# Exploring the Potential of Sensing for Breast Cancer Detection
## 5 Biosensors for Breast Cancer Detection  
### Surface Plasmon Resonance Imaging  
The surface plasmon resonance (SPR) method has been used to study molecular interactions in real time [90; 91; 92; 93]. The direction of light propagation is rigorously dependent on the medium, especially on the density of the medium in which light is propagated and the associated incident angle. When light propagates to the interface of two media, the light can undergo total internal reflection due to the specific incident angle and the density of both media [93]. For a plain polarized light undergoing total internal reflection, if a thin piece of metal is placed on the top, then some refracted light is absorbed. Due to this absorption, a dark zone appears in the refracted light. This absorption of light is carried out by the electrons present in the metal. Because of the absorption of energy, the electrons start oscillating. It is not a single electron, but a group of electrons collectively absorbing the energy of light [93]. These electrons undergoing oscillation because of energy absorption are called plasmons. The refractive index (RI) associated with the medium near the metal surface is responsible for changing the surface plasmon resonant angle, and a tiny change in RI can cause a significant change in the SPR angle [93]. SPR is widely used to study the interaction between two molecules, for example, the interaction between a ligand and its receptor. The interaction between a ligand and its receptor can be easily detected just by measuring the change in SPR angle. Biomolecules on a surface interaction between DNA and RNA can also be detected [93]. Figure 2 represents the experimental setup of SPR.  
In [90], the authors developed an SPR immunosensor, in which gold nanoparticles were used to amplify the signal, to detect the ErbB2 biomarker associated with breast cancer, and they observed the following: (a) a limit of detection (LOD) of 180 pg mL\({}^{-1}\); (b) from breast cancer cell line graphs, it was shown that for different levels of ErbB2 (MDA-MB-436, SK-BR-3, MCF-7, etc.), the SK-BR3 cell line contains approximately 10 times more copies of the ErbB2 gene than both MDA-MB-436 and MCF-7 cell lines. In [91], the concentration of a carcinoembryonic antigen was determined by employing an SPR immunosensor, and the findings were as follows: (a) 3 \(\mu\)L of serum or plasma sample is required; (b) the LOD is 0.12 ng mL\({}^{-1}\), and the quantification limit is 0.40 ng mL\({}^{-1}\); (c) a linear response was observed from 0.40 ng mL\({}^{-1}\) to 20 ng mL\({}^{-1}\). This method is substantially suitable for the detection of cancer biomarkers. In [95], a photonic crystal fiber (PCF) biosensor based on SPR was developed, and the authors observed a high optical sensitivity of 6214.28 nm/RIU for the quasi-transverse magnetic mode and an optical sensitivity of 6000 nm/RIU for the quasi-transverse magnetic mode for breast cancer cells. In [96], the authors designed an SPR-based PCF biosensor to detect a cancer biomarker and found that (a) the refractive index (RI) varied from 1.392 to 1.401 for six different types of cancerous cells and (b) the maximum sensitivity was 7142.86 nm/RIU for MCF-7.  
Figure 2: SPR experimental setup [94].