Plasmonic Nanostructures for Biosensing Applications

The aim of this work is the study, the design and the nanofabrication of innovative plasmonic nanostructured materials to develop label-free optical biosensors. Noble metalbased nanostructures have gained interest in the last years due to their extraordinary optical properties, which allow to develop optical biosensors able to detect very low concentrations of specific biomolecules, called analyte, down to the picomolar range. Such biosensors rely on the Surface Plasmon Resonance (SPR) excitation which occurs under specific conditions that depend both on the morphology of the nanostructure and on the adjacent dielectric medium. Therefore, the binding of the biomolecules to metal surfaces is revealed as a change in the SPR condition. Four kinds of nanostructures are investigated in this work: ordered and disordered nanohole array (o-NHA, d-NHA), nanoprism array (NPA) and nanodisk array (NDA). The o-NHA and d-NHA consist of a thin metallic film (50 - 100 nm) patterned with, respectively, a hexagonal and a disordered array of circular holes. The NPA consists of a honeycomb lattice of triangle shaped nanoprisms with edges of about 100 - 200 nm and height of 40 - 80 nm. Finally, the NDA consists of a disordered array of non-interacting disks with 100 - 300 nm diameter and 40 - 80 nm height. The first two support the Extended-SPR whereas the last two, due to their three-dimensional confinement, present Localized-SPR property. Two colloidal techniques are employed for the scalable and cost-effective synthesis of wide areas of nanostructures that allow a fine control of the morphology: NanoSphere Lithography (NSL) and Sparse Colloidal Lithography (SCL). Ordered arrays were nanofabricated by NSL (i.e., NPA and o-NHA) whereas disordered nanostructures were synthesized by the SCL (i.e., NDA and d-NHA). Firstly, the nanostructures are simulated by Finite Element Method (FEM) computations and their performances in revealing small variations of the dielectric medium at the interface is evaluated as a function of their geometrical parameters. Simulated local sensitivities range from 3.1 nm/RIU of the o-NHA up to 13.6 nm/RIU of the NPA. Afterwards, the sensing performances are evaluated experimentally with nanofabricated samples and comparable but slightly smaller sensitivities are obtained. Secondly, a proof-of-concept protocol for the detection assay, that relies on the binding of streptavidin protein to the biotinylated gold surfaces, is exploited to test the nanostructures as biosensors. A 4.4 nM limit of detection is reached with the best performing biosensor (NPA) and picomolar ones are expected for NPA and NDA with a suitable improvement of the functionalization protocol. Finally, complementary single stranded RNA molecules were used, respectively, as bioreceptor and analyte. Revealing short sequences of non-coding RNA, called microRNA, is fundamental for the medical research since these oligonucleotides act as biomarkers for specific diseases, like tumors. Signals of about 13 nm are obtained from the binding of bioreceptor to the nanostructure and from the hybridization of the analyte oligonucleotide at saturation concentrations (∼ 1 μM), indicating that for the moment the developed protocol is quite effective down to the 100 nM range. Of course, for reading the nm or even sub-nM range further optimizations are needed

