Enhancement of Single Molecule Raman Scattering using Sprouted Potato Shaped Bimetallic Nanoparticles

Observation of reversible and irreversible charge transfer processes in dye-monolayer graphene systems using Raman spectroscopy as a tool

Herein, we report the Raman spectroscopy of crystal violet (CV) and IR-780 Iodide molecules dispersed on the monolayer graphene film (MGF). In the CV-MGF system, the enhancement in the Raman scattering of CV molecules is observed irrespective of the location probed during the spectral measurements. This enhancement is due to the charge transfer from the MGF to CV molecules. However, in the case of the IR-780 Iodide - MGF system, the enhancement of Raman scattering of dye molecules or MGF is observed strongly depending upon the probed location. These observations indicate that the charge transfer is irreversible and reversible in the CV-MGF and IR-780 Iodide-MGF systems, respectively. Importantly, for the first time, this experimental study revealed that enhancing the Raman scattering of MGF is possible through the "chemical mechanism" with suitable dye molecules apart from the "electromagnetic mechanism" with plasmonic hot spots of the metal nanoparticles and photonic nanojets of single dielectric microparticles.

Plasmonic substrates for biochemical applications of surface-enhanced Raman spectroscopy

Due to its great practical importance, the detection and determination of many biomolecules in body fluids and other samples is carried out in a large number of laboratories around the world. One of the most promising analytical techniques now being widely introduced into medical analysis is surface-enhanced Raman scattering (SERS) spectroscopy. SERS is one of the most sensitive analytical methods, and in some cases, a good quality SERS spectrum dominated by the contribution of even a single molecule can be obtained. Highly sensitive SERS measurements can only be carried out on substrates generating a very high SERS enhancement factor and a low Raman spectral background, and so using of right nanomaterials is a key element in the success of SERS biochemical analysis. In this review article, we present progress that has been made in the preparation of nanomaterials used in SERS spectroscopy for detecting various kinds of biomolecules. We describe four groups of nanomaterials used in such measurements: nanoparticles of plasmonic metals and deposits of plasmonic nanoparticles on macroscopic substrates, nanocomposites containing plasmonic and non-plasmonic parts, nanostructured macroscopic plasmonic metals, and nanostructured macroscopic non-plasmonic materials covered by plasmonic films. We also describe selected SERS biochemical analyses that utilize the nanomaterials presented. We hope that this review will be useful for researchers starting work in this fascinating field of ​​science and technology.

Electrochemical DNA Cleavage Sensing for EcoRV Activity and Inhibition with an ERGO Electrode

An electrochemically reduced graphene oxide (ERGO) electrode-based electrochemical assay was developed for rapid, sensitive, and straightforward analysis of both activity and inhibition of the endonuclease EcoRV. The procedure uses a DNA substrate designed for EcoRV, featuring a double-stranded DNA (dsDNA) region labeled with methylene blue (MB) and a single-stranded DNA (ssDNA) region immobilized on the ERGO surface. The ERGO electrode, immobilized with the DNA substrate, was subsequently exposed to a sample containing EcoRV. Upon enzymatic hydrolysis, the cleaved dsDNA fragments were detached from the ERGO surface, leading to a decrease in the MB concentration near the electrode. This diminished the electron transfer efficiency for MB reduction, resulting in a decreased reduction current. This assay demonstrates excellent specificity and high sensitivity, with a limit of detection (LOD) of 9.5 × 10-3 U mL-1. Importantly, it can also measure EcoRV activity in the presence of aurintricarboxylic acid, a known inhibitor, highlighting its potential for drug discovery and clinical diagnostic applications.

