Surface Modification for Improving the Stability of Nanoparticles in Liquid Media †

Today, nanoparticles have already became an indispensable material for industries because of their unique size dependent properties such as electrical, magnetic, mechanical, optical and chemical properties, which largely differ from those of their bulk materials. Since nanoparticles have different surface structures and surface interactions compared to the sub-micron sized particles, nanoparticles have an extremely high tendency of adhesion and aggregation. Thus, it is quite important to develop techniques to control the dispersion/aggregation phenomena of nanoparticles to apply them into functional materials and products. In terms of aqueous media dispersions, DLVO theory is a quite useful tool to control and characterize the dispersion phenomena. However, it is still a challenging issue to control the stability of suspension with highly loaded nanoparticles or suspensions in organic media. Surface modification of nanoparticles is one of the mostly accepted methods to improve the dispersion stability of nanoparticles in those challenging conditions. On modifying nanoparticle surface, it is quite important to design the surface structure based on the type of nanoparticles and the liquid media. In this review, we will first introduce some reasons why nanoparticles form aggregates so easily. Then, various surface modification techniques will be reviewed for the dispersion of various nanoparticles into various liquid media.


Introduction
Today, nanoparticles have already became an indispensable material for industries because of their unique size dependent properties such as electrical, magnetic, mechanical, optical and chemical properties, which largely differ from those of their bulk materials [1][2][3][4][5] . Snce nanoparticles have different surface structures and surface interactions compared to the sub-micron sized particles, nanoparticles have an extremely high tendency of adhesion and aggregation.Thus, it is quite important to develop techniques to control the dispersion/aggregation phenomena of nanoparticles to apply them into functional materials and products.In terms of aqueous media dispersions, DLVO theor y is a quite useful tool to control and characterize the dispersion phenomena.However, it is still a challenging issue to control the stability of suspension with highly loaded nanoparticles or suspensions in organic media.Surface modification of nanoparticles is one of the mostly accepted methods to improve the dispersion stability of nanoparticles in those challenging conditions.On modifying nanoparticle surface, it is quite important to design the surface structure based on the type of nanoparticles and the liquid media.In this review, we will first introduce some reasons why nanoparticles form aggregates so easily.Then, various surface modification techniques will be reviewed for the dispersion of various nanoparticles into various liquid media.

Difficulties of Handling Nanoparticles
Brownian motion is one of the inneglectable phenomena for handling nano-sized particles.The three dimensional mean transfer distance (Δx) by the Brownian motion can be stated as eq.1, where DB and Δt is Brownian diffusion coefficient and time, respectively.The Brownian diffusion coefficient in Eq.1 can be stated as eq. 2 where k is Boltzmann constant, T is the absolute temperature, μ is the viscosity of media and dp is particle size.From eq.1 and eq.2, it is clear that the Brownian motion increases as the particle decreases.Thus, nanoparticles have ver y high chances to make an approach and collision with other nanoparticles whatever the media is.Since van der Waals attractive force also works among nanoparticles, the generation of potential barrier among nanoparticles is necessary to prevent strong aggregations.
The DLVO theory 6) , which involves van der Waals attractive force and electro static interactions from electrical double layer, is one of the tools to control potential barrier between particles in aqueous media.
In the case of sub-micron sized particles, it is quite easy to control their dispersion stability by the electrical double layer which can be tuned by the surface potential of particles and the counter ion concentration.However, in the case of nano-sized particles, it is difficult to improve their dispersion stability only by the DLVO interactions because of the small potential barrier.For an example, Fig. 1 shows the effect of particle size and the surface potential on the potential cur ves calculated by the DLVO theor y.The x-axis and the y-axis is the total potential (VT) normalized by kT and the surface distance, respectively.The counter ion concentration was fixed to be 1.0 mM.When the particle size and the surface potential was 100 nm and 88.5 mV, respectively, a large potential value, which VT/kT>30, was calculated.This VT/kT value is large enough to prevent the nanoparticles from aggregation.Contrary, when the particle size was reduced down to 30 nm with same surface potential value, the normalized surface potential reduces to VT/kT<10.In this condition, since the potential barrier generated by the electric double layer is small, it is expected that the particles will strongly aggregate.In order to obtain the large potential barrier as VT/kT>30 for 30 nm nanoparticles, it is necessary to increase the surface potential up to 177 mV, which is difficult to obtain even by adsorbing polyelectrolytes.
From the potential curve calculated by the DLVO theory as shown in Fig. 1, we can estimate the required surface potential to let the particle be in the dispersed state.In the case of high concentration suspensions, it is also important to take in account the surface distances among the particles.In Fig. 1, we can observe that the maximum potential was obtained when the surface distance is several nanometers.When the particle concentration is high and the particles stay near than this distance, the particles will be trapped in the van der Waals force and form aggregates. Fig. 2 shows the relationships between the volume fraction of particles and mean surface distance calculated by eq.3 which was reported by Woodcock 7) .In the case of sub-micron sized particles, the mean particle surface distance will approach near to several nanometers when the volume fraction is over 60 vol%.Contrary, in the case of nanoparticles, the mean particle surface distance will reduce to several nanometers when the volume fraction is only 20 vol%.The mean particle surface distance will be smaller at low solid concentration in the case of nanoparticles.An additional surface repulsive force such as steric repulsive force will be necessary at this condition where the mean particle surface distance approaches near than several nanometers.
Nanoparticles also possess unique surface structure compared to the sub-micron sized particles [8][9] .
For an example, we have synthesized SiO2 particles with various diameters by sol-gel method and characterized their surface structure by FT-IR.It was found that the peak of free silanol groups increases as the particle size reduces.It is expected that the large cur vature of nanopar ticles increased the average distance of silanol groups and redused the possibility of hydrogenation.This difference of surface structure affected largely the surface force interactions  and dispersion stabilities.Fig. 3 shows the surface force interaction curves of SiO2 particles with various diameters measured by AFM colloid probe method.The theoretical force cur ve calculated by DVLO theory is also shown in this figure.In all particle diameters shown in Fig. 3, it can be observed that the measured force curves fits with the theoretical force cur ves until the surface distance approaches near to several nanometers.However, in case of particles larger than 60 nm, the van der Waals attractive was not detected when the particles were approached near than several nanometers.This was due to the repulsive force from the hydrogenated silanol groups.This repulsive force from hydrogenated silanol groups was effective to improve the stability of particles in aqueous media.Contrary, in the case of 8 nm SiO2 nanoparticles, the repulsive force from hydrogenated silanol groups were not detected so that there is only less chance to improve the dispersion stability in liquid media.

