Quantitative SERS using the sequestration of small molecules inside precise plasmonic nanoconstructs

Plasmonic trimers designed as SERS-active chemical traps for subtyping of lung tumors

Plasmonic materials can generate strong electromagnetic fields to boost the Raman scattering of surrounding molecules, known as surface-enhanced Raman scattering. However, these electromagnetic fields are heterogeneous, with only molecules located at the 'hotspots', which account for ≈ 1% of the surface area, experiencing efficient enhancement. Herein, we propose patterned plasmonic trimers, consisting of a pair of plasmonic dimers at the bilateral sides and a trap particle positioned in between, to address this challenge. The trimer configuration selectively directs probe molecules to the central traps where 'hotspots' are located through chemical affinity, ensuring a precise spatial overlap between the probes and the location of maximum field enhancement. We investigate the Raman enhancement of the Au@Al2O3-Au-Au@Al2O3 trimers, achieving a detection limit of 10-14 M of 4-methylbenzenethiol, 4-mercaptopyridine, and 4-aminothiophenol. Moreover, single-molecule SERS sensitivity is demonstrated by a bi-analyte method. Benefiting from this sensitivity, our approach is employed for the early detection of lung tumors using fresh tissues. Our findings suggest that this approach is sensitive to adenocarcinoma but not to squamous carcinoma or benign cases, offering insights into the differentiation between lung tumor subtypes.

Surface-Enhanced Raman Spectroscopy: Current Understanding, Challenges, and Opportunities

While surface-enhanced Raman spectroscopy (SERS) has experienced substantial advancements since its discovery in the 1970s, it is an opportunity to celebrate achievements, consider ongoing endeavors, and anticipate the future trajectory of SERS. In this perspective, we encapsulate the latest breakthroughs in comprehending the electromagnetic enhancement mechanisms of SERS, and revisit CT mechanisms of semiconductors. We then summarize the strategies to improve sensitivity, selectivity, and reliability. After addressing experimental advancements, we comprehensively survey the progress on spectrum-structure correlation of SERS showcasing their important role in promoting SERS development. Finally, we anticipate forthcoming directions and opportunities, especially in deepening our insights into chemical or biological processes and establishing a clear spectrum-structure correlation.

Visualization-enhanced under-oil open microfluidic system for in situ characterization of multi-phase chemical reactions

Under-oil open microfluidic system, utilizing liquid-liquid boundaries for confinements, offers inherent advantages including clogging-free flow channels, flexible access to samples, and adjustable gas permeation, making it well-suited for studying multi-phase chemical reactions that are challenging for closed microfluidics. However, reports on the novel system have primarily focused on device fabrication and functionality demonstrations within biology, leaving their application in broader chemical analysis underexplored. Here, we present a visualization-enhanced under-oil open microfluidic system for in situ characterization of multi-phase chemical reactions with Raman spectroscopy. The enhanced system utilizes a semi-transparent silicon (Si) nanolayer over the substrate to enhance visualization in both inverted and upright microscope setups while reducing Raman noise from the substrate. We validated the system's chemical stability and capability to monitor gas evolution and gas-liquid reactions in situ. The enhanced under-oil open microfluidic system, integrating Raman spectroscopy, offers a robust open-microfluidic platform for label-free molecular sensing and real-time chemical/biochemical process monitoring in multi-phase systems.

Observing π-Au Interaction between Aromatic Molecules and Single Au Nanodimers with a Subnanometer Gap by SERS

Interface interaction between aromatic molecules and noble metals plays a prominent role in fundamental science and technological applications. However, probing π-metal interactions under ambient conditions remains challenging, as it requires characterization techniques to have high sensitivity and molecular specificity without any restrictions on the sample. Herein, the interactions between polycyclic aromatic hydrocarbon (PAH) molecules and Au nanodimers with a subnanometer gap are investigated by surface-enhanced Raman spectroscopy (SERS). A cleaner and stronger plasmonic field of subnanometer gap Au nanodimer structures was constructed through solvent extraction. High sensitivity and strong π-Au interaction between PAHs and Au nanodimers are observed. Additionally, the density functional theory calculation confirmed the interactions of PAHs physically absorbed on the Au surface; the binding energy and differential charge further theoretically indicated the correlation between the sensitivity and the number of PAH rings, which is consistent with SERS experimental results. This work provides a new method to understand the interactions between aromatic molecules and noble metal surfaces in an ambient environment, also paving the way for designing the interfaces in the fields of catalysis, sensors, and molecular electronics.

