Polyvinylpyrrolidone, graphene oxide and their composites as potential fluorescence sensing materials for nitrate and nitrite ions

Abstract

The existence of toxic nitrate (NO3 -) and nitrite (NO2 -) ions above the permissible level causes environmental pollution and human health hazard. Therefore, many studies have been carried out to improve sensitivity and selectivity of sensors for the ion detections. In this study, polyvinylpyrrolidone (PVP), graphene oxide (GO), and polyvinylpyrrolidonegraphene oxide (PVP-GO) were prepared, characterized, and tested for their ability to detect nitrate and nitrite ions. A series of PVP with concentration of 1-10% was prepared by dissolvation in deionized water. The PVP has –C=O and –N–C sensing sites, with excitation wavelength of 285 nm and 330 nm and emission wavelength of 408 nm and 410 nm, respectively. All the PVP sensing materials showed greater sensitivity towards nitrite than nitrate. It was found that 7% PVP showed the best sensitivity for the analytes detection at both sensing sites. Nitrate preferred the –C=O site, while nitrite preferred the –N–C site. The selectivity tests demonstrated that 7% PVP gave great selectivity towards analytes even in the presence of SO4 2-, HCO3 -, or Cl-, but not OH- ion. The high interference from OH- could be due to hydrogen bond formation. Computational simulation for PVP and analytes was investigated using B3LYP/6-311G(d,p). The simulation studies revealed that PVP formed greater interaction with nitrite than nitrate. Characterization results showed that the GO was successfully prepared by the improved Hummers’ method. GO showed greater sensitivity for the detection to nitrite than nitrate in the range of 0-100 mM. Selectivity tests found that GO showed great selectivity tawards analytes even in the presence of SO4 2- or Cl-, but low selectivity in the presence of HCO3 - or OH- ion, due to the formation of hydrogen bond. Simulation results demonstrated that GO formed greater interaction with nitrite compared to nitrate. The high binding energies between hydroxyl or carboxyl site and the analytes showed that they might be the possible sensing sites in GO. A series of PVP-GO(x) composite was prepared by mixing and sonication treatment of 7% PVP (100 mL) and various amounts of GO (x = 0.0075-0.03 g). The characterizations supported the successful formation of the composites. All composites showed superior sensitivity towards nitrite than nitrate. Among the composites, the PVP-GO(0.01) showed the highest sensitivity for the detection of both analytes. For the selectivity tests, PVP-GO(0.01) showed great selectivity for the detection of analytes even in the presence of SO4 2-, HCO3 -, or Cl-, but not for OH- ion. The simulation tests exhibited that the –C=O site of PVP interacted with hydroxyl site of GO to form PVP-GO composite. The PVP-GO showed greater interaction with nitrite compared to nitrate. All computational results matched with the experimental results. The addition of GO to the PVP was found to increase the sensitivity and selectivity for nitrate detection, but not for nitrite detection. However, the composite gave better limit of detection (LOD) than the 7% PVP and GO. This study showed that among all the investigated materials, 7% PVP was the most potential fluorescence sensor for nitrate detection with LOD of 4.00 mM at –C=O site, while PVP-GO(0.01) was the most potential one for nitrite detection with LOD of 0.26 mM at –N–C site. Real sample testing using UTM lake water demonstrated the potential application of 7% PVP as a fluorescence sensor.