Characterization of novel solar based nitrogen doped titanium dioxide photocatalytic membrane for wastewater treatment

The increasing presence of microbial and emerging organic contaminants pose detrimental effects on the environment and ecosystem such as diseases, pandemics and toxicity. Most of these synthetic pollutants are biorecalcitrant and therefore persist in the environment. Conventional water treatment methods are not effective thereby necessitating the development of advanced techniques such as photocatalysis and membrane processes. In this study, visible light-driven photocatalytic membrane was synthesized through the immobilization of nitrogen-doped nanoparticles onto the polyvinylidene fluoride (PVDF) membrane and performance evaluated with E.coli microbial contaminant removal. Characterization was done using Fourier transform infrared spectra, X-ray diffraction (XRD), water contact angle, Scanning Electron Microscopy-Energy Dispersive X-ray (SEM-EDX). The Nitrogen-doping of titanium dioxide red-shifted the light absorption to a visible range of 440 nm from 400 nm. Nitrogen dopant was detected at 1420 cm-1and 1170 cm-1 for nitrogen doped nanoparticles and 1346-1417 cm-1 for nitrogen doped titanium dioxide PVDF membrane. SEM-EDX confirmed presences of nitrogen in nitrogen doped titanium dioxide nanoparticles on membrane surface with nitrogen elemental composition of 0.01 % wt. The water contact angle reduced by 81.39o from 120.14o to 38.75o because of PVA immobilization of nitrogen-doped titanium dioxide and glutaraldehyde crosslinking. Nitrogen doping resulted in visible light active photocatalytic membranes with better hydrophilicity and fouling resistance. 8.42 E.coli log removal and a relative flux of 0.35 was obtained within 75 min. The developed photocatalytic membrane enables the use of sunlight hence a less costly method for decontamination of wastewater.

On-chip microscale isoelectric focusing enhances protein detection limit

Enhancing the detection limit in protein analysis is essential for a wide range of biomedical applications. In typical fluorescent protein assays, this limit is constrained by the detection capacity of the photon detector. Here, we develop an approach that significantly enhances the protein detection threshold by using microscale isoelectric focusing implemented directly at the detection site on a protein sensor chip. We demonstrate that by electrically generating a localized pH environment within a radius of ∼60 μm, protein molecules can be concentrated within this range and be detected at levels over four times lower than those achieved by measurements without on-chip isoelectric focusing. We find that this detection-limit enhancement results from a dual effect: the concentrating of the protein molecules and a reduction in the diffusion-induced fluctuation. Our approach offers a simple, yet highly effective ultra-low-power all-electronic solution for substantially improving protein analysis detection limits for diverse applications, including healthcare, clinical diagnostics, and therapeutics.

Antibody Binding at the Liposome-Water Interface: A FRET Investigation toward a Liposome-Based Assay

Different signal amplification strategies to improve the detection sensitivity of immunoassays have been applied which utilize enzymatic reactions, nanomaterials, or liposomes. The latter are very attractive materials for signal amplification because liposomes can be loaded with a large amount of signaling molecules, leading to a high sensitivity. In addition, liposomes can be used as a cell-like "bioscaffold" to directly test recognition schemes aiming at cell-related processes. This study demonstrates an easy and fast approach to link the novel hydrophobic optical probe based on [1,3]dioxolo[4,5-f]-[1,3]benzodioxole (DBD dye mm239) with tunable optical properties to hydrophilic recognition elements (e.g., antibodies) using liposomes for signal amplification and as carrier of the hydrophobic dye. The fluorescence properties of mm239 (e.g., long fluorescence lifetime, large Stokes shift, high photostability, and high quantum yield), its high hydrophobicity for efficient anchoring in liposomes, and a maleimide bioreactive group were applied in a unique combination to build a concept for the coupling of antibodies or other protein markers to liposomes (coupling to membranes can be envisaged). The concept further allowed us to avoid multiple dye labeling of the antibody. Here, anti-TAMRA-antibody (DC7-Ab) was attached to the liposomes. In proof-of-concept, steady-state as well as time-resolved fluorescence measurements (e.g., fluorescence depolarization) in combination with single molecule detection (fluorescence correlation spectroscopy, FCS) were used to analyze the binding interaction between DC7-Ab and liposomes as well as the binding of the antigen rhodamine 6G (R6G) to the antibody. Here, the Förster resonance energy transfer (FRET) between mm239 and R6G was monitored. In addition to ensemble FRET data, single-molecule FRET (PIE-FRET) experiments using pulsed interleaved excitation were used to characterize in detail the binding on a single-molecule level to avoid averaging out effects.

