Elemental ratios for characterization of quantum-dots populations in complex mixtures by asymmetrical flow field-flow fractionation on-line coupled to fluorescence and inductively coupled plasma mass spectrometry

Electrical asymmetrical flow field-flow fractionation: Fundamentals, evolution, applications, and prospects

Among the plethora of techniques that conforms the Field-Flow Fractionation (FFF) family, electrical field-flow fractionation (ElFFF) was designed to separate different analytes based on their size and electrophoretic mobility (µ). However, major technical and operational issues made this technique to fall into oblivion. Many of those drawbacks can be circumvented if another field is employed as the main driving force for the elution in the same channel, such as the most successful and useful FFF-related technique, asymmetrical flow field-flow fractionation (AF4). The combination results in a new member of the FFF family termed as electrical asymmetrical flow field-flow fractionation (EAF4). This technique was conceptualized in 2015, and has gained attention, especially in the study of biomolecules, nanomaterials, their interactions and transformations, due to its ability to determine size-resolved electrophoretic mobility of this kind of analytes, which is not directly accessible by batch techniques based on e.g., electrophoretic light scattering. This review gives an overview of this novel analytical technique, including a summary of the theoretical elements behind this approach, followed by a discussion of the possibility to provide multiple information through its coupling with a variety of detectors. Also, the applications of EAF4 to analytes from the nano to the micro scale in diverse matrices are presented. Finally, the challenges that EAF4 faces today and trends for the near future are addressed.

An in-depth physicochemical investigation of drug-loaded core-shell UiO66 nanoMOFs

Nanosized UiO66 are among the most studied MOF materials. They have been extensively applied in various areas, such as catalysis, gas absorption, electrochemistry, chemical sensing, and biomedical applications. However, the preparation of stable nano-sized UiO66 for drug delivery applications is challenging because of the high tendency of UiO66 to aggregate during storage. To address this issue, we coated UiO66 with oligomers made of crosslinked cyclodextrins. The coated UiO66 exhibited a good stability upon storage for more than three weeks, even for low quantities of coating materials. The resulting core-shell UiO66 were characterized using a set of complementary methods including microscopies, spectroscopies, X-ray diffraction, and thermogravimetric investigations. Size distribution was assessed by orthogonal methods. Cisplatin was loaded in the core-shell nanoparticles, followed by an in-depth analysis by asymmetric flow field-flow fractionation (AF4) hyphenated with inductively coupled plasma-mass spectrometry (ICP-MS). This method combines the extremely high elemental selectivity and ultratrace detection limits of mass spectrometry with the capacity of AF4 to differentiate the diverse populations present in the sample. Free cisplatin and UiO66-associated cisplatin could be well separated by AF4. AF4-ICP-MS/MS analysis provided the exact drug loading, without the need of separating the nanoparticles from their suspension media. These data suggest the potential of AF4-ICP-MS/MS in the optimization of drug delivery systems.

AF4-UV/VIS-MALS-ICPMS/MS for the characterization of the different nanoparticulated species present in oligonucleotide-gold nanoparticle conjugates

In-depth characterization of functionalized nanomaterials is still a remaining challenge in nanobioanalytical chemistry. In this work, we propose the online coupling of Asymmetric Flow Field-Flow Fractionation (AF4) with UV/Vis, Multiangle Light Scattering (MALS) and Inductively Coupled Plasma-Tandem Mass Spectrometry (ICP-MS/MS) detectors to carry out, in less than 10 min and directly in the functionalization reaction mixture, the complete characterization of gold nanoparticles (AuNPs) functionalized with oligonucleotides and surface-modified with polyethylene glycol (PEG). AF4 separation provided full separation of the bioconjugates from the original AuNPs while P/Au and S/Au ICP-MS/MS ratios in the bioconjugate fractographic peaks could be used to compute the corresponding stoichiometries, oligonucleotide/AuNP and PEG/AuNPs. MALS detection clearly showed the coexistence of two distinct nanoparticulated populations in the bioconjugation mixture, which were demonstrated to be different not only in size but in functionality as well. The major bioconjugate population showed lower hydrodynamic ratios (18 nm) with higher and steadier oligonucleotides/AuNPs (92) and PEG/AuNPs (2350) stoichiometries, in comparison to the minor abundant population (54 nm, 51 and 1877, respectively). Moreover, the ratio between the absorbance signals measured at 520 nm and 650 nm reflects a lower AuNP aggregation in the major (10.5) than in the minor (4.5) population. Results obtained prove the benefits of a detailed characterization to find out if subsequent purification of functionalized AuNP-oligonucleotides is required to design more efficiently their final bioanalytical application.

