Untitled Quiz

Biomedical Nanotechnology Lecture 2

• Lecture topic: Inorganic Nanostructures: Properties and Synthesis
• Lecturer: Duan Hongwei
• Office: N1.3-B3-12
• Office hours: Teams meetings by request

Framework

• Biomedical Nanotechnology
• Introduction
• Characterization
• Nanostructure
• Properties
• Synthesis
• Medical Application
• Diagnostics
• Therapeutics

Key Points

• Quantum size effect (semiconductor QDs)
• Optical properties of quantum dots (QDs)
• Surface plasmon resonance (SPR) & factors controlling the SPR of metal nanostructures
• Properties of magnetic nanoparticles
• Wet-chemistry for inorganic nanoparticle synthesis
• Stabilizing mechanisms of nanoparticles

Colors: Visible Light Spectrum

• Visible light spectrum ranges from 400-700 nm
• Longer wavelength = lower energy
• Shorter wavelength = higher energy

Topic 1: Semiconductor Quantum Dots (QDs)

• QD sizes affect emission wavelength
• Larger QDs emit longer wavelengths

Fluorescence Imaging

• Traditional fluorophores:
  • Organic dye
  • Fluorescent proteins
• Nanoscale fluorophores:
  • Semiconductor quantum dots

Fluorescence of Organic Dyes

• Fluorescence is the emission of light after absorbing light.
• Emitted light has a longer wavelength and lower energy than the absorbed light.
• Absorption (excitation): Higher energy (shorter wavelength)
• Fluorescence (emission): Lower energy (longer wavelength)
• Stokes shift: Difference between excitation and emission spectra

Fluorescent Dyes & Proteins

• Green laser excites fluorescent dyes and proteins.
• Fluorescent proteins are labelled and visualized

Semiconductor Quantum Dots (QDs)

• Photograph of different-sized QDs (irradiated by 365nm UV light)
• Emission wavelength of QDs increases with their sizes (1-10 nm)

Quantum Size Effect

• When QD size < double Bohr radius of exciton (electron & hole), energy band gap is size-dependent.
• Smaller QDs have a larger band gap, and subsequently, shorter wavelength emission
• Larger QDs: smaller band gap, longer wavelength emission

QD: Optical Properties

• Excitation: Narrow & symmetric emission, broad absorption profile
• Emission: Single wavelength excitation of multi-colored QDs.
• Superior photo-stability

QDs: Chemical Composition

• Periodic table showing elements used in QD synthesis.

Organic Dye: Excitation (absorption) and Emission (Fluorescence)

• Light absorption: electron transition to excited state (S1)
• Non-radiative transition: S1 → S0 (Heat release)
• Fluorescence: Electron transition from S1 → S0 and light emission
• Stokes shift: Difference between absorption and emission spectra.

QDs vs Organic Fluorophores

• Excitation: Multiple excitation peaks for dyes, single peak for QDs.
• Emission: Broad emission (dye), narrow emission (QDs)
• Stability: QDs: superior photo-stability, dyes: poor photo-stability

Size Exclusion Chromatography (SEC)

• Bigger QDs elute faster than smaller QDs.
• Longer wavelength emission corresponds to faster elution times.

Topic 2: Metal Nanostructures

The "Color" of Gold

• Color depends on gold nanoparticle size (smaller is purplish-red, larger is red).
• Gold nanoparticles are colored because of surface plasmon resonance.

Metal Nanostructures

• Dietary supplements
• Antioxidant
• Source of metal elements

Metallic Bonding & Free Electrons

• Free electron model: electrons are delocalized, forming a cloud around the cation core.
• Metal nanoparticles consist of an ion matrix and free electrons.

Surface Plasmon Resonance

• Localized surface plasmon resonance (LSPR) is the collective oscillation of electrons in metal nanoparticles when excited by light.
• The wavelength of the absorbed light depends on the size and shape of the nanoparticle

Why are Gold Nanoparticle Dispersion Red

• Absorption of specific wavelengths causes colors.
• The absorbed light energy is converted into heat or dissipated.
• The unabsorbed light is what we see as color.

Surface Plasmon Resonance: Size Dependence

• Surface plasmon resonance red-shifts with increasing nanoparticle size.
• Uneven excitation leads to retardation effect.

Surface Plasmon Resonance: Shape Effect

• Au nanorods exhibit both transverse and longitudinal surface plasmon resonances.
• The peaks are sensitive to size/aspect ratios, and can cover wider spectral ranges.

