A general strategy to incorporate a wide range of metallic salts into ring-like organized nanostructures via polymer self-assembly

Abstract

A new approach for surface nanostructuring was innovated by means of metallic nanoparticles organized in a thin homopolymer film (PMMA) on a silicon substrate. The originality of this simple self-assembly is that a film of polymer drilled by metallic nanoparticle (MNP) rings was fabricated in one step, in addition no reducing agent was used nor was preliminary functionalization of the surface carried out. Moreover, the deep understanding of the self-assembly mechanism and the influence of various physicochemical parameters involved tend to introduce into the fabrication process a wide range of metallic salts “Mⁿ⁺(NO₃⁻)_n” where Mⁿ⁺ is Ag⁺, Au³⁺, Mn²⁺, Mg²⁺, Cr³⁺, Cu²⁺, Ni²⁺, Eu³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Tm³⁺, Ca²⁺, Fe³⁺, Co²⁺, Zn²⁺, Al³⁺. Finally, to illustrate the potential of our nanofabrication approach, an example of the SERS application of Silver NPs is given, where the detection limit of crystal violet was shown to reach 10⁻¹⁵ M.

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1. Introduction

Ring-like nanostructures (RLN) have been widely studied in recent years as a result of their potential applications as optical,^1–3 magnetic and electronic resonators,^4–7 and sensors.⁸ The size-dependent properties of RLN by simply varying their diameter and wall thickness have made them the subject of many theoretical and experimental investigations.^9–11 For example, (i) plasmonic nanorings have been investigated for ultrasensitive bio- and chemical sensing due to the facility to tune their plasmonic resonance band,^8,12,13 (ii) nanoscale ring resonators made from semiconductors can be used to fabricate nanolasers with fine tenability of the emission wavelength,^14,15 (iii) magnetic nanorings have also specific response making them important candidates for applications in high-density information storage.^16–18 Several ways have been recently explored to fabricate RLN including nano-sphere and e-beam lithography,^19,20 template-based techniques,^21–24 molecular-beam epitaxy,²⁵ thermal evaporation process,²⁶ etc. Nanoring formation, however, is presently limited by a lack of convenient, versatile, inexpensive, rapid and material-independent fabrication methods.²⁷ Among the above mentioned methods, the template-based approach remains the most frequently since it is high-throughput, simple, low-cost. More precisely, the self-assembly approach using amphiphilic diblock copolymers as templates, have been introduced in the synthesis of RLN.²⁸ On this basis, block copolymer vesicles are formed due to the incompatibility between the polymer diblocks, which results into a micro-phase separation and creates self-organized nanoscale polymer particles.²⁹ Loading these vesicles with metallic precursor and then spin coating them on a stiff substrate followed by a chemical reduction step allows the formation of MNPs in the porous polymer film.^30,31 However, this process includes several steps to obtain pure MNPs firmly attached to the substrate surface including the selective functionalization of the polymer, the reduction of the metallic salt and the etching of the copolymer via oxygen plasma.³² Moreover, extending the self-assembly method to a large variety of MNPs is still a big challenge.

The complexity and limitations to specific metals of copolymer templated techniques motivated us to develop a general strategy to incorporate a wide range of metallic salts into ring-like organized nanostructures via polymer self-assembly (Scheme 1). In this new nanofabrication way, MNPs are formed in a “one step” upon spin coating, on conductor or n-doped semi-conductor substrate, of a dispersion of homopolymer (PMMA) and metallic salt. The potential and novelty in this synthesis is that no reducing agent or copolymer was used nor preliminary functionalization of the surface was done. In addition, the MNPs are fabricated directly on the substrate's surface; consequently there is no need for polymer film etching.

Scheme 1 Illustration of our approach to incorporate a wide variety of metallic salts into nanorings dispersed in a PMMA matrix. (A) Mixture of PMMA dissolved in toluene and Mⁿ⁺(NO₃⁻)_n dissolved in isopropanol (non-solvent of PMMA) showing phase separation. (B) The mixture becomes transparent after adding acetone (co-solvent). (C) Spin coating of the mixture on silicon substrate. (D) During evaporation of solvents, vesicles are formed at the substrate surface and (E) get exploded after complete solvents evaporation giving rise to ring-like nanostructures containing metallic salts.

