Construction And Sensing Application Of Ultra-sensitive Fluorescent Nanoprobes

Fluorescence methods have become popular and attracted significant research interest in various fields of applications.This is due to their lower cost of analysis,fairly easy operation,rapid response,and higher sensitivity.Fluorescence sensors became a method of choice in environmental,clinical,chemical,and biochemical analyses.To date,designing a new fluorescence sensor with excellent sensitivity and selectivity is a research hotspot.Our thesis is mainly concerned on designing and constructing ultra-sensitive fluorescence sensors to detect various target analytes of interest.Below,we have summarized the main activities and findings of our investigations.1.Thrombin is a useful biological marker and the change in thrombin concentration in the blood could be used as an indication of various health problems.Therefore,thrombin detection with high sensitivity and selectivity is highly desired.Having this in mind,we have developed a sensitive and selective fluorescence method for thrombin detection.The as-proposed sensor was constructed based on enzyme-free signal-amplified target-triggered catalytic hairpin assembly.The detection system consisted of two thrombin aptamers(aptamer-1 and aptamer-2).The aptamer-1 was functionalized with fluorescein(FAM)at one end and chemically bound to AuNPs at the other via Au-S chemistry.In the absence of thrombin,aptamer-1 forms a duplex via complementary units and brings fluorescein to proximity to AuNPs resulting in quenching of fluorescence emission.However,in the presence of thrombin and aptamer-2,a catalytic hairpin assembly can be initiated,and also aptamer-1 forms a duplex with aptamer-2 and eventually placing FAM far from the quencher and hence resulting in the recovery of the fluorescence.Moreover,the as-displaced target thrombin could initiate another catalytic hairpin reaction and eventually amplify the fluorescence signal.The amount of fluorescence recovery was concentration-dependent and showed linearity in the concentration range from 0.2 to 100 nM with a limit of detection(LOD)of 165 pM.In addition,the potential applicability of the as-constructed fluorescence sensor for thrombin detection in the complex matrix was proved.2.To date,graphene quantum dots(GQDs)have attracted significant research interest in fluorescence sensing applications due to their unique optical properties.In this regard,we have also synthesized GQDs with excellent optical properties by employing a simple and one-pot solid-phase pyrolysis method.In the synthesis procedure,citric acid was used as a carbon source and thiourea as a nitrogen and sulfur source.The as-synthesized nitrogen and sulfur-doped graphene quantum dots(N/S-GQDs)showed excellent optical properties such as bright blue fluorescence,high relative quantum yield,and excellent photo-stability.In addition,the N/S-GQDs showed excellent water dispersibility,good biocompatibility,and excellent stability under extreme conditions(i.e.,ionic strength,pH,and temperature).Moreover,based on inner filter effect(IFE),the potential applicability of the as-synthesized quantum dots as a fluorescent probe for the detection of Cr(VI)ions was investigated.In this regard,a paper-and solution-based detection procedure was developed.The as-constructed paper-as well as solution-based fluorescence sensors,showed excellent sensitivity and selectivity for the detection of Cr(VI)ions with a LOD of 0.4 and 0.01 μM,respectively.3.Fluorescence sensors can be constructed in signal on-off or off-on sensing modes.Compared to on-off sensing modes,signal off-on sensing strategies have attracted significant research attention due to their excellent sensitivity.Although a lot of research work has been reported on signal off-on fluorescence sensing mode,most of these studies employed nanomaterial-based quencher which requires complex and tedious synthesis procedures.Herein,we have proposed a simple,yet effective N/S-GQDs-KMnO4system as a fluorescent signal off-on probe for sensitive and selective detection of ascorbic acid.The as-proposed sensor was based on the effective quenching effect of KMnO4 on the fluorescence intensity of N/S-GQDs.In the presence of KMnO4,the fluorescence emission of N/S-GQDs could be effectively quenched(signal-off)due to static and inner filter effects.However,the addition of target ascorbic acid could reduce permanganate into a lower valent oxidation state,and hence recovers the fluorescence emission(signal-on)of N/S-GQDs.Hence,the developed N/S-GQDs-KMnO4 system can be used as an effective fluorescence donor and quencher pair to construct signal off-on sensors for ascorbic acid detection.The as-constructed fluorescence sensor showed a concentration-dependent fluorescence restoration with a linear range from 0.01 to 25 μM and a LOD of 8.3 nM.In addition,the as-constructed sensor showed promising application for ascorbic acid detection in real samples.Moreover,compared to nanomaterial-based quenchers,KMnO4 has many advantages as it shows higher quenching efficiency,eliminates tedious and complex quencher synthesis procedures,and is a common laboratory reagent.