Label-Free SERS Detection of Small Proteins Modified to Act as Bifunctional Linkers

Experimental and Theoretical Screening of Core Gold Nanoparticles and Their Binding Mechanism to an Anticancer Drug, 2-Thiouracil

This study demonstrated the capability of two readily available optical spectroscopy tools, namely UV-Vis absorption spectrophotometry and Raman/surface-enhanced Raman spectroscopy, to select in a rapid and noninvasive manner the most homogenous gold nanoparticle (AuNP) models and to identify their chemical binding mechanism to 2-thiouracil (2-TU). 2-TU is an anticancer drug of great promise in the antiproliferative and photothermal therapies of cancer. The citrate-capped AuNPs emerged as the most stable as well as time- and cost-effective AuNP model out of the three widely used colloidal nanocores (citrate-, borohydride-citrate-, and sodium dodecyl sulfate (SDS)-capped AuNPs) that were examined. 2-TU chemically attached to the relatively monodispersed AuNPs via a chemisorption mechanism. The 2-TU-AuNPs complex formed through the covalent bonding of the S atom of 2-TU to the nanosurface in a vertical orientation. The spectroscopic results were then confirmed with the help of density functional theory (DFT) calculations and other physicochemical characterization tools for nanomaterials such as transmission electron microscopy (TEM), dynamic light scattering (DLS), and zeta potential. Overall, the purified 2-TU-AuNPs were found to be spherical, had an average diameter of 25 ± 2 nm, a narrow size distribution (1-30 nm), a sharp localized surface plasmon resonance (LSPR) peak at 525 nm, and a negative surface charge (-14 mV).

Surface Functionalization and Escherichia coli Detection Using Surface-Enhanced Raman Spectroscopy Driven by Functional Organic Polymer/Gold Nanofilm-Based Microfluidic Chip

In this work, a microfluidic prototype based on polymeric materials was developed to monitor surface processes using surface-enhanced Raman spectroscopy (SERS), keeping the reagents free of environmental contamination. The prototype was fabricated on poly(methyl methacrylic acid) (PMMA). A micrometric membrane of a functional organic polymer (FOP) based on p-terphenyl and bromopyruvic acid monomers was formed on the PMMA surface to promote the formation of metal nanoclusters. Au nanosized film was deposited on the FOP membrane to give rise to the SERS effect. A microchannel was formed on another piece of PMMA using micromachining. A representative 3D model of the prototype layer arrangement was built and simulated in COMSOL Multiphysics® to approximate the electric field distribution and calculate the power enhancement factor as the Au film changes over time. The fabrication process was characterized using UV-visible and Raman spectroscopies and XPS. The prototype was tested using a Raman microscope and liquid solutions of cysteamine and Escherichia coli (E. coli). The simulation results demonstrated that the morphological characteristics of the Au layer give rise to the SERS effect, and the power enhancement factor reaches values as high as 8.8 × 105 on the FOP surface. The characterization results showed the formation of the FOP and the Au film on PMMA and the surface functionalization with amine groups. The Raman spectra of the prototype showed temporal evolution as different compounds were deposited on the upper wall of the microchannel. Characteristic peaks associated with these compounds were detected with continuous monitoring over time. This prototype offers many benefits for applications like monitoring biological processes. Some advantages include timely surface evaluation while avoiding environmental harm, decreased use of reagents and samples, minimal interference with the process by measuring, and detecting microorganisms in just 1 h, as demonstrated with the E. coli sample.

A review of applications of surface-enhanced raman spectroscopy laser for detection of biomaterials and a quick glance into its advances for COVID-19 investigations

Surface-enhanced Raman spectroscopy (SERS) is one of the most sensitive analytical tools. In some cases, it is possible to record a high-quality SERS spectrum in which even a single molecule is involved. Therefore, SERS is considered a significantly promising option as an alternative to routine analytical techniques used in food, environmental, biochemical, and medical analyzes. In this review, the definitive applications of SERS developed to identify biochemically important species (especially medical and biological) from the simplest to the most complex are briefly discussed. Moreover, the potential capability of SERS for being used as an alternative to routine methods in diagnostic and clinical cases is demonstrated. In addition, this article describes how SERS-based sensors work, addresses its advancements in the last 20 years, discusses its applications for detecting Coronavirus Disease 2019 (COVID-19), and finally describes future works. The authors hope that this article will be useful for researchers who want to enter this amazing field of research.

