**2. Quantum dots in medical applications**

Researchers are excited up for new nano-theranostic stages including quantum dots in bio-detecting, as they can detect, picture, and treat simultaneously. Their benefit over old advancements lies in the control of conductive properties through crystal size. However, quantum dots enjoy many benefits, they have huge constraints. For instance, they are just comprised of a few molecules, and they are difficult to eliminate from the body. Researchers are attempting to take care of these issues so quantum dots can be utilized in additional clinical applications. The remarkable optical properties of QDs have prompted their utilization in various fields, including imaging, following, diagnostics, drug delivery, tissue designing, malignant growth treatment, multicolor optical coding, and single molecule probes. For example, quality treatment can fix infections brought about by hereditary anomalies, like malignant growth or diabetes. QDs can stop quality action, and furthermore track down application in RNA advancements. For instance, in situ hybridization (ISH) can utilize QDs to identify mRNA particles, and RNA mediation might join siRNAs with CdSSe/ZnS quantum dot—polyethylenimine (QDs-PEI) to really target qualities more successfully [20]. Various benefits of utilizing QDs are investigated by different research groups because of their adaptable properties that are featured in **Table 1**.

Radiotherapy (RT) is a typical malignant growth treatment technique, with a few incidental effects. Researchers have fostered nanozymes by doping Mn (II) and Silver Selenide quantum dots particles (Ag2Se QDs) transmitting in the second near-infrared window. These nanozymes are attached with cancer targeting arginine-glycine-aspartate (RGD) tripeptides and polyethylene glycol and built into in vivo nanoformulations for NIR-II imaging-directed radiation treatment of malignancy [21]. Tumor particularity and NIR-II radiating abilities of the nanoprobes empower exact confinement, giving exact RT. The ultra-stability of these nanoprobes in the living body likewise improves RT adequacy through persistent creation of oxygen and help from hypoxia of cancers. Nanoprobe-intervened RT supported by real-time, enhanced clarity imaging advances against growth invulnerability and altogether hinders cancers or fixes them totally. Another investigation discovered that Mercaptopropionic corrosive (MPA)-coated QDs were exceptionally


#### **Table 1.**

*The advantages of using QDs in various biomedical applications.*

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

biocompatible and enact the lysosomal pathway, which clears cell garbage. The decreased ROS could assist the cells with adapting to nanomaterial-initiated pressure, clearing the way for malignant growth treatment. Scientists fostered a new pseudo-homogeneous vector for gene delivery that uses the cationic CQDs got from chitosan to make a non-viral gene exchange framework [22]. This new vector shows better uptake in cells. Another sort of ZnO QDs nanoplatforms to deliver genes that assist with treating Parkinson's illness has been grown as of late. Glutathione changed ZnO QDs composites stacked with quality and NGF to safeguard the brain and reverse the impacts of neurodegenerative issues, which are normal in Parkinson's patients. QDs were utilized to address the reasons for later-stage nearsightedness (myopia). Carboxylated CuInS/ZnS QDs (ZCIS QDs) were added to intraocular lenses by a facial initiation-immersion technique to warm up the lenses and keep the cells from sticking to the lens surface. This technique could assist with regarding cataract development as well as posterior capsule opacification (PCO) [21].

#### **3. Bio-sensing application**

The utility of semiconductor QDs in biosensing applications has been growing, as they offer several advantages over other types of sensors. All the more significantly, it is not difficult to present nucleic acid enhancement methodologies or potentially nanomaterials to work on the conjugation of aptamer-based detecting structures. In this manner, the composite of QDs attached with aptamers acquires added open doors bioanalytical methods. QD-based fluorescent nanoformulations are generally utilized for bio-sensing for DNA and protein discoveries [23]. Heavy metal cytotoxicity was minimized or eliminated by developing metal-free quantum dots. QDs synthesized using silicon quantum dots (Si QDs), graphene quantum dots (GQDs), carbon dots (C-dots), and near-infrared (NIR) QDs (silver selenide, silver sulfide, (Ag2Se and Ag2S) are the alternate possibilities. Apart from this, metal nanoclusters have exhibited noticeable benefits as fluorescence probes for optical bio-sensing and imaging procedures in current biomedical research [24]. QD-based biosensors are molecular networks made of organic receptors and are proficient substitutes for conventional sensors [25]. These biosensors have gained significant attention, because of their function in bridging the gap between pure organic receptors and inorganic materials [25]. The physical properties of photo-sensors can be altered by the environment of surfacecoated and biological ligand-attached quantum dots. This includes heat, ions, and pH of solution as well as confirms the use of QDs in bio-sensor development. The chemical sensors are receptor molecules that show selective responses to particular ions (cations/ anions) or neutral groups. For the selection and accounting of the compounds present in biological environment, development of chemosensors is significant, particularly those specific ions that are possibly dangerous to surroundings and individuals.

