Surface-enhanced Raman scattering on colloidal nanostructures

A CRISPR/Cas12a-Assisted SERS Nanosensor for Highly Sensitive Detection of HPV DNA

The lack of timely and effective screening and diagnosis is a major contributing factor to the high mortality rate of cervical cancer in low-income countries and resource-limited regions. Therefore, the development of a rapid, sensitive, and easily deployable diagnostic tool for HPV DNA is of critical importance. In this study, we present a novel high-sensitivity and high-specificity detection method for HPV16 and HPV18 by integrating the CRISPR/Cas12a system with surface-enhanced Raman scattering (SERS) technology. This method leverages the trans-cleavage activity of the CRISPR/Cas12a system, which cleaves biotin-modified spherical nucleic acids (Biotin-SNA) in the presence of target DNA, releasing free Biotin-DNA. The released Biotin-DNA preferentially binds to streptavidin-modified magnetic beads (SAV-MB), reducing the capture of Biotin-SNA by SAV-MB and thereby significantly enhancing detection sensitivity. This method offers the potential for point-of-care diagnostics as it operates efficiently at 37 °C without the need for thermal cycling. Using standard DNA samples, we demonstrated that this biosensor achieved detection limits as low as 209 copies/μL and 444 copies/μL within 95 min. When combined with recombinase polymerase amplification (RPA), the sensor demonstrated enhanced sensitivity, enabling detection of target DNA at concentrations as low as 1 copy/μL within approximately 50 min. Furthermore, validation with clinical samples confirmed the feasibility and practical applicability of this method. This novel SERS-based sensor offers a new and effective tool in the prevention and detection of cervical cancer.

Light-driven in situ deposited Au nanoparticles on ZnO substrate with ultrasensitive SERS enhancement for molecular detection

Uniform, reproducible and stable SERS substrates with high detection sensitivity are crucial for their successful commercial applications. Here, we introduce a plasmonic SERS substrate based on gold nanoparticles (AuNPs) fabricated by using a straightforward light-driven in-situ method for highly sensitive molecular detection. A dense array of zinc oxide nanorods (ZnO NRs) was used as a support surface for the in-situ growth of Au nanoparticles (AuNPs). The SERS performance of the fabricated Au-ZnO substrates was evaluated by using rhodamine 6G (R6G) dye as a model Raman probe, where the distribution of the AuNPs on the substrate was found to play an important role defining the SERS activities The Au-ZnO substrates exhibited exceptional homogeneity (RSD = 3.95%), a detection limit (LOD) of 9 × 10-11 M, and signal enhancement in the order of 106. Additionally, these substrates demonstrated good stability over a period of 4 weeks when stored under standard room conditions, maintaining more than 80% of the initial Raman signal intensity. When tested for antibiotic residue detection in water using amoxicillin as a model antibiotic, the Au-ZnO substrates revealed LOD in the order of 10-9 M with linear detection over a wide concentration range of amoxicillin in water. The present work offers a straightforward and inexpensive solution-processed fabrication approach for SERS substrates that holds great potential for the development of extremely sensitive and reliable SERS-based detection and sensor systems.

Requirements for fast multianalyte detection and characterisation via electrochemical-assisted SERS in a reusable and easily manufactured flow cell

Surface-enhanced Raman spectroscopy (SERS) is a highly sensitive analytical technique that captures vibrational spectra of analytes adsorbed to rough coin metal surfaces with remarkable signal intensities. However, its wider application is limited by challenges in substrate range, quantification, and the disposable nature of SERS substrates partly due to irreversible analyte adsorption-commonly referred to as the 'memory effect'. Overcoming these limitations and achieving real-time analysis in flow-through systems remains a key challenge for the advancement of SERS. This study presents a SERS flow cell incorporating an Ag-based SERS substrate and a Pt counter-electrode, enabling the investigation of how electrochemical methods can address existing challenges. Our approach demonstrates that signal intensities can be both enhanced and spectroelectrochemically modified. Additionally, the combination of constant solvent flow and electrochemical potentials enhances the longevity of the SERS substrate, facilitating multianalyte measurements while mitigating the memory effect. Key parameters have been systematically studied, including SERS substrate materials (silver and copper), solvents, buffers, supporting electrolytes, and electrochemical protocols. We achieved consistent and reproducible electrochemical tuning of SERS signals by using halogen-free electrolytes in polar solvents commonly used in techniques like HPLC. The versatility of the system was validated through the analysis of several model compounds and the sequential detection of multiple analytes. We also successfully applied the system to detect and characterise contaminants and pharmaceuticals, highlighting its potential for a wide range of analytical applications.

Metallic and Non-Metallic Plasmonic Nanostructures for LSPR Sensors

Localized surface plasmonic resonance (LSPR) provides a unique scheme for light management and has been demonstrated across a large variety of metallic nanostructures. More recently, non-metallic nanostructures of two-dimensional atomic materials and heterostructures have emerged as a promising, low-cost alternative in order to generate strong LSPR. In this paper, a review of the recent progress made on non-metallic LSPR nanostructures will be provided in comparison with their metallic counterparts. A few applications in optoelectronics and sensors will be highlighted. In addition, the remaining challenges and future perspectives will be discussed.

