Magnetic and fluorescent core-shell nanoparticles for ratiometric pH sensing

This paper describes the preparation of nanoparticles composed of a magnetic core surrounded by two successive silica shells embedding two fluorophores, showing uniform nanoparticle size (50-60 nm in diameter) and shape, which allow ratiometric pH measurements in the pH range 5-8. Uncoated iron oxide magnetic nanoparticles (～10 nm in diameter) were formed by the coprecipitation reaction of ferrous and ferric salts. Then, they were added to a water-in-oil microemulsion where the hydrophilic silica shells were obtained through hydrolysis and condensation of tetraethoxyorthosilicate together with the corresponding silylated dye derivatives-a sulforhodamine was embedded in the inner silica shell and used as the reference dye while a pH-sensitive fluorescein was incorporated in the outer shell as the pH indicator. The magnetic nanoparticles were characterized using vibrating sample magnetometry, dynamic light scattering, transmission electron microscopy, x-ray diffraction and Fourier transform infrared spectroscopy. The relationship between the analytical parameter, that is, the ratio of fluorescence between the sensing and reference dyes versus the pH was adjusted to a sigmoidal fit using a Boltzmann type equation giving an apparent pK(a) value of 6.8. The fluorescence intensity of the reference dye did not change significantly (～3.0%) on modifying the pH of the nanoparticle dispersion. Finally, the proposed method was statistically validated against a reference procedure using samples of water and physiological buffer with 2% of horse serum, indicating that there are no significant statistical differences at a 95% confidence level.     

Red/orange dual-emissive carbon dots for pH sensing and cell imaging

The dual-emissive N, S co-doped carbon dots (N, S-CDs) with a long emission wavelength were synthesized via solvothermal method. The N, S-CDs possess relatively high photoluminescence (PL) quantum yield (QY) (35.7%) towards near-infrared fluorescent peak up to 648 nm. With the advanced characterization techniques including X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), etc. It is found that the doped N, S elements play an important role in the formation of high QY CDs. The N, S-CDs exist distinct pH-sensitive feature with reversible fluorescence in a good linear relationship with pH values in the range of 1.0–13.0. What is more, N, S-CDs can be used as an ultrasensitive Ag⁺ probe sensor with the resolution up to 0.4 μM. This finding will expand the application of as prepared N, S-CDs in sensing and environmental fields.

     

Wavelength demodulation of ultrabright green light-emitting diodes for electrical current sensing

We report on a novel electrical current-sensing principle based on wavelength-encoded modulation of the ultrabright green (at 525 nm) light-emitting diode (LED) transducers. It complies with the optical subsystem of a hybrid current transformer . Real-time wavelength demodulation is performed with the passive spectral edge filter OG 530. Linear calibration plots were achieved with -0.33 nm/mA for dc and +0.99 mA/sup -1/ for ac current sensitivity, respectively. A measurement accuracy of 1.3% for 28.4-mA ac peak current range is achieved. A simple theoretical model is outlined. Issues such as electronic and thermal effects on stability performance are also addressed.     

Photoluminescence properties of N-doped carbon dots prepared in different solvents and applications in pH sensing

Two kinds of nitrogen-doped carbon dots (N-CDs) were synthesized by choosing citric acid, l-serine and monoethanolamine as the precursors, respectively, in organic solvent N,N-dimethylformamide (DMF) and deionized water. The FTIR, UV–Vis, HRTEM and XRD were used to characterize the surface molecular structures of these two N-CDs. It was found that different reaction systems made the as-prepared N-CDs possess varied surface states and thus exhibit distinctive photoluminescence features. For the N-CDs prepared in DMF, its fluorescent property showed pH dependent and color-switchable in visible light and could be reversibly changing over in both alkaline and acidic environments due to the protonation and de-protonation of the carboxyl groups on the surface of the N-CDs. Also, its fluorescence intensity exhibited a linear fashion over the pH range from 1.5 to 7.5, indicating its potential applications in pH quantitative detection under acidic condition.     

