Elsevier

Applied Surface Science

Volume 292, 15 February 2014, Pages 128-136
Applied Surface Science

Controlling formation of gold nanoparticles generated in situ at a polymeric surface

https://doi.org/10.1016/j.apsusc.2013.11.099Get rights and content

Highlights

  • In situ formation of gold nanoparticles at a polymeric surface was studied.

  • Reduction of surface-bound Au ions forms nanoparticles at and below the interface.

  • The size of the nanoparticles depends upon the reducing agent used.

  • Bulky amines confine ion-binding and nanoparticle formation to the interface.

  • Surface-bound gold ions are present as both Au(III) and Au(I).

Abstract

This work shows that in situ reduction of metal ions bound at a polymer surface can form nanoparticles within the polymer matrix as well as at the interface, and the size and distribution of nanoparticles between the interface and subsurface depends upon the choice of reagents and reaction conditions. Tetrachloroaurate ions were bound to cross-linked SU-8 films that were functionalized using a variety of multi-functional amines, then reduced using one of several reagents. Reduction using sodium borohydride or sodium citrate generates bands of interspersed gold nanoparticles as much as 40 nm deep within the polymer, indicating that both the Au ions and the reducing agent can penetrate the surface enabling formation of nanoparticles within the polymer matrix. Nanoparticle formation can be confined nearer to the polymer interface by reducing with hydroquinone, or by processing the polymer film in aqueous media using high molecular-weight multifunctional amines that confine the gold ions at the interface.

Introduction

Gold nanoparticles (Au NPs) and clusters or arrays of surface-bound Au NPs have a wide variety of applications, including nanofabrication, optical devices, and catalysis [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11]. Au NPs and nanoparticle-aggregates are effective for enhancing signal via surface plasmon resonance in various spectroscopic and sensing methods [12], [13]. Metal nanoparticles (NPs) are used as nucleation sites in electroless metallization, which is a promising approach for creating metallized micro-electromechanical structures (MEMS) and optical MEMS devices [7], [14], [15], [16], [17], [18]. Such applications frequently involve Au–NP functionalization of a polymeric surface or structure. As such, NP synthesis and surface functionalization continue to be active areas of research that impact many established and emerging technologies.

The common approach for functionalizing surfaces with NPs involves first synthesizing colloidal particles in solution and subsequently binding them to the surface of interest, using the Turkevich method [7], [8], [12], [18], [19], [20], [21] or sodium borohydride (NaBH4) reduction [13], [20], [22], [23], [24], [25]. In contrast, in-situ reduction of gold ions adsorbed onto a surface is an alternative means for generating Au NP-functionalized surfaces and devices [1], [2], [26], [27], [28], [29], [30]. This approach offers some advantages, including the possibility for generating smaller particles, stabilization of the NPs through surface attachment, and decreased aggregation due to immobilization on the surface [1], [30], [31]. Additionally, surface-bound NPs can be readily isolated from the synthesis medium or further derivitized by simple physical transfer of the supporting substrate. There are several pioneering reports of in-situ synthesis of Au NPs at polymeric surfaces [1], [2], [3], [28], [30], [31], [32], [33]. Yet in comparison to the conventional approach of separately synthesizing NPs then binding them to a surface, in-situ synthesis of Au NPs at polymeric surface remains far less explored.

In this work we show how the choice of gold-ion surface-linker and reducing agent affects in-situ formation of Au NPs at the surface of a polymer known as “SU-8”. SU-8 is a cross-linkable epoxide which is increasingly employed for patterning micro- and nano-scale surfaces and creating functional devices [25], [34], [35], [36]. The key findings of this work are (1) NPs formed by in-situ reduction do not reside exclusively at the liquid-polymer interface; (2) varying the reducing agent can affect both the size of the NPs and their location relative to the interface; and (3) formation of Au NPs can be confined to the liquid-polymer interface by judicious choice of the amine binding agent and how it is processed.

Section snippets

Preparation of gold nanoparticle functionalized films

All commercial materials were reagent grade and used as received unless otherwise indicated. Deionized water (18 MΩ) was used to rinse all samples and prepare all aqueous solutions. The process of polymer surface modification is illustrated in Fig. 1, for the case of the binding agent ethylenediamine (ED) and reducing agent NaBH4. All reactions and solution preparation were carried out under ambient conditions unless otherwise stated.

Square glass coverslips (25 mm, no. 1 thickness) served as

Nanoparticle characteristics and surface distribution

Fig. 3 shows a plan-view bright-field TEM image of Au NPs generated by reduction of gold ions bound at the polymer surface using ED then reduced using NaBH4. It is important to note that this imaging mode shows features in projection, so the nanoparticles visible may be bound at the interface or within the polymer film. Elemental analysis line scans obtained during plan-view imaging, as well as cross-sectional imaging discussed below, consistently indicate the round high-contrast features are

Conclusion

This work shows that synthesizing Au NPs at a cross-linked polymer surface by in-situ reduction of metal cations does not necessarily generate particles solely at the liquid-polymer interface. Reducing agents commonly used to synthesize NPs in solution, such as NaBH4 and citrate, can generate particles within the cross-linked polymer SU-8 as much as 40 nm below the surface. In contrast, hydroquinone appears to generate NPs only at the interface. It is proposed that diffusion of the metal cation

Acknowledgments

This work was supported by NSF CAREER grant DMR/CHE-0748712 and NSF grant CHE-0809821. CJK was supported at the University of Central Florida (UCF) by the Beckman Scholars Program. DJF was supported by NSF grant nos. 0525429 and 0806931. CNG was supported by an REU supplement to NSF grant no. 0748712. MAH was supported by a UCF SURF Scholarship. We thank Dr. Florencio E. Hernandez for helpful discussions concerning potential applications of this work, Dr. Andre Gesquiere and Mr. Ernie

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