Plasmonic circular dichroism of Peptide-functionalized gold nanoparticles

Engineering the Blackbody Absorption of the Au-Branch-on-Au-Plate Heterostructures

Utilizing the strong ligand control effects of l-cysteine (l-Cys), the growth of Au on Au triangular nanoplate (AuTN) seeds was continuously tuned from layer-by-layer (the Frank-van der Merwe) to layer-plus-island (the Stranski-Krastanov), and island (the Volmer-Weber) growth modes, leading to the formation of a series of Au-on-AuTN heterostructures. Within the window of VW growth mode (featured by the growth of Au spikes and branches on AuTNs), the effective localized surface plasmon resonance (LSPR) coupling led to the selective strengthening of the "valley" absorptions, leading to smooth and flat absorption curves. Interestingly, through engineering the number/density, size, and branching degree of the Au branches, except for the black color, full spectrum absorption within 400-1300 nm wavelength was achieved on Au-branch-on-AuTN structures. Mechanistic studies revealed that the blackbody absorption property of the Au-branch-on-AuTN originates from the well-balanced intraparticle LSPR couplings among the neighboring Au branches. The tunable blackness and the full spectrum absorption property made the Au-branch-on-AuTN heterostructure a suitable candidate for various plasmonic-related applications, such as a wide spectrum light absorber, photoacoustic imaging contrast agent, and photothermal therapy medium. In addition, our strong ligand control in Au-branch-on-AuTN heterostructures could be extended to other hybrid systems with diverse material combinations, so long as to find the proper strong ligand.

Trace-Amount Detection of Chiral Molecules Based on Plasmonic Racemic Arrays Fabricated via Direct Laser Writing

Superchiral fields, supported by chiral plasmonic structures, have shown outstanding performance for chiral molecule sensing via enhanced chiral light-matter interaction. However, this sensing capability cannot fully reveal the chiral origin of the molecules as the chiroptic response of the molecules is intertwined with the chiroptic response of the chiral plasmonic nanostructures, which can potentially be excluded by using a plasmonic racemic mixture. Such a plasmonic racemic mixture is not easily attainable, as it normally requires complex fabrication and expensive instrumentation, whose structural fineness is limited by the fabrication precision. Here, we demonstrate trace-amount chiral molecule detection with plasmonic racemic arrays fabricated by direct laser writing with vector beams, which is facile, cost-effective, and highly controllable. The racemic arrays present no inherent circular differential scattering but a large local superchiral field, which reflects the intrinsic chiral features of the chiral molecules. They are further applied to discriminate enantiomers of phenylalanine with a limit of detection (LOD) of 10.0 ± 2.8 μM, which is an order of magnitude smaller than the LOD of conventional circular dichroism spectroscopy. The strong local superchiral field provided by the plasmonic racemic arrays enlightens the design of a superior sensing platform, which holds promising applications for biomedical detection and enantioselective drug development.

Collective chiroptical activity through the interplay of excitonic and charge-transfer effects in localized plasmonic fields

The collective light-matter interaction of chiral supramolecular aggregates or molecular ensembles with confined light fields remains a mystery beyond the current theoretical description. Here, we programmably and accurately build models of chiral plasmonic complexes, aiming to uncover the entangled effects of excitonic correlations, intra- and intermolecular charge transfer, and localized surface plasmon resonances. The intricate interplay of multiple chirality origins has proven to be strongly dependent on the site-specificity of chiral molecules on plasmonic nanoparticle surfaces spanning the nanometer to sub-nanometer scale. This dependence is manifested as a distinct circular dichroism response that varies in spectral asymmetry/splitting, signal intensity, and internal ratio of intensity. The inhomogeneity of the surface-localized plasmonic field is revealed to affect excitonic and charge-transfer mixed intermolecular couplings, which are inherent to chirality generation and amplification. Our findings contribute to the development of hybrid classical-quantum theoretical frameworks and the harnessing of spin-charge transport for emergent applications.

Synthesis, characterisation and in vitro anticancer activity of conjugated protease inhibitor-silver nanoparticles (AgNPs-PI) against human breast MCF-7 and prostate PC-3 cancer cell lines

The conjugated silver nanoparticles using biomolecules have attracted great attention of researchers because physical dimensions and surface chemistry play important roles in toxicity and biocompatibility of AgNPs. Hence, in the current study, synthesis of bio-conjugated AgNPs with protein protease inhibitor (PI) isolated from Streptomyces spp. is reported. UV-visible spectra of PI and AgNPs showed stronger peaks at 280 and 405 nm, confirming the synthesis of conjugated AgNPs-PI. TEM and SEM images of AgNPs-PI showed spherical-shaped nanoparticles with a slight increase in particle size and thin amorphous layer around the surface of silver nanomaterial. Circular dichroism, FT-IR and fluorescence spectral studies confirmed AgNPs-PI conjugation. Conjugated AgNPs-PI showed excellent anticancer potential than AgNPs and protease inhibitor separately on human breast MCF-7 and prostate PC-3 cell lines. The findings revealed that surface modification of AgNPs with protein protease inhibitor stabilised the nanomaterial and increased its anticancer activity.

Tuning the Geometry and Optical Chirality of Pentatwinned Au Nanoparticles with 5-Fold Rotational Symmetry

Chirality transfer from chiral molecules to chiral nanomaterials represents an important topic for exploring the origin of chirality in many natural and artificial systems. Moreover, developing a promising class of chiral nanomaterials holds great significance for various applications, including sensing, photonics, catalysis, and biomedicine. Here we demonstrate the geometric control and tunable optical chirality of chiral pentatwinned Au nanoparticles with 5-fold rotational symmetry using the seed-mediated chiral growth method. A distinctive growth pathway and optical chirality are observed using pentatwinned decahedra as seeds, in comparison with the single-crystal Au seeds. By employing different peptides as chiral inducers, pentatwinned Au nanoparticles with two distinct geometric chirality (pentagonal nanostars and pentagonal prisms) are obtained. The intriguing formation and evolution of geometric chirality with the twinned structure are analyzed from a crystallographic perspective upon maneuvering the interplay of chiral molecules, surfactants, and reducing agents. Moreover, the interesting effects of the molecular structure of peptides on tuning the geometric chirality of pentatwinned Au nanoparticles are also explored. Finally, we theoretically and experimentally investigate the far-field and near-field optical properties of chiral pentatwinned Au nanoparticles through numerical simulations and single-particle chiroptical measurements. The ability to tune the geometric chirality in a controlled manner represents an important step toward the development of chiral nanomaterials with increasing architectural complexity for chiroptical applications.

