Abstract
Gold nanorods (GNRs) are of particular interest for biomedical applications due to their unique size-dependent longitudinal surface plasmon resonance band in the visible to near-infrared. Purified GNRs are essential for the advancement of technologies based on these materials. Used in concert, asymmetric-flow field flow fractionation (A4F) and single particle inductively coupled mass spectrometry (spICP-MS) provide unique advantages for fractionating and analyzing the typically complex mixtures produced by common synthetic procedures. A4F fractions collected at specific elution times were analyzed off-line by spICP-MS. The individual particle masses were obtained by conversion of the ICP-MS pulse intensity for each detected particle event, using a defined calibration procedure. Size distributions were then derived by transforming particle mass to length assuming a fixed diameter. The resulting particle lengths correlated closely with ex situ transmission electron microscopy. In contrast to our previously reported observations on the fractionation of low-aspect ratio (AR) GNRs (AR < 4), under optimal A4F separation conditions the results for high-AR GNRs of fixed diameter (≈20 nm) suggest normal, rather than steric, mode elution (i.e., shorter rods with lower AR generally elute first). The relatively narrow populations in late eluting fractions suggest the method can be used to collect and analyze specific length fractions; it is feasible that A4F could be appropriately modified for industrial scale purification of GNRs.
Highlights
Nanotechnology is having a substantial impact on a broad range of applications, from consumer products to environmental remediation to advanced electronics and coatings
Our research team has previously tackled the major challenges of separating gold nanorods (GNRs) with A4F by optimizing mobile phase conditions, determining the best channel and cross flow (Vx) for separation and recovery, and overcoming limitations of the diode array detector (DAD) [36]
While it is possible to synthesize high-aspect ratio (AR) GNRs, it is nearly impossible to obtain a finished product without non-cylindrical byproducts [8,36,37,38,39,40]
Summary
Nanotechnology is having a substantial impact on a broad range of applications, from consumer products to environmental remediation to advanced electronics and coatings. The quality and purity of GNRs, with respect to their length and diameter, is essential to their utilization and commercial viability, because the characteristic longitudinal SPR (LSPR) is highly sensitive to aspect ratio (AR). There has been a substantial effort to develop optimized synthesis conditions that yield these highly prized nanomaterials in purified form. While methods such as electrochemical deposition in anodic membranes [9] provide well defined and dispersed nanorods, the synthesis method is labor intensive and can only produce small-scale quantities of GNRs (i.e., it is not scaled up to commercial production levels). Large-scale (mostly wet chemical) synthesis methods exist, such as seed mediated growth, that have the capacity to yield liters of product, but the as-produced dispersions inevitably contain a mixture of polydispersed GNRs and unintentional byproducts comprising other geometries
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