Citation: Messina, T.C.; Srijanto, B.R.;
Collier, C.P.; Kravchenko, I.I.;
Richards, C.I. Gold Ion Beam Milled
Gold Zero-Mode Waveguides.
Nanomaterials 2022, 12, 1755. https://
doi.org/10.3390/nano12101755
Academic Editors: Ki-Hyun Kim
and Deepak Kukkar
Received: 26 April 2022
Accepted: 17 May 2022
Published: 21 May 2022
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Article
Gold Ion Beam Milled Gold Zero-Mode Waveguides
Troy C. Messina
1,
*
,†
, Bernadeta R. Srijanto
2,†
, Charles Patrick Collier
2,†
, Ivan I. Kravchenko
2,†
and Christopher I. Richards
3
1
Department of Physics, Berea College, 101 Chestnut Street, Berea, KY 40404, USA
2
Center for Nanophase Materials Science, Oak Ridge National Labs, Oak Ridge, TN 37831, USA;
srijantobr@ornl.gov (B.R.S.); colliercp@ornl.gov (C.P.C.); kravchenkoii@ornl.gov (I.I.K.)
3
Department of Chemistry, University of Kentucky, 209 Chemistry-Physics Building,
Lexington, KY 40202, USA; chris.richards@uky.edu
* Correspondence: messinat@berea.edu; Tel.: +1-859-985-3326
† These authors contributed equally to this work.
Abstract:
Zero-mode waveguides (ZMWs) are widely used in single molecule fluorescence mi-
croscopy for their enhancement of emitted light and the ability to study samples at physiological
concentrations. ZMWs are typically produced using photo or electron beam lithography. We re-
port a new method of ZMW production using focused ion beam (FIB) milling with gold ions. We
demonstrate that ion-milled gold ZMWs with 200 nm apertures exhibit similar plasmon-enhanced
fluorescence seen with ZMWs fabricated with traditional techniques such as electron beam lithography.
Keywords:
single molecule spectroscopy; nanostructures; single molecule; sub-wavelength apertures;
zero-mode waveguides
1. Introduction
Single molecule fluorescence spectroscopy (SMS) is widely used as an experimental
technique to examine the real-time dynamics of biological systems with high spatial and
temporal resolution [
1
–
4
]. Spatial resolution has been pushed to its limits with a variety of
techniques such as laser scanning confocal microscopy and super-resolution techniques
such as Stochastic Optical Reconstruction Microscopy (STORM) and Photoactivated Lo-
calization Microscopy (PALM) [5]. Both STORM and PALM rely on fluorescent molecules
that switch between dark and emitting states so that overlapping diffraction-limited single
molecule emission spots can be distinguished. The temporal resolution of single molecule
studies is primarily limited by the photon flux of individual emitters. Non-radiative relax-
ation from the excited state reduces the fluorescence quantum yields of molecular probes
used in fluorescence experiments, which affects the maximum rate at which information
is obtained by detected photons. Amelioration comes from advances being made with
techniques such as near-field scanning optical microscopes [
6
,
7
], the use of nanostructures
such as nano-antennas [
8
–
13
], and zero-mode waveguides (ZMW) [
14
]. Nano-antennas
and zero-mode waveguides enhance fluoresence by creating a surface plasmon resonance
that is confined to an active region within the nanostructure. The surface plasmon electric
field interacts with the electric dipole of a fluorescent molecule occupying the active region
of the nanostructure. This interaction affects both the excitation and the emission of the
fluorescent molecule [
15
]. Experimental dissection of the contributions to excitation and
emission have been made [
16
]. A theory of spontaneous emission related to nanostructures
and their substrates has also been developed [
17
]. Our interest in this work is with ZMW
because of their potential to improve the temporal resolution in SMS.
Zero-mode waveguides are arrays of thousands to millions of sub-wavelength diam-
eter holes, typically 50–250 nm, in a thin metallic film of 100–200 nm thickness. Having
a diameter smaller than the wavelength of excitation light, the ZMW acts as a plasmonic
Nanomaterials 2022, 12, 1755. https://doi.org/10.3390/nano12101755 https://www.mdpi.com/journal/nanomaterials
