Reproducible Surface Enhanced Raman Spectroscopy Research Articles

Three-dimensional Ag NPs-cellulose fiber/polyacrylamide hydrogels as a novel SERS platform for the efficient determination of thiram in fruits and juice

The preparation of sensitive, homogeneous and reproducible surface-enhanced Raman spectroscopy (SERS) platforms is imperative for rapid in situ measurements, but remains a formidable task. We constructed a high-performance, stable and homogeneous Ag NPs-TOCNF/PAAM hydrogel SERS platform using a cellulose-based hydrogel with a porous network structure composed of TEMPO-oxidized cellulose nanofibers (TOCNF) and polyacrylamide (PAAM) to attach, coordinate and protect silver nanoparticles (Ag NPs). The -COO- group in the TOCNF is coordinated to the silver atoms, resulting in a more uniform distribution of Ag NPs. The encapsulation of hydrogel effectively protects the Ag NPs from oxidation and improves the stability of the substrate. The volume shrinkage treatment of the hydrogel can adjust the structural gap of the Ag NPs, thus exhibiting high sensitivity. The SERS performance of the hydrogel was evaluated using the molecule R6G. Its enhancement factor is up to 7.56 × 10−7. In addition, SERS activity remained above 85% even after 50 days. Finally, the Ag NPs-TOCNF/PAAM hydrogel platform was used to detect thiram in fruit peels with a detection limit of 10−6 M. In conclusion, the platform shows great promise for the determination of hazardous chemicals in the field environment.

Evaporation-induced self-assembly of gold nanorods on a hydrophobic substrate for surface enhanced Raman spectroscopy applications

The controllable assembly of plasmonic nanoparticles has developed as one of the most significant approaches for surface enhanced Raman spectroscopy (SERS) applications. This study developed a simple approach to improve a large-scale ordered assembly of gold nanorods (GNRs) by controlling the droplet evaporation mode on hydrophobic substrates. The hydrophobic substrate was efficiently produced by spin coating the silicone oil onto the glass slides and annealing them. The analyte molecule rhodamine (R6G) was employed as a surface-enhanced Raman scattering probe to demonstrate the potential effects of the synthesized arrays. This hydrophobic platform enables the concentration and delivery of analyte molecules into the surface enhanced Raman spectroscopy sensitive site while suppressing the coffee ring effect generated by the smooth contraction motion of the base contact radius of the droplet without any pinning. Thus, the limit of detection (LOD) of the R6G analyte was lowered to 10−10 M and the homogenous dispersion of surface enhanced Raman spectroscopy hotspots within the self-assembly reproducible surface enhanced Raman spectroscopy signal. This new method enables a broad range of packing patterns and mechanisms by changing the host nanoparticles in the dispersion.

Plasmonic rhenium trioxide self-assembled microtubes for highly sensitive, stable and reproducible surface-enhanced Raman spectroscopy detection

Compared with noble metals, improving the sensitivity of semiconducting surface-enhanced Raman scattering (SERS) substrates is of great significance to their fundamental research and practical application of Raman spectroscopy. Herein, a simple chemical method is developed to synthesize a rhenium trioxide (ReO3) microtubes assembled with highly crystalline nanoparticles. The ReO3 microtubes show a strong and well-defined surface plasmon resonance (SPR) behavior in visible region, which is rare for non-noble metals. As a low-cost SERS substrate, the plasmonic ReO3 microtubes exhibit a Raman enhancement factor of 8.9 × 105 and a lowest detection limit of 1.0 × 10−9 mol/L for phenolic pollutants. Moreover, these ReO3 microtubule SERS substrates show excellent chemical stability and can resist the corrosion of strong acids and bases.

Plasmonic Superlattice Membranes Based on Bimetallic Nano-Sea Urchins as High-Performance Label-Free Surface-Enhanced Raman Spectroscopy Platforms.

