Alexandre François

Optical biosensor based on whispering gallery mode excitations in clusters of microparticles

A new concept for an optical biosensor based on whispering gallery mode (WGM) excitations in clusters of spherical microresonators is presented. Clusters of microresonators offer the advantage to exhibit specific WGM spectra that can be considered as their fingerprint. Therefore, individual clusters can be traced throughout an experiment even without knowledge of their precise positions. Polyelectrolyte adsorption onto clusters of 10μm polystyrene spheres is monitored in situ. It is shown that the WGMs shift to the same amount as those of a single microresonator and thus sensitivity does not depend on the number of microresonators present in the cluster.

Whispering gallery mode biosensor operated in the stimulated emission regime

Whispering gallery modes (WGMs) are generated in fluorescent polymer microparticles in phosphate buffered saline (PBS) above the threshold for stimulated emission and compared to their characteristics below threshold. The WGM microresonators show an eightfold improvement of their signal-to-noise ratio and a threefold increase in their quality factor when operated above threshold. In an investigation on the benefits for biochemical sensing, a real-time adsorption kinetics of bovine serum albumin in PBS is monitored and compared with those kinetics acquired by means of a WGM microresonator operated below threshold as well as by surface plasmon resonance (SPR).

Highly efficient excitation and detection of whispering gallery modes in a dye-doped microsphere using a microstructured optical fiber

A technique for the excitation of whispering gallery modes (WGMs) has been demonstrated using a dye-doped microsphere positioned onto the tip of a suspended core microstructured optical fiber. With this configuration, we have shown that both the excitation and collection efficiency of the WGMs modulated fluorescence spectra of the dye are greatly improved compared to a more conventional excitation scheme; an overall efficiency increase by a factor of 200 is demonstrated. It is also shown that positioning the resonator onto the fiber tip does not impact its sensitivity, providing a compact and robust architecture for applications such as localized in-vivo/vitro biosensing.

In-vitro sensing of biomechanical forces in live cells by a whispering gallery mode biosensor

Direct measurement of the biomechanical stress induced by a live cell during endocytosis is reported. Fluorescent dye-doped polystyrene microspheres were used as microscopic remote optical sensors applying whispering gallery modes (WGMs) as transducer mechanism. Monitoring of the WGMs throughout the incorporation of the microsphere into the cell enabled the determination of the deformation experienced by the microsphere, characterized by both a broadening and a blue shift of the resonances, and consequently the stress induced by the cell. The results reveal an unexpectedly high stress with a magnitude of up to five times that of the passive cortical tension, which can be only explained by a so far undetermined active stress component induced by the cytoskeletal machinery during particle incorporation. The method is adaptable to the study of any other kind of phagocyte and thus provides a novel research tool of high interest for cell biology.

Collection mode surface plasmon fibre sensors: a new biosensing platform.

Sensors based on surface plasmon resonance (SPR) allow rapid, label-free, highly sensitive detection, and indeed this phenomenon underpins the only label-free optical biosensing technology that is available commercially. In these sensors, the existence of surface plasmons is inferred indirectly from absorption features that correspond to the coupling of light into a thin metallic film. Although SPR is not intrinsically a radiative process, when the metallic coating which support the plasmonic wave exhibits a significant surface roughness, the surface plasmon can itself couple to the local photon states, and emit light. Here we show that using silver coated optical fibres, this novel SPR transducing mechanism offers significant advantages compare to traditional reflectance based measurements such as lower dependency on the metallic thickness and higher signal to noise ratio. Furthermore, we show that more complex sensor architectures with multiple sensing regions scattered along a single optical fibre enable multiplexed detection and dynamic self referencing of the sensing signal. Moreover, this alternative approach allows to combine two different sensing technologies, SPR and fluorescence sensing within the same device, which has never been demonstrated previously. As a preliminary proof of concept of potential application, this approach has been used to demonstrate the detection of the seasonal influenza A virus.