Silver induced chirality controlled spin filtration observed in ss-DNA functionalized with MoS2

Chiral molecules can exhibit strong spin-orbit coupling, which can result in a large spin polarization. This is due to the fact that the energy levels of the electrons in a chiral molecule are strongly influenced by the chiral structure of the molecule, which can result in the separation of the energy levels for electrons with different spin orientations. We report a controlled spin-selective transmission of electrons through 20 base-paired poly-cytosine molecules functionalized with MoS2 flakes on ITO glass via the quantum mechanical tunneling effect. A reversion in spin polarization was observed after the silver ions interact with poly-cytosine due to the strong coordination of Ag(I) with cytosine-cytosine (C-C) mismatches, indicating the formation of duplex structural motifs, as confirmed by the circular dichroism spectroscopy at room temperature. Manipulating the spin of an electron through such a small molecule merely controlled by special cations could pave the way for major advances in spin-independent charge transport, advanced bioanalytical system design, and related applications.

Unraveling the Chirality Transfer from Circularly Polarized Light to Single Plasmonic Nanoparticles

Due to their broken symmetry, chiral plasmonic nanostructures have unique optical properties and numerous applications. However, there is still a lack of comprehension regarding how chirality transfer occurs between circularly polarized light (CPL) and these structures. Here, we thoroughly investigate the plasmon-assisted growth of chiral nanoparticles from achiral Au nanocubes (AuNCs) via CPL without the involvement of any chiral molecule stimulators. We identify the structural chirality of our synthesized chiral plasmonic nanostructures using circular differential scattering (CDS) spectroscopy, which is correlated with scanning electron microscopy imaging at both the single-particle and ensemble levels. Theoretical simulations, including hot-electron surface maps, reveal that the plasmon-induced chirality transfer is mediated by the asymmetric distribution of hot electrons on achiral AuNCs under CPL excitation. Furthermore, we shed light on how this plasmon-induced chirality transfer can also be utilized for chiral growth in bimetallic systems, such as Ag or Pd on AuNCs. The results presented here uncover fundamental aspects of chiral light-matter interaction and have implications for the future design and optimization of chiral sensors and chiral catalysis, among others.

Universal Chiral-Plasmon-Induced Upward and Downward Transfer of Circular Dichroism to Achiral Molecules

Electromagnetic chirality transfer represents an effective means of the nanoscale manipulation of optical chirality. While most of the previous reports have exclusively focused on the circular dichroism (CD) transfer from UV-responsive chiral molecules toward visible-resonant achiral colloidal nanoparticles, here we demonstrate a reverse process in which plasmonic chirality can be transferred to achiral molecules, either upward from visible to UV or downward from visible to near infrared (NIR). By hybridizing achiral UV- or NIR-responsive dye molecules with chiral metal nanoparticles in solution, we observe a chiral-plasmon-induced CD (CPICD) signal at the intrinsically achiral molecular absorption bands. Full-wave electromagnetic modeling reveals that both near-field Coulomb interaction and far-field radiative coupling contribute to the observed CPICD, indicating that the mechanism considered here is universal for different material systems and types of optical resonances. Our study provides a set of design guidelines for broadband nanophotonic chiral sensing from the UV to NIR spectral regime.

Recent Advances in the 3D Chiral Plasmonic Nanomaterials

From the view of geometry, chirality is that an object cannot overlap with its mirror image, which has been a fundamental scientific problem in biology and chemistry since the 19th century. Chiral inorganic nanomaterials serve as ideal templates for investigating chiral transfer and amplification mechanisms between molecule and bulk materials, garnering widespread attentions. The chiroptical property of chiral plasmonic nanomaterials is enhanced through localized surface plasmon resonance effects, which exhibits distinctive circular dichroism (CD) response across a wide wavelength range. Recently, 3D chiral plasmonic nanomaterials are becoming a focal research point due to their unique characteristics and planar-independence. This review provides an overview of recent progresses in 3D chiral plasmonic nanomaterials studies. It begins by discussing the mechanisms of plasmonic enhancement of molecular CD response, following by a detailed presentation of novel classifications of 3D chiral plasmonic nanomaterials. Finally, the applications of 3D chiral nanomaterials such as biology, sensing, chiral catalysis, photology, and other fields have been discussed and prospected. It is hoped that this review will contribute to the flourishing development of 3D chiral nanomaterials.

Multicomponent chiral plasmonic hybrid nanomaterials: recent advances in synthesis and applications

Chiral hybrid nanomaterials with multiple components provide a highly promising approach for the integration of desired chirality with other functionalities into one single nanoscale entity. However, precise control over multicomponent chiral plasmonic hybrid nanomaterials to enable their application in diverse and complex scenarios remains a significant challenge. In this review, our focus lies on the recent advances in the preparation and application of multicomponent chiral plasmonic hybrid nanomaterials, with an emphasis on synthetic strategies and emerging applications. We first systematically elucidate preparation methods for multicomponent chiral plasmonic hybrid nanomaterials encompassing the following approaches: physical deposition approach, galvanic replacement reaction, chiral molecule-mediated, chiral heterostructure, circularly polarized light-mediated, magnetically induced, and chiral assembly. Furthermore, we highlight emerging applications of multicomponent chiral plasmonic hybrid nanomaterials in chirality sensing, enantioselective catalysis, and biomedicine. Finally, we provide an outlook on the challenges and opportunities in the field of multicomponent chiral plasmonic hybrid nanomaterials. In-depth investigations of these multicomponent chiral hybrid nanomaterials will pave the way for the rational design of chiral hybrid nanostructures with desirable functionalities for emerging technological applications.