Trimer structures formed by target-triggered AuNPs self-assembly inducing electromagnetic hot spots for SERS-fluorescence dual-signal detection of intracellular miRNAs

Accurate quantitative, in situ and temporal tracking imaging of tumor-associated miRNAs in living cells could provide a basis for cancer diagnosis and prognosis. In this strategy, a surface-enhanced Raman scattering (SERS)-fluorescence (FL) dual-spectral sensor (DSS) was constructed based on the nanoscale photophysical properties of AuNPs, mediated by functionalized DNA, to achieve rapid imaging of FL and accurate SERS quantification of intracellular miRNAs. The dual-spectrum sensor in the strategy is highly sensitive, specific and reproducibly stable. The LOD values of the dual spectra were 3.58 pM (SERS) as well as 11.8 pM (FL) with RSD values less than 2.69%. The bispectral sensor self-assembled into a trimer by the lapidation of Y-type DNA under the excitation of the target, generating a stable enhanced electric field coupling; and selected adenine located in the enhanced electric field as the reporter molecule, simplifying the labeling process and variables of the Raman reporter molecule, distinguishing it from other traditional methods. This strategy successfully achieved accurate tracking and quantification of miR-21 in cancer cells and showed good stability in the cells. The reported probes are potential tools for reliable monitoring of biomolecular dynamics in living cells.

Reflective FT-NIR and SERS studies of HER-II breast cancer biomarker using plasmonic-active nanostructured thin film immobilized oriented antibody

We describe the fabrication of plasmonic-active nanostructured thin film substrate as a label-free surface-enhanced Raman scattering (SERS)-based biosensor immobilized covalently with monoclonal HER-II antibody (mAb) to detect overexpressed HER-II as a biomarker in breast cancer serum (BCS). Oriented conjugation of mAb via hydrazone linkage to provide higher mAb accessibility was characterized by UV-vis and reflective Fourier transform near-infrared (FT-NIR) spectroscopic techniques. The interaction of BCS with mAb was studied by FT-NIR and nonresonant SERS at 637 nm. The results showed detection of glycoprotein content at different laser powers including a rise in amino acid and glycan content with varying results at higher power. With nonresonant SERS we observed nonlinear behavior of peak intensity. Analysis of variance was implemented to determine the effect of laser power which was found not to be a contributing factor. However, at the nanoscale, factors including the heating effect and aggregation of molecules can contribute to the nonlinearity of peak intensity.

Non-invasive optical characterization and detection of CA 15-3 breast cancer biomarker in blood serum using monoclonal antibody-conjugated gold nanourchin and surface-enhanced Raman scattering

A proof-of-concept of colloidal surface-enhanced Raman scattering (SERS) substrate for rapid selective detection of overexpressed CA 15-3 biomarker in breast cancer serum (BCS) is suggested using PEGylated gold nanourchins (GNUs) conjugated with anti-CA 15-3 monoclonal antibody (mAb). UV-vis spectroscopy provided conformational information about mAb where the initial aromatic amino acid peak was red-shifted from 271 to 291 nm. The fluorescence peak of tyrosine in mAb was reduced by ≈ 77%, and red-shifted by ≈ 3 nm after incubation in BCS. Fourier transform near-infrared spectroscopy and SERS were used to study the composition and the molecular structure of the mAb and BCS. Some of the most dominant Raman shifts after GNU-PEG-mAb interaction with BCS are 498, 736, 818, 1397, 1484, 2028, 2271, and 3227 cm-1 mainly corresponding to C-N-C in amines, vibrational modes of amino acids, C-H out-of-plane bend, C-O stretching carboxylic acid, the vibrational mode in phospholipids, NH3+ amine salt, C≡N stretching in nitriles, and O-H stretching. The intensity of SERS signals varied per trial due to the statistical behavior of GNU in BCS, agglomeration, laser power, and the heating effect. Despite very small amount of plasmonic heating, the result of the ANOVA test demonstrated that under our experimental conditions, the heating effect on signal variation is negligible and that the differences in the laser power are insignificant for all SERS observations (p > 0.6); thus, other parameters are responsible. The absorbance of mAb-conjugated GNU was decreased after five minutes of irradiation at 8 mW in the BCS.

Dynamics of Citrate Coordination on Gold Nanoparticles Under Low Specific Power Laser-Induced Heating

SERS evolution recorded over a drop-coated coffee-ring pattern of citrate-capped gold colloids was investigated as a function of time under low-specific laser power. Spectral changes caused by plasmon-induced reaction could not be detected, but a long-term transient original spectral profile showing additional lines was observed. We performed deep qualitative and quantitative SERS intensity variation analysis based on the complementary use of extreme deviation and cross-correlation statistics, which provided further insights on the behavior of citrate-capping layers of gold nanoparticles upon laser illumination. More precisely, the cross-correlation analysis made possible to follow the so-called individual events denoting particular resonance structures, in which groups of modes were assigned to an evolution of citrate coordination on gold surface driven by photo-activation. As a consequence, the detection limit was increased and new lines were related to the presence of a very low amount of dicarboxy-acetone (DCA), which was already present in the system.

