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

The first investigation of silver nanoparticle (AgNP) calorimetric 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 mu M (R-2=0.992). The limit of naked-eye detection is 7.8 nM or 0.5 mu g 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 mu m. The use of different flow directions in the PADs and mu PADs 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 mu M (tap water) and 3.2 +/- 0.30 mu M (pond water), respectively, and were within error of the values measured using an atomic absorption spectrometer (2.8 +/- 0.08 mu M in tap water, and 3.4 +/- 0.04 mu M 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. (C) 2012 Elsevier B.V. All rights reserved.