Synthesis of Porous Ultrathin Graphitic Carbon Nitride for Ultrasensitive Fluorescence Detection of 2,4,6-Trinitrophenol in Environmental Water and Mechanism
Ultrasensitive detection of 2,4,6-trinitrophenol (TNP) in environmental security is important, but challenging. Graphitic carbon nitride (g-C3N4) is promising for fluorescence sensing of TNP. It is highly desired to greatly promote the adsorption of TNP onto g-C3N4 for improving the fluorescence detection sensitivity. Herein, the porous ultrathin g-C3N4 (~1.3 nm) nanosheets are successfully synthesized via combining secondly calcining with HNO3 treating processes. Under the optimized conditions of excitation wavelength (Ex=350 nm) and solution pH value (pH=3), the wide detection range of TNP from 4 μM to 54 μM is confirmed by the widely-adopted normal Stern-Volmer equation (SVE). Interestingly, the limit of detection is as low as 0.04 nM according to the linear range of 0.1 nM to 4 μM by the generally neglected double logarithmic (DL) SVE, which is much competitive for TNP detection compared with the reported results. It is confirmed based on the isothermal adsorption curves, the time-resolved fluorescence and surface photovoltage spectra that the multilayer adsorption of TNP on resulting g-C3N4 at low concentration mainly leads to the fluorescence decay by the inner filter effect (IFE), while the monolayer adsorption at ultralow concentration makes the fluorescence decay by the combination effects of the IFE and the photoinduced electron transfer. It is suggested that the normal and DL SVEs are applicable to the fluorescence decay processes respectively determined by the single and double factors. The obtained ultrasensitive detection is attributed to the greatly promoted adsorption of TNP via the hydrogen bonding by enlarging the surface area from the porous nanosheet structure and by increasing the surface -OH sites from HNO3 treatment.This work helps to deeply understand the SVE related to the fluorescence decay processes, and provides a feasible route to develop an ultrasensitive detection of TNP with g-C3N4 nanosheets for environmental waters monitoring and security inspection.