Plasmonic Nanostructures for Biosensing Applications

The aim of this work is the study, the design and the nanofabrication of innovative plasmonic nanostructured materials to develop label-free optical biosensors. Noble metalbased nanostructures have gained interest in the last years due to their extraordinary optical properties, which allow to develop optical biosensors able to detect very low concentrations of specific biomolecules, called analyte, down to the picomolar range. Such biosensors rely on the Surface Plasmon Resonance (SPR) excitation which occurs under specific conditions that depend both on the morphology of the nanostructure and on the adjacent dielectric medium. Therefore, the binding of the biomolecules to metal surfaces is revealed as a change in the SPR condition. Four kinds of nanostructures are investigated in this work: ordered and disordered nanohole array (o-NHA, d-NHA), nanoprism array (NPA) and nanodisk array (NDA). The o-NHA and d-NHA consist of a thin metallic film (50 - 100 nm) patterned with, respectively, a hexagonal and a disordered array of circular holes. The NPA consists of a honeycomb lattice of triangle shaped nanoprisms with edges of about 100 - 200 nm and height of 40 - 80 nm. Finally, the NDA consists of a disordered array of non-interacting disks with 100 - 300 nm diameter and 40 - 80 nm height. The first two support the Extended-SPR whereas the last two, due to their three-dimensional confinement, present Localized-SPR property. Two colloidal techniques are employed for the scalable and cost-effective synthesis of wide areas of nanostructures that allow a fine control of the morphology: NanoSphere Lithography (NSL) and Sparse Colloidal Lithography (SCL). Ordered arrays were nanofabricated by NSL (i.e., NPA and o-NHA) whereas disordered nanostructures were synthesized by the SCL (i.e., NDA and d-NHA). Firstly, the nanostructures are simulated by Finite Element Method (FEM) computations and their performances in revealing small variations of the dielectric medium at the interface is evaluated as a function of their geometrical parameters. Simulated local sensitivities range from 3.1 nm/RIU of the o-NHA up to 13.6 nm/RIU of the NPA. Afterwards, the sensing performances are evaluated experimentally with nanofabricated samples and comparable but slightly smaller sensitivities are obtained. Secondly, a proof-of-concept protocol for the detection assay, that relies on the binding of streptavidin protein to the biotinylated gold surfaces, is exploited to test the nanostructures as biosensors. A 4.4 nM limit of detection is reached with the best performing biosensor (NPA) and picomolar ones are expected for NPA and NDA with a suitable improvement of the functionalization protocol. Finally, complementary single stranded RNA molecules were used, respectively, as bioreceptor and analyte. Revealing short sequences of non-coding RNA, called microRNA, is fundamental for the medical research since these oligonucleotides act as biomarkers for specific diseases, like tumors. Signals of about 13 nm are obtained from the binding of bioreceptor to the nanostructure and from the hybridization of the analyte oligonucleotide at saturation concentrations (∼ 1 μM), indicating that for the moment the developed protocol is quite effective down to the 100 nM range. Of course, for reading the nm or even sub-nM range further optimizations are needed

Tunable Third-Order Nonlinear Optical Response in \u3f5-Near-Zero Multilayer Metamaterials

Metamaterials with properly engineered linear and nonlinear optical response are of great interest for many advanced applications in nanophotonics and quantum optics. In the present work, we perform a detailed spectral investigation of the third-order nonlinear optical properties (nonlinear refractive index and nonlinear absorption coefficient) of epsilon-near-zero Au/Al2O3 multilayer metamaterials in a broad range of the visible spectrum across their epsilon-near-zero (ENZ) wavelength, at different incidence angles with TE and TM-polarized light. Multilayers with different gold filling fractions (16 and 33%) are produced by magnetron sputtering to tune the spectral position of the -near-zero wavelength. The results demonstrate that a continuous modulation of the linear and nonlinear optical parameters of these metamaterials can be obtained as a function of the angle of incidence, with a peak of the nonlinear optical coefficients close to the ENZ wavelength. A model is proposed to describe the nonlinear optical response of the metamaterials, and an optimal agreement between experimental and simulated results is obtained in all the configurations explored. This model represents a useful tool to design multilayer metamaterials with tailored nonlinear optical properties, to be used in different experimental configurations

Selective Control of Eu3+ Radiative Emission by Hyperbolic Metamaterials

In recent years the quest for novel materials possessing peculiar abilities of manipulating light at the nanoscale has been significantly boosted due to the strict demands of advanced nanophotonics and quantum technologies. In this framework radiative decay engineering of quantum emitters is of paramount importance for developing efficient single-photon sources or nanolasers. Hyperbolic metamaterials stand out among the best cutting-edge candidates for photoluminescence control owing to their potentially unlimited photonic density of states and their ability to sustain high-k modes that allow us to strongly enhance the radiative decay rate of quantum light emitters. The aim of the present paper is to show how Au/Al2O3 hyperbolic multilayers can be used to selectively control the photoluminescence of coupled Eu3+ emitters. We point out an enhancement of the Eu3+ transitions when they are in the hyperbolic regime of the metamaterials and a significant alteration of the ED and MD branching ratios by changing the emitter–metamaterial distance

