Abstract
Silver-copper alloy nanoparticles were explored for metal enhanced luminescence (MEL) applications in this study. Surface plasmon resonance (SPR) wavelengths of these Ag-Cu nanoparticles were tuned in the visible and near infrared regions by changing annealing temperature. We observed strong emission enhancement of luminophores (141.48 ± 19.20 times for Alexa Fluor 488 and 23.91 ± 12.37 times for Alexa Fluor 594) when the SPR spectrum of Ag-Cu nanoparticles was tuned to result in maximum spectral overlap of the emission and absorption spectra of the luminophores with the SPR spectrum of the nanoparticles. This finding opens new avenues for utilization of metal alloy nanoparticles in MEL-based applications.
Introduction
Luminescence based measurements and devices are currently widely used methods in different fields such as biology, chemistry, materials science and medicine. Strong luminescence intensity is one of the most important desired properties for luminophores for these applications. It is possible to design and synthesis luminophores with desired spectral properties. The absorption and emission peaks of any luminophores can be predicted by analogy of known luminophores.
Introduction
But it is difficult to design luminophores with desired luminescence intensity. At the vicinity of conducting metallic nanoparticles such as those of silver and gold, the emission intensity of luminophores is known to be significantly influenced [1] [2] [3] [4] . Using nanoparticles platform, it can be possible to increase the quantum yield of weakly luminescent probes by increasing it's emission efficiency by modifying radiative decay rate or by coupling the emission with surface plasmon resonance. It can also improve the intensity of luminophores that already have near unit quantum yield by enhancing their absorption by increasing the local field. Distance between the fluorophore molecules and nanostructured platform is also an important parameter in fluorescence enhancement 5 This work is focused on the use of alloy nanoparticles in the MEL. We demonstrate that SPR spectra of alloy nanoparticles can easily be tuned by manipulating only one experimental condition to result in maximum spectral overlap of the emission and absorption spectra of the luminophores with the SPR spectrum of the nanoparticles. We observed enhanced fluorescence emission from two thiol-reactive dyes, Alexa Fluor 594 and Alexa Fluor 488 (obtained from Molecular Probes, Invitrogen, Portland, OR), at the proximity of Ag-Cu alloy nanoparticles.
Experimental procedure
In this study, Ag or Ag-Cu nanoparticles were deposited on 22 × 22 mm glass cover slips (Fisher finest cover glass, thickness approximately 140 microns) by using DC magnetron sputtering (Plasma Sciences CRC-100 Sputter Tool). Before the depositions, the cover slips were cleaned by air plasma (Harrick PDC-32G) for 10 garybaker@example.org watts power setting. During deposition, the background pressure was of the order of 10 -6 Torr, the Ar pressure was 5 mTorr 
Result and Discussion
Transmission electron microscopy of the Ag-Cu alloy nanoparticles indicated the average size to be 7.03 nm ± 1.97 nm (derived from a population of 100 particles) ( 

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