Near-Field Detection of Aβ Proteins Using Micro Beads

Abstract

Alzheimer's disease (AD) is a critical social problem as the number of older aged individuals is increasing in society. It is classified by a common cause of dementia, which is a loss of cognitive function and a behavior disorder. Also, AD is characterized by two types of structures in patient's brains: senile plaques and neurofibrillary tangles (1). Senile plaques are deposits of Aǰ protein in the central nervous system, and neurofibrillary tangles are an accumulation of hyperphosphorylated tau protein in the neuron (2,3). Many senile plaques in the brain or especially detected in Alzheimer's patients, accelerating nerve cell death. An early diagnosis for AD is the pathologically progressed quantitative detection of Beta-amyloid (Aǰ ) deposits. The main isoforms of Aǰ found in AD patients brain are Aǰ40 and Aǰ42 peptides of amino acids. The concentration of Aǰ42 is approxi- mately 10% of that regarding Aǰ40 and the ratio of Aǰ42 to Aǰ40 may be discriminating patients with AD (4). The detection and real-time monitoring of Aǰ , in vivo, is widely used Thioflavin-T (Th-T) (5). The dye Th-T can be easily binded to Aǰfibril deposits that are 4-10 nm in diameter. Then, it fluoresces strongly with emission at 490 nm. However, Th-T ineffectively passes the blood-brain barrier (BBB) and improved Th-T based compounds have been reported (6). Meanwhile, detection and capture of Aǰ outside the central nervous system has been under study. For example, Aǰdeposits are observed within the blood, olfactory bulb, and saliva (7-9). The results of those studies represent that, in vitro diagnosis of AD is possible. Quantitative detection of Aǰhas been attempted to Enzyme-linked immunosorbent assay (ELISA) (10). In this paper, we propose the possibility of quantification analysis for Aǰusing Near-filed detection. Aǰare selectively captured using treated micro beads to form antigen-antibody complexes. Near-field generates the evanescent field upon the surface from the prism with the incident angle of total internal reflection. Micro particles, which are on the evanescent field, generate the illumination through the internal reflections inside the particle. Therefore, micro bead or Aǰdeposit does not require any tagging materials for detection in this system. This method can be used for not only quantitative analysis for the number of Aǰ , but the binding ratio of Aǰto micro beads as well. Abstract In this paper, we present the possibility of quantification analysis for Aǰ captured by micro beads using Near-filed detection. In order to evaluate detection efficiency, detected signals were compared with different sizes of micro beads and a varied number of micro beads. Also, Aǰdeposits and Aǰbinding to micro beads were measured, therefore, we observed the Aǰdeposit and light scattering around the surface of micro beads induced by attached Aǰ . This method can be used for quantitative analysis for not only the number of Aǰ , but also the binding ratio of Aǰ to micro beads.