The micro-, submicron-, and nanoplastic hunt: A review of detection methods for plastic particles

Plastic particle pollution has been shown to be almost completely ubiquitous within our surrounding environment. This ubiquity in combination with a variety of unique properties (e.g. density, hydrophobicity, surface functionalization, particle shape and size, transition temperatures, and mechanical properties) and the ever-increasing levels of plastic production and use has begun to garner heightened levels of interest within the scientific community. However, as a result of these properties, plastic particles are often reported to be challenging to study in complex (i.e. real) environments. Therefore, this review aims to summarize research generated on multiple facets of the micro- and nanoplastics field; ranging from size and shape definitions to detection and characterization techniques to generating reference particles; in order to provide a more complete understanding of the current strategies for the analysis of plastic particles. This information is then used to provide generalized recommendations for researchers to consider as they attempt to study plastics in analytically complex environments; including method validation using reference particles obtained via the presented creation methods, encouraging efforts towards method standardization through the reporting of all technical details utilized in a study, and providing analytical pathway recommendations depending upon the exact knowledge desired and samples being studied.

Biofunctionalization of Porous Ti Substrates Coated with Ag Nanoparticles for Potential Antibacterial Behavior

Ti prosthesis have shown better biological compatibility, mechanical performance, and resistance to corrosion in cases of bone replacements. Nevertheless, fully dense Ti in connection with bone-host tissues show stress-shielding phenomenon that, together with the development of frequent undesirable microbial infections, may lead to implant failures. To overcome these issues, the present study aimed at the development of a novel combination of a chemically functionalized porous Ti substrate with a potentially therapeutic AgNPs coating. Fully dense and porous Ti substrates (30 and 60 vol.%, 100–200 and 355–500 μm, as spacer particles) were studied. Ti surface was treated with acid or basic medium followed by silanization and deposition of AgNPs by “submerged” and “in situ” methods. In general, for similar porosity, mechanical resistance decreased as pore size increased. Acidic reagent and submerged methodology were the best combination for fully dense Ti substrates. Hence, they were also employed for porous Ti substrates. Depending on the porosity of the substrates, variations can be observed both in the size and degree of agglomeration of the deposited AgNPs, entailing differences in the antibacterial behavior of the samples.

Plasmonics meets super-resolution microscopy in biology

Super-resolution microscopy can reveal the subtle biological processes hidden behind the optical diffraction barrier. Plasmonics is a key nanophotonic that combines electronics and photonics through the interaction of light with the metallic nanostructure. In this review, we survey the recent progresses on plasmonic-assisted super-resolution microscopy. The strong electromagnetic field enhancement trapped near metallic nanostructures offers a unique opportunity to manipulate the illumination scheme for overcoming the diffraction limit. Plasmonic nanoprobes, exploited as surface-enhanced Raman scattering (SERS) and plasmon-enhanced fluorescence nanoparticles, are a major category of contrast agent in super-resolution microscopy. The outstanding challenges, future developments, and potential biological applications are also discussed.

Real-time dynamic SERS detection of galectin using glycan-decorated gold nanoparticles

We present the application of surface-enhanced Raman scattering (SERS) spectroscopy for the fast, sensitive and highly specific detection of the galectin-9 (Gal-9) protein in binding buffer (mimicking natural conditions). The method involves the use of specifically designed nanotags comprising glycan-decorated gold nanoparticles encoded with 4-mercaptobenzoic acid. At fast time scales Gal-9 can be detected down to a concentration of 1.2 nM by monitoring the SERS signal of the reporter, driven by aggregation of the functionalized Au NPs tags, induced by Gal-9 recognition. We additionally demonstrate that the sensitivity and concentration working range of the sensor can be tuned via control of aggregation dynamics and cluster size distribution.

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.

Superparamagnetic nanoarchitectures for disease-specific biomarker detection

Synthesis, bio-functionalization, and multifunctional activities of superparamagnetic-nanostructures have been extensively reviewed with a particular emphasis on their uses in a range of disease-specific biomarker detection and associated challenges.

Probing Membrane Receptors with Enhanced Raman Imaging

Our lab has shown that nanoparticles functionalized with short peptides can selectively bind to receptor proteins in vitro. Our results indicate that the Raman signals observed from purified receptors in surface enhanced Raman scattering (SERS) experiments match those observed with tip-enhanced Raman scattering (TERS) experiments performed on membrane receptors in intact cell membranes. Analysis of the observed Raman signals suggest the signals arise from the amino-acids in the protein receptor responsible for binding and recognition of the ligand attached to the nanoparticle probe. Further experiments show the variance in the data correlates with affinity of the nanoparticle probe with a specific receptor. This result illustrates a new approach to studying membrane receptors.

