Optimization and characterization of self-assembled monolayer multilayer surface-enhanced Raman scattering substrate for immuno-nanosensors.

摘要

Surface-enhanced Raman scattering (SERS) is a powerful analytical tool and the recent expansion of substrates for SERS measurement broadens its field of applications, including SERS-based immuno-nanosensing. This dissertation describes the optimization and characterization of self-assembled monolayer (SAM) multilayer SERS substrates for improving performance of different types of substrates. SERS substrates derived from metal film on nanostructures were modified with multiple metal films interspaced with SAM dielectric spacers to achieve multilayered SERS substrates. The fundamental concept of this substrate geometry exploited the cumulative effect of the multiple electromagnetic fields and various properties of SAMs to optimize SERS enhancement of multilayer SERS substrates to about 20-fold compared to conventional substrates.;SAMs of less bulky terminal groups of alkylthiols with relatively small dielectric constant, strong attractive interaction, and good electron donating ability enhanced multilayer SERS. Compared to SFON SERS substrates, alkyl-terminated SAM substrates exhibited about 7-fold SERS enhancement. SAM formation conditions that enhanced strong intermolecular interactions improved SERS enhancement. Thus, -COOH-terminated SAM multilayer substrates fabricated in acidic conditions exhibited SERS enhancement up to 10-fold compared to similar substrates fabricated in basic conditions.;It was observed that the distance of separation of the adjacent metal films (2 nm) was important for optimum interaction of EM fields generated on these metal films. SAM multilayer SERS substrates formed from 7 or 8 carbon atoms of alkyl-terminated alkylthiols with separation distances of 1.2 and 1.4 nm respectively exhibited about 14 to 16-fold SERS enhancement relative to SFON. A systematic increase of the separation distance between adjacent metal films beyond 2 nm led to a decrease in the SERS enhancement, indicating decreasing interactive effect between the adjacent EM fields.;In general, there was an increase in the SERS enhancement with increase in the number of SERS-active layers. Correspondingly, the relative standard deviation decreased to about 5% for multilayered substrates compared to 14% for conventional single layered substrates, thereby improving multilayer SERS reproducibility while increasing the SERS enhancement. Overall, SERS enhancement of conventional SERS substrates can be optimized using multiple SERS-active surfaces well separated with appropriate dielectric spacers and optimum separation distance.