Effect of Hydrophilic and Hydrophobic Nanoparticles on the Surface

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ARTICLE pubs.acs.org/JPCC

Effect of Hydrophilic and Hydrophobic Nanoparticles on the Surface Pressure Response of DPPC Monolayers Eduardo Guzm
an,* Libero Liggieri, Eva Santini, Michele Ferrari, and Francesca Ravera CNR—Istituto per l’Energetica e le Interfasi, UOS Genova, Via De Marini 6, 16149 Genoa, Italy ABSTRACT: The study of the interaction between Langmuir monolayers of 1,2-dipalmitoyl-sn-glycerol-3-phosphocholine (DPPC), as the major component of lung surfactant (LS), and nanoparticles of different nature, hydrophilic silica (SiO2) and hydrophobic carbon black (CB), has been carried out by measuring the compression Π
A isotherms and the response of the surface pressure to harmonic variations of the interfacial area simulating respiratory cycles in a Langmuir trough. The change of the monolayer interfacial structure induced by nanoparticles was monitored by Brewster angle microscopy. The results point out that nanoparticles incorporating into the monolayers influence the interfacial organization of the molecules and induce important modifications in both the phase behavior and the mechanical properties. Silica has stronger effects on DPPC phase behavior, compared to carbon black, while both affect the monolayer elasticity, the collapse conditions, and the nonlinearity of the surface pressure response to area expansion
compression simulating the respiratory cycles. With DPPC being the major component of pulmonary surfactant, the results here obtained are relevant in the framework of wider studies on the effect of nanoparticles on the pulmonary surfactant interfacial properties.

1. INTRODUCTION In the last decades, the release of nanoparticles in the environment has undergone an important increase due to industrial activities. The main sources of these nanoparticulate materials are the combustion processes of fossil fuels in power plants, vehicles, and heat generation systems which inject continuously carbonaceous particulate into the atmosphere.1 Additionally, the production and utilization of nanomaterials in several industrial fields related to the continuous development of nanotechnologies must be considered as a new source of nanoparticles with raising interest in recent years.2 The increasing exposure to these micro/nanoparticles makes a deeper comprehension necessary of the potential risk and hazards related to these materials for both the environment and health. Among the potential risks, the effects on the respiratory function are particularly important, lungs being a major entry point for microscopic particulates.3 The deposition along the respiratory tract of inhaled particles has been known for many years.4 This deposition is a size-controlled process where the geometry of the nanoparticles plays a key role in its distribution. Typically, the smaller particles (hydrodynamic diameter 1

Δσ1

ð3Þ

where Δσk are the k-Fourier coefficients, which are the amplitude of the k-order harmonics in the representation of the signal as a Fourier sum, and Δσ1 is, in particular, the amplitude of the fundamental harmonic. According to its definition, linear systems present a response with vanishing THD, while larger values are associated with a nonlinear behavior. Thus, the THD value can be effectively utilized to discriminate and quantify different regimes in the monolayer response.

3. RESULTS AND DISCUSSION To investigate the interaction of SiO2 nanoparticles with the DPPC monolayer, the latter is spread using a chloroform solution on a 1% in weight aqueous particle dispersion. The nanoparticle dispersion is stabilized by the sole particle charge and does not show any surface activity. As discussed elsewhere,39,40 the initially hydrophilic nanoparticles transfer into the DPPC monolayer, driven by the electrostatic interaction between the ammonium groups of the DPPC hydrophilic heads, oriented toward the aqueous phase, and the dissociated silanol groups of the silica nanoparticles, with negative charge.41 This attractive interaction leads to the formation of partially hydrophobic nanoparticle
DPPC complexes which are incorporated into the DPPC monolayer. The interaction between the DPPC monolayer and CB is instead investigated by adding a different amount of nanoparticles from dispersion in chloroform on the DPPC monolayer already spread on pure water. In this case, the interaction between nanoparticles and DPPC molecules is expected to be controlled by the CB hydrophobicity and the hydrophobic tails of DPPC molecules. Note that the systems containing CB cannot be considered strictly as mixed monolayers, since the spreading of DPPC and CB is not made together from a premixed dispersion (first, the DPPC is spread at the pure air/water interface, and then, the spreading of the CB is made on the preformed DPPC monolayer). However, for simplicity, these monolayers are called below mixed monolayers. It is important to emphasize that the nanoparticle
DPPC interactions depend on the particle nature, charged hydrophilic particles or hydrophobic, involving different parts of the lipid molecules. Thus, the DPPC
CB interaction is expected to mainly involve the DPPC hydrophobic tails.25 21717

