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V Lakshminarayanan

Publications and source records attributed to V Lakshminarayanan.

At least 19 recordsLinked to original sources
PubMed2006

Self-assembled monolayers of alkanethiols on gold prepared in a hexagonal lyotropic liquid crystalline phase of Triton X-100/water system.

In this paper, we have reported a new method of preparing self-assembled monolayers (SAMs) of decanethiol and hexadecanethiol on gold surface by using a lyotropic liquid crystalline phase as an adsorbing medium. The stability and blocking ability of these SAMs were characterized using grazing angle Fourier transform infrared (FTIR) spectroscopy and electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy. The lyotropic liquid crystalline medium possesses a hexagonal structure consisting of a nonionic surfactant Triton X-100, water, and the corresponding thiol, which provides a highly hydrophobic environment to solubilize the alkanethiols and later to facilitate their delivery to the gold surface. We find that the SAMs formed from the hexagonal liquid crystalline phase are highly compact and have excellent electrochemical blocking ability towards the redox probes compared to conventional SAMs prepared from commonly used organic solvents such as ethanol. From the impedance studies, we have determined the capacitance of the monolayer-coated electrodes and the surface coverage of the SAM, which has been found to be >99.98% on gold surface. We have also estimated the extent of ionic permeability through the film and measured the rate constants for the redox reactions on the SAM-modified electrodes. Our results show that the rate constants of [Fe(CN)6](3-/4-) and [Ru(NH3)6](2+/3+) redox couples are very much lower in the case of monolayers prepared in liquid crystalline phase compared to the SAM formed in 1 mM thiol in ethanol solution, suggesting a better blocking ability of the SAMs in the former case. From the grazing angle FTIR spectroscopic studies and capacitance measurements, we have ruled out any coadsorption of surfactant molecules on the Au surface. These results suggest that SAMs of very low defect density and extremely low ionic permeability can be obtained when a hexagonal lyotropic liquid crystalline phase is used as an adsorbing medium.

Journal Article↗
PubMed2005

Self-assembled monolayers (SAMs) of alkoxycyanobiphenyl thiols on gold--a study of electron transfer reaction using cyclic voltammetry and electrochemical impedance spectroscopy.

Self-assembled monolayers (SAMs) of liquid crystalline thiol-terminated alkoxycyanobiphenyl molecules with different alkyl chain lengths on Au surface have been studied for the first time using electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The barrier property of the SAM-modified surfaces was evaluated using two different redox probes, namely potassium ferro/ferri cyanide and hexaammineruthenium(III) chloride. It was found that for short-length alkyl chain thiol (C5) the electron transfer reaction of hexaammineruthenium(III) chloride takes place through tunneling mechanism. In contrast, redox reaction of potassium ferro/ferri cyanide is almost completely blocked by the SAM-modified Au surface. From the impedance data, a surface coverage value of >99.9% was calculated for all the thiol molecules.

Journal Article↗
PubMed2005

Scanning tunneling microscopy, Fourier transform infrared spectroscopy, and electrochemical characterization of 2-naphthalenethiol self-assembled monolayers on the Au surface: a study of bridge-mediated electron transfer in Ru(NH3)6(2+)/Ru(NH3)6(3+) redox reactions.

We have studied the structure, adsorption kinetics, and barrier properties of self-assembled monolayers of 2-naphthalenethiol on Au using electrochemical techniques, grazing-angle Fourier transform infrared (FTIR) spectroscopy, and scanning tunneling microscopy (STM). The results of cyclic voltammetric and impedance measurements using redox probes show that 2-naphthalenethiol on Au forms a stable and reproducible, but moderately blocking, monolayer. Annealing of the self-assembled monolayer (SAM)-modified surface at 72 +/- 2 degrees C remarkably improves the blocking property of the monolayer of 2-naphthalenethiol on Au. From the study of kinetics of SAM formation, we find that the self-assembly follows Langmuir adsorption isotherm. Our STM and FTIR results show that the molecules are adsorbed with the naphthalene ring tilted from the surface normal by forming a square root 3 x 3 R30 degrees overlayer structure. From our studies, we conclude that the electron-transfer reaction of ferro/ferricyanide in the freshly formed monolayer occurs predominantly through the pinholes and defects present in the monolayer. However, in the case of thermally annealed specimen, although the ferro/ferricyanide reaction is almost completely blocked, the electron-transfer reaction of hexaammineruthenium(III) chloride is not significantly inhibited. It is proposed that the electron-transfer reaction in the case of the ruthenium redox couple takes place by a tunneling mechanism through the high-electron-density aromatic naphthalene ring acting as a bridge between the monolayer-modified electrode and the ruthenium complex.

