Study On Dual-signal Optical Sensors Based On Fluorescent Nanomaterials And Their Applications In Environmental And Food Detection

The development of analytical methods with excellent performance is important for environmental monitoring and protection,food quality,and safety control.Spectral analysis methods,such as fluorescence analysis,light scattering analysis,and ultraviolet-visible spectrophotometry,have been widely used for their fast response,high sensitivity,and simple operation.In recent years,the development of nanotechnology has provided a new perspective for the development of various disciplines and fields.Optical sensors based on nanomaterials also play an increasingly important role in environmental monitoring and food quality and safety monitoring.In this paper,several new fluorescent carbon dots and non-conjugated polymer nanoparticles were prepared based on environmental and food monitoring applications.Their chemical and optical properties were studied by means of electron microscopy,spectroscopy,and surface analysis.Then,based on these two kinds of fluorescent nanomaterials and combining with molecular spectroscopy analysis technology,a dual-signal optical sensor was constructed for the selective and sensitive detection of environmental pollutants and food ingredients,and the response mechanism of the sensing strategy was explored.The main research contents of this paper are summarized as follows:1.Regulation and visualization of redox carbon nanodotsNovel CNDs with tunable redox abilities and emission are synthesized via one-pot hydrothermal method.The distinct optical properties of blue-and green-emitting CNDs(b-CNDs,g-CNDs)are demonstrated due to the dominated effect of graphited core.In addition,the redox abilities of CNDs are regulated by the change of surface states.By virtue of a reagent indicator of HAu Cl₄,the adjustable reducibility of CNDs is visually identified.Gold salts are reduced to Au nanobelts under the reduction and stabilization of b-CNDs,resulting in the plasmons-induced color variations,whereas g-CNDs possess no effect on Au³⁺.2.An aggregation-induced scattering enhanced ratiometric sensing strategy based on reduced carbon nanodots for the detection of Au(III)Based on the redox reaction between the reduced carbon point(b-CNDs)and the target,a ratio sensing strategy for the detection of Au(III)was constructed.Under anion induction,Au(III)was rapidly reduced and aggregated by b-CNDs,resulting in a significant increase in second order scattering(SOS).Meanwhile,electron transfer between b-CNDs and Au(III)caused fluorescence(FL)quenching of b-CNDs.Therefore,a ratio sensing method for Au(III)selective detection was developed by combining SOS and FL signal changes.The detection range of Au(III)by this ratio strategy was 0.2-230?M,and the detection limit(LOD)was as low as 67 n M.Compared with single signal FL or SOS method,this ratio sensor has a wider linear range,higher sensitivity,and better selectivity.Moreover,the sensing method converts Au(III)into metallic Au and aggregates it while detecting Au(III),effectively reducing the toxicity of Au(III).3.A fluorescent and digital image colorimetric dual-mode nanosensor based on novel green fluorescent carbon dots for rapid and highly selective detection of2,4,6-trinitrophenolA digital image colorimetric and fluorescence dual-mode nanoscale sensing platform based on fluorescent carbon points was created for the rapid and accurate quantitative detection of 2,4,6-trinitrophenyl(TNP).Carbon dots(CDs)as response elements were prepared by a one-step hydrothermal method with noradrenaline(NE)and ethylenediamine(EDA).The addition of TNP can significantly change the color of CDs solution and make CDs fluorescence quenching.Thus,the colorimetric and fluorescence dual mode nanosensors are established to detect TNP.In order to solve the problem that visual colorimetry cannot realize quantitative detection,the change of fluorescence color is quantified by using smartphones,and TNP is accurately detected by digital image colorimetry.The Value(V)of HSV color space was used as the color feature to measure TNP concentration.The detection range was 3.7-36.9?M,and the detection limit was 1.5?M.The colorimetric mode detects TNP,and the response is fast,accurate,and portable.The fluorescence mode based on fluorescence spectrophotometer can also detect TNP sensitively and selectively,with detection range from 0.1 to 15.8?M and LOD as low as 27 n M.It was found that the selective