Supplementary Material (ESI) for Journal of Materials Chemistry
This journal is © The Royal Society of Chemistry 2001
Electronic Supplementary Information
Fast, reversible optical sensing of NO2 using
5,10,15,20-tetrakis(3,4-bis[2-ethylhexyloxy]phenyl)21H,23H-porphine assemblies
Colin M Dooling, Oliver Worsfold and Tim H Richardson*
Applied Molecular Engineering Group, Department of Physics & Astronomy,
University of Sheffield, Hounsfield Road, Sheffield S3 7RH (UK).
Rose Tregonning, M.O. Vysotsky and Chris A Hunter
Department of Chemistry, University of Sheffield, Dainton Building, Brook Hill,
Sheffield S3 7HF (UK).
Keizo Kato, Kazunari Shinbo and Futao Kaneko
Graduate School of Science and Technology, Niigata University, Niigata, Japan.
*Corresponding Author: t.richardson@sheffield.ac.uk
Notes on Ultra-fast LB Deposition
One of the disadvantages of the principal layer-by-layer deposition techniques,
such as LB deposition and poly-ion adsorption of polyelectrolytes, is the slow rate of
substrate coverage that can be achieved. Conventional LB deposition involves linear
deposition rates of typically 0.1 – 10 mm per minute per monolayer, and poly-ion
adsorption rates correspond to complete monolayer formation in around 10-20
minutes per layer. Very few attempts have ever been made to perform Langmuir film
transfer at fast rates since this often leads to wetting of the substrate and imperfections
in the built-up multilayer films.
However, for at least one application – toxic gas sensing - highly
homogeneous, ordered and dense molecular films are often actually detrimental in
achieving good performance. Researchers in the Applied Molecular Electronics
Group at the University of Sheffield have recently achieved ultra-fast deposition of
certain porphyrin monolayers to produce semi-ordered aggregated assemblies that act
as highly sensitive toxic gas sensors. These coatings consist of isolated porphyrin
“islands” distributed over the substrate in a fairly disordered manner. The multilayer
structure of these islands is built up layer-by-layer and is not present before deposition
within the Langmuir layer (Figure 1). It should be noted that deposition was not found
possible for the title porphyrin at conventional transfer rates. Remarkably, these films
are deposited at linear rates of 50 – 1500 mm minute-1. Multilayer deposition for
many cycles is possible - for example, an assembly produced by cycling a 25 mm x
25 mm substrate through the floating Langmuir film 50 times takes less than two
minutes to prepare. The deposition is achieved using a custom-built NIMA Fast
Supplementary Material (ESI) for Journal of Materials Chemistry
This journal is © The Royal Society of Chemistry 2001
Dipper deposition arm that can achieve translational linear deposition rates in the
range 0.1 – 2000 mm minute-1.
The Effect of Speed on Deposition
2
1000 mm/min
1.8
1400 mm/min
5 mm/min
1.6
1.4
Absorbance
1.2
1
0.8
0.6
0.4
0.2
0
0
2
4
6
8
10
12
Number of Excursions
Figure 1
For the application of gas-sensing, the most important design feature of a
material is that the gas must have easy access into and out of the molecular assembly
in order to achieve fast response and recovery rates. It is well-known that the surface
adsorption and desorption from conventional LB films can often be quite fast; the
slow diffusive processes in relatively dense, close-packed LB films reduce their
effectiveness as real sensors. Until now, researchers have focused their efforts in
trying to identify materials which provide increased sensitivity whilst hardly
bothering to change the way in which the films themselves are made. Assemblies
prepared using the ultra-fast method have a large surface area to volume ratio – hence
the surface adsorption process can become dominant and the disadvantage of the slow
diffusion is reduced.
This work shows that LB film deposition rates can be dramatically increased
for certain materials with a corresponding enhancement in materials performance for
certain applications.