CRISPR-Cas systems mediated biosensing and applications in food safety detection

Food safety, closely related to economic development of food industry and public health, has become a global concern and gained increasing attention worldwide. Effective detection technology is of great importance to guarantee food safety. Although several classical detection methods have been developed, they have some limitations in portability, selectivity, and sensitivity. The emerging CRISPR-Cas systems, uniquely integrating target recognition specificity, signal transduction, and efficient signal amplification abilities, possess superior specificity and sensitivity, showing huge potential to address aforementioned challenges and develop next-generation techniques for food safety detection. In this review, we focus on recent progress of CRISPR-Cas mediated biosensing and their applications in food safety monitoring. The properties and principles of commonly used CRISPR-Cas systems are highlighted. Notably, the frequently coupled nucleic acid amplification strategies to enhance their selectivity and sensitivity, especially isothermal amplification methods, as well as various signal output modes are also systematically summarized. Meanwhile, the application of CRISPR-Cas systems-based biosensors in food safety detection including foodborne virus, foodborne bacteria, food fraud, genetically modified organisms (GMOs), toxins, heavy metal ions, antibiotic residues, and pesticide residues is comprehensively described. Furthermore, the current challenges and future prospects in this field are tentatively discussed.

Highly Sensitive Flexible SERS-Based Sensing Platform for Detection of COVID-19

COVID-19 continues to spread and has been declared a global emergency. Individuals with current or past infection should be identified as soon as possible to prevent the spread of disease. Surface-enhanced Raman spectroscopy (SERS) is an analytical technique that has the potential to be used to detect viruses at the site of therapy. In this context, SERS is an exciting technique because it provides a fingerprint for any material. It has been used with many COVID-19 virus subtypes, including Deltacron and Omicron, a novel coronavirus. Moreover, flexible SERS substrates, due to their unique advantages of sensitivity and flexibility, have recently attracted growing research interest in real-world applications such as medicine. Reviewing the latest flexible SERS-substrate developments is crucial for the further development of quality detection platforms. This article discusses the ultra-responsive detection methods used by flexible SERS substrate. Multiplex assays that combine ultra-responsive detection methods with their unique biomarkers and/or biomarkers for secondary diseases triggered by the development of infection are critical, according to this study. In addition, we discuss how flexible SERS-substrate-based ultrasensitive detection methods could transform disease diagnosis, control, and surveillance in the future. This study is believed to help researchers design and manufacture flexible SERS substrates with higher performance and lower cost, and ultimately better understand practical applications.

Microfluidics and surface-enhanced Raman spectroscopy, a win-win combination?

With the continuous development in nanoscience and nanotechnology, analytical techniques like surface-enhanced Raman spectroscopy (SERS) render structural and chemical information of a variety of analyte molecules in ultra-low concentration. Although this technique is making significant progress in various fields, the reproducibility of SERS measurements and sensitivity towards small molecules are still daunting challenges. In this regard, microfluidic surface-enhanced Raman spectroscopy (MF-SERS) is well on its way to join the toolbox of analytical chemists. This review article explains how MF-SERS is becoming a powerful tool in analytical chemistry. We critically present the developments in SERS substrates for microfluidic devices and how these substrates in microfluidic channels can improve the SERS sensitivity, reproducibility, and detection limit. We then introduce the building materials for microfluidic platforms and their types such as droplet, centrifugal, and digital microfluidics. Finally, we enumerate some challenges and future directions in microfluidic SERS. Overall, this article showcases the potential and versatility of microfluidic SERS in overcoming the inherent issues in the SERS technique and also discusses the advantage of adding SERS to the arsenal of microfluidics.

Surface Enhanced Raman Spectroscopy: Applications in Agriculture and Food Safety

Recent global warming has resulted in shifting of weather patterns and led to intensification of natural disasters and upsurges in pests and diseases. As a result, global food systems are under pressure and need adjustments to meet the change—often by pesticides. Unfortunately, such agrochemicals are harmful for humans and the environment, and consequently need to be monitored. Traditional detection methods currently used are time consuming in terms of sample preparation, are high cost, and devices are typically not portable. Recently, Surface Enhanced Raman Scattering (SERS) has emerged as an attractive candidate for rapid, high sensitivity and high selectivity detection of contaminants relevant to the food industry and environmental monitoring. In this review, the principles of SERS as well as recent SERS substrate fabrication methods are first discussed. Following this, their development and applications for agrifood safety is reviewed, with focus on detection of dye molecules, melamine in food products, and the detection of different classes of pesticides such as organophosphate and neonicotinoids.

Surface enhanced Raman spectroscopic (SERS) behavior of phenylpyruvates used in heterogeneous catalytic asymmetric cascade reaction

The strength and geometry of adsorption of substituted phenylpyruvates on silver surface was studied by means of surface enhanced Raman spectroscopy (SERS) using silver sol. 2'-nitrophenylpyruvates were used as starting materials in a newly developed heterogeneous catalytic asymmetric cascade reaction to produce substituted quinoline derivatives. Substituents on the aromatic ring of the starting materials had significant influence on the yield of the desired quinoline derivatives. Product selectivity of the transformation of nitrophenylpyruvates were enhanced by the acid added. The geometry and the strength of the adsorption are assumed to play an important role in the outcome of this reaction, so we have tried to find correlation between the structure of adsorbed phenylpyruvates and their catalytic performance. Based on the results of our spectroscopic measurements, the enol form is predominant in the series of phenylpyruvates in solid form and methanol solutions. Stronger adsorption of phenylpyruvates in acidic media through oxygen atoms was indicated by the increased enhancement in the SERS spectrum. The nitro group of 2'-nitrophenylpyruvates has no direct role in the adsorption on Ag surface. This observation has explained why the hydrogenation of the keto group (presumably via the enol form) occurs preferentially and why the formation of the undesired indole derivatives requiring reduction of the nitro group is suppressed. The SERS behavior has helped to get a closer look on the first step of adsorption of starting materials contributing to a better understanding of the cascade reaction studied, thus providing a better flexibility in catalyst design.