A Raman-Based Imaging Method for Characterizing the Molecular Adsorption and Spatial Distribution of Silver Nanoparticles on Hydrated Mineral Surfaces

Although minerals are known to affect the environmental fate and transformation of heavy-metal ions, little is known about their interaction with the heavily exploited silver nanoparticles (AgNPs). Proposed here is a combination of hitherto under-utilized micro-Raman-based mapping and chemometric methods for imaging the distribution of AgNPs on various mineral surfaces and their molecular interaction mechanisms. The feasibility of the Raman-based imaging method was tested on two macro- and microsized mineral models, muscovite [KAl₂(AlSi₃O₁₀)(OH)₂] and corundum (α-Al₂O₃), under key environmental conditions (ionic strength and pH). Both AgNPs^- and AgNPs⁺ were found to covalently attach to corundum (pH_pzc = 9.1) through the formation of Ag-O-Al- bonds and thereby to potentially experience reduced environmental mobility. Because label-free Raman imaging showed no molecular interactions between AgNPs^- and muscovite (pH_pzc = 7.5), a label-enhanced Raman imaging approach was developed for mapping the scarce spatial distribution of AgNPs^- on such mineral surfaces. Raman maps comprising of n = 625-961 spectra for each sample/control were rapidly analyzed in Vespucci, a free open-source software, and the results were confirmed via ICP-OES, AFM, and SEM-EDX. The proposed Raman-based imaging requires minimum to no sample preparation; is sensitive, noninvasive, cost-effective; and might be extended to other environmentally relevant systems.

Bioanalytical applications of surface-enhanced Raman spectroscopy: de novo molecular identification

Surface enhanced Raman scattering (SERS) has become a powerful technique for trace analysis of biomolecules. The use of SERS-tags has evolved into clinical diagnostics, the enhancement of the intrinsic signal of biomolecules on SERS active materials shows tremendous promise for the analysis of biomolecules and potential biomedical assays. The detection of the de novo signal from a wide range of biomolecules has been reported to date. In this review, we examine different classes of biomolecules for the signals observed and experimental details that enable their detection. In particular, we survey nucleic acids, amino acids, peptides, proteins, metabolites, and pathogens. The signals observed show that the interaction of the biomolecule with the enhancing nanostructure has a significant influence on the observed spectrum. Additional experiments demonstrate that internal standards can correct for intensity fluctuations and provide quantitative analysis. Experimental methods that control the interaction at the surface are providing for reproducible SERS signals. Results suggest that combining advances in methodology with the development of libraries for SERS spectra may enable the characterization of biomolecules complementary to other existing methods.

Silver nanoflowers for single-particle SERS with 10 pM sensitivity

Surface-enhanced Raman scattering (SERS) has received considerable attention as a noninvasive optical sensing technique with ultrahigh sensitivity. While numerous types of metallic particles have been actively investigated as SERS substrates, the development of new SERS agents with high sensitivity and their reliable characterization are still required. Here we report the preparation and characterization of flower-shaped silver (Ag) nanoparticles that exhibit high-sensitivity single-particle SERS performance. Ag nanoflowers (NFs) with bud sizes in the range 220-620 nm were synthesized by the wet synthesis method. The densely packed nanoscale petals with thicknesses in the range 9-22 nm exhibit a large number of hot spots that significantly enhance their plasmonic activity. A single Ag NF particle (530-620 nm) can detect as little as 10^-11 M 4-mercaptobenzoic acid, and thus provides a sensitivity three orders of SERS magnitude greater than that of a spherical Ag nanoparticle. The analytical enhancement factors for single Ag NF particles were found to be as high as 8.0 × 10⁹, providing unprecedented high SERS detectivity at the single particle level. Here we present an unambiguous and systematic assessment of the SERS performances of the Ag NFs and demonstrate that they provide highly sensitive sensing platforms by single SERS particle.

SERS Quantification and Characterization of Proteins and Other Biomolecules

Changes in protein expression levels and protein structure may indicate genomic mutations and may be related to some diseases. Therefore, the precise quantification and characterization of proteins can be used for disease diagnosis. Compared with several other alternative methods, surface-enhanced Raman scattering (SERS) spectroscopy is regarded as an excellent choice for the quantification and structural characterization of proteins. Herein, we review the main advance of using plasmonic nanostructures as SERS sensing platform for this purpose. Three design approaches, including direct SERS, indirect SERS, and SERS-encoded nanoparticles, are discussed in the direction of developing new precise approaches of quantification and characterization of proteins. While this Review is focused on proteins, in order to highlight concepts of SERS-based sensors also detection of other biomolecules will be discussed.

