Spectroscopic Insights into Nanodiamond–Doxorubicin Interactions in Drug Delivery Systems for Potential Cancer Treatment: “What Is Essential Is Invisible to the Eye”

Figures

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  Figure 1. The spectra of a serial of nanodiamond concentration (10, 25, 50, 75, 100, 125, and 150 μg/mL, from bottom to top, respectively).

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  Figure 2. A preliminary spectroscopic scan of DOX (2.5 μg/mL) and ND/DOX combination (50 μg/mL/2.5 μg/mL) bottom to top, respectively, pointing at two prominent absorption peaks at the wavelengths of 500 and 580 nm.

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  Figure 3. The resulting difference spectra of subtracted corresponding ND spectra from ND/DOX spectra. DOX concentration is constant in all ND/DOX samples, ND concentration is increasing from top to bottom (10, 25, 50, 75, 100, 125, and 150 μg/mL).

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  Figure 4. Absorbance of nanodiamonds as a function of concentration at 500 nm (A500, black) and 580 nm (A580, red). The linear fits were used to determine the extinction coefficients according to the Lambert–Beer law.

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  Figure 5. Effect of centrifugation on the absorbance spectra of (a) DOX (2.5 μg/mL), (b) ND (50 μg/mL) and (c) ND/DOX combination (50 μg/mL/2.5 μg/mL). The upper curves correspond to spectra acquired prior to centrifugation.

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  Figure 6. Theoretical verification of Lambert–Beer law additivity at 500 nm before (no cf) and after (cf) centrifugation for different ND/DOX ratios.

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  Figure 7. (a) Pure DOX and ND/DOX raw absorption spectra, (b) theoretically modelled DOX and ND/DOX absorption spectra, (c) pre-assigned absorbing species from the deconvoluted spectrum of the ND/DOX.

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  Figure 8. Shift in the equilibrium between free and self-associated DOX species with increasing nanodiamond concentration in ND/DOX mixtures (a) before centrifugation, (b) after centrifugation.

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  Figure 9. Comparative overview of the ratios of different doxorubicin forms in the presence of increasing nanodiamond concentrations, before and after (cf) centrifugation.

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  Figure 10. Relative contributions of labile and stable ND/DOX as a function of ND concentration.

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  Figure 11. (a) Time-dependent UV–VIS absorbance spectra of DOX the ND/DOX recorded over 24 h, (b) Reaction kinetics of the ND/DOX system over 24 h, showing changes in absorbance at 500 nm.

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  Figure 12. Short-time kinetics of the ND/DOX interaction monitored at (a) 15 s (1 h), (b) 5 s (30 min), and (c) 1 s (3 min) intervals at 500 nm.