Exploration of MOF nanoparticle sizes using various physical characterization methods – is what you measure what you get?

Abstract

While the size of nanoparticles (NPs) seems to be a concept established in the field of NPs and is commonly used to characterize them, its definition is not that trivial as different "sizes" have to be distinguished depending on the physical characterization technique performed to measure them. Metal-organic frameworks (MOFs) are known for their crystallinity, their large variety of compositions due to a huge number of inorganic building blocks that can be combined with almost endless organic linkers, their tunable pore structure, their ultrahigh porosity, and the different ways their backbones can be functionalised. The combination of these features with the nanoworld offers manifold perspectives for the synthesis of welldefined MOF nanoparticles (NPs), whose size attribute should be accurately determined as it strongly influences their physicochemical properties (at this length scale). In order to elucidate size determination, we synthesised zirconium fumarate metal-organic framework nanoparticles (Zr-fum MOF NPs) and characterized them using various common characterization methods. Herein, we compare the results of different solid-state methods, including powder X-ray diffraction (PXRD), atomic force microscopy (AFM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to data obtained from dispersionbased methods, such as fluorescence correlation spectroscopy (FCS) and dynamic light scattering (DLS). In doing so, we illustrate the challenge of finding the appropriate method for obtaining a MOF NP size that is meaningful in the context of the desired application. Moreover, we demonstrate the importance of applying multiple complementary techniques as soon as the MOF NP size is considered. Throughout this paper, we highlight and define some reasonable recommendations of how the MOF NP size should be explored.

Selective Surface PEGylation of UiO-66 Nanoparticles for Enhanced Stability, Cell Uptake, and pH-Responsive Drug Delivery

“…Scanning electron microscopy (SEM) showed that UiO-66-L1 forms well-defined nanoparticles around 100–200 nm in size (Figure S6) in a manner similar to a recently reported modulated synthesis of a related zirconium fumarate MOF 69, 70. In contrast, UiO-66-L2 has a larger particle size, around 400–600 nm (Figure S7), suggesting that click modulation can be used to control nanoparticle size as well as surface chemistry.…”

Nanoparticle Characterization: What to Measure?

“…The nanoparticle size distribution is, therefore, an intrinsic measure of the control and quality of the synthesis procedure, while the estimated size value only refers to an averaged quantity derived from this distribution. Different techniques present different sensitivity to particles of varying dimensions, and care has to be taken when comparing results obtained from different characterization methods . Techniques with single‐particle resolution, such as high‐resolution microscopy and nanoparticle tracking analysis, are able to provide a number‐based average of particle size, where each particle is assigned an equal weight.…”

“…This endeavor is limited by our ability to fully investigate the nanoscale realm: Different characterization techniques are based on different physical properties, therefore only providing a partial picture of the nanoparticle characteristics. Making matters more challenging yet, the characterization methods themselves can directly affect the measured quantities …”

Metal‐Organic Framework Mediated Multifunctional Nanoplatforms for Cancer Therapy

“…For instance, how to standardize some physicochemical characterizations of MOFs in terms of size distribution, shape, colloidal stability, and surface chemistry, and how to obtain the effective synergistic effect in combined therapy strategies. When it comes to the size of MOFs, the state of the sample should also be considered and multiple characterization methods should be adopted . Moreover, the designs of cheaper and less toxic MOFs, such as Mg(II) and Ca(II) MOFs, are an important issue in research on applicable MOFs.…”