Mg2+-induced DNA compaction, condensation, and phase separation in gene delivery vehicles based on zwitterionic phospholipids: a dynamic light scattering and surface-enhanced Raman spectroscopic study

Despite the significant efforts towards applying improved non-destructive and label-free measurements of biomolecular structures of lipid-based gene delivery vectors, little is achieved in terms of their structural relevance in gene transfections. Better understanding of structure-activity relationships of lipid-DNA complexes and their gene expression efficiencies thus becomes an essential issue. Raman scattering offers a complimentary measurement technique for following the structural transitions of both DNA and lipid vesicles employed for their transfer. This work describes the use of SERS coupled with light scattering approaches for deciphering the bioelectrochemical phase formations between nucleic acids and lipid vesicles within lipoplexes and their surface parameters that could influence both the uptake of non-viral gene carriers and the endocytic routes of interacting cells. As promising non-viral alternatives of currently employed risky viral systems or highly cytotoxic cationic liposomes, complexations of both nucleic acids and zwitterionic lipids in the presence of Mg²⁺ were studied applying colloidal Ag nanoparticles. It is shown that the results could be employed in further conformational characterizations of similar polyelectrolyte gene delivery systems.

Site-Specific SERS Assay for Survivin Protein Dimer: From Ensemble Experiments to Correlative Single-Particle Imaging

An assay for Survivin, a small dimeric protein which functions as modulator of apoptosis and cell division and serves as a promising diagnostic biomarker for different types of cancer, is presented. The assay is based on switching on surface-enhanced Raman scattering (SERS) upon incubation of the Survivin protein dimer with Raman reporter-labeled gold nanoparticles (AuNP). Site-specificity is achieved by complexation of nickel-chelated N-nitrilo-triacetic acid (Ni-NTA) anchors on the particle surface by multiple histidines (His₆ -tag) attached to each C-terminus of the centrosymmetric protein dimer. Correlative single-particle analysis using light sheet laser microscopy enables the simultaneous observation of both elastic and inelastic light scattering from the same sample volume. Thereby, the SERS-inactive AuNP-protein monomers can be directly discriminated from the SERS-active AuNP-protein dimers/oligomers. This information, i.e. the percentage of SERS-active AuNP in colloidal suspension, is not accessible from conventional SERS experiments due to ensemble averaging. The presented correlative single-particle approach paves the way for quantitative site-specific SERS assays in which site-specific protein recognition by small chemical and in particular supramolecular ligands can be tested.

Bioanalytical Measurements Enabled by Surface-Enhanced Raman Scattering (SERS) Probes

Since its discovery in 1974, surface-enhanced Raman scattering (SERS) has gained momentum as an important tool in analytical chemistry. SERS is used widely for analysis of biological samples, ranging from in vitro cell culture models, to ex vivo tissue and blood samples, and direct in vivo application. New insights have been gained into biochemistry, with an emphasis on biomolecule detection, from small molecules such as glucose and amino acids to larger biomolecules such as DNA, proteins, and lipids. These measurements have increased our understanding of biological systems, and significantly, they have improved diagnostic capabilities. SERS probes display unique advantages in their detection sensitivity and multiplexing capability. We highlight key considerations that are required when performing bioanalytical SERS measurements, including sample preparation, probe selection, instrumental configuration, and data analysis. Some of the key bioanalytical measurements enabled by SERS probes with application to in vitro, ex vivo, and in vivo biological environments are discussed.

Raman Imaging in Cell Membranes, Lipid-Rich Organelles, and Lipid Bilayers

Raman-based optical imaging is a promising analytical tool for noninvasive, label-free chemical imaging of lipid bilayers and cellular membranes. Imaging using spontaneous Raman scattering suffers from a low intensity that hinders its use in some cellular applications. However, developments in coherent Raman imaging, surface-enhanced Raman imaging, and tip-enhanced Raman imaging have enabled video-rate imaging, excellent detection limits, and nanometer spatial resolution, respectively. After a brief introduction to these commonly used Raman imaging techniques for cell membrane studies, this review discusses selected applications of these modalities for chemical imaging of membrane proteins and lipids. Finally, recent developments in chemical tags for Raman imaging and their applications in the analysis of selected cell membrane components are summarized. Ongoing developments toward improving the temporal and spatial resolution of Raman imaging and small-molecule tags with strong Raman scattering cross sections continue to expand the utility of Raman imaging for diverse cell membrane studies.

Highly sensitive and scalable AAO-based nano-fibre SERS substrate for sensing application

Well-ordered periodic nanostructures are excellent substrates for many surface-enhanced Raman spectroscopy (SERS) applications. Conventional fabrication approaches such as high precision electron beam lithography or focused ion beam produce high resolution nano-features with great reproducibility at the expense of low throughput. In this work, a highly sensitive and scalable AAO-nano-fibre (ANF) SERS substrate is demonstrated by optimising the second anodisation time of the standard two-step anodisation of aluminium and performing an additional wet etching step on the resulting AAO substrate. The optimised ANF substrate exhibits SERS sensitivity that surpasses the AAO nanoholes and the metal-film-on-nanoparticles substrates. A detection limit of 0.1 nM is achieved with a signal-to-noise ratio of 2.6-3 using a low excitation power of 0.1 mW. The ANF substrate exhibits an enhancement factor of 9.28 × 106 and a standard deviation of no more than 8%. The results indicate that the highly sensitive and scalable ANF substrate is a promising substrate for commercial SERS application.

