Per- and polyfluoroalkyl substances (PFAS), such as perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), are persistent and harmful pollutants whose detection at trace levels is extremely challenging due to their amphiphilic nature, chemical inertness, and intrinsically low Raman scattering cross-section. Here, we present a novel PFAS detection method featuring silver nanostructures with strong surface-enhanced Raman scattering (SERS) activity that are electrochemically deposited on self-assembled graphene nanoplatelet (GNP) hydrogels. The morphology and plasmonic activity of the silver nanostructure on the GNP hydrogel scaffolds were studied as a function of deposition parameters, such as applied voltage, frequency, and deposition time. Optimal conditions (5 V at 5 Hz for 60 min) yielded branched silver nanostructures with high surface coverage (∼89.6%). The substrate demonstrated reproducible SERS performance, with a relative standard deviation of 6% for peak intensity across multiple substrates. Moreover, we introduce an electric field-assisted co-deposition strategy that facilitates analyte positioning at SERS hotspots, thus enabling reliable detection for both PFOA and PFOS down to 30 ppt. Experimental findings were supported by finite-difference time-domain (FDTD) simulations, offering insights into the synergistic electromagnetic enhancement provided by the hybrid dendritic-nanoparticle architecture. Overall, this work demonstrates a reproducible hydrogel-based SERS-active substrate with potential for on-site PFAS detection and monitoring.

We present a novel SERS-based biosensing methodology for selective detection of proteins that combines plasmonic activity and molecular recognition. Detection is performed using a surface-based plasmonic sensor featuring electrochemically deposited core-shell Ag-Au dendritic nanostructures that are chemically stable and amenable to biofunctionalization. Target selectivity to the sensor is imparted by means of antibodies immobilized on the surface of these SERS-active nanostructures. Moreover, SERS signal amplification is uniquely achieved by superimposing silver nanoparticles onto the captured analyte with the aid of an electric field. We find that such ”sandwich” architecture favors SERS signal enhancement from the top molecular layer. The sensor’s ability for sensitive and specific detection is assessed in physiologically relevant (HSA-spiked artificial urine) and complex biological media (calf serum) by employing green fluorescent protein (GFP) as the model analyte. Our results pave the way for the development of a new class of hybrid surface-based biosensors (SERS-based immunosensors) that permit sensitive detection of target proteins in complex biological matrices.

Removal of tritiated water from heavy water (D2O) in a cost-effective, energy efficient and environmentally friendly manner is essential for the safe operation and sustainability of nuclear plants. Graphene oxide (GO) membranes have emerged as promising candidates for this application due to their selective permeability and tunable structure. Using a H2O/D2O mixture as a model for tritium removal, we test and compare the separation performance of GO membranes in vapor-phase and liquid-phase filtration experiments. We find that vapor-phase filtration yields overall better results under the same experimental conditions; however, its performance significantly benefits from mixture vaporization (vapor-liquid equilibrium), which is an energy intensive and not GO membrane-specific effect. Conversely, liquid-phase filtration emerges as a more practical and environmentally sustainable option. Membranes used in this study were produced through the filtration of GO suspensions. The membrane preparation method was found to play a key role in separation performance. Membranes made through the filtration of GO suspensions under high pressures (Delta P range: 1-4 bar) were found to be more compact and with lower permeability than membranes produced through vacuum filtration. For membranes prepared under the same filtration pressure, increasing GO loadings resulted in membranes with higher permeability. Overall, higher GO suspension filtration pressures and GO loadings have a beneficial role in the membrane's separation performance and a negative impact on the transmembrane flux. GO membranes fabricated at Delta P = 4 bar demonstrated the highest separation efficiency (7.86%) in vapor-phase filtration at 110 degrees C, while liquid-phase filtration at room temperature yielded 4.72%. These findings offer valuable insights into the optimization of GO membranes for hydrogen isotopic separation and pave the way for their application in realworld nuclear reactor systems.

Surface-enhanced Raman scattering (SERS) pushes the boundaries of Raman spectroscopy as an analytical technique, allowing improved sensitivity and high discriminatory ability in analyte detection. Here, we introduce a SERS substrate using reduced graphene oxide aerogels as scaffolds. Reduced graphene oxide aerogels are hydrophobic, electrically conductive, and easily formable, providing a versatile platform for silver dendritic nanostructure growth via electrochemical deposition. We show that the electrochemical growth conditions (applied voltage, reduction time) have a significant effect on both the morphology and coverage of the silver nanostructures, which in turn have a strong effect on the SERS performance of the substrate. The importance of Ag dendrite morphology to the SERS substrate's performance is also confirmed by finite-difference time-domain simulations. Under silver growth conditions of 10 V applied voltage at 10 Hz for 120 min, we obtained a limit of detection of 3.16 × 10-5 ppm for thiram, which is lower than the testing requirements set by food and environmental regulatory agencies. Moreover, the substrates showed high silver coverage (85.6%), reproducibility (relative standard deviation ∼6% for substrates produced under the same conditions), and relative stability (∼20% change) of the obtained signal over one month. In view of their SERS capabilities and relative ease of preparation, we consider this new class of substrates a strong candidate for meeting detection and quantification challenges for a broad spectrum of analytes.

