This work presents the proof of concept of the detection of global and surface optical index variations by surface plasmon resonance (SPR) thanks to optical fiber bundles. This work is the first necessary step for the future design of a lab-on-fiber tool dedicated to molecular analysis for endoscopic diagnosis. Our approach is based on nanostructured optical fiber bundles comprising several thousands of individual optical fibers. These nanostructures were coated by a thin gold layer in order to gain interesting optical properties such like SPR. The sensitivity and resolution of the bundle to global optical index changes were measured in retro-reflection. We performed numerical simulations in order to optimize the fiber tip geometry, gold coating thickness and finally enhance their analytical performances. We achieved a resolution of 10-4 refractive index unit, which is fully compatible with the detection of biological interactions involving large proteins or bacteria. Finally, we proved that our sensor was sensitive to surface optical index variations and able to detect the adsorption of a thin self-assembled molecular layer.
Single biomolecule behaviour can reveal crucial information about processes not accessible by ensemble
measurements. It thus represents a real biotechnological challenge. Common optical microscopy approaches
require pico- to nano-molar concentrations in order to isolate an individual molecule in the observation
volume. However, biologically relevant conditions often involve micromolar concentrations, which impose a
drastic reduction of the conventional observation volume by at least three orders of magnitude. This
confinement is also crucial for mapping sub-wavelength heterogeneities in cells, which play an important role
in many biological processes. We propose an original approach, which couples Fluorescence Correlation
Spectroscopy (FCS), a powerful tool to retrieve essential information on single molecular behaviour, and
nano-fakir substrates with strong field enhancements and confinements at their surface. These
electromagnetic singularities at nanometer scale, called "hotspots", are the result of the unique optical
properties of surface plasmons. They provide an elegant means for studying single-molecule dynamics at
high concentrations by reducing dramatically the excitation volume and enhancing the fluorophore signal by
several orders of magnitude. The nano-fakir substrates used are obtained from etching optical fiber bundles
followed by sputtering of a gold thin-film. It allows one to design reproducible arrays of nanotips.
Ordered arrays of nanometer-sized optical probes with electrochemiluminescent properties were developed on the
distal face of imaging fiber bundles. The fabrication steps are adapted from SNOM probes and nanoelectrodes
methodologies and allow to produce high-density arrays of opto-electrochemical probes which retain the initial
architecture of the bundle. Apertureless probe arrays and also nanoaperture arrays have thus been prepared. The
angular distribution of the far-field intensity transmitted through such nanostructured arrays depends both on their
respective architectures and on the characteristic dimensions of the nanoprobes. The subwavelength aperture arrays
show a diffracting behavior which is a function of the optical aperture size. The far-field analysis demonstrates their
potential application as a parallel near-field optical array in both apertureless and aperture configurations. In addition,
each optical nanoaperture is surrounded by a ring-shaped gold nanoelectrode. The electrochemical response of the
array is sigmoidal in shape indicating that the nanoelectrodes forming the array are diffusively independent. In other
words, each nanoelectrode of the array probes electrochemically a different micro-environment. We show also that the
nanoaperture array can be used as an electrochemiluminescent nanosensor array for NADH. Eventually, the arrays
keep the imaging properties at both nanometer and micrometer scales. Indeed, each nanoprobe can explore optically a
near-field region, whereas the global array allows imaging simultaneously a large micrometric area. This optical array
format plays therefore a bridging role by interrelating optical and electrochemiluminescent information obtained
concomitantly at the nanometer and micrometer scales.
Ordered arrays of nanometer-sized optical probes were developed on the distal face of coherent optical fiber bundles. The fabrication steps derived from SNOM probes methodology and allowed to produce high-density microarrays of nanoapertures with adjustable dimensions. The angular profile of the far-field intensity transmitted through the nanostructured arrays shows a diffracting behavior which is a function of the nanoaperture sizes. Nearfield optical behavior was thus established in an array format where each element is equivalent to a single nanoaperture. It means that each subwavelength aperture of the array is optically independent in the far-field regime. This analysis demonstrates its potential use as a near-field optical array. Since the initial architecture of the bundle is retained, the array format allows imaging a sample concomitantly at both nanometric and micrometric scales. Therefore such an array plays a bridging role between these 2 fundamental scales.