Portable Surface-Enhanced Raman Scattering Sensor for Rapid Detection of Aniline and Phenol Derivatives by On-Site Electrostatic Preconcentration

Portable Copper-Based Electrochemical SERS Sensor for Point-of-Care Testing of Paraquat and Diquat by On-Site Electrostatic Preconcentration

With the advent of portable Raman spectrometers, the deployment of surface-enhanced Raman spectroscopy (SERS) in point-of-care testing (POCT) has been initiated. Within any analytical framework employing SERS, the acuity and selectivity inherent to the SERS substrate are of paramount importance. In this article, we utilize in situ electrochemical passivation technology to fabricate CuI passivation film, which serves as a flexible copper-based SERS substrate. Furthermore, portable electrochemical SERS (EC-SERS) sensors were prepared by combining this with laser direct writing technology. The detection signal was amplified using electrostatic preconcentration technology, showcasing impressive sensitivity, selectivity, and stability in pesticide detection. The detected concentrations of paraquat and diquat in tea reached as low as 3.36 and 2.43 μg/kg, respectively. Furthermore, the application of electrostatic preconcentration facilitated selective target molecule aggregation on the SERS sensor, markedly increasing Raman signal strength and enabling single-molecule detection. This research introduces an innovative POCT method for pesticides, promising to advance environmental monitoring's analytical capabilities.

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.

All-in-One Electrokinetic Strategy Coupled with a Miniaturized Chip for SERS Detection of Multipesticides

Complexed and tiresome pretreatment processes have significantly impeded in-field analysis of environmental specimens. Herein, an all-in-one sample separation and enrichment strategy based on a compact charge-selective capture/nanoconfined enrichment (CSC/NCE) device is exploited for marker-free surface-enhanced Raman spectroscopy (SERS) detection of charged pesticides in matrix specimens. This tactic incorporating in situ separations, seizing, and nanoconfined enhancement can greatly elevate the effectiveness of sample pretreatment. Importantly, CSC/NCE with excellent adsorption performances and excellent plasmonic features facilitates concentration and signal amplification of electrically charged pesticides. With the introduction of an electric field on this integrated CSC/NCE, the matrix effect in samples could be significantly eradicated, and a distinct SERS response is witnessed for targeted analytes. Accurate quantification of multipesticides is achieved by synergizing the CSC/NCE chip and chemometrics, and the contents found by the CSC/NCE-based sensing strategy agree with those obtained from chromatography assays with relative deviations lower than 10%. The facile and versatile all-in-one tactic infused in a compact chip exhibits enormous potential for field-test application in chemical measurement and food safety.

Advances in surface-enhanced Raman spectroscopy-based sensors for detection of various biomarkers

Surface enhanced Raman spectroscopy (SERS) allows the ultrasensitive detection of analytes present in traces or even single molecule levels by the generation of electromagnetic fields. It is a powerful vibrational spectroscopic method that is capable to detect traces of chemical and biological analytes. SERS technique is involved in the extremely sophisticated studies of molecules with high specificity and sensitivity. In the vicinity of nanomaterials decorated surfaces, SERS can monitor extremely low concentrations of analytes in a non-destructive manner with narrow line widths. This review article is focused on some recently developed SERS-based sensors for distinct types of analytes like disease-related biomarkers, organic and inorganic molecules, various toxins, dyes, pesticides, bacteria as well as single molecules. This study aims to enlighten the arising sensing approaches based on the SERS technique. Apart from this, some basics of the SERS technique like their mechanism, detection strategy, and involvement of some specific nanomaterials are also highlighted herein. Finally, the study concluded with some discussion of applications of SERS in various fields like food and environmental analysis.

Recent State and Challenges in Spectroelectrochemistry with Its Applications in Microfluidics

This review paper presents the recent developments in spectroelectrochemical (SEC) technologies. The coupling of spectroscopy and electrochemistry enables SEC to do a detailed and comprehensive study of the electron transfer kinetics and vibrational spectroscopic fingerprint of analytes during electrochemical reactions. Though SEC is a promising technique, the usage of SEC techniques is still limited. Therefore, enough publicity for SEC is required, considering the promising potential in the analysis fields. Unlike previously published review papers primarily focused on the relatively frequently used SEC techniques (ultraviolet-visible SEC and surface-enhanced Raman spectroscopy SEC), the two not-frequently used but promising techniques (nuclear magnetic resonance SEC and dark-field microscopy SEC) have also been studied in detail. This review paper not only focuses on the applications of each SEC method but also details their primary working mechanism. In short, this paper summarizes each SEC technique's working principles, current applications, challenges encountered, and future development directions. In addition, each SEC technique's applicative research directions are detailed and compared in this review work. Furthermore, integrating SEC techniques into microfluidics is becoming a trend in minimized analysis devices. Therefore, the usage of SEC techniques in microfluidics is discussed.

