SILVER MIRROR SUBSTRATE AND ROLLING METHOD FOR IMPROVED SURFACE-ENHANCED RAMAN SPESCTROSCOPIC ANALYSIS IN FOOD

Surface-enhanced Raman spectroscopy, short for SERS, is an emerging technology with great potential in food analysis due to rapid detection, high sensitivity, portable instrumentation, and simple sample preparation. However, it is always a bottleneck to obtain reproducible SERS measurements in real analytical cases due to the complicity of food systems and inhomogeneous aggregation of colloidal nanoparticles. To improve its performance for practical applications in food analysis, efforts have been made in improving the reproducibility, enhancing the selectivity and reducing the matrix interference to the analyte. Herein, a self-assembly silver nanoparticles mirror substrate was fabricated to improve the and the quantitative ability and effectiveness of sample preparation for different applications in food analysis, including pesticides detection in beverages, chemical profiling of red wines, and headspace analysis of garlic. The AgNPs mirror was fabricated using the interface between polar and non-polar solvents and showed a uniform arrangement of nanoparticles under the microscope. It demonstrated a great reliability for the detection of a pesticide fonofos in beverages (i.e., apple juice and green tea). AgNPs mirror can also be in situ fabricated in red wines to generate a comprehensive spectrum constituted by signals of five wine phytochemicals, which provided a great potential of SERS in the differentiation, authentication, and quality/safety control of red wines. AgNPs mirror also showed great potential in the headspace characterization of aromatic compounds from Allium species plants. Additionally, a facile rolling method was developed to enrich analyte (i.e., chlordane pesticide) and to amplify its weak SERS activity and a mathematic model was generated and successfully quantified the chlordane in a complicated crude oil sample with a very good recovery. Overall, AgNPs mirror and the rolling method overcame the reproducibility and sensitivity problems for SERS in several challenging food matrices. With these improvements, SERS can be much more reliable for analytical applications and its range of targets can be widely expanded

INKJET PRINTED PAPER SURFACE ENHANCED RAMAN SPECTROSCOPY DEVICES FOR TRACE CHEMICAL ANALYSIS

The needs of an ever growing human population are fueling demands for better and cheaper sensors for the early detection of harmful chemicals, pathogens and diseases markers from a variety of sources such as food, water, bodily fluids and contaminated surfaces. To address this, recent innovations utilize Microelectromechanical Systems (MEMS) technology to integrate multiple laboratory functions onto millimeter-sized chips to form Micro Total Analysis Systems (µTAS) or Lab-on-chip (LOC) devices. While sophisticated and powerful, the use of these devices for chemical and biological sensing is limited by complicated fabrication processes, high cost and robustness of the sensors. In this work we have developed a simple and inexpensive but exceptionally sensitive portable chemical and biological sensing platform through the innovative use of paper combined with Surface Enhanced Raman spectroscopy (SERS). Paper is functionalized with plasmonic nanostructures to transform it into a SERS substrate, while the natural properties of paper are leveraged for sample collection, cleanup, and analyte concentration in user-friendly formats such as wipes, dipsticks, and filters. The use of simple deposition methods such as inkjet printing for sensor fabrication combined with paper as the construction material means that sensors can be made at a very low cost. Additionally, the ability to be printed on demand eliminates issues with sensor shelf-life, while the absence of mechanical components makes these paper sensors much more robust than conventional sensors. In this work, practical applications of paper SERS sensors for the detection of food contaminants, narcotics, pesticides and other chemicals at trace levels are presented. Paper SERS sensors, by virtue of their low cost, simplicity of fabrication, high sensitivity and ease of use, promises to make chemical and biological sensing more accessible to the common user

Development of Filter-Based Surface Enhanced Raman Spectroscopic Assays for Rapid Detection of Chemical and Biological Contaminants in Water

Surface enhanced Raman spectroscopy (SERS) has been widely applied for rapid and sensitive detection of various chemical and biological targets. Here, we incorporated a syringe filter system into the SERS method to detect pesticides, protein toxins and bacteria in water. For the detection of chemical and protein targets, silver nanoparticles (Ag NPs) were aggregated by sodium chloride (NaCl) to form nanoclusters that could be trapped in the pores of the filter membrane to from the SERS-active membrane. Then a coating of capture (e.g. aptamer) was integrated on the nanoparticle substrate if needed. Then samples were filtered through the membrane. After capturing the target, the membrane was taken out and air dried before measuring by a Raman instrument. The developed filter SERS method was able to detect fungicide ferbam as low as 2.5 ppb level and had a good quantitative capability, which could also be carried out on site using a portable Raman instrument. The aptamer integrated filter SERS was able to detect ricin b chain in water at 100 ppb level. The filter membrane was then applied to detect bacteria E.Coli with the integration of 4-mpba as a capture and indicator. With SERS mapping, we can detect E.Coli down to 101 CFU/ml and the viability of bacteria on the membrane could be confirmed by incubating the membrane on TS agar down to 102 CFU/ml. This study shows the filter based SERS methods improve the detection capability in water samples, with a great versatility for various types of assays

