biomimetics Article Surfactant Interactions with Protein-Coated Surfaces: Comparison between Colloidal and Macroscopically Flat Surfaces Helena Mateos 1,* , Alessandra Valentini 2, Francesco Lopez 3 and Gerardo Palazzo 4 1 CSGI (Center for Colloid and Surface Science), via Orabona 4, 70125 Bari, Italy 2 School of Chemical Engineering, University of Birmingham, Birmingham B15 2TT, UK; [email protected] 3 Department of Agricultural, Environmental and Food Sciences (DiAAA) and CSGI, University of Molise, Via De Sanctis, 86100 Campobasso, Italy; [email protected] 4 Department of Chemistry, University of Bari, via Orabona 4, 70125 Bari, Italy; [email protected] * Correspondence: [email protected] Received: 16 May 2020; Accepted: 29 June 2020; Published: 1 July 2020 Abstract: Surface interactions with polymers or proteins are extensively studied in a range of industrial and biomedical applications to control surface modification, cleaning, or biofilm formation. In this study we compare surfactant interactions with protein-coated silica surfaces differing in the degree of curvature (macroscopically flat and colloidal nanometric spheres). The interaction with a flat surface was probed by means of surface plasmon resonance (SPR) while dynamic light scattering (DLS) was used to study the interaction with colloidal SiO2 (radius 15 nm). First, the adsorption of bovine serum albumin (BSA) with both SiO2 surfaces to create a monolayer of coating protein was studied. Subsequently, the interaction of these BSA-coated surfaces with a non-ionic surfactant (a decanol ethoxylated with an average number of eight ethoxy groups) was investigated. A fair comparison between the results obtained by these two techniques on different geometries required the correction of SPR data for bound water and DLS results for particle curvature. Thus, the treated data have excellent quantitative agreement independently of the geometry of the surface suggesting the formation of multilayers of C10PEG over the protein coating. The results also show a marked different affinity of the surfactant towards BSA when the protein is deposited on a flat surface or individually dissolved in solution. Keywords: protein-coated surfaces; surfactant; DLS; ζ potential; BSA; colloids 1. Introduction Protein/surfactant interactions find important applications in industry and science. Their behavior at the air/liquid or liquid/liquid interface has been widely investigated due to their importance in the formation and stabilization of foams and emulsions [1–7]. Protein adsorption to solid surfaces is also related to dirt cleaning, or ultimately, it can be associated with biofilm formation, in which case, it is important to minimize or remove proteins adsorbed to the solid surface [8–10]. Studies of adsorption and protein/surfactant interaction at the liquid/solid interface are, however, not common. Furthermore, predicting protein/surfactant interactions at a solid surface is often difficult and not intuitive, especially when dealing with soft and flexible proteins that might change conformation when in contact with a surface. The subject of this paper is to investigate the potential use of colloidal surfaces instead of flat macromolecular ones for the research of interactions at the liquid/solid interface. The use of colloidal surfaces in combination with dynamic light scattering (DLS) could present many potential advantages since it can be applied to a wide range of materials including those that are unsuited for the external Biomimetics 2020, 5, 31; doi:10.3390/biomimetics5030031 www.mdpi.com/journal/biomimetics Biomimetics 2020, 5, x FOR PEER REVIEW 2 of 13 Biomimetics 2020, 5, 31 2 of 13 (DLS) could present many potential advantages since it can be applied to a wide range of materials including those that are unsuited for the external coatings. Furthermore, working with colloidal solutionscoatings. increases Furthermore, substantially working the with analyzed colloidal surface solutions area, increases which could substantially lead to more the analyzedrepresentative surface results.area, which could lead to more representative results. WeWe have have studied studied thethe interactioninteraction between between bovine bovine serum serum albumin albumin (BSA) (BSA) and the and non-ionic the non surfactant‐ionic C10PEG, at the glass/water interface in flat and colloidal forms (Scheme1). Using glass is an interesting surfactant C10PEG, at the glass/water interface in flat and colloidal forms (Scheme 1). Using glass is ancase interesting scenario case since scenario it is a very since common it is a very household common surface. household On the surface. other On hand, the BSA’s other adsorptionhand, BSA’s at a variety of interfaces has been extensively studied previously [10–15] and C10PEG is a common and adsorption at a variety of interfaces has been extensively studied previously [10–15] and C10PEG is a commonrelevant and surfactant relevant widely surfactant used widely in industry. used Inin thisindustry. paper, In we this compare paper, thewe resultscompare of the interactionsresults of thebetween interactions both between solid-surface both conformationssolid‐surface conformations using surface using plasmon surface resonance plasmon (SPR), resonance dynamic (SPR), light dynamicscattering light (DLS), scattering and laser (DLS), Doppler and laser electrophoresis Doppler electrophoresis (LDE) measurements. (LDE) measurements. SchemeScheme 1. Graphical 1. Graphical representation representation of ofthe the interaction interaction between between bovine bovine serum serum albumin albumin (BSA) (BSA)-coated‐coated SiOSiO2 and2 and surfactant surfactant on onboth both flat flat (A) ( Aand) and colloidal colloidal surfaces surfaces (B). (B ). 2. 2.Materials Materials and and Methods Methods BovineBovine serum serum albumin albumin (BSA) (BSA) protein protein (purity (purity > 98%)> 98%) and and Ludox Ludox AM AM colloidal colloidal silica silica system system were were purchasedpurchased from from Sigma Sigma-Aldrich‐Aldrich (St. (St. Louis, Louis, MO, MO, USA) USA) and and used used without without any any further further purification. purification. Na HPO and NaH PO were purchased from Fluka (Buchs, Switzerland) and J. T. Baker (Phillipsburg, Na2HPO2 4 and4 NaH22PO44 were purchased from Fluka (Buchs, Switzerland) and J. T. Baker (Phillipsburg,NJ, USA), respectively. NJ, USA), respectively. The water was The purified water was by means purified of aby Milli-Q means system of a Milli (Millipore‐Q system Corp., (Millipore Bedford, Corp.,MA, Bedford, USA) and MA, hereafter USA) is and denoted hereafter as MQ is denoted water. Aqueous as MQ water. phosphate Aqueous buffer phosphate was prepared buffer from was MQ preparedwater. The from surfactant MQ water. (decanol The surfactant ethoxylated (decanol with an ethoxylated average number with of an eight average ethoxy number (EO) groups) of eight was ethoxysupplied (EO) by groups) Sasol with was asupplied purity grade by Sasol of 95%. with a purity grade of 95%. WeWe used used surface surface plasmon plasmon resonance resonance (SPR) (SPR) to to evaluate evaluate the the interaction interaction of of C10 CPEG10PEG on on a previously a previously depositeddeposited BSA BSA layer layer on on a a flat, flat, macroscopic silica-coatedsilica‐coated sensor sensor surface surface (the (the thickness thickness of of the the silica silica layer layerwas was 10 nm). 10 nm). A dual-wavelength A dual‐wavelength (670 (670 and and 785 785 nm), nm), multiparametric multiparametric SPR SPR instrument instrument (MP–SPR (MP–SPR Navi Navi200, 200, BioNavis BioNavis Ltd., Tampere,Ltd., Tampere, Finland) Finland) was used was to recordused to the record SPR spectra. the SPR Contrary spectra. to traditionalContrary SPR,to traditionalMP SPR allowsSPR, MP the SPR measurement allows the measurement up to 5 µm above up to the 5 μ solidm above surface. the solid Spectra surface. were acquiredSpectra were in the acquiredrange of in 50 the◦ torange 77.5 ◦of. The50° to fluidic 77.5°. system The fluidic was system cleaned was before cleaned and afterbefore each and experiment after each experiment using a glass usingslide a inglass the slide place in of the the place sensor. of A the flow sensor. of 200 Aµ flowL/min of of 200 purified μL/min MQ of waterpurified was MQ set water for 10 minwas followedset for 10by min a 1%followed SDS solution by a 1% in SDS water solution for 10 in min water and for the 10 final min flow and of the MQ final water flow for of other MQ water 10 min. for BSA other and 10surfactant min. BSA and samples surfactant were preparedsamples were in phosphate-bu prepared inff phosphateered saline‐buffered (PBS) (pH saline 7.2) (PBS) and injected (pH 7.2) (250 andµ L injectedinjection (250 loops) μL injection on a silica-coated loops) on a sensorsilica‐coated in kinetic sensor titration in kinetic mode titration at a constant mode at flow a constant of 20 µ flowL/min. of All20 μ theL/min. experiments All the experiments were done in were triplicate. done in The triplicate. affinity The constants affinity were constants estimated were using estimated Winspall using 3.02 Winspall(Max Planck 3.02 Institute(Max Planck for Polymer Institute Research, for Polymer Mainz, Germany)Research, andMainz, OriginPro Germany) (OriginLab and Corporation,OriginPro (OriginLabNorthampton, Corporation, MA, USA) Northampton, software. MA, USA) software. TheThe interaction interaction between between colloidal colloidal particles particles and and BSA BSA was was studied