Design of multi-metal nanoarrays for optical sensors

Abstract

In the past ten years, a considerable effort was made to develop protocols for designing SERS platforms, employing metallic structures such as Ag, Au, Cu, metallic oxides or graphene. The potentiality of these materials has been demonstrated by enhancing the Raman signal as much as 10(10) times. However, considering the practical application of SERS platforms for sensing complex samples, it is also desirable the association of further practical advantages such as the response to a wide excitation wavelength range. This can be done by introducing nanoarrays of multi metals on the sensing surface. Particularly the obtaining of isolated metals such as Au, Ag and Cu with specific spatial localization is of major importance, as multiple optimized laser excitation wavelengths are expected, resulting in the versatility of high-enhancing, fluorescence quenching and resonant effects of several molecules covered by the same platform. Complex samples of environmental interest containing aromatic or heterocyclic rings can also benefit from multiple stable metal nanoparticles to adsorb, as Raman profiles can be acquired with a diversity of options, since a conservative low-energy excitation on gold to a more energetic blue/green excitation on copper for resonant enhancement.We believe this versatile sensing platform can be designed by combining the template-assisted electrodeposition technique mastered at USP with the light-activated electrochemistry technique developed at UNSW.As previously demonstrated by the candidate, a simple methodology for preparing an array of Au structures with 200 nm in diameter can be achieved on a conductive substrate by employing a spin-coated porous PMMA mask to template the electrodeposition of the metal. The smaller size of the features is important for sensing applications and the electrochemistry is a powerful tool for tunnelling the roughness required for SERS. This is possible due to the fine electrochemical control of the kinetics during electrodeposition. However, plating of metals inside the pores of a mask deposited on a conductive substrate occurs without spatial resolution. This prevents the electrodeposition of other metals separately in distinct pores on the same surface.An alternative rises from employing the light-activated electrochemistry (LAE), which is a technique where electrochemistry can be spatially resolved on a semiconductor electrode using light. This was demonstrated in aqueous solutions by employing depleted silicon electrodes modified with well-defined organic monolayers. It was shown that (i) an interface can be switched from insulating to conductive by using a stimulus that is inherently clean and easy to deliver with high spatial and time control, (ii) the position of each stimulating element is freed of predetermined geometric arrangements.It is proposed herein the usage of light-activated electrochemistry for the electrodeposition of multiple metals with spatial control on semiconductor electrodes covered with porous templates. With this kind of combination, light is the driving agent for electrodeposition of a certain metal (e.g. Au) in a determined two-dimensional geometry. Further, a second solution containing another metal (e.g. Ag) can then be used for deposition in localized spots, once again relying on the two-dimensional control on top of the electrode surface. LAE has been employed for electrodepositing polymers and metals (data not published) using the light-addressability. However, the dimensions of the electrodeposited constructs are mainly determined by the arrival of the minority carriers at the depletion layer, which depends on the size of the illuminated area and the lateral diffusion of minority carriers. This has provided poor uniformity and restricted the dimensions of the final structured to the micrometric scale. The template-assisted electrodeposition can confine the growth of the metallic constructs to a restricted area. (AU)