Microwave Synthesis of Au Nanoparticles in the Presence of Tetrahydrothiophenocucurbituril

The preparation of gold nanoparticles (AuNPs) from tetrachloroauric acid in the presence of tetrahydrothiophenocucurbit[n]uril (THTmQ[n]) has been effectively achieved in a microwave reactor. The reaction was performed in the presence of an excess of the tetrahydrothiopheno function in a partial reductant role, while the remainder formed AuNP-THTmQ[n] conjugates after the reduction was completed with formic acid. An affinity for the AuNPs by the THTmQ[n] was observed in the purification of the NPs via centrifugation, removal of the supernatant and resuspension of the conjugate.

Supramolecular Click Chemistry for Surface Modification of Quantum Dots Mediated by Cucurbit[7]uril

Cucurbiturils (CBs), barrel-shaped macrocyclic molecules, are capable of self-assembling at the surface of nanomaterials in their native state, via their carbonyl-ringed portals. However, the symmetrical two-portal structure typically leads to aggregated nanomaterials. We demonstrate that fluorescent quantum dot (QD) aggregates linked with CBs can be broken-up, retaining CBs adsorbed at their surface, via inclusion of guests in the CB cavity. Simultaneously, the QD surface is modified by a functional tail on the guest, thus the high affinity host-guest binding (logKa > 9) enables a non-covalent, click-like modification of the nanoparticles in aqueous solution. We achieved excellent modification efficiency in several functional QD conjugates as protein labels. Inclusion of weaker-binding guests (logKa = 4-6) enables subsequent displacement with stronger binders, realising modular switchable surface chemistries. Our general "hook-and-eye" approach to host-guest chemistry at nanomaterial interfaces will lead to divergent routes for nano-architectures with rich functionalities for theranostics and photonics in aqueous systems.

SERS Sensing of Dopamine with Fe(III)-Sensitized Nanogaps in Recleanable AuNP Monolayer Films

Sensing of neurotransmitters (NTs) down to nm concentrations is demonstrated by utilizing self-assembled monolayers of plasmonic 60 nm Au nanoparticles in close-packed arrays immobilized onto glass substrates. Multiplicative surface-enhanced Raman spectroscopy enhancements are achieved by integrating Fe(III) sensitizers into the precisely-defined <1 nm nanogaps, to target dopamine (DA) sensing. The transparent glass substrates allow for efficient access from both sides of the monolayer aggregate films by fluid and light, allowing repeated sensing in different analytes. Repeated reusability after analyte sensing is shown through oxygen plasma cleaning protocols, which restore pristine conditions for the nanogaps. Examining binding competition in multiplexed sensing of two catecholamine NTs, DA and epinephrine, reveals their bidentate binding and their interactions. These systems are promising for widespread microfluidic integration enabling a wide range of continuous biofluid monitoring for applications in precision health.

High-Yield Gold Nanohydrangeas on Three-Dimensional Carbon Nanotube Foams for Surface-Enhanced Raman Scattering Sensors

Recently, carbon nanomaterial-supported plasmonic nanocrystals used as high-performance surface-enhanced Raman scattering (SERS) substrates have attracted increasing attention due to their ultra-high sensitivity of detection. However, most of the work focuses on the design of 2-D planar substrates with traditional plasmonic structures, such as nanoparticles, nanorods, nanowires, and so forth. Here, we report a novel strategy for the preparation of high-yield Au nanohydrangeas on three-dimensional porous polydopamine (PDA)/polyvinyl alcohol (PVA)/carbon nanotube (CNT) foams. The structures and growth mechanisms of these specific Au nanocrystals are systematically investigated. PDA plays the role of both a reducing agent as well as an anchoring site for Au nanohydrangeas' growth. We also show that the ratio of surfactant KBr to the gold precursor (HAuCl4) is key to obtain these structures in a manner of high production. Moreover, the substrate of the CNT foam-Au nanohydrangea hybrid can be employed as SERS sensors and can detect the analytes down to 10-9 M.

Cucurbit[8]uril-mediated SERS plasmonic nanostructures with sub-nanometer gap for the identification and determination of estrogens

The SERS intensity of analytes is primarily influenced by the density and distribution of hotspots, which are often difficult to manipulate or regulate. In this study, cucurbit[8]uril (CB[8]), a kind of rigid macrocyclic molecule, was introduced to achieve ~ 1-nm nanogap between gold nanoparticles to increase the density of SERS hotspots. Three kinds of estrogens (estrone (E1), bisphenol A (BPA), and hexestrol (DES)) which are molecules with weak SERS signals were targeted in the hotspots by CB[8] to further improve the sensitivity and selectivity of SERS. It was demonstrated that CB[8] can link gold nanoparticles together through carbonyl groups. In addition, the host-guest interaction of CB[8] and estrogens was proved from the nuclear magnetic resonance hydrogen and infrared spectra. In the presence of CB[8], the SERS intensities of E1, BPA, and DES were increased to 19-fold, 74-fold, and 4-fold, respectively, and the LOD is 3.75 µM, 1.19 µM, and 8.26 µM, respectively. Furthermore, the proposed SERS method was applied to actual milk sample analysis with recoveries of E1 (85.0 ~ 112.8%), BPA (83.0 ~ 103.7%), and DES (62.6 ~ 132.0%). It is expected that the proposed signal enlarging strategy can be applied to  other analytes after further development.

