Detection And Catalytic Reduction Of Typical Pollutants In Aqueous Solution By Functionalized Gold Nanoparticles

The continuous progress of urbanization,excessive use of chemical fertilizers and wanton discharge of industrial and domestic sewage have caused irreversible impacts on people's life and the ecological environment.Water pollution has caused the global attention and become the problem that needs to be solved urgently.As typical pollutants in water,heavy metals and organic pollutants can spread through water and accumulate by food chain,which will cause a serious threat to food safety,ecological environment and human health.Hence,it is urgent to search for efficient,simple and green method and green,secure,and highly stable materials to detct and remove these pollutants.Gold nanoparticles?AuNPs?with large specific surface area,good optical property,small size effect,and quantum effect are good candicates for this purpose.AuNPs have gained wide attention in sensing,catalysis,synthesis,and medicine field sience they were discovered.In this paper,two kinds of typical water pollutants including Hg²⁺and kanamycin?KA?were chosen,and AuNPs were used to construct colorimetic sensors for the detection of Hg²⁺and KA.The detection mechanisms were also investigated.For the removal of typical pollutants,some nitrophenols and azo dyes were chosen as template target molecules.Different carbon materials supported Au catalysts were prepared to reduce nitrophenols and azo dyes.The catalytic efficiency of these catalysts and mechanism of catalytic reduction were further investigated.The structure and main contents of the paper are as follows:The first part is the introduction of the research background and significance of the subject.The definition,characteristics,and hazard of heavy metals and organic pollutants are introduced in detail.The research status about the detection and control methods of these pollutants are also stated.In addition,the properties,preparation and application of functionalized AuNPs and AuNPs catalysts were discussed.Finally,the purpose and content of the paper are clarified.The second part described the rational design of a sensitive and selective colorimetric sensor for mercury ions(Hg²⁺)detection by using papain and 2,6-Pyridinedicarboxylic acid?PDCA?functionalized AuNPs?P-PDCA-AuNPs?.Papain is a protein with seven cystein residues and 212 amino acid residues,which can be anchored on the surface of AuNPs and can also combine with Hg²⁺on the other side.PDCA is a chelating ligand with one mercapto group?-SH?,which has strong chelating affinity with Hg²⁺and can combine with AuNPs via strong Au-S bond.Thus,the proposed sensor can sensitivel y and selectively detect Hg²⁺in aqueous solution as the synergistic effect of papain and PDCA.The linear range of Hg²⁺conentration is from 0.01?M to 14?M and the detection limit could be as low as 9 nM?1.8 ppb?,which met the maximum allowable standa rd of Environmental Protection Agency?EPA?set in drinking water.It demonstrated the high sensitivity of P-PDCA-AuNPs.This sensor was also used to detect Hg²⁺from real samples of tap water,river water,and pond water spiked with Hg²⁺ions,and the results showed good agreement with the concentration determined by an atomic fluorescence spectrometry.Functionalized AuNPs sensor proposed in this section has theoretical and practical significance for the construction of colorimetric sensor.The third section was inspired and simplified on the basis of the second section,and constrcted a simpler and more stable colorimetric sensor avoiding the introduction of many kinds of ligands for AuNPs functionalization.The authors heredescribedavisualdetectionstrategyemploying4-amino-3-hydrazino-5-mercapto-1,2,4-triazole?AHMT?functionalized AuNPs to detect KA in various samples.AHMT with one-SH was self-assembled onto the surface of AuNPs via Au-S bond and AHMT-AuNPs aggregated when KA existed owing to the hydrogen-bonding interaction between KA and AHMT.As a result,KA could be quantitatively and sensitively detected by the proposed sensor in the linear range of 0.005 to 0.1?M and 0.1 to 20?M,with the detection limit as low as0.004?M which is much lower than the maximum contamination level for KA in milk defined by the European Union.Furthermore,the proposed sensor was not affected by the interference chemicals including common amine acid,antibiotics,and metal ions.The sensor was also used to detect KA from various real samples,and the results were excellent in accord with the values measured by the high performance liquid chromatography?HPLC?.This sensor exhibited a promising potential for simple and real-time detection of KA in various real