Side‐by‐Side and End‐to‐End Gold Nanorod Assemblies for Environmental Toxin Sensing

Abstract

Controllable assembly of nanoscale building blocks (monomers) is a necessary part of practical realization of the unique optical, electrical, magnetic, and chemical properties of nanoscale matter in macroscale materials. Such assemblies also contain much fundamental information about collective behavior of nanocolloids, which we are just beginning to understand. The key decisive factors for the successful assembly of nanocolloids is the anisotropy of nanoscale interactions, which stems from both the shape of nanocolloids and unequal distribution of organic molecules on their surface. Gold nanorods (Au NRs) have both geometrical and chemical anisotropy components and demonstrate strong optical extinction in the range of visible and near-infrared (NIR) wavelengths convenient for both research and practical purposes. Au NRs can be assembled by interactions with organic molecules, polymers, an antibody–antigen reaction, biotin–strepavidin connectors, and DNA, leading to superstructures with different degree of organization and complexity of collective behavior. Besides the utilization of NR monomers in non-linear optics, cellular imaging, and cancer therapy, optical effects corresponding to monomer!superstructure transitions allowed preparation of excellent biosensors because of large changes in oscillation frequencies of plasmons when NR pairs are formed. These studies mostly targeted biomedical applications. Simultaneously, their unique sensing capabilities have been virtually unexplored for the needs of environmental detection and monitoring. These challenges and impact can equal or exceed those encountered in detection of cancer. A better understanding of methods for the realization of speed/selectivity/sensitivity detection of common environmental pollutants is thus of great importance. Therefore, we decided to explore the potential of NR assemblies taking a pervasive environmental toxin, namely microcystin-LR (MC-LR), as the model while also addressing the general questions about the choice of different assembly motif for different sensing tasks. MC-LR is common in both developed and developing countries, with recorded cases of mass poisoning. MC-LR originates from common bluegreen algae and causes rapid liver failure; prolonged exposure to small concentrations of MC-LR in drinking water causes liver cancer. Herein, we describe the successful use of Au NRs for detection of MC-LR, which is significantly more sensitive than the traditional techniques, such as ELISA, yielding detection limit of 5 pgmL . It is also much simpler and faster than any other methods. These two factors are critical for environmental monitoring and have been a long-standing challenge. The pattern of the assembly strongly affects the sensitivity parameters for MC-LR detection. To realize different modes of assembly, such as side-byside and end-to-end motifs, with a degree of control sufficient for conclusive evaluation of sensing implications, two kinds of protein-carrying Au NRs were synthesized (Figure 1). One type of NR carried MC-LR antibodies (ABs) preferentially on the sides, while the other type carried antibodies located almost exclusively in the ends. These motifs were formed by using either electrostatic binding or covalent attachment of the antibodies mediated by a bifinctional linker, thioctic acid (TA).When electrostatic forces govern the placement of ABs, they attach primarily to the sides of NRs due to the larger area of contact and thus stronger electrostatic interactions. When a TA anchor covalently binds by a S Au bond, the conjugation of the ABs occurs predominantly in the ends of the rods due to better accessibility of the gold surface to the reactive thiol end. Variation of pH also allows varying repulsion or attraction of NRs and MC-LR, modality of attachment, and geometrical characteristics of assemblies (see the Supporting Information). The number of AB molecules on the surface of one Au NR was estimated to be 31 and 10 for the side-by-side [*] L. Wang, Y. Zhu, L. Xu, Dr. W. Chen, H. Kuang, L. Liu, Prof. C. Xu School of Food Science and Technology, State Key Laboratory of Food Science and Technology Jiangnan University, Wuxi, 214122 (China) E-mail: xcl@jiangnan.edu.cn Dr. W. Chen, A. Agarwal, Prof. N. A. Kotov Department of Chemical Engineering, Department of Biomedical Engineering Department of Materials Science and Engineering University of Michigan, Ann Arbor, MI 48109 (USA) E-mail: kotov@umich.edu [] These authors contributed equally to this paper.