Reshaping and induction of optical activity in gold@silver nanocuboids by chiral glutathione molecules

Core-shell gold-silver cuboidal nanoparticles were produced, with either concave or straight facets. Their incubation with a low concentration of chiral l-glutathione (GSH) biomolecules was found to produce near UV plasmonic extinction and induced circular dichroism (CD) peaks. The effect is sensitive to the silver shell thickness. The GSH molecules were found to cause redistribution of silver in the shell, removing silver atoms from edges/corners and re-depositing them at the nanocuboid facets, probably through some redox and complexation processes between the silver and thiol group of the GSH. Other thiolated chiral biomolecules (and drug molecules) did not show this effect. The emerging near UV surface plasmon resonance is a silver slab resonance, which might also possess some multipolar resonance nature. The concave-shaped nanocuboids exhibited stronger induced plasmonic CD relative to the nanocuboids with straight facets.

Nanophotonic Enhanced Chiral Sensing and Its Biomedical Applications

Chiral sensing is crucial in the fields of biology and the pharmaceutical industry. Many naturally occurring biomolecules, i.e., amino acids, sugars, and nucleotides, are inherently chiral. Their enantiomers are strongly associated with the pharmacological effects of chiral drugs. Owing to the extremely weak chiral light-matter interactions, chiral sensing at an optical frequency is challenging, especially when trace amounts of molecules are involved. The nanophotonic platform allows for a stronger interaction between the chiral molecules and light to enhance chiral sensing. Here, we review the recent progress in nanophotonic-enhanced chiral sensing, with a focus on the superchiral near-field and enhanced circular dichroism (CD) spectroscopy generated in both the dielectric and in plasmonic structures. In addition, the recent applications of chiral sensing in biomedical fields are discussed, including the detection and treatment of difficult diseases, i.e., Alzheimer's disease, diabetes, and cancer.

Single Gold Nanobipyramids Sensing the Chirality of Amyloids

Plasmonic nanoparticles, due to their sensitivity to small changes in their closest environment and plasmon resonance, can sense the chirality of the surrounding molecules. Therefore, plasmonic nanoparticles can be applied as a next-generation biosensor for peptides or proteins. In this work, we explore the interaction between chiral, ordered protein aggregates (amyloids) and small gold nanobipyramids. We show how the morphology, structure, and chiroptical properties of amyloids induce circular dichroism in the plasmon resonance wavelengths from individual plasmonic nanoparticles upon binding to the chiral amyloid template. Moreover, using the data from microscopic and spectroscopic analyses of formed heterostructures, we propose the most probable mechanism behind the induction of chirality in this system and discuss which specific feature of insulin protein aggregates is sensed by nanobipyramids.

Functional mimicry of sea urchin biomineralization proteins with CaCO3-binding peptides selected by phage display

The intricate process of biomineralization, e.g. in sea urchins, involves the precise interplay of highly regulated mineralization proteins and the spatiotemporal coordination achieved through compartmentalization. However, the investigation of biomineralization effector molecules, e.g. proteins, is challenging, due to their very low abundance. Therefore, we investigate the functional mimicry in the bioinspired precipitation of calcium carbonate (CaCO3) with artificial peptides selected from a peptide library by phage display based on peptide-binding to calcite and aragonite, respectively. The structure-directing effects of the identified peptides were compared to those of natural protein mixes isolated from skeletal (test) structures of two sea urchin species (Arbacia lixula and Paracentrotus lividus). The calcium carbonate samples deposited in the absence or presence of peptides were analyzed with a set of complementary techniques with regard to morphology, polymorph, and nanostructural motifs. Remarkably, some of the CaCO3-binding peptides induced morphological features in calcite that appeared similar to those obtained in the presence of the natural protein mixes. Many of the peptides identified as most effective in exerting a structure-directing effect on calcium carbonate crystallization were rich in basic amino acid residues. Hence, our in vitro mineralization study further highlights the important, but often neglected, role of positively charged soluble organic matrices associated with biological and bioinspired CaCO3 deposition.

Plasmon-enhanced circular dichroism spectroscopy of chiral drug solutions

We investigate the potential of surface plasmon polaritons at noble metal interfaces for surface-enhanced chiroptical sensing of dilute chiral drug solutions with nl volume. The high quality factor of surface plasmon resonances in both Otto and Kretschmann configurations enables the enhancement of circular dichroism differenatial absorption thanks to the large near-field intensity of such plasmonic excitations. Furthermore, the subwavelength confinement of surface plasmon polaritons is key to attain chiroptical sensitivity to small amounts of drug volumes placed around ≃100 nm by the metal surface. Our calculations focus on reparixin, a pharmaceutical molecule currently used in clinical studies for patients with community-acquired pneumonia, including COVID-19 and acute respiratory distress syndrome. Considering realistic dilute solutions of reparixin dissolved in water with concentration ≤5 mg/ml and nl volume, we find a circular-dichroism differential absorption enhancement factor of the order ≃20 and chirality-induced polarization distortion upon surface plasmon polariton excitation. Our results are relevant for the development of innovative chiroptical sensors capable of measuring the enantiomeric imbalance of chiral drug solutions with nl volume.

Plexcitonic optical chirality in the chiral plasmonic structure-microcavity-exciton strong coupling system

Chiral plexcitonic systems exhibit a novel chiroptical phenomenon, which can provide a new route to design chiroptical devices. Reported works focused on the two-mode strong coupling between chiral molecules and nanoparticles, while multiple-mode coupling can provide richer modulation. In this paper, we proposed a three-mode coupling system consisting of a chiral Au helices array, a Fabry-Pérot cavity, and monolayer WSe2, which can provide an extra chiral channel, a more widely tunable region, and more tunable methods compared to two-mode coupled systems. The optical response of this hybrid system was investigated based on the finite element method. Mode splitting observed in the circular dichroism (CD) spectrum demonstrated that the chiroptical response successfully shifted from the resonant position of the chiral structure to three plexcitons through strong coupling, which provided a new route for chiral transfer. Furthermore, we used the coupled oscillator model to obtain the energy and Hopfield coefficients of the plexciton branches to explain the chiroptical phenomenon of the hybrid system. Moreover, the tunability of the hybrid system can be achieved by tuning the temperature and period of the helices array. Our work provides a feasible strategy for chiral sensing and modulation devices.

