Long-range surface plasmon resonance sensor with liquid micro-channels to maintain the symmetry condition of the refractive index

Abstract

We propose a method to maintain the symmetry of the refractive index with respect to an Au film, in which the refractive indices are the same near both surfaces of the Au film, for LRSPR (long-range surface plasmon resonance) sensors. Maintenance of the symmetry is necessary for exciting the LRSPR mode. However, because the buffer layer under the Au film is usually made of a solid dielectric material with a constant refractive index, the quality of the measurement is reduced when the refractive index of the analyte used is dramatically different from that of the buffer layer. To solve this problem, the proposed sensor is equipped with liquid channels under the Au film. The Au film used in this study was supported by a thin (100 nm) polymer film forming parallel, one-dimensional liquid channels with a 29 µm pitch. Because the analyte solution flows in the channels, both surfaces of the Au film face the same analyte. Thus, this configuration automatically satisfies the symmetry condition for analytes with a wide range of refractive indices. We examined the validity of the sensor and compared it to that of a conventional sensor by measuring the LRSPR for five analyte solutions with different refractive indices. LRSPR dips were clearly observed for all of the analytes tested. The dip of the conventional LRSPR sensor became shallow when the refractive index increased, with the final dip depth being 65% of the initial dip depth for a refractive index of 1.358. In contrast, the dip depth of the proposed LRSPR sensor remained constant over the entire measured refractive index range of 1.331 to 1.358. These results indicate that the proposed sensor maintains the symmetry condition and confirm that the proposed method is effective for highly sensitive LRSPR measurements for a wide variety of analyte species.