Abstract
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.
Highlights
IntroductionProteins play a key role in repairing and maintenance processes of human cells, tissues and organs
Among all the biomolecules, proteins play a key role in repairing and maintenance processes of human cells, tissues and organs
paper-analytical devices (PADs) performances have been investigated to some extent in [6], where different assays have been exploited (Biuret [21], Lowry [22,23], Bicinchoninic acid (BCA) [24]), Bradford assay [12], Bromocresol green (BCG) and Tetrabromophenol blue (TBPB) assays [25] respectively, tested on Bovine Serum Albumin (BSA) as protein standard [6]
Summary
Proteins play a key role in repairing and maintenance processes of human cells, tissues and organs. Total protein analysis is carried out by colorimetric protein assays [8,9,10,11], which are based on colorimetric reactions promoted when the protein solution reacts with a specific dye-based reagent They lead to the formation of a dye-protein complex responsible for shifting the dye absorbance by a detectable amount depending on the protein concentration. For unknown BSA dilution, the measurement result error was estimated to be lower respect the standard method, such as 72.8 μg/mL in chip and 73.4 μg/mL in tube, respectively [6] These results clearly demonstrate that PADs are of great interest but unsuccessful when nL or pL protein solutions are under investigation. They are not suitable for monitoring the protein synthesis protocol especially when the latter is made of several critical steps and a long time of execution
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