Buffer Components Incorporate into the Framework of Polyserotonin Nanoparticles and Films during Synthesis

The paper reports, in Section 3.4: "for all buffer systems, the PSe film coating makes glass more hydrophilic, decreasing the water contact angle by over 15°." The effect is reported as common to all four synthesis buffers; no per-buffer water contact angle is given in the main text.

Surface energy of glass increases from ~47 to ~61 mJ/m² when coated with PSe films (Table S1). These are OWRK model outputs computed from the three static contact angles (Section 2.2).

The paper notes that "although the PSe film increased the overall surface energy of the glass, there is a higher contribution from the increase in the dispersive energy component than the polar energy component (Table S1)." Its stated significance is a contrast with polydopamine: "This is significantly different from polydopamine films, where an increase in surface energy was largely driven by increasing polar energy component, independent of the substrate used." The polydopamine comparison is drawn from prior literature (Bourmaud et al. 2009; Jiang et al. 2011), not from measurements made in this study.

The paper's title finding. Nanoparticles synthesised in HEPES showed FTIR peaks at 1369, 1056 and 838 cm⁻¹ absent from the other three buffers; the first two are assigned to S=O stretching from the sulfur group of HEPES. High-resolution XPS confirmed a S2p sulfur peak in the HEPES particles only. The paper is precise about the limit of this evidence: "HEPES, out of the four buffer components, has a unique sulfur atom that allows this difference to be identified." Direct spectroscopic evidence therefore exists for one buffer of four; for the other three the authors infer — "we posit that other buffer components are also likely to become trapped". The XPS survey spectrum showed no difference in oxygen, nitrogen or carbon composition between buffers.

All four buffer components bind favourably to a serotonin monomer at the pH 9 synthesis condition: Bicine −3.745 eV, HEPES −3.281 eV, Tris −1.640 eV, DEA −1.469 eV. Binding is weaker at pH 7 for all four, and the HEPES/Bicine order reverses (HEPES −2.638 eV, Bicine −1.733 eV). The calculation's limits are stated by the authors: "we used the structure of serotonin monomer since the mechanism of polyserotonin formation is currently unknown, and the structures of the early intermediates of nanoparticle formation have not been investigated." They note that further work is under way: "We are currently investigating whether the buffer components interact with early intermediates of polyserotonin formation, which could then lead to physical entrapment during nanoparticle growth."

Three null results, all stated by the authors. Elasticity: "we did not observe a significant difference in the Young's modulus of the particles as a function of the buffer system" — apparent Young's modulus was ~400 MPa across all four. Radical species: "an EPR analysis showed no difference in g-values (2.0038–2.0042) across all four buffer types further indicating the formation of the same semiquinone radical species, regardless of the buffer used." Bulk elemental composition: the XPS survey spectrum "showed no difference in the atomic composition of oxygen, nitrogen, and carbon".

Against an APTES-functionalised (positively charged) AFM tip, adhesion force on PSe films was 1.6 ± 0.3 nN for Tris and 1.8 ± 0.5 nN for HEPES, versus 1.2 ± 0.2 nN for both DEA and Bicine, and 0.4 ± 0.1 nN for bare glass (n = 153). Work of adhesion showed a similar pattern: Tris and HEPES 0.07 fJ, DEA 0.04, Bicine 0.03, glass 0.01 (n = 145, one-way ANOVA, p ≤ 0.001). Film thickness ranged from 1.9 ± 0.9 nm in Tris to 9.1 ± 2.1 nm in HEPES. The paper's Conclusions cite these film results, together with the nanoparticle zeta potentials, when describing how buffer choice affects the materials.

Static contact angles were collected using 20 µL drops of milli-Q water, glycerol, and diiodomethane, with camera-based imaging and in-house angle extraction feeding an OWRK surface-energy calculation.

The reported outcome for PSe film-coated glass is a reduction in water contact angle by over 15° — and the paper reports this "for all buffer systems", with no per-buffer difference given in the main text.

The reported total surface energy increases from ~47 to ~61 mJ/m² when glass is coated with PSe films (Table S1) — an OWRK calculation from the three measured angles, not a directly measured quantity.

The study attributes more of the surface-energy increase to the dispersive component than to the polar component (Table S1) — the reverse of what the polydopamine literature reports, which is why the authors flag it.

The protocol was 20 µL drops placed manually and imaged with a 0.3 Mp camera, with angles analyzed by an in-house program and surface energies calculated with the OWRK model. Replicate count, temperature and humidity for these measurements are not stated in the paper. The paper uses these measurements to compare PSe film-coated glass with uncoated glass, where the water contact angle decreased by over 15° (Section 3.4).