Construction Of Nanocellulose-Based Fluorescent Sensors And Their Applications Research

In recent years,with the continuous development of globalization,food safety has become a major global issue that all human beings pay close attention to and urgently need to solve.Food safety not only refers to the guarantee of quantity,but also means the guarantee of quality and safety.However,traditional food quality assessment and testing technology can no longer meet people’s requirements for the food diversity and safety.In addition,the traditional food testing technologies have many disadvantages such as complex operations,expensive equipment,and time-consuming pretreatment of samples.Therefore,it is an importance way to promote the development of sensitive,simple,fast,efficient,low-cost and reliable food safety detecting methods,which can effectively solve food safety problems.Fluorescence sensors have attracted much attention due to their simple operation,strong specificity,low cost,and high sensitivity.They are widely used in the detection of various food contaminants and are one of the most potential detection methods.As the natural polymer material with the largest reserves on earth,cellulose has the advantages of being renewable and biodegradable.The abundant hydroxyl groups on its surface make it a favorable raw material for the production of fluorescent materials.A highly selective,environmentally friendly and low-cost cellulose-based fluorescence sensors can be prepared by functional modification of hydroxyl groups,expanding the application value of cellulose in the sensor field.In this context,based on the research idea of structure design-functional assembly-application research,this paper uses specific chemical modification technology to carry out structural design and functional assembly of nanocellulose for the fluorescence sensing mechanism of different types of pollutants,and successfully constructs A series of nanocellulose-based fluorescent sensors were developed,and the feasibility and mechanism of the obtained nanocellulose-based fluorescent sensors in pollutant detection applications were discussed,which provided a theory for the in-depth development and comprehensive application of nanocellulose-based fluorescent sensors.Fundamentals and research ideas.It mainly includes the following aspects of work:（1）For four types of representative pollutants,for the purpose of constructing fluorescent sensors,bleached softwood pulp was used as raw material,and mechanical method-maleic anhydride acetylation,mechanical method-3-aminopropyl-triethyl Carboxyl-containing CNFs-MA,amino-containing CNFs@APTES,carboxyl-containing TCNFs and easily soluble in ACNCs in organic solvents.The surface chemical structure of nanocellulose was analyzed by FTIR,NMR and XPS,and it was confirmed that nanocellulose with four chemical structures was successfully prepared.The carboxyl contents of CNFs-MA and TCNFs are 1.599 mmol/g and 1.980 mmol/g,respectively,and the degrees of substitution between CNFs@APTES and ACNCs are 0.5280 and 2.1450,respectively,which are suitable for the subsequent construction of single emission,colorimetric/single emission dual mode,and dual emission,The three-emission nanocellulose-based fluorescent sensor provides the raw material basis..（2）In order to solve the self-quenching problem of carbon dots（CDs）and improve the fluorescence efficiency of CDs,chitosan was used as a precursor to prepare N-doped carbon dots（N-CDs）containing amino groups with high fluorescence intensity by a hydrothermal method.Acetylated CNFs/CDs single-emission fluorescent sensors（CNFs-MA-CDs）were fabricated by anchoring N-CDs on the previously prepared CNFs-MA surface through chemical bonds.The results showed that N-CDs were successfully prepared,which were regular spherical particles with uniform size and an average particle size of 1.8±0.4 nm.Under the synergistic effect of molecular chain dilution and chemical bond anchoring,CNFs-MA effectively prevents the aggregation and quenching of CDs and improves the fluorescence efficiency.quantum yield（34%）.For the representative anion pollutant NO₂-,CNFs-MA-CDs have high selectivity and sensitivity,and have good detection performance(in the concentration range of 1～500μM,the detection limit（LOD）is 7.3×10^-8 M),and its fluorescence detection mechanism is the internal filter effect mechanism（IFE）.In order to further expand the application range of the single-emission fluorescent sensor,it was introduced into the chitosan system to construct the fluorescent hydrogel（CMCGC）.The chemical model and the Langmuir model are suitable for describing the adsorption behavior of NO₂-on the surface of CMCGC.