Construction Of Fluorescent Probes Based On Layered Rare Earth Hydroxide Nanosheets And Bioiogical Detection Applications

Layered rare-earth hydroxide（LRH）is a new type of positively charged layered compound.The interlayer anions can be flexibly regulated,and show a synergistic effect with the rare-earth ions of the laminates,controlling the properties of the compounds.As a result,LRH has broad application prospects in the fields of fluorescence display,biomedicine,catalysis and so on.In addition,fluorescence detections can also be realized according to the luminescence characteristics of the interlayer luminescent guest or the rare-earth ions of the laminate.Fluorescence sensors are widely used in the field of analysis and detection due to the advantages of simple equipment operation,fast detection speed and high sensitivity.In recent years,the design and development of fluorescence sensors based on mechanisms such as fluorescence resonance energy transfer,antenna effect,and inner filter effect have received extensive attention.In this work,the construction and biodetection application of fluorescent probes based on layered rare-earth hydroxide nanosheets were systematically studied,and the energy transfer mechanism between organic guest molecules and layers,detection substances and response signals were explored respectively.It specifically involves the following three parts:（1）Layered terbium hydroxide（Cl-LTbH）was the precursor and the organic light-emitting molecule 3-aminobenzenesulfonic acid（AS）was the guest to synthesize the composite material AS-LTbH containing two emission centers by ion-exchange method.The layered composite material was exfoliated into single-layer nanosheets in formamide,and a fluorescent probe was constructed based on this unit.AS with strong emission and anti-interference ability can be used as a reference.Tb³⁺can be sensitized by 2,6-pyridine dicarboxylic acid（DPA）based on the antenna effect,which can be acted as a recognition and signal output unit.Utilizing the different emission colors of AS and Tb³⁺,the visual detection from blue to green was realized.According to the data,the ratio of fluorescence emission intensity(I₅₄₄/I₃₆₀)of the probe exhibited a positive linear relationship with the concentration of DPA in the detection concentration range of 0.05～5.0μM,with a detection limit of 3.9 nM.Moreover,the detection limit detected by this probe was much lower than the detection limit of Tb³⁺single-signal response（11.86 nM）.（2）Layered europium hydroxide（Cl-LEuH）was used as the precursor,and organic 4,4’-stilbene dicarboxylic acid（SDC）and sodium dodecyl sulfate（SDS）were used as the guests to synthesize nanocomposite SDC_0.05SDS_0.95-LEuH with dual emission centers.The single-layer nanosheets were obtained by the exfoliation in formamide,which acted as a unit to assemble a fluorescent“on-off”sensor.Taking advantage of the fluorescence emission characteristics of SDC and Eu³⁺,a ratiometric fluorescent nanoprobe FNP(SDC_0.05SDS_0.95-LEuH)was constructed,which realized the detection of DPA and Cu²⁺in the same system.Based on the effect of fluorescence resonance energy transfer,the emission intensity of SDC gradually decreased,and the emission of Eu³⁺gradually increased with the addition of DPA,which benefited from the strong coordination and energy transfer between DPA and Eu³⁺.When Cu²⁺was added,the emission of Eu³⁺was gradually weakened,which could be ascribed to the charge transfer between Cu²⁺and Eu-DPA.As a result,the fluorescence change of Eu³⁺could be acted as the identification and output signal for detecting DPA and Cu²⁺.The experimental results showed that the ratio of fluorescence emission intensity(I₆₁₉/I₃₉₄)of the probe had a positive linear relationship with the concentration of DPA,and a negative linear relationship with the concentration of Cu²⁺,thus realizing the high sensitivity detection of DPA and a wide detection range of Cu²⁺.（3）Layered europium hydroxide（NO₃-LEuH）was the precursor,the organic light-emitting molecule 8-naphthylamine-1-naphthalene sulfonic acid（ANS）and the surface-active molecule 1-octane sulfonate sodium（OS）were as the guests to synthesize fluorescent nanomaterial OS_0.2ANS_0.8-LEuH with novel emission properties.Then,single-layer nanosheets were prepared by dispersing in a mixed solvent of formamide and water,and a fluorescence-colorimetric dual-output dopamine（DA）sensor was constructed.The fluorescence of the probe was quenched due to the inner filter effect and electron transfer.At the same time,the easy oxidation of DA in an alkaline environment led to a color deepening of the probe,which enabled the probe to emit a signal with enhanced absorbance.Based on this research,the dual signal output detection of fluorescence and colorimetry for DA was realized,and the fluorescence quenching efficiency F₀/F and absorbance displayed a good positive linear relationship with the DA concentration.In the 0.001～0.20 mM range,the detection limit of fluorescence response was 0.29μM,and the detection limit of colorimetric response was 0.058μM.The dual-signal output sensing strategy greatly improves detection accuracy and reliability.In conclusion,the ratiometric or dual-signal output probes based on LRH nanosheets show excellent performances in biological detection.High precision,strong anti-interference,high sensitivity,high stability,and a green-friendly preparation procedure are all benefits of the probe,which can expand the detection range or achieve a lower detection limit by adjusting the ratio of nanosheets and analytes.This research work not only expands the application fields of LRH but also puts forward new ideas for the construction of fluorescence sensors.