General approach to surface-accessible plasmonic Pickering emulsions for SERS sensing and interfacial catalysis

Pickering emulsions represent an important class of functional materials with potential applications in sustainability and healthcare. Currently, the synthesis of Pickering emulsions relies heavily on the use of strongly adsorbing molecular modifiers to tune the surface chemistry of the nanoparticle constituents. This approach is inconvenient and potentially a dead-end for many applications since the adsorbed modifiers prevent interactions between the functional nanosurface and its surroundings. Here, we demonstrate a general modifier-free approach to construct Pickering emulsions by using a combination of stabilizer particles, which stabilize the emulsion droplet, and a second population of unmodified functional particles that sit alongside the stabilizers at the interface. Freeing Pickering emulsions from chemical modifiers unlocks their potential across a range of applications including plasmonic sensing and interfacial catalysis that have previously been challenging to achieve. More broadly, this strategy provides an approach to the development of surface-accessible nanomaterials with enhanced and/or additional properties from a wide range of nano-building blocks including organic nanocrystals, carbonaceous materials, metals and oxides.

Film-based fluorescent sensor for visual monitoring and efficient removal of aniline in solutions and gas phase

Aniline has attracted much concern for its long degradation half-life and huge toxicity to the environment and human beings. Therefore, the development of a multi-functional device for visual detection and efficient removal of aniline was highly anticipated. In our work, the small-size Eu@UiO-66(COOH) was obtained by post-synthesis modification (PSM), and then the film-based fluorescent sensor was prepared by crosslinking reaction. The films not only showed incredible mechanical stability and potential for large-scale preparation, but also have excellent fluorescence response to aniline in solutions and gas phase. As the concentration of aniline increased, the fluorescence of films gradually increased at 350 nm, while the fluorescence gradually quenching at 620 nm, and the detection limits (LOD) of aniline in water and air were 0.27 ppb and 0.086 ppb, respectively. In addition, the adsorption performance of the film for aniline has also been confirmed and the maximum adsorption capacity was 32.6 mg/g, which is a strong guarantee for the realization of ultra-trace detection and toxicity reduction of aniline. In summary, the multi-functional film sensor has been designed for ultra-trace detection and efficient removal of aniline in solutions and gas phase, and have significant value for pollutant treatment, ecological restoration and early prevention.

Organic Anion Detection with Functionalized SERS Substrates via Coupled Electrokinetic Preconcentration, Analyte Capture, and Charge Transfer

Detecting ultralow concentrations of anionic analytes in solution by surface-enhanced Raman spectroscopy (SERS) remains challenging due to their low affinity for SERS substrates. Two strategies were examined to enable in situ, liquid phase detection using 5(6)-carboxyfluorescein (5(6)-FAM) as a model analyte: functionalization of a gold nanopillar substrate with cationic cysteamine self-assembled monolayer (CA-SAM) and electrokinetic preconcentration (EP-SERS) with potentials ranging from 0 to +500 mV. The CA-SAM did not enable detection without an applied field, likely due to insufficient accumulation of 5(6)-FAM on the substrate surface limited by passive diffusion. 5(6)-FAM could only be reliably detected with an applied electric field with the charged molecules driven by electroconvection to the substrate surface and the SERS intensity following the Langmuir adsorption model. The obtained limits of detection (LODs) with an applied field were 97.5 and 6.4 nM on bare and CA-SAM substrates, respectively. For the CA-SAM substrates, both the ligand and analyte displayed an ∼15-fold signal enhancement with an applied field, revealing an additional enhancement due to charge-transfer resonance taking place between the metal and 5(6)-FAM that improved the LOD by an order of magnitude.

Rapid Determination of the 'Legal Highs' 4-MMC and 4-MEC by Spectroelectrochemistry: Simultaneous Cyclic Voltammetry and In Situ Surface-Enhanced Raman Spectroscopy

The synthetic cathinones mephedrone (4-MMC) and 4-methylethcathinone (4-MEC) are two designer drugs that represent the rise and fall effect of this drug category within the stimulants market and are still available in several countries around the world. As a result, the qualitative and quantitative determination of 'legal highs', and their mixtures, are of great interest. This work explores for the first time the spectroelectrochemical response of these substances by coupling cyclic voltammetry (CV) with Raman spectroscopy in a portable instrument. It was found that the stimulants exhibit a voltammetric response on a gold screen-printed electrode while the surface is simultaneously electro-activated to achieve a periodic surface-enhanced Raman spectroscopy (SERS) substrate with high reproducibility. The proposed method enables a rapid and reliable determination in which both substances can be selectively analyzed through the oxidation waves of the molecules and the characteristic bands of the electrochemical SERS (EC-SERS) spectra. The feasibility and applicability of the method were assessed in simulated seized drug samples and spiked synthetic urine. This time-resolved spectroelectrochemical technique provides a cost-effective and user-friendly tool for onsite screening of synthetic stimulants in matrices with low concentration analytes for forensic applications.

