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

The zeta potential (ζ) of a particle is a surface charge density (σ)-dependent parameter. If a change in σ can be induced by surface modification, the number of molecules modified on the particle can be detected as a measurable change in ζ. 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 ζ change. Protein modification of the PS particles changes σ because the negatively charged carboxy group is used for protein binding, and proteins also have charged amino acids. The pH dependence of ζ 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 (<i>n</i>_pro/PS) leads to a decrease in ζ, which is consistent with the results estimated by the proposed model. The detection limits of <i>n</i>_BSA/PS, <i>n</i>_Mb/PS, and <i>n</i>_Lyz/PS are 1.17 × 10⁴, 1.22 × 10⁴, and 1.20 × 10⁴ 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.