Surface Modification of Nanoparticles for the Improvement of their Dispersion Stability in
Various Iiquid Media.
In section 2, we have described the difficulties of stabilizing nanoparticles in liquid media.In order to improve the dispersion stability of nanoparticles in liquid media, it is necessar y to modify the particle surface by polymeric surfactants or other modifiers to generate an ef fective repulsive force between nanoparticles.

Post-synthesis surface modifications 3.1.1 Polymer dispersants
The adsorption of polymeric dispersant is one of the simplest surface modification techniques to improve the dispersion stability of nanopar ticles in liquid media.In terms of dispersing hydrophilic nanopar ticles into aqueous media or in organic solvents with high polarities, anionic polymer dispersants or cationic polymer dispersants are widely used to generate steric repulsive force from the polymer chains and to increase the surface charge.
For an example for anionic surfactants, various types of polycarboxylic acids and their salts such as polyacrylic acid (PAA) [10][11][12][13] , polyacrylic acid sodium salts (PAA-Na) [14][15][16] , and co-polymers of polyacr ylic acid and maleic acid 17) are used to disperse many kinds of oxide nanoparticles as like BaTiO3 10) ,TiO2 , MgO 12) , and Fe2O3 15) .For an example of cationic surfactant, polyethyleneimine (PEI) is widely accepted [18][19] .The relationships among pH of suspension, solid fraction of the suspension, dissociation ratio of polymer dispersant, molecular weight of polymer surfactant, surface charge of nanoparticles and the size of the particle are quite essential to the adsorption ratio of surfactants and the degree of steric repulsive force. Kakui t al. [17][18] have reported the effect of particle size and the molecular weight of branched PEI on the viscosity of Al2O3/ethanol suspension at fixed solid concentration.It was found that in the case of sub-micron sized par ticles, the smallest viscosity was obtained when the molecular weight of PEI was 10,000. Conrary, in the case of 7 nm Al2O3 nanoparticles, the suspension viscosity reduced drastically when the molecular weight was 1800.As shown in Fig. 2, the mean surface distance will drastically reduced at same solid fraction when the particle size is reduced to several ten nanometers from sub microns. It s expected that large sized polymer surfactant were not able to travel among the particles when the particle size was nanosized. Thus, wen the high molecular weight surfactant was used for nanoparticles, the surfactant was not able to be adsorbed on the surface and could not possess an effective repulsive force to improve the dispersion stability of the suspension.Similar effects are also reported on the TiO2 aqueous suspension. The efect of solid fraction of TiO2 nanoparticles and the molecular weight of PAA-Na on the suspension viscosity were reported.14) Fig. 4 shows the apparent viscosity of TiO2 suspension of various solid fractions treated with PAA-Na with various molecular weights.It can be observed that the optimum molecular weight to obtain the minimum apparent viscosity of suspension reduces as the solid fraction increases.It was found that the suspension viscosity drastically reduces when the size of polymeric dispersants adsorbed on TiO2 were near to the mean surface distance calculated by Eq.3.Even if we are using the same TiO2 nanoparticles, the smaller molecular weighted surfactant reduces the suspension stability at high solid content where the larger molecular weighted surfactant reduces the suspension stability at low solid content.Not only the molecular weight but also the structure of the polymer sur factant can af fect on the dispersion stability of nanoparticles.One example is the use of polymer dispersant with hydrophilic group and hydrophobic group 20) .Fig. 5(a) shows the force cur ve between TiO2 nanoparticles in water treated by PAA-NH4 + or a copolymer of PAA-NH4 + and polyacrylic acid methyl, measured by AFM colloid probe method.It was measured that TiO2 modified by PAA-NH4 + had larger steric repulsive force and reduced the apparent suspension viscosities (Fig. 5(b)).It is expected that the loop-train structure can be controlled by tuning the ratio of hydrophilic site and the hydrophobic site.Another example is the use of comb type polymers such as PAA backbones with PEO chain 10), 16), 21).These comb polymers are also applied to improve the stability of various oxides, such as BaTiO3 and Al2O3, and the particles adsorbed with these polymers are reported to be well stabilized under wide range of pH and ion concentrations.On dispersing hydrophobic nanoparticles, such as SiC, CNT, and coals, in aqueous media, the copolymers with hydrophilic group and hydrophobic group is often used for anionic surfactants.[22][23][24] The hydrophobic segments are for the adsorption of dispersant on hydrophobic particles and the hydrophilic segments are for the improvement of compatibility with aqueous media.The hydrophilic segment also plays an important role for the generation of effective repulsive force by the electrical double layer interactions. For th hydrophobic segments, an aromatic compound such as styrene is used to make an effect adsorption on hydrophobic particles by hydrophobic interactions and pi-pi interactions.In the case of the use of cationic polymers, PEI is also applicable to hydrophobic particles and various reports can be found which improves the stability of SiC and CNT in aqueous media by PEI.25)

