Mona B. Mohamed

The `lightning' gold nanorods: fluorescence enhancement of over a million compared to the gold metal

Abstract Gold nanorods capped with micelles and having an aspect ratio of 2.0–5.4 are found to fluoresce with a quantum yield which is over a million times that of the metal. For rods of the same width, the yield is found to increase quadratically while the wavelength maximum increases linearly with the length. We assign this emission to the electron and hole interband recombination. The increase in the emission yield results from the enhancement effect of the incoming and outgoing electric fields via coupling to the surface plasmon resonance in the rods. This is similar to the previously proposed fluorescence and the Raman enhancement on noble metal rough surfaces.

Crystallographic facets and shapes of gold nanorods of different aspect ratios

Abstract Crystal structures of gold nanorods synthesized electrochemically using micelles as a capping material have been studied by high-resolution transmission electron microscopy. Short gold nanorods with aspect ratios of 3–7 are enclosed mainly by {100} and {110} facets and their axial growth direction is [001], whereas long gold nanorods of aspect ratios 20–35 are dominated by {111} and {110} facets and their growth axial direction is 〈112〉. The short rods are the dominant constituents, whereas the long rods are observed occasionally. Spherical-like Au particles with equivalent mass to the short rods are dominated by {111} and {100} facets with shapes of truncated octahedra, icosahedra and decahedra. The unique {110} facets of Au nanorods are expected to have interesting surface properties.

Relative enhancement of ultrafast emission in gold nanorods

Time-resolved fluorescence spectroscopy has been employed to probe the ultrafast emission observed in gold nanospheres and nanorods. The decay of the emission was found to be ≤50 fs. The ultrafast emission from nanorods has been directly compared to that for nanospheres and the dispersion of the relative local field enhancement factor was obtained and compared with calculations. Measurements with both visible and ultraviolet excitations have been analyzed. These results give the first observation and analysis of the mechanism for the local field-enhanced ultrafast emission in gold nanorods and nanospheres.

Hot electron and phonon dynamics of gold nanoparticles embedded in a gel matrix

Abstract Using pump-probe technique, the dynamics of the hot carriers in metallic nanodots induced by femtosecond laser pulses are investigated in gold nanoparticles embedded in hydrogel and in organic gel and compared to that in aqueous solution. We found that changing the surrounding matrix from aqueous solution to hydrogel and then to organic gel leads to a large increase in the relaxation time of both the electron–phonon (e–ph) and the phonon–phonon (ph–ph) coupling. Furthermore, the ph–ph relaxation time becomes sensitive to the type of the organic solvent trapped in the gel network. This indicates that the relaxation dynamics depend on the thermal conductivity, chemical structure and the molecular dynamics of the surrounding medium.

Some properties of spherical and rod-shaped semiconductor and metal nanocrystals

In this paper, we summarize our results on the effect of changing size and shape of semiconductor and metallic nanocrystals. The change in size (from 3.5 to 1.1 nm) and shape (from spherical to rod) of CdSe nanocrystals is found to affect both their optical and nonradiative properties. In gold quasi-spherical nanocrystals, results on the dependence of the electron-phonon relaxation of the surrounding medium are presented. For gold nanorods, results and conclusions on the mechanism of their self-assembly are presented. Enhanced Raman scattering on their surfaces is studied and compared with that on spherical particle surfaces.

The pump power dependence of the femtosecond relaxation of CdSe nanoparticles observed in the spectral range from visible to infrared

The pump power dependence of the relaxation dynamics of CdSenanoparticles (NPs) was studied with femtosecond pump probe spectroscopy at observation wavelengths of the first exciton transition at 560 nm, the near infrared (NIR) absorption at 2 μm, and the transient mid-infrared (IR) absorption at 4.5 μm. Excitation with less than one photon per particle leads to bleaching of the excitonic transitions, and the bleach intensity is initially linear to the pump power. At higher pump power the bleach intensity levels off, when complete saturation of the excitonic transition is reached. At the same time, increasing pump power causes an acceleration of the bleach decay, which is due to additional Auger processes when multiple excitons are formed in the NPs. In addition, the pump power effect was investigated for the NIR and IR regions, at 2 and 4.5 μm wavelength, respectively. Whereas the IR transients are very similar to the ones observed for the bleach, the NIR transients behave completely different. No pump power dependence was found for the transients at 2 μm when pumped in a power range from 0.5 to 5 μJ per pulse. The results show that the fs transients in the visible (bleach) and IR (absorption) regions are due to electron relaxation in the conduction band and the NIR transients are due to the relaxation of the hole. Furthermore, it suggests that in the investigated CdSe NPs, Auger processes act much more efficiently on the electrons than for the holes.

