2D Attogram Surface Plasmon Resonance Imaging
24th April 2014

Array Development and fluorescence enhanced Imaging

The PRP array research seeks to explore the optical plasmon of the particles as a function of their size, shape and proximity for the application to biological problems. The interaction between nano-features on the surface and the PRP has led to the report of enhancements in fluorescence, Surface Enhanced Raman Spectroscopy (SERS) and optical absorption spectroscopy. Our goal is to construct an array of 'hot spots', Figure 2, to enhance the sensitivity of the techniques and produce the attogram ml-1 sensitivity previously seen in e-CRDS measurements.

Technical Challenge - Fabricating the nano-structured surfaces. We will use SRIF2 funded e-beam/focussed ion-beam lithographies at Exeter for nano-structuring since these techniques allow exquisite nanometre control, allowing us for example to investigate whether an array of metallic posts or holes provides a better surface for our purposes. Second, we will use self-assembly techniques such as colloids and nanosphere lithography, already we have considerable expertise in using both of these techniques to make sub-wavelength scale structures. In addition, we will also explore the use of photo-modifying the fabrication of the metallic nanoparticles so as to tailor their optical response. Structural information about our metal surfaces and scaffolds will be obtained via SEM (Exeter) and AFM (Nottingham) images.

Fig: Candidate nanostructured surfaces (a) coated nanospheres, (b) posts, and (c) hole arrays. In each case, contact between the metallic surface (red) and the molecule coated metal particle (yellow) produces hot spots that lead to dramatic modifications of the optical response.

Understanding the optical response. We will need to develop an understanding of the underlying physics through systematic and innovative investigations of the plasmon modes supported by the nanostructured surfaces, metal-particle coated nanostructured surfaces, and molecule filled hot-spots of the metal-particle coated nanostructured surfaces. We will make intensive use of fluorescence from test molecules placed on our structures to explore the plasmon modes they support. This will involve imaging the fluorescence and making spectral and time resolved measurements. Fluorescence hot-spots of sub-wavelength size will be identified by their small size, brightness, and radically reduced fluorescence lifetimes. Interrogation of the surface enhanced fluorescence spectrum will form part of the control of the design cycle of the array and will require the constant use of a confocal microscope. Particle sizes will be fabricated and tested using the fluorescence and e-CRDS technologies to calibrate the response of the array. Simple protein binding studies using bovine serum albumin (BSA) and fibrinogen will be used to optimise the particle size and shape and hence plasmon frequency. The array will be further tested using DNA hybridisation. The benchmark for performance will be the ability to monitor hybridisation of DNA sequences of known length, shorter lengths denoting better performance.