Derrick Yong

In-fiber fluorospectroscopy based on a spectral decomposition method

We report a simplified model for the computation of light-fluorescence interactions within photonic crystal fibers (PCFs). It involved the plotting of ray trajectories confined by total internal reflection within a geometrically simplified PCF core. This was followed by the calculation of absorption and fluorescence emission at each point of reflection, which were subsequently summed and averaged over all the launched rays. The computation of these components for two specified wavelengths (peak excitation and emission) produced a dimensionless ratiometric relationship for varying concentrations of fluorescence dye. This hence eliminated the need for optical filters and minimized the effects of intensity fluctuations. Modeled results were demonstrated to concur well with that obtained experimentally for two PCFs with different microstructured cores.

In-fiber photo- immobilization of a bioactive surface

Abstract. We demonstrate the first in-fiber light-induced bioactive biotin-functionalization via photobleaching fluorophore-conjugated biotin. Photobleaching the fluorophores generated free radicals that bind to the albumin-passivated inner surface of pure silica photonic crystal fiber. The subsequent attachment of dye-conjugated streptavidin to the bound biotin qualified the photo-immobilization process and demonstrated a potential for the construction of in-fiber macromolecular assemblies or multiplexes. Compared with other in-fiber bioactive coating methods, the proposed light-induced technique requires only a low-power light source, without the need for additional preactivation steps or toxic chemical reagents. This method, hence, enables a simple and compact implementation for potential biomedical applications.

Activation and inactivation of Bacillus pumilus spores by kiloelectron volt X-ray irradiation

In this study, we investigated the inactivation efficacy of endospore-forming bacteria, Bacillus pumilus, irradiated by low-energy X-rays of different beam qualities. The different low-energy X-rays studied had cut-off energies of 50, 100 and 150 keV. Bacillus pumilus spores (in biological indicator strips) were irradiated at step doses between 6.5 to 390 Gy. The resulting bacteria populations were then quantified by a pour plate method. Results showed that X-rays of lower energies were more effective in inactivating bacterial spores. In addition, an increment in bacterial population was observed at doses below 13Gy. We attributed this increase to a radiation-induced activation of bacterial spores. Four kinetic models were then evaluated for their prediction of bacterial spore behavior under irradiation. This included: (i) first-order kinetics model; (ii) Shull model; (iii) Sapru model; and (iv) probabilistic model. From R2 and AIC analyses, we noted that the probabilistic model performed the best, followed by the Sapru model. We highlighted that for simplicity in curve fitting the Sapru model should be used instead of the probabilistic model. A 12-log reduction in bacterial population (corresponding to a sterility assurance level of 10−6 as required in the sterilization of medical devices) was computed to be achievable at doses of 1000, 1600 and 2300 Gy for the three different X-ray cut-off energies respectively. These doses are an order in magnitude lesser than that required in gamma irradiation. This highlights the applicability of cheaper and safer table-top X-ray sources for sterilization application.

Two-Phase Photobleaching Dequenching in Dye-Loaded Liposomes

A two-phase fluorescence dequenching was for the first time demonstrated in dye-loaded liposomes upon exposure to a prolonged duration of excitation. Liposomes encapsulating self-quenching concentrations (100 mM) of carboxyfluorescein were subjected to illumination under a 405 nm continuous-wave laser. Emission spectra collected over the span of exposure exhibited a peculiar behavior--two consecutive hikes in peak emission over time that were subsequently attributed to photobleaching induced dequenching. A mathematical model considering the effects of photobleaching on fluorescence quantum yield and absorption then accounted for the observed peaking. Specifically, the dequenching of dye in the outermost and bulk inner regions of the core, were ascribed to the first (fast, primary) and second (slow, secondary) peaking event, correspondingly. The occurrence of the latter was attributed to the combined effects of lower photobleaching rates and anomalous diffusion, stemming to a slower dequenching event. Results not only provided insight to future application of these liposomes in random lasing, but also highlight a potential method of optically tuning the extent of self-quenching.

