Highly Sensitive, Uniform, and Reproducible Surface-Enhanced Raman Spectroscopy Substrate with Nanometer-Scale Quasi-periodic Nanostructures

Recent Development and Applications of Stretchable SERS Substrates

Surface-enhanced Raman scattering (SERS) is a cutting-edge technique for highly sensitive analysis of chemicals and molecules. Traditional SERS-active nanostructures are constructed on rigid substrates where the nanogaps providing hot-spots of Raman signals are fixed, and sample loading is unsatisfactory due to the unconformable attachment of substrates on irregular sample surfaces. A flexible SERS substrate enables conformable sample loading and, thus, highly sensitive Raman detection but still with limited detection capabilities. Stretchable SERS substrates with flexible sample loading structures and controllable hot-spot size provide a new strategy for improving the sample loading efficiency and SERS detection sensitivity. This review summarizes and discusses recent development and applications of the newly conceptual stretchable SERS substrates. A roadmap of the development of SERS substrates is reviewed, and fabrication techniques of stretchable SERS substrates are summarized, followed by an exhibition of the applications of these stretchable SERS substrates. Finally, challenges and perspectives of the stretchable SERS substrates are presented. This review provides an overview of the development of SERS substrates and sheds light on the design, fabrication, and application of stretchable SERS systems.

Surface-enhanced Raman spectroscopy charged probes under inverted superhydrophobic platform for detection of agricultural chemicals residues in rice combined with lightweight deep learning network

In this study, surface-enhanced Raman spectroscopy (SERS) charged probes and an inverted superhydrophobic platform were used to develop a detection method for agricultural chemicals residues (ACRs) in rice combined with lightweight deep learning network. First, positively and negatively charged probes were prepared to adsorb ACRs molecules to SERS substrate. An inverted superhydrophobic platform was prepared to alleviate the coffee ring effect and induce tight self-assembly of nanoparticles for high sensitivity. Chlormequat chloride of 15.5-0.05 mg/L and acephate of 100.2-0.2 mg/L in rice were measured with the relative standard deviation of 4.15% and 6.25%. SqueezeNet were used to develop regression models for the analysis of chlormequat chloride and acephate. And the excellent performances were obtained with the coefficients of determination of prediction of 0.9836 and 0.9826 and root-mean-square errors of prediction of 0.49 and 4.08. Therefore, the proposed method can realize sensitive and accurate detection of ACRs in rice.

Au Nanorings with Intertwined Triple Rings

We designed complex Au nanorings with intertwined triple rings (ANITs) in a single entity to amplify the efficacy of near-field focusing. Such a complex and unprecedented morphology at the nanoscale was realized through on-demand multistepwise reactions. Triangular nanoprisms were first sculpted into circular nanorings, followed by a series of chemical etching and deposition reactions eventually leading to ANITs wherein thin metal bridges hold the structure together without any linker molecules. In the multistepwise reaction, the well-faceted growth pattern of Au, which induces the growth of two distinctive flat facets in a lateral direction, is important to evolve the morphology from single to multiple nanorings. Although our synthesis proceeds through multiple steps in one batch without purification steps, it shows a remarkably high yield (>∼90%) at the final stage. The obtained high degree of homogeneity (in both shape and size) of the resulting ANITs allowed us to systematically investigate the corresponding localized surface plasmon resonance (LSPR) coupling with varying nanoring arrangements and observe their single-particle surface enhanced Raman scattering (SERS). Surprisingly, individual ANITs exhibited an enormously large enhancement factor (∼10⁹), which confirms their superior near-field focusing relative to other reported nanoparticles.

Fabrication and Applications of 3D Nanoarchitectures for Advanced Electrocatalysts and Sensors

For the last few decades, nanoscale materials and structures have been extensively studied and developed, making a huge impact on human sustainability. For example, the introduction of nanostructures has brought substantial development in electrocatalysts and optical sensing applications. However, there are still remaining challenges that need to be resolved to further improve their performance, reliability, and cost-effectiveness. Herein, long-range ordered 3D nanostructures and their design principles are introduced with an emphasis on electrocatalysts for energy conversion and plasmonic nanostructures for optical sensing. Among the various fabrication techniques, sequential solvent-injection-assisted nanotransfer printing is suggested as a practical fabrication platform for tunable long-range ordered 3D nanostructures composed of ultrahigh-resolution building blocks. Furthermore, the importance of understanding and controlling the 3D design parameters is discussed to realize more efficient energy conversion as well as effective surface-enhanced Raman spectroscopy analyses, suggesting new solutions for clean energy and healthcare issues.

