Chemical sensors respond to the presence of a specific analyte in a variety of ways. One of the most convenient is a change in optical properties, and in particular a visually perceptible colour change. Here we report the preparation of a material that changes colour in response to a chemical signal by means of a change in diffraction (rather than absorption) properties. Our material is a crystalline colloidal array of polymer spheres (roughly 100 nm diameter) polymerized within a hydrogel that swells and shrinks reversibly in the presence of certain analytes (here metal ions and glucose). The crystalline colloidal array diffracts light at (visible) wavelengths determined by the lattice spacing, which gives rise to an intense colour. The hydrogel contains either a molecular-recognition group that binds the analyte selectively (crown ethers for metal ions), or a molecular-recognition agent that reacts with the analyte selectively. These recognition events cause the gel to swell owing to an increased osmotic pressure, which increases the mean separation between the colloidal spheres and so shifts the Bragg peak of the diffracted light to longer wavelengths. We anticipate that this strategy can be used to prepare 'intelligent' materials responsive to a wide range of analytes, including viruses.

We have directly determined the amide band resonance Raman spectra of the "average" pure alpha-helix, beta-sheet, and unordered secondary structures by exciting within the amide pi-->pi* transitions at 206.5 nm. The Raman spectra are dominated by the amide bands of the peptide backbone. We have empirically determined the average pure alpha-helix, beta-sheet, and unordered resonance Raman spectra from the amide resonance Raman spectra of 13 proteins with well-known X-ray crystal structures. We demonstrate that we can simultaneously utilize the amide I, II, and III bands and the Calpha-H amide bending vibrations of these average secondary structure spectra to directly determine protein secondary structure. The UV Raman method appears to be complementary, and in some cases superior, to the existing methods, such as CD, VCD, and absorption spectroscopy. In addition, the spectra are immune to the light-scattering artifacts that plague CD, VCD, and IR absorption measurements. Thus, it will be possible to examine proteins in micelles and other scattering media.

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We report the development of a novel sensing material that reports on analyte concentrations via diffraction of visible light from a polymerized crystalline colloidal array (PCCA). The PCCA is a mesoscopically periodic crystalline colloidal array (CCA) of spherical polystyrene colloids polymerized within a thin, intelligent polymer hydrogel film. CCAs are brightly colored, and they efficiently diffract visible light meeting the Bragg condition. The intelligent hydrogel incorporates chemical molecular recognition agents that cause the gel to swell in response to the concentration of particular analytes; the gel volume is a function of the analyte concentration. The color diffracted from the hydrogel film is, thus, a function of analyte concentration: the swelling of the gel changes the periodicity of the CCA, which results in a shift in the diffracted wavelength. We have fabricated a sensor, utilizing a crown ether as the recognition agent, that detects Pb 2+ in the 0.1 µM-20mM (∼20 ppb-∼4000 ppm) concenration range. We have also fabricated glucose and galactose sensors, utilizing glucose oxidase or β-D-galactosidase as the recognition elements. The glucose oxidase sensor detects glucose in the 0.1-0.5 mM (18-90 ppm) concentration range in the presence of oxygen and detects as little as 10 -12 M glucose (0.18 ppt) in the absence of oxygen. In addition, this sensor reports on dissolved oxygen concentration from ∼1 to 6 ppm in the presence of constant glucose concentrations.

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