Liesbet Lagae

Gold nanoring as a sensitive plasmonic biosensor for on-chip DNA detection

We report an on-chip nanosensor based on the localized surface plasmon resonance (LSPR) property of gold nanorings. The fabricated nanorings using nanosphere lithography showed highly tunable optical properties and were optimized to boost LSPR-based biosensing. The LSPR extinction spectra of the nanosensor agreed well with the theoretical calculations using a finite-difference time-domain model. Such an on-chip nanoring-based biosensor showed a refractive index sensitivity of 350u2009nm/refractive index unit with a figure of merit of 3.1 and was utilized to detect DNA in a real-time and label-free manner.

Manipulation of magnetic particles on chip by magnetophoretic actuation and dielectrophoretic levitation

The prospect of using magnetic particles for biomedical purposes in lab-on-a-chip systems compels accurate and flexible particle manipulation. Toward such a goal we designed a microdevice comprising a pair of meander-shaped current carrying conductors, which enable simultaneous magnetophoresis and dielectrophoresis by generating both a traveling magnetic field and an ac electric field. Therefore, both the in-plane and out-of-plane movements of magnetic particles can be electrically controlled. A transport speed of tens of μm∕s was achieved with actuation forces at piconewton scale. The enhanced control of particle movement avoids the contact and nonspecific adhesion between the particle and device.

Localized surface plasmon resonance biosensor integrated with microfluidic chip

A sensitive and low-cost microfluidic integrated biosensor is developed based on the localized surface plasmon resonance (LSPR) properties of gold nanoparticles, which allows label-free monitoring of biomolecular interactions in real-time. A novel quadrant detection scheme is introduced which continuously measures the change of the light transmitted through the nanoparticle-coated sensor surface. Using a green light emitting diode (LED) as a light source in combination with the quadrant detection scheme, a resolution of 10−4 in refractive index units (RIU) is determined. This performance is comparable to conventional LSPR-based biosensors. The biological sensing is demonstrated using an antigen/antibody (biotin/anti-biotin) system with an optimized gold nanoparticle film. The immobilization of biotin on a thiol-based self-assembled monolayer (SAM) and the subsequent affinity binding of anti-biotin are quantitatively detected by the microfluidic integrated biosensor and a detection limit of 270xa0ng/mL of anti-biotin was achieved. The microfluidic chip is capable of transporting a precise amount of biological samples to the detection areas to achieve highly sensitive and specific biosensing with decreased reaction time and less reagent consumption. The obtained results are compared with those measured by a surface plasmon resonance (SPR)-based Biacore system for the same binding event. This study demonstrates the feasibility of the integration of LSPR-based biosensing with microfluidic technologies, resulting in a low-cost and portable biosensor candidate compared to the larger and more expensive commercial instruments.

On-chip separation of magnetic particles with different magnetophoretic mobilities

Recent integrations of giant magnetoresistive sensor into laboratory-on-a-chip systems enable the direct detection of biological entities such as cells coated with magnetic particles on chip. However, before detection the different biological entities need to be separated. As a model system, we investigated the separation of two types of magnetic particles (4.5 and 2μm in diameter). The motion of the particles was studied when actuated using an alternating traveling magnetic field produced by four-phase conductors on chip. Different magnetic particles migrate with different speeds in the same traveling magnetic field. By carefully choosing the frequency of the magnetic field, different magnetic particles can be separated in a microfluidic system.

Tuning the Fano Resonance Between Localized and Propagating Surface Plasmon Resonances for Refractive Index Sensing Applications

Localized and propagating surface plasmon resonances are known to show very pronounced interactions if they are simultaneously excited in the same nanostructure. Here, we study the Fano interference that occurs between localized surface plasmon resonance (LSPR) and propagating surface plasmon polariton (SPP) modes by means of phase-sensitive spectroscopic ellipsometry. The sample structures consist of periodic gratings of gold nanodisks on top of a continuous gold layer and a thin dielectric spacer, in which the structural dimensions were tuned in such a way that the dipolar LSPR mode and the propagating SPP modes are excited in the same spectral region. We observe pronounced anti-crossing and strongly asymmetric line shapes when both modes move to each other’s vicinity, accompanied of largely increased phase differences between the respective plasmon resonances. Moreover, we show that the anti-crossing can be exploited to increase the refractive index sensitivity of the localized modes dramatically, which result in largely increased values for the figure-of-merit which reaches values between 24 and 58 for the respective plasmon modes.

