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.