（3）To solve the problem of false positive results caused by the low signal-to-noise ratio of single-emission fluorescent sensors,a rhodamine derivative（RG2）with fluorescent properties was designed and synthesized using rhodamine 6G,hydrazine hydrate and glyoxal as initial raw materials,and the derivative was reacted with the previously prepared amino-containing CNFs@APTES for aldo-amine reaction,and a dual-mode sensor（CR-1）based on silanized CNFs/rhodamine 6G colorimetric/single emission fluorescence was successfully prepared.The results showed that the rhodamine derivative RG2 was successfully designed and synthesized.The addition of RG2 did not destroy the crystal structure of CR-1,and improved the thermal stability of CR-1,and the substitution degree of CR-1 was 0.3031.For the representative heavy metal ion pollutant Fe³⁺,CR-1 has high selectivity and sensitivity,and can be used as a colorimetric/single emission fluorescence dual-mode sensor.In the colorimetric mode,the LOD is 4.79×10^-7 M,and in the single-emission fluorescence mode,the LOD is 3.36×10^-7 M.The two modes can be mutually verified to make the results more reliable,effectively solving the false positive of the single-emission fluorescence sensor problem,and the detection performance of CR-1 is not affected by other competing metal ions,and its detection mechanism belongs to the intramolecular charge transfer（ICT）mechanism.In addition,a novel porous fluorescent hydrogel（PB-CR-1）was successfully prepared by combining CR-1 with PVA-borax system,and the results showed that PB-CR-1 could be prepared as a colorimetric detection card(Fe³⁺:0～100 mg/L),and has good adsorption performance（281.41 mg/g）,and its adsorption process is controlled by chemical adsorption and monolayer adsorption.（4）Aiming at the problem that the fluorescence signal change of a single wavelength will be affected by environmental interference factors,using citric acid and ethylenediamine as precursors and europium chloride as dopant,europium-doped carbon dots were synthesized by hydrothermal method（Eu-CDs）,and the Eu-CDs were successfully modified to the previously prepared carboxyl-containing TCNFs molecular chains by amidation reaction,and the Tempo oxidized CNFs/Eu3+/CDs dual-emission ratiometric fluorescent sensor（TCNFs-Eu-CDs）was successfully constructed..The results showed that Eu-CDs were successfully prepared with a diameter of 1.8±0.4 nm and a Zeta potential of-3.77 e V.The optimal excitation wavelength and maximum emission wavelength of TCNFs-Eu-CDs are 390 nm and 455 nm,respectively,the quantum yield is 31%,and TCNFs-Eu-CDs are negatively charged.For the representative antibiotic pollutant tetracycline（TC）,TCNFs-Eu-CDs has good sensitivity and high selectivity,the optimal detection p H value and the optimal detection time are 8 and 10 min,respectively,and at 455 nm and 615 nm It shows two fluorescence signal changes,effectively improving the anti-interference ability of environmental factors of the dual-emission ratiometric fluorescence sensor.In addition,the fluorescence color of TCNFs-Eu-CDs showed a gradient from blue to red with the target concentration,and the LOD was 1.75×10^-8 M,compared with the single emission fluorescent sensor TCNFs-CDs(2.64×10^-8 M),TCNFs-Eu-CDs have anti-environment interference ability and higher sensitivity.In addition,novel porous fluorescent air condensation beads（TECGP）were prepared by combining TCNFs-Eu-CDs with 3-glycidoxypropyltrimethoxysilane（GPTMS）and polyethyleneimine（PEI）.The results showed that TECGP air beads had good adsorption performance（140.35 mg/g）,and the optimal adsorption conditions were p H=8 and T=25℃.Chemisorption and monolayer adsorption could explain the adsorption process of TC on TECGP.（5）In order to further increase the range of visual color changes and improve the sensitivity of the ratiometric fluorescent sensor,based on the theory of three primary colors of color and light,1-pyrenecarboxylic acid（PCA）and protoporphyrin（Pp IX）were used as blue and red fluorescent internal standards,Using fluorescein isothiocyanate（FITC）as the green fluorescent responder,it was grafted onto the molecular chain skeleton of ACNCs by homogeneous reaction to prepare three kinds of ACNCs-based fluorescent materials with different fluorescent colors（CA-Pyr,CA-FITC,CA-Pp IX）,and a CA-FITC/CA-Pyr/CA-Pp IX three-emission ratiometric fluorescence sensor was obtained by physical blending.The results showed that CA-FITC,CA-Pp IX and CA-Pyr were successfully prepared with substitution degrees of 0.0122,0.0029 and0.0452,respectively.CA-FITC,CA-Pp IX and CA-Pyr can be excited by the excitation light at the same wavelength,and emit green（500 nm）,red（600～720 nm）and blue fluorescence（400nm）respectively,proving that CA-Pyr,CA-FITC and CA-Pp IX can be prepared by simple physical blending of three emission ratio fluorescent sensors,and the optimal blending ratio is1:16.67:83.33.The CA-FITC/CA-Pyr/CA-Pp IX three-emission ratiometric fluorescent sensor has good response to representative aquatic pollutants biogenic amines,and its fluorescent color shows a transition color from red to green,and its three fluorescent signals The intensity ratio IFITC/（IPyr+IPp IX）has a linear relationship with the logarithm of the concentration of ammonia（0-30000 ppm）,a model biogenic amine,and the composite material can be reused.In addition,using cellulose quantitative filter paper as a template,a three-emission ratio fluorescence sensor indicator label was prepared by solution impregnation method,realizing the visualized application of the three-emission ratio fluorescence sensor.In order to explore the role of nanocellulose surface groups in the construction of fluorescent sensors,this paper based on the fluorescent sensing mechanism of different pollutants,using the molecular structure and easy modification of cellulose polyhydroxyl groups,successfully constructed a sensor using different modification methods.A series of nanocellulose-based fluorescent sensors have laid the experimental foundation for nanocellulose materials in the field of pollutant detection,and provided a scientific basis for the in-depth development and application of nanocellulose functional materials.