Advanced Optical Sensing of Phenolic Compounds for Environmental Applications

Phenolic compounds are particularly dangerous due to their ability to remain in the environment for a long period of time and their toxic effects. They enter in the environment in different ways, such as waste from paper manufacturing, agriculture (pesticides, insecticides, herbicides), pharmaceuticals, the petrochemical industry, and coal processing. Conventional methods for phenolic compounds detection present some disadvantages, such as cumbersome sample preparation, complex and time-consuming procedures, and need of expensive equipment. Therefore, there is a very large interest in developing sensors and new sensing schemes for fast and easy-to-use methods for detecting and monitoring the phenolic compound concentration in the environment, with special attention to water. Good analytical properties, reliability, and adaptability are required for the developed sensors. The present paper aims at revising the most generally used optical methods for designing and fabricating biosensors and sensors for phenolic compounds. Some selected examples of the most interesting applications of these techniques are also proposed.

Recent Developments in Plasmonic Sensors of Phenol and Its Derivatives

Many scientists are increasingly interested in on-site detection methods of phenol and its derivatives because these substances have been universally used as a significant raw material in the industrial manufacturing of various chemicals of antimicrobials, anti-inflammatory drugs, antioxidants, and so on. The contamination of phenolic compounds in the natural environment is a toxic response that induces harsh impacts on plants, animals, and human health. This mini-review updates recent developments and trends of novel plasmonic resonance nanomaterials, which are assisted by various optical sensors, including colorimetric, fluorescence, localized surface plasmon resonance (LSPR), and plasmon-enhanced Raman spectroscopy. These advanced and powerful analytical tools exhibit potential application for ultrahigh sensitivity, selectivity, and rapid detection of phenol and its derivatives. In this report, we mainly emphasize the recent progress and novel trends in the optical sensors of phenolic compounds. The applications of Raman technologies based on pure noble metals, hybrid nanomaterials, and metal–organic frameworks (MOFs) are presented, in which the remaining establishments and challenges are discussed and summarized to inspire the future improvement of scientific optical sensors into easy-to-operate effective platforms for the rapid and trace detection of phenol and its derivatives.

Development of shipboard automatic flow injection analysis-Surface-enhanced Raman spectroscopy instrument toward onsite detection of trace polycyclic aromatic hydrocarbons in water environment

The qualitative and quantitative analysis of polycyclic aromatic hydrocarbons (PAHs) has been important for the environmental control of persistent organic pollutants for decades. Considering the potential risk of deterioration, degradation, and external pollution during transportation, the development of rapid and onsite detection of trace PAHs is in demand. Here, taking the advantage of high sensitivity of surface-enhanced Raman spectroscopy (SERS), we developed a shipboard instrument by combining a portable Raman instrument and a flow injection device, integrating the sample pretreatment and target detection step by step. The feasibility of the instrument was demonstrated by detecting trace benzo[a]pyrene from different water environments with the lowest detection concentration less than 1 µg/l. The reliable stability and repeatability indicate that in the case of emergency response, the developed flow injection analysis-SERS instrument is very promising for the quantitative and qualitative analysis of diverse organic pollutants other than PAHs in water environments.

Ultrasensitive SERS detection of exhaled biomarkers of lung cancer using a multifunctional solid phase extraction membrane

