Planar model catalysts were prepared by deposition of size-selected gold clusters containing up to seven atoms on rutile TiO2 (110). Molecular oxygen is observed to bind inefficiently to the surface, probably at oxygen vacancies, and some oxygen also appears to bind to the gold clusters. Stable CO binding is observed atop gold for catalysts prepared by Au and Au2 deposition, but not for larger Aun. CO oxidation activity is strongly dependent on cluster size, with Au7-prepared samples >50 times more reactive than samples prepared by Au or Au2 deposition

Photostability, inherent fluorescence brightness, and optical modulation of fluorescence are key attributes distinguishing silver nanoclusters as fluorophores. DNA plays a central role both by protecting the clusters in aqueous environments and by directing their formation. Herein, we characterize a new near infrared-emitting cluster with excitation and emission maxima at 750 and 810 nm, respectively that is stabilized within C 3 AC 3 AC 3 TC 3 A. Following chromatographic resolution of the near infrared species, a stoichiometry of 10 Ag/oligonucleotide was determined. Combined with excellent photostability, the cluster's 30% fluorescence quantum yield and 180,000 M −1 cm −1 extinction coefficient give it a fluorescence brightness that significantly improves on that of the organic dye Cy7. Fluorescence correlation analysis shows an optically accessible dark state that can be directly depopulated with longer wavelength co-illumination. The coupled increase in total fluorescence demonstrates that enhanced sensitivity can be realized through Synchronously Amplified Fluorescence Image Recovery (SAFIRe), which further differentiates this new fluorophore. Keywordsnear infrared fluorescence; few-atom silver clusters; DNA templates; Ag nanodot; optical modulation Improved fluorescence sensitivity, largely through background reduction, continues to motivate the development of fluorescence contrast agents in the near infrared spectral region. 1 From 700-1000 nm, not only is scattering diminished relative to shorter wavelengths, but light absorption by hemoglobin, lipids, and water is also minimized. [2][3] Furthermore, endogenous chromophores typically have electronic transitions in the ultraviolet and visible spectral regions, so background autofluorescence is also drastically reduced using near infrared excitation. 1 These spectroscopic features in conjunction with cost-effective instrumentation suggest the great promise of near-infrared based molecular diagnostics. 4 However, the true potential of near infrared contrast agents is restricted by fluorophores with low sustained emission rates at low excitation (brightness), small numbers of emitted photons (photostability), and/or limited compatibility with biological environments. 5 jeff.petty@furman.edu and dickson@chemistry.gatech.edu. Supporting Information Available: Detailed experimental procedures and supplemental figures. This material is available free of charge via the Internet at http://pubs.acs.org. Due to their small size (~1 kDa), organic, transition metal, and lanthanide fluorophores both minimize perturbation of biomolecular interactions and enable high labeling densities to increase detection sensitivity. 6 Also contributing to their prevalence is amenability to synthetic modifications, thereby permitting covalent attachment to specific biomolecules, enhanced aqueous solubility, and modified spectral properties. 5,7-9 Genetically expressed fluorescent proteins are also attractive fluorophores that enable direct and specific correlations of fluores...

Few-atom silver clusters harbored by DNA are promising fluorophores due to their high molecular brightness along with their long- and short-term photostability. Furthermore, their emission rate can be enhanced when co-illuminated with low-energy light that optically depopulates the fluorescence-limiting dark state. The photophysical basis for this effect is evaluated for two near infrared-emitting clusters. Clusters emitting at ~800 nm form with C3AC3AC3TC3A and C3AC3AC3GC3A and both exhibit a trap state with λmax ~ 840 nm and an absorption cross section of 5–6 × 10−16 cm2/molec that can be optically depopulated. Transient absorption spectra, complemented by fluorescence correlation spectroscopy studies, show that the dark state has an inherent lifetime of 3–4 μs and that absorption from this state is accompanied by photoinduced crossover back to the emissive manifold of states with an action cross section of ~2 × 10−18 cm2/molec. Relative to C3AC3AC3TC3A, C3AC3AC3GC3A produces a longer-lived trap state and permits more facile passage back to the emissive manifold. With the C3AC3AC3AC3G template, a spectrally distinct cluster forms having emission at ~900 nm and its trap state has a ~four-fold shorter lifetime. These studies of optically-gated fluorescence bolster the critical role of the nucleobases on both the formation and excited state dynamics of these highly emissive metallic clusters.