Furthermore, some metal salts such as (silver, gold, aluminum, chromium, copper) could be converted into metal nanoparticles which are well known as plasmonic materials. So far, coupling plasmonic nanoparticles to each other's in order to increase the local enhancement of electromagnetic field and tuning the plasmon resonance band over the visible and near infrared range, is highly desirable for a broad range of applications. In this context, we show the possibility to fabricate silver nanorings with tunable size and which are highly sensitive as SERS substrates for the detection of femtomolar concentrations of organic molecules as shown in the last part of this paper.

On the other hand, other metal salts like Zn²⁺ have been introduced into nanoring-like structures and could be converted into ZnO. This makes our method interesting for the fabrication of nanoring resonators. Additionally, the same method allows us to introduce a wide number of rare earth elements including photoluminescent ions such as Eu³⁺ and Er³⁺. Magnetic nano-rings could be also fabricated by the same method using Ni²⁺ or Co²⁺ ions as precursor.

More interestingly, by mixing 2 or 3 metallic salts in the same solution, our versatile method could be used to fabricate multifunctional hybrid materials (polymer, plasmonic, luminescent, magnetic, etc.).

In this paper, we show the possibility to incorporate more than 20 nitrate metallic salts into the ring-like organized nanostructures. To make it possible, the influence of several physicochemical parameters (solvent of metallic salt or non-solvent of PMMA, co-solvent, solvent of PMMA, evaporation dynamics of the solvents, and the surface–mixture interaction) on the dimensions of metallic nanorings and a proposal mechanism is demonstrated below.

2. Experimental

The fabrication procedure is illustrated in Scheme 1. A reference mixture of PMMA/Mⁿ⁺(NO₃⁻)_n salt was prepared by mixing a solution of Mⁿ⁺(NO₃⁻)_n/isopropanol and a solution of PMMA/toluene. After the mixing, a phase separation takes place. Consequently, to eliminate this separation and to obtain a stable mixture, a co-solvent (acetone) was added which is a good solvent for PMMA and miscible with the both solvents. The composition of the so called reference mixture is described in the ESI (Table S1†). The resulting mixture was then deposited by spin coating on n-doped silicon substrates that allows the reduction of Mn⁺ into M⁰. Unless otherwise stated, all the samples were prepared with the following spin coating parameters: (time = 30 s, speed = 3000 rpm and acceleration = 3000 rpm s⁻¹). The composition of all the mixtures used in this study is reported in the ESI (Tables S1–S5†).

2.1. Surface treatment

The substrates used in the Section 3.2 were treated either by chemical functionalization or by physical coating of the surface.

Chemical functionalization³³. The glass substrates were cleaned by immersion into Piranha solution [H₂SO₄, 98%/H₂O₂, 30% 2 : 1 (v/v)] for one hour, then rinsed with deionized water and dried in an oven. To obtain alkyl-terminated surface, the glass substrate was immersed in a 1% solution of undecyltrichlorosilane for 24 h in anhydrous toluene under argon atmosphere. For the amine-terminated silane, the substrate was immersed in a 1% solution of 3-aminopropyldimethylethoxysilane for 24 h in anhydrous toluene under argon atmosphere.

Physical coating. 5 nm of silver, gold, chromium and SiO₂ films were deposited over silicon substrates by physical vapor deposition using a PLASSYS 400 MEB evaporator.

2.2. SERS experiments

A Dilor Jobin-Yvon Spex instrument from Horiba equipped with a 632.8 nm laser, a CCD camera and an X10 (NA: 0.3) objective on a downright configuration was used. All shown SERS spectra were acquired independently, at full laser power (2 mW), and employing 10 s of acquisition time. For the SERS measurements, a 15 mL drop Crystal Violet (CV) was deposited on the substrate and the spectra acquisitions were rapidly conducted before the drop was dried.

3. Results and discussion

Both AFM and SEM images in Fig. 1 show the formation of Ag nano-rings in a porous film of PMMA. The MNPs are located on the outline of the pores (Fig. 1B). The optical images in reflected dark field illumination (Fig. 1D–G) showing the scattering of light from the nano-rings confirm the presence of metallic NPs. Thus, the reduction of the metallic precursor into NPs and its dispersion took place spontaneously during the deposition of the mixture on the surface.

