Mechanism Of Chirality In Self-assembled Nanoparticle Pyramids And The Application In Ultrasensitive Biosensors

In this paper, three-dimensional chiral nanoparticle pyramids were achieved based onDNA self-assembly technology combined with variety of nanoparticles materials; Takingadvantage of the stability and programmability of DNA and multiplexed elements of spatialpyramids, a series of new methods of simple, rapid, ultra-sensitive biosensor detection wereestablished by changing the tetrahedral conformation.Firstly, five different nanoparticles were synthesized, which were10nm,15nm and25nm gold nanoparticles,10nm silver nanoparticles and5nm fluorescent quantum dots, thesematerials showed uniform morphology, good dispersion and stability in aqueous solution.Three-dimensional pyramids were assembled using four single-stranded DNA (ssDNA)modified with sulfhydryl or amino group, and nanoparticles were attached on each top ofpyramids by coupling with the sulfhydryl or amino group of ssDNA. By changing the types orsizes of nanoparticle attached on the top of pyramids, six different types of three-dimensionalpyramids with well dispersion and stability were constructed successfully with the yieldexceeding75%. A variety of optical instruments were used to characterize the spatial structureof pyramids. Circular dichroism spectroscopy showed that, in addition to the first class ofpyramid, the others all exhibited significant circular dichroism (CD) signals in the visibleregion (300-800nm), in particular, the fifth and the sixth pyramids emerged almostcompletely symmetric chiral spectrum. By comparing the spatial structures of six differenttypes of pyramids, we found that pyramids formed with four identical particles were achiral(the first type), with the addition of different nanoparticles, the CD intensity of nanoparticlespyramids were gradually increased, the chiral signal of the fifth and sixth pyramids were thestrongest. The origin of chirality of nanoparticle pyramids can be interpreted from thefollowing aspects:1) the addition of different nanoparticles breaks the symmetric frame ofpyramids;2) the interactions between plasmical nanoparticles with different shapes and size;3) the chirality of DNA molecules was transferred to the nanoparticles in pyramids.Secondly, gold nanoparticles heterodimers were assembled with a chiral (DNA) and anachiral molecule (NaCl), respectively. Because the dimers are the simplest structure innanoparticle assemblies, they could be used as a respensentative material to study the originof chirality, it is easy to clearly observe the relationship between the conformational changeand the CD signal of assemblies. It is found that these two types of heterodimers both showedsignificant CD signals at the visible region of500nm, the value of CD signal from DNAassembled dimers was negative and the value of CD signal from NaCl assembled dimers waspositive, the intensity of CD signal from the former was stronger than the later，which wasagreed well with the software simulation dates. The results showed that chiral molecules werenot the origin of chirality of heterodimers and the structure of heterodimers may be the originof chirality. Cryo-electron microscopy combined with simulation software were used to studythe spatial structure of the two types of dimers, it was found that the angles between twoparticles in heterodimers was the origin of the CD signal. A new theory of the origin ofchirality in nanoparticle assemblies was first proposed. Thirdly, two different three-dimensional pyramids (chiral and achiral) were constructed,with the addition of target DNA, the conformations of two different pyramids were changedwhich altered the CD signals of the two pyramids, one CD signal was gradually increased andthe other one was gradually decreased. Various methods were used to characterize theconformation changes of these two pyramids. Based on the relationship between theconformation change and the CD signals of pyramids, two different methods were establishedto detection of DNA molecules. Under optimal conditions, a limit of detection (LOD) forDNA was as low as3.4aM, which was much lower than other optical sensors. Here, a rapid,ultra-sensitive chiral sensor for DNA detection was established.Lastly, uniform silver nanoparticles pyramids were prepared with high yields, and thenDNA aptamers were inserted in each side of nanoparticles pyramids, they can bind with itsaptamer and form a DNA stem-loop structure and produced a strong "hot spots" between thesilver nanoparticles, so the signal of the Raman reporter molecular existed in this region wassignificantly enhanced. Based on this principle， an ultrasensitive Raman sensor forsulfadimethoxine (SDM) and prostate-specific protein (PSA) were constructed using Ag NPspyramids, respectively, in which amino thiophenol (4-ATP) was used as Raman reportermolecule. Based on the relationship between the intensity of Raman signals and theconcentration of SDM and PSA, a standard curve for these two molecules detection has beenestablished, the LOD of SDM and PSA were82.6pM and40zM respectively, which was theultrasensitive method for SDM or PSA detection using surface-enhanced Raman scattering(SERS). Taking advantage of the multi-element of pyramids, a multiplexed Raman sensor forbio-marker proteins was constructed based on three Raman reporter molecules and threeprotein aptamers. Three targets used in this work were prostate-specific protein, thrombin, andmucin-1, the corresponding LOD were1.2aM,158aM and26aM, respectively. Comparedwith other SERS sensors, this method showed well stability and high sensitivity.