Hemolysin coregulated protein 1 as a molecular gluing unit for the assembly of nanoparticle hybrid structures

Hybrid nanoparticle (NP) structures containing organic building units such as polymers, peptides, DNA and proteins have great potential in biosensor and electronic applications. The nearly free modification of the polymer chain, the variation of the protein and DNA sequence and the implementation of functional moieties provide a great platform to create inorganic structures of different morphology, resulting in different optical and magnetic properties. Nevertheless, the design and modification of a protein structure with functional groups or sequences for the assembly of biohybrid materials is not trivial. This is mainly due to the sensitivity of its secondary, tertiary and quaternary structure to the changes in the interaction (e.g., hydrophobic, hydrophilic, electrostatic, chemical groups) between the protein subunits and the inorganic material. Here, we use hemolysin coregulated protein 1 (Hcp1) from Pseudomonas aeruginosa as a building and gluing unit for the formation of biohybrid structures by implementing cysteine anchoring points at defined positions on the protein rim (Hcp1_cys3). We successfully apply the Hcp1_cys3 gluing unit for the assembly of often linear, hybrid structures of plasmonic gold (Au NP), magnetite (Fe3O4 NP), and cobalt ferrite nanoparticles (CoFe2O4 NP). Furthermore, the assembly of Au NPs into linear structures using Hcp1_cys3 is investigated by UV-vis spectroscopy, TEM and cryo-TEM. One key parameter for the formation of Au NP assembly is the specific ionic strength in the mixture. The resulting network-like structure of Au NPs is characterized by Raman spectroscopy, showing surface-enhanced Raman scattering (SERS) by a factor of 8·10(4) and a stable secondary structure of the Hcp1_cys3 unit. In order to prove the catalytic performance of the gold hybrid structures, they are used as a catalyst in the reduction reaction of 4-nitrophenol showing similar catalytic activity as the pure Au NPs. To further extend the functionality of the Hcp1_cys3 gluing unit, Fe3O4 and CoFe2O4 NPs are aligned in a magnetic field and connected by utilization of cysteine-modified Hcp1. After lyophilization, a fiber-like material of micrometer scale length can be observed. The Fe3O4 Hcp1_cys3 fibers show superparamagnetic behavior with a decreasing blocking temperature and an increasing remanent magnetization leading to a higher squareness value of the hysteresis curve. Thus the Hcp1_cys3 unit is shown to be very versatile in the formation of new biohybrid materials with enhanced magnetic, catalytic and optical properties.

Sorting of Single Biomolecules based on Fourier Polar Representation of Surface Enhanced Raman Spectra

Surface enhanced Raman scattering (SERS) spectroscopy becomes increasingly used in biosensors for its capacity to detect and identify single molecules. In practice, a large number of SERS spectra are acquired and reliable ranking methods are thus essential for analysing all these data. Supervised classification strategies, which are the most effective methods, are usually applied but they require pre-determined models or classes. In this work, we propose to sort SERS spectra in unknown groups with an alternative strategy called Fourier polar representation. This non-fitting method based on simple Fourier sine and cosine transforms produces a fast and graphical representation for sorting SERS spectra with quantitative information. The reliability of this method was first investigated theoretically and numerically. Then, its performances were tested on two concrete biological examples: first with single amino-acid molecule (cysteine) and then with a mixture of three distinct odorous molecules. The benefits of this Fourier polar representation were highlighted and compared to the well-established statistical principal component analysis method.

Thermodynamic and conformational changes of protein toward interaction with nanoparticles: a spectroscopic overview

Nanoparticles (NPs) in different forms have been widely used in medicine and pharmaceutics for diagnosis and drug delivery.

Multiplex in vitro detection using SERS

This review focuses on the recent advances in SERS and its potential to detect multiple biomolecules in clinical samples.

Graphene Oxide-Supported Ag Nanoplates as LSPR Tunable and Reproducible Substrates for SERS Applications with Optimized Sensitivity

Nanoparticles and nanohybrids with well-defined structures, along with tunable localized surface plasmon resonance (LSPR) properties and optimized sensitivity, are crucial and highly desired for surface-enhanced Raman spectroscopy (SERS) applications. In this article, we report on a very promising and flexible SERS platforms with a tunable LSPR response and sensitivity based on Ag nanoplates and graphene oxide (GO). The SERS detection sensitivity can be easily optimized and significantly improved by fine-tuning the LSPR band of the Ag nanoplate/GO substrates (to enhance the SERS response) during sample preparation. We applied the as-prepared SERS platform for sensitive SERS detection of 4-mercaptobenzoic acid and 4-aminothiophenol and found that the SERS signal varied markedly (by ∼10-15-fold) with the fine-tuning of the LSPR band. The SERS enhancement factor of the Ag nanoplate/GO complexes was more than 10(4) times larger than that obtained using spherical Ag nanoparticles. The as-prepared Ag nanoplate/GO platforms, because of their excellent stability and tunable LSPR properties, will find promising practical SERS applications.