Stereoselective Plasmonic Interaction in Peptide-tethered Photopolymerizable Diacetylenes Doped with Chiral Gold Nanoparticles

Designing polymeric systems with ultra-high optical activity is instrumental in the pursuit of smart artificial chiroptical materials, including the fundamental understanding of structure/property relations. Herein, we report a diacetylene (DA) moiety flanked by chiral D- and L-FF dipeptide methyl esters that exhibits efficient topochemical photopolymerization in the solid phase to furnish polydiacetylene (PDA) with desired control over the chiroptical properties. The doping of the achiral gold nanoparticles provides plasmonic interaction with the PDAs to render asymmetric shape to the circular dichroism bands. With the judicious design of the chiral amino acid ligand appended to the AuNPs, we demonstrate the first example of selective chiral amplification mediated by stereo-structural matching of the polymer-plasmonic AuNP hybrid pairs. Such ordered self-assembly aided by topochemical polymerization in peptide-tethered PDA provides a smart strategy to produce soft responsive materials for applications in chiral photonics.

Geometric Control and Optical Properties of Intrinsically Chiral Plasmonic Nanomaterials

Intrinsically chiral plasmonic nanomaterials exhibit intriguing geometry-dependent chiroptical properties, which is due to the combination of plasmonic features with geometric chirality. Thus, chiral plasmonic nanomaterials have become promising candidates for applications in biosensing, asymmetric catalysis, biomedicine, photonics, etc. Recent advances in geometric control and optical tuning of intrinsically chiral plasmonic nanomaterials have further opened up a unique opportunity for their widespread applications in many emerging technological areas. Here, the recent developments in the geometric control of chiral plasmonic nanomaterials are reviewed with special attention given to the quantitative understanding of the chiroptical structure-property relationship. Several important optical spectroscopic tools for characterizing the optical chirality of plasmonic nanomaterials at both ensemble and single-particle levels are also discussed. Three emerging applications of chiral plasmonic nanomaterials, including enantioselective sensing, enantioselective catalysis, and biomedicine, are further highlighted. It is envisioned that these advanced studies in chiral plasmonic nanomaterials will pave the way toward the rational design of chiral nanomaterials with desired optical properties for diverse emerging technological applications.

Molecular-Induced Chirality Transfer to Plasmonic Lattice Modes

Molecular chirality plays fundamental roles in biology. The chiral response of a molecule occurs at a specific spectral position, determined by its molecular structure. This fingerprint can be transferred to other spectral regions via the interaction with localized surface plasmon resonances of gold nanoparticles. Here, we demonstrate that molecular chirality transfer occurs also for plasmonic lattice modes, providing a very effective and tunable means to control chirality. We use colloidal self-assembly to fabricate non-close packed, periodic arrays of achiral gold nanoparticles, which are embedded in a polymer film containing chiral molecules. In the presence of the chiral molecules, the surface lattice resonances (SLRs) become optically active, i.e., showing handedness-dependent excitation. Numerical simulations with varying lattice parameters show circular dichroism peaks shifting along with the spectral positions of the lattice modes, corroborating the chirality transfer to these collective modes. A semi-analytical model based on the coupling of single-molecular and plasmonic resonances rationalizes this chirality transfer.

Trace detection of chiral J-aggregated molecules adsorbed on single Au nanorods

Trace detection of chiral molecules, which is of great significance in chemical, biological, medical and pharmaceutical sciences, requires microscopic techniques at the single-particle or single-molecule level. Although ensemble experiments show that the circular dichroism of chiral molecules can be amplified by plasmonic nanocrystals, trace detection of small chiral molecules remains challenging due to weak signals that are far below the detection limit. Herein, we demonstrate trace detection of chiral J-aggregated molecules adsorbed on individual Au nanorods (NRs) using single-particle circular differential scattering (CDS) spectroscopy. Through measuring the single-particle CDS spectra, we identified dip-peak bisignatures and further determined the chirality by matching them with calculations modelled with chiral media. We therefore find that plasmonic nanocrystals can dramatically amplify the circular dichroism of strongly coupled molecules to a detectable level so that the detection limit is as low as 3.9 × 10³ molecules on an individual plasmonic nanoparticle, whereas 2.5 × 10¹² molecules free in solution are barely detectable using a commercial circular dichroism instrument, suggesting a significant amplification factor of 10⁸. Our method provides a promising strategy with a high amplification factor, shedding light on the trace detection of chiral molecules using optical microscopic methods.

Coupling of plasmonic hot spots with shurikens for superchiral SERS-based enantiomer recognition

Detection of enantiomers is a challenging problem in drug development as well as environmental and food quality monitoring where traditional optical detection methods suffer from low signals and sensitivity. Application of surface enhanced Raman scattering (SERS) for enantiomeric discrimination is a powerful approach for the analysis of optically active small organic or large biomolecules. In this work, we proposed the coupling of disposable chiral plasmonic shurikens supporting the chiral near-field distribution with SERS active silver nanoclusters for enantio-selective sensing. As a result of the plasmonic coupling, significant difference in SERS response of optically active analytes is observed. The observations are studied by numerical simulations and it is hypothesized that the silver particles are being excited by superchiral fields generated at the surface inducing additional polarizations in the probe molecules. The plasmon coupling phenomena was found to be extremely sensitive to slight variations in shuriken geometry, silver nanostructured layer parameters, and SERS excitation wavelength(s). Designed structures were able to discriminate cysteine enantiomers at concentrations in the nanomolar range and probe biomolecular chirality, using a common Raman spectrometer within several minutes. The combination of disposable plasmonic substrates with specific near-field polarization can make the SERS enantiomer discrimination a commonly available technique using standard Raman spectrometers.

