Livia Angeloni

Magnetic force microscopy: Quantitative issues in biomaterials

Magnetic force microscopy (MFM) is an atomic force microscopy (AFM) based technique in which an AFM tip with a magnetic coating is used to probe local magnetic fields with the typical AFM spatial resolution, thus allowing one to acquire images reflecting the local magnetic properties of the samples at the nanoscale. Being a well established tool for the characterization of magnetic recording media, superconductors and magnetic nanomaterials, MFM is finding constantly increasing application in the study of magnetic properties of materials and systems of biological and biomedical interest. After reviewing these latter applications, three case studies are presented in which MFM is used to characterize: (i) magnetoferritin synthesized using apoferritin as molecular reactor; (ii) magnetic nanoparticles loaded niosomes to be used as nanocarriers for drug delivery; (iii) leukemic cells labeled using folic acid-coated core-shell superparamagnetic nanoparticles in order to exploit the presence of folate receptors on the cell membrane surface. In these examples, MFM data are quantitatively analyzed evidencing the limits of the simple analytical models currently used. Provided that suitable models are used to simulate the MFM response, MFM can be used to evaluate the magnetic momentum of the core of magnetoferritin, the iron entrapment efficiency in single vesicles, or the uptake of magnetic nanoparticles into cells.

Thickness measurement of soft thin films on periodically patterned magnetic substrates by phase difference magnetic force microscopy

The need for accurate measurement of the thickness of soft thin films is continuously encouraging the development of techniques suitable for this purpose. We propose a method through which the thickness of the film is deduced from the quantitative measurement of the contrast in the phase images of the sample surface acquired by magnetic force microscopy, provided that the film is deposited on a periodically patterned magnetic substrate. The technique is demonstrated by means of magnetic substrates obtained from standard floppy disks. Colonies of Staphylococcus aureus adherent to such substrates were used to obtain soft layers with limited lateral (a few microns) and vertical (hundreds of nanometers) size. The technique is described and its specific merits, limitations and potentialities in terms of accuracy and measurable thickness range are discussed. These parameters depend on the characteristics of the sensing tip/cantilever as well as of the substrates, the latter in terms of spatial period and homogeneity of the magnetic domains. In particular, with the substrates used in this work we evaluated an uncertainty of about 10%, a limit of detection of 50-100 nm and an upper detection limit (maximum measurable thickness) of 1 μm, all obtained with standard lift height values (50-100 nm). Nonetheless, these parameters can be easily optimized by selecting/realizing substrates with suitable spacing and homogeneity of the magnetic domains. For example, the upper detection limit can be increased up to 25-50 μm while the limit of detection can be reduced to a few tens of nanometers or a few nanometers.

Removal of electrostatic artifacts in magnetic force microscopy by controlled magnetization of the tip: application to superparamagnetic nanoparticles.

Magnetic force microscopy (MFM) has been demonstrated as valuable technique for the characterization of magnetic nanomaterials. To be analyzed by MFM techniques, nanomaterials are generally deposited on flat substrates, resulting in an additional contrast in MFM images due to unavoidable heterogeneous electrostatic tip-sample interactions, which cannot be easily distinguished from the magnetic one. In order to correctly interpret MFM data, a method to remove the electrostatic contributions from MFM images is needed. In this work, we propose a new MFM technique, called controlled magnetization MFM (CM-MFM), based on the in situ control of the probe magnetization state, which allows the evaluation and the elimination of electrostatic contribution in MFM images. The effectiveness of the technique is demonstrated through a challenging case study, i.e., the analysis of superparamagnetic nanoparticles in absence of applied external magnetic field. Our CM-MFM technique allowed us to acquire magnetic images depurated of the electrostatic contributions, which revealed that the magnetic field generated by the tip is sufficient to completely orient the superparamagnetic nanoparticles and that the magnetic tip-sample interaction is describable through simple models once the electrostatic artifacts are removed.

Experimental issues in magnetic force microscopy of nanoparticles

The development of magnetic nanoparticles for biomedical applications requires a detailed characterization of their magnetic properties, with relation not only to their chemical structure, but also their morphology and size. Magnetic force microscopy (MFM), thanks to its nanometric lateral resolution and its capability to detect weak magnetic fields, appears as a powerful tool for the characterization of the magnetic properties of single nanoparticles, together with their morphological characteristics. Nevertheless, the application of MFM to the quantitative measurements of magnetic properties at the nanoscale is still an open issue because of a certain incongruence between experimental data and existing theoretical models of the tip-sample magnetic interactions. In this work, MFM data acquired on different magnetic nanoparticles in different experimental conditions (magnetized and not magnetized probes, out-of-field and in-field measurements) are analyzed, with the aim of individuating the possible phenomena affecting MFM measurements. These include topography-induced artifacts resulting from the tip-sample capacitive coupling, which we propose here for the first time. In case of measurements performed in presence of an external magnetic field, much more intense MFM signals were detected as it produces the saturation of the magnetization of the nanoparticles, which is not completely obtained by the sole stray field produced by the tip. Nevertheless, even in in-field measurements, the results evidenced the presence of significant electrostatic effects in MFM images, which, therefore, appear as an important factor to be taken into account for the quantitative interpretation of MFM data.