Cross-Wavelength Hierarchical Metamaterials Enabled for Trans-Scale Molecules Detection Simultaneously

Metamaterials have attracted increasing attention in sensing applications. However, the critical feature sizes of meta-atom span several orders of magnitude in length scale, almost all the metamaterials are designed to operate at limited bands. It is challenging for a single type of meta-atom with ultra-broadband adaptability. Inspired by the natural hierarchical architectures, herein, the authors introduce a new constructing scheme of cross-wavelength hierarchical metamaterials with a single type of meta-atom that can realize enhancement of terahertz (THz) resonance and surface-enhanced Raman scattering (SERS) at the same time. By combining multiple subwavelength structures at different hierarchical levels into a single meta-atom, the obtained metamaterial can operate in two frequencies and realize multiple functionalities. Armed with this hierarchical metamaterial, detecting analytes as small as sub-nanoscale chemical molecules or as big as microscale biomolecules simultaneously can be realized in one single metamaterial for the first time. As a proof-of-concept example, a smart sensory packaging is developed, which allowed them to real-time monitor the kinetic growth of pathogenic bacteria and their metabolites in food without opening the packaging. They believe that their work will provide a valuable example that satisfies the unmet need for multiscale functional meta-devices.

Microfluidics and surface-enhanced Raman spectroscopy, a win-win combination?

With the continuous development in nanoscience and nanotechnology, analytical techniques like surface-enhanced Raman spectroscopy (SERS) render structural and chemical information of a variety of analyte molecules in ultra-low concentration. Although this technique is making significant progress in various fields, the reproducibility of SERS measurements and sensitivity towards small molecules are still daunting challenges. In this regard, microfluidic surface-enhanced Raman spectroscopy (MF-SERS) is well on its way to join the toolbox of analytical chemists. This review article explains how MF-SERS is becoming a powerful tool in analytical chemistry. We critically present the developments in SERS substrates for microfluidic devices and how these substrates in microfluidic channels can improve the SERS sensitivity, reproducibility, and detection limit. We then introduce the building materials for microfluidic platforms and their types such as droplet, centrifugal, and digital microfluidics. Finally, we enumerate some challenges and future directions in microfluidic SERS. Overall, this article showcases the potential and versatility of microfluidic SERS in overcoming the inherent issues in the SERS technique and also discusses the advantage of adding SERS to the arsenal of microfluidics.

Revealing unconventional host-guest complexation at nanostructured interface by surface-enhanced Raman spectroscopy

Interfacial host-guest complexation offers a versatile way to functionalize nanomaterials. However, the complicated interfacial environment and trace amounts of components present at the interface make the study of interfacial complexation very difficult. Herein, taking the advantages of near-single-molecule level sensitivity and molecular fingerprint of surface-enhanced Raman spectroscopy (SERS), we reveal that a cooperative effect between cucurbit[7]uril (CB[7]) and methyl viologen (MV2+2I-) in aggregating Au NPs originates from the cooperative adsorption of halide counter anions I-, MV2+, and CB[7] on Au NPs surface. Moreover, similar SERS peak shifts in the control experiments using CB[n]s but with smaller cavity sizes suggested the occurrence of the same guest complexations among CB[5], CB[6], and CB[7] with MV2+. Hence, an unconventional exclusive complexation model is proposed between CB[7] and MV2+ on the surface of Au NPs, distinct from the well-known 1:1 inclusion complexation model in aqueous solutions. In summary, new insights into the fundamental understanding of host-guest interactions at nanostructured interfaces were obtained by SERS, which might be useful for applications related to host-guest chemistry in engineered nanomaterials.

Dual-Purpose SERS Sensor for Selective Determination of Polycyclic Aromatic Compounds via Electron Donor-Acceptor Traps

