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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We have examined the UV resonance Raman and the VUV absorption spectra of aqueous glycylglycine and other dipeptides. We observe strong resonance Raman enhancement of the amide I, II, and III bands and the amide CαH bending mode in a manner similar to that we observed previously with excitation within the π→π* transition of N-methylacetamide (Chen, X. G.; Asher, S. A.; Schweitzer-Stenner, R.; Mirkin, N. G.; Krimm, S. J. Am. Chem. Soc. 1995, 117, 2884). However, in addition, we observe strong resonance Raman enhancement of the ca. 1400 cm-1 symmetric COO- stretching vibration, whose 206.5 nm Raman cross section is increased 20-fold compared to that of the carboxylate in sodium acetate, for example. Addition of a methylene spacer between the amide and carboxylate groups causes the resonance Raman enhancement of this symmetric COO- stretch to disappear. The UV resonance Raman excitation profiles, the Raman depolarization ratio dispersion, and the VUV absorption spectra of glycylglycine and other dipeptides demonstrate the existence of a new 197 nm charge transfer band which involves electron transfer from a nonbonding carboxylate orbital to the amide-like π* orbital. This transition occurs at the penultimate carboxylate end of all peptides and proteins.

Over the past decade, we have been working to develop intelligent photonic-crystal materials with unique properties, which will be useful in a number of technological areas. These photonic-crystal materials utilize mesoscopically periodic arrays of spherical particles as their active optical elements and are easily fabricated chemically by the use of crystalline-colloidal-array (CCA) self-assembly techniques.Crystalline colloidal arrays are mesoscopically periodic fluid materials, which efficiently diffract light meeting the Bragg condition. These photonic-crystal materials consist of arrays of colloidal particles that self-assemble in solution into either face-centered-cubic (fcc) or body-centered-cubic (bcc) crystalline arrays (Figure 1), with lattice constants in the mesoscale size range (50-500 nm). Just as atomic crystals diffract x-rays that meet the Bragg condition, CCAs diffract ultraviolet, visible, and near-infrared light, depending on the lattice spacing; the diffraction phenomena resemble those of opals, which are close-packed arrays of monodisperse silica spheres.The CCA however can be prepared as macroscopically ordered arrays of non-close-packed spheres. This self-assembly is the result of electrostatic repulsions between colloidal particles, each of which has numerous charged surface functional groups. We have concentrated on the development of CCAs that diffract light in the visible spectral region and generally utilize colloidal particles of ~100-nm diameter. These particles have thousands of surface charges, which result from the ionization of surface sulfonate groups. The nearest-neighbor distances are often >200 nm.

We have measured the polarized nonresonance and resonance Raman as well as FTIR spectra of the model peptides glycylglycine and N-acetylglycine in H2O and D2O at pH/pD values between 1.5 and 12.0 with visible, near UV, and far UV excitation wavelengths. The spectra were self-consistently analyzed to obtain reliable spectral parameters of even strongly overlapping bands. Additionally, we have analyzed the polarized nonresonance and preresonance Raman spectra of glycylglycine single crystals. The most important result of this analysis is that for glycylglycine all amide bands as well as the symmetric carboxyl stretch band at ca. 1400 cm-1 are doublets. As shown in an earlier study (Sieler, G.; Schweitzer-Stenner, R. J. Am. Chem. Soc. 1997, 119, 1720) the amide I doublet results from vibrational coupling of the delocalized H2O bending mode with internal coordinates of the amide I mode. The amide III doublet is interpreted to result from vibrational coupling between the twisting mode of the Cα methylene group and internal coordinates which normally give rise to the amide III vibration (i.e., CN and Cα 1C stretching). In contrast, the amide II and carboxylate subbands are assigned to different conformers with respect to the torsional coordinate of the carboxylate group. While the higher frequency subband of the amide II and carboxylate bands may reflect a parallel orientation of the latter with respect to the peptide, which could be stabilized by hydrogen bonding to NH, the lower frequency band may reflect different orientations in which the carboxylate is hydrogen bonded to water. For N-acetylglycine we also observe two subbands underlying amide I and the carboxyl symmetric stretch band, which again reflects vibrational mixing with water and multiple rotational substates of the carboxylate, respectively.