A novel label-free colorimetric assay for DNA concentration in solution

• A polymer-based etalon was fabricated and used for DNA sensing.
• The device was shown to be sensitive to micromolar levels of DNA with no assay optimization.
• Assay was selective to a target DNA sequence in the presence of interfering DNA with 2 and 4 base pair mismatches.

Optical devices were fabricated by sandwiching a “monolithic” poly(N-isopropylacrylamide-co-N-(3-aminopropyl) methacrylamide hydrochloride) (pNIPAm-co-APMAH) microgel layer between two semitransparent Au layers. These devices, referred to as etalons, exhibit characteristic multipeak reflectance spectra, and the position of the peaks in the spectra primarily depends on the distance between the Au surfaces mediated by the microgel layer thickness. Here, we show that the positively charged microgel layer can collapse in the presence of negatively charged single stranded DNA (ssDNA) due to ssDNA induced microgel crosslinking. The collapse results in a change in the etalon's optical properties, which can be used to detect target DNA in a complex mixture.

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