The pH Effect on Thermal Response of Fluorescence Spectroscopy of Graphene Quantum Dots for Nanoscale Thermal Characterization

Accurate and sensitive nanoscale thermal probing for thermophysical property characterization is appealing but still a challenge to date. Previous studies have revealed that graphene quantum dots (GQDs) are good temperature markers for their small dimension and superior fluorescence excitation. In this work, we show that the thermal response of fluorescence spectrum of GQDs is strongly pH-dependent. Significant decrease (about 56% to 30%) for temperatureinduced intensity reduction within a small range of 75°C under different excitation wavelengths of 370 nm, 390 nm, and 410 nm is observed as pH value increases from pH = 1 to pH = 13. The temperature coefficients of peak wavelength change from positive to negative with the increase of pH value, meaning that the blue shift happens as the condition is changed from acidity to alkalinity. Temperature dependence of peak width is also studied with the largest coefficient of 0.2255nm/°C, which is remarkable. These suggest that when using GQDs in nanoscale thermal probing, the pH value is an important factor that should be considered besides the excitation wavelength. Regarding the superior biocompatibility and low cytotoxicity, GQDs could play an important role in thermal probing or mapping in a complex biology system such as a cell, and help to develop novel treatments and diagnoses.