Self-assembly-based integration of Ag@Au oligomers and core/shell nanoparticles on polymer chips for efficient sensing devices

Two distinct Ag/Au nanocomposites, namely, hetero-oligomers and eccentric core/shells were obtained by one-step polymer self-assembly-based fabrication. The Ag concentration-dependent, facet-specific passivation, and presence or absence of anisotropic facets were the main factors responsible for controlling the structures of the final products. Based on an understanding of the role of Ag⁺ ions in controlling the shape of anisotropic gold nanoparticles (AuNPs), tailored concentrations of Ag⁺ were applied to design nanoparticles with desired anisotropic surface facets to allow site-specific Ag coatings on AuNPs. The Ag additives acted as shape-directing agents due to an underpotential deposition (UPD) that was responsible for stabilizing the various surface facets that enclose the AuNPs. The morphology differences between the substrate samples resulted in discrete plasmonic and sensing features. In surface-enhanced Raman spectroscopy (SERS) studies, we showed that the site-selective deposition of Ag on anisotropic gold nanohexagons (AuNHs) delivers more advantages as compared to their hetero-oligomer nanostructured counterparts due to synergistic effects.

A review of cardiac troponin I detection by surface enhanced Raman spectroscopy: Under the spotlight of point-of-care testing

Cardiac troponin I (cTnI) is a biomarker widely related to acute myocardial infarction (AMI), one of the leading causes of death around the world. Point-of-care testing (POCT) of cTnI not only demands a short turnaround time for its detection but the highest accuracy levels to set expeditious and adequate clinical decisions. The analytical technique Surface-enhanced Raman spectroscopy (SERS) possesses several properties that tailor to the POCT format, such as its flexibility to couple with rapid assay platforms like microfluidics and paper-based immunoassays. Here, we analyze the strategies used for the detection of cTnI by SERS considering POCT requirements. From the detection ranges reported in the reviewed literature, we suggest the diseases other than AMI that could be diagnosed with this technique. For this, a section with information about cardiac and non-cardiac diseases with cTnI release, including their release kinetics or cut-off values are presented. Likewise, POCT features, the use of SERS as a POCT technique, and the biochemistry of cTnI are discussed. The information provided in this review allowed the identification of strengths and lacks of the available SERS-based point-of-care tests for cTnI and the disclosing of requirements for future assays design.

Picosecond Laser-Ablated Nanoparticles Loaded Filter Paper for SERS-Based Trace Detection of Thiram, 1,3,5-Trinitroperhydro-1,3,5-triazine (RDX), and Nile Blue

Recently, filter paper (FP)-based surface-enhanced Raman scattering (SERS) substrates have stimulated significant attention owing to their promising advantages such as being low-cost, easy to handle, and practically suitable for real-field applications in comparison to the solid-based substrates. Herein, a simple and versatile approach of laser-ablation in liquid for the fabrication of silver (Ag)-gold (Au) alloy nanoparticles (NPs). Next, the optimization of flexible base substrate (sandpaper, printing paper, and FP) and the FP the soaking time (5−60 min) was studied. Further, the optimized FP with 30 min-soaked SERS sensors were exploited to detect minuscule concentrations of pesticide (thiram-50 nM), dye (Nile blue-5 nM), and an explosive (RDX-1,3,5-Trinitroperhydro-1,3,5-triazine-100 nM) molecule. Interestingly, a prominent SERS effect was observed from the Au NPs exhibiting satisfactory reproducibility in the SERS signals over ~1 cm2 area for all of the molecules inspected with enhancement factors of ~105 and relative standard deviation values of <15%. Furthermore, traces of pesticide residues on the surface of a banana and RDX on the glass slide were swabbed with the optimized FP substrate and successfully recorded the SERS spectra using a portable Raman spectrometer. This signifies the great potential application of such low-cost, flexible substrates in the future real-life fields.

Biosensing Using SERS Active Gold Nanostructures

Surface-enhanced Raman spectroscopy (SERS) has become a powerful tool for biosensing applications owing to its fingerprint recognition, high sensitivity, multiplex detection, and biocompatibility. This review provides an overview of the most significant aspects of SERS for biomedical and biosensing applications. We first introduced the mechanisms at the basis of the SERS amplifications: electromagnetic and chemical enhancement. We then illustrated several types of substrates and fabrication methods, with a focus on gold-based nanostructures. We further analyzed the relevant factors for the characterization of the SERS sensor performances, including sensitivity, reproducibility, stability, sensor configuration (direct or indirect), and nanotoxicity. Finally, a representative selection of applications in the biomedical field is provided.