Optimization and performance analysis of SERS-active suspended core photonic crystal fibers

Recently, surface enhanced Raman spectroscopy (SERS)-active photonic crystal fiber (PCFs) probes have gained great interest for biosensing applications due to the tremendous advantages it has over the conventional planar substrate based SERS measurements, with improvements on the detection sensitivity and reliability in measurements. So far, two main approaches were employed to get the analyte molecule in the vicinity of nanoparticles (NPs) inside PCFs in order to achieve the SERS effect. In the first case, analyte and NPs are pre-mixed and injected inside the holes of the PCF prior to the measurement. In the second approach, controlled anchoring of the NPs inside the inner walls of the PCF was achieved prior to the incorporation of the analyte. Although many studies have been conducted using one configuration or the other, no clear trend is emerging on which one would be the best suited for optimizing the biosensing properties offered by SERS active-PCF. In this paper, we investigate the performances of both configurations along with their interplays with the core size of the PCF probe. We have fabricated several samples of a standard PCF design with different core sizes, and SERS measurements of a standard Raman-active molecule are realized in the same conditions for enabling direct comparisons of the SERS intensity and measurement reliabilities between each configuration, yielding clear directions on the optimization of the SERS-active PCF probe. We envision that this study will pave the way for next-generation clinical biosensors for body fluid analysis, as it exhibits high sensitivity and excellent reliability.

Development of highly reliable SERS-active photonic crystal fiber probe and its application in the detection of ovarian cancer biomarker in cyst fluid

Conventionally Surface-enhanced Raman spectroscopy (SERS) is realized by adsorbing analytes onto nano-roughened planar substrate coated with noble metals (silver or gold) or their colloidal nanoparticles (NPs). Nanoscale irregularities in such substrates/NPs could lead to SERS sensors with poor reproducibility and repeatability. Herein, we demonstrate a suspended core photonic crystal fiber (PCF) based SERS sensor with extremely high reproducibility and repeatability in measurement with a relative SD of only 1.5% and 4.6%, respectively, which makes it more reliable than any existing SERS sensor platforms. In addition, our platform could improve the detection sensitivity owing to the increased interaction area between the guided light and the analyte, which is incorporated into the holes that runs along the length of the PCF. Numerical calculation established the significance of the interplay between light coupling efficiency and evanescent field distribution, which could eventually determine the sensitivity and reliability of the developed SERS active-PCF sensor. As a proof of concept, using this sensor, we demonstrated the detection of haptoglobin, a biomarker for ovarian cancer, contained within the ovarian cyst fluid, which facilitated in differentiating the stages of cancer. We envision that with necessary refinements, this platform could potentially be translated as a next-generation highly sensitive SERS-active opto-fluidic biopsy needle for the detection of biomarkers in body fluids.

Plasmon-enhanced linear and second-order surface nonlinear optical response of silver nanoparticles fabricated using a femtosecond pulse

We present the plasmon-enhanced linear and second-order surface nonlinear optical response of silver nanoparticles (Ag NPs) fabricated using a femtosecond pulse. Theoretical analysis indicates Ag NPs with a diameter of ∼100 nm have excellent linear response within the visible band, and the electric field intensity enhancement factor reaches ∼10⁵ under excitation of continuous light of 632.8 nm. Meanwhile, the simulation result of second-order surface nonlinear optical response shows that the second harmonic conversion efficiency of the Ag NPs dimer is two orders of magnitude higher than that of a single Ag NP, under excitation of a femtosecond pulse. In experiment, the linear response of Ag NPs is examined using surface-enhanced Raman spectroscopy (SERS) with a Raman enhancement factor of ∼1.7 × 10¹⁰, revealing the excellent linear optical response of Ag NPs. Moreover, the spectra of the second harmonic can be measured clearly under conditions of an average pump power of 40 μW, revealing the excellent second-order surface nonlinear optical response of Ag NPs.