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The Journal of Physical Chemistry C 3.1. Surface Pressure
Area Isotherms (Π
A). The quasiequilibrium compression of the monolayer area provides the Π
A isotherms which, besides being an important characteristic related to the functionality of LS, gives also information about the phase behavior of these monolayers. Figure 1 reports the Π
A isotherms for DPPC spread on the silica nanoparticle dispersion and for CB
DPPC mixtures spread on water. The isotherm of pure DPPC is also shown which is in agreement with those previously reported.8,17
19 At large areas per molecule, the DPPC isotherm presents a nearly vanishing surface pressure which starts to raise smoothly with the compression, indicating an increasing packing of the homogeneous liquid-expanded (LE) phase. At area per molecule ALE ≈ 66 Å2, the isotherm presents the typical quasi-flat feature attributed42 to the coexistence of a LE fluid phase and a highly ordered liquid-condensed phase (LC). When the layer is further compressed below a certain value of the area per molecule, the surface pressure increases with a larger slope, indicating that only the LC phase is present. Further compression leads to a new change in the slope of the isotherm corresponding to the appearance of the solid-like state21 followed by the collapse of the monolayer that occurs at a surface pressure around 70 mN 3 m
1. As a general trend, the presence of nanoparticles, independently of its nature and concentration, shifts the DPPC isotherm to higher areas per molecule. The incorporation of either CB or silica nanoparticles into the monolayer, in fact, reducing the free area available for the lipid molecules, increases the real surface density and favors an earlier molecular packing at the interface. As a consequence of the penetration of the nanoparticles into the lipid layer, the lift-off in the isotherm related to the compression of the LE phase occurs at higher areas per molecule compared to DPPC alone. This effect is evident both for silica nanoparticles and CB at the lowest concentration (10:1 DPPC
CB), while it becomes less important at larger CB weight fraction, xCB. This can be explained considering that the deposition of the CB takes place on preformed DPPC monolayers and, above a certain amount of CB nanoparticles, the available interfacial area could be insufficient for a monolayer distribution. Thus, it is possible that stacks of nanoparticles grow onto the preformed mixed film, leading to the formation of folds toward the aqueous subphase in a similar way to that proposed by Lin et al.43 for PAMAM dendrimers
DPPC systems. This folding of the interfacial layer increases the available area per DPPC molecule and, for this reason, may contract the above-discussed shift of the isotherm, observed for silica nanoparticles and CB at the lowest concentration. Concerning the LE
LC coexistence region, characterized by the plateau in the DPPC isotherm, it is noteworthy that CB particles do not change its surface pressure value significantly, whereas silica particles modify it substantially, making that zone less flat. This is probably related to the hindering of the phase transition. In fact, the attractive electrostatic interaction between the negative charged silica particles and the ammonium group of the DPPC head leads to the formation of lipid
nanoparticle complexes which, penetrating into the monolayer, may contribute to the modification of the dipolar moment of the DPPC molecules44 and consequently of its orientation at the water surface, affecting the molecular packing in the monolayer. Thus, these complexes disrupt the structure of the monolayer, hindering as a matter of fact the nucleation of the LC phase within the LE matrix. In contrast to silica particles, the hydrophobic interaction between CB and DPPC does not affect the electrostatic properties of

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Figure 2. Collapse surface pressure of mixed DPPC
CB monolayers on water as a function of the weight fraction of CB (O) and of the DPPC monolayer spread on the 1 wt % silica dispersions (9).