Tunable LSPR of silver/gold bimetallic nanoframes and their SERS activity for methyl red detection

Ag/Au bimetallic nanostructures have received much attention in surface-enhanced Raman scattering (SERS). However, the synthesis of this nanostructure type still remains a challenge. In the present research, Ag/Au nanoframes were synthesized via a simple room temperature solution phase chemical reduction method using pre-synthesized triangular Ag nanoplates as templates in the presence of appropriate amounts of HAuCl4. Controlling experimental parameters was applied for understanding of the growth mechanism. The galvanic exchange reaction resulted in a uniform deposition of the Au shell on the Ag nanoplates and the Ag core was removed which generated triangular hollow nanoframes. It is found that the amount of HAuCl4 added to the growth solution played a key role in controlling the Ag/Au nanoframes. The resultant silver/gold nanoframes with average size of 50 nm were applied in detecting methyl red (MR) in the solution-phase using an excitation wavelength laser of 532 nm. The SERS signal was greatly enhanced owing to the tunable plasmonic peaks in the visible region (400-650 nm). The limit of detection (LOD) of MR in diluted solution was 10-6 M. The enhancement factor (EF) was about 8 × 104 toward 10-5 M of MR. Interestingly, the linear dependence between the logarithm of the SERS signal intensity (log I) and the logarithm of the MR concentration (log C) occurred in the range from 10-6 to 10-4 M. Our work promises the application of Ag/Au nanoframes as a chemical sensor in detecting MR molecules at low concentration with high performance.

Study of surface enhanced Raman scattering of IR-780 Iodide molecules using Au-Ag bimetallic nanostructures with blunt and sharp sprouts

Au-Ag bimetallic nanostructures with blunt and sharp sprouts are synthesized using a high yield one-step synthesis process. For the first time, these nanostructures are obtained at different growth times in the same synthesis process. The synthesized nanostructures are characterized using a field emission-scanning electron microscope, transmission electron microscope, energy dispersive X-ray analyzer, and UV-Visible spectrometer. The plasmon-active substrates are fabricated using synthesized nanostructures with ease. The Raman probe (IR-780 Iodide) molecules are dispersed on the surface of plasmon-active substrates by drop-casting 10 μl of dye solution of concentration ranging from 1 μM to 1 picomolar (pM) on the substrates. The surface enhanced Raman scattering (SERS) spectra are recorded for each concentration. The nanostructures with blunt sprouts are found useful only up to 100 pM. However, this limitation is brought down to 1 pM using nanostructures with sharp sprouts. The normal Raman scattering spectra of molecules and microcrystals are also recorded and compared with the SERS spectra of molecules. The experimental SERS enhancement factor (EF) is found around 1 × 10⁹ for the Raman probe solution with 1 pM concentration. Finite Element Method (FEM) simulations are performed for estimating the possible single molecule SERS enhancement.

Electrodeposition of plasmonic bimetallic Ag-Cu nanodendrites and their application as surface-enhanced Raman spectroscopy (SERS) substrates

Plasmonic bimetallic Ag-Cu nanodendrites were synthesized by an electrodeposition process and their potential as surface-enhanced Raman scattering (SERS) substrates was studied. We demonstrated a facile and efficient way for the preparation of highly sensitive SERS substrates. The electrodeposition time was an important parameter in the formation of Ag-Cu dendrites onto the Al sheet. The Ag-Cu dendrites showed an excellent response detecting Rhodamine 6 G at ultra-low concentrations such as 1 × 10^-15 mol l^-1. This Ag-Cu substrate possesses an excellent SERS activity and it could be used for the detection of molecules at trace level. This electrodeposition process could be extended for the fabrication of other plasmonic bimetallic dendrites.

