Nanoporous gold nanoparticles (NPG-NP) showcase tunable pore and ligament sizes ranging from nanometers to microns. The nanoporous structure and sub-wavelength nanoparticle shape contribute to its unique LSPR properties. NPG-NP features large specific surface area and high-density plasmonic field enhancement known as “hot-spots”. Hence, NPG-NP has found many applications in nanoplasmonic sensor development. In our recent studies, we have shown that NPG-NP array chip can be utilized for high-sensitivity detection by various enhanced spectroscopic modalities, as photothermal agents, and for disease biomarker detection.
To date, array-format, substrate-bound NPGN has been fabricated by either colloidal nanosphere lithography or random nucleation during the sputtering deposition process. Although highly cost-effective, these techniques cannot provide precise control of individual particle size and location. In this paper, we report the development of a new fabrication technique based on electron-beam lithography (EBL).
Herein, a customized EBL technique is utilized to pattern larger areas (several square millimeters) of randomly distributed NPGN by careful design of the shot pattern, which limits the writing time to the acceptable level. Since the position, size, and shape of a huge number of features need to be generated and stored individually, memory limitations of this unique EBL technique constitutes an additional challenge, which is normally not present if small areas are to be patterned with features on an ordered lattice. This issue is solved by programmatically generating random feature positions within a simulation cell of carefully chosen size and implementing periodic boundary conditions.
Plasmonic metal nanostructures have shown great potential in sensing, photovoltaics, imaging and biomedicine, principally due to enhancement of the local electric field by light-excited surface plasmons, the collective oscillation of conduction band electrons. Thin films of nanoporous gold have received a great deal of interest due to the unique 3- dimensional bicontinuous nanostructures with high specific surface area. However, in the form of semi-infinite thin films, nanoporous gold exhibits weak plasmonic extinction and little tunability in the plasmon resonance, because the pore size is much smaller than the wavelength of light. Here we show that by making nanoporous gold in the form of disks of sub-wavelength diameter and sub-100 nm thickness, these limitations can be overcome. Nanoporous gold disks (NPGDs) not only possess large specific surface area but also high-density, internal plasmonic “hot-spots” with impressive electric field enhancement, which greatly promotes plasmon-matter interaction as evidenced by spectral shifts in the surface plasmon resonance. In addition, the plasmonic resonance of NPGD can be easily tuned from 900 to 1850 nm by changing the disk diameter from 300 to 700 nm. The coupling between external and internal nanoarchitecture provides a potential design dimension for plasmonic engineering. The synergy of large specific surface area, high-density hot spots, and tunable plasmonics would profoundly impact applications where plasmonic nanoparticles and non-plasmonic mesoporous nanoparticles are currently employed, e.g., in in-vitro and in-vivo biosensing, molecular imaging, photothermal contrast agents, and molecular cargos.