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Adsorption of NGF and BDNF derived peptides on gold surfaces Giuseppe Forte,a Alessio Travaglia,b Antonio Magrì,c Cristina Satriano,*d and Diego La e 5 Mendola* Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x

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S1. Lengthy experimental details. ……………………………………………………………………..2 1.1 X-ray photoelectron spectroscopy (XPS) ……………………………………………………………………………….2 1.2 Atomic Force Microscopy (AFM) ……………………………………………………………………………………….2 1.3 Water contact angle (WCA) and Surface free energy (SFE) measurements …………………………………………2 1.4 Computational methods ………………………………………………………………………………………………….2

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S2. Chemical and topographical characterization of the gold substrates…………………………..3 Figure S1. Water contact angle (left hand side axis; WCA) and surface free energy components for as received (untreated) and surfacecleaned gold substrates……………………………………………………………………………………………………………3 20

Figure S2. XPS survey spectrum of the treated gold surfaces immediately before the peptide adsorption experiments………..3 Figure S3. High-resolution photoelectron peaks of Au4f and O1s of the base piranha-treated gold surfaces. …………………4 25

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S1. Lengthy experimental details. 1.1 X-ray photoelectron spectroscopy (XPS) X-ray photoelectron spectroscopy (XPS) analyses were performed by using a PHI ESCA/SAM 5600 multitechnique spectrometer. Experiments were carried out with the standard AlK radiation source (h = 1486.6 eV) at a base pressure of 2 x 10-9 Torr. XPS spectra were collected at a photoelectron take-off angle of 45°, which, according to the effective attenuation length of 3.8 nm from Au in organic films [D.Y. Petrovykh et al., Langmuir 2004, 20, 429-440], corresponds roughly to an actual sampling depth of about 8 nm. Both survey and high-resolution region scans were recorded, namely O 1s, N 1s, C 1s and Au 4f peaks, at pass energy and 10 incremental step size of 150 eV/1 eV for survey and 11.85 eV/0.05 eV for the narrow scans, respectively. The XPS signals were analysed with a peak synthesis program based on a non-linear background and experimental bounds fitting by Gaussian components. The atomic elemental compositions were evaluated using sensitivity factors provided by the software (Indago_Instrument Version_ 3.1.0.77). All binding energies were referenced to C 1s hydrocarbon carbon peak at 285 eV. 5

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1.2 Atomic Force Microscopy (AFM) The surface morphology was measured with a Multimode/Nanoscope IIIA Atomic Force Microscope (Veeco) in tapping mode in air with a standard silicon tip. The relative room humidity was 30% and the room temperature was 23 °C. Data were acquired on square frames having edges of 10 m and 1 m. Images were recorded using 20 height and phase-shift, channels with 512x512 measurement points (pixels). Measurements were made twice or three times on different zones of each sample. 1.3 Water contact angle (WCA) and Surface free energy (SFE) measurements Half automatic video-based measurements of contact angle were performed at 25 °C by using a ThetaLite instrument (Biolin Scientific). Measurements of surface free energy were performed evaluating static contact angles of three different liquids onto the untreated and treated surfaces. With the sessile drop method, liquid drops of 2 l of volume were applied on different zones of each sample surface and by digital image analysis, WCAs were measured on both sides of the two-dimensional projection of the droplet. At least five measurements were made for each ample and then averaged. The surface free energies, in terms of apolar 30 Lifshitz–van der Waals (LW) and polar Lewis acid (+) and basic (-) components, were evaluated by using the Good-van Oss model, with the three following liquids: ultrapure Millipore water, glycerol, and tricresyl phosphate (Aldrich). 25

