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

The first investigation of silver nanoparticle (AgNP) colorimetric sensing of Cu(2+) by paper-based analytical devices (PADs) is reported here. AgNP colorimetric sensing for the detection of Cu(2+) 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 -NH(2) functional groups have strong affinity to Cu(2+) 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 Cu(2+). Paper devices coated with the modified AgNP solution changed from yellow to orange and green-brown color after the addition of Cu(2+) due to nanoparticle aggregation. The color intensity change as a function of Cu(2+) concentration gave a linear response in the range of 7.8-62.8 μM (R(2)=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 Cu(2+) can be clearly differentiated from the other metals (As(3+), Cd(2+), Co(2+), Hg(2+), Ni(2+), Pb(2+), Zn(2+), Mg(2+), Mn(2+), Ca(2+), Fe(3+), Na(+), and K(+)) at 15.7 μM. The use of different flow directions in the PADs and μPADs for Cu(2+) detection was also demonstrated. Levels of Cu(2+) 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.