Toxic, carcinogenic, and mutagenic properties of polycyclic aromatic hydrocarbons (PAHs) and environmental pollution caused by polycyclic aromatic sulfur heterocycles (PASHs) postulate the importance of their selective and sensitive determination in environmental and oil fuel samples. Surface-enhanced Raman spectroscopy (SERS) opens up an avenue toward multiplex analysis of complex mixtures, however not every molecule gives high enhancement factors and, thus, cannot be reliably detected via SERS. However, the sensitivity can be drastically increased by additional resonant enhancement as a result of the analyte absorption band overlapping with the surface plasmon band of nanoparticles (NPs) and the laser excitation wavelength. Using this idea, we developed a dual-purpose SERS sensor based on trapping the target PAHs and PASHs into colored charge-transfer complexes (CTCs) with selected organic π-acceptor molecules on the surface of AgNPs. Studying, computing, and then comparing stability constants of the formed CTC served as a powerful explanation and prediction tool for a wise choice of π-acceptor indicator systems for the further silver surface modification. Moreover, we show that CTC formation can be effectively utilized for increasing both selectivity and sensitivity by simple liquid-liquid extraction prior to SERS measurements. For the first time, the dual-purpose SERS sensor allowed determination of two different classes of polycyclic aromatic fuel components down to 10 nM concentration, lower than that restricted by the ASTM regulation, and demonstrated multi-purpose capabilities of the developed approach.

Plasmonic Electronic Raman Scattering as Internal Standard for Spatial and Temporal Calibration in Quantitative Surface-Enhanced Raman Spectroscopy

Ultrasensitive surface-enhanced Raman spectroscopy (SERS) still faces difficulties in quantitative analysis because of its susceptibility to local optical field variations at plasmonic hotspots in metallo-dielectric nanostructures. Current SERS calibration approaches using Raman tags have inherent limitations due to spatial occupation competition with analyte molecules, spectral interference with analyte Raman peaks, and photodegradation. Herein, we report that plasmon-enhanced electronic Raman scattering (ERS) signals from metal can serve as an internal standard for spatial and temporal calibration of molecular Raman scattering (MRS) signals from analyte molecules at the same hotspots, enabling rigorous quantitative SERS analysis. We observe a linear dependence between ERS and MRS signal intensities upon spatial and temporal variations of excitation optical fields, manifesting the |E|4 enhancements for both ERS and MRS processes at the same hotspots in agreement with our theoretical prediction. Furthermore, we find that the ERS calibration's performance limit can result from orientation variations of analyte molecules at hotspots.

Toward Understanding CB[7]-Based Supramolecular Diels-Alder Catalysis

Cucurbiturils (CBs) are robust and versatile macrocyclic compounds, often used as molecular hosts in complex supramolecular systems. In previous work, remarkable catalytic activity has been observed for asymmetric cycloadditions under very mild conditions. Herein, we investigate the nature of supramolecular catalysis using DFT calculations and QM/MM techniques. We discuss induced conformational changes, electrostatic shielding effects from the highly polar aqueous environment and cooperativity in hydrogen bonding of the substrates in explicit water using QM/MM simulation techniques. Our results show little specificity for the chosen molecules, suggesting an excellent opportunity to expand the scope for catalytic use of these supramolecular macrocyclic containers.

Capture of Phenylalanine and Phenylalanine-Terminated Peptides Using a Supramolecular Macrocycle for Surface-Enhanced Raman Scattering Detection

The cucurbit[n]uril (CB[n]) family of macrocycles are known to bind a variety of small molecules with high affinity. These motifs thus have promise in an ever-growing list of trace detection methods. Surface-enhanced Raman scattering (SERS) detection schemes employing CB[n] motifs exhibit increased sensitivity due to selective concentration of the analyte at the nanoparticle surface, coupled with the ability of CB[n] to facilitate the formation of well-defined electromagnetic hot spots. Herein, we report a CB[7] SERS assay for quantification of phenylalanine (Phe) and further demonstrate its utility for detecting peptides with an N-terminal Phe. The CB[7]-guest interaction improves the sensitivity 5-25-fold over direct detection of Phe using citrate-capped silver nanoparticle aggregates, enabling use of a portable Raman system. We further illustrate detection of insulin via binding of CB[7] to the N-terminal Phe residue on its B-chain, suggesting a general strategy for detecting Phe-terminated peptides of clinically relevant biomolecules.

Nanometer control in plasmonic systems through discrete layer-by-layer macrocycle-cation deposition

In this work, we demonstrate that coordination interactions between Fe3+ and cucurbit[7]uril (CB[7]) can be utilised to build up defined nanoscale spacing layers in metallic nanosystems. We begin by characterising the layer-by-layer deposition of CB[7] and FeCl3·6H2O coordination layers through the use of a Quartz-Crystal Microbalance (QCM) and contact angle measurements. We then apply this layered structure to accurately control the spacing, and thus optical properties, of gold nanoparticles in a Nanoparticle-on-Mirror (NPoM) structure, which is demonstrated via normalising plasmon resonance spectroscopy.

