The New Youth of the In Situ Transmission Electron Microscopy

The idea of in situ transmission electron microscopy (TEM) and its possible ramifications were proposed at the very dawn of electron microscopy, but the translation from theory to practice encountered many technological setbacks, which hindered the feasibility of the most elaborated approaches until recent times. However, the several technological improvements achieved in the last 10–15 years filled this gap, allowing the direct observation of the dynamic response of materials to external stimuli under a vast range of conditions going from vacuum to gaseous or liquid environment. This resulted in a blossoming of the in situ TEM and scanning TEM (STEM) techniques to a new youth for a vast, growing range of applications, which cannot be rightfully detailed in a short span; therefore, this chapter should be intended as a guide highlighting a selection of the most inspiring, recently achieved results

Direct protein quantification in complex sample solutions by surface-engineered nanorod probes

Detecting biomarkers from complex sample solutions is the key objective of molecular diagnostics. Being able to do so in a simple approach that does not require laborious sample preparation, sophisticated equipment and trained staff is vital for point-of-care applications. Here, we report on the specific detection of the breast cancer biomarker sHER2 directly from serum and saliva samples by a nanorod-based homogeneous biosensing approach, which is easy to operate as it only requires mixing of the samples with the nanorod probes. By careful nanorod surface engineering and homogeneous assay design, we demonstrate that the formation of a protein corona around the nanoparticles does not limit the applicability of our detection method, but on the contrary enables us to conduct in-situ reference measurements, thus further strengthening the point-of-care applicability of our method. Making use of sandwich assays on top of the nanorods, we obtain a limit of detection of 110 pM and 470 pM in 10-fold diluted spiked saliva and serum samples, respectively. In conclusion, our results open up numerous applications in direct protein biomarker quantification, specifically in point-of-care settings where resources are limited and ease-of-use is of essenceThis research was supported by the European Commission FP7 NAMDIATREAM project (EU NMP4-LA-2010–246479), and the German Research Foundation (DFG grant PA 794/25-1)S

Homogeneous Biosensing Based on Magnetic Particle Labels

The growing availability of biomarker panels for molecular diagnostics is leading to an increasing need for fast and sensitive biosensing technologies that are applicable to point-of-care testing. In that regard, homogeneous measurement principles are especially relevant as they usually do not require extensive sample preparation procedures, thus reducing the total analysis time and maximizing ease-of-use. In this review, we focus on homogeneous biosensors for the in vitro detection of biomarkers. Within this broad range of biosensors, we concentrate on methods that apply magnetic particle labels. The advantage of such methods lies in the added possibility to manipulate the particle labels by applied magnetic fields, which can be exploited, for example, to decrease incubation times or to enhance the signal-to-noise-ratio of the measurement signal by applying frequency-selective detection. In our review, we discriminate the corresponding methods based on the nature of the acquired measurement signal, which can either be based on magnetic or optical detection. The underlying measurement principles of the different techniques are discussed, and biosensing examples for all techniques are reported, thereby demonstrating the broad applicability of homogeneous in vitro biosensing based on magnetic particle label actuation

Synthesizing Iron Oxide Nanostructures: The Polyethylenenemine (PEI) Role

Controlled synthesis of anisotropic iron oxide nanoparticles is a challenge in the field of nanomaterial research that requires an extreme attention to detail. In particular, following up a previous work showcasing the synthesis of magnetite nanorods (NRs) using a two-step approach that made use of polyethylenenemine (PEI) as a capping ligand to synthesize intermediate β-FeOOH NRs, we studied the effect and influence of the capping ligand on the formation of β-FeOOH NRs. By comparing the results reported in the literature with those we obtained from syntheses performed (1) in the absence of PEI or (2) by using PEIs with different molecular weight, we showed how the choice of different PEIs determines the aspect ratio and the structural stability of the β-FeOOH NRs and how this affects the final products. For this purpose, a combination of XRD, HRTEM, and direct current superconducting quantum interference device (DC SQUID) magnetometry was used to identify the phases formed in the final products and study their morphostructural features and related magnetic behavior

Surface Compositional Change of Iron Oxide Porous Nanorods: A Route for Tuning their Magnetic Properties

The capability of synthesizing specific nanoparticles (NPs) by varying their shape, size and composition in a controlled fashion represents a typical set of engineering tools that tune the NPs magnetic response via their anisotropy. In particular, variations in NP composition mainly affect the magnetocrystalline anisotropy component, while the different magnetic responses of NPs with isotropic (i.e., spherical) or elongated shapes are mainly caused by changes in their shape anisotropy. In this context, we propose a novel route to obtain monodispersed, partially hollow magnetite nanorods (NRs) by colloidal synthesis, in order to exploit their shape anisotropy to increase the related coercivity; we then modify their composition via a cation exchange (CE) approach. The combination of a synthetic and post-synthetic approach on NRs gave rise to dramatic variations in their magnetic features, with the pores causing an initial magnetic hardening that was further enhanced by the post-synthetic introduction of a manganese oxide shell. Indeed, the coupling of the core and shell ferrimagnetic phases led to even harder magnetic NRs

Synthesizing Iron Oxide Nanostructures: The Polyethylenenemine (PEI) Role

Controlled synthesis of anisotropic iron oxide nanoparticles is a challenge in the field of nanomaterial research that requires an extreme attention to detail. In particular, following up a previous work showcasing the synthesis of magnetite nanorods (NRs) using a two-step approach that made use of polyethylenenemine (PEI) as a capping ligand to synthesize intermediate β-FeOOH NRs, we studied the effect and influence of the capping ligand on the formation of β-FeOOH NRs. By comparing the results reported in the literature with those we obtained from syntheses performed (1) in the absence of PEI or (2) by using PEIs with different molecular weight, we showed how the choice of different PEIs determines the aspect ratio and the structural stability of the β-FeOOH NRs and how this affects the final products. For this purpose, a combination of XRD, HRTEM, and direct current superconducting quantum interference device (DC SQUID) magnetometry was used to identify the phases formed in the final products and study their morphostructural features and related magnetic behavior

