Efficient Radioisotope Energy Transfer by Gold Nanoclusters for Molecular Imaging

Abstract

Beta-emitting isotopes Fluorine-18 and Yttrium-90 are tested for their potential to stimulate gold nanoclusters conjugated with blood serum proteins (AuNCs). AuNCs excited by either medical radioisotope are found to be highly effective ionizing radiation energy transfer mediators, suitable for in vivo optical imaging. AuNCs synthesized with protein templates convert beta-decaying radioisotope energy into tissue-penetrating optical signals between 620 and 800 nm. Optical signals are not detected from AuNCs incubated with Technetium-99m, a pure gamma emitter that is used as a control. Optical emission from AuNCs is not proportional to Cerenkov radiation, indicating that the energy transfer between the radionuclide and AuNC is only partially mediated by Cerenkov photons. A direct Coulombic interaction is proposed as a novel and significant mechanism of energy transfer between decaying radionuclides and AuNCs.

Keywords: cerenkov radiation; gold nanoclusters; imaging, radioisotope energy transfer; optical imaging; radioisotopes.

Figure 1.

Characterization of BSA-conjugated AuNCs. (a) Dynamic light scattering data displaying size distribution of AuNCs at pH~7. HR-TEM analysis confirms the existence of gold clusters with interplanar spacing of ~2.35Å. (b) Absorption and photoemission (AuNC λ_ex 450 nm) at room and physiological temperature. (c) Photoemission of AuNC excited in “blue” Cerenkov region (300–500nm). (d) Spectrum of Cerenkov photons emitted per unit wavelength in the water for E_max of ¹⁸F and ⁹⁰Y (taken from Ross et al.³⁸).

Figure 2.

RET spectrum. Spectral data of controls, i.e. water (H₂O), gold chloride (HAuCl₄) and BSA only, and BSA-conjugated AuNCs. (a, b) AuNC were excited by ¹⁸F (120 μCi ) and ⁹⁰Y (20 μCi), respectively, data was recorded in the range 460–720nm. (c) Spectrum measurements of control experiment with γ-emitter ^99mTc (300 μCi). (d-f) Captions are shown for blue (~460nm) and red region of spectrum (~700nm). All RET data is taken at 37°C. Data was normalized to the decay value of radiotracers in the water.

Figure 3.

Verification of RET in vitro. (a, c) Dose-dependence of ¹⁸F and ⁹⁰Y in the presence of constant amount of BSA-conjugated AuNCs with atomic gold concentration of ~1μM. (b, d) Concentration dependence of AuNCs in the presence of a constant amount of ¹⁸F (200 μCi) and ⁹⁰Y (20μCi). All data taken at 37°C.

Figure 4.

Phenomenological model of AuNC – Radioisotope interactions. (a) Theoretical energy spectra for ¹⁸F and ⁹⁰Y (normalized to have equal area under the curves). Only beta particles with energies above 0.25MeV can produce CR photons in water. (b) Direct beta (β) and indirect beta (β) interactions are the major contributors to AuNC excitation. The effect of gamma-photons was confirmed to have negligible contributions.

Figure 5.

In vivo and Ex vivo RET studies. (a) Mice bearing subcutaneous breast carcinomas were injected with 750μCi ¹⁸F-FDG and 1xPBS (left, n=4) and 750μCi ¹⁸F-FDG and BSA-conjugated AuNCs (right, n=7) Optical readings were acquired with Cy5.5 bandpass filter for 5 min. (b) In vivo quantitative analysis of the signal in the tumor before and after AuNC injection (n=7). The ROI (region of interest) was selected from the readings with open filter and used for data analysis at the selected wavelengths. The signal was compensated for radioactive decay and Cerenkov background by comparison to control mice. (c) Ex vivo quantitative analysis of the signal in the tumor. Optical readings were acquired in 695–770 nm range for 5 minutes. The measured activity per tumor was ~2 μCi. Average tumor weight was ~0.15 mg. Tumors harvested from control mice (n=4) were compared to the tumors harvested from mice injected with AuNCs (n=7). (d) RET signal correlation with tumor activity up-take.