1.4 Computational methods A procedure of equilibration was applied for each of the eight system configuration here studied, namely: 1) entangled NGF(1-14) and BDNF(1-12) polypeptides; 2) entangled NGF(1-14) and NGF(1-14) polypeptides; 3) entangled BDNF(1-12) and BDNF(1-12) polypeptides; 4) NGF(1-14) physisorbed onto Au2O3 surface in water ; 5) BDNF(1-12) physisorbed onto Au2O3 surface; 6) entangled NGF(1-14) and BDNF(1-12) physisorbed onto Au2O3 surface; 7) entangled NGF(1-14) and NGF(1-14) physisorbed onto Au2O3 surface; 8) entangled BDNF(140 12) and BDNF(1-12) physisorbed onto Au2O3 surface. All the systems were placed into a box, and soaked with water molecules as solvent. The equilibration procedure consists of: (i) 5000 steps of Smart Minimizer minimization algorithm applied to the water molecules with a fixed solute, (ii) followed by 20-ns of molecular dynamic (MD) of the solvent at 298 K while the solute is still fixed, and (iii) another 80-ns MD simulation, at the same temperature, removing the constraint of the solute. 35

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S2. Chemical and topographical characterization of the gold substrates Contact angle measurements were carried out prior to the peptide adsorption experiments in order to determine the wettability character and surface free energies of the used substrates. Figure S1 illustrates the contact angle measurements and the calculated surface free energy, for both 5 dispersive Lifshitz van der Waals and polar acid-base components, taken from untreated gold crystals and those prepared for the peptide adsorption experiments, i.e., piranha and UV-ozone treated gold surfaces. The contact angles and therefore the hydrophilicity of the prepared surfaces are increased after the cleaning procedure. In particular, the acid-base polar component of free energy rises over the dispersive Lifshitz van der Waals one (the AB/LW ratio changes from 0.4 to 0.66), due to the 4.4-fold increase of the Lewis-base 10 component. This change is related to the removal from the gold surface of contaminative hydrocarbon species and the introduction of electron donor oxygen-containing species, due to the UV ozone treatment.

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Figure S1. Water contact angle (left hand side axis; WCA) and surface free energy components (right hand side axis: LW: Lifshitz van der Waals, AB polar acid base, + = Lewis acid component, - = Lewis base component) for as received (untreated) and surface-cleaned gold substrates.

According to the surface chemical analysis by XPS and the topography investigation at the sub-micron scale by AFM (Figure S2), the chemical structure of the used substrates is consistent with the stoichiometry of Au2O3, and a relatively flat topography corresponding to the measured root mean squared roughness of about 15 nm.

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Figure S2. XPS survey spectrum of the treated gold surfaces immediately before the peptide adsorption experiments. The photoelectron peaks of oxygen and gold are displayed. In the inset the topographical AFM image for 1 m x1 m scan size.

In particular, the high-resolution spectra of Au 4f and O 1s (Figure S3) evidence the presence of the oxygen incorporated in a gold oxide surface layer. The O1 and O2 components for oxygen, at the binding energy (BE) values of 528.8  0.2 eV and 530.5  0.2 eV are attributed to the oxygen atoms respectively in two- and threedimensional gold oxide Au2O3 [A.I. Stadnichenko et al., Moscow Univ. Chem. Bull., 2007, 62, 343–349]. Moreover, the two Auox peaks (at BEs respectively of 86.2  0.2 eV and 89.5  0.2 eV) indicate, according to literature, that gold is bonded to O in an Au2O3 structure [A. Krozer et al. J. Vac. Sci. Technol. A 15 (1997) 30 1704; D.E. King, J. Vac. Sci. Technol. A 13 (1995) 1247; B. Koslowki et al., Surface Science 475 (2001) 1-10]. 25


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It is to note that such a gold oxide layer is thinner than the sampled depth of analysis at the used conditions (which is of about 8 nm, see S1 section), therefore the most intense peaks in the Au photoelectron spectrum are those assigned to metallic gold (Au peaks at BEs respectively of 84.5  0.2 eV and 88.2  0.2 eV, representing the spin-orbit splitted components of the Au-4f level in the pure Au metal). Finally, the other peak components 5 evidenced in Figure S3b point to the presence of atomic oxygen adsorbed at metal surfaces, as well as peroxide and superoxide species deriving from the combined base piranha-UV ozone treatments (O3 component, at BE= 531.8  0.2 eV), OH groups or water molecules on the gold surface (O4 component, at BE= 533.3  0.2 eV), and weakly bound oxygen species (O5 component, at BE= 535.8  0.2 eV).

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Figure S3. High-resolution photoelectron peaks of Au4f (a) and O1s (b) of the base piranha-treated gold surfaces.

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