# An Alkanethiol-Surfactant Bilayer To Prevent Shape Conversion of Anisotropic Silver Nanoparticles and Probe Their Formation Mechanisms

Nouha El Amri, Kevin Roger

► To cite this version:

Nouha El Amri, Kevin Roger. An Alkanethiol-Surfactant Bilayer To Prevent Shape Conversion of Anisotropic Silver Nanoparticles and Probe Their Formation Mechanisms. Langmuir, 2023, 39 (25), pp.8623-8628. 10.1021/acs.langmuir.3c00398. hal-04276708

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# An Alkanethiol-Surfactant Bilayer To Prevent Shape Conversion of Anisotropic Silver Nanoparticles and Probe Their Formation Mechanisms

Nouha El Amri and Kevin Roger*

**ABSTRACT:** Anisotropic nanoparticles can be synthesized under kinetic control but may undergo subsequent shape changes due to atomic reorganization. Furthermore, their synthesis involves rapid steps, which are challenging to monitor in situ. Here, we show how a nanoemulsion of alkanethiols with an ethoxylated surfactant, easily prepared and metastable for months, can simultaneously prevent shape reorganization and arrest reaction kinetics. We illustrate this generic method on the silver nanoplates synthesized in concentrated acetic acid aqueous solutions, in which rapid shape reorganization occurs. We show that there exists an optimum thiol concentration corresponding to full coverage of all silver surface atoms, which can be simply calculated from particle dimensions. Furthermore, we demonstrate that arresting nanoparticle formation can be achieved within milliseconds using a tandem rapid mixers scheme in a continuous flow setup, allowing ex situ monitoring of the reaction.

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## INTRODUCTION

Synthesizing nanoparticles in aqueous solutions is a relatively straightforward process; nonetheless, probing their formation mechanism and maintaining their structural integrity are much more challenging. Indeed, nanoparticles are nonequilibrium objects obtained along nonequilibrium pathways, which simultaneously involve a usually fast nucleation step and a tendency to structurally evolve in order to reach thermodynamic equilibrium. Reaching a metastable state is thus crucial to both obtain well-controlled nanoparticles and arrest their formation in order to probe their underlying mechanism. In practice, this requires metastability barriers at both colloidal and molecular scales. Indeed, interparticle repulsions are necessary to hinder aggregation,¹⁻³ but it is also necessary to restrain molecular reorganization that may arise from nonspherical structures. Shape anisotropy is typically achievable with many metal and oxide nanoparticles but causes a strong deviation from thermodynamic equilibrium, which promotes such molecular reorganization and is a well-documented issue in their long-term storage.⁴⁻⁹

Interesting strategies were devised that use different amphiphilic molecules, usually together with thiols. For instance, Jiang et al. added alkanethiols to shape-stabilize silver nanoplates, together with the AOT surfactant.⁷ Miesen et al. rather used a combination of citrate, phospholipids and alkanethiols.¹⁰ Such strategies were shown to efficiently preserve anisotropic shapes and prevent oxidation.¹¹,¹² However, the use of ethanol is required to solubilize the alkanethiols, while the amphiphiles are not well-adapted to

large ionic strengths in the case of AOT or require rather complex preparation protocols in the case of phospholipids, which are also water-insoluble.

This work presents a simple stabilization method that provides colloidal metastability and prevents any molecular reorganization of anisotropic silver nanoparticles. This method rests on the formation of an interfacial bilayer made of alkanethiols and surfactants, which are brought as a nanoemulsion into the aqueous reaction medium. The stabilizing nanoemulsion is prepared via spontaneous self-assembly in water¹³,¹⁴ and imparts any responsivity of the surfactant to the silver nanoparticles. We provide a quantitative rationale strategy to adjust its optimum amount based on particle dimensions. Furthermore, we show that adding this nanoemulsion effectively arrests the nanoprecipitation reaction at its current stage, trapping nanostructures in a metastable state, which allows the ex-situ monitoring of the reaction mechanism. Rapid reaction steps can thus be effectively characterized ex-situ when injecting this stabilizing nanoemulsion within milliseconds, using a continuous-flow scheme.
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Shape reorganization of silver nanoplates over time, without adding thiols. (A) Picture of the continuous flow setup used for the synthesis of silver nanoplates (adapted from ref 15 with permission, copyright 2022 Elsevier). We used tangential vortex mixers as depicted in the inset, which operate at high Reynolds numbers. (B) Scheme of the reactive sequence used to produce silver nanoplates. It is possible to add alkanethiols nanoemulsified with a surfactant after synthesis as shown in Figure 2. (C) The formed silver nanoplates do not aggregate or undergo significant size changes over time, but they experience shape reorganization, as reflected in the progressive merging of the three plasmonic bands into a single one, corresponding to isotropic particles.</figcaption></figure>

