Comparative electron and photon excitation of localized surface plasmon resonance in lithographic gold arrays for enhanced Raman scattering

Recent progress in Surface-Enhanced Raman Spectroscopy detection of biomarkers in liquid biopsy for breast cancer

Breast cancer is the most commonly diagnosed cancer in women globally and a leading cause of cancer-related mortality. However, current detection methods, such as X-rays, ultrasound, CT scans, MRI, and mammography, have their limitations. Recently, with the advancements in precision medicine and technologies like artificial intelligence, liquid biopsy, specifically utilizing Surface-Enhanced Raman Spectroscopy (SERS), has emerged as a promising approach to detect breast cancer. Liquid biopsy, as a minimally invasive technique, can provide a temporal reflection of breast cancer occurrence and progression, along with a spatial representation of overall tumor information. SERS has been extensively employed for biomarker detection, owing to its numerous advantages such as high sensitivity, minimal sample requirements, strong multi-detection ability, and controllable background interference. This paper presents a comprehensive review of the latest research on the application of SERS in the detection of breast cancer biomarkers, including exosomes, circulating tumor cells (CTCs), miRNA, proteins and others. The aim of this review is to provide valuable insights into the potential of SERS technology for early breast cancer diagnosis.

Imaging of Antiferroelectric Dark Modes in an Inverted Plasmonic Lattice

Plasmonic lattice nanostructures are of technological interest because of their capacity to manipulate light below the diffraction limit. Here, we present a detailed study of dark and bright modes in the visible and near-infrared energy regime of an inverted plasmonic honeycomb lattice by a combination of Au+ focused ion beam lithography with nanometric resolution, optical and electron spectroscopy, and finite-difference time-domain simulations. The lattice consists of slits carved in a gold thin film, exhibiting hotspots and a set of bright and dark modes. We proposed that some of the dark modes detected by electron energy-loss spectroscopy are caused by antiferroelectric arrangements of the slit polarizations with two times the size of the hexagonal unit cell. The plasmonic resonances take place within the 0.5-2 eV energy range, indicating that they could be suitable for a synergistic coupling with excitons in two-dimensional transition metal dichalcogenides materials or for designing nanoscale sensing platforms based on near-field enhancement over a metallic surface.

Machine Learning-Aided Design of Gold Core-Shell Nanocatalysts toward Enhanced and Selective Photooxygenation

We demonstrate the use of the machine learning (ML) tools to rapidly and accurately predict the electric field as a guide for designing core-shell Au-silica nanoparticles to enhance ¹O₂ sensitization and selectivity of organic synthesis. Based on the feature importance analysis, obtained from a deep neural network algorithm, we found a general and linear dependent descriptor (θ ∝ aD^0.25t^-1, where a, D, and t are the shape constant, size of metal nanoparticles, and distance from the metal surface) for the electric field around the core-shell plasmonic nanoparticle. Directed by the new descriptor, we synthesized gold-silica nanoparticles and validated their plasmonic intensity using scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS) mapping. The nanoparticles with θ = 0.40 demonstrate an ∼3-fold increase in the reaction rate of photooxygenation of anthracene and 4% increase in the selectivity of photooxygenation of dihydroartemisinic acid (DHAA), a long-standing goal in organic synthesis. In addition, the combination of ML and experimental investigations shows the synergetic effect of plasmonic enhancement and fluorescence quenching, leading to enhancement for ¹O₂ generation. Our results from time-dependent density functional theory (TD-DFT) calculations suggest that the presence of an electric field can favor intersystem crossing (ISC) of methylene blue to enhance ¹O₂ generation. The strategy reported here provides a data-driven catalyst preparation method that can significantly reduce experimental cost while paving the way for designing photocatalysts for organic drug synthesis.

Exosome detection via surface-enhanced Raman spectroscopy for cancer diagnosis

As nanoscale extracellular vesicles, exosomes are secreted by various cell types, and they are widely distributed in multiple biological fluids. Studies have shown that tumor-derived exosomes can carry a variety of primary tumor-specific molecules, which may represent a novel tool for the early detection of cancer. However, the clinical translation of exosomes remains a challenge due to the requirement of large quantities of samples when enriching the cancer-related exosomes in biological fluids, the insufficiency of traditional techniques for exosome subpopulations, and the complex exosome isolation of the current commercially available exosome phenotype profiling approaches. The evolving surface-enhanced Raman scattering (SERS) technology, with properties of unique optoelectronics, easy functionalization, and the particular interaction between light and nanoscale metallic materials, can achieve sensitive detection of exosomes without large quantities of samples and multiplexed phenotype profiling, providing a new mode of real-time and noninvasive analysis for cancer patients. In the present review, we mainly discussed exosome detection based on SERS, especially SERS immunoassay. The basic structure and function of exosomes were firstly introduced. Then, recent studies using the SERS technique for cancer detection were critically reviewed, which mainly included various SERS substrates, biological modification of SERS substrates, SERS-based exosome detection, and the combination of SERS and other technologies for cancer diagnosis. This review systematically discussed the essential aspects, limitations, and considerations of applying SERS technology in the detection and analysis of cancer-derived exosomes, which could provide a valuable reference for the early diagnosis of cancer through SERS technology. STATEMENT OF SIGNIFICANCE: Surface-enhanced Raman scattering (SERS) has been applied to exosomes detection to obtain better diagnostic results. In past three years, several reviews have been published in exosome detection, which were narrowly focus on methods of exosome detection. Selection and surface functionalization of the substrate and the combination detection with different methods based on SERS will provide new strategies for the detection of exosomes. This review will focus on the above aspects. This emerging detection method is constantly evolving and contributing to the early discovery of diseases in the future.

