Tetra(4-carboxyphenyl)porphyrin (TCPP) adsorbs strongly onto nanoparticulate TiO2 and serves as an efficient photosensitizer for solar-energy conversion by TCPP-sensitized TiO2 electrodes. Nanoparticulate TiO2 electrodes were prepared from Degussa P25 TiO2 powder in the standard manner for a Grätzel cell. Adsorption studies of TCPP onto these sintered TiO2 electrodes gave a saturation surface coverage of 47 μmol/g. Adsorption studies of TCPP onto colloidal dispersions of Degussa P25 in ethanol gave a saturation surface coverage of 77 μmol/g. The difference between the saturation coverages is attributed to the reduction of the available surface area in the TiO2 films after sintering, from 55 m2/g as a free colloid to about 34 m2/g as a sintered electrode. The nature of the binding of TCPP onto the TiO2 electrodes was investigated using X-ray photoelectron spectroscopy (XPS) and Resonance Raman Spectroscopy (RRS). In the XPS spectra of TiO2 with adsorbed TCPP, the O (1s) and Ti (2p3/2) peaks of TiO2 were shifted to a higher binding energy value, by about 0.3 eV, and the O (1s) and N (1s) peaks of TCPP were shifted to a higher binding energy, by about 0.7 eV. Upon adsorption of TCPP, one of the Ti (2p3/2) peaks of TiO2 disappeared, suggesting complexation and removal of surface states. The RRS results indicated that for cases in which TCPP was adsorbed onto TiO2 films from ethanolic solutions of about 1 μM concentration, the porphyrin spectrum showed distinctive interactions with the surface, but for cases in which it was adsorbed from higher concentrations, the RRS spectra were similar to spectra of TCPP powder, indicating the dominance of porphyrin−porphyrin interactions. We conclude that lateral interactions between adsorbed TCPP are significant upon adsorption from all but the lowest (micromolar) initial concentrations. Photovoltaic cells with TCPP-sensitized TiO2 electrodes gave good solar-energy conversion efficiencies. At light simulating one sun (AM 1.5), a cell sensitized by TCPP gives a short-circuit photocurrent of about 6 mA/cm2 and an open-circuit photopotential of 485 mV. The incident photon-to-current conversion efficiency was 55% at the Soret peak and 25−45% at the Q-band peaks; the cells have a fill factor of 60−70% and an overall energy conversion efficiency of about 3%.

Although resonating microcantilevers are demonstrated to be excellent mass sensors, adsorption-induced changes in the spring constant result in errors in the calculation of adsorbed mass from shifts in resonance frequencies. However, simultaneous measurement of resonance frequency and adsorption-induced cantilever bending can be used to determine the variation in spring constant. Plotting the change in surface stress as a function of analyte concentration, the surface excess of adsorbed molecules and, therefore, the mass adsorbed can be determined. Here, we demonstrate this concept for adsorption of Na+ ions on microcantilevers in NaCl solutions where a change in the spring constant was found to increase from 9.5×10−4 to 7.5×10−3 N/m as the NaCl concentration increased from 0.05 to 0.8 M.

Microcantilevers, such as those used in atomic force microscopy, undergo Brownian motion due to mechanical thermal noise. The root mean square amplitude of the Brownian motion of a cantilever typically ranges from 0.01–0.1 nm, which limits its use in practical applications. Here we describe a technique by which the Brownian amplitude and the Q factor in air and water can be amplified by three and two orders of magnitude, respectively. This technique is similar to a positive feedback oscillator, wherein the Brownian motion of the vibrating cantilever controls the frequency output of the oscillator. This technique can be exploited to improve sensitivity of microcantilever-based chemical and biological sensors, especially for sensors in liquid environments.

The reversible partitioning of molecules between immiscible phases, such as in solvent extraction, is usually controlled by chemical composition of one or both phases, temperature, or pH. An additional means for controlling the equilibrium distribution of a solute between phases, the use of light and suitable photochromic molecules, such as those drawn from the spiropyran family, is described. Partitioning of 1′,3′,3′-trimethyl-6-nitrospiro[2H-1]-benzopyran-2,2′ indoline (1′-methyl 6-NO 2 BIPS) and its derivatives between toluene and water phases is shown to be controlled by the wavelength of incident light, the ionogenic functional groups on the molecule, and the aqueous solution pH. At pH 2, both 1′-methyl 6-NO 2 BIPS and 1′-(3-carbomethoxypropyl) 6-NO 2 BIPS partition reversibly and preferentially into the aqueous phase when irradiated with ultraviolet light in the 365-370 nm region but only slightly into the aqueous phase when illuminated with wavelengths greater than or equal to 480 nm. The partition coefficient of these spiropyrans at pH 2 and under irradiation with ultraviolet light is 25 times larger than when irradiated with visible light. For aqueous solutions of 1′-(3-carboxypropyl) 6-NO 2 BIPS, the partition coefficient at pH 4 and under UV light irradiation is 22 times larger than that under visible light irradiation, whereas at pH 6 the partition coefficient under UV irradiation is only 2 times greater than under visible light irradiation. These partition coefficient variations are explained by applying chemical equilibrium theory to the reversible, irradiation-induced structural changes in spiropyran molecules. The fit of theory to the data confirms the importance of interface speciation.

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