C&EN: TODAY'S HEADLINES - GETTING SILVER OUT OF PHOTOS

MICHAEL FREEMANTLE

A novel type of photographic system employs a single sheet coated with layers containing all the chemical components needed to create and develop a fixed image.

The film, designed by chemists John L. Marshall and Stephen J. Telfer and coworkers at Polaroid Corp., captures a latent image consisting of a "primary acid" that forms in regions exposed to visible light. Heating the film generates a larger amount of a "secondary acid" that combines with indicator dyes to form a permanent image [Science, published online Aug. 8, http://www.sciencemag.org/cgi/content/
abstract/1074583v1].


	STABLE This color image was formed without addition or subtraction of chemical reagents. COURTESY OF POLAROID

"Conventional silver halide-based photographic systems are environmentally problematic, as they either use processing solutions into which the silver salts are extracted or use two-sheet media in which one of the sheets--that containing the residual silver--is discarded," Telfer explains. "The latter process is used in Polaroid peel-apart photography and in some dry silver systems."

"The AAI [acid amplification imaging] system uses a single sheet which can be exposed to light and developed using a thermal process," he continues. "Addition and removal of chemical reagents are not required to develop and fix the image."

The film consists of a series of photoacid-generating and dye layers stacked on a substrate. When it is exposed to light, small amounts of the primary acid are generated in the photoacid-generating layers by the light-induced decomposition of an iodonium salt sensitized by cationic dyes.

During thermal processing, the primary acid catalyzes the formation of the secondary acid from a phosphate ester, known as the acid amplifier. The secondary acid then protonates pH-sensitive indicator dyes in the dye layers. The dyes are colorless in their neutral forms, but absorb blue, green, or red light in their protonated forms.

To avoid unwanted absorption of visible light in the final image when it is viewed, sensitizing dyes and the iodonium salts remaining in the unexposed areas of the photoacid-generating layers are treated with a base that diffuses from the dye layers during the heating process.

Although the sensitivity of the AAI system is low by silver halide standards, Telfer tells C&EN that a range of potential printing applications can be foreseen. "The primary goal of this program is not to make a film that can be put into a camera, but to use the process for digital imaging," he says. "For this type of imaging, we can use a brighter light source than ambient light. The film is readily exposed, for example, by organic light-emitting diode arrays, lasers, and digital light-valve projection systems."

Possible applications of the new film include digital printing of transparent or reflective images and in situ formation of color filters for liquid-crystal displays. And since high resolution is possible with AAI, the system could lead to applications in information storage.