Nanoparticles for Biomedical Applications Part I: Preparation & Stabilization Jingwu Zhang 5/3/06

Nanoparticles for biomedical applications Imaging agents Gold, Silver, Quantum Dots, Magnetic Nanoparticles Chemical sensors DNA Modified Au particles Drug delivery devices Nanocapsules Therapeutic agents (?) Conjugated Au particles stick to cancer cells A cluster of gold nanoparticles 50 nanometers in diameter created a much larger crater in ice Nano-sized delivery systems based on lipids and amphiphilic block copolymers

Outline Preparation of monodisperse nanoparticles Gold nanoparticles Methods for achieving uniform particle size Colloidal Stability in electrolyte solutions Surface charge DLVO theory Schulze-Hardy rule Stabilization of Nanoparticles by polymers Polymer adsorption Stabilization mechanisms

Preparation of Monodisperse Nanoparticles

Possible Applications of Colloidal Gold (C. W Possible Applications of Colloidal Gold (C.W.Corti et al, Gold Bulletin 2002, 35/4 11-118) (B.Chaudhuri and S. Raychaudhuri, IVD Technology 2001 March)                                               Microwire Nanoswitch

Making Colloidal Gold: 1857 Faraday prepared gold colloids by reduction of gold chloride with phosphorus. "Experimental relations of gold (and other metals) to light." In: Philosophical Transactions, 147, Part I, pp. 145-181, [1]. London Taylor & Francis 1857. 1861 Thomas Graham coined the word “colloid” to describe systems which exhibited slow rates of diffusion through a porous membrane. Zsigmody (Nobel Prize, 1925) developed “seed” method to produce uniform and stable gold sols. 1908 Mie interpreted the vivid color of colloidal gold (Verification of Mie theory for light scattering). 1951 Turkevich, et. al. studied nucleation and growth of gold particles in sodium citrate (Discussions Faraday Soc. 1951, No. 11 55-75) 1973 Frens developed a simple sodium citrate reduction method to produce colloidal gold of uniform and controlled size. Ref: M.A. Hayat “ Colloidal Gold” Vol 1, 1989

Colloidal Gold Synthesis (Turkevich, et. al. Discussions Faraday Soc Colloidal Gold Synthesis (Turkevich, et. al. Discussions Faraday Soc. 1951, No. 11 55-75) Solution color varies extensively with particle size Usually a deep red, but also dark brown/purple to light orange/yellow Colloid size can be controlled by Au:Citrate ratios Anywhere between 1nm - 100nm Extremely stable Cit- H2AuCl4 H2O, 100OC

Reduction by Citrate (Frens,1973) Boil 50mL 0.01% HAuCl4 (0.29mM) Add 1.75mL 1% Na3Citrate Keep boiling for a few minutes Mean particle size is 12nm (CV 20%)

TEM images gold nanoparticles Produced by citrate reduction

Homegeneous Nucleation Interface Energy r2 Volume Free Energy r3 r* DG DGr r DGr* Free energy change for formation of bulk Saturation Ratio: S=C/Cs C=concentration; Cs=solubility Free energy change for generating the surface: ΔGs=4πr2σ=4π(r/a)2γ γ=surface energy per atomic site Maximum Gibbs free energy for nucleation

Homogeneous Nucleation Size Critical Nucleus Nucleation Rate Activation Energy Sm

Preparation of Uniform Particles Strategy 1: Control of nucleation Monodisperse nanoparticles can be produced by confining the formation of nuclei to a very short period, so that the particle number remains constant and all grow together to the same size. This strategy was first used by La Mer to produce highly monodisperse sulfer sols.

Steps for making Au nanocrystals 1: HAuCl4 + 3e- = Au 2: Supersaturation build-up 3: Homogeneous Nucleation 4: Growth of Nuclei 5: Stabilization by Dispersants [Au] Metastable Zone: S=1 to Sm

Preparation of Uniform Particles Strategy 2: Seeded Growth Preparation of seed crystals Growth on seeds in meta-stable zone Growth 2:1 3:2 Diameter ratio growth 4:3 The particle size distribution becomes narrower with time. This strategy was first used by Zsigmondy to produce monodisperse gold sols

Preparation of Uniform Particles Strategy 3: Aggregation of Nanosized Precursors This strategy has been employed by Matijevic and co-workers to make a variety of transition metal oxide by controlled hydrolysis techniques

Hematite (α-Fe2O3) Prepared by Forced Hydrolysis (Matijević and Schneider, 1978) pHiep=9.2 50nm J. Zhang & J. Buffle, J. Coll Int. Sci 174 (1995) 500-509

Preparation of silver particles Stabilizing agent AgNO3 + NaBH4 Na3Citrate NaOH Ag Reducing agent pH Control

Colloidal Stability in Electrolyte Solutions

Mechanism of surface Charge Generation OH O- COO- Ionization of functional groups at surface Ion adsorption from solution Crystal lattice defects (clay mineral system, due to isomorphous replacement of one ionic species by another of lower charge) X- X- X- Si(IV) Al(III)- Al(III)

Electrical double layer Helmholtz Model Guouy-Chapman Model Stern Model

Colloidal Stability: DLVO Theory Derjaguin-Landau (1941) & Verwey-Overbeek(1948) Van der Waals Attraction R s Electrostatic Repulsion Energy Maximum Sm = 3 nm for hematite

Total interaction free energy VT=VA+VR+VS VS = steric repulsion

Influence of electrolyte concentration on particle-particle interaction energy Debye Parameter К ~ I1/2 ~ electrolyte concentration Double layer thickness (unit: Å): К-1 = 3.04/I1/2

Size Evolution vs. Ionic Strength Fe2O3: 10mg/L (2. 4x1013/L), pH=3 Size Evolution vs. Ionic Strength Fe2O3: 10mg/L (2.4x1013/L), pH=3.0, 25.0±0.3°C Critical coagulation concentration (CCC): The concentration of an electrolyte about which aggregation occurs rapidly

CCC for selected sols

Schulze-Hardy rule (recognized at end of 19th century) The CCC for similar electrolyte solutions is similar but not identical. It is the valency of the counter ion that is of paramount importance in determining the coagulation concentration. According to DLVO theory: CCC~1/z6

Stabilization of Nanoparticles by Polymers

Colloidal stabilization by polymers

Examples of polymers Synthetic polymers Biopolymers: protein, DNA, Polysaccharide

Isotherm of polymer adsorption Configuration of polymer chain on surface (a) A typical high-affinity polymer adsorption isotherm (b) Langmuir adsorption isotherm, usually followed by small molecules

Mechanisms of Colloid stabilization by polymers Increase in electrostatic repulsion Decrease in attraction energy Decrease in Hamaker Constant Steric repulsion Volume restriction Osmotic effect H2O

Destabilization of Colloids by Polymers Polymer bridging Charge Neutralization Electrostatic Patch Model

Double roles of polymers Flocculation and Stabilization Steric stabilization Charge reversal Electrostatic & steric Increasing polymer concentration

Aggregation of Hematite by PAA (Mw=1.36x106) Polymer Concentration (ppm) Collision efficiency factor Zeta-potential DLA Pre-DLA Post-DLA

Effect of Molecular Weight Collision efficiency factor Zeta-potential

Q&A

Methods for determining particle size

Dynamic Light Scattering (PCS)                        PM With this method each size measurement takes less than a minute. Thus it is especially useful for aggregation kinetics studies.

Bonding Type IV – Van De Walls Force from permanent and induced Dipole