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

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.