Effects of Gold Nanoparticles on the Stability of Microbubbles

Aug 28, 2012 - Surfactant-coated microbubbles are utilized in a wide variety of applications, from wastewater purification to contrast agents in medic...
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Effects of Gold Nanoparticles on the Stability of Microbubbles Graciela Mohamedi,† Mehrdad Azmin,‡ Isabel Pastoriza-Santos,§ Victoria Huang,‡ Jorge Pérez-Juste,§ Luis M. Liz-Marzán,§ Mohan Edirisinghe,‡ and Eleanor Stride*,† †

Institute of Biomedical Engineering, Department of Engineering Science, University of Oxford, Old Road Campus, Headington OX3 7DQ, U.K. ‡ Department of Mechanical Engineering, University College London, Torrington Place, London WC1E 7JE, U.K. § Departamento de Química-Física, Universidade de Vigo, 36310 Vigo, Spain S Supporting Information *

ABSTRACT: Surfactant-coated microbubbles are utilized in a wide variety of applications, from wastewater purification to contrast agents in medical ultrasound imaging. In many of these applications, the stability of the microbubbles is crucial to their effectiveness. Controlling this, however, represents a considerable challenge. In this study, the potential for stabilizing microbubbles using solid nanoparticles adsorbed onto their surfaces was explored. A new theoretical model has been developed to describe the influence of interfacially adsorbed solid particles upon the dissolution of a gas bubble in a liquid. The aim of this work was to test experimentally the prediction of the model that the presence of the nanoparticles would inhibit gas diffusion and coalescence/disproportionation, thus increasing the life span of the bubbles. Near-monodisperse microbubbles (∼100 μm diameter) were prepared using a microfluidic device and coated with a surfactant, with and without the addition of a suspension of spherical gold nanoparticles (∼15 nm diameter). The experimental results confirmed the theoretical predictions that as the surface concentration of gold nanoparticles increased the bubbles underwent negligible changes in their size and size distribution over a period of 30 days at the ambient temperature and pressure. Under the same conditions, bubbles coated with the same surfactant but no nanoparticles survived only a matter of hours.

1. INTRODUCTION Suspensions of microbubbles stabilized by a surfactant or polymer coating are widely used in applications ranging from drag reduction in marine vehicle design to food science.1−3 In medical imaging they are used as contrast agents in ultrasound scanning, and their therapeutic applications are also under investigation.4−6 The presence of the coating both lowers the interfacial tension at the bubble surface and slows gas diffusion, thus preventing rapid dissolution and/or coalescence and disproportionation within the suspension.7,8 It has been shown in a previous study that the adsorption of solid nanoparticles on to the bubble surface can potentially improve the efficacy of microbubbles as ultrasound contrast agents by increasing the nonlinear character of the microbubble acoustic response at low excitation amplitudes.9 This is due to the “jamming” of particles when the bubble undergoes compression, leading to an asymmetric volume oscillation and nonlinear acoustic radiation. During this study it was also observed that the stability of the microbubbles was significantly enhanced by the presence of the nanoparticles, in a manner akin to the well-known phenomenon of Pickering stabilization in liquid−liquid emulsions10,11 and which has also been observed in foams.12 In particular, a number of recent studies have demonstrated that ultrastable foams can be generated through the use of solid particles13−16 © 2012 American Chemical Society

with applications ranging from food engineering to cosmetics and the biotech and ceramic industries. As described in the above references and the review by Horozov,17 a range of different materials including polymeric, metallic, and ceramic particles have all been shown to be highly effectiveas have different shapes, from long rod shaped particles to spheres. The sizes of particles used have ranged from a few tens of nanometers to the micrometer scale with cell diameters similarly varying from tens to hundreds of micrometers. Foams have been shown to be stable for periods of several weeks including under harsh ambient conditions. This was of considerable potential interest because the stability of microbubbles in biomedical applications, as well as in other areas, is of great importance. In the case of contrast agents, it determines the period over which diagnostic information may be acquired, and it is equally important in drug delivery to ensure accurate dosing and minimize unwanted side effects. Moreover, in addition to tailoring the acoustic response, there are other incentives for coating microbubbles with nanoparticles to achieve enhanced functionality.18,19 Received: July 3, 2012 Revised: August 24, 2012 Published: August 28, 2012 13808

