A ZnO Quantum Dots Based Ratiometric Fluorescent Sensor For Anthrax Biomarker Detection

ZnO quantum dots(QDs)are excellent fluorenscent sensors with many superior properties such as high brightness,excellent photostability,outstanding colloidal stability and good biocompatibility.These unique properties give researchers inspiration to use ZnO QDs in drug delivery,biological labeling and biosensing.However,none report on the fluorescent sensor for B.anthracis using ZnO QDs as a internal reference can be found.Optical sensing of bacterial spore are based on detection of calcium dipicolinate(Ca DPA),a unique biomarker and major component for bacillus spores.It is noteworthy that lanthanide ions(Ln3+)exhibit highly enhanced luminescence upon bonding with Ca DPA due to the antenna effect.The luminescence of Ln3+ has the unique features including large Stokes shift,narrow emission bands,and long fluorescence lifetime.To date,the europium(Eu)and terbium(Tb)are the commonly used lanthanide ions in fluorescent sensor because of their bright fluorescence,sharp line-like emission bands,and long fluorescence lifetime.In our work,the selection of europium ion(Eu3+)comes from the facts as follows: The strongest emission of Eu3+ locates at around 616 nm which will be not interfered by the ZnO QDs emission at 530 nm.By contrast,the peak at 544 nm domains the emission spectrum of Tb3+,which overlaps the emission band the ZnO QDs.Moreover,the red emission of Eu3+ is very desirable for the naked eye observation.The europium-based sensor can decrease the false-positive results arising from the nonselective binding of aromatic compounds to Tb3+.Therefore,Eu ion-functionalized fluorescent sensor has been considered a promising method for the rapid,sensitive and selective detection of CaDPA.The results of this paper are as follows:1)The zinc oxide(ZnO)quantum dots(QDs)with hexagonal wurtzite structure have been successfully prepared.ZnO QDs with the average particle size of ~5 nm have good water solubility,strong yellow emission,high quantum yield(31.98%)and good luminous stability.At the same time,ZnO QDs have less expensive,ecofriendly,favorably biocompatible and low cytotoxicity.Because of the surface of ZnO QDs with-NH2 group,it is easy to surface modification to design and development of the excellent performance of nano-fluorescent probe.It will play a vital role in the biomedical imaging,disease diagnosis and treatment.2)A hybrid nanostructure based on ZnO quantum dots(QDs)has been fabricated for ratiometric detection of Bacillus anthracis spores,where yellow-emitting ZnO QDs are employed as the internal reference and europium ions(Eu3+)are chelated on the surface of the ZnO QDs as the signal report unit.The Eu3+ exhibits enhanced red luminescence upon bonding with calcium dipicolinate(Ca DPA),an important biomarker of Bacillus anthracis spores,while the fluorescence of the ZnO QDs will not be altered.Accordingly,the increased Ca DPA levels can lead to variations of the two fluorescence intensity ratios of the ZnO / Eu hybrid nanostructure.3)The time-dependent fluorescence response reveals that the reaction can be completed within 8 s,thus enabling a rapid detection of Bacillus anthracis spores.The fluorescence intensity of the ZnO / Eu nanostructure increases linearly with the concentration of the Ca DPA in the investigated range from 0 to 4 M.The limit of detection(LOD)for Ca DPA is calculated to be ~3 n M according to the method suggested by IUPAC,which is six orders of magnitude lower than an infectious dosage of Bacillus anthracis spores for human being(60 ?M).The built-in correction of the ratiometric design effectively eliminates the potential operation and environmental effects,thus significantly improving the sensitivity and accuracy of sensors.Interestingly,the color change can be identified by unaided eyes under UV illumination,even at a Ca DPA concentration as low as 0.5 ?M.Additionally,the sensor shows a remarkable selectivity for Ca DPA over other aromatic ligands,amino acids and common cellular ions.The fast response speed as well as good sensitivity and selectivity makes the ZnO/Eu nanostructure great potential application in clinical analysis.