Semiconducting Polymer Nanoparticles Enable Light‐Controlled Bidirectional Modulation of Nitric Oxide in Endothelial Cells

ABSTRACT Endothelial‐derived nitric oxide (·NO) is a key signaling molecule in the vascular system, exerting concentration‐dependent control over critical cellular functions such as angiogenesis, vascular tone, and endothelial barrier integrity. Tools for achieving reversible, spatiotemporally resolved modulation of intracellular ·NO, without pharmacological or genetic manipulation, are currently lacking. Here, we present a light‐activated, nanoparticle (NP)‐based strategy enabling bidirectional control of endogenous ·NO in endothelial cells. Composite NPs, based on poly(3‐hexylthiophene), P3HT, and poly(3,4‐ethylene‐dioxythiophene):poly(styrenesulfonate), PEDOT:PSS polymers, are efficiently internalized in human (HUVEC) and murine (H5V) endothelial cells. In dark, NP uptake induces a ROS‐dependent, intracellular ·NO increase (+50% and +100% in HUVEC and H5V, respectively, vs. controls), a metabolic shift toward glycolysis and upregulation of both endothelial nitric oxide synthase (eNOS, +50%) and induced nitric oxide synthase (iNOS, +40%). NP photostimulation reverses this response, decreasing ·NO below basal levels, up to −40% in HUVEC and H5V, via ROS‐mediated scavenging and iNOS downregulation (−40%), partially restoring oxidative phosphorylation metabolism. Importantly, the photoexcitation protocol is compatible with perspective in vivo use, in terms of source type (LEDs) and power density (6 mW/cm 2 ). Our approach represents an innovative strategy for bidirectional endothelial ·NO modulation, providing new opportunities in the emerging field of photo‐redox medicine.