This paper deals with the quantification of proteins by implementing the Bradford protein assay method in a portable opto-microfluidic platform for protein concentrations lower than 1.4 mg/mL. Absorbance is measured by way of optical waveguides integrated to a cross-junction microfluidic circuit on a single lithium niobate substrate. A new protocol is proposed to perform the protein quantification based on the high correlation of the light absorbance at 595 nm, as commonly used in the Bradford method, with the one achieved at 633 nm with a cheap commercially available diode laser. This protocol demonstrates the possibility to quantify proteins by using nL volumes, 1000 times less than the standard technique such as paper-analytical devices. Moreover, it shows a limit of quantification of at least 0.12 mg/mL, which is four times lower than the last literature, as well as a better accuracy (98%). The protein quantification is obtained either by using one single microfluidic droplet as well by performing statistical analysis over ensembles of several thousands of droplets in less than 1 min. The proposed methodology presents the further advantage that the protein solutions can be reused for other investigations and the same pertains to the opto-microfluidic platform.
A comprehensive description of all the optical phenomena occurring when light interacts with a moving dispersed phase in a constrained environment such as a real microfluidic channel is needed to perform a quantitative analysis as well as predictive one. This requires identifying fingerprints in the detected optical signal that are doubtlessly correlated with the shape and content type of the dispersed phase from those connected to uncertainties of the optical detection systems and/or instabilities in the microfluidics apparatus leading to dispersed phase size distribution. This article aims to model all the droplet-induced optical effects in an opto-microfluidic cross-configuration system and quantify how diffraction, transmission, absorbance, and reflection contribute to the overall response in the detected intensity after light-matter interaction. The model has been tested in the case of water droplets dispersed in hexadecane continuous phase as generated in an opto-microfluidic platform where optical waveguides are fully integrated with the microfluidic channels, so that light illuminates the flowing droplets from the channel wall and collected on the opposite side. A critical discussion of the impact of geometry and constrains is proposed as well as the impact of each contribute in terms of fingerprints in the detected signal. The good agreement obtained demonstrates the potentialities of both the derived model and the cross-configuration, getting information on droplet characteristics from the intensity arising from its light interaction.
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