Forming Antifouling Organic Multilayers on Porous Silicon Rugate Filters Towards In Vivo/Ex Vivo Biophotonic Devices

Advances in medical diagnostics towards in vivo and ex vivo devices relies heavily on development of transducers that are minimally invasive, highly sensitive and selective to analytes of interest and stable to biological enviroments. Minimally invasive optical devices have been developed for use intravascularly, extravascularly and subcutaneously with medical applications for monitoring during surgery, at the bedside and in point of care diagnosis. One drawback to many optical systems is the need for molecular species at the biointerface that require excitation, often limiting the choice of analyte or requiring introduction of labelling compounds to the system. Furthermore, sophisticated microscopy instrumentation is required and the emitted light oftentimes interacts adversely with tissue, confounding accurate detection and read-out. Minimally invasive fibreoptic devices using external light sources and lasers to interrogate the refractive index change at the biorecognition interface have shown promise, obviating the need for labelling schemes. Although methods using evanescent fields such as surface plasmon resonance or resonant mirror techniques eliminate the need for labels, external contact with the device remains a requirement. Non-invasive read-out of implantable photonic structures is an interesting alternative to these methods, made possible by engineering devices which respond within the optical window of tissue (near-IR, 700–1000 nm). Using the intrinsic properties of photonic materials, detection relies on a change in the wavelength of reflected light induced by refractive index changes upon biorecognition. This approach allows more control over optical path lengths, avoiding light scattering effects common to fiberoptic transduction whilst enabling a simple method for real time monitoring. Detection of molecular species in a patients blood stream (in vivo, ex vivo) or subcutaneous environement can then be performed merely by irradiating the area with a light source whilst collecting the reflected spectrum. One material that has enormous potential for implantable photonic devices is porous silicon. The interest in porous silicon is due to the ease and quality of manufacturing, a large internal surface area for analyte binding, an inherant biocompatability and the ability to tune the optical properties of porous silicon photonic crystals with unprecedented control. Porous silicon is formed by anodic etching of silicon in ethanolic hydrofluoric acid solution. The porosity of the material is directly proportional to the applied current density during the electrochemical etch, thus allowing control over the average refractive index of the material. Porous silicon based rugate filters are a class of multilayered photonic crystal with a sinusoidal refractive index distribution normal to the surface. Light incident on the surface of a rugate filter will be reflected in a narrow spectral range, the spectral position of which is dependent on the