Alberto Sinibaldi

A full ellipsometric approach to optical sensing with Bloch surface waves on photonic crystals

We report on the investigation on the resolution of optical sensors exploiting Bloch surface waves sustained by one dimensional photonic crystals. A figure of merit is introduced to quantitatively assess the performance of such sensors and its dependency on the geometry and materials of the photonic crystal. We show that the figure of merit and the resolution can be improved by adopting a full ellipsometric phase-sensitive approach. The theoretical predictions are confirmed by experiments in which, for the first time, such type of sensors are operated in the full ellipsometric scheme.

Combining label-free and fluorescence operation of Bloch surface wave optical sensors

We report on the design, fabrication, and characterization of optical sensors based on Bloch surface waves propagating at the truncation edge of one-dimensional photonic crystals. The sensors can be simultaneously operated in both a label-free mode, where small refractive index changes at the surface are detected, and a fluorescence mode, where the fluorescence from a novel heptamethyne dye label in the proximity of the surface is collected. The two modes operate in the near-infrared spectral range with the same configuration of the optical reading apparatus. The limit of detection is shown to be smaller than that of equivalent surface plasmon sensors and the fluorescence collection efficiency is such that it can be efficiently analyzed by the same camera sensor used for label-free operation.

Probing losses of dielectric multilayers by means of Bloch surface waves

We exploit the excitation of electromagnetic surface waves on high-quality dielectric multilayers to measure the very low extinction coefficient of the structures, with a resolution down to 4·10(-7) and in a simple optical configuration. The effect of exposition to a rhodamine 6G solution in water and ethanol is also reported, including dye adsorption in the layers and bleaching upon resonant excitation.

Hydrogenated amorphous silicon nitride photonic crystals for improved-performance surface electromagnetic wave biosensors

We exploit the properties of surface electromagnetic waves propagating at the surface of finite one dimensional photonic crystals to improve the performance of optical biosensors with respect to the standard surface plasmon resonance approach. We demonstrate that the hydrogenated amorphous silicon nitride technology is a versatile platform for fabricating one dimensional photonic crystals with any desirable design and operating in a wide wavelength range, from the visible to the near infrared. We prepared sensors based on photonic crystals sustaining either guided modes or surface electromagnetic waves, also known as Bloch surface waves. We carried out for the first time a direct experimental comparison of their sensitivity and figure of merit with surface plasmon polaritons on metal layers, by making use of a commercial surface plasmon resonance instrument that was slightly adapted for the experiments. Our measurements demonstrate that the Bloch surface waves on silicon nitride photonic crystals outperform surface plasmon polaritons by a factor 1.3 in terms of figure of merit.

Label-Free Detection of Tumor Angiogenesis Biomarker Angiopoietin 2 Using Bloch Surface Waves on One Dimensional Photonic Crystals

We describe the design and fabrication of biochips based on 1-D photonic crystals supporting Bloch surface waves for label-free optical biosensing. The optical properties of Bloch surface waves are studied in relation to the geometry of the photonic crystals and on the properties of the dielectric materials used for the fabrication. The planar stacks of the biochips are composed of silica, tantala, and titania that were deposited using plasma-ion-assisted evaporation under high-vacuum conditions. The biochip surfaces were functionalized by silanization, and appropriate fluidic cells were designed to operate in an automated platform. An angularly resolved ellipsometric optical sensing apparatus was assembled to carry out the sensing studies. The angular operation is obtained by a focused laser beam at a fixed wavelength and detection of the angular reflectance spectrum by means of an array detector. The results of the experimental characterization of the physical properties of the fabricated biochips show that their characteristics, in terms of sensitivity and figure of merit, match those expected from the numerical simulations. Practical application of the sensor was demonstrated by detecting a specific glycoprotein, Angio-poietin 2, that is involved in angiogenesis and inflammation processes. The protocol used for the label-free detection of Angiopoietin 2 is described, and the results of an exemplary assay, carried out at a relatively high concentration of 1 μg/ml, are given and confirm that an efficient detection can be achieved. The limit of detection of the biochips for Angiopoietin 2, based on the protocol used, is 1.5 pg/mm2 in buffer solution. The efficiency of the label-free assay is confirmed by independent measurements carried out by means of confocal fluorescence microscopy.