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## RESULTS AND DISCUSSION

**Shape-Reorganization of Silver Nanoplates without Molecular Stabilization.** We synthesized silver nanoplates through the reduction of silver ions by both sodium borohydride $NaBH_4$ and ascorbic acid. Purified¹⁶ polyvinylpyrrolidone (PVP) was added to ensure colloidal metastability. The synthesis was performed in the presence of high concentrations of acetic acid to obtain silver nanoplates.¹⁵ Figure 1 C shows that this synthesis yielded a red-purple liquid that displays three plasmonic bands characteristic of silver nanoplates,¹⁷ as confirmed by TEM imaging. Over time, color changes were observed until a yellow tint was obtained corresponding to a single absorption peak, which is characteristic of isotropic particles. Since we did not observe either macroscopic aggregation or significant changes in light scattering, these changes must originate from molecular reorganization and shape changes. Such a synthesis is thus a representative instance in which one might want to prevent shape reorganization to retain anisotropy.

**Forming an Alkanethiol/Nonionic Surfactant Bilayer To Prevent Shape Changes.** We prepared a stabilizing aqueous solution by dissolving dodecanethiol in swollen $C_{16}EO_8$ micelles.¹³,¹⁴ This solution was then added to a freshly prepared silver nanoplates dispersion. The idea was thus to coat nanoparticles with a hydrophobic alkane layer attached through S-Ag bonds, on which nonionic surfactant adsorbed, thus providing colloidal metastability.

To assess the efficiency of this stabilization strategy and provide a rational tool to choose the adequate concentration, we systematically varied the stabilizer concentration and monitored the resulting plasmonic changes over time, as displayed in Figure 2 A. These data are extracted at a single

wavelength to evidence changes over time at various concentrations in Figure 2 B. Overall, adding this stabilization solution had no significant impact on the plasmonic spectrum of silver nanoplates, which means that their nanostructure remains preserved at all concentrations following the immediate addition of the stabilization solution. Still, a minor wavelength shift of the main plasmonic band is observed and displayed in Figure 2 C (green diamonds), a general feature of thiol adsorption on metallic nanoparticles.¹⁸ Strikingly, the wavelength first increases (red-shift) and then decreases (blue-shift) upon increasing the thiol concentration. Interestingly, this nonmonotonic behavior is also observed when plotting the long-term change of transmittance as also shown in Figure 2 C (blue dots). At intermediate concentrations, both the visual aspect and plasmonic spectra remain practically unchanged over time even after 100 days, which shows that our stabilizing method is indeed efficient to preserve anisotropic shapes. However, decreasing concentration leads to a progressive collapse of molecular metastability with significant reorganization taking place. Yet, the same applies to a lesser extent when increasing too much concentration. Crucially, for each of these two different phenomena, relating for the first to the interfacial thiol coverage and for the second to the long-term structural evolution, the optimal thiol concentration is the same.

To understand what sets this optimum concentration, we proceeded with a simple calculation of the number of thiols needed to cover all silver surfaces so that each surface silver atom bonds with a thiol molecule. The surface area per silver atom, $s_{Ag}$, can be estimated from its atomic radius of 144 pm as 0.065 nm². Nanoplates' dimensions measured by TEM (mean diameter 16 nm and thickness 8 nm) yield both their mean
<figure><img src="image_5.png" /><figcaption>**Figure 2.** Assessing the optimum thiol concentration required to protect silver nanoplates from shape reorganization. (A) UV-visible monitoring of silver nanoplate dispersions stabilized with increasing concentrations of a dodedanethiol/surfactant mixture, injected 30 s after the start of the reaction. Kinetics of shape changes first decrease with increasing thiol concentration and then increase again. (B) Transmission at 522 nm over time for increasing concentrations of a dodedanethiol/surfactant mixture. (C) (blue dots) Transmission at 522 nm, 100 days after preparation, as a function of the dodedanethiol/surfactant mixture concentration; (green diamonds) wavelength at the minimal transmittance immediately after thiol addition. A bell shape is observed for these two different quantities, and its maximum corresponds to the calculated concentration required to cover all silver surface atoms with dodedanethiol.</figcaption></figure>