Fig. 1 AFM (A) and SEM (B) images of the film obtained from the reference mixture (Table S1†). The inset in the SEM image is a zoom on one metallic ring (C) profile of the AFM image. (D) Optical image in reflected dark field illumination showing the diffusion of light from silver nanoparticles. (E, F and G) Zoom on one ring from (D) showing respectively orange, green and red light color.

The same experiment was done this time without metallic salt and the result obtained shows an absence of any structuring of PMMA. This indicates that the metallic salt is a key factor in the polymer self-assembly mechanism and thus in the MNPs features.

3.1. Influence of the interactions within the mixture

We studied the role of various physicochemical parameters which influence the size and the distribution of nano-rings as well as the structure and the organization of the MNPs during the deposition of the mixture on the surface.

Influence of the interactions between solvents. During the addition of PMMA/toluene dispersion into the Ag⁺/isopropanol solution, the mixture becomes turbid in relation with phase separation. Thus, PMMA aggregates are formed due to repulsive interactions between the PMMA on one side and the couple Ag⁺/non-solvent on the other side ending in the increase of the attractive interactions between the PMMA chains.

To better understand the origin of this phase separation mechanism and to show the influence of the intermolecular interactions on the structuring of the PMMA, four solutions were prepared in which the quantity of isopropanol/Ag⁺ increases from 1 to 4 and the ratio of PMMA/toluene is kept constant. Simultaneously, the quantity of acetone increases in order to reach a certain threshold that allows the elimination of phase separation inside the mixture. The four solutions were deposited over silicon substrates and then the polymer surface was analysed by AFM, which shows the progressive evolution of polymer nanostructuring as a function of isopropanol proportion in the whole mixture (Fig. 2). As the proportion of isopropanol increases, the nano-rings get closer from each other (Fig. 2, image 2), which is attributed to increased repulsive interaction between isopropanol and PMMA. Higher amount of isopropanol results in stronger interactions. Consequently aggregations were obtained on the film instead of nano-rings (Fig. 2, images 3 and 4).

Fig. 2 Evolution of the quantity of acetone added to the solution in order to obtain stable mixture of PMMA/toluene/Ag⁺/isopropanol, according to the quantity of isopropanol. The ratios PMMA/toluene and AgNO₃/isopropanol were kept constant. The composition of each mixture is shown in Table S2.† The inset is the AFM images for the corresponding solutions after spin coating on silicon.

Indeed, surface structuring in the form of rings requires a relatively weak interaction between the PMMA and the couple isopropanol/salt. The increase of the quantity of the last couple beyond a certain limit will cause a loss in the polymer nanostructuring.

Influence of the co-solvent. As assessed above, acetone plays an important role in stabilizing the mixture. To evidence its influence on the nanostructuring, 3 mixtures of PMMA/toluene/AgNO₃/water were prepared with different weight percentages of acetone ((A) 67.2, (B) 80.4 and (C) 86.0). These mixtures were deposited on silicon substrate by spin coating and AFM analyzes were done (Fig. 3).

Fig. 3 AFM images showing the influence of acetone on the size distribution of metallic ring. (A, B and C) are obtained from mixtures that contain respectively 67.2, 80.4 and 86.0 weight percentage of acetone. The ratios PMMA/toluene, Ag⁺/water and water/toluene were kept constant. The complete composition of each mixture is shown in Table S3.†

The role of acetone is to adjust the repulsive interaction forces between the PMMA and the couple non-solvent/salt. Higher amounts of acetone allow a better dispersion of the polymer chains and then to better disseminate the couple non-solvent/salt. This was confirmed by a decrease in the rings size and an increase of their density as the percentage of acetone increases. Therefore co-solvent plays a major role in the control of MNPs characteristics by tuning the repulsive interactions in the mixture.

Influence of the nature of non-solvent. As done for the solvent, the interaction between the non-solvent and the polymer was studied. For this reason, four solutions containing different non-solvents (isopropanol, ethanol, methanol and water) were prepared (Table S4†).