Optical nano antennas: state of the art, scope and challenges as a biosensor along with human exposure to nano-toxicology

The concept of optical antennas in physical optics is still evolving. Like the antennas used in the radio frequency (RF) regime, the aspiration of optical antennas is to localize the free propagating radiation energy, and vice versa. For this purpose, optical antennas utilize the distinctive properties of metal nanostructures, which are strong plasmonic coupling elements at the optical regime. The concept of optical antennas is being advanced technologically and they are projected to be substitute devices for detection in the millimeter, infrared, and visible regimes. At present, their potential benefits in light detection, which include polarization dependency, tunability, and quick response times have been successfully demonstrated. Optical antennas also can be seen as directionally responsive elements for point detectors. This review provides an overview of the historical background of the topic, along with the basic concepts and parameters of optical antennas. One of the major parts of this review covers the use of optical antennas in biosensing, presenting biosensing applications with a broad description using different types of data. We have also mentioned the basic challenges in the path of the universal use of optical biosensors, where we have also discussed some legal matters.

Silver@graphene oxide nanocomposite-based optical sensor platform for biomolecules

A silver incorporated graphene oxide nanocomposite was prepared and used for the detection of biomolecules using an absorbance based optical sensing method.

Allosteric transition induced by Mg²⁺ ion in a transactivator monitored by SERS

We demonstrate the utility of the surface-enhanced Raman spectroscopy (SERS) to monitor conformational transitions in protein upon ligand binding. The changes in protein's secondary and tertiary structures were monitored using amide and aliphatic/aromatic side chain vibrations. Changes in these bands are suggestive of the stabilization of the secondary and tertiary structure of transcription activator protein C in the presence of Mg(2+) ion, whereas the spectral fingerprint remained unaltered in the case of a mutant protein, defective in Mg(2+) binding. The importance of the acidic residues in Mg(2+) binding, which triggers an overall allosteric transition in the protein, is visualized in the molecular model. The present study thus opens up avenues toward the application of SERS as a potential tool for gaining structural insights into the changes occurring during conformational transitions in proteins.

Influence of protein size on surface-enhanced Raman scattering (SERS) spectra in binary protein mixtures

The size-dependent interactions of eight blood proteins with silver nanoparticles (AgNPs) in their binary mixtures were investigated using surface-enhanced Raman scattering (SERS). Principal component analysis (PCA) was performed on the SERS spectra of each binary mixture, and the differentiation ability of the mixtures was tested. It was found that the effect of relative concentration change on the SERS spectra of the binary mixtures of small proteins could be detected using PCA. However, this change was not observed with the binary mixtures of large proteins. This study demonstrated that the relative interactions of the smaller proteins with an average size of 50 nm AgNPs smaller than the large proteins could be monitored, and this information can be used for the detection of proteins in protein mixtures.

The chemical origin of enhanced signals from tip-enhanced Raman detection of functionalized nanoparticles

Here we present results that investigate the origins of signals observed in tip-enhanced Raman (TERS) measurements of functionalized nanoparticles. Surface enhanced Raman scattering (SERS) is known to give the largest enhancements in gap junctions. Similarly, gap-mode TERS also produces significant enhancements. The methodology developed here provides gap-mode like enhancements in TERS measurements without the need for a metal surface. Using a combination of aggregated nanoparticle SERS and TERS detection of functionalized nanoparticles, we assess the chemical origins of the observed peaks and show that molecules outside of gap junctions are also enhanced using our methodology. Our experiments use biotin and streptavidin as a model system for protein-ligand binding. Different size functionalized nanoparticles (20, 50, 80 nm) show changes in intensity in both SERS and TERS measurements. SERS measurements indicate that streptavidin has a larger Raman cross-section than biotin and is preferentially observed. The specific streptavidin peaks observed by TERS vary depending on whether streptavidin is attached to the nanoparticle and located in the gap or bound to the substrate surface. This methodology suggests a route to enhancing TERS signals associated with protein receptors in biological systems that cannot be isolated to a metallic surface.