On the basis of an abundance of elemental plasmonic nanocrystals identifiable by their unique morphology and intrinsic optoelectronic properties, it is necessary to rationally tailor the structural parameters to optimize the functionalities of nanoassemblies for application as plasmonic circuits/devices. Among them, the plasmonic superlattice membrane has emerged as a novel optically active metamaterial, which is constructed by nanocrystals at a two-dimensional (2D) plane with a highly ordered structure and strong plasmonic coupling interactions. Here, we report on the fabrication of a novel plasmonic superlattice membrane using bimetallic core-shell nano-sea urchins (Nano-SEUs) as meta-atoms. Under the guidance of soft-ligand balancing in conjugation with drying-mediated self-assembly at the air/water interface, well-defined giant 2D superlattices with total lateral dimensions of up to 5 mm wide and 80 nm thick have been synthesized, corresponding to an aspect ratio of 62 500. Programmable morphology control over the Nano-SEUs has been achieved in high yield by rationally tuning the spiky branches as well as the thickness of the silver shell, allowing systematic variation of the plasmonic properties of the membrane. Such superlattice membranes exhibited a strong and reproducible surface-enhanced Raman spectroscopy (SERS) signal that originates from interparticle coupling and electric (E)-field enhancement, enabling an enhancement factor of up to 106. We also demonstrated that the fabricated membrane allows the label-free SERS detection of dopamine from 0.1 nM to 1 μM. Thus, this giant Nano-SEU assembled superlattice membrane can be used as a SERS substrate for on-spot biomarker detection, which paves a robust and inexpensive avenue for highly sensitive and reliable biomedical sensing and diagnostics.

Hybrid Metal-Dielectric-Metal Sandwiches for SERS Applications.

The development of efficient plasmonic nanostructures with controlled and reproducible surface-enhanced Raman spectroscopy (SERS) signals is an important task for the evolution of ultrasensitive sensor-related methods. One of the methods to improving the characteristics of nanostructures is the development of hybrid structures that include several types of materials. Here, we experimentally investigate ultrathin gold films (3–9 nm) near the percolation threshold on Si/Au/SiO2 and Si/Au/SiO2/graphene multilayer structures. The occurring field enhanced (FE) effects were characterized by a recording of SERS signal from Crystal Violet dye. In this geometry, the overall FE principally benefits from the combination of two mechanisms. The first one is associated with plasmon excitation in Au clusters located closest to each other. The second is due to the gap plasmons’ excitation in a thin dielectric layer between the mirror and corrugated gold layers. Experimentally obtained SERS signals from sandwiched structures fabricated with Au film of 100 nm as a reflector, dielectric SiO2 spacer of 50 nm and ultrathin gold atop could reach SERS enhancements of up to around seven times relative to gold films near the percolation threshold deposited on a standard glass substrate. The close contiguity of the analyte to graphene and nanostructured Au efficiently quenches the fluorescent background of the model compound. The obtained result shows that the strategy of combining ultrathin nano-island gold films near the percolation threshold with gap plasmon resonances is promising for the design of highly efficient SERS substrates for potential applications in ultrasensitive Raman detection.

Plasmonic Core-Shell-Satellites with Abundant Electromagnetic Hotspots for Highly Sensitive and Reproducible SERS Detection.

In this work, we develop a Ag@Al2O3@Ag plasmonic core–shell–satellite (PCSS) to achieve highly sensitive and reproducible surface-enhanced Raman spectroscopy (SERS) detection of probe molecules. To fabricate PCSS nanostructures, we employ a simple hierarchical dewetting process of Ag films coupled with an atomic layer deposition (ALD) method for the Al2O3 shell. Compared to bare Ag nanoparticles, several advantages of fabricating PCSS nanostructures are discovered, including high surface roughness, high density of nanogaps between Ag core and Ag satellites, and nanogaps between adjacent Ag satellites. Finite-difference time-domain (FDTD) simulations of the PCSS nanostructure confirm an enhancement in the electromagnetic field intensity (hotspots) in the nanogap between the Ag core and the satellite generated by the Al2O3 shell, due to the strong core–satellite plasmonic coupling. The as-prepared PCSS-based SERS substrate demonstrates an enhancement factor (EF) of 1.7 × 107 and relative standard deviation (RSD) of ~7%, endowing our SERS platform with highly sensitive and reproducible detection of R6G molecules. We think that this method provides a simple approach for the fabrication of PCSS by a solid-state technique and a basis for developing a highly SERS-active substrate for practical applications.

Economically reproducible surface-enhanced Raman spectroscopy of different compounds in thin film

We report herein an economically cheap and functionally stable surfaceenhanced Raman scattering (SERS) protocol of two photoactive pigments Rhodamine 6G (R6G) and Kiton Red (KR), implemented in thin films of silver (Ag) and gold (Au) nanoparticles (AgNPs and AuNPs). Both commercially available and chemically synthesized nanoparticles were used. The suitability of the nanoparticles toward SERS activity was tested through UV-visible absorption spectroscopy and scanning electron microscopy (SEM). The AgNPs and AuNPs based SERS substrates in the form of films were fabricated onto square-sized aluminum(Al) plates by simple drop deposition of colloidal nanoparticles solution onto their polished surfaces. The prepared nanoparticle films were sufficiently dried and coated further with the probe (R6G and KR) molecules by employing the identical deposition technique. The enhanced Raman signals of R6G and KR in such composite film structures were then recorded through a custom-built dispersive Raman spectrometer with He-Ne laser excitation at 632.8 nm. Our AgNPsfilm-based SERS protocol could yield the magnitude of the Raman signal enhancement up to 104 times for both R6G and KR. Moreover, AuNPs-based film was found to be less efficient toward the Raman enhancement of both compounds. Our SERS substrates can be easily fabricated, and SERS spectra are reproducible and stable, allowing one to consistently get a reproducible result even after 6 months.
 J. Bangladesh Acad. Sci. 45(1); 1-11: June 2021