The Potential of ICP-MS as a Complementary Tool in Nanoparticle-Protein Corona Analysis

The prolific applicability of nanomaterials has made them a common citizen in biological systems, where they interact with proteins forming a biological corona complex. These complexes drive the interaction of nanomaterials with and within the cells, bringing forward numerous potential applications in nanobiomedicine, but also arising toxicological issues and concerns. Proper characterization of the protein corona complex is a great challenge typically handled with the combination of several techniques. Surprisingly, despite inductively coupled plasma mass spectrometry (ICP-MS) being a powerful quantitative technique whose application in nanomaterials characterization and quantification has been consolidated in the last decade, its application to nanoparticle-protein corona studies is scarce. Furthermore, in the last decades, ICP-MS has experienced a turning point in its capabilities for protein quantification through sulfur detection, hence becoming a generic quantitative detector. In this regard, we would like to introduce the potential of ICP-MS in the nanoparticle protein corona complex characterization and quantification complementary to current methods and protocols.

Mass spectrometry for multi-dimensional characterization of natural and synthetic materials at the nanoscale

Characterization of materials at the nanoscale plays a crucial role in in-depth understanding the nature and processes of the substances. Mass spectrometry (MS) has characterization capabilities for nanomaterials (NMs) and nanostructures by offering reliable multi-dimensional information consisting of accurate mass, isotopic, and molecular structural information. In the last decade, MS has emerged as a powerful nano-characterization technique. This review comprehensively summarizes the capabilities of MS in various aspects of nano-characterization that greatly enrich the toolbox of nano research. Compared with other characterization techniques, MS has unique capabilities for real-time monitoring and tracking reaction intermediates and by-products. Moreover, MS has shown application potential in some novel aspects, such as MS imaging of the biodistribution and fate of NMs in animals and humans, stable isotopic tracing of NMs, and risk assessment of NMs, which deserve update and integration into the current knowledge framework of nano-characterization.

Capping of Mn-Doped ZnS Quantum Dots with DHLA for Their Stabilization in Aqueous Media: Determination of the Nanoparticle Number Concentration and Surface Ligand Density

Colloidal Mn²⁺-doped ZnS quantum dots (QDs) were synthesized, surface modified, and thoroughly characterized using a pool of complementary techniques. Cap exchange of the native l-cysteine coating of the QDs with dihydrolipoic acid (DHLA) ligands is proposed as a strategy to produce nanocrystals with a strong phosphorescent-type emission and improved aqueous stability. Moreover, such a stable DHLA coating can facilitate further bioconjugation of these QDs to biomolecules using established reagents such as cross-linker molecules. First, a structural and morphological characterization of the l-cysteine QD core was performed by resorting to complementary techniques, including X-ray powder diffraction (XRD) and microscopy tools. XRD patterns provided information about the local structure of ions within the nanocrystal structure and the number of metal atoms constituting the core of a QD. The judicious combination of the data obtained from these complementary characterization tools with the analysis of the QDs using inductively coupled plasma-mass spectrometry (ICP-MS) allowed us to assess the number concentration of nanoparticles in an aqueous sample, a key parameter when such materials are going to be used in bioanalytical or toxicological studies. Asymmetric flow field-flow fractionation (AF4) coupled online to ICP-MS detection proved to be an invaluable tool to compute the number of DHLA molecules attached to the surface of a single QD, a key feature that is difficult to estimate in nanoparticles and that critically affects the behavior of nanoparticles when entering the biological media (e.g., cellular uptake, biodistribution, or protein corona formation). This hybrid technique also allowed us to demonstrate that the elemental composition of the nanoparticle core remains unaffected after the ligand exchange process. Finally, the photostability and robustness of the DHLA-capped QDs, critical parameters for bioanalytical applications, were assessed by molecular luminescence spectroscopy.