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Figure 1. Bubble preparation using a T-junction: basic device schematic and image of gold-coated microbubbles emerging from the lower capillary prior to collection.

and out of the bubble as well as reducing the interfacial tension. Both of these phenomena are dependent on the concentration of the surfactant on the bubble surface. Second, the presence of the nanoparticles on the bubble surface will reduce the surface area available for diffusion. Third, as the bubble contracts, there will come a point when the nanoparticles will reach their packing density, and the bubble will be prevented from further reduction in size. This is assuming that the forces present between the particles are not large enough to overcome the attraction force between the particles and the interface and dislodge the particles from the bubble surface.22 If eq 2 is rederived to account for these effects, the following equation is obtained:

A theoretical model has therefore been derived to describe the effects of the nanoparticles on the interfacial diffusivity and surface tension.20 The aim of the present study was to compare the predictions of the model with experimental results and thereby investigate the use of nanoparticles as a complementary and/or alternative strategy for bubble stabilization.

2. THEORY Full details of the model derivation may be found in Azmin et al.20 and in the Supporting Information, but briefly: microbubbles are inherently unstable in liquids due to the effects of capillary pressure acting on the bubble surface as given by the Laplace equation: Pcap =

2σ R

(1)

where σ is the interfacial tension and R is the radius of curvature (equal to the radius of the bubble when spherical). For an uncoated bubble suspended in a liquid at a constant pressure and temperature, the rate of dissolution depends upon the magnitude of the interfacial tension, the size of the bubble, and the concentration and diffusivity of the gas in the liquid. Assuming the effect of convection to be negligible, an equation can be derived following Epstein and Plesset21 for the rate of change of bubble size under constant interfacial tension and with a constant coefficient of diffusion, D: ⎤ D(C i − Csat(R )) ⎡ 1 dR 1 ⎢ ⎥ = + 2M 2σ dt (πDt )1/2 ⎦ ρ(∞) + 3BT R ⎣ R

dR =d dt

f−1− 1+

⎡

2τ ⎢σ 3Rρ(∞) ⎣ 0

⎡ D(R ) ⎢ + ⎢⎣ R

+

τσ (R ) Rρ(∞)

K Γ0 x + 1 ⎡ R 2(x + 1) x R0 x+1 ⎢ ⎣

( )

⎛ R ⎞2 ⎤ D(R ) ⎤⎡ ⎥⎢1 − fp0 ⎜ 0 ⎟ ⎥ ⎝R⎠⎦ πt ⎥⎦⎣

⎤⎤ + 1⎥⎥ ⎦⎦

(3)

where D(R) = aD0 exp(b(1−(R0/R)2)), Γ0 is the initial surfactant concentration on the bubble surface at which σ = σ0, K, x, D0, a, and b are constants characterizing a given surfactant,23 and R0 is the initial bubble radius. f p0 is the initial fractional coverage of particles on the bubble surface. In addition, the following relations were used, again following Epstein and Plesset:21

(2)

In eq 2, Ci and Csat are the initial and saturation concentrations of the dissolved gas in the liquid, respectively, t is time, M is the molecular weight of the gas, B is the universal gas constant, T is the gas temperature, and ρ(∞) is the gas density at a zero curvature interface. The addition of a surfactant layer on the bubble surface together with the embedded nanoparticles will affect the dissolution of the bubble in several ways: First, the surfactant film creates a barrier to the mass transfer of the gas in

f=

Ci , Csat(∞)

d=

13809

τ=

2M , BT

Csat(∞) C (R ) Csat(R ) = sat = τσ(R ) ρ (∞ ) ρ (R ) ρ (∞ ) + R dx.doi.org/10.1021/la302674g | Langmuir 2012, 28, 13808−13815