Optical tweezers-controlled hotspot for sensitive and reproducible surface-enhanced Raman spectroscopy characterization of native protein structures

Surface-enhanced Raman spectroscopy (SERS) has emerged as a powerful tool to detect biomolecules in aqueous environments. However, it is challenging to identify protein structures at low concentrations, especially for the proteins existing in an equilibrium mixture of various conformations. Here, we develop an in situ optical tweezers-coupled Raman spectroscopy to visualize and control the hotspot between two Ag nanoparticle-coated silica beads, generating tunable and reproducible SERS enhancements with single-molecule level sensitivity. This dynamic SERS detection window is placed in a microfluidic flow chamber to detect the passing-by proteins, which precisely characterizes the structures of three globular proteins without perturbation to their native states. Moreover, it directly identifies the structural features of the transient species of alpha-synuclein among its predominant monomers at physiological concentration of 1 μM by reducing the ensemble averaging. Hence, this SERS platform holds the promise to resolve the structural details of dynamic, heterogeneous, and complex biological systems.

Development of a spectroscopic technique that enables the saliva based detection of COVID-19 at safe distances

Research activities are in full swing globally to translate the use of saliva as a non-invasive and highly potential specimen for clinical diagnostics, particularly for COVID-19 detection. Being comprised of a pool of biomarkers also enriched with ACE-2 receptors, saliva can provide vital information regarding the state of the human body. Advancements in biophotonics tools for saliva investigation may offer promise for developing rapid, highly objective, optical modalities for COVID- 19 detection. This article presents concept/design study, which propose the use of Raman/laser induced fluorescence spectroscopic device that have the potential for viral detection via saliva from a safer distance. Noticeable changes of biomarkers present in saliva in response to viral infection can reflect the pathological state, thus can altogether affect the Raman spectral pattern. Monitoring these spectral patterns of saliva, which are further enhanced by using cost effective and reproducible Surface Enhanced Raman Spectroscopy substrates can be a viable option for sensitive and non-invasive viral detection. The spectral information acquired from the optical device can be processed using various multivariate statistical analytical tools, which ultimately facilitate effective viral detection in few minutes. This method doesn’t demand the necessity of qualified professionals and sample processing with reagents unlike in RT-PCR test. The proposed optical device can be further modified into a portable form, which can be easily transported for field applications. The stand-off observation, contactless and highly non-invasive technique can be of paramount importance in the current context, where the safer screening of a large population for viral infection by maintaining social distances is a necessity. The proposed stand-off spectroscopic technique can also address the major concern of nosocomial viral transmission amongst healthcare workers during sample collection in a pandemic scenario.

Sensitive and reproducible surface-enhanced raman spectroscopy (SERS) with arrays of dimer-nanopillars

It is challenge for most metallic nanostructures to produce both reproducible and high sensitive SERS enhancement, which hinder practical applications of SERS technology. In this work, we report that gold-coated silicon (Au@Si) dimer-nanopillar arrays can be used as SERS substrates with both high-sensitivity and reproducibility. The substrates consist of dimer-nanopillars with height of 480 nm and gaps of 8−10 nm fabricated with electron-beam lithography and followed plasma etching. The dimer-nanopillars have demonstrated SERS enhancement factors (EFs) of 4.3−8.0 × 109 with SERS signals relative standard deviation (RSD) of 3.2−5.6 % (reproducibility). The EFs are 2–3 orders of magnitude higher in comparison with that of other types of ordered nanostructures and are comparable with those of chemically synthesized nanoparticle aggregates. Meanwhile, the reproducibility is among the best ever reported. The dimer-nanopillar arrays were applied for detection of component of rapeseed oil. Cooked and uncooked oil can be clearly identified from the SERS spectra. Our study demonstrated that the dimer-nanopillars are promising to develop highly enhancing and reproducible SERS substrates for SERS sensing applications.

