Milan Vala

Compact and low-cost biosensor based on novel approach to spectroscopy of surface plasmons

A high-performance surface plasmon resonance (SPR) sensor based on a novel approach to spectroscopy of surface plasmons is reported. This approach employs a special diffraction grating structure (referred to as surface plasmon resonance coupler and disperser, SPRCD) which simultaneously couples light into a surface plasmon and disperses the diffracted light for spectral readout of SPR signal. The developed SPRCD sensor consists of a miniature cartridge integrating the diffraction grating and microfluidics and a compact optical system which simultaneously acquires data from four independent sensing channels in the cartridge. It is demonstrated that the SPRCD sensor is able to measure bulk refractive index changes as small as 3 x 10(-7) RIU (refractive index units) and to detect short oligonucleotides in concentrations down to 200 pM.

Compact surface plasmon-enhanced fluorescence biochip

A new concept of compact biochip for surface plasmon-enhanced fluorescence assays is reported. It takes advantage of the amplification of fluorescence signal through the coupling of fluorophore labels with confined and strongly enhanced field intensity of surface plasmons. In order to efficiently excite and collect the emitted fluorescence light via surface plasmons on a metallic sensor surface, (reverse) Kretschmann configuration is combined with diffractive optical elements embedded on the chip surface. These include a concentric relief grating for the imaging of highly directional surface plasmon-coupled emission to a detector. Additional linear grating is used for the generating of surface plasmons at the excitation wavelength on the sensor surface in order to increase the fluorescence excitation rate. The reported approach offers the increased intensity of fluorescence signal, reduced background, and compatibility with nanoimprint lithography for cost-effective preparation of sensor chip. The presented approach was implemented for biosensing in a model immunoassay experiment in which the limit of detection of 11 pM was achieved.

Flexible method based on four-beam interference lithography for fabrication of large areas of perfectly periodic plasmonic arrays

A novel nanofabrication technique based on 4-beam interference lithography is presented that enables the preparation of large macroscopic areas (>50 mm2) of perfectly periodic and defect-free two-dimensional plasmonic arrays of nanoparticles as small as 100 nm. The technique is based on a special interferometer, composed of two mirrors and a sample with photoresist that together form a right-angled corner reflector. In such an interferometer, the incoming expanded laser beam is split into four interfering beams that yield an interference pattern with rectangular symmetry. The interferometer allows setting the periods of the array from about 220 nm to 1500 nm in both directions independently through the rotation of the corner-reflector assembly around horizontal and vertical axes perpendicular to the direction of the incident beam. Using a theoretical model, the implementation of the four-beam interference lithography is discussed in terms of the optimum contrast as well as attainable periods of the array. Several examples of plasmonic arrays (on either glass or polymer substrate layers) fabricated by this technique are presented.

Real-time label-free monitoring of the cellular response to osmotic stress using conventional and long-range surface plasmons.

Cell volume and its regulation are key factors for cellular integrity and also serve as indicators of various cell pathologies. SPR sensors represent an efficient tool for real-time and label-free observations of changes in cell volume and shape. Here, we extend this concept by employing the use of long-range surface plasmons (LRSP). Due to the enhanced penetration depth of LRSP (~1μm, compared to ~0.4μm of a conventional surface plasmon), the observation of refractive index changes occurring deeper inside the cells is possible. In this work, the responses of a confluent normal rat kidney (NRK) epithelial cell layer to osmotic stress are studied by both conventional and long-range surface plasmons. Experiments are conducted in parallel using cell layers grown and stimulated under the same conditions to enable direct comparison of the results and discrimination of the osmotic stress-induced effects in different parts of the cell.

Toward single-molecule detection with sensors based on propagating surface plasmons

Surface plasmon resonance (SPR) sensors are known to be able to detect very low surface concentrations of (bio)molecules on macroscopic areas. To explore the potential of SPR biosensors to achieve single-molecule detection, we have minimized the read-out area (to ~64 μm2) by employing a sensor system based on spectroscopy of surface plasmons generated on a diffractive structure via a microscope objective and light collection through a small aperture. This approach allows for decreasing the number of detected molecules by 3 orders of magnitude compared to state-of-the-art SPR sensors. A protein monolayer has been shown to produce a response of 5000 times the baseline noise, suggesting that as few as ~500 proteins could be detected by the sensor.

Diffraction grating-coupled surface plasmon resonance sensor based on spectroscopy of long-range and short-range surface plasmons

We report a SPR biosensor based on long-range (LR) and short-range (SR) surface plasmon (SP) modes excited simultaneously on a diffraction grating. Employing the LRSP and SRSP in the grating-coupled SPR sensor offers several interesting features such as extended probe depth of the LRSP and ability to distinguish sensor response caused by bulk and surface refractive index changes. Prototype device based on wavelength interrogation of SPs was developed and be tested in model refractometric experiment. This paper presents results of theoretical analysis and experimental characterization of the sensor. Sensitivity of the laboratory prototype of the sensor agrees well with the theory. The sensor is shown to be able to detect changes in the refractive index as small as 3.5 x 10-6 RIU (refractive index unit).

Surface plasmon resonance biosensors for detection of foodborne pathogens and toxins

In the last decade surface plasmon resonance (SPR) biosensors have made great strides both in terms of technology and its applications. SPR biosensors have become a central tool for study of molecular interactions and have been widely used for detection of chemical and biological analytes. Food analysis belongs to major areas of potential applications of SPR biosensors. Therefore, numerous SPR biosensors for detection of analytes implicated in food safety (e.g. pathogens, toxins, drug residues, vitamins, hormones, chemical contaminants, and allergens) have been developed. This paper reviews recent developments in the field of SPR biosensors for food safety, in particular, for detection of foodborne pathogens and toxins.

Compact multi-channel high-sensitivity biosensor based on spectroscopy of surface plasmons

We report a compact multi-channel biosensor based on diffraction grating-coupled SPR for the most demanding detection applications in the field or home environments. The sensor utilizes special diffraction grating (referred to as surface plasmon coupler and disperser - SPRCD) for coupling light into the surface plasmon and its simultaneous wavelength dispersion through a different diffraction order. This approach combines most of the optical instrumentation on a single SPR chip produced by stamper hot-embossing technique which is fully compatible with mass production. The sensor consists of a disposable cartridge (SPR chip and microfluidics) and a compact SPR instrument with the footprint which includes optical system of SPR sensor, supporting and data acquisition electronics, microfluidics delivering sample into six independent sensing channels in the cartridge, and temperature stabilization. We demonstrate that the sensor is able to measure changes in the refractive index as low as 2x10-7 refractive index units (RIU) and to detect the binding of antibodies to the antigen-coated sensor surface.

Multiple beam interference lithography: A tool for rapid fabrication of plasmonic arrays of arbitrary shaped nanomotifs

A novel method enabling rapid fabrication of 2D periodic arrays of plasmonic nanoparticles across large areas is presented. This method is based on the interference of multiple coherent beams originating from diffraction of large-diameter collimated beam on a transmission phase mask. Mutual orientation of the interfering beams is determined by parameters of the used phase mask. Herein, parameters of the phase mask (periods and modulation depth) are selected to yield an interference pattern with high contrast and narrow well-separated maxima. Finally, multiple beam interference lithography (MBIL)-based fabrication of periodic plasmonic arrays with selected nanomotifs including discs, disc dimers, rods and bowtie antennas is demonstrated.