Design and Synthesis of Model Transparent Aqueous Colloids with Optimal Scattering Properties

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Soft Matter Keywords

colloids, hydrogels, methods for studying self-assembly, fluorescent tracers

Summary

Perro et al. designed a colloidal particle that allows for precision position tracking while minimizing optical scattering in an aqueous solution. By allowing tuning of both the diameter and the optical properties of the particle, this colloid can be optimized to function as a tracer for microrheology or studies of real-space phase behavior [1]. This is achieved though a core-shell particle structure; the fluorescent core can be imaged via confocal microscopy to achieve high spatial resolution, while the shell is index matched with the solution (water).

Fig 1: The flourescent core/minimally scattering shell particles suspended in water; note how only the flourescent cores are visible using standard bright-field microscopy. The inset shows the same sample after the water has been allowed to evaporate, and the PNIPAM-co-AAc shells --which were index-matched to water over visible wavelengths-- become visible. [1]

The particles are composed of a fluorescent polystyrene (PS) core and a hydrogel shell {poly(N-isopropylacrylamide-co-acrylic acid), or PNIPAM-co-AAc}. Other tracers using similar core-shell structures have already been synthesized, but this formulation minimizes scattering by maximizing shell thickness (producing effectively transparent <math>\mu</math>m scale particles). This study first explored the optical properties of the shell compound; in particular, the effect of cross-linker and comonomer concentration on the particle's size and optical properties. Adding acrylic acid (AAc) changed both doubled particle diameter and lowered its index of refraction (the AAc probably lowered the refractive index by reducing the density of the hydrogel). In addition, the shell's index of refraction was observed to increase by increasing the cross-linker concentration, so an adjustment of these parameters to match the refractive index of water, allowing the preparation of suspensions that are completely transparent (see Fig. 1)

Fig. 3: A) positions of flourescent cores in crystallite, obtained by confocal microscopy. B) x-y distributions of cores in a single z-section[1].
Fig 2: Left:Camera images of light reflected off crystallites obtained after 1 week of crystallite growth. Right: Demonstration of the transparency of the suspension after 1 week of crystallite growth; the quarter is visible through a 400 <math>\mu</math>m cell.


A possible application is use as a model colloid for real-space studies. At high volume fraction and after the addition of 60nm PNIPAM particles, the sample separates into two phases. The large particles formed a crystalline solid and the small particles remained fluid. After 1 week of crystal growth, the sample is still transparent, but an image of light reflected from the sample reveals the crystallites (see Fig. 2). Because the cores are fluorescently tagged, this crystal structure can be resolved in 3D using confocal microscopy (see Fig. 3) [1].

References

[1] Perro, A., Meng, G., Fung, J., & Manoharan, V. N. (2009). Design and Synthesis of Model Transparent Aqueous Colloids with Optimal Scattering Properties. Langmuir, 25(19), 11295-11298