Large-Scale, Lithography-Free Production of Transparent Nanostructured Surface for Dual-Functional Electrochemical and SERS Sensing

Seeing an Unobservable Fe(III)/Fe(IV) Redox Process of the Nonheme Iron N4Py Complex by High-Speed Surface-Enhanced Raman Spectroelectrochemistry

High-valent iron oxido species, central to many enzymatic and biomimetic catalyzed organic oxidative transformations, can be generated by direct electrochemical oxidation, circumventing high-energy O atom donor reagents. Electrochemical generation necessitates knowledge of the redox potentials involved, which is hindered by the lack of well-defined Fe(III)/Fe(IV) redox waves in the voltammetry of many iron-based catalysts. Hence, other approaches including chemical oxidation and bulk spectro(electro)chemical methods need to be taken. In the case of the well-studied oxidation catalyst, [ ( N 4 P y ) F e ( I I ) O H 2 ] 2 + , where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine, estimates of the Fe(III/IV) redox potentials range from 0.4 to 1.3 V vs SCE. Here, we show that electrochemical surface-enhanced Raman scattering spectroscopy reveals "hidden" redox waves, and hence redox potentials, when coupled with cyclic voltammetry. Rapid spectral acquisition (>2 Hz) of surface-enhanced Raman spectra at electrochemically roughened gold electrodes enables real-time spectral acquisition during cyclic voltammetry. We show that the Fe(III)/Fe(IV) redox potential of [ ( N 4 P y ) F e ( I I ) O H 2 ] 2 + is close to that determined earlier by chemical redox titrations (0.85 V vs SCE). Furthermore, comproportionation and adsorption processes are shown to impact the rates of electron transfer observed, which rationalizes the absence of a distinct Fe(III)/Fe(IV) redox wave in its cyclic voltammetry.

Selective Detection of Fungal and Bacterial Glycans with Galactofuranose (Galf) Residues by Surface-Enhanced Raman Scattering and Machine Learning Methods

Specific monosaccharide residue, β-D-galactofuranose (Galf) featuring a five-membered ring structure, is found in the glycans of fungi and bacteria, but is normally absent in healthy mammals and humans. In this study, synthetic oligosaccharides mimicking bacterial and fungal glycans were investigated by SERS (Surface-Enhanced Raman Scattering) techniques for the first time to distinguish between different types of glycan chains. SERS spectra of oligosaccharides related to fungal α-(1→2)-mannan, β-(1→3)-glucan, β-(1→6)-glucan, galactomannan of Aspergillus, galactan I of Klebsiella pneumoniae, and diheteroglycan of Enterococcus faecalis were measured. To analyze the spectra, a number of machine learning methods were used that complemented each other: principal component analysis (PCA), confidence interval estimation (CIE), and logistic regression with L1 regularization. Each of the methods has shown own effectiveness in analyzing spectra. Namely, PCA allows the visualization of the divergence of spectra in the principal component space, CIE visualizes the degree of overlap of spectra through confidence interval analysis, and logistic regression allows researchers to build a model for determining the belonging of the analyte to a given class of carbohydrate structures. Additionally, the methods complement each other, allowing the determination of important features representing the main differences in the spectra containing and not containing Galf residue. The developed mathematical models enabled the reliable identification of Galf residues within glycan compositions. Given the high sensitivity of SERS, this spectroscopic technique serves as a promising basis for developing diagnostic test systems aimed at detecting biomarkers of fungal and bacterial infections.

Solid-phase extraction coupled to automated centrifugal microfluidics SERS: Improving quantification of therapeutic drugs in human serum

Surface-enhanced Raman spectroscopy (SERS) is a powerful method in analytical chemistry, but its application in real-life medical settings has been limited due to technical challenges. In this work, we introduce an innovative approach that is meant to advance the automation of microfluidics SERS to improve reproducibility and label-free quantification of two widely used therapeutic drugs, methotrexate (MTX) and lamotrigine (LTG), in human serum. Our methodology involves a miniaturized solid-phase extraction (μ-SPE) method coupled to a centrifugal microfluidics disc with incorporated SERS substrates (CD-SERS). The CD-SERS platform enables simultaneous controlled sample wetting and accurate SERS mapping. Together with the assay we implemented a machine learning method based on Partial Least Squares Regression (PLSR) for robust data analysis and drug quantification. The results indicate that combining μ-SPE with CD-SERS (μ-SPE to CD-SERS) led to a substantial improvement in the signal-to-noise ratio compared to combining CD-SERS with ultrafiltration or protein precipitation. The PLSR model enabled us to obtain the limit of detection and quantification for MTX as 2.90 and 8.92 μM, respectively, and for LTG as 10.76 and 32.29 μM. We also validated our μ-SPE to CD-SERS method for MTX against HPLC and immunoassay (p-value <0.05), using patient samples undergoing MTX therapy. In addition, we achieved a satisfactory recovery rate (80%) for LTG when quantifying it in patient samples. Our results show the potential of this newly developed approach as a strategy for therapeutic drugs in point-of-care clinical settings and highlight the benefits of automating label-free SERS assays.