Urchin-like ZnO-nanorod arrays templated growth of ordered hierarchical Ag/ZnO hybrid arrays for surface-enhanced Raman scattering

Here, a universal strategy for the controllable synthesis of three dimensional (3D) hierarchical Ag/ZnO hybrid arrays based on the urchin-like ZnO-nanorod array template is presented. The urchin-like ZnO-nanorod arrays are first achieved by electrodepositing a high density of ZnO-nanorods onto the surface of highly hexagonally arranged arrays of polystyrene (PS) microspheres, and then Ag-nanoparticles (Ag-NPs) are assembled onto the surface of each ZnO-nanorod via photochemical reaction, ion sputtering, galvanic cell reaction deposition and electrochemical deposition, forming the ordered hierarchical Ag/ZnO hybrid arrays. The urchin-like Ag/ZnO hybrid arrays with well-ordered hierarchical morphology and high density 'hot spots' located in the sub-10 nm gaps between neighboring Ag-NPs on both the same ZnO-nanorod and neighboring ZnO-nanorods can be directly utilized as hybrid surface-enhanced Raman scattering (SERS) substrates with high SERS activity. This work provides a strategy for the rational assembly of well-ordered hierarchical noble metal/semiconductor hybrid arrays, which may open up many opportunities in areas such as catalysis, SERS, and biosensing.

Surface-enhanced Raman spectroscopy: benefits, trade-offs and future developments

Surface-enhanced Raman spectroscopy (SERS) is a vibrational spectroscopy technique with sensitivity down to the single molecule level that provides fine molecular fingerprints, allowing for direct identification of target analytes. Extensive theoretical and experimental research, together with continuous development of nanotechnology, has significantly broadened the scope of SERS and made it a hot research field in chemistry, physics, materials, biomedicine, and so on. However, SERS has not been developed into a routine analytical technique, and continuous efforts have been made to address the problems preventing its real-world application. The present minireview focuses on analyzing current and potential strategies to tackle problems and realize the SERS performance necessary for translation to practical applications.

Inhibiting Analyte Theft in Surface-Enhanced Raman Spectroscopy Substrates: Subnanomolar Quantitative Drug Detection

Quantitative applications of surface-enhanced Raman spectroscopy (SERS) often rely on surface partition layers grafted to SERS substrates to collect and trap-solvated analytes that would not otherwise adsorb onto metals. Such binding layers drastically broaden the scope of analytes that can be probed. However, excess binding sites introduced by this partition layer also trap analytes outside the plasmonic “hotspots”. We show that by eliminating these binding sites, limits of detection (LODs) can effectively be lowered by more than an order of magnitude. We highlight the effectiveness of this approach by demonstrating quantitative detection of controlled drugs down to subnanomolar concentrations in aqueous media. Such LODs are low enough to screen, for example, urine at clinically relevant levels. These findings provide unique insights into the binding behavior of analytes, which are essential when designing high-performance SERS substrates.

Selective Detection of Nitroexplosives Using Molecular Recognition within Self-Assembled Plasmonic Nanojunctions

We demonstrate that the reproducibility of sensors for nitroaromatics based on surface-enhanced Raman spectroscopy (SERS) can be significantly improved via a hierarchical aqueous self-assembly approach mediated by the multifunctional macrocyclic molecule cucurbit[7]uril (CB[7]). Our approach is enabled by the novel host-guest complexation between CB[7] and an explosive marker 2,4-dinitrotoluene (DNT). Binding studies are performed using experimental and computation techniques to quantify key binding parameters for the first time. This supramolecular complexation allows DNT to be positioned in close proximity to the plasmonic hotspots within aggregates of CB[7] and gold nanoparticles, resulting in significant SERS signals with a detection limit of ∼1 μM. The supramolecular ensemble is selective against a structurally similar nitroaromatics owing to the molecular-recognition nature of the complexation as well as tolerant against the presence of model organic contaminants that bind strongly to the SERS substrates.

Quantitative SERS by hot spot normalization - surface enhanced Rayleigh band intensity as an alternative evaluation parameter for SERS substrate performance

The performance of surface-enhanced Raman spectroscopy (SERS) substrates is typically evaluated by calculating an enhancement factor (EF). However, it is challenging to accurately calculate EF values since the calculation often requires the use of model analytes and requires assumptions about the number of analyte molecules within the laser excitation volume. Furthermore, the measured EF values are target analyte dependent and thus it is challenging to compare substrates with EF values obtained using different analytes. In this study, we propose an alternative evaluation parameter for SERS substrate performance that is based on the intensity of the surface plasmon enhanced Rayleigh band (I_Rayleigh) that originates from the amplified spontaneous emission (ASE) of the laser. Compared to the EF, I_Rayleigh reflects the enhancing capability of the substrate itself, is easy to measure without the use of any analytes, and is universally applicable for the comparison of SERS substrates. Six SERS substrates with different states (solid, suspended in liquid, and hydrogel), different plasmonic nanoparticle identities (silver and gold), as well as different nanoparticle sizes and shapes were used to support our hypothesis. The results show that there are excellent correlations between the measured SERS intensities and I_Rayleigh as well as between the SERS homogeneity and the variation of I_Rayleigh acquired with the six SERS substrates. These results suggest that I_Rayleigh can be used as an evaluation parameter for both SERS substrate efficiency and reproducibility.

