Silicon Quantum Dots-based Fluorescence Probes For Detecting Butyrylcholinesterase Activity In Human Serum

Human blood plays a key role in transport of essential nutrients and energy for normal activities of the human body.Variations of these physiological substances in blood can point out the health states or diseases of the human body.Therefore,the rapid and accurate detection of physiological substances in human blood is necessary for clinical diagnosis.Fluorescence probe has been widely used in bioassays,and their accuracy and sensitivity are mainly determined by the optical properties as well as the response mechanisms of fluorescence materials.In this work,silicon quantum dots（Si QDs）were prepared and combined with manganese dioxide（Mn O₂）nanosheets and gold nanoclusters（Au NCs）to construct two fluorescence probes for detecting the activity of butyrylcholinesterase（BCh E）in human serum.1.Blue-emission Si QDs were prepared via a one-pot hydrothermal method using glutathione（GSH）and N-[3-（trimethoxysiyl）propyl]-ethylenediamine（DAMO）as the reagents.The product of the hydrolysis reaction of S-Butyrylthiocholine iodide（BTCh）catalyzed by BCh E,thiocholine（TCh）would reduce Mn O₂ nanosheets to Mn²⁺cations,which consequently inhibits the inner filter effect（IFE）between Mn O₂ nanosheets and Si QDs,and results in the enhancement of fluorescence emission of Si QDs.By measuring the variations of the fluorescence intensity of Si QDs,a fluorometric assay was developed for detecting the activity of BCh E in human serum.The feasibility of this assay was further confirmed by using fluorescence and UV-Vis absorption spectroscopies.The linear range is 0.2 U/L-40 U/L.Limit of detection（LOD,3σ/S）is0.0425 U/L.The testing results obtained by this assay were consistent with those obtained by standard Ellman method.The inorganic ions,enzymes and biological molecules in human serum exhibited little interference for this probe.The probe demonstrates broad potential for future applications in clinical diagnosis.2.A ratiometric fluorometry based on Si QDs and gold nanoclusters（Au NCs）was established for detecting the activity of BCh E in human serum.By using BTCh as the substrate of BCh E-catalyzed hydrolysis reaction,variation of fluorescence emitted from Au NCs was employed as an indicator of BCh E activity since one of the hydrolysis products,thiocholine（TCh）,would influence the aggregation state of Au NCs and consequently led to the change of fluorescence quantum efficiency of Au NCs.It is interesting that there are two mechanisms working for the fluorescence emission of aggregated Au NCs:the aggregation-induced emission enhancement（AIEE）and the aggregation-caused quenching（ACQ）with the presence of TCh at very low and higher concentration levels,respectively.Although both of these mechanisms can be utilized for sensing BCh E,their opposite influence on the fluorescence emission of aggregated Au NCs should be worthy of attention,especially in the process of developing fluorescence methods for detecting trace targets by using Au NCs.In order to eliminate the fluctuation of fluorophotometer,Si QDs was chosen as the reference fluorophore to develop by ratiometric fluorescence methods in this work.Additionally,obvious aggregation of Au NCs induces significant decrease of IFE on the fluorescence emitted from Si QDs,while mild aggregation of Au NCs demonstrates little IFE.The linear ranges for detecting activity of BCh E were 0.004 U/L-0.05 U/L and 0.5 U/L-20 U/L by ratiometric fluorometry based on the AIEE and ACQ,respectively.The limit of detection（LOD,3σ/S）is 0.019 U/L.The very different responses originated from AIEE and ACQ of Au NCs would respectively make their own contributions to the determination of BCh E activities at very low or high levels,which facilitate the developments of enhanced or quenched fluorescence methods.However,the detection of BCh E activities at medium levels might suffer from the combination of AIEE and ACQ with ambiguous fractions.Therefore,it must be careful during the processes of developing and applying fluorescence methods based on the AIEE and ACQ of Au NCs.The above finding in this work can be utilized to help the understanding and the interpreting of some ambiguous or even conflicting response signals obtained during specific applications of fluorescent nanoparticles and the performance improvement of these applications.