Gold nanocages, invented by Younan Xia and colleagues at Washington University, are new types of nanostructures that have hollow interiors and ultrathin porous walls. They are typically between 30–100 nm in diameter. The structures can be designed to absorb strongly and scatter light in the near-infrared (IR) region of the electromagnetic spectrum (from 700–900 nm). Light at these wavelengths can penetrate deeply into soft biological tissue and so is perfect for optical imaging.

The researchers, led by Xia and colleague Lihong Wang, begin by injecting a solution of gold nanocages into the front paw of a rat. The nanocages then travel through the lymphatic system and accumulate in the lymph nodes of the animal. Using reflection-mode photoacoustic imaging (a technique developed in Wang's lab), the researchers identified successfully where the nanocages were in these lymph nodes. The technique relies on pulsing the area of interest with near-IR light from a laser and analysing the photoacoustic signals generated subsequently by the nanocages therein.

The sentinel lymph node
Tumours spread by invading adjacent issue and cancerous cells propagate throughout the lymphatic system and then into the blood stream. The closest lymph node that drains from a tumour is called the sentinel lymph node (SLN) and this is most likely the area from which metastasis starts.

The new visualisation technique is non-invasive compared with conventional methods (based on injecting organic dyes and hazardous, radioactive colloids) that require surgery. Moreover, the technique can pinpoint the location of the nanocages accumulated in an SLN at up to 70 nm deep in tissue.

Wang says that the method could replace “sentinel lymph node biopsy” – an invasive procedure that is used routinely in hospitals these days, and which has become the standard for axillary staging. “SLN mapping is the prerequisite for sentinel lymph node biopsy,” he explained, “but we believe that our non-invasive photoacoustic SLN mapping based on gold nanocages could be used to identify SLNs for axillary staging of breast-cancer patients, for example, and would greatly aid minimally invasive methods such as fine-needle aspiration biopsy.”

Nanocages with modified surfaces
The team now plans to investigate further how gold nanocages travel through the lymphatic system. They will also examine how nanocages with modified surfaces behave. Indeed, previous work showed that combining the gold nanostructures with specific antibodies or peptides allows them to damage targeted cancerous tissue when illuminated with light at wavelengths around the absorption peak of the gold nanoparticles.

“For example, we will attach fluorescent dyes to the surface of the nanocages via an enzyme-cleavable peptide,” Xia told nanotechweb.org. “The dye molecules should then cleave and release from the surface of the nanocage in the presence of a specific protease (a biomarker for certain cancer cells), and thus we will be able to measure their fluorescence.” In practice, the distribution of gold nanocages in lymph nodes can be mapped by photoacoustic imaging while the protease activity can be monitored by fluorescence spectroscopy. Combining the two will provide critical information for diagnosing cancer metastasis," he added.

The present work is detailed in ACS Nano.