Nanoparticle metrology of silica colloids and super-resolution studies using the ADOTA fluorophore

We describe how a new fluorescent dye, methyl ADOTA (N-methyl-azadioxatriangulenium tetrafluoroborate), is an improvement on dyes reported previously for measuring silica nanoparticle size in sols using the decay of fluorescence anisotropy. Me(thyl)-ADOTA possesses the unusual combination of having a red emission and a long fluorescence lifetime of ∼20 ns, leaving it better-placed to reveal particle sizes at the upper end of the 1–10 nm measurement range. For stable LUDOX colloids, Me-ADOTA is shown to offer higher measurement precision in  ⩽1/30th of the measurement time required for dyes previously used. In measurement times of only ∼20 min nanoparticle radii for LUDOX SM-AS, AM and AS-40 of 4.6  ±  0.3 nm, 5.9  ±  0.2 nm and 11.1  ±  1.1 nm, are in good agreement with two of the manufacturer's values of 3.5 nm, 6 nm and 11 nm respectively. Unlike the Si-ADOTA (N-(4-(triethoxysilylethyl)urea-phenyl-) ADOTA tetrafluoroborate) derivative containing a reactive trimetoxysilane group, Me-ADOTA is shown to not induce aggregation of colloidal silica. Measurements on nanoparticles growing in an acidic silica hydrogel at pH 0.94, prior to the gel time of ∼50 h, reveals an average nanoparticle size up to ∼6.3 nm, significantly larger than the 4.5 nm reported previously. The difference is most certainly due to the longer fluorescence lifetime of Me-ADOTA (∼20 ns) revealing the presence of larger particles. Studies of growing silica clusters in an alcogel of tetraethyl orthosilicate (TEOS) were able to resolve a monotonically increasing average radius of 1.42  ±  0.10 nm to 1.81  ±  0.14 nm over a period of 48 h. We have also assessed a carboxylic acid derivative of ADOTA (N-(3-carboxypropylene)-ADOTA tetrafluoroborate-Acid-ADOTA) using dSTORM super-resolution microscopy. Although demonstrating high photochemical stability and blinking, its lower brightness and relative propensity to aggregate limits Acid-ADOTA's use for dSTORM. (paper)