Building Composite Iron–Manganese Oxide Flowerlike Nanostructures: A Detailed Magnetic Study

Here we show that it is possible to produce different magnetic core−multiple shell heterostructures from monodisperse Fe3O4 spherical magnetic seeds by finely controlling the amount of a manganese precursor and using, in a smart and simple way, a cation-exchange synthetic approach. In particular, by increasing the amount of precursor, we were able to produce nanostructures ranging from Fe3O4/ manganese ferrite core−single-shell nanospheres to larger, flowerlike Fe3O4/manganese ferrite/Mn3O4 core−double-shell nanoparticles. We first demonstrate how formation of the initial thin manganese ferrite shell determines a dramatic reduction of the superficial disorder in the starting Fe3O4, bringing nanomagnets with lower hardness. Then, the growth of the second and most external manganese oxide shell causes magnetic hardening of the heterostructures, while its magnetic exchange coupling with the rest of the heterostructure can be either antiferromagentic or ferromagnetic, depending on the strength of the applied external magnetic field. This response is similar to that of an iron oxide−manganese oxide core−shell system but differs from what is observed in multiple-shell heterostructures. Finally, we report that the most external shell becomes magnetically irrelevant above the ferrimagnetic−paramagnetic transition of the manganese oxide, and the resulting magnetic behavior of the flowerlike structures is studied in depth

Building Composite Iron-Manganese Oxide Flowerlike Nanostructures: A Detailed Magnetic Study

Here we show that it is possible to produce different magnetic core multiple shell heterostructures from monodisperse Fe3O4 spherical magnetic seeds by finely controlling the amount of a manganese precursor and using, in a smart and simple way, a cation-exchange synthetic approach. In particular, by increasing the amount of precursor, we were able to produce nanostructures ranging from Fe3O4/manganese ferrite core single-shell nanospheres to larger, flowerlike Fe3O4/manganese ferrite/Mn3O4 core double-shell nanoparticles. We first demonstrate how formation of the initial thin manganese ferrite shell determines a dramatic reduction of the superficial disorder in the starting Fe3O4, bringing nanomagnets with lower hardness. Then, the growth of the second and most external manganese oxide shell causes magnetic hardening of the heterostructures, while its magnetic exchange coupling with the rest of the heterostructure can be either antiferromagentic or ferromagnetic, depending on the strength of the applied external magnetic field. This response is similar to that of an iron oxide manganese oxide core shell system but differs from what is observed in multiple-shell heterostructures. Finally, we report that the most external shell becomes magnetically irrelevant above the ferrimagnetic paramagnetic transition of the manganese oxide, and the resulting magnetic behavior of the flowerlike structures is studied in depth

Unexpected Insights about Cation-Exchange on Metal Oxide Nanoparticles and Its Effect on Their Magnetic Behavior

Tuning the magnetic behavior of nanoparticles via the control of their features has always been challenging because these features are mostly intertwined. In the last years, a novel synthetic approach based on cation-exchange has been reported, and one of its main advantages is to maintain the shape and size of nanoparticles. However, such a synthetic strategy has been seldom applied to iron oxide magnetic nanoparticles, where the substitution of iron with diverse transition element cations was described as occurring in their whole volume. Surprisingly, we found results quite discordant from the few ones so far published in exploiting again this approach. We show here that it unavoidably leads to core/shell structures with only the shell undergoing the cation-exchange. Moreover, the starting phase of iron oxide strongly dictates the number of iron cations that could be replaced: if it is structurally free of vacancies, like magnetite, the maximum amount of exchanged cations is low and only affects the nanoparticles most external, disordered layers. Conversely, the cation-exchange is boosted if the iron oxide phase is structurally prone to vacancies, like wustite, and the shell where the iron cations have been partly substituted becomes quite thicker. These findings are further corroborated by the materials magnetic properties

Modeling and Development of a Biosensor Based on Optical Relaxation Measurements of Hybrid Nanoparticles

Schrittwieser S, Ludwig F, Dieckhoff J, et al. Modeling and Development of a Biosensor Based on Optical Relaxation Measurements of Hybrid Nanoparticles. ACS Nano. 2012;6(1):791-801.We present a new approach for homogeneous real-time immunodiagnostics (denoted as "PlasMag") that can be directly carried out in sample solutions such as serum, thus promising to circumvent the need of sample preparation. It relies on highly sensitive plasmon-optical detection of the relaxation dynamics of magnetic nanoparticles immersed in the sample solution, which changes when target molecules bind to the surfaces of the nanoparticles due to the increase in their hydrodynamic radii. This method requires hybrid nanoparticles that combine both magnetic and optical anisotropic properties. Our model calculations show that core shell nanorods with a cobalt core diameter of 6 nm, a cobalt core length of 80 nm, and a gold shell thickness of 5 nm are ideally suited as nanoprobes. On the one hand, the spectral position of the longitudinal plasmon resonance of such nanoprobes lies in the near-infrared, where the optical absorption in serum is minimal. On the other hand, the expected change in their relaxation properties on analyte binding is maximal for rotating magnetic fields as excitation in the lower kHz regime. In order to achieve high alignment ratios of the nanoprobes, the strength of the magnetic field should be around 5 mT. While realistic distributions of the nanoprobe properties result in a decrease of their mean optical extinction, the actual relaxation signal change on analyte binding is largely unaffected. These model calculations are supported by measurements on plain cobalt nanorod dispersions, which are the base component of the aspired core-shell nanoprobes currently under development