Hollow NCQDs@HSiO₂/MPT Nanohybrids as Ratiometric Fluorescent pH Sensor

The ratiometric fluorescence system has garnered significant attention as it offers the capability to correct for environmental interferences and mitigate fluctuations in light excitation intensity. This study presents the development of a dual-emission system achieved through the decoration of nitrogen-doped carbon quantum dots (NCQDs) on the surface of hollow silica spheres (HSiO₂), which were further modified with 5-methoxy-2-(pyridyl)thiazole (MPT) to function as a ratiometric fluorescent sensor for pH measurements. When subjected to single wavelength excitation, the resulting nanohybrid sensor exhibited two distinct emission bands at approximately 480 nm and 360 nm, likely arising from the intrinsic characteristics of the NCQDs and 5-methoxy-2-(pyridyl)thiazole, respectively. By integrating the highly pH-sensitive NCQDs with a fluorophore group exhibiting minimal pH response, we successfully designed a ratiometric pH sensor capable of quantitatively monitoring pH values in aqueous solutions. An increase in pH resulted in a notable quenching of fluorescence at 480 nm, while the fluorescence intensity at 360 nm remained relatively constant. The intensity ratio (I₄₈₀ nm/I₃₆₀ nm) was measured against pH values ranging from 3 to 9 in buffer solutions, thereby demonstrating its effectiveness as a ratiometric sensor for pH determination. Additionally, the designed sensor exhibited exceptional reversibility and photostability during pH measurements. The nanohybrid sensor discussed herein shows considerable promise as a model for the development of future sensing applications.