Optical sensing: Holes in gold (inverse nanoparticle arrays) form efficient optical sensor

Newly fabricated mesoporous gold films can create plasmonic effects useful for optical sensing applications.

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Researchers at the International Center for Materials Nanoarchitectonics (MANA) within the National Institute for Materials Science (NIMS; Tsukuba, Japan), collaborating with the Japan Science and Technology Agency (JST; Saitama, Japan), Bilkent University (Ankara, Turkey), and the National Institute of Advanced Industrial Science and Technology (AIST; Nagoya, Japan), have fabricated mesoporous gold films that behave much like inverse nanoparticle arrays in their ability to create plasmonic effects useful for optical sensing applications.1 Furthermore, the films are far simpler and less expensive to fabricate than arrays.

Tunable holes in gold

Although colloidal crystal templating and self-assembly methods have been used to fabricate macroporous (pores with tens-of-microns diameter) gold films, smaller mesoporous structures (2–50 nm diameter pores) are needed to generate surface-plasmon resonances strong enough for sensing applications, as dictated by the Babinet principle wherein, as noted in Wikipedia, "... the diffraction pattern from an opaque body is identical to that from a hole of the same size and shape except for the overall forward beam intensity."

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Scanning electron microscope images show mesoporous gold films ([a] and [b] on different scales) prepared with an electrolytic solution that creates micelles critical to film fabrication. Different electrolyte compositions produce films with different or "tunable" pore sizes (c, d, and e). (Image credit: NIMS)

Recognizing that a mesoporous gold material with tunable, uniform pores can be constructed as an inverse gold nanocrystal ensemble, the researchers prepared mesoporous gold films using spherical micelles of polystyrene-block-poly(oxyethylene) (PS-b-PEO) diblock copolymers within an electrolytic solution of gold and chlorine ions (AuCl4-). Essentially, these micelles—aggregate spheres of molecules that form in aqueous solution—act as soft templates for hydrogen bonding of the gold within the AuCl4- ions. Other solutions can be used as well.

By controlling the electrolyte compositions, the micelle size effectively controls the pore size of the resultant metallic gold films, which correspondingly dictates the electric-field amplitude and frequency of the local surface-plasmon resonance (LSPR). The LSPR effect can be observed inside or at the perimeter of the mesopores and can be optically tailored for molecular sensing applications such as surface-enhanced Raman scattering (SERS) or surface-enhanced infrared absorption (SEIRA).

Sensing performance

By comparing the spectrum from a sputtered gold film without mesopores to that of mesoporous gold films with various pore diameters via a custom-made dark-field spectroscopic microscope, the researchers found that scattered LSPR increases with increasing pore size. It is surmised that the charge induced at the surface of the pore becomes larger as the pore size increases because the gold volume exposed to the electric field of the incoming light becomes larger.

The stability of these mesoporous gold films in water and the strong affinity of gold surfaces to proteins and amino acids make them an excellent substrate for biosensing. Using Nile blue molecules adsorbed on the surface of varying-pore-size gold films, the SERS signal increased significantly compared to a sputtered gold film and the SERS enhancement factor was calculated to be on the order of 1.2 × 105.

"When light is incident on nanostructured gold, a greatly enhanced electromagnetic field is usually generated due to collective oscillation of the electrons," says Yusuke Yamauchi, an independent scientist at MANA. "In addition to Babinet's principle in which the inverse mesoporous structure leads to local electromagnetic field enhancements, we found we could induce tunable enhancement by controlling the pore sizes. This discovery highly depends on our success in controlling pore sizes of the mesoporous gold films."

REFERENCE

1. C. Li et al., NatureCommun., 6, 6608 (2015); doi:10.1038/ncomms7608.

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