Quantum Dot Sensors

Quantum dots, shortly QDs are small semiconducting particles with typical size ranging from 1 nm to 10 nm. These are artificial clusters of semi conductive atoms that have the ability to confine electron motion due to their small size. One of the most important properties of quantum dots is the ability to tune their band gap and therefore to control their absorbance and emission frequencies.

Just for the information. In solid state physics, a band gap also called an energy gap or band gap is an energy range in solid where no electron state can exist. If you look at graphs of electronic band structure of solids, the band gap generally refers to the energy difference in electron volts between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. It is the energy required to promote a valence electron bound to an atom to become a conduction electron, which is free and serve as a charge carrier to conduct electric current.

Figure 1 - Representation of band gaps in crystal and quantum dots

The ability of tuning band gaps in quantum dots is due to energy level quantization. In this way it is possible for their optical and electrical properties to be adjusted as needed.

Quantum dots absorbs photons of light and then re-emit wavelength photons for a period of time. The high controllability of quantum dot size provides very precise control over the wavelength of the re-emitted photon. Therefore the color of the light emitted from the quantum dot can be manipulated without significant cost or the use of high end technology. Following this procedure, a full range of quantum dots can be manufactured, each with a narrow distinct emission spectrum.

Besides quantum dots there are also quantum wires and quantum wells. Quantum wires, confines electrons or holes in two spatial dimensions and allow free propagation in the third dimension, and quantum wells which confine electrons or holes in one dimension and allow free propagation in two dimensions.

 Quantum Size Effect

To describe and understand mechanism of quantum dot electrical characteristic let’s explain electric characteristics of bulk semiconductors. In these materials, there is forbidden range of energy levels known as a band gap.

                                            

Figure 2 – Semiconductor band structure    

As seen on Figure 2 the lower energy level below the forbidden band gap levels is called Valence band, and the energy level above the band gap is called the conduction band. If the valence band is completely full and the conduction band is completely empty, than electrons cannot move in the solid, however if some electrons transfer from the valence to the conduction band, then current can flow. Therefore the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are called insulators, those with smaller gaps are called semiconductors, while conductors either have very small band gaps or none, because the valence and conduction band overlap.

In order for an electron to jump from a valence band to a conduction band, it requires a specific minimum amount of energy for the transition. The required energy differs with different materials.  The required energy differs with different materials. Electron can gain enough energy to jump from valence band to conduction band by absorbing either a phonon (heat) or a photon light.

Phonon – is a quantum of energy or a quasiparticle associated with a compressional waves such as sound or vibration of a crystal lattice.

Photon – is an elementary particle, the quantum of all forms of electromagnetic radiation including light.

A semiconductor is a material with a small but non-zero band gap that behaves as an insulator at absolute zero but allows thermal excitation of electrons into its conduction band at temperatures that are below its melting point. The average distance between an electron and a hole in an exciton is called the exciton Bohr radius. When the size of the semiconductor falls below the Bohr radius, the semiconductor is called quantum dot.

Just to clarify the “Excitons” are electron – hole pairs that are formed when light interacts with certain materials. An exciton is formed when a photon is absorbed by a semiconductor. This excites an electron from the valence band to the conduction band as a result, a hole is created in the valence band. Bohr radius is a physical constant, approximately equal to the most probable distance between the proton and electron in a hydrogen atom in its ground state. This radius is named after Niels Bohr, due to its role in the Bohr model of an atom. Its value is equal to 5.2917721067 x 10^-11 m.

An exciton has a limited lifetime and eventually the electron returns to the valence band and recombine with the hole. The recombination process is usually a radiation process which includes photon release, called fluorescence. These absorption and fluorescence process can be measured. It is important to point out in this context that confinement in quantum dots can also arise from electrostatic potentials. Generally the smaller the size of the crystal, the larger the band gap, and the energy difference between the highest valence band and the lowest conduction band becomes greater. That means that more energy is needed to excite the dot, and simultaneously, more energy is released when the crystal returns to its resting state. This equates to higher frequency light emitted after excitation of the dot as the crystal size grows smaller, resulting in a color shift from red to blue in the light emitted. In addition to such tuning, a main advantage with quantum dots is that, because of the high level of control possible over the size of the crystals produced, it is possible to have very precise control over the conductive properties of the material.

Here is an interesting and informative Youtube video about Quantum Dots.

As mentioned in the video Quantum dots are made from the elements in the second and sixth group of the period system – cadmium chalcogenides ( CdS, CdSe, CdTe), zinc (ZnSe, ZnS, ZnTe) and the third and fifth groups, phosphides and indium arsenide.

The engineering of quantum dots to fluoresce at different wavelengths based on their physical dimensions. By use of different colloidal quantum dots for the different parts of the visible spectrum, the entire range of the spectrum can be synthesized as shown in following figure.

Figure 3 – Solutions of colloidal QDs of varying size and composition

Functionalization of Quantum Dots

The ability to functionalize quantum dots has the potential to change their properties, including the solubility of the quantum dots in a variety of solvents and the ability of quantum dots to specifically bind to targets.

Quantum dots can be coated with a monolayer of hydrophilic thiols which are generally ionically stabilized in solution. They can also be coated with cross-linked silica shell, which in turn can be readily modified with a variety of organic functionalities using well developed silane chemistry. An additional option is to encapsulate quantum dots in amphiphilic polymers, which form high stable, micelle – like structures. In this case, quantum dots may additionally be modified to contain polyethylene glycol (PEG) to decrease surface charge and increase colloidal stability.

Water-soluble quantum dots may be covalently or electrostatically bound to a wide range of biologically active molecules to render specificity to a biological target. For example, quantum dots that are conjugated to antibodies can yield specificity for a variety of antigens, and are often prepared through the reaction between reduced antibody fragments with maleimide-PEG-activated quantum dots. They can also be cross-linked to small molecule ligands, inhibitors, peptides, or aptamers that can bind with high specificity to many different cellular receptors and targets. A third example is the possibility to conjugate quantum dots to cationic peptides, such as the HIV Tat peptide, which can allow quick association with cells and internalization via endocytosis.

Quantum dots have been used to detect the presence of biomolecules using intricate probe designs incorporating energy donors or acceptors. For example, quantum dots can be adapted to sense the presence of the sugar maltose by conjugating the maltose binding protein to the nanocrystal surface as seen in following figure. By initially incubating the quantum dots with an energy-accepting dye that is conjugated to a sugar recognized by the receptor, blue excitation of the quantum dots yields little fluorescence, as the energy is non-radiatively transferred (grey) to the dye. Upon addition of maltose, the quencher–sugar conjugate is displaced, restoring fluorescence (green) in a concentration-dependent manner.

Figure 4 – Quantum Dots sensors for the detection of biomolecules.

Quantum dots can also be sensors for specific DNA sequences. For example, this can be achieved by mixing the targeted single-stranded DNA with acceptor fluorophores conjugated to a DNA fragment complementary to one end of the target DNA. In other instance, the DNA detection could be achieved by mixing the targeted single-stranded DNA with a biotinylated DNA fragment complementary to the opposite end of the target DNA. In this case, these nucleotides hybridize to yield a biotin–DNA–fluorophore conjugate. Upon mixing this conjugate with quantum dots, a green fluorescence is quenched via non-radiative energy transfer (grey) to the fluorophore conjugate. This dye acceptor then becomes fluorescent (in red color), indicating the presence of the target DNA. Finally, quantum dots conjugated to the luciferase enzyme can non-radiatively accept energy from the enzymatic bioluminescent oxidation of luciferins on the quantum dots surface, exciting the quantum dots without the need for external illumination.