Preparation And Properties Of Highly Sensitive Fluorescent Sensors Based On CdSe/Cd_xZn_1-xS Quantum Dots

As an important neurotransmitter,dopamine（DA）is implicated in a variety of neurological diseases,such as neuroendocrine tumors,and physiological processes.The epidemiological investigation shows that in recent 30 years,the incidence rate of neuroendocrine tumors has increased 5 times,which becoming an increasingly serious threat to human health.Therefore,rapid and sensitive detection of DA holds high importance and urgent practical significance for early disease diagnosis and human health protection.Fluorescent sensors are a kind of sensing system that can convert the concentration of target analytes into fluorescence signal that is easy to read.Compared with the traditional detection technology based on large instruments and professional technicians,it shows significant advantages in preparation process,response time,operation steps,cost of detection and so on.In the recent years,semiconductor quantum dots（QDs）have been widely used in the construction of fluorescence sensors because of their high fluorescence quantum yield and adjustable surface structure.The combination of functional QD-based fluorescent probe and solid substrate is conductive to improve portability,optical stability,simplify operation steps and realize naked eye recognition.In this paper,sensitive and stable fluorescent probes were prepared by surface modification and functionalization of Cd Se/Cd_xZn_1-xS QDs.Further,dip-coating technology and inkjet printing technology are used to deposit the fluorescent probes on the surface of solid substrates.Therefore,a series of fluorescent sensors with high sensitivity,fast response,portability and naked-eye recognition are obtained and applied for DA detection.The main research contents and results are as follows:1.QDs-MPBA was prepared by ligand exchange of QDs by thiophenylboric acid and used as the DA-sensitive fluorescent probe.Ag/PA6 nanofibrous membrane was prepared by electrospinning,and then combined the fluorescent probes on the surface of nanofibers by dip coating method to prepare the fluorescent film sensor.The particle size of QDs-MPBA was about 6 nm and there was no obvious aggregation.The size of Ag/PA6 nanofibers is about 20-55 nm,and the size of Ag nanoparticles in the fibers is about 2～4 nm.With the increase of Ag content,the size of Ag nanoparticles in the fibers gradually increases,the size of nanofibers gradually decreases,and the surface roughness gradually increases.QDs-MPBA can uniformly combine on the fibers after dip-coating process.There is a strong linear correlation between the fluorescence quenching efficiency and concentration of DA at 0-1.0μM and 2.0-20.0μM.The detection limit of DA by naked eye observation was as low as0.4μM with a rapid response time of about 10s.Besides,in the real sample detection with artificial sweat and artificial urine as the background,the recovery is in the range of 94-110%.2.To improve the sensing sensitivity,PEI functionalized with phenylboric acid（PEIBA）,prepared by nucleophilic substitution reaction,was used as functional ligands to modified QDs and prepare QDs-PEIBA fluorescent probes.Three kinds of fluorescence detection chips were prepared by inkjet printing QDs-PEIBA based inks on the surface of the microfluidic channel of the substrate.The prepared QDs-PEIBA can be evenly dispersed in water and the fluorescence intensity remained 89%compared with that before modification.The fluorescence of QDs-PEIBA can be quenched by DA.When the concentration of DA is at 0-5μM,ratio of the fluorescence intensity,I/I₀,has a linear relationship with DA concentration.The results of fluorescence microscope show that the fluorescent ink prepared with QDs-PEIBA can clearly print the fluorescent pattern with a diameter of only 8 mm,demonstrating excellent printability.Based on this property,QDs-MPBA can be uniformly combined on the channel by inkjet printing to prepare fluorescent sensing chips.In single channel fluorescence detection chips,DA aqueous solution can significantly reduce the fluorescence in the channel.The fluorescence changes can be observed with naked eyes with the detection limit of s 0.05μM.In multi-channel fluorescence sensing chips,it shows different fluorescence quenching distances in the eight channels when dropped DA with different concentration.The fluorescence quenching region of the channel extends outward from the sample inlet and grows with the increase of DA concentration.The detection concentration range is 0.2～5.0μM.In the fluorescence sensing chips based on QR code,when the DA concentration is greater than or equal to 0.5μM,the smart phone cannot recognize QR code because the fluorescence of the sensing part is weakened.Therefore,the concentration of DA can be obtain by a smart phone.3.Fluorescent Au NCs were prepared by a typical template method and combined with QDs-PEIBA to construct dual fluorescence system to realize sensitive naked-eye detection.Then,the fluorescent sensing chips with dual fluorescent signals were prepared by inkjet printing of the QDs/Au NCs based fluorescent inks.The prepared Au NCs were spherical,with a size of 2～3 nm.The fluorescence intensity at547 nm of the QDs/Au NCs gradually decreases with the increase of DA concentration from 0 to 16μM,while the fluorescence intensity at 650 nm gradually increases.The fluorescence color of the system changes from yellow green to orange gradually,showing high sensitivity in naked-eye detection,as shown in CIE1931 color coordinate diagrams.The QDs/Au NCs could be evenly combined on the channel.With the diffusion of DA aqueous solution in the channel,the fluorescence in the channel have three kinds of changes,including the fluorescence changes from green to orange red,the fluorescence of orange red is gradually quenched,and the fluorescence quenching distance increases,when DA concentration ranges from 0.1μM to 2.0μM.Therefore,the concentration of DA can be obtained by observing the changes of fluorescence color and quenching distance.4.The QDs-PEIBA@POSS was prepared by combining octa-（tetramethylammonium）polysilsesquioxane（TMA-POSS）with octahedral structure on the surface of QDs-PEIBA through electrostatic interaction.Flexible hydrogel substrates were prepared by acrylic acid,acrylamide and Au NRs by ultraviolet irradiation crosslinking method.A flexible sensor was prepared by inkjet printing the QDs-PEIBA@POSS on the surface of hydrogel.The QDs-PEIBA@POSS can be dispersed in water without significant aggregation and has a improved fluorescence intensity,which is 70%higher than that of QDs-PEIBA.It can be seen that the fluorescence can be quenched by DA.When the concentration of DA is at 0～30μM,the ratio of the fluorescence intensity,I/I₀,has a linear relationship with DA concentration.The hydrogel possesses three dimensional porous structure with the micron-level the pore diameter,which is conductive to absorb and transport the detection samples.A fluorescent pattern is printed on the surface of hydrogel by inkjet printing QDs-PEIBA@POSS based fluorescent inks.Further,a wearable fluorescent chip contains the hydrophilic region prepared by hydrogel,the hydrophobic region prepared by the silicone rubber and the sensing region prepared by inkjet printing QDs-PEIBA@POSS on the hydrogel.To simulate the detection of DA in sweat on skin,the surface without QDs-PEIBA@POSS was used as the surface in contact with skin and contacted with the sample.The results show that the fluorescence on the upper surface can be quenched after 5 minutes,and the detection concentration range is 0～0.2 m M.It demonstrates a great potential in wearable flexible fluorescent sensor for monitoring of DA in sweat.