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

Hazel L Stewart, Philip Yip, Martin Rosenberg, Thomas Just Sørensen, Bo W Laursen, Alex E Knight, David J S Birch

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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 mins 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 hr, 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 hr. 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.
Original languageEnglish
Article number045007
Number of pages12
JournalMeasurement Science and Technology
Issue number4
Publication statusPublished - 14 Mar 2016


  • fluorescence lifetime
  • super-resolution microscopy
  • dSTORM
  • silica sol-gel
  • nanoparticle metrology
  • fluorescence anisotropy


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  • Instrumentation for fluorescence lifetime measurement using photon counting

    Birch, D. J. S., Hungerford, G., McLoskey, D., Sagoo, K. & Yip, P., 2 Jul 2019, Fluorescence in Industry. Pedras, B. (ed.). Cham, Switzerland: Springer, Vol. 18. p. 103-133 31 p. (Springer Series on Fluorescence; vol. 18).

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