Chemical surface modifications
Surface modifying the particle surface chemically is an also useful technique to improve the stability of nanoparticles in various liquid media.On modifying the oxide nanoparticle surface, silane coupling agent, which has 1-3 alkoxy groups and 3-1 organic functional groups, are used from the 1960s.Metal-OH group on the particle surface is used as a reaction site.The first purpose of the silane coupling agents was to improve the compatibility of hydrophilic particle surface with hydrophobic polymer surface by introducing various organic functional groups on particle surface.Ever since Plueddemann have reported the concept of silane coupling agents 26) , numerous researchers have modified the particle surface to improve the compatibility of particle/polymer surface and to improve the properties of composite materials [27][28]. The surface modification of nanoparticles by silane coupling agents is also useful to improve the dispersion stability in organic media.The first example is grafting various polymers on particle surface by using silane coupling agents.Typically, various reactive groups such as amines, epoxides and vinyls are firstly introduced on the particle surface by silane coupling agents, and then polymers are grafted from or grafted to the particle surface.For examples, the particle surface was modified by aminopropylsilane, and then PEG chains were fur ther grafted to the particle by using epoxy terminated PEG 29) .Various radical polymers such as polyvinylpyrrolidone (PVP) can be grafted on vinyl functionalized particle surface.30) Radical polymer brushes such as PMMA can also be grafted from the amino functionalized surface by reversible addition−fragmentation chain transfer polymerization (RAFT) [31][32] .
Another example is designing steric silane networks on particle surface by controlling the reaction of silane alkoxides.Chaimberg et al., have reported that the amount of silane coupling agent chemisorbed on oxide supports differs drastically by changing the surface modification procedures. 33)The type of solvents, pH, and amount adsorbed water on particles largely affected to the chemisorbed content of silane coupling agents.Having an impression from this report, we have modified the surface of fumed silica nanoparticles in MEK with slight amount addition of pH controlled water. 34)It was found that when 3-glycidoxypropyltrimethoxysilane were modified with slight amount addition of acidic water, a relatively large steric repulsive force was measured by colloid probe AFM method while that modified with slight amount addition of base water possessed small steric repulsive force.It was also reported that the MEK suspension of particles with large measured steric force had lower viscosity and the silane network on the particle surface played an important role for improving the suspension stability.
On chemically modifying hydrophobic particles such as carbides and carbon related materials, it is necessary to design the surface by a chemistry that entails the reaction with the surface functional group on the par ticle surface.In case of carbon related particles, the unsaturated hydrocarbon which mostly related to the defect of graphite rings are one of the useful functional groups.For example, we have modified the surface of SiC nanoparticles by using various azo radical initiators such as 2,2 -azobisisobutyronitrile (AIBN) and 2,2 -azobis (2-methylpropionamidine) dihydrochloride (AMPA). 35)It is reported that the intensity ratio of D band and G band (D/G) measured by raman spectroscopy reduced by modifying the SiC nanoparticle, which is an evidence of reaction between unsaturated hydrocarbon on SiC surface and radical initiators.By choosing the structure of azo radical initiators, the surface properties of SiC nanoparticles can also be tuned.When AIBN was introduced on the SiC surface, the particles possessed anionic characteristics that they were stable in basic aqueous solution while that possessed cationic characteristics and were stable in acidic aqueous solution then they were modified by AMPA (Fig. 6).By applying this possibility of radical reactions at the surface of carbon related particles, various polymers can be also grafted on the particle surface.For example, ATRP radical initiators were first generated on the surface of CNT and then PMMA was grafted from the surface. 36)There are also examples of surface modification techniques which directly react with the graphite ring on the particle surface.It is reported that biradical groups such as nitrene compounds can be reacted with double bonds on carbon related materials. 37)The particle surface can be tuned by applying various nitrenes with reactive functional groups such as amine, carboxyl, and bromide groups. 38)The use of 1,3-dipolar cycloaddition of azomethine ylides, which can be generated by condensation of an R-amino acid and an aldehyde, are also applicable to fictionalization of carbon related materials. 39)n the case of post-synthesis surface modifications, it is possible to improve the dispersion stability of nanoparticles in various solvents.However, there are large difficulties in redispersing them into solvents near to their primary particle size.This is because nanoparticles strongly aggregate when they were collected as dried powder.In order to redisperse this aggregated dr y powder into solvents near to their primary particle size, the mechanical milling method using small bead has recently developed. 40)As reducing the bead size down to 15-30 μm, the aggregation size reduced to their primary particle size as about 10 nm where the par ticles were still aggregated when the bead size was larger than 100 μm in diameter.This method and other physical technique, for example, ultrasonic dispersion, can applied to redisperse various nanoparticle into liquid media by the simultaneous processing of the surface modification introduced above and the bead milling.

