# Exploring the Potential of Sensing for Breast Cancer Detection
## 5 Biosensors for Breast Cancer Detection
### Field-Effect Biosensor  
Semiconductor-based biosensors (SBBs) can be directly applied to extract information associated with different cancers from biological experiments [135; 136]. These SBBs have been designed to observe a sample's electrical property variation. The results that are found from these experiments are used to determine the status of the cancer and diagnose the condition. A field-effect transistor (FET) is a semiconductor device with three terminals: source, gate, and drain. The FET controls the conductivity between the drain and source by considering the electric field or the application of a voltage to the gate [135]. A field-effect biosensor (FEB) is a device manufactured by following the principle of the FET that can detect the existence of biological molecules, such as proteins or DNA, in a sample [137; 138; 139]. Currently, several FEBs are available, viz., ion-sensitive field-effect transistors, metal-oxide-semiconductor field-effect transistors, organic field-effect transistors, and nanowire field-effect transistors [140]. Currently, ultra-sensitive nanowires can be utilized for developing lab-on-a-chip devices. The epitaxial method has been employed by considering nanowires for designing three-five SBBs. In that case, a metal droplet is deposited over the surface of the semiconductor, and nanowires vertically grow from the metal droplets [140; 141; 142]. The nanowire behaves like a junction, and due to the ultra-sensitive properties of nanowires, it can easily capture and transmit the optical and electrical signals to the next portion of the device. Nanowires are so thin that the free charges exist at the nanowires' surface [140; 141; 142]. So, when target molecules contact the nanowires, the surface free charges from the nanowires bond to the target molecules, and a change in the nanowires' surface charge density is observed. So, the interaction between the nanowires and target molecules can change the surface charge density of the nanowires, and this change in the charge density can indicate the existence of different biomarkers [140; 141; 142]. So, the idea of FET is an ultra-sensitive approach to recognizing the existence of specific biomarkers, and a carbon nanotube (CNT) FET device with high sensitivity down to the fM range can be regenerated in an acidic environment, enabling the reusability of the sensor device [140; 141; 142].  
In [135], the author considered a DNA-functionalized CNT-based FET biosensor in which gold nanoparticles are considered linkers to anchor probe molecules to detect breast cancer exosomal miRNA. Initially, exosomes are collected from the breast, and specific miR21 is extracted from the exosomes. Hybridizing the target miRNA and immobilized DNA probe establishes a current flow. So, before and after the hybridization, the variation in the current flow can predict the existence of exosomal miRNA21. The CNT-based FET biosensor can directly convert the hybridization of biomolecules with a sensing interface into an electrical signal.  
In [136], a single-nanotube FET array was considered to detect a change in the electrical signals due to the development of a bond between the receptor proteins and their corresponding antibodies. In that antibody device experiment, the authors considered two antibodies (IGF1R and Her2) at different levels. They established an interaction with intact MCF7 and BT474 collected from human breast cancer cells, and a 60% conductivity decrease was observed when BT474 or MCF7 breast cancer cells interacted with IGF1R and Her2 antibodies. The antigen-antibody interaction changed the system's free energy, increased the band gap, and decreased the system's conductivity. A single cell with a diameter 1000 times smaller than a cancer cell produced a bond with a single-nanotube FET, changing the conductivity.