Circular Dichroism Studies on Plasmonic Nanostructures

In recent years, optical chirality of plasmonic nanostructures has aroused great interest because of innovative fundamental understanding as well as promising potential applications in optics, catalysis and sensing. Herein, state‐of‐the‐art studies on circular dichroism (CD) characteristics of plasmonic nanostructures are summarized. The hybrid of achiral plasmonic nanoparticles (NPs) and chiral molecules is explored to generate a new CD response at the plasmon resonance as well as the enhanced CD intensity of chiral molecules in the UV region, owing to the Coulomb static and dynamic dipole interactions between plasmonic NPs and chiral molecules. As for chiral assembly of plasmonic NPs, plasmon–plasmon interactions between the building blocks are found to induce generation of intense CD response at the plasmon resonance. Three‐dimensional periodical arrangement of plasmonic NPs into macroscale chiral metamaterials is further introduced from the perspective of negative refraction and photonic bandgap. A strong CD signal is also discerned in achiral planar plasmonic nanostructures under illumination of circular polarized plane wave at oblique incidence or input vortex beam at normal incidence. Finally perspectives, especially on future investigation of time‐resolved CD responses, are presented.     

Chiral nanocrystals: plasmonic spectra and circular dichroism

The life is inherently chiral. Consequently, chirality plays a pivotal role in biochemistry and the evolution of life itself. Optical manifestation of chirality of biomolecules, so-called circular dichroism, is a remarkable but relatively weak effect appearing typically in the UV. In contrast to the biomolecules, plasmonic nanocrystals offer an interesting opportunity to create strong circular dichroism (CD) in the visible spectral range. Here we describe plasmonic properties of single chiral nanocrystals and focus on a new mechanism of optical chirality originating from a chiral shape of a nanocrystal. After careful examination, we found that this CD mechanism is induced by the mixing between different plasmon harmonics and is qualitatively different from the previously described dipolar CD effect in chiral assemblies of spherical nanoparticles. Chiral plasmonic nanocrystals studied here offer a new approach for the creation of nanomaterials with strong chiroptical responses in the visible spectral interval.     

Circular dichroism thermal lens microscope for sensitive chiral analysis on microchip

A novel chiral detector, a circular-dichroism thermal lens microscope (CD-TLM), was developed to realize sensitive and selective detection of small volume chiral samples on a microchip. To realize chiral recognition on TLM, an excitation beam was phase-modulated at a frequency of 1.2 kHz, and left-circularly polarized light (LCPL) and right-circularly polarized light (RCPL) were generated. Then, the differential light absorption between LCPL and RCPL, which is the CD effect, was detected as thermal lens signal intensity and phase. As a standard sample, optically active tris(ethylenediamine)cobalt(III) [Co-(en)3]3+I3- aqueous solutions were used for performance evaluations. First, we verified the basic principle for selective chiral analysis by comparing the signals in intensity-modulation and phase-modulation modes of the excitation beam. Also, we found that the g-factor, which is significant for determining enantiomeric excess, agreed well with the value obtained by the CD spectrometer. The limit of detection (LOD) for enantiopure [Co-(en)3]3+I3- was 6.3 x 10(-5) M (1.9 x 10(-7) abs) for (-)-Co(en)3(3+), and the sensitivity in absorbance units was more than 250 times higher than that in a CD spectrophotometer. Finally, we demonstrated enantiomeric excess determination on a microchip. The LOD was 1.7% (8.5 x 10(-7) abs) for (-)-Co(en)3(3+) and at least one order superior to the LOD of a CD spectrometer. The applicability of CD-TLM for sensitive chiral analysis on a microchip was verified, and CD-TLM is expected to be promising for microchip-based chiral synthesis and analysis systems.     

Bioassisted capture and release of nanoparticles on nanolithographed ZnO films

Using an artificial peptide library, we have identified a peptide that has strict selective affinity for ZnO surfaces. The binding affinity of the peptide on the ZnO surface can be controlled simply through changes in phosphate concentration at constant pH and temperature. In this study, we functionalized inorganic nanoparticles by orderly conjugating ZnO-binding peptides (ZnOBPs) on the surface of cadmium selenide (CdSe) nanoparticles and performed spontaneous and reversible nanopatterning of ZnOBP-displayed nanoparticles on lithographed ZnO films. Conjugation of ZnOBPs on CdSe nanoparticles caused spontaneous adsorption of the nanoparticles on a ZnO film, and fluorescence and cathodoluminescence images clearly showed specific adsorption of nanoparticles on the ZnO films lithographed on nano-?and micrometer scales. The selectively bound nanoparticles on ZnO films were completely released by changing the phosphate concentration in solution; such release did not require heat or mechanical applications. Repeated capture and release of nanoparticles were achieved on the micrometer scale. Our results show the potential of material-binding peptides for nanopatterning and dynamic microarrays.     

