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

Surface Plasmon Field-Enhanced Raman Scattering Co-Excited by P-Polarized and S-Polarized Light Based on Waveguide-Coupled Surface Plasmon Resonance Configuration

We constructed a waveguide-coupled surface plasmon resonance (WCSPR) structure to enhance Raman scattering. In this structure, P-polarized and S-polarized incident lasers can simultaneously coexcite the evanescent field, thereby further enhancing Raman scattering. This configuration is a five-phase Kretschmann resonance setup that consists of a SF10 prism/inner Ag film/SiO2 film/outer Ag film/water structure. The WCSPR configuration effectively concentrates and confines the evanescent field excited by the incident light. Ag nanoparticles assembled on the outer Ag film surface enhance the evanescent field further by means of surface plasmon resonance. By finely tuning the thickness of the Ag and SiO2 films, it is possible to achieve a coincidence between the SPR angle of P-polarized light and that of S-polarized light. At this angle, both P- and S-polarized light can jointly elevate the evanescent field intensity, leading to the simultaneous enhancement of the electric fields at the upper, lower, left, and right parts of the silver nanoparticles and generating maximum evanescent field enhancement. We achieved an electric field enhancement of up to 103 around the nanoparticles, leading to more SERS hotspots and comparable SERS enhancement capability to gap-type hotspots. Our WCSPR structure combined with the nanoparticles offers a feasible strategy for the SERS detection of large molecules that cannot be placed in traditional gap-type hotspots. It is highly convenient for SERS detection of large molecules.

Pharmaceutical applications of a nanospectroscopic technique: Surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is a very sensitive technique offering unique opportunities for detection and identification of molecules and molecular structures at extremely low concentrations even in complex sample matrixes. Since a nanostructured noble metal surface is required for the enhancement of Raman scattering, the acquired spectral information naturally originates from nanometer size domains making it a nanospectroscopic technique by breaking the diffraction limit of light. In this review, first Raman spectroscopy, its comparison to other related techniques, its modes and instrumentation are briefly introduced. Then, the SERS mechanism, substrates and the parameters influencing a SERS experiment are discussed. Finally, its applications in pharmaceuticals including drug discovery, drug metabolism, multifunctional chemo-photothermal-therapy-delivery-release-imaging, drug stability and drug/metabolite detection in complex biological samples are summarized and elaborated.

Comprehensive review of conventional and state-of-the-art detection methods of Cryptosporidium

Cryptosporidium is a critical waterborne protozoan pathogen found in water resources that have been a major cause of death and serious illnesses worldwide, costing millions of dollars annually for its detection and treatment. Over the past several decades, substantial efforts have been made towards developing techniques for the detection of Cryptosporidium. Early diagnostic techniques were established based on the existing tools in laboratories, such as microscopes. Advancements in fluorescence microscopy, immunological, and molecular techniques have led to the development of several kits for the detection of Cryptosporidium spp. However, these methods have several limitations, such as long processing times, large sample volumes, the requirement for bulky and expensive laboratory tools, and the high cost of reagents. There is an urgent need to improve these existing techniques and develop low-cost, portable and rapid detection tools for applications in the water quality industry. In this review, we compare recent advances in nanotechnology, biosensing and microfluidics that have facilitated the development of sophisticated tools for the detection of Cryptosporidium spp.Finally, we highlight the advantages and disadvantages, of these state-of-the-art detection methods compared to current analytical methodologies and discuss the need for future developments to improve such methods for detecting Cryptosporidium in the water supply chain to enable real-time and on-site monitoring in water resources and remote areas.