surface, $S_{nano}$, and volume, $V_{nano}$. The thiol concentration that corresponds to the bonding of all surface silver atoms can thus be expressed as

$$ [\text{thiol}]_{\text{monolayer}} = \frac{[\text{Ag}]M_{\text{Ag}}S_{\text{nano}}}{N_{\text{A}}\rho_{\text{Ag}}S_{\text{Ag}}V_{\text{nano}}} \quad (1) $$

with [Ag] the silver concentration, $M_{Ag}$ the silver molar mass, $\rho_{Ag}$ the silver density, and $N_{A}$ the Avogadro's constant.

For the experiment displayed in Figure 2 C, we calculate $[\text{thiol}]_{\text{monolayer}} = 0.057$ mM, which is displayed as the orange area. This calculated concentration neatly matches the optimum stabilization concentration. Such an agreement highlights that our method indeed relies on the complete surface coverage of surface silver atoms by thiols to protect them from reorganization. It is also consistent with the easy removal of PVP by thiols as documented in the literature.¹⁹ Furthermore, this formula is also interesting in practice, as it yields a simple evaluation of the optimum thiol concentration once particle dimensions are known.

Below this optimum concentration, some silver surface atoms are unprotected and reorganization may thus occur.

Around this optimum concentration, all silver surface atoms are bonded with thiols and thus protected. Above this concentration, there is an excess of thiols, which is somewhat detrimental to long-term stability. We interpret this as the formation of silver sulfide that extends throughout the particle and thus modifies its plasmonic signature.²⁰⁻²² Using the right amount of thiol, not too little and not too much, is thus crucial to protect the initial nanostructure. At the optimal concentration, it is possible to achieve outstanding colloidal and molecular metastability for anisotropic silver particles, even at high ionic strengths and low pH.

An important point at this stage is to notice that the area per silver atom is 1 order of magnitude smaller than the typical area occupied by an adsorbed surfactant molecule, typically 0.5 ± 0.1 nm². Indeed, surfactants at an interface involve molecular rather than atomic dimensions. This is why the simpler solution of adding amphiphilic thiols, such as PEGylated ones, was observed to be unsuccessful at preventing reorganization. Most silver atoms indeed remained unbonded to thiols in this case. Note that our calculation also shows that much less surfactant than the 1:1 stoichiometry with alkanethiol can be used. In the present experiment, a 0.0074 mM surfactant
concentration would be sufficient due to the much larger area per surfactant. However, we chose the 1:1 stoichiometry to ease alkanethiol dissolution in swollen micelles and did not observe any detrimental effects from using a surfactant excess. The exact quantity can be further optimized by conducting a solubilization study.

Finally, a large array of surfactants can be chosen depending on the desired application. Here, we would like to stress that the use of ethoxylated surfactants, with a poly(ethylene oxide) headgroup (PEO, PEG), imparts temperature responsiveness to coating. Upon heating, hydrophilic heads dehydrate, which allows the destabilization of nanoparticles. We verified that heating above 80 °C for this surfactant leads to macroscopic precipitation of all silver nanoparticles, which is a convenient way to recycle and handle silver nanoparticle dispersions.

**Arresting Nanosynthesis during Nanoparticle Formation.** This stabilization solution can be used as a tool to arrest the synthesis during the intermediate stages of the reaction, which allowed us to illustrate that this strategy can produce a powerful ex situ probe into nanoparticle formation mechanisms, especially when coupled with a tandem mixing setup. Figure 3 A displays the continuous flow schematic in

