Use of scanners for colorimetric analysis of microfluidic paper-based analytical devices (μPADs): A practical guide

Office style scanners are often used by researchers for the colorimetric detection in the development of micro-fluidic paper-based analytical devices (mu PADs). However, little attention has been paid to nuances in calibration curve responses caused by varying settings of a single image-capturing device. Hence, in this study, the effect of parameters, such as the scanner operation conditions, the method of data analysis used (e.g., choice of colour space system, suitable channel), and the influence of the physical characteristics of the mu PAD (e.g., template colour, type of lamination pouches used) on the colorimetric detection were evaluated for the first time. Combinations of the scanning parameters were thoroughly examined and their impact on the quality of a mock calibration curve assessed. The most sensitive calibration curve was achieved only by the selection of the optimal scanning conditions combined with an appropriate choice of colorimetric system. A flatbed scanner, used in this study, showed greater variation of signal with varying scanning setup, whereas a portable scanner provided nearly identical responses under diverse scanning conditions. Gamma correction, scanning resolution, region of interest (ROI) surrounding colour (i.e., template colour), bit-depth, image file type, and the number of analysed pixels were found not to markedly affect the dynamics of the calibration curve when colour in the ROI is homogeneous. Parameters that had an effect on the colorimetric detection were the scanner settings, colour of the blank, image acquisition mode (colour vs greyscale), and mu PAD covering (e.g., lamination). Based on these findings, generic guidelines for optimal data acquisition and mu PAD manipulation with respect to mu PAD calibration have been formulated.