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 $\lambda = 1.3\,\,\mu \text{m}$ , $\lambda = 1.45\,\,\mu \text{m}$ , and $\lambda = 2.15\,\,\mu \text{m}$ ) 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 $\lambda = 2.15\,\,\mu \text{m}$ . 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 $\lambda = 1.3\,\,\mu \text{m}$ is mainly affected by the fluid refractive index (real part). Around $\lambda = 2.15\,\,\mu \text{m}$ , 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 $\lambda = 2.15\,\,\mu \text{m}$ , its contribution can be accounted and compensated for by measuring the transmittance in the wavelength range around $\lambda = 1.45\,\,\mu \text{m}$ where water only exhibits a strong absorption band. By introducing $R_{1.45}(C)$ and $R_{2.15}(C)$ as the ratios between the output signal in presence of the solution and the output signal collected on the empty channel, at $\lambda = 1.45\,\,\mu \text{m}$ and $\lambda = 2.15\,\,\mu \text{m}$ , respectively, we have demonstrated that their ratio $\upsilon (C)\,\,= R_{1.45}(C)/R_{2.15}(C)$ is highly specific for urea detection with a sensitivity $S \sim 7$ (g/ml) ⁻¹ and limit of detection (LoD) $\approx ~0.006$ (g/ml).