Dynamic contrast in optical imaging using stimuli-responsive agents

Qingshan Wei, Purdue University

Abstract

Optical imaging has become an indispensable tool for cancer diagnostics and treatment. However, separating optical signals from high levels of background noise is a persistent challenge. This thesis focuses on the development of dynamic contrast, a novel approach for enhancing image quality by the periodic modulation of optical signals and the subsequent reduction in background via signal demodulation processes. Chapter 1 introduces the basic concept of dynamic contrast and its application to optical imaging, with illustrations by several recent examples. Some important optical properties of gold nanoparticles (GNPs) are also described, namely plasmon-resonant scattering and photothermal (PT) effect, as a prelude to discussions in Chapter 2 and 4, respectively. The remaining chapters contain examples of dynamic optical contrast generated from stimuli-responsive probes. Chapter 2 discusses how dynamic contrast can be generated to enhance the optical quality of darkfield images, using gold nanostars (NSTs) with superparamagnetic cores as polarization-sensitive contrast agents. The scattering from NSTs produces a periodic "twinkling" in response to a rotating magnetic field gradient. The modulations produced by gyromagnetic (GM) NSTs are then recovered as signals in the Fourier domain, with a dramatic reduction in background. Similarly, linear magnetic field gradients can produce magnetomotive (MM) signals from NSTs as well as Fe3O4@Au core–shell nanoparticles (NPs), although using different mechanisms of signal generation. Whereas NSTs can produce the polarization-dependent scattering, core–shell NPs create dynamic contrast via lateral displacement. Both GM and MM imaging are applied toward the detection of NSTs or core–shell NPs inside of tumor cells and macrophages, with greatly enhanced signal-to-noise (SNR) ratios compared with conventional darkfield imaging. In Chapter 3, I investigate how the motion of GM-active NSTs can also be gated by chemical or biological recognition, mediated by small molecules such as neurotransmitters. I demonstrate that NSTs coated with polyMV (a polymer ligand bearing methyl viologen groups) can be "handcuffed" to surfaces coated with polyNp (a polymer ligand bearing 2-naphthol ethers) in the presence of excess cucurbit[8]uril (CB[8]) via supramolecular recognition. The tethering of NSTs on polyNp-coated surfaces can be competitively inhibited by neurotransmitters such as serotonin (5-HT) and dopamine (DA), by preventing formation of the ternary MV2+•Np•CB[8] complex. Reactivation of the GM signal of "handcuffed" NSTs is also observed upon the addition of free 2-naphthol or DA. In Chapter 4, dynamic contrast is applied to fluorescence thermometric imaging for enhanced temperature resolution. Conventional ratiometric fluorescence thermometry (RaFT) has a temperature resolution of 0.1 to 1.0 K, but a dynamic form of RaFT (DRaFT) can provide greater sensitivity by introducing periodic modulations in laser-induced heating, followed by demodulation of the temperature-dependent fluorescence signals by Fourier transform. The results show that the temperature resolution of DRaFT imaging can be as low as 0.02 K, an order of magnitude better than that produced by RaFT. DRaFT imaging can be used to map thermal gradients with optical resolution, as demonstrated with planar gold substrates, micron-sized islands of gold nanorods, and within live tumor cells.

Degree

Ph.D.

Advisors

Wei, Purdue University.

Subject Area

Analytical chemistry|Nanoscience|Nanotechnology

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