We present a novel plasmonic biosensing method for on-chip detection on viral particles featuring a micro- electrode platform that integrates accelerated sampling of virus with surface-enhanced Raman scattering (SERS). We show experimentally that our approach can produce spectacular results owing to the unique incorporation of two key features: (1) Concentration amplification of virus on the SERS-active substrate; (2) Local plasmonic activity enhancement due to the targeted superimposition of silver nanoparticles on the captured virus sites. When tested for the detection of the M13 bacteriophage our "sandwich" assay yielded excellent reproducibility (signal variation <6%) and a very low limit of detection (1.13 x 102 pfu/ml). Compared with the performance of our standard SERS substrates, SERS signals stronger by at least one order of magnitude are typically achieved. In addition to experimental results, our work also includes finite element (COMSOL Multiphysics) and finite- difference time-domain (FDTD) simulations that provide insights into the mechanisms of concentration amplification and plasmonic activity enhancement, respectively.

Antimicrobial resistance (AMR) poses a significant global health threat, necessitating rapid and precise detection methods. One widespread mechanism of AMR involves bacterial production of β-lactamase enzymes which render β-lactam antibiotics ineffective. The ability of β-lactamases with carbapenemase activity to degrade carbapenems, β-lactams used as antibiotics of last resort, is of particular concern. Carbapenemase-producing organisms (CPOs) cause infections with high mortality rates, hence, their timely detection is of utmost importance. Here, we applied surface-enhanced Raman scattering (SERS) to the detection of carbapenemase activity, where our data reveal that enzyme-catalyzed carbapenem hydrolysis results in distinct spectral fingerprint changes. We capitalize on this finding by illustrating an experimental methodology implementing SERS that permits the detection of CPOs.

In this work, an analytical solution for the hydrodynamic forces that transport silver ions between microelectrodes of axially variable shape is presented. Four different microelectrode shapes were employed to explore the use of passive methods for silver dendritic growth. The results indicate that a fractal microelectrode shape promotes better silver dendritic formation due to lightning rod effect, while in all configurations, a condition of no-contact between dendrites is reached due to an induced pressure field.

This article provides a proof of concept on how Surface-enhanced Raman Scattering (SERS) can be combined with microfluidics towards the development of an in situ chemical detection method. A microfluidic device prototype was designed, fabricated, and characterized to demonstrate the feasibility of microfluidics in achieving analyte transport from the sample introduction site to the detection site. Subsequently, the microfluidic setup was used for monitoring the adsorption kinetics of the molecule rhodamine 6G (R6G) onto a SERS-active substrate; an application, for which only scare information is available in the literature. Moreover, this work demonstrates how the actual concentration of adsorbed R6G (moles m-2) onto the SERS substrate can be quantified in real time. SERS-active substrates, such the ones used in this study, can be integrated in a microfluidic device for achieving reproducible monitoring of the adsorption kinetics of a molecule in a liquid sample, or for detecting and reporting the concentration of an unknown target molecule during a diagnostic assay.

In this work, an analytic solution for the hydrodynamic dispersion of silver colloidal nanoparticles released into an oscillatory electroosmotic flow between microelectrodes of axially variable shape is presented. The long-time colloid concentration response is derived using the homogenization method together with multiple-scale analysis. The results indicate that the deposition of nanoparticles onto the surface of the microelectrodes depends on the rate constant β of solute reaction at the wall, on the angular frequency ω, and mainly on the induced pressure gradient that arises due to the variable geometrical shape of the walls. For suitable values of the previous parameters, we show that colloidal nanoparticle concentration can be enhanced as well as choosing the location where it will happen.

Although easily scalable, the production of graphene nanoplatelets (GNP) by the means of liquid-phase exfoliation of graphite flakes (GF) remains an energy- and time-intensive process. In this work, we demonstrate that significant time and energy can be saved in GNP production when employing expanded graphite (EG) in a surfactant-assisted liquid phase exfoliation process. Owing to its increased interlayer distance, the exfoliation of EG can be accomplished in a much shorter time (<30 min) compared to GF (approximately 7 h in the present case). Moreover, the energy required for the EG exfoliation is close to 80-fold lower than that for GF exfoliation. Monitoring of the mean lateral dimension, specific surface area, and graphite flake-to-GNP transition during exfoliation was performed experimentally using several analytical techniques. The EG-derived GNPs are produced much faster and require less energy for exfoliation compared to GF, thus making it a more efficient alternative technique.