Chitosan Glutamate-Coated Niosomes: A Proposal for Nose-to-Brain Delivery

The aim of this in vitro study is to prepare and characterize drug free and pentamidine loaded chitosan glutamate coated niosomes for intranasal drug delivery to reach the brain through intranasal delivery. Mucoadhesive properties and stability testing in various environments were evaluated to examine the potential of these formulations to be effective drug delivery vehicles for intranasal delivery to the brain. Samples were prepared using thin film hydration method. Changes in size and ζ-potential of coated and uncoated niosomes with and without loading of pentamidine in various conditions were assessed by dynamic light scattering (DLS), while size and morphology were also studied by atomic force microscopy (AFM). Bilayer properties and mucoadhesive behavior were investigated by fluorescence studies and DLS analyses, respectively. Changes in vesicle size and ζ-potential values were shown after addition of chitosan glutamate to niosomes, and when in contact with mucin solution. In particular, interactions with mucin were observed in both drug free and pentamidine loaded niosomes regardless of the presence of the coating. The characteristics of the proposed systems, such as pentamidine entrapment and mucin interaction, show promising results to deliver pentamidine or other possible drugs to the brain via nasal administration.

In Situ control and modification of the probe magnetization state for accurate magnetic force microscopy

Electrostatic tip-sample interactions currently represent the main limitation to accurate quantitative analysis of magnetic force microscopy (MFM) data. Controlled magnetization MFM (CM-MFM) represents a smart solution to overcome this limitation as it allows one to identify electrostatic artifacts and to subtract them from standard MFM images, thus enabling the quantitative investigation of magnetic properties of materials at the nanometer scale. CM-MFM, however, requires not only the magnetization, but also the in situ accurate demagnetization of the MFM probe. In particular, the latter represents a crucial step for the complete removal of electrostatic artifacts. In this work, we describe two different methods to depolarize the MFM tip, based on the application of the coercive remanent magnetic field of the tip and on a damped alternating magnetic field, respectively. The two techniques are escribed and compared to emphasize their specific advantages and limitations.

Identification of nanoparticles and nanosystems in biological matrices with scanning probe microscopy

Identification of nanoparticles and nanosystems into cells and biological matrices is a hot research topic in nanobiotechnologies. Because of their capability to map physical properties (mechanical, electric, magnetic, chemical, or optical), several scanning probe microscopy based techniques have been proposed for the subsurface detection of nanomaterials in biological systems. In particular, atomic force microscopy (AFM) can be used to reveal stiff nanoparticles in cells and other soft biomaterials by probing the sample mechanical properties through the acquisition of local indentation curves or through the combination of ultrasound-based methods, like contact resonance AFM (CR-AFM) or scanning near field ultrasound holography. Magnetic force microscopy can detect magnetic nanoparticles and other magnetic (bio)materials in nonmagnetic biological samples, while electric force microscopy, conductive AFM, and Kelvin probe force microscopy can reveal buried nanomaterials on the basis of the differences between their electric properties and those of the surrounding matrices. Finally, scanning near field optical microscopy and tip-enhanced Raman spectroscopy can visualize buried nanostructures on the basis of their optical and chemical properties. Despite at a still early stage, these methods are promising for detection of nanomaterials in biological systems as they could be truly noninvasive, would not require destructive and time-consuming specific sample preparation, could be performed in vitro, on alive samples and in water or physiological environment, and by continuously imaging the same sample could be used to dynamically monitor the diffusion paths and interaction mechanisms of nanomaterials into cells and biological systems. This article is categorized under: Diagnostic Tools > In Vivo Nanodiagnostics and Imaging Nanotechnology Approaches to Biology > Nanoscale Systems in Biology.

Nanomechanical characterization of K-basalt from Roman comagmatic province: A preliminary study

Based on atomic force microscopy (AFM), contact resonance AFM (CR-AFM) is a nondestructive technique that allows one to perform single point measurements as well as surface mapping of the indentation modulus of a material. In this work we exploit the possibility to use CR-AFM to study synthetic materials representative of K-basalt from Roman comagmatic Province. Having observed the presence of subsurface voids and inclusions at micrometer and sub-micrometer scale, a preliminary study has been conducted to verify the capability of CR-AFM nanomechanical mapping to nondestructively detect these features.Based on atomic force microscopy (AFM), contact resonance AFM (CR-AFM) is a nondestructive technique that allows one to perform single point measurements as well as surface mapping of the indentation modulus of a material. In this work we exploit the possibility to use CR-AFM to study synthetic materials representative of K-basalt from Roman comagmatic Province. Having observed the presence of subsurface voids and inclusions at micrometer and sub-micrometer scale, a preliminary study has been conducted to verify the capability of CR-AFM nanomechanical mapping to nondestructively detect these features.