Study of fluorescent nanodiamonds concentrations in aqueous solutions for biological applications

Commercial fluorescent nanodiamonds constitute a new class of nanoprobes with excellent perspectives for biomedical applications. They produce a broad emission spectrum partially matching the biological window for high tissue penetration, which can be excited in the visible region (380–780 nm) and in the biological window (1000–2500 nm) using two-photon excitation processes, giving a chance for high-resolution multi-photon fluorescence imaging at a sub-cellular resolution scale. Because of their carbon-based structure, nanodiamonds are highly biocompatible and display low toxicity if adequately coated. The nanodiamonds fluorescence spectrum arises due to the so-called Nitrogen-Vacancy (NV) color centers, which are highly photostable and are formed by local charge density compensating defects because of the typical nitrogen presence impurity ions in the host nanocrystals. In this work, a study of different nanodiamonds’ concentrations in terms of their fluorescent activity was investigated in the range of 0.1–10 mg/mL by means of fluorescence spectroscopy with the purpose of determining which concentration of nanodiamonds suspended in milliQ water achieves the maximum intensity, in which it was found that the 5 mg/mL was the most intense in our experimental observations. Additionally, a non-linear fit of the experimental observations of intensity was performed to predict the fluorescence intensity following an expression of a fourth order polynomial with respect to concentration, with a figure of merit, FOM = 0.03–0.04 (3–4%). Based on this effect, the optimal nanodiamonds/aqueous media solution has been determined by means of cell viability assays, finding that concentrations ≤0.1 mg/mL are as biocompatible as milliQ water.