Printable QR code paper microfluidic colorimetric assay for screening volatile biomarkers

We present a QR code paper microfluidic colorimetric assay that can exploit the hardware and software on mobile devices, and circumvent sample preparation by directly targeting volatile biomarkers. Our platform is a printable microarray of well-defined reaction regions, which outputs an instant diagnosis by directing the user to a URL containing their test result, while simultaneously storing epidemiological data for remote access and bioinformatics. To assist in the rapid identification of Escherichia coli in bloodstream infections, we employed an existing colorimetric reagent (p-dimethylaminocinnamaldehyde) and adapted its use to detect volatile indole, a biomarker produced by E. coli. Our assay was able to quantitatively detect indole in the headspace of E. coli culture after 12 h of growth (27.0 ± 3.1 ppm), assisting in species-level identification hours earlier than existing methods. Results were confirmed with headspace solid-phase microextraction (HS-SPME) two-dimensional gas chromatography time-of-flight mass spectrometry (GC×GC-ToFMS), which estimated indole concentration in E. coli culture to average 32.3 ± 5.2 ppm after 12 h of growth. This QR paper microfluidic platform represents a novel development in both telemedicine and diagnostics using volatile biomarkers. We envision that our QR code platform can be extended to other colorimetric assays for real-time diagnostics in low-resource environments.