Fluorescence sensor based on metal-free polymeric carbon nitride for detection of nitrite ions

Abstract

Recently, we discovered that the polymeric metal-free carbon nitride (CN) is a potential fluorescence sensor for detection of nitrate (NO3-) ions. However, since the NO3- usually exists together with nitrite ions (NO2-), development of fluorescence sensor for detection of both NO3- and NO2- is an important issue. In this study, we evaluated the performance of the CN as the fluorescence sensor for NO2- detection. The CN was prepared through thermal polymerization of urea precursor at 823 K in closed condition and was characterized by using diffuse reflectance ultraviolet-visible (DR UV-Vis), Fourier transform infrared (FTIR), and fluorescence spectroscopies. The DR UV-Vis spectrum confirmed that CN could absorb light up to 450 nm. The FTIR spectrum revealed the presence of graphitic CN single and double bond characters in the 800 to 1700 cm-1 area, while the fluorescence spectroscopy confirmed that only one emission peak was found at 453 nm when CN was excited at wavelength 278, 310 and 369 nm which due to the presence of C=N, C=O and N-C groups, respectively. In this study, the capability of CN as fluorescence sensor was investigated via fluorescence quenching of different concentration of NO2- in the range of 0.5 to 4 ?M. It was confirmed that the emission intensities of all emission sites of CN were quenched linearly with the concentration of NO2-. Based on the linear Stern-Volmer plots, the interactions between emission sites and the NO2- would occur more favourably at C=N and C=O sites than N-C sites. The quenching rate on the C=N sites towards the NO2- was determined to be 0.14 ʯM-1, which was much higher than the reported value for the NO3- detection. This study demonstrated that CN was more sensitive towards the detection of the NO2-. Reproducibility tests were carried out to study the repeatability as the fluorescence sensor. It was confirmed that the CN showed good repeatability, supporting its application as the sensitive fluorescence sensor towards the NO2.