The chemical dynamics of nanosensors capable of single-molecule detection.

Recent advances in nanotechnology have produced the first sensor transducers capable of resolving the adsorption and desorption of single molecules. Examples include near infrared fluorescent single-walled carbon nanotubes that report single-molecule binding via stochastic quenching. A central question for the theory of such sensors is how to analyze stochastic adsorption events and extract the local concentration or flux of the analyte near the sensor. In this work, we compare algorithms of varying complexity for accomplishing this by first constructing a kinetic Monte Carlo model of molecular binding and unbinding to the sensor substrate and simulating the dynamics over wide ranges of forward and reverse rate constants. Methods involving single-site probability calculations, first and second moment analysis, and birth-and-death population modeling are compared for their accuracy in reconstructing model parameters in the presence and absence of noise over a large dynamic range. Overall, birth-and-death population modeling was the most robust in recovering the forward rate constants, with the first and second order moment analysis very efficient when the forward rate is large (>10(-3) s(-1)). The precision decreases with increasing noise, which we show masks the existence of underlying states. Precision is also diminished with very large forward rate constants, since the sensor surface quickly and persistently saturates.

Near-infrared fluorescent sensors based on single-walled carbon nanotubes for life sciences applications.

Many properties of single-walled carbon nanotubes (SWCNTs) make them ideal candidates for sensors, particularly for biological systems. Both their fluorescence in the near-infrared range of 820-1600 nm, where absorption by biological tissues is often minimal, and their inherent photostability are desirable attributes for the design of in vitro and in vivo sensors. The mechanisms by which a target molecule can selectively alter the fluorescent emission include primarily changes in emission wavelength (i.e., solvatochromism) and intensity, including effects such as charge-transfer transition bleaching and exciton quenching. The central challenge lies in engineering the nanotube interface to be selective for the analyte of interest. In this work, we review the recent development in this area over the past few years, and describe the design rules that we have developed for detecting various analytes, ranging from stable small molecules and reactive oxygen species (ROS) or reactive nitrogen species (RNS) to macromolecules. Applications to in vivo sensor measurements using these sensors are also described. In addition, the emerging field of SWCNT-based single-molecule detection using band gap fluorescence and the recent efforts to accurately quantify and utilize this unique class of stochastic sensors are also described in this article.

Single-molecule detection of H2O2 mediating angiogenic redox signaling on fluorescent single-walled carbon nanotube array.

Reactive oxygen species, specifically hydrogen peroxide (H(2)O(2)), activate signal transduction pathways during angiogenesis and therefore play an important role in physiological development as well as various pathophysiologies. Herein, we utilize a near-infrared fluorescent single-walled carbon nanotube (SWNT) sensor array to measure the single-molecule efflux of H(2)O(2) from human umbilical vein endothelial cells (HUVEC) in response to angiogenic stimulation. Two angiogenic agents were investigated: the pro-angiogenic cytokine, vascular endothelial growth factor A (VEGF-A) and the recently identified inorganic pro-angiogenic factor, europium(III) hydroxide in nanorod form. The nanosensor array consists ofa SWNT embedded within a collagen matrix that exhibits high selectivity and sensitivity to single molecules of H(2)O(2). A calibration from 12.5 to 400 nM quantifies the production of H(2)O(2) at nanomolar concentration in HUVEC with 1 s temporal and 300 nm spatial resolutions. We find that the production of H(2)O(2) following VEGF stimulation is elevated outside of HUVEC, but not for stimulation via nanorods, while increased generation is observed in the cytoplasm for both cases, suggesting two distinct signaling pathways.