Silver SERS Adenine Sensors with a Very Low Detection Limit

Exploring Surface-Enhanced Raman Spectroscopy of Pyrazine-2-Carbonitrile for Indirect Label-Free Albumin Quantification in an In Vitro Endothelium Permeability Assay

Accurate, label-free quantification of proteins, and more specifically albumin, is essential in studies aimed at monitoring transport across biological barrier tissues in vitro. Surface-enhanced Raman scattering (SERS) can deliver the sensitivity and specificity needed for such studies at physiologically relevant conditions, however, direct detection of albumin is not typically feasible at such concentrations. Here we use a small-molecule reporter (pyrazine-2-carbonitrile, PCN) that can interact both with albumin and a SERS substrate to facilitate albumin quantification. The nanoparticle surface/PCN and albumin/PCN interactions are sufficiently balanced to yield the sensitivity and specificity needed for in vitro tissue studies. The major challenge in using SERS for such assays is that the spectra of analytes can differ from their nonresonant Raman spectra, due to distinct species forming at and near the surface of the nanoparticles. Specifically, the binding of PCN to gold nanoparticles, formation of Au-PCN complexes, as well as PCN itself, contribute to the SERS spectra. We elucidate the nature of these interactions through concentration dependence studies and computational methods. Ultimately, we show that understanding these different interactions is key to quantification of albumin, at physiologically relevant albumin concentrations ranging from 0.4 to 4.4 μM using SERS spectroscopy. These data compare well with the state-of-the-art spectroscopic method, i.e., the transport of fluorescently labeled albumin across cell layers. We anticipate that this assay will stimulate analysis in in vitro models, such as organ-on-a-chip models and flow systems.

Effects of Cosolvent on the Intermolecular Interactions between an Analyte and a Gold Nanostar Surface Studied Using SERS

For surface-enhanced Raman scattering (SERS), reproducible solution-phase results are typically obtained using nanoparticles functionalized with surface-stabilizing molecules that can prevent the adsorption of analyte molecules with surface affinities lower than those of the stabilizing agent. Herein, we investigate the effects of intermolecular interactions between a nonthiolated analyte and cosolvent to facilitate and modulate analyte adsorption to gold nanostars. SERS, extinction spectroscopy, transmission electron microscopy (TEM), and density functional theory (DFT) calculations are employed in this regard. Tetrahydrofuran (THF) is utilized as a cosolvent in water to facilitate the detection of acetylsalicylic acid (aspirin) on anisotropic gold nanoparticles. Intermolecular interactions between the analyte, solvent, and surface are modulated by changing the solution composition to understand how THF facilitates the SERS detection of aspirin in THF-water cosolvents. SERS signals for 5 mM aspirin arise only in the presence of THF at and above 60 mM, while no signal with or without THF below 60 mM is observed. SERS detection of aspirin is hypothesized to depend on THF forming a hydrogen-bonded complex with aspirin that reduces aspirin hydrophobicity, thus stabilizing the acid form of the molecule and allowing it to weakly interact with the gold nanoparticles. The aspirin-THF complex adsorbs to the gold surface through π-orbital overlap between the aromatic ring and gold, where additional THF weakens orbital overlap. Understanding the mechanism by which organic cosolvents facilitate the SERS detection of nonthiolated analytes such as aspirin, in aqueous media, allows other cosolvents, nonthiolated analytes, and other surfaces to obtain a SERS signal in a variety of systems.

Stability and SERS signal strength of laser-generated gold, silver, and bimetallic nanoparticles at different KCl concentrations

Noble metal nanoparticles, specifically gold and silver, are extensively utilized in sensors, catalysts, surface-enhanced Raman scattering (SERS), and optical-electronic components due to their unique localized surface plasmon resonance (LSPR) properties. The production of these nanoparticles involves various methods, but among the environmentally friendly approaches, laser ablation stands out as it eliminates the need for toxic chemicals during purification. However, nanoparticle aggregation poses a challenge in laser ablation, necessitating the addition of extra materials that contaminate the otherwise clean process. In this study, we investigate the effectiveness of a biocompatible material, potassium chloride (KCl), in preventing particle aggregation. Although salt is known to trigger aggregation, we observed that certain concentrations of KCl can slow down this process. Over an eight-week period, we examined the aggregation rate, extinction behavior, and stability of gold, silver, and hybrid nanoparticles generated in different KCl concentrations. Extinction spectra, SEM images, SERS signal strength, and zeta potential were analyzed. Our results demonstrate that laser ablation in water and salt solutions yields nanoparticles with a spherical shape and a negative zeta potential. Importantly, we identified the optimal concentration of potassium chloride salt that maintains solution stability and SERS signal strength. Adsorbed chloride ions on silver nanoparticles were evidenced by low-frequency SERS band near 242 cm-1. A better understanding of the effect of KCl concentration on the properties of noble metal nanoparticles can lead to improved generation protocols and the development of tailored nanoparticle systems with enhanced stability and SERS activity.