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Figure 2. Comparison of the stability of surfactant coated microbubbles without (A−C) and with the addition of gold nanoparticles (D−F). Examples of bubble suspensions at (A) t = 0 min, (B) t = 60 min, (C) t = 24 h, (D) t = 0, (E) t = 24 h, and (F) t = 72 h. ratios of PEG40S and DSPC molecules to gold nanoparticles in the solution were respectively 104:1 and 102:1. Unfortunately, it was not possible to quantify the relative quantities adsorbed onto the surfaces of individual bubbles. Stable monodisperse microbubbles were produced using the previously described T-junction at a constant flow rate of 0.4 mL/ min and constant gas pressure of 42 500 Pa using the prepared suspensions with and without the gold nanoparticles. The bubbles were collected in clean glass vials and samples suspended in an aqueous solution of glycerol (80 vol %) on glass slides. 3.2. Optical Microscopy. The size and stability of the bubbles were characterized using a Nikon Eclipse ME600 optical microscope. The bubble size distribution in each case was measured as a function of time via optical microscopy based on a sample of 200 bubbles from each image set. Images were taken every 5 min for 2 h and additionally at 24, 48, and 72 h. Three images were taken at each time interval and analyzed using Image J. It is important to mention that the purpose of this experiment was to compare experimental results against the theoretical predictions. Larger bubbles than those used for ultrasound contrast enhancement were therefore studied in order to be able obtain measurements of bubble size to an acceptable precision under optical microscopy. Typical contrast agent microbubbles are of the order of 1−3 μm in diameter and will shrink to sizes below the optical resolution limit over the time scales involved in this study. In its current form, the model is equally valid for bubbles with diameters between hundreds of nanometers and hundreds of micrometers, and the latter were therefore investigated. The implicit assumptions will be discussed further below. 3.3. Electron Microscopy. Deposition of gold nanoparticles on to the bubbles’ surfaces was expected as described above, and this was confirmed using backscattered electron (BSE) imaging with a Hitachi S-3400N scanning electron microscope (SEM) and an Oxford instrument of energy dispersive spectroscopy (EDS). EDS enables the energy of X-rays emitted from a selected region of the sample being imaged to be measured. This energy depends on the atomic structure of the element(s) present and thus enables the chemical composition of the sample region to be determined. Bubble samples were dried for 24 h and then carbon coated. The dried bubble structure was then examined and analyzed using EDS.

3. MATERIALS AND METHODS 3.1. Microbubble Preparation. Microbubbles were produced using a specially designed T-junction9 as shown in Figure 1. Three polyether ether ketone (PEEK) capillaries with an internal diameter of 150 μm were embedded in a rigid acrylic poly(methyl methacrylate) (PMMA) block as shown in Figure 1. The ends of the upper and lower (parallel) capillaries were separated in the center of the device by a distance of ∼70 μm, with the middle capillary aligned centrally with this gap. The capillaries were held in place using standard highpressure liquid chromatography (HPLC) connectors and ferrules. All tubing and ferrules were purchased from Gilson Scientific Ltd., Luton, UK. The upper capillary was connected to a nitrogen cylinder, which supplied the gas at a constant pressure of 43.5 ± 2 MPa, as measured by a digital manometer (Digitron 2026P - Digitron manometer 0−10 bar). The middle capillary was connected to a digital syringe pump (Harvard Apparatus Standard Infuse/Withdraw PHD 4400 Hpsi programmable syringe pump), which allowed for constant measurable nonpulsatile fluid flow. The lower capillary was used to collect the microbubbles after formation. An aqueous gold citrate suspension (0.51 mM) was used as the base for the microbubble coating. Small gold colloids with an average diameter of ∼15 nm ± 10% were synthesized according to the sodium citrate reduction method14,24 by boiling 95 mL of 5 × 10−4 M tetrachloroauric acid (HAuCl4·4H2O), adding 5 mL of a warm sodium citrate solution (1 wt %), and boiling for 15 min. This yielded a solution containing ∼1015 nanoparticles/mL. Suspensions containing different nanoparticle concentrations were prepared by diluting this suspension with filtered deionized water by 75%, 50%, and 25%. To each gold suspension, 0.5 vol % (5 mg/mL) poly(ethylene glycol) 40 stearate (PEG40S) and 0.01 vol % (0.67 mg/mL) 1,2-distearoyl-snglycero-3-phosphocholine (DSPC), purchased from Sigma-Aldrich Co., were added. All chemicals were purchased from Sigma-Aldrich Ltd. (Poole, Dorset, UK), and nitrogen was used as the gas in all cases. It is hypothesized that the deposition of gold nanoparticles on to the surface of the microbubble was effected first by adsorption of DSPC molecules onto the negatively charged surface of the nanoparticles. The resulting hydrophobic nanoparticles were then subsequently adsorbed at the gas/liquid interface and stabilized by further adsorption of surfactant from the surrounding liquid. The approximate 13810