The construction and clinical application of a surface-enhanced Raman scattering (SERS) platform for the early diagnosis of lung cancer could improve the survival rate of patients and would be of great significance. Nevertheless, a sensitive and reusable method for the detection of aldehydes, as biomarkers of lung cancer, in exhaled breath is still an enormous challenge. Aldehydes generally have a low cross section in Raman scattering and have a weak specific affinity to plasmonic nanoparticle surfaces, meaning that sensing them at low concentrations is incredibly difficult. Herein, an ultrasensitive SERS strategy, that can be recycled for further use, for the detection of lung cancer biomarkers in the form of aldehydes was realized by fabrication of a multifunctional Ag NPs@ZIF-67/g-C₃N₄ solid phase extraction (SPE) membrane. Based on the change in the vibrational fingerprints of 4-ATP before and after reaction with the aldehydes, the SPE membrane was successfully used for the ultrasensitive detection of aldehydes with a detection limit of 1.35 nM. The excellent SERS performance was attributed to the synergistic effect of the densely and closely distributed Ag NPs (providing SERS "hot spots"), ZIF-67 (concentrating the analyte molecules) and g-C₃N₄ (forming a membrane to prolong the contact time between the aldehydes and the substrate). In addition, recycling of the SPE membrane was achieved by utilizing the self-cleaning ability of the Ag NPs@ZIF-67/g-C₃N₄ membrane originating from the photocatalytic properties of g-C₃N₄. The proposed SERS membrane was easy to operate, rapid and portable, thus providing a potential tool for a point-of-care test in clinical and diagnostic practice.

Nanostructure-Based Surface-Enhanced Raman Spectroscopy Techniques for Pesticide and Veterinary Drug Residues Screening

Pesticide and veterinary drug residues in food and environment pose a threat to human health, and a rapid, super-sensitive, accurate and cost-effective analysis technique is therefore highly required to overcome the disadvantages of conventional techniques based on mass spectrometry. Recently, the surface-enhanced Raman spectroscopy (SERS) technique emerges as a potential promising analytical tool for rapid, sensitive and selective detections of environmental pollutants, mostly owing to its possible simplified sample pretreatment, gigantic detectable signal amplification and quick target analyte identification via finger-printing SERS spectra. So theoretically the SERS detection technology has inherent advantages over other competitors especially in complex environmental matrices. The progress in nanostructure SERS substrates and portable Raman appliances will promote this novel detection technology to play an important role in future rapid on-site assay. This paper reviews the advances in nanostructure-based SERS substrates, sensors and relevant portable integrated systems for environmental analysis, highlights the potential applications in the detections of synthetic chemicals such as pesticide and veterinary drug residues, and also discusses the challenges of SERS detection technique for actual environmental monitoring in the future.

Combined Paper Centrifugal Chromatographic Separation and SERS Detection for Multicomponent Substances

The separation and chemical analysis of mixtures in an emergency situation represent major challenges, especially in remote or poverty-stricken areas. A novel method was developed for the rapid separation and detection of multiple components via paper centrifugal chromatography, which costs as little as $2.26 US. The method was realized based on the combination of portable paper centrifugal chromatography and surface-enhanced Raman scattering (SERS) detection. This coupled technique was successfully implemented for the separation and qualitative analysis of a rhodamine 6G-crystal violet mixture and a colorless aniline-pyrocatechol-benzidine mixture. A chromatographic mobile phase was collected using absorbent cotton, which was demonstrated to have no effect on the SERS results. The optimized device achieved rapid and effective separation of the colorless aniline-pyrocatechol-benzidine mixture with a high centrifugal force (0.3303π² N). The newly developed method involving multicomponent paper centrifugal chromatography-SERS detection will be of great value for emergency-related substance separation and analysis in remote and poor areas.

Spiropyran based recognitions of amines: UV-Vis spectra and mechanisms

As one of the important photochromic molecules, spiropyran (SP) compounds are widely used as detectors and fluorescence probes in the environment and bio-imaging field. Although great achievements have been attained for various sophisticated spiropyrans in metal ion sensing, less success is achieved in sensing organic molecules due to the weak interaction between the spiropyran and the target of the organic molecule. In this study, a spiropyran derivative containing a hydroxyl group (SPOH) was employed for the recognition of four kinds of amines via ultraviolet-visible (UV-Vis) spectra. The aliphatic primary amines, aromatic primary amines, aliphatic secondary and tertiary amines, aromatic secondary and tertiary amines were successfully distinguished according to the shapes and trends of their UV-Vis absorption spectra. The chemical reaction between aliphatic, aromatic primary amines and SPOH as well as alkalinity are two vital interaction mechanisms for the recognition process which are testified by Fourier Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR). Although SP is generally water-insoluble, it is easy to achieve soluble by fixing SPOH inside micelle or vesicle and thus the results in this study is meaningful for amine recognition utility in environments and biological systems.