Simultaneous SERS detection of illegal food additives rhodamine B and basic orange II based on Au nanorod-incorporated melamine foam

In food safety assessment, surface-enhanced Raman spectroscopy (SERS) is a novel detection method with the advantages of being fast, easy, and of high sensitivity. However, many SERS substrate synthesis methods are complex, and there are only a few studies on the simultaneous detection of multiple substances. In this study, a new, simple, low-cost SERS substrate was synthesised for the first time to simultaneously detect illegal food additives rhodamine B and basic orange II in chilli products. A lightweight, porous, and low-cost material of melamine foam (MF) was employed as the SERS synthesis template. The substrate's SERS effect on, and sensitivity to, rhodamine B and basic orange II were demonstrated. The molecular vibration and SERS enhancement mechanisms of the two target molecules were analysed by density functional theory (DFT) calculations. The results reveal that this fabricated substrate has great application potential for the supervision and testing industry.

DNA-assisted synthesis of Ortho-NanoDimer with sub-nanoscale controllable gap for SERS application

Nanostructure with precisely controllable narrow gap width remains a great challenge, especially at the sub-nanoscale level. Here, a versatile strategy named as DNA-assisted synthesis of ortho-nanodimer (DaSON) is proposed to fabricate Ag (Au) nanodimers with a uniform gap width from nanometers to angstroms. In such a strategy, two nanoparticles are constrained by the equilibrium state of the DNA hybridization and electrostatic repulsion to form zipper-like ortho-nanostructures with an extremely uniform gap whose width can be finely adjusted at nanoscale or sub-nanoscale by changing the DNA sequence and the surface charge of nanoparticles. The inherent strong electromagnetic coupling in the uniform sub-nanometer gap can generates an unparalleled SERS enhancement together with an extraordinary reproducibility. Compared with conventional DNA-based nano-gap fabrication strategy, the DaSON strategy enhances the SERS intensity for more than two orders of magnitude with a detection limit of 100 aM for DNA, and significantly improves the reproducibility in both labeled and label-free SERS sensing applications. Moreover, the DaSON strategy holds wide applicability for arbitrary kinds of DNA-modifiable nanoparticles. Therefore, we believe that the DaSON strategy provides an innovative method for the synthesis of nanostructures with controllable nanogaps and has a promising future in multiple fields including nanotechnology, catalysis and photonics.

Recent progress in mycotoxins detection based on surface-enhanced Raman spectroscopy

Mycotoxins are toxic compounds naturally produced by certain types of fungi. The contamination of mycotoxins can occur on numerous foodstuffs, including cereals, nuts, fruits, and spices, and pose a major threat to humans and animals by causing acute and chronic toxic effects. In this regard, reliable techniques for accurate and sensitive detection of mycotoxins in agricultural products and food samples are urgently needed. As an advanced analytical tool, surface-enhanced Raman spectroscopy (SERS), presents several major advantages, such as ultrahigh sensitivity, rapid detection, fingerprint-type information, and miniaturized equipment. Benefiting from these merits, rapid growth has been observed under the topic of SERS-based mycotoxin detection. This review provides a comprehensive overview of the recent achievements in this area. The progress of SERS-based label-free detection, aptasensor, and immunosensor, as well as SERS combined with other techniques, has been summarized, and in-depth discussion of the remaining challenges has been provided, in order to inspire future development of translating the techniques invented in scientific laboratories into easy-to-operate analytic platforms for rapid detection of mycotoxins.

Nano-substructured plasmonic pore arrays: a robust, low cost route to reproducible hierarchical structures extended across macroscopic dimensions

Plasmonic nanostructures are important across diverse applications from sensing to renewable energy. Periodic porous array structures are particularly attractive because such topography offers a means to encapsulate or capture solution phase species and combines both propagating and localised plasmonic modes offering versatile addressability. However, in analytical spectroscopic applications, periodic pore arrays have typically reported weaker plasmonic signal enhancement compared to particulate structures. This may be addressed by introducing additional nano-structuring into the array to promote plasmonic coupling that promotes electric field-enhancement, whilst retaining pore structure. Introducing nanoparticle structures into the pores is a useful means to promote such coupling. However, current approaches rely on either expensive top-down methods or on bottom-up methods that yield random particle placement and distribution. This report describes a low cost, top-down technique for preparation of nano-sub-structured plasmonic pore arrays in a highly reproducible manner that can be applied to build arrays extending over macroscopic areas of mm2 to cm2. The method exploits oxygen plasma etching, under controlled conditions, of the cavity encapsulated templating polystyrene (PS) spheres used to create the periodic array. Subsequent metal deposition leads to reproducible nano-structuring within the wells of the pore array, coined in-cavity nanoparticles (icNPs). This approach was demonstrated across periodic arrays with pore/sphere diameters ranging from 500 nm to 3 μm and reliably improved the plasmonic properties of the substrate across all array dimensions compared to analogous periodic arrays without the nano-structuring. The enhancement factors achieved for metal enhanced emission and surface enhanced Raman spectroscopy depended on the substrate dimensions, with the best performance achieved for nanostructured 2 μm diameter pore arrays, where a more than 104 improvement over Surface Enhanced Raman Spectroscopy (SERS) and 200-fold improvement over Metal Enhanced Fluorescence (MEF) were observed for these substrates compared with analogous unmodified pore arrays. The experiments were supported by Finite-Difference Time-Domain (FDTD) calculations used to simulate the electric field distribution as a function of pore nano-structuring.