Empirical pseudo-potential studies on electronic structure of semiconducting quantum dots

Theoretical investigations of electronic structure of quantum dots is of current interest in nanophase materials. Empirical theories such as effective mass approximation, tight binding methods and empirical pseudo-potential method are capable of explaining the experimentally observed optical properties. We employ the empirical pseudo-potential to calculate the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) as a function of shape and size of the quantum dots. Our studies explain the building up of the bulk band structure when the size of the dot is much larger than the bulk Bohr exciton radius. We present our investigations of HOMO-LUMO gap variation with size, for CdSe, ZnSe and GaAs quantum dots. The calculated excitonic energies are sensitive to the shape and size of quantum dots and are in good agreement with experimental HOMO-LUMO gaps for CdSe quantum dots. The agreement improves as experimentally observed lattice contraction is incorporated in pseudo-potential calculations for ZnSe quantum dots. Electronic structure evolution, as the size of quantum dot increases, is presented for CdSe, ZnSe and GaAs quantum dots.     

Peroxynitrous-acid-induced chemiluminescence of fluorescent carbon dots for nitrite sensing

In this work, chemiluminescent (CL) property of the carbon dots in the presence of peroxynitrous acid was studied. Peroxynitrous acid is formed by online mixing of nitrite and acidified hydrogen peroxide. The CL intensity was increased linearly with nitrite concentration in the range from 1.0 × 10(-7) M to 1.0 × 10(-5) M, and the detection limit was 5.3 × 10(-8) M (signal-to-noise ratio of S/N = 3). This method has been successfully applied to the determination of nitrites in pond water, river water, and pure milk, with recoveries in the range of 98%-108%. The CL mechanism of the peroxynitrous acid-carbon dots system was investigated using the CL, ultraviolet-visible light (UV-vis), and electron paramagnetic resonance (EPR) spectra. The electron-transfer annihilation of hole-injected and electron-injected carbon dots could mainly account for the CL emission, which sheds new light on the optical properties of the carbon dots.     

Heavy metallic oxide nanoparticles for enhanced sensitivity in semiconducting polymer x-ray detectors

Semiconducting polymers have previously been used as the transduction material in x-ray dosimeters, but these devices have a rather low detection sensitivity because of the low x-ray attenuation efficiency of the organic active layer. Here, we demonstrate a way to overcome this limitation through the introduction of high density nanoparticles having a high atomic number (Z) to increase the x-ray attenuation. Specifically, bismuth oxide (Bi(2)O(3)) nanoparticles (Z?=?83 for Bi) are added to a poly(triarylamine) (PTAA) semiconducting polymer in the active layer of an x-ray detector. Scanning electron microscopy (SEM) reveals that the Bi(2)O(3) nanoparticles are reasonably distributed in the PTAA active layer. The reverse bias dc current-voltage characteristics for PTAA-Bi(2)O(3) diodes (with indium tin oxide (ITO) and Al contacts) have similar leakage currents to ITO/PTAA/Al diodes. Upon irradiation with 17.5?keV x-ray beams, a PTAA device containing 60?wt% Bi(2)O(3) nanoparticles demonstrates a sensitivity increase of approximately 2.5 times compared to the plain PTAA sensor. These results indicate that the addition of high-Z nanoparticles improves the performance of the dosimeters by increasing the x-ray stopping power of the active volume of the diode. Because the Bi(2)O(3) has a high density, it can be used very efficiently, achieving a high weight fraction with a low volume fraction of nanoparticles. The mechanical flexibility of the polymer is not sacrificed when the inorganic nanoparticles are incorporated.     

Biofunctionalised quantum dots for sensing and identification of waterborne bacterial pathogens

Harmful bacteria are the most common cause of food- and waterborne illnesses. Infection often leads to bloody diarrhoea, and occasionally to kidney failure. Several strains of the bacteria Escherichia coli produce a powerful toxin which causes serious illness. Food and water can be contaminated with other bacteria like Salmonella, Coliform, Pseudomonas, etc. Hence, it has become important to rapidly detect and identify infectious bacteria. Colloidal luminescent semiconductor nanocrystals or quantum dots (QDs) have elicited a great deal of interest in the biosensing community due to their unique fluorescent properties. Here ZnS?:?Mn²⁺ QDs are synthesised and biofunctionalised with chitosan. They are attached to the anionic cell wall of E. coli bacteria and different properties of this compound system are studied. These nanocrystals may offer cost effective and quicker alternative to detect single bacterium compared to other conventional methods. The process of the synthesis of QDs, biofunctionalisation and detection of bacteria have been characterised by XRD, UV-Vis spectroscopy, FTIR, photoluminescence spectroscopy, AFM, high-resolution transmission electron microscopy and confocal microscopy. The particle size calculated is approximately 8–10?nm. The blue shift of PL peak has been observed after the bacteria get attached.     

