Single-Molecule Fluorescence Probes Interactions between Photoactive Protein-Silver Nanowire Conjugate and Monolayer Graphene

Abstract

In this work, we apply single-molecule fluorescence microscopy and spectroscopy to probe plasmon-enhanced fluorescence and Förster resonance energy transfer in a nanoscale assemblies. The structure where the interplay between these two processes was present consists of photoactive proteins conjugated with silver nanowires and deposited on a monolayer graphene. By comparing the results of continuous-wave and time-resolved fluorescence microscopy acquired for this structure with those obtained for the reference samples, where proteins were coupled with either a graphene monolayer or silver nanowires, we find clear indications of the interplay between plasmonic enhancement and the energy transfer to graphene. Namely, fluorescence intensities calculated for the structure, where proteins were coupled to graphene only, are less than for the structure playing the central role in this study, containing both silver nanowires and graphene. Conversely, decay times extracted for the latter are shorter compared to a protein-silver nanowire conjugate, pointing towards emergence of the energy transfer. Overall, the results show that monitoring the optical properties of single emitters in a precisely designed hybrid nanostructure provides an elegant way to probe even complex combination of interactions at the nanoscale.

Keywords: energy transfer; fluorescence; graphene; plasmonic interaction; silver nanowire.

Figure 1

Wide-field fluorescence images measured for single-PCP complexes: (a) on glass; (b) on graphene; (c) conjugated with AgNWs on glass; and (d) conjugated with AgNWs on graphene. The corresponding optical transmission images are included in the Supplementary Material (Figure S2). The intensity scale is different for the data obtained with and without the AgNWs.

Figure 2

The intensity profiles are as follows: (a) single PCP on glass (black) and graphene (red), (b) single PCP conjugated with AgNWs on glass (blue) and graphene (green). The profiles for (b) were made along AgNWs.

Figure 3

Histograms of fluorescence intensities extracted for the following: (a) single PCP on glass (black) and graphene (red), (b) single-PCP conjugated with AgNWs on glass (blue) and graphene (green). The average intensity values are included.

Figure 4

Confocal fluorescence microscopy imaging results are as follows: (a) single PCP and (b) single PCP conjugated with AgNWs. Graphene–glass boundary was marked with green contour. (c) Time-resolved measurements results—fluorescence intensity decays of single PCP: on glass (black), on graphene (red), @AgNWs on glass (blue) and @AgNWs on graphene (green). The presented intensity decays are the averages for all measured data.

Figure 5

Histograms of fitted average fluorescence decay times (left), integrated fluorescence intensity-average fluorescence decay time correlation plot (right). The data are marked as follows: single PCP on glass (black) and on graphene (red); single PCP@AgNWs on glass (blue) and on graphene (green).