**1. Introduction**

Nanotechnology is a part of applied science and innovation that deals with the control of matter on a nuclear and sub-atomic scale, ranging from 1 to 100 nm. This is a profoundly interdisciplinary field that advantages from the endeavors

and progressions of many disciplines, including applied physical science, materials science, point of interaction and colloid science, supramolecular science, compound designing, mechanical designing, organic designing, and electrical designing [1–5]. Nanotechnology has prospered since the introduction of group science and the development of the scanning tunneling microscope in the 1980s, with the capacity to gauge and imagine novel peculiarities, as well as control and production materials and gadgets with nanostructures of 100 nm or more modest. Semiconductor nanocrystals, otherwise called quantum dots, are a recently arising nanomaterial that has provoked the curiosity of numerous scientists. QDs are ordinarily synthesized from periodic table elements in groups III−V and II−VI [6–8]. Nonetheless, ongoing improvements in nanotechnology and biomedical utilization of nanomaterials have driven us to expand the meaning of QDs to incorporate more extensive classifications of nanoparticles (like carbon, silicon, and gold) which show the quantum-confinement occurrence related to a sensational alteration in electrons' way of behaving at the limit of the Bohr radius. Because of their tiny size (contrasted with most cell structures or biomolecules), they can communicate with biological structures on the nuclear scale. However, a few scientists use "traditional" semiconductors III−V and II−VI nanocrystals, these are as yet the significant short utilized in research and biomedical applications for a few goal reasons. Firstly, they have remarkable physical characteristics that recognize them from different kinds of QDs; second, they have been effectively used in medical procedures, ultimately, proceeded with use by industry requirements the broad assessment of dangers related with exposure. Electrons are depicted by quantum confinement effects in terms of their energy levels, likely wells, valence band, conduction band, and electron energy band [9]. At the point when the molecule's size is too little to possibly be similar to the frequency of the electron, the quantum confinement effect is noticed. Clearly, the confinement of an electron and a hole in nanocrystals is profoundly subject to material properties, explicitly the Bohr radius aB. These impacts rely upon material properties, [10] special to greater nanocrystals, and are Electrons in bulk dielectric materials can be portrayed by energy bands or electron energy levels (bigger than 10 nm). There are different energy levels or bands for electrons. These energy levels are suggested to as continuous since the energy difference is tiny in mass materials. The main part of electrons oscillates in the valence band under a prohibited energy level, known as the band gap, as they balance out at different energy levels. There are no electron states in this energy range. Fewer energy levels that are over the restricted hole make up the conduction band. The Bohr span is equivalent to 2.34 nm [11]. Materials' electrical and optical qualities contrast emphatically with mass materials when they are at the nanoscale. As the material gets more uncertain until it comes to the nanoscale, the confining aspect normally gets more unassertive. The properties, in any case, are presently at the quanta level and consequently discrete as [12] opposed to arriving at the midpoint of by mass and accordingly constant. All in all, the energy range becomes discrete, estimated as quanta, as opposed to consistent, as in mass materials. The bandgap, or the small and limited space between energy levels, results thus. Discrete energy levels are the situation that is alluded to as quantum confinement. Electrons in mass dielectric materials can be described by energy groups or electron energy levels (bigger than 10 nm). There are different energy levels or groups for electrons. These energy levels are indicated as persistent since the energy distinction is little in bulk materials. The majority of electrons waver in valence groups under a forbidden energy level,

#### *Application of Quantum Dots in Bio-Sensing, Bio-Imaging, Drug Delivery, Anti-Bacterial… DOI: http://dx.doi.org/10.5772/intechopen.107018*

known as the band gap, as they balance out at different energy levels. There are no electron states in this energy range.

Fewer energy levels that are over the restricted gap make up the conduction band. Moreover, disconnected islands of electrons that can shape at the designed connection point of two unique semiconducting materials make up quantum confinement distinctiveness. Normally, electrons are contained in plate molded regions called quantum dots. As was at that point referenced, electron control in these frameworks significantly changes how they collaborate with electromagnetic radiation. The bandgap limits are modified by the expansion or evacuation of a couple of iotas on the grounds that the electron energy levels of quantum spots are discrete instead of constant. Changes in the quantum dots surface shape additionally influence the bandgap energy as a result of the speck's little size and the impacts of quantum confinement [13, 14].

Quantum dots, which have excitons bound in every one of the three spatial dimensions, offer attributes somewhere between those of discrete particles and bulk semiconductors. Quantum dots can have their structure and molecule size modified subsequent to being excited by a reasonable laser pulse by exploiting the notable impacts of the quantum size confinement effect. When contrasted with quantum dots, fluorophores, (for example, rhodamine 6G and fluorescein) have various disadvantages in biosensing applications [15–17]. High yields, broad absorption, restricted size-dependent emission spectra, and incredible photo quenching obstruction are a couple of advantages of QDs' extraordinary optical elements [18]. The quantum confinement phenomenon gives an upsurge to the dimension-tunable absorbance and luminescence bands of QDs. At the point when the size of the semiconductor meets the material's Exciton Bohr Radius (EBR), the discrete electron band-gap energy levels are denoted as being under quantum confinement. On the off chance that the size of the QDs diminishes, the absorption and photoluminescence will go through a hyperchromatic shift and the energy of the band-gap expands [19]. Subsequently, QDs of similar material composition yet different sizes can emit fluorescence at a pool of frequencies. With innumerable applications, this optoelectronic nature of QDs gives a higher multiplexing potential. These QDs can possibly carry out numerous roles subsequent to being altered with explicit affinity ligands (antibody, peptide, aptamers, etc.). Subsequently, they might have the option to fulfill the rules for the ideal theranostic framework, including yet not restricted to the characteristics mentioned beneath.

Targeting specific cell types, diminished cytotoxicity, effective intracellular trafficking, conquering intracellular boundaries, responding to external or internal stimuli, delivering curative agents, and bearing a symptomatic specialist (optical or MRI) all add to a medication's capacity to treat a patient progressively. Thus, QDs can be utilized for drug delivery, biosensing, and bio-imaging. Creating QDs that are not poisonous to cells could have an enormous clinical effect. Given the adaptability of QDs, research is in the works to foster tests that can follow continuous cell processes, exhibiting selectivity and particularity towards cells and the capacity to beat excretory issues, for example, non-poisonousness. The examination into QD/ drug nanoparticle plans will show huge development in different medical servicesrelated regions, for example, diagnostics and therapeutics for conditions like malignant growth and heart and immune system problems/illnesses. In this chapter, the role of QDs in various medical applications like bio-sensing, bio-imaging, drug delivery applications, anti-bacterial activity, photo-thermal, photodynamic therapy, and optoelectronic devices were discussed in detail. Furthermore, their specific

targeting abilities were also discussed when they are formulated with multifunctional moieties, they offer a versatile platform that is suitable for theranostics.