Nanocomposites based on highly luminescent nanocrystals and semiconducting conjugated polymer for inkjet printing

In this work nanocomposites based on organic-capped semiconductor nanocrystals formed of a core of CdSe coated with a shell of ZnS (CdSe@ZnS), with different sizes, and a semiconducting conjugated polymer, namely poly[(9,9-dihexylfluoren-2,7-diyl)-alt- (2,5-dimethyl-1,4-phenylene)] (PF-DMB) have been investigated. The nanocomposites are prepared by mixing the pre-synthesized components in organic solvents, thereby assisting the dispersion of the organic-coated nano-objects in the polymer host. UV-vis steady state and time-resolved spectroscopy along with (photo)electrochemical techniques have been performed to characterize the obtained materials. The study shows that the embedded nanocrystals increase the PF-DMB stability against oxidation and, at the same time, extend the light harvesting capability to the visible spectral region, thus resulting in detectable photocurrent signals. The nanocomposites have been dispensed by means of a piezo-actuated inkjet system. Such inks present viscosity and surface tension properties well suited for stable and reliable drop-on-demand printing using an inkjet printer. The fabrication of arrays of single-color pixels made of the nanocomposites and micrometers in size has been performed. Confocal and atomic force microscopy have confirmed that inkjet-printed microstructures present the intrinsic emission properties of both the embedded nanocrystals and PF-DMB, resulting in a combined luminescence. Finally, the morphology of the printed pixels is influenced by the embedded nanofillers.     

Ferromagnetic microwires enabled polymer composites for sensing applications

In the present work, sensing functionalities are introduced into structural composites via embedded magnetic microwires. A systematic study on the structure and functionalities of microwires and their composites is performed. The single-wire composite shows a significant giant magnetoimpedance (GMI) effect of up to 320% in a frequency range of 1–100 MHz due to stress enhanced transverse magneto-anisotropy. With increasing quantities of embedded wires from 1 to 3, the maximum GMI ratio is enhanced significantly by more than 35%, making the resultant composite favourable for field sensing applications. The microwire-composite also shows superior stress-sensing resolution as high as 134.5 kHz/microstrain, which is about 26 times higher than the recently proposed SRR-based sensor. As evidenced by the structural examination and tensile tests, the extremely small volume fraction of microwires (～0.01 vol.%) allows the wire-composites to retain their mechanical integrity and performance.     

Patterned layers of a semiconducting polymer via imprinting and microwave-assisted grafting

Enhancements in both the rate and extent of grafting of poly(9,9'-n-dihexyl fluorene) (PDHF) onto flat and nanopatterned crosslinked photopolymer films are described. Reactivity of the surfaces toward grafting via the Yamamoto-type Ni(0)-mediated coupling reaction is increased by synthesizing and incorporating 2,7-dibromo-9-fluorenyl methacrylate (DBFM, 2) as a new grafting agent. Varying the concentration of surface-embedded DBFM is shown to control both overall graft formation and fluorescence with a maximum thickness of up to 30 nm and peak emission at 407 nm for 40 wt% loading. In addition, microwave irradiation is introduced as an effective means to drive graft formation and thus allows fabrication of PDHF-functionalized surfaces in as little as 30 min. Both forms of improvement are extended to DBFM-embedded, nanocontact-molded features ranging in size from 100 microm to 100 nm in width and 60 nm in height. Microwave-assisted grafting from these patterned surfaces produces fluorescent features as imaged by optical microscopy and a corresponding increase in feature height as measured by atomic force microscopy.     

Photoluminescence quenching of semiconducting polymer nanoparticles in presence of Au nanoparticles

In this report, we have demonstrated the photoluminescence quenching and energy transfer properties of semiconducting polymer nanoparticles, poly (N-vinylcarbazole) (PVK) in presence of different sized Au nanoparticles by steady state and time-resolved spectroscopy. We have described the quenching phenomena by sphere of action static quenching mechanism and both dynamic and static quenching processes are found in these systems. PL quenching values are 24· 22% and 57· 3% for 14?nm and 18?nm Au nanoparticles, respectively. It is found that the radiative and nonradiative decay have been modified with the size of Au nanoparticles. PL quenching and shortening of decay time regarding polymer nanoparticles in presence of Au nanoparticles suggest the nonradiative energy transfer process. The values of energy transfer are 6· 7%, 49· 5% and 53· 38% from PVK polymer nanoparticles to 3?nm, 14?nm and 18?nm Au nanoparticles, respectively. Using FRET and SET equations we have calculated the average distance of donor PVK polymer nanoparticles and acceptor Au nanoparticles.     

