materials Review Surface Modifications of Nanoparticles for Stability in Biological Fluids Luca Guerrini 1, Ramon A. Alvarez-Puebla 1,2,* and Nicolas Pazos-Perez 1,* ID 1 Departamento de Quimica Fisica e Inorganica and EMaS, Universitat Rovira i Virgili Carrer de Marcel•lí Domingo s/n, 43007 Tarragona, Spain; [email protected] 2 Institución Catalana de Investigación y Estudios Avanzados, Passeig Lluís Companys 23, 08010 Barcelona, Spain * Correspondence: [email protected] (R.A.A.-P.); [email protected] (N.P.-P.) Received: 13 June 2018; Accepted: 2 July 2018; Published: 6 July 2018 Abstract: Due to the high surface: volume ratio and the extraordinary properties arising from the nanoscale (optical, electric, magnetic, etc.), nanoparticles (NPs) are excellent candidates for multiple applications. In this context, nanoscience is opening a wide range of modern technologies in biological and biomedical fields, among others. However, one of the main drawbacks that still delays its fast evolution and effectiveness is related to the behavior of nanomaterials in the presence of biological fluids. Unfortunately, biological fluids are characterized by high ionic strengths which usually induce NP aggregation. Besides this problem, the high content in biomacromolecules—such as lipids, sugars, nucleic acids and, especially, proteins—also affects NP stability and its viability for some applications due to, for example, the formation of the protein corona around the NPs. Here, we will review the most common strategies to achieve stable NPs dispersions in high ionic strength fluids and, also, antifouling strategies to avoid the protein adsorption. Keywords: nanoparticles; biological fluids; colloidal stability; surface modification; protein corona; antifouling; plasmonics; quantum dots; magnetism 1. Introduction The use of inorganic nanoparticles (NPs) for biological and medical applications has attracted great attention in recent decades. This is clearly demonstrated by the large increase in publications reporting the use of nanotechnology for biomedical purposes [1–10], which can be summarized in three main properties arising from the nanoscale: (i) the similar size to biomacromolecules allows for a better interaction of NPs with cells and biomolecules (nucleic acids, proteins, lipid membranes, etc.) [11]; (ii) the high NP surface: volume ratio facilitates the incorporation of a high density of functional moieties [12]; and, (iii) the unique physicochemical properties (optical, electric, and magnetic) derived from the nanoscale size. The possibility of tailoring these properties on demand by modifying NP composition, size, and/or shape enables obtaining responsive materials toward very specific external electromagnetic stimuli [13,14]. All these features can be exploited in many potential applications such as MRI imaging [15], precise therapy at the cellular and subcellular levels with remotely triggered drug delivery (or hyperthermia) [12,16,17], early diagnostics of infectious diseases and cancer [18], detection of genetic mutations [19,20], or improving implant performance with antibacterial activity [21]. Therefore, the full understanding of the properties and behavior of NPs in biological media is critical. For conventional sols, the colloidal stability in such complex environments is usually compromised, resulting in aggregation and flocculation [22–24]. Broadly speaking, two main approaches are typically used for imparting colloidal stabilization. The most common and simplest strategy relies on the stabilization through electrostatic repulsion (e.g., citrate-stabilized Materials 2018, 11, 1154; doi:10.3390/ma11071154 www.mdpi.com/journal/materials Materials 2018, 11, 1154 2 of 28 Materials 2018, 11, x FOR PEER REVIEW 2 of 27 NPs).NPs). At At low low ionic ionic strengths,strengths, thethe diffuse double double layer layer (DDL) (DDL) extends extends far far from from the the particle particle surface, surface, facilitatingfacilitating particle–particle particle–particle repulsions. repulsions. However, at at high high ionic ionic strengths, strengths, the the NP NP DDL DDL is compressed is compressed andand neutralized neutralized with with the subsequentthe subsequent aggregation aggregatio duen todue van to der van Waals der forces Waals [25 forces,26]. Thus,[25,26]. electrostatic Thus, stabilizationelectrostatic largely stabilization fails to largely provide fails sufficient to provide colloidal sufficient stability colloidal in biological stability media.in biological The second media. method The consistssecond in method the generation consists of in a physicalthe generation barrier of at thea ph NPysical surface barrier (i.e., at steric the stabilization).NP surface (i.e., For instance,steric polymersstabilization). attached