Rapid and sensitive identification of uropathogenic Escherichia coli using a surface-enhanced-Raman-scattering-based biochip

Rapid, selective and sensitive sensing of bacteria remains challenging. We report on a highly sensitive and reproducible surface-enhanced Raman spectroscopy (SERS)-based sensing approach for the detection of uropathogenic Escherichia coli (E. coli) bacteria in urine. The assay is based on the specific capture of the bacteria followed by interaction with cetyltrimethylammonium bromide (CTAB)-stabilised gold nanorods (Au NRS) as SERS markers. High sensitivity up to 10 CFU mL-1 is achieved by optimizing the capture interface based on hydrogenated amorphous silicon a-Si:H thin films. The integration of CH3O-PEG750 onto a-Si:H gives the sensing interface an efficient anti-fouling character, while covalent linkage of antibodies directed against the major type-1 fimbrial pilin FimA of the human pathogen E. coli results in the specific trapping of fimbriated E. coli onto the SERS substrate and their spectral fingerprint identification.

Assembly engineering of Ag@ZnO hierarchical nanorod arrays as a pathway for highly reproducible surface-enhanced Raman spectroscopy applications

Semiconductor-based surface-enhanced Raman spectroscopy (SERS) substrates with high stability and reproducibility are an important analytical tool in the field of SERS. However, compared with metal substrates, even though semiconductor SERS substrates are cheaper but the progress in SERS enhancement is limited by their insufficient performance. Conventional SERS chips costly and are produced by complicated fabrication methods, while the sample handling processes are not biocompatible and efficient. To address the aforementioned challenges, large-area hexagonal-wrapped ZnO nanorod (ZnO NR) arrays incorporating Ag nanoparticles (Ag NPs) have been employed resulting in efficient SERS-active platforms. This study presents a new design pathway to fabricate highly sensitive and homogeneous SERS substrates that are simple and cost-effective with no aggregations on the surface. This method provides not only ultrasensitive target detection with highly reproducible and uniform SERS signals but also has a self-cleaning capability and reactivating SERS-behaviour under UV light irradiation. We found significantly strong and uniform SERS signal enhancement in the detection of R6G up to ∼4.2 × 107 due to the charge-transfer process between Ag NPs, ZnO NRs, and analyte molecules based on the participation of “hot electrons,” engineering nanogaps of Ag NPs, which may be attributed to the matching of the surface plasmon wavelength resonance with the laser excitation wavelength of 532 nm. Ag NPs assembled onto ZnO NRs may also serve as SERS active platforms for promising applications in chemical and biomedical detection.

Spectroelectrochemical Nanosensor for the Determination of Cystatin C in Human Blood.

The detection of protein biomarkers for the clinical diagnosis of diseases requires selective and sensitive methodologies and biosensors that can be easily used at pathology laboratories and points of care. An ideal methodology would be able to conduct multimode screening of low and high concentrations of proteins in biological fluids using recyclable platforms. In this work, we demonstrate a novel nanosensing methodology for the dual detection of cystatin C (CST-C), as a protein biomarker model, in blood plasma by surface-enhanced Raman spectroscopy and electrochemistry. The new methodology utilizes the thiol chemistry of biomolecules to develop a target-specific and recyclable extractor chip for the rapid isolation of protein biomarkers from blood plasma. This is followed by the rapid reduction of the disulfide bonds within the isolated protein to influence its oriented immobilization onto a conductive gold coated silicon nanopillar substrate via stable gold-sulfur (Au-S) bonds. The oriented immobilization led to reproducible surface-enhanced Raman spectroscopy (SERS) measurements of the reduced protein (RSD = 3.8%) and allowed for its direct electrochemical determination. After the SERS measurement, differential pulse voltammetry (DPV) was used to desorb the analyte from the substrate and generate a reduction current that is proportional to its concentration. CST-C was determined down to 1 pM and 62.5 nM by SERS and DPV, respectively, which satisfies the requirements for monitoring Alzheimer's and kidney failure diseases. The new dual nanosensing methodology has strong potential for miniaturization in a lab-on-a-chip platform for the screening of many protein biomarkers that have a disulfide bond structure.

