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.
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.
Metal (especially gold) nanoparticles exhibit unique electronic, optical, and catalytic properties. In order to utilize these
properties, an integration of the particles into technical setups such as a chip surface is helpful. We develop techniques to
use (bio) molecular tools in order to address and control the positioning of particles on microstructured chips. These
techniques are utilized for novel DNA detection schemes using optical or electrical principles. Plasmonic properties of
the particles and the combination of nano-apertures with particles are promising fields for further bioanalytical
developments.
On the other hand, methods for defined positioning of single molecules or molecular constructs in parallel approaches
are under development, in order to provide needed defined nanostructures for applications in nanoelectronics.
Connecting DNA with nanoparticles, metallization of DNA or positioning of individual DNA-structures over
microstructured electrode gap including subsequent metal particle binding are important steps in this direction. The
utilization of (bio) molecular tools and principles based on highly specific binding and self-assembly represent a
promising development in order to realize novel nanoparticle-based devices for bioanalytics, nano-optics and - electronics.
Based on their various interesting properties metal nanoparticles show the potential as analytical tool in electronic,
optical, and catalytical applications. The different properties depending on composition, shape, and size of the single
particles were utilized in many different approaches such as optics, magnetics and laser technology1.
We present a way for enzymatic deposition of silver nanoparticles and a bioanalytical application in DNA microarray
technology for this method. The technology consists of a microstructured chip with 10&mgr;m broad electrode gaps on the
surface and specially designed readout device2. In principle we immobilize gold nanoparticle-labelled DNA in a gap
between two electrodes. Afterwards a silver deposition on the bound gold nanoparticles generates a conductive layer
between the electrodes. The measured drop in the resistance serves as signal for the chip-based electrical detection of
DNA3.
To further optimize this system the gold nanoparticles as seed are replaced by the enzyme horseradish peroxidase. For a
better understanding of the enzymatically silver deposition process the formed silver particles were analyzed by
spectroscopic characterization on a single particle level. Further investigations of these particles by AFM and SEM
should give a hint to the connection between size/shape and the plasmonic properties at individual particles.
Chemical approaches allow for the synthesis of highly defined metal heterostructures, such as core-shell nanoparticles.
As the material of metal nanoparticles determines the plasmon resonance-induced absorption band, the control of
particle composition results in control of the absorption maximum position.
Metal deposition on gold or silver nanoparticles was used to prepare core-shell particles with modified optical properties
with respect to monometal nanoparticles. UV-vis spectroscopy on solution-grown and immobilized particles was
conducted as ensemble measurements, complemented by single particle spectroscopy of selected structures. Increasing
layers of a second metal, connected to a dominant contribution of the shell material to the extinction spectrum, lead to a
shift in the absorption band. The extent of shell growth could be controlled by reaction time or the concentration of
either the metal salt or the reducing agent. Additional to the optical characterization, the utilization of AFM, SEM and
TEM yielded important information about the ultrastructure of the nanoparticle complexes.
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