Surface-Enhanced Raman Spectroscopy (SERS)-Based Sensors for Deoxyribonucleic Acid (DNA) Detection

Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful technique for the detection and analysis of biomolecules due to its high sensitivity and selectivity. In recent years, SERS-based sensors have received significant attention for the detection of deoxyribonucleic acid (DNA) molecules, offering promising applications in fields such as medical diagnostics, forensic analysis, and environmental monitoring. This paper provides a concise overview of the principles, advancements, and potential of SERS-based sensors for DNA detection. First, the fundamental principles of SERS are introduced, highlighting its ability to enhance the Raman scattering signal by several orders of magnitude through the interaction between target molecules with metallic nanostructures. Then, the fabrication technologies of SERS substrates tailored for DNA detection are reviewed. The performances of SERS substrates previously reported for DNA detection are compared and analyzed in terms of the limit of detection (LOD) and enhancement factor (EF) in detail, with respect to the technical parameters of Raman spectroscopy (e.g., laser wavelength and power). Additionally, strategies for functionalizing the sensor surfaces with DNA-specific capture probes or aptamers are outlined. The collected data can be of help in selecting and optimizing the most suitable fabrication technology considering nucleotide sensing applications with Raman spectroscopy.

New insights into the surface Enhanced Raman Scattering (SERS) response of adenine using chemometrics

The SERS response of adenine is one of the most studied, due to its outstanding exaltation. However, the spectra obtained strongly differ according to the experimental conditions and still remain not well understood. To be able to search for the presence of this molecule in complex environments, it is essential to better understand the SERS response of adenine alone. After a brief presentation of the literature on the subject, we present results suggesting that the experimental spectra would result from the overlap of different spectroscopic signatures, that may probably be due to different non-covalent interactions or different electromagnetic fields experienced by adenine molecules. An independent component analysis is reported. Our results underline the difficulty to precisely analyze the experimental data, the need to continue this research and to constitute data banks that would allow comparing the spectra obtained in different laboratories according to the experimental conditions.

Strongly coupled plasmonic metal nanoparticles with reversible pH-responsiveness and highly reproducible SERS in solution

We report a facile method to prepare polymer-grafted plasmonic metal nanoparticles (NPs) that exhibit pH-responsive surface-enhanced Raman scattering (SERS). The concept is based on the use of pH-responsive polymers, such as poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH), as multidentate ligands to wrap around the surface of NPs instead of forming polymer brushes. Upon changing the solvent quality, the grafted pH-responsive polymers would drive reversible aggregation of NPs, leading to a decreased interparticle distance. This creates numerous hot spots, resulting in a secondary enhancement of SERS as compared to the SERS from discrete NPs. For negatively charged PAA-grafted NPs, the SERS response at pH 2.5 showed a secondary enhancement of up to 104-fold as compared to the response for discrete NPs at pH 12. Similarly, positively charged PAH-grafted AuNPs showed an opposite response to pH. We demonstrated that enhanced SERS with thiol-containing and charged molecular probes was indeed from the pH-driven solubility change of polymer ligands. Our method is different from the conventional SERS sensors in the solid state. With pH-responsive polymer-grafted NPs, SERS can be performed in solution with high reproducibility and sensitivity but without the need for sample pre-concentration. These findings could pave the way for innovative designs of polymer ligands for metal NPs where polymer ligands do not compromise interparticle plasmon coupling.

Multiwavelength SERS of Magneto-Plasmonic Nanoparticles Obtained by Combined Laser Ablation and Solvothermal Methods

