Adaptive chemistry of bifunctional gold nanoparticles at the air/water interface. A synchrotron X-ray study of giant amphiphiles

A series of ligand stabilized gold nanoparticles with diameters close to 3 nm were studied as Langmuir monolayers at the air/water interface by synchrotron X-ray diffraction and reflectivity.

Alkylthiols with different length and/or terminal functional group (hydrophilic or hydrophobic) were introduced into the ligand shell by ligand place exchange reactions.

Synchrotron grazing incidence X-ray diffraction (GIXD) and specular X-ray reflectivity reveal the well known hexagonally packed monolayers.

In addition the mixed hydrophilic/hydrophobic ligand shell nanoparticles show a high degree of environmental responsiveness, as they adapt to an amphiphilic distribution of ligands around the gold core when spread at the water surface.

Likewise nanoparticles of mixed long and short alkyl chains respond to lateral pressure by adopting a structure where the short alkyl chains determine the nearest neighbor distance while the long alkyl chains determine the film thickness.

Based on X-ray reflectivity measurements, which quantitatively account for the electron density in the monolayers, combined with GIXD we calculate the average size and number of atoms of the individual gold particle cores, and estimate the number of passivating ligand on the particle surface.

Typical values are dAu-core = 16 Å, dnanoparticle = 26–37 Å depending on the ligands, MW = 30–40 000 g mol–1, number of ligands = 40–60.

The thickness of the monolayers was determined by AFM after transfer of the monolayers to a solid support using the Langmuir–Schaefer technique.

The combination of the different techniques produce a very consistent picture of the structure and adaptive chemical nature of the nanoparticles studied, and reveal a surprisingly monodisperse particle distribution centered around 140 atoms in the gold core.


Monolayer protected clusters (MPCs) are promising nanometer sized objects for potential applications in such diverse areas as catalysis, transducers for chemical and biological sensing and as building blocks for nanoscale optical and electronic devices.1

In particular thiol-derivatized gold clusters have received a lot of attention due to their remarkable stability and ease of preparation and characterization.2

Such MPCs can be regarded as individual large molecules, the properties of which are governed by their size and the chemical nature of the ligand shell.

The protecting ligands can undergo place exchange reactions with other thiols in solution,3,4 and one may thereby introduce different functionalities into the ligand shell.

The ligand exchange is assumed to occur through an associative reaction pathway with a 1 : 1 stoichiometric relationship between the incoming and leaving thiol.5

Furthermore the ligands are able to move on the particle surface through diffusion,6 presenting the possibility of constructing environmentally responsive systems, as elegantly exemplified by Boal and Rotello.7

The collective properties of self-organized Au nanoparticle systems and the formation of nanoparticle superlattices have also been studied extensively.2

In particular, the construction of organized two-dimensional nanoparticle assemblies at interfaces has attracted a lot of attention.8,9

When a solution of relatively monodisperse Au-nanoparticles is allowed to evaporate on a surface, the particles typically form hexagonal monolayer patterns.10,11

The interparticle spacing of such assemblies is, to some extent, adjustable by the length of the ligands.

The air/water interface is often used in studies of two-dimensional self-assembly processes.12

The water surface functions as an ideally flat and well-defined substrate.

The development of synchrotron sources, providing very intense X-ray beams, has allowed diffraction experiments to be performed directly on monolayer films at the air/water interface, and permits the in-plane structure of such films to be elucidated.13

Here we present the structural characterization of a number of ligand stabilized gold nanoparticles at the water air interface.

In particular, it is demonstrated that clusters containing both hydrophobic and hydrophilic moieties in the ligand shell can adapt to form a giant amphiphilic structure as a response to the asymmetric environment of the air/water interface by reorganizing the ligand shell so that the hydrophilic groups are in contact with the water.

This leads to asymmetric and highly oriented nanoparticles, which could be of great value as building blocks in nanostructure self-assembly.


The preparation of the nanoparticles 1, 3 and 6 (Table 1),containing only one type of ligand, followed a slightly modified literature14 procedure, i.e. a two-phase (toluenewater) reduction of HAuCl4 in the presence of the stabilizing ligand.

In a typical experiment 500 mg of HAuCl4 was dissolved in 50 mL water, and transferred to 200 mL toluene by adding 2.5 g tetraoctylammonium bromide while stirring.

A volume of alkanethiol (C5H11SH, C6H13SH or C12H25SH) corresponding to a molar ratio AuCl4/RSH ≈ 3.5 was added.

