Simple silver nanoparticle colorimetric sensing for copper by paper-based devices

The first investigation of silver nanoparticle (AgNP) colorimetric sensing of Cu2+ by paper-based analytical devices (PADs) is reported here. AgNP colorimetric sensing for the detection of Cu2+ was first characterized by UV–visible spectroscopy. The –SH groups on homocysteine (Hcy) and dithiothreitol (DTT) were used to modify the AgNP surface whereas the –COOH and –NH2 functional groups have strong affinity to Cu2+ relative to other ions in solution. The plasmon resonance absorption peak intensity at 404 nm decreased and a new red-shifted band at 502 nm occurred in the presence of Cu2+. Paper devices coated with the modified AgNP solution changed from yellow to orange and green-brown color after the addition of Cu2+ due to nanoparticle aggregation. The color intensity change as a function of Cu2+ concentration gave a linear response in the range of 7.8–62.8 μM (R2=0.992). The limit of naked-eye detection is 7.8 nM or 0.5 μg L−1. A color change observed by the naked eye with the addition of Cu2+ can be clearly differentiated from the other metals (As3+, Cd2+, Co2+, Hg2+, Ni2+, Pb2+, Zn2+, Mg2+, Mn2+, Ca2+, Fe3+, Na+, and K+) at 15.7 μM. The use of different flow directions in the PADs and μPADs for Cu2+ detection was also demonstrated. Levels of Cu2+ in real water samples were measured using the paper devices to be 2.9±0.24 μM (tap water) and 3.2±0.30 μM (pond water), respectively, and were within error of the values measured using an atomic absorption spectrometer (2.8±0.08 μM in tap water, and 3.4±0.04 μM in pond water). Thus, this work shows the successful integration of paper devices and AgNP colorimetric sensing as a simple, rapid, easy-to-use, inexpensive and portable alternative point-of-measurement monitoring.