Study of conformational changes and protein aggregation of bovine serum albumin in presence of Sb(III) and Sb(V)

Antimony is a metalloid that affects biological functions in humans due to a mechanism still not understood. There is no doubt that the toxicity and physicochemical properties of Sb are strongly related with its chemical state. In this paper, the interaction between Sb(III) and Sb(V) with bovine serum albumin (BSA) was investigated in vitro by fluorescence spectroscopy, and circular dichroism (CD) under simulated physiological conditions. Moreover, the coupling of the separation technique, asymmetric flow field-flow fractionation, with elemental mass spectrometry to understand the interaction of Sb(V) and Sb(III) with the BSA was also used. Our results showed a different behaviour of Sb(III) vs. Sb(V) regarding their effects on the interaction with the BSA. The effects in terms of protein aggregates and conformational changes were higher in the presence of Sb(III) compared to Sb(V) which may explain the differences in toxicity between both Sb species in vivo. Obtained results demonstrated the protective effect of GSH that modifies the degree of interaction between the Sb species with BSA. Interestingly, in our experiments it was possible to detect an interaction between BSA and Sb species, which may be related with the presence of labile complex between the Sb and a protein for the first time.

Precise determination of the nanoparticle concentration and ligand density of engineered water-soluble HgSe fluorescent nanoparticles

Fluorescent HgSe NPs were synthesized and water-stabilized. Full characterization, including ligand density and nanoparticle concentration, makes them ideal candidates as standards to investigate HgSe NPs metabolism and toxicity.

Asymmetric flow field-flow fractionation coupled to inductively coupled plasma mass spectrometry for the quantification of quantum dots bioconjugation efficiency

Hyphenation of asymmetric flow field-flow fractionation (AF4) to an on-line elemental detection (inductively coupled plasma-mass spectrometry, ICP-MS) is proposed as a powerful diagnostic tool for quantum dots bioconjugation studies. In particular, conjugation effectiveness between a "model" monoclonal IgG antibody (Ab) and CdSe/ZnS core-shell Quantum Dots (QDs), surface-coated with an amphiphilic polymer, has been monitored here by such hybrid AF4-ICP-MS technique. Experimental conditions have been optimized searching for a proper separation between the sought bioconjugates from the eventual free reagents excesses employed during the bioconjugation (QDs and antibodies). Composition and pH of the carrier have been found to be critical parameters to ensure an efficient separation while ensuring high species recovery from the AF4 channel. An ICP-MS equipped with a triple quadropole was selected as elemental detector to enable sensitive and reliable simultaneous quantification of the elemental constituents, including sulfur, of the nanoparticulated species and the antibody. The hyphenated technique used provided nanoparticle size-based separation, elemental detection, and composition analysis capabilities that turned out to be instrumental in order to investigate in depth the Ab-QDs bioconjugation process. Moreover, the analytical strategy here proposed allowed us not only to clearly identify the bioconjugation reaction products but also to quantify nanoparticle:antibodies bioconjugation efficiency. This is a key issue in future development of analytical and bioanalytical photoluminescent QDs applications.

Inductively coupled plasma - Tandem mass spectrometry (ICP-MS/MS): A powerful and universal tool for the interference-free determination of (ultra)trace elements – A tutorial review

This paper is intended as a tutorial review on the use of inductively coupled plasma - tandem mass spectrometry (ICP-MS/MS) for the interference-free quantitative determination and isotope ratio analysis of metals and metalloids in different sample types. Attention is devoted both to the instrumentation and to some specific tools and procedures available for advanced method development. Next to the more typical reaction gases, e.g., H2, O2 and NH3, also the use of promising alternative gases, such as CH3F, is covered, and the possible reaction pathways with those reactive gases are discussed. A variety of published applications relying on the use of ICP-MS/MS are described, to illustrate the added value of tandem mass spectrometry in (ultra)trace analysis.