For each solution, a certain threshold quantity of acetone was added in order to eliminate the PMMA chains aggregation.

To quantify the interaction which took place between non-solvent and PMMA, the parameter of interaction X_{PMMA/non-solvent} was calculated by applying the following equation:³⁴

Ø₃: corresponds to the volume fraction of the PMMA in the ternary system PMMA/non-solvent/acetone.

As shown in (Fig. 4), the calculated X_{PMMA/non-solvent} varies in the following order: X_PMMA/water > X_{PMMA/methanol} > X_PMMA/ethanol > X_{PMMA/isopropanol}. This indicates that the affinity between the PMMA and the non-solvent increases as follows: water < methanol < ethanol < isopropanol and it was confirmed by the behaviour of the threshold quantity of acetone added to every solution, which is appreciably very similar to that of the parameter of interaction.

Fig. 4 Evolution of the quantity of acetone (left) and the parameter of interaction X (PMMA/non-solvent) (right) with the non-solvent. In inset the corresponding AFM images after spin coating. (A) Isopropanol; (B) ethanol; (C) methanol and (D) water. The composition of each mixture is shown in Table S4.†

It seems clear from the AFM images of Fig. 4 that the average diameter of the rings increases considerably in the presence of water (Fig. 4D). Some rings with big diameters were observed in the presence of methanol (Fig. 4C) while in ethanol (Fig. 4B) there is an absence of an important effect on the nano-structuring in comparison with isopropanol (Fig. 4A).

Consequently, after the evaporation of acetone, the repulsive polymer/non-solvent interaction would lead to the formation of nano-vesicles containing the non-solvent and the metallic salt. As this interaction becomes stronger, these vesicles prefer to assemble to each other in order to minimize their interaction with PMMA; as a result nano-rings of larger size will be formed.

Influence of the nature of PMMA solvent. In order to understand the role of the PMMA solvent on the structuring mechanism, four reference mixtures of PMMA/solvent/acetone/Ag⁺/isopropanol were prepared with different solvents of PMMA: MIBK, THF, acetone, and toluene. The quantity of each component was kept constant. The AFM images of the corresponding films are shown in Fig. 5.

Fig. 5 AFM images showing the surface structuring of the mixture in the presence of various solvents of PMMA. The composition of each mixture is shown in Table S5.†

THF and acetone show a different behaviour compared with toluene and MIBK. In particular, both THF and acetone do not give rise to ring-like nanostructures and the PMMA do not form any continuous phase on the substrate. On the other hand, it is interesting to note the difference of saturated vapour pressure (in mbar at 25 °C) between these solvents: MIBK (21), THF (235), acetone (228) and toluene (29). It appears clearly that the saturated vapour pressures of THF and acetone are 10 times higher than that of the MIBK and toluene. This parameter is directly related to the volatility of the solvent, which seems to be an important key parameter in the mechanism of structuring.

3.2. Influence of the nature of the surface

So far, the structuring of the mixture showed that it is influenced by the physical interactions between the various constituents. To highlight and estimate the contribution of the substrate surface, two types of functionalization were done.

Chemical treatment of the surface. To study the interactions between the mixture and the surface, a reference solution was deposited on two different substrates, for which the surface was modified chemically by handling hydrophobic group (–CH₃) on the first substrate, and hydrophilic group (–NH₃⁺) on the second one. The AFM images of these two substrates are shown in (Fig. 6).

Fig. 6 AFM images showing the influence of the nature of the functionalized surface on the nanostructuring of the reference mixture (Table S1†). (A) Hydrophobic. (B) Hydrophilic.

In the case of hydrophobic surface (Fig. 6A), the diameters of Ag rings are larger than those obtained with a hydrophilic surface (Fig. 6B). The explanation is attributed to the assembly of nano-vesicles in the presence of hydrophobic surface. In case of –CH₃ groups, the repulsive interaction is very strong making the vesicles attracting to each other's in order to minimize their interaction with the hydrophobic surface. This promotes their assembly and results in rings of bigger size. In the contrary, the vesicles tend to spread on the hydrophilic surface containing –NH₃⁺ groups.

