Scientists in the United States have produced a simple dipstick test for detecting lead levels in paints. Easy-to-use biosensors are important for detection of highly toxic trace metal ions in the environment. Cross-linked gold nanoparticles modified with metal-specific DNAzymes have been used in solution to create highly sensitive and selective colorimetric metal sensors based on the colour change between aggregated (blue) and dispersed (red) gold nanoparticles. However, in solution the colour change can be difficult to distinguish and nanoparticle stability is poor, explains Yi Lu at the University of Illinois at Urbana Champaign. Lu and colleagues have developed a sensor that uses...