Liquid crystal sol containing oleophilic Pd nanoparticles for liquid crystal device

We have succeeded in preparation of liquid crystal sol containing oleophilic Pd nanoparticles (NPs) stabilized by multidentate copolymer and fabrication of the twisted nematic liquid crystal devices (TN-LCDs) by using Pd NP-containing liquid crystal sol. Oleophilic Pd NPs were prepared by refluxing Pd acetate solution in toluene/ethanol containing poly(N-vinyl-2-pyroridone-co-styrene). Oleophilic Pd NPs showed better solubility in liquid crystal medium than poly(N-vinyl-2-pyrrolidone)-stabilized NPs. The TN-LCDs were fabricated by using two kinds of practical liquid crystal materials doped with oleophilic Pd NPs. The NP-doped LCD showed 22% faster response than non-doped one at -20 degrees C without a chiral dopant. However, LCDs fabricated by liquid crystal materials with a chiral dopant were not affected by NPs. These results suggest that the effect of NPs on the electro-optic performance of LCD is incompatible with that of a chiral dopant.     

Chiral Au₂₅ nanospheres and nanorods: synthesis and insight into the origin of chirality

Chirality in nanoparticles is an intriguing phenomenon. Herein, we have devised a well-defined gold nanoparticle system for investigating the origin of chirality in nanoparticles. We have designed chiral thiols (R- and S-isomers) and synthesized chiral gold nanoparticles composed of 25 gold atoms and 18 ligands, referred to as Au(25)(pet)(18), where pet represents chirally modified phenylethylthiolate -SCH(2)CH(CH(3))Ph at the 2-position. These optically active nanoparticles are close analogues of the optically nonactive phenylethylthioalte-capped Au(25)(pet)(18) nanoparticles, and the latter's crystal structure is known. On the basis of the atomic and electronic structures of these well-defined Au(25) nanoparticles, we have explicitly revealed that the ligands and surface gold atoms of Au(25)(pet)(18) play a critical role in effecting the circular dichroism responses from the nanoparticles. Similar effects are also observed in chiral Au(25) rods. The mixing of electronic states of ligands with those of surface gold atoms constitutes the fundamental origin of chirality in such nanoparticles.     

Mixed monolayer-protected gold nanoclusters as selective peptide extraction agents for MALDI-MS analysis

Cationic and anionic nanoparticles selectively target peptides with low and high isoelectric points, respectively. Additionally, their high surface area-to-volume ratios make these nanoparticles (approximately 2-nm core diameter) very efficient extraction and concentration agents. Upon extraction, the peptide-bound nanoparticles can be analyzed by MALDI-MS to provide highly sensitive detection of the targeted peptides. We demonstrate that MALDI-MS can detect peptide concentrations as low as 500 pM from 250-microL solutions using these nanoparticle scaffolds as extraction and concentration agents.     

Stereoselective determination of amino acids in beta-amyloid peptides and senile plaques

A novel method for the determination of the enantiomeric composition of peptides is presented. In this paper, the focus has been on beta-amyloid peptides from deceased Alzheimer's disease patients. The peptides are hydrolyzed using mineral acid. The free amino acids are derivatized with the chiral reagent (+)- or (-)-1-(9-anthryl)-2-propyl chloroformate and subsequently separated using micellar electrokinetic chromatography (MEKC) and detected using laser-induced fluorescence (LIF) detection. The high separation efficiency of the MEKC-LIF system, yielding approximately 1 million theoretical plates/m for most amino acids, facilitates the simultaneous chiral determination of nine amino acids. The samples that have been analyzed were standard 1-40 beta-amyloid peptides, in vitro precipitated beta-amyloid fibrils, and human senile plaque samples.     