We present a novel and effective sensing method that combines surface-enhanced Raman scattering (SERS) with electric fields for achieving sensitive and label-free detection of proteins in physiologically relevant media. Using as an example the detection and quantification of human serum albumin (HSA) and creatinine in artificial urine, we combine experiments and simulations to demonstrate how properly applied electric field effects facilitate accelerated transport and deterministic capture of proteins and silver nanoparticles onto the SERS-active substrate. Our experimental approach yields a sandwich assay that amplifies protein concentration and creates higher surface density of hotspots on SERS-active substrates. We call our method “LESS” for brevity, an acronym that captures its key attributes: Label-free, Electric field-assisted, Sandwich-based, and Surface-enhanced Raman scattering. LESS was compared with a standard SERS assay and was shown to produce over 10-fold signal enhancement. Moreover, it offers protein detection and identification even in cases where the standard SERS assay produces no results. Reproducible SERS signals and detection thresholds well below the physiological levels of these proteins in urine were achieved. When combined with chemometric analysis, LESS allows us to extract diagnostic information by classifying SERS spectra acquired from solutions of the individual proteins or their mixtures.

Electroencephalography has garnered interest for applications in mobile healthcare, human–machine interfaces, and Internet of Things. Conventional electroencephalography relies on wet and dry electrodes. Despite favorable interface impedance of wet electrodes and skin, the application of a large amount of gel at their interface with skin limits the electroencephalography spatial resolution, increases the risk of shorting between electrodes, and makes them unsuited for long‐term mobile recording. In contrast, dry electrodes are better suited for long‐term recordings but susceptible to motion artifacts. In addition, both wet and dry electrodes are non‐adhesive to the hairy scalp and mechanical support, or chemical adhesives are used to hold them in place. Herein, a conical microstructure array (CMSA) based sensor made of carbon nanotube‐polydimethylsiloxane composite is reported. The CMSA sensor is fabricated using the innovative, cost‐effective, and scalable method of viscosity‐controlled dip‐pull process. The sensor adheres to the hairy scalp by generating negative pressure in its conical microstructures when it is pressed against scalp. Aided by the application of a trace amount of gel, CMSA sensor establishes good electrical contact with the skin, enabling its applications in mobile electroencephalography over extended periods. Notably, the signal quality of CMSA sensors is comparable to that of medical‐grade wet gel electrodes.

The rapid increase in illicit drug use and its adverse health effects and socio-economic consequences have reached alarming proportions in recent years. Surface-enhanced Raman scattering (SERS) has emerged as a highly sensitive analytical tool for the detection of low dosages of drugs in liquid and solid samples. In the present article, we review the state-of-the-art use of SERS for chemical analysis of illicit drugs in aqueous and complex biological samples, including saliva, urine, and blood. We also include a review of the types of SERS substrates used for this purpose, pointing out recent advancements in substrate fabrication towards quantitative and qualitative detection of illicit drugs. Finally, we conclude by providing our perspective on the field of SERS-based drug detection, including presently faced challenges. Overall, our review provides evidence of the strong potential of SERS to establish itself as both a laboratory and in situ analytical method for fast and sensitive drug detection and identification.

Reduced graphene oxide (rGO) membranes have huge potential for use in the separation of isotopic water mixtures due to the narrow interlayer spacing forming between the rGO sheets. Although there have been several studies on water purification using graphene oxide (GO) and rGO membranes, no experimental work has been performed towards understanding the role of rGO's oxygen-containing groups in the successful filtration of isotopes. The present work investigates key factors and functional groups that govern the separation performance of rGO membranes. The latter are produced by reducing GO membranes in hydroiodic acid vapor at low temperature (40 degrees C). The membranes are subsequently tested in filtration experiments using D2O/H2O mixtures and their selectivity and transmembrane flux are recorded as functions of time and temperature. Chemical characterization of the membranes reveals that the hydroxyl and epoxide groups are primarily responsible for the observed selective separation of isotopic water mixtures. Partially reduced GO membranes were found to provide a D2O rejection efficiency that is comparable to their GO counterparts but exhibit higher mechanical robustness that permits longer filtration cycles. Experimental evidence provided herein proves that rGO membranes can be an effective and promising means of isotopic water mixture separation.