In-situ surface modifications 3.2.1 Synthesis of nanoparticles redispersible near to their primar y particle size
In order to re-disperse nanoparticles into solvents without aggregations, an in-situ surface modification route, which conduct surface modification during the nanoparticle synthesis, have been investigated by many researchers.A reverse micelle method is one of such method that particle synthesis reaction occurs in the reverse micelle structures. 41)Various metals [42][43] , oxides [44][45] , and non oxide semiconductors [46][47] can be synthesized by this method. Snce the particles are synthesized in the reverse micelle structure, the obtained nanoparticles are redispersible into organic solvents with low-polarities.A thermal decomposition method, which involves the thermal decomposition of ogranometallic compounds in high boiling solvents are also widely accepted methods to obtain nanoparticles with high crystallinity and sharp size distributions. 48) Th] In the former two cases, capping agents such as trioctylphosphine oxides (TOPO), oleic acid and oleyl amines are dissolved in the synthesis solution to prevent the aggregation of nanoparticles.In the case of surfactant-metal uses such as metal oleate, the surfactant acts as the capping agents.It is reported that these thermal decomposition routes has possibility of particle size tuning in nanometer scale.53) Not only metals, 52,54) oxides, 49,52,53) and nonoxide semiconductors, [55][56] but also complicated compounds such as fluorides 57) and phosphine nanoparticles 50) are able to be synthesized by these methods.Since the prepared particles are capped with surfactants, they are typically redispersible into low-polar organic solvents.Nonhydrolytic sol-gel reaction, [58][59][60] which is a reaction between metal halides and alkoxides or ethers, with the presence of capping agents such as TOPO and fatty acids are another accepted routes to prepare nanoparticles redispersble into low-polar solvents.
][63][64] Typically, nanoparticle reagents those soluble in high polar solvents, such as metal salts and metal complexes, are dissolved in polyol media (e.g.ethylene glycol, diethylene glycol, triethylene glycol) and then aged at temperature near to the boiling temperature.Many oxides [61][62] , sulfides 63) and flurodes 64) those redispersible into high polar solvents can be prepared by this method.
Based on abovementioned methods, various nanoparticles including metals, oxides, sulfides, and fluorides that are redispersible into many solvents can be synthesized by in-situ surface modification techniques.Contrary, a large difficulty lies in terms of engineering an in-situ surface modification technique to carbon related nanoparticles, because the particle synthesis temperature is extremely high.However recently, there are several reports of synthesis of carbon related materials those dispersible in solvents near to their primary particle size.The first example is the synthesis of carbogenic nanoparticles by thermal decomposition of various ammonium citrate salts such as octadecyl ammonium citrate salts and 2-(2-amonoethoxy)-ethanol salts. 65)It was reported that the citrate groups decomposes into carbogenetic nanoparticles while the organic ammonium salts act as a surface modifier.Hydrophilic and hydrophobic carbogeneic nanoparticles about 7 nm were prepared by using octadecyl ammonium salts and and 2-(2-amonoethoxy)-ethanol salts, respectively.Another example is the application of laser irradiation method.Graphite powders were dispersed into PEG and Nd:YAG laser was irradiated to the suspension. 