All measurements were performed in a dark compartment at room temperature. Figure 6 Typical fluorescence intensity trajectories of single QDs. On the (a) Au-NP-modified AFM probe, (b) glass surface, and (c) 65-nm Au film. The photoblinking phenomenon, or fluorescence intermittency, is an important characteristic of QDs [19]. The term refers to the
temporal disappearance of emitted light when molecules or particles undergo reversible transitions between ‘on’ and ‘off’ states. Single QDs on glass https://www.selleckchem.com/products/JNJ-26481585.html clearly demonstrate this phenomenon, leading to bimodal variations in intensity (Figure 6b). This study demonstrated that through the appropriate coupling of Au-NP to the modified AFM probe, single QDs exhibit suppressed blinking and quenched fluorescence intensity (approximately 2-fold) (Figure 6a). Single QDs on the 65-nm Au film (Figure 6c) also exhibited suppressed blinking behavior; however, fluorescence
intensity was increased (approximately 1.5-fold). Applying QDs on a 10-nm Au film surface resulted in the enhancement of fluorescence intensity approximately 3-fold (see Additional file 1). These results support those of previous studies, in which the intensity of photoluminescence is either enhanced or quenched on roughened and smooth metal surfaces [20–25], respectively. However, conjugating QDs to the Au-NP modified-AFM probe presented a slightly different situation, which may be attributed to the effect of the nanoenvironment associated with the QD. These results are similar to those of Ratchford et al. [26]
and Bharadwaj and Selleckchem ACY-738 Novotny [27]. In these studies, an Au-NP was pushed proximal to a CdSe/ZnS QDs resulting in the quenching of fluorescence intensity (approximately 2.5-fold [26] and approximately 20-fold [27], respectively). Our results provide evidence of the existence of a small Au-film on the AFM tip. Mechanism: evaporation and electromigration One possible mechanism involved in the attachment of a 1.8-nm Au-NP to an AFM tip under a pulse of electrical voltage may be the evaporation and electromigration of Au-NPs induced by the strong electric field, resulting in a small area of Au film coating the AFM tip (an Au film roughly 4 nm in diameter coating the tip without a visible Au particle). In this scenario, an Au-NP GPX6 is melted and attracted to the tip apex through a sudden increase in the electric field due to a voltage pulse. Au has a vapor pressure of 10-5 Torr (estimated from bulk Au and is presumably lower for Au nanoparticles). As a result, Au is first evaporated and the Au vapor is then guided by the electrical field between the AFM apex and the substrate to be deposited over a limited region of the AFM apex. The energy 4SC-202 required to transfer Au vapor is very small and can be disregarded. Throughout the Au-NP evaporation process, the energy supplied to the system can be estimated as i 0 V s t.