Interactions between pulsed laser radiation and nanostructured materials, with dimensions ranging from 1 nm to 500 nm, can result in enhanced desorption and ionization of organic and biomolecular adsorbates. When the critical dimensions of the nanostructures fall below the characteristic lengths for the involved transport processes, novel regimes of ion production are observed. Systems with dimensions commensurate with the wavelength of the laser radiation are the basis of photonic ion sources with unique properties, including polarization dependent ion yields and fragmentation. The main characteristics of these systems are often governed by altered modes of transport, e.g., ballistic vs. diffusive, energy confinement, plasmon resonances, and local field enhancements. Some structures offer control over the internal energy and the active fragmentation channels for the produced ions. Emerging applications of photonic ion sources in mass spectrometry benefit from ultrahigh sensitivity, a wide dynamic range for detection and quantitation, and a broad coverage of adsorbates ranging from small organic molecules to biopolymers, as well as to highly complex samples like single cells.

Nanostructures that have dimensions commensurate with the wavelength of the electromagnetic radiation exhibit near-field effects and, as optical antennas, can couple laser radiation to the local environment. Laser-induced silicon microcolumn arrays behave as nanophotonic ion sources that can be modulated by rotating the plane of light polarization. However, the limited range of surface morphologies available for these substrates makes it difficult to study the underlying mechanism that governs ion production. Here we demonstrate that nanopost arrays (NAPAs) can be tailored to exhibit resonant ion production. Ion yields from posts with subwavelength diameter show sharp resonances at high aspect ratios. The resonant enhancement in ion intensities can be modulated by adjusting the periodicity. In addition to strong molecular ion formation, the presence of high-energy fragmentation channels is observed. Ion yields from NAPAs exhibit dramatic differences for p- and s-polarized laser beams, indicating that energy coupling is similar to antenna arrays. These nanophotonic ion sources can control the degree of ion fragmentation and could eventually be integrated with micromachined mass spectrometers and microfluidic devices.

Recent mechanistic studies have indicated that at subwavelength post diameters and selected aspect ratios nanopost arrays (NAPA) exhibit ion yield resonances ( Walker , B. N. , Stolee , J. A. , Pickel , D. L. , Retterer , S. T. , and Vertes , A. J. Phys. Chem. C 2010 , 114 , 4835 - 4840 ). In this contribution we explore the analytical utility of these optimized structures as matrix-free platforms for laser desorption ionization mass spectrometry (LDI-MS). Using NAPA, we show that high ionization efficiencies enable the detection of ultratrace amounts of analytes (e.g., ∼800 zmol of verapamil) with a dynamic range spanning up to 4 orders of magnitude. Due to the clean nanofabrication process and the lack of matrix material, minimal background interferences are present in the low-mass range. We demonstrate that LDI from NAPA ionizes a broad class of small molecules including pharmaceuticals, natural products, metabolites, and explosives. Quantitation of resveratrol in red wine samples shows that the analysis of targeted analytes in complex mixtures is feasible with minimal sample preparation using NAPA-based LDI. We also describe how multiple metabolite species can be directly detected in single yeast cells deposited on the NAPA chip. Twenty-four metabolites, or 4% of the yeast metabolome, were identified in the single-cell spectra.