Digital Concentration Readout of DNA by Absolute Quantification of Optically Countable Gold Nanorods

Isolation and amplification procedures are indispensable for traditional single-molecule detection methods applied in low-abundance biomolecule analysis. Here, we describe a method for ultrasensitive detection of DNA through encoding a single target molecule with a single-countable nanometer-sized substitute (gold nanorod, AuNR) and enrichment of the substitute AuNRs into a limited region followed by accurate microscopic enumeration. The enrichment and the bright distinct color allow the AuNRs to be efficiently counted from as few as 1-2 to tens of thousands in 3 μL of test solution, which demonstrates the ability of rapid digital concentration readout of the target DNA. On this basis, a detection limit of 6.5 aM was achieved for DNA associated with human papillomavirus (HPV). Notably, our method requires neither complicated isolation and amplification procedures nor extremely expensive instruments and consumables.