Timothée Toury

SERS detection of biomolecules using lithographed nanoparticles towards a reproducible SERS biosensor.

In this paper we highlight the accurate spectral detection of bovine serum albumin and ribonuclease-A using a surface-enhanced Raman scattering (SERS) substrate based on gold nanocylinders obtained by electron-beam lithography (EBL). The nanocylinders have diameters from 100 to 180 nm with a gap of 200 nm. We demonstrate that optimizing the size and the shape of the lithographed gold nanocylinders, we can obtain SERS spectra of proteins at low concentration. This SERS study enabled us to estimate high enhancement factors (10(5) for BSA and 10(7) for RNase-A) of important bands in the protein Raman spectrum measured for 1 mM concentration. We demonstrate that, to reach the highest enhancement, it is necessary to optimize the SERS signal and that the main parameter of optimization is the LSPR position. The LSPR have to be suitably located between the laser excitation wavelength, which is 632.8 nm, and the position of the considered Raman band. Our study underlines the efficiency of gold nanocylinder arrays in the spectral detection of proteins.

Resonances of individual lithographic gold nanowires in the infrared

With infrared spectroscopic microscopy using synchrotron radiation, we systematically studied resonant light scattering from electron-beam lithographically produced gold nanowires (nanostripes) with diameters in the 100 nm range and with various lengths below 1 to about 2.5 μm. Similar to electrochemically grown cylindrical wires of high crystalline quality, clear antennalike plasmon resonances were observed for these stripelike and less-perfect wires. The resonance wavelength shifts with length as theoretically predicted for cylindrical gold antennas in the optical range. Surprisingly, also the extinction cross section of the nanostripes is equal to that measured for highly crystalline cylinders.

Design of nanostructures for imaging and biomedical applications by plasmonic optimization

An adapted method of optimization of coated metallic nanoparticles is introduced to perform the optimal choice of material and sizes for better scattering or absorption efficiency. This design of nanoshells, involving plasmon resonance, is achieved to maximize the efficiency factors. The presented method is turned to tune the efficiency of nanoshells for biomedical applications and an increasing of the efficiency factors by 1 or 2 orders of magnitude is predicted with realistic materials.

Optimized plasmonic nanostructures for improved sensing activities.

The paper outlines the optimization of plasmonic nanostructures in order to improve their sensing properties such as their sensitivity and their ease of manipulation. The key point in this study is the optimization of the localized surface plasmon resonance (LSPR) properties essential to the sensor characteristics, and more especially for surface-enhanced Raman scattering (SERS). Two aspects were considered in order to optimize the sensing performance: apolar plasmonic nanostructures for non polarization dependent detection and improvements of SERS sensitivity by using a molecular adhesion layer between gold nanostructures and glass. Both issues could be generalized to all plasmon-resonance-based sensing applications.

Fast and reliable fabrication of gold tips with sub-50 nm radius of curvature for tip-enhanced Raman spectroscopy

We have developed a new electrochemical etching procedure to fabricate gold tips with sub-50 nm apical radius of curvature with a production yield of 80% and production time lower than 5 min. The technique is based on a two-step self-terminating process in which a gold wire is first quickly (<1 min) pre-etched in an hydrochloric acid (HCl)∕ethanol solution at high voltage (10 VDC), and then slowly (2-4 min) etched at lower voltages (<2.5 VDC). The first step occurs under intense bubbling conditions and allows us to thin rapidly the wire. This reduces the time required by subsequent low-voltage process during which the tips are formed at the liquid∕air interface. A statistical analysis of the surface morphology has been carried out on a set of 60 tips by scanning electron microscopy. The results show that the surface roughness and the sharpness of the final tip are critically influenced by the intrinsic granularity of the gold wires. Moreover, there is a correlation between the tip quality and the time elapse required to complete the low-voltage etching step. Tips featuring smooth surfaces and radii of curvature <50 nm are produced whenever the etching times are lower than 250 s, while etching times larger than 300 s typically yield rough, blunt tips. Such a correlation can be used as a screening criterion to select sharp tips during production with an 80% yield. The high quality of the gold tips produced with such method is confirmed by the electromagnetic field enhancement measured both in tip-enhanced Raman scattering and surface-enhanced Raman scattering on the tip apex experiments.

