Miniaturized Thermal-Assisted Purge-and-Trap Technique Coupling with Surface-Enhanced Raman Scattering for Trace Analysis of Complex Samples

Portable purge and trap-microplasma optical emission spectrometric device for field detection of iodine in water

Iodine is essential for human growth and can enter the body through food, water, and air. Analyzing its presence in the environment is crucial for ensuring healthy human development. However, current large-scale instruments have limitations in the field analysis of iodine. Herein, a miniaturized purge and trap point discharge microplasma optical emission spectrometric (P&T-μPD-OES) device was developed for the field analysis of iodine in water. Volatile iodine molecules were produced from total inorganic iodine (TII) through a basic redox reaction under acidic conditions, then the purge and trap module effectively separated and preconcentrated iodine molecules. The iodine molecules were subsequently atomized and excited by the integrated point discharge microplasma and an iodine atomic emission line at 206.24 nm was monitored by the spectrometer. Under optimal conditions, this proposed method had a detection limit of 16.2 μg L-1 for iodine and a precision better than 4.8%. Besides, the accuracy of the portable device was validated by successful analysis of surface and groundwater samples and a comparison of the mass spectrometry method. This proposed portable, low-power device is expected to support rapid access to iodine levels and distribution in water.

Silver nanoparticles modified sulfur-containing POSS polymer membrane substrate for adsorption and surface-enhanced Raman scattering analysis of chrysoidine in food samples

In analysis of complex samples, the stability and sensitivity of surface-enhanced Raman scattering (SERS) substrates may be compromised by matrix interference. To address this issue, a membrane substrate was prepared for fast enrichment, separation, and detection of chrysoidine all-in-one. The silver nanoparticles modified sulfur-containing POSS polymer (AgNPs/POSS-P-S) SERS membrane substrate was fabricated using polyhedral oligomeric silsesquioxane (POSS) as support materials. Through in-situ growth, AgNPs were uniformly modified on POSS-P-S to ensure the stability and SERS activity of the membrane substrate. The enhancement factor of the malachite green was up to 5.3 × 105. By loading the AgNPs/POSS-P-S on membrane, on the other hand, the SERS membrane substrate can also serve as an adsorption medium for separating chrysoidine from sample matrix. Furthermore, the specific sensing mechanism of AgNPs/POSS-P-S for chrysoidine was investigated and a fast, sensitive, and selective method for its quantification was established, with a linear range of 0.010-2.0 mg/L and the limits of detection at 3.7 μg/L. In addition, the SERS method was successfully applied for the analysis of chrysoidine in beverages and chili products with the recoveries in the range of 83.5%-113.4 % and the relative standard deviations in 3.2%-9.0 %. The proposed AgNPs/POSS-P-S membrane based SRES method has great potential for rapid chrysoidine analysis in food samples.

ATP-Responsive Strand Displacement Coupling with DNA Origami/AuNPs Strategy for the Determination of Microcystin-LR Using Surface-Enhanced Raman Spectroscopy

The DNA origami-mediated self-assembly strategy has emerged as a powerful tool in surface-enhanced Raman spectroscopy (SERS). However, these self-assembly approaches typically do not possess high detection specificity. Herein, a novel strategy based on adenosine triphosphate (ATP)-responsive strand displacement (ARSD) coupling with DNA origami/AuNPs for SERS analysis of microcystin-LR (MC-LR) is presented. In the presence of MC-LR and ATP molecules, nucleic acid sensing structures fabricated with anti-MC-LR aptamer (T1) and ATP aptamer (T2) were triggered to release the remaining ATP. In addition, DNA origami-assisted assembly results in the formation of homogeneous plasmonic nanostructures for Raman enhancement via strong plasmonic coupling. After the binding in the gaps of functionalized DNA origami/AuNPs, the Raman shift of the ATP molecules becomes detectable, leading to increased SERS intensity in 734 cm^-1. A linear response to MC-LR was obtained in the concentration range of 1.56-50 μg·L^-1, and the limit of detection (LOD) was 0.29 μg·L^-1. Combined with the solid-phase extraction sample pretreatment for extraction and 10-fold concentration, this proposed method was successfully used to detect MC-LR type in real lake-water samples with good recoveries of 98.4-116% and relative standard deviations of 1.9-6.7%. Furthermore, for the detection of MC-LR in contaminated lake-water samples, the results of the developed method and ultrahigh-performance liquid chromatography-tandem mass spectrometry were found to be in agreement with relative errors between -12 and 2.4%. The proposed strategy provides a sensitive recognition and signal amplification platform for trace MC-LR analysis as well as innovative nucleic acid sensing structures for toxin analysis more generally.