The steady-state rate r CO 2 of CO2 formation from CO + O2 on an epitaxially grown RuO2(110) single-crystal surface was recorded as a function of the partial pressures in the 10-7−10-6 mbar range. The RuO2(110) surface exposes singly coordinatively unsaturated “Ru-cus” sites and 2-fold coordinatively unsaturated “Ru-bridge” sites. Normally, the Ru-bridge sites are saturated by oxygen (O-bridge) which can be replaced by CO. The stage of the surface was controlled by vibrational spectroscopy (HREELS), and analysis of the kinetic data was based on previous information about the structural and adsorptive properties of this surface. Measurements of the rate as a function of temperature up to ≈350 K in a 1:1 mixture of CO and O2, each with 10-7 mbar partial pressure, revealed most remarkable agreement with data reported by Zang and Kisch13 with small supported RuO2 particles prepared from aqueous solution for 1 bar total pressure. Under these conditions of temperature and ratio of partial pressures the reaction will essentially proceed between CO and O species adsorbed at Ru-cus. The total pressure becomes largely insignificant:  whereas the pressure changes by 10 orders of magnitude, r CO 2 is estimated to vary only within 1 order of magnitude so that the pressure gap is bridged here.

Highlights d PVN CRH neurons are rapidly inhibited by natural reward d Reward relieves stress-induced behavioral and hormonal responses d Reward rebalances the synaptic homeostasis of PVN CRH neurons d Reward abolishes stress-induced burst firing in PVN CRH neurons

Model catalysts were prepared by deposition of size-selected Au(n) (n = 1-7) on rutile TiO2(110), and characterized by a combination of electron spectroscopy, ion scattering, temperature-programmed desorption, and pulse-dosing mass spectrometry. CO oxidation activity was found to vary strongly with deposited cluster size, with significant activity appearing at Au3. Activity is not obviously correlated with affinity for CO, or with cluster morphology, but is strongly correlated with the clusters' ability to bind oxygen (during O2 exposure) on top of the gold. The temporal dependence of CO2 evolution in reaction of O2 pre-exposed samples with CO pulses shows an interesting cluster size dependence. For Au5 and Au6, the peak CO2 production is coincident with the peak CO flux, but for Au3, Au4, and Au7, there are significant induction periods for CO2 evolution. In addition, it is observed that some of the most active cluster sizes have the slowest CO2 evolution rates. Several mechanistic scenarios capable of accounting for the observations are laid out.

Chemiresistive sensor devices using two-dimensional (2D) materials have been extensively studied.

Fluorescence modulation offers the opportunity to detect low concentration fluorophore signals within high background. Applicable from the single molecule to bulk levels, we demonstrate long-wavelength optical depopulation of dark states that otherwise limit Cy5 fluorescence intensity. By modulated excitation of a long wavelength Cy5 transient absorption, we dynamically modulate Cy5 emission. The frequency dependence enables specificaton of the dark state timescales enabling optical demodulation-based signal recovery from high background. These dual laser illumination schemes for high sensitivity fluorescence signal recovery easily improve signal to noise ratios by well over an order of magnitude, largely by discrimination against background. Previously limited to very specialized dyes, our utilization of long-lived dark states in Cy5 enables selective detection of this very common single molecule and bulk fluorophore. Although, in principle, the “dark state” can arise from any photoinduced process, we demonstrate that cis-trans photoisomerization, with its unique transient absorption and lifetime enables this sensitivity boosting, long-wavelength modulation to occur in Cy5. Such studies underscore the need for transient absorption studies on common fluorophores to extend the impact of fluorescence modulation for high sensitivity fluorescence imaging in a much wider array of applications.