Journal Article↗
PubMed2004

Inclusion of gold nanoparticles into a discotic liquid crystalline matrix.

The thermophysical properties of mixtures of hexanethiolate-capped gold nanoparticles and three types of discotic liquid crystals, investigated using polarizing optical microscopy, differential scanning calorimetry and DC conductivity, indicate inclusion of gold nanoparticles into a matrix of triphenylene-based discotic liquid crystals.

Journal Article↗
PubMed2003

Cyclic voltammetric and electrochemical impedance studies on the structure, adsorption kinetics, and barrier properties of some organic dithiol self-assembled monolayers on gold.

We have studied the orientation and barrier properties of self-assembled monolayers (SAMs) of two alkanedithiols, hexanedithiol, octanedithiol, and an aromatic dithiol, 1,4-benzene dimethanethiol (BDMT), on gold in acetonitrile. From our studies, we conclude that BDMT molecules can form more organized monolayers on gold than aliphatic dithiol SAMs due to extremely strong lateral van der Waals interaction among the phenyl rings in the former. A study of the adsorption kinetics of octanedithiol in ethanol indicates that the adsorption rate law is concentration dependent just as for alkanethiols. However, the rate of adsorption is considerably faster than for simple alkanethiols.

Journal Article↗
PubMed2002

Existence of a hydrophobic gap at the alkanethiol SAM-water interface: an interfacial capacitance study.

The highly hydrophobic neat alkanethiol-coated SAM on evaporated gold shows an unusually low interfacial capacitance in aqueous media. This result cannot be explained by a simple parallel plate model of the double layer with the alkanethiol monolayer as a sole dielectric separator. Interestingly, a hydrophilic SAM prepared from a neat hydroxy thiol does not show any such capacitance lowering in aqueous media. Our results suggest the existence of a "hydrophobic gap" between the alkanethiol SAM-water interface. Such a model is also very much consistent with the predictions of Lum, Chandler, and Weeks theory of length scale dependent hydrophobicity.

Letter↗
PubMed2001

Reactive oxygen intermediates induce monocyte chemotactic protein-1 in vascular endothelium after brief ischemia.

Chemokine expression is associated with reperfusion of infarcted myocardium in the setting of tissue necrosis, intense inflammation, and inflammatory cytokine release. The specific synthesis of monocyte chemotactic protein (MCP)-1 mRNA by cardiac venules in reperfused infarcts corresponded to the region where leukocytes normally localize. MCP-1 could be induced by exogenous tumor necrosis factor (TNF)-alpha or by postischemic cardiac lymph containing TNF-alpha. However, the release of TNF-alpha during early reperfusion did not explain the venular localization of MCP-1 induction. To better understand the factors mediating MCP-1 induction, we examined the role of ischemia/reperfusion in a model of brief coronary occlusion in which no necrosis or inflammatory response is seen. Adult mongrel dogs were subjected to 15 minutes of coronary occlusion and 5 hours of reperfusion. Ribonuclease protection assay revealed up-regulation of MCP-1 mRNA only in ischemic segments of reperfused canine myocardium. Pretreatment with the reactive oxygen scavenger N-(2-mercaptopropionyl)-glycine completely inhibited MCP-1 induction. In situ hybridization localized MCP-1 message to small venular endothelium in ischemic areas without myocyte necrosis. Gel shift analysis of nuclear extracts from the ischemic area showed enhanced DNA binding of the transcription factors AP-1 and nuclear factor (NF)-kappaB, crucial for MCP-1 expression, in ischemic myocardial regions. Immunohistochemical staining demonstrated reperfusion-dependent nuclear translocation of c-Jun and NF-kappaB (p65) in small venular endothelium, only in the ischemic regions of the myocardium, that was inhibited by N-(2-mercaptopropionyl)-glycine. In vitro, treatment of cultured canine jugular vein endothelial cells with the reactive oxygen intermediate H2O2 induced a concentration-dependent increase in MCP-1 mRNA levels, which was inhibited by the antioxidant N-acetyl-L-cysteine, a precursor of glutathione, but not pyrrolidine dithiocarbamate, an inhibitor of NF-kappaB and activator of AP-1. In contrast to our studies with infarction, incubation of canine jugular vein endothelial cells with postischemic cardiac lymph did not induce MCP-1 mRNA expression suggesting the absence of cytokine-mediated MCP-1 induction after a sublethal ischemic period. These results suggest that reactive oxygen intermediate generation, after a brief ischemic episode, is capable of inducing MCP-1 expression in venular endothelium through AP-1 and NF-kappaB. Short periods of ischemia/reperfusion, insufficient to produce a myocardial infarction, induce MCP-1 expression, potentially mediating angiogenesis in the ischemic noninfarcted heart.