samples.The fourth section investigated the removal behaviours of nitrophenols and azo dyes by polydopamine modified carbon nitride supported Au catalyst?PDA-g-C₃N₄/Au?and NaBH₄.PDA is a kind of neurotransmitter and served as the reductant and stabilizer for AuNPs reduction,avoiding the use of chemical reductant and stabilizer that may result in secondary contamination.g-C₃N₄ not only acted as the support but also provided compatibility for AuNPs deposition,enhancing the stability and deposition of AuNPs,which improved the catalytic activity.Different experimental parameters including the amount of Au loading,concentration of NaBH₄,and dosage of catalyst were studied.Results showed that PDA-g-C₃N₄/Au_?3?revealed higher catalytic activity with a rate constant of 0.0514s^-1 and TOF of 545.60 h^-1 for 4-nitrophenol?4-NP?reduction.In addition,the catalyst was highly efficient in reduction of other nitrophenols and azo dyes and the reduction rates of these compounds were found as the sequence:methyl orange?MO?>2-nitrophenol?2-NP?>2,4-dinitrophenol?2,4-DNP?>Erichrome Black T?EBT?>Congo red?CR?,which demonstrates the good universality.Moreover,the PDA-g-C₃N₄/Au_?3?catalyst kept high stability and excellent conversion efficiency over ten reduction cycles.The practical application on different real water samples suggests that this Au catalyst has promising application in environmental w ater purification.The simple and green synthetic Au catalyst expands the range of application and provides potential application on environmental remediation.The fifth section,four kinds of porous carbon materials including carbon black?CB?,activated coke?ACk?,activated carbon?AC?,and multi-walled carbon nanotubes?MWCNTs?were used as supports to anchor AuNPs by a polyol reduction method.The catalytic reduction of 4-NP was employed as a model reaction to evaluate the property and catalytic activity of these catalysts.Results identified that CB showed hierarchical porous structure,which is beneficial to the high catalytic activity of Au catalyst.In this,CB restricted-Au catalyst provided large specific surface area,small AuNPs size,high dispe rsion of AuNPs,and low cost,which showed better catalytic activity for 4-nitrophenol reduction as compared with that of other porous carbon supported Au catalysts.Besides,with the increase of Au loadings,the catalytic activity of Au/CB was enhanced an d the1.2 wt%of Au loading exhibited the highest catalytic activity with the rate of0.8302 min^-1 and the turnover frequency of 492.50 h^-1.Universality and real water application demonstrated that the as-prepared Au/CB catalyst was promising candidate for other phenols and azo dyes reduction and had great potential for practical application.Furthermore,after ten cycles,Au/CB still retained satisfying stability and activity.These results suggest that the larger specific surface area and smaller particle size attributed to the porosity of CB are conducive to improving the catalytic activity of Au catalysts.This design shows high potential of hierarchical porous carbon materials for highly catalytic reaction in many fields,especially the water purification.In the last section,on the basis of section five,HNO₃-modified CB supported Ni-Au bimetallic nanocatalysts?HCB-Ni-Au?with different Ni/Au molar ratio were synthesized for reduction of phenols and azo dyes.Results showed that the reaction rate of 4-NP reduced by HCB-Ni₆-Au₁ reached 1.9617 min^-1,which was 15 and 38times higher than that of Ni and Au monometallic nanocatalyst,respectively.The synergistic effect between bimetallic nanoparticles and HCB,Ni and Au nanoparticles contributed to the high catalytic efficiency of HCB-Ni-Au.The reaction mechanism and pathway investigation exhibited that nitroaromatics were reduced by cleavage of–N=O bond and azo linkage?–N=N–?and uptake of H from Ni-H and Au-H bonds.Density functional theory?DFT?theoretical calculation showed 4-NP with higher free energy was easier to be catalyzed.The activation of enthalpy for MO was 65.7 kcal·mol^-1,which displayed the highest catalytic rate of2.1055 min^-1.HCB-Ni₆-Au₁ with good structural stability could be well reused and applied in tap water,distilled water,river water,and lake water samples.This study creatively combined experimental results and DFT theoretical calculation to investigate the reason for different catalytic efficiency of different target mo lecules.This design not only overcomes the low catalytic activity of Ni-based catalyst,but also reduces the usage of Au,which controls the cost and provides a theoretical basis for the application of different technologies in environmental remediation a nd large-scale preparation of catalysts.