Protein-Inspired Polymers with Metal-Site-Regulated Ordered Conformations

Metal ions play critical roles in facilitating peptide folding and inducing conformational transitions, thereby impacting on the biological activity of many proteins. However, the effect of metal sites on the hierarchical structures of biopolymers is still poorly understood. Herein, inspired by metalloproteins, we report an order-to-order conformational regulation in synthetic polymers mediated by a variety of metal ions. The copolymers are decorated with clinically available desferrioxamine (DFO) as an exogenous ligand template, which presents a geometric constraint toward peptide backbone via short-range hydrogen bonding interactions, thus dramatically altering the secondary conformations and self-assembly behaviors of polypeptides and allowing for a controllable β-sheet to α-helix transition modulated by metal-ligand interactions. These metallopolymers could form ferritin-inspired hierarchical structures with high stability and membrane activity for efficient brain delivery across the blood-brain barrier (BBB) and long-lasting magnetic resonance imaging (MRI) in vivo.

Chiral plasmonic Au-Ag core shell nanobipyramid for SERS enantiomeric discrimination of biologically relevant small molecules

The identification of enantiomers is of great importance in chiral separations and medicinal chemistry. While Surface-enhanced Raman spectroscopy (SERS) is a technique that provides vibrational fingerprints of analytes. The enantiomers identification relies on the SERS difference between left and right-handed circularly polarized light or additional selectors for indirect distinction. In this work, Au-Ag core shell nanobipyramid (L/D-Au@Ag BPs) were synthesized guiding by chiral encoder of L/D-cysteine. L/D-Au@Ag BPs produced plasmon-induced circular dichroism signals in the plasmon resonance absorption band, which can be tuned by modulation the amount of cysteine. Moreover, the chiral anisotropy factor of L/D-Au@Ag BPs at 532 nm can reach 5.11 × 10^-3. Due to the selective resonance coupling between L/D-Au@Ag BPs and different enantiomers, L/D-Au@Ag BPs were further used as SERS substrates for efficient discrimination of biologically relevant small molecules. Chiral Au@Ag BPs display the potential for chiral drug identification.

On the stability of peptide secondary structures on the TiO2 (101) anatase surface: a computational insight

The biological activity of proteins is partly due to their secondary structures and conformational states. Peptide chains are rather flexible so that finding ways inducing protein folding in a well-defined state is of great importance. Among the different constraint techniques, the interaction of proteins with inorganic surfaces is a fruitful strategy to stabilize selected folded states. Surface-induced peptide folding can have potential applications in different biomedicine areas, but it can also be of fundamental interest in prebiotic chemistry since the biological activity of a peptide can turn-on when folded in a given state. In this work, periodic quantum mechanical simulations (including implicit solvation effects) at the PBE-D2* level have been carried out to study the adsorption and the stability of the secondary structures (α-helix and β-sheet) of polypeptides with different chemical composition (i.e., polyglycine, polyalanine, polyglutamic acid, polylysine, and polyarginine) on the TiO₂ (101) anatase surface. The computational cost is reduced by applying periodic boundary conditions to both the surface and the peptides, thus obtaining full periodic polypeptide/TiO₂ surface systems. At variance with polyglycine, the interaction of the other polypeptides with the surface takes place with the lateral chain functionalities, leaving the secondary structures almost undistorted. Results indicate that the preferred conformation upon adsorption is the α-helix over the β-sheet, with the exception of the polyglutamic acid. According to the calculated adsorption energies, the affinity trend of the polypeptides with the (101) anatase surface is: polyarginine ≈ polylysine > polyglutamic acid > polyglycine ≈ polyalanine, both when adsorbed in gas phase and in presence of the implicit water solvent, which is very similar to the trend for the single amino acids. A set of implications related to the areas of surface-induced peptide folding, biomedicine and prebiotic chemistry are finally discussed.

Biomolecule-Based Optical Metamaterials: Design and Applications

Metamaterials are broadly defined as artificial, electromagnetically homogeneous structures that exhibit unusual physical properties that are not present in nature. They possess extraordinary capabilities to bend electromagnetic waves. Their size, shape and composition can be engineered to modify their characteristics, such as iridescence, color shift, absorbance at different wavelengths, etc., and harness them as biosensors. Metamaterial construction from biological sources such as carbohydrates, proteins and nucleic acids represents a low-cost alternative, rendering high quantities and yields. In addition, the malleability of these biomaterials makes it possible to fabricate an endless number of structured materials such as composited nanoparticles, biofilms, nanofibers, quantum dots, and many others, with very specific, invaluable and tremendously useful optical characteristics. The intrinsic characteristics observed in biomaterials make them suitable for biomedical applications. This review addresses the optical characteristics of metamaterials obtained from the major macromolecules found in nature: carbohydrates, proteins and DNA, highlighting their biosensor field use, and pointing out their physical properties and production paths.

Gold-Nanoparticle-Based Chiral Plasmonic Nanostructures and Their Biomedical Applications

As chiral antennas, plasmonic nanoparticles (NPs) can enhance chiral responses of chiral materials by forming hybrid structures and improving their own chirality preference as well. Chirality-dependent properties of plasmonic NPs broaden application potentials of chiral nanostructures in the biomedical field. Herein, we review the wet-chemical synthesis and self-assembly fabrication of gold-NP-based chiral nanostructures. Discrete chiral NPs are mainly obtained via the seed-mediated growth of achiral gold NPs under the guide of chiral molecules during growth. Irradiation with chiral light during growth is demonstrated to be a promising method for chirality control. Chiral assemblies are fabricated via the bottom-up assembly of achiral gold NPs using chiral linkers or guided by chiral templates, which exhibit large chiroplasmonic activities. In describing recent advances, emphasis is placed on the design and synthesis of chiral nanostructures with the tuning and amplification of plasmonic circular dichroism responses. In addition, the review discusses the most recent or even emerging trends in biomedical fields from biosensing and imaging to disease diagnosis and therapy.

