I‑Motif\nDNA Based Fluorescent Ratiometric Microneedle\nSensing Patch for Sensitive Response of Small pH Variations in Interstitial\nFluid

Detection of slight pH changes in\nskin interstitial fluid (ISF)\nis crucial yet challenging for studying pathological processes and\nunderstanding personal health conditions. In this work, we construct\nan i-motif DNA based fluorescent ratiometric microneedle sensing patch\n(IFR-pH MN patch) strategy that enables minimally invasive, high-resolution,\nand sensitive transdermal monitoring of small pH variations in ISF.\nThe IFR-pH MN patch with advanced integration of both ISF sampling\nand pH sensing was fabricated from the cross-linking of gelatin methacryloyl\nand methacrylated hyaluronic acid, wrapping with pH-sensitive hairpin-containing\n i-motif DNA based fluorescent ratiometric probes in the matrix. Because\nit is mechanically robust for skin penetration and has high swelling\nability, the IFR-pH MN patch could be quickly extracted as sufficient\nliquid from agarose gel (∼56.4 μL in 10 min). Benefiting\nfrom conformation changes of the hairpin-containing i-motif DNA under\npH variation and ratiometric fluorescence signal readout, the IFR-pH\nMN patch could quantitate pH over a small range between pH 6.2 and\n6.9 with an accuracy of 0.2 pH units in the mimic skin model. Furthermore, <i>in vivo</i> testing on wound and tumor mouse models indicated\nthe ability of the biocompatible IFR-pH MN patch to penetrate the\nskin for obtaining transdermal pH values, demonstrating the potential\napplications in monitoring and intervention of pathological states.