Accelerating Sample Preparation for the Analysis of Complex Samples

Sample preparation (that is, separation and enrichment) is a critical step in complex sample analysis that affects the sensitivity, selectivity, speed, and accuracy of analytical results, especially in rapid analysis. From chaos to order, the entropy reduction procedure of sample preparation cannot happen spontaneously. Given that sample preparation consumes over two thirds of analysis time, sample preparation becomes the bottleneck issue in analytical chemistry, resulting in the urgent necessity of developing accelerated sample preparation techniques.

Flexible membrane composite based on sepiolite/chitosan/(silver nanoparticles) for enrichment and surface-enhanced Raman scattering determination of sulfamethoxazole in animal-derived food

A sepiolite/chitosan/silver nanoparticles (Sep/CTs/AgNPs) membrane substrate has been developed for the fast separation, enrichment, and surface-enhanced Raman scattering (SERS) determination of sulfamethoxazole all-in-one. The Sep/CTs/AgNPs membrane substrate possessed the ability of rapid separation and enrichment to simplify the process for pretreatment and improve the efficiency of analysis. The grown AgNPs can provide abundant hot spots and plasmonic areas to amplify the Raman signals of target molecules effectively. The membrane substrate exhibited good stability with relative standard deviations of 5.8% and 7.1% to same batch and different batches membrane substrate, respectively,  by detecting sulfamethoxazole. The SERS method based on Sep/CTs/AgNPs membrane substrate was used for the determination of sulfamethoxazole with a linear range of 0.05-2.0 mg/L, and the limit of detection was 0.020 mg/L. The established SERS method was finally applied to the quantification of sulfamethoxazole in animal-derived food samples. Sulfamethoxazole was actually found in crucian sample with 12.4 μg/kg, and the result was confirmed by a high-performance liquid chromatography method with relative error of 5.3%. The whole process of analysis can be finished within 25 min with recoveries of 89.3-102.2%. The SERS method based on Sep/CTs/AgNPs membrane substrate provided an integrated strategy for rapid and accurate SERS analysis in food safety issues.

Simultaneous and Accurate Quantification of Multiple Antibiotics in Aquatic Samples by Surface-Enhanced Raman Scattering Using a Ti3C2Tx/DNA/Ag Membrane Substrate

Rapid and accurate analysis of multiple targets in complex samples is still a big challenge in the fast detection field. Herein, we developed a rapid and accurate strategy for simultaneous quantification of trace multiple antibiotic residues in complex aquatic samples by surface-enhanced Raman scattering (SERS) using a Ti₃C₂T_x/DNA/Ag membrane substrate. This membrane substrate was proven to have good uniformity, reproducibility, stability, and SERS activity by a series of characterizations. Also, this substrate combined excellent electromagnetic enhancement and chemical enhancement effects, which endowed it with good sensitivity and selectivity during SERS analysis. It achieved the integration of multitarget separation, enrichment, and in situ detection, which significantly improved the selectivity, sensitivity, accuracy, and detection throughput by membrane substrate coupling with SERS for real-sample analysis. Finally, this rapid SERS analysis strategy was successfully applied to the simultaneous quantification of trace nitrofurantoin (NFT) and ofloxacin (OFX) in aquatic samples. It was observed that trace NFT and OFX were actually detected and simultaneously quantified to be 8.0-13.7 and 42.6-49.1 μg/kg in aquatic samples, respectively, with good recoveries of 88.0-107% and relative standard deviations of 0.3-5.5%. The results were verified by a traditional high-performance liquid chromatography method with relative errors of -9.8 to 5.3%. This strategy provided a methodological reference for accurate SERS quantification of multiple targets in complex samples.

UiO-66 metal-organic frameworks/gold nanoparticles based substrates for SERS analysis of food samples

Recently, metal-organic frameworks (MOFs) based substrates have shown great potential for the quantitative analysis of food samples by surface-enhanced Raman scattering (SERS) due to their unique properties. Herein, we developed two UiO-66 MOFs/gold nanoparticles (AuNPs) based substrates by self-assembly, including UiO-66/AuNPs suspension substrate and UiO-66(NH₂)/AuNPs/Nylon-66 flexible membrane substrate, for quantitative analysis of complex food samples by SERS. UiO-66/AuNPs suspension substrate was prepared for SERS-based determination of a carcinogenic heterocyclic amine in barbecue meat. UiO-66(NH₂)/AuNPs/Nylon-66 membrane substrate was fabricated for the simultaneous separation, enrichment, and in situ analysis of Sudan Red 7B in chilli products. The heterocyclic amine and Sudan dye in real samples could be detected and quantified with the recoveries of 82.3-110% and 84.5-114% and relative standard deviations (RSDs) of 3.1-11.0% and 1.9-5.6% (n = 3) by use of these two substrates, respectively. These two UiO-66/AuNPs based substrates combined molecular enrichment and SERS activity, achieving excellent analytical accuracy and widening SERS application in practical food safety analysis.

