Xingde Li

In vivo ultrahigh-resolution optical coherence tomography

Ultrahigh-resolution optical coherence tomography (OCT) by use of state of the art broad-bandwidth femtosecond laser technology is demonstrated and applied to in vivo subcellular imaging. Imaging is performed with a Kerr-lens mode-locked Ti:sapphire laser with double-chirped mirrors that emits sub-two-cycle pulses with bandwidths of up to 350 nm, centered at 800 nm. Longitudinal resolutions of ~1mum and transverse resolution of 3mum, with a 110-dB dynamic range, are achieved in biological tissue. To overcome depth-of-field limitations we perform zone focusing and image fusion to construct a tomogram with high transverse resolution throughout the image depth. To our knowledge this is the highest longitudinal resolution demonstrated to date for in vivo OCT imaging.

Ultrahigh-resolution optical coherence tomography using continuum generation in an air–silica microstructure optical fiber

We demonstrate ultrahigh-resolution optical coherence tomography (OCT) using continuum generation in an air-silica microstructure fiber as a low-coherence light source. A broadband OCT system was developed and imaging was performed with a bandwidth of 370 nm at a 1.3-mu;m center wavelength. Longitudinal resolutions of 2.5 microm in air and ~2 microm in tissue were achieved. Ultrahigh-resolution imaging in biological tissue in vivo was demonstrated.

Facile synthesis of Ag nanocubes and Au nanocages

This protocol describes a method for the synthesis of Ag nanocubes and their subsequent conversion into Au nanocages via the galvanic replacement reaction. The Ag nanocubes are prepared by a rapid (reaction time < 15 min), sulfide-mediated polyol method in which Ag(I) is reduced to Ag(0) by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) and a trace amount of Na2S. When the concentration of Ag atoms reaches supersaturation, they agglomerate to form seeds that then grow into Ag nanostructures. The presence of both PVP and Na2S facilitate the formation of nanocubes. With this method, Ag nanocubes can be prepared and isolated for use within approximately 3 h. The Ag nanocubes can then serve as sacrificial templates for the preparation of Au nanocages, with a method for their preparation also described herein. The procedure for Au nanocage preparation and isolation requires approximately 5 h.

Fluorescence lifetime imaging in turbid media

The lifetime of a f luorophore generally varies in different environments, making the molecule a sensitive indicator of tissue oxygenation, pH, and glucose. However, lifetime measurements are complicated when the f luorophore is embedded in an optically thick, highly scattering medium such as human tissue. We formulate the inverse problem for f luorescence lifetime tomography using diffuse photon density waves, and we demonstrate the technique by deriving spatial images of heterogeneous f luorophore distribution and lifetime, using simulated measurements in heterogeneous turbid media.

Imaging needle for optical coherence tomography

We describe a miniature optical coherence tomography (OCT) imaging needle that can be inserted into solid tissues and organs to permit interstitial imaging of their internal microstructures with micrometer scale resolution and minimal trauma. A novel rotational coupler with a glass capillary tube is also presented that couples light from a rotating single-mode fiber to a stationary one. A prototype needle with a 27-gauge (approximately 410-microm) outer diameter has been developed and is demonstrated for in vivo imaging. The OCT needle can be integrated with standard excisional biopsy devices and used for OCT-guided biopsy.

A quantitative study on the photothermal effect of immuno gold nanocages targeted to breast cancer cells

Gold nanocages with an average edge length of 65 +/- 7 nm and a strong absorption peak at 800 nm were conjugated with monoclonal antibodies (anti-HER2) to target breast cancer cells (SK-BR-3) through the epidermal growth factor receptor (in this case, HER2), which is overexpressed on the surfaces of the cells. Both the number of immuno Au nanocages immobilized per cell and the photothermal therapeutic effect were quantified using flow cytometry. The targeted cells were irradiated with a pulsed near-infrared laser, and by varying the power density, the duration of laser exposure, and the time of response after irradiation, we were able to optimize the treatment conditions to achieve effective destruction of the cancer cells. We found that cells targeted with the immuno Au nanocages responded immediately to laser irradiation and that the cellular damage was irreversible at power densities greater than 1.6 W/cm(2). The percentage of dead cells increased with increasing exposure time up to 5 min and then became steady. By quantifying the photothermal effect of immuno Au nanocages, critical information with regards to both the optimal dosage of nanocages and parameters of the laser irradiation has been garnered and will be applied to future in vivo studies.

Fiber-optic scanning two-photon fluorescence endoscope

We report on the development of a miniature, flexible, fiber-optic scanning endoscope for two-photon fluorescence imaging. The endoscope uses a tubular piezoelectric actuator for achieving two-dimensional beam scanning and a double-clad fiber for delivery of the excitation light and collection of two-photon fluorescence. Real-time imaging of fluorescent beads and cancer cells has been performed.

Gold nanocages as contrast agents for spectroscopic optical coherence tomography

We describe gold nanocages as a new class of potential contrast agent for spectroscopic optical coherence tomography (OCT). Monodispersed gold nanocages of an approximately 35 nm edge length exhibit strong optical resonance, with the peak wavelength tunable in the near-infrared range. We characterized the optical properties of the nanocage by using OCT experiments along with numerical calculations, revealing an absorption cross section approximately 5 orders of magnitude larger than conventional dyes. Experiments with tissue phantoms demonstrated that the nanocages provide enhanced contrast for spectroscopic as well as conventional intensity-based OCT imaging.

Rapid-scanning forward-imaging miniature endoscope for real-time optical coherence tomography.

We developed a miniature endoscope that is capable of rapid lateral scanning and is suitable for real-time forward-imaging optical coherence tomography (OCT). The endoscope has an outer diameter of 2.4 mm, consisting of a miniature tubular lead zirconate titanate (PZT) actuator, a single-mode fiber-optic cantilever, and a graded-index lens. Rapid lateral scanning at 2.8 kHz is achieved when the fiber-optic cantilever is resonated with the PZT actuator. This allows OCT imaging to be performed by fast lateral beam scanning followed by slow depth scanning, which is different from the conventional OCT imaging sequence. Real-time OCT imaging with the endoscope operated in the new image acquisition sequence at 6 frames/s is demonstrated.

Gold nanocages for cancer detection and treatment

Gold nanocages represent a novel class of biocompatible vectors with potential applications in drug delivery, tumor/tissue imaging and photothermal therapy. They are prepared through the galvanic-replacement reaction between Ag nanostructures and HAuCl(4). By controlling the amount of HAuCl(4) added, we can tune the surface-plasmon resonance peaks of the Au nanocages into the near-infrared, where the attenuation of light by blood and soft tissue is relatively low. Here, we highlight recent advances in the synthesis and utilization of Au nanocages for cancer detection and treatment. We have tailored the optical properties of Au nanocages for use as contrast agents in optical coherence tomography and as transducers for the selective photothermal ablation of cancer cells. Our results show improved optical coherence tomography image contrast when Au nanocages are added to tissue phantoms as well as the selective photothermal destruction of breast cancer cells in vitro when immunotargeted Au nanocages are used.