Adsorption/Desorption Behaviors and SERS Chemical Enhancement of 6-Mercaptopurine on a Nanostructured Gold Surface: The Au20 Cluster Model

Computational approaches are employed to elucidate the binding mechanism and the SERS phenomenon of 6-mercaptopurine (6MP) adsorbed on the tetrahedral Au20 cluster as a simple model for a nanostructured gold surface. Computations are carried out in both vacuum and aqueous environments using a continuum model. In the gaseous phase and neutral conditions, interaction of 6MP with the gold cluster is mostly dominated by a covalent Au-S bond and partially stabilized by the Au⋅⋅⋅H-N coupling. However, in acidic solution, the nonconventional Au⋅⋅⋅H-S hydrogen-bond becomes the most favorable binding mode. The 6MP affinity for gold clusters decreases in the order of vacuum > neutral solution > acidic medium. During the adsorption, the energy gap of Au20 substantially declines, leading to an increase in its electrical conductivity, which can be converted to an electrical noise. Moreover, such interaction is likely a reversible process and triggered by either the low pH in sick tissues or the presence of cysteine residues in protein matrices. While N-H bending and stretching vibrations play major roles in the SERS phenomenon of 6MP on gold surfaces in neutral solution, the strongest enhancement in acidic environment is mostly due to an Au⋅⋅⋅H-S coupling, rather than an aromatic ring-gold surface π overlap as previously proposed.

Long-Range Surface Plasmon Resonance Configuration for Enhancing SERS with an Adjustable Refractive Index Sample Buffer to Maintain the Symmetry Condition

We propose a method to maintain the symmetry condition of the refractive index with respect to a dielectric buffer layer for a long-range surface plasmon resonance (LRSPR) configuration. The symmetry condition was maintained by changing the concentration of the ethylene glycol aqueous solution (sample buffer layer) to match the refractive index of the MgF₂ film. Maintenance of the symmetry condition is necessary for exciting the LRSPR mode and increasing the electric field intensity near the film. We used a four-phase Kretschmann resonance setup composed of a K9 prism, MgF₂ film, Ag film, and sample buffer layer. The incident angle-dependent surface-enhanced Raman scattering (SERS) spectra were measured in the evanescent field. At the SPR angle, the SERS signal of the symmetric configuration was 60 times higher than that of the conventional SPR configuration. Moreover, the electric field penetration depth of the symmetric long-range surface plasmon configuration (>1000 nm) was longer than that of their asymmetric counterparts. The enhancement factor of the symmetric configuration was 8.6 × 10⁷, which corresponded to the lowest detectable concentration for 4-mercaptopyridine, reaching 1.0 × 10^-10 M at the resonance angle. Thus, the symmetric LRSPR configuration has great potential for label-free sensing and detection of macromolecules and biomolecules.

Biosensing based on surface-enhanced Raman spectroscopy as an emerging/next-generation point-of-care approach for acute myocardial infarction diagnosis

Cardiovascular disease is a major global health issue. In particular, acute myocardial infarction (AMI) requires urgent attention and early diagnosis. The use of point-of-care diagnostics has resulted in the improved management of cardiovascular disease, but a major drawback is that the performance of POC devices does not rival that of central laboratory tests. Recently, many studies and advances have been made in the field of surface-enhanced Raman scattering (SERS), including the development of POC biosensors that utilize this detection method. Here, we present a review of the strengths and limitations of these emerging SERS-based biosensors for AMI diagnosis. The ability of SERS to multiplex sensing against existing POC detection methods are compared and discussed. Furthermore, SERS calibration-free methods that have recently been explored to minimize the inconvenience and eliminate the limitations caused by the limited linear range and interassay differences found in the calibration curves are outlined. In addition, the incorporation of artificial intelligence (AI) in SERS techniques to promote multivariate analysis and enhance diagnostic accuracy are discussed. The future prospects for SERS-based POC devices that include wearable POC SERS devices toward predictive, personalized medicine following the Fourth Industrial Revolution are proposed.

Raman Spectroscopy and Aptamers for a Label-Free Approach: Diagnostic and Application Tools

Raman spectroscopy is a powerful optical technique based on the inelastic scattering of incident light to assess the chemical composition of a sample, including biological ones. Medical diagnostic applications of Raman spectroscopy are constantly increasing to provide biochemical and structural information on several specimens, being not affected by water interference, and potentially avoiding the constraint of additional labelling procedures. New strategies have been recently developed to overcome some Raman limitations related, for instance, to the need to deal with an adequate quantity of the sample to perform a reliable analysis. In this regard, the use of metallic nanoparticles, the optimization of fiber optic probes, and other approaches can actually enhance the signal intensity compared to spontaneous Raman scattering. Moreover, to further increase the potential of this investigation technique, aptamers can be considered as a valuable means, being synthetic, short, single, or double-stranded oligonucleotides (RNAs or DNAs) that fold up into unique 3D structures to specifically bind to selected molecules, even at very low concentrations, and thus allowing an early diagnosis of a possible disease. Due to the paramount relevance of the topic, this review focuses on the main Raman spectroscopy techniques combined with aptamer arrays in the label-free mode, providing an overview on different applications to support healthcare management.

