Lucia Fornasari

Multiplexed label-free optical biosensor for medical diagnostics

Abstract. This paper describes a new multiplexed label-free biosensor. The detection technology is based on nanostructured gold-polymer surfaces. These surfaces support surface plasmon resonance modes that can be probed by a miniaturized optical setup. The optical characterization of the sensing chip shows the sensitivity and the limit-of-detection to refractive index changes. Moreover, by studying the progressive adhesion of molecular monolayers of polyelectrolytes, the decay of the plasmonic mode electric field above the surface has been reconstructed. A multiplexed label-free biosensing device is then described and characterized in terms of sensitivity, lateral resolution, and sensitivity to a model biological assay. The sensitivity in imaging mode of the device is of the order of 10−6 refractive index units, while the measured lateral resolution is 6.25 μm within a field of view of several tenths of mm2, making the instrument unique in terms of multiplexing capability. Finally, the proof-of-concept application of the technology as a point-of-care diagnostic tool for an inflammatory marker is demonstrated.

Optical Sensitivity Gain in Silica-Coated Plasmonic Nanostructures.

Ultrathin films of silica realized by sol-gel synthesis and dip-coating techniques were successfully applied to predefined metal/polymer plasmonic nanostructures to spectrally tune their resonance modes and to increase their sensitivity to local refractive index changes. Plasmon resonance spectral shifts up to 100 nm with slope efficiencies of ∼8 nm/nm for increasing layer thickness were attained. In the ultrathin layer regime (<10 nm), which could be reached by suitable dilution of the silica precursors and optimization of the deposition speed, the sensitivity of the main plasmonic resonance to refractive index changes in aqueous solution could be increased by over 50% with respect to the bare plasmonic chip. Numerical simulations supported experimental data and unveiled the mechanism responsible for the optical sensitivity gain, proving an effective tool in the design of high-performance plasmonic sensors.

Self-assembled InAs nanowires as optical reflectors

Subwavelength nanostructured surfaces are realized with self-assembled vertically-aligned InAs nanowires, and their functionalities as optical reflectors are investigated. In our system, polarization-resolved specular reflectance displays strong modulations as a function of incident photon energy and angle. An effective-medium model allows one to rationalize the experimental findings in the long wavelength regime, whereas numerical simulations fully reproduce the experimental outcomes in the entire frequency range. The impact of the refractive index of the medium surrounding the nanostructure assembly on the reflectance was estimated. In view of the present results, sensing schemes compatible with microfluidic technologies and routes to innovative nanowire-based optical elements are discussed.

Synergic combination of the sol-gel method with dip coating for plasmonic devices

Summary Biosensing technologies based on plasmonic nanostructures have recently attracted significant attention due to their small dimensions, low-cost and high sensitivity but are often limited in terms of affinity, selectivity and stability. Consequently, several methods have been employed to functionalize plasmonic surfaces used for detection in order to increase their stability. Herein, a plasmonic surface was modified through a controlled, silica platform, which enables the improvement of the plasmonic-based sensor functionality. The key processing parameters that allow for the fine-tuning of the silica layer thickness on the plasmonic structure were studied. Control of the silica coating thickness was achieved through a combined approach involving sol–gel and dip-coating techniques. The silica films were characterized using spectroscopic ellipsometry, contact angle measurements, atomic force microscopy and dispersive spectroscopy. The effect of the use of silica layers on the optical properties of the plasmonic structures was evaluated. The obtained results show that the silica coating enables surface protection of the plasmonic structures, preserving their stability for an extended time and inducing a suitable reduction of the regeneration time of the chip.

Lab on Chip: Portable Optical Device for On-site Multi-parametric Analysis

Recently the demand dramatically grew up for portable biosensors, providing an on-site multi-parametric measurement. Present instruments, however, are limited by large size and consumption (which prevents portability) and costs (which prevents their usage in some Countries). In this paper, we propose a compact and portable device based on a nano-structured array biochip featuring the Surface Plasmonic Resonance, lighted by a suitable optics and equipped with an 830 nm irradiating LED. The reflected image is detected by an Aptina CMOS sensor and managed by an ARM9 processor which is responsible of the acquisition end processing, performed within 14 sec form the application of the assy. The processor evaluates the average grey level pixel ratio between suitable biochip areas so as to be independent on illumination fluctuations and external noise. Preliminary results indicate a sensitivity close to 10-4 RIU change in the refractive index and applicability of the device to different applications and fields (waste water and food pollution analysis, among the main ones).

