Photodeposition of silver can result in metal-enhanced fluorescence

Abstract

Chemically deposited silver particles are widely used for surface-enhanced Raman scattering (SERS) and more recently for surface-enhanced fluorescence (SEF), also known as metal-enhanced fluorescence (MEF). We now show that metallic silver deposited by laser illumination results in an approximately 7-fold increased intensity of locally bound indocyanine green. The increased intensity is accompanied by a decreased lifetime and increased photostability. These results demonstrate the possibility of photolithographic preparation of surfaces for enhanced fluorescence in microfluidics, medical diagnostics, and other applications.

Fig. 1

Chemical structure of indocyanine green (ICG).

Fig. 2

Experimental setup for laser deposition of silver on APS coated glass microscope slides.

Fig. 3

Absorption spectrum of a 0.5% v/v APS coated glass slide after 5 min illumination with a 442 nm HeCd laser.

Fig. 4

(Top) Fluorescence intensity of HSA–ICG coated glass, I_G, and laser deposited silver, I_S (442 nm, 15 min exposure). (Bottom) Fluorescence intensities normalized to the intensity on silver.

Fig. 5

Time-dependent intensity decays of ICG–HSA in solution (buffer), bound to glass, and on laser-deposited (LD) silver. (RF) Instrumental response function <40 ps FWHM.

Fig. 6

(Top) Photostability of ICG–HSA on (G) glass, and (S) laser-deposited silver, measured with the same excitation power at 760 nm, and (Bottom) with the laser power at 760 nm adjusted for the same initial fluorescence intensity. Laser-deposited samples were made by focusing 442 nm laser light onto APS coated glass slides immersed in a AgNO₃ citrate solution for 15 min. The OD of the sample was ~0.3.

Fig. 7

(Top) Inverted Axiovert 135 TV micrcoscope with epi-illumination for LD and, (Bottom) image of silver spots, produced by LD, using transmitted light illumination. The diameter of a spot is typically 50 μm. The irradiance was 560 W/cm² with a 40×, NA 1.2 objective. Images (A–F) were taken 5 s apart.