Analysis of selenium nanoparticles in human plasma

Nanoparticles (NPs) have been proposed more and more often during recent times in the biomedical field as active agents, diagnostics, and carriers in drug delivery systems. To a significant part, research has focused on metal-based NPs, with gold and silver NPs being the most investigated. Despite the vast research efforts in this direction, relatively few metal NP systems have been approved by medical agencies. Key reasons are lack of understanding and control of interactions with the biological environment, cytotoxicity owing to accumulation in tissues and insufficient quantitative analysis to follow transport, distribution and fate. Selenium nanoparticles have been proposed as alternatives, due to their antimicrobial efficiency and the general non-toxic behaviour towards mammalian cells. Anyhow, to study the interactions of SeNPs with biological environments, analytical methods are needed to characterize them and determine their distribution, degradation and biotransformation. While a range of complementary methods is available to characterize NPs, there is a lack of appropriate methods for the analysis of complex biological matrices.   

A group of Danish researchers now aimed at filling this gap by demonstrating the use of capillary electrophoresis hyphenated to inductively coupled plasma mass spectrometry (CE-ICP-MS) for the separation of Se NPs from degradation and transformation products in human plasma. PVA-coated SeNPs prepared in-house and a commercially available SeNP product were used as model NPs.  

All CE-ICP-MS/MS measurements were performed using an Agilent 8800 ICP-MS Triple Quad instrument using oxygen as cell gas. For separation, a 7100 CE instrument was hyphenated to the ICP-MS via a capillary outlet. Samples were introduced via an electrospray ionization source sprayer operated with a sheath liquid supplied by a syringe pump.  CE conditions such as voltage, pressure and capillary positioning at the sprayer were optimized.   

A first CE-ICP-MS method was developed for the analysis of aqueous samples using bare fused silica capillaries and a 30 mM tetraborate buffer as background electrolyte (BGE). After optimization, baseline separation of three low molecular weight Se species was achieved within 7 min., with PVA-coated SeNPs appearing at a migration time of 3.7 min. The analysis of a Q-SeNP product, that was open already for some months, indicated substantial degradation toward selenite in the order of more than 30 %. The exact percentage was not easy to evaluate, since the response from the NPs is lower than from the dissolved species due to aerosol transport efficiency differences.

The CE-ICP-MS method for the analysis of aqueous samples had to be modified for accepting the more complex plasma samples. To overcome the influence of the high protein content and ionic strength, capillaries were coated with a commercial electrostatically adsorbed highly polar neutral coating.   

Using this method, PVA-SeNPS were analysed after incubation in 50% plasma. In order to investigate their degradation, samples were analysed at 5h, 48 h, and 5 days after incubation. After 48 h, a signal with a migration time of 11 min. indicating selenite appeared  and the peak increased during the 5 days following. The fraction of SeNPs being degraded to selenite was estimated to less than 5% after 5 days. In contrast, a suspension of PVA-SeNPs stored in Milli-Q water showed no degradation over the same time.  

The authors concluded, that their preliminary study highlights the potential of CE-ICP-MS for the quantitative characterization of SeNP in biological media.

L. Bendahl, B. Gammelgaard, O. Jons, O. Farver, S.H. Hansen. Interfacing capillary electrophoresis with inductively coupled plasma mass spectrometry by direct injection nebulization for selenium speciation. J. Anal. At. Spectrom., 16/1 (2001) 38–42. DOI: 10.1039/b007137o.