Portable and benchtop Raman spectrometers coupled to cluster analysis to identify quinine sulfate polymorphs in solid dosage forms and antimalarial drug quantification in solution by AuNPs-SERS with MCR-ALS

This paper proposes for the first time: (a) a qualitative analytical method based on portable and benchtop backscattering Raman spectrometers coupled to hierarchical cluster analysis (HCA) and multivariate curve resolution - alternating least-squares (MCR-ALS) to identify two polymorphs of antimalarial quinine sulfate in commercial pharmaceutical tablets in their intact forms and (b) a quantitative analytical method based on gold nanoparticles (AuNPs) as active substrates for surface-enhanced Raman scattering (SERS) in combination with MCR-ALS to quantify quinine sulfate in commercial pharmaceutical tablets in solution. The pure concentration and spectral profiles recovered by MCR-ALS proved that both formulations present different polymorphs. These results were also confirmed by two clusters observed in the HCA model, according to their similarities within and among the samples that provided useful information about the homogeneity of different pharmaceutical manufacturing processes. AuNPs-SERS coupled to MCR-ALS was able to quantify quinine sulfate in the calibration range from 150.00 to 200.00 ng mL-1 even with the strong overlapping spectral profile of the background SERS signal, proving that it is a powerful ultrahigh sensitivity analytical method. This reduced linearity was validated throughout a large calibration range from 25.00 to 175.00 μg mL-1 used in a reference analytical method based on high performance liquid chromatography with a diode array detector (HPLC-DAD) coupled to MCR-ALS for analytical validation purposes, even in the presence of a coeluted compound. The analytical methods developed herein are fast, because second-order chromatographic data and first-order SERS spectroscopic data were obtained in less than 6 and 2 min, respectively. Concentrations of quinine sulfate were estimated with low root mean square error of prediction (RMSEP) values and a low relative error of prediction (REP%) in the range 1.8-4.5%.

Sperm quality of rats exposed to difenoconazole using classical parameters and surface-enhanced Raman scattering: classification performance by machine learning methods

Difenoconazole is a fungicide extensively used in agriculture. The aim of this study was to evaluate the effects of difenoconazole fungicide on the sperm quality of rats. Wistar rats were divided into four groups: control and exposed to 5 (D5), 10 (D10), or 50 mg^-1 kg bw^-1day (D50) of difenoconazole for 30 days, by gavage. Classical sperm parameters and surface-enhanced Raman scattering (SERS) were performed. Progressive motility, acrosomal integrity, and percentage of morphologically normal spermatozoa were reduced in the D10 and D50 groups in comparison with the control group. Sperm viability was reduced only in the D50 group. Sperm number in the testis and caput/corpus epididymis and daily sperm production were reduced in the three exposed groups. SERS measurements showed changes in the spectra of spermatozoa from D50 group, suggesting DNA damage. In addition, machine learning (ML) methods were used to evaluate the performance of three classification algorithms (artificial neural network-ANN, K-nearest neighbors-K-NN, and support vector machine-SVM) in the identification task of the groups exposed to difenoconazole. The results obtained by ML algorithms were very promising with accuracy ≥ 90% and validated the hypothesis of the exposure to difenoconazole reduces sperm quality. In conclusion, exposure of rats to different doses of the fungicide difenoconazole may impair sperm quality, with a recognizable classification pattern of exposure groups.

Structural analysis of the neuropeptide substance P by using vibrational spectroscopy

Substance P (SP) is one of the most studied peptide hormones and knowing the relationship between its structure and function may have important therapeutic applications in the treatment of a variety of stress-related illnesses. In order to obtain a deeper insight into its folding, the effects of different factors, such as pH changes, the presence of Ca²⁺ ions, and the substitution of the Met-NH₂ moiety in the SP structure, was studied by Raman and infrared spectroscopies. SP has a pH-dependent structure. Under acidic-neutral conditions, SP possesses a prevalent β-sheet structure although also other secondary structure elements are present. By increasing pH, a higher orderliness in the SP secondary structure is induced, as well as the formation of strongly bound intermolecular β-strands with a parallel alignment, which favour the self-assembly of SP in β-aggregates. The substitution of the Met-NH₂ moiety with the acidic functional group in the SP sequence, giving rise to a not biologically active SP analogue, results in a more disordered folding, where the predominant contribution comes from a random coil. Conversely, the presence of Ca²⁺ ions affects slightly but sensitively the folding of the polypeptide chain, by favouring the α-helical content and a different alignment of β-strands; these are structural elements, which may favour the SP biological activity. In addition, the capability of SERS spectroscopy to detect SP in its biologically active form was also tested by using different metal nanoparticles. Thanks to the use of silver NPs prepared by reduction of silver nitrate with hydroxylamine hydrochloride, SP can be detected at very low peptide concentration (~ 90 nM). However, the SERS spectra cannot be obtained under alkaline conditions since both the formation of SP aggregates and the lack of ion pairs do not allow a strong enough interaction of SP with silver NPs. Graphical abstract.

