Tuning the magnetic properties of nanoparticles Kolhatkar, Arati G; Jamison, Andrew C; Litvinov, Dmitri ...
International Journal of Molecular Sciences,
07/2013, Volume:
14, Issue:
8
Journal Article, Book Review
Peer reviewed
Open access
The tremendous interest in magnetic nanoparticles (MNPs) is reflected in published research that ranges from novel methods of synthesis of unique nanoparticle shapes and composite structures to a ...large number of MNP characterization techniques, and finally to their use in many biomedical and nanotechnology-based applications. The knowledge gained from this vast body of research can be made more useful if we organize the associated results to correlate key magnetic properties with the parameters that influence them. Tuning these properties of MNPs will allow us to tailor nanoparticles for specific applications, thus increasing their effectiveness. The complex magnetic behavior exhibited by MNPs is governed by many factors; these factors can either improve or adversely affect the desired magnetic properties. In this report, we have outlined a matrix of parameters that can be varied to tune the magnetic properties of nanoparticles. For practical utility, this review focuses on the effect of size, shape, composition, and shell-core structure on saturation magnetization, coercivity, blocking temperature, and relaxation time.
"The Sixties." The powerful images conveyed by those two words have
become an enduring part of American cultural and political history.
But where did Sixties radicalism come from? Who planted the
...intellectual seeds that brought it into being? These questions are
answered with striking clarity in Andrew Jamison and Ron Eyerman's
book. The result is a combination of history and biography that
vividly portrays an entire culture in transition. The authors focus
on specific individuals, each of whom in his or her distinctive way
carried the ideas of the 1930s into the decades after World War II,
and each of whom shared in inventing a new kind of intellectual
partisanship. They begin with C. Wright Mills, Hannah Arendt, and
Erich Fromm and show how their work linked the "old left" of the
Thirties to the "new left" of the Sixties. Lewis Mumford, Rachel
Carson, and Fairfield Osborn laid the groundwork for environmental
activism; Herbert Marcuse, Margaret Mead, and Leo Szilard
articulated opposition to the postwar "scientific-technological
state." Alternatives to mass culture were proposed by Allen
Ginsberg, James Baldwin, and Mary McCarthy; and Saul Alinsky,
Dorothy Day, and Martin Luther King, Jr., made politics personal.
This is an unusual book, written with an intimacy that brings to
life both intellect and emotion. The portraits featured here
clearly demonstrate that the transforming radicalism of the Sixties
grew from the legacy of an earlier generation of thinkers. With a
deep awareness of the historical trends in American culture, the
authors show us the continuing relevance these partisan
intellectuals have for our own age. "In a time colored by
'political correctness' and the ascendancy of market liberalism, it
is well to remember the partisan intellectuals of the 1950s. They
took sides and dissented without becoming dogmatic. May we be able
to say the same about ourselves." -from Chapter 7
Surface dipoles arise from differences in the distribution of electron density of interfacial molecular structures as expressed by charge separation. The direction and magnitude of the associated ...dipole moments directly impact a variety of interfacial phenomena. For example, the wettability of thin film-coated solid surfaces toward polar contacting liquids can be systematically adjusted by reorienting the direction of an array of interfacial dipoles, while the vector sum total of all of the dipole moments associated with such thin films can be used to tune the work function of a metal. One method of producing such dipole arrays is by coating a surface with a self-assembled monolayer (SAM), which is a thin organic film of amphiphilic adsorbates that spontaneously assemble on a surface. The interfacial properties of SAMs can be menu-selected by choice of adsorbate structure using ω-terminated thiols on gold surfaces as a convenient system for studying and utilizing these properties. In this Account, we describe the impact of an array of oriented surface dipoles upon the interfacial energy of the thin film bearing such an array. Our analysis of these films divides the subject of surface dipole arrays into three types: (1) those directing a well-defined electronegative pole toward the interface, (2) those incorporating an invertable polar group, and (3) those directing a well-defined electropositive pole toward the interface. With regard to the first category, we analyze the impact of permanent dipoles on the wettability of alkanethiolate SAMs generated from adsorbates possessing well-defined transitions between terminal fluorocarbon and underlying hydrocarbon chain segments. The second category covers recent reports of light-responsive SAMs formed from azobenzene-based adsorbates. Finally, the third category explores a unique example of a dipole array that exposes the positive ends of the interfacial dipoles formed from CH3-terminated fluorocarbon tailgroups. Our analysis of the SAMs formed from these carefully crafted adsorbates encompassing several series of fluorocarbon-containing thiols provides support for a conclusion that oriented surface dipoles exert a significant influence on interfacial energetics and wettability. In contrast to the limited distance from the interface that a surface dipole array will have upon contacting liquids, the work function of a thin film reflects the influence of all the polar groups within the film. Therefore, we also explore the change in the substrate work function for n-alkanethiol-modified gold surfaces as a function of molecular length and for other adsorbates as a function of their chemical composition.