Encapsulation of Hydrophilic and Hydrophobic Peptides into Hollow Mesoporous Silica Nanoparticles for Enhancement of Antitumor Immune Response

Codelivery of combinational antigenic peptides and adjuvant to antigen presenting cells is expected to amplify tumor specific T lymphocytes immune responses while minimizing the possibility of tumor escaping and reducing immune tolerance to single antigenic peptide. However, the varied hydrophobicities of these multivariant derived short antigenic peptides limit their codelivery efficiency in conventional delivery systems. Here, a facile yet effective route is presented to generate monodisperse and stable hollow mesoporous silica nanoparticles (HMSNs) for codelivering of HGP100_25–33 and TRP2_180–188, two melanoma‐derived peptides with varied hydrophobicities. The HMSNs with large pore size can improve the encapsulation efficiency of both HGP100 and TRP2 after ? NH₂ modification on the inner hollow core and ? COOH modification in the porous channels. HGP100 and TRP2 loaded HMSNs (HT@HMSNs) are further enveloped within monophosphoryl lipid A adjuvant entrapped lipid bilayer (HTM@HMLBs), for improved stability/biocompatibility and codelivery efficiency of multiple peptides, adjuvant, and enhanced antitumor immune responses. HTM@HMLBs increase uptake by dendritic cells (DCs) and stimulate DCs maturation efficiently, which further induce the activation of both tumor specific CD8⁺ and CD4⁺ T lymphocytes. Moreover, HTM@HMLBs can significantly inhibit tumor growth and lung metastasis in murine melanoma models with good safety profiles. HMSNs enveloped with lipid bilayers (HMLBs) are believed to be a promising platform for codelivery of multiple peptides, adjuvant, and enhancement of antitumor efficacy of conventional vaccinations.     

Chiral self-assembly/disassembly transition of supramolecular aggregates characterized by variable temperature circular dichroism spectroscopy

Self-assembly and disassembly behavior of a series of chiral sorbitol-based derivatives in n-octanol were investigated by using the variable temperature circular dichroism (CD) technique. An attempt has been made to establish a novel strategy for the characterization of the self-assembly/disassembly transition by measuring the difference of CD signals of chiral aggregates.     

Artificial Chiral Probes and Bioapplications

The development of artificial chiral architectures, especially chiral inorganic nanostructures, has greatly promoted research into chirality in nanoscience. The nanoscale chirality of artificial chiral nanostructures offers many new application opportunities, including chiral catalysis, asymmetric synthesis, chiral biosensing, and others that may not be allowed by natural chiral molecules. Herein, the progress achieved during the past decade in chirality-associated biological applications (biosensing, biolabeling, and bioimaging) combined with individual chiral nanostructures (such as chiral semiconductor nanoparticles and chiral metal nanoparticles) or chiral assemblies is discussed.     

Selective extraction and enrichment of multiphosphorylated peptides using polyarginine-coated diamond nanoparticles

Despite recent advances in phosphopeptide research, detection and characterization of multiply phosphorylated peptides have been a challenge. This work presents a new strategy that not only can effectively extract phosphorylated peptides from complex samples but also can selectively enrich multiphosphorylated peptides for direct matrix-assisted laser desorption/ionization time-of-flight mass spectrometric analysis. Polyarginine-coated diamond nanoparticles are the solid-phase extraction supports used for this purpose. The supports show an exceptionally high affinity for multiphosphorylated peptides due to multiple arginine-phosphate interactions. The efficacy of this method was demonstrated by analyzing a small volume (50 microL) of tryptic digests of proteins such as beta-casein, alpha-casein, and nonfat milk at a concentration as low as 1 x 10 (-9) M. The concentration is markedly lower than that can be achieved by using other currently available technologies. We quantified the enhanced selectivity and detection sensitivity of the method using mixtures composed of mono- and tetraphosphorylated peptide standards. This new affinity-based protocol is expected to find useful applications in characterizing multiple phosphorylation sites on proteins of interest in complex and dilute analytes.     

Nonexclusive Fluorescent Sensing for l/d Enantiomers Enabled by Dynamic Nanoparticle-Nanorod Assemblies

Fluorescence sensing of enantiomers is a much needed yet very challenging task due to nearly identical chemical and physical properties of the chiral isomers also known as chiral equivalence. In this study, we propose a novel strategy for fluorescence sensing of enantiomers using chiral nanoparticles and their ability to form dynamic assemblies. Fluorescence resonance energy transfer (FRET) in nanoscale assemblies consisting of either l-cysteine- or d-cysteine-modified quantum dots (QDs) and gold nanorods (GNRs) was found to be strongly dependent on traces of cysteine. This occurs due to high sensitivity of dynamic assemblies to the weak internanoparticle interactions that can exponentially increase energy transfer efficiencies from QDs to GNRs. Comprehensive analysis of the fluorescence responses in the two types of chiral nanoscale assemblies enables accurate determination of both concentration and enantiomeric composition of the analyte, i.e., cysteine. The described method can quantify the composition of a chiral sample, even the content of one enantiomer is as low as 10% in the mixture. Exceptional selectivity in respect to d/l-cysteine in comparison to analogous small molecules was observed. Versatility of nanoparticle-nanorod assemblies and tunability of intermolecular interactions in them open the road to adaptation of this sensing platform to other chiral analytes.     

