Counting single native biomolecules and intact viruses with color-coded nanoparticles

Nanometer-sized particles such as semiconductor quantum dots and energy-transfer nanoparticles have novel optical properties such as tunable light emission, signal brightness, and multicolor excitation that are not available from traditional organic dyes and fluorescent proteins. Here we report the use of color-coded nanoparticles and dual-color fluorescence coincidence for real-time detection of single native biomolecules and viruses in a microfluidic channel. Using green and red nanoparticles to simultaneously recognize two binding sites on a single target, we demonstrate that individual molecules of genes, proteins, and intact viruses can be detected and identified in complex mixtures without target amplification or probe/target separation. Real-time coincidence analysis of single-photon events allows rapid detection of bound targets and efficient discrimination of excess unbound probes. Quantitative studies indicate that the counting results are remarkably precise when the total numbers of counted molecules are more than 10. The use of bioconjugated nanoparticle probes for single-molecule detection is expected to have important applications in ultrasensitive molecular diagnostics, bioterrorism agent detection, and real-time imaging and tracking of single-molecule processes inside living cells.