A cost-effective, point of care (POC) device based on highly oriented CNT arrays was developed as an electrochemical assay for real-time and sensitive detection of glucose in complex samples. A low-cost, microcontroller-based potentiostat consisting of Arduino Due and LMP9100-EVM was developed to perform electrochemical measurements such as cyclic voltammetry (CV) and amperometry. A syringe pump based on open-source electronics was designed to direct the flow through a microfluidic chip. Vertically aligned carbon nanotube (VACNT) sensor arrays, in combination with the miniature potentiostat and the syringe pumps, were utilized as a POC device for the rapid and accurate detection of glucose. The structure and morphology of samples were characterized by field emission scanning electron microscopy (FESEM) and attenuated total reflectance Fourier transform infrared spectrometry (ATR-FTIR). CV as well as electrochemical impedance spectroscopy (EIS) was performed to investigate the electrochemical behavior of the electrode with respect to different diffusion regimes. The mediator-less biosensor had a limit of detection of 23 μM and sensitivity of 1462 μA mM−1 cm−2 and 1050 μA mM−1 cm−2 at the linear range of 1.2–7.8 mM and 7.8–11.2 mM, respectively. The presence of other biological compounds such as uric acid (UA) and ascorbic acid (AA) did not interfere with the detection of glucose. Finally, the designed POC device was successfully applied for the determination of glucose in human blood plasma samples.

North America's opioid epidemic has created a need for drug checking technologies (DCT) that can detect potent opioids such as fentanyl and its analogues in drugs, and thus reduce the risk of accidental overdose. Here, a novel silver surface-enhanced Raman scattering (SERS) substrate is evaluated on its suitability for use as a DCT in point-of-care harm reduction applications. These substrates are assembled by electrochemically reducing silver (Ag) ions onto a silicon (Si) chip imprinted with gold (Au) microelectrodes, forming SERS-active Ag nanostructures. SERS analysis of lab-grade fentanyl analogues found that the substrates produced spectra of fentanyl, furanylfentanyl, and carfentanil that could be distinguished using principal component analysis (PCA). The limit of detection of fentanyl on the substrates was determined to be within a practical range, however estimations of fentanyl concentrations to an order of magnitude were achieved with 70% accuracy. The substrates also demonstrated the ability to detect trace fentanyl in high concentrations of heroin and caffeine, respectively. Notably, the substrates are compatible with handheld Raman spectrometers, allowing for powerful chemical detection on a compact, portable device. The performance and usability demonstrated by these SERS substrates validates their potential for use as DCTs that could be vital to the safety of people who use drugs.

In a world that increasingly demands answers in real-time, there exists a distinct need for chemical sensors that can quickly and efficiently detect substances with high sensitivity and selectivity. To address this need, we use surface-enhanced Raman scattering (SERS) as a powerful analytical technique that can provide ultrasensitive and versatile chemical detection on a mobile platform when implemented on a handheld Raman spectrometer. However, the large laser spot size of handheld Raman spectrometers requires SERS substrates of sufficient surface area. Here, we present a facile method for electrodepositing nanostructured silver (Ag) SERS substrates onto silicon microchips. In this method, silver ions are continuously reduced from a large volume of solution in an apparatus resembling a batch electrochemical reactor. The straightforward protocol is scalable, fast, and reproducible. Further, we investigate the influence of temperature and fluid agitation on the growth of Ag nanostructures with the intention of maximizing surface area coverage. We observe an increase in lateral nanostructure growth from heating due to an increase in the diffusion coefficient. However, no significant increase in lateral nanostructure growth is observed from stirring the reagent solution. Despite the absence of trends in lateral growth, we find that high agitation levels promote the growth of extraneous Ag structures on top of the nanostructured film, indicating the presence of a boundary layer at the silicon surface. Further, we find that increased diffusion rates at high temperatures shift the reaction towards the limits of the mass transfer-controlled regime.

The preparation of aqueous graphene nanoplatelet (GNP) dispersions is a challenging task due to their tendency to agglomerate. In contrast, graphene oxide (GO), an oxidized form of GNP, has amphiphilic characteristics which allow it to disperse in an aqueous milieu without the addition of a surfactant. Therefore, we utilize GO as a dispersing agent to prepare highly-concentrated aqueous GNP dispersions, which can be used for various applications. The nature of the dispersed phase is investigated by various material characterization methods. Furthermore, we measure the zeta potentials of the dispersed phased as well as the contact angles of dispersions on different substrates. The latter data are used in the extended DerjaguineLandaueVerweyeOverbeek theory to gain insight into the interactions that determine dispersion stability. We find that the stability of such dispersions mainly depends on the pH value as well as the ratio of GO and GNP concentrations. The rheology of the dispersions is investigated and different nematic phases are identified. Finally, highly-concentrated dispersions are used for printing of graphene films on a flexible substrate. After reduction of the minor GO fraction, the films have a high conductivity of more than 4500 S m(-1). (C) 2020 Elsevier Ltd. All rights reserved.