Acetylcysteine↗
PubMed2001

Stiles-Crawford effect of the first kind: assessment of photoreceptor alignments following dark patching.

Properties of presumed mechanisms controlling photoreceptor alignments are partially defined. A phototropic mechanism normally dominates alignment, but do modest changes in orientations occur with dark patching? Here, new photopic Stiles-Crawford (SCE-I) determinations were made before patching (pre-patch), just after 8-days of dark-patching (post-patch), and 3 days after patch removal (recovery test). We tested at 0, 11 and 22 degrees in the temporal retina of both eyes. Ten eyes of adult subjects were tested. SCE-I peak positions and Stile's parameter 'rho' were assessed. Dark-patching effects were small. Observations revealed meaningful corrective alignment overshoots with recovery in the light. Results suggest (1) the presence of multiple weak mechanisms affecting receptor alignments in the dark; (2) the phototropic mechanism to be dominant in the light; (3) the need for multiple test loci to be sampled in such studies, and (4) small changes in the SCE-I in the pupil plane can reflect meaningful events occurring at the retina.

Adaptation, Ocular↗
PubMed2001

Brief murine myocardial I/R induces chemokines in a TNF-alpha-independent manner: role of oxygen radicals.

Early chemokine induction in the area at risk of an ischemic-reperfused (I/R) myocardium is first seen in the venular endothelium. Reperfusion is associated with several induction mechanisms including increased extracellular tumor necrosis factor (TNF)-alpha, reactive oxygen intermediate (ROI) species formation, and adhesion of leukocytes to the venular endothelium. To test the hypothesis that chemokine induction in cardiac venules can occur by ROIs in a TNF-alpha-independent manner, and in the absence of leukocyte accumulation, we utilized wild-type (WT) and TNF-alpha double-receptor knockout mice (DKO) in a closed-chest mouse model of myocardial ischemia (15 min) and reperfusion (3 h), in which there is no infarction. We demonstrate that a single brief period of I/R induces significant upregulation of the chemokines macrophage inflammatory protein (MIP) -1 alpha, -1 beta, and -2 at both the mRNA and protein levels. This induction was independent of TNF-alpha, whereas levels of these chemokines were increased in both WT and DKO mice. Chemokine induction was seen predominantly in the endothelium of small veins and was accompanied by nuclear translocation of nuclear factor-kappa B and c-Jun (AP-1) in venular endothelium. Intravenous infusion of the oxygen radical scavenger N-2-mercaptopropionyl glycine (MPG) initiated 15 min before ischemia and maintained throughout reperfusion obviated chemokine induction, but MPG administration after reperfusion had begun had no effect. The results suggest that ROI generation in the reperfused myocardium rapidly induces C-C and C-X-C chemokines in the venular endothelium in the absence of infarction or irreversible cellular injury.

Animals↗
PubMed2000

Is inflammation good for the ischemic heart--perspectives beyond the ordinary.

