Differentiation between Shallow and Deep Charge Trap States on Single Poly(3‐hexylthiophene) Chains through Fluorescence Photon Statistics

Blinking of the photoluminescence (PL) emitted from individual conjugated polymer chains is one of the central observations made by single‐molecule spectroscopy (SMS). Important information, for example regarding excitation energy transfer, can be extracted by evaluating dynamic quenching. However, the nature of trap states, which are responsible for PL quenching, often remains obscured. We present a detailed investigation of the photon statistics of single poly(3‐hexylthiophene) (P3HT) chains obtained by SMS. The photon statistics provide a measure of the number and brightness of independently emitting areas on a single chain. These observables can be followed during blinking. A decrease in PL intensity is shown to be correlated with either 1) a decrease in the average brightness of the emitting sites; or 2) a decrease in the number of emitting regions. We attribute these phenomena to the formation of 1) shallow charge traps, which can weakly affect all emitting areas of a single chain at once; and 2) deep traps, which have a strong effect on small regions within the single chains.     

A luminescence-based probe for sensitive detection of hydrogen peroxide in seconds

Here, we present a fast and simple hydrogen peroxide assay that is based on time-resolved fluorescence. The emission intensity of a complex consisting of terbium ions (Tb³⁺) and phthalic acid (PA) in HEPES buffer is quenched in the presence of H₂O₂ and this quenching is concentration-dependent. The novel PATb assay detects hydrogen peroxide at a pH range from 7.5 to 8.5 and with a detection limit of 150 nmol L⁻¹ at pH 8.5. The total assay time is less than 1 min. The linear range of the assay can be adapted by a pH adjustment of the aqueous buffer and covers a concentration range from 310 nmol L⁻¹ to 2.56 mmol L⁻¹ in total which encompasses four orders of magnitude. The assay is compatible with high concentrations of all 47 tested inorganic and organic compounds. The PATb assay was applied to quantify H₂O₂ in polluted river water samples. In conclusion, this fast and easy-to-use assay detects H₂O₂ with high sensitivity and precision.     

Reduction of water wettability of nanofibrillated cellulose by adsorption of cationic surfactants

Adsorption isotherms of single and double chain cationic surfactants with different chain length (cetyltrimethyl-, didodecyl- and dihexadecyl ammonium bromide) onto cellulose nanofibrils were determined. Nanofibrillated cellulose, also known as microfibrillated cellulose (MFC), with varying contents of carboxyl groups (different surface charge) was prepared by TEMPO-mediated oxidation followed by mechanical fibrillation. The fibril charge was characterized by potentiometric and conductometric titration. Surfactant adsorption was verified by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray Photoelectron Spectroscopy (XPS). Wetting and adhesion of water onto fibril films was determined by contact angle measurements. Small aggregates (admicelles) of surfactant were shown to form on the nanofibril surfaces, well below critical micelle concentrations. The results demonstrate the possibility of using cationic surfactants to systematically control the degree of water wettability of cellulose nanofibrils.     

A valveless microfluidic device for integrated solid phase extraction and polymerase chain reaction for short tandem repeat (STR) analysis

A valveless microdevice has been developed for the integration of solid phase extraction (SPE) and polymerase chain reaction (PCR) on a single chip for the short tandem repeat (STR) analysis of DNA from a biological sample. The device consists of two domains--a SPE domain filled with silica beads as a solid phase and a PCR domain with an ~500 nL reaction chamber. DNA from buccal swabs was purified and amplified using the integrated device and a full STR profile (16 loci) resulted. The 16 loci Identifiler? multiplex amplification was performed using a non-contact infrared (IR)-mediated PCR system built in-house, after syringe-driven SPE, providing an ~80-fold and 2.2-fold reduction in sample and reagent volumes consumed, respectively, as well as an ~5-fold reduction in the overall analysis time in comparison to conventional analysis. Results indicate that the SPE-PCR system can be used for many applications requiring genetic analysis, and the future addition of microchip electrophoresis (ME) to the system would allow for the complete processing of biological samples for forensic STR analysis on a single microdevice.     

Structure of nanofibrillated cellulose layers at the o/w interface

The nature of layers formed by cellulose nanofibrils that had been surface modified (hydrophobized) at the oil/water (o/w) interface was investigated. The aim of the study was to clarify the mechanism underlying the excellent ability of these nanoparticles to stabilize emulsions. Layers of hydrophobized nanofibrillated cellulose spread at the o/w interface were deposited on glass slides by the Langmuir-Blodgett deposition technique. Overall evaluation of layer structures was performed by image analysis based on a Quadtree decomposition of images obtained from a flatbed scanner. A more detailed characterization of the layer structures was performed by Atomic Force Microscopy (AFM), and Field-Emission Scanning Electron Microscopy (FE-SEM). The results show that nanofibrils that were able to stabilize emulsions occur as single, dispersed fibrils or form large, network-like aggregates at the o/w interface. Fibrils that were insufficiently hydrophobized and therefore did not stabilize emulsions were only partially deposited and formed small, compact aggregates. We conclude that it is likely that the network formation is the main mechanism by which the fibrils prevent coalescence of emulsion droplets.     

A Connection between Singular Stochastic Control and Optimal Stopping

We show that the value function of a singular stochastic control problem is equal to the integral of the value function of an associated optimal stopping problem. The connection is proved for a general class of diffusions using the method of viscosity solutions.     

Complexation by N-(3,6,9-trioxadecyl)monoaza-12-crown-4 lariat ether: a “calabash” complex of a potassium cation by a synthetic macrocycle containing a total of only seven donor atoms

Structural and cation binding data for the title compound demonstrate that the large value of K_s results from complete K⁺ cation encapsulation by one nitrogen and six oxygen atoms despite the presence of a twelve-membered macroring.     

A forest building process on simple graphs

Consider the following process on a simple graph without isolated vertices: order the edges randomly and keep an edge if and only if it contains a vertex which is not contained in some preceding edge. The resulting set of edges forms a spanning forest of the graph.The probability of obtaining $k$ components in this process for complete bipartite graphs is determined as well as a formula for the expected number of components in any graph. A generic recurrence and some additional basic properties are discussed.