A review on recent advances in the applications of surface-enhanced Raman scattering in analytical chemistry

This review is focused on recent developments of surface-enhanced Raman scattering (SERS) applications in Analytical Chemistry. The work covers advances in the fabrication methods of SERS substrates, including nanoparticles immobilization techniques and advanced nanopatterning with metallic features. Recent insights in quantitative and sampling methods for SERS implementation and the development of new SERS-based approaches for both qualitative and quantitative analysis are discussed. The advent of methods for pre-concentration and new approaches for single-molecule SERS quantification, such as the digital SERS procedure, has provided additional improvements in the analytical figures-of-merit for analysis and assays based on SERS. The use of metal nanostructures as SERS detection elements integrated in devices, such as microfluidic systems and optical fibers, provided new tools for SERS applications that expand beyond the laboratory environment, bringing new opportunities for real-time field tests and process monitoring based on SERS. Finally, selected examples of SERS applications in analytical and bioanalytical chemistry are discussed. The breadth of this work reflects the vast diversity of subjects and approaches that are inherent to the SERS field. The state of the field indicates the potential for a variety of new SERS-based methods and technologies that can be routinely applied in analytical laboratories.

Plasmonic Gold Nanohole Array for Surface-Enhanced Raman Scattering Detection of DNA Methylation

Surface-enhanced Raman spectroscopy (SERS), which utilizes nanogaps between noble-metal nanostructures as hot spots to yield ultrasensitive SERS signals, is an outstanding label-free and straightforward tool for DNA methylation analysis. Herein, a plasmonic gold nanohole array (PGNA) with well-controlled hot spots and an open surface was designed as a SERS substrate for DNA methylation detection. A finite-difference time-domain (FDTD) simulation was first employed to investigate the electric field distributions of the PGNA as a function of the geometric parameters. The plasmonic response was tuned to 785 cm^-1 to match the ring breathing vibrational band of cytosine, the intensity change of which was revealed to be a marker of DNA methylation. Then, guided by the FDTD simulation results, the PGNA was fabricated via the electron beam lithography (EBL) technique. The fabricated PGNA had an open and easily accessible surface topology, a SERS enhancement factor of ∼10⁶, and a relative standard deviation (RSD) of 7.1% for 500 repetitions over an area of 20 × 20 μm² using 1 μM Rhodamine 6G as the Raman reporter. The fabricated PGNA was further used as a platform for determining DNA methylation. The proposed method exhibited a sensitivity for detecting 1% of methylation changes. Moreover, insight into the dynamic information on methylation events was obtained by combining principal component analysis (PCA) with 2D correlation spectroscopy analysis. Finally, clear discrimination of the different methylation sites, such as 5-methylcytosine and N6-methyladenine, was demonstrated.

Sub-10 nm Monolayer MoS2 Transistors Using Single-Walled Carbon Nanotubes as an Evaporating Mask

Transition-metal dichalcogenides are promising challengers to conventional semiconductors owing to their remarkable electrical performance and suppression of short-channel effects (SCEs). In particular, monolayer molybdenum disulfide has exhibited superior suppression of SCEs owing to its atomic thickness, high effective carrier mass, and low dielectric constant. However, difficulties still remain in large-scale stable fabrication of nanometer-scale channels. Herein, a method to fabricate electrodes with sub-10 nm gaps was demonstrated using horizontally aligned single-walled carbon nanotubes as an evaporation mask. The widths of the nanogaps exhibit robust stability to various process parameters according to the statistical results. Based on these nanogaps, ultrashort-channel length monolayer MoS₂ field-effect transistors were produced. Monolayer MoS₂ devices with a 7.5 nm channel length and a 10 nm thick HfO₂ dielectric layer exhibited excellent performances with an ON/OFF ratio up to 10⁷, a mobility of 17.4 cm²/V·s, a subthreshold swing of about 120 mV/dec, and a drain-induced barrier lowering of about 140 mV/V, all of which suggest a superior suppression of SCEs. This work provides a universal and stable method for large-scale fabrication of ultrashort-channel 2D-material transistors.

