# Nanomaterials for Healthcare Biosensing Applications
## 1 Introduction  
The International Union of Pure and Applied Chemistry (IUPAC) defines a biosensor as, \"a device that uses specific biochemical reactions mediated by isolated enzymes, immunosystems, tissues, organelles or whole cells to detect chemical compounds, usually by electrical, thermal or optical signals\" [1]. This definition facilitates an insight into what a biosensor entails. Biosensors have three imperative constituents [2]:  
1. A receptor that specifically binds to an analyte;
2. A transducer that generates a signal following the binding event;
3. A detection system to quantify the signal and transform it into utile information. These detection methods may be electrochemical, optical, or piezoelectric in nature. In contrast to conventional screening techniques, such as enzyme linked immunosorbent assays (ELISA), biosensors can be fully automated, show enhanced reproducibility, allow real-time and rapid analysis, and often show a possibility for re-use as a result of surface regeneration [3]. Biosensing plays a key role in a multitude of fields, such as medical diagnostics [2, 4], food toxicity [5], fermentations [6], environmental safety [7], biodefense [8], and plant biology [9]. Ischaemic heart disease, lung cancer, cirrhosis, and similar infectious diseases are the leading causes of death worldwide [10]. Successful and inexpensive remedies are impeded by a lack of early diagnosis. Biosensors have thus gained prominence in the field of healthcare diagnostics by providing user-friendly, economical, reliable, and rapid sensing platforms [2]. Biosensing technology has considerable merits in comparison to conventional detection techniques involving spectroscopy or chromatography. These include an elimination of the need for skilled operating personnel, quicker response times, portability, and higher sensitivity [3]. For instance, the required detection time of pathogens such as anthrax has reduced from 2-3 days to 5 min with the help of modern biosensors [11]. Materials with at least one of their dimensions measuring 1-100 nm are termed nanomaterials [12]. Due to their small size, most of their constituent atoms or molecules are located on the surface of such materials, giving rise to remarkable distinction in their fundamental physicochemical properties from the bulk of the same materials. Another factor causing significant differences in the characteristics of nanomaterials is the quantum effects arising from discontinuous behaviour because of the quantum confinement of delocalised electrons. Since the number of atoms on the surface of these nanoparticles is much higher than the bulk, they show less binding energy, thus exhibiting a lower melting point. The shape of these particles is crucial to their properties. For instance, nanorods may have significantly different properties to nanospheres of the same material [2]. The increased surface area per unit mass also results in an approximately 1000-fold increase in the chemical reactivity [13]. Synthetic nanostructures such as quantum dots rely on the exploitation of the quantum effects observed in nanoparticles. They act as artificial atoms, since their electronic behaviour is very similar to that of small molecules or individual atoms, as the spatial confinement of electrons at nanoscale generates a quantised energy spectrum. Similarly, owing to multiple unpaired electron spins from hundreds of atoms, nanoparticles possess magnetic moments, showing their best performance at 10-29 nm sizes because of supermagnetism, and are therefore suitable as contrast agents in magnetic resonance imaging (MRI) [12; 13; 14; 15]. Due to all these factors, there are various possible classifications of nanomaterials. On the basis of chemical constitution, nanomaterials can mainly be classified into: (1) carbon allotrope-based nanomaterials consisting of only carbon atoms, (2) inorganic nanomaterials made up of metallic or non-metallic constituents such as Au, Ag, SiO\({}_{2}\), and (3) organic nanomaterials majorly comprising of polymeric nanomaterials.