Physical treatment of the surface. The change of the physical properties of the surface could also contribute in the nano-structuring of the mixture, so a treatment of the surface was done in order to assure its intervening. Four materials: silver, gold, chromium, and silica were chosen to adjust the affinity between the surface and the metallic salt. A layer of 5 nm of each material was evaporated on four substrates respectively. Then, the reference mixture was deposited on each substrate by spin coating. The obtained results presented in (Fig. 7) shows that the PMMA structuring is possible in the presence of the four types of layers. Metallic nano-rings are infrequent in the presence of silica (SiO₂) this is because it has the non-conducting character which prevents the reduction of the Ag salt and their binding on the substrate. While in the remaining three layers, clear nano-rings were observed particularly in the silver layer that gives rise to a very strong density of nano-rings due to the high affinity between vesicles that contains Ag⁺ and the silver layer. The density of these rings increases in the order SiO₂ < Cr < Au < Ag following in the same order the electric conductivity (in S m⁻¹) of the materials deposited on the surface of the substrates: silica (2.52 × 10⁻⁴) < chromium (7.74 × 10⁶) < gold (45.2 × 10⁶) < silver (63 × 10⁶). Indeed after the evaporation of the solvents, the conducting surface provides electrons to the metallic precursor and then allows the fabrication of metallic nanoparticles upon a simple electroless reduction of the metallic salt. This phenomenon has been already reported by Kalkan et al.³⁵ and investigated by Bhatt et al.³⁶

Fig. 7 AFM images showing the influence of the nature of surface on the nanostructuring of the reference mixture (Table S1†).

3.3. Influence of the evaporation dynamics of the solvents

Studies carried out until now suggest us that the nanostructuring takes place during the spin-coating. Indeed, after the addition of the acetone, the solution is completely transparent and no reduction of the metallic salt is observed. During the spin-coating, the various solvents begin to evaporate more or less quickly according to their volatility and compatibility with the rest of the mixture. It was supposed that the non-solvent inside the vesicles will burst in order to evaporate which in turns leaves holes surrounded with MNPs on the PMMA film. Therefore, the influence of the kinetics of phase separation should be studied to understand more deeply the mechanism of nanostructuring. By varying the speed in which vesicles would be formed and assembled before bursting, it will be possible to study the kinetics of micro-phase separation and its influence on the dimensional characteristics of the nano films. For that purpose the reference mixture was used to prepare three samples by using three different speeds of spreading: 500, 1000, and 5000 rpm. The results of AFM images in (Fig. 8) show clearly the influence of the spin coating speed on the structuring of the PMMA film and the size of metallic rings. It is noticed clearly that the diameter of the metallic rings increases considerably when the speed of spin coating passes from 5000 to 500 rpm. Simultaneously, we observe that at 5000 rpm, nano-rings are rather monodisperse contrary to those obtained in 1000 and in 500 rpm.

Fig. 8 AFM images showing the influence of the spreading speed on the nanostructuring of the reference mixture (Table S1†).

An explanation can be given to this behaviour by using the model of vesicles proposed above. The latter are formed when the acetone begins to evaporate. Simultaneously, their repulsive interaction with the PMMA and/or the surface is a factor that promotes their assembly as it is evidenced in Fig. 9. However, faster evaporation will stop this assembly and limit their size.

Fig. 9 Optical microscope images showing the formation of micro-vesicles and their growth during the evaporation of a droplet (reference mixture) on a glass substrate. The images (A to I) were successively taken during the solvents evaporation.