Surface-Enhanced Raman Scattering: A Technique of Choice for Molecular Detection

Although surface-enhanced Raman scattering (SERS) has crossed its infancy long ago, it is yet to persuade different challenges to make it available in day-to-day applications. SERS is being criticized mainly due to the quality of the SERS analyses that uses substrates to get the giant enhancement for respective Raman signal of the target molecule. Hence, understanding the phenomena behind substrates, cost-effective development and optimization of such substrates for routine analytical purposes and utilization of modern modalities to get the insights out has become a very wide-spreading and interesting area of research. In this piece of work, several key terminologies related to SERS have been presented in brief. Since SERS is a localized surface plasmon resonance (LSPR) mediated signal-enhancing phenomena, it is indispensable to understand the correlation between LSPR excitations originated from substrate and SERS signal originated from molecules. A wide range of SERS-active substrates including scattered nanoaggregates, anisotropic assembly, two-dimensional nanostructure, multi-layered nanostructure of gold nanoparticles and colloidal approach have been used to interpret such correlation between LSPR excitations and SERS characteristics. Few exemplary applications of SERS have been also mentioned followed by typical simulative work how nanoobject behaves at different excitations and polarizations.

Surface-enhanced Raman scattering: an emerging label-free detection and identification technique for proteins

The detection and identification of biologically important molecules has critical importance in several fields such as medicine, biotechnology, and pharmacology. Surface-enhanced Raman scattering (SERS) is a powerful emerging vibrational spectroscopic technique that allows not only for the characterization, but also for the identification and detection of biomacromolecules in a very short time. In this review, efforts to utilize SERS for label-free protein detection and identification is summarized after a short introduction of proteins and the technique.

Molecularly-mediated assemblies of plasmonic nanoparticles for Surface-Enhanced Raman Spectroscopy applications

In recent years, Surface-Enhanced Raman Spectroscopy (SERS) has experienced a tremendous increase of attention in the scientific community, expanding to a continuously wider range of diverse applications in nanoscience, which can mostly be attributed to significant improvements in nanofabrication techniques that paved the way for the controlled design of reliable and effective SERS nanostructures. In particular, the plasmon coupling properties of interacting nanoparticles are extremely intriguing due to the concentration of enormous electromagnetic enhancements at the interparticle gaps. Recently, great efforts have been devoted to develop new nanoparticle assembly strategies in suspension with improved control over hot-spot architecture and cluster structure, laying the foundation for the full exploitation of their exceptional potential as SERS materials in a wealth of chemical and biological sensing. In this review we summarize in an exhaustive and systematic way the state-of-art of plasmonic nanoparticle assembly in suspension specifically developed for SERS applications in the last 5 years, focusing in particular on those strategies which exploited molecular linkers to engineer interparticle gaps in a controlled manner. Importantly, the novel advances in this rather new field of nanoscience are organized into a coherent overview aimed to rationally describe the different strategies and improvements in the exploitation of colloidal nanoparticle assembly for SERS application to real problems.

Label-free in situ detection of individual macromolecular assemblies by surface enhanced Raman scattering

We demonstrate label-free detection of lipid vesicles and polystyrene beads freely diffusing in aqueous solution using surface enhanced Raman scattering (SERS). The signals observed enable real-time identification and monitoring of individual particles interacting with the SERS substrate. SERS is demonstrated as a label-free method capable of monitoring transient species in solution on the millisecond time scale.

Surface-Enhanced Raman Scattering as an Emerging Characterization and Detection Technique

While surface-enhanced Raman spectroscopy (SERS) has been attracting a continuously increasing interest of scientific community since its discovery, it has enjoyed a particularly rapid growth in the last decade. Most notable recent advances in SERS include novel technological approaches to SERS substrates and innovative applications of SERS in medicine and molecular biology. While a number of excellent reviews devoted to SERS appeared in the literature over the last two decades, we will focus this paper more specifically on several promising trends that have been highlighted less frequently. In particular, we will briefly overview strategies in designing and fabricating SERS substrates using deterministic patterning and then cover most recent biological applications of SERS.

Localized surface plasmon resonance: nanostructures, bioassays and biosensing--a review

Localized surface plasmon resonance (LSPR) is an optical phenomena generated by light when it interacts with conductive nanoparticles (NPs) that are smaller than the incident wavelength. As in surface plasmon resonance, the electric field of incident light can be deposited to collectively excite electrons of a conduction band, with the result being coherent localized plasmon oscillations with a resonant frequency that strongly depends on the composition, size, geometry, dielectric environment and separation distance of NPs. This review serves to describe the physical theory of LSPR formation at the surface of nanostructures, and the potential for this optical technology to serve as a basis for the development bioassays and biosensing of high sensitivity. The benefits and challenges associated with various experimental designs of nanoparticles and detection systems, as well as creative approaches that have been developed to improve sensitivity and limits of detection are highlighted using examples from the literature.