<figure><img src="image_6.png" /><figcaption>**Figure 3.** Arresting the formation of silver nanoplates with the rapid addition of the alkanethiol/surfactant solution. (A) Scheme of the tandem mixer setup used to arrest reactions at the desired time. (B) Ex situ UV-visible monitoring at increasing arrest times.</figcaption></figure>

which the reaction is initiated by mixing reagents at the first mixer, while the stabilizing solution is injected at a second mixer. By changing the tubing length between mixers 1 and 2, it is thus possible to tune the time at which the stabilizing solution is injected. Indeed, this solution immediately arrests the reaction. We also prepared a reference solution by mixing silver ions with the stabilizing mixture and did not observe any significant nanoobject formation, using small-angle X-ray scattering, or any plasmonic signature, using UV-visible spectroscopy. Typical results are displayed in Figure 3 B with ex situ UV monitoring. We see that the plasmonic signature progressively develops over time. We first observe a peak around 350 nm, which corresponds to the out-of-plane quadrupole resonance, developing simultaneously with a peak around 430 nm, which corresponds to the out-of-plane dipole

resonance and in-plane quadrupole resonance. A third band emerges around 570 nm and corresponds to the in-plane dipole resonance. Anisotropy thus develops progressively and is discernible even at 160 ms after reaction initiation.

This experiment paves the way to a powerful ex situ probing of nanoparticle formation mechanisms, with the advantage of being able to perform many characterization techniques while still probing kinetics as fast as the millisecond. This technique rests on arresting the reaction, which requires both bonding all surface silver atoms and complexing all the remaining silver ions to avoid a secondary reduction and precipitation. In the early stages, the thiol concentration should thus be chosen equal to the initial silver ion concentration, while at the latter stages, a lower thiol concentration can be used closer to the final optimal concentration calculated in this article. In practice, for a first experiment, the best choice is thus to use the largest thiol concentration, equal to the silver ion initial concentration. Still, this may entail the formation of a thicker silver sulfide layer, which can slightly impact the outcome of some characterization techniques.

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## CONCLUSION

In this work, we presented a method that provides both molecular and colloidal metastability to anisotropic silver nanoparticles. This method consists of coating silver nano-particles surfaces with a bilayer, with the inner layer composed of dodecanethiol covalently bonded to silver atoms on which is adsorbed a surfactant monolayer. This can be viewed as coating silver particles with an oil layer that is chemically bonded to the silver surface, on which surfactant adsorbs as it would for oil droplets. As when formulating an emulsion, this allows the use of a broad range of surfactants and thus provides a powerful array of surface functionalizations. The stabilization method notably transfers any surfactant responsivity to silver nanoparticles themselves, which can be used to control the dispersion's metastability. In practice, both the alkanethiol and the surfactant are delivered without the use of any additional solvent or species but rather as a nanoemulsion obtained from superswollen surfactant micelles with alkanethiols. We demonstrated the existence of an optimum alkanethiol concentration, which corresponds to the concentration of surface silver atoms and can be easily calculated. Nanoparticle surfaces are then fully covered at the atomic level by a monolayer of alkanethiol. Moving away from this optimum was shown to be detrimental, as too little thiol does not prevent reorganization, while too much thiol leads to the formation of a surface layer of silver sulfide. Apart from stabilizing efficiently anisotropic nanoparticles, we also demonstrated that this method presents a powerful means to probe formation mechanisms ex situ when coupled with a rapid tandem mixing scheme. This continuous flow scheme combines two mixers in a series arrangement, with the reaction triggered at the first mixer by mixing reagents and arrested at the second mixer by the addition of the alkanethiol/surfactant nanoemulsion. This generic and simple approach allows monitoring nanoparticle formation within milliseconds, without requiring in situ characterization tools but rather common ex situ ones. Thus, such a generic and simple approach should be useful when investigating the rapid formation and structural details of nanoparticles, particularly those exhibiting a complex shape.
## EXPERIMENTAL SECTION

**Materials.** Silver nitrate (AgNO₃, 99.9999%), polyvinylpyrrolidone PVP (MW<sub>supplier</sub> = 55 kg mol⁻¹), ascorbic acid (99%), acetic acid, and 1-dodecanethiol (99%) were purchased from Sigma-Aldrich. Octaethylenehexadecyl ether (C₁₆EO₈) was purchased from Nikko Chemicals. Sodium hydroxide solution (NaOH, extra pure, 50% w/v solution) was purchased from Acros Organics. Dialysis membranes (Spectra/por, 100–500 g·mol⁻¹ cutoff mass) were purchased from Spectrum Chemical. All aqueous solutions were prepared using milli-Q water (obtained from a purelab option Q device) in new 16 mL glass vials with butyl/PTFE caps. All the solutions were prepared at room temperature.

