Abstract
The refractive index (RI) is a fundamental parameter of materials that can be used to distinguish and sort materials of different nature. Although the RI of a virus is required for many optics-based biosensing applications, RIs of animal viruses have never been measured. Here we have developed a technique that can measure the RI of individual viruses in aqueous media with high precision. This technique is based on optical trapping of single virions and works by relating the size and RI of a single virus to the stiffness of an optical trap. We have derived an analytic expression to quantitatively describe the optical trapping of these particles. We have validated this equation using nanoparticles of known RI, and measured the RI of individual human immunodeficiency viruses type-1, which yielded a value of 1.42 at 830 nm with less than 2% coefficient of variation. This value is much lower than the RI typically assumed for viruses, but very close to that of 2.0 M sucrose solution in water. To the best of our knowledge, this is the first report on the experimental measurement of the RI for a single animal virus in aqueous media. This technique does not require prior knowledge on the diameter of the nanoparticles, and can be applied to other viruses or nanoparticles for accurate measurement of RI that is critical for the label-free detection of these particles in various settings.
Highlights
The refractive index (RI), defined as the ratio between the speeds of light in vacuum relative to that in the material of interest, is a fundamental parameter of materials
We present a new method based on optical tweezers that allows experimental measurement of the RI of a spherical nanoparticle with high precision, and apply it to individual human immunodeficiency viruses type-1 (HIV-1) virions
To demonstrate the quantitative dependence of optical trap stiffness on the RI of the trapped nanoparticle, we have chosen to work with nanospheres made of polystyrene or silica, whose size are close to those of animal viruses but with known RI
Summary
The refractive index (RI), defined as the ratio between the speeds of light in vacuum relative to that in the material of interest, is a fundamental parameter of materials. Because RI is related to the composition of materials, it potentially offers a label-free parameter for distinction and sorting of materials of different nature [1,2,3]. Biological materials are known to display heterogeneity in RI [5]. As a result, it is difficult and often not straightforward to measure the RI of biological materials with high precision [6, 7]. As a matter of fact, there is no experimental measurement on the RI for individual virus particles. How heterogeneous these particles are in their RI values is not known
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