The determination of soluble silicates is essential for environmental monitoring and industrial water treatment, while indicating water quality and geochemical processes in natural waters. This work introduces the first ratiometric fluorescence method for detecting soluble silicates using nitrogen and sulfur co-doped carbon dots (N, S-CDs) that emit at two distinct wavelengths: 500 nm and 650 nm. When silicates react to form the blue molybdenum complex, this blue color selectively blocks the 650 nm signal while leaving the 500 nm reference signal unchanged, creating a built-in calibration system. This dual-wavelength approach overcomes major limitations of current methods that rely on measuring single-wavelength color changes, which are easily affected by sample turbidity and interference. The method demonstrated excellent analytical performance with a linear response range of 0.05–0.8 mg Si/L, a detection limit of 0.018 mg Si/L, and recovery rates between 96.0 and 99.0 % with relative standard deviations below 3.03 % in the water matrices studied. The method was successfully validated for real-world applications through analysis of Nile River water and groundwater samples, demonstrating excellent recovery rates and precision in complex environmental matrices. Additionally, a smartphone-based colorimetric detection system was integrated to enable field measurements by capturing the fluorescence color changes under UV illumination, providing a portable and user-friendly alternative for on-site silicate monitoring.