This ratio is expected to yield relatively small nanoparticles15 (around 1.5 nm core size).

An amount of 1 g of NaBH4 dissolved in a small amount of water was added and the mixture was stirred overnight.

Excess NaBH4 was removed by washing with 2 M H2SO4, and the water phase was separated from the organic phase in a separation funnel.

The organic phase was washed a few times with water, and finally vacuum evaporated to near dryness.

The nanoparticles were precipitated by adding a large amount of ethanol, collected and washed three times with ethanol, once with 2-propanol and finally dried.

Chloroform or toluene solutions of the final products were dark brown, characteristic of particles with a core diameter below 3 nm.

The nanoparticles were used as-prepared in all the subsequent experiments, and no further attempts were made to narrow their size distribution.

The functionalized nanoparticles 2a–c and 4 (Table 1) were prepared from 1 and 3 respectively, using a ligand place exchange procedure.3–5

Approximately 20 mg of Au nanoparticles (1 or 3) was added to chloroform solutions of 11-mercapto-undecanol (HS(CH2)11OH).

The molar ratio between particle bound thiolate and thiol in solution ranged between 0.05 and 5 in the various experiments.

The solutions were stirred at room temperature for 24 h, after which the chloroform was removed by vacuum evaporation.

The product was washed several times with ethanol and finally with 2-propanol in order to remove unreacted thiols.

Subsequently the product was redissolved in CDCl3 for spectroscopic analysis.

The ratio of exchanged C11H22OH to non-exchanged C5H11SH or C12H25SH ligands was estimated using 1H-NMR by comparing the methyl resonance of the alkylthiol ligands at 0.9 ppm to the broad resonance around 3.5–3.6 ppm originating from the methylene group carrying the –OH functionality.

The NMR spectra were recorded on either a 250 MHz Bruker AM250 or a 400 MHz Varian Unity 400 spectrometer.

The –OH functionalized nanoparticles were stable in toluene or chloroform solution for extended periods of time.

When dried, however, they had a tendency to aggregate and become insoluble after a period of 24 h or less.

This may be due to a reorganization of the ligand shells leading to segregation of hydrophobic and hydrophilic ligands (see below).

Nanoparticles 5 were prepared by a similar approach as described above.

A 6 mg amount of dodecylthiol was added to a 20 mg chloroform solution of 1 (C5–Au) and stirred for 24 h.

The solvent was removed with vacuum, and the product was washed several times with ethanol, dried and re-dissolved in chloroform.

For the Langmuir and Langmuir–Schaefer experiments, chloroform solutions of the nanoparticles were spread onto the purified water surface (MILLI-Q 18.2 MΩ cm) of a Langmuir trough and slowly compressed at constant rate (5 mm min–1) while monitoring the surface pressure.

The LB troughs (KSV 5000 or KSV minitrough) were equipped with symmetrical double barriers and a platinum Wilhelmy plate.

The resulting monolayers of some of the nanoparticles were transferred horizontally onto freshly cleaved mica by the Langmuir–Schaefer technique, and subsequently investigated by AFM, using a Digital Instruments Nanoscope IIIa microscope operating in tapping mode.

The X-ray experiments involved two complementary techniques: Grazing incidence X-ray diffraction (GIXD), probing the in-plane structure, and specular reflectivity, probing the out-of-plane electron density profile.

The measurements were performed on the liquid surface diffractometer at the undulator beamline BW1 at the synchrotron facility DESY in Hamburg, Germany.

The setup consisted of a sealed, thermostated box that was purged of air by flowing Helium, containing a Langmuir trough which was equipped with a single movable barrier and a Wilhelmy plate for monitoring the surface pressure.

The setup is more thoroughly described elsewhere.13,16,17

In all the experiments the wavelength of the X-ray radiation was 1.3038 Å.

In the GIXD measurements the angle of incidence was held slightly below the critical angle for total external reflection (αi = 0.85αc).

By virtue of the total reflection that occurs, no transmitted wave travels downwards into the subphase.

Instead an evanescent wave travels parallel to the interface, penetrating less than ca. 100 Å into the subphase.

Thus surface sensitivity is greatly enhanced.

The horizontal scattering angle (2θxy) was resolved with a Soller collimator.

GIDX data were typically recorded at several points along the Langmuir isotherm, ranging from expanded to fully compressed films.

The reflectivity measurements were typically performed on rather compressed films, with a surface pressure not far from the collapse point.

