Sanna Uusitalo

Polymeric dual-slab waveguide interferometer for biochemical sensing applications.

A polymer based dual-slab waveguide Youngs interferometer was demonstrated for biochemical sensing. Evanescent field is utilized for probing the binding events of biomolecules on the waveguide surface. Refractive index sensing in analyte and protein adsorption on the sensing surface were investigated with glucose de-ionized water solution and bovine serum albumin, immunoglobulin G solutions in phosphate buffered saline buffer. A detection limit of 10(-5) RIU and 4 pg/mm(2) was achieved for homogeneous and surface sensing, respectively. Also, the influence of water absorption inside the polymeric device on the measurement stability was evaluated. The results indicate that the waveguide polymer sensor fabricated with the spin coating technique can achieve a satisfactory sensitivity for homogeneous refractive index sensing and, as well, for monitoring molecular binding events on the surface.

Manipulation of optical field distribution in layered composite polymeric-inorganic waveguides

We discuss the manipulation of optical field distribution in a low-refractive index polymeric waveguide by depositing a thin high refractive index Ta2O5 film on top of the waveguide. According to microstructure studies, the sputtered Ta2O5 thin films deposited on the polymer layer were very smooth with root mean square surface roughness value of 0.58 nm, had amorphous phase, and were optimal for integrated optical devices. Both computational and experimental optical studies suggest that the inorganic-polymeric composite waveguide design greatly increases the intensity distribution of the propagating mode at the surface. Consequently, the interaction of the optical field with the ambient surrounding medium is enhanced by a factor of about 1.7 in order of magnitude.

Detection of Listeria innocua on roll-to-roll produced SERS substrates with gold nanoparticles

The rapid and accurate detection of food pathogens plays a critical role in the early prevention of foodborne epidemics. Current bacteria identification practices, including colony counting, polymerase chain reaction (PCR) and immunological methods, are time consuming and labour intensive; they are not ideal for achieving the required immediate diagnosis. Different SERS substrates have been studied for the detection of foodborne microbes. The majority of the approaches are either based on costly patterning techniques on silicon or glass wafers or on methods which have not been tested in large scale fabrication. We demonstrate the feasibility of analyte specific sensing using mass-produced, polymer-based low-cost SERS substrate in analysing the chosen model microbe with biological recognition. The use of this novel roll-to-roll fabricated SERS substrate was combined with optimised gold nanoparticles to increase the detection sensitivity. Distinctive SERS spectral bands were recorded for Listeria innocua ATCC 33090 using an in-house build (785 nm) near infra red (NIR) Raman system. Results were compared to both those found in the literature and the results obtained from a commercial time-gated Raman system with a 532 nm wavelength laser excitation. The effect of the SERS enhancer metal and the excitation wavelength on the detected spectra was found to be negligible. The hypothesis that disagreements within the literature regarding bacterial spectra results from conditions present during the detection process has not been supported. The sensitivity of our SERS detection was improved through optimization of the concentration of the sample inside the hydrophobic polydimethylsiloxane (PDMS) wells. Immunomagnetic separation (IMS) beads were used to assist the accumulation of bacteria into the path of the beam of the excitation laser. With this combination we have detected Listeria with gold enhanced SERS in a label free manner from such low sample concentrations as 104 CFU ml−1.

Bare laser-synthesized Au-based nanoparticles as nondisturbing surface-enhanced Raman scattering probes for bacteria identification

The ability of noble metal-based nanoparticles (NPs) (Au, Ag) to drastically enhance Raman scattering from molecules placed near metal surface, termed as surface-enhanced Raman scattering (SERS), is widely used for identification of trace amounts of biological materials in biomedical, food safety and security applications. However, conventional NPs synthesized by colloidal chemistry are typically contaminated by nonbiocompatible by-products (surfactants, anions), which can have negative impacts on many live objects under examination (cells, bacteria) and thus decrease the precision of bioidentification. In this article, we explore novel ultrapure laser-synthesized Au-based nanomaterials, including Au NPs and AuSi hybrid nanostructures, as mobile SERS probes in tasks of bacteria detection. We show that these Au-based nanomaterials can efficiently enhance Raman signals from model R6G molecules, while the enhancement factor depends on the content of Au in NP composition. Profiting from the observed enhancement and purity of laser-synthesized nanomaterials, we demonstrate successful identification of 2 types of bacteria (Listeria innocua and Escherichia coli). The obtained results promise less disturbing studies of biological systems based on good biocompatibility of contamination-free laser-synthesized nanomaterials.

