hydroxysuccinimide FA Folic Acid FOI Fluorescence Optical Imaging FPC-NCs Fluorescent Porous Carbon-Nanocapsules GA-GQDs Gum Arabic-based Graphene Quantum Dots Gem Gemcitabine gGQDs Gold-doped Graphene Quantum Dots GQDs Graphene Quantum Dots HA Hyaluronic Acid L-Arg L-Arginine MIPs Molecularly Imprinted Polymers MRI Magnetic Resonance Imaging NIR Near-Infrared NMR Nuclear Magnetic Resonance NPCP Fluorescent Iron Oxide Nanoparticle PD Parkinson’s Disease Pt (IV) Platinum(IV) PTX Paclitaxel RGD Arginylglycylaspartic Acid ROS Reactive Oxygen Species Sil Silibinin TMZ Temozolomide ZIF-8 Zeolitic Imidazolate Framework-8

and electronic properties [2, 3]. They possess small dimensions, typically less than 10 nm, and exhibit quantum confinement effects that result in size-dependent optical properties and excellent biocompatibility [ 4]. These attributes, coupled with their capacity for surface functionalization, make CQDs highly desirable for biomedical applications, particularly in the realm of drug delivery [5]. The efficacy of drug delivery systems is pivotal in modern medicine as they facilitate targeted delivery, controlled release, and improved therapeutic effectiveness while minimizing adverse effects [6]. Traditional drug delivery methods often encounter challenges related to stability,solubility, and non-specific targeting [7]. CQDs present promising solutions to overcome these limitations and enhance drug delivery efficiency [8]. The excep­tional features of CQDs, including their substantial surface area, adjustable fluo­rescence, and capability to encapsulate therapeutic agents, position them as prime candidates for drug delivery applications [9]. By functionalizing CQDs with specific ligands, antibodies, or targeting molecules, active targeting to specific cells or tissues can be achieved, facilitating site-specific drug delivery. Furthermore, their excellent biocompatibility and low toxicity profile further bolster their potential as carriers for various therapeutic agents [10]. This book chapter explores the fascinating field of CQDs in the context of drug delivery. It discusses various approaches to creating CQDs, techniques for altering their surfaces, and how they can improve the efficiency of drug delivery. Additionally, recent advancements and breakthroughs in utilizing CQDs as carriers for diverse therapeutic agents, encompassing small molecules, proteins, nucleic acids, and imaging agents, are highlighted. Through harnessing the unique properties of CQDs, researchers are unlocking fresh possibilities for designing intelligent and efficient drug delivery systems that hold tremendous promise in revo­lutionizing the field of medicine. The exploration of CQDs in drug delivery appli­cations represents an exciting frontier, brimming with potential to improve patient outcomes and advance the field of personalized medicine.

Several synthesis techniques exist for generating CQDs, broadly classified into top­down and bottom-up approaches. Here, we will discuss some commonly employed synthesis methods.

Top-down approach involves the breakdown of larger carbon structures into smaller CQDs. This can be done through methods such as laser ablation, electrochemical oxidation, and arc discharge. Top-down approaches are typically more scalable than bottom-up approaches, but they can also produce CQDs with a wider range of sizes and shapes [11].

This approach entails the aggregation of smaller carbon molecules to form larger CQDs. This can be done through methods such as hydrothermal synthesis, microwave-assisted synthesis, and solvothermal synthesis. Bottom-up approaches are typically more versatile than top-down approaches, and they can produce CQDs with more uniform sizes and shapes [12].

2.2.1 Hydrothermal/Solvothermal Method

Electrochemical method is a bottom-up approach that uses an electric current to initiate the carbonization and fragmentation of carbon electrodes. This method offers good control over the size and properties of the produced CQDs [15]. Electro­chemical synthesis involves the application of an electric potential to carbon elec­trodes immersed in a suitable electrolyte solution. The electric current initiates the carbonization and fragmentation of the electrodes, leading to the generation of CQDs. This method offers good control over the size and properties of the produced CQDs [17].