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Figure 3. Time study of bubbles: (A) t = 0 min, (B) t = 60 min, (C) t = 24 h, (D) t = 48 h, (E) t = 72 h, and (F) t = 30 days. (A), (C), and (E) are reproduced from Figure 2 for comparison.

Figure 4. (a) SEM of dried bubble residue and (b) EDS analysis confirming the presence of gold at the bubble surface. m−2. This reflects the initial concentration required in nongold coated microbubbles, which would result in zero surface tension when stabilized after some shrinkage.

3.4. Theoretical Simulations and Data Analysis. Equation 3 was recast in terms of the new variables x′2 = 2dD0t/R02 and ε = R/R0 and solved numerically using a fourth-order Runge−Kutta method. This was achieved via the function ODE45 in MATLAB (R2010b, MathWorks Inc.). A line of best fit was initially obtained for the goldcoated microbubbles by determining the combination of values for a, b, f, and f p0 which resulted in the lowest value of the sum of the squared errors (i.e., the difference between the experimental and theoretical values at each measurement point). Subsequently, in order to simulate the dissolution of the microbubbles coated with surfactant only, f p0 was set to 0.00, and the values of a, b, and f were not changed (parameter set (a)). It became clear, however, that using the same set of variables between the two experiments would result in a poor fit, the reasons for which will be discussed below. A second curve was therefore plotted which represented the best fit to the data for the bubbles coated with surfactant only (parameter set (b)). The constant d = 0.02 was assumed, following Epstein and Plesset.21 Assuming that the particles are monodisperse rigid spheres, the maximum packing density25 on the surface is ∼0.84. However values for maximum close packing density and various approaches to its determination have been challenged.26 Thus, the value of f pmax should be lower than or equal to 0.84. The surfactant specific nondimensional constant K was set at 1.5 × 10−18 following Stride.27 Assuming that the surfactant adsorption dynamics for bubbles with and without nanoparticles present would be the same, the initial surfactant concentration Γ0 was set at 4.0 × 1015

4. RESULTS AND DISCUSSION Five different bubble suspensions were tested, containing different concentrations of gold nanoparticles. Each of the bubble populations, with and without nanoparticles, was nearmonodisperse immediately upon emerging from the Tjunction. In the time taken to transfer the bubbles from the device to the adjacent microscope, however, the bubbles coated with surfactant only had undergone a considerable reduction in mean size, unlike the bubbles prepared from the gold suspension (Figure 2A,D). Over the course of an hour at the ambient temperature and pressure (22.5 °C and normal atmospheric pressure), significant Ostwald ripening was observed in the surfactant-only bubble suspension, causing the size distribution of the bubbles to broaden greatly (Figure 2B). By the end of 24 h, both the concentration and size of the bubbles were considerably reduced, and after 72 h only a negligible quantity was still detectable under the microscope. The bubbles prepared from the gold suspension, however, remained near-monodisperse and underwent a much smaller 13811

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Figure 5. (a) Change in mean bubble size over time for bubbles prepared from suspensions containing different concentrations of gold nanoparticles (n = 200 bubbles). (b) Ratio of the standard deviation to mean diameter as a function of time for different nanoparticle concentrations. 100% refers to the 0.51 mM gold suspension used for the earlier experiments (Figures 3 and 4) and 25%, 50%, and 75% to volume dilutions of this suspension.