Stability optimization of microbial surface-enhanced Raman spectroscopy detection with immunomagnetic separation beads

Abstract. Immunomagnetic separation (IMS) beads with antibody coating are an interesting option for biosensing applications for the identification of biomolecules and biological cells, such as bacteria. The paramagnetic properties of the beads can be utilized with optical sensing by migrating and accumulating the beads and the bound analytes toward the focus depth of the detection system by an external magnetic field. The stability of microbial detection with IMS beads was studied by combining a flexible, inexpensive, and mass producible surface-enhanced Raman spectroscopy (SERS) platform with gold nanoparticle detection and antibody recognition by the IMS beads. Listeria innocua ATCC 33090 was used as a model sample and the effect of the IMS beads on the detected Raman signal was studied. The IMS beads were deposited into a hydrophobic sample well and accumulated toward the detection plane by a neodymium magnet. For the first time, it was shown that the spatial stability of the detection could be improved up to 35% by using IMS bead capture and sample well placing. The effect of a neodymium magnet under the SERS chip improved the temporal detection and significantly reduced the necessary time for sample stabilization for advanced laboratory testing.

Integrated dual-slab waveguide interferometer for glucose concentration detection in the physiological range

This paper presents a label-free optical biosensor based on a Youngs interferometer configuration that uses a vertically integrated dual-slab waveguide interferometer as sensing element. In this element, linearly polarized light is coupled into a dual-slab waveguide chip from the input end-face, and the in-coupled zeroth order mode propagates in separated upper and lower waveguides. At the output end-face, the two closely spaced coherent beams diffract out and produce an interference fringe pattern. An evanescent wave field, generated on the surface of the upper waveguide, probes changes in the refractive index of the studied sample, causing a phase shift in the fringe pattern. Compared to a conventional integrated Youngs interferometer utilizing a Y-junction as the beam splitter, the dual-slab waveguide Youngs interferometer has the advantage of easy fabrication and large tolerance to the input-coupling beam. This paper builds a measurement system to investigate sensor performance using glucose solutions with various concentrations. These glucose concentration measurements are performed within the physiological range of 30mg/dl ~ 500mg/dl. The results indicate that a dual-slab waveguide interferometer yields an average phase resolution of 0.002 rad, which corresponds to an effective refractive index change of 4×10-8 with an interaction path length of 15 mm.

Effect of water absorption on the stability of a polymer based Young's interferometric sensor

In this work the refractive index change of a polymer (Ormocer) based waveguide interferometer under the influence of water absorption was analyzed. The water absorption caused refractive index change was up to 10−4. After half an hour contacting with deionized water a pseudo-equilibrium state was reached and the response of the sensor tended to be stabilized. We performed refractive index measurement with glucose water dilutions. The measured refractive index change showed a linear relationship with the glucose concentration.

PDMS surface modification in the application of waveguide claddings for evanescent field sensing

We fabricated SU-8 based slab waveguides on surface-modified poly(dimethyl siloxane) PDMS lower claddings for application in evanescent field sensing. In this application, higher sensitivity is obtained by generating stronger penetrating power above the waveguide into the analyte. This can be achieved by reducing the refractive index of the substrate. Compared with glass substrates that have a refractive index of 1.5, PDMS has a refractive index of 1.42 at 633 nm, thus serving as a better lower cladding material for high-sensitivity sensing with an evanescent field or as claddings in multilayer waveguide applications. In order to increase the adhesion of PDMS surfaces for successful SU-8 application we treated PDMS thin films in low-frequency (40 kHz) oxygen plasma for varied length of exposure time. The treatment process made PDMS hydrophilic and created nano-structures on the surfaces. The resultant surface topography with different exposure time was studied by an interferometric profiler on PDMS lower claddings and the later spin-coated SU-8 waveguides. Measurement results showed that longer plasma treatment on PDMS claddings significantly improved the uniformity and waviness of the waveguides. Light propagation tests performed with a prism coupler and an end-butt coupling setup proved that PDMS can be used as a proper material for SU-8 waveguides.