Enantiospecific Molecular Fingerprinting Using Potential-Modulated Surface-Enhanced Raman Scattering to Achieve Label-Free Chiral Differentiation

Ultrasensitive Surface-Enhanced Raman Scattering Platform for Protein Detection via Active Delivery to Nanogaps as a Hotspot

Surface-enhanced Raman scattering (SERS) is an attractive technique in molecular detection with high sensitivity and label-free characteristics. However, its use in protein detection is limited by the large volume of proteins, hindering its approach to the narrow spaces of hotspots. In this study, we fabricated a Au nanoTriangle plate Array on Gel (AuTAG) as an SERS substrate by attaching a Au nanoTriangle plate (AuNT) arrangement on a thermoresponsive hydrogel surface. The AuTAG acts as an actively tunable plasmonic device, on which the interparticle distance is altered by controlling temperature via changes in hydrogel volume. Further, we designed a Gel Filter Trapping (GFT) method as an active protein delivery strategy based on the characteristics of hydrogels, which can absorb water and separate biopolymers through their three-dimensional (3D) polymer networks. On the AuTAGs, fabricated with AuNTs modified with charged surface ligands to prevent the nonspecific adsorption of analytes to particles, the GFT method helped the delivery of proteins to hotspot areas on the AuNT arrangement. This combination of a AuTAG substrate and the GFT method enables ultrahigh sensitivity for protein detection by SERS up to a single-molecule level as well as a wide quantification concentration range of 6 orders due to their geometric advantages.

Cu2+-Dominated Chirality Transfer from Chiral Molecules to Concave Chiral Au Nanoparticles

Foreign ions as additives are of great significance for realizing excellent control over the morphology of noble metal nanostructures in the state-of-the-art seed-mediated growth method; however, they remain largely unexplored in chiral synthesis. Here, we report on a Cu2+-dominated chiral growth strategy that can direct the growth of concave chiral Au nanoparticles with C3-dominant chiral centers. The introduction of trace amounts of Cu2+ ions in the seed-mediated chiral growth process is found to dominate the chirality transfer from chiral molecules to chiral nanoparticles, leading to the formation of chiral nanoparticles with a concave VC geometry. Both experimental and theoretical results further demonstrate the correlation between the nanoparticle structure and optical chirality for the concave chiral nanoparticle. The Cu2+ ion is found to dominate the chiral growth by selectively activating the deposition of Au atoms along the [110] and [111] directions, facilitating the formation of the concave VC. We further demonstrate that the Cu2+-dominated chiral growth strategy can be employed to generate a variety of concave chiral nanoparticles with enriched geometric chirality and desired chiroptical properties. Concave chiral nanoparticles also exhibit appealing catalytic activity and selectivity toward electrocatalytic oxidation of enantiomers in comparison to helicoidal nanoparticles. The ability to tune the geometric chirality in a controlled manner by simply manipulating the Cu2+ ions as additives opens up a promising strategy for creating chiral nanomaterials with increasing architectural diversity for chirality-dependent optical and catalytic 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.

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.

Microfluidic chip and chiroptical gold nanoparticle-based colorimetric sensor for enantioselective detection of L-tryptophan

Herein, we introduce a novel integrated system that merges an enantio-discriminative bio-MOF-packed centrifugal microfluidic chip made from PDMS with a user-friendly on-site colorimetric sensor. This innovative approach enables the precise enantioselective recognition of L-tryptophane (L-Trp). This chiral recognition probe was successfully synthesized through meticulous control of nano-ovals-shaped gold nanoparticles morphology and surface passivation. The operational factor of this methodology was optimized to ensure simplicity, practicality, and efficiency. This optimization led to reduced reagent consumption and instantaneous analytical feedback. The integrated system was effectively applied for enantioselective separation and quantification of L-Trp across an extensive linear range of 50 μM-1.5 mM, impressive limit of detection as low as 15 μM. It is noteworthy that this integrated system demonstrated desirable selectivity even in the presence of similar biomolecules, showcasing its robust performance and rapid detection capability. Further extended the application of this strategy to exceptional performance across enantioselective sensing of L-Trp in various sample matrices, comprising bovine serum albumin, bovine milk, blood plasma and urine samples. This integrated microfluidic sample pretreatment, chiroptical sensing, and on-site signal recording with a smartphone hold tremendous potential for widespread implementation, practical applications engaging healthcare and environmental, food safety, and point-of-needs analysis, facilitating successive solution mixing and colorimetric detection.