SERS based detection of multiple analytes from dye/explosive mixtures using picosecond laser fabricated gold nanoparticles and nanostructures

Surface enhanced Raman spectroscopy (SERS) is a cutting edge analytical tool for trace analyte detection due to its highly sensitive, non-destructive and fingerprinting capability. Herein, we report the detection of multiple analytes from various mixtures using gold nanoparticles (NPs) and nanostructures (NSs) as SERS platforms. NPs and NSs were achieved through the simple approach of laser ablation in liquids (LAL) and their morphological studies were conducted with a UV-Visible absorption spectrometer, a high resolution transmission electron microscope (HRTEM) and a field emission scanning electron microscope (FESEM). The fabricated NPs/NSs allowed the sensitive and selective detection of different mixed compounds containing (i) rhodamine 6G (Rh6G) and methylene blue (MB), (ii) crystal violet (CV) and malachite green (MG), (iii) picric acid (explosive) and MB (dye), (iv) picric acid and 3-nitro-1,2,4- triazol-5-one (explosive, NTO) and (v) picric acid and 2,4-dinitrotoluene (explosive, DNT) using a portable Raman spectrometer. Thus, the obtained results demonstrate the capability of fabricated SERS substrates in identifying explosives and dyes from various mixtures. This could pave a new way for simultaneous detection of multiple analytes in real field applications.

Femtosecond Laser-Induced, Nanoparticle-Embedded Periodic Surface Structures on Crystalline Silicon for Reproducible and Multi-utility SERS Platforms

Fabrication of reproducible and versatile surface-enhanced Raman scattering (SERS) substrates is crucial for real-time applications such as explosive detection for human safety and biological imaging for cancer diagnosis. However, it still remains a challenging task, even after several methodologies were developed by various research groups, primarily due to (a) a lack of consistency in detection of a variety of molecules (b) cost-effectiveness of the SERS substrates prepared, and (c) byzantine preparation procedures, etc. Herein, we establish a procedure for preparing reproducible SERS-active substrates comprised of laser-induced nanoparticle-embedded periodic surface structures (LINEPSS) and metallization of silicon (Si) LINEPSS. LINEPSS were fabricated using the technique of femtosecond laser ablation of Si in acetone. The versatile SERS-active substrates were then achieved by two ways, including the drop casting of silver (Ag)/gold (Au) nanoparticles (NPs) on Si LINEPSS and Ag plating on the Si LINEPSS structures. By controlling the LINEPSS grating periodicity, the effect of plasmonic nanoparticles/plasmonic plating on the Si NPs embedded periodic surface structures enormously improved the SPR strength, resulting in the consistent and superior Raman enhancements. The reproducible SERS signals were achieved by detecting the molecules of Methylene Blue (MB), 2,4-dinitrotoluene (DNT), and 5-amino-3-nitro-l,2,4-triazole (ANTA). The SERS signal strength is determined by the grating periodicity, which, in turn, is determined by the input laser fluence. The SERS-active platform with grating periodicity of 130 ± 10 nm and 150 ± 5 nm exhibited strong Raman enhancements of ∼10⁸ for MB and ∼10⁷ for ANTA molecules, respectively, and these platforms are demonstrated to be capable, even for multiple usages.

Controlling surface morphology and sensitivity of granular and porous silver films for surface-enhanced Raman scattering, SERS

