Spontaneous assembly and synchronous scan spectra of gold nanoparticle monolayer Janus film with thiol-terminated polystyrene

Abstract

This communication presents a facile method for preparing ordered hydrophilic metal nanoparticles into gold nanoparticle monolayer Janus film (top face solvent-phobic polystyrene and bottom face solvent-philic nanoparticles) with thiol-terminated polystyrene. It also reveals the enhanced light source spectrum properties of the gold nanoparticle monolayer Janus film.

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Assembly of nanoparticles with asymmetric, hybrid structures into hierarchical structures is of considerable interest because of their already widespread and potential applications in electronics, optical devices, and sensing devices.¹ The size, shape, and surface functionality of gold nanoparticles can be controlled using polymer ligands, which affect performance of the nanoparticles.² For example, on aggregation, the surface plasmon resonance for each GNP becomes delocalized, altering the optical properties.³ The gold nanoparticles show selective and efficient destruction of cancerous cells because of the significant thermo–plasmonic effect of their nanostructures.⁴

A novel assembly procedure to form close-packed two-dimensional nanoparticle films with high regularity by converting nanoparticle repulsive force into van der Waals interactions through in situ coating of nanoparticles on the interface with alkane–thiols has been presented.⁵ However, the alkane–thiol-terminated single nanoparticle facilely rolls over under Brownian motion because the size and quality of the alkane chain are very small. To obtain a homogeneous and bulk monolayer film of nanoparticles, it has been suggested that polymer–thiols are used to fix gold nanoparticles to form a Janus film (top face solvent-phobic polystyrene and bottom face solvent-philic nanoparticles) on the interface.⁶ As is known, Janus substances can have varying assembly behaviors and properties because of the many categories and variety of shapes.⁷ Great effort has been devoted to investigation of Janus substances in recent years, focusing both on novel preparation strategies and on theory of simulation.⁸

Also in recent years, much experimental work and theoretical calculations have reported the fluorescent properties of gold nanoparticles.⁹ However, the quantum efficiency of such AuNPs is very low, limiting the widespread use in biosensing.¹⁰ Functionalizing AuNPs is a powerful tool to drive assembly of groups with optical properties thanks to their large absorption and scattering cross-sections at visible plasmon resonance frequencies.¹¹ Synchronous scan spectra have received intense attention because of their high sensitivity and simplicity. They have been used in successful analysis of complicated multi-component samples with severely overlapping emission and/or excitation spectra, which limit the ability of single-wavelength fluorescence measurement.¹² Therefore, functionalizing AuNPs could be utilized in synchronous scan spectra to enlarge the tremendous scope for applications.

To obtain a homogeneous and bulk monolayer film of nanoparticles in this communication, polymer–thiols are suggested to form Janus film on the interface with gold nanoparticles. A facile method is presented for preparing ordered hydrophilic metal nanoparticles into gold nanoparticle monolayer Janus film with thiol-terminated polystyrene at the interface of toluene and water. Synchronous scan spectra show the enhanced light source spectrum properties of the gold nanoparticle monolayer Janus film. Gold nanoparticles, destabilized as a result of addition of alcohol, are adsorbed into an interface at which the surface of entrapped Au nanoparticles was in situ coated with thiol-terminated polystyrene present in a transition toluene layer.

A schematic representation of the AuNP nanoparticle Janus film formation at the liquid–liquid interface and the synchronous scan spectra obtained are shown in Fig. 1. Gold nanoparticle monolayer Janus film with thiol-terminated polystyrene (PS–SH) was facilely prepared. The surface enhanced spectral characteristics of the AuNP–PS asymmetric particle film are of interest in the fields of optical devices and sensing devices.

Fig. 1 Schematic representation of the AuNP nanoparticle Janus film formation at liquid–liquid interface and the synchronous scan spectra obtained.

We chose large AuNPs for this work because the larger colloids will result in stronger enhancement in the light source spectrum than smaller colloids.¹³ Fig. 2a is the TEM image of the AuNP in colloid with 3.0 mL of sodium citrate solution (0.034 mol L⁻¹). As can be seen, the gold particle dispersion and the particle shape are uniform. The average size of AuNP is about 11.0–15.0 nm (ESI, Fig. S2†). The preparation of AuNPs is presented with different amounts of reductant (ESI†).

Fig. 2 TEM image of the gold colloid (a) and IR spectra of PS, PS–SH and PS–AuNP (b).

The polystyrene mentioned above in this communication was synthesized by atom transfer radical polymerization (ATRP). Then thiol-terminated polystyrene was synthesized by reaction of thiourea with the bromide-terminated polymer (ESI†). Fig. 2b shows IR spectra of PS and PS–SH. The peak at wave number 3600 cm⁻¹ is the N–H bond absorption peak; the peak at wave number 3080–2920 cm⁻¹ is the benzene ring absorption peak; the peaks at wave numbers 1600 cm⁻¹, 1492 cm⁻¹, and 1462 cm⁻¹ are benzene ring skeleton vibration absorption peaks; the peaks at wave numbers 756 cm⁻¹ and 696 cm⁻¹ suggest that the reaction product is mono-substituted at the benzene ring. Therefore, it was confirmed that the resulting product was PS. The IR spectra of the particles and of PS–SH are similar, which indicates that thiol is also part of the composite.¹⁴

As can be seen from Fig. 2b, all the absorption peaks on the IR spectra of PS–Au and PS present peculiar peaks; it means that the polymer obtained is polystyrene. The intensity of absorption peak of the obtained product becomes lower compared with that of PS, which indicates that the polystyrene was thiolated gold nanoparticles.