Surface Topographical Engineering of Chiral Au Nanocrystals with Chiral Hot Spots for Plasmon-Enhanced Chiral Discrimination

Surface roughness in chiral plasmonic nanostructures generates asymmetrical localized electromagnetic fields, which hold great promise for applications in chiral recognition, chiroptical spectroscopic sensing, and enantioselective photocatalysis. In this study, we develop a surface topographical engineering approach to precisely manipulate the surface structures of chiral Au nanocrystals. Through carefully controlling the amounts of l- or d-cystine (Cys) and the seed solution in the growth process, we successfully synthesize chiral Au nanocrystals with highly disordered, ordered, and less ordered wrinkled surfaces. An underlying principle governing the relationship between surface roughness, orderliness, and chiroptical response is also proposed. More importantly, the chiral ordered wrinkles on the surfaces of the nanocrystals generate asymmetrical localized electronic fields with enhanced intensity, which achieve excellent plasmon-enhanced chiral discrimination ability for penicillamine (Pen) enantiomers. This work offers exciting prospects for manipulating the surface structures of chiral nanocrystals and designing highly sensitive plasmon-enhanced enantioselective sensors with chiral hot spots.

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.

Semi-wrapped gold nanoparticles for surface-enhanced Raman scattering detection

Researchers have struggled to develop highly reliable and sensitive surface-enhanced Raman scattering (SERS) substrates for detecting compounds in complicated systems. In this work, a strategy by constructing Au cores with incompletely wrapped Prussian blue (PB) for highly reliable and sensitive SERS substrate is proposed. The wrapped PB layers can provide the internal standard (IS) to calibrate the SERS signal floatation, whereas the exposed surface of Au cores offers the enhancement effect. The balance between the signal self-calibration and enhancement (hence the trade-off between SERS reliability and sensitivity) is obtained by the approximate semi-wrapping configuration of PB layers on Au cores (i.e., SW-Au@PB). The proposed SW-Au@PB nanoparticles (NPs) exhibit the similar enhancement factor as the pristine Au NPs and contribute to the ultralow RSD (8.55%) of calibrated SERS signals using R6G as probe molecules. The simultaneously realized reliability and sensitivity of SW-Au@PB NPs also enables the detection of hazardous pesticide residues such as paraquat and thiram in herbal plants, with the average detection accuracy up to 92%. Overall, this work mainly provides a controllable synthetic strategy for incompletely wrapped NPs, and most importantly, explores the potential with a proof-of-concept practical application in accurate and sensitive Raman detection of hazardous substances with varying solubility.

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.

Tunable Reversal of Circular Dichroism in the Seed-Mediated Growth of Bichiral Plasmonic Nanoparticles

Plasmonic nanoparticles with an intrinsic chiral structure have emerged as a promising chiral platform for applications in biosensing, medicine, catalysis, separation, and photonics. Quantitative understanding of the correlation between nanoparticle structure and optical chirality becomes increasingly important but still represents a significantly challenging task. Here we demonstrate that tunable signal reversal of circular dichroism in the seed-mediated chiral growth of plasmonic nanoparticles can be achieved through the hybridization of bichiral centers without inverting the geometric chirality. Both experimental and theoretical results demonstrated the opposite sign of circular dichroism of two different bichiral geometries. Chiral molecules were found to not only contribute to the chirality transfer from molecules to nanoparticles but also manipulate the structural evolution of nanoparticles that synergistically drive the formation of two different chiral centers. By deliberately adjusting the concentration of chiral molecules and other synthetic parameters, such as the reducing agent concentration, the capping surfactant concentration, and the amount of Au precursor, we have been able to fine-tune the circular dichroism reversal of bichiral Au nanoparticles. We further demonstrate that the structure of chiral molecules and the crystal structure of Au seeds play crucial roles in the formation of Au nanoparticles with bichiral centers. The insights gained from this work not only shed light on the underlying mechanisms dictating the intriguing geometric and chirality evolution of bichiral plasmonic nanoparticles but also provide an important knowledge framework that guides the rational design of bichiral plasmonic nanostructures toward chiroptical applications.

