week 4 - gold nanoparticles

Lecture 9: Gold Nanoparticles

Introduction to Gold Nanoparticles

  • Historical Context:
      - The first scientific report describing the production of colloidal gold nanoparticles was published in 1857 by Michael Faraday.
      - Reference: Dreaden, E. C., Alkilany, A. M., Huang, X., Murphy, C. J.,
    and El-Sayed, M. A. (2012). Chem. Soc. Rev., 41, 2740-2779.

Properties of Gold at Different Scales

  • The properties of matter are influenced by scale:
      - As objects approach the nanoscale, their physical properties, including optical, electronic, and chemical characteristics, change significantly.
      - Example sizes include:
        - Bulk Properties (gold jewellery and leaf): ~0.1µm
        - Nanolayer Properties (gold on CDs): 50-100 nm
        - Colloidal Gold Particles in Solution: 13-20 nm

Size vs Colour

  • Color Dependence on Particle Size:
      - As the size of gold nanoparticles decreases, they interact with electromagnetic radiation differently, leading to various color absorptions and transmissions.
      - Example: Different colorations observed in stained glass windows are due to the size of gold particles; larger particles display a lustrous color while smaller ones exhibit different hues.

  • Electrical Conductivity:
      - Gold behaves as a semiconductor at the nanoscale; it displays an electrical conductivity that falls between that of a conductor and an insulator.

Surface Plasmon Resonance (SPR)

  • Definition:
      - Surface Plasmon Resonance is the resonance of free electrons in a metal particle, specifically how they move in response to light's electromagnetic field, forming plasmon oscillations.

  • Behavior:
      - When light interacts with these surface electrons, they slosh back and forth at a frequency characteristic to their size and composition, leading to unique optical responses compared to bulk gold.

  • Color Interaction:
      - Nanogold particles are known to absorb specific colors of light (e.g., they absorb yellow and blue and reflect red).

Application of Surface Plasmon Resonance

  • Biacore Systems:
      - SPR technology, utilized in Biacore systems, monitors molecular binding events without the need for labeling, providing insights into binding kinetics, affinity, specificity, and concentration.

Types of Gold Nanoparticles in Biomedicine

  • Commonly used types include:
      - Gold Nanorods
      - Gold Core-Shell Nanoparticles
      - Gold Nanocages
      - The intense color of these nanoparticles stems from collective excitation of conduction electrons, varying with aspect ratio, shell thickness, and galvanic displacement (the deposition of a metal from a solution).

Surface Functionalisation of Gold Nanoparticles

  • Methods for conjugating gold nanoparticles with biofunctional molecules:
      - Hydrophobic Entrapment
      - Electrostatic Adsorption
      - Covalent Cross-Coupling: methods include carbodiimide coupling, maleimide coupling, and click chemistry.

Applications of Gold Nanoparticles

  • Multifunctional Platforms:
      - A multifunctional gold nanoparticle platform can integrate multiple receptor targeting, multimodal imaging, and various therapeutic agents tailored for specific applications.

  • Photothermal Therapy:
      - Heat is generated as excited electrons lose energy to the surrounding media, which can be used in targeted cancer therapies.
        - Specifically designed to absorb light in the range of ~800–1200 nm, efficient at increasing local temperature enough to kill nearby cells (e.g., cancer cells or pathogens).
      - Near-Infrared light (NIR) offers the best tissue penetration depth (approximately 3 cm).

  • Experiments:
      - Study by Dykman and Khlebtsov (2012) showed successful photothermal destruction of tumors in mice using functionalized gold nanorods.

Potential Toxicity

  • Clinical Observations:
      - Studies indicate that the average lifespan of mice receiving gold nanoparticles (8-37 nm in diameter) was reduced compared to those receiving smaller or larger nanoparticles.

  • Toxicity Assessment:
      - Chen Y.S., Hung Y.C., Liau I and Huang G.S (2009) assessed in vivo toxicity of gold nanoparticles published in Nanoscale Research Letters.