Results and discussion

The gold nanoparticles studied are shown in Table 1.

Langmuir film crystallographic data are summarized in Table 2

Langmuir films

Toluene or chloroform solutions of nanoparticles 1–6 were spread onto the surface of purified water in a Langmuir trough.

The recorded pressure-area isotherms of nanoparticles 1, 3 and 4 are shown as examples in Fig. 1.

Nanoparticles 1 and 3 are covered solely with hydrophobic C5H11SH or C12H25SH ligands, while 4 contains a mixture of hydrophobic C12H25SH and hydroxyl-terminated HSC11H22OH ligands.

The compression isotherms show that Langmuir films even of the purely hydrophobic nanoparticles act as well-behaved monolayers, and are capable of sustaining a reasonable surface pressure.

The compression proceeds without any notable increase in surface pressure at large mean molecular areas, suggesting the absence of any liquid phase in the monolayers, as is otherwise observed for many organic surfactants, such as e.g. phospholipids.18

Instead the nanoparticles are believed to form small compact rafts, which are being compressed without any occurrence of a phase transition from a liquid to a solid phase.

This notion is confirmed by GIXD, as discussed later.

Using molecular weights calculated based on GIXD and X-ray reflectivity data (cf. below and Table 2) we observe that the surface pressure increases rather steeply around 1100 Å2 particle–1 for C12H25–SAu and 600 Å2 particle–1 for C5H11–SAu.

Further compression results in the collapse of the monolayers, followed by the formation of multilayer structures in some parts of the films, which can sometimes be observed directly by the naked eye.

C12H25Au monolayers collapse at a surface pressure of 15 mN m–1, whereas monolayers of the mixed C12H25SH/C11H22OH particles collapse at a much higher surface pressure, around 30 mN m–1, demonstrating that the introduction of ≈20% hydroxyl groups into the ligand shell strongly alters the chemical properties of the individual nanoparticles.

The –OH clusters also wet the surface better when spread on the water, as can be observed by the more gradual increase in surface pressure as compared tomonolayers of 3, and thus exhibit a closer resemblance to surfactant systems.

We attribute this effect to an increase in the hydrophilicity of each nanoparticle, caused by the end group –OH functionality.

We have previously described this difference in hydrophilicity for systems consisting of nanoparticles embedded in phospholipid monolayers.19

The effect of the ligand chain length on the properties of nanoparticle Langmuir films is reflected in the higher collapse pressure of the C5H11SH particles (at 30 mN m–1) as compared to the C12H25SH particles, indicating that shorter ligands increase the monolayer stability on the water surface, although the size dispersity of the different nanoparticle preparations is also believed to contribute significantly to this effect.

The smaller particle diameter of the short-chain particles 1 is evidenced by the observation that the onset of the increase in surface pressure occurs at a lower mean molecular area (≈600 Å2 particle–1) than for the longer-chain particles e.g.3 (1100 Å2 particle–1).

Grazing incidence X-ray diffraction (GIXD)

The in-plane structure of Langmuir monolayers has been determined for nanoparticles 1–6 by synchrotron X-ray diffraction.

Fig. 2 shows the Bragg reflections observed from a monolayer of 1 on water as a function of the in-plane component Qxy of the scattering vector.

The data are recorded at a surface pressure of Π = 5 mN m–1, and a temperature of 20 °C.

Two peaks are present in the diffractogram (i) an intense narrow small-angle peak centered at Qxy = 0.274 Å–1, which corresponds to a lattice spacing of d = 2π/Qxy = 22.9 Å and (ii) a broad wide angle peak centered around Qxy = 2.67 Å–1, corresponding to a lattice spacing of d = 2.35 Å.

The small-angle peak originates from the monolayer superstructure, being the (10) reflection from the hexagonal packing arrangement of the Au-nanoparticles in the Langmuir film.

The lateral distance between the centers of neighboring nanoparticles including their ligand shells is thus a = d/cos(30°) = 22.9 Å/cos(30°) = 26.4 Å.

The broader wide-angle peak arises from the X-ray scattering from the presumed fcc (111) lattice planes of the gold atoms that constitute the cores of each of the individual nanoparticles.

Use of the Scherrer formula (L = 2π × 0.9/FWHM(Qxy)) allows for the coherence lengths of each of the scattering domains to be estimated.

The small-angle peak in Fig. 2 has a coherence length of L ≈ 250 Å, corresponding to an average of about 10 particles in both directions.