Detection of soluble ERBB2 in breast cancer cell lysates using a combined label-free/fluorescence platform based on Bloch surface waves

We report on the use of one-dimensional photonic crystals to detect clinically relevant concentrations of ERBB2/neu/Her2 in cell lysates. ERBB2 is a pivotal breast cancer biomarker and targetable oncogenic driver associated with aggressive breast cancer subtypes. To quantitate soluble ERBB2, we developed an optical platform that combines label-free and fluorescence detection modes. Such platform makes use of a sandwich assay in which the one-dimensional photonic crystals sustaining Bloch surface waves are tailored with a monoclonal antibody for highly specific biological recognition (BSW biochip). In a second step, a second antibody to ERBB2 quantitatively detects the bound analyte. The strategy of the present approach takes advantage of the combination of label-free and fluorescence techniques, making bio-recognition more robust and sensitive. In the fluorescence operation mode, the platform can attain the limit of detection 0.3ng/mL (1.5pM) for ERBB2 in cell lysates. Such resolution meets the international guidelines and recommendations (15ng/mL) for diagnostic ERBB2 assays that in the future may help to more precisely assign therapies counteracting cancer cell proliferation and metastatic spread.

Improving the sensitivity of optical biosensors by means of Bloch surface waves

One factor limiting the performance of Surface Plasmon Resonance (SPR) biosensors is the width of the resonance that basically originates from metal absorption. In contrast, comparable sensing schemes based upon the concept of Bloch surface waves (BSW) have been proposed. As such sensors are prepared from dielectric materials only, losses as well as the resonance width decrease significantly in comparison to SPR. By preparing BSW stacks that feature a similar resonance position as Gold based SPR sensors, both approaches are compared directly using one detection system only. We obtain an increased performance of BSW although utilizing an optical system optimized for SPR analysis.

Effect of thickness disorder on the performance of photonic crystal surface wave sensors.

We investigated experimentally and numerically the robustness of optical sensors based on Bloch waves at the surface of periodic one-dimensional photonic crystals. The distributions of sensor characteristics caused by the fabrication uncertainties in dielectric layer thicknesses have been analyzed and robustness criteria have been set forth and discussed. We show that the performance of the surface wave sensors is sufficiently robust with respect to the changes of the photonic crystal layer thicknesses. Layer thickness optimization of the photonic crystal, carried out to achieve low limit of detection, leads to an improvement of the robustness of the surface wave sensors that is attributed to Bloch states lying deeper in the photonic band gap.

Bloch surface wave enhanced biosensor for the direct detection of Angiopoietin-2 tumor biomarker in human plasma

Quantitative detection of angiogenic biomarkers provides a powerful tool to diagnose cancers in early stages and to follow its progression during therapy. Conventional tests require trained personnel, dedicated laboratory equipment and are generally time-consuming. Herein, we propose our developed biosensing platform as a useful tool for a rapid determination of Angiopoietin-2 biomarker directly from patient plasma within 30 minutes, without any sample preparation or dilution. Bloch surface waves supported by one dimensional photonic crystal are exploited to enhance and redirect the fluorescence arising from a sandwich immunoassay that involves Angiopoietin-2. The sensing units consist of disposable and low-cost plastic biochips coated with the photonic crystal. The biosensing platform is demonstrated to detect Angiopoietin-2 in plasma samples at the clinically relevant concentration of 6 ng/mL, with an estimated limit of detection of approximately 1 ng/mL. This is the first Bloch surface wave based assay capable of detecting relevant concentrations of an angiogenic factor in plasma samples. The results obtained by the developed biosensing platform are in close agreement with enzyme-linked immunosorbent assays, demonstrating a good accuracy, and their repeatability showed acceptable relative variations.

Bloch Surface Waves Biosensors for High Sensitivity Detection of Soluble ERBB2 in a Complex Biological Environment

We report on the use of one-dimensional photonic crystals to detect clinically relevant concentrations of the cancer biomarker ERBB2 in cell lysates. Overexpression of the ERBB2 protein is associated with aggressive breast cancer subtypes. To detect soluble ERBB2, we developed an optical set-up which operates in both label-free and fluorescence modes. The detection approach makes use of a sandwich assay, in which the one-dimensional photonic crystals sustaining Bloch surface waves are modified with monoclonal antibodies, in order to guarantee high specificity during the biological recognition. We present the results of exemplary protein G based label-free assays in complex biological matrices, reaching an estimated limit of detection of 0.5 ng/mL. On-chip and chip-to-chip variability of the results is addressed too, providing repeatability rates. Moreover, results on fluorescence operation demonstrate the capability to perform high sensitive cancer biomarker assays reaching a resolution of 0.6 ng/mL, without protein G assistance. The resolution obtained in both modes meets international guidelines and recommendations (15 ng/mL) for ERBB2 quantification assays, providing an alternative tool to phenotype and diagnose molecular cancer subtypes.