the DPPC monolayer. For this reason, we can assume that, in this region of the isotherm, the compression leads to the packing of DPPC molecules around the CB particles without an important effect on the surface reorientation of the lipid molecules. This is in agreement with the results reported by Wang et al.25 for mixed Langmuir monolayers of fullerenes and DPPC and also confirmed by the BAM images reported in Figure 1. Thus, even if the presence of CB particles disrupts the DPPC monolayer, inducing in some cases a folding of the monolayer, it does not affect the local nucleation of LC domains. From BAM images of Figure 1, it is evident that, while for DPPC spread on silica dispersion the LC nucleation is delayed (smaller LC domains), in the case of DPPC
CB mixtures the observed LC domains present shape and size similar to those obtained for pure DPPC, that are beanshape domains with a size distribution around 10
15 μm in agreement with what was obtained by other authors.45,46 As mentioned above, advancing with the compression, the DPPC monolayer passes from the LC to a solid-like phase, increasing the surface pressure before reaching the conditions for the monolayer collapse, corresponding to a maximum in the surface pressure isotherm. From Figure 1, it is evident that the presence of silica nanoparticles reduced the slope of this part of the isotherm, while it did not change the value of the surface pressure at the collapse. As discussed in ref 39, this is probably due to the squeezing out of particles
DPPC complexes, which reduces the DPPC content and allows compressing the monolayer until a smaller area than for the pure DPPC. For mixed DPPC
CB monolayers, a decrease of the collapse pressure (ΠC) is instead observed with the increase of the concentration of nanoparticles (Figure 2). As the formation of complexes does not occur in this case, this effect may be due to the formation of close-packed monolayers, driven by the increase of the cohesive forces between nanoparticles and DPPC, as a result of the many-body interactions.47 However, this weakens the average cohesive interactions between DPPC molecules at the interfacial layer. The relative packing density of DPPC at the 21718

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The Journal of Physical Chemistry C

Figure 3. Equilibrium elasticity, ε0, for DPPC and mixed DPPC
nanoparticle monolayers: (a) DPPC spread on 1 wt % silica dispersion; (b) DPPC
CB mixtures on water.

interface is thus reduced, and the collapse consequently occurs at surface pressures lower than those of pure DPPC. The nanoparticle effect on the collapse pressure is an important aspect related to the ability of LS of reducing the surface tension. From Figure 2, one can conclude that the increase of the CB concentration may present a negative effect in the LS functionality which seems to be more important than in the case of SiO2. The influence of the nature of the nanoparticles on the collapse pressure allows different scenarios for the nanoparticle incorporation into the LS layer to be proposed. The incorporation of the silica can be considered reversible. In fact, after a refinement process mediated by the squeezing out of DPPC
nanoparticle complexes, the monolayer before the collapse loses a large part of particles and thus achieves values of surface pressure similar to those of pure DPPC. On the contrary, as mixed DPPC
CB monolayers are stabilized by lateral hydrophobic interactions, the refinement process which has the effect of expelling the nanoparticles from the monolayer does not occur and these latter have to be considered irreversibly attached at the interface. This hinders the formation of close-packed DPPC monolayers and the reaching of high surface pressure before the collapse. 3.2. Equilibrium Elasticity. The equilibrium elasticity, ε0, obtained under quasi-static compression of the surface area is an important characteristic of the monolayer related to its rigidity and to the capability to store elastic energy.48,49 This quantity, for isothermal compression of the interfacial layer, is defined as   ∂Π ð4Þ ε0 ¼
A ∂A T and consequently can be obtained from the Π
A isotherms. The values of ε0 vs the surface pressure, evaluated by the numerical derivate of the Π
A isotherms, for the different investigated monolayers, are plotted in Figure 3. The two maxima

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Figure 4. Maximum values of ε0 in the LE and LC phases for mixed DPPC
CB monolayers on water as a function of the weight fraction of CB (open symbols) and of the DPPC monolayer spread on the 1 wt % silica dispersions (filled symbols). The lines are eye guidelines.

Figure 5. Surface pressure response to a sinusoidal variation of the area at frequency ν = 0.05 Hz for a dilation amplitude of ΔA/A0 = 0.01 (a) and ΔA/A0 = 0.4 (b), obtained for pure DPPC monolayer (right) and mixed 2:1 in weight DPPC
CB monolayers on water (left). In both cases, the reference surface pressure for the oscillation is Π0 = 40 mN/m.

in the plots located around 4 and 40 mN/m correspond, respectively, to the LE and LC phases. As evidenced in Figure 4, where these two maximum elasticity values are reported versus the weight fraction and nature of the nanoparticles, the elasticity of the LE phase is not affected by the presence of nanoparticles which instead appreciably modifies the elasticity of the LC phase. This is because the LE phases are in general characterized by an intrinsic disorder where the orientation of the lipid chains hinders, as a matter of fact, the cohesion between the molecules. This leads to a reduced value of the equilibrium elasticity of the monolayer which is not appreciably changed by the presence of nanoparticles, even after the formation of complexes, because the state of disorder typical of the LE state is not essentially changed. In contrast, the highly ordered LC phase 21719

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Figure 6. FFT spectra of the surface pressure signals of Figure 5. DPPC (right) and mixed 2:1 in weight DPPC
CB (left).

