"Dipstick" Colorimetric Detection of Metal Ions Based on Immobilization of DNAzyme and Gold Nanoparticles onto a Lateral Flow Device.

Real-time, on-site detection and quantification of different trace analytes is a challenge that requires both searching a general class of molecules to recognize a broad range of contaminants and translating this recognition to easily detectable signals. Functional nucleic acids, which include DNAzymes (DNA with catalytic activity) and aptamers (nucleic acids that bind an analyte), are ideal candidates for the target recognition. These nucleic acids can be selected by a combinatorial biology method called in vitro selection to interact with a particular analyte with high specificity and sensitivity. Furthermore, they can be incorporated into sensors by attaching signaling molecules. Due to the high extinction coefficients and distance-dependent optical properties, metallic nanoparticles such as the commonly used gold nanoparticles have been shown to be very attractive in converting analyte-specific functional DNA into colorimetric sensors. DNAzyme directed assembly of gold nanoparticles has been used to make colorimetric sensors for metal ions such as lead, uranium, and copper. To make the operation even easier and less vulnerable to operator's errors, dipstick tests have been constructed. Here, we describe protocols for the preparation of DNAzyme-linked gold nanoparticles (AuNP) that are then immobilized on to lateral flow devices to make easy-to-use dipstick tests for metal ions.