INTRODUCTION
Liquid crystals (LCs) are excellent detection materials in which the intrinsic properties of long-range orientational order and short-range molecule interactions can be used to convert biomolecular binding events into visible macroscopic optical signals (Brake, Daschner, Luk, & Abbott, 2003; Gupta, Skaife, Dubrovsky, & Abbott, 1998; Tan, Li, Liao, Yu, & Wu, 2014). These materials have unique properties such as self-orientation, dielectric, optical anisotropy and light-transmitting (Gupta et al., 1998; Kim, Kim, Kim, Oh, & Choi, 2005; S. Yang et al., 2012). Furthermore, the orientational changes of the LCs can be easily observed under a polarized optical microscope due to their unique birefringence (Artiga et al., 1998; Koenig Jr et al., 2009; Li, Li, Yang, Chen, & Xiong, 2015). Therefore, the LC-based biosensor system has been recently became a promising platform for chemical and biological detection, which can be used for real-time and unlabeled detection with high sensitivity and without any complex techniques (Li et al., 2015; Zapp, Westphal, Gallardo, de Souza, & Vieira, 2014). In 1998, Abbott and coworkers initiated a field of study using LCs as sensing elements in the detection of biomolecules (Gupta et al., 1998; S. Yang et al., 2012). In recent days, LC-based biosensors are widely used in organophosphate detection, enzymatic activity assays, bacteria, and viruses detection and to investigate protein-protein binding events (Cadwell et al., 2007; Chen & Zhong, 2013; Clare & Abbott, 2005; Li et al., 2015; Park & Abbott, 2008; K.-L. Yang, Cadwell, & Abbott, 2005; Zhong & Jang, 2014; Zhu, Shih, & Shih, 2013)
LC-based biosensors are widely used in the detection of neurodegenerative disorders, such as Parkinson’s, Alzheimer’s diseases. Alzheimer’s disease (AD) is an age-related, progressive and unremitting form of neurodegenerative disease that is estimated to affect 1 in 85 people worldwide by 2050 (Brookmeyer, Johnson, Ziegler-Graham, & Arrighi, 2007; C.-C. Liu, Kanekiyo, Xu, & Bu, 2013; Masters & Bateman, 2015). This disease induces severe loss of memory and cognitive decline, selective neuron death, and abnormal plaque formation in the cerebral cortex (Hardy & Selkoe, 2002; Y. Liu et al., 2014; Yu et al., 2015; Zhao, Wu, Xie, Ke, & Yung, 2010). However, thus far, no effective treatment for Alzheimer’s disease has been found and developed (Yu et al., 2015). The neuropathological hallmarks of AD are the deposition of extracellular neuritic plaques containing amyloid-beta peptide (Aβ) and intracellular neurofibrillary tangles (NFTs) (L. Liu et al., 2013; Rauk, 2009; Rolinski, Amaro, & Birch, 2010; Yu et al., 2015). Experimental evidence from both in vitro and in vivo studies have shown that amyloid-beta (Aβ) aggregation induces pathogenic cascade that leads to neuronal loss and dementia at the onset of Alzheimer’s disease (Glenner & Wong, 1984; Masters & Bateman, 2015; Yu et al., 2015). Aβ fragment is a normal product of Amyloid Precursor Protein (APP) metabolism (Masters & Bateman, 2015). APP is a transmembrane protein located in neuronal and glial cells of the brain (Müller & Zheng, 2012). Under physiological conditions, the APP sequentially cleaved by α secretase and β secretase enzymes, resulting in the formation of extracellular non-toxic soluble fragment and APP intracellular domain (AICD) (Overk et al., 2014; Schott et al., 2016). Under pathological conditions, the APP protein is cleaved by β secretase (BACE) and γ secretase enzymes and extracellular amyloid- beta-42 (Aβ42) fragment monomers are formed (Cole & Vassar, 2008). These Aβ42 monomers then draw closer to form β-sheets structure (Breydo et al., 2016). The sheets ultimately form ordered fibrils known as toxic Aβ42 plaques ( Roeters et al., 2017). The formed Aβ42 plaques accumulate in the synapses of the brain, leading to loss of signal transmission between the connecting neurons (Haass & Selkoe, 2007; Madav et al., 2019). Therefore, the Aβ42 monomer and its aggregates are considered promising important biomarkers in plasma and cerebral spinal fluid (CSF) for the diagnosis and prognosis of AD (Ammar et al., 2013; Choi, Islam, Lee, Song, & Oh, 2011; Doong, Lee, & Anitha, 2010; Golde, Eckman, & Younkin, 2000; Haes, Chang, Klein, & Van Duyne, 2005; Li et al., 2015; L. Liu et al., 2013). According to recent clinical studies, it is known that the Aβ42 level in the plasma of Alzheimer’s patients is 36-140 pg/ml, and the Aβ42 level in the CSF is 652 ± 235 pg/ml (Fagan et al., 2009; Roher et al., 2009). Currently, the detection of Aβ42 is performed using different techniques, such as mass spectrometry (MS), scanning tunneling microscopy (STM), surface plasmon resonance (SPR), electrophoresis and the enzyme-linked immunosorbent assay (ELISA) (Dai, Molazemhosseini, & Liu, 2017; Haes et al., 2005; Hestekin, Kurtz, & Lutz-Rechtin, 2014; Kang, Lee, Oh, & Choi, 2009; Picou, Moses, Wellman, Kheterpal, & Gilman, 2010; Wang, Sweeney, Gandy, & Sisodia, 1996). ELISA technique is commonly used for Alzheimer’s detection due to its high sensitivity, selectivity and reliability features. However, this method is expensive, time-consuming, labor-intensive, elaborate and requires a skilled operator (Dai et al., 2017; Yu et al., 2015). The fact that the current techniques used for the detection of Aβ, both physiologically and pathologically, has many disadvantages that encourage the researchers to develop a biosensor which is faster, more sensitive, selective and easily applicable detection on the patient.
In this study, we have established a new LC-based approach for the highly sensitive detection of Aβ42. In the first stage of our study, Dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium chloride (DMOAP) was coated on a glass surface of microscope slides in order to obtain the vertical alignment of LC molecules. The LC molecules were then dropped onto DMOAP coated glass slides and a LC film was formed. The different concentrations of Aβ42 antibody and Aβ42 peptide were prepared. Afterward, the Aβ42 antibody immunocomplex was formed immobilizing the Aβ42 antibody and Aβ42 peptide on the LC film surface. The binding of immunocomplex to LC molecules leads to a change in the orientation of the LC which can be observed via a polarized optical microscope (POM). After observation of the optical textures by using POM, spectrometric analysis was performed between 200nm and 900 nm via Ocean Optics spectrometer and the detection limit of our biosensor was determined.