For toinstance, the particle polymers surface attached (e.g., to poly(ethylene the particle surface glycol), (e.g., PEG poly(ethylene or poly(vinyl glycol), pyrrolidone), PEG PVP)or poly(vinyl [27,28] are pyrrolidone), often used toPVP) increase [27,28] NPs are stabilityoften used in to suspensions. increase NPs The stability hydrophilic in suspensions. nature of The these hydrophilic nature of these polymers also induces an extra stabilization through the short-range polymers also induces an extra stabilization through the short-range repulsive hydration forces [24]. repulsive hydration forces [24]. Thus, steric stabilization results more adequate for biological systems. Thus, steric stabilization results more adequate for biological systems. In contrast with the electrostatic In contrast with the electrostatic stabilization, where the charge is defined during the synthesis steps stabilization, where the charge is defined during the synthesis steps of the NPs, steric stabilization of the NPs, steric stabilization usually requires an additional step of functionalization of the usually requires an additional step of functionalization of the preformed colloids (Figure1)[ 29,30]. preformed colloids (Figure 1) [29,30]. Also, another major challenge in the use of colloids in biofluids Also,is related another to majorthe tendency challenge of incertain the use compositio of colloidsnal incomponents biofluids is(mainly related proteins) to the tendency of the fluid of certain to compositionalunspecific coat components the particles (mainly yielding proteins) the so-called of the “protein fluid to corona” unspecific [31–35]. coat theThis particles corona can yielding cause the so-calledtwo main “protein issues: corona”particle destabilization [31–35]. This corona and surf canace cause inertization. two main These issues: phenomena particle destabilizationcan decrease the and surfacecirculatory inertization. lifetime Theseof NPs phenomena in blood or inhibit can decrease their ability the circulatory to bind the lifetimespecific target of NPs receptors in blood on or cells inhibit theirand ability tissues to [36]. bind the specific target receptors on cells and tissues [36]. FigureFigure 1.1. StrategiesStrategies to achieve achieve stable stable NPs NPs in in biofluids. biofluids. Herein, we will revise the most common strategies used to achieve stable inorganic NPs in Herein, we will revise the most common strategies used to achieve stable inorganic NPs in biological fluids and their interactions with proteins. Although special attention will be devoted to biologicalPEG coating fluids of andNPs, their as the interactions most widely with used proteins. approach, Although other strategies special using attention zwitterionic will be devotedligands, to PEGlipid coating bilayers, of NPs,proteins, as the glycans, most widely antibodies, used aptamers, approach, or other amphiphilic strategies polymers using zwitterionic and/or the ligands,NPs lipidimmobilization bilayers, proteins, on larger glycans, colloidal antibodies, templates will aptamers, be also discussed. or amphiphilic polymers and/or the NPs immobilizationOverall, several on larger established colloidal methods templates are will available be also for discussed. imparting colloidal stability to inorganic NPsOverall, in biological several media. established However, methods they typically are available exploit surface for imparting ligands colloidallacking of stability functional to groups inorganic NPswith in targeting biological specificity. media. These However, functionalities they typically are, in exploit fact, commonly surface ligandsintroduced lacking a posteriori of functional, in a groupssecond with modification targeting step. specificity. On the Theseother hand, functionalities the used stabilizing are, in fact, ligands commonly can significantly introduced increment a posteriori , inthe a second hydrodynamic modification diameter step. of Onthe theNPs, other thereby hand, altering the usedNPs properties stabilizing such ligands as mobility, can significantly blood incrementcirculation the times, hydrodynamic or cellular diameteruptake. For of theinstance, NPs, therebysmaller alteringhydrodynamic NPs properties radii have such much as mobility,lower blooddegrees circulation of opsonization. times, or cellularAlso, for uptake. efficient For transmembrane instance, smaller permeation hydrodynamic and excretion radii have small much lowerhydrodynamic degrees of radius opsonization. are required. Also, Th forerefore, efficient is obvious transmembrane that size is permeation a very important and excretion factor in the small hydrodynamicbiodistribution radius of NPs are [37]. required. Another Therefore,nontrivial aspect is obvious in the that selection