Katharina K. Strelau

SERS as tool for the analysis of DNA-chips in a microfluidic platform

A sequence-specific detection method of DNA is presented combining a solid chip surface for immobilisation of capture DNAs with a microfluidic platform and a readout of the chip based on SERS. The solid chip surface is used for immobilisation of different capture DNAs, where target strands can be hybridised and unbound surfactants can be washed away. For the detection via SERS, short-labelled oligonucleotides are hybridised to the target strands. This technique is combined with a microfluidic platform that enables a fast and automated preparation process. By applying a chip format, the problems of sequence-specific DNA detection in solution phase by means of SERS can be overcome. With this setup, we are able to distinguish between different complementary and non-complementary target sequences in one sample solution.

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.

Novel Bottom-Up SERS Substrates for Quantitative and Parallelized Analytics

Surface-enhanced Raman spectroscopy (SERS) is an emerging technology in the field of analytics. Due to the high sensitivity in connection with specific Raman molecular fingerprint information SERS can be used in a variety of analytical, bioanalytical, and biosensing applications. However, for the SERS effect substrates with metal nanostructures are needed. The broad application of this technology is greatly hampered by the lack of reliable and reproducible substrates. Usually the activity of a given substrate has to be determined by time-consuming experiments such as calibration or ultramicroscopic studies. To use SERS as a standard analytical tool, cheap and reproducible substrates are required, preferably with a characterization technique that does not interfere with the subsequent measurements. Herein we introduce an innovative approach to produce low-cost and large-scale reproducible substrates for SERS applications, which allows easy and economical production of micropatterned SERS active surfaces on a large scale. This approach is based on an enzyme-induced growth of silver nanostructures. The special structural feature of the enzymatically deposited silver nanoparticles prevents the breakdown of SERS activity even at high particle densities (particle density >60%) that lead to a conductive layer. In contrast to other approaches, this substrate exhibits a relationship between electrical conductivity and the resulting SERS activity of a given spot. This enables the prediction of the SERS activity of the nanostructure ensemble and therewith the controllable and reproducible production of SERS substrates of enzymatic silver nanoparticles on a large scale, utilizing a simple measurement of the electrical conductivity. Furthermore, through a correlation between the conductivity and the SERS activity of the substrates it is possible to quantify SERS measurements with these substrates.

Functionalization of Microstructured Optical Fibers by Internal Nanoparticle Mono-Layers for Plasmonic Biosensor Applications

For fully integrated next-generation plasmonic devices, microstructured optical fibers (MOFs) represent a promising platform technology. This paper describes the use of a dynamic technique to demonstrate the wet chemical deposition of gold and silver nanoparticles (NPs) within MOFs. The plasmonic structures were realized on the internal capillary walls of a three-hole suspended core fiber. Electron micrographs, taken of the inside of the fiber holes, confirm the even distribution of the NP in the MOF over a length of up to 6 m. Accordingly, this procedure is highly productive and makes the resulting MOF-based sensors potentially (very) cost efficient. In proof-of-principle experiments with liquids of different refractive indices, the dependence of the localized surface plasmon resonance (LSPR) on the surroundings was confirmed. Comparing Raman spectra of MOFs with and without NP layers, each one filled with crystal violet, a significant signal enhancement demonstrates the usability of such functionalized MOFs for surface-enhanced Raman spectroscopy (SERS) experiments.

Raman spectroscopic determination of norbixin and tartrazine in sugar

In this paper, a method for the detection of norbixin and tartrazine in sugar by means of resonance Raman spectroscopy is presented. The extraction was done in four steps using methanol and the measurements were performed in aqueous solution. The excitation wavelength was 514 nm for norbixin and 488 nm for tartrazine samples. The characteristic resonance Raman signals of the dyes were fitted by different functions. Depending on the R 2 values of the different fits, each spectrum was classified as positive or negative response. A detection limit of 250 ng g−1 for norbixin and 989 ng g−1 for tartrazine in solid sugar samples could be reached by logistic regression.

A microfluidic platform for chip-based DNA detection using SERS and silver colloids

Within this contribution, we demonstrate a combination of microarrays, microfluidics and SERS to enable a sequence specific detection of DNA. In this combination, the microarray allows for the immobilisation of DNA sequences as well as the removal of unbound DNA, microfluidics permit the automation of the process and SERS provides a highly sensitive detection by means of an interaction between an analyte molecule and the enhanced electromagnetic field in the proximity of metallic nanostructured surfaces such as spherical nanoparticles. With this setup, we are able to distinguish between different complementary and non-complementary target sequences in one sample solution.

Easy characterization of SERS substrates of enzymatically produced silver nanoparticles and their applications in the area of bioanalytics

The broad application of surface-enhanced Raman spectroscopy (SERS) is greatly hampered by the lack of reliable and reproducible substrates; usually the activity of a given substrate has to be determined by time-consuming experiments such as calibration studies or ultramicroscopy. To use SERS as a standard analytical tool, cheap and reproducible substrates are required, preferably characterizable with a technique that does not interfere with the subsequent measurements. Here, we introduce an innovative approach to produce low cost and large scale reproducible substrates for SERS applications, which allows an easy and economical production of micropatterned SERS active surfaces based on an enzyme induced growth of silver nanostructures. The special structural feature of the enzymatically deposited silver nanoparticles prevents the breakdown of SERS activity even at high particle densities and exhibits a relationship between electrical conductivity and resulting SERS activity of a given spot. This enables the prediction of the SERS activity of the nanostructure ensemble and therewith the controllable and reproducible production of SERS substrates of enzymatic silver nanoparticles on a large scale. Furthermore, the presented substrate shows a high reproducibility and is appropriate for various applications.

Plasmonic nanostructures for biophotonic applications

Within this contribution we convincingly demonstrate that the enhancement of the intrinsically weak Raman signals through an interaction between an analyte molecule and enhanced electromagnetic fields in the vicinity of metallic nanostructured surfaces is an extremely potent tool in bioanalytical science because such a SERS approach comprises high sensitivity with molecular specificity. In particular innovative approaches to realize reproducible plasmonic nanostructures i.e. SERS substrates like e.g. lithographically produced nanostructured gold surfaces or the defined deposition of silver nanoparticles through an enzymatic reaction are introduced.