The present study introduces a novel method for the synthesis of magneto-plasmonic nanoparticles (MPNPs) with enhanced functionality for surface-enhanced Raman scattering (SERS) applications. By employing pulsed laser ablation in liquid (PLAL) to synthesize plasmonic nanoparticles and wet chemistry to synthesize magnetic nanoparticles, we successfully fabricated chemically pure hybrid Fe3O4@Au and Fe3O4@Ag nanoparticles. We demonstrated a straightforward approach of an electrostatic attachment of the plasmonic and magnetic parts using positively charged polyethylenimine. The MPNPs displayed high SERS sensitivity and reproducibility, and the magnetic part allowed for the controlled separation of the nanoparticles from the reaction mixture, their subsequent concentration, and their precise deposition onto a specified surface area. Additionally, we fabricated alloy based MPNPs from AgxAu100-x (x = 50 and 80 wt %) targets with distinct localized surface plasmon resonance (LSPR) wavelengths. The compositions, morphologies, and optical properties of the nanoparticles were characterized by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), UV-vis spectroscopy, and multiwavelength Raman spectroscopy. A standard SERS marker, 4-mercaptobenzoic acid (4-MBA), validated the enhancement properties of the MPNPs and found an enhancement factor of 2 × 108 for the Fe3O4@Ag nanoparticles at 633 nm excitation. Lastly, we applied MPNP-enhanced Raman spectroscopy for the analysis of the biologically relevant molecule adenine and found a limit of detection of 10-7 M at 785 nm excitation. The integration of PLAL and wet chemical methods enabled the relatively fast and cost-effective production of MPNPs characterized by high SERS sensitivity and signal reproducibility that are required in various fields, including biomedicine, food safety, materials science, security, and defense.

Intracellular metabolic profiling of drug resistant cells by surface enhanced Raman scattering

BACKGROUND

Intracellular metabolic profiling reveals real-time metabolic information useful for the study of underlying mechanisms of cells in particular conditions such as drug resistance. However, mass spectrometry (MS), one of the leading metabolomics technologies, usually requires a large number of cells and complex pretreatments. Surface enhanced Raman scattering (SERS) has an ultrahigh detection sensitivity and specificity, favorable for metabolomics analysis. However, some targeted SERS methods focus on very limited metabolite without global bioprofiling, and some label-free approaches try to fingerprint the metabolic response based on whole SERS spectral classification, but comprehensive interpretation of biological mechanisms was lacking. (95) RESULTS: We proposed a label-free SERS technique for intracellular metabolic profiling in complex cellular lysates within 3 min. We first compared three kinds of cellular lysis methods and sonication lysis shows the highest extraction efficiency of metabolites. To obtain comprehensive metabolic information, we collected a spectral set for each sample and further qualified them by the Pearson correlation coefficient (PCC) to calculate how many spectra should be acquired at least to gain the adequate information from a statistical and global view. In addition, according to our measurements with 10 pure metabolites, we can understand the spectra acquired from complex cellular lysates of different cell lines more precisely. Finally, we further disclosed the variations of 22 SERS bands in enzalutamide-resistant prostate cancer cells and some are associated with the androgen receptor signaling activity and the methionine salvage pathway in the drug resistance process, which shows the same metabolic trends as MS. (149) SIGNIFICANCE: Our technique has the capability to capture the intracellular metabolic fingerprinting with the optimized lysis approach and spectral set collection, showing high potential in rapid, sensitive and global metabolic profiling in complex biosamples and clinical liquid biopsy. This gives a new perspective to the study of SERS in insightful understanding of relevant biological mechanisms. (54).

The Role of GaN in the Heterostructure WS2/GaN for SERS Applications

In the application of WS₂ as a surface-enhanced Raman scattering (SERS) substrate, enhancing the charge transfer (CT) opportunity between WS₂ and analyte is an important issue for SERS efficiency. In this study, we deposited few-layer WS₂ (2-3 layers) on GaN and sapphire substrates with different bandgap characteristics to form heterojunctions using a chemical vapor deposition. Compared with sapphire, we found that using GaN as a substrate for WS₂ can effectively enhance the SERS signal, with an enhancement factor of 6.45 × 10⁴ and a limit of detection of 5 × 10^-6 M for probe molecule Rhodamine 6G according to SERS measurement. Analysis of Raman, Raman mapping, atomic force microscopy, and SERS mechanism revealed that The SERS efficiency increased despite the lower quality of the WS₂ films on GaN compared to those on sapphire, as a result of the increased number of transition pathways present in the interface between WS₂ and GaN. These carrier transition pathways could increase the opportunity for CT, thus enhancing the SERS signal. The WS₂/GaN heterostructure proposed in this study can serve as a reference for enhancing SERS efficiency.

Black Silicon: Breaking through the Everlasting Cost vs. Effectivity Trade-Off for SERS Substrates

Black silicon (bSi) is a highly absorptive material in the UV-vis and NIR spectral range. Photon trapping ability makes noble metal plated bSi attractive for fabrication of surface enhanced Raman spectroscopy (SERS) substrates. By using a cost-effective room temperature reactive ion etching method, we designed and fabricated the bSi surface profile, which provides the maximum Raman signal enhancement under NIR excitation when a nanometrically-thin gold layer is deposited. The proposed bSi substrates are reliable, uniform, low cost and effective for SERS-based detection of analytes, making these materials essential for medicine, forensics and environmental monitoring. Numerical simulation revealed that painting bSi with a defected gold layer resulted in an increase in the plasmonic hot spots, and a substantial increase in the absorption cross-section in the NIR range.