Colloidal dendritic nanostructures of gold and silver for SERS analysis of water pollutants

Surface-Enhanced Raman Scattering (SERS) using colloidal metal (Ag, Au) nanoparticles has been regarded as a powerful method for detecting organic pollutants at vestigial levels. Although less investi- gated, the controlled synthesis of binary nanostructures comprising two metals provides an alternative route to SERS platforms with tuned surface plasmon resonances. Here, we demonstrate that the use of dendrimers allows the formation of distinct combinations of Ag:Au nanostructures that are composed of smaller metal nanocrystals. Our research highlights the role of the dendrimer macromolecules as a multipurpose ligand in the generation of such hybrid nanostructure, including as a reducing agent, an effective long-term colloidal stabilizer and as a molecular glue for interconnecting the primary metal nanocrystals. Noteworthy, the dendrimer-based Ag:Au hybrid nanostructures are more SERS sensitive as compared to the corresponding colloidal blends or to the single-phase metals, as revealed by using molecular pesticides as analytes in spiked water samples. We suggest that the high SERS sensitivity of the hybrid nanostructures is due to interparticle plasmonic coupling occurring between the primary metal nanoparticle aggregates, whose arrangement is templated by the presence of the dendrimer macromolecules.publishe

Recent advances in surface enhanced Raman spectroscopy (SERS): finite difference time domain (FDTD) method for SERS and sensing applications

There have been significant advancements in the field of surface-enhanced Raman spectroscopy (SERS). Despite being an ultra-sensitive analytical technique, challenges, such as how to get a proper match between the SERS substrate and light for better signal enhancement to obtain a stable, sensitive SERS substrate, prevent its widespread applications. Finite-difference time-domain (FDTD) method, a numerical tool for modeling computational electrodynamics, has recently been used to investigate SERS for understanding the underlying physics, and optimally design and fabricate SERS substrates for molecular analysis. In this review, we summarize the trend of using FDTD method in SERS studies by providing an introduction of fundamental principles, the studies of optical responses, electromagnetic (EM) field distribution, enhancement factor (EF) of SERS, the application in design and fabrication of SERS substrates, and SERS for biosensing and environmental analysis. Finally, the critical issues of using inherently approximate FDTD method and future improvement for solving EM problems and SERS applications are discussed

Novel Approaches to Prepare and Utilize SERS Substrates: Multiplex Microfluidics and Nanotransfer Printing

Over the past few decades, surface enhanced Raman spectroscopy (SERS) has garnered respect as an analytical technique with significant chemical and biological applications. SERS is important for the life sciences because it can provide trace level detection and a high level of molecular structure information. The development of quantitative, highly sensitive substrates requires control over size, shape, and position of metal nanoparticles which function as the SERS active medium. Thus, creating and successfully implementing a sensitive, reproducible, and robust SERS active substrate continues to be a challenging ask. Its future development depends critically on techniques for lithography and nanofabrication. Herein, we report a novel method for SERS that is based upon using colloidal silver nanoparticles in a multiplexed microfluidics (MMFs) platform. The MMF is created in polydimethylsiloxane (PDMS) polymer material and used to perform parallel, high throughput, and sensitive detection/identification of single or various analytes under easily manipulated conditions. A facile passive pumping method is used to deliver samples into the channels under flowing conditions that are highly conducive for SERS measurments. Also an unconventional nanofabrication approach is modified to produce efficient SERS substrates. Metallic nanopatterns of silver discs are transferred from a stamp onto PDMS to create nanocomposite substrates with regular periodic morphologies. The stamp with periodic arrays of square, triangular, and elliptical pillars is created via Electron Beam Lithography of ma-N 2403 resist. A modified cyclodextrin is thermally evaporated on the stamp to overcome the adhesive nature of the ebeam resist and to function as a releasing layer. Subsequently, the stamp is over coated with Ag by physical vapor deposition at a controlled rate and thickness and used directly for nanotransfer printing (nTP). Stamps, substrates, and the efficiency of the nTP process were explored by SEM. Ag nano-disc-PDMS substrates are studied by SERS using Rhodamine 6G as the probe analyte. The SERS response of metallic nano-discs of various shapes/sizes on the original stamp is compared to the corresponding nTP substrates. We demonstrate that physical manipulation of the PDMS post nTP can be used to alter morphology. Additionally, stamps are shown to be reusable after the nTP process

