Nanoparticles have become ubiquitous. They are present in various consumer products, from skincare to food. Meanwhile, massive quantities of nanoparticles continuously generated by industrial processes and vehicles have raised serious concerns about human health and the environment. The World Health Organization estimates that, each year, around two million premature deaths worldwide are caused by airborne chemical and biological particles. As interest in nanoparticle applications and safety concerns grow, there is an urgent need to assess their benefits and risks. This requires a technology that can rapidly detect and measure nanoparticles of varying size, shape, and chemical composition in diverse environments, including air, water, blood, and serum. Such a technology would also be pivotal in detecting and measuring pathogens (e.g., viruses, bacteria, microbes, and toxic nanoparticles).
A portable, real-time, in situ sensing system with single-nanoparticle resolution and nanometer sensitivity is key to investigating the properties and kinetic behavior of ultrasmall particles and structures as well as the physics resulting from their interactions. Such a system would help us to understand properties of the particles under various conditions. It would also help us to develop strategies against threats from hazardous chemical agents, biochemical warfare, or an outbreak of airborne contagious disease.
Recent progress in nanoelectromechanical and optical technologies has boosted the development of sensors with unprecedented capabilities. Among the many techniques and schemes, whispering-gallery-mode (WGM) optical resonators and nanocantilevers have shown great promise in tackling the challenges in achieving compact sensors with high sensitivity and reliability.1,2 The resonance structures enhance interactions between the sensing field and the target, which boosts the signal-to-noise ratio and increases sensitivity dramatically compared with waveguide-based sensing devices whose interaction time is limited by the physical length of the waveguide. The conventional approach to resonator-based sensing devices centers on monitoring the frequency shift of a single resonance. However, this method has several inherent limitations. The resonance shift varies with the position of the targets on the sensor. Furthermore, it is affected by instrument noise and many types of disturbances in the environment (e.g., temperature variation, mechanical instability, and irrelevant molecules), any of which can cause ‘false signals’ to be detected.
We have developed a novel self-referencing and position-independent sensing technique that overcomes the limitations of current resonator-based sensors while preserving the advantages offered by resonant structures.3 Specifically, we used mode splitting,4,5 a phenomenon resulting from the interaction of nanoparticles and an ultra-high-quality optical micro-resonator, to achieve an ultrasensitive, real-time, in situ sensing concept (see Figure 1). We have shown that a nanoparticle can ‘split’ a single resonance into a doublet in a microtoroid resonator. Although each of the split modes is affected by the particle position, applying the information from both modes allows us to develop a position-independent sensing scheme to accurately derive the size information of the nanoparticle. The two split modes, which reside in the same resonator and experience the same noise, form a self-referencing system that is more immune to noise. We have detected and measured single nanoparticles down to 30nm in radius using an on-chip WGM resonator for the first time. We have also found that it is possible to push the value down to several nanometers.3
Figure 1. Fiber-taper coupled ultra-high-quality microtoroid resonators on a silicon chip for single-nanoparticle detection and measurement. The nanoparticles in the evanescent field of the resonator (bright spots) lift the twofold degeneracy of a single whispering-gallery mode, which leads to two orthogonal standing waves (split modes) around the periphery of the toroid. The periodic patterns of the standing waves in the resonators are imaged on the reflecting silicon substrate.
Our recent experiments demonstrated nanoparticle-induced mode splitting in an aqueous environment, which could lead to the construction of mode-splitting-based biological and chemical sensors in liquid solutions. The next step is to further improve the design of the sensing system to approach the theoretical detection limit of measurable size for different types of nanoparticles and pathogens in various media. Our long-term goal is to achieve a new class of compact and inexpensive sensors capable of real-time monitoring and in situ characterization of nanoparticles with ultrahigh sensitivity and reliability.
Lan Yang, Jiangang Zhu, Sahin Kaya Ozdemir, Lina He, Woosung Kim, Faraz Monifi
Department of Electrical and Systems Engineering
Washington University in St. Louis
Saint Louis, MO
Lan Yang is an assistant professor. She received her PhD in applied physics from the California Institute of Technology. Her current research focuses on new functional photonic materials, ultra-high-quality optical resonators and on-chip photonic devices. In 2010 she received a National Science Foundation CAREER Award.
School of Physics
Lin Li, Da-ren Chen
Department of Energy, Environmental, and Chemical Engineering
Washington University in St. Louis
Saint Louis, MO
3. J. Zhu, S. K. Ozdemir, Y.-F. Xiao, L. Li, L. He, D.-R. Chen, L. Yang, On-chip single nanoparticle detection and sizing by mode splitting in an ultrahigh-Q microresonator, Nat. Photon. 4, pp. 46-49, 2010.