A simple strategy to improve the sensitivity of probe molecules on SERS substrates

A single atom cobalt anchored MXene bifunctional platform for rapid, label-free and high-throughput biomarker analysis and tissue imaging

Few of single-atom materials have been served as platform to analyze small molecules for surface assisted laser desorption/ionization mass spectrometry (SALDI-MS). Herein, a novel single Co atom-anchored MXene (Co-N-Ti3C2) is prepared to achieve enhanced SALDI-MS and mass spectrometry imaging (MSI) performance for the first time. The Co-N-Ti3C2 films were prepared by a simple in situ self-assembly strategy to generate an efficient SALDI-MS platform. Compared to typical inorganic/organic matrices, Co-N-Ti3C2 films exhibit superior performance in small molecules detection with ultra-high sensitivity (LOD at amol level), excellent repeatability (CV <4%), clean background and wide analyte coverage, enabling accurate quantitative analysis of various low-concentration metabolites from 1 μL biofluid in seconds. Its usage efficiently enhanced SALDI-MS detection of various small-molecule biomarkers such as amino acids, succinic acid, itaconic acid, arachidonic acid, citrulline, prostaglandin E2, creatinine, uric acid, glutamine, D-mannose, cholesterol and inositol in positive ion mode. The blood glucose level in humans was successfully determined from a linearity concentration range (0.25-10 mM). Notably, the Co-N-Ti3C2 assisted SALDI-MSI enables study the spatial distribution of small molecules covering the range central to metabolomics at a high resolution on a tissue section. Furthermore, Co-N-Ti3C2 platform revealed a specific peak profile that distinguishes osteoarthritis (OA) from rheumatoid arthritis (RA) tissue. Density functional theory theoretical investigation revealed that single Co atoms anchored on Ti3C2 could highly enhanced the ionization ability of metabolites, resulting in high-sensitivity and heterogeneous metabolome coverage.

Flexible PDMS-Based SERS Substrates Replicated from Beetle Wings for Water Pollutant Detection

The flexible surface-enhanced Raman scattering (SERS) sensor, which has the bionic 3D nanoarray structure of a beetle-wing substrate (BWS), was successfully prepared by replicated technology and thermal evaporation. The bionic structure was replicated with polydimethylsiloxane (PDMS) and then silver (Ag) nanoisland thin films were deposited by thermal evaporation. The deposition times and thicknesses (25-40 nm) of the Ag thin films were manipulated to find the optimal SERS detection capability. The Ag nanoisland arrays on the surface of the bionic replicated PDMS were observed by scanning electron microscope (SEM), X-ray diffraction (XRD), and contact angle, which can generate strong and reproducible three-dimensional hotspots (3D hotspots) to enhance Raman signals. The water pollutant, rhodamine 6G (R6G), was used as a model molecule for SERS detection. The results show that 35 nm Ag deposited on a PDMS-BWS SERS substrate displays the strongest SERS intensity, which is 10 times higher than that of the pristine BWS with 35 nm Ag coating, due to the excellent 3D bionic structure. Our results demonstrate that bionic 3D SERS sensors have the potential to be applied in wearable devices and sensors to detect biomolecules and environmental pollutants, such as industrial wastewater, in the future.

Innovative Application of SERS in Food Quality and Safety: A Brief Review of Recent Trends

Innovative application of surface-enhanced Raman scattering (SERS) for rapid and nondestructive analyses has been gaining increasing attention for food safety and quality. SERS is based on inelastic scattering enhancement from molecules located near nanostructured metallic surfaces and has many advantages, including ultrasensitive detection and simple protocols. Current SERS-based quality analysis contains composition and structural information that can be used to establish an electronic file of the food samples for subsequent reference and traceability. SERS is a promising technique for the detection of chemical, biological, and harmful metal contaminants, as well as for food poisoning, and allergen identification using label-free or label-based methods, based on metals and semiconductors as substrates. Recognition elements, including immunosensors, aptasensors, or molecularly imprinted polymers, can be linked to SERS tags to specifically identify targeted contaminants and perform authenticity analysis. Herein, we highlight recent studies on SERS-based quality and safety analysis for different foods categories spanning the whole food chain, ‘from farm to table’ and processing, genetically modified food, and novel foods. Moreover, SERS detection is a potential tool that ensures food safety in an easy, rapid, reliable, and nondestructive manner during the COVID-19 pandemic.

Trapping analytes into dynamic hot spots using Tyramine-medicated crosslinking chemistry for designing versatile sensor

HYPOTHESIS

Due to the intrinsic nature of the surface-enhanced Raman scattering (SERS), the detection of molecules with weak binding affinities toward metal substrates is critical for development of a universal SERS sensing platform. We hypothesized the physical trapping of small pesticide molecules for active hot spot generation using tyramine-mediated crosslinking chemistry and silver nanoparticles (Ag NPs) enhances SERS detection sensitivity.

