Synthesis Of Silicon Quantum Dots And Their Biological Effects On Vegetables

As a representative fluorescent nanomaterial,silicon quantum dots have several excellent and unique properties,including low toxic,light stability,good biocompatibility and surface modification,and been widely used in biological and biomedical fields.Studies have shown that silicon quantum dots can be degraded in the body into low or nontoxic products,and that can be excreted by the kidneys,making them a favorite in biology.Crops are the basis of human survival.Productivities of crops are usually improved through genetic breeding,but still face many problems.Based on the unique physical,chemical and optical properties of silicon quantum dots,in this work,we used a simple and fast method to synthesize the multicolor fluorescent silicon quantum dots,which were used as a new nano-tool to explore their absorption,distribution in plants and their application in plant growth,photosynthesis and molecular detection.In addition,during this process,we found that CDs derived from biomass salvia miltiorrhiza has a strong antioxidant capacity,which can simulate the antioxidant system in plants to scavenge reactive oxygen species.The main research contents are summarized as follows:（1）Maximizing the utilization of the ultraviolet light in the solar spectrum has always been an optimal goal in plant growth to minimize the harmful and reap the benefits.For this reason,we fabricate fluorescent,amine-functionalized,and water-soluble silicon quantum dots（Si QDs）with a mono-dispersed size of 2.4 nm.To enhance the photosynthesis in Italian lettuce,Si QDs were used as artificial antennas to amplify the light harvesting ability.Upon ultraviolet excitation,the intense blue emission from the Si QDs well matches the blue absorption of chloroplasts,allowing for the ultraviolet portion in solar radiation to be effectively utilized.The consumed optical energy enhances the photosystem II activity,which makes amplifying photosynthesis possible.More importantly,in vivo,the Si QDs significantly promoted Italian lettuce seedling growth at concentrations below 30 mg·L^-1on the root length,seedling height,and biomass and significantly enhance soluble sugar and water content by 49.8%and 40.9%at an optimized concentration.Interestingly,the content of chlorophyll a and b is constantly increasing up to 41.0 and 114.8%,respectively,with no inhibition,even at the highest dose of 200 mg·L^-1.This work provides a new perspective on the use of Si QDs to amplify photosynthesis by utilizing ultraviolent light in agriculture.（2）In this work,a biomimetic synthetic strategy for red fluorescent silicon quantum dots（Si QDs）was developed.Unicellular algae of diatoms were used as the reaction precursor.The resultant nontoxic Si QDs exhibits remarkable red luminescence,high quantum yield（～15%）and narrow full width at half maximum（FWHM,～35 nm）with the size of～2.0 nm.To investigate the effect of Si QDs on plsnts growth,they were used to cultivat cucumber seedlings.The results show that the as-prepared Si QDs have the potential to regulate plant growth.The growth of cucumber seedlings were significant promoted within a wide range of Si QDs concentrations of 10-300 mg·L^-1.Compared with the control group,the total weight of cucumber seedlings significantly increased by 51.91%（P<0.001）after incubation for 10 days.Moreover,it was found that the growth of cucumber seedlings was positively correlated with the water uptake rate of roots.The water uptake rate of roots was increased by 74.6%at the optimum concentration after 5 days incubation.According to real-time fluorescence quantitative polymerase chain reaction（RT-PCR）analysis,it was found that Si QDs significant increased the expression of aquaporin gene in cucumber roots.Therefore,it can be concluded that,Si QDs can stimulate the overexpression of aquaporin gene in cucumber seedlings and promote their growth.The results widen the range of applications of silicon quantum dots in plant science.（3）An“off-on”assay system for H₂O₂ determination was developed based on assembling ultra-bright fluorescent silicon quantum dots（Si QDs）and PEG-Mn O₂nanosheets.Among them,Si QDs acted as fluorometric reporter,which can effectively eliminate the interference of plant pigments under excitation of 365 nm.PEG-Mn O₂nanosheets played dual function of nanoquencher and H₂O₂ recognizer.Unlike previous reports,the quenching mechanism of Si QDs by PEG-Mn O₂nanosheets is attributed to both the associative effect of inner filter effect and the static quenching effect.Thus,the fluorescence intensity of Si QDs at 445 nm decreased with the increasing concentration of PEG-Mn O₂ nanosheets.After addition of H₂O₂,PEG-Mn O₂ nanosheets were reduced to Mn²⁺,consequently resulting in the recovery of the fluorescence of Si QDs.Combined with these properties,we made an off-on fluorescent method for determination of H₂O₂ in plant leaves with high sensitivity and selectivity.The present method has two linear ranges:from0.05 to 1μM with a detection limit of 0.09μM and from 1 to 80μM with a detection limit of 4.04μM.（4）Effectively scavenging the excess reactive oxygen species（ROS）from plants is very important to minimize the biotic and abiotic stress and increase the crop yield in agricultural.Although efficient natural ROS scavenging enzymes already exist,which are sensitive to environmental conditions and difficult to produce on a large scale.As a result,enormous efforts have been made to develop artificial antioxidant enzymes,but most of them have side effects that limit their using in biology.In this study,using biomass salvia miltiorrhiza as the precursor,fluorescence carbon dots（CDs）were synthesized in large scale by one-step hydrothermal method.Salvia miltiorrhiza forms core-shell carbon dots through a series of structural changes such as dehydration,polymerization and carbonation.Importantly,the surface of CDs still retains salvia miltiorrhiza-like polymer,thus endowing the carbon dots with high antioxidant capacity.The results show that the CDs have multiple enzyme activities with a strong antioxidant activity,which can significantly scavenge DPPH·,O₂^·-and·OH.The scavenging efficiency was up to 88.9%,81.8%,71.4%,respectively.Moreover,corresponding enzyme activities are superior to the natural antioxidant Vc.The synthesized CDs not only have a good scavenging effect on ROS in vitro,but also can effectively scavenging ROS in vivo to reduce the damage of plants to salt stress.These results indicate that the synthesized CDs are promising nanoantioxidant enzyme for attenuating plant biotic and abiotic stress.