In recent years, early reperfusion of the myocardial infarct has become the mainstay of optimal therapeutic management since it limits ventricular injury and infarct expansion and improves patient survival [14]. It has become clear that reperfusion promotes effective tissue repair and decreases ventricular remodeling even under circumstances where reperfusion is effected at too late a time to limit myocardial necrosis [4]. One of the striking differences between reperfused and non-reperfused myocardial infarctions is that the early intense inflammatory reaction which ensues immediately upon reperfusion has been demonstrated to potentially extend myocardial injury [9]. Substantial evidence suggests that this inflammatory reaction is important in tissue repair, leading to an obvious paradox with regard to the role of the inflammatory reaction. This communication takes the position that inflammatory injury occurring upon reperfusion of the infarcted myocardium results from the overwhelming of tissue defenses against inflammation resulting from the sudden entry of large amounts of neutrophils to an area of infarction where chemotactic factors have built up to high concentration. However, we hypothesize that inflammation fundamentally evolved as a protective mechanism to promote tissue healing and protect jeopardized myocardium. In this presentation, we will discuss our approach and data designed to evaluate potential protective roles of the inflammatory reaction after reperfusion.

Animals↗
PubMed2000

A generalized perceptual space.

Based upon experimental results on certain visual spatial discrimination tasks, we have developed a formalism for a non-Euclidian perceptual space. In this formalism, the transformation from real to perceptual space is given by a generalized covariant tensor. The concept of parallel transport of vectors in a curved space is used to prove that closed loops in real space imply closed loops with the same orientation in the perceptual space also.

Discrimination Learning↗
PubMed2000

Performance on the three-point Vernier alignment or acuity test as a function of age: measurement extended to ages 5 to 9 years.

The three-point Vernier alignment (or acuity) test was conducted on children from 5 to 9 years old. There is a significant difference between the 5-9 year old subjects and those in age groups 10 to 19 and 20 to 29 years in Vernier performance. These data were also compared with previously published data from older subjects (up to age 70). We conclude that Vernier function has not fully matured within the age range of 5 to 9 years; this finding is consistent with previous results reported in the literature.

Adolescent↗
PubMed2000

A chronic mouse model of myocardial ischemia-reperfusion: essential in cytokine studies.

Reperfusion of the ischemic myocardium is associated with a cytokine cascade that reflects a cellular response to injury. We studied this cascade in the mouse and found that acute surgical trauma in sham-operated animals obscured early changes in cytokine induction that occur during myocardial ischemia-reperfusion (MI/R). Therefore, we utilized a new implantable device that allows occlusion and reperfusion of the left anterior descending coronary artery in a closed-chest mouse at any time after instrumentation. Induction of interleukin (IL)-6 and tumor necrosis factor (TNF)-alpha mRNA in the whole heart was examined by RNase protection assay and quantitated by Phosphor- Imager. At 3 h after instrumentation, levels of IL-6 mRNA in sham-operated animals increased above those of control naive hearts, whereas this increase did not occur until after 1 day for TNF-alpha mRNA. The surgical trauma led to exaggeration of I/R cytokine induction with greater variance in response. At 3 days and 1 wk after instrumentation, levels of both IL-6 and TNF-alpha mRNA in sham-operated animals were comparable to those of naive hearts and induction responses in I/R were much less variant. We also found that 1 h of ischemia and 2 h of reperfusion at all time points of recovery (i.e., 3 h and 1, 3, and 7 days after instrumentation) led to a significant increase in IL-6 and TNF-alpha mRNA levels. In addition, 3 h of permanent occlusion, which did not induce any mRNA increase after 1 wk postinstrumentation, caused marked upregulation of IL-6 mRNA in an acutely prepared animal. This study of early cytokine responses evoked by MI/R highlights the need for dissipation of acute surgical trauma by using a chronic, closed-chest mouse preparation.

Animals↗
PubMed2000

Duplication of unique optical effects of ancient Egyptian lenses from the IV/V Dynasties: lenses fabricated ca 2620-2400 BC or roughly 4600 years ago.

Ancient (in fact, first known) lenses (ca. 4600 years ago) mainly from the IVth and Vth Dynasties of Egypt had truly remarkable and unique optical properties. These were parts of equally fascinating eye structures. These structures were fabricated as separate assemblies for insertion into funerary statues during certain brief windows of time (roughly from 2620-2400 BC, and 1750-1700 BC). These "eyes" appear to follow the observer as he/she rotated in any direction about these statues. In this paper, by simple means, we have recreated the optical properties of these unique lenses in the laboratory in order to help understand their special properties.

