In Situ Investigation Into Graphene Quantum Dots Effects On The Adsorption And Photodegradation Behavior Of Nitrated Polycyclic Aromatic Hydrocarbons Adsorbed Onto Crop Leaf Surfaces

Nitrated polycyclic aromatic hydrocarbons(NPAHs)are a group of derivatives of PAHs.Studies have shown that the occurrence of NPAHs in ambient air is much lower than those of their PPAHs is,but their toxicity is much higher than that of PPAHs.Plants are the first way for NPAHs to enter the food chain.What's more,NPAHs can exchange dynamically between the atmosphere and the plant interface.Therefore,in order to better evaluate the situation of human exposure to NPAHs through dietary pathways,in-depth study on the environmental behavior of NPAHs in crops is worthy of special attention.Up to now,there have been a wealth of in-situ studies on the environmental behavior and mechanism of PAHs on the surface of living plants.However,the major environmental chemical processes and mechanism of NPAHs on the surface of living crops' leaves are still unclear.In the present study,a novel method for in situ detection of NPAHs adsorbed onto the leaf surfaces of living crop seedlings was established based on the fiber-optic fluorimetry combined with graphene quantum dots(GQDs)as a fluorescent probe.Using the novel method,the adsorption and photochemical degradation of individual NPAHs on the surfaces of living crop leaf,as well as the effects of GQDs,were in situ investigated.Main results were summarized as followings:(1)A novel method based on the fiber-optic fluorimetry combined with graphene quantum dots(GQDs)as a fluorescent probe was utilized for in situ detection of 9-NAnt and 3-NFla adsorbed onto the leaf surfaces of living soybean and maize seedlings.Experimental results showed that the linear dynamic ranges for the in situ determination of the 9-NAnt and 3-NFla adsorbed onto the leaf surfaces of soybean/maize were 2.9-410/3.4-435 ng/spot and 1.7-370/2.1-390 ng/spot.The experimental recoveries for the soybean/maize were 87.8%-103.4%/96.5%-113.9%,84.1%-109.1%/93.0%-104.8%,respectively.The relative standard deviations were less than 7.9%,5.71%(n = 9),respectively.The detection limits for the in situ quantification of the two adsorbed NPAHs by fiber-optic fluorimetry and fluorescence quenching method ranged from 7.8-11.9 ng/spot and 0.8 to 1.6 ng/spot,respectively.These results clearly confirmed the feasibility of the fiber-optic fluorimetry and fluorescence quenching method for the in situ determination of adsorbed 9-NAnt and 3-NFla.The sensitivity of fluorescence quenching method is about 8-10 times higher than that of fiber fluorescence method.(2)Using the novel method,the effects of GQDs on the adsorption of individual 9-NAnt and 3-NFla by the living soybean and maize leaf surfaces were in situ investigated.The presence of GQDs altered the adsorption mechanism from the sole film diffusion to the combination of film diffusion and intra-particle diffusion,and shortened the time required to achieving adsorption equilibrium by 15.8-21.7%.Significant inter-species and interchemical variability existed in terms of the equilibrated adsorption capacity(qe)with the sequence of soybean > maize and 3-NFla > 9-NAnt.The occurrence of GQDs enlarged the qe values of 9-NAnt and 3-NFla by 22.8% versus 28.7% for soybean,and 16.2% versus 20.3% for maize,respectively,which was largely attributed to GQDs-induced expansion to the surface area for adsorbing NPHAs and the stronger electrostatic interaction between the-NO2 of NPAH molecules and the functional groups(e.g.,-COOH,-OH)of GQDs outer surfaces.And,the varied enhancement degrees in the order of 3-NFla > 9-NAnt might be explained by the steric effects that resulted in the easier accessibility of-NO2 of 3-NFla to the outer surface of GQDs.Summarily,the GQDs increased the retention of NPAHs on crop leaf surfaces,potentially threatening the crop security.(3)In this study,based on the fiber-optic fluorimetry,a fluorescence quenching method using graphene quantum dots(GQDs)as a fluorescent probe was applied for the in vivo quantification of 9-NAnt and 1-NPyr adsorbed onto the leaf surfaces of living lettuce(Lactuca sativa L.)seedlings.Using the established method,the photolysis kinetics of the two adsorbed NPAHs and related mechanism were determined.The detection limits for 9-NAnt and 1-NPyr reached as low as 1.2 ng/spot and 0.9 ng/spot,respectively.Photochemical degradation played the dominant role in the clearance of NPAHs from the lettuce leaf surfaces,and the photolytic kinetics followed pseudo-first-order equation.Inter-chemical variability was observed in terms of photolysis rates(kP)with the sequence of 9-NAnt(0.2969 ± 0.0017 h-1)> 1-NPyr(0.0944 ± 0.0004 h-1),as well as the photodegradation half-lives(t1/2)with mean values of 2.3 ± 0.1 h for 9-NAnt and 7.4 ± 0.2 h for 1-NPyr.The photostability of NPAHs depends on the planar arrangement of-NO2 groups and aromatic rings.The photolysis mechanism of NPAHs could not be determined because no photolysis products of NPAHs were detected.