The coherence length of the wide-angle peak is much shorter, L ≈ 13 Å.

This reflects the small domains of crystalline order inside the individual nanoparticle Au-cores.

We note that the position of this peak is close to the (111) reflection of bulk fcc gold.

The (200) appears to be absent in our data, indicating a strong deviation from simple powder diffraction from Au fcc powder.

Based on the peak width the diameter of the Au-core is estimated as Dcore ≈ 13 Å, but could be slightly larger since the outer shell atoms of the gold core might not be packed completely commensurate with the bulk due to surface reconstruction, and would therefore not contribute to the scattering intensity of the fcc lattice.

As described later, this does indeed seem to be the case.

Nanoparticles 2, 3, 4 and 6 follow the same general pattern as 1 in the diffraction experiments, as they all give rise to a single narrow small-angle peak and a broad wide-angle peak.

The width and position of the small-angle superstructure peak varies to some extent between the different nanoparticles, depending on the nature of the ligand shell.

Shorter ligands result in a more compact hexagonal lattice, and as a consequence the superstructure peak shifts towards larger angles (larger Qxy) in the diffraction patterns as compared to nanoparticles decorated with longer ligands.

The coherence lengths are observed to increase in the series 1 < 2a < 2b < 2c, indicating that the packing efficiency of the Langmuir films is progressively improved as the ligand shell becomes increasingly hydrophilic.

The position of the wide-angle peak of the different nanoparticles is unaffected by the nature of the stabilizing ligand, since it reflects only the packing of the Au-atoms in the particle cores.

The width of this peak however is not necessarily the same in all experiments, because the gold cores from the different nanoparticle preparations could differ slightly in size.

Common to all the Langmuir films of 1–4 and 6 is that the position of the diffraction peaks in the GIXD experiments is almost entirely unaffected by the pressure applied to the monolayers by the LB-trough barrier.

This again is a clear indication that the nanoparticles spontaneously aggregate into small, densely packed rafts when spread at the water surface, and these rafts are being pushed together as the area decreases without any significant change in their packing motifs.

Table 2 summarizes the lattice spacings and the corresponding coherence lengths of the two peaks observed for the nanoparticles 1–4 and 6.

The area per nanoparticle in the hexagonally ordered monolayer is given by: A = a2cos(30°).

For the C5H22SHAu this value is 604 Å2.

We have previously shown that compressed Langmuir films of the nanoparticles contain small holes in the monolayers, probably due to packing defects.20

These holes typically cover less than 5% of the total area.

Based on the small variations in values of LAu (Table 2) it is reasonable to assume that the gold core size is roughly the same for all three nanoparticle preparations 1, 3 and 6.

The difference in unit cell length is hence governed only by the length of the stabilizing ligands.

The unit cell parameter a as a function of the length of the extended thiol ligands (as determined by the Chem3D program) is shown in Fig. 3.

A linear fit of |a| = Dcore + s × ligand-length yields a core diameter Dcore = 16.1 Å, which is only slightly larger than the values listed in Table 2 for the coherent scattering lengths of the Au atoms in the nanoparticle core.

If the ligands were fully extended and non-interdigitated the slope of the fit would be s = 2 because two ligands separate the gold cores.

The observed value s = 1.14 is smaller, implying that the nanoparticle ligands are either tilted with respect to the particle surface or interdigitated with the ligands from neighbouring nanoparticles or a combination of both.

If this reduced effective ligand shell thickness is caused by ligand tilting, they would be inclined at an angle of about α = cos–1(1.14/2) = 55° with respect to the surface normal.

A very notable observation that can be made from the GIXD measurements is that the in-plane distance between neighboring nanoparticles in monolayers of 2a–2c is practically unaffected by the amount of long HSC11H22OH ligands present in their ligand shell.

Even at a ligand exchange mol percent of up to 25% (a = 26.3 Å–26.6 Å), the lattice spacing is still very close to the value found for the purely C5H11SH covered particles (i.e.1, a = 26.4 Å).

This strongly indicates that the relatively long HSC11H22OH ligands do not take up lateral space between the gold cores, and suggests that the gold particles adapt to the asymmetric environment by the formation of amphiphilic clusters with a reorganised ligand shell.

This is illustrated schematically in Fig. 4, and further corroborated by AFM measurements (see Fig. 8 and discussion below).

The nanoparticles with mixed C5H11SH/C12H25SH ligands differ markedly from the rest of the particles in this study since the GIXD experiments give rise to two small-angle superstructure peaks, instead of one.

