Construction Of Highly Sensitive Upconversion Nanosensors And Rapid Detection Applications

Upconversion nanoparticles（UCNPs）convert low-energy near-infrared（NIR）excitation light into high-energy visible or ultraviolet light.Near-infrared excited UCNPs have the advantages of near-infrared excitation,excellent photostability,high signal-to-noise ratio,and multicolor emission,which have wide applications in biomedicine,environmental detection,food safety,and other fields.However,traditional upconversion-based nanoprobes suffer from some disadvantages such as low luminescence resonance energy transfer（LRET）efficiency and low upconversion luminescence（UCL）quantum yield,limiting further development and related practical applications.Based on the above analysis,this paper firstly optimizes the nanostructure of UCNPs,confining the luminescent center to the intermediate shell,and reducing the distance between the energy donor and acceptor to improve the LRET efficiency.What’s more,the quantum yield of UCNPs was enhanced by increasing the sensitizer concentration.Secondly,a highly sensitive nanoprobe was designed for the detection of ascorbic acid based on the as-prepared UCNPs.Finally,based on the successful preparation of highly bright UCNPs and highly sensitive nanoprobes,several upconversion nanoprobes,with no background fluorescence,good photostability and high sensitivity,were constructed for the detection of heparin,semicarbazide and SO₂ derivatives,expanding the application of upconversion nanoprobes in fields of life safety,food safety and environmental safety.The specific research is as follows:（1）Sandwich-structured UCNPs（SWUCNPs）greatly improve the efficiency of LRET by confining the light-emitting layer to the intermediate shell.However,this also reduces the UCL intensity of SWUCNPs,which affects their wide application.In this work,we synthesized SWUCNPs（NaYbF4:（30%Gd）@NaYbF4:Er（2%）@NaYF₄）by doping high-concentration of Yb³⁺.SWUCNPs reduced the distance between Er³⁺ and the surface quencher of UCNPs by confining the luminescent center of Er³⁺in the inner shell to maintain high LRET efficiency.Meanwhile,the doping of a high concentration of sensitizer ions increased the absorption of 980 nm excitation light,thereby increasing the UCL intensity.Compared with the ordinary UCNPs of NaYF₄@NaYF₄:Yb/Er@NaYF₄,the UCL intensity of the prepared SWUCNPs was greatly improved,with the UCL at 540 nm increased by 56 times while that at 655 nm increased by 117 times.（2）Excessive intake or deficiency of ascorbic acid（AA）causes scurvy,Parkinson’s disease,cancer,diabetes and other serious diseases.Therefore,the development of sensitive and reliable analytical methods for AA is of great significance.The SWUCNPs prepared in the previous chapter were used as energy donors,while CoOOH was modified on the surface,used as the energy acceptor to quench the UCL.After the addition of AA,a specific reaction between AA and CoOOH leads to its decomposition,resulting in the recovery of UCL.The SWUCNPs greatly improved the UCL efficiency of UCNPs by retaining high LRET efficiency.Therefore,the designed upconversion nanosensor exhibited a highly sensitive response to AA with a LOD as low as 38 nM.In addition,the practical application performance of the nanosensor was verified by detecting AA in serum and food.（3）Heparin is an important anticoagulant in clinics,which requires regular detection and dose adjustment.Fluorimetry is a powerful tool considering the advantages of fast response and high sensitivity while most fluorescent sensors suffer from spontaneous fluorescence for excitation under short-wavelength light,the occurrence of false positives,complex and time-consuming preparation,as well as expensive detection instruments.Herein,we developed upconversion nanoparticles（UCNPs）based nanosensor for fluorescent and colorimetric detection of heparin.The nanosensor exhibited sensitive detection for heparin with the low limit of detection（LOD）of 0.1 nM in fluorescent mode and 0.3 nM in colorimetric mode.In addition,we used a smartphone-sensing platform for quantitative detection of heparin with the LOD of 2 nM.Depending on long-wavelength excitation and fluorescent-colorimetric dual-response detection signal,the designed nanosensor could eliminate the autofluorescence,which effectively increased the accuracy of the sensor,thus extending the applications for bed-side heparin assay and related medical safety.（4）Semicarbazide（SEM）is a widespread carcinogenic and neurotoxic food contaminant,originating from the metabolite of antibiotic nitrofurazone,which is used in aquaculture,or thermal decomposition byproduct of a flour blowing agent azodicarbonamide.Although optical detection technologies are powerful tools considering the advantages of fast response and visualization detection,there are few optical nanosensors for highly sensitive and visual assays of SEM due to no luminescence response and UV absorbance of SEM.Herein,an upconversion luminescence（UCL）-based nanosensor was designed for visual detection of SEM with high sensitivity and good selectivity.The nanosensor was constructed by combining upconversion nanoparticles（UCNPs）and phosphomolybdic acid（PMA）,which was used as the specific recognition element of SEM.The developed nanosensor exhibited selective absorbance enhancement and UCL quenching behavior with the addition of SEM based on the inner filter effect（IFE）.Since the change in absorbance translated into an exponential change in luminescence,the sensitivity of the nanosensor was greatly improved.The nanosensor realized a highly sensitive and visual response to SEM in the linear range of 0.5-16 μM with a low limit of detection of 58 nM.Moreover,satisfactory recovery values ranging from 90 to 112%in spiked real samples indicated the practical applicability of the nanosensor.The nanosensor designed here provides a sensitive and convenient sensing strategy for visual detection of hazardous substances and is expected to develop the upconversion sensing application in food safety.（5）SO₂ derivatives（sulfites and bisulfites）cause a series of physiological diseases such as respiratory diseases,asthma,chronic obstructive pulmonary disease,and lung tumors.Therefore,the detection of SO₂ derivatives is important for protecting human health and monitoring air quality,while the upconversion nanoprobes used for the detection of SO₂ derivatives are generally limited by the complex and time-consuming synthesis process of the specific recognition unit,or the low quenching efficiency of UCL.In this paper,a high-efficient UCL nanosensor was developed for SO₂ derivatives,using the commercial dye of parafuchsin hydrochloride（PAR）as the specific recognition unit and energy acceptor and Yb³⁺ and Er³⁺ co-doped UCNPs as the energy donor.PAR significantly quenched the green emission of UCNPs by IFE,while the red emission remained unchanged.After the addition of sulfite,the nucleophilic addition reaction between PAR and sulfite occurred,resulting in the destruction of the chromophore structure of PAR and the recovery of the green luminescence.The relative standard deviation in the standard addition experiment was low,and the recovery was satisfactory,showing excellent real sample detection performance of the prepared nanosensor.