Prediction for the separation efficiency of a pair of enantiomers during chiral high-performance liquid chromatography using a quartz crystal microbalance

A simple and rapid screening method of the chiral stationary phase during high-performance liquid chromatography (HPLC) utilizing a quartz crystal microbalance (QCM) has been developed for the chiral separation of a pair of enantiomers. The outline of the method is as follows: a self-assembled monolayer (SAM) is constructed on the gold electrodes of the QCM sensor chips by utilizing the interaction between thiols and gold. The chiral selectors used as chiral stationary phases in the HPLC are then immobilized, and a pseudostationary phase is constructed on the electrodes. Subsequently, the sensors are equilibrated in the solutions, the targeted chiral samples are injected, and the frequency changes are observed. Four kinds of chiral molecules and three kinds of chiral stationary phases were examined in this study. When chiral separation is possible using the chiral stationary phase immobilized on the sensors, significant differences in the frequency changes are observed because the intensities based on interactions differ among the isomers. The developed method can predict not only the possibility for chiral separation but also the elution order from the chiral stationary phase column. Furthermore, the degree of the mutual separation of a pair of enantiomers seems to be roughly predictable from the difference in the frequency change (DeltaF) and first-order association rate constant (k(obs)). The method does not require several different kinds of chiral columns that are more expensive than achiral ones such as the octadecylsilica (ODS) column. The required amounts of the chiral stationary phases are extremely small, and the sensors with immobilized chiral selectors are reusable. In addition, the method requires only a few minutes to complete the analysis. Thus, considerable reductions in both cost and time are realized. By applying the developed method to many chiral molecules and chiral stationary phases, its superiority may be corroborated; thus, it is expected that the method can be effectively used for the selection of chiral stationary phases.     

Targeted biodegradable nanoparticles for drug delivery to smooth muscle cells

Targeted delivery of therapeutic agents to prevent smooth muscle cell (SMC) proliferation is important in averting restenosis (a narrowing of blood vessels). Since platelet derived growth factor (PDGF) receptors are over-expressed in proliferating SMCs after injury from cardiovascular interventions, such as angioplasty and stent implantation, our hypothesis is that conjugation of PDGF-BB (platelet-derived growth factor BB (homodimer)) peptides to biodegradable poly (D,L-lactic-co-glycolide) (PLGA) nanoparticles (NPs) would exhibit an increased uptake of these NPs by proliferating SMCs. In this study, poly (D,L-lactide-co-glycolide) (PLGA) nanoparticles containing dexamethasone were formulated and conjugated with PDGF-BB peptides. These NPs were stable, biocompatible, and exhibited a sustained drug release over 14 days. Various particle uptake studies using HASMCs (human aortic smooth muscle cells) demonstrated that PDGF-BB peptide-conjugated nanoparticles significantly increased cellular uptake and decreased proliferation of HASMCs compared to control nanoparticles (without conjugation of PDGF-BB peptides). These NPs were internalized primarily by clathrin-mediated endocytosis and macropinocytosis. Our in vitro results suggest that PDGF-BB peptide-conjugated NPs could represent as an effective targeted, sustained therapeutic delivery system to reduce restenosis and neointimal hyperplasia.     

手性聚苯胺的制备及其电磁学性能研究

采用二次掺杂法制备了具有手征特性的聚苯胺,利用傅立叶红外光谱(FT-IR)、X射线衍射仪(XRD)、圆二色谱仪(CD)和紫外一可见光分光光度计(UV-vis)等分析手段对聚苯胺的结构、性能和手征特性进行了表征.结果表明,通过过硫酸铵作引发剂、盐酸掺杂获得了导电的盐酸掺杂态聚苯胺(PANI-HCI);通过氨水脱掺杂后得到本征态聚苯胺(EB),EB通过手性樟脑磺酸(CSA)诱导,形成了手征性螺旋构型聚苯胺.电磁性能测试表明,与非手性聚苯胺相比,手性聚苯胺具有较优越的吸波性能.     