Chiral Detection of Glucose: An Amino Acid-Assisted Surface-Enhanced Raman Scattering Strategy Showing Opposite Enantiomeric Effects on SERS Signals

It is of considerable concern to establish chiral detection methods for revealing enantioselective interactions among chiral molecules. Surface-enhanced Raman scattering (SERS) spectroscopy is sensitive to molecular interaction due to bond variations. However, its application in chiral detection is underexplored. Inspired by the chiral selectivity toward glucose and amino acids in life, we herein propose a SERS strategy based on molecular interactions for the discrimination of d- and l-glucose (Glu) using chiral phenylalanine (Phe) decorated on gold nanoparticles as a chirality selector and Raman reporter. Interestingly, the SERS signal of l-Phe is enhanced by d-Glu but suppressed by l-Glu. In contrast, the SERS signal of d-Phe is increased by l-Glu but decreased by d-Glu. According to the above-observed intensity change (ΔI) of the SERS signal of Phe induced by Glu, it is easy to determine the chiral configurations (judged by the positive or negative sign of ΔI), enantiomeric excess (ee) values, and concentrations (estimated by the magnitude of ΔI) of Glu. Taking advantage of the high SERS enhancement and opposite enantiomeric effects on SERS signals, the proposed strategy enables enantiomeric discrimination at a low Glu concentration (10^-6 mol/L) and is further exerted for the noninvasive detection of d-/l-Glu in saliva samples. In contrast, the common chiroptical analysis tool of circular dichroism (CD) spectroscopy failed to directly detect Glu enantiomers.

Chiral molecular imprinting-based SERS detection strategy for absolute enantiomeric discrimination

Chiral discrimination is critical in environmental and life sciences. However, an ideal chiral discrimination strategy has not yet been developed because of the inevitable nonspecific binding entity of wrong enantiomers or insufficient intrinsic optical activities of chiral molecules. Here, we propose an "inspector" recognition mechanism (IRM), which is implemented on a chiral imprinted polydopamine (PDA) layer coated on surface-enhanced Raman scattering (SERS) tag layer. The IRM works based on the permeability change of the imprinted PDA after the chiral recognition and scrutiny of the permeability by an inspector molecule. Good enantiomer can specifically recognize and fully fill the chiral imprinted cavities, whereas the wrong cannot. Then a linear shape aminothiol molecule, as an inspector of the recognition status is introduced, which can only percolate through the vacant and nonspecifically occupied cavities, inducing the SERS signal to decrease. Accordingly, chirality information exclusively stems from good enantiomer specific binding, while nonspecific recognition of wrong enantiomer is curbed. The IRM benefits from sensitivity and versatility, enabling absolute discrimination of a wide variety of chiral molecules regardless of size, functional groups, polarities, optical activities, Raman scattering, and the number of chiral centers.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical applications.

High-Throughput Surface-Enhanced Raman Scattering for Screening Chemical Sensor Candidates Enabled by Bipolar Electrochemistry

A variety of hydrothermal or electrochemical methods have been explored to prepare noble metal nanostructures as surface-enhanced Raman scattering (SERS) substrates. However, most of those metallic nanoarrays are structurally homogeneous, which makes it laborious to select the high-performance substrates for particular Raman sensing purposes. Here, a high-throughput SERS imaging strategy is demonstrated for the first time for screening chemical sensors with sub-nanomolar sensitivities. Bipolar electrochemistry was applied to generate Au or Au-Ag gradient nanoarrays with diverse chemical compositions, morphologies, and particle dimensions ranging from several nanometers to micrometers. The selected "hot-spots" on the Au-Ag alloy nanoarray exhibited a 660-fold enhancement in SERS intensity compared to those on the pure Au gradient nanoarray. The SERS screening of 4-aminothiophenol, 4-nitrothiophenol, and 4-mercaptobenzoic acid was carried out that provided a limit of detection (LOD) between 1 and 5 pM. The distinctive LODs among three thiophenolic Raman probes are ascribed to the differences in the affinity of the probe to the alloy, orientation of the metal-ligand monolayer, or plasmonic environment of the nanoarray surface. As a continuous, rapid, and cost-effective manner to fabricate transitional nanostructures and screen out SERS responsive sites, this method not only facilitates controllable synthesis of noble metal nanoarrays but has the potential to provide an alternative tool for ultrasensitive chemical sensing on a wide range of bimetallic substrates.

