Monitoring Spatiotemporal Dynamics of Extracellular pH in Breast Tumor Spheroid and Tissue by SERS Microsensors

Abstract Extracellular pH (pHe) acidification is a hallmark of solid tumors, linked to cancer initiation, progression, and drug resistance. Non‐invasive monitoring of pHe spatiotemporal dynamics during cancer growth and in response to drug is crucial for understanding cancer mechanisms and developing novel therapeutic strategies. Here, a micro‐sized pH sensor is developed by encapsulating Au@Ag@MBA nanosensors within biocompatible agarose hydrogel microspheres. These Aga microsensors in a size range of 3–10 µm, may avoid cellular endocytosis and stably reside in the extracellular environment. pHe dynamics are monitored via surface‐enhanced Raman scattering (SERS) mapping in tumor spheroids and solid tumor tissues over 7 and 25‐day periods, respectively. Distinct pH gradients are observed between the outer and core regions of the tumor spheroid, which evolved over time and are disrupted by doxorubicin treatment. In vivo, a decrease of average tumor pHe from ≈6.2 to ≈5.5 is observed with tumor progression, and doxorubicin administration resulted in a pHe elevation of ≈0.6 units. This microsensor‐based SERS sensing approach provides a facile method for long‐term monitoring of spatiotemporal dynamics within the disease microenvironment.