Craig McDougall

Hybrid gold-iron oxide nanoparticles as a multifunctional platform for biomedical application

BackgroundIron oxide nanoparticles (IONPs) have increasing applications in biomedicine, however fears over long term stability of polymer coated particles have arisen. Gold coating IONPs results in particles of increased stability and robustness. The unique properties of both the iron oxide (magnetic) and gold (surface plasmon resonance) result in a multimodal platform for use as MRI contrast agents and as a nano-heater.ResultsHere we synthesize IONPs of core diameter 30 nm and gold coat using the seeding method with a poly(ethylenimine) intermediate layer. The final particles were coated in poly(ethylene glycol) to ensure biocompatibility and increase retention times in vivo. The particle coating was monitored using FTIR, PCS, UV–vis absorption, TEM, and EDX. The particles appeared to have little cytotoxic effect when incubated with A375M cells. The resultant hybrid nanoparticles (HNPs) possessed a maximal absorbance at 600 nm. After laser irradiation in agar phantom a ΔT of 32°C was achieved after only 90 s exposure (50 μgmL-1). The HNPs appeared to decrease T2 values in line with previously clinically used MRI contrast agent Feridex®.ConclusionsThe data highlights the potential of these HNPs as dual function MRI contrast agents and nano-heaters for therapies such as cellular hyperthermia or thermo-responsive drug delivery.

Laser-nucleated acoustic cavitation in focused ultrasound

Acoustic cavitation can occur in therapeutic applications of high-amplitude focused ultrasound. Studying acoustic cavitation has been challenging, because the onset of nucleation is unpredictable. We hypothesized that acoustic cavitation can be forced to occur at a specific location using a laser to nucleate a microcavity in a pre-established ultrasound field. In this paper we describe a scientific instrument that is dedicated to this outcome, combining a focused ultrasound transducer with a pulsed laser. We present high-speed photographic observations of laser-induced cavitation and laser-nucleated acoustic cavitation, at frame rates of 0.5×10(6) frames per second, from laser pulses of energy above and below the optical breakdown threshold, respectively. Acoustic recordings demonstrated inertial cavitation can be controllably introduced to the ultrasound focus. This technique will contribute to the understanding of cavitation evolution in focused ultrasound including for potential therapeutic applications.

Directed jetting from collapsing cavities exposed to focused ultrasound

We demonstrate directed jetting from pulsed laser-induced cavities subjected to a burst of focused ultrasound. Alignment of the ultrasound focus and the pressure amplitudes in the vicinity of the cavity dictate the direction and length of the resulting jet, respectively. We interpret our observations in terms of radiation forces exerted on the cavity, due to the pressure gradient introduced to the ultrasound focus by its presence. We support our hypothesis with a linear analysis of the force distribution across the cavity surface, at the moment of maximum inflation, which shows reasonable predictive agreement with the observed jet characteristics.

Characterizing conical refraction optical tweezers

Conical refraction occurs when a beam of light travels through an appropriately cut biaxial crystal. By focusing the conically refracted beam through a high numerical aperture microscope objective, conical refraction optical tweezers can be created, allowing for particle manipulation in both Raman spots, and in the Lloyd/Poggendorff rings. We present a thorough quantification of the trapping properties of such a beam, focusing on the trap stiffness, and how this varies with trap power and trapped particle location. We show that the lower Raman spot can be thought of as a single-beam optical gradient force trap, while radiation pressure dominates in the upper Raman spot, leading to optical levitation rather than trapping. Particles in the Lloyd/Poggendorff rings experience a lower trap stiffness than particles in the lower Raman spot, but benefit from rotational control.

Flexible particle manipulation techniques with conical refraction-based optical tweezers

We present an optimized optical tweezers system based upon the conical refraction of circularly polarized light in a biaxial crystal. The described optical arrangement avoids distortions to the Lloyd plane rings that become apparent when working with circularly polarized light in conventional optical tweezers. We demonstrate that the intensity distribution of the conically diffracted light permits optical manipulation of high and low refractive index particles simultaneously. Such trapping is in three dimensions and not limited to the Lloyd plane rings. By removal of a quarter waveplate the system also permits the study of linearly polarized conical refraction. We show that particle position in the Raman plane is determined by beam power, and indicates that true optical tweezing is not taking place in this part of the beam.

Acoustic trapping in bubble-bounded micro-cavities

Abstract We present a method for controllably producing longitudinal acoustic trapping sites inside microfluidic channels. Air bubbles are injected into a micro-capillary to create bubble-bounded ‘micro-cavities’. A cavity mode is formed that shows controlled longitudinal acoustic trapping between the two air/water interfaces along with the levitation to the centre of the channel that one would expect from a lower order lateral mode. 7 μm and 10 μm microspheres are trapped at the discrete acoustic trapping sites in these micro-cavities.We show this for several lengths of micro-cavity.

HoloHands: games console interface for controlling holographic optical manipulation

The increased application of holographic optical manipulation techniques within the life sciences has sparked the development of accessible interfaces for control of holographic optical tweezers. Of particular interest are those that employ familiar, commercially available technologies. Here we present the use of a low cost games console interface, the Microsoft Kinect for the control of holographic optical tweezers and a study into the effect of using such a system upon the quality of trap generated.