M. Cather Simpson

Phosphorus as a carbon copy and as a photocopy: New conjugated materials featuring multiply bonded phosphorus*

Phosphaalkenes (RP=CR2) and diphosphenes (RP=PR) are main group analogues of alkenes (R2C=CR2). Molecules featuring such multiply bonded phosphorus functionalities often display structural features and chemical reactivities that mimic their purely organic counterparts, lending credence to the claim that these compounds are “carbon copies”. We have been expanding this analogy to include oligomers and polymers with extended conjugation that directly involve P=C and P=P units. Many of these materials, however, display little or no photoluminescence (PL). This article summarizes our efforts to understand P=C and P=P photobehavior and to produce materials having significant PL that mimic or “photocopy” the PL properties of the phosphorus-free systems. Recent materials based on benzoxaphospholes (BOPs), benzobisoxaphospholes (BBOPs), and higher analogues having significant fluorescence quantum yields are covered.

Imaging deep skeletal muscle structure using a high-sensitivity ultrathin side-viewing optical coherence tomography needle probe

We have developed an extremely miniaturized optical coherence tomography (OCT) needle probe (outer diameter 310 µm) with high sensitivity (108 dB) to enable minimally invasive imaging of cellular structure deep within skeletal muscle. Three-dimensional volumetric images were acquired from ex vivo mouse tissue, examining both healthy and pathological dystrophic muscle. Individual myofibers were visualized as striations in the images. Degradation of cellular structure in necrotic regions was seen as a loss of these striations. Tendon and connective tissue were also visualized. The observed structures were validated against co-registered hematoxylin and eosin (H&E) histology sections. These images of internal cellular structure of skeletal muscle acquired with an OCT needle probe demonstrate the potential of this technique to visualize structure at the microscopic level deep in biological tissue in situ.

Screening for Adulterants in Liquid Milk Using a Portable Raman Miniature Spectrometer with Immersion Probe

A portable Raman system with an immersion fiber optic probe was assessed for point-of-collection screening for the presence of adulterants in liquid milk. N-rich adulterants and sucrose were measured in this proof-of-concept demonstration. Reproducibility, limit of detection range and other figures of merit such as specificity, sensitivity, ratio of predicted to standard deviation, standard error of prediction and root mean squared error for cross validation were determined from partial least squares (PLS) and partial least squares with discriminant analysis (PLS-DA) calibrations of milk mixtures containing 50–1000 ppm (parts per million) of melamine, ammonium sulphate, Dicyandiamide, urea and sucrose. The spectra were recorded by immersing the fiber optic probe directly in the milk solutions. Despite the high scattering background which was easily and reliably estimated and subtracted, the reproducibility for four N-rich compounds averaged to 11% residual standard deviation (RSD) and to 5% RSD for sucrose. PLS calibration models predicted the concentrations of separate validation sets with standard errors of prediction of between 44 and 76 ppm for the four N-rich compounds and 0.17% for sucrose. The sensitivity and specificity of the PLS-DA calibration were 92% and 89%, respectively. The study shows promise for use of portable mini Raman systems for routine rapid point-of-collection screening of liquid milk for the presence of adulterants, without the need for sample preparation or addition of chemicals.

Critical setup parameter for ultrafast whitelight coherent antistokes raman scattering spectroscopy of living plankton in sea water

The health and growth rates of shellfish depends critically on the precise chemical and biological makeup of the surrounding seawater. Rapid and continuous sensing of the marine environment enables improved harvests and better profits. However, underwater spectroscopy is still in its infancy. Our research aim is to develop a new sensor that is well-suited for marine environments and one that is capable of working at depths of up to 20 metres. We discuss a modified high intensity femtosecond coherent anti-Stokes Raman Scattering (CARS) system, which removes the usual limitations such as a limited wavenumber range and a low signal-to-noise ratio (SNR) due to non-resonant background. A particular set of Probe and Stokes pulses aim to investigate the spectra of various living plankton in seawater. It also determines critical parameters for the underwater CARS system such as signal dependent on the pulse energies, vibrational dephasing times and numerical aperture dependencies in forward CARS. This links to theory and greatly expands the understanding of CARS parameters for future underwater CARS experiments.