Fig. 5 shows a small-angle scan of 5 measured at a surface pressure Π = 0.9 mN m–1, and 20 °C.

The intensity profile (solid line) has been decomposed into two Gaussian peaks (dashed line), centered at Qxy(i) = 0.22 and Qxy(ii) = 0.27, which corresponds to lattice spacings of d(i) = 28.6 Å and d(ii) = 23.3 Å.

These lattice spacings translate into hexagonal in-plane distances of a(i) = 33.0 Å and a(ii) = 26.9 Å respectively.

Contrary to the other nanoparticles studied here, the GIXD measurements on nanoparticle 5 Langmuir films show changes in the intensity profile as a function of the surface pressure.

We believe this behaviour is due to the coexistence of two crystalline phases in the monolayer films. i) a random phase with a hexagonal lattice spacing of a(i) = 33 Å in which the long and short ligands are randomly distributed across the entire surface of the gold core, and ii) a ligand segregated phase with a hexagonal lattice spacing of a(ii) = 26.9 Å in which only the short ligands are present in the monolayer plane, while the long ligands are present only on the top and the bottom of the particles.

When the pressure exerted on the monolayer is increased the ligand segregated phase becomes more pronounced.

This is shown in the Fig. 5 inset, which plots the ratio between the integrated areas of the two superstructure peaks as a function of the lateral surface pressure of the monolayer.

The area of the peak corresponding to the ligand segregated phase is thus increasing compared to the random phase peak as the monolayer is compressed.

This indicates that the nanoparticles respond to the changes in surface pressure by expelling the long-chain ligands from the plane of the monolayer.

Specular X-ray reflectivity

Specular X-ray reflectivity measurements have been performed on Langmuir films of some of the nanoparticles from Table 1 in order to determine the electron density profile perpendicular to the water surface.

As an example Fig. 6 shows the measured reflectivity, R, normalized to RF the Fresnel reflectivity of bare water (R/RF squares) of 1 recorded at the surface pressure Π = 6 mN m–1.

The data were inverted to yield ρ(z) (Fig. 6B) by expressing ρ(z) in terms of cubic splines and fitting the corresponding R/RF (full line in Fig. 6A) to the data with a smoothness constraint on the ρ(z)-curve.21

The electron density profile of the monolayer is completely dominated by the very dense Au particle core.

The profile indicates that the monolayer thickness is slightly less than 30 Å, and it reaches a maximum relative electron density value of ρparticle/ρwater = 5.

By combining the information obtained from the in-plane GIXD experiments with the out-of-plane data provided by the reflectivity measurements, it is possible to construct a three-dimensional representation of the Au nanoparticle monolayers on the water surface.

The X-ray reflectivity data allows deduction of the total number of electrons in each nanoparticle, since it yields, on an absolute scale, the entire electron density normal to the surface, without the requirement of crystalline order in the measured sample.

It is therefore possible to estimate the number of gold atoms in each nanoparticle if the unit cell and electron density profile is known (c.f. below).

Table 2 lists the resulting values for the number of gold atoms in nanoparticles 1, 2c and 3.

As described in the GIXD section each unit cell contains one Au-nanoparticle in a hexagonal packing arrangement.

Each unit cell can be considered as consisting of a volume corresponding to the nanoparticle gold core, and a volume corresponding to the organic ligand shell.

This can be written as: Vtot = VAu + Vlig

The total volume of a unit cell can also be estimated by assuming that the monolayer height, normal to the water surface, is equal to the in-plane unit cell length (a), i.e.: Vtot = a3cos(30 °)

The total number of electrons in each nanoparticle is found by multiplying the unit cell area with the integrated electron density profile (Fig. 6) normal to the water surface: #electot = a2cos(30°) ∫ρ(z)dz

The symbol #tot denotes the total number of electrons in one unit cell (nanoparticle) and ρ(z) is the electron density profile of the nanoparticle monolayer.

The total number of electrons can be divided into contributions from the gold core and the organic ligand shell, as described by: #electot = VAuρAu + Vligρlig

ρAu and ρlig is the electron density of gold and the organic molecules respectively, defined as ρAu = #Au-electrons/VAu; ρlig = #lig-electrons/Vlig.

Combination of eqns. (1)–(4) results in an expression for the volume of gold in one nanoparticle:

The total number of electrons in one nanoparticle is established from the electron density profile of the monolayer by use of eqn. (3).

