Detection, counting, and imaging of single nanoparticles.

Abstract

Applications of nanomaterials, including nanoparticles, in analytical chemistry continue to grow in recent years. Consequently, many excellent reviews have been published, covering different aspects of nanomaterials for chemical analysis.1–6 The present review focuses on applications of single nanoparticles in analytical chemistry. Detecting, counting, imaging, and tracking of single nanoparticles represent a critically important capability in analytical chemistry. In addition to revealing basic properties of nanoparticle that could be otherwise washed out in the ensemble analysis of nanoparticles, the capability of single nanoparticle analysis offers many unique applications. Due to limited space, this review includes mainly works published from 2011 to 2013. Nanoparticles have been used in many analytical assays because of their extraordinary physical and chemical properties. Studies have shown that the physical (e.g., light emission and absorption7) and chemical (e.g., catalytic reactions) properties of a nanoparticle depend not only on its chemical composition, but also on its size and shape. For example, the optical responses of 10 different gold nanorods (AuNRs) modified with aptamers were found to vary by 3–4 times.8 In another study, highly anisotropic nanoparticles were found to be more sensitive to the binding to Hg2+ compared to the isotropic nanoparticles.9 By analyzing over 100 nanoparticles, Kim et al.10 discovered a correlation between the maximum extinction wavelength of a single AuNR and its sensitivity to a change in the surrounding refractive index. Yi et al.11 studied the evolution of optical scattering spectrum of Au nanoparticle-catalyzed reduction of 4-nitrophenol, and observed faster electron transfer rates in high-index elongated tetrahexahedral Au nanoparticles compared with those of low-index AuNRs. The size of a nanoparticle also affects its properties. For example, in a study of antibody binding to prostate-specific antigen-modified Au nanoparticles, smaller nanoparticles were found to be more sensitive than the larger nanoparticles.12 These results were consistent with the study of polarized nanoparticles, such as nanoplates13 and nanorods,14 whose corners and ends exhibited higher responses to ligand binding. These findings demonstrate a need for single nanoparticle analysis. The present review contains four sections, covering sensing, counting, imaging and tracking of single nanoparticles, respectively. In the section of single nanoparticle-based sensors, we discuss chemical sensing applications, where the signal transduction or readout is based on detecting a physical property, such as optical absorption, of each individual nanoparticles. In contrast, in the section of counting single nanoparticles, signals from different nanoparticles are detected as a function of time, which are identified individually because they are separated in time domain. In the section of imaging chemical processes of single nanoparticles, individual nanoparticles are resolved spatially and analyzed by different imaging techniques. Finally, the last section is devoted to single nanoparticle tracking, which is based on the dynamic movements of single nanoparticles revealed by time resolved images.