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Biomedical subjects

L F Cohen

Publications and source records attributed to L F Cohen.

At least 19 recordsLinked to original sources
PubMed2006

On the experimental estimation of surface enhanced raman scattering (SERS) cross sections by vibrational pumping.

We present an in-depth analysis of the experimental estimation of cross-sections in surface enhanced raman scattering (SERS) by vibrational pumping. The paper highlights the advantages and disadvantages of the technique, pinpoints the main aspects and limitations, and provides the underlying physical concepts to interpret the experimental results. Examples for several commonly used SERS probes are given, and a discussion on future possible developments is also presented. Obtaining good estimates of SERS cross-sections is, in general, an extremely hard problem and has been a longstanding ambition of the SERS community for reasons that go from the purely applied (quantification of signals) to the more fundamental (comparisons of theoretical electromagnetic enhancement factors with experiment). Any method that can produce a standard protocol for the estimation of cross-sections is, accordingly, of great interest and an effort to understand its principles and limitations is required.

Journal Article↗
PubMed2006

A conclusive demonstration of vibrational pumping under surface enhanced Raman scattering conditions.

We provide a conclusive demonstration of vibrational pumping under Surface Enhanced Raman Scattering (SERS) conditions by performing anti-Stokes/Stokes ratio measurements over a large spatial area and low power density, down to 10 K with dried silver colloids, the dye rhodamine 6G, and 676 nm laser excitation. The method we propose allows for the measurement of the cross sections for different modes and also provides the determination of the asymmetry between the anti-Stokes and Stokes SERS cross sections for each mode.

Journal Article↗
PubMed2006

Temperature-dependent anti-stokes/stokes ratios under surface-enhanced Raman scattering conditions.

We make systematic measurements of Raman anti-Stokes/Stokes (aS/S) ratios using two different laser excitations (514 and 633 nm) of rhodamine 6G (RH6G) on dried Ag colloids over a wide range of temperatures (100 to 350 K). We show that a temperature scan allows the separation of the contributions to the aS/S ratios from resonance effects and heating/pumping, thus decoupling the two main aspects of the problem. The temperature rise is found to be larger when employing the 633 nm laser. In addition, we find evidence for mode specific vibrational pumping at higher laser power densities. We analyze our results in the framework of ongoing discussion on laser heating/pumping under surface-enhanced Raman scattering (SERS) conditions.

Journal Article↗
PubMed2006

Spin relaxation by transient monopolar and bipolar optical orientation.

We have used two-color time-resolved spectroscopy to measure the relaxation of electron spin polarizations in a bulk semiconductor. The circularly polarized pump beam induces a polarization either by direct excitation from the valence band, or by free-carrier (Drude) absorption when tuned to an energy below the band gap. We find that the spin relaxation time, measured with picosecond time resolution by resonant induced Faraday rotation in both cases, increases in the presence of photogenerated holes. In the case of the material chosen, n-InSb, the increase was from 14 to 38 ps.

Journal Article↗
PubMed2006

A study of local heating of molecules under surface enhanced Raman scattering (SERS) conditions using the anti-Stokes/Stokes ratio.

We make systematic measurements of the anti-Stokes/Stokes (aS/S) ratios using low power lasers (0.5 mW at 514 and 633 nm) of rhodamine 6G (RH6G) on dried silver colloids over a wide range of temperatures from 140 to 350 K. We show that a scan in temperature allows the extraction of the contributions to the anti-Stokes/Stokes ratio from resonance effects and heating independently, thus decoupling the two aspects of the problem.

Journal Article↗
PubMed2005

Resonance contributions to anti-Stokes/Stokes ratios under surface enhanced Raman scattering conditions.

Anti-Stokes/Stokes asymmetries under surface enhanced Raman scattering (SERS) conditions are studied for a wide variety of SERS-active media and different analytes. Evidence is provided for the existence of underlying resonances that create these asymmetries. We show here that these resonances are associated with the electromagnetic coupling between the analyte (probe) and the metal. The work demonstrates the use of the anti-Stokes/Stokes ratio as a tool to understand the hierarchy of resonances in the SERS problem, which is essential for quantification purposes.

