Elucidating Protein/Ligand Recognition with Combined Surface Plasmon Resonance and Surface Enhanced Raman Spectroscopy

Perspective on Tailored Nanostructure-Dominated SPP Effects for SERS

Localized surface plasmon resonance (LSPR) excited by an incident light can normally produce strong surface-enhanced Raman scattering (SERS) at the nanogaps among plasmonic nano-objects (so-called hot spots), which is extensively explored. In contrast, surface plasmon polaritons (SPPs) that can be generated by an incident beam via particular structures with a conservation of wave vectors can excite SERS effects as well. SPPs actually play an indispensable role in high-performance SERS devices but receive much less attention. In this perspective, SPPs and their couplings with LSPR for SERS excitations with differing effectiveness through particular plasmonic/dielectric structures/configurations, along with relevant fabrication approaches, are profoundly reviewed and commented on from a unique perspective from in situ to ex situ excitations of SERS enabled by spatiotemporally separated multiple processes of SPPs. Quantitative design of particular configurations/architectures enabling highly efficient and effective multiple processes of SPPs is particularly emphasized as one giant leap toward ultimate full quantitative design of intrinsically high-performance SERS chips and very critical for their batch manufacturability and applications as well. The viewpoints and prospects about innovative SERS devices based on tailored structure-dominated SPPs effects and their coupling with LSPR are presented and discussed.

Interfacial Profiling of MicroRNAs at Patterned Nanogaps for an Integrated Microfluidic-SERS Liquid Biopsy

A versatile microfluidic-SERS barcoding system is developed for sensitive and multiplexed imaging of circulating microRNAs through interfacial probing of encoded nanorod aggregates at diverse patterned nanogaps. The use of a single-layer, vertically oriented nanorod array creates a plasmonic coupling-based electromagnetic field with enormously enhanced Raman outputs. The introduction of the herringbone micromixer with circulated microflow sampling accelerates the hybridization and capture of nanorod aggregates on the plasmonic substrate. The method is able to achieve ideal sensitivities at subfemtomolar levels for four miRNAs, with multiplexed assay capability for an integrated liquid biopsy. The on-chip digital profiling of serum miRNAs in mapping and barcoding formats enable both clear discrimination of untreated cancer patients from the healthy cohort and precise classification of tumor stages, metastatic conditions, and subtypes, with an overall accuracy of 94%. The SERS-based microfluidic barcoding system therefore holds great promise in early cancer screening, diagnosis, and prognosis.

Investigation of a Highly Sensitive Surface-Enhanced Raman Scattering Substrate Formed by a Three-Dimensional/Two-Dimensional Graphene/Germanium Heterostructure

Three-dimensional graphene (3D-graphene) is used in surface-enhanced Raman spectroscopy (SERS) because of its plasmonic nanoresonator structure and good ability to interact with light. However, a thin (3-5 nm) layer of amorphous carbon (AC) inevitably appears as a template layer between the 3D-graphene and object substrate when the 3D-graphene layer is synthesized, weakening the enhancement factor. Herein, two-dimensional graphene (2D-graphene) is employed as a template layer to directly synthesize 3D-graphene on a germanium (Ge) substrate via plasma-assisted chemical vapor deposition, bypassing the formation of an AC layer. The interaction and photoinduced charge transfer ability of the 3D-graphene/Ge heterojunction with incident light are improved. Moreover, the high density of electronic states close to the Fermi level of the heterojunction induces the adsorbed probe molecules to efficiently couple to the 3D-graphene-based SERS substrate. Our experimental results imply that the lowest concentrations of rhodamine 6G and rhodamine B that can be detected on the 3D/2D-graphene/Ge SERS substrate correspond to 10^-10 M; for methylene blue, it is 10^-8 M. The detection limits of the 3D/2D-graphene/Ge SERS substrate with respect to 3-hydroxytyramine hydrochloride and melamine (in milk) are both less than 1 ppm. This work may provide a viable and convenient alternative method for preparing 3D-graphene SERS substrates. It also constitutes a new approach to developing SERS substrates with remarkable performance levels.

