Multiwavelength Fluidic Sensing of Water-Based Solutions in a Channel Microslide with SWIR LEDs

A multiwavelength instrumental configuration, combining short-wave infra-red (SWIR) LEDs (with emission bands centered at = 1.3 um, = 1.45 um, and = 2.15 um) and InGaAs amplified photodiodes has been applied for contactless fluidic sensing by measuring light intensity transmittance across a channel microslide. The main goal is the proof of the principle of urea detection in water-based solutions, exploiting urea absorption band around = 2.15 um. The functionality of the system has been tested on urea water solutions with urea concentrations up to 0.2 g/ml (20 wt%), an interesting range for biomedical applications. Transmittance in the spectral range around = 1.3 um is mainly affected by the fluid refractive index (real part). Around = 2.15 um, the transmittance is conditioned by the absorption increment due to an increasing fraction of urea in the solution. Since even water (solvent of all tested solutions) absorbs around = 2.15 um, its contribution can be accounted and compensated for by measuring the transmittance in the wavelength range around = 1.45 um where water only exhibits a strong absorption band. By introducing R1.45(C) and R2.15(C) as the ratios between the output signal in presence of the solution and the output signal collected on the empty channel, at = 1.45 um and = 2.15 um, respectively, we have demonstrated that their ratio (C) = R1.45(C)/R2.15(C) is highly specific for urea detection with a sensitivity S 7 (g/ml)-1 and limit of detection (LoD) 0.006 (g/ml).