Karina Weber

Surface-enhanced Raman spectroscopy (SERS): progress and trends.

AbstractSurface-enhanced Raman spectroscopy (SERS) combines molecular fingerprint specificity with potential single-molecule sensitivity. Therefore, the SERS technique is an attractive tool for sensing molecules in trace amounts within the field of chemical and biochemical analytics. Since SERS is an ongoing topic, which can be illustrated by the increased annual number of publications within the last few years, this review reflects the progress and trends in SERS research in approximately the last three years. The main reason why the SERS technique has not been established as a routine analytic technique, despite its high specificity and sensitivity, is due to the low reproducibility of the SERS signal. Thus, this review is dominated by the discussion of the various concepts for generating powerful, reproducible, SERS-active surfaces. Furthermore, the limit of sensitivity in SERS is introduced in the context of single-molecule spectroscopy and the calculation of the ‘real’ enhancement factor. In order to shed more light onto the underlying molecular processes of SERS, the theoretical description of SERS spectra is also a growing research field and will be summarized here. In addition, the recording of SERS spectra is affected by a number of parameters, such as laser power, integration time, and analyte concentration. To benefit from synergies, SERS is combined with other methods, such as scanning probe microscopy and microfluidics, which illustrates the broad applications of this powerful technique. FigureVarious SERS substrates visualized using scanning electron microscopy

Isolation and identification of bacteria by means of Raman spectroscopy

Bacterial detection is a highly topical research area, because various fields of application will benefit from the progress being made. Consequently, new and innovative strategies which enable the investigation of complex samples, like body fluids or food stuff, and improvements regarding the limit of detection are of general interest. Within this review the prospects of Raman spectroscopy as a reliable tool for identifying bacteria in complex samples are discussed. The main emphasis of this work is on important aspects of applying Raman spectroscopy for the detection of bacteria like sample preparation and the identification process. Several approaches for a Raman compatible isolation of bacterial cells have been developed and applied to different matrices. Here, an overview of the limitations and possibilities of these methods is provided. Furthermore, the utilization of Raman spectroscopy for diagnostic purposes, food safety and environmental issues is discussed under a critical view.

Bioanalytical application of surface- and tip-enhanced Raman spectroscopy

Due to its fingerprint specificity and trace‐level sensitivity, surface‐enhanced Raman spectroscopy (SERS) is an attractive tool in bioanalytics. This review reflects the research in this highly interesting topic of the last 3–4 years. The detection of the SERS signature of biomolecules up to microorganisms and cells is introduced. Labeling using modified nanoparticles (SERS tags) is also introduced. In order to establish biomedical applications, SERS analysis is performed in complex matrices such as body fluids. Furthermore, the SERS technique is combined with other methods such as microfluidic devices for online monitoring and scanning probe microscopy (i.e. tip‐enhanced Raman spectroscopy, TERS) to investigate nanoscaled features. The present review illustrates the broad application fields of SERS and TERS in bioanalytics and shows the great potential of these methods for biomedical diagnostics.

SERS-based detection of biomolecules

Abstract In order to detect biomolecules, different approaches using for instance biological, spectroscopic or imaging techniques are established. Due to the broad variety of these methods, this review is focused on surface enhanced Raman spectroscopy (SERS) as an analytical tool in biomolecule detection. Here, the molecular specificity of Raman spectroscopy is combined with metallic nanoparticles as sensor platform, which enhances the signal intensity by several orders of magnitude. Within this article, the characterization of diverse biomolecules by means of SERS is explained and moreover current application fields are presented. The SERS intensity and as a consequence thereof the reliable detection of the biomolecule of interest is effected by distance, orientation and affinity of the molecule towards the metal surface. Furthermore, the great capability of the SERS technique for cutting-edge applications like pathogen detection and cancer diagnosis is highlighted. We wish to motivate by this comprehensive and critical summary researchers from various scientific background to create their own ideas and schemes for a SERS-based detection and analysis of biomolecules.

The morphology of silver nanoparticles prepared by enzyme-induced reduction

Summary Silver nanoparticles were synthesized by an enzyme-induced growth process on solid substrates. In order to customize the enzymatically grown nanoparticles (EGNP) for analytical applications in biomolecular research, a detailed study was carried out concerning the time evolution of the formation of the silver nanoparticles, their morphology, and their chemical composition. Therefore, silver-nanoparticle films of different densities were investigated by using scanning as well as transmission electron microscopy to examine their structure. Cross sections of silver nanoparticles, prepared for analysis by transmission electron microscopy were additionally studied by energy-dispersive X-ray spectroscopy in order to probe their chemical composition. The surface coverage of substrates with silver nanoparticles and the maximum particle height were determined by Rutherford backscattering spectroscopy. Variations in the silver-nanoparticle films depending on the conditions during synthesis were observed. After an initial growth state the silver nanoparticles exhibit the so-called desert-rose or nanoflower-like structure. This complex nanoparticle structure is in clear contrast to the auto-catalytically grown spherical particles, which maintain their overall geometrical appearance while increasing their diameter. It is shown, that the desert-rose-like silver nanoparticles consist of single-crystalline plates of pure silver. The surface-enhanced Raman spectroscopic (SERS) activity of the EGNP structures is promising due to the exceptionally rough surface structure of the silver nanoparticles. SERS measurements of the vitamin riboflavin incubated on the silver nanoparticles are shown as an exemplary application for quantitative analysis.

