Table 2. Comparative performance of optical sensors for wearable sweat pH monitoring.

Sensor type	Sensing mechanism	Advantages	Limitations
SERS	Raman spectral shift	Label-free detection; ultra-high sensitivity; excellent molecular specificity; minimal sample volume	Requires Raman spectrometer; high cost; limited portability; not suitable for long-term continuous monitoring
Fluorescence	Fluorescence intensity/wavelength shift	High sensitivity and selectivity; rapid response; capable of detecting small pH variations	Requires excitation source and optical filters; higher power consumption; susceptible to ambient light interference
Colorimetric	pH-responsive dyes	Low cost; lightweight; simple readout; no complex electronics; suitable for continuous and outdoor monitoring	Moderate sensitivity; discrete data; limited temporal resolution; affected by lighting conditions