A simulation-guided fluorescence correlation spectroscopy tool to investigate the protonation dynamics of cytochrome c oxidase

Fluorescence correlation spectroscopy (FCS) is a single molecule based technique to temporally resolve rate-dependent processes by correlating the fluorescence fluctuations of individual molecules traversing through a confocal volume. In addition, chemical processes like protonation or intersystem crossing can be monitored in the sub-microsecond range. FCS thereby provides an excellent tool for investigations of protonation dynamics in proton pumps like cytochrome c oxidase (CcO). To achieve this, the pH-dependent fluorescent dye fluorescein was attached as a protonation probe to the CcO surface via site-specific labeling of single reactive cysteines that are located close to the entry point of a proton input channel (K-pathway). The analysis of protonation dynamics is complicated by overlapping triplet and protonation rates of the fluorophore. A Monte Carlo simulation based algorithm was developed to facilitate discrimination of these temporally overlapping processes thus allowing for improved protonation reaction rate determination. Using this simulation-guided approach we determined precise local proton association and dissociation rates and provide information about protein surface effects, such as proton collecting antennae, on the transport properties of proton transfer channels.

Imaging fluorescence fluctuation spectroscopy: new tools for quantitative bioimaging

Fluorescence fluctuation spectroscopy (FFS) techniques provide information at the single-molecule level with excellent time resolution. Usually applied at a single spot in a sample, they have been recently extended into imaging formats, referred to as imaging FFS. They provide spatial information at the optical diffraction limit and temporal information in the microsecond to millisecond range. This review provides an overview of the different modalities in which imaging FFS techniques have been implemented and discusses present imaging FFS capabilities and limitations. A combination of imaging FFS and nanoscopy would allow one to record information with the detailed spatial information of nanoscopy, which is ∼20 nm and limited only by fluorophore size and labeling density, and the time resolution of imaging FFS, limited by the fluorescence lifetime. This combination would provide new insights into biological events by providing spatiotemporal resolution at unprecedented levels.

Time-averaged fluorescence intensity analysis in fluorescence fluctuation polarization sensitive experiments

In fluorescence fluctuation polarization sensitive experiments, the limitations associated with detecting the rotational timescale are usually eliminated by applying fluorescence correlation spectroscopy analysis. In this paper, the variance of the time-averaged fluorescence intensity extracted from the second moment of the measured fluorescence intensity is analyzed in the short time limit, before fluctuations resulting from rotational diffusion average out. Since rotational correlation times of fluorescence molecules are typically much lower than the temporal resolution of the system, independently of the time bins used, averaging over an ensemble of time-averaged trajectories was performed in order to construct the time-averaged intensity distribution, thus improving the signal-to-noise ratio. Rotational correlation times of fluorescein molecules in different viscosities of the medium within the range of the anti-bunching time (1-10 ns) were then extracted using this method.

Quantitative measurement of intracellular transport of nanocarriers by spatio-temporal image correlation spectroscopy

Spatio-temporal image correlation spectroscopy (STICS) is a powerful technique for assessing the nature of particle motion in complex systems although it has been rarely used to investigate the intracellular dynamics of nanocarriers so far. Here we introduce a method to characterize the mode of motion of nanocarriers and to quantify their transport parameters on different length scales from single-cell to subcellular level. Using this strategy we were able to study the mechanisms responsible for the intracellular transport of DOTAP-DOPC/DNA and DC-Chol-DOPE/DNA lipoplexes in CHO-K1 live cells. Measurement of both diffusion coefficients and velocity vectors (magnitude and direction) averaged over regions of the cell revealed the presence of distinct modes of motion. Lipoplexes diffused slowly on the cell surface (diffusion coefficient, D ≈ 0.003 µm²/s). In the cytosol, the lipoplexes' motion was characterized by active transport with average velocity ν ≈ 0.03 µm/s and random motion. The method permitted us to generate intracellular transport map showing several regions of concerted motion of lipoplexes.

Note: a 4 ns hardware photon correlator based on a general-purpose field-programmable gate array development board implemented in a compact setup for fluorescence correlation spectroscopy

We present a fast hardware photon correlator implemented in a field-programmable gate array (FPGA) combined with a compact confocal fluorescence setup. The correlator has two independent units with a time resolution of 4 ns while utilizing less than 15% of a low-end FPGA. The device directly accepts transistor-transistor logic (TTL) signals from two photon counting detectors and calculates two auto- or cross-correlation curves in real time. Test measurements demonstrate that the performance of our correlator is comparable with the current generation of commercial devices. The sensitivity of the optical setup is identical or even superior to current commercial devices. The FPGA design and the optical setup both allow for a straightforward extension to multi-color applications. This inexpensive and compact solution with a very good performance can serve as a versatile platform for uses in education, applied sciences, and basic research.

Photon arrival timing with sub-camera exposure time resolution in wide-field time-resolved photon counting imaging

We demonstrate that an ultra-fast CMOS camera combined with a photon counting image intensifier can be used to determine photon arrival times well below the exposure time of the camera. We can obtain a time resolution down to around 1% of the exposure time, i.e. of the order of 40 ns with microsecond exposure times. This is achieved by exploiting the invariant phosphor decay of the image intensifier's phosphor screen: Developing a suitable mathematical framework, we show that the relative intensities of the phosphor decay in successive frames following the photon detection uniquely determine the photon arrival time. This approach opens a way to measuring fast luminescence decays in parallel in many pixels. Possible applications include oxygen and ion concentration imaging using probes with luminescence lifetimes in the range of 100 ns to microseconds.