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There is a high demand of monitoring in the era of QbD in industrial scale require new approaches to gain data rapidly and of sufficient quality in real time. Raman spectroscopy technology has great potential but not yet shown it fully in process on-line monitoring due to limitations such as i) uncomplete separation between cells and growth media alone, ii) general weak Raman signals of analytes in complex solutions and iii) strong background signals such as the auto-fluorescence, cosmic rays and surrounding lights overlapping the weak Raman signals. Here we demonstrate a Proof-of-Concept on an the example lactic acid bacteria process using a Streptococcus thermophiles fermentation. Results from three different Raman approaches are presented: 1) Time-Gated Raman Spectroscopy (TG-Raman), 2) Surface Enhanced Raman Spectroscopy (SERS) and 3) Raman process spectroscopy with NIR excitation combined with multivariate data analysis (MVDA) using Principal Component Analysis (PCA) and Partial Least Squares Regression (PLSR).
Multifunctional composite nanoprobes consisting of iron oxide nanoparticles linked to silver and gold nanoparticles, Ag–Magnetite and Au–Magnetite, respectively, were introduced by endocytic uptake into cultured fibroblast cells. The cells containing the non-toxic nanoprobes were shown to be displaceable in an external magnetic field and can be manipulated in microfluidic channels. The distribution of the composite nanostructures that are contained in the endosomal system is discussed on the basis of surfaceenhanced Raman scattering (SERS) mapping, quantitative laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) micromapping, and cryo soft X-ray tomography (cryo soft-XRT). Cryo soft-XRT of intact, vitrified cells reveals that the composite nanoprobes form intra-endosomal aggregates. The nanoprobes provide SERS signals from the biomolecular composition of their surface in the endosomal environment. The SERS data indicate the high stability of the nanoprobes and of their plasmonic properties in the harsh environment of endosomes and lysosomes. The spectra point at the molecular composition at the surface of the Ag–Magnetite and Au–Magnetite nanostructures that is very similar to that of other Composite structures, but different from the composition of pure silver and gold SERS nanoprobes used for intracellular investigations. As shown by the LA-ICP-MS data, the uptake efficiency of the magnetite composites is approximately two to three times higher than that of the pure gold and silver nanoparticles.
Paper-based SERS test strips
(2016)
For the non-destructive chemical analysis of organic compounds, several different methods such as NMR, UV-vis absorption, IR, Raman, or fluorescence spectroscopy are available. However, all available methods have some restrictions such as the necessity of a large sample amount, interferences in the presence of water, or overlapping signals from the analytes or matrix. Surface enhanced Raman scattering (SERS) allows to observe analytes directly without labelling in low concentrations in aqueous solutions and to identify them by their spectral fingerprint. Therefore, in this work we use SERS as a detection method for different analytes in low concentrations in combination with paper-based test strips as SERS substrates and for sample preparation.
We present a spray method for the preparation of SERS test stripes.[1] With this spray method, nanoparticle solution was deposited on cellulose and glass fibre paper as SERS substrate. The prepared paper-based test strips were tested with classical SERS reporter molecules, e.g. rhodamine 6G, 4-aminothiophenol, and adenine. For the quantification of analytes, highly reproducible signal intensities are necessary, which can be realized with the test strips in acceptable quality. Moreover, employing intensity vs concentration calibration for the analytes, data analysis revealed a behaviour that was best described by a Langmuir isotherm, stressing the strong distance dependence of the SERS effect.
For an easier identification of analytes in a mixture of compounds, the paper-based test strips were functionalised with hydrophobic barriers by wax printing. With these microfluidic paper-based analytical devices (µPAD) the sample mixture can be separated by the chromatographic effects of the paper and the different analytes can be separately detected and identified by SERS.
[1] A. Bolz, U. Panne, K. Rurack, M. Buurman, Glass fibre paper-based test strips for sensitive SERS sensing, Anal. Methods, 2016, 8, 1313-1318.
Surface enhanced Raman scattering (SERS) is a fast and sensitive spectroscopic method for the identification of analytes. With available portable Raman spectrometers, on-site analysis is possible. However, for on-site analysis, SERS substrates, which are cheap, easy to prepare, and simple in sample handling are necessary. Relevant analytes in the addressable concentration region for SERS are e.g. antibiotics and pesticides. Here, we present paper-based test strips for SERS analysis which are coated with silver nanoparticles. The coating was realized with different deposition methods of nanoparticle solutions. The papers were also functionalised with hydrophobic barriers to create μPADs. The strips were tested with selected analytes (e.g., adenine, rhodamine-6G) over a broad concentration range. The focus of our study lay on reproducibility and optimum SERS signal intensity.
For the quantification of analytes, highly reproducible signal intensities are necessary. We have realized this reproducibility in acceptable quality. Moreover, employing intensity vs concentration calibration for the analytes, data analysis revealed a behaviour that was best described by a Langmuir isotherm, stressing the strong distance dependence of the SERS effect. For a fast and reproducible analysis of the data, a Labview program was finally compiled, which was fed with the calibration data and derived the concentration of analyte unknowns accordingly.
Surface-enhanced Raman scattering (SERS) exploits the enhancement of electromagnetic fields in close vicinity of plasmonic nanostructures, enabling characterization of analytes at the single-molecule level. The nanometer-scale spatial arrangement of plasmonic metal nanoparticles and analyte molecules has a significant effect on the observed signal enhancements and represents a great challenge in this technique.
In our work, DNA origami is used as platform for precise positioning of gold nanoparticles (AuNPs). Especially high sensitivities are expected for gold nanolenses (AuNLs), consisting of rows of three or more differently-sized AuNPs. We assembled different AuNL designs and determined respective SERS enhancement factors by collecting Raman spectra from single AuNLs. Finite difference time domain calculations estimate attainable electromagnetic field enhancements. Ultimately, we aim to develop a versatile platform for various SERS applications.
