Rodney R. Agayan

Indocyanine-green-embedded PEBBLEs as a contrast agent for photoacoustic imaging.

Nanoparticles 100 nm in diameter containing indocyanine green (ICG) have been developed as a contrast agent for photoacoustic (PA) imaging based on (photonic explorers for biomedical use by biologically localized embedding PEBBLE) technology using organically modified silicate (ormosil) as a matrix. ICG is an FDA-approved dye with strong optical absorption in the near-infrared (NIR) region, where light can penetrate deepest into biological tissue. A photoacoustic imaging system was used to study image contrast as a function of PEBBLE concentration in phantom objects. ICG-embedded ormosil PEBBLEs showed improved stability in aqueous solution compared with free ICG dye. The particles were conjugated with HER-2 antibody for breast cancer and prostate cancer cell targeting. Initial in vitro characterization shows high contrast and high efficiency for binding to prostate cancer cells. ICG can also be used as a photosensitizer (generating toxic oxygen by illumination) for photodynamic therapy. We have measured the photosensitization capability of ICG-embedded ormosil nanoparticles. This feature can be utilized to combine detection and therapeutic functions in a single agent.

Optical trapping near resonance absorption

Expressions for radiation-induced forces are presented for the case of a Rayleigh particle near the focus of a Gaussian laser beam at near-resonant conditions. Classical electromagnetic theory was used to obtain the dependence of the scattering and gradient forces on the incident laser frequency, the beam convergence angle, and the spatial position of the particle with respect to the focus. Approximative numerical analysis performed for particles with a single resonant absorption peak demonstrates the occurrence of up to 50-fold enhanced trapping forces at near-resonant frequencies. The use of this technique of gradient force enhancement may provide optical tweezers with enhanced trapping strengths and a degree of specificity.

Single bacterial cell detection with nonlinear rotational frequency shifts of driven magnetic microspheres

Shifts in the nonlinear rotational frequency of magnetic microspheres, driven by an external magnetic field, offer a dynamic approach for the detection of single bacterial cells. We demonstrate this capability by optically measuring such frequency shifts when an Escherichia coli attaches to the surface of a 2.0μm magnetic microsphere, thereby affecting the drag of the system. From this change in drag, the nonlinear rotation rate was reduced, on average, by a factor of 3.8. Sequential bacterial cell attachments were also monitored.

Optical manipulation of metal-silica hybrid nanoparticles

Metallic nanoparticles are known to experience enhanced optical trap strengths compared to dielectric particles due to the increased optical volume, or polarizability. In our experience, larger metallic particles (~micron) are not easily trapped because momentum effects due to reflection become significant. Hybrid particles comprised of both metal and dielectric materials can circumvent this limitation while still utilizing a larger polarizability. Heterogeneous nanosystems were fabricated by embedding/coating silica nanoparticles with gold or silver in varying amounts and distributions. These compound particles were manipulated via optical tweezers, and their trapping characteristics quantitatively and qualitatively compared to homogeneous particles of comparable size. The parameters explored include the dependence of the trapping force on the percentage of loading of gold, the size of the embedded colloids, and the distribution of metal within the surrounding matrix or on its surface.

Slipping friction of an optically and magnetically manipulated microsphere rolling at a glass-water interface

The motion of submerged magnetic microspheres rolling at a glass-water interface has been studied using magnetic rotation and optical tweezers combined with bright field microscopy particle tracking techniques. Individual microspheres of varying surface roughness were magnetically rotated both in and out of an optical trap to induce rolling along either plain glass cover slides or glass cover slides functionalized with polyethylene glycol. It has been observed that the manipulated microspheres exhibited nonlinear dynamic rolling-while-slipping motion characterized by two motional regimes: At low rotational frequencies, the speed of microspheres free rolling along the surface increased proportionately with magnetic rotation rate; however, a further increase in the rotation frequency beyond a certain threshold revealed a sharp transition to a motion in which the microspheres slipped with respect to the external magnetic field, resulting in decreased rolling speeds. The effects of surface-microsphere interac...

Single Cell Detection with Driven Magnetic Beads

Shifts in the nonlinear rotational frequency of magnetic beads (microspheres) offer a new and dynamic approach for the detection of single cells. We present the first demonstration of this capability by measuring the changes in the nonlinear rotational frequency of magnetic beads driven by an external magnetic field. The presence of an Escherichia coli bacterium on the surface of a 2.0 μm magnetic bead affects the drag of the system, thus changing the nonlinear rotation rate. Measurement of this rotational frequency is straight-forward utilizing standard microscopy techniques.

Laser tweezing near resonance absorption

Expressions for radiation-induced forces are presented for the case of a Rayleigh particle at the focus of a Gaussian laser beam. Classical electromagnetic theory was used to obtain the dependence of the scattering and gradient forces on the incident laser frequency, beam convergence angle, and spatial position of the particle in the focus. Numerical analysis performed for particles with a single resonant absorption peak demonstrates the occurrence of enhanced trapping forces at near-resonant frequencies. An indication of the breakdown of the paraxial Gaussian approximation for large convergence angles is also shown by calculation of the scattering force. Use of this technique of gradient force enhancement may provide optical tweezers with enhanced trapping strengths and a degree of specificity.