Bolei Xu

Activation of Thiols at a Silver Nanoparticle Surface

Adsorption of organosulfur (thiol) molecules on noble metal surfaces has been widely used to fabricate self-assembled monolayers or modify surface properties in a broad range of novel technological applications. Adsorption of thiols on the surface of noble metals, such as gold or silver, has been intensively studied to gain fundamental understandings on the adsorption process, including its mechanism and rates. It is generally accepted that thiol adsorption onto the gold/ silver surfaces consists of many steps, starting with physical adsorption, followed by the sulfur–metal bonding reaction, the reorientation of the adsorbed thiol molecules, and the formation of a compact self-assembled layer. On the other hand, both diffusion-limited and non-diffusion-limited adsorptions have been reported, though the conditions for the two scenarios have not been consistently defined. 13–16] Thiol adsorption has been widely used to influence the properties of metallic nanoparticles, such as controlling their biomedical functions, tuning optical properties, changing the attached chromophores, and the synthesis of nanoparticles/nanocages of different shapes and size. The adsorption of thiol molecules on metallic nanoparticles have been studied mostly using optical techniques. For example, a sum frequency vibrational spectroscopy study has revealed that long-chain thiol molecules on gold nanoparticles of varying sizes have different conformations. The adsorption free energies of several thiol molecules on gold and silver nanoparticles have been determined using fluorescence or second harmonic generation (SHG) measurements. More than a decade ago, hyper Rayleigh scattering from gold nanoparticles was pursued and first reported as an incoherent second harmonic method for examining nanoparticles, as it was thought that there is an inherent size restriction in SHG such that the method could not be detected from particles with diameters much smaller than the optical wavelength. We have subsequently demonstrated that SHG from the surface of nanometer size particles can be detected at specific scattering angles where phase matching conditions are satisfied. This advancement has led to the detection of SHG from silver nanoparticles (AgNPs). In principle, the SHG signal may arise from the surface and/or bulk of the nanoparticles. In the case of AgNPs, the surface contribution is identified by the response in the SHG signal to the formation of surface bonding that would diminish the polarizability of the free electrons at the surface. The surface portion of the SHG signal can then be used for probing processes occurring at the surface. It was illustrated in our work that the adsorption of thiols onto the AgNP surface can be directly monitored with time using SHG. The SHG intensity generated at the surface layer of the silver particle is sensitive to the formation of the S Ag bonds at the nanoparticle surface. It was found that the majority of the SHG signal from AgNPs of 80 nm diameter can be quenched by the adsorption of thiol molecules as the formation of the S Ag bonds localizes the Ag electrons that are responsible for the nonlinear susceptibility. The change of the SHG intensity can be quantitatively related to the thiol coverage on the surface, and used for the determination of the adsorption and desorption rates as well as the free energy change of the adsorptive reaction process. If there is a barrier associated with the ratelimiting process in the adsorption, temperature dependence of the adsorption rate should enable the determination of the activation energy and reveal which is the activation process. Herein we present experimental observations indicating that the thiol reactions at the silver nanoparticle surface is an activated process and that temperature can be used to control the reaction rates. For the case of 1,2-benzenedithiol adsorption onto a silver nanoparticle, the activation energy was determined as 8.4 kcalmol . This energy barrier is likely associated with the formation of the transition state during the formation of sulfur–silver bond and not from the diffusion-limited process. Figure 1 shows that as 1,2-benzenedithiol at different concentrations was added into Ag colloids at 293 K, the intensity of the SHG light scattered from silver nanoparticles decreased. The decrease is resulted from the reduction of the nonlinear susceptibility of the Ag surface atoms as S Ag bonds form. As indicated by the rate of the SHG intensity decay, which is proportional to the square of the thiol coverage on the surface, the rate of adsorption of thiol molecules is faster when thiol concentration is higher. Figure 1 shows three of the five experimental measurements conducted with added thiol concentrations varying from 0.1 to 0.5 mm. The Langmuir model can be used to describe the adsorption kinetics with adsorption and desorption rate constants ka and kd:

Dynamics of the triplet-pair state reveals the likely coexistence of coherent and incoherent singlet fission in crystalline hexacene

The absorption of a photon usually creates a singlet exciton (S1) in molecular systems, but in some cases S1 may split into two triplets (2×T1) in a process called singlet fission. Singlet fission is believed to proceed through the correlated triplet-pair 1(TT) state. Here, we probe the 1(TT) state in crystalline hexacene using time-resolved photoemission and transient absorption spectroscopies. We find a distinctive 1(TT) state, which decays to 2×T1 with a time constant of 270 fs. However, the decay of S1 and the formation of 1(TT) occur on different timescales of 180 fs and <50 fs, respectively. Theoretical analysis suggests that, in addition to an incoherent S1→1(TT) rate process responsible for the 180 fs timescale, S1 may couple coherently to a vibronically excited 1(TT) on ultrafast timescales (<50 fs). The coexistence of coherent and incoherent singlet fission may also reconcile different experimental observations in other acenes.