Diboron Porphyrins: The Raman Signature of the In-Plane Tetragonal Elongation of the Macrocycle

We describe an unusual in-plane type of porphyrin core distortion, tetragonal elongation (TE), observed experimentally in diboron porphyrins. The vibrational spectra of several of these complexes exhibit shifts that we have assigned to this TE distortion by comparing experimental spectra with DFT computational findings. The influence of TE in porphyrin systems was isolated using DFT analysis of the well-known model compounds Ni(II)porphine and Zn(II)porphine, with the macrocycle ring constrained to eliminate the influence of out-of-plane (OOP) distortions. A significant down-shift in frequencies was observed for porphyrin normal vibrational modes, particularly the in-plane A1g/B1g modes that are dominated by contributions from stretching and bending of Cα-Cm coordinates. In contrast, TE had little effect on the v(Pyrhalfring) and δ(Pyrdef) modes, though the lowered symmetry of the system resulted in significant splitting of the B2u and B3u modes. The impact of the TE distortion upon the diboron porphyrin vibrational spectrum was probed experimentally using Raman spectroscopy of B2O2(BCl3)2(TTP), B2OF2(TTP), and B2OPhOH2(TTP) (TTP = 5,10,15,20-(tetra- p-tolyl)porphyrin). Comparing the experimentally obtained spectral signatures to the computational findings allowed us to assign the large shifts observed for the v2 and v3 modes to the TE distortion in diboron porphyrins.

Ultrafast pulsed Bessel beams for enhanced laser ablation of bone tissue for applications in LASSOS

Using a femtosecond pulsed laser system (pulse width = 100fs, repetition rate = 500 Hz, λ=800nm), a zero-order Bessel beam was generated using a LCOS-Spatial light modulator (LCOS-SLM) with an effective cone angle of 4.56°. Ablation threshold studies of fresh bovine and ovine load bearing cortical bone was identified using the method of least damage and found to be identical at φth = 0.15 ± 0.03 J cm-2, irrespective of the target species. The ablation threshold is significantly reduced compared to the ablation threshold determined for Gaussian beams in bovine and ovine cortical bone (Load Bearing: φth = 0.91 ± 0.03 J cm-2, Skull: φth = 1.19 ± 0.06 J cm-2). Incubation effects were investigated and the incubation coefficient was determined to be ζ = 0.93 ± 0.06, indicating no incubation effects are present. The relationship between tissue removal and the number of pulses applied was explored. By altering the translation rate of the sample under the Bessel region of the incident laser, the number of pulses applied at each point along the linear ablation features was varied. Cross sections of ablation features were measured using scanning electron microscopy (SEM) and maximum depths of the ablation features measured. The ablation rate of bovine and ovine cortical was found to be 2.69 – 13.21 ± 0.05 μm pulse-1 and 2.49 – 12.79 ± 0.03 μm pulse-1 respectively for fluence values ranging from 25.0 – 2.5 Jcm-2, significantly higher than those of Gaussian beams. Structural analysis of the ablation features using SEM and optical microscopy showed no signs of heat affected zone (HAZ) in the form of thermal shockwave cracking, molten debris deposition or charring of the tissue.

Ultrafast laser patterning and defect generation in titania nanotubes for the enhancement of optical and photocatalytic properties

This study demonstrates the first known instance of the templating of titanium dioxide nanotube arrays by laser induced periodic surface structures (LIPSS) and subsequent electrochemical anodization. Titanium dioxide is an established photocatalyst, however it suffers from poor visible light absorption, thus limiting its use under solar irradiation. Thermal annealing can enhance the visible light absorption, with the downside of introducing more defect traps that reduce the lifetime of the charge separated state. Hence, this study proposes an alternative to chemical methods, by modulating the surface profile of the nanotube array to trap visible light. The enhanced visible light absorption is predicted via computational modelling and the morphological evolution of the anodization process was investigated. This study provides the basis for further work into LIPSS templating of other anodized transition metal oxide materials.

