Resettable, Multi‐Readout Logic Gates Based on Controllably Reversible Aggregation of Gold Nanoparticles

Abstract

In recent years, more and more chemical systems have been employed to perform Boolean logic operations, resulting in the remarkable progress of various molecule-based logic systems, such as AND, OR, XOR, NAND, NOR, INHIBIT, half-adder, and half-subtractor. The set–reset function in molecular digital systems is an important step to construct memory elements of sequential logic operations that have the possibility to store information in a write–read–erasable form. Most of reported molecular logic gates have the following disadvantages: 1) they are not resettable; and 2) they employ small molecules or biomacromolecules as inputs and a fluorescent signal as the output, which relies on advanced instruments for the readout. Gold nanoparticles (AuNPs) have recently attracted considerable attention because of their many distinctive physical and optical properties. In particular, the surface plasmon resonance (SPR) of AuNPs can be changed by modulating the distance between AuNPs, along with the color change of AuNPs solutions. Therefore, by using AuNPs, the molecular events in logic systems can be easily transformed into color changes, which can be monitored by the naked eye. Moreover, the change of dispersion states of AuNPs can be further monitored by other tools such as UV/Vis spectra, zeta potential, and dynamic light scattering (DLS), which in turn can be applied as output signals. Herein we present a resettable and multi-readout logic system capable of several types of logic operations based on the aggregation of spiropyran-modified gold nanoparticles (spiropyran-AuNPs) in aqueous media. Spiropyrans are an important class of photoswitchable organic molecules. Under dark conditions or exposure to visible light, the spiropyrans exist in their colorless, nonplanar, and closed form (SP), which can be isomerized to the red, planar, and open merocyanine form (MC) with UV light irradiation by the heterocyclic ring cleavage of the spiro C O bond. The MC isomer has a phenolate group, which can readily bind with metal ions. The chelated metal ions can be released by visible light irradiation. We expect the closed form of spiropyran-AuNPs to be monodispersed in solutions that exhibit red color; after UV light irradiation followed with the addition of 10 mm of Cu, the formation of a sandwich between Cu and MC on different AuNPs can cause open merocyanine-modified AuNPs (MC-AuNPs) to aggregate, with a color change from red to purple. We expect that the combination of UV light irradiation and the subsequent addition of Cu to spiropyran-AuNPs solutions could be applied to construct an AND logic gate. Moreover, we can apply this system to construct other kinds of logic gates based on various input signals such as different kinds of metal ions and chelating ligands. The aggregates of MC-AuNPs can be redispersed upon exposure to visible light irradiation owing to the open-to-closed isomerization of spiropyran and the release of the “bridge” (metal ions), which generates a resettable AuNP-based logic system (Scheme 1). To realize the spiropyran-AuNP-based logic gates, we synthesized spiropyran-terminated alkanethiols (Supporting Information, Scheme S1), and tested the isomerization of spiropyran thiols in solution. We prepared the solution of the spiropyran thiols in ethanol at a concentration of 1 10 m. Spiropyran thiols were stored in the dark overnight at 0 8C, and the majority of the spiropyran molecules existed as the closed colorless isomer, which was switched into the openMC form with UV light irradiation; the maximum absorption at 550 nm was obtained after persistent irradiation for 20 seconds (Supporting Information, Figure S1). Addition of Cu (1 10 m) into the MC solution resulted in a change in the absorbance spectrum, with a new peak appearing at 420 nm along with the decrease of the absorption at 550 nm, which is due to the formation of the MC–ion complex (Supporting Information, Figure S2). AuNPs were prepared and modified with an alkanethiol terminated in triethylene glycol (EG3) and another alkanethiol terminated in spiropyran (EG3SP; Scheme 1). EG3 thiols were used to mix with EG3SP thiols to increase the water solubility of spiropyran-AuNPs and decrease the density of EG3SP on AuNP surface. The mixed alkanethiols adsorbed onto gold surfaces by means of ligand exchange. We found that if the ratio of EG3SP to EG3 was higher than 1:10, the AuNPs can readily aggregate in the process of preparing spiropyran-AuNPs. Upon decreasing the ratios from 1:10 to 1:100, the spiropyran-AuNPs can be well[*] D. B. Liu, W. W. Chen, K. Sun, Prof. K. Deng, Prof. W. Zhang, Prof. Z. Wang, Prof. X. Y. Jiang CAS Key Lab for Biological Effects of Nanomaterials and Nanosafety, National Center for NanoScience and Technology 11 Beiyitiao, ZhongGuanCun, Beijing 100190 (China) Fax: (+86)10-8254-5631 E-mail: wangz@nanoctr.cn xingyujiang@nanoctr.cn