Fabrication Of Ratiometric Probes Based On Rare Earth-carbon Dots And Their Analytical Application

As an emerging kind of functional materials,rare earth-carbon dots（RE-CDs）nanomaterials have shown great promise in an extensive range of applications.RE-CDs nanomaterials have higher luminescence stability and better water-solubility than single RE composites,and also have more specific binding sites than pure CDs,which makes RE-CDs nanomaterials have quickly become a promising functional material in biochemical analysis.More interestingly,RE-CDs have the merits of simultaneously preserving the luminescence of both RE³⁺and CDs,suggesting the potential to construct ratiometric probes.Compared with the single-signal readout mode,ratiometric probes with two signals can effectively eliminate environmental interference,showing better sensitivity and accuracy of signal response to the target.Generally,the luminescent RE-CDs are fabricated via simple one-step doping method,direct mixing coordination and assistance with ligand or matrix.In this paper,based on RE-CDs nanomaterials,four ratiometric probes were fabricated for the analysis of food and environmental samples.The main research contents are as follows:（1）A novel ratiometric probe（CDs/Eu-DPA）based on the self-assembly of blue-emitting carbon dots（CDs）and 2,6-dipicolinic acid-sensitized Eu³⁺（Eu-DPA）was constructed to realize the one-step direct detection of ascorbic acid（AA）in foods.The probe used CDs as the reference signal and Eu-DPA as the response signal of single-wavelength excitation.After adding AA,the fluorescence emission of Eu³⁺at 615 nm was obviously extinguished,and the fluorescence emission of CDs at 440 nm changed relatively little.The fluorescence ratio(F₆₁₅/F₄₄₀)showed a good linear relationship with the AA concentration in the range of 0.5-820μM,and the detection limit was 60 n M.The method has high selectivity and anti-interference,and can be well applied to the detection of AA in actual food samples.（2）A new kind of water-soluble RE-CDs nanocomposite(CDs@Ca F₂:Eu³⁺)was developed for the ratiometric determination of doxycycline（DOX）.The CDs@Ca F₂:Eu³⁺displayed blue emission of CDs at 440 nm and no obvious emission of Eu³⁺under the excitation at 365 nm.With the addition of DOX,substantial fluorescence quenching of the CDs at 440 nm and enhancement of Eu³⁺at 613 nm were observed,resulting in a ratiometric fluorescent response toward DOX.A wide linear range from 0.1μM to 30μM was achieved in the detection of DOX with a lowest detection limit of 43 n M.In particular,the probe could discriminate DOX from other tetracycline antibiotics through unique fluorescence response.The probe also showed excellent performance for spiked recovery of DOX in real food samples such as milk and honey.（3）To facilitate the exploration of the interaction mechanisms for RE-CDs,same precursors（folic acid and terbium nitrate）were used to synthesize RE-CDs via two most common fabrication methods（doping and direct coordination）in this study.The morphologies,surface groups,and optical properties of the as-prepared terbium-doped CDs（Tb-CDs）and terbium-functionalized CDs（Tb@CDs）were comprehensively compared and analyzed.The anthrax biomarker（2,6-dipicolinic acid,DPA）sensing performances of both RE-CDs were also evaluated.Taking the original CDs without terbium as control,the Tb-CDs had higher quantum yield（QY）and longer fluorescence lifetime,and the fluorescence emission peak was red-shifted,suggesting the effective passivation of surface defects.On the contrary,QY and fluorescence lifetime of Tb@CDs decreased,and the emission peak remained unchanged,indicating that interaction between free Tb³⁺and CDs was similar to the fluorescence quenching behavior of metal ions on CDs.Further characterization also helped in demonstrating the different interaction mechanisms.Moreover,for the relatively better performance in DPA detection,Tb-CDs was chosen as the probe for the follow-up application.The Tb-CDs exhibited a sensitive response to DPA in the concentration range of 0.05–8μM with the detection limit of 44 n M.In addition,it performed well in the spiked recovery of real samples such as running water and lake water.（4）A design strategy for the ratiometric detection of phosphate（Pi）based on blue-emitting CDs（CDs）and yellow-emitting terbium-doped CDs（Tb-CDs）was developed.Due to electrostatic attraction and strong coordination ability of Tb³⁺,CDs/Tb-CDs composite were successfully constructed.Meanwhile,the fluorescence of CDs was partially quenched by Tb-CDs through fluorescence resonance energy transfer（FRET）.In the presence of Pi,the FRET process was interrupted due to the stronger affinity of Tb³⁺and Pi,so the fluorescence of CDs was recovered,while the yellow fluorescence of Tb-CDs remained unchanged,thereby realizing the ratio response of Pi.The linear detection range of this probe for Pi was 0.5-12μM,and the detection limit was about 0.41μM.In addition,the probe has good selectivity and good spike recovery in lake water and running water samples.In summary,five kinds of RE-CDs nanomaterials were synthesized by assistance with ligand,assistance with matrix,direct mixing coordination and one-step doping method,and on this basis,four ratiometric probes were fabricated for the detection of ascorbic acid,doxycycline in food samples,and the anthrax biomarker,phosphate in environmental samples.The work realized the rapid and visual detection of target substances in complex matrix samples.The synthesis and application of RE-CDs nanomaterials were expanded,and the binding mechanism of RE-CDs was further discussed.It also provided a new idea for the research and development of RE-CDs nanomaterials,and a new study direction of novel ratiometric fluorescent probes.