Figure 7. Fourier coefficients vs the relative dilation amplitude for the surface pressure signals of Figure 5. DPPC (right) and mixed 2:1 in weight DPPC
CB (left).

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obtained for pure DPPC presents different characteristics when nanoparticles are incorporated in the monolayer. This phase is characterized by strong intermolecular cohesion between the DPPC molecules (close-packed phase). The introduction of nanoparticles leads to the formation of monolayers with the packing of the lipid molecules disturbed by the presence of regions of the interfacial layer where the lipid molecules are replaced by nanoparticles. This leads to the reduction of the rigidity of the monolayer as a consequence of the reduction of the average cohesive interactions in the film (cohesive lipid
lipid interactions are stronger than lipid
nanoparticle). Notice that the decrease in ε0 caused by silica particles is equivalent to that caused by CB at a weight fraction around xCB = 0.75, in accordance with the different degrees of structural disruption induced by the nanoparticles. The more important disrupting effect of silica particles, compared with CB, is related to the electrostatic interactions which influence the molecular reorientation of the molecules in the monolayer, reducing the order of the phase and, consequently, the molecular cohesion and the rigidity of the film. Moreover, it is noteworthy that the value of Π corresponding to the LC maxima slightly decreases with the increase of the xCB. This confirms a scenario where the nanoparticles reduce the available area for DPPC molecules, leading to an earlier local packing of DPPC molecules in the monolayer. 3.3. Effect of Nanoparticles on Simulated Respiratory Cycles. An important aspect in relation to the potential physiological implications concerns the dilational rheology of the DPPC monolayer and the effect that nanoparticles of different nature may have on it, under quasi-realistic respiratory conditions. During the respiratory cycle, the LS film in the alveoli is subjected to a periodic area perturbation of about 30
40%, at a frequency within ν ∼ 0.04
0.2 Hz, around a reference state characterized by a value of Π ∼ 35
40 mN/m.8 To investigate this aspect, we analyzed the Π response of the DPPC monolayer, with and without nanoparticles, to sinusoidal changes of the interfacial area at a fixed frequency ν ∼ 0.05 Hz and different amplitudes of the area change, ΔA/A0, from 1 up to 40%. In all cases, both for pure DPPC and DPPC
particle monolayers, with increasing ΔA/A0, the response becomes nonlinear (Figure 5). This is evident by observing the Fourier spectra of

Figure 8. Total harmonic distortion (THD) vs the relative dilational amplitude for the surface pressure response to a harmonic area deformation at frequency ν0 = 0.05 Hz, for pure DPPC and mixed DPPC
nanoparticle monolayers. DPPC spread on 1 wt % silica nanoparticle dispersion (a); DPPC
CB mixtures spread on water (b). 21720

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The Journal of Physical Chemistry C such signals in Figure 6, where harmonics of higher order appear in addition to the fundamental ones. Figure 7 reports the values of the nonzero Fourier coefficients obtained by the FFT of the surface pressure responses obtained for different amplitudes ΔA/A0. It is clear that the number of nonzero values (corresponding to the appearance of higher order harmonics) and their values increase with the increase of the deformation amplitude. Therefore, the incorporation of nanoparticles in the DPPC monolayer promotes the onset of the nonlinear regime in the oscillatory Π response. In fact, for given perturbation amplitudes and frequency, the harmonics of higher order end up being more important in the presence of nanoparticles. The total harmonic distortion (THD), calculated on the basis of the Fourier coefficients from Figure 7, allows for a more compact representation of the nonlinearity. As shown in Figure 8, the TDH increases with the dilation amplitude already for pure DPPC but achieves a plateau above ΔA/A0 = 0.2. In the presence of silica nanoparticles (Figure 8a) instead, the THD does not level off, achieving values significantly larger than for pure DPPC monolayers, evidencing an increase of the nonlinearity. The same occurs for DPPC
CB monolayers at a ratio of 2:1 in weight, and higher CB concentration (Figure 8b), while for lower CB concentration the nonlinearity of the system is almost unaffected. This may be explained in terms of the degree of disruption that nanoparticles induce in the monolayer structure. In general, the nonlinear behavior of a lipid monolayer is related to the reduced efficiency of the surface diffusion to redistribute material during expansion after important compression of the surface area.50 For a pure DPPC monolayer, this is caused by the presence of microdomains. Nanoparticles cause a further hindering of surface diffusion due to the formation, during compression, of heterogeneous particle
DPPC structures in the monolayer. Figure 8 also shows how the THD value can be used to discriminate different regimes of the system behavior and consequently could be conveniently utilized to classify nanoparticles in relation to potential adverse effects on the lung functionality, based on dilational rheology measurements.