concentration gradient with respect to the surroundings. After a certain period of time (∼60 min) the rate at which the bubble size was changing became negligibly small. It is hypothesized that this corresponds to the point at which the gold particles reach their packing density on the bubble surface, thereby both minimizing the surface area available for further gas diffusion and significantly reducing the effective interfacial tension due to a decrease in the surface to volume ratio.20 There was no marked effect of the suspension nanoparticle concentration upon the rate of change of mean bubble size observed. The main effect of varying the gold nanoparticle concentration in the suspension was seen in the standard deviation in the mean bubble size, which increased with decreasing concentration (Figure 5b). This may have been due to the fact that particle adsorption on to the bubbles’ surfaces took more time in the more dilute suspensions so that there was less inhibition of diffusion initially. In all cases the change in standard deviation reduced significantly over time, and this further supports the above hypothesis. Having confirmed the improved stability of the microbubbles prepared from the gold nanoparticle suspension and that the gold had been deposited at the bubble surface, comparison was

reduction in mean size over 72 h (Figure 2E,F). Over a much longer measurement period of 30 days, a reduction in bubble concentration and increase in polydispersity were observed in the suspension (Figure 3). The changes were still much less significant than those seen in the surfactant-only bubble suspension in just 24 h, however. To examine the composition of the bubble surface, a dried sample was viewed and analyzed using SEM and EDS (Figure 4). EDS was run several times at random intervals on the sample in order to conclusively determine the presence of gold at the surface of the bubbles. The EDS spectrogram clearly shows a gold peak, which was verified through several scans at randomly selected points at the bubble interface. Once the initial stability studies were completed, additional experiments were conducted to determine if the concentration of gold nanoparticles in the original suspension would affect the bubble stability. Figure 5 shows how the mean diameter and standard deviation varied with time for samples of 200 bubbles prepared using gold nanoparticle suspensions with varying concentrations. As may be seen, in all cases the bubbles undergo a reduction in radius under the influence of interfacial tension and the gas 13812

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Figure 6. Experimental results fitted by the diffusion eq 3 for microbubbles coated with surfactant and gold nanoparticles (fitting parameters were as follows: f p0 = 0.52, a = 0.8, b = 9, f = 0.85). Average error for experimental measurements ±1.34%. Radii are nondimensionalised with respect to the mean initial value.

Figure 7. Experimental results fitted by the diffusion eq 3 for microbubbles coated only with surfactant. Parameter set (a) refers to Figure 6; parameter set (b) refers to the least mean squares best fit to the experimental data ( f p0 = 0.0, a = 0.8, b = 4, f = 0.5). Average error for experimental measurements (parameter set (b)): ±33.7%. Radii are nondimensionalised with respect to the mean initial value.

made between the experimental data and the predictions from the theoretical model. Figures 6 and 7 show how the mean size and standard deviation varied with time for samples of 200 bubbles prepared from the surfactant with gold nanoparticle and surfactant only suspensions, respectively. The rate of shrinkage of the surfactant-only bubbles also reduced considerably after ∼20 min (Figure 7), and this is thought to be due to an analogous increase in the concentration

of surfactant molecules on the bubble surface and hence reduction in diffusivity and interfacial tension. The change is less significant than that effected by solid nanoparticles, however, and the bubbles continued to shrink with time until complete dissolution occurs. As mentioned above, the parameters characterizing the bubble (a, b, f p0, and f) that provided the best fit to the experimental data were found to be different for the bubbles 13813

