been widely applied in resource-limited settings due to their easy-to-operate and equipment-independent characteristics with minimum user intervention. [9] However, LFAs are often limited to qualitative results readout by providing "yes" or "no" signal, which is insufficient in some application scenarios, e.g., a clinical action will be made in clinics according to the accurate amount of targets. Recently, the emerging µPADs have gained considerable interests due to their excellent capability for outputting quantitative results. [10] Nowadays, quantitative paper-based assays are mainly based on electrochemical detection, [11] electrochemiluminescence detection, [12] and fluorescent detection. [13,14] Although sensitive and quantitative detection can be obtained, special equipment for quantification of detection signal is usually involved. For instance, electrochemical workstation is needed for performing electrochemical detection on paper, while a special light-emitting device is indispensable for fluorescent testing. For addressing these limitations, some handheld equipments, such as glucose meter, [15] multimeter, [16] and smartphone, [17,18] have been introduced as readers for outputting quantitative measurements with improved portability. However, the accessibility and cost of these handheld equipments may still be a big challenge for their application in resource-poor areas, where the quantitative detection in an equipment-free manner is preferred. Hence, the exploitation of equipment-free paper-based analytical devices shows broad practical application prospects in POCT field.Colorimetric detection holds great potential for developing equipment-free detection methods through direct readout by naked eye. [19] Conventionally, the color intensity analysis has been widely used to realize quantification of analytes, whereas a camera is needed to capture images and a computer-based software for image processing. [20] Hence, these quantitative methods based on color intensity analysis are unsatisfactory for exploiting equipment-free quantitative strategies. Fortunately, thanks to the novel signal transformation design in paper-based colorimetric detection, different types of equipment-free quantitative paper devices have already been proposed, where the detection signal has been transformed into the measurement of color change distance, [21] the counting of colored detection zones, [22] the measurement of testing time, [23] and the reading of displayed text. [24] These equipment-free strategies have been demonstrated to be capable of detecting a variety of analytes with acceptable performance, such as metal ions, nucleic acids, bacteria, and cells, whose detection shows great attraction in POCT, thus exhibiting great feasibility and versatility for POCT applications.Equipment-free paper-based devices with quantitative readout have gained considerable interests in recent years, due to the features of remarkable cheapness, prominent portability, and excellent user friendliness. These equipment-free strategies have b...
