Detection of nanoscale objects is highly desirable in various fields such as early-stage disease diagnosis, environmental monitoring and homeland security. Optical microcavity sensors are renowned for ultrahigh sensitivities due to strongly enhanced light-matter interaction. This review focuses on single nanoparticle detection using optical whispering gallery microcavities and photonic crystal microcavities, both of which have been developing rapidly over the past few years. The reactive and dissipative sensing methods, characterized by light-analyte interactions, are explained explicitly. The sensitivity and the detection limit are essentially determined by the cavity properties, and are limited by the various noise sources in the measurements. On the one hand, recent advances include significant sensitivity enhancement using techniques to construct novel microcavity structures with reduced mode volumes, to localize the mode field, or to introduce optical gain. On the other hand, researchers attempt to lower the detection limit by improving the spectral resolution, which can be implemented by suppressing the experimental noises. We also review the methods of achieving a better temporal resolution by employing mode locking techniques or cavity ring up spectroscopy. In conclusion, outlooks on the possible ways to implement microcavity-based sensing devices and potential applications are provided.

In this paper, a self-sensing carbon nanotube (CNT)/cement composite is investigated for traffic monitoring. The cement composite is filled with multi-walled carbon nanotubes whose piezoresistive properties enable the detection of mechanical stresses induced by traffic flow. The sensing capability of the self-sensing CNT/cement composite is explored in laboratory tests and road tests. Experimental results show that the fabricated self-sensing CNT/cement composite presents sensitive and stable responses to repeated compressive loadings and impulsive loadings, and has remarkable responses to vehicular loadings. These findings indicate that the self-sensing CNT/cement composite has great potential for traffic monitoring use, such as in traffic flow detection, weigh-in-motion measurement and vehicle speed detection.

Ultrasensitive nanoparticle detection holds great potential for early-stage diagnosis of human diseases and for environmental monitoring. In this work, we report for the first time, to our knowledge, single nanoparticle detection by monitoring the beat frequency of split-mode Raman lasers in high-Q optical microcavities. We first demonstrate this method by controllably transferring single 50-nm-radius nanoparticles to and from the cavity surface using a fiber taper. We then realize real-time detection of single nanoparticles in an aqueous environment, with a record low detection limit of 20 nm in radius, without using additional techniques for laser noise suppression. Because Raman scattering occurs in most materials under practically any pump wavelength, this Raman laser-based sensing method not only removes the need for doping the microcavity with a gain medium but also loosens the requirement of specific wavelength bands for the pump lasers, thus representing a significant step toward practical microlaser sensors.stimulated Raman scattering | optical microcavity | mode splitting | optical sensor | label free S timulated Raman scattering holds great potential for various photonic applications, such as label-free high-sensitivity biomedical imaging (1) and for extending the wavelength range of existing lasers (2), as well as for generating ultra-short light pulses (3). In high Q microcavities (4), stimulated Raman scattering, also called Raman lasing, has been experimentally demonstrated to possess ultra-low thresholds (5-12), due to the greatly increased light density in microcavities (13). Such microcavity Raman lasers hold great potential for sensing applications. In principle, Raman lasing initially occurs in the two initially degenerate counter propagating traveling cavity modes. These two modes couple to each other due to backscattering when a nanoscale object binds to the cavity surface. For a sufficiently strong coupling, in which the photon exchange rate between the two initial modes becomes larger than the rates of all of the loss mechanisms in the system, two new split cavity modes form (14-18) and lase simultaneously. Thus, by monitoring the beat frequency of the split-mode Raman lasers, ultrasensitive nanoparticle detection can be realized.In this work, we report, to our knowledge, the first experimental demonstration of single nanoparticle detection using split-mode microcavity Raman lasers. The sensing principle is first demonstrated in air, by controllably binding or removing single 50-nm-radius polystyrene (PS) nanoparticles to and from the cavity surface using a fiber taper (19) and measuring the changes in the beat frequency of the two split Raman lasers. Real-time single nanoparticle detection is then performed in an aqueous environment by monitoring the discrete changes in beat frequency of the Raman lasers, and a detection limit of 20 nm in particle radius is realized. This microcavity Raman laser sensing method holds several advantages. On the one hand, the beat frequency of the Raman las...

Abstract. We have investigated the mechanisms of assembly and transport to the cell surface of the mouse muscle nicotinic acetylcholine receptor (AM) in transiently transfected COS cells. In cells transfected with all four subunit cDNAs, AChR was expressed on the surface with properties resembling those seen in mouse muscle cells (Gu, Y., A . F. Franco, Jr., P D. Gardner, J. B. Lansman, J. R. Forsayeth, and Z. W. Hall. 1990. Neuron . 5:147-157) . When incomplete combinations of AChR subunits were expressed, surface binding of 'III-a-bungarotoxin was not detected except in the case of aOy which expressed <15% of that seen with all four subunits. Immunoprecipitation and sucrose gradient sedimentation experiments showed T RANSMEMBRANE ion channels comprise several families of proteins with a common structural design in which homologous subunits or protein domains surround a central aqueous pore (Unwin, 1989) . The simplest oligomeric channels are homopolymers ; others contain as many as four different polypeptide subunits. Although the structure and function of many ofthese channels is well understood, relatively little is known about how they are assembled . Indeed, the mechanisms of assembly of only a few oligomeric membrane proteins of any type have been extensively investigated (Carlin and Merlie, 1987;Rose and Doms, 1988;Hurtley and Helenius, 1989) .The most completely studied ion channel is the nicotinic actylcholine receptor (AChR)' from vertebrate muscle or from Torpedo electric organ (McCarthy et al., 1986;Claudio, 1989). The AChR is a pentamer with four different subunits whose stoichiometry is a2,Qyb . The subunits have highly homologous sequences and are presumably evolved from a common ancestor that formed a homo-oligomeric channel in which all of the subunits were interchangeable (Raftery et al ., 1980;Noda et al., 1983 ;Numa et al., 1983) . Each of the subunits is made as a single polypeptide chain (Anderson and Blobel, 1981), and the four are assembled into the complete oligomer in the endoplasmic reticulum (Smith et al., 1987;Gu et al., 1989b) . After synthesis of the polypeptide, the a chain undergoes a maturational step be- that in cells expressing pairs of subunits, aS and ay heterodimers were formed, but a# was not . When three subunits were expressed, abo and ayf complexes were formed. Variation of the ratios of the four subunit cDNAs used in the transfection mixture showed that surface AChR expression was decreased by high concentrations of S or y cDNAs in a mutually competitive manner. High expression of 6 or y subunits also each inhibited formation of a heterodimer with a and the other subunit . These results are consistent with a defined pathway for AChR assembly in which aS and ay heterodimers are formed first, followed by association with the a subunit and with each other to form the complete AM.

This paper reports a dramatic reduction in plasmon resonance line width of a single Au nanorod by coupling it to a whispering gallery cavity of a silica microfiber. With fiber diameter below 6 μm, strong coupling between the nanorod and the cavity occurs, leading to evident mode splitting and spectral narrowing. Using a 1.46-μm-diameter microfiber, we obtained single-band 2-nm-line-width plasmon resonance in an Au nanorod around a 655-nm-wavelength, with a quality factor up to 330 and extinction ratio of 30 dB. Compared to an uncoupled Au nanorod, the strongly coupled nanorod offers a 30-fold enhancement in the peak intensity of plasmonic resonant scattering.