Optimal geometry for plasmonic sensing with non-interacting Au nanodisk arrays

Combining finite elements method electrodynamic simulations and cost-effective and scalablenanofabrication techniques, we carried out a systematic investigation and optimization of the sensingproperties of non- interacting gold nanodisk arrays. Such plasmonic nanoarchitectures offer a veryeffective platform for fast and simple, label-free, optical bio- and chemical-sensing. We varied their maingeometrical parameters (diameter and height) to monitor the plasmonic resonance position and tofindthe configurations that maximize the sensitivity to small layers of an analyte (local sensitivity) or to thevariation of the refractive index of an embedding medium (bulk sensitivity). The spectral position of theplasmonic resonance can be tuned over a wide range from the visible to the near-IR region (500–1300nm) and state-of-the-art performances can be obtained using the optimized nanodisks; we obtainedlocal and bulk sensitivities of S0=11.9 RIU^-11and Sbulk=662 nm/RIU, respectively. Moreover, theresults of the simulations are compared with the performances of experimentally synthesized non-interacting Au nanodisk arrays fabricated by combining sparse colloidal lithography and hollow masklithography, with the parameters obtained by the sensitivity numerical optimization. An excellentagreement between the experimental and the simulated results is demonstrated, confirming that theoptimization performed with the simulations is directly applicable to nanosensors realized with cost-effective methods, due to the quite large stability basin around the maximum sensitivities

Double-Langmuir model for optimized nanohole array-based plasmonic biosensors

The sensing mechanism of plasmonic nanohole arrays is investigated and a novel model is proposed to interpret their optical response over a wide dynamic range of concentrations (10^-13 - 10^-5 M), based on a double- Langmuir model. This model describes the signal response of the analyte binding as the sum of two independent contributions which are related to two different surface regions of the biosensor, namely the top gold surface of the nanohole array and the lateral gold area inside the nanoholes. Numerical simulations highlight the different near-field behaviour of these two regions and their very different refractive index sensitivities, which both support the double-Langmuir model. This is corroborated by experimental biosensing measurements with gold nanohole arrays with hexagonal symmetry, synthesized by nanosphere lithography. Their sensing performances are optimized by numerical simulations by changing their geometrical parameters (i.e., lattice constant, nanohole diameter and height) in order to achieve a maximum sensitivity. For the biosensing experiments, the biotin-streptavidin complex is used as a benchmark test for the optimized nanohole array and a robust calibration is provided by the double-Langmuir model obtaining a limit of detection of 0.3 ng/mL, which corresponds to an absolute analyte quantity of 0.02 fmol

Quasi-BIC Modes in All-Dielectric Slotted Nanoantennas for Enhanced Er3+ Emission

In the quest for new and increasingly efficient photon sources, the engineering of the photonic environment at the subwavelength scale is fundamental for controlling the properties of quantum emitters. A high refractive index particle can be exploited to enhance the optical properties of nearby emitters without decreasing their quantum efficiency, but the relatively modest Q-factors (Q ∼ 5–10) limit the local density of optical states (LDOS) amplification achievable. On the other hand, ultrahigh Q-factors (up to Q ∼ 109) have been reported for quasi-BIC modes in all-dielectric nanostructures. In the present work, we demonstrate that the combination of quasi-BIC modes with high spectral confinement and nanogaps with spacial confinement in silicon slotted nanoantennas lead to a significant boosting of the electromagnetic LDOS in the optically active region of the nanoantenna array. We observe an enhancement of up to 3 orders of magnitude in the photoluminescence intensity and 2 orders of magnitude in the decay rate of the Er3+ emission at room temperature and telecom wavelengths. Moreover, the nanoantenna directivity is increased, proving that strong beaming effects can be obtained when the emitted radiation couples to the high Q-factor modes. Finally, via tuning the nanoanntenna aspect ratio, a selective control of the Er3+ electric and magnetic radiative transitions can be obtained, keeping the quantum efficiency almost unitary

Bidimensional ordered plasmonic nanoarrays for nonlinear optics, nanophotonics and biosensing applications

The capability to develop novel nanomaterials with properly tailored optical properties is of great interest for many nanophotonic applications. In the present work, 2D-ordered plasmonic nanoarrays with different morphologies, as nanoprism, nanoparticle, nanohole and semi-nanoshell arrays, are realized by nanosphere lithography, combining metal depositions, reactive ion etching and thermal treatments. By controlling the processing parameters, the plasmonic properties of the nanoarrays can be tailored, engineering the optical functionalities of the plasmonic nanosystems. Some selected examples are presented to show the potentialities of the synthesized nanoarrays in different fields: to realize ultra-fast and tunable nonlinear optical materials, to enhance the quantum efficiency of nearby rare-earth emitters and to develop high-sensitivity, label-free biosensors. The versatility of the fabrication technique gives the possibility to design different configurations and many other applications can be envisaged for these plasmonic nanoarrays in advanced nanophotonic devices