The unit cell volume is determined from the diffraction data by eqn. (2).

If we assume that the electron density of the nanoparticle gold core and ligand shell is identical to the values for bulk gold and alkylthiol respectively, then it becomes possible to determine the volume of the gold core by means of eqn. (5).

The number of gold atoms is found by multiplying the Au core volume with the mass density of gold followed by division with the molar mass of gold and multiplication with Avogadro’s number.

The surface area of the gold core available for the ligand shell can now be estimated by assuming that it forms either a spherical shape, which probably underestimates the surface area, or a truncated octahedral type shape, which probably overestimates the accessible area for the ligands.

The projected area of an alkylthiol is 21.4 Å2,22 and this value is used to determine the maximum number of ligands on each nanoparticle, assuming that the gold core is completely covered with the thiols.

The number of Au atoms and ligands calculated by the above method is given in Table 2 for the nanoparticles 1, 2c and 3.

Based on these values a molecular mass of the particles has been calculated, and the mass ratio between gold and organic ligand shell molecules is also tabulated.

Particles 1 and 2c are both from the same original preparation and the calculations correspondingly yield the same number of Au atoms (i.e. 140) for them both.

Nanoparticles 3 however are from a different preparation but the calculation still (within the experimental error) results in the same number of gold core atoms.

If the particle gold core is assumed to be perfectly spherical, the number of possible ligands is 41, or 59 if the gold core is believed to have a truncated octahedral shape.15

The above treatment of the nanoparticles yields an average number of gold atoms in the particle core #Au = 140 ± 5.

We note that this is close to a “magic number” for truncated octahedra according to Murray.15

It also shows that different nanoparticle preparations can yield very similar products if identical reaction conditions are employed.

Langmuir–Schaefer films

Transfer of nanoparticle monolayers from the water surface to a solid support is possible using a horizontal lifting method (Langmuir–Schaefer).

The support (mica) is submerged into the water subphase prior to spreading the monolayer.

After compression of the Langmuir film to the desired surface pressure, the support is slowly (0.3 mm/min) lifted through the water surface leaving a nanoparticle monolayer film on top of it.

Fig. 7 shows an AFM image of a monolayer LB-film of 2c on mica transferred at 5 mN m–1.

We have previously reported that monolayer LB-films of gold nanoparticles on mica contain a number of holes scattered randomly across the surface20 presumably due to packing defects originating from the imperfect packing of the spontaneously formed small rafts.

Depending on the lateral size of the holes, their depths can be used as a measure of the monolayer thickness.

If the holes are too small, i.e. corresponding to the absence of only a few nanoparticles in the monolayer, the size of the AFM-tip prevents it from reaching the bottom of the hole, whereas larger defects in the monolayer do not cause this problem.

The measured thicknesses of monolayers of 1, 2c and 3 on mica are shown in Table 2.

The C5H11SH and C12H25SH monolayer thicknesses are both fairly close to the in-plane unit cell length determined in the GIXD experiments.

The mixed C5H11SH/HSC11H22OH particles, however, have a notably larger monolayer thickness than in-plane particle spacing.

These findings are illustrated in Fig. 8, which compares the in-plane unit cell parameter of the nanoparticles 1, 2a–c and 3 obtained from the GIXD experiments with their out-of-plane heights measured with tapping mode AFM.

This result agrees very well with the apparent amphiphilic distribution of ligands around the gold core, as illustrated in Fig. 4, since the reorganization of the ligand shell induced by the water surface makes it highly anisotropic, with the molecular long-axis along oriented perpendicular to the air/water interface.


We have shown how the introduction of alkylthiols with different length and/or terminal functionality into the ligand shell of nanosized MPCs induces a large extent of anisotropic molecular reorganization of ligands around a nanoparticle core when spread as monolayers at the air/water interface.

The mixed shell hydrophobic/hydrophilic nanoparticles respond to the water surface by adapting to an amphiphilic distribution of ligands around the gold core.

Similarly nanoparticles with mixed long and short alkylthiol ligands respond to lateral pressure in the monolayer by expelling the longer chains from the in-plane structure.

The amphiphilic nature of the mixed shell hydrophilic/hydrophobic nanoparticles makes them an interesting candidate for self-assembly on nanoscale patterned hydrophilic/hydrophobic substrates23 for e.g. fabrication of integrated circuit structures.

We have also demonstrated that the combination of synchrotron diffraction and reflectivity techniques permits detailed information on the nanoparticle size and molecular composition to be obtained.