Optical Waveguide for Common Path Simultaneous Refractive Index and Broadband Absorption Measurements in Small Volumes

Simultaneous refractive index monitoring and absorption spectroscopy in small volumes common to both measurements using single light source and detector are beneficial, but challenging to perform. This work presents an optical device consisting of a porous waveguide deposited on glass, which is called dye-doped leaky waveguide (DDLW), for this purpose. The waveguide was made of agarose doped with a dye, reactive blue 4. Glycerol and rhodamine 6G were used as an exemplar system to demonstrate the feasibility of DDLW to perform simultaneous refractive index and broadband absorption measurements in a small common volume of 3.5 nL. In this case, the immobilised reactive blue 4 permits visualisation of the resonance angle of the waveguide for refractive index monitoring, while the additional absorption caused by the free rhodamine 6G was determined by measuring the wavelength dependent additional losses in the reflectivity profile of the DDLW. The refractive index sensitivity and limit of detection of the DDLW was 106.32 ± 0.97° RIU−1 and 2.82 × 10−6 respectively. By combining the two measurements, the DDLW was shown to be suitable to monitor the absorption spectrum and anomalous dispersion of rhodamine 6G as well as the nature of its interactions with reactive blue 4. These interactions resulted in enhancing the concentration of the rhodamine 6G in the waveguide by a factor between 119 and 191. The DDLW was shown to be suitable to obtain the absorption spectrum of rhodamine 6G at concentrations as low as 1 μM. Future work will focus on the application of the DDLW to characterise analytes/determine parameters of biological significance such as iron loading per molecule of serum ferritin.