Plasmonic Sensors on 2D Ordered Structures

Colloidal lithography with polystyrene spheres allows for the fabrication of hybrid polymer/metal two-dimensional ordered surfaces. They consist of a hexagonal lattice of plasma-polymerized acrylic acid (ppAA) or poly(methyl methacrylate) (PMMA) pillars embedded in an optically thick gold film deposited on a glass substrate. Such a kind of nanostructured system has been shown to support either propagating Surface Plasmon Polaritons or “Mie-like” localized resonances, and appears to be particularly interesting for sensing applications. Tuning the structural parameters, a strong interaction among delocalized and localized plasmonic modes can be obtained together with a good coupling with light. This opens the way towards an optical biomolecular sensor system in which a modification/adhesion on the free nanostructured surface can be easily detected by a simple, near normal reflectance measurement performed from the substrate side, despite the relatively large gold thickness. The simple configuration allows for a surface plasmon resonance (SPR) imaging configuration and enables the real-time multiplexed detection of several analytes. The sensing performance of the surfaces (sensitivity to refractive index change and to the adhesion of molecular monolayers) has been tested using standard spectroscopic techniques. The electromagnetic field’s spatial distribution within the nanostructures and its intensity enhancement have been numerically calculated by finite difference time domain (FDTD) simulations. The results, including the calculated reflectance spectra, are in good agreement with the experimentaldata.

A multiplexed label free plasmonic nano-device for near infrared applications

The development of a new surface plasmon resonance (SPR) imaging biosensor is reported. The biosensor exploits the optical properties of a nano-structured gold-polymer chip, which allows for the coupling of the SPR with the incident light. The spectral characterization of the chip permits to analyze the plasmonic response to a refractive index change near its free surface. The nano-structured surface features are presented together with an exemplifying biological tests which demonstrate the multiplexing label-free detection capability of the proposed device.

Plasmonic Structures for Sensing and Emitting Devices

We report on the study of a plasmonic nanostructure that could be adopted as platform for emitting and sensing applications. Several devices have been prepared and characterized by atomic force microscopy (AFM) and Fourier transform micro-reflectance (FT- pR) techniques. In addition, a modelling via finite-difference time-domain (FDTD) simulations have been developed in order to interpret the morphological shape and the optical response of the considered structures. Until now, remarkable performances as surface plasmon resonance (SPR) based optical sensor have been founded. Moreover, we are performing preliminary trials in order to establish a coupling between photoluminescence (PL) features of suitable emitters with respect to the plasmonic resonances.

Thickness controlled sol-gel silica films for plasmonic bio-sensing devices

Plasmonics has recently received considerable interest due to its potentiality in many fields as well as in nanobio-technology applications. In this regard, various strategies are required for modifying the surfaces of plasmonic nanostructures and to control their optical properties in view of interesting application such as bio-sensing, We report a simple method for depositing silica layers of controlled thickness on planar plasmonic structures. Tetraethoxysilane (TEOS) was used as silica precursor. The control of the silica layer thickness was obtained by optimizing the sol-gel method and dip-coating technique, in particular by properly tuning different parameters such as pH, solvent concentration, and withdrawal speed. The resulting films were characterized via atomic force microscopy (AFM), Fourier-transform (FT) spectroscopy, and spectroscopic ellipsometry (SE). Furthermore, by performing the analysis of surface plasmon resonances before and after the coating of the nanostructures, it was observed that the position of the resonance structures could be properly shifted by finely controlling the silica layer thickness. The effect of silica coating was assessed also in view of sensing applications, due to important advantages, such as surface protection of the plasmonic structure.