Light incident on nanoscale metal-insulator-metal (MIM) plasmonic gratings generates surface plasmon polaritons (SPPs) which resonate and propagate within the grating structure. The SPP resonant wavelength can be altered by introducing a gradient in the width of the MIM grooves. Specifically, a symmetrically graded grating with a narrow central groove leads to a gradient in the effective refractive index such that the index increases in the direction of the central groove. This index gradient guides non-localized SPP waves towards the grating center. This waveguiding of SPPs along with localized SPP modes within the narrow central grooves give rise to multi-wavelength electric field enhancement at the grating center. SPP coupling across the grooves leads to an increase in electromagnetic field strength with decreasing groove width. However, standard nanofabrication techniques limit the minimum width of the grooves to approximately 50nm, preventing maximum field enhancement. Herein we report on the development of a novel nanoplasmonic graded grating with a 10 nm central groove flanked by increasingly wider grooves on either side, which are fabricated using thin film RF magnetron sputter deposition technique. These structures are studied using COMSOL Multiphysics modelling in which we vary the groove width, groove separation and groove depth, and thus demonstrate localization of broadband incident light. Raman spectroscopy and fluorescence microscopy are used to demonstrate field enhancement at several visible wavelengths.
We report on a new class of plasmonic nanogratings in which the gradient in the groove-width enables facile fabrication of multiwavelength surface-enhanced Raman spectroscopy (SERS) substrates. These substrates have the potential of achieving unprecedented detection sensitivity, specificity and speed. The structure of these nano-gratings consist of metal-insulator-metal grooves with a 40 nm central groove width flanked by a series of grooves on either side with gradually increasing width. Groove widths increase in steps of 5 nm up to a maximum width of 200 nm positioned farthest from the central groove on either side. The gradient in groove width in turn produces a gradient in the effective refractive index of the grating determined by the groove width at each location. Together, multiple laser wavelengths can be simultaneously confined to the centrally situated narrow grooves, with the neighbouring larger grooves guiding the nonlocalized waves toward the grating center from both directions. This generates a maximally enhanced plasmonic field over a broad range of wavelengths on the surface of the nanograting which can be used to increase the Raman scattering efficiency of a sample molecule distributed over the structure. The structures were fabricated using electron beam lithography, reactive ion etching, and sputter-deposition techniques. Experimental results demonstrated up to four orders of magnitude enhancement in the SERS intensity of 1 mM phospholipid samples deposited over the graded nano-gratings. In addition, characterization of the phospholipids in aqueous phase flowing over the nano-gratings integrated within a microfluidic device revealed that the Raman peaks were only detectable with the enhancement introduced by the grating. These results were obtained using 532, 638, and 785 nm lasers, demonstrating the multispectral sensing capability of the graded gratings for static and dynamic characterization of low concentration species.
Perfluorocarbon droplets containing nanoparticles (NPs) have recently been investigated as theranostic and dual-mode contrast agents. These droplets can be vaporized via laser irradiation or used as photoacoustic contrast agents below the vaporization threshold. This study investigates the photoacoustic mechanism of NP-loaded droplets using photoacoustic frequencies between 100 and 1000 MHz, where distinct spectral features are observed that are related to the droplet composition. The measured photoacoustic spectrum from NP-loaded perfluorocarbon droplets was compared to a theoretical model that assumes a homogenous liquid. Good agreement in the location of the spectral features was observed, which suggests the NPs act primarily as optical absorbers to induce thermal expansion of the droplet as a single homogenous object. The NP size and composition do not affect the photoacoustic spectrum; therefore, the photoacoustic signal can be maximized by optimizing the NP optical absorbing properties. To confirm the theoretical parameters in the model, photoacoustic, ultrasonic, and optical methods were used to estimate the droplet diameter. Photoacoustic and ultrasonic methods agreed to within 1.4%, while the optical measurement was 8.5% higher; this difference decreased with increasing droplet size. The small discrepancy may be attributed to the difficulty in observing the small droplets through the partially translucent phantom.
