Palladium-porphyrin-crosslinked phosphorescenct hydrogels for oxygen sensing
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Porphyrins chelated with palladium group metals can serve as effective oxygen-responsive biosensors based on near infrared (NIR) luminescence reporting. Recently, more and more papers were published for researching on different kinds of metalloporphyrin derivatives and developing their biological and medical applications. However, development of Pd/Pt porphyrins as implantable biosensors for subcutaneous oxygen monitoring applications has proved challenging due to difficulties in permanently immobilizing the porphyrins in a biocompatible matrix that prevents dye aggregation yet maintains high enough concentration for transdermal detection. Here, we demonstrate that Pd-porphyrins could be used as a PEG crosslinkers to generate an oxygen responsive polyamide hydrogel with extreme porphyrin density (~5 mM). With the stoichiometric ratio, the yield of Pd-mTCPP hydrogel can reach over 95%. Dye aggregation was avoided due to the spatially constraining 3-D mesh formed by the porphyrins themselves. The hydrogel could reversibly equilibrate with surrounding oxygen levels within minutes and phosphorescence lifetimes ranged over three orders of magnitude from 20 ns in pure oxygen to 45 ms in the absence of oxygen. This kind of oxygen sensor shows perfect linear Stern-Volmer relationship and good reproducibility. To further facilitate oxygen detection using steady state techniques, we developed an oxygen non-responsive companion hydrogel by blending copper and free base porphyrins to have intensity-matched luminescence. The hydrogels could be implanted and used for non-invasive optical ratiometric measurement of subcutaneous oxygen levels in mice in vivo during wound closure and exposure to high oxygen levels. The subcutaneous oxygen partial pressure changes could be shown directly by the different phosphorescence intensity of Pd-mTCPP hydrogel.