Semiconductor Nanocrystals Quantum Dots - Properties and Biological Applications

Semiconductor physics Electrons are confined to a number of bands of energy, and forbidden from other regions. The term "band gap" refers to the energy difference between the top of the valence band and the bottom of the conduction band; electrons are able to jump from one band to another. Valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature. Conduction band is the range of electron energies enough to free an electron from binding with its atom to move freely within the atomic lattice of the material. In order for an electron to jump from a valence band to a conduction band, it requires a specific amount of energy for the transition. The required energy differs with different materials. The conductivity of intrinsic semiconductors is strongly dependent on the band gap. The only available carriers for conduction are the electrons which have enough thermal energy to be excited across the band gap.

Band gaps Atomic orbital: a discrete set of energy levels. If several atoms are brought together into a molecule, their atomic orbitals split, as in a coupled oscillation  A number of molecular orbitals proportional to the number of atoms. When a large number of atoms (of order 1020 or more) are brought together to form a solid, the number of orbitals becomes exceedingly large.  Difference in energy between them becomes very small  The levels may be considered to form continuous bands of energy rather than the discrete energy levels of the atoms in isolation. However, some intervals of energy contain no orbitals, no matter how many atoms are aggregated, forming band gaps.

A stimulus (assume electromagnetic radiation) of bandgap energy or higher can excite an electron into the conduction band. In reality and at room temperature, there are practically no electrons in the conduction band compared to the number in the valence band. In reality, the distance between energy levels in a band is practically zero compared to the size of the band gap (in this diagram, the distance between energy levels has been blown up for visual ease).

Bulk Semiconductors - A Fixed Range of Energies Relaxing Electron Emits Fixed Radiation Electrons relax back to the top edge of the valence band from the bottom edge of the conduction band. This causes the fixed emission peak of semiconductors.

Quantum Confinement If the size of a semiconductor crystal becomes small enough that it approaches the size of the material's Exciton Bohr Radius, the electron energy levels can no longer be treated as continuous - treated as discrete, namely there is a small and finite separation between energy levels. This situation of discrete energy levels is called quantum confinement. Bohr radius: Physical constant : the most probable distance between the proton and electron in a hydrogen atom in its ground state Exciton : A bound state of an electron and hole which are attracted to each other by the electrostatic Coulomb force : Electron-hole pair An exciton can form when a photon is absorbed by a semiconductor.  This excites an electron from the valence band into the conduction band

A Feel for the Size of a Quantum Dot Each dot is between 2 and 10 nm (10 and 50 atoms) in diameter. Lined end to end, 2 million dots would be 1 cm long. In reality, most applications of quantum dots involve attaching molecules to their surface and suspending the dots in a liquid, gel, or solid matrix.

A Tunable Bandgap According to size of quantum dot semiconductor will measurably alter the bandgap energy – a tunable bandgap! This is possible as long as the size of the dot is close to or below the Exciton Bohr Radius. 보조설명!!! Changing the geometry of the surface of the quantum dot  changes the bandgap energy, owing again to the small size of the dot, and the effects of quantum confinement. The bandgap in a quantum dot will always be energetically larger;  we refer to the radiation from quantum dots to be "blue shifted" reflecting the fact that electrons must fall a greater distance in terms of energy and thus produce radiation of a shorter, and therefore "bluer" wavelength.

Size dependent color Quantum dots, the size of the bandgap is controlled simply by adjusting the size of the dot. Because the emission frequency of a dot is dependent on the bandgap, it is therefore possible to control the output wavelength of a dot Ref. http://www.evidenttech.com/qdot-definition/quantum-dot-introduction.php

500Å Quantum Dots – Semiconductor Nanocrystals 65Å CdSe Nanocrystal QDs: Artificial Atom - Nanosized semiconductor materials II-VI Semiconductor CdS CdSe CdTe PbS PbSe PbTe III-V Semiconductor AlSb GaP GaAs GaSb InP InAS InSb 65Å 500Å CdSe Nanocrystal

Structure of CdSe Quantum Dots - CdSe core crystal are capped with surface stabilizing capping molecules

Synthesis of CdSe/ZnS (Core/Shell) QDs 5.5 nm (red) Step 1 CdO + Se CdSe Solvent : TOPO, HAD, TOP Surfactant : TDPA, dioctylamine Step 2 ZnS ZnEt2 + S(TMS)2 CdSe Growth temperature  140 ℃ (green) 200 ℃ (red) Thermocouple Se solution CdO solution 320 ℃ 20 nm Bawendi et al. J. Am. Chem. Soc. (1994)

Optical Properties Of Quantum Dots a) Multiple colors b) Photostability (Tellurium) c) Wide absorption and narrow emission d) High quantum yield Quantum Yield ≥ 60 ~ 70 % Single source excitation