The spontaneous adsorption of organic molecules on a variety of planar and nonplanar substrates, that is, self assembly, can generate films just one molecule thick. These nanoscale, self-assembled ...monolayer (SAM) films have been extensively used to engineer surfaces with well-defined properties. Their utility has been demonstrated in a wide range of applications, including wetting, adhesion, lubrication, patterning, and molecular recognition. Many SAM systems have been investigated, but alkanethiols adsorbed on gold are the most successful combination. This pairing offers a variety of advantages, including the ability to tune precisely the interfacial properties of a surface through the well-established organic synthetic methodologies that have been developed for preparing custom ω-terminated alkanethiols. Alkanethiolate monolayers are moderately stable at room temperature; however, these films degrade over time and readily desorb upon moderate heating. This shortcoming limits the use of SAMs in applications involving elevated temperatures or harsh environments. Accordingly, new adsorbates with multiple bonding moieties have been created to enhance the stability and versatility of SAMs. In this Account, we examine a variety of multidentate adsorbate structures that have been used to generate SAMs on planar substrates and on nanoparticles. Each of these chelating adsorbates (bidentates and tridentates) has been designed to generate well-defined organic monolayer films with multiple attachment points to the underlying substrate. This bonding arrangement allows the formation of SAMs with enhanced stability through the entropy-driven “chelate effect”. The research examined here demonstrates that multidentate adsorbates provide robust films: they enable the use of SAMs under conditions that are incompatible with SAMs derived from normal alkanethiols. Another advantage offered by multidentate adsorbates is the capacity for new paradigms in thin-film composition. In particular, appropriately designed chelating adsorbates can be engineered to have two or more chemically distinct terminal groups that are covalently linked to the same underlying headgroup, without adding steric bulk that might prove detrimental to the resultant assembly. This strategy allows the generation of homogeneously mixed multicomponent surfaces, overcoming the problem of phase separation or “islanding” that is pervasive when two or more chemically distinct adsorbates are used to form mixed SAMs. Such homogeneously mixed films offer the opportunity to fine-tune the interfacial properties of a substrate and to create unique heterogeneous interfaces that are well defined by the chemical composition of the tailgroups exposed at the surface. The insight derived from these studies opens the door to new uses for SAMs, both in surface engineering applications (such as corrosion resistance and soft lithographic patterning) and in the stabilization and manipulation of nanoparticles.
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► A historical perspective of self-assembled monolayers (SAMs) is provided. ► The use of SAMs as protective coatings for flat substrates and nanoparticles is examined. ► The thermal ...and chemical stabilities of the SAMs are reviewed. ► Various strategies for enhancing the stability of SAMs are described.
This review examines the use of self-assembled monolayers (SAMs) on gold as nanoscale protective coatings for both nominally flat substrates and for nanoparticles. The thermal and chemical stabilities of the SAMs are discussed, and the various strategies used by researchers to generate thin films with enhanced resistance to corrosion and/or decomposition are described. The use of multidentate adsorbates to achieve the desired objectives is emphasized.
Background
This article explores the contradiction that has developed between market‐driven and academic strategies of response to the challenges facing engineering and engineering education. In ...these contending response strategies there are deep‐seated, underlying historical tensions at work.
Purpose
This article aims to create a theoretical and conceptual framework, which allows engineering educators to reflect on their perceptions and practices in regard to institutional change and reform.
Design
A historical review traces the history of the tensions between different traditions of engineering education from their medieval roots through the institutionalization of engineering education in the nineteenth and twentieth centuries and discusses the different conceptions of engineering and engineering knowledge on which they are based. We then relate the historical review to educational conceptualizations of the university as well as curriculum models.
Results
On the basis of a synthesis of the historical and educational perspectives, we present an analytical distinction between three modes of engineering education. We propose a transformation to an integrative mode that is less prominent historically, but growing in importance. By introducing the term hybrid learning, we furthermore outline important aspects to be considered in the process of transforming engineering education.
Conclusions
We conclude by inviting engineering educators, students, administrators, and policy makers to consult our theoretical and conceptual framework and consider a transformation to an integrative mode. This integrative mode is designed to foster hybrid learning, a contextual, transformative, collaborative, and situated learning approach that holds potential in facing the increasing complexity of engineering.