5-Fluorouracil Induces Plasmonic Coupling in Gold Nanospheres: New Generation of Chemotherapeutic Agents

Loading 5- Fluorouracil (5-FU) into gold nanoparticles (AuNPs) could enhance its activity as anticancer drug hugely by enhancing its ability for penetration through the cell membrane. Accordingly, this work is devoted to loading 5-FU into AuNPs surface and studying the binding mechanism of the drug to the surface of the gold nanoparticles. Our finding indicates that new absorption band appears at longer wavelength upon loading 5-FU into gold nanospheres capped with citrate. This near IR band is due to induced surface plasmon coupling via hydrogen bonding between 5-FU and surface capping AuNPs. This leads to great enhancement of the drug action as chemotherapeutic as well as photothermal agents. Factors which affect the binding between 5-FU and the AuNPs such as pH, time after mixing the ndrug with AuNPs, concentration of the 5-FU, have been studied in detail. Accordingly, the binding interaction is proven to be via hydrogen bonding. Upon the investigation of thermal and photo satiability, the formed composite 5-FU@ AuNPs showed high stability towards these factors. The spectral and morphological studies were measured via UV-VIS spectroscopy and Transmission Electron Microscopy (HR-TEM). Remarkable increases in the drug anticancer activity upon loading into AuNPs were observed for the cell viability test of human colon cancer (HCT16).

Cervical Pedicle Screw Fixation: Anatomic Feasibility of Pedicle Morphology and Radiologic Evaluation of the Anatomical Measurements

Study Design All parameters were measured manually and with a computed tomography (CT) scanner. For the manual measurements, a Vernier scale instrument was used. Purpose This study evaluates quantitatively pedicles of middle and lower cervical spine (C3 to C7) and to evaluate the possibilities of using these structures as anchors in posterior cervical fusion. Overview of Literature Pedicle screws may be an alternative fixation technique for posterior cervical instrumentation. Methods Twenty-two bony sets of adult cervical spines were studied (110 vertebrae, 220 pedicles) from C3 down to C7. Results CT measurement of cervical pedicles appeared to be accurate and valuable for preoperative planning of cervical pedicle screw instrumentation. The study showed a high correlation between the values obtained by manual and CT measurements of pedicle dimensions. The technical challenge of insertion is the obvious theoretical drawback of the use of cervical pedicle screws. Many technical factors are important to consider, namely, the point of screw entry, the pedicle dimensions, the screw direction according to the pedicle angle and orientation, the screw diameter and length, and the method of screw introduction. Conclusions Transpedicular screw fixation of the cervical spine appears to be promising. Anatomic limitations should be clear to the surgeon. Further clinical and biomechanical studies are needed to settle this technique.

Dynamic Cervical Implant (DCI) In Single Level Cervical Disc Disease

Introduction: This study evaluated the safety and effectiveness of Dynamic Cervical Implants (DCI) as a form of dynamic instrumentation to treat single level cervical disc disease. This is a prospective case study of 15 consecutive patients, treated by the author, with single level cervical disc disease. Study duration was 3 years from October 2009 to October 2012. These 15 cases included 10 men and 5 women. Patients ages ranged from 35 to 54 years (mean, 47 years). All patients had anterior cervical discectomy with interbody DCI implantation, at a single level from C3 to C7. Placement of the DCI implant aimed at reconstruction of the anterior column to preserve a controlled flexion and extension motion. Materials and Methods: Among the 15 cases, herniated cervical disc (HCD) accounted for 7 and cervical degenerative disc disease (DDD) for 8. Neck pain was present in all cases, radiculopathy in 9 cases, radiculomyelopathy in 4 cases and myelopathy in 2 cases. Preoperative evaluation included plain cervical x-rays including dynamic views. Magnetic reso- nance imaging (MRI) was carried out routinely to confirm the clinical diagnosis. Results: Our study showed that the procedure was safe and easy. There were no major complications. The most common DCI prosthesis used in this study was the 6M size with a width of 12 mm, a depth of 12 mm, and a height of 9 mm. No implant related complications were reported. This procedure has the advantages of shorter operative time; minimal blood loss; and early postoperative recovery. Neck pain and radiculopathy improved in 86.7% of cases, and myelopathy in 50% of cases. Hetertopic ossification indicating fusion was not observed. Conclusion: Disc replacement with DCI is a new strategy, in between ACDF & ADR. It is an intermediate solution in the spectrum of management strategies of cervical disc diseases. Immediate dynamic stability with good clinical response and no implant-related morbidity or complications are the main advantages of this implant.