Development of Potentiometric Lspr Sensors with Au and Au@TiO2 Nanoparticles
Authors: Akiyoshi Kazutaka , Tatsuma Tetsu 
Published: February 2020

Introduction

Light incident on metal thin films or nanoparticles (NPs) may induce a collective oscillation of conduction electrons that can lead to generation of strong electromagnetic fields in the vicinity of the metal surfaces. This phenomenon is commonly referred to as surface plasmon resonance (SPR) for metal films and localized surface plasmon resonance (LSPR) for metal NPs. These plasmon resonances have been applied to affinity-based sensing due to their sharp response to refractive index changes in the vicinity of the metal surface. Although LSPR sensors can be smaller and less expensive than SPR sensors, they are not suitable for colored or turbid sample solutions in general because light must pass through the sample to an optical detector. In this study, LSPR sensors that directly output potential changes were developed (Figure 1).1 The sensors are based on plasmon-induced charge separation (PICS),2in which excited conduction electrons in metal NPs transfer to the semiconductor in direct contact with the NPs. Since light need not pass through the sample solution, the sensor can be applied to colored and turbid sample liquids.

Experimental

TiO2 thin films (~70 nm thick) were prepared on ITO-coated glass plates by a spray pyrolysis method from 2-propanol containing titanium diisopropoxide bis(acetylacetonate) followed by annealing at 500 °C for 30 min. Citrate-protected Au NPs (40 nm) were adsorbed onto the TiO2 surface by immersing the substrate in a Au NP dispersion at pH 2.7 for 6 h. Au@TiO2 core-shell NPs were also used instead of Au NPs. The Au NPs were covered with 5–7 nm-thick TiO2shells.
The open-circuit potential (OCP) measurements were conducted with the Au NP/TiO2/ITO working electrode in water (refractive index n = 1.33) and a Ag|AgCl reference electrode. The working electrode was irradiated with light of different wavelengths, and a negative potential shift upon the light irradiation was measured. Refractive index of the solution was changed by adding glycerol (n= 1.47) to water and peak shifts in the photopotential action spectra were detected.

Results and Discussion

a) Sensors with Au NPs. The Au NP/TiO2/ITO electrode was irradiated with light and a negative shift of the OCP was observed. The photoresponse is ascribed to injection of electrons from resonant Au NPs to the conduction band of TiO2 due to PICS. In Figure 2, typical potential shifts –ΔE at t = 60 s (light irradiation was started at t = 0 s) are plotted against the energy of irradiated photons. The photopotential action spectrum is in good agreement with the LSPR absorption spectrum in shape. Furthermore, the peak in the action spectrum was redshifted in response to an increase in the refractive index of the sample solution. This peak wavelength shift was plotted against the refractive index of solutions, and a linear relationship was obtained. The refractive index sensitivity was calculated to be 33 ± 4 nm RIU-1 (refractive index unit). The sensor was applied to coffee as a colored and turbid sample. The Au/TiO2film in coffee did not show any extinction peak but a clear photopotential peak.
b) Sensors with Au@TiO2 NPs. Since the refractive index sensitivity was not sufficiently high, we replaced the Au NPs with Au@TiO2 NPs. As a result, the sensitivity was increased to 72 nm RIU-1. In the case of the bare Au NPs, electron oscillation is somewhat localized at the bottom of each NP, at which the NP is in contact with TiO2, so that certain volume of the sensing region is occupied by the TiO2 substrate. On the other hand, the electron oscillation is delocalized over the whole NP surface for the Au@TiO2 NPs because the Au core is fully covered with the TiO2 shell, and therefore the masking by the TiO2substrate is less significant.

Conclusion and Future Plan

The LSPR sensor which directly output electrical signals from TiO2loaded with Au NPs was developed. The photopotential peaks were obtained on the basis of PICS. The response peak redshifted linearly as the refractive index of the sample solution increased. Since light need not pass through a sample solution, the sensor can be applied to a colored and turbid sample liquid. Work is currently underway to improve the sensor performance by using anisotropic NPs.

References

(1) T. Tatsuma, Y. Katagi, S. Watanabe, K. Akiyoshi, T. Kawawaki, H. Nishi, and E. Kazuma, Chem. Commun., 51, 6100 (2015).
(2) Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 127, 7632 (2005).

Figure Caption. (a) Mechanism of the potentiometric LSPR sensor. (b) Typical photopotential action spectra of a Au/TiO2 film.

Figure 1