Tailored Silicon Nanopost Arrays for Resonant Nanophotonic Ion Production
Bennett N. Walker1, Jessica A. Stolee1, Deanna L. Pickel2
, Scott T. Retterer2
and Akos Vertes1
Fine control over the production and properties of ions is a prerequisite for the efficient analysis of biomolecules by mass spectrometry. The interactions of laser light with nanostructured arrays having periodicities on the order of the wavelength of the electromagnetic field can be utilized to couple the radiation to the local environment. When the nanostructures are covered with biomolecules, the energy deposition can result in ion production from the adsorbates. We have shown that tailored silicon nanopost arrays (NAPA) are efficient photonic ion sources for mass spectrometry that exhibit high sensitivity and resolution for small organic molecules and biomolecules. Due to the significance of this discovery at the intersection of the emerging field of photonics and molecular sciences, it is featured on the cover of the March 25, 2010, issue of Journal of Physical Chemistry C (see Figure).
|Figure. Cover of the March 25, 2010, issue of J. Phys. Chem. C depicting our initial NAPA results.|
Nanofabrication enabled accurate control over the diameter, height, and periodicity of the NAPA. By tailoring the NAPA dimensions, we explored the mechanism of ion production and the relationship between NAPA dimensions and their ability to transfer laser energy to the adsorbates. The ion yields as a function of NAPA dimensions showed resonances with large enhancements for high aspect ratio NAPA structures. This was explained in terms of strong near-fields around the posts and elevated surface temperatures for thinner posts due to radial energy confinement. In addition to molecular ion formation, structure-specific fragment ions were produced and the degree of fragmentation was changed by adjusting the laser fluence and the dimensions of the posts. Finally, unique to nanophotonic ion sources, a dramatic dependence of the ion yields on the plane of polarization was observed. These achievements demonstrated that the laser light-nanostructure interaction could be tuned to efficiently produce ions with a mass-to-charge ratio (m/z) less than 2,000 for laser desorption ionization mass spectrometry (LDI-MS).
Biochemical analysis is essential to understanding how biological systems, e.g., plants or microorganisms, capture and convert energy. Our results provide new insight into nanophotonic ion production for the analysis of biomolecules by LDI-MS. Due to the strong polarization dependence, ion yields from these newly discovered ion sources can be rapidly modulated. Furthermore, the degree of adsorbate fragmentation can be easily adjusted in NAPA systems, enabling efficient exploration of biomolecule structures. These unique nanostructured ion sources provide an enhanced control of ion production on the nanometer scale. Due to their size and fabrication technology, NAPA are amenable for integration with microfluidic lab-on-a-chip devices and miniaturized mass spectrometers.
|Figure 1. (A) Schematic of NAPA with post height H, diameter D, and periodicity P, indicates the relationship between the electric field vector of the incident laser beam propagating in k direction, Ei, its orthogonal projections, and the current, J, induced in the post. (B) SEM image of a NAPA segment with D ) 100 nm, H ) 1,000 nm, and P ) 350 nm shows uniform periodicity and height. The posts have vertical walls with minimum tapering|