Stuart A. Brittle

Detection of Volatile Organic Compounds Using Porphyrin Derivatives

Seven different porphyrin compounds have been investigated as colorimetric gas sensors for a wide range of volatile organic compounds. The porphyrins examined were the free base and Mg, Sn, Zn, Au, Co, and Mn derivatives of 5,10,15,20-tetrakis[3,4-bis(2-ethylhexyloxy)phenyl]-21H,23H-porphine. Chloroform solutions of these materials were prepared and changes in their absorption spectra induced by exposure to various organic compounds measured. The porphyrins that showed strong responses in solution were selected, and Langmuir-Blodgett films were prepared and exposed to the corresponding analytes. This was done to determine whether they are useful materials for solid state thin film colorimetric vapor sensors. Porphyrins that readily coordinate extra ligands are shown to be suitable materials for colorimetric volatile organic compound detectors. However, porphyrins that already have bound axial ligands when synthesized only show a sensor response to those analytes that can substitute these axial ligands. The Co porphyrin displays a considerably larger response than the other porphyrins investigated which is attributed to a switch between Co(II) and Co(III) resulting in a large spectral change.

Alkylamine Sensing Using Langmuir-Blodgett Films of n-Alkyl-N-phenylamide-Substituted Zinc Porphyrins

Two porphyrin compounds, zinc(II) 5,10,15,20-tetrakis(3,5,5-trimethyl- N-phenylhexanamide)porphyrin and zinc(II) 5,10,15,20-tetrakis(2,2-dimethyl- N-phenylpropanamide)porphyrin, have been investigated as possible candidates for the detection of alkylamines. UV-visible spectroscopy has shown that their solution absorption spectra are significantly modified upon interaction with a range of organic analytes, including acetic acid, butanone, ethylacetate, hexanethiol, octanal, octanol, alkylamines, and trimethylphosphite. Large spectral changes are observed for the family of alkylamines as a result of the specific affinity between zinc and the amine moiety. Langmuir-Blodgett (LB) films of the porphyrins have been fabricated in order to assess their solid-state sensing capability toward amines. The surface pressure-area (Pi- A) isotherms reveal a clear three-phase Langmuir film behavior and show that these monolayer films may be compressed to a relatively high surface pressure ( approximately 40-50 mN m (-1)). The isotherm data alongside molecular modeling suggest a relatively flat orientation of the porphyrin rings of both compounds: that is, a mutually parallel alignment of the plane of the porphyrin ring and that of the water surface. LB films deposited at 15 mN m (-1) have been exposed to alkylamine vapor (carried by N 2). A red shift and increase in intensity of the Soret band absorbance is observed which can be reversed by flowing pure N 2 over the gently heated sample (60 degrees C) after exposure. Primary amines were expected to invoke the greatest sensing response due to (i) their larger association constants with these porphyrins compared to secondary and tertiary amines and (ii) the ease of diffusion of amines which is expected to follow the order primary > secondary > tertiary due to the steric hindrance arising from the bulky secondary and tertiary amines. However, the magnitude of the absorbance change is largest for exposure to the secondary amines, dipropylamine and dibutylamine, for both porphyrins, compared to primary and tertiary amines. This trend follows that observed when the amines were added to solutions of the porphyrins. The rate of response of the porphyrin LB films falls as the molecular weight of the diffusing alkylamine increases. Furthermore, a greater rate of response is observed for the phenylhexanamide porphyrin compared to the phenylpropanamide porphyrin due to its lower molecular density within the LB film and therefore more porous structure.

Tuning free base tetraphenylporphyrins as optical sensing elements for volatile organic analytes

Changes in the visible absorption spectra of ten free base porphyrin compounds in solution and thin film form upon exposure to a range of volatile organic compounds have been investigated. The functional groups attached to the phenyl rings varied in their electron donating/withdrawing strength. These porphyrins were studied with a view to determining their suitability for use in the fabrication of an optical based gas sensor. It has been found that the sensitivity of these sensor materials depends upon the electron donating/withdrawing strength of the functional groups attached to the phenyl rings.

