Coimmunocapture and Electrochemical Quantitation of Total and Phosphorylated Amyloid-β Monomers.

Phosphorylated proteins play important roles in the pathogenesis of Alzheimer's disease (AD). The most abundant constituent in AD's brain deposit is the amyloid-β peptide (Aβ). Based on it, the degree of phosphorylated Aβ in body fluids (e.g., cerebrospinal fluid, CSF), which is defined by the ratio of phosphorylated Aβ to total Aβ (pAβ/tAβ), is anticipated to be an index for early diagnosis of AD. The major challenge in pAβ/tAβ detection is the large concentration difference between two Aβ forms in the real samples, which usually requires multichannel equipment and complicated detection process. In this paper, we revealed the unexpected close affinities of the anti-Aβ antibody to Aβ (40.2 nM) and to pAβ (42.3 nM). Based on it, a convenient coimmunocapture and electrochemical quantitation of tAβ and pAβ was achieved on an anti-Aβ antibody immobilized Au electrode (anti-Aβ/Au). Once Aβ and pAβ were synchronously captured on the anti-Aβ/Au electrode, the tAβ levels in CSF samples were quantified with electrochemical impedance spectroscopy. With the signal amplification from Cd/Ti-functionalized titanium phosphate nanospheres (Cd/Ti@TiP) which was selective conjugated to pAβ, concentrations of low abundant pAβ as low as 1 fM were readily measured by square wave voltammetry. Our results reveal that despite the concentrations of tAβ and pAβ fluctuate in each individual case, the concentration ratios of pAβ/tAβ in CSF samples from AD patients are significant larger than those from healthy donors. It demonstrates that the degree of phosphorylated Aβ is hopeful to be an effective index for evaluating the AD progress and improving the accuracy of clinic AD diagnosis.