Plasmonic-enhanced fluorescence emission using D-shape microstructured optical fiber

Highly sensitive side-polished D-shaped optical fiber sensors have been fabricated based on surface plasmon resonance (SPR) technology. Current techniques in plasmonic-enhanced total internal reflection microscopy (TIRM) and evanescent wave microscopy, although advantageous, require cumbersome set-ups and encompass large coupling losses. A gold coated D-shaped optical fiber was demonstrated to provide fluorescence enhanced spectroscopy. Comparison was made between a gold coated and uncoated D-shaped microstructured optical fiber (MOF) with respect to excitation of Rhodamine B (Rh B). Results highlighted improved fluorescence emission intensity and heightened sensitivity in fluorescence spectroscopy in the gold coated device, indicating potential in enhanced bio-imaging applications.

Fluorospectroscopy of Dye-Loaded Liposomes in Photonic Crystal Fibers

The immobilization and probing of liposomes within photonic crystal fibers was demonstrated for the first time. A bioactive surface was used to tether the liposomes. This bioactive surface consisted of streptavidin bound to a photochemically functionalized biotin layer. Bound streptavidin, hence, enabled the further binding of biotinylated dye-loaded liposomes. In-fiber fluorescence spectroscopy was used to quantify the streptavidin coverage density. The same method was also used to characterize the surface-tethered liposomes. The further observation of a unique phenomenon-photobleaching dequenching-was used for the first time as an indication of liposomal content retention. This indicated no rupturing of liposomes, highlighting them as bioderived analogues to dye-doped nanoparticles. The demonstrated integration of liposomes with optical waveguides shows potential as a biointegrated photonic device.

Photonic crystal fiber integrated microfluidic chip for highly sensitive real-time chemical sensing

Photonic crystal fibers (PCFs), although a highly effective platform for sensing, encounter difficulties with coupling as well as infiltration and evacuation. A PCF integrated microfluidic chip has therefore been fabricated to demonstrate improved coupling for real-time chemical sensing. Furthermore, an extremely sensitive dip-shifting analysis was employed for the detection regime. Results eventually demonstrated its notable sensitivity and a refractive index resolution of 10-7 RIU, rendering it suitable for utilization in highly sensitive sensing applications.

Lab-in-fiber platform for plasmonic photothermal study

A lab-in-fiber platform, comprising a photonic crystal fiber component for light-sample interaction, was experimentally demonstrated to be effective as a sensor and micro-reactor. Specifically, it enabled the discrimination between free and liposome-encapsulated fluorophores as well as allowed for the excitation of in-fiber plasmonic photothermal effects, by alternating between different fiber-coupled inputs. The significant increase in fluorescence emissions upon release of fluorophores, encapsulated within liposomes at self-quenching concentrations, was perceived as a shoulder in the device’s spectral output that otherwise only comprises the input excitation. Markedly, the observed shoulder was only discernible when the photonic crystal fiber was placed in a bent orientation. This was explained to be associated with the bending-induced refractive index profile changes in the fiber cross section that led to increased amounts of evanescent fields for light-sample interactions. Results highlighted the viability of the lab-in-fiber platform as an alternative to current lab-on-a-chip devices.

Side Excitation of Dye/Liposome-Filled Hollow Core Photonic Crystal Fibers

Side excitation of hollow-core photonic crystal fibers infiltrated with dye or dye-liposomal mixture was studied. It was found that there is a redshift in the emission spectra as excitation length increases due to reabsorption.

Photonic Bandgap Fiber for Infiltration-Free Refractive-Index Sensing

We report a novel photonic bandgap sensor, comprising a D-shaped fiber fabricated via side polishing. It takes advantage of measurand-elicited refractive-index profile changes upon exposure of its side-polished region to an ambient liquid. The bandgap shift can be induced by perturbations in close proximity to the fiber core, such as the change of refractive index in the surrounding. The side-polished all-solid photonic bandgap fiber utilized offers an infiltration-free sensing mechanism, which eliminates difficult air-hole infiltration and evacuation processes experienced by most photonic crystal fiber-based index sensors. Experimental results demonstrated the repeatability and linearity in refractive-index sensing from 1.31 to 1.43 with a sensitivity of 10 -5 refractive-index units (RIUs). Additional study was also conducted to identify the significance of polishing depths in relation to the sensor performance.