Construction Of Plant-Derived Nanoprobes For Colorimetric/Fluorescent Sensing

Optical sensing technologies including colorimetry and fluorimetry have becoming increasingly attractive due to their on-site,real-time,rapid,sensitive and noninvasive response characteristics.Compared with synthetic probes,plant-derived probes show great superiority for their wide sources,diverse organic frameworks,abundant active functional groups,broad-spectrum pharmacological activities and high biocompatibility.At present,various plant-derived nanoprobes have been developed for sensing,such as carbon dots/gold nanoparticles（CDs/Au NPs）,nanozymes,and fluorescent nanoparticles.However,the development of novel plant-derived probes is greatly hindered owning to the following challenges:（1）The role of solvent in the growth and assembly of Au NPs by CDs reduction is still unknown;（2）The construction and regulation mechanisms of nanozyme with multi-enzyme like activities need to be further studied;（3）The development and construction of novel fluorescent nanoprobes and the discovery of new luminescence mechanisms are still challenging.Herein,Plant-derived CDs or fluorescent molecules were adopted for the fabrication of nanoprobes.The mechanisms on their growth,assembly or defect/electronic structures change,as well as the sensing performance were elucidated.Finally,the developed plant-derived colorimetric/fluorescent nanoprobes were applied for sensing and bioimaging successfully.This work will open new perspectives for the construction and application of novel plant-derived nanoprobes.The main research contents are as follows:（1）The mechanism of in situ growth and self-assembly of Au NPs on the surface of plant-derived CDs（prepared from citric acid in lemon and urea）in H₂O-ethanol（Et OH）system was investigated,and a colorimetric probe based on Au NPs@CDs for non-enzymatic Et OH content sensing was developed.In H₂O,hydrophilic CDs have stronger reducibility and are oxidized into carboxyl-rich CDs_（H2O）to stabilize Au⁰.The formed core-shell Au NPs@CDs_（H2O）are stabilized through the electrostatic repulsion（zeta potential,-23.5 m V）.Then characteristic absorption peak of monodisperse Au NPs at 528 nm（color of solution,red）appears.When Et OH content increased,CDs with weaker reducibility are transferred into CDs_{（Et OH）}with lower contents of carboxyl groups.The Au NPs@CDs_{（Et OH）}aggregate due to the strong hydrogen-bonding interaction between carboxyl dimers.Then characteristic absorption peak at 690 nm appears（color of solution,blue）.Thereby,colorimetric detection of Et OH was realized with linear range of 20.0～90.0%（v/v,R²=0.999）,and limit of detection（LOD）of 11.6%.Real sample analysis of liquors certified the simplicity,accuracy and reproducibility of the method.This work will provide valuable information for the surface chemistry and applications of Au NPs-based colorimetric sensors.（2）CDs-Fe（Ⅱ）/Fe（Ⅲ）-MOF with dual enzyme-like activities was designed for colorimetric point-of-care sensing of glucose and alkaline phosphatase（ALP）in human serum based on paper-based platform.The modification of CDs greatly improves the water stability,Fe（Ⅱ）/Fe（Ⅲ）cycling and electron transfer efficiency of CDs-Fe（Ⅱ）/Fe（Ⅲ）-MOF.Thereby,higher oxidase and peroxidase-like activities are obtained.Then,a paper-based platform based on glucose oxidase/CDs-Fe（Ⅱ）/Fe（Ⅲ）-MOF and pyrophosphoric acid/CDs-Fe（Ⅱ）/Fe（Ⅲ）-MOF system was designed.And colorimetric sensing of glucose（linear range 0.01～1.0 m M;LOD,0.004 m M）and ALP(linear range,20.0～200.0 U·L^-1;LOD,17.1 U·L^-1)with high selectivity and accuracy（recovery,89.4～108.9%）was achieved.This work certifies great potential of CDs-Fe（Ⅱ）/Fe（Ⅲ）-MOF in the rapid on-site analysis of complex biological samples.What is more,it provides novel strategy for the design and the regulation of catalytic activity of plant-derived nanozymes.（3）A stable fluorescent probe,defective neomangiferin–ZIF-8,was designed and de novo synthesized with neomangiferin（widely distributes in mango leaves）for naked-eye differentiation of aprotic/protic solvents and real-time sensing of trace water in organic solvents and air.Experimental and theoretical investigations reveal the unique ligand-to-ligand charge transfer with large dipole moment existing in defective neomangiferin-ZIF-8.And solvents affect the intermolecular electron-transfer transition through coordination,dipole–dipole and/or hydrogen-bonding interactions,which induce solvatochromic behavior and hydrogen-bonding induced quenching.Due to the unique fluorescent characteristics of neomangiferin-ZIF-8,ultrafast（0.1 s）visual differentiation of aprotic/protic solvents,rapid（8 s）and sensitive（LOD,0.01～0.30%（v/v））detection of trace water in organic solvents,as well as on-site and reversible sensing of relative humidity in air were realized.This study will stimulate extensive research on the rationally design and multifunctional applications of novel plant-derived luminescent MOFs.（4）Natural quercetin-3-O-β-D-glucuronide（obtained from Bupleurum scorzonerifolium）was proposed for"turn-on"sensing ofβ-glucuronidase（GUS）.GUS can specifically hydrolyze quercetin-3-O-β-D-glucuronide.Then hydrophobic quercetin nanoparticles with aggregation-induced emission and excited state intramolecular proton transfer are produced in situ.A new keto emission with large Stokes shift（175 nm）at540 nm appears.The fluorescent intensity is linearly correlated with GUS activity(linear range,5.0～60.0μg·m L^-1,R²=0.9904)with high sensitivity(LOD,0.7μg·m L^-1),high precision（relative standard deviation values<3.0%for intra-day and inter-day variations）and superior selectivity.Then,a"turn-on"platform for facile GUS inhibitors screening was constructed.Molecular docking results confirmed the feasibility of the method.Moreover,in situ imaging of GUS in E.coli noninvasively with low background noise was realized.Results demonstrate the great potential of quercetin-3-O-β-D-glucuronide in high-throughput inhibitors screening,GUS-related drug discovery,and disease diagnosis.