Surface-Enhanced Raman Spectroscopy and Electrochemistry: The Ultimate Chemical Sensing and Manipulation Combination

One of the lessons we learned from the COVID-19 pandemic is that the need for ultrasensitive detection systems is now more critical than ever. While sensors' sensitivity, portability, selectivity, and low cost are crucial, new ways to couple synergistic methods enable the highest performance levels. This review article critically discusses the synergetic combinations of optical and electrochemical methods. We also discuss three key application fields-energy, biomedicine, and environment. Finally, we selected the most promising approaches and examples, the open challenges in sensing, and ways to overcome them. We expect this work to set a clear reference for developing and understanding strategies, pros and cons of different combinations of electrochemical and optical sensors integrated into a single device.

Reproducible SERS substrates manipulated by interparticle spacing and particle diameter of gold nano-island array using in-situ thermal evaporation

Gold (Au) nano-island arrays were deposited on the glass substrate to fabricate surface-enhanced Raman scattering (SERS) substrates by in-situ thermal evaporation (deposited and annealed samples at the same time). The optimal SERS intensity deposited by various thicknesses and in-situ annealing temperatures of Au nano-island arrays would be investigated. The biomolecules (adenine) were dropped on the well-designed SERS substrate for precise and quantitative SERS detection. The characterization of Au nano-island arrays SERS substrate would be evaluated by scanning electron microscope (SEM) and Raman spectroscopy. The results showed that the optimal deposition thickness and annealing temperature of Au nano-island arrays SERS substrate is about 14 nm and 200 °C respectively, which can construct the smallest interparticle spacing (W)/ particle diameter (D) ratio and the lowest reflection (%) and transmittance (%) to form the strongest SERS intensity. Moreover, finite-difference time-domain (FDTD) simulation of the electromagnetic field distributions on Au nano-island arrays displays the similar trend with the experimental results. The 14 nm deposition with 200 °C in-situ annealing temperature would display the highest density of hot-spots by FDTD simulation. The reproducible Au nano-island arrays SERS substrates with tunable surface roughness, W/D ratio, and lower reflection and transmittance show promising potential for SERS detection of biomolecules, bacteria, and viruses.

SERS and EC-SERS detection of local anesthetic procaine using Pd loaded highly reduced graphene oxide nanocomposite substrate

The localized surface plasmon resonance (LSPR) excitations are critical towards achieving sizable spectral enhancements of the Raman scattered light. Herein, the synthesis of palladium-based highly reduced graphene oxide (Pd-HRG) with LSPR properties as an effective surface-enhanced Raman scattering (SERS) substrate and its utility in the highly sensitive detection of procaine are reported. The concentration detection of procaine samples was optimized by applying a set of pre-concentration parameters. The Pd-HRG nanocomposite showed a remarkable LSPR response with a Raman enhancement factor of 8.7 × 10². The Pd-HRG is employed to modify fluorine doped tin oxide electrode (Pd-HRG/FTO), resulted with an enhancement factor of 7.5 × 10⁴ corresponding to the EC-SERS technique. The electronic and surface properties of synthesized Pd-HRG and functionalized FTO electrode were evaluated using Raman, infrared, EIS, XRD, FESEM and EDX techniques. Quantum chemical calculations were carried out to elaborate on the nature of interaction of procaine molecules with a nanostructured surface model. Pd-HRG, with an efficient and cost-effective fabrication, can be considered as a promising EC-SERS substrate for the detection of organic therapeutic drugs.

Review on combining surface-enhanced Raman spectroscopy and electrochemistry for analytical applications

The discovery of surface enhanced Raman scattering (SERS) from an electrochemical (EC)-SERS experiment is known as a historic breakthrough. Five decades have passed and Raman spectroelectrochemistry (SEC) has developed into a common characterization tool that provides information about the electrode-electrolyte interface. Recently, this technique has been successfully explored for analytical purposes. EC was found to highly improve the performances of SERS sensors, providing, among others, controlled adsorption of analytes and increased reproducibility. In this review, we highlight the potential of EC-SERS sensors to be implemented for point-of-need (PON) analyses as miniaturized devices, and their ability to revolutionize fields like quality control, diagnosis or environmental and food safety. Important developments have been achieved in Raman spectroelectrochemistry, which now represents a promising alternative to conventional analytical methods and interests more and more researchers. The studies included in this review open endless possibilities for real-life EC-SERS analytical 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.