Molecular and plasmonic resonances on tip-enhanced Raman spectroscopy

Plasmon has been widely investigated and applied, because it can greatly enhance molecular Raman spectral intensity. In this study, the resonance Raman effect of the tetra-tert-butylnaphthalocyanine (TTBN) is analyzed, including the Raman wave number shift and enhancement factor, resulting from light of different incident wavelengths. Furthermore, the optical properties of TTBN are obtained, such as charge transfer, the electronic circular dichroism (ECD) spectrum, etc. Lastly, we study the tip-enhanced Raman spectroscopy (TERS) by adjusting the parameters of the metal tip to achieve the highest electromagnetic enhancement at different incident wavelengths. Combining the resonance excitation effect and the tip enhanced Raman effect, the enhancement factor of TERS can reach up to 10⁸-10⁹. This study provides significant help for a profound understanding of the TERS mechanism.

Gold Nanoparticles Generated Using the Nanosecond Laser Treatment of Multilayer Films and Their Use for SERS Applications

Surface-enhanced Raman spectroscopy (SERS) substrates fabricated using a repeated laser treatment of thin gold films are demonstrated. The presented SERS substrates consist of the gold nanoparticles, whose density and size depend on the used film thickness and number of treated films. The larger number of the treated gold film layers increases the amount of larger nanoparticles (size >20 nm). However, a large number of small nanoparticles (5-20 nm) in all cases is also observed. The manufactured SERS substrates exhibit a high enhancement factor, which is in the range of 10⁶. The enhancement factor can be increased by adding an additional Au coating on the top of nanoparticles generated from a single gold layer. The demonstrated laser-based fabrication approach of large-scale SERS substrates is simple, reliable, without the use of chemicals for the reduction and stabilization of nanoparticles, and cost-effective.

Gap-Dependent Surface-Enhanced Raman Scattering (SERS) Enhancement Model of SERS Substrate-Probe Combination Using a Polyelectrolyte Nanodroplet as a Distance Controller

The development of surface-enhanced Raman scattering (SERS) biosensor platforms based on the sandwich combination of an SERS substrate and Raman reporter coated gold nanoparticle (AuNP) labeled with antibody has been widely performed for highly sensitive detection of biomolecules. The size of biomolecules located between these SERS-active materials dictates the sensitivity enhancement of the sensor. However, no suitable molecular size is provided. In this study, we report the gap-dependent SERS enhancement model using the combination of two SERS-active materials of 2D arrays of gold core-silver shell nanoparticles (Au@Ag core-shell NPs) as SERS-active substrates and mercaptobenzoic acid (MBA)-labeled AuNPs as SERS probes. The distance between these two materials is finely tuned using layer-by-layer assembled polyelectrolyte multilayer films. The morphology of the polyelectrolyte spacer is controlled into a droplet nanostructure, which is assumed to have a comparable shape with globular biomolecules. The well-controlled height or thickness of polyelectrolyte nanodroplet was achieved by changing number of deposition cycles. By increasing the thickness of the polyelectrolyte nanodroplet, MBA SERS intensities gradually decreased until at 40 nm-thick nanodroplet film and maintained afterward. This spacer thickness defined the limit of plasmonic coupling effect from this SERS probe-substrate combination. The SERS enhancement capability of this model was compared to conventional SERS immunoassay using three different antigen-antibody complex sizes of prostate-specific antigen, carcinoembryonic antigen, and carbohydrate antigen 19-9. Good agreement of the limitation of plasmon coupling as a function of the distance between the SERS substrate-probe combination using this developed model and SERS immunoassay was found. The finding provides valuable guidelines for immune-system selection in SERS immunosensors based on SERS substrate-probe combination.

Exploring valence states of abnormal mineral deposits in biological tissues using correlative microscopy and spectroscopy techniques: A case study on ferritin and iron deposits from Alzheimer's disease patients

Abnormal accumulation of inorganic trace elements in a human brain, such as iron, zinc and aluminum, oftentimes manifested as deposits and accompanied by a chemical valence change, is pathologically relevant to various neurodegenerative diseases. In particular, Fe²⁺ has been hypothesized to produce free radicals that induce oxidative damage and eventually cause Alzheimer's disease (AD). However, traditional biomedical techniques, e.g. histology staining, are limited in studying the chemical composition and valence states of these inorganic deposits. We apply commonly used physical (phys-) science methods such as X-ray energy dispersive spectroscopy (EDS), focused-ion beam (FIB) and electron energy loss spectroscopy (EELS) in transmission electron microscopy in conjunction with magnetic resonance imaging (MRI), histology and optical microscopy (OM) to study the valence states of iron deposits in AD patients. Ferrous ions are found in all deposits in brain tissues from three AD patients, constituting 0.22-0.50 of the whole iron content in each specimen. Such phys-techniques are rarely used in medical science and have great potential to provide unique insight into biomedical problems.