response mechanism of TNP was due to the internal filtration effect between CDs and TNP.Finally,the TNP detection in environmental water samples was successfully realized by the strategy.4.A fluorescent and digital image colorimetric dual-mode detection platform based on biomass carbon dots for p H sensingA digital image colorimetric and fluorescent dual-mode nano-sensing platform based on biomass fluorescent carbon dots(CDs)was created for p H monitoring of environmental water samples.CDs are prepared by microwave method with onion juice as precursor.They have good optical properties and excellent stability,and the synthesis method is simple,green and efficient.Since the solution color and fluorescence intensity of CDs can be changed with p H value,and a colorimetric and fluorescent dual-mode sensor is constructed to monitor the p H.Under the optimized detection conditions,the colorimetric mode with integrated smartphone can realize quantitative monitoring of p H in the range of 6.1–13.6.Compared with the traditional visual colorimetric method,this method considers portability and accuracy.The fluorescence mode based on the fluorescence spectrophotometer can achieve accurate p H sensing in the ranges of p H2.0–6.1 and 6.1–13.6.In addition,the response behavior of both modes is reversible.The response mechanism is as follows.As the p H increases,protonated groups such as hydroxy,carboxyl,and amino groups on the surface of the carbon dots deprotonate to form hydrogen bonds.Hydrogen bonding causes the carbon dots to aggregate,resulting in fluorescence quenching and color change.Finally,the method is used to monitor the p H of river water,rainwater and tap water samples.The results are very close to the results of p H meter test,which indicates that the method is highly accurate.5.Multifunctional binding strategy on non-conjugated polymer nanoparticles for ratiometric detection and effective removal of mercury ionsNovel non-conjugated polymer nanoparticles(PNPs)called mercaptosuccinic acid-thiosemicarbazide PNPs(MT-PNPs)with appealing fluorescence and stability are synthesized via facile one-step hydrothermal treatment for attractive sensing and simultaneous removal of mercury(II).Interestingly,aggregation-induced fluorescence switch-off and scattering enhancement are found upon the addition of Hg²⁺,rendering MT-PNPs as a ratiometric sensor for selective and accurate Hg²⁺monitoring.A wide linear range(0.1-1470.6?M)and a low detection limit(95 n M)are obtained.This dual-signal opposite responses triggered by Hg²⁺originate from the formation of MT-PNPs-Hg²⁺congeries via the multi-site binding between S,N,O-containing groups of MT-PNPs and mercury.Meanwhile,target-induced aggregation renders an effective Hg²⁺separation from contaminative aqueous media by MT-PNPs,which exhibit satisfactory absorption efficiency of 99.27%within 50 min.Upon the simple Na₂S treatment,the MT-PNPs can be regenerated and reused.This work thus delivers an applicable method for the ratiometric detection and effective removal of mercury with novel non-conjugated PNPs,offering potential in tackling the problem of heavy metal ions pollution for environmental monitoring and remediation.6.A ratiometric optical strategy for halide ions sensing based on targets-induced competitive coordination of metal-organic nano-systemA novel ratiometric assay platform coupled with FL and scattering has been constructed for the selective and accurate quantization of Br^–/I^–.The ratiometric sensor is based on a metal-organic nano-system,which is constructed via coordination binding-induced self-assembly between the non-conjugated polymer nanoparticles(NC-PNPs)and Hg²⁺,consequently displaying fluorescence and strong SOS signals.In the response system,the NC-PNPs function as a signal unit,and Hg²⁺ion serves as a recognition element.Upon the introduction of Br^–/I^–,the high affinity with Hg²⁺renders Br^–/I^–snatch Hg²⁺from the NC-PNPs-Hg²⁺nano-system,causing the nanocomplex to disassemble.The target-triggered disaggregation results in a decrease in the SOS,meanwhile the Hg-Br^–or Hg-I^–compounds produced in site reduce the FL of NC-PNPs via static quenching,suggesting the feasibility of ratiometric Br^–/I^–detection.Finally,the FL-SOS dual-signal strategy towards Br^–/I^–is successfully applied to real water/food samples.The ratiometric analytical platform coupled with FL and SOS offers a new perspective for the application based on target-induced aggregation and disaggregation behavior.