Rapid and accurate determination of trace volatile sulfur compounds in human halitosis by an adaptable active sampling system coupling with gas chromatography

Halitosis with the main components of trace volatile sulfur compounds widely affects the quality of life. In this study, an adaptable active sampling system with two sample-collection modes of direct injection and solid-phase microextraction was developed for the rapid and precise determination of trace volatile sulfur compounds in human halitosis coupled with gas chromatography-flame photometric detection. The active sampling system was well designed and produced for efficiently sampling and precisely determining trace volatile targets in halitosis under the optimized sampling and detection conditions. The analytical method established was successfully applied for the determination of trace targets in halitosis. The limits of detection of H₂ S, CH₃ SH, and CH₃ SCH₃ by direct injection were 0.0140-23.0 μg/L with good recoveries ranging from 82.2 to 118% and satisfactory relative standard deviations of 0.4-9.5% (n = 3), respectively. The limit of detections of CH₃ SH and CH₃ SCH₃ by solid-phase microextraction were 2.03 and 0.186 × 10^-3  μg/L with good recoveries ranging from 98.3 to 108% and relative standard deviations of 5.9-9.0% (n = 3). Trace volatile targets in positive real samples could be actually found and quantified by combination of direct injection and solid-phase microextraction. This method was reliable and efficient for the determination of trace volatile sulfur compounds in halitosis.

Miniaturized array gas membrane separation strategy for rapid analysis of complex samples by surface-enhanced Raman scattering

It remains a significant challenge for fast and high-throughput detection of trace analytes in complex samples with surface-enhanced Raman scattering (SERS) strategy due to the severe interference from matrices. In this work, a miniaturized array gas membrane separation (AGMS) device coupled with SERS was designed and drew up to eliminate matrix influence and improve the reproducibility of SERS signal during real sample analysis. The design of miniaturized AGMS tube was optimized based on quantitative calculation of its air permeability by computational fluid dynamics simulation. A 10 mm height tube was selected as an optimized design with a recovery of 98.3% for acetaldehyde. The practical feasibility of miniaturized AGMS was validated based on the applications in biochemical analysis and food analysis, such as albuminuria and acetaldehyde in urine sample and metaldehyde and thiram in food samples. The results showed that SERS responses of all analytes dramatically increased by eliminating sample matrices after miniaturized AGMS process. Acetaldehyde, albuminuria, metaldehyde and thiram in real samples could be accurately quantified with recoveries of 82.0-123.3%, and the analytical results were validated by corresponding standard methods with relative error ranging from -4.8% to 5.3%. Time consumption of miniaturized AGMS-SERS for one real sample analysis including sample preparation and determination was less than 20 min and could treat 96 samples with 45 min in one run. It is potential that the miniaturized AGMS technique automated by implementation with a robotic arm could greatly expand the range and accelerate the speed of SERS analysis.

Circumventing silver oxidation induced performance degradation of silver surface-enhanced Raman scattering substrates

Surface-enhanced Raman scattering (SERS) has been recognized as a promising sensing technique in biomedical/biosensing applications and analytical chemistry. Silver (Ag) nanostructures have the strongest SERS enhancement, but suffer from severe enhancement degradation induced by oxidation. Here, we introduce electrochemical reduction of silver oxide to produce Ag SERS substrates on request to partially circumvent the SERS enhancement degradation problem of Ag SERS substrates. Silver oxide nanostructures were first prepared in pure silver citrate aqueous solutions with controllable morphologies depending on the electrodeposition parameters. The transition process from silver oxide to Ag was investigated by density functional theory calculations. Based on the understanding of the transition mechanism, heating treatment, applying reducing agent, and electrochemical reduction were adopted to transform silver oxide to Ag. Notably, no organic agents were introduced neither in the electrodeposition of silver oxide nor electrochemical transformation of silver oxide to Ag. The electrochemical reduction strategy could produce Ag SERS substrates with a 'clean' surface with outstanding SERS performance in a simple as well as cost and time effective manner. Ag SERS substrates can be used in biomedical/biosensing fields. The approach through electrochemical reduction of silver oxide to generate Ag SERS substrate may push forward practical application process of Ag SERS substrates.

SERS Sensors: Recent Developments and a Generalized Classification Scheme Based on the Signal Origin

Owing to its extreme sensitivity and easy execution, surface-enhanced Raman spectroscopy (SERS) now finds application for a wide variety of problems requiring sensitive and targeted analyte detection. This widespread application has prompted a proliferation of different SERS-based sensors, suggesting the need for a framework to classify existing methods and guide the development of new techniques. After a brief discussion of the general SERS modalities, we classify SERS-based sensors according the origin of the signal. Three major categories emerge from this analysis: surface-affinity strategy, SERS-tag strategy, and probe-mediated strategy. For each case, we describe the mechanism of action, give selected examples, and point out general misconceptions to aid the construction of new devices. We hope this review serves as a useful tutorial guide and helps readers to better classify and design practical and effective SERS-based sensors.