3.4. The proposed mechanism for the formation of ring-like nanostructures

The mixture contains thermodynamically incompatible molecules where the repulsive interaction between them is hidden due to the presence of acetone. During the spreading of the mixture on the surface, the various solvents began to evaporate. Being the most volatile, acetone will evaporate first, so that a thermodynamic instability of the mixture will take place. This happens because metallic salts are strongly dissolved in the alcohol which is a non-solvent of the PMMA and not compatible with the polymer solution (PMMA/toluene). The repulsive interaction between the couple non-solvent/metallic salt and the PMMA chains results in a micro-phase separation. This micro-phase separation manifests itself through the appearance of vesicles containing the couple non-solvent/Ag⁺, which are then immobilized on substrate surface after the complete evaporation of the PMMA solvent and acetone (Fig. 10B). After that, explosion of the vesicles will happen (Fig. 10C) to allow the evaporation of the non-solvent, so that rings containing metallic salts will be formed. The strong concentration of the salt on the outline of the pores and the formation of nano-rings indicates that the metallic salt would be located at the PMMA/non-solvent interface of the vesicle. This later contains esters groups, and since the substrate is a conducting or n-doped semiconducting surface, some salts like silver nitrate, undergo spontaneous reduction and form metallic nanoparticles organized in rings (Fig. 10E and F).

Fig. 10 (A) Schematic representation of the self-assembled vesicles before complete solvents evaporation; the inset shows one vesicle containing inside the non-solvent and stabilized in the presence of Mⁿ⁺(NO₃⁻)_n that limit the interaction with PMMA chains. (B) Vesicles explosion after complete evaporation of acetone and toluene allowing for the non-solvent to be evaporated. (C) Ring-like nanostructures containing metallic salts or in some cases self-organized metallic nanoparticles; (D) SEM picture showing exploded vesicles; (E) SEM picture showing a ring-like silver nanoparticles. (F) SEM picture showing after removing the PMMA showing self-organized silver nanoparticles.

The study of the various parameters explained above and their influence on the nanostructuring process gives us a good understanding for the mechanism of the self-assembly. As a result we introduced, into the fabrication process, a wide range of metallic salts of nitrate counter ion Mn⁺(NO₃⁻)_n where Mn⁺ is Ag⁺, Au³⁺, Mn²⁺, Mg²⁺, Cr³⁺, Cu²⁺, Ni²⁺, Eu³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Tb³⁺, Dy³⁺, Tm³⁺, Ca²⁺, Fe³⁺, Co²⁺, Zn²⁺, Al³⁺. Some examples are given in Fig. 11.

Fig. 11 AFM and SEM images showing the nanostructuring of PMMA in the presence of 12 different metallic precursors. Whatever the metallic salt, the spin coating of the mixture on the substrate results in a film of ring-like nanostructures confirming the high versatility of our approach.

3.5. Detection of femtomolar levels of crystal violet (CV)

The reliance on the above process allowed us to fabricate highly sensitive SERS substrates and to enhance the detection limit of some organic molecules. In particular, a strong enhancement of Crystal Violet's (CV) Raman signal was observed using the substrate of Ag NPs fabricated following the above reported method. The variation of the Raman spectrum of CV deposited on the Ag NPs substrate according to its concentration is represented in the (Fig. 12) which shows a detection limit for the CV at a threshold concentration about 10⁻¹⁵ M.

Fig. 12 (A) Raman spectra of the Crystal Violet (CV) of various concentrations on a substrate of Ag NPs. (B) The evolution of the SERS intensity of the peak at 1374 cm⁻¹ according to the CV concentration.

4. Conclusions

In summary, we have reported a new versatile technique of hybrid self-assembly owed to the physicochemical interactions between polymer, metallic salts, solvents and substrate's surface. This “one step” concept has been applied on a wide range of metal salts resulting in polymer nanostructures that contain metallic precursors. In some cases, reduction of metallic precursor is spontaneous leading to the formation of ring-like self-organized metal NPs. The ring diameter is adjustable in a wide range starting from 100 nm to few microns. This innovative way is promising to fabricate multifunctional materials (plasmonic, luminescent, magnetic, etc.) by simply changing the type of precursor loaded polymer vesicles. An example of sensing applications was demonstrated, where femtomolar concentration of crystal violet was detected using silver NPs as SERS substrate.

We believe that this technique and the present study allowing for the structural dimensions to be tuned will stimulate further works given the broad spectrum of potential applications that can take advantage of its adaptability to a very large number of metal salts.

Acknowledgements

Financial support of the “Conseil régional Champagne-Ardenne” (projet «Nanomatériaux 3D»), NanoMat (www.nanomat.eu) by the “Ministère de l'enseignement supérieur et de la recherche”, and COST Action MP1302 NanoSpectroscopy is acknowledged.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra24505f

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