Investigating the Origins of Toxic Response in TiO₂ Nanoparticle-Treated Cells

Titanium dioxide nanoparticles (TiO₂ NPs) are widely used in sunscreens, cosmetics and body implants, and this raises toxicity concerns. Although a large number of reports claim that they are safe to use, others claim that they induce reactive oxygen species formation and can be carcinogenic. In this study, the origins of toxic response to TiO₂ NPs were investigated by using Surface-enhanced Raman spectroscopy (SERS) which provides multidimensional information on the cellular dynamics at single cell level without any requirement for cell fixation. Three cell lines of vein (HUVEC), lung carcinoma (A549) and skin (L929) origin were tested for their toxic response upon exposure to 20, 40, 80 and 160 µg/mL anatase-TiO₂ NPs for 24 h. It was demonstrated that the level of toxic response is both cell line and dose-dependent. L929 fibroblasts were the most resistant cell line to oxidative stress whereas in HUVEC and A549, cell lines collagen and lipid deformation were observed, respectively.

Recent achievements in colorectal cancer diagnostic and therapy by the use of nanoparticles

Colorectal cancer is a major public health issue, being the third most common cancer in men and the second in women. It is one of the leading causes of cancer deaths. Nanomedicine is an emerging field of interest, many of its aspects being linked to cancer research. Chemotherapy has a well-established role in colorectal cancer management, unfortunately being limited by inability to have a selective distribution, by multidrug resistance and adverse effects. Researches carried out in recent years about nanotechnologies aimed, among others, to resolve the issues mentioned above. Targeted and localized delivery of the chemotherapeutic drugs, using nanoparticles, with selective destruction of cancerous cells would minimize the toxicity on healthy tissues. Also, the use of nanomaterials as contrast agent could improve sensitivity and specificity of diagnosis. The purpose of this review is to highlight the recent achievements of cancer research by use of nanomaterials, in the idea of finding the ideal composite, capable to simultaneous diagnostic and treat cancer.

Analysis of Genomic DNAs from Nine Rosaceae Species Using Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) of genomic DNA was used to determine genetic relationships and species identification of nine plants from three subfamilies of Rosaceae. Genomic DNA was extracted, and the SERS spectra were obtained by using a nanosilver collosol at an excitation wavelength of 785 nm. Adenine and ribodesose were the active sites of genomic DNAs in the silver surface-enhanced Raman spectra. The strong peak at 714 cm(-1) was assigned to the stretching vibration of adenine, the strong peak at 1011cm(-1) contributed to the stretching vibration of the deoxyribose and the scissoring vibrations of cytosine, and the strong peak at 625 cm(-1) is the stretching vibration of glycosidic bond and the scissoring vibrations of guanine. The three-dimensional plot of the first, second, and third principal components showed that the nine species could be classified into three categories (three subfamilies), consistent with the traditional classification. The model of the hierarchical cluster combined with the principal component of the second derivative was more reasonable. The results of the cluster analysis showed that apricot (Prunus armeniaca L.) and cherry (Prunus seudocerasus Lindl.) were clustered into one category (Prunoideae); firethorn (Firethorn fortuneana Li.), loquat (Eriobotrya japonica Lindl.), apple (Malus pumila Mill.), and crabapple (Malus hallianna Koehne.) were clustered into a second category (Pomoideae); and potentilla (Potentilla fulgens Wall.), rose (Rosa chinensis Jacd.), and strawberry (Fragaria chiloensis Duchesne.) were clustered into a third category (Rosoideae). These classifications were in accordance with the traditional classification with a correction rate of clustering of 100%. The correct rate of species identification was 100%. These five main results indicate that the genetic relationship and species identification of nine Rosaceae species could be determined by using SERS spectra of their genomic DNAs.

Raman spectroscopy for cancer diagnosis: how far have we come?