Journal Article↗
PubMed2004

Physics of single molecule fluctuations in surface enhanced Raman spectroscopy active liquids.

Surface enhanced Raman spectroscopy (SERS) of dyes in solution allows the study of the differences between ensemble averaged spectroscopic signals and single molecular events. We address several outstanding issues in single molecule detection via SERS; in particular, evidence for single molecule vibrational pumping and/or single molecule laser heating, the statistics of hotspots in the liquid, and anti-Stokes/Stokes anomalies. We demonstrate that anti-Stokes/Stokes ratios are a very unreliable measure of temperature, because the two processes are affected differently by the underlying frequency-dependent plasmon resonances. Subtle hints of vibrational pumping and/or heating in single molecules can only obtained via careful cross correlations between the parameters (frequency position, width, and intensity) of the Stokes signals for different excitation lasers. We introduce the use of single-peak parameter cross correlations for the study of these phenomena.

Journal Article↗
PubMed2004

Stokes/anti-Stokes anomalies under surface enhanced Raman scattering conditions.

The possibility of achieving anti-Stokes stimulation and/or pumping under surface enhanced Raman scattering conditions has been the source of intense controversies in the literature. With the aim of clarifying some of the aspects of this problem, we study theoretically and experimentally the situation in a model system which highlights some of the difficulties in the interpretation of the data. We show that many of the assumptions often presumed in the literature need to be assessed with care in each case. Through careful experiments we show, in particular, that the anti-Stokes/Stokes ratio for a specific mode in the same sample can depend on the chosen laser wavelength. This latter effect is a manifestation of the influence of the internal plasmon resonances (hot spots) in the result. Different possibilities and further research directions are highlighted and discussed.

Journal Article↗
PubMed2004

First order magnetic transition in doped CeFe2 alloys: phase coexistence and metastability.

First order ferromagnetic (FM) to antiferromagnetic (AFM) phase transition in doped CeFe2 alloys is studied with the micro-Hall probe technique. Clear visual evidence of magnetic phase coexistence on micrometer scales and the evolution of this phase coexistence as a function of temperature, magnetic field, and time across the first order FM-AFM transition is presented. Such phase coexistence and metastability arise as a natural consequence of an intrinsic disorder-influenced first order transition. The generality of these phenomena involving other classes of materials is discussed.

Journal Article↗
PubMed2001

Enhancement of the high-magnetic-field critical current density of superconducting MgB2 by proton irradiation.

Magnesium diboride, MgB2, has a relatively high superconducting transition temperature, placing it between the families of low- and high-temperature (copper oxide based) superconductors. Supercurrent flow in MgB2 is unhindered by grain boundaries, making it potentially attractive for technological applications in the temperature range 20-30 K. But in the bulk material, the critical current density (Jc) drops rapidly with increasing magnetic field strength. The magnitude and field dependence of the critical current are related to the presence of structural defects that can 'pin' the quantized magnetic vortices that permeate the material, and a lack of natural defects in MgB2 may be responsible for the rapid decline of Jc with increasing field strength. Here we show that modest levels of atomic disorder induced by proton irradiation enhance the pinning of vortices, thereby significantly increasing Jc at high field strengths. We anticipate that either chemical doping or mechanical processing should generate similar levels of disorder, and so achieve performance that is technologically attractive in an economically viable way.

Journal Article↗
PubMed2001

Vortex dynamics in superconducting MgB2 and prospects for applications.

