Plasmon-less surface enhanced Raman spectra induced by self-organized networks of silica nanoparticles produced by femtosecond lasers

Raman spectroscopy is the workhorse for label-free analysis of molecules. It relies on the inelastic scattering of incoming monochromatic light impinging molecules of interest. This effect leads to a very weak emission of light spectrum that provides a signature of the molecules being observed. Considerable efforts have been made over the last decades, in particular with the development of Surface Enhanced Raman Spectroscopy (SERS), to enhance the intensity of the emitted signal so that ultimately, traces of molecules can be detected. Here, we show that dense self-organized networks of quasi-monodisperse nanoparticles redepositing during femtosecond laser ablation of trenches in fused silica can lead to a significant field enhancement effect, enabling the Raman detection of a single-molecule layer deposited on the surface (so called monolayer). Unlike previously reported for SERS experiments, here, there is no metal layer promoting plasmonics effects causing localized field enhancement. The method for producing SERS substrates is therefore quite straightforward and low cost. (C) 2017 Optical Society of America

Label-free Analytics by Transmission Localized-SPR and its Application to Small Molecules Monitoring in Serum

Giulia Cappi

In the frame of therapeutic drug monitoring and personalized medicine, point-of-care systems (POCs) that can help overcome long waiting times for results and costly procedures of clinical tests are highly desirable. At present, the detection of low-molecular-weight molecules with portable systems remains a challenge. In addition, measurements in serum have always been more complicated with respect to buffer solutions due to the occurrence of non-specific binding. This thesis presents a portable transmission-localized SPR (T-LSPR)-based setup that is intended to work as a POC for the detection of small molecules in serum. As a biorecognition element, a selected DNA aptamer specific to tobramycin (467 Da) has been used to functionalize a gold nanoislands (NIs) fluorine-doped tin oxide (FTO) covered glass that acts as a biosensor. As a proof of concept, real-time detection in TE buffer was performed by monitoring concentrations down to 0.5 µM and enabling the observation of association and dissociation phases. The extracted parameters match those obtained with a high-end commercial SPR system. Concentrations of tobramycin in undiluted serum down to 10 µM were measured. The interesting effect of the serum is that it masks the association kinetics of the tobramycin to the DNA aptamer. The quantification of the captured tobramycin is calculated at the beginning of the dissociation phase and leads to a linear calibration curve for the concentrations in the clinical range tested. The reason why the binding of tobramycin is hindered by the serum remains under investigation. The T-LSPR system employs low-cost, off-the-shelf components that make it possible to scale down the system to a palm size. The CMOS image sensor, employed as a light detector, forced the choice of the NIs to have resonance in the visible spectrum in order to match the sensitivity of the light detector. Despite the low sensitivity of the NIs FTO-coated glass slides, justified by the irregularity in size and pattern of the NIs and of the FTO substrate, the NIs exhibit extremely high stability in high-ionic solutions, standing continuous regeneration cycles without altering their sensing properties and without denaturation of the DNA aptamer on their surface. An algorithm for the extraction of the plasmon peak location of the resonance was developed. To increase the speed of data elaboration and to allow the real-time display of the results, hue was studied and used as an alternative parameter for plasmonic evaluation.

EPFL2014

Label-free Analytics by Transmission Localized-SPR and its Application to Small Molecules Monitoring in Serum

Giulia Cappi

EPFL2014