Plasmonic tunable Ag-coated gold nanorod arrays as reusable SERS substrates for multiplexed antibiotics detection

Antibiotic contaminants in aqueous media pose a serious threat to human and ecological environments. Therefore, it is necessary to develop robust strategies to detect antibiotic residues. For this purpose, a self-assembly and in situ electrochemical reduction method is utilized to tailor silver nanoparticles (AgNPs)-coated GNRs (AgNPs/GNRs) large-scale vertical arrays. These AgNPs/GNRs arrays exhibit outstanding surface-enhanced Raman scattering (SERS) activities because of abundant Raman hot-spots among the adjacent AgNPs and GNRs, but also excellent stability and reproducibility due to the close-packed arrayed nanostructure. These remarkable features validate this arrayed substrate for high-sensitivity 4-aminothiophenol analysis with a detection limit of 0.35 pM and self-cleaning via electrochemical stripping of the adsorbed analytes and AgNPs from the GNRs arrays, therefore realizing renewable SERS applications. Moreover, the distinct SERS performance of AgNPs/GNRs arrays is verified via the analysis of multiplexed antibiotics at tens of picomolar level and no apparent changes of SERS activities are observed when recyclability is explored. The result demonstrates that the proposed AgNPs/GNRs arrays provide a novel strategy for avoiding conventional, disposable SERS substrates, as well as expanding SERS applications for simultaneous sensing and stripping of environmental contaminants.

Strategies for SERS Detection of Organochlorine Pesticides

Organochlorine pesticides (OCPs) embody highly lipophilic hazardous chemicals that are being phased out globally. Due to their persistent nature, they are still contaminating the environment, being classified as persistent organic pollutants (POPs). They bioaccumulate through bioconcentration and biomagnification, leading to elevated concentrations at higher trophic levels. Studies show that human long-term exposure to OCPs is correlated with a large panel of common chronic diseases. Due to toxicity concerns, most OCPs are listed as persistent organic pollutants (POPs). Conventionally, separation techniques such as gas chromatography are used to analyze OCPs (e.g., gas chromatography coupled with mass spectrometry (GC/MS)) or electron capture detection (GC/ECD). These are accurate, but expensive and time-consuming methods, which can only be performed in centralized lab environments after extensive pretreatment of the collected samples. Thus, researchers are continuously fueling the need to pursue new faster and less expensive alternatives for their detection and quantification that can be used in the field, possibly in miniaturized lab-on-a-chip systems. In this context, surface enhanced Raman spectroscopy (SERS) represents an exceptional analytical tool for the trace detection of pollutants, offering molecular fingerprint-type data and high sensitivity. For maximum signal amplification, two conditions are imposed: an efficient substrate and a high affinity toward the analyte. Unfortunately, due to the highly hydrophobic nature of these pollutants (OCPs,) they usually have a low affinity toward SERS substrates, increasing the challenge in their SERS detection. In order to overcome this limitation and take advantage of on-site Raman analysis of pollutants, researchers are devising ingenious strategies that are synthetically discussed in this review paper. Aiming to maximize the weak Raman signal of organochlorine pesticides, current practices of increasing the substrate's performance, along with efforts in improving the selectivity by SERS substrate functionalization meant to adsorb the OCPs in close proximity (via covalent, electrostatic or hydrophobic bonds), are both discussed. Moreover, the prospects of multiplex analysis are also approached. Finally, other perspectives for capturing such hydrophobic molecules (MIPs-molecularly imprinted polymers, immunoassays) and SERS coupled techniques (microfluidics-SERS, electrochemistry-SERS) to overcome some of the restraints are presented.

Electrokinetic Preseparation and Molecularly Imprinted Trapping for Highly Selective SERS Detection of Charged Phthalate Plasticizers

Nonspecific binding and weak spectral discernment are the main challenges for surface-enhanced Raman scattering (SERS) detection, especially in real sample analysis. Herein, molecularly imprinted polymer (MIP)-based core-shell AuNP@polydopamine (AuNP@PDA-MIP) nanoparticles (NPs) are designed and immobilized on an electrochemically reduced MoS₂-modified screen-printed electrode (SPE). This portable electrochemical-Raman interface offers the dual functions of electrokinetic preseparation (EP) and MIP trapping of charged molecules so that a reliable SERS recognition with molecular selectivity and high sensitivity can be achieved. Core-shell AuNP@PDA-MIP NPs can be controllably synthesized, possess predesigned specific recognition, and provide "hot spots" at the junction of NPs. The introduction of an electric field enables the autonomous exclusion and separation of similarly charged molecules as well as attraction and concentration of the oppositely charged molecules by electrostatic attraction. Subsequently, the specific MIP recognition cavities allow selective adsorption of targets on the interface without the interference of analogues. Owing to the distinctive design of the multiple coupling separation, trapping, and enrichment strategies, the MIP-based SERS-active interface can be used for label-free detection of charged molecules in real samples without pretreatment. As a proof-of-concept study, label-free SERS detection of charged phthalate plasticizers (PAEs) was demonstrated with a detection limit as low as 2.7 × 10^-12 M for dimethyl phthalate (DMP) and 2.3 × 10^-11 M for di(2-ethylhexyl) phthalate (DEHP). This sensing strategy for in situ SERS analysis of charged pollutants or toxins holds vast promises for a wide range of in-field applications.