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either: the curvature of the bubble surface between the particles becomes convex and the Laplace pressure becomes sufficiently negative that the direction of gas diffusion is reversed or the close-packed particle structure buckles. Other studies, albeit with much larger particle to bubble diameter ratios, have also shown that nonspherical bubble shapes can be stably maintained31 over long periods of time. As mentioned above, it should be noted that while the bubbles observed in this study had an initial diameter of ∼150 μm, smaller bubbles can be produced using this type of preparation method.32 This will be important for further testing of the response of gold-coated microbubbles to ultrasound excitation, since in terms of both patient safety and eliciting a resonant response from the microbubbles it is necessary that the bubble diameter is in the range ∼1−8 μm.4

with and without the gold nanoparticles (see captions to Figures 6 and 7). This is despite the fact that the bubble preparation and measurement protocols were identical in both cases. It is perhaps less surprising however when the ideal system considered in deriving eq 3 is compared with the actual experimental conditions. Equation 3 describes a spherical single bubble stationary in an infinite volume of liquid. The experimental measurements, however, were conducted on multiple bubbles closely packed and near the surface of a finite volume of liquid. Although the microbubbles remained largely spherical, their close proximity inevitably will result in inconsistencies with the model. For example there will be a constant flux of gas from the bubbles to the liquid and from the liquid to the surrounding air. Therefore, f will be time dependent. Controlling or even measuring f in such small volumes was not found to be practical. Therefore, it is plausible for the effective value of f to have differed between the experiments. In addition, the diffusion coefficient will be strongly influenced by the amount of solution around the microbubble as well as the extent to which it has mixed with the glycerol during collection. This would be manifest in some variation in parameter b. In fitting the data for the different particle concentrations it was found that the least mean-square error was obtained by keeping b constant and varying f p0 from 0.48 for 25% to 0.59 for 50% and 75%. The corresponding values for f were 0.8, 0.91, and 0.91 for 25%, 50%, and 75%, respectively. The increase in initial particle surface concentration is commensurate with the higher concentration in the surrounding liquid if it is assumed that some time is required to achieve a close-packed structure (as the diffusion curves indicate). A further significant factor is the absence of any bubble interactions in the theoretical model. In the surfactant-only bubble suspensions, however, the increase in the standard deviation in bubble size can be attributed at least in part to size disproportionation or Ostwald ripening. This phenomenon occurs as a result of suspended droplets, in this case bubbles, minimizing their surface to volume ratio and hence their surface free energy through transport of material from small to larger droplets, with an accompanying increase in the mean droplet size with time.8,28 While the mean size of the droplets increases, the population density will decrease as will the interfacial energy, which decreases with increasing droplet size. In order to slow the ripening process, a small amount of a second component with a very low solubility can be incorporated into the system, which results in a difference in composition between large and small droplets. This difference may counterbalance the driving force for Ostwald ripening and eventually result in a termination.29,30 As seen in Figure 3, after 30 days the bubbles are no longer spherical, with some bubbles adopting highly irregular shapes even before this (cf. panel E). At this time the rates of both gas diffusion and Ostwald ripening have become negligible, and the gold particles are assumed to have reached their packing density and thus significantly inhibit further mass transfer.9,23 The loss of sphericity also indicates a significant reduction in interfacial tension which may be due to the reduction in curvature of the gas/water interface between the close packed particles. As discussed in Azmin et al.,15 it is implicitly assumed in the theoretical treatment that the bubble remains spherical and that the nanoparticles are rigid. Thus, the predicted radius of the bubble cannot undergo further change. The amount of gas inside the bubble could, however, continue to decrease until

5. CONCLUSIONS Microbubbles prepared from a liquid suspension of gold nanoparticles were found to have significantly enhanced stability compared with bubbles coated only with surfactant. The former remained monodisperse for upward of 72 h and stable for 30 days compared with 24 h for the latter. These findings are in agreement with the predictions from a new theoretical model derived to describe the influence of interfacially adsorbed solid particles upon the dissolution of a gas bubble in a liquid. The mechanisms of stabilization are thought to relate to the inhibition of gas diffusion due to the physical presence of the solid nanoparticles and the reduction in interfacial tension, which also limits Ostwald ripening.

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ASSOCIATED CONTENT

* Supporting Information S

Derivation of eq 3. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel +44(0)1865 617 747; Fax +44(0)1865 617 728; e-mail [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Engineering and Physical Sciences Research Council for supporting this work through grant EP/ G031754/1, Joshua Owen at the Oxford Institute of Biomedical Engineering for helpful discussions, and Kevin Reeves and the UCL Institute of Archaeology for assistance with the electron microscopy.

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REFERENCES

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