Stabilized phase detection of heterodyne sum frequency generation for interfacial studies

We present a collinear-geometry heterodyne sum frequency generation (HD-SFG) method for interfacial studies. The HD detection is based on a collinear SFG configuration, in which picosecond visible and femtosecond IR beams are used to first produce a strong local oscillator and then to generate weak SFG signals from an interface. A time-delay compensator, consisting of an MgF2 window, is placed before the sample to introduce the time delay between the local oscillator and the interfacial SFG signals for spectral interferometry. Our HD-SFG method exhibits advantages of long-time phase stability. It is not sensitive to sample heights, does not require reflection correction, and is easy to implement.

Gram's Stain Does Not Cross the Bacterial Cytoplasmic Membrane.

For well over a century, Hans Christian Grams famous staining protocol has been the standard go-to diagnostic for characterizing unknown bacteria. Despite continuous and ubiquitous use, we now demonstrate that the current understanding of the molecular mechanism for this differential stain is largely incorrect. Using the fully complementary time-resolved methods: second-harmonic light-scattering and bright-field transmission microscopy, we present a real-time and membrane specific quantitative characterization of the bacterial uptake of crystal-violet (CV), the dye used in Grams protocol. Our observations contradict the currently accepted mechanism which depicts that, for both Gram-negative and Gram-positive bacteria, CV readily traverses the peptidoglycan mesh (PM) and cytoplasmic membrane (CM) before equilibrating within the cytosol. We find that not only is CV unable to traverse the CM but, on the time-scale of the Gram-stain procedure, CV is kinetically trapped within the PM. Our results indicate that CV, rather than dyes which rapidly traverse the PM, is uniquely suited as the Gram stain.

Observation of Organic Molecules at the Aerosol Surface

Organic molecules at the gas-particle interface of atmospheric aerosols influence the heterogeneous chemistry of the aerosol and impact climate properties. The ability to probe the molecules at the aerosol particle surface in situ therefore is important but has been proven challenging. We report the first successful observations of molecules at the surface of laboratory-generated aerosols suspended in air using the surface-sensitive technique second harmonic light scattering (SHS). As a demonstration, we detect trans-4-[4-(dibutylamino)styryl]-1-methylpyridinium iodide and determine its population and adsorption free energy at the surface of submicron aerosol particles. This work illustrates a new and versatile experimental approach for studying how aerosol composition may affect the atmospheric properties.

Effects of Molecular Structure and Solvent Polarity on Adsorption of Carboxylic Anchoring Dyes onto TiO2 Particles in Aprotic Solvents

Interactions of molecules with the surface of TiO2 particles are of fundamental and technological importance. One example is that the adsorption density and energy of the dye molecules on TiO2 particles affect the efficiency of dye-sensitized solar cells (DSSC). In this work, we present measurements characterizing the adsorption of the two isomers, para-ethyl red (p-ER) and ortho-ethyl red (o-ER), of a dye molecule potentially applicable for DSSC onto TiO2 particles by second harmonic scattering (SHS). It is found that while at the wavelengths used here o-ER has a much bigger molecular hyperpolarizability, p-ER exhibits strong SHS responses but o-ER gives no detectable SHS when the dyes are added to the TiO2 colloids, respectively. This observation indicates that o-ER does not adsorb onto TiO2, likely due to steric hindrance. Furthermore, we investigate how solvents affect the surface adsorption strength and density of p-ER onto TiO2 in four aprotic solvents with varying polarity. The absolute magnitude of the adsorption free energy was found to increase with the specific solvation energy that represents the ability of accepting electrons and solvent polarity. It is likely that resolvation of the solvent molecules displaced by the adsorption of the dye molecule at the surface in stronger electron-accepting and more polar solvents results in a larger adsorption free energy for the dye adsorption.

Super Bright Luminescent Metallic Nano-Particles

It is found that, by curing the surface defects that quench photoexcited carriers, luminescence efficiency of metallic nanoparticles can be dramatically increased. For Ag nanoparticles, as much as 300 times increase in photoexcitation induced luminescence is observed upon surface adsorption of ethanethiol. The same treatment increases Au nanoparticle luminescence efficiency by a factor of 3. A model based on the elimination of surface defects by the sulfur-metal bond formed upon thiol adsorption can quantitatively account for the observations, which also indicate that nanoparticles without proper surface treatment typically have low luminescence quantum yields.