Nanosphere lithography fabricated plasmonic materials and their applications
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Nanosphere lithography fabricated plasmonic materials and their applications Xiaoyu Zhang,a) Chanda Ranjit Yonzon,a) and Richard P. Van Duyneb) Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113 (Received 4 November 2005; accepted 23 January 2006)
Nanosphere lithography fabricated nanostructures have highly tunable localized surface plasmons, which have been used for important sensing and spectroscopy applications. In this work, the authors focus on biological applications and technologies that utilize two types of related plasmonic phenomena: localized surface plasmon resonance (LSPR) spectroscopy and surface-enhanced Raman spectroscopy (SERS). Two applications of these plasmonic materials are presented: (i) the development of an ultrasensitive nanoscale optical biosensor based on LSPR wavelength-shift spectroscopy and (ii) the SERS detection of an anthrax biomarker.
I. INTRODUCTION
The last two decades have seen a tremendous advancement of noble metal nanostructures as optical chemosensing and biosensing platforms in environmental protection,1,2 medical diagnostics,3–5 drug screening,6 food safety,2,7 and homeland security.8 The optical properties, especially the plasmonic properties, of noble metal nanostructures are used in the development of a new class of optical biosensors. On the most elementary level, surface plasmons exist in two simple forms: propagating plasmons and localized surface plasmons. Propagating surface plasmons are evanescent electromagnetic waves bounded by flat, smooth metal-dielectric interfaces and arise from oscillations of the conduction electrons in the metal.9 When surface plasmons are confined on periodic,10 colloidal,11 or other nanosystems,8 localized optical modes are observed. Localized surface plasmon resonance (LSPR) results in wavelength-selective absorptions with extremely large molar extinction coefficients (∼1011 m−1 cm−1),12 resonant Rayleigh scattering with efficiency equivalent to that of 106 fluorophores,13 and enhanced local electromagnetic fields near the
a)
These authors contributed equally to this work. Address all correspondence to this author. e-mail: [email protected] This paper was selected as the Outstanding Meeting Paper for the 2005 MRS Spring Meeting Symposium R Proceedings, Vol. 876E. DOI: 10.1557/JMR.2006.0136 b)
J. Mater. Res., Vol. 21, No. 5, May 2006
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surface of the nanoparticle that are responsible for the intense signals observed in surface-enhanced Raman spectroscopy (SERS).14 Strong electromagnetic fields are generated when the LSPR of nanoscale roughness features on a silver, gold, or copper substrate is excited by light.15,16 When the Raman scatterer is subjected to these intensified electromagnetic fields, the magnitude of the induced dipole increases, and accordingly, the intensity of the inelastic scattering increases.14 This phenomenon is known as surface-enhanced Raman scattering and can be exploited for sensitive and selective molecular id
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