Label-free genosensor based on immobilized DNA hairpins on gold surface

In this report, we demonstrate a label-free genosensor based on DNA hairpins coupled to gold coated sensor surfaces. The hairpin probes were labeled with a thiolated moiety for immobilization at the 5 end and with a fluorophore for signal transduction at the 3 end. In the absence of the complement, the fluorophore is quenched by energy transfer to the gold surface. Addition of the target sequence leads to the hairpin unfolding, and releases the fluorescent signal. This built-in property, using a gold film as both the immobilizing substrate and quenching agent, has the advantage of simplicity in design and ease of further integration. Our results showed that lengths of both the stem and the loop structures have significant effects on the sensor performance. Hybridization kinetics was investigated for various probe/target lengths and concentrations. An optimized hairpin probe gave a fluorescent signal increase of 39 folds after hybridization, which is much higher than the earlier reported results. A limit of detection (LOD) down to 0.3 nM for the complementary target DNA detection has been achieved. The developed sensor was further successfully applied for the detection of single-base mismatch targets, as well as for the direct detection of PCR products.

Single cell viability observation in cell dielectrophoretic trapping on a microchip

We reported a microfluidic integrated dielectrophoretic (DEP) device for single MCF-7 cell trapping and studied the different effects of applied electric field on the viability of the trapped cell. The cell remained alive when DEP voltage was 3 V. Above 3u2009V, cell viability significantly decreased when increasing stimulation time. At 8u2009V, the cell was rapidly lysed by the electric field. The high transmembrane potential induced was found to be the major cause of cell damage. The obtained results indicated that an operational electric field below than 2u2009kV/cm was safe for cell viability when using DEP for cell manipulation.

Microscope-on-chip: combining lens-free microscopy with integrated photonics

Lens-free in-line Holographic Microscopy (LHM) is a promising imaging technique for many biomedical and industrial applications. The main advantage of the technique is the simplicity of the imaging hardware, requiring no lenses nor high-precision mechanical components. Nevertheless, the LHM systems achieve high imaging performance only in combination with a high-quality and complex illumination. Furthermore, to achieve truly high-throughput imaging capabilities, many applications require a complete on-chip integration. We demonstrate the strength, versatility and scalability of our integrated approach on two microscopes-on-chip instances that combine image sensor technologies with photonics (and micro-fluidics): a fully integrated Point-Source (PS) LHM module for in-flow cell inspection and Large Field-of-View (LFoV) microscope with on-chip photonic illumination for large-area imaging applications. The proposed PS-LHM module consists of a photonic illumination, a micro-fluidic channel and an imager, integrated in a total volume smaller than 0.5 mm3. A low-loss single-mode photonic waveguide is adapted to generate a high- NA illumination spot. Experimental results show strong focusing capabilities and sufficient overall coupling efficiency. Current PS-LHM prototype reaches imaging resolution below 600nm. Our LFoV-LHM system is extremely vertically compact as it consists of only one 1mm-thick illumination chip and one 3mm-thick imaging module. The illumination chip is based on fractal-layout phase-matched waveguides designed to generate multiple light sources that create a quasi-planar illumination wavefront over an area few square millimeter large. Current illumination prototype has active area of approximately 1.2×1.2mm2. Our LFoV-LHM prototype reaches imaging resolution of 870nm using image sensor with 1.12μm pixel pitch with maximum FoV of 16.47mm2.

Raman spectroscopy and optical trapping of 20 nm polystyrene particles in plasmonic nanopores

The detection and identification of nanoparticles has caught the attention in the last decade due to its potential application on small bio-particles. Raman spectroscopy stands out as a label-free technique for the detection of such particles. However, it may require a high concentration of particles. In a solution with low concentration of particles to detect Raman spectroscopy, the number of particles in the detection area can be increased by optical trapping. The optical trapping force applied to a dielectric nanoparticle is proportional to the gradient of the optical intensity field. Plasmonic nanopores are efficient platforms for trapping nanoparticles due to highly enhanced localized field and its high gradient. Here, we report our work on the optical trapping and assembly of 20 nm polystyrene nanoparticles in a plasmonic nanopore and its detection by Raman spectroscopy.

Microelectronics-Based Biosensors for the Detection of Proteins and Nucleic Acids

Novel biosensor concepts are being explored for the transduction of affinity-based interactions into electrical signals. The specificity of affinity-based biosensors depends on the molecular recognition properties of receptors, for example antibodies, or on the hybridization of complementary nucleic acids. In this chapter, various transducer technologies for the detection of affinity-based interactions will be discussed, focusing on the exploitation of microelectronics. Several examples of label-free detection will be given and impedimetric and plasmonic biosensors will be discussed into detail. In addition, one particular example where the use of labels makes sense will be highlighted by elaborating on magnetic biosensors. In this approach, magnetic particles can be used for sample preparation as well as detection, which allows an entry into the lab-on-chip field.