Colloids and Surfaces A: Physicochemical and Engineering Aspects
Time dependent spectral change upon potential step perturbation for Au nanoparticles immobilized on an organic monolayer-modified ITO electrode
Introduction
Nanoparticles of noble metals such as Au and Ag exhibit a plasmon absorption band in the visible wavelength region. The absorption spectrum is sensitive to various factors such as particle size [1], [2], [3] and shape [4], [5], particle charge [6], [7], [8], [9], [10], aggregation state [11], [12], and physicochemical micro-environment of the surrounding medium [13], [14], [15], [16]. The increase of the electron density on the particle results in the increase of the plasma frequency of the electrons in the particle, leading to a blue shift of the plasmon absorption band. Particle aggregation causes the appearance of an additional low-energy absorption peak, which is ascribable to coupled plasmons of interacting particles. The increase of the refractive index of the surrounding medium results in a red shift in plasmon band position. Regulation of the spectrum through the construction of nano-ordered two-dimensional lattices is among the current topics.
Clarification of the spectral change of a metal particle at an electrified interface is of importance in the application to chemical sensors. In the sensor application, shortening of the response time is highly desirable as well as the sensitization and quantification of the plasmon absorption change. To mention just one example, Nath and Chilkoti detected steptavidin at a detection limit of 16 nM using the absorbance change of the localized surface plasmon resonance for the biotin-functionalized Au colloidal particle immobilized on a siloxane monolayer-modified glass substrate (Au-biotin) [17]. When they monitored the absorbance change at a wavelength of 550 nm due to the local refractive index change upon the initiation of the binding between streptavidin and Au-biotin, the full absorbance change typically took over 30 min. In a number of reports of the sensor application of the plasmon absorption, however, the response time has not been described explicitly.
Immobilization of metal nanoparticles on an optically transparent electrode is well-suited to measure the potential dependence of the plasmon absorption spectrum of the particles both statically and dynamically. Especially, potential dependent particle charge, aggregation state, and microenvironment can be highlighted. In our previous report, the potential dependence of plasmon absorption band was described using the results of the measurements of sine-wave potential-modulated UV–vis transmission–absorption (PMTA) signal for Au nanoparticles on a modified ITO electrode in 0.1 M phosphate buffer solution at pH 7.0 [10]. The observed spectral change was mainly due to the charging–discharging process of the particles. Negative change of the electrode potential results in the increase of the electron density on the particle. This leads to the increase of the plasma frequency of the electrons in the particle. As a result, the plasmon absorption is enhanced and shifted to blue at more negative potentials. Our combined analysis of frequency dependencies of PMTA signal and charging current has demonstrated that the absorbance change at frequencies higher than 5 Hz is due to the charging–discharging. The spectral change well corresponds to the charging of a 11 nm particles by a rate of 1500 electrons V−1, in good agreement with the Mie-Drude optical theory [6].
We herein focus on the response time of the plasmon spectral change of Au nanoparticles under potential control. As mentioned above, the accurate grasp of the apparent end-point or equilibrium point of the spectral change is highly important in sensing business. The initial event of opto-electronic process should be usually a rapid process (≤10−15 s). Apparent slow response may originate from interfacial electron transfer, chemical process, mass transfer, or other processes.
In our recent extension of the study to confirm the time-dependent spectral change as a response to potential step for the Au nanoparticles on an ITO electrode, we have found a very slow relaxation taking over 400 s under certain conditions. We herein describe the nature of the slow relaxation in detail using the results of potential step measurements of the transients of the absorbance at various conditions.
Section snippets
Chemicals
Hydrogen tetrachloroaurate(III) tetrahydrate (Wako), trisodium citrate dihydrate (Kanto), 4-aminobutyltriethoxysilane (Gelest), and 3-mercaptopropyltrimethoxysilane (TCI) as well as all other chemicals were of reagent grade and used as received. Water was purified through a Milli-Q Plus Ultrapure water system coupled with an Elix-5 kit (Millipore). Its resistivity was over 18 MΩ cm.
Preparation of Au particles
Citrate-stabilized Au nanoparticles (TEM diameter, 11.5 ± 1.1 nm; plasmon absorption maximum of the colloidal solution,
Results
An ABSiO-ITO electrode (Fig. 1A), on which Au nanoparticles of an amount of 1.2 × 1012 particles cm−2 were immobilized, was subjected to absorption spectral measurements at various constant potentials in Pi solution. Representative spectra at two different potentials are shown in Fig. 1B. The plasmon absorption band was observed with its maximum at a slightly longer wavelength than that of the Au colloidal solution (line c). Within the full potential range used (−0.6 V ≤ E ≤ 1.0 V), monotonic change of
Discussion
A very slow relaxation of the absorption spectrum of Au nanoparticles immobilized on two different siloxane-modified ITO electrodes was found in the potential step response. The features of the relaxation of the absorbance can be summarized as follows. The absorbance transient obtained by potentials step is described by Eq. (1), which involves a term of a single exponential component with a long relaxation time constant ranging from 44 s to over 200 s. Both the content ratio of slow component and
Conclusion
We have described herein the time-dependent spectral change as a response to potential step for the Au nanoparticles immobilized on an ITO electrode. The transient of absorbance in response to the potential step was a sum of a fast component and a slow single exponential component. The half-life of the slow component and the ratio of the two components depended on the incident wavelength and the direction of the potential step. The content of the slow relaxation in the total spectral change was
Acknowledgements
This work was financially supported in part by a Grant-in-Aid for Scientific Research on Priority Areas (Area no. 417 for T.S.) and Scientific Research B (no. 16350077 for T.S.) from the Ministry of Education, Culture, Sports, Science, and Technology. Financial supports from Yazaki Memorial Foundation for Science and Technology and Iketani Science and Technology Foundation are also acknowledged.
References (31)
- et al.
Chem. Phys. Lett.
(2001) - et al.
J. Electroanal. Chem.
(2004) - et al.
Electrochim. Acta
(1998) - et al.
Electrochim. Acta
(2002) - et al.
J. Electroanal. Chem.
(1996) - et al.
Surf. Sci.
(2004) - et al.
Surf. Sci.
(2000) - et al.
Surf. Sci.
(2000) - et al.
Phys. Stat. Sol. (a)
(1999) - et al.
J. Phys. Chem. B
(1999)
Langmuir
J. Am. Chem. Soc.
Adv. Mater.
Langmuir
Langmuir
Cited by (5)
Long range self-organisations of small metallic nanocrystals for SERS detection of electrochemical reactions
2020, Journal of Electroanalytical ChemistryCitation Excerpt :In the map presented in Fig. 3a, a non-monotonous shift of ca 40 nm is observed for the peak, a red-shift being observed below 0.10 V. Fig. 3b reports the maximum position as a function of potential to pinpoint this evolution. Previously, Schiffrin and Sagara observed a blue shift for decreasing potentials as expected from the theoretical considerations presented above [24–28]. In comparison to the results obtained with silver, the important and reversible changes suggest that electrolyte can better penetrate in the ligand layer that encapsulates the NCs, leading to a larger interfacial capacitance in comparison with dodecanethiol.
Influence of ions on dynamic response of surface plasmon resonance fiber optic sensor
2013, Sensors and Actuators, B: ChemicalCitation Excerpt :Firstly, the surface energy of thin gold film is higher than that of bulk gold [18]. Cl− anions experience spontaneous oxidation and transfer electron to gold and form almost a zerovalent chloride adsorption layer [10,13,19,20]. Cl− anions undergoes a specific adsorption on gold surface.
Time-dependent scattering of ultrathin gold film under potential perturbation
2012, ACS Applied Materials and Interfaces