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A highly sensitive and selective colorimetric lead biosensor based on DNAzyme-directed assembly of gold nanoparticles is reported. It consists of a DNAzyme and its substrate that can hybridize to a 5'-thio-modified DNA attached to gold nanoparticles. The hybridization brings gold nanoparticles together, resulting in a blue-colored nanoparticle assembly. In the presence of lead, the DNAzyme catalyzes specific hydrolytic cleavage, which prevents the formation of the nanoparticle assembly, resulting in red-colored individual nanoparticles. The detection level can be tuned to several orders of magnitude, from 100 nM to over 200 muM, through addition of an inactive variant of the DNAzyme. The concept developed here can be applied to the design of nucleic acid enzyme/nanoparticle sensors for analytes that are subject to in vitro selection, and thus can significantly expand the scope of nanomaterial applications and provide a novel approach to designing simple colorimetric biosensors.

Blue Type I Copper Centers vs Purple Cu A Centers 4439 5. Enzymes Employing a Combination of Different Types of Electron Transfer Centers 4439 5.1. Enzymes Using Both Heme and Cu as Electron Transfer Centers 4439 5.1.1. Cytochrome c and Cu A as Redox Partners to Cytochrome c Oxidases 4439 5.1.2. Cu A and Heme b as Redox Partners to Nitric Oxide Reductases 4440 5.1.3. Cytochrome c and Cu A as Redox Partners to Nitrous Oxide Reductases 4440 5.2. Enzymes Using Both Heme and Iron−Sulfur Clusters as Electron Transfer Centers 4440 5.2.1. As Redox Partners to the Cytochrome bc 1 Complex 4440 5.2.2. As Redox Partners to the Cytochrome b 6 f Complex 4442 5.2.3. As Redox Centers in Formate Dehydrogenases 4443 5.2.4. As Redox Centers in Nitrate Reductase 4443 6. Summary and Outlook 4443 Author Information 4445 Corresponding Author 4445 Author Contributions 4445 Notes 4445 Biographies 4445 Acknowledgments 4447 Abbreviations 4447 References 4447

Aptamers are single-stranded DNA or RNA molecules that can bind target molecules with high affinity and specificity. The conformation of an aptamer usually changes upon binding to its target analyte, and this property has been used in a wide variety of sensing applications, including detection based on fluorescence intensity, polarization, energy transfer, electrochemistry or color change. Colorimetric sensors are particularly important because they minimize or eliminate the necessity of using expensive and complicated instruments. Among the many colorimetric sensing strategies, metallic nanoparticle-based detection is desirable because of the high extinction coefficients and strong distance-dependent optical properties of the nanoparticles. Here, we describe a protocol for the preparation of aptamer-linked gold nanoparticle purple aggregates that undergo fast disassembly into red dispersed nanoparticles upon binding of target analytes. This method has proved to be generally applicable for colorimetric sensing of a broad range of analytes. The time range for the entire protocol is approximately 5 d, including synthesis and functionalization of nanoparticles, preparation of nanoparticle aggregates and sensing.

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