Recent progress in mycotoxins detection based on surface-enhanced Raman spectroscopy

Mycotoxins are toxic compounds naturally produced by certain types of fungi. The contamination of mycotoxins can occur on numerous foodstuffs, including cereals, nuts, fruits, and spices, and pose a major threat to humans and animals by causing acute and chronic toxic effects. In this regard, reliable techniques for accurate and sensitive detection of mycotoxins in agricultural products and food samples are urgently needed. As an advanced analytical tool, surface-enhanced Raman spectroscopy (SERS), presents several major advantages, such as ultrahigh sensitivity, rapid detection, fingerprint-type information, and miniaturized equipment. Benefiting from these merits, rapid growth has been observed under the topic of SERS-based mycotoxin detection. This review provides a comprehensive overview of the recent achievements in this area. The progress of SERS-based label-free detection, aptasensor, and immunosensor, as well as SERS combined with other techniques, has been summarized, and in-depth discussion of the remaining challenges has been provided, in order to inspire future development of translating the techniques invented in scientific laboratories into easy-to-operate analytic platforms for rapid detection of mycotoxins.

Fabrication of liquid–liquid self-assembled Ag arrays on disposable screen-printed electrodes and their application in the identification and analysis of the adsorption behavior of organic carboxylates through in situ electrochemical surface-enhanced Raman scattering

A disposable Ag array@screen-printed electrode (SPE) was fabricated for the identification and analysis of the adsorption behavior of organic carboxylates in in situ electrochemical surface-enhanced Raman scattering (EC-SERS).

A microfluidic device enabling surface-enhanced Raman spectroscopy at chip-integrated multifunctional nanoporous membranes

A three-dimensional microfluidic chip that combines sample manipulation and SERS detection on-chip was developed. This was successfully achieved by chip integration of a nanoporous polycarbonate track-etched (PCTE) membrane which connects microfluidic channels on two different levels with each other. The membrane fulfills two functions at the same time. On the one hand, it enables sample enrichment by selective electrokinetic transport processes through the membrane. On the other hand, the silver nanoparticle-coated backside of the same membrane enables SERS detection of the enriched analytes. The SERS substrate performance and the electrokinetic transport phenomena were studied using Rhodamine B (RhB) by Raman microscopy and fluorescence video microscopy. After system validation, the approach was attested by on-chip processing of a complex food sample. In a proof-of-concept study, the microfluidic device with the SERS substrate membrane was used to detect a concentration of 1 ppm melamine (705 cm^-1) in whole milk. Electrokinetic transport across the nanoporous SERS substrate facilitates the extraction of analyte molecules from a sample channel into a detection channel via a potential gradient, thus easily removing obscuring compounds present in the sample matrix. The SERS signal of the analyte could be significantly increased by on-target sample drying. This was achieved by guiding an additional gas flow over the membrane which further extends the microfluidic functionality of the chip device. The proposed method possesses the advantages of combining a rapid (within 15 min) sample clean-up using electrokinetic transport in a three-dimensional microfluidic device which is highly suitable for sensitive and selective SERS detection of chemical and biological analytes. Graphical Abstract.

Quantitative TLC-SERS detection of histamine in seafood with support vector machine analysis

Scombroid fish poisoning caused by histamine intoxication is one of the most prevalent allergies associated with seafood consumption in the United States. Typical symptoms range from mild itching up to fatal cardiovascular collapse seen in anaphylaxis. In this paper, we demonstrate rapid, sensitive, and quantitative detection of histamine in both artificially spoiled tuna solution and real spoiled tuna samples using thin layer chromatography in tandem with surface-enhanced Raman scattering (TLC-SERS) sensing methods, enabled by machine learning analysis based on support vector regression (SVR) after feature extraction with principal component analysis (PCA). The TLC plates used herein, which were made from commercial food-grade diatomaceous earth, served simultaneously as the stationary phase to separate histamine from the blended tuna meat and as ultra-sensitive SERS substrates to enhance the detection limit. Using a simple drop cast method to dispense gold colloidal nanoparticles onto the diatomaceous earth plate, we were able to directly detect histamine concentration in artificially spoiled tuna solution down to 10 ppm. Based on the TLC-SERS spectral data of real tuna samples spoiled at room temperature for 0 to 48 hours, we used the PCA-SVR quantitative model to achieve superior predictive performance exceling traditional partial least squares regression (PLSR) method. This work proves that diatomaceous earth based TLC-SERS technique combined with machine-learning analysis is a cost-effective, reliable, and accurate approach for on-site detection and quantification of seafood allergen to enhance food safety.