The recently discovered superconductor magnesium diboride, MgB2, has a transition temperature, Tc, approaching 40 K, placing it intermediate between the families of low- and high-temperature superconductors. In practical applications, superconductors are permeated by quantized vortices of magnetic flux. When a supercurrent flows, there is dissipation of energy unless these vortices are 'pinned' in some way, and so inhibited from moving under the influence of the Lorentz force. Such vortex motion ultimately determines the critical current density, Jc, which the superconductor can support. Vortex behaviour has proved to be more complicated in high-temperature superconductors than in low-temperature superconductors and, although this has stimulated extensive theoretical and experimental research, it has also impeded applications. Here we describe the vortex behaviour in MgB2, as reflected in Jc and in the vortex creep rate, S, the latter being a measure of how fast the 'persistent' supercurrents decay. Our results show that naturally occurring grain boundaries are highly transparent to supercurrents, a desirable property which contrasts with the behaviour of the high-temperature superconductors. On the other hand, we observe a steep, practically deleterious decline in Jc with increasing magnetic field, which is likely to reflect the high degree of crystalline perfection in our samples, and hence a low vortex pinning energy.

Journal Article↗
PubMed2000

Identification and evaluation of axillary sentinel lymph nodes in patients with breast carcinoma treated with neoadjuvant chemotherapy.

Sentinel lymph node (SLN) biopsy has been shown to predict axillary metastases accurately in early stage breast cancer. Some patients with locally advanced breast cancer receive preoperative (neoadjuvant) chemotherapy, which may alter lymphatic drainage and lymph node structure. In this study, we examined the feasibility and accuracy of SLN mapping in these patients and whether serial sectioning and keratin immunohistochemical (IHC) staining would improve the identification of metastases in lymph nodes with chemotherapy-induced changes. Thirty-eight patients with stage II or III breast cancer treated with neoadjuvant chemotherapy were included. In all patients, SLN biopsy was attempted, and immediately afterward, axillary lymph node dissection was performed. If the result of the SLN biopsy was negative on initial hematoxylin and eosin-stained sections, all axillary nodes were examined with three additional hematoxylin and eosin sections and one keratin IHC stain. SLNs were identified in 31 (82%) of 38 patients. The SLN accurately predicted axillary status in 28 (90%) of 31 patients (three false negatives). On examination of the original hematoxylin and eosin-stained sections, 20 patients were found to have tumor-free SLNs. With the additional sections, 4 (20%) of these 20 patients were found to have occult lymph node metastases. These metastatic foci were seen on the hematoxylin and eosin staining and keratin IHC staining. Our findings indicate that lymph node mapping in patients with breast cancer treated with neoadjuvant chemotherapy can identify the SLN, and SLN biopsy in this group accurately predicts axillary nodal status in most patients. Furthermore, serial sectioning and IHC staining aid in the identification of occult micrometastases in lymph nodes with chemotherapy-induced changes.

Adenocarcinoma, Mucinous↗
PubMed1997

Streptococcus bovis infection of the central nervous system: report of two cases and review.

Streptococcus bovis is an uncommon cause of meningitis and subdural empyema. We report one case each of meningitis and subdural empyema in which S. bovis biotype II was isolated from both the spinal fluid and blood. In one case, the organisms were seen on a gram-stained preparation of cerebrospinal fluid. The first patient presented with gastrointestinal symptoms of unknown etiology, was immunosuppressed, and recovered. The second patient presented with syncope, developed a subdural empyema, and died; at autopsy, a colonic adenoma was found. A review of the English-language literature revealed only 14 previously reported cases of meningitis due to S. bovis and no cases of subdural empyema due to S. bovis. These cases indicate the importance of complete laboratory identification of specific organisms and confirm the need for a thorough neurological examination and search for underlying gastrointestinal disease in cases of S. bovis infection.

Aged↗
PubMed1997

A new staining method for identification of Helicobacter pylori and simultaneous visualization of gastric morphologic features.

