1 Introduction
Electrospray scanning mobility particle sizer (ES-SMPS) or electrospray ionization-gas-phase electrophoretic mobility molecular analysis (ESI-GEMMA) is a common method for size characterization in the field of aerosols and nanoparticles (NPs) science [1-5]. The size differentiation of NPs is based on the size-dependent, single-charged electrophoretic mobility of NPs so that NPs with various diameter will be separated under a series of voltages in a differential mobility analyzer (DMA). DMA works in a size range from 1 nm to 500 nm and the downstream condensed particle counter (CPC) quantifies the number of NPs screened by DMA. Consequently, both size and amount of NPs could be obtained with a scan time of few minutes.
Except for the applicability in NP measurement, ES-SMPS has demonstrated its performance in size measurement of bio-nanoparticls (bio-NPs) including proteins varies of sizes [6-8], viruses and virus-like particles [9-11], liposome [12], and etc. The correlation between mobility size and molecular weight of biomolecules has been reported. [6,13], and the aggregate of proteins can accordingly characterized [14,15]. Another advantage of the nanoscale ES-SMPS is the real time scanning of sample, which make the technique fascinating for biomanufacturing process, nanomedicine and pharmaceutical development. Taking the drug progress for Alzheimer’s disease for instance, the fast monitoring of ES-SMPS verified the oligomerization of the 42-residue amyloid-β peptide and provided the knowledge of the aggregation mechanism [16]. For the quantification of absolute number concentration for bio-NPs, Li et al. proposed the method by evaluating the droplet size, measuring the number concentration of NP formed as different oligomers for the total sample number summarization [17]; the internal calibration for the electrospray transmission efficiency proposed by Clouet-Foraison et al. [18].
To obtain accurate and reliable size measurements via ES-SMPS, the absolute calibration with proper reference materials (RMs) is needed. Commercially available RMs including polystyrene latex, metallic nanoparticles, and metal oxides in the solution are frequently selected for calibration. However, the presence of additives such as surfactant hinders the size calibration of ES-SMPS for NPs having size < 20 nm due to the formation of non-volatile residue coated on the pristine NPs (Figure 1 ). Alternatively, bio-NPs possess highly repeatable size and shape and long-term stability, and have been considered as candidate RMs for ES-SMPS recently. You et al. systematically evaluated five different bio-NPs illustrated their capability of offering superior calibration of ES-SMPS over conventional NP standards with high stability over several weeks [19]. Even though the consideration of bio-NPs as candidate RMs, research focusing on the selection of bio-NPs for calibration.is hardly found.
In this study, the suitable RM for the sub-10 nm size calibration for ES-SMPS was evaluated. AuNPs with diameter of 10 nm and 5 nm and four biomolecules involving bovine serum albumin (BSA), ubiquitin, low-density lipoprotein (LDL) and high-density lipoprotein (HDL) were selected for the size measurement. The pre-treatments of AuNPs by centrifugation and heating were applied for the removal of additives. Those treatment methods presented little effect on the accurate improvement of detected diameter and the difference was up to 112 % in Au-5 compared to the information from electron microscopy. On the contrary, BSA and ubiquitin performed consistent diameters with 6.9 nm and 3.6 nm, respectively, and showed comparable size distribution that referred to good stability as a function of time after dissolving in electrolyte. LDL and HDL contained multiple peaks in size distribution and were similar with that observed in previous study [20]. The results of this research revealed the advantages of using the self-disperse proteins and suggested the suitability of BSA and ubiquitin as the reference nanoparticles for sub-10 nm calibration for ES-SMPS.