We report a new sensitive bead-based chemcial assay based on sensitivity of the red blood cell dielectrophoretic mobility to chemcial modification of its surface proteins. We demonstrate a glutaraldehyde cell-crosslinking fixation and cell membrane-reducing sodium borohydride condensation reaction that allows for sensitive titration-like adjustment of cell membrane conductivity and quantification of membrane surface amine groups consistent with the aldehyde-amine stoichiometry. Due to the core-shell nature of the highly conductivity cytoplasm and surrounding low conductive cell membrane, untreated red blood cells have two measurable cross-over frequencies. It is shown that the addition of glutaraldehyde cross-links the surface amine groups which render the membrane more conductive than the suspending electrolyte, and is illustrated by the observation of only one cross-over frequency and an increase in measured cell zeta-potential. Through the addition of sodium borohydride, a condensation reaction takes place at the nitrogen-carbon double bond, which is responsible for a decrease in membrane conductivity and an observed decrease in cell zeta potential. This is seen through the existence of two cross-over frequencies, a condition that can only exist when the cell membrane has a lower conductivity that the surrounding media. The concentration of the chemical can be calibrated to the shift in the cross-over frequencies via reaction stoichiometry. This work could hence lead to a new dielectrophoretic chemical sensing platform with blood cell probes and pico-molar sensitivities.