Leonel Marques

Thin-film optoacoustic transducers for subcellular Brillouin oscillation imaging of individual biological cells

At low frequencies ultrasound is a valuable tool to mechanically characterize and image biological tissues. There is much interest in using high-frequency ultrasound to investigate single cells. Mechanical characterization of vegetal and biological cells by measurement of Brillouin oscillations has been demonstrated using ultrasound in the GHz range. This paper presents a method to extend this technique from the previously reported single-point measurements and line scans into a high-resolution acoustic imaging tool. Our technique uses a three-layered metal-dielectric-metal film as a transducer to launch acoustic waves into the cell we want to study. The design of this transducer and measuring system is optimized to overcome the vulnerability of a cell to the exposure of laser light and heat without sacrificing the signal-to-noise ratio. The transducer substrate shields the cell from the laser radiation, efficiently generates acoustic waves, facilitates optical detection in transmission, and aids with heat dissipation away from the cell. This paper discusses the design of the transducers and instrumentation and presents Brillouin frequency images on phantom, fixed, and living cells.

High resolution 3D imaging of living cells with sub-optical wavelength phonons

Label-free imaging of living cells below the optical diffraction limit poses great challenges for optical microscopy. Biologically relevant structural information remains below the Rayleigh limit and beyond the reach of conventional microscopes. Super-resolution techniques are typically based on the non-linear and stochastic response of fluorescent labels which can be toxic and interfere with cell function. In this paper we present, for the first time, imaging of live cells using sub-optical wavelength phonons. The axial imaging resolution of our system is determined by the acoustic wavelength (λa = λprobe/2n) and not on the NA of the optics allowing sub-optical wavelength acoustic sectioning of samples using the time of flight. The transverse resolution is currently limited to the optical spot size. The contrast mechanism is significantly determined by the mechanical properties of the cells and requires no additional contrast agent, stain or label to image the cell structure. The ability to breach the optical diffraction limit to image living cells acoustically promises to bring a new suite of imaging technologies to bear in answering exigent questions in cell biology and biomedicine.

Optically excited nanoscale ultrasonic transducers

In order to work at higher ultrasonic frequencies, for instance, to increase the resolution, it is necessary to fabricate smaller and higher frequency transducers. This paper presents an ultrasonic transducer capable of being made at a very small size and operated at GHz frequencies. The transducers are activated and read optically using pulsed lasers and without physical contact between the instrumentation and the transducer. This removes some of the practical impediments of traditional piezoelectric architectures (such as wiring) and allows the devices to be placed immediately on or within samples, reducing the significant effect of attenuation which is very strong at frequencies above 1 GHz. The transducers presented in this paper exploit simultaneous optical and mechanical resonances to couple the optical input into ultrasonic waves and vice versa. This paper discusses the mechanical and optical design of the devices at a modest scale (a few μm) and explores the scaling of the transducers toward the sub-micron scale. Results are presented that show how the transducers response changes depending on its local environment and how the resonant frequency shifts when the transducer is loaded by a printed protein sample.

Sensitive protein detection using an optical fibre long period grating sensor anchored with silica core gold shell nanoparticles

An optical fibre long period grating (LPG), modified with a coating of silica gold (SiO2:Au) core/shell nanoparticles (NPs) deposited using the layer-by-layer (LbL) method, was employed for the development of a bio-sensor. The SiO2:Au NPs were electrostatically assembled onto the LPG with the aid of a poly(hydrochloride ammonium) (PAH) polycation layer. The LPG sensor operates at the phase matching turning point to provide the highest sensitivity. The SiO2:Au NPs were modified with biotin, which was used as a ligand for streptavidin (SV) detection. The sensing mechanism is based on the measurement of the refractive index change induced by the binding of the SV to the biotin. The lowest detected concentration of SV was 19 nM using an LPG modified with a 3 layer (PAH/SiO2:Au) thin film.

