# Hybrid Quantum Dot as Promising Tools for Theranostic Application in Cancer
## 1 Introduction  
Cancer is the leading cause of death worldwide, accounting for nearly 10 million deaths in 2020 [1]. The correct diagnosis is crucial for accurate and effective treatment because all tumors need specific treatments such as surgery, radiotherapy, and chemotherapy. Novel therapeutic interventions and early diagnosis of cancer are major concerns for scientists, physicians, and healthcare professionals. In the last few decades, nanotechnology has been at the forefront of most cutting-edge research in many fields, such as medicine, diagnostics, bioimaging, and other biomedical applications [2]. Among the delivery approach of medicines in different cancers, various nanoparticles (NPs) are the most exploited carrier system for drug delivery in different cancer [3, 4]. These carrier systems have different structures and dimensions, which may vary in size range from 1 to 100 nm, particularly for drug delivery in cancer. This gives them different physical and chemical properties that can be exploited for various purposes in cancer. The specific physicochemical properties of nanomaterials are exploited for focused ultrasonic heating therapy [5, 6, 7], radiofrequency (RF) ablation [8, 9, 10], magnetic fluid hyperthermia [11, 12, 13], and magneticparticle imaging [14; 15] in cancer. NPs are widely exploited for bioimaging [16], drug delivery systems [17], therapeutic agents for photodynamic therapy (PDT) [18], photothermal therapy (PTT) [19], regenerative medicine [20], and smart biomaterials [21]. Furthermore, metallic NPs, especially gold [22], silver [23], platinum [24], and palladium [25], are the most investigated biocompatible NPs, for the diagnosis and treatment of cancer. Along with imaging agents to treat a wide range of diseases, including different carcinomas, the delivery of synthetic drugs, therapeutic peptides, and genes has raised interest in the the transostic approach, which can be simultaneously utilized for both diagnosis and treatment through a single system. Different nanomaterials have high penetrating efficiency to biological membranes, good biocompatibility, and can perform multiple functions due to their small size and ability to functionalization. This makes the utilization of nanomaterials a good candidate for theranostic application in different cancers [26]. Quantum dots (QDs) are a very small nanoparticulate system of organic (such as carbon-QDs, graphene-QDs) and inorganic nature (such as zinc sulfide-QDs, cadmium telluride-QDs, cadmium selenide-QDs) and vary in size ranges from 2 to 10 nm [27]. The small nano dimension is helpful to impart specific optical (high brightness, high quantum yield, high extinction coefficient, intermittent fluorescence signals, high stability against photobleaching) [28] and electronic properties that are exploited in different biomedical applications [27; 28; 29]. Its nanocrystal is distinguished by an energy band gap, required to excite an electron from one electronic band, i.e., a lower energy level, into another band, i.e., a higher energy level. Because they are so small, these nanocarriers of semiconductor origin can easily move electrons to a higher energy state, even when they are exposed to UV light. These properties of QDs are used in the diagnosis and treatment of various diseases, including cancer. This excitation scheme ultimately creates an electron-hole pair known as an exciton. The exciton gives off energy in the form of a fluorescent photon when it goes back to its ground state [30]. The 2-10 nm sized semiconducting nanocrystal started to act like the bulk Bohr exciton radius, and the particle's electrical and optical properties changed [31]. The inverse relationship between nanocrystal size and energy band gap is well-documented and understood. The inverse property says that as the size of the nanocrystal decreases, the energy band gap increases, and the corresponding excitation/emission wavelengths decrease. The size of the particle can be changed to change the color of the light that the QDs give off when they are exposed to UV light.