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.