Unlocking the secret of lignin-enzyme interactions: Recent advances in developing state-of-the-art analytical techniques

Bioconversion of renewable lignocellulosics to produce liquid fuels and chemicals is one of the most effective ways to solve the problem of fossil resource shortage, energy security, and environmental challenges. Among the many biorefinery pathways, hydrolysis of lignocellulosics to fermentable monosaccharides by cellulase is arguably the most critical step of lignocellulose bioconversion. In the process of enzymatic hydrolysis, the direct physical contact between enzymes and cellulose is an essential prerequisite for the hydrolysis to occur. However, lignin is considered one of the most recalcitrant factors hindering the accessibility of cellulose by binding to cellulase unproductively, which reduces the saccharification rate and yield of sugars. This results in high costs for the saccharification of carbohydrates. The various interactions between enzymes and lignin have been explored from different perspectives in literature, and a basic lignin inhibition mechanism has been proposed. However, the exact interaction between lignin and enzyme as well as the recently reported promotion of some types of lignin on enzymatic hydrolysis is still unclear at the molecular level. Multiple analytical techniques have been developed, and fully unlocking the secret of lignin-enzyme interactions would require a continuous improvement of the currently available analytical techniques. This review summarizes the current commonly used advanced research analytical techniques for investigating the interaction between lignin and enzyme, including quartz crystal microbalance with dissipation (QCM-D), surface plasmon resonance (SPR), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, atomic force microscopy (AFM), nuclear magnetic resonance (NMR) spectroscopy, fluorescence spectroscopy (FLS), and molecular dynamics (MD) simulations. Interdisciplinary integration of these analytical methods is pursued to provide new insight into the interactions between lignin and enzymes. This review will serve as a resource for future research seeking to develop new methodologies for a better understanding of the basic mechanism of lignin-enzyme binding during the critical hydrolysis process.

Ligand-receptor-mediated attachment of lipid vesicles to a supported lipid bilayer

The interaction of exosomes (cell-secreted [Formula: see text]100 nm-sized extracellular vesicles) or membrane-enveloped virions with cellular lipid membranes is often mediated by relatively weak ligand-receptor bonds. Interactions of this type can be studied using vesicles and observing their attachment to receptors located in a lipid bilayer formed at a solid surface. The contact region between a vesicle and the supported lipid bilayer and accordingly the number of ligand-receptor pairs there can be increased by deforming a vesicle. Herein, I (i) estimate theoretically the corresponding deformation energy assuming a disk-like or elongated shape of vesicles, (ii) present the equations allowing one to track such deformations by employing total internal reflection fluorescence microscopy and surface plasmon resonance, and (iii) briefly discuss some related experimental studies.

Advances in surface-enhanced Raman spectroscopy (SERS) substrates for lipid and protein characterization: sensing and beyond

Surface-enhanced Raman spectroscopy (SERS) has become an essential ultrasensitive analytical tool for biomolecular analysis of small molecules, macromolecular proteins, and even cells. SERS enables label-free, direct detection of molecules through their intrinsic Raman fingerprint. In particular, protein and lipid bilayers are dynamic three-dimensional structures that necessitate label-free methods of characterization. Beyond direct detection and quantitation, the structural information contained in SERS spectra also enables deeper biophysical characterization of biomolecules near metallic surfaces. Therefore, SERS offers enormous potential for such systems, although making measurements in a nonperturbative manner that captures the full range of interactions and activity remains a challenge. Many of these challenges have been overcome through advances in SERS substrate development, which have expanded the applications and targets of SERS for direct biomolecular quantitation and biophysical characterization. In this review, we will first discuss different categories of SERS substrates including solution-phase, solid-supported, tip-enhanced Raman spectroscopy (TERS), and single-molecule substrates for biomolecular analysis. We then discuss detection of protein and biological lipid membranes. Lastly, biophysical insights into proteins, lipids and live cells gained through SERS measurements of these systems are reviewed.