Egypt, Ancient↗
PubMed2000

The three point vernier alignment or acuity test (3Pt VA test): an analysis of variance.

A very useful clinical vernier acuity or vernier alignment test has been developed in this and associated laboratories over a span of two decades. We seek to refine further this test, to seek internal inconsistencies and to optimize parameters used in a variety of devices and environments. Vernier testing, and many aspects of this work have been considered by numerous able scientists over a long time period (not reviewed here), but none have encountered conditions faced by this group, particularly in the developing world. The three point vernier alignment (acuity) threshold test (3Pt VeA test) employed here has broad applications; it can be used as a clinical test of vision status, for triage, and as a reference standard for vision testing (in general). As one example, if sufficient luminance is employed, the test is applicable in the presence of truly dense media disorders, even advanced leucomas, mature cataracts, intraocular bleeds, as well as combinations of these disorders, with or without a window to the retina. Only retinal and centrally-based neural disorders, eccentricity from fixation, and about 50% of hypermature cataracts affect outcomes. With minor alterations, this test can be employed to assess the visual field. An analysis of variance (ANOVA) was performed on a defined set of parameters, to enable us to understand better the special properties and requirements associated with this test. Results are not significantly altered between ages 10-94 years (not assessed here). Employing settings well above threshold for test spot detection and well separated individual test spots, changes in test spot luminance, or background luminance (or adaptation level), contrast, veiling glare, test spot size, clear and variously degraded images, seem not to affect meaningfully measured outcomes. If an observer can define well a center of gravity for each test spot viewed, he can align the three points with remarkable precision.

Adult↗
PubMed1999

Stimulus-specific regulation of chemokine expression involves differential activation of the redox-responsive transcription factors AP-1 and NF-kappaB.

The promoters of the IL-8, MCP-1, and RANTES genes contain binding sites for the redox-responsive transcription factors AP-1 and NF-kappaB, which have been shown to be important for their expression. In this overview, we present evidence from our laboratories that the stimulus-specific regulation of these chemokines by the reactive oxidant H2O2, the proinflammatory cytokine TNF-alpha, and respiratory syncytial virus (RSV) is mediated in a cell type-specific manner involving different patterns of AP-1 and NF-kappaB binding activity. Our results demonstrate that H2O2 induction of IL-8 gene expression is linked with the selective binding of AP-1 to the IL-8 promoter, whereas TNF-alpha and RSV induction of IL-8 correlates with the activation of NF-kappaB binding. We propose that the differential activation and binding of inducible transcription factors to the promoter regions of chemokine genes provides a critical regulatory mechanism by which the CXC and CC chemokines can be selectively expressed in a cell type-specific and stimulus-specific manner. Such a regulatory mechanism of differential chemokine expression could critically influence the site-specific recruitment of distinct subsets of leukocytes to sites of inflammation and injury.

Animals↗
PubMed1999

Forever young: visual functions not affected or minimally affected by aging: a review.

Six visual functions, once developed to adult levels of performance, have been noted to exhibit little or no alteration with aging (also see Appendix, Note 1). Those selected for more substantial discussions in this article are: (a) the Stiles-Crawford effect of the first kind (SCE-I), also known as the "directional sensitivity of the retina"; (b) specific vernier acuity paradigms (including alignment of two lines one with the other, and two- and three-point vernier alignment tasks); and (c) color vision-related perceptual constancies. Each of these functions has rather different origins in the visual system. The SCE incorporates optical waveguide photoreceptor properties and has both physical and physiological origins; vernier acuity (one of the hyperacuities) is largely the result of neural data processing mechanisms; and the color vision-related effects have their origins in retinal neural processes. Descriptions of additional visual functions minimally affected by age are presented as well. This recent research raises many questions. How can these visual responses be so stable, when so many other visual responses show decrements with aging? What does it mean if anomalous responses within the more stable functions are encountered in individuals? Can these age-resistant functions be employed to help sustain other functions in aging individuals? Are such relatively invariant functions limited to the visual system? Because of the stability of the reported responses with aging, these same relationships can be used as test controls for other studies of aging, and as benchmarks to distinguish between "normal" aging and disease processes.

Aged↗