Vertical arrays of gold nanorods achieved

(a) Illustration of the experimental setup. The micropatterned substrate is placed in the centre of an Eppendorf tube and immersed in a concentrated colloidal solution of GNR’s. The substrate is left to dry under isothermal conditions. b) Schematic representation of the formation of standing arrays of GNR’s on a template upon solvent evaporation. 

 

Transmission SAXS pattern recorded on a 100 μm × 250 μm section. Gray lines indicate the positions of the hcp reflections (labels are Miller indices (hk) for the reflections; not all are shown for clarity). The inset shows the associated diffraction pattern. 

 

 

October 2011

Fabrication of novel self-assembly metallic nanostructures shows great potential for applications in biosensing, optical analysis, computing and solar energy conversion.

Working collaboratively with Instrument Manager Matteo Altissimo, Thibaut Thai of Monash University has combined bottom-up self-assembly processes with top-down techniques to generate vertical arrays of gold nanorods on patterned substrates.

Gold nanorods (GNRs) are of particular interest to researchers as they display unique yet highly variable optical properties. One such property is their remarkable sensing ability, which is exploited during Surface Enhancement Raman Spectroscopy (SERS). During this process GNR’s are closely packed together in a hexaganol-type arrangement forming high density “hot- spots”. If a molecule is within this “hot-spot”, its Raman signal will be greatly amplified.

The Raman signal then acts as a molecular fingerprint and is key in characterising a materials’ chemical structure, temperature or frequency mode.

In past research, controlling the self-assembly process has been a challenge due to the anisotropic nature of the nanorods.

Using a method developed at MCN, researchers were able to control the orientation of the assembly and select a precise location to express the desired features. The mask aligner, reactive ion etching process, electron-beam evaporator and scanning electron microscope were all critical components during the fabrication process.

The accuracy of the micro-patterns was confirmed using small-angle X-ray scattering (SAXS), a characterisation process undertaken using the SAXS beamline at the Australian Synchrotron National Research Facility.

In addition to showcasing the vast array of fabrication and characterisation tools available within MCN, the work highlights the unique relationship between the Australian Synchrotron and MCN, acting in unison as a hub for world-class research.

Manipulating hot spots to increase biosensor sensitivity

(a) A 2μm2 AFM image of gold nanoparticles on a flat substrate. (b) SEM micrograph showing measurements of nanoparticles under a polymer film.

 

October 2011

Surface Plasmon Resonance (SPR) can be described as the resonant, collective movement of electrons activated by incident light that travels in a direction parallel to a surface. When two nanoparticles are placed in close proximity, the electric field is greatly enhanced creating a ‘hot spot.’ These hot spots are of particular interest to researcher Soon Ng, of Monash University. Working collaboratively with MCN’s Varsha Lal and Matteo Altissimo, Soon is manipulating these electric field hot spots to increase the sensitivity of his biosensors.

The JPK Nanowizard II AFM at the MCN was used to accurately measure the height of the particles as well as the overall topography. One of the key features of this instrument is that it allows imaging in both air and liquid.

Additionally, the FEG-SEM was used to measure the size of particles and to aid in the calculation of surface coverage. Future work on this project will utilise electron beam lithography to create templates for nanoparticle absorption in order to study the spatial configurations related to the hot spots. As well as having applications in biosensing, chemical detection and solar cell technologies, the properties of surface plasmon resonance may be used during characterisation and imaging processes.

This is an example of a project where multiple capabilities available at MCN were used in sequence and assembly and characterisation.