Intelligent design of the chiral metasurfaces for flexible targets: combining a deep neural network with a policy proximal optimization algorithm

Recently, deep reinforcement learning (DRL) for metasurface design has received increased attention for its excellent decision-making ability in complex problems. However, time-consuming numerical simulation has hindered the adoption of DRL-based design method. Here we apply the Deep learning-based virtual Environment Proximal Policy Optimization (DE-PPO) method to design the 3D chiral plasmonic metasurfaces for flexible targets and model the metasurface design process as a Markov decision process to help the training. A well trained DRL agent designs chiral metasurfaces that exhibit the optimal absolute circular dichroism value (typically, ∼ 0.4) at various target wavelengths such as 930 nm, 1000 nm, 1035 nm, and 1100 nm with great time efficiency. Besides, the training process of the PPO agent is exceptionally fast with the help of the deep neural network (DNN) auxiliary virtual environment. Also, this method changes all variable parameters of nanostructures simultaneously, reducing the size of the action vector and thus the output size of the DNN. Our proposed approach could find applications in efficient and intelligent design of nanophotonic devices.

Resonant Plasmonic-Biomolecular Chiral Interactions in the Far-Ultraviolet: Enantiomeric Discrimination of sub-10 nm Amino Acid Films

Resonant plasmonic-molecular chiral interactions are a promising route to enhanced biosensing. However, biomolecular optical activity primarily exists in the far-ultraviolet regime, posing significant challenges for spectral overlap with current nano-optical platforms. We demonstrate experimentally and computationally the enhanced chiral sensing of a resonant plasmonic-biomolecular system operating in the far-UV. We develop a full-wave model of biomolecular films on Al gammadion arrays using experimentally derived chirality parameters. Our calculations show that detectable enhancements in the chiroptical signals from small amounts of biomolecules are possible only when tight spectral overlap exists between the plasmonic and biomolecular chiral responses. We support this conclusion experimentally by using Al gammadion arrays to enantiomerically discriminate ultrathin (<10 nm thick) films of tyrosine. Notably, the chiroptical signals of the bare films were within instrumental noise. Our results demonstrate the importance of using far-UV active metasurfaces for enhancing natural optical activity.

Site-directed dual bioprobes inducing single-step nano-sandwich assay for the detection of cardiac troponin I

Bioreceptor functionalized metallic nano-colloids have been identified as effective nanobioprobes to realize the detection of an analyte based on a common phenomenon of salt-induced aggregation. In marked contrast to this, we describe a nano-sandwich assay integrating the novel match-pair of aptamer and peptide functionalized gold nanoparticles. The site-directed biomolecular interaction of high affinity aptamer and peptide bioreceptors directed towards distinct sites of cardiac biomarker troponin I; this was found to form a nano-sandwich assay in a peculiar manner. The gold nanoconjugates interact with specific and distant regions of troponin I to result in collision of probes upon target identification. In the presence of TnI, both nanobioprobes bind at their respective sites forming a nano-sandwich pair providing a visual color change from red to blue. Thus, the presence of target TnI itself causes instant agglomeration in just a single-step without addition of any external aggregator. The assay imparts 100% specificity and 90% sensitivity in a dynamic concentration range of 0.1-500 ng/mL troponin I with detection limit as low as 0.084 ng/mL. The applicability of the assay has been validated in clinical samples of acute myocardial infarction patients thus establishing a promising point-of-care detection of TnI.

Thermoresponsive chiral plasmonic nanoparticles

Chiral metallic nanoparticles can exhibit novel plasmonic circular dichroism (PCD) in the ultraviolet and visible range of the electromagnetic spectrum. Here, we investigate how thermoresponsive dielectric nanoenvironments will influence such PCD responses through poly(N-isopropylacrylamide) (PNIPAM) modified chiral gold nanorods (AuNRs). We observed the temperature-dependent chiral plasmonic responses distinctly from unmodified counterparts. As for the modified systems, the PCD peaks for both L-AuNRs and D-AuNRs at 50 °C red shifted simultaneously with enhanced intensities compared to the results at 20 °C. In contrast, the unmodified L-AuNRs and D-AuNRs exhibited no peak shift with reduced intensities. Subsequent simulation and experimental studies demonstrated that the enhanced PCD was attributed to PNIPAM chain collapse causing the increase of the refractive index by expelling minute water out of the corona surrounding chiral plasmonic AuNRs. Notably, such thermoresponsive chiral plasmonic responses are reversible, general, and extendable to other types of chiral plasmonic nanoparticles.

Insight on Chirality Encoding from Small Thiolated Molecule to Plasmonic Au@Ag and Au@Au Nanoparticles

Chiral plasmonic nanomaterials exhibiting intense optical activity are promising for numerous applications. In order to prepare those nanostructures, one strategy is to grow metallic nanoparticles in the presence of chiral molecules. However, in such approach the origin of the observed chirality remains uncertain. In this work, we expand the range of available chiral plasmonic nanostructures and we propose another vision of the origin of chirality in such colloidal systems. For that purpose, we investigated the synthesis of two core-shell Au@Ag and Au@Au systems built from gold nanobipyramid cores, in the presence of cysteine. The obtained nanoparticles possess uniform shape and size and show plasmonic circular dichroism in the visible range, and were characterized by electron microscopy, circular dichroism, and UV-vis-NIR spectroscopy. Opto-chiral responses were found to be highly dependent on the morphology and the plasmon resonance. It revealed (i) the importance of the anisotropy for Au@Au nanoparticles and (ii) the role of the multipolar modes for Au@Ag nanoparticles on the way to achieve intense plasmonic circular dichroism. The role of cysteine as shaping agent and as chiral encoder was particularly evaluated. Our experimental results, supported by theoretical simulations, contrast the hypothesis that chiral molecules entrapped in the nanoparticles determine the chiral properties, highlighting the key role of the outmost part of the nanoparticles shell on the plasmonic circular dichroism. Along with these results, the impact of enantiomeric ratio of cysteine on the final shape suggested that the presence of a chiral shape or chiral patterns should be considered.

