Solvo-driven Constructions Of Spherical Nucleic Acid Probes And Strongly Coupled Raman Enhancement Interfaces

The past decades have witnessed an accelerated development of DNA nanotechnology toward accurate structural manipulating and functional tailoring of matters on the nanometer scale.DNA can be facilely bonded with optically,electrically,or magnetically active nanoparticles to produce DNA-conjugated building blocks that undergo highly accurate,DNA-programmed assembly into functional metamaterials via base-pair complementarity.However,two fundamental issues have to be considered,including(1)how to realize rapid,reliable,and efficient DNA grafting on nanomaterials,and(2)how to achieve strong physical/chemical coupling between DNA-interconnected inorganic nanomodules.The synthesis of DNA-nanomaterial conjugates is often carried out in a strongly ionic solution in order to reduce electrostatic repulsions between DNA and nanoparticles,which also incurs a risk of nanoparticle agglomeration.To address this problem,stepwise salt-addition with extended aging time is usually adopted.The whole process takes several to tens of hours to complete,with great caution to avoid an operation failure that may lead to irreversible nanoparticle aggregation.Therefore,it is a great challenge to achieve high DNA loading on nanoparticles in a very short period of time.Generally,electrostatic repulsions between nanoparticles and steric hindrances of surface ligands such as DNA are responsible for the difficulty of strong physical/chemical coupling within DNA-directed nanostructures,which severely limits the pursuit of assembly-enabled functions and applications.Research in this thesis is therefore directed toward addressing the above problems,with the major findings and achievements being summarized as follows.1.Flash synthesis of spherical nucleic acid probes with ultrahigh DNA density.A method is developed to produce spherical nucleic acids(SNAs)with record-high DNA loadings in seconds by taking advantage of an instantaneous dehydration in butanol(INDEBT).This process generates a highly concentrated,dehydrated,and homogeneously mixed "solid solution" of DNA/gold nanoparticles,which greatly promotes DNA conjugation on gold nanoparticles(AuNPs)via Au-S bonding.Accordingly,new records regarding DNA conjugation speed and density are simultaneously set in comparison with literature results.The INDEBT strategy is suitable for oligonucleotides with different chain lengths and base sequences,as well as AuNPs with different sizes and shapes.The significantly intensified DNA packing densities on AuNPs and gold nanorods(AuNRs)by INDEBT are unambiguously revealed by agarose gel electrophoresis and fluorometric quantifications,which are up to 2-3 times of what previously reported.Based on this process,a one-pot preparation of fluorescent nanoflares is realized.The resulting nanoflares exhibit a rapid fluorescence recovery upon addition of a DNA target.The DNA target is capable of displacing and releasing fluorophore-tagged DNA strands from the nanoflares to cut off the energy transfers from the fluorescent tags to the AuNPs,regenerating the light-emitting functions of the fluorophores on DNA.Benefiting from the greatly boosted DNA coverage on the nanoflares,a much widened dynamic range of fluorescence signals is achieved.In addition,the abundant hybridization sites on the INDEBT-derived SNAs are highly desirable for DNA-programmable nanoassemblies such as core-satellite structures.As well,the INDEBT process is found especially effective in promoting the reductive cleavage and oxidative coupling reactions of a dithiol linkage flanked by DNA sequences,which is of great value for DNA-directed chemical reactions.The resulting SNAs are useful for DNA-programmable assembly,and provide an unprecedented chance to probe their unique chemical and biological effects associated with the ultra-high DNA density.2.Solvent-driven nanodimeric Raman enhancement hotspots with strong plasmonic coupling under DNA direction.Strong and reversible coupling between AuNPs is made possible for DNA-linked AuNP dimers,which is driven by less polar water-miscible solvents such as ethanol,acetone,isopropanol,and acetonitrile.In the case of ethanol,an interparticle gap as small as 1.25 nm is achieved in a homogeneous solution.Extinction spectra indicate that the resonance wavelength of the coupled plasmon is independent of the DNA linker length and the ethanol concentration(above a certain threshold).These facts along with various control experiments point to a direct surface-to-surface coupling mechanism of AuNP dimers,which is favored by enhanced double-layer neutralization and reduced water repulsion in organic solvents.The strongly coupled dimers obtained in ethanol can be fixed and transferred to an aqueous solution via a previously developed Ag ion soldering(AIS)strategy or SiO2 encapsulation.Benefiting from a dramatically boosted electric field localized at the dimeric gap,the closely and dielectrically interfaced dimer represents an excellent substrate for surface-enhanced Raman scattering(SERS).In this regard,the coupling reversibility makes it possible to capture target molecules on the nanoparticle surface before the hotspot is formed such that large molecules can easily access the Raman hots-pot upon solvo-driven coupling.The new type of hotspots allow for a highly reproducible in-solution Raman quantification benefiting from the homogeneous sample distribution and internal signal references of the solvent molecules.This work makes a significant contribution to the development of highly reliable Raman sensing nanoprobes toward applications in chemical and biological fluidic environments.3.DNA-linked AuNP dimers as pH/ion responsive plasmon nanorulers.Gold nanoparticles bearing carboxylic acid ligands(lipoic acid,tris(2-carboxyethyl)phosphine,and thiol-PEG-acid)are prepared by ligand exchange with as-synthesized citrate-capped gold nanoparticles.The resulting nanoparticles with suitable colloidal stability are sparsely grafted with DNA strands of specific base sequences.Hybridization of DNA-functionalized AuNPs results in discrete nanoparticle assemblies linked by DNA,from which dimeric products are obtained based on agarose gel electrophoretic isolation.The resulting AuNP dimers displaying different carboxylic acid molecules can form strongly coupled configurations upon being transferred to an ethanol solution,leading to different interparticle gap sizes determined by the geometries of surface ligands.Furthermore,Coulombic repulsions between nanoparticles can be dynamically tuned by protonation and deprotonation of the carboxyl groups to form strongly coupled and decoupled plasmonic nanodimers in a reversible fashion.In addition to pH adjustment,studies show that divalent metal cations have strong affinity with the carboxyl groups,which effectively shield the negative charges on the nanoparticles or form“ionic bridges" interconnecting two carboxyl-bearing surfaces.Either of the two roles supports the formation of strongly coupled dimers in an aqueous environment.In all cases,the resonance wavelengths of coupled plasmons are rationally correlated with the sizes of carboxylic acid ligands and their counter ions with different extents of hydration.This investigation delivers a reliable way to construct plasmon-coupling nanoruler that accurately measures a gap size change at sub-nanometer resolution.Besides,it provides a novel nanoplasmonic sensing device responding to counter ions base on their sizes and carboxylic affinities.