Nanoimprint Lithography with UV-Curable Hyperbranched Polymer Nanocomposites for Optical Biosensing Applications
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Nanoimprint lithography with UV-curable hyperbranched polymer nanocomposites for optical biosensing applications Valérie Geiser1, Yves Leterrier1, Jan-Anders E. Månson1, Rosendo Sanjines2, Guy Voirin3 and Max Wiki4 1 Laboratoire de Technologie des Composites et Polymères (LTC), Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 12, 1015 Lausanne, Switzerland. 2 Laboratoire de Nanostructures et Nouveaux Matériaux Electroniques (LNNME), Ecole Polytechnique Fédérale de Lausanne (EPFL), Station 3, 1015 Lausanne, Switzerland. 3 Centre Suisse d’Electronique et de Microtechnique SA (CSEM), Jaquet-Droz 1, 2002 Neuchâtel, Switzerland. 4 Dynetix AG, Bahnhofstrasse 1, 7302 Landquart, Switzerland. ABSTRACT Polymer nanocomposite gratings with a 363 nm period and a 12 nm step height were replicated using a glass master in a rapid, low-pressure imprint process. The composite materials were based on a UV-curable acrylated hyperbranched polymer and nanosized SiO2 particles. The influence of particle fraction up to 25 vol%, process pressure and UV intensity on the grating geometry was analyzed using atomic force microscopy. The period of the grating was found to be identical to that of the glass master for all investigated conditions. It was shown that the gel point of the nanocomposite was an important factor that determined the stability as well as the dimensions of the imprinted structure. However, a distortion of the grating was observed with increasing fraction of SiO2, which was correlated to the increased internal stress of the composite. Wavelength interrogated optical sensors were produced by depositing a high refractive index TiO2 layer on the composite gratings. The laser signal strength of the polymer sensors was equal to that of the reference high precision glass sensor with 10-12 g/mm2 sensitivity. The strength was lower for the nanocomposites due to propagation losses argued to result from residual porosity. INTRODUCTION Food safety is of great concern within the general public health discussion. Antibiotic residues are among the most frequently detected contaminants in diary products, because antibiotics are not only used for the treatment of bacterial infections, but also to improve productivity of foodstuffs. A strict legislation has emerged to regulate the presence of these undesired substances, hence to prevent their negative impact on human health [1]. The wavelength interrogated optical biosensing platform (WIOS) [2] was developed to detect label-free molecular binding with sensitivity as high as 10-12 g/mm2 and was applied to the detection of trace contaminants in liquid food. This label-free sensor contains an optical nanograting through which a light wave is coupled into a high refractive index waveguide layer, to form a sensitive optical resonator. The resonance condition at which coupling occurs is interrogated with a tunable laser at 763 nm wavelength. This resonant coupling condition is very sensitive to minute changes in the refractive index caused by the interaction of the analytical
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