Electrochemical Molecular Recognition System Utilizing Current Amplification of Metal Complexes Assembled on Gold Nanoparticles
Authors: Endo Akira , Hashimoto Takeshi , Hayashita Takashi 
Published: February 2020

Introduction

Because gold nanoparticles (GNP) have a characteristic color and the color changes by their aggregation, the optical molecular recognitions are widely investigated by using a color change. However, we reported that the electrochemical sugar recognition system using tris(acetylacetonato)ruthenium complex and boronic acid assembled on the GNP showed a glucose-selectivity and higher sensitivity in 10-6 M order than optical recognition.1) It is considered that one of the reasons of high sensitivity may be the amplification of the reduction current of tris(acetylacetonato)ruthenium complex by assembly on the GNP. This electrochemical molecular recognition system has a great advantage that the molecular recognition site can exchange to others according to the target molecules. Furthermore, the metal complex can also change to the others according to avoid the disturbance from coexistent redox active species. That is, the redox potential can select by changing the redox active complex.

Purpose

For the purpose of the verification of the current amplification and the utility of this electrochemical molecular recognition system using the GNP, in this work, ferrocene (Fc), tris(acetylacetonato)ruthenium ([Ru(acac)3]), bis(acetylacetonato)ruthenium ([Ru(acac)(BP)2], BP = pyridine boronic acid), and bipylidine ruthenium ([Ru(py)(bpy)(trpy)]) were assembled on the GNP as electroactive site. Boronic acid (BA) for recognition of sugar, dipicolilamine (DPY) for recognition of phosphate derivatives (ADP, ATP, etc.), succinimide (SCI) for recognition of dopamine (DA), and crown ether (CE) for recognition of cesium ion were assembled on the GNP. Thus, the six kinds of GNP complexes (GNP/[Ru(acac)(BP)2]/BA, GNP/Fc/BA, GNP/[Ru(py)(bpy)(trpy)]/BA, GNP/[Ru(acac)3]/DPY, GNP/[Ru(acac)3]/CE), and GNP/[Ru(acac)3]/SCI) prepared and investigated molecular recognition functions. Osteryoung square wave voltammetry (OSWV) or differential pulse voltammetry (DPV) were used for the electrochemical measurements.

Results

1. Current Amplification: In order to confirm the current amplification, the concentration dependence of the reduction current of the [Ru(acac)3] and of GNP/[Ru(acac)3] were measured and their slopes were compared. The slope for GNP/[Ru(acac)3] was ca. 29 times larger than that for [Ru(acac)3]. For oxidation current of ferrocene, the slope for GNP/Fc was ca. 7 times larger than that for Fc. In the silver nanoparticle system, further current amplification was observed, so it is considered that the kind of nanoparticle seem to affect the current amplification. The mechanism of the current amplification is not yet clear, but the electron tunneling due to the electron conductivity of the GNP may be considered as main factor of the current amplification mechanism.
2. Molecular Recognition: In the electrochemical measurement, the current values decreased by adding the target molecules. In the case of sugar recognition by BA, GNP/[Ru(acac)3]/BA showed highest sensitivity among three types of GNP complexes (GNP/[Ru(acac)3]/BA, GNP/Fc/BA, and GNP/[Ru(py)(bpy)(trpy)]/BA). Sensitivity of [Ru(py)(bpy)(trpy)] was slightly less than two metal complexes. Each complex showed different selectivity for three kinds of sugars. For GNP/[Ru(acac)(BP)2]/BAsystem (recognition site (BP) is on the Ru complex), sensitivity is higher than that of GNP/[Ru(acac)3]/BA, although the selectivity did not show for three kinds of sugar.

GNP/[Ru(acac)3]/(DPY+Zn2+) could be recognized phosphate derivatives such as AMP, ADP, and ATP with high sensitivity (10-6 M order) in the presence of Zn2+. Furthermore, slight ATP selectivity was obtained.

GNP/[Ru(acac)3]/CE showed an excellent sensitivity for cesium ion (10-9 M order). In this case, it was consider that the aggregation caused by CE ring sandwiching for cesium ion increased the sensitivity, because the CE (18-crown-6) ring size was smaller than cesium ion diameter. In this case, two types of GNP (type A and type B) complexes were prepared. In type A, CE probe has longer alkyl chain than [Ru(acac)3] probe, and in type B CE probe has same length as [Ru(acac)3] probe. Type A may aggregate more than type B, because there is no steric obstruction by [Ru(acac)3]. As a result, the detection limit of the type A is 0.4 nM and 10 times as sensitive as type B.

For the recognition of dopamine, the aggregation of GNP is brought about by the complexation of dopamine in the presence of Fe3+. As a result, high sensitivity was attained (LOD = 3 nmol dm-3).
By using this electrochemical molecular recognition system, target molecules could be recognized with high sensitivity of mM – nM. The sensitivity and selectivity was depending on the kind of electroactive metal complex. Thus, it was verified the utility of this electrochemical molecular recognition system using metal complexes assembled on gold nanoparticles.
1) A.Endo, M. Kimura, T. Hashimoto, and T. Hayashita, Anal. Methods, 2014, 6(22), 8874-8877.

Figure 1