Binary Thiol-Capped Gold Nanoparticle Monolayer Films for Quantitative Surface-Enhanced Raman Scattering Analysis

Surface-enhanced Raman scattering (SERS) can provide fingerprint information of analyte molecules with unparalleled sensitivity. However, quantitative analysis using SERS has remained one of the major challenges owing to the difficulty of obtaining reproducible SERS substrates with high-density hotspots. Here, we report the rational design and fabrication of a binary thiol-capped gold nanoparticle (AuNP) monolayer film (MLF) as a substrate for highly sensitive and quantitative SERS analysis. The two thiol ligands chemically bonded to the AuNPs play different roles: dodecanethiol with a long alkyl chain controls the interparticle gaps and electromagnetic coupling among AuNPs and 4-mercaptopyridine works as a Raman internal standard (IS). The binary thiol-capped AuNPs can self-assemble into an ordered MLF with high-density hotspots and uniformly distributed IS. The as-prepared MLF has been demonstrated as a reliable SERS substrate for quantitative detection of fungicide malachite green in aqueous solution, with a high enhancement factor (up to 3.3 × 10⁷) and a low detection limit (100 pM). Moreover, the MLF SERS substrate is flexible and transparent, which has enabled in situ detection of trace fungicide residues in a shrimp tissue.

Anomalously Large Spectral Shifts near the Quantum Tunnelling Limit in Plasmonic Rulers with Subatomic Resolution

The resonance wavelength of a coupled plasmonic system is extremely sensitive to the distance between its metallic surfaces, resulting in "plasmon rulers". We explore this behavior in the subnanometer regime using self-assembled monolayers of bis-phthalocyanine molecules in a nanoparticle-on-mirror (NPoM) construct. These allow unprecedented subangstrom control over spacer thickness via choice of metal center, in a gap-size regime at the quantum-mechanical limit of plasmonic enhancement. A dramatic shift in the coupled plasmon resonance is observed as the gap size is varied from 0.39 to 0.41 nm. Existing theoretical models are unable to account for the observed spectral tuning, which requires inclusion of the quantum-classical interface, emphasizing the need for new treatments of light at the subnanoscale.

Control of Silver Coating on Raman Label Incorporated Gold Nanoparticles Assembled Silica Nanoparticles

Signal reproducibility in surface-enhanced Raman scattering (SERS) remains a challenge, limiting the scope of the quantitative applications of SERS. This drawback in quantitative SERS sensing can be overcome by incorporating internal standard chemicals between the core and shell structures of metal nanoparticles (NPs). Herein, we prepared a SERS-active core Raman labeling compound (RLC) shell material, based on Au⁻Ag NPs and assembled silica NPs (SiO₂@Au@RLC@Ag NPs). Three types of RLCs were used as candidates for internal standards, including 4-mercaptobenzoic acid (4-MBA), 4-aminothiophenol (4-ATP) and 4-methylbenzenethiol (4-MBT), and their effects on the deposition of a silver shell were investigated. The formation of the Ag shell was strongly dependent on the concentration of the silver ion. The negative charge of SiO₂@Au@RLCs facilitated the formation of an Ag shell. In various pH solutions, the size of the Ag NPs was larger at a low pH and smaller at a higher pH, due to a decrease in the reduction rate. The results provide a deeper understanding of features in silver deposition, to guide further research and development of a strong and reliable SERS probe based on SiO₂@Au@RLC@Ag NPs.

Plasmon-induced optical control over dithionite-mediated chemical redox reactions

External-stimuli controlled reversible formation of radical species is of great interest for synthetic and supramolecular chemistry, molecular machinery, as well as emerging technologies ranging from (photo)catalysis and photovoltaics to nanomedicine. Here we show a novel hybrid colloidal system for light-driven reversible reduction of chemical species that, on their own, do not respond to light. This is achieved by the unique combination of photo-sensitive plasmonic aggregates and temperature-responsive inorganic species generating radicals that can be finally accepted and stabilised by non-photo-responsive organic molecules. In this system Au nanoparticles (NPs) self-assembled via sub-nm precise molecular spacers (cucurbit[n]urils) interact strongly with visible light to locally accelerate the decomposition of dithionite species (S₂O₄^2-) close to the NP interfaces. This light-driven process leads to the generation of inorganic radicals whose electrons can then be reversibly picked up by small organic acceptors, such as the methyl viologen molecules (MV²⁺) used here. During light-triggered plasmon- and heat-assisted generation of radicals, the S₂O₄^2- species work as a chemical 'fuel' linking photo-induced processes at the NP interfaces with redox chemistry in the surrounding water environment. By incorporating MV²⁺ as a Raman-active reporter molecule, the resulting optically-controlled redox processes can be followed in real-time.

