A colloidal metal−insulator−metal ensemble chemiresistor sensor based on a monolayer stabilized metal nanocluster transducer film is described. In the example
presented, the thin transducer film is composed of 2-nm
gold clusters encapsulated by octanethiol monolayers and
is deposited on an interdigital microelectrode.
Responses
to organic vapor exposures are large (resistance changes
up to 2-fold or more), fast (90% response in less than 1
s), reversible, and selective. Chemiresistor sorption
isotherms for toluene, tetrachloroethylene, 1-propanol,
and water vapors are nonlinear and illustrate the high
sensitivity and selectivity (ppm detection for toluene and
tetrachloroethylene; negligible response for 1-propanol
and water).
A series of gold nanoclusters stabilized by ligands containing short ethylene oxide oligomers
of fixed length were prepared and characterized. The thiols CH3(OCH2CH2)
n
SH (where n =
2, 3, and 4) were substituted onto the surface of 1.8-nm hexanethiol-capped gold clusters by
a thiol-exchange reaction, and the resulting nanoclusters were characterized by NMR, FTIR,
and UV/vis spectroscopies; TGA; and TEM analysis. A degree of ligand exchange greater
than 99% was achieved, and the gold core diameter remained unchanged in the final material.
Of particular interest, the cluster with n = 2 was water-insoluble, whereas those with n =
3 or 4 were water-soluble. The thin-film electrical conductivities of these clusters were
compared with those of alkanethiol-capped clusters of similar chain lengths and found to be
roughly 1 order of magnitude greater. In a chemical vapor sensor configuration, this series
of clusters displayed strong electrical responses that showed a correlation between the length
of the ethylene oxide ligand and the polarity of the vapor.
Procedures for the preparation of metal-free phthalocyanine network polymers from oxygen-, sulfur-, and selenium-bridged bis(phthalonitrile) monomers were investigated on the basis of phthalocyanine model compounds derived from phenoxy-, (phenylthio)-, and (phenylseleno) phthalonitrile compounds. The oxygen-and sulfur-substituted phthalonitrile compounds could be converted in high yield to the corresponding metal-free phthalocyanine compounds by reaction with tetrahydropyridine, hydroquinone, or 4,4'-biphenol. With an optimum quantity of coreactant, the phthalocyanine yield ranged from near-quantitative to 65% to no conversion for the respective oxygen, sulfur, and selenium phthalonitriles. A side reaction to a triazine structure was also investigated. The model phthalocyanine compounds were characterized by IR, electronic, NMR, and X-ray diffraction spectroscopies and TGA, from which an analysis of the corresponding phthalocyanine network polymers was made. Spectroscopic analysis and sulfuric acid insolubility indicated a significantly higher phthalocyanine content in the oxygen-bridged network polymer. Both phthalocyanine model compounds and network polymers had very high electrical resistivities, and the polymers were not dopable with iodine.
The performance of an optical limiter based on Pb-tetrakis(cumylphenoxy)phthalocyanine, a robust organic material with a large χ(3) and figure of merit, χ(3)/α0, is described. In an f/5 limiter with a sample transmission of 0.68, the threshold for limiting was 8±2 nJ and the dynamic range was greater than a factor of 103. The threshold for the PbPc(CP)4 limiter was ∼15 times smaller and the high intensity transmission ∼4–5 times lower than an equivalent limiter based on a thermal nonlinearity.
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