Application Of Smartphone-based Single-atom Nanoenzyme Colorimetric Sensor For Rapid Detection Of Common Contaminants In Water

Water is the source of life and an important medium for food,human development,energy production and industry.However,industrialization,globalization and population growth have caused serious water pollution problems,and the increasing number of pollutants in water posed a serious threat to ecosystems and human health.Therefore,the development of effective detection methods is of great importance to ensure the safety of water quality.Among many analytical methods,colorimetric methods have the advantages of simple operation,low cost,short time,less dependence on analytical instruments,and visualization of results,which make them suitable for point-of-care testing（POCT）.Nanozymes have been widely used in the construction of colorimetric sensors because of their low cost,high stability,good biocompatibility and easy modification.However,traditional nanozymes-based technologies still face many challenges such as inhomogeneous size,difficult control of the exposed crystalline surface,complex composition,surface-dependent catalysis and inherently low active sites,unclear catalytic mechanism,and difficult precise regulation.To break through the above limitations,researchers have developed a novel single-atom nanoenzyme（SAzyme）with a uniformly dispersed single-atom structure and a good coordination environment.SAzyme has some advantages,such as simple composition,high catalytic activity,good selectivity,clear active site,and adjustable design,which is an ideal material for developing colorimetric sensors.The development of SAzyme with high enzyme-like activity for the construction of colorimetric sensors can greatly improve the detection sensitivity,specificity and stability of the colorimetric sensing platform.In this thesis,three kinds of SAzymes with high enzyme-like activity were synthesized by using a biological bionic method,taking different metals as the active centers and modulating the surface electron and geometric structures by heteroatom doping（P,S,etc.）.In addition,three smartphone-based colorimetric sensors were constructed using prepared SAzyme as colorimetric sensing elements for the rapid and sensitive detection of contaminants in water(heavy metals Hg²⁺,sulfides,and organic phosphorus).The main work is as follows:First,Fe-N/S-C SAzyme with high oxidase-like activity was synthesized using waste peanut shells as carbon,nitrogen and sulfur sources.Fe-N/S-C SAzyme could oxidize 3,3’,5,5’-tetramethylbenzidine（TMB）to blue oxTMB,but glutathione（GSH）could inhibit TMB oxidation,leading to blue fading.However,when Hg²⁺was added to the above system,due to the high affinity of Hg²⁺for thiols such as GSH,generating Hg²⁺-SH complexes,which eventually lead to the recovery of blue color.Based on this phenomenon,an"on-off-on"colorimetric sensor was constructed and applied to multi-mode detection of Hg²⁺in water samples with a detection range of 1 n M-10μM and an LOD of 0.17 n M,and finally,this sensor was successfully applied to the detection of Hg²⁺in actual water samples.Second,a three-dimensional and porous Fe-P/N-C SAzyme with high peroxidase（POD）activity was synthesized using the high-yielding sweet potato as the template.A colorimetric sensor based Fe-P/N-C with the high POD activity was constructed for the rapid and highly sensitive detection of sulfide.In the presence of H₂O₂,Fe-P/N-C oxidizes TMB to blue oxTMB.The addition of S^2-inhibits the POD activity of Fe-P/N-C and competes with TMB for H₂O₂,and the reduction of oxTMB by S^2-together weaken the colorimetric reaction triggered by Fe-P/N-C,resulting in the fading of blue color and the decrease of absorbance value.Based on this principle,a colorimetric sensor based the smartphone for sulfide detection was constructed.This sensor could detect S^2-in the range of 1-800μM with an LOD of 0.23μM,and was successfully used for the quantitative detection of S^2-in water.Third,Ni-N-C SAzyme with excellent POD-like activity was synthesized.Based on the fact that acetylcholinesterase（AChE）can catalyze acetylthiocholine（ATCh）to produce strong reducing thiocholine（TCh）,which can reduce the POD activity of Ni-N-C and decrease the production of oxTMB.When organophosphorus（OPs）is added,the enzymatic activity of AChis inhibited due to the formation of covalent bonds between OPs and specific serine groups on the active site of AChE,resulting in a decrease of TChcontent.Therefore,the inhibitory degree of POD activity of Ni-N-C decreases,and the catalytic TMB of Ni-N-C shows obvious blue color.Using this principle,a Ni-N-C SAzyme-based colorimetric sensor was constructed to detect OPs pesticide residues.In this study,phoxiphos was used as a model of OPs,and the detection range of this colorimetric sensor for phoxim was 5-600 ng/m L and the LOD was 0.27 ng/m L,and finally proposed sensor was successfully applied to the detection of phoxim residues in water.