A chemical sensor based on a photonic-crystal L3 nanocavity defined in a silicon-nitride membrane

The application of a silicon-nitride based L3 optical nanocavity as a chemical sensor is explored. It is shown that by adjusting the thickness of an ultra-thin Lumogen Red film deposited onto the nanocavity surface, the fundamental optical mode undergoes a progressive red-shift as the layer-thickness increases, with the cavity being able to detect the presence of a single molecular monolayer. The optical properties of a nanocavity whose surface is coated with a thin layer of a porphyrin-based polymer are also explored. On exposure of the cavity to an acidic-vapour, it is shown that changes in the optical properties of the porphyrin-film (thickness and refractive index) can be detected through a reversible shift in the cavity mode wavelength. Such effects are described using a finite difference time-domain model.

Macroscopic expansion of tetraphenylporphyrin Langmuir layers stimulated by protonation

We report the relationship between the degree of areal expansion of floating Langmuir layers of tetraphenylporphyrins and their molecular structure, and show that their realisable expansion depends on the ratio of their experimental area per molecule (footprint) in the unexpanded film to their modelled molecular area when in the flat, face-down configuration in which the plane of the porphyrin ring is parallel to the plane of the water surface.

Physical Pendulum Experiments to Enhance the Understanding of Moments of Inertia and Simple Harmonic Motion.

This paper describes a set of experiments aimed at overcoming some of the difficulties experienced by students learning about the topics of moments of inertia and simple harmonic motion, both of which are often perceived to be complex topics amongst students during their first-year university courses. By combining both subjects in a discussion about physical pendula, in which the oscillation time periods for the periodic motion of several objects (a tennis ball, a thin beam, a hoop and a solid disc) are measured and compared, students are able to understand both topics at a higher level and also experience the synergistic effect of combining two or more physics themes in order to accelerate their learning whilst simultaneously raising their motivation. Special attention is given to the ‘ball and stick’ pendulum in which a block of material (treated as a point mass) can be moved along a shaft to create a composite pendulum whose time period exhibits a minimum value at a certain separation between the block and the rotation axis.

Macroscopic manifestation of a vapour-induced molecular switching phenomenon

We report a molecular switching event that leads to a reversible large-scale expansion of a floating porphyrin Langmuir layer residing at the air–water interface. This vapour-induced protonation–deprotonation mechanism, responsible for the expansion–contraction cycle, could potentially be exploited in applications in which a physical shape-change is triggered by exposure to an acidic or basic gas.

Development of an optical microscopy system for automated bubble cloud analysis.

Recently, the number of uses of bubbles has begun to increase dramatically, with medicine, biofuel production, and wastewater treatment just some of the industries taking advantage of bubble properties, such as high mass transfer. As a result, more and more focus is being placed on the understanding and control of bubble formation processes and there are currently numerous techniques utilized to facilitate this understanding. Acoustic bubble sizing (ABS) and laser scattering techniques are able to provide information regarding bubble size and size distribution with minimal data processing, a major advantage over current optical-based direct imaging approaches. This paper demonstrates how direct bubble-imaging methods can be improved upon to yield high levels of automation and thus data comparable to ABS and laser scattering. We also discuss the added benefits of the direct imaging approaches and how it is possible to obtain considerable additional information above and beyond that which ABS and laser scattering can supply. This work could easily be exploited by both industrial-scale operations and small-scale laboratory studies, as this straightforward and cost-effective approach is highly transferrable and intuitive to use.

Development of an optical microscopy system for automated bubble cloud analysis: publisher's note.

This note reports changes to the author list and additional funding sources for [Appl. Opt.55, 6102 (2016)].APOPAI0003-693510.1364/AO.55.006102.