Fluorescence Enhancement and Quenching Properties of Dyes Positioned on Plasmonic Copper Arrays: Comparision with Those on Plasmonic Gold Arrays

Abstract

The metal-enhanced fluorescence (MEF) based on the localized surface plasmon resonance (LSPR) of metal nanoparticles (nanostructures) is one of useful optical phenomenon to achieve the low detection limits of fluorescence-based biosensors. The enhancement mechanism has been suggested to be due to a photoexcitation enhancement and a plamson-exciton coupling effect. However, commonly-used metal nanoparticles in this research field have been composed of only expensive metal, e.g. Au and Ag, because these generate strong local electromagnetic fields within the visible wavelengths, which can significantly enhance the molecular fluorescence. The high material cost is not suitable for the practical application. The Cu metal is suitable for the industrial use because of the low material cost. Furthermore, the LSPR of the Cu nanoparticles is induced within the visible region. Therefore, the use of the Cu nanostructures as a light-harvesting center is worthy of attention. However, the two critical problems should be resolved for the utilization of the Cu LSPR. First, the LSPR of the traditional spherical Cu nanoparticles generate at around 570 nm where the interband transition of Cu from 3d to the Fermi level is occurred. The strong interband transition damps the LSPR excitation. Therefore, the generating wavelength of the Cu LSPR was tuned to the longer wavelength, i.e. above 590 nm, where the interband transition efficiency is very low. Second, the Cu is susceptible to the spontaneous oxidation in air. The resultant Cu2O layers also damp the Cu LSPR. Therefore, the development of the efficient oxidation suppression technique of the Cu surfaces is important. Recently, we succeeded in significantly enhancing the fluorescence of porphyrin immobilized on the periodic Cu structures, which were fabricated using a two-dimensional colloidal crystals of silica particles as a template (denoted as Cu half-shell arrays: CuHSs(d); dis the underlying silica diameter). However, the fluorescence enhancement ability of the plasmonic Cu nanostructures, as compared with the Au nanostructures known as a traditional fluorescence enhancement platform is still not clear. Our goal in this study is to evaluate the enhancement ability of the Cu nanostructures from the comparison with that of Au nanostructures. The silica colloidal crystals on glass plates (25 × 18 mm)were prepared by a previously-reported bottom-up procedure using silica particles with diameter of 226, 346, and 552 nm.1 The Cu with 100 nm in thickness was deposited onto the crystal surfaces by thermal deposition via a modification of mercaptopropyltriethoxysilane. As a reference sample, a planar Cu plate (CuP) was prepared by depositing Cu on a bare glass plate. The Au half-shell arrays (AuHSs(d)) were prepared by depositing Au with the same thickness as Cu in the same manner. Tetracarboxyphenyporphyrin (TCPP) as a fluorescence probe molecule was immobilized on the arrays by a surface sol-gel method viaa modification of the self-assembled monolayer of mercaptohexadecanoic acid (MHA) as a surface oxidation-suppressing ultrathin films (denoted as TCPP/CuHS(AuHS)). In order to calculate the local electromagnetic fields of the arrays, finite difference time domain (FDTD) method were used. The strong local electromagnetic fields were expected to be generated at the spatial gap region between the neighboring silica particles in the CuHS as well as the AuHS. The LSPR generating wavelength was red-shifted to longer wavelengths with increasing the underlying silica diameter for both of the CuHS and the AuHS. (586, 630, and 552 nm for the CuHS(226), CuHS(346), and CuHS(552), respectively and 594, 670, and 890 nm for AuHS(226), AuHS(346), and AuHS(552), respectively). We confirmed from the fluorescence excitation spectra of the TCPP/CuHSs (and AuHSs) at the fluorescence wavelength of 715 nm that the molecular fluorescence was significantly enhanced in all of the samples. Also, the fluorescence signals from the TCPP/CuHS were 10-fold weaker than those from the TCPP/AuHS. On the other hand, quenching efficiency of the fluorescence on the CuP was significantly higher than Au. Therefore, the Cu nanostructures have the combination of fluorescence enhancement and quenching abilities. As a result, we demonstrated that the Cu LSPR was useful for the fluorescence-based switching sensing and device applications. 1. K.Sugawa et al. ACS Nano 2013, 7, 9997 Figure 1