Effective photothermal treatments for cancer

Cancer accounts for a quarter of all deaths in the United States,¹ and most of these cancers derive from epithelial tissues (which line blood vessels and organs). Although the majority of cancers are treated through chemotherapy or radiotherapy, both of these approaches have associated problems. Chemotherapy can result in systemic toxicity,^{2, 3} whereas large radiotherapy doses in regions close to a tumor can cause (unavoidable) collateral damage to healthy tissue (e.g., radiation burns).⁴ Hyperthermic treatments are another option, but poor targeting capabilities are an issue and burns remain a possibility.^{5, 6} In addition, excessive hyperthermic doses can cause injury to neighboring (healthy) tissue and insufficient doses are not therapeutically active. It is therefore critical to accurately determine the optimum dose of heat that should be used in these treatments. As such, thermal dissipation in the surrounding medium, which causes killing of surrounding cells, needs to be considered and compensated for.

Upconverting nanoparticles (UCNPs) are excited at near-IR wavelengths and emit fluorescent radiation in the visible spectrum. They offer good photostability and a high signal-to-noise ratio, because there is no tissue autofluorescence.^{7, 8} UCNPs are therefore suited to 3D imaging and monitoring of biological processes over long periods. Combined with the large penetration depth and low background of two-photon fluorescence^9–11 and active targeting,¹² UCNPs can be used as ideal 3D diagnostic probes. It has also been demonstrated that UCNPs do not affect cellular processes and that they are relatively insensitive to physiological changes (e.g., salt concentrations,¹³ and pH levels) during cellular temperature monitoring.^{14, 15} The nanothermometer properties of UCNPs are caused by the emission from an ion of the rare earth element erbium (i.e., Er³⁺).^{16, 17} The emission is connected to the relative intensity of two green emission bands. This intensity ratio is inversely proportional to the temperature of most biological systems.¹⁵ Other nanoparticles—gold nanorods (AuNRs)—have strong light absorption and scattering properties, as well as local field enhancements that are caused by their surface plasmons. These AuNR characteristics lead to a strong plasmonic photothermal (PPT) heating effect, which can be exploited to kill surrounding cancer cells.^18–21 In addition, the large scattering cross section of AuNRs means that they are commonly used in immunoassays, as biochemical sensors, in surface-enhanced spectroscopies, and for photothermally activated killing of cancer cells. AuNRs can be used to selectively target cancer cells by inducing localized heating (i.e., allowing more healthy cells to be preserved than with other conventional thermal therapies).²² In general, nanoparticles can thus provide several benefits, e.g., targeted delivery of correct dosages and controlled release/activation.^23–25

We have developed a new approach in which we harness the advantages of UCNPs and AuNRs for cancer treatment and produce a single nanoplatform (see Figure 1).²⁶ We take advantage of the powerful diagnostic and thermal sensing capacities of UCNPs (see Figure 2), as well as the known therapeutic properties of AuNRs. By coupling the AuNRs and UCNPs together, we are able to increase the upconverted emission brightness and enhance their diagnostic strength.

Figure 1. Transmission electron microscope image of 200nm-sized upconverting nanoparticle (UCNP) that is coupled to a single 975nm-resonant gold nanorod (AuNR).

Figure 2. Schematic illustration of the coupled UCNP (green) and AuNR (yellow) nanodevices during surface temperature detection. The cytoplasm (pink) and nuclei (purple) of HeLa cells can also be seen.

Molecules or fluorophorescent emitters that are attached to, or near to, an AuNR surface will experience an emission enhancement. This is caused by the strong surface plasmon resonance (SPR) on the surface of the AuNRs. In particular, the enhancement can be created by tuning the gold SPRs so that they are closer to the excitation of the ytterbium/erbium-doped (Yb³⁺/Er³⁺) core.²⁷ In our experiments, we have observed an emission enhancement of our nanodevice, which is caused by the AuNR coupling (see Figure 3). This enhancement is produced by the localized SPR of the AuNR at about 980nm, which is resonant with the absorption of the Yb³⁺/Er³⁺ ions in the doped core. Our results clearly show an equal enhancement of the green (540nm) and red (650nm) emissions (i.e., a three- to four-fold increase in emission intensity).

Figure 3. Single particle upconverted emission spectra from a single UCNP-AuNR device with excitation at 975nm. The AuNR coupling gives rise to a three- to four-fold increase in emission intensity. a.u.: Arbitrary units.

We have also conducted time-resolved studies to investigate the enhancement effect of our nanodevices. In our preliminary work, we have studied the influence of AuNR coupling on the time-resolved decay of the upconversion emissive states. Our data (see Figure 4) shows a stronger dependence for green emission than for red emission on the AuNR coupling. We observe no change in the rise time for the decay of either the green or red emissions. This indicates that there is very little absorption enhancement at near-IR wavelengths.

Figure 4. Single particle time-resolved decay for (a) green emission, 545nm, and (b) red emission, 650nm, from a UCNP-AuNR device (with excitation at 975nm).

To achieve effective photothermal treatments—so that protein destruction occurs—the temperature of the heated regions must rise to at least 40–45°C.^{24, 28} With the nanothermometer property of our UCNPs, we are also able to quantify the local temperature of the coupled photothermally excited AuNRs. We can thus determine the laser fluence and time duration that is needed to generate the required temperature for killing the appropriate cancer cells. Combined with the 3D thermal sensing property of the UCNPs, our approach can potentially ensure that cancer lesions are completely destroyed.

We have developed and experimentally tested new nanodevices that can be used for effective photothermal cancer treatments. In our devices, we couple upconverting nanoparticles and gold nanorods. As such, we can achieve brighter emissions, as well as improved diagnostic and thermal sensing capabilities compared with conventional photothermal treatment approaches. In our upcoming work, we will map the transport of photothermal excitation between two separate UCNP-AuNR particles that are embedded within a tissue phantom. We will also map the 3D temperature profile that leads to tissue-wide cell damage.

We acknowledge financial support from the National Science Foundation Division of Chemical, Bioengineering, Environmental, and Transport Systems (grant 1067508).