Catheter-related infections (CRIs) are associated with the formation of pathogenic biofilms on the surfaces of silicone catheters, which are ubiquitous in medicine. These biofilms provide protection ...against antimicrobial agents and facilitate the development of bacterial resistance to antibiotics. The application of photothermal agents on catheter surfaces is an innovative approach to overcoming biofilm-generated CRIs. Gold nanoshells (AuNSs) represent a promising photothermal tool, because they can be used to generate heat upon exposure to near-infrared (NIR) radiation, are biologically inert at physiological temperatures, and can be engineered for the photothermal ablation of cells and tissue. In this study, AuNSs functionalized with carboxylate-terminated organosulfur ligands were attached to model catheter surfaces and tested for their effectiveness at killing adhered Enterococcus faecalis (E. faecalis) bacteria. The morphology of the AuNSs was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), while the elemental composition was characterized by energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). Furthermore, optical and photothermal properties were acquired by ultraviolet–visible (UV-vis) spectroscopy and thermographic imaging with an infrared camera, respectively. Bacterial survival studies on AuNS-modified surfaces irradiated with and without NIR light were evaluated using a colony-formation assay. These studies demonstrated that AuNS-modified surfaces, when illuminated with NIR light, can effectively kill E. faecalis on silicone surfaces.
Mixed self-assembled monolayers (SAMs) generated from perfluoro-, n-alkyl-, and oligo(ethylene glycol)-terminated alkanedithiols were utilized to prepare two-dimensional interfaces with precise ...composition and wettability. Interfacial control was afforded simply by adjusting the mole fraction of the adsorbates present in the development solutions and was modulated by the dual contributions of the tailgroups of the bidentate thiols. In contrast, the composition and wettability of mixed SAMs generated from traditional monodentate thiols bearing analogous tailgroups failed to track systematically with the mole fractions of the adsorbates in the development solutions, reflecting a greater dependence on solvent/adsorbate interactions. All of the SAMs were thoroughly characterized by contact angle goniometry, ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS). Furthermore, solution-phase displacement tests performed to evaluate the stability of the adsorbates as a function of the adsorbate composition of the mixed alkanedithiolate films, provided evidence of the markedly enhanced stability associated with mixed SAMs formed from bidentate adsorbates.
The ability to manipulate interfaces at the nanoscale via a variety of thin-film technologies offers a plethora of avenues for advancing surface applications. These include surfaces with remarkable ...antibiofouling properties as well as those with tunable physical and electronic properties. Molecular self-assembly is one notably attractive method used to decorate and modify surfaces. Of particular interest to surface scientists has been the thiolate–gold system, which serves as a reliable method for generating model thin-film monolayers that transform the interfacial properties of gold surfaces. Despite widespread interest, efforts to tune the interfacial properties using mixed adsorbate systems have frequently led to phase-separated domains of molecules on the surface with random sizes and shapes depending on the structure and chemical composition of the adsorbates. This feature article highlights newly emerging methods for generating mixed thin-film interfaces, not only to enhance the aforementioned properties of organic thin films, but also to give rise to interfacial compositions never before observed in nature. An example would be the development of monolayers formed from bidentate adsorbates and other unique headgroup architectures that provide the surface bonding stability necessary to allow the assembly of interfaces that expose mixtures of chains that are fundamentally different in character (i.e., either phase-incompatible or structurally dissimilar), producing compositionally “conflicted” interfaces. By also exploring the prior efforts to produce such homogeneously blended interfaces, this feature article seeks to convey the relationships between the methods of film formation and the overall properties of the resulting interfaces.
A new bromoisobutyrate-terminated alkanethiol, 16-(3,5-bis(mercaptomethyl)phenoxy)hexadecyl 2-bromo-2-methylpropanoate (BMTBM), was designed as a bidentate adsorbate to form thermally stable ...bromoisobutyrate-terminated self-assembled monolayers (SAMs) on flat gold surfaces to conduct atom-transfer radical polymerizations (ATRPs). The monolayers derived from BMTBM were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS) and compared to the monolayers formed from 16-mercaptohexadecyl 2-bromo-2-methylpropanoate (MBM), 16-(3-(mercaptomethyl)phenoxy)hexadecyl 2-bromo-2-methyl-propanoate (MTBM), and octadecanethiol (C18SH). In this study, although the monolayer derived from BMTBM was less densely packed than those derived from MBM and MTBM, the bidentate adsorbates demonstrated much higher thermal stability in solution-phase thermal desorption tests, owing to the “chelate effect”. The enhanced stability of the BMTBM SAMs ensured the development of thick brushes of poly(methyl methacrylate) and polystyrene at elevated temperatures (60, 90, 105, and 120 °C). In contrast, SAMs derived from MBM and MTBM failed to grow polymer brushes at temperatures above 100 °C.
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