Investigation of DMOAP coated Glasses
To observe the success of DMOAP coatings, 2 µl of 5CB liquid crystal was
dropped onto DMOAP- coated microscope glass slide. Figure. 2 a. shows
the LC structure can be seen on the non-coated DMOAP glass slide. Figure
2 b, c, d show the LC structure cannot be seen on the DMOAP coated glass
slides, since LC molecules have homeotropic orientation through the
DMOAP coating.
Determination of Optimum Anti-amyloid beta-42 (Aβ42) Antibody
Concentration
In order to determine the optimum antibody concentration, six different
samples, which are shown in Table 1, were prepared and then each sample
was dropped onto the LC film surface. Before Aβ42 antibodies interacted
with Aβ42 peptides, DMSO was dropped onto the Aβ42 antibody surface. No
effect of DMSO on the orientation of liquid crystals has been observed.
Subsequently, antibodies at these six different concentrations were
interacted with three Aβ42 peptide concentrations of 100 pg/ml, 50 pg/ml
and 25 pg/ml, respectively.
The formed antibody-antigen immune complex leads to a change in the
orientation LCs from homeotropic to a random one. The changes
corresponding to the random orientation of the LC molecules were
observed by a polarized optical microscope (POM). The POM images showed
that a notable change when the samples including the Aβ42 peptide
concentrations of 100 pg/ml, 50 pg/ml, 25 pg/ml interacted with 100
µg/ml, 50 µg/ml and 25 µg/ml Aβ42 antibody concentrations as seen in
Figure. 3, Figure. 4, Figure 5. The POM images prove that 25 µg/ml Aβ42
antibody concentration can be selected (Sample 3) as the optimum
antibody concentration.
Then the selected optimum antibody concentration was interacted with
different Aβ42 peptide concentrations as of 1000 pg/ml, 500 pg/ml, 200
pg/ml, 100 pg/ml, 50 pg/ml, 25 pg/ml, 20 pg/ml, 15 pg/ml, 10 pg/ml, 5
pg/ml, and 1 pg/ml, which are listed in Table 2, to determine the
detection limit of the biosensor.
Detection of Amyloid beta-42 (Aβ42) Peptide Concentration
1 µl of 25 μg∕ml anti- Aβ42 antibody was immobilized on the glass with
DMOAP alignment layer and then interacted with various concentrations of
Aβ42 peptide for 1.5 hours at room temperature. Figure 6 and Figure 7
show that the changes in the orientation of LC direction determined via
a POM. In the absence of peptides or antibodies, the images as seen in
Figure 6a-e & Figure 7f-k were black because the alignment of nematic
LCs was perpendicular to the glass surface with the DMOAP monolayer.
Figure. 6a1-e1 & Figure 7f1-k1 show that the immobilized Aβ42
antibodies were insufficient to change the homeotropic alignment of LCs.
As the Aβ42 peptide concentration was gradually increased, the number of
antigen-antibody immunocomplexes also increased, which caused
significant orientational changes in LCs alignment due to their larger
size of the immunocomplex (Figure 6a2-e2 & Figure 7f2-k2). Therefore,
the optical textures of LCs have changed significantly to reflect the
amount of available immunocomplexes. This LC - based optical biosensor
was examined to determine the minimum concentration of Aβ42 peptide that
could induce an orientational change of LCs. The biosensor was incubated
with Aβ42 peptide concentrations in the range of 1000 pg/ml - 1 pg/ml.
The change in LC optical textures was considerable when the biosensor
was incubated with all the concentrations of Aβ42 peptide as shown in
Fig. 6 and Fig. 7. Moreover, these results indicate that a minimum
concentration of 1 pg/ml Aβ42 peptide was required to change of LCs
orientational on the surface. Therefore, the LOD of this LC-based
optical biosensor was determined as 1 pg/ml Aβ42 peptide. Dai and
co-workers fabricated that simple in vitro biosensor for the detection
of β-amyloid 42 in phosphate-buffered saline (PBS) and undiluted human
serum. The concentration range of β-amyloid 42 in this study was from
0.0675 µg/ml to 0.5 µg/ml. Yu and co-workers have been developed
gelsolin-bound-Aβ1–40/1–42 detection assay for detecting level
variations of Aβ1–40/1–42 associated with AD progress. Compared to
other studies, a very low detection limit was achieved in our optical
sensor.