66)After 2hours of irradiation, the suspension was centrifugated and the resulted supernatant contained PEG modified carbon nanoparticles which diameter was about 3 nm.The surfactant capped nanoparticles which were synthesized by in-situ surface modification procedure can also further be modified in order to tune their surface properties.Ligand exchange method, which exchanges the capped surfactant, is a typically accepted method and has a large benefit in controlling the particle surface by keeping their dispersion stability.The surface structure can be controlled by exchanging various capping agents which has thermo-sensitivity 67) and that increases compatibility with aqueous solution [68][69] , polymers 70) , or biomolecules [71][72] .For examples, hydrophobic ligands on Fe3O4 67) , and FePt 72) nanoparticles were exchanged to PEG based ligand to improve the biocompatibility of nanoparticles. 2-mrcaptothanol capped ZnS nanoparticles were modified with a tailor-made surfactant which contains polymerizable functional group in order to improve the compatibility with radical polymerizable polymers. 70) Siane coupling agent, which was introduced above, can also be attached by ligand exchange route.For example, oleic acid stabilized ferrite magnetic nanoparticles were ligand exchanged by various silane couling agents witch has amine, carboxylic aicd, or PEG group 73) . Aftr the ligand exchange procedure, ferrite magnetic nanoparticles were able to redisperse in aqueous media without strong aggregations. Thi ligand exchange by silane coupling agents can also applied to various metals and oxides such as Au, Ag, and Fe3O4.74) The use of mixed silane alkoxides are also an useful tool to tune the surface properties for their redispersion in various types of solvents.75) For an example, we have modified the surface of TiO2 nanoparticles by mixed silane alkoxides with hydrophobic group (decyltrimethoxysilane: DES) and hydrophilic group (3-aminoproyltrimethoxysilane: APTMS).When TiO2 were only modified by DES, the surface modified particles were only redispersible into low polar solvents such as toluene while that became redispersible into high polar solvents by modifying the particles by DES and APTMS (Fig. 7).By applying these methods, various reactive groups can be introduced on the particle surface without formation of strong aggregations.Since the particles are controlled to be not aggregated during the entire process, from the particle synthesis to surface modification procedure, it is a useful process to engineer the particle surface for nanotechnology applications.

Conclusion
The reason for the dif ficulties for dispersing nanoparticles in liquid media, and various surface modification technique to overcome the difficulties of nanoparticle dispersions have briefly reviewed.The post synthesis surface modification have a benefit for easy processing whereas the controlled in-situ surface modification have a benefits for the possibility of engineering the nanoparticle surface without aggregations, e.g.surface modified nanoparticles are redispersible into liquid media near to their primary particle size.By selecting the surface modification procedure by time and circumstances, a nanoparticle suspension which are useful for further material processing can be designed.

Fig. 2
Fig. 2 Relation between solid volume fraction and mean particle surface distance.

Fig. 1
Fig. 1 Example of the potential curves calculated by DLVO theory.

Fig. 3
Fig. 3Force curves between sol-gel derived SiO 2 particles with different particle diameter measured by colloid probe AFM.

Fig. 4
Fig.4 Effect of molecular weight on the TiO 2 /water suspension viscosity at various solid fractions.

Fig. 5 (
Fig. 5 (a) Force curves measured between TiO 2 nanoparticles treated by anionic polymer dispersants and the (b) apparent viscosity of corresponding TiO 2 /water suspensions.

Fig. 6
Fig. 6 Surface structure and the stability of surface modified SiC nanoparticle in pH controlled water after modified by (a) AIBN and (b) AMPA.