CTAB-triggered Ag aggregates for reproducible SERS analysis of urinary polycyclic aromatic hydrocarbon metabolites

We have demonstrated a simple SERS assay for urinary hydroxylated polycyclic aromatic hydrocarbons (OH-PAHs) with high sensitivity and reproducibility.

Exploiting the chemical differences between Ag and Au colloids allows dramatically improved SERS detection of "non-adsorbing" molecules

In colloidal SERS only analytes that can spontaneously adsorb onto nanoparticles are detected. Therefore, considerable effort has been placed on modifying the surface properties of colloidal particles, particularly Ag particles, to promote the absorption of "difficult" analytes which do not spontaneously adsorb to as-prepared nanoparticles. In contrast, much less attention has been paid to the role which the identity of the underlying metal plays in the absorption since it is widely believed that the chemical properties of Ag and Au are very similar. This leads to the assumption that molecules which do not adsorb to Ag, such as hydrocarbons, will also not adsorb to aggregated Au colloids for SERS measurements. Here, we challenge this common perception by showing that SERS detection of "difficult" aromatic targets such as naphthalene, trinitrotoluene and 3,4-methylenedioxymethamphetamine which cannot be achieved even at >10^-3 M concentrations with bare aggregated Ag colloids is possible at ≥10^-8 M with unmodified aggregated Au colloids. For naphthalene and 3,4-methylenedioxymethamphetamine the detection limit obtained in this work with bare citrate-capped Au particles exceeds the previous best limit of detection obtained with surface-modified nanoparticles by an order of magnitude.

Synthesis of Spiked Plasmonic Nanorods with an Interior Nanogap for Quantitative Surface-Enhanced Raman Scattering Analysis

Realizing quantitative surface-enhanced Raman scattering (SERS) analysis is extremely helpful and challenging. Here, we utilize a facile method to synthesize spiked plasmonic nanorods with an interior gap. The Raman signal from the molecules embedded in the gap can be dramatically enhanced, leading to strong, stable, and reproducible SERS signals that can be used as an internal reference for quantitative SERS analysis. We demonstrate that the rough exterior surface has a good performance in enhancing the Raman signal of polycyclic aromatic hydrocarbon molecules adsorbed on the surface. The result shows that this method is applicable for a large range of analyte concentrations and there is an excellent linear relationship between the SERS intensity ratio and the analyte concentration (0.5-100 μM).

Quantitative Nanoplasmonics

Plasmonics, the study of the interactions between photons and collective oscillations of electrons, has seen tremendous advances during the past decade. Controllable nanometer- and sub-nanometer-scale engineering in plasmonic resonance and electromagnetic field localization at the subwavelength scale have propelled diverse studies in optics, materials science, chemistry, biotechnology, energy science, and various applications in spectroscopy. However, for translation of these accomplishments from research into practice, major hurdles including low reproducibility and poor controllability in target structures must be overcome, particularly for reliable quantification of plasmonic signals and functionalities. This Outlook introduces and summarizes the recent attempts and findings of many groups toward more quantitative and reliable nanoplasmonics, and discusses the challenges and possible future directions.

Cucurbituril mediated single molecule detection and identification via recognition tunneling

Recognition tunneling (RT) is an emerging technique for investigating single molecules in a tunnel junction. We have previously demonstrated its capability of single molecule detection and identification, as well as probing the dynamics of intermolecular bonding at the single molecule level. Here by introducing cucurbituril as a new class of recognition molecule, we demonstrate a powerful platform for electronically investigating the host-guest chemistry at single molecule level. In this report, we first investigated the single molecule electrical properties of cucurbituril in a tunnel junction. Then we studied two model guest molecules, aminoferrocene and amantadine, which were encapsulated by cucurbituril. Small differences in conductance and lifetime can be recognized between the host-guest complexes with the inclusion of different guest molecules. By using a machine learning algorithm to classify the RT signals in a hyper dimensional space, the accuracy of guest molecule recognition can be significantly improved, suggesting the possibility of using cucurbituril molecule for single molecule identification. This work enables a new class of recognition molecule for RT technique and opens the door for detecting a vast variety of small molecules by electrical measurements.