EXPERIMENTS

Tyramine-mediated crosslinking chemistry for sensor application was validated by ultraviolet-visible absorption spectroscopy, scanning electron microscopy, dynamic light scattering, and Raman spectroscopy. SERS sensing platform using tyramine-mediated crosslinking reaction was systematically studied for detection of 1,4-dyethylnylbenzene as a model analyte. This sensor system was applied to detect two other pesticides, thiabendazole and 1,2,3,5-tetrachlorobenzene, which have different binding affinities toward metal surfaces.

FINDINGS

The SERS signal of 1,4-dyethylnylbenzene obtained using this sensor system was 3.6 times stronger than that obtained using the Ag colloidal due to the nanogap of approximately 1.3 nm within the generated hot spots. This sensor system based on tyramine-mediated crosslinked Ag NPs was evaluated as a promising tool to achieve a solution based sensitive detection of various pesticide molecules that cannot be adsorbed on the surfaces of typical SERS substrates such as metal nanoparticles.

A Sensitive SERS Method for Determination of Pymetrozine in Apple and Cabbage Based on an Easily Prepared Substrate

Residual pesticides are one of the major food safety concerns around the world. There is a demand for simple and reliable methods to monitor pesticide residues in foods. In this study, a sensitive method for determination of pymetrozine in apple and cabbage samples using surface-enhanced Raman spectroscopy (SERS) based on decanethiol functionalized silver nanoparticles was established. The proposed method performed satisfactorily with the linear detection range of 0.01-1.00 mg/L and limit of detection (LOD) of 0.01 mg/L in methanol. In addition, it was successfully used to detect pymetrozine in apple and cabbage samples, the LOD was 0.02 and 0.03 mg/L, respectively, and the recoveries of spiked cabbage and apple ranged 70.40-104.00%, with relative standard deviations below 12.18% and 10.33% for intra-day and inter-day tests. Moreover, the results of the correlation test with real cabbage samples of liquid chromatography-tandem mass spectrometry showed that they were highly correlated (slope = 0.9895, R2 = 0.9953). This study provides a sensitive approach for detection of pymetrozine in apple and cabbage, which has great potential for determination of pymetrozine residues in food products.

Detection of Chlortetracycline Hydrochloride in Milk with a Solid SERS Substrate Based on Self-assembled Gold Nanobipyramids

This paper described how a high-yield, monodisperse Au nanobipyramids (Au NBs) sol was prepared by a seed-mediated method, and gold nanoparticles were assembled on the surface of a silicon wafer by self-assembly technology to obtain a solid SERS substrate. Scanning electron microscopy (SEM) showed that the average length of Au NBs was 34.31 nm, and the analysis enhancement factor (AEF) was approximately 7.3 × 10⁵ with rhodamine 6G (R6G) used as a probe. SERS detection of chlortetracycline hydrochloride (CCH) in milk was performed utilizing the prepared Au NBs substrate, and the limit of detection was 0.01 mg/mL. In the range of 0.01 - 1 mg/mL, the mass concentration of CCH and the SERS signal intensity satisfied the linear relationship y = 258.467x + 150.501; the value of the correlation coefficient was 0.9785. In addition, the recovery of spiked samples fluctuated between 96.80 to 111.38%. These results proved that the method is simple and fast, and it is promising to be applied to the field detection of antibiotics in milk.

Au Sputtered Paper Chromatography Tandem Raman Platform for Sensitive Detection of Heavy Metal Ions

Surface-enhanced Raman scattering (SERS) is a powerful technique for sensitive detection, but it normally has difficulty in multicomponent detection in a complex system, especially for simultaneous analysis of mixture of heavy metal ions. In this work, a simple paper chromatography tandem SERS (PC-SERS) separation/detection platform is proposed by ion-sputtering gold on a filter paper. Based on SEM results, the great electromagnetic field inside nanogaps of Au nanoislands on the paper surface is evaluated with FDTD simulation. It is found that the PC-SERS platform has good uniformity (RSD = 10.12%) and long-time stability. The as-prepared PC-SERS platform was applied to efficiently separate and detect a mixture of pesticides (MG, MB, and CV) in pond water without any pretreatment process, and the limits of detection (LODs) were down to 10 nM. As a crucial application for food safety, several heavy metal ions such as Cd²⁺, Cu²⁺, and Ni²⁺ in grinded rice were successfully detected by the PC-SERS method taking advantage of the sandwich structure based on 4-mercaptobenzoic acid (4-MBA) molecules, which were modified onto sputtering the Au filter paper and gold nanoparticles (Au NPs) to link metal ions and acted as Raman signal molecules. All the LODs for metal ions were down to 1 μM. Due to the easiness of fabrication, good reproducibility, and simple pretreatment step, the PC-SERS platform holds promise in multicomponent detection in a real sample.

Controllable fabrication of Ag-NP-decorated porous ZnO nanosheet arrays with superhydrophobic properties for high performance SERS detection of explosives

An efficient hydrophobic condensation surface was developed and used as an ultrasensitive and stable SERS sensor based on ZnO–Ag hybrid mesoporous nanosheet (MNS) arrays for natural explosive sample detection.