DEVELOPMENT OF HEADSPACE ANALYSIS OF LIVING AND POSTHARVEST FRESH PRODUCE USING SURFACE-ENHANCED RAMAN SPECTROSCOPY (SERS)

The increasing market demand for fresh produce promotes a keen interest in developing a rapid, sensitive and reliable method for monitoring plant health and determining the shelf-life of postharvest produce. The objective of this study is to explore the capability of Surface-enhanced Raman spectroscopy (SERS) in these applications. SERS integrates Raman spectroscopy which measures molecular vibrations and nanotechnology which enhances the weak Raman signals. Herein, we developed two SERS methods based on a surface detection approach using nanoparticles solution and a headspace detection approach using gold nanoparticles (AuNPs) fibers, to detect biochemical changes during postharvest storage of arugula leaves. Compared with surface detection, the headspace detection revealed significant spectral changes during the storage, particularly in the shifts around 500, 950 and 1030 cm-1. These changes analyzed using principal component analysis (PCA) to establish a prediction model for shelf-life determination. Through analyzing reference standard compounds, we identified the dimethyl disulfide (DMDS), 1-propanethiol and methanethiol (MT) were most likely to account for the signature spectra of headspace arugula at the late storage period due to the activities of spoilage bacteria. The headspace detection method was also applied to monitor the stress responses of living basil to abiotic stresses (pesticide/salinity). However, the volatile analysis of the basil plants response to abiotic stresses (pesticide/salinity) showed indistinctive results. In conclusion, the headspace detection based on SERS provides a new strategy for quality monitoring of fresh produce in the food industry

Expanding the Applicability of Raman Spectroscopy for Monitoring Photocatalytic Degradation

Compared to other types of wastewater pollutants, dangerous chemical compounds such as pharmaceuticals, pesticides, and herbicides are difficult to remove and consequently being detected (at least in part because detection limits have decreased) in drinking water at increasing concentrations. Photocatalytic degradation degrades harmful compounds to innocuous end products using energy from light. Although it is effective and cost-efficient, the underlying chemical mechanisms are not understood well enough to ensure that dangerous intermediate products are not formed during the degradation process. Raman spectroscopy can be used to analyze photocatalytic degradation reactions in real time, identifying intermediate products based on spectral features. Due to fast data acquisition, Raman studies can identify those intermediate products which are short-lived and could be missed by slower conventional methods. In the current research, colloidal gold nanoparticles were introduced to increase sensitivity via surface-enhanced Raman spectroscopy (SERS), and later modified to maintain signal intensity over a longer period of time. Additionally, an internal standard was introduced for ratiometric determination of analyte concentration. These procedural modifications serve to expand the applicability of Raman spectroscopy for in-situ reaction monitoring

Rapid detection of pesticide residues in foods using surface-enhanced raman spectroscopy coupled with gold nanostars

Constant monitoring pesticide residues in foods is an essential part of food safety. In recent years, there is a growing concern about food issues in agricultural products. Tradition testing methods such as high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS) demand time-consuming sample preparations and well-trained operators. Therefore, this research aimed to establish a novel, simple, and rapid testing technique. In this study, the SERS performance of gold nanostars was evaluated by detecting two commonly used pesticides, thiabendazole (TBZ) and paraquat in the real food samples. Gold nanostars were used as a surface-enhanced Raman spectroscopy (SERS) substrate due to their highly branched structure, which provide many SERS hot spots for generating intensified Raman signals from the target analytes. Additionally, the rough topography of gold nanostars has large surface area, which can enable good interactions between the substrate and analyte molecules. The UV-vis spectrometer, electron microscopes and Zetasizer were utilized for characterization. The detection limits of this SERS method are 5 ppm for TBZ in apple juice and 0.2 ppm for paraquat in green tea. These results indicate that SERS coupled with gold nanostars is a facile approach and has great potential to be applied for qualification and quantification of trace contaminants in foods.Includes bibliographical references (pages 48-55)