Simple and cost-effective microfabrication of flexible and stretchable electronics for wearable multi-functional electrophysiological monitoring

The fabrication of flexible and stretchable electronics is a critical requirement for the successful application of wearable healthcare devices. Although such flexible electronics have been commonly fabricated by microelectromechanical system (MEMS) technologies, they require a specialised equipment for vacuum deposition, photolithography, and wet and dry etching. A photolithography-free simple patterning method using a desktop plotter cutter has been proposed; however, the metal formation and electrode opening still rely on the MEMS technology. To address this issue, we demonstrate a simple, rapid, cost-effective, and a complete microfabrication process for flexible and stretchable sensor platforms encompassing conductor formation and patterning to encapsulate and open sensing windows, which only require an economic plotter cutter and readily available supplies. Despite its simplicity, the proposed process could stably create microscale features of 200 μm wide conductor lines and 1 mm window openings, which are in the useful range for various wearable applications. The feasibility of the simple fabrication of multi-functional sensors for various physiological monitoring applications was successfully demonstrated in electrochemical (glucose), electrical (electrocardiogram), mechanical (strain), and thermal (body temperature) modalities.

Block Copolymer Derived Vertically Coupled Plasmonic Arrays for Surface-Enhanced Raman Spectroscopy

A surface-enhanced Raman spectroscopy sensing template consisting of gold-covered nanopillars is developed. The plasmonic slab consists of a perforated gold film at the base of the nanopillars and a Babinet complementary dot array on top of the pillars. The nanopillars were fabricated by the incorporation of an iron salt precursor into a self-assembled block copolymer thin film and subsequent reactive ion etching. The preparation is easy, scalable, and cost-effective. We report on the increase in surface-enhanced Raman scattering efficiency for smaller pillar heights and stronger coupling between the dot array and perforated gold film with average enhancement factors as high as 10⁷. In addition, the block copolymer-derived templates show an excellent relative standard deviation of 8% in the measurement of the Raman intensity. Finite difference time domain simulations were performed to investigate the nature of the electromagnetic near-field enhancement and to identify plasmonic hot spots.

Tethered molecular redox capacitors for nanoconfinement-assisted electrochemical signal amplification

Nanostructured materials offer the potential to drive future developments and applications of electrochemical devices, but are underutilized because their nanoscale cavities can impose mass transfer limitations that constrain electrochemical signal generation. Here, we report a new signal-generating mechanism that employs a molecular redox capacitor to enable nanostructured electrodes to amplify electrochemical signals even without an enhanced reactant mass transfer. The surface-tethered molecular redox capacitor engages diffusible reactants and products in redox-cycling reactions with the electrode. Such redox-cycling reactions are facilitated by the nanostructure that increases the probabilities of both reactant-electrode and product-redox-capacitor encounters (i.e., the nanoconfinement effect), resulting in substantial signal amplification. Using redox-capacitor-tethered Au nanopillar electrodes, we demonstrate improved sensitivity for measuring pyocyanin (bacterial metabolite). This study paves a new way of using nanostructured materials in electrochemical applications by engineering the reaction pathway within the nanoscale cavities of the materials.

Present and Future of Surface-Enhanced Raman Scattering

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

Injection-Molded Microfluidic Device for SERS Sensing Using Embedded Au-Capped Polymer Nanocones

To enable affordable detection and diagnostic, there is a need for low-cost and mass producible miniaturized sensing platforms. We present a fully polymeric microfluidic lab-on-a-chip device with integrated gold (Au)-capped nanocones for sensing applications based on surface-enhanced Raman spectroscopy (SERS). All base components of the device were fabricated via injection molding (IM) and can be easily integrated using ultrasonic welding. The SERS sensor array, embedded in the bottom of a fluidic channel, was created by evaporating Au onto IM nanocone structures, resulting in densely packed Au-capped SERS active nanostructures. Using a Raman active model analyte, trans-1,2-bis-(4-pyridyl)-ethylene, we found a surface-averaged SERS enhancement factor of ∼5 × 10⁶ with a relative standard deviation of 14% over the sensor area (2 × 2 mm²), and a 18% signal variation among substrates. This reproducible fabrication method is cost-effective, less time consuming, and allows mass production of fully integrated polymeric, microfluidic systems with embedded high-density and high-aspect ratio SERS sensor.

Giant Gold Nanowire Vesicle-Based Colorimetric and SERS Dual-Mode Immunosensor for Ultrasensitive Detection of Vibrio parahemolyticus

Conventional methods for the detection of Vibrio parahemolyticus (VP) usually need tedious, labor-intensive processes, and have low sensitivity, which further limits their practical applications. Herein, we developed a simple and efficient colorimetry and surface-enhanced Raman scattering (SERS) dual-mode immunosensor for sensitive detection of VP, by employing giant Au vesicles with anchored tiny gold nanowires (AuNW) as a smart probe. Due to the larger specific surface and special hollow structure of giant Au vesicles, silver staining would easily lead to vivid color change for colorimetric analysis and further amplify SERS signals. The t-test was further used to determine if two sets of data from colorimetry and SERS were significantly different from each other. The result shows that there was no significant difference between data from the two methods. Two sets of data can mutually validate each other and avoid false positive and negative detection. The designed colorimetry-SERS dual-mode sensor would be very promising in various applications such as food safety inspection, personal healthcare, and on-site environmental monitoring.