Vibrational and electronic spectroscopic detection and quantification of carminic acid in candies

Carminic acid (CA) contained in one kind of gummy candy was detected and quantified by using vibrational and electronic spectroscopy, respectively; the proposed methodology in solution is simple and rapid without sample pretreatment as usually used and reported. The identification of CA in candies was performed through the Raman and surface enhanced Raman scattering (SERS) spectra along with the equivalent spectral data from the natural CA dye. A modified silver colloidal solution was used in order to obtain SERS activity of CA at very low concentration in colloidal aqueous solution. Theoretical calculations allow infer about both the CA/silver surface interaction nature and on the orientation of CA on the surface. The electronic spectroscopic (UV-Vis) data allowed quantify CA in candies; the amount resulted nearly identical to that determined from HPLC traditional measurements. The present results should contribute to the health of children consumers.

A Review on Surface-Enhanced Raman Scattering

Surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical, material and life sciences, owing to its intrinsic features (i.e., fingerprint recognition capabilities and high sensitivity) and to the technological advancements that have lowered the cost of the instruments and improved their sensitivity and user-friendliness. We provide an overview of the most significant aspects of SERS. First, the phenomena at the basis of the SERS amplification are described. Then, the measurement of the enhancement and the key factors that determine it (the materials, the hot spots, and the analyte-surface distance) are discussed. A section is dedicated to the analysis of the relevant factors for the choice of the excitation wavelength in a SERS experiment. Several types of substrates and fabrication methods are illustrated, along with some examples of the coupling of SERS with separation and capturing techniques. Finally, a representative selection of applications in the biomedical field, with direct and indirect protocols, is provided. We intentionally avoided using a highly technical language and, whenever possible, intuitive explanations of the involved phenomena are provided, in order to make this review suitable to scientists with different degrees of specialization in this field.

Stimulated Adsorption of Humic Acids on Capped Plasmonic Ag Nanoparticles Investigated by Surface-Enhanced Optical Techniques

The adsorption of humic substances on Ag nanoparticles (AgNPs) is of crucial environmental importance and determines the toxicity of these NPs and the structure of adsorbed organic matter. In this work, the adsorption of two standard soil and leonardite International Humic Substances Society humic acids was studied on AgNPs of different sizes, shapes (spherical and star-like), and interfacial chemical compositions. Surface-enhanced optical (Raman and fluorescence) spectroscopies were used to follow the specific chemical groups involved in this adsorption. By means of the latter optical techniques, information regarding the binding mechanism and the macromolecular aggregation can be deduced. The influence of the surface chemical composition induced by the different functionalizations of the interfaces of these NPs is highly important regarding the chemical interactions of these complex organic macromolecules. The surface functionalization with positively charged alkyl diamines led to a large increase in the adsorption as well as a strong structural rearrangement of the macromolecule once adsorbed onto the surface.

Highly Localized SERS Measurements Using Single Silicon Nanowires Decorated with DNA Origami-Based SERS Probe

Surface enhanced Raman spectroscopy (SERS) measurements are conventionally performed using assemblies of metal nanostructures on a macro- to micro-sized substrate or by dispersing colloidal metal nanoparticles directly onto the sample of interest. Despite intense use, these methods allow neither the removal of the nanoparticles after a measurement nor a defined confinement of the SERS measurement position. So far, tip enhanced Raman spectroscopy is still the key technique in this regard but not adequate for various samples mainly due to diminished signal enhancement compared to other techniques, poor device fabrication reproducibility, and cumbersome experimental setup requirements. Here, we demonstrate that a rational combination of only four gold nanoparticles (AuNPs) on a DNA origami template, and single silicon nanowires (SiNWs) yield functional optical amplifier nanoprobes for SERS. These nanoscale SERS devices offer a spatial resolution below the diffraction limit of light and still a high electric field intensity enhancement factor ( EF) of about 10⁵ despite of miniaturization.

Tuning plasmons layer-by-layer for quantitative colloidal sensing with surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy (SERS) is well known for its high sensitivity that emerges due to the plasmonic enhancement of electric fields typically on gold and silver nanostructures. However, difficulties associated with the preparation of nanostructured substrates with uniform and reproducible features limit reliability and quantitation using SERS measurements. In this work we use layer-by-layer (LbL) self-assembly to incorporate multiple functional building blocks of collaborative assemblies of nanoparticles on colloidal spheres to fabricate SERS sensors. Gold nanoparticles (AuNPs) are packaged in discrete layers, effectively 'freezing nano-gaps', on spherical colloidal cores to achieve multifunctionality and reproducible sensing. Coupling between layers tunes the plasmon resonance for optimum SERS signal generation to achieve a 10 nM limit of detection. Significantly, using the layer-by-layer construction, SERS-active AuNP layers are spaced out and thus optically isolated. This uniquely allows the creation of an internal standard within each colloidal sensor to enable highly reproducible self-calibrated sensing. By using 4-mercaptobenzoic acid (4-MBA) as the internal standard adenine concentrations are quantified to an accuracy of 92.6-99.5%. Our versatile approach paves the way for rationally designed yet quantitative colloidal SERS sensors and their use in a variety of sensing applications.