The design of efficient substrates for surface-enhanced Raman spectroscopy (SERS) for large-scale fabrication at low cost is an important issue in further enhancing the use of SERS for routine chemical analysis. Here, we systematically investigate the effect of different radio frequency (rf) plasmas (argon, hydrogen, nitrogen, air and oxygen plasma) as well as combinations of these plasmas on the surface morphology of thin silver films. It was found that different surface structures and different degrees of surface roughness could be obtained by a systematic variation of the plasma type and condition as well as plasma power and treatment time. The differently roughened silver surfaces act as efficient SERS substrates showing greater enhancement factors compared to as prepared, sputtered, but untreated silver films when using rhodamine B as Raman probe molecule. The obtained roughened silver films were fully characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron (XPS and Auger) and ultraviolet-visible spectroscopy (UV-vis) as well as contact angle measurements. It was found that different morphologies of the roughened Ag films could be obtained under controlled conditions. These silver films show a broad range of tunable SERS enhancement factors ranging from 1.93 × 102 to 2.35 × 105 using rhodamine B as probe molecule. The main factors that control the enhancement are the plasma gas used and the plasma conditions, i.e., pressure, power and treatment time. Altogether this work shows for the first time the effectiveness of a plasma treatment for surface roughening of silver thin films and its profound influence on the interface-controlled SERS enhancement effect. The method can be used for low-cost, large-scale production of SERS substrates.

Fabrication and simulation of V-shaped Ag nanorods as high-performance SERS substrates

Bending straight Ag nanorods (AgNRs) into V-shaped structures can generate a higher surface-enhanced Raman scattering (SERS) performance. Numerical simulations showed that V-shaped AgNRs with a total length between 300 nm and 800 nm were more sensitive than equal-length straight AgNRs under a 785 nm laser in most cases. It was found that at a laser wavelength between 500 nm and 1000 nm, the Raman enhancement factor (EF) of a V-shaped AgNR's 3rd plasmon mode was not only optimal among the other major plasmon modes, but also outperformed the plasmon modes of straight AgNRs. Besides, a linear relationship between the resonance wavelength of the V-shaped AgNR's 3rd mode and its length was observed both numerically and experimentally, which was beneficial for the optimization of SERS substrates. Under 785 nm laser excitation, V-shaped AgNR substrates with a single arm length between 330 nm and 340 nm possessed the highest SERS efficiency. This work took AgNR array substrates one step closer to practical applications.

Flexible, transparent and highly sensitive SERS substrates with cross-nanoporous structures for fast on-site detection

A flexible and transparent film assembled from the cross-nanoporous structures of Au on PET (CNS of Au@PET) is developed as a versatile and effective SERS substrate for rapid, on-site trace analysis with high sensitivity. The fabrication of the CNS of Au can be achieved on a large scale at low cost by employing an etching process with super-aligned carbon nanotubes as a mask, followed by metal deposition. A strongly enhanced Raman signal with good uniformity can be obtained, which is attributed to the excitation of "hot spots" around the metal nanogaps and sharp edges. Using the CNS of Au@PET film as a SERS platform, real-time and on-site SERS detection of the food contaminant crystal violet (CV) is achieved, with a detection limit of CV solution on a tomato skin of 10-7 M. Owing to its ability to efficiently extract trace analytes, the resulting substrate also achieves detection of 4-ATP contaminants and thiram pesticides by swabbing the skin of an apple. A SERS detection signal for 4-ATP has a relative standard deviation of less than 10%, revealing the excellent reproducibility of the substrate. The flexible, transparent and highly sensitive substrates fabricated using this simple and cost-effective strategy are promising for practical application in rapid, on-site SERS-based detection.

Surface-enhanced Raman spectroscopy with Au-nanoparticle substrate fabricated by using femtosecond pulse

Au-nanoparticle (Au-NP) substrates for surface-enhanced Raman spectroscopy (SERS) were fabricated by grid-like scanning a Au-film using a femtosecond pulse. The Au-NPs were directly deposited on the Au-film surface due to the scanning process. The experimentally obtained Au-NPs presented local surface plasmon resonance effect in the visible spectral range, as verified by finite difference time domain simulations and measured reflection spectrum. The SERS experiment using the Au-NP substrates exhibited high activity and excellent substrate reproducibility and stability, and a clearly present Raman spectra of target analytes, e.g. Rhodamine-6G, Rhodamine-B and Malachite green, with concentrations down to 10^-9 M. This work presents an effective approach to producing Au-NP SERS substrates with advantages in activity, reproducibility and stability, which could be used in a wide variety of practical applications for trace amount detection.