Anal. Chem. 2003, 75, 6759
A Sorptive Behavior of Monolayer-Protected Gold Nanoparticle Films Containing Alkanethiols and Alkanedithiols Recently we reported data on vapor uptake by films of monolayer-protected gold nanoparticles (MPNs), using the responses of MPN-coated thickness shear mode (TSM) devices to calibrated vapor streams.1 These uptake measurements were compared to those of sorptive polymers to determine whether these new nanomaterials were more or less sorptive than typical polymers. We found them to be less sorptive than the selected polymers on the basis of the mass of the vapor sorbed per mass of sorptive material. In terms of the mass of vapor sorbed per volume of sorptive material, considering that the nanoparticle materials are more dense than typical organic polymers, the nanoparticle films and polymer films were comparable. The vapor concentrations in these studies were reported in standard units of milligrams per cubic meter. In the course of the review of the manuscript, we were asked to report more quantitative data, which were provided in Table 3 of our revised manuscript as sensitivities in units of hertz per milligrams per cubic meter. A reviewer also suggested that we compare our results to those reported by Zhong et al.2 The latter authors had prepared networked nanoparticle films by using dithiols to displace monothiols from MPNs, and they measured both resistance changes and vapor uptake in response to a number of vapors. Vapor uptake data from TSM measurements were plotted as -“∆f (Hz)” versus “C (ppm)” in the figures in their paper, and the slopes were tabulated as “∆f/∆C”, with no units provided, in Table 3 of their paper. The experimental section of their paper says that, “The ppm concentration was calculated from the partial vapor pressure and the mixing ratio”. Within the text, concentrations are denoted with Cppm. We took ppm to be the standard ppm by volume typically used for analytical gas-phase vapor concentrations and compared the toluene vapor uptake properties of a monothiolderivatized dodecanethiol-protected gold nanoparticle film as measured in our experiments with a dithiol-derivatized nonanedithiol-protected gold nanoparticle film as reported by Zhong et al. We reported that, “The toluene sensitivities of these networked alkanedithiol-protected nanoparticle films were reported as -0.18 and -0.24, apparently in hertz per ppm, for films with 5- and 2-nm gold cores, respectively. In vapor concentrations of milligrams per cubic meter, these sensitivities become -0.048 and -0.064 Hz per mg/m3. ...The toluene (1) Grate, J. W.; Nelson, D. A.; Skaggs, R. Anal. Chem. 2003, 75, 1868-1879. (2) Han, L.; Daniel, D. R.; Maye, M. M.; Zhong, C.-J. Anal. Chem. 2001, 73, 4441-4449. 10.1021/ac030280b CCC: $25.00 Published on Web 10/23/2003
© 2003 American Chemical Society
sensitivity of the dodecanethiol-protected nanoparticle film in the present study was -0.006 Hz per mg/m3.” We concluded that “the nanostructured networked films may yield greater vapor sorption than films of the simpler alkanethiol-protected gold nanoparticles” and that the comparison “suggests that the detailed nanostructure may influence the sorptive strength of the nanoparticle materials”. On a careful reexamination of the Experimental Section of the paper of Zhong et al., it is apparent that the ppm units are unlikely to have been ppm by volume. Concentrations from a saturated vapor source, in ppm by volume, are calculated from the partial pressure of the saturated vapor using eq 1,3 where P is the total experimental pressure (typically 1 atm) and pn is the partial vapor pressure of the nth component.
Cppm ) (106pn)/P
(1)
Using the saturated partial vapor pressure of 0.0333, as reported by Zhong et al., the Cppm by eq 1 is 33 300 ppm.4 Experimentally, Zhong et al. reported bubbling dry nitrogen through a bubbler containing the vapor solvent at flow rates from 5 to 50 mL/min, with additional nitrogen added to a total of 100 mL/ min. Therefore, their toluene vapor concentration range in ppm by volume would be 1660-16 600 ppm. By contrast, the data plots indicate toluene concentrations spanning a decade with a maximum concentration of about 700. These numbers are consistent with ppm defined as moles per liter of toluene in the gas phase times 106. Units of ppm by the latter method differ from ppm by volume by a factor of 24.4. Given these considerations, the vapor uptake sensitivities of the networked nanoparticle films containing dithiols are approximately -0.002 and -0.003 Hz per mg/m3 rather than -0.048 and -0.064 Hz per mg/m3 and, thus, somewhat less than those of films of the dodecanethiol-protected gold nanoparticles reported as 0.006 Hz per mg/m3 in our own paper.5 Therefore, our conclusion that the “the nanostructured networked films may yield greater vapor sorption than films of the simpler alkanethiol-protected gold nanoparticles” was incorrect, and we do not have evidence to conclude that the detailed nanostructure is responsible for differences in vapor uptake between the considered materials. This correspondence is provided as a correction to the conclusions we reached assuming that ppm in Zhong’s paper meant ppm by volume.
Jay W. Grate
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352 AC030280B (3) Nelson, G. Gas Mixtures Preparation and Control; Lewis Publishers: Boca Raton, FL, 1992. (4) We agree that 0.0333 atm is a reasonable number for the saturated vapor pressure of toluene at room temperature. (5) For example, from Figure 3 in Zhong’s paper, it is seen that there is a response of 140 Hz at the highest test concentration indicated as about 700 ppm in his units. Assuming the highest concentration was in fact 16 600 ppm in conventional ppm by volume units, this response would become 140 Hz per 16 600 ppm in ppm by volume, which would be 140 Hz per 62 500 mg/m3, or 0.0022 Hz per mg/m3.
Analytical Chemistry, Vol. 75, No. 23, December 1, 2003 6759