Local Growth Mediated by Plasmonic Hot Carriers: Chirality from Achiral Nanocrystals Using Circularly Polarized Light

Plasmonic nanocrystals and their assemblies are excellent tools to create functional systems, including systems with strong chiral optical responses. Here we study the possibility of growing chiral plasmonic nanocrystals from strictly nonchiral seeds of different types by using circularly polarized light as the chirality-inducing mechanism. We present a novel theoretical methodology that simulates realistic nonlinear and inhomogeneous photogrowth processes in plasmonic nanocrystals, mediated by the excitation of hot carriers that can drive surface chemistry. We show the strongly anisotropic and chiral growth of oriented nanocrystals with lowered symmetry, with the striking feature that such chiral growth can appear even for nanocrystals with subwavelength sizes. Furthermore, we show that the chiral growth of nanocrystals in solution is fundamentally challenging. This work explores new ways of growing monolithic chiral plasmonic nanostructures and can be useful for the development of plasmonic photocatalysis and fabrication technologies.

Recent developments in the chiroptical properties of chiral plasmonic gold nanostructures: bioanalytical applications

The presence of excess L-amino acid in the Murchison meteorite, circular polarization effect in the genesis of stars and existence of chirality in interstellar molecules contribute to the origin of life on earth. Chiral-sensitive techniques have been employed to untangle the secret of the symmetries of the universe, designing of effective secure drugs and investigation of chiral biomolecules. The relationship between light and chiral molecules was employed to probe and explore such molecules using spectroscopy techniques. The mutual interaction between electromagnetic spectrum and chirality of matter give rise to distinct optical response, which advances vital information contents in chiroptical spectroscopy. Chiral plasmonic gold nanoparticle exhibits distinctive circular dichroism peaks in broad wavelength range thereby crossing the limits of its characterization. The emergence of strong optical activity of gold nanosystem is related to its high polarizability, resulting in plasmonic and excitonic effects on incident photons. Inspired by the development of advanced chiral plasmonic nanomaterials and exploring its properties, this review gives an overview of various chiral gold nanostructures and the mechanism behind its chiroptical properties. Finally, we highlight the application of different chiral gold nanomaterials in the field of catalysis and medical applications with special emphasis to biosensing and biodetection.

Nanophotonic Approaches for Chirality Sensing

Chiral nanophotonic materials are promising candidates for biosensing applications because they focus light into nanometer dimensions, increasing their sensitivity to the molecular signatures of their surroundings. Recent advances in nanomaterial-enhanced chirality sensing provide detection limits as low as attomolar concentrations (10^-18 M) for biomolecules and are relevant to the pharmaceutical industry, forensic drug testing, and medical applications that require high sensitivity. Here, we review the development of chiral nanomaterials and their application for detecting biomolecules, supramolecular structures, and other environmental stimuli. We discuss superchiral near-field generation in both dielectric and plasmonic metamaterials that are composed of chiral or achiral nanostructure arrays. These materials are also applicable for enhancing chiroptical signals from biomolecules. We review the plasmon-coupled circular dichroism mechanism observed for plasmonic nanoparticles and discuss how hotspot-enhanced plasmon-coupled circular dichroism applies to biosensing. We then review single-particle spectroscopic methods for achieving the ultimate goal of single-molecule chirality sensing. Finally, we discuss future outlooks of nanophotonic chiral systems.

Emergent chiroptical properties in supramolecular and plasmonic assemblies

This tutorial provides a comprehensive description of the origin of chiroptical properties of supramolecular and plasmonic assemblies in the UV-visible region of the electromagnetic spectrum. The photophysical concepts essential for understanding chiroptical signatures are presented in the first section. Just as the oscillator strength (a positive quantity) is related to absorption, the rotational strength (either a positive or a negative quantity) defines the emergence of chiroptical signatures in molecular/plasmonic systems. In supramolecular systems, induced circular dichroism (ICD) originates through the off-resonance coupling of transition dipoles in chiral inclusion complexes, while exciton coupled circular dichroism (ECD) originates through the on-resonance exciton coupling of transition dipoles in chiral assemblies resulting in the formation of a bisignated CD signal. In bisignated ECD spectra, the sign of the couplet is determined not only by the handedness of chiral supramolecular assemblies, but also by the sign of the interaction energy between transition dipoles. Plasmonic chirality is briefly addressed in the last section, focusing on inherent chirality, induced chirality, and surface plasmon-coupled circular dichroism (SP-CD). The oscillator strength is of the order of 1 in molecular systems, while it becomes very large (10⁴-10⁵) in plasmonic systems due to the collective plasmonic excitations, resulting in intense CD signals, which can be exploited for the design of plasmonic metamaterial platforms for chiral sensing applications.

Diverse axial chiral assemblies of J-aggregates in plexcitonic nanoparticles

Plexcitonic hybrids, consisting of metal nanoparticles and J-aggregates, are effective nanostructures to achieve a strong coupling regime. The chirality of the exciton in the strong coupled plexcitons provides more potential for the design of advanced optoelectronic devices. Here, we experimentally measured the circular dichroism (CD) spectra of plexcitonic hybrids, and researched the diverse chirality of J-aggregates assembled on the surface of the achiral Au nanorods. We found that the chirality of J-aggregates is not only related to the quantity of dye molecules in the plexcitonic, but also to the distribution in different positions of the nanorods, by analyzing the composition of the CD spectra with a quasistatic theory. The J-aggregates assembled on both ends and both sides of the nanorods had opposite chirality. The interaction between the longitudinal localized surface plasmon resonance (LLSPR) of the nanorods and J-aggregates achieved the strong coupling regime, and Rabi splitting of about 198.3 meV was observed. The research into the chirality of the plexcitons provided more detail on the chiral J-aggregates assembly on the nanoparticles, and give a perspective on the development of the strong coupling interactions and the design of optoelectronic systems.

Comparative study of an antimicrobial peptide and a neuropeptide conjugated with gold nanorods for the targeted photothermal killing of bacteria

There are certain disadvantages in treating bacterial infections through conventional methods. For this reason, the current study does focus on combating bacterial wound infections by photothermal therapy assisted by gold nanorod-peptide conjugates (GNR-peptide conjugates). Two peptides, the cationic antimicrobial peptide LL-37 and neuropeptide ANGIOPEP-2 both with specificity for targeted bacterial binding, were conjugated with GNR surface through electrostatic interactions. The GNR-peptide conjugates showed good biocompatibility, sufficient stability, enhanced targeting, potential photothermal killing of bacteria, and possible acceleration of wound healing. The photo-biomodulation properties of NIR improved the wound closure rates through enhanced cell migration. The multifunctional LL37-conjugated GNRs significantly enhanced photothermal therapeutic outcomes based on bacterial targeting with promising wound healing properties.