The structural transition of bimetallic Ag-Au from core/shell to alloy and SERS application

It is well-known that Ag-Au bimetallic nanoplates have attracted significant research interest due to their unique plasmonic properties and surface-enhanced Raman scattering (SERS). In recent years, there have been many studies on the fabrication of bimetallic nanostructures. However, controlling the shape, size, and structure of bimetallic nanostructures still has many challenges. In this work, we present the results of the synthesis of silver nanoplates (Ag NPls), and Ag-Au bimetallic core/shell and alloy nanostructures, using seed-mediated growth under green LED excitation and a gold salt (HAuCl4) as a precursor of gold. The results show that the optical properties and crystal structure strongly depend on the amount of added gold salt. Interestingly, when the amount of gold(x) in the sample was less than 0.6 μmol (x < 0.6 μmol), the structural nature of Ag-Au was core/shell, in contrast x > 0.6 μmol gave the alloy structure. The morphology of the obtained nanostructures was investigated using the field emission scanning electron microscopy (FESEM) technique. The UV-Vis extinction spectra of Ag-Au nanostructures showed localized surface plasmon resonance (LSPR) bands in the spectral range of 402-627 nm which changed from two peaks to one peak as the amount of gold increased. Ag-Au core/shell and alloy nanostructures were utilized as surface enhanced Raman scattering (SERS) substrates to detect methylene blue (MB) (10-7 M concentration). Our experimental observations indicated that the highest enhancement factor (EF) of about 1.2 × 107 was obtained with Ag-Au alloy. Our detailed investigations revealed that the Ag-Au alloy exhibited significant EF compared to pure metal Ag and Ag-Au core/shell nanostructures. Moreover, the analysis of the data revealed a linear dependence between the logarithm of concentration (log C) and the logarithm of SERS signal intensity (log I) in the range of 10-7-10-4 M with a correlation coefficient (R 2) of 0.994. This research helps us understand better the SERS mechanism and the application of Raman spectroscopy on a bimetallic surface.

Latest Novelties on Plasmonic and Non-Plasmonic Nanomaterials for SERS Sensing

An explosion in the production of substrates for surface enhanced Raman scattering (SERS) has occurred using novel designs of plasmonic nanostructures (e.g., nanoparticle self-assembly), new plasmonic materials such as bimetallic nanomaterials (e.g., Au/Ag) and hybrid nanomaterials (e.g., metal/semiconductor), and new non-plasmonic nanomaterials. The novel plasmonic nanomaterials can enable a better charge transfer or a better confinement of the electric field inducing a SERS enhancement by adjusting, for instance, the size, shape, spatial organization, nanoparticle self-assembly, and nature of nanomaterials. The new non-plasmonic nanomaterials can favor a better charge transfer caused by atom defects, thus inducing a SERS enhancement. In last two years (2019-2020), great insights in the fields of design of plasmonic nanosystems based on the nanoparticle self-assembly and new plasmonic and non-plasmonic nanomaterials were realized. This mini-review is focused on the nanoparticle self-assembly, bimetallic nanoparticles, nanomaterials based on metal-zinc oxide, and other nanomaterials based on metal oxides and metal oxide-metal for SERS sensing.

Retracted Article: Synergistic action of star-shaped Au/Ag nanoparticles decorated on AgFeO2 for ultrasensitive SERS detection of a chemical warfare agent on real samples

It is a great challenge to design and fabricate a cost-effective surface-enhanced Raman spectroscopy (SERS) substrate with excellent reproducibility and sensitivity for reliable environmental analysis. In this work, we have synthesized silver ferrite (AgFeO₂) interlinked with star-shaped gold/silver (Au/Ag) bimetallic (BM) nanoparticles (NPs) by a simple physical method for the effective detection of an acetylcholinesterase (AchE) inhibitor, paraoxon ethyl (PE). The successful construction of AgFeO₂@Au/Ag NPs was confirmed by UV-Vis spectroscopy, X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy and Raman spectroscopy. The enhancement of the SERS signal is achieved by the synergistic effect of the charge transfer mechanism and electromagnetic mechanism. The Raman peak centered at 1357 cm^-1 was selected as an ideal peak for the quantitative analysis of PE. The AgFeO₂@Au/Ag NPs can detect PE down to 1 × 10^-8 M with a high analytical enhancement factor of 3.53 × 10⁶ and excellent uniformity, as determined randomly from 14 spots (relative standard deviation, RSD, <15%). The recovery values of PE in tap water and tomato juice were from 93.16% to 99.16%. All these results suggest that our proposed SERS substrate has promising potential for the detection of PE. The proposed simple strategy for PE detection by SERS using AgFeO₂@Au/Ag NPs paves the way for future reliable environmental analysis and real sample monitoring.