The AuNPs Janus film formed by self-assembly of AuNP at liquid–liquid interface is depicted structurally in Scheme 1 (ESI†). Because of the addition of ethanol, the driving force for the entrapment of gold nanoparticles at the interface is at the hexane–water interface.¹⁵ Reincke and colleagues suggested this process on the basis of a thermodynamic evaluation of the charged nanoparticle self-assembly at the water–oil interface.¹⁶ With the addition of ethanol, the electrostatic repulsive force between nanoparticles was weak; entrapped nanoparticles would form an irreversible aggregate of nanoparticles at the interface. However, hydrophilic thiol-terminated polystyrene prone to the interface from toluene extends to the aqueous phase, resulting in reaction of gold nanoparticles with the styrene–thiol groups in the oil phase and water, which prevents the entrapped nanoparticles from aggregating. Therefore, the decrease in electrostatic repulsive force leads to formation of a close-packed nanoparticle film by counteracting the van der Waals interaction. Because of the attractive force among the trapped nanoparticles, as there is polystyrene–thiol in the toluene layer, the density of nanoparticles at the interface increases. After toluene evaporation, the repulsive force disappears and the distance between nanoparticles decreases. The monolayer gold nanoparticle film was obtained (ESI†). In this process, the interface allows half of the surface gold nanoparticles to react with the thiol-terminated polystyrene, and the other half do not participate in the reaction.

Scheme 1 AuNPs Janus film formed by self-assembly of AuNP at liquid–liquid interface depicted by chemical structural formula.

Although it is possible to assemble nanoparticles into monolayers on the interfaces, void areas appear between the nanoparticle aggregates. These are caused by the broad size distribution and competitive electrostatic repulsion against long-range van der Waals interactions.⁵

Field emission transmission electron microscopy (FETEM) and scanning electron microscopy (FESEM) images were obtained using a JEM-200CX TEM and JSM-6700F SEM, respectively (ESI†). Fig. 3 shows FETEM (A and B) and FESEM (C and D) images of the Au–PS asymmetric particle films. There is a monolayer of nanoparticles with lots of voids as thiolated polystyrene was added (Fig. 3A and C). As previously reported, in the film close-packed gold nanoparticles coexist with large void areas.⁵ However, Fig. 3B and D shows corresponding enlarged FETEM and FESEM images, presenting the monolayer nature. As shown in the TEM image, nanoparticles are in direct contact with each other. Fig. 3B and D show close and ordered nanoparticle films without taking into account the void area.

Fig. 3 TEM (A and B) and SEM (C and D) images of the Au–PS asymmetric particle films. The average diameter of the Au nanoparticles is about 13.0 nm.

The synchronous scan spectra were performed at room temperature on a RF-5301PC spectrofluorophotometer (Shimadzu Corporation, Japan) equipped with a 150 W Xenon lamp, a recorder and dual monochromators (ESI†). The sample was adjusted under the condition of 45° angle geometry in the direction of light. The enhanced light spectra of excitation were obtained at a wavelength of 636 nm and the enhanced light spectrum of emission at a wavelength of 515 nm. The enhanced light spectra of the Au–PS asymmetric particle Janus films are shown in Fig. 4a. The maximum enhanced spectra intensity of excitation and emission are at wavelengths of 598 nm and 637 nm, respectively. A similar result with AgNPs was demonstrated by Lukomska et al.¹³

Fig. 4 (a) Enhanced light spectra of excitation (dot line, λ_em = 636 nm) and emission (solid line, λ_ex = 599 nm) of the Au–PS asymmetric particle films on quartz glass; (b) synchronous scan spectra of the Au–PS asymmetric particle films on quartz glass (solid line) and quartz glass (dot line, Δλ = 57 nm).

The enhanced light spectra of excitation and emission overlap. To eliminate the interference influence of adjacent peaks and obtain a sharp, simple, and precise value of enhanced light spectra intensity, synchronous scan spectrometry is made.

The enhanced light spectra of the Au–PS asymmetric particle films on the quartz glass, and of quartz glass are shown in Fig. 4b. The surface light spectrum peaks of quartz glass are slight. However, the nanoparticle films on a quartz substrate prepared in the presence of PS–Au show a sharp enhanced light peak centered at a wavelength of 634 nm. This is caused by the large surface plasmon characteristics of these films.⁵

In conclusion, we present a facile method for preparing ordered metal nanoparticles in a gold nanoparticle monolayer Janus film. Firstly, AuNPs and PS–SH were obtained, then the gold nanoparticles were reacted with polystyrene–thiol groups at the interface between the oil phase and water because of hydrophilic polystyrene extending to the aqueous phase from toluene. The addition of alcohol caused the destabilized gold nanoparticles to be adsorbed into the interface where the surface of entrapped Au nanoparticle was in situ coated with the thiol-terminated polystyrene in a transition toluene layer. After toluene evaporation, the monolayer gold nanoparticle film was obtained. The neatly arranged gold particles on this nanoparticle Janus film were observed by TEM and SEM. As a result of the introduction of functional polystyrene–thiols and orderly nanoparticles, the synchronous scan spectra showed that the gold nanoparticle monolayer Janus film had enhanced light source spectrum properties. It is expected that preparation of this Janus film would be facile, and that there is tremendous scope for applications of this ordering and surface functionalizing, such as in electronic, optical, and sensing devices.

Acknowledgements

We gratefully acknowledge support from National Natural Science Foundation of China (Grant no. 51203088), Innovative Foundation of Shanghai University and Special Research Foundation of Selecting and Cultivating Excellent Young University Teachers in Shanghai.

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Footnote

† Electronic supplementary information (ESI) available: Detailed chemicals and materials, preparation of AuNPs, synthesis of PS and PS–SH, self-assembly of AuNP Janus film at liquid–liquid interface and characterization. See DOI: 10.1039/c4ra10811f

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