Disulphide linkage: To get cleaved or not? Bulk and nano copper based SERS of cystine

Different nano-structures of noble metals have been the conventional substrates for carrying out Surface Enhanced Raman Spectroscopy (SERS). In this paper we examine electrodeposited copper (Cu) nano-structures on pencil graphite as novel substrate to carry out SERS measurements by considering l-cystine (Cys-Cys) (dimer of the amino acid cysteine) as the probe. The formation of monolayer of the probe molecule on the substrates was confirmed using cyclic voltammetric measurements. Mode of adsorption of Cys-Cys was observed to be different on bulk Cu (taken in the wire form) and nano-structured Cu on pencil graphite. Whereas in the former the disulphide bond of Cys-Cys remained intact, it got cleaved when Cys-Cys was adsorbed on electrodeposited copper indicating the activated nature of the nano-structure compared to bulk copper. CS stretching mode of vibration underwent blue shift in Cys-Cys adsorbed on Cu on pencil graphite vis-à-vis Cys-Cys adsorbed on Cu wire. Further evidence on the cleavage of the CS bond on an activated substrate was obtained by considering a bimetallic substrate comprising of silver on copper which was electrodeposited on pencil graphite. Our studies have demonstrated that nano‑copper surface is an excellent substrate for SERS giving 200 μM as lower detection limit for Cys-Cys.

Tuning plasmons layer-by-layer for quantitative colloidal sensing with surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is well known for its high sensitivity that emerges due to the plasmonic enhancement of electric fields typically on gold and silver nanostructures. However, difficulties associated with the preparation of nanostructured substrates with uniform and reproducible features limit reliability and quantitation using SERS measurements. In this work we use layer-by-layer (LbL) self-assembly to incorporate multiple functional building blocks of collaborative assemblies of nanoparticles on colloidal spheres to fabricate SERS sensors. Gold nanoparticles (AuNPs) are packaged in discrete layers, effectively 'freezing nano-gaps', on spherical colloidal cores to achieve multifunctionality and reproducible sensing. Coupling between layers tunes the plasmon resonance for optimum SERS signal generation to achieve a 10 nM limit of detection. Significantly, using the layer-by-layer construction, SERS-active AuNP layers are spaced out and thus optically isolated. This uniquely allows the creation of an internal standard within each colloidal sensor to enable highly reproducible self-calibrated sensing. By using 4-mercaptobenzoic acid (4-MBA) as the internal standard adenine concentrations are quantified to an accuracy of 92.6-99.5%. Our versatile approach paves the way for rationally designed yet quantitative colloidal SERS sensors and their use in a variety of sensing applications.

Organic Solvent as Internal Standards for Quantitative and High-Throughput Liquid Interfacial SERS Analysis in Complex Media

Liquid-state interfacial nanoparticle arrays for surface-enhanced Raman scattering (SERS) promises a practical, substrate-free, and rapid analysis but faces a great challenge to develop a batch and uniform fabrication strategy with stable internal standards (IS) because of the difficulties in precisely locating both the IS tags and analytes in the same local structure under the harsh conditions of biphasic liquid interface. Here, we develop a fast batch preparation of self-ordered dense Au nanoparticle (GNP) arrays on cyclohexane/water biphasic interface in 96-well plates with the assist of acetone as the phase-crossing inducer. The acetone can extract the pesticide molecules via a simple dipping sample peels and can rapidly capture and locate the pesticide molecule into the plasmonic hotspots. Meanwhile, this phase-crossing solvent, acetone itself, generates stable SERS signal and is used as the IS tags to calibrate the signal fluctuation. This platform presents an excellent uniformity with a relative standard deviation (RSD) of 5.9% compared to the RSD of 14.5% without the IS's correction and a good sensitivity with a limit of detection (LOD) of 1 nM thiram. This high-throughput strategy for analyzing pesticide residues at fruit peels reached detection levels of nanograms per square centimeter (ng/cm²). Combined with the 96-well plates, this platform greatly facilitates the self-assembly and multiplex sampling. The self-ordered arrays at two immiscible phases interface evidenced the detection of both the oil-soluble thiabendazole and the water-soluble thiram molecules and also realized the multiplex and two-phase detection of these two pesticides. This platform offers vast possibilities for on-site sensing of various analytes and paves a new way for the quantitative and high-throughput SERS analyzer just as convenient as the microplate reader.

Competitive Binding Investigations and Quantitation in Surface-Enhanced Raman Spectra of Binary Dye Mixtures

This research presents a study in surface-enhanced Raman quantitation of dyes present in mixtures of alizarin and purpurin using standard calibration curves and Langmuir isotherm calibration models. Investigations of the nature of competitive adsorption onto silver nanoparticles by centrifugation indicates that both dyes in the mixture interact with the nanoparticles simultaneously, but only the stronger adsorbing one is seen to dominate the spectral characteristics. Calibration can be carried out by careful selection of peaks characteristic to each dye in the mixture. Comparisons of peak height and peak area calibrations reveal that peak heights, when selected by the maximum value and accounting for peak shifts, prove the better model for quantitation. It is also shown that the microwave nanoparticle synthesis method produces stable nanoparticles with a shelf-life of at least one year that give very little variation within and between uses.