Complexity of fatty acid distribution inside human macrophages on single cell level using Raman micro-spectroscopy

AbstractMacrophages are phagocytic cells which are involved in the non-specific immune defense. Lipid uptake and storage behavior of macrophages also play a key role in the development of atherosclerotic lesions within walls of blood vessels. The allocation of exogenous lipids such as fatty acids in the blood stream dictates the accumulation and quantity of lipids within macrophages. In case of an overexposure, macrophages transform into foam cells because of the large amount of lipid droplets in the cytoplasm. Raman micro-spectroscopy is a powerful tool for studying single cells due to the combination of microscopic imaging with spectral information. With a spatial resolution restricted by the diffraction limit, it is possible to visualize lipid droplets within macrophages. With stable isotopic labeling of fatty acids with deuterium, the uptake and storage of exogenously provided fatty acids can be investigated. In this study, we present the results of time-dependent Raman spectroscopic imaging of single THP-1 macrophages incubated with deuterated arachidonic acid. The polyunsaturated fatty acid plays an important role in the cellular signaling pathway as being the precursor of icosanoids. We show that arachidonic acid is stored in lipid droplets but foam cell formation is less pronounced as with other fatty acids. The storage efficiency in lipid droplets is lower than in cells incubated with deuterated palmitic acid. We validate our results with gas chromatography and gain information on the relative content of arachidonic acid and its metabolites in treated macrophages. These analyses also provide evidence that significant amounts of the intracellular arachidonic acid is elongated to adrenic acid but is not metabolized any further. The co-supplementation of deuterated arachidonic acid and deuterated palmitic acid leads to a non-homogenous storage pattern in lipid droplets within single cells. Figure aᅟ

A new calibration concept for a reproducible quantitative detection based on SERS measurements in a microfluidic device demonstrated on the model analyte adenine

This study demonstrates a new concept of calibrating surface enhanced Raman scattering (SERS) intensities without using additional substances as an internal standard and explores factors such as laser fluctuation and different Ag substrates, which affect the results of quantitative analyses based on SERS. To demonstrate the capabilities of the concept, the model analyte adenine has been chosen. A lab-on-a-chip device is applied for the measurements to guarantee consistent data recording. In order to simulate varied measuring conditions, two varying silver colloids (batch 1 and 2) are utilized as a SERS substrate and two different laser power levels (25 or 55 mW) are applied on the sample. A concentration gradient was generated which allows the use of the analyte itself for the correction of the resulting SERS spectra regarding intensity deviations caused by different ambient conditions. In doing so, a vast improvement in the quantification using SERS, especially in view of the comparability, reproducibility and repeatability, is demonstrated.

Towards SERS based applications in food analytics: Lipophilic sensor layers for the detection of Sudan III in food matrices

Food safety is a topic of great importance for our society which places high demands on analytical methods. Surface enhanced Raman spectroscopy (SERS) meets the requirements for a rapid, sensitive and specific detection technique. The fact that metallic colloids, one of the most often used SERS substrates, are usually prepared in aqueous solution makes the detection of water-insoluble substances challenging. In this paper we present a SERS based approach for the detection of water-insoluble molecules by applying a hydrophobic surface modification onto the surface of enzymatic generated silver nanoparticles. By this approach the detection of the illegal water-insoluble food dyes, such as Sudan III in presence of riboflavin, as water-soluble competitor, is possible. Moreover, we demonstrate the usability of this kind of SERS substrates for determination of Sudan III out of spiked paprika extracts.

LOC-SERS: A Promising Closed System for the Identification of Mycobacteria

A closed droplet based lab-on-a-chip (LOC) device has been developed for the differentiation of six species of mycobacteria, i.e., both Mycobacterium tuberculosis complex (MTC) and nontuberculous mycobacteria (NTM), using surface-enhanced Raman spectroscopy (SERS). The combination of a fast and simple bead-beating module for the disruption of the bacterial cell with the LOC-SERS device enables the application of an easy and reliable system for bacteria discrimination. Without extraction or further treatment of the sample, the obtained SERS spectra are dominated by the cell-wall component mycolic acid. For the differentiation, a robust data set was recorded using a droplet based LOC-SERS device. Thus, more than 2100 individual SERS spectra of the bacteria suspension were obtained in 1 h. The differentiation of bacteria using LOC-SERS provides helpful information for physicians to define the conditions for the treatment of individual patients.

LOC-SERS: towards point-of-care diagnostic of methotrexate

Therapeutic drug monitoring is of major importance in the case of medication with a narrow therapeutic range as well as when pharmacokinetic/pharmacodynamic variability is suspected. Methotrexate (MTX), an antifolate antibiotic, proved to be toxic regardless of the chosen treatment schedule. In this contribution, a new analytical method was used for the detection of MTX. A linear response was achieved in the 0.2–2 μM concentration range, with a limit of detection ≈ 0.17 μM. The lab-on-a-chip surface enhanced Raman spectroscopy (LOC-SERS) approach combines the fingerprint specificity and high sensitivity of SERS with the high sample throughput of a microfluidic platform. Additionally, it is shown that due to the chemical affinity of the MTX molecules towards Ag nanostructures, the pH value of the solving medium highly affects the obtained SERS signal. More specifically, SERS signals with well resolved bands can be obtained from deprotonated MTX molecules due to their binding to the metallic surface via the amine groups of the aromatic ring.