Metal nanoparticle (NP) colloids with diameters (d) between 1-100 nm posses a unique optical absorption that is related to the oscillation of surface electrons. This surface plasmon resonance (SPR) property is dependent on the size and shape of the NP as well as the surrounding medium. Since the SPR band is a characteristic of the NP composition and morphology, it is often the first property tested to judge synthesis success. Subsequently, the SPR is used to monitor a number of characteristics, such as concentration. Gold NPs (AuNPs) have extinction coefficients >107 M-1cm-1, when d > 10 nm, making them very strongly absorbing. It is therefore difficult to measure absorbance of highly concentrated samples using a 1 cm pathlength. In addition, since many of these AuNPs are used in self-assembly studies, in which the NPs have undergone tedious functionalization and purification, often only small volumes are available.
Thus, an instrument capable of measuring high AuNP concentrations at small sample volumes is ideal. The Thermo Scientific NanoDrop 2000 UV-Vis Spectrophotometer has proven to be particularly useful in this regard because of the instrument’s short pathlengths (0.05 -1.0 mm) and microliter sample volume requirement. Here, we show that it is possible to accurately quantify the SPR band of two AuNPs (d = 13 nm, d = 52 nm) over a wide range of concentrations.
The AuNPs used in these experiments were synthesized following two procedures. The AuNP with d = 13 nm were synthesized via a standard sodium citrate reduction of gold (III) chloride. The AuNP with d = 52 nm were synthesized by a seed-mediated growth approach. The NanoDrop™ 2000 was first used to determine the approximate diameter by measuring the SPR band of each AuNP. For example, the AuNP with d = 13 nm had a SPR maximum of 520 nm, whereas the AuNP with d = 52 nm showed SPR of 533 nm, as previously reported.