Enhanced thermal lens effect using surface plasmon resonance of gold nanoparticle for detection of fluorescence molecules

A photo thermal lens microscope (PTLM) is a recent promising development of thermal lens spectrometry (TLS) toward miniaturization. A TLM not only has the similar advantage of high sensitivity as the conventional TLS, but also has its unique characteristics such as high temporal (ms) and spatial resolutions (∼μm), which enable it for high-sample-throughput and small-volume detection of a variety of compounds with low sample/reagent consumption when it is coupled to lab-on chip chemistry, such as in microfluidic chips or miniaturized microliter plates [1]. In the TLM, when the analyte molecules in the confocal region absorb the excitation beam, they emit heat via radiationless processes. The temperature at the confocal region increases, and usually the refractive index (RI) decreases because of the heat. The laser intensity profile is nearly a Gaussian distribution, and the heat source distribution is in proportion to it. The RI distribution becomes a nearly Gaussian distribution and acts as a concave lens. Resonant energy transfer (RET) systems consisting of organic dye molecules and noble metal nanoparticles have recently gained considerable interest in biophotonics [2]. If the donor molecule is placed in the vicinity of a metal surface instead of an organic acceptor, not only resonant energy transfer takes place but also the radiative lifetime of the donor molecule changes [4]. In this work, a combined photo thermal lens microscopy (PTLM) system was assembled for highly sensitive determination of dye molecule vicinity of gold nanoparticle as acceptor. We used Triazene component as donor molecules in the vicinity of AuGNs. Triazene component has a fluorescence emission peak at 510 nm when excited at 405 nm. We show Gold nanoparticles in focal point make the overlap of the molecule’s emission with the nanoparticle’s absorption spectrum that cause temperature at the focal point increases and enhance thermal lens signal of triazene. The detection volume is equal to the focal volume of the excitation beam; the focal volume was calculated to be 1.18 fL. The results show the possibility of direct determination of 7 to 100 triazene molecules in the vicinity of 2.5 AuGNs in the focal volume of the excitation beam.