Flow-through Capture and in Situ Amplification Can Enable Rapid Detection of a Few Single Molecules of Nucleic Acids from Several Milliliters of Solution.

Detecting nucleic acids (NAs) at zeptomolar concentrations (few molecules per milliliter) currently requires expensive equipment and lengthy processing times to isolate and concentrate the NAs into a volume that is amenable to amplification processes, such as PCR or LAMP. Shortening the time required to concentrate NAs and integrating this procedure with amplification on-device would be invaluable to a number of analytical fields, including environmental monitoring and clinical diagnostics. Microfluidic point-of-care (POC) devices have been designed to address these needs, but they are not able to detect NAs present in zeptomolar concentrations in short time frames because they require slow flow rates and/or they are unable to handle milliliter-scale volumes. In this paper, we theoretically and experimentally investigate a flow-through capture membrane that solves this problem by capturing NAs with high sensitivity in a short time period, followed by direct detection via amplification. Theoretical predictions guided the choice of physical parameters for a chitosan-coated nylon membrane; these predictions can also be applied generally to other capture situations with different requirements. The membrane is also compatible with in situ amplification, which, by eliminating an elution step enables high sensitivity and will facilitate integration of this method into sample-to-answer detection devices. We tested a wide range of combinations of sample volumes and concentrations of DNA molecules using a capture membrane with a 2 mm radius. We show that for nucleic acid detection, this approach can concentrate and detect as few as ∼10 molecules of DNA with flow rates as high as 1 mL/min, handling samples as large as 50 mL. In a specific example, this method reliably concentrated and detected ∼25 molecules of DNA from 50 mL of sample.