Chemical photosensitivity

E. coli, when compared to that of bacteria suspended in MQW. This type of water probably contains, in addition of natural anions, chemical photosensitizers (organic or inorganic) which increase the detrimental action of light on bacteria. 

In the conventional applications of photodynamic therapy (PDT), fight-activated chemicals (photosensitizers) are used to destroy fast-growing cells and tissues. The commonly used sensitizers are based upon porphyrin-like molecules, e.g., porphyrins, chlorins, bacteriocholins, and phthalocyanines. These substances collect in... 

PDT is based on the administration of a chemical (photosensitizer), systemically or locally. This chemical accumulates in tissues for a certain amount of time before irradiation at a suitable wavelength. Herein lies one major problem with PDT. To obtain a clinically significant effect, irradiation must take place after an optimal delay following the administration of the photosensitizer. Since the duration of the irradiation is very short, relative to the incubation time (i.e., the injection-irradiation interval), it is necessary to be aware of the optimal delay in order to maximize the clinical therapeutic effect. However, this delay varies greatly from one experimental model to another, from one cell to another within a given model, and also probably between cells within a single tumor. An issue that is of little importance in cancer chemotherapy, where kinetics have little influence on efficacy, is of critical importance in PDT where efficacy depends crucially upon the sensitizer concentration and the mumber of cancer cells loaded with a lethal amount of sensitizer at the time of irradiation. Simple tools for the monitoring of tissue concentration still do not exist. 

Diborane Disilane Silane chemical warfare Benzyl chloride Crotonaldehyde chemical, photosensitive paper coatings N-Ethyl-N-hydroxyethyl-m-toluidine 2-(N-Methylanilino) ethanol Phenylethylethanolamine chemical/pharmaceutical machinery Copper... 

Many occupationally related cases of photosensitivity have been reported in workers in outdoor occupations [4]. However, this is not always the case, since the quantity of radiation necessary to induce a reaction can be quite small, as little and 20-30 min of natural sun exposure [3]. In addition, it must be remembered that almost all chemical photosensitization has its action spectrum in the long wave ultraviolet (UV) A (320-400 nm) and visible (400-800 nm) ranges [3]. As such, window glass will not protect an individual whose skin is harboring a photosensitizer. Patients might easily develop a reaction while riding in an automobile to and from work. 

Dry-Film Resists Based on Radical Photopolymerization. Photoinitiated polymerization (PIP) is widely practiced ia bulk systems, but special measures must be taken to apply the chemistry ia Hthographic appHcations. The attractive aspect of PIP is that each initiator species produced by photolysis launches a cascade of chemical events, effectively forming multiple chemical bonds for each photon absorbed. The gain that results constitutes a form of "chemical amplification" analogous to that observed ia silver hahde photography, and illustrates a path for achieving very high photosensitivities. 

In resists of this class, the imaging layer contains a multifunctional monomer that can form an intercormected network upon polymerization, and a photosensitizer to generate a flux of initiating free radicals. Although not stricdy required for imaging, the composition usually includes a polymeric binder (typically an acryhc copolymer) to modify the layer s physical properties. Figure 7b shows the chemical stmctures of typical components. 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cycHzed mbber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenoHc stmcture renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxyHc acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubiHty is controUed by chemical and polarity differences rather than molecular size. 

One potential approach extends the idea of chemical amplification introduced in our preceding description of dry-film resists. In 1982, Ito and co-workers (37,38) recognized that if a photosensitizer producing an acidic product is photolyzed in a polymer matrix containing acid-labile groups, the acid will serve as a spatially localized catalyst for the formation or cleavage of chemical bonds. 

The chemical pathways leading to acid generation for both direct irradiation and photosensitization (both electron transfer and triplet mechanisms) are complex and at present not fully characterized. Radicals, cations, and radical cations aH have been proposed as reactive intermediates, with the latter two species beHeved to be sources of the photogenerated acid (Fig. 20) (53). In the case of electron-transfer photosensitization, aromatic radical cations (generated from the photosensitizer) are beHeved to be a proton source as weU (54). 

The main suppHers and manufacturers of polymethine dyes are Aldrich Chemical Company, Eastman Organic Chemicals (U.S.), Japanese Institute for Photosensitizing Dyes, NK Dyes (Japan), Riedel deHaen (Germany), Institute of Organic Chemistry of the National Academy of Sciences (Ukraine), and NIIKhim-EotoProekt (Russia). 

Similar types of cross-linking reactions are observed for polymers to which photosensitive molecules ate chemically attached to the backbone of the polymer stmcture (Fig. 7). Radiation curing of polymers using uv and visible light energies is used widely in photoimaging and photoresist technology (Table 8) (58,59). 

The detection of spectral sensitizing action often depends on amplification methods such as photographic or electrophotographic development or, alternatively, on chemical or biochemical detection of reaction products. Separation of the photosensitization reaction from the detection step or the chemical reaction allows selection of the most effective spectral sensitizers. Prime considerations for spectral sensitizing dyes include the range of wavelengths needed for sensitization and the absolute efficiency of the spectrally sensitized process. Because both sensitization wavelength and efficiency are important, optimum sensitizers vary considerably in their stmctures and properties. 

Resists. Resists are temporary, thin coatings appHed to the surface of the copper-clad laminate. After patterning, these films act as masks that are chemically resistant to the cleaning, plating, and etching solutions used to define the circuit traces of the PWB. Both nonphotosensitive and photosensitive types are used. 

Since the optical transitions near the HOMO-LUMO gap are symmetry-forbidden for electric dipole transitions, and their absorption strengths are consequently very low, study of the absorption edge in Ceo is difficult from both an experimental and theoretical standpoint. To add to this difficulty, Ceo is strongly photosensitive, so that unless measurements arc made under low light intensities, photo-induced chemical reactions take place, in some cases giving rise to irreversible structural changes and polymerization of the... 

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