Raman spectroscopy is increasingly investigated for cancer diagnosis. As the potential of the technique is explored and realized, it is slowly making its way into clinics. There are more reports in recent years showing promise that it can help clinicians for cancer diagnosis. However, a number of challenges remain to be overcome, especially in vivo cancer diagnosis. In this article, the recent progress of the technique toward clinical cancer diagnosis is discussed from a critical perspective.

Surface-enhanced spectroscopies

This themed issue presents a collection of articles on surface-enhanced spectroscopies.

Vibrational spectroscopy of biofluids for disease screening or diagnosis: translation from the laboratory to a clinical setting

There remains a need for objective and cost-effective approaches capable of diagnosing early-stage disease in point-of-care clinical settings. Given an increasingly ageing population resulting in a rising prevalence of chronic diseases, the need for screening to facilitate the personalising of therapies to prevent or slow down pathology development will increase. Such a tool needs to be robust but simple enough to be implemented into clinical practice. There is interest in extracting biomarkers from biofluids (e.g., plasma or serum); techniques based on vibrational spectroscopy provide an option. Sample preparation is minimal, techniques involved are relatively low-cost, and data frameworks are available. This review explores the evidence supporting the applicability of vibrational spectroscopy to generate spectral biomarkers of disease in biofluids. We extend the inter-disciplinary nature of this approach to hypothesise a microfluidic platform that could allow such measurements. With an appropriate lightsource, such engineering could revolutionize screening in the 21(st) century.

Label-free detection of native proteins by surface-enhanced Raman spectroscopy using iodide-modified nanoparticles

Proteins perform vital functional and structural duties in living systems, and the in-depth investigation of protein in its native state is one of the most important challenges in the postgenomic era. Surface-enhanced Raman spectroscopy (SERS) can provide the intrinsic fingerprint information of samples with ultrahigh sensitivity but suffers from the reproducibility and reliability issues. In this paper, we proposed an iodide-modified Ag nanoparticles method (Ag IMNPs) for label-free detection of proteins. The silver nanoparticles provide the huge enhancement to boost the Raman signal of proteins, and the coated iodide layer offers a barrier to prevent the direct interaction between the proteins and the metal surface, helping to keep the native structures of proteins. With this method, highly reproducible and high-quality SERS signals of five typical proteins (lysozyme, avidin, bovine serum albumin, cytochrome c, and hemoglobin) have been obtained, and the SERS features of the proteins without chromophore were almost identical to the respective normal Raman spectra. This unique feature allows the qualitative identification of them by simply taking the intensity ratio of the Raman peaks of tryptophan to phenylalanine residues. We further demonstrated that the method can also be used for label-free multiplex analysis of protein mixture as well as to study the dynamic process of protein damage stimulated by hydrogen peroxide. This method proves to be very promising for further applications in proteomics and biomedical research.

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.

Spectroscopy of scattered light for the characterization of micro and nanoscale objects in biology and medicine

The biomedical uses for the spectroscopy of scattered light by micro and nanoscale objects can broadly be classified into two areas. The first, often called light scattering spectroscopy (LSS), deals with light scattered by dielectric particles, such as cellular and sub-cellular organelles, and is employed to measure their size or other physical characteristics. Examples include the use of LSS to measure the size distributions of nuclei or mitochondria. The native contrast that is achieved with LSS can serve as a non-invasive diagnostic and scientific tool. The other area for the use of the spectroscopy of scattered light in biology and medicine involves using conducting metal nanoparticles to obtain either contrast or electric field enhancement through the effect of the surface plasmon resonance (SPR). Gold and silver metal nanoparticles are non-toxic, they do not photobleach, are relatively inexpensive, are wavelength-tunable, and can be labeled with antibodies. This makes them very promising candidates for spectrally encoded molecular imaging. Metal nanoparticles can also serve as electric field enhancers of Raman signals. Surface enhanced Raman spectroscopy (SERS) is a powerful method for detecting and identifying molecules down to single molecule concentrations. In this review, we will concentrate on the common physical principles, which allow one to understand these apparently different areas using similar physical and mathematical approaches. We will also describe the major advancements in each of these areas, as well as some of the exciting recent developments.