Polymer modified carbon nitride composites as potential fluorescence sensors for detection of nitrite and nitrate ions

Abstract

The detection of nitrite (N0 2 ) and nitrate (N03 ) ions is important since these ions are closely related to environmental remediation and human health. Rapid, sensitive, and yet reusable sensors are of great importance for such applications. In the present study, novel materials were developed as fluorescence sensors for detection of N 0 2 and N 0 3 . Metalfree carbon nitride (CN) was modified with several different polymers, namely polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG) and polyaniline (PANI). The series of polymer(x%)-CN composites (x = 1, 3 and 5 w/w%) were prepared by a simple impregnation method using two types of CN, namely bulk CN (BCN) and mesoporous CN (MCN). X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy results revealed that the chemical structures of the BCN and the MCN did not change with the addition of polymers. In contrast, the specific surface area of the BCN and the MCN decreased with the addition of the polymers, suggesting the presence of the polymers in the composites. Fluorescence spectroscopy of both BCN and the MCN revealed three absorption and excitation peaks, which corresponded to N=C, C=0, and terminal C-N sensing sites. Depending on the type of added polymer, the emission intensities of BCN and MCN were found to either decrease or increase, suggesting the occurrence of interactions between the emission sites and the added polymers. The fluorescence sensor capabilities of the prepared samples were investigated using the quenching test for the detection of N 0 2 and N 0 3 in the range of 5-40 and 3-18 x 103 mol, respectively. All polymer(x%)-CN composites gave almost linear Stem-Volmer plots for quenching of N 0 2 and N 0 3 . suggesting their potential ability as fluorescence sensors for detection of both N 0 2 and N 0 3~. Interestingly, all composites showed superior sensitivity towards N 0 2 than N 0 3 . While specific surface area might not be the direct crucial factor, porosity and larger surface area of MCN than BCN might provide better dispersion of polymers, which in turn resulted in better sensing performance. Among the composites, the PVP(3)-MCN showed the highest sensitivity towards both N 0 2 and N 0 3 . As for the detection of N 0 2 . PVP(3)-MCN gave the highest quenching efficiency with Stem-Volmer quenching constant (Ksi ) values of 13.5 x 1CT3 m o l1, 12.5 x 1CT3 m o l1, and 10.5 x 10~3 m o l1 at N=C, C=0, and terminal C-N sensing sites, respectively. As for the detection of N 0 3~, PVP(3)-MCN gave the highest KSr values of 21.3 x 10~6 m o l1, 23.9 x 10~6 m o l1, and 24.2 x 10~6 m o l1 at N=C, C=0 and terminal C-N sensing sites, respectively. Since the KSr value for N 0 2 detection was much higher than that for N0 3 detection, this study showed that the composite was more sensitive for the detection of N 0 2~. These could be due to the smaller size and higher polarity of N 0 2 compared to N 0 3~. Reproducibility, limit of detection (LOD), reusability, and selectivity studies were carried out on the best sensor (PVP(3)-MCN) for the detection of N 0 2 . It was revealed that PVP(3)-MCN composite showed good reproducibility with LODs of 2.9, 4.6 and 5.2 mol at N=C, C=0 and terminal C-N sensing sites, respectively. The reusability test showed that the quenching efficiency of the PVP(3)-MCN did not change much after four cycles of quenching test for N 0 2 detection. PVP(3)-MCN also showed great selectivity for the detection of N 0 2~ in the presence of Cl . P04 or S 0 42 . These results demonstrated the potential capability of PVP(3)-MCN as a fluorescence sensor for N 0 2 .