Capture of Single Silver Nanoparticles in Nanopore Arrays Detected by Simultaneous Amperometry and Surface-Enhanced Raman Scattering

The attoliter volumes and confinement abilities of zero-dimensional nanopore-electrode arrays (NEAs) hold considerable promise for examining the behavior of single nanoparticles. In this work, we use surface-enhanced Raman scattering (SERS) in tandem with amperometry in order to monitor single Ag Raman-sentinel nanoparticles transported to and captured in single nanopores. To that end, highly ordered solid-state NEAs were fabricated to contain periodic arrays of nanopores, each housing a single recessed Au-ring electrode. These were used to electrostatically capture and trap single silver nanoparticles (AgNPs) functionalized with the electrochemically stable Raman reporter, 1,4-bis(2-methylstyryl)benzene (bis-MSB). Transport and capture of the bis-MSB-tagged AgNPs in the nanopores was followed by simultaneous amperometry and SERS signals characteristic of AgNP oxidation and enhanced Raman scattering by bis-MSB at silver-gold hot spots, respectively. The frequency and magnitude of oxidation-current spikes increased with stepwise increases in DC voltage, and characteristic bis-MSB SERS spectra were observed. Under AC excitation, on the other hand, two distinctly different types of SERS signals were observed, independent of frequency and amplitude: (1) strong, transient (<10 s) spectra and (2) slow (>100 s) monotonically diminishing spectra. We hypothesize that the former behavior results from AgNP aggregates, whereas the latter occurs as a result of multiple incomplete AgNP-oxidation events in succession. These results show that attoliter-volume NEAs are competent for acquiring concurrent SERS spectra and for amperometry of single nanoparticles and that together these measurements can illuminate the collision dynamics of nanoparticles in confined environments.

Advancements in fractal plasmonics: structures, optical properties, and applications

Fractal nanostructures exhibit optical properties that span the visible to far-infrared and are emerging as exciting structures for plasmon-mediated applications.

DNA-fueled target recycling-induced two-leg DNA walker for amplified electrochemical detection of nucleic acid

Taking advantage of the homogeneous and heterogeneous electrochemical biosensors, a simple, sensitive, and selective electrochemical biosensor is constructed by combining entropy-driven amplification (EDA) with DNA walker. This electrochemical biosensor realizes the biorecognition and EDA operation in homogeneous solution, which is beneficial to improve the recognition and amplification efficiency. A two-leg DNA walker generated by EDA can walk on the surface of gold electrode for cleaving the immobilized substrate DNA and releasing the electroactive labels, giving rise to a significant decrease of the electrochemical signal. The immobilization of the electroactive labels ensures the reproducibility and reliability of the biosensor. The present cascade amplification assay can be applied to detect target DNA with a detection limit of 0.29 fM, and base mutations can be easily distinguished. Moreover, the proposed electrochemical biosensor shows a satisfactory performance for the detection of target DNA in human serum. Thus, the novel electrochemical biosensor holds promising potential for a future application in disease diagnosis.

Plasmonic molecular assays: Recent advances and applications for mobile health

Plasmonics-based biosensing assays have been extensively employed for biomedical applications. Significant advancements in use of plasmonic assays for the construction of point-of-care (POC) diagnostic methods have been made to provide effective and urgent health care of patients, especially in resourcelimited settings. This rapidly progressive research area, centered on the unique surface plasmon resonance (SPR) properties of metallic nanostructures with exceptional absorption and scattering abilities, has greatly facilitated the development of cost-effective, sensitive, and rapid strategies for disease diagnostics and improving patient healthcare in both developed and developing worlds. This review highlights the recent advances and applications of plasmonic technologies for highly sensitive protein and nucleic acid biomarker detection. In particular, we focus on the implementation and penetration of various plasmonic technologies in conventional molecular diagnostic assays, and discuss how such modification has resulted in simpler, faster, and more sensitive alternatives that are suited for point-of-use. Finally, integration of plasmonic molecular assays with various portable POC platforms for mobile health applications are highlighted.