Design and fabrication of ultrasonic transducers with nanoscale dimensions

The development of nanometre sized ultrasonic transducers is important in both biological and industrial applications. The small size can be important in its own right or necessary in order to generate acoustic waves with nanometric wavelengths. Potential applications of nanotransducers range from embedded sensors through to sub optical wavelength acoustic imaging. In this paper, we show the generation and detection of ultra high frequency acoustic waves using nanometre scaled optical ultrasonic transducers. The optical and mechanical properties of these devices have been modelled using finite element modelling (FEM) and analytical techniques. The models allow the fine tuning of the design parameters to enhance both the acoustic and optical performance of the transducers. The devices were fabricated by evaporating the required metal and transparent layers onto a substrate, and then surface patterning of the device was created by laser machining or photolithography, thus allowing close comparison between model and experiment. We discuss the transducer design process and the effect of the coating parameters and how these affect the operating frequency and efficiency of the devices. We discuss the possibility of using molecular self assembly to produce even smaller devices.

CHOTs optical transducers

Laser ultrasonics conventionally use direct absorption in the sample to generate ultrasound and monitor the sample to detect the ultrasound. However, in some circumstances there are significant advantages to using an optical transducer – a device to facilitate the conversion of optical energy into acoustic energy or to facilitate the modulation of light by an acoustic wave. These devices are known as ‘cheap optical transducers’ (CHOTs). For some applications, they offer considerable advantages over conventional laser ultrasonics or conventional contact ultrasonics. In this paper, the design, operation and applications of CHOTs are discussed, and the concept is extended to the use of superCHOTs that can achieve amplitudes and sensitivities above the usual laser ultrasound limits, nano-scale devices capable of the generation and the detection of ultrasound with wavelengths smaller than that of visible light and devices capable of generating modes not normally accessible to laser ultrasound.

Novel Highly Sensitive Protein Sensors Based on Tapered Optical Fibres Modified with Au-Based Nanocoatings

Novel protein sensors based on tapered optical fibres modified with Au coatings deposited using two different procedures are proposed. Au-based coatings are deposited onto a nonadiabatic tapered optical fibre using (i) a novel facile method composed of layer-by-layer deposition consisting of polycation (poly(allylamine hydrochloride), PAH) and negatively charged SiO₂ nanoparticles (NPs) followed by the deposition of the charged Au NPs and (ii) the sputtering technique.The Au NPs and Au thin film surfaces are then modified with biotin in order to bind streptavidin (SV) molecules and detect them. The sensing principle is based on the sensitivity of the transmission spectrum of the device to changes in the refractive index of the coatings induced by the SV binding to the biotin. Both sensors showed high sensitivity to SV, with the lowest measured concentration levels below 2.5 nM. The calculated binding constant for the biotin-SV pair was 2.2×10‾¹¹ M‾¹ when a tapered fibre modified with the LbL method was used, with a limit of detection (LoD) of 271 pM. The sensor formed using sputtering had a binding constant of 1.01 × 10‾¹⁰ M‾¹ with a LoD of 806 pM. These new structures and their simple fabrication technique could be used to develop other biosensors.

Sound of nano

Ultrasound is widely used for imaging, measurement and diagnostics in the MHz region and is perhaps most familiar as a medical or non-destructive imaging or measurement tool. In the MHz frequency range the wavelength is typically measured in microns and is many times longer than the wavelength of visible light, limiting its resolution to objects much larger than the nano-scale. It is possible to perform ultrasonic imaging and measurement at much higher frequencies, in the GHz region. Here the acoustic wavelength is typically less than that of light permitting the higher resolutions than optical microscopy and the ability to probe micro and nano-scale objects. At these high frequencies ultrasonics has much to offer the nano-world as a powerful diagnostic tool: it could be used in circumstances where optical microscopy, electron microscopy and probe microscopy cannot, such as inside living objects. Despite the potential that ultrasonics offers for imaging and measurement at the micro and nano-scale, performing ultrasonics at the nano-scale is hampered by many problems that render the techniques typically used in the MHz region impractical. In this paper we discuss some of the practical problems standing in the way of nano-ultrasonics and some of the solutions, especially the use of pico-second laser ultrasonics and the development of nano-ultrasonic transducers and their application to ultrasonic imaging inside living cells.