Perfluorocarbon droplets containing optical absorbing nanoparticles have been developed for use as theranostic agents
(for both imaging and therapy) and as dual-mode contrast agents. Droplets can be used as photoacoustic contrast agents,
vaporized via optical irradiation, then the resulting bubbles can be used as ultrasound imaging and therapeutic agents.
The photoacoustic signals from micron-sized droplets containing silica coated gold nanospheres were measured using
ultra-high frequencies (100-1000 MHz). The spectra of droplets embedded in a gelatin phantom were compared to a
theoretical model which calculates the pressure wave from a spherical homogenous liquid undergoing thermoelastic
expansion resulting from laser absorption. The location of the spectral features of the theoretical model and experimental
spectra were in agreement after accounting for increases in the droplet sound speed with frequency. The agreement
between experiment and model indicate that droplets (which have negligible optical absorption in the visible and
infrared spectra by themselves) emitted pressure waves related to the droplet composition and size, and was independent
of the physical characteristics of the optical absorbing nanoparticles. The diameter of individual droplets was calculated
using three independent methods: the time domain photoacoustic signal, the time domain pulse echo ultrasound signal,
and a fit to the photoacoustic model, then compared to the diameter as measured by optical microscopy. It was found the
photoacoustic and ultrasound methods calculated diameters an average of 2.6% of each other, and 8.8% lower than that
measured using optical microscopy. The discrepancy between the calculated diameters and the optical measurements
may be due to the difficulty in resolving the droplet edges after being embedded in the translucent gelatin medium.
An acoustic and photoacoustic characterization of micron-sized perfluorocarbon (PFC) droplets is presented. PFC
droplets are currently being investigated as acoustic and photoacoustic contrast agents and as cancer therapy agents.
Pulse echo measurements at 375 MHz were used to determine the diameter, ranging from 3.2 to 6.5 μm, and the sound
velocity, ranging from 311 to 406 m/s of nine droplets. An average sound velocity of 379 ± 18 m/s was calculated for
droplets larger than the ultrasound beam width of 4.0 μm. Optical droplet vaporization, where vaporization of a single
droplet occurred upon laser irradiation of sufficient intensity, was verified using pulse echo acoustic methods. The
ultrasonic backscatter amplitude, acoustic impedance and attenuation increased after vaporization, consistent with a
phase change from a liquid to gas core. Photoacoustic measurements were used to compare the spectra of three droplets
ranging in diameter from 3.0 to 6.2 μm to a theoretical model. Good agreement in the spectral features was observed
over the bandwidth of the 375 MHz transducer.
Computed tomography (CT) enables high resolution, whole-body imaging with excellent depth penetration. The
development of new targeted radiopaque CT contrast agents can provide the required sensitivity and localization for the
successful detection and diagnosis of smaller lesions representing earlier disease. Nanoscale, perfluorooctylbromide
(C8F17Br, PFOB) droplets have previously been used as untargeted contrast agents in X-ray imaging, and form the basis
of a promising new group of agents that can be developed for targeted CT imaging. For successful targeting to disease
sites, new PFOB droplet formulations tailored for ideal in vivo performance (e.g., biodistribution, toxicity, and
pharmacokinetics) must be developed. However, the direct assessment of PFOB agents in biological environments early
in their development is difficult using CT, as its sensitivity is not adequate for identification of single probes in vitro or
in vivo. In order to allow single droplet interactions with cells to be directly assessed using standard cellular imaging
tools, we integrate an optical marker within the PFOB agent. In this work, a new method to label a PFOB agent with
fluorescent quantum dot (QD) nanoparticles is presented. These composite PFOB-QD droplets loaded into macrophage
cells result in fluorescence on a cellular level that correlates well to the strong CT contrast exhibited in corresponding
tissue-mimicking cell pellets. QD loading within the PFOB droplet core allows optical labeling without influencing the
surface-dependent properties of the PFOB droplets in vivo, and may be used to follow PFOB localization from in vitro
cell studies to histopathology.