# Exploring the Potential of Sensing for Breast Cancer Detection
## 5 Biosensors for Breast Cancer Detection
### Fluorescence Biosensor  
If an external light is applied to fluorophores or fluorescent molecules, then the fluorophores or fluorescent molecules absorb this external light and are internally excited [114]. The excited fluorophores jump to higher unstable energy states [115]. Most of the time, the higher energy states of any system are inconsistent, and the fluorophores tend to return to the most stable lower energy states and become stable by releasing the excited energy. When the excited fluorophores tend to move the original lower stable energy states from the higher unstable conditions, the exciting energy is emitted by a fluorophore with a different color from the original absorbing light's color. The process of absorption, excitation, jumping to higher unstable states, and becoming stable after releasing and again jumping to the lower stable energy states is known as fluorescence. In fluorescence experiments, laser light is applied to fluorophores to excite them in a correct specific range [115]. After absorbing the energy from the laser light, these fluorophores jump to the higher energy states and again to the lower stable energy states. Finally, the detector detects the corresponding emitted light from the fluorophores [115]. In biological experiments, fluorescent dyes or antibodies have been used to detect the structure and processes of proteins, tissues, organisms, etc. The existence and number of fluorescent molecules can be determined using microscopy or flow cytometry techniques [115]. The fluorescence method is regularly considered due to its ultra-high sensitivity and process producing a quantitative result. DAPI, which has been used to detect DNA, and ethidium bromide, which has been used to intercalate DNA in agarose gels, are common fluorescent molecules. Green fluorescent protein can be found in jellyfish. A biological response has been converted to a measurable signal by a biosensor by combining enzymes or antibodies with a transducer. In that case, fluorescence molecules are considered as the transducer in fluorescence biosensors to produce a measurable signal.  
In [116], the authors developed a complex 3D polymeric network, called a hydrogel, by combining carbon dots for early detection of cancer cells. For this purpose, the authors fabricated a nanocomposite hydrogel and functionalized it with an ssDNA probe to detect human breast cancer MCF-7 cell lines. In that case, they considered the DNA hydrogel bioassay strategy. They observed fluorescence spectra for three nanocomposite hydrogels (viz., GA-CDs-CH, NB-CDs-CH, and NB-CDs-CH) for a different concentration of microRNA-21. For a particular condition, the decreased fluorescence intensity, and the logarithm of microRNA-21 concentration provided a linear relationship at 0.1 to 125 fM for GA-CDs-CH and NB-CDs-CH hydrogel and 0.1 to 26.3 fM for B-CDs-CH hydrogel. These nanocomposite hydrogels are applicable for multicolor imaging of MCF-7 cancer cells. In [117], the authors employed the fluorescence emission method to detect biomarkers (microRNAs, miR-21, miR-195, and miR-155). They considered label- and enzyme-free methods and fabricated a fluorescence reporter based on DNA-templated copper nanoclusters to detect biomarkers. They found a linearity range of 500 nM to 3 uM for a detection limit of 1.7 pM. This fluorescent spectrum method is suitable for detecting microRNAs without any linkers and enzymes.