Internal Calibration Potentiometric Aptasensors for Simultaneous Detection of Hg2+, Cd2+, and As3+ Based on a Screen-Printed Carbon Electrodes Array

An all-solid-state potentiometric aptasensor array based on a multichannel disposable screen-printed carbon electrode (SPCE) was demonstrated for the simultaneous detection of Hg²⁺, Cd²⁺, and As³⁺ by open circuit potential (OCP) technology. The potential of the channel with an internal calibration DNA sequence (IC-DNA) was employed as the internal calibration potential (ICP) to subtract the background signal generated by the detection system, providing a built-in correction methodology. As a result, the developed aptasensor array showed high sensitivity and accuracy for detecting Hg²⁺, Cd²⁺, and As³⁺ without mutual interference or interference from other ions. The linear response ranged from 2.5 pM to 2.5 μM, and the detection limits for Hg²⁺, Cd²⁺, and As³⁺ were 2.0, 0.62, and 0.17 pM, respectively. Furthermore, the potentiometric aptasensor array was successfully applied for the simultaneous detection of three ions in real samples. The results obtained from the developed approach agreed well with the results obtained from inductively coupled plasma mass spectrometry.

Photochemical device for selective detection of phenol in aqueous solutions

We demonstrate that a lab-on-a-chip device (hereafter termed a photochemical phenol sensor) that integrates a photocatalytic long-period fiber grating (PLPFG), fiber Bragg grating (FBG), polymer membrane, ultraviolet (UV) visible light, and microchannels can be exploited to selectively detect phenol in aqueous solutions. The novel PLPFG consisted of a thinned long-period fiber grating (LPFG) and a UV-visible-light-driven Er3+:YAlO3/SiO2/TiO2 (EYST) coating. The polymer membrane with high phenol permselectivity was synthesized using PEBA2533 doped with β-cyclodextrin and was wrapped around the EYST surface, thus forming a microchannel between the membrane and PLPFG to enable the injection and outflow of standard analytes. Subsequently, a Z-shaped microchannel in a PMMA plate was fabricated and employed as a storage chamber for phenol analytes. To realize the EYST photocatalyst, UV-visible-light was irradiated using a tapered UV optical array. Thereafter, to eliminate the effect of temperature on the device, a FBG sensor as a temperature-compensating element was presented. To demonstrate the sensitivity and selectivity of the proposed device, we investigated the effects of the EYST coating's thickness, phenol-based analytes and temperature on the sensitivity and accuracy of the device for measuring phenol concentrations. The results of our present study suggest that the photochemical sensor is effective over a wide range of concentrations (7.5 μg L-1 to 100 mg L-1), pH values (2.0 to 14.0), and temperatures (10 to 48 °C) for selective detection of phenol in aqueous solutions. Thus, the proposed lab-on-a-chip device may be useful for accurate determination of phenol concentrations in real samples.

Surface-enhanced Raman spectroscopic analysis of N6-benzylaminopurine residue quantity in sprouts with gold nanoparticles

A rapid and quantitative method for the determination of N⁶-Benzylademine (N⁶-BA) was established through the application of surface-enhanced Raman spectroscopy (SERS). The Raman peak intensities of N⁶-BA at 1002 cm^-1 positively correlated to N⁶-BA concentrations in sprout extracts. The R² reached 0.99, and RSDs calculated below 10% at the concentration range of 0.1 ∼5μg mL^-1. The average recoveries were 80.0% ∼ 98.2% for blank samples intentionally contaminated at differing levels of 0.04, 0.4, and 1 μg g^-1. The whole procedure, including sample preparation and SERS detection, did not exceed 30 min for a set of 6 samples. This study indicates that SERS is a promising technique for rapid tracing analysis and on-site testing of N⁶-BA.

A simple way to synthesize large-scale Cu2O/Ag nanoflowers for ultrasensitive surface-enhanced Raman scattering detection

Here, we report a simple strategy to prepare highly sensitive surface-enhanced Raman spectroscopy (SERS) substrates based on Ag decorated Cu₂O nanoparticles by combining two common techniques, viz, thermal oxidation growth of Cu₂O nanoparticles and magnetron sputtering fabrication of a Ag nanoparticle film. Methylene blue is used as the Raman analyte for the SERS study, and the substrates fabricated under optimized conditions have very good sensitivity (analytical enhancement factor ∼10⁸), stability, and reproducibility. A linear dependence of the SERS intensities with the concentration was obtained with an R ² value >0.9. These excellent properties indicate that the substrate has great potential in the detection of biological and chemical substances.