Surface Plasmon Resonance: Aggregation State

• Surface plasmon band red-shifts upon nanoparticle aggregation due to interparticle coupling.
• Strong color change (red-to-blue) is used in biosensors.

Topic 3: Magnetic Nanostructures

Magnetic Nanoparticles

• Size-dependent magnetic properties due to surface effect.
• Bigger nanoparticles have stronger magnetic properties.

Magnetic Nanoparticles- Polymerization

• Polymerization around aligned magnetic nanoparticles in external magnetic field leads to structural fixation, forming nanochains.

Magnetic Nanochains

• SEM and optical microscopy images show random/aligned nanochain structures.

Magnetic Nanoparticles: Heating

• Nanoparticles generate heat in high-frequency oscillating magnetic fields due to magnetic relaxation.

Topic 4: Synthesis of Nanoparticles

Synthesis of Nanoparticles

• Components: synthesis, surface engineering, and bio-functionalization.

Nanoparticles & Surface Coating Ligands

• Surface atoms in nanoparticles differ from bulk atoms, leading to high surface energy.
• Surface coating ligands guide growth and stabilize structures.

Gold Nanoparticle Synthesis: Citrate Reduction

• Citrate is a reducing agent and coating ligand.
• Leads to controlled synthesis of 5-100nm gold nanoparticles.

Nanocrystal Synthesis

• Precursor compound decomposes or reduces to zerovalent atoms.
• Atom concentration increases over time.
• Nucleation: Atoms aggregate into nuclei.
• Growth: Nuclei grow into nanocrystals of increasing size

Quantum Dots Synthesis

• Monodisperse high-quality QDs are synthesized through controlled conditions.

Size-Controlled Synthesis of Nanocrystals

• Controlling the number of nuclei controls the nanocrystal size.

CoPt₃ Nanocrystal Synthesis

• Temperature controls nucleation rate.
• Higher temperature → faster nucleation → smaller particles.

Shape-Controlled Synthesis of Nanocrystals

• Controlled growth leads to varieties of nanocrystal shapes like spheres, rods, and wires.

Gold Nanorods: The Seed Growth Method

• Preferential ligand binding leads to anisotropic growth (different directions).
• Ligands (e.g., CTAB) control the growth direction and facets.

Topic 5: Surface Engineering

Why Surface Engineering?

• Important for nanoparticle properties
• Water solubility, colloidal stability, surface functionality, nanoparticle-cell interactions, biodistribution and pharmacokinetics are key facets.

Stabilizing Mechanisms for Nanoparticles in Aqueous Medium

• Surface charge (e.g., functional groups like -SH) prevents aggregation.
• Stable polymers (e.g., PEG) create steric repulsion.

Charge-Stabilized Nanoparticles

• Repulsion of surface charges prevents aggregation.
• Salt concentration reduces repulsion, affecting colloidal stability.

Polymer Ligands: Steric Stabilization

• Polymer coatings create steric repulsion between nanoparticles.
• Solubility and density of polymers affect stability.

Surface Modification by Ligand Exchange Reaction

• Functional modifications (e.g., thiols).

Self-Assembled Monolayer

• Functional headgroups, hydrocarbon chains, and assembly groups for various functionalities, including protein binding.

Surface Modification by Ligand Exchange Reaction

• Coating with thermosensitive polymers (e.g. PNIPAM) allows for temperature-control.

Protein Corona on Nanoparticles: Non-Specific Binding

• Biological fluids coat nanoparticles with proteins.
• Non-specific protein binding can impact function.

PEGylation: One Way to Reduce Non-Specific Binding

• PEGylation reduces protein adsorption by providing steric hindrance.

Topic 6: Carbon Nanomaterials

Carbon Nanomaterials

• Carbon nanomaterials include zero-dimensional (buckyballs), one-dimensional (carbon nanotubes), two-dimensional (graphene), and three-dimensional (graphite).

Buckyball

• Discovered in 1985
• Fused-ring structure similar to a soccer ball

Carbon Nanotube

• Discovered in 1991
• Single-walled and multi-walled nanotubes (SWNTs, MWNTs)

Synthesis of CNTs: Chemical Vapor Deposition (CVD)

• Metal catalysts are used to grow CNTs on a substrate

Graphene

• A single layer of carbon atoms arranged in a honeycomb lattice
• Isolated in 2004