Fabrication of Gold Nanoparticles on Porous Silicon and Its Application to (Bio) Chemical Sensing Platforms
Authors: Ko Euna , Kim Ji Hye , Pham Xuan-Hung , Huynh-Nguyen Buu-Chau , Le Thanh-Hai , Seong Gi Hun 
Published: February 2020

Traditional electrodes, such as glassy carbon, gold, and ITO, are most commonly used, but they are difficult to fabricate, limited in sensor design, and expensive to manufacture. In contrast, nano-scaled silicon has attracted considerable interest due to its potential advantages such as controllable pore size, low toxicity, and recyclable materials. It has been studied to explore its immense potential applications in electronics, actuators, and sensors. In particular, porous silicon has emerged as a promising material for applications in drug delivery, diagnostics, and (bio) chemical sensing owing to its high surface area, tunable pore structure, and biocompatibility [1]. In recent, to fabricate and assess the utilities of nanoparticles hybrid device, research efforts have focused on using novel metal nanoparticles to modify electrode surface in order to enhance the electrocatalytic activity of the electrode. In this study, we present the patterning of gold nanoparticles (AuNPs) on porous silicon using electrochemical deposition. The fabricated electrodes were used as the electrochemical sensing platforms for hydroxylamine detection and as surface-enhanced Raman spectroscopy (SERS) substrates.
The formation of porous structure was performed by electrochemical etching of n-type crystalline silicon. The morphology and structure of porous silicon could be easily controlled by changing electrochemical etching conditions such as HF concentration, current density, anodization time, and illumination. The morphologies of the fabricated porous silicon substrates were investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) (Figures 1A and B). To extend the application of the porous silicon substrates to sensing platforms, gold nanoparticles were deposited electrochemically, and used as the electrodes for hydroxylamine detection. The size and density of AuNPs could easily be adjusted, and the morphologies of the AuNPs deposited porous silicon (AuNPs-porous silicon) substrates were investigated by scanning electron microscopy (Figures 1C and D).

Figure 1 Images of porous silicon by (A) SEM and (B) AFM. SEM images of (C) AuNPs-crystalline silicon and (D) AuNPs-porous silicon. The scale bars are 5μm.
Hydroxylamine plays an important role as a reducing agent and an intermediate in industry and biological processing, respectively.However, hydroxylamine is well- known mutagen, moderately toxic and harmful to microorganisms that could interfere with biological sewage plant performance. Because of the industrial and pharmacological significance, the highly sensitive detection of hydroxylamine is interested in this study. Figure 2A show the cyclic voltammetry for the oxidation of 1 mM hydroxylamine. The AuNPs-porous silicon electrodes exhibited higher anodic peaks compared to AuNPs-crystalline silicon electrodes. The anodic peaks were attributed to the oxidation of hydroxylamine. Hydroxylamine oxidation on the AuNPs-porous silicon electrode occurred at 0.45 V (curve f), which was lower than that on the AuNPs-crystalline silicon electrode (curve e).
As another application, AuNPs-porous silicon substrates were used as platforms for the SERS. Among biophysical and biochemical analysis, SERS is expected as a new active tool because of its high sensitivity, the wide applicability, little to no sample preparation requirement, and straight forward sample identification [2]. The SERS activity was investigated by measuring the Raman signal of Rhodamine 6G (R6G) absorbed onto the as-prepared substrates; AuNPs-porous silicon and AuNPs-crystalline silicon. The SERS spectra showed that the AuNPs-porous silicon substrate emitted a strong R6G Raman signal compared to AuNPs-crystalline silicon substrate due to the large active area of AuNPs on porous silicon substrates. In addition, the Raman signals on silver NPs-porous silicon were higher than those on AuNPs-porous silicon.
We expect that the developed AuNPs-porous silicon substrates can be used as various types of (bio) chemical sensing platforms.

Figure 2 (A) Cyclic voltammetry of the oxidation of 1.08 mM Hydroxylamine in 0.1 M PBS (pH 7.0) on (a) crystalline silicon, (b) AuNPs-crystalline silicon, (c) porous silicon, and (d) AuNPs-porous silicon at a scan rate of 30mV/s. (B) SERS spectra of 300μM R6G on (a) AuNPs-porous silicon (red) and (b) AuNPs-crystalline silicon (blue).

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