Solvent additive control of morphology and crystallization in semiconducting polymer blends

4-bromoanisole is used as a very versatile processing additive to control the phase separation and phase purity of organic photovoltaic devices. Polymer-polymer blends based on P3HT as donor and a wide range of acceptor materials (F8TBT, PCDTBT,…?) are investigated. The additive promotes the aggregation of P3HT which improves the morphology for both initially mixed and demixed blends.     

CdTe quantum dots and polymer nanocomposites for x-ray scintillation and imaging

Investigations are reported on the x-ray scintillation and imaging application of CdTe quantum dots (QDs) and their polymer nanocomposites. Aqueous CdTe QDs with emissions ranging between 510 and 680 nm were prepared and incorporated into polyvinyl alcohol or polymethyl methacrylate polymer matrices. The x-ray luminescent properties were evaluated and a resolution of 5 lines∕mm was obtained from the nanocomposite films. Additionally, the fast decay time, nonafterglow, and superior spectral match to conventional charge coupled devices, show that CdTe QD nanocomposites have high promise for x-ray imaging applications.     

Fluorescence lifetime spectroscopy for pH sensing in scattering media

Fluorescence lifetime spectroscopy in the presence of tissuelike scattering is demonstrated from measurements of phase and modulation ratio as a function of modulation frequency using a pH-sensitive dye, Carboxy Seminaphthofluorescein-1 (C-SNAFL-1). From the optical diffusion equation describing the propagation and generation of fluorescence within solutions of 0.5 microM C-SNAFL-1 containing 2.0% (by volume) of Intralipid as a scatterer, the values of the average lifetime of C-SNAFL-1 were determined as the solution pH varied between 5 and 9. Average lifetime values were found to match those measured using traditional phase-modulation measurement in nonscattering media. Furthermore, the robustness of the spectroscopic technique was demonstrated by conducting lifetime measurements at varying scatterer concentrations (1.5-3.0 vol % Intralipid). These results confirm the approach for analytical sensing in scattering media via fluorescence lifetime kinetics in order to track changes in analyte concentrations.     

Micelle nanoparticles for FRET-based ratiometric sensing of mercury ions in water, biological fluids and living cells

A fluorescence resonance energy transfer (FRET) based ratiometric sensing system for mercury ions is built in nano-sized core/corona micelles formed by a poly(ethylene oxide)-b-polystyrene diblock copolymer. For this system, a hydrophobic fluorescein derivative (FLS-C12), which serves as the energy transfer donor, is incorporated into the micelle core during the micelle formation; and a spirolactam-rhodamine derivative (RhB-CS) as a probe for mercury ions is located at the micelle core/corona interface. An efficient ring-opening reaction of RhB-CS induced by mercury ions generates the long-wavelength rhodamine B fluorophore which can act as the energy acceptor, affording the micelle nanoparticles the water-dispersible FRET-based ratiometric detection system for mercury ions, with a detection limit of 0.1 μM in water. The donor and the probe fluorophores, with their structure being appropriately modified, can strongly bind (non-covalently) to the specific sites of the micelles and form a stable ratiometric sensor in water and in some biological fluids. In addition, with the water-soluble and biocompatible poly(ethylene oxide) (PEO) as the corona of the micelles, the nano-sized sensing system can readily permeate through cell membrane and detect intracellular Hg(2+) level changes.     

Coupled electrorotation of polymer microspheres for microfluidic sensing and mixing

We show that coupled electrorotation (CER) of microscopic particles using microfabricated electrodes can be used for localized sensing and mixing. The effective use of microelectromechanical systems and micro total analysis systems requires many types of control. These include the abilityto (1) manipulate objects within microchannels by noncontact means, (2) mix fluids, and (3) sense local chemical parameters. Coupled electrorotation, in which the interactions between induced electric dipoles of adjacent particles lead to particle rotation, addresses aspects of all three challenges simultaneously. CER is a simple means of controlling the rotation of dielectric objects using homogeneous external radio frequency electric fields. CER is sensitive to several chemical and physical parameters such as the solution conductivity, pH, and viscosity. As a step toward integrating CER devices into microfluidic systems, a simple chip was designed to induce local mixing and to detect local changes in salt concentration, pH, and viscosity.