Recent progress in SERS biosensing

This perspective gives an overview of recent developments in surface-enhanced Raman scattering (SERS) for biosensing. We focus this review on SERS papers published in the last 10 years and to specific applications of detecting biological analytes. Both intrinsic and extrinsic SERS biosensing schemes have been employed to detect and identify small molecules, nucleic acids, lipids, peptides, and proteins, as well as for in vivo and cellular sensing. Current SERS substrate technologies along with a series of advancements in surface chemistry, sample preparation, intrinsic/extrinsic signal transduction schemes, and tip-enhanced Raman spectroscopy are discussed. The progress covered herein shows great promise for widespread adoption of SERS biosensing.

Uniform gold nanoarray formed by controlled IP6 micelles for chemical mapping

A uniform Au nanoarray is successfully formed at an indium tin oxide (ITO) glass surface modified with well-distributed inositol hexakisphosphoric (IP(6)) micelle layers by controlling the pH of the medium at 10. When Rhodamine 6G (R6G) and 2-mercaptopyridine (2-MPy) are used as the Raman probes, the uniform Au nanoarray presents a sound surface enhanced Raman scattering (SERS) efficiency and a reproducible Raman signal in two dimensions. The relative standard deviation (RSD) of Raman intensities of R6G or 2-MPy on the uniform Au nanoarray recorded by point to point is less than 12%, which is beneficial to its application for chemical mapping or imaging. A case of Raman point-mapping for onion epidermis is demonstrated in the present work. A uniform IP(6)-Au nanoarray might be mass-produced by this protocol.

Detection of proteins on silica-silver core-shell substrates by surface-enhanced Raman spectroscopy

We have employed the proposed Silica-Silver Core-Shell (SSCS) SERS-active substrates to detect four model proteins: lysozyme (a protein without chromophore), cytochrome c (a protein with chromophore of heme), fluorescein isothiocyanate (FITC)-anti human IgG (labeled with FITC) and atto610-biotin/avidin (recognition with labeled small molecules). SERS spectra of these proteins and Raman labels on the SSCS substrates show both high sensitivity and reproducibility, which are due to electromagnetic SERS enhancement with additional localization field within closely packed Ag nanoparticles decorated on the SiO(2) nanoparticles and the aggregation of SiO(2)@Ag particles. We have found that the SERS intensities of atto610-biotin/avidin adsorbed on the SSCS substrates are about 20 times stronger than those from Ag plating on Au-decorated substrate. Moreover, the broad surface plasmon resonance (SPR) of the proposed substrates will extend SERS applications to more biological molecules with different laser excitations.

Label-free Raman mapping of surface distribution of protein a and IgG biomolecules

We have demonstrated a nanoengineered substrate composed of micropatterned silver nanoparticles to be used for the label-free mapping of adsorbed biomolecules. We utilized surface-enhanced Raman scattering (SERS) phenomenon to monitor the known bioanalytes, protein A and human immunoglobulin G (IgG). The SERS substrate was composed of a poly(alylamine hydrochloride) (PAH)/poly(styrenesulfonate) (PSS) layer-by-layer (LbL) nanocoating micropatterned with silver nanoparticles confined to microscopic stripes. Selective adsorption of biomacromolecules is facilitated by the amine-terminated LbL nanocoating, which prevents the surface adsorption of positively charged protein A across the surface except on the patterned regions containing negatively charged silver nanoparticles. Furthermore, adsorption of IgG on predetermined regions is facilitated by the selective binding of the Fc region of IgG to protein A. This label-free SERS approach provides accurate, selective, and fast detection of protein A and IgG solutions with a nanomolar concentration, down to below 1 nM for IgG in solution. This method could also be utilized for the facile detection of proteins in field conditions as well as in clinical, forensic, industrial, and environmental laboratories.

A review on the fabrication of substrates for surface enhanced Raman spectroscopy and their applications in analytical chemistry

This work reviews different types of substrates used for surface-enhanced Raman scattering (SERS) that have been developed in the last 10 years. The different techniques of self-assembly to immobilize metallic nanoparticles on solid support are covered. An overview of SERS platforms developed using nanolithography methods, including electron-beam (e-beam) lithography and focused ion beam (FIB) milling are also included, together with several examples of template-based methodologies to generate metallic nano-patterns. The potential of SERS to impact several aspects of analytical chemistry is demonstrated by selected examples of applications in electrochemistry, biosensing, environmental analysis, and remote sensing. This review shows that highly enhancing SERS substrates with a high degree of reliability and reproducibility can now be fabricated at relative low cost, indicating that SERS may finally realize its full potential as a very sensitive tool for routine analytical applications.

Protein-ligand binding investigated by a single nanoparticle TERS approach

We report TERS imaging of individual 50 nm, biotin-labeled gold nanoparticles bound to a streptavidin-derivatized glass slide. Individual gold nanoparticles detected by a nanoparticle TERS tip generate Raman enhancements in both the biotin and streptavidin signals. These results indicate that nanoparticles are capable of investigating nanoscale spatial and chemical environments with non-resonant Raman enhancements.