Advanced photons in science

Advanced Photon SourceDavid Bradley worked with Argonne National Laboratory on a series of articles for the annual report of the ANL's Advanced Photon Source. For the latest issue of Elemental Discoveries, we offer a sneak preview of the advanced science featured in the report.

Are films ferroelectric? | Gold nanocrystals | Photosynthetic system | Dissecting the atom | Catalytic clues Digging in the dirt | Folding Protein Sensors | X-ray movies

SAXS and smart materials

Smart materials containing tiny channels, a thousandth the thickness of a human hair could carry drug molecules direct to their target in the body and release them when the material reaches body temperature. The heat-responsive materials would allow drugs that are otherwise inactive when taken by mouth to be administered without the patient getting the needle. Related materials might also act as artificial enzymes, speeding up the conversion of the trapped molecules into useful products.

Millicent Firestone at the Argonne National Laboratory's Advanced Photon Source and her colleagues have designed several polymeric materials containing these tiny water and membrane channels. Their studies using BESSRC/XOR sector 12 are revealing how chemical fine-tuning of the polymer's structure can change the size of the channels so that different molecules can be held inside selectively for subsequent ejection.

The chemical driver behind the research is the possibility of certain molecules self-assembling into more complex chemical structures without further intervention by the chemist. This allows chemists to design a building block that will then build itself into the desired material. Firestone and her colleagues explain that this is one way of making materials with a controlled structure on the nanoscale that respond to different external stimuli, such as light, temperature, and pH. By fine-tuning the chemistry of the building blocks it is then possible to make such nanostructures with different response levels to the stimulus. For instance, when a polymer called PEG is mixed with a fatty, phospholipid molecule in water containing a surfactant (a soap-like molecule), the building blocks rearrange themselves into microscopic bubbles known as micelles. The resulting material is gel-like and doubly refracts light - it is birefringent. By changing the length of the chemical side chains on the PEG component it is possible to change the level of birefringence. Such materials are of interest for researchers working in optoelectronics.

Firestone and her colleagues, however, have spotted the potential of the related polymer N-isopropylacrylamide (PNIPAM) in making, not microscopic bubbles, but channels in a similar mixture of ingredients. These channels are essentially "endless" bubbles into which water and other molecules can fit. While other researchers have found they can change the size of micelles by tweaking the chemistry of the building blocks, the Firestone team set out to make the size of their water channels tunable using an external stimulus, such as a change in temperature. Other researchers have already demonstrated that PNIPAM undergoes dramatic, changes in its chemical conformation when the temperature rises, so it seemed the perfect starting material for making a thermally-responsive material.

Dan Hay, a post-doctoral research associate synthesised a series of new materials in which a PNIPAM group was grafted on to the head of a phospholipid. They used several spectroscopic techniques to demonstrate that the chemicals had self-assembled into the structures they were expecting and optical microscopy to confirm birefringence in these new materials. They found that the mixture was a transparent gel at room temperature (22 Celsius), but when warmed to 32 Celsius it became an opaque fluid.

The researchers then turned to the beamline to obtain a detailed view of the water channels using SAXS, small-angle x-ray scattering. They carried out SAXS at different temperatures. At room temperature, the diffraction pattern for the material carries three peaks, which the researchers explain is likely due to a layered, or lamellar, structure composed of alternating layers of the PNIPAM-based material and water layers. The SAXS also suggest that the stacking of these layers is directional alluding to the channel-like nature of the layering. When the gel is warmed above 32 Celsius, the diffraction pattern changes dramatically, indicating that the layered structure collapses and the size of the water channels are compressed.

See: Daniel N. T. Hay,1 Paul G. Rickert, 2 Sönke Seifert, 3 and Millicent A. Firestone1 "Thermoresponsive Nanostructures by Self-Assembly of a Poly(N-isopropylacrylamide)-Lipid Conjugate," J. Am. Chem. Soc. 126, 2290-2291 (2004).

Author affiliations: 1Materials Science Division, 2Chemistry Division, 3Advanced Photon Source Division, Argonne National Laboratory, Argonne, IL 60439, USA

This work was supported by the United States DOE-BES, Div. of Material Sciences under Contract W-31-109-ENG-38 to the University of Chicago.

In Issue 76
Are films ferroelectric?
Discipline for gold nanocrystals
X-rays shed light on machinery of photosynthesis

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