Detection of volatile-organic-compounds (VOCs) in solution using cantilever-based gas sensors

Micromechanical resonant sensor offers many advantages for chemical detection, but it fails to maintain high quality factor (Q-factor) when working directly in liquid because of the viscous damping. To solve the problem, a gas/liquid separated sensing method is introduced to detect volatile organic compounds (VOCs) in solution with a resonant cantilever gas sensor. With the help of a waterproof and breathable expanded polytetrafluoroethylene (ePTFE) film, the resonant sensor can be physically isolated from the analyte solution. Thus, the sensor can resonate in gas phase environment with a high Q-factor, meanwhile the interference from the solvent emission can be significantly suppressed. Loaded with the sensing-group functionalized mesoporous-silica nanoparticles (MSNs), the resonant cantilever can detect the target VOC molecules that permeate from the flowing solution sample at the other side of the film. Two typical kind of resonant microcantilever VOC sensors are tested to verify the proposed method, which are loaded with carboxyl (-COOH) and amino (-NH₂) sensing groups functionalized MSNs, respectively. The sensors exhibit highly sensitive (mg/L level resolution) and reproducible detection ability to aniline and acetic-acid solution, respectively. This gas/liquid separated sensing technique is promising in various on-site chemical detection applications.

Au@Ag core–shell nanoparticles with a hidden internal reference promoted quantitative solid phase microextraction-surface enhanced Raman spectroscopy detection

Structural representation of the SPME-SERS fiber with an internal reference and the SERS detection.

Cyclodextrin-based ordered rotaxane-monolayers at gold surfaces

Complexation-to-deaggregation effect of cyclodextrin was applied to achieve ordered functional monolayers on a gold surface.

Green method to fabricate porous microspheres for ultrasensitive SERS detection using UV light

A green method to fabricate porous polymer microspheres with SERS activity for ultrasensitive SERS detection using UV light.

Colorimetric and fluorometric dual-mode detection of aniline pollutants based on spiropyran derivatives

The selective UV-vis absorption of spiropyran and the fluorescence quenching of SP-cellulose material induced by aniline were observed and exploited for rapid detection and accurate quantitative determination of aniline pollutants in the environment.

A gas-diffusion microfluidic paper-based analytical device (μPAD) coupled with portable surface-enhanced Raman scattering (SERS): facile determination of sulphite in wines

A novel facile method for on-site determination of sulphite in wine using a gas-diffusion microfluidic paper-based analytical device (μPAD) combined with surface-enhanced Raman spectroscopy (SERS) was explored.

Highly sensitive SERS-based immunoassay of aflatoxin B1 using silica-encapsulated hollow gold nanoparticles

Aflatoxin B1 (AFB1) is a well-known carcinogenic contaminant in foods. It is classified as an extremely hazardous compound because of its potential toxicity to the human nervous system. AFB1 has also been extensively used as a biochemical marker to evaluate the degree of food spoilage. In this study, a novel surface-enhanced Raman scattering (SERS)-based immunoassay platform using silica-encapsulated hollow gold nanoparticles (SEHGNs) and magnetic beads was developed for highly sensitive detection of AFB1. SEHGNs were used as highly stable SERS-encoding nano tags, and magnetic beads were used as supporting substrates for the high-density loading of immunocomplexes. Quantitative analysis of AFB1 was performed by monitoring the intensity change of the characteristic peaks of Raman reporter molecules. The limit of detection (LOD) of AFB1, determined by this SERS-based immunoassay, was determined to be 0.1 ng/mL. This method has some advantages over other analytical methods with respect to rapid analysis (less than 30 min), good selectivity, and reproducibility. The proposed method is expected to be a new analytical tool for the trace analysis of various mycotoxins.

Hydrophobic gold nanostructures via electrochemical deposition for sensitive SERS detection of persistent toxic substances

The hydrophobic gold nanostructures were used for direct SERS detection of PTS with high sensitivity.

Silver nanoparticle aggregates on metal fibers for solid phase microextraction–surface enhanced Raman spectroscopy detection of polycyclic aromatic hydrocarbons

Silver–copper fibers loaded with silver nanoparticles are used for SPME–SERS detection of polycyclic aromatic hydrocarbons, which can be further confirmed by GC-MS.