# Exploring the Potential of Sensing for Breast Cancer Detection
## 5 Biosensors for Breast Cancer Detection
### Surface-Enhanced Raman Spectroscopy  
The Raman spectroscopy (RS) [99] technique has been employed to study the intrinsic properties of molecules and molecular mixtures in a liquid, slurry, paste, or solid phase. The vibrational transition of molecules has been considered to examine the structural properties of molecules by considering Raman spectroscopy, and a similar method has also been considered for infrared spectroscopy (IR) [100]. The basic difference between RS and IR is that RS is related to scattering while IR is related to absorption [101]. There are two types of scattering, Rayleigh, and Raman [101]. Rayleigh is elastic scattered energy, meaning the frequency of the scattered photon is the same as the excitation photon frequency. Rayleigh scattering is inadequate for finding detailed information about the chemical composition of different molecules. Ramen scattering, however, is inelastic photon scattering and is adequate for finding detailed information about the chemical composition of other molecules. RS requires two steps to occur for a molecule to Raman scatter. In the first step, the excitation-source photons excite the molecule into a virtual energy state. In the second step, the molecule relaxes by releasing scattered photons to a ground state. RS is rare, with only 1 in 10 million scattered photons being Raman scattered. Current technological advancements associated with solid-state lasers, gratings, and detectors make RS valuable for identifying and monitoring compounds in a reaction or crystallization. RS can be categorized in two ways. The first is Stokes RS, where scattered photons have higher energy than the incident exciting photons. The second is anti-Stokes RS, where scattered photons have lower energy than the incident exciting photons. Stokes RS generally has been considered for designing current Raman instruments. The interaction between the laser and the structure of the different chemical bonds among other atoms is responsible for changing the frequency of the incident photon. RS can address the specific frequency changes after the interaction of the incident laser with the chemical bond of the molecules and has been applied to recognize the bond structure of different molecules. The graph of Raman shift versus intensity associated with different chemical bonds among the molecules can express the deviation of the frequency of the scattered photon from the incident photon. The intensity associated with the Raman signal increases with the increase in the value of a chemical bond number. So, RS is a quantitative treatment for the chemical bond number. Additionally, when coupling a probe to a Raman system, measuring these bonds and their changes in real time is possible. This is valuable for understanding reaction characteristics such as kinetics initiation, endpoint, intermediates, crystal form, molecular backbone, andmechanical information. The Raman scattering mechanism and the outcomes of experiment can be seen in Figure 3.  
In [100], the surface-enhanced Raman scattering (SERS) technique was utilized to determine the existence of breast cancer cells/stem cells. For their experiment, the authors considered two material types, namely carbon and gold nanoparticles. The concentration of \(100\ \upmu\text{g}/1\times 10^{4}\) exhibited a 5.4 increment for breast cancer cells and a 4.8-fold increment for stem cells associated with breast cancer. In [103], the authors studied the heterogeneity of HER2 associated with breast cancer. In that case, they observed four different cell lines, viz., SKBR3, MCF-7, T47D, and MDA-MB-231, from high to low HER2 expression levels. In [103], the authors suggested a functionalized silver nanostructured plate with the SERS technique, and this platform could be used for cells with different HER2 expression levels and is mostly based on the identification of the most prominent Raman peak at 1002/cm.