4. Sensing transmembrane receptors for cancer diagnosis
In this section, we report on the use of gold-coated TFBGs for
selective cellular detection through membrane protein targeting.
The focus is made on the epithelial growth factor receptor (EGFR),
which is a transmembrane receptor from the 4-tyrosine kinase
receptors family. It is an important biomarker and therapeutic
target that it is over-expressed by numerous cancer cells.
The sensor surface selectivity was ensured by bio-functionalization
through a two-step approach. First, the clean waveguide
surface was activated with the Carboxylic acid-SAM formation
reagent (cat. n: C488) obtained from Dojindo (Japan). Then,
monoclonal mouse immunoglobulin G (IgG) raised against the
human epidermoid carcinoma cell line (Santa Cruz Biotechnology
Inc. and American Type Culture Collection (USA)) were immobilized
on the surface through carbodiimide covalent biochemistry using
Dojindo’s amine coupling kit (cat. n: A515-10). This antibody was
diluted in the Dojindo’s reaction buffer to 0.01 mg/mL for efficient
covalent immobilization during 30 min. The manufacturer’s instructions
were followed for each step for both processes.
Two cell lines were used: the first one with overexpressed
human epidermal growth factor receptors (EGFRs)-A431 cell line,
(EGFR positive, EGFR (þ)). The second cell line was EGFR negative
– OCM1 cell line, (EGFR ()). The culture process will not be described
here. It was done at 37 °C in a humidified incubator with
5% CO2 atmosphere. No antibiotic was used and cell cultures were
free of mycoplasma and pathogenic viruses. Cells suspensions
were then prepared for SPR experiments. For this, A431 and OCM1
cells from a confluent monolayer were detached mechanically by a
gentle scraping of cells from the growth surface into ice-cold
phosphate-buffered saline (PBS). Cells were then washed three
times in cold PBS by repeated centrifugation (at 2500 RPM and
4 °C for 3 min). Pellets were then again suspended to a concentration
of 2–5 106 cells/mL. 0.5 mL volumes were use for
experiments. A handheld automated cell counter from Millipore
(Scepter 2.0, PHCC00000) was used to count the cells.
For immunosensing experiments, gold-coated TFBGs were hold
straight between two clamps and were then approached towards
the cells suspensions, using a vertical translation stage. Fig. 5 depicts
a picture taken with a microscope (100 magnification)
positioned above the cells suspension when the fiber section
containing the TFBG comes into contact with the cells. This picture
figures out the actual size of the cells, which is typically 2–3 mm.
With respect to this size, it is particularly important to guarantee a
sufficient penetration depth of the plasmon wave (evanescent
field) at the metal-dielectric interface. As the latter is proportional
to a fraction of the operating wavelength [4], near-infrared TFBGs
appear more competitive than other configurations based on
visible light.