Improvement strategies on colorimetric performance and practical applications of Paper-based analytical devices

• Strategies for improving colorimetric performance of paper-based analytical devices. • Review of color quality enhancement techniques based on printing principles. • First proposing the selectivity of paper assaying environment for a reaction. • Review of Image processing methods towards detection sensitivity improvement. • Designs that enhance the portability of colorimetric PADs for field use. Colorimetric paper-based analytical devices (PADs) have been widely fabricated due to the ease of construction of colorimetric reactions on paper and the high compatibility with low-cost and easy-to-use imaging devices for on-site analysis and results recording. However, the further advance of colorimetric PADs is limited by the detection sensitivity and other intrinsic limitations of paper regarding the color quality. This review summarizes the strategies for improving the colorimetric performances (color intensity and uniformity) of PADs. In particular, the principles of controlling liquid-paper interaction for the color intensity enhancement, developed in printing research, are discussed in relation to PAD designs. The influence of the paper assaying environment to the paper assaying selectivity is discussed through a case study for the first time. Image capture techniques and image process algorithms are related to the accuracy and sensitivity of the colored results, which are carefully introduced. Finally, based on the tendency of improvement of colorimetric PADs for practical applications, we summarized the designs that enhance the portability of colorimetric PADs. It is believed that the new links and opinions provided in this review may be useful to researchers to address the challenges in designing future colorimetric PADs with improved sensitivities for field use.