Exploring the influence of Diels-Alder linker length on photothermal molecule release from gold nanorods

We studied the photothermal release of carboxyfluorescein (CF) linked to the gold surface of gold nanorods (GNRs) by two Diels-Alder adducts of different lengths (n = 4 and n = 9). The functionalized GNRs were irradiated with infrared light to produce photothermal release of CF by a retro-Diels-Alder reaction. The adducts were chemisorbed on the GNRs and the functionalized nanoparticles were characterized by UV-vis, DLS, zeta potential and Raman and surface-enhanced Raman spectroscopy (SERS). On the basis of the degree of nanoparticle functionalization and the SERS results, we inferred the orientation of CF on the surface of the gold nanoparticle. Moreover, we determined the photothermal release profiles of CF from the gold surface by laser irradiation. The release was faster for the longer linker (n = 9). SERS revealed that, for the shorter linker (n = 4), molecules are oriented perpendicularly with respect to the gold surface, thereby maintaining the CF far from the surface. In contrast, the longer linker was observed to be tilted, thus maintaining CF close to the gold surface and therefore potentially favoring the photothermal transfer of energy. These results are relevant for the future development of the spatial and temporal controlled release of drugs by means of gold nanoparticles.

Cellular imaging by targeted assembly of hot-spot SERS and photoacoustic nanoprobes using split-fluorescent protein scaffolds

The in cellulo assembly of plasmonic nanomaterials into photo-responsive probes is of great interest for many bioimaging and nanophotonic applications but remains challenging with traditional nucleic acid scaffolds-based bottom-up methods. Here, we address this quandary using split-fluorescent protein (FP) fragments as molecular glue and switchable Raman reporters to assemble gold or silver plasmonic nanoparticles (NPs) into photonic clusters directly in live cells. When targeted to diffusing surface biomarkers in cancer cells, the NPs self-assemble into surface-enhanced Raman-scattering (SERS) nanoclusters having hot spots homogenously seeded by the reconstruction of full-length FPs. Within plasmonic hot spots, autocatalytic activation of the FP chromophore and near-field amplification of its Raman fingerprints enable selective and sensitive SERS imaging of targeted cells. This FP-driven assembly of metal colloids also yields enhanced photoacoustic signals, allowing the hybrid FP/NP nanoclusters to serve as contrast agents for multimodal SERS and photoacoustic microscopy with single-cell sensitivity.

Different binding sites of serum albumins in the protein corona of gold nanoparticles

Binding sites of albumins on gold nanoparticles were characterized by surface-enhanced Raman scattering.

Properties of in situ generated gold nanoparticles in the cellular context

Gold nanostructures that serve as probes for nanospectroscopic analysis of eukaryotic cell cultures can be obtained by the in situ reduction of tetrachloroauric acid (HAuCl₄). To understand the formation process of such intracellularly grown particles depending on the incubation medium, the reaction was carried out with 3T3 fibroblast cells in three different incubation media, phosphate buffer, Dulbecco's Modified Eagle Medium (DMEM), and standard cell culture medium (DMEM with fetal calf serum). The size, the optical properties, the biomolecular corona, and the localization of the gold nanoparticles formed in situ vary for the different conditions. The combination of surface-enhanced Raman scattering (SERS) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) microscopic mapping and transmission electron microscopy (TEM) provides complementary perspectives on plasmonic nanoparticles and non-plasmonic gold compounds inside the cells. While for the incubation with HAuCl₄ in PBS, gold particles provide optical signals from the nucleus, the incubation in standard cell culture medium leads to scavenging of the toxic molecules and the formation of spots of high gold concentration in the cytoplasm without formation of SERS-active particles inside the cells. The biomolecular corona of nanoparticles formed in situ after incubation in buffer and DMEM differs, suggesting that different intracellular molecular species serve for reduction and stabilization. Comparison with data obtained from ready-made gold nanoparticles suggests complementary application of in situ and ex situ generated nanostructures for optical probing.

Negligible interaction of [Ru(Phen)3]2+ with human serum albumin makes it promising for a reliable invivo assessment of the tissue oxygenation