#### **3.1 Core-ligand interaction**

Connecting biomolecules like ligands, antibodies, peptides, or nucleic acids with nanoparticles have drawn broad interest for researchers in the biosensing region as it offers practical bionanomaterials for targeting and drug delivery applications [26]. Semiconductor QDs are broadly utilized in the biosensing region due to their one-ofa-kind property, for example, confined and symmetric emission with colors that are adjustable, considerable quantum yield, appropriate stability, and very much directed shape and size [27]. Among these ligands, aptamers show a few advantages containing lesser aspects, great synthetic stability, and straightforward cycle in combination with high cluster to-bunch homogeneity and more flexibility. Further to this, it is not difficult to present nucleic acid intensification strategies and nanomaterials that can offer significantly better sensitivity. Subsequently, the mix of semiconductor quantum dots and aptamers gets added possibilities in bio examination. In this way, nanoformulation offers a few applications in different sign transducing components, including optical, electrochemical, and electro-produced discharge of light because of synthetic response (chemiluminescence) approaches. These two unique parts connect at this basic biomolecular-materials point of interaction and offer further developed action and promising qualities. The interactions are represented by different factors, for example, Förster reverberation energy move (FRET), the presence of electrostatic and other useful appealing powers (energy electron move) between the biomolecule and the QD surface.

A comprehension of these basic collaborations at this point of interaction can yield a bunch of methodologies that will allow the reasonable plan of ensuing researcher high-movement bionanocomposites and theranostic nanoformulations [28]. Four significant methodologies can be utilized to adjust aptamers onto ODs. (i) Self-gathering among DNA and QDs. (ii) Biospecific collaborations, e.g., biotin-avidin (or streptavidin) cooperation. (iii) Covalent interactions. (iv) Nucleic acid hybridization [29]. The blend of Quantum dots and aptamers will give different identification stages, including optical, electrochemical, and electrochemical luminescence (**Figure 1**).

**Figure 1.** *QD based various biomolecule detection platforms [26].*

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

This takes into consideration the identification of an assortment of analytes like proteins, little particles, and cancer cells [30]. Advanced signal level intensification systems incorporate utilizing nanoscaled materials as devices for ultrasensitive bioanalysis. High-level sign enhancement procedures take into account ultrasensitive bioassays. These procedures likewise consider exceptionally unambiguous biosensing-recognizing follow measures of a wide assortment of analytes in clinical, ecological, or modern applications. The quick advancement in the utilization of other utilitarian materials for detecting and the clarification of their novel photosensitive qualities proposes that by conjugating the aptamer-QD with graphitic-carbon nanoformulation, novel detecting and identification stages might be planned. In what way to integrate these cross breeds into cells and complete in vivo detecting might be a test from here on out.

### **4. Bio-imaging application**

Water-soluble quantum dots (QDs) can be conjugated with various biological molecules like peptides, proteins, aptamers, drugs, and antibodies. The benefits of QD-based biomolecule conjugation help to deal with several immunohistochemistry, labeling and imaging the single-molecule and tracks that are well explored in numerous studies [31–33]. The exclusive wavelength tunable properties of QDs offer wide prospects for designing systems for numerous analyses by multi-colored imaging for the concurrent recognition of numerous targets. Linking drugs with QDs or their combination into QD-based drug delivery molecules marks it a potential candidate for monitoring the drug release and undertaking image-guided therapy. In light of the versatile nature of their photosensitive properties, QDs emanating in the NIR region have turned into an interesting device for intense tissue single photon and multiphoton *in-vivo* imaging. Conventional green fluorescent proteins and fluorescent biological dyes have several restrictions such as spectral crossing, low signal intensity, photo-bleaching when compared to QDs which have substantial benefits in chemosensors and biosensor development [34]. In recent times, QDs are demonstrated to have a significant candidate for fluorescent probes and tags in several organic procedures, ranging from molecular-level histopathological studies to whole-body diagnosis [35, 36]. While a small number of studies have tried the utility of QDs for *in-vitro* or *invivo* diagnostic imaging studies, the important restriction on possible harmfulness of group II–IV QDs (like cadmium telluride and cadmium selenide) have hampered their practical application in science and medication. The leakage of the ions belonging to the heavy metal present inside the core of these quantum dots by physiochemical reactions may contribute to the harmful toxic effect, which raises several questions about the utility and biocompatibility of these QDs [37, 38]. Furthermore, the longterm cumulative effects of these ions' augmentation and prolonged periods within the body may produce unnecessary damage to tissues and organs. At the same time, the quick clearance by the reticuloendothelial system (RES) or getting into the liver and spleen can result in noise in the image and thus the quality may get affected. The application of quantum dots as contrast agents for in vivo imaging has been an area of high prospect since they were initially studied, and they have recently been granted repeated attention. The optical imaging system even at present utilizes these conventional fluorescent molecules for animal imaging. The NIR wavelengths (~730 nm) are significant, as they have reduced light scattering and low tissue absorption also relatively easy to detect with apparatuses [39]. Different types of quantum dots emit

 **Figure 2.**

 *QDs plausible medical utility and cautiousness.* 

different wavelengths—one type emits red light (730 nm). QD bioconjugates are widely reported in several studies for their application in in-vitro imaging and diagnostics, like immuno-fluorescent labeling of tissues and cells, intracellular delivery of QDs, tracking of single-molecule in living cells, and in-vivo tracking by fluorescence labeling drug localization and biodistribution [ 29 ]. Hence, quantum dots hold great potential for molecular and cellular imaging both in vitro and in vivo conditions. In recent times, researchers have used QDs for imaging stem cells in mice embryo. This unlocks the opportunity to use quantum dots for imaging stem cell and stem cell therapy tracking in a fast and accurate manner ( **Figure 2** ). Stem cell therapy is the most common way of infusing stem cells into a living being with the expectation that they will separate and supplant harmed tissue or develop new organs.