4. CONCLUSIONS The interfacial properties of layers of DPPC spread on silica aqueous dispersions and of mixed layers of DPPC
carbon black spread on water have been investigated as models for the incorporation of hydrophobic and hydrophilic particles into the lung surfactant. Investigations were carried out by measuring the compression Π
A isotherms and the response of the surface pressure to harmonic variations of the interfacial area simulating respiratory cycles in a Langmuir trough. In addition, the structure of the monolayers was investigated by Brewster angle microscopy. The results point out that the presence of nanoparticles induces significant changes in both the phase behavior and the dynamic response of DPPC monolayers. These modifications have been attributed to the reduction of the available area to the DPPC molecules at the interface and to the disruption of the interfacial structure of the monolayer caused by the incorporation of the nanoparticles. Significant differences in the interfacial properties of these mixed systems have been evidenced depending on the surface nature and on the concentration of the nanoparticles. Since the investigated nanoparticles are representative of those realistically present in the environment, these results have implications for the assessment of the toxicological effects of nanoparticles on the lung surfactant physiology. Besides the specific results, this study offers interesting working elements to

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develop in vitro toxicology assays dealing with the LS mechanical response. In particular, the use of the THD as a quantitative parameter to evaluate the effect of nanoparticles on the surface pressure response of LS monolayers during simulated respiratory cycles can be considered promising to rank the nanoparticles for a potential adverse impact.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by IIT—Istituto Italiano di Tecnologia within the Project SEED 2009 “Nanoparticle Impact of Pulmonary Surfactant Interfacial Properties—NIPS” and by European Science Foundation, ESF-COST Action D43 “Colloid and Interface Chemistry for Nanotechnologies”. ’ REFERENCES (1) Morawska, L.; Bofinger, N. F.; Kocis, L.; Nwankwola, A. Environ. Sci. Technol. 1998, 32, 2033–2042 . (2) Card, J. W.; Zeldin, D. C.; Bonner, J. C.; Nestmann, E. R. Am. J. Physiol. 2008, 295, L400–L411. (3) Kondej, D.; Sosnowski, T. R. Chem. Eng. Trans. 2010, 19, 315–320. (4) Heyder, J.; Gebhart, J.; Rudolf, G.; Schiller, C. F.; Stahlofen, W. J. Aerosol Sci. 1986, 5, 811–825. (5) Geiser, M.; Sch€urch, S.; Gehr, P. J. Appl. Physiol. 2003, 94, 1793–1801. (6) Johansson, J.; Gustafsson, M.; Palmblad, M.; Zaltash, S.; Robertson, B.; Curstedt, T. Biol. Neonate 1998, 74, 9–14. (7) Veldhuizen, R.; Nag, K.; Orgeig, S.; Possmayer, F. Biochim. Biophys. Acta 1998, 1408, 90–108. (8) W€ustneck, R.; Perez-Gil, J.; W€ustneck, N.; Cruz, A.; Fainerman, V. B.; Pison, U. Adv. Colloid Interface Sci. 2005, 117, 33–58. (9) Bachofen, H.; Schurch, S. Comp. Biochem. Physiol., Part A: Mol. Integr. Physiol. 2001, 129, 183–193. (10) Zuo, Y. Y.; Veldhuizen, R. A. W.; Neumann, A. W.; Petersen, N. O.; Possmayer, F. Biochim. Biophys. Acta 2008, 1778, 1947–1977. (11) Ravera, F.; Santini, E.; Loglio, G.; Ferrari, M.; Liggieri, L. J. Phys. Chem. B 2006, 110, 19543–19551. (12) Ravera, F.; Ferrari, M.; Liggieri, L.; Loglio, G.; Santini, E.; Zanobini, A. Colloids Surf., A 2008, 323, 99–108. (13) Liggieri, L.; Santini, E.; Guzm
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