nanoparticles - quantum dots

Definition of Light Wavelength:
  - Light is identified by its wavelength. For example:
    - 650 nm corresponds to red light.
    - 470 nm corresponds to blue light.
  - Energy Relationship:
    - Light with shorter wavelengths has higher energy.
    - Blue light (shorter wavelength) possesses more energy than red light (longer wavelength).
    - Ultraviolet (UV) and X-rays exhibit even greater energy levels as their wavelengths decrease further.

Fundamental Concepts:
  - Traditional fluorophores are small organic molecules.
  - In fluorescence, a species absorbs a photon, exciting it from its ground electronic state to various vibrational states in an excited electronic state.
  - Through collisions with other molecules, the excited molecule loses vibrational energy until reaching the lowest vibrational state of the excited state.
Typical Fluorescence Microscopy Techniques:
- Rely on the absorption of light at a specific wavelength (excitation), followed by emission of secondary fluorescence at a longer wavelength.
- The excitation and emission wavelengths are separated by tens to hundreds of nanometers.

Definition:
  - Quantum dots (QDs) are semiconductor nanocrystals, a term coined by Mark Reed in 1988.
  - They are semiconductor inorganic nanomaterials ranging between 1-10 nm and contain elements from:
    - Groups II–IV (e.g., CdSe, CdTe, CdS, ZnSe)
    - Groups III–V (e.g., InP, InAs) of the periodic table.
  - Feature superior fluorescent properties with less photobleaching compared to conventional chromophores.

Nobel Prize in Chemistry 2023:
  - Awarded to:
    - Moungi G. Bawendi (MIT)
    - Alexei I. Ekimov (Nanocrystals Technology Inc.)
    - Louis E. Brus (Columbia University)
  - Recognized for the discovery and synthesis of quantum dots, marking a significant contribution to nanotechnology.
Technological Features of Quantum Dots:
  - QDs possess widely tunable optical, electrical, chemical, and physical properties.
  - Applications include:
    - Energy harvesting
    - Illumination
    - Displays
    - Cameras
    - Sensors
    - Communication and information technology
    - Biology and medicine
  - QDs have been utilized to develop efficient lasers, displays, and biotags, and are emerging in:
    - Photovoltaics
    - Sensing
    - Quantum information

Research Example:
- Study by Igor L. Medintz et al. on microtubules labeled with QD 630–streptavidin (red) while nuclear antigens were stained green with Alexa 488.
- Quenching is defined as any process that decreases fluorescence intensity of a substance.

Definition of Band Gap:
- 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.
- The band gap is essentially a forbidden region for electrons; larger band gaps restrict electron movement more than smaller ones.
Size Influence on Band Gap:
  - The band gap in a quantum dot inversely correlates with its size:
    - Larger quantum dots exhibit smaller band gaps, emitting longer wavelength photons (red-shift).
    - Smaller quantum dots emit higher energy light (bluer in color).

Dimensional Constraints on Carriers:
  - 3-D Systems: Carriers act as free carriers in all directions.
  - 2-D (Quantum Wells): Carriers act freely in a plane, first observed in semiconductor systems.
  - 1-D (Quantum Wires): Carriers can move only along the wire's length.
  - 0-D (Quantum Dots): Carriers are confined in all dimensions (no free carriers).

Unique Photo-Physical Properties of QD Probes:
  - Narrow size-tunable light emission allows precise control over probe color by varying nanoparticle size.
  - Ability to absorb high-energy (UV-blue) light and emit fluorescence enables effective separation from fluorescent background.
  - High photostability facilitates real-time monitoring and quantitative analysis, overcoming limitations seen in organic dyes.

Toxicity and Biocompatibility:
  - The core of quantum dots consists of semiconductor material responsible for light emission, while the shell typically comprises an insulator that protects this property.
  - The shell would be functionalized with biocompatible materials, such as PEG or lipid layers.
  - Additional functionalization can serve various purposes (e.g., drug delivery, antibody assembly for target specificity).

Ligand-Exchange Process:
- Replaces native hydrophobic surface ligands (e.g., TOPO) with hydrophilic ones by direct anchoring of ligands to the QD surface.
Encapsulation Procedures:
- Preserve native QD structure, over-coating QDs with amphiphilic molecules (e.g., polymers, lipids) through hydrophobic interactions.

Requirements for Aqueous Biological Conditions:
  - Efficient fluorescence, colloidal stability, and low non-specific adsorption.
  - Challenges:
    - Tendency to aggregate and adsorb non-specifically due to hydrophobic organic ligands on their surface.
  - Approaches:
    - Monolayer and multilayer shell strategies for stability and control of coating processes.
    - Overcoating with proteins followed by other layers for bioconjugation.

Definition:
- A technique to gauge distance between two chromophores (donor-acceptor pair), effective only if the separation distance is smaller than 10 nanometers.
- Mechanism involves the transfer of excitation energy from a donor fluorophore in an excited state to a nearby acceptor chromophore in a non-radiative manner through dipole-dipole interactions.

Advantages of Quantum Dots:
  - More resistant to degradation versus organic dyes, enabling tracking of cellular processes for extended periods.
  - Broad absorption spectrum with static emission wavelength allows for versatile labeling and tracking of biological processes.
  - Potential use for monitoring cancerous cells and integrating therapeutic strategies (theranostics) with tumor-fighting agents.

Labeling Applications:
  - Surface and intracellular targets can be labeled with QD probes for various biological research.
  - Simultaneous two-color staining to visualize nuclear antigens and Her2 receptors.
  - Non-invasive imaging capabilities of implanted QD-tagged tumor cells, distinguishing quantum dot signals from organic dye autofluorescence.

Comparison of imaging depths:
  - MRI: No limit
  - PET/SPECT: No limit
  - X-ray, CT: 2-4 cm depth limit
  - Laser emitting diodes: Limited by resolution of < 1 mm3

Pros:
- Tunability from UV to IR, high photostability, and possibility for significant photon yield.
- Conjugation capabilities for targeting ligands.
Cons:
- Potential toxicity to normal tissues/organs, including reproductive systems.
- Myths surrounding simple synthesis, broad absorption capabilities, and ultra-thin biocompatible coatings.