Applications of surface-enhanced Raman spectroscopy in environmental detection

As the human population grows, the anthropogenic impacts from various agricultural and industrial processes produce unwanted contaminants in the environment. The accurate, sensitive and rapid detection of such contaminants is vital for human health and safety. Surface-enhanced Raman spectroscopy (SERS) is a valuable analytical tool with wide applications in environmental contaminant monitoring. The aim of this review is to summarize recent advancements within SERS research as it applies to environmental detection, with a focus on research published or accessible from January 2021 through December 2021 including early-access publications. Our goal is to provide a wide breadth of information that can be used to provide background knowledge of the field, as well as inform and encourage further development of SERS techniques in protecting environmental quality and safety. Specifically, we highlight the characteristics of effective SERS nanosubstrates, and explore methods for the SERS detection of inorganic, organic, and biological contaminants including heavy metals, pharmaceuticals, plastic particles, synthetic dyes, pesticides, viruses, bacteria and mycotoxins. We also discuss the current limitations of SERS technologies in environmental detection and propose several avenues for future investigation. We encourage researchers to fill in the identified gaps so that SERS can be implemented in a real-world environment more effectively and efficiently, ultimately providing reliable and timely data to help and make science-based strategies and policies to protect environmental safety and public health.

Emerging nanosensor platforms and machine learning strategies toward rapid, point-of-need small-molecule metabolite detection and monitoring

Speedy, point-of-need detection and monitoring of small-molecule metabolites are vital across diverse applications ranging from biomedicine to agri-food and environmental surveillance. Nanomaterial-based sensor (nanosensor) platforms are rapidly emerging as excellent candidates for versatile and ultrasensitive detection owing to their highly configurable optical, electrical and electrochemical properties, fast readout, as well as portability and ease of use. To translate nanosensor technologies for real-world applications, key challenges to overcome include ultralow analyte concentration down to ppb or nM levels, complex sample matrices with numerous interfering species, difficulty in differentiating isomers and structural analogues, as well as complex, multidimensional datasets of high sample variability. In this Perspective, we focus on contemporary and emerging strategies to address the aforementioned challenges and enhance nanosensor detection performance in terms of sensitivity, selectivity and multiplexing capability. We outline 3 main concepts: (1) customization of designer nanosensor platform configurations via chemical- and physical-based modification strategies, (2) development of hybrid techniques including multimodal and hyphenated techniques, and (3) synergistic use of machine learning such as clustering, classification and regression algorithms for data exploration and predictions. These concepts can be further integrated as multifaceted strategies to further boost nanosensor performances. Finally, we present a critical outlook that explores future opportunities toward the design of next-generation nanosensor platforms for rapid, point-of-need detection of various small-molecule metabolites.

Overview of the current status on emerging, multi-faceted nanosensor platform designs and data analysis strategies for rapid, point-of-need detection and monitoring of small-molecule metabolites.

Improved discrimination of phenylalanine enantiomers by surface enhanced Raman scattering assay: molecular insight into chiral interaction

Chiral interaction-based SERS discrimination of phenylalanine (Phe) enantiomers, with the Raman scattering enhancement degree of d-Phe being 50-fold greater than that of l-Phe.

Selective area grown AlInGaN nanowire arrays with core-shell structures for photovoltaics on silicon

To pave the way for InGaN-on-Si integrated photovoltaics, uniform and close-packed n-GaN/(Al)InGaN/p-GaN nanowire (NW) arrays with a ∼0.29 μm thick absorption segment of ∼2.35 eV energy bandgap were fabricated on a Si substrate using Ti-mask selective area growth (SAG) in a molecular beam epitaxy (MBE) chamber. Instead of using thick and insulting buffer layers, this SAG process was realized by employing a 3 nm AlN/GaN: Ge buffer layer to facilitate electrical and thermal conduction between NWs and Si. Scanning transmission electron microscopy and high-resolution electron energy loss spectroscopy mapping revealed the discontinuities of AlN and the embedments of GaN:Ge which contribute to a negligible resistance of the NWs-on-Si interface. AlInGaN active segment exhibits core-shell structures, which suppress nonradiative surface recombination at NW surfaces. Working of AlInGaN core-shell NW solar cells was demonstrated with improved open-circuit voltage (Voc) and higher energy conversion efficiency (η) than those reported for InGaN NW solar cells. Stable output characteristics including the Voc of 1.41 V and η of 2.46% were obtained under 30-Sun illuminations. Such NWs-on-Si devices use Si substrate as the bottom electrode. With a low series resistance of ∼1 Ω, this work paves the way to monolithically integrate MBE-SAG III-nitride devices and Si-based electronics, such as Si solar cells and CMOS devices.