Rapid, sensitive and reproducible method for point-of-collection screening of liquid milk for adulterants using a portable Raman spectrometer with novel optimized sample well

Point-of-care diagnostics are of interest in the medical, security and food industry, the latter particularly for screening food adulterated for economic gain. Milk adulteration continues to be a major problem worldwide and different methods to detect fraudulent additives have been investigated for over a century. Laboratory based methods are limited in their application to point-of-collection diagnosis and also require expensive instrumentation, chemicals and skilled technicians. This has encouraged exploration of spectroscopic methods as more rapid and inexpensive alternatives. Raman spectroscopy has excellent potential for screening of milk because of the rich complexity inherent in its signals. The rapid advances in photonic technologies and fabrication methods are enabling increasingly sensitive portable mini-Raman systems to be placed on the market that are both affordable and feasible for both point-of-care and point-of-collection applications. We have developed a powerful spectroscopic method for rapidly screening liquid milk for sucrose and four nitrogen-rich adulterants (dicyandiamide (DCD), ammonium sulphate, melamine, urea), using a combined system: a small, portable Raman spectrometer with focusing fibre optic probe and optimized reflective focusing wells, simply fabricated in aluminium. The reliable sample presentation of this system enabled high reproducibility of 8% RSD (residual standard deviation) within four minutes. Limit of detection intervals for PLS calibrations ranged between 140 - 520 ppm for the four N-rich compounds and between 0.7 - 3.6 % for sucrose. The portability of the system and reliability and reproducibility of this technique opens opportunities for general, reagentless adulteration screening of biological fluids as well as milk, at point-of-collection.

Enhancement of laser machining resolution using the two photon absorption effect

One of the main strengths of ultrafast laser machining is its ability to produce very high resolution cuts. This is useful in a range of fields for manufacturing purposes, however being able to incorporate two photon absorption (TPA) to further enhance this would allow even smaller fabrication. In this study we look at the effect of doping amorphous polycarbonate with two different molecules (R1 and R2) which have been identified to have a high TPA cross-section. Using the diameter regression (D2) technique, we observe a notable decrease in the ablation thresholds as dopant concentration increases from 0 – 10%. In addition, these results provide a measurement of effective beam waist (ωeff). Beam waist values for R2 doped samples show a decrease to half the reference, which can be linked to an increase in the occurrence of multiphoton absorption. Linear ablation features produced at identical peak powers and pulse numbers show improved machining resolution as dopant concentration increases. This increase in resolution is more prolific for R2 than R1 despite their similar TPA cross sections. We believe that this is due to the relative bandgaps of the materials causing three photon absorption for R2 whereas multiphoton absorption is limited to the second order process for R1. This means by doping appropriate materials with these molecules it is possible to improve resolution whilst maintaining most properties of the main material.

Investigating parylene-HT as a substrate for human cell patterning

We demonstrate, for the first time, how parylene-HT on SiO2 substrates can be used as a human cell patterning platform. We demonstrate this platform with hNT astrocytes, derived from the human NTera2.D1 cell line. We show how hNT astrocytes are attracted to Parylene-HT and repelled by the SiO2 and are shown to adopt a similar morphology as that attained on standard tissue culture polystyrene. Furthermore, parylene-HT was capable of patterning the astrocytes achieving a ratio of 8:1 for cells on parylene compared to SiO2. Thus, as parylene-HT has similar physical properties to parylene-C with the addition of UV and thermal resistance, parylene-HT represents a desirable alternative substrate for human cell patterning.