Helicobacter pylori (HP) is prevalent in the general population and is associated with chronic active gastritis, peptic ulcers, gastric carcinoma, and lymphoma. Different methods to diagnose HP colonization include the urea breath test, serologic analysis, and gastric biopsy. Many different staining methods, including silver and Giemsa-based stains, have been used to demonstrate these organisms in gastric biopsy specimens. The tissue morphologic features, however, are often obscured, thus mandating evaluation of both hematoxylin and eosin (H&E) and special stains. This study evaluates the ability of a new staining method that allows simultaneous identification of HP and visualization of tissue morphologic features. We examined formalin-fixed, paraffin-embedded archival tissues from 184 consecutive gastric biopsy specimens using our new staining method, and we compared our results with those previously obtained from H&E and Warthin-Starry (WS) stains. Our new stain uses periodic acid-Schiff, Coleman's Feulgen solution, Mayer's hematoxylin, and methylene blue. Our method is technically simpler than the WS stain and can be completed in approximately 9 minutes. We found HP organisms in 76 (41%) of 184 biopsy specimens by H&E and/or WS staining. Our new staining method identified HP in 83 specimens (45%). The organisms stained bright blue and were easily visualized (compared with the H&E-stained specimens) because of the contrasting magenta mucin. Tissue morphologic features consisted of well-defined cells with dark blue nuclei and pale cytoplasm. Intestinal metaplasia was highlighted magenta Our new staining method revealed chronic active gastritis in 71 cases (39%), intestinal metaplasia in 18 cases (10%), and lymphoid aggregates in 35 cases (19%). These findings were similar to the results obtained with H&E-stained sections. We conclude that our new staining method is inexpensive, quick, and easy to perform and interpret. It has a sensitivity comparable to that of conventional staining methods and simultaneously demonstrates both tissue morphologic features and the presence of HP.

Biopsy↗
PubMed1995

Development of degenerative spondylosis of the lumbar spine after partial discectomy. Comparison of laminotomy, discectomy, and posterolateral discectomy.

STUDY DESIGN: The development of degenerative spondylosis after successful operative decompression of the affected nerve root was prospectively evaluated in a comparative case series of 100 patients with a herniated lumbar nucleus pulposus. OBJECTIVES: The objective of this study was to compare the relative incidence of degenerative spondyloarthrosis after successful posterior laminotomy and discectomy and posterolateral extradural discectomy for decompression of a compromised lumbar nerve root. SUMMARY OF BACKGROUND DATA: The relationship between the radiographic appearance of degenerative spondylosis and prior operative procedures has been controversial and at times contradictory. The posterior operative approach with a partial discectomy has been associated with a significant incidence of postoperative degenerative spondylosis and intraneural and perineural fibrosis, complications that may be minimized, or perhaps even eliminated, with the posterolateral evacuation of the offending intervertebral disc fragment. METHODS: Each patient had: 1) not responded favorably to nonoperative treatment, 2) a persistent radiculopathy, 3) correlative imaging studies with no preoperative spondyloarthrosis and 4) minimum 2-year follow-up. Fifty patients were treated by posterior laminotomy and discectomy and fifty were treated by a posterolateral extradural discectomy. Postoperative spondylosis was graded based on the radiographic presence or absence of osteophytes, the intervertebral disc height, the vertebral body alignment and the facet joint changes. RESULTS: At an average postoperative follow-up of 65 months, the incidence of a one grade increase in degenerative spondylosis was 80% of the laminotomy and discectomy patients as compared to 39% of the posterolateral discectomy patients. CONCLUSIONS: The increased incidence of spondyloarthrosis with the posterior approach suggests that minimally invasive posterolateral extradural procedures should be considered for the decompression of a compromised lumbar nerve root.

Diskectomy↗
PubMed1993

Arthroscopic microdiscectomy versus nucleotomy techniques.

Arthroscopic microdiscectomy represents an alternative method of treatment for herniated lumbar disc and its associated radiculopathy. In contrast to nucleotomy procedures that allow for only central evacuation of the nuclear mass, in arthroscopic microdiscectomy, the emphasis is placed on evacuation of the posterior collagenized fragments, which are responsible for mechanical compression of the nerve roots. Over a period of 10 years, 250 patients with unremitting sciatica due to herniated nucleus pulposus have been treated at the Graduate Hospital through a posterolateral approach via a 6.5-mm cannula. Our overall successful results have been 87.5%, and we have not encountered any neurovascular or other serious complications.

Arthroscopes↗