Plasmonic nanoparticles track dynamic behavior of molecules in live cells

Abstract

Distance-dependent coupling of plasmon resonances between closely spaced metal nanoparticles offers an attractive approach for imaging molecular interactions.Molecular interactions govern a myriad of normal and pathologic human processes. Developing methods that allow us to see how biomolecules behave in live cells will lead to better understanding of the cellular and molecular underpinnings of devastating diseases such as cancer. To date, fluorescence-resonance energy transfer (FRET) has been the fundamental tool in imaging and understanding inter-and intramolecular interactions. 1, 2 However, FRET has a number of shortcomings that limit its application to a broader range of biomedical problems, including photobleaching and low efficiency. Moreover, the technique is sensitive only to molecular interactions that occur at short distances (typically 5nm), which requires painstaking chemical labeling protocols to precisely position the donor and acceptor fluorophors within the molecules of interest.Distance-dependent coupling of plasmon resonances between closely spaced metal nanoparticles offers an attractive alternative for imaging molecular interactions. The advantages of plasmon coupling include a dramatic nonlinear increase in scattering cross-section per interacting particle, 3, 4 alterations in plasmon resonance frequency (color change), 4 and depolarization of linearly polarized light. 5 The distances over which coupling is significant can be as large as three times the particle radius, thus extending the range of detectable protein interaction distance more than an order of magnitude relative to FRET. 6,7 Consequently, interactions can be imaged across cellular membranes. They can also be detected between two biomarkers that may be separated by one or more intermediate or adapter proteins linking them together. Plasmonic nanoparticles offer other advantages. They have optical cross-sections that are many times larger than fluorescent dyes, green fluorescent protein, or even quantum dots. 8,9 They are chemically inert. 10 They have stable signal intensity because of a lack of photobleaching or blinking effects. 9 Finally, they are amenable to surface modification and functionalization strategies that allow synthesis of multifunctional nanoparticles. [11][12][13] The tremendous Figure 1. Dark-field image of a live cell 15min after labeling with anti-epidermal growth-factor-receptor (EGFR) gold nanoparticles (left). Over time, the labeling pattern undergoes progressive color change from green (at 15min) to yellow ( 30min), and finally to orange-red (>50min), as shown in the middle column (the images display the region that is highlighted by the white box on the left). The color changes are well correlated with the trafficking dynamics of EGFR, which include receptor-molecule dimerization (pairing) and aggregation in the plasma membrane, followed by endocytosis (uptake) into early endosomes (intracellular compartments) and, finally, formation of late endosomes. The transmission-electron-microscope (TEM) pictur...