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Measuring the size and charge of single nanoscale objects in solution using an electrostatic fluidic trap

Abstract

Measuring the size and charge of objects suspended in solution, such as dispersions of colloids or macromolecules, is a significant challenge. Measurements based on light scattering are inherently biased to larger entities, such as aggregates in the sample1, because the intensity of light scattered by a small object scales as the sixth power of its size. Techniques that rely on the collective migration of species in response to external fields (electric or hydrodynamic, for example) are beset with difficulties including low accuracy and dispersion-limited resolution2,3,4. Here, we show that the size and charge of single nanoscale objects can be directly measured with high throughput by analysing their thermal motion in an array of electrostatic traps5. The approach, which is analogous to Millikan's oil drop experiment, could in future be used to detect molecular binding events6 with high sensitivity or carry out dynamic single-charge resolved measurements at the solid/liquid interface.

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Figure 1: Experimental set-up and device geometry.
Figure 2: Analysis of the three-dimensional motion of particles trapped in a harmonic potential well.
Figure 3: Dependence of trapping free energy on particle charge.
Figure 4: MSD measurements and deducing the charge on a single particle.

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Acknowledgements

M.K. acknowledges support from the European Commission in the form of a Marie Curie Research Fellowship. T. Savin and R. Klemm are thanked for fruitful discussions and D. Weitz for hospitality at the time of writing. The authors thank V. Sandoghdar for sustained support.

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N.M. performed the experiments, analysed data and participated in writing of the manuscript. M.K. conceived the project, performed the theoretical analysis, analysed data and wrote the manuscript.

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Correspondence to Madhavi Krishnan.

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The authors declare no competing financial interests.

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Mojarad, N., Krishnan, M. Measuring the size and charge of single nanoscale objects in solution using an electrostatic fluidic trap. Nature Nanotech 7, 448–452 (2012). https://doi.org/10.1038/nnano.2012.99

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