Biotechnological applications of QDs Requirements under aqueous biological conditions - efficient fluorescence - colloidal stability - low non-specific adsorption Main challenge - QDs have hydrophobic organic ligands coating their surface  Organophilic ligands should be exchanged with more polar ones to make QDs biocompatible Approach - Monolayer shells: reproducible, rapid, well-oriented, thin-coating low colloidal stability - Multilayer shells : high stable in vitro, long coating process, difficult to control the coating process ex) overcoating with proteins followed by other layers for bioconjugation overcoating of the outer shell with surfactants or polymers Drawbacks : tends to aggregate and adsorb non-specifically

QD Surface Coating for Biocompatibility ① Encapsulation with the hydrophobic core of a micell Coating with PC Coating of the outer shell with ZnS CdSe QDs CdSe/ZnS core-shell Quantum Dots Encapsulated in Phospholipid Micelles PEG-PE (n-poly(ethylenglycol)phosphatidylethanolamine): micell-forming hydrophilic polymer-grafted lipids  comparable to natural lipoproteins PEG : low immunogenic and antigenic, low non-specific protein binding PC : Phosphatidylcholine Dubertret et al. Science (2002)

QD Surface Coating for Biocompatibility ② (Trioctylphosphine Oxide) 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide N-hydroxysulfosuccinimide EDC/NHS Wu et al. Nature Biotech. (2003) ; QdotTM Corporation

Bioconjugation Method with Quantum Dots S. Nie, Science 1998, 281, 1016 H. Mattoussi, J. Am. Chem. Soc. 2000, 122, 12142 X. Wu, Nature Biotech. 2003, 21, 41 S. Nie, Nature Biotech. (2001) 1-Ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) : a water- soluble derivative of carbodiimide. Carbodiimide catalyzes the formation of amide bonds between carboxylic acids or phosphates and amines by activating carboxyl or phosphate to form an O-urea derivative EDC coupling is enhanced in the presence of N-hydroxysulfosuccinimide (Sulfo-NHS) A. P. Alivisatos, Science (1998)

Bioconjugaion Using Chemical Linker Molecules CdSe IgG NH2 CdSe NH2 NH2 sulfo-NHS / EDC CdSe COOH streptavidin CdSe COOH sulfo-NHS / EDC COOH EDC : Ethyl-3-(dimethylaminopropyl)carbodiimide NHS (Sulfo-NHS) : N-hydroxysulfosuccinimide

Commercially Available QD Bioconjugates Core Core Shell Shell Polymer Coating Polymer Coating Biomolecule (Biotin, Protein A…) Streptavidin 525 565 585 605 655 From QdotTM Corporation

Y Y + In Vivo Cell Imaging QD Organelle QD Organelle QD-Antibody conjugates Antigen Organelle QD Y ▲ 3T3 cell nucleus stained with red QDs and microtubules with green QDs - Multiple Color Imaging - Stronger Signals Wu et al. Nature Biotech. (2003)

In Vivo Cell Imaging Live Cell Imaging Cell Motility Imaging Quantum Dot Injection ▶ Red Quantum Dot locating a tumor in a live mouse Cell Motility Imaging 10um ◀ Green QD filled vesicles move toward to nucleus (yellow arrow) in breast tumor cell Alivisatos et al., Adv. Mater.(2002)

– In vivo Cell Imaging Xenopus embryos B ~E : different stages from injection into a cell F : axon & somites G : Nucleus H: Neural crest cells I: Gut of an embryo “In Vivo Imaging of Quantum Dots Encapsulated in Phospholipid Micelles” Science (2002)

Quantum Dot Microarrays DNA-microarray based application QD QD-DNA conjugates QD DNA chip Human oncogene p53 Very High Signal to Noise ratio (>100) No Cross talk High Sensitivity Human hepatitis virus B Human hepatitis virus C Gerion et al., Anal. Chem.(2003)

Biobarcode Made of Quantum Dots Fluorescence Intensity-based Molecular Probes QDs with different color ratio coated with silica beads and are linked to probe DNA which can hybridize with target DNA. These QD barcodes can be read by fluorescence profiles QDs in Silica Beads Nie et al., Nature Biotech.(2001)

Particle in a box In many species, the lowest excited state (the lowest unfilled orbital (LUO)) is more than 300 kJ/mole above the ground state (the highest filled orbital (HFO)) and no visible spectrum is observed. Application of the Schroedinger equation to this problem results in the well known expressions for the wavefunctions and energies, namely:     n ; the quantum number (n= 1, 2, 3,....) L ; the 'length' of the (one dimensional) molecular box m ; the mass of the particle (electron) h ; Planck's constant The particle may only occupy certain positive energy levels. Likewise, it can never have zero energy, meaning that the particle can never "sit still". It is more likely to be found at certain positions than at others, depending on its energy level.