**Transmission Electron Microscopy.** On a carbon-coated copper grid, 10 µL of each solution was placed and allowed to dry under ambient conditions. TEM imaging was performed using a JEOL JEM 1400 operated at 120 kV. Approximately 50 particles were measured using ImageJ software to determine the average dimensions.

**UV-Visible Spectroscopy.** UV-visible spectra were recorded using a Cary 60 UV-vis spectrophotometer. The aqueous solution of silver nanoparticles was placed into 2 mm quartz cuvettes, and the baseline was set using milli-Q water.

**Preparation of the Stabilization Solution.** Eight milligrams of 1-dodecanethiol and 23.78 mg of C₁₆EO₈ were added to 1 mL of Milli-Q water under stirring. The resulting mixture was turbid, indicating that the thiol was not fully dissolved. It was thus heated up to 80 °C, leading to a decrease of the C₁₆EO₈ spontaneous curvature. As a result, micelles swelled with alkanethiol and were trapped in a metastable state by fast cooling to ambient temperature, which corresponds to a sub-PIT cycle as described by Roger et al.,¹³,¹⁴ with PIT being the phase inversion temperature of the surfactant/water/alkanethiol mixture. This stock solution was then diluted to reach the different 1-dodecanethiol concentrations used in this work.

**Silver Nanoplate Synthesis.** Nanoparticles were synthesized using a continuous flow setup that associates three rapid mixing units.¹⁵ Each unit consists of four inlets meeting tangentially in a mixing chamber and exiting orthogonally, as first described by Hartridge and Roughton.²³ In the first mixer, the first inlet was used for the injection of a 2.2 mM silver nitrate aqueous solution, the second inlet was used for the injection of a 1.57 M aqueous solution of acetic acid, and the other two inlets were connected with a T-junction in which a 2.3 mM purified PVP aqueous solution is added. The output of the first mixer was connected to two inputs of the second mixer via a T-junction. The two other inlets were connected through a T-junction in which a 0.045 mM NaBH₄ solution, prepared in pH 12 NaOH aqueous solution, was added. The output of the second mixer was connected to two inlets of a third mixer using a T-junction. The other two inlets were connected through a T-junction in which a 2.2 mM ascorbic acid aqueous solution was added. All the solutions were freshly prepared and used within 5 min. They were injected at the same flow rate of 300 µL/s. The final flow rate at the third mixer exit was thus equal to 1500 µL/s. The yellow-tinted solution existing from the last mixer evolved further until reaching a red-purple color after a few seconds.

**Arresting the Synthesis at Different Stages.** We also prepared silver nanoplates by mixing reagents simultaneously in a single mixer. The first inlet was used for the injection of a 2.2 mM aqueous solution of silver nitrate. The second inlet was used to inject 2.2 mM ascorbic acid solution prepared in a 1.57 M aqueous acetic acid solution. The third inlet was used for the injection of a 0.045 mM NaBH₄ solution prepared in a pH 12 NaOH aqueous solution. The fourth inlet was used for the injection of a 2.3 mM purified PVP aqueous solution. To arrest the synthesis, a second mixer was installed at the exit to rapidly introduce the surfactant/alkanethiol solution to the reacting mixture. In practice, the exit of the first mixer was connected to two inlets of the second mixer through a T-junction. The other two inlets were used for the addition of a 3.9 mM aqueous solution of 1-dodecanethiol in the presence of octaethylenehexadecyl ether.

## AUTHOR INFORMATION

### Corresponding Author

Kevin Roger – Laboratoire de Génie Chimique, Université de Toulouse, CNRS, Institut National Polytechnique de Toulouse, Université Paul Sabatier, Toulouse 31432, France;
*orcid.org/0000-0001-7914-0951; Email: kevin.roger@cnrs.fr

### Author

Nouha El Amri – Laboratoire de Génie Chimique, Université de Toulouse, CNRS, Institut National Polytechnique de Toulouse, Université Paul Sabatier, Toulouse 31432, France

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.langmuir.3c00398

### Notes

The authors declare no competing financial interest.

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