Recent advances of Au@Ag core-shell SERS-based biosensors

The methodological advancements in surface-enhanced Raman scattering (SERS) technique with nanoscale materials based on noble metals, Au, Ag, and their bimetallic alloy Au-Ag, has enabled the highly efficient sensing of chemical and biological molecules at very low concentration values. By employing the innovative various type of Au, Ag nanoparticles and especially, high efficiency Au@Ag alloy nanomaterials as substrate in SERS based biosensors have revolutionized the detection of biological components including; proteins, antigens antibodies complex, circulating tumor cells, DNA, and RNA (miRNA), etc. This review is about SERS-based Au/Ag bimetallic biosensors and their Raman enhanced activity by focusing on different factors related to them. The emphasis of this research is to describe the recent developments in this field and conceptual advancements behind them. Furthermore, in this article we apex the understanding of impact by variation in basic features like effects of size, shape varying lengths, thickness of core-shell and their influence of large-scale magnitude and morphology. Moreover, the detailed information about recent biological applications based on these core-shell noble metals, importantly detection of receptor binding domain (RBD) protein of COVID-19 is provided.

DEEP Surveillance of Brain Cancer Using Self-Functionalized 3D Nanoprobes for Noninvasive Liquid Biopsy

Brain cancers, one of the most fatal malignancies, require accurate diagnosis for guided therapeutic intervention. However, conventional methods for brain cancer prognosis (imaging and tissue biopsy) face challenges due to the complex nature and inaccessible anatomy of the brain. Therefore, deep analysis of brain cancer is necessary to (i) detect the presence of a malignant tumor, (ii) identify primary or secondary origin, and (iii) find where the tumor is housed. In order to provide a diagnostic technique with such exhaustive information here, we attempted a liquid biopsy-based deep surveillance of brain cancer using a very minimal amount of blood serum (5 μL) in real time. We hypothesize that holistic analysis of serum can act as a reliable source for deep brain cancer surveillance. To identify minute amounts of tumor-derived material in circulation, we synthesized an ultrasensitive 3D nanosensor, adopted SERS as a diagnostic methodology, and undertook a DEEP neural network-based brain cancer surveillance. Detection of primary and secondary tumor achieved 100% accuracy. Prediction of intracranial tumor location achieved 96% accuracy. This modality of using patient sera for deep surveillance is a promising noninvasive liquid biopsy tool with the potential to complement current brain cancer diagnostic methodologies.

Deep Learning-Based Spectral Extraction for Improving the Performance of Surface-Enhanced Raman Spectroscopy Analysis on Multiplexed Identification and Quantitation

Surface-enhanced Raman spectroscopy (SERS) has been recognized as a promising analytical technique for its capability of providing molecular fingerprint information and avoiding interference of water. Nevertheless, direct SERS detection of complicated samples without pretreatment to achieve the high-efficiency identification and quantitation in a multiplexed way is still a challenge. In this study, a novel spectral extraction neural network (SENN) model was proposed for synchronous SERS detection of each component in mixed solutions using a demonstration sample containing diquat dibromide (DDM), methyl viologen dichloride (MVD), and tetramethylthiuram disulfide (TMTD). A SERS spectra dataset including 3600 spectra of DDM, MVD, TMTD, and their mixtures was first constructed to train the SENN model. After the training step, the cosine similarity of the SENN model can achieve 0.999, 0.997, and 0.994 for DDM, MVD, and TMTD, respectively, which means that the spectra extracted from the mixture are highly consistent with those collected by the SERS experiment of the corresponding pure samples. Furthermore, a convolutional neural network model for quantitative analysis is combined with the SENN, which can simultaneously and rapidly realize the qualitative and quantitative SERS analysis of mixture solutions with lower than 8.8% relative standard deviation. The result demonstrates that the proposed strategy has great potential in improving SERS analysis in environmental monitoring, food safety, and so on.

Ascorbic acid functionalized anti-aggregated Au nanoparticles for ultrafast MEF and SERS detection of tartrazine: an ultra-wide piecewise linear range study

The enhancement mechanism of MEF and SERS.

