Carbon Dots As Theranostic Agents

Madhuri Sharon, Ashmi Mewada

• Publisher: Wiley-Scrivener
• Year: 2018
• ISBN:
  9781119460459
• Edition: 1
• Topic: Technik & Maschinenbau
• Subtopic: Werkstoffwissenschaft

This book is designed for researchers and students interested in carbon dots applications in health care, especially as a theranostic agent.

Carbon Dots as Theranostic Agents focuses on the fundamental understanding along with the applications of this unique fluorescent nano-biomachine. The book begins with the explanation that carbon dots fall between the usual daily macro or bulk physics and the quantum mechanics and covers their unique properties like quantum mechanics and quantum confinement. It then encompasses the domain of various physical, chemical and biological methods that efficiently synthesizes the carbon dots and their desired properties. The basic characterization techniques used for carbon dots is also covered in this book. Conjugation of carbon dots with different moieties is another aspect that enhances its applications, hence this is highlighted too. The book also details how to maneuver the carbon dots for their use in targeted drug delivery with emphasis on cancer and neurodegenerative disease as well as cellular imaging and diagnostics. One of the unique features of this book is that it reports on the use of carbon dots to act as a nano-fertilizer, as a drug/antibiotic delivery vehicle to diseased plants through foliar application.

Chapter 1
Carbon Dots: Discovery, Synthesis and Characterization

The saddest aspect of life right now is that science gathers knowledge faster than society gathers wisdom.

Isaac Asimov

1.1 Background

Owing to never-ending human curiosity for search of answers to unsolved complex natural phenomena, Max Plank, a German scientist, first introduced the concept of discretization to explain black body radiation in a slightly different way from the already available (classical mechanics) explanation [1]. In the race to explain atoms, Rutherford’s atomic model unintentionally implied that atoms could not exist because the electron should eventually fall into the nucleus as it loses energy in its orbital motion around the nucleus. Niels Bohr first used Plank’s concept to explain an acceptable and stable atomic model [2, 3]. The origin of the new concept of scientific theory or quantum theory or quantum mechanics had begun, as this theory could explain various natural phenomena in an acceptable way. Quantum mechanics generally is used to explain atomic and subatomic interactions with the help of rigorous mathematical derivations formulated by the Austrian scientist Erwin Schrödinger [4].

Since the discovery of fullerene [5], carbon nanoscience has emerged. It has extended up to one-dimensional (1D) carbon nanotubes (CNTs) [6, 7] and most recently two-dimensional (2D) graphene [8]. The physics of these nanomaterials could be explained by quantum mechanics. It has been observed that there is a drastic change in optical and electronic properties of materials once they are in the order of a few nanometers as compared to the properties of bulk materials. This change in properties of nanomaterials is attributed to a term called “quantum confinement,” i.e., when material becomes as small as the de Broglie wavelength of electron wave function [9–12]. This factor can be explained using rigorous mathematical derivations. The mathematical explanation coined by Schrödinger constituted quantum mechanics or quantum physics. In quantum mechanics all the experimentally measurable quantities of a particular system, generally expressed in terms of position and momentum, are replaced by operators which operate on the wave function of the system. For example, the Hamiltonian operator operating on a given wave function in quantum mechanics corresponds to total energy of a system similar to the Hamiltonian one of a given system in classical mechanics.

1.2 Introduction to QD

The term QD takes us to a difficult domain of Physics where a material follows wave theory concept that introduces us to very small particles. A fluorescent colloidal semiconductor nanocrystal was first discovered in the 1980s by Louis E. Brus of Columbia University and was later named as quantum dot (QD) by Mark Reed in 1988 [13] because of quantum confinement. Since then thousands of people have contributed to the synthesis, understanding, physics, chemistry and various applications of QD.

Generally, QDs fall in the size range of 1–10 nm. QDs are known for various optical and electronic properties like excitation of multiple fluorescence spectra, enhanced photostability, high quantum yield, photocatalysis, biological imaging, diagnostics and molecular histopathology [14–18]. Inorganic QDs of various types have been reported to date which include CdTe, CdSe, PbSe, GaAs, GaN, InP, and InAs [19–21]. However, major drawbacks in employing these QDs for biological applications are related to their toxicity as well as insolubility in water [22–24]; and also the tedious and expensive procedure of QDs synthesis. To overcome the above-mentioned drawbacks related to QDs, researchers are constantly trying their hands at various directions in pursuit of more compatible materials.

Do you know that we see these particles in everyday life when we sit in front of LED or LCD television? The TV screen has incredibly small crystals of 2–20 nm, called QDs. These QDs have a unique property that depending on their size they emit different colors, e.g., smaller QDs emit blue light and larger ones emit red light. When QDs are used in TVs they give better color accuracy. But their uses are not limited to TVs only. They have found applications in transistors, solar cells, LEDs, diode lasers and they can charge smart phones in 30 seconds, etc.

The shortest possible explanation of QDs is that they are semiconductor crystals having their excitons or electron and holes confined in all three dimensions of space. Consequently, QDs have electronic properties that are intermediate between those of bulk semiconductors and those of discrete molecules. Being very small in size, they possess larger band gap. This is because of the discreet energy level, which depends on the size and shape of the QD. Smaller QDs need more energy to confine excitons to a smaller volume and they spread out more, thus having higher band gap energy. These very small semiconductor particles can be compared to the Bohr radius of the excitons, i.e., separation of electron and holes. A QD is usually 1–10 nm in size that encompasses ~10–50 atoms in diameter. Structure-wise, QD includes a Core—the active part of QD—surrounded by a protective layer called the Shell, which is responsible for enhancing the stability and emission efficiency of the core, and Surface ligands, which are the outermost passivating, protective and chemically active layer.

Apart from QD there are a few other quantum confinement structures, i.e., Quantum well and Quantum wire, but the difference is that unlike QD, which has electron or hole confinement in three dimensions of the space, in the Quantum wire they are confined in two spatial dimensions and in Quantum well electrons or holes are confined in one direction that can freely propagate in one dimension.

1.2.1 What is Quantum Mechanics?

The QDs are nanometer scale particles that follow the quantum mechanical principle of confinement. To understand the quantum mechanics, let’s look at the physics of QD (semiconductor). The band gap of QD varies from those of bulk material, conducting material and insulators.

What is band gap? The difference in energy between the conduction band and the valance band is called the Energy gap or Band gap Figure 1.1. The electrons can be excited across this band gap into the conduction band with an applied voltage or a photon or any other energy source. When the electron is excited from the valance band it leaves an empty space called a hole. The electron-hole pair is called an exciton. Once excited, the electron can move around in the conduction band, and the hole can move around in the valance band.

Figure 1.1 Band gap and exciton.

The energy bands are quantized at the atomic level, which is broadened into bands of energy. The levels inside the bands are very closely spaced. Hence, they are not considered to be discrete. Conducting materials conduct at room temperature because their band gaps are in the range of KT energy (~0.25 eV). Because of their low band gap, electrons from the valence band easily get excited to conduction band creating a corresponding number of holes in the valence band. These holes assist in making materials conductors even at room temperature.

In insulators, though the valence band is full of electrons and the conduction band is completely empty of electrons, but both these bands are separated by a large energy (large band gap), which makes it difficult for electrons of the valance band to go to conduction band. A semiconductor is like an insulator, but the only difference is that the two energy bands (i.e., band gap) are...