Fabrication of Au-Ag nanorod SERS substrates by co-sputtering technique and dealloying with selective chemical etching
Graphical abstract
Introduction
Surface enhanced Raman scattering is a powerful analytical method that allows for trace level analysis and even detection of single molecules [1], [2]. It is a productive technique used in a wide range of applications including chemical sensing [3], [4], biosensing [5], [6], medical applications [7], [8], environmental analysis [9], [10], forensics [11], food safety [12] and for pesticide detection [13], [14]. Many researches have attempted to prepare SERS substrates with controllable structure, reproducibility, high sensitivity and long shelf life by application of engineered nanometallic structures. More specifically, plasmonic materials that function well as SERS substrates are silver (Ag) and gold (Au) nanoparticles due to their strong absorption in the visible region of the electromagnetic spectrum [15].
It is well known that Ag exhibits superior SERS performance but is highly sensitive toward oxidation under ambient conditions. The oxide coating suppresses enhancement effects and limits further use in detection. On the other hand Au, even though it demonstrates lower SERS activity, but it is chemically stable and resistant to oxidation [16]. Hence, nanoalloy compositions containing both Au and Ag particle domains have recently drawn attention in SERS development due superior optical, thermal, chemical and especially tunable plasmonic properties [17], [18], [19].
Several modes of fabrication of Au-Ag alloy SERS substrate have been reported including co-reduction of gold salts with silver nitrate [20], [21], laser ablation [22], [23], solid-state ionics [24], electrodeposition [25], [26], [27], electron-beam lithography [17] and sputtering deposition [28], [29]. Among these methods, the sputtering technique is a facilitative method to fabricate alloyed nanostructures due to resultant purity, control of morphological and alloy composition, and scale-up potential. Recently, there have been reports on the fabrication of Au-Ag dense films by sputtering and dealloying treatment to create interconnected nanoporous gold films with inner pores [29], [30]. These complicated ligaments and bicontinuous nanostructures with small pore sizes provide high-density hotspots due to strong near-field coupling effect of ligaments resulting in excellent SERS activities. However, the inner pore of ligament can coarse during the dealloying process leading to a degraded SERS performance [31], [32] Here, we report design of SERS substrates in the form of Au-Ag nanorods film using oblique angle deposition (OAD) by the co-sputtering technique and create surface roughness on each nanorods by a chemical dealloying process. Series of Au-Ag alloy nanorods were fabricated with uniform structure and controllable alloy compositions. The dealloying process consisted of immersing Au-Ag alloyed substrates into an etching solution made of methanol, hydrogen peroxide and ammonium at varied times Finally, SERS measurements were performed to investigate the substrates in terms of sensitivity, uniformity, reproducibility and stability.
Section snippets
Materials
Ag and Au sputtering targets were purchased from K.J. Lesker (Purity 99.99%, 2 in. diameter, 0.25 in. thick). 4-Mercaptobenzoic acid (HO2C-C6H4-SH, 4-MBA, purity 99%) was used as Raman probe molecules to investigate SERS performance. Analytical grade 30% hydrogen peroxide (H2O2) was purchased from Chem-Supply; 30% ammonium hydroxide (NH4OH) was obtained from CARLO ERBA Reagents; and analytical grade methanol (CH3OH) was purchased from RCI Labscan.
Synthesis of Au-Ag alloyed and dealloyed substrates
Au-Ag alloy nanostructures were fabricated on
SERS substrates characterization
Morphology and Mass Depletion: Fig. 1A-N shows surface and cross-section SEM images of Au-Ag alloyed/dealloyed substrates of various etching times starting with 0 (control) up to 120 s and at an interval of 20 s. Please note that the 40 s etched substrates were selected due to optimal Raman performance (data to be explained later). All Au-Ag alloyed nanorods showed a 45° tilt from the normal with respect to substrates, a consequence of the OAD sputtering procedure. Variations in surface
Conclusion
Au-Ag dealloyed nanorod substrates were successfully fabricated using the co-sputter OAD technique and further dealloying by chemical etching. The dealloying resulted in higher surface roughness and provided good SERS activity with an average EF of 1.5 × 106 which is stronger than that of the Au-Ag alloy substrates by an order of magnitude. Dealloyed substrates also showed ca. an order of magnitude more adsorption capability of 4-MBA, as verified by traditional titration method. The dealloyed
CRediT authorship contribution statement
U. Waiwijit: Investigation, Writing - original draft. C. Chananonnawathorn: Investigation, Writing - original draft. P. Eimchai: Formal analysis. T. Bora: Methodology, Writing - review & editing. G.L. Hornyak: Visualization, Writing - review & editing. N. Nuntawong: Conceptualization, Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The work is financial support from integrated platform of the National Science and Technology Development Agency (NSTDA).
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