-based detection limits are 722, 376, and 371 pM, respectively. We expect that the present method will be a strategy for the detection of molecules on particles.

Automated summary

In this study, protein detection at zmol or pM levels on carboxy-functionalized polystyrene microparticles using zeta change was successfully demonstrated. The pH dependence of zeta for the protein-modified particles was well-explained, and an increase in the binding number of proteins per single PS particle led to a decrease in zeta, consistent with the proposed model. The detection limits for different proteins were determined.