Controlling the assembly of nanoparticles using surface grafted molecular and macromolecular gradients

Abstract

We report on the generation of assemblies comprising number density gradients of nanoparticles in two (2D) and three (3D) dimensions. These structures are fabricated by creating a surface-bound organic template which directs the spatial arrangement of gold nanoparticles. The 2D template is made of amine-terminated organosilane with a concentration gradient along the solid substrate. The 3D matrix comprises surface-anchored poly(acryl amide), whose molecular weight changes gradually on the specimen. In both cases, the composite is assembled at low pH, where the positively charged-NH + 3 groups within the organic scaffold attract negatively charged gold nanoparticles. We use a battery of experimental tools to determine the number density of particles along the gradient substrate and in the case of 3D structures also their spatial distribution. For 2D gradient assemblies, we show that gold nanoparticle coverage on the surface decreases gradually as the concentration of substrate-bound aminosilane decreases. The number of particles in the polymer brush/particle hybrid is found to increase with increasing polymer molecular weight. We show that for a given grafting density of polymer brush, larger particles predominantly stay near the brush-air interface. In contrast, smaller nanoparticles penetrate deeper into the polymer brush, thus forming a 3D structure. Finally, we discuss possible applications of these nanoparticle gradient assemblies.

Stimuli-Responsive Polyelectrolyte Brushes As a Matrix for the Attachment of Gold Nanoparticles: The Effect of Brush Thickness on Particle Distribution

“…Brushes can be used as matrices for the immobilization of nanoparticles, resulting in nanocomposite materials with interesting features, such as tunable optical, magnetical or mechanical behavior. In the present study, polyelectrolyte brushes are used as a matrix for the immobilization of gold nanoparticles (AuNPs), which induce optical properties, due to their surface plasmon resonance (SPR) [34]. This can be exploited to construct a nanosensor, where the swelling/shrinking of responsive brushes can be monitored by detecting the surface plasmon resonance of the particles [20,22].…”

“…If the brushes are long enough, 3D ordering of particles should be observed, i.e., the particles should interpenetrate into the brush system. Bhat et al [34] studied the assembly of AuNPs in two and three dimensions by generating concentration gradients of a self-assembled monolayer as a 2D template and molecular weight gradients of poly(acryl amide) as the 3D template. Using scanning force microscopy and UV-Vis spectroscopy, they found an increased particle uptake with increasing polymer molecular weight [34].…”

Structural changes in gradient colloidal thin gold films deposited from aqueous solution

“…Deposition methods of choice include vapour deposition techniques Bhat et al, 2003), sputtering Bauer et al, 2003), pulsed laser deposition (Krebs et al, 2003) and solution casting (Park & Xia, 1999;Su, 2004;Narayanan et al, 2004). All these methods aim to produce a large and homogeneous surface area.…”

Surface-chemical and -morphological gradients

“…The oxygenated species were used as grafting sites for various polymers, such as PEG 58 or others. [59][60][61] Brush-like polymer gradients prepared by these techniques have been used to immobilise nanoparticles 62,63 and to study protein adsorption and cell adhesion.…”

“…One widely used approach to create morphological gradients is to bind particles onto a smooth substrate. Genzer and his colleagues 62,66,[91][92][93] have established a simple but highly efficient procedure to create nanofeature gradients by creating gradients of polymers and then adsorbing gold particles. They have employed two basic principles to create one-dimensional gradients or a combination of both to prepare two-dimensional gradients.…”