The interaction between a ruthenium - based water soluble oxygen probe ([Ru(Phen)₃]²⁺, phen - phenanthroline) and human serum albumin (HSA) was investigated with the aim of describing the influence of HSA on the [Ru(Phen)₃]²⁺ luminescence properties. Nowadays, several oxygen sensitive luminescent probes are used to determine the oxygen level in different compartments of living organisms. However, they can interact, depending on their hydrophilic/hydrophobic characters, with various serum proteins, and/or lipids, during their utilization for invivo oxygen measurement. Since HSA is the most abundant serum protein in most biological organisms, its presence may affect the spectral properties of the employed probes and, consequently, the determination of the oxygen concentration. Having this in mind, we have applied several spectroscopic and calorimetric techniques to study [Ru(Phen)₃]²⁺ - HSA mixtures. Only a negligible effect of HSA on the absorption and luminescence spectra of [Ru(Phen)₃]²⁺ was observed. In addition, differential scanning calorimetric studies showed that [Ru(Phen)₃]²⁺ does not significantly influence HSA thermal stability. Importantly, [Ru(Phen)₃]²⁺ retained a reliable luminescence lifetime sensitivity to the oxygen concentration in solutions supplemented with HSA and in U87 MG cancer cells. Finally, the biodistribution of [Ru(Phen)₃]²⁺ in the presence of serum proteins in the blood stream of chick embryo's chorioallantoic membrane (CAM) was investigated. Fast [Ru(Phen)₃]²⁺ and similar extravasations were observed in the presence or absence of CAM-serum. We can conclude that HSA-[Ru(Phen)₃]²⁺ complex interaction does not significantly influence the potential of [Ru(Phen)₃]²⁺ to be a suitable candidate for a reliable oxygen probe in living organisms.

In situ synthesis of Au-shelled Ag nanoparticles on PDMS for flexible, long-life, and broad spectrum-sensitive SERS substrates

A simple, fast, one-step fabrication of silver nanoparticles with atomically thin gold coatings on polydimethylsiloxane affords oxidation-resistant and highly sensitive surface enhanced Raman scattering (SERS) substrates.

Quantitative detection of codeine in human plasma using surface-enhanced Raman scattering via adaptation of the isotopic labelling principle

In this study surface-enhanced Raman scattering (SERS) combined with the isotopic labelling (IL) principle has been used for the quantification of codeine spiked into both water and human plasma.

Plant-Mediated Synthesis of Silver Nanoparticles: Their Characteristic Properties and Therapeutic Applications

Interest in "green nanotechnology" in nanoparticle biosynthesis is growing among researchers. Nanotechnologies, due to their physicochemical and biological properties, have applications in diverse fields, including drug delivery, sensors, optoelectronics, and magnetic devices. This review focuses on the green synthesis of silver nanoparticles (AgNPs) using plant sources. Green synthesis of nanoparticles is an eco-friendly approach, which should be further explored for the potential of different plants to synthesize nanoparticles. The sizes of AgNPs are in the range of 1 to 100 nm. Characterization of synthesized nanoparticles is accomplished through UV spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, and scanning electron microscopy. AgNPs have great potential to act as antimicrobial agents. The green synthesis of AgNPs can be efficiently applied for future engineering and medical concerns. Different types of cancers can be treated and/or controlled by phytonanotechnology. The present review provides a comprehensive survey of plant-mediated synthesis of AgNPs with specific focus on their applications, e.g., antimicrobial, antioxidant, and anticancer activities.

Monitoring of anatabine release by methyl jasmonate elicited BY-2 cells using surface-enhanced Raman scattering

A new application of surface-enhanced Raman scattering (SERS) in the field of plant material analysis is proposed in this study. The aim was to monitor the release of anatabine by methyl jasmonate (MeJa) elicited Bright Yellow-2 (BY-2) cells. Gold nanoparticles (AuNps) were used as SERS substrate. The first step was to study the SERS activity of anatabine in a complex matrix comprising the culture medium and BY-2 cells. The second step was the calibration. This one was successfully performed directly in the culture medium in order to take into account the matrix effect, by spiking the medium with different concentrations of anatabine, leading to solutions ranging from 250 to 5000µgL(-1). A univariate analysis was performed, the intensity of a band situated at 1028cm(-1), related to anatabine, was plotted against the anatabine concentration. A linear relationship was observed with a R(2) of 0.9951. During the monitoring study, after the MeJa elicitation, samples were collected from the culture medium containing BY-2 cells at 0, 24h, 48h, 72h and 96h and were analysed using SERS. Finally, the amount of anatabine released in the culture medium was determined using the response function, reaching a plateau after 72h of 82µg of anatabine released/g of fresh weight (FW) MeJa elicited BY-2 cells.

Graphene Dendrimer-stabilized silver nanoparticles for detection of methimazole using Surface-enhanced Raman scattering with computational assignment

Graphene functionalized with polyamidoamine dendrimer, decorated with silver nanoparticles (G-D-Ag), was synthesized and evaluated as a substrate with surface-enhanced Raman scattering (SERS) for methimazole (MTZ) detection. Sodium borohydride was used as a reducing agent to cultivate silver nanoparticles on the dendrimer. The obtained G-D-Ag was characterized by using UV-vis spectroscopy, scanning electron microscope (SEM), high-resolution transmission electron microscope (TEM), Fourier-transformed infrared (FT-IR) and Raman spectroscopy. The SEM image indicated the successful formation of the G-D-Ag. The behavior of MTZ on the G-D-Ag as a reliable and robust substrate was investigated by SERS, which indicated mostly a chemical interaction between G-D-Ag and MTZ. The bands of the MTZ normal spectra at 1538, 1463, 1342, 1278, 1156, 1092, 1016, 600, 525 and 410 cm(-1) were enhanced due to the SERS effect. Correlations between the logarithmical scale of MTZ concentrations and SERS signal intensities were established, and a low detection limit of 1.43 × 10(-12) M was successfully obtained. The density functional theory (DFT) approach was utilized to provide reliable assignment of the key Raman bands.