Comment pubs.acs.org/ac
Reply to Comment on “Development of Measurement Technique for Ion Distribution in Extended Nanochannel by Super Resolution-Laser Induced Fluorescence” Yutaka Kazoe, Chih-Chang Chang, Kazuma Mawatari, and Takehiko Kitamori* Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-8656, Japan
Anal. Chem. 2011, 83 (21), 8152−8157. DOI: 10.1021/ac201654r
A
nzo and Okada1 commented on our results2 from a viewpoint of a conventional model of an electric double layer, which is a diffuse ion layer on a charged surface. We have developed a spatially resolved measurement method to study ion behavior in 100 nm-order space (extended nanospace) by using stimulated emission depletion (STED) microscopy and fluorescein as a pH indicator and reported that proton distribution in a 400 nm fused-silica square nanochannel (width = depth) was obtained for the first time.2 Anzo and Okada estimated our experimental results based on the Poisson−Boltzmann theory with the Gouy−Chapman model.3,4 Their fitting method based on the Poisson− Boltzmann theory well predicted the double layer with the proton distribution, when the double layer thickness estimated by the Debye length is sufficiently thin compared to the channel size. However, they suggested that the Gouy− Chapman model cannot interpret the result in case of pure water, where the double layers with 100 nm-order thickness overlap in the nanochannel. We consider that their Comment is very supportive and extends our report to conventional surface and solution chemistry. In the Gouy−Chapman model, the electric double layer has been considered as a charge distribution at an interface region in homogeneous liquid that is approximated to be the liquid at infinity.3,4 An important suggestion of Anzo and Okada is that this conventional model cannot totally explain our experimental results, especially when the double layer is comparable to the size of an extended nanochannel. Their claim is reasonable from the viewpoint of chemistry considering the far-field liquid, because there are still several points to be discussed such as a chemical effect of fluorescein molecules on the water. However, from our recent studies, we suggest the importance of near-field liquid structure and charge behavior at the interface region in extended nanospace. Our previous studies have revealed various unique properties of aqueous liquid confined in extended space, including higher viscosity, lower permittivity, and higher proton mobility.5−7 On the basis of these experimental results, we have considered a proton transfer phase with an approximately 50 nm scale of loosely coupled liquid molecules by a hydrogen bond, which exists between a 1−10 nm adsorption phase of oriented liquid molecules on the solid surface and bulk phase of a far-field liquid.8,9 This near-field polar protic solvent model, which we call the three phase model, suggests that the liquid near the solid surface has a heterogeneous structure of a 100 nm-order scale. Especially, when the Debye length is a 100 nm-order, the © 2012 American Chemical Society
proton transfer phase becomes dominant for the electric double layer. Therefore, the heterogeneity of aqueous liquid in an extended nanospace should be taken into consideration, in order to predict the electric double layer. Our group is establishing a numerical approach for proton distribution based on the Poisson−Boltzmann theory combined with the site dissociation model, which considers the specific properties in extended nanospace. The effect of specific liquid properties on formation of an electric double layer and proton distribution is discussed by comparing with the experimental results obtained by STED microscopy. This study will be published in the near future.
■
AUTHOR INFORMATION
Corresponding Author
*Phone: +81-3-5841-7231. Fax: +81-3-5841-6039. E-mail:
[email protected].. Notes
The authors declare no competing financial interest.
■
REFERENCES
(1) Anzo, K.; Okada, T. Anal. Chem. 2012, DOI: 10.1021/ ac302013b. (2) Kazoe, Y.; Mawatari, K.; Sugii, Y.; Kitamori, T. Anal. Chem. 2011, 83, 8152−8157. (3) Hunter, R. J. Zeta Potential in Colloid Science, Academic Press: London, 1981. (4) Probstein, R. F. Physicochemical Hydrodynamics: An Introduction, 2nd ed.; John Willy & Sons: New York, 1994. (5) Tsukahara, T.; Mawatari, K.; Kitamori, T. Chem. Soc. Rev. 2010, 39, 1000−1013. (6) Morikawa, K.; Mawatari, K.; Kazoe, Y.; Tsukahara, T.; Kitamori, T. Appl. Phys. Lett. 2011, 99, 123115. (7) Chinen, H.; Mawatari, K.; Pihosh, Y.; Morikawa, K.; Kazoe, Y.; Tsukahara, T.; Kitamori, T. Angew. Chem., Int. Ed. 2012, 51, 3573− 3577. (8) Tsukahara, T.; Hibara, A.; Ikeda, Y.; Kitamori, T. Angew. Chem., Int. Ed. 2007, 46, 1180−1183. (9) Tsukahara, T.; Mizutani, W.; Mawatari, K.; Kitamori, T. J. Phys. Chem. B 2009, 113, 10808−10816.
Published: November 12, 2012 10855
dx.doi.org/10.1021/ac302482g | Anal. Chem. 2012, 84, 10855−10855