SERSNet: Surface-Enhanced Raman Spectroscopy Based Biomolecule Detection Using Deep Neural Network

Surface-Enhanced Raman Spectroscopy (SERS)-based biomolecule detection has been a challenge due to large variations in signal intensity, spectral profile, and nonlinearity. Recent advances in machine learning offer great opportunities to address these issues. However, well-documented procedures for model development and evaluation, as well as benchmark datasets, are lacking. Towards this end, we provide the SERS spectral benchmark dataset of Rhodamine 6G (R6G) for a molecule detection task and evaluate the classification performance of several machine learning models. We also perform a comparative study to find the best combination between the preprocessing methods and the machine learning models. Our best model, coined as the SERSNet, robustly identifies R6G molecule with excellent independent test performance. In particular, SERSNet shows 95.9% balanced accuracy for the cross-batch testing task.

A Review on the Development of Gold and Silver Nanoparticles-Based Biosensor as a Detection Strategy of Emerging and Pathogenic RNA Virus

The emergence of highly pathogenic and deadly human coronaviruses, namely SARS-CoV and MERS-CoV within the past two decades and currently SARS-CoV-2, have resulted in millions of human death across the world. In addition, other human viral diseases, such as mosquito borne-viral diseases and blood-borne viruses, also contribute to a higher risk of death in severe cases. To date, there is no specific drug or medicine available to cure these human viral diseases. Therefore, the early and rapid detection without compromising the test accuracy is required in order to provide a suitable treatment for the containment of the diseases. Recently, nanomaterials-based biosensors have attracted enormous interest due to their biological activities and unique sensing properties, which enable the detection of analytes such as nucleic acid (DNA or RNA), aptamers, and proteins in clinical samples. In addition, the advances of nanotechnologies also enable the development of miniaturized detection systems for point-of-care (POC) biosensors, which could be a new strategy for detecting human viral diseases. The detection of virus-specific genes by using single-stranded DNA (ssDNA) probes has become a particular interest due to their higher sensitivity and specificity compared to immunological methods based on antibody or antigen for early diagnosis of viral infection. Hence, this review has been developed to provide an overview of the current development of nanoparticles-based biosensors that target pathogenic RNA viruses, toward a robust and effective detection strategy of the existing or newly emerging human viral diseases such as SARS-CoV-2. This review emphasizes the nanoparticles-based biosensors developed using noble metals such as gold (Au) and silver (Ag) by virtue of their powerful characteristics as a signal amplifier or enhancer in the detection of nucleic acid. In addition, this review provides a broad knowledge with respect to several analytical methods involved in the development of nanoparticles-based biosensors for the detection of viral nucleic acid using both optical and electrochemical techniques.

Selective Fluorescent Sensing of Adenine Via the Emissive Enhancement of a Simple Cobalt Porphyrin

Porphyrins absorb strongly in the visible region and are also excellent fluorophores that emit in the visible region that make them excellent candidates for fluorescence sensing and in vivo imaging. This work describes the fluorescence determination of adenine using cobalt complex of a simple porphyrin. Tetraphenylporphyrin (TPP) and tetraphenylpophyrinatocobalt(II) (CoTPP) were synthesized and characterised. TPP on metallation with cobalt resulted in the red shift of fluorescence emission in the region 652 nm and 716 nm and showed an enhancement in the emission peaks with the addition of the nucleobase, adenine. CoTPP is found to be an efficient fluorescent sensor for adenine in DMF solvent. The fluorescence enhancement is due to the formation of the ground state complex formation between adenine and CoTPP, which is supported by experimental evidences from UV- visible spectra, time resolved fluorescence life time measurements etc. The detection limit of adenine was found to be 4.2 μM using the CoTPP fluorescent probe. The proposed sensor is found to be highly selective for adenine in presence of other nitrogen bases like guanine, cytosine, uracil, thymine, alanine, histidine etc. in 1:1 concentration.

Silver-Based SERS Pico-Molar Adenine Sensor

Adenine is an important molecule for biomedical and agricultural research and applications. The detection of low concentration adenine molecules is thus desirable. Surface-enhanced Raman scattering (SERS) is a promising label-free detection and fingerprinting technique for molecules of significance. A novel SERS sensor made of clusters of silver nanostructures deposited on copper bumps in valleys of an etched silicon substrate was previously reported to exhibit a low and reproducible detection limit for a 10⁻¹¹ M neutral adenine aqueous solution. Reflection of laser illumination from the silicon surface surrounding a valley provides additional directions of laser excitation to adenine molecules adsorbing on a silver surface for the generation of enhanced SERS signal strength leading to a low detection limit. This paper further reports a concentration dependent shift of the ring-breathing mode SERS adenine peak towards 760 cm⁻¹ with decreasing concentration and its pH-dependent SERS signal strength. For applications, where the pH value can vary, reproducible detection of 10⁻¹² M adenine in a pH 9 aqueous solution is feasible, making the novel SERS structure a desirable pico-molar adenine sensor.