Assembled Plasmonic Asymmetric Heterodimers with Tailorable Chiroptical Response

Directed nanocrystal (NC) heteroassemblies could potentially achieve tailorable multiplex circular dichroism (CD) bands. Here, for the first time, we developed assembly of nanoparticle (NP)‐nanorod (NR) chiral heterodimers with chiral molecules to explore their chiroptical activities. The experimental results revealed that plasmonic CD responses were in the region from 520 to 750 nm, which was in agreement with the theoretical simulation. Importantly, the CD band could be regulated by controlling the gaps between adjacent NCs and altering the building blocks of the assemblies. These results show that the plasmonic chiroptical response of NP‐NR heterodimers could come from the finger‐crossed chiral construction of adjacent NC in the heterodimers and the formation of plasmonic hot‐spots in the assemblies could further enhance the plasmonic CD. This work provides a new opportunity to create heterogeneous nanoscale plasmonic objects with tailorable chiroptical response for application in biosensors, in vivo chiral medical carriers and negative refractive index materials.     

Dynamic Spirals of Nanoparticles in Light‐Responsive Polygonal Fields

Nanoparticles tend to aggregate once integrated into soft matter and consequently, self‐assembling nanoparticles into large‐scale, regular, well‐defined, and ultimately chiral patterns remains an ongoing challenge toward the design and realization of organized superstructures of nanoparticles. The patterns of nanoparticles that are reported in liquid crystals so far are all static, and this lack of responsiveness extends to assemblies of nanoparticles formed in topological singularities and other localized structures of anisotropic matter. Here, it is shown that gold nanoparticles form spiral superstructures in polygonal fields of cholesteric liquid crystals. Moreover, when the cholesteric liquid crystals incorporate molecular photoswitches in their composition, the pitch of the nanoparticulate spirals follows the light‐induced reorganization of the cholesteric liquid crystals. These experimental findings indicate that chiral liquid crystals can be used as chiral and dynamic templates for soft photonic nanomaterials. Controlling the geometry of these spirals of nanoparticles will ultimately allow modulating the plasmonic signature of hybrid and chiral systems.     

Zeolite Nanosheets Stabilize Catalyst Particles to Promote the Growth of Thermodynamically Unfavorable,Small‐Diameter Carbon Nanotubes

A challenge in the synthesis of single‐wall carbon nanotubes (SWCNTs) is the lack of control over the formation and evolution of catalyst nanoparticles and the lack of control over their size or chirality. Here, zeolite MFI nanosheets (MFI‐Ns) are used to keep cobalt (Co) nanoparticles stable during prolonged annealing conditions. Environmental transmission electron microscopy (ETEM) shows that the MFI‐Ns can influence the size and shape of nanoparticles via particle/support registry, which leads to the preferential docking of nanoparticles to four or fewer pores and to the regulation of the SWCNT synthesis products. The resulting SWCNT population exhibits a narrow diameter distribution and SWCNTs of nearly all chiral angles, including sub‐nm zigzag (ZZ) and near‐ZZ tubes. Theoretical simulations reveal that the growth of these unfavorable tubes from unsupported catalysts leads to the rapid encapsulation of catalyst nanoparticles bearing them; their presence in the growth products suggests that the MFI‐Ns prevent nanoparticle encapsulation and prologue ZZ and near‐ZZ SWCNT growth. These results thus present a path forward for controlling nanoparticle formation and evolution, for achieving size‐ and shape‐selectivity at high temperature, and for controlling SWCNT synthesis.     

Chiroptically Active Plasmonic Nanoparticles Having Hidden Helicity and Reversible Aqueous Solvent Effect on Chiroptical Activity

The geometrical prerequisite for forming a helix is P (helical pitch) > d (wire diameter). Limited by the current development of nanofabrication techniques, it is difficult to minimize d and consequently P to the sub‐10 nm molecule‐comparable scale, preventing the study of chiral plasmonics at dimensions approaching the physical limit. Herein, glancing angle deposition is operated at substrate temperature of 0 °C and high speed of substrate rotation to generate silver nanoparticles (AgNPs) with nominal P < d. The AgNPs have intrinsic chiroptical activity characterized by circular dichroism (CD), originating from the hidden helicity. With increasing P from 3 to 66 nm, the plasmonic mode barely shifts but shows a logarithmic increase in CD amplitude. Immersing AgNPs in water causes the plasmonic mode to redshift and rise in CD amplitude, i.e., a water effect on chiroptical activity. Hydrophilic AgNP arrays with low array porosity show a reversible water effect, but hydrophobic Ag nanospiral arrays with P > d and high array porosity have an irreversible water effect. This work introduces a cost‐effective, facile approach to minimize P to sub‐10 nm at a regular substrate temperature, paving the way to study chiral plasmonics approaching the physical limit and exploit chirality‐related bioapplications typically operated in aqueous solutions to tackle significant health and environmental problems.