Fluorescent polymer coated capillaries as optofluidic refractometric sensors

A capillary microresonator platform for refractometric sensing is demonstrated by coating the interior of thick-walled silica capillaries with a sub-wavelength layer of high refractive index, dye-doped polymer. No intermediate processing, such as etching or tapering, of the capillary is required. Side illumination and detection of the polymer layer reveals a fluorescence spectrum that is periodically modulated by whispering gallery mode resonances within the layer. Using a Fourier technique to calculate the spectral resonance shifts, the fabricated capillary resonators exhibited refractometric sensitivities up to approximately 30 nm/RIU upon flowing aqueous glucose through them. These sensors could be readily integrated with existing biological and chemical separation platforms such as capillary electrophoresis and gas chromatography where such thick walled capillaries are routinely used with polymer coatings. A review of the modelling required to calculate whispering gallery eigenmodes of such inverted cylindrical resonators is also presented.

Polymer based whispering gallery mode laser for biosensing applications

Whispering gallery mode lasers are of interest for a wide range of applications and especially biological sensing, exploiting the dependence of the resonance wavelengths on the surrounding refractive index. Upon lasing, the Q factors of the resonances are greatly improved, enabling measurements of wavelength shifts with increased accuracy. A way forward to improve the performance of the refractive index sensing mechanism is to reduce the size of the optical resonator, as the refractive index sensitivity is inversely proportional to the resonator dimensions. However, as the lasing threshold is believed to depend on the Q factor among other parameters, and the reduction of the microresonator size results in lower Q, this poses additional challenges for reaching the lasing threshold. In this letter, we demonstrate lasing in 10 μm diameter dye doped polystyrene microspheres in aqueous solution, the smallest polystyrene microsphere lasers ever reported in these conditions. We also investigate the dependence of the lasing threshold on the Q factor by changing the refractive index surrounding the sphere, highlighting a much stronger dependency than initially reported.

Radiative-surface plasmon resonance for the detection of apolipoprotein E in medical diagnostics applications

UNLABELLED Surface plasmon resonance (SPR)-based sensors enable the rapid, label-free and highly sensitive detection of a large range of biomolecules. We have previously shown that, using silver-coated optical fibers with a high surface roughness, re-scattering of light from the surface plasmons is possible, turning SPR into a radiative process. The efficacy of this platform has proven for the detection of large biomolecules such as viruses, proteins and enzymes. Here, we demonstrate that by bringing together this novel emission-based fiber SPR platform with an improved surface functionalization process aimed at properly orienting the antibodies, it is possible to rapidly and specifically detect the regulation of human apolipoprotein E (apoE), a low-molecular-weight protein (~39 kDa) known to be involved in cardiovascular diseases, Alzheimers disease and gastric cancer. The results obtained clearly show that this new sensing platform has the potential to serve as a tool for point-of-decision medical diagnostics. FROM THE CLINICAL EDITOR In this study, a novel emission-based surface plasmon resonance platform using silver-coated optical fibers is described. Properly orienting antibodies on the surface enables rapid and specific detection of human apolipoprotein E (apoE).

Optical Sensors Based on Whispering Gallery Modes in Fluorescent Microbeads: Response to Specific Interactions

Whispering gallery modes (WGMs) in surface-fixated fluorescent polystyrene microbeads are studied in view of their capability of sensing the formation of biochemical adsorption layers on their outer surface with the well-established biotin-streptavidin specific binding as the model system. Three different methods for analysis of the observed shifts in the WGM wavelength positions are applied and used to quantify the adsorbed mass densities, which are then compared with the results of a comparative surface plasmon resonance (SPR) study.

Plasmonic Fiber Optic Refractometric Sensors: From Conventional Architectures to Recent Design Trends

Surface Plasmon Resonance (SPR) fiber sensor research has grown since the first demonstration over 20 year ago into a rich and diverse field with a wide range of optical fiber architectures, plasmonic coatings, and excitation and interrogation methods. Yet, the large diversity of SPR fiber sensor designs has made it difficult to understand the advantages of each approach. Here, we review SPR fiber sensor architectures, covering the latest developments from optical fiber geometries to plasmonic coatings. By developing a systematic approach to fiber-based SPR designs, we identify and discuss future research opportunities based on a performance comparison of the different approaches for sensing applications.