CDDA: extension and analysis of the discrete dipole approximation for chiral systems

Discrete dipole approximation (DDA) is a computational method broadly used to solve light scattering problems. In this work, we propose an extension of DDA that we call Chiral-DDA (CDDA), to study light-chiral matter interactions with the capability of describing the underlying physics behind. Here, CDDA is used to solve and analyze the interaction of a nanoantenna (either metallic or dielectric) with a chiral molecule located in its near field at different positions. Our method allowed to relate near field interactions with far field spectral response of the system, elucidating the role that the nanoantenna electric and magnetic polarizabilities play in the coupling with a chiral molecule. In general, this is not straightforward with other methods. We believe that CDDA has the potential to help researchers revealing some of the still unclear mechanisms responsible for the chiral signal enhancements induced by nanoantennas.

Solution NMR of Nanoparticles in Serum: Protein Competition Influences Binding Thermodynamics and Kinetics

The spontaneous formation of a protein corona on a nanoparticle surface influences the physiological success or failure of the synthetic nanoparticle as a drug carrier or imaging agent used in vivo. A quantitative understanding of protein-nanoparticle interactions is therefore critical for the development of nanoparticle-based therapeutics. In this perspective, we briefly discuss the challenges and limitations of current approaches used for studying protein-nanoparticle binding in a realistic biological medium. Subsequently, we demonstrate that solution nuclear magnetic resonance (NMR) spectroscopy is a powerful tool to monitor protein competitive binding in a complex serum medium in situ. Importantly, when many serum proteins are competing for a gold nanoparticle (AuNP) surface, solution NMR is able to detect differences in binding thermodynamics, and kinetics of a tagged protein. Combined with other experimental approaches, solution NMR is an invaluable tool to understand protein behavior in the nanoparticle corona.

Boosting chiral amplification in plasmon-coupled circular dichroism using discrete silver nanorods as amplifiers

Remarkable chiral amplification in plasmon-coupled circular dichroism spectroscopy (CD) is demonstrated by using discrete Ag nanorods as amplifiers. An unprecedented CD enhancement factor of over 3000 times is achieved without resonant or near-resonant exciton-plasmon couplings.

Strong Light-Matter Interactions in Chiral Plasmonic-Excitonic Systems Assembled on DNA Origami

The exploitation of strong light-matter interactions in chiral plasmonic nanocavities may enable exceptional physical phenomena and lead to potential applications in nanophotonics, information communication, etc. Therefore, a deep understanding of strong light-matter interactions in chiral plasmonic-excitonic (plexcitonic) systems constructed by a chiral plasmonic nanocavity and molecular excitons is urgently needed. Herein, we systematically studied the strong light-matter interactions in gold nanorod-based chiral plexcitonic systems assembled on DNA origami. Rabi splitting and anticrossing behavior were observed in circular dichroism spectra, manifesting chiroptical characteristic hybridization. The bisignate line shape of the circular dichroism (CD) signal allows the accurate discrimination of hybrid modes. A large Rabi splitting of ∼205/∼199 meV for left-handed/right-handed plexcitonic nanosystems meets the criterion of strong coupling. Our work deepens the understanding of light-matter interactions in chiral plexcitonic nanosystems and will facilitate the development of chiral quantum optics and chiroptical devices.

Long- and short-ranged chiral interactions in DNA-assembled plasmonic chains

Circular dichroism (CD) has long been used to trace chiral molecular states and changes of protein configurations. In recent years, chiral plasmonic nanostructures have shown potential for applications ranging from pathogen sensing to novel optical materials. The plasmonic coupling of the individual elements of such metallic structures is a crucial prerequisite to obtain sizeable CD signals. We here identify and implement various coupling entities-chiral and achiral-to demonstrate chiral transfer over distances close to 100 nm. The coupling is realized by an achiral nanosphere situated between a pair of gold nanorods that are arranged far apart but in a chiral fashion using DNA origami. The transmitter particle causes a strong enhancement of the CD response, the emergence of an additional chiral feature at the resonance frequency of the nanosphere, and a redshift of the longitudinal plasmonic resonance frequency of the nanorods. Matching numerical simulations elucidate the intricate chiral optical fields in complex architectures.

DNA Origami-Enabled Plasmonic Sensing

The reliable programmability of DNA origami makes it an extremely attractive tool for bottom-up self-assembly of complex nanostructures. Utilizing this property for the tuned arrangement of plasmonic nanoparticles holds great promise particularly in the field of biosensing. Plasmonic particles are beneficial for sensing in multiple ways, from enhancing fluorescence to enabling a visualization of the nanoscale dynamic actuation via chiral rearrangements. In this Perspective, we discuss the recent developments and possible future directions of DNA origami-enabled plasmonic sensing systems. We start by discussing recent advancements in the area of fluorescence-based plasmonic sensing using DNA origami. We then move on to surface-enhanced Raman spectroscopy sensors followed by chiral sensing, both utilizing DNA origami nanostructures. We conclude by providing our own views on the future prospects for plasmonic biosensors enabled using DNA origami.

X-ray-Based Techniques to Study the Nano-Bio Interface

X-ray-based analytics are routinely applied in many fields, including physics, chemistry, materials science, and engineering. The full potential of such techniques in the life sciences and medicine, however, has not yet been fully exploited. We highlight current and upcoming advances in this direction. We describe different X-ray-based methodologies (including those performed at synchrotron light sources and X-ray free-electron lasers) and their potentials for application to investigate the nano-bio interface. The discussion is predominantly guided by asking how such methods could better help to understand and to improve nanoparticle-based drug delivery, though the concepts also apply to nano-bio interactions in general. We discuss current limitations and how they might be overcome, particularly for future use in vivo.