Particles able to absorb or scatter light of well-defined colors have been used in applications involving detection, absorption, or scattering of light, for example medical diagnostic imaging. Such particles are typically colloidal metal particles. The term colloidal conventionally refers to the size of the particles, generally denoting particles having a size between about 1 nanometer and about 1 micron.
Small particles made from certain metals that are in the size range of colloidal metal particles tend to have a particularly strong interaction with light, termed a resonance, with a maximum at a well-defined wavelength. Such metals include gold, silver, platinum, and, to a lesser extent, others of the transition metals. Light at the resonance wavelength excites particular collective modes of electrons, termed plasma modes, in the metal. Hence the resonance is termed the plasmon resonance.
By selecting the metal material of a colloidal particle, it possible to vary the wavelength of the plasmon resonance. When the plasmon resonance involves the absorption of light, this gives a solution of absorbing particles a well-defined color, since color depends on the wavelength of light that is absorbed. Solid gold colloidal particles have a characteristic absorption with a maximum at 500-530 nanometers, giving a solution of these particles a characteristic red color. The small variation in the wavelength results from a particle size dependence of the plasmon resonance. Alternatively, solid silver colloidal particles have a characteristic absorption with a maximum at 390-420 nanometers, giving a solution of these particles a characteristic yellow color.
Using small particles of various metals, particles can be made that exhibit absorption or scattering of selected characteristic colors across a visible spectrum. However, a solid metal colloidal particle absorbing in the infrared is not known. Optical extinction, in particular absorption or scattering, in the infrared is desirable for imaging methods that operate in the infrared. Further, optical communications, such as long distance phone service that is transmitted over optical fibers, operate in the infrared.
It has been speculated since the 1950's that it would be theoretically possible to shift the plasmon resonance of a metal to longer wavelengths by forming a shell of that metal around a core particle made of a different material. In particular, the full calculation of scattering from a sphere of arbitrary material was solved by Mie, as described in G. Mie, Ann. Phys. 24, 377 (1908). This solution was extended to concentric spheres of different materials, using simplifying assumptions regarding the dielectric properties of the materials, by Aden and Kerker, as described in A. L. Aden and M. Kerker, J. of Applied Physics, 22, 10, 1242 (1951). The wavelength of the plasmon resonance would depend on the ratio of the thickness of the metal shell to the size, such as diameter of a sphere, of the core. In this manner, the plasmon resonance would be geometrically tunable, such as by varying the thickness of the shell layer. A disadvantage of this approach was its reliance on bulk dielectric properties of the materials. Thus, thin metal shells, with a thickness less than the mean free path of electrons in the shell, were not described.
Despite the theoretical speculation, early efforts to confirm tunability of the plasmon resonance were unsuccessful due to the inability to make a particle having a metal shell on a dielectric core with sufficient precision so as to have well-defined geometrical properties. In these earlier methods, it was difficult to achieve one or both of monodispersity of the dielectric core and a well-defined controllable thickness of a metal shell, both desirable properties for tuning the plasmon resonance. Thus, attempts to produce particles having a plasmon resonance in keeping with theoretical predictions tended to result instead in solutions of those particles having broad, ill-defined absorption spectra. In many instances this was because the methods of making the particles failed to produce smooth uniform metal shells. Rather, the methods tended to produce isotropic, non-uniform shells, for example shells having a bumpy surface.
However, one of the present inventors co-developed a novel method of making metallized nanoparticles (particles with a size between about 1 nanometer and about 5 microns) that was successful in producing metal-coated particles having narrow well-defined spectra. Further, one of the present inventors co-discovered that improved agreement with theoretical modeling of the metallized nanoparticles resulted from the incorporation in the theory of a non-bulk, size-dependent value of the electron mean free path. That is, improved agreement with theory was achieved by developing an improved theory applicable to thin metallization layers. Thus, in the improved theory a dependence of the width of the plasmon resonance on the thickness of the metallization layer was described.
Particles having at least one substantially uniform metallization layer have been termed metal nanoshells. Nanoshell structures that exhibit structural tunability of optical resonance's from the visible into the infrared can currently be fabricated. For example, complete nanoparticle metallization shell layers containing gold have been demonstrated. Gold has the advantage of a strong plasmon resonance that can be tuned by varying the thickness of the coating. More generally, the resonance may be tuned by varying either the core thickness or the thickness of the coating, in turn affecting the ratio of the thickness of the coating to the thickness of the core. This ratio determines the wavelength of the plasmon resonance. A further advantage of gold-coated particles is that they have shown promise as materials with advantages in imaging and diagnostics. In particular, they have utility as band-pass optical filters, impeding the photo-oxidation of conjugated polymers, and in conjunction with sensing devices based on surface enhanced Raman substrates.
Present methods for making nanoshells involve purifying suspensions of various intermediates, as well as purifying a suspension of the nanoshell products. Methods of making nanoshells are disclosed, for example, in U.S. Pat. No. 6,344,272 and in S. Oldenburg, R. D. Averitt, S. Westcott, and N. J. Halas, “Nanoengineering of Optical Resonances”, Chemical Physics Letters 288, 243-247 (1998), each hereby incorporated herein by reference. In particular, a method for making nanoshells may include coating linkers onto substrate particles so as to form linker-coated nanoparticles, seeding metal colloids onto the linkers so as to form colloid-seeded nanoparticles, and reducing metal onto the metal colloids so as to form nanoshells. Further, at various stages, the method includes purification of linker-coated nanoparticle suspensions, purification of colloid-seeded nanoparticle suspensions, and purification of nanoshell suspensions. Corresponding undesirable byproducts correspondingly include, for example, excess linkers, excess metal colloids, and excess metal ions, respectively.
Present methods for purifying nanoshells and intermediates thereof rely on a technique for purifying product or intermediate thereof that involves centrifugation and redispersal. The centrifugation and redispersal may be repeated a suitable number of times to achieve the desired level of purity. A disadvantage of this conventional laboratory method is that it is difficult to scale up to commercial scale. Thus, a scalable method of purifying nanoshells and intermediates thereof is desirable.
Replacement of combined centrifugation and redispersal with filtration is known to those skilled in the art of purification. Filtration has the advantage that it is known to be a scalable method of purification, in particular filtration may be scaled up to commercial scale. However, previous laboratory scale attempts to use filtration to separate excess colloid from colloid-seeded nanoparticles serving as intermediates in making nanoshells have been unsuccessful. This lack of success occurred despite the typical size disparity between excess metal colloid, typically between about 1 nm and 5 nm, and colloid-seeded nanoparticles, typically between 50 nm and 5 μm. In particular, attempts by the present inventors to crossflow filter a suspension, as it results from the seeding reaction, of colloid-seeded nanoparticles and excess metal colloid using a filter having a 50 nm nominal pore size have demonstrated ineffective passage of the excess colloids through the filter in the filtrate. Rather, sufficient excess colloids were found to remain in the retentate with the colloid-seeded nanoparticles that subsequent reduction of metal so as to form metal nanoshells was unsuccessful.
Thus, notwithstanding the above-described teachings, there remains a need for a scalable method of making nanoshells, particularly when the method involves intermediate colloid-seeded nanoparticles.