Highly sensitive and well reproducible Surface-enhanced Raman spectroscopy from silver triangular platelets

Recently, considerable efforts have been made to prepare Surface-enhanced Raman scattering (SERS) nanostructures with high sensitivity and good reproducibility, which are always difficult and costly. In this article, we provide a simple route on the synthesis of silver particles with a novel single {111} plane and triangular morphology by reducing silver nitrate with poly(vinyl pyrrolidone) (PVP) in N,N-dimethylformamide (DMF) solutions. The results show that the Raman intensities of rhodamine 6G (R6G) decrease with decreasing the loading concentration and that R6G loading follows the first-order adsorption kinetics. Even at a low concentration of 10−12M, the signatures of R6G in Raman spectrum are still clearly observed. It indicates that the silver triangular platelets (i.e. AgTP) can offer sufficient active sites to efficiently capture the R6G molecules from the loading solution, and exhibit high SERS sensitivity. In addition, the relative standard deviation (RSD) of band at 1650cm−1 is calculated to be 13.7%, 12.7%, 15.7%, 15.1%, 13.3%, and 13.1%, respectively, corresponding with the concentration of R6G from 10−6M to 10−12M, which reveals the nanometer-scale AgTP can provide abundant hot spots and exhibit excellent reproducibility. This method offers an easy and low-cost way to prepare AgTP substrates and makes SERS detection more practicable.

Assemble of high-density gold nanodots on TiO2 substrate for surface-e nhanced Raman spectroscopy

The cost-effective assemble of Au nanodots on TiO2 substrate over a large domain for high-sensitivity and reproducible surface enhanced Raman spectroscopy (SERS) is demonstrated. Au nanoparticles of defined size are immobilized on TiO2 to form high-density Au nanodots with removal of surface ligand meanwhile. The Au nanodots exhibit pronounced surface Plasmon resonance (SPR) due to strong interparticle coupling and hot spots formed in these gaps which enable large amplification of SERS signals. Using the Au nanodots (3.8nm) assemble a 5 times stronger SERS enhancement than larger Au nanoparticles (NPs) of same Au amount is experimentally demonstrated. The facile fabrication of SERS-active substrate over a large area and cost-effective character enable this high-density Au nanodots on TiO2 substrate practical application for SERS detection of trace amount of molecule of interest.

Hotspots: Hotspot‐Engineered 3D Multipetal Flower Assemblies for Surface‐Enhanced Raman Spectroscopy (Adv. Mater. 34/2014)

On page 5924, N. Park, M. Choi, and co-workers present novel 3D multipetal flower structures consisting of nanoparticles for uniform and reproducible surface enhanced Raman spectroscopy (SERS) with high field enhancement sufficient for single-molecule detection. They demonstrate that the intensities and locations of hotspots can be engineered by designing 3D nanostructures with changing numbers of petals.

Hotspot-engineered 3D multipetal flower assemblies for surface-enhanced Raman spectroscopy.

Novel 3D metallic structures composed of multipetal flowers consisting of nanoparticles are presented. The control of surface plasmon hotspots is demonstrated in terms of location and intensity as a function of petal number for uniform and reproducible surfaceenhanced Raman spectroscopy (SERS) with high field enhancement.

Quantitative SERS Using the Sequestration of Small Molecules Inside Precise Plasmonic Nanoconstructs

We show how the macrocyclic host, cucurbit[8]uril (CB[8]), creates precise subnanometer junctions between gold nanoparticles while its cavity simultaneously traps small molecules; this enables their reproducible surface-enhanced Raman spectroscopy (SERS) detection. Explicit shifts in the SERS frequencies of CB[8] on complexation with guest molecules provides a direct strategy for absolute quantification of a range of molecules down to 10(-11) M levels. This provides a new analytical paradigm for quantitative SERS of small molecules.

Electrodeposition of Well-Adhered Multifarious Au Particles at a Solid|Toluene|Aqueous Electrolyte Three-Phase Junction

In order to obtain uniform and reproducible surface enhanced Raman spectroscopy (SERS) platforms, a novel method for deposition of well-adhered multifarious gold particles on a tin-doped indium oxide electrode through electrogeneration at an electrode|gold compound in toluene|aqueous-electrolyte three-phase junction was developed. The electrodeposition was carried out both by double-potential-step chronoamperometry with one pulse for nucleation and one for growth of the particles, and by potentiostatic single-potential-step chronoamperometry. Both procedures give angular, multifarious Au particles with a diameter of 150 ± 40 nm. The size of the particles is independent of deposition time, after an initial growth phase, and controlled by the formation of a microemulsion at the three-phase junction. The particles are likely deposited from the microdroplets and their size is determined by the amount of gold salt in a droplet. The mechanism involves electoreduction of tetraoctylammonium tetrachloroaurate at t...

Gold nanothorns–macroporous silicon hybrid structure: a simple and ultrasensitive platform for SERS

We synthesised a novel gold-on-porous silicon hybrid material that exhibits a highly sensitive and reproducible surface-enhanced Raman spectroscopy (SERS) response. The material was fabricated simply by reducing gold chloride with hydrofluoric acid on the surface of macro-porous silicon (macro-PSi). The material consists of thorn-shaped gold nanocrystals with characteristic shapes and sizes on the surface of macro-PSi.