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