The modified AgNPls at 0.8mL were dropped onto the detection
zone and control zone, and allowed to dry. For Cu2+ measurement,
20mL of the standard/sample solutions was added to the sample
application zone and then the solution flowed into the detection
zone. The color change at the test zone can be observed within
2 min. For quantitative analysis, the photograph of the results on
the paper-based sensor was recorded by a digital camera (Cannon
EOS 1000 D1, Japan) in a light control box. Then, color intensity of
the testing area on the device was measured using ImageJ 1.45s
(National Institutes of Health, USA). Finally, the color intensity
values were used to obtain a calibration curve