DNA origami nanostructures are a versatile tool to arrange metal nanostructures and other chemical entities with nanometer precision. In this way gold nanoparticle dimers with defined distance can be constructed, which can be exploited as novel substrates for surface enhanced Raman scattering (SERS). We have optimized the size, composition and arrangement of Au/Ag nanoparticles to create intense SERS hot spots, with Raman enhancement up to 10^10, which is sufficient to detect single molecules by Raman scattering. This is demonstrated using single dye molecules (TAMRA and Cy3) placed into the center of the nanoparticle dimers. In conjunction with the DNA origami nanostructures novel SERS substrates are created, which can in the future be applied to the SERS analysis of more complex biomolecular targets, whose position and conformation within the SERS hot spot can be precisely controlled.
The fast identification and quantification of analytes in the field of food safety or environmental analysis is difficult. Surface enhanced Raman scattering (SERS) is an analytical method which can be used simultaneously for the rapid identification and concentration determination of trace analytes,[1,2] usually covering a large dynamic range from nanomolar up to molar concentrations. The identification of the molecules is accomplished through the specific fingerprint of a molecule’s Raman spectrum.
For facile and straightforward SERS measurements, we present here a combination of paper-based SERS test strips with microfluidic systems on paper as a microfluidic paper-based analytical device (μPAD). The SERS μPAD is thus principally suited for cheap, fast, non-destructive, label-free and portable detection of analytes. In this system basically, the use of the microfluidic structured paper increases the sensitivity and suppresses background signals of the SERS assay.
Deposition of the SERS substrate on the test strips is simple and relies on an inkjet printer. For the optimization of the reproducibility and intensity of the SERS signal, we tested different nanoparticles, different numbers of print cycles and different paper types. The nanoparticle solutions used in the μPAD preparation were gold and silver nanoparticle solutions. The paper types were cellulose and glass fiber. SERS arrays were prepared by printing and compared to arrays prepared by spraying. The optimized μPAD was used for the identification and quantification of pure analyte solutions (e.g., adenine) and mixtures of compounds, the concentration series following Langmuir isotherms.
Relevant analytes in the field of food safety are antibiotics and pesticides. We apply the SERS microfluidic paper-based analytical devices for the detection of antibiotics (enoxacin, enrofloxacin) and pesticides.
Different approaches have been proposed to treat cancer cells using gold nanoparticles (AuNPs) in combination with radiation ranging from infrared lasers to high-energy ion beams. Here we study the decomposition of the DNA/RNA nucleobases thymine (T) and uracil (U) and the well-known radiosensitizer 5-bromouracil (BrU) in close vicinity to AuNPs, which are irradiated with a nanosecond pulsed laser (532 nm) matching the surface plasmon resonance of the
AuNPs. The induced damage of nucleobases is analyzed by UV−vis Absorption spectroscopy and surface-enhanced Raman scattering (SERS). A clear DNA damage is observed upon laser irradiation. SERS spectra indicate the fragmentation of the aromatic ring system of T and U as the dominant form of damage, whereas with BrU mainly the cleavage of the Br−C bond and formation of Br− ions is observed. This is accompanied by a partial transformation of BrU into U. The observed damage is at least partly ascribed to the intermediate formation of lowenergy electrons from the laser-excited AuNPs and subsequent dissociative electron attachment to T, U, and BrU. These reactions represent basic DNA damage pathways occurring on the one hand in plasmon-assisted cancer therapy and on the other hand in conventional cancer radiation therapy using AuNPs as sensitizing agents.
Surface-enhanced Raman scattering (SERS) hybrid probes are characterized by the typical spectrum of a reporter molecule. In addition, they deliver information from their biological environment. Here, we report SERS hybrid probes generated by conjugating different reporter molecules to bovine serum albumin (BSA) and using gold nanoparticles as plasmonic core. Advantages of the BSA-conjugate hybrid nanoprobes over other SERS nanoprobes are a high biocompatibility, stabilization of the gold nanoparticles in the biological environment, stable reporter signals, and easy preparation. The coupling efficiencies of the BSA–reporter conjugates were determined by MALDI-TOF-MS. The conjugates' characteristic SERS spectra differ from the spectra of unbound reporter molecules. This is a consequence of the covalent coupling, which leads to altered SERS enhancement and changes in the chemical structures of the reporter and of BSA. The application of the BSA–reporter conjugate hybrid probes in 3T3 cells, including duplex imaging, is demonstrated. Hierarchical cluster analysis and principal components analysis were applied for multivariate imaging using the SERS signatures of the incorporated SERS hybrid nanoprobes along with the spectral information from biomolecules in endosomal structures of cells. The results suggest more successful applications of the SERS hybrid probes in cellular imaging and other unordered high-density bioanalytical sensing.
Thin metal-filled polyterafluoroethylene films with various metal concentration were produced by co-deposition in vacuum. Metal nanocluster size increased with metal concentration. Films were heated up to 300 degrees C, their optical spectra were recorded during heating. The changes in plasmon band shape and wavelength of the nanocluster ensemble during heating are not linearly related with metal concentration and heating temperature. This is caused by different thermal behavior of the complex processes, which are taking place in each of the two materials present in the film. The metal cluster size and optical properties of the whole ensemble can be purposefully formed by varying metal nature, its concentration and annealing temperature of the film. Nano- and micro-domains with properties different from original film were generated by focused excimer laser or electron beam. Gold-filled PTFE nano-structured films were used as substrate for surface enhanced Raman scattering measurements of ultrathin film of Rhodamine 6G dye.