New Gold Nanoparticles Adhesion Process Opening the Way of Improved and Highly Sensitive Plasmonics Technologies

Gold nanostructures have very suitable physical properties for plasmonic applications but do not stick on glass substrates. One usually uses a chromium adhesion layer that gives good mechanical adhesion but quench the plasmon. We developed a new adhesion process that permits a covalent bonding between gold and glass thanks to an MPTMS molecular layer throughout nanolithography process. We demonstrate that this new adhesion layer allows an improvement of the optical properties of the gold nanoparticles as well as an essential improvement of their surface-enhanced Raman scattering performances.

Comparison of adhesion layers of gold on silicate glasses for SERS detection

Gold is one of the most widely used metals for building up plasmonic devices. Although slightly less efficient than silver for producing sharp resonance, its chemical properties make it one of the best choices for designing sensors. Sticking gold on a silicate glass substrate requires an adhesion layer, whose effect has to be taken into account. Traditionally, metals (Cr or Ti) or dielectric materials (TiO2 or Cr2O3) are deposited between the glass and the nanoparticle. Recently, indium tin oxide and (3-mercaptopropyl)trimethoxysilane (MPTMS) were used as a new adhesion layer. The aim of this work is to compare these six adhesion layers for surface-enhanced Raman scattering sensors by numerical modeling. The near-field and the far-field optical responses of gold nanocylinders on the different adhesion layers are then calculated. It is shown that MPTMS leads to the highest field enhancement, slightly larger than other dielectric materials. We attributed this effect to the lower refractive index of MPTMS compared with the others.

Novel Apolar Plasmonic Nanostructures with Extended Optical Tunability for Sensing Applications

This paper outlines the design of complex nanostructures with apolar behavior which pave the way to a wider range of plasmon resonance tuning and applications requiring higher enhancement. These new nanostructure families are simply defined by symmetry considerations. An irreducible decomposition of optical response tensor demonstrates that nanoparticles which belong to Cn, with n ≥ 3, symmetry point group for at least one scale have an optical response insensitive on the light polarization. This is experimentally confirmed by extinction and surface-enhanced Raman-scattering measurements.

Fabrication of an atrazine acoustic immunosensor based on a drop-deposition procedure

Among the various novel analytical systems, immunosensors based on acoustic waves are of emerging interest because of their good sensitivity, real-time monitoring capability, and experimental simplicity. In this work, piezoelectric immunosensors were constructed for the detection of atrazine through the immobilization of specific monoclonal anti-atrazine antibodies on thiolated modified quartz crystal microbalances (QCMs). The immunoassay was conducted by a novel drop-deposition procedure using different atrazine dilutions in phosphate buffer solution ranging from 10-10 to 10-1 mg/mL. The immunoreactions between varying contents of atrazine and its antibody were dynamically exhibited through in situ monitoring of the frequency and motional resistance changes over 20 min. Thus, atrazine recognition by the anti-atrazine antibody leads to a decrease of the resonant frequency that is proportional to a given atrazine concentration. Interestingly, the motional resistance also increased proportionally during the measurements, which could be attributed to the specific viscoelastic properties and/or conformation changes of the antibodies once the immunoreactions occurred. By combining the measurements of frequency with those of motional resistance, additional information was provided about the interaction between the atrazine-named antigen and its respective antibody. Finally, the analytical specificity of the immunosensor to atrazine was evaluated through the response to a nonspecific anti-human IgG antibody-modified QCM crystal under the same drop conditions.

Surface enhanced infrared absorption by nanoantenna on chalcogenide glass substrates

In recent years, nanowires have been proven efficient to enhanced IR absorption of molecules and opened prospects of new ultrasensitive IR sensors. The development of integrated components requires the use of special IR glasses such as chalcogenide or silver halide glasses. In this study